Samudra Kallol Mukherji

Project Report on Study of the processes involved in the manufacturing of Solar Cells amp Modules and

an overview of Solar Energy in general

A compilation of the raw data accumulated after the conclusion of a one month internship at

Webel SL Energy Systems Pvt Ltd

By

Bengal Engineering and Science University Shibpur

June July 2009

ACKNOWLEDGEMENT

We would also like to thank all the employees of Webel SL for their help and cooperation which went a long way in enriching this report

Special thanks are due to the Managing Director Mr SL Agarwal for giving us the opportunity

to pursue our summer training in Webel SL

Acknowledgement

CONTENTS

bull Introduction helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip1 bull An Overview of a Solar CellModule Manufacturing Unit helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip2 bull Need for Solar Energy helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip3 bull Raw materials needed helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip5 bull History of Photovoltaics helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip6 bull Solar cell manufacturing processes helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip7

Etching helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip8 Texturisation helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip10 Diffusion helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip12 Plasma Etching helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip16 PSG Removal helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip19 PECVD (Plasma Enhanced Chemical Vapor Deposition) helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip20 Screen Printing helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip22 Cell Testing and Sorting helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip25 Cell Testing Parameters helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip26

bull Solar Module helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip28 Module Manufacturing and Testing helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip29 Uses of Module helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip31

bull Uses of Solar Energy helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip33 bull Solar Energy Advantages Opportunities and Challenges helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip35 bull Conclusion helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip37

1

INTRODUCTION In recent years the world has taken huge strides towards accepting renewable sources of energy as valid sources of power With fast diminishing fossil fuel reserves and the polluting nature of coal and petroleum solar energy has emerged as a clean environmentally acceptable source of energy The mechanism of photoelectric power generation is simple from a theoretical point of view It owes its existence to the photon generated excitation of electrons across a semiconductor p-n junction The practical implementation of this useful phenomenon has given rise to the solar power industry In a series of manufacturing processes the raw semiconductor wafers are transformed to printed solar cells which are further integrated to solar modules and arrays As the world hurdles into an energy crisis and the fragile ecology comes under increasing strain due to pollution it is imperative that we look at alternative renewable sources of energy This report looks into the theoretical and practical aspects of Module manufacturing and the role solar energy has to play in the near and distant future Some of the appealing characteristics of solar energy are strikingly obvious these include no fuel consumption no pollution wide power handling capabilities and high power to weight ratio A more in-depth analysis is done in the pages that follow

2

AN OVERVIEW OF A SOLAR CELLMODULE MANUFACTURING UNIT

Manufacture of solar modules follows a series of well defined sequences There is scope for much modification and choice of raw materials andor processes that a manufacturing firm can adopt while producing modules However the basic methodology is the same no matter how different individual steps may be During the course of production engineers may tweak process parameters to achieve peak efficiency However the fundamental infrastructure that the firm must invest in are essentially constant among manufacturers The following paragraphs aim to provide the brief outline of any manufacturing unit regardless of the manufacturer A DI UNIT A Deionised (DI) Water plant is fundamental to any unit It provides water free from charged particles to all units that require it B ETCHING UNIT A series of polypropylene tanks form the etching and texturisation unit All equipment and tanks are labeled This has mechanized and automated robot arms that simplify the immersion and handling of wafer chests C DIFFUSION UNIT The diffusion unit has a multi tube diffusion furnace with its accessory units Diffusion system consists of the furnace load station cross flow box gas cabinet and control system The wafers are inserted in batches in quartz boats After diffusion the formation of the p-n junction has been completed we move into the plasma etching process This performs edge isolation which is crucial to prevent short circuits between the front and back surfaces of the wafer D PLASMA UNIT Plasma etching is done in an evacuated chamber in humidity controlled room Freon gas and RF power are the inputs along with the wafer Time varies from 28 minutes to 40 minutes This is followed by rinsing and drying E PSG Removal removes the phosphosilicate glass The next step leads to the manufacture of solar modules F PECVD (Plasma Enhanced Chemical Vapor Deposition) A solar module plant has a PECVD unit This deposits an antireflective layer on the wafer Silane and Ammonia are combined to give Silicon Nitride Hydrogen provides excellent surface and bulk passivization by saturating dangling bonds An abatement system is provided to detoxify harmful wastes To form the Metallization and front and back electrodes the process of SCREEN PRINTING is carried out This is typically a 3 phase process AgAl Ag and Al are printed in paste form and allowed to dry This is followed by the testing and sorting process whereby cells are arranged into various classes depending on their I-V curve (and hence efficiency) The entire process is automated The work of a Photovoltaic firm does not end with the creation of cells After Cell manufacturing the MODULE PRODUCTION UNIT comes into play As each cell is not sufficient to provide enough power for household and industrial purposes they are strung together to form modules Tabbing is done to provide connections for the stringing of cells This is followed by lamination to form a multilayered structure that protects the cell Framing is done to provide structural rigidity A junction box is provided for the tapping of electrical power Module testing is done to check if each module meets expectations In short these are the essential units and processes common to all photovoltaic cellmodule manufacturing plants

3

NEED FOR SOLAR ENERGY According to the World Health Organization 3 million people are killed worldwide by outdoor air pollution annually from vehicles and industrial emissions and 16 million indoors through the use of solid fuel Although technology has made oil extraction more efficient the world has to struggle to provide oil by using increasingly costly and less productive methods such as deep sea drilling and developing environmentally sensitive areas such as the Arctic National Wildlife Refuge Global surface temperature increased 074 plusmn 018 degC (133 plusmn 032 degF) during the last century The Intergovernmental Panel on Climate Change (IPCC) concludes that increasing greenhouse gas concentrations resulting from human activity such as fossil fuel burning and deforestation are responsible for most of the observed temperature increase since the middle of the 20th century Mitigation of global warming is accomplished through reductions in the rate of man-made greenhouse gas release The worlds population continues to grow at a quarter of a million people per day increasing the consumption of energy Currently fossil fuels provided around 66 of the worlds electrical power and 95 of the worlds total energy demands

The 1973 Oil crisis due to the OPEC embargo The 1979 (or second) oil crisis (due to the Iranian Revolution) the 1990 oil price shock (due to the First Gulf War) lead to drastic rise in the price of crude oil the most

4

indispensable source of energy for modern industry to function From the mid-1980s to September 2003 the inflation-adjusted price of a barrel of crude oil on NYMEX was generally under $25barrel During 2003 the price rose above $30 reached $60 by August 11 2005 and peaked at $14730 in July 2008

Thus fossil fuels are proven to be polluting and severely detrimental to human health We are paying the price for it in terms of respiratory diseases and a looming energy crisis As the oil prices are volatile and are steadily but unpredictably rising it is expedient to look beyond polluting fossil fuels as the sole source of power

Todays solar systems for solar heating are highly efficient and easy to install The initial cost is high but the payoff is long lasting Solar systems pay for themselves in about half their lifetime It is a free source of energy unlike oil coal or natural gas The environment impact of solar power is low Solar is clean power There are no emissions no maintenance costs and no unstable fuel costs Solar power is reliable with no outages There are no moving parts to replace or break Solar panels can withstand most extreme weather conditions Saving money energy conservation and clean power are the best benefits of solar energy

In response to the petroleum crisis the principles of green energy and sustainable living movements gain popularity This has led to increasing interest in alternate powerfuel research The Sun is 150 million kilometers away but is amazingly powerful Covering 4 of the worlds desert area with photovoltaic could supply all of the worlds electricity In such a situation development of solar energy should be a priority for every nation

5

RAW MATERIALS NEEDED

Along with the enormous capital investment required to successfully set up a solar cellmodule manufacturing plant it also requires a steady supply of raw materials The most basic raw materials is the silicon wafer itself which is usually bought from another company rather than prepared in-house Two types of crystalline silicon solar cell are single crystalline silicon solar cell which is known as monocrystalline silicon solar cell made by slicing wafers and polycrystalline silicon solar cell made by sawing a block of silicon first into bars and then wafers Other semiconductors such as gallium arsenide cadmium telluride and copper indium diselenide etc have both single crystalline and polycrystalline forms The solar cell which made by gallium arsenide has high efficiency ranging from 20-25 The first process Etching requires a number of chemicals HF HCL NaOH which may vary depending on the etching methodology (Acidic etching or Alkali etching) Deionised water is obtained from the DI plant IPA is used for cleaning purposes For diffusion an MRL diffusion furnace can be used which utilizes N2 amp O2

N2 flows through a bubbler of POCl3

for Phosphorus diffusion HF HCl and DI water are also used to clean the components (bubbler quartz tube etc) used in Diffusion For plasma etching we need a gas like Freon to provide the ions used in edge isolation Silane and Ammonia in gaseous form are needed to perform PECVD in order to deposit the anti reflective coating Silicon Nitride is formed due to the reaction of Silane and Ammonia Screen Printing is a three phase process The first phase is AgAl which could use the paste Dupont PV505 The second front side printing uses an Ag pastes (Dupont PV145) Third phase uses Aluminium paste (Analog 12D)

Module manufacturing requires its own set of raw materials It needs Tedlar EVA and Glass to form the multilayered structure Aluminium frames are needed to provide structural integrity and a junction box for output terminals Thus a manufacturing plant needs a variety of raw materials at the right time in the right quantity quality and purity A fall is any of these parameters will severely hamper the manufacturing process and negatively effect margins Thus ensuring a proper supply of this varied mix of materials is of paramount importance to the managers

6

HISTORY OF PHOTOVOLTAICS Most people are surprised to learn that photovoltaic technology actually dates back over 160 years French physicist Edmond Becquerel first described the photovoltaic (PV) effect in 1839 but it remained a curiosity of science for the next three quarters of a century At only 19 Becquerel found that certain materials would produce small amounts of electric current when exposed to light The effect was first studied in solids such as selenium by Heinrich Hertz in the 1870s Soon afterward selenium PV cells were converting light to electricity at 1 to 2 efficiency As a result selenium was quickly adopted in the emerging field of photography for use in light-measuring devices Major steps toward commercializing PV were taken in the 1940s and early 1950s when the Czochralski process was developed for producing highly pure crystalline silicon In 1954 scientists at Bell Laboratories depended on the Czochralski process to develop the first crystalline silicon photovoltaic cell which had an efficiency of 4 Although a few attempts were made in the 1950s to use silicon cells in commercial products it was the new space program that gave the technology its first major application In the 1970s research drives PV costs down 80 allowing for applications such as offshore navigation warning lights and horns lighthouses railroad crossings and remote use where utility-grid connections are too costly Despite these advances PV devices in 1970 were still too expensive for most mass market uses But in the mid-1970s rising energy costs sparked by a world oil crisis renewed interest in making PV technology more affordable Since then the federal government industry and research organizations have invested billions of dollars in research development and production Often industry and the government work together sharing the cost of PV research and development In the 1980s photovoltaic became a popular power source for consumer electronic devices including calculators watches radios lanterns and other small battery-charging applications

Todays commercial PV systems can convert from 11 to 17 of sunlight into electricity They are highly reliable and last 20 years or longer The cost of PV-generated electricity has dropped 15- to 20-fold and PV modules now cost around $6 per watt (W) and produce electricity for as little as 25 to 50 cents per kilowatt-hour (kWh)

7

Solar Cell Manufacturing Processes

8

ETCHING

INTRODUCTION

Etching is the process of using strong acid or mordant to cut into the unprotected parts of a metal surface to create a design in intaglio in the metal (the original processmdashin modern manufacturing other chemicals may be used on other types of material) As an intaglio method of printmaking it is along with engraving the most important technique for old master prints and remains widely used today But in wafer fabrication Etching refers to a process by which material is removed from the wafer ie either from the silicon substrate itself or from any film or layer of material on the wafer

DEFINITION

The treatment of a prepared metal surface with acid or other chemical reagent which by differential attack reveals the structure is known as etching In wafer fabrication etching process is generally classified in two classes Dry etching and Wet etching

Normally dry etching is not being used in this solar cells and module manufacturing unit So we will just go on a brief look on wet etching process

WET ETCHING It is a process that utilizes liquid chemicals or etchants to remove materials from the wafer usually in specific patterns defined by photo resist masks on the wafer Materials not covered by these masks are etched away by the chemicals while those covered by the masks are left almost intact A simple wet etching process may just consist of dissolution of the material to be removed in a liquid solvent without changing the chemical nature of the dissolved material In general however a wet etching process involves one or more chemical reactions that consume the original reactants and produce new species A basic wet etching process may be broken down in three steps as follows

1 Diffusion of the etchant to the surface for removal 2 Reaction between the etchant and the material being removed and 3 Diffusion of the reaction byproducts from the reacted surface

Wet etching can be both isotropic ie it proceeds in all directions at the same rate and anisotropic ie it proceeds in only one direction Isotropic Wet Etching A process which proceeds at the same rate in all directions is known as isotropic wet etching The etchants used in this process are isotropic in nature that etches away even the portion of material that is directly under the mask (usually in the shape of a quarter-circle) since its horizontal etching rate is the same as its vertical rate When an isotropic etchant eats away a portion of the material under the mask the etched film is said to have undercut the mask The amount of undercutting is a measure of an etching parameter known as the bias Bias is simply defined as the difference between the lateral dimensions of the etched image and the masked image Thus the mask used in etching must compensate for whatever bias an etchant is known to produce in order to create the desired feature on the wafer

Figure 1 Isotropic wet etching

9

Anisotropic Wet Etching A process which proceeds in only one direction is known as anisotropic wet etching It removes material in the vertical direction only since it will follow the mask patterns on the wafer very faithfully leaving any material covered by mask material basically untouched Since isotropic wet etching process has high bias values which are not practical for use in pattern images which has features of measuring less than 3 microns thus wafer patterns of less than 3 microns are wet etched anisotropically instead of isotropic

Figure 2 Anisotropic wet etching

Advantages of wet etching 1 Low cost 2 High reliability 3 High throughput 4 Excellent selectivity in most cases with respect to both mask and substrate materials

Disadvantages of wet etching 1 Limited resolution 2 Higher safety risks due to the direct chemical exposure of the personnel 3 High cost of etchants in some cases 4 Problems related to the resists loss of adhesion to the substrate 5 Problems related to the formation of bubbles which inhibit the etching process where they are present and 6 Problems related to incomplete or non-uniform etching

Before moving on to texturization we first of all must do Saw damage removal which is also called as 1st Etch First Etch When ingots obtained from the Czochralski method are sliced into wafers by saw then it leaves some damaged marks on the wafers if it is not being sliced properly which makes the surface non-uniform So to make the surface uniform and free of error it becomes mandatory to do saw damage removal or 1st

1 First wafers are kept in the solution of 300ml DI water and 10kg NaOH(RSE) within the temperature range of 75⁰plusmn 2⁰C for 1min The reaction which takes place in this step is

etching Steps involved are

Si + 2NaOH + H2O (DI) -gt Na3SiO2 + H2 + H2

2 Finally these etched wafers are washed with DI water O

10

TEXTURIZATION INTRODUCTION Since 1960 for the improvement of the cell performance texturization has been attempted Texturization of front andor back surfaces of a solar cell surfaces improve their performances The reflection from chemically textured wafer reduced from 34 for a polished silicon surface to 11 Many researchers have reported the selective etching of silicon of lt100gt orientation of using sodium or potassium hydroxide solution OBJECTIVES OF TEXTURIZATION The major objectives of texturization are-

1 Reduction of front surface reflectance 2 Increase in the path length of the entered light of lower wavelengths by oblique trajectory within the

cell and absorption of light closer to the junction 3 Optical trapping of weakly absorbed light by multiple internal reflections from the top and bottom

surfaces of the cell thereby also increasing the optical path length The increased path length with an oblique trajectory leads to an effective increase in the absorption co-efficient

All these effects lead to realization of higher performance thick cells and also efficient solar cells of comparatively lower thickness which significantly reduces the material cost without degrading the cell performance MODELS OF TEXTURIZATION To reduce the reflection co-efficient and to trap weak absorbed light various texturing geometries are being employed The different texturing geometries include

1 Lambertian geometry 2 Upright pyramid geometry 3 Slat or Micro-grooved geometry 4 Inverted pyramid geometry 5 Simple prism pyramid geometry 6 Grating geometry 7 Three perpendicular planes (3PP) geometry 8 Perpendicular slat geometry 9 Porous silicon

Now in large number of manufacturing industries Inverted pyramid texturing geometry scheme is being used these days Letrsquos have a look on this scheme very briefly Inverted Pyramid Geometry The light trapping properties of the inverted pyramid geometry has been first investigated by Smith and Rohatgi The increase in current for the type of textured cell is due to incorporation of al the three effects necessary for an efficient light trapping design The front surface reflectance is reduced by providing the opportunity for a portion of the incoming light to undergo a triple bounce thereby reducing the overall front surface reflectance The increase in path length and light trapping efficiency means that a larger fraction of the light which has entered the cell will be absorbed before exiting the cell The short wavelength spectral response analysis at the texturing angle of 5375ordm indicates that 37 of the incoming light experiences a triple bounce on the front surface of the inverted pyramid geometry and the rest percentage gets double bounce The inverted pyramids on a 100microm cell with a two layer AR coating is estimated to give 402mAcm2 and cell efficiency of 24 with realistic cell design and material parameters

11

This texturization method can be employed on both mono-crystalline and multi-crystalline substrates Letrsquos have a look on this Mono-crystalline substrates For mono-crystalline substrate it is advantageous to make use of the anisotropic etching properties of Si in an alkaline solution As the 111 planes get etched more slowly than other crystal planes 111 facets are developed On lt100gtwafers this leads to pyramidal shapes at the surface that are particularly effective in reducing reflectance A regular array of pyramidal pits with facets at 547ordm to the horizontal plane mdash called inverted pyramids mdash can be formed using oxide etch mask with reflectances as low as 8 without ARC

Multi-crystalline Substrates Due to the anisotropic nature of multi-crystalline silicon substrate alkaline random texturing is not effective Some grains remain untextured leading to a high average reflectance So for texturing multi-crystalline silicon wafers should be etched in an acid mixture of HF and HNO3 because HNO3 tends to oxidize the surface while the HF etches the oxide away These acidic iso-texturing results in lower reflection than traditional anisotropic etching on multi-crystalline material and better conversion efficiency

STANDARD NAOH-IPA TEXTURIZATION APPROACH In order to achieve good uniformity of pyramidal structure on the silicon surface a mixture of NaOH or KOH and isopropyl alcohol (IPA) is generally used for texturization of mono-crystalline silicon solar cell For better texturization the interfacial energy between silicon and ionized electrolyte chemical solution should be reduced to achieve sufficient wettability for the silicon surface which will enhance the pyramid nucleation From a manufacturability point of view it is appealing to combine the saw damage removal and alkaline texturing in one single step in which case a trade-off has to be made between process speed and quality of the surface treatment

Fig1 Inverted Pyramid Textured Surface

Fig3 Textured multi-crystalline Si surface

Fig2 Anisotropic Etching of Mono-crystalline Si

12

DIFFUSION

INTRODUCTION

Diffusion the movement of a chemical species from an area of high concentration to an area of lower concentration is one of the two major processes by which chemical species or dopants are introduced into a semiconductor (the other one being ion implantation) The controlled diffusion of dopants into silicon to alter the type and level of conductivity of semiconductor materials is the foundation of forming a p-n junction The mathematics that govern the mass transport phenomena of diffusion are based on Ficks laws There are two laws according to Fick Letrsquos have a look on these laws First law Whenever an impurity concentration gradient partCpartx exists in a finite volume of a matrix substance (the silicon substrate in this context) the impurity material will have the natural tendency to move in order to distribute itself more evenly within the matrix and decrease the gradient Given enough time this flow of impurities will eventually result in homogeneity within the matrix causing the net flow of impurities to stop The mathematics of this transport mechanism was formalized in 1855 by Fick who postulated that the flux of material across a given plane is proportional to the concentration gradient across the plane Thus Ficks First Law states

J = -D ( partC(xt)partx ) Where J is the flux D is the diffusion constant for the material that is diffusing in the specific solvent and partC(xt)partx is the concentration gradient The diffusion constant of a material is also referred to as diffusion coefficient or simply diffusivity It is expressed in units of length2time such as microm2hour The negative sign of the right side of the equation indicates that the impurities are flowing in the direction of lower concentration Second Law Ficks First Law does not consider the fact that the gradient and local concentration of the impurities in a material decreases with an increase in time an aspect thats important to diffusion processes The flux J1 of impurities entering a section of a bar with a concentration gradient is different from the flux J2 of impurities leaving the same section From the law of conservation of matter the difference between J1 and J2 must result in a change in the concentration of impurities within the section (assuming that no impurities are formed or consumed in the section) This is Ficks Second Law which states that the change in impurity concentration over time is equal to the change in local diffusion flux or

partC(xt)partt = - partJpartx or from Ficks First Law

partC(xt)partt = part(DpartC(xt)partx)partx If the diffusion coefficient is independent of position such as when the impurity concentration is low then Ficks Second Law may be further simplified into the following equation

partC(xt)partt = D part2C(xt)partx2 The most important property of diffusion process in the manufacture of silicon solar cell is the formation of p-n junction by doping n-type material ie phosphorous on the p-type ie boron doped silicon textured wafer playing an essential role for photovoltaic effect Let us have a brief descriptive look on the properties of p-n junction P-N JUNCTION A p-n junction is a junction formed by combining p-type and n-type semiconductors together in very close contact

13

Principle of p-n junction The most common type of solar cell is a large p-n junction where the free carrier pairs emitted by light energy are separated by the junction and contribute to the current In its simplest form it consists of a junction formed between n-type and p-type semiconductors either of the same material (homo-junction) or different materials (hetero-junction) The band-structure of the two differently doped sides with respect to their Fermi levels can be seen in adjacent figure1

When the two halves are brought together the Fermi levels on either side are forced in to coincidence causing the valence and conduction bands to bend (Figure 2)

These bent bands represent a built-in electric field over what is referred to as the depletion region When a photon with a energy greater than the band-gap of the semiconductor passes through the solar cell it may be absorbed by the material This absorption takes the form of a band-to-band electronic transition so an electronhole pair is produced If these carriers can diffuse to the depletion region before they recombine then they are separated by the electric field causing one quantum of charge to flow through an external load This is the origin of the solar cells photocurrent and is shown in Figure 3

14

SOLAR CELL AS A P-N JUNCTION A solar cell is essential a p-n junction with large surface area The n type material is kept thin to allow light to pass through to the p-n junction

Light travels in packets of energy called photons The generation of electric current happens inside the depletion zone of the PN junction The depletion region as explained previously with the diode is the area around the PN junction where the electrons from the N-type silicon have diffused into the holes of the P-type material When a photon of light is absorbed by one of these atoms in the N-Type silicon it will dislodge an electron creating a free electron and a hole The free electron and hole has sufficient energy to jump out of the depletion zone If a wire is connected from the cathode (N-type silicon) to the anode (P-type silicon) electrons will flow through the wire The electron is attracted to the positive charge of the P-type material and travels through the external load (meter) creating a flow of electric current The hole created by the dislodged electron is attracted to the negative charge of N-type material and migrates to the back electrical contact As the electron enters the P-type silicon from the back electrical contact it combines with the hole restoring the electrical neutrality

DIFFUSION PROCESS

The first layer formation after doing texturization of wafer is performed in this process by doping the substrate with n-type dopant N-doping can be accomplished by depositing n dopant onto the substrate and then heating the substrate to drive the n dopant into the substrate Gaseous diffusion (for example POCl3 horizontal tube diffusion including temperatures of from 700 to 1000degC preferably 800 to 900degC) can be used to deposit the n-dopant onto the substrate surface to create an n-doped layer and a shallow p-n junction proximal to the substrate surface In this process if wafers are loaded onto the quartz boat back to back in the same lot then double diffusion process takes place and if single wafers are being loaded then single diffusion takes place On the quartz boat wafers can be loaded in two methods

1 Parallel diffusion loading of wafers parallel to the gas flow 2 Perpendicular diffusion loading of wafers perpendicular to the gas flow

Diffusion process is comprised of the following steps 1 Idle state-In this state the load center and source zone are set at 800ordmC and no process is

carried out in the furnace 2 Load-During this sequence the silicon carbide paddle moves at a speed of 45 rpm and the

maximum time taken is 7min 3 Ramp-The cold wafers loaded into the tubes bring down the temperature of the tube The tube

is then reheated to reach the set point temperature and the time taken is 7min

15

4 Stabilization-During this phase any overshooting of temperature during ramp-up phase is controlled to the predetermined set value and the time taken is 10min

5 Pre-deposition-Any excess POCL3 present in vapour form inside the quartz bubbler is vent out before the deposition starts by N2

6 Deposition- N purging in 5 sec

2 is bubbled through liquid POCL3 in presence of O2 to form P2O5

7 Post-deposition-During this phase O in paste form

2 and N2

8 N

flows to drive in the phosphorous from wafer surface to the junction

2

9 Unload-During this phase the SiC paddle moves slowly out of the furnace at set speed of 45rpm and maximum time taken is 7min

Purge-This phase is also known as drive in diffusion which is designed to evenly distribute the impurities into wafer and reduce the generally undesirable high concentration in the wafer surface and the maximum time taken is 7min

10 End-The tube is set to 800⁰C temperature in idle state

16

PLASMA ETCHING

INTRODUCTION

Plasma etching is one type of dry etching which uses plasma to produce chemically reactive species from inert gases The reactive gases are then made to react with the material to be etched The main objective of this process is to remove edge shunts present on the cell which leads to 80 loss and thus it is also termed as Edge isolation method Since edge isolation is the main motive of this process so first let us see that why edge isolation is so important

Edge isolation

In the standard processing sequence the entire wafer is phosphorous diffused The contact to the p-type base is made by firing an Al containing paste that produces a high doping of Al thus over-compensating the shallow emitter diffusion However there is still a direct conductive path from the emitter on the front side to the rear p contact Therefore a large portion of the photo-generated current can directly flow through the emitter to the back contact instead of flowing through the external circuit This current path is modeled in the two-diode model of the solar cell as the parallel resistance R

where R

p

s and Rp are the series and parallel resistance respectively j01 and j02 represent the first and second diode saturation current densities respectively n1 and n2 are the first and second ideality diode factors respectively Vth = ktq is the thermal voltage and jL

In this model the first diode accounts for emitter and bulk recombination with a recombination rate assumed to be proportional to the excess carrier density This leads to an ideality factor of n

is the light generated current density The two diode model is as shown below

1=1 The second diode is associated with the recombination in the space charge region that is assumed to be independent of the injection level leading to n2=2 The effect of increased j02 and Rp can be shown by model calculations for ideal solar cells in the figure below for a fill factor potential of 84 j02 must be less than 2x10-9 Acm2 and Rp must be greater than approximately 2000Ω cm2 if the respective parameters contribute solely to the fill factor loss

17

This increase in Rp is achieved by the edge isolation process that interrupts the parasitic current path as shown in fig2 below

Thus edge isolation is required in solar cell to increase open circuit voltage and thus leading to greater efficiency

Now we will move on to plasma etching process to see that how it works actually in solar cell and module manufacturing industries

Plasma Etching Process

After diffusion a layer of phosphosilicate glass is formed on the edges and rear surface of diffused wafer The process generally requires adequate physical masking of the front surface while exposing the full back of the cell to the dry plasma environment In an embodiment of this process cells are slotted face to face

Fig2Sketch of an industrial screen-printed solar cell The emitter diffusion is present on the complete wafer surface and gets locally overcompensated by printing and firing Al containing metal paste as back contact Without edge-isolation light generated e-s can flow through the emitter region to the back contact which corresponds to the small series resistance in the two diode model

18

into inert carriers so that the plasma chemistry can effectively react with the back of the cells thereby removing excess material from the second coating that might have reached the back of the cell This process is preferably performed via the generation of an RF (13 6 MHz) plasma using a mix of Freon 14 (CF4) with oxygen (O2) and or nitrogen (N2) in a ratio which depends on the chamber and load geometries

19

PHOSPHOSILICATE GLASS REMOVAL INTRODUCTION In diffusion process during deposition phase N2 gas is bubbled through liquid POCL3

The reaction related to this phase is given as follows

1 4POCL3 + 3O2 -gt 2P2O5 + 6Cl 2 2P

2 2O5 + 5Si -gt 5SiO2

+ 4P (PSG)

P2O5

So it becomes mandatory to remove phosphosilicate glass by chemicals which may act as recombination sites or may contribute towards the surface density defect and hence all these defects may contribute to a lower voltage and current

obtained in first reaction step is in paste form and is transferred to the Teflon duct to the process tube which then reacts with silicon of which the wafer is made And thus when this phosphorous pentoxide reaches silicon surface then second reaction takes place and thus a silicon dioxide layer and phosphosilicate glass ie PSG forms on the wafer surface Formation of this dead layer on the wafer surface contributes to a lower efficiency

PROCESS OF REMOVAL Following are the steps that should be taken into consideration for the removal of phosphosilicate glass (PSG) from the wafer surface

1 Firstly wafers are washed combinedly with HF HCL and DI water in a tank for 1min 2 Again wafers are washed with HF and DI water for 2min in the other tank 3 For another 1 min DI water is being sprayed on the diffused wafers 4 Next wafers are washed with DI water subsequently on an interval of 30 sec 5 Finally wafers are kept in the OVEN for 10mins within the temperature range of 95plusmn5ordmC

20

PLASMA ENHANCED CHEMICAL VAPOUR DEPOSITION INTRODUCTION In recent years the low-temperature (le 400ordmC) surface passivation of crystalline silicon solar cells by means of PECVD silicon nitride films has attracted an increasing attention of the PV community These films have proven to be capable of combining an outstanding surface passivation quality with excellent anti-reflection properties On typical p-type silicon solar cells substrate it has been shown that the surface passivation quality of very silicon rich Si3N4

we must first know that whatrsquos the use of silicon nitride coating on the surface of the wafer

films is superior to that of state-of-the-art high temperature silicon dioxide Before proceeding to the details of PECVD system

WHY SILICON NITRIDE SURFACE COATING IS PREFERRED IN PECVD PROCESS It is preferred as a surface coating because it functions as An effective diffusion mask An effective metallization diffusion barrier An anti-reflective coating Provide surface and bulk passivation

Preferably hydrogen is trapped in such a silicon nitride surface coating PECVD SYSYTEM PECVD or Plasma Enhanced Chemical Vapor Deposition is one of the methods of different Chemical Vapor Deposition (CVD) technique In PECVD plasma is being used to decompose a reactant gas such as silane (SiH4) to produce reaction products that precipitate on the surface of the substrate as a new layer

Working Principle Inside a PECVD reactor a strong electric field ignites plasma between two electrodes one of which holds the substrate This plasma ignition cracks the molecular bonds of the process gas which in turn are able to crack more process gas molecules before reaching the surface of the substrate The glow discharge used in this reactor is created by applying an RF field to a low-pressure gas creating free electrons within the discharge region The electrons are sufficiently energized by the electric field that gas-phase dissociation and ionization of the reactant gases occur when the free electrons collide with them Energetic species are then adsorbed on the film surface where they are subjected to ion and electron bombardment rearrangements reactions with other species new bond formation and film formation and growth Eventually a new layer is deposited on the substrate For example when silane is used as precursor plasma ignition frees up Si and SiH radicals which also crack more silane molecules on their way to the surface of the substrate where silicon is deposited PROCESS DETAILS OF PECVD In this system wafers coming from the previous state ie PSG removal step are being loaded onto the graphite boat where the individual wafers are held by three pins inside the boat It is absolutely necessary for the wafers to touch all the three pins otherwise there will remain a possibility of poor deposition in which

Figure Anti-reflective coating made out of silicon nitride deposited by the PECVD technique whereby silane and ammonia is reacted in a furnace The major color of coating looks blue but the color could also look like purple blue or light blue due to different thickness of coating

21

the HF generator fails to generate the required power for process deposition While loading the double diffusion wafers it should be ensured that deposition side should be correctly loaded The wafers are being loaded parallel to the boat After loading the trolley along with the boat is placed inside the machine and then after filling the input parameters of the CMI interface used here and the process tube press the RUN button and then machine will be in auto mode The handling mechanism automatically picks up the boat and places it on the desired paddle corresponding to the selected for the specified job And then process STARTS

22

SCREEN PRINTING INTRODUCTION In screen printing on wafer-based solar PV cells the mesh and buses of silver are printed on the front furthermore the buses of silver are printed on the back Subsequently aluminum paste is dispensed over the whole surface of the back for passivation and surface reflection [14] One of the parameters that can vary and can be controlled in screen printing is the thickness of the print This makes it useful for some of the techniques of printing solar cells electronics etc Screen printing is one of the most critical processes to maintain high yield Solar wafers are becoming thinner and larger so careful printing is required to maintain a lower breakage rate On the other hand high throughput at the printing stage improves the throughput of the whole cell production line PROCESS This process is divided in three phase-

1 Phase I- AgAl printing 2 Phase II- Ag printing 3 Phase III- Al printing

PHASE I- AgAl PRINTING This phase is required to print the back bus bar for tabbing purpose The AgAl paste used in this printing is Dupont (PV-505) and the consumption of the paste per wafer is 011 gm And the screen type used is of the following specification

1 Mesh size- 200microm 2 Tension- 24 N-cm 3 Mesh angle- 225ordm 45ordm 4 Emulsion thickness- 24microm

Process details During printing the paste is poured on the screen and is spread over it with the help of polypropylene spatula Then the wafers (ARC coated) are pasted with the sunny side down on the loader and picked one by one automatically with the start of the print command Once printing starts the wafers are placed one by one between the tool paste and the screen and then the squeegee moves over the screen and allow the paste through the exposed parts on the screen and gets pasted on the wafers Thus in this way the wafers are printed one by one Two most important parameters to be controlled during this printing are

1 Squeegee Pressure 2 Snap off distance

The next step after printing is drying step DRYING In this step wafers are passed through the dryer with a temperature range of 180ordm-400ordmC PHASE II- Ag PRINTING This phase is required to print the front bus bar for tabbing purpose The Ag paste used in this printing is Dupont (PV-145) and the consumption of the paste per wafer is 014 gm And the screen type used is of the following specification

1 Mesh size- 325microm 2 Tension- 24 N-cm 3 Mesh angle- 225ordm 4 Emulsion thickness- 24microm

ACKNOWLEDGEMENT

We would also like to thank all the employees of Webel SL for their help and cooperation which went a long way in enriching this report

Special thanks are due to the Managing Director Mr SL Agarwal for giving us the opportunity

to pursue our summer training in Webel SL

Acknowledgement

CONTENTS

bull Introduction helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip1 bull An Overview of a Solar CellModule Manufacturing Unit helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip2 bull Need for Solar Energy helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip3 bull Raw materials needed helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip5 bull History of Photovoltaics helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip6 bull Solar cell manufacturing processes helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip7

Etching helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip8 Texturisation helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip10 Diffusion helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip12 Plasma Etching helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip16 PSG Removal helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip19 PECVD (Plasma Enhanced Chemical Vapor Deposition) helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip20 Screen Printing helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip22 Cell Testing and Sorting helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip25 Cell Testing Parameters helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip26

bull Solar Module helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip28 Module Manufacturing and Testing helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip29 Uses of Module helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip31

bull Uses of Solar Energy helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip33 bull Solar Energy Advantages Opportunities and Challenges helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip35 bull Conclusion helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip37

1

INTRODUCTION In recent years the world has taken huge strides towards accepting renewable sources of energy as valid sources of power With fast diminishing fossil fuel reserves and the polluting nature of coal and petroleum solar energy has emerged as a clean environmentally acceptable source of energy The mechanism of photoelectric power generation is simple from a theoretical point of view It owes its existence to the photon generated excitation of electrons across a semiconductor p-n junction The practical implementation of this useful phenomenon has given rise to the solar power industry In a series of manufacturing processes the raw semiconductor wafers are transformed to printed solar cells which are further integrated to solar modules and arrays As the world hurdles into an energy crisis and the fragile ecology comes under increasing strain due to pollution it is imperative that we look at alternative renewable sources of energy This report looks into the theoretical and practical aspects of Module manufacturing and the role solar energy has to play in the near and distant future Some of the appealing characteristics of solar energy are strikingly obvious these include no fuel consumption no pollution wide power handling capabilities and high power to weight ratio A more in-depth analysis is done in the pages that follow

2

AN OVERVIEW OF A SOLAR CELLMODULE MANUFACTURING UNIT

Manufacture of solar modules follows a series of well defined sequences There is scope for much modification and choice of raw materials andor processes that a manufacturing firm can adopt while producing modules However the basic methodology is the same no matter how different individual steps may be During the course of production engineers may tweak process parameters to achieve peak efficiency However the fundamental infrastructure that the firm must invest in are essentially constant among manufacturers The following paragraphs aim to provide the brief outline of any manufacturing unit regardless of the manufacturer A DI UNIT A Deionised (DI) Water plant is fundamental to any unit It provides water free from charged particles to all units that require it B ETCHING UNIT A series of polypropylene tanks form the etching and texturisation unit All equipment and tanks are labeled This has mechanized and automated robot arms that simplify the immersion and handling of wafer chests C DIFFUSION UNIT The diffusion unit has a multi tube diffusion furnace with its accessory units Diffusion system consists of the furnace load station cross flow box gas cabinet and control system The wafers are inserted in batches in quartz boats After diffusion the formation of the p-n junction has been completed we move into the plasma etching process This performs edge isolation which is crucial to prevent short circuits between the front and back surfaces of the wafer D PLASMA UNIT Plasma etching is done in an evacuated chamber in humidity controlled room Freon gas and RF power are the inputs along with the wafer Time varies from 28 minutes to 40 minutes This is followed by rinsing and drying E PSG Removal removes the phosphosilicate glass The next step leads to the manufacture of solar modules F PECVD (Plasma Enhanced Chemical Vapor Deposition) A solar module plant has a PECVD unit This deposits an antireflective layer on the wafer Silane and Ammonia are combined to give Silicon Nitride Hydrogen provides excellent surface and bulk passivization by saturating dangling bonds An abatement system is provided to detoxify harmful wastes To form the Metallization and front and back electrodes the process of SCREEN PRINTING is carried out This is typically a 3 phase process AgAl Ag and Al are printed in paste form and allowed to dry This is followed by the testing and sorting process whereby cells are arranged into various classes depending on their I-V curve (and hence efficiency) The entire process is automated The work of a Photovoltaic firm does not end with the creation of cells After Cell manufacturing the MODULE PRODUCTION UNIT comes into play As each cell is not sufficient to provide enough power for household and industrial purposes they are strung together to form modules Tabbing is done to provide connections for the stringing of cells This is followed by lamination to form a multilayered structure that protects the cell Framing is done to provide structural rigidity A junction box is provided for the tapping of electrical power Module testing is done to check if each module meets expectations In short these are the essential units and processes common to all photovoltaic cellmodule manufacturing plants

3

NEED FOR SOLAR ENERGY According to the World Health Organization 3 million people are killed worldwide by outdoor air pollution annually from vehicles and industrial emissions and 16 million indoors through the use of solid fuel Although technology has made oil extraction more efficient the world has to struggle to provide oil by using increasingly costly and less productive methods such as deep sea drilling and developing environmentally sensitive areas such as the Arctic National Wildlife Refuge Global surface temperature increased 074 plusmn 018 degC (133 plusmn 032 degF) during the last century The Intergovernmental Panel on Climate Change (IPCC) concludes that increasing greenhouse gas concentrations resulting from human activity such as fossil fuel burning and deforestation are responsible for most of the observed temperature increase since the middle of the 20th century Mitigation of global warming is accomplished through reductions in the rate of man-made greenhouse gas release The worlds population continues to grow at a quarter of a million people per day increasing the consumption of energy Currently fossil fuels provided around 66 of the worlds electrical power and 95 of the worlds total energy demands

The 1973 Oil crisis due to the OPEC embargo The 1979 (or second) oil crisis (due to the Iranian Revolution) the 1990 oil price shock (due to the First Gulf War) lead to drastic rise in the price of crude oil the most

4

indispensable source of energy for modern industry to function From the mid-1980s to September 2003 the inflation-adjusted price of a barrel of crude oil on NYMEX was generally under $25barrel During 2003 the price rose above $30 reached $60 by August 11 2005 and peaked at $14730 in July 2008

Thus fossil fuels are proven to be polluting and severely detrimental to human health We are paying the price for it in terms of respiratory diseases and a looming energy crisis As the oil prices are volatile and are steadily but unpredictably rising it is expedient to look beyond polluting fossil fuels as the sole source of power

Todays solar systems for solar heating are highly efficient and easy to install The initial cost is high but the payoff is long lasting Solar systems pay for themselves in about half their lifetime It is a free source of energy unlike oil coal or natural gas The environment impact of solar power is low Solar is clean power There are no emissions no maintenance costs and no unstable fuel costs Solar power is reliable with no outages There are no moving parts to replace or break Solar panels can withstand most extreme weather conditions Saving money energy conservation and clean power are the best benefits of solar energy

In response to the petroleum crisis the principles of green energy and sustainable living movements gain popularity This has led to increasing interest in alternate powerfuel research The Sun is 150 million kilometers away but is amazingly powerful Covering 4 of the worlds desert area with photovoltaic could supply all of the worlds electricity In such a situation development of solar energy should be a priority for every nation

5

RAW MATERIALS NEEDED

Along with the enormous capital investment required to successfully set up a solar cellmodule manufacturing plant it also requires a steady supply of raw materials The most basic raw materials is the silicon wafer itself which is usually bought from another company rather than prepared in-house Two types of crystalline silicon solar cell are single crystalline silicon solar cell which is known as monocrystalline silicon solar cell made by slicing wafers and polycrystalline silicon solar cell made by sawing a block of silicon first into bars and then wafers Other semiconductors such as gallium arsenide cadmium telluride and copper indium diselenide etc have both single crystalline and polycrystalline forms The solar cell which made by gallium arsenide has high efficiency ranging from 20-25 The first process Etching requires a number of chemicals HF HCL NaOH which may vary depending on the etching methodology (Acidic etching or Alkali etching) Deionised water is obtained from the DI plant IPA is used for cleaning purposes For diffusion an MRL diffusion furnace can be used which utilizes N2 amp O2

N2 flows through a bubbler of POCl3

for Phosphorus diffusion HF HCl and DI water are also used to clean the components (bubbler quartz tube etc) used in Diffusion For plasma etching we need a gas like Freon to provide the ions used in edge isolation Silane and Ammonia in gaseous form are needed to perform PECVD in order to deposit the anti reflective coating Silicon Nitride is formed due to the reaction of Silane and Ammonia Screen Printing is a three phase process The first phase is AgAl which could use the paste Dupont PV505 The second front side printing uses an Ag pastes (Dupont PV145) Third phase uses Aluminium paste (Analog 12D)

Module manufacturing requires its own set of raw materials It needs Tedlar EVA and Glass to form the multilayered structure Aluminium frames are needed to provide structural integrity and a junction box for output terminals Thus a manufacturing plant needs a variety of raw materials at the right time in the right quantity quality and purity A fall is any of these parameters will severely hamper the manufacturing process and negatively effect margins Thus ensuring a proper supply of this varied mix of materials is of paramount importance to the managers

6

HISTORY OF PHOTOVOLTAICS Most people are surprised to learn that photovoltaic technology actually dates back over 160 years French physicist Edmond Becquerel first described the photovoltaic (PV) effect in 1839 but it remained a curiosity of science for the next three quarters of a century At only 19 Becquerel found that certain materials would produce small amounts of electric current when exposed to light The effect was first studied in solids such as selenium by Heinrich Hertz in the 1870s Soon afterward selenium PV cells were converting light to electricity at 1 to 2 efficiency As a result selenium was quickly adopted in the emerging field of photography for use in light-measuring devices Major steps toward commercializing PV were taken in the 1940s and early 1950s when the Czochralski process was developed for producing highly pure crystalline silicon In 1954 scientists at Bell Laboratories depended on the Czochralski process to develop the first crystalline silicon photovoltaic cell which had an efficiency of 4 Although a few attempts were made in the 1950s to use silicon cells in commercial products it was the new space program that gave the technology its first major application In the 1970s research drives PV costs down 80 allowing for applications such as offshore navigation warning lights and horns lighthouses railroad crossings and remote use where utility-grid connections are too costly Despite these advances PV devices in 1970 were still too expensive for most mass market uses But in the mid-1970s rising energy costs sparked by a world oil crisis renewed interest in making PV technology more affordable Since then the federal government industry and research organizations have invested billions of dollars in research development and production Often industry and the government work together sharing the cost of PV research and development In the 1980s photovoltaic became a popular power source for consumer electronic devices including calculators watches radios lanterns and other small battery-charging applications

Todays commercial PV systems can convert from 11 to 17 of sunlight into electricity They are highly reliable and last 20 years or longer The cost of PV-generated electricity has dropped 15- to 20-fold and PV modules now cost around $6 per watt (W) and produce electricity for as little as 25 to 50 cents per kilowatt-hour (kWh)

7

Solar Cell Manufacturing Processes

8

ETCHING

INTRODUCTION

Etching is the process of using strong acid or mordant to cut into the unprotected parts of a metal surface to create a design in intaglio in the metal (the original processmdashin modern manufacturing other chemicals may be used on other types of material) As an intaglio method of printmaking it is along with engraving the most important technique for old master prints and remains widely used today But in wafer fabrication Etching refers to a process by which material is removed from the wafer ie either from the silicon substrate itself or from any film or layer of material on the wafer

DEFINITION

The treatment of a prepared metal surface with acid or other chemical reagent which by differential attack reveals the structure is known as etching In wafer fabrication etching process is generally classified in two classes Dry etching and Wet etching

Normally dry etching is not being used in this solar cells and module manufacturing unit So we will just go on a brief look on wet etching process

WET ETCHING It is a process that utilizes liquid chemicals or etchants to remove materials from the wafer usually in specific patterns defined by photo resist masks on the wafer Materials not covered by these masks are etched away by the chemicals while those covered by the masks are left almost intact A simple wet etching process may just consist of dissolution of the material to be removed in a liquid solvent without changing the chemical nature of the dissolved material In general however a wet etching process involves one or more chemical reactions that consume the original reactants and produce new species A basic wet etching process may be broken down in three steps as follows

1 Diffusion of the etchant to the surface for removal 2 Reaction between the etchant and the material being removed and 3 Diffusion of the reaction byproducts from the reacted surface

Wet etching can be both isotropic ie it proceeds in all directions at the same rate and anisotropic ie it proceeds in only one direction Isotropic Wet Etching A process which proceeds at the same rate in all directions is known as isotropic wet etching The etchants used in this process are isotropic in nature that etches away even the portion of material that is directly under the mask (usually in the shape of a quarter-circle) since its horizontal etching rate is the same as its vertical rate When an isotropic etchant eats away a portion of the material under the mask the etched film is said to have undercut the mask The amount of undercutting is a measure of an etching parameter known as the bias Bias is simply defined as the difference between the lateral dimensions of the etched image and the masked image Thus the mask used in etching must compensate for whatever bias an etchant is known to produce in order to create the desired feature on the wafer

Figure 1 Isotropic wet etching

9

Anisotropic Wet Etching A process which proceeds in only one direction is known as anisotropic wet etching It removes material in the vertical direction only since it will follow the mask patterns on the wafer very faithfully leaving any material covered by mask material basically untouched Since isotropic wet etching process has high bias values which are not practical for use in pattern images which has features of measuring less than 3 microns thus wafer patterns of less than 3 microns are wet etched anisotropically instead of isotropic

Figure 2 Anisotropic wet etching

Advantages of wet etching 1 Low cost 2 High reliability 3 High throughput 4 Excellent selectivity in most cases with respect to both mask and substrate materials

Disadvantages of wet etching 1 Limited resolution 2 Higher safety risks due to the direct chemical exposure of the personnel 3 High cost of etchants in some cases 4 Problems related to the resists loss of adhesion to the substrate 5 Problems related to the formation of bubbles which inhibit the etching process where they are present and 6 Problems related to incomplete or non-uniform etching

Before moving on to texturization we first of all must do Saw damage removal which is also called as 1st Etch First Etch When ingots obtained from the Czochralski method are sliced into wafers by saw then it leaves some damaged marks on the wafers if it is not being sliced properly which makes the surface non-uniform So to make the surface uniform and free of error it becomes mandatory to do saw damage removal or 1st

1 First wafers are kept in the solution of 300ml DI water and 10kg NaOH(RSE) within the temperature range of 75⁰plusmn 2⁰C for 1min The reaction which takes place in this step is

etching Steps involved are

Si + 2NaOH + H2O (DI) -gt Na3SiO2 + H2 + H2

2 Finally these etched wafers are washed with DI water O

10

TEXTURIZATION INTRODUCTION Since 1960 for the improvement of the cell performance texturization has been attempted Texturization of front andor back surfaces of a solar cell surfaces improve their performances The reflection from chemically textured wafer reduced from 34 for a polished silicon surface to 11 Many researchers have reported the selective etching of silicon of lt100gt orientation of using sodium or potassium hydroxide solution OBJECTIVES OF TEXTURIZATION The major objectives of texturization are-

1 Reduction of front surface reflectance 2 Increase in the path length of the entered light of lower wavelengths by oblique trajectory within the

cell and absorption of light closer to the junction 3 Optical trapping of weakly absorbed light by multiple internal reflections from the top and bottom

surfaces of the cell thereby also increasing the optical path length The increased path length with an oblique trajectory leads to an effective increase in the absorption co-efficient

All these effects lead to realization of higher performance thick cells and also efficient solar cells of comparatively lower thickness which significantly reduces the material cost without degrading the cell performance MODELS OF TEXTURIZATION To reduce the reflection co-efficient and to trap weak absorbed light various texturing geometries are being employed The different texturing geometries include

1 Lambertian geometry 2 Upright pyramid geometry 3 Slat or Micro-grooved geometry 4 Inverted pyramid geometry 5 Simple prism pyramid geometry 6 Grating geometry 7 Three perpendicular planes (3PP) geometry 8 Perpendicular slat geometry 9 Porous silicon

Now in large number of manufacturing industries Inverted pyramid texturing geometry scheme is being used these days Letrsquos have a look on this scheme very briefly Inverted Pyramid Geometry The light trapping properties of the inverted pyramid geometry has been first investigated by Smith and Rohatgi The increase in current for the type of textured cell is due to incorporation of al the three effects necessary for an efficient light trapping design The front surface reflectance is reduced by providing the opportunity for a portion of the incoming light to undergo a triple bounce thereby reducing the overall front surface reflectance The increase in path length and light trapping efficiency means that a larger fraction of the light which has entered the cell will be absorbed before exiting the cell The short wavelength spectral response analysis at the texturing angle of 5375ordm indicates that 37 of the incoming light experiences a triple bounce on the front surface of the inverted pyramid geometry and the rest percentage gets double bounce The inverted pyramids on a 100microm cell with a two layer AR coating is estimated to give 402mAcm2 and cell efficiency of 24 with realistic cell design and material parameters

11

This texturization method can be employed on both mono-crystalline and multi-crystalline substrates Letrsquos have a look on this Mono-crystalline substrates For mono-crystalline substrate it is advantageous to make use of the anisotropic etching properties of Si in an alkaline solution As the 111 planes get etched more slowly than other crystal planes 111 facets are developed On lt100gtwafers this leads to pyramidal shapes at the surface that are particularly effective in reducing reflectance A regular array of pyramidal pits with facets at 547ordm to the horizontal plane mdash called inverted pyramids mdash can be formed using oxide etch mask with reflectances as low as 8 without ARC

Multi-crystalline Substrates Due to the anisotropic nature of multi-crystalline silicon substrate alkaline random texturing is not effective Some grains remain untextured leading to a high average reflectance So for texturing multi-crystalline silicon wafers should be etched in an acid mixture of HF and HNO3 because HNO3 tends to oxidize the surface while the HF etches the oxide away These acidic iso-texturing results in lower reflection than traditional anisotropic etching on multi-crystalline material and better conversion efficiency

STANDARD NAOH-IPA TEXTURIZATION APPROACH In order to achieve good uniformity of pyramidal structure on the silicon surface a mixture of NaOH or KOH and isopropyl alcohol (IPA) is generally used for texturization of mono-crystalline silicon solar cell For better texturization the interfacial energy between silicon and ionized electrolyte chemical solution should be reduced to achieve sufficient wettability for the silicon surface which will enhance the pyramid nucleation From a manufacturability point of view it is appealing to combine the saw damage removal and alkaline texturing in one single step in which case a trade-off has to be made between process speed and quality of the surface treatment

Fig1 Inverted Pyramid Textured Surface

Fig3 Textured multi-crystalline Si surface

Fig2 Anisotropic Etching of Mono-crystalline Si

12

DIFFUSION

INTRODUCTION

Diffusion the movement of a chemical species from an area of high concentration to an area of lower concentration is one of the two major processes by which chemical species or dopants are introduced into a semiconductor (the other one being ion implantation) The controlled diffusion of dopants into silicon to alter the type and level of conductivity of semiconductor materials is the foundation of forming a p-n junction The mathematics that govern the mass transport phenomena of diffusion are based on Ficks laws There are two laws according to Fick Letrsquos have a look on these laws First law Whenever an impurity concentration gradient partCpartx exists in a finite volume of a matrix substance (the silicon substrate in this context) the impurity material will have the natural tendency to move in order to distribute itself more evenly within the matrix and decrease the gradient Given enough time this flow of impurities will eventually result in homogeneity within the matrix causing the net flow of impurities to stop The mathematics of this transport mechanism was formalized in 1855 by Fick who postulated that the flux of material across a given plane is proportional to the concentration gradient across the plane Thus Ficks First Law states

J = -D ( partC(xt)partx ) Where J is the flux D is the diffusion constant for the material that is diffusing in the specific solvent and partC(xt)partx is the concentration gradient The diffusion constant of a material is also referred to as diffusion coefficient or simply diffusivity It is expressed in units of length2time such as microm2hour The negative sign of the right side of the equation indicates that the impurities are flowing in the direction of lower concentration Second Law Ficks First Law does not consider the fact that the gradient and local concentration of the impurities in a material decreases with an increase in time an aspect thats important to diffusion processes The flux J1 of impurities entering a section of a bar with a concentration gradient is different from the flux J2 of impurities leaving the same section From the law of conservation of matter the difference between J1 and J2 must result in a change in the concentration of impurities within the section (assuming that no impurities are formed or consumed in the section) This is Ficks Second Law which states that the change in impurity concentration over time is equal to the change in local diffusion flux or

partC(xt)partt = - partJpartx or from Ficks First Law

partC(xt)partt = part(DpartC(xt)partx)partx If the diffusion coefficient is independent of position such as when the impurity concentration is low then Ficks Second Law may be further simplified into the following equation

partC(xt)partt = D part2C(xt)partx2 The most important property of diffusion process in the manufacture of silicon solar cell is the formation of p-n junction by doping n-type material ie phosphorous on the p-type ie boron doped silicon textured wafer playing an essential role for photovoltaic effect Let us have a brief descriptive look on the properties of p-n junction P-N JUNCTION A p-n junction is a junction formed by combining p-type and n-type semiconductors together in very close contact

13

Principle of p-n junction The most common type of solar cell is a large p-n junction where the free carrier pairs emitted by light energy are separated by the junction and contribute to the current In its simplest form it consists of a junction formed between n-type and p-type semiconductors either of the same material (homo-junction) or different materials (hetero-junction) The band-structure of the two differently doped sides with respect to their Fermi levels can be seen in adjacent figure1

When the two halves are brought together the Fermi levels on either side are forced in to coincidence causing the valence and conduction bands to bend (Figure 2)

These bent bands represent a built-in electric field over what is referred to as the depletion region When a photon with a energy greater than the band-gap of the semiconductor passes through the solar cell it may be absorbed by the material This absorption takes the form of a band-to-band electronic transition so an electronhole pair is produced If these carriers can diffuse to the depletion region before they recombine then they are separated by the electric field causing one quantum of charge to flow through an external load This is the origin of the solar cells photocurrent and is shown in Figure 3

14

SOLAR CELL AS A P-N JUNCTION A solar cell is essential a p-n junction with large surface area The n type material is kept thin to allow light to pass through to the p-n junction

Light travels in packets of energy called photons The generation of electric current happens inside the depletion zone of the PN junction The depletion region as explained previously with the diode is the area around the PN junction where the electrons from the N-type silicon have diffused into the holes of the P-type material When a photon of light is absorbed by one of these atoms in the N-Type silicon it will dislodge an electron creating a free electron and a hole The free electron and hole has sufficient energy to jump out of the depletion zone If a wire is connected from the cathode (N-type silicon) to the anode (P-type silicon) electrons will flow through the wire The electron is attracted to the positive charge of the P-type material and travels through the external load (meter) creating a flow of electric current The hole created by the dislodged electron is attracted to the negative charge of N-type material and migrates to the back electrical contact As the electron enters the P-type silicon from the back electrical contact it combines with the hole restoring the electrical neutrality

DIFFUSION PROCESS

The first layer formation after doing texturization of wafer is performed in this process by doping the substrate with n-type dopant N-doping can be accomplished by depositing n dopant onto the substrate and then heating the substrate to drive the n dopant into the substrate Gaseous diffusion (for example POCl3 horizontal tube diffusion including temperatures of from 700 to 1000degC preferably 800 to 900degC) can be used to deposit the n-dopant onto the substrate surface to create an n-doped layer and a shallow p-n junction proximal to the substrate surface In this process if wafers are loaded onto the quartz boat back to back in the same lot then double diffusion process takes place and if single wafers are being loaded then single diffusion takes place On the quartz boat wafers can be loaded in two methods

1 Parallel diffusion loading of wafers parallel to the gas flow 2 Perpendicular diffusion loading of wafers perpendicular to the gas flow

Diffusion process is comprised of the following steps 1 Idle state-In this state the load center and source zone are set at 800ordmC and no process is

carried out in the furnace 2 Load-During this sequence the silicon carbide paddle moves at a speed of 45 rpm and the

maximum time taken is 7min 3 Ramp-The cold wafers loaded into the tubes bring down the temperature of the tube The tube

is then reheated to reach the set point temperature and the time taken is 7min

15

4 Stabilization-During this phase any overshooting of temperature during ramp-up phase is controlled to the predetermined set value and the time taken is 10min

5 Pre-deposition-Any excess POCL3 present in vapour form inside the quartz bubbler is vent out before the deposition starts by N2

6 Deposition- N purging in 5 sec

2 is bubbled through liquid POCL3 in presence of O2 to form P2O5

7 Post-deposition-During this phase O in paste form

2 and N2

8 N

flows to drive in the phosphorous from wafer surface to the junction

2

9 Unload-During this phase the SiC paddle moves slowly out of the furnace at set speed of 45rpm and maximum time taken is 7min

Purge-This phase is also known as drive in diffusion which is designed to evenly distribute the impurities into wafer and reduce the generally undesirable high concentration in the wafer surface and the maximum time taken is 7min

10 End-The tube is set to 800⁰C temperature in idle state

16

PLASMA ETCHING

INTRODUCTION

Plasma etching is one type of dry etching which uses plasma to produce chemically reactive species from inert gases The reactive gases are then made to react with the material to be etched The main objective of this process is to remove edge shunts present on the cell which leads to 80 loss and thus it is also termed as Edge isolation method Since edge isolation is the main motive of this process so first let us see that why edge isolation is so important

Edge isolation

In the standard processing sequence the entire wafer is phosphorous diffused The contact to the p-type base is made by firing an Al containing paste that produces a high doping of Al thus over-compensating the shallow emitter diffusion However there is still a direct conductive path from the emitter on the front side to the rear p contact Therefore a large portion of the photo-generated current can directly flow through the emitter to the back contact instead of flowing through the external circuit This current path is modeled in the two-diode model of the solar cell as the parallel resistance R

where R

p

s and Rp are the series and parallel resistance respectively j01 and j02 represent the first and second diode saturation current densities respectively n1 and n2 are the first and second ideality diode factors respectively Vth = ktq is the thermal voltage and jL

In this model the first diode accounts for emitter and bulk recombination with a recombination rate assumed to be proportional to the excess carrier density This leads to an ideality factor of n

is the light generated current density The two diode model is as shown below

1=1 The second diode is associated with the recombination in the space charge region that is assumed to be independent of the injection level leading to n2=2 The effect of increased j02 and Rp can be shown by model calculations for ideal solar cells in the figure below for a fill factor potential of 84 j02 must be less than 2x10-9 Acm2 and Rp must be greater than approximately 2000Ω cm2 if the respective parameters contribute solely to the fill factor loss

17

This increase in Rp is achieved by the edge isolation process that interrupts the parasitic current path as shown in fig2 below

Thus edge isolation is required in solar cell to increase open circuit voltage and thus leading to greater efficiency

Now we will move on to plasma etching process to see that how it works actually in solar cell and module manufacturing industries

Plasma Etching Process

After diffusion a layer of phosphosilicate glass is formed on the edges and rear surface of diffused wafer The process generally requires adequate physical masking of the front surface while exposing the full back of the cell to the dry plasma environment In an embodiment of this process cells are slotted face to face

Fig2Sketch of an industrial screen-printed solar cell The emitter diffusion is present on the complete wafer surface and gets locally overcompensated by printing and firing Al containing metal paste as back contact Without edge-isolation light generated e-s can flow through the emitter region to the back contact which corresponds to the small series resistance in the two diode model

18

into inert carriers so that the plasma chemistry can effectively react with the back of the cells thereby removing excess material from the second coating that might have reached the back of the cell This process is preferably performed via the generation of an RF (13 6 MHz) plasma using a mix of Freon 14 (CF4) with oxygen (O2) and or nitrogen (N2) in a ratio which depends on the chamber and load geometries

19

PHOSPHOSILICATE GLASS REMOVAL INTRODUCTION In diffusion process during deposition phase N2 gas is bubbled through liquid POCL3

The reaction related to this phase is given as follows

1 4POCL3 + 3O2 -gt 2P2O5 + 6Cl 2 2P

2 2O5 + 5Si -gt 5SiO2

+ 4P (PSG)

P2O5

So it becomes mandatory to remove phosphosilicate glass by chemicals which may act as recombination sites or may contribute towards the surface density defect and hence all these defects may contribute to a lower voltage and current

obtained in first reaction step is in paste form and is transferred to the Teflon duct to the process tube which then reacts with silicon of which the wafer is made And thus when this phosphorous pentoxide reaches silicon surface then second reaction takes place and thus a silicon dioxide layer and phosphosilicate glass ie PSG forms on the wafer surface Formation of this dead layer on the wafer surface contributes to a lower efficiency

PROCESS OF REMOVAL Following are the steps that should be taken into consideration for the removal of phosphosilicate glass (PSG) from the wafer surface

1 Firstly wafers are washed combinedly with HF HCL and DI water in a tank for 1min 2 Again wafers are washed with HF and DI water for 2min in the other tank 3 For another 1 min DI water is being sprayed on the diffused wafers 4 Next wafers are washed with DI water subsequently on an interval of 30 sec 5 Finally wafers are kept in the OVEN for 10mins within the temperature range of 95plusmn5ordmC

20

PLASMA ENHANCED CHEMICAL VAPOUR DEPOSITION INTRODUCTION In recent years the low-temperature (le 400ordmC) surface passivation of crystalline silicon solar cells by means of PECVD silicon nitride films has attracted an increasing attention of the PV community These films have proven to be capable of combining an outstanding surface passivation quality with excellent anti-reflection properties On typical p-type silicon solar cells substrate it has been shown that the surface passivation quality of very silicon rich Si3N4

we must first know that whatrsquos the use of silicon nitride coating on the surface of the wafer

films is superior to that of state-of-the-art high temperature silicon dioxide Before proceeding to the details of PECVD system

WHY SILICON NITRIDE SURFACE COATING IS PREFERRED IN PECVD PROCESS It is preferred as a surface coating because it functions as An effective diffusion mask An effective metallization diffusion barrier An anti-reflective coating Provide surface and bulk passivation

Preferably hydrogen is trapped in such a silicon nitride surface coating PECVD SYSYTEM PECVD or Plasma Enhanced Chemical Vapor Deposition is one of the methods of different Chemical Vapor Deposition (CVD) technique In PECVD plasma is being used to decompose a reactant gas such as silane (SiH4) to produce reaction products that precipitate on the surface of the substrate as a new layer

Working Principle Inside a PECVD reactor a strong electric field ignites plasma between two electrodes one of which holds the substrate This plasma ignition cracks the molecular bonds of the process gas which in turn are able to crack more process gas molecules before reaching the surface of the substrate The glow discharge used in this reactor is created by applying an RF field to a low-pressure gas creating free electrons within the discharge region The electrons are sufficiently energized by the electric field that gas-phase dissociation and ionization of the reactant gases occur when the free electrons collide with them Energetic species are then adsorbed on the film surface where they are subjected to ion and electron bombardment rearrangements reactions with other species new bond formation and film formation and growth Eventually a new layer is deposited on the substrate For example when silane is used as precursor plasma ignition frees up Si and SiH radicals which also crack more silane molecules on their way to the surface of the substrate where silicon is deposited PROCESS DETAILS OF PECVD In this system wafers coming from the previous state ie PSG removal step are being loaded onto the graphite boat where the individual wafers are held by three pins inside the boat It is absolutely necessary for the wafers to touch all the three pins otherwise there will remain a possibility of poor deposition in which

Figure Anti-reflective coating made out of silicon nitride deposited by the PECVD technique whereby silane and ammonia is reacted in a furnace The major color of coating looks blue but the color could also look like purple blue or light blue due to different thickness of coating

21

the HF generator fails to generate the required power for process deposition While loading the double diffusion wafers it should be ensured that deposition side should be correctly loaded The wafers are being loaded parallel to the boat After loading the trolley along with the boat is placed inside the machine and then after filling the input parameters of the CMI interface used here and the process tube press the RUN button and then machine will be in auto mode The handling mechanism automatically picks up the boat and places it on the desired paddle corresponding to the selected for the specified job And then process STARTS

22

SCREEN PRINTING INTRODUCTION In screen printing on wafer-based solar PV cells the mesh and buses of silver are printed on the front furthermore the buses of silver are printed on the back Subsequently aluminum paste is dispensed over the whole surface of the back for passivation and surface reflection [14] One of the parameters that can vary and can be controlled in screen printing is the thickness of the print This makes it useful for some of the techniques of printing solar cells electronics etc Screen printing is one of the most critical processes to maintain high yield Solar wafers are becoming thinner and larger so careful printing is required to maintain a lower breakage rate On the other hand high throughput at the printing stage improves the throughput of the whole cell production line PROCESS This process is divided in three phase-

1 Phase I- AgAl printing 2 Phase II- Ag printing 3 Phase III- Al printing

PHASE I- AgAl PRINTING This phase is required to print the back bus bar for tabbing purpose The AgAl paste used in this printing is Dupont (PV-505) and the consumption of the paste per wafer is 011 gm And the screen type used is of the following specification

1 Mesh size- 200microm 2 Tension- 24 N-cm 3 Mesh angle- 225ordm 45ordm 4 Emulsion thickness- 24microm

Process details During printing the paste is poured on the screen and is spread over it with the help of polypropylene spatula Then the wafers (ARC coated) are pasted with the sunny side down on the loader and picked one by one automatically with the start of the print command Once printing starts the wafers are placed one by one between the tool paste and the screen and then the squeegee moves over the screen and allow the paste through the exposed parts on the screen and gets pasted on the wafers Thus in this way the wafers are printed one by one Two most important parameters to be controlled during this printing are

1 Squeegee Pressure 2 Snap off distance

The next step after printing is drying step DRYING In this step wafers are passed through the dryer with a temperature range of 180ordm-400ordmC PHASE II- Ag PRINTING This phase is required to print the front bus bar for tabbing purpose The Ag paste used in this printing is Dupont (PV-145) and the consumption of the paste per wafer is 014 gm And the screen type used is of the following specification

1 Mesh size- 325microm 2 Tension- 24 N-cm 3 Mesh angle- 225ordm 4 Emulsion thickness- 24microm

Acknowledgement

CONTENTS

bull Introduction helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip1 bull An Overview of a Solar CellModule Manufacturing Unit helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip2 bull Need for Solar Energy helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip3 bull Raw materials needed helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip5 bull History of Photovoltaics helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip6 bull Solar cell manufacturing processes helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip7

Etching helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip8 Texturisation helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip10 Diffusion helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip12 Plasma Etching helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip16 PSG Removal helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip19 PECVD (Plasma Enhanced Chemical Vapor Deposition) helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip20 Screen Printing helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip22 Cell Testing and Sorting helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip25 Cell Testing Parameters helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip26

bull Solar Module helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip28 Module Manufacturing and Testing helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip29 Uses of Module helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip31

bull Uses of Solar Energy helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip33 bull Solar Energy Advantages Opportunities and Challenges helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip35 bull Conclusion helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip37

1

INTRODUCTION In recent years the world has taken huge strides towards accepting renewable sources of energy as valid sources of power With fast diminishing fossil fuel reserves and the polluting nature of coal and petroleum solar energy has emerged as a clean environmentally acceptable source of energy The mechanism of photoelectric power generation is simple from a theoretical point of view It owes its existence to the photon generated excitation of electrons across a semiconductor p-n junction The practical implementation of this useful phenomenon has given rise to the solar power industry In a series of manufacturing processes the raw semiconductor wafers are transformed to printed solar cells which are further integrated to solar modules and arrays As the world hurdles into an energy crisis and the fragile ecology comes under increasing strain due to pollution it is imperative that we look at alternative renewable sources of energy This report looks into the theoretical and practical aspects of Module manufacturing and the role solar energy has to play in the near and distant future Some of the appealing characteristics of solar energy are strikingly obvious these include no fuel consumption no pollution wide power handling capabilities and high power to weight ratio A more in-depth analysis is done in the pages that follow

2

AN OVERVIEW OF A SOLAR CELLMODULE MANUFACTURING UNIT

Manufacture of solar modules follows a series of well defined sequences There is scope for much modification and choice of raw materials andor processes that a manufacturing firm can adopt while producing modules However the basic methodology is the same no matter how different individual steps may be During the course of production engineers may tweak process parameters to achieve peak efficiency However the fundamental infrastructure that the firm must invest in are essentially constant among manufacturers The following paragraphs aim to provide the brief outline of any manufacturing unit regardless of the manufacturer A DI UNIT A Deionised (DI) Water plant is fundamental to any unit It provides water free from charged particles to all units that require it B ETCHING UNIT A series of polypropylene tanks form the etching and texturisation unit All equipment and tanks are labeled This has mechanized and automated robot arms that simplify the immersion and handling of wafer chests C DIFFUSION UNIT The diffusion unit has a multi tube diffusion furnace with its accessory units Diffusion system consists of the furnace load station cross flow box gas cabinet and control system The wafers are inserted in batches in quartz boats After diffusion the formation of the p-n junction has been completed we move into the plasma etching process This performs edge isolation which is crucial to prevent short circuits between the front and back surfaces of the wafer D PLASMA UNIT Plasma etching is done in an evacuated chamber in humidity controlled room Freon gas and RF power are the inputs along with the wafer Time varies from 28 minutes to 40 minutes This is followed by rinsing and drying E PSG Removal removes the phosphosilicate glass The next step leads to the manufacture of solar modules F PECVD (Plasma Enhanced Chemical Vapor Deposition) A solar module plant has a PECVD unit This deposits an antireflective layer on the wafer Silane and Ammonia are combined to give Silicon Nitride Hydrogen provides excellent surface and bulk passivization by saturating dangling bonds An abatement system is provided to detoxify harmful wastes To form the Metallization and front and back electrodes the process of SCREEN PRINTING is carried out This is typically a 3 phase process AgAl Ag and Al are printed in paste form and allowed to dry This is followed by the testing and sorting process whereby cells are arranged into various classes depending on their I-V curve (and hence efficiency) The entire process is automated The work of a Photovoltaic firm does not end with the creation of cells After Cell manufacturing the MODULE PRODUCTION UNIT comes into play As each cell is not sufficient to provide enough power for household and industrial purposes they are strung together to form modules Tabbing is done to provide connections for the stringing of cells This is followed by lamination to form a multilayered structure that protects the cell Framing is done to provide structural rigidity A junction box is provided for the tapping of electrical power Module testing is done to check if each module meets expectations In short these are the essential units and processes common to all photovoltaic cellmodule manufacturing plants

3

NEED FOR SOLAR ENERGY According to the World Health Organization 3 million people are killed worldwide by outdoor air pollution annually from vehicles and industrial emissions and 16 million indoors through the use of solid fuel Although technology has made oil extraction more efficient the world has to struggle to provide oil by using increasingly costly and less productive methods such as deep sea drilling and developing environmentally sensitive areas such as the Arctic National Wildlife Refuge Global surface temperature increased 074 plusmn 018 degC (133 plusmn 032 degF) during the last century The Intergovernmental Panel on Climate Change (IPCC) concludes that increasing greenhouse gas concentrations resulting from human activity such as fossil fuel burning and deforestation are responsible for most of the observed temperature increase since the middle of the 20th century Mitigation of global warming is accomplished through reductions in the rate of man-made greenhouse gas release The worlds population continues to grow at a quarter of a million people per day increasing the consumption of energy Currently fossil fuels provided around 66 of the worlds electrical power and 95 of the worlds total energy demands

The 1973 Oil crisis due to the OPEC embargo The 1979 (or second) oil crisis (due to the Iranian Revolution) the 1990 oil price shock (due to the First Gulf War) lead to drastic rise in the price of crude oil the most

4

indispensable source of energy for modern industry to function From the mid-1980s to September 2003 the inflation-adjusted price of a barrel of crude oil on NYMEX was generally under $25barrel During 2003 the price rose above $30 reached $60 by August 11 2005 and peaked at $14730 in July 2008

Thus fossil fuels are proven to be polluting and severely detrimental to human health We are paying the price for it in terms of respiratory diseases and a looming energy crisis As the oil prices are volatile and are steadily but unpredictably rising it is expedient to look beyond polluting fossil fuels as the sole source of power

Todays solar systems for solar heating are highly efficient and easy to install The initial cost is high but the payoff is long lasting Solar systems pay for themselves in about half their lifetime It is a free source of energy unlike oil coal or natural gas The environment impact of solar power is low Solar is clean power There are no emissions no maintenance costs and no unstable fuel costs Solar power is reliable with no outages There are no moving parts to replace or break Solar panels can withstand most extreme weather conditions Saving money energy conservation and clean power are the best benefits of solar energy

In response to the petroleum crisis the principles of green energy and sustainable living movements gain popularity This has led to increasing interest in alternate powerfuel research The Sun is 150 million kilometers away but is amazingly powerful Covering 4 of the worlds desert area with photovoltaic could supply all of the worlds electricity In such a situation development of solar energy should be a priority for every nation

5

RAW MATERIALS NEEDED

Along with the enormous capital investment required to successfully set up a solar cellmodule manufacturing plant it also requires a steady supply of raw materials The most basic raw materials is the silicon wafer itself which is usually bought from another company rather than prepared in-house Two types of crystalline silicon solar cell are single crystalline silicon solar cell which is known as monocrystalline silicon solar cell made by slicing wafers and polycrystalline silicon solar cell made by sawing a block of silicon first into bars and then wafers Other semiconductors such as gallium arsenide cadmium telluride and copper indium diselenide etc have both single crystalline and polycrystalline forms The solar cell which made by gallium arsenide has high efficiency ranging from 20-25 The first process Etching requires a number of chemicals HF HCL NaOH which may vary depending on the etching methodology (Acidic etching or Alkali etching) Deionised water is obtained from the DI plant IPA is used for cleaning purposes For diffusion an MRL diffusion furnace can be used which utilizes N2 amp O2

N2 flows through a bubbler of POCl3

for Phosphorus diffusion HF HCl and DI water are also used to clean the components (bubbler quartz tube etc) used in Diffusion For plasma etching we need a gas like Freon to provide the ions used in edge isolation Silane and Ammonia in gaseous form are needed to perform PECVD in order to deposit the anti reflective coating Silicon Nitride is formed due to the reaction of Silane and Ammonia Screen Printing is a three phase process The first phase is AgAl which could use the paste Dupont PV505 The second front side printing uses an Ag pastes (Dupont PV145) Third phase uses Aluminium paste (Analog 12D)

Module manufacturing requires its own set of raw materials It needs Tedlar EVA and Glass to form the multilayered structure Aluminium frames are needed to provide structural integrity and a junction box for output terminals Thus a manufacturing plant needs a variety of raw materials at the right time in the right quantity quality and purity A fall is any of these parameters will severely hamper the manufacturing process and negatively effect margins Thus ensuring a proper supply of this varied mix of materials is of paramount importance to the managers

6

HISTORY OF PHOTOVOLTAICS Most people are surprised to learn that photovoltaic technology actually dates back over 160 years French physicist Edmond Becquerel first described the photovoltaic (PV) effect in 1839 but it remained a curiosity of science for the next three quarters of a century At only 19 Becquerel found that certain materials would produce small amounts of electric current when exposed to light The effect was first studied in solids such as selenium by Heinrich Hertz in the 1870s Soon afterward selenium PV cells were converting light to electricity at 1 to 2 efficiency As a result selenium was quickly adopted in the emerging field of photography for use in light-measuring devices Major steps toward commercializing PV were taken in the 1940s and early 1950s when the Czochralski process was developed for producing highly pure crystalline silicon In 1954 scientists at Bell Laboratories depended on the Czochralski process to develop the first crystalline silicon photovoltaic cell which had an efficiency of 4 Although a few attempts were made in the 1950s to use silicon cells in commercial products it was the new space program that gave the technology its first major application In the 1970s research drives PV costs down 80 allowing for applications such as offshore navigation warning lights and horns lighthouses railroad crossings and remote use where utility-grid connections are too costly Despite these advances PV devices in 1970 were still too expensive for most mass market uses But in the mid-1970s rising energy costs sparked by a world oil crisis renewed interest in making PV technology more affordable Since then the federal government industry and research organizations have invested billions of dollars in research development and production Often industry and the government work together sharing the cost of PV research and development In the 1980s photovoltaic became a popular power source for consumer electronic devices including calculators watches radios lanterns and other small battery-charging applications

Todays commercial PV systems can convert from 11 to 17 of sunlight into electricity They are highly reliable and last 20 years or longer The cost of PV-generated electricity has dropped 15- to 20-fold and PV modules now cost around $6 per watt (W) and produce electricity for as little as 25 to 50 cents per kilowatt-hour (kWh)

7

Solar Cell Manufacturing Processes

8

ETCHING

INTRODUCTION

Etching is the process of using strong acid or mordant to cut into the unprotected parts of a metal surface to create a design in intaglio in the metal (the original processmdashin modern manufacturing other chemicals may be used on other types of material) As an intaglio method of printmaking it is along with engraving the most important technique for old master prints and remains widely used today But in wafer fabrication Etching refers to a process by which material is removed from the wafer ie either from the silicon substrate itself or from any film or layer of material on the wafer

DEFINITION

The treatment of a prepared metal surface with acid or other chemical reagent which by differential attack reveals the structure is known as etching In wafer fabrication etching process is generally classified in two classes Dry etching and Wet etching

Normally dry etching is not being used in this solar cells and module manufacturing unit So we will just go on a brief look on wet etching process

WET ETCHING It is a process that utilizes liquid chemicals or etchants to remove materials from the wafer usually in specific patterns defined by photo resist masks on the wafer Materials not covered by these masks are etched away by the chemicals while those covered by the masks are left almost intact A simple wet etching process may just consist of dissolution of the material to be removed in a liquid solvent without changing the chemical nature of the dissolved material In general however a wet etching process involves one or more chemical reactions that consume the original reactants and produce new species A basic wet etching process may be broken down in three steps as follows

1 Diffusion of the etchant to the surface for removal 2 Reaction between the etchant and the material being removed and 3 Diffusion of the reaction byproducts from the reacted surface

Wet etching can be both isotropic ie it proceeds in all directions at the same rate and anisotropic ie it proceeds in only one direction Isotropic Wet Etching A process which proceeds at the same rate in all directions is known as isotropic wet etching The etchants used in this process are isotropic in nature that etches away even the portion of material that is directly under the mask (usually in the shape of a quarter-circle) since its horizontal etching rate is the same as its vertical rate When an isotropic etchant eats away a portion of the material under the mask the etched film is said to have undercut the mask The amount of undercutting is a measure of an etching parameter known as the bias Bias is simply defined as the difference between the lateral dimensions of the etched image and the masked image Thus the mask used in etching must compensate for whatever bias an etchant is known to produce in order to create the desired feature on the wafer

Figure 1 Isotropic wet etching

9

Anisotropic Wet Etching A process which proceeds in only one direction is known as anisotropic wet etching It removes material in the vertical direction only since it will follow the mask patterns on the wafer very faithfully leaving any material covered by mask material basically untouched Since isotropic wet etching process has high bias values which are not practical for use in pattern images which has features of measuring less than 3 microns thus wafer patterns of less than 3 microns are wet etched anisotropically instead of isotropic

Figure 2 Anisotropic wet etching

Advantages of wet etching 1 Low cost 2 High reliability 3 High throughput 4 Excellent selectivity in most cases with respect to both mask and substrate materials

Disadvantages of wet etching 1 Limited resolution 2 Higher safety risks due to the direct chemical exposure of the personnel 3 High cost of etchants in some cases 4 Problems related to the resists loss of adhesion to the substrate 5 Problems related to the formation of bubbles which inhibit the etching process where they are present and 6 Problems related to incomplete or non-uniform etching

Before moving on to texturization we first of all must do Saw damage removal which is also called as 1st Etch First Etch When ingots obtained from the Czochralski method are sliced into wafers by saw then it leaves some damaged marks on the wafers if it is not being sliced properly which makes the surface non-uniform So to make the surface uniform and free of error it becomes mandatory to do saw damage removal or 1st

1 First wafers are kept in the solution of 300ml DI water and 10kg NaOH(RSE) within the temperature range of 75⁰plusmn 2⁰C for 1min The reaction which takes place in this step is

etching Steps involved are

Si + 2NaOH + H2O (DI) -gt Na3SiO2 + H2 + H2

2 Finally these etched wafers are washed with DI water O

10

TEXTURIZATION INTRODUCTION Since 1960 for the improvement of the cell performance texturization has been attempted Texturization of front andor back surfaces of a solar cell surfaces improve their performances The reflection from chemically textured wafer reduced from 34 for a polished silicon surface to 11 Many researchers have reported the selective etching of silicon of lt100gt orientation of using sodium or potassium hydroxide solution OBJECTIVES OF TEXTURIZATION The major objectives of texturization are-

1 Reduction of front surface reflectance 2 Increase in the path length of the entered light of lower wavelengths by oblique trajectory within the

cell and absorption of light closer to the junction 3 Optical trapping of weakly absorbed light by multiple internal reflections from the top and bottom

surfaces of the cell thereby also increasing the optical path length The increased path length with an oblique trajectory leads to an effective increase in the absorption co-efficient

All these effects lead to realization of higher performance thick cells and also efficient solar cells of comparatively lower thickness which significantly reduces the material cost without degrading the cell performance MODELS OF TEXTURIZATION To reduce the reflection co-efficient and to trap weak absorbed light various texturing geometries are being employed The different texturing geometries include

1 Lambertian geometry 2 Upright pyramid geometry 3 Slat or Micro-grooved geometry 4 Inverted pyramid geometry 5 Simple prism pyramid geometry 6 Grating geometry 7 Three perpendicular planes (3PP) geometry 8 Perpendicular slat geometry 9 Porous silicon

Now in large number of manufacturing industries Inverted pyramid texturing geometry scheme is being used these days Letrsquos have a look on this scheme very briefly Inverted Pyramid Geometry The light trapping properties of the inverted pyramid geometry has been first investigated by Smith and Rohatgi The increase in current for the type of textured cell is due to incorporation of al the three effects necessary for an efficient light trapping design The front surface reflectance is reduced by providing the opportunity for a portion of the incoming light to undergo a triple bounce thereby reducing the overall front surface reflectance The increase in path length and light trapping efficiency means that a larger fraction of the light which has entered the cell will be absorbed before exiting the cell The short wavelength spectral response analysis at the texturing angle of 5375ordm indicates that 37 of the incoming light experiences a triple bounce on the front surface of the inverted pyramid geometry and the rest percentage gets double bounce The inverted pyramids on a 100microm cell with a two layer AR coating is estimated to give 402mAcm2 and cell efficiency of 24 with realistic cell design and material parameters

11

This texturization method can be employed on both mono-crystalline and multi-crystalline substrates Letrsquos have a look on this Mono-crystalline substrates For mono-crystalline substrate it is advantageous to make use of the anisotropic etching properties of Si in an alkaline solution As the 111 planes get etched more slowly than other crystal planes 111 facets are developed On lt100gtwafers this leads to pyramidal shapes at the surface that are particularly effective in reducing reflectance A regular array of pyramidal pits with facets at 547ordm to the horizontal plane mdash called inverted pyramids mdash can be formed using oxide etch mask with reflectances as low as 8 without ARC

Multi-crystalline Substrates Due to the anisotropic nature of multi-crystalline silicon substrate alkaline random texturing is not effective Some grains remain untextured leading to a high average reflectance So for texturing multi-crystalline silicon wafers should be etched in an acid mixture of HF and HNO3 because HNO3 tends to oxidize the surface while the HF etches the oxide away These acidic iso-texturing results in lower reflection than traditional anisotropic etching on multi-crystalline material and better conversion efficiency

STANDARD NAOH-IPA TEXTURIZATION APPROACH In order to achieve good uniformity of pyramidal structure on the silicon surface a mixture of NaOH or KOH and isopropyl alcohol (IPA) is generally used for texturization of mono-crystalline silicon solar cell For better texturization the interfacial energy between silicon and ionized electrolyte chemical solution should be reduced to achieve sufficient wettability for the silicon surface which will enhance the pyramid nucleation From a manufacturability point of view it is appealing to combine the saw damage removal and alkaline texturing in one single step in which case a trade-off has to be made between process speed and quality of the surface treatment

Fig1 Inverted Pyramid Textured Surface

Fig3 Textured multi-crystalline Si surface

Fig2 Anisotropic Etching of Mono-crystalline Si

12

DIFFUSION

INTRODUCTION

Diffusion the movement of a chemical species from an area of high concentration to an area of lower concentration is one of the two major processes by which chemical species or dopants are introduced into a semiconductor (the other one being ion implantation) The controlled diffusion of dopants into silicon to alter the type and level of conductivity of semiconductor materials is the foundation of forming a p-n junction The mathematics that govern the mass transport phenomena of diffusion are based on Ficks laws There are two laws according to Fick Letrsquos have a look on these laws First law Whenever an impurity concentration gradient partCpartx exists in a finite volume of a matrix substance (the silicon substrate in this context) the impurity material will have the natural tendency to move in order to distribute itself more evenly within the matrix and decrease the gradient Given enough time this flow of impurities will eventually result in homogeneity within the matrix causing the net flow of impurities to stop The mathematics of this transport mechanism was formalized in 1855 by Fick who postulated that the flux of material across a given plane is proportional to the concentration gradient across the plane Thus Ficks First Law states

J = -D ( partC(xt)partx ) Where J is the flux D is the diffusion constant for the material that is diffusing in the specific solvent and partC(xt)partx is the concentration gradient The diffusion constant of a material is also referred to as diffusion coefficient or simply diffusivity It is expressed in units of length2time such as microm2hour The negative sign of the right side of the equation indicates that the impurities are flowing in the direction of lower concentration Second Law Ficks First Law does not consider the fact that the gradient and local concentration of the impurities in a material decreases with an increase in time an aspect thats important to diffusion processes The flux J1 of impurities entering a section of a bar with a concentration gradient is different from the flux J2 of impurities leaving the same section From the law of conservation of matter the difference between J1 and J2 must result in a change in the concentration of impurities within the section (assuming that no impurities are formed or consumed in the section) This is Ficks Second Law which states that the change in impurity concentration over time is equal to the change in local diffusion flux or

partC(xt)partt = - partJpartx or from Ficks First Law

partC(xt)partt = part(DpartC(xt)partx)partx If the diffusion coefficient is independent of position such as when the impurity concentration is low then Ficks Second Law may be further simplified into the following equation

partC(xt)partt = D part2C(xt)partx2 The most important property of diffusion process in the manufacture of silicon solar cell is the formation of p-n junction by doping n-type material ie phosphorous on the p-type ie boron doped silicon textured wafer playing an essential role for photovoltaic effect Let us have a brief descriptive look on the properties of p-n junction P-N JUNCTION A p-n junction is a junction formed by combining p-type and n-type semiconductors together in very close contact

13

Principle of p-n junction The most common type of solar cell is a large p-n junction where the free carrier pairs emitted by light energy are separated by the junction and contribute to the current In its simplest form it consists of a junction formed between n-type and p-type semiconductors either of the same material (homo-junction) or different materials (hetero-junction) The band-structure of the two differently doped sides with respect to their Fermi levels can be seen in adjacent figure1

When the two halves are brought together the Fermi levels on either side are forced in to coincidence causing the valence and conduction bands to bend (Figure 2)

These bent bands represent a built-in electric field over what is referred to as the depletion region When a photon with a energy greater than the band-gap of the semiconductor passes through the solar cell it may be absorbed by the material This absorption takes the form of a band-to-band electronic transition so an electronhole pair is produced If these carriers can diffuse to the depletion region before they recombine then they are separated by the electric field causing one quantum of charge to flow through an external load This is the origin of the solar cells photocurrent and is shown in Figure 3

14

SOLAR CELL AS A P-N JUNCTION A solar cell is essential a p-n junction with large surface area The n type material is kept thin to allow light to pass through to the p-n junction

Light travels in packets of energy called photons The generation of electric current happens inside the depletion zone of the PN junction The depletion region as explained previously with the diode is the area around the PN junction where the electrons from the N-type silicon have diffused into the holes of the P-type material When a photon of light is absorbed by one of these atoms in the N-Type silicon it will dislodge an electron creating a free electron and a hole The free electron and hole has sufficient energy to jump out of the depletion zone If a wire is connected from the cathode (N-type silicon) to the anode (P-type silicon) electrons will flow through the wire The electron is attracted to the positive charge of the P-type material and travels through the external load (meter) creating a flow of electric current The hole created by the dislodged electron is attracted to the negative charge of N-type material and migrates to the back electrical contact As the electron enters the P-type silicon from the back electrical contact it combines with the hole restoring the electrical neutrality

DIFFUSION PROCESS

The first layer formation after doing texturization of wafer is performed in this process by doping the substrate with n-type dopant N-doping can be accomplished by depositing n dopant onto the substrate and then heating the substrate to drive the n dopant into the substrate Gaseous diffusion (for example POCl3 horizontal tube diffusion including temperatures of from 700 to 1000degC preferably 800 to 900degC) can be used to deposit the n-dopant onto the substrate surface to create an n-doped layer and a shallow p-n junction proximal to the substrate surface In this process if wafers are loaded onto the quartz boat back to back in the same lot then double diffusion process takes place and if single wafers are being loaded then single diffusion takes place On the quartz boat wafers can be loaded in two methods

1 Parallel diffusion loading of wafers parallel to the gas flow 2 Perpendicular diffusion loading of wafers perpendicular to the gas flow

Diffusion process is comprised of the following steps 1 Idle state-In this state the load center and source zone are set at 800ordmC and no process is

carried out in the furnace 2 Load-During this sequence the silicon carbide paddle moves at a speed of 45 rpm and the

maximum time taken is 7min 3 Ramp-The cold wafers loaded into the tubes bring down the temperature of the tube The tube

is then reheated to reach the set point temperature and the time taken is 7min

15

4 Stabilization-During this phase any overshooting of temperature during ramp-up phase is controlled to the predetermined set value and the time taken is 10min

5 Pre-deposition-Any excess POCL3 present in vapour form inside the quartz bubbler is vent out before the deposition starts by N2

6 Deposition- N purging in 5 sec

2 is bubbled through liquid POCL3 in presence of O2 to form P2O5

7 Post-deposition-During this phase O in paste form

2 and N2

8 N

flows to drive in the phosphorous from wafer surface to the junction

2

9 Unload-During this phase the SiC paddle moves slowly out of the furnace at set speed of 45rpm and maximum time taken is 7min

Purge-This phase is also known as drive in diffusion which is designed to evenly distribute the impurities into wafer and reduce the generally undesirable high concentration in the wafer surface and the maximum time taken is 7min

10 End-The tube is set to 800⁰C temperature in idle state

16

PLASMA ETCHING

INTRODUCTION

Plasma etching is one type of dry etching which uses plasma to produce chemically reactive species from inert gases The reactive gases are then made to react with the material to be etched The main objective of this process is to remove edge shunts present on the cell which leads to 80 loss and thus it is also termed as Edge isolation method Since edge isolation is the main motive of this process so first let us see that why edge isolation is so important

Edge isolation

In the standard processing sequence the entire wafer is phosphorous diffused The contact to the p-type base is made by firing an Al containing paste that produces a high doping of Al thus over-compensating the shallow emitter diffusion However there is still a direct conductive path from the emitter on the front side to the rear p contact Therefore a large portion of the photo-generated current can directly flow through the emitter to the back contact instead of flowing through the external circuit This current path is modeled in the two-diode model of the solar cell as the parallel resistance R

where R

p

s and Rp are the series and parallel resistance respectively j01 and j02 represent the first and second diode saturation current densities respectively n1 and n2 are the first and second ideality diode factors respectively Vth = ktq is the thermal voltage and jL

In this model the first diode accounts for emitter and bulk recombination with a recombination rate assumed to be proportional to the excess carrier density This leads to an ideality factor of n

is the light generated current density The two diode model is as shown below

1=1 The second diode is associated with the recombination in the space charge region that is assumed to be independent of the injection level leading to n2=2 The effect of increased j02 and Rp can be shown by model calculations for ideal solar cells in the figure below for a fill factor potential of 84 j02 must be less than 2x10-9 Acm2 and Rp must be greater than approximately 2000Ω cm2 if the respective parameters contribute solely to the fill factor loss

17

This increase in Rp is achieved by the edge isolation process that interrupts the parasitic current path as shown in fig2 below

Thus edge isolation is required in solar cell to increase open circuit voltage and thus leading to greater efficiency

Now we will move on to plasma etching process to see that how it works actually in solar cell and module manufacturing industries

Plasma Etching Process

After diffusion a layer of phosphosilicate glass is formed on the edges and rear surface of diffused wafer The process generally requires adequate physical masking of the front surface while exposing the full back of the cell to the dry plasma environment In an embodiment of this process cells are slotted face to face

Fig2Sketch of an industrial screen-printed solar cell The emitter diffusion is present on the complete wafer surface and gets locally overcompensated by printing and firing Al containing metal paste as back contact Without edge-isolation light generated e-s can flow through the emitter region to the back contact which corresponds to the small series resistance in the two diode model

18

into inert carriers so that the plasma chemistry can effectively react with the back of the cells thereby removing excess material from the second coating that might have reached the back of the cell This process is preferably performed via the generation of an RF (13 6 MHz) plasma using a mix of Freon 14 (CF4) with oxygen (O2) and or nitrogen (N2) in a ratio which depends on the chamber and load geometries

19

PHOSPHOSILICATE GLASS REMOVAL INTRODUCTION In diffusion process during deposition phase N2 gas is bubbled through liquid POCL3

The reaction related to this phase is given as follows

1 4POCL3 + 3O2 -gt 2P2O5 + 6Cl 2 2P

2 2O5 + 5Si -gt 5SiO2

+ 4P (PSG)

P2O5

So it becomes mandatory to remove phosphosilicate glass by chemicals which may act as recombination sites or may contribute towards the surface density defect and hence all these defects may contribute to a lower voltage and current

obtained in first reaction step is in paste form and is transferred to the Teflon duct to the process tube which then reacts with silicon of which the wafer is made And thus when this phosphorous pentoxide reaches silicon surface then second reaction takes place and thus a silicon dioxide layer and phosphosilicate glass ie PSG forms on the wafer surface Formation of this dead layer on the wafer surface contributes to a lower efficiency

PROCESS OF REMOVAL Following are the steps that should be taken into consideration for the removal of phosphosilicate glass (PSG) from the wafer surface

1 Firstly wafers are washed combinedly with HF HCL and DI water in a tank for 1min 2 Again wafers are washed with HF and DI water for 2min in the other tank 3 For another 1 min DI water is being sprayed on the diffused wafers 4 Next wafers are washed with DI water subsequently on an interval of 30 sec 5 Finally wafers are kept in the OVEN for 10mins within the temperature range of 95plusmn5ordmC

20

PLASMA ENHANCED CHEMICAL VAPOUR DEPOSITION INTRODUCTION In recent years the low-temperature (le 400ordmC) surface passivation of crystalline silicon solar cells by means of PECVD silicon nitride films has attracted an increasing attention of the PV community These films have proven to be capable of combining an outstanding surface passivation quality with excellent anti-reflection properties On typical p-type silicon solar cells substrate it has been shown that the surface passivation quality of very silicon rich Si3N4

we must first know that whatrsquos the use of silicon nitride coating on the surface of the wafer

films is superior to that of state-of-the-art high temperature silicon dioxide Before proceeding to the details of PECVD system

WHY SILICON NITRIDE SURFACE COATING IS PREFERRED IN PECVD PROCESS It is preferred as a surface coating because it functions as An effective diffusion mask An effective metallization diffusion barrier An anti-reflective coating Provide surface and bulk passivation

Preferably hydrogen is trapped in such a silicon nitride surface coating PECVD SYSYTEM PECVD or Plasma Enhanced Chemical Vapor Deposition is one of the methods of different Chemical Vapor Deposition (CVD) technique In PECVD plasma is being used to decompose a reactant gas such as silane (SiH4) to produce reaction products that precipitate on the surface of the substrate as a new layer

Working Principle Inside a PECVD reactor a strong electric field ignites plasma between two electrodes one of which holds the substrate This plasma ignition cracks the molecular bonds of the process gas which in turn are able to crack more process gas molecules before reaching the surface of the substrate The glow discharge used in this reactor is created by applying an RF field to a low-pressure gas creating free electrons within the discharge region The electrons are sufficiently energized by the electric field that gas-phase dissociation and ionization of the reactant gases occur when the free electrons collide with them Energetic species are then adsorbed on the film surface where they are subjected to ion and electron bombardment rearrangements reactions with other species new bond formation and film formation and growth Eventually a new layer is deposited on the substrate For example when silane is used as precursor plasma ignition frees up Si and SiH radicals which also crack more silane molecules on their way to the surface of the substrate where silicon is deposited PROCESS DETAILS OF PECVD In this system wafers coming from the previous state ie PSG removal step are being loaded onto the graphite boat where the individual wafers are held by three pins inside the boat It is absolutely necessary for the wafers to touch all the three pins otherwise there will remain a possibility of poor deposition in which

Figure Anti-reflective coating made out of silicon nitride deposited by the PECVD technique whereby silane and ammonia is reacted in a furnace The major color of coating looks blue but the color could also look like purple blue or light blue due to different thickness of coating

21

the HF generator fails to generate the required power for process deposition While loading the double diffusion wafers it should be ensured that deposition side should be correctly loaded The wafers are being loaded parallel to the boat After loading the trolley along with the boat is placed inside the machine and then after filling the input parameters of the CMI interface used here and the process tube press the RUN button and then machine will be in auto mode The handling mechanism automatically picks up the boat and places it on the desired paddle corresponding to the selected for the specified job And then process STARTS

22

SCREEN PRINTING INTRODUCTION In screen printing on wafer-based solar PV cells the mesh and buses of silver are printed on the front furthermore the buses of silver are printed on the back Subsequently aluminum paste is dispensed over the whole surface of the back for passivation and surface reflection [14] One of the parameters that can vary and can be controlled in screen printing is the thickness of the print This makes it useful for some of the techniques of printing solar cells electronics etc Screen printing is one of the most critical processes to maintain high yield Solar wafers are becoming thinner and larger so careful printing is required to maintain a lower breakage rate On the other hand high throughput at the printing stage improves the throughput of the whole cell production line PROCESS This process is divided in three phase-

1 Phase I- AgAl printing 2 Phase II- Ag printing 3 Phase III- Al printing

PHASE I- AgAl PRINTING This phase is required to print the back bus bar for tabbing purpose The AgAl paste used in this printing is Dupont (PV-505) and the consumption of the paste per wafer is 011 gm And the screen type used is of the following specification

1 Mesh size- 200microm 2 Tension- 24 N-cm 3 Mesh angle- 225ordm 45ordm 4 Emulsion thickness- 24microm

Process details During printing the paste is poured on the screen and is spread over it with the help of polypropylene spatula Then the wafers (ARC coated) are pasted with the sunny side down on the loader and picked one by one automatically with the start of the print command Once printing starts the wafers are placed one by one between the tool paste and the screen and then the squeegee moves over the screen and allow the paste through the exposed parts on the screen and gets pasted on the wafers Thus in this way the wafers are printed one by one Two most important parameters to be controlled during this printing are

1 Squeegee Pressure 2 Snap off distance

The next step after printing is drying step DRYING In this step wafers are passed through the dryer with a temperature range of 180ordm-400ordmC PHASE II- Ag PRINTING This phase is required to print the front bus bar for tabbing purpose The Ag paste used in this printing is Dupont (PV-145) and the consumption of the paste per wafer is 014 gm And the screen type used is of the following specification

1 Mesh size- 325microm 2 Tension- 24 N-cm 3 Mesh angle- 225ordm 4 Emulsion thickness- 24microm

1

INTRODUCTION In recent years the world has taken huge strides towards accepting renewable sources of energy as valid sources of power With fast diminishing fossil fuel reserves and the polluting nature of coal and petroleum solar energy has emerged as a clean environmentally acceptable source of energy The mechanism of photoelectric power generation is simple from a theoretical point of view It owes its existence to the photon generated excitation of electrons across a semiconductor p-n junction The practical implementation of this useful phenomenon has given rise to the solar power industry In a series of manufacturing processes the raw semiconductor wafers are transformed to printed solar cells which are further integrated to solar modules and arrays As the world hurdles into an energy crisis and the fragile ecology comes under increasing strain due to pollution it is imperative that we look at alternative renewable sources of energy This report looks into the theoretical and practical aspects of Module manufacturing and the role solar energy has to play in the near and distant future Some of the appealing characteristics of solar energy are strikingly obvious these include no fuel consumption no pollution wide power handling capabilities and high power to weight ratio A more in-depth analysis is done in the pages that follow

2

AN OVERVIEW OF A SOLAR CELLMODULE MANUFACTURING UNIT

Manufacture of solar modules follows a series of well defined sequences There is scope for much modification and choice of raw materials andor processes that a manufacturing firm can adopt while producing modules However the basic methodology is the same no matter how different individual steps may be During the course of production engineers may tweak process parameters to achieve peak efficiency However the fundamental infrastructure that the firm must invest in are essentially constant among manufacturers The following paragraphs aim to provide the brief outline of any manufacturing unit regardless of the manufacturer A DI UNIT A Deionised (DI) Water plant is fundamental to any unit It provides water free from charged particles to all units that require it B ETCHING UNIT A series of polypropylene tanks form the etching and texturisation unit All equipment and tanks are labeled This has mechanized and automated robot arms that simplify the immersion and handling of wafer chests C DIFFUSION UNIT The diffusion unit has a multi tube diffusion furnace with its accessory units Diffusion system consists of the furnace load station cross flow box gas cabinet and control system The wafers are inserted in batches in quartz boats After diffusion the formation of the p-n junction has been completed we move into the plasma etching process This performs edge isolation which is crucial to prevent short circuits between the front and back surfaces of the wafer D PLASMA UNIT Plasma etching is done in an evacuated chamber in humidity controlled room Freon gas and RF power are the inputs along with the wafer Time varies from 28 minutes to 40 minutes This is followed by rinsing and drying E PSG Removal removes the phosphosilicate glass The next step leads to the manufacture of solar modules F PECVD (Plasma Enhanced Chemical Vapor Deposition) A solar module plant has a PECVD unit This deposits an antireflective layer on the wafer Silane and Ammonia are combined to give Silicon Nitride Hydrogen provides excellent surface and bulk passivization by saturating dangling bonds An abatement system is provided to detoxify harmful wastes To form the Metallization and front and back electrodes the process of SCREEN PRINTING is carried out This is typically a 3 phase process AgAl Ag and Al are printed in paste form and allowed to dry This is followed by the testing and sorting process whereby cells are arranged into various classes depending on their I-V curve (and hence efficiency) The entire process is automated The work of a Photovoltaic firm does not end with the creation of cells After Cell manufacturing the MODULE PRODUCTION UNIT comes into play As each cell is not sufficient to provide enough power for household and industrial purposes they are strung together to form modules Tabbing is done to provide connections for the stringing of cells This is followed by lamination to form a multilayered structure that protects the cell Framing is done to provide structural rigidity A junction box is provided for the tapping of electrical power Module testing is done to check if each module meets expectations In short these are the essential units and processes common to all photovoltaic cellmodule manufacturing plants

3

NEED FOR SOLAR ENERGY According to the World Health Organization 3 million people are killed worldwide by outdoor air pollution annually from vehicles and industrial emissions and 16 million indoors through the use of solid fuel Although technology has made oil extraction more efficient the world has to struggle to provide oil by using increasingly costly and less productive methods such as deep sea drilling and developing environmentally sensitive areas such as the Arctic National Wildlife Refuge Global surface temperature increased 074 plusmn 018 degC (133 plusmn 032 degF) during the last century The Intergovernmental Panel on Climate Change (IPCC) concludes that increasing greenhouse gas concentrations resulting from human activity such as fossil fuel burning and deforestation are responsible for most of the observed temperature increase since the middle of the 20th century Mitigation of global warming is accomplished through reductions in the rate of man-made greenhouse gas release The worlds population continues to grow at a quarter of a million people per day increasing the consumption of energy Currently fossil fuels provided around 66 of the worlds electrical power and 95 of the worlds total energy demands

The 1973 Oil crisis due to the OPEC embargo The 1979 (or second) oil crisis (due to the Iranian Revolution) the 1990 oil price shock (due to the First Gulf War) lead to drastic rise in the price of crude oil the most

4

indispensable source of energy for modern industry to function From the mid-1980s to September 2003 the inflation-adjusted price of a barrel of crude oil on NYMEX was generally under $25barrel During 2003 the price rose above $30 reached $60 by August 11 2005 and peaked at $14730 in July 2008

Thus fossil fuels are proven to be polluting and severely detrimental to human health We are paying the price for it in terms of respiratory diseases and a looming energy crisis As the oil prices are volatile and are steadily but unpredictably rising it is expedient to look beyond polluting fossil fuels as the sole source of power

Todays solar systems for solar heating are highly efficient and easy to install The initial cost is high but the payoff is long lasting Solar systems pay for themselves in about half their lifetime It is a free source of energy unlike oil coal or natural gas The environment impact of solar power is low Solar is clean power There are no emissions no maintenance costs and no unstable fuel costs Solar power is reliable with no outages There are no moving parts to replace or break Solar panels can withstand most extreme weather conditions Saving money energy conservation and clean power are the best benefits of solar energy

In response to the petroleum crisis the principles of green energy and sustainable living movements gain popularity This has led to increasing interest in alternate powerfuel research The Sun is 150 million kilometers away but is amazingly powerful Covering 4 of the worlds desert area with photovoltaic could supply all of the worlds electricity In such a situation development of solar energy should be a priority for every nation

5

RAW MATERIALS NEEDED

Along with the enormous capital investment required to successfully set up a solar cellmodule manufacturing plant it also requires a steady supply of raw materials The most basic raw materials is the silicon wafer itself which is usually bought from another company rather than prepared in-house Two types of crystalline silicon solar cell are single crystalline silicon solar cell which is known as monocrystalline silicon solar cell made by slicing wafers and polycrystalline silicon solar cell made by sawing a block of silicon first into bars and then wafers Other semiconductors such as gallium arsenide cadmium telluride and copper indium diselenide etc have both single crystalline and polycrystalline forms The solar cell which made by gallium arsenide has high efficiency ranging from 20-25 The first process Etching requires a number of chemicals HF HCL NaOH which may vary depending on the etching methodology (Acidic etching or Alkali etching) Deionised water is obtained from the DI plant IPA is used for cleaning purposes For diffusion an MRL diffusion furnace can be used which utilizes N2 amp O2

N2 flows through a bubbler of POCl3

for Phosphorus diffusion HF HCl and DI water are also used to clean the components (bubbler quartz tube etc) used in Diffusion For plasma etching we need a gas like Freon to provide the ions used in edge isolation Silane and Ammonia in gaseous form are needed to perform PECVD in order to deposit the anti reflective coating Silicon Nitride is formed due to the reaction of Silane and Ammonia Screen Printing is a three phase process The first phase is AgAl which could use the paste Dupont PV505 The second front side printing uses an Ag pastes (Dupont PV145) Third phase uses Aluminium paste (Analog 12D)

Module manufacturing requires its own set of raw materials It needs Tedlar EVA and Glass to form the multilayered structure Aluminium frames are needed to provide structural integrity and a junction box for output terminals Thus a manufacturing plant needs a variety of raw materials at the right time in the right quantity quality and purity A fall is any of these parameters will severely hamper the manufacturing process and negatively effect margins Thus ensuring a proper supply of this varied mix of materials is of paramount importance to the managers

6

HISTORY OF PHOTOVOLTAICS Most people are surprised to learn that photovoltaic technology actually dates back over 160 years French physicist Edmond Becquerel first described the photovoltaic (PV) effect in 1839 but it remained a curiosity of science for the next three quarters of a century At only 19 Becquerel found that certain materials would produce small amounts of electric current when exposed to light The effect was first studied in solids such as selenium by Heinrich Hertz in the 1870s Soon afterward selenium PV cells were converting light to electricity at 1 to 2 efficiency As a result selenium was quickly adopted in the emerging field of photography for use in light-measuring devices Major steps toward commercializing PV were taken in the 1940s and early 1950s when the Czochralski process was developed for producing highly pure crystalline silicon In 1954 scientists at Bell Laboratories depended on the Czochralski process to develop the first crystalline silicon photovoltaic cell which had an efficiency of 4 Although a few attempts were made in the 1950s to use silicon cells in commercial products it was the new space program that gave the technology its first major application In the 1970s research drives PV costs down 80 allowing for applications such as offshore navigation warning lights and horns lighthouses railroad crossings and remote use where utility-grid connections are too costly Despite these advances PV devices in 1970 were still too expensive for most mass market uses But in the mid-1970s rising energy costs sparked by a world oil crisis renewed interest in making PV technology more affordable Since then the federal government industry and research organizations have invested billions of dollars in research development and production Often industry and the government work together sharing the cost of PV research and development In the 1980s photovoltaic became a popular power source for consumer electronic devices including calculators watches radios lanterns and other small battery-charging applications

Todays commercial PV systems can convert from 11 to 17 of sunlight into electricity They are highly reliable and last 20 years or longer The cost of PV-generated electricity has dropped 15- to 20-fold and PV modules now cost around $6 per watt (W) and produce electricity for as little as 25 to 50 cents per kilowatt-hour (kWh)

7

Solar Cell Manufacturing Processes

8

ETCHING

INTRODUCTION

Etching is the process of using strong acid or mordant to cut into the unprotected parts of a metal surface to create a design in intaglio in the metal (the original processmdashin modern manufacturing other chemicals may be used on other types of material) As an intaglio method of printmaking it is along with engraving the most important technique for old master prints and remains widely used today But in wafer fabrication Etching refers to a process by which material is removed from the wafer ie either from the silicon substrate itself or from any film or layer of material on the wafer

DEFINITION

The treatment of a prepared metal surface with acid or other chemical reagent which by differential attack reveals the structure is known as etching In wafer fabrication etching process is generally classified in two classes Dry etching and Wet etching

Normally dry etching is not being used in this solar cells and module manufacturing unit So we will just go on a brief look on wet etching process

WET ETCHING It is a process that utilizes liquid chemicals or etchants to remove materials from the wafer usually in specific patterns defined by photo resist masks on the wafer Materials not covered by these masks are etched away by the chemicals while those covered by the masks are left almost intact A simple wet etching process may just consist of dissolution of the material to be removed in a liquid solvent without changing the chemical nature of the dissolved material In general however a wet etching process involves one or more chemical reactions that consume the original reactants and produce new species A basic wet etching process may be broken down in three steps as follows

1 Diffusion of the etchant to the surface for removal 2 Reaction between the etchant and the material being removed and 3 Diffusion of the reaction byproducts from the reacted surface

Wet etching can be both isotropic ie it proceeds in all directions at the same rate and anisotropic ie it proceeds in only one direction Isotropic Wet Etching A process which proceeds at the same rate in all directions is known as isotropic wet etching The etchants used in this process are isotropic in nature that etches away even the portion of material that is directly under the mask (usually in the shape of a quarter-circle) since its horizontal etching rate is the same as its vertical rate When an isotropic etchant eats away a portion of the material under the mask the etched film is said to have undercut the mask The amount of undercutting is a measure of an etching parameter known as the bias Bias is simply defined as the difference between the lateral dimensions of the etched image and the masked image Thus the mask used in etching must compensate for whatever bias an etchant is known to produce in order to create the desired feature on the wafer

Figure 1 Isotropic wet etching

9

Anisotropic Wet Etching A process which proceeds in only one direction is known as anisotropic wet etching It removes material in the vertical direction only since it will follow the mask patterns on the wafer very faithfully leaving any material covered by mask material basically untouched Since isotropic wet etching process has high bias values which are not practical for use in pattern images which has features of measuring less than 3 microns thus wafer patterns of less than 3 microns are wet etched anisotropically instead of isotropic

Figure 2 Anisotropic wet etching

Advantages of wet etching 1 Low cost 2 High reliability 3 High throughput 4 Excellent selectivity in most cases with respect to both mask and substrate materials

Disadvantages of wet etching 1 Limited resolution 2 Higher safety risks due to the direct chemical exposure of the personnel 3 High cost of etchants in some cases 4 Problems related to the resists loss of adhesion to the substrate 5 Problems related to the formation of bubbles which inhibit the etching process where they are present and 6 Problems related to incomplete or non-uniform etching

Before moving on to texturization we first of all must do Saw damage removal which is also called as 1st Etch First Etch When ingots obtained from the Czochralski method are sliced into wafers by saw then it leaves some damaged marks on the wafers if it is not being sliced properly which makes the surface non-uniform So to make the surface uniform and free of error it becomes mandatory to do saw damage removal or 1st

1 First wafers are kept in the solution of 300ml DI water and 10kg NaOH(RSE) within the temperature range of 75⁰plusmn 2⁰C for 1min The reaction which takes place in this step is

etching Steps involved are

Si + 2NaOH + H2O (DI) -gt Na3SiO2 + H2 + H2

2 Finally these etched wafers are washed with DI water O

10

TEXTURIZATION INTRODUCTION Since 1960 for the improvement of the cell performance texturization has been attempted Texturization of front andor back surfaces of a solar cell surfaces improve their performances The reflection from chemically textured wafer reduced from 34 for a polished silicon surface to 11 Many researchers have reported the selective etching of silicon of lt100gt orientation of using sodium or potassium hydroxide solution OBJECTIVES OF TEXTURIZATION The major objectives of texturization are-

1 Reduction of front surface reflectance 2 Increase in the path length of the entered light of lower wavelengths by oblique trajectory within the

cell and absorption of light closer to the junction 3 Optical trapping of weakly absorbed light by multiple internal reflections from the top and bottom

surfaces of the cell thereby also increasing the optical path length The increased path length with an oblique trajectory leads to an effective increase in the absorption co-efficient

All these effects lead to realization of higher performance thick cells and also efficient solar cells of comparatively lower thickness which significantly reduces the material cost without degrading the cell performance MODELS OF TEXTURIZATION To reduce the reflection co-efficient and to trap weak absorbed light various texturing geometries are being employed The different texturing geometries include

1 Lambertian geometry 2 Upright pyramid geometry 3 Slat or Micro-grooved geometry 4 Inverted pyramid geometry 5 Simple prism pyramid geometry 6 Grating geometry 7 Three perpendicular planes (3PP) geometry 8 Perpendicular slat geometry 9 Porous silicon

Now in large number of manufacturing industries Inverted pyramid texturing geometry scheme is being used these days Letrsquos have a look on this scheme very briefly Inverted Pyramid Geometry The light trapping properties of the inverted pyramid geometry has been first investigated by Smith and Rohatgi The increase in current for the type of textured cell is due to incorporation of al the three effects necessary for an efficient light trapping design The front surface reflectance is reduced by providing the opportunity for a portion of the incoming light to undergo a triple bounce thereby reducing the overall front surface reflectance The increase in path length and light trapping efficiency means that a larger fraction of the light which has entered the cell will be absorbed before exiting the cell The short wavelength spectral response analysis at the texturing angle of 5375ordm indicates that 37 of the incoming light experiences a triple bounce on the front surface of the inverted pyramid geometry and the rest percentage gets double bounce The inverted pyramids on a 100microm cell with a two layer AR coating is estimated to give 402mAcm2 and cell efficiency of 24 with realistic cell design and material parameters

11

This texturization method can be employed on both mono-crystalline and multi-crystalline substrates Letrsquos have a look on this Mono-crystalline substrates For mono-crystalline substrate it is advantageous to make use of the anisotropic etching properties of Si in an alkaline solution As the 111 planes get etched more slowly than other crystal planes 111 facets are developed On lt100gtwafers this leads to pyramidal shapes at the surface that are particularly effective in reducing reflectance A regular array of pyramidal pits with facets at 547ordm to the horizontal plane mdash called inverted pyramids mdash can be formed using oxide etch mask with reflectances as low as 8 without ARC

Multi-crystalline Substrates Due to the anisotropic nature of multi-crystalline silicon substrate alkaline random texturing is not effective Some grains remain untextured leading to a high average reflectance So for texturing multi-crystalline silicon wafers should be etched in an acid mixture of HF and HNO3 because HNO3 tends to oxidize the surface while the HF etches the oxide away These acidic iso-texturing results in lower reflection than traditional anisotropic etching on multi-crystalline material and better conversion efficiency

STANDARD NAOH-IPA TEXTURIZATION APPROACH In order to achieve good uniformity of pyramidal structure on the silicon surface a mixture of NaOH or KOH and isopropyl alcohol (IPA) is generally used for texturization of mono-crystalline silicon solar cell For better texturization the interfacial energy between silicon and ionized electrolyte chemical solution should be reduced to achieve sufficient wettability for the silicon surface which will enhance the pyramid nucleation From a manufacturability point of view it is appealing to combine the saw damage removal and alkaline texturing in one single step in which case a trade-off has to be made between process speed and quality of the surface treatment

Fig1 Inverted Pyramid Textured Surface

Fig3 Textured multi-crystalline Si surface

Fig2 Anisotropic Etching of Mono-crystalline Si

12

DIFFUSION

INTRODUCTION

Diffusion the movement of a chemical species from an area of high concentration to an area of lower concentration is one of the two major processes by which chemical species or dopants are introduced into a semiconductor (the other one being ion implantation) The controlled diffusion of dopants into silicon to alter the type and level of conductivity of semiconductor materials is the foundation of forming a p-n junction The mathematics that govern the mass transport phenomena of diffusion are based on Ficks laws There are two laws according to Fick Letrsquos have a look on these laws First law Whenever an impurity concentration gradient partCpartx exists in a finite volume of a matrix substance (the silicon substrate in this context) the impurity material will have the natural tendency to move in order to distribute itself more evenly within the matrix and decrease the gradient Given enough time this flow of impurities will eventually result in homogeneity within the matrix causing the net flow of impurities to stop The mathematics of this transport mechanism was formalized in 1855 by Fick who postulated that the flux of material across a given plane is proportional to the concentration gradient across the plane Thus Ficks First Law states

J = -D ( partC(xt)partx ) Where J is the flux D is the diffusion constant for the material that is diffusing in the specific solvent and partC(xt)partx is the concentration gradient The diffusion constant of a material is also referred to as diffusion coefficient or simply diffusivity It is expressed in units of length2time such as microm2hour The negative sign of the right side of the equation indicates that the impurities are flowing in the direction of lower concentration Second Law Ficks First Law does not consider the fact that the gradient and local concentration of the impurities in a material decreases with an increase in time an aspect thats important to diffusion processes The flux J1 of impurities entering a section of a bar with a concentration gradient is different from the flux J2 of impurities leaving the same section From the law of conservation of matter the difference between J1 and J2 must result in a change in the concentration of impurities within the section (assuming that no impurities are formed or consumed in the section) This is Ficks Second Law which states that the change in impurity concentration over time is equal to the change in local diffusion flux or

partC(xt)partt = - partJpartx or from Ficks First Law

partC(xt)partt = part(DpartC(xt)partx)partx If the diffusion coefficient is independent of position such as when the impurity concentration is low then Ficks Second Law may be further simplified into the following equation

partC(xt)partt = D part2C(xt)partx2 The most important property of diffusion process in the manufacture of silicon solar cell is the formation of p-n junction by doping n-type material ie phosphorous on the p-type ie boron doped silicon textured wafer playing an essential role for photovoltaic effect Let us have a brief descriptive look on the properties of p-n junction P-N JUNCTION A p-n junction is a junction formed by combining p-type and n-type semiconductors together in very close contact

13

Principle of p-n junction The most common type of solar cell is a large p-n junction where the free carrier pairs emitted by light energy are separated by the junction and contribute to the current In its simplest form it consists of a junction formed between n-type and p-type semiconductors either of the same material (homo-junction) or different materials (hetero-junction) The band-structure of the two differently doped sides with respect to their Fermi levels can be seen in adjacent figure1

When the two halves are brought together the Fermi levels on either side are forced in to coincidence causing the valence and conduction bands to bend (Figure 2)

These bent bands represent a built-in electric field over what is referred to as the depletion region When a photon with a energy greater than the band-gap of the semiconductor passes through the solar cell it may be absorbed by the material This absorption takes the form of a band-to-band electronic transition so an electronhole pair is produced If these carriers can diffuse to the depletion region before they recombine then they are separated by the electric field causing one quantum of charge to flow through an external load This is the origin of the solar cells photocurrent and is shown in Figure 3

14

SOLAR CELL AS A P-N JUNCTION A solar cell is essential a p-n junction with large surface area The n type material is kept thin to allow light to pass through to the p-n junction

Light travels in packets of energy called photons The generation of electric current happens inside the depletion zone of the PN junction The depletion region as explained previously with the diode is the area around the PN junction where the electrons from the N-type silicon have diffused into the holes of the P-type material When a photon of light is absorbed by one of these atoms in the N-Type silicon it will dislodge an electron creating a free electron and a hole The free electron and hole has sufficient energy to jump out of the depletion zone If a wire is connected from the cathode (N-type silicon) to the anode (P-type silicon) electrons will flow through the wire The electron is attracted to the positive charge of the P-type material and travels through the external load (meter) creating a flow of electric current The hole created by the dislodged electron is attracted to the negative charge of N-type material and migrates to the back electrical contact As the electron enters the P-type silicon from the back electrical contact it combines with the hole restoring the electrical neutrality

DIFFUSION PROCESS

The first layer formation after doing texturization of wafer is performed in this process by doping the substrate with n-type dopant N-doping can be accomplished by depositing n dopant onto the substrate and then heating the substrate to drive the n dopant into the substrate Gaseous diffusion (for example POCl3 horizontal tube diffusion including temperatures of from 700 to 1000degC preferably 800 to 900degC) can be used to deposit the n-dopant onto the substrate surface to create an n-doped layer and a shallow p-n junction proximal to the substrate surface In this process if wafers are loaded onto the quartz boat back to back in the same lot then double diffusion process takes place and if single wafers are being loaded then single diffusion takes place On the quartz boat wafers can be loaded in two methods

1 Parallel diffusion loading of wafers parallel to the gas flow 2 Perpendicular diffusion loading of wafers perpendicular to the gas flow

Diffusion process is comprised of the following steps 1 Idle state-In this state the load center and source zone are set at 800ordmC and no process is

carried out in the furnace 2 Load-During this sequence the silicon carbide paddle moves at a speed of 45 rpm and the

maximum time taken is 7min 3 Ramp-The cold wafers loaded into the tubes bring down the temperature of the tube The tube

is then reheated to reach the set point temperature and the time taken is 7min

15

4 Stabilization-During this phase any overshooting of temperature during ramp-up phase is controlled to the predetermined set value and the time taken is 10min

5 Pre-deposition-Any excess POCL3 present in vapour form inside the quartz bubbler is vent out before the deposition starts by N2

6 Deposition- N purging in 5 sec

2 is bubbled through liquid POCL3 in presence of O2 to form P2O5

7 Post-deposition-During this phase O in paste form

2 and N2

8 N

flows to drive in the phosphorous from wafer surface to the junction

2

9 Unload-During this phase the SiC paddle moves slowly out of the furnace at set speed of 45rpm and maximum time taken is 7min

Purge-This phase is also known as drive in diffusion which is designed to evenly distribute the impurities into wafer and reduce the generally undesirable high concentration in the wafer surface and the maximum time taken is 7min

10 End-The tube is set to 800⁰C temperature in idle state

16

PLASMA ETCHING

INTRODUCTION

Plasma etching is one type of dry etching which uses plasma to produce chemically reactive species from inert gases The reactive gases are then made to react with the material to be etched The main objective of this process is to remove edge shunts present on the cell which leads to 80 loss and thus it is also termed as Edge isolation method Since edge isolation is the main motive of this process so first let us see that why edge isolation is so important

Edge isolation

In the standard processing sequence the entire wafer is phosphorous diffused The contact to the p-type base is made by firing an Al containing paste that produces a high doping of Al thus over-compensating the shallow emitter diffusion However there is still a direct conductive path from the emitter on the front side to the rear p contact Therefore a large portion of the photo-generated current can directly flow through the emitter to the back contact instead of flowing through the external circuit This current path is modeled in the two-diode model of the solar cell as the parallel resistance R

where R

p

s and Rp are the series and parallel resistance respectively j01 and j02 represent the first and second diode saturation current densities respectively n1 and n2 are the first and second ideality diode factors respectively Vth = ktq is the thermal voltage and jL

In this model the first diode accounts for emitter and bulk recombination with a recombination rate assumed to be proportional to the excess carrier density This leads to an ideality factor of n

is the light generated current density The two diode model is as shown below

1=1 The second diode is associated with the recombination in the space charge region that is assumed to be independent of the injection level leading to n2=2 The effect of increased j02 and Rp can be shown by model calculations for ideal solar cells in the figure below for a fill factor potential of 84 j02 must be less than 2x10-9 Acm2 and Rp must be greater than approximately 2000Ω cm2 if the respective parameters contribute solely to the fill factor loss

17

This increase in Rp is achieved by the edge isolation process that interrupts the parasitic current path as shown in fig2 below

Thus edge isolation is required in solar cell to increase open circuit voltage and thus leading to greater efficiency

Now we will move on to plasma etching process to see that how it works actually in solar cell and module manufacturing industries

Plasma Etching Process

After diffusion a layer of phosphosilicate glass is formed on the edges and rear surface of diffused wafer The process generally requires adequate physical masking of the front surface while exposing the full back of the cell to the dry plasma environment In an embodiment of this process cells are slotted face to face

Fig2Sketch of an industrial screen-printed solar cell The emitter diffusion is present on the complete wafer surface and gets locally overcompensated by printing and firing Al containing metal paste as back contact Without edge-isolation light generated e-s can flow through the emitter region to the back contact which corresponds to the small series resistance in the two diode model

18

into inert carriers so that the plasma chemistry can effectively react with the back of the cells thereby removing excess material from the second coating that might have reached the back of the cell This process is preferably performed via the generation of an RF (13 6 MHz) plasma using a mix of Freon 14 (CF4) with oxygen (O2) and or nitrogen (N2) in a ratio which depends on the chamber and load geometries

19

PHOSPHOSILICATE GLASS REMOVAL INTRODUCTION In diffusion process during deposition phase N2 gas is bubbled through liquid POCL3

The reaction related to this phase is given as follows

1 4POCL3 + 3O2 -gt 2P2O5 + 6Cl 2 2P

2 2O5 + 5Si -gt 5SiO2

+ 4P (PSG)

P2O5

So it becomes mandatory to remove phosphosilicate glass by chemicals which may act as recombination sites or may contribute towards the surface density defect and hence all these defects may contribute to a lower voltage and current

obtained in first reaction step is in paste form and is transferred to the Teflon duct to the process tube which then reacts with silicon of which the wafer is made And thus when this phosphorous pentoxide reaches silicon surface then second reaction takes place and thus a silicon dioxide layer and phosphosilicate glass ie PSG forms on the wafer surface Formation of this dead layer on the wafer surface contributes to a lower efficiency

PROCESS OF REMOVAL Following are the steps that should be taken into consideration for the removal of phosphosilicate glass (PSG) from the wafer surface

1 Firstly wafers are washed combinedly with HF HCL and DI water in a tank for 1min 2 Again wafers are washed with HF and DI water for 2min in the other tank 3 For another 1 min DI water is being sprayed on the diffused wafers 4 Next wafers are washed with DI water subsequently on an interval of 30 sec 5 Finally wafers are kept in the OVEN for 10mins within the temperature range of 95plusmn5ordmC

20

PLASMA ENHANCED CHEMICAL VAPOUR DEPOSITION INTRODUCTION In recent years the low-temperature (le 400ordmC) surface passivation of crystalline silicon solar cells by means of PECVD silicon nitride films has attracted an increasing attention of the PV community These films have proven to be capable of combining an outstanding surface passivation quality with excellent anti-reflection properties On typical p-type silicon solar cells substrate it has been shown that the surface passivation quality of very silicon rich Si3N4

we must first know that whatrsquos the use of silicon nitride coating on the surface of the wafer

films is superior to that of state-of-the-art high temperature silicon dioxide Before proceeding to the details of PECVD system

WHY SILICON NITRIDE SURFACE COATING IS PREFERRED IN PECVD PROCESS It is preferred as a surface coating because it functions as An effective diffusion mask An effective metallization diffusion barrier An anti-reflective coating Provide surface and bulk passivation

Preferably hydrogen is trapped in such a silicon nitride surface coating PECVD SYSYTEM PECVD or Plasma Enhanced Chemical Vapor Deposition is one of the methods of different Chemical Vapor Deposition (CVD) technique In PECVD plasma is being used to decompose a reactant gas such as silane (SiH4) to produce reaction products that precipitate on the surface of the substrate as a new layer

Working Principle Inside a PECVD reactor a strong electric field ignites plasma between two electrodes one of which holds the substrate This plasma ignition cracks the molecular bonds of the process gas which in turn are able to crack more process gas molecules before reaching the surface of the substrate The glow discharge used in this reactor is created by applying an RF field to a low-pressure gas creating free electrons within the discharge region The electrons are sufficiently energized by the electric field that gas-phase dissociation and ionization of the reactant gases occur when the free electrons collide with them Energetic species are then adsorbed on the film surface where they are subjected to ion and electron bombardment rearrangements reactions with other species new bond formation and film formation and growth Eventually a new layer is deposited on the substrate For example when silane is used as precursor plasma ignition frees up Si and SiH radicals which also crack more silane molecules on their way to the surface of the substrate where silicon is deposited PROCESS DETAILS OF PECVD In this system wafers coming from the previous state ie PSG removal step are being loaded onto the graphite boat where the individual wafers are held by three pins inside the boat It is absolutely necessary for the wafers to touch all the three pins otherwise there will remain a possibility of poor deposition in which

Figure Anti-reflective coating made out of silicon nitride deposited by the PECVD technique whereby silane and ammonia is reacted in a furnace The major color of coating looks blue but the color could also look like purple blue or light blue due to different thickness of coating

21

the HF generator fails to generate the required power for process deposition While loading the double diffusion wafers it should be ensured that deposition side should be correctly loaded The wafers are being loaded parallel to the boat After loading the trolley along with the boat is placed inside the machine and then after filling the input parameters of the CMI interface used here and the process tube press the RUN button and then machine will be in auto mode The handling mechanism automatically picks up the boat and places it on the desired paddle corresponding to the selected for the specified job And then process STARTS

22

SCREEN PRINTING INTRODUCTION In screen printing on wafer-based solar PV cells the mesh and buses of silver are printed on the front furthermore the buses of silver are printed on the back Subsequently aluminum paste is dispensed over the whole surface of the back for passivation and surface reflection [14] One of the parameters that can vary and can be controlled in screen printing is the thickness of the print This makes it useful for some of the techniques of printing solar cells electronics etc Screen printing is one of the most critical processes to maintain high yield Solar wafers are becoming thinner and larger so careful printing is required to maintain a lower breakage rate On the other hand high throughput at the printing stage improves the throughput of the whole cell production line PROCESS This process is divided in three phase-

1 Phase I- AgAl printing 2 Phase II- Ag printing 3 Phase III- Al printing

PHASE I- AgAl PRINTING This phase is required to print the back bus bar for tabbing purpose The AgAl paste used in this printing is Dupont (PV-505) and the consumption of the paste per wafer is 011 gm And the screen type used is of the following specification

1 Mesh size- 200microm 2 Tension- 24 N-cm 3 Mesh angle- 225ordm 45ordm 4 Emulsion thickness- 24microm

Process details During printing the paste is poured on the screen and is spread over it with the help of polypropylene spatula Then the wafers (ARC coated) are pasted with the sunny side down on the loader and picked one by one automatically with the start of the print command Once printing starts the wafers are placed one by one between the tool paste and the screen and then the squeegee moves over the screen and allow the paste through the exposed parts on the screen and gets pasted on the wafers Thus in this way the wafers are printed one by one Two most important parameters to be controlled during this printing are

1 Squeegee Pressure 2 Snap off distance

The next step after printing is drying step DRYING In this step wafers are passed through the dryer with a temperature range of 180ordm-400ordmC PHASE II- Ag PRINTING This phase is required to print the front bus bar for tabbing purpose The Ag paste used in this printing is Dupont (PV-145) and the consumption of the paste per wafer is 014 gm And the screen type used is of the following specification

1 Mesh size- 325microm 2 Tension- 24 N-cm 3 Mesh angle- 225ordm 4 Emulsion thickness- 24microm

2

AN OVERVIEW OF A SOLAR CELLMODULE MANUFACTURING UNIT

Manufacture of solar modules follows a series of well defined sequences There is scope for much modification and choice of raw materials andor processes that a manufacturing firm can adopt while producing modules However the basic methodology is the same no matter how different individual steps may be During the course of production engineers may tweak process parameters to achieve peak efficiency However the fundamental infrastructure that the firm must invest in are essentially constant among manufacturers The following paragraphs aim to provide the brief outline of any manufacturing unit regardless of the manufacturer A DI UNIT A Deionised (DI) Water plant is fundamental to any unit It provides water free from charged particles to all units that require it B ETCHING UNIT A series of polypropylene tanks form the etching and texturisation unit All equipment and tanks are labeled This has mechanized and automated robot arms that simplify the immersion and handling of wafer chests C DIFFUSION UNIT The diffusion unit has a multi tube diffusion furnace with its accessory units Diffusion system consists of the furnace load station cross flow box gas cabinet and control system The wafers are inserted in batches in quartz boats After diffusion the formation of the p-n junction has been completed we move into the plasma etching process This performs edge isolation which is crucial to prevent short circuits between the front and back surfaces of the wafer D PLASMA UNIT Plasma etching is done in an evacuated chamber in humidity controlled room Freon gas and RF power are the inputs along with the wafer Time varies from 28 minutes to 40 minutes This is followed by rinsing and drying E PSG Removal removes the phosphosilicate glass The next step leads to the manufacture of solar modules F PECVD (Plasma Enhanced Chemical Vapor Deposition) A solar module plant has a PECVD unit This deposits an antireflective layer on the wafer Silane and Ammonia are combined to give Silicon Nitride Hydrogen provides excellent surface and bulk passivization by saturating dangling bonds An abatement system is provided to detoxify harmful wastes To form the Metallization and front and back electrodes the process of SCREEN PRINTING is carried out This is typically a 3 phase process AgAl Ag and Al are printed in paste form and allowed to dry This is followed by the testing and sorting process whereby cells are arranged into various classes depending on their I-V curve (and hence efficiency) The entire process is automated The work of a Photovoltaic firm does not end with the creation of cells After Cell manufacturing the MODULE PRODUCTION UNIT comes into play As each cell is not sufficient to provide enough power for household and industrial purposes they are strung together to form modules Tabbing is done to provide connections for the stringing of cells This is followed by lamination to form a multilayered structure that protects the cell Framing is done to provide structural rigidity A junction box is provided for the tapping of electrical power Module testing is done to check if each module meets expectations In short these are the essential units and processes common to all photovoltaic cellmodule manufacturing plants

3

NEED FOR SOLAR ENERGY According to the World Health Organization 3 million people are killed worldwide by outdoor air pollution annually from vehicles and industrial emissions and 16 million indoors through the use of solid fuel Although technology has made oil extraction more efficient the world has to struggle to provide oil by using increasingly costly and less productive methods such as deep sea drilling and developing environmentally sensitive areas such as the Arctic National Wildlife Refuge Global surface temperature increased 074 plusmn 018 degC (133 plusmn 032 degF) during the last century The Intergovernmental Panel on Climate Change (IPCC) concludes that increasing greenhouse gas concentrations resulting from human activity such as fossil fuel burning and deforestation are responsible for most of the observed temperature increase since the middle of the 20th century Mitigation of global warming is accomplished through reductions in the rate of man-made greenhouse gas release The worlds population continues to grow at a quarter of a million people per day increasing the consumption of energy Currently fossil fuels provided around 66 of the worlds electrical power and 95 of the worlds total energy demands

The 1973 Oil crisis due to the OPEC embargo The 1979 (or second) oil crisis (due to the Iranian Revolution) the 1990 oil price shock (due to the First Gulf War) lead to drastic rise in the price of crude oil the most

4

indispensable source of energy for modern industry to function From the mid-1980s to September 2003 the inflation-adjusted price of a barrel of crude oil on NYMEX was generally under $25barrel During 2003 the price rose above $30 reached $60 by August 11 2005 and peaked at $14730 in July 2008

Thus fossil fuels are proven to be polluting and severely detrimental to human health We are paying the price for it in terms of respiratory diseases and a looming energy crisis As the oil prices are volatile and are steadily but unpredictably rising it is expedient to look beyond polluting fossil fuels as the sole source of power

Todays solar systems for solar heating are highly efficient and easy to install The initial cost is high but the payoff is long lasting Solar systems pay for themselves in about half their lifetime It is a free source of energy unlike oil coal or natural gas The environment impact of solar power is low Solar is clean power There are no emissions no maintenance costs and no unstable fuel costs Solar power is reliable with no outages There are no moving parts to replace or break Solar panels can withstand most extreme weather conditions Saving money energy conservation and clean power are the best benefits of solar energy

In response to the petroleum crisis the principles of green energy and sustainable living movements gain popularity This has led to increasing interest in alternate powerfuel research The Sun is 150 million kilometers away but is amazingly powerful Covering 4 of the worlds desert area with photovoltaic could supply all of the worlds electricity In such a situation development of solar energy should be a priority for every nation

5

RAW MATERIALS NEEDED

Along with the enormous capital investment required to successfully set up a solar cellmodule manufacturing plant it also requires a steady supply of raw materials The most basic raw materials is the silicon wafer itself which is usually bought from another company rather than prepared in-house Two types of crystalline silicon solar cell are single crystalline silicon solar cell which is known as monocrystalline silicon solar cell made by slicing wafers and polycrystalline silicon solar cell made by sawing a block of silicon first into bars and then wafers Other semiconductors such as gallium arsenide cadmium telluride and copper indium diselenide etc have both single crystalline and polycrystalline forms The solar cell which made by gallium arsenide has high efficiency ranging from 20-25 The first process Etching requires a number of chemicals HF HCL NaOH which may vary depending on the etching methodology (Acidic etching or Alkali etching) Deionised water is obtained from the DI plant IPA is used for cleaning purposes For diffusion an MRL diffusion furnace can be used which utilizes N2 amp O2

N2 flows through a bubbler of POCl3

for Phosphorus diffusion HF HCl and DI water are also used to clean the components (bubbler quartz tube etc) used in Diffusion For plasma etching we need a gas like Freon to provide the ions used in edge isolation Silane and Ammonia in gaseous form are needed to perform PECVD in order to deposit the anti reflective coating Silicon Nitride is formed due to the reaction of Silane and Ammonia Screen Printing is a three phase process The first phase is AgAl which could use the paste Dupont PV505 The second front side printing uses an Ag pastes (Dupont PV145) Third phase uses Aluminium paste (Analog 12D)

Module manufacturing requires its own set of raw materials It needs Tedlar EVA and Glass to form the multilayered structure Aluminium frames are needed to provide structural integrity and a junction box for output terminals Thus a manufacturing plant needs a variety of raw materials at the right time in the right quantity quality and purity A fall is any of these parameters will severely hamper the manufacturing process and negatively effect margins Thus ensuring a proper supply of this varied mix of materials is of paramount importance to the managers

6

HISTORY OF PHOTOVOLTAICS Most people are surprised to learn that photovoltaic technology actually dates back over 160 years French physicist Edmond Becquerel first described the photovoltaic (PV) effect in 1839 but it remained a curiosity of science for the next three quarters of a century At only 19 Becquerel found that certain materials would produce small amounts of electric current when exposed to light The effect was first studied in solids such as selenium by Heinrich Hertz in the 1870s Soon afterward selenium PV cells were converting light to electricity at 1 to 2 efficiency As a result selenium was quickly adopted in the emerging field of photography for use in light-measuring devices Major steps toward commercializing PV were taken in the 1940s and early 1950s when the Czochralski process was developed for producing highly pure crystalline silicon In 1954 scientists at Bell Laboratories depended on the Czochralski process to develop the first crystalline silicon photovoltaic cell which had an efficiency of 4 Although a few attempts were made in the 1950s to use silicon cells in commercial products it was the new space program that gave the technology its first major application In the 1970s research drives PV costs down 80 allowing for applications such as offshore navigation warning lights and horns lighthouses railroad crossings and remote use where utility-grid connections are too costly Despite these advances PV devices in 1970 were still too expensive for most mass market uses But in the mid-1970s rising energy costs sparked by a world oil crisis renewed interest in making PV technology more affordable Since then the federal government industry and research organizations have invested billions of dollars in research development and production Often industry and the government work together sharing the cost of PV research and development In the 1980s photovoltaic became a popular power source for consumer electronic devices including calculators watches radios lanterns and other small battery-charging applications

Todays commercial PV systems can convert from 11 to 17 of sunlight into electricity They are highly reliable and last 20 years or longer The cost of PV-generated electricity has dropped 15- to 20-fold and PV modules now cost around $6 per watt (W) and produce electricity for as little as 25 to 50 cents per kilowatt-hour (kWh)

7

Solar Cell Manufacturing Processes

8

ETCHING

INTRODUCTION

Etching is the process of using strong acid or mordant to cut into the unprotected parts of a metal surface to create a design in intaglio in the metal (the original processmdashin modern manufacturing other chemicals may be used on other types of material) As an intaglio method of printmaking it is along with engraving the most important technique for old master prints and remains widely used today But in wafer fabrication Etching refers to a process by which material is removed from the wafer ie either from the silicon substrate itself or from any film or layer of material on the wafer

DEFINITION

The treatment of a prepared metal surface with acid or other chemical reagent which by differential attack reveals the structure is known as etching In wafer fabrication etching process is generally classified in two classes Dry etching and Wet etching

Normally dry etching is not being used in this solar cells and module manufacturing unit So we will just go on a brief look on wet etching process

WET ETCHING It is a process that utilizes liquid chemicals or etchants to remove materials from the wafer usually in specific patterns defined by photo resist masks on the wafer Materials not covered by these masks are etched away by the chemicals while those covered by the masks are left almost intact A simple wet etching process may just consist of dissolution of the material to be removed in a liquid solvent without changing the chemical nature of the dissolved material In general however a wet etching process involves one or more chemical reactions that consume the original reactants and produce new species A basic wet etching process may be broken down in three steps as follows

1 Diffusion of the etchant to the surface for removal 2 Reaction between the etchant and the material being removed and 3 Diffusion of the reaction byproducts from the reacted surface

Wet etching can be both isotropic ie it proceeds in all directions at the same rate and anisotropic ie it proceeds in only one direction Isotropic Wet Etching A process which proceeds at the same rate in all directions is known as isotropic wet etching The etchants used in this process are isotropic in nature that etches away even the portion of material that is directly under the mask (usually in the shape of a quarter-circle) since its horizontal etching rate is the same as its vertical rate When an isotropic etchant eats away a portion of the material under the mask the etched film is said to have undercut the mask The amount of undercutting is a measure of an etching parameter known as the bias Bias is simply defined as the difference between the lateral dimensions of the etched image and the masked image Thus the mask used in etching must compensate for whatever bias an etchant is known to produce in order to create the desired feature on the wafer

Figure 1 Isotropic wet etching

9

Anisotropic Wet Etching A process which proceeds in only one direction is known as anisotropic wet etching It removes material in the vertical direction only since it will follow the mask patterns on the wafer very faithfully leaving any material covered by mask material basically untouched Since isotropic wet etching process has high bias values which are not practical for use in pattern images which has features of measuring less than 3 microns thus wafer patterns of less than 3 microns are wet etched anisotropically instead of isotropic

Figure 2 Anisotropic wet etching

Advantages of wet etching 1 Low cost 2 High reliability 3 High throughput 4 Excellent selectivity in most cases with respect to both mask and substrate materials

Disadvantages of wet etching 1 Limited resolution 2 Higher safety risks due to the direct chemical exposure of the personnel 3 High cost of etchants in some cases 4 Problems related to the resists loss of adhesion to the substrate 5 Problems related to the formation of bubbles which inhibit the etching process where they are present and 6 Problems related to incomplete or non-uniform etching

Before moving on to texturization we first of all must do Saw damage removal which is also called as 1st Etch First Etch When ingots obtained from the Czochralski method are sliced into wafers by saw then it leaves some damaged marks on the wafers if it is not being sliced properly which makes the surface non-uniform So to make the surface uniform and free of error it becomes mandatory to do saw damage removal or 1st

1 First wafers are kept in the solution of 300ml DI water and 10kg NaOH(RSE) within the temperature range of 75⁰plusmn 2⁰C for 1min The reaction which takes place in this step is

etching Steps involved are

Si + 2NaOH + H2O (DI) -gt Na3SiO2 + H2 + H2

2 Finally these etched wafers are washed with DI water O

10

TEXTURIZATION INTRODUCTION Since 1960 for the improvement of the cell performance texturization has been attempted Texturization of front andor back surfaces of a solar cell surfaces improve their performances The reflection from chemically textured wafer reduced from 34 for a polished silicon surface to 11 Many researchers have reported the selective etching of silicon of lt100gt orientation of using sodium or potassium hydroxide solution OBJECTIVES OF TEXTURIZATION The major objectives of texturization are-

1 Reduction of front surface reflectance 2 Increase in the path length of the entered light of lower wavelengths by oblique trajectory within the

cell and absorption of light closer to the junction 3 Optical trapping of weakly absorbed light by multiple internal reflections from the top and bottom

surfaces of the cell thereby also increasing the optical path length The increased path length with an oblique trajectory leads to an effective increase in the absorption co-efficient

All these effects lead to realization of higher performance thick cells and also efficient solar cells of comparatively lower thickness which significantly reduces the material cost without degrading the cell performance MODELS OF TEXTURIZATION To reduce the reflection co-efficient and to trap weak absorbed light various texturing geometries are being employed The different texturing geometries include

1 Lambertian geometry 2 Upright pyramid geometry 3 Slat or Micro-grooved geometry 4 Inverted pyramid geometry 5 Simple prism pyramid geometry 6 Grating geometry 7 Three perpendicular planes (3PP) geometry 8 Perpendicular slat geometry 9 Porous silicon

Now in large number of manufacturing industries Inverted pyramid texturing geometry scheme is being used these days Letrsquos have a look on this scheme very briefly Inverted Pyramid Geometry The light trapping properties of the inverted pyramid geometry has been first investigated by Smith and Rohatgi The increase in current for the type of textured cell is due to incorporation of al the three effects necessary for an efficient light trapping design The front surface reflectance is reduced by providing the opportunity for a portion of the incoming light to undergo a triple bounce thereby reducing the overall front surface reflectance The increase in path length and light trapping efficiency means that a larger fraction of the light which has entered the cell will be absorbed before exiting the cell The short wavelength spectral response analysis at the texturing angle of 5375ordm indicates that 37 of the incoming light experiences a triple bounce on the front surface of the inverted pyramid geometry and the rest percentage gets double bounce The inverted pyramids on a 100microm cell with a two layer AR coating is estimated to give 402mAcm2 and cell efficiency of 24 with realistic cell design and material parameters

11

This texturization method can be employed on both mono-crystalline and multi-crystalline substrates Letrsquos have a look on this Mono-crystalline substrates For mono-crystalline substrate it is advantageous to make use of the anisotropic etching properties of Si in an alkaline solution As the 111 planes get etched more slowly than other crystal planes 111 facets are developed On lt100gtwafers this leads to pyramidal shapes at the surface that are particularly effective in reducing reflectance A regular array of pyramidal pits with facets at 547ordm to the horizontal plane mdash called inverted pyramids mdash can be formed using oxide etch mask with reflectances as low as 8 without ARC

Multi-crystalline Substrates Due to the anisotropic nature of multi-crystalline silicon substrate alkaline random texturing is not effective Some grains remain untextured leading to a high average reflectance So for texturing multi-crystalline silicon wafers should be etched in an acid mixture of HF and HNO3 because HNO3 tends to oxidize the surface while the HF etches the oxide away These acidic iso-texturing results in lower reflection than traditional anisotropic etching on multi-crystalline material and better conversion efficiency

STANDARD NAOH-IPA TEXTURIZATION APPROACH In order to achieve good uniformity of pyramidal structure on the silicon surface a mixture of NaOH or KOH and isopropyl alcohol (IPA) is generally used for texturization of mono-crystalline silicon solar cell For better texturization the interfacial energy between silicon and ionized electrolyte chemical solution should be reduced to achieve sufficient wettability for the silicon surface which will enhance the pyramid nucleation From a manufacturability point of view it is appealing to combine the saw damage removal and alkaline texturing in one single step in which case a trade-off has to be made between process speed and quality of the surface treatment

Fig1 Inverted Pyramid Textured Surface

Fig3 Textured multi-crystalline Si surface

Fig2 Anisotropic Etching of Mono-crystalline Si

12

DIFFUSION

INTRODUCTION

Diffusion the movement of a chemical species from an area of high concentration to an area of lower concentration is one of the two major processes by which chemical species or dopants are introduced into a semiconductor (the other one being ion implantation) The controlled diffusion of dopants into silicon to alter the type and level of conductivity of semiconductor materials is the foundation of forming a p-n junction The mathematics that govern the mass transport phenomena of diffusion are based on Ficks laws There are two laws according to Fick Letrsquos have a look on these laws First law Whenever an impurity concentration gradient partCpartx exists in a finite volume of a matrix substance (the silicon substrate in this context) the impurity material will have the natural tendency to move in order to distribute itself more evenly within the matrix and decrease the gradient Given enough time this flow of impurities will eventually result in homogeneity within the matrix causing the net flow of impurities to stop The mathematics of this transport mechanism was formalized in 1855 by Fick who postulated that the flux of material across a given plane is proportional to the concentration gradient across the plane Thus Ficks First Law states

J = -D ( partC(xt)partx ) Where J is the flux D is the diffusion constant for the material that is diffusing in the specific solvent and partC(xt)partx is the concentration gradient The diffusion constant of a material is also referred to as diffusion coefficient or simply diffusivity It is expressed in units of length2time such as microm2hour The negative sign of the right side of the equation indicates that the impurities are flowing in the direction of lower concentration Second Law Ficks First Law does not consider the fact that the gradient and local concentration of the impurities in a material decreases with an increase in time an aspect thats important to diffusion processes The flux J1 of impurities entering a section of a bar with a concentration gradient is different from the flux J2 of impurities leaving the same section From the law of conservation of matter the difference between J1 and J2 must result in a change in the concentration of impurities within the section (assuming that no impurities are formed or consumed in the section) This is Ficks Second Law which states that the change in impurity concentration over time is equal to the change in local diffusion flux or

partC(xt)partt = - partJpartx or from Ficks First Law

partC(xt)partt = part(DpartC(xt)partx)partx If the diffusion coefficient is independent of position such as when the impurity concentration is low then Ficks Second Law may be further simplified into the following equation

partC(xt)partt = D part2C(xt)partx2 The most important property of diffusion process in the manufacture of silicon solar cell is the formation of p-n junction by doping n-type material ie phosphorous on the p-type ie boron doped silicon textured wafer playing an essential role for photovoltaic effect Let us have a brief descriptive look on the properties of p-n junction P-N JUNCTION A p-n junction is a junction formed by combining p-type and n-type semiconductors together in very close contact

13

Principle of p-n junction The most common type of solar cell is a large p-n junction where the free carrier pairs emitted by light energy are separated by the junction and contribute to the current In its simplest form it consists of a junction formed between n-type and p-type semiconductors either of the same material (homo-junction) or different materials (hetero-junction) The band-structure of the two differently doped sides with respect to their Fermi levels can be seen in adjacent figure1

When the two halves are brought together the Fermi levels on either side are forced in to coincidence causing the valence and conduction bands to bend (Figure 2)

These bent bands represent a built-in electric field over what is referred to as the depletion region When a photon with a energy greater than the band-gap of the semiconductor passes through the solar cell it may be absorbed by the material This absorption takes the form of a band-to-band electronic transition so an electronhole pair is produced If these carriers can diffuse to the depletion region before they recombine then they are separated by the electric field causing one quantum of charge to flow through an external load This is the origin of the solar cells photocurrent and is shown in Figure 3

14

SOLAR CELL AS A P-N JUNCTION A solar cell is essential a p-n junction with large surface area The n type material is kept thin to allow light to pass through to the p-n junction

Light travels in packets of energy called photons The generation of electric current happens inside the depletion zone of the PN junction The depletion region as explained previously with the diode is the area around the PN junction where the electrons from the N-type silicon have diffused into the holes of the P-type material When a photon of light is absorbed by one of these atoms in the N-Type silicon it will dislodge an electron creating a free electron and a hole The free electron and hole has sufficient energy to jump out of the depletion zone If a wire is connected from the cathode (N-type silicon) to the anode (P-type silicon) electrons will flow through the wire The electron is attracted to the positive charge of the P-type material and travels through the external load (meter) creating a flow of electric current The hole created by the dislodged electron is attracted to the negative charge of N-type material and migrates to the back electrical contact As the electron enters the P-type silicon from the back electrical contact it combines with the hole restoring the electrical neutrality

DIFFUSION PROCESS

The first layer formation after doing texturization of wafer is performed in this process by doping the substrate with n-type dopant N-doping can be accomplished by depositing n dopant onto the substrate and then heating the substrate to drive the n dopant into the substrate Gaseous diffusion (for example POCl3 horizontal tube diffusion including temperatures of from 700 to 1000degC preferably 800 to 900degC) can be used to deposit the n-dopant onto the substrate surface to create an n-doped layer and a shallow p-n junction proximal to the substrate surface In this process if wafers are loaded onto the quartz boat back to back in the same lot then double diffusion process takes place and if single wafers are being loaded then single diffusion takes place On the quartz boat wafers can be loaded in two methods

1 Parallel diffusion loading of wafers parallel to the gas flow 2 Perpendicular diffusion loading of wafers perpendicular to the gas flow

Diffusion process is comprised of the following steps 1 Idle state-In this state the load center and source zone are set at 800ordmC and no process is

carried out in the furnace 2 Load-During this sequence the silicon carbide paddle moves at a speed of 45 rpm and the

maximum time taken is 7min 3 Ramp-The cold wafers loaded into the tubes bring down the temperature of the tube The tube

is then reheated to reach the set point temperature and the time taken is 7min

15

4 Stabilization-During this phase any overshooting of temperature during ramp-up phase is controlled to the predetermined set value and the time taken is 10min

5 Pre-deposition-Any excess POCL3 present in vapour form inside the quartz bubbler is vent out before the deposition starts by N2

6 Deposition- N purging in 5 sec

2 is bubbled through liquid POCL3 in presence of O2 to form P2O5

7 Post-deposition-During this phase O in paste form

2 and N2

8 N

flows to drive in the phosphorous from wafer surface to the junction

2

9 Unload-During this phase the SiC paddle moves slowly out of the furnace at set speed of 45rpm and maximum time taken is 7min

Purge-This phase is also known as drive in diffusion which is designed to evenly distribute the impurities into wafer and reduce the generally undesirable high concentration in the wafer surface and the maximum time taken is 7min

10 End-The tube is set to 800⁰C temperature in idle state

16

PLASMA ETCHING

INTRODUCTION

Plasma etching is one type of dry etching which uses plasma to produce chemically reactive species from inert gases The reactive gases are then made to react with the material to be etched The main objective of this process is to remove edge shunts present on the cell which leads to 80 loss and thus it is also termed as Edge isolation method Since edge isolation is the main motive of this process so first let us see that why edge isolation is so important

Edge isolation

In the standard processing sequence the entire wafer is phosphorous diffused The contact to the p-type base is made by firing an Al containing paste that produces a high doping of Al thus over-compensating the shallow emitter diffusion However there is still a direct conductive path from the emitter on the front side to the rear p contact Therefore a large portion of the photo-generated current can directly flow through the emitter to the back contact instead of flowing through the external circuit This current path is modeled in the two-diode model of the solar cell as the parallel resistance R

where R

p

s and Rp are the series and parallel resistance respectively j01 and j02 represent the first and second diode saturation current densities respectively n1 and n2 are the first and second ideality diode factors respectively Vth = ktq is the thermal voltage and jL

In this model the first diode accounts for emitter and bulk recombination with a recombination rate assumed to be proportional to the excess carrier density This leads to an ideality factor of n

is the light generated current density The two diode model is as shown below

1=1 The second diode is associated with the recombination in the space charge region that is assumed to be independent of the injection level leading to n2=2 The effect of increased j02 and Rp can be shown by model calculations for ideal solar cells in the figure below for a fill factor potential of 84 j02 must be less than 2x10-9 Acm2 and Rp must be greater than approximately 2000Ω cm2 if the respective parameters contribute solely to the fill factor loss

17

This increase in Rp is achieved by the edge isolation process that interrupts the parasitic current path as shown in fig2 below

Thus edge isolation is required in solar cell to increase open circuit voltage and thus leading to greater efficiency

Now we will move on to plasma etching process to see that how it works actually in solar cell and module manufacturing industries

Plasma Etching Process

After diffusion a layer of phosphosilicate glass is formed on the edges and rear surface of diffused wafer The process generally requires adequate physical masking of the front surface while exposing the full back of the cell to the dry plasma environment In an embodiment of this process cells are slotted face to face

Fig2Sketch of an industrial screen-printed solar cell The emitter diffusion is present on the complete wafer surface and gets locally overcompensated by printing and firing Al containing metal paste as back contact Without edge-isolation light generated e-s can flow through the emitter region to the back contact which corresponds to the small series resistance in the two diode model

18

into inert carriers so that the plasma chemistry can effectively react with the back of the cells thereby removing excess material from the second coating that might have reached the back of the cell This process is preferably performed via the generation of an RF (13 6 MHz) plasma using a mix of Freon 14 (CF4) with oxygen (O2) and or nitrogen (N2) in a ratio which depends on the chamber and load geometries

19

PHOSPHOSILICATE GLASS REMOVAL INTRODUCTION In diffusion process during deposition phase N2 gas is bubbled through liquid POCL3

The reaction related to this phase is given as follows

1 4POCL3 + 3O2 -gt 2P2O5 + 6Cl 2 2P

2 2O5 + 5Si -gt 5SiO2

+ 4P (PSG)

P2O5

So it becomes mandatory to remove phosphosilicate glass by chemicals which may act as recombination sites or may contribute towards the surface density defect and hence all these defects may contribute to a lower voltage and current

obtained in first reaction step is in paste form and is transferred to the Teflon duct to the process tube which then reacts with silicon of which the wafer is made And thus when this phosphorous pentoxide reaches silicon surface then second reaction takes place and thus a silicon dioxide layer and phosphosilicate glass ie PSG forms on the wafer surface Formation of this dead layer on the wafer surface contributes to a lower efficiency

PROCESS OF REMOVAL Following are the steps that should be taken into consideration for the removal of phosphosilicate glass (PSG) from the wafer surface

1 Firstly wafers are washed combinedly with HF HCL and DI water in a tank for 1min 2 Again wafers are washed with HF and DI water for 2min in the other tank 3 For another 1 min DI water is being sprayed on the diffused wafers 4 Next wafers are washed with DI water subsequently on an interval of 30 sec 5 Finally wafers are kept in the OVEN for 10mins within the temperature range of 95plusmn5ordmC

20

PLASMA ENHANCED CHEMICAL VAPOUR DEPOSITION INTRODUCTION In recent years the low-temperature (le 400ordmC) surface passivation of crystalline silicon solar cells by means of PECVD silicon nitride films has attracted an increasing attention of the PV community These films have proven to be capable of combining an outstanding surface passivation quality with excellent anti-reflection properties On typical p-type silicon solar cells substrate it has been shown that the surface passivation quality of very silicon rich Si3N4

we must first know that whatrsquos the use of silicon nitride coating on the surface of the wafer

films is superior to that of state-of-the-art high temperature silicon dioxide Before proceeding to the details of PECVD system

WHY SILICON NITRIDE SURFACE COATING IS PREFERRED IN PECVD PROCESS It is preferred as a surface coating because it functions as An effective diffusion mask An effective metallization diffusion barrier An anti-reflective coating Provide surface and bulk passivation

Preferably hydrogen is trapped in such a silicon nitride surface coating PECVD SYSYTEM PECVD or Plasma Enhanced Chemical Vapor Deposition is one of the methods of different Chemical Vapor Deposition (CVD) technique In PECVD plasma is being used to decompose a reactant gas such as silane (SiH4) to produce reaction products that precipitate on the surface of the substrate as a new layer

Working Principle Inside a PECVD reactor a strong electric field ignites plasma between two electrodes one of which holds the substrate This plasma ignition cracks the molecular bonds of the process gas which in turn are able to crack more process gas molecules before reaching the surface of the substrate The glow discharge used in this reactor is created by applying an RF field to a low-pressure gas creating free electrons within the discharge region The electrons are sufficiently energized by the electric field that gas-phase dissociation and ionization of the reactant gases occur when the free electrons collide with them Energetic species are then adsorbed on the film surface where they are subjected to ion and electron bombardment rearrangements reactions with other species new bond formation and film formation and growth Eventually a new layer is deposited on the substrate For example when silane is used as precursor plasma ignition frees up Si and SiH radicals which also crack more silane molecules on their way to the surface of the substrate where silicon is deposited PROCESS DETAILS OF PECVD In this system wafers coming from the previous state ie PSG removal step are being loaded onto the graphite boat where the individual wafers are held by three pins inside the boat It is absolutely necessary for the wafers to touch all the three pins otherwise there will remain a possibility of poor deposition in which

Figure Anti-reflective coating made out of silicon nitride deposited by the PECVD technique whereby silane and ammonia is reacted in a furnace The major color of coating looks blue but the color could also look like purple blue or light blue due to different thickness of coating

21

the HF generator fails to generate the required power for process deposition While loading the double diffusion wafers it should be ensured that deposition side should be correctly loaded The wafers are being loaded parallel to the boat After loading the trolley along with the boat is placed inside the machine and then after filling the input parameters of the CMI interface used here and the process tube press the RUN button and then machine will be in auto mode The handling mechanism automatically picks up the boat and places it on the desired paddle corresponding to the selected for the specified job And then process STARTS

22

SCREEN PRINTING INTRODUCTION In screen printing on wafer-based solar PV cells the mesh and buses of silver are printed on the front furthermore the buses of silver are printed on the back Subsequently aluminum paste is dispensed over the whole surface of the back for passivation and surface reflection [14] One of the parameters that can vary and can be controlled in screen printing is the thickness of the print This makes it useful for some of the techniques of printing solar cells electronics etc Screen printing is one of the most critical processes to maintain high yield Solar wafers are becoming thinner and larger so careful printing is required to maintain a lower breakage rate On the other hand high throughput at the printing stage improves the throughput of the whole cell production line PROCESS This process is divided in three phase-

1 Phase I- AgAl printing 2 Phase II- Ag printing 3 Phase III- Al printing

PHASE I- AgAl PRINTING This phase is required to print the back bus bar for tabbing purpose The AgAl paste used in this printing is Dupont (PV-505) and the consumption of the paste per wafer is 011 gm And the screen type used is of the following specification

1 Mesh size- 200microm 2 Tension- 24 N-cm 3 Mesh angle- 225ordm 45ordm 4 Emulsion thickness- 24microm

Process details During printing the paste is poured on the screen and is spread over it with the help of polypropylene spatula Then the wafers (ARC coated) are pasted with the sunny side down on the loader and picked one by one automatically with the start of the print command Once printing starts the wafers are placed one by one between the tool paste and the screen and then the squeegee moves over the screen and allow the paste through the exposed parts on the screen and gets pasted on the wafers Thus in this way the wafers are printed one by one Two most important parameters to be controlled during this printing are

1 Squeegee Pressure 2 Snap off distance

The next step after printing is drying step DRYING In this step wafers are passed through the dryer with a temperature range of 180ordm-400ordmC PHASE II- Ag PRINTING This phase is required to print the front bus bar for tabbing purpose The Ag paste used in this printing is Dupont (PV-145) and the consumption of the paste per wafer is 014 gm And the screen type used is of the following specification

1 Mesh size- 325microm 2 Tension- 24 N-cm 3 Mesh angle- 225ordm 4 Emulsion thickness- 24microm

3

NEED FOR SOLAR ENERGY According to the World Health Organization 3 million people are killed worldwide by outdoor air pollution annually from vehicles and industrial emissions and 16 million indoors through the use of solid fuel Although technology has made oil extraction more efficient the world has to struggle to provide oil by using increasingly costly and less productive methods such as deep sea drilling and developing environmentally sensitive areas such as the Arctic National Wildlife Refuge Global surface temperature increased 074 plusmn 018 degC (133 plusmn 032 degF) during the last century The Intergovernmental Panel on Climate Change (IPCC) concludes that increasing greenhouse gas concentrations resulting from human activity such as fossil fuel burning and deforestation are responsible for most of the observed temperature increase since the middle of the 20th century Mitigation of global warming is accomplished through reductions in the rate of man-made greenhouse gas release The worlds population continues to grow at a quarter of a million people per day increasing the consumption of energy Currently fossil fuels provided around 66 of the worlds electrical power and 95 of the worlds total energy demands

The 1973 Oil crisis due to the OPEC embargo The 1979 (or second) oil crisis (due to the Iranian Revolution) the 1990 oil price shock (due to the First Gulf War) lead to drastic rise in the price of crude oil the most

4

indispensable source of energy for modern industry to function From the mid-1980s to September 2003 the inflation-adjusted price of a barrel of crude oil on NYMEX was generally under $25barrel During 2003 the price rose above $30 reached $60 by August 11 2005 and peaked at $14730 in July 2008

Thus fossil fuels are proven to be polluting and severely detrimental to human health We are paying the price for it in terms of respiratory diseases and a looming energy crisis As the oil prices are volatile and are steadily but unpredictably rising it is expedient to look beyond polluting fossil fuels as the sole source of power

Todays solar systems for solar heating are highly efficient and easy to install The initial cost is high but the payoff is long lasting Solar systems pay for themselves in about half their lifetime It is a free source of energy unlike oil coal or natural gas The environment impact of solar power is low Solar is clean power There are no emissions no maintenance costs and no unstable fuel costs Solar power is reliable with no outages There are no moving parts to replace or break Solar panels can withstand most extreme weather conditions Saving money energy conservation and clean power are the best benefits of solar energy

In response to the petroleum crisis the principles of green energy and sustainable living movements gain popularity This has led to increasing interest in alternate powerfuel research The Sun is 150 million kilometers away but is amazingly powerful Covering 4 of the worlds desert area with photovoltaic could supply all of the worlds electricity In such a situation development of solar energy should be a priority for every nation

5

RAW MATERIALS NEEDED

Along with the enormous capital investment required to successfully set up a solar cellmodule manufacturing plant it also requires a steady supply of raw materials The most basic raw materials is the silicon wafer itself which is usually bought from another company rather than prepared in-house Two types of crystalline silicon solar cell are single crystalline silicon solar cell which is known as monocrystalline silicon solar cell made by slicing wafers and polycrystalline silicon solar cell made by sawing a block of silicon first into bars and then wafers Other semiconductors such as gallium arsenide cadmium telluride and copper indium diselenide etc have both single crystalline and polycrystalline forms The solar cell which made by gallium arsenide has high efficiency ranging from 20-25 The first process Etching requires a number of chemicals HF HCL NaOH which may vary depending on the etching methodology (Acidic etching or Alkali etching) Deionised water is obtained from the DI plant IPA is used for cleaning purposes For diffusion an MRL diffusion furnace can be used which utilizes N2 amp O2

N2 flows through a bubbler of POCl3

for Phosphorus diffusion HF HCl and DI water are also used to clean the components (bubbler quartz tube etc) used in Diffusion For plasma etching we need a gas like Freon to provide the ions used in edge isolation Silane and Ammonia in gaseous form are needed to perform PECVD in order to deposit the anti reflective coating Silicon Nitride is formed due to the reaction of Silane and Ammonia Screen Printing is a three phase process The first phase is AgAl which could use the paste Dupont PV505 The second front side printing uses an Ag pastes (Dupont PV145) Third phase uses Aluminium paste (Analog 12D)

Module manufacturing requires its own set of raw materials It needs Tedlar EVA and Glass to form the multilayered structure Aluminium frames are needed to provide structural integrity and a junction box for output terminals Thus a manufacturing plant needs a variety of raw materials at the right time in the right quantity quality and purity A fall is any of these parameters will severely hamper the manufacturing process and negatively effect margins Thus ensuring a proper supply of this varied mix of materials is of paramount importance to the managers

6

HISTORY OF PHOTOVOLTAICS Most people are surprised to learn that photovoltaic technology actually dates back over 160 years French physicist Edmond Becquerel first described the photovoltaic (PV) effect in 1839 but it remained a curiosity of science for the next three quarters of a century At only 19 Becquerel found that certain materials would produce small amounts of electric current when exposed to light The effect was first studied in solids such as selenium by Heinrich Hertz in the 1870s Soon afterward selenium PV cells were converting light to electricity at 1 to 2 efficiency As a result selenium was quickly adopted in the emerging field of photography for use in light-measuring devices Major steps toward commercializing PV were taken in the 1940s and early 1950s when the Czochralski process was developed for producing highly pure crystalline silicon In 1954 scientists at Bell Laboratories depended on the Czochralski process to develop the first crystalline silicon photovoltaic cell which had an efficiency of 4 Although a few attempts were made in the 1950s to use silicon cells in commercial products it was the new space program that gave the technology its first major application In the 1970s research drives PV costs down 80 allowing for applications such as offshore navigation warning lights and horns lighthouses railroad crossings and remote use where utility-grid connections are too costly Despite these advances PV devices in 1970 were still too expensive for most mass market uses But in the mid-1970s rising energy costs sparked by a world oil crisis renewed interest in making PV technology more affordable Since then the federal government industry and research organizations have invested billions of dollars in research development and production Often industry and the government work together sharing the cost of PV research and development In the 1980s photovoltaic became a popular power source for consumer electronic devices including calculators watches radios lanterns and other small battery-charging applications

Todays commercial PV systems can convert from 11 to 17 of sunlight into electricity They are highly reliable and last 20 years or longer The cost of PV-generated electricity has dropped 15- to 20-fold and PV modules now cost around $6 per watt (W) and produce electricity for as little as 25 to 50 cents per kilowatt-hour (kWh)

7

Solar Cell Manufacturing Processes

8

ETCHING

INTRODUCTION

Etching is the process of using strong acid or mordant to cut into the unprotected parts of a metal surface to create a design in intaglio in the metal (the original processmdashin modern manufacturing other chemicals may be used on other types of material) As an intaglio method of printmaking it is along with engraving the most important technique for old master prints and remains widely used today But in wafer fabrication Etching refers to a process by which material is removed from the wafer ie either from the silicon substrate itself or from any film or layer of material on the wafer

DEFINITION

The treatment of a prepared metal surface with acid or other chemical reagent which by differential attack reveals the structure is known as etching In wafer fabrication etching process is generally classified in two classes Dry etching and Wet etching

Normally dry etching is not being used in this solar cells and module manufacturing unit So we will just go on a brief look on wet etching process

WET ETCHING It is a process that utilizes liquid chemicals or etchants to remove materials from the wafer usually in specific patterns defined by photo resist masks on the wafer Materials not covered by these masks are etched away by the chemicals while those covered by the masks are left almost intact A simple wet etching process may just consist of dissolution of the material to be removed in a liquid solvent without changing the chemical nature of the dissolved material In general however a wet etching process involves one or more chemical reactions that consume the original reactants and produce new species A basic wet etching process may be broken down in three steps as follows

1 Diffusion of the etchant to the surface for removal 2 Reaction between the etchant and the material being removed and 3 Diffusion of the reaction byproducts from the reacted surface

Wet etching can be both isotropic ie it proceeds in all directions at the same rate and anisotropic ie it proceeds in only one direction Isotropic Wet Etching A process which proceeds at the same rate in all directions is known as isotropic wet etching The etchants used in this process are isotropic in nature that etches away even the portion of material that is directly under the mask (usually in the shape of a quarter-circle) since its horizontal etching rate is the same as its vertical rate When an isotropic etchant eats away a portion of the material under the mask the etched film is said to have undercut the mask The amount of undercutting is a measure of an etching parameter known as the bias Bias is simply defined as the difference between the lateral dimensions of the etched image and the masked image Thus the mask used in etching must compensate for whatever bias an etchant is known to produce in order to create the desired feature on the wafer

Figure 1 Isotropic wet etching

9

Anisotropic Wet Etching A process which proceeds in only one direction is known as anisotropic wet etching It removes material in the vertical direction only since it will follow the mask patterns on the wafer very faithfully leaving any material covered by mask material basically untouched Since isotropic wet etching process has high bias values which are not practical for use in pattern images which has features of measuring less than 3 microns thus wafer patterns of less than 3 microns are wet etched anisotropically instead of isotropic

Figure 2 Anisotropic wet etching

Advantages of wet etching 1 Low cost 2 High reliability 3 High throughput 4 Excellent selectivity in most cases with respect to both mask and substrate materials

Disadvantages of wet etching 1 Limited resolution 2 Higher safety risks due to the direct chemical exposure of the personnel 3 High cost of etchants in some cases 4 Problems related to the resists loss of adhesion to the substrate 5 Problems related to the formation of bubbles which inhibit the etching process where they are present and 6 Problems related to incomplete or non-uniform etching

Before moving on to texturization we first of all must do Saw damage removal which is also called as 1st Etch First Etch When ingots obtained from the Czochralski method are sliced into wafers by saw then it leaves some damaged marks on the wafers if it is not being sliced properly which makes the surface non-uniform So to make the surface uniform and free of error it becomes mandatory to do saw damage removal or 1st

1 First wafers are kept in the solution of 300ml DI water and 10kg NaOH(RSE) within the temperature range of 75⁰plusmn 2⁰C for 1min The reaction which takes place in this step is

etching Steps involved are

Si + 2NaOH + H2O (DI) -gt Na3SiO2 + H2 + H2

2 Finally these etched wafers are washed with DI water O

10

TEXTURIZATION INTRODUCTION Since 1960 for the improvement of the cell performance texturization has been attempted Texturization of front andor back surfaces of a solar cell surfaces improve their performances The reflection from chemically textured wafer reduced from 34 for a polished silicon surface to 11 Many researchers have reported the selective etching of silicon of lt100gt orientation of using sodium or potassium hydroxide solution OBJECTIVES OF TEXTURIZATION The major objectives of texturization are-

1 Reduction of front surface reflectance 2 Increase in the path length of the entered light of lower wavelengths by oblique trajectory within the

cell and absorption of light closer to the junction 3 Optical trapping of weakly absorbed light by multiple internal reflections from the top and bottom

surfaces of the cell thereby also increasing the optical path length The increased path length with an oblique trajectory leads to an effective increase in the absorption co-efficient

All these effects lead to realization of higher performance thick cells and also efficient solar cells of comparatively lower thickness which significantly reduces the material cost without degrading the cell performance MODELS OF TEXTURIZATION To reduce the reflection co-efficient and to trap weak absorbed light various texturing geometries are being employed The different texturing geometries include

1 Lambertian geometry 2 Upright pyramid geometry 3 Slat or Micro-grooved geometry 4 Inverted pyramid geometry 5 Simple prism pyramid geometry 6 Grating geometry 7 Three perpendicular planes (3PP) geometry 8 Perpendicular slat geometry 9 Porous silicon

Now in large number of manufacturing industries Inverted pyramid texturing geometry scheme is being used these days Letrsquos have a look on this scheme very briefly Inverted Pyramid Geometry The light trapping properties of the inverted pyramid geometry has been first investigated by Smith and Rohatgi The increase in current for the type of textured cell is due to incorporation of al the three effects necessary for an efficient light trapping design The front surface reflectance is reduced by providing the opportunity for a portion of the incoming light to undergo a triple bounce thereby reducing the overall front surface reflectance The increase in path length and light trapping efficiency means that a larger fraction of the light which has entered the cell will be absorbed before exiting the cell The short wavelength spectral response analysis at the texturing angle of 5375ordm indicates that 37 of the incoming light experiences a triple bounce on the front surface of the inverted pyramid geometry and the rest percentage gets double bounce The inverted pyramids on a 100microm cell with a two layer AR coating is estimated to give 402mAcm2 and cell efficiency of 24 with realistic cell design and material parameters

11

This texturization method can be employed on both mono-crystalline and multi-crystalline substrates Letrsquos have a look on this Mono-crystalline substrates For mono-crystalline substrate it is advantageous to make use of the anisotropic etching properties of Si in an alkaline solution As the 111 planes get etched more slowly than other crystal planes 111 facets are developed On lt100gtwafers this leads to pyramidal shapes at the surface that are particularly effective in reducing reflectance A regular array of pyramidal pits with facets at 547ordm to the horizontal plane mdash called inverted pyramids mdash can be formed using oxide etch mask with reflectances as low as 8 without ARC

Multi-crystalline Substrates Due to the anisotropic nature of multi-crystalline silicon substrate alkaline random texturing is not effective Some grains remain untextured leading to a high average reflectance So for texturing multi-crystalline silicon wafers should be etched in an acid mixture of HF and HNO3 because HNO3 tends to oxidize the surface while the HF etches the oxide away These acidic iso-texturing results in lower reflection than traditional anisotropic etching on multi-crystalline material and better conversion efficiency

STANDARD NAOH-IPA TEXTURIZATION APPROACH In order to achieve good uniformity of pyramidal structure on the silicon surface a mixture of NaOH or KOH and isopropyl alcohol (IPA) is generally used for texturization of mono-crystalline silicon solar cell For better texturization the interfacial energy between silicon and ionized electrolyte chemical solution should be reduced to achieve sufficient wettability for the silicon surface which will enhance the pyramid nucleation From a manufacturability point of view it is appealing to combine the saw damage removal and alkaline texturing in one single step in which case a trade-off has to be made between process speed and quality of the surface treatment

Fig1 Inverted Pyramid Textured Surface

Fig3 Textured multi-crystalline Si surface

Fig2 Anisotropic Etching of Mono-crystalline Si

12

DIFFUSION

INTRODUCTION

Diffusion the movement of a chemical species from an area of high concentration to an area of lower concentration is one of the two major processes by which chemical species or dopants are introduced into a semiconductor (the other one being ion implantation) The controlled diffusion of dopants into silicon to alter the type and level of conductivity of semiconductor materials is the foundation of forming a p-n junction The mathematics that govern the mass transport phenomena of diffusion are based on Ficks laws There are two laws according to Fick Letrsquos have a look on these laws First law Whenever an impurity concentration gradient partCpartx exists in a finite volume of a matrix substance (the silicon substrate in this context) the impurity material will have the natural tendency to move in order to distribute itself more evenly within the matrix and decrease the gradient Given enough time this flow of impurities will eventually result in homogeneity within the matrix causing the net flow of impurities to stop The mathematics of this transport mechanism was formalized in 1855 by Fick who postulated that the flux of material across a given plane is proportional to the concentration gradient across the plane Thus Ficks First Law states

J = -D ( partC(xt)partx ) Where J is the flux D is the diffusion constant for the material that is diffusing in the specific solvent and partC(xt)partx is the concentration gradient The diffusion constant of a material is also referred to as diffusion coefficient or simply diffusivity It is expressed in units of length2time such as microm2hour The negative sign of the right side of the equation indicates that the impurities are flowing in the direction of lower concentration Second Law Ficks First Law does not consider the fact that the gradient and local concentration of the impurities in a material decreases with an increase in time an aspect thats important to diffusion processes The flux J1 of impurities entering a section of a bar with a concentration gradient is different from the flux J2 of impurities leaving the same section From the law of conservation of matter the difference between J1 and J2 must result in a change in the concentration of impurities within the section (assuming that no impurities are formed or consumed in the section) This is Ficks Second Law which states that the change in impurity concentration over time is equal to the change in local diffusion flux or

partC(xt)partt = - partJpartx or from Ficks First Law

partC(xt)partt = part(DpartC(xt)partx)partx If the diffusion coefficient is independent of position such as when the impurity concentration is low then Ficks Second Law may be further simplified into the following equation

partC(xt)partt = D part2C(xt)partx2 The most important property of diffusion process in the manufacture of silicon solar cell is the formation of p-n junction by doping n-type material ie phosphorous on the p-type ie boron doped silicon textured wafer playing an essential role for photovoltaic effect Let us have a brief descriptive look on the properties of p-n junction P-N JUNCTION A p-n junction is a junction formed by combining p-type and n-type semiconductors together in very close contact

13

Principle of p-n junction The most common type of solar cell is a large p-n junction where the free carrier pairs emitted by light energy are separated by the junction and contribute to the current In its simplest form it consists of a junction formed between n-type and p-type semiconductors either of the same material (homo-junction) or different materials (hetero-junction) The band-structure of the two differently doped sides with respect to their Fermi levels can be seen in adjacent figure1

When the two halves are brought together the Fermi levels on either side are forced in to coincidence causing the valence and conduction bands to bend (Figure 2)

These bent bands represent a built-in electric field over what is referred to as the depletion region When a photon with a energy greater than the band-gap of the semiconductor passes through the solar cell it may be absorbed by the material This absorption takes the form of a band-to-band electronic transition so an electronhole pair is produced If these carriers can diffuse to the depletion region before they recombine then they are separated by the electric field causing one quantum of charge to flow through an external load This is the origin of the solar cells photocurrent and is shown in Figure 3

14

SOLAR CELL AS A P-N JUNCTION A solar cell is essential a p-n junction with large surface area The n type material is kept thin to allow light to pass through to the p-n junction

Light travels in packets of energy called photons The generation of electric current happens inside the depletion zone of the PN junction The depletion region as explained previously with the diode is the area around the PN junction where the electrons from the N-type silicon have diffused into the holes of the P-type material When a photon of light is absorbed by one of these atoms in the N-Type silicon it will dislodge an electron creating a free electron and a hole The free electron and hole has sufficient energy to jump out of the depletion zone If a wire is connected from the cathode (N-type silicon) to the anode (P-type silicon) electrons will flow through the wire The electron is attracted to the positive charge of the P-type material and travels through the external load (meter) creating a flow of electric current The hole created by the dislodged electron is attracted to the negative charge of N-type material and migrates to the back electrical contact As the electron enters the P-type silicon from the back electrical contact it combines with the hole restoring the electrical neutrality

DIFFUSION PROCESS

The first layer formation after doing texturization of wafer is performed in this process by doping the substrate with n-type dopant N-doping can be accomplished by depositing n dopant onto the substrate and then heating the substrate to drive the n dopant into the substrate Gaseous diffusion (for example POCl3 horizontal tube diffusion including temperatures of from 700 to 1000degC preferably 800 to 900degC) can be used to deposit the n-dopant onto the substrate surface to create an n-doped layer and a shallow p-n junction proximal to the substrate surface In this process if wafers are loaded onto the quartz boat back to back in the same lot then double diffusion process takes place and if single wafers are being loaded then single diffusion takes place On the quartz boat wafers can be loaded in two methods

1 Parallel diffusion loading of wafers parallel to the gas flow 2 Perpendicular diffusion loading of wafers perpendicular to the gas flow

Diffusion process is comprised of the following steps 1 Idle state-In this state the load center and source zone are set at 800ordmC and no process is

carried out in the furnace 2 Load-During this sequence the silicon carbide paddle moves at a speed of 45 rpm and the

maximum time taken is 7min 3 Ramp-The cold wafers loaded into the tubes bring down the temperature of the tube The tube

is then reheated to reach the set point temperature and the time taken is 7min

15

4 Stabilization-During this phase any overshooting of temperature during ramp-up phase is controlled to the predetermined set value and the time taken is 10min

5 Pre-deposition-Any excess POCL3 present in vapour form inside the quartz bubbler is vent out before the deposition starts by N2

6 Deposition- N purging in 5 sec

2 is bubbled through liquid POCL3 in presence of O2 to form P2O5

7 Post-deposition-During this phase O in paste form

2 and N2

8 N

flows to drive in the phosphorous from wafer surface to the junction

2

9 Unload-During this phase the SiC paddle moves slowly out of the furnace at set speed of 45rpm and maximum time taken is 7min

Purge-This phase is also known as drive in diffusion which is designed to evenly distribute the impurities into wafer and reduce the generally undesirable high concentration in the wafer surface and the maximum time taken is 7min

10 End-The tube is set to 800⁰C temperature in idle state

16

PLASMA ETCHING

INTRODUCTION

Plasma etching is one type of dry etching which uses plasma to produce chemically reactive species from inert gases The reactive gases are then made to react with the material to be etched The main objective of this process is to remove edge shunts present on the cell which leads to 80 loss and thus it is also termed as Edge isolation method Since edge isolation is the main motive of this process so first let us see that why edge isolation is so important

Edge isolation

In the standard processing sequence the entire wafer is phosphorous diffused The contact to the p-type base is made by firing an Al containing paste that produces a high doping of Al thus over-compensating the shallow emitter diffusion However there is still a direct conductive path from the emitter on the front side to the rear p contact Therefore a large portion of the photo-generated current can directly flow through the emitter to the back contact instead of flowing through the external circuit This current path is modeled in the two-diode model of the solar cell as the parallel resistance R

where R

p

s and Rp are the series and parallel resistance respectively j01 and j02 represent the first and second diode saturation current densities respectively n1 and n2 are the first and second ideality diode factors respectively Vth = ktq is the thermal voltage and jL

In this model the first diode accounts for emitter and bulk recombination with a recombination rate assumed to be proportional to the excess carrier density This leads to an ideality factor of n

is the light generated current density The two diode model is as shown below

1=1 The second diode is associated with the recombination in the space charge region that is assumed to be independent of the injection level leading to n2=2 The effect of increased j02 and Rp can be shown by model calculations for ideal solar cells in the figure below for a fill factor potential of 84 j02 must be less than 2x10-9 Acm2 and Rp must be greater than approximately 2000Ω cm2 if the respective parameters contribute solely to the fill factor loss

17

This increase in Rp is achieved by the edge isolation process that interrupts the parasitic current path as shown in fig2 below

Thus edge isolation is required in solar cell to increase open circuit voltage and thus leading to greater efficiency

Now we will move on to plasma etching process to see that how it works actually in solar cell and module manufacturing industries

Plasma Etching Process

After diffusion a layer of phosphosilicate glass is formed on the edges and rear surface of diffused wafer The process generally requires adequate physical masking of the front surface while exposing the full back of the cell to the dry plasma environment In an embodiment of this process cells are slotted face to face

Fig2Sketch of an industrial screen-printed solar cell The emitter diffusion is present on the complete wafer surface and gets locally overcompensated by printing and firing Al containing metal paste as back contact Without edge-isolation light generated e-s can flow through the emitter region to the back contact which corresponds to the small series resistance in the two diode model

18

into inert carriers so that the plasma chemistry can effectively react with the back of the cells thereby removing excess material from the second coating that might have reached the back of the cell This process is preferably performed via the generation of an RF (13 6 MHz) plasma using a mix of Freon 14 (CF4) with oxygen (O2) and or nitrogen (N2) in a ratio which depends on the chamber and load geometries

19

PHOSPHOSILICATE GLASS REMOVAL INTRODUCTION In diffusion process during deposition phase N2 gas is bubbled through liquid POCL3

The reaction related to this phase is given as follows

1 4POCL3 + 3O2 -gt 2P2O5 + 6Cl 2 2P

2 2O5 + 5Si -gt 5SiO2

+ 4P (PSG)

P2O5

So it becomes mandatory to remove phosphosilicate glass by chemicals which may act as recombination sites or may contribute towards the surface density defect and hence all these defects may contribute to a lower voltage and current

obtained in first reaction step is in paste form and is transferred to the Teflon duct to the process tube which then reacts with silicon of which the wafer is made And thus when this phosphorous pentoxide reaches silicon surface then second reaction takes place and thus a silicon dioxide layer and phosphosilicate glass ie PSG forms on the wafer surface Formation of this dead layer on the wafer surface contributes to a lower efficiency

PROCESS OF REMOVAL Following are the steps that should be taken into consideration for the removal of phosphosilicate glass (PSG) from the wafer surface

1 Firstly wafers are washed combinedly with HF HCL and DI water in a tank for 1min 2 Again wafers are washed with HF and DI water for 2min in the other tank 3 For another 1 min DI water is being sprayed on the diffused wafers 4 Next wafers are washed with DI water subsequently on an interval of 30 sec 5 Finally wafers are kept in the OVEN for 10mins within the temperature range of 95plusmn5ordmC

20

PLASMA ENHANCED CHEMICAL VAPOUR DEPOSITION INTRODUCTION In recent years the low-temperature (le 400ordmC) surface passivation of crystalline silicon solar cells by means of PECVD silicon nitride films has attracted an increasing attention of the PV community These films have proven to be capable of combining an outstanding surface passivation quality with excellent anti-reflection properties On typical p-type silicon solar cells substrate it has been shown that the surface passivation quality of very silicon rich Si3N4

we must first know that whatrsquos the use of silicon nitride coating on the surface of the wafer

films is superior to that of state-of-the-art high temperature silicon dioxide Before proceeding to the details of PECVD system

WHY SILICON NITRIDE SURFACE COATING IS PREFERRED IN PECVD PROCESS It is preferred as a surface coating because it functions as An effective diffusion mask An effective metallization diffusion barrier An anti-reflective coating Provide surface and bulk passivation

Preferably hydrogen is trapped in such a silicon nitride surface coating PECVD SYSYTEM PECVD or Plasma Enhanced Chemical Vapor Deposition is one of the methods of different Chemical Vapor Deposition (CVD) technique In PECVD plasma is being used to decompose a reactant gas such as silane (SiH4) to produce reaction products that precipitate on the surface of the substrate as a new layer

Working Principle Inside a PECVD reactor a strong electric field ignites plasma between two electrodes one of which holds the substrate This plasma ignition cracks the molecular bonds of the process gas which in turn are able to crack more process gas molecules before reaching the surface of the substrate The glow discharge used in this reactor is created by applying an RF field to a low-pressure gas creating free electrons within the discharge region The electrons are sufficiently energized by the electric field that gas-phase dissociation and ionization of the reactant gases occur when the free electrons collide with them Energetic species are then adsorbed on the film surface where they are subjected to ion and electron bombardment rearrangements reactions with other species new bond formation and film formation and growth Eventually a new layer is deposited on the substrate For example when silane is used as precursor plasma ignition frees up Si and SiH radicals which also crack more silane molecules on their way to the surface of the substrate where silicon is deposited PROCESS DETAILS OF PECVD In this system wafers coming from the previous state ie PSG removal step are being loaded onto the graphite boat where the individual wafers are held by three pins inside the boat It is absolutely necessary for the wafers to touch all the three pins otherwise there will remain a possibility of poor deposition in which

Figure Anti-reflective coating made out of silicon nitride deposited by the PECVD technique whereby silane and ammonia is reacted in a furnace The major color of coating looks blue but the color could also look like purple blue or light blue due to different thickness of coating

21

the HF generator fails to generate the required power for process deposition While loading the double diffusion wafers it should be ensured that deposition side should be correctly loaded The wafers are being loaded parallel to the boat After loading the trolley along with the boat is placed inside the machine and then after filling the input parameters of the CMI interface used here and the process tube press the RUN button and then machine will be in auto mode The handling mechanism automatically picks up the boat and places it on the desired paddle corresponding to the selected for the specified job And then process STARTS

22

SCREEN PRINTING INTRODUCTION In screen printing on wafer-based solar PV cells the mesh and buses of silver are printed on the front furthermore the buses of silver are printed on the back Subsequently aluminum paste is dispensed over the whole surface of the back for passivation and surface reflection [14] One of the parameters that can vary and can be controlled in screen printing is the thickness of the print This makes it useful for some of the techniques of printing solar cells electronics etc Screen printing is one of the most critical processes to maintain high yield Solar wafers are becoming thinner and larger so careful printing is required to maintain a lower breakage rate On the other hand high throughput at the printing stage improves the throughput of the whole cell production line PROCESS This process is divided in three phase-

1 Phase I- AgAl printing 2 Phase II- Ag printing 3 Phase III- Al printing

PHASE I- AgAl PRINTING This phase is required to print the back bus bar for tabbing purpose The AgAl paste used in this printing is Dupont (PV-505) and the consumption of the paste per wafer is 011 gm And the screen type used is of the following specification

1 Mesh size- 200microm 2 Tension- 24 N-cm 3 Mesh angle- 225ordm 45ordm 4 Emulsion thickness- 24microm

Process details During printing the paste is poured on the screen and is spread over it with the help of polypropylene spatula Then the wafers (ARC coated) are pasted with the sunny side down on the loader and picked one by one automatically with the start of the print command Once printing starts the wafers are placed one by one between the tool paste and the screen and then the squeegee moves over the screen and allow the paste through the exposed parts on the screen and gets pasted on the wafers Thus in this way the wafers are printed one by one Two most important parameters to be controlled during this printing are

1 Squeegee Pressure 2 Snap off distance

The next step after printing is drying step DRYING In this step wafers are passed through the dryer with a temperature range of 180ordm-400ordmC PHASE II- Ag PRINTING This phase is required to print the front bus bar for tabbing purpose The Ag paste used in this printing is Dupont (PV-145) and the consumption of the paste per wafer is 014 gm And the screen type used is of the following specification

1 Mesh size- 325microm 2 Tension- 24 N-cm 3 Mesh angle- 225ordm 4 Emulsion thickness- 24microm

4

indispensable source of energy for modern industry to function From the mid-1980s to September 2003 the inflation-adjusted price of a barrel of crude oil on NYMEX was generally under $25barrel During 2003 the price rose above $30 reached $60 by August 11 2005 and peaked at $14730 in July 2008

Thus fossil fuels are proven to be polluting and severely detrimental to human health We are paying the price for it in terms of respiratory diseases and a looming energy crisis As the oil prices are volatile and are steadily but unpredictably rising it is expedient to look beyond polluting fossil fuels as the sole source of power

Todays solar systems for solar heating are highly efficient and easy to install The initial cost is high but the payoff is long lasting Solar systems pay for themselves in about half their lifetime It is a free source of energy unlike oil coal or natural gas The environment impact of solar power is low Solar is clean power There are no emissions no maintenance costs and no unstable fuel costs Solar power is reliable with no outages There are no moving parts to replace or break Solar panels can withstand most extreme weather conditions Saving money energy conservation and clean power are the best benefits of solar energy

In response to the petroleum crisis the principles of green energy and sustainable living movements gain popularity This has led to increasing interest in alternate powerfuel research The Sun is 150 million kilometers away but is amazingly powerful Covering 4 of the worlds desert area with photovoltaic could supply all of the worlds electricity In such a situation development of solar energy should be a priority for every nation

5

RAW MATERIALS NEEDED

Along with the enormous capital investment required to successfully set up a solar cellmodule manufacturing plant it also requires a steady supply of raw materials The most basic raw materials is the silicon wafer itself which is usually bought from another company rather than prepared in-house Two types of crystalline silicon solar cell are single crystalline silicon solar cell which is known as monocrystalline silicon solar cell made by slicing wafers and polycrystalline silicon solar cell made by sawing a block of silicon first into bars and then wafers Other semiconductors such as gallium arsenide cadmium telluride and copper indium diselenide etc have both single crystalline and polycrystalline forms The solar cell which made by gallium arsenide has high efficiency ranging from 20-25 The first process Etching requires a number of chemicals HF HCL NaOH which may vary depending on the etching methodology (Acidic etching or Alkali etching) Deionised water is obtained from the DI plant IPA is used for cleaning purposes For diffusion an MRL diffusion furnace can be used which utilizes N2 amp O2

N2 flows through a bubbler of POCl3

for Phosphorus diffusion HF HCl and DI water are also used to clean the components (bubbler quartz tube etc) used in Diffusion For plasma etching we need a gas like Freon to provide the ions used in edge isolation Silane and Ammonia in gaseous form are needed to perform PECVD in order to deposit the anti reflective coating Silicon Nitride is formed due to the reaction of Silane and Ammonia Screen Printing is a three phase process The first phase is AgAl which could use the paste Dupont PV505 The second front side printing uses an Ag pastes (Dupont PV145) Third phase uses Aluminium paste (Analog 12D)

Module manufacturing requires its own set of raw materials It needs Tedlar EVA and Glass to form the multilayered structure Aluminium frames are needed to provide structural integrity and a junction box for output terminals Thus a manufacturing plant needs a variety of raw materials at the right time in the right quantity quality and purity A fall is any of these parameters will severely hamper the manufacturing process and negatively effect margins Thus ensuring a proper supply of this varied mix of materials is of paramount importance to the managers

6

HISTORY OF PHOTOVOLTAICS Most people are surprised to learn that photovoltaic technology actually dates back over 160 years French physicist Edmond Becquerel first described the photovoltaic (PV) effect in 1839 but it remained a curiosity of science for the next three quarters of a century At only 19 Becquerel found that certain materials would produce small amounts of electric current when exposed to light The effect was first studied in solids such as selenium by Heinrich Hertz in the 1870s Soon afterward selenium PV cells were converting light to electricity at 1 to 2 efficiency As a result selenium was quickly adopted in the emerging field of photography for use in light-measuring devices Major steps toward commercializing PV were taken in the 1940s and early 1950s when the Czochralski process was developed for producing highly pure crystalline silicon In 1954 scientists at Bell Laboratories depended on the Czochralski process to develop the first crystalline silicon photovoltaic cell which had an efficiency of 4 Although a few attempts were made in the 1950s to use silicon cells in commercial products it was the new space program that gave the technology its first major application In the 1970s research drives PV costs down 80 allowing for applications such as offshore navigation warning lights and horns lighthouses railroad crossings and remote use where utility-grid connections are too costly Despite these advances PV devices in 1970 were still too expensive for most mass market uses But in the mid-1970s rising energy costs sparked by a world oil crisis renewed interest in making PV technology more affordable Since then the federal government industry and research organizations have invested billions of dollars in research development and production Often industry and the government work together sharing the cost of PV research and development In the 1980s photovoltaic became a popular power source for consumer electronic devices including calculators watches radios lanterns and other small battery-charging applications

Todays commercial PV systems can convert from 11 to 17 of sunlight into electricity They are highly reliable and last 20 years or longer The cost of PV-generated electricity has dropped 15- to 20-fold and PV modules now cost around $6 per watt (W) and produce electricity for as little as 25 to 50 cents per kilowatt-hour (kWh)

7

Solar Cell Manufacturing Processes

8

ETCHING

INTRODUCTION

Etching is the process of using strong acid or mordant to cut into the unprotected parts of a metal surface to create a design in intaglio in the metal (the original processmdashin modern manufacturing other chemicals may be used on other types of material) As an intaglio method of printmaking it is along with engraving the most important technique for old master prints and remains widely used today But in wafer fabrication Etching refers to a process by which material is removed from the wafer ie either from the silicon substrate itself or from any film or layer of material on the wafer

DEFINITION

The treatment of a prepared metal surface with acid or other chemical reagent which by differential attack reveals the structure is known as etching In wafer fabrication etching process is generally classified in two classes Dry etching and Wet etching

Normally dry etching is not being used in this solar cells and module manufacturing unit So we will just go on a brief look on wet etching process

WET ETCHING It is a process that utilizes liquid chemicals or etchants to remove materials from the wafer usually in specific patterns defined by photo resist masks on the wafer Materials not covered by these masks are etched away by the chemicals while those covered by the masks are left almost intact A simple wet etching process may just consist of dissolution of the material to be removed in a liquid solvent without changing the chemical nature of the dissolved material In general however a wet etching process involves one or more chemical reactions that consume the original reactants and produce new species A basic wet etching process may be broken down in three steps as follows

1 Diffusion of the etchant to the surface for removal 2 Reaction between the etchant and the material being removed and 3 Diffusion of the reaction byproducts from the reacted surface

Wet etching can be both isotropic ie it proceeds in all directions at the same rate and anisotropic ie it proceeds in only one direction Isotropic Wet Etching A process which proceeds at the same rate in all directions is known as isotropic wet etching The etchants used in this process are isotropic in nature that etches away even the portion of material that is directly under the mask (usually in the shape of a quarter-circle) since its horizontal etching rate is the same as its vertical rate When an isotropic etchant eats away a portion of the material under the mask the etched film is said to have undercut the mask The amount of undercutting is a measure of an etching parameter known as the bias Bias is simply defined as the difference between the lateral dimensions of the etched image and the masked image Thus the mask used in etching must compensate for whatever bias an etchant is known to produce in order to create the desired feature on the wafer

Figure 1 Isotropic wet etching

9

Anisotropic Wet Etching A process which proceeds in only one direction is known as anisotropic wet etching It removes material in the vertical direction only since it will follow the mask patterns on the wafer very faithfully leaving any material covered by mask material basically untouched Since isotropic wet etching process has high bias values which are not practical for use in pattern images which has features of measuring less than 3 microns thus wafer patterns of less than 3 microns are wet etched anisotropically instead of isotropic

Figure 2 Anisotropic wet etching

Advantages of wet etching 1 Low cost 2 High reliability 3 High throughput 4 Excellent selectivity in most cases with respect to both mask and substrate materials

Disadvantages of wet etching 1 Limited resolution 2 Higher safety risks due to the direct chemical exposure of the personnel 3 High cost of etchants in some cases 4 Problems related to the resists loss of adhesion to the substrate 5 Problems related to the formation of bubbles which inhibit the etching process where they are present and 6 Problems related to incomplete or non-uniform etching

Before moving on to texturization we first of all must do Saw damage removal which is also called as 1st Etch First Etch When ingots obtained from the Czochralski method are sliced into wafers by saw then it leaves some damaged marks on the wafers if it is not being sliced properly which makes the surface non-uniform So to make the surface uniform and free of error it becomes mandatory to do saw damage removal or 1st

1 First wafers are kept in the solution of 300ml DI water and 10kg NaOH(RSE) within the temperature range of 75⁰plusmn 2⁰C for 1min The reaction which takes place in this step is

etching Steps involved are

Si + 2NaOH + H2O (DI) -gt Na3SiO2 + H2 + H2

2 Finally these etched wafers are washed with DI water O

10

TEXTURIZATION INTRODUCTION Since 1960 for the improvement of the cell performance texturization has been attempted Texturization of front andor back surfaces of a solar cell surfaces improve their performances The reflection from chemically textured wafer reduced from 34 for a polished silicon surface to 11 Many researchers have reported the selective etching of silicon of lt100gt orientation of using sodium or potassium hydroxide solution OBJECTIVES OF TEXTURIZATION The major objectives of texturization are-

1 Reduction of front surface reflectance 2 Increase in the path length of the entered light of lower wavelengths by oblique trajectory within the

cell and absorption of light closer to the junction 3 Optical trapping of weakly absorbed light by multiple internal reflections from the top and bottom

surfaces of the cell thereby also increasing the optical path length The increased path length with an oblique trajectory leads to an effective increase in the absorption co-efficient

All these effects lead to realization of higher performance thick cells and also efficient solar cells of comparatively lower thickness which significantly reduces the material cost without degrading the cell performance MODELS OF TEXTURIZATION To reduce the reflection co-efficient and to trap weak absorbed light various texturing geometries are being employed The different texturing geometries include

1 Lambertian geometry 2 Upright pyramid geometry 3 Slat or Micro-grooved geometry 4 Inverted pyramid geometry 5 Simple prism pyramid geometry 6 Grating geometry 7 Three perpendicular planes (3PP) geometry 8 Perpendicular slat geometry 9 Porous silicon

Now in large number of manufacturing industries Inverted pyramid texturing geometry scheme is being used these days Letrsquos have a look on this scheme very briefly Inverted Pyramid Geometry The light trapping properties of the inverted pyramid geometry has been first investigated by Smith and Rohatgi The increase in current for the type of textured cell is due to incorporation of al the three effects necessary for an efficient light trapping design The front surface reflectance is reduced by providing the opportunity for a portion of the incoming light to undergo a triple bounce thereby reducing the overall front surface reflectance The increase in path length and light trapping efficiency means that a larger fraction of the light which has entered the cell will be absorbed before exiting the cell The short wavelength spectral response analysis at the texturing angle of 5375ordm indicates that 37 of the incoming light experiences a triple bounce on the front surface of the inverted pyramid geometry and the rest percentage gets double bounce The inverted pyramids on a 100microm cell with a two layer AR coating is estimated to give 402mAcm2 and cell efficiency of 24 with realistic cell design and material parameters

11

This texturization method can be employed on both mono-crystalline and multi-crystalline substrates Letrsquos have a look on this Mono-crystalline substrates For mono-crystalline substrate it is advantageous to make use of the anisotropic etching properties of Si in an alkaline solution As the 111 planes get etched more slowly than other crystal planes 111 facets are developed On lt100gtwafers this leads to pyramidal shapes at the surface that are particularly effective in reducing reflectance A regular array of pyramidal pits with facets at 547ordm to the horizontal plane mdash called inverted pyramids mdash can be formed using oxide etch mask with reflectances as low as 8 without ARC

Multi-crystalline Substrates Due to the anisotropic nature of multi-crystalline silicon substrate alkaline random texturing is not effective Some grains remain untextured leading to a high average reflectance So for texturing multi-crystalline silicon wafers should be etched in an acid mixture of HF and HNO3 because HNO3 tends to oxidize the surface while the HF etches the oxide away These acidic iso-texturing results in lower reflection than traditional anisotropic etching on multi-crystalline material and better conversion efficiency

STANDARD NAOH-IPA TEXTURIZATION APPROACH In order to achieve good uniformity of pyramidal structure on the silicon surface a mixture of NaOH or KOH and isopropyl alcohol (IPA) is generally used for texturization of mono-crystalline silicon solar cell For better texturization the interfacial energy between silicon and ionized electrolyte chemical solution should be reduced to achieve sufficient wettability for the silicon surface which will enhance the pyramid nucleation From a manufacturability point of view it is appealing to combine the saw damage removal and alkaline texturing in one single step in which case a trade-off has to be made between process speed and quality of the surface treatment

Fig1 Inverted Pyramid Textured Surface

Fig3 Textured multi-crystalline Si surface

Fig2 Anisotropic Etching of Mono-crystalline Si

12

DIFFUSION

INTRODUCTION

Diffusion the movement of a chemical species from an area of high concentration to an area of lower concentration is one of the two major processes by which chemical species or dopants are introduced into a semiconductor (the other one being ion implantation) The controlled diffusion of dopants into silicon to alter the type and level of conductivity of semiconductor materials is the foundation of forming a p-n junction The mathematics that govern the mass transport phenomena of diffusion are based on Ficks laws There are two laws according to Fick Letrsquos have a look on these laws First law Whenever an impurity concentration gradient partCpartx exists in a finite volume of a matrix substance (the silicon substrate in this context) the impurity material will have the natural tendency to move in order to distribute itself more evenly within the matrix and decrease the gradient Given enough time this flow of impurities will eventually result in homogeneity within the matrix causing the net flow of impurities to stop The mathematics of this transport mechanism was formalized in 1855 by Fick who postulated that the flux of material across a given plane is proportional to the concentration gradient across the plane Thus Ficks First Law states

J = -D ( partC(xt)partx ) Where J is the flux D is the diffusion constant for the material that is diffusing in the specific solvent and partC(xt)partx is the concentration gradient The diffusion constant of a material is also referred to as diffusion coefficient or simply diffusivity It is expressed in units of length2time such as microm2hour The negative sign of the right side of the equation indicates that the impurities are flowing in the direction of lower concentration Second Law Ficks First Law does not consider the fact that the gradient and local concentration of the impurities in a material decreases with an increase in time an aspect thats important to diffusion processes The flux J1 of impurities entering a section of a bar with a concentration gradient is different from the flux J2 of impurities leaving the same section From the law of conservation of matter the difference between J1 and J2 must result in a change in the concentration of impurities within the section (assuming that no impurities are formed or consumed in the section) This is Ficks Second Law which states that the change in impurity concentration over time is equal to the change in local diffusion flux or

partC(xt)partt = - partJpartx or from Ficks First Law

partC(xt)partt = part(DpartC(xt)partx)partx If the diffusion coefficient is independent of position such as when the impurity concentration is low then Ficks Second Law may be further simplified into the following equation

partC(xt)partt = D part2C(xt)partx2 The most important property of diffusion process in the manufacture of silicon solar cell is the formation of p-n junction by doping n-type material ie phosphorous on the p-type ie boron doped silicon textured wafer playing an essential role for photovoltaic effect Let us have a brief descriptive look on the properties of p-n junction P-N JUNCTION A p-n junction is a junction formed by combining p-type and n-type semiconductors together in very close contact

13

Principle of p-n junction The most common type of solar cell is a large p-n junction where the free carrier pairs emitted by light energy are separated by the junction and contribute to the current In its simplest form it consists of a junction formed between n-type and p-type semiconductors either of the same material (homo-junction) or different materials (hetero-junction) The band-structure of the two differently doped sides with respect to their Fermi levels can be seen in adjacent figure1

When the two halves are brought together the Fermi levels on either side are forced in to coincidence causing the valence and conduction bands to bend (Figure 2)

These bent bands represent a built-in electric field over what is referred to as the depletion region When a photon with a energy greater than the band-gap of the semiconductor passes through the solar cell it may be absorbed by the material This absorption takes the form of a band-to-band electronic transition so an electronhole pair is produced If these carriers can diffuse to the depletion region before they recombine then they are separated by the electric field causing one quantum of charge to flow through an external load This is the origin of the solar cells photocurrent and is shown in Figure 3

14

SOLAR CELL AS A P-N JUNCTION A solar cell is essential a p-n junction with large surface area The n type material is kept thin to allow light to pass through to the p-n junction

Light travels in packets of energy called photons The generation of electric current happens inside the depletion zone of the PN junction The depletion region as explained previously with the diode is the area around the PN junction where the electrons from the N-type silicon have diffused into the holes of the P-type material When a photon of light is absorbed by one of these atoms in the N-Type silicon it will dislodge an electron creating a free electron and a hole The free electron and hole has sufficient energy to jump out of the depletion zone If a wire is connected from the cathode (N-type silicon) to the anode (P-type silicon) electrons will flow through the wire The electron is attracted to the positive charge of the P-type material and travels through the external load (meter) creating a flow of electric current The hole created by the dislodged electron is attracted to the negative charge of N-type material and migrates to the back electrical contact As the electron enters the P-type silicon from the back electrical contact it combines with the hole restoring the electrical neutrality

DIFFUSION PROCESS

The first layer formation after doing texturization of wafer is performed in this process by doping the substrate with n-type dopant N-doping can be accomplished by depositing n dopant onto the substrate and then heating the substrate to drive the n dopant into the substrate Gaseous diffusion (for example POCl3 horizontal tube diffusion including temperatures of from 700 to 1000degC preferably 800 to 900degC) can be used to deposit the n-dopant onto the substrate surface to create an n-doped layer and a shallow p-n junction proximal to the substrate surface In this process if wafers are loaded onto the quartz boat back to back in the same lot then double diffusion process takes place and if single wafers are being loaded then single diffusion takes place On the quartz boat wafers can be loaded in two methods

1 Parallel diffusion loading of wafers parallel to the gas flow 2 Perpendicular diffusion loading of wafers perpendicular to the gas flow

Diffusion process is comprised of the following steps 1 Idle state-In this state the load center and source zone are set at 800ordmC and no process is

carried out in the furnace 2 Load-During this sequence the silicon carbide paddle moves at a speed of 45 rpm and the

maximum time taken is 7min 3 Ramp-The cold wafers loaded into the tubes bring down the temperature of the tube The tube

is then reheated to reach the set point temperature and the time taken is 7min

15

4 Stabilization-During this phase any overshooting of temperature during ramp-up phase is controlled to the predetermined set value and the time taken is 10min

5 Pre-deposition-Any excess POCL3 present in vapour form inside the quartz bubbler is vent out before the deposition starts by N2

6 Deposition- N purging in 5 sec

2 is bubbled through liquid POCL3 in presence of O2 to form P2O5

7 Post-deposition-During this phase O in paste form

2 and N2

8 N

flows to drive in the phosphorous from wafer surface to the junction

2

9 Unload-During this phase the SiC paddle moves slowly out of the furnace at set speed of 45rpm and maximum time taken is 7min

Purge-This phase is also known as drive in diffusion which is designed to evenly distribute the impurities into wafer and reduce the generally undesirable high concentration in the wafer surface and the maximum time taken is 7min

10 End-The tube is set to 800⁰C temperature in idle state

16

PLASMA ETCHING

INTRODUCTION

Plasma etching is one type of dry etching which uses plasma to produce chemically reactive species from inert gases The reactive gases are then made to react with the material to be etched The main objective of this process is to remove edge shunts present on the cell which leads to 80 loss and thus it is also termed as Edge isolation method Since edge isolation is the main motive of this process so first let us see that why edge isolation is so important

Edge isolation

In the standard processing sequence the entire wafer is phosphorous diffused The contact to the p-type base is made by firing an Al containing paste that produces a high doping of Al thus over-compensating the shallow emitter diffusion However there is still a direct conductive path from the emitter on the front side to the rear p contact Therefore a large portion of the photo-generated current can directly flow through the emitter to the back contact instead of flowing through the external circuit This current path is modeled in the two-diode model of the solar cell as the parallel resistance R

where R

p

s and Rp are the series and parallel resistance respectively j01 and j02 represent the first and second diode saturation current densities respectively n1 and n2 are the first and second ideality diode factors respectively Vth = ktq is the thermal voltage and jL

In this model the first diode accounts for emitter and bulk recombination with a recombination rate assumed to be proportional to the excess carrier density This leads to an ideality factor of n

is the light generated current density The two diode model is as shown below

1=1 The second diode is associated with the recombination in the space charge region that is assumed to be independent of the injection level leading to n2=2 The effect of increased j02 and Rp can be shown by model calculations for ideal solar cells in the figure below for a fill factor potential of 84 j02 must be less than 2x10-9 Acm2 and Rp must be greater than approximately 2000Ω cm2 if the respective parameters contribute solely to the fill factor loss

17

This increase in Rp is achieved by the edge isolation process that interrupts the parasitic current path as shown in fig2 below

Thus edge isolation is required in solar cell to increase open circuit voltage and thus leading to greater efficiency

Now we will move on to plasma etching process to see that how it works actually in solar cell and module manufacturing industries

Plasma Etching Process

After diffusion a layer of phosphosilicate glass is formed on the edges and rear surface of diffused wafer The process generally requires adequate physical masking of the front surface while exposing the full back of the cell to the dry plasma environment In an embodiment of this process cells are slotted face to face

Fig2Sketch of an industrial screen-printed solar cell The emitter diffusion is present on the complete wafer surface and gets locally overcompensated by printing and firing Al containing metal paste as back contact Without edge-isolation light generated e-s can flow through the emitter region to the back contact which corresponds to the small series resistance in the two diode model

18

into inert carriers so that the plasma chemistry can effectively react with the back of the cells thereby removing excess material from the second coating that might have reached the back of the cell This process is preferably performed via the generation of an RF (13 6 MHz) plasma using a mix of Freon 14 (CF4) with oxygen (O2) and or nitrogen (N2) in a ratio which depends on the chamber and load geometries

19

PHOSPHOSILICATE GLASS REMOVAL INTRODUCTION In diffusion process during deposition phase N2 gas is bubbled through liquid POCL3

The reaction related to this phase is given as follows

1 4POCL3 + 3O2 -gt 2P2O5 + 6Cl 2 2P

2 2O5 + 5Si -gt 5SiO2

+ 4P (PSG)

P2O5

So it becomes mandatory to remove phosphosilicate glass by chemicals which may act as recombination sites or may contribute towards the surface density defect and hence all these defects may contribute to a lower voltage and current

obtained in first reaction step is in paste form and is transferred to the Teflon duct to the process tube which then reacts with silicon of which the wafer is made And thus when this phosphorous pentoxide reaches silicon surface then second reaction takes place and thus a silicon dioxide layer and phosphosilicate glass ie PSG forms on the wafer surface Formation of this dead layer on the wafer surface contributes to a lower efficiency

PROCESS OF REMOVAL Following are the steps that should be taken into consideration for the removal of phosphosilicate glass (PSG) from the wafer surface

1 Firstly wafers are washed combinedly with HF HCL and DI water in a tank for 1min 2 Again wafers are washed with HF and DI water for 2min in the other tank 3 For another 1 min DI water is being sprayed on the diffused wafers 4 Next wafers are washed with DI water subsequently on an interval of 30 sec 5 Finally wafers are kept in the OVEN for 10mins within the temperature range of 95plusmn5ordmC

20

PLASMA ENHANCED CHEMICAL VAPOUR DEPOSITION INTRODUCTION In recent years the low-temperature (le 400ordmC) surface passivation of crystalline silicon solar cells by means of PECVD silicon nitride films has attracted an increasing attention of the PV community These films have proven to be capable of combining an outstanding surface passivation quality with excellent anti-reflection properties On typical p-type silicon solar cells substrate it has been shown that the surface passivation quality of very silicon rich Si3N4

we must first know that whatrsquos the use of silicon nitride coating on the surface of the wafer

films is superior to that of state-of-the-art high temperature silicon dioxide Before proceeding to the details of PECVD system

WHY SILICON NITRIDE SURFACE COATING IS PREFERRED IN PECVD PROCESS It is preferred as a surface coating because it functions as An effective diffusion mask An effective metallization diffusion barrier An anti-reflective coating Provide surface and bulk passivation

Preferably hydrogen is trapped in such a silicon nitride surface coating PECVD SYSYTEM PECVD or Plasma Enhanced Chemical Vapor Deposition is one of the methods of different Chemical Vapor Deposition (CVD) technique In PECVD plasma is being used to decompose a reactant gas such as silane (SiH4) to produce reaction products that precipitate on the surface of the substrate as a new layer

Working Principle Inside a PECVD reactor a strong electric field ignites plasma between two electrodes one of which holds the substrate This plasma ignition cracks the molecular bonds of the process gas which in turn are able to crack more process gas molecules before reaching the surface of the substrate The glow discharge used in this reactor is created by applying an RF field to a low-pressure gas creating free electrons within the discharge region The electrons are sufficiently energized by the electric field that gas-phase dissociation and ionization of the reactant gases occur when the free electrons collide with them Energetic species are then adsorbed on the film surface where they are subjected to ion and electron bombardment rearrangements reactions with other species new bond formation and film formation and growth Eventually a new layer is deposited on the substrate For example when silane is used as precursor plasma ignition frees up Si and SiH radicals which also crack more silane molecules on their way to the surface of the substrate where silicon is deposited PROCESS DETAILS OF PECVD In this system wafers coming from the previous state ie PSG removal step are being loaded onto the graphite boat where the individual wafers are held by three pins inside the boat It is absolutely necessary for the wafers to touch all the three pins otherwise there will remain a possibility of poor deposition in which

Figure Anti-reflective coating made out of silicon nitride deposited by the PECVD technique whereby silane and ammonia is reacted in a furnace The major color of coating looks blue but the color could also look like purple blue or light blue due to different thickness of coating

21

the HF generator fails to generate the required power for process deposition While loading the double diffusion wafers it should be ensured that deposition side should be correctly loaded The wafers are being loaded parallel to the boat After loading the trolley along with the boat is placed inside the machine and then after filling the input parameters of the CMI interface used here and the process tube press the RUN button and then machine will be in auto mode The handling mechanism automatically picks up the boat and places it on the desired paddle corresponding to the selected for the specified job And then process STARTS

22

SCREEN PRINTING INTRODUCTION In screen printing on wafer-based solar PV cells the mesh and buses of silver are printed on the front furthermore the buses of silver are printed on the back Subsequently aluminum paste is dispensed over the whole surface of the back for passivation and surface reflection [14] One of the parameters that can vary and can be controlled in screen printing is the thickness of the print This makes it useful for some of the techniques of printing solar cells electronics etc Screen printing is one of the most critical processes to maintain high yield Solar wafers are becoming thinner and larger so careful printing is required to maintain a lower breakage rate On the other hand high throughput at the printing stage improves the throughput of the whole cell production line PROCESS This process is divided in three phase-

1 Phase I- AgAl printing 2 Phase II- Ag printing 3 Phase III- Al printing

PHASE I- AgAl PRINTING This phase is required to print the back bus bar for tabbing purpose The AgAl paste used in this printing is Dupont (PV-505) and the consumption of the paste per wafer is 011 gm And the screen type used is of the following specification

1 Mesh size- 200microm 2 Tension- 24 N-cm 3 Mesh angle- 225ordm 45ordm 4 Emulsion thickness- 24microm

Process details During printing the paste is poured on the screen and is spread over it with the help of polypropylene spatula Then the wafers (ARC coated) are pasted with the sunny side down on the loader and picked one by one automatically with the start of the print command Once printing starts the wafers are placed one by one between the tool paste and the screen and then the squeegee moves over the screen and allow the paste through the exposed parts on the screen and gets pasted on the wafers Thus in this way the wafers are printed one by one Two most important parameters to be controlled during this printing are

1 Squeegee Pressure 2 Snap off distance

The next step after printing is drying step DRYING In this step wafers are passed through the dryer with a temperature range of 180ordm-400ordmC PHASE II- Ag PRINTING This phase is required to print the front bus bar for tabbing purpose The Ag paste used in this printing is Dupont (PV-145) and the consumption of the paste per wafer is 014 gm And the screen type used is of the following specification

1 Mesh size- 325microm 2 Tension- 24 N-cm 3 Mesh angle- 225ordm 4 Emulsion thickness- 24microm

5

RAW MATERIALS NEEDED

Along with the enormous capital investment required to successfully set up a solar cellmodule manufacturing plant it also requires a steady supply of raw materials The most basic raw materials is the silicon wafer itself which is usually bought from another company rather than prepared in-house Two types of crystalline silicon solar cell are single crystalline silicon solar cell which is known as monocrystalline silicon solar cell made by slicing wafers and polycrystalline silicon solar cell made by sawing a block of silicon first into bars and then wafers Other semiconductors such as gallium arsenide cadmium telluride and copper indium diselenide etc have both single crystalline and polycrystalline forms The solar cell which made by gallium arsenide has high efficiency ranging from 20-25 The first process Etching requires a number of chemicals HF HCL NaOH which may vary depending on the etching methodology (Acidic etching or Alkali etching) Deionised water is obtained from the DI plant IPA is used for cleaning purposes For diffusion an MRL diffusion furnace can be used which utilizes N2 amp O2

N2 flows through a bubbler of POCl3

for Phosphorus diffusion HF HCl and DI water are also used to clean the components (bubbler quartz tube etc) used in Diffusion For plasma etching we need a gas like Freon to provide the ions used in edge isolation Silane and Ammonia in gaseous form are needed to perform PECVD in order to deposit the anti reflective coating Silicon Nitride is formed due to the reaction of Silane and Ammonia Screen Printing is a three phase process The first phase is AgAl which could use the paste Dupont PV505 The second front side printing uses an Ag pastes (Dupont PV145) Third phase uses Aluminium paste (Analog 12D)

Module manufacturing requires its own set of raw materials It needs Tedlar EVA and Glass to form the multilayered structure Aluminium frames are needed to provide structural integrity and a junction box for output terminals Thus a manufacturing plant needs a variety of raw materials at the right time in the right quantity quality and purity A fall is any of these parameters will severely hamper the manufacturing process and negatively effect margins Thus ensuring a proper supply of this varied mix of materials is of paramount importance to the managers

6

HISTORY OF PHOTOVOLTAICS Most people are surprised to learn that photovoltaic technology actually dates back over 160 years French physicist Edmond Becquerel first described the photovoltaic (PV) effect in 1839 but it remained a curiosity of science for the next three quarters of a century At only 19 Becquerel found that certain materials would produce small amounts of electric current when exposed to light The effect was first studied in solids such as selenium by Heinrich Hertz in the 1870s Soon afterward selenium PV cells were converting light to electricity at 1 to 2 efficiency As a result selenium was quickly adopted in the emerging field of photography for use in light-measuring devices Major steps toward commercializing PV were taken in the 1940s and early 1950s when the Czochralski process was developed for producing highly pure crystalline silicon In 1954 scientists at Bell Laboratories depended on the Czochralski process to develop the first crystalline silicon photovoltaic cell which had an efficiency of 4 Although a few attempts were made in the 1950s to use silicon cells in commercial products it was the new space program that gave the technology its first major application In the 1970s research drives PV costs down 80 allowing for applications such as offshore navigation warning lights and horns lighthouses railroad crossings and remote use where utility-grid connections are too costly Despite these advances PV devices in 1970 were still too expensive for most mass market uses But in the mid-1970s rising energy costs sparked by a world oil crisis renewed interest in making PV technology more affordable Since then the federal government industry and research organizations have invested billions of dollars in research development and production Often industry and the government work together sharing the cost of PV research and development In the 1980s photovoltaic became a popular power source for consumer electronic devices including calculators watches radios lanterns and other small battery-charging applications

Todays commercial PV systems can convert from 11 to 17 of sunlight into electricity They are highly reliable and last 20 years or longer The cost of PV-generated electricity has dropped 15- to 20-fold and PV modules now cost around $6 per watt (W) and produce electricity for as little as 25 to 50 cents per kilowatt-hour (kWh)

7

Solar Cell Manufacturing Processes

8

ETCHING

INTRODUCTION

Etching is the process of using strong acid or mordant to cut into the unprotected parts of a metal surface to create a design in intaglio in the metal (the original processmdashin modern manufacturing other chemicals may be used on other types of material) As an intaglio method of printmaking it is along with engraving the most important technique for old master prints and remains widely used today But in wafer fabrication Etching refers to a process by which material is removed from the wafer ie either from the silicon substrate itself or from any film or layer of material on the wafer

DEFINITION

The treatment of a prepared metal surface with acid or other chemical reagent which by differential attack reveals the structure is known as etching In wafer fabrication etching process is generally classified in two classes Dry etching and Wet etching

Normally dry etching is not being used in this solar cells and module manufacturing unit So we will just go on a brief look on wet etching process

WET ETCHING It is a process that utilizes liquid chemicals or etchants to remove materials from the wafer usually in specific patterns defined by photo resist masks on the wafer Materials not covered by these masks are etched away by the chemicals while those covered by the masks are left almost intact A simple wet etching process may just consist of dissolution of the material to be removed in a liquid solvent without changing the chemical nature of the dissolved material In general however a wet etching process involves one or more chemical reactions that consume the original reactants and produce new species A basic wet etching process may be broken down in three steps as follows

1 Diffusion of the etchant to the surface for removal 2 Reaction between the etchant and the material being removed and 3 Diffusion of the reaction byproducts from the reacted surface

Wet etching can be both isotropic ie it proceeds in all directions at the same rate and anisotropic ie it proceeds in only one direction Isotropic Wet Etching A process which proceeds at the same rate in all directions is known as isotropic wet etching The etchants used in this process are isotropic in nature that etches away even the portion of material that is directly under the mask (usually in the shape of a quarter-circle) since its horizontal etching rate is the same as its vertical rate When an isotropic etchant eats away a portion of the material under the mask the etched film is said to have undercut the mask The amount of undercutting is a measure of an etching parameter known as the bias Bias is simply defined as the difference between the lateral dimensions of the etched image and the masked image Thus the mask used in etching must compensate for whatever bias an etchant is known to produce in order to create the desired feature on the wafer

Figure 1 Isotropic wet etching

9

Anisotropic Wet Etching A process which proceeds in only one direction is known as anisotropic wet etching It removes material in the vertical direction only since it will follow the mask patterns on the wafer very faithfully leaving any material covered by mask material basically untouched Since isotropic wet etching process has high bias values which are not practical for use in pattern images which has features of measuring less than 3 microns thus wafer patterns of less than 3 microns are wet etched anisotropically instead of isotropic

Figure 2 Anisotropic wet etching

Advantages of wet etching 1 Low cost 2 High reliability 3 High throughput 4 Excellent selectivity in most cases with respect to both mask and substrate materials

Disadvantages of wet etching 1 Limited resolution 2 Higher safety risks due to the direct chemical exposure of the personnel 3 High cost of etchants in some cases 4 Problems related to the resists loss of adhesion to the substrate 5 Problems related to the formation of bubbles which inhibit the etching process where they are present and 6 Problems related to incomplete or non-uniform etching

Before moving on to texturization we first of all must do Saw damage removal which is also called as 1st Etch First Etch When ingots obtained from the Czochralski method are sliced into wafers by saw then it leaves some damaged marks on the wafers if it is not being sliced properly which makes the surface non-uniform So to make the surface uniform and free of error it becomes mandatory to do saw damage removal or 1st

1 First wafers are kept in the solution of 300ml DI water and 10kg NaOH(RSE) within the temperature range of 75⁰plusmn 2⁰C for 1min The reaction which takes place in this step is

etching Steps involved are

Si + 2NaOH + H2O (DI) -gt Na3SiO2 + H2 + H2

2 Finally these etched wafers are washed with DI water O

10

TEXTURIZATION INTRODUCTION Since 1960 for the improvement of the cell performance texturization has been attempted Texturization of front andor back surfaces of a solar cell surfaces improve their performances The reflection from chemically textured wafer reduced from 34 for a polished silicon surface to 11 Many researchers have reported the selective etching of silicon of lt100gt orientation of using sodium or potassium hydroxide solution OBJECTIVES OF TEXTURIZATION The major objectives of texturization are-

1 Reduction of front surface reflectance 2 Increase in the path length of the entered light of lower wavelengths by oblique trajectory within the

cell and absorption of light closer to the junction 3 Optical trapping of weakly absorbed light by multiple internal reflections from the top and bottom

surfaces of the cell thereby also increasing the optical path length The increased path length with an oblique trajectory leads to an effective increase in the absorption co-efficient

All these effects lead to realization of higher performance thick cells and also efficient solar cells of comparatively lower thickness which significantly reduces the material cost without degrading the cell performance MODELS OF TEXTURIZATION To reduce the reflection co-efficient and to trap weak absorbed light various texturing geometries are being employed The different texturing geometries include

1 Lambertian geometry 2 Upright pyramid geometry 3 Slat or Micro-grooved geometry 4 Inverted pyramid geometry 5 Simple prism pyramid geometry 6 Grating geometry 7 Three perpendicular planes (3PP) geometry 8 Perpendicular slat geometry 9 Porous silicon

Now in large number of manufacturing industries Inverted pyramid texturing geometry scheme is being used these days Letrsquos have a look on this scheme very briefly Inverted Pyramid Geometry The light trapping properties of the inverted pyramid geometry has been first investigated by Smith and Rohatgi The increase in current for the type of textured cell is due to incorporation of al the three effects necessary for an efficient light trapping design The front surface reflectance is reduced by providing the opportunity for a portion of the incoming light to undergo a triple bounce thereby reducing the overall front surface reflectance The increase in path length and light trapping efficiency means that a larger fraction of the light which has entered the cell will be absorbed before exiting the cell The short wavelength spectral response analysis at the texturing angle of 5375ordm indicates that 37 of the incoming light experiences a triple bounce on the front surface of the inverted pyramid geometry and the rest percentage gets double bounce The inverted pyramids on a 100microm cell with a two layer AR coating is estimated to give 402mAcm2 and cell efficiency of 24 with realistic cell design and material parameters

11

This texturization method can be employed on both mono-crystalline and multi-crystalline substrates Letrsquos have a look on this Mono-crystalline substrates For mono-crystalline substrate it is advantageous to make use of the anisotropic etching properties of Si in an alkaline solution As the 111 planes get etched more slowly than other crystal planes 111 facets are developed On lt100gtwafers this leads to pyramidal shapes at the surface that are particularly effective in reducing reflectance A regular array of pyramidal pits with facets at 547ordm to the horizontal plane mdash called inverted pyramids mdash can be formed using oxide etch mask with reflectances as low as 8 without ARC

Multi-crystalline Substrates Due to the anisotropic nature of multi-crystalline silicon substrate alkaline random texturing is not effective Some grains remain untextured leading to a high average reflectance So for texturing multi-crystalline silicon wafers should be etched in an acid mixture of HF and HNO3 because HNO3 tends to oxidize the surface while the HF etches the oxide away These acidic iso-texturing results in lower reflection than traditional anisotropic etching on multi-crystalline material and better conversion efficiency

STANDARD NAOH-IPA TEXTURIZATION APPROACH In order to achieve good uniformity of pyramidal structure on the silicon surface a mixture of NaOH or KOH and isopropyl alcohol (IPA) is generally used for texturization of mono-crystalline silicon solar cell For better texturization the interfacial energy between silicon and ionized electrolyte chemical solution should be reduced to achieve sufficient wettability for the silicon surface which will enhance the pyramid nucleation From a manufacturability point of view it is appealing to combine the saw damage removal and alkaline texturing in one single step in which case a trade-off has to be made between process speed and quality of the surface treatment

Fig1 Inverted Pyramid Textured Surface

Fig3 Textured multi-crystalline Si surface

Fig2 Anisotropic Etching of Mono-crystalline Si

12

DIFFUSION

INTRODUCTION

Diffusion the movement of a chemical species from an area of high concentration to an area of lower concentration is one of the two major processes by which chemical species or dopants are introduced into a semiconductor (the other one being ion implantation) The controlled diffusion of dopants into silicon to alter the type and level of conductivity of semiconductor materials is the foundation of forming a p-n junction The mathematics that govern the mass transport phenomena of diffusion are based on Ficks laws There are two laws according to Fick Letrsquos have a look on these laws First law Whenever an impurity concentration gradient partCpartx exists in a finite volume of a matrix substance (the silicon substrate in this context) the impurity material will have the natural tendency to move in order to distribute itself more evenly within the matrix and decrease the gradient Given enough time this flow of impurities will eventually result in homogeneity within the matrix causing the net flow of impurities to stop The mathematics of this transport mechanism was formalized in 1855 by Fick who postulated that the flux of material across a given plane is proportional to the concentration gradient across the plane Thus Ficks First Law states

J = -D ( partC(xt)partx ) Where J is the flux D is the diffusion constant for the material that is diffusing in the specific solvent and partC(xt)partx is the concentration gradient The diffusion constant of a material is also referred to as diffusion coefficient or simply diffusivity It is expressed in units of length2time such as microm2hour The negative sign of the right side of the equation indicates that the impurities are flowing in the direction of lower concentration Second Law Ficks First Law does not consider the fact that the gradient and local concentration of the impurities in a material decreases with an increase in time an aspect thats important to diffusion processes The flux J1 of impurities entering a section of a bar with a concentration gradient is different from the flux J2 of impurities leaving the same section From the law of conservation of matter the difference between J1 and J2 must result in a change in the concentration of impurities within the section (assuming that no impurities are formed or consumed in the section) This is Ficks Second Law which states that the change in impurity concentration over time is equal to the change in local diffusion flux or

partC(xt)partt = - partJpartx or from Ficks First Law

partC(xt)partt = part(DpartC(xt)partx)partx If the diffusion coefficient is independent of position such as when the impurity concentration is low then Ficks Second Law may be further simplified into the following equation

partC(xt)partt = D part2C(xt)partx2 The most important property of diffusion process in the manufacture of silicon solar cell is the formation of p-n junction by doping n-type material ie phosphorous on the p-type ie boron doped silicon textured wafer playing an essential role for photovoltaic effect Let us have a brief descriptive look on the properties of p-n junction P-N JUNCTION A p-n junction is a junction formed by combining p-type and n-type semiconductors together in very close contact

13

Principle of p-n junction The most common type of solar cell is a large p-n junction where the free carrier pairs emitted by light energy are separated by the junction and contribute to the current In its simplest form it consists of a junction formed between n-type and p-type semiconductors either of the same material (homo-junction) or different materials (hetero-junction) The band-structure of the two differently doped sides with respect to their Fermi levels can be seen in adjacent figure1

When the two halves are brought together the Fermi levels on either side are forced in to coincidence causing the valence and conduction bands to bend (Figure 2)

These bent bands represent a built-in electric field over what is referred to as the depletion region When a photon with a energy greater than the band-gap of the semiconductor passes through the solar cell it may be absorbed by the material This absorption takes the form of a band-to-band electronic transition so an electronhole pair is produced If these carriers can diffuse to the depletion region before they recombine then they are separated by the electric field causing one quantum of charge to flow through an external load This is the origin of the solar cells photocurrent and is shown in Figure 3

14

SOLAR CELL AS A P-N JUNCTION A solar cell is essential a p-n junction with large surface area The n type material is kept thin to allow light to pass through to the p-n junction

Light travels in packets of energy called photons The generation of electric current happens inside the depletion zone of the PN junction The depletion region as explained previously with the diode is the area around the PN junction where the electrons from the N-type silicon have diffused into the holes of the P-type material When a photon of light is absorbed by one of these atoms in the N-Type silicon it will dislodge an electron creating a free electron and a hole The free electron and hole has sufficient energy to jump out of the depletion zone If a wire is connected from the cathode (N-type silicon) to the anode (P-type silicon) electrons will flow through the wire The electron is attracted to the positive charge of the P-type material and travels through the external load (meter) creating a flow of electric current The hole created by the dislodged electron is attracted to the negative charge of N-type material and migrates to the back electrical contact As the electron enters the P-type silicon from the back electrical contact it combines with the hole restoring the electrical neutrality

DIFFUSION PROCESS

The first layer formation after doing texturization of wafer is performed in this process by doping the substrate with n-type dopant N-doping can be accomplished by depositing n dopant onto the substrate and then heating the substrate to drive the n dopant into the substrate Gaseous diffusion (for example POCl3 horizontal tube diffusion including temperatures of from 700 to 1000degC preferably 800 to 900degC) can be used to deposit the n-dopant onto the substrate surface to create an n-doped layer and a shallow p-n junction proximal to the substrate surface In this process if wafers are loaded onto the quartz boat back to back in the same lot then double diffusion process takes place and if single wafers are being loaded then single diffusion takes place On the quartz boat wafers can be loaded in two methods

1 Parallel diffusion loading of wafers parallel to the gas flow 2 Perpendicular diffusion loading of wafers perpendicular to the gas flow

Diffusion process is comprised of the following steps 1 Idle state-In this state the load center and source zone are set at 800ordmC and no process is

carried out in the furnace 2 Load-During this sequence the silicon carbide paddle moves at a speed of 45 rpm and the

maximum time taken is 7min 3 Ramp-The cold wafers loaded into the tubes bring down the temperature of the tube The tube

is then reheated to reach the set point temperature and the time taken is 7min

15

4 Stabilization-During this phase any overshooting of temperature during ramp-up phase is controlled to the predetermined set value and the time taken is 10min

5 Pre-deposition-Any excess POCL3 present in vapour form inside the quartz bubbler is vent out before the deposition starts by N2

6 Deposition- N purging in 5 sec

2 is bubbled through liquid POCL3 in presence of O2 to form P2O5

7 Post-deposition-During this phase O in paste form

2 and N2

8 N

flows to drive in the phosphorous from wafer surface to the junction

2

9 Unload-During this phase the SiC paddle moves slowly out of the furnace at set speed of 45rpm and maximum time taken is 7min

Purge-This phase is also known as drive in diffusion which is designed to evenly distribute the impurities into wafer and reduce the generally undesirable high concentration in the wafer surface and the maximum time taken is 7min

10 End-The tube is set to 800⁰C temperature in idle state

16

PLASMA ETCHING

INTRODUCTION

Plasma etching is one type of dry etching which uses plasma to produce chemically reactive species from inert gases The reactive gases are then made to react with the material to be etched The main objective of this process is to remove edge shunts present on the cell which leads to 80 loss and thus it is also termed as Edge isolation method Since edge isolation is the main motive of this process so first let us see that why edge isolation is so important

Edge isolation

In the standard processing sequence the entire wafer is phosphorous diffused The contact to the p-type base is made by firing an Al containing paste that produces a high doping of Al thus over-compensating the shallow emitter diffusion However there is still a direct conductive path from the emitter on the front side to the rear p contact Therefore a large portion of the photo-generated current can directly flow through the emitter to the back contact instead of flowing through the external circuit This current path is modeled in the two-diode model of the solar cell as the parallel resistance R

where R

p

s and Rp are the series and parallel resistance respectively j01 and j02 represent the first and second diode saturation current densities respectively n1 and n2 are the first and second ideality diode factors respectively Vth = ktq is the thermal voltage and jL

In this model the first diode accounts for emitter and bulk recombination with a recombination rate assumed to be proportional to the excess carrier density This leads to an ideality factor of n

is the light generated current density The two diode model is as shown below

1=1 The second diode is associated with the recombination in the space charge region that is assumed to be independent of the injection level leading to n2=2 The effect of increased j02 and Rp can be shown by model calculations for ideal solar cells in the figure below for a fill factor potential of 84 j02 must be less than 2x10-9 Acm2 and Rp must be greater than approximately 2000Ω cm2 if the respective parameters contribute solely to the fill factor loss

17

This increase in Rp is achieved by the edge isolation process that interrupts the parasitic current path as shown in fig2 below

Thus edge isolation is required in solar cell to increase open circuit voltage and thus leading to greater efficiency

Now we will move on to plasma etching process to see that how it works actually in solar cell and module manufacturing industries

Plasma Etching Process

After diffusion a layer of phosphosilicate glass is formed on the edges and rear surface of diffused wafer The process generally requires adequate physical masking of the front surface while exposing the full back of the cell to the dry plasma environment In an embodiment of this process cells are slotted face to face

Fig2Sketch of an industrial screen-printed solar cell The emitter diffusion is present on the complete wafer surface and gets locally overcompensated by printing and firing Al containing metal paste as back contact Without edge-isolation light generated e-s can flow through the emitter region to the back contact which corresponds to the small series resistance in the two diode model

18

into inert carriers so that the plasma chemistry can effectively react with the back of the cells thereby removing excess material from the second coating that might have reached the back of the cell This process is preferably performed via the generation of an RF (13 6 MHz) plasma using a mix of Freon 14 (CF4) with oxygen (O2) and or nitrogen (N2) in a ratio which depends on the chamber and load geometries

19

PHOSPHOSILICATE GLASS REMOVAL INTRODUCTION In diffusion process during deposition phase N2 gas is bubbled through liquid POCL3

The reaction related to this phase is given as follows

1 4POCL3 + 3O2 -gt 2P2O5 + 6Cl 2 2P

2 2O5 + 5Si -gt 5SiO2

+ 4P (PSG)

P2O5

So it becomes mandatory to remove phosphosilicate glass by chemicals which may act as recombination sites or may contribute towards the surface density defect and hence all these defects may contribute to a lower voltage and current

obtained in first reaction step is in paste form and is transferred to the Teflon duct to the process tube which then reacts with silicon of which the wafer is made And thus when this phosphorous pentoxide reaches silicon surface then second reaction takes place and thus a silicon dioxide layer and phosphosilicate glass ie PSG forms on the wafer surface Formation of this dead layer on the wafer surface contributes to a lower efficiency

PROCESS OF REMOVAL Following are the steps that should be taken into consideration for the removal of phosphosilicate glass (PSG) from the wafer surface

1 Firstly wafers are washed combinedly with HF HCL and DI water in a tank for 1min 2 Again wafers are washed with HF and DI water for 2min in the other tank 3 For another 1 min DI water is being sprayed on the diffused wafers 4 Next wafers are washed with DI water subsequently on an interval of 30 sec 5 Finally wafers are kept in the OVEN for 10mins within the temperature range of 95plusmn5ordmC

20

PLASMA ENHANCED CHEMICAL VAPOUR DEPOSITION INTRODUCTION In recent years the low-temperature (le 400ordmC) surface passivation of crystalline silicon solar cells by means of PECVD silicon nitride films has attracted an increasing attention of the PV community These films have proven to be capable of combining an outstanding surface passivation quality with excellent anti-reflection properties On typical p-type silicon solar cells substrate it has been shown that the surface passivation quality of very silicon rich Si3N4

we must first know that whatrsquos the use of silicon nitride coating on the surface of the wafer

films is superior to that of state-of-the-art high temperature silicon dioxide Before proceeding to the details of PECVD system

WHY SILICON NITRIDE SURFACE COATING IS PREFERRED IN PECVD PROCESS It is preferred as a surface coating because it functions as An effective diffusion mask An effective metallization diffusion barrier An anti-reflective coating Provide surface and bulk passivation

Preferably hydrogen is trapped in such a silicon nitride surface coating PECVD SYSYTEM PECVD or Plasma Enhanced Chemical Vapor Deposition is one of the methods of different Chemical Vapor Deposition (CVD) technique In PECVD plasma is being used to decompose a reactant gas such as silane (SiH4) to produce reaction products that precipitate on the surface of the substrate as a new layer

Working Principle Inside a PECVD reactor a strong electric field ignites plasma between two electrodes one of which holds the substrate This plasma ignition cracks the molecular bonds of the process gas which in turn are able to crack more process gas molecules before reaching the surface of the substrate The glow discharge used in this reactor is created by applying an RF field to a low-pressure gas creating free electrons within the discharge region The electrons are sufficiently energized by the electric field that gas-phase dissociation and ionization of the reactant gases occur when the free electrons collide with them Energetic species are then adsorbed on the film surface where they are subjected to ion and electron bombardment rearrangements reactions with other species new bond formation and film formation and growth Eventually a new layer is deposited on the substrate For example when silane is used as precursor plasma ignition frees up Si and SiH radicals which also crack more silane molecules on their way to the surface of the substrate where silicon is deposited PROCESS DETAILS OF PECVD In this system wafers coming from the previous state ie PSG removal step are being loaded onto the graphite boat where the individual wafers are held by three pins inside the boat It is absolutely necessary for the wafers to touch all the three pins otherwise there will remain a possibility of poor deposition in which

Figure Anti-reflective coating made out of silicon nitride deposited by the PECVD technique whereby silane and ammonia is reacted in a furnace The major color of coating looks blue but the color could also look like purple blue or light blue due to different thickness of coating

21

the HF generator fails to generate the required power for process deposition While loading the double diffusion wafers it should be ensured that deposition side should be correctly loaded The wafers are being loaded parallel to the boat After loading the trolley along with the boat is placed inside the machine and then after filling the input parameters of the CMI interface used here and the process tube press the RUN button and then machine will be in auto mode The handling mechanism automatically picks up the boat and places it on the desired paddle corresponding to the selected for the specified job And then process STARTS

22

SCREEN PRINTING INTRODUCTION In screen printing on wafer-based solar PV cells the mesh and buses of silver are printed on the front furthermore the buses of silver are printed on the back Subsequently aluminum paste is dispensed over the whole surface of the back for passivation and surface reflection [14] One of the parameters that can vary and can be controlled in screen printing is the thickness of the print This makes it useful for some of the techniques of printing solar cells electronics etc Screen printing is one of the most critical processes to maintain high yield Solar wafers are becoming thinner and larger so careful printing is required to maintain a lower breakage rate On the other hand high throughput at the printing stage improves the throughput of the whole cell production line PROCESS This process is divided in three phase-

1 Phase I- AgAl printing 2 Phase II- Ag printing 3 Phase III- Al printing

PHASE I- AgAl PRINTING This phase is required to print the back bus bar for tabbing purpose The AgAl paste used in this printing is Dupont (PV-505) and the consumption of the paste per wafer is 011 gm And the screen type used is of the following specification

1 Mesh size- 200microm 2 Tension- 24 N-cm 3 Mesh angle- 225ordm 45ordm 4 Emulsion thickness- 24microm

Process details During printing the paste is poured on the screen and is spread over it with the help of polypropylene spatula Then the wafers (ARC coated) are pasted with the sunny side down on the loader and picked one by one automatically with the start of the print command Once printing starts the wafers are placed one by one between the tool paste and the screen and then the squeegee moves over the screen and allow the paste through the exposed parts on the screen and gets pasted on the wafers Thus in this way the wafers are printed one by one Two most important parameters to be controlled during this printing are

1 Squeegee Pressure 2 Snap off distance

The next step after printing is drying step DRYING In this step wafers are passed through the dryer with a temperature range of 180ordm-400ordmC PHASE II- Ag PRINTING This phase is required to print the front bus bar for tabbing purpose The Ag paste used in this printing is Dupont (PV-145) and the consumption of the paste per wafer is 014 gm And the screen type used is of the following specification

1 Mesh size- 325microm 2 Tension- 24 N-cm 3 Mesh angle- 225ordm 4 Emulsion thickness- 24microm

6

HISTORY OF PHOTOVOLTAICS Most people are surprised to learn that photovoltaic technology actually dates back over 160 years French physicist Edmond Becquerel first described the photovoltaic (PV) effect in 1839 but it remained a curiosity of science for the next three quarters of a century At only 19 Becquerel found that certain materials would produce small amounts of electric current when exposed to light The effect was first studied in solids such as selenium by Heinrich Hertz in the 1870s Soon afterward selenium PV cells were converting light to electricity at 1 to 2 efficiency As a result selenium was quickly adopted in the emerging field of photography for use in light-measuring devices Major steps toward commercializing PV were taken in the 1940s and early 1950s when the Czochralski process was developed for producing highly pure crystalline silicon In 1954 scientists at Bell Laboratories depended on the Czochralski process to develop the first crystalline silicon photovoltaic cell which had an efficiency of 4 Although a few attempts were made in the 1950s to use silicon cells in commercial products it was the new space program that gave the technology its first major application In the 1970s research drives PV costs down 80 allowing for applications such as offshore navigation warning lights and horns lighthouses railroad crossings and remote use where utility-grid connections are too costly Despite these advances PV devices in 1970 were still too expensive for most mass market uses But in the mid-1970s rising energy costs sparked by a world oil crisis renewed interest in making PV technology more affordable Since then the federal government industry and research organizations have invested billions of dollars in research development and production Often industry and the government work together sharing the cost of PV research and development In the 1980s photovoltaic became a popular power source for consumer electronic devices including calculators watches radios lanterns and other small battery-charging applications

Todays commercial PV systems can convert from 11 to 17 of sunlight into electricity They are highly reliable and last 20 years or longer The cost of PV-generated electricity has dropped 15- to 20-fold and PV modules now cost around $6 per watt (W) and produce electricity for as little as 25 to 50 cents per kilowatt-hour (kWh)

7

Solar Cell Manufacturing Processes

8

ETCHING

INTRODUCTION

Etching is the process of using strong acid or mordant to cut into the unprotected parts of a metal surface to create a design in intaglio in the metal (the original processmdashin modern manufacturing other chemicals may be used on other types of material) As an intaglio method of printmaking it is along with engraving the most important technique for old master prints and remains widely used today But in wafer fabrication Etching refers to a process by which material is removed from the wafer ie either from the silicon substrate itself or from any film or layer of material on the wafer

DEFINITION

The treatment of a prepared metal surface with acid or other chemical reagent which by differential attack reveals the structure is known as etching In wafer fabrication etching process is generally classified in two classes Dry etching and Wet etching

Normally dry etching is not being used in this solar cells and module manufacturing unit So we will just go on a brief look on wet etching process

WET ETCHING It is a process that utilizes liquid chemicals or etchants to remove materials from the wafer usually in specific patterns defined by photo resist masks on the wafer Materials not covered by these masks are etched away by the chemicals while those covered by the masks are left almost intact A simple wet etching process may just consist of dissolution of the material to be removed in a liquid solvent without changing the chemical nature of the dissolved material In general however a wet etching process involves one or more chemical reactions that consume the original reactants and produce new species A basic wet etching process may be broken down in three steps as follows

1 Diffusion of the etchant to the surface for removal 2 Reaction between the etchant and the material being removed and 3 Diffusion of the reaction byproducts from the reacted surface

Wet etching can be both isotropic ie it proceeds in all directions at the same rate and anisotropic ie it proceeds in only one direction Isotropic Wet Etching A process which proceeds at the same rate in all directions is known as isotropic wet etching The etchants used in this process are isotropic in nature that etches away even the portion of material that is directly under the mask (usually in the shape of a quarter-circle) since its horizontal etching rate is the same as its vertical rate When an isotropic etchant eats away a portion of the material under the mask the etched film is said to have undercut the mask The amount of undercutting is a measure of an etching parameter known as the bias Bias is simply defined as the difference between the lateral dimensions of the etched image and the masked image Thus the mask used in etching must compensate for whatever bias an etchant is known to produce in order to create the desired feature on the wafer

Figure 1 Isotropic wet etching

9

Anisotropic Wet Etching A process which proceeds in only one direction is known as anisotropic wet etching It removes material in the vertical direction only since it will follow the mask patterns on the wafer very faithfully leaving any material covered by mask material basically untouched Since isotropic wet etching process has high bias values which are not practical for use in pattern images which has features of measuring less than 3 microns thus wafer patterns of less than 3 microns are wet etched anisotropically instead of isotropic

Figure 2 Anisotropic wet etching

Advantages of wet etching 1 Low cost 2 High reliability 3 High throughput 4 Excellent selectivity in most cases with respect to both mask and substrate materials

Disadvantages of wet etching 1 Limited resolution 2 Higher safety risks due to the direct chemical exposure of the personnel 3 High cost of etchants in some cases 4 Problems related to the resists loss of adhesion to the substrate 5 Problems related to the formation of bubbles which inhibit the etching process where they are present and 6 Problems related to incomplete or non-uniform etching

Before moving on to texturization we first of all must do Saw damage removal which is also called as 1st Etch First Etch When ingots obtained from the Czochralski method are sliced into wafers by saw then it leaves some damaged marks on the wafers if it is not being sliced properly which makes the surface non-uniform So to make the surface uniform and free of error it becomes mandatory to do saw damage removal or 1st

1 First wafers are kept in the solution of 300ml DI water and 10kg NaOH(RSE) within the temperature range of 75⁰plusmn 2⁰C for 1min The reaction which takes place in this step is

etching Steps involved are

Si + 2NaOH + H2O (DI) -gt Na3SiO2 + H2 + H2

2 Finally these etched wafers are washed with DI water O

10

TEXTURIZATION INTRODUCTION Since 1960 for the improvement of the cell performance texturization has been attempted Texturization of front andor back surfaces of a solar cell surfaces improve their performances The reflection from chemically textured wafer reduced from 34 for a polished silicon surface to 11 Many researchers have reported the selective etching of silicon of lt100gt orientation of using sodium or potassium hydroxide solution OBJECTIVES OF TEXTURIZATION The major objectives of texturization are-

1 Reduction of front surface reflectance 2 Increase in the path length of the entered light of lower wavelengths by oblique trajectory within the

cell and absorption of light closer to the junction 3 Optical trapping of weakly absorbed light by multiple internal reflections from the top and bottom

surfaces of the cell thereby also increasing the optical path length The increased path length with an oblique trajectory leads to an effective increase in the absorption co-efficient

All these effects lead to realization of higher performance thick cells and also efficient solar cells of comparatively lower thickness which significantly reduces the material cost without degrading the cell performance MODELS OF TEXTURIZATION To reduce the reflection co-efficient and to trap weak absorbed light various texturing geometries are being employed The different texturing geometries include

1 Lambertian geometry 2 Upright pyramid geometry 3 Slat or Micro-grooved geometry 4 Inverted pyramid geometry 5 Simple prism pyramid geometry 6 Grating geometry 7 Three perpendicular planes (3PP) geometry 8 Perpendicular slat geometry 9 Porous silicon

Now in large number of manufacturing industries Inverted pyramid texturing geometry scheme is being used these days Letrsquos have a look on this scheme very briefly Inverted Pyramid Geometry The light trapping properties of the inverted pyramid geometry has been first investigated by Smith and Rohatgi The increase in current for the type of textured cell is due to incorporation of al the three effects necessary for an efficient light trapping design The front surface reflectance is reduced by providing the opportunity for a portion of the incoming light to undergo a triple bounce thereby reducing the overall front surface reflectance The increase in path length and light trapping efficiency means that a larger fraction of the light which has entered the cell will be absorbed before exiting the cell The short wavelength spectral response analysis at the texturing angle of 5375ordm indicates that 37 of the incoming light experiences a triple bounce on the front surface of the inverted pyramid geometry and the rest percentage gets double bounce The inverted pyramids on a 100microm cell with a two layer AR coating is estimated to give 402mAcm2 and cell efficiency of 24 with realistic cell design and material parameters

11

This texturization method can be employed on both mono-crystalline and multi-crystalline substrates Letrsquos have a look on this Mono-crystalline substrates For mono-crystalline substrate it is advantageous to make use of the anisotropic etching properties of Si in an alkaline solution As the 111 planes get etched more slowly than other crystal planes 111 facets are developed On lt100gtwafers this leads to pyramidal shapes at the surface that are particularly effective in reducing reflectance A regular array of pyramidal pits with facets at 547ordm to the horizontal plane mdash called inverted pyramids mdash can be formed using oxide etch mask with reflectances as low as 8 without ARC

Multi-crystalline Substrates Due to the anisotropic nature of multi-crystalline silicon substrate alkaline random texturing is not effective Some grains remain untextured leading to a high average reflectance So for texturing multi-crystalline silicon wafers should be etched in an acid mixture of HF and HNO3 because HNO3 tends to oxidize the surface while the HF etches the oxide away These acidic iso-texturing results in lower reflection than traditional anisotropic etching on multi-crystalline material and better conversion efficiency

STANDARD NAOH-IPA TEXTURIZATION APPROACH In order to achieve good uniformity of pyramidal structure on the silicon surface a mixture of NaOH or KOH and isopropyl alcohol (IPA) is generally used for texturization of mono-crystalline silicon solar cell For better texturization the interfacial energy between silicon and ionized electrolyte chemical solution should be reduced to achieve sufficient wettability for the silicon surface which will enhance the pyramid nucleation From a manufacturability point of view it is appealing to combine the saw damage removal and alkaline texturing in one single step in which case a trade-off has to be made between process speed and quality of the surface treatment

Fig1 Inverted Pyramid Textured Surface

Fig3 Textured multi-crystalline Si surface

Fig2 Anisotropic Etching of Mono-crystalline Si

12

DIFFUSION

INTRODUCTION

Diffusion the movement of a chemical species from an area of high concentration to an area of lower concentration is one of the two major processes by which chemical species or dopants are introduced into a semiconductor (the other one being ion implantation) The controlled diffusion of dopants into silicon to alter the type and level of conductivity of semiconductor materials is the foundation of forming a p-n junction The mathematics that govern the mass transport phenomena of diffusion are based on Ficks laws There are two laws according to Fick Letrsquos have a look on these laws First law Whenever an impurity concentration gradient partCpartx exists in a finite volume of a matrix substance (the silicon substrate in this context) the impurity material will have the natural tendency to move in order to distribute itself more evenly within the matrix and decrease the gradient Given enough time this flow of impurities will eventually result in homogeneity within the matrix causing the net flow of impurities to stop The mathematics of this transport mechanism was formalized in 1855 by Fick who postulated that the flux of material across a given plane is proportional to the concentration gradient across the plane Thus Ficks First Law states

J = -D ( partC(xt)partx ) Where J is the flux D is the diffusion constant for the material that is diffusing in the specific solvent and partC(xt)partx is the concentration gradient The diffusion constant of a material is also referred to as diffusion coefficient or simply diffusivity It is expressed in units of length2time such as microm2hour The negative sign of the right side of the equation indicates that the impurities are flowing in the direction of lower concentration Second Law Ficks First Law does not consider the fact that the gradient and local concentration of the impurities in a material decreases with an increase in time an aspect thats important to diffusion processes The flux J1 of impurities entering a section of a bar with a concentration gradient is different from the flux J2 of impurities leaving the same section From the law of conservation of matter the difference between J1 and J2 must result in a change in the concentration of impurities within the section (assuming that no impurities are formed or consumed in the section) This is Ficks Second Law which states that the change in impurity concentration over time is equal to the change in local diffusion flux or

partC(xt)partt = - partJpartx or from Ficks First Law

partC(xt)partt = part(DpartC(xt)partx)partx If the diffusion coefficient is independent of position such as when the impurity concentration is low then Ficks Second Law may be further simplified into the following equation

partC(xt)partt = D part2C(xt)partx2 The most important property of diffusion process in the manufacture of silicon solar cell is the formation of p-n junction by doping n-type material ie phosphorous on the p-type ie boron doped silicon textured wafer playing an essential role for photovoltaic effect Let us have a brief descriptive look on the properties of p-n junction P-N JUNCTION A p-n junction is a junction formed by combining p-type and n-type semiconductors together in very close contact

13

Principle of p-n junction The most common type of solar cell is a large p-n junction where the free carrier pairs emitted by light energy are separated by the junction and contribute to the current In its simplest form it consists of a junction formed between n-type and p-type semiconductors either of the same material (homo-junction) or different materials (hetero-junction) The band-structure of the two differently doped sides with respect to their Fermi levels can be seen in adjacent figure1

When the two halves are brought together the Fermi levels on either side are forced in to coincidence causing the valence and conduction bands to bend (Figure 2)

These bent bands represent a built-in electric field over what is referred to as the depletion region When a photon with a energy greater than the band-gap of the semiconductor passes through the solar cell it may be absorbed by the material This absorption takes the form of a band-to-band electronic transition so an electronhole pair is produced If these carriers can diffuse to the depletion region before they recombine then they are separated by the electric field causing one quantum of charge to flow through an external load This is the origin of the solar cells photocurrent and is shown in Figure 3

14

SOLAR CELL AS A P-N JUNCTION A solar cell is essential a p-n junction with large surface area The n type material is kept thin to allow light to pass through to the p-n junction

Light travels in packets of energy called photons The generation of electric current happens inside the depletion zone of the PN junction The depletion region as explained previously with the diode is the area around the PN junction where the electrons from the N-type silicon have diffused into the holes of the P-type material When a photon of light is absorbed by one of these atoms in the N-Type silicon it will dislodge an electron creating a free electron and a hole The free electron and hole has sufficient energy to jump out of the depletion zone If a wire is connected from the cathode (N-type silicon) to the anode (P-type silicon) electrons will flow through the wire The electron is attracted to the positive charge of the P-type material and travels through the external load (meter) creating a flow of electric current The hole created by the dislodged electron is attracted to the negative charge of N-type material and migrates to the back electrical contact As the electron enters the P-type silicon from the back electrical contact it combines with the hole restoring the electrical neutrality

DIFFUSION PROCESS

The first layer formation after doing texturization of wafer is performed in this process by doping the substrate with n-type dopant N-doping can be accomplished by depositing n dopant onto the substrate and then heating the substrate to drive the n dopant into the substrate Gaseous diffusion (for example POCl3 horizontal tube diffusion including temperatures of from 700 to 1000degC preferably 800 to 900degC) can be used to deposit the n-dopant onto the substrate surface to create an n-doped layer and a shallow p-n junction proximal to the substrate surface In this process if wafers are loaded onto the quartz boat back to back in the same lot then double diffusion process takes place and if single wafers are being loaded then single diffusion takes place On the quartz boat wafers can be loaded in two methods

1 Parallel diffusion loading of wafers parallel to the gas flow 2 Perpendicular diffusion loading of wafers perpendicular to the gas flow

Diffusion process is comprised of the following steps 1 Idle state-In this state the load center and source zone are set at 800ordmC and no process is

carried out in the furnace 2 Load-During this sequence the silicon carbide paddle moves at a speed of 45 rpm and the

maximum time taken is 7min 3 Ramp-The cold wafers loaded into the tubes bring down the temperature of the tube The tube

is then reheated to reach the set point temperature and the time taken is 7min

15

4 Stabilization-During this phase any overshooting of temperature during ramp-up phase is controlled to the predetermined set value and the time taken is 10min

5 Pre-deposition-Any excess POCL3 present in vapour form inside the quartz bubbler is vent out before the deposition starts by N2

6 Deposition- N purging in 5 sec

2 is bubbled through liquid POCL3 in presence of O2 to form P2O5

7 Post-deposition-During this phase O in paste form

2 and N2

8 N

flows to drive in the phosphorous from wafer surface to the junction

2

9 Unload-During this phase the SiC paddle moves slowly out of the furnace at set speed of 45rpm and maximum time taken is 7min

Purge-This phase is also known as drive in diffusion which is designed to evenly distribute the impurities into wafer and reduce the generally undesirable high concentration in the wafer surface and the maximum time taken is 7min

10 End-The tube is set to 800⁰C temperature in idle state

16

PLASMA ETCHING

INTRODUCTION

Plasma etching is one type of dry etching which uses plasma to produce chemically reactive species from inert gases The reactive gases are then made to react with the material to be etched The main objective of this process is to remove edge shunts present on the cell which leads to 80 loss and thus it is also termed as Edge isolation method Since edge isolation is the main motive of this process so first let us see that why edge isolation is so important

Edge isolation

In the standard processing sequence the entire wafer is phosphorous diffused The contact to the p-type base is made by firing an Al containing paste that produces a high doping of Al thus over-compensating the shallow emitter diffusion However there is still a direct conductive path from the emitter on the front side to the rear p contact Therefore a large portion of the photo-generated current can directly flow through the emitter to the back contact instead of flowing through the external circuit This current path is modeled in the two-diode model of the solar cell as the parallel resistance R

where R

p

s and Rp are the series and parallel resistance respectively j01 and j02 represent the first and second diode saturation current densities respectively n1 and n2 are the first and second ideality diode factors respectively Vth = ktq is the thermal voltage and jL

In this model the first diode accounts for emitter and bulk recombination with a recombination rate assumed to be proportional to the excess carrier density This leads to an ideality factor of n

is the light generated current density The two diode model is as shown below

1=1 The second diode is associated with the recombination in the space charge region that is assumed to be independent of the injection level leading to n2=2 The effect of increased j02 and Rp can be shown by model calculations for ideal solar cells in the figure below for a fill factor potential of 84 j02 must be less than 2x10-9 Acm2 and Rp must be greater than approximately 2000Ω cm2 if the respective parameters contribute solely to the fill factor loss

17

This increase in Rp is achieved by the edge isolation process that interrupts the parasitic current path as shown in fig2 below

Thus edge isolation is required in solar cell to increase open circuit voltage and thus leading to greater efficiency

Now we will move on to plasma etching process to see that how it works actually in solar cell and module manufacturing industries

Plasma Etching Process

After diffusion a layer of phosphosilicate glass is formed on the edges and rear surface of diffused wafer The process generally requires adequate physical masking of the front surface while exposing the full back of the cell to the dry plasma environment In an embodiment of this process cells are slotted face to face

Fig2Sketch of an industrial screen-printed solar cell The emitter diffusion is present on the complete wafer surface and gets locally overcompensated by printing and firing Al containing metal paste as back contact Without edge-isolation light generated e-s can flow through the emitter region to the back contact which corresponds to the small series resistance in the two diode model

18

into inert carriers so that the plasma chemistry can effectively react with the back of the cells thereby removing excess material from the second coating that might have reached the back of the cell This process is preferably performed via the generation of an RF (13 6 MHz) plasma using a mix of Freon 14 (CF4) with oxygen (O2) and or nitrogen (N2) in a ratio which depends on the chamber and load geometries

19

PHOSPHOSILICATE GLASS REMOVAL INTRODUCTION In diffusion process during deposition phase N2 gas is bubbled through liquid POCL3

The reaction related to this phase is given as follows

1 4POCL3 + 3O2 -gt 2P2O5 + 6Cl 2 2P

2 2O5 + 5Si -gt 5SiO2

+ 4P (PSG)

P2O5

So it becomes mandatory to remove phosphosilicate glass by chemicals which may act as recombination sites or may contribute towards the surface density defect and hence all these defects may contribute to a lower voltage and current

obtained in first reaction step is in paste form and is transferred to the Teflon duct to the process tube which then reacts with silicon of which the wafer is made And thus when this phosphorous pentoxide reaches silicon surface then second reaction takes place and thus a silicon dioxide layer and phosphosilicate glass ie PSG forms on the wafer surface Formation of this dead layer on the wafer surface contributes to a lower efficiency

PROCESS OF REMOVAL Following are the steps that should be taken into consideration for the removal of phosphosilicate glass (PSG) from the wafer surface

1 Firstly wafers are washed combinedly with HF HCL and DI water in a tank for 1min 2 Again wafers are washed with HF and DI water for 2min in the other tank 3 For another 1 min DI water is being sprayed on the diffused wafers 4 Next wafers are washed with DI water subsequently on an interval of 30 sec 5 Finally wafers are kept in the OVEN for 10mins within the temperature range of 95plusmn5ordmC

20

PLASMA ENHANCED CHEMICAL VAPOUR DEPOSITION INTRODUCTION In recent years the low-temperature (le 400ordmC) surface passivation of crystalline silicon solar cells by means of PECVD silicon nitride films has attracted an increasing attention of the PV community These films have proven to be capable of combining an outstanding surface passivation quality with excellent anti-reflection properties On typical p-type silicon solar cells substrate it has been shown that the surface passivation quality of very silicon rich Si3N4

we must first know that whatrsquos the use of silicon nitride coating on the surface of the wafer

films is superior to that of state-of-the-art high temperature silicon dioxide Before proceeding to the details of PECVD system

WHY SILICON NITRIDE SURFACE COATING IS PREFERRED IN PECVD PROCESS It is preferred as a surface coating because it functions as An effective diffusion mask An effective metallization diffusion barrier An anti-reflective coating Provide surface and bulk passivation

Preferably hydrogen is trapped in such a silicon nitride surface coating PECVD SYSYTEM PECVD or Plasma Enhanced Chemical Vapor Deposition is one of the methods of different Chemical Vapor Deposition (CVD) technique In PECVD plasma is being used to decompose a reactant gas such as silane (SiH4) to produce reaction products that precipitate on the surface of the substrate as a new layer

Working Principle Inside a PECVD reactor a strong electric field ignites plasma between two electrodes one of which holds the substrate This plasma ignition cracks the molecular bonds of the process gas which in turn are able to crack more process gas molecules before reaching the surface of the substrate The glow discharge used in this reactor is created by applying an RF field to a low-pressure gas creating free electrons within the discharge region The electrons are sufficiently energized by the electric field that gas-phase dissociation and ionization of the reactant gases occur when the free electrons collide with them Energetic species are then adsorbed on the film surface where they are subjected to ion and electron bombardment rearrangements reactions with other species new bond formation and film formation and growth Eventually a new layer is deposited on the substrate For example when silane is used as precursor plasma ignition frees up Si and SiH radicals which also crack more silane molecules on their way to the surface of the substrate where silicon is deposited PROCESS DETAILS OF PECVD In this system wafers coming from the previous state ie PSG removal step are being loaded onto the graphite boat where the individual wafers are held by three pins inside the boat It is absolutely necessary for the wafers to touch all the three pins otherwise there will remain a possibility of poor deposition in which

Figure Anti-reflective coating made out of silicon nitride deposited by the PECVD technique whereby silane and ammonia is reacted in a furnace The major color of coating looks blue but the color could also look like purple blue or light blue due to different thickness of coating

21

the HF generator fails to generate the required power for process deposition While loading the double diffusion wafers it should be ensured that deposition side should be correctly loaded The wafers are being loaded parallel to the boat After loading the trolley along with the boat is placed inside the machine and then after filling the input parameters of the CMI interface used here and the process tube press the RUN button and then machine will be in auto mode The handling mechanism automatically picks up the boat and places it on the desired paddle corresponding to the selected for the specified job And then process STARTS

22

SCREEN PRINTING INTRODUCTION In screen printing on wafer-based solar PV cells the mesh and buses of silver are printed on the front furthermore the buses of silver are printed on the back Subsequently aluminum paste is dispensed over the whole surface of the back for passivation and surface reflection [14] One of the parameters that can vary and can be controlled in screen printing is the thickness of the print This makes it useful for some of the techniques of printing solar cells electronics etc Screen printing is one of the most critical processes to maintain high yield Solar wafers are becoming thinner and larger so careful printing is required to maintain a lower breakage rate On the other hand high throughput at the printing stage improves the throughput of the whole cell production line PROCESS This process is divided in three phase-

1 Phase I- AgAl printing 2 Phase II- Ag printing 3 Phase III- Al printing

PHASE I- AgAl PRINTING This phase is required to print the back bus bar for tabbing purpose The AgAl paste used in this printing is Dupont (PV-505) and the consumption of the paste per wafer is 011 gm And the screen type used is of the following specification

1 Mesh size- 200microm 2 Tension- 24 N-cm 3 Mesh angle- 225ordm 45ordm 4 Emulsion thickness- 24microm

Process details During printing the paste is poured on the screen and is spread over it with the help of polypropylene spatula Then the wafers (ARC coated) are pasted with the sunny side down on the loader and picked one by one automatically with the start of the print command Once printing starts the wafers are placed one by one between the tool paste and the screen and then the squeegee moves over the screen and allow the paste through the exposed parts on the screen and gets pasted on the wafers Thus in this way the wafers are printed one by one Two most important parameters to be controlled during this printing are

1 Squeegee Pressure 2 Snap off distance

The next step after printing is drying step DRYING In this step wafers are passed through the dryer with a temperature range of 180ordm-400ordmC PHASE II- Ag PRINTING This phase is required to print the front bus bar for tabbing purpose The Ag paste used in this printing is Dupont (PV-145) and the consumption of the paste per wafer is 014 gm And the screen type used is of the following specification

1 Mesh size- 325microm 2 Tension- 24 N-cm 3 Mesh angle- 225ordm 4 Emulsion thickness- 24microm

7

Solar Cell Manufacturing Processes

8

ETCHING

INTRODUCTION

Etching is the process of using strong acid or mordant to cut into the unprotected parts of a metal surface to create a design in intaglio in the metal (the original processmdashin modern manufacturing other chemicals may be used on other types of material) As an intaglio method of printmaking it is along with engraving the most important technique for old master prints and remains widely used today But in wafer fabrication Etching refers to a process by which material is removed from the wafer ie either from the silicon substrate itself or from any film or layer of material on the wafer

DEFINITION

The treatment of a prepared metal surface with acid or other chemical reagent which by differential attack reveals the structure is known as etching In wafer fabrication etching process is generally classified in two classes Dry etching and Wet etching

Normally dry etching is not being used in this solar cells and module manufacturing unit So we will just go on a brief look on wet etching process

WET ETCHING It is a process that utilizes liquid chemicals or etchants to remove materials from the wafer usually in specific patterns defined by photo resist masks on the wafer Materials not covered by these masks are etched away by the chemicals while those covered by the masks are left almost intact A simple wet etching process may just consist of dissolution of the material to be removed in a liquid solvent without changing the chemical nature of the dissolved material In general however a wet etching process involves one or more chemical reactions that consume the original reactants and produce new species A basic wet etching process may be broken down in three steps as follows

1 Diffusion of the etchant to the surface for removal 2 Reaction between the etchant and the material being removed and 3 Diffusion of the reaction byproducts from the reacted surface

Wet etching can be both isotropic ie it proceeds in all directions at the same rate and anisotropic ie it proceeds in only one direction Isotropic Wet Etching A process which proceeds at the same rate in all directions is known as isotropic wet etching The etchants used in this process are isotropic in nature that etches away even the portion of material that is directly under the mask (usually in the shape of a quarter-circle) since its horizontal etching rate is the same as its vertical rate When an isotropic etchant eats away a portion of the material under the mask the etched film is said to have undercut the mask The amount of undercutting is a measure of an etching parameter known as the bias Bias is simply defined as the difference between the lateral dimensions of the etched image and the masked image Thus the mask used in etching must compensate for whatever bias an etchant is known to produce in order to create the desired feature on the wafer

Figure 1 Isotropic wet etching

9

Anisotropic Wet Etching A process which proceeds in only one direction is known as anisotropic wet etching It removes material in the vertical direction only since it will follow the mask patterns on the wafer very faithfully leaving any material covered by mask material basically untouched Since isotropic wet etching process has high bias values which are not practical for use in pattern images which has features of measuring less than 3 microns thus wafer patterns of less than 3 microns are wet etched anisotropically instead of isotropic

Figure 2 Anisotropic wet etching

Advantages of wet etching 1 Low cost 2 High reliability 3 High throughput 4 Excellent selectivity in most cases with respect to both mask and substrate materials

Disadvantages of wet etching 1 Limited resolution 2 Higher safety risks due to the direct chemical exposure of the personnel 3 High cost of etchants in some cases 4 Problems related to the resists loss of adhesion to the substrate 5 Problems related to the formation of bubbles which inhibit the etching process where they are present and 6 Problems related to incomplete or non-uniform etching

Before moving on to texturization we first of all must do Saw damage removal which is also called as 1st Etch First Etch When ingots obtained from the Czochralski method are sliced into wafers by saw then it leaves some damaged marks on the wafers if it is not being sliced properly which makes the surface non-uniform So to make the surface uniform and free of error it becomes mandatory to do saw damage removal or 1st

1 First wafers are kept in the solution of 300ml DI water and 10kg NaOH(RSE) within the temperature range of 75⁰plusmn 2⁰C for 1min The reaction which takes place in this step is

etching Steps involved are

Si + 2NaOH + H2O (DI) -gt Na3SiO2 + H2 + H2

2 Finally these etched wafers are washed with DI water O

10

TEXTURIZATION INTRODUCTION Since 1960 for the improvement of the cell performance texturization has been attempted Texturization of front andor back surfaces of a solar cell surfaces improve their performances The reflection from chemically textured wafer reduced from 34 for a polished silicon surface to 11 Many researchers have reported the selective etching of silicon of lt100gt orientation of using sodium or potassium hydroxide solution OBJECTIVES OF TEXTURIZATION The major objectives of texturization are-

1 Reduction of front surface reflectance 2 Increase in the path length of the entered light of lower wavelengths by oblique trajectory within the

cell and absorption of light closer to the junction 3 Optical trapping of weakly absorbed light by multiple internal reflections from the top and bottom

surfaces of the cell thereby also increasing the optical path length The increased path length with an oblique trajectory leads to an effective increase in the absorption co-efficient

All these effects lead to realization of higher performance thick cells and also efficient solar cells of comparatively lower thickness which significantly reduces the material cost without degrading the cell performance MODELS OF TEXTURIZATION To reduce the reflection co-efficient and to trap weak absorbed light various texturing geometries are being employed The different texturing geometries include

1 Lambertian geometry 2 Upright pyramid geometry 3 Slat or Micro-grooved geometry 4 Inverted pyramid geometry 5 Simple prism pyramid geometry 6 Grating geometry 7 Three perpendicular planes (3PP) geometry 8 Perpendicular slat geometry 9 Porous silicon

Now in large number of manufacturing industries Inverted pyramid texturing geometry scheme is being used these days Letrsquos have a look on this scheme very briefly Inverted Pyramid Geometry The light trapping properties of the inverted pyramid geometry has been first investigated by Smith and Rohatgi The increase in current for the type of textured cell is due to incorporation of al the three effects necessary for an efficient light trapping design The front surface reflectance is reduced by providing the opportunity for a portion of the incoming light to undergo a triple bounce thereby reducing the overall front surface reflectance The increase in path length and light trapping efficiency means that a larger fraction of the light which has entered the cell will be absorbed before exiting the cell The short wavelength spectral response analysis at the texturing angle of 5375ordm indicates that 37 of the incoming light experiences a triple bounce on the front surface of the inverted pyramid geometry and the rest percentage gets double bounce The inverted pyramids on a 100microm cell with a two layer AR coating is estimated to give 402mAcm2 and cell efficiency of 24 with realistic cell design and material parameters

11

This texturization method can be employed on both mono-crystalline and multi-crystalline substrates Letrsquos have a look on this Mono-crystalline substrates For mono-crystalline substrate it is advantageous to make use of the anisotropic etching properties of Si in an alkaline solution As the 111 planes get etched more slowly than other crystal planes 111 facets are developed On lt100gtwafers this leads to pyramidal shapes at the surface that are particularly effective in reducing reflectance A regular array of pyramidal pits with facets at 547ordm to the horizontal plane mdash called inverted pyramids mdash can be formed using oxide etch mask with reflectances as low as 8 without ARC

Multi-crystalline Substrates Due to the anisotropic nature of multi-crystalline silicon substrate alkaline random texturing is not effective Some grains remain untextured leading to a high average reflectance So for texturing multi-crystalline silicon wafers should be etched in an acid mixture of HF and HNO3 because HNO3 tends to oxidize the surface while the HF etches the oxide away These acidic iso-texturing results in lower reflection than traditional anisotropic etching on multi-crystalline material and better conversion efficiency

STANDARD NAOH-IPA TEXTURIZATION APPROACH In order to achieve good uniformity of pyramidal structure on the silicon surface a mixture of NaOH or KOH and isopropyl alcohol (IPA) is generally used for texturization of mono-crystalline silicon solar cell For better texturization the interfacial energy between silicon and ionized electrolyte chemical solution should be reduced to achieve sufficient wettability for the silicon surface which will enhance the pyramid nucleation From a manufacturability point of view it is appealing to combine the saw damage removal and alkaline texturing in one single step in which case a trade-off has to be made between process speed and quality of the surface treatment

Fig1 Inverted Pyramid Textured Surface

Fig3 Textured multi-crystalline Si surface

Fig2 Anisotropic Etching of Mono-crystalline Si

12

DIFFUSION

INTRODUCTION

Diffusion the movement of a chemical species from an area of high concentration to an area of lower concentration is one of the two major processes by which chemical species or dopants are introduced into a semiconductor (the other one being ion implantation) The controlled diffusion of dopants into silicon to alter the type and level of conductivity of semiconductor materials is the foundation of forming a p-n junction The mathematics that govern the mass transport phenomena of diffusion are based on Ficks laws There are two laws according to Fick Letrsquos have a look on these laws First law Whenever an impurity concentration gradient partCpartx exists in a finite volume of a matrix substance (the silicon substrate in this context) the impurity material will have the natural tendency to move in order to distribute itself more evenly within the matrix and decrease the gradient Given enough time this flow of impurities will eventually result in homogeneity within the matrix causing the net flow of impurities to stop The mathematics of this transport mechanism was formalized in 1855 by Fick who postulated that the flux of material across a given plane is proportional to the concentration gradient across the plane Thus Ficks First Law states

J = -D ( partC(xt)partx ) Where J is the flux D is the diffusion constant for the material that is diffusing in the specific solvent and partC(xt)partx is the concentration gradient The diffusion constant of a material is also referred to as diffusion coefficient or simply diffusivity It is expressed in units of length2time such as microm2hour The negative sign of the right side of the equation indicates that the impurities are flowing in the direction of lower concentration Second Law Ficks First Law does not consider the fact that the gradient and local concentration of the impurities in a material decreases with an increase in time an aspect thats important to diffusion processes The flux J1 of impurities entering a section of a bar with a concentration gradient is different from the flux J2 of impurities leaving the same section From the law of conservation of matter the difference between J1 and J2 must result in a change in the concentration of impurities within the section (assuming that no impurities are formed or consumed in the section) This is Ficks Second Law which states that the change in impurity concentration over time is equal to the change in local diffusion flux or

partC(xt)partt = - partJpartx or from Ficks First Law

partC(xt)partt = part(DpartC(xt)partx)partx If the diffusion coefficient is independent of position such as when the impurity concentration is low then Ficks Second Law may be further simplified into the following equation

partC(xt)partt = D part2C(xt)partx2 The most important property of diffusion process in the manufacture of silicon solar cell is the formation of p-n junction by doping n-type material ie phosphorous on the p-type ie boron doped silicon textured wafer playing an essential role for photovoltaic effect Let us have a brief descriptive look on the properties of p-n junction P-N JUNCTION A p-n junction is a junction formed by combining p-type and n-type semiconductors together in very close contact

13

Principle of p-n junction The most common type of solar cell is a large p-n junction where the free carrier pairs emitted by light energy are separated by the junction and contribute to the current In its simplest form it consists of a junction formed between n-type and p-type semiconductors either of the same material (homo-junction) or different materials (hetero-junction) The band-structure of the two differently doped sides with respect to their Fermi levels can be seen in adjacent figure1

When the two halves are brought together the Fermi levels on either side are forced in to coincidence causing the valence and conduction bands to bend (Figure 2)

These bent bands represent a built-in electric field over what is referred to as the depletion region When a photon with a energy greater than the band-gap of the semiconductor passes through the solar cell it may be absorbed by the material This absorption takes the form of a band-to-band electronic transition so an electronhole pair is produced If these carriers can diffuse to the depletion region before they recombine then they are separated by the electric field causing one quantum of charge to flow through an external load This is the origin of the solar cells photocurrent and is shown in Figure 3

14

SOLAR CELL AS A P-N JUNCTION A solar cell is essential a p-n junction with large surface area The n type material is kept thin to allow light to pass through to the p-n junction

Light travels in packets of energy called photons The generation of electric current happens inside the depletion zone of the PN junction The depletion region as explained previously with the diode is the area around the PN junction where the electrons from the N-type silicon have diffused into the holes of the P-type material When a photon of light is absorbed by one of these atoms in the N-Type silicon it will dislodge an electron creating a free electron and a hole The free electron and hole has sufficient energy to jump out of the depletion zone If a wire is connected from the cathode (N-type silicon) to the anode (P-type silicon) electrons will flow through the wire The electron is attracted to the positive charge of the P-type material and travels through the external load (meter) creating a flow of electric current The hole created by the dislodged electron is attracted to the negative charge of N-type material and migrates to the back electrical contact As the electron enters the P-type silicon from the back electrical contact it combines with the hole restoring the electrical neutrality

DIFFUSION PROCESS

The first layer formation after doing texturization of wafer is performed in this process by doping the substrate with n-type dopant N-doping can be accomplished by depositing n dopant onto the substrate and then heating the substrate to drive the n dopant into the substrate Gaseous diffusion (for example POCl3 horizontal tube diffusion including temperatures of from 700 to 1000degC preferably 800 to 900degC) can be used to deposit the n-dopant onto the substrate surface to create an n-doped layer and a shallow p-n junction proximal to the substrate surface In this process if wafers are loaded onto the quartz boat back to back in the same lot then double diffusion process takes place and if single wafers are being loaded then single diffusion takes place On the quartz boat wafers can be loaded in two methods

1 Parallel diffusion loading of wafers parallel to the gas flow 2 Perpendicular diffusion loading of wafers perpendicular to the gas flow

Diffusion process is comprised of the following steps 1 Idle state-In this state the load center and source zone are set at 800ordmC and no process is

carried out in the furnace 2 Load-During this sequence the silicon carbide paddle moves at a speed of 45 rpm and the

maximum time taken is 7min 3 Ramp-The cold wafers loaded into the tubes bring down the temperature of the tube The tube

is then reheated to reach the set point temperature and the time taken is 7min

15

4 Stabilization-During this phase any overshooting of temperature during ramp-up phase is controlled to the predetermined set value and the time taken is 10min

5 Pre-deposition-Any excess POCL3 present in vapour form inside the quartz bubbler is vent out before the deposition starts by N2

6 Deposition- N purging in 5 sec

2 is bubbled through liquid POCL3 in presence of O2 to form P2O5

7 Post-deposition-During this phase O in paste form

2 and N2

8 N

flows to drive in the phosphorous from wafer surface to the junction

2

9 Unload-During this phase the SiC paddle moves slowly out of the furnace at set speed of 45rpm and maximum time taken is 7min

Purge-This phase is also known as drive in diffusion which is designed to evenly distribute the impurities into wafer and reduce the generally undesirable high concentration in the wafer surface and the maximum time taken is 7min

10 End-The tube is set to 800⁰C temperature in idle state

16

PLASMA ETCHING

INTRODUCTION

Plasma etching is one type of dry etching which uses plasma to produce chemically reactive species from inert gases The reactive gases are then made to react with the material to be etched The main objective of this process is to remove edge shunts present on the cell which leads to 80 loss and thus it is also termed as Edge isolation method Since edge isolation is the main motive of this process so first let us see that why edge isolation is so important

Edge isolation

In the standard processing sequence the entire wafer is phosphorous diffused The contact to the p-type base is made by firing an Al containing paste that produces a high doping of Al thus over-compensating the shallow emitter diffusion However there is still a direct conductive path from the emitter on the front side to the rear p contact Therefore a large portion of the photo-generated current can directly flow through the emitter to the back contact instead of flowing through the external circuit This current path is modeled in the two-diode model of the solar cell as the parallel resistance R

where R

p

s and Rp are the series and parallel resistance respectively j01 and j02 represent the first and second diode saturation current densities respectively n1 and n2 are the first and second ideality diode factors respectively Vth = ktq is the thermal voltage and jL

In this model the first diode accounts for emitter and bulk recombination with a recombination rate assumed to be proportional to the excess carrier density This leads to an ideality factor of n

is the light generated current density The two diode model is as shown below

1=1 The second diode is associated with the recombination in the space charge region that is assumed to be independent of the injection level leading to n2=2 The effect of increased j02 and Rp can be shown by model calculations for ideal solar cells in the figure below for a fill factor potential of 84 j02 must be less than 2x10-9 Acm2 and Rp must be greater than approximately 2000Ω cm2 if the respective parameters contribute solely to the fill factor loss

17

This increase in Rp is achieved by the edge isolation process that interrupts the parasitic current path as shown in fig2 below

Thus edge isolation is required in solar cell to increase open circuit voltage and thus leading to greater efficiency

Now we will move on to plasma etching process to see that how it works actually in solar cell and module manufacturing industries

Plasma Etching Process

After diffusion a layer of phosphosilicate glass is formed on the edges and rear surface of diffused wafer The process generally requires adequate physical masking of the front surface while exposing the full back of the cell to the dry plasma environment In an embodiment of this process cells are slotted face to face

Fig2Sketch of an industrial screen-printed solar cell The emitter diffusion is present on the complete wafer surface and gets locally overcompensated by printing and firing Al containing metal paste as back contact Without edge-isolation light generated e-s can flow through the emitter region to the back contact which corresponds to the small series resistance in the two diode model

18

into inert carriers so that the plasma chemistry can effectively react with the back of the cells thereby removing excess material from the second coating that might have reached the back of the cell This process is preferably performed via the generation of an RF (13 6 MHz) plasma using a mix of Freon 14 (CF4) with oxygen (O2) and or nitrogen (N2) in a ratio which depends on the chamber and load geometries

19

PHOSPHOSILICATE GLASS REMOVAL INTRODUCTION In diffusion process during deposition phase N2 gas is bubbled through liquid POCL3

The reaction related to this phase is given as follows

1 4POCL3 + 3O2 -gt 2P2O5 + 6Cl 2 2P

2 2O5 + 5Si -gt 5SiO2

+ 4P (PSG)

P2O5

So it becomes mandatory to remove phosphosilicate glass by chemicals which may act as recombination sites or may contribute towards the surface density defect and hence all these defects may contribute to a lower voltage and current

obtained in first reaction step is in paste form and is transferred to the Teflon duct to the process tube which then reacts with silicon of which the wafer is made And thus when this phosphorous pentoxide reaches silicon surface then second reaction takes place and thus a silicon dioxide layer and phosphosilicate glass ie PSG forms on the wafer surface Formation of this dead layer on the wafer surface contributes to a lower efficiency

PROCESS OF REMOVAL Following are the steps that should be taken into consideration for the removal of phosphosilicate glass (PSG) from the wafer surface

1 Firstly wafers are washed combinedly with HF HCL and DI water in a tank for 1min 2 Again wafers are washed with HF and DI water for 2min in the other tank 3 For another 1 min DI water is being sprayed on the diffused wafers 4 Next wafers are washed with DI water subsequently on an interval of 30 sec 5 Finally wafers are kept in the OVEN for 10mins within the temperature range of 95plusmn5ordmC

20

PLASMA ENHANCED CHEMICAL VAPOUR DEPOSITION INTRODUCTION In recent years the low-temperature (le 400ordmC) surface passivation of crystalline silicon solar cells by means of PECVD silicon nitride films has attracted an increasing attention of the PV community These films have proven to be capable of combining an outstanding surface passivation quality with excellent anti-reflection properties On typical p-type silicon solar cells substrate it has been shown that the surface passivation quality of very silicon rich Si3N4

we must first know that whatrsquos the use of silicon nitride coating on the surface of the wafer

films is superior to that of state-of-the-art high temperature silicon dioxide Before proceeding to the details of PECVD system

WHY SILICON NITRIDE SURFACE COATING IS PREFERRED IN PECVD PROCESS It is preferred as a surface coating because it functions as An effective diffusion mask An effective metallization diffusion barrier An anti-reflective coating Provide surface and bulk passivation

Preferably hydrogen is trapped in such a silicon nitride surface coating PECVD SYSYTEM PECVD or Plasma Enhanced Chemical Vapor Deposition is one of the methods of different Chemical Vapor Deposition (CVD) technique In PECVD plasma is being used to decompose a reactant gas such as silane (SiH4) to produce reaction products that precipitate on the surface of the substrate as a new layer

Working Principle Inside a PECVD reactor a strong electric field ignites plasma between two electrodes one of which holds the substrate This plasma ignition cracks the molecular bonds of the process gas which in turn are able to crack more process gas molecules before reaching the surface of the substrate The glow discharge used in this reactor is created by applying an RF field to a low-pressure gas creating free electrons within the discharge region The electrons are sufficiently energized by the electric field that gas-phase dissociation and ionization of the reactant gases occur when the free electrons collide with them Energetic species are then adsorbed on the film surface where they are subjected to ion and electron bombardment rearrangements reactions with other species new bond formation and film formation and growth Eventually a new layer is deposited on the substrate For example when silane is used as precursor plasma ignition frees up Si and SiH radicals which also crack more silane molecules on their way to the surface of the substrate where silicon is deposited PROCESS DETAILS OF PECVD In this system wafers coming from the previous state ie PSG removal step are being loaded onto the graphite boat where the individual wafers are held by three pins inside the boat It is absolutely necessary for the wafers to touch all the three pins otherwise there will remain a possibility of poor deposition in which

Figure Anti-reflective coating made out of silicon nitride deposited by the PECVD technique whereby silane and ammonia is reacted in a furnace The major color of coating looks blue but the color could also look like purple blue or light blue due to different thickness of coating

21

the HF generator fails to generate the required power for process deposition While loading the double diffusion wafers it should be ensured that deposition side should be correctly loaded The wafers are being loaded parallel to the boat After loading the trolley along with the boat is placed inside the machine and then after filling the input parameters of the CMI interface used here and the process tube press the RUN button and then machine will be in auto mode The handling mechanism automatically picks up the boat and places it on the desired paddle corresponding to the selected for the specified job And then process STARTS

22

SCREEN PRINTING INTRODUCTION In screen printing on wafer-based solar PV cells the mesh and buses of silver are printed on the front furthermore the buses of silver are printed on the back Subsequently aluminum paste is dispensed over the whole surface of the back for passivation and surface reflection [14] One of the parameters that can vary and can be controlled in screen printing is the thickness of the print This makes it useful for some of the techniques of printing solar cells electronics etc Screen printing is one of the most critical processes to maintain high yield Solar wafers are becoming thinner and larger so careful printing is required to maintain a lower breakage rate On the other hand high throughput at the printing stage improves the throughput of the whole cell production line PROCESS This process is divided in three phase-

1 Phase I- AgAl printing 2 Phase II- Ag printing 3 Phase III- Al printing

PHASE I- AgAl PRINTING This phase is required to print the back bus bar for tabbing purpose The AgAl paste used in this printing is Dupont (PV-505) and the consumption of the paste per wafer is 011 gm And the screen type used is of the following specification

1 Mesh size- 200microm 2 Tension- 24 N-cm 3 Mesh angle- 225ordm 45ordm 4 Emulsion thickness- 24microm

Process details During printing the paste is poured on the screen and is spread over it with the help of polypropylene spatula Then the wafers (ARC coated) are pasted with the sunny side down on the loader and picked one by one automatically with the start of the print command Once printing starts the wafers are placed one by one between the tool paste and the screen and then the squeegee moves over the screen and allow the paste through the exposed parts on the screen and gets pasted on the wafers Thus in this way the wafers are printed one by one Two most important parameters to be controlled during this printing are

1 Squeegee Pressure 2 Snap off distance

The next step after printing is drying step DRYING In this step wafers are passed through the dryer with a temperature range of 180ordm-400ordmC PHASE II- Ag PRINTING This phase is required to print the front bus bar for tabbing purpose The Ag paste used in this printing is Dupont (PV-145) and the consumption of the paste per wafer is 014 gm And the screen type used is of the following specification

1 Mesh size- 325microm 2 Tension- 24 N-cm 3 Mesh angle- 225ordm 4 Emulsion thickness- 24microm

8

ETCHING

INTRODUCTION

Etching is the process of using strong acid or mordant to cut into the unprotected parts of a metal surface to create a design in intaglio in the metal (the original processmdashin modern manufacturing other chemicals may be used on other types of material) As an intaglio method of printmaking it is along with engraving the most important technique for old master prints and remains widely used today But in wafer fabrication Etching refers to a process by which material is removed from the wafer ie either from the silicon substrate itself or from any film or layer of material on the wafer

DEFINITION

The treatment of a prepared metal surface with acid or other chemical reagent which by differential attack reveals the structure is known as etching In wafer fabrication etching process is generally classified in two classes Dry etching and Wet etching

Normally dry etching is not being used in this solar cells and module manufacturing unit So we will just go on a brief look on wet etching process

WET ETCHING It is a process that utilizes liquid chemicals or etchants to remove materials from the wafer usually in specific patterns defined by photo resist masks on the wafer Materials not covered by these masks are etched away by the chemicals while those covered by the masks are left almost intact A simple wet etching process may just consist of dissolution of the material to be removed in a liquid solvent without changing the chemical nature of the dissolved material In general however a wet etching process involves one or more chemical reactions that consume the original reactants and produce new species A basic wet etching process may be broken down in three steps as follows

1 Diffusion of the etchant to the surface for removal 2 Reaction between the etchant and the material being removed and 3 Diffusion of the reaction byproducts from the reacted surface

Wet etching can be both isotropic ie it proceeds in all directions at the same rate and anisotropic ie it proceeds in only one direction Isotropic Wet Etching A process which proceeds at the same rate in all directions is known as isotropic wet etching The etchants used in this process are isotropic in nature that etches away even the portion of material that is directly under the mask (usually in the shape of a quarter-circle) since its horizontal etching rate is the same as its vertical rate When an isotropic etchant eats away a portion of the material under the mask the etched film is said to have undercut the mask The amount of undercutting is a measure of an etching parameter known as the bias Bias is simply defined as the difference between the lateral dimensions of the etched image and the masked image Thus the mask used in etching must compensate for whatever bias an etchant is known to produce in order to create the desired feature on the wafer

Figure 1 Isotropic wet etching

9

Anisotropic Wet Etching A process which proceeds in only one direction is known as anisotropic wet etching It removes material in the vertical direction only since it will follow the mask patterns on the wafer very faithfully leaving any material covered by mask material basically untouched Since isotropic wet etching process has high bias values which are not practical for use in pattern images which has features of measuring less than 3 microns thus wafer patterns of less than 3 microns are wet etched anisotropically instead of isotropic

Figure 2 Anisotropic wet etching

Advantages of wet etching 1 Low cost 2 High reliability 3 High throughput 4 Excellent selectivity in most cases with respect to both mask and substrate materials

Disadvantages of wet etching 1 Limited resolution 2 Higher safety risks due to the direct chemical exposure of the personnel 3 High cost of etchants in some cases 4 Problems related to the resists loss of adhesion to the substrate 5 Problems related to the formation of bubbles which inhibit the etching process where they are present and 6 Problems related to incomplete or non-uniform etching

Before moving on to texturization we first of all must do Saw damage removal which is also called as 1st Etch First Etch When ingots obtained from the Czochralski method are sliced into wafers by saw then it leaves some damaged marks on the wafers if it is not being sliced properly which makes the surface non-uniform So to make the surface uniform and free of error it becomes mandatory to do saw damage removal or 1st

1 First wafers are kept in the solution of 300ml DI water and 10kg NaOH(RSE) within the temperature range of 75⁰plusmn 2⁰C for 1min The reaction which takes place in this step is

etching Steps involved are

Si + 2NaOH + H2O (DI) -gt Na3SiO2 + H2 + H2

2 Finally these etched wafers are washed with DI water O

10

TEXTURIZATION INTRODUCTION Since 1960 for the improvement of the cell performance texturization has been attempted Texturization of front andor back surfaces of a solar cell surfaces improve their performances The reflection from chemically textured wafer reduced from 34 for a polished silicon surface to 11 Many researchers have reported the selective etching of silicon of lt100gt orientation of using sodium or potassium hydroxide solution OBJECTIVES OF TEXTURIZATION The major objectives of texturization are-

1 Reduction of front surface reflectance 2 Increase in the path length of the entered light of lower wavelengths by oblique trajectory within the

cell and absorption of light closer to the junction 3 Optical trapping of weakly absorbed light by multiple internal reflections from the top and bottom

surfaces of the cell thereby also increasing the optical path length The increased path length with an oblique trajectory leads to an effective increase in the absorption co-efficient

All these effects lead to realization of higher performance thick cells and also efficient solar cells of comparatively lower thickness which significantly reduces the material cost without degrading the cell performance MODELS OF TEXTURIZATION To reduce the reflection co-efficient and to trap weak absorbed light various texturing geometries are being employed The different texturing geometries include

1 Lambertian geometry 2 Upright pyramid geometry 3 Slat or Micro-grooved geometry 4 Inverted pyramid geometry 5 Simple prism pyramid geometry 6 Grating geometry 7 Three perpendicular planes (3PP) geometry 8 Perpendicular slat geometry 9 Porous silicon

Now in large number of manufacturing industries Inverted pyramid texturing geometry scheme is being used these days Letrsquos have a look on this scheme very briefly Inverted Pyramid Geometry The light trapping properties of the inverted pyramid geometry has been first investigated by Smith and Rohatgi The increase in current for the type of textured cell is due to incorporation of al the three effects necessary for an efficient light trapping design The front surface reflectance is reduced by providing the opportunity for a portion of the incoming light to undergo a triple bounce thereby reducing the overall front surface reflectance The increase in path length and light trapping efficiency means that a larger fraction of the light which has entered the cell will be absorbed before exiting the cell The short wavelength spectral response analysis at the texturing angle of 5375ordm indicates that 37 of the incoming light experiences a triple bounce on the front surface of the inverted pyramid geometry and the rest percentage gets double bounce The inverted pyramids on a 100microm cell with a two layer AR coating is estimated to give 402mAcm2 and cell efficiency of 24 with realistic cell design and material parameters

11

This texturization method can be employed on both mono-crystalline and multi-crystalline substrates Letrsquos have a look on this Mono-crystalline substrates For mono-crystalline substrate it is advantageous to make use of the anisotropic etching properties of Si in an alkaline solution As the 111 planes get etched more slowly than other crystal planes 111 facets are developed On lt100gtwafers this leads to pyramidal shapes at the surface that are particularly effective in reducing reflectance A regular array of pyramidal pits with facets at 547ordm to the horizontal plane mdash called inverted pyramids mdash can be formed using oxide etch mask with reflectances as low as 8 without ARC

Multi-crystalline Substrates Due to the anisotropic nature of multi-crystalline silicon substrate alkaline random texturing is not effective Some grains remain untextured leading to a high average reflectance So for texturing multi-crystalline silicon wafers should be etched in an acid mixture of HF and HNO3 because HNO3 tends to oxidize the surface while the HF etches the oxide away These acidic iso-texturing results in lower reflection than traditional anisotropic etching on multi-crystalline material and better conversion efficiency

STANDARD NAOH-IPA TEXTURIZATION APPROACH In order to achieve good uniformity of pyramidal structure on the silicon surface a mixture of NaOH or KOH and isopropyl alcohol (IPA) is generally used for texturization of mono-crystalline silicon solar cell For better texturization the interfacial energy between silicon and ionized electrolyte chemical solution should be reduced to achieve sufficient wettability for the silicon surface which will enhance the pyramid nucleation From a manufacturability point of view it is appealing to combine the saw damage removal and alkaline texturing in one single step in which case a trade-off has to be made between process speed and quality of the surface treatment

Fig1 Inverted Pyramid Textured Surface

Fig3 Textured multi-crystalline Si surface

Fig2 Anisotropic Etching of Mono-crystalline Si

12

DIFFUSION

INTRODUCTION

Diffusion the movement of a chemical species from an area of high concentration to an area of lower concentration is one of the two major processes by which chemical species or dopants are introduced into a semiconductor (the other one being ion implantation) The controlled diffusion of dopants into silicon to alter the type and level of conductivity of semiconductor materials is the foundation of forming a p-n junction The mathematics that govern the mass transport phenomena of diffusion are based on Ficks laws There are two laws according to Fick Letrsquos have a look on these laws First law Whenever an impurity concentration gradient partCpartx exists in a finite volume of a matrix substance (the silicon substrate in this context) the impurity material will have the natural tendency to move in order to distribute itself more evenly within the matrix and decrease the gradient Given enough time this flow of impurities will eventually result in homogeneity within the matrix causing the net flow of impurities to stop The mathematics of this transport mechanism was formalized in 1855 by Fick who postulated that the flux of material across a given plane is proportional to the concentration gradient across the plane Thus Ficks First Law states

J = -D ( partC(xt)partx ) Where J is the flux D is the diffusion constant for the material that is diffusing in the specific solvent and partC(xt)partx is the concentration gradient The diffusion constant of a material is also referred to as diffusion coefficient or simply diffusivity It is expressed in units of length2time such as microm2hour The negative sign of the right side of the equation indicates that the impurities are flowing in the direction of lower concentration Second Law Ficks First Law does not consider the fact that the gradient and local concentration of the impurities in a material decreases with an increase in time an aspect thats important to diffusion processes The flux J1 of impurities entering a section of a bar with a concentration gradient is different from the flux J2 of impurities leaving the same section From the law of conservation of matter the difference between J1 and J2 must result in a change in the concentration of impurities within the section (assuming that no impurities are formed or consumed in the section) This is Ficks Second Law which states that the change in impurity concentration over time is equal to the change in local diffusion flux or

partC(xt)partt = - partJpartx or from Ficks First Law

partC(xt)partt = part(DpartC(xt)partx)partx If the diffusion coefficient is independent of position such as when the impurity concentration is low then Ficks Second Law may be further simplified into the following equation

partC(xt)partt = D part2C(xt)partx2 The most important property of diffusion process in the manufacture of silicon solar cell is the formation of p-n junction by doping n-type material ie phosphorous on the p-type ie boron doped silicon textured wafer playing an essential role for photovoltaic effect Let us have a brief descriptive look on the properties of p-n junction P-N JUNCTION A p-n junction is a junction formed by combining p-type and n-type semiconductors together in very close contact

13

Principle of p-n junction The most common type of solar cell is a large p-n junction where the free carrier pairs emitted by light energy are separated by the junction and contribute to the current In its simplest form it consists of a junction formed between n-type and p-type semiconductors either of the same material (homo-junction) or different materials (hetero-junction) The band-structure of the two differently doped sides with respect to their Fermi levels can be seen in adjacent figure1

When the two halves are brought together the Fermi levels on either side are forced in to coincidence causing the valence and conduction bands to bend (Figure 2)

These bent bands represent a built-in electric field over what is referred to as the depletion region When a photon with a energy greater than the band-gap of the semiconductor passes through the solar cell it may be absorbed by the material This absorption takes the form of a band-to-band electronic transition so an electronhole pair is produced If these carriers can diffuse to the depletion region before they recombine then they are separated by the electric field causing one quantum of charge to flow through an external load This is the origin of the solar cells photocurrent and is shown in Figure 3

14

SOLAR CELL AS A P-N JUNCTION A solar cell is essential a p-n junction with large surface area The n type material is kept thin to allow light to pass through to the p-n junction

Light travels in packets of energy called photons The generation of electric current happens inside the depletion zone of the PN junction The depletion region as explained previously with the diode is the area around the PN junction where the electrons from the N-type silicon have diffused into the holes of the P-type material When a photon of light is absorbed by one of these atoms in the N-Type silicon it will dislodge an electron creating a free electron and a hole The free electron and hole has sufficient energy to jump out of the depletion zone If a wire is connected from the cathode (N-type silicon) to the anode (P-type silicon) electrons will flow through the wire The electron is attracted to the positive charge of the P-type material and travels through the external load (meter) creating a flow of electric current The hole created by the dislodged electron is attracted to the negative charge of N-type material and migrates to the back electrical contact As the electron enters the P-type silicon from the back electrical contact it combines with the hole restoring the electrical neutrality

DIFFUSION PROCESS

The first layer formation after doing texturization of wafer is performed in this process by doping the substrate with n-type dopant N-doping can be accomplished by depositing n dopant onto the substrate and then heating the substrate to drive the n dopant into the substrate Gaseous diffusion (for example POCl3 horizontal tube diffusion including temperatures of from 700 to 1000degC preferably 800 to 900degC) can be used to deposit the n-dopant onto the substrate surface to create an n-doped layer and a shallow p-n junction proximal to the substrate surface In this process if wafers are loaded onto the quartz boat back to back in the same lot then double diffusion process takes place and if single wafers are being loaded then single diffusion takes place On the quartz boat wafers can be loaded in two methods

1 Parallel diffusion loading of wafers parallel to the gas flow 2 Perpendicular diffusion loading of wafers perpendicular to the gas flow

Diffusion process is comprised of the following steps 1 Idle state-In this state the load center and source zone are set at 800ordmC and no process is

carried out in the furnace 2 Load-During this sequence the silicon carbide paddle moves at a speed of 45 rpm and the

maximum time taken is 7min 3 Ramp-The cold wafers loaded into the tubes bring down the temperature of the tube The tube

is then reheated to reach the set point temperature and the time taken is 7min

15

4 Stabilization-During this phase any overshooting of temperature during ramp-up phase is controlled to the predetermined set value and the time taken is 10min

5 Pre-deposition-Any excess POCL3 present in vapour form inside the quartz bubbler is vent out before the deposition starts by N2

6 Deposition- N purging in 5 sec

2 is bubbled through liquid POCL3 in presence of O2 to form P2O5

7 Post-deposition-During this phase O in paste form

2 and N2

8 N

flows to drive in the phosphorous from wafer surface to the junction

2

9 Unload-During this phase the SiC paddle moves slowly out of the furnace at set speed of 45rpm and maximum time taken is 7min

Purge-This phase is also known as drive in diffusion which is designed to evenly distribute the impurities into wafer and reduce the generally undesirable high concentration in the wafer surface and the maximum time taken is 7min

10 End-The tube is set to 800⁰C temperature in idle state

16

PLASMA ETCHING

INTRODUCTION

Plasma etching is one type of dry etching which uses plasma to produce chemically reactive species from inert gases The reactive gases are then made to react with the material to be etched The main objective of this process is to remove edge shunts present on the cell which leads to 80 loss and thus it is also termed as Edge isolation method Since edge isolation is the main motive of this process so first let us see that why edge isolation is so important

Edge isolation

In the standard processing sequence the entire wafer is phosphorous diffused The contact to the p-type base is made by firing an Al containing paste that produces a high doping of Al thus over-compensating the shallow emitter diffusion However there is still a direct conductive path from the emitter on the front side to the rear p contact Therefore a large portion of the photo-generated current can directly flow through the emitter to the back contact instead of flowing through the external circuit This current path is modeled in the two-diode model of the solar cell as the parallel resistance R

where R

p

s and Rp are the series and parallel resistance respectively j01 and j02 represent the first and second diode saturation current densities respectively n1 and n2 are the first and second ideality diode factors respectively Vth = ktq is the thermal voltage and jL

In this model the first diode accounts for emitter and bulk recombination with a recombination rate assumed to be proportional to the excess carrier density This leads to an ideality factor of n

is the light generated current density The two diode model is as shown below

1=1 The second diode is associated with the recombination in the space charge region that is assumed to be independent of the injection level leading to n2=2 The effect of increased j02 and Rp can be shown by model calculations for ideal solar cells in the figure below for a fill factor potential of 84 j02 must be less than 2x10-9 Acm2 and Rp must be greater than approximately 2000Ω cm2 if the respective parameters contribute solely to the fill factor loss

17

This increase in Rp is achieved by the edge isolation process that interrupts the parasitic current path as shown in fig2 below

Thus edge isolation is required in solar cell to increase open circuit voltage and thus leading to greater efficiency

Now we will move on to plasma etching process to see that how it works actually in solar cell and module manufacturing industries

Plasma Etching Process

After diffusion a layer of phosphosilicate glass is formed on the edges and rear surface of diffused wafer The process generally requires adequate physical masking of the front surface while exposing the full back of the cell to the dry plasma environment In an embodiment of this process cells are slotted face to face

Fig2Sketch of an industrial screen-printed solar cell The emitter diffusion is present on the complete wafer surface and gets locally overcompensated by printing and firing Al containing metal paste as back contact Without edge-isolation light generated e-s can flow through the emitter region to the back contact which corresponds to the small series resistance in the two diode model

18

into inert carriers so that the plasma chemistry can effectively react with the back of the cells thereby removing excess material from the second coating that might have reached the back of the cell This process is preferably performed via the generation of an RF (13 6 MHz) plasma using a mix of Freon 14 (CF4) with oxygen (O2) and or nitrogen (N2) in a ratio which depends on the chamber and load geometries

19

PHOSPHOSILICATE GLASS REMOVAL INTRODUCTION In diffusion process during deposition phase N2 gas is bubbled through liquid POCL3

The reaction related to this phase is given as follows

1 4POCL3 + 3O2 -gt 2P2O5 + 6Cl 2 2P

2 2O5 + 5Si -gt 5SiO2

+ 4P (PSG)

P2O5

So it becomes mandatory to remove phosphosilicate glass by chemicals which may act as recombination sites or may contribute towards the surface density defect and hence all these defects may contribute to a lower voltage and current

obtained in first reaction step is in paste form and is transferred to the Teflon duct to the process tube which then reacts with silicon of which the wafer is made And thus when this phosphorous pentoxide reaches silicon surface then second reaction takes place and thus a silicon dioxide layer and phosphosilicate glass ie PSG forms on the wafer surface Formation of this dead layer on the wafer surface contributes to a lower efficiency

PROCESS OF REMOVAL Following are the steps that should be taken into consideration for the removal of phosphosilicate glass (PSG) from the wafer surface

1 Firstly wafers are washed combinedly with HF HCL and DI water in a tank for 1min 2 Again wafers are washed with HF and DI water for 2min in the other tank 3 For another 1 min DI water is being sprayed on the diffused wafers 4 Next wafers are washed with DI water subsequently on an interval of 30 sec 5 Finally wafers are kept in the OVEN for 10mins within the temperature range of 95plusmn5ordmC

20

PLASMA ENHANCED CHEMICAL VAPOUR DEPOSITION INTRODUCTION In recent years the low-temperature (le 400ordmC) surface passivation of crystalline silicon solar cells by means of PECVD silicon nitride films has attracted an increasing attention of the PV community These films have proven to be capable of combining an outstanding surface passivation quality with excellent anti-reflection properties On typical p-type silicon solar cells substrate it has been shown that the surface passivation quality of very silicon rich Si3N4

we must first know that whatrsquos the use of silicon nitride coating on the surface of the wafer

films is superior to that of state-of-the-art high temperature silicon dioxide Before proceeding to the details of PECVD system

WHY SILICON NITRIDE SURFACE COATING IS PREFERRED IN PECVD PROCESS It is preferred as a surface coating because it functions as An effective diffusion mask An effective metallization diffusion barrier An anti-reflective coating Provide surface and bulk passivation

Preferably hydrogen is trapped in such a silicon nitride surface coating PECVD SYSYTEM PECVD or Plasma Enhanced Chemical Vapor Deposition is one of the methods of different Chemical Vapor Deposition (CVD) technique In PECVD plasma is being used to decompose a reactant gas such as silane (SiH4) to produce reaction products that precipitate on the surface of the substrate as a new layer

Working Principle Inside a PECVD reactor a strong electric field ignites plasma between two electrodes one of which holds the substrate This plasma ignition cracks the molecular bonds of the process gas which in turn are able to crack more process gas molecules before reaching the surface of the substrate The glow discharge used in this reactor is created by applying an RF field to a low-pressure gas creating free electrons within the discharge region The electrons are sufficiently energized by the electric field that gas-phase dissociation and ionization of the reactant gases occur when the free electrons collide with them Energetic species are then adsorbed on the film surface where they are subjected to ion and electron bombardment rearrangements reactions with other species new bond formation and film formation and growth Eventually a new layer is deposited on the substrate For example when silane is used as precursor plasma ignition frees up Si and SiH radicals which also crack more silane molecules on their way to the surface of the substrate where silicon is deposited PROCESS DETAILS OF PECVD In this system wafers coming from the previous state ie PSG removal step are being loaded onto the graphite boat where the individual wafers are held by three pins inside the boat It is absolutely necessary for the wafers to touch all the three pins otherwise there will remain a possibility of poor deposition in which

Figure Anti-reflective coating made out of silicon nitride deposited by the PECVD technique whereby silane and ammonia is reacted in a furnace The major color of coating looks blue but the color could also look like purple blue or light blue due to different thickness of coating

21

the HF generator fails to generate the required power for process deposition While loading the double diffusion wafers it should be ensured that deposition side should be correctly loaded The wafers are being loaded parallel to the boat After loading the trolley along with the boat is placed inside the machine and then after filling the input parameters of the CMI interface used here and the process tube press the RUN button and then machine will be in auto mode The handling mechanism automatically picks up the boat and places it on the desired paddle corresponding to the selected for the specified job And then process STARTS

22

SCREEN PRINTING INTRODUCTION In screen printing on wafer-based solar PV cells the mesh and buses of silver are printed on the front furthermore the buses of silver are printed on the back Subsequently aluminum paste is dispensed over the whole surface of the back for passivation and surface reflection [14] One of the parameters that can vary and can be controlled in screen printing is the thickness of the print This makes it useful for some of the techniques of printing solar cells electronics etc Screen printing is one of the most critical processes to maintain high yield Solar wafers are becoming thinner and larger so careful printing is required to maintain a lower breakage rate On the other hand high throughput at the printing stage improves the throughput of the whole cell production line PROCESS This process is divided in three phase-

1 Phase I- AgAl printing 2 Phase II- Ag printing 3 Phase III- Al printing

PHASE I- AgAl PRINTING This phase is required to print the back bus bar for tabbing purpose The AgAl paste used in this printing is Dupont (PV-505) and the consumption of the paste per wafer is 011 gm And the screen type used is of the following specification

1 Mesh size- 200microm 2 Tension- 24 N-cm 3 Mesh angle- 225ordm 45ordm 4 Emulsion thickness- 24microm

Process details During printing the paste is poured on the screen and is spread over it with the help of polypropylene spatula Then the wafers (ARC coated) are pasted with the sunny side down on the loader and picked one by one automatically with the start of the print command Once printing starts the wafers are placed one by one between the tool paste and the screen and then the squeegee moves over the screen and allow the paste through the exposed parts on the screen and gets pasted on the wafers Thus in this way the wafers are printed one by one Two most important parameters to be controlled during this printing are

1 Squeegee Pressure 2 Snap off distance

The next step after printing is drying step DRYING In this step wafers are passed through the dryer with a temperature range of 180ordm-400ordmC PHASE II- Ag PRINTING This phase is required to print the front bus bar for tabbing purpose The Ag paste used in this printing is Dupont (PV-145) and the consumption of the paste per wafer is 014 gm And the screen type used is of the following specification

1 Mesh size- 325microm 2 Tension- 24 N-cm 3 Mesh angle- 225ordm 4 Emulsion thickness- 24microm

9

Anisotropic Wet Etching A process which proceeds in only one direction is known as anisotropic wet etching It removes material in the vertical direction only since it will follow the mask patterns on the wafer very faithfully leaving any material covered by mask material basically untouched Since isotropic wet etching process has high bias values which are not practical for use in pattern images which has features of measuring less than 3 microns thus wafer patterns of less than 3 microns are wet etched anisotropically instead of isotropic

Figure 2 Anisotropic wet etching

Advantages of wet etching 1 Low cost 2 High reliability 3 High throughput 4 Excellent selectivity in most cases with respect to both mask and substrate materials

Disadvantages of wet etching 1 Limited resolution 2 Higher safety risks due to the direct chemical exposure of the personnel 3 High cost of etchants in some cases 4 Problems related to the resists loss of adhesion to the substrate 5 Problems related to the formation of bubbles which inhibit the etching process where they are present and 6 Problems related to incomplete or non-uniform etching

Before moving on to texturization we first of all must do Saw damage removal which is also called as 1st Etch First Etch When ingots obtained from the Czochralski method are sliced into wafers by saw then it leaves some damaged marks on the wafers if it is not being sliced properly which makes the surface non-uniform So to make the surface uniform and free of error it becomes mandatory to do saw damage removal or 1st

1 First wafers are kept in the solution of 300ml DI water and 10kg NaOH(RSE) within the temperature range of 75⁰plusmn 2⁰C for 1min The reaction which takes place in this step is

etching Steps involved are

Si + 2NaOH + H2O (DI) -gt Na3SiO2 + H2 + H2

2 Finally these etched wafers are washed with DI water O

10

TEXTURIZATION INTRODUCTION Since 1960 for the improvement of the cell performance texturization has been attempted Texturization of front andor back surfaces of a solar cell surfaces improve their performances The reflection from chemically textured wafer reduced from 34 for a polished silicon surface to 11 Many researchers have reported the selective etching of silicon of lt100gt orientation of using sodium or potassium hydroxide solution OBJECTIVES OF TEXTURIZATION The major objectives of texturization are-

1 Reduction of front surface reflectance 2 Increase in the path length of the entered light of lower wavelengths by oblique trajectory within the

cell and absorption of light closer to the junction 3 Optical trapping of weakly absorbed light by multiple internal reflections from the top and bottom

surfaces of the cell thereby also increasing the optical path length The increased path length with an oblique trajectory leads to an effective increase in the absorption co-efficient

All these effects lead to realization of higher performance thick cells and also efficient solar cells of comparatively lower thickness which significantly reduces the material cost without degrading the cell performance MODELS OF TEXTURIZATION To reduce the reflection co-efficient and to trap weak absorbed light various texturing geometries are being employed The different texturing geometries include

1 Lambertian geometry 2 Upright pyramid geometry 3 Slat or Micro-grooved geometry 4 Inverted pyramid geometry 5 Simple prism pyramid geometry 6 Grating geometry 7 Three perpendicular planes (3PP) geometry 8 Perpendicular slat geometry 9 Porous silicon

Now in large number of manufacturing industries Inverted pyramid texturing geometry scheme is being used these days Letrsquos have a look on this scheme very briefly Inverted Pyramid Geometry The light trapping properties of the inverted pyramid geometry has been first investigated by Smith and Rohatgi The increase in current for the type of textured cell is due to incorporation of al the three effects necessary for an efficient light trapping design The front surface reflectance is reduced by providing the opportunity for a portion of the incoming light to undergo a triple bounce thereby reducing the overall front surface reflectance The increase in path length and light trapping efficiency means that a larger fraction of the light which has entered the cell will be absorbed before exiting the cell The short wavelength spectral response analysis at the texturing angle of 5375ordm indicates that 37 of the incoming light experiences a triple bounce on the front surface of the inverted pyramid geometry and the rest percentage gets double bounce The inverted pyramids on a 100microm cell with a two layer AR coating is estimated to give 402mAcm2 and cell efficiency of 24 with realistic cell design and material parameters

11

This texturization method can be employed on both mono-crystalline and multi-crystalline substrates Letrsquos have a look on this Mono-crystalline substrates For mono-crystalline substrate it is advantageous to make use of the anisotropic etching properties of Si in an alkaline solution As the 111 planes get etched more slowly than other crystal planes 111 facets are developed On lt100gtwafers this leads to pyramidal shapes at the surface that are particularly effective in reducing reflectance A regular array of pyramidal pits with facets at 547ordm to the horizontal plane mdash called inverted pyramids mdash can be formed using oxide etch mask with reflectances as low as 8 without ARC

Multi-crystalline Substrates Due to the anisotropic nature of multi-crystalline silicon substrate alkaline random texturing is not effective Some grains remain untextured leading to a high average reflectance So for texturing multi-crystalline silicon wafers should be etched in an acid mixture of HF and HNO3 because HNO3 tends to oxidize the surface while the HF etches the oxide away These acidic iso-texturing results in lower reflection than traditional anisotropic etching on multi-crystalline material and better conversion efficiency

STANDARD NAOH-IPA TEXTURIZATION APPROACH In order to achieve good uniformity of pyramidal structure on the silicon surface a mixture of NaOH or KOH and isopropyl alcohol (IPA) is generally used for texturization of mono-crystalline silicon solar cell For better texturization the interfacial energy between silicon and ionized electrolyte chemical solution should be reduced to achieve sufficient wettability for the silicon surface which will enhance the pyramid nucleation From a manufacturability point of view it is appealing to combine the saw damage removal and alkaline texturing in one single step in which case a trade-off has to be made between process speed and quality of the surface treatment

Fig1 Inverted Pyramid Textured Surface

Fig3 Textured multi-crystalline Si surface

Fig2 Anisotropic Etching of Mono-crystalline Si

12

DIFFUSION

INTRODUCTION

Diffusion the movement of a chemical species from an area of high concentration to an area of lower concentration is one of the two major processes by which chemical species or dopants are introduced into a semiconductor (the other one being ion implantation) The controlled diffusion of dopants into silicon to alter the type and level of conductivity of semiconductor materials is the foundation of forming a p-n junction The mathematics that govern the mass transport phenomena of diffusion are based on Ficks laws There are two laws according to Fick Letrsquos have a look on these laws First law Whenever an impurity concentration gradient partCpartx exists in a finite volume of a matrix substance (the silicon substrate in this context) the impurity material will have the natural tendency to move in order to distribute itself more evenly within the matrix and decrease the gradient Given enough time this flow of impurities will eventually result in homogeneity within the matrix causing the net flow of impurities to stop The mathematics of this transport mechanism was formalized in 1855 by Fick who postulated that the flux of material across a given plane is proportional to the concentration gradient across the plane Thus Ficks First Law states

J = -D ( partC(xt)partx ) Where J is the flux D is the diffusion constant for the material that is diffusing in the specific solvent and partC(xt)partx is the concentration gradient The diffusion constant of a material is also referred to as diffusion coefficient or simply diffusivity It is expressed in units of length2time such as microm2hour The negative sign of the right side of the equation indicates that the impurities are flowing in the direction of lower concentration Second Law Ficks First Law does not consider the fact that the gradient and local concentration of the impurities in a material decreases with an increase in time an aspect thats important to diffusion processes The flux J1 of impurities entering a section of a bar with a concentration gradient is different from the flux J2 of impurities leaving the same section From the law of conservation of matter the difference between J1 and J2 must result in a change in the concentration of impurities within the section (assuming that no impurities are formed or consumed in the section) This is Ficks Second Law which states that the change in impurity concentration over time is equal to the change in local diffusion flux or

partC(xt)partt = - partJpartx or from Ficks First Law

partC(xt)partt = part(DpartC(xt)partx)partx If the diffusion coefficient is independent of position such as when the impurity concentration is low then Ficks Second Law may be further simplified into the following equation

partC(xt)partt = D part2C(xt)partx2 The most important property of diffusion process in the manufacture of silicon solar cell is the formation of p-n junction by doping n-type material ie phosphorous on the p-type ie boron doped silicon textured wafer playing an essential role for photovoltaic effect Let us have a brief descriptive look on the properties of p-n junction P-N JUNCTION A p-n junction is a junction formed by combining p-type and n-type semiconductors together in very close contact

13

Principle of p-n junction The most common type of solar cell is a large p-n junction where the free carrier pairs emitted by light energy are separated by the junction and contribute to the current In its simplest form it consists of a junction formed between n-type and p-type semiconductors either of the same material (homo-junction) or different materials (hetero-junction) The band-structure of the two differently doped sides with respect to their Fermi levels can be seen in adjacent figure1

When the two halves are brought together the Fermi levels on either side are forced in to coincidence causing the valence and conduction bands to bend (Figure 2)

These bent bands represent a built-in electric field over what is referred to as the depletion region When a photon with a energy greater than the band-gap of the semiconductor passes through the solar cell it may be absorbed by the material This absorption takes the form of a band-to-band electronic transition so an electronhole pair is produced If these carriers can diffuse to the depletion region before they recombine then they are separated by the electric field causing one quantum of charge to flow through an external load This is the origin of the solar cells photocurrent and is shown in Figure 3

14

SOLAR CELL AS A P-N JUNCTION A solar cell is essential a p-n junction with large surface area The n type material is kept thin to allow light to pass through to the p-n junction

Light travels in packets of energy called photons The generation of electric current happens inside the depletion zone of the PN junction The depletion region as explained previously with the diode is the area around the PN junction where the electrons from the N-type silicon have diffused into the holes of the P-type material When a photon of light is absorbed by one of these atoms in the N-Type silicon it will dislodge an electron creating a free electron and a hole The free electron and hole has sufficient energy to jump out of the depletion zone If a wire is connected from the cathode (N-type silicon) to the anode (P-type silicon) electrons will flow through the wire The electron is attracted to the positive charge of the P-type material and travels through the external load (meter) creating a flow of electric current The hole created by the dislodged electron is attracted to the negative charge of N-type material and migrates to the back electrical contact As the electron enters the P-type silicon from the back electrical contact it combines with the hole restoring the electrical neutrality

DIFFUSION PROCESS

The first layer formation after doing texturization of wafer is performed in this process by doping the substrate with n-type dopant N-doping can be accomplished by depositing n dopant onto the substrate and then heating the substrate to drive the n dopant into the substrate Gaseous diffusion (for example POCl3 horizontal tube diffusion including temperatures of from 700 to 1000degC preferably 800 to 900degC) can be used to deposit the n-dopant onto the substrate surface to create an n-doped layer and a shallow p-n junction proximal to the substrate surface In this process if wafers are loaded onto the quartz boat back to back in the same lot then double diffusion process takes place and if single wafers are being loaded then single diffusion takes place On the quartz boat wafers can be loaded in two methods

1 Parallel diffusion loading of wafers parallel to the gas flow 2 Perpendicular diffusion loading of wafers perpendicular to the gas flow

Diffusion process is comprised of the following steps 1 Idle state-In this state the load center and source zone are set at 800ordmC and no process is

carried out in the furnace 2 Load-During this sequence the silicon carbide paddle moves at a speed of 45 rpm and the

maximum time taken is 7min 3 Ramp-The cold wafers loaded into the tubes bring down the temperature of the tube The tube

is then reheated to reach the set point temperature and the time taken is 7min

15

4 Stabilization-During this phase any overshooting of temperature during ramp-up phase is controlled to the predetermined set value and the time taken is 10min

5 Pre-deposition-Any excess POCL3 present in vapour form inside the quartz bubbler is vent out before the deposition starts by N2

6 Deposition- N purging in 5 sec

2 is bubbled through liquid POCL3 in presence of O2 to form P2O5

7 Post-deposition-During this phase O in paste form

2 and N2

8 N

flows to drive in the phosphorous from wafer surface to the junction

2

9 Unload-During this phase the SiC paddle moves slowly out of the furnace at set speed of 45rpm and maximum time taken is 7min

Purge-This phase is also known as drive in diffusion which is designed to evenly distribute the impurities into wafer and reduce the generally undesirable high concentration in the wafer surface and the maximum time taken is 7min

10 End-The tube is set to 800⁰C temperature in idle state

16

PLASMA ETCHING

INTRODUCTION

Plasma etching is one type of dry etching which uses plasma to produce chemically reactive species from inert gases The reactive gases are then made to react with the material to be etched The main objective of this process is to remove edge shunts present on the cell which leads to 80 loss and thus it is also termed as Edge isolation method Since edge isolation is the main motive of this process so first let us see that why edge isolation is so important

Edge isolation

In the standard processing sequence the entire wafer is phosphorous diffused The contact to the p-type base is made by firing an Al containing paste that produces a high doping of Al thus over-compensating the shallow emitter diffusion However there is still a direct conductive path from the emitter on the front side to the rear p contact Therefore a large portion of the photo-generated current can directly flow through the emitter to the back contact instead of flowing through the external circuit This current path is modeled in the two-diode model of the solar cell as the parallel resistance R

where R

p

s and Rp are the series and parallel resistance respectively j01 and j02 represent the first and second diode saturation current densities respectively n1 and n2 are the first and second ideality diode factors respectively Vth = ktq is the thermal voltage and jL

In this model the first diode accounts for emitter and bulk recombination with a recombination rate assumed to be proportional to the excess carrier density This leads to an ideality factor of n

is the light generated current density The two diode model is as shown below

1=1 The second diode is associated with the recombination in the space charge region that is assumed to be independent of the injection level leading to n2=2 The effect of increased j02 and Rp can be shown by model calculations for ideal solar cells in the figure below for a fill factor potential of 84 j02 must be less than 2x10-9 Acm2 and Rp must be greater than approximately 2000Ω cm2 if the respective parameters contribute solely to the fill factor loss

17

This increase in Rp is achieved by the edge isolation process that interrupts the parasitic current path as shown in fig2 below

Thus edge isolation is required in solar cell to increase open circuit voltage and thus leading to greater efficiency

Now we will move on to plasma etching process to see that how it works actually in solar cell and module manufacturing industries

Plasma Etching Process

After diffusion a layer of phosphosilicate glass is formed on the edges and rear surface of diffused wafer The process generally requires adequate physical masking of the front surface while exposing the full back of the cell to the dry plasma environment In an embodiment of this process cells are slotted face to face

Fig2Sketch of an industrial screen-printed solar cell The emitter diffusion is present on the complete wafer surface and gets locally overcompensated by printing and firing Al containing metal paste as back contact Without edge-isolation light generated e-s can flow through the emitter region to the back contact which corresponds to the small series resistance in the two diode model

18

into inert carriers so that the plasma chemistry can effectively react with the back of the cells thereby removing excess material from the second coating that might have reached the back of the cell This process is preferably performed via the generation of an RF (13 6 MHz) plasma using a mix of Freon 14 (CF4) with oxygen (O2) and or nitrogen (N2) in a ratio which depends on the chamber and load geometries

19

PHOSPHOSILICATE GLASS REMOVAL INTRODUCTION In diffusion process during deposition phase N2 gas is bubbled through liquid POCL3

The reaction related to this phase is given as follows

1 4POCL3 + 3O2 -gt 2P2O5 + 6Cl 2 2P

2 2O5 + 5Si -gt 5SiO2

+ 4P (PSG)

P2O5

So it becomes mandatory to remove phosphosilicate glass by chemicals which may act as recombination sites or may contribute towards the surface density defect and hence all these defects may contribute to a lower voltage and current

obtained in first reaction step is in paste form and is transferred to the Teflon duct to the process tube which then reacts with silicon of which the wafer is made And thus when this phosphorous pentoxide reaches silicon surface then second reaction takes place and thus a silicon dioxide layer and phosphosilicate glass ie PSG forms on the wafer surface Formation of this dead layer on the wafer surface contributes to a lower efficiency

PROCESS OF REMOVAL Following are the steps that should be taken into consideration for the removal of phosphosilicate glass (PSG) from the wafer surface

1 Firstly wafers are washed combinedly with HF HCL and DI water in a tank for 1min 2 Again wafers are washed with HF and DI water for 2min in the other tank 3 For another 1 min DI water is being sprayed on the diffused wafers 4 Next wafers are washed with DI water subsequently on an interval of 30 sec 5 Finally wafers are kept in the OVEN for 10mins within the temperature range of 95plusmn5ordmC

20

PLASMA ENHANCED CHEMICAL VAPOUR DEPOSITION INTRODUCTION In recent years the low-temperature (le 400ordmC) surface passivation of crystalline silicon solar cells by means of PECVD silicon nitride films has attracted an increasing attention of the PV community These films have proven to be capable of combining an outstanding surface passivation quality with excellent anti-reflection properties On typical p-type silicon solar cells substrate it has been shown that the surface passivation quality of very silicon rich Si3N4

we must first know that whatrsquos the use of silicon nitride coating on the surface of the wafer

films is superior to that of state-of-the-art high temperature silicon dioxide Before proceeding to the details of PECVD system

WHY SILICON NITRIDE SURFACE COATING IS PREFERRED IN PECVD PROCESS It is preferred as a surface coating because it functions as An effective diffusion mask An effective metallization diffusion barrier An anti-reflective coating Provide surface and bulk passivation

Preferably hydrogen is trapped in such a silicon nitride surface coating PECVD SYSYTEM PECVD or Plasma Enhanced Chemical Vapor Deposition is one of the methods of different Chemical Vapor Deposition (CVD) technique In PECVD plasma is being used to decompose a reactant gas such as silane (SiH4) to produce reaction products that precipitate on the surface of the substrate as a new layer

Working Principle Inside a PECVD reactor a strong electric field ignites plasma between two electrodes one of which holds the substrate This plasma ignition cracks the molecular bonds of the process gas which in turn are able to crack more process gas molecules before reaching the surface of the substrate The glow discharge used in this reactor is created by applying an RF field to a low-pressure gas creating free electrons within the discharge region The electrons are sufficiently energized by the electric field that gas-phase dissociation and ionization of the reactant gases occur when the free electrons collide with them Energetic species are then adsorbed on the film surface where they are subjected to ion and electron bombardment rearrangements reactions with other species new bond formation and film formation and growth Eventually a new layer is deposited on the substrate For example when silane is used as precursor plasma ignition frees up Si and SiH radicals which also crack more silane molecules on their way to the surface of the substrate where silicon is deposited PROCESS DETAILS OF PECVD In this system wafers coming from the previous state ie PSG removal step are being loaded onto the graphite boat where the individual wafers are held by three pins inside the boat It is absolutely necessary for the wafers to touch all the three pins otherwise there will remain a possibility of poor deposition in which

Figure Anti-reflective coating made out of silicon nitride deposited by the PECVD technique whereby silane and ammonia is reacted in a furnace The major color of coating looks blue but the color could also look like purple blue or light blue due to different thickness of coating

21

the HF generator fails to generate the required power for process deposition While loading the double diffusion wafers it should be ensured that deposition side should be correctly loaded The wafers are being loaded parallel to the boat After loading the trolley along with the boat is placed inside the machine and then after filling the input parameters of the CMI interface used here and the process tube press the RUN button and then machine will be in auto mode The handling mechanism automatically picks up the boat and places it on the desired paddle corresponding to the selected for the specified job And then process STARTS

22

SCREEN PRINTING INTRODUCTION In screen printing on wafer-based solar PV cells the mesh and buses of silver are printed on the front furthermore the buses of silver are printed on the back Subsequently aluminum paste is dispensed over the whole surface of the back for passivation and surface reflection [14] One of the parameters that can vary and can be controlled in screen printing is the thickness of the print This makes it useful for some of the techniques of printing solar cells electronics etc Screen printing is one of the most critical processes to maintain high yield Solar wafers are becoming thinner and larger so careful printing is required to maintain a lower breakage rate On the other hand high throughput at the printing stage improves the throughput of the whole cell production line PROCESS This process is divided in three phase-

1 Phase I- AgAl printing 2 Phase II- Ag printing 3 Phase III- Al printing

PHASE I- AgAl PRINTING This phase is required to print the back bus bar for tabbing purpose The AgAl paste used in this printing is Dupont (PV-505) and the consumption of the paste per wafer is 011 gm And the screen type used is of the following specification

1 Mesh size- 200microm 2 Tension- 24 N-cm 3 Mesh angle- 225ordm 45ordm 4 Emulsion thickness- 24microm

Process details During printing the paste is poured on the screen and is spread over it with the help of polypropylene spatula Then the wafers (ARC coated) are pasted with the sunny side down on the loader and picked one by one automatically with the start of the print command Once printing starts the wafers are placed one by one between the tool paste and the screen and then the squeegee moves over the screen and allow the paste through the exposed parts on the screen and gets pasted on the wafers Thus in this way the wafers are printed one by one Two most important parameters to be controlled during this printing are

1 Squeegee Pressure 2 Snap off distance

The next step after printing is drying step DRYING In this step wafers are passed through the dryer with a temperature range of 180ordm-400ordmC PHASE II- Ag PRINTING This phase is required to print the front bus bar for tabbing purpose The Ag paste used in this printing is Dupont (PV-145) and the consumption of the paste per wafer is 014 gm And the screen type used is of the following specification

1 Mesh size- 325microm 2 Tension- 24 N-cm 3 Mesh angle- 225ordm 4 Emulsion thickness- 24microm

10

TEXTURIZATION INTRODUCTION Since 1960 for the improvement of the cell performance texturization has been attempted Texturization of front andor back surfaces of a solar cell surfaces improve their performances The reflection from chemically textured wafer reduced from 34 for a polished silicon surface to 11 Many researchers have reported the selective etching of silicon of lt100gt orientation of using sodium or potassium hydroxide solution OBJECTIVES OF TEXTURIZATION The major objectives of texturization are-

1 Reduction of front surface reflectance 2 Increase in the path length of the entered light of lower wavelengths by oblique trajectory within the

cell and absorption of light closer to the junction 3 Optical trapping of weakly absorbed light by multiple internal reflections from the top and bottom

surfaces of the cell thereby also increasing the optical path length The increased path length with an oblique trajectory leads to an effective increase in the absorption co-efficient

All these effects lead to realization of higher performance thick cells and also efficient solar cells of comparatively lower thickness which significantly reduces the material cost without degrading the cell performance MODELS OF TEXTURIZATION To reduce the reflection co-efficient and to trap weak absorbed light various texturing geometries are being employed The different texturing geometries include

1 Lambertian geometry 2 Upright pyramid geometry 3 Slat or Micro-grooved geometry 4 Inverted pyramid geometry 5 Simple prism pyramid geometry 6 Grating geometry 7 Three perpendicular planes (3PP) geometry 8 Perpendicular slat geometry 9 Porous silicon

Now in large number of manufacturing industries Inverted pyramid texturing geometry scheme is being used these days Letrsquos have a look on this scheme very briefly Inverted Pyramid Geometry The light trapping properties of the inverted pyramid geometry has been first investigated by Smith and Rohatgi The increase in current for the type of textured cell is due to incorporation of al the three effects necessary for an efficient light trapping design The front surface reflectance is reduced by providing the opportunity for a portion of the incoming light to undergo a triple bounce thereby reducing the overall front surface reflectance The increase in path length and light trapping efficiency means that a larger fraction of the light which has entered the cell will be absorbed before exiting the cell The short wavelength spectral response analysis at the texturing angle of 5375ordm indicates that 37 of the incoming light experiences a triple bounce on the front surface of the inverted pyramid geometry and the rest percentage gets double bounce The inverted pyramids on a 100microm cell with a two layer AR coating is estimated to give 402mAcm2 and cell efficiency of 24 with realistic cell design and material parameters

11

This texturization method can be employed on both mono-crystalline and multi-crystalline substrates Letrsquos have a look on this Mono-crystalline substrates For mono-crystalline substrate it is advantageous to make use of the anisotropic etching properties of Si in an alkaline solution As the 111 planes get etched more slowly than other crystal planes 111 facets are developed On lt100gtwafers this leads to pyramidal shapes at the surface that are particularly effective in reducing reflectance A regular array of pyramidal pits with facets at 547ordm to the horizontal plane mdash called inverted pyramids mdash can be formed using oxide etch mask with reflectances as low as 8 without ARC

Multi-crystalline Substrates Due to the anisotropic nature of multi-crystalline silicon substrate alkaline random texturing is not effective Some grains remain untextured leading to a high average reflectance So for texturing multi-crystalline silicon wafers should be etched in an acid mixture of HF and HNO3 because HNO3 tends to oxidize the surface while the HF etches the oxide away These acidic iso-texturing results in lower reflection than traditional anisotropic etching on multi-crystalline material and better conversion efficiency

STANDARD NAOH-IPA TEXTURIZATION APPROACH In order to achieve good uniformity of pyramidal structure on the silicon surface a mixture of NaOH or KOH and isopropyl alcohol (IPA) is generally used for texturization of mono-crystalline silicon solar cell For better texturization the interfacial energy between silicon and ionized electrolyte chemical solution should be reduced to achieve sufficient wettability for the silicon surface which will enhance the pyramid nucleation From a manufacturability point of view it is appealing to combine the saw damage removal and alkaline texturing in one single step in which case a trade-off has to be made between process speed and quality of the surface treatment

Fig1 Inverted Pyramid Textured Surface

Fig3 Textured multi-crystalline Si surface

Fig2 Anisotropic Etching of Mono-crystalline Si

12

DIFFUSION

INTRODUCTION

Diffusion the movement of a chemical species from an area of high concentration to an area of lower concentration is one of the two major processes by which chemical species or dopants are introduced into a semiconductor (the other one being ion implantation) The controlled diffusion of dopants into silicon to alter the type and level of conductivity of semiconductor materials is the foundation of forming a p-n junction The mathematics that govern the mass transport phenomena of diffusion are based on Ficks laws There are two laws according to Fick Letrsquos have a look on these laws First law Whenever an impurity concentration gradient partCpartx exists in a finite volume of a matrix substance (the silicon substrate in this context) the impurity material will have the natural tendency to move in order to distribute itself more evenly within the matrix and decrease the gradient Given enough time this flow of impurities will eventually result in homogeneity within the matrix causing the net flow of impurities to stop The mathematics of this transport mechanism was formalized in 1855 by Fick who postulated that the flux of material across a given plane is proportional to the concentration gradient across the plane Thus Ficks First Law states

J = -D ( partC(xt)partx ) Where J is the flux D is the diffusion constant for the material that is diffusing in the specific solvent and partC(xt)partx is the concentration gradient The diffusion constant of a material is also referred to as diffusion coefficient or simply diffusivity It is expressed in units of length2time such as microm2hour The negative sign of the right side of the equation indicates that the impurities are flowing in the direction of lower concentration Second Law Ficks First Law does not consider the fact that the gradient and local concentration of the impurities in a material decreases with an increase in time an aspect thats important to diffusion processes The flux J1 of impurities entering a section of a bar with a concentration gradient is different from the flux J2 of impurities leaving the same section From the law of conservation of matter the difference between J1 and J2 must result in a change in the concentration of impurities within the section (assuming that no impurities are formed or consumed in the section) This is Ficks Second Law which states that the change in impurity concentration over time is equal to the change in local diffusion flux or

partC(xt)partt = - partJpartx or from Ficks First Law

partC(xt)partt = part(DpartC(xt)partx)partx If the diffusion coefficient is independent of position such as when the impurity concentration is low then Ficks Second Law may be further simplified into the following equation

partC(xt)partt = D part2C(xt)partx2 The most important property of diffusion process in the manufacture of silicon solar cell is the formation of p-n junction by doping n-type material ie phosphorous on the p-type ie boron doped silicon textured wafer playing an essential role for photovoltaic effect Let us have a brief descriptive look on the properties of p-n junction P-N JUNCTION A p-n junction is a junction formed by combining p-type and n-type semiconductors together in very close contact

13

Principle of p-n junction The most common type of solar cell is a large p-n junction where the free carrier pairs emitted by light energy are separated by the junction and contribute to the current In its simplest form it consists of a junction formed between n-type and p-type semiconductors either of the same material (homo-junction) or different materials (hetero-junction) The band-structure of the two differently doped sides with respect to their Fermi levels can be seen in adjacent figure1

When the two halves are brought together the Fermi levels on either side are forced in to coincidence causing the valence and conduction bands to bend (Figure 2)

These bent bands represent a built-in electric field over what is referred to as the depletion region When a photon with a energy greater than the band-gap of the semiconductor passes through the solar cell it may be absorbed by the material This absorption takes the form of a band-to-band electronic transition so an electronhole pair is produced If these carriers can diffuse to the depletion region before they recombine then they are separated by the electric field causing one quantum of charge to flow through an external load This is the origin of the solar cells photocurrent and is shown in Figure 3

14

SOLAR CELL AS A P-N JUNCTION A solar cell is essential a p-n junction with large surface area The n type material is kept thin to allow light to pass through to the p-n junction

Light travels in packets of energy called photons The generation of electric current happens inside the depletion zone of the PN junction The depletion region as explained previously with the diode is the area around the PN junction where the electrons from the N-type silicon have diffused into the holes of the P-type material When a photon of light is absorbed by one of these atoms in the N-Type silicon it will dislodge an electron creating a free electron and a hole The free electron and hole has sufficient energy to jump out of the depletion zone If a wire is connected from the cathode (N-type silicon) to the anode (P-type silicon) electrons will flow through the wire The electron is attracted to the positive charge of the P-type material and travels through the external load (meter) creating a flow of electric current The hole created by the dislodged electron is attracted to the negative charge of N-type material and migrates to the back electrical contact As the electron enters the P-type silicon from the back electrical contact it combines with the hole restoring the electrical neutrality

DIFFUSION PROCESS

The first layer formation after doing texturization of wafer is performed in this process by doping the substrate with n-type dopant N-doping can be accomplished by depositing n dopant onto the substrate and then heating the substrate to drive the n dopant into the substrate Gaseous diffusion (for example POCl3 horizontal tube diffusion including temperatures of from 700 to 1000degC preferably 800 to 900degC) can be used to deposit the n-dopant onto the substrate surface to create an n-doped layer and a shallow p-n junction proximal to the substrate surface In this process if wafers are loaded onto the quartz boat back to back in the same lot then double diffusion process takes place and if single wafers are being loaded then single diffusion takes place On the quartz boat wafers can be loaded in two methods

1 Parallel diffusion loading of wafers parallel to the gas flow 2 Perpendicular diffusion loading of wafers perpendicular to the gas flow

Diffusion process is comprised of the following steps 1 Idle state-In this state the load center and source zone are set at 800ordmC and no process is

carried out in the furnace 2 Load-During this sequence the silicon carbide paddle moves at a speed of 45 rpm and the

maximum time taken is 7min 3 Ramp-The cold wafers loaded into the tubes bring down the temperature of the tube The tube

is then reheated to reach the set point temperature and the time taken is 7min

15

4 Stabilization-During this phase any overshooting of temperature during ramp-up phase is controlled to the predetermined set value and the time taken is 10min

5 Pre-deposition-Any excess POCL3 present in vapour form inside the quartz bubbler is vent out before the deposition starts by N2

6 Deposition- N purging in 5 sec

2 is bubbled through liquid POCL3 in presence of O2 to form P2O5

7 Post-deposition-During this phase O in paste form

2 and N2

8 N

flows to drive in the phosphorous from wafer surface to the junction

2

9 Unload-During this phase the SiC paddle moves slowly out of the furnace at set speed of 45rpm and maximum time taken is 7min

Purge-This phase is also known as drive in diffusion which is designed to evenly distribute the impurities into wafer and reduce the generally undesirable high concentration in the wafer surface and the maximum time taken is 7min

10 End-The tube is set to 800⁰C temperature in idle state

16

PLASMA ETCHING

INTRODUCTION

Plasma etching is one type of dry etching which uses plasma to produce chemically reactive species from inert gases The reactive gases are then made to react with the material to be etched The main objective of this process is to remove edge shunts present on the cell which leads to 80 loss and thus it is also termed as Edge isolation method Since edge isolation is the main motive of this process so first let us see that why edge isolation is so important

Edge isolation

In the standard processing sequence the entire wafer is phosphorous diffused The contact to the p-type base is made by firing an Al containing paste that produces a high doping of Al thus over-compensating the shallow emitter diffusion However there is still a direct conductive path from the emitter on the front side to the rear p contact Therefore a large portion of the photo-generated current can directly flow through the emitter to the back contact instead of flowing through the external circuit This current path is modeled in the two-diode model of the solar cell as the parallel resistance R

where R

p

s and Rp are the series and parallel resistance respectively j01 and j02 represent the first and second diode saturation current densities respectively n1 and n2 are the first and second ideality diode factors respectively Vth = ktq is the thermal voltage and jL

In this model the first diode accounts for emitter and bulk recombination with a recombination rate assumed to be proportional to the excess carrier density This leads to an ideality factor of n

is the light generated current density The two diode model is as shown below

1=1 The second diode is associated with the recombination in the space charge region that is assumed to be independent of the injection level leading to n2=2 The effect of increased j02 and Rp can be shown by model calculations for ideal solar cells in the figure below for a fill factor potential of 84 j02 must be less than 2x10-9 Acm2 and Rp must be greater than approximately 2000Ω cm2 if the respective parameters contribute solely to the fill factor loss

17

This increase in Rp is achieved by the edge isolation process that interrupts the parasitic current path as shown in fig2 below

Thus edge isolation is required in solar cell to increase open circuit voltage and thus leading to greater efficiency

Now we will move on to plasma etching process to see that how it works actually in solar cell and module manufacturing industries

Plasma Etching Process

After diffusion a layer of phosphosilicate glass is formed on the edges and rear surface of diffused wafer The process generally requires adequate physical masking of the front surface while exposing the full back of the cell to the dry plasma environment In an embodiment of this process cells are slotted face to face

Fig2Sketch of an industrial screen-printed solar cell The emitter diffusion is present on the complete wafer surface and gets locally overcompensated by printing and firing Al containing metal paste as back contact Without edge-isolation light generated e-s can flow through the emitter region to the back contact which corresponds to the small series resistance in the two diode model

18

into inert carriers so that the plasma chemistry can effectively react with the back of the cells thereby removing excess material from the second coating that might have reached the back of the cell This process is preferably performed via the generation of an RF (13 6 MHz) plasma using a mix of Freon 14 (CF4) with oxygen (O2) and or nitrogen (N2) in a ratio which depends on the chamber and load geometries

19

PHOSPHOSILICATE GLASS REMOVAL INTRODUCTION In diffusion process during deposition phase N2 gas is bubbled through liquid POCL3

The reaction related to this phase is given as follows

1 4POCL3 + 3O2 -gt 2P2O5 + 6Cl 2 2P

2 2O5 + 5Si -gt 5SiO2

+ 4P (PSG)

P2O5

So it becomes mandatory to remove phosphosilicate glass by chemicals which may act as recombination sites or may contribute towards the surface density defect and hence all these defects may contribute to a lower voltage and current

obtained in first reaction step is in paste form and is transferred to the Teflon duct to the process tube which then reacts with silicon of which the wafer is made And thus when this phosphorous pentoxide reaches silicon surface then second reaction takes place and thus a silicon dioxide layer and phosphosilicate glass ie PSG forms on the wafer surface Formation of this dead layer on the wafer surface contributes to a lower efficiency

PROCESS OF REMOVAL Following are the steps that should be taken into consideration for the removal of phosphosilicate glass (PSG) from the wafer surface

1 Firstly wafers are washed combinedly with HF HCL and DI water in a tank for 1min 2 Again wafers are washed with HF and DI water for 2min in the other tank 3 For another 1 min DI water is being sprayed on the diffused wafers 4 Next wafers are washed with DI water subsequently on an interval of 30 sec 5 Finally wafers are kept in the OVEN for 10mins within the temperature range of 95plusmn5ordmC

20

PLASMA ENHANCED CHEMICAL VAPOUR DEPOSITION INTRODUCTION In recent years the low-temperature (le 400ordmC) surface passivation of crystalline silicon solar cells by means of PECVD silicon nitride films has attracted an increasing attention of the PV community These films have proven to be capable of combining an outstanding surface passivation quality with excellent anti-reflection properties On typical p-type silicon solar cells substrate it has been shown that the surface passivation quality of very silicon rich Si3N4

we must first know that whatrsquos the use of silicon nitride coating on the surface of the wafer

films is superior to that of state-of-the-art high temperature silicon dioxide Before proceeding to the details of PECVD system

WHY SILICON NITRIDE SURFACE COATING IS PREFERRED IN PECVD PROCESS It is preferred as a surface coating because it functions as An effective diffusion mask An effective metallization diffusion barrier An anti-reflective coating Provide surface and bulk passivation

Preferably hydrogen is trapped in such a silicon nitride surface coating PECVD SYSYTEM PECVD or Plasma Enhanced Chemical Vapor Deposition is one of the methods of different Chemical Vapor Deposition (CVD) technique In PECVD plasma is being used to decompose a reactant gas such as silane (SiH4) to produce reaction products that precipitate on the surface of the substrate as a new layer

Working Principle Inside a PECVD reactor a strong electric field ignites plasma between two electrodes one of which holds the substrate This plasma ignition cracks the molecular bonds of the process gas which in turn are able to crack more process gas molecules before reaching the surface of the substrate The glow discharge used in this reactor is created by applying an RF field to a low-pressure gas creating free electrons within the discharge region The electrons are sufficiently energized by the electric field that gas-phase dissociation and ionization of the reactant gases occur when the free electrons collide with them Energetic species are then adsorbed on the film surface where they are subjected to ion and electron bombardment rearrangements reactions with other species new bond formation and film formation and growth Eventually a new layer is deposited on the substrate For example when silane is used as precursor plasma ignition frees up Si and SiH radicals which also crack more silane molecules on their way to the surface of the substrate where silicon is deposited PROCESS DETAILS OF PECVD In this system wafers coming from the previous state ie PSG removal step are being loaded onto the graphite boat where the individual wafers are held by three pins inside the boat It is absolutely necessary for the wafers to touch all the three pins otherwise there will remain a possibility of poor deposition in which

Figure Anti-reflective coating made out of silicon nitride deposited by the PECVD technique whereby silane and ammonia is reacted in a furnace The major color of coating looks blue but the color could also look like purple blue or light blue due to different thickness of coating

21

the HF generator fails to generate the required power for process deposition While loading the double diffusion wafers it should be ensured that deposition side should be correctly loaded The wafers are being loaded parallel to the boat After loading the trolley along with the boat is placed inside the machine and then after filling the input parameters of the CMI interface used here and the process tube press the RUN button and then machine will be in auto mode The handling mechanism automatically picks up the boat and places it on the desired paddle corresponding to the selected for the specified job And then process STARTS

22

SCREEN PRINTING INTRODUCTION In screen printing on wafer-based solar PV cells the mesh and buses of silver are printed on the front furthermore the buses of silver are printed on the back Subsequently aluminum paste is dispensed over the whole surface of the back for passivation and surface reflection [14] One of the parameters that can vary and can be controlled in screen printing is the thickness of the print This makes it useful for some of the techniques of printing solar cells electronics etc Screen printing is one of the most critical processes to maintain high yield Solar wafers are becoming thinner and larger so careful printing is required to maintain a lower breakage rate On the other hand high throughput at the printing stage improves the throughput of the whole cell production line PROCESS This process is divided in three phase-

1 Phase I- AgAl printing 2 Phase II- Ag printing 3 Phase III- Al printing

PHASE I- AgAl PRINTING This phase is required to print the back bus bar for tabbing purpose The AgAl paste used in this printing is Dupont (PV-505) and the consumption of the paste per wafer is 011 gm And the screen type used is of the following specification

1 Mesh size- 200microm 2 Tension- 24 N-cm 3 Mesh angle- 225ordm 45ordm 4 Emulsion thickness- 24microm

Process details During printing the paste is poured on the screen and is spread over it with the help of polypropylene spatula Then the wafers (ARC coated) are pasted with the sunny side down on the loader and picked one by one automatically with the start of the print command Once printing starts the wafers are placed one by one between the tool paste and the screen and then the squeegee moves over the screen and allow the paste through the exposed parts on the screen and gets pasted on the wafers Thus in this way the wafers are printed one by one Two most important parameters to be controlled during this printing are

1 Squeegee Pressure 2 Snap off distance

The next step after printing is drying step DRYING In this step wafers are passed through the dryer with a temperature range of 180ordm-400ordmC PHASE II- Ag PRINTING This phase is required to print the front bus bar for tabbing purpose The Ag paste used in this printing is Dupont (PV-145) and the consumption of the paste per wafer is 014 gm And the screen type used is of the following specification

1 Mesh size- 325microm 2 Tension- 24 N-cm 3 Mesh angle- 225ordm 4 Emulsion thickness- 24microm

11

This texturization method can be employed on both mono-crystalline and multi-crystalline substrates Letrsquos have a look on this Mono-crystalline substrates For mono-crystalline substrate it is advantageous to make use of the anisotropic etching properties of Si in an alkaline solution As the 111 planes get etched more slowly than other crystal planes 111 facets are developed On lt100gtwafers this leads to pyramidal shapes at the surface that are particularly effective in reducing reflectance A regular array of pyramidal pits with facets at 547ordm to the horizontal plane mdash called inverted pyramids mdash can be formed using oxide etch mask with reflectances as low as 8 without ARC

Multi-crystalline Substrates Due to the anisotropic nature of multi-crystalline silicon substrate alkaline random texturing is not effective Some grains remain untextured leading to a high average reflectance So for texturing multi-crystalline silicon wafers should be etched in an acid mixture of HF and HNO3 because HNO3 tends to oxidize the surface while the HF etches the oxide away These acidic iso-texturing results in lower reflection than traditional anisotropic etching on multi-crystalline material and better conversion efficiency

STANDARD NAOH-IPA TEXTURIZATION APPROACH In order to achieve good uniformity of pyramidal structure on the silicon surface a mixture of NaOH or KOH and isopropyl alcohol (IPA) is generally used for texturization of mono-crystalline silicon solar cell For better texturization the interfacial energy between silicon and ionized electrolyte chemical solution should be reduced to achieve sufficient wettability for the silicon surface which will enhance the pyramid nucleation From a manufacturability point of view it is appealing to combine the saw damage removal and alkaline texturing in one single step in which case a trade-off has to be made between process speed and quality of the surface treatment

Fig1 Inverted Pyramid Textured Surface

Fig3 Textured multi-crystalline Si surface

Fig2 Anisotropic Etching of Mono-crystalline Si

12

DIFFUSION

INTRODUCTION

Diffusion the movement of a chemical species from an area of high concentration to an area of lower concentration is one of the two major processes by which chemical species or dopants are introduced into a semiconductor (the other one being ion implantation) The controlled diffusion of dopants into silicon to alter the type and level of conductivity of semiconductor materials is the foundation of forming a p-n junction The mathematics that govern the mass transport phenomena of diffusion are based on Ficks laws There are two laws according to Fick Letrsquos have a look on these laws First law Whenever an impurity concentration gradient partCpartx exists in a finite volume of a matrix substance (the silicon substrate in this context) the impurity material will have the natural tendency to move in order to distribute itself more evenly within the matrix and decrease the gradient Given enough time this flow of impurities will eventually result in homogeneity within the matrix causing the net flow of impurities to stop The mathematics of this transport mechanism was formalized in 1855 by Fick who postulated that the flux of material across a given plane is proportional to the concentration gradient across the plane Thus Ficks First Law states

J = -D ( partC(xt)partx ) Where J is the flux D is the diffusion constant for the material that is diffusing in the specific solvent and partC(xt)partx is the concentration gradient The diffusion constant of a material is also referred to as diffusion coefficient or simply diffusivity It is expressed in units of length2time such as microm2hour The negative sign of the right side of the equation indicates that the impurities are flowing in the direction of lower concentration Second Law Ficks First Law does not consider the fact that the gradient and local concentration of the impurities in a material decreases with an increase in time an aspect thats important to diffusion processes The flux J1 of impurities entering a section of a bar with a concentration gradient is different from the flux J2 of impurities leaving the same section From the law of conservation of matter the difference between J1 and J2 must result in a change in the concentration of impurities within the section (assuming that no impurities are formed or consumed in the section) This is Ficks Second Law which states that the change in impurity concentration over time is equal to the change in local diffusion flux or

partC(xt)partt = - partJpartx or from Ficks First Law

partC(xt)partt = part(DpartC(xt)partx)partx If the diffusion coefficient is independent of position such as when the impurity concentration is low then Ficks Second Law may be further simplified into the following equation

partC(xt)partt = D part2C(xt)partx2 The most important property of diffusion process in the manufacture of silicon solar cell is the formation of p-n junction by doping n-type material ie phosphorous on the p-type ie boron doped silicon textured wafer playing an essential role for photovoltaic effect Let us have a brief descriptive look on the properties of p-n junction P-N JUNCTION A p-n junction is a junction formed by combining p-type and n-type semiconductors together in very close contact

13

Principle of p-n junction The most common type of solar cell is a large p-n junction where the free carrier pairs emitted by light energy are separated by the junction and contribute to the current In its simplest form it consists of a junction formed between n-type and p-type semiconductors either of the same material (homo-junction) or different materials (hetero-junction) The band-structure of the two differently doped sides with respect to their Fermi levels can be seen in adjacent figure1

When the two halves are brought together the Fermi levels on either side are forced in to coincidence causing the valence and conduction bands to bend (Figure 2)

These bent bands represent a built-in electric field over what is referred to as the depletion region When a photon with a energy greater than the band-gap of the semiconductor passes through the solar cell it may be absorbed by the material This absorption takes the form of a band-to-band electronic transition so an electronhole pair is produced If these carriers can diffuse to the depletion region before they recombine then they are separated by the electric field causing one quantum of charge to flow through an external load This is the origin of the solar cells photocurrent and is shown in Figure 3

14

SOLAR CELL AS A P-N JUNCTION A solar cell is essential a p-n junction with large surface area The n type material is kept thin to allow light to pass through to the p-n junction

Light travels in packets of energy called photons The generation of electric current happens inside the depletion zone of the PN junction The depletion region as explained previously with the diode is the area around the PN junction where the electrons from the N-type silicon have diffused into the holes of the P-type material When a photon of light is absorbed by one of these atoms in the N-Type silicon it will dislodge an electron creating a free electron and a hole The free electron and hole has sufficient energy to jump out of the depletion zone If a wire is connected from the cathode (N-type silicon) to the anode (P-type silicon) electrons will flow through the wire The electron is attracted to the positive charge of the P-type material and travels through the external load (meter) creating a flow of electric current The hole created by the dislodged electron is attracted to the negative charge of N-type material and migrates to the back electrical contact As the electron enters the P-type silicon from the back electrical contact it combines with the hole restoring the electrical neutrality

DIFFUSION PROCESS

The first layer formation after doing texturization of wafer is performed in this process by doping the substrate with n-type dopant N-doping can be accomplished by depositing n dopant onto the substrate and then heating the substrate to drive the n dopant into the substrate Gaseous diffusion (for example POCl3 horizontal tube diffusion including temperatures of from 700 to 1000degC preferably 800 to 900degC) can be used to deposit the n-dopant onto the substrate surface to create an n-doped layer and a shallow p-n junction proximal to the substrate surface In this process if wafers are loaded onto the quartz boat back to back in the same lot then double diffusion process takes place and if single wafers are being loaded then single diffusion takes place On the quartz boat wafers can be loaded in two methods

1 Parallel diffusion loading of wafers parallel to the gas flow 2 Perpendicular diffusion loading of wafers perpendicular to the gas flow

Diffusion process is comprised of the following steps 1 Idle state-In this state the load center and source zone are set at 800ordmC and no process is

carried out in the furnace 2 Load-During this sequence the silicon carbide paddle moves at a speed of 45 rpm and the

maximum time taken is 7min 3 Ramp-The cold wafers loaded into the tubes bring down the temperature of the tube The tube

is then reheated to reach the set point temperature and the time taken is 7min

15

4 Stabilization-During this phase any overshooting of temperature during ramp-up phase is controlled to the predetermined set value and the time taken is 10min

5 Pre-deposition-Any excess POCL3 present in vapour form inside the quartz bubbler is vent out before the deposition starts by N2

6 Deposition- N purging in 5 sec

2 is bubbled through liquid POCL3 in presence of O2 to form P2O5

7 Post-deposition-During this phase O in paste form

2 and N2

8 N

flows to drive in the phosphorous from wafer surface to the junction

2

9 Unload-During this phase the SiC paddle moves slowly out of the furnace at set speed of 45rpm and maximum time taken is 7min

Purge-This phase is also known as drive in diffusion which is designed to evenly distribute the impurities into wafer and reduce the generally undesirable high concentration in the wafer surface and the maximum time taken is 7min

10 End-The tube is set to 800⁰C temperature in idle state

16

PLASMA ETCHING

INTRODUCTION

Plasma etching is one type of dry etching which uses plasma to produce chemically reactive species from inert gases The reactive gases are then made to react with the material to be etched The main objective of this process is to remove edge shunts present on the cell which leads to 80 loss and thus it is also termed as Edge isolation method Since edge isolation is the main motive of this process so first let us see that why edge isolation is so important

Edge isolation

In the standard processing sequence the entire wafer is phosphorous diffused The contact to the p-type base is made by firing an Al containing paste that produces a high doping of Al thus over-compensating the shallow emitter diffusion However there is still a direct conductive path from the emitter on the front side to the rear p contact Therefore a large portion of the photo-generated current can directly flow through the emitter to the back contact instead of flowing through the external circuit This current path is modeled in the two-diode model of the solar cell as the parallel resistance R

where R

p

s and Rp are the series and parallel resistance respectively j01 and j02 represent the first and second diode saturation current densities respectively n1 and n2 are the first and second ideality diode factors respectively Vth = ktq is the thermal voltage and jL

In this model the first diode accounts for emitter and bulk recombination with a recombination rate assumed to be proportional to the excess carrier density This leads to an ideality factor of n

is the light generated current density The two diode model is as shown below

1=1 The second diode is associated with the recombination in the space charge region that is assumed to be independent of the injection level leading to n2=2 The effect of increased j02 and Rp can be shown by model calculations for ideal solar cells in the figure below for a fill factor potential of 84 j02 must be less than 2x10-9 Acm2 and Rp must be greater than approximately 2000Ω cm2 if the respective parameters contribute solely to the fill factor loss

17

This increase in Rp is achieved by the edge isolation process that interrupts the parasitic current path as shown in fig2 below

Thus edge isolation is required in solar cell to increase open circuit voltage and thus leading to greater efficiency

Now we will move on to plasma etching process to see that how it works actually in solar cell and module manufacturing industries

Plasma Etching Process

After diffusion a layer of phosphosilicate glass is formed on the edges and rear surface of diffused wafer The process generally requires adequate physical masking of the front surface while exposing the full back of the cell to the dry plasma environment In an embodiment of this process cells are slotted face to face

Fig2Sketch of an industrial screen-printed solar cell The emitter diffusion is present on the complete wafer surface and gets locally overcompensated by printing and firing Al containing metal paste as back contact Without edge-isolation light generated e-s can flow through the emitter region to the back contact which corresponds to the small series resistance in the two diode model

18

into inert carriers so that the plasma chemistry can effectively react with the back of the cells thereby removing excess material from the second coating that might have reached the back of the cell This process is preferably performed via the generation of an RF (13 6 MHz) plasma using a mix of Freon 14 (CF4) with oxygen (O2) and or nitrogen (N2) in a ratio which depends on the chamber and load geometries

19

PHOSPHOSILICATE GLASS REMOVAL INTRODUCTION In diffusion process during deposition phase N2 gas is bubbled through liquid POCL3

The reaction related to this phase is given as follows

1 4POCL3 + 3O2 -gt 2P2O5 + 6Cl 2 2P

2 2O5 + 5Si -gt 5SiO2

+ 4P (PSG)

P2O5

So it becomes mandatory to remove phosphosilicate glass by chemicals which may act as recombination sites or may contribute towards the surface density defect and hence all these defects may contribute to a lower voltage and current

obtained in first reaction step is in paste form and is transferred to the Teflon duct to the process tube which then reacts with silicon of which the wafer is made And thus when this phosphorous pentoxide reaches silicon surface then second reaction takes place and thus a silicon dioxide layer and phosphosilicate glass ie PSG forms on the wafer surface Formation of this dead layer on the wafer surface contributes to a lower efficiency

PROCESS OF REMOVAL Following are the steps that should be taken into consideration for the removal of phosphosilicate glass (PSG) from the wafer surface

1 Firstly wafers are washed combinedly with HF HCL and DI water in a tank for 1min 2 Again wafers are washed with HF and DI water for 2min in the other tank 3 For another 1 min DI water is being sprayed on the diffused wafers 4 Next wafers are washed with DI water subsequently on an interval of 30 sec 5 Finally wafers are kept in the OVEN for 10mins within the temperature range of 95plusmn5ordmC

20

PLASMA ENHANCED CHEMICAL VAPOUR DEPOSITION INTRODUCTION In recent years the low-temperature (le 400ordmC) surface passivation of crystalline silicon solar cells by means of PECVD silicon nitride films has attracted an increasing attention of the PV community These films have proven to be capable of combining an outstanding surface passivation quality with excellent anti-reflection properties On typical p-type silicon solar cells substrate it has been shown that the surface passivation quality of very silicon rich Si3N4

we must first know that whatrsquos the use of silicon nitride coating on the surface of the wafer

films is superior to that of state-of-the-art high temperature silicon dioxide Before proceeding to the details of PECVD system

WHY SILICON NITRIDE SURFACE COATING IS PREFERRED IN PECVD PROCESS It is preferred as a surface coating because it functions as An effective diffusion mask An effective metallization diffusion barrier An anti-reflective coating Provide surface and bulk passivation

Preferably hydrogen is trapped in such a silicon nitride surface coating PECVD SYSYTEM PECVD or Plasma Enhanced Chemical Vapor Deposition is one of the methods of different Chemical Vapor Deposition (CVD) technique In PECVD plasma is being used to decompose a reactant gas such as silane (SiH4) to produce reaction products that precipitate on the surface of the substrate as a new layer

Working Principle Inside a PECVD reactor a strong electric field ignites plasma between two electrodes one of which holds the substrate This plasma ignition cracks the molecular bonds of the process gas which in turn are able to crack more process gas molecules before reaching the surface of the substrate The glow discharge used in this reactor is created by applying an RF field to a low-pressure gas creating free electrons within the discharge region The electrons are sufficiently energized by the electric field that gas-phase dissociation and ionization of the reactant gases occur when the free electrons collide with them Energetic species are then adsorbed on the film surface where they are subjected to ion and electron bombardment rearrangements reactions with other species new bond formation and film formation and growth Eventually a new layer is deposited on the substrate For example when silane is used as precursor plasma ignition frees up Si and SiH radicals which also crack more silane molecules on their way to the surface of the substrate where silicon is deposited PROCESS DETAILS OF PECVD In this system wafers coming from the previous state ie PSG removal step are being loaded onto the graphite boat where the individual wafers are held by three pins inside the boat It is absolutely necessary for the wafers to touch all the three pins otherwise there will remain a possibility of poor deposition in which

Figure Anti-reflective coating made out of silicon nitride deposited by the PECVD technique whereby silane and ammonia is reacted in a furnace The major color of coating looks blue but the color could also look like purple blue or light blue due to different thickness of coating

21

the HF generator fails to generate the required power for process deposition While loading the double diffusion wafers it should be ensured that deposition side should be correctly loaded The wafers are being loaded parallel to the boat After loading the trolley along with the boat is placed inside the machine and then after filling the input parameters of the CMI interface used here and the process tube press the RUN button and then machine will be in auto mode The handling mechanism automatically picks up the boat and places it on the desired paddle corresponding to the selected for the specified job And then process STARTS

22

SCREEN PRINTING INTRODUCTION In screen printing on wafer-based solar PV cells the mesh and buses of silver are printed on the front furthermore the buses of silver are printed on the back Subsequently aluminum paste is dispensed over the whole surface of the back for passivation and surface reflection [14] One of the parameters that can vary and can be controlled in screen printing is the thickness of the print This makes it useful for some of the techniques of printing solar cells electronics etc Screen printing is one of the most critical processes to maintain high yield Solar wafers are becoming thinner and larger so careful printing is required to maintain a lower breakage rate On the other hand high throughput at the printing stage improves the throughput of the whole cell production line PROCESS This process is divided in three phase-

1 Phase I- AgAl printing 2 Phase II- Ag printing 3 Phase III- Al printing

PHASE I- AgAl PRINTING This phase is required to print the back bus bar for tabbing purpose The AgAl paste used in this printing is Dupont (PV-505) and the consumption of the paste per wafer is 011 gm And the screen type used is of the following specification

1 Mesh size- 200microm 2 Tension- 24 N-cm 3 Mesh angle- 225ordm 45ordm 4 Emulsion thickness- 24microm

Process details During printing the paste is poured on the screen and is spread over it with the help of polypropylene spatula Then the wafers (ARC coated) are pasted with the sunny side down on the loader and picked one by one automatically with the start of the print command Once printing starts the wafers are placed one by one between the tool paste and the screen and then the squeegee moves over the screen and allow the paste through the exposed parts on the screen and gets pasted on the wafers Thus in this way the wafers are printed one by one Two most important parameters to be controlled during this printing are

1 Squeegee Pressure 2 Snap off distance

The next step after printing is drying step DRYING In this step wafers are passed through the dryer with a temperature range of 180ordm-400ordmC PHASE II- Ag PRINTING This phase is required to print the front bus bar for tabbing purpose The Ag paste used in this printing is Dupont (PV-145) and the consumption of the paste per wafer is 014 gm And the screen type used is of the following specification

1 Mesh size- 325microm 2 Tension- 24 N-cm 3 Mesh angle- 225ordm 4 Emulsion thickness- 24microm

12

DIFFUSION

INTRODUCTION

Diffusion the movement of a chemical species from an area of high concentration to an area of lower concentration is one of the two major processes by which chemical species or dopants are introduced into a semiconductor (the other one being ion implantation) The controlled diffusion of dopants into silicon to alter the type and level of conductivity of semiconductor materials is the foundation of forming a p-n junction The mathematics that govern the mass transport phenomena of diffusion are based on Ficks laws There are two laws according to Fick Letrsquos have a look on these laws First law Whenever an impurity concentration gradient partCpartx exists in a finite volume of a matrix substance (the silicon substrate in this context) the impurity material will have the natural tendency to move in order to distribute itself more evenly within the matrix and decrease the gradient Given enough time this flow of impurities will eventually result in homogeneity within the matrix causing the net flow of impurities to stop The mathematics of this transport mechanism was formalized in 1855 by Fick who postulated that the flux of material across a given plane is proportional to the concentration gradient across the plane Thus Ficks First Law states

J = -D ( partC(xt)partx ) Where J is the flux D is the diffusion constant for the material that is diffusing in the specific solvent and partC(xt)partx is the concentration gradient The diffusion constant of a material is also referred to as diffusion coefficient or simply diffusivity It is expressed in units of length2time such as microm2hour The negative sign of the right side of the equation indicates that the impurities are flowing in the direction of lower concentration Second Law Ficks First Law does not consider the fact that the gradient and local concentration of the impurities in a material decreases with an increase in time an aspect thats important to diffusion processes The flux J1 of impurities entering a section of a bar with a concentration gradient is different from the flux J2 of impurities leaving the same section From the law of conservation of matter the difference between J1 and J2 must result in a change in the concentration of impurities within the section (assuming that no impurities are formed or consumed in the section) This is Ficks Second Law which states that the change in impurity concentration over time is equal to the change in local diffusion flux or

partC(xt)partt = - partJpartx or from Ficks First Law

partC(xt)partt = part(DpartC(xt)partx)partx If the diffusion coefficient is independent of position such as when the impurity concentration is low then Ficks Second Law may be further simplified into the following equation

partC(xt)partt = D part2C(xt)partx2 The most important property of diffusion process in the manufacture of silicon solar cell is the formation of p-n junction by doping n-type material ie phosphorous on the p-type ie boron doped silicon textured wafer playing an essential role for photovoltaic effect Let us have a brief descriptive look on the properties of p-n junction P-N JUNCTION A p-n junction is a junction formed by combining p-type and n-type semiconductors together in very close contact

13

Principle of p-n junction The most common type of solar cell is a large p-n junction where the free carrier pairs emitted by light energy are separated by the junction and contribute to the current In its simplest form it consists of a junction formed between n-type and p-type semiconductors either of the same material (homo-junction) or different materials (hetero-junction) The band-structure of the two differently doped sides with respect to their Fermi levels can be seen in adjacent figure1

When the two halves are brought together the Fermi levels on either side are forced in to coincidence causing the valence and conduction bands to bend (Figure 2)

These bent bands represent a built-in electric field over what is referred to as the depletion region When a photon with a energy greater than the band-gap of the semiconductor passes through the solar cell it may be absorbed by the material This absorption takes the form of a band-to-band electronic transition so an electronhole pair is produced If these carriers can diffuse to the depletion region before they recombine then they are separated by the electric field causing one quantum of charge to flow through an external load This is the origin of the solar cells photocurrent and is shown in Figure 3

14

SOLAR CELL AS A P-N JUNCTION A solar cell is essential a p-n junction with large surface area The n type material is kept thin to allow light to pass through to the p-n junction

Light travels in packets of energy called photons The generation of electric current happens inside the depletion zone of the PN junction The depletion region as explained previously with the diode is the area around the PN junction where the electrons from the N-type silicon have diffused into the holes of the P-type material When a photon of light is absorbed by one of these atoms in the N-Type silicon it will dislodge an electron creating a free electron and a hole The free electron and hole has sufficient energy to jump out of the depletion zone If a wire is connected from the cathode (N-type silicon) to the anode (P-type silicon) electrons will flow through the wire The electron is attracted to the positive charge of the P-type material and travels through the external load (meter) creating a flow of electric current The hole created by the dislodged electron is attracted to the negative charge of N-type material and migrates to the back electrical contact As the electron enters the P-type silicon from the back electrical contact it combines with the hole restoring the electrical neutrality

DIFFUSION PROCESS

The first layer formation after doing texturization of wafer is performed in this process by doping the substrate with n-type dopant N-doping can be accomplished by depositing n dopant onto the substrate and then heating the substrate to drive the n dopant into the substrate Gaseous diffusion (for example POCl3 horizontal tube diffusion including temperatures of from 700 to 1000degC preferably 800 to 900degC) can be used to deposit the n-dopant onto the substrate surface to create an n-doped layer and a shallow p-n junction proximal to the substrate surface In this process if wafers are loaded onto the quartz boat back to back in the same lot then double diffusion process takes place and if single wafers are being loaded then single diffusion takes place On the quartz boat wafers can be loaded in two methods

1 Parallel diffusion loading of wafers parallel to the gas flow 2 Perpendicular diffusion loading of wafers perpendicular to the gas flow

Diffusion process is comprised of the following steps 1 Idle state-In this state the load center and source zone are set at 800ordmC and no process is

carried out in the furnace 2 Load-During this sequence the silicon carbide paddle moves at a speed of 45 rpm and the

maximum time taken is 7min 3 Ramp-The cold wafers loaded into the tubes bring down the temperature of the tube The tube

is then reheated to reach the set point temperature and the time taken is 7min

15

4 Stabilization-During this phase any overshooting of temperature during ramp-up phase is controlled to the predetermined set value and the time taken is 10min

5 Pre-deposition-Any excess POCL3 present in vapour form inside the quartz bubbler is vent out before the deposition starts by N2

6 Deposition- N purging in 5 sec

2 is bubbled through liquid POCL3 in presence of O2 to form P2O5

7 Post-deposition-During this phase O in paste form

2 and N2

8 N

flows to drive in the phosphorous from wafer surface to the junction

2

9 Unload-During this phase the SiC paddle moves slowly out of the furnace at set speed of 45rpm and maximum time taken is 7min

Purge-This phase is also known as drive in diffusion which is designed to evenly distribute the impurities into wafer and reduce the generally undesirable high concentration in the wafer surface and the maximum time taken is 7min

10 End-The tube is set to 800⁰C temperature in idle state

16

PLASMA ETCHING

INTRODUCTION

Plasma etching is one type of dry etching which uses plasma to produce chemically reactive species from inert gases The reactive gases are then made to react with the material to be etched The main objective of this process is to remove edge shunts present on the cell which leads to 80 loss and thus it is also termed as Edge isolation method Since edge isolation is the main motive of this process so first let us see that why edge isolation is so important

Edge isolation

In the standard processing sequence the entire wafer is phosphorous diffused The contact to the p-type base is made by firing an Al containing paste that produces a high doping of Al thus over-compensating the shallow emitter diffusion However there is still a direct conductive path from the emitter on the front side to the rear p contact Therefore a large portion of the photo-generated current can directly flow through the emitter to the back contact instead of flowing through the external circuit This current path is modeled in the two-diode model of the solar cell as the parallel resistance R

where R

p

s and Rp are the series and parallel resistance respectively j01 and j02 represent the first and second diode saturation current densities respectively n1 and n2 are the first and second ideality diode factors respectively Vth = ktq is the thermal voltage and jL

In this model the first diode accounts for emitter and bulk recombination with a recombination rate assumed to be proportional to the excess carrier density This leads to an ideality factor of n

is the light generated current density The two diode model is as shown below

1=1 The second diode is associated with the recombination in the space charge region that is assumed to be independent of the injection level leading to n2=2 The effect of increased j02 and Rp can be shown by model calculations for ideal solar cells in the figure below for a fill factor potential of 84 j02 must be less than 2x10-9 Acm2 and Rp must be greater than approximately 2000Ω cm2 if the respective parameters contribute solely to the fill factor loss

17

This increase in Rp is achieved by the edge isolation process that interrupts the parasitic current path as shown in fig2 below

Thus edge isolation is required in solar cell to increase open circuit voltage and thus leading to greater efficiency

Now we will move on to plasma etching process to see that how it works actually in solar cell and module manufacturing industries

Plasma Etching Process

After diffusion a layer of phosphosilicate glass is formed on the edges and rear surface of diffused wafer The process generally requires adequate physical masking of the front surface while exposing the full back of the cell to the dry plasma environment In an embodiment of this process cells are slotted face to face

Fig2Sketch of an industrial screen-printed solar cell The emitter diffusion is present on the complete wafer surface and gets locally overcompensated by printing and firing Al containing metal paste as back contact Without edge-isolation light generated e-s can flow through the emitter region to the back contact which corresponds to the small series resistance in the two diode model

18

into inert carriers so that the plasma chemistry can effectively react with the back of the cells thereby removing excess material from the second coating that might have reached the back of the cell This process is preferably performed via the generation of an RF (13 6 MHz) plasma using a mix of Freon 14 (CF4) with oxygen (O2) and or nitrogen (N2) in a ratio which depends on the chamber and load geometries

19

PHOSPHOSILICATE GLASS REMOVAL INTRODUCTION In diffusion process during deposition phase N2 gas is bubbled through liquid POCL3

The reaction related to this phase is given as follows

1 4POCL3 + 3O2 -gt 2P2O5 + 6Cl 2 2P

2 2O5 + 5Si -gt 5SiO2

+ 4P (PSG)

P2O5

So it becomes mandatory to remove phosphosilicate glass by chemicals which may act as recombination sites or may contribute towards the surface density defect and hence all these defects may contribute to a lower voltage and current

obtained in first reaction step is in paste form and is transferred to the Teflon duct to the process tube which then reacts with silicon of which the wafer is made And thus when this phosphorous pentoxide reaches silicon surface then second reaction takes place and thus a silicon dioxide layer and phosphosilicate glass ie PSG forms on the wafer surface Formation of this dead layer on the wafer surface contributes to a lower efficiency

PROCESS OF REMOVAL Following are the steps that should be taken into consideration for the removal of phosphosilicate glass (PSG) from the wafer surface

1 Firstly wafers are washed combinedly with HF HCL and DI water in a tank for 1min 2 Again wafers are washed with HF and DI water for 2min in the other tank 3 For another 1 min DI water is being sprayed on the diffused wafers 4 Next wafers are washed with DI water subsequently on an interval of 30 sec 5 Finally wafers are kept in the OVEN for 10mins within the temperature range of 95plusmn5ordmC

20

PLASMA ENHANCED CHEMICAL VAPOUR DEPOSITION INTRODUCTION In recent years the low-temperature (le 400ordmC) surface passivation of crystalline silicon solar cells by means of PECVD silicon nitride films has attracted an increasing attention of the PV community These films have proven to be capable of combining an outstanding surface passivation quality with excellent anti-reflection properties On typical p-type silicon solar cells substrate it has been shown that the surface passivation quality of very silicon rich Si3N4

we must first know that whatrsquos the use of silicon nitride coating on the surface of the wafer

films is superior to that of state-of-the-art high temperature silicon dioxide Before proceeding to the details of PECVD system

WHY SILICON NITRIDE SURFACE COATING IS PREFERRED IN PECVD PROCESS It is preferred as a surface coating because it functions as An effective diffusion mask An effective metallization diffusion barrier An anti-reflective coating Provide surface and bulk passivation

Preferably hydrogen is trapped in such a silicon nitride surface coating PECVD SYSYTEM PECVD or Plasma Enhanced Chemical Vapor Deposition is one of the methods of different Chemical Vapor Deposition (CVD) technique In PECVD plasma is being used to decompose a reactant gas such as silane (SiH4) to produce reaction products that precipitate on the surface of the substrate as a new layer

Working Principle Inside a PECVD reactor a strong electric field ignites plasma between two electrodes one of which holds the substrate This plasma ignition cracks the molecular bonds of the process gas which in turn are able to crack more process gas molecules before reaching the surface of the substrate The glow discharge used in this reactor is created by applying an RF field to a low-pressure gas creating free electrons within the discharge region The electrons are sufficiently energized by the electric field that gas-phase dissociation and ionization of the reactant gases occur when the free electrons collide with them Energetic species are then adsorbed on the film surface where they are subjected to ion and electron bombardment rearrangements reactions with other species new bond formation and film formation and growth Eventually a new layer is deposited on the substrate For example when silane is used as precursor plasma ignition frees up Si and SiH radicals which also crack more silane molecules on their way to the surface of the substrate where silicon is deposited PROCESS DETAILS OF PECVD In this system wafers coming from the previous state ie PSG removal step are being loaded onto the graphite boat where the individual wafers are held by three pins inside the boat It is absolutely necessary for the wafers to touch all the three pins otherwise there will remain a possibility of poor deposition in which

Figure Anti-reflective coating made out of silicon nitride deposited by the PECVD technique whereby silane and ammonia is reacted in a furnace The major color of coating looks blue but the color could also look like purple blue or light blue due to different thickness of coating

21

the HF generator fails to generate the required power for process deposition While loading the double diffusion wafers it should be ensured that deposition side should be correctly loaded The wafers are being loaded parallel to the boat After loading the trolley along with the boat is placed inside the machine and then after filling the input parameters of the CMI interface used here and the process tube press the RUN button and then machine will be in auto mode The handling mechanism automatically picks up the boat and places it on the desired paddle corresponding to the selected for the specified job And then process STARTS

22

SCREEN PRINTING INTRODUCTION In screen printing on wafer-based solar PV cells the mesh and buses of silver are printed on the front furthermore the buses of silver are printed on the back Subsequently aluminum paste is dispensed over the whole surface of the back for passivation and surface reflection [14] One of the parameters that can vary and can be controlled in screen printing is the thickness of the print This makes it useful for some of the techniques of printing solar cells electronics etc Screen printing is one of the most critical processes to maintain high yield Solar wafers are becoming thinner and larger so careful printing is required to maintain a lower breakage rate On the other hand high throughput at the printing stage improves the throughput of the whole cell production line PROCESS This process is divided in three phase-

1 Phase I- AgAl printing 2 Phase II- Ag printing 3 Phase III- Al printing

PHASE I- AgAl PRINTING This phase is required to print the back bus bar for tabbing purpose The AgAl paste used in this printing is Dupont (PV-505) and the consumption of the paste per wafer is 011 gm And the screen type used is of the following specification

1 Mesh size- 200microm 2 Tension- 24 N-cm 3 Mesh angle- 225ordm 45ordm 4 Emulsion thickness- 24microm

Process details During printing the paste is poured on the screen and is spread over it with the help of polypropylene spatula Then the wafers (ARC coated) are pasted with the sunny side down on the loader and picked one by one automatically with the start of the print command Once printing starts the wafers are placed one by one between the tool paste and the screen and then the squeegee moves over the screen and allow the paste through the exposed parts on the screen and gets pasted on the wafers Thus in this way the wafers are printed one by one Two most important parameters to be controlled during this printing are

1 Squeegee Pressure 2 Snap off distance

The next step after printing is drying step DRYING In this step wafers are passed through the dryer with a temperature range of 180ordm-400ordmC PHASE II- Ag PRINTING This phase is required to print the front bus bar for tabbing purpose The Ag paste used in this printing is Dupont (PV-145) and the consumption of the paste per wafer is 014 gm And the screen type used is of the following specification

1 Mesh size- 325microm 2 Tension- 24 N-cm 3 Mesh angle- 225ordm 4 Emulsion thickness- 24microm

13

Principle of p-n junction The most common type of solar cell is a large p-n junction where the free carrier pairs emitted by light energy are separated by the junction and contribute to the current In its simplest form it consists of a junction formed between n-type and p-type semiconductors either of the same material (homo-junction) or different materials (hetero-junction) The band-structure of the two differently doped sides with respect to their Fermi levels can be seen in adjacent figure1

When the two halves are brought together the Fermi levels on either side are forced in to coincidence causing the valence and conduction bands to bend (Figure 2)

These bent bands represent a built-in electric field over what is referred to as the depletion region When a photon with a energy greater than the band-gap of the semiconductor passes through the solar cell it may be absorbed by the material This absorption takes the form of a band-to-band electronic transition so an electronhole pair is produced If these carriers can diffuse to the depletion region before they recombine then they are separated by the electric field causing one quantum of charge to flow through an external load This is the origin of the solar cells photocurrent and is shown in Figure 3

14

SOLAR CELL AS A P-N JUNCTION A solar cell is essential a p-n junction with large surface area The n type material is kept thin to allow light to pass through to the p-n junction

Light travels in packets of energy called photons The generation of electric current happens inside the depletion zone of the PN junction The depletion region as explained previously with the diode is the area around the PN junction where the electrons from the N-type silicon have diffused into the holes of the P-type material When a photon of light is absorbed by one of these atoms in the N-Type silicon it will dislodge an electron creating a free electron and a hole The free electron and hole has sufficient energy to jump out of the depletion zone If a wire is connected from the cathode (N-type silicon) to the anode (P-type silicon) electrons will flow through the wire The electron is attracted to the positive charge of the P-type material and travels through the external load (meter) creating a flow of electric current The hole created by the dislodged electron is attracted to the negative charge of N-type material and migrates to the back electrical contact As the electron enters the P-type silicon from the back electrical contact it combines with the hole restoring the electrical neutrality

DIFFUSION PROCESS

The first layer formation after doing texturization of wafer is performed in this process by doping the substrate with n-type dopant N-doping can be accomplished by depositing n dopant onto the substrate and then heating the substrate to drive the n dopant into the substrate Gaseous diffusion (for example POCl3 horizontal tube diffusion including temperatures of from 700 to 1000degC preferably 800 to 900degC) can be used to deposit the n-dopant onto the substrate surface to create an n-doped layer and a shallow p-n junction proximal to the substrate surface In this process if wafers are loaded onto the quartz boat back to back in the same lot then double diffusion process takes place and if single wafers are being loaded then single diffusion takes place On the quartz boat wafers can be loaded in two methods

1 Parallel diffusion loading of wafers parallel to the gas flow 2 Perpendicular diffusion loading of wafers perpendicular to the gas flow

Diffusion process is comprised of the following steps 1 Idle state-In this state the load center and source zone are set at 800ordmC and no process is

carried out in the furnace 2 Load-During this sequence the silicon carbide paddle moves at a speed of 45 rpm and the

maximum time taken is 7min 3 Ramp-The cold wafers loaded into the tubes bring down the temperature of the tube The tube

is then reheated to reach the set point temperature and the time taken is 7min

15

4 Stabilization-During this phase any overshooting of temperature during ramp-up phase is controlled to the predetermined set value and the time taken is 10min

5 Pre-deposition-Any excess POCL3 present in vapour form inside the quartz bubbler is vent out before the deposition starts by N2

6 Deposition- N purging in 5 sec

2 is bubbled through liquid POCL3 in presence of O2 to form P2O5

7 Post-deposition-During this phase O in paste form

2 and N2

8 N

flows to drive in the phosphorous from wafer surface to the junction

2

9 Unload-During this phase the SiC paddle moves slowly out of the furnace at set speed of 45rpm and maximum time taken is 7min

Purge-This phase is also known as drive in diffusion which is designed to evenly distribute the impurities into wafer and reduce the generally undesirable high concentration in the wafer surface and the maximum time taken is 7min

10 End-The tube is set to 800⁰C temperature in idle state

16

PLASMA ETCHING

INTRODUCTION

Plasma etching is one type of dry etching which uses plasma to produce chemically reactive species from inert gases The reactive gases are then made to react with the material to be etched The main objective of this process is to remove edge shunts present on the cell which leads to 80 loss and thus it is also termed as Edge isolation method Since edge isolation is the main motive of this process so first let us see that why edge isolation is so important

Edge isolation

In the standard processing sequence the entire wafer is phosphorous diffused The contact to the p-type base is made by firing an Al containing paste that produces a high doping of Al thus over-compensating the shallow emitter diffusion However there is still a direct conductive path from the emitter on the front side to the rear p contact Therefore a large portion of the photo-generated current can directly flow through the emitter to the back contact instead of flowing through the external circuit This current path is modeled in the two-diode model of the solar cell as the parallel resistance R

where R

p

s and Rp are the series and parallel resistance respectively j01 and j02 represent the first and second diode saturation current densities respectively n1 and n2 are the first and second ideality diode factors respectively Vth = ktq is the thermal voltage and jL

In this model the first diode accounts for emitter and bulk recombination with a recombination rate assumed to be proportional to the excess carrier density This leads to an ideality factor of n

is the light generated current density The two diode model is as shown below

1=1 The second diode is associated with the recombination in the space charge region that is assumed to be independent of the injection level leading to n2=2 The effect of increased j02 and Rp can be shown by model calculations for ideal solar cells in the figure below for a fill factor potential of 84 j02 must be less than 2x10-9 Acm2 and Rp must be greater than approximately 2000Ω cm2 if the respective parameters contribute solely to the fill factor loss

17

This increase in Rp is achieved by the edge isolation process that interrupts the parasitic current path as shown in fig2 below

Thus edge isolation is required in solar cell to increase open circuit voltage and thus leading to greater efficiency

Now we will move on to plasma etching process to see that how it works actually in solar cell and module manufacturing industries

Plasma Etching Process

After diffusion a layer of phosphosilicate glass is formed on the edges and rear surface of diffused wafer The process generally requires adequate physical masking of the front surface while exposing the full back of the cell to the dry plasma environment In an embodiment of this process cells are slotted face to face

Fig2Sketch of an industrial screen-printed solar cell The emitter diffusion is present on the complete wafer surface and gets locally overcompensated by printing and firing Al containing metal paste as back contact Without edge-isolation light generated e-s can flow through the emitter region to the back contact which corresponds to the small series resistance in the two diode model

18

into inert carriers so that the plasma chemistry can effectively react with the back of the cells thereby removing excess material from the second coating that might have reached the back of the cell This process is preferably performed via the generation of an RF (13 6 MHz) plasma using a mix of Freon 14 (CF4) with oxygen (O2) and or nitrogen (N2) in a ratio which depends on the chamber and load geometries

19

PHOSPHOSILICATE GLASS REMOVAL INTRODUCTION In diffusion process during deposition phase N2 gas is bubbled through liquid POCL3

The reaction related to this phase is given as follows

1 4POCL3 + 3O2 -gt 2P2O5 + 6Cl 2 2P

2 2O5 + 5Si -gt 5SiO2

+ 4P (PSG)

P2O5

So it becomes mandatory to remove phosphosilicate glass by chemicals which may act as recombination sites or may contribute towards the surface density defect and hence all these defects may contribute to a lower voltage and current

obtained in first reaction step is in paste form and is transferred to the Teflon duct to the process tube which then reacts with silicon of which the wafer is made And thus when this phosphorous pentoxide reaches silicon surface then second reaction takes place and thus a silicon dioxide layer and phosphosilicate glass ie PSG forms on the wafer surface Formation of this dead layer on the wafer surface contributes to a lower efficiency

PROCESS OF REMOVAL Following are the steps that should be taken into consideration for the removal of phosphosilicate glass (PSG) from the wafer surface

1 Firstly wafers are washed combinedly with HF HCL and DI water in a tank for 1min 2 Again wafers are washed with HF and DI water for 2min in the other tank 3 For another 1 min DI water is being sprayed on the diffused wafers 4 Next wafers are washed with DI water subsequently on an interval of 30 sec 5 Finally wafers are kept in the OVEN for 10mins within the temperature range of 95plusmn5ordmC

20

PLASMA ENHANCED CHEMICAL VAPOUR DEPOSITION INTRODUCTION In recent years the low-temperature (le 400ordmC) surface passivation of crystalline silicon solar cells by means of PECVD silicon nitride films has attracted an increasing attention of the PV community These films have proven to be capable of combining an outstanding surface passivation quality with excellent anti-reflection properties On typical p-type silicon solar cells substrate it has been shown that the surface passivation quality of very silicon rich Si3N4

we must first know that whatrsquos the use of silicon nitride coating on the surface of the wafer

films is superior to that of state-of-the-art high temperature silicon dioxide Before proceeding to the details of PECVD system

WHY SILICON NITRIDE SURFACE COATING IS PREFERRED IN PECVD PROCESS It is preferred as a surface coating because it functions as An effective diffusion mask An effective metallization diffusion barrier An anti-reflective coating Provide surface and bulk passivation

Preferably hydrogen is trapped in such a silicon nitride surface coating PECVD SYSYTEM PECVD or Plasma Enhanced Chemical Vapor Deposition is one of the methods of different Chemical Vapor Deposition (CVD) technique In PECVD plasma is being used to decompose a reactant gas such as silane (SiH4) to produce reaction products that precipitate on the surface of the substrate as a new layer

Working Principle Inside a PECVD reactor a strong electric field ignites plasma between two electrodes one of which holds the substrate This plasma ignition cracks the molecular bonds of the process gas which in turn are able to crack more process gas molecules before reaching the surface of the substrate The glow discharge used in this reactor is created by applying an RF field to a low-pressure gas creating free electrons within the discharge region The electrons are sufficiently energized by the electric field that gas-phase dissociation and ionization of the reactant gases occur when the free electrons collide with them Energetic species are then adsorbed on the film surface where they are subjected to ion and electron bombardment rearrangements reactions with other species new bond formation and film formation and growth Eventually a new layer is deposited on the substrate For example when silane is used as precursor plasma ignition frees up Si and SiH radicals which also crack more silane molecules on their way to the surface of the substrate where silicon is deposited PROCESS DETAILS OF PECVD In this system wafers coming from the previous state ie PSG removal step are being loaded onto the graphite boat where the individual wafers are held by three pins inside the boat It is absolutely necessary for the wafers to touch all the three pins otherwise there will remain a possibility of poor deposition in which

Figure Anti-reflective coating made out of silicon nitride deposited by the PECVD technique whereby silane and ammonia is reacted in a furnace The major color of coating looks blue but the color could also look like purple blue or light blue due to different thickness of coating

21

the HF generator fails to generate the required power for process deposition While loading the double diffusion wafers it should be ensured that deposition side should be correctly loaded The wafers are being loaded parallel to the boat After loading the trolley along with the boat is placed inside the machine and then after filling the input parameters of the CMI interface used here and the process tube press the RUN button and then machine will be in auto mode The handling mechanism automatically picks up the boat and places it on the desired paddle corresponding to the selected for the specified job And then process STARTS

22

SCREEN PRINTING INTRODUCTION In screen printing on wafer-based solar PV cells the mesh and buses of silver are printed on the front furthermore the buses of silver are printed on the back Subsequently aluminum paste is dispensed over the whole surface of the back for passivation and surface reflection [14] One of the parameters that can vary and can be controlled in screen printing is the thickness of the print This makes it useful for some of the techniques of printing solar cells electronics etc Screen printing is one of the most critical processes to maintain high yield Solar wafers are becoming thinner and larger so careful printing is required to maintain a lower breakage rate On the other hand high throughput at the printing stage improves the throughput of the whole cell production line PROCESS This process is divided in three phase-

1 Phase I- AgAl printing 2 Phase II- Ag printing 3 Phase III- Al printing

PHASE I- AgAl PRINTING This phase is required to print the back bus bar for tabbing purpose The AgAl paste used in this printing is Dupont (PV-505) and the consumption of the paste per wafer is 011 gm And the screen type used is of the following specification

1 Mesh size- 200microm 2 Tension- 24 N-cm 3 Mesh angle- 225ordm 45ordm 4 Emulsion thickness- 24microm

Process details During printing the paste is poured on the screen and is spread over it with the help of polypropylene spatula Then the wafers (ARC coated) are pasted with the sunny side down on the loader and picked one by one automatically with the start of the print command Once printing starts the wafers are placed one by one between the tool paste and the screen and then the squeegee moves over the screen and allow the paste through the exposed parts on the screen and gets pasted on the wafers Thus in this way the wafers are printed one by one Two most important parameters to be controlled during this printing are

1 Squeegee Pressure 2 Snap off distance

The next step after printing is drying step DRYING In this step wafers are passed through the dryer with a temperature range of 180ordm-400ordmC PHASE II- Ag PRINTING This phase is required to print the front bus bar for tabbing purpose The Ag paste used in this printing is Dupont (PV-145) and the consumption of the paste per wafer is 014 gm And the screen type used is of the following specification

1 Mesh size- 325microm 2 Tension- 24 N-cm 3 Mesh angle- 225ordm 4 Emulsion thickness- 24microm

14

SOLAR CELL AS A P-N JUNCTION A solar cell is essential a p-n junction with large surface area The n type material is kept thin to allow light to pass through to the p-n junction

Light travels in packets of energy called photons The generation of electric current happens inside the depletion zone of the PN junction The depletion region as explained previously with the diode is the area around the PN junction where the electrons from the N-type silicon have diffused into the holes of the P-type material When a photon of light is absorbed by one of these atoms in the N-Type silicon it will dislodge an electron creating a free electron and a hole The free electron and hole has sufficient energy to jump out of the depletion zone If a wire is connected from the cathode (N-type silicon) to the anode (P-type silicon) electrons will flow through the wire The electron is attracted to the positive charge of the P-type material and travels through the external load (meter) creating a flow of electric current The hole created by the dislodged electron is attracted to the negative charge of N-type material and migrates to the back electrical contact As the electron enters the P-type silicon from the back electrical contact it combines with the hole restoring the electrical neutrality

DIFFUSION PROCESS

The first layer formation after doing texturization of wafer is performed in this process by doping the substrate with n-type dopant N-doping can be accomplished by depositing n dopant onto the substrate and then heating the substrate to drive the n dopant into the substrate Gaseous diffusion (for example POCl3 horizontal tube diffusion including temperatures of from 700 to 1000degC preferably 800 to 900degC) can be used to deposit the n-dopant onto the substrate surface to create an n-doped layer and a shallow p-n junction proximal to the substrate surface In this process if wafers are loaded onto the quartz boat back to back in the same lot then double diffusion process takes place and if single wafers are being loaded then single diffusion takes place On the quartz boat wafers can be loaded in two methods

1 Parallel diffusion loading of wafers parallel to the gas flow 2 Perpendicular diffusion loading of wafers perpendicular to the gas flow

Diffusion process is comprised of the following steps 1 Idle state-In this state the load center and source zone are set at 800ordmC and no process is

carried out in the furnace 2 Load-During this sequence the silicon carbide paddle moves at a speed of 45 rpm and the

maximum time taken is 7min 3 Ramp-The cold wafers loaded into the tubes bring down the temperature of the tube The tube

is then reheated to reach the set point temperature and the time taken is 7min

15

4 Stabilization-During this phase any overshooting of temperature during ramp-up phase is controlled to the predetermined set value and the time taken is 10min

5 Pre-deposition-Any excess POCL3 present in vapour form inside the quartz bubbler is vent out before the deposition starts by N2

6 Deposition- N purging in 5 sec

2 is bubbled through liquid POCL3 in presence of O2 to form P2O5

7 Post-deposition-During this phase O in paste form

2 and N2

8 N

flows to drive in the phosphorous from wafer surface to the junction

2

9 Unload-During this phase the SiC paddle moves slowly out of the furnace at set speed of 45rpm and maximum time taken is 7min

Purge-This phase is also known as drive in diffusion which is designed to evenly distribute the impurities into wafer and reduce the generally undesirable high concentration in the wafer surface and the maximum time taken is 7min

10 End-The tube is set to 800⁰C temperature in idle state

16

PLASMA ETCHING

INTRODUCTION

Plasma etching is one type of dry etching which uses plasma to produce chemically reactive species from inert gases The reactive gases are then made to react with the material to be etched The main objective of this process is to remove edge shunts present on the cell which leads to 80 loss and thus it is also termed as Edge isolation method Since edge isolation is the main motive of this process so first let us see that why edge isolation is so important

Edge isolation

In the standard processing sequence the entire wafer is phosphorous diffused The contact to the p-type base is made by firing an Al containing paste that produces a high doping of Al thus over-compensating the shallow emitter diffusion However there is still a direct conductive path from the emitter on the front side to the rear p contact Therefore a large portion of the photo-generated current can directly flow through the emitter to the back contact instead of flowing through the external circuit This current path is modeled in the two-diode model of the solar cell as the parallel resistance R

where R

p

s and Rp are the series and parallel resistance respectively j01 and j02 represent the first and second diode saturation current densities respectively n1 and n2 are the first and second ideality diode factors respectively Vth = ktq is the thermal voltage and jL

In this model the first diode accounts for emitter and bulk recombination with a recombination rate assumed to be proportional to the excess carrier density This leads to an ideality factor of n

is the light generated current density The two diode model is as shown below

1=1 The second diode is associated with the recombination in the space charge region that is assumed to be independent of the injection level leading to n2=2 The effect of increased j02 and Rp can be shown by model calculations for ideal solar cells in the figure below for a fill factor potential of 84 j02 must be less than 2x10-9 Acm2 and Rp must be greater than approximately 2000Ω cm2 if the respective parameters contribute solely to the fill factor loss

17

This increase in Rp is achieved by the edge isolation process that interrupts the parasitic current path as shown in fig2 below

Thus edge isolation is required in solar cell to increase open circuit voltage and thus leading to greater efficiency

Now we will move on to plasma etching process to see that how it works actually in solar cell and module manufacturing industries

Plasma Etching Process

After diffusion a layer of phosphosilicate glass is formed on the edges and rear surface of diffused wafer The process generally requires adequate physical masking of the front surface while exposing the full back of the cell to the dry plasma environment In an embodiment of this process cells are slotted face to face

Fig2Sketch of an industrial screen-printed solar cell The emitter diffusion is present on the complete wafer surface and gets locally overcompensated by printing and firing Al containing metal paste as back contact Without edge-isolation light generated e-s can flow through the emitter region to the back contact which corresponds to the small series resistance in the two diode model

18

into inert carriers so that the plasma chemistry can effectively react with the back of the cells thereby removing excess material from the second coating that might have reached the back of the cell This process is preferably performed via the generation of an RF (13 6 MHz) plasma using a mix of Freon 14 (CF4) with oxygen (O2) and or nitrogen (N2) in a ratio which depends on the chamber and load geometries

19

PHOSPHOSILICATE GLASS REMOVAL INTRODUCTION In diffusion process during deposition phase N2 gas is bubbled through liquid POCL3

The reaction related to this phase is given as follows

1 4POCL3 + 3O2 -gt 2P2O5 + 6Cl 2 2P

2 2O5 + 5Si -gt 5SiO2

+ 4P (PSG)

P2O5

So it becomes mandatory to remove phosphosilicate glass by chemicals which may act as recombination sites or may contribute towards the surface density defect and hence all these defects may contribute to a lower voltage and current

obtained in first reaction step is in paste form and is transferred to the Teflon duct to the process tube which then reacts with silicon of which the wafer is made And thus when this phosphorous pentoxide reaches silicon surface then second reaction takes place and thus a silicon dioxide layer and phosphosilicate glass ie PSG forms on the wafer surface Formation of this dead layer on the wafer surface contributes to a lower efficiency

PROCESS OF REMOVAL Following are the steps that should be taken into consideration for the removal of phosphosilicate glass (PSG) from the wafer surface

1 Firstly wafers are washed combinedly with HF HCL and DI water in a tank for 1min 2 Again wafers are washed with HF and DI water for 2min in the other tank 3 For another 1 min DI water is being sprayed on the diffused wafers 4 Next wafers are washed with DI water subsequently on an interval of 30 sec 5 Finally wafers are kept in the OVEN for 10mins within the temperature range of 95plusmn5ordmC

20

PLASMA ENHANCED CHEMICAL VAPOUR DEPOSITION INTRODUCTION In recent years the low-temperature (le 400ordmC) surface passivation of crystalline silicon solar cells by means of PECVD silicon nitride films has attracted an increasing attention of the PV community These films have proven to be capable of combining an outstanding surface passivation quality with excellent anti-reflection properties On typical p-type silicon solar cells substrate it has been shown that the surface passivation quality of very silicon rich Si3N4

we must first know that whatrsquos the use of silicon nitride coating on the surface of the wafer

films is superior to that of state-of-the-art high temperature silicon dioxide Before proceeding to the details of PECVD system

WHY SILICON NITRIDE SURFACE COATING IS PREFERRED IN PECVD PROCESS It is preferred as a surface coating because it functions as An effective diffusion mask An effective metallization diffusion barrier An anti-reflective coating Provide surface and bulk passivation

Preferably hydrogen is trapped in such a silicon nitride surface coating PECVD SYSYTEM PECVD or Plasma Enhanced Chemical Vapor Deposition is one of the methods of different Chemical Vapor Deposition (CVD) technique In PECVD plasma is being used to decompose a reactant gas such as silane (SiH4) to produce reaction products that precipitate on the surface of the substrate as a new layer

Working Principle Inside a PECVD reactor a strong electric field ignites plasma between two electrodes one of which holds the substrate This plasma ignition cracks the molecular bonds of the process gas which in turn are able to crack more process gas molecules before reaching the surface of the substrate The glow discharge used in this reactor is created by applying an RF field to a low-pressure gas creating free electrons within the discharge region The electrons are sufficiently energized by the electric field that gas-phase dissociation and ionization of the reactant gases occur when the free electrons collide with them Energetic species are then adsorbed on the film surface where they are subjected to ion and electron bombardment rearrangements reactions with other species new bond formation and film formation and growth Eventually a new layer is deposited on the substrate For example when silane is used as precursor plasma ignition frees up Si and SiH radicals which also crack more silane molecules on their way to the surface of the substrate where silicon is deposited PROCESS DETAILS OF PECVD In this system wafers coming from the previous state ie PSG removal step are being loaded onto the graphite boat where the individual wafers are held by three pins inside the boat It is absolutely necessary for the wafers to touch all the three pins otherwise there will remain a possibility of poor deposition in which

Figure Anti-reflective coating made out of silicon nitride deposited by the PECVD technique whereby silane and ammonia is reacted in a furnace The major color of coating looks blue but the color could also look like purple blue or light blue due to different thickness of coating

21

the HF generator fails to generate the required power for process deposition While loading the double diffusion wafers it should be ensured that deposition side should be correctly loaded The wafers are being loaded parallel to the boat After loading the trolley along with the boat is placed inside the machine and then after filling the input parameters of the CMI interface used here and the process tube press the RUN button and then machine will be in auto mode The handling mechanism automatically picks up the boat and places it on the desired paddle corresponding to the selected for the specified job And then process STARTS

22

SCREEN PRINTING INTRODUCTION In screen printing on wafer-based solar PV cells the mesh and buses of silver are printed on the front furthermore the buses of silver are printed on the back Subsequently aluminum paste is dispensed over the whole surface of the back for passivation and surface reflection [14] One of the parameters that can vary and can be controlled in screen printing is the thickness of the print This makes it useful for some of the techniques of printing solar cells electronics etc Screen printing is one of the most critical processes to maintain high yield Solar wafers are becoming thinner and larger so careful printing is required to maintain a lower breakage rate On the other hand high throughput at the printing stage improves the throughput of the whole cell production line PROCESS This process is divided in three phase-

1 Phase I- AgAl printing 2 Phase II- Ag printing 3 Phase III- Al printing

PHASE I- AgAl PRINTING This phase is required to print the back bus bar for tabbing purpose The AgAl paste used in this printing is Dupont (PV-505) and the consumption of the paste per wafer is 011 gm And the screen type used is of the following specification

1 Mesh size- 200microm 2 Tension- 24 N-cm 3 Mesh angle- 225ordm 45ordm 4 Emulsion thickness- 24microm

Process details During printing the paste is poured on the screen and is spread over it with the help of polypropylene spatula Then the wafers (ARC coated) are pasted with the sunny side down on the loader and picked one by one automatically with the start of the print command Once printing starts the wafers are placed one by one between the tool paste and the screen and then the squeegee moves over the screen and allow the paste through the exposed parts on the screen and gets pasted on the wafers Thus in this way the wafers are printed one by one Two most important parameters to be controlled during this printing are

1 Squeegee Pressure 2 Snap off distance

The next step after printing is drying step DRYING In this step wafers are passed through the dryer with a temperature range of 180ordm-400ordmC PHASE II- Ag PRINTING This phase is required to print the front bus bar for tabbing purpose The Ag paste used in this printing is Dupont (PV-145) and the consumption of the paste per wafer is 014 gm And the screen type used is of the following specification

1 Mesh size- 325microm 2 Tension- 24 N-cm 3 Mesh angle- 225ordm 4 Emulsion thickness- 24microm

15

4 Stabilization-During this phase any overshooting of temperature during ramp-up phase is controlled to the predetermined set value and the time taken is 10min

5 Pre-deposition-Any excess POCL3 present in vapour form inside the quartz bubbler is vent out before the deposition starts by N2

6 Deposition- N purging in 5 sec

2 is bubbled through liquid POCL3 in presence of O2 to form P2O5

7 Post-deposition-During this phase O in paste form

2 and N2

8 N

flows to drive in the phosphorous from wafer surface to the junction

2

9 Unload-During this phase the SiC paddle moves slowly out of the furnace at set speed of 45rpm and maximum time taken is 7min

Purge-This phase is also known as drive in diffusion which is designed to evenly distribute the impurities into wafer and reduce the generally undesirable high concentration in the wafer surface and the maximum time taken is 7min

10 End-The tube is set to 800⁰C temperature in idle state

16

PLASMA ETCHING

INTRODUCTION

Plasma etching is one type of dry etching which uses plasma to produce chemically reactive species from inert gases The reactive gases are then made to react with the material to be etched The main objective of this process is to remove edge shunts present on the cell which leads to 80 loss and thus it is also termed as Edge isolation method Since edge isolation is the main motive of this process so first let us see that why edge isolation is so important

Edge isolation

In the standard processing sequence the entire wafer is phosphorous diffused The contact to the p-type base is made by firing an Al containing paste that produces a high doping of Al thus over-compensating the shallow emitter diffusion However there is still a direct conductive path from the emitter on the front side to the rear p contact Therefore a large portion of the photo-generated current can directly flow through the emitter to the back contact instead of flowing through the external circuit This current path is modeled in the two-diode model of the solar cell as the parallel resistance R

where R

p

s and Rp are the series and parallel resistance respectively j01 and j02 represent the first and second diode saturation current densities respectively n1 and n2 are the first and second ideality diode factors respectively Vth = ktq is the thermal voltage and jL

In this model the first diode accounts for emitter and bulk recombination with a recombination rate assumed to be proportional to the excess carrier density This leads to an ideality factor of n

is the light generated current density The two diode model is as shown below

1=1 The second diode is associated with the recombination in the space charge region that is assumed to be independent of the injection level leading to n2=2 The effect of increased j02 and Rp can be shown by model calculations for ideal solar cells in the figure below for a fill factor potential of 84 j02 must be less than 2x10-9 Acm2 and Rp must be greater than approximately 2000Ω cm2 if the respective parameters contribute solely to the fill factor loss

17

This increase in Rp is achieved by the edge isolation process that interrupts the parasitic current path as shown in fig2 below

Thus edge isolation is required in solar cell to increase open circuit voltage and thus leading to greater efficiency

Now we will move on to plasma etching process to see that how it works actually in solar cell and module manufacturing industries

Plasma Etching Process

After diffusion a layer of phosphosilicate glass is formed on the edges and rear surface of diffused wafer The process generally requires adequate physical masking of the front surface while exposing the full back of the cell to the dry plasma environment In an embodiment of this process cells are slotted face to face

Fig2Sketch of an industrial screen-printed solar cell The emitter diffusion is present on the complete wafer surface and gets locally overcompensated by printing and firing Al containing metal paste as back contact Without edge-isolation light generated e-s can flow through the emitter region to the back contact which corresponds to the small series resistance in the two diode model

18

into inert carriers so that the plasma chemistry can effectively react with the back of the cells thereby removing excess material from the second coating that might have reached the back of the cell This process is preferably performed via the generation of an RF (13 6 MHz) plasma using a mix of Freon 14 (CF4) with oxygen (O2) and or nitrogen (N2) in a ratio which depends on the chamber and load geometries

19

PHOSPHOSILICATE GLASS REMOVAL INTRODUCTION In diffusion process during deposition phase N2 gas is bubbled through liquid POCL3

The reaction related to this phase is given as follows

1 4POCL3 + 3O2 -gt 2P2O5 + 6Cl 2 2P

2 2O5 + 5Si -gt 5SiO2

+ 4P (PSG)

P2O5

So it becomes mandatory to remove phosphosilicate glass by chemicals which may act as recombination sites or may contribute towards the surface density defect and hence all these defects may contribute to a lower voltage and current

obtained in first reaction step is in paste form and is transferred to the Teflon duct to the process tube which then reacts with silicon of which the wafer is made And thus when this phosphorous pentoxide reaches silicon surface then second reaction takes place and thus a silicon dioxide layer and phosphosilicate glass ie PSG forms on the wafer surface Formation of this dead layer on the wafer surface contributes to a lower efficiency

PROCESS OF REMOVAL Following are the steps that should be taken into consideration for the removal of phosphosilicate glass (PSG) from the wafer surface

1 Firstly wafers are washed combinedly with HF HCL and DI water in a tank for 1min 2 Again wafers are washed with HF and DI water for 2min in the other tank 3 For another 1 min DI water is being sprayed on the diffused wafers 4 Next wafers are washed with DI water subsequently on an interval of 30 sec 5 Finally wafers are kept in the OVEN for 10mins within the temperature range of 95plusmn5ordmC

20

PLASMA ENHANCED CHEMICAL VAPOUR DEPOSITION INTRODUCTION In recent years the low-temperature (le 400ordmC) surface passivation of crystalline silicon solar cells by means of PECVD silicon nitride films has attracted an increasing attention of the PV community These films have proven to be capable of combining an outstanding surface passivation quality with excellent anti-reflection properties On typical p-type silicon solar cells substrate it has been shown that the surface passivation quality of very silicon rich Si3N4

we must first know that whatrsquos the use of silicon nitride coating on the surface of the wafer

films is superior to that of state-of-the-art high temperature silicon dioxide Before proceeding to the details of PECVD system

WHY SILICON NITRIDE SURFACE COATING IS PREFERRED IN PECVD PROCESS It is preferred as a surface coating because it functions as An effective diffusion mask An effective metallization diffusion barrier An anti-reflective coating Provide surface and bulk passivation

Preferably hydrogen is trapped in such a silicon nitride surface coating PECVD SYSYTEM PECVD or Plasma Enhanced Chemical Vapor Deposition is one of the methods of different Chemical Vapor Deposition (CVD) technique In PECVD plasma is being used to decompose a reactant gas such as silane (SiH4) to produce reaction products that precipitate on the surface of the substrate as a new layer

Working Principle Inside a PECVD reactor a strong electric field ignites plasma between two electrodes one of which holds the substrate This plasma ignition cracks the molecular bonds of the process gas which in turn are able to crack more process gas molecules before reaching the surface of the substrate The glow discharge used in this reactor is created by applying an RF field to a low-pressure gas creating free electrons within the discharge region The electrons are sufficiently energized by the electric field that gas-phase dissociation and ionization of the reactant gases occur when the free electrons collide with them Energetic species are then adsorbed on the film surface where they are subjected to ion and electron bombardment rearrangements reactions with other species new bond formation and film formation and growth Eventually a new layer is deposited on the substrate For example when silane is used as precursor plasma ignition frees up Si and SiH radicals which also crack more silane molecules on their way to the surface of the substrate where silicon is deposited PROCESS DETAILS OF PECVD In this system wafers coming from the previous state ie PSG removal step are being loaded onto the graphite boat where the individual wafers are held by three pins inside the boat It is absolutely necessary for the wafers to touch all the three pins otherwise there will remain a possibility of poor deposition in which

Figure Anti-reflective coating made out of silicon nitride deposited by the PECVD technique whereby silane and ammonia is reacted in a furnace The major color of coating looks blue but the color could also look like purple blue or light blue due to different thickness of coating

21

the HF generator fails to generate the required power for process deposition While loading the double diffusion wafers it should be ensured that deposition side should be correctly loaded The wafers are being loaded parallel to the boat After loading the trolley along with the boat is placed inside the machine and then after filling the input parameters of the CMI interface used here and the process tube press the RUN button and then machine will be in auto mode The handling mechanism automatically picks up the boat and places it on the desired paddle corresponding to the selected for the specified job And then process STARTS

22

SCREEN PRINTING INTRODUCTION In screen printing on wafer-based solar PV cells the mesh and buses of silver are printed on the front furthermore the buses of silver are printed on the back Subsequently aluminum paste is dispensed over the whole surface of the back for passivation and surface reflection [14] One of the parameters that can vary and can be controlled in screen printing is the thickness of the print This makes it useful for some of the techniques of printing solar cells electronics etc Screen printing is one of the most critical processes to maintain high yield Solar wafers are becoming thinner and larger so careful printing is required to maintain a lower breakage rate On the other hand high throughput at the printing stage improves the throughput of the whole cell production line PROCESS This process is divided in three phase-

1 Phase I- AgAl printing 2 Phase II- Ag printing 3 Phase III- Al printing

PHASE I- AgAl PRINTING This phase is required to print the back bus bar for tabbing purpose The AgAl paste used in this printing is Dupont (PV-505) and the consumption of the paste per wafer is 011 gm And the screen type used is of the following specification

1 Mesh size- 200microm 2 Tension- 24 N-cm 3 Mesh angle- 225ordm 45ordm 4 Emulsion thickness- 24microm

Process details During printing the paste is poured on the screen and is spread over it with the help of polypropylene spatula Then the wafers (ARC coated) are pasted with the sunny side down on the loader and picked one by one automatically with the start of the print command Once printing starts the wafers are placed one by one between the tool paste and the screen and then the squeegee moves over the screen and allow the paste through the exposed parts on the screen and gets pasted on the wafers Thus in this way the wafers are printed one by one Two most important parameters to be controlled during this printing are

1 Squeegee Pressure 2 Snap off distance

The next step after printing is drying step DRYING In this step wafers are passed through the dryer with a temperature range of 180ordm-400ordmC PHASE II- Ag PRINTING This phase is required to print the front bus bar for tabbing purpose The Ag paste used in this printing is Dupont (PV-145) and the consumption of the paste per wafer is 014 gm And the screen type used is of the following specification

1 Mesh size- 325microm 2 Tension- 24 N-cm 3 Mesh angle- 225ordm 4 Emulsion thickness- 24microm

16

PLASMA ETCHING

INTRODUCTION

Plasma etching is one type of dry etching which uses plasma to produce chemically reactive species from inert gases The reactive gases are then made to react with the material to be etched The main objective of this process is to remove edge shunts present on the cell which leads to 80 loss and thus it is also termed as Edge isolation method Since edge isolation is the main motive of this process so first let us see that why edge isolation is so important

Edge isolation

In the standard processing sequence the entire wafer is phosphorous diffused The contact to the p-type base is made by firing an Al containing paste that produces a high doping of Al thus over-compensating the shallow emitter diffusion However there is still a direct conductive path from the emitter on the front side to the rear p contact Therefore a large portion of the photo-generated current can directly flow through the emitter to the back contact instead of flowing through the external circuit This current path is modeled in the two-diode model of the solar cell as the parallel resistance R

where R

p

s and Rp are the series and parallel resistance respectively j01 and j02 represent the first and second diode saturation current densities respectively n1 and n2 are the first and second ideality diode factors respectively Vth = ktq is the thermal voltage and jL

In this model the first diode accounts for emitter and bulk recombination with a recombination rate assumed to be proportional to the excess carrier density This leads to an ideality factor of n

is the light generated current density The two diode model is as shown below

1=1 The second diode is associated with the recombination in the space charge region that is assumed to be independent of the injection level leading to n2=2 The effect of increased j02 and Rp can be shown by model calculations for ideal solar cells in the figure below for a fill factor potential of 84 j02 must be less than 2x10-9 Acm2 and Rp must be greater than approximately 2000Ω cm2 if the respective parameters contribute solely to the fill factor loss

17

This increase in Rp is achieved by the edge isolation process that interrupts the parasitic current path as shown in fig2 below

Thus edge isolation is required in solar cell to increase open circuit voltage and thus leading to greater efficiency

Now we will move on to plasma etching process to see that how it works actually in solar cell and module manufacturing industries

Plasma Etching Process

After diffusion a layer of phosphosilicate glass is formed on the edges and rear surface of diffused wafer The process generally requires adequate physical masking of the front surface while exposing the full back of the cell to the dry plasma environment In an embodiment of this process cells are slotted face to face

Fig2Sketch of an industrial screen-printed solar cell The emitter diffusion is present on the complete wafer surface and gets locally overcompensated by printing and firing Al containing metal paste as back contact Without edge-isolation light generated e-s can flow through the emitter region to the back contact which corresponds to the small series resistance in the two diode model

18

into inert carriers so that the plasma chemistry can effectively react with the back of the cells thereby removing excess material from the second coating that might have reached the back of the cell This process is preferably performed via the generation of an RF (13 6 MHz) plasma using a mix of Freon 14 (CF4) with oxygen (O2) and or nitrogen (N2) in a ratio which depends on the chamber and load geometries

19

PHOSPHOSILICATE GLASS REMOVAL INTRODUCTION In diffusion process during deposition phase N2 gas is bubbled through liquid POCL3

The reaction related to this phase is given as follows

1 4POCL3 + 3O2 -gt 2P2O5 + 6Cl 2 2P

2 2O5 + 5Si -gt 5SiO2

+ 4P (PSG)

P2O5

So it becomes mandatory to remove phosphosilicate glass by chemicals which may act as recombination sites or may contribute towards the surface density defect and hence all these defects may contribute to a lower voltage and current

obtained in first reaction step is in paste form and is transferred to the Teflon duct to the process tube which then reacts with silicon of which the wafer is made And thus when this phosphorous pentoxide reaches silicon surface then second reaction takes place and thus a silicon dioxide layer and phosphosilicate glass ie PSG forms on the wafer surface Formation of this dead layer on the wafer surface contributes to a lower efficiency

PROCESS OF REMOVAL Following are the steps that should be taken into consideration for the removal of phosphosilicate glass (PSG) from the wafer surface

1 Firstly wafers are washed combinedly with HF HCL and DI water in a tank for 1min 2 Again wafers are washed with HF and DI water for 2min in the other tank 3 For another 1 min DI water is being sprayed on the diffused wafers 4 Next wafers are washed with DI water subsequently on an interval of 30 sec 5 Finally wafers are kept in the OVEN for 10mins within the temperature range of 95plusmn5ordmC

20

PLASMA ENHANCED CHEMICAL VAPOUR DEPOSITION INTRODUCTION In recent years the low-temperature (le 400ordmC) surface passivation of crystalline silicon solar cells by means of PECVD silicon nitride films has attracted an increasing attention of the PV community These films have proven to be capable of combining an outstanding surface passivation quality with excellent anti-reflection properties On typical p-type silicon solar cells substrate it has been shown that the surface passivation quality of very silicon rich Si3N4

we must first know that whatrsquos the use of silicon nitride coating on the surface of the wafer

films is superior to that of state-of-the-art high temperature silicon dioxide Before proceeding to the details of PECVD system

WHY SILICON NITRIDE SURFACE COATING IS PREFERRED IN PECVD PROCESS It is preferred as a surface coating because it functions as An effective diffusion mask An effective metallization diffusion barrier An anti-reflective coating Provide surface and bulk passivation

Preferably hydrogen is trapped in such a silicon nitride surface coating PECVD SYSYTEM PECVD or Plasma Enhanced Chemical Vapor Deposition is one of the methods of different Chemical Vapor Deposition (CVD) technique In PECVD plasma is being used to decompose a reactant gas such as silane (SiH4) to produce reaction products that precipitate on the surface of the substrate as a new layer

Working Principle Inside a PECVD reactor a strong electric field ignites plasma between two electrodes one of which holds the substrate This plasma ignition cracks the molecular bonds of the process gas which in turn are able to crack more process gas molecules before reaching the surface of the substrate The glow discharge used in this reactor is created by applying an RF field to a low-pressure gas creating free electrons within the discharge region The electrons are sufficiently energized by the electric field that gas-phase dissociation and ionization of the reactant gases occur when the free electrons collide with them Energetic species are then adsorbed on the film surface where they are subjected to ion and electron bombardment rearrangements reactions with other species new bond formation and film formation and growth Eventually a new layer is deposited on the substrate For example when silane is used as precursor plasma ignition frees up Si and SiH radicals which also crack more silane molecules on their way to the surface of the substrate where silicon is deposited PROCESS DETAILS OF PECVD In this system wafers coming from the previous state ie PSG removal step are being loaded onto the graphite boat where the individual wafers are held by three pins inside the boat It is absolutely necessary for the wafers to touch all the three pins otherwise there will remain a possibility of poor deposition in which

Figure Anti-reflective coating made out of silicon nitride deposited by the PECVD technique whereby silane and ammonia is reacted in a furnace The major color of coating looks blue but the color could also look like purple blue or light blue due to different thickness of coating

21

the HF generator fails to generate the required power for process deposition While loading the double diffusion wafers it should be ensured that deposition side should be correctly loaded The wafers are being loaded parallel to the boat After loading the trolley along with the boat is placed inside the machine and then after filling the input parameters of the CMI interface used here and the process tube press the RUN button and then machine will be in auto mode The handling mechanism automatically picks up the boat and places it on the desired paddle corresponding to the selected for the specified job And then process STARTS

22

SCREEN PRINTING INTRODUCTION In screen printing on wafer-based solar PV cells the mesh and buses of silver are printed on the front furthermore the buses of silver are printed on the back Subsequently aluminum paste is dispensed over the whole surface of the back for passivation and surface reflection [14] One of the parameters that can vary and can be controlled in screen printing is the thickness of the print This makes it useful for some of the techniques of printing solar cells electronics etc Screen printing is one of the most critical processes to maintain high yield Solar wafers are becoming thinner and larger so careful printing is required to maintain a lower breakage rate On the other hand high throughput at the printing stage improves the throughput of the whole cell production line PROCESS This process is divided in three phase-

1 Phase I- AgAl printing 2 Phase II- Ag printing 3 Phase III- Al printing

PHASE I- AgAl PRINTING This phase is required to print the back bus bar for tabbing purpose The AgAl paste used in this printing is Dupont (PV-505) and the consumption of the paste per wafer is 011 gm And the screen type used is of the following specification

1 Mesh size- 200microm 2 Tension- 24 N-cm 3 Mesh angle- 225ordm 45ordm 4 Emulsion thickness- 24microm

Process details During printing the paste is poured on the screen and is spread over it with the help of polypropylene spatula Then the wafers (ARC coated) are pasted with the sunny side down on the loader and picked one by one automatically with the start of the print command Once printing starts the wafers are placed one by one between the tool paste and the screen and then the squeegee moves over the screen and allow the paste through the exposed parts on the screen and gets pasted on the wafers Thus in this way the wafers are printed one by one Two most important parameters to be controlled during this printing are

1 Squeegee Pressure 2 Snap off distance

The next step after printing is drying step DRYING In this step wafers are passed through the dryer with a temperature range of 180ordm-400ordmC PHASE II- Ag PRINTING This phase is required to print the front bus bar for tabbing purpose The Ag paste used in this printing is Dupont (PV-145) and the consumption of the paste per wafer is 014 gm And the screen type used is of the following specification

1 Mesh size- 325microm 2 Tension- 24 N-cm 3 Mesh angle- 225ordm 4 Emulsion thickness- 24microm

17

This increase in Rp is achieved by the edge isolation process that interrupts the parasitic current path as shown in fig2 below

Thus edge isolation is required in solar cell to increase open circuit voltage and thus leading to greater efficiency

Now we will move on to plasma etching process to see that how it works actually in solar cell and module manufacturing industries

Plasma Etching Process

After diffusion a layer of phosphosilicate glass is formed on the edges and rear surface of diffused wafer The process generally requires adequate physical masking of the front surface while exposing the full back of the cell to the dry plasma environment In an embodiment of this process cells are slotted face to face

Fig2Sketch of an industrial screen-printed solar cell The emitter diffusion is present on the complete wafer surface and gets locally overcompensated by printing and firing Al containing metal paste as back contact Without edge-isolation light generated e-s can flow through the emitter region to the back contact which corresponds to the small series resistance in the two diode model

18

into inert carriers so that the plasma chemistry can effectively react with the back of the cells thereby removing excess material from the second coating that might have reached the back of the cell This process is preferably performed via the generation of an RF (13 6 MHz) plasma using a mix of Freon 14 (CF4) with oxygen (O2) and or nitrogen (N2) in a ratio which depends on the chamber and load geometries

19

PHOSPHOSILICATE GLASS REMOVAL INTRODUCTION In diffusion process during deposition phase N2 gas is bubbled through liquid POCL3

The reaction related to this phase is given as follows

1 4POCL3 + 3O2 -gt 2P2O5 + 6Cl 2 2P

2 2O5 + 5Si -gt 5SiO2

+ 4P (PSG)

P2O5

So it becomes mandatory to remove phosphosilicate glass by chemicals which may act as recombination sites or may contribute towards the surface density defect and hence all these defects may contribute to a lower voltage and current

obtained in first reaction step is in paste form and is transferred to the Teflon duct to the process tube which then reacts with silicon of which the wafer is made And thus when this phosphorous pentoxide reaches silicon surface then second reaction takes place and thus a silicon dioxide layer and phosphosilicate glass ie PSG forms on the wafer surface Formation of this dead layer on the wafer surface contributes to a lower efficiency

PROCESS OF REMOVAL Following are the steps that should be taken into consideration for the removal of phosphosilicate glass (PSG) from the wafer surface

1 Firstly wafers are washed combinedly with HF HCL and DI water in a tank for 1min 2 Again wafers are washed with HF and DI water for 2min in the other tank 3 For another 1 min DI water is being sprayed on the diffused wafers 4 Next wafers are washed with DI water subsequently on an interval of 30 sec 5 Finally wafers are kept in the OVEN for 10mins within the temperature range of 95plusmn5ordmC

20

PLASMA ENHANCED CHEMICAL VAPOUR DEPOSITION INTRODUCTION In recent years the low-temperature (le 400ordmC) surface passivation of crystalline silicon solar cells by means of PECVD silicon nitride films has attracted an increasing attention of the PV community These films have proven to be capable of combining an outstanding surface passivation quality with excellent anti-reflection properties On typical p-type silicon solar cells substrate it has been shown that the surface passivation quality of very silicon rich Si3N4

we must first know that whatrsquos the use of silicon nitride coating on the surface of the wafer

films is superior to that of state-of-the-art high temperature silicon dioxide Before proceeding to the details of PECVD system

WHY SILICON NITRIDE SURFACE COATING IS PREFERRED IN PECVD PROCESS It is preferred as a surface coating because it functions as An effective diffusion mask An effective metallization diffusion barrier An anti-reflective coating Provide surface and bulk passivation

Preferably hydrogen is trapped in such a silicon nitride surface coating PECVD SYSYTEM PECVD or Plasma Enhanced Chemical Vapor Deposition is one of the methods of different Chemical Vapor Deposition (CVD) technique In PECVD plasma is being used to decompose a reactant gas such as silane (SiH4) to produce reaction products that precipitate on the surface of the substrate as a new layer

Working Principle Inside a PECVD reactor a strong electric field ignites plasma between two electrodes one of which holds the substrate This plasma ignition cracks the molecular bonds of the process gas which in turn are able to crack more process gas molecules before reaching the surface of the substrate The glow discharge used in this reactor is created by applying an RF field to a low-pressure gas creating free electrons within the discharge region The electrons are sufficiently energized by the electric field that gas-phase dissociation and ionization of the reactant gases occur when the free electrons collide with them Energetic species are then adsorbed on the film surface where they are subjected to ion and electron bombardment rearrangements reactions with other species new bond formation and film formation and growth Eventually a new layer is deposited on the substrate For example when silane is used as precursor plasma ignition frees up Si and SiH radicals which also crack more silane molecules on their way to the surface of the substrate where silicon is deposited PROCESS DETAILS OF PECVD In this system wafers coming from the previous state ie PSG removal step are being loaded onto the graphite boat where the individual wafers are held by three pins inside the boat It is absolutely necessary for the wafers to touch all the three pins otherwise there will remain a possibility of poor deposition in which

Figure Anti-reflective coating made out of silicon nitride deposited by the PECVD technique whereby silane and ammonia is reacted in a furnace The major color of coating looks blue but the color could also look like purple blue or light blue due to different thickness of coating

21

the HF generator fails to generate the required power for process deposition While loading the double diffusion wafers it should be ensured that deposition side should be correctly loaded The wafers are being loaded parallel to the boat After loading the trolley along with the boat is placed inside the machine and then after filling the input parameters of the CMI interface used here and the process tube press the RUN button and then machine will be in auto mode The handling mechanism automatically picks up the boat and places it on the desired paddle corresponding to the selected for the specified job And then process STARTS

22

SCREEN PRINTING INTRODUCTION In screen printing on wafer-based solar PV cells the mesh and buses of silver are printed on the front furthermore the buses of silver are printed on the back Subsequently aluminum paste is dispensed over the whole surface of the back for passivation and surface reflection [14] One of the parameters that can vary and can be controlled in screen printing is the thickness of the print This makes it useful for some of the techniques of printing solar cells electronics etc Screen printing is one of the most critical processes to maintain high yield Solar wafers are becoming thinner and larger so careful printing is required to maintain a lower breakage rate On the other hand high throughput at the printing stage improves the throughput of the whole cell production line PROCESS This process is divided in three phase-

1 Phase I- AgAl printing 2 Phase II- Ag printing 3 Phase III- Al printing

PHASE I- AgAl PRINTING This phase is required to print the back bus bar for tabbing purpose The AgAl paste used in this printing is Dupont (PV-505) and the consumption of the paste per wafer is 011 gm And the screen type used is of the following specification

1 Mesh size- 200microm 2 Tension- 24 N-cm 3 Mesh angle- 225ordm 45ordm 4 Emulsion thickness- 24microm

Process details During printing the paste is poured on the screen and is spread over it with the help of polypropylene spatula Then the wafers (ARC coated) are pasted with the sunny side down on the loader and picked one by one automatically with the start of the print command Once printing starts the wafers are placed one by one between the tool paste and the screen and then the squeegee moves over the screen and allow the paste through the exposed parts on the screen and gets pasted on the wafers Thus in this way the wafers are printed one by one Two most important parameters to be controlled during this printing are

1 Squeegee Pressure 2 Snap off distance

The next step after printing is drying step DRYING In this step wafers are passed through the dryer with a temperature range of 180ordm-400ordmC PHASE II- Ag PRINTING This phase is required to print the front bus bar for tabbing purpose The Ag paste used in this printing is Dupont (PV-145) and the consumption of the paste per wafer is 014 gm And the screen type used is of the following specification

1 Mesh size- 325microm 2 Tension- 24 N-cm 3 Mesh angle- 225ordm 4 Emulsion thickness- 24microm

18

into inert carriers so that the plasma chemistry can effectively react with the back of the cells thereby removing excess material from the second coating that might have reached the back of the cell This process is preferably performed via the generation of an RF (13 6 MHz) plasma using a mix of Freon 14 (CF4) with oxygen (O2) and or nitrogen (N2) in a ratio which depends on the chamber and load geometries

19

PHOSPHOSILICATE GLASS REMOVAL INTRODUCTION In diffusion process during deposition phase N2 gas is bubbled through liquid POCL3

The reaction related to this phase is given as follows

1 4POCL3 + 3O2 -gt 2P2O5 + 6Cl 2 2P

2 2O5 + 5Si -gt 5SiO2

+ 4P (PSG)

P2O5

So it becomes mandatory to remove phosphosilicate glass by chemicals which may act as recombination sites or may contribute towards the surface density defect and hence all these defects may contribute to a lower voltage and current

obtained in first reaction step is in paste form and is transferred to the Teflon duct to the process tube which then reacts with silicon of which the wafer is made And thus when this phosphorous pentoxide reaches silicon surface then second reaction takes place and thus a silicon dioxide layer and phosphosilicate glass ie PSG forms on the wafer surface Formation of this dead layer on the wafer surface contributes to a lower efficiency

PROCESS OF REMOVAL Following are the steps that should be taken into consideration for the removal of phosphosilicate glass (PSG) from the wafer surface

1 Firstly wafers are washed combinedly with HF HCL and DI water in a tank for 1min 2 Again wafers are washed with HF and DI water for 2min in the other tank 3 For another 1 min DI water is being sprayed on the diffused wafers 4 Next wafers are washed with DI water subsequently on an interval of 30 sec 5 Finally wafers are kept in the OVEN for 10mins within the temperature range of 95plusmn5ordmC

20

PLASMA ENHANCED CHEMICAL VAPOUR DEPOSITION INTRODUCTION In recent years the low-temperature (le 400ordmC) surface passivation of crystalline silicon solar cells by means of PECVD silicon nitride films has attracted an increasing attention of the PV community These films have proven to be capable of combining an outstanding surface passivation quality with excellent anti-reflection properties On typical p-type silicon solar cells substrate it has been shown that the surface passivation quality of very silicon rich Si3N4

we must first know that whatrsquos the use of silicon nitride coating on the surface of the wafer

films is superior to that of state-of-the-art high temperature silicon dioxide Before proceeding to the details of PECVD system

WHY SILICON NITRIDE SURFACE COATING IS PREFERRED IN PECVD PROCESS It is preferred as a surface coating because it functions as An effective diffusion mask An effective metallization diffusion barrier An anti-reflective coating Provide surface and bulk passivation

Preferably hydrogen is trapped in such a silicon nitride surface coating PECVD SYSYTEM PECVD or Plasma Enhanced Chemical Vapor Deposition is one of the methods of different Chemical Vapor Deposition (CVD) technique In PECVD plasma is being used to decompose a reactant gas such as silane (SiH4) to produce reaction products that precipitate on the surface of the substrate as a new layer

Working Principle Inside a PECVD reactor a strong electric field ignites plasma between two electrodes one of which holds the substrate This plasma ignition cracks the molecular bonds of the process gas which in turn are able to crack more process gas molecules before reaching the surface of the substrate The glow discharge used in this reactor is created by applying an RF field to a low-pressure gas creating free electrons within the discharge region The electrons are sufficiently energized by the electric field that gas-phase dissociation and ionization of the reactant gases occur when the free electrons collide with them Energetic species are then adsorbed on the film surface where they are subjected to ion and electron bombardment rearrangements reactions with other species new bond formation and film formation and growth Eventually a new layer is deposited on the substrate For example when silane is used as precursor plasma ignition frees up Si and SiH radicals which also crack more silane molecules on their way to the surface of the substrate where silicon is deposited PROCESS DETAILS OF PECVD In this system wafers coming from the previous state ie PSG removal step are being loaded onto the graphite boat where the individual wafers are held by three pins inside the boat It is absolutely necessary for the wafers to touch all the three pins otherwise there will remain a possibility of poor deposition in which

Figure Anti-reflective coating made out of silicon nitride deposited by the PECVD technique whereby silane and ammonia is reacted in a furnace The major color of coating looks blue but the color could also look like purple blue or light blue due to different thickness of coating

21

the HF generator fails to generate the required power for process deposition While loading the double diffusion wafers it should be ensured that deposition side should be correctly loaded The wafers are being loaded parallel to the boat After loading the trolley along with the boat is placed inside the machine and then after filling the input parameters of the CMI interface used here and the process tube press the RUN button and then machine will be in auto mode The handling mechanism automatically picks up the boat and places it on the desired paddle corresponding to the selected for the specified job And then process STARTS

22

SCREEN PRINTING INTRODUCTION In screen printing on wafer-based solar PV cells the mesh and buses of silver are printed on the front furthermore the buses of silver are printed on the back Subsequently aluminum paste is dispensed over the whole surface of the back for passivation and surface reflection [14] One of the parameters that can vary and can be controlled in screen printing is the thickness of the print This makes it useful for some of the techniques of printing solar cells electronics etc Screen printing is one of the most critical processes to maintain high yield Solar wafers are becoming thinner and larger so careful printing is required to maintain a lower breakage rate On the other hand high throughput at the printing stage improves the throughput of the whole cell production line PROCESS This process is divided in three phase-

1 Phase I- AgAl printing 2 Phase II- Ag printing 3 Phase III- Al printing

PHASE I- AgAl PRINTING This phase is required to print the back bus bar for tabbing purpose The AgAl paste used in this printing is Dupont (PV-505) and the consumption of the paste per wafer is 011 gm And the screen type used is of the following specification

1 Mesh size- 200microm 2 Tension- 24 N-cm 3 Mesh angle- 225ordm 45ordm 4 Emulsion thickness- 24microm

Process details During printing the paste is poured on the screen and is spread over it with the help of polypropylene spatula Then the wafers (ARC coated) are pasted with the sunny side down on the loader and picked one by one automatically with the start of the print command Once printing starts the wafers are placed one by one between the tool paste and the screen and then the squeegee moves over the screen and allow the paste through the exposed parts on the screen and gets pasted on the wafers Thus in this way the wafers are printed one by one Two most important parameters to be controlled during this printing are

1 Squeegee Pressure 2 Snap off distance

The next step after printing is drying step DRYING In this step wafers are passed through the dryer with a temperature range of 180ordm-400ordmC PHASE II- Ag PRINTING This phase is required to print the front bus bar for tabbing purpose The Ag paste used in this printing is Dupont (PV-145) and the consumption of the paste per wafer is 014 gm And the screen type used is of the following specification

1 Mesh size- 325microm 2 Tension- 24 N-cm 3 Mesh angle- 225ordm 4 Emulsion thickness- 24microm

19

PHOSPHOSILICATE GLASS REMOVAL INTRODUCTION In diffusion process during deposition phase N2 gas is bubbled through liquid POCL3

The reaction related to this phase is given as follows

1 4POCL3 + 3O2 -gt 2P2O5 + 6Cl 2 2P

2 2O5 + 5Si -gt 5SiO2

+ 4P (PSG)

P2O5

So it becomes mandatory to remove phosphosilicate glass by chemicals which may act as recombination sites or may contribute towards the surface density defect and hence all these defects may contribute to a lower voltage and current

obtained in first reaction step is in paste form and is transferred to the Teflon duct to the process tube which then reacts with silicon of which the wafer is made And thus when this phosphorous pentoxide reaches silicon surface then second reaction takes place and thus a silicon dioxide layer and phosphosilicate glass ie PSG forms on the wafer surface Formation of this dead layer on the wafer surface contributes to a lower efficiency

PROCESS OF REMOVAL Following are the steps that should be taken into consideration for the removal of phosphosilicate glass (PSG) from the wafer surface

1 Firstly wafers are washed combinedly with HF HCL and DI water in a tank for 1min 2 Again wafers are washed with HF and DI water for 2min in the other tank 3 For another 1 min DI water is being sprayed on the diffused wafers 4 Next wafers are washed with DI water subsequently on an interval of 30 sec 5 Finally wafers are kept in the OVEN for 10mins within the temperature range of 95plusmn5ordmC

20

PLASMA ENHANCED CHEMICAL VAPOUR DEPOSITION INTRODUCTION In recent years the low-temperature (le 400ordmC) surface passivation of crystalline silicon solar cells by means of PECVD silicon nitride films has attracted an increasing attention of the PV community These films have proven to be capable of combining an outstanding surface passivation quality with excellent anti-reflection properties On typical p-type silicon solar cells substrate it has been shown that the surface passivation quality of very silicon rich Si3N4

we must first know that whatrsquos the use of silicon nitride coating on the surface of the wafer

films is superior to that of state-of-the-art high temperature silicon dioxide Before proceeding to the details of PECVD system

WHY SILICON NITRIDE SURFACE COATING IS PREFERRED IN PECVD PROCESS It is preferred as a surface coating because it functions as An effective diffusion mask An effective metallization diffusion barrier An anti-reflective coating Provide surface and bulk passivation

Preferably hydrogen is trapped in such a silicon nitride surface coating PECVD SYSYTEM PECVD or Plasma Enhanced Chemical Vapor Deposition is one of the methods of different Chemical Vapor Deposition (CVD) technique In PECVD plasma is being used to decompose a reactant gas such as silane (SiH4) to produce reaction products that precipitate on the surface of the substrate as a new layer

Working Principle Inside a PECVD reactor a strong electric field ignites plasma between two electrodes one of which holds the substrate This plasma ignition cracks the molecular bonds of the process gas which in turn are able to crack more process gas molecules before reaching the surface of the substrate The glow discharge used in this reactor is created by applying an RF field to a low-pressure gas creating free electrons within the discharge region The electrons are sufficiently energized by the electric field that gas-phase dissociation and ionization of the reactant gases occur when the free electrons collide with them Energetic species are then adsorbed on the film surface where they are subjected to ion and electron bombardment rearrangements reactions with other species new bond formation and film formation and growth Eventually a new layer is deposited on the substrate For example when silane is used as precursor plasma ignition frees up Si and SiH radicals which also crack more silane molecules on their way to the surface of the substrate where silicon is deposited PROCESS DETAILS OF PECVD In this system wafers coming from the previous state ie PSG removal step are being loaded onto the graphite boat where the individual wafers are held by three pins inside the boat It is absolutely necessary for the wafers to touch all the three pins otherwise there will remain a possibility of poor deposition in which

Figure Anti-reflective coating made out of silicon nitride deposited by the PECVD technique whereby silane and ammonia is reacted in a furnace The major color of coating looks blue but the color could also look like purple blue or light blue due to different thickness of coating

21

the HF generator fails to generate the required power for process deposition While loading the double diffusion wafers it should be ensured that deposition side should be correctly loaded The wafers are being loaded parallel to the boat After loading the trolley along with the boat is placed inside the machine and then after filling the input parameters of the CMI interface used here and the process tube press the RUN button and then machine will be in auto mode The handling mechanism automatically picks up the boat and places it on the desired paddle corresponding to the selected for the specified job And then process STARTS

22

SCREEN PRINTING INTRODUCTION In screen printing on wafer-based solar PV cells the mesh and buses of silver are printed on the front furthermore the buses of silver are printed on the back Subsequently aluminum paste is dispensed over the whole surface of the back for passivation and surface reflection [14] One of the parameters that can vary and can be controlled in screen printing is the thickness of the print This makes it useful for some of the techniques of printing solar cells electronics etc Screen printing is one of the most critical processes to maintain high yield Solar wafers are becoming thinner and larger so careful printing is required to maintain a lower breakage rate On the other hand high throughput at the printing stage improves the throughput of the whole cell production line PROCESS This process is divided in three phase-

1 Phase I- AgAl printing 2 Phase II- Ag printing 3 Phase III- Al printing

PHASE I- AgAl PRINTING This phase is required to print the back bus bar for tabbing purpose The AgAl paste used in this printing is Dupont (PV-505) and the consumption of the paste per wafer is 011 gm And the screen type used is of the following specification

1 Mesh size- 200microm 2 Tension- 24 N-cm 3 Mesh angle- 225ordm 45ordm 4 Emulsion thickness- 24microm

Process details During printing the paste is poured on the screen and is spread over it with the help of polypropylene spatula Then the wafers (ARC coated) are pasted with the sunny side down on the loader and picked one by one automatically with the start of the print command Once printing starts the wafers are placed one by one between the tool paste and the screen and then the squeegee moves over the screen and allow the paste through the exposed parts on the screen and gets pasted on the wafers Thus in this way the wafers are printed one by one Two most important parameters to be controlled during this printing are

1 Squeegee Pressure 2 Snap off distance

The next step after printing is drying step DRYING In this step wafers are passed through the dryer with a temperature range of 180ordm-400ordmC PHASE II- Ag PRINTING This phase is required to print the front bus bar for tabbing purpose The Ag paste used in this printing is Dupont (PV-145) and the consumption of the paste per wafer is 014 gm And the screen type used is of the following specification

1 Mesh size- 325microm 2 Tension- 24 N-cm 3 Mesh angle- 225ordm 4 Emulsion thickness- 24microm

20

PLASMA ENHANCED CHEMICAL VAPOUR DEPOSITION INTRODUCTION In recent years the low-temperature (le 400ordmC) surface passivation of crystalline silicon solar cells by means of PECVD silicon nitride films has attracted an increasing attention of the PV community These films have proven to be capable of combining an outstanding surface passivation quality with excellent anti-reflection properties On typical p-type silicon solar cells substrate it has been shown that the surface passivation quality of very silicon rich Si3N4

we must first know that whatrsquos the use of silicon nitride coating on the surface of the wafer

films is superior to that of state-of-the-art high temperature silicon dioxide Before proceeding to the details of PECVD system

WHY SILICON NITRIDE SURFACE COATING IS PREFERRED IN PECVD PROCESS It is preferred as a surface coating because it functions as An effective diffusion mask An effective metallization diffusion barrier An anti-reflective coating Provide surface and bulk passivation

Preferably hydrogen is trapped in such a silicon nitride surface coating PECVD SYSYTEM PECVD or Plasma Enhanced Chemical Vapor Deposition is one of the methods of different Chemical Vapor Deposition (CVD) technique In PECVD plasma is being used to decompose a reactant gas such as silane (SiH4) to produce reaction products that precipitate on the surface of the substrate as a new layer

Working Principle Inside a PECVD reactor a strong electric field ignites plasma between two electrodes one of which holds the substrate This plasma ignition cracks the molecular bonds of the process gas which in turn are able to crack more process gas molecules before reaching the surface of the substrate The glow discharge used in this reactor is created by applying an RF field to a low-pressure gas creating free electrons within the discharge region The electrons are sufficiently energized by the electric field that gas-phase dissociation and ionization of the reactant gases occur when the free electrons collide with them Energetic species are then adsorbed on the film surface where they are subjected to ion and electron bombardment rearrangements reactions with other species new bond formation and film formation and growth Eventually a new layer is deposited on the substrate For example when silane is used as precursor plasma ignition frees up Si and SiH radicals which also crack more silane molecules on their way to the surface of the substrate where silicon is deposited PROCESS DETAILS OF PECVD In this system wafers coming from the previous state ie PSG removal step are being loaded onto the graphite boat where the individual wafers are held by three pins inside the boat It is absolutely necessary for the wafers to touch all the three pins otherwise there will remain a possibility of poor deposition in which

Figure Anti-reflective coating made out of silicon nitride deposited by the PECVD technique whereby silane and ammonia is reacted in a furnace The major color of coating looks blue but the color could also look like purple blue or light blue due to different thickness of coating

21

the HF generator fails to generate the required power for process deposition While loading the double diffusion wafers it should be ensured that deposition side should be correctly loaded The wafers are being loaded parallel to the boat After loading the trolley along with the boat is placed inside the machine and then after filling the input parameters of the CMI interface used here and the process tube press the RUN button and then machine will be in auto mode The handling mechanism automatically picks up the boat and places it on the desired paddle corresponding to the selected for the specified job And then process STARTS

22

SCREEN PRINTING INTRODUCTION In screen printing on wafer-based solar PV cells the mesh and buses of silver are printed on the front furthermore the buses of silver are printed on the back Subsequently aluminum paste is dispensed over the whole surface of the back for passivation and surface reflection [14] One of the parameters that can vary and can be controlled in screen printing is the thickness of the print This makes it useful for some of the techniques of printing solar cells electronics etc Screen printing is one of the most critical processes to maintain high yield Solar wafers are becoming thinner and larger so careful printing is required to maintain a lower breakage rate On the other hand high throughput at the printing stage improves the throughput of the whole cell production line PROCESS This process is divided in three phase-

1 Phase I- AgAl printing 2 Phase II- Ag printing 3 Phase III- Al printing

PHASE I- AgAl PRINTING This phase is required to print the back bus bar for tabbing purpose The AgAl paste used in this printing is Dupont (PV-505) and the consumption of the paste per wafer is 011 gm And the screen type used is of the following specification

1 Mesh size- 200microm 2 Tension- 24 N-cm 3 Mesh angle- 225ordm 45ordm 4 Emulsion thickness- 24microm

Process details During printing the paste is poured on the screen and is spread over it with the help of polypropylene spatula Then the wafers (ARC coated) are pasted with the sunny side down on the loader and picked one by one automatically with the start of the print command Once printing starts the wafers are placed one by one between the tool paste and the screen and then the squeegee moves over the screen and allow the paste through the exposed parts on the screen and gets pasted on the wafers Thus in this way the wafers are printed one by one Two most important parameters to be controlled during this printing are

1 Squeegee Pressure 2 Snap off distance

The next step after printing is drying step DRYING In this step wafers are passed through the dryer with a temperature range of 180ordm-400ordmC PHASE II- Ag PRINTING This phase is required to print the front bus bar for tabbing purpose The Ag paste used in this printing is Dupont (PV-145) and the consumption of the paste per wafer is 014 gm And the screen type used is of the following specification

1 Mesh size- 325microm 2 Tension- 24 N-cm 3 Mesh angle- 225ordm 4 Emulsion thickness- 24microm

21

the HF generator fails to generate the required power for process deposition While loading the double diffusion wafers it should be ensured that deposition side should be correctly loaded The wafers are being loaded parallel to the boat After loading the trolley along with the boat is placed inside the machine and then after filling the input parameters of the CMI interface used here and the process tube press the RUN button and then machine will be in auto mode The handling mechanism automatically picks up the boat and places it on the desired paddle corresponding to the selected for the specified job And then process STARTS

22

SCREEN PRINTING INTRODUCTION In screen printing on wafer-based solar PV cells the mesh and buses of silver are printed on the front furthermore the buses of silver are printed on the back Subsequently aluminum paste is dispensed over the whole surface of the back for passivation and surface reflection [14] One of the parameters that can vary and can be controlled in screen printing is the thickness of the print This makes it useful for some of the techniques of printing solar cells electronics etc Screen printing is one of the most critical processes to maintain high yield Solar wafers are becoming thinner and larger so careful printing is required to maintain a lower breakage rate On the other hand high throughput at the printing stage improves the throughput of the whole cell production line PROCESS This process is divided in three phase-

1 Phase I- AgAl printing 2 Phase II- Ag printing 3 Phase III- Al printing

PHASE I- AgAl PRINTING This phase is required to print the back bus bar for tabbing purpose The AgAl paste used in this printing is Dupont (PV-505) and the consumption of the paste per wafer is 011 gm And the screen type used is of the following specification

1 Mesh size- 200microm 2 Tension- 24 N-cm 3 Mesh angle- 225ordm 45ordm 4 Emulsion thickness- 24microm

Process details During printing the paste is poured on the screen and is spread over it with the help of polypropylene spatula Then the wafers (ARC coated) are pasted with the sunny side down on the loader and picked one by one automatically with the start of the print command Once printing starts the wafers are placed one by one between the tool paste and the screen and then the squeegee moves over the screen and allow the paste through the exposed parts on the screen and gets pasted on the wafers Thus in this way the wafers are printed one by one Two most important parameters to be controlled during this printing are

1 Squeegee Pressure 2 Snap off distance

The next step after printing is drying step DRYING In this step wafers are passed through the dryer with a temperature range of 180ordm-400ordmC PHASE II- Ag PRINTING This phase is required to print the front bus bar for tabbing purpose The Ag paste used in this printing is Dupont (PV-145) and the consumption of the paste per wafer is 014 gm And the screen type used is of the following specification

1 Mesh size- 325microm 2 Tension- 24 N-cm 3 Mesh angle- 225ordm 4 Emulsion thickness- 24microm

22

SCREEN PRINTING INTRODUCTION In screen printing on wafer-based solar PV cells the mesh and buses of silver are printed on the front furthermore the buses of silver are printed on the back Subsequently aluminum paste is dispensed over the whole surface of the back for passivation and surface reflection [14] One of the parameters that can vary and can be controlled in screen printing is the thickness of the print This makes it useful for some of the techniques of printing solar cells electronics etc Screen printing is one of the most critical processes to maintain high yield Solar wafers are becoming thinner and larger so careful printing is required to maintain a lower breakage rate On the other hand high throughput at the printing stage improves the throughput of the whole cell production line PROCESS This process is divided in three phase-

1 Phase I- AgAl printing 2 Phase II- Ag printing 3 Phase III- Al printing

PHASE I- AgAl PRINTING This phase is required to print the back bus bar for tabbing purpose The AgAl paste used in this printing is Dupont (PV-505) and the consumption of the paste per wafer is 011 gm And the screen type used is of the following specification

1 Mesh size- 200microm 2 Tension- 24 N-cm 3 Mesh angle- 225ordm 45ordm 4 Emulsion thickness- 24microm

Process details During printing the paste is poured on the screen and is spread over it with the help of polypropylene spatula Then the wafers (ARC coated) are pasted with the sunny side down on the loader and picked one by one automatically with the start of the print command Once printing starts the wafers are placed one by one between the tool paste and the screen and then the squeegee moves over the screen and allow the paste through the exposed parts on the screen and gets pasted on the wafers Thus in this way the wafers are printed one by one Two most important parameters to be controlled during this printing are

1 Squeegee Pressure 2 Snap off distance

The next step after printing is drying step DRYING In this step wafers are passed through the dryer with a temperature range of 180ordm-400ordmC PHASE II- Ag PRINTING This phase is required to print the front bus bar for tabbing purpose The Ag paste used in this printing is Dupont (PV-145) and the consumption of the paste per wafer is 014 gm And the screen type used is of the following specification

1 Mesh size- 325microm 2 Tension- 24 N-cm 3 Mesh angle- 225ordm 4 Emulsion thickness- 24microm

23

Process details During printing the paste is poured on the screen and is spread over it with the help of polypropylene spatula Then the AgAl printed and dried wafers are pasted with the sunny side up on the loader and picked one by one automatically with the start of the print command Once printing starts the wafers are placed one by one between the tool paste and the screen and then the squeegee moves over the screen and allow the paste through the exposed parts on the screen and gets pasted on the wafers Thus in this way the wafers are printed one by one Two most important parameters to be controlled during this printing are

1 Squeegee Pressure 2 Snap off distance

The next step after printing is drying step DRYING In this step wafers are passed through the dryer with a temperature range of 200ordm-420ordmC PHASE III- Al PRINTING This phase is required to print the back side of the Ag printed and dried wafer The Al paste used in this printing is Analog pasc 12D and the consumption of the paste per wafer is 095 gm And the screen type used is of the following specification

1 Mesh size- 280microm 2 Tension- 24 N-cm 3 Mesh angle- 45ordm 4 Emulsion thickness- 24microm

Process details During printing the paste is poured on the screen and is spread over it with the help of polypropylene spatula Then the Ag printed and dried wafers are pasted with the sunny side down on the loader and picked one by one automatically with the start of the print command Once printing starts the wafers are placed one by one between the tool paste and the screen and then the squeegee moves over the screen and allow the paste through the exposed parts on the screen and gets pasted on the wafers Thus in this way the wafers are printed one by one Two most important parameters to be controlled during this printing are

1 Squeegee Pressure 2 Snap off distance

The next step consists of drying and firing process DRYING PROCESS The dryer consists of air heaters also called fan heaters It consists of six zones and the printed wafers are initially passed through the dryer which is simply used to dry the paste The drying temperature is maintained within 350-420ordmC FIRING PROCESS The firing furnace consists of six zones and here wafers undergo an ohmic metallic contact formation both on the front and back side And the pre-firing temperature is maintained at 580ordmC in order to ensure that all the aluminum melts before the sintering or firing process as Al and Si mixes to form the eutectic point During the firing process there is a sudden increase in temperature from 580ordmC to 840ordmC Three phenomenons happen during firing process

1 The Al forms contact with the p-layer at the back surface thereby forming the back surface field 2 At the front surface Ag forms the contact with the n-layer 3 The H2 which is present in the nitride layers gets distributed throughout the wafers in less than a

sec depending on the temperature and the belt speed of the firing furnace The hydrogen distribution throughout the wafer on the surface is one important parameter for saturating the Si

24

dangling bonds and consequently lowering the recombination on the wafer surface towards the better voltage and current

The figure shown below is a schematic diagram of the standard screen printed silicon solar cell

25

CELL TESTING AND SORTING Photovoltaic cells are sorted according to their electrical performance tested under simulated sunlight Depending on the efficiency the mechanized automated sorting machine groups them into different categories Current industry standard of efficiency is 165 and above A computer with user-friendly software adjusts the lamp intensity controls the measurement process and cell handling and acquires cell performance data The data as a full I-V curve allows sorting by a variety of selectable criteria The computer plots the I-V curve and displays a variety of cell characteristics Curves and data can be printed and stored on disk

Figure A typical solar cell I-V curve

26

CELL TESTING PARAMETERS

The short circuit current ISC

I (at V=0) = I

corresponds to the short circuit condition when the impedance is low and is calculated when the voltage equals 0

SC

I

SC

I

occurs at the beginning of the forward-bias sweep and is the maximum current value in the power quadrant For an ideal cell this maximum current value is the total current produced in the solar cell by photon excitation

SC = IMAX = Iℓ

The open circuit voltage (V

for forward-bias power quadrant

OC

V (at I=0) = V

) occurs when there is no current passing through the cell

OC

V

OC

V

is also the maximum voltage difference across the cell for a forward-bias sweep in the power quadrant

OC= VMAX

The power produced by the cell in Watts can be easily calculated along the I-V sweep by the equation P=IV At the I

for forward-bias power quadrant

SC and VOC points the power will be zero and the maximum value for power will occur between the two The voltage and current at this maximum power point are denoted as VMP and IMP

respectively

The Fill Factor (FF) is essentially a measure of quality of the solar cell It is calculated by comparing the maximum power to the theoretical power (PT) that would be output at both the open circuit voltage and short circuit current together FF can also be interpreted graphically as the ratio of the rectangular areas depicted in Figure 5

Getting the Fill Factor from the I-V Sweep

A larger fill factor is desirable and corresponds to an I-V sweep that is more square-like Typical fill factors range from 05 to 082 Fill factor is also often represented as a percentage

Efficiency is the ratio of the electrical power output Pout compared to the solar power input Pin into the PV cell Pout can be taken to be PMAX since the solar cell can be operated up to its maximum power output to get the maximum efficiency

27

Pin is taken as the product of the irradiance of the incident light measured in Wm2 or in suns (1000 Wm2) with the surface area of the solar cell [m2] The maximum efficiency (ηMAX

) found from a light test is not only an indication of the performance of the device under test but like all of the I-V parameters can also be affected by ambient conditions such as temperature and the intensity and spectrum of the incident light

28

Solar Module

29

Module Manufacturing and Testing

In the field of photovoltaic a photovoltaic module or photovoltaic panel is a packaged interconnected assembly of photovoltaic cells also known as solar cells

Steps in Module Manufacturing

1 Classified Cells 2 Ribbon Soldering on the front surface 3 Cell interconnection (stringing) 4 Electrical circuit assembly 5 Inspection visual and electrical 6 Laminate Sandwich assembly 7 Laminate Curing 8 Module finishing

9 High Voltage Test and I-V Curve

We can connect (34 36 68 54 72 88) cells in series Generally we make modules of nominal voltage 12V 18V 24V 29V 36V

TABBING amp STRINGING

In the tabbing amp stringing process cells are soldered to strings connected in parallel both on the front and back of the cell With the help of copper tinned wires the cells are soldered at the front side of the bus bar which is known as front tabbing The remaining part of the copper tinned wire is soldered at the back side of another cell which is known as back tabbing Precise alignment and careful soldering is necessary to maintain a low breakage rate and also to ensure long life and high quality of solar module Any breakage of the solar wafer will have to be detected before assembly

LAY-UP

In the lay-up station a string of soldered cells glass plates and foils are placed on the top of another after alignment The alignment has to be accurate to enhance solar module efficiency Each cell and glass plate and foils are inspected before module is assembled

LAMINATION

Lamination consists of sealing the cells in an encapsulation For electrical insulation the cells are encapsulated by an ethyl-vinyl-acetate (EVA) and fiber glass layer At the front a tempered glass sheet with high light transmittance and good mechanical strength is applied On the back an opaque substrate Tedlar is applied This sandwich is put in an oven in which the EVA foils polymerize become transparent and guarantee the sealing To laminate a module uniformity of temperature is required across the full module

FRAMING

In the framing process a lightweight Aluminum frame is used to give mechanical support to the cells

JUNCTION BOX

The junction box is mounted in the defined place on the backside of the module It protects the output wiring of the terminal that connects the module to the balance of the system

30

MODULE TESTING

This operation consists in subjecting the module to luminous exposure simulating sunlight for the purposes of measuring recording and processing the characteristic parameters of the module

TECHNICAL DATA The most important module parameters include a short circuit current an open circuit voltage and a nominal voltage at 1000 Wm2 solar radiation current and rated power at 1000 Wm2 solar radiation value Module parameters are measured at standard test conditions (STC) - solar radiation 1000 Wm2 air mass (AM) 15 and temperature 25o

Peak power - W

C The following parameters can usually be found in module datasheets

p Open circuit voltage - Voc Short circuit current - Isc Voltage at maximum power - Vmp Current at maximum power - Imp Current at battery operating voltage - I Nominal operating cell temperature (NOCT) - oC Wind loading of surface pressure - Nm2 (kmh) Impact resistance - mm at kmh Maximum system voltage - Vmax Storage and operating temperature - o

C

31

USES OF MODULES

Solar modules can be used singly or interconnected in solar arrays for a variety of applications from charging batteries and driving motors to powering entire communities Sample configurations and uses (courtesy of GE Power website) include

Directly connected systems

bull

Solar module mounting hardware DC motor or pump and disconnect switch or circuit breaker

Applications Remote water pumping a ceiling or attic fan or a solar thermal (hot water) circulation pumps

The solar module produces current that is used immediately by a motor As sunlight rises and falls current and voltage rise and fall and the motor speeds up and slows down proportionally The motor operates slowly during cloudy or stormy weather and does not operate at night

bull Solar module fuse andor fused disconnect switch

Applications Trickle charging of vehicle starting batteries (fleet vehicles seasonal road equipment like snowploughs) and boat batteries

A small current flows from the solar module through a starting battery to counteract any inherent self-discharge in the battery A trickle charge flows only during daylight hours but on average offsets any self-discharge

Stand-alone systems

bull

Solar modules and mounting hardware a charge regulator storage batteries and disconnect switches or circuit breakers

bull

A solar array produces DC current that passes through the charge controller into storage batteries The charge regulator reduces or stops charging current to prevent battery overcharge Small DC loads may be connected to the charge regulator which can then prevent battery over-discharge The battery operates loads at night and during overcast or stormy days Solar modules recharge the batteries when average or good weather returns Applications Remote industrial areas (telecommunications navigational aids cathodic protection and traffic systems) and remote home systems

Above components with the addition of an inverter and an AC distribution centre

The inverter draws power from the battery and changes DC to AC current and voltage For safety power is sent to the distribution centre which houses circuit breakers for individual AC circuits The inverter operates from battery energy day or night Applications Remote home systems

bull Above components with the addition of a fuel generator (gasoline diesel or propane) a rectifier and a sophisticated hybrid system controller

The system controller monitors the battery voltage When the voltage drops to a safe but low level the generator is turned on AC output is converted to DC power and recharges the battery AC output can also be used directly to power AC loads When the battery reaches an almost full recharge level the generator is turned off The solar array can be sized to supply average GE needs throughout the year and the generator is used to fill in during seasonal low output periods and prolonged bad weather Applications Village power systems

32

Grid-connected systems

bull

Solar modules and mounting hardware disconnect switches or circuit breakers and a grid interactive inverter

bull

The solar array produces DC current that passes through inverter which converts to AC current and voltage Power is sent to the utility meter and is either consumed immediately by home or business loads or is sent out to the general utility grid network The utility meter spins backwards or two meters are used to record incoming and outgoing power At night loads operate from utility power since the solar power system does not produce power The inverter shuts down automatically in case of utility power failure for safety and reconnects automatically when utility power resumes Applications Urban residential and commercial systems and utility-scale power plants

Above components with the addition of a battery bank charge regulator and bi-directional inverter

The solar array charges the battery bank through a charge regulator DC power from the battery passes through the inverter and is converted to AC current and voltage Power is sent to the utility meter and is either consumed immediately by home or business loads or is sent out to the general utility grid network The utility meter spins backwards or two meters are used to record incoming and outgoing power At night loads operate and the battery bank is kept trickle charged from utility power since the solar power system does not produce power In case of utility power failure the direct connection to the utility meter is shut down for safety Selected circuits in the home or business that are connected to a special secondary inverter output continue to operate drawing energy from battery bank The solar array recharges the battery each day until normal utility power resumes Applications Urban residential and commercial systems

33

USES OF SOLAR ENERGY India receives solar energy equivalent to over 5000 trillion kWhyear which is far more than the total energy consumption of the country The big question is how to harness this power and where are its main application areas The energy in solar radiation can be used directly or indirectly for all of our energy needs in daily life including heating cooling lighting electrical power transportation and even environmental cleanup Many such applications are already cost-competitive with conventional energy sources for example photovoltaic (PV) electricity in remote applications is replacing diesel generator sets Some applications such as photovoltaic and solar heating are better known and popular while others such as solar detoxification of contaminated waters or solar distillation are less known Some basic facts Form of Energy Thermal energy This energy is used for CookingHeating DryingTimber seasoning Distillation ElectricityPower generation Cooling Refrigeration Cold storage Moreover application areas include (a) Domestic lighting (b) Street lighting (c) Village electrification (d) Water pumping (e) Desalination of salty water (f) Powering of remote telecommunication repeater stations and (g) Railway signals Some of the gadgets and other devices Solar cooker Flat plate solar cookers Concentrating collectors Solar hot water systems (Domestic and Industrial) Solar pond Solar hot air systems Solar Dryers Solar timber kilns solar stills Solar photovoltaic systems Solar pond Concentrating collectors Power Tower Air conditioning Solar collectors Refrigeration systems Heliostats For many years Solar Energy has been the power supply of choice for Industrial applications where power is required at remote locations Most systems in individual uses require a few kilowatts of power The examples are powering repeater stations for microwave TV and radio telemetry and radio telephones Solar energy is also frequently used on transportation signaling eg offshore navigation buoys lighthouses aircraft warning lights on pylons or structures and increasingly in road traffic warning signals Recent years have seen rapid growth in the number of installations of PV on to buildings that are connected to the electricity grid This area of demand has been stimulated in part by government subsidy programmes (especially Japan and Germany) and by green pricing policies of utilities or electricity service providers (eg in Switzerland and the USA) The central driving force though comes from the desire of individuals or companies to obtain their electricity from a clean non-polluting renewable source for which they are prepared to pay a small premium In these grid-connected systems PV System supplies electricity to the building and any day-time excess may be exported to the grid Batteries are not required because the grid supplies any extra demand However if one wants to be independent of the grid supply one will need battery storage to provide power outside daylight hours

34

Solar PV modules can be retrofitted on to a pitched roof above the existing roof-tiles or the tiles replaced by specially designed PV roof-tiles or roof-tiling systems Apart from off-grid homes other remote buildings such as schools community halls and clinics can all benefit from electrification with Solar Energy This can power TV video telephony and a range of refrigeration equipment

Perhaps the most exciting solar energy applications involve transportation Solar boats have been constructed some of which circumnavigate and solar cars have been built and raced Satellites depend on solar PV arrays for power Early communication satellites were severely limited by the lack of suitable power sources This severely limited the output power of the satellite transmitter The only source of power available within early weight restrictions was a very inefficient panel of solar cells without battery backup A major disadvantage of this type of power source is that the satellite has no power when it is in eclipse For continuous communications this outage is unacceptable A combination of solar cells (Using III-V Direct Band GaAs Semiconductor) and storage batteries is a better prime power source This is a practical choice even though the result is far from an ideal power source About ten percent of the energy of the sunlight that strikes the solar cells is converted to electrical power This low rate is sometimes decreased even further Early satellites had over 8500 solar cells mounted on the surface of the satellite which supplied about 42 watts of power No battery backup was provided in these satellites Newer communications satellites have about 32000 solar cells mounted on the surface of the satellite and they supply about 520 watts A nickel cadmium battery is used for backup power during eclipses

35

SOLAR ENERGY ADVANTAGES OPPORTUNITIES and CHALLENGES

Solar energy is by nature environmentally viable and non-polluting and often considered to be the best among such energy sources In particular the eco-friendly nature of this source is expected to come to the fore in the future The sun provides heat and light during the day in addition electricity can also be made available during the night provided the solar power system is grid-tied or has a battery-back There are other benefits of solar power systems as well

1 Saves money

Solar Energy Advantages

bull After the initial investment has been recovered the energy from the sun is practically FREE bull The recovery payback period for this investment can be very short depending on how much

electricity a household uses bull Financial incentives are available form the government that will reduce the cost bull Solar energy does not require any fuel bull Its not affected by the supply and demand of fuel and is therefore not subjected to the ever-

increasing price of gasoline bull The savings are immediate and for many years to come bull The use of solar energy indirectly reduces health costs

2 Environmentally friendly

bull Solar Energy is clean renewable (unlike gas oil and coal) and sustainable helping to protect our environment

bull It does not pollute our air by releasing carbon dioxide nitrogen oxide sulphur dioxide or mercury into the atmosphere like many traditional forms of electrical generations does

bull Therefore Solar Energy does not contribute to global warming acid rain or smog bull It actively contributes to the decrease of harmful green house gas emissions bull By not using any fuel Solar Energy does not contribute to the cost and problems of the recovery

and transportation of fuel or the storage of radioactive waste

3 Independent semi-independent

bull Solar Energy can be utilized to offset utility-supplied energy consumption It does not only reduce your electricity bill but will also continue to supply your home business with electricity in the event of a power outage

bull A Solar Energy system can operate entirely independent not requiring a connection to a power or gas grid at all Systems can therefore be installed in remote locations (like holiday log cabins) making it more practical and cost-effective than the supply of utility electricity to a new site

bull The use of Solar Energy reduces our dependence on foreign andor centralized sources of energy influenced by natural disasters or international events and so contributes to a sustainable future

bull Solar Energy supports local job and wealth creation fuelling local economies

4 Low no maintenance

bull Solar Energy systems are virtually maintenance free and will last for decades bull Once installed there are no recurring costs

36

bull They operate silently have no moving parts do not release offensive smells and do not require you to add any fuel

bull More solar panels can easily be added in the depending on need

bull The initial cost is the main disadvantage of installing a solar energy system largely because of the high cost of the semi-conducting materials used in building one

Solar Energy DisadvantagesChallenges

bull It is available most abundantly in areas with a high number of sunshine hours Where it is needed most cold countries in high northern or southern latitudes it is less easily captured and used

bull The cost of solar energy is also high compared to non-renewable utility-supplied electricity As energy shortages are becoming more common solar energy is becoming more price-competitive

bull Solar panels require quite a large area for installation to achieve a good level of efficiency bull The efficiency of the system also relies on the location of the sun although this problem can be

overcome with the installation of certain components bull The production of solar energy is influenced by the presence of clouds or pollution in the air bull Similarly no solar energy will be produced during nighttime although a battery backup system will

solve this problem bull DC power is produced by solar cells which must be converted to AC power before it can be used bull It is not entirely pollution free always While having much better credentials than fossil fuel for

polluting emissions the environmental costs of manufacturing and constructing solar energy appliances must not be forgotten

In the main these disadvantages are really engineering problems for which solutions are becoming increasingly available and efficient They are not serious reasons to dismiss the value or utility of solar energy

37

CONCLUSION LET THERE BE LIGHT Anyone still thinking that solar energy is mainly for heating water or cooking purposes is in for a huge shock Advances in materials and production modalities are enabling new products including those that integrate PV with building materials and others that provide novel sources of power for cell phones and note book computers Hence it came as a mild surprise to industry watchers and PV experts when NanoMarkets an industry analyst firm reported that the market for thin film and organic Photovoltaic (PV) will be worth over $72 billion by the year 2015

However the current situation is not so rosy Off the renewable energy options solar energy is believed to offer a very long-term potential but is currently a high-cost niche solution High initial investment costs and relatively low plant-load factors make it costlier than conventional sources of energy This is the primary reason for the low pick-up in solar power generation capacity Inadequate returns for investors and limited involvement of private companies that have the know-how and financial capability to undertake such projects have also stymied the industryrsquos growth However with support from large capital subsidies and grant funds from governments and developmental bodies globally the solar market has been growing at 30-40 a year Annual installations are projected to increase from 25gigawatt (GW) in 2007 to about 10 GW in 2012 and to 30 GW or more by 2025 By 2015 solar power is expected to achieve grid paritymdashthe point at which solar power becomes equal to or cheaper than grid powermdashin sunny climates for peak day-time demand

The solar market is far more complex than first envisioned This complexity is rooted in the fact that

despite apparent social and environmental benefits solar energy remains unaffordable to large populations in most developing countries

A successful solar venture should be structured with a narrow well-defined target market so that the investment offering can adequately address a relatively homogenous set of needs

Economies of scale play a big role in making a solar project sustainable Without a sizeable service population a private solar company cannot financially sustain the cost

A strong local presence and knowledge of the local market and regulations is important

38

A supportive legal environment is critical These include factors like incentives for solar energy or absence of competing subsidised electricity

India is one of the nations around the world that is blessed with a vast amount of solar insolation every year The challenging aim of rural electrification can be achieved by producing energy locally Photovoltaic systems offer the possibility of exploiting an energy source available everywhere while at the same time respecting the environment It can be safely said that India is churning out innovative incentive schemes for mass adoption of solar power Government policies will be implemented for market adoption and growth such as financing schemes for individuals investing to absorb the high upfront costs With PV market on a roll like never before several multi-national companies are bracing themselves to get a share of this rapidly expanding solar pie India is proving to be a pioneer in the realms of incentives for setting up a base and sustaining operations as well as being a huge market in itself India is among those few countries in Asian region that have a govt department exclusively dedicated to renewable energys promotion amp support There has been a continuous decline in the costs of photovoltaic For companies considering offshore location for designing and developing PV systems India may be the best answer This report provides a basic insight into the manufacturing processes that go into the creation of a cell or module A blend of theory and ground reality was adopted to elucidate the important concepts to the reader Although various modifications are possible and each firm has it own set of unique attributes the basic process is the same as reviewed in the previous pages The entire project has been an immense learning experience for the authors and we hope to enshrine some of the relevant information through this project This report should provide the fundamental block to a further more in-depth study of photovoltaic manufacturing in general Based on a review of the ongoing research in solar energy technologies it is clear that they will continue to improve promising higher efficiencies and lower costs Examples of such promising new technologies beyond the horizon include continued development of new thin-film technologies nano-scale antennas for conversion of sunlight to electricity

• TITLE PAGE
• ACKNOWLEDGEMENTS
• contents
• INTRODUCTION
• AN OVERVIEW OF A SOLAR CELLMODULE MANUFACTURING UNIT
• NEED FOR SOLAR ENERGY
• RAW MATERIALS NEEDED
• HISTORY OF PHOTOVOLTAICS
• Solar cell manufacturing processes
• Etching
• texturization
• DIFFUSION
• PLASMA ETCHING
• PSG removal
• PECVD
• SCREENPRINTING
• CELL TESTING AND SORTING
• CELL TESTING PARAMETERS
• Solar Module
• Module manufacturing and testing
• uses of modules
• USES OF SOLAR ENERGY
• SOLAR ENERGY ADVANTAGESOPPORTUNITIES and CHALLENGES
• Conclusions

24

dangling bonds and consequently lowering the recombination on the wafer surface towards the better voltage and current

The figure shown below is a schematic diagram of the standard screen printed silicon solar cell

25

CELL TESTING AND SORTING Photovoltaic cells are sorted according to their electrical performance tested under simulated sunlight Depending on the efficiency the mechanized automated sorting machine groups them into different categories Current industry standard of efficiency is 165 and above A computer with user-friendly software adjusts the lamp intensity controls the measurement process and cell handling and acquires cell performance data The data as a full I-V curve allows sorting by a variety of selectable criteria The computer plots the I-V curve and displays a variety of cell characteristics Curves and data can be printed and stored on disk

Figure A typical solar cell I-V curve

26

CELL TESTING PARAMETERS

The short circuit current ISC

I (at V=0) = I

corresponds to the short circuit condition when the impedance is low and is calculated when the voltage equals 0

SC

I

SC

I

occurs at the beginning of the forward-bias sweep and is the maximum current value in the power quadrant For an ideal cell this maximum current value is the total current produced in the solar cell by photon excitation

SC = IMAX = Iℓ

The open circuit voltage (V

for forward-bias power quadrant

OC

V (at I=0) = V

) occurs when there is no current passing through the cell

OC

V

OC

V

is also the maximum voltage difference across the cell for a forward-bias sweep in the power quadrant

OC= VMAX

The power produced by the cell in Watts can be easily calculated along the I-V sweep by the equation P=IV At the I

for forward-bias power quadrant

SC and VOC points the power will be zero and the maximum value for power will occur between the two The voltage and current at this maximum power point are denoted as VMP and IMP

respectively

The Fill Factor (FF) is essentially a measure of quality of the solar cell It is calculated by comparing the maximum power to the theoretical power (PT) that would be output at both the open circuit voltage and short circuit current together FF can also be interpreted graphically as the ratio of the rectangular areas depicted in Figure 5

Getting the Fill Factor from the I-V Sweep

A larger fill factor is desirable and corresponds to an I-V sweep that is more square-like Typical fill factors range from 05 to 082 Fill factor is also often represented as a percentage

Efficiency is the ratio of the electrical power output Pout compared to the solar power input Pin into the PV cell Pout can be taken to be PMAX since the solar cell can be operated up to its maximum power output to get the maximum efficiency

27

Pin is taken as the product of the irradiance of the incident light measured in Wm2 or in suns (1000 Wm2) with the surface area of the solar cell [m2] The maximum efficiency (ηMAX

) found from a light test is not only an indication of the performance of the device under test but like all of the I-V parameters can also be affected by ambient conditions such as temperature and the intensity and spectrum of the incident light

28

Solar Module

29

Module Manufacturing and Testing

In the field of photovoltaic a photovoltaic module or photovoltaic panel is a packaged interconnected assembly of photovoltaic cells also known as solar cells

Steps in Module Manufacturing

1 Classified Cells 2 Ribbon Soldering on the front surface 3 Cell interconnection (stringing) 4 Electrical circuit assembly 5 Inspection visual and electrical 6 Laminate Sandwich assembly 7 Laminate Curing 8 Module finishing

9 High Voltage Test and I-V Curve

We can connect (34 36 68 54 72 88) cells in series Generally we make modules of nominal voltage 12V 18V 24V 29V 36V

TABBING amp STRINGING

In the tabbing amp stringing process cells are soldered to strings connected in parallel both on the front and back of the cell With the help of copper tinned wires the cells are soldered at the front side of the bus bar which is known as front tabbing The remaining part of the copper tinned wire is soldered at the back side of another cell which is known as back tabbing Precise alignment and careful soldering is necessary to maintain a low breakage rate and also to ensure long life and high quality of solar module Any breakage of the solar wafer will have to be detected before assembly

LAY-UP

In the lay-up station a string of soldered cells glass plates and foils are placed on the top of another after alignment The alignment has to be accurate to enhance solar module efficiency Each cell and glass plate and foils are inspected before module is assembled

LAMINATION

Lamination consists of sealing the cells in an encapsulation For electrical insulation the cells are encapsulated by an ethyl-vinyl-acetate (EVA) and fiber glass layer At the front a tempered glass sheet with high light transmittance and good mechanical strength is applied On the back an opaque substrate Tedlar is applied This sandwich is put in an oven in which the EVA foils polymerize become transparent and guarantee the sealing To laminate a module uniformity of temperature is required across the full module

FRAMING

In the framing process a lightweight Aluminum frame is used to give mechanical support to the cells

JUNCTION BOX

The junction box is mounted in the defined place on the backside of the module It protects the output wiring of the terminal that connects the module to the balance of the system

30

MODULE TESTING

This operation consists in subjecting the module to luminous exposure simulating sunlight for the purposes of measuring recording and processing the characteristic parameters of the module

TECHNICAL DATA The most important module parameters include a short circuit current an open circuit voltage and a nominal voltage at 1000 Wm2 solar radiation current and rated power at 1000 Wm2 solar radiation value Module parameters are measured at standard test conditions (STC) - solar radiation 1000 Wm2 air mass (AM) 15 and temperature 25o

Peak power - W

C The following parameters can usually be found in module datasheets

p Open circuit voltage - Voc Short circuit current - Isc Voltage at maximum power - Vmp Current at maximum power - Imp Current at battery operating voltage - I Nominal operating cell temperature (NOCT) - oC Wind loading of surface pressure - Nm2 (kmh) Impact resistance - mm at kmh Maximum system voltage - Vmax Storage and operating temperature - o

C

31

USES OF MODULES

Solar modules can be used singly or interconnected in solar arrays for a variety of applications from charging batteries and driving motors to powering entire communities Sample configurations and uses (courtesy of GE Power website) include

Directly connected systems

bull

Solar module mounting hardware DC motor or pump and disconnect switch or circuit breaker

Applications Remote water pumping a ceiling or attic fan or a solar thermal (hot water) circulation pumps

The solar module produces current that is used immediately by a motor As sunlight rises and falls current and voltage rise and fall and the motor speeds up and slows down proportionally The motor operates slowly during cloudy or stormy weather and does not operate at night

bull Solar module fuse andor fused disconnect switch

Applications Trickle charging of vehicle starting batteries (fleet vehicles seasonal road equipment like snowploughs) and boat batteries

A small current flows from the solar module through a starting battery to counteract any inherent self-discharge in the battery A trickle charge flows only during daylight hours but on average offsets any self-discharge

Stand-alone systems

bull

Solar modules and mounting hardware a charge regulator storage batteries and disconnect switches or circuit breakers

bull

A solar array produces DC current that passes through the charge controller into storage batteries The charge regulator reduces or stops charging current to prevent battery overcharge Small DC loads may be connected to the charge regulator which can then prevent battery over-discharge The battery operates loads at night and during overcast or stormy days Solar modules recharge the batteries when average or good weather returns Applications Remote industrial areas (telecommunications navigational aids cathodic protection and traffic systems) and remote home systems

Above components with the addition of an inverter and an AC distribution centre

The inverter draws power from the battery and changes DC to AC current and voltage For safety power is sent to the distribution centre which houses circuit breakers for individual AC circuits The inverter operates from battery energy day or night Applications Remote home systems

bull Above components with the addition of a fuel generator (gasoline diesel or propane) a rectifier and a sophisticated hybrid system controller

The system controller monitors the battery voltage When the voltage drops to a safe but low level the generator is turned on AC output is converted to DC power and recharges the battery AC output can also be used directly to power AC loads When the battery reaches an almost full recharge level the generator is turned off The solar array can be sized to supply average GE needs throughout the year and the generator is used to fill in during seasonal low output periods and prolonged bad weather Applications Village power systems

32

Grid-connected systems

bull

Solar modules and mounting hardware disconnect switches or circuit breakers and a grid interactive inverter

bull

The solar array produces DC current that passes through inverter which converts to AC current and voltage Power is sent to the utility meter and is either consumed immediately by home or business loads or is sent out to the general utility grid network The utility meter spins backwards or two meters are used to record incoming and outgoing power At night loads operate from utility power since the solar power system does not produce power The inverter shuts down automatically in case of utility power failure for safety and reconnects automatically when utility power resumes Applications Urban residential and commercial systems and utility-scale power plants

Above components with the addition of a battery bank charge regulator and bi-directional inverter

The solar array charges the battery bank through a charge regulator DC power from the battery passes through the inverter and is converted to AC current and voltage Power is sent to the utility meter and is either consumed immediately by home or business loads or is sent out to the general utility grid network The utility meter spins backwards or two meters are used to record incoming and outgoing power At night loads operate and the battery bank is kept trickle charged from utility power since the solar power system does not produce power In case of utility power failure the direct connection to the utility meter is shut down for safety Selected circuits in the home or business that are connected to a special secondary inverter output continue to operate drawing energy from battery bank The solar array recharges the battery each day until normal utility power resumes Applications Urban residential and commercial systems

33

USES OF SOLAR ENERGY India receives solar energy equivalent to over 5000 trillion kWhyear which is far more than the total energy consumption of the country The big question is how to harness this power and where are its main application areas The energy in solar radiation can be used directly or indirectly for all of our energy needs in daily life including heating cooling lighting electrical power transportation and even environmental cleanup Many such applications are already cost-competitive with conventional energy sources for example photovoltaic (PV) electricity in remote applications is replacing diesel generator sets Some applications such as photovoltaic and solar heating are better known and popular while others such as solar detoxification of contaminated waters or solar distillation are less known Some basic facts Form of Energy Thermal energy This energy is used for CookingHeating DryingTimber seasoning Distillation ElectricityPower generation Cooling Refrigeration Cold storage Moreover application areas include (a) Domestic lighting (b) Street lighting (c) Village electrification (d) Water pumping (e) Desalination of salty water (f) Powering of remote telecommunication repeater stations and (g) Railway signals Some of the gadgets and other devices Solar cooker Flat plate solar cookers Concentrating collectors Solar hot water systems (Domestic and Industrial) Solar pond Solar hot air systems Solar Dryers Solar timber kilns solar stills Solar photovoltaic systems Solar pond Concentrating collectors Power Tower Air conditioning Solar collectors Refrigeration systems Heliostats For many years Solar Energy has been the power supply of choice for Industrial applications where power is required at remote locations Most systems in individual uses require a few kilowatts of power The examples are powering repeater stations for microwave TV and radio telemetry and radio telephones Solar energy is also frequently used on transportation signaling eg offshore navigation buoys lighthouses aircraft warning lights on pylons or structures and increasingly in road traffic warning signals Recent years have seen rapid growth in the number of installations of PV on to buildings that are connected to the electricity grid This area of demand has been stimulated in part by government subsidy programmes (especially Japan and Germany) and by green pricing policies of utilities or electricity service providers (eg in Switzerland and the USA) The central driving force though comes from the desire of individuals or companies to obtain their electricity from a clean non-polluting renewable source for which they are prepared to pay a small premium In these grid-connected systems PV System supplies electricity to the building and any day-time excess may be exported to the grid Batteries are not required because the grid supplies any extra demand However if one wants to be independent of the grid supply one will need battery storage to provide power outside daylight hours

34

Solar PV modules can be retrofitted on to a pitched roof above the existing roof-tiles or the tiles replaced by specially designed PV roof-tiles or roof-tiling systems Apart from off-grid homes other remote buildings such as schools community halls and clinics can all benefit from electrification with Solar Energy This can power TV video telephony and a range of refrigeration equipment

Perhaps the most exciting solar energy applications involve transportation Solar boats have been constructed some of which circumnavigate and solar cars have been built and raced Satellites depend on solar PV arrays for power Early communication satellites were severely limited by the lack of suitable power sources This severely limited the output power of the satellite transmitter The only source of power available within early weight restrictions was a very inefficient panel of solar cells without battery backup A major disadvantage of this type of power source is that the satellite has no power when it is in eclipse For continuous communications this outage is unacceptable A combination of solar cells (Using III-V Direct Band GaAs Semiconductor) and storage batteries is a better prime power source This is a practical choice even though the result is far from an ideal power source About ten percent of the energy of the sunlight that strikes the solar cells is converted to electrical power This low rate is sometimes decreased even further Early satellites had over 8500 solar cells mounted on the surface of the satellite which supplied about 42 watts of power No battery backup was provided in these satellites Newer communications satellites have about 32000 solar cells mounted on the surface of the satellite and they supply about 520 watts A nickel cadmium battery is used for backup power during eclipses

35

SOLAR ENERGY ADVANTAGES OPPORTUNITIES and CHALLENGES

Solar energy is by nature environmentally viable and non-polluting and often considered to be the best among such energy sources In particular the eco-friendly nature of this source is expected to come to the fore in the future The sun provides heat and light during the day in addition electricity can also be made available during the night provided the solar power system is grid-tied or has a battery-back There are other benefits of solar power systems as well

1 Saves money

Solar Energy Advantages

bull After the initial investment has been recovered the energy from the sun is practically FREE bull The recovery payback period for this investment can be very short depending on how much

electricity a household uses bull Financial incentives are available form the government that will reduce the cost bull Solar energy does not require any fuel bull Its not affected by the supply and demand of fuel and is therefore not subjected to the ever-

increasing price of gasoline bull The savings are immediate and for many years to come bull The use of solar energy indirectly reduces health costs

2 Environmentally friendly

bull Solar Energy is clean renewable (unlike gas oil and coal) and sustainable helping to protect our environment

bull It does not pollute our air by releasing carbon dioxide nitrogen oxide sulphur dioxide or mercury into the atmosphere like many traditional forms of electrical generations does

bull Therefore Solar Energy does not contribute to global warming acid rain or smog bull It actively contributes to the decrease of harmful green house gas emissions bull By not using any fuel Solar Energy does not contribute to the cost and problems of the recovery

and transportation of fuel or the storage of radioactive waste

3 Independent semi-independent

bull Solar Energy can be utilized to offset utility-supplied energy consumption It does not only reduce your electricity bill but will also continue to supply your home business with electricity in the event of a power outage

bull A Solar Energy system can operate entirely independent not requiring a connection to a power or gas grid at all Systems can therefore be installed in remote locations (like holiday log cabins) making it more practical and cost-effective than the supply of utility electricity to a new site

bull The use of Solar Energy reduces our dependence on foreign andor centralized sources of energy influenced by natural disasters or international events and so contributes to a sustainable future

bull Solar Energy supports local job and wealth creation fuelling local economies

4 Low no maintenance

bull Solar Energy systems are virtually maintenance free and will last for decades bull Once installed there are no recurring costs

36

bull They operate silently have no moving parts do not release offensive smells and do not require you to add any fuel

bull More solar panels can easily be added in the depending on need

bull The initial cost is the main disadvantage of installing a solar energy system largely because of the high cost of the semi-conducting materials used in building one

Solar Energy DisadvantagesChallenges

bull It is available most abundantly in areas with a high number of sunshine hours Where it is needed most cold countries in high northern or southern latitudes it is less easily captured and used

bull The cost of solar energy is also high compared to non-renewable utility-supplied electricity As energy shortages are becoming more common solar energy is becoming more price-competitive

bull Solar panels require quite a large area for installation to achieve a good level of efficiency bull The efficiency of the system also relies on the location of the sun although this problem can be

overcome with the installation of certain components bull The production of solar energy is influenced by the presence of clouds or pollution in the air bull Similarly no solar energy will be produced during nighttime although a battery backup system will

solve this problem bull DC power is produced by solar cells which must be converted to AC power before it can be used bull It is not entirely pollution free always While having much better credentials than fossil fuel for

polluting emissions the environmental costs of manufacturing and constructing solar energy appliances must not be forgotten

In the main these disadvantages are really engineering problems for which solutions are becoming increasingly available and efficient They are not serious reasons to dismiss the value or utility of solar energy

37

CONCLUSION LET THERE BE LIGHT Anyone still thinking that solar energy is mainly for heating water or cooking purposes is in for a huge shock Advances in materials and production modalities are enabling new products including those that integrate PV with building materials and others that provide novel sources of power for cell phones and note book computers Hence it came as a mild surprise to industry watchers and PV experts when NanoMarkets an industry analyst firm reported that the market for thin film and organic Photovoltaic (PV) will be worth over $72 billion by the year 2015

However the current situation is not so rosy Off the renewable energy options solar energy is believed to offer a very long-term potential but is currently a high-cost niche solution High initial investment costs and relatively low plant-load factors make it costlier than conventional sources of energy This is the primary reason for the low pick-up in solar power generation capacity Inadequate returns for investors and limited involvement of private companies that have the know-how and financial capability to undertake such projects have also stymied the industryrsquos growth However with support from large capital subsidies and grant funds from governments and developmental bodies globally the solar market has been growing at 30-40 a year Annual installations are projected to increase from 25gigawatt (GW) in 2007 to about 10 GW in 2012 and to 30 GW or more by 2025 By 2015 solar power is expected to achieve grid paritymdashthe point at which solar power becomes equal to or cheaper than grid powermdashin sunny climates for peak day-time demand

The solar market is far more complex than first envisioned This complexity is rooted in the fact that

despite apparent social and environmental benefits solar energy remains unaffordable to large populations in most developing countries

A successful solar venture should be structured with a narrow well-defined target market so that the investment offering can adequately address a relatively homogenous set of needs

Economies of scale play a big role in making a solar project sustainable Without a sizeable service population a private solar company cannot financially sustain the cost

A strong local presence and knowledge of the local market and regulations is important

38

A supportive legal environment is critical These include factors like incentives for solar energy or absence of competing subsidised electricity

India is one of the nations around the world that is blessed with a vast amount of solar insolation every year The challenging aim of rural electrification can be achieved by producing energy locally Photovoltaic systems offer the possibility of exploiting an energy source available everywhere while at the same time respecting the environment It can be safely said that India is churning out innovative incentive schemes for mass adoption of solar power Government policies will be implemented for market adoption and growth such as financing schemes for individuals investing to absorb the high upfront costs With PV market on a roll like never before several multi-national companies are bracing themselves to get a share of this rapidly expanding solar pie India is proving to be a pioneer in the realms of incentives for setting up a base and sustaining operations as well as being a huge market in itself India is among those few countries in Asian region that have a govt department exclusively dedicated to renewable energys promotion amp support There has been a continuous decline in the costs of photovoltaic For companies considering offshore location for designing and developing PV systems India may be the best answer This report provides a basic insight into the manufacturing processes that go into the creation of a cell or module A blend of theory and ground reality was adopted to elucidate the important concepts to the reader Although various modifications are possible and each firm has it own set of unique attributes the basic process is the same as reviewed in the previous pages The entire project has been an immense learning experience for the authors and we hope to enshrine some of the relevant information through this project This report should provide the fundamental block to a further more in-depth study of photovoltaic manufacturing in general Based on a review of the ongoing research in solar energy technologies it is clear that they will continue to improve promising higher efficiencies and lower costs Examples of such promising new technologies beyond the horizon include continued development of new thin-film technologies nano-scale antennas for conversion of sunlight to electricity

• TITLE PAGE
• ACKNOWLEDGEMENTS
• contents
• INTRODUCTION
• AN OVERVIEW OF A SOLAR CELLMODULE MANUFACTURING UNIT
• NEED FOR SOLAR ENERGY
• RAW MATERIALS NEEDED
• HISTORY OF PHOTOVOLTAICS
• Solar cell manufacturing processes
• Etching
• texturization
• DIFFUSION
• PLASMA ETCHING
• PSG removal
• PECVD
• SCREENPRINTING
• CELL TESTING AND SORTING
• CELL TESTING PARAMETERS
• Solar Module
• Module manufacturing and testing
• uses of modules
• USES OF SOLAR ENERGY
• SOLAR ENERGY ADVANTAGESOPPORTUNITIES and CHALLENGES
• Conclusions

25

CELL TESTING AND SORTING Photovoltaic cells are sorted according to their electrical performance tested under simulated sunlight Depending on the efficiency the mechanized automated sorting machine groups them into different categories Current industry standard of efficiency is 165 and above A computer with user-friendly software adjusts the lamp intensity controls the measurement process and cell handling and acquires cell performance data The data as a full I-V curve allows sorting by a variety of selectable criteria The computer plots the I-V curve and displays a variety of cell characteristics Curves and data can be printed and stored on disk

Figure A typical solar cell I-V curve

26

CELL TESTING PARAMETERS

The short circuit current ISC

I (at V=0) = I

corresponds to the short circuit condition when the impedance is low and is calculated when the voltage equals 0

SC

I

SC

I

occurs at the beginning of the forward-bias sweep and is the maximum current value in the power quadrant For an ideal cell this maximum current value is the total current produced in the solar cell by photon excitation

SC = IMAX = Iℓ

The open circuit voltage (V

for forward-bias power quadrant

OC

V (at I=0) = V

) occurs when there is no current passing through the cell

OC

V

OC

V

is also the maximum voltage difference across the cell for a forward-bias sweep in the power quadrant

OC= VMAX

The power produced by the cell in Watts can be easily calculated along the I-V sweep by the equation P=IV At the I

for forward-bias power quadrant

SC and VOC points the power will be zero and the maximum value for power will occur between the two The voltage and current at this maximum power point are denoted as VMP and IMP

respectively

The Fill Factor (FF) is essentially a measure of quality of the solar cell It is calculated by comparing the maximum power to the theoretical power (PT) that would be output at both the open circuit voltage and short circuit current together FF can also be interpreted graphically as the ratio of the rectangular areas depicted in Figure 5

Getting the Fill Factor from the I-V Sweep

A larger fill factor is desirable and corresponds to an I-V sweep that is more square-like Typical fill factors range from 05 to 082 Fill factor is also often represented as a percentage

Efficiency is the ratio of the electrical power output Pout compared to the solar power input Pin into the PV cell Pout can be taken to be PMAX since the solar cell can be operated up to its maximum power output to get the maximum efficiency

27

Pin is taken as the product of the irradiance of the incident light measured in Wm2 or in suns (1000 Wm2) with the surface area of the solar cell [m2] The maximum efficiency (ηMAX

) found from a light test is not only an indication of the performance of the device under test but like all of the I-V parameters can also be affected by ambient conditions such as temperature and the intensity and spectrum of the incident light

28

Solar Module

29

Module Manufacturing and Testing

In the field of photovoltaic a photovoltaic module or photovoltaic panel is a packaged interconnected assembly of photovoltaic cells also known as solar cells

Steps in Module Manufacturing

1 Classified Cells 2 Ribbon Soldering on the front surface 3 Cell interconnection (stringing) 4 Electrical circuit assembly 5 Inspection visual and electrical 6 Laminate Sandwich assembly 7 Laminate Curing 8 Module finishing

9 High Voltage Test and I-V Curve

We can connect (34 36 68 54 72 88) cells in series Generally we make modules of nominal voltage 12V 18V 24V 29V 36V

TABBING amp STRINGING

In the tabbing amp stringing process cells are soldered to strings connected in parallel both on the front and back of the cell With the help of copper tinned wires the cells are soldered at the front side of the bus bar which is known as front tabbing The remaining part of the copper tinned wire is soldered at the back side of another cell which is known as back tabbing Precise alignment and careful soldering is necessary to maintain a low breakage rate and also to ensure long life and high quality of solar module Any breakage of the solar wafer will have to be detected before assembly

LAY-UP

In the lay-up station a string of soldered cells glass plates and foils are placed on the top of another after alignment The alignment has to be accurate to enhance solar module efficiency Each cell and glass plate and foils are inspected before module is assembled

LAMINATION

Lamination consists of sealing the cells in an encapsulation For electrical insulation the cells are encapsulated by an ethyl-vinyl-acetate (EVA) and fiber glass layer At the front a tempered glass sheet with high light transmittance and good mechanical strength is applied On the back an opaque substrate Tedlar is applied This sandwich is put in an oven in which the EVA foils polymerize become transparent and guarantee the sealing To laminate a module uniformity of temperature is required across the full module

FRAMING

In the framing process a lightweight Aluminum frame is used to give mechanical support to the cells

JUNCTION BOX

The junction box is mounted in the defined place on the backside of the module It protects the output wiring of the terminal that connects the module to the balance of the system

30

MODULE TESTING

This operation consists in subjecting the module to luminous exposure simulating sunlight for the purposes of measuring recording and processing the characteristic parameters of the module

TECHNICAL DATA The most important module parameters include a short circuit current an open circuit voltage and a nominal voltage at 1000 Wm2 solar radiation current and rated power at 1000 Wm2 solar radiation value Module parameters are measured at standard test conditions (STC) - solar radiation 1000 Wm2 air mass (AM) 15 and temperature 25o

Peak power - W

C The following parameters can usually be found in module datasheets

p Open circuit voltage - Voc Short circuit current - Isc Voltage at maximum power - Vmp Current at maximum power - Imp Current at battery operating voltage - I Nominal operating cell temperature (NOCT) - oC Wind loading of surface pressure - Nm2 (kmh) Impact resistance - mm at kmh Maximum system voltage - Vmax Storage and operating temperature - o

C

31

USES OF MODULES

Solar modules can be used singly or interconnected in solar arrays for a variety of applications from charging batteries and driving motors to powering entire communities Sample configurations and uses (courtesy of GE Power website) include

Directly connected systems

bull

Solar module mounting hardware DC motor or pump and disconnect switch or circuit breaker

Applications Remote water pumping a ceiling or attic fan or a solar thermal (hot water) circulation pumps

The solar module produces current that is used immediately by a motor As sunlight rises and falls current and voltage rise and fall and the motor speeds up and slows down proportionally The motor operates slowly during cloudy or stormy weather and does not operate at night

bull Solar module fuse andor fused disconnect switch

Applications Trickle charging of vehicle starting batteries (fleet vehicles seasonal road equipment like snowploughs) and boat batteries

A small current flows from the solar module through a starting battery to counteract any inherent self-discharge in the battery A trickle charge flows only during daylight hours but on average offsets any self-discharge

Stand-alone systems

bull

Solar modules and mounting hardware a charge regulator storage batteries and disconnect switches or circuit breakers

bull

A solar array produces DC current that passes through the charge controller into storage batteries The charge regulator reduces or stops charging current to prevent battery overcharge Small DC loads may be connected to the charge regulator which can then prevent battery over-discharge The battery operates loads at night and during overcast or stormy days Solar modules recharge the batteries when average or good weather returns Applications Remote industrial areas (telecommunications navigational aids cathodic protection and traffic systems) and remote home systems

Above components with the addition of an inverter and an AC distribution centre

The inverter draws power from the battery and changes DC to AC current and voltage For safety power is sent to the distribution centre which houses circuit breakers for individual AC circuits The inverter operates from battery energy day or night Applications Remote home systems

bull Above components with the addition of a fuel generator (gasoline diesel or propane) a rectifier and a sophisticated hybrid system controller

The system controller monitors the battery voltage When the voltage drops to a safe but low level the generator is turned on AC output is converted to DC power and recharges the battery AC output can also be used directly to power AC loads When the battery reaches an almost full recharge level the generator is turned off The solar array can be sized to supply average GE needs throughout the year and the generator is used to fill in during seasonal low output periods and prolonged bad weather Applications Village power systems

32

Grid-connected systems

bull

Solar modules and mounting hardware disconnect switches or circuit breakers and a grid interactive inverter

bull

The solar array produces DC current that passes through inverter which converts to AC current and voltage Power is sent to the utility meter and is either consumed immediately by home or business loads or is sent out to the general utility grid network The utility meter spins backwards or two meters are used to record incoming and outgoing power At night loads operate from utility power since the solar power system does not produce power The inverter shuts down automatically in case of utility power failure for safety and reconnects automatically when utility power resumes Applications Urban residential and commercial systems and utility-scale power plants

Above components with the addition of a battery bank charge regulator and bi-directional inverter

The solar array charges the battery bank through a charge regulator DC power from the battery passes through the inverter and is converted to AC current and voltage Power is sent to the utility meter and is either consumed immediately by home or business loads or is sent out to the general utility grid network The utility meter spins backwards or two meters are used to record incoming and outgoing power At night loads operate and the battery bank is kept trickle charged from utility power since the solar power system does not produce power In case of utility power failure the direct connection to the utility meter is shut down for safety Selected circuits in the home or business that are connected to a special secondary inverter output continue to operate drawing energy from battery bank The solar array recharges the battery each day until normal utility power resumes Applications Urban residential and commercial systems

33

USES OF SOLAR ENERGY India receives solar energy equivalent to over 5000 trillion kWhyear which is far more than the total energy consumption of the country The big question is how to harness this power and where are its main application areas The energy in solar radiation can be used directly or indirectly for all of our energy needs in daily life including heating cooling lighting electrical power transportation and even environmental cleanup Many such applications are already cost-competitive with conventional energy sources for example photovoltaic (PV) electricity in remote applications is replacing diesel generator sets Some applications such as photovoltaic and solar heating are better known and popular while others such as solar detoxification of contaminated waters or solar distillation are less known Some basic facts Form of Energy Thermal energy This energy is used for CookingHeating DryingTimber seasoning Distillation ElectricityPower generation Cooling Refrigeration Cold storage Moreover application areas include (a) Domestic lighting (b) Street lighting (c) Village electrification (d) Water pumping (e) Desalination of salty water (f) Powering of remote telecommunication repeater stations and (g) Railway signals Some of the gadgets and other devices Solar cooker Flat plate solar cookers Concentrating collectors Solar hot water systems (Domestic and Industrial) Solar pond Solar hot air systems Solar Dryers Solar timber kilns solar stills Solar photovoltaic systems Solar pond Concentrating collectors Power Tower Air conditioning Solar collectors Refrigeration systems Heliostats For many years Solar Energy has been the power supply of choice for Industrial applications where power is required at remote locations Most systems in individual uses require a few kilowatts of power The examples are powering repeater stations for microwave TV and radio telemetry and radio telephones Solar energy is also frequently used on transportation signaling eg offshore navigation buoys lighthouses aircraft warning lights on pylons or structures and increasingly in road traffic warning signals Recent years have seen rapid growth in the number of installations of PV on to buildings that are connected to the electricity grid This area of demand has been stimulated in part by government subsidy programmes (especially Japan and Germany) and by green pricing policies of utilities or electricity service providers (eg in Switzerland and the USA) The central driving force though comes from the desire of individuals or companies to obtain their electricity from a clean non-polluting renewable source for which they are prepared to pay a small premium In these grid-connected systems PV System supplies electricity to the building and any day-time excess may be exported to the grid Batteries are not required because the grid supplies any extra demand However if one wants to be independent of the grid supply one will need battery storage to provide power outside daylight hours

34

Solar PV modules can be retrofitted on to a pitched roof above the existing roof-tiles or the tiles replaced by specially designed PV roof-tiles or roof-tiling systems Apart from off-grid homes other remote buildings such as schools community halls and clinics can all benefit from electrification with Solar Energy This can power TV video telephony and a range of refrigeration equipment

Perhaps the most exciting solar energy applications involve transportation Solar boats have been constructed some of which circumnavigate and solar cars have been built and raced Satellites depend on solar PV arrays for power Early communication satellites were severely limited by the lack of suitable power sources This severely limited the output power of the satellite transmitter The only source of power available within early weight restrictions was a very inefficient panel of solar cells without battery backup A major disadvantage of this type of power source is that the satellite has no power when it is in eclipse For continuous communications this outage is unacceptable A combination of solar cells (Using III-V Direct Band GaAs Semiconductor) and storage batteries is a better prime power source This is a practical choice even though the result is far from an ideal power source About ten percent of the energy of the sunlight that strikes the solar cells is converted to electrical power This low rate is sometimes decreased even further Early satellites had over 8500 solar cells mounted on the surface of the satellite which supplied about 42 watts of power No battery backup was provided in these satellites Newer communications satellites have about 32000 solar cells mounted on the surface of the satellite and they supply about 520 watts A nickel cadmium battery is used for backup power during eclipses

35

SOLAR ENERGY ADVANTAGES OPPORTUNITIES and CHALLENGES

Solar energy is by nature environmentally viable and non-polluting and often considered to be the best among such energy sources In particular the eco-friendly nature of this source is expected to come to the fore in the future The sun provides heat and light during the day in addition electricity can also be made available during the night provided the solar power system is grid-tied or has a battery-back There are other benefits of solar power systems as well

1 Saves money

Solar Energy Advantages

bull After the initial investment has been recovered the energy from the sun is practically FREE bull The recovery payback period for this investment can be very short depending on how much

electricity a household uses bull Financial incentives are available form the government that will reduce the cost bull Solar energy does not require any fuel bull Its not affected by the supply and demand of fuel and is therefore not subjected to the ever-

increasing price of gasoline bull The savings are immediate and for many years to come bull The use of solar energy indirectly reduces health costs

2 Environmentally friendly

bull Solar Energy is clean renewable (unlike gas oil and coal) and sustainable helping to protect our environment

bull It does not pollute our air by releasing carbon dioxide nitrogen oxide sulphur dioxide or mercury into the atmosphere like many traditional forms of electrical generations does

bull Therefore Solar Energy does not contribute to global warming acid rain or smog bull It actively contributes to the decrease of harmful green house gas emissions bull By not using any fuel Solar Energy does not contribute to the cost and problems of the recovery

and transportation of fuel or the storage of radioactive waste

3 Independent semi-independent

bull Solar Energy can be utilized to offset utility-supplied energy consumption It does not only reduce your electricity bill but will also continue to supply your home business with electricity in the event of a power outage

bull A Solar Energy system can operate entirely independent not requiring a connection to a power or gas grid at all Systems can therefore be installed in remote locations (like holiday log cabins) making it more practical and cost-effective than the supply of utility electricity to a new site

bull The use of Solar Energy reduces our dependence on foreign andor centralized sources of energy influenced by natural disasters or international events and so contributes to a sustainable future

bull Solar Energy supports local job and wealth creation fuelling local economies

4 Low no maintenance

bull Solar Energy systems are virtually maintenance free and will last for decades bull Once installed there are no recurring costs

36

bull They operate silently have no moving parts do not release offensive smells and do not require you to add any fuel

bull More solar panels can easily be added in the depending on need

bull The initial cost is the main disadvantage of installing a solar energy system largely because of the high cost of the semi-conducting materials used in building one

Solar Energy DisadvantagesChallenges

bull It is available most abundantly in areas with a high number of sunshine hours Where it is needed most cold countries in high northern or southern latitudes it is less easily captured and used

bull The cost of solar energy is also high compared to non-renewable utility-supplied electricity As energy shortages are becoming more common solar energy is becoming more price-competitive

bull Solar panels require quite a large area for installation to achieve a good level of efficiency bull The efficiency of the system also relies on the location of the sun although this problem can be

overcome with the installation of certain components bull The production of solar energy is influenced by the presence of clouds or pollution in the air bull Similarly no solar energy will be produced during nighttime although a battery backup system will

solve this problem bull DC power is produced by solar cells which must be converted to AC power before it can be used bull It is not entirely pollution free always While having much better credentials than fossil fuel for

polluting emissions the environmental costs of manufacturing and constructing solar energy appliances must not be forgotten

In the main these disadvantages are really engineering problems for which solutions are becoming increasingly available and efficient They are not serious reasons to dismiss the value or utility of solar energy

37

CONCLUSION LET THERE BE LIGHT Anyone still thinking that solar energy is mainly for heating water or cooking purposes is in for a huge shock Advances in materials and production modalities are enabling new products including those that integrate PV with building materials and others that provide novel sources of power for cell phones and note book computers Hence it came as a mild surprise to industry watchers and PV experts when NanoMarkets an industry analyst firm reported that the market for thin film and organic Photovoltaic (PV) will be worth over $72 billion by the year 2015

However the current situation is not so rosy Off the renewable energy options solar energy is believed to offer a very long-term potential but is currently a high-cost niche solution High initial investment costs and relatively low plant-load factors make it costlier than conventional sources of energy This is the primary reason for the low pick-up in solar power generation capacity Inadequate returns for investors and limited involvement of private companies that have the know-how and financial capability to undertake such projects have also stymied the industryrsquos growth However with support from large capital subsidies and grant funds from governments and developmental bodies globally the solar market has been growing at 30-40 a year Annual installations are projected to increase from 25gigawatt (GW) in 2007 to about 10 GW in 2012 and to 30 GW or more by 2025 By 2015 solar power is expected to achieve grid paritymdashthe point at which solar power becomes equal to or cheaper than grid powermdashin sunny climates for peak day-time demand

The solar market is far more complex than first envisioned This complexity is rooted in the fact that

despite apparent social and environmental benefits solar energy remains unaffordable to large populations in most developing countries

A successful solar venture should be structured with a narrow well-defined target market so that the investment offering can adequately address a relatively homogenous set of needs

Economies of scale play a big role in making a solar project sustainable Without a sizeable service population a private solar company cannot financially sustain the cost

A strong local presence and knowledge of the local market and regulations is important

38

A supportive legal environment is critical These include factors like incentives for solar energy or absence of competing subsidised electricity

India is one of the nations around the world that is blessed with a vast amount of solar insolation every year The challenging aim of rural electrification can be achieved by producing energy locally Photovoltaic systems offer the possibility of exploiting an energy source available everywhere while at the same time respecting the environment It can be safely said that India is churning out innovative incentive schemes for mass adoption of solar power Government policies will be implemented for market adoption and growth such as financing schemes for individuals investing to absorb the high upfront costs With PV market on a roll like never before several multi-national companies are bracing themselves to get a share of this rapidly expanding solar pie India is proving to be a pioneer in the realms of incentives for setting up a base and sustaining operations as well as being a huge market in itself India is among those few countries in Asian region that have a govt department exclusively dedicated to renewable energys promotion amp support There has been a continuous decline in the costs of photovoltaic For companies considering offshore location for designing and developing PV systems India may be the best answer This report provides a basic insight into the manufacturing processes that go into the creation of a cell or module A blend of theory and ground reality was adopted to elucidate the important concepts to the reader Although various modifications are possible and each firm has it own set of unique attributes the basic process is the same as reviewed in the previous pages The entire project has been an immense learning experience for the authors and we hope to enshrine some of the relevant information through this project This report should provide the fundamental block to a further more in-depth study of photovoltaic manufacturing in general Based on a review of the ongoing research in solar energy technologies it is clear that they will continue to improve promising higher efficiencies and lower costs Examples of such promising new technologies beyond the horizon include continued development of new thin-film technologies nano-scale antennas for conversion of sunlight to electricity

• TITLE PAGE
• ACKNOWLEDGEMENTS
• contents
• INTRODUCTION
• AN OVERVIEW OF A SOLAR CELLMODULE MANUFACTURING UNIT
• NEED FOR SOLAR ENERGY
• RAW MATERIALS NEEDED
• HISTORY OF PHOTOVOLTAICS
• Solar cell manufacturing processes
• Etching
• texturization
• DIFFUSION
• PLASMA ETCHING
• PSG removal
• PECVD
• SCREENPRINTING
• CELL TESTING AND SORTING
• CELL TESTING PARAMETERS
• Solar Module
• Module manufacturing and testing
• uses of modules
• USES OF SOLAR ENERGY
• SOLAR ENERGY ADVANTAGESOPPORTUNITIES and CHALLENGES
• Conclusions

26

CELL TESTING PARAMETERS

The short circuit current ISC

I (at V=0) = I

corresponds to the short circuit condition when the impedance is low and is calculated when the voltage equals 0

SC

I

SC

I

occurs at the beginning of the forward-bias sweep and is the maximum current value in the power quadrant For an ideal cell this maximum current value is the total current produced in the solar cell by photon excitation

SC = IMAX = Iℓ

The open circuit voltage (V

for forward-bias power quadrant

OC

V (at I=0) = V

) occurs when there is no current passing through the cell

OC

V

OC

V

is also the maximum voltage difference across the cell for a forward-bias sweep in the power quadrant

OC= VMAX

The power produced by the cell in Watts can be easily calculated along the I-V sweep by the equation P=IV At the I

for forward-bias power quadrant

SC and VOC points the power will be zero and the maximum value for power will occur between the two The voltage and current at this maximum power point are denoted as VMP and IMP

respectively

The Fill Factor (FF) is essentially a measure of quality of the solar cell It is calculated by comparing the maximum power to the theoretical power (PT) that would be output at both the open circuit voltage and short circuit current together FF can also be interpreted graphically as the ratio of the rectangular areas depicted in Figure 5

Getting the Fill Factor from the I-V Sweep

A larger fill factor is desirable and corresponds to an I-V sweep that is more square-like Typical fill factors range from 05 to 082 Fill factor is also often represented as a percentage

Efficiency is the ratio of the electrical power output Pout compared to the solar power input Pin into the PV cell Pout can be taken to be PMAX since the solar cell can be operated up to its maximum power output to get the maximum efficiency

27

Pin is taken as the product of the irradiance of the incident light measured in Wm2 or in suns (1000 Wm2) with the surface area of the solar cell [m2] The maximum efficiency (ηMAX

) found from a light test is not only an indication of the performance of the device under test but like all of the I-V parameters can also be affected by ambient conditions such as temperature and the intensity and spectrum of the incident light

28

Solar Module

29

Module Manufacturing and Testing

In the field of photovoltaic a photovoltaic module or photovoltaic panel is a packaged interconnected assembly of photovoltaic cells also known as solar cells

Steps in Module Manufacturing

1 Classified Cells 2 Ribbon Soldering on the front surface 3 Cell interconnection (stringing) 4 Electrical circuit assembly 5 Inspection visual and electrical 6 Laminate Sandwich assembly 7 Laminate Curing 8 Module finishing

9 High Voltage Test and I-V Curve

We can connect (34 36 68 54 72 88) cells in series Generally we make modules of nominal voltage 12V 18V 24V 29V 36V

TABBING amp STRINGING

In the tabbing amp stringing process cells are soldered to strings connected in parallel both on the front and back of the cell With the help of copper tinned wires the cells are soldered at the front side of the bus bar which is known as front tabbing The remaining part of the copper tinned wire is soldered at the back side of another cell which is known as back tabbing Precise alignment and careful soldering is necessary to maintain a low breakage rate and also to ensure long life and high quality of solar module Any breakage of the solar wafer will have to be detected before assembly

LAY-UP

In the lay-up station a string of soldered cells glass plates and foils are placed on the top of another after alignment The alignment has to be accurate to enhance solar module efficiency Each cell and glass plate and foils are inspected before module is assembled

LAMINATION

Lamination consists of sealing the cells in an encapsulation For electrical insulation the cells are encapsulated by an ethyl-vinyl-acetate (EVA) and fiber glass layer At the front a tempered glass sheet with high light transmittance and good mechanical strength is applied On the back an opaque substrate Tedlar is applied This sandwich is put in an oven in which the EVA foils polymerize become transparent and guarantee the sealing To laminate a module uniformity of temperature is required across the full module

FRAMING

In the framing process a lightweight Aluminum frame is used to give mechanical support to the cells

JUNCTION BOX

The junction box is mounted in the defined place on the backside of the module It protects the output wiring of the terminal that connects the module to the balance of the system

30

MODULE TESTING

This operation consists in subjecting the module to luminous exposure simulating sunlight for the purposes of measuring recording and processing the characteristic parameters of the module

TECHNICAL DATA The most important module parameters include a short circuit current an open circuit voltage and a nominal voltage at 1000 Wm2 solar radiation current and rated power at 1000 Wm2 solar radiation value Module parameters are measured at standard test conditions (STC) - solar radiation 1000 Wm2 air mass (AM) 15 and temperature 25o

Peak power - W

C The following parameters can usually be found in module datasheets

p Open circuit voltage - Voc Short circuit current - Isc Voltage at maximum power - Vmp Current at maximum power - Imp Current at battery operating voltage - I Nominal operating cell temperature (NOCT) - oC Wind loading of surface pressure - Nm2 (kmh) Impact resistance - mm at kmh Maximum system voltage - Vmax Storage and operating temperature - o

C

31

USES OF MODULES

Solar modules can be used singly or interconnected in solar arrays for a variety of applications from charging batteries and driving motors to powering entire communities Sample configurations and uses (courtesy of GE Power website) include

Directly connected systems

bull

Solar module mounting hardware DC motor or pump and disconnect switch or circuit breaker

Applications Remote water pumping a ceiling or attic fan or a solar thermal (hot water) circulation pumps

The solar module produces current that is used immediately by a motor As sunlight rises and falls current and voltage rise and fall and the motor speeds up and slows down proportionally The motor operates slowly during cloudy or stormy weather and does not operate at night

bull Solar module fuse andor fused disconnect switch

Applications Trickle charging of vehicle starting batteries (fleet vehicles seasonal road equipment like snowploughs) and boat batteries

A small current flows from the solar module through a starting battery to counteract any inherent self-discharge in the battery A trickle charge flows only during daylight hours but on average offsets any self-discharge

Stand-alone systems

bull

Solar modules and mounting hardware a charge regulator storage batteries and disconnect switches or circuit breakers

bull

A solar array produces DC current that passes through the charge controller into storage batteries The charge regulator reduces or stops charging current to prevent battery overcharge Small DC loads may be connected to the charge regulator which can then prevent battery over-discharge The battery operates loads at night and during overcast or stormy days Solar modules recharge the batteries when average or good weather returns Applications Remote industrial areas (telecommunications navigational aids cathodic protection and traffic systems) and remote home systems

Above components with the addition of an inverter and an AC distribution centre

The inverter draws power from the battery and changes DC to AC current and voltage For safety power is sent to the distribution centre which houses circuit breakers for individual AC circuits The inverter operates from battery energy day or night Applications Remote home systems

bull Above components with the addition of a fuel generator (gasoline diesel or propane) a rectifier and a sophisticated hybrid system controller

The system controller monitors the battery voltage When the voltage drops to a safe but low level the generator is turned on AC output is converted to DC power and recharges the battery AC output can also be used directly to power AC loads When the battery reaches an almost full recharge level the generator is turned off The solar array can be sized to supply average GE needs throughout the year and the generator is used to fill in during seasonal low output periods and prolonged bad weather Applications Village power systems

32

Grid-connected systems

bull

Solar modules and mounting hardware disconnect switches or circuit breakers and a grid interactive inverter

bull

The solar array produces DC current that passes through inverter which converts to AC current and voltage Power is sent to the utility meter and is either consumed immediately by home or business loads or is sent out to the general utility grid network The utility meter spins backwards or two meters are used to record incoming and outgoing power At night loads operate from utility power since the solar power system does not produce power The inverter shuts down automatically in case of utility power failure for safety and reconnects automatically when utility power resumes Applications Urban residential and commercial systems and utility-scale power plants

Above components with the addition of a battery bank charge regulator and bi-directional inverter

The solar array charges the battery bank through a charge regulator DC power from the battery passes through the inverter and is converted to AC current and voltage Power is sent to the utility meter and is either consumed immediately by home or business loads or is sent out to the general utility grid network The utility meter spins backwards or two meters are used to record incoming and outgoing power At night loads operate and the battery bank is kept trickle charged from utility power since the solar power system does not produce power In case of utility power failure the direct connection to the utility meter is shut down for safety Selected circuits in the home or business that are connected to a special secondary inverter output continue to operate drawing energy from battery bank The solar array recharges the battery each day until normal utility power resumes Applications Urban residential and commercial systems

33

USES OF SOLAR ENERGY India receives solar energy equivalent to over 5000 trillion kWhyear which is far more than the total energy consumption of the country The big question is how to harness this power and where are its main application areas The energy in solar radiation can be used directly or indirectly for all of our energy needs in daily life including heating cooling lighting electrical power transportation and even environmental cleanup Many such applications are already cost-competitive with conventional energy sources for example photovoltaic (PV) electricity in remote applications is replacing diesel generator sets Some applications such as photovoltaic and solar heating are better known and popular while others such as solar detoxification of contaminated waters or solar distillation are less known Some basic facts Form of Energy Thermal energy This energy is used for CookingHeating DryingTimber seasoning Distillation ElectricityPower generation Cooling Refrigeration Cold storage Moreover application areas include (a) Domestic lighting (b) Street lighting (c) Village electrification (d) Water pumping (e) Desalination of salty water (f) Powering of remote telecommunication repeater stations and (g) Railway signals Some of the gadgets and other devices Solar cooker Flat plate solar cookers Concentrating collectors Solar hot water systems (Domestic and Industrial) Solar pond Solar hot air systems Solar Dryers Solar timber kilns solar stills Solar photovoltaic systems Solar pond Concentrating collectors Power Tower Air conditioning Solar collectors Refrigeration systems Heliostats For many years Solar Energy has been the power supply of choice for Industrial applications where power is required at remote locations Most systems in individual uses require a few kilowatts of power The examples are powering repeater stations for microwave TV and radio telemetry and radio telephones Solar energy is also frequently used on transportation signaling eg offshore navigation buoys lighthouses aircraft warning lights on pylons or structures and increasingly in road traffic warning signals Recent years have seen rapid growth in the number of installations of PV on to buildings that are connected to the electricity grid This area of demand has been stimulated in part by government subsidy programmes (especially Japan and Germany) and by green pricing policies of utilities or electricity service providers (eg in Switzerland and the USA) The central driving force though comes from the desire of individuals or companies to obtain their electricity from a clean non-polluting renewable source for which they are prepared to pay a small premium In these grid-connected systems PV System supplies electricity to the building and any day-time excess may be exported to the grid Batteries are not required because the grid supplies any extra demand However if one wants to be independent of the grid supply one will need battery storage to provide power outside daylight hours

34

Solar PV modules can be retrofitted on to a pitched roof above the existing roof-tiles or the tiles replaced by specially designed PV roof-tiles or roof-tiling systems Apart from off-grid homes other remote buildings such as schools community halls and clinics can all benefit from electrification with Solar Energy This can power TV video telephony and a range of refrigeration equipment

Perhaps the most exciting solar energy applications involve transportation Solar boats have been constructed some of which circumnavigate and solar cars have been built and raced Satellites depend on solar PV arrays for power Early communication satellites were severely limited by the lack of suitable power sources This severely limited the output power of the satellite transmitter The only source of power available within early weight restrictions was a very inefficient panel of solar cells without battery backup A major disadvantage of this type of power source is that the satellite has no power when it is in eclipse For continuous communications this outage is unacceptable A combination of solar cells (Using III-V Direct Band GaAs Semiconductor) and storage batteries is a better prime power source This is a practical choice even though the result is far from an ideal power source About ten percent of the energy of the sunlight that strikes the solar cells is converted to electrical power This low rate is sometimes decreased even further Early satellites had over 8500 solar cells mounted on the surface of the satellite which supplied about 42 watts of power No battery backup was provided in these satellites Newer communications satellites have about 32000 solar cells mounted on the surface of the satellite and they supply about 520 watts A nickel cadmium battery is used for backup power during eclipses

35

SOLAR ENERGY ADVANTAGES OPPORTUNITIES and CHALLENGES

Solar energy is by nature environmentally viable and non-polluting and often considered to be the best among such energy sources In particular the eco-friendly nature of this source is expected to come to the fore in the future The sun provides heat and light during the day in addition electricity can also be made available during the night provided the solar power system is grid-tied or has a battery-back There are other benefits of solar power systems as well

1 Saves money

Solar Energy Advantages

bull After the initial investment has been recovered the energy from the sun is practically FREE bull The recovery payback period for this investment can be very short depending on how much

electricity a household uses bull Financial incentives are available form the government that will reduce the cost bull Solar energy does not require any fuel bull Its not affected by the supply and demand of fuel and is therefore not subjected to the ever-

increasing price of gasoline bull The savings are immediate and for many years to come bull The use of solar energy indirectly reduces health costs

2 Environmentally friendly

bull Solar Energy is clean renewable (unlike gas oil and coal) and sustainable helping to protect our environment

bull It does not pollute our air by releasing carbon dioxide nitrogen oxide sulphur dioxide or mercury into the atmosphere like many traditional forms of electrical generations does

bull Therefore Solar Energy does not contribute to global warming acid rain or smog bull It actively contributes to the decrease of harmful green house gas emissions bull By not using any fuel Solar Energy does not contribute to the cost and problems of the recovery

and transportation of fuel or the storage of radioactive waste

3 Independent semi-independent

bull Solar Energy can be utilized to offset utility-supplied energy consumption It does not only reduce your electricity bill but will also continue to supply your home business with electricity in the event of a power outage

bull A Solar Energy system can operate entirely independent not requiring a connection to a power or gas grid at all Systems can therefore be installed in remote locations (like holiday log cabins) making it more practical and cost-effective than the supply of utility electricity to a new site

bull The use of Solar Energy reduces our dependence on foreign andor centralized sources of energy influenced by natural disasters or international events and so contributes to a sustainable future

bull Solar Energy supports local job and wealth creation fuelling local economies

4 Low no maintenance

bull Solar Energy systems are virtually maintenance free and will last for decades bull Once installed there are no recurring costs

36

bull They operate silently have no moving parts do not release offensive smells and do not require you to add any fuel

bull More solar panels can easily be added in the depending on need

bull The initial cost is the main disadvantage of installing a solar energy system largely because of the high cost of the semi-conducting materials used in building one

Solar Energy DisadvantagesChallenges

bull It is available most abundantly in areas with a high number of sunshine hours Where it is needed most cold countries in high northern or southern latitudes it is less easily captured and used

bull The cost of solar energy is also high compared to non-renewable utility-supplied electricity As energy shortages are becoming more common solar energy is becoming more price-competitive

bull Solar panels require quite a large area for installation to achieve a good level of efficiency bull The efficiency of the system also relies on the location of the sun although this problem can be

overcome with the installation of certain components bull The production of solar energy is influenced by the presence of clouds or pollution in the air bull Similarly no solar energy will be produced during nighttime although a battery backup system will

solve this problem bull DC power is produced by solar cells which must be converted to AC power before it can be used bull It is not entirely pollution free always While having much better credentials than fossil fuel for

polluting emissions the environmental costs of manufacturing and constructing solar energy appliances must not be forgotten

In the main these disadvantages are really engineering problems for which solutions are becoming increasingly available and efficient They are not serious reasons to dismiss the value or utility of solar energy

37

CONCLUSION LET THERE BE LIGHT Anyone still thinking that solar energy is mainly for heating water or cooking purposes is in for a huge shock Advances in materials and production modalities are enabling new products including those that integrate PV with building materials and others that provide novel sources of power for cell phones and note book computers Hence it came as a mild surprise to industry watchers and PV experts when NanoMarkets an industry analyst firm reported that the market for thin film and organic Photovoltaic (PV) will be worth over $72 billion by the year 2015

However the current situation is not so rosy Off the renewable energy options solar energy is believed to offer a very long-term potential but is currently a high-cost niche solution High initial investment costs and relatively low plant-load factors make it costlier than conventional sources of energy This is the primary reason for the low pick-up in solar power generation capacity Inadequate returns for investors and limited involvement of private companies that have the know-how and financial capability to undertake such projects have also stymied the industryrsquos growth However with support from large capital subsidies and grant funds from governments and developmental bodies globally the solar market has been growing at 30-40 a year Annual installations are projected to increase from 25gigawatt (GW) in 2007 to about 10 GW in 2012 and to 30 GW or more by 2025 By 2015 solar power is expected to achieve grid paritymdashthe point at which solar power becomes equal to or cheaper than grid powermdashin sunny climates for peak day-time demand

The solar market is far more complex than first envisioned This complexity is rooted in the fact that

despite apparent social and environmental benefits solar energy remains unaffordable to large populations in most developing countries

A successful solar venture should be structured with a narrow well-defined target market so that the investment offering can adequately address a relatively homogenous set of needs

Economies of scale play a big role in making a solar project sustainable Without a sizeable service population a private solar company cannot financially sustain the cost

A strong local presence and knowledge of the local market and regulations is important

38

A supportive legal environment is critical These include factors like incentives for solar energy or absence of competing subsidised electricity

India is one of the nations around the world that is blessed with a vast amount of solar insolation every year The challenging aim of rural electrification can be achieved by producing energy locally Photovoltaic systems offer the possibility of exploiting an energy source available everywhere while at the same time respecting the environment It can be safely said that India is churning out innovative incentive schemes for mass adoption of solar power Government policies will be implemented for market adoption and growth such as financing schemes for individuals investing to absorb the high upfront costs With PV market on a roll like never before several multi-national companies are bracing themselves to get a share of this rapidly expanding solar pie India is proving to be a pioneer in the realms of incentives for setting up a base and sustaining operations as well as being a huge market in itself India is among those few countries in Asian region that have a govt department exclusively dedicated to renewable energys promotion amp support There has been a continuous decline in the costs of photovoltaic For companies considering offshore location for designing and developing PV systems India may be the best answer This report provides a basic insight into the manufacturing processes that go into the creation of a cell or module A blend of theory and ground reality was adopted to elucidate the important concepts to the reader Although various modifications are possible and each firm has it own set of unique attributes the basic process is the same as reviewed in the previous pages The entire project has been an immense learning experience for the authors and we hope to enshrine some of the relevant information through this project This report should provide the fundamental block to a further more in-depth study of photovoltaic manufacturing in general Based on a review of the ongoing research in solar energy technologies it is clear that they will continue to improve promising higher efficiencies and lower costs Examples of such promising new technologies beyond the horizon include continued development of new thin-film technologies nano-scale antennas for conversion of sunlight to electricity

• TITLE PAGE
• ACKNOWLEDGEMENTS
• contents
• INTRODUCTION
• AN OVERVIEW OF A SOLAR CELLMODULE MANUFACTURING UNIT
• NEED FOR SOLAR ENERGY
• RAW MATERIALS NEEDED
• HISTORY OF PHOTOVOLTAICS
• Solar cell manufacturing processes
• Etching
• texturization
• DIFFUSION
• PLASMA ETCHING
• PSG removal
• PECVD
• SCREENPRINTING
• CELL TESTING AND SORTING
• CELL TESTING PARAMETERS
• Solar Module
• Module manufacturing and testing
• uses of modules
• USES OF SOLAR ENERGY
• SOLAR ENERGY ADVANTAGESOPPORTUNITIES and CHALLENGES
• Conclusions

27

Pin is taken as the product of the irradiance of the incident light measured in Wm2 or in suns (1000 Wm2) with the surface area of the solar cell [m2] The maximum efficiency (ηMAX

) found from a light test is not only an indication of the performance of the device under test but like all of the I-V parameters can also be affected by ambient conditions such as temperature and the intensity and spectrum of the incident light

28

Solar Module

29

Module Manufacturing and Testing

In the field of photovoltaic a photovoltaic module or photovoltaic panel is a packaged interconnected assembly of photovoltaic cells also known as solar cells

Steps in Module Manufacturing

1 Classified Cells 2 Ribbon Soldering on the front surface 3 Cell interconnection (stringing) 4 Electrical circuit assembly 5 Inspection visual and electrical 6 Laminate Sandwich assembly 7 Laminate Curing 8 Module finishing

9 High Voltage Test and I-V Curve

We can connect (34 36 68 54 72 88) cells in series Generally we make modules of nominal voltage 12V 18V 24V 29V 36V

TABBING amp STRINGING

In the tabbing amp stringing process cells are soldered to strings connected in parallel both on the front and back of the cell With the help of copper tinned wires the cells are soldered at the front side of the bus bar which is known as front tabbing The remaining part of the copper tinned wire is soldered at the back side of another cell which is known as back tabbing Precise alignment and careful soldering is necessary to maintain a low breakage rate and also to ensure long life and high quality of solar module Any breakage of the solar wafer will have to be detected before assembly

LAY-UP

In the lay-up station a string of soldered cells glass plates and foils are placed on the top of another after alignment The alignment has to be accurate to enhance solar module efficiency Each cell and glass plate and foils are inspected before module is assembled

LAMINATION

Lamination consists of sealing the cells in an encapsulation For electrical insulation the cells are encapsulated by an ethyl-vinyl-acetate (EVA) and fiber glass layer At the front a tempered glass sheet with high light transmittance and good mechanical strength is applied On the back an opaque substrate Tedlar is applied This sandwich is put in an oven in which the EVA foils polymerize become transparent and guarantee the sealing To laminate a module uniformity of temperature is required across the full module

FRAMING

In the framing process a lightweight Aluminum frame is used to give mechanical support to the cells

JUNCTION BOX

The junction box is mounted in the defined place on the backside of the module It protects the output wiring of the terminal that connects the module to the balance of the system

30

MODULE TESTING

This operation consists in subjecting the module to luminous exposure simulating sunlight for the purposes of measuring recording and processing the characteristic parameters of the module

TECHNICAL DATA The most important module parameters include a short circuit current an open circuit voltage and a nominal voltage at 1000 Wm2 solar radiation current and rated power at 1000 Wm2 solar radiation value Module parameters are measured at standard test conditions (STC) - solar radiation 1000 Wm2 air mass (AM) 15 and temperature 25o

Peak power - W

C The following parameters can usually be found in module datasheets

p Open circuit voltage - Voc Short circuit current - Isc Voltage at maximum power - Vmp Current at maximum power - Imp Current at battery operating voltage - I Nominal operating cell temperature (NOCT) - oC Wind loading of surface pressure - Nm2 (kmh) Impact resistance - mm at kmh Maximum system voltage - Vmax Storage and operating temperature - o

C

31

USES OF MODULES

Solar modules can be used singly or interconnected in solar arrays for a variety of applications from charging batteries and driving motors to powering entire communities Sample configurations and uses (courtesy of GE Power website) include

Directly connected systems

bull

Solar module mounting hardware DC motor or pump and disconnect switch or circuit breaker

Applications Remote water pumping a ceiling or attic fan or a solar thermal (hot water) circulation pumps

The solar module produces current that is used immediately by a motor As sunlight rises and falls current and voltage rise and fall and the motor speeds up and slows down proportionally The motor operates slowly during cloudy or stormy weather and does not operate at night

bull Solar module fuse andor fused disconnect switch

Applications Trickle charging of vehicle starting batteries (fleet vehicles seasonal road equipment like snowploughs) and boat batteries

A small current flows from the solar module through a starting battery to counteract any inherent self-discharge in the battery A trickle charge flows only during daylight hours but on average offsets any self-discharge

Stand-alone systems

bull

Solar modules and mounting hardware a charge regulator storage batteries and disconnect switches or circuit breakers

bull

A solar array produces DC current that passes through the charge controller into storage batteries The charge regulator reduces or stops charging current to prevent battery overcharge Small DC loads may be connected to the charge regulator which can then prevent battery over-discharge The battery operates loads at night and during overcast or stormy days Solar modules recharge the batteries when average or good weather returns Applications Remote industrial areas (telecommunications navigational aids cathodic protection and traffic systems) and remote home systems

Above components with the addition of an inverter and an AC distribution centre

The inverter draws power from the battery and changes DC to AC current and voltage For safety power is sent to the distribution centre which houses circuit breakers for individual AC circuits The inverter operates from battery energy day or night Applications Remote home systems

bull Above components with the addition of a fuel generator (gasoline diesel or propane) a rectifier and a sophisticated hybrid system controller

The system controller monitors the battery voltage When the voltage drops to a safe but low level the generator is turned on AC output is converted to DC power and recharges the battery AC output can also be used directly to power AC loads When the battery reaches an almost full recharge level the generator is turned off The solar array can be sized to supply average GE needs throughout the year and the generator is used to fill in during seasonal low output periods and prolonged bad weather Applications Village power systems

32

Grid-connected systems

bull

Solar modules and mounting hardware disconnect switches or circuit breakers and a grid interactive inverter

bull

The solar array produces DC current that passes through inverter which converts to AC current and voltage Power is sent to the utility meter and is either consumed immediately by home or business loads or is sent out to the general utility grid network The utility meter spins backwards or two meters are used to record incoming and outgoing power At night loads operate from utility power since the solar power system does not produce power The inverter shuts down automatically in case of utility power failure for safety and reconnects automatically when utility power resumes Applications Urban residential and commercial systems and utility-scale power plants

Above components with the addition of a battery bank charge regulator and bi-directional inverter

The solar array charges the battery bank through a charge regulator DC power from the battery passes through the inverter and is converted to AC current and voltage Power is sent to the utility meter and is either consumed immediately by home or business loads or is sent out to the general utility grid network The utility meter spins backwards or two meters are used to record incoming and outgoing power At night loads operate and the battery bank is kept trickle charged from utility power since the solar power system does not produce power In case of utility power failure the direct connection to the utility meter is shut down for safety Selected circuits in the home or business that are connected to a special secondary inverter output continue to operate drawing energy from battery bank The solar array recharges the battery each day until normal utility power resumes Applications Urban residential and commercial systems

33

USES OF SOLAR ENERGY India receives solar energy equivalent to over 5000 trillion kWhyear which is far more than the total energy consumption of the country The big question is how to harness this power and where are its main application areas The energy in solar radiation can be used directly or indirectly for all of our energy needs in daily life including heating cooling lighting electrical power transportation and even environmental cleanup Many such applications are already cost-competitive with conventional energy sources for example photovoltaic (PV) electricity in remote applications is replacing diesel generator sets Some applications such as photovoltaic and solar heating are better known and popular while others such as solar detoxification of contaminated waters or solar distillation are less known Some basic facts Form of Energy Thermal energy This energy is used for CookingHeating DryingTimber seasoning Distillation ElectricityPower generation Cooling Refrigeration Cold storage Moreover application areas include (a) Domestic lighting (b) Street lighting (c) Village electrification (d) Water pumping (e) Desalination of salty water (f) Powering of remote telecommunication repeater stations and (g) Railway signals Some of the gadgets and other devices Solar cooker Flat plate solar cookers Concentrating collectors Solar hot water systems (Domestic and Industrial) Solar pond Solar hot air systems Solar Dryers Solar timber kilns solar stills Solar photovoltaic systems Solar pond Concentrating collectors Power Tower Air conditioning Solar collectors Refrigeration systems Heliostats For many years Solar Energy has been the power supply of choice for Industrial applications where power is required at remote locations Most systems in individual uses require a few kilowatts of power The examples are powering repeater stations for microwave TV and radio telemetry and radio telephones Solar energy is also frequently used on transportation signaling eg offshore navigation buoys lighthouses aircraft warning lights on pylons or structures and increasingly in road traffic warning signals Recent years have seen rapid growth in the number of installations of PV on to buildings that are connected to the electricity grid This area of demand has been stimulated in part by government subsidy programmes (especially Japan and Germany) and by green pricing policies of utilities or electricity service providers (eg in Switzerland and the USA) The central driving force though comes from the desire of individuals or companies to obtain their electricity from a clean non-polluting renewable source for which they are prepared to pay a small premium In these grid-connected systems PV System supplies electricity to the building and any day-time excess may be exported to the grid Batteries are not required because the grid supplies any extra demand However if one wants to be independent of the grid supply one will need battery storage to provide power outside daylight hours

34

Solar PV modules can be retrofitted on to a pitched roof above the existing roof-tiles or the tiles replaced by specially designed PV roof-tiles or roof-tiling systems Apart from off-grid homes other remote buildings such as schools community halls and clinics can all benefit from electrification with Solar Energy This can power TV video telephony and a range of refrigeration equipment

Perhaps the most exciting solar energy applications involve transportation Solar boats have been constructed some of which circumnavigate and solar cars have been built and raced Satellites depend on solar PV arrays for power Early communication satellites were severely limited by the lack of suitable power sources This severely limited the output power of the satellite transmitter The only source of power available within early weight restrictions was a very inefficient panel of solar cells without battery backup A major disadvantage of this type of power source is that the satellite has no power when it is in eclipse For continuous communications this outage is unacceptable A combination of solar cells (Using III-V Direct Band GaAs Semiconductor) and storage batteries is a better prime power source This is a practical choice even though the result is far from an ideal power source About ten percent of the energy of the sunlight that strikes the solar cells is converted to electrical power This low rate is sometimes decreased even further Early satellites had over 8500 solar cells mounted on the surface of the satellite which supplied about 42 watts of power No battery backup was provided in these satellites Newer communications satellites have about 32000 solar cells mounted on the surface of the satellite and they supply about 520 watts A nickel cadmium battery is used for backup power during eclipses

35

SOLAR ENERGY ADVANTAGES OPPORTUNITIES and CHALLENGES

Solar energy is by nature environmentally viable and non-polluting and often considered to be the best among such energy sources In particular the eco-friendly nature of this source is expected to come to the fore in the future The sun provides heat and light during the day in addition electricity can also be made available during the night provided the solar power system is grid-tied or has a battery-back There are other benefits of solar power systems as well

1 Saves money

Solar Energy Advantages

bull After the initial investment has been recovered the energy from the sun is practically FREE bull The recovery payback period for this investment can be very short depending on how much

electricity a household uses bull Financial incentives are available form the government that will reduce the cost bull Solar energy does not require any fuel bull Its not affected by the supply and demand of fuel and is therefore not subjected to the ever-

increasing price of gasoline bull The savings are immediate and for many years to come bull The use of solar energy indirectly reduces health costs

2 Environmentally friendly

bull Solar Energy is clean renewable (unlike gas oil and coal) and sustainable helping to protect our environment

bull It does not pollute our air by releasing carbon dioxide nitrogen oxide sulphur dioxide or mercury into the atmosphere like many traditional forms of electrical generations does

bull Therefore Solar Energy does not contribute to global warming acid rain or smog bull It actively contributes to the decrease of harmful green house gas emissions bull By not using any fuel Solar Energy does not contribute to the cost and problems of the recovery

and transportation of fuel or the storage of radioactive waste

3 Independent semi-independent

bull Solar Energy can be utilized to offset utility-supplied energy consumption It does not only reduce your electricity bill but will also continue to supply your home business with electricity in the event of a power outage

bull A Solar Energy system can operate entirely independent not requiring a connection to a power or gas grid at all Systems can therefore be installed in remote locations (like holiday log cabins) making it more practical and cost-effective than the supply of utility electricity to a new site

bull The use of Solar Energy reduces our dependence on foreign andor centralized sources of energy influenced by natural disasters or international events and so contributes to a sustainable future

bull Solar Energy supports local job and wealth creation fuelling local economies

4 Low no maintenance

bull Solar Energy systems are virtually maintenance free and will last for decades bull Once installed there are no recurring costs

36

bull They operate silently have no moving parts do not release offensive smells and do not require you to add any fuel

bull More solar panels can easily be added in the depending on need

bull The initial cost is the main disadvantage of installing a solar energy system largely because of the high cost of the semi-conducting materials used in building one

Solar Energy DisadvantagesChallenges

bull It is available most abundantly in areas with a high number of sunshine hours Where it is needed most cold countries in high northern or southern latitudes it is less easily captured and used

bull The cost of solar energy is also high compared to non-renewable utility-supplied electricity As energy shortages are becoming more common solar energy is becoming more price-competitive

bull Solar panels require quite a large area for installation to achieve a good level of efficiency bull The efficiency of the system also relies on the location of the sun although this problem can be

overcome with the installation of certain components bull The production of solar energy is influenced by the presence of clouds or pollution in the air bull Similarly no solar energy will be produced during nighttime although a battery backup system will

solve this problem bull DC power is produced by solar cells which must be converted to AC power before it can be used bull It is not entirely pollution free always While having much better credentials than fossil fuel for

polluting emissions the environmental costs of manufacturing and constructing solar energy appliances must not be forgotten

In the main these disadvantages are really engineering problems for which solutions are becoming increasingly available and efficient They are not serious reasons to dismiss the value or utility of solar energy

37

CONCLUSION LET THERE BE LIGHT Anyone still thinking that solar energy is mainly for heating water or cooking purposes is in for a huge shock Advances in materials and production modalities are enabling new products including those that integrate PV with building materials and others that provide novel sources of power for cell phones and note book computers Hence it came as a mild surprise to industry watchers and PV experts when NanoMarkets an industry analyst firm reported that the market for thin film and organic Photovoltaic (PV) will be worth over $72 billion by the year 2015

However the current situation is not so rosy Off the renewable energy options solar energy is believed to offer a very long-term potential but is currently a high-cost niche solution High initial investment costs and relatively low plant-load factors make it costlier than conventional sources of energy This is the primary reason for the low pick-up in solar power generation capacity Inadequate returns for investors and limited involvement of private companies that have the know-how and financial capability to undertake such projects have also stymied the industryrsquos growth However with support from large capital subsidies and grant funds from governments and developmental bodies globally the solar market has been growing at 30-40 a year Annual installations are projected to increase from 25gigawatt (GW) in 2007 to about 10 GW in 2012 and to 30 GW or more by 2025 By 2015 solar power is expected to achieve grid paritymdashthe point at which solar power becomes equal to or cheaper than grid powermdashin sunny climates for peak day-time demand

The solar market is far more complex than first envisioned This complexity is rooted in the fact that

despite apparent social and environmental benefits solar energy remains unaffordable to large populations in most developing countries

A successful solar venture should be structured with a narrow well-defined target market so that the investment offering can adequately address a relatively homogenous set of needs

Economies of scale play a big role in making a solar project sustainable Without a sizeable service population a private solar company cannot financially sustain the cost

A strong local presence and knowledge of the local market and regulations is important

38

A supportive legal environment is critical These include factors like incentives for solar energy or absence of competing subsidised electricity

India is one of the nations around the world that is blessed with a vast amount of solar insolation every year The challenging aim of rural electrification can be achieved by producing energy locally Photovoltaic systems offer the possibility of exploiting an energy source available everywhere while at the same time respecting the environment It can be safely said that India is churning out innovative incentive schemes for mass adoption of solar power Government policies will be implemented for market adoption and growth such as financing schemes for individuals investing to absorb the high upfront costs With PV market on a roll like never before several multi-national companies are bracing themselves to get a share of this rapidly expanding solar pie India is proving to be a pioneer in the realms of incentives for setting up a base and sustaining operations as well as being a huge market in itself India is among those few countries in Asian region that have a govt department exclusively dedicated to renewable energys promotion amp support There has been a continuous decline in the costs of photovoltaic For companies considering offshore location for designing and developing PV systems India may be the best answer This report provides a basic insight into the manufacturing processes that go into the creation of a cell or module A blend of theory and ground reality was adopted to elucidate the important concepts to the reader Although various modifications are possible and each firm has it own set of unique attributes the basic process is the same as reviewed in the previous pages The entire project has been an immense learning experience for the authors and we hope to enshrine some of the relevant information through this project This report should provide the fundamental block to a further more in-depth study of photovoltaic manufacturing in general Based on a review of the ongoing research in solar energy technologies it is clear that they will continue to improve promising higher efficiencies and lower costs Examples of such promising new technologies beyond the horizon include continued development of new thin-film technologies nano-scale antennas for conversion of sunlight to electricity

• TITLE PAGE
• ACKNOWLEDGEMENTS
• contents
• INTRODUCTION
• AN OVERVIEW OF A SOLAR CELLMODULE MANUFACTURING UNIT
• NEED FOR SOLAR ENERGY
• RAW MATERIALS NEEDED
• HISTORY OF PHOTOVOLTAICS
• Solar cell manufacturing processes
• Etching
• texturization
• DIFFUSION
• PLASMA ETCHING
• PSG removal
• PECVD
• SCREENPRINTING
• CELL TESTING AND SORTING
• CELL TESTING PARAMETERS
• Solar Module
• Module manufacturing and testing
• uses of modules
• USES OF SOLAR ENERGY
• SOLAR ENERGY ADVANTAGESOPPORTUNITIES and CHALLENGES
• Conclusions

28

Solar Module

29

Module Manufacturing and Testing

In the field of photovoltaic a photovoltaic module or photovoltaic panel is a packaged interconnected assembly of photovoltaic cells also known as solar cells

Steps in Module Manufacturing

1 Classified Cells 2 Ribbon Soldering on the front surface 3 Cell interconnection (stringing) 4 Electrical circuit assembly 5 Inspection visual and electrical 6 Laminate Sandwich assembly 7 Laminate Curing 8 Module finishing

9 High Voltage Test and I-V Curve

We can connect (34 36 68 54 72 88) cells in series Generally we make modules of nominal voltage 12V 18V 24V 29V 36V

TABBING amp STRINGING

In the tabbing amp stringing process cells are soldered to strings connected in parallel both on the front and back of the cell With the help of copper tinned wires the cells are soldered at the front side of the bus bar which is known as front tabbing The remaining part of the copper tinned wire is soldered at the back side of another cell which is known as back tabbing Precise alignment and careful soldering is necessary to maintain a low breakage rate and also to ensure long life and high quality of solar module Any breakage of the solar wafer will have to be detected before assembly

LAY-UP

In the lay-up station a string of soldered cells glass plates and foils are placed on the top of another after alignment The alignment has to be accurate to enhance solar module efficiency Each cell and glass plate and foils are inspected before module is assembled

LAMINATION

Lamination consists of sealing the cells in an encapsulation For electrical insulation the cells are encapsulated by an ethyl-vinyl-acetate (EVA) and fiber glass layer At the front a tempered glass sheet with high light transmittance and good mechanical strength is applied On the back an opaque substrate Tedlar is applied This sandwich is put in an oven in which the EVA foils polymerize become transparent and guarantee the sealing To laminate a module uniformity of temperature is required across the full module

FRAMING

In the framing process a lightweight Aluminum frame is used to give mechanical support to the cells

JUNCTION BOX

The junction box is mounted in the defined place on the backside of the module It protects the output wiring of the terminal that connects the module to the balance of the system

30

MODULE TESTING

This operation consists in subjecting the module to luminous exposure simulating sunlight for the purposes of measuring recording and processing the characteristic parameters of the module

TECHNICAL DATA The most important module parameters include a short circuit current an open circuit voltage and a nominal voltage at 1000 Wm2 solar radiation current and rated power at 1000 Wm2 solar radiation value Module parameters are measured at standard test conditions (STC) - solar radiation 1000 Wm2 air mass (AM) 15 and temperature 25o

Peak power - W

C The following parameters can usually be found in module datasheets

p Open circuit voltage - Voc Short circuit current - Isc Voltage at maximum power - Vmp Current at maximum power - Imp Current at battery operating voltage - I Nominal operating cell temperature (NOCT) - oC Wind loading of surface pressure - Nm2 (kmh) Impact resistance - mm at kmh Maximum system voltage - Vmax Storage and operating temperature - o

C

31

USES OF MODULES

Solar modules can be used singly or interconnected in solar arrays for a variety of applications from charging batteries and driving motors to powering entire communities Sample configurations and uses (courtesy of GE Power website) include

Directly connected systems

bull

Solar module mounting hardware DC motor or pump and disconnect switch or circuit breaker

Applications Remote water pumping a ceiling or attic fan or a solar thermal (hot water) circulation pumps

The solar module produces current that is used immediately by a motor As sunlight rises and falls current and voltage rise and fall and the motor speeds up and slows down proportionally The motor operates slowly during cloudy or stormy weather and does not operate at night

bull Solar module fuse andor fused disconnect switch

Applications Trickle charging of vehicle starting batteries (fleet vehicles seasonal road equipment like snowploughs) and boat batteries

A small current flows from the solar module through a starting battery to counteract any inherent self-discharge in the battery A trickle charge flows only during daylight hours but on average offsets any self-discharge

Stand-alone systems

bull

Solar modules and mounting hardware a charge regulator storage batteries and disconnect switches or circuit breakers

bull

A solar array produces DC current that passes through the charge controller into storage batteries The charge regulator reduces or stops charging current to prevent battery overcharge Small DC loads may be connected to the charge regulator which can then prevent battery over-discharge The battery operates loads at night and during overcast or stormy days Solar modules recharge the batteries when average or good weather returns Applications Remote industrial areas (telecommunications navigational aids cathodic protection and traffic systems) and remote home systems

Above components with the addition of an inverter and an AC distribution centre

The inverter draws power from the battery and changes DC to AC current and voltage For safety power is sent to the distribution centre which houses circuit breakers for individual AC circuits The inverter operates from battery energy day or night Applications Remote home systems

bull Above components with the addition of a fuel generator (gasoline diesel or propane) a rectifier and a sophisticated hybrid system controller

The system controller monitors the battery voltage When the voltage drops to a safe but low level the generator is turned on AC output is converted to DC power and recharges the battery AC output can also be used directly to power AC loads When the battery reaches an almost full recharge level the generator is turned off The solar array can be sized to supply average GE needs throughout the year and the generator is used to fill in during seasonal low output periods and prolonged bad weather Applications Village power systems

32

Grid-connected systems

bull

Solar modules and mounting hardware disconnect switches or circuit breakers and a grid interactive inverter

bull

The solar array produces DC current that passes through inverter which converts to AC current and voltage Power is sent to the utility meter and is either consumed immediately by home or business loads or is sent out to the general utility grid network The utility meter spins backwards or two meters are used to record incoming and outgoing power At night loads operate from utility power since the solar power system does not produce power The inverter shuts down automatically in case of utility power failure for safety and reconnects automatically when utility power resumes Applications Urban residential and commercial systems and utility-scale power plants

Above components with the addition of a battery bank charge regulator and bi-directional inverter

The solar array charges the battery bank through a charge regulator DC power from the battery passes through the inverter and is converted to AC current and voltage Power is sent to the utility meter and is either consumed immediately by home or business loads or is sent out to the general utility grid network The utility meter spins backwards or two meters are used to record incoming and outgoing power At night loads operate and the battery bank is kept trickle charged from utility power since the solar power system does not produce power In case of utility power failure the direct connection to the utility meter is shut down for safety Selected circuits in the home or business that are connected to a special secondary inverter output continue to operate drawing energy from battery bank The solar array recharges the battery each day until normal utility power resumes Applications Urban residential and commercial systems

33

USES OF SOLAR ENERGY India receives solar energy equivalent to over 5000 trillion kWhyear which is far more than the total energy consumption of the country The big question is how to harness this power and where are its main application areas The energy in solar radiation can be used directly or indirectly for all of our energy needs in daily life including heating cooling lighting electrical power transportation and even environmental cleanup Many such applications are already cost-competitive with conventional energy sources for example photovoltaic (PV) electricity in remote applications is replacing diesel generator sets Some applications such as photovoltaic and solar heating are better known and popular while others such as solar detoxification of contaminated waters or solar distillation are less known Some basic facts Form of Energy Thermal energy This energy is used for CookingHeating DryingTimber seasoning Distillation ElectricityPower generation Cooling Refrigeration Cold storage Moreover application areas include (a) Domestic lighting (b) Street lighting (c) Village electrification (d) Water pumping (e) Desalination of salty water (f) Powering of remote telecommunication repeater stations and (g) Railway signals Some of the gadgets and other devices Solar cooker Flat plate solar cookers Concentrating collectors Solar hot water systems (Domestic and Industrial) Solar pond Solar hot air systems Solar Dryers Solar timber kilns solar stills Solar photovoltaic systems Solar pond Concentrating collectors Power Tower Air conditioning Solar collectors Refrigeration systems Heliostats For many years Solar Energy has been the power supply of choice for Industrial applications where power is required at remote locations Most systems in individual uses require a few kilowatts of power The examples are powering repeater stations for microwave TV and radio telemetry and radio telephones Solar energy is also frequently used on transportation signaling eg offshore navigation buoys lighthouses aircraft warning lights on pylons or structures and increasingly in road traffic warning signals Recent years have seen rapid growth in the number of installations of PV on to buildings that are connected to the electricity grid This area of demand has been stimulated in part by government subsidy programmes (especially Japan and Germany) and by green pricing policies of utilities or electricity service providers (eg in Switzerland and the USA) The central driving force though comes from the desire of individuals or companies to obtain their electricity from a clean non-polluting renewable source for which they are prepared to pay a small premium In these grid-connected systems PV System supplies electricity to the building and any day-time excess may be exported to the grid Batteries are not required because the grid supplies any extra demand However if one wants to be independent of the grid supply one will need battery storage to provide power outside daylight hours

34

Solar PV modules can be retrofitted on to a pitched roof above the existing roof-tiles or the tiles replaced by specially designed PV roof-tiles or roof-tiling systems Apart from off-grid homes other remote buildings such as schools community halls and clinics can all benefit from electrification with Solar Energy This can power TV video telephony and a range of refrigeration equipment

Perhaps the most exciting solar energy applications involve transportation Solar boats have been constructed some of which circumnavigate and solar cars have been built and raced Satellites depend on solar PV arrays for power Early communication satellites were severely limited by the lack of suitable power sources This severely limited the output power of the satellite transmitter The only source of power available within early weight restrictions was a very inefficient panel of solar cells without battery backup A major disadvantage of this type of power source is that the satellite has no power when it is in eclipse For continuous communications this outage is unacceptable A combination of solar cells (Using III-V Direct Band GaAs Semiconductor) and storage batteries is a better prime power source This is a practical choice even though the result is far from an ideal power source About ten percent of the energy of the sunlight that strikes the solar cells is converted to electrical power This low rate is sometimes decreased even further Early satellites had over 8500 solar cells mounted on the surface of the satellite which supplied about 42 watts of power No battery backup was provided in these satellites Newer communications satellites have about 32000 solar cells mounted on the surface of the satellite and they supply about 520 watts A nickel cadmium battery is used for backup power during eclipses

35

SOLAR ENERGY ADVANTAGES OPPORTUNITIES and CHALLENGES

Solar energy is by nature environmentally viable and non-polluting and often considered to be the best among such energy sources In particular the eco-friendly nature of this source is expected to come to the fore in the future The sun provides heat and light during the day in addition electricity can also be made available during the night provided the solar power system is grid-tied or has a battery-back There are other benefits of solar power systems as well

1 Saves money

Solar Energy Advantages

bull After the initial investment has been recovered the energy from the sun is practically FREE bull The recovery payback period for this investment can be very short depending on how much

electricity a household uses bull Financial incentives are available form the government that will reduce the cost bull Solar energy does not require any fuel bull Its not affected by the supply and demand of fuel and is therefore not subjected to the ever-

increasing price of gasoline bull The savings are immediate and for many years to come bull The use of solar energy indirectly reduces health costs

2 Environmentally friendly

bull Solar Energy is clean renewable (unlike gas oil and coal) and sustainable helping to protect our environment

bull It does not pollute our air by releasing carbon dioxide nitrogen oxide sulphur dioxide or mercury into the atmosphere like many traditional forms of electrical generations does

bull Therefore Solar Energy does not contribute to global warming acid rain or smog bull It actively contributes to the decrease of harmful green house gas emissions bull By not using any fuel Solar Energy does not contribute to the cost and problems of the recovery

and transportation of fuel or the storage of radioactive waste

3 Independent semi-independent

bull Solar Energy can be utilized to offset utility-supplied energy consumption It does not only reduce your electricity bill but will also continue to supply your home business with electricity in the event of a power outage

bull A Solar Energy system can operate entirely independent not requiring a connection to a power or gas grid at all Systems can therefore be installed in remote locations (like holiday log cabins) making it more practical and cost-effective than the supply of utility electricity to a new site

bull The use of Solar Energy reduces our dependence on foreign andor centralized sources of energy influenced by natural disasters or international events and so contributes to a sustainable future

bull Solar Energy supports local job and wealth creation fuelling local economies

4 Low no maintenance

bull Solar Energy systems are virtually maintenance free and will last for decades bull Once installed there are no recurring costs

36

bull They operate silently have no moving parts do not release offensive smells and do not require you to add any fuel

bull More solar panels can easily be added in the depending on need

bull The initial cost is the main disadvantage of installing a solar energy system largely because of the high cost of the semi-conducting materials used in building one

Solar Energy DisadvantagesChallenges

bull It is available most abundantly in areas with a high number of sunshine hours Where it is needed most cold countries in high northern or southern latitudes it is less easily captured and used

bull The cost of solar energy is also high compared to non-renewable utility-supplied electricity As energy shortages are becoming more common solar energy is becoming more price-competitive

bull Solar panels require quite a large area for installation to achieve a good level of efficiency bull The efficiency of the system also relies on the location of the sun although this problem can be

overcome with the installation of certain components bull The production of solar energy is influenced by the presence of clouds or pollution in the air bull Similarly no solar energy will be produced during nighttime although a battery backup system will

solve this problem bull DC power is produced by solar cells which must be converted to AC power before it can be used bull It is not entirely pollution free always While having much better credentials than fossil fuel for

polluting emissions the environmental costs of manufacturing and constructing solar energy appliances must not be forgotten

In the main these disadvantages are really engineering problems for which solutions are becoming increasingly available and efficient They are not serious reasons to dismiss the value or utility of solar energy

37

CONCLUSION LET THERE BE LIGHT Anyone still thinking that solar energy is mainly for heating water or cooking purposes is in for a huge shock Advances in materials and production modalities are enabling new products including those that integrate PV with building materials and others that provide novel sources of power for cell phones and note book computers Hence it came as a mild surprise to industry watchers and PV experts when NanoMarkets an industry analyst firm reported that the market for thin film and organic Photovoltaic (PV) will be worth over $72 billion by the year 2015

However the current situation is not so rosy Off the renewable energy options solar energy is believed to offer a very long-term potential but is currently a high-cost niche solution High initial investment costs and relatively low plant-load factors make it costlier than conventional sources of energy This is the primary reason for the low pick-up in solar power generation capacity Inadequate returns for investors and limited involvement of private companies that have the know-how and financial capability to undertake such projects have also stymied the industryrsquos growth However with support from large capital subsidies and grant funds from governments and developmental bodies globally the solar market has been growing at 30-40 a year Annual installations are projected to increase from 25gigawatt (GW) in 2007 to about 10 GW in 2012 and to 30 GW or more by 2025 By 2015 solar power is expected to achieve grid paritymdashthe point at which solar power becomes equal to or cheaper than grid powermdashin sunny climates for peak day-time demand

The solar market is far more complex than first envisioned This complexity is rooted in the fact that

despite apparent social and environmental benefits solar energy remains unaffordable to large populations in most developing countries

A successful solar venture should be structured with a narrow well-defined target market so that the investment offering can adequately address a relatively homogenous set of needs

Economies of scale play a big role in making a solar project sustainable Without a sizeable service population a private solar company cannot financially sustain the cost

A strong local presence and knowledge of the local market and regulations is important

38

A supportive legal environment is critical These include factors like incentives for solar energy or absence of competing subsidised electricity

India is one of the nations around the world that is blessed with a vast amount of solar insolation every year The challenging aim of rural electrification can be achieved by producing energy locally Photovoltaic systems offer the possibility of exploiting an energy source available everywhere while at the same time respecting the environment It can be safely said that India is churning out innovative incentive schemes for mass adoption of solar power Government policies will be implemented for market adoption and growth such as financing schemes for individuals investing to absorb the high upfront costs With PV market on a roll like never before several multi-national companies are bracing themselves to get a share of this rapidly expanding solar pie India is proving to be a pioneer in the realms of incentives for setting up a base and sustaining operations as well as being a huge market in itself India is among those few countries in Asian region that have a govt department exclusively dedicated to renewable energys promotion amp support There has been a continuous decline in the costs of photovoltaic For companies considering offshore location for designing and developing PV systems India may be the best answer This report provides a basic insight into the manufacturing processes that go into the creation of a cell or module A blend of theory and ground reality was adopted to elucidate the important concepts to the reader Although various modifications are possible and each firm has it own set of unique attributes the basic process is the same as reviewed in the previous pages The entire project has been an immense learning experience for the authors and we hope to enshrine some of the relevant information through this project This report should provide the fundamental block to a further more in-depth study of photovoltaic manufacturing in general Based on a review of the ongoing research in solar energy technologies it is clear that they will continue to improve promising higher efficiencies and lower costs Examples of such promising new technologies beyond the horizon include continued development of new thin-film technologies nano-scale antennas for conversion of sunlight to electricity

• TITLE PAGE
• ACKNOWLEDGEMENTS
• contents
• INTRODUCTION
• AN OVERVIEW OF A SOLAR CELLMODULE MANUFACTURING UNIT
• NEED FOR SOLAR ENERGY
• RAW MATERIALS NEEDED
• HISTORY OF PHOTOVOLTAICS
• Solar cell manufacturing processes
• Etching
• texturization
• DIFFUSION
• PLASMA ETCHING
• PSG removal
• PECVD
• SCREENPRINTING
• CELL TESTING AND SORTING
• CELL TESTING PARAMETERS
• Solar Module
• Module manufacturing and testing
• uses of modules
• USES OF SOLAR ENERGY
• SOLAR ENERGY ADVANTAGESOPPORTUNITIES and CHALLENGES
• Conclusions

29

Module Manufacturing and Testing

In the field of photovoltaic a photovoltaic module or photovoltaic panel is a packaged interconnected assembly of photovoltaic cells also known as solar cells

Steps in Module Manufacturing

1 Classified Cells 2 Ribbon Soldering on the front surface 3 Cell interconnection (stringing) 4 Electrical circuit assembly 5 Inspection visual and electrical 6 Laminate Sandwich assembly 7 Laminate Curing 8 Module finishing

9 High Voltage Test and I-V Curve

We can connect (34 36 68 54 72 88) cells in series Generally we make modules of nominal voltage 12V 18V 24V 29V 36V

TABBING amp STRINGING

In the tabbing amp stringing process cells are soldered to strings connected in parallel both on the front and back of the cell With the help of copper tinned wires the cells are soldered at the front side of the bus bar which is known as front tabbing The remaining part of the copper tinned wire is soldered at the back side of another cell which is known as back tabbing Precise alignment and careful soldering is necessary to maintain a low breakage rate and also to ensure long life and high quality of solar module Any breakage of the solar wafer will have to be detected before assembly

LAY-UP

In the lay-up station a string of soldered cells glass plates and foils are placed on the top of another after alignment The alignment has to be accurate to enhance solar module efficiency Each cell and glass plate and foils are inspected before module is assembled

LAMINATION

Lamination consists of sealing the cells in an encapsulation For electrical insulation the cells are encapsulated by an ethyl-vinyl-acetate (EVA) and fiber glass layer At the front a tempered glass sheet with high light transmittance and good mechanical strength is applied On the back an opaque substrate Tedlar is applied This sandwich is put in an oven in which the EVA foils polymerize become transparent and guarantee the sealing To laminate a module uniformity of temperature is required across the full module

FRAMING

In the framing process a lightweight Aluminum frame is used to give mechanical support to the cells

JUNCTION BOX

The junction box is mounted in the defined place on the backside of the module It protects the output wiring of the terminal that connects the module to the balance of the system

30

MODULE TESTING

This operation consists in subjecting the module to luminous exposure simulating sunlight for the purposes of measuring recording and processing the characteristic parameters of the module

TECHNICAL DATA The most important module parameters include a short circuit current an open circuit voltage and a nominal voltage at 1000 Wm2 solar radiation current and rated power at 1000 Wm2 solar radiation value Module parameters are measured at standard test conditions (STC) - solar radiation 1000 Wm2 air mass (AM) 15 and temperature 25o

Peak power - W

C The following parameters can usually be found in module datasheets

p Open circuit voltage - Voc Short circuit current - Isc Voltage at maximum power - Vmp Current at maximum power - Imp Current at battery operating voltage - I Nominal operating cell temperature (NOCT) - oC Wind loading of surface pressure - Nm2 (kmh) Impact resistance - mm at kmh Maximum system voltage - Vmax Storage and operating temperature - o

C

31

USES OF MODULES

Solar modules can be used singly or interconnected in solar arrays for a variety of applications from charging batteries and driving motors to powering entire communities Sample configurations and uses (courtesy of GE Power website) include

Directly connected systems

bull

Solar module mounting hardware DC motor or pump and disconnect switch or circuit breaker

Applications Remote water pumping a ceiling or attic fan or a solar thermal (hot water) circulation pumps

The solar module produces current that is used immediately by a motor As sunlight rises and falls current and voltage rise and fall and the motor speeds up and slows down proportionally The motor operates slowly during cloudy or stormy weather and does not operate at night

bull Solar module fuse andor fused disconnect switch

Applications Trickle charging of vehicle starting batteries (fleet vehicles seasonal road equipment like snowploughs) and boat batteries

A small current flows from the solar module through a starting battery to counteract any inherent self-discharge in the battery A trickle charge flows only during daylight hours but on average offsets any self-discharge

Stand-alone systems

bull

Solar modules and mounting hardware a charge regulator storage batteries and disconnect switches or circuit breakers

bull

A solar array produces DC current that passes through the charge controller into storage batteries The charge regulator reduces or stops charging current to prevent battery overcharge Small DC loads may be connected to the charge regulator which can then prevent battery over-discharge The battery operates loads at night and during overcast or stormy days Solar modules recharge the batteries when average or good weather returns Applications Remote industrial areas (telecommunications navigational aids cathodic protection and traffic systems) and remote home systems

Above components with the addition of an inverter and an AC distribution centre

The inverter draws power from the battery and changes DC to AC current and voltage For safety power is sent to the distribution centre which houses circuit breakers for individual AC circuits The inverter operates from battery energy day or night Applications Remote home systems

bull Above components with the addition of a fuel generator (gasoline diesel or propane) a rectifier and a sophisticated hybrid system controller

The system controller monitors the battery voltage When the voltage drops to a safe but low level the generator is turned on AC output is converted to DC power and recharges the battery AC output can also be used directly to power AC loads When the battery reaches an almost full recharge level the generator is turned off The solar array can be sized to supply average GE needs throughout the year and the generator is used to fill in during seasonal low output periods and prolonged bad weather Applications Village power systems

32

Grid-connected systems

bull

Solar modules and mounting hardware disconnect switches or circuit breakers and a grid interactive inverter

bull

The solar array produces DC current that passes through inverter which converts to AC current and voltage Power is sent to the utility meter and is either consumed immediately by home or business loads or is sent out to the general utility grid network The utility meter spins backwards or two meters are used to record incoming and outgoing power At night loads operate from utility power since the solar power system does not produce power The inverter shuts down automatically in case of utility power failure for safety and reconnects automatically when utility power resumes Applications Urban residential and commercial systems and utility-scale power plants

Above components with the addition of a battery bank charge regulator and bi-directional inverter

The solar array charges the battery bank through a charge regulator DC power from the battery passes through the inverter and is converted to AC current and voltage Power is sent to the utility meter and is either consumed immediately by home or business loads or is sent out to the general utility grid network The utility meter spins backwards or two meters are used to record incoming and outgoing power At night loads operate and the battery bank is kept trickle charged from utility power since the solar power system does not produce power In case of utility power failure the direct connection to the utility meter is shut down for safety Selected circuits in the home or business that are connected to a special secondary inverter output continue to operate drawing energy from battery bank The solar array recharges the battery each day until normal utility power resumes Applications Urban residential and commercial systems

33

USES OF SOLAR ENERGY India receives solar energy equivalent to over 5000 trillion kWhyear which is far more than the total energy consumption of the country The big question is how to harness this power and where are its main application areas The energy in solar radiation can be used directly or indirectly for all of our energy needs in daily life including heating cooling lighting electrical power transportation and even environmental cleanup Many such applications are already cost-competitive with conventional energy sources for example photovoltaic (PV) electricity in remote applications is replacing diesel generator sets Some applications such as photovoltaic and solar heating are better known and popular while others such as solar detoxification of contaminated waters or solar distillation are less known Some basic facts Form of Energy Thermal energy This energy is used for CookingHeating DryingTimber seasoning Distillation ElectricityPower generation Cooling Refrigeration Cold storage Moreover application areas include (a) Domestic lighting (b) Street lighting (c) Village electrification (d) Water pumping (e) Desalination of salty water (f) Powering of remote telecommunication repeater stations and (g) Railway signals Some of the gadgets and other devices Solar cooker Flat plate solar cookers Concentrating collectors Solar hot water systems (Domestic and Industrial) Solar pond Solar hot air systems Solar Dryers Solar timber kilns solar stills Solar photovoltaic systems Solar pond Concentrating collectors Power Tower Air conditioning Solar collectors Refrigeration systems Heliostats For many years Solar Energy has been the power supply of choice for Industrial applications where power is required at remote locations Most systems in individual uses require a few kilowatts of power The examples are powering repeater stations for microwave TV and radio telemetry and radio telephones Solar energy is also frequently used on transportation signaling eg offshore navigation buoys lighthouses aircraft warning lights on pylons or structures and increasingly in road traffic warning signals Recent years have seen rapid growth in the number of installations of PV on to buildings that are connected to the electricity grid This area of demand has been stimulated in part by government subsidy programmes (especially Japan and Germany) and by green pricing policies of utilities or electricity service providers (eg in Switzerland and the USA) The central driving force though comes from the desire of individuals or companies to obtain their electricity from a clean non-polluting renewable source for which they are prepared to pay a small premium In these grid-connected systems PV System supplies electricity to the building and any day-time excess may be exported to the grid Batteries are not required because the grid supplies any extra demand However if one wants to be independent of the grid supply one will need battery storage to provide power outside daylight hours

34

Solar PV modules can be retrofitted on to a pitched roof above the existing roof-tiles or the tiles replaced by specially designed PV roof-tiles or roof-tiling systems Apart from off-grid homes other remote buildings such as schools community halls and clinics can all benefit from electrification with Solar Energy This can power TV video telephony and a range of refrigeration equipment

Perhaps the most exciting solar energy applications involve transportation Solar boats have been constructed some of which circumnavigate and solar cars have been built and raced Satellites depend on solar PV arrays for power Early communication satellites were severely limited by the lack of suitable power sources This severely limited the output power of the satellite transmitter The only source of power available within early weight restrictions was a very inefficient panel of solar cells without battery backup A major disadvantage of this type of power source is that the satellite has no power when it is in eclipse For continuous communications this outage is unacceptable A combination of solar cells (Using III-V Direct Band GaAs Semiconductor) and storage batteries is a better prime power source This is a practical choice even though the result is far from an ideal power source About ten percent of the energy of the sunlight that strikes the solar cells is converted to electrical power This low rate is sometimes decreased even further Early satellites had over 8500 solar cells mounted on the surface of the satellite which supplied about 42 watts of power No battery backup was provided in these satellites Newer communications satellites have about 32000 solar cells mounted on the surface of the satellite and they supply about 520 watts A nickel cadmium battery is used for backup power during eclipses

35

SOLAR ENERGY ADVANTAGES OPPORTUNITIES and CHALLENGES

Solar energy is by nature environmentally viable and non-polluting and often considered to be the best among such energy sources In particular the eco-friendly nature of this source is expected to come to the fore in the future The sun provides heat and light during the day in addition electricity can also be made available during the night provided the solar power system is grid-tied or has a battery-back There are other benefits of solar power systems as well

1 Saves money

Solar Energy Advantages

bull After the initial investment has been recovered the energy from the sun is practically FREE bull The recovery payback period for this investment can be very short depending on how much

electricity a household uses bull Financial incentives are available form the government that will reduce the cost bull Solar energy does not require any fuel bull Its not affected by the supply and demand of fuel and is therefore not subjected to the ever-

increasing price of gasoline bull The savings are immediate and for many years to come bull The use of solar energy indirectly reduces health costs

2 Environmentally friendly

bull Solar Energy is clean renewable (unlike gas oil and coal) and sustainable helping to protect our environment

bull It does not pollute our air by releasing carbon dioxide nitrogen oxide sulphur dioxide or mercury into the atmosphere like many traditional forms of electrical generations does

bull Therefore Solar Energy does not contribute to global warming acid rain or smog bull It actively contributes to the decrease of harmful green house gas emissions bull By not using any fuel Solar Energy does not contribute to the cost and problems of the recovery

and transportation of fuel or the storage of radioactive waste

3 Independent semi-independent

bull Solar Energy can be utilized to offset utility-supplied energy consumption It does not only reduce your electricity bill but will also continue to supply your home business with electricity in the event of a power outage

bull A Solar Energy system can operate entirely independent not requiring a connection to a power or gas grid at all Systems can therefore be installed in remote locations (like holiday log cabins) making it more practical and cost-effective than the supply of utility electricity to a new site

bull The use of Solar Energy reduces our dependence on foreign andor centralized sources of energy influenced by natural disasters or international events and so contributes to a sustainable future

bull Solar Energy supports local job and wealth creation fuelling local economies

4 Low no maintenance

bull Solar Energy systems are virtually maintenance free and will last for decades bull Once installed there are no recurring costs

36

bull They operate silently have no moving parts do not release offensive smells and do not require you to add any fuel

bull More solar panels can easily be added in the depending on need

bull The initial cost is the main disadvantage of installing a solar energy system largely because of the high cost of the semi-conducting materials used in building one

Solar Energy DisadvantagesChallenges

bull It is available most abundantly in areas with a high number of sunshine hours Where it is needed most cold countries in high northern or southern latitudes it is less easily captured and used

bull The cost of solar energy is also high compared to non-renewable utility-supplied electricity As energy shortages are becoming more common solar energy is becoming more price-competitive

bull Solar panels require quite a large area for installation to achieve a good level of efficiency bull The efficiency of the system also relies on the location of the sun although this problem can be

overcome with the installation of certain components bull The production of solar energy is influenced by the presence of clouds or pollution in the air bull Similarly no solar energy will be produced during nighttime although a battery backup system will

solve this problem bull DC power is produced by solar cells which must be converted to AC power before it can be used bull It is not entirely pollution free always While having much better credentials than fossil fuel for

polluting emissions the environmental costs of manufacturing and constructing solar energy appliances must not be forgotten

In the main these disadvantages are really engineering problems for which solutions are becoming increasingly available and efficient They are not serious reasons to dismiss the value or utility of solar energy

37

CONCLUSION LET THERE BE LIGHT Anyone still thinking that solar energy is mainly for heating water or cooking purposes is in for a huge shock Advances in materials and production modalities are enabling new products including those that integrate PV with building materials and others that provide novel sources of power for cell phones and note book computers Hence it came as a mild surprise to industry watchers and PV experts when NanoMarkets an industry analyst firm reported that the market for thin film and organic Photovoltaic (PV) will be worth over $72 billion by the year 2015

However the current situation is not so rosy Off the renewable energy options solar energy is believed to offer a very long-term potential but is currently a high-cost niche solution High initial investment costs and relatively low plant-load factors make it costlier than conventional sources of energy This is the primary reason for the low pick-up in solar power generation capacity Inadequate returns for investors and limited involvement of private companies that have the know-how and financial capability to undertake such projects have also stymied the industryrsquos growth However with support from large capital subsidies and grant funds from governments and developmental bodies globally the solar market has been growing at 30-40 a year Annual installations are projected to increase from 25gigawatt (GW) in 2007 to about 10 GW in 2012 and to 30 GW or more by 2025 By 2015 solar power is expected to achieve grid paritymdashthe point at which solar power becomes equal to or cheaper than grid powermdashin sunny climates for peak day-time demand

The solar market is far more complex than first envisioned This complexity is rooted in the fact that

despite apparent social and environmental benefits solar energy remains unaffordable to large populations in most developing countries

A successful solar venture should be structured with a narrow well-defined target market so that the investment offering can adequately address a relatively homogenous set of needs

Economies of scale play a big role in making a solar project sustainable Without a sizeable service population a private solar company cannot financially sustain the cost

A strong local presence and knowledge of the local market and regulations is important

38

A supportive legal environment is critical These include factors like incentives for solar energy or absence of competing subsidised electricity

India is one of the nations around the world that is blessed with a vast amount of solar insolation every year The challenging aim of rural electrification can be achieved by producing energy locally Photovoltaic systems offer the possibility of exploiting an energy source available everywhere while at the same time respecting the environment It can be safely said that India is churning out innovative incentive schemes for mass adoption of solar power Government policies will be implemented for market adoption and growth such as financing schemes for individuals investing to absorb the high upfront costs With PV market on a roll like never before several multi-national companies are bracing themselves to get a share of this rapidly expanding solar pie India is proving to be a pioneer in the realms of incentives for setting up a base and sustaining operations as well as being a huge market in itself India is among those few countries in Asian region that have a govt department exclusively dedicated to renewable energys promotion amp support There has been a continuous decline in the costs of photovoltaic For companies considering offshore location for designing and developing PV systems India may be the best answer This report provides a basic insight into the manufacturing processes that go into the creation of a cell or module A blend of theory and ground reality was adopted to elucidate the important concepts to the reader Although various modifications are possible and each firm has it own set of unique attributes the basic process is the same as reviewed in the previous pages The entire project has been an immense learning experience for the authors and we hope to enshrine some of the relevant information through this project This report should provide the fundamental block to a further more in-depth study of photovoltaic manufacturing in general Based on a review of the ongoing research in solar energy technologies it is clear that they will continue to improve promising higher efficiencies and lower costs Examples of such promising new technologies beyond the horizon include continued development of new thin-film technologies nano-scale antennas for conversion of sunlight to electricity

• TITLE PAGE
• ACKNOWLEDGEMENTS
• contents
• INTRODUCTION
• AN OVERVIEW OF A SOLAR CELLMODULE MANUFACTURING UNIT
• NEED FOR SOLAR ENERGY
• RAW MATERIALS NEEDED
• HISTORY OF PHOTOVOLTAICS
• Solar cell manufacturing processes
• Etching
• texturization
• DIFFUSION
• PLASMA ETCHING
• PSG removal
• PECVD
• SCREENPRINTING
• CELL TESTING AND SORTING
• CELL TESTING PARAMETERS
• Solar Module
• Module manufacturing and testing
• uses of modules
• USES OF SOLAR ENERGY
• SOLAR ENERGY ADVANTAGESOPPORTUNITIES and CHALLENGES
• Conclusions

30

MODULE TESTING

This operation consists in subjecting the module to luminous exposure simulating sunlight for the purposes of measuring recording and processing the characteristic parameters of the module

TECHNICAL DATA The most important module parameters include a short circuit current an open circuit voltage and a nominal voltage at 1000 Wm2 solar radiation current and rated power at 1000 Wm2 solar radiation value Module parameters are measured at standard test conditions (STC) - solar radiation 1000 Wm2 air mass (AM) 15 and temperature 25o

Peak power - W

C The following parameters can usually be found in module datasheets

p Open circuit voltage - Voc Short circuit current - Isc Voltage at maximum power - Vmp Current at maximum power - Imp Current at battery operating voltage - I Nominal operating cell temperature (NOCT) - oC Wind loading of surface pressure - Nm2 (kmh) Impact resistance - mm at kmh Maximum system voltage - Vmax Storage and operating temperature - o

C

31

USES OF MODULES

Solar modules can be used singly or interconnected in solar arrays for a variety of applications from charging batteries and driving motors to powering entire communities Sample configurations and uses (courtesy of GE Power website) include

Directly connected systems

bull

Solar module mounting hardware DC motor or pump and disconnect switch or circuit breaker

Applications Remote water pumping a ceiling or attic fan or a solar thermal (hot water) circulation pumps

The solar module produces current that is used immediately by a motor As sunlight rises and falls current and voltage rise and fall and the motor speeds up and slows down proportionally The motor operates slowly during cloudy or stormy weather and does not operate at night

bull Solar module fuse andor fused disconnect switch

Applications Trickle charging of vehicle starting batteries (fleet vehicles seasonal road equipment like snowploughs) and boat batteries

A small current flows from the solar module through a starting battery to counteract any inherent self-discharge in the battery A trickle charge flows only during daylight hours but on average offsets any self-discharge

Stand-alone systems

bull

Solar modules and mounting hardware a charge regulator storage batteries and disconnect switches or circuit breakers

bull

A solar array produces DC current that passes through the charge controller into storage batteries The charge regulator reduces or stops charging current to prevent battery overcharge Small DC loads may be connected to the charge regulator which can then prevent battery over-discharge The battery operates loads at night and during overcast or stormy days Solar modules recharge the batteries when average or good weather returns Applications Remote industrial areas (telecommunications navigational aids cathodic protection and traffic systems) and remote home systems

Above components with the addition of an inverter and an AC distribution centre

The inverter draws power from the battery and changes DC to AC current and voltage For safety power is sent to the distribution centre which houses circuit breakers for individual AC circuits The inverter operates from battery energy day or night Applications Remote home systems

bull Above components with the addition of a fuel generator (gasoline diesel or propane) a rectifier and a sophisticated hybrid system controller

The system controller monitors the battery voltage When the voltage drops to a safe but low level the generator is turned on AC output is converted to DC power and recharges the battery AC output can also be used directly to power AC loads When the battery reaches an almost full recharge level the generator is turned off The solar array can be sized to supply average GE needs throughout the year and the generator is used to fill in during seasonal low output periods and prolonged bad weather Applications Village power systems

32

Grid-connected systems

bull

Solar modules and mounting hardware disconnect switches or circuit breakers and a grid interactive inverter

bull

The solar array produces DC current that passes through inverter which converts to AC current and voltage Power is sent to the utility meter and is either consumed immediately by home or business loads or is sent out to the general utility grid network The utility meter spins backwards or two meters are used to record incoming and outgoing power At night loads operate from utility power since the solar power system does not produce power The inverter shuts down automatically in case of utility power failure for safety and reconnects automatically when utility power resumes Applications Urban residential and commercial systems and utility-scale power plants

Above components with the addition of a battery bank charge regulator and bi-directional inverter

The solar array charges the battery bank through a charge regulator DC power from the battery passes through the inverter and is converted to AC current and voltage Power is sent to the utility meter and is either consumed immediately by home or business loads or is sent out to the general utility grid network The utility meter spins backwards or two meters are used to record incoming and outgoing power At night loads operate and the battery bank is kept trickle charged from utility power since the solar power system does not produce power In case of utility power failure the direct connection to the utility meter is shut down for safety Selected circuits in the home or business that are connected to a special secondary inverter output continue to operate drawing energy from battery bank The solar array recharges the battery each day until normal utility power resumes Applications Urban residential and commercial systems

33

USES OF SOLAR ENERGY India receives solar energy equivalent to over 5000 trillion kWhyear which is far more than the total energy consumption of the country The big question is how to harness this power and where are its main application areas The energy in solar radiation can be used directly or indirectly for all of our energy needs in daily life including heating cooling lighting electrical power transportation and even environmental cleanup Many such applications are already cost-competitive with conventional energy sources for example photovoltaic (PV) electricity in remote applications is replacing diesel generator sets Some applications such as photovoltaic and solar heating are better known and popular while others such as solar detoxification of contaminated waters or solar distillation are less known Some basic facts Form of Energy Thermal energy This energy is used for CookingHeating DryingTimber seasoning Distillation ElectricityPower generation Cooling Refrigeration Cold storage Moreover application areas include (a) Domestic lighting (b) Street lighting (c) Village electrification (d) Water pumping (e) Desalination of salty water (f) Powering of remote telecommunication repeater stations and (g) Railway signals Some of the gadgets and other devices Solar cooker Flat plate solar cookers Concentrating collectors Solar hot water systems (Domestic and Industrial) Solar pond Solar hot air systems Solar Dryers Solar timber kilns solar stills Solar photovoltaic systems Solar pond Concentrating collectors Power Tower Air conditioning Solar collectors Refrigeration systems Heliostats For many years Solar Energy has been the power supply of choice for Industrial applications where power is required at remote locations Most systems in individual uses require a few kilowatts of power The examples are powering repeater stations for microwave TV and radio telemetry and radio telephones Solar energy is also frequently used on transportation signaling eg offshore navigation buoys lighthouses aircraft warning lights on pylons or structures and increasingly in road traffic warning signals Recent years have seen rapid growth in the number of installations of PV on to buildings that are connected to the electricity grid This area of demand has been stimulated in part by government subsidy programmes (especially Japan and Germany) and by green pricing policies of utilities or electricity service providers (eg in Switzerland and the USA) The central driving force though comes from the desire of individuals or companies to obtain their electricity from a clean non-polluting renewable source for which they are prepared to pay a small premium In these grid-connected systems PV System supplies electricity to the building and any day-time excess may be exported to the grid Batteries are not required because the grid supplies any extra demand However if one wants to be independent of the grid supply one will need battery storage to provide power outside daylight hours

34

Solar PV modules can be retrofitted on to a pitched roof above the existing roof-tiles or the tiles replaced by specially designed PV roof-tiles or roof-tiling systems Apart from off-grid homes other remote buildings such as schools community halls and clinics can all benefit from electrification with Solar Energy This can power TV video telephony and a range of refrigeration equipment

Perhaps the most exciting solar energy applications involve transportation Solar boats have been constructed some of which circumnavigate and solar cars have been built and raced Satellites depend on solar PV arrays for power Early communication satellites were severely limited by the lack of suitable power sources This severely limited the output power of the satellite transmitter The only source of power available within early weight restrictions was a very inefficient panel of solar cells without battery backup A major disadvantage of this type of power source is that the satellite has no power when it is in eclipse For continuous communications this outage is unacceptable A combination of solar cells (Using III-V Direct Band GaAs Semiconductor) and storage batteries is a better prime power source This is a practical choice even though the result is far from an ideal power source About ten percent of the energy of the sunlight that strikes the solar cells is converted to electrical power This low rate is sometimes decreased even further Early satellites had over 8500 solar cells mounted on the surface of the satellite which supplied about 42 watts of power No battery backup was provided in these satellites Newer communications satellites have about 32000 solar cells mounted on the surface of the satellite and they supply about 520 watts A nickel cadmium battery is used for backup power during eclipses

35

SOLAR ENERGY ADVANTAGES OPPORTUNITIES and CHALLENGES

Solar energy is by nature environmentally viable and non-polluting and often considered to be the best among such energy sources In particular the eco-friendly nature of this source is expected to come to the fore in the future The sun provides heat and light during the day in addition electricity can also be made available during the night provided the solar power system is grid-tied or has a battery-back There are other benefits of solar power systems as well

1 Saves money

Solar Energy Advantages

bull After the initial investment has been recovered the energy from the sun is practically FREE bull The recovery payback period for this investment can be very short depending on how much

electricity a household uses bull Financial incentives are available form the government that will reduce the cost bull Solar energy does not require any fuel bull Its not affected by the supply and demand of fuel and is therefore not subjected to the ever-

increasing price of gasoline bull The savings are immediate and for many years to come bull The use of solar energy indirectly reduces health costs

2 Environmentally friendly

bull Solar Energy is clean renewable (unlike gas oil and coal) and sustainable helping to protect our environment

bull It does not pollute our air by releasing carbon dioxide nitrogen oxide sulphur dioxide or mercury into the atmosphere like many traditional forms of electrical generations does

bull Therefore Solar Energy does not contribute to global warming acid rain or smog bull It actively contributes to the decrease of harmful green house gas emissions bull By not using any fuel Solar Energy does not contribute to the cost and problems of the recovery

and transportation of fuel or the storage of radioactive waste

3 Independent semi-independent

bull Solar Energy can be utilized to offset utility-supplied energy consumption It does not only reduce your electricity bill but will also continue to supply your home business with electricity in the event of a power outage

bull A Solar Energy system can operate entirely independent not requiring a connection to a power or gas grid at all Systems can therefore be installed in remote locations (like holiday log cabins) making it more practical and cost-effective than the supply of utility electricity to a new site

bull The use of Solar Energy reduces our dependence on foreign andor centralized sources of energy influenced by natural disasters or international events and so contributes to a sustainable future

bull Solar Energy supports local job and wealth creation fuelling local economies

4 Low no maintenance

bull Solar Energy systems are virtually maintenance free and will last for decades bull Once installed there are no recurring costs

36

bull They operate silently have no moving parts do not release offensive smells and do not require you to add any fuel

bull More solar panels can easily be added in the depending on need

bull The initial cost is the main disadvantage of installing a solar energy system largely because of the high cost of the semi-conducting materials used in building one

Solar Energy DisadvantagesChallenges

bull It is available most abundantly in areas with a high number of sunshine hours Where it is needed most cold countries in high northern or southern latitudes it is less easily captured and used

bull The cost of solar energy is also high compared to non-renewable utility-supplied electricity As energy shortages are becoming more common solar energy is becoming more price-competitive

bull Solar panels require quite a large area for installation to achieve a good level of efficiency bull The efficiency of the system also relies on the location of the sun although this problem can be

overcome with the installation of certain components bull The production of solar energy is influenced by the presence of clouds or pollution in the air bull Similarly no solar energy will be produced during nighttime although a battery backup system will

solve this problem bull DC power is produced by solar cells which must be converted to AC power before it can be used bull It is not entirely pollution free always While having much better credentials than fossil fuel for

polluting emissions the environmental costs of manufacturing and constructing solar energy appliances must not be forgotten

In the main these disadvantages are really engineering problems for which solutions are becoming increasingly available and efficient They are not serious reasons to dismiss the value or utility of solar energy

37

CONCLUSION LET THERE BE LIGHT Anyone still thinking that solar energy is mainly for heating water or cooking purposes is in for a huge shock Advances in materials and production modalities are enabling new products including those that integrate PV with building materials and others that provide novel sources of power for cell phones and note book computers Hence it came as a mild surprise to industry watchers and PV experts when NanoMarkets an industry analyst firm reported that the market for thin film and organic Photovoltaic (PV) will be worth over $72 billion by the year 2015

However the current situation is not so rosy Off the renewable energy options solar energy is believed to offer a very long-term potential but is currently a high-cost niche solution High initial investment costs and relatively low plant-load factors make it costlier than conventional sources of energy This is the primary reason for the low pick-up in solar power generation capacity Inadequate returns for investors and limited involvement of private companies that have the know-how and financial capability to undertake such projects have also stymied the industryrsquos growth However with support from large capital subsidies and grant funds from governments and developmental bodies globally the solar market has been growing at 30-40 a year Annual installations are projected to increase from 25gigawatt (GW) in 2007 to about 10 GW in 2012 and to 30 GW or more by 2025 By 2015 solar power is expected to achieve grid paritymdashthe point at which solar power becomes equal to or cheaper than grid powermdashin sunny climates for peak day-time demand

The solar market is far more complex than first envisioned This complexity is rooted in the fact that

despite apparent social and environmental benefits solar energy remains unaffordable to large populations in most developing countries

A successful solar venture should be structured with a narrow well-defined target market so that the investment offering can adequately address a relatively homogenous set of needs

Economies of scale play a big role in making a solar project sustainable Without a sizeable service population a private solar company cannot financially sustain the cost

A strong local presence and knowledge of the local market and regulations is important

38

A supportive legal environment is critical These include factors like incentives for solar energy or absence of competing subsidised electricity

India is one of the nations around the world that is blessed with a vast amount of solar insolation every year The challenging aim of rural electrification can be achieved by producing energy locally Photovoltaic systems offer the possibility of exploiting an energy source available everywhere while at the same time respecting the environment It can be safely said that India is churning out innovative incentive schemes for mass adoption of solar power Government policies will be implemented for market adoption and growth such as financing schemes for individuals investing to absorb the high upfront costs With PV market on a roll like never before several multi-national companies are bracing themselves to get a share of this rapidly expanding solar pie India is proving to be a pioneer in the realms of incentives for setting up a base and sustaining operations as well as being a huge market in itself India is among those few countries in Asian region that have a govt department exclusively dedicated to renewable energys promotion amp support There has been a continuous decline in the costs of photovoltaic For companies considering offshore location for designing and developing PV systems India may be the best answer This report provides a basic insight into the manufacturing processes that go into the creation of a cell or module A blend of theory and ground reality was adopted to elucidate the important concepts to the reader Although various modifications are possible and each firm has it own set of unique attributes the basic process is the same as reviewed in the previous pages The entire project has been an immense learning experience for the authors and we hope to enshrine some of the relevant information through this project This report should provide the fundamental block to a further more in-depth study of photovoltaic manufacturing in general Based on a review of the ongoing research in solar energy technologies it is clear that they will continue to improve promising higher efficiencies and lower costs Examples of such promising new technologies beyond the horizon include continued development of new thin-film technologies nano-scale antennas for conversion of sunlight to electricity

• TITLE PAGE
• ACKNOWLEDGEMENTS
• contents
• INTRODUCTION
• AN OVERVIEW OF A SOLAR CELLMODULE MANUFACTURING UNIT
• NEED FOR SOLAR ENERGY
• RAW MATERIALS NEEDED
• HISTORY OF PHOTOVOLTAICS
• Solar cell manufacturing processes
• Etching
• texturization
• DIFFUSION
• PLASMA ETCHING
• PSG removal
• PECVD
• SCREENPRINTING
• CELL TESTING AND SORTING
• CELL TESTING PARAMETERS
• Solar Module
• Module manufacturing and testing
• uses of modules
• USES OF SOLAR ENERGY
• SOLAR ENERGY ADVANTAGESOPPORTUNITIES and CHALLENGES
• Conclusions

31

USES OF MODULES

Solar modules can be used singly or interconnected in solar arrays for a variety of applications from charging batteries and driving motors to powering entire communities Sample configurations and uses (courtesy of GE Power website) include

Directly connected systems

bull

Solar module mounting hardware DC motor or pump and disconnect switch or circuit breaker

Applications Remote water pumping a ceiling or attic fan or a solar thermal (hot water) circulation pumps

The solar module produces current that is used immediately by a motor As sunlight rises and falls current and voltage rise and fall and the motor speeds up and slows down proportionally The motor operates slowly during cloudy or stormy weather and does not operate at night

bull Solar module fuse andor fused disconnect switch

Applications Trickle charging of vehicle starting batteries (fleet vehicles seasonal road equipment like snowploughs) and boat batteries

A small current flows from the solar module through a starting battery to counteract any inherent self-discharge in the battery A trickle charge flows only during daylight hours but on average offsets any self-discharge

Stand-alone systems

bull

Solar modules and mounting hardware a charge regulator storage batteries and disconnect switches or circuit breakers

bull

A solar array produces DC current that passes through the charge controller into storage batteries The charge regulator reduces or stops charging current to prevent battery overcharge Small DC loads may be connected to the charge regulator which can then prevent battery over-discharge The battery operates loads at night and during overcast or stormy days Solar modules recharge the batteries when average or good weather returns Applications Remote industrial areas (telecommunications navigational aids cathodic protection and traffic systems) and remote home systems

Above components with the addition of an inverter and an AC distribution centre

The inverter draws power from the battery and changes DC to AC current and voltage For safety power is sent to the distribution centre which houses circuit breakers for individual AC circuits The inverter operates from battery energy day or night Applications Remote home systems

bull Above components with the addition of a fuel generator (gasoline diesel or propane) a rectifier and a sophisticated hybrid system controller

The system controller monitors the battery voltage When the voltage drops to a safe but low level the generator is turned on AC output is converted to DC power and recharges the battery AC output can also be used directly to power AC loads When the battery reaches an almost full recharge level the generator is turned off The solar array can be sized to supply average GE needs throughout the year and the generator is used to fill in during seasonal low output periods and prolonged bad weather Applications Village power systems

32

Grid-connected systems

bull

Solar modules and mounting hardware disconnect switches or circuit breakers and a grid interactive inverter

bull

The solar array produces DC current that passes through inverter which converts to AC current and voltage Power is sent to the utility meter and is either consumed immediately by home or business loads or is sent out to the general utility grid network The utility meter spins backwards or two meters are used to record incoming and outgoing power At night loads operate from utility power since the solar power system does not produce power The inverter shuts down automatically in case of utility power failure for safety and reconnects automatically when utility power resumes Applications Urban residential and commercial systems and utility-scale power plants

Above components with the addition of a battery bank charge regulator and bi-directional inverter

The solar array charges the battery bank through a charge regulator DC power from the battery passes through the inverter and is converted to AC current and voltage Power is sent to the utility meter and is either consumed immediately by home or business loads or is sent out to the general utility grid network The utility meter spins backwards or two meters are used to record incoming and outgoing power At night loads operate and the battery bank is kept trickle charged from utility power since the solar power system does not produce power In case of utility power failure the direct connection to the utility meter is shut down for safety Selected circuits in the home or business that are connected to a special secondary inverter output continue to operate drawing energy from battery bank The solar array recharges the battery each day until normal utility power resumes Applications Urban residential and commercial systems

33

USES OF SOLAR ENERGY India receives solar energy equivalent to over 5000 trillion kWhyear which is far more than the total energy consumption of the country The big question is how to harness this power and where are its main application areas The energy in solar radiation can be used directly or indirectly for all of our energy needs in daily life including heating cooling lighting electrical power transportation and even environmental cleanup Many such applications are already cost-competitive with conventional energy sources for example photovoltaic (PV) electricity in remote applications is replacing diesel generator sets Some applications such as photovoltaic and solar heating are better known and popular while others such as solar detoxification of contaminated waters or solar distillation are less known Some basic facts Form of Energy Thermal energy This energy is used for CookingHeating DryingTimber seasoning Distillation ElectricityPower generation Cooling Refrigeration Cold storage Moreover application areas include (a) Domestic lighting (b) Street lighting (c) Village electrification (d) Water pumping (e) Desalination of salty water (f) Powering of remote telecommunication repeater stations and (g) Railway signals Some of the gadgets and other devices Solar cooker Flat plate solar cookers Concentrating collectors Solar hot water systems (Domestic and Industrial) Solar pond Solar hot air systems Solar Dryers Solar timber kilns solar stills Solar photovoltaic systems Solar pond Concentrating collectors Power Tower Air conditioning Solar collectors Refrigeration systems Heliostats For many years Solar Energy has been the power supply of choice for Industrial applications where power is required at remote locations Most systems in individual uses require a few kilowatts of power The examples are powering repeater stations for microwave TV and radio telemetry and radio telephones Solar energy is also frequently used on transportation signaling eg offshore navigation buoys lighthouses aircraft warning lights on pylons or structures and increasingly in road traffic warning signals Recent years have seen rapid growth in the number of installations of PV on to buildings that are connected to the electricity grid This area of demand has been stimulated in part by government subsidy programmes (especially Japan and Germany) and by green pricing policies of utilities or electricity service providers (eg in Switzerland and the USA) The central driving force though comes from the desire of individuals or companies to obtain their electricity from a clean non-polluting renewable source for which they are prepared to pay a small premium In these grid-connected systems PV System supplies electricity to the building and any day-time excess may be exported to the grid Batteries are not required because the grid supplies any extra demand However if one wants to be independent of the grid supply one will need battery storage to provide power outside daylight hours

34

Solar PV modules can be retrofitted on to a pitched roof above the existing roof-tiles or the tiles replaced by specially designed PV roof-tiles or roof-tiling systems Apart from off-grid homes other remote buildings such as schools community halls and clinics can all benefit from electrification with Solar Energy This can power TV video telephony and a range of refrigeration equipment

Perhaps the most exciting solar energy applications involve transportation Solar boats have been constructed some of which circumnavigate and solar cars have been built and raced Satellites depend on solar PV arrays for power Early communication satellites were severely limited by the lack of suitable power sources This severely limited the output power of the satellite transmitter The only source of power available within early weight restrictions was a very inefficient panel of solar cells without battery backup A major disadvantage of this type of power source is that the satellite has no power when it is in eclipse For continuous communications this outage is unacceptable A combination of solar cells (Using III-V Direct Band GaAs Semiconductor) and storage batteries is a better prime power source This is a practical choice even though the result is far from an ideal power source About ten percent of the energy of the sunlight that strikes the solar cells is converted to electrical power This low rate is sometimes decreased even further Early satellites had over 8500 solar cells mounted on the surface of the satellite which supplied about 42 watts of power No battery backup was provided in these satellites Newer communications satellites have about 32000 solar cells mounted on the surface of the satellite and they supply about 520 watts A nickel cadmium battery is used for backup power during eclipses

35

SOLAR ENERGY ADVANTAGES OPPORTUNITIES and CHALLENGES

Solar energy is by nature environmentally viable and non-polluting and often considered to be the best among such energy sources In particular the eco-friendly nature of this source is expected to come to the fore in the future The sun provides heat and light during the day in addition electricity can also be made available during the night provided the solar power system is grid-tied or has a battery-back There are other benefits of solar power systems as well

1 Saves money

Solar Energy Advantages

bull After the initial investment has been recovered the energy from the sun is practically FREE bull The recovery payback period for this investment can be very short depending on how much

electricity a household uses bull Financial incentives are available form the government that will reduce the cost bull Solar energy does not require any fuel bull Its not affected by the supply and demand of fuel and is therefore not subjected to the ever-

increasing price of gasoline bull The savings are immediate and for many years to come bull The use of solar energy indirectly reduces health costs

2 Environmentally friendly

bull Solar Energy is clean renewable (unlike gas oil and coal) and sustainable helping to protect our environment

bull It does not pollute our air by releasing carbon dioxide nitrogen oxide sulphur dioxide or mercury into the atmosphere like many traditional forms of electrical generations does

bull Therefore Solar Energy does not contribute to global warming acid rain or smog bull It actively contributes to the decrease of harmful green house gas emissions bull By not using any fuel Solar Energy does not contribute to the cost and problems of the recovery

and transportation of fuel or the storage of radioactive waste

3 Independent semi-independent

bull Solar Energy can be utilized to offset utility-supplied energy consumption It does not only reduce your electricity bill but will also continue to supply your home business with electricity in the event of a power outage

bull A Solar Energy system can operate entirely independent not requiring a connection to a power or gas grid at all Systems can therefore be installed in remote locations (like holiday log cabins) making it more practical and cost-effective than the supply of utility electricity to a new site

bull The use of Solar Energy reduces our dependence on foreign andor centralized sources of energy influenced by natural disasters or international events and so contributes to a sustainable future

bull Solar Energy supports local job and wealth creation fuelling local economies

4 Low no maintenance

bull Solar Energy systems are virtually maintenance free and will last for decades bull Once installed there are no recurring costs

36

bull They operate silently have no moving parts do not release offensive smells and do not require you to add any fuel

bull More solar panels can easily be added in the depending on need

bull The initial cost is the main disadvantage of installing a solar energy system largely because of the high cost of the semi-conducting materials used in building one

Solar Energy DisadvantagesChallenges

bull It is available most abundantly in areas with a high number of sunshine hours Where it is needed most cold countries in high northern or southern latitudes it is less easily captured and used

bull The cost of solar energy is also high compared to non-renewable utility-supplied electricity As energy shortages are becoming more common solar energy is becoming more price-competitive

bull Solar panels require quite a large area for installation to achieve a good level of efficiency bull The efficiency of the system also relies on the location of the sun although this problem can be

overcome with the installation of certain components bull The production of solar energy is influenced by the presence of clouds or pollution in the air bull Similarly no solar energy will be produced during nighttime although a battery backup system will

solve this problem bull DC power is produced by solar cells which must be converted to AC power before it can be used bull It is not entirely pollution free always While having much better credentials than fossil fuel for

polluting emissions the environmental costs of manufacturing and constructing solar energy appliances must not be forgotten

In the main these disadvantages are really engineering problems for which solutions are becoming increasingly available and efficient They are not serious reasons to dismiss the value or utility of solar energy

37

CONCLUSION LET THERE BE LIGHT Anyone still thinking that solar energy is mainly for heating water or cooking purposes is in for a huge shock Advances in materials and production modalities are enabling new products including those that integrate PV with building materials and others that provide novel sources of power for cell phones and note book computers Hence it came as a mild surprise to industry watchers and PV experts when NanoMarkets an industry analyst firm reported that the market for thin film and organic Photovoltaic (PV) will be worth over $72 billion by the year 2015

However the current situation is not so rosy Off the renewable energy options solar energy is believed to offer a very long-term potential but is currently a high-cost niche solution High initial investment costs and relatively low plant-load factors make it costlier than conventional sources of energy This is the primary reason for the low pick-up in solar power generation capacity Inadequate returns for investors and limited involvement of private companies that have the know-how and financial capability to undertake such projects have also stymied the industryrsquos growth However with support from large capital subsidies and grant funds from governments and developmental bodies globally the solar market has been growing at 30-40 a year Annual installations are projected to increase from 25gigawatt (GW) in 2007 to about 10 GW in 2012 and to 30 GW or more by 2025 By 2015 solar power is expected to achieve grid paritymdashthe point at which solar power becomes equal to or cheaper than grid powermdashin sunny climates for peak day-time demand

The solar market is far more complex than first envisioned This complexity is rooted in the fact that

despite apparent social and environmental benefits solar energy remains unaffordable to large populations in most developing countries

A successful solar venture should be structured with a narrow well-defined target market so that the investment offering can adequately address a relatively homogenous set of needs

Economies of scale play a big role in making a solar project sustainable Without a sizeable service population a private solar company cannot financially sustain the cost

A strong local presence and knowledge of the local market and regulations is important

38

A supportive legal environment is critical These include factors like incentives for solar energy or absence of competing subsidised electricity

India is one of the nations around the world that is blessed with a vast amount of solar insolation every year The challenging aim of rural electrification can be achieved by producing energy locally Photovoltaic systems offer the possibility of exploiting an energy source available everywhere while at the same time respecting the environment It can be safely said that India is churning out innovative incentive schemes for mass adoption of solar power Government policies will be implemented for market adoption and growth such as financing schemes for individuals investing to absorb the high upfront costs With PV market on a roll like never before several multi-national companies are bracing themselves to get a share of this rapidly expanding solar pie India is proving to be a pioneer in the realms of incentives for setting up a base and sustaining operations as well as being a huge market in itself India is among those few countries in Asian region that have a govt department exclusively dedicated to renewable energys promotion amp support There has been a continuous decline in the costs of photovoltaic For companies considering offshore location for designing and developing PV systems India may be the best answer This report provides a basic insight into the manufacturing processes that go into the creation of a cell or module A blend of theory and ground reality was adopted to elucidate the important concepts to the reader Although various modifications are possible and each firm has it own set of unique attributes the basic process is the same as reviewed in the previous pages The entire project has been an immense learning experience for the authors and we hope to enshrine some of the relevant information through this project This report should provide the fundamental block to a further more in-depth study of photovoltaic manufacturing in general Based on a review of the ongoing research in solar energy technologies it is clear that they will continue to improve promising higher efficiencies and lower costs Examples of such promising new technologies beyond the horizon include continued development of new thin-film technologies nano-scale antennas for conversion of sunlight to electricity

• TITLE PAGE
• ACKNOWLEDGEMENTS
• contents
• INTRODUCTION
• AN OVERVIEW OF A SOLAR CELLMODULE MANUFACTURING UNIT
• NEED FOR SOLAR ENERGY
• RAW MATERIALS NEEDED
• HISTORY OF PHOTOVOLTAICS
• Solar cell manufacturing processes
• Etching
• texturization
• DIFFUSION
• PLASMA ETCHING
• PSG removal
• PECVD
• SCREENPRINTING
• CELL TESTING AND SORTING
• CELL TESTING PARAMETERS
• Solar Module
• Module manufacturing and testing
• uses of modules
• USES OF SOLAR ENERGY
• SOLAR ENERGY ADVANTAGESOPPORTUNITIES and CHALLENGES
• Conclusions

32

Grid-connected systems

bull

Solar modules and mounting hardware disconnect switches or circuit breakers and a grid interactive inverter

bull

The solar array produces DC current that passes through inverter which converts to AC current and voltage Power is sent to the utility meter and is either consumed immediately by home or business loads or is sent out to the general utility grid network The utility meter spins backwards or two meters are used to record incoming and outgoing power At night loads operate from utility power since the solar power system does not produce power The inverter shuts down automatically in case of utility power failure for safety and reconnects automatically when utility power resumes Applications Urban residential and commercial systems and utility-scale power plants

Above components with the addition of a battery bank charge regulator and bi-directional inverter

The solar array charges the battery bank through a charge regulator DC power from the battery passes through the inverter and is converted to AC current and voltage Power is sent to the utility meter and is either consumed immediately by home or business loads or is sent out to the general utility grid network The utility meter spins backwards or two meters are used to record incoming and outgoing power At night loads operate and the battery bank is kept trickle charged from utility power since the solar power system does not produce power In case of utility power failure the direct connection to the utility meter is shut down for safety Selected circuits in the home or business that are connected to a special secondary inverter output continue to operate drawing energy from battery bank The solar array recharges the battery each day until normal utility power resumes Applications Urban residential and commercial systems

33

USES OF SOLAR ENERGY India receives solar energy equivalent to over 5000 trillion kWhyear which is far more than the total energy consumption of the country The big question is how to harness this power and where are its main application areas The energy in solar radiation can be used directly or indirectly for all of our energy needs in daily life including heating cooling lighting electrical power transportation and even environmental cleanup Many such applications are already cost-competitive with conventional energy sources for example photovoltaic (PV) electricity in remote applications is replacing diesel generator sets Some applications such as photovoltaic and solar heating are better known and popular while others such as solar detoxification of contaminated waters or solar distillation are less known Some basic facts Form of Energy Thermal energy This energy is used for CookingHeating DryingTimber seasoning Distillation ElectricityPower generation Cooling Refrigeration Cold storage Moreover application areas include (a) Domestic lighting (b) Street lighting (c) Village electrification (d) Water pumping (e) Desalination of salty water (f) Powering of remote telecommunication repeater stations and (g) Railway signals Some of the gadgets and other devices Solar cooker Flat plate solar cookers Concentrating collectors Solar hot water systems (Domestic and Industrial) Solar pond Solar hot air systems Solar Dryers Solar timber kilns solar stills Solar photovoltaic systems Solar pond Concentrating collectors Power Tower Air conditioning Solar collectors Refrigeration systems Heliostats For many years Solar Energy has been the power supply of choice for Industrial applications where power is required at remote locations Most systems in individual uses require a few kilowatts of power The examples are powering repeater stations for microwave TV and radio telemetry and radio telephones Solar energy is also frequently used on transportation signaling eg offshore navigation buoys lighthouses aircraft warning lights on pylons or structures and increasingly in road traffic warning signals Recent years have seen rapid growth in the number of installations of PV on to buildings that are connected to the electricity grid This area of demand has been stimulated in part by government subsidy programmes (especially Japan and Germany) and by green pricing policies of utilities or electricity service providers (eg in Switzerland and the USA) The central driving force though comes from the desire of individuals or companies to obtain their electricity from a clean non-polluting renewable source for which they are prepared to pay a small premium In these grid-connected systems PV System supplies electricity to the building and any day-time excess may be exported to the grid Batteries are not required because the grid supplies any extra demand However if one wants to be independent of the grid supply one will need battery storage to provide power outside daylight hours

34

Solar PV modules can be retrofitted on to a pitched roof above the existing roof-tiles or the tiles replaced by specially designed PV roof-tiles or roof-tiling systems Apart from off-grid homes other remote buildings such as schools community halls and clinics can all benefit from electrification with Solar Energy This can power TV video telephony and a range of refrigeration equipment

Perhaps the most exciting solar energy applications involve transportation Solar boats have been constructed some of which circumnavigate and solar cars have been built and raced Satellites depend on solar PV arrays for power Early communication satellites were severely limited by the lack of suitable power sources This severely limited the output power of the satellite transmitter The only source of power available within early weight restrictions was a very inefficient panel of solar cells without battery backup A major disadvantage of this type of power source is that the satellite has no power when it is in eclipse For continuous communications this outage is unacceptable A combination of solar cells (Using III-V Direct Band GaAs Semiconductor) and storage batteries is a better prime power source This is a practical choice even though the result is far from an ideal power source About ten percent of the energy of the sunlight that strikes the solar cells is converted to electrical power This low rate is sometimes decreased even further Early satellites had over 8500 solar cells mounted on the surface of the satellite which supplied about 42 watts of power No battery backup was provided in these satellites Newer communications satellites have about 32000 solar cells mounted on the surface of the satellite and they supply about 520 watts A nickel cadmium battery is used for backup power during eclipses

35

SOLAR ENERGY ADVANTAGES OPPORTUNITIES and CHALLENGES

Solar energy is by nature environmentally viable and non-polluting and often considered to be the best among such energy sources In particular the eco-friendly nature of this source is expected to come to the fore in the future The sun provides heat and light during the day in addition electricity can also be made available during the night provided the solar power system is grid-tied or has a battery-back There are other benefits of solar power systems as well

1 Saves money

Solar Energy Advantages

bull After the initial investment has been recovered the energy from the sun is practically FREE bull The recovery payback period for this investment can be very short depending on how much

electricity a household uses bull Financial incentives are available form the government that will reduce the cost bull Solar energy does not require any fuel bull Its not affected by the supply and demand of fuel and is therefore not subjected to the ever-

increasing price of gasoline bull The savings are immediate and for many years to come bull The use of solar energy indirectly reduces health costs

2 Environmentally friendly

bull Solar Energy is clean renewable (unlike gas oil and coal) and sustainable helping to protect our environment

bull It does not pollute our air by releasing carbon dioxide nitrogen oxide sulphur dioxide or mercury into the atmosphere like many traditional forms of electrical generations does

bull Therefore Solar Energy does not contribute to global warming acid rain or smog bull It actively contributes to the decrease of harmful green house gas emissions bull By not using any fuel Solar Energy does not contribute to the cost and problems of the recovery

and transportation of fuel or the storage of radioactive waste

3 Independent semi-independent

bull Solar Energy can be utilized to offset utility-supplied energy consumption It does not only reduce your electricity bill but will also continue to supply your home business with electricity in the event of a power outage

bull A Solar Energy system can operate entirely independent not requiring a connection to a power or gas grid at all Systems can therefore be installed in remote locations (like holiday log cabins) making it more practical and cost-effective than the supply of utility electricity to a new site

bull The use of Solar Energy reduces our dependence on foreign andor centralized sources of energy influenced by natural disasters or international events and so contributes to a sustainable future

bull Solar Energy supports local job and wealth creation fuelling local economies

4 Low no maintenance

bull Solar Energy systems are virtually maintenance free and will last for decades bull Once installed there are no recurring costs

36

bull They operate silently have no moving parts do not release offensive smells and do not require you to add any fuel

bull More solar panels can easily be added in the depending on need

bull The initial cost is the main disadvantage of installing a solar energy system largely because of the high cost of the semi-conducting materials used in building one

Solar Energy DisadvantagesChallenges

bull It is available most abundantly in areas with a high number of sunshine hours Where it is needed most cold countries in high northern or southern latitudes it is less easily captured and used

bull The cost of solar energy is also high compared to non-renewable utility-supplied electricity As energy shortages are becoming more common solar energy is becoming more price-competitive

bull Solar panels require quite a large area for installation to achieve a good level of efficiency bull The efficiency of the system also relies on the location of the sun although this problem can be

overcome with the installation of certain components bull The production of solar energy is influenced by the presence of clouds or pollution in the air bull Similarly no solar energy will be produced during nighttime although a battery backup system will

solve this problem bull DC power is produced by solar cells which must be converted to AC power before it can be used bull It is not entirely pollution free always While having much better credentials than fossil fuel for

polluting emissions the environmental costs of manufacturing and constructing solar energy appliances must not be forgotten

In the main these disadvantages are really engineering problems for which solutions are becoming increasingly available and efficient They are not serious reasons to dismiss the value or utility of solar energy

37

CONCLUSION LET THERE BE LIGHT Anyone still thinking that solar energy is mainly for heating water or cooking purposes is in for a huge shock Advances in materials and production modalities are enabling new products including those that integrate PV with building materials and others that provide novel sources of power for cell phones and note book computers Hence it came as a mild surprise to industry watchers and PV experts when NanoMarkets an industry analyst firm reported that the market for thin film and organic Photovoltaic (PV) will be worth over $72 billion by the year 2015

However the current situation is not so rosy Off the renewable energy options solar energy is believed to offer a very long-term potential but is currently a high-cost niche solution High initial investment costs and relatively low plant-load factors make it costlier than conventional sources of energy This is the primary reason for the low pick-up in solar power generation capacity Inadequate returns for investors and limited involvement of private companies that have the know-how and financial capability to undertake such projects have also stymied the industryrsquos growth However with support from large capital subsidies and grant funds from governments and developmental bodies globally the solar market has been growing at 30-40 a year Annual installations are projected to increase from 25gigawatt (GW) in 2007 to about 10 GW in 2012 and to 30 GW or more by 2025 By 2015 solar power is expected to achieve grid paritymdashthe point at which solar power becomes equal to or cheaper than grid powermdashin sunny climates for peak day-time demand

The solar market is far more complex than first envisioned This complexity is rooted in the fact that

despite apparent social and environmental benefits solar energy remains unaffordable to large populations in most developing countries

A successful solar venture should be structured with a narrow well-defined target market so that the investment offering can adequately address a relatively homogenous set of needs

Economies of scale play a big role in making a solar project sustainable Without a sizeable service population a private solar company cannot financially sustain the cost

A strong local presence and knowledge of the local market and regulations is important

38

A supportive legal environment is critical These include factors like incentives for solar energy or absence of competing subsidised electricity

India is one of the nations around the world that is blessed with a vast amount of solar insolation every year The challenging aim of rural electrification can be achieved by producing energy locally Photovoltaic systems offer the possibility of exploiting an energy source available everywhere while at the same time respecting the environment It can be safely said that India is churning out innovative incentive schemes for mass adoption of solar power Government policies will be implemented for market adoption and growth such as financing schemes for individuals investing to absorb the high upfront costs With PV market on a roll like never before several multi-national companies are bracing themselves to get a share of this rapidly expanding solar pie India is proving to be a pioneer in the realms of incentives for setting up a base and sustaining operations as well as being a huge market in itself India is among those few countries in Asian region that have a govt department exclusively dedicated to renewable energys promotion amp support There has been a continuous decline in the costs of photovoltaic For companies considering offshore location for designing and developing PV systems India may be the best answer This report provides a basic insight into the manufacturing processes that go into the creation of a cell or module A blend of theory and ground reality was adopted to elucidate the important concepts to the reader Although various modifications are possible and each firm has it own set of unique attributes the basic process is the same as reviewed in the previous pages The entire project has been an immense learning experience for the authors and we hope to enshrine some of the relevant information through this project This report should provide the fundamental block to a further more in-depth study of photovoltaic manufacturing in general Based on a review of the ongoing research in solar energy technologies it is clear that they will continue to improve promising higher efficiencies and lower costs Examples of such promising new technologies beyond the horizon include continued development of new thin-film technologies nano-scale antennas for conversion of sunlight to electricity

• TITLE PAGE
• ACKNOWLEDGEMENTS
• contents
• INTRODUCTION
• AN OVERVIEW OF A SOLAR CELLMODULE MANUFACTURING UNIT
• NEED FOR SOLAR ENERGY
• RAW MATERIALS NEEDED
• HISTORY OF PHOTOVOLTAICS
• Solar cell manufacturing processes
• Etching
• texturization
• DIFFUSION
• PLASMA ETCHING
• PSG removal
• PECVD
• SCREENPRINTING
• CELL TESTING AND SORTING
• CELL TESTING PARAMETERS
• Solar Module
• Module manufacturing and testing
• uses of modules
• USES OF SOLAR ENERGY
• SOLAR ENERGY ADVANTAGESOPPORTUNITIES and CHALLENGES
• Conclusions

33

USES OF SOLAR ENERGY India receives solar energy equivalent to over 5000 trillion kWhyear which is far more than the total energy consumption of the country The big question is how to harness this power and where are its main application areas The energy in solar radiation can be used directly or indirectly for all of our energy needs in daily life including heating cooling lighting electrical power transportation and even environmental cleanup Many such applications are already cost-competitive with conventional energy sources for example photovoltaic (PV) electricity in remote applications is replacing diesel generator sets Some applications such as photovoltaic and solar heating are better known and popular while others such as solar detoxification of contaminated waters or solar distillation are less known Some basic facts Form of Energy Thermal energy This energy is used for CookingHeating DryingTimber seasoning Distillation ElectricityPower generation Cooling Refrigeration Cold storage Moreover application areas include (a) Domestic lighting (b) Street lighting (c) Village electrification (d) Water pumping (e) Desalination of salty water (f) Powering of remote telecommunication repeater stations and (g) Railway signals Some of the gadgets and other devices Solar cooker Flat plate solar cookers Concentrating collectors Solar hot water systems (Domestic and Industrial) Solar pond Solar hot air systems Solar Dryers Solar timber kilns solar stills Solar photovoltaic systems Solar pond Concentrating collectors Power Tower Air conditioning Solar collectors Refrigeration systems Heliostats For many years Solar Energy has been the power supply of choice for Industrial applications where power is required at remote locations Most systems in individual uses require a few kilowatts of power The examples are powering repeater stations for microwave TV and radio telemetry and radio telephones Solar energy is also frequently used on transportation signaling eg offshore navigation buoys lighthouses aircraft warning lights on pylons or structures and increasingly in road traffic warning signals Recent years have seen rapid growth in the number of installations of PV on to buildings that are connected to the electricity grid This area of demand has been stimulated in part by government subsidy programmes (especially Japan and Germany) and by green pricing policies of utilities or electricity service providers (eg in Switzerland and the USA) The central driving force though comes from the desire of individuals or companies to obtain their electricity from a clean non-polluting renewable source for which they are prepared to pay a small premium In these grid-connected systems PV System supplies electricity to the building and any day-time excess may be exported to the grid Batteries are not required because the grid supplies any extra demand However if one wants to be independent of the grid supply one will need battery storage to provide power outside daylight hours

34

Solar PV modules can be retrofitted on to a pitched roof above the existing roof-tiles or the tiles replaced by specially designed PV roof-tiles or roof-tiling systems Apart from off-grid homes other remote buildings such as schools community halls and clinics can all benefit from electrification with Solar Energy This can power TV video telephony and a range of refrigeration equipment

Perhaps the most exciting solar energy applications involve transportation Solar boats have been constructed some of which circumnavigate and solar cars have been built and raced Satellites depend on solar PV arrays for power Early communication satellites were severely limited by the lack of suitable power sources This severely limited the output power of the satellite transmitter The only source of power available within early weight restrictions was a very inefficient panel of solar cells without battery backup A major disadvantage of this type of power source is that the satellite has no power when it is in eclipse For continuous communications this outage is unacceptable A combination of solar cells (Using III-V Direct Band GaAs Semiconductor) and storage batteries is a better prime power source This is a practical choice even though the result is far from an ideal power source About ten percent of the energy of the sunlight that strikes the solar cells is converted to electrical power This low rate is sometimes decreased even further Early satellites had over 8500 solar cells mounted on the surface of the satellite which supplied about 42 watts of power No battery backup was provided in these satellites Newer communications satellites have about 32000 solar cells mounted on the surface of the satellite and they supply about 520 watts A nickel cadmium battery is used for backup power during eclipses

35

SOLAR ENERGY ADVANTAGES OPPORTUNITIES and CHALLENGES

Solar energy is by nature environmentally viable and non-polluting and often considered to be the best among such energy sources In particular the eco-friendly nature of this source is expected to come to the fore in the future The sun provides heat and light during the day in addition electricity can also be made available during the night provided the solar power system is grid-tied or has a battery-back There are other benefits of solar power systems as well

1 Saves money

Solar Energy Advantages

bull After the initial investment has been recovered the energy from the sun is practically FREE bull The recovery payback period for this investment can be very short depending on how much

electricity a household uses bull Financial incentives are available form the government that will reduce the cost bull Solar energy does not require any fuel bull Its not affected by the supply and demand of fuel and is therefore not subjected to the ever-

increasing price of gasoline bull The savings are immediate and for many years to come bull The use of solar energy indirectly reduces health costs

2 Environmentally friendly

bull Solar Energy is clean renewable (unlike gas oil and coal) and sustainable helping to protect our environment

bull It does not pollute our air by releasing carbon dioxide nitrogen oxide sulphur dioxide or mercury into the atmosphere like many traditional forms of electrical generations does

bull Therefore Solar Energy does not contribute to global warming acid rain or smog bull It actively contributes to the decrease of harmful green house gas emissions bull By not using any fuel Solar Energy does not contribute to the cost and problems of the recovery

and transportation of fuel or the storage of radioactive waste

3 Independent semi-independent

bull Solar Energy can be utilized to offset utility-supplied energy consumption It does not only reduce your electricity bill but will also continue to supply your home business with electricity in the event of a power outage

bull A Solar Energy system can operate entirely independent not requiring a connection to a power or gas grid at all Systems can therefore be installed in remote locations (like holiday log cabins) making it more practical and cost-effective than the supply of utility electricity to a new site

bull The use of Solar Energy reduces our dependence on foreign andor centralized sources of energy influenced by natural disasters or international events and so contributes to a sustainable future

bull Solar Energy supports local job and wealth creation fuelling local economies

4 Low no maintenance

bull Solar Energy systems are virtually maintenance free and will last for decades bull Once installed there are no recurring costs

36

bull They operate silently have no moving parts do not release offensive smells and do not require you to add any fuel

bull More solar panels can easily be added in the depending on need

bull The initial cost is the main disadvantage of installing a solar energy system largely because of the high cost of the semi-conducting materials used in building one

Solar Energy DisadvantagesChallenges

bull It is available most abundantly in areas with a high number of sunshine hours Where it is needed most cold countries in high northern or southern latitudes it is less easily captured and used

bull The cost of solar energy is also high compared to non-renewable utility-supplied electricity As energy shortages are becoming more common solar energy is becoming more price-competitive

bull Solar panels require quite a large area for installation to achieve a good level of efficiency bull The efficiency of the system also relies on the location of the sun although this problem can be

overcome with the installation of certain components bull The production of solar energy is influenced by the presence of clouds or pollution in the air bull Similarly no solar energy will be produced during nighttime although a battery backup system will

solve this problem bull DC power is produced by solar cells which must be converted to AC power before it can be used bull It is not entirely pollution free always While having much better credentials than fossil fuel for

polluting emissions the environmental costs of manufacturing and constructing solar energy appliances must not be forgotten

In the main these disadvantages are really engineering problems for which solutions are becoming increasingly available and efficient They are not serious reasons to dismiss the value or utility of solar energy

37

CONCLUSION LET THERE BE LIGHT Anyone still thinking that solar energy is mainly for heating water or cooking purposes is in for a huge shock Advances in materials and production modalities are enabling new products including those that integrate PV with building materials and others that provide novel sources of power for cell phones and note book computers Hence it came as a mild surprise to industry watchers and PV experts when NanoMarkets an industry analyst firm reported that the market for thin film and organic Photovoltaic (PV) will be worth over $72 billion by the year 2015

However the current situation is not so rosy Off the renewable energy options solar energy is believed to offer a very long-term potential but is currently a high-cost niche solution High initial investment costs and relatively low plant-load factors make it costlier than conventional sources of energy This is the primary reason for the low pick-up in solar power generation capacity Inadequate returns for investors and limited involvement of private companies that have the know-how and financial capability to undertake such projects have also stymied the industryrsquos growth However with support from large capital subsidies and grant funds from governments and developmental bodies globally the solar market has been growing at 30-40 a year Annual installations are projected to increase from 25gigawatt (GW) in 2007 to about 10 GW in 2012 and to 30 GW or more by 2025 By 2015 solar power is expected to achieve grid paritymdashthe point at which solar power becomes equal to or cheaper than grid powermdashin sunny climates for peak day-time demand

The solar market is far more complex than first envisioned This complexity is rooted in the fact that

despite apparent social and environmental benefits solar energy remains unaffordable to large populations in most developing countries

A successful solar venture should be structured with a narrow well-defined target market so that the investment offering can adequately address a relatively homogenous set of needs

Economies of scale play a big role in making a solar project sustainable Without a sizeable service population a private solar company cannot financially sustain the cost

A strong local presence and knowledge of the local market and regulations is important

38

A supportive legal environment is critical These include factors like incentives for solar energy or absence of competing subsidised electricity

India is one of the nations around the world that is blessed with a vast amount of solar insolation every year The challenging aim of rural electrification can be achieved by producing energy locally Photovoltaic systems offer the possibility of exploiting an energy source available everywhere while at the same time respecting the environment It can be safely said that India is churning out innovative incentive schemes for mass adoption of solar power Government policies will be implemented for market adoption and growth such as financing schemes for individuals investing to absorb the high upfront costs With PV market on a roll like never before several multi-national companies are bracing themselves to get a share of this rapidly expanding solar pie India is proving to be a pioneer in the realms of incentives for setting up a base and sustaining operations as well as being a huge market in itself India is among those few countries in Asian region that have a govt department exclusively dedicated to renewable energys promotion amp support There has been a continuous decline in the costs of photovoltaic For companies considering offshore location for designing and developing PV systems India may be the best answer This report provides a basic insight into the manufacturing processes that go into the creation of a cell or module A blend of theory and ground reality was adopted to elucidate the important concepts to the reader Although various modifications are possible and each firm has it own set of unique attributes the basic process is the same as reviewed in the previous pages The entire project has been an immense learning experience for the authors and we hope to enshrine some of the relevant information through this project This report should provide the fundamental block to a further more in-depth study of photovoltaic manufacturing in general Based on a review of the ongoing research in solar energy technologies it is clear that they will continue to improve promising higher efficiencies and lower costs Examples of such promising new technologies beyond the horizon include continued development of new thin-film technologies nano-scale antennas for conversion of sunlight to electricity

• TITLE PAGE
• ACKNOWLEDGEMENTS
• contents
• INTRODUCTION
• AN OVERVIEW OF A SOLAR CELLMODULE MANUFACTURING UNIT
• NEED FOR SOLAR ENERGY
• RAW MATERIALS NEEDED
• HISTORY OF PHOTOVOLTAICS
• Solar cell manufacturing processes
• Etching
• texturization
• DIFFUSION
• PLASMA ETCHING
• PSG removal
• PECVD
• SCREENPRINTING
• CELL TESTING AND SORTING
• CELL TESTING PARAMETERS
• Solar Module
• Module manufacturing and testing
• uses of modules
• USES OF SOLAR ENERGY
• SOLAR ENERGY ADVANTAGESOPPORTUNITIES and CHALLENGES
• Conclusions

34

Solar PV modules can be retrofitted on to a pitched roof above the existing roof-tiles or the tiles replaced by specially designed PV roof-tiles or roof-tiling systems Apart from off-grid homes other remote buildings such as schools community halls and clinics can all benefit from electrification with Solar Energy This can power TV video telephony and a range of refrigeration equipment

Perhaps the most exciting solar energy applications involve transportation Solar boats have been constructed some of which circumnavigate and solar cars have been built and raced Satellites depend on solar PV arrays for power Early communication satellites were severely limited by the lack of suitable power sources This severely limited the output power of the satellite transmitter The only source of power available within early weight restrictions was a very inefficient panel of solar cells without battery backup A major disadvantage of this type of power source is that the satellite has no power when it is in eclipse For continuous communications this outage is unacceptable A combination of solar cells (Using III-V Direct Band GaAs Semiconductor) and storage batteries is a better prime power source This is a practical choice even though the result is far from an ideal power source About ten percent of the energy of the sunlight that strikes the solar cells is converted to electrical power This low rate is sometimes decreased even further Early satellites had over 8500 solar cells mounted on the surface of the satellite which supplied about 42 watts of power No battery backup was provided in these satellites Newer communications satellites have about 32000 solar cells mounted on the surface of the satellite and they supply about 520 watts A nickel cadmium battery is used for backup power during eclipses

35

SOLAR ENERGY ADVANTAGES OPPORTUNITIES and CHALLENGES

Solar energy is by nature environmentally viable and non-polluting and often considered to be the best among such energy sources In particular the eco-friendly nature of this source is expected to come to the fore in the future The sun provides heat and light during the day in addition electricity can also be made available during the night provided the solar power system is grid-tied or has a battery-back There are other benefits of solar power systems as well

1 Saves money

Solar Energy Advantages

bull After the initial investment has been recovered the energy from the sun is practically FREE bull The recovery payback period for this investment can be very short depending on how much

electricity a household uses bull Financial incentives are available form the government that will reduce the cost bull Solar energy does not require any fuel bull Its not affected by the supply and demand of fuel and is therefore not subjected to the ever-

increasing price of gasoline bull The savings are immediate and for many years to come bull The use of solar energy indirectly reduces health costs

2 Environmentally friendly

bull Solar Energy is clean renewable (unlike gas oil and coal) and sustainable helping to protect our environment

bull It does not pollute our air by releasing carbon dioxide nitrogen oxide sulphur dioxide or mercury into the atmosphere like many traditional forms of electrical generations does

bull Therefore Solar Energy does not contribute to global warming acid rain or smog bull It actively contributes to the decrease of harmful green house gas emissions bull By not using any fuel Solar Energy does not contribute to the cost and problems of the recovery

and transportation of fuel or the storage of radioactive waste

3 Independent semi-independent

bull Solar Energy can be utilized to offset utility-supplied energy consumption It does not only reduce your electricity bill but will also continue to supply your home business with electricity in the event of a power outage

bull A Solar Energy system can operate entirely independent not requiring a connection to a power or gas grid at all Systems can therefore be installed in remote locations (like holiday log cabins) making it more practical and cost-effective than the supply of utility electricity to a new site

bull The use of Solar Energy reduces our dependence on foreign andor centralized sources of energy influenced by natural disasters or international events and so contributes to a sustainable future

bull Solar Energy supports local job and wealth creation fuelling local economies

4 Low no maintenance

bull Solar Energy systems are virtually maintenance free and will last for decades bull Once installed there are no recurring costs

36

bull They operate silently have no moving parts do not release offensive smells and do not require you to add any fuel

bull More solar panels can easily be added in the depending on need

bull The initial cost is the main disadvantage of installing a solar energy system largely because of the high cost of the semi-conducting materials used in building one

Solar Energy DisadvantagesChallenges

bull It is available most abundantly in areas with a high number of sunshine hours Where it is needed most cold countries in high northern or southern latitudes it is less easily captured and used

bull The cost of solar energy is also high compared to non-renewable utility-supplied electricity As energy shortages are becoming more common solar energy is becoming more price-competitive

bull Solar panels require quite a large area for installation to achieve a good level of efficiency bull The efficiency of the system also relies on the location of the sun although this problem can be

overcome with the installation of certain components bull The production of solar energy is influenced by the presence of clouds or pollution in the air bull Similarly no solar energy will be produced during nighttime although a battery backup system will

solve this problem bull DC power is produced by solar cells which must be converted to AC power before it can be used bull It is not entirely pollution free always While having much better credentials than fossil fuel for

polluting emissions the environmental costs of manufacturing and constructing solar energy appliances must not be forgotten

In the main these disadvantages are really engineering problems for which solutions are becoming increasingly available and efficient They are not serious reasons to dismiss the value or utility of solar energy

37

CONCLUSION LET THERE BE LIGHT Anyone still thinking that solar energy is mainly for heating water or cooking purposes is in for a huge shock Advances in materials and production modalities are enabling new products including those that integrate PV with building materials and others that provide novel sources of power for cell phones and note book computers Hence it came as a mild surprise to industry watchers and PV experts when NanoMarkets an industry analyst firm reported that the market for thin film and organic Photovoltaic (PV) will be worth over $72 billion by the year 2015

However the current situation is not so rosy Off the renewable energy options solar energy is believed to offer a very long-term potential but is currently a high-cost niche solution High initial investment costs and relatively low plant-load factors make it costlier than conventional sources of energy This is the primary reason for the low pick-up in solar power generation capacity Inadequate returns for investors and limited involvement of private companies that have the know-how and financial capability to undertake such projects have also stymied the industryrsquos growth However with support from large capital subsidies and grant funds from governments and developmental bodies globally the solar market has been growing at 30-40 a year Annual installations are projected to increase from 25gigawatt (GW) in 2007 to about 10 GW in 2012 and to 30 GW or more by 2025 By 2015 solar power is expected to achieve grid paritymdashthe point at which solar power becomes equal to or cheaper than grid powermdashin sunny climates for peak day-time demand

The solar market is far more complex than first envisioned This complexity is rooted in the fact that

despite apparent social and environmental benefits solar energy remains unaffordable to large populations in most developing countries

A successful solar venture should be structured with a narrow well-defined target market so that the investment offering can adequately address a relatively homogenous set of needs

Economies of scale play a big role in making a solar project sustainable Without a sizeable service population a private solar company cannot financially sustain the cost

A strong local presence and knowledge of the local market and regulations is important

38

A supportive legal environment is critical These include factors like incentives for solar energy or absence of competing subsidised electricity

India is one of the nations around the world that is blessed with a vast amount of solar insolation every year The challenging aim of rural electrification can be achieved by producing energy locally Photovoltaic systems offer the possibility of exploiting an energy source available everywhere while at the same time respecting the environment It can be safely said that India is churning out innovative incentive schemes for mass adoption of solar power Government policies will be implemented for market adoption and growth such as financing schemes for individuals investing to absorb the high upfront costs With PV market on a roll like never before several multi-national companies are bracing themselves to get a share of this rapidly expanding solar pie India is proving to be a pioneer in the realms of incentives for setting up a base and sustaining operations as well as being a huge market in itself India is among those few countries in Asian region that have a govt department exclusively dedicated to renewable energys promotion amp support There has been a continuous decline in the costs of photovoltaic For companies considering offshore location for designing and developing PV systems India may be the best answer This report provides a basic insight into the manufacturing processes that go into the creation of a cell or module A blend of theory and ground reality was adopted to elucidate the important concepts to the reader Although various modifications are possible and each firm has it own set of unique attributes the basic process is the same as reviewed in the previous pages The entire project has been an immense learning experience for the authors and we hope to enshrine some of the relevant information through this project This report should provide the fundamental block to a further more in-depth study of photovoltaic manufacturing in general Based on a review of the ongoing research in solar energy technologies it is clear that they will continue to improve promising higher efficiencies and lower costs Examples of such promising new technologies beyond the horizon include continued development of new thin-film technologies nano-scale antennas for conversion of sunlight to electricity

• TITLE PAGE
• ACKNOWLEDGEMENTS
• contents
• INTRODUCTION
• AN OVERVIEW OF A SOLAR CELLMODULE MANUFACTURING UNIT
• NEED FOR SOLAR ENERGY
• RAW MATERIALS NEEDED
• HISTORY OF PHOTOVOLTAICS
• Solar cell manufacturing processes
• Etching
• texturization
• DIFFUSION
• PLASMA ETCHING
• PSG removal
• PECVD
• SCREENPRINTING
• CELL TESTING AND SORTING
• CELL TESTING PARAMETERS
• Solar Module
• Module manufacturing and testing
• uses of modules
• USES OF SOLAR ENERGY
• SOLAR ENERGY ADVANTAGESOPPORTUNITIES and CHALLENGES
• Conclusions

35

SOLAR ENERGY ADVANTAGES OPPORTUNITIES and CHALLENGES

Solar energy is by nature environmentally viable and non-polluting and often considered to be the best among such energy sources In particular the eco-friendly nature of this source is expected to come to the fore in the future The sun provides heat and light during the day in addition electricity can also be made available during the night provided the solar power system is grid-tied or has a battery-back There are other benefits of solar power systems as well

1 Saves money

Solar Energy Advantages

bull After the initial investment has been recovered the energy from the sun is practically FREE bull The recovery payback period for this investment can be very short depending on how much

electricity a household uses bull Financial incentives are available form the government that will reduce the cost bull Solar energy does not require any fuel bull Its not affected by the supply and demand of fuel and is therefore not subjected to the ever-

increasing price of gasoline bull The savings are immediate and for many years to come bull The use of solar energy indirectly reduces health costs

2 Environmentally friendly

bull Solar Energy is clean renewable (unlike gas oil and coal) and sustainable helping to protect our environment

bull It does not pollute our air by releasing carbon dioxide nitrogen oxide sulphur dioxide or mercury into the atmosphere like many traditional forms of electrical generations does

bull Therefore Solar Energy does not contribute to global warming acid rain or smog bull It actively contributes to the decrease of harmful green house gas emissions bull By not using any fuel Solar Energy does not contribute to the cost and problems of the recovery

and transportation of fuel or the storage of radioactive waste

3 Independent semi-independent

bull Solar Energy can be utilized to offset utility-supplied energy consumption It does not only reduce your electricity bill but will also continue to supply your home business with electricity in the event of a power outage

bull A Solar Energy system can operate entirely independent not requiring a connection to a power or gas grid at all Systems can therefore be installed in remote locations (like holiday log cabins) making it more practical and cost-effective than the supply of utility electricity to a new site

bull The use of Solar Energy reduces our dependence on foreign andor centralized sources of energy influenced by natural disasters or international events and so contributes to a sustainable future

bull Solar Energy supports local job and wealth creation fuelling local economies

4 Low no maintenance

bull Solar Energy systems are virtually maintenance free and will last for decades bull Once installed there are no recurring costs

36

bull They operate silently have no moving parts do not release offensive smells and do not require you to add any fuel

bull More solar panels can easily be added in the depending on need

bull The initial cost is the main disadvantage of installing a solar energy system largely because of the high cost of the semi-conducting materials used in building one

Solar Energy DisadvantagesChallenges

bull It is available most abundantly in areas with a high number of sunshine hours Where it is needed most cold countries in high northern or southern latitudes it is less easily captured and used

bull The cost of solar energy is also high compared to non-renewable utility-supplied electricity As energy shortages are becoming more common solar energy is becoming more price-competitive

bull Solar panels require quite a large area for installation to achieve a good level of efficiency bull The efficiency of the system also relies on the location of the sun although this problem can be

overcome with the installation of certain components bull The production of solar energy is influenced by the presence of clouds or pollution in the air bull Similarly no solar energy will be produced during nighttime although a battery backup system will

solve this problem bull DC power is produced by solar cells which must be converted to AC power before it can be used bull It is not entirely pollution free always While having much better credentials than fossil fuel for

polluting emissions the environmental costs of manufacturing and constructing solar energy appliances must not be forgotten

In the main these disadvantages are really engineering problems for which solutions are becoming increasingly available and efficient They are not serious reasons to dismiss the value or utility of solar energy

37

CONCLUSION LET THERE BE LIGHT Anyone still thinking that solar energy is mainly for heating water or cooking purposes is in for a huge shock Advances in materials and production modalities are enabling new products including those that integrate PV with building materials and others that provide novel sources of power for cell phones and note book computers Hence it came as a mild surprise to industry watchers and PV experts when NanoMarkets an industry analyst firm reported that the market for thin film and organic Photovoltaic (PV) will be worth over $72 billion by the year 2015

However the current situation is not so rosy Off the renewable energy options solar energy is believed to offer a very long-term potential but is currently a high-cost niche solution High initial investment costs and relatively low plant-load factors make it costlier than conventional sources of energy This is the primary reason for the low pick-up in solar power generation capacity Inadequate returns for investors and limited involvement of private companies that have the know-how and financial capability to undertake such projects have also stymied the industryrsquos growth However with support from large capital subsidies and grant funds from governments and developmental bodies globally the solar market has been growing at 30-40 a year Annual installations are projected to increase from 25gigawatt (GW) in 2007 to about 10 GW in 2012 and to 30 GW or more by 2025 By 2015 solar power is expected to achieve grid paritymdashthe point at which solar power becomes equal to or cheaper than grid powermdashin sunny climates for peak day-time demand

The solar market is far more complex than first envisioned This complexity is rooted in the fact that

despite apparent social and environmental benefits solar energy remains unaffordable to large populations in most developing countries

A successful solar venture should be structured with a narrow well-defined target market so that the investment offering can adequately address a relatively homogenous set of needs

Economies of scale play a big role in making a solar project sustainable Without a sizeable service population a private solar company cannot financially sustain the cost

A strong local presence and knowledge of the local market and regulations is important

38

A supportive legal environment is critical These include factors like incentives for solar energy or absence of competing subsidised electricity

India is one of the nations around the world that is blessed with a vast amount of solar insolation every year The challenging aim of rural electrification can be achieved by producing energy locally Photovoltaic systems offer the possibility of exploiting an energy source available everywhere while at the same time respecting the environment It can be safely said that India is churning out innovative incentive schemes for mass adoption of solar power Government policies will be implemented for market adoption and growth such as financing schemes for individuals investing to absorb the high upfront costs With PV market on a roll like never before several multi-national companies are bracing themselves to get a share of this rapidly expanding solar pie India is proving to be a pioneer in the realms of incentives for setting up a base and sustaining operations as well as being a huge market in itself India is among those few countries in Asian region that have a govt department exclusively dedicated to renewable energys promotion amp support There has been a continuous decline in the costs of photovoltaic For companies considering offshore location for designing and developing PV systems India may be the best answer This report provides a basic insight into the manufacturing processes that go into the creation of a cell or module A blend of theory and ground reality was adopted to elucidate the important concepts to the reader Although various modifications are possible and each firm has it own set of unique attributes the basic process is the same as reviewed in the previous pages The entire project has been an immense learning experience for the authors and we hope to enshrine some of the relevant information through this project This report should provide the fundamental block to a further more in-depth study of photovoltaic manufacturing in general Based on a review of the ongoing research in solar energy technologies it is clear that they will continue to improve promising higher efficiencies and lower costs Examples of such promising new technologies beyond the horizon include continued development of new thin-film technologies nano-scale antennas for conversion of sunlight to electricity

• TITLE PAGE
• ACKNOWLEDGEMENTS
• contents
• INTRODUCTION
• AN OVERVIEW OF A SOLAR CELLMODULE MANUFACTURING UNIT
• NEED FOR SOLAR ENERGY
• RAW MATERIALS NEEDED
• HISTORY OF PHOTOVOLTAICS
• Solar cell manufacturing processes
• Etching
• texturization
• DIFFUSION
• PLASMA ETCHING
• PSG removal
• PECVD
• SCREENPRINTING
• CELL TESTING AND SORTING
• CELL TESTING PARAMETERS
• Solar Module
• Module manufacturing and testing
• uses of modules
• USES OF SOLAR ENERGY
• SOLAR ENERGY ADVANTAGESOPPORTUNITIES and CHALLENGES
• Conclusions

36

bull They operate silently have no moving parts do not release offensive smells and do not require you to add any fuel

bull More solar panels can easily be added in the depending on need

bull The initial cost is the main disadvantage of installing a solar energy system largely because of the high cost of the semi-conducting materials used in building one

Solar Energy DisadvantagesChallenges

bull It is available most abundantly in areas with a high number of sunshine hours Where it is needed most cold countries in high northern or southern latitudes it is less easily captured and used

bull The cost of solar energy is also high compared to non-renewable utility-supplied electricity As energy shortages are becoming more common solar energy is becoming more price-competitive

bull Solar panels require quite a large area for installation to achieve a good level of efficiency bull The efficiency of the system also relies on the location of the sun although this problem can be

overcome with the installation of certain components bull The production of solar energy is influenced by the presence of clouds or pollution in the air bull Similarly no solar energy will be produced during nighttime although a battery backup system will

solve this problem bull DC power is produced by solar cells which must be converted to AC power before it can be used bull It is not entirely pollution free always While having much better credentials than fossil fuel for

polluting emissions the environmental costs of manufacturing and constructing solar energy appliances must not be forgotten

In the main these disadvantages are really engineering problems for which solutions are becoming increasingly available and efficient They are not serious reasons to dismiss the value or utility of solar energy

37

CONCLUSION LET THERE BE LIGHT Anyone still thinking that solar energy is mainly for heating water or cooking purposes is in for a huge shock Advances in materials and production modalities are enabling new products including those that integrate PV with building materials and others that provide novel sources of power for cell phones and note book computers Hence it came as a mild surprise to industry watchers and PV experts when NanoMarkets an industry analyst firm reported that the market for thin film and organic Photovoltaic (PV) will be worth over $72 billion by the year 2015

However the current situation is not so rosy Off the renewable energy options solar energy is believed to offer a very long-term potential but is currently a high-cost niche solution High initial investment costs and relatively low plant-load factors make it costlier than conventional sources of energy This is the primary reason for the low pick-up in solar power generation capacity Inadequate returns for investors and limited involvement of private companies that have the know-how and financial capability to undertake such projects have also stymied the industryrsquos growth However with support from large capital subsidies and grant funds from governments and developmental bodies globally the solar market has been growing at 30-40 a year Annual installations are projected to increase from 25gigawatt (GW) in 2007 to about 10 GW in 2012 and to 30 GW or more by 2025 By 2015 solar power is expected to achieve grid paritymdashthe point at which solar power becomes equal to or cheaper than grid powermdashin sunny climates for peak day-time demand

The solar market is far more complex than first envisioned This complexity is rooted in the fact that

despite apparent social and environmental benefits solar energy remains unaffordable to large populations in most developing countries

A successful solar venture should be structured with a narrow well-defined target market so that the investment offering can adequately address a relatively homogenous set of needs

Economies of scale play a big role in making a solar project sustainable Without a sizeable service population a private solar company cannot financially sustain the cost

A strong local presence and knowledge of the local market and regulations is important

38

A supportive legal environment is critical These include factors like incentives for solar energy or absence of competing subsidised electricity

India is one of the nations around the world that is blessed with a vast amount of solar insolation every year The challenging aim of rural electrification can be achieved by producing energy locally Photovoltaic systems offer the possibility of exploiting an energy source available everywhere while at the same time respecting the environment It can be safely said that India is churning out innovative incentive schemes for mass adoption of solar power Government policies will be implemented for market adoption and growth such as financing schemes for individuals investing to absorb the high upfront costs With PV market on a roll like never before several multi-national companies are bracing themselves to get a share of this rapidly expanding solar pie India is proving to be a pioneer in the realms of incentives for setting up a base and sustaining operations as well as being a huge market in itself India is among those few countries in Asian region that have a govt department exclusively dedicated to renewable energys promotion amp support There has been a continuous decline in the costs of photovoltaic For companies considering offshore location for designing and developing PV systems India may be the best answer This report provides a basic insight into the manufacturing processes that go into the creation of a cell or module A blend of theory and ground reality was adopted to elucidate the important concepts to the reader Although various modifications are possible and each firm has it own set of unique attributes the basic process is the same as reviewed in the previous pages The entire project has been an immense learning experience for the authors and we hope to enshrine some of the relevant information through this project This report should provide the fundamental block to a further more in-depth study of photovoltaic manufacturing in general Based on a review of the ongoing research in solar energy technologies it is clear that they will continue to improve promising higher efficiencies and lower costs Examples of such promising new technologies beyond the horizon include continued development of new thin-film technologies nano-scale antennas for conversion of sunlight to electricity

• TITLE PAGE
• ACKNOWLEDGEMENTS
• contents
• INTRODUCTION
• AN OVERVIEW OF A SOLAR CELLMODULE MANUFACTURING UNIT
• NEED FOR SOLAR ENERGY
• RAW MATERIALS NEEDED
• HISTORY OF PHOTOVOLTAICS
• Solar cell manufacturing processes
• Etching
• texturization
• DIFFUSION
• PLASMA ETCHING
• PSG removal
• PECVD
• SCREENPRINTING
• CELL TESTING AND SORTING
• CELL TESTING PARAMETERS
• Solar Module
• Module manufacturing and testing
• uses of modules
• USES OF SOLAR ENERGY
• SOLAR ENERGY ADVANTAGESOPPORTUNITIES and CHALLENGES
• Conclusions

37

CONCLUSION LET THERE BE LIGHT Anyone still thinking that solar energy is mainly for heating water or cooking purposes is in for a huge shock Advances in materials and production modalities are enabling new products including those that integrate PV with building materials and others that provide novel sources of power for cell phones and note book computers Hence it came as a mild surprise to industry watchers and PV experts when NanoMarkets an industry analyst firm reported that the market for thin film and organic Photovoltaic (PV) will be worth over $72 billion by the year 2015

However the current situation is not so rosy Off the renewable energy options solar energy is believed to offer a very long-term potential but is currently a high-cost niche solution High initial investment costs and relatively low plant-load factors make it costlier than conventional sources of energy This is the primary reason for the low pick-up in solar power generation capacity Inadequate returns for investors and limited involvement of private companies that have the know-how and financial capability to undertake such projects have also stymied the industryrsquos growth However with support from large capital subsidies and grant funds from governments and developmental bodies globally the solar market has been growing at 30-40 a year Annual installations are projected to increase from 25gigawatt (GW) in 2007 to about 10 GW in 2012 and to 30 GW or more by 2025 By 2015 solar power is expected to achieve grid paritymdashthe point at which solar power becomes equal to or cheaper than grid powermdashin sunny climates for peak day-time demand

The solar market is far more complex than first envisioned This complexity is rooted in the fact that

despite apparent social and environmental benefits solar energy remains unaffordable to large populations in most developing countries

A successful solar venture should be structured with a narrow well-defined target market so that the investment offering can adequately address a relatively homogenous set of needs

Economies of scale play a big role in making a solar project sustainable Without a sizeable service population a private solar company cannot financially sustain the cost

A strong local presence and knowledge of the local market and regulations is important

38

A supportive legal environment is critical These include factors like incentives for solar energy or absence of competing subsidised electricity

India is one of the nations around the world that is blessed with a vast amount of solar insolation every year The challenging aim of rural electrification can be achieved by producing energy locally Photovoltaic systems offer the possibility of exploiting an energy source available everywhere while at the same time respecting the environment It can be safely said that India is churning out innovative incentive schemes for mass adoption of solar power Government policies will be implemented for market adoption and growth such as financing schemes for individuals investing to absorb the high upfront costs With PV market on a roll like never before several multi-national companies are bracing themselves to get a share of this rapidly expanding solar pie India is proving to be a pioneer in the realms of incentives for setting up a base and sustaining operations as well as being a huge market in itself India is among those few countries in Asian region that have a govt department exclusively dedicated to renewable energys promotion amp support There has been a continuous decline in the costs of photovoltaic For companies considering offshore location for designing and developing PV systems India may be the best answer This report provides a basic insight into the manufacturing processes that go into the creation of a cell or module A blend of theory and ground reality was adopted to elucidate the important concepts to the reader Although various modifications are possible and each firm has it own set of unique attributes the basic process is the same as reviewed in the previous pages The entire project has been an immense learning experience for the authors and we hope to enshrine some of the relevant information through this project This report should provide the fundamental block to a further more in-depth study of photovoltaic manufacturing in general Based on a review of the ongoing research in solar energy technologies it is clear that they will continue to improve promising higher efficiencies and lower costs Examples of such promising new technologies beyond the horizon include continued development of new thin-film technologies nano-scale antennas for conversion of sunlight to electricity

• TITLE PAGE
• ACKNOWLEDGEMENTS
• contents
• INTRODUCTION
• AN OVERVIEW OF A SOLAR CELLMODULE MANUFACTURING UNIT
• NEED FOR SOLAR ENERGY
• RAW MATERIALS NEEDED
• HISTORY OF PHOTOVOLTAICS
• Solar cell manufacturing processes
• Etching
• texturization
• DIFFUSION
• PLASMA ETCHING
• PSG removal
• PECVD
• SCREENPRINTING
• CELL TESTING AND SORTING
• CELL TESTING PARAMETERS
• Solar Module
• Module manufacturing and testing
• uses of modules
• USES OF SOLAR ENERGY
• SOLAR ENERGY ADVANTAGESOPPORTUNITIES and CHALLENGES
• Conclusions

38

A supportive legal environment is critical These include factors like incentives for solar energy or absence of competing subsidised electricity

India is one of the nations around the world that is blessed with a vast amount of solar insolation every year The challenging aim of rural electrification can be achieved by producing energy locally Photovoltaic systems offer the possibility of exploiting an energy source available everywhere while at the same time respecting the environment It can be safely said that India is churning out innovative incentive schemes for mass adoption of solar power Government policies will be implemented for market adoption and growth such as financing schemes for individuals investing to absorb the high upfront costs With PV market on a roll like never before several multi-national companies are bracing themselves to get a share of this rapidly expanding solar pie India is proving to be a pioneer in the realms of incentives for setting up a base and sustaining operations as well as being a huge market in itself India is among those few countries in Asian region that have a govt department exclusively dedicated to renewable energys promotion amp support There has been a continuous decline in the costs of photovoltaic For companies considering offshore location for designing and developing PV systems India may be the best answer This report provides a basic insight into the manufacturing processes that go into the creation of a cell or module A blend of theory and ground reality was adopted to elucidate the important concepts to the reader Although various modifications are possible and each firm has it own set of unique attributes the basic process is the same as reviewed in the previous pages The entire project has been an immense learning experience for the authors and we hope to enshrine some of the relevant information through this project This report should provide the fundamental block to a further more in-depth study of photovoltaic manufacturing in general Based on a review of the ongoing research in solar energy technologies it is clear that they will continue to improve promising higher efficiencies and lower costs Examples of such promising new technologies beyond the horizon include continued development of new thin-film technologies nano-scale antennas for conversion of sunlight to electricity

• TITLE PAGE
• ACKNOWLEDGEMENTS
• contents
• INTRODUCTION
• AN OVERVIEW OF A SOLAR CELLMODULE MANUFACTURING UNIT
• NEED FOR SOLAR ENERGY
• RAW MATERIALS NEEDED
• HISTORY OF PHOTOVOLTAICS
• Solar cell manufacturing processes
• Etching
• texturization
• DIFFUSION
• PLASMA ETCHING
• PSG removal
• PECVD
• SCREENPRINTING
• CELL TESTING AND SORTING
• CELL TESTING PARAMETERS
• Solar Module
• Module manufacturing and testing
• uses of modules
• USES OF SOLAR ENERGY
• SOLAR ENERGY ADVANTAGESOPPORTUNITIES and CHALLENGES
• Conclusions