Chapter - I General Introduction -...

Chapter - I

General Introduction

In this chapter, starting from the basic need to pursue solar photovoltaics, a review about the basic

requirements of a semiconductor to be suitable for use in a solar cell is presented. The basic

construction and the energy conversion process in a solar cell are also presented in elaborate manner

taking an example of the simplest p-n junction silicon solar cell. Different technologies available for

production of solar cells are reviewed in brief. This includes the conventional single crystal wafer

technology, various thin film technologies, organic and dye sensitized solar cell technologies and

advanced solar cell technologies.

More emphasis is given on the understanding of the material properties of a-Si:H and nc-Si:H and the

properties owing to solar cell application of the same. A light is thrown in various angles on the

structural and opto-electronic properties of nc-Si:H material, which is main part of the study presented

in this thesis. The important aspect of the light induced degradation, the Staebler-Wronski Effect

(SWE) is also covered in this chapter. And finally the outline of the thesis is given at end of the chapter.

Chapter I 1

Adinath M. Funde Ph. D. Thesis, Pune University (November 2010)

11..11:: RREENNEEWWAABBLLEE EENNEERRGGYY--BBRRIIEEFF IINNTTRROODDUUCCTTIIOONN

Energy is an important input for the economic development of a nation. Presently, the world’s

energy supply is largely based on fossil fuels and nuclear power. These sources of energy will not

last forever and have proven to be contributors to our environmental problems. It has been

accepted that in order to fuel economic progress, energy generation capacity has to grow

drastically by at least the factor of two in the forthcoming two-three decades. It is becoming clear

everyday by the 1973 oil crisis, oil price shock of 2008 and the ever increasing oil prices since

then, we are approaching the ceiling of the oil supply. This declining availability of the fossil fuels

and the sustainability criterion has led to the fast development of renewable energy resources

such as biomass, biofuels, wind, solar, geothermal, and hydro energies etc, [1].

The world’s energy system is at a crossroads. Current global trends in energy supply and

consumption are patently unsustainable-environmentally, economically, socially but this situation

can be changed if we can secure the supply of reliable and affordable energy; and effect a rapid

transformation to low-carbon, efficient and environmentally gracious system of energy supply [2].

Since exhaustible energy sources are limited, there is an urgent need to focus attention on

development of renewable energy sources and use of energy efficient technologies. Now, more

than ever, countries all over the world have fully recognized the imperative to promote wide the

adoption of renewable energy into their country’s energy sources to boost sustained economic

growth, social development and environmental protectorship. Following hydro, wind and biomass,

all renewable energy technologies are becoming cost-competitive with other low-carbon

technologies in electricity generation. With increasing scope, scale, research and development,

the costs of renewable energy technologies will come down allowing renewable energy to make

major contributions to electricity generations, heating, cooling and transport. It is also estimated

that renewable energy can contribute at least half of all electric power in each of the large

economies by 2050, even those with significantly higher electricity demand. Also, as energy

independence is an increasingly important factor for economic stability and political security.

Renewable energy, especially solar energy has potential to provide energy independence and

security of supply to every economy. Figure 1.1 depicts the physical potential of the renewable

energy sources. The present global primary energy consumption (GPEC) is ~ 15 TW. If we look

Chapter I 2


at the potential of renewable energy sources compared to GPEC, the multiples of each are

shown in the figure. Giant potential is of solar energy with highest multiplicity factor of 1800

followed by wind and biomass with multiplicity factor 200 and 20 each. The solar energy can be

harnessed in different ways. Among the available renewable energy sources Solar Photovoltaics

(PV) is the most attractive candidate with tremendous potential and advantages. The contents

hereafter will be restricted to the PV technology only.

Figure 1.1: Physical potential of different renewable energy sources (Source: www.setfor2020.eu)

The exploitation of solar energy has become an essential measure to address present energy

shortages and environmental problems. We have several reasons to be optimistic as there is

great excitement about the possibilities opening up before scientific community in the field of solar

PV. Looking at the present contribution of solar PV, it appears that the target ahead to be

accomplished is very hard to achieve and will take a great deal of efforts and perhaps also a

great deal of time. In addition to the proven PV technologies, new ideas are opening up like

possible applications of nanotechnology in PV. For instance, the PV applications of CNTs, Si

nano-wires, CdSe nano-rods and TiO2 nanocrystals have been extensively investigated [3].

Biggest advantage for the exploitation of nanotechnology in harvesting solar energy is the present

challenge in the field. The top-down and the bottom-up approaches at the nanometer scale have

shown convergence for first time in history when it is within reach to design a macroscopic

functional material by controlling the composition of matter on every length scale right from the

atom up. In the realm of nanoscience and nanotechnology, many important properties of

materials are controlled or limited by behavior on the nanometer scale, and so there are great

Chapter I 3


opportunities opening up the doors in this area. As solar energy has enormous potential on the

long term, it is expected to grow strongly in the coming years. More recent reports on PV by using

nanotechnology to utilize minimum material and trap maximum incident solar light using

advanced light management concepts keeps the strong hopes for breakthrough in the PV

technology [4]. This solar cell design based on Si microwires achieves efficient absorption of

sunlight using only 1% of the active material used in conventional designs. Having enough

curiosity about it, let us understand the PV and the basic building blocks of it, the solar cells.

11..22:: SSOOLLAARR PPHHOOTTOOVVOOLLTTAAIICCSS ((PPVV))

11..22..11:: IInnttrroodduuccttiioonn Photovoltaic solar electricity is the most elegant method to produce electricity without moving

parts, emissions or noise and all this by converting abundant sunlight without practical limitations.

The relevance of solar energy specifically PV can be justified mainly with the factors like

scalability, environmental impact and the security of source. The scalability means the abundant

availability of the solar radiation to be utilized for PV. Solar cells are zero emission electricity

generators, which prove its environment friendliness. And the security of the source means

individual and the country does not have to rely on others for source unlike fossil fuels, nuclear

power etc. This is the reason why there is a worldwide major push to solar PV, despite of its

higher generation cost compared to the conventional counterparts. The hurdle of the higher

production cost can be eased and is successfully demonstrated by countries like Germany with

the concept of feed-in-tariff [5]. Feed-in-tariff is essentially a policy mechanism designed to

encourage adoption of renewable energy sources. In Japan PV is already cost effective, when

the conventional electricity is charged differentially depending on the peak demand hours.

11..22..22:: TThhee bbiigg ppiiccttuurree ooff PPhhoottoovvoollttaaiicc iinndduussttrryy

The year 2008 was an exceptional year for solar PV. As seen in the figure 1.2 at the end of 2008,

with more than 16 (sixteen!) GW of generation capacity, the global PV market has grown six fold

compared to 2004 [6]. The USA has passed Waxman-Markey Clean Energy Bill 2009 aiming

electric utilities to meet 20 % of their electricity demand through renewable energy sources and

energy efficiency by 2020 [7]. The same ambitious plan of European Union (EU) is targeting 20

Chapter I 4


% share of renewable energies by 2020. As an example of the momentum, Spain alone has

added more than 2 GW PV electricity followed by Germany (1.5 GW).

Figure 1.2: Solar Photovoltaics, existing world capacity 1995-2008 (Source: www1.eere.energy.gov)

India also, in the beginning of January 2010 has launched major solar initiative namely

Jawaharlal Nehru National Solar Mission. The Mission recommends the implementation in 3

stages leading up to an installed capacity of 20 GW by the end of the 13th

11..22..33:: BBeesstt rreesseeaarrcchh--cceellll eeffffiicciieenncciieess

Five Year Plan in 2022.

Though it also includes the solar thermal technology, PV will have major contribution [8].

Other than numerous advantages for preferring PV, the recent worldwide move towards it is

mainly driven by the criteria of eco-friendliness, sustainability and self reliability of the energy

source. If one looks into the history of PV, there is tremendous progress in the cost effectiveness

and efficiency. Figure 1.3 summarizes the progress of in efficiency of various PV technologies.

This was possible only through aggressive efforts both at research and manufacturing technology

levels. There are several methods used to manufacture different semiconductor materials for PV

application. In the early stages of PV, when it was introduced as a technology to power satellites

solar cells were made up of c-Si only. But at present several PV technologies are available.

Chapter I 5


Figure 1.3: Best research-cell efficiencies (Source: www.en.wikipedia.org/wiki/Solar_cell)

The PV maestros and researchers worldwide believe in dividing the solar cell technologies till

date into three generations, i) First Generation consisting of Si wafer based technology ii) Second

Generation with low-cost approach of thin film solar cells of Si and other materials like CdS,

CdTe, CIGS etc. and iii) Third Generation based on high efficiency thin film technology. Tandem

solar cells are the examples of the third generation PV technology [9-11]. Another definition as

introduced by Luque and Marti-High-efficiency devices that can ideally surpass the Shockley-

Queisser efficiency limit also gives nice idea of various generations [12-14]. The common driving

force to pursue either research in the existing technology or to introduce new PV technology is to

improve efficiency and to make it cost effective.

11..33:: CCRRYYSSTTAALLLLIINNEE SSIILLIICCOONN ((cc--SSii)) SSOOLLAARR CCEELLLLSS

11..33..11:: EEnneerrggyy ccoonnvveerrssiioonn pprroocceessss iinn aa ssoollaarr cceellll--TThhee pphhoottoovvoollttaaiicc eeffffeecctt

The energy conversion in solar cell is based on two important steps namely photo-generation and

charge separation [15, 16]. In the first step, semiconductor material absorbs the incoming

photons and generates electron-hole pairs (EHPs). In this step, the decisive parameter is the

Chapter I 6


band gap energy (Eg) of semiconductor. In an ideal case, no photons with an energy hν < Eg will

contribute to the photo-generation, whereas all photons with an energy hν > Eg will each

contribute to the photo-generation of EHPs, with the excess energy (hν - Eg) being very rapidly

lost because of thermalization. The maximum limit for the photo generated electric current density

(Jph) is therefore given by the flux of photons with an energy hν > Eg. Thus, Jph decreases with

increasing Eg. At the same time, the net energy transferred to each EHP increases, as it is equal

to (hυ - Eg). There exists an optimum for Eg in this first step which is ~ 1.1 eV. At this band gap,

roughly half of the incident solar energy is transferred and utilizes for generation of EHPs. This

maximum limit will only be approached if optical losses due to reflections, shading by grid

patterns, and so forth are minimized and if the semiconductor is thick enough to absorb all

incident photons. In the second step of the energy conversion process, the photo-generated

EHPs are separated, with electrons drifting to one of the electrodes and holes drifting to the other

electrode, because of the internal electric field created by the p-n junction in a diode structure of

the solar cell. In case of thin film silicon solar cells the junction must be p-i-n type structure. The

generation of internal electric field is governed by so called built-in voltage (Vbi) which is always

smaller than (Eg/q) where q is the fundamental charge. This second step of the charge separation

is achieved at the price of the additional energy loss; about one third to one half of the remaining

energy has to be sacrificed. The charge separation process is evidently more efficient, if the

internal electric field is stronger i. e. if Vbi

11..33..22:: TThhee pp--nn jjuunnccttiioonn

is higher. This means that in the optimal case, the

charge process is more efficient if the band gap energy is higher. Therefore the optimum band

gap energy for maximizing the energy conversion efficiency of the whole solar cell, including the

second step of charge separation, is shifted to higher values, when compared to the optimum for

the first step alone. The optimum for the energy conversion process is around 1.5 eV.

To understand a solar cell p-n junction, the junction of Si is the most simple to understand. The p-

n junction consists of a layer of n-type Si joined to a layer of p-type Si, with an uninterrupted Si

crystal structure across the junction. Schematic of typical p-n c-Si solar cell is shown in figure 1.4.

The n-layer has an abundance of free electrons and the p-layer has an abundance of free holes.

Chapter I 7


Since there cannot be abrupt separation of charges at the p-n junction, both electrons and holes

are subject to random diffusion within the Si crystalline structure, so each tends to diffuse from

regions of high concentration to regions of low concentration.

Figure 1.4: Schematic of typical p-n junction c-Si solar cell structure

Figure 1.5: The p-n junction showing electron and hole drift and diffusion

The enormous concentration differences of hole and electron between the n-side and the p-side

of the junction cause large concentration gradients across the junction. The net result is that the

electrons diffuse across the junction into the p-region and the holes diffuse across the junction

into the n-region, which is shown in figure 1.5. Thus, as electrons diffuse to the p-side of the

junction, they leave behind positively charged electron donor ions that are covalently bound to the

Si lattice. As holes diffuse to the n-side of the junction, they leave behind negatively charged

hole-donor ions that are covalently bound to the Si lattice on the p-side of the junction. The

---------

+ +

+ +

+ +

+ +

+ +

P-type

Positive donar ions left

behind at junction Negative acceptor ions left

behind at junction

Holes diffuse

Electrons diffuse

Holes drift

Electrons drift

EBuilt-in

N-type

Space charge layer

n-type c-Si

Back contact

p-type c-Si

Chapter I 8


diffusion of charge carriers across the junction thus creates an electric field across the junction,

directed from the positive ions on the n-side to the negative ions on the p-side.

Electric fields exert forces on charged particles according to the familiar relationship. This

force causes the charge carriers to drift. In the case of the positively charged holes, they drift in

the direction of the electric field i. e. from the n-side to the p-side of the junction. The negatively

charged electrons drift in the direction opposite the field i. e. from the p-side to the n-side of the

junction.

11..33..33:: IIlllluummiinnaatteedd pp--nn jjuunnccttiioonn

Figure 1.6 illustrates the effect of photons impinging upon the junction area. The energy of a

photon is given by equation,

Where is the wavelength of the photon, is Planck’s constant (6.625x10-34 J.s), and c is the

speed of light (3x108

m/s). The energy of a photon in electron-volts (eV) becomes 1240/ , if is

in nm. If a photon has an energy that equals or exceeds the semiconductor band gap energy of

the p-n junction material, then it is capable of creating an electron-hole pair (EHP). For Si, the

band gap is 1.1 eV, so if the photon wavelength > 1130 nm, which is in near IR region, then the

photon will have sufficient energy to generate an EHP.

Figure 1.6: Illuminated p-n junction showing desirable geometry and the creation of electron-hole pairs

Although photons with energies higher than the band gap energy can be absorbed, one photon

can create only one EHP. The excess energy of the photon is wasted as heat. As photons enter a

material, the intensity of the beam depends upon a wavelength-dependent absorption constant

Chapter I 9


(α). The intensity I of the photon beam as a function of penetration depth x into the material is

given by where, is the initial intensity of photon impinging on the material.

Optimization of photon capture, thus, suggests that the junction should be within (1/α) of the

surface to ensure transmission of photons to within a diffusion length of the p-n junction, as

shown in figure 1.6. If an EHP is created within one minority carrier diffusion length , of the

junction, then, on the average, the EHP will contribute to current flow in an external circuit [17].

The diffusion length is defined to be , where and are the minority carrier

diffusivity and lifetime respectively for electrons in the p-region if = n. If and are the

minority carrier diffusivity and lifetime [18] for holes in the n-region if = p. So the idea is to

quickly move the electron and hole of the EHP to the junction before either has a chance to

recombine with a majority charge carrier. In figure 1.6, points A, B, and C represent EHP

generation within a minority carrier diffusion length of the junction. But if an EHP is generated at

point D, it is highly unlikely that the electron will diffuse to the junction before it recombines.

11..44:: LLIIMMIITTAATTIIOONNSS OOFF cc--SSii SSOOLLAARR CCEELLLLSS

Using c-Si solar cells, it has been estimated that the maximum conversion efficiency that can be

obtained is 25 % [19]. However, the highest conversion efficiency that has been achieved in the

laboratory is 20 % [20]. The c-Si solar cells have a long history and proved that it is the most

reliable power source in remote applications such as space craft [21]. Despite of numerous

advantages, there are serious limitations on c-Si solar cells. Some major limitations are,

i) High processing cost of single crystal Si material: When one is thinking of terrestrial

applications on large scales cost effectiveness is more important than the reliability. The

cost analysis and results of feasibility calculations of solar cells made from c-Si show that

the material cost itself accounts for about 50 % of the total cost of solar cell. Thus, if these

solar cells are made from c-Si become viable energy source compare with conventional

energy sources, material cost should be reduced.

ii) Kerf losses: The losses incurred in the process of obtaining the c-Si called kerf losses.

The c-Si solar cells are made from Si ingots, this need slicing the ingot into wafers by

Chapter I 10


diamond wire saws. During this slicing costly pure Si is wasted [22]. At the same time it

requires surface treatments to the wafers cut in the above step, this also adds up to the

cost of solar cell. In addition to this, Si is very brittle material so it needs to be handled

carefully and one cannot go below wafer thickness of 200 µm.

iii) Low absorption coefficient: One of the major limitations of c-Si is that it has a low-

absorption coefficient (~103 cm-1

iv) Long energy payback period: The estimated energy payback period of c-Si solar cell is

15-20 years. This is a fairy long period and if the solar cell does not last long then the

system becomes energy consuming than the energy producing.

) (see figure 1.7). This requires that the thickness of the

absorber part of solar cell to be 300 µm to absorb the incident solar radiations completely.

One can use efficient light trapping mechanism to reduce the material thickness.

All these considerations show that c-Si solar cells may not be able to compete with conventional

power sources and hence it will not serve as viable option for the electrical power generation on a

large scale. Most of the c-Si material merely acts as a mechanical carrier for the solar cell device

with the largest part of the optical absorption taking place in the upper 30 µm region [23]. Moving

to thinner Si wafers to reduce Si consumption represents one option, but there are obvious

concerns about process yield, showing up when producing cells in Si wafers with thickness below

200 µm. Si has band gap (Eg) ~ 1.1 eV, which is below than the optimum Eg

~1.5 eV required for

efficient solar-electric conversion using a single-junction solar cell. Thus, using the Shockley-

Queisser limit for the maximum efficiency of single junction c-Si is 28 % [24]. To overcome the

limitations of wafer based silicon solar cells, thin films solar cells were introduced.

11..55:: AADDVVAANNTTAAGGEESS OOFF TTHHIINN FFIILLMM SSOOLLAARR CCEELLLLSS

A method to reduce the cost of solar cell is utilization of thin film semiconductor materials than the

bulk materials. Thin film is a material created ab initio by the random nucleation and growth

processes of individually condensing/reacting atomic/ionic/molecular species on a substrate [25].

The structural, chemical, metallurgical and physical properties of such material are strongly

dependent on a large number of deposition parameters and may also be thickness dependent.

Thin films may encompass a considerable thickness range, varying from a few nanometers to

Chapter I 11


tens of micrometers and thus are best defined in terms of the production processes rather than by

thickness. Following features of thin-film processes have been shown to be of interest for PV

technologies [25, 26].

i) A variety of physical, chemical, electro-chemical, plasma based and hybrid techniques

are available for depositing thin-films of the same material.

ii) Microstructure of the films of most materials can be varied from one extreme of

amorphous/nano-crystalline to highly oriented and/or epitaxial growth, depending on the

technique, deposition parameters and substrate.

iii) A wide choice of shapes, sizes, areas and substrates are available.

iv) Because of relaxed solubility conditions and phase diagram, doping and alloying with

compatible as also, in many cases, incompatible materials can be obtained.

v) Surface and grain boundaries can be passivated with suitable materials.

vi) Different types of electronic junctions, single and tandem junctions, are feasible.

vii) The estimated energy payback time of the thin-film PV is 3-5 years.

viii) Graded band gap, graded composition, graded lattice constants etc., can be obtained to

meet the requirements of designer of the solar cell.

ix) In case of multi-component materials, composition, and hence band gap and other opto-

electronic properties, can be graded in desired manner.

x) Surfaces and interfaces can be modified to provide an interlayer diffusion barrier and

surface electric field.

xi) Surfaces can be modified to achieve desired optical reflectance/transmission

characteristics, haze and optical trapping effects.

xii) Integration of unit processes for manufacturing solar cells and integration of individual

solar cells can be easily accomplished.

xiii) Flexible and lightweight PV facilitates several attractive applications.

xiv) Besides conservation of energy and materials, thin-film processes are in general eco-

friendly and are thus green processes.

xv) High-absorption coefficient (> 105 cm-1) of the absorber materials which is about 100

times higher than c-Si thus about 1-2 µm of material thickness is sufficient to harness

Chapter I 12


more than 90 % of the incident solar light. This helps in reducing the material mass

significantly to make modules cost-effective.

xvi) Formation of hetero-junction and better device engineering for reduction of photon

absorption losses and enhanced collection of photo-generated carriers are possible.

xvii) Large-area deposition (order of m2

xviii) Roll-to-roll manufacturing of flexible solar modules is possible. This gives high

throughput and thus can reduce the energy payback time significantly.

), along with the monolithic integration is possible

which minimizes area losses, handling, and packaging efforts.

xix) Tandem/multijunction devices could be realized to utilize the full solar spectrum to

achieve higher-efficiency (> 50 %) devices.

But, keeping in mind that all good things come for a price, the ability to tailor numerous properties

of thin-films required for an efficient solar cell demands good understanding of the material

produced with the help of a range of monitoring and analytic facilities.

Figure 1.7: The optical absorption coefficient (α) versus photon energy of c-Si and other prominent light

absorbing materials that are used in thin film solar cells

One also has to keep in mind that the high sensitivity of film properties to deposition parameters

can produce a huge amount of undesired results; thus thin-film materials has be treated with due

respect and understanding. The thin film PV is also characterized by flexibility to deposition by

several techniques and several possible materials. And one of the important features of thin film

PV is the low requirement of the material, shown due to the high absorption coefficient in the

Chapter I 13


energy region of interest to use in solar cell. Absorption coefficient of various materials is shown

in figure 1.7 above.

11..66:: TTHHIINN FFIILLMM SSOOLLAARR CCEELLLL MMAATTEERRIIAALLSS

11..66..11:: GGrroouupp IIIIII--VV mmaatteerriiaallss III-V compound materials like Gallium Arsenide (GaAs), Indium Phosphate (InP) and their

derived alloys and compounds, which most often have a direct band gap character, are ideal for

PV applications, but are too expensive for large-scale commercial applications, because of the

high cost of the necessary precursors for the deposition and the deposition systems itself. The

deposition systems for these materials are either based on molecular beam epitaxy (MBE) or

metal-organic chemical vapor deposition (MO-CVD). The band gap of GaAs is 1.43 eV, which is

nearly ideal for single-junction solar cell. In addition, it has a high absorptivity and requires a cell

only a few microns thick to absorb maximum sunlight. The GaAs cells are relatively insensitive to

heat, so they are used in solar concentrator PV for terrestrial applications. Alloys made from

GaAs using Al, P, Sb, or In have characteristics complementary to those of GaAs, allowing great

flexibility in high-efficiency cell design like tandem solar cells. The GaAs is very resistant to

radiation damage so the cells are used in satellite solar panels.

11..66..22:: CCooppppeerr iinnddiiuumm ddii--sseelleenniiddee aanndd rreellaatteedd mmaatteerriiaallss

Copper indium di-selenide, CuInSe2 (CIS) and copper indium gallium di-selenide, CuInGaSe2

(CIGS) are direct-band gap polycrystalline semiconductors with very high optical absorption

coefficients and are presently being widely studied for application in solar cells. CIS and CIGS are

p-type and are always used in a hetero-junction structure, mostly with very thin n-type cadmium

sulfide (CdS) layers. In terms of stability, CIS and CIGS solar cells do not have a problem of light-

induced degradation. However, they do have a problem of instability in hot and humid

environment. The low recombination activity at the grain boundaries allows high solar cell

efficiencies even when the material is polycrystalline with grain sizes in the order of only a few

μm. This is to be contrasted with c-Si where grain boundaries are normally characterized by a

high recombination velocity. Moreover, the polycrystallinity allows a large number of substrates

(glass, stainless steel etc.) and is compatible with low temperature deposition techniques, as

Chapter I 14


there is no need for epitaxial growth or high temperatures to obtain large grain sizes. A second

important property is the possibility to tailor the band gap e. g. replacing Se by S in CuInSe2

11..66..33:: GGrroouupp IIII--VVII ccoommppoouunndd mmaatteerriiaallss

results in a material with a higher band gap. This property allows one to build in band gap

gradients aiding the collection of excess carriers and, ultimately, could even be used to develop

multi-junction solar cells.

More appealing from the point of view of ease of processing and cost of material and deposition

are the II-VI compound materials like Cadmium Telluride (CdTe). It is a semiconductor with a

direct band gap, which almost fully absorbs the visible light within ~ 1 µm. The band gap energy

of CdTe (∼ 1.45 eV) is very near the optimum value for single-junction solar cells, simultaneously

yielding both high current densities Jsc (up to 26 mA/cm2) and high voltages Voc

11..66..44:: DDyyee--sseennssiittiizzeedd ssoollaarr cceellllss

(up to 850 mV).

Being a binary compound, CdTe solar cells and modules are easier to fabricate. A typical CdTe

solar cell structure consists of an n-CdS and p-CdTe hetero-junction deposited on a glass

substrate coated with a TCO. The highly doped CdS layer is a very thin n-type window layer used

for barrier formation and is photo-electrically inactive. The deposition of such a very thin CdS

layer (to minimize the loss in blue light response) with sufficient uniformity is one of the critical

issues for large-area devices. The most carriers are generated in the underlying p-type layer (on

the CdTe p-type layer, very close to the n-p interface), which means the n-p heterojunction

interface is a critical region that can cause efficiency, stability problems, or both, if the deposition

technology is not fully mastered. The possibility of a large number of substrates and compatible

with low-cost temperature deposition techniques is also applicable for the material.

An interesting alternative to inorganic semiconductor absorbers are organic semiconductors,

which combine interesting opto-electronic properties with the excellent mechanical and

processing properties of polymeric/plastic materials. In organic semiconductors, absorption of

photons leads to the creation of bound electron–hole pairs (excitons) with a binding energy of 0.5

eV rather than free charges. The excitons carry energy, but no net charge, and have to diffuse to

dissociation sites where their charges can be separated and transported to the contacts. In most

Chapter I 15


organic semiconductors, only a small portion (30 %) of the incident light is absorbed because the

majority of semiconductor polymers have band gaps higher than 2.0 eV. The typically low charge-

carrier and exciton mobility require the active absorber layer thickness to be less than 100 nm.

This thickness is sufficient to absorb most of the incident solar photons if light trapping is used,

although the low refractive index calls for adapted approaches. More importantly, organic

semiconductors can be processed from solutions at or near room temperature on flexible

substrates using simple, cheap and low-energy deposition methods such as spin coating or

screen printing thereby yielding cheaper devices. Even though the efficiency of these devices is

poor at present, they may find immediate applications for disposable solar cell based small power

applications. Among the major issues to be addressed before reasonable market penetration of

the organic devices takes place are the improvement of the stability of conjugate polymers, and

the matching of the band gap of the organic materials with the solar spectrum for higher

conversion efficiency by using blended/composite polymers and suitable dyes.

11..77:: AAMMOORRPPHHOOUUSS SSIILLIICCOONN ((aa--SSii)) AANNDD HHYYDDRROOGGEENNAATTEEDD AAMMOORRPPHHOOUUSS SSIILLIICCOONN ((aa--SSii::HH))

In crystalline form, Si has four fold coordinated, diamond like structure and, an indirect band gap

of 1.1 eV. The indirect gap character indicates that three particles (photon, phonon and electron)

are required in optical excitation of electron from valence band to conduction band. Due to this, c-

Si has low absorption coefficient of about 103 cm-1 for red light. Amorphous silicon (a-Si), in

contrast to the c-Si, does not have a long-range order. The absence of periodicity results in

relaxation of requirement of conservation of momentum in electronic transition processes.

Amorphous silicon has a disordered lattice showing localized tetrahedral bonding schemes but

with broken Si-Si bonds of random orientation. These broken (or unsaturated) bonds are called

dangling bonds and contribute to the defect density in the material. Because of disorder, the

momentum conservation rules are relaxed and a higher absorption coefficient (α) is observed in

a-Si materials. The absorption coefficient of a-Si is about two-three orders of magnitude higher

than c-Si, thus it only requires a couple of microns of thickness for effective absorption and

utilization of the solar spectrum. However, due to its predominant disordered structure, high

densities (~1019 cm-3) of localized defect states are created within the energy gap that causes the

Chapter I 16


Fermi-level pinning. Hence, the material cannot be doped because the defects states act as a

trap for all free carriers generated in the material and hence cannot be used in solar cell

application. One effective way to overcome this problem is to passivate the unsaturated bonds of

a-Si with the help of small atoms that could get into the crystal and attach themselves with the

available dangling bonds as shown in figure 1.8.

Figure 1.8: Two dimensional schematic of the atomic structure of a-Si:H

The hydrogenated amorphous silicon (a-Si:H) may be considered as an alloy of silicon with

hydrogen. Thus, we have moved from unsuitable amorphous silicon (a-Si) to potential a-Si:H

material for device application. This is precisely done by adding 5-10 % atomic hydrogen into a-

Si, which attaches itself to the uncoordinated bonds due to its high activity; this reduces the

dangling bonds density from ~1019 to ~1015 cm-3. The distortion of the bond length and bond

angle after passivation with hydrogen modifies the defect distribution and consequently changes

the optical and electronic properties. Thermodynamically, a-Si:H, is in a metastable state. The

real structure can be varied experimentally in many ways and therefore the material properties

strongly depend on the preparation conditions and on the treatment of the amorphous samples

after deposition. Thermal annealing was shown to produce changes of practically all material

properties (enthalpy, electrical properties, defect densities, optical properties) [27]. At the reduced

defect density of the order 1015 cm-3 the doping of material is possible and the material can be

made as p- or n-type using boron and phosphorous as dopants and extended for device

applications.

Chapter I 17


The discovery of doping in a-Si:H by Spear and LeComber in 1975 was therefore a welcome

surprise, and a great boost to the device potential of the material [28]. This was achieved with

PE-CVD deposition of a-Si:H using silane (SiH4

1.7.1: Density of states of hydrogenated amorphous silicon (a-Si:H)

) as a source gas of silicon. In the dissociation

process in radio-frequency (RF) induced plasma of silane, it creates atomic hydrogen and

complexes of hydrogen with silicon that are contributing towards the thin film growth of a-Si:H.

The energy distribution of states, called the density of states (DOS) gives information about the

distributions and concentrations of charge carriers in a semiconductor material. For an ideal

intrinsic Si, the valence band and the conduction band are separated by a well defined band gap

(Eg

), and there are no allowed energy states in the band gap. Due to long range disorder in the

atomic structure of a-Si:H, the energy states of the valence band and conduction band spread

into the band gap and form regions of states that are called band tails. Figure 1.9 describes the

schematic of DOS in a-Si:H. In general, the energy distribution of states in a-Si:H is characterized

by three different regions: (i) extended states above the mobility edge of conduction band, (ii)

extended states below the mobility edge of the valence band and (iii) localized states between the

mobility edges.

Figure1.9: Schematic presentation of density of states in a-Si:H [51]

The continuous distribution of the localized states is a superposition of conduction and valence

band tail states and the defect states. In addition, the defects introduce allowed energy states that

are located in the central region between valence band and conduction band states. This means

Chapter I 18


that there is a continuous distribution of density of states in a-Si:H and that there is no well

defined band gap between the valence band and the conduction band.

The energy states in which the charge carriers can be considered as free carriers are described

by wave functions that extend over the whole atomic structure. These states are non-localized

and are called extended states. The disorder in a-Si:H causes the wave functions of the tail and

defect states to become localized within the atomic network. These states are called localized

states. Consequently, mobility that characterizes transport of carriers through the localized states

is strongly reduced. This feature of a sharp drop in the mobility of carriers in the localized states

in comparison to the extended states is used to define the band gap in a-Si:H. This band gap is

denoted by the term mobility gap (Emob

11..77..22:: SSttaaeebblleerr--WWrroonnsskkii eeffffeecctt ((SSWWEE))

) because the presence of a considerable density of states

in the mobility gap is in conflict with the classical concept of a band gap without any allowed

energy states. The energy levels that separate the extended states from the localized states in a-

Si:H are called the valence band and the conduction band mobility edges. The mobility gap of a-

Si:H is larger than the band gap of c-Si and has a typical value between 1.7 eV and 1.8 eV.

The Staebler-Wronski Effect (SWE) refers to light-induced metastable changes in the properties

of a-Si:H. Short after the development of the first solar cell based on a-Si material in 1976, a light

induced degradation of performances was reported by Staebler and Wronski in 1977 [29]. They

showed that the dark conductivity and photoconductivity of a-Si:H can be reduced significantly by

prolonged illumination of light. However, on heating the samples to above 150 °C, they could

reverse the effect. Practically, the efficiency of an a-Si:H solar cell typically drops during the first

few months of operation. This drop may be in the range from 10 % up to 30 % depending on the

material quality and device design [30]. After this initial drop, the effect reaches equilibrium and

causes little further degradation. Though advances have been made in the understanding of

SWE, as of yet there is still no general accord on the exact nature of the light-induced defects or

the mechanisms responsible for their creation. [31]. There are various models proposed to

explain the light induced metastability [32]. Historically, the most favored model has been the

hydrogen bond switching model. It proposes that an EHP formed by the incident light may

recombine near a weak Si–Si bond, releasing energy sufficient to break the bond. A neighboring

Chapter I 19


H atom then forms a new bond with one of the Si atoms, leaving a dangling bond. These dangling

bonds can trap EHPs, thus reducing the current that can pass through. However, new

experimental evidence is projecting doubt on this model.

In the hydrogen flip model, Biswas et al. [33] demonstrated a higher-energy metastable state is

formed when H is flipped to backside of the Si-H bond at mono-hydride site, shown in figure1.11.

Figure 1.10: Hydrogen flip model (a) Normal bonding configuration for a H atom at a silicon site (Si1) in a-Si:H

network. (b) Higher energy H-flip defect where the H is at the backplane of Si1. The Si1 also moves .0.5 Å. [34]

They have proposed that metastability occurs in three steps [34], (i) Illumination creates photo-

excited electrons and holes. Non-radiative recombination of EHP can break a weak silicon bond

and generate a dangling bond–floating bond (FB) pair, analogous to a vacancy-interstitial pair in

c-Si. (ii) The mobile FB diffuses away from the location of the DB. (iii) Migrating FBs recombine

or annihilate, accompanied by local H rearrangement. Weak silicon bonds are then broken and

DBs are created at spatially separated parts of the network. They find that non-radiative

recombination of an EHP can break a weak silicon bond. It also provides a very low energy path

for bond breaking. A weak bond (WB) can easily trap a hole in a band-tail state. A mobile photo-

excited electron can be trapped in the vicinity of this hole. In summary, metastable defect creation

is driven by the breaking of weak silicon bonds and the re-bonding of both silicon and H sites.

Chang and Zhang [35-37] suggested that the dissociation of a two-hydrogen interstitial complex,

(H2*), into separate and more mobile H atoms, caused by carriers localized on the H2*, is a

mechanism for the metastable phenomena. In the hydrogen collision model proposed by Branz

[38], the recombination-induced emission of H from the Si-H bond creates mobile H and dangling

bonds, and these newly created dangling bonds become metastable when two mobile H atoms

(a) (b)

Chapter I 20


collide to form a complex containing two Si-H bonds. The two-phase model of Zafar and Schiff

[39, 40] explained thermal stability data, exploited the concept of paired hydrogen, and later

merged with the Branz model and invoked di-hydride bonding [41].

The microscopic origin of the SWE is still not clear and many mechanisms have been put forward

to explain it. However, the degradation appears to cause by the recombination process in which

the excess energy is dissipated in the lattice, resulting in creation of dangling bonds. Preparing a-

Si:H films at low hydrogen content with controlling the process parameters can reduce the SWE.

Another major drawback of a-Si:H based solar cell is that the material has high band gap as

compare to c-Si. Due to high band gap, the long wavelength region of the solar spectrum cannot

be efficiently utilized by solar cells. This limits conversion efficiency of a-Si:H based solar cells.

11..88:: PPOOSSSSIIBBLLEE SSOOLLUUTTIIOONNSS

As mention earlier, low absorption efficiency in the long wavelength region and the degradation

caused by the SWE are the two main drawbacks of a-Si:H solar cells. The efficiency limitations

due to low absorption in the long wavelength region can be overcome by utilizing a suitable

multilayer solar structure called tandem solar cell. One way to reduce the SWE is to produce

device quality a-Si:H with minimum possible hydrogen content.

11..88..11:: TTaannddeemm ssoollaarr cceellllss

The efficiency limitations due to the low absorption efficiency in the long wavelength region can

be overcome by utilizing a suitable solar structure called tandem solar cell. One method of

utilizing solar energy more effectively to obtain greatly improved conversion efficiency and

minimizing the SW effect is to connect several solar cells in series. This structure is called as

tandem structure. In a tandem solar cell, the top solar cell contains a material with high band gap

as an active layer and the bottom solar cell contains a material with lower band gap as an active

layer. Tandem structures are more suitable in amorphous silicon solar cells because the

amorphous nature of the material provides freedom in material selection by the fact that the

materials may be combined without any constraints imposed by the requirement of lattice

matching. A typical tandem solar cell structure is shown in figure 1.11.

Chapter I 21


Figure 1.11: A typical tandem solar cell structure

In tandem solar cells long wavelength region of solar spectrum can be utilized by a low band gap

material like a-SiGe:H. Thickness of each photosensitive layer in tandem solar cell is less than

that in a single junction solar cell, light induced degradation effects are less and, strong internal

electric field prevents carrier recombination and maintains higher fill factor after illumination.

11..88..22:: HHyyddrrooggeennaatteedd nnaannooccrryyssttaalllliinnee ssiilliiccoonn ((nncc--SSii::HH))

The nc-Si:H have attracted a great deal of research interest in recent years because of their

potential applications in low cost solar cells. Figure 1.12 show the schematic of structure of nc-

Si:H. It is considered as a mixture of crystallites, a-Si and grain boundaries. By controlling the

deposition conditions, µc/nc-Si:H can be obtained that has different properties than a-Si:H form.

Figure 1.12: The hydrogenated nanocrystalline silicon (nc-Si:H) structure

Some major advantages of nc-Si:H for solar cell applications are listed below.

1) Transition from c-Si, poly-Si, a-Si to nc-Si:H: The conventional Si technology based on

c-Si wafers and polycrystalline Si thin films is incompatible with large area electronics

Top Solar Cell

Bottom Solar Cell

Back contact

Glass

TCC

Chapter I 22


because of high processing temperatures and limited sizes of substrates. This is the

reason for the development of new technique and material that could permit the Si to be

obtained by deposition over large areas at low temperatures. To overcome these

problems, a-Si:H has been introduced. However, the low cell efficiency and deterioration

of material and the decrease in its efficiency due to the SWE are still major problems

associated with a-Si:H based solar cells.

2) Prevention of charge carrier recombination: The nc-Si:H composed of small Si

crystallites of 10~100 nm embedded in a-Si sea. Each crystallite island is bounded by H

coating, which prevents photo-excited charge carriers from recombination.

3) Tailorable band gap: The absorption in nc-Si:H is due to absorptions from both the

amorphous as well as the crystalline fraction present in it [42]. The amorphous and

crystalline fraction in the film and thus the band gap can be controlled by process

parameters. Furthermore, the absorption coefficient (α) of c-Si and µc-Si:H have more or

less the same onset of transition, but nc-Si:H has a higher α in the low-wavelength region.

Since α for nc-Si:H is lower than that of a-Si:H, thicker nc-Si:H layers are required

compared to a-Si:H for the absorption of the solar spectrum. A stacked combination of the

two- nc-Si:H and a-Si:H layers is attractive for absorption of the most useful part of the

spectrum in thin layers.

4) Better stability against the prolonged light illumination: The nc-Si:H does not show

light-induced degradation in its opto-electronic properties [43]. Under long term

illumination, efficiency of nc-Si:H solar cells degrade less (< 5 %) compare to a-Si:H solar

cells which usually degrade more than 10 % [44].

5) High intrinsic conductivity: The nc-Si:H have columnar structure and is good for solar

cells because the grains are oriented parallel to the primary flow of current [45].

6) Higher doping efficiency: Similar to a-Si:H, the nc-Si:H can be easily doped by adding

B2H6 and PH3

7) Availability of various deposition methods: Many techniques are available for

synthesis of nc-Si:H. Each technique has its own advantages and disadvantages.

in source gases. The doping efficiency in nc-Si:H is higher than a-Si:H

[46].

Chapter I 23


11..99:: AAIIMM AANNDD OOUUTTLLIINNEE OOFF TTHHEE TTHHEESSIISS

In view of the above discussion, the nc-Si:H, active or intrinsic layer in solar cell, the topic of the

present thesis has many advantages over a-Si:H material. On small scale it has proven to

produce device quality nc-Si:H films by plasma enhanced chemical vapor deposition (PE-CVD)

and hot wire chemical vapor deposition (HW-CVD). The aim of the present study is to understand

the PE-CVD and HW-CVD deposition techniques and to optimize the process parameters so as

to get the device quality nc-Si:H films. The thesis is divided into two parts. The first part deal with

the synthesis and characterization of nc-Si:H films using existing PE-CVD technique. The second

part comprise the establishment of dual chamber HW-CVD technique in the laboratory and

synthesis and characterization of nc-Si:H films using it.

There are different deposition techniques of preparing nc-Si:H films and its alloys and all of these

techniques have their own advantages and disadvantages. All these techniques are briefly

discussed mentioning their merits and demerits in chapter II. Since thin films of nc-Si:H reported

in this thesis are deposited using PE-CVD technique, more detailed discussion on this technique

is given. The same chapter also focuses on effect of different process parameters of PE-CVD on

the film properties. The growth mechanism of nc-Si:H films and advantages of PE-CVD technique

are also discussed in this chapter. Furthermore, the characterization techniques and data

analysis methods used for the analysis of nc-Si:H are discussed in detail. This includes a detailed

overview of thickness measurement, Dark conductivity and Photoconductivity measurements,

Raman spectroscopy (RS), Low angle x-ray diffraction (low-XRD), Atomic force microscopy

(AFM), Fourier transform infrared (FTIR) spectroscopy, UV-Visible Spectroscopy etc.

In preparation of nc-Si:H films by PE-CVD method, various process parameters and dilution of

the source gas, silane (SiH4) with hydrogen (H2) have a strong influence on the structure and

morphology of the films. Some of the reports indicated that the nc-Si:H films deposited by PE-

CVD do not show any systematic correlation between the process parameters and the resulting

film properties [48] due to the heterogeneity of grown films. On the other hand, some reports [49,

50] have shown that the crystallite size and height, as well as their density can be controlled by

deposition time, process pressure, rf power and substrate temperature. Therefore, more detail

Chapter I 24


and careful investigations of the synthesis and characterization of PE-CVD grown nc-Si:H films

are needed. With this motivation, an attempt has been made to synthesise undoped nc-Si:H thin

films by conventional PE-CVD method on insulator glass substrate at low substrate temperature

in view of their use in solar cells. In chapter III, we present results on synthesis and

characterization of nc-Si:H films prepared directly by conventional PE-CVD method at low

substrate temperature of 200 0C. The influence of inter electrode separation (ds-e) and process

pressure (Pr

The influence of dilution of source gas, silane (SiH

) on structural, optical and electrical properties have been discussed in detail.

4) with hydrogen (H2) and noble gases like

Argon (Ar) and Helium (He) have a strong influence on the structure and morphology of nc-Si:H

films synthesized by PE-CVD method. In chapter IV we have discussed the structural, optical

and electrical properties of nc-Si:H as a function of H2, Ar and He dilutions of SiH4

During last more than 10 years nc-Si:H has been studied extensively as a low cost material for

thin film PV applications. The PE-CVD technique has been established for industrial applications.

However, device quality nc-Si:H films prepared by PE-CVD show undesirable meta-stable effects

and lower growth rates. The HW-CVD technique has received considerable attention in recent

years as an alternative deposition method for nc-Si:H because it is capable of improving film

stability and of achieving higher deposition rates. In addition, the structural ordering of these films

is better than PE-CVD deposited films. In chapter V, we have described the establishment of

dual chamber HW-CVD technique. This dual chamber HW-CVD system is indigenously designed,

developed and successfully commissioned in the laboratory. The technique is fully automated to

enhance the repeatability of deposited material. All aspects of design of dual chamber HW-CVD

technique to increase deposition rate, deposition area and to enhance the repeatability are

discussed in this chapter. The growth mechanism and influence of various process parameters

on deposition of nc-Si:H films are also discussed. In addition, major advantages of HW-CVD over

the conventional PE-CVD technique are also mentioned. Chapter V also describes synthesis and

characterization of nc-Si:H films by recently established HW-CVD system. We end the thesis by

discussing the perspectives of HW-CVD and PE-CVD deposited nc-Si:H material for the future

solar cell applications.

.

Chapter I 25


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