1

TABLE OF CONTENTS

Page no.

List of Figures 3

List of Tables 4

Abstract 5

Chapter 1:Introduction 6

Chapter 2: Literature Survey 8

2.1 Dye sensitized solar cell 8

2.1.1 Detailed description of the materials used in DSSC 9

2.1.2 Parallel module for DSSC 10

2.2 Silver paste for Dye sensitized solar cell 11

2.2.1 Silver metal powder 11

2.2.1.1 Preparation of silver nano particles 11

2.2.1.2 Stabilization of metal nano particles against aggregation 13

2.2.1.3 Chemical reduction method for silver nano particle

synthesis

13

2.2.1.4 Effect of pH on reducing power of the reducing agent 14

2.2.2 Glass frit 15

2.2.2.1 Effect of glass content on different properties of silver

paste

16

2

2.2.3 Organic compounds 17

2.3 Preparation of silver paste 17

2.4 Screen Printing Technology 17

2.4.1 Main components of the screen printing process 18

2.4.2 General procedure for contact formation by screen printing 19

2.4.3 Important parameters involved in screen printing process 20

2.5 Drying and Co-firing process 20

2.6 Importance of rheological behavior of metal paste 21

Chapter: 3 Work carried out 23

3.1 Silver nitrate preparation 23

3.2 Preparation of silver nanoparticles 24

Chapter: 4 Future plan of work 27

References 28

Bibliography 29

Acknowledgement 30

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List of figures

Figure no. Description Page no.

1.1 Schematic cross-section view of grid type DSSC module 7

2.1 Schematic of typical Dye sensitized solar cell 8

2.2 Design scheme of parallel DSC module 10

2.3 Schematic preparation methods for nano particles 12

2.4 (a) Solution pH as a function of time with different additions of

[Na2CO3]/[AgNO3] = (a) 0; (b) 1; (c) 2

(b) Effect of [Na2CO3]/[AgNO3] ratio on silver average size and

its standard deviation

15

2.5 (a) Dependence of sheet resistance of fired film samples on the

glass weight percentage

(b) Dependence of adhesion strength of fired film samples on the

glass weight percentage

(c) Dependence of Vickers hardness of fired film samples on the

glass weight percentage

16

2.6 Printing cycle steps, main parts of screen-printing machine are

detailed along with the process sequence followed to define the

contact

19

2.7 Evolution of viscosity and shear rate during printing 21

3.1 Silver bar (50gm) brought from HDFC bank 23

3.2 XRD plot for AgNO3 powder 24

3.3 Flow chart explaining synthesis of silver nano particles using formaldehyde as reducing agent

26

4

List of tables

Table no. Description Page no.

2.1 The compositions (in mol %), thermal properties and crystal

phase of Bi2O3-B2O3-ZnO glass samples 16

3.1 Amounts of different raw materials required when 250ml and

500ml of 0.01M AgNO3 solution are used respectively 25

5

Abstract

Dye sensitized solar cell (DSSC) is emerging technology which can be a good alternative to

conventional Si based solar cell technology due to their cost effectiveness and simple fabrication.

During up scaling of DSSCs, in parallel module, the energy efficiency gets degraded due to high

sheet resistance of large area FTO coated glass substrate. Incorporation of metal fingers, similar

to the case of silicon solar cells, would lower the ohmic losses. Silver is the most preferred

material for metal fingers due to its good conductivity. Typically, metal paste contains metal

particles, glass frit and organic vehicle. The metal paste can be screen printed on to the substrate

followed by sintering at appropriate temperature. Using silver nanoparticles along with low

melting glass frit such as Bi2O3-B2O3-ZnO glass would lower the sintering temperature of

printed silver paste. Silver paste should exhibit psuedoplastic behavior with minimum slumping

in order to get desired dimensions of metal fingers on the substrate. Silver paste is to be

characterized for rheology (Rheometer), conductivity (four probe method) and FE-SEM for

connectivity with substrate after sintering. The goal is to screen print silver paste with desired

properties on the glass substrate to achieve the metal fingers of desired dimensions.

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Chapter 1: Introduction

In todays world, energy is an important factor for maintaining economic, environmental and

political stability. Non renewable sources of energy such as fossil fuels are depleting fast. It

urges the need to use the cost effective solar energy. Conventional solar cell modules have an

efficiency of 15%, but it requires ultra pure material, high temperature processing which makes

this technology expensive [1]. Dye sensitized solar cell (DSSC) is emerging technology which

can be a good alternative conventional Si based solar cell technology due to their cost

effectiveness and simple fabrication. Laboratory scale DSSC (using of nano size titanium

dioxide (TiO2)) provided reasonable energy conversion efficiency of about 11% [1]. This caused

increased interest in the possible large-scale application of DSSCs. Even though the performance

of DSSCs is inferior to that of conventional solar cells in terms of efficiency, a simple production

method at relatively low temperature makes them a good alternative. So, industrial researchers

are trying to scale up the DSSC technology from laboratory level to stand alone power

generators. In general, scaling up DSSCs is possible in two ways: series monolithic or

interconnect modules and parallel grid module. Series interconnect modules are prepared by

connecting individual small cells in series. Since each unit cell is prepared under ideal

conditions, it is to be expected that the performance will be the same as that of the unit cell. On

the other hand, parallel grid modules are prepared on large TCO glass substrates. A device with

an active area of a few square centimeters prepared on a fluorine-doped tin oxide (FTO) glass

substrate showed an energy conversion efficiency of only 0.3% because of ohmic losses

associated with the large substrate [2]. One way to decrease ohmic losses and thereby increase

the overall efficiency is to use current collecting metal fingers on FTO coated glass substrate.

Metal fingers are used for the same purpose in silicon based solar cells. Among many conductive

metals, silver is studied most extensively due to its low electrical resistivity (15.87 n-m at

20C). But, silver gets corroded by I-/I3- redox electrolyte. Thus, it has to be protected from

electrolyte. Many researchers tried to protect it with surlyn, overcoat of glass frit [3] etc.

The main components of conductive silver paste are silver powder for conductivity, glass frit to

bind silver particles together and organic compounds are used as a vehicle to transport the

suspended silver and glass particle. Silver inks with PbO based glass frit have been extensively

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used for making electrical contacts in solar cells and other electronic devices due to its excellent

electrical properties [4]. Recently, PbO-based glass in silver thick films has been restricted as it

is dangerous to health and environment. This has been the motivation behind replacement of Pb

based glass frit. Generally, the firing temperature of Pb-free frit is higher than that of PbO glass

frit. [4] Thus, it is difficult to sinter silver paste at lower temperatures using Pb-free frit. Densification of silver paste after printing is important to achieve good conductivity. Nano-sized

particles can help the substance to sinter at lower temperatures due to their higher surface energy [4]. Therefore, it is possible to prepare Pb-free silver paste at lower firing temperatures by adding

silver nanoparticles.

Fig 1.1 - Schematic cross-section view of grid type DSSC module [5]

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Chapter 2: Literature Survey 2.1 Dye sensitized solar cell The dye sensitized solar cell comprises of

A nanocrystalline titanium dioxide (TiO2) electrode

A ruthenium based dye

Counter-electrode usually Platinum (Pt)

An electrolyte usually solution with a dissolved iodide/triiodide (I-/I3-) ion redox couple

in the space between the two electrodes

In a dye sensitized solar cell a large band-gap material namely nanocrystalline TiO2 serves as

charge transport medium. A monolayer of dye is chemically adsorbed on this material. When

light falls on dye, it causes injection of electrons into the conduction band of TiO2. The electrons

then diffuse towards a transparent conducting oxide (TCO) substrate. The dye is regenerated

through receiving electrons from an iodide/triiodide (I/I3) redox couple dissolved in the

electrolyte. After passing through a desired load, the electrons enter the cell through a counter

electrode to reduce the triiodide ions and thus complete the circuit.

Fig 2.1 - Schematic of typical Dye sensitized solar cell [6]

9

2.1.1 Detailed description of the materials used in DSSC

(a) TiO2 Photoelectrode

TiO2 is used as photoelectrode which acts as a semiconductor material in DSSC. TiO2 is

preferred because of its good chemical stability under visible irradiation in solution, its low cost,

easy availability and non toxic nature. Generally, TiO2 layer of 2-20m is deposited on TCO

(Transparent conducting oxide) coated glass [6]. Nanocrystalline TiO2 porous layer coated on

TCO gives good absorption and PV performance. This is because nanosized grains have larger

area for dye absorption and porosity within nanoparticles ensures mobility of charge carriers.

TCO generally used are Fluorine doped Tin oxide (FTO) or indium tin oxide (ITO) which has

high transmittance coupled with low resistance.

(b) Dyes

The dye absorbs the incoming light and thus, controls the light harvesting efficiency and thereby

the overall efficiency of DSSC. Dye material should such that it should absorb all the light below

the threshold wavelength of about 920nm [6]. Its redox potential should be high enough so that it

can be regenerated via electron transfer by electrolyte. It should get attached firmly to the

semiconductor material (TiO2). It should be stable to sustain large number of redox steps during

the lifetime of cell.

Two types of dyes have been used: Organic and Inorganic.

Inorganic dyes include metal complexes, such as polypyridyl complexes of ruthenium and

osmium, metal porphyrin, phthalocyanine. While organic dyes include natural and synthetic

organic dyes. Compared with organic dyes, inorganic dyes especially metal complexes have high

thermal and chemical stability. Polypyridyl ruthenium dyes were widely used and investigated as

they have good stability and redox properties along with good response to natural visible

sunlight.

(c) Counter electrode

Counter electrode consists of TCO coated glass. This is generally coated with Platinum which

acts as a catalyst for an electrolyte system containing iodide/tri-iodide (I-/I3-) redox couple. But,

due to higher cost of Pt, a low cost carbon nanomaterial based TCO is studied. Carbon

nanomaterials possess properties like large surface area, chemical stability, good conductivity

and high chemical activity.

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(d) Electrolytes

Electrolyte is used to transfer electron from counter-electrode to the oxidized dye. This process

completes the circuit and also regenerates the dye. The electrolyte must reduce the dye cation

before the dye cation recombines with an electron in the photo-electrode. The electrolyte should

not allow its oxidized form to intercept an electron from the photo-electrode. Generally I3-/I-

couple is used as electrolyte in most of the DSSC modules. There are few problems associated

with liquid electrolytes such as sealing and stability of cells. Thus, research is going on to replace

liquid electrolytes with solid or gel electrolyte. The main substitutes are gel electrolytes and

polymer electrolytes.

(e) Sealants

In order to make DSSC suitable for indoor and outdoor applications long term stability of

the module is required. By having a sealant, evaporation of electrolyte and intrusion of foreign

material from outside atmosphere in DSSC modules is avoided. Sealing can be done in many

ways. Polymer sealing can be done by surlyn, Bynel, etc. Also, glass frit and waterproofed plastic can also be used to seal the module [6].

2.1.2 Parallel module for DSSC

Fig 2.2 - Design scheme of parallel DSSC module [7]

In parallel module of DSSC, a single large FTO coated glass electrode is used. A device with an

active area of a few square centimeters prepared on a fluorine-doped tin oxide (FTO) glass

substrate showed an energy conversion efficiency of only 0.3% [2]. This low efficiency is due

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ohmic losses associated with the large glass substrate. Thus, in this module, current collecting

metal fingers (shown in figure) are used same as that in silicon solar cells. The materials tried for

these fingers are Ag, Au, Cu, Al, etc. But, silver is the most researched metal due to its low

electrical resistivity (15.87 n-m at 20 C), low dark current. Silver paste is having constituents

like silver powder, glass frit and organic vehicle is used for contact formation.

2.2 Silver paste for Dye sensitized solar cell Typical properties of silver paste used for DSSC

High conductivity and low contact resistance for minimizing series losses

Good line resolution to reduce shadowing effect

Excellent adhesion to variety of TCOs

Should be screen printable for mass production

Silver thick film paste mainly consists of three constituents:

2.2.1 Silver metal powder

Silver is the most preferred due to its good conductivity. In a given commercial silver paste, the

pure silver powder present is about 70-85% of the total weight. Nano sized silver particles are

used along with micro sized silver particles in different proportions in preparing silver paste.

Silver powder in the commercial pastes is also present in different shaped particles such as

spherical powder, grains or flakes. The shape and size of silver powder particles are responsible

for the paste conductivity and final adhesion with the contact.

2.2.1.1 Preparation of silver nano particles

Metal nano particles can be prepared by two ways:

(a) Top down

(b) Bottom up

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Fig 2.3 - Schematic preparation methods for nano particles [8]

(a)Top down method: Top-down, or physical methods are usually high-energy methods, in

which bulk metal or microscopic particles are converted to nanosized particles. It includes

physical gas-phase methods based on the rapid condensation of metal vapor. Metal powder is

usually obtained by thermal heating or plasma excitation of metal plates, powders or wires. The

metal powder is then transported with a stream of an inert gas (N2, Ar, He) onto a solid substrate

or into a liquid containing a stabilizing agent in order to accumulate, stock, and handle the metal

particles. Top down method also includes laser ablation which enables dispersion of bulk metal

in either organic or aqueous medium. This technique was employed for preparation of Ag

Nanoparticles in the size range from 4 to 80 nm [9].

The main disadvantages of gas-phase and laser ablation methods are the difficulties in obtaining

uniform and stable NPs. In addition, these methods are expensive, energy-consuming, and

require sophisticated equipment.

(b)Bottom up method: In this method, metal nano particles are obtained from precursor ions or

molecules in the presence of suitable reducing agent. The process is performed in a liquid

medium which can be either water or organic solvents or ionic liquids.

For the synthesis of Ag (Standard reduction potential: E0 = 0.8 V) nanoparticles in aqueous

medium, borohydride, hydrazine, ascorbic acid, and citrate are usually used as reducing agents

[9].

For the formation of Ag nanoparticles in organic solvents, ethanol and N, N-dimethylformamide

can function as reducing agents [9].

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The polyol method can be used for preparation of metal nano particles. In this method, the liquid

polyol (ethylene glycol, propylene glycol, diethylene glycol) acts both, as the solvent of the

metal precursor and as the reducing agent [9].

2.2.1.2 Stabilization of metal nano particles against aggregation

Metal nano particles in colloidal dispersions approach each other and collide due to Brownian

motion. This may result in their aggregation followed by irreversible coagulation and

sedimentation, as the density of metal is very high compared to the density of typical liquids. The

best way to avoid this problem is to prevent aggregation of metal nanoparticles at the early stages

of their formation, while synthesizing the metal ink precursor. Thus, selection of stabilizer, liquid

vehicle, and stabilization procedure are of great importance as it decides the shelf life and

performance of the metal paste.

Stabilization can be done in two ways:

(a)Electrostatic stabilization: It is a result of electrostatic repulsion between electrical double

layers surrounding interacting particles. Stability of dispersions increases with increase in the

value of the electric potential of the NPs. Zeta potential () is used as a measure of the electrical

potential of a particle. But, the electrostatic mechanism is not effective in organic vehicles, and

that requires an additional or different stabilization mechanism.

(b)Steric stabilization: To overcome the problem, steric stabilization is frequently used as an alternative. Steric stabilization is achieved by surrounding the particle with an adsorbed layer of

sterically bulky molecules, such as polymers, mostly nonionic. The most frequently used stabilizing agent is poly (N-vinyl-2-pyrrolidone) (PVP) of various

molecular weights. It exhibits a highly effective protective function in both organic and aqueous

media. PVP has been shown to be an effective stabilizer of aqueous dispersions of Ag

nanoparticles as for organic solvents as well as in polyol synthesis [9]

2.2.1.3 Chemical reduction method for silver nano particle synthesis

For chemical reduction method, choice of reducing agent is of prime importance. The precursor

used in most cases is silver nitrate. Reducing agents like formaldehyde (HCHO) [10], hydrazine

monohydrate (N2H4) [11], [4], glucose [12], ammonium formate (HCOONH4) [12], sodium

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borohydrate (NaBH4) [12] can be used. The reducing ability will determine the formation

kinetics and thus the reaction temperature. The reaction can be carried out in aqueous or organic

solvents in the presence of suitable protective agent such as PVP, PVA, etc. [10].

2.2.1.4 Effect of pH on reducing power of the reducing agent

Solution alkalinity plays important role in deciding the reducing power of the reducing agent.

Suitable amount of base such as NaOH, Na2CO3, and NH3.H2O can be added in order to

maintain the pH during the reduction reaction of silver nitrate.

During reduction of silver nitrate with formaldehyde, two reaction mechanisms are proposed in

[11,10].

2Ag+ + HCHO + 3OH = 2Ag + HCOO + 2H2O

2Ag+ + HCHO + OH = Ag + HCOOH + H2

They studied the effect of solution pH during formation of silver nanoparticles. The ratio of

formaldehyde to silver nitrate is maintained at 4 (after trying ratio from 1 to 120) so that

sufficient formaldehyde should be present to reduce silver nitrate. Using only formaldehyde gave

very slow reduction rates at room temperature due decrease in pH of the solution as the reaction

proceeds. Thus, pH is maintained at 7 by using Na2CO3 (Na2CO3/AgNO3=1) which favored the

reduction process. Although higher pH favors the reduction process, it affects the particle size

adversely. Use of powerful base such as NaOH caused adverse effect on particle size. This

prompted them to use Na2CO3 to replace NaOH. Also, addition of too much of Na2CO3 caused

the particles to grow as shown in figure. This is because, PVP (protective agent) molecules are

capable of combining with hydroxyl ions. This reduces the effective quantity of PVP to protect

silver colloids. Thus, maintaining pH to the optimum level is important to get good reduction

rate and particle size.

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(a) (b)

Fig 5 (a) Solution pH as a function of time with different additions of [Na2CO3]/[AgNO3] = (a)

0; (b) 1; (c) 2: [AgNO3]D0.005 M, [HCHO]/[AgNO3] = 4, [NaOH]/[AgNO3] = 1, PVP/AgNO3 =

9.27) (b) Effect of [Na2CO3]/[AgNO3] ratio on silver average size and its standard deviation

(other conditions (other conditions same as that in fig (a)) [10]

2.2.2 Glass frit

Glass frits are metal oxides that play an important role in the formation of the contact. It acts as a

permanent binder and also helps during sintering of metal particles.

PbO based paste were extensively used for contact formation in solar cells. The toxic nature of

PbO based paste has been the motivation behind its replacement with lead free paste. Generally,

the firing temperature of Pb-free frit is higher than that of PbO glass frit [4]. So, it is difficult to

sinter silver paste at lower temperatures using Pb-free frit. Thus, many researchers have studied

the low melting lead free glass systems. One of the low melting lead free glass systems is Bi2O3-

B2O3-ZnO ternary system which can be considered as an alternative to lead oxide glasses

because the bismate glasses have unique thermal and optical properties [13]. The thermal

properties (Tg, Tm) and crystal phase information for Bi2O3-B2O3-ZnO glass with different

compositions is shown in the table.

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Table 2.1 - The compositions (in mol %), thermal properties and crystal phase of Bi2O3-B2O3-

ZnO glass samples [13]

2.2.2.1 Effect of glass content on different properties of silver paste

In general, the sheet resistance increases with increase in glass content of silver paste. The sheet

resistance of substrate for the case where silver paste (with different glass content) was prepared

using different shaped micron size silver particles is shown in figure [14].

However, the adhesive strength and Vickers hardness increases with increase in glass content in

silver paste. Thus, it is important to optimize the glass content in silver paste to get a good

balance between sheet resistance and adhesive strength.

(a) (b)

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(c)

Fig. 2.5 (a) Dependence of sheet resistance of fired film samples on the glass weight percentage

(b) Dependence of adhesion strength of fired film samples on the glass weight percentage

(c) Dependence of Vickers hardness of fired film samples on the glass weight percentage [14]

2.2.3 Organic compounds

Organic compounds are used as a vehicle to transport the suspended silver and glass particles.

The organic compound plays an important role in deciding the desired rheological properties to

the paste. During sintering, the organic compounds get evaporated leaving a dense mixture of

silver particles and glass frit. Organic vehicle used in silver paste is a mixture of ethyl cellulose

and terpineol.

2.3 Preparation of silver paste The silver paste is prepared by mixing silver powder (nano and micron size or only nano size

particles), glass frit, and organic vehicle in required proportion in a three roll mill so that the

individual components get mixed thoroughly. This paste needs to be printed on the FTO coated

glass substrate. The printing process is generally done with screen printer.

2.4 Screen Printing Technology Screen printing is the printing technique which uses a mesh in order to print an ink on to the

substrate. The ink is spread over the mesh which contains open areas for ink transfer. The

roller or squeegee is moved onto the upper surface of the mesh so as to transfer the ink on the

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substrate through mesh openings. Screen printing is one of the worlds oldest methods of

printing words and images. Screen printing first appeared in a recognizable form in China long

back in 960 AD. But, this technology was first introduced in photovoltaic industry only few

decades back in 1970s. In 1976, Spectrolab, Inc. combined screen-printing with a laminated

module design to print metal contacts on the surface of solar cell [15].

Screen printed contact technology is very rapid and cost effective metallization process. The

screen printing equipment is robust, simple, and inexpensive and can be easily automated. It also

produces less chemical waste and thus does not affect the environment very much. Thus, screen

printing is very effective in mass production of solar cells. Most solar cell manufactures use the

basic screen printing technique for metallization. In fact, over 90% of all crystalline silicon

modules are manufactured using screen printing and about 60% of flexible thin film modules use

screen printing in the manufacturing process [16]. Other methods of printing are pad printing,

photolithography, ink-jet printing. But, screen printing stands out as the most popular printing

methods due to its advantages.

2.4.1 Main components of the screen printing process

(a) A screen with desired mesh size

(b) A squeegee

(c) The metal paste of desired composition

(a) Screen

Usually nylon mesh is used for screen printing metal paste on glass substrate. The size of the

mesh in the screen determines the minimum width of the fingers. In case of fine line printing on

the substrate, the mesh with small opening is required. The mesh with fine mesh size experiences

higher tension. Thus, stainless steel mesh is used which provides higher strength.

(b)Squeegee

A squeegee is used to provide shear stress which causes the viscosity of the paste to decrease

which ensures free flow of the paste through the mesh openings. The pressure applied by the

squeegee affects the final efficiency of solar cell [17].

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(c)Metal Pastes

The metal paste which is used should have desired properties such as viscosity for getting a good

print on the substrate.

2.4.2 General procedure for contact formation by screen printing

The screen printing process consists of three steps. In the first step, the screen is covered with

paste by moving a squeegee over the surface of the screen-printing mask in the filling phase.

Secondly, the squeegee moves over the screen, again applying an additional vertical force that

presses the screen on the wafer and the paste through the meshes of the mask. In the last step the

mask is lifted, creating distance to the solar cells surface, which is known as the snap-off

distance. Emulsion over mesh (EOM) is used to increase the theoretical height of the printed

contacts, thus, increasing the aspect ratio. The screen printing process is summarized in figure 1.

Fig 2.6 - Printing cycle steps, main parts of screen-printing machine are detailed along with the

process sequence followed to define the contact [17].

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2.4.3 Important parameters involved in screen printing process

Screen printing is one of process which decides the final efficiency of solar cell. There are

several factors which affect the screen printing process which include snap off distance,

squeegee pressure and squeegee speed [1].

Snap off distance

The snap off distance is the distance between the screen and the wafer. When the paste is

printed onto the screen, a downward force is applied to the screen. As the screen is

elastic, it restores its shape and this upward movement causes the paste to deposit onto

the surface. If the snap off distance is too high, then pressure will have to be applied to

force the paste onto the wafer. On the contrary, if it is too low, the paste might not get

released from the screen.

Squeegee pressure

The pressure applied also plays an important role in the deposition of the paste. When

very high pressure is applied, excessive paste from the screen could be deposited which

can break the wafer. On the contrary, when too little pressure is applied, the paste might

not get released from the screen at all.

Squeegee speed

The speed of the squeegee movement also determines the print quality. If the speed is too

high, the paste can miss many holes and lead to a non-uniform deposition.

2.5 Drying and Co-firing process

The firing process, also referred to as sintering, is one of the key steps in making contacts in a

solar cell. In this process, the thick film paste is dried at about 150C to remove much of the

solvents. The presence of solvents can cause excessive out gassing which can lead to cracks and

voids. The dried substrates are then fired inside a firing furnace. The firing temperature will

depend on the substrate used and the metal paste. In case of glass substrate, the firing

temperature is typically between 500C to 600C if micro size silver particles [14] are used and

will decrease to temperatures 250C to 450C [11] in case of nano sized silver particles. The

sintering temperature will vary from 400C to 550C [4] if the mixture of micro and nano size

particles is used.

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2.6 Importance of rheological behavior of metal paste

Paste rheology plays an important role during contact formation. For fine line printing on thick

film, it is important that the paste can be printed easily and produces the required fine and

continuous lines with a high aspect ratio. Thus, the paste rheology should be such that it must be

easy to squeegee the paste over the surface and through the small openings of the screen and

deposit the paste on the silicon substrate. For this purpose, the paste must exhibit a shear thinning

or pseudo-plastic behavior. Pseudo-plastic behavior of paste is defined as fall in viscosity with

increase in shear stress. To avoid slumping or spreading of the paste, the paste must completely

release from the screen openings and regain its structure fast. Slumping causes the printed line to

lose height and gain width which is undesirable as it decreases the aspect ratio of the contact line.

Slumping of the paste can be stopped by proper use of the drying step.

As a rheological parameter the dynamic viscosity h at a certain shear rate g is commonly used.

The typical dynamic viscosity values for the screen printing paste range from 900 to 2000 Pa-sec

at a shear rate of 0.1 reciprocal seconds [18]

Fig 2.7 - Evolution of viscosity and shear rate during printing [18]

Rheological changes in the paste during printing are time and shear dependent and specifically

the actions during and just after the print stroke are framed in milli-seconds. Figure 2.7 shows the

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changes in viscosity and shear rate prior to, during and after screen printing. If stirring is applied

prior to printing, the viscosity of the paste is reduced. Further reduction in viscosity is caused by

the squeegee action when the paste is pushed by squeegee over the screen surface. The steep

change in viscosity and shear rate is observed when the paste is forced through the screen

openings. After the paste is released from the screen opening, the shear rate returns to original

proportions and the viscosity increases. Thus the paste recovers its original structure. The speed

of the recovery of the paste structure determines the slumping character of the paste. In case of

using a snap-off the screen is lifted directly behind the squeegee. Thus, the paste has very little

time to recover the structure. If no snap-off is used, the paste has little more time to regain its

structure, but it will be disturbed again when the screen is lifted.

The paste flow behavior during the squeegee movement is not understood in detail and is

dependent on different assumptions. As indicated in figure 2.7, when the screen openings are

filled, the shear rate has a maximum value, which is supposed to be more than 1000 reciprocal

seconds. In general commercially available pastes can be printed fairly easily, but show

extensive slumping actions, broadening the printed lines by over 50% [18]. If, on the other hand,

a paste is produced for less slumping, then it is extremely difficult or impossible to print and to

keep the printing quality under control in mass production. This often leads to blockages in the

screen (clogging) or interruptions in the printed lines. Thus, a paste with optimum slumping

character is recommended.

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Chapter: 3 Work carried out

3.1 Silver nitrate preparation

Silver bar (50gm) was brought from HDFC bank having 99.99% purity. It was cut into five

pieces of approximately 10gm. One of the silver pieces was used to prepare silver nitrate

powder.

A silver piece was put into silver nitrate solution and the solution was heated to about 95C. This

process was carried out with continuous stirring till all the silver was dissolved. Then, the

heating was stopped and the solution was allowed to cool which caused crystallization of silver

nitrate. Silver nitrate powder was separated from liquor solution by filtration. The powder was

dried at around 100C for 2 hours to remove all the moisture.

After drying, powder was characterized with X-ray diffraction. The XRD pattern of the AgNO3

sample (fig 3.2) showed a typical AgNO3 structure corresponding to JCPDS 00-043-0649. The

peaks which have circles over it, match with the data in the above JCPDS file.

Fig 3.1 - Silver bar (50gm) brought from HDFC bank

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Fig 3.2 - XRD plot for AgNO3 powder 3.2 Preparation of silver nanoparticles

Raw materials used

Silver nitrate powder

Formaldehyde solution (37-41wt% from Merck) as reducing agent

PVP(Polyvinyl Pyrrolidione K30 from Spectrochem) as protective agent

Sodium carbonate (anhydrous from Chemport) as pH controller

The process developed by [10] was followed. Silver nitrate solution of 0.01M prepared from

silver nitrate powder. Formaldehyde solution (37% by wt. or 10.05M) was used to make 2M

formaldehyde solution. As described in [10], the quantities of formaldehyde, PVP and sodium

carbonate were taken in the ratio:

[AgNO3]/[HCHO] = 4, [AgNO3]/[PVP] = 9.27, [AgNO3]/[Na2CO3] = 1.

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Compound Amount for 250ml 0.01M AgNO3

solution

Amount for 0.01M 500ml AgNO3

solution

AgNO3 0.43gm 0.85gm

HCHO 28.33ml (2M) 56.66ml(2M)

Na2CO3 0.425gm 0.85gm

PVP 3.94gm 7.88gm

Table 3.1 - Amounts of different raw materials required when 250ml and 500ml of 0.01M

AgNO3 solution are used respectively

The procedure for making silver nano particle is explained in a flow chart. Due to small size of

silver particles, colloid is quite difficult to separate from the solution. In the last step, acetone of

the same volume as that of the solution has to be added [10] and then the colloid is separated

from the solution through centrifuge and filtration. Due to low solubility of PVP in acetone, PVP

protected silver colloids agglomerate and can be separated easily. If the precipitate is kept wet, it

can be re-dispersed by de-ionized water. Only the size of the silver nanoparticles might increase

marginally during the re-dispersion process [10].

26

Fig 3.3 - Flow chart explaining synthesis of silver nano particles using formaldehyde as reducing agent

Take 0.01M AgNO3 solution

Add required amount of poly (N-vinyl-2-pyrrolidone) PVP

Add required amount of 2M formaldehyde

Add required amount of sodium carbonate

Stir the solution for around an hour

Add same amount of acetone as that of the solution

Separate the colloid from the solution via centrifuge and

filtration and then dry it

27

Chapter: 4 Future plan of work

Silver nanoparticles

Prepare silver nanoparticles using different reducing agents such as hydrazine monohydrate (N2H4), sodium borohydrate (NaBH4) to get low particle size and narrow size distribution

Characterize nano particles with particle size analyzer to get average particle size and particle size distribution

Characterize morphology of nano particles with FE-SEM and X ray diffraction for crystallinity

Glass frit

Prepare low melting Bi2O3-B2O3-ZnO glass by conventional melt quench technique and frit it with the help of three roll mill

Find the glass transition temperature of the formed glass by DSC

Try different combinations of Bi2O3-B2O3-ZnO glass to obtain good compatibility with FTO coated glass in terms of properties of the paste

Silver Paste

Prepare silver paste by adding silver nanoparticles, glass frit, and organic vehicle in proper proportions

Characterize the paste for rheology using a rheometer with cone and plate geometry

Screen print the paste with desired dimensions on FTO coated glass

Drying and sintering the paste at different temperatures (depending on Tg of glass frit and

size of nano particles) after screen printing

Characterize sintered paste for sheet resistance with four probe method and FEG-SEM

for connectivity with substrate

Optimize properties of the paste by try different proportions of the above components

28

References

[1] Won Jae Lee, Easwaramoorthi Ramasamy, Dong Yoon Lee, Jae Sung Song, Dye-sensitized solar cells: Scale up and currentvoltage characterization, Solar Energy Materials & Solar Cells 91 (2007) 16761680

[2] Easwaramoorthi Ramasamy, Won Jae Lee, Dong Yoon Lee, Jae Sung Song, Portable, parallel grid dye-sensitized solar cell module prepared by screen printing, Journal of Power Sources 165 (2007) 446449

[3] Won Jae Lee, Easwaramoorthi Ramasamy, Dong Yoon Lee, Jae Sung Song, Glass frit overcoated silver grid lines for nano-crystalline dye sensitized solar cells, Journal of Photochemistry and Photobiology A: Chemistry 183 (2006) 133137

[4] Sunghyun Park, Dongseok Seo, Jongkook Lee, Preparation of Pb-free silver paste containing nanoparticles, Colloids and Surfaces A: Physicochem. Eng. Aspects 313314 (2008) 197201

[5] Won Jae Lee, Easwaramoorthi Ramasamy, Dong Yoon Lee, Jae Sung Song, Grid type dye-sensitized solar cell module with carbon counter electrode, Journal of Photochemistry and Photobiology A: Chemistry 194 (2008) 2730

[6] H. Aguilar Ribeiro, P. M. Sommeling, J. M. Kroon, A. Mendes, C. A. V. Costa, Dye sensitized Solar Cells: Novel Concepts, Materials, and State-of-the-Art Performances, International Journal of Green Energy, Vol 6: 3 (2009), 245 256

[7] Lei Wang, Xiaoming Fang, Zhengguo Zhang, Design methods for large scale dye-sensitized solar modules and the progress of stability research, Renewable and Sustainable Energy Reviews 14 (2010) 31783184

[8] Matthias Hrteis, Fine-Line Printed Contacts on Crystalline Silicon Solar Cells, PhD Thesis

[9] Alexander Kamyshny, Joachim Steinke and Shlomo Magdassi, Metal-based Inkjet Inks for

Printed Electronics, The Open Applied Physics Journal, 2011, 4, 19-36

[10] Kan-Sen Chou, Chiang-Yuh Ren, Synthesis of nanosized silver particles by chemical

reduction method, Materials Chemistry and Physics 64 (2000) 241246

[11] Keunju Park, Dongseok Seo, Jongkook Lee, Conductivity of silver paste prepared from

nanoparticles, Colloids and Surfaces A: Physicochem. Eng. Aspects 313314 (2008) 351354

29

[12] Qian Peng, Yuelin Zhu, Three reductions used in preparation of the silver nano-particle for

crystalline silicon solar cell and its characterization, Advanced Materials Research Vols. 399-

401 (2012) pp 1524-1527

[13] Yu-Jin Kim, Seong-Jin Hwang and Hyung-Sun Kim, Thermal Properties of Bi2O3-B2O3-ZnO Glass System, Materials Science Forum Vols. 510-511 (2006) pp 578-581

[14] Luo Shiyong, Wang Ning, Xu Wencai, Lv Yong, Preparation and rheological behavior of lead free silver conducting paste, Materials Chemistry and Physics 111 (2008) 2023

[15] Guoxiao Yao, High efficiency metal stencil printed silicon solar cells, PhD thesis, page 22-

23

[16] Tina E. Rardin, Renmei Xu, Printing Processes Used to Manufacture Photovoltaic Solar

Cells, The Journal of Technology Study, Volume 37, number 2, Fall 2007

[17] Dr. Luis Jaime Caballero, Solar Energy, edited by Radu D Rugescu, Chapter 16 Contact

Definition in Industrial Silicon Solar Cells

[18] Jaap Hoornstra, Arthur W. Weeber, Hugo H.C. de Moor, Wim C. Sinke, The importance of

paste rheology in improving fine line, thick film screen printing of front side metallization, 14th

European photovoltaic solar energy conference and exhibition, 30 June - 4 July, 1997, Barcelona

Bibliography

[1] Screen-printed solar cells,http://pveducation.org/pvcdrom/manufacturing/screen-printed

solar-cells. (Accessed on 10/9/2011)

30

Acknowledgement

I would like to take this opportunity to express a deep sense of gratitude to my guide, Prof. Parag

Bhargava, Department of Metallurgical Engineering and Materials Science, Indian Institute of

Technology, Bombay. His continuous encouragement and competent guidance motivated me and

instilled a sense of confidence in me. I would like to thank Prof. Rajiv Dusane, HOD,

Department of Metallurgical Engineering & Materials Science, IIT Bombay. I would also like to

thank all the faculty & students from our department for their help & support.

Screen printed contact technology is very rapid and cost effective metallization process. The screen printing equipment is robust, simple, and inexpensive and can be easily automated. It also produces less chemical waste and thus does not affect the e...