TITANIUM DIOXIDE THICK FILM PRINTING PASTE FOR DYE SENSITIZED SOLAR CELL

by

CHENG-LUN YU

Submitted in partial fulfillment of the requirements

For the degree of Master of Science

Thesis advisor: Dr. Chung-Chiun Liu

Department of Material Science and Engineering

CASE WESTERN RESERVE UNIVERSITY

January, 2011 CASE WESTERN RESERVE UNIVERSITY

SCHOOL OF GRADUATE STUDIES

We hereby approve the thesis/dissertation of

______

candidate for the ______degree *.

(signed)______(chair of the committee)

______

______

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(date) ______

*We also certify that written approval has been obtained for any proprietary material contained therein. TABLE OF CONTENT

Table of content i

List of tables iii

List of figures iv

Acknowledgement vi

Abstract viii

Chapter 1 : Introduction 1

1.1 : General background information 1

1.2 : DSSC working principle 1

1.3 : Screen printing manufacturing processing 4

1.4 : Sensitizer materials for DSSC application 9

1.5 : TiO2 electrode 14

Chapter 2 : Materials, experimental and results 16

2.1 : Design of the research plan 16

2.2 : Chemicals and equipments 17

2.3 : Experimental procedures of the DSSC electrode

paste manufacturing 20

2.4.1 : Testing flow 22

2.4.2 : Four point probe conductivity measurement 23 i

2.4.3 : Sunlight simulation of Xenon arc lamp testing 24

2.4.4 : Introduction of laser profile scanner and software for thickness

measurement in this study 26

2.5 : Curing temperature testing of the printed TiO2 paste on

glass substrate 28

2.6 : The binders tested in this DSSC TiO2 electrode paste study 31

2.6.1 : Polyethylene glycol (PEG) and ethylene glycol (EG) 31

2.6.2 : Polyvinyl alcohol (PVA) and Polyvinyl butyral (PVB) 32

2.6.3 : Terpineol 34

2.6.4 : Testing results of commercial inorganic binders from Aremco 41

2.6.5 : Tetraethyl orthosilicate (TEOS) 45

2.6.6 : Water glass binders 47

Chapter 3 : Analysis and result discussion 50

Chapter 4 : Conclusion 59

Chapter 5 : Future work 61

ii

LIST OF TABLES Chapter 2

Table 1. Chemicals and their usage in this DSSC study 18

Table 2. Equipments and their usage in this DSSC study 19

Table 3. Curing temperature results of PVB/PVA inks (photos taken at 50X) 29

Table 4. The TiO2 paste recipes based on PEG and EG binders 32

Table 5. TiO2 paste with different PVA/PVB ratio ink and their testing results 33

Table 6. The recipes of the TiO2 pastes in table 5 34

Table 7. Dyesol thick film printing TiO2 paste recipe for DSSC usage 35

Table 8. TiO2 electrode paste recipes with different components

in this chemical evaluating study 35

Table 9. Comparison and results of the properties of the testing

results base on the TiO2 films recipes in Table 6. 36

Table 10. Aremco paste properties 42

Table 11. Aremco paste heating process 43

Table 12. TiO2 paste recipes based on Aremco inorganic binders 44

Table 13. Testing results and recipes of TiO2 film based on TEOS for

binder material. 46

Table 14. Testing results and recipes of TiO2 film based on water glass binder

materials 47

iii

LIST OF FIGURES

Chapter 1

Figure 1. Schematic representation of Gratzel cell working principle.

The numbers indicate the reactions take place in sequence. 4

Figure 2. A schematic illustration of screen printing procedures and part. 6

Figure 3. Incident photon to current conversion efficiency as a function of the

wavelength for the standard ruthenium sensitizers N3 (red line),

the black dye N749 (black curve), and the blank nanocrystalline

TiO2 film (blue curve) 11

Figure 4. Ruthenium compound sensitizers and the TiO2 film samples with

dye adhesion 11

Figure 5. Directions and pathways of metal to ligand charge transfer between

dye and TiO2 interface 13

Chapter 2

Figure 6. Doctor blade TiO2 paste on ITO glass substrate with squeegee 21

Figure 7. (a) Dimensions of the self-made four point probe base and gold ink

electrode (b) TiO2 paste printing area (grey strip) on the four point

probe base 23

Figure 8. Demonstration the functions of the four point probes when

contacted to substrate 24 iv

Figure 9. Comparison of Xe flash lamp spectrum and Solar AM 1.5 standard 26

Figure 10. Measuring process of laser profilometer provided by Optical

Gapping Product, Inc.. 27

v

ACKNOWLEDGEMENT

Two years ago, I entered Case Western University and began the graduate

student study. A lot of memories came across my mind at the moment when I started

writing this acknowledgement. Fortunately, many people shared their experience with

me not only in academic, research, but also American life.

I wish to thank Prof. C.C. Liu, my advisor, who lead me into all of the researches,

and always support me in these two years. Without his generous help, my graduate

student life should be very different. I also want to thank Laurie Dudik, the managing

engineer who gave me a lot of advises in my research. I still remember the day Dr.

Liu and Laurie brought me to the office because it was the day that started everything

in these two years.

I want to thank Prof. Clements Burda, and his research group. Half of the

experiments in this study are finished in their laboratory. He’s always encouraging

this research and sharing his experience. I would like to thank Yixin Zhao, Yajun Ji

and Keng-Chu Lin who help me measuring the experimental data and providing

useful information, especially Keng-Chu Lin who gives me a lot of hands during the

past two years. Also, I have to thank Dr. Mark De Guire and Dr. Gerhard E. Welsch

who agree to be the committee members for the thesis defence and sharing their

opinions.

I wish to thank Dr. Wei-Hung Chaing who not only gives me comments about vi the experiment, academics, but also helps me get used to American life. No matter what kind of difficult situations I face, those friends who always support me, wherever them are, gives me confidence. I want to thank you all.

Finally, I have to thank my parents and the family members. They are always with me and encourage me, especially when I feel frustrated. Because of their support,

I have this opportunity to study in Case Western Reserve University and meet the peoples here. Thank you for everything you do for me. I’m always grateful to have your support in my life.

vii

TITANIUM DIOXIDE THICK FILM PRINTING PASTE FOR DYE SENSITIZED SOLAR CELL

Abstract

By

CHENG-LUN YU

Comparing to the narrow band gap material, dye sensitized solar cells (DSSC)

provide a less expensive manufacturing method. TiO2 nano-crystalline anode

material highly improves the DSSC efficiency as Gratzel reported in 1991. Today

the DSSC efficiency is about 12% based on TiO2 and ruthenium compound sensitizer.

The development of a thick film screen printable paste for the DSSC TiO2 anode is the objective of this study, since thick film printing technique is appropriate for

lowering the manufacturing cost of DSSC via mass production. An ideal TiO2

electrode has to achieve moderate thickness, good light transmittance, high degree of

roughness and good electrical connection between the dyes and the TiO2 layer.

Thick film printing pastes for the TiO2 layer are prepared tested and evaluated in this viii study, and their properties are experimentally examined. Experimental methods

and experimental protocols for the characterization of the TiO2 thick film printable paste are established which can be useful in the advancement of the manufacturing of

DSSC.

ix

I. INTRODUCTION

1.1 General background information

Today, the price of oil continues to rise. Energy crisis is an unavoidable issue

which we have to face and solve. Solar cell is an alternative renewable energy

source which is potentially viable to replace fossil fuel, at least partially. Dye

sensitized solar cell (DSSC) is one energy source in the photo-electrochemical cell

family. DSSC is a solar cell using wide band gap semiconductor and sensitizer

loading to adsorb light, then, converting the light energy into electricity. Comparing

to the silicon based solar cell, DSSC manufacturing process is easier and less

expensive. Wide band gap semiconductor is also able to provide water splitting

energy for further solar fuel because it is more stable in liquid comparing to the

silicon semiconductors and higher output voltage. The dye sensitize theory was first

applied for solar cell in 1970’s, but the efficiency was only 0.1% [1]. In 1991,

Michael Gratzel described a DSSC with nano-crystalline TiO2, which had a 15 nm

average size, for the anode material, and a ruthenium complex sensitizer with high

turnover number, achieving a 7% efficiency [2]. Gratzel established a basic DSSC structure which most DSSC investigators adapted or modified.

1.2 DSSC working principle

In dye sensitized solar cell (DSSC), light is captured by a dye first and then passes through the nano-crystalline anode material to a conductive glass substrate, 1

such as tin-doped indium oxide (ITO) or fluorine-doped tin oxide (FTO) glasses.

According to the equation E=hc/λ (where E stands for the energy of light, h stands for the Planck’s constant, c stands for the speed of light and λ stands for the

wavelength of light), anatase TiO2 can absorb ultra-violet light which has wavelength

less than 387 nm because it has a band gap of 3.2 ev, but it cannot absorb visible light

which is between 400 nm and 700 nm. In order to increase the efficiency, dye is

needed to capture visible light. Dye will be excited by sun light and charges can then

be transferred from the metal center of the dye, such as ruthenium, to TiO2 by ligand

attached on the TiO2. The excited energy level of the dye is higher than the TiO2

anode conduction band and the electrons will inject into the semiconductor with

femto-seconds speed which is a very fast rate [3]. The back electron transfer speed

is much lower and the electrolyte to oxide dye charge transfer is also very fast in order

to regenerate the oxide dye. That is the principle that the device is able to collect

electrons in nano-semiconductor powder. An ideal dye should have larger excited

energy level comparing to the TiO2, wide absorption range for visible light and high turnover number [3]. Ruthenium complex is the most widely used dye, which also

has best solar cell efficiency at present assembled with TiO2 electrode and volatile

iodide/triodide electrolyte [2]. After electrons inject into the nano-crystalline TiO2, electrons will move by diffusion to the conductive glass and reach the outer circuit to

2

the cathode, which is usually made by platinum. Finally the reduction and oxidation

of electrolyte, which already supply electrons to the oxide dyes, are completed, and

- - the circuit can be showed in figure 1. Typical electrolyte is I /I3 because it provides

fast dye regeneration speed and it is chemically stable. The most common cathode

material is platinum which has good conductivity and also reflects incident light to

dye-TiO2 layer, and this enhances the efficiency. In figure 1, the TiO2 Fermi level

and the oxide electrolyte energy level will determine the maximum voltage.

The chemical reactions of this DSSC can be given as follows:

(1)Dye is excited by light and the electrons jump to the excited state form the ground state.

S + hv = S* (eq.1)

S: ground state of dye S*: excited state of dye S+: Oxidized state of dye

(2)Electrons inject into the conduction band of semiconductor from the dye excited

state.

* - + S =e (in TiO2 conduction band) + S (eq.2)

(3)Electrolyte redox couple reaction is.

+ - - S + 3I = S + I3 (eq.3)

(4) Recombination between electrons in TiO2 conduction band and oxide dye occurs

+ - S + e (in TiO2 conduction band) = S (eq.4)

3

(5)Electrons in conduction band conduct to outer circuit

- - e (in TiO2 conduction band) = e (Back contact) (eq.5)

- (6)Electrons in nano-crystalline semiconductor combine with I3 ions

- - I3 + 2e = 3I (eq.6)

- (7) I3 ions spread to the counter electrode and electrons lead to the re-generation of

I- ions

- - I3 + 2e (in TiO2 conduction band) = 3 I (eq.7)

Figure 1 Schematic representation of Gratzel cell working principle. The numbers in this figure indicate the reactions take place in sequence. [2]

1.3 Screen printing manufacturing processing

DSSC is competitive because its manufacture process is less expensive, less

4

contamination and most of the materials are plenty comparing to those used in silicon based solar cell. Using thick film printing technique such as screen printing, the

DSSC electrode can be massively manufactured and the film thickness is able to be

controlled between 10 to 15 microns, a desirable thickness. Our objective in this

study is to develop and select a screen printing TiO2 paste for DSSC anode material.

When manufacturing a DSSC, the anode semiconductor material is usually printed on a conductive glass and then cure at 450oC. Sensitizers, the dyes, are first

dispersed in solution such as acetonitrile. Place the cured TiO2 into sensitizer solution overnight for dye adsorption. Finally, electrolyte is injected if using liquid electrolyte and the solar cell is sealed with spacer material. Screen printing

technique is often used to print the thick film TiO2, which is ideally about 10 microns,

on a conductive glass, such as FTO or ITO.

This thick film screen printing technology has been developed for the electronic

industries to produce miniature and robust electronics circuits in a cost-effective

manner, as this technology is massive and automated. This technology can produce

well-defined, highly reproducible structures, and these characteristics are very

desirable for manufacturing DSSC. The printing procedure can be described as

shown in figure 2.

5

Figure 2: A schematic illustration of screen printing procedures and parts [4]

A thick film printing paste is first pressed onto the substrate by a mechanical squeegee through the openings of a screen. The pattern from the screen is then transferred onto the substrate. The thick film screen printing ink or paste normally consists of a solvent, a binder and the material of interest. The binder can be an

organic or inorganic salt, which binds the material of interest, such as the TiO2 particles in our case, onto the substrate after the firing process. The solvent serves as a vehicle to

provide a homogeneous mixture of the ink for the printing. The screen consists of a

finely woven mesh of stainless steel, nylon or polyester, mounted under tension on a

metal frame. During printing, the substrate is held at a distance from one side of the screen, while the ink is placed on the opposite side of the screen and a squeegee traverses the screen under pressure. The screen is thereby brought into contact with the substrate and also the ink is forced through the open area of the mesh. The required device 6

pattern from the screen is thus left on the substrate. The next step is to dry the substrate removing the solvent, most likely an organic component. This solvent drying step is

usually accomplished at a temperature approximately 120oC. After drying, the printed

film is relatively mechanically stable and the substrates can then be handled easily. In order to remove the nonvolatile portion of the solvent, firing under high temperature is

required. Further screen printing layers may be added after firing if necessary.

The most popular screen printable ink recipe contains ethanol for solvent, terpineol for dispersant, ethyl cellulose for thickener and Triton X-100 for surfactant [13]. The boiling point of these components can be given as follows: Terpineol is about 200oC,

ethanol is about 80oC. Organic solvent is more popular than water for solvent because

the water solvent film is more likely to have cracking problem. Terpineol component is

added for long term stability and ethyl cellulose is helpful for TiO2 morphology, which is

important for dye adsorption. If the TiO2 topography has more room for dye loading,

the cell efficiency will increase. The heating procedure is to evaporate the organic

o solvents and then to sinter TiO2 for half to one hour at 450 to 500 C. The sintering step

will enhance the solar cell efficiency by modifying TiO2 porous structure electronically

interconnected.

When preparing a screen printing ink, the dispersion of nano-particles is difficult,

but it is also an important step because it will affect the TiO2 structure and reduce the

7

efficiency of DSSC if the dispersion is incomplete. The agglomerates will lower the

TiO2 film integrity, particle connection and increase dark current as the result of higher

trap state density and reduce total output photo-current. Trap state is a state existing

between conduction band and valence band. When electrons go to the trap state, they will

stay here without jump into the conduction band, and the electrons will have more chance

to recombine with oxide sensitizer or electrolyte. The charge recombination will

contribute to the dark current, which will lower the current output and decrease the solar

cell efficiency. In order to disperse nano-particles, physical methods and chemical

methods are both important techniques. There are several approaches to disperse

TiO2 particles into the ink, for example, grinding the powders with ethanol in mortar

with pestle or using mixers such as bio-homogenizer to stir the mixture. After that,

magnetic stir bar can be used in the paste and further disperse the nano-particles.

Ultra-sonic bath is the one of most effective physical method to reduce agglomerates.

However, it will heat up the paste during ultra-sonic process degrading the quality of

TiO2 paste. In order to avoid the heating problem, we keep the ultra-sonic processing time within 20 minutes each time.

In order to better disperse the TiO2 particles, especially 20 nm size particles,

chemical dispersion method is necessary. Surfactant is frequently used in a chemical

method. The use of surfactant leads to an increase in surface roughness increasing and

8

better dispersing the individual particles which enable better porous conductive structure.

Large agglomerations formed by Van der Waals' forces between nano-particles in the

case that surfactant is absent. Surfactant molecules attached on the TiO2 particle

surface will modify the surface property and alter the interaction between particles and particle to liquid to increase the repulsive force between each particle [5].

In order to enhance the adhesion between conductive glass and TiO2 film, a common strategy is to form -OH bonding on the interface. It is possible to supply

-OH bonding by polymer additives in screen printing paste or rinse the conductive

glass with chemicals before curing the TiO2 paste. A successful cured TiO2 film

should be crack-free and cannot peel off from the substrate easily after the 450oC

sintering process. The ink recipe and curing procedure will highly affect the

efficiency because the defects in TiO2 layer will contribute to the trap state density.

The cracks in TiO2 film will increase dark current because of the contact between

conductive glass and electrolyte, as a result, lowering the solar cell efficiency.

Furthermore, the thickness, roughness and surface area of the semiconductor film will determine the dye loading. If the dye loading is higher, the solar conversion efficiency will be better as well.

1.4 Sensitizer materials for DSSC application

The theoretical requirements of DSSC sensitizer are: higher energy level of the

9

excited dye comparing to the TiO2 or other anode semiconductor conduction band

edge, slow back electron transfer, but fast dye to TiO2 transfer. The electron transfer

from electrolyte to oxide dye must be fast enough to regenerate it to complete the

overall electron transfer loop. The turn-over number must be high for long term

usage.

The sensitizers with highest efficiency are ruthenium complex today.

Commercial available ruthenium based dye such as cis-Di-(thiocyanato) bis (2,

2¢-bipyridyl)-4, 4¢-dicarboxylate) ruthenium-(II), coded as N3 or N-719 dye

depending on whether it contains four or two protons [6,7], have been found to be

outstanding solar light absorbers and charge-transfer sensitizers. The performance

of this red ruthenium complex is unmatched for a long time by any other dyestuff.

However, a black dye has been discovered that shows a performance comparable to

that of N3 as a charge-transfer sensitizer in the DSSC a few years ago [8]. The structure of these sensitizers is shown in Figure 3. The adsorption spectrum can be changed by manipulating the ligands. It is one of the most powerful parts of

sensitizer investigation. Figure 3 shows the adsorption spectrum of TiO2 and these

commercial available dyes.

10

Figure 3: Incident photon to current conversion efficiency as a function of the wavelength for the standard ruthenium sensitizers N3 (red line), the black dye N749 (black curve), and the blank nanocrystalline TiO2 film (blue curve) [9]

Figure 4: Ruthenium compound sensitizers and

the TiO2 film samples with dye adhesion [7].

There is a summary made by Gratzel [3] about the description of charge

behaviors between sensitizer and TiO2:

1. Electron injection into TiO2: following the light absorption of the Ru complex, the electron injection into the conduction band is in the sub-ps to ps range.

11

2. Back electron transfer: the rate constant for the back electron transfer (dark reduction in the absence of externally added electron donors) however is much smaller for several reasons, typically several μs.

- 3. Reduction of triiodide by ecb : another important recombination process is

- reduction of I3 in the electrolyte by conduction band electrons. The exchange

current density, jo of the reverse saturation current of this process has been

measured in the range 10 to 11x10-9 A cm-2, depending on the electrolyte.

Surface treatment of the electrode can alter these values drastically.

4. The electron movement (percolation) in the nanocrystalline TiO2 electrode to

the back contact is significantly slower than in single crystal TiO2. Studies have

shown that the photocurrent transients, following UV excitation of TiO2

particles from a ns pulsed laser, decay in the ms to s range.

5. The exchange current density for the reduction of triiodide at the counter

electrode ITO coated with a catalytic amount of Pt, has been measured to be

0.01 to 2x10-1 A cm-2.

6. Reduction of the oxidized dye by iodide occurs on a timescale of 10-8 s.

The metal to ligand charge transfer is important because it will determine the

injection speed and backward electron transfer rate. As mentioned before, the

charges can be separate due to the huge speed difference [3]. Usually carboxylate

12 group is applied to attach the ruthenium dye on the TiO2 surface, like N3, N719 and black dye. The ruthenium center will be excited after lighting and inject the electrons through the metal to ligand charge transfer. Figure 5 shows the basic concept of metal to ligand charge transfer.

The reason that the backward charge transfer is much slower because the back reaction of the electrons with the oxidized ruthenium complex involves a d orbital

localized on the ruthenium metal. Its electronic overlap with the TiO2 conduction band is small and is further reduced by the spatial contraction of the wave function upon oxidation of Ru (II) to Ru (III). Thus, the electronic coupling element for the back reaction is 1-2 orders of magnitude smaller for the back electron transfer as compared to injection reducing the back reaction rate by the same factor.

Figure 5: Directions and pathways of metal to

ligand charge transfer between dye and TiO2 interface

13

1.5 TiO2 electrode

The nano-crystalline semiconductor anode is not only where the sensitizers adhere, but also where the charge transfer occurs. Consequently, it is acknowledgeable that the quality of the semiconductor electrode directly affects the efficiency of DSSC. Reducing charge recombination is the most direct means to

improve DSSC efficiency, and the quality of TiO2 film is one of the most important

key. There are four fundamental requirements of the electrode material: large

surface area and roughness, sponge-like nano-crystalline structure with good

electronic contact. The anode material structure should allow electrolyte redox

couple regenerates the oxide dye and charge should also be able to rapidly inject into

the semiconductor. At the meantime, backward charge recombination should be the

slowest step. TiO2 is an ideal material with appropriate band gap and electronic

properties as mentioned. It has additional advantages like non-toxic, stable in water

and better anti-corrosion property comparing to narrow band gap semiconductor

material like silicon. TiO2 has three kinds of crystal structure: anatase, rutile and brookite. Brookite only exists in natural ores and it cannot be synthesize in industry.

Anatase has a band gap of 3 ev and rutile has a band gap of 3.2 ev. The conduction band density of anatase in Ti 3d is higher than that of rutile, which makes it easier to accept electrons form the top of valence band (O 2p). That is the reason that anatase

14

TiO2 has better performance comparing to rutile [10, 28, 29].

These mechanics will determine the efficiency of the DSSC. TiO2 electrode is

one of the most important key to increase the efficiency because it is where sensitizers

load and the electrons transfer happen. The requirements of the TiO2 film is relate to

these mentioned concepts. The purpose of this study is to manufacture a thick film

printable TiO2 paste which is appropriate for mass production and able to lower the

DSSC cost.

15

II. MATERIALS , EXPERIMENTAL & RESULTS

2.1 Design of the research plan

In this study, the objective is to prepare a suitable thick film printing paste for the application of manufacturing DSSC. Specifically, this paste will be applied for the anode of a dye sensitized solar cell (DSSC). The thick film printing electrode must have the following basic requirements: sensitizer adhesion, film conductivity and sufficient mechanical strength in contact with electrolyte. When we first attempt to reproduce reported screen printing pastes by other researchers, we have found that the adhesion between the substrate and the anode needs to be further improved. Thus, an appropriate binder material which can enhance the adhesion property of the paste is then undertaken. The evaluating procedure is shown in the flow chart 1.

Chart 1: Preparing TiO2 paste

Doctor blade paste on substrate, such as conductive glass and 4 point probe bases

Test mechanical strength after paste is dried at room temperature

o Sinter the TiO2 film at 450 C before conductivity test

Test conductivity on four point probe base

Test conductivity under Xe light source by source meter

Measure thickness by laser profilometer 16

All components are first mixed in a glass container and the mixture needs to be

as homogeneous as possible. The doctor blade method is applied in our experiment

instead of screen printing to place the paste on substrate. The TiO2 film is kept dry

at room temperature and then use a tape, such as the Scotch magic tape of 3M, to test

the adhesion strength between the substrate and the film. If the strength is sufficient, the film will be sintered at 450oC, in order to test the temperature durability. Source

meter and four point probe base are applied for conductivity measurement of the film

after sintering. A Xenon arc lamp is used to test the impedance change of the TiO2 film during illumination. The film thickness is measured in the last step because the thickness variation after sintering will depend on the recipes. The thickness without

solvent is the actual thickness of TiO2 film.

2.2 Chemicals and equipment

In order to produce a thick film printing paste for DSSC electrode material,

chemicals for different purpose are necessary. For example, binders are intended to

increase the adhesion and viscosity properties of the paste, dispersant to disperse and

maintain the long term shelf life stability of the paste. Also, an appropriate solvent to disperse the solids is needed. The chemicals used in this DSSC study are listed in

Table 1.

17

Chemicals Manufacturer Purity Usage

P25 Titanium Dioxide Degussa, Japan TiO2 powder

Triton X-100 Sigma Chemical Co.., USA Surfactant

Acetone A18SK-4 Fisher 98% Clean substrate

Ethanol Alcohol 200 Decon labs 99% Solvent

Heptanol Acros 98% Solvent

Terpineol Fisher Dispersant/Binder

TEOS Acros 98% Binder

Aremco 642 Aremco Binder

Aremco 542 Aremco Binder

Aremco 644-A Aremco Binder

Aremco 644-S Aremco Binder

Aremco 643 Aremco Binder

Aremco 830 Aremco Binder

Ethyl Cellulose Aldrich Binder

Sodium silicate SS338-1 Fisher Binder

Potassium silicate 50828816 Pfaltz and Bauer Binder

Table 1: Chemicals and their usage in this DSSC study

18

There are equipments applied in this study to make the TiO2 pastes and films, such are dispersing and sintering. Equipment was used for the measurements of the thickness, conductivity and morphology of the film and the experimental data acquisition. All of the apparatus are listed in Table 2.

Equipments Manufacturer Model number Usage

Optical microscopy Olympus BX-60 Observation

Thickness Laser profileometer Optical gaping products inc Cobra measurement

Magnetic plate Thermo scientific 50094596 Disperse Paste

Hot plate Corning PC-320 Heating, stirring

Tube furnace Lindberg/Blue TF55035A-1 Sintering

Lab Oven Fisher scientific Isotemp Heating

Ultrasonic cleaner Branson 2510 Disperse Paste

Homogenizer Biospec products inc 1281 Disperse Paste

Arc lamp Newport 67005 Light source

Conductivity Source meter Keithley 2400 Measurement

FTO glass Hartford TEC8 (8ohm) Paste substrate

Table 2: Equipment and their usage in this DSSC study

19

2.3 Experimental procedures of the DSSC electrode paste manufacturing.

Depending on the recipes selected, the procedures of the formulation of the paste

o are not exactly identical. In general, TiO2 particles are preheated at 100 C for 30

minutes to remove the absorbed water vapor inside the particles. Cellulose powder is added into ethanol within a glass vial, a magnetic stir bar is then used to disperse

the ethyl cellulose. If the recipe contains cellulose, the cellulose is dissolved first in

ethanol before mixing with the other components because the cellulose may not

dissolve in other solvents. The other organic solutions, such as dispersant and

surfactant, are added in the recipe into the cellulose/ethanol solution. The paste

mixture is then stirred with a magnetic stir bar overnight. A homogenizer will be

applied if the TiO2 dispersion is incomplete, for example, agglomerates exist. In the

cases of using water glass and commercial inorganic binder, which will be mentioned

in the following chapter, only water is added as solvent. The glass slide substrate is

cleaned by acetone, ethanol and distilled water in sequence. Then, an air spray is

used (Fisher catalog number 23-022523) to dry the slide before the paste is applied.

Doctor blade means a film smoothing method using any steel, rubber, plastic, or

other type of blade used to apply or remove a liquid substance from another surface.

The term "doctor blade" is derived from the name of a blade used in conjunction with

20 the ductor roll on the letterpress press. The term "ductor blade" eventually mutates into the term "doctor blade." Based on the doctor blade methods [25, 26, 27] and the experience of Burda’s laboratory at CWRU, the doctor blade method used in this

study employs a glass bar to remove extra TiO2 paste on the conductive side of the glass substrate where the area is immobilized by 3M Scotch magic tape strips on it.

Figure 6 shows the doctor blade method using squeegee to smooth the taped area [25].

A glass bar can be used instead of squeegee in this study.

Figure 6: Doctor blade TiO2 paste on ITO glass substrate with squeegee [25]

The 3M Scotch magic tape is used to create a one centimeter square area on a cleaned microscope slide, (Fisher catalog number 12-55C). The 3M tape is squeezed

with a metal spatula to remove the air bubble within the taped area. The TiO2 paste is then added using a glass pipette placing it into the area. The doctor blade method is used to obtain smooth paste surface and control the thickness with a glass rod.

The ideal thickness of TiO2 is approximately 10 microns [11]. Higher thickness of

21 the film is able to ensure more dye loading for higher efficiency. However, there are

trap states existing in the TiO2 film, which will capture electrons and make it easier to recombine with electrolyte or oxide dye. When the thickness of the film increases, the possibilities of charge recombination also raise. On the other hand, if the thickness of the film is too thin, it will lower the dye loading which could affect adversely of the overall efficiency of the DSSC. That is the reason of choosing an optimum film thickness of about 10 microns based on the experimental results.

The prepared film is then kept at room temperature with a glass cap over the

glass slide resulting in a relatively low evaporation rate. Otherwise, the TiO2 films will crack after curing in the oven at room temperature. When the film is dried, there should be no crack on it and the film should not flake off from the glass slide substrate, neither. Furthermore, the cured film should be translucent or transparent

because the TiO2 electrode film will face the light source directly. If the cured film shows solid white color, the incident light could be blocked out of the dyes lowering the efficiency of the DSSC.

2.4.1 Testing flow

If there is no visible crack on the film and the paste adhesion appears to be sufficiently good to the glass substrate, the following test procedure is then undertaken. The first step in this evaluation is visual inspection with an optical

22 microscopy to inspect any crack of the film and the agglomeration of TiO2 particles.

2.4.2 Four point probe conductivity measurement

The second step of this testing is to determine the conductivity of the film. A four point probe test base is cleaned with acetone, ethanol and distill water before doctor blade the paste. Similar to the procedure described previously, the area of the film deposited on the four point probe base is defined by the tape and the film is then cured at room temperature under a glass cap. The thickness of the film is measured by laser profileometer after the sample is dried. The conductivity of the film is tested with a source meter connected to a toothless copper alligator clip to measure the impedance of the film. The structure of the 4 point probe base is showed in figure 7. The four point probe base used is on a thin aluminum oxide substrate. The electrodes are screen-printed using a commercial gold ink from Electroscience

Laboratories, and the part number is 8884-A.

(a) 0.5mm 8mm

10mm

2x1.2mm2

23

(b) 12mm

3mm

Figure 7: (a) Dimensions of the self-made four point probe base and gold ink

electrode (b) TiO2 paste printing area (grey strip) on the four point probe base

The four point probe is designed for resistance measurement, especially thin

films. To match the four point probe base, there will be four corresponding probes

in a linear arrangement with a current injected into the film via the outer two

electrodes. The electric potential distribution will be measured by the inner two

electrodes. Figure 8 shows the demonstration of the four point probe measurement

[12]. In our experiment, we use copper clipper to contact with the gold electrode instead of using four point probes and read the conductivity by source meter.

Figure 8: Demonstration the functions of the four point probe when contacted to substrate [12].

2.4.3 Sunlight simulation of Xenon arc lamp testing 24

If the paste resistance is less than 10 G ohm and has response to arc lamp light, indicating that the film has a reasonable conductivity, next testing step will then be

carried out, namely: the Xenon light test. Xenon arc lamp is a commonly used

sunlight simulation equipment, the comparison of Xenon spectrums and AM 1.5 are

shown in figure 9. AM1.5 is a most widely applied standard for solar cells which is

about 1000 W/m2. In this study, Newport 67005 Arc lamp is applied. If the resistance is too high, it might be the effect of cracks or excessive binders. The lamp

wattage is between 50 - 500 W and the transmittance range of lens material is between

200 - 2500 nm. A water tank is placed in front of the Xenon arc lamp in order to

adjust the light wavelength similar to daylight (350～1800nm, 100mW). The

measured distance from Xenon arc lamp to specimen holder is about 22 cm. The

Xenon arc lamp light test is basically testing the response after TiO2 film is

light-activated. Even though the TiO2 cannot absorb visible light, it is able to absorb

UV light and to excite electrons. If the resistance of the film is reduced, which

means the electrons are excited and conducting in the circuit. The measured

resistance change should be at least more than 1 G ohm when we fix the voltage at

210V, and the 210V will be fixed in every conductivity measurement in the source

meter setting. Along with the lighting, photo-electrons will be increasing and the

resistance will decrease. We record the conductivity data before and right after the

25

light first contact with the TiO2 film. The most important thing in this conductivity

measurement is the photo-response of the TiO2 film. Initial resistance is highly

dependent for thickness or surface defects, such as cracks, so that the range of

impedance change is more important than the initial or the final resistance numbers.

Figure 9: Comparison of Xe flash lamp spectrum and Solar AM 1.5 standard

2.4.4 Introduction of laser profile scanner and software for thickness measurement in this study

The Cobra laser profile scanner is used in the thickness measurement of TiO2 film. This equipment is designed from Optical Gaping Product , Inc. (OGP). A

Scan-X software is also provided by the company for thickness measurement operation. The laser profile scanner has a digital range sensor (DRS) to receive the reflect laser beam from the measuring substrate. Figure 10 is a demonstration of

DRS measurement. This is a non-contact method which can prevent the film from

26 surface damage. The DRS-500 sensor equipped on the Cobra 2D has a resolution of

0.125 microns with 500 microns range, which is appropriate to measure the objective

TiO2 film around 10 microns.

Figure 10: Measuring process of laser profileometer provided by Optical Gaping Product, Inc..

Since the substrate thickness are not the same, laser beam alignment is necessary.

The Scan-X software provides easy alignment indications for lateral calibration. The minimum scanning step size is 0.001mm. Higher scanning step size shows better resolution, but the scanning rate is slower. A 0.02mm step size is applied in this thickness measurement for shorter scanning time, but also provides sufficient resolution, which the measured accuracy error is less than 0.1 micron. A slope on

the surface of the TiO2 film prepared with doctor blade method is common, instead of completely flat plane. It may be the result of unparallel force applied to the substrate.

The viscosity of the TiO2 paste will highly affect not only the thickness, but also the

27

surface curvature of the film, owing to the surface tension of the paste.

After the laser scanning process, there are several options to select the reference

bases to measure the thickness in the interested area. The dual differential is applied

in this study to measure the printed film on the substrate. In order to obtain an

average thickness of the film, laser scanning is carried out three times per sample in

the area between each gold ink electrodes.

2.5 Curing temperature testing of the printed TiO2 paste on glass substrate.

In the next testing step, the paste is placed into the taped area on the glass

substrate by glass pipette after the TiO2 particles are well mixed. The paste is doctor

bladed with a glass bar to smooth the surface of the TiO2 film. The curing temperature and method are important keys to produce a crack-free film. This step

o carried out immediately, before the 450 C sintering process of TiO2 electrode.

Several curing temperatures have been used and the results are shown in table 3.

The testing recipe is Polyvinyl Alcohol (PVA)/ Polyvinyl Butyral (PVB) binder based

paste with pentanol as solvent, which has a boiling point of about 120oC.

Our experimental results show that a crack-free film can be achieved at a room

temperature curing. We believe that a slow evaporation rate is essential to produce a

crack-free film at room temperature. In order to further reduce the solvent

28

evaporating rate, a glass lid is then placed over the glass substrate, which can also protect the wet film from dust in the environment.

Table 3.Curing temperature results of PVB/PVA inks(photos taken at 50X)

Cured at 60oC Center of the sample PVA/PVB ink recipe: 1-pentanol 17.883g, pioloform BN18 2.22g and Luvikol K30 2.22g

1.5 g TiO2, PVA/PVB ink 5g, 1-pentanol 2g and 0.2g X-100 Mixed Titanium powder, 1-pentanol and PVA/PVB ink in a glass jar and homogenize it for 30 seconds Cured at 60oC for 2 minutes Thickness: 16.01 micrometer Cured at 35 oC Stirred for 29 days by magnetic stirring bar PVA/PVB ink recipe: 1-pentanol 17.883g, pioloform BN18 2.22g and Luvikol K30 2.22g

1.5 g TiO2, PVA/PVB ink 5g, 1-pentanol 2g and 0.2g X-100 Mixed Titanium powder, 1-pentanol and PVA/PVB ink in a glass jar. Homogenize it for 30 seconds, then stirred for 29 days by a magnetic stirring bar Cured at 35oC for 8 minutes Thickness: 13 micrometer

29

Agglomerates are highly reduced

Cured at 25oC Stirred for 30 by magnetic stirring bar PVA/PVB ink recipe: 1-pentanol 17.883g, pioloform BN18 2.22g and Luvikol K30 2.22g

1.5 g TiO2, PVA/PVB ink 5g, 1-pentanol 2g and 0.2g X-100 Mixed Titanium powder, 1-pentanol and PVA/PVB ink in a glass jar. Homogenize it for 30 seconds, then stirred for 30 days by a magnetic stirring bar Cured at 25oC for 23 minutes Thickness: 13 micrometer Cured at room temperature This photo is taken at 50X PVA/PVB ink recipe: 1-pentanol 17.883g, pioloform BN18 2.22g and Luvikol K30 2.22g

1.5 g TiO2, PVA/PVB ink 5g, 1-pentanol 2g and 0.2g X-100 Mixed Titanium powder, 1-pentanol and PVA/PVB ink in a glass jar. Homogenize it for 30 seconds, then stirred for 30 days by a magnetic stirring bar Cured at room temperature with a lid on the sample Thickness is 9 micrometer on glass substrate, but 5 to 6 micrometer on ITO substrate 30

2.6 The binders tested in this DSSC TiO2 electrode paste study

The first TiO2 paste recipe tested is water solvent based, and it does not use any binder material, but solvent and surfactant. However, for thick film printing paste application, binders are necessary to enhance the film adhesion. Based on published screen printing information [13], terpineol and ethyl cellulose binder based recipes are used and, these two components are frequently applied in screen printing DSSC

manufacturing. In order to prepare a paste which can tolerate 450oC high

temperature sintering, inorganic silicate binders are also used and evaluated in this

study.

2.6.1 Polyethylene glycol (PEG) and ethylene glycol (EG)

Polyethylene glycol (PEG) is used in industry as surfactants, dispersing agents,

and solvents. PEG is a common additive for screen printing paste as a binder. It is

also a common binder for DSSC usage [14, 30, 31]. Ethylene glycol (EG) is also

reported to replace PEG as a binder because it is easier to be removed after 450oC

sintering [14]. PEG products have many different molecule weights, ranging from

400 to 20000 g/mol. For screen printing purpose, high molecule weight PEG works

better as previous literature reported, and the PEG recipe is basically reproducing a recipe from literature [14]. The only difference is that we use PEG 10000 g/mol

31

instead of PEG 20000 g/mol. The EG recipe only replaces the PEG with EG. The

experimental result shows that the PEG and EG help to increase the viscosity of the

TiO2 paste, but the interface adhesion does not improve.

Recipes

TiO2-0.5g, Ethanol-8ml, Ethyl cellulose-0.3g, PEG (10000)-0.3g PEG Acetyl acetone-0.1ml, Terpineol-0.5ml, HCl-0.1g

TiO2-0.5g, Ethanol-8ml, Ethyl cellulose-0.3g, EG-0.3g EG Acetyl acetone-0.1ml, Terpineol-0.5ml, HCl-0.1g

Table 4: the TiO2 paste recipes based on PEG and EG binders

2.6.2 Polyvinyl alcohol (PVA) and polyvinyl butyral (PVB)

Polyvinyl Alcohol (PVA) and Polyvinyl Butyral (PVB) have melting points of about 200oC, and they are relatively high temperature resin binders. Polyvinyl

alcohol has excellent film forming, emulsifying, and adhesive properties. It has both

hydrophilic and hydrophobic groups which give it a surfactant property. Polyvinyl

butyral is a resin usually used for applications that require strong binding, optical

clarity, adhesion to many surfaces, toughness and flexibility. It is also applied for

photovoltaic thin film solar modules.

We use an established PVA/PVB ink recipe formulated in the Electronics Design

Center CWRU. A magnetic stir plate is used in the dissolution process which

32

normally takes at least one hour to complete. A binder solution containing up to 15

weight percent of binder material can be prepared using this method. This recipe

works for many kinds of metal powders.

In order to properly prepare the ink, it is easier to mix the powders in a larger batch. Equal quantities by mass of PVA (Luviskol K30) and PVB (Pioloform BN-18)

are placed in a glass jar first. The ratio between the solvent and the powders is

approximately 18 grams to 4 grams, almost a 9 to 2 ratio. A magnetic stir bar is then

placed inside the glass jar and the glass jar is put on the magnetic stir plate. The

mixture is stirred for approximately 1 hour or longer until the solid powders are well

dispersed into the solution, depending on the size of the PVA/PVB ink batch. The

well dispersed ink should be a light yellow color and transparent.

Property Resistance Resistance Thickness Topography Recipe (before lighting) (after lighting) Crack after 1 1.1GΩ 1.1GΩ 7.1 microns sintering

Crack before 2 No data No data No data sintering

Crack after 3 61MΩ 57MΩ 4.5 microns sintering

Crack after 4 1.2GΩ 1.09GΩ 6.3 microns sintering

Crack before 5 No data No data No data sintering

Table 5: TiO2 paste with different PVA/PVB ratio ink and their testing results 33

In the following test, the amount of PVA/PVB powder is doubled or tripled based

on the mentioned regular PVA/PVB ink as shown in the recipe 1 of table 6. The

ratio of the binder material is increased in order to enhance the film adhesion strength,

however, there are small crack present on the sintered TiO2 films and the adhesion is

no better than the paste recipe 1. It turns out that the doubled and triple amount of

PVA/PVB is too much and the adhesion strength decreases.

TiO2-0.1g, PVA/PVB ink-0.5g, Ethyl Cellulose-0.2g, 1 Ethanol-1g, Terpineol-1g

TiO2-0.1g, double binder PVA/PVB ink-0.5g, Ethyl Cellulose-0.2g, 2 Ethanol-1g, Terpineol-0.5g

TiO2-0.1g, triple binder PVA/PVB ink-0.5g, Ethyl Cellulose-0.2g, 3 Ethanol-1g, Terpineol-0.5g

TiO2-0.1g, double binder PVA/PVB ink-0.5g, Ethyl Cellulose-0.3g, 4 Ethanol-1g, Terpineol-0.5g

TiO2-0.1g, PVA/PVB ink-0.5g, Ethyl Cellulose-0.2g, 5 Ethanol-1g, Terpineol-1g

Table 6: the recipes of the TiO2 pastes in table 5

2.6.3 Terpineol

Terpineol and ethyl cellulose based pastes are the most common recipes reported in the literatures [13, 15, 16, 18]. Based on a commercial paste recipe from Dyesol, an Australia company shown in table 7, several different component adjustments are

carried out in order to evaluate the chemicals in this recipe.

34

Table 7: Dyesol thick film printing TiO2 paste recipe for DSSC

Titanium dioxide 1317-70-0 10 - 30 %

Ethyl cellulose 9004-57-3 5 - 15 %

Terpineol 8006-39-1 8001-47-7 50 - 70%

Organic plasticizer 5 - 20%

In order to evaluate the effect of the TiO2 films with each chemical, 9 samples

with different recipes are tested to confirm their influence of the electrode film. The

TiO2 thick film printing paste recipes are listed in table 8 and the experimental results are listed in table 9.

Table 8: TiO2 electrode paste recipes with different components in this chemical evaluating study

TiO2-0.2g, Terpineol-0.75g 1 Ethyl Cellulose-0.05g, Heptanol-1.3g, Triton X100-0.1g

TiO2-0.2g, Terpineol-0.75g 2 Ethyl Cellulose-0.05g, Heptanol-1.3g

TiO2-0.3g, Terpineol-0.4g 3 Ethyl Cellulose-0.1g, Heptanol-1.6g

TiO2-0.3g, Terpineol-0.3g 4 Ethyl Cellulose-0.05g, Heptanol-1.6g

TiO2-0.3g, Terpineol-0.6g 5 Ethyl Cellulose-0.05g, Heptanol-1.6g

TiO2-0.2g, Terpineol-0.75g 6 Ethyl Cellulose-0.05g, Heptanol-1.3g, CH3COOH(ph=2)-0.04g

35

TiO2-0.2g, Terpineol-0.75g 7 Ethyl Cellulose-0.05g, Heptanol-1.3g, HCl(ph=2)-0.03g

TiO2-0.3g, Terpineol-0.6g 8(5) Ethyl Cellulose-0.05g, Heptanol-1.6g, Triton X100-0.1g

TiO2-0.3g, Terpineol-0.4g 9(3) Ethyl Cellulose-0.1g, Heptanol-1.6g, Triton X100-0.1g

The electrode film properties of conductivity, thickness and the topography observed under optical microscopy are recorded in table 9. Resistance change before and after the Xenon arc lamp lighting is tested to determine if the electron

conducts in the electrode itself after light activated. In some cases, the TiO2 film has no response to light, and it means that the film has poor conductivity.

Property Resistance Resistance Thickness Topography Recipe (before lighting) (after lighting) Very few cracks 1 8.3GΩ 6.6 11.3μm Particles

Small cracks 2 16 15 7.3 Particles

Crack free 3 2 2 11.8 Few particles

Few cracks 4 14.4 13.4 6.1 Few particles

Crack free 5 5 5 5.5 Transparent

Crack free 6 80 20 6.3 Transparent

36

Crack 7 33 22 9.2 Particles

Crack(thickness) 8 150 130 10.5 transparent

Crack free 9 10 5 9.5 Transparent

Table 9: Properties of the testing results base on the TiO2 films recipes in table 6.Comparison and Results:

1) Surfactant: Among the recipes 1 to 5, recipe 1 is the only specimen shows a

decrease in resistance after exposing to light. Comparing the component among the

recipes, recipe 1 is the only sample with surfactant X-100. It appears that the

surfactant is important to better disperse the particles and its effect may be more than

the phenomena observed under optical microscopy.

2) Terpineol: Comparing the recipes 4 and 5, the difference is that the recipe 5 has

double amount of terpineol. Although the result shows no resistance change after exposing to lighting, but the initial resistance are different. The recipe 4 has a higher thickness, and a larger resistance result due to the cracks present. It appears that the recipe 4 requires more solvent to disperse the particle and to reduce the cracking

problem. The recipe 5 contains more terpineol, but the thickness is less than the

result of recipe 4. It suggests that terpineol does not effectively produce thicker film as a binder material. There are reports stating that terpineol is used as dispersion agent, and not as a binder material. Our experimental testing results show that

37

terpineol does not perform well as an appropriate binder to enhance adhesion of the

TiO2 film to the glass substrate.

3) Ethyl Cellulose: Assessment of the thickness of the film prepared using recipe 3 and recipe 4 have been carried out, and the film prepared by recipe 3 shows higher thickness. There is a 0.1 gram difference in the quantity of terpineol used, however, as mentioned above, and terpineol does not appear to affect the film thickness.

Consequently, the thickness of the TiO2 film may be altered by adding more ethyl

cellulose. The film produced from recipe 4 shows slightly response to light, even its

initial resistance is higher, indicating the ethyl cellulose may block electron

transportation. This might cause by the residue of ethyl cellulose, which cannot be

totally removed after 450oC sintering processing.

4) Acid: According to various reports [13, 18, 19, 22], adding small amount of acid

may enhance TiO2 dispersion. Burda’s lab at the Chemistry department of CWRU

shows that HCl works better than the other acid. Based on the recipe form Gratzel’s

screen printable TiO2 paste [13] as an example, they also use acetic acid. The

recipes of 6 and 7 are prepared to evaluate the effect of acid presence. They are based

on recipe 2 with different kinds of acid, but the same pH value (pH=2). The

38

thickness varies within 2 micron which may result from the doctor blade process.

Initial resistances of the films produced from these two recipes are very different. It

might be caused by the degradation of the four point probe base, which can be

affected by the presence of acid and also may be affected by the blockage of electron

transportation. However, the films show response to light comparing to the film

prepared from recipe 2.

5) Confirm surfactant effect: Triton X-100 is a water-soluble liquid, nonionic

surfactant that has been recognized as the performance standard among similar

products. It is an octylphenol ethoxylate with an average of 9 to 10 moles of

ethylene oxide and it is a 100-percent active product. According to literature

research [20, 21], surfactants such as Triton X-100 is proved to improve the

morphology of TiO2 film. In order to support the idea that Triton X-100 really increases the conductivity as a surfactant, Triton X-100 is added to recipes 3 and 5 which the films show no response to light as mentioned in table 9. Consequently,

the addition of Triton X-100 to recipes 5 and 3 leads to the formulation of recipes 8

and 9 respectively. The result of the initial resistance in the recipe 8 is abnormally

high, after we compare the initial impedance of recipes 3 and 5. Although the higher

thickness may be a part of the reason, but the initial resistance change between the

39 recipes 8 and 5 is much larger than the difference between recipes 9 and 3. Even though the films prepared using recipes 8 and 9 show response to light, and the films prepared with recipes 3 and 5 do not. Comparing the films prepared with recipes 5 and 8, the thickness is increased by Triton X-100, and the initial impedance is higher, which indicate that the binders will affect conductivity if they cannot be removed well after sintering process.

6) Processing step of the film preparation: All of the pastes prepared are homogenized using a homogenizer with at least 20 seconds per minute for approximately 20 times after the initial mixed using a metal spatula. The rest of the paste is stored with magnetic stir bar inside the glass vial. Sintering process is

o carried out by heating up the room temperature cured TiO2 film to 130 C for 1 hour and then to 450oC for 1 hour using the tube furnace. The furnace temperature rise

2oC every minute during the heating process. 130oC is near the boiling temperature of the solvent used and it is heptanol in this case. Sample films are dried at room

temperature for 48 hours before sintering with a glass cap over the TiO2 paste substrate. Depending on the boiling point of the solvent, the curing time may be longer. For example, increasing the amount of terpineol will prolong the curing time at room temperature.

40

7) Adhesion: The strength of TiO2 film to substrate adhesion is not as good as

PVA/PVB paste and Aremco 542, but stronger than terpineol TiO2 paste without ethyl cellulose. The experimental results show that the addition of ethyl cellulose will slightly improve the interface adhesion between the electrode and substrate. The

topography improvement may cause the TiO2 film structure stronger owing to better

TiO2 particle connections and pore size within the electrode is controlled, so that the cracks can be diminished.

2.6.4 Testing results of commercial inorganic binders from Aremco

Aremco Cerama-bind binders from Aremco are applied as high temperature binders. These commercial binders are designed for adhesives, coatings, sealants and putties for applications up to 1760oC. Aremco inorganic binders are able to withstand at least 650oC, which will survive after 450oC sintering. We have tested 6

types of Aremco products, but only 3 types of them are stable with TiO2. Depending

on different binders, Aremco has provided corresponding recommended sintering

process. Although the binder to powder weight ratios of 4:1 to 1:1 are recommended

when formulating the pastes, but the recommendations are not designed for

nano-scale TiO2. Some of the binders such as Aremco 642 and 880 have viscosities

41

about 370 cps and 480 cps respectively, which are very viscous. When we test the

binders with Aremco’s recommend ratio, the P25 powders are much more than the

binder, so that it has problem to produce a paste properly. In order to produce the

paste easier for printing on glass substrate, distilled water is added to dilute the binder.

DI water is the only recommended solvent to dilute the binders from Aremco because

the Aremco products contain distilled water.

Aremco inorganic binders are diluted with distilled water, and then slowly mixed

using a magnetic stir bar to avoid generating air bubbles in the mixture. P25 TiO2

powders are added after the binder dilution is completed. Several film properties are

recorded in table 8. Agglomerates appear in the Aremco binder based pastes after

they are stored several days, which is also cited in table 10.

Table 10: Aremco paste properties

Property Crack& Thickness Conductivity Remark Binder Adhesion

Aremco Crack free 5 low Agglomerates 642 Good adhesion

Aremco Crack free No data low Agglomerates 643 Good adhesion

Aremco Crack free Only stable for 6 low 542 Good adhesion 3 to 4 days

Aremco Crack free Only stable for 8 low 644-A bad adhesion 3 to 4 days

42

Aremco Crack free No data No data Agglomerates 644-S bad adhesion

Dries too fast Aremco Crack free No data No data For thickness 830 bad adhesion less than 25 μ

All paste are homogenized until paste turns to uniform Sintering at 450oC with recommended procedure (Aremco Product)

The recommending heating procedure of Aremco inorganic binders is provided by Aremco. Depending on different binders and their designed working temperature, the heating process varies. The heating procedure in this part of study is completely following their instruction as shown in table 11.

Table 11: Aremco paste heating process

Heating process:

Aremco Air dries at room temperature for 1 hour. Cure at 200F (93oC) for 2 642 hours and 400F (205oC) for 1 hour

Aremco Air dries at room temperature for 1 hour. Then ramp up to 643 operating temperature at moderate rate of 200F per hour.

Aremco Air dries at room temperature for 1 to 4 hours. Cure at 200F, 400F 542 and 700 F for 2 hours each.

Aremco Air dry at room temperature for 1 hr, then ramp up to operating 644-A temperature at a moderate rate of 200F per hour.

43

Aremco Air dries at room temperature for 1 hour. Then ramp up to 644-S operating temperature at moderate rate of 200F per hour.

Air dries at room temperature for 1 hour. Then ramp up to Aremco operating temperature. (This binder is designed for room 830 temperature. Does not require a heat cure)

The quantity of TiO2 powder used is fixed, however, the amount of water applies depending on the viscosity of the binders. Less water is added for the low viscous binders, such as Aremco 644-A, Aremco 644-S and Aremco 830. In table 12, the

Aremco binder based TiO2 paste recipes are listed.

Table 12: TiO2 paste recipes based on Aremco inorganic binders

Recipes

Aremco TiO2-0.5g, Aremco 642-3g, DI water-2g 642

Aremco TiO2-0.5g, Aremco 643-2g, DI water-1g 643

Aremco TiO2-0.5g, Aremco 542-0.6g, DI water-3g 542

Aremco TiO2-0.5g, Aremco 644A-0.5g, DI water-1g 644-A

Aremco TiO2-0.5g, Aremco 644S-1g, DI water 1g 644-S

44

Aremco TiO2-0.5g, Aremco 830 2g, DI water 0.5g 830

2.6.5 Tetraethyl orthosilicate (TEOS)

Tetraethyl orthosilicate is a silicate binder which has -77oC melting point and

166 to 169oC boiling point. It is a solvent-borne which is able to mix with other organic solutions like terpineol, ethyl cellulose/ethanol mixture and Triton X-100.

However, TEOS cannot provide interface adhesion after room temperature curing, which is showed in the TEOS recipe 1. In order to increase the adhesion, we try to employ its remarkable property of easily converting into silicon dioxide. There are two reactions of TEOS to silicon dioxide transformation. The first one is a hydrolysis reaction, which proceeds via a series of condensation reactions that convert the TEOS molecule into a mineral-like solid via the formation of Si-O-Si linkages.

The rates of this conversion are sensitive to the presence of acids and bases, both of which serve as catalysts. The function is cited in equation 8.

Si (OC2H5)4 + 2 H2O → SiO2 + 4 C2H5OH. (Eq. 8)

45

Distilled water is added into the paste solution, however, the hydrolysis reaction

rate is too fast to doctor blade the TiO2 film on the glass substrate. Thus, the paste is

rather difficult to form a suitable and useful film in our study.

The other way to transfer TEOS to silicon dioxide is to carry out the

transformation at elevated temperature. The reaction in equation 9 occurs at the

temperature more than 600oC. 600oC is lower than the rutile phase transformation

temperature 750oC. The testing result (TEOS 2) shows cracks on the surface after

o o 600 C sintering. The TiO2 flakes off from the four point probe base after 600 C sintering, but conductivity test shows photo-response.

Si (OC2H5)4 → SiO2 + 2O (C2H5)2. (Eq. 9)

The testing results in table 13 are the results after 450oC sintering. There is no

water added in the paste during testing procedure. Although a translucent agglomerate-free film is obtained in this method, but the interfacial adhesion between film and substrate is relatively poor.

Table 13: Testing results and recipes of TiO2 film based on TEOS for binder material.

Property Crack& Thickness Conductivity Remark Binder Adhesion

Crack free TEOS(1) 10 No data stable Poor adhesion

46

Recipe TiO2‐0.5g, TEOS‐0.5g, pentanol 1.8g, Triton X100‐0.1g

Cracks 1.9 GΩ to TEOS(2) 16.7 stable Poor adhesion 1.3GΩ

Recipe TiO2‐0.2g, TEOS‐0.5g, ethanol‐1g, Ethyl cellulose‐0.2g, terpinel‐1g.

2.6.6 Water glass binders

Most of the organic binders will evaporate within 200oC heating. Inorganic

binders may have a better chance to keep binding on the substrate after 450oC

sintering. Water glass binders are frequently used in photo-catalyst literatures to

o bind TiO2 particles, and withstand 500 C sintering temperature [23, 24]. Sodium silicate and potassium silicate are common water-borne binders. The melting point of sodium silicate is 1088 °C (anhydrous) and potassium silicate offer higher temperature resistance for insulation applications, up to 50oC higher than sodium

silicates. Testing results turn out that the water glass binder will not dissolve in solutions other than distilled water. Water glass binders will solidify itself once in

contact with solvents like ethanol and terpineol. The TiO2 film recipes based on

sodium silicate and potassium silicate binders and the film properties are list in table

14 after the films pass 450oC sintering process.

47

Table 14: Testing results and recipes of TiO2 film based on water glass binder materials

Property Crack& Thickness Conductivity Remark Binder Adhesion

Crack free Sodium silicate moderate 10 No data stable adhesion

Recipe TiO2‐0.2g, sodium silicate‐0.5g, DI water‐1.5g

Crack free potassium moderate 10 No data stable silicate adhesion

Recipe TiO2‐0.2g, potassium silicate‐0.5g, DI water‐1.5g

These two water glass binders show good interface adhesion performance,

however, the adhesion strength decreases after 450oC sintering process, and some

TiO2 powders in the surface can be removed by tape. Water glass binders have the same problem in Aremco products: agglomerates,which appears after three to four

days after all of the components are mixed together.

Comparing to the Aremco products, such as Aremco 543, the water glass paste

are less viscous and easier to form a clear film without TiO2 agglomerations.

However, the water glass cannot be added into terpineol and ethanol solution, which

means the morphology modification agents cannot be added with water glass at the

same time.

48

III. ANALYSIS AND RESULT DISCUSSION

Essential properties for the determination of an optimal TiO2 anode film in DSSC

are the thickness, the conductivity and the transparency of the film. However, it is

rather difficult to have a TiO2 anode film containing all the optimal values of the

properties. For instance, if the thickness of the TiO2 anode film increases, it will

lower the transparency easily, and the cracks will be more likely to occur. The

presence of the cracks will lower the conductivity, so that these properties affect each

other adversely. In order to solve this dilemma, the most direct and effective way is

to better disperse the TiO2 paste. Physical and chemical methods are used to

disperse the TiO2 paste. Homogenizer, magnetic stir bars are physical methods, and surfactants, acid are chemical methods. After comparing the recipes list in the previous chapter, the recipes which provide best performance in different properties are summarized as follows:

Considering the optimal thickness of the anode TiO2 film, which is about 10

microns [11], the terpineol binder based paste gives the best result. In table 9, the

films terpineol binder based pastes are easier to reach 10 microns thickness.

Thickness is highly relative to the viscosity of the paste and the viscosity can be enhanced from ethyl cellulose, Triton X-100, and even terpineol itself. The viscous

paste contributes to achieve the targeted thickness and film morphology for thick film

49

printing and doctor blade smoothing process. If the paste viscosity is too low, the

film will be difficult to achieve the targeted thickness and the film thickness cannot be

compromised with thicker surrounding tape layers in our experience. When a low

viscosity film is printed smoothly, but the thickness of the film will not be uniform

after curing at room temperature. The low viscosity films usually have much higher

thickness at the edge of the film comparing to the center owing to the surface tension.

The addition of binders also prevents the thick film from cracks. The experimental

investigation indicates that when the TiO2 powder to solvent ratio is high, which

means TiO2 powders are excessive, cracks will be most likely to occur owing to

dispersion difficulty. In order to better disperse TiO2 powders into solution, the

amount of solvent and the other solutions have to be controlled. Binders such as

ethyl cellulose provide a media that allows the film to achieve the targeted thickness

and ideal shape. The binders like cellulose materials will be burned out after 450oC sintering process, and the particles connection will be improved after sintering [11], which shows better overall solar cell efficiency. In this study, the terpineol and ethyl

cellulose based TiO2 paste give the best results in terms of the desirable thickness of

the film, a good transparency and light response. However, the film adhesion is not

as good as Dyesol’s product DSL 90-T. The TiO2 paste recipe of Dyesol is shown in

table 7. Our experimental results indicate that the plasticizer is the key component

50

for the film adhesion. Another difference between our paste and Dyesol’s product is

that the color of the sintered TiO2 films. Our sintered film is translucent white color,

but Dyesol’s product is totally transparent and clear after sintering. It indicates that

Dyesol’s paste is very well dispersed. Even we tried to disperse the paste with

physical methods, such as magnetic stir bar, homogenizer and ultra-sonic cleaner, but the film never shows the same color, or even close to it. There are likely reasons

causing the difference between our TiO2 film and the Dyesol’s product. Regardless

the use of physical or chemical dispersion method, the particle sizes are not exactly

the same between the Degussa P25 and Dyesol’s anatase powder. The TiO2 powders

used in this study is P25 from Degussa, the powder has a 25 nanometer average

particle size which contains both anatase and rutile phase TiO2 powders with the 3:1

ratio and 25:75 nanometers particle sizes. Dyesol’s product description shows that

the DSL 90-T only contains anatase TiO2 particles with 20 nanometer average size.

The anatase and rutile phase particles has been proved separately exist in P25 with

different particles [32]. The TiO2 film made with the recipes of table 8 is similar to the result of Gratzel’s report [13], however, the thickness we can achieve is better.

The DSL 90-T shows an orange color before sintering. Since the color of terpineol,

ethyl cellulose and TiO2 are not orange, the color could come from the other additives,

for example, the plastisizer.

51

In conclusion, the different TiO2 powders and the chemicals additive may be the reasons that affect the paste dispersion. A transmittance test has been done by

Gratzel [13] showing that the average particle size which will affect the light transmittance as well, and the P25 has large aggregates itself and it is not pure anatase particles. Thus, the efficiency will not be better than 100%-anatase nanoparticles

made by fumed-TiCl4 synthesis method.

The best result between the films made with the recipes in table 9 is the recipe

number 9. The film is crack free and the initial resistance is within 10 G ohm.

Furthermore, the thickness is about 10 microns and it shows light response. The

recipe 9 is based on recipe 3, and the Triton X-100 contributes to the favorable results

especially to the light response. There is no light response for the film made with

recipe 3, but the film starts to have light response after Triton X-100 is added, which

confirms the effect of surfactant in the TiO2 paste. The film produced with recipe 9 in table 8 shows the best overall result in this study.

In order to produce a high adhesion strength film, several binders are used and

tested. The adhesion strength in the films made with terpineol based paste shows

good interfacial adhesion strength after PVA/PVB ink is added, but the

particle-to-particle binding strength is relatively poor. The result shows that the TiO2 powders on the surface of the film can be removed slightly by tape. If the adhesion

52

of the film to the substrate needs to be improved, a binder which can bind the particles

is necessary. The inorganic binders are tested and are listed in table 10, and the film

made with Aremco 542 shows the best adhesion strength among all of the films tested.

However, the paste is relatively difficult to disperse with physical methods, such as homogenizing comparing to the organic pastes. Furthermore, the particles tend to agglomerate themselves, so that the paste cannot be stored longer than 5 days, and the agglomerates start to appear on the top of the paste. On the other hand, the results of the inorganic binders show poor conductivity and the thickness is about 6 microns as

shown in table 10. Under this preparation condition, the TiO2 film adhesion is

improved, and the conductivity and the thickness of the film are adversely affected.

Also, the morphology modification agents, such as ethyl cellulose, cannot be added

into any of the inorganic binders.

The results show that the inorganic binders are not appropriate binder materials for the DSSC electrode paste, including Aremco’s products and the water glass binders.

Comparing all of the films made of thick film printing recipes in this study, the binder-free materials usually have better conductivity . This should be the binders

o attaching on the TiO2 particles after 450 C sintering process, which lower the

electrical connection. The efficiency of DSSC is highly depended on the charge

transfer between particles to the dye and the electrolyte, and the electron transport rate

53

directly affects the charge recombination. Thus, the residue of binder materials will

be likely to hinder the efficiency because of blocking the overall charge transportation

and increase the possibility of charge recombination. The binder residue molecule

will also possibly decrease the dye loading and lower the overall efficiency of DSSC.

However, the film made with binder material shows better light transmittance and

better film adhesion strength to the substrate after 450oC sintering. In order to lower the charge recombination, finding a binder material that can be easily remove after

450oC sintering is very important. The films with adhesion strength improvement in

this study, such as the films made of inorganic binders or PVA/PVB binders, show

poor conductivity when the ratio of binder is increased. The binders that can keep

binding strength after 450oC sintering are usually difficult to be removed, and it might

lower the DSSC efficiency. The results turn out that finding a binder material to

improve adhesion strength and not reducing the conductivity is not easy. Finding an

appropriate binder recipe with moderate binder ratio can overcome the drawback of

conductivity loss.

Most of the binders mentioned in TiO2 recipes can contribute to crack-free film.

The heating procedure is the process which crack presents most easily. Usually, cracks can be reduce by better dispersion, and the most effective way is to control the

amount of solvent added into the TiO2 paste recipe. Sufficient physical dispersion is

54

also very important for making a crack-free film. As the Brownian movement

described, the smaller particle size powders will move faster in the solution. Thus, the

nano-particles tend to form agglomerations to reduce the individual surface energy.

When the agglomeration size becomes bigger, the possibility to form cracks will be higher. Hence, the physical dispersion is necessary to provide energy to separate the agglomerations and further reduction of the cracks. The results in chapter 2 show that the dispersion time is binder material dependant. The high viscosity binders, such as Aremco product take much more time to disperse the visible agglomerates in the paste mixture. Furthermore, it is not stable for long term storage since the agglomerates presents after about 5 days when the paste is done. The influence of physical dispersion reflects on the conductivity test results. For films with no light response, increasing physical dispersion time, especially vigorous stirring and ultrasonic bath, sometimes do improve the light response. The reduction of crack is

very important for TiO2 film manufacturing. When the crack presents, it is difficult

to verify the reasons which lead to lower conductivity and light response. As

mentioned, the physical dispersion is very important, however, it also produces heat in

the process, so the dispersion time cannot be continuous. A moderate intermittence

time is necessary to avoid heat accumulated in the formulation of the TiO2 paste.

After the nano-crystalline TiO2 particles are well dispersed into the paste,

55

keeping the paste stable without agglomerates themselves is also important. The

dispersant is helpful for the long term paste stabilization. In order to keep the nano

particles stable in solution, two methods are applied: electrostatic stabilization and

steric stabilization. Electrostatic stabilization prevents the particle contact by

increasing the same charge on the surface of nano-pasticles. The nano-particles

basically repulse each other, but the nano-particles will attract each other once the

energy barrier is overcome when the spacing between particles are getting closer.

Thus, increasing the energy barrier will keep the paste more stable. The energy can

be adjusted by altering the pH value. Using dispersant is a steric stabilization

method, which build up a shell like layer around the nano-particles with non-ionic

material. Instead of increasing the energy barrier, the steric stabilization method

uses polymer molecules attaching on the nano-particle, ensuring the spacing between

two particles are more than the critical distance where the energy barrier is. The

advantages of using dispersant are: not sensitive to electrolyte, appropriate for

non-water solution, high solid ratio of the paste and reversible flocculation process.

Therefore, the steric stabilization is more appropriate for the preparation of the TiO2 screen printing paste. The dispersant helps increasing microstructure roughness by

better separating each TiO2 particles [35]. It may be interpreted that the better conductivity performance of the films made of organic pastes, especially the pastes

56 consist of terpineol, comparing to the inorganic binder pastes.

57

IV. CONCLUSION

This study focuses on the preparation and characterization of TiO2 thick film

printing paste manufacturing process. Film properties, such as, thickness, conductivity,

and light transmittance are studied in this study, the conclusions are listed as follows:

1. Ethyl cellulose as an added component in the preparation of the TiO2 printing paste

contributes well to the formation of the film for the targeted thickness of the film,

which is about 10 microns. Comparing to the other recipes in this study, the films

prepared with ethyl cellulose based TiO2 paste achieve the target thickness and the

adhesion strength between the TiO2 film and the glass substrate is also increased

slightly.

2. Terpineol is a good dispersant which has a boiling point about 200oC, the –OH

bond of terpineol allows it to attach on the TiO2 particle to steric stabilizing the

nano-crystalline TiO2 paste enhancing the long term paste stablility.

3. Since the nano-particles tend to agglomerate themselves to reduce the surface

energy, surfactant is necessary to better disperse the particles. Triton X-100 is

confirmed to increase the light response of the TiO2 film, which means the

conductivity can be enhanced using Triton X-100 as an added component in the

formulation of the thick film printed paste..

4. Using organic binders, such as ethyl cellulose, and dispersant, such as terpineol, is

helpful to obtain better light transmittance film in this study. These two chemicals also 58

help in the improvement of the morphology of the TiO2 film.

In this study, we have established good experimental methods and experimental

protocols for the characterization of the TiO2 thick film printed paste for potential

DSSC applications. The importance of chemical components that will affect directly to the properties of the film prepared has been identified and experimental assessed through various recipes of the past preparation. Our results will be useful for the further advancement of the thick film screen printing approach of future

DSSC.

59

V. FUTURE WORK

After testing the properties of the TiO2 binders, basic film properties are obtained.

In order to acquire more information, building a DSSC prototype is necessary. The

highest efficiency DSSC reported is based on TiO2 electrode material, ruthenium

compound sensitizer, and liquid iodide/triiodide electrolyte, which achieved 11.5%

DSSC efficiency [33]. This efficiency will serve as a basis for comparison of the

DSSC fabricated using our thick film printable TiO2 material. The next step will be

using the thick film printing TiO2 paste anode material to construct a DSSC with

commercial sensitizer and electrolyte, in order to compare the DSSC efficiency

performance reported, and the effect of the TiO2 screen printing film can then be

further studied. The solaronix products are popular in literatures, especially

Ruthenium 535-bis TBA dye, which is also known as N-719, and Iodolyte TG-50 for liquid electrolyte. In the proposed future work, these components will be used for the preparation of a DSSC for comparison purpose.

When we try to build a DSSC using our TiO2 paste, the high volatile electrolyte is a concern because it evaporates too fast and cannot be sealed in between the

electrodes. In order to keep the DSSC efficiency stable, an appropriate sealing

material is needed. The sealing material not only prevents the electrolyte leakage,

but also provides spacing between the TiO2 electrode and the Pt counter electrode [34].

Finding an appropriate sealing material and sealing method will be one of the most 60 important future works. The TiO2 electrode microstructure is highly depending on the dispersant and SEM observations can be helpful. After the optimal formulation of

the TiO2 paste is established, the above suggested future work can then be pursued with high degree of success and practical potential.

61

REFERENCE

[1] K, Honda & A, Fujishima. Nature 238, 37-39 (1972)

[2] B. O’Regan & M. Gratzel. Nature 353, 737-739 (1991)

[3] K. Kalyanasundaram & M. Gratzel. Coordination Chemistry Reviews 77, 347-414 (1998)

[4] M. J.Madou. Fundamentals of Microfabrication: The Science of Miniaturization, (2002)

[5] J. Sun, L. Gao. and J.K.Fuo. Nanostructured Mater 10, 1081-1086 (1998)

[6] S. Nakade, W. Kubo, Y. Saito, T. Kanzaki, T. Kitamura, Y. Wada, S. Yanagida J. Phys. Chem. B, 107 (2003)

[7] M. Gratzel, Inorganic Chemistry, Vol. 44, No. 20, 6841-6851 (2005)

[8] M.K. Nazeeruddin, A. Kay, I. Rodicio, R. Humphrey-Baker, E. Muller, P. Liska, N. Vlachopoulos, M. Gratzel. J. Am. Chem. Soc. 115, 6382. (1993)

[9] M. Gratzel, Acc. Chem. Res. vol. 42 (11) 1788-1798 (2009)

[10] C.C. Ma. Organic solar cells and plastic solar cells (2007)

[11] P. Balraju, P. Suresh, Manish Kumar, M.S. Roy, G.D. Sharma. Journal of Photochemistry and Photobiology A: Chemistry 206 53–63 (2009)

[12] W. Lewis, C. Brown, and W. J. Geerts. Am. J. Phys. 72 No2, February 149-153 (2004)

[13] S. Ito, P. Chen, P. Comte, M. K. Nazeeruddin, P. Liska, P. Pechy and M. Gratzel. Prog. Photovolt: Res. Appl. 15, 603–612, (2007)

[14] K. Fan, M. Liu, T. Peng, L. Ma and K. Dai. Renewable Energy 35, 555-561, (2009)

62

[15] P. Wang, S.M. Zakeeruddin, P. Comte, R. Charvet, R. Humphry-Baker, M. Gratzel. Journal of Physical Chemistry B, 107: 14336–14341. (2003)

[16] S. Ito, M.K. Nazeeruddin, P. Liska, P. Comte, R. Charvet, P. Pechy, M. Jirousek, A. Kay, S.M. Zakeeruddin, M. Gratzel. Progress in Photovoltaics, 14: 589–601. (2006)

[17] M.K. Nazeeruddin, F. De Angelis, S. Fantacci, A. Selloni, G. Viscardi, P. Liska, S. Ito, B. Takeru, M. Gratzel. Combined Journal of the American Chemical Society, 127: 16835–16847. (2005)

[18] A. Mills, N. Elliott, G. Hill, D. Fallis, J.R. Durrant and R.L. Willis. Photochem. Photobiol. Sci., 2, 591–596 (2003)

[19] H.P. Kyung, K.H. Chang. Electrochemistry Communications 10 1187-1190 (2008)

[20] L. Kavan, J. Rathousky, M. Gratzel, V. Shklover and A. Zukal. J. Phys. Chem. B, 104, 12012-12020, (2000)

[21] H.J. An, S.R. Jang, R. Vittal, J. Lee, K.J. Kima, Electrochimica Acta 50, 2713–2718, (2005)

[22] P. Chrysicopoulou, D. Davazoglou, Chr. Trapalis, G. Kordas. Thin solid films 323, 188-193 (1998)

[23] C. Hachem , F. Bocquillon, O. Zahraa, M. Bouchy. Dyes and Pigments 49, 117-125, (2001)

[24] M. A. Behnajady, N. Modirshahla, M. Mirzamohammady, B. Vahid, B. Behnajady. Journal of Hazardous Materials 160, 508-513, (2008)

[25] I. Bernacka-Wojcik, R. Senadeera, P. J. Wojcik, L. B. Silva, G. Doria, P. Baptista, H. Aguas, E. Fortunato, R. Martins. Biosensors and Bioelectronics 25, 1229–1234, (2010)

63

[26] I.M. Arabatzis, S. Antonaraki, T. Stergiopoulos, A. Hiskia, E. Papaconstantinou, M.C. Bernard, P. Falaras. Journal of Photochemistry and Photobiology A: Chemistry 149, 237–245, (2002)

[27] A. I. Kontos, A. G. Kontos, D. S. Tsoukleris, M.C. Bernard, N. Spyrellis, P. Falaras, journal of materials processing technology 1 9 6, 243–248, (2008)

[28] X. Chen and S. S. Mao. Chem. Rev, 107, 2891-2959, (2007)

[29] N.G. Park, J. van de Lagemaat, and A. J. Frank. J. Phys. Chem. B, 104, 8989-8994, (2000)

[30] D.S. Tsoukleris, I.M. Arabatzis, E. Chatzivasiloglou, A.I. Kontos, V. Belessi, M.C. Bernard, P. Falaras. Solar Energy 79, 422–430 (2005)

[31] C. J. Barbe, F. Arendse, P. Comte, M. Jirousek, F. Lenzmann,V. Shklover, and M. Gratzel. J. Am. Ceram. Soc., 80, 12, 3157-3171 (1997)

[32] T. Ohno, K. Sarukawa, K. Tokieda, and M. Matsumura. Journal of Catalysis 203, 82–86 (2001)

[33] C.Y. Chen, M. Wang, J.Y. Li, N. Pootrakulchote, L. Alibabaei, C. Ngocle, J.D. Decoppet, J.H. Tsai, C. Gratzel, C.C. Wu, S. M. Zakeeruddin, and M. Gratzel. Vol. 3, No. 10, 3103–3109, (2009)

[34] S. Ito, T.N. Murakami, P. Comte, P. Liska, C. Grätzel, M. K. Nazeeruddin, M. Grätzel. Thin Solid Films 516, 4613–4619, (2008)

[35]Y.Q. Liu, L. Gao and L. Yu. J. Colloid and Interface Sci., 227, 164-170, (2000)

64