<%BANNER%>

Deposition of Indium Tin Oxide for Opto-Electronics

Permanent Link: http://ufdc.ufl.edu/UFE0044308/00001

Material Information

Title: Deposition of Indium Tin Oxide for Opto-Electronics
Physical Description: 1 online resource (58 p.)
Language: english
Creator: Srinivasan, Aishwarya
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2012

Subjects

Subjects / Keywords: electrodeposition -- ito
Chemical Engineering -- Dissertations, Academic -- UF
Genre: Chemical Engineering thesis, M.S.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Abstract: Production of clean and affordable energy is one of the major challenges the world is facing today. Photo-voltaic devices work on the principle of generation of electricity by utilizing energy from the sun. DSSCs are the most important third generation photo-voltaic cell and there have been novel attempts to modify the morphology of the photo-anode by the introduction of high aspect ratio nanowires. A suitable material for fabrication of DSSCs is Indium Tin oxide (ITO) because of high conductivity and transparency. This work focuses on the electrochemical fabrication of ITO films as an initial step to generate nanowires. In the first part, electrophoretic deposition of ITO nano-particles on aluminum substrates is discussed. Films were examined for their thickness, phase and conductivity. This part of the study could not be successfully applied to our system due to some limitations, which have been described. As an alternative, a novel approach for the electro-deposition of ITO is examined. The effect of dopant level on deposition was studied and films prepared with 10wt% tin showed a sheet resistance of 8.048x10-4 ?cm. It was observed that the sheet resistance increased with dopant level. Hence, an approximate 10 wt% or less tin was fixed for optimum conductivity. The transparency of the films has been an issue for both the mentioned techniques and it was realized that the lack of transparency could be due to the surface roughness of aluminum substrates and chloride precursor materials used in preparation of solutions.
General Note: In the series University of Florida Digital Collections.
General Note: Includes vita.
Bibliography: Includes bibliographical references.
Source of Description: Description based on online resource; title from PDF title page.
Source of Description: This bibliographic record is available under the Creative Commons CC0 public domain dedication. The University of Florida Libraries, as creator of this bibliographic record, has waived all rights to it worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law.
Statement of Responsibility: by Aishwarya Srinivasan.
Thesis: Thesis (M.S.)--University of Florida, 2012.
Local: Adviser: Ziegler, Kirk.

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2012
System ID: UFE0044308:00001

Permanent Link: http://ufdc.ufl.edu/UFE0044308/00001

Material Information

Title: Deposition of Indium Tin Oxide for Opto-Electronics
Physical Description: 1 online resource (58 p.)
Language: english
Creator: Srinivasan, Aishwarya
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2012

Subjects

Subjects / Keywords: electrodeposition -- ito
Chemical Engineering -- Dissertations, Academic -- UF
Genre: Chemical Engineering thesis, M.S.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Abstract: Production of clean and affordable energy is one of the major challenges the world is facing today. Photo-voltaic devices work on the principle of generation of electricity by utilizing energy from the sun. DSSCs are the most important third generation photo-voltaic cell and there have been novel attempts to modify the morphology of the photo-anode by the introduction of high aspect ratio nanowires. A suitable material for fabrication of DSSCs is Indium Tin oxide (ITO) because of high conductivity and transparency. This work focuses on the electrochemical fabrication of ITO films as an initial step to generate nanowires. In the first part, electrophoretic deposition of ITO nano-particles on aluminum substrates is discussed. Films were examined for their thickness, phase and conductivity. This part of the study could not be successfully applied to our system due to some limitations, which have been described. As an alternative, a novel approach for the electro-deposition of ITO is examined. The effect of dopant level on deposition was studied and films prepared with 10wt% tin showed a sheet resistance of 8.048x10-4 ?cm. It was observed that the sheet resistance increased with dopant level. Hence, an approximate 10 wt% or less tin was fixed for optimum conductivity. The transparency of the films has been an issue for both the mentioned techniques and it was realized that the lack of transparency could be due to the surface roughness of aluminum substrates and chloride precursor materials used in preparation of solutions.
General Note: In the series University of Florida Digital Collections.
General Note: Includes vita.
Bibliography: Includes bibliographical references.
Source of Description: Description based on online resource; title from PDF title page.
Source of Description: This bibliographic record is available under the Creative Commons CC0 public domain dedication. The University of Florida Libraries, as creator of this bibliographic record, has waived all rights to it worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law.
Statement of Responsibility: by Aishwarya Srinivasan.
Thesis: Thesis (M.S.)--University of Florida, 2012.
Local: Adviser: Ziegler, Kirk.

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2012
System ID: UFE0044308:00001


This item has the following downloads:


Full Text

PAGE 1

1 DEPOS I TION OF INDIUM TIN OXIDE FOR OPTO ELECTRONICS By AI S HWARYA SRINIVASAN A THESIS PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE UNIVERSITY OF FLORIDA 2012

PAGE 2

2 2012 Aishwarya Srinivasan

PAGE 3

3 To my parents and friends

PAGE 4

4 ACKNOWLEDGMENTS I would like to thank my advisor, Dr. Kirk J. Ziegler and my committee member Dr. Peng Jiang, for their valuable inputs and guidance. I would also like to thank my fellow lab mates who have assisted me with my work. I would like to take this opportunity to acknowledge the faculty and staff at Major Analytical instrumentation Center, Particl e and Engineering research Center and Microfabritech at University of Florida especially Dr. Mark Davidson and D r. Valentin Crucin for their time and inputs.

PAGE 5

5 TABLE OF CONTENTS page ACKNOWLEDGMENTS ................................ ................................ ................................ .. 4 LIST OF TABLES ................................ ................................ ................................ ............ 7 LIST OF FIGURES ................................ ................................ ................................ .......... 8 LIST OF ABBREVIATI ONS ................................ ................................ ............................. 9 ABSTRACT ................................ ................................ ................................ ................... 10 CHAPTER 1 INTRODUCTION ................................ ................................ ................................ .... 12 1.1 Aim and Objectives ................................ ................................ ........................... 13 1.2 Thesis Layout ................................ ................................ ................................ ... 13 2 RESEARCH BACKGROUND ................................ ................................ ................. 15 2.1 Tin doped Indium Oxide ................................ ................................ .................... 15 2.1.1 Structure and Properties of ITO ................................ ............................... 15 2.1.2 Applications of ITO ................................ ................................ .................. 17 2.2 Ph otovoltaic Devices ................................ ................................ ......................... 18 2.2.1 History of DSSCs ................................ ................................ ..................... 19 2.2.2 Components and Operation of DSSC ................................ ...................... 20 3 REVIEW OF LITERATURE ................................ ................................ .................... 24 3.1 Nanowi re DSSCs ................................ ................................ .............................. 25 3.2 ITO Deposition Techniques ................................ ................................ .............. 26 3.2.1 Physical Vapor Deposition ................................ ................................ ....... 27 3.2.2 Spray Pyrolysis ................................ ................................ ........................ 28 3.2.3 Scre en Printing ................................ ................................ ........................ 28 3.2.4 Electro chemical Deposition ................................ ................................ .... 29 3.2.4.1 Electrophoretic Deposition ................................ ............................. 29 3.2.4.2 Electro deposition ................................ ................................ .......... 30 3.3 Thesis Organization ................................ ................................ .......................... 32 4 EXPERIMENTAL PROCEDURE AND RESULTS ................................ .................. 33 4.1 Indium Tin Oxide Sol ................................ ................................ ......................... 33 4.1.1 Introduction ................................ ................................ .............................. 33 4.1.2 Synthesis of Tin doped Indium Oxide Nanoparticle Sol ........................... 33 4.1.2.1 Materials ................................ ................................ ........................ 33

PAGE 6

6 4.1.2.2 Experimental ................................ ................................ .................. 34 4.1.2.3 Characterization ................................ ................................ ............. 35 4.1.2.4 Results and Discussion ................................ ................................ .. 35 4.2 Electrodeposition of Indium Tin Oxide ................................ .............................. 37 4.2.1 Indium Oxide Electrodeposition ................................ ............................... 37 4.2.1.1 Materials ................................ ................................ ........................ 37 4.2.1.2 Results and Discussions ................................ ................................ 38 4.2.2 Electrodeposition of tin ................................ ................................ ............ 39 4. 2.2.1 Materials ................................ ................................ ........................ 39 4.2.2.2 Results and Discussions ................................ ................................ 39 4.2.3 Indium Tin Oxide solution ................................ ................................ ........ 40 4.2.3.1 Materials ................................ ................................ ........................ 40 4.2.3.2 Results and Discussions ................................ ................................ 40 5 CONCLUSIONS AND SCOPE FOR FUTURE WORK ................................ ........... 52 LIST OF REFERENCES ................................ ................................ ............................... 54 BIOGRAPHICAL SKETCH ................................ ................................ ............................ 58

PAGE 7

7 LIST OF TABLES Table page 4 1 Comparison of lattice spacing of electro phoretically deposited ITO with values obtained from diffraction card. ................................ ................................ ............ 44 4 2 Particle size measurement of ITO nanoparticles in sol by dynamic light scattering. ................................ ................................ ................................ ........... 44 4 3 Comparison of lattice spacing (d) of electrodeposited In 2 O 3 with values obtained from diffraction card. ................................ ................................ ............ 44 4 4 Comparison of lattice spacing (d) of electrodeposited tin with values obtained from diffraction card. ................................ ................................ ........................... 44 4 5 Comparison of lattice spacing (d) of electrodeposited ITO91 and ITO82 with indium oxide. ................................ ................................ ................................ ...... 45 4 6 Thickness and Sheet resistance of samples. ................................ ..................... 45

PAGE 8

8 LIST OF FIGURES Figure page 2 1 Cubic bix byite structure of Indium oxide ................................ ............................. 23 2 2 Conduction and Valence band of doped and undoped In 2 O 3 [27] ...................... 23 4 1 X ray diffractogram of electrophoretically deposited ITO. ................................ ... 45 4 2 Chronoamperometry response of electro phoretically deposited ITO nano particles. ................................ ................................ ................................ ............. 46 4 3 Cyclic Voltammetry response for indium oxide electro deposition. ..................... 46 4 4 X ray diffractogram of electrodeposited film of indium oxi de. ............................. 47 4 5 Cyclic Voltammetry response for electrodeposition of tin. ................................ .. 47 4 6 X ray diffractogram of electrodeposited film of tin. ................................ .............. 48 4 7 Cyclic Voltammetry response for electrodeposition of ITO91. ............................ 48 4 8 X ray diffractogram of electro depos ited ITO91. ................................ ................ 49 4 9 X ray diffractogram of electro deposited ITO82.. ................................ ................ 49 4 10 X ray diffractogram of electro deposited ITO73.. ................................ ................ 50 4 11 Change in d spacing with increase in dopant level. ................................ ............ 50 4 12 Change in sheet resistance with increase in d opant level. ................................ 51

PAGE 9

9 LIST OF ABBREVIATIONS AAO Anodized Aluminum Oxide DSSC Dye Sensitized Solar Cells ED Electro deposition EPD Electro phoretic Deposition ITO Indium T in Oxide RF Radio Frequency TCO Transparent Conductive Oxide UV Ultra violet XRD X ray Diffraction

PAGE 10

10 Abstract of Thesis Presented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Master of Science DEPOSITION OF INDIUM TIN OXIDE FOR OPTO ELECTRONIC S By Aishwarya Srinivasan May 2012 Chair: Kirk Jeremy Ziegler Major: Chemical Engineering Production of clean and affordable energy is one of the major challenges the world is facing today. Photo voltaic devices work on the principle of generation of electricity by utilizing energy from the sun DSSCs are the most important third generation photo voltaic cell and there have been novel attempts to modify the morphology of the photo anode by the introduction of high aspect ratio nanowires. A suitable materi al for fabri cation of DSSCs is indium t in oxide (ITO) because of high conductivity and transparency. This work focuses on the electrochemical fabrication of ITO films as an initial step to generate nanowires. In the first part, electrophoretic deposition of ITO nano particles on a luminum substrates is discussed. Films were examined for their thickness, phase and conductivity. This part of the study could not be successfully applied to our system due to some limitations, which have been described. As an alternative, a novel approach for the electro deposition of ITO is examined. The effect of dopant level on deposition was studied and films prepared with 10wt% tin showed a sheet resistance of 8.048 x 10 4 It was observed that the sheet resistance increased with dopa nt level. Hence an approximate 10 wt% or less tin was fixed for optimum conductivity. The transparency of

PAGE 11

11 the films has been an issue for both the mentioned techniques and it was realized that the lack of transparency could be due to the surface roughness of aluminum substrates and chloride precursor materials used in preparation of solutions.

PAGE 12

12 CHAPTER 1 INTRODUCTION Dye Sensitized Solar Cells (DSSCs) are photo electrochemical cells and belongs to the category of third generation solar cells. Being the most efficient third generation solar technology available currently, they generate electricity by converting energy f rom light absorbed by a monolayer of the dye. The research interest in this technology is rapidly increasing due to its promising advantages such as inexpensive manufacturing steps, large scale feasibility and higher performance to price ratio. DSSCs curre ntly reach 12.3% energy conversion efficiency under standard conditions in liquid junction devices. The performance of a DSSC largely relies on provid ing an optimal conduction path for electrons so that they reach the current collector, thereby avoiding c harge recombination Poor efficiency of DSSCs based on nano crystalline films is due to the immediate recombination of the injected electrons. Hence, maximum area of the dye oxide interface is desired for improved conductance. Recent research focuses on na nowires of high aspect ratios, which provides large surface area resulting in efficient dye adsorption. Nanowires provide a direct conduction path for electrons and increase their diffusion length before they reach the collection electrode. They can be gen erated by several techniques such as pulsed layer deposition, sputtering, e beam deposition, thermal evaporation and spray pyrolysis. The above mentioned techniques, though very efficient, are very expensive to manufacture solar cells on a commercial scale Alternative, simple and less expensive methods are electro deposition and electro phoretic deposition of several materials into the pores of a template to grow nanowires. Electrochemical techniques are very versatile in their applicability and easily ada ptable

PAGE 13

13 to commercial production The target material in this research work is i ndium t in oxide. ITO is a highly conductive and transparent oxide which makes it the most desirable material for DSSC. Due to its high conductance, it can efficiently deliver in jected electrons and its high transparency increases the absorption of light in the visible spectrum. The focus of this research is to electrochemically fabricate ITO films for DSSCs. 1.1 A im and Objectives One of the primary objectives of this research is t o fabricat e a nano structured ITO electrode of high aspect ratio for DSSCs The first step in the growth of nanowires is the successful deposition of films. With this goal in mind, the electrophoretic deposition of ITO thin films was first studied to par ticularly understand the effects of deposition conditions on film properties. An effort was made to list out the issues associated with this technique and some suitable alternatives to these problems. Further work was carried to explore the electro deposit ion of indium oxide films followed by doping with tin to obtain transparent and conductive films. The main goal of the study is to successfully grow high aspect ratio nanowires of different class of materials, namely, polymer, semiconducting and metallic, for 3D DSSCs and evaluate their performance. The motivation behind this work is to determine if electrodeposited nanostructures of specific dimensions could be a suitable alternative to other expensive techniques for fabrication of efficient solar cells. 1 .2 Thesis Layout In this thesis, a detailed background theory is presented in C hapter 2 followed by a review of literature in Chapter 3 T he experimental techniques involved and important

PAGE 14

14 results are discussed in C hapter 4 Chapter 5 includes the conclusions of the whole work and scope for future research.

PAGE 15

15 CHAPTER 2 RESEARCH BACKGROUND 2. 1 Tin doped Indium Oxide Tin doped indium oxide (ITO) is a highly transparent and conductive oxide. Due to this property, it is the commonly used transparent c onductive oxides (TCO) for optoelectro nic applications. In 1907, the first TCO of cadmium was reported. Since then, the re search has focused more o n transparent condu ctors [1]. Novel materials with unique properties have attracted technological interest in these materials and there has been an increased production and numerous techniques have been evolv ed for their d eposition Studies conducted on doped films of zinc, cadmium and indium oxides have shown high optical transmittance (about 90%) and approximate metallic conductivity can be attained [1] ITO films have reported a high optical transmittance and electrical conductivity of ~ 95% and 1 0 ^4 W 1 cm 1 respectively [2]. Such favorable properties of ITO have made it one of the most commonly used TCO for many opto electronic and mechanical applications W ith increased application in electronic technology there is a need to u nderstand the electr ical and opti cal properties of ITO in great detail. Although the amount of concise and accurate knowledge available is less the research done till date implies that many properties can be understood by having a control o ver the operating conditions and parameters influencing deposition. 2. 1 1 Structure and Properties of ITO Indium tin oxide is a n type semiconductor with a band gap between 3.5 and 4.3 eV and a maximum charge carrier concentration of approximately 10 21 cm 3 [38] It is primaril y formed when tin dopants replace the In 3+ atoms from indium oxide. The crystal

PAGE 16

16 structures of In 2 O 3 and ITO are both of cubic bix byite structure as shown in figure 2 1 [4,5] Each cation res t s at the center of a distorted cube. The oxygen anions occupy the six corner positions The two remaining corners ar e oxygen vacancies and have a major influence in the defect chemistry of ITO The b site cation is coordinated to six oxygen anions at a distance of 2.18 The two oxyge n interstitial positions lie along the body diagonal of the cube. The d cation exh ibits less symmetry and is coordinated to six oxygen anions at three distances (2.13, 2.19 and 2.23 ) and to two oxygen interstitial sites along a face diagonal of the cube [5]. When a Sn 4+ cation incorporates into the bixbyite structure by substituting an In 3+ cation, a free elec tron from the Sn atom is available for electrical conduction The direct optical band gap of an ITO film is generally greater than 3.75 eV and values ranging from 3.5 to 4.06 eV have been publish ed by previous researchers [1,2 7 ] The absorpti on edge of ITO lies in the ultra violet region of solar radiation. As the carrier concentration increas es, the absorption edge shifts to shorter wavelengths [30,31] Figure 2 2 shows the parabolic band structure of ITO with the conduction band and valence band for doped and undoped indium oxide. T he f ermi level is positioned at the center of the ba ndgap for indium oxide. When indium oxide is doped with tin, donor states are form ed below the conduction band. A t a critical dopant density, the donor states com bine with the conduction band and this value is calculated to be 2.3 10 19 cm 3 [2,28]. If the density of electrons increases beyond the critical density, the material is conductive and presents free electron properties If the carrier concentration is gr eater than the critical density, mutual exchange and c oulomb ic interactions in the ITO

PAGE 17

17 films causes the conduction band and the valence band to merge. This reduces the bandgap [2]. The trans parency of ITO films to visible light is also an important property that is dependent on a number of parameters, among which quality of surface precursor composition and optical in homogeneity normal to the film surface are most important [27,29] Being a wide band gap semiconductor, ITO has a very h igh optical transmittance. As a result it is mostly transparent to visible light In the ultraviolet region it is opaque because a UV photon can excite an electron from the valence b and to the conduction band. It is also opaque in the near infrared spectrum because a photon can excite an electron from near the bottom of the conduction band to higher levels within the co nduction band [2]. 2.1.2 Applications of ITO ITO films are the most widely used TCO for its excellent performance. Some applications include e lectrodes in flat panel displays heat mirrors, electro chro mic windows and solar cells [2]. Convention ally some of the common uses include transparent conductive coating s for liquid crystal displays, touch panels antistatic coatings, heat reflectin g mirrors, electronic ink applications and high temperature gas sensors [2,6]. R ecent applications include t ransparent contact s in advanced optoelectronic devices such as solar cells, photo transistors and lasers [2,13]. Thus ITO is becoming an int rinsic part of cutting edge electronic technology and showing an increased ability to improv e the performance of light sensitive devices. Due to its electrical and optical properties, extensive research is in progress to produce efficient, new generation solar cells.

PAGE 18

18 2.2 Photovoltaic D evices Photovoltaic devices convert direct light into electricity by using the energy f rom an absorbed photon to excite an electron across a p n junction [14]. This field has been largely dominated by p n junction devices, generally made of silicon, because of its efficiency and material availability [13,17]. Due to the high ratio of price t o performance, the inorganic solid state junction devices are now being confronted by the outset of a new generation of solar cells. Based on advanced and novel materials, they are particularly attractive, because of low fabrication cost and usage of simpl e materials, which is facilitating their market entry and production [15,17]. Solar cells are categorized into three different generations according to the increasing order in which they gained importance. Presently, active research is ongoing in all the t hree generations. First Generation Solar Cells First g eneration s olar cells are high quality, single junction devices that dominate the solar panel market because of their efficiency. These solar cells, generally based on single crystal silicon, are appro aching the theoretical limiting efficiency of 33% but it is unlikely to reach less than US$1 per Watt [14]. It is expensive due to the input it requires in terms of money and labor to extract silicon from sand and then purify silicon before growing the cry stals. Second Generation Solar Cells Second generation solar cells are developed to overcome the efficiency and production costs of first generation cells. They are manufactured by vapor deposition, electro deposition and ultrasonic nozzles. Due to the abs ence of high temperature processing, production costs are reduced significantly [14,15]. In addition to their low cost, these cells are more flexible and lightweight. The most successful second

PAGE 19

19 generation materials have been Cadmium Telluride (CdTe), Coppe r indium gallium selenide (CIGS) and amorphous silicon [14]. Third Generation Solar Cells Being the most recent invention in the solar field, they aim to improve the performance efficiency of second generation at low production costs. They do not need the traditional p n junction necessary in first generation cells [14,15]. They include a broad gamut of innovations, such as organic solar cells, nanocrystalline material cells, and dye sensitized solar cells (DSSCs). One of our target applications is to fabri cate nanowire based DSSCs. They are an attractive option as it can be made from low cost materials and less elaborate apparatus. The overall conversion efficiency for DSSCs is 11% [33]. 2. 2.1 History of DSSCs The photovoltaic effect was discovered by Frenc h scientist Edmond Becquerel in 1839. Since then, the direct conversion of solar energy to electricity has been a were covered with 10% efficient solar cells our energ y needs would be fulfilled. Years later, in 1883, German photochemist Hermann Carl Vogel extended the sensitivity of photographic emulsions to longer wavelengths by adding dyes [20]. The concept was further expanded to photo electrochemical cells by dying silver halide electrodes The first modern solar cell was produced at Be ll labs in 1954 and since then c onventional solid state photovoltaic devices that employ the electrochemical properties of a p n semiconductor junction have dominated the field of sola r conversion and devices have been produced that exhibit energy conversion efficiencies better than 25% [ 1 7,18,19 ]. Almost a century later, in 1965, at The Institute of Physical and Chemical Research in

PAGE 20

20 Tokyo Namba and Hishki reported on the sensitization of zinc oxide semiconductor electrodes with cyanine dyes. It was discovered that photons could be converted to electricity by charge injection of excited dye molecules into a wide bandgap semiconductor. But, a single layer of dye molecules could only abso rb upto 1% of the incident light and hence the efficiency was low [21,22]. Transition metal complexes (dyes) chemisorbed to semiconductor substrates, such as titanium di oxide, transferred electrons efficiently [2 2 ,2 3 ]. In 1991, Gratzel and coworkers anno unced the first operational DSSC with an efficiency of approximately 7%, as an alternative to p n junction devices [22,23]. Since then, DSSCs have garnered attention as a low cost alternative to solid state solar conversion devices and the best research ce lls today achieve an efficiency of 12.3%. 2. 2 2 Components and Operation of DSSC DSSCs consist of a photoanode, a cathode and an electrolyte or mediator solution [13,17]. The photoanode is the heart of DSSC and consists of a mesoporous layer of nanometer s ized TiO 2 particles that is deposited onto a transparent conducting oxide (TCO) glass substrate. Due to its highly organized structure, the mesoporous semiconductor layer has an actual surface area that is two to times larger than the geometric surface are a. A monolayer of dye is chemisorbed on the semi conductor surface by inserting the electrode in a saturated dye solution for 12 to 18 hours [22,17]. Due to sensitization, dye absorbs an appreciable amount of sunlight for conversion to electricity. Commonl y used dye as a sensitizer in DSSCs is cis di(thiocyanato) bis(2,2 bipyridyl 4,4 dicarboxylate)ruthenium(II), usually referred to as the N3 dye [23]. The cathode typically used in a DSSC is a platinum electrode fabricated by depositing platinum as either a thin film or dispersed as nanoparticles on a TCO

PAGE 21

21 substrate. The mediator solution consists of a redox couple that is dissolved in a low viscosity, low vapor pressure solvent. The function of the redox mediator is to reduce the photo oxidized dye and shutt le the resulting hole (the oxidized form of the mediator) to the cathode [17, 24]. Typical DSSCs utilize the iodide/tri iodide (I /I 3 ) redox couple due to convergence of necessary kinetics. Operation of DSSC The mesoporous semiconductor oxide layer, consisting of nano particles is sintered together for improved electronic conduction. The nano crystalline semiconductor film is covered by a single layer of the charge transfer dye. When light falls on the surface of this dye, the dye is oxidized and injects an electron. The photo oxidized dye is reduced by the redox couple, present in the mediator solution. Electrons that have been injected into the conduction band travel to the underlying TCO substrate via a diffusional random walk pathway [13]. Once collected in the TCO substrate electrons can flow through an external circuit, creating an electrical current capable of doing useful work. Finally, the dye is reduced by receiving an electron from the electrolyte. The electrolyte is regenerated by the reduction of tri iodide at the counter electrode and charge neutrality is maintained by electron migration through external circuit. This causes the generation of power and the voltage produced under illumination is the difference bet ween the Fermi level of the electron in the solid and the redox potential of the electrolyte [17,24]. Thus, a DSSC can generate power from solar energy without any significant loss or chemical change. Some inhibiting reactions in a DSSC reduce the performa nce efficiency. For instance, when the photo excited dye injects an electron, it can relax back to the ground

PAGE 22

22 state, enter into the conduction band of semiconductor or back react with the electrolyte. Ideally, we would want the electron to enter the semico nductor layer, but the kinetics of electron injection is much slower than kinetics of back reaction. Thus, there is a high possibility of recombination of electrons. Also, the reduction of the oxidized form of the mediator by the conduction band electron r educes the efficiency and capability of a DSSC, but active research is ongoing to overcome these issues.

PAGE 23

23 Figure 2 1. Cubic bixbyite structure of indium oxide. Blue spheres are the oxygen atoms, red sphere is the tin atom and brown sphere is indium. ITO is formed by replacing one of the indium atoms with tin. Figure 2 2 Conduction and valence band of d oped and undoped In 2 O 3 [27 ]

PAGE 24

24 CHAPTER 3 REVIEW OF LITERATURE Majority of the solar cells manufactured in the world today are silicon solar cells, which include bulk silicon solar cells, thin film solar cells and non silicon compound semiconducto Energy and Industrial Technology Development Organization (NEDO) has predicted that by 2030, the generation of solar power will increase by 100 times, which will cover nearly half the electric power required for household use and about 10% of total electric power [15]. Since the invention of the first DSSC in 1991, there has been active research in improving the performance of photo anode, dye and cathode to achieve comparable performance to amo rphous silicon thin films. Currently, studies focus on improving four major parts of the cell, namely the dye, counter electrode, photoelectrode and electrolyte. The dye plays an important role in expanding the range of wavelengths in the spectrum. The dye that was first used in the Grtzel cell is a ruthenium (Ru) bipyridine complex with a carboxyl group. Since ruthenium is an expensive metal, efforts to produce ruthenium free dyes are ongoing. Studies are also conducted on the stability of the dye [17,25 ]. An electrolyte solution of acetonitrile is reported to produce the highest conversion efficiency in a DSSC. The solidification of the electrolyte is also required due to concern about physical damage to the cell. A quasi solid electrolyte made of a com bination of a nonvolatile ionic liquid and a gel with a conversion efficiency of above 7% was reported [25,26]. Inorganic compounds are being used to produce a fully solid electrolyte.

PAGE 25

25 3.1 Nanowire DSSCs Research has been made to improve the conductivity of TiO 2 through morphological control, such as the formation of nanowires and nanorods, as well as to coat the surface of TiO 2 with a different type of oxide, such as niobium oxide (Nb 2 O 5 ), to suppress electron leakage Materials other than titanium oxide and composite materials with other oxide semiconductors are being explored t o improve the charge separation efficiency. Such efforts, however, have not been successful in providing characteristics superior to those of simple titanium oxide [25] As for the method of manufacturing the photoelectrode, the uniform formation of the electrode is essential to favorable characteristics. The photoelectrode must have a high surface area to maximize dye absorption which will result in efficient electron t ransport to the c ounter electrode without charge recombination A close knit networ k of wide band gap nanowires with high aspect ratio (diameter less than 50 nm and lengths of m scale or more) may be beneficial because the nanowire morphology imparts a hi gh surface area, resulting in efficient dye adsorption It increases the diffusion length of electrons and provides a direct conduction path for electrons to flow from the point of generatio n to the collection electrode. Thus, the probability of charge rec ombination is less. Hill et al. developed a bulk charge transport model to show improved conversion efficiency of photoanodes made of high density and high aspect ratio nanowires. Such nanowires may be produced by several techniques including vapor liquid solid (VLS) method, self assembly from nanosized building blocks, seeded growth, template synthesis of nanowires, lithography and etching techniques. Martin et al. introduced the template synthesis of nanowires, where an anodized aluminum oxide (AAO) or p oly carbonate (PC) template is used as a working substrate to grow nanostructures of the desired

PAGE 26

26 material by electro chemical deposition. Yang et al. has developed a novel 3D ITO electrode with better solar conversion efficiency by RF sputtering. Although these approaches are efficient there are some issues of concern associated with them such as orientation control, nanowire spacing and precision control over desired structures over long ranges. Templated electro deposition is a well known, inexpensive pr ocess for the fabrication of such nanowires. Wang et al. described the electrophoretic deposition of ITO/TiO 2 core shell nanowire arrays, which is a design developed by Cao et al.. 3.2 ITO Deposition Techniques Several techniques have been developed to deposit thin films of ITO. The choice is governed by certain parameters, such as surface roughness, performance quality and reproducibility of th e film, cost and availability, as well as adverse s e condary effects and drawbacks pertaining to each approach. T he microstructure [30] stoichiometry [31] and the nature of the impuritie s present [31] also significantly affect the properties of ITO films. Thus, each deposition technique yield films with different characteristics and it is wise to choose an appropriate technique based on the intended application The deposition techniques that are suitable for the production of thin films of ITO include magnetron sputtering [29], thermal evaporation deposition [12], spray pyrolysis [9], electron beam evaporation [16], screen printing, dip coating and pulsed layer deposition [32]. T ransmission electron microscopy (T EM ) and electron diffraction studies of radio frequency (RF) sputtered ITO films on glass substrates ind icate that films grown at room temperature have crystallographic defects and exhibit an amorphous structure However, it is known that annealing improves the crystallinity of sample and hence increasing th e growth temperature to 3 00C can r esult in near si ngle crystallinity and

PAGE 27

27 uniform grain size producing highly conductive nanostructures [6]. Some of the commonly used techniques are described below. 3.2.1 Physical V apor D eposition Physical vapor deposition (PVD) describes a collection of techniques to dep osit thin films Basically, in PVD, vaporized form of target precursor material s is condensed on various substrates to produce the deposit It includ es purely physical processes such as h igh temperature vacuum evaporation or plasma sputter bombardment to coat the s urface Ca thodic a rc d eposition electron beam physical vapor deposition, e vaporative deposition p ulsed laser deposition and s putter d eposition are some of the variants of PVD. Queen et al. have grown thin films of ITO by thermal evaporation, with resistivity 2.4 x10 4 cm and transparency greater than 95% [12,16]. Sputtering is one of the most commonly used deposition tech nique and is described below. Sputtering The target material is eroded by accelerated ions from gaseous plasma and a n atom or molecule is knock ed out which is ejected as individual or clusters of atoms or molecules. Sputtering can further be classified as reactive or magnetron. In reactive sputtering, a chemical reaction modifies the precursor material during the trans fer from the source t o the substrate. It can also be achieved by a DC field combined with a magnet radio frequency (RF ) a nd by ion beams Thus, depending on the type of process, they are classified as mag ne tron and reactive RF sputtering [7,8] S puttering pressure, pre conditioning and film thickness influence the quality of ITO film RF sputtering has a concern over the damage associated with the rate at which un desir ed high velocity electrons are guided away from the substrate [30]. Alternatively,

PAGE 28

28 m agnetron sputter ing can produce high deposition rat es, thereby minimizing th e damage [2] Deng et al. obtained RF sputtered ITO films with a sheet resistance less than 50 and the solar cells obtained from these films had an overall efficiency of 8.8%. 3.2 .2 Spray Pyrolysis Spray p yrolysis (SP) involves the thermal decomposition of s ource solution, which results in small droplets to splash and vaporize on the substrate. A n alcoholic solution of an indium and tin precursor material is employed. Spr aying is carried out in a furnace in the absence of vacuum and at a temperature of 400C. One of its primary advantages is its fast rate of deposition, which is approximately > 1000 /sec. Some c ritical parameters that influence deposition in clude substrat e positioning and the chemical composition of the spray solution [2] R esistivities of 1 x10 3 Wcm for a 420 nm thick ITO film with trans parency greater than 90% at a wavelength of 550 nm has been published by Ashok et al. while Haitjema et al obtained val ues of 3 x10^ 4 Wcm and 85% respectively [9,10] 3.2.3 Screen Printing Screen printing t echnique (SPT) is appropriat e for industrial production to manufacture thick layers of ITO. It is suited for non device orientate d applications such as flat panel displays and anti static coatings [2]. In a t ypical SPT films are deposited in the range 10 to 30 mm thick, with a cry stallization temperature as high as 600C Although the ITO film is conductive its transparency is markedly lower (< 80%) because of com pression of several layers of ITO [11].

PAGE 29

29 3.2.4 Electro chemical Deposition With an increasing interest in the fabrication and application on nano sized mate rials, recent studies have been focusing on the electro chemical synthesis of nanowire arrays. Elec trochemical techniques are very versatile in their applicability and hence are gaining more importance for advanced materials and combinations. It is a very cost effective and simple approach that requires basic equipment. An important advantage of this te chnique is its ability to be scaled up to large volumes and adaptability to commercial production. Two important techniques used and described in detail in this work are electrophoretic deposition and electrodeposition. 3.2 .4.1 Electrophoretic Deposition E lectrophoretic deposition (EPD) is an electrochemical technique gaining rapid interest for the production of thin films on conductive substrates. In 1808, Russian scientist Ruess discovered EPD and in 1933, it was practically tested to deposit thorium oxid e particles for electron tube applications [39]. In the past few years, it was primarily used ceramic processing, and the EPD of m etal and semiconductor materials was a less explored area of research. However, there has been a renewed interest in this appl ication for the producing advanced materials. N ovel and advanced material combinations in the micro and nano scale can be analyzed. EPD is flexible, in terms of its applicability to a variety of macroscopic geometries with various dimensions and orientatio ns Recently, EPD has been employe d for the deposition of nanoparticles and carbon nanotubes to produce advanced nanostructured materials Wang et al describes the preparation and characterization of ZnO arrays by EPD. Limmer et al has reported a citric acid sol route to electrophoretically grow n anowires within a template [34].

PAGE 30

30 EPD is a process where charged colloidal particles suspended in a liquid migrate under the influence of an e lectric field and get deposited onto an electrode. A schematic is shown in Fig. 2 3 1. EPD occurs in two basic steps. F irst ly, an electric field is applied between t he working and counter electrode Due to external force, c harged particles suspended in the medium get attracted toward s the oppositely charged electrode. This motion of dispersed particles relative to a fluid under the influence of a spatially uniform electric field i s known as electrophoresis [39]. T he second step involves particle accumulat ion a nd d eposition to f orm a relatively firm and homogeneous film T his technique largely depends on the produc tion of a stable suspension (sol) so that the charged particles are free to move on the application of a n electric field. A number of theories have been proposed to und erstand the mechanism of EPD. It has been largely described in the framework of Derjaguin Landau Verwey Overbeek (DLVO) theory and particle double layer distortion under the application of a DC electric field [39]. F locculation by particle accumulation, pa rticle charge neutralization, electrochemical particle coagulation, electrical double layer (EDL) distortion and thinning mechanism also explain particle interactions and the kinetics of deposition. Further research and modeling is necessary to understand the mechanisms that govern deposition and its effect on interactions between particles, solvent, and electric charge. 3.2.4.2 Electro deposition Electrodeposition is a process where a conductive substrate is placed in a liquid solution (electrolyte) and the application of an electrical potential between a working substrate and a counter electrode results in a chemical redox process causing a thin

PAGE 31

31 layer of material to dep osit on the surface. Basically, electrodeposition can be said to occur in four simple steps: Migration of ions in the solution under the influence of applied potential Diffusion of the ion through the double layer Electron transfer and adsorption on the su rface of the substrate. Diffusion of atom to the growth point on substrate Electrodeposition is carried out in an electrochemical cell consisting of a reaction vessel and two or three electrodes. In the two electrode cell the reactions are controlled by th e current applied between a working electrode (substrate) and a counter electrode. In a three electrode cell, a reference electrode is used to control or measure the potential of the working electrode, and depositions are carried out by controlling either current or potential. It has been used as a thin film deposition technique, especially for metals, for more than 150 years [24]. More recently, it has been more intensively studied for the preparation of compound semiconductors. This technique is more adva ntageous, in certain aspects, with respect to other commonly employed solution based methods as it is relatively easily scalable and cost effective. As the current focus is on the commercial production of DSSCs, ED is preferred due to its effectiveness in both small and large scales. In addition, substrates with various sizes and shapes may be used without the involvement of toxic gaseous precursors. It is also easier to control since the film compositions are not easily affected by small deviations in the precursor concentrations. Few attempts have been made to produce ITO films and nanowires by low cost solution based methods. Nina et al. described the assisted electrochemical deposition of ITO films, which involves a fast one step cathodic deposition of In Sn hydroxide film followed by thermal conversion to ITO [40]. Chang et al. described the deposition and characterization of thin films of tin by a novel process [41]. The deposition of indium and

PAGE 32

32 indium oxide films is a less explored subject and the chemistry of indium is yet to be explored completely. Various physical deposition methods including reactive evaporation, sol gel technique and sputtering have been investigated for the deposition of indium oxide films. Sharma et al. showed the applicatio n of indium oxide films for DSSCs with an efficiency of 0.5%. It was suggested that the efficiency could be improved by increasing the porosity and dye absorption [36]. 3.3 Thesis Organization The research work presented here focuses on producing indium t in oxide films on aluminum substrates by electro phoretic deposition and electrodeposition. The target material in this research work is indium tin oxide. Till date, not much work has been targeted on the electro chemical fabrication of ITO. Hence, it has been a challenge to prepare films with the required properties. To start with, films of ITO are deposited electrophoretically from a sol of ITO nanoparticles. Some of the film properties are discussed and limitiations associated with the applicability of t his approach to our system is listed. Possible suggestions to overcome the issues are also put forward. The work is further extended to electrodeposit ITO films. This technique is relatively simple, but in order to be applicable, it has to be understood in detail for every specific system. Thus, the electro deposition of ITO was split into two sub tasks. First, to produce Indium oxide films by electrodeposition and second was to deposit films of tin. The idea was to combine the two techniques at different r atio of In:Sn to determine the optimum dopant level that gives the best conductivity and transparency. The films were characterized for their, thickness, phase and conductivity. The relation between the dopant level and film properties is discussed.

PAGE 33

33 CHAPTER 4 EXPERIMENTAL PROCEDU RE AND RESULTS 4.1 Indium T in Oxide Sol 4 .1.1 Introduction Indium tin oxide is becoming increasingly important in optoelectronic applications because of their high electrical c onductivity combined with optical transparency. The prep aration conditions and deposition parameters have a major influence on optical and electrical p roperties of ITO films. C onductive and transparent ITO thin films have been prepared by several methods and more recently, by e lectro chemical techniques Fabricating ITO films from well dispersed nano particles is a good alternative due to several factors such as, adaptability to complex shapes and geome tries better control over dop ant level and concentration or precursor materials without using elaborat e and complicated equipment in compar ison to other methods. The performance of ITO material relies mostly on the preparation of homogeneously mono dispe rsed ITO nanoparticles. Among the various wet chemical methods to synthesize ITO nanoparticles, hydrothermal micro emulsion and sol gel, conventional co precipitation method s are widely used commercially. A citric acid based sol method is applied for for mation of complex and stable indium tin oxide sol [37]. The research work presented here adapt s this route for the preparation of ITO s ol for electro phoretic d eposition. 4 .1.2 Synthesis of Tin doped Indium Oxide Nanoparticle Sol 4.1.2.1 Materials Chemical s used in preparation of the sol were indium chloride (Sigma Aldrich), tin chloride (Sigma Aldrich), citric acid monohydrate (Arcos), ethylene glycol, ethanol and

PAGE 34

34 de ionized water and were used as received. Nitrogen gas was received from Airgas and used as received. 4.1.2.2 Experimental A target volume of 50 mL was achieved by heating 30 mL of ethylene glycol and 20 mL of ethanol to 40 o C. Citric acid monohydrate was dissolved in this solution such that the molar amount of citric acid is twice that of indiu m and tin combined together. The calculated amount of indium chloride was added into the solution and stirring was continued at 200 rpm for about 90 min. During the final 30 min, tin chloride pentahydrate was added into the solution such that the molar rat io of In:Sn is maintained at 9:1. At the end of stirring, water was added into the solution to achieve a molar ratio of water: (In+Sn) of 3.1:1. The moles of water added include the water of hydration in the tin chloride and citric acid. After stirring for about 30 minutes, the sol was allowed to naturally cool to room temperature. It is then vacuum filtered with 1 m Whatmann filter paper. This sol was then used for electrophoretic deposition. Cyclic Voltammetry was used to determine the potential for depos ition of ITO. The deposition was carried out in a three electrode electrochemical cell at room temperature. The working substrate was a sheet of 99.99% aluminum, which was degreased prior to deposition by sonication of the substrates with soapy water, foll owed by acetone and ethanol. The cleaned substrates were then dried with nitrogen gas. One side of the substrate was made non conductive by applying a thin layer of nail polish and the conductive side was attached to the copper electrode with copper metall ic tape. A carbon electrode was used as the counter electrode and a saturated calomel electrode (SCE) was used as the reference electrode. Deposition was performed at a potential of 4 V and at room temperature for 8000 s. The film deposited was annealed

PAGE 35

35 i n a tube furnace at 300 o C for 1 h to improve the density of deposit and particle inter connectivity. 4.1.2.3 Characterization X Ray diffraction (XRD Philips APD, Cu source) was used to verify the formation of ITO after annealing. 4 point probe measurement s were performed to measure the electrical conductivity of the samples. Dynamic light scattering (Brookhaven Zeta Plus) was used to measure the size of particles dispersed in the sol. The thickness of films was characterized using a profilometer to have a n estimate of the deposition rate. 4.1.2.4 Results and Discussion The film was characterized using XRD to determine if the desired ITO phase was present. The XRD spectrum is shown in Figure 4 1. Peaks corresponding to cubic In 2 O 3 and cubic In 2 SnO 5 were observed. The peaks match well with the diffraction data of cubic indium oxide. o which corresponds to reflection from (321) planes in ITO. P hases corresponding to tin or other tin oxides were not detected indicating that the desired phase of ITO is produced and that all tin ions have come into the lattice to substitute for indium ions. A profilometer was used to determine the film thickness and it was found to be 7.5 m thick. Films were not very uniform and had a high surface roughness of ~1 m. 4 point probe measurements were conducted to determine the electrical conductivity of the films and the films were conductive with a sheet resistance of 8.098x10 4 cm, which is slightly lower than the val ue reported in literature [37]. Though the films showed good conductance, the transparency of the film was not high enough for visible light to pass through. The films had a gray tinge to it, which made it less suitable for optical applications. Thus, furt her experiments were conducted to improve the optical

PAGE 36

36 transparency of the film. It is known from literature that indium tin oxide is transparent and colorless in thin layers but acquires a yellowish to grey color in bulk form or when compressed. Also, the thickness of the film was high enough for several layers of ITO to get compressed, thus explaining the color of the film obtained. Film thickness was reduced by altering the deposition time from 8000 s to 3600 s. Films were deposited at room tem perature and at a potential of 4V. The thickness of the films obtained was 3.5 m. Though the films were light gray in color, it was not possible to produce completely transparent films. It was noticed that the surface roughness of the film was ~1.5 m an d this roughness is due to the aluminum surface, which causes preferential orientation and deposition of ITO nanoparticles. This resulted in a surface roughness of ~1.5 m and produced opaque and non uniform films. Thus, it was not possible to achieve fil ms with a thickness less than 3 m. Nevertheless, since the films were conductive, the approach was extended to deposit ITO nanowires within an AAO template. The AAO template was fabricated with dimensions of 30 nm in diameter and 10 m in length. The depo sition was carried out for about 5000 s in order to overfill the template. The chronoamperometric response is shown in Figure 4 2. The nanowires subsequently were characterized by scanning electron microscopy. It was observed that the successful growth of nanowires could not be achieved. The deposition occurred on top of the porous template, indicating that the material could not penetrate into the bottom of the template. One possible reason could be due to the size of ITO particle in sol. Therefore, dynami c light scattering was used to measure the particle size and the results are presented in Table 4 2. The diameter of ITO nanoparticles was found to be ~94 nm. As the particle size is larger than the pore

PAGE 37

37 size of the template, nanowire growth was not achiev ed and deposition was observed on top of the template. The issue of particle size can be solved by alternative techniques to make nanoparticles of desirable size. One such method is to prepare a nanopowder of ITO, followed by dispersion in a solvent. Nano particles with diameter of 5 nm have been synthesized by co precipitation and sol gel methods [38]. Due to the issue of transparency and particle size, electro phoretic deposition of ITO did not yield the desired properties. Hence, an alternative method is employed, which involves the deposition of thin films of indium oxide, followed by doping them with the desired amount of tin. Electro deposition technique, unlike EPD, is the reduction/oxidation of ions to form a thin film on the substrate. Hence, it is unlikely to encounter particle size issues with this approach. It is a relatively simple technique, but in order to apply, it has to be understood in detail for every specific system. Thus, the electro deposition of ITO was split into two sub tasks. First, to produce indium oxide films by electrodeposition and second was to deposit films of tin. The idea was to combine the two techniques at different ratio of In:Sn to produce ITO with the desired properties. 4.2 Electrodeposition of Indium Tin Oxide 4.2.1 I ndium Oxide Electrodeposition 4.2.1.1 Materials The chemicals used in the preparation of solution are indium chloride, citric acid monohydrate and distilled water. Aluminum substrates were used as the working electrode and were degreased prior to depositio n. The deposition bath was prepared by dissolving 4.022 g of citric acid monohydrate in 150 mL distilled water. After dissolution,

PAGE 38

38 0.575 g of indium chloride was added to the solution and it was stirred for 30 min to produce a homogenous solution. Cyclic voltammetry is used to determine the potential for deposition and is shown in Figure 4 3. Chronoamperometry was perfomed to electrodeposit indium oxide films. The deposition is carried out at 65 o C and at a potential of 0.6V with a carbon counter electrod e and a SCE reference electrode. The duration of deposition was 3600 s. The films were annealed in air for 1 h at 300 o C. X Ray diffraction (XRD Philips APD, Cu source), with a diffraction angle between 20 o and 80 0 was used to verify the formation of indiu m oxide after annealing. 4 point probe measurements were performed to measure the electrical conductivity of the samples. The thickness of films was characterized using a profilometer. 4.2.1.2 Results and Discussions The XRD spectrum of indium oxide films are shown in Figure 4 4. It is known from literature that indium oxide exists in two phases, the cubic bixbyite and the rhombohedral type. The rhombohedral type is produced only at temperatures greater than 1000 o C an d at high pressures [39]. XRD results confirm the formation of cubic type indium oxide and the peak obtained at 2 = 33.11 o is identified as the reflection from the (321) plane of indium oxide using JCPDF diffraction data. The calculated values of inter pl listed in Table 4 3. It can be observed that the calculated values closely match the standard values from JCPDF data, which confirms the formation of indium oxide. The thickness of th e samples was found to be ~3.5 m with a sheet resistance of 2.37x10 3 cm respectively.

PAGE 39

39 4.2.2 Electrodeposition of tin 4.2.2.1 Materials Tin chloride pentahydrate (Sigma Aldrich) and concentrated sulfuric acid (Acros Organics) were used as received to ma ke the electrodeposition solution. A 0.1 wt% solution of tin was prepared by dissolving the appropriate amount of tin chloride pentahydrate in de ionized water. The solution was stirred for 30 min. Concentrated sulfuric acid is added dropwise to reduce the pH to approximately 2. Stirring is continued for another 30 min to produce the final solution. Degreased 99.99% aluminum substrates were used as the working electrode. These substrates were attached to a copper holder with a copper metallic tape. A carbo n electrode and SCE was used as the counter and reference electrode respectively. Cyclic voltammetry was used to determine the potential for deposition and is shown in Figure 4 5. Chronoamperometry was performed to electrodeposit tin films. Deposition was carried out at room temperature and a potential of 0.7V is applied across the working electrode and a carbon counter electrode. Duration of deposition was 3600 s. The films were dried at 100 o C for 30 min. The films were metallic gray in color. 4.2.2.2 Re sults and Discussions Figure 4 6 shows the X ray difractogram for tin films deposited at 0.7 V. All peaks could be assigned to the tetragonal rutile structure of Sn and no other peak was observed which indicates absence of impurities. The calculated valu es of inter planar 5.The thickness and sheet resistance of the films, as measured were 4.3 m and 4.87x10 4 cm, respectively.

PAGE 40

40 4.2.3 Indium Tin Oxide solution 4.2.3.1 Materials This secti on describes a simple procedure to make a solution of indium tin oxide from solutions of indium chloride and tin chloride. A solution of indium chloride was prepared by dissolving 4.022 g of citric acid monohydrate in 150 mL de ionized water. Once the citr ic acid was dissolved, 0.575 gms of indium chloride was added into the solution and was stirred for 30 min to form a homogenous solution. Tin chloride pentahydrate was added into this solution such that the molar ratio of In:Sn = 9:1. Concentrated sulfuric acid is added into this solution to reduce the pH~2 and is stirred for 2 h. Solutions with different ratios of In:Sn (8:2 and 7:3) are prepared by varying the amount of tin added in the solution. The solutions are stirred for 2 hours prior to deposition. Cyclic voltametry (Figure 4 7) and chronoamperometry were performed for the deposition of ITO. Deposition was carried out at 65 o C and a potential of 0.65 V was applied with a carbon counter electrode and SCE reference electrode. The duration of depositi on was 3600 s and the films were annealed in air at 300 o C for I h. 4.2.3.2 Results and Discussions The deposited films were characterized for phase, thickness and conductivity and were compared to indium oxide films and electrophoretically deposited ITO films. The XRD for ITO with In:Sn = 9:1 (hereafter called ITO91) is shown in Figure 4 8. As can be seen from the figure, the peak positions are very similar to that obtained for indium oxide. However, on close observation, the peaks have a s light shift and a significant change in the peak intensity can be recognized. These changes clearly indicate that there is a change in the d spacing of indium oxide and that tin has been incorporated

PAGE 41

41 into the crystal structure. In addition, no peaks of tin or tin oxide were identified. To further confirm the formation of ITO, the d spacing for the peaks of indium oxide and ITO91 has been calculated and listed in Table 4 5. Figure 4 11 shows the effect of dopant level on the lattice spacing for three differe nt lattice planes. It can be observed that there is a slight increase in the d spacing of ITO91 in comparison to indium oxide. This indicates the formation of indium tin oxide. The peak positions also match with the diffraction data from JCPDF card. The h kl values of ITO91 also match with that obtained from an ion beam sputtered ITO film. The thickness and conductivity of films were measured. The films were conductive with a resistance of 4.8x10 3 and a thickness of 3.7 m. The sheet resistance was calcu lated to be 8.048x10 4 cm, which is less than films obtained from other techniques [37]. Similarly, deposition was carried out for ITO82 and ITO73 at the same conditions as ITO91. The duration of deposition was 3600 s and the films obtained were annealed in air at 300 o C for 1 h. Similar characterization was performed to understand the effect of dopant level on the properties of ITO. The XRD spectrum of ITO82 and ITO73 are shown in Figure 4 9 and Figure 4 10. It can be seen that there is a loss in crystal linity in the sample and only some peaks of ITO could be identified. No peaks of indium oxide or tin were found. This can be attributed to the increase in the tin concentration, which tends to occupy the interstices in the cubic structure and some tin atom s may also occupy the oxygen vacancies in ITO lattice and form crystal imperfections, which explains the reason why some peaks could not be identified. Manifacier and Kim et al. have also observed an increase in disorder with higher dopant levels in ITO fi lms, which seems to be in agreement with our results [40].

PAGE 42

42 Few peaks in ITO73 were identified to be of ITO, but none of the peaks were of t in or indium oxide. This may be because of Sn over doping. When excess amount of tin is introduced, a Sn ion may occ upy oxygen vacancies in the crystal lattice and these interactions can lead to crystal defects and reduction in electrical conductivity [40]. The thickness and conductivity of the films deposited were measured and are reported in Table 4 3. Figure 4 12 sho ws the effect of dopant level on the sheet resistance of the sample. It can be observed that there is a drastic reduction in the resistance of ITO91 as compared to In 2 O 3 This is related to the inclusion of tin, which generates a free electron for conducti on. Unexpectedly, a slight increase in the resistance was observed when the dopant level was increased. This may be because of Sn ions getting trapped by the oxygen vacancies leading to imperfections in the lattice, which reduces the electronic conductivit y. Some concerns associated with this approach include the transparency and composition of films. As the deposition rate of indium oxide and tin are different, an important property to measure would be the ratio of In:Sn in the films obtained. This can be done using X ray Fluorescence Spectroscopy (XRF). A better control over deposition rate and composition can be obtained with an in depth analysis of this method. The transparency of films has also been a problem with both the techniques. It may be due to t he surface roughness of aluminum substrate which causes preferential orientation and deposition. Electro polishing the surface may help in reducing the roughness of aluminum surface and thereby producing a uniform deposit. Currently, my focus is on produci ng transparent films by depositing ITO on a thin layer of gold. The exact cause

PAGE 43

43 for the lack of transparency is not known and further studies need to be formulated to get a better understanding.

PAGE 44

44 Table 4 1. Compariso n of lattice spacing of electro phoretically deposited ITO with values obtained from diffraction card Peak Positions 2 (degree) ITO d spacing from JCPDF card() Calculated d spacing() 30.94 2.921 2.886 33.34 2.704 2.6842 35.7 2.529 2.512 39.7 2.262 2.267 51.22 1.79 1.781 54.82 1.686 1.6726 57.02 1.6 1.6132 63.5 1.46 1.463 69.5 1.352 1.35 Table 4 2. Particle size measurement of ITO nanoparticles in sol by dynamic light scattering Trial Effective Diameter (nm) Half width(nm) Polydispersity Sample quality 1 92.4 32.5 0.103 9.9 2 94.7 25.5 0.072 8.9 3 97.4 32.0 0.108 9.5 Table 4 3 Comparison of lattice spacing (d) of electrodeposited In 2 O 3 with values obtained from diffraction card Peak Positions 2 (degree) d spacing from JCPDF card() Calculated d spacing() 33.25 2.704 2.691 39.61 2.262 2.269 54.67 1.686 1.678 63.43 1.46 1.464 69.39 1.352 1.3527 Table 4 4. Comparison of lattice spacing (d) of electrodeposited tin with values obtained from diffraction card Peak Positions 2 (degree) d spacing from JCPDF card () Calculated d spacing() 30.8 2.91 2.899 32.16 2.79 2.789 43.94 2.062 2.058 55.58 1.659 1.651 62.82 1.484 1.479 64.16 1.442 1.450

PAGE 45

45 Table 4 5. Comparison of lattice spacing (d) of electrodeposited ITO91 and ITO82 with indium oxide Peak Positions 2 (degree) Calculated d spacing() In 2 O 3 ITO91 ITO82 In 2 O 3 ITO91 ITO82 33.23 33.2 32.99 2.691 2.6961 2.7128 39.39 39.38 37.01 2.2855 2.286 2.4268 54.67 54.6 1.677 1.679 67.23 67.20 64.21 1.3913 1.3918 1.4492 Table 4 6. Thickness and Sheet resistance of samples Material Thickness (m) Sheet Resistance x10 ITO 7.5 8.098 In 2 O 3 3.5 23.7 Sn 4.3 4.87 ITO91 3.7 8.048 ITO82 4.74 8.169 ITO73 5.3 8.406 Figure 4 1. X ray diffractogram of e lectrophoretically deposited ITO. The peaks correspond to the cubic structure of i ndium o xide and i ndium tin oxide

PAGE 46

46 Figure 4 2. Chronoamperometry response of electro phoretically deposited ITO nano particles Figure 4 3. Cyclic Voltammetry response for indium oxide electro deposition

PAGE 47

47 Figure 4 4 X ray diffractogram of e lectrodeposited film of i ndium oxide. The peaks correspond to the cubic structure of i ndium oxide Figure 4 5 Cyclic Voltammetry response for electrodeposition of tin

PAGE 48

48 Figure 4 6. X ray diffractogram of electrodeposited film of tin. The peaks correspond to the tetragonal rutile structure of metallic tin Figure 4 7. Cyclic Voltammetry response for electrodeposition of ITO9 1.

PAGE 49

49 Figure 4 8. X ray diffractogram of e lectro deposited ITO91. The molar ratio of In:Sn = 9:1 The peaks correspond to the cubic structure of i ndium o xide and i ndium tin oxide Figure 4 9. X ray diffractogram of elect ro deposited ITO82. The molar ratio of In:Sn =8:2. 321 400 420 600 611 444 721

PAGE 50

50 Figure 4 10. X ray diffractogram of electro deposited ITO73. The molar ratio of In:Sn =7:3 Figure 4 11. Change in d spacing with increase in dopant level.

PAGE 51

51 Figure 4 12. Change in sheet resistance with increase in dopant level 1 In 2 O 3 2 ITO91 3 ITO82 4 ITO73

PAGE 52

52 C HAPTER 5 CONCLUSIONS AND SCOP E FOR FUTURE WORK The work presented in this research examines the electro phoretic deposi tion and electro deposition of i ndium t in oxide films fo r nanowire based solar cells. Electro chemical fabrication of ITO films has been a less explored technique and this research sets the foundation for growth of ITO nanowires and presents some issues involved with the approach. An ITO nanoparticle sol was p repared for the electrophoretic deposition of ITO films and nanowires. As a first step, films were deposited on a luminum substrates. It was found that the films were conductive but lacked transparency. The approach was extended to grow nanowires and was ob served that successful deposition of nanowires could not be achieved as the particle size was greater than the pore size of the templates. Alternatively, nano particles of diameter less than 5 nm can be synthesized by new co precipitation routes and by dis persing the nanoparticles in a solvent, EPD can be performed. This way we may be able to solve the issue of particle size. Another idea was to electrodeposit i ndium tin oxide, which involves the deposition of ions, rather than particles. Indium tin oxide s olution was prepared by a unique method and the molar ratio of In:Sn was varied. It was found that the deposition of ITO films occurred for a In:Sn molar ratio of 9:1 The films were conductive with a sheet resistance of 8.098x10 4 cm. It was observed th at transparency of films was still an issue. It may be due to the chloride precursor materials and hence, acetate or nitrate salts may be used to prepare solution. It is also recommended to electropolish the aluminum substrates prior to deposition to obtai n a uniform and smooth deposit. It was also noticed that the aluminum substrates were attacked or anodized by the solutions if the pH~1 or

PAGE 53

53 temperature was greater than 75 C. Hence, careful control over the operating conditions is required. Electrodepositi on from solutions with In:Sn = 8:2 and 7:3 was also carried out, but successful deposition of ITO films was not achieved. This may be due to the increase in the amount of tin in the solution which creates disorders and defects in the system. It can also b e confirmed from XRD diffractograms that none of the peaks could be attributed to ITO or Indium oxide. The thickness of the films increases with an increase in the amount of tin. The conductivity of ITO91 film is the maximum, with a sheet resistance of 8.0 48x10 4 cm. It was also observed that the conductivity decreased slightly with increase in tin content, which may also be due to the increase in crystal defects in the film. This research could further be extended to deposit nano wires of ITO through elec tro deposition within the pores of a template. To achieve this successfully, w e must determine the amount of i ndium and tin present in the ITO f ilm. As the deposition rate of i ndium is different from that of t in, one sh ould expect varying amounts of i ndium and tin in the deposited films. X ray Fluoresence should be performed to get a better idea on the composition of the films. Once we have a control over the deposition rate, ITO nano wires can be electrodeposited. These ITO nano wires are further coat ed with a layer of t itanium dioxide to form a core of ITO and a shell of TiO 2 which forms the photoanode for DSSC.

PAGE 54

54 LIST OF REFERENCES [1 ] Chopra K. L. S. Major et al (1983) Transparent Conductors A Status Review, Thin Solid Films, Volume 102, Issue 1 [2] Shabbir A Bashar, Study of Indium Tin Oxide for Novel Optoelectronic Devices, PhD Thesis, Department of Elect ronic Engineering, University of London, 1998. [3] Gonzalez G.B T. O. Mason, J. P. Quintana, O. Warschkow, D. E. Ellis, J. H. Hwang, J. P. Hodges, J. D. Jorgensen (2004) Defect structure studies of bulk and nano indium tin oxide, J. Appl. Phys. [4 ] Freeman A.J K. R. Poeppelmeier, T. O. Mason, R. P. H. Chang, T. J. Marks (2000) Chemical and ThinFilm Strategies for New Transparent Conducting Oxides MRS Bulletin [5] J ianhua Ba, Non aqueous Synthes i s of metal oxide Nanoparticles and t heir a ssembly into m esoporous m aterials, PhD d issertation, Universitt Potsdam 2006 [6] Kerkache L. A. Layadi, F.Hadjersi, E. Dogheche,A. Gokarna, A. Stolz, M. Halbwax, J.P. Vilcot, D.Decoster B. El Zein, S. S. Habib (2010) Sputtered Indium Tin Oxide th in films deposited on glass substrate for photovoltaic application International Conference on Renewable Energies and Power Quality [7] Higuchi M. S. Ue kusa, R. Nakano and K. Yokogawa ( 1994 ) Post Deposition Annealing Influence on Sputtered Indium Tin Oxide Film Characteristics Japanese Journal of Applied Physics [8 ] Bregm an J. Y. Shapira and H. Aharoni ( 1990 ), Effects of Oxygen Partial Pressure d uring Deposition on the Properties of Ion Beam Sputte red Indium Tin Oxide Thin Films Journal of App lied Physics [9 ] Ashok S. P. P. Sharma and S. J. Fonash (1980) Spray Deposited ITO Silicon SIS Heterojunction Solar Cells IEEE Transactions on Electron Devices [10 ] Haitjema H. J. J. Elich (1991) Physical Properties of Pyrolitically Sprayed Tin Doped Indium Oxide Coatings, Thin Solid Films [11 ] Bessa is B. N. Mliki, R. Bennaceur (1993) Technological, Structural and Morphological Aspects of Screen Printed IT O Used in ITO/Si Type Structure Semiconductor Science and Technology [12 ] Queen D. R., J. Morford Transparent indium tin oxide films prepared by reactive thermal evaporation U.C Berkeley Microfab [13 ] Michael Grtzel (2003) Review Dye sensitized solar cells Journal of Photochemistry and Photobiology Photochemistry Reviews

PAGE 55

55 [14 ] http://www.top alternative energy sources.com/solar cells.html [15] Tomohiro Nagata, Hirohiko Murakami (2009) Development of Dye sensitized Solar Cells U lvac T echnical J our nal. [ 16 ] Wang R X C D Beling, A B Djuri si c, S Li and S Fung (2004) Properties of ITO thin films deposited on amorphous and crystalline substrates with e beam evaporation Semicond. Sci. Technol. [17] Gratzel M (2001) Photo Electrochemical cells, Nature, 414, 338. [18 ] Chapin D.M, C.S Fuller, G.L Pearson (1954) A new Silicon P N Jun ction photocell for converting s olar radiation into electrical power, Journal of App lied physics [19 ] Hagfield A. M. Grat zel (2000) Molecular Photovoltaics, Accoun ts of Chemical Research [20 ] West.W (1974) First Hundred years of spectral sensitization, Photographic Science and Engineering [21 ] Namba, S., Y. Hishki, (1980), C olor Sensitization of Zinc Oxide wi th Cyanine Dyes. Discussions 285 298. [22 ] Dare Edwar ds M.P, J.B Goodenough, A. Hamnett, K.R Seddon, R.D Wright (1980) Sensitization of semiconducting electrodes with ruthenium based dyes Faraday Discussions. [23 ] Hamnett, A. M.P Dare Edwards, K.R Goodenough, R.D Wright, (1979) Photosensitization of Tita Bipyridine) Ruthenium(II) Chloride Surfa ce States of Titanium(IV) Oxide, Journal of Physical Chemistry [24 ] Brenner A, (1963), Electrodeposition of Alloys, Berichte der Bunsengesellschaft fr physikalische Chemie Volume 68, Issue 3. [25 ] Jin Kawakita (2010) Trends of Research and Development of Dye Sensitized Solar Cells, Science and Technolo gy trends, quarterly review 35. [26 ] Shuji Hayase, Hiroyasu Sumino, Shinji Murai, Tomo Mikoshiba, (2001) IEICE Tec hnical Report. [27 ] J. C C. Fan, Goodenough J.B (1977), X Ray Photoemission Spectroscopy Studies of Sn doped Indium Oxide Films, Journal of Applied Physics [28 ] L. Gupta, Mansingh A., Srivastava P.K (1989), Band Gap Narrowing and the Band Structure of Tin Doped Indium Oxide Films, Thin Solid Films,

PAGE 56

56 [29 ] K. Sreenivas, Sundarsena Ra o T. Mansnigh A., Chandra T. (1985) Preparation and Characterization of r.f. Sputtered Indium Tin Oxide Films, Jou rnal of Applied Physics [30 ] L. R. Cruz Legnani C. (2004), Influence of pressure and annealing on the microstructural and electro optical properties of RF magn etron sputtered ITO thin films, Materials Research Bulletin 39(7 8): 993 1003 [3 1 ] J. H. Hwang, Edwards D.D. Kammler D.R. Mason T.O. (2000) Point defec ts and electrical properties of Sn doped In based transparent conducting oxides, Solid State Ionics. [32 ] Young Sang Cho, Gi Ra Yi, Jeong Jin Hong, Sung Hoon Jang, Seung Man Yang, (2006), Colloidal indium tin oxide nanoparticles for transparent and conduc tive films, Thin Solid Films. [33 ] American Chemical Society (2006, September 18). Ultrathin, Dye sensitized Solar Cells Called Most Efficient To Date. ScienceDaily [34] S.J Limmer, Cruz S.V. Cao G.Z. (2004), Films and nanorods of transparent conducti ng oxide ITO by a citri c acid sol route, Appl. Phys. [35] Ja Eun Song, Don Keun Lee, Young Hwan Kim & Young Soo Kang (2006), Preparation of Water Dispersed Indium Tin Oxide Sol Solution, Molecular Crystals and L iquid Crystals. [36] Ramphal Sharma, Rajaram S. Mane, Sun Ki Min, Sung Hwan Han, (2009) Optimization of growth of In 2 O 3 nano spheres thin films by electrodeposition for dye sensitized solar cells, Journal of Alloys and Compounds. [37] H. Kima Gilmore C.M. (1999), Electrical, optical, and structural properties of indium tin oxide thin films for organic light emitting devices, J ournal of Applie d Physics. [38] Yong Gan J. L., Shengnan Zeng, (2006) Transparent conductive indium tin oxide film fabricated by dip coating technique from colloid p recursor, Surface and Coatings Technolog y [39] Ilaria Corni, Mary P. Ryan, Aldo R. Boccaccini (2008), Electrophoretic deposition: From traditional ceramics to nanotechnology Journal o f the European Ceramic Society. [40] Nina I. Kovtyukhova, Thomas E. Mallouk, (2011), Conductive indium tin oxide nanowire and nanotube arrays made by electrochemically assisted deposition in template membranes: switching between wire and tube growth modes by surface chemical modification of the template Nanoscale.

PAGE 57

57 [41 ] S. T. Chang, Leuz I.C. Hon M.H. (2002), Preparation and Characterization of Nanostructured Tin Oxide Films by Electrochemical Deposition, Electrochemical and Solid State Letters

PAGE 58

58 BIOGRAPHICAL SKETCH Aishwarya Srinivasan received her bachelor s in c hemi cal e ngineering in 2009 with recognized academic accomplishments. She worked as a r esearch a ssistant in Indian Institute of T echnology, Madras, India, where she focused on preparation and testing of polymer fuel cell membranes. She later joined Ion Exchang e India limited in January 2010 as a Process Engineer Trainee. She worked on multiple projects and assisted in the large scale production of hydrated ferric oxide nano adorbents for heavy metal removal from water. She continued her research in nano materia ls at the University of Florida and receiv ed her m aster s degree in c hemical e ngineering in the s pring of 2012. She plans to be involved in the industry sector and work o n application of nano materials.