I hereby declare that the work presented in this ...€¦ · To conclude, sol-gel spin-coating...

Title

Optoelectronic and Mechanical Properties of Sol-Gel Derived Multi-Layer

ITO Thin Films Improved by Elemental Doping, Carbon Nanotubes and

Nanoparticles

Hatem Taha, BSc, MSc

Thesis submitted in a partial fulfillment of the requirements for the degree of Doctor of

Philosophy in the School of Engineering & Information Technology at

Murdoch University

2018

II

DECLARATION

I hereby declare that the work presented in this dissertation is my own version of

research and contains as its main content work which has not been previously

submitted elsewhere for the award of a degree at any tertiary education institute.

Hatem Taha

III

Copyright

Copyright by

@ Murdoch University, Australia

2018

IV

Dedication

This noble study has been dedicated to my parents, brothers, wife and children the

most loving people in this planet for your love, understanding, encouragement,

patience and prayers.

V

ABSTRACT

Transparent conductors (TCs) are an essential ingredient in numerous new applications which

are emerging in the 21st century including high efficiency solar cells, rigid and tactile displays,

light emitting diodes, photonics for communications and computing, energy efficient and smart

windows and gas sensors, since they allow efficient light transmission while electric signals are

applied or collected. So far, indium tin oxide (ITO) reflects the best trade-off between low

electrical resistivity and high optical transparency, making it the first candidate as transparent

conductor for most optoelectronics technologies despite its drawbacks such as expensiveness and

poor mechanical characteristics. However, due to the intricacy of ITO, the coating characteristics

strongly depend on the deposition conditions. Despite many developments in ITO-based

transparent conductive coatings; these coating are yet to be commercialized for optoelectronic

applications. Many challenges still exist in terms of the fabrication of high quality ITO-based

transparent conductive coatings, in order to meet the criteria of better, cost-effectiveness and

environmentally-friendly characteristics, especially in the context of transparent conductive

electrodes.

In this study, the deposition conditions along with incorporating thin ITO films with transition

metals (Ti and Ag), carbon nanotubes (e.g., SWCNTs) and metals nanoparticles (Ag and Au

nanoparticles) were optimized to synthesize high quality ITO based TCs via a facile,

environmentally friendly and cost-effective sol-gel spin-coating method. ITO thin films were

fabricated with different Ti and Ag doping concentration, different annealing temperature and

different geometry such as single layer, bi-layer and multi-layer. The structural, surface

morphology and composition, electrical, optical and mechanical properties were characterized

using a wide range of complementary techniques, namely, X-ray diffraction (XRD), field

emission scanning electron microscopy (FESEM), electron dispersive X-ray (EDX), atomic force

microscopy (AFM), X-ray photoelectron spectroscopy (XPS), four point probes and Hall Effect,

UV-Vis, nanoindentation and FEM modeling.

All the fabricated ITO-based TCs showed a nano-sized grain-like morphology forming a

homogenous surface structure. XRD results demonstrated a relatively good crystallinity of ITO-

based thin film coatings after applying a suitable heat treatment. XPS and EDX analysis

corroborated the existence of the main elements for each case of thin ITO coating. In the case of

VI

pure ITO and (Ti- and Ag-) doped ITO thin films with a thickness of 350 ±5 nm and 500 C

annealing temperature, the highest optical transparency was determined to be 92% for pure ITO,

4 at.% Ti-doped ITO and 2 at.% Ag-doped ITO thin films, while the lowest electrical resistivity

of 1.6×10-4

Ωcm was achieved for the ITO film prepared with 4 at.% Ti content. However, these

thin films exhibit mechanical characteristics namely hardness and Young’s modulus in the range

of (5.3 – 6.8) GPa and (128 – 148) GPa, respectively.

In order to enhance their mechanical characteristics while maintaining their optoelectronic

properties, SWCNTs were incorporated with ITO with different films thicknesses (i.e. 150, 210,

250 and 320) ± 5 nm, and the characterizations were carried out with respect to the film

thickness. The hardness and Young’s modulus for SWCNTs/ITO thin films were in the range of

(22 – 28) GPa and (254 – 306) GPa, respectively. The lowest electrical resistivity of 4.6×10-4

(Ω

cm) was achieved for the thicker film, while the highest transmittance of 91.5 % was obtained

from the thinner film. Obtained results show a significant improvement in the mechanical

properties of SWCNT/ITO thin films compared with their counterparts of pure and doped ITO

thin films, along with distinctively good optoelectronic properties.

To minimize the consumption of indium, ITO thin films were combined with (very thin metal-,

metal nanoparticles- and metal oxide-) layers in bi-layered and tri-layered geometries of (AuI),

((Au)nI), ((Ag)nI) and ((AgO)I), and (IAuI), (I(Au)nI), (I(Ag)nI) and (I(AgO)I), respectively, with

films thicknesses (~ 130 ± 5 nm). For these coating systems, the lowest electrical resistivity

1.2×10-4

Ωcm was for ITO/Au/ITO thin film, while the highest optical transparency ~ 91.5% was

for ITO/AgO/ITO thin film. Two thin films with the configurations of ITO\AgO\ITO and

AgO\ITO for tri-layer and bi-layer coatings, respectively with the best optoelectronic performance

were nominated as transparent conductive electrodes in designing inverted organic solar cells, and

compared with pure ITO thin films. Power conversion efficiencies of 4.9%, 4.2% and 3.8% were

found for ITO/AgO/ITO, AgO/ITO and ITO thin film coatings, respectively.

To conclude, sol-gel spin-coating derived ITO based transparent conductive coatings present

high quality crystal structure, distinctively good optoelectronic properties as well as reasonably

mechanical characteristics, and comparable with those achieved from other sophisticated

VII

fabrication techniques such as sputtering, pulsed laser deposition, electron beam evaporation etc.

All these attributes render the ITO-based coatings promising as transparent conductors in many

industrial and technological applications.

VIII

Table of Contents

Title .................................................................................................................................................. I

DECLARATION ............................................................................................................................ II

Copyright ...................................................................................................................................... III

Dedication ..................................................................................................................................... IV

ABSTRACT ................................................................................................................................... V

Table of Contents ....................................................................................................................... VIII

List of Figures ............................................................................................................................ XIV

List of Tables ............................................................................................................................ XVII

Glossary of Abbreviations and Technical Symbols ................................................................. XVIII

Acknowledgments..................................................................................................................... XXII

List of publications ................................................................................................................. XXIV

Conference papers ................................................................................................................... XXVI

1. CHAPTER 1 General Introduction and Review of the Early Works ...................................... 1

1.1. Transparent Conductive Materials ................................................................................... 1

1.2. Literature Review of Transparent Conductive Materials ................................................. 3

1.3. Recent Advances in Metal Oxide Based Transparent Conductive Coatings ................... 4

1.4. Strategies to Obtain High Carrier Concentration in TCO Materials ................................ 6

1.5. Strategies to Obtain High Carrier Mobility in TCO Materials ................................. 7

1.6. P-Type Transparent Conductors .................................................................................... 10

1.7. Introduction to ITO Thin Films ...................................................................................... 11

1.7.1. Indium Oxide and Indium Tin Oxide Structure ............................................................. 11

1.7.1.1. Indium Oxide and Indium Tin Oxide Band Structures ........................................... 14

1.7.1.2. Recent Advances in ITO Based TCOs .................................................................... 15

1.7.1.3. Survey of the progress of sol-gel derived ITO based TCO coatings ...................... 20

1.7.2. Recent Advances of Some Characterization Techniques for ITO Based TCOs Coatings

21

1.7.2.1. XRD Analysis of ITO Based TCOs ........................................................................ 21

1.7.2.2. Synchrotron X-ray Scattering Analysis of ITO Based TCOs ................................. 21

1.7.2.3. XPS Analysis of ITO-Based TCOs......................................................................... 22

IX

1.7.2.4. High Resolution Transmission Electron Microscopy (HRTEM) Analysis of ITO

Based TCOs............................................................................................................................... 22

1.7.2.5. Computational Modeling Analysis of ITO Based TCOs ........................................ 23

1.7.2.6. Mechanical Characteristics and Finite Element Modeling (FE) for ITO Based

TCOs 23

1.8. Alternatives to Metal Oxides Based TCMs.................................................................... 24

1.9. Industrial Applications of TCO ...................................................................................... 27

1.9.1. Transparent Conductive Electrodes in Solar Cells ......................................................... 27

1.9.2. Transparent Conductive Electrodes in Information Displays ........................................ 27

1.9.3. Transparent Conductive Electrodes in Architectural Glass ........................................... 28

1.10. Challenges and Significances of the Study .................................................................... 29

1.11. Research Questions Addressed in this Study ................................................................. 29

1.12. Methodology .................................................................................................................. 30

1.13. Outline of the Dissertation ............................................................................................. 32

2. CHAPTER 2 Background Theory of Transparent Conductive Materials, Sol-Gel Process

and Characterization Techniques .................................................................................................. 33

2.1. Transparent Conductive Materials ................................................................................. 33

2.1.1. Material Types................................................................................................................ 33

2.1.2. Semiconductors .............................................................................................................. 33

2.1.3. Fermi-Dirac Statistics and Fermi Level ......................................................................... 36

2.1.4. Semiconductors and Transparent Conducting Oxides ................................................... 38

2.1.5. Electrical Properties of ITO Films ................................................................................. 39

2.1.5.1. Carrier Concentration.............................................................................................. 39

2.1.5.1.1. Defect Models ......................................................................................................... 40

2.1.5.1.2. Limitation of Free Carrier Density ......................................................................... 42

2.1.5.2. Free Carrier Mobility of ITO Films ........................................................................ 43

2.1.5.2.1. Scattering Mechanism ............................................................................................. 43

2.1.5.2.2. Limitation of Carrier Mobility ................................................................................ 44

2.1.6. Optical Properties of ITO Films ..................................................................................... 45

2.1.6.1. Optical Constants .................................................................................................... 46

2.2. Sol-Gel Spin Coating Process, Fundamental, Theory and Advantages ......................... 49

2.2.1. General Overview .......................................................................................................... 49

X

2.2.2. Sol-Gel Process .............................................................................................................. 49

2.2.3. Sol-Gel Chemistry .......................................................................................................... 51

2.2.4. Film Formation ............................................................................................................... 52

2.2.4.1. Dip-Coating Process ............................................................................................... 52

2.2.4.2. Spin Coating Process .............................................................................................. 53

2.2.4.3. Deposition of the Solution via Spin Coating .......................................................... 54

2.2.4.3.1. Spin Up ................................................................................................................... 54

2.2.4.3.2. Spin Off ................................................................................................................... 55

2.2.4.3.3. Drying of the Film .................................................................................................. 55

2.3. ITO Thin Films Synthesis .............................................................................................. 56

2.4. Instrumentations and Characterizations Techniques ...................................................... 58

2.4.1. XRD Measurements ....................................................................................................... 58

2.4.1.1. Analysis of XRD Data ............................................................................................ 59

2.4.1.2. Identifications of Phases ......................................................................................... 59

2.4.1.3. Determination of Crystallite Size from XRD Data ................................................. 60

2.4.1.4. Determination of Lattice Parameter ........................................................................ 60

2.4.1.5. X-ray Diffraction Measurements Technique .......................................................... 60

2.4.2. X-ray Photoelectron Spectroscopy (XPS) ...................................................................... 61

2.4.2.1. Applications of XPS Technique.............................................................................. 62

2.4.2.2. Experimental Procedure of X-ray Photoelectron Spectroscopy (XPS) analysis .... 63

2.4.3. Scanning Electron Microscopy (SEM), Energy Dispersive Spectroscopy (EDS) and

Field Emission Scanning Electron Microscopy (FESEM) ........................................................ 63

2.4.3.1. Experimental Technique of SEM............................................................................ 64

2.4.4. Atomic Force Microscopy (AFM) ................................................................................. 65

2.4.5. Raman Analysis.............................................................................................................. 66

2.4.5.1. Experimental Technique of Raman Analysis ......................................................... 67

2.4.6. Optical Properties of Thin Films .................................................................................... 67

2.4.6.1. Measurements of Optical Transmittance via UV-Vis Spectroscopy ...................... 68

2.4.6.2. Band Gap Calculations ........................................................................................... 68

2.4.7. Electrical Properties of Thin Film Coatings ................................................................... 69

2.4.7.1. Four-Point Probes Measurements ........................................................................... 70

XI

2.4.7.2. Hall Effects Measurements ..................................................................................... 71

2.4.8. Mechanical Characterization: Nanoindentation Measurements and Finite Element

Modeling (FEM) of Thin Film Coatings ................................................................................... 72

2.4.8.1. Experimental Technique of Nanoindentation Measurement .................................. 74

2.4.8.2. Finite Element Modeling (FEM) ............................................................................ 75

3. CHAPTER 3 Improving the Optoelectronic Properties of Titanium Doped Indium Tin Oxide

Thin Films ..................................................................................................................................... 76

3.1. Abstract .......................................................................................................................... 77

3.2. Introduction .................................................................................................................... 78

3.3. Experimental Techniques ............................................................................................... 79

3.3.1. Thin Films Deposition .................................................................................................... 79

3.3.2. Sol-Gel and Thin Films Preparation............................................................................... 79

3.3.3. Analysis Techniques ...................................................................................................... 80

3.4. Results and Discussions ................................................................................................. 80

3.4.1. Structural Properties ....................................................................................................... 80

3.4.2. Surface Morphology Properties ..................................................................................... 86

3.4.3. Electrical Properties ....................................................................................................... 88

3.4.4. Optical Properties ........................................................................................................... 91

3.5. Conclusions .................................................................................................................... 95

4. CHAPTER 4: Probing the Effects of Thermal Treatment on the Electronic Structure and

Mechanical Properties of Ti-Doped ITO Thin Films ................................................................... 96

4.1. Abstract .......................................................................................................................... 97

4.2. Introduction .................................................................................................................... 98

4.3. Experimental Details ...................................................................................................... 99

4.3.1. Thin Films Deposition .................................................................................................... 99

4.3.2. Characterization Techniques ........................................................................................ 100

4.4. Results and Discussions ............................................................................................... 101

4.4.1. Raman Analysis............................................................................................................ 101

4.4.2. Atomic Compositions and Surface Chemical Bonding States ..................................... 103

4.4.3. Mechanical Properties (Hardness, Young’s modulus and Wear resistance) ................ 115

4.5. Conclusions .................................................................................................................. 120

XII

5. CHAPTER 5: Improved Mechanical Properties of Sol-Gel Derived ITO Thin Films via Ag

Doping......................................................................................................................................... 121

5.1. Abstract ........................................................................................................................ 122

5.2. Introduction .................................................................................................................. 123

5.3. Experimental Methods ................................................................................................. 124

5.3.1. Raw Materials .............................................................................................................. 124

5.3.2. Sol-Gel and Thin Films Preparation............................................................................. 124

5.3.3. Analysis Techniques .................................................................................................... 125


5.4.1. Crystallographic Structure Properties .......................................................................... 126

5.4.2. Morphological and EDX Analysis ............................................................................... 130

5.4.3. Electrical Properties ..................................................................................................... 132

5.4.4. Optical Properties ......................................................................................................... 136

5.4.5. Determination of Optical Constants ............................................................................. 141

5.4.6. Mechanical Characteristics .......................................................................................... 144

5.5. Conclusions .................................................................................................................. 154

6. CHAPTER 6: Novel Approach for Fabricating Transparent and Conducting SWCNTs/ITO

Thin Films for Optoelectronic Applications ............................................................................... 156

6.1. Abstract ........................................................................................................................ 157

6.2. Introduction .................................................................................................................. 158

6.3. Experimental Section ................................................................................................... 159

6.3.1. Raw Materials .............................................................................................................. 159

6.3.2. SWCNTs/ITO Sol Preparation and Samples Deposition ............................................. 160

6.3.3. Analysis Techniques .................................................................................................... 161


6.4.1. Structural Properties ..................................................................................................... 162

6.4.2. Elemental Compositions and Surface Chemical Bonding States ................................. 164

6.4.3. Raman Studies .............................................................................................................. 167

6.4.4. Surface Morphology and Topography ......................................................................... 168



XIII

6.4.7. Mechanical Characteristics .......................................................................................... 179

6.5. Conclusions .................................................................................................................. 182

7. CHAPTER 7: ITO-Based Bi-Layer and Tri-Layer Thin Film Coatings ............................. 183

7.1. Abstract ........................................................................................................................ 183

7.2. Introduction .................................................................................................................. 184

7.3. Experimental Details .................................................................................................... 186

7.3.1. Sol Preparation ............................................................................................................. 186

7.3.2. Substrate Cleaning........................................................................................................ 187

7.3.3. Thin Films Depositions ................................................................................................ 187

7.3.4. Fabrication of Organic Solar Cells (OSCs) .................................................................. 187

7.3.5. Characterization Techniques ........................................................................................ 188


7.4.1. Characterizations of Au-NPs and Ag-NPs Colloidal Solutions ................................... 189

7.4.2. XRD Analysis of The Thin Films ................................................................................ 191

7.4.3. Surface Morphology ..................................................................................................... 194

7.4.4. Atomic Compositions and Surface Chemical Bonding States ..................................... 196



7.4.7. Characteristics of OSCs ............................................................................................... 209

7.5. Conclusions .................................................................................................................. 210

8. CHAPTER 8: Conclusions and Recommendations............................................................. 212

8.1. Conclusions .................................................................................................................. 212

8.2. Recommendations ........................................................................................................ 216

References .................................................................................................................................. 217

XIV

List of Figures

Figure 1.1 Bixbiyte crystal structure of In2O3 ..................................................................................... 12

Figure 1.2 In2O3 structure with two non-equivalent cataionic b and d sites ....................................... 13

Figure 1.3 Schematic diagram of the energy band model for ITO adopted (a) low doping level and

(b) high doping level ............................................................................................................................ 15

Figure 2.1 Band gap structures of conductor, semiconductor and insulator materials ........................ 34

Figure 2.2 Fermi-Dirac distribution function plot .............................................................................. 37

Figure 2.3 Location of Fermi level for intrinsic semiconductor and n-type and p-type extrinsic

semiconductors ................................................................................................................................... 38

Figure 2.4 Dependence of the free carrier density Ne on the Sn concentration c. Dots represent

experimental points; drawn curves represent (a) NInc, (b) NInc(1-c)8 ................................................. 42

Figure 2.5 Typical spectral dependence of transparent conductive oxides .......................................... 46

Figure 2.6 Complex refractive index (left) and dielectric constant (right) of ITO thin films ............. 48

Figure 2.7 The main steps of spin coating .......................................................................................... 53

Figure 2.8 Flow diagram of sol-gel spin-coating process .................................................................... 57

Figure 2.9 Incident and scattered X-rays in XRD analysis ................................................................. 59

Figure 2.10 Schematic diagram of SEM technique with the CRT screen .......................................... 64

Figure 2.11 Typical Tauc plot for estimating the band gap energy of a thin film ............................... 69

Figure 2.12 A geometry defining the sheet resistance calculation ...................................................... 69

Figure 2.13 Configuration of four point probes measurements .......................................................... 71

Figure 2.14 A schematic diagram of Hall Effect measurements ........................................................ 72

Figure 2.15 Typical loading and unloading curves of the nanoindentation ........................................ 74

Figure 3.1 The XRD patterns of as deposited and post annealed ITO thin films ................................ 82

Figure 3.2 The XRD patterns of Ti-doped ITO films prepared at 2at% and 4 at% Ti concentrations

and post annealed at different temperatures ......................................................................................... 83

Figure 3.3 The annealing temperature dependent grain size ............................................................... 85

Figure 3.4 Peaks shift of (222) for the ITO and Ti-doped ITO thin films annealed at 500 °C ............ 85

Figure 3.5 FESEM images of prepared and post annealed ITO thin films .......................................... 87

Figure 3.6 FESEM images of Ti-doped ITO thin films, (a1-a4) for 2 at% and (b1-b4) for 4 at% Ti

concentrations and post annealed at different temperatures ................................................................ 87

Figure 3.7 Electrical resistivity of pure ITO and Ti-doped ITO thin films for 2 at% and 4 at% Ti

concentrations and post annealed at different temperatures ................................................................ 89

Figure 3.8 Variations of carrier concentration and Hall mobility of ITO and Ti-doped ITO thin films

for 2 at% and 4 at% Ti concentrations with annealing temperature .................................................... 90

Figure 3.9 Optical transmittance of pure ITO and Ti-doped ITO thin films annealed at different

temperatures ......................................................................................................................................... 92

Figure 3.10 Estimated optical band gaps of pure ITO and Ti-doped ITO thin films for 2 at% and 4

at% Ti concentrations post annealed at different ................................................................................. 94

Figure 4.1 Raman shift of Ti-doped ITO films (a): 2 at % and (b): 4 at % of Ti, annealed at different

temperatures ....................................................................................................................................... 103

XV

Figure 4.2 XPS survey scans of Ti-doped ITO films (a): 2 at % and (b): 4 at % of Ti, annealed at

different temperatures ........................................................................................................................ 104

Figure 4.3 In 3d XPS spectra of Ti doped ITO films (a): 2 at % and (b): 4 at % Ti, annealed at


Figure 4.4 Sn 3d XPS spectra of Ti doped ITO films (a): 2 at % and (b): 4 at % Ti, annealed at


Figure 4.5 Ti 2p XPS spectra of Ti doped ITO films (a): 2 at % and (b): 4 at % Ti, annealed at


Figure 4.6 O 1s XPS spectra of Ti doped ITO films (a): 2 at % and (b): 4 at % Ti, annealed at


Figure 4.7 Cl 2p XPS spectra of Ti-doped ITO films (a): 2 at % and (b): 4 at % Ti, annealed at


Figure 4.8 Load-displacement curves of Ti-doped ITO films (a): 2 at % and (b): 4 at % Ti, annealed

at different temperatures .................................................................................................................... 116

Figure 4.9 Hardness and Young’s modulus of Ti doped ITO films (a): 2 at % and (b): 4 at % Ti,

annealed at different temperatures ..................................................................................................... 119

Figure 5.1 The XRD patterns of as deposited and annealed ITO thin films ...................................... 127

Figure 5.2 The XRD patterns of as deposited and annealed Ag-doped ITO films prepared at different

Ag concentrations .............................................................................................................................. 128

Figure 5.3 Variations the crystallite size of as deposited and annealed ITO and Ag-doped ITO thin

films with Ag concentration ............................................................................................................... 130

Figure 5.4 FESEM images of annealed ITO and Ag-doped ITO thin films at different Ag

concentrations .................................................................................................................................... 131

Figure 5.5 EDX analyses of annealed (a): ITO and (b): Ag-doped ITO thin films ........................... 131

Figure 5.6 Electrical resistivity and conductivity of ITO and Ag-doped ITO thin films with different

Ag concentration, (a): as deposited and (b): annealed ....................................................................... 134

Figure 5.7 Mobility and carrier concentration of ITO and Ag-doped ITO thin films with different Ag

concentration, (a): as deposited and (b): annealed ............................................................................. 135

Figure 5.8 Transmittance spectra of ITO and Ag-doped ITO thin films with different Ag


Figure 5.9 Optical band gap of ITO and Ag-doped ITO thin films with different Ag concentration,

(a): as deposited and (b): annealed ..................................................................................................... 139

Figure 5.10 Variation the absorption coefficient of ITO and Ag-doped ITO thin films with different


Figure 5.11 Variation the refractive index of ITO and Ag-doped ITO thin films with different Ag


Figure 5.12 Variation the extinction coefficient of ITO and Ag-doped ITO thin films with different


Figure 5.13 Typical load-displacement curves of ITO and Ag-doped ITO thin films with different Ag


Figure 5.14 Variation the hardness and Young’s modulus of ITO and Ag-doped ITO thin films with

different Ag concentration, (a): as deposited and (b): annealed ........................................................ 148

XVI

Figure 5.15 Variation of hardness with grain size (D-1/2) of ITO and Ag-doped ITO thin films with

different Ag concentration, (a): as deposited and (b): annealed ........................................................ 150

Figure 5.16 (a): FEM modelling result of von Mises stress distribution of as-deposited ITO film and

(b): a comparison of displacement in two different films under study, top: ITO with 6% Ag film and

bottom: pure ITO film ........................................................................................................................ 153

Figure 6.1 XRD pattern of SWCNT/ITO thin films prepared at various thicknesses and annealed at

350 °C ................................................................................................................................................ 163

Figure 6.2 Effect of film thickness on the grain crystallite size of SWCNTs/ITO thin films ........... 164

Figure 6.3 XPS wide scans of SWCNTs/ITO thin films prepared at various thicknesses and annealed

at 350 °C ............................................................................................................................................ 165

Figure 6.4 High resolution scans of (a)- In3d, (b)- Sn3d, (c)- O1s and (d)-C1s of SWCNTs/ITO thin

films ................................................................................................................................................... 167

Figure 6.5 Raman spectra of SWCNTs/ITO thin film ....................................................................... 168

Figure 6.6 FESEM images of SWCNTs/ITO thin films prepared at various thicknesses and annealed

at 350 °C ............................................................................................................................................ 169

Figure 6.7 Elemental distributions of SWCNTs/ITO thin films (annealed at 350 °C) by EDX ........ 170

Figure 6.8 AFM images of SWCNTs/ITO thin films annealed at 350 °C ......................................... 171

Figure 6.9 Variations of (a) electrical resistivity and conductivity, (b) mobility and carrier

concentration of SWCNTs/ITO thin films with film thickness ......................................................... 175

Figure 6.10 Schematic diagram of free carrier injection from ITO network to SWCNTs and carrier

transfer in SWCNTs/ITO thin film .................................................................................................... 176

Figure 6.11 Optical transmittance spectra of SWCNT/ITO thin films annealed at 350 °C ............... 177

Figure 6.12 Absorption coefficients of SWCNT/ITO thin films annealed at 350 °C ........................ 177

Figure 6.13 Tauc plot of SWCNT/ITO thin films annealed at 350 °C .............................................. 179

Figure 6.14 Hardness and Young’s modulus of SWCNTs/ITO thin films annealed at 350 °C ......... 181

Figure 7.1 Absorption spectra of Ag-NPs and Au-NPs colloidal solutions ....................................... 191

Figure 7.2 XRD patterns of (a) bi-layer and (b) tri-layer thin films annealed at 500 °C ................... 193

Figure 7.3 Top-view images of bi-layer (a1 – a4) and tri-layer (b1 – b4) thin films. Cross-sectional

images for (c1) - IAuI and (c2) - I(Au)nI thin films ........................................................................... 195

Figure 7.4 Schematic diagram shows the differences in the formation of the mid-layer thin film

coatings .............................................................................................................................................. 196

Figure 7.5 High resolution scans of (a)- In 3d, (b)- Sn 3d and (c)- O 1s of IAuI thin film ............... 198

Figure 7.6 Transmittance spectra of (a)- bi-layer and (b)- tri-layer thin films .................................. 202

Figure 7.7 Absorption coefficient of (a)- bi-layer and (b)- tri-layer thin films .................................. 203

Figure 7.8 Variations of the band gap energy of (a)- bi-layer and (b)- tri-layer thin films ............... 205

Figure 7.9 Refractive index spectra of (a)- bi-layer and (b)- tri-layer thin films ............................... 207

Figure 7.10 Extinction coefficient spectra of (a)- bi-layer and (b)- tri-layer thin films ..................... 208

7.11 (a)- Device architecture and (b)- J-V characteristic curves of the fabricated and compared OSCs

in this investigation ............................................................................................................................ 210

XVII

List of Tables

Table 1.1 Typical TCO materials and their related dopants and compounds ................................. 9

Table 2.1 Examples of semiconductor materials .......................................................................... 35

Table 2.2 Comparison of some important binary TCOs and their corresponding preparation

methods and optoelectronic properties ......................................................................................... 48

Table 3.1 Structural parameters for the ITO and Ti-doped ITO thin films at 2 at.% and 4 at.% Ti

concentration and post annealed at different temperatures ........................................................... 86

Table 3.2 The optical and electrical parameters of as prepared and post annealed ITO and Ti-

doped ITO thin films for 2 at.% and 4 at.% Ti concentrations ..................................................... 93

Table 4.1 Elemental compositions of Ti doped ITO thin films at 2 at.% and 4 at.% Ti

concentrations after being annealed at different temperatures ................................................... 105

Table 4.2 Fitting results of the XPS data of Ti doped ITO films for the core level binding

energies ....................................................................................................................................... 113

Table 4.3 Mechanical properties of Ti doped ITO films after being annealed at different

temperatures ................................................................................................................................ 119

Table 5.1 Structural parameters for as deposited and annealed ITO and Ag-doped ITO thin films

..................................................................................................................................................... 128

Table 5.2 Electrical and optical properties for as deposited and annealed ITO and Ag-doped ITO

thin films ..................................................................................................................................... 133

Table 5.3 Mechanical properties for as deposited and annealed ITO and Ag-doped ITO thin films

..................................................................................................................................................... 147

Table 6.1 Elemental compositions of SWCNTs/ITO thin films annealed at 350 °C ................. 165

Table 6.2 Electrical properties of SWCNTs/ITO, pure ITO and Ti-doped ITO thin films ........ 173

Table 6.3 Structural, and optical properties of SWCNTs/ITO, pure ITO and Ti-doped ITO thin

films ............................................................................................................................................ 178

Table 6.4 Mechanical properties of SWCNTs/ITO thin films annealed at 350 °C and Ti-doped

ITO thin film annealed at 400°C ................................................................................................. 181

Table 7.1 Structural parameters of the fabricated bi-layer and tri-layer thin films .................... 194

Table 7.2 Variation of the atomic percentage of the main elements for bi-layer and tri-layer thin

films ............................................................................................................................................ 196

Table 7.3 3 Electric properties of fabricated bi-layer and tri-layer thin films ............................ 200

Table 7.4 Optical, sheet resistance and FOM values of synthesized thin films.......................... 204

Table 7.5 Photovoltaic performance parameters of fabricated OSCs ......................................... 210

Table 8.1 A comparison of the best achieved results of ITO-based TCOs thin films fabricated in

this study ..................................................................................................................................... 215

XVIII

Glossary of Abbreviations and Technical Symbols

LEDs Light Emitting Diodes

TCMs Transparent Conductive Materials

T Optical Transparency

ρ Electrical Resistivity

TCOs Transparent Conductive Oxides

HDTVs High Definition Televisions

CNTs Carbon Nanotubes

CVD Chemical Vapour Deposition

Ω Ohm

PVA Polyvinyl Alcohol

HOMO Highest Occupied Molecular Orbital

LUMO Lowest Unoccupied Molecular Orbital

PEDOT:PSS poly (3, 4- ethylenedioxythiophene): poly (styrenesulfonate)

DMSO Dimethyl Sulfoxide

Cu-NWs Copper Nanowires

Ag-NWs Silver Nanowires

eV Electron Volt

e Electron Charge

VB Valance Band

CB Conduction band

Eg Electronic Energy Band Gap

Ne Free Electron Concentration

N(E) Density of Allowed Energy States

d(E) Incremental Energy Range

Ec Bottom of the Conduction Band

Etop Top of the Conduction Band

KB Boltzmann Constant

F(E) Fermi-Dirac Distribution Function

XIX

EF Fermi Energy

T Absolute Temperature

Å Angstrom

ESCA Electron Spectroscopy for Chemical Analysis

RT Room Temperature

σ Electrical Conductivity

μ Free Carrier Mobility

λ Wavelength

h Plank’s Constant

me Electron Mass

τ Average Collision Time

°C Degree Celsius

Ni Density of Impurity Scattering Centers

ɛ0 Permittivity of Free Space

ɛr Low Frequency Relative Permittivity of Low-Loss Dielectrics

ɡ(x) Screening Function

m* Electron Effective Mass

V Volt

s Second

λgap Band Gap Absorption

λp Plasma Edge or Plasma Wavelength

UV Ultraviolet

IR Infrared

ωp Frequency of Plasma Absorption

ωgap Frequency of Band Gap Absorption

˂ n ˃ Complex Refractive Index

n Refractive Index of Low-Loss Dielectrics

no Refractive Index of Air

n1 Refractive Index of the Substrate

c Speed of Light in Space

v Speed of Light in A Medium

XX

k Extinction Coefficient

ɛ Complex Dielectric Constant

t Thin Film Thickness

ɛ1 Real Part of Dielectric Constant

ɛ2 Imaginary Part of Dielectric Constant

α Absorption Coefficient

C0 Initial Solvent Concentration

ω Spinning Speed

RF Radio Frequency

DC Direct Current

2D, 3D Two Dimensions, Three Dimensions

PLD Pulsed Laser Deposition

FDTD Finite-Difference Time-Domain

LCD Liquid Crystal Display

R2R Roll-to-Roll

OSCs Organic Solar Cells

XRD X-ray Diffraction

XPS X-ray Photoelectron Spectroscopy

HRTEM High Resolution Transmission Electron Microscopy

TD-DFT Time Dependent Density Function Theory

HF Hartree Fock

FEM Finite Element Modeling

PVP Polyvinylpyrrolidone

HTL Hole Transport Layer

ETL Electron Transport Layer

PCE Power Conversion Efficiency

Voc Open Circuit Voltage

FF Fill Factor

PET Polyethylene terephthalate

DI Deionized

rpm Revolution per Minute

XXI

GO Graphene Oxide

D Crystal Size

Β, (FWHM) Full Width at Half Maximum

a Lattice Constant

VH Hall Voltage

B Magnetic flux Density

H Hardness

E Elastic (Young’s) Modulus

XXII

Acknowledgments

Firstly, all thanks to almighty Allah (swt), the creator, the sustained, the most merciful and the

controller of the universe, who command has made everything possible for me.

The research work presented in this dissertation reflects the help and support of many people

whom I wish to give noteworthy salutations here.

I would like to thank my supervisors, Dr Zhong-Tao Jiang, Dr David J. Henry, Dr Chun-Yang

Yin and Dr Amun Amri for their inspiration, encouragement and support. Many thanks to each

of them, and I deeply appreciate the motivation and commitment they have instilled in me

throughout this journey towards achieving my PhD degree.

I would also like to acknowledge Dr Jean-Pierre Veder and Dr Elaine Miller (John de Laeter

Centre, Curtin University), Dr David Ralph (postgraduate research Director of School of

Engineering and information Technology), Dr Marc Hampton, Mr. Kenneth Seymour, Jacqueline

Briggs and Caitlin Sweeney (School of Engineering and Information Technology, Murdoch

University), for their assistance, valuable comments and discussions during my research work.

I give my sincere thanks to Dr Xiaoli Zhao (School of Engineering, Edith Cowan University) for

his assistance, appreciated discussions and comments during the preparation of the journal

articles.

Appreciation is extended to my colleagues: M. Mahbubur Rahman, Oday H. Ahmed, Khalil

Ibrahim, Hussein Jan, Zainab Jaf, Ehsan Mohammadpour, Hantarto Widjaja and Nick Mondinos

for their support and friendship.

I pay my deepest love, respects and thanks to my parents, brothers and other family members for

their patience, love and prayer. With their support, and encouragement, I have been able to

achieve my dream of completing my PhD study.

Special thanks and love to my wife who has love, support, inspire and encourage me during the

past four years of my study. Her belief in me has inspired me to force the hard time that I

experienced, to stay firm and to keep going through this very hard journey of research work.

Thank you for believing in me, for loving me, for supporting me; and for the memorable and

XXIII

lovely past, present and future moments. My love has also payed to my sweet daughter Maryam

and to my sweet son Muhammad, the loveliest gifts from Allah (swt).

I am immensely grateful to all of my friends here in Australia, especially Mr Emad Al-Shwany

and Mr Nacer Fellah and his family for their assistance, support and incredible friendship. I am

sure I will miss you, but will never forget you.

Last but not least, I would like to thank the Iraqi Government represented by the Ministry of

Higher Education and Scientific Research / University of Baghdad/ College of Education for

Pure Science, Ibn Al-Haitham, for providing the PhD scholarship.

Hatem Taha

Murdoch University

March 2018

XXIV

List of publications

1. Hatem Taha, Zhong-Tao Jiang, David J. Henry, Amun Amri, Chun-Yang Yin, M.

Mahbubur Rahman, Improving the Optoelectronic Properties of Titanium-Doped Indium Tin

Oxide Thin Films, Semiconductor Science and Technology, 32 (2017) 065011-065021.

2. Hatem Taha, David J. Henry, Chun-Yang Yin, Amun Amri, Xiaoli Zhao, Syaiful Bahri,

Le Minh Cam, Nguyen Ngoc Ha, M. Mahbubur Rahman, Zhong-Tao Jiang, Probing the Effects

of Thermal Treatment on the Electronic Structure and Mechanical Properties of Ti-doped ITO

Thin Films, Journal of Alloys and Compounds, 721 (2017) 333-346.

3. Hatem Taha, Zhong-Tao Jiang, David J. Henry, Amun Amri, Chun-Yang Yin, Afishah

Binti Alias, Xiaoli Zhao, Improved Mechanical Properties of Sol-Gel Derived ITO Thin Films

via Ag Doping, Materials Today Communications, 14 (2018) 210-224.

4. Hatem Taha, Zhong-Tao Jiang, Chun-Yang Yin, David J. Henry, Xiaoli Zhao,

Geoffrey Trotter, Amun Amri, Novel Approach for Fabricating Transparent and Conducting

SWCNTs/ITO Thin Films for Optoelectronic Applications, The Journal of Physical Chemistry C,

122 (2018) 3014-3027.

5. Tapos Kumar Bromho, Khalil Ibrahim, Humayun Kabir, M. Mahbubur Rahman, Kamrul

Hasan, Tahmina Ferdous, Hatem Taha, Mohammednoor Altarawneh, Zhong-Tao Jiang,

Understanding the Impacts of Al+3

-Substitutions on the Enhancement of Magnetic, Dielectric

and Electrical Behaviours of Ceramic Processed Nickel Zinc Mixed Ferrites: FTIR assisted

studies, Materials Research Bulletin, 97 (2018) 444-451.

6. Khalil Ibrahim, Hatem Taha, M. Mahbubur Rahman, Humayun Kabir, Zhong-Tao Jiang,

Solar Selective Performance of Metal Nitride/Oxynitride Based Magnetron Sputtered Thin Film

Coatings: A Comprehensive Review, Journal of optics, 20 (2018) 033001-033016.

XXV

7. Khalil Ibrahim, M Mahbubur Rahman, E. Mohammadpour, Hatem Taha, Zhifeng Zhou,

Chun-Yang Yin, Aleksandar Nikoloski, Zhong-Tao Jiang, Structural, Morphological, and

Optical Characterizations of Mo, CrN and Mo:CrN Sputtered Coatings for Potential Solar

Selective Applications, Applied Surface Science, 440 (2018) 1001-1010.

8. Khalil Ibrahim, Xiaoli Zhao, Hatem Taha, M Mahbubur Rahman, Zhi-Feng Zhou,

Mohmmednoor Altarawneh, T.S.Y. Moh, Aleksandar Nikoloski, Zhong-Tao Jiang, Tailoring the

Structural, Morphological, Opto-Dielectric and Mechanical Behaviours of Mo-Doped CrN

Coatings: Experimental Studies and Finite Element Modelling, Submitted to Surface and

Coatings Technology, January (2018).

9. Khalil Ibrahim, Syed Mahedi Hasan, Hatem Taha, M Mahbubur Rahman, Muna

S. Kassim, Chun-Yang Yin, Zhong-Tao Jiang, A First-principle Study of Electronic Structural

and Optical Properties of CrN and Mo:CrN Clusters, Submitted to Journal of Alloys and

compounds, January (2018).

XXVI

Conference papers

1. Hatem Taha, David J. Henry, Zhong-Tao Jiang, Compositional analysis and mechanical

properties of sol-gel derived Ti-doped ITO thin films, Murdoch University Annual Research

Symposium, November 2017, Murdoch University, WA, Australia.

2. Hatem Taha, Henry, D., Rahman, M.M. and Jiang, Z-T (2016) Optoelectronic properties

of spin coated titanium doped indium tin oxide thin films. Australian Institute of Physics (AIP)

Postgraduate Students Conference, October 2016, University of Western Australia, WA,

Australia.

3. Hatem Taha, Zhong-Tao Jiang, David Henry, Amun Amri, Chun-Yang Yin, M.

Mahbubur Rahman, Hantarto Widjaja, Ehsan Mohammadpour, Nicholas Mondinos "Structural

and optical properties of spin coating ITO thin films". Australian Institute of Physics (AIP)

Postgrad Student Conference, October 2015, University of Western Australia, WA, Australia.

4. Ehsan Mohammadpour, Zhong-Tao Jiang, Mohammednoor Altaraweh, Bogdan Z.

Dlugogorski, Mohammad Rahman, Hantarto Widjaja, Hatem Taha, "Study the structural

changes in CrAlN coatings by in-situ synchrotron radiation X-ray diffraction". Australian

Institute of Physics (AIP) Postgrad Student Conference, October 2015, University of Western

Australia, WA, Australia.

5. M. Mahbubur Rahman, Zhong-Tao Jiang, Zhi-feng Zhou, Hatem Taha, Hantarto

Widjaja, Ehsan Mohammadpour, Chun Yang Yin, Nick Mondinos, Mohammednoor Altarawneh,

Bogdan Z. Dlugogorski "Solar Selective Absorbing Characteristics and Thermal Stability of

TiAlSiN Coatings". Australian Institute of Physics (AIP) Postgrad Student Conference, October

2015, University of Western Australia, WA, Australia.

6. Hantarto Widjaja, Zhong-Tao Jiang, M. Mahbubur Rahman, Ehsan Mohammadpour,

Hatem Taha, Mohammednoor Altarawneh, Chun-Yang Yin, Nicholas Mondinos, and Bogdan

Z. Dlugogorski"Geometrical investigations on the electronic structure of elemental adsorption on

graphene". Australian Institute of Physics (AIP) Postgrad Student Conference, October 2015,

University of Western Australia, WA, Australia.

XXVII

Statement of Contribution

Journal Publications

1. Hatem Taha, Zhong-Tao Jiang, David J. Henry, Amun Amri, Chun-Yang Yin, M.

Mahbubur Rahman, Improving the Optoelectronic Properties of Titanium-Doped Indium Tin

Oxide Thin Films, Semiconductor Science and Technology, 32 (2017) 065011-065021.

http://iopscience.iop.org/article/10.1088/1361-6641/aa6e3f/pdf

Author’s Name

Contribution

Overall

Percentage

(%)

Signature

Hatem Taha

Fabricating the Ti-doped ITO

thin films, analysis and

interbretation of the results,

writing the manuscript and

related submition documents.

70

Zhong-Tao Jiang

Project leader

Data analysis and disucssion

Manuscript preparation and

submission

Corresponding author

David J Henry

PhD Supervisor.

Advisor on sol-gel process and

chemistry. Result discussion

and manuscript preparation

30

Amun Amri

Result discussion and

manuscript preparation

Chun-Yang Yin



CY Yin

M. Mahbubur

Rahman



http://iopscience.iop.org/article/10.1088/1361-6641/aa6e3f/pdf

XXVIII



2. Hatem Taha, David J. Henry, Chun-Yang Yin, Amun Amri, Xiaoli Zhao, Syaiful Bahri,


of Thermal Treatment on the Electronic Structure and Mechanical Properties of Ti-doped ITO

Thin Films, Journal of Alloys and Compounds, 721 (2017) 333-346.

http://www.elsevier.com/locate/jalcom

Author’s Name

Contribution

Overall

Percentage

(%)

Signature

Hatem Taha

Fabricating the Ti-doped ITO





70

David J Henry

PhD Supervisor.




Chun-Yang Yin



30

CY Yin

Amun Amri



Xiaoli Zhao

Machenical property analysis

and

Manuscript preparation

Syaiful Bahri


XXIX

Le Minh Cam


Nguyen Ngoc Ha


M. Mahbubur

Rahman



Zhong-Tao Jiang

Project leader



submission


XXX



3. Hatem Taha, Zhong-Tao Jiang, David J. Henry, Amun Amri, Chun-Yang Yin, Afishah

Binti Alias, Xiaoli Zhao, Improved Mechanical Properties of Sol-Gel Derived ITO Thin Films

via Ag Doping, Materials Today Communications, 14 (2018) 210-224.

www.elsevier.com/locate/mtcomm

Author’s Name

Contribution

Overall

Percentage

(%)

Signature

Hatem Taha

Fabricating the Ag-doped ITO





70

Zhong-Tao Jiang

Project leader



submission


David J Henry

PhD Supervisor.




30

Amun Amri



Chun-Yang Yin



CY Yin

XXXI

Afishah Binti

Alias

Manuscript prepartion

Xiaoli Zhao


and


XXXII



4. Hatem Taha, Zhong-Tao Jiang, Chun-Yang Yin, David J. Henry, Xiaoli Zhao,

Geoffrey Trotter, Amun Amri, Novel Approach for Fabricating Transparent and Conducting

SWCNTs/ITO Thin Films for Optoelectronic Applications, The Journal of Physical Chemistry C,

122 (2018) 3014-3027.

https://pubs.acs.org/doi/abs/10.1021/acs.jpcc.7b10977

Author’s Name

Contribution

Overall

Percentage

(%)

Signature

Hatem Taha

Fabricating the SWCNTs/ITO





70

Zhong-Tao Jiang

Project leader



submission


Chun-Yang Yin


Discussion

30

CY Yin

David J Henry

PhD Supervisor.




Xiaoli Zhao


and


Geoffrey Trotter


Amun Amri



1

1. CHAPTER 1 General Introduction and Review of the Early

Works

1.1. Transparent Conductive Materials

In many daily optoelectronic devices, such as touch screens, solar cells, displays, smart widows,

sensors and light emitting diodes (LEDs), light needs to be efficiently transmitted while applying

or collecting electrical signals. This function is recognized by specific groups of materials named

as transparent conductive materials (TCMs) whose characteristics impact the performance of the

optoelectronic devices and accordingly their specifications differ based on their requirements.

For instance, the optical scattering (haze) can decrease the clearness in displays but increases the

efficiency in solar cells. On the other hand, the surface roughness and work function are very

crucial for LEDs, yet they are less important in touch screens. Mechanical flexibility is

indispensable for flexible applications but not important for devices fabricated on rigid

substrates. However, both optical transparency (T) and electrical resistivity (ρ) are essential

factors for all the aforementioned optoelectronic devices. Each TCM has a trade-off between

optical transparency and electrical resistivity, since the higher the TCM film thickness the lower

both T and ρ will be.

Materials like metals are usually electrically conductive but do not transmit light, whereas,

materials like glass are highly transparent but electrical insulating. The challenge of fabricating

TCMs is to decouple these two characteristics in such a way that one can tune the optical

transparency without impacting the electrical properties and vice versa. To attain this, most

efforts have been in developing dielectric transparent materials into conductive ones without

significantly affecting their optical transparency. E.g. transparent conductive oxides (TCOs) [1].

So far, the most important applications of TCMs are as electrodes for photovoltaic cells, displays

and LEDs [2]. The key technical drivers of commercial products are cost effectiveness, low

power consumption, large-sized devices, minimum reflection, simplified value chain, thinness

and robustness, flexibility and ease of patterning.

Over the past few decades, the majority of TCO materials were based on three main n-type

TCOs, In2O3, ZnO and SnO2 thin films. These mainly experimental studies were accomplished

2

with the aims of improving the performance of these TCOs to meet ever-increasing industrial

demands with various specific requirements.

Within these aforementioned TCOs, In2O3 and/or tin-doped In2O3 (named as indium tin oxide or

ITO) are the most widely used. ITO has reached the industrial level, especially with the use of

the sputtering deposition process. However, thin ITO films can also be fabricated by other

physical and/or chemical methods. For optoelectronic applications and devices, countless

research has been carried out on improving either the electrical conductivity or the transparency

of thin ITO films. The optical transparency can be optimized by introducing a proper level of

oxygen gas into the sputtering chamber or heating up the substrates during or after the

fabrication process [3, 4]. Alternatively, the electrical conductivity can be improved by

increasing the film thickness, thermal treatment (pre-heating the substrates or post annealing

process) and introducing foreign atoms as dopants to the host structure. However, there is a

trade-off between the electrical conductivity and the transmittance of thin ITO film. For

example, the electrical resistivity of ITO might be increased when an excess amount of oxygen is

present or it severely annealed (i.e., ˃ 500 °C), despite the enhancement of its optical

transparency. In general, the physical properties of semiconductors are characteristic of both the

properties of the host structure and the presence of impurities and defects. Dopant impurities

which mainly substitute the host atoms provide electronic states in the band-gap close to the

conduction and valence band edges and hence control the conductivity and the type of the

material. These electronic states (so-called shallow level impurities) facilitate a wide range of

semiconductor characteristics. On the other hand, other impurities along with crystal structure

defects introduce deeper electronic states (named as deep level defects) in the band-gap that can

also tune the properties of the semiconductor but may make a semiconductor inappropriate for its

proposed applications. It is therefore important to control carefully the deposition process to fine-

tune the desired characteristics of ITO based TCO thin films.

It is believed that the performance of ITO thin films in an optoelectronic device is linked to its

structural, morphological, electrical and optical characteristics. High quality ITO films are

mainly deposited within high vacuum systems via physical processes including chemical vapor

deposition or sputtering techniques. Both methods are energy intensive due to their high vacuum

3

system requirement, their high squandered energy through sample fabrication and their high cost

of maintenance [5].

Recent development in the synthesis of TCOs highlight the need for materials with good

optoelectronic properties along with a cost-effective and environmentally-freindly fabrication

process. In this context, the sol-gel process is potentially a very promising technique and can

meet these criteria [6, 7]. The sol-gel process is a well-known simple, low-cost and

environmentally friendly fabrication technique resulting in a thin film with uniform chemical

composition. Also, it is a wet chemistry process where the precursors are commonly in the form

of colloidal solutions that transform into extensive networks of discrete or continuously-

connected molecules. The sol-gel method facilitates fine-tuning of coating parameters including,

homogeneity, thickness, particle size distribution, absorber particle size and chemical

composition of the synthesized thin film. All these eventually make the sol-gel technique a good

prospect for scaling up to an industrial level [8-10]. Duta et. al. [11] carried out a comparitive

study of sol-gel versus sputtering ITO thin films, and they proposed that a simple and low-cost

sol-gel method can effectively replace the expensive sputtering technique in fabricating high

quality ITO thin films. The most important benefit of the sol-gel process over conventional

coating techniques is its capability to tailor the microstructure of the fabricated coatings at low

temperatures [12].

1.2. Literature Review of Transparent Conductive Materials

The past few decades have witnessed an extraordinary increase in both volume and variety of

optoelectronic applications in to the consumer-electronic markets. Along with this growth, there

has also been a switch to devices that provide eye-catching features including energy efficiency,

transparency, tactility, stretchability and flexibility [13]. Most optoelectronic devices with these

properties necessitate transparent electrodes that facilitate these functionalities. Sustainable

production of transparent electrodes with easily available raw materials is also an important

benchmark for successful commercialization. The well-known TCMs include metal oxide thin

films so-called transparent conductive oxides (TCOs), graphene, carbon nanotubes, metal nano (-

particles, –wires and rods) and conductive polymers. This section includes comprehensive

4

literature reviews, fundamental theories and principles, and recently published studies on the

importance, characteristics and applications of metal oxides based transparent conductive

coatings for their optoelectronic and mechanical properties.

1.3. Recent Advances in Metal Oxide Based Transparent Conductive Coatings

Electrically conductive and optically transparent TCOs thin films are an extraordinary class of

material that achieves high value of electrical conductivity while maintaining a high optical

transparency in the visible region of the light spectrum. Since the first realization of such type of

material, vast amount of research and development have been gone into marketable these thin

films. The combination of optical transparency and electrical conductivity of TCOs is usually not

observed in stoichiometric materials, but can be successfully achieved by fabricating these

materials with a non-stoichiometric composition or by incorporating appropriate dopants to the

host lattice.

The most common commercial TCM products so far are based on TCOs materials. In 1907

Baedeker discovered that he could produce a transparent thin cadmium film while sustaining

electrical conductivity by thermally oxidizing cadmium to produce a cadmium oxide (CdO) thin

film. In 1937, a tin oxide thin film was fabricated by post-oxidation of an evaporated metal film.

With the development of the chemical deposition process (pyrolysis) in 1940s, tin oxide (SnO2)

from SnCl4 and indium oxide from InCl2 were synthesized and the indium oxide thin film had

electrical conductivity of 10 S/cm. The early uses of TCOs were as transparent electrodes and

anti-static coatings in electroluminescent panels and as transparent electrical heaters for airplane

windshield de-frosting in World War 2 [14]. With the development of vacuum deposition

systems in the late 1950s and 1960s, ITO thin films were successfully fabricated via vacuum

evaporation of In/Sn metals and via direct current (DC) and radio frequency (RF) sputtering of

metal In/Sn alloy targets. Thin film deposition of metal oxide compounds using vacuum

evaporation was found to be a more controllable process starting with fully oxidized

(stoichiometric) or partially oxidized (sub-oxide) materials, than starting with metal targets and

supplying the required amount of oxygen for reaction. Studies on preparing and sintering

powders of metal oxides as sputtering targets to fabricate TCO thin films started, in order to

attain an easy and scalable process. Vossen in 1970s [15] used oxidized targets and an RF

sputtering system to fabricate ITO and SnO2 films. Investigation of doped binary compounds

5

such as indium oxide and tin oxide were continued in the 1980s along with huge developments in

vacuum deposition hardware and equipment.

Most TCOs are binary or ternary compounds, containing one or two metallic elements. Their

resistivity can be as low as 10-4

Ωcm, and their wide optical band gap (Eg) can be greater than 3

eV. Although, this combination of conductivity and transparency is usually impossible in

intrinsic stoichiometric oxides, it can be achieved by producing a non-stoichiometric material or

by doping.

The usefulness of transparent conductive oxide thin films depends on both their electrical and

optical properties. Therefore, both these parameters have to be considered together with other

parameters such as stability, electron work function, corrosion resistance, mechanical flexibility

and compatibility with substrate as appropriate for the applications [16, 17]. Other considerations

are the readily available raw materials and their recyclability along with economical and

sustainable fabrication method which are important criteria in choosing the appropriate TCO

materials for successful commercialization [18].

The demand for TCOs has risen recently due to their role as key components of electrodes in

touch screen technology, liquid crystal displays and photovoltaic solar cells [19-21]. Other

applications include waveguides in plasmonic devices, bio-analytical devices, antireflective

coatings and organic light emitting devices [22-25]. So far, the highest dollar-value market for

TCOs is as transparent electrodes in flat panel displays, valued at $US 97 billion in 2014 and

expected to reach $US 135 billion in 2020 [26].

During the last three to four decades, countless TCOs were developed from this interesting field.

While some of the TCOs had long been known as intrinsic n-type semiconductors, their potential

as transparent conductive materials came to be realized later by numerous research groups [27].

Efforts speedily moved on from intrinsically (un-doped) TCOs in which the conductivity is

related to oxygen vacancies, defects and lattice interstitial, to extrinsically (doped) TCOs such as

ITO and indium doped tin oxide [28, 29]. The most common TCOs have been so far tin oxide

(SnO2), indium oxide (In2O3), indium tin oxide (In2O3:Sn or ITO), zinc oxide (ZnO) and

aluminium doped zinc oxide (AZO), which have found application in electronic and

6

optoelectronic devices[30-33]. Of these TCOs, ITO is the most important TCO used in the last

few decades.

1.4. Strategies to Obtain High Carrier Concentration in TCO Materials

Works have been carried out to improve the performance of ITO based transparent conductive

coatings via enhancing the electrical and optical characteristics. Most of the efforts were focused

on improving the electrical properties while maintaining the optical transparency [34]. This can

be achieved throughout different processes, systems and mechanisms including doping, thermal

treatment, or incorporating with other materials, etc. The most effective way was found to be by

using a suitable dopant atom which results in increasing the carrier concentration in the film

material [35]. Both carrier concentration and carrier mobility were reported to be crucial factors

for improving electrical conductivity [36-38].

The number of electrons in TCO materials can be increased by doping them with elements that

have higher valence electron count than that of the host. For instance, tin atoms with a valence of

four acts as a cationic dopant in the indium oxide lattice when replacing indium. Since indium

has a valence of three, the Sn can donate an electron to the conduction band results in n-doping

of the lattice. Furthermore, there are studies on the use of tellurium (Te) [39], erbium (Er) [40,

41], germanium (Ge) [42, 43], silicon (Si) [44], fluorine (F) [45-47], sulphur (S) [48] and zinc

(Zn) [49-51] as dopants for indium. Nevertheless, most studies have been carried out on tin-

doped indium oxide [52-54]. All the aforementioned studies aimed to maximize the carrier

concentration, thereby improving the electrical conductivity of the film coatings without altering

their carrier mobility. The relatively low impurity concentration in the host matrix (~ 1 - 5 at %)

is a trade-off between carrier concentration and increasing scattering centres at the dopant sites.

When the carrier concentration in a TCO is very low compared to a metal element, the mobility

of these carriers becomes a very important factor in determining the electrical conductivity of the

TCO [55]. Other macroscopic parameters to optimise the doping level have also been postulated.

For example, a high dopant level leads to distortion of the crystal structure, therefore neither the

generation of oxygen vacancies nor the generation of Sn4+

ions on substitutional sites will be

effective [56]. Also, excessive distortion in the form of an inter-granular amorphous phase is

observed [57, 58]. On the other hand, it has been reported that excessive tin oxide inversely

affected the characteristics of ITO coatings through the formation of a continuous SnO2

7

precipitate at the grain boundaries [59]. Since doping is the most effective mechanism to

improve the optoelectronic characteristics of ITO, different types of elements were employed

recently for this purpose. While some of these elements have different ionic radiuses relative to

the host ions, others were nominated based on their physical or chemical nature, stability,

robustness, high work function, etc. These elements include silver (Ag) [60], copper (Cu) [61],

titanium (Ti) [62], molybdenum (Mo) [63] and chromium (Cr) [64], to name a few. Introduction

of an impurity atom with smaller ionic radius than that of the host ensures that the original

crystalline structure will not change, however, this dopant could enhance the crystalline quality

of the host [65].

The effect of thermal treatment onto the electrical properties has also been extensively studied.

Heat treatment can be performed either through the deposition process (pre-heating the substrate)

or after deposition process (post annealing). Currently, the majority of the fabrication techniques

include a relatively high substrate temperature (≥300 °C). It has been reported that the sputtered

ITO film exhibits a structure changing from amorphous to polycrystalline after annealing [66].

Also, thermal annealing can oxidize the non-stoichiometric compositions of both indium oxide

and tin oxide and results in a re-arrangement of the atoms in the form of stable polycrystalline

thin films [67]. Beside the traditional heat treatment process, post annealing in vacuum and in oil

bath was also reported. An ITO thin film annealed at 350 °C in silicon oil was reported to show a

better crystalline structure with grain size of 51 nm compared to the same thin film annealed at

same temperature under vacuum [68].

1.5. Strategies to Obtain High Carrier Mobility in TCO Materials

Conventionally, the carrier mobility has been increased by enhancing the crystalline quality by

means of appropriate thermal treatment, choice of suitable deposition technique and choice of

substrate. It has been widely reported that TCO thin films fabricated without pre heating the

substrate or post annealing process show low carrier mobility due to scattering by point defects

and/or grain boundaries [69, 70]. However, a post deposition calcination process even at ambient

conditions decreases point and dislocation defects in amorphous or poorly crystallised TCO thin

films, by enlarging their crystallite size and improving the overall crystallite quality [71].

Another very important factor that controls the properties of TCO thin films is selecting a

suitable deposition method and its corresponding conditions. For example, TCO thin films

8

prepared with highly energetic particles may have large crystallite size, however, an extreme

transfer of momentum could deteriorate the film structure and therefore the carrier mobility [72].

For instance, the discharge voltage at the surface of the target (during sputtering deposition

process) repels both neutral argon atoms and negative oxygen ions that reach the substrate with

kinetic energy (~100eV) and distorts the crystal lattice of the TCO thin film [73]. Therefore, RF

sputtering with low discharge voltage is preferred for obtaining TCO films with high values of

carrier mobility [74]. A typical system of pulsed laser deposition with kinetic energy of particles

varied from (1eV – 100eV) was reported to be easier to control than sputtering, and resulted in

better TCO thin film structures and higher carrier mobility [75]. Control of crystal structure of

the TCO thin film by using a highly oriented substrate can also increase the carrier mobility by

improving the film crystallinity. ITO thin films are often grown on yttria-stabilised zirconia

(YSZ) substrate because the lattice constant matching between ITO or indium oxide (10.18Å)

and YSZ (10.20Å) is excellent. Kamie et al. have used an electron beam deposition technique to

grow epitaxial ITO layers onto (111) YSZ substrate [76]. Their results indicated that the ITO

layers have excellent crystallinity as confirmed by reflective high energy electron diffraction,

low energy electron diffraction and X-ray photoelectron diffraction. These ITO layers also

demonstrated carrier mobility in the range of (30 - 40) cm2 V

-1 s

-1 at room temperature.

So far, the manufacturing of TCOs has been limited to three metal oxides, In2O3, SnO2 and ZnO

along with mixtures of these such as ITO and zinc-doped In2O3 (IZO) [77] or with other dopants

such as fluorine (F), antimony (Sb), gallium (Ga), aluminium (Al), boron (B) etc. that yield F-

doped SnO2 [78, 79], Sb-doped SnO2 [55, 80], F-doped ZnO [81, 82], Ga-doped ZnO [83, 84],

Al-doped ZnO [85, 86], B-doped ZnO [87, 88] etc. As for other conventional TCOs, these

materials are formed with metal atoms that have electronic configuration of (n-1)d10

ns2 [89, 90].

The excellent interaction of oxygen p states and metal s states leads to metal-oxide orbital

overlap in these structures and gives rise to low effective electron masses, thereby, the formation

of a wide band gap energy (Eg ˃ 3 eV), permitting the transmittance of light in the visible region

and below. All these materials are very important host lattices for optoelectronic applications

[91, 92]. Recently, other materials with TCO characteristics were reported including titanium

oxide (TiO2) and calcium aluminate (12CaO.7Al2O3) [93, 94]. Binary and ternary compounds

and solid solutions with suitable optical, electrical and mechanical characteristics have also been

developed for numerous applications [95-97]. Since early in the 1990s, multi-cation TCOs which

9

consist of metal ions beyond the conventional In, Sn, Zn and Cd have been developed. These

materials which include Ga2O3, GaInO3 and MgIn2O4 have attracted wide attention [98]. Multi-

cation oxides of InGaZnO4, InGaMgO4, InAlMgO4 and InAlCdO4 fabricated in the form of

layered structures which have identified the hybrid nature of the conduction band due to the

strong hybridization between the states of each cation and its neighbouring oxygen atoms in the

matrix [99-101]. An interesting approach for improving the TCOs performance was based on

employing correlated metals such as SrVO3 and CaVO3, where strong electron-electron

interaction leads to a significant increase in the density of effective electron mass in the TCO

matrix and hence decrease the plasma frequency at fixed electrical resistivity [102]. Table 1.1

summarizes some important TCO materials and their related dopants and compounds.

Table 1.1 Typical TCO materials and their related dopants and compounds

Materials Dopants or compounds Ref.

In2O3 Sn, Ti, Mo, Zr, Ge, Ta, Hf, W, Te, F, Nb [30, 103-105]

SnO2 As, Nb, F, Ta [106, 107]

ZnO Al, Ga, In, B, F, Ti, Zr, Y, S, Hf, V, Sc, Ge [108-110]

Ga2O3 Sn [111, 112]

CdO In, Sn, Sm [113, 114]

TiO2 Nb, V, Ta [115-118]

In2O3-ZnO Zn2In2O5, Zn3In2O6 [119]

In2O3-SnO2 In4Sn3O12 [120]

ZnO-SnO2 Zn2SnO4, ZnSnO3 [121, 122]

GaInO3, (Ga, In)2O3 Sn, Ge [123]

Zn-In2O3-SnO2 Zn2In2O5-In4Sn3O12 [77]

CdO-In2O3-SnO2 CdIn2O4-Cd2SnO4 [124]

CdSb2O6 Y [125]

10

1.6. P-Type Transparent Conductors

All high performance TCOs used in industrial applications are n-type binary systems with

polycrystalline features and doped with halides, group XIII or XV elements. Equivalent p-type

transparent conductive materials have yet to be industrialized. Recently, there have been some

reports on p-type transparent conductors in the literature; however, their electrical conductivities

and figures of merit are still orders of magnitudes less than those of n-type transparent

conductors. Transparent p-type conductors could be useful in numerous applications including,

interconnection layers (recombination junctions) in tandem photovoltaic, electrodes provide

carrier selectivity and carrier collection in photovoltaic devices and electrodes for fully

transparent devices [126]. Fabricating an optimal un-doped or doped p-type TCO seems to be

more difficult than n-type TCOs. This difficulty stems from the ionicity of the metallic atoms

and the electronic structure of the wide band gap metal oxide with strong localization of holes

(large effective mass) at oxygen 2p states [127]. These localized holes require high energy to

overcome a high ionization potential in order to move through the crystal lattice, resulting in low

hole-mobility [128]. To cope with this result and improve the hole-mobility, most p-type TCOs

similar to their n-type counterparts, require high thermal treatment to enhance their crystal

quality. The highest p-type conductivity of 220 Scm-1

was reported for Mg-doped CuCrO2 thin

film [129]. High p-type conductivity has also been investigated practically in perovskites-

structured TCOs such as SrInxTi1-xO3 and SrxLa1-xCrO3 [130, 131], spinel-structured TCOs such

as NiCo2O4 [132] and composite or amorphous oxides such as In-doped MoO3 and ZnORh2O3

[133, 134].

An alternative strategy is to look beyond oxides to materials such as CuI [135, 136]. Non-oxide

wide band gap materials with N, S or P anions have also attracted much attention for p-type

materials due to their more delocalized valence bands resulting in lower hole effective masses

[128]. This new strategy has been established experimentally via Cu-based transparent

chalcogenides and oxychalcogenides include BaCu2S2, CuAlS2, BaCu2(S, Se)F and Cu-Zn-S

which resolve the issues of localised O 2p orbital, achieving electrical conductivity higher than

10 Scm-1

[137-139].

The majority of the p-type transparent conducting materials discussed previously have carrier

concentrations of the order of (1017

- 1019

cm-3

) and usually conduct by hopping mechanism or an

11

activated transport mechanism. For instance, the chalcogenide-based systems are degenerately

doped semiconductors with a conductivity found to be independent of temperature, limited by

ionized impurity scattering and have smaller band gap energy compared to oxides.

Compensation by deep donors could decrease the hole density and the mobility through

increasing ionized impurity scattering. This trade-off was discovered theoretically for some

oxide based materials investigations [140]; however, a more general process directing non-

oxides would be valuable. To avoid this trade-off, Varley et al. proposed a system of boron

phosphide with wide band gap larger than 3.1 eV and low hole mass [141]. To conclude, while

n-type TCOs can achieve high conductivity even in their amorphous state, p-type TCOs show

lower conductivity at quite high temperature.

1.7. Introduction to ITO Thin Films

Indium Oxide and Indium Tin Oxide (ITO) are very interesting transparent conducting oxides

(TCOs). Although band theory predicts them to have wide direct band gaps (> 3.5 eV), they are

electrically conductive n-type semiconductors. The direct wide band gap makes the transmission

window between wavelengths 300 - 1500 nm, depending on the processing conditions. In the

ultraviolet region - high energy (short wavelengths) electronic transitions from the valence band

to the empty states in the conduction band limit the transmission up to the band gap. In the

infrared region - low energy (long wavelengths) light is reflected due to the quasi-free electron

plasma. Therefore, the wavelength cut-off in the infrared region depends on the charge carrier

density. Weiher and Ley [142] have calculated the indirect band gap for ITO which is about

2.62eV, but it is still not widely reported in the literature. The combination of high transmittance

in the visible range and high electrical conductivity, make ITO thin films suitable for

optoelectronic and electroluminescence applications. Typical ITO films have transmittance close

to 90% in the range of 400 - 700 nm and electrical resistivity below 2×10-4

Ωcm. ITO films also

have high luminous transmittance, high infrared reflectance, excellent substrate adherence, quite

good hardness (~ 5 – 6 GPa), and chemical inertness.

1.7.1. Indium Oxide and Indium Tin Oxide Structure

The structure of Indium tin oxide (ITO) and indium oxide (In2O3) are closely related. During the

synthesis of In2O3, which is classified as an ionically bound semiconductor oxide, some defects

12

are formed and these defects involve interstitial indium atoms and oxygen vacancies. Since these

defects are electrically charged, ionized impurity scattering is unavoidable in In2O3 and ITO.

In2O3 crystallizes in a cubic bixbyite structure (C-type rare-earth sesquioxide structure) with a

space group Ia3 and a standard lattice parameter of 10.118 Å (JCBDS card no. 06-0416). The

In2O3 unit cell consists of 80 atoms, which show a fluorite-related superstructure Figure 1.1.

Figure 1.1 Bixbiyte crystal structure of In2O3 [143]

One quarter of the oxygen anions residing along the four >111< axes are missing, 32 sites are

occupied by cations located in two non-equivalent six-fold coordinated sites, 8 In3+

ions are

located on trigonally compressed octahedral sites named as (b site) and the rest of the In3+

ions

are located at highly distorted octahedral sites (d site). Both b and d sites can be described as a

cube with eight corners and the indium atoms exist in the centre of this distorted cube. Six

corners occupied by oxygen atoms and the remaining two corners are empty. In the b sites, the

oxygen vacancies are located along the body diagonal while, they are located along the face

13

diagonal for the d sites. Figure 1.2 shows the schematic representation of cubic In2O3 structure

with two non-equivalent cataionic b and d sites.

Based on his in-situ electrical property measurements, De Wit [144] reported that In2O3 is an

anion-deficient n-type semiconductor where the intrinsic oxygen vacancies population (~ 1%)

limits the electron concentration. This electron concentration deficiency continues at high

temperature even under equilibrium deposition conditions.

Figure 1.2 In2O3 structure with two non-equivalent cataionic b and d sites [145]

Indium Tin Oxide (ITO) is produced by doping In2O3 with tin (Sn) atoms which replace the In3+

atoms from the cubic bixbyite structure of indium oxide. ITO films have a lattice parameter in

the range of (10.12 - 10.31 Å) close to that of In2O3 (10.118 Å). The Sn atoms create an

interstitial bond with oxygen and form SnO or SnO2. Thus, they have a valency of +2 or +4,

respectively, which has a direct effect on the conductivity of ITO. The SnO2 (Sn4+

) acts as an n-

type donor releasing electrons to the conduction band. On the other hand, the lower valence state

occurs due to the reduction in carrier concentration which results because a hole is created and

acts as a trap and reduces conductivity. However, in ITO, not only the tin atoms but also oxygen

vacancies contribute to the carrier density and hence to the high conductivity, where in the ideal

case, each doubly charged oxygen vacancy contributes two free electrons. It has been observed

that the Sn dopant concentration can play an important role in controlling defects that are

14

responsible for lattice stress, which occurs when the film suffers transition from compressive

stress to tensile stress. These defects create impurity states which overlap the conduction band,

creating degenerate semiconductors with behaviour similar to semi-metals.

Theoretical electronic structure calculations by Odaka et al. [146] used a primitive unit cell

having 40 atoms (similar to that of In2O3 crystal). With one In atom either at b-site or d-site

substituted by an Sn atom, resulting in a system of 2.5 at.% Sn concentration along with

approximately 1 × 1021

cm−3

free electron concentration, if each substituted Sn atom donated one

electron. The latter theoretical calculation is in good agreement with the experimental results

reported by Shigesato et al. [36] on ITO thin films having the same free electron concentration

along with very low electrical resistivity. This implies that the super-cell model appears to be a

useful approximation of real ITO thin film coatings.

1.7.1.1. Indium Oxide and Indium Tin Oxide Band Structures

Despite the observed high sensitivity of their characteristics to growth conditions [147], it is

generally accepted that the electronic band structure is a key factor for understanding the unique

interplay between electrical and optical properties of TCOs. However, the band structure of

In2O3 and ITO is very complicated to model due to the complexity of both the lattice structure

and electronic interaction in these materials. Therefore, the characteristics of In2O3 and ITO are

commonly described and discussed on the basis of an assumed band structure comprising of an

isotropic parabolic conduction band [146].

Based on Electron Spectroscopy for Chemical Analysis (ESCA)/XPS measurements, Fan and

Goodenough [148] were the first to report a typical schematic energy band model for In2O3 and

ITO as presented in Figure 1.3. Although this band model is generally simplified, it is still

widely used and the only one offering a qualitative explanation of the observed concurrent high

electrical conductivity and optical transparency in ITO thin films. This band model shows band

gap energy of (3.5eV) along with high optical transparency in the visible region. The conduction

15

band was assumed to be principally from indium 5s electrons while the valence band from

oxygen 2p electrons and the Fermi level is very close to the conduction band.

Figure 1.3 Schematic diagram of the energy band model for ITO adopted (a) low doping level

and (b) high doping level [148]

At low doping level, the density of states for donor atoms is low and is formed below the

conduction band while the Fermi level lies between the donor level and the bottom band edge of

the conduction band (Figure 1.3a). On the other hand, at high doping level, the donor density

increases and the donor states overlap the conduction band (Figure 1.3b).

1.7.1.2. Recent Advances in ITO Based TCOs

As stated earlier, ITO remains the key component in most optoelectronic devices that require

transparent conductive electrodes owing to its two key characteristics, electrical conductivity and

16

optical transparency in the visible range. ITO has been fabricated in different forms, shapes and

structures including thin films, nanoparticles, nanowires, nanorods, 2D and 3D structures. Of

these, ITO thin films have been widely fabricated and studied as a function of deposition

technique and conditions, dopant types, dopant concentration, substrate types, film thickness, etc.

As for other TCOs, all these parameters were found to play significant roles in improving the

performance of TCO thin films through enhancing both electrical conductivity and optical

transparency which also strongly depend on the crystalline structure of the ITO thin film. It has

been extensively observed that heat treatment led to enhanced crystalline quality of ITO thin

films along with their grain size, thereby, improving the optoelectronic performance [149]. The

properties of ITO thin films are strongly dependent on different parameters including thin film

nature, geometry (thickness), which are also associated with deposition techniques and their

corresponding conditions and post deposition processes. Recently, Amalathas and Alkaisi have

reported that the electrical and optical properties of sputtered ITO thin films are strongly

dependent on the film thickness. They found that when the film thickness increased from 75 nm

to 225 nm, the minimum sheet resistance assigned to be 27.5 Ω/sq along with carrier

concentration of 1.12 1021

cm-3

were achieved [150]. Further investigations on the effect of

ITO film thickness on the optoelectronic characteristics have also been carried out by many

groups of researchers [52, 151, 152]. Additionally, efforts on employing an appropriate dopant to

improve the carrier mobility of ITO thin films were made. Other dopants including Mo, W, Ti

and Nb have also shown promise. A Mo-doped ITO thin film with a molar ratio (In:Sn:Mo =

9.6:2:2) was prepared via aerosol-assisted chemical vapor deposition (AACVD) and exhibited a

mobility of the order of 119 cm2 V

-1 s

-1 compared with the same thin film without Mo which

showed a mobility of 25 cm2 V

-1 s

-1. With further increase of the Mo (mol%) up to 5(mol%), the

mobility dropped to 109 cm2 V

-1 s

-1 while the carrier concentration increased to 7 10

20 cm

-3

yielding the lowest resistivity of 8.3 10-5

Ω cm [153]. This implies that higher conductivity is

best achieved through combining high mobility with relatively low carrier concentration rather

than low mobility with higher carrier concentration.

Since the deposition technique also exerts a significant impact on the performance of an ITO thin

film, different techniques have been employed to fabricate high quality ITO coatings and the

competition between these techniques continues up to the present. DC sputtering and RF

17

sputtering [154, 155] are the most widely used techniques for fabricating ITO based TCO

coatings for industrial development, due to high deposition rate, possibility of using available

sputtering system and good reproducibility [5]. Other techniques such as chemical vapor

deposition (CVD), Pulsed laser deposition (PLD), thermal evaporation, electron beam

evaporation, spray pyrolysis, sol-gel process, to name a few, have also been employed for this

purpose [6, 156-160]. Usually, magnetron sputtering depositions are implemented at high

substrate temperatures which achieve the best results in terms of thin film conductivity and

transparency. However, numerous applications such as flexible devices require a quite low

deposition temperature. For this reason, the RF sputtering technique was mostly employed by

researchers to investigate this challenging task.

The increasing demands for advanced devices requires new electrodes with a resistivity lower

than that achieved before and with optical properties higher than those of the present generation

of materials. Minimizing the consumption of indium while maintaining the optoelectronic

properties may be achieved by combining ITO films with other thin metal films that have a

resistivity in the range of 10−6

Ω.cm. Bi-layer and multi-layer systems have been reported to be

an efficient strategy for obtaining high quality ITO based TCO thin films. Of these systems,

combining a metal layer with ITO thin films (i.e., ITO/metal or metal/ITO), insertion of a metal

layer between two ITO thin films (ITO/metal/ITO) or even encapsulating ITO thin film by two

metal layers (metal/ITO/metal) have been proposed as new strategies for TCM production.

Although (metal/ITO/metal) and metal/ITO structure offer very low electrical resistivity, they

suffer from a great reduction in optical transparency due to the single or double opaque metal

layer. Recently, bi-layer ITO/metal thin films were reported by Ng et al. [161]. The authors

investigated the effect of different types of metal layers such as (Ag, AuSn, In, Ni, Sn and SiO2)

on the optical and electrical properties of ITO thin films. Their results showed a decrease in the

electrical resistivity from 16.2 × 10-4 to 7.58 × 10

-4 Ω cm by inserting a thin metal layer

underneath the ITO film. TCO/metal/TCO multilayer structures have also been proposed to be

interesting TCOs due to their higher electrical and optical properties relative to those of a single-

layered TCO thin film. This multi-layer structure gives excellent optoelectronic properties

despite its quite low thickness. Most of these structures were fabricated to be 100 nm thick or

less from similar or even different TCOs thin films encapsulating a thin metal layer. This metal

layer is the key aspect to determine the lowest available resistivity. Silver is the most widely

18

used metal in these systems due to its very low electrical resistance compared to other elements

[162].

Optimization of the electrical and optical characteristics of sputtered indium gallium zinc

oxide/Ag/indium gallium zinc oxide (IGZO/Ag/IGZO) multilayer thin films with respect to

IGZO film thickness has been investigated by Kim and his research group [163]. In their study,

the authors reported that the transmission edge slightly broadened and red shifted toward lower

energies and the free carrier density decreased while the mobility was almost stable along with

increasing IGZO thickness. Also, for further understanding the optical properties, the authors re-

calculated the transmittance spectra in the wavelength range (300 – 800 nm) using Finite-

Difference Time-Domain (FDTD) modeling. Theoretical results for transmittance spectra were

fairly similar to the experimental ones. Nickel doped indium zinc oxide (NIZO) multilayer thin

films with Ag layer were deposited via RF magnetron sputtering technique and studied as

effective transparent conductive electrodes for liquid crystal display (LCD) devices. Despite the

loss of optical transparency, the Ag layer brought improvement in the electrical resistivity and

figure of merit values [164]. Besides Ag, other conductive elements including gold (Au), copper

(Cu), chromium (Cr), platinum (Pt) etc., to name a few, have also been employed in such thin

film systems [165-168]. The majority of recent thin film solar cells and flat panel displays are

based on glass substrates which offer a rigid support and can withstand high temperatures

required for consecutive fabricating processes [169-172]. However, it is important to replace the

rigid substrates by flexible plastics which provide easier handling than panes of glass. Polymer

foils and/or flexible substrates are light, bendable and have a price advantage relative to rigid

glass, thereby, allowing cost-effective mass production by a roll-to-roll (R2R) manufacturing

processes [20]. However, they cannot withstand the high temperature required for industrial mass

production processes. On the other hand, it is very difficult to fabricate high quality transparent

electrodes on these substrates due to the quite high temperature required for obtaining high

electrical conductivity. Compared to a single-layered traditional ITO thin film, the tri-layer

systems have exhibited high conductivity, low absorption in the visible range and room

temperature deposition process. The latter has encouraged researchers around the world to

develop low temperature thin film coatings [173-175]. The most recent investigation in this sense

was made by Kim et al. on sputtered ITO/Ag/ITO thin film at room temperature as electrodes for

organic solar cells [176]. The authors have reported superior results of this coating system

19

compared with those of a high-temperature-fabricated single-layered ITO thin film. Silver oxide

has also been reported as a substitute for the metal layer in tri-layer thin film coatings for flexible

electronic applications. The observed optoelectronic results of ITO/AgOx/ITO are comparable

with those of common ITO/Ag/ITO; however, an improvement in the power-conversion

efficiency was achieved for highly flexible organic solar cells (OSCs) [177]. All the

aforementioned multilayer thin films were fabricated using Dc or RF sputtering system.

However, other techniques such as thermal evaporation and direct laser patterning have also been

used for depositing such coatings [178-180].

Recently, ITO has been investigated in different forms such as nanoparticles, nanowires,

nanorod, 2D and 3D structure. The XRD analysis carried out by Pramod et al. on indium oxide

nanoparticles confirms the formation of polycrystalline features similar to that of layered ITO

thin films [181]. Also, the authors discussed and calculated the optical properties in terms of

transparency, direct and indirect optical band gap. Further investigations on the gas sensing

properties of the prepared thin films have also been reported by the authors. Surface modification

of ITO nanoparticles have been reported by Lee and co-authors [182]. In their study, the authors

demonstrated that effective surfactant removal can improve the electronic performance of ITO

nanoparticles, which were uniformly assembled onto glass and silicon substrates via a spin-

coating process. ITO nanorods were employed as an anode in polymer light emitting diodes

(LEDs). The optical characteristics of these nanorods were investigated via dark-field optical

microscopy and transmission spectroscopy, and the results revealed that the optical transparency

and the light scattering properties of ITO nanorods were better than those of ITO thin films

[183]. Additionally, multifunctional single crystalline ITO nanowires with controlled Sn doping

were fabricated through a vapor transport process by Gao et al. [184]. The authors have reported

these ITO nanowires as promising TCOs where a thin film of these nanowires exhibits a sheet

resistance as low as 6.4 Ω/sq and measurements on single nanowire show a resistivity of the

order of 2.4 10-4

Ω cm along with free carrier density up to 2.6 1020

cm-3

. ITO thin films are

also fabricated and characterized as a three dimensionally structured electrode for current

collection applications in lithium ion (Li-ion) batteries [185]. Further investigations on the

electrochemical stability of this 3D structure of ITO with WO3 as a negative electrode

demonstrates the 3D ITO structure can successfully perform as a suitable current collector.

20

1.7.1.3. Survey of the progress of sol-gel derived ITO based TCO coatings

So far, the sol-gel process has been developed and extensively used as alternative technique for

producing different kinds of transparent conductive thin films. This section presents a survey of

using the sol-gel process for fabricating ITO based TCO thin films.

Kesim and Durucan [186] have been reported the effect of incorporating oxalic acid dihydrate in

ITO solution on to the optoelectronic properties of ITO thin films. The oxalic acid concentration

in the ITO solution was varied (0.05, 0.1, 0.3, 0.5 or 1.0) M. Their results show that the film

formation efficiency, homogeneity and surface coverage were improved with the addition of

oxalic acid. Also, a sheet resistance of 3.8 kΩ/sq and optical transparency of 93% were achieved

for 45 nm ITO film. Eco-friendly pure aqueous sol-gel process was modified by Misra [187] and

co-authors to fabricate ITO thin films. DI-water was used as an alternative to organic solvents for

preparing the ITO solution, and the resulted thin films were annealed at (200 – 400 °C) in step of

50 °C, and treated with argon plasma. The authors reported that the sheet resistance of 45 nm

ITO thin film was considerably decreased to 443 Ω/sq after 2h of plasma treatment. In addition,

the XPS results established that the number of oxygen vacancies in thin ITO film increased with

increasing annealing temperature and applying the plasma treatment. Ren et al [188] have

studied the effect of using benzoylacetone (BzAch) as a chemical modifier in the preparation of

nanocrystalline ITO films with plasmon electrochromic properties. The BzAch was found to

decrease the crystallite size of ITO thin film, and the growth of these crystals was governed by

varying the BzAcH concentration in the ITO film materials. Also, ITO film shows significant

localized surface plasmon resonance absorption features that can be influenced via

electrochemical doping. The plasmonic mechamism in ITO films revealed a significant

reversible and persistent variation in their optical transparency in the near-infrared range of the

electromagnetic radiation.

21

1.7.2. Recent Advances of Some Characterization Techniques for ITO Based TCOs

Coatings

1.7.2.1. XRD Analysis of ITO Based TCOs

The crystallographic characteristics are a key factor in improving the performance of ITO. The

XRD spectra of the ITO thin films fabricated on glass substrates at room temperature have been

widely reported to be amorphous in nature [189]; however, XRD analysis of ITO films

synthesized at different temperatures shows the formation of crystalline structure. Bragg peaks

observed around 31.5°, 35.5°, 51.2°, 60.8° were identified as a body centred cubic bixbyite

structure with space group of la3/cubic and a lattice parameter of the order of 10.20Å similar to

In2O3 (JCPDS card 06-0416). Features for polycrystalline ITO thin films are demonstrated by the

(222), (400), (411), (332), (440) and (622) reflection directions of In2O3. Also, at high annealing

temperature, the degree of crystallinity of ITO phase was improved along the (222) plane [190-

192]. Both the cubic and rhombohedral structures with broadened diffraction lines were observed

by Korosi and his research group [193] for spin coated ITO thin films treated with NH3 and

annealed at 550 °C. These broadened peaks reflect the overlap of several reflection lines of the

aforementioned crystal phases.

1.7.2.2. Synchrotron X-ray Scattering Analysis of ITO Based TCOs

The structural evolution of transparent ITO/Ag/ITO thin films has been investigated as a

function of annealing temperature by means of synchrotron X-ray scattering. The results showed

that the as deposited and 200 °C annealed ITO/Ag/ITO thin films have similar weak (222) and

(400) reflection planes of ITO. This shows that the synthesized thin films at low temperature

consisted of nanocrystalline ITO and Ag phases and there was no significant change in the

crystalline structure below 200 °C. With increasing annealing temperature above 300 °C, the

ITO/Ag/ITO thin films shown strong crystalline structure with peaks featured at (211), (222),

(400) and (440) reflection orientations similar to ITO, indicating high crystalline quality of the

top and bottom ITO layers. Also, strong Ag peaks observed at (111) and (220) planes

demonstrated enhanced crystal structure of the Ag layer after annealing [194].

22

1.7.2.3. XPS Analysis of ITO-Based TCOs

XPS analysis has been implemented to investigate the surface electronic structure of ITO

coatings. Multiple oxidation states for indium e.g., In0 and In

3+ and tin e.g., Sn

2+, 4+ were

observed, while oxygen was from lattice, surface and subsurface. Residual carbon was also

observed at the film surface even after etching. Removing the impurities from the top surface of

sol-gel derived ITO thin film coatings has been reported to decrease the electrical resistivity of

such coatings [195]. It was noted that the increase of tin concentration in the ITO matrix

promoted the reduction of In3+

to In0 which is similar to oxygen being etched out of the film

surface through argon sputtering [196-198].

1.7.2.4. High Resolution Transmission Electron Microscopy (HRTEM) Analysis of

ITO Based TCOs

HRTEM analysis was performed to study the degradation mechanism of post annealed

ITO/Ag/ITO thin films. The cross-sectional HRTEM image of as deposited ITO/Ag/ITO thin

film shows well-defined top ITO layer, mid Ag layer and bottom ITO layer without any

observable interfacial layers, and the mid layer appears as a continuously connected layer

between both bottom and top ITO layers. On the other hand, cross sectional HRTEM images of

600 °C annealed ITO/Ag/ITO thin films show that discrete Ag islands were formed between the

crystalline top and bottom ITO layers. Decreased optical transparency and increased electrical

resistivity of annealed thin films was attributed to the formation of these discrete Ag islands

which gave rise to light and electron scattering. The high magnification images of the as

deposited and annealed samples expose the transformation of continuous Ag layer for as

deposited sample to discrete Ag islands due to the agglomeration of Ag atoms during the

annealing process. At high annealing temperature, the top ITO layer exhibited the formation of

grain boundaries that act as an oxygen diffusion path from this top layer to the Ag layer. Further

investigations reveal that in some regions, both the top and bottom layers were connected with

the absence of the Ag layer due to lateral diffusion of Ag atoms. All these processes eventually

23

result in degradation of the electrical and optical properties of ITO/Ag/ITO thin films at high

annealing temperature (600 °C) [194].

1.7.2.5. Computational Modeling Analysis of ITO Based TCOs

Computational modelling analysis of different materials and their corresponding structures and

properties can be very beneficial in interpreting experimental results and identifying different

types of materials for specific applications. So far, several investigations have focused on the

principles and parameters required in development of ITO based TCOs.

ITO and indium oxide are reported to have similar body centred cubic (BCC) structure. The

electronic characteristics including excitation energy, dipole momentum, frontier molecular

orbital energies, absorption wavelengths, along with the complete vibrational analysis and the

optimized structural parameter calculations of indium oxide were obtained from Time Dependent

Density Function Theory (TD-DFT) and the Hartree-Fock approximation (HF), as reported by

Panneerdoss et al. [199]. The authors have confirmed that indium oxide composites have BCC

frame work with C2v point group symmetry, and the most stable structure has a preferred (222)

reflection plane of the bixbyite structure with symmetrical In-O bonds. In and O atoms are

coupled linearly, where the O atom is centred and two In atoms bonded symmetrically with equal

inter-nuclear distance to form a V-shape structure. Also, the calculated highest occupied

molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energies

confirm the inter charge transformation within the structure, resulting in enhanced band gap

energy.

1.7.2.6. Mechanical Characteristics and Finite Element Modeling (FE) for ITO

Based TCOs

ITO is likely to be susceptible to cracking and delamination due to its brittle nature and to the

large misfit in hardness and Young’s modulus values between the ITO coating and the substrate.

Therefore, it is believed that the mechanical properties of ITO coatings play a significant role in

determining the performance of the optoelectronic device. However, there is little information on

the mechanical characteristics of ITO based TCO coatings particularly in the (nano- or micro-)

structure scales. Recently, an interesting combination of experimental nanoindentation

measurements and theoretical finite element modelling were performed by Gupta et al. to

24

investigate the mechanical behaviour of thick sputter deposited ITO thin films grown on Si

substrate [200]. In their investigations, the contributions of the substrate and the film coating

were considered in a power law formulation based nonlinear material model. Experimental p-h

curves were effectively simulated by finite element modelling. Also, the FE model successfully

predicted the stress-strain curves, the vertical deformation distribution throughout the film

thickness and the Von-Mises stress distribution with respect to the horizontal distance from the

indenter to the axis of symmetry. The FE model also predicted yield strength of 2.2 GPa and 7.75

GPa for ITO thin film and silicon substrate, respectively.

1.8. Alternatives to Metal Oxides Based TCMs

With the vast demand for TCMs in recent decades, research on new materials which are non-

metal oxide based transparent conductors has increased. These materials include graphene,

CNTs, metal nanowires, conductive polymers and perovskite materials. AS for the TCOs, these

materials have also witnessed a gradual development since their first discovery.

Two-dimensional (2D) materials have been extensively proposed as transparent conducting

electrodes in electronic and optoelectronic devices due to their atomic monolayer thickness (that

allowing high optical transparency over the visible range of the electromagnetic radiation) and

the probability of ballistic transport (without electrons scattering even at room temperature).

Among these materials, graphene layers are arguably the most widely used as potential TCMs

for their high optical transparency (~2.3% visible light absorbed per single graphene layer), their

high carrier mobility (103-10

4 cm

2V

-1s

-1) and their mechanical flexibility [201-203]. High

electron mobility have been achieved for graphene thin films grown by CVD on boron nitride

substrates [204]. Despite this high mobility, the electrical conductivity is quite low compared

with other TCMs, which is largely due to its inherently low carrier density (˂ 1013

cm-3

) [203].

Many investigations were carried out aiming to increase the carrier density via a multilayer

graphene thin film growth [205], impurity doping [206] and a recent method using intercalation

doping [207]. Also, graphene has proven itself operable in flexible PV devices under bending of

138° compared to operable bending of 60° in a brittle ITO cell [208]. It has been reported that

four layers of graphene are required to obtain good optical transparency along with a sheet

25

resistance as low as that for ITO of the order of 30 Ω/sq [209]. CNT networks are also one of the

materials recently developed as an efficient substitute to ITO and other TCO materials in the

state of the art of optoelectronic devices, particularly, those for flexible electronics, due to the

extraordinary mechanical and electrical (even under extreme bending conditions) characteristics

these networks possess [210]. CNTs have been extensively adopted for production of composite

materials for printed electronics. Nano-composite films of CNTs were electrochemically

deposited on conducting ITO and Ag grid printed on PET substrate, and the results revealed that

the composite films exhibit non-linear optical characteristics along with antireflective

performance over a visible to long IR range, allowing their potential application as optical

materials [211]. Besides that, Mo and co-workers [212] have reported a high performance TC

coating made by combining an Ag grid with CNTs. These thin films exhibit tremendous overall

performance with a sheet resistance of 14.8 Ω/sq and 82.6% transmittance at room temperature.

Their analysis also indicates that the improvement in the conductivity of the hybrid Ag

grid/CNTs coatings was related to the filling spaces between the Ag grids and interconnecting

the CNTs with Ag nanoparticles. Moreover, the hybrid coatings exhibit high flexibility along

with excellent mechanical strength compared to an ITO coating. The electrical and optical

properties of CNT thin films were evaluated by Hwang et al. [213] as a function of post (fuming-

and immersion-) nitric acid treatments. The treated CNT thin films possessed lower sheet

resistance and improved optical transparency over the visible range of 210 Ω/sq and 90%,

respectively, compared to those thin films that had not undergone acid treatment. However, the

mechanism by which acid treatment improved the optoelectronic properties of CNT based thin

films is still unclear. Earlier reports attributed this improvement to the effective removal of

residual amorphous carbon on the surface of CNTs and the decreasing of the cross-junction

resistance between CNTs [214-216]. Additionally, a random mesh structure of metallic nanowire

film coatings, especially Ag-NWs has attracted considerable attention as a high performance

flexible TC film. These TC films have been extensively adopted in solar cells, displays and

touchscreens to achieve new-generation optoelectronic devices that can be bent, compressed,

stretched, twisted and deformed into complex, non-planar shapes while maintaining high

conductive performance, integration and reliability. Lee et al. [217] have demonstrated Ag-NWs

based flexible transparent conductive composite thin films comprise of a spin-coated Ag-NW

layer followed by a buffer layer of sputtered indium doped zinc oxide (IZO). The latter has a

26

significant potential in prohibiting the surface oxidation of the Ag-NWs, thereby, successfully

avoiding undesirable changes in the conductivity. In their study, a significant enhancement in the

sheet resistance value of pristine Ag-NWs was achieved after being buffered with IZO layers,

(19.84 Ω/sq and 10.35 Ω/sq) for Ag-NWs and Ag-NWs/IZO thin films, respectively. Recently,

transparent conducting films (TCFs) based on Ag-NWs were successfully fabricated as a

function of spraying times, annealing time and solution concentration through a simple spraying

process by Hu and co-workers [218]. Polyvinylpyrrolidone (PVP) was used to modify the

interface of the substrate. A quite stable low sheet resistance of 30 Ω/sq was obtained in this

study, even after 500 cycles of bend testing with bending radius of 5 mm. Conductive polymers

(so-called intrinsically conducting polymers) are organic polymers that possess excellent

electrical conductivity. The superior potential of these materials is their simple processability,

mainly by dispersion. PEDOT:PSS is the most extensively used and commercially available

conducting polymer due to its high optical transparency, suitable thermal stability, high

mechanical flexibility and easy solution based processability [219-222]. Unfortunately, it has

low electrical conductivity compared to other conductive polymers. However, the conductivity

of pure PEDOT:PSS has been improved through solvent treatment including dimethyl sulfoxide

(DMSO), sulphuric acid (H2SO4) and ethylene glycol (EG) [223-225]. Introduction of

appropriate dopant atoms has also proven to be beneficial in improving the conductivity of

PEDOT:PSS films. In this sense, copper bromide (CuBr2) has been adopted to dope

PEDOT:PSS as the HTL in polymer solar cells [226]. A significant improvement in the

conductivity and the power conversion efficiency (PCE) of doped PEDOT:PSS has been

observed by the authors (5.6 × 10-2

Scm-1

and 7.05%, respectively) compared to those of un-

doped (1.9× 10-4

Scm-1

and 5.84%, respectively). This improvement has been attributed to two

mechanisms, (1) the weakening of the columbic attractions between PEDOT and PSS

components, and (2) the increase of the work function value of PEDOT:PSS with CuBr2 doping

which results in increase of both the open circuit voltage (Voc) and the filling factor (FF) of the

solar cell. Further investigations to improve the performance of optoelectronic devices based on

hybrid TCFs are continuing.

27

1.9. Industrial Applications of TCO

TCOs have been extensively used in various industrial applications and ITO seems to be the

most widespread TCOs employed due to its advantages discussed previously, despite its cost

perspective. The main three commercial applications of TCOs thin films in terms of the surface

area covered and their total values are solar cells, flat panel displays and coatings on architectural

glass.

1.9.1. Transparent Conductive Electrodes in Solar Cells

TCO films that act as transparent electrodes are employed in almost solar cells. The main

considerations in selecting the appropriate TCO for this purpose, besides transparency and lateral

conductivity, are processing requirements, electronic compatibility with adjacent layers in the

solar cell and stability under environmental conditions. Design of a conventional photovoltaic

(PV) solar cell consists of deposition of different layers consequently on rigid or flexible

substrates such as glass or PET. Of course, some of these layers are TCO electrodes (anode and

cathode). The TCO anode is deposited on glass substrate, followed by hole transparent layer

(HTL) such as poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS). The

photoactive layer which is directly connected to a low-work function and reflective metallic

cathode is deposited upon the HTL. In some cases, there is no need for an intermediary electron

transport layers (ETL). ITO is the most widely proposed TCO anode in a PV cell.

1.9.2. Transparent Conductive Electrodes in Information Displays

Since the mid-1990s, transparent organic light emitting diode (TOLED) displays have been in

development [227]. Evolution in the fields of TCs and active matrix organic light emitting diode

(AMOLED) displays has resulted in their combining to establish a new and exciting generation

of transparent AMOLED displays. Early TOLED displays consisted of a quite high work

function TCO anode (typically ITO) with a low work function metal cathode such as Au, Ca or

Ag-Mg, encapsulating an organic semiconductor layer along with ETL and HTL transport layers.

While even a thin metallic cathode layer can hinder the optical transparency by 50% [228],

prototype transparent televisions have been developed by Samsung through adoption of twin ITO

electrodes [229]. However, the challenge continues to discover a cathode layer with comparable

28

optoelectronic properties to ITO but a lower work function for efficient electron injection.

Recently, it has been observed both theoretically [230] and experimentally [231] that doping of

ZnO with Ga or Al can effectively reduce its work function and increase its surface carrier

concentration. Similarly, in order to enhance band alignment with HTL layer and hence overall

device efficiency, most recent investigations have also intended to increase the work function of

the ITO anode [232]. The usual industrial deposition of ITO via sputtering could damaging the

underlying organic layers of the OLED device [233]. However, deposition of additional

protective layers via thermal evaporation, prior to the sputtering process, has thus far been able

to address the damage issue [229]. Additionally, it has been reported that with many TCO thin

films such as F-doped SnO2, Sb-doped SnO2 and B-doped ZnO, the chemical vapour deposition

technique can often produce films with superior optoelectronic properties to those achieved via

sputtering [78, 79, 87, 88, 234]. In recent years, many results have been reported for

experimental investigations and computational modelling concerning the modification of the

work function of doped TCOs [92, 230, 232], and it is predicted that the ongoing correlation

between experimental and theoretical approaches can provide tangible development to this

quickly growing field.

1.9.3. Transparent Conductive Electrodes in Architectural Glass

TCO coatings are widely applied to architectural glass as part of multilayer stacks. In this

context, the electrical conductivity is immaterial, but the high infrared reflectivity is exploited, to

minimize heating costs in the winter and air conditioning costs in the summer. Almost 25% of

plane glass is coated, and energy preserving coatings are required in various areas of

applications. The most common technique for applying such coatings is via a very stable and

low-cost, atmospheric pressure chemical vapour deposition (APCVD), along with the float glass

fabrication route. However, the APCVD technique is not very adaptable due to limitations of the

available options for the coating architecture. A more expensive, but also more flexible process

is magnetron sputtering. Usually, many rotating targets (e.g. 20-60) are placed in a long modular

vacuum chamber and multilayer stacks are deposited when the glass plates are moved

underneath various cathodes.

29

1.10. Challenges and Significances of the Study

Both the optical and electrical properties of ITO are correlated and could be tailored to a wide

range of optoelectronic applications. ITO thin films are very sensitive to microstructure and

crystallinity, and show brittle surface structure. Increasing ongoing demands from industrial and

technological applications for high quality optoelectronic devices showing higher performance

limits involve maintaining the structure properties including (crysatllinity, phases and

stoichiometry) as well as the morphology properties including (texture and crystallite size),

which govern the electrical, optical and mechanical properties of thin polycrystalline ITO films.

The nanostructure of ITO is itself sensitive to fabrication conditions. Therefore, controlling this

nanostructure is necessary to produce high quality thin films with suitable optoelectronic and

mechanical characteristics for such applications.

Most of the industrial and technological applications are looking for fabricating high quality ITO

thin films either from low-cost materials or by using simple, clean and efficient fabricating

techniques. However, high quality ITO thin films were mostly fabricated via physical vapour

deposition techniques provided with complicated high vacuum systems and control unit such as

sputtering, pulsed laser deposition, electron beam evaporation, chemical vapour deposition, etc.

These techniques when combined with expensive indium result in high-cost end products.

Therefore, fabricating high quality ITO thin films using a low-cost technique still represent the

main challenge for researchers over the globe. In this study a simple, low-cost and

environmentally friendly sol-gel spin-coating method was used to fabricate high crystalline

quality thin ITO films comparable to those fabricated from complicated physical vapour

deposition techniques. The optoelectronic and mechanical properties of ITO thin films were

improved by using suitable dopants and annealing temperatures. Also, ITO-based multi-layer

thin film coatings were successfully fabricated via sol-gel spin-coating method, and used as

electrodes in OSCs application with improved power conversion efficiency.

1.11. Research Questions Addressed in this Study

Can a simple, cost effective and environmentally friendly method be used to produce

high quality thin ITO films with properties comparable to those prepared from

30

complicated and costly physical vapour deposition techniques such as sputtering, pulsed

laser deposition, electron beam evaporation, etc.?

Is it possible to control the fabrication conditions of the sol-gel spin-coating process to

fabricate high crystalline quality thin film coatings?

Can suitable annealing temperatures be identified that will not significantly change the

structural properties of ITO but improve the network?

Can the carrier concentration of sol-gel derived ITO-based thin films be increased by

doping with metals that have higher valency and work function values compared to

indium? This increment could be made either by substituting the host atoms by dopant

atoms with high valency that contribute extra free charge carriers in the ITO network, or

by enhancing the injection of the free carriers from metals that have high work function

values.

Can the carrier mobility of sol-gel derived ITO-based thin films be enhanced through

improvement of the crystalline quality of these films by doping and/or heat treatment, and

by combining the ITO with conducting material such as CNTs that may provide highly

conducting pathways to the free electrons within the ITO structure?

Can the sol-gel process be beneficial in fabricating the ITO thin films with different

materials in bi-layer and tri-layer coatings with low films thicknesses that offer

optoelectronic properties comparable to thick single ITO film?

Can the mechanical properties of brittle ITO thin films be improved via doping, applying

heat treatments and incorporating a robust material such as CNTs in the ITO matrix?

1.12. Methodology

The methodology developed in this experimental work was designed to address each of the

research questions posed in the previous section.

The sol-gel process was used as low-cost alternative to physical vapour deposition techniques to

prepare ITO thin films. This was then combined with a spin-coating approach to prepare uniform

thin films on suitable substrates such as soda-lime glass slides. In addition to the low-cost and

ease of operation, the sol-gel spin-coating approach has the potential to be scaled-up for large

surface thin film coatings.

31

A number of experimental variables were manipulated in the sol-gel spin-coating process to

achieve high crystalline quality thin films including, solution concentration, solution viscosity,

spin-coater dispensing rate, spin-coater rotation speed, calcining process, film thickness. The

spin-coating deposition parameters consist of three main steps with different spinning speed and

spinning time. These steps include: (1) dispensing the sol onto the rotated substrates, (2)

spreading the sol uniformly and (3) drying the thin films. These steps were followed by a

calcined process at specific temperature which depends on the solution concentration, and the

deposition process was repeated until the desired film thickness was achieved. The thin films

thicknesses were varied in the range of (150 – 350 ± 5 nm).

Annealing was carried out in the range of (350 – 600 °C) throughout the present study to

improve the crystalline quality and enhance the homogeneity of the prepared thin films.

Doping the ITO thin films was achieved by incorporating selected 3d transition metal salts in the

sol-gel process including titanium (IV) isopropoxide (Ti[OCH(CH3)2]4), silver nitrate (AgNO3)

and CNTs. The level of Ti and Ag dopants was varied in the range of (2 – 10 at.%) , while CNTs

were incorporated at a level of 0.25 wt.%. Combining of the dopant level and annealing

temperatures were explored to achieve high carrier concentration and mobility as well as good

mechanical properties in the fabricated ITO thin films.

ITO thin films were combined with metal-, metal nanoparticles- and metal oxide- layers in the

geometries of bi-layered and tri-layered thin film coatings with thicknesses of ~ 130 ± 10 nm.

These combined layers could facilitate obtaining optoelectronic properties comparable to thick

single ITO film (≥ 350 nm).

Structural, surface morphology and roughness, surface composition/electronic structure,

electrical, optical and mechanical properties were investigated using a variety of

complementary techniques, such as X-ray diffraction (XRD), X-ray photoelectron spectroscopy

(XPS), Raman analysis, scanning electron microscopy (SEM) and field emission scanning

electron microscopy (FESEM), energy dispersive X-ray spectroscopy (EDS), atomic force

measurements (AFM), 4-point probes, Hall effect measurements, UV-Vis spectrophotometry,

and nanoindentation and FEM measurements.

32

1.13. Outline of the Dissertation

This dissertation consists of eight chapters. Chapter 1 provides general introduction together

with a review of relevant literature, challenges, significance, research questions, and

methodology used in the current study. Chapter 2 includes the background aspects of ITO and

its correlated optoelectronic characteristics along with the theory of sol-gel spin coating method

and the equipment and characterization techniques used in this work. Chapters 3 to 7 comprise

the main findings obtained from the characterization of the synthesized thin film coatings with

further discussions and commentaries. In particular, Chapter 3 includes the results obtained

from Ti doping including the effects of Ti concentration and annealing temperature on the

structural, morphological, electrical and optical properties of Ti-doped ITO thin films. The

contents of this chapter have been published in Semiconductor Science and Technology (Journal

article 1). Chapter 4 describes the surface chemical bonding states and mechanical properties of

Ti-doped ITO thin films. The major results of this chapter have been published in Journal of

Alloys and compounds (Journal article 2). The results presented in Chapter 5 cover the effects

of Ag additive on the mechanical, structural and optoelectronic properties of ITO thin film

coatings. The results of this chapter have been published in Materials Today Communications

(Journal article 3). The major results of incorporating SWCNTs with ITO thin films have been

extensively described in Chapter 6 in terms of the structural, morphological, optoelectronic and

mechanical characteristics. The findings of this chapter have been published in The Journal of

Physical Chemistry C (Journal article 4). Chapter 7 presents the optoelectronic properties of bi-

layered and tri-layered ITO-based thin films. A summary of the present work, the need for

further developments and scope of further investigations are presented in Chapter 8 as

Conclusions and future works. Since this is a thesis by publications, the reader may find some

repeated information, especially in the introductions of some chapters and/or in the discussion of

the results.

33

2. CHAPTER 2 Background Theory of Transparent Conductive

Materials, Sol-Gel Process and Characterization Techniques

2.1. Transparent Conductive Materials

Transparent and conductive materials (TCMs) are a class of materials, which are characterized

by very low electrical sheet resistance and simultaneously high optical transparency. These

materials have attracted much attention so far on account of their extraordinary and

tremendously useful characteristics. [92, 235].

2.1.1. Material Types

Solid state materials can be categorized with respect to their electrical conductance into three

main categories: conductors, semiconductors and insulators, and the electrical characteristics of

given materials depend on the electronic populations of the different allowed bands. While

conductors have abundance of free electrons for electric conduction, insulators have no free

electrons under atmospheric conditions for electrical conduction. Semiconductors have electrical

characteristics that lie in between those of conductors and insulators [236]. The band gap

structures of conductors, semiconductors and insulators are shown in Figure 2.1. Among the

aforementioned materials, semiconductors are the one of great interest. Therefore, the

semiconductor materials will be discussed in more details in the next section.

2.1.2. Semiconductors

In this type of material, the free electrons can be easily excited from the almost filled valence

band to the almost empty conduction band because they generally have small band gap energy (~

1 - 3eV). Semiconductors behave like insulators at zero Kelvin where there are no free electrons

in the conduction band. However, with increasing temperature, some of the free electrons are

excited in to the conduction band and the electrical conductivity of semiconductor materials

increases. In addition, the excited electrons leave behind positive holes in the valence band. The

resultant semiconductor current is determined by summing both electron and hole currents that

flowing in opposite directions.

34

Figure 2.1 Band gap structures of conductor, semiconductor and insulator materials

Semiconductor materials can be categorized into three main categories including: elemental

semiconductor, composite semiconductor (III-V composites and II-VI composites) and alloy

semiconductor. Since early nineteenth century when the research on semiconductor materials

started, many semiconductors have been discovered and investigated. The most well-known

elemental semiconductor materials are silicon (Si) and germanium (Ge) which both belong to

group 14 of the periodic table of elements. In the mid of last century, germanium was the most

employed semiconductor in photodiodes and rectifiers. However, it was not suitable for

applications at high temperature or of weak current consumption. Later, silicon replaced

germanium semiconductors owing to its low cost and low power consumption. Composite

semiconductors were built with two or more different elements such as gallium Ga (III) and

arsenide (As) to form semiconductor gallium arsenide GaAs (III-V) and aluminum Al (III), Ga

and As in the form of AlxGa1-xAs. These composite and alloy semiconductors were demonstrated

to have optical and electrical characteristics that were not achievable with elemental

semiconductors. Table 2.1 shows examples of each type of semiconductor material.

Full VB

Empty CB

Eg ˃ 4eV

Insulator

e- e

- e

- e

- e

- e

- e

-

Almost full VB

Almost empty CB

e- e

- e

-

Semiconductor

Almost full CB

VB

Conductor

35

Table 2.1 Examples of semiconductor materials

Semiconductor materials can also be classified as intrinsic or extrinsic based on stoichiometry.

Intrinsic semiconductors or undoped semiconductors are pure semiconductors i.e., there is no

foreign atom in their crystalline structures. The number of free carriers is therefore determined

by the characteristics of the semiconductor itself. In such type of materials, the number of

excited electrons and the number of holes are equal (n = p). In general, the population of

thermally excited free carriers in most intrinsic semiconductor materials at room temperature is

very low. Therefore, intrinsic semiconductor materials have high electrical resistivity and hence

low electrical conductivity [237]. Extrinsic semiconductors can be defined as modified intrinsic

semiconductors through adding impurity atoms which improve their number of free carriers and

therefore electrical conductivity. Depending on the desired carrier type, a selected impurity atom

(so-called dopant atom) can be added into the intrinsic semiconductor. Dopant atoms are atoms

of a different element which have either more or less valence electrons than the intrinsic

semiconductor. These dopant atoms can act as acceptors or donors to pure semiconductors.

Acceptor dopant atoms have less valence electrons than the replaced atoms in the host lattice.

Thus, they provide excess holes to the host resulting in increased hole carrier concentration,

thereby forming a p-type semiconductor. On the other hand, donor dopant atoms have more

valence electron than the replaced atoms in the intrinsic semiconductor lattice. These donor

atoms donate free electrons to the conduction band of the host material, lead to excess free

electrons to the intrinsic semiconductor and hence creating an n-type semiconductor [238].

General classification Specific examples

1- Elemental semiconductors Si, Ge

2a- III-V composite semiconductors GaAs, GaSb, AlP, GaN, AlAs, GaP, InP, InSb, InAs,

SiC

2b- II-IV composite semiconductors CdS, CdTe, CdSe, HgS, ZnO, ZnS, ZnTe, ZnSe

3- Alloys AlxGa1-xAs, GaAs1-xP, GaIn1-xAs1-yPy, Hg1-xCdxTe

36

2.1.3. Fermi-Dirac Statistics and Fermi Level

Fermi-Dirac statistics or Fermi-Dirac distribution function can be applied to determine the

distribution of electrons over energy states in any solid state material. In the case of

semiconductor materials at thermal equilibrium, the electron energy distribution is described by

the law of Fermi-Dirac distribution function and therefore provides carrier concentration at a

certain temperature without external excitations such as electric field, pressure or light. At

certain temperatures, continuous thermal agitation leads to the excitation of electrons from the

valence band to the conduction band [239]. The density of electrons (the number of electrons per

unit volume) in an intrinsic semiconductor is determined by the electron density n(E) in an

incremental energy range dE [240, 241]. The electron density is also given by multiplying the

density of states N(E) which represent the density of allowed energy states per unit volume per

energy range by the probability of occupying that energy range F(E) as in the following

relations:

∫ ( )

∫ ( ) ( )

(2.1)

The integration limit is taken from the bottom of the conduction band (Ec) (which is taken to be

E = 0 for simplicity) to the top of the conduction band (Etop).

The probability of an electron to occupying an electronic state of energy E is given by the Fermi-

Dirac distribution function.

( )

( ) ⁄ (2.2)

where K is the Boltzmann constant, T the absolute temperature in Kelvin and EF is called the

Fermi energy or the Fermi level which is defined as the energy state where the probability of

being occupied by an electron is equal to one-half.

In the high energy region (i.e., E – EF ˃˃ KT), the probability of a state being occupied decreases

exponentially with increasing E and equation (2.2) can be approximated as:

( ) ( ) ⁄ (2.3)

Equation (2.3) is known as the Boltzmann approximation.

37

In the low energy region (i.e., E – EF ˂˂ KT), the occupation probability of a state approaches 1.

Therefore, the low energy states tend to be fully occupied and equation (2.2) can also be

approximated with:

( ) ( ) ⁄ (2.4)

Also, in the low energy region, the probability of a state being occupied by holes is:

( ) ( ) ⁄ (2.5)

Figure 2.2 shows the Fermi-Dirac distribution function [241].

Figure 2.2 Fermi-Dirac distribution function plot [241]

It is very important to understand that there is only one Fermi level in a system at thermal

equilibrium. The F(E) is symmetrical around the Fermi level. When the number of energy states

in the conduction band is equal to the number of energy states in the valence band, and the

number of electrons in the conduction band is the same as the number of holes in the valence

band, the Fermi level will lie in the middle of the band gap. For an n-type semiconductor

material where the number of electrons in the conduction band is more than the number of holes

38

in the valence band, the Fermi level will be closer to the bottom of the conduction band.

Similarly, for p-type semiconductor materials where the number of holes in the valence band is

more than the number of electrons in the conduction band, the Fermi level will be closer to the

top of the valence band. Figure 2.3 illustrates the location of the Fermi level for an intrinsic

semiconductor and n-type and p-type extrinsic semiconductors [241].

Figure 2.3 Location of Fermi level for intrinsic semiconductor and n-type and p-type extrinsic

semiconductors [241]

2.1.4. Semiconductors and Transparent Conducting Oxides

A subset of semiconductor materials, transparent conducting oxides have become of great

interest so far, due to their combining of high optical transparency and electrical conductivity.

Among TCOs, ITO is still the most extensively used material in daily optoelectronic

applications. The physical properties of semiconductors in general and TCOs in particular are

determined by the properties of the host crystal, the impurities and crystalline defects. Dopant

atoms which substitute for host crystal atoms introduce electronic states in the band gap energy

close to the conduction and valence band edges and hence determine the conductivity type of the

material as discussed previously. These so-called shallow level defects facilitate a wide range of

TCOs properties and their corresponding applications. However, foreign impurities along with

crystalline defects of the host which generate electronic states deeper in the band gap energy

39

(known as deep level defects), can also modify the properties of the TCOs and therefore make

these materials unsuitable for their intended purposes. It is then important to control carefully the

doping level to fine-tune the required characteristics.

2.1.5. Electrical Properties of ITO Films

2.1.5.1. Carrier Concentration

Generally, the transport process in a polycrystalline structure is more complex than in a single

crystal structure. Thin films of ITO may consist of crystalline or amorphous phases or a mix of

them. In general, polycrystalline thin films have better optoelectronic properties than amorphous

ones [242]. This especially occurs when thin films are deposited on preheated substrates or after

annealing the amorphous thin films [243, 244]. In ITO thin films, introducing the Sn atoms leads

to reduce electrical resistivity if these atoms are thermally activated (i.e., diffusion of Sn atoms

from interstitials and/or grain boundaries to In sites), which increases the number of the free

electrons in the conduction band [245, 246]. However, for as deposited ITO films, this process is

not efficient enough at room temperature (RT). As a result, the resistivity at low substrate

temperatures is generally higher compared to In2O3 thin films because the Sn atoms can act as

scattering centres [247].

The electrical conductivity (σ) basically depends on both the concentration (Ne) and mobility (μ)

of the free carriers as follows:

(2.6)

where e is the electron charge.

Therefore, high carrier concentration and carrier mobility should be simultaneously considered to

obtain high conductivity ITO thin films. In2O3 acts as an insulator in its stochiometric form.

However, indium oxide can achieve high n-type doping levels (~ 1019

cm-3

) due to its intrinsic

defects (so-called oxygen vacansies (Vo)) if fabricated in its oxygen decient form. The

generation of oxygen vacancies contributes two free electrons to the In2O3 matrix as follows:

40

( )

. Consequently, different electrical properties can be achieved

depending on the stoichiometry of In2O3 (i.e., without doping) [4].

Most research efforts have been directed to increase the electrical conductivity of In2O3 thin

films by increasing the number of free carriers via tin doping. Despite some success, this process

is self- limiting, especially when the dopant atoms occupy random sites in the host matrix (eg. at

the grain boundaries) which impair the carrier mobility while increasing the carrier

concentration. Therefore, achieving the lowest electrical resistivity is a trade-off between carrier

concentration and carrier moility. Johnson and Lark-Horovitz [248] deduced a relation for a

completely degenerate semiconductor as follows:

( ⁄ )( ⁄ ) ⁄ ⁄ ⁄ (2.7)

This relation shows that the carrier concentration and mobility are no longer independent.

Moreover, at high doping levels, the observed carrier concentration of ITO thin films is lower

than expected. This implies that some of the tin atoms remain electrically inactive.

2.1.5.1.1. Defect Models

As mentioned before, the free carriers for ITO films are due to two different defects; oxygen

vacancies and substitutional tetravalent tin atoms which govern and control the free carrier

density in the bixbyite structure. Frank and Köstlin [249] performed numerous experiments and

analyses on ITO films with different tin contents, synthesized via chemical vapor deposition

(spray pyrolysis) and treated in reducing and oxidizing atmospheres. Accordingly, they proposed

the formation of the following lattice defects.

A. Neutral defect (Sn2*Oi

")

This neutral defect is formed when two Sn4+

ions which are not on nearest neighbor

positions, are loosely bound to an interstitial oxygen anion. This interstitial defect

dissociates on annealing under reducing conditions and Oi may drift away:

( )

(2.8)

41

(2.9)

B. Neutral defect (Sn2O4)x

This neutral defect is formed at high doping level when two nearest neighbor Sn4+

ions

are bound to three nearest neighbors on regular anion sites and an interstitial oxygen ion

on a nearest quasianion site. Here, the Sn-O bond is quite strong; hence, it is a

detrimental effect since it may not be reduced by heat treatment.

C. Defect (Sn2Oi)(Sn2O4)x

It is composed of both of the above defects including loosely and strongly bound extra

oxygen and it is a common phenomenon in very highly doped systems.

(

) ( ) ((

)( )

) (2.10)

D. Impurity ion-substitutional tin (Sn*)

Tin atom can act as a cationic dopant and substitutes the indium in the In2O3 lattice. Since

indium has a valence of three, the tin doping is providing an electron to the conduction

band, resulting in n-type doping of the lattice. Therefore, the overall charge neutrality is

preserved.

(2.11)

E. Oxygen vacancy VO**

The oxygen vacancies behave as doubly ionized donors and contribute two electrons to

the electrical conductivity as shown in the following relation:

( )

(2.12)

42

Fabrication of ITO films in an oxygen-rich environment will lead to a film which is saturated

with oxygen. Increasing heat treatment will shift both equations (2.9) and (2.10) to the left,

resulting in free electrons, dissolving interstitial oxygen and substitutional Sn. Interstitial oxygen

(Oi) can effectively diffuse within the ITO matrix and reach the grain boundaries. At the grain

boundaries, absorbed oxygen (O2(a)) can be formed and desorbed. Depending on the ambient

conditions, the reactions can also be reversed.

2.1.5.1.2. Limitation of Free Carrier Density

The plot of free carrier density versus the doping concentration of Sn is shown in Figure 2.4.

Theoretically, maximum carrier density Ne due to only Sn doping in typical ITO thin film is Ne =

NIn c, where NIn = 3.0 10

22 cm

-3 the concentration of indium atoms and c is the concentration

of tin atoms. As seen in Figure 2.4 (curve a), at very low tin concentration (c < 4 at. %) the free

carrier density tracks a straight line, indicating that each substitutional Sn atom acts as a donor,

which donates one free electron to the ITO matrix. On the other hand, at higher Sn concentration,

the free carrier density rises up to a maximum at about 10 at.% tin, and then drops with further

increasing c following the relation Ne = NIn c (1 - c)

8 (curve b). This surmises that a portion of

the tin atoms are inactive at high doping level. This is because the higher the tin concentration,

the more possible the tin ions occupy the nearest-neighbouring anion sites, resulting in the

formation of a neutral defect (Sn2O4) as discussed previously.

Figure 2.4 Dependence of the free carrier density Ne on the Sn concentration c. Dots represent

experimental points; drawn curves represent (a) NInc, (b) NInc(1-c)8 [250]

43

2.1.5.2. Free Carrier Mobility of ITO Films

2.1.5.2.1. Scattering Mechanism

For ITO thin films used as transparent conductive electrodes, it is essential to make a

compromise between optical transparency and electrical conductivity. Reduction of the electrical

resistivity consists of either an increase in the free carrier concentration or in the free carrier

mobility. However, increasing the first can also results in an increase in the visible absorption.

Therefore, increasing the mobility is very important for ITO thin films for high electrical and

optical characteristics.

The free carrier mobility µ can be defined as:

(2.13)

where me is the effective electron mass in the conduction band and is the average collision time

of electrons.

Weiher [251] and Groth [245, 252] were the first to report high Hall mobilities (160 and 170

cm2/V s) for single crystal In2O3 and zirconium doped In2O3, respectively. ITO thin films

mobility is strongly dependent on the disorder of the In2O3 structure and on the modification of

the network due to the tin doping process. Therefore, a strong scattering process on the free

carrier could occur at high doping concentrations. There are many sources of electron scattering

which may influence the mobility, such as ionized impurity scattering, neutral impurity

scattering, grain boundary and external surface scattering, acoustical phonon scattering, defect

lattice scattering, etc. In the case of ITO films, which have good crystallinity, the electron

scattering due to the structural disorder and dislocations is predicted to be less important and can

be neglected. The scattering by acoustical phonons seemingly is also of a little importance in

ITO films where there is no significant temperature dependence detected between 100 and

500⁰C. Furthermore, the free carrier mobility is not affected by surface scattering unless the

mean free path is comparable to the film thickness. However, for ITO films having small

crystallite size, grain boundary scattering is supposed to be an important factor which contributes

to decreasing the carrier mobility. It is well-known that grain boundaries act as sites for the

impurities and these sites act as scattering centers for free carriers. However, for a heavily

44

degenerate semiconductor, the mean free path of electrons is much smaller than the crystallite

size. Hence, grain boundary scattering is perhaps insignificant at high carrier densities. Weijtens

[253] has reported that the mobility is mainly determined by grain boundary scattering when the

carrier density is below 7 1020

cm-3

. Kulkarni and Knickerbocker [254] also proposed that, for

polycrystalline materials, the grain boundary scattering may predominate at very high carrier

concentration (>1 1020

cm-3

) due to enhancement of the grain boundary potential.

2.1.5.2.2. Limitation of Carrier Mobility

Generally, the conductivity of practical transparent conductive oxides with relatively high free

carrier concentrations is intrinsically limited since the free carrier concentration and the mobility

cannot be independently increased. One important scattering mechanism that cannot be ignored

is scattering against ionized donor impurities. These ions are important for preserving charge

neutrality in highly doped thin films. However, the Coulomb interactions between the free

carriers and these impurities provide a source of scattering, which is intrinsic to the doped

semiconductors leading to reduced carrier mobility. This drastically affects the conductivity and

the optical transmission near the infrared edge. The contribution by ionized impurities to the

electrical resistivity is based on the Coulomb interaction and the relation below has been used by

many researchers to define the effect of these impurity scattering centers on the mobility

[ ( )

] [ ( ) ] (2.14)

Where N is the carrier concentration, ni is the density of impurities scattering centers, m* is the

electron effective mass, ɛ0 and ɛr are permittivity of free space and low frequency relative

permittivity respectively, Z is the charge of the ionized centers, ɡ(x) is the screening function and

h is Planck’s constant.

For metal oxides with high free carrier density, some absorption of incident radiation takes place

by interaction with the electron gas, which increases with increasing carrier concentration.

Therefore, at carrier densities greater than 2×1021

cm−3

, the TCO exhibits plasma frequencies

which shift to visible light from absorbing infrared wavelengths, and reduce the transparency in

the visible region. Recently, developed TCO materials, including un-doped and doped binary,

ternary, and quaternary compounds suffer from the above limitations. A few exceptional samples

have a resistivity of ≤1×10-4

Ωcm. However, it is difficult to approach a resistivity of 4×10−5

45

Ω.cm because additional scattering processes result from grain boundaries, neutral impurities,

and other forms of structural disorder, which depend on the materials and the preparation

techniques [255].

2.1.6. Optical Properties of ITO Films

The optical properties of ITO films are governed and modified by the band structure and

defects. The spectral range of interest for the optical properties of ITO films lies between 200-

3000 nm, and it is controlled by three different kinds of electronic excitation in different spectral

regions. The electronic excitations include inter-band transitions from the valence band into the

conduction band, intra-band transitions for the free carriers within the conduction band and band

gap transitions. Consequently, there are three different regions that can be recognized for the

transmittance curve as follow:

Ultraviolet (UV) region: In this region, ITO films exhibit high UV absorption related to

interband transitions. This absorption is due to the fundamental direct band gap around 3.5 eV.

The band gap absorption (λgap), named the absorption edge, strongly depends on the preparation

technique [256], and it lies in the range of (3.5 - 4.1) eV. Manifacier et al. [257] observed that

the higher the free electron density, the shorter the wavelength of the absorption edge. When the

free electron density is very high the conduction band is partially filled up, thus the first

noticeable transition happens for higher photon energy. Therefore, the free electron density is a

crucial factor in defining the optical properties in this region.

Infrared (IR) region: In this region, the ITO films enter a reflecting area with a plasma edge or

plasma wavelength λp that is defined by reflectance = transmittance, with metallic properties

where the metal-like infrared reflectance equals the dielectric-like transmittance [250]. Classical

Drude theory can be used to explain the optical phenomenon in this region [258]. The absorption

process for the free carriers in this region is important for reflectance and transmittance of the

ITO films. Therefore, as in the UV region, the free electron density is a crucial factor in defining

the optical properties in this region as well.

46

Visible region: In between the UV and IR regions (λgap and λp), there is a very high

transmittance region for around 90% of the visible wavelengths that depends on the preparation

conditions. Figure 2.5 shows the typical spectral dependence of transparent conductive oxides.

ωp and ωgap are the frequencies at which the plasma absorption and the band gap absorption takes

place, respectively. The wide optical band gap <3.5eV prohibits inter-band transitions thereby

making it transparent in this region. The transmittance window is defined by the absorption edge

toward the shorter wavelengths and by the plasma edge toward the longer wavelengths.

Figure 2.5 Typical spectral dependence of transparent conductive oxides

2.1.6.1. Optical Constants

The optical properties of the materials are defined by the complex refractive index ˂ n ˃ and the

complex dielectric constant (ε). The complex refractive index describes the velocity of light in

the medium (v) following the relation: n = c/v where c is the speed of light in vacuum (Eq.

2.15). The complex dielectric constant is given by equation (2.16).

(2.15)

(2.16)

47

The relation between refractive index and dielectric constant is:

√ (2.17)

Where,

, (2.18)

and k is the extinction coefficient which describes the attenuation of light wave as it goes through

a material. k is directly correlated to the absorption coefficient (α) of a medium and is shown as:

(2.19)

where λ is the wavelength of light in vacuum.

The transmittance (T) is then given by:

(

) ( ) (2.20)

and

(

)

(( ) ) (( )

) (2.21)

no, n and n1 are the refractive indices of air, film and substrate, respectively and t is the film

thickness.

As for the optical spectra, the dielectric constant also consists of three regions. In the UV region,

the imaginary part of the dielectric constant increases sharply. In the visible region, the

imaginary part approaches zero and in the IR region, increases while, decreases until it

crosses zero at the plasma frequency and strongly reaches its negative value. Figure 2.6 shows

the complex refractive index and dielectric constant of an ITO film.

48

Figure 2.6 Complex refractive index (left) and dielectric constant (right) of ITO thin films [145]

Table 2.2 Comparison of some important binary TCOs and their fabrication techniques and

relevant optoelectronic properties

Species Deposition method ρ ×10-4

(Ω.cm) T% Ref.

ITO PLD 0.77 85 [259]

ITO DC sputtering 1.5 92 [260]

ITO RF sputtering 1.29 85 [189]

ITO Sol-gel 3 80 [261]

SnO2:F CVD 5.2 80 [78]

SnO2:Sb RF sputtering 20 85 [234]

SnO2:N RF sputtering 9.1 80 [262]

ZnO:Al PLD 0.85 88 [85]

ZnO:Al RF sputtering 2 80 [86]

ZnO:Ga PLD 0.51 85 [84]

ZnO:Ga RF sputtering 2.7 85 [83]

TiO2:F PLD 16 95 [263]

49

2.2. Sol-Gel Spin Coating Process, Fundamental, Theory and Advantages

2.2.1. General Overview

Many techniques have been proposed for fabricating high quality thin film coatings. But, these

generally fall into two main categories, depending on whether the process is physical or chemical

[264]. Physical vapor deposition produces a thin film coating by means of thermodynamic,

mechanical or electromechanical processes. Common physical deposition systems require a low

pressure environment along with a complicated controlling system to functionalize thin film

coatings properly. Examples of physical vapor deposition techniques include, thermal

evaporation, sputtering, electron beam evaporation, pulsed laser deposition and molecular beam

epitaxy [265]. Chemical deposition processes can involve gas-phase or solution-phase

precursors. Gas-phase precursor-based processes include chemical vapor deposition (often

involving a hydride or halide elements) and plasma enhanced chemical vapor deposition (using

ionized vapor as precursor). The solution-phase precursor-based process is also known as wet

chemical deposition, and the prepared thin films tend to be conformal. Examples of wet chemical

deposition processes include, electroplating, spray pyrolysis and sol-gel methods [242]. Among

these wet chemical processes, sol-gel is of great interest and widely used because of its low

temperature and ease of handling. In the next sections, the fundamental, theory and advantages

of the sol-gel process are discussed.

2.2.2. Sol-Gel Process

Fabricating thin film coatings via a sol-gel process represents the oldest application of this

technique. The first patent on sol-gel thin film formation was granted to Schott and Gen in 1939

for their dip coated silicate thin film. Sol-gel derived thin film coatings of: mirror coatings have

been in production since 1953, antireflection coatings since 1964 and design coatings since 1969

[266]. Over the past few decades, sol-gel thin film coatings have been widely considered for a

variety of additional applications including, optical coatings [267, 268], high and low dielectric

constant thin films [269], sensors [270], electro-chromic coatings [271], semiconducting anti-

static coatings [272], strengthening and protective layers [273, 274], superconducting films

[275], planarization and passivation layers [276] and ferroelectric coatings [277, 278]. The sol-

50

gel process is mainly used for the production of uniform metal oxides thin film coatings using

metal alkoxides precursors. Samples fabricated via this process normally do not require

annealing temperature as high as those fabricated via conventional techniques such as sputtering,

thermal evaporation, chemical vapor deposition, to name a few. This makes the sol-gel process

suitable for fabricating samples that are unstable at high temperatures. Compared to other

conventional thin film fabricating techniques, this process requires considerably less equipment,

is generally very simple, cost effective and free of difficult technical challenges that may result

from using specialized equipment. This process also offers many other advantages [279]:

Low processing temperature that allows the use of volatile components.

The intimate mixing of the starting precursors, results in a high degree of coating

homogeneity.

Dopants can be easily introduced and controlled even at a trace level.

The viscosity and thereby the film thickness can be adjusted by adequate choice of

concentration, solvent, chelating agent, etc.

Large area sample coating along with desired thickness and composition are obtainable.

The sol-gel method can be classified based on the solvent type into two different routes, aqueous

and non-aqueous processes [280]. In aqueous sol-gel process, oxygen which supplied by water is

essential for the formation of metal oxide. The key aspects such as hydrolysis, condensation and

drying which are take place concurrently leading to some difficulties including, controlling the

particle morphology and reproducibility of the final protocol during the sol-gel process. In non-

aqueous sol-gel process, oxygen required for the formation of metal oxide is supplied from the

organic solvents such as ketones, alcohols, aldehydes and/or by the metal precursors. These

organic solvents not only supply the oxygen but also provide a versatile tool for controlling

many other parameters such as surface properties and morphology, crystallite size and

composition of the resultant metal oxide [281].

Non-aqueous process can also be divided into solvent- and surfactant- controlled approaches

[282]. Surfactant-controlled processes include the transformation of the precursor species into

the oxidic compound in the presence of stabilizing ligands in a specific temperature range of 250

to 350 °C. An alternative to surfactants is the use of organic solvents that act as reactant as well

51

as control agent for particle growth, enabling the synthesis of high purity nanomaterials in

surfactant-free medium. In comparison to the synthesis of metal oxides using the surfactants, the

solvent-controlled approaches are simpler because the initial reaction mixture only contains the

metal oxide precursors and a common organic solvent. The synthesis temperature is in the range

of 50 to 200 °C, which is also lower than in the surfactant-controlled routes. Despite solvent-

controlled approaches result in some agglomeration, the dispersibility properties of the

nanoparticles can be improved by post-synthetic steps [283].

Preliminary syntheses are often carried out under ambient conditions, usually without any special

environment. Initial precursors are mixed together in a solution and interact easily to form a

homogeneous mix [284]. This process comprises the conversion of a colloidal solution of

molecular precursors via chemical reactions to a sol or gel through drying followed by post

annealing at high temperature. This assists the transformation of formed sol/gel into a

polycrystalline structure thin film [285]. Typical sol-gel processes for thin film coatings

fabrication consist of three main steps: (a) preparation of the precursor solution, (b) depositing of

the prepared solution onto the substrate and (c) annealing the xerogel film. The xerogel is

defined as the dried gel at ambient conditions, while the aerogel is the dried gel in supercritical

conditions [286].

2.2.3. Sol-Gel Chemistry

The sol-gel process comprises the use of metal-organic or solid inorganic precursors in a solvent

which forming a colloidal dispersion. The precursors are hydrolyzed and condensed in the

organic solvent resulting in inorganic polymers that consist of oxo (M-O-M) bonds. For many

inorganic precursors, the hydrolysis proceeds via a proton removal from an aquo ion

[M(OH2)n]z+

resulting in the formation of an oxo (M=O) or hydroxo (M-OH) ligand (M=metal)

[287]:

[ ] [ ]

( )

(2.22)

Condensation reactions of hydroxo ligands give rise to the formation of inorganic polymers in

which metals are bridged by oxo (M-O-M) or hydroxyl (M-μ(OH)-M) bonds [12]. The most

often used metal organic precursors are metal alkoxides M(OR)z, where R is an alkyl group

CxH2x+1. Usually the alkoxide is dissolved in alcohol and hydrolyzed by adding water under

52

acidic, neutral or basic conditions. Hydrolysis leads to the substitution of an alkoxide ligand by a

hydroxyl ligand [12]:

( ) ( ) ( ) (2.23)

Condensation then follows a similar pattern to that of hydrolyzed inorganic precursors. Basically,

the preparation of a sol-gel solution consists of the use of metal organic or inorganic precursor,

aqueous, organic and/or alcoholic solvent, along with the addition of an acid/base as a catalyst

[288]. The structure of the growing polymers for both metal-organic and inorganic precursors

depends on the functionality of the metal and the degree of hydrolysis [289].

2.2.4. Film Formation

Several deposition options for sol-gel technique include dip-, spin-, spray-, roll- and flow-coating

used to manufacture various materials in different forms: powders, fibers, monoliths and/or thin

films [290]. Dip-coating and/or spin coating are the major processes adopted for manufacturing

coatings via the sol-gel technique. Additionally, the sol-gel process facilitates the fabricating of

multi-component film coatings with a complex structure [291].

2.2.4.1. Dip-Coating Process

After preparing the desired precursor solution, the substrate is dipped vertically into the solution

for a certain time (dwell time). Then the coated sample is withdrawn slowly and steadily. Both

the dipping rate and withdrawal rate are to be considered and maintained in the film fabrication.

The withdrawn sample is dried for a short period of time on a hot plate or in a conventional oven,

and then the coating procedure repeated until the desired thickness is achieved. Finally, thermal

treatment is applied on the coated sample. Therefore, the sol-gel dip-coating process can be

summarized in the following main stages: substrate preparation, sol preparation, dipping,

withdrawing the coated sample and annealing.

53

2.2.4.2. Spin Coating Process

Spin coating is the predominant process employed for fabricating homogeneous and uniform thin

film coatings with thicknesses of the order of nanometers or micrometers. The first attempts of

the spin coating process were performed by Emslie and co-others [292] who considered the

spreading of an axisymmetric thin film coating of Newtonian sol onto a flat substrate spinning

with a constant angular velocity. Usually, the coating material is applied onto the substrate in the

form of a sol from which the solvent vaporizes [293]. In this process, centrifugal force spreads

the solution radially outward while the surface tension and viscous force result in a thin film to

be retained on the planar substrate. The spin coating process can be classified into four major

steps including, deposition of the sol, spin up, spin off and film drying [294]. The deposition of

the sol, spin up and spin off occur consecutively, while the drying step occurs throughout the

process. Figure 2.7 shows the main steps of spin coating process.

Figure 2.7 The main steps of spin coating [295]

54

2.2.4.3. Deposition of the Solution via Spin Coating

Different techniques have been used to apply the solution onto the substrate and depend on the

type of spin coating machine. Some machines have specific electronic injectors where the

amount (volume) of solution can be controlled and maintained while the others are use syringes

or dropers for this purpose. In the deposition step, the solution can be applied in different ways

[296]:

(1) As a bolus on the center of the substrate with the solution flowing over the rest of it.

(2) In the form of heavy rain that floods the substrate.

(3) As a continuous stream at the center of the substrate with the solution spreading outward over

the substrate.

(4) As a continuous stream from a raised distribution port that moves radially over the substrate.

The different ways for applying the solution onto the substrate depend on the concentration and

viscousity of the sol, which can be achieved either by pouring the sol onto the substrate for a

period of time before spinning or introduce the sol while the substarte is rotating at a constant

angular velocity. In both cases the amount of solution deposited is such that the substrate top

surface receives a high enough excess of the solution to be completely covered [297].

2.2.4.3.1. Spin Up

This stage starts when the substarte accelerates up to the final desired rotation velocity resulting

in an aggressive sol rejection from the substrate. Spiral whirlpools may exist during this step due

to the initial depth of the sol on the surface of the substrate. These spiral whirlpools form because

of the twisting motion caused by the inertia that the top of the sol layer wields while the substrate

below rotates very fast. Eventually, the resultant film thickness is thin enough and completely

uniform and homogenuous to be co-spinning with the substrate at the desired speed, where the

viscosity balances the spinning accelerations. The final film thickness is found to be independent

of the way of sol dispensing, the amount of sol applied to the substrate and the acceleration level

of the substrate to the final rotating velocity [298, 299]. The resultant film thickness might

55

depend on the aforementioned conditions if the amount of sol is insufficient to fully cover the

substrate surface or if the substrate accelerates slowly [300]. Due to the importance and

complexity of the spin coating process, a huge effort has been focused on establishing

experimental relationships that describe the dependence of spin coating technique on process

factors. The main result of these studies was that the final film thickness (t) was found to be

proportional to a function of the initial solvent concentration and the power of the spinning speed

[301, 302] as seen in the following relation:

( ) (2.24)

where C0 is the initial solvent concentration, ω is the spinning speed and b was found to be -1/2

and it depends on the viscosity level and evaporation conditions.

2.2.4.3.2. Spin Off

This step starts when the substrate is rotating at a constant speed and the viscous force dominates

the sol thinning behavior. The centrifugal force works to drive the sol outward off the edge of the

substrate obstructed only by the viscous force. The radial drift rapidly decreases because the sol

becomes very thin and solvent evaporation increases the viscosity by several orders of

magnitude. Therefore, it is convenient to describe the spin coating process by two discrete

mechanisms with an abrupt conversion from pure centrifugal sol-drift sol flow to pure solvent

evaporation [293, 303-306]. The idea behind this separation is that the film thickness is

controlled first by the centrifugal force and second by the solvent evaporation [295, 307] due to

the strong concentration dependence of the viscosity, the solvent evaporation may play an

important role through the flow dominated system. It was reported by Jenekhe [308] that the

main effect of solvent evaporation is through altering rheological characteristics of the regime.

2.2.4.3.3. Drying of the Film

This step starts when the spin off step ends. Through this step, the centrifugal solution out flow

halts and further shrinkage of the film arises due to solvent loss alone. As mentioned before, the

solvent concentration profile depended on solution convection flow, the cross term in the solvent

conservation equation (see Eq. 2.23). Nevertheless, the solvent conservation could be considered

56

independently when the spinning speed reaches zero and the solvent concentration profile

dependence becomes unimportant. At this stage, the solvent evaporation rules the film thinning

behaviour. Through the drying step, the dissolved or suspended materials become concentrated at

the surface of the sol so as to produce a low diffusivity, high viscosity or solid skin. This leads to

the formation of a thin film onto the substrate.

2.3. ITO Thin Films Synthesis

Thin ITO films preparation consists of four main steps: (1) preparation of the glass substrates, (2)

sol-gel solution preparation, (3) thin film deposition and (4) thermal treatment at different

temperatures of the deposited thin films. Soda-lime glass substrates were used in this study,

which were cleaned carefully by using ethanol and DI-water and then sonicated in an ultra-sound

bath. The concentration of ITO solution was maintained at (In:Sn = 90:10 at.%) in preparing all

pure, doped and incorporated ITO thin films. Details of thin film deposition process (spinning

speed and spinning time) using a Polos spin-coater was varied based on the type of dopant and/or

incorporated materials used with ITO. In general, the deposition steps consist of: dispensing the

prepared solution onto rotated substrate, spreading the solution and drying the thin film, followed

by the calcination of the thin films on a hot plate at a suitable temperature. These steps were

repeated until the desired film thickness was achieved. Annealing was performed on the resultant

thin films. Details of the specific experimental conditions for fabricating ITO based transparent

thin film coatings are provided in subsequent chapters. Figure 2.8 shows the flow diagram of the

sol-gel spin-coating procedure.

57

In(NO3)3.5H2

O

Ethanol

Refluxed separately

Mixed and refluxed

SnCl2.2H2O

Doping or

incorporating

material

Ageing for 24 hours

Film deposition by

spin-coater

Drying

Post annealing

Resulted thin films

Figure 2.8 Flow diagram of sol-gel spin-coating process

58

2.4. Instrumentations and Characterizations Techniques

Since nanoscale materials are quite different from their bulk counterparts, high precision devices

and instruments are needed to study, quantify and characterize these kinds of materials. The

crystalline structure and film coating surface were confirmed by XRD, XPS and EDX, whereas,

the surface morphology and topography were imaged by SEM, FESEM and AFM. The electrical

properties were investigated by using 4-point probe method and Hall-effect measurements, while

the optical properties were characterised by UV-VIS spectroscopy. The mechanical properties

were determined by a nanoindentation technique. The instruments and the characterisation

techniques used in this study will be briefly discussed in the following sections.

2.4.1. XRD Measurements

XRD is one of the oldest and most effective analysis techniques for investigating the

crystallographic and atomic arrangements in a crystal structure [309]. X-ray wavelengths are

found to be of the same order of magnitude as the lattice constants of the crystals [310]. XRD is

a non-destructive tool which works basically on the interaction between the periodical crystal

lattices and the incident monochromatic X-ray radiation. This interaction happens when the X-

ray beam with a certain specific incident angle (θ) and wavelength (λ) hits the surface of a

crystalline specimen yielding a reflected radiation pattern. Therefore, intense peaks (which are

known as Bragg peaks) are produced from the reflected radiation. Bragg expressed this result by

suggesting that each crystal generally consist of discrete parallel planes separated by constant

distance named as inter-planar d-spacing. It was assumed that, Bragg peaks occurred if the

reflected X-ray radiation from different crystal planes interfered constructively according to

Bragg’s law [311].

(2.25)

where n is an integer determined by the reflection order, λ is the wavelength of the incident X-

ray radiation, d is the inter-planar distance (d-spacing) and θ is the angle between the incident

radiation and the surface of the specimen which is also called (Bragg’s angle). Figure 2.9 shows

the incident and the scattered X-ray radiation.

59

Figure 2.9 Incident and scattered X-rays in XRD analysis [309, 312]

It is believed that most materials are not single crystals but consist of billions of small crystallites

randomly orientated and are referred to as polycrystalline structures or powder form. Therefore,

when these materials are examined by X-ray radiation, many inter-atomic planes from different

crystallites will be realized. However, the diffraction from each plane will take place at specific

diffraction angles (θ).

2.4.1.1. Analysis of XRD Data

The value of 2θ is estimated for each diffraction peak after the XRD pattern appears and the d-

spacing (dhkl) values calculated from 2θ values via Bragg’s law. These values are used to derive

crystallographic parameters of thin films such as, identification of phases, calculation of the

crystallite size, lattice parameter determination, etc.

2.4.1.2. Identifications of Phases

The XRD technique has proven itself a powerful tool to identify the phases of elements,

compounds and materials, as well as, provide important information on the dimensions of the

unit cell. In a crystalline structure, electrons cause a scattering to the incident X-ray radiation

resulting in XRD pattern with maxima and minima peaks following Bragg’s law. From the XRD

pattern, the position and the intensity of the peaks, the characteristics of crystal structure and

atomic composition of material can be determined. By comparing the positions of Bragg’s peaks

with standard crystalline XRD cards, the phase identification and existence of multiple phases in

60

a compound can be established and quantified. Furthermore, the XRD approach can also afford

precise results for lattice parameters, unit cell volume, constituents of the coatings and crystal

symmetry with atom positions.

2.4.1.3. Determination of Crystallite Size from XRD Data

The Debye-Scherrer equation is derived from Bragg’s law and can be used to calculate the

crystallite size as follow:

(2.26)

where, K is Scherrer constant (constant of proportionality) which is depends on the shape of the

crystal, β full width at half maximum (FWHM) of Bragg peak and λ wavelength of X-ray beam.

2.4.1.4. Determination of Lattice Parameter

The lattice parameters of a crystal structure can be easily calculated by using Bragg’s law to

determine dhkl value, the standard Bravias lattice relation for a cubic system and Miller indices

(hkl) related to the reflection orientation as follow:

(2.27)

where λ = 0.154 ± 10-5

nm for Cu K radiation source, d inter-planar spacing and a lattice

constant

2.4.1.5. X-ray Diffraction Measurements Technique

The X-ray diffraction data can be obtained by either reflection or transmission geometry. In this

study, the XRD data was collected using a GBC EMMA X-ray Diffractometer supplied with

LynxEye detector which collects the data in reflection mode. The XRD analysis was conducted

at room temperature using a Cu-Kα radiation with a primary beam power of 35 kV and 28 mA.

Cu-Kα primary beam radiation has a wavelength of 1.5406 Å. The fundamental diffraction peaks

of the samples were measured over a 2θ range from 20° and 70° with a step size of 0.02° and a

61

scanning rate of 2°/min. Both receiving and divergence slits were used to control the output

intensity and the irradiated beam area from the sample. In order to get parallel X-ray beams, two

solar slits were used; one after the tube and the other in front of the detector. QualX program was

used for the XRD analysis in this study by inquiring the PDF-2 and PDF-4 databases and the

new freely POW-COD database which has been developed by the authors of QualX program.

The POW-COD database was generated based on the structure information contained in the

Crystallography Open Database.

2.4.2. X-ray Photoelectron Spectroscopy (XPS)

XPS is well known as a powerful surface analysis tool that yields information over 5 – 10 nm

probing depth, making it an accurate surface analysis technique. For instance, XPS analysis

offers detection limits of 0.2-0.4 at.% for nitrogen, oxygen and carbon, 0.03 at.% for lighter

transition metals and 0.001-0.005 at.% for heavy metals [313]. XPS is based on the photoelectric

effect to investigate the electronic structure and the chemical composition of sample materials. It

was first developed by K. Siegbahn and his research group in mid the 1960s. The fundamental

idea of XPS phenomenon is the discharge of electrons from a surface which has been bombarded

by energetic photons. Due to the short range of photoelectrons released out of bombarding

surface, XPS is considered a highly sensitive surface analysis technique. XPS analysis is also

defined in some literature as electron spectroscopy for chemical analysis (ESCA). XPS is the

most common surface analysis technique to determine the chemical and electronic bonding states

and the elemental composition of surfaces. XPS surface analysis is made by using soft X-rays

(i.e., monochromatic photons of energy (~200 - 2000 eV) to investigate the inner shell (core

level) electrons. The most widely used X-ray sources are Al Kα radiation (hν = 1486.6 eV) and

Mg Kα radiation (hν = 1253.6 eV). By illuminating the atoms of a surface, placed in an ultrahigh

vacuum system, by a monochromatic soft X-ray photons, a photoelectron in an inner shell is

ejected with a kinetic energy (Ek) expressed by the following relation [314]:

(2.28)

62

where is the kinetic energy of X-ray photon, is the binding energy of the orbital from

which the electron is ejected and is the work function of the sample. The emitted

photoelectrons are moved through a hemispherical photoelectron energy analyser with energy

governed by electrostatic fields prior to hitting the detector.

A typical XPS spectrum is obtained by plotting the electron count rate (y-axis) versus binding

energy (x-axis). The specific binding energy indicates the element while the intensity of the peak

is related to the concentration of the element in the specimen. Each element has a specific

binding energy that relates to each core atomic orbital, resulting in a series of separate peaks in

the photoelectron spectrum [315]. The composition of the sample can be obtained by utilising the

peak height and the peak area, while the variations in the oxidation states and/or the structural

configuration of the elements can be recognized by noting the shifts in the binding energies of a

particular core level. Therefore, the elemental identity, elemental composition, chemical states

and electronic states of the studied surface area of the sample can be analysed by the XPS

technique.

2.4.2.1. Applications of XPS Technique

XPS analysis technique is mostly used in different types of material characterizations including:

- Identification of the stoichiometry of the coatings,

- Changes in surface composition of materials due to absorption, heating and radiation,

- Detection of surface contamination,

- Identification of the interface properties of nanomaterials,

- Studying multilayer film structure,

- Identification of material corrosion and degradation,

- Identification of interface reactions and diffusions, and

- Determination of the problems of protective coating caused by segregation of materials.

63

2.4.2.2. Experimental Procedure of X-ray Photoelectron Spectroscopy (XPS) analysis

XPS measurements were carried out via a kratos axis ultra-X-ray photoelectron spectrometer

installed at Curtin University. The samples were mounted horizontally on steel sample holders

using double-sided sticky copper tape and the probed surfaces were normal to the electrostatic

lens. A charge neutralizer was used to balance the electron shortage on the sample surface during

the measurements. The analyser entrance and electrostatic lens mode of the XPS machine were

set to slot mode and hybrid (Irs = 0.6 and aperture = 49), respectively. The background pressure

of the analyser chamber was less than 10-9

Torr, which was maintained during data collection.

The voltage and current of X-ray beam radiation were 12 kV and 12 mA, respectively. Primary

survey scans were probed with pass energy of 80 eV, while high resolution scans were carried

out with pass energy of 10 eV and step size of 0.1 eV.

The peak positions in XPS spectra are affected by both the sample surface and the spectrometer

conditions. Therefore, calibration of the binding energy before XPS peak identification is

important to overcome the charging effects. This is commonly done by using the C 1s peak of

saturated carbon (C-H or C-C) at a binding energy of 284.6 eV. The conducted XPS data was

analysed via CASA XPS software (version 2.3.15) with Shirley background subtraction.

2.4.3. Scanning Electron Microscopy (SEM), Energy Dispersive Spectroscopy (EDS) and

Field Emission Scanning Electron Microscopy (FESEM)

Scanning electron microscopy (SEM) analysis is one of the most powerful electron microscope

techniques. SEM images materials having microscopic structure by scanning the surface of the

specimen with much greater depth and much higher resolution than confocal microscopes. This

technique is used to image the morphology and the surface microstructure by scanning the

surface of the sample with a well-focused high energy electron beam. Based on the interactions

between the primary electron beam, which is generated from an electron gun or filament and the

sample, a number of signals such as secondary electrons, back scattering electrons and X-rays

will be produced [316]. Both secondary electrons and back scattering electrons are of great

interest because they provide information about the microstructure and the morphology of the

scanned area. A photomultiplier detector is used to detect and record these signals to regenerate

64

an image for the surface of interest which is displayed on a screen. Figure 2.10 shows a

schematic diagram of SEM technique.

Figure 2.10 Schematic diagram of SEM technique with the CRT screen [316]

Both elemental compositions and thickness of a thin film deposited on a substrate can be

accurately determined by measuring different X-ray intensities at various accelerating voltages.

When the accelerated electron beam interacts with atoms in a sample, the energy generated by X-

rays is dependent on the atomic structure. In other words, each atom shows a characteristic X-ray

emission spectrum. Therefore, these X-rays are characteristic of the elements present in the

sample. Also, these rays can be separated in the energy spectrum to identify the elemental

composition of the sample. This phenomenon is known as Energy Dispersive X-ray spectroscopy

(EDX). This technique provides quick analysis of the elemental composition of the sample

because a spectrum in the range of 0.1 to 20 KeV can be recorded in a short time (10 ~ 100 s).

2.4.3.1. Experimental Technique of SEM

SEM has been widely used in elemental and morphological analysis of materials. It also provides

very clear images of samples that are outside the capabilities of commonplace microscopes. An

65

SEM image is generated by a focused electron beam which hits and probes over the surface of a

sample. The most important advantage of SEM is its ability to form three-dimensional images

due to its great depth of field which can reach the order of micrometres at 104 × magnification.

Also, the SEM technique is relatively simply operated and maintained, compared with

transmission electron microscopy (TEM). In the present study, a JCM-6000 scanning electron

microscope supplied with an EDX analysis column was used to determine thin film thicknesses

and the elemental compositions of the samples.

FESEM is similar to a SEM instrument, except that the FESEM is supplied with an extra field

emission cathode in the electron gun. The electrons ejected from the cathode are accelerated by

high electric field then focused and deflected by specific electronic lenses in a high vacuum

column yielding a narrow beam that hits the specimen surface. Accordingly, secondary electrons

are released from the specimen with velocities and angles related to the surface structure of the

specimen. The FESEM technique provides better images compared with SEM due to the sharper

probing beam at high and low electron energy, yielding both enhanced resolution of the image

and minimising the damage and charging of the sample [317]. In this work, surface

morphologies of synthesized thin films were examined via a PHILIPS XL 20 SEM equipped

with a Zeiss Neon 40EsB FESEM with a maximum extra high tension (EHT) voltage field

emission gun of 30 kV. SE2 detectors and InLens were used to obtain FESEM images at

different magnifications. For FESEM analysis, samples were mounted on a sample holder using

double-sided carbon tape and coated with platinum (3 nm thick) to minimise the charging effect

prior to imaging.

2.4.4. Atomic Force Microscopy (AFM)

Atomic force microscopy is a common and powerful tool to image and analyse the roughness

and topography of coating surfaces. In comparison with the optical and electron microscopes that

image the coating’s surface, AFM involves measuring the deflection of a sharp force-sensing tip

in contact with the surface. This sharp tip is usually made from Si or SiN. The changes in the

coating’s surface contours result in moving the cantilever vertically (i.e., z-axis) for each (x, y)

point. A four segment-photodiode detector is used to monitor the changes in the deflection of the

66

cantilever and these changes are recorded and processed by computer to create a topographic

image for the coating’s surface [318].

In this study, the AFM images of the fabricated thin films were obtained via a Bruker Dimension

(Santa Barbara, CA) fast scan atomic force microscope. The thin films were mounted on

adhesive tape prior to AFM scans, and the images were performed in tapping mode using a

standard tapping mode probe (type NCHV with resonant frequency of 300 kHz and spring

constant of 40N/m).

2.4.5. Raman Analysis

Raman spectroscopy is a common vibrational spectroscopy technique that is used for material

characterization. It provides specific structural information at the atomic scale on organic and

inorganic compounds [319]. Raman Effect, discovered by C. V. Raman in 1928, is based on the

phenomenon of electromagnetic radiation scattering (Raman scattering) by molecules of the

material [320]. When a photon of light of single wavelength interacts with a molecule in a

sample, the photon may be absorbed and/or scattered. The absorption process occurs when the

resonance condition is satisfied (i.e., the energy of the incident photon and the energy difference

between two states of the molecule are equal), and transitions between these states are

accompanied by a change in the dipole moment of the molecule. Photons which are not absorbed

will be scattered elastically and/or in-elastically. Elastic scattering occurs if the scattered photon

has the same energy (frequency) and therefore, wavelength as that of the incident radiation. This

phenomenon is commonly known as Rayleigh scattering [321]. Inelastic scattering, referred to as

Raman scattering, occurs when the energy of scattered and incident radiations are no longer

equal. In Raman scattering, the molecule may either lose energy, or gain energy from the photon

of light. When the scattered photon has energy lower than the incident photon, this phenomenon

is known as Stokes Raman scattering. Conversely, when the scattered photon has energy higher

than the incident photon, this phenomenon is known as anti-stokes Raman scattering. When a

molecular vibration encourages a change in the polarizability of molecule’s electric field,

vibrational Raman scattering takes place [322]. The selection rules for Raman active vibrational

modes are related to molecular symmetry and enable detection of vibration modes which alter

67

the polarizability of the molecule. A Raman spectrum can be obtained by plotting the intensity of

Raman scattered photon versus its frequency difference from the incident photon (Stokes-shifted

frequencies). This difference in frequency is referred to as Raman shift, which is independent of

the frequency of incident photon. Frequency shifts between the Raman scattered photons and the

incident photon relate to the vibrational energy levels of the crystal and/or molecule. Several

parameters such as symmetry of the crystal, molecular geometry, structural disorder, crystal

strain, atomic mass, bond order, local environment and atomic environment may affect the

vibrational energy. Therefore, Raman spectroscopy can help to investigate the crystal lattice

vibrations, intra-molecular vibrations and other types of motions of extended solids [322].

2.4.5.1. Experimental Technique of Raman Analysis

In this study, Raman analysis was conducted using a Nicolet 6700 Fourier transform infrared

(FT-IR) spectrophotometer equipped with NXR FT-Raman module. Raman spectra were

collected using a Helium-Neon (He-Ne) monochromatic laser with 1064 nm excitation

wavelength and 0.204 W operation power, CaF2 beam splitter, InGaAs detector with 90°

detection angle, optical velocity of 0.3165, gain of 1, aperture of 150, and 16 scans with

resolution of 8 cm-1

in the range of 0 - 4000 cm-1

.

2.4.6. Optical Properties of Thin Films

The optical characteristics of TCO thin film coatings are very important in optoelectronic

applications. Thin film coatings are classified as transparent if they show a typical transparency

greater than 80% in the entire visible range of the electromagnetic spectrum. Transmission or

transparency is defined as the ratio of transmitted light to the incident light. The optical

properties of a solid can be categorized into three phenomena named as transmission, reflection

and absorption [323]. These interactions depend on the nature of the solid matter and the

wavelength of the incident light. The amount of light transmitted mainly depends on the

reflectivity from the front and back surface as well as the way the light is absorbed within the

medium [324]. The optical properties of ITO thin films prepared in this study were characterized

by means of optical transmittance measurements. The main optical properties included optical

band gap energy and optical constants such as refractive index, extinction coefficient and

dielectric constant that cannot be directly obtained from the measurements.

68

2.4.6.1. Measurements of Optical Transmittance via UV-Vis Spectroscopy

In this study, a Perkin Elmer lambda 650 UV-Vis spectrophotometer was used to collect the

transmittance data of the prepared thin films. This instrument consists of a double beam light

source, double monochromators with a diffraction grating, ratio recording optical system and

detector (photodiode). A halogen lamp is often used as the radiation sources to cover the working

wavelength range (190 - 900 nm). All reflecting optical components of this instrument are coated

with silica (SiO2) for durability. Prior to each optical measurement, light intensity calibration

was performed and comprised in taking a baseline spectrum without sample in the measurement

chamber (air blank) then uncoated substrate was also measured as a reference. This calibration

process facilitates the suppression of residual noise. The transmittance spectrum of the film

coatings at a desired wavelength was then conducted and compared with the reference one

(uncoated substrate). For a transmission measurement, the coated side of the sample was

irradiated by incident monochromatic light perpendicular to the sample surface. The optical

transmittance data presented in this study consists of the transmittance of both the thin film

coating and the substrate.

2.4.6.2. Band Gap Calculations

Transmission data can be used to determine thin film optical band gap energy. Jan Tauc

reported that the transmittance data of a thin film coating can be used to calculate the absorption

coefficient (α) of a coating along the entire wavelength. Plotting (αhυ)1/n

versus photon energy

results in a Tauc plot, which shows the linear drop of the absorption edge. The value of n

depends on the type of electron transition whether it is direct (n = 1/2) or indirect (n = 2). By

extrapolating the linear part of Tauc plot toward X-axis, the optical band gap energy is obtained

[37]. Figure 2.11 shows a typical Tauc plot for estimating the band gap energy.

69

2.4.7. Electrical Properties of Thin Film Coatings

The electrical performance of the thin film coating is evaluated by means of electrical resistivity

(given in Ω.cm) or sheet resistance (given in Ω/sq.) of the film. Figure 2.12 shows the geometry

defining the sheet resistance of a thin film with thickness t. The sheet resistance is then given by

the following relation [325]:

(2.29)

where t is the film thickness and A is the effective cross-sectional area (A=b×h).

t

b

I

b

3 3.1 3.2 3.3 3.4 3.5 3.6 3.7 3.8 3.9 4

Photon energy (eV)

( 𝛼 𝑣) ( 𝑐𝑚 𝑒𝑉

)

Figure 2.11 Typical Tauc plot for estimating the band gap energy of a thin film

Figure 2.12 A geometry defining the sheet resistance calculation

70

Electrical conductivity is the inverse of resistivity (σ = 1/ρ) and is expressed in Siemens per

meter (S/m). In this study, the electrical properties including resistivity, conductivity, carrier

concentration and Hall mobility of the synthesized thin films were investigated by means of four-

point probes and Hall effects measurements. Since the electrical properties of the material are

temperature dependent, all the electrical measurements adopted in this work were carried out at

room temperature.

2.4.7.1. Four-Point Probes Measurements

The 4-point probe system is a powerful tool commonly used to measure the electrical resistivity

and sheet resistance of semiconductor materials. Compared to traditional 2-point probe, 4-point

probe gives more accurate results of material resistivity because it eliminates the contact and

probe resistance. This device generally comprises of four uniformly spaced probes organized in

linear configuration (see Figure 2.13). The spaces between each two neighbouring probes are

1mm. An electric current is passed through the two outer probes, while the voltage recorded

from the two inner probes, which allows the measurement of the film resistivity, provided the

thickness is known. The electrical resistivity is given by the following relation:

( ) (

) (2.30)

where t is the film thickness, V is the voltage across the inner two probes and I is the current

passed through the outer two probes. For accurate resistivity measurements, the four probes must

perfectly attached the film surface and the contact diameter should be smaller than the spaces

between the probes [325].

71

Figure 2.13 Configuration of four point probes measurements [326]

2.4.7.2. Hall Effects Measurements

Hall effect is one of the most commonly used methods for characterizing electrical properties of

thin film coating and materials including charge carrier concentration, carrier mobility and type

of conductivity ( n- or p- type). The Hall Effect is the product of a transverse voltage difference

(Hall voltage VH) resulting from the deflection of the charge carrier by Lorentz force, when an

electric current flows through a conductor placed in a magnetic field [327]. The current and

magnetic field are perpendicular to each other and the resultant Hall voltage is perpendicular to

both of them. Figure (2.14) shows the scheme of Hall Effect. The sign of the Hall voltage,

whether it is positive or negative, reflect the induced Lorentz force direction as well as the carrier

type. The Hall voltage is expressed by the following relation:

(2.31)

where I and B are respectively the electrical current and the magnetic field, n is the carrier

concentration (density), e is the electron charge and t is the film thickness.

72

Figure 2.14 A schematic diagram of Hall Effect measurements [279]

Therefore, by calculating VH from identified values of B, I, q and t, the carrier concentration can

be determined. The Hall mobility is given by the following relation:

(2.32)

where Rs is the sheet resistance, which is determined via four-point probe.

Thus, a combination of Hall measurements and resistivity is needed to obtain both the carrier

concentration and hall mobility. This is known as the Van Der Pauw method [241].

2.4.8. Mechanical Characterization: Nanoindentation Measurements and Finite Element

Modeling (FEM) of Thin Film Coatings

Determination of the mechanical characteristics of different types of thin film coatings are very

important for many electronic and optoelectronic systems. Mechanical characteristics including

hardness (H) and Young’s modulus (E) can identify strengthening and deformation mechanisms

73

on a small scale [328]. In addition, the hardness and Young’s modulus are important aspects to

estimate the wear resistance of thin film based transparent conductive materials. The stability and

the performance of thin film coatings could be maintained by monitoring the wear resistance

behaviours of the film coatings [329]. Nanoindentation is a well-established technique performed

to investigate the mechanical properties of thin film coatings at a nanometre scale. A load (P) is

applied through a diamond head, initially in contact with the film surface. Both load and

penetration depth or displacement (h) are controlled during the loading and un-loading

measurement processes, and the penetration depth of the diamond head into the film coating is

recorded, along with the applied load. Different parameters such as hardness, Young’s modulus,

residual stress and fracture toughness can be investigated from the load-penetration depth curve

(P-h curve) [330]. The most important part in the nanoindentation system is the indenter which

has to have well-known geometry and high precision. Most nanoindentation systems use a

diamond indenter with specific geometry such as flat-ended cylindrical, spherical or sharp. Of

these, the sharp indenter (including Vickers, cube corner and Berkovich) is preferred, especially

for nano and micro-scale measurements of thin films.

Oliver and Pharr proposed that the hardness and Young’s modulus could be determined directly

from a set of indentation load and penetration depth (p-h) measurements, recorded in both

loading and unloading modes without the need to image the hardness impression [331]. Three

important quantities can be estimated from the p-h plot: maximum load (Pmax), the maximum

displacement (hmax) and elastic unloading stiffness (S) (also known as the contact stiffness),

which is represented by the slope of the higher part of the unloading curve within the early stage

of unloading (See Figure 2.15). The hardness which is defined as a measure of how resistant a

solid is to deform permanently under an applied compressive force can be calculated by the

following relation:

(2.33)

where Pmax is the maximum load and A is the area of the hardness impression (contact area).

Young’s modulus is known as the ratio of stress to strain within the elastic (Hooke) limit which

can be determined from the values of contact stiffness S and contact area A via the following

relation:

74

√ √ (2.34)

where β is a dimensionless constant taken as unity and Eeff can be expressed as:

(

) (2.35)

where σ and E are the Poisson’s ratio and Young’s modulus of the thin film, respectively. σi and

Ei are the Poisson’s ratio and Young’s modulus of the indenter, respectively.

Figure 2.15 Typical loading and unloading curves of the nanoindentation [331]

2.4.8.1. Experimental Technique of Nanoindentation Measurement

The mechanical characteristics of the thin film coatings can be determined either in lateral or

depth geometry without removing the thin film from the substrate. The nanoindentation system

measures the depth of pentration by a capacitance or inductance displacement sensor. The main

part of this system is the indenter. In this work, the mechanical properties of the thin film

coatings were investigated via a nanoindentation workstation (Ultra-Micro Indentation System

75

2000, CSIRO, Sydney,Austarlia) supplied with a sharp Berkovich-type indenter of 5μm radius

according to Oliver and Pharr model [332]. Prior to the nanoindentation measurements, the area

of the indenter head was calibrated with a standard fused silica substrate. The nanoindentatuion

results were performed under a load control method with a maximum load of 200 nN. The low

peak loading is based on the consideration that the maximum displacement during indentation

should be no more than 10% of the coating thickness, and that high loading may result in micro-

cracks in the coating, which is thought to be relatively brittle, compared with metal coatings. To

obtain better resolution, the number of measuring points were increased to 15 during loading,

and 20 during unloading, respectively.

2.4.8.2. Finite Element Modeling (FEM)

FEM was performed to study the stress distribution throughout the thin films and the substrates

underneath the indentation tip and to evaluate the mechanical response of the coatings to

external loading. Experimental results of the mechanical properties obtained from the

nanoindentation measurements were used as input parameters in FEM modelling. COMSOL

multiphysics version 3.5a software was used in the simulation, and a two dimensional (2D)

axisymmetric model was built with the loading direction along the z-axis. Details of this model

set-up are breifly outlined here. A spherical indenter with a radius of 5 μm loaded onto a film of

2 μm thickness placed on top of glass substrate, with a maximum indentation depth of 0.2 - 0.6

μm, was modelled. Note that this film thickness, which is different from those of our

experimental thin films, was used here for a general assessment and comparison, and is designed

to match the loading conditions. The consideration here is that, under real application

environments, loadings with a dimension in the order of μm are more accurate than smaller and

sharper ones.

76

3. CHAPTER 3 Improving the Optoelectronic Properties of

Titanium Doped Indium Tin Oxide Thin Films

Paper I

Hatem Taha, Zhong-Tao Jiang, David J. Henry, Amun Amri, Chun-Yang Yin,

M. Mahbubur Rahman, Improving The Optoelectronic Properties of Titanium-

Doped Indium Tin Oxide Thin Films. Semicond. Sci. Technol. 2017, 32, 065011-

065021.

77

3.1. Abstract

The focus of this study is on a sol–gel method combined with spin-coating to prepare high

quality transparent conducting oxide (TCO) films. Structural, morphological, optical, and

electrical properties of sol-gel derived pure and Ti-doped indium tin oxide (ITO) thin films were

studied as a function of Ti concentration (i.e., 0, 2 and 4 at.%) and annealing temperatures (150 –

600 °C). FESEM measurements indicate that all the films are 350 nm thick. XRD analysis

confirmed the cubic bixbyite structure of the polycrystalline indium oxide phase for all of the

thin films. Increase of the Ti ratio, as well as the annealing temperature, improved the

crystallinity of the films. The highly crystallite structures were obtained at 500 C, with average

grain sizes of about 50, 65 and 80 nm for Ti doping of 0 at.%, 2 at.% and 4 at.%, respectively.

Electrical and optical properties improved as the annealing temperature increased, with an

enlarged electronic energy band gap and the optical absorption edge below 280 nm. In particular,

the optical transmittance and electrical resistivity of samples with 4 at.% Ti content improved

from 87% and 7.10×10-4

Ωcm to 92% and 1.6×10-4

Ωcm, respectively. The conductivity,

especially for the annealing temperature at 150 °C, is acceptable for many applications such as

flexible electronics. These results demonstrate that, unlike the more expensive and complex

vacuum sputtering process, high quality Ti-doped ITO films can be achieved by a fast

processing, simple wet-chemistry, and easy doping level control with the possibility of producing

films with high scalability.

78

3.2. Introduction

Research on transparent conductive oxide (TCO) materials has received significant attention in

order to develop a good combination of unique optoelectronic properties including high visible

light transmittance and low electrical resistivity. A large number of metal oxides such as indium

oxide (In2O3), tin oxide (SnO2), zinc oxide (ZnO), titanium dioxide (TiO2), cadmium oxide

(CdO), aluminium doped zinc oxide (AZO) and indium tin oxide (ITO) are widely utilized as

transparent conductive materials [126, 333, 334]. Amongst this family of materials ITO has a

large number of favourable properties such as low electrical resistivity, high transparency in the

visible light region, great film-substrate adhesion and good chemical stability. Therefore, ITO

films are of great interest and represent the prime candidate for numerous applications in

optoelectronic devices such as flat panel displays, solar cells, organic light-emitting diodes,

sensors, functional glass and electro-chromic devices [153, 335-341]. Numerous techniques are

employed for synthesizing ITO films such as, DC and RF magnetron sputtering, thermal

evaporation, pulsed laser deposition, electron beam evaporation, screen printing and sol-gel

processes [342-346]. Among the existing methods, some of them require sophisticated

equipment including high vacuum systems coupled with physical vapour deposition (PVD)

processes that consume a large amount of bulk material and are detrimental to the environment,

which results in very expensive end products. Therefore, finding an alternative to the PVD

process to synthesize high quality transparent conductive films becomes an important objective

for materials scientists around the world. The sol–gel method combined with spin-coating and/or

dip-coating offers certain advantages such as lower processing time, cost effectiveness, no

vacuum requirements, easy control of the doping level, and producing films with high scalability,

along with the flexibility of achieving desired shapes and surface areas [196, 347, 348].

The electrical conductivity of TCO films can be improved by increasing either its carrier

concentration or carrier mobility. Doping In2O3 or ITO with some transition-metal elements, e.g.,

Mo, Ti, Nb and W, can improve their optoelectronic properties significantly as a result of (1) the

smaller ionic radii and the higher mobility of these elements [349], and (2) facilitating the

production of transparent conductive films with low resistivity, high electron mobility and

superior near-infrared transmittance [350].

79

In this study, simple and low cost spin-coating derived Ti-doped ITO thin films were fabricated

with two different Ti concentrations and various post annealing temperatures (400 - 600) C. The

films were then compared with pure ITO prepared under the same conditions. The roles of Ti

additives and annealing temperature were examined for the effects on structural, morphological,

electrical and optical characteristics, as these are effects which have not been previously looked

at in detail especially that annealed at high temperature of 600 °C.

3.3. Experimental Techniques

3.3.1. Thin Films Deposition

ITO and Ti-doped ITO solutions were prepared using hydrated indium nitrate (In(NO3)3.5H2O,

purity 99.9%, Alfa Aesar), tin chloride dihydrate (SnCl2.2H2O, purity 98%, Chem-supply) and

titanium(IV) isopropoxide (Ti[OCH(CH3)2]4, purity 99.9%, Sigma Aldrich). All these chemicals

were used as received. Absolute ethanol was used as a solvent and the ITO films were deposited

onto soda-lime glass slides.

3.3.2. Sol-Gel and Thin Films Preparation

In order to prepare an ITO solution with concentration of 0.1M, the required amounts of

hydrated indium nitrate In(NO3)3.5H2O and tin chloride dihydrate (SnCl2.2H2O) were dissolved

in absolute ethanol and stirred for 1 hour. The resultant solutions were then mixed and refluxed

at 85 °C for 1 hour. The percentages of In was kept constant (~ 90 at.%) in all initial solutions.

Two different varieties of the Ti-doped ITO solutions were prepared with 2 at.% and 4 at.% Ti

contents with respect to 0.1M of ITO solutions. In separate beakers, hydrated indium nitrate, tin

chloride dihydrate and titanium (IV) isopropoxide (Ti[OCH(CH3)2]4 were first dissolved in

absolute ethanol and stirred for 1 hour. The resultant solutions were then mixed together and

refluxed at 85 °C for 1 hour. The final solutions were aged for 24 hours at room temperature. In

order to obtain uniform and homogenous thin films, glass substrates were first cleaned with

detergent and rinsed in deionized water (DI water). Then, the substrates were sonicated in

ethanol at 60 °C for 10 minutes, and DI water cleaning was repeated. The final step was the

sonication of substrates in DI water at 60 °C for 10 minutes. All the glass substrates (soda–lime–

80

silicate with 25×25 mm dimensions) were dried in a conventional vacuum drying oven at 100 °C

for 20 minutes prior to the coating process. Spin coating experimental procedures were (1)

dispensing the solution on to the substrate at 300 rpm for 15 seconds, (2) spreading the solution

at 2500 rpm for 20 seconds, (3) drying the solution at 4000 rpm for 20 seconds, and (4) heat

treatment on a hot plate at 150 °C for 10 minutes. These steps were repeated until the desired

thickness was achieved. Fabricated samples were then annealed at the temperatures of 400, 500

or 600 °C for 2 hours in air atmosphere.

3.3.3. Analysis Techniques

The structural properties of the sol–gel derived thin films were investigated via X-ray diffraction

(XRD) analysis using a GBC EMMA diffractometer with CuKα radiation (=0.154nm). The

diffraction angle (2) of measurement was within the range of 20° - 70° with a scanning rate of

2°/min and 2 step size of 0.01. The surface morphology was examined by using Zeiss Neon

40EsB field emission scanning electron microscopy (FESEM). For FESEM, the samples were

mounted onto the substrate holder using double-sided carbon sticky tape and then coated with

platinum (3nm thick) to reduce the charging effects prior to FESEM imaging. The electrical

properties of prepared films were measured by a 4-point probe method (Dasol Eng. FPP-HS8)

and the Hall-effect measurements were performed on specimens of (10×10 mm). Au contacts

were thermally evaporated on the coated samples in a system of four probes. The measurements

were carried out using ECOPIA HMS-2000 system with Polytec Bipolar Operation Power

supplies (BPO) at 0.9 Tesla of magnetic field. The optical properties were obtained by using UV-

VIS Perkin Elmer spectrometer in a wavelength range of 250 - 800 nm.

3.4. Results and Discussions

3.4.1. Structural Properties

Figures 3.1 and 3.2 show the XRD spectra for the ITO and Ti-doped ITO coatings as deposited

(150 °C heat treatment only) and annealed at different temperatures, respectively. The XRD data

indicate that all the films are polycrystalline with a body centred cubic bixbyite structure similar

to In2O3 with space group of Ia3/cubic (JCPDS card 06-0416) as expected and there are no

impure phases related to tin or titanium compounds, indicating that the prepared solutions were

homogeneous. The main peaks from as deposited and post annealed thin films at temperatures of

81

400, 500 or 600 °C are assigned to be (222), (400), (411), (332), (440) and (622) reflection

planes of In2O3. These results confirm the observation that the polycrystalline feature of In2O3

may act as a network with a body centred cubic bixpyite structure with lattice parameter of

10.118 Å [351, 352]. Both as deposited ITO and Ti-doped ITO thin films show poor crystallinity

because of the crystallization temperature of ITO in the range of 150-160 °C [68, 353]. This is

also consistent with Yang et al. who reported the same crystalline properties for as deposited Ti-

doped ITO films [354]. As the ionic radii of Titanium (Ti4+

), tin (Sn4+

) and indium (In3+

) are

0.06, 0.07 and 0.09 nm, respectively [65], the small ionic radii of Sn4+

and Ti4+

imply that the

crystalline structure may not change if these elements are used as dopant into In2O3 and/or ITO

frameworks. As such, we postulate that enhancing the preferred orientation and enhancing the

crystal growth of the 3-d metal-doped ITO thin films could be attributed to introduction of Ti as

a dopant for these thin films [244, 355]. Figures 3.1 and 3.2 also show that for both ITO and Ti-

doped ITO films the (222) and (400) peaks around 30.5° and 35.2°, respectively, are featured

prominently. This indicates the coexistence of <100> and <111> textures. The films have further

observable characteristics as the annealing temperature was increased, indicating enhanced

crystalline quality. When the annealing temperature was increased from 150 °C to 600 °C, the

ITO thin film exhibits the same (222) orientation, while the (400) diffraction peak becomes very

strong for both Ti concentrations, indicating a preferred orientation in <111> direction. It has

been observed by Suzuki et al. and Latz et al. that ITO films prepared by sputtering exhibit a

change in the orientation direction from (222) to (400) when the substrate temperature was

increased during the sputtering process [243, 356]. Kumar and Mansingh also reported that the

structure and the orientation direction for ITO samples was governed by the energy of the

sputtered particles reaching the substrate and they assumed that the higher energy sputtered

particles orientate in the (400) direction while the thermalized sputtered particles prefer the (222)

orientation [357]. However, the pure ITO thin films did not show the tendency of changing the

orientation direction from (222) to (400) reported by the aforementioned studies of Suzuki et al.

and Latz, et al. [243, 356] which may be attributed to the nature of sol-gel spin coating process.

Regarding the Ti-doped ITO thin films adopted in this work, the change of the major orientation

direction from (222) to (400) with the increase of annealing temperature is in good agreement

with Pu et al. results for vacuum synthesised Ti-doped ITO films at different substrate

82

temperatures. They reported that the high substrate temperatures provided the ad-atoms with

sufficient energy to migrate along the (400) direction [244].

25 30 35 40 45 50 55 60 65

Inte

nsi

ty (

a.u.)

2θ (degree)

(222

)

(400)

(440)

(411)

(622)

(332)

150°C

400°C

500°C

600°C

Figure 3.1 The XRD patterns of as deposited and post annealed ITO thin films

83

25 30 35 40 45 50 55 60 65

Inte

nsi

ty (

a.u

.)

2θ (degree)

(222)

(400

)

(440

)

(332

)

(622

)

150°C

400°C

500°C

600°C

(2 at.%)

25 30 35 40 45 50 55 60 65

Inte

nsi

ty (

a.uu.)

2θ (degree)

(22

2)

(40

0)

(44

0)

(33

2)

(62

2)

(4 at%)

600°C

400°C

500°C

150°C

Figure 3.2 The XRD patterns of Ti-doped ITO films prepared at 2at.% and 4 at.%

Ti concentrations and post annealed at different temperatures

84

The Scherrer equation (Eq. 2.26) was used to calculate the grain size from the line broadening or

the full width at half maximum (FWHM) of the main peak (222). While, Bragg's law (Eq. 2.25)

was used to estimate the inter-planar spacing (d spacing) and the standard Bravais lattice for a

cubic system (Eq. 2.27) was used to obtain the lattice constant.

From Table 3.1 and Figure 3.3, it is clearly shown that the grain sizes of the thin films are

enlarged with the increase in Ti-contents. This is because the additional Ti atoms act as

nucleation centres that help the early coalescence of the particles, resulting in formation of larger

crystals [358]. It is also found that with gradual increase in annealing temperature the grain sizes

of the films are enlarged as well, reaching a maximum at 500 °C and then decreasing (see data at

600 C). Both Ti doping level and annealing temperature enhanced the peak intensities and

hence the crystalline structure for ITO and Ti-doped ITO coatings, through substitution of In

atoms by both Sn and Ti atoms. Also, the vibration of the doping atoms could also result in an

annealing effect that supports nucleation and growth of the film along the preferred orientations.

For the as deposited and post annealed thin films, the average calculated lattice constants of both

ITO and 2 at.% and 4 at.% Ti-doped ITO films were estimated to be 10.13 Å, 10.14 Å and 10.17

Å respectively, which are slightly greater than the reported value for un-doped indium oxide of

10.118 Å. This increment in the lattice constant for both ITO and Ti-doped ITO thin films is

related to the small ionic radius of both Sn and Ti relative to the substituted In atoms in the In2O3

matrix. Also, it appears that the extent of Sn and Ti incorporation is intensely temperature

dependent, resulting in an increase in the lattice constants for ITO and Ti-doped ITO thin films

compared with un-doped In2O3. This in turn, has a noticeable impact on the film structure

leading to improved electron mobility in optimally doped films. Bourlange and co-authors

attributed the increase of the lattice constant to the occupation of conduction band states which

are In-O anti-bonding [359]. Also, the alteration in the lattice parameters could be associated

with the growth induced stress near the film surface during doping and annealing [360], which

may be raised from the oxygen deficiency and thermal expansion coefficient mismatch between

the film and the substrate [361, 362]. Figure 3.4 shows the shifting of the main peaks (i.e., (222)

orientation) for ITO and Ti-doped ITO thin films annealed at 500 °C. The shifts of (222) toward

lower 2θ value indicates distortion in the lattice structure takes place due to Ti additives, and thus

leading to an expansion in calculated lattice parameters.

85

10

20

30

40

50

60

70

80

100 200 300 400 500 600 700

Gra

in s

ize

(nm

)

Annealing temperature (°C)

4 at%

2 at%

ITO

28 28.5 29 29.5 30 30.5 31 31.5 32 32.5 33

Inte

nsi

ty (

a.u.)

2θ (degree)

2θ =30.52° 2θ =30.44°

4 at% Ti

2 at% Ti

ITO

Figure 3.3 The annealing temperature dependent grain size

Figure 3.4 Peaks shift of (222) for the ITO and Ti-doped

ITO thin films annealed at 500 °C

86

Table 3.1 Structural parameters for the ITO and Ti-doped ITO thin films at 2 at.% and 4 at.% Ti

concentration and post annealed at different temperatures

3.4.2. Surface Morphology Properties

The FESEM images of the synthesized ITO and Ti-doped ITO films as deposited (150 °C) and

after being annealed at 400, 500 and 600 °C are presented in Figures 3.5 and 3.6. The surfaces of

all the thin films are smooth, homogeneous, and composed of uniform coalesced clusters, and

show a grain structure demonstrating the nanocrystalline features. However, the as deposited thin

film of 4 at.% Ti shows a smoother surface and larger grains compared to those of pure ITO and

2 at.% Ti-doped ITO that may be due to the enhancement of crystallinity with Ti ratio. Also,

increasing the annealing temperature up to 500 °C resulted in an increase in grain size and

decreased grain boundaries which are in a good agreement with the results obtained from XRD

measurements. As the annealing temperature was further increased to 600 °C, micro-cracks were

formed on the surfaces of all the films due to induced stress resulting from possible different

thermal expansions between the film and its substrate.

Ti at.% T°C 2θ (in degrees) d-spacing Å (h k l) Lattice constant Å Crystallite size nm 150 30.56 2.921 (222) 10.12 17

0 at.% 400 30.52 2.925 10.13 31

500 30.52 2.925 10.13 50

600 30.52 2.925 10.13 46

150 30.52 2.925 (222) 10.13 19

2 at.% 400 30.50 2.925 10.14 38

500 30.52 2.925 10.13 65

600 30.48 2.923 10.15 60

150 30.42 2.934 (222) 10.16 21

4 at.% 400 30.34 2.942 10.19 47

500 30.44 2.933 10.16 80

600 30.46 2.931 10.15 68

87

a4 a3

a2 a1 150°C 400°C

500°C 600°C

600°C b4 500°C b3

b2 400°C b1 150°C

600°C 500°C

400°C 150°C

Figure 3.5 FESEM images of prepared and post annealed ITO thin films

Figure 3.6 FESEM images of Ti-doped ITO thin films, (a1-a4) for 2 at.% and (b1-b4) for 4

at.% Ti concentrations and post annealed at different temperatures

88

3.4.3. Electrical Properties

The dependence of electrical resistivity, carrier concentration and carrier mobility on Ti dopant

concentration and thermal annealing temperature are shown in Figures 3.7 and 3.8 respectively.

Figure 3.7 indicates that the resistivity for as deposited thin films with 4 at.% Ti content is lower

than those for as deposited pure ITO and 2 at.% Ti thin films. This is most likely attributed to the

level of Ti doping which contribute to a large number of electrons and hence increasing the

number of free carries in the thin films. Furthermore, during the sol-gel spin coating process, the

formation of both defects and oxygen vacancies (those are considered as electron donors) were

introduced easily which also results in a decrease in the film resistivity. Additionally, the

resistivity of the films decreased with increasing annealing temperature for both pure ITO and

Ti-doped ITO at both Ti concentrations. The lowest resistivity values (1.6×10-4

Ωcm and

1.8×10-4

Ωcm) were obtained from samples annealed at 500 °C with 4 at.%, 2 at.% Ti

concentrations and pure ITO, respectively. These results are as good as that fabricated using

vacuum sputtering techniques, e. g., 1.8×10-4

Ωcm [363]. It was reported by Chopra et al. that

the low resistivity and/or the high conductivity of the ITO films is related to carrier

concentration, which is linked to the dopant level and/or oxygen vacancies [14, 342, 364]. The

annealing temperature leads to enhanced substitution of indium atoms by titanium and tin atoms,

yielding an increase in carrier concentration in the thin films. Moreover, increased annealing

temperature assisted desorption of interstitial oxygen atoms at the grain boundaries which has led

to improved crystal structure and reduced the grain boundaries of the films. Therefore, the

trapped electrons at the grain boundaries were released which contributed to increase the carrier

concentration, as well as conductivity. This is evident in Figure 3.8, where the carrier

concentration and the Hall mobility increased to their maximum values of (7×1020

, 7.4×1020

and

8.6×1020

) cm

-3 for pure ITO, 2 at.% and 4 at.% Ti concentrations, respectively. Similarly, the

Hall mobility is strongly depended by the disordered of the crystallite structure, which can be

manipulated by a number of approaches, such as doping and annealing temperature. It is

interesting to note that, however, the as deposited film with 4 at.% Ti has a higher carrier

concentration but lower Hall mobility compared with those of pure ITO and 2 at.% Ti thin films.

This is probably attributed by the solubility limits of Sn and Ti dopants resulting in the increase

of occupational random sites in the ITO matrix. With increasing the annealing temperature up to

89

1

2

3

4

5

6

7

8

100 200 300 400 500 600 700

Res

isti

vit

y×

10

-4(Ω

.cm

)


Pure ITO

1

2

3

4

5

6

7

8

100 200 300 400 500 600 700

Res

isti

vit

y×

10

-4(Ω

.cm

)


4 at% Ti

2 at% Ti

500 °C, the highest Hall mobility values (39 cm2/Vs, 42 cm

2/Vs and 49 cm

2/Vs) and lowest

resistivity were accomplished for all the films studied here.

As the annealing temperature was further increased to 600 °C, the resistivity increased while the

carrier concentration and the carrier mobility decreased for both Ti concentrations. This

behaviour is expected, due to oxygen chemisorbed on the sample surface [364, 365]. Also, all the

thin films annealed at this temperature show some micro-cracks, see FESEM images above.

These cracks may be due to the mismatch of thermal coefficients between films and substrates

which causes a significant increase of compressive stress in the films. Eventually these micro-

cracks increase resistivity of the films. A summary of electrical properties of these films are

presented in Table 3.2.

Figure 3.7 Electrical resistivity of pure ITO and Ti-doped ITO thin films for 2 at.%

and 4 at.% Ti concentrations and post annealed at different temperatures

90

4

5

6

7

8

9

10

100 200 300 400 500 600 700

Car

rier

co

nce

ntr

atio

n×

10

20(c

m-3

)


4 at% Ti

2 at% Ti

15

25

35

45

55

0 100 200 300 400 500 600 700

Mo

bil

ity (

cm2/V

s)


4 at% Ti

2 at% Ti

3.5

4.5

5.5

6.5

7.5

10

20

30

40

50

0 100 200 300 400 500 600 700

Car

rier

con

cen

trat

ion

× 1

02

0 (

cm-3

)

Mo

bil

ity (

cm2/V

s)


Pure ITO

Figure 3.8 Variations of carrier concentration and Hall mobility of ITO and Ti-doped

ITO thin films for 2 at.% and 4 at.% Ti concentrations with annealing temperature

91

3.4.4. Optical Properties

The optical transmittance spectra of ITO and Ti-doped ITO thin films are shown in Figure 3.9.

The optical transmittance of as deposited ITO and Ti-doped ITO thin films are very similar;

however, the ITO film doped with 4 at.% Ti exhibits a higher optical transmittance of 87% over

the entire wavelength range. This may be due to the enlargement of the grain size and Ti

additive, as explained in the XRD and FESEM results (see section 3.4.1 and section 3.4.2).

Moreover, decreasing the level of defects, which could act as scattering centres, may contribute

to a higher optical transparency in the thin films. The highest optical transmittance of 92% was

achieved for the thin film annealed at 500 °C. Higher annealing temperature leads to higher

optical transmittance due to the improvement in the film quality through the increase in the

crystallinity and grain size of the film (see the XRD results) as well as reduced grain boundaries.

All these factors eventually reduce light scattering yielding better transmittance. It is also

observed from Figure 3.9 that there is a shift in the absorption edge to shorter wavelengths as the

annealing temperature is increased up to 500 °C which is related to the Burstein-Moss shift (shift

of Fermi level due to increasing the carrier density in the conduction band) [366, 367]. This

carrier concentration increment is comparable with the results obtained from electrical studies

(section 3.4.3). Thus, the higher optical transmittance and the move of the optical absorption

edge toward higher photon energy from Ti-doped ITO thin films essentially confirmed the

overall improvement of the optical properties of TCO films.

To obtain the band gap energy (Eg) from the transmittance data, we used the relation:

(αhv)2=A(hv-Eg) (3.1)

where α is the absorption coefficient, hv photon energy, A the proportionality coefficient. Plots

of photon energy hv versus (αhv)2 are shown in Figure 3.10. The energy band gap was obtained

by extrapolating the straight line part of the plotted curves along the energy axis. Table 3.2

summarizes the optical and the electrical properties of pure ITO and Ti-doped ITO thin films at

different annealing temperatures.

92

0

10

20

30

40

50

60

70

80

90

100

200 300 400 500 600 700 800

Tra

nsm

itta

nce

%

Wavelength (nm)

150°C

400⁰C

500⁰C

600⁰C

2 at.% Ti

contents

0

10

20

30

40

50

60

70

80

90

100

200 300 400 500 600 700 800

Tra

nsm

itta

nce

%

Wavelength (nm)

150°C

400⁰C 500⁰C 600⁰C

4 at.% Ti

contents

0

10

20

30

40

50

60

70

80

90

100

200 300 400 500 600 700 800

Tra

nsm

itta

nce

%

Wavelength (nm)

150°C

400⁰C

500⁰C

600⁰C

Pure ITO

Figure 3.9 Optical transmittance of pure ITO and Ti-doped ITO thin films

annealed at different temperatures

93

Table 3.2 The optical and electrical parameters of as prepared and post annealed ITO and Ti-

doped ITO thin films for 2 at.% and 4 at.% Ti concentrations

Table 3.2 shows that the optical band gaps of as deposited (150 °C) ITO films in both pure and

Ti-doped vary between 3.21 and 3.4 eV. Both pure and Ti-doped ITO thin films exhibit the

largest optical energy band-gap after being annealed at the temperature of 500 °C, i.e., 3.7, 3.73

and 3.77 eV for pure ITO, 2 at.% and 4 at.% Ti dopant levels of ITO thin films, respectively. A

gradual increase in the optical energy band gap due to the annealing temperature can be

associated with the change of grain size and modified carrier concentration (as seen in XRD

results) [368]. This can be explained by noting that free electrons are normally trapped in the

grain boundaries and by increasing the annealing temperature the grain size increases and the

volume of grain boundaries decreases. Consequently, fewer carriers may be trapped in the grain

boundary regions leading to an overall increase in the free carrier concentration, and as a result, a

higher electrical conductivity is obtained (see section 3.3). Furthermore, the enhancement of

optical band gap can also be explained through the (ΔEgBM

) Burstein-Moss relation [369, 370]:

(

)

(3.2)

where is the electron effective mass calculated from mv and mc. mv and mc are the effective

electron mass in valance and conduction bands, respectively. n is the carrier concentration. The

results are consistent with the prediction given by equation (3.2), and in good agreement with

Hall measurement results as well [244, 371].

Ti at.% T (°C) T (%) Eg (eV) ρ ×10-4

(Ω.cm) n ×1020

(cm-3

) μ (cm/Vs)

150 86.7 3.4 7.6 3.9 20

0 at.% 400 89 3.5 3.6 5.8 25

500 90.5 3.7 2.2 7 39

600 90 3.55 4.5 6 33

150 86.7 3.21 7.25 4.3 20

2 at.% 400 88.9 3.54 3.78 6 27

500 90.2 3.73 1.8 7.4 42

600 89.8 3.62 3.2 6.4 31

150 87 3.4 7.1 4.6 19

4 at.% 400 89 3.59 3.3 6.4 30

500 92 3.77 1.6 8.6 49

600 90.5 3.7 2.87 6.8 32

94

3 3.1 3.2 3.3 3.4 3.5 3.6 3.7 3.8 3.9 4 4.1 4.2 4.3

Photon energy (eV)

150⁰C

400⁰C

500⁰C

600⁰C


) 4 at.% Ti

3 3.1 3.2 3.3 3.4 3.5 3.6 3.7 3.8 3.9 4 4.1 4.2 4.3

Photon energy (eV)

150⁰C

400⁰C

500⁰C

600⁰C


) 2 at.% Ti

3 3.1 3.2 3.3 3.4 3.5 3.6 3.7 3.8 3.9 4

Photon energy (eV)

150⁰C

400⁰C

500⁰C

600⁰C


) Pure ITO

Figure 3.10 Estimated optical band gaps of pure ITO and Ti-doped ITO thin

films for 2 at.% and 4 at.% Ti concentrations post annealed at different

95

3.5. Conclusions

In this work, transparent conductive oxide thin films of pure ITO and Ti-doped ITO were

fabricated on to soda-lime glass substrates using an efficient and low cost sol-gel spin coating

technique. The effects of doping concentrations (i.e., 0 at.%, 2 at.% and 4 at.% Ti) and annealing

temperature (ranging of 150 – 600 °C) on the structural, morphological, electrical and optical

properties were studied. The conductivity and optical transparency of Ti doped ITO, particularly

those for as deposited (i.e., at 150 °C), are acceptable for many applications such as flexible

electronics. All these properties were improved as the Ti concentration was increased from 0% to

4% and the annealing temperature increased up to 500 °C. Increasing both Ti doping level and

annealing temperature leads to an enhanced substitution of indium atoms by titanium and tin

atoms, yielding an increase in carrier concentration in the thin films. Moreover, increased

annealing temperature assists desorption of interstitial oxygen atoms at the grain boundaries

which has led to improved crystal structure and reduced the grain boundaries of the films.

Likewise, higher Ti ratio and annealing temperature can lead to a higher optical transmittance,

due to the improvement in the film quality through the increase in the crystallinity and grain size.

The best performing Ti-doped ITO thin films were achieved with 4 at.% Ti concentration,

annealed at 500 °C. In particular, these films exhibited low resistivity (1.6×10-4

Ωcm), high

carrier concentration and mobility of (8.63×1020

cm-3

and 48.7 cm2/Vs, respectively), high

transmittance 92%) and a high optical band gap (3.77 eV). These results confirm that high

quality Ti-doped ITO thin films, suitable for various optoelectronic applications, can be

synthesized by a simple sol-gel and spin coating process.

96

4. CHAPTER 4: Probing the Effects of Thermal Treatment on

the Electronic Structure and Mechanical Properties of Ti-Doped

ITO Thin Films

Paper II

Hatem Taha, David J. Henry, Chun Yang Yin, Amun Amri, Xiaoli Zhao, Syaiful Bahri,


of Thermal Treatment on The Electronic Structure and Mechanical Properties of Ti-doped ITO

Thin Films. J. Alloys Compd. 2017, 721, 333-346.

97

4.1. Abstract

Titanium-doped indium tin oxide thin films were synthesized via a sol-gel spin coating process.

Surface chemical bonding states and mechanical properties have been investigated as a function

of titanium content (2 and 4 at.%) and annealing temperature ranging from 400 to 600 °C with

increments of 100 °C. Raman analysis was performed to study the phonon vibrations for the

prepared samples and the results revealed the existence of ITO vibrational modes. The elemental

compositions, bonding states and binding energies of the film materials were investigated using

X-ray photoelectron spectroscopy (XPS) technique. The XPS results indicated that the ratio of

the metallic elements (In, Sn, Ti) to the oxygen on the surface of the thin film coatings decreased

due to the increase of the oxide layer on the surface of the thin films. Also, by increasing the

annealing temperature up to 600 °C, the Ti 2p and Cl 2p signals were no longer detected for both

2 and 4 at.% Ti contents, due to the thicker surface oxidation layer. Mechanical properties of the

synthesized films were also evaluated using a nanoindentation process. Variations in the

hardness (H) and the elastic modulus (E) were observed with different Ti at.% and annealing

temperatures. The hardness is within the range of 6.3 - 6.6 GPa and 6.7 - 6.8 GPa for 2 and 4

at.% Ti content samples, respectively, while the elastic modulus is within the ranges of 137 - 143

and 139 - 143 GPa for 2 at.% and 4 at.% Ti contents samples, respectively. A combination of the

highest H and E was achieved in the sample of 4% Ti content annealed at 600 °C. Furthermore,

the H/E ratio ranges from 4.5 10-2 to 5.0 10

-2 which reflects a reasonable level of wear

resistance.

98

4.2. Introduction

Transparent conductive oxides (TCOs) receive widespread attention from researchers due to

their unique combination of very low electrical resistivity and high optical transparency over the

visible spectral region [336, 372-374]. They exhibit favorable properties which include high

infrared reflection, good chemical stability and good mechanical properties along with low

electrical contact resistance and high optical transparency in the visible light region. Indium

oxide (In2O3) and/or indium tin oxide (ITO) based TCOs are the most commonly used

transparent conductive oxides compared with zinc oxide (ZnO), aluminium doped zinc oxide

(AZO), tin oxide (SnO2), and cadmium oxide (CdO) [159, 367]. ITO-based TCO films can

exhibit different electro-optical, mechanical, electronic structure and composition properties

depending on manufacturing technique. Many deposition techniques are used to prepare TCO

coatings such as Dc and/or RF sputtering, chemical vapour deposition, reactive evaporation,

molecular beam epitaxy, sol-gel method, electron beam evaporation and pulsed laser deposition

[193, 243, 259, 375-379]. However, the sol-gel technique has many advantages including cost

effectiveness, short processing time, simplicity and the opportunity to fabricate coatings with

high scalability along with the flexibility of fine-tuning coatings to have preferred shapes and

surface areas [196, 347, 348, 380].

Novel ITO composites doped with 3-d and/or 4-d transition metals, such as (Ti, Ag, Cr, Mo, W,

Ta and Cu), are being considered as potential candidates in a wide range of optoelectronic

applications such as organic and inorganic solar cells, touch screens, organic light-emitting

diodes, sensors and electro-chromic devices [153, 217, 335, 338, 381, 382]. In order to satisfy

the demands for some of these applications, TCO composites with improved durability are

required. The durability of ITO based coatings depends on their physical, mechanical and

structural properties. Therefore, it is expected that the mechanical properties of ITO based films

play a significant role in the performance of optoelectronic devices. It has been reported that the

bare ITO films have a brittle surface and seem to be susceptible to cracking and delamination. So

far, there has been scarce information on the mechanical properties of ITO thin films,

particularly, their micro or nanostructure. The elastic modulus and the hardness of 240 nm thick

sputtered ITO thin film coatings were reported by Chen et al. [20] to be 140 GPa and 12 GPa

respectively. The hardness and elastic modulus were enhanced by introducing hydrogen into the

99

gas mixture during the deposition process [383]. We propose that incorporating a transition metal

such as titanium into the ITO nanostructure may enhance the mechanical properties of this

material. Therefore, the main objective of the present study is to evaluate the influence of

variations in the Ti concentration and annealing temperature on the chemical bonding states and

the mechanical properties, including Young’s modulus (E) and Hardness (H) of spin coated Ti-

doped ITO thin films, technical aspects which have not been reported thus far, to the best of the

authors’ knowledge.

4.3. Experimental Details

4.3.1. Thin Films Deposition

Hydrated indium nitrate (In(NO3)35H2O, purity 99.9%, Alfa Aesar) titanium(IV) isopropoxide

(Ti[OCH(CH3)2]4, purity 99.9%, Sigma Aldrich), and tin chloride dihydrate (SnCl22H2O, purity

98%, Chem-supply) were used as received as precursors to produce Ti-doped ITO films via a

sol-gel spin coating process. Absolute ethanol was used as a solvent and the ITO films were

deposited onto soda-lime glass slides.

ITO solutions (0.1 M) were prepared by dissolving appropriate amounts of In(NO3)35H2O and

SnCl22H2O separately in absolute ethanol. The solutions were combined and stirred vigorously

for 1 hour. Required amounts of (Ti[OCH(CH3)2]4) were then dissolved in ethanol and added to

ITO solutions in order to obtain solutions of Ti doped ITO with 2 and 4 at.% Ti contents,

respectively. The resultant solutions were then refluxed at 85 °C for 1 hour and aged for 24 hours

at room temperature. Since uniformity and homogeneity of the thin films mainly depends on the

cleanness of the substrates, the glass substrates with (25×25 mm dimensions) were first washed

with soap and rinsed repeatedly in deionized water. Then, the substrates were sonicated at 60 C

for 10 min in ethanol and DI water, respectively and dried in a vacuum drying oven at 100 °C,

prior to the coating process. A Polos spin coater was used to fabricate Ti-doped ITO films based

on three main spin steps. Firstly, the solution was dispensed on to the substrate at 300 rpm for 15

seconds followed by spreading at 2500 rpm for 20 seconds and finally dried at 4000 rpm for 20

seconds. The sample was calcined on a conventional hot plate at 150 C for 10 minutes. These

100

steps were repeated until the desired coating thickness (300 – 400 nm) was achieved. Thermal

annealing in air atmosphere was performed upon the fabricated samples at (400, 500 and 600) C

for 2h.

4.3.2. Characterization Techniques

Raman analysis was performed on a Nicolet 6700 Fourier transform infrared (FT-IR)

spectrophotometer attached with an NXR FT-Raman module. Raman spectra were obtained by

using the following settings: Helium-Neon (He-Ne) laser beam with excitation wavelength and

operation power of 1064 nm and 0.204 W, respectively, InGaAs detector with 90° detection

angle, optical velocity of 0.3165, CaF2 beam splitter, gain of 1, aperture of 150, maximum peak

to peak signal along with optimum focusing and side-to-side values, and 32 scans with resolution

of 8 cm-1

in the range of 0 - 4000 cm-1

. Information of elemental compositions, chemical

structures and bonding states of the thin film coating surfaces were probed using a Kratos Axis

Ultra-X-ray Photoelectron Spectrometer. The X-ray source was Al-Kα monochromatic radiation

(hv = 1486.6 eV) with a power of 15 kV and 10 mA. The base pressure of the analysis chamber

maintained at 2.9 × 10-9

Torr. The XPS survey and high resolution scans were collected before

etching. Typical high resolution XPS core level spectra were focused on the regions of In3d,

Sn3d, Ti2p, O1s and C1s. The deconvolution of high resolution spectra was carried by

employing the CASA XPS software (version 2.3.1.5) which provides information for the

analyses of chemical bonding states.

The elastic modulus (E) and the hardness (H) of the coatings were measured using a

nanoindentation workstation (Ultra-Micro Indentation System 2000, CSIRO, Sydney, Australia),

equipped with a diamond Berkovich indenter [332]. The area function of the indenter tip was

calibrated using a standard fused silica specimen. Load control method was used with a

maximum loading of 200 nN. The low peak loading was based on the consideration that the

maximum displacement during indentation should be no more than 10% of the coating thickness,

and that high loading may result in micro-cracks in the coating, which is thought to be relatively

brittle compared with metal coatings. To obtain good resolution, 15 measuring points were used

during loading and 20 during unloading.

101


4.4.1. Raman Analysis

The structural and morphological properties of the synthesized and annealed Ti-doped ITO thin

film coatings were characterized via x-ray diffraction (XRD) and field emission scanning

electron microscopy (FESEM) techniques and were discussed in our previous work [384]. The

XRD results confirm the formation of body centred cubic bixbiyte structure with space group of

Ia3/cubic similar to In2O3 (JCPDS card 06-0416). The ionic radiuses of tin (Sn4+

), Titanium

(Ti4+

) and indium (In3+

) are 0.07, 0.06 and 0.09 nm, respectively. The small ionic radii of Sn4+

and Ti4+

imply that the crystalline features of In2O3 and/or ITO frameworks should not change if

these elements are used as dopants. Grain sizes increase with the introduction of Ti dopants

along with increasing annealing temperature, reaching the maximum value of 80 nm for the film

prepared with 4 at.% Ti concentration and post-annealed at 500 °C. Therefore, we propose that

enhancing the crystal growth and the preferred orientation of the 3-d metal doped ITO thin films

could be attributed to introduction of Ti as a dopant for these thin films.

The FESEM images of the synthesized ITO and Ti-doped ITO films as deposited (150 °C) and

after being annealed at 400, 500 and 600 °C show that the surfaces of all the thin films are

smooth, homogeneous, and composed of uniform coalesced clusters, and show a grain structure

demonstrating nanocrystalline features. However, the as deposited thin film of 4 at.% Ti shows a

smoother surface and possesses larger grains compared to those of pure ITO and 2 at.% Ti-doped

ITO, which may be attributed to the enhancement of crystallinity with Ti ratio.

Raman spectroscopy was used to study the effect of Ti contents and annealing temperature on

the electronic and mechanical properties of Ti doped ITO thin films. It is well known that ITO

based materials have a body centred cubic structure that belongs to Ia3 space group, similar to

that of In2O3. Two types of cation are present in the ITO based structure: (1) 8 In3+

with 3 sides

symmetry at b-sites and (2) 24 In3+

with 2-fold point symmetry at d-sites. Moreover, the 48

oxygen atoms in this structure occupy general e-sites with no symmetry. Thus, six possible

vibration modes may be identified [321]:

4Ag + 4Eg + 14Tg + 5Au + 5Eu + 16Tu

102

The Ag, Eg and Tg symmetry vibrations are Raman active and infrared (IR) inactive modes;

whereas, Tu vibration modes are Raman inactive and IR active. The Au and Eu vibration modes are

inactive for both Raman and IR measurements. Figure 4.1 presents the results of Raman active

modes for the Ti doped ITO thin films at 2% and 4% Ti concentrations after being annealed at

the temperatures of 400, 500 and 600 °C. The observed vibrational modes correspond to 106,

135, 176, 275, 367, 432, 584, 633 and 704 cm-1

which represent an unequivocal finger print for

In2O3 and/or ITO cubic structure. These results are in good agreement with Raman active modes

observed from In2O3 and ITO based materials [385, 386]. The peaks recorded at a wavenumbers

below 500 cm-1

are attributed by In-O stretching vibrational modes in the ITO matrix. However,

the line at 633 cm-1

is due to the overlapping of the contributions from In-O, Sn-O and O-Ti-O

vibrational modes with frequencies of 630, 633 and 636 cm-1

respectively. Indeed, these

observed vibrational modes in Ti doped ITO structure may be ascribed to a good incorporation

of Ti and Sn dopants into the In2O3 lattice. It is also noticed that, along with the increase in

annealing temperature from 400 to 500 C, the intensity of the high frequency peaks at 633 and

704 cm−1

was gradually enhanced. However, the intensity of the low frequency peak at 175 cm−1

decreased. For the sample annealed at 600 C, the intensity of the high frequency peaks

decreased and the low frequency peak at 135 cm−1

disappeared. Increasing the annealing

temperature up to 600 °C also led to the disappearance of the peaks at 367 and 432 cm-1

.

103

50 150 250 350 450 550 650 750 850

Inte

nsi

ty (

a.u)

Raman shift (cm-1)

(a)

50 150 250 350 450 550 650 750 850

Inte

nsi

ty (

a.u)

Raman shift (cm-1)

(b)

400°C

600°C

600°C

500°C

500°C

400°C

Figure 4.1 Raman shift of Ti-doped ITO films (a): 2 at % and (b): 4 at % of Ti, annealed at

different temperatures

4.4.2. Atomic Compositions and Surface Chemical Bonding States

The elemental compositions of Ti-doped ITO thin films were obtained via XPS survey scans.

Figure 4.2 (a, b) shows the survey scan results for the synthesized Ti doped ITO thin films for

both Ti contents. The photoelectron peaks for In 3d, Sn 3d, Ti 2p, O 1s, Cl 2p and C 1s in the

binding energy range of 0 - 1200 eV were observed. The XPS survey spectra confirm the

existence of the principal elements (In, Sn, Ti, Cl and O), as well as carbon, in the related sample

104

0100200300400500600

Inte

nsi

ty (

a.u)

Binding energy (eV)

Cl2p C1s

In3d

Ti2p

Sn3d O1s

(a)

600 °C

500 °C

400 °C

0100200300400500600

Inte

nsi

ty (

a.u)

Binding energy (eV)

Cl2p

C1s

In3d

Ti2p

Sn3d

O1s

(b)

400 °C

600 °C

500 °C

coatings. Table 4.1 lists the atomic compositions of the Ti doped ITO thin films at 2% and 4% Ti

concentrations after being annealed at 400, 500 and 600 °C. The results imply that the surface

elemental composition of the film materials was significantly affected by annealing temperature.

As the annealing temperature increased, the ratio of atomic percentages of the principal elements

(In, Sn and Ti) to oxygen on the surface of the thin film coatings decreased for both levels of Ti

doping. At 600 °C the Ti 2p and Cl 2p signals were completely absent for both 2 and 4 at % Ti

concentrations, respectively, indicating that surface oxidation is taking place at high temperature.

As the oxygen content becomes higher at high temperatures, the surface oxidation layer should

become thicker. In order to compensate for any charge shifts, the XPS energy scale was

calibrated by the C1s (C-H) line at 284.60 eV (bonding energy).

Figure 4.2 XPS survey scans of Ti-doped ITO films (a): 2 at % and (b): 4 at % of Ti, annealed at


105

Table 4.1 Elemental compositions of Ti doped ITO thin films at 2 at.% and 4 at.% Ti

concentrations after being annealed at different temperatures

The XPS spectra for these elements are shown in Figures 4.3 – 4.7. The surface chemical

bonding states of the annealed Ti doped ITO films were characterised by de-convoluting the high

resolution In 3d, Sn 3d, Ti 2p, Cl 2p and O 1s photoelectron lines using the Gaussian

distribution, in order to appraise the possible chemical bonding states of these atoms in the

composites. The parameters, derived from the analysis of the XPS spectra, are listed in Table 4.2

and include: the photoelectron lines, bonding states and their corresponding binding energies,

full width at half maximum (FWHM) values and atomic percentages of the elemental

compositions present in the films after being annealed at 400, 500 and 600 °C as evaluated from

XPS curve fittings.

The de-convoluted curves of the high resolution XPS spectra of In 3d5/2 photoelectron lines are

shown in Figure 4.3 (a, b). It is clear that the de-convoluted In3d5/2 spectrum is assigned to three

different bonding states in the range of 443.6 - 445.4eV. The lower energy peaks placed within

the range 443.6 – 443.8 eV (labelled i) is linked to In° bonding state, precisely In-In bonds, while

the mid energy peaks located in the range 444.1 – 444.6 eV (labelled ii) corresponds to In3+

bonding state from In2O3 [387, 388]. The higher energy peaks in the range 444.7 – 445.4 eV

(labelled iii) could be attributed to the In-Cl bonds from InCl.

Ti

Concentration

Annealing

Temperature °C

Atomic percentage of the elements

In Sn Ti O Cl C

400 8.0 1.9 4.4 37.7 1.0 47.0

2 at.% 500 7.7 2.8 2.9 39.6 1.8 45.2

600 7.1 1.6 0.0 50.8 1.0 39.5

400 9.2 3.2 7.4 35.9 1.2 43.1

4 at.% 500 8.3 2.1 5.0 39.7 1.6 43.3

600 6.6 1.9 4.0 52.0 0.0 35.5

106

Figure 4.3 In 3d XPS spectra of Ti doped ITO films (a): 2 at % and (b): 4 at % Ti, annealed at


441442443444445446447448

CP

S/I

nte

nsi

ty (

a.u

)

Binding energy (ev)

400°C

(i)

(ii)

(iii)

441442443444445446447448

CP

S/I

nte

nsi

ty (

a.u

)

Binding energy (ev)

500°C

(i) (ii)

(iii)

441442443444445446447448

CP

S/I

nte

nsi

ty (

a.u

)

Binding energy (ev)

600°C

(i)

(ii)

(iii)

(a)

441442443444445446447448

CP

S/I

nte

nsi

ty (

a.u

)

Binding energy (ev)

400°C

(i)

(ii)

(iii)

441442443444445446447448

CP

S/I

nte

nsi

ty (

a.u

)

Binding energy (ev)

500°C

(ii) (i)

(iii)

441442443444445446447448

CP

S/I

nte

nsi

ty (

a.u

)

Binding energy (ev)

600°C

(ii)

(i)

(iii)

(b)

107

Figure 4.4 (a, b) displays the de-convoluted curves of high resolution XPS spectra for Sn 3d5/2

photoelectron lines. Three bonding states in the range of 485.6 - 487.2 eV have been allocated.

The first components are obtained within the range 485.6 - 485.8 eV (labelled ‘i’) correspond to

Sn4+

in SnO2, whereas the second features (labelled ‘ii’) observed in the range 486.0 - 486.5 eV

are related to Sn2+

from SnO. It has been reported by Fan and Goodenough that the Sn3d peak

for Sn2+

in SnO was seen at a binding energy around 0.5eV higher than that for Sn4+

in SnO2

[148, 389]. The SnO phase is thermodynamically less stable than the SnO2 phase; therefore, the

Sn4+

is thought to be the predominant state in the ITO based materials. The last components

(labelled ‘iii’) seen in the range 486.6 - 487.2 eV may be attributed to the SnCl2 bonding state.

The Ti 2p photoelectron lines of Ti-doped ITO films are presented in Figure 4.5 (a, b). The curve

fitting of Ti2p core level XPS spectra assigned three components within the energy range of

457.3-458.5 eV. The lower constituent in the energy range of 457.3 - 457.5 eV (labelled ‘i’)

corresponds to Ti3+

in Ti2O3 bonding state [390]. The second component within the energy range

457.9 - 458.0 eV could be attributed to TiCl3; while the higher energy components in the range

458.4 - 458.5 eV correspond to Ti4+

from TiO2 [391].

108

483484485486487488489

CP

S/I

nte

nsi

ty (

a.u

)

Binding energy (eV)

(i)

(ii)

(iii)

483484485486487488489

CP

S/I

nte

nsi

ty (

a.u

)

Binding energy (eV)

(i)

(ii)

(iii)

483484485486487488489

CP

S/I

nte

nsi

ty (

a.u

)

Binding energy (eV)

(i) (iii)

(ii)

(b)

400°C

600°C

500°C

483484485486487488489

CP

S/I

nte

nsi

ty (

a.u

)

Binding energy (eV)

(i)

(ii) (iii)

483484485486487488489

CP

S/I

nte

nsi

ty (

a.u

)

Binding energy (eV)

(i) (ii)

(iii)

483484485486487488489

CP

S/I

nte

nsi

ty (

a.u

)

Binding energy (eV)

(iii)

(i)

(ii)

(a)

400°C

600°C

500°C

Figure 4.4 Sn 3d XPS spectra of Ti doped ITO films (a): 2 at % and (b): 4 at

% Ti, annealed at different temperatures

109

Figure 4.5 Ti 2p XPS spectra of Ti doped ITO films (a): 2 at % and (b): 4 at % Ti, annealed at


456457458459460

CP

S/I

nte

nsi

ty (

a.u

)

Binding energy (eV)

456457458459460

CP

S/I

nte

nsi

ty (

a.u

)

Binding energy (eV)

(i)

(ii)

(iii)

(a)

456457458459460

CP

S/I

nte

nsi

ty (

a.u

)

Binding energy (eV)

(i)

(ii) (iii)

456457458459460

CP

S/I

nte

nsi

ty (

a.u

)

Binding energy (eV)

(i)

(ii) (iii)

(b)

500°C

500°C

400°C

400°C

456457458459460

CP

S/I

nte

nsi

ty (

a.u

)

Binding energy (eV)

(i)

(ii) (iii)

600°C

(i)

(iii)

(ii)

110

The de-convolution of O 1s spectra exhibits three sub-peaks within the energy range of 529.3 -

531.8 eV as shown in Figure 4.6 (a, b). The first components in the range of 529.3 - 529.6 eV

correspond to O-In bonds and O-Ti bonds, in In2O3 and Ti2O3. The second components within

the range 530.3 - 530.8 eV are attributed to TiO2 and SnO2 bonding states. Finally, the other

components in the range of 531.4 - 531.8 eV could be due to the existence of O-Sn bonds [387,

388, 391]. For In2O3-based oxide semiconductors, the role of oxygen vacancies (Vo) on the

electrical properties has inspired significant debate. Using first principle calculations and density

function theory, Agoston et.al reported that the oxygen vacancies in indium oxide lattice are

shallow, suitable for generating free electrons in the conduction band. Existence of oxygen

vacancies in indium oxide system and its derivative materials indicate experimentally that these

defects are the major source of n-type conductivity [392].

Figure 4.7 (a, b) shows the results from curve fitting of high resolution XPS spectra for Cl 2p

photon lines. Here also three components were obtained in the range of 197.6 – 199.9 eV. The

first components are observed in the energy range of 197.6 - 198.0 eV (labelled ‘i’) related to

InCl bonding state [393], the second components within 198.5 - 198.7 eV (labelled ‘ii’) may be

attributed to SnCl2 bonding states, while the last components in the range of 199.6 – 199.9 eV

(labelled ‘iii’) corresponds to TiCl3 bonding states [391]. Table 3.2 details the de-convoluting

results of the XPS data of spin coated Ti-doped ITO thin films as a function of Ti concentrations

and annealing temperature.

111

(a)

527528529530531532533534

CP

S/I

nte

nsi

ty (

a.u

)

Binding energy (eV)

(i)

(ii) (iii)

527528529530531532533534

CP

S/I

nte

nsi

ty (

a.u

)

Binding energy (eV)

(i

(iii) (ii)

527528529530531532533534

CP

S/I

nte

nsi

ty (

a.u

)

Binding energy (eV)

(i)

(iii) (ii

(b)

527528529530531532533534

CP

S/I

nte

nsi

ty (

a.u

)

Binding energy (eV)

(iii)

(ii)

(ii

527528529530531532533534

CP

S/I

nte

nsi

ty (

a.u

)

Binding energy (eV)

(iii

(i)

(ii)

527528529530531532533534

CP

S/I

nte

nsi

ty (

a.u

)

Binding energy (eV)

(iii

(i)

(ii)

400°C

600°C

500°C

600°C

500°C

400°C

Figure 4.6 O 1s XPS spectra of Ti doped ITO films (a): 2 at % and (b): 4 at % Ti, annealed at


112

196197198199200201202

CP

S/I

nte

nsi

ty (

a.u

)

Binding energy (eV)

(i) (ii)

(iii)

196197198199200201

CP

S/I

nte

nsi

ty (

a.u

)

Binding energy (eV)

(i) (ii)

(iii)

(b)

195196197198199200201202

CP

S/I

nte

nsi

ty (

a.u

)

Binding energy (eV)

(i)

(ii)

(iii)

196197198199200201

CP

S/I

nte

nsi

ty (

a.u

)

Binding energy (eV)

(i) (ii)

(iii)

196197198199200201202

CP

S/I

nte

nsi

ty (

a.u

)

Binding energy (eV)

(i)

(ii) (iii)

(a)

600°C

500°C

500°C

400°C

400°C

Figure 4.7 Cl 2p XPS spectra of Ti-doped ITO films (a): 2 at % and (b): 4 at

% Ti, annealed at different temperatures

113

Table 4.2 Fitting results of the XPS data of Ti doped ITO films for the core level binding

energies

Ti

concentration

%

Annealing

temperature °C

Photoelectrons

lines

Bonding states

Binding

energy (eV)

FWHM (eV)

Percentages of

the

components %

In°(In-In bonds) 443.7 1.1 49.5

In 3d5/2 In2O3 444.4 1.2 41.9

InCl/InO× 445.1 1.3 8.6

SnO2 485.8 1.3 45.0

Sn3d5/2 SnO/SnO×/Sn 486.3 1.2 41.6

400 SnCl2 486.9 1.3 13.4

Ti2O3 457.4 0.5 28.8

Ti 2p3/2 TiO2 457.9 0.5 43.7

TiCl3 458.4 0.5 27.5

TiO2/In2O3 529.4 1.2 52.6

O 1s InO×/SnO 530.4 2.2 31.1

Ti2O3/SnO2 531.5 1.8 16.3

InCl 198.0 1.3 32.7

Cl 2p3/2 SnCl2 198.6 1.2 32.1

TiCl3 199.7 1.4 35.2

In°(In-In bonds) 443.7 1.2 59.4

In 3d5/2 In2O3 444.3 1.0 24.0

InCl/InO× 444.9 1.2 16.6

SnO2 485.8 1.2 61.7

Sn3d5/2 SnO/SnO×/Sn 486.5 1.0 30.7

2 at.% 500 SnCl2 487.2 1.0 7.6

Ti2O3 457.4 0.5 31.9

Ti 2p3/2 TiO2 457.9 0.6 58.6

TiCl3 458.5 0.3 9.5

TiO2/In2O3 529.5 1.4 66.1

O 1s InO×/SnO 530.8 1.6 20.3

Ti2O3/SnO2 531.8 1.7 13.6

InCl 197.9 0.7 46.6

Cl 2p3/2 SnCl2 198.5 0.9 23.4

TiCl3 199.6 1.0 30.0

In°(In-In bonds) 443.6 1.4 72.1

In 3d5/2 In2O3 444.5 1.1 15.6

InCl/InO× 445.2 1.8 12.3

SnO2 485.7 1.3 41.2

Sn3d5/2 SnO/SnO×/Sn 486.3 1.5 55.4

600 SnCl2 487.0 1.3 3.4

TiO2/In2O3 529.4 1.3 41.8

O 1s InO×/SnO 530.6 2.1 41.5

Ti2O3/SnO2 531.6 2.2 16.7

InCl 197.7 1 39.5

Cl 2p3/2 SnCl2 198.5 0.9 30.1

TiCl3 199.8 0.9 30.4

In°(In-In bonds) 443.7 1.3 64.4

In 3d5/2 In2O3 444.5 1.1 23.1

InCl/InO× 445.3 1.5 12.5

SnO2 485.7 0.9 30.3

Sn3d5/2 SnO/SnO×/Sn 486.3 0.8 37.7

400 SnCl2 486.9 1.2 32.0

Ti2O3 457.5 0.8 25.0

Ti 2p3/2 TiO2 458.0 0.7 33.8

TiCl3 458.5 0.9 41.2

TiO2/In2O3 529.6 1.2 58.7

114

O 1s InO×/SnO 530.7 1.7 25.9

In2O3 531.7 1.7 15.4

InCl 197.9 0.9 30.7

Cl 2p3/2 SnCl2 198.7 1.2 39.4

TiCl3 199.9 1.2 29.9

In°(In-In bonds) 443.6 1.2 53.5

In 3d5/2 In2O3 444.1 1.0 33.8

InCl/InO× 444.7 1.1 12.7

SnO2 485.6 1.2 35.3

Sn3d5/2 SnO/SnO×/Sn 486.0 1.2 40.7

4 at. % 500 SnCl2 486.6 1.3 24.0

Ti2O3 457.4 0.8 41.9

Ti 2p3/2 TiO2 457.9 0.6 39.7

TiCl3 458.5 0.5 18.4

TiO2/In2O3 529.3 1.2 63.0

O 1s InO×/SnO 530.3 1.7 23.8

TiO2/In2O3 531.4 1.7 13.2

InCl 197.6 1.2 42.6

Cl 2p3/2 SnCl2 198.6 1.3 37.1

TiCl3 199.7 1.2 20.3

In°(In-In bonds) 443.8 1.3 75.7

In 3d5/2 In2O3 444.6 1.1 19.5

InCl/InO× 445.4 1.0 4.8

SnO2 485.7 1.1 31.8

Sn3d5/2 SnO/SnO×/Sn 486.1 1.2 44.6

600 SnCl2 486.7 1.2 23.6

Ti2O3 457.3 0.7 43.0

Ti 2p3/2 TiO2 457.9 0.5 40.4

TiCl3 458.4 0.5 16.6

TiO2/In2O3 529.4 1.3 66.3

O 1s InO×/SnO 530.5 1.8 23.5

TiO2/In2O3 531.6 1.7 10.2

115

4.4.3. Mechanical Properties (Hardness, Young’s modulus and Wear resistance)

The thickness of the TCO films prepared for this study is within the range of 300 - 400 nm. It has

previously been reported that the indentation depth should be less than 10% of the film thickness

to avoid substrate effects. Therefore, in order to determine the hardness and elastic modulus

values for our Ti-doped ITO thin films, the measurements were taken at a maximum indenter

depth of around 13 nm, which is less than one-tenth our samples thickness.

Figure 4.8 (a, b) shows the load-displacement curves determined from the nanoindentation

measurements corresponding to the Ti-doped ITO samples of both Ti contents and after being

annealed. It was reported by Jian and co-worker that establishment of cracking in films

underneath the nanoindentor resulted in a distinct discontinuity in the loading part of the loading

displacement curve [394]. In this study, the experimental nanoindentation results presented in

Figure 4.8 (a, b) confirmed that continuous loading curves were obtained for all the films,

indicating that the phenomenon of cracking was absent in the Ti-doped ITO films at different Ti

contents and annealing temperatures. Hardness and Elastic (Young’s) modulus can be calculated

directly from the measurements of indentation load and penetration depth for both loading and

unloading process. From a typical load-penetration depth (p-h) curve, maximum load, maximum

displacement and unloading stiffness (also called contact stiffness S) can be determined. The

value of how resistant solid matter is to deformation under an applied force is known as the

hardness which was expressed by (Eq. 2.33). Young’s modulus can also be calculated from (Eq.

2.34) and (Eq. 2.35).

116

Figure 4.8 Load-displacement curves of Ti-doped ITO films (a): 2 at % and (b): 4 at % Ti,

annealed at different temperatures

255075

100125150175200225

0 3 6 9 12 15

Ind

enta

tion l

oad

(μN

)

Indentation depth (μm)

600 °C

255075

100125150175200225

0 3 6 9 12 15

Ind

enta

tio

n l

oad

(μ

N)


500 °C

255075

100125150175200225

0 3 6 9 12 15

Inden

tati

on l

oad

(μN

)


400 °C

(b)

255075

100125150175200225

0 3 6 9 12 15

Ind

enta

tio

n l

oad

(μ

N)

Indentation depth (nm)

400 °C

255075

100125150175200225

0 3 6 9 12 15

Ind

enta

tio

n l

oad

(μN

)


500 °C

255075

100125150175200225

0 3 6 9 12 15

Ind

enta

tio

n l

oad

(μ

N)


600 °C

(a)

117

The hardness (H) and elastic modulus (E) values of the Ti-doped ITO films derived from

nanoindentation experiments are shown in Figure 4.9 (a, b). A difference is observed between

the trends in the hardness results of the two sets of samples. For the 2 at.% samples, the hardness

decreased with increasing annealing temperature, changing from 6.6 to 6.3 GPa when the

annealing temperature increased from 400 to 600 °C. In contrast, for the 4 at.% samples, the

hardness seems to stabilise around 6.8 GPa after annealing at different temperatures, only

showing a slight decrease after annealing at 500 °C. Overall, the results show that the ITO films

with higher Ti concentrations have the highest average hardness. The hardness of the Ti-doped

ITO thin films measured in the present study are generally consistent with those reported by

Zeng et.al (6.5 ± 1.6 GPa) for 250 nm thick sputtered ITO thin films onto glass substrate using a

Berkovich indenter with 6 mN load [395]. Similar trends were also observed by Yen et.al for

ZnO thin films deposited by atomic layer deposition on Si substrates and then post annealed at

different temperatures. They suggested that the decreasing hardness of the ZnO thin films with

increasing annealing temperature can be related to the increment of the grain size with increasing

temperature [396]. Therefore, it seems here that there is no effect of the grain size to the hardness

of the films prepared with 4 at% Ti since they mostly possess the same hardness. However, the

films prepared with 2 at% Ti show a slight improve in the hardness with decreasing their grain

sizes (as seen in section 3.4.1). However they are still less hard than the 4 at.% samples. Also,

the reduction in the hardness could be ascribed to the changes in elemental compositions

resulting from the diffusion process in the coatings, softening of the substrate, stress relaxations

and formation of new phases. As such, the decrease in the hardness of 2 at.% Ti-doped ITO films

with increasing annealing temperature could be attributed to a number of different mechanisms.

In this work, the samples were annealed in atmospheric environment, so it was expected that the

composition of the surfaces would change due to oxidation. Therefore, it is reasonable to assume

that the change in the hardness of Ti-doped ITO samples is induced by the difference in the

oxygen content on the surface of the film with increasing temperature, as proved in the XPS

results (section 4.4.1). These results are in general agreement with those reported by Biswas and

co-authors, which show that there is a relation between the hardness and the oxygen contents in

the ITO film materials. They found that when the oxygen content increased from 6 to 10 wt% in

the precursor solutions, the hardness of the corresponding ITO films decreased from 9.6 ± 0.9

GPa to 1.6 ± 0.1 GPa [397]. Barna et.al and Petrov et.al reported that, oxygen, impurities and

other defects may lead to grain refinement and then change the morphology of the films [398,

118

399]. As a summary, a higher annealing temperature leads to larger grain size (as mentioned

previously in Section 4.4.1), which, when combined with substitutions, interstitials, and/or the

formation of oxygen vacancies and other point defects, may cause a hardness decrement,

although this effect is not so significant in our 4 at.% samples. In relation to the annealing

temperature increment, the Young’s modulus of Ti-doped ITO films decreased for the films of 2

at.% Ti, while it increased for the films of 4 at.% as shown in Figure 4.9 (a and b). However,

both thin films with 2 at.% and 4 at.% Ti contents present similar Young’s modulus in the range

of 137 - 143 GPa, which is comparable to the reported results given by Zeng et.al (120 - 160

GPa) [38] and Chen and Bull (141 GPa)[383, 395].

Elastic modulus along with hardness of material can influence wear behaviour. The wear of a

material coupled with elastic limit define the ability of this material to deform under an applied

stress and regain its initial state without being deformed permanently [400]. The ratio, H/E,

which is obtained from nanoindentation measurements can be used to evaluate the wear

resistance of the coating. The larger this ratio is the higher the wear resistance will be [329]. The Ti-

doped ITO thin films exhibited a decrease in H/E values which are correlated to the reduction of

wear resistance through high annealing temperature, as shown in Table 4.3. It is clear from Table

4.3 that both the hardness and Young’s modulus depends on the annealing temperature. As

discussed previously, the prepared Ti-doped ITO thin films were annealed in air atmosphere

resulting in changes of the composition of the sample surface due to an oxidation process. Thus,

the decrease in the hardness of Ti-doped ITO samples along with increasing annealing

temperature could be attributed to increasing the oxygen contents at the surface of the film. A

similar trend for Young’s modulus was observed with increasing annealing temperature. Thus, it

is possible to surmise that the annealing temperature exerts a negative effect on the mechanical

properties of the Ti-doped ITO film coatings. The H/E values in this study are in the range of

4.5 10-2 – 4.6 10

-2 and 4.8 10-2 - 5 10

-2, for the films of 2 at.% and 4 at.%, respectively. These

values represent a reasonable level of wear resistance, which is even better than other metal

oxide ceramics with H/E values of 3 10-2 - 4 10

-2 [401, 402].

119

6.1

6.2

6.3

6.4

6.5

6.6

6.7

6.8

135

136

137

138

139

140

141

142

143

144

300 400 500 600 700

Har

dnes

s (G

Pa)

Yo

ung's

mo

dulu

s (G

Pa)


Young's modulus

Hardness (H)

(E)

(a)

6.5

6.6

6.7

6.8

6.9

135

136

137

138

139

140

141

142

143

144

300 400 500 600 700

Har

dnes

s (G

Pa)

Yo

ung's

mo

dulu

s (G

Pa)


Young's modulus

Hardness (H)

(E)

(b)

Table 4.3 Mechanical properties of Ti doped ITO films after being annealed at different

temperatures

Ti

concentration

(at.%)

Annealing

temperature °C

Hardness

(GPa)

Young’s

modulus (GPa)

Wear resistance

(H/E 10-2

)

400 6.6 143 4.6

2 500 6.4 141 4.5

600 6.3 139 4.5

400 6.8 137 5.0

4 500 6.7 141 4.8

600 6.8 143 4.8

Figure 4.9 Hardness and Young’s modulus of Ti doped ITO films (a): 2 at % and (b):

4 at % Ti, annealed at different temperatures

120

4.5. Conclusions

Ti-doped ITO thin films were fabricated on glass substrates using a low cost and efficient sol-gel

spin coating method. The effects of Ti concentrations (i.e., 2 at.% and 4 at.% Ti) and annealing

temperature (ranging from 400 - 600 °C) on the phonon vibrational modes, surface chemical

bonding states and mechanical properties of Ti doped ITO thin films were studied using Raman

spectroscopy, XPS and nanoindentation techniques. Raman analysis revealed the existence of

ITO and/or In2O3 vibrational modes. By increasing the annealing temperature some peaks were

inversely altered. The XPS results show that the main component atomic percentages such as In,

Sn and Ti decreased as the annealing temperature increased from 400 to 600 °C, while the O

atomic percentages increased. Also, at 600 °C, the Ti2p and Cl2p signals were completely absent

for 2 at % and 4 at % Ti concentration, respectively, indicating that surface oxidation takes place

at high temperature.

The nanoindentation load-displacement curves confirmed that all the films exhibited continuous

loading curves, indicating that the phenomenon of cracking was absent in the Ti doped ITO films

at both Ti contents. Variations in the hardness (H) and the elastic modulus (E) were observed

with different Ti at.% and annealing temperature. The hardness is within the range of 6.3-6.6

GPa and 6.7-6.8 GPa for 2 at.% and 4 at.% Ti content samples, respectively. In most cases the

hardness values are negatively affected with increasing annealing temperature, and the films with

higher Ti concentration in the ITO films possess the highest average hardness. Synthesized Ti-

doped ITO thin films at different Ti contents show the same Young’s modulus values in the

range of 137 - 143 GPa. A combination of the highest H and E were achieved in the sample of

4% Ti content annealed at 600 °C. All the Ti-doped ITO films appear to possess high wear

resistance. Combining with optimised control over the doping and annealing processes, the

results from this work are expected to help facilitate the engineering design of customizable Ti-

doped ITO thin films for various industrial applications.

121

5.9

6.1

6.3

6.5

6.7

6.9

128

132

136

140

144

148

0 2 4 6 8 10 12

Har

dnes

s (G

Pa)

Yo

ung's

mo

dulu

s (G

Pa)

Ag concentration (%)

Young's modulus

Hardness (H)

60

65

70

75

80

85

90

95

100

300 350 400 450 500 550 600 650 700 750 800

Tra

nsm

itta

nce

%

Wavelength (nm)

Ag-doped ITO

Mechanical

UVvis

5. CHAPTER 5: Improved Mechanical Properties of Sol-Gel

Derived ITO Thin Films via Ag Doping

Paper III

Hatem Taha, Zhong-Tao Jiang, David J. Henry, Amun Amri, Chun-Yang Yin, Afishah Binti

Alias, Xiaoli Zhao, Improved mechanical properties of sol-gel derived ITO thin films via Ag doping.

Mat. Tod. Com., 2018, 14, 210-224.

122

5.1. Abstract

In this work, indium tin oxide (ITO) and silver-doped ITO thin films at different Ag

concentrations (i.e. 2, 4, 6 and 10 at. %) were fabricated via a sol-gel spin coating technique. The

thin film coatings were post annealed at 500 °C. Structural, mechanical, surface morphology,

electrical and optical characteristics of the ITO and Ag-doped ITO thin films were studied as a

function of Ag concentrations. XRD results are consistent with the presence of body centered

cubic structure of the indium oxide polycrystalline phase with a new peak for Ag at 2θ ≈ 38.2°,

indicating that by introducing Ag atoms they do not remarkably change the structural

characteristics of the ITO films. FESEM images display the formation of ITO and Ag-doped ITO

nanocrystals and the average crystallite size increases after annealing treatment as well as the

incorporation of Ag concentration. The mechanical properties of the thin films were

characterised by nanoindentation technique, with significant improvement observed through a

combination of Ag doping and annealing treatment, which helps enhancing the effectiveness of

the grain boundary in obstructing the displacement movements. The observed improvement in E

and H is 14% and 28% respectively, with the hardness and Young’s modulus finally reaching 6.7

GPa and 148 GPa, respectively. Load carrying capability and wear resistance are also enhanced

by doping and annealing treatment. After annealing at 500 C, the thin film coatings with the

electrical resistivity of 2.4×10-4

Ωcm, carrier concentration of 6.8×1020

cm-3

and carrier mobility

of 37 cm2/V s were achieved when the Ag doping level was 4 at.%. On the other hand, the

maximum optical transparency was found to be 92% for both ITO and 2% Ag-doped ITO thin

films after being annealed at 500 °C. The variation of optical constants, such as absorption

coefficient, refractive index and extinction coefficient with different Ag concentrations, were

also studied.

123

5.2. Introduction

Transparent conductive films (TCFs) have received significant attention owing to their desirable

optoelectronic characteristics such as high optical transparency and low electrical resistivity. So

far, indium oxide (In2O3) and/or tin-doped In2O3 (ITO) are well-known materials that have been

widely used as transparent conductive oxides (TCOs) in touch screens, solar cells, liquid crystal

displays, smart windows, light emitting diodes (LEDs) and sensors [217, 403-409]. Other than

possessing good electrical and optical properties, ITO thin films also have good chemical and

thermal stability as well as great adhesion and etching characteristics, compared to other TCOs

such as ZnO, TiO2 and SnO2 [199, 410-413]. Many techniques are used for manufacturing ITO

based thin films such as, thermal evaporation, DC and RF magnetron sputtering, electron beam

evaporation, pulsed laser deposition, screen printing, spray pyrolysis and sol-gel methods [26,

212, 414-419]. The most common fabricating methods are based on sophisticated equipment

such as high vacuum systems combined with physical vapour deposition (PVD) processes

supplied with high calcine temperature and complicated controlling unit. These complicated and

harsh processes, which result in very high end products, have greatly encouraged the

development of vacuum-free and low-cost techniques. The sol-gel method combined with a spin

coating process has certain advantages such as cost effectiveness (no vacuum requirements), low

temperature, easy control of the doping level, shorter processing time, and the possibility of

producing coatings with high scalability along with the opportunity of fine-tuning the preferred

shapes and surface areas has been widely used so far for fabricating TCFs [182, 261, 420-422].

To improve the performance of ITO thin films, numerous studies have been carried out to

improve either their electrical conductivity or optical transparency. Introducing suitable foreign

dopants such as Ti, Cr, Cu, Mo, Fe, Nb and Ag to the ITO matrix were found to be beneficial for

improving both the electrical conductivity and optical transparency [423-425]. Among these

dopants, silver (Ag) was found to be favoured due to its low electrical resistivity compared to

other bulk materials [426-429]. However, there have been only limited efforts and attempts to

improve the mechanical characteristics of ITO thin films. In this study, simple and low cost spin-

coating derived ITO and Ag-doped ITO thin films were produced. The concentration of the ITO

solution was fixed at 0.1 M, while the silver (additive) doping concentration varied (2%, 4% 6%

and 10%). All the samples were post annealed at 500 °C. Owing to the brittle surface of ITO, it

is thought that by introducing a foreign metal element with better mechanical properties than

124

indium in ITO matrix may improve the mechanical properties of the thin films. A review of the

literature indicated that both high Ag doping concentration (10%) and high annealing

temperature (500 °C) for Ag-doped ITO thin films have not been investigated in detail. We

assume that higher Ag concentration may result in enhancing the mechanical properties along

with maintaining the optoelectronic characteristics of ITO thin films. Furthermore, based on our

previous study [430], a better optoelectronic characteristic can be achieved by applying

annealing process. Therefore, the main objective of the present investigation is to study the role

of Ag additive and annealing temperature on the mechanical and optoelectronic properties of sol-

gel synthesized Ag-doped ITO thin films.

5.3. Experimental Methods

5.3.1. Raw Materials

ITO and Ag-doped ITO solutions were prepared by using hydrated indium nitrate

(In(NO3)3.5H2O, Alfa Aesar, 99.9%), tin chloride dihydrate (SnCl2.2H2O, Chem-supply, 98%,),

Silver nitrate (AgNO3, Chem-supply, 99.5%) and polyvinylpyrrolidone (PVP, M = 40000g/mol).

All of these chemicals were used as received. Absolute ethanol was employed as a solvent and

the ITO and Ag-doped ITO thin films were deposited on soda-lime glass substrates.

5.3.2. Sol-Gel and Thin Films Preparation

A 40 ml solution of ITO (0.1M) was prepared by separately dissolving the required amounts of

In(NO3)3.5H2O and SnCl2.2H2O into two separate beakers with 20 ml absolute ethanol. The two

solutions were stirred for 1 hour, then mixed together and refluxed at 60 °C for 1 hour until a

transparent solution was obtained. Then, the resultant solution was divided into five separate

beakers each with 8 ml of ITO solution. The required amounts of AgNO3 and PVP were then

added to four of the previous ITO solutions to make Ag-doped ITO solutions with 2, 4, 6 and 10

at.% Ag concentrations then refluxed at 60 °C for 1 hour. The role of PVP is to provide a

hydrophobic layer to prevent the agglomeration of Ag particles through the preparation of

solutions. The final solutions were stored at room temperature for 24 hours prior to deposition

processes. Soda-lime glass substrates (25×25 mm) were sonicated in absolute ethanol followed

by milliQ water at 60 °C for 10 minutes, then dried under vacuum in a drying oven at 100 °C for

125

30 minutes. The deposition process included: (1) dispensing the solution at 300 rpm for 15

seconds, (2) spreading the solution at 2500 rpm for 20 seconds and (3) drying the solution at

4000 rpm for 20 seconds. After each deposition step, the sample was calcined at 150 °C for 10

minutes. These steps were repeated until the desired film thickness was achieved. Samples

referred to as deposited samples were calcined but not annealed. The prepared samples were then

annealed at 500 °C for 2 hours in air atmosphere.


The crystallographic structure of the ITO and Ag-doped ITO thin films were examined by X-ray

diffraction (XRD) analysis via a GBC EMMA diffractometer with CuKα radiation (=0.154nm).

All the XRD measurements were carried out at a diffraction angle (2) in the range of 25° - 70°

with a scanning rate of 2°/min and 2 step size of 0.02. The surface morphology was observed

via Philips XL 20 SEM equipped with an electron dispersive X-ray (EDX) analysis column and a

Zeiss Neon 40EsB Field Emission Scanning Electron Microscopy (FESEM). Prior to FESEM

imaging, the samples were first mounted onto the substrates holder using carbon tape followed

by coating with platinum (about 3nm thick) to minimize the charging effects. A 4-point probe

(Dasol Eng. FPP-HS8) was used to measure the electrical properties of prepared films and the

Hall-effect measurements were carried out on specimens of (10×10 mm) using ECOPIA HMS-

2000 system at 0.9 Tesla of magnetic field and Polytec Bipolar Operation Power supplies (BPO).

Au contacts were thermally evaporated on the coated samples in a system of four probes. The

transmittance spectrum was obtained by using UV-VIS Perkin Elmer spectrometer in a

wavelength range of 250 - 800 nm. The mechanical characterisation was performed using a

nanoindentation workstation (Ultra-Micro Indentation system 2000, CSIRO, Sydney, Australia),

equipped with a diamond Berkovich indenter. The area function of the indenter tip was

calibrated using a standard fused silica specimen. The measurements were collected with

maximum loading of 200 nN and maximum penetration depth less than one-tenth of the coating

thickness. In order to attain better resolution, the number of measuring points has been increased

to 15 during loading and 20 during unloading.

126


5.4.1. Crystallographic Structure Properties

In this investigation, the films prepared with 4 at.% and 6 at.% Ag concentration exhibit quit

similar electro-optical characteristics. Therefore, only the results related to 4 at.% Ag are

discussed in terms of structural, morphological, electrical and optical properties. Figures 5.1 and

5.2 show the XRD patterns for the as deposited (150 °C) and post annealed spin coated ITO and

Ag-doped ITO thin films, respectively. The XRD data for all the samples are consistent with the

presence of polycrystalline features of a body centred cubic bixbyite structure with space group

of la3/cubic similar to In2O3 (JCPDS card 06-0416) with a lattice parameter of 10.118 Å. The

main features for the prepared and post annealed thin films are assigned to be (222), (400), (411),

(332), (440) and (622) ITO reflection directions of In2O3 as well as (111) Ag reflection direction

located around (2θ ≈ 38.2°) for the films of Ag-doped ITO at all Ag concentrations. These results

confirm the observation that the polycrystalline feature of In2O3 may act as a network with a

body centred cubic bixbyite structure and the Ag atoms are well incorporated in the ITO matrix

without any observable changes in the crystal structure of the host. Furthermore, there aren’t any

adulterated phases related to tin compounds observed in the XRD patterns of ITO and Ag-doped

ITO thin films. Both as deposited ITO and Ag-doped ITO thin films show poor crystallinity due

to the crystallization temperature of ITO to be in the range of 150-160 °C [68, 353]. Since the

variation of Ag concentration has not altered the as-deposited crystalline structure for Ag-doped

ITO samples, one XRD spectrum of the thin film with 4 at.% Ag was used as an example in

Figure 5.2. After the annealing process, the increase of Ag concentration slightly enhanced the

Ag phase (111) reflection direction as expected, see Figure 5.2. Mirzaee et al. [431] have

proposed that doping of ITO with silver leads to formation of silver nanoparticles within the

structure of In2O3. They suggest that the reduction of silver ions is facilitated by a redox reaction

with tin as described in the following relation:

Sn2+

+ 2Ag+ → Sn

4+ +2Ag

0 (5.1)

It is also clear from Figures 5.1 and 5.2 that the ITO and Ag-doped ITO thin films had improved

crystalline quality, evident by the intensity of the (222) peak as the annealing temperature (500

127

°C) was applied. However, there is a slight shift in the main peak (i.e., (222)) position toward

smaller angle of 2θ for the synthesized thin films compared to the standard JCPDS of In2O3. This

indicates that the occurrence of lattice expansion is possibly due to the substitution of In3+

atoms

by both Sn4+

and Ag atoms having different atomic radii. The atomic radii for In, Sn and Ag are

0.09, 0.07 and 0.126 nm, respectively. This phenomenon can be attributed to the fact that

electrons are thermally excited from the donor impurities bands to the conduction band after

applying annealing temperature. These electrons are mainly characterized as Sn and Ag 5s state

with a Sn-O and Ag-O anti-bonding character. The occupations in these anti-bonding states

enhance the reduction of Sn and Ag, which is lower than the forces between Sn and O, Ag and O

as well as the overall energy of the crystal structure, thus inducing the lattice expansion [432,

433]. With the combined effects of both ionic radii and occupations in the anti-bonding states,

the lattice variation can impact the electronic band structure, thereby the electronic properties.

Another consideration for this shift could be related to the strain effect due to thermal expansion

misfit between the substrate and the thin film.

20 25 30 35 40 45 50 55 60 65

Inte

nsi

ty (

a.u.)

2θ (degree)

Annealed at 500° C

As deposited

(22

2)

(62

2)

(44

0)

(41

1) (4

00

)

Figure 5.1 The XRD patterns of as deposited and annealed ITO thin films

128

Table 5.1 Structural parameters for as deposited and annealed ITO and Ag-doped ITO thin films

Ag

content

(at.%)

2θ (in

degrees)

d-spacing

(Å)

(h k l) Lattice

constant (Å)

Crystallite

size (nm)

0 30.44 2.933 (222) 10.16 23

2 30.46 2.932 10.15 31

As deposited 4 30.52 2.925 10.13 26

10 30.54 2.923 10.12 24

0 30.38 2.938 (222) 10.17 41

2 30.42 2.935 10.16 54

Annealed at 500°C 4 30.44 2.933 10.16 43

10 30.50 2.927 10.14 33

25 30 35 40 45 50 55 60 65

Inte

nsi

ty (

a.u.)

2θ (degree)

(22

2)

(40

0)

(44

0)

(41

1)

(33

2)

(62

2)

As deposited

10 at.% Ag

4 at.% Ag

2 at.% Ag

(11

1)

Ag

Figure 5.2 The XRD patterns of as deposited and annealed Ag-doped ITO films

prepared at different Ag concentrations

129

Debye Scherrer equation (Eq. 2.26) was used to determine the crystallite size (D) from the full

width at half maximum (FWHM) of the main peak (222).

Also, the lattice constant and the related inter-planar spacing (d-spacing) were calculated using

Bragg's law and related standard Bravias lattice for cubic system (Eq. 2.25) and (Eq. 2.27),

respectively. The calculated results are presented in Table 5.1.

From Table 5.1 and Figure 5.3, it is clearly shown that the calculated crystallite size of the ITO

and Ag-doped ITO thin films at different Ag content is increased after being annealed at 500 °C.

However, the Ag-doped ITO samples showed bigger crystallite sizes than those for pure ITO.

This can be related to the effect of Ag additive that may act as nucleation centres, resulting in the

formation of large crystallites. Also, by enlarging the crystallite sizes for ITO and Ag-doped ITO

thin films along with applying annealing temperature suggest that annealing can improve the

crystallization and enhance the substitution of In3+

by Sn4+

and Ag ions [61]. Furthermore, it has

been reported that the Ag atoms may also vibrate resulting in an annealing effect in the ITO

structure which promotes the nucleation process and growth of the thin film along the preferred

direction [60]. We also note that crystallite sizes in our study are substantially larger than those

reported by Mirzaee et al. annealed at 350 °C [431]. Indeed, the average calculated lattice

constants for all the as deposited and post annealed ITO and Ag-doped ITO thin films were

estimated to be slightly larger than the reported value for un-doped indium oxide of 10.118Å.

The variation in the lattice constants could be due to the size differences of the dopant atoms

(i.e., Sn and Ag) as well as the induced stress near the film surface through both doping and

annealing process. Furthermore, since Ag is an amphoteric dopant, it can be considered that

some of these ions may occupy interstitial sites or become scattered or clustered centres in the

ITO matrix.

130

5.4.2. Morphological and EDX Analysis

Figures 5.4 presents high magnification FESEM images of the synthesized ITO and Ag-doped

ITO at (2, 4 and 10 at.% Ag content) thin films after being annealed at 500 °C. All thin films

show uniform, dense, smooth and homogeneous surfaces with a grain structure indicating the

nanocrystalline features. However, the Ag-doped ITO thin films prepared at different Ag

contents have larger crystallite sizes compared to that of pure ITO. This can be related to the

effect of Ag dopants that may agglomerate around the preferred orientation resulting in the

formation of large crystallites, thereby improving the crystalline quality of Ag-doped ITO thin

films as noticed previously in the XRD results. These results are in good agreement with those

reported by Jeong et al. [434] for Ag doped ZnO. Figure 5.5 show the EDX analysis of the ITO

and Ag- doped ITO thin films. The results reveal the existence of the main elements (In, Sn, Ag,

and O) in each sample coatings.

0

10

20

30

40

50

60

0 2 4 6 8 10 12

Cry

stal

lite

siz

e (n

m)

Ag content (at.%)

As-deposited

Annealed at 500 °C

Figure 5.3 Variations the crystallite size of as deposited and annealed ITO

and Ag-doped ITO thin films with Ag concentration

131

4%

ITO

10%

2%

0 1 2 3 4 5

Co

unts

Energy (KeV)

Ag Mg

Na

O

In Sn

In

Si

Al

Pure ITO

4 at% Ag-doped ITO

Figure 5.4 FESEM images of annealed ITO and Ag-doped ITO thin films at different Ag

concentrations

Figure 5.5 EDX analyses of annealed (a): ITO and (b): 4 at% Ag-doped ITO thin films

132


The electrical properties of ITO and Ag- doped ITO thin films were investigated by means of 4-

point probes and Hall-Effect measurements. Figure 5.6 presents the variation of electrical

resistivity and conductivity (σ = 1/ρ) with Ag concentration. The as deposited ITO thin film has

a resistivity of 7.4×10-4

Ωcm with carrier concentration and mobility values of 2.8×1020

cm-3

and 29 cm2/Vs, respectively, as shown in Figure 5.7a. The resistivity of as deposited Ag-doped

ITO thin films decreased from 7.1 to 5.8 ×10-4

Ωcm as the Ag concentration increased from 2

at.% to 4 at.% respectively. Also, it is observed from Figure 5.6a that the resistivities for as

deposited thin films with 2 at.% and 4 at.% Ag concentration were lower than that from pure

ITO thin films. When Ag concentration increased to 10 at.%, the resistivity showed a rapid

increase reaching its maximum value of 8×10-4

Ωcm as seen in Figure 5.6a. This increment in

the electrical resistivity at higher Ag concentration is likely to be related to the solubility limits

of Ag in the ITO matrix. Therefore, the Ag atoms may occupy random sites in the ITO structure,

resulting in an increase of lattice distortion. Also, at higher Ag doping concentration, the increase

of ionized impurities scattering and electron-electron scattering leads to increasing the resistivity

of Ag-doped ITO as well [431]. Ag doping contributes a large number of electrons, thereby,

increasing the carrier concentration in the thin film. This is seen in Figure 5.7a where the as

deposited film of 4 at.% Ag concentration has a high carrier concentration of 3.9×1020

cm-3

.

However, higher dopant concentration can impair the carrier mobility since these dopants may

act as trapping centres for the free carriers. Therefore, the carrier concentration and mobility

were found to be lower than expected for the film prepared with higher Ag doping rate [435].

This suggests that a portion of these dopants may remain electrically inactive, hence, the carrier

concentration and the mobility of the as deposited film of 10 at.% Ag content decreased to values

of 3.5×1020

cm-3

and 22 cm2/Vs, respectively, as seen in Figure 5.7a.

When the annealing temperature was applied on the fabricated samples, the resistivity for ITO

film decreased to 2.7×10-4

Ωcm, while the carrier concentration and the mobility increased to

6.5×1020

cm-3

and 35 cm2/Vs, respectively. The Ag-doped ITO samples showed the lowest

resistivity value of 2.4×10-4

Ωcm for the film of 4 at.% Ag content as shown in Figure 5.6b.

Annealing temperature leads to an improvement in the crystalline quality along with increasing

the crystallite size of fabricated samples as evident in the XRD results. As a result, the grain

133

boundaries decreased and the trapped electrons were released which led to an increase in the

carrier concentration. For the films of Ag-doped ITO, increasing the crystallite size after

annealing treatment may be due to the growth of the coalescence of Ag particles in the film

matrix, which may provide effective conducting paths for free carriers [179, 436]. Moreover,

annealing treatment enhances substitution of indium atoms by tin and silver atoms, yielding an

increase in carrier concentration as well as conductivity in the thin films. This is evident in

Figure 5.7b where the carrier concentration and the mobility of Ag-doped ITO thin films

increased especially for that of 4 at.% Ag contents reaching their maximum values of 6.8×1020

cm-3

and 37 cm2/Vs, respectively. The film with 10 at.% Ag content exhibits an increase in the

carrier concentration reaching 6.3×1020

cm-3

after being annealed at 500 °C, compared with

6.0×1020

cm-3

obtained from the 2 at.% Ag sample. Meanwhile, in the former thin film the carrier

mobility decreased to 22 cm2/Vs due to the increase of electron-electron scattering and

impurities scattering, as discussed before which in turn reduced the conductivity. It is interesting

to notice that, the film with 10 at.% Ag content has a higher carrier concentration; however, it

also has a higher electrical resistivity compared to that prepared with 2 at.% Ag content. This is

because the lower resistivity and/or the higher conductivity basically depends on both carrier

concentration (N) and mobility (μ) based on the relation of σ = N e μ.

Table 5.2 Electrical and optical properties for as deposited and annealed ITO and Ag-doped ITO

thin films

Ag

(at.%)

T

(%)

Eg

(eV)

ρ ×10-4

(Ωcm)

n ×1020

(cm-3

)

μ

(cm/Vs)

Φ×10-3

(Ω-1

)

0 88.0 3.69 7.4 2.8 29 13

2 88.5 3.71 7.1 3.2 26 14

As deposited 4 88.0 3.68 5.8 3.9 27 16

10 85.0 3.67 8.0 3.5 22 8

0 92.0 3.74 2.7 6.5 35 56

2 92.0 3.75 3.2 6.0 32 47

Annealed at 500°C 4 90.0 3.72 2.4 6.8 37 50

10 87.0 3.67 4.3 5.6 25 20

134

0

1000

2000

3000

4000

5000

1

2

3

4

5

6

7

8

9

0 2 4 6 8 10 12

Co

nd

uct

ivit

y (

S.c

m-1

)

Res

isti

vit

y×

10

-4(Ω

.cm

)

Ag content (at.%)

0

1000

2000

3000

4000

5000

1

2

3

4

5

0 2 4 6 8 10 12

Co

nd

uct

ivit

y (

S.c

m-1

)

Res

isti

vit

y×

10

-4(Ω

.cm

)

Ag content (at.%)

(b)

(a)

Figure 5.6 Electrical resistivity and conductivity of ITO and Ag-doped ITO thin films with

different Ag concentration, (a): as deposited and (b): annealed

135

2.8

3

3.2

3.4

3.6

3.8

4

0

5

10

15

20

25

30

0 2 4 6 8 10 12

Car

rier

co

nce

ntr

atio

n×

10

20 (

cm-3

)

Mo

bil

ity (

cm/V

s)

Ag content (at.%)

5.8

6

6.2

6.4

6.6

6.8

7

20

25

30

35

40

0 2 4 6 8 10 12C

arri

er c

on

cen

trat

ion×

10

20 (

cm-3

)

Mo

bil

ity (

cm/V

s)

Ag content (at.%)

(a)

(b)

Figure 5.7 Mobility and carrier concentration of ITO and Ag-doped ITO thin films with different

Ag concentration, (a): as deposited and (b): annealed

136


The transmittance spectra of as deposited and annealed pure ITO and Ag-doped ITO thin films

prepared at different Ag content (i.e., 2, 4, and 10) at.% are shown in Figure 5.8. The optical

transparency of as deposited ITO thin film was found to be 88%. When the Ag dopants were

introduced to the ITO film materials, the optical transparency of as deposited Ag-doped ITO thin

film with 2 at.% Ag content was slightly enhanced achieving 88.5 % due to the enlargement of

the crystallite size. This is proved by the XRD and FESEM results while, the film with 4 at.% Ag

appears to have the same transparency as the ITO thin film. However, with further increase of

the Ag content to 10 at.%, the optical transparency of Ag-doped ITO thin film decreased to a

value of 85% as seen in Figure 5.8a, which could be related to increasing both the electrons and

impurities scattering along with increasing the lattice distortion at high doping concentrations.

When the annealing temperature of 500 °C was applied on the deposited ITO and Ag-doped ITO

thin film, the transparency was increased in all of the samples reaching its maximum value of

92% for both pure ITO and 2 at.% Ag-doped ITO thin films (Figure 5.8b). Higher annealing

temperature leads to an improvement of the crystalline quality of the prepared samples through

increasing the crystallite size and decreasing the grain boundaries (as noticed previously in the

XRD analysis), and then higher optical transparency. It is also observed from Figure 5.8 that the

transmittance threshold for as deposited and annealed Ag-doped ITO thin films shift to lower

energy compared with that for pure ITO spectra. This can be related to the Burstein-Moss shift

(shift of Fermi level due to high carrier concentration in the conduction band) [437, 438].

137

60

65

70

75

80

85

90

95

100

300 350 400 450 500 550 600 650 700 750 800

Tra

nsm

itta

nce

%

Wavelength (nm)

10%

4%

2%

ITO

60

65

70

75

80

85

90

95

100

300 350 400 450 500 550 600 650 700 750 800

Tra

nsm

itta

nce

%

Wavelength (nm)

10%

4%

2%

ITO

(a)

(b)

Figure 5.8 Transmittance spectra of ITO and Ag-doped ITO thin films with different Ag

concentration, (a): as deposited and (b): annealed

138

The band gap energy (Eg) was obtained from the transmittance data using (Eq. (3.1).

The Tauc Plot of photon energy hv versus (αhv)2

was used to obtain the band gap energies of the

synthesised ITO and Ag-doped ITO thin films by extrapolating the linear part of the plotted

curves along with (αhv = 0).

The estimated energy band gap of ITO and Ag-doped ITO thin films for both as deposited and

after being annealed at 500 °C are presented in Figure 5.9. The results show that the as deposited

ITO and Ag-doped ITO thin films have smaller energy band gaps than those annealed at 500 °C

indicating a band gap broadening occurring after thermal treatment. The energy band gap

increment is possibly due to the improvement of crystalline structure and increasing the

crystallite size, as well as carrier concentration, in the films. The optical band gap energy for ITO

thin film increased from 3.69 eV for as deposited film to 3.74 eV after being annealed at 500 °C.

Thin film with lower Ag concentration, such as 2 at.%, exhibits slightly larger energy band gap

than that of ITO, i.e., 3.71 eV and 3.75 eV for as deposited and annealed thin films, respectively.

However, with the increase of Ag concentration, the optical band gap decreased due to the

increase of electron and ion impurities scattering at high doping concentration (Burstein-Moss

effect) [162, 439]. The films with 10 at.% Ag content appears to have the lowest optical band

gap energy of 3.67 eV for both as deposited and after being annealed at 500 °C. Relevant optical

properties are listed in Table 5.2.

139

Figure 5.9 Optical band gap of ITO and Ag-doped ITO thin films with different Ag

concentration, (a): as deposited and (b): annealed

3.5 3.55 3.6 3.65 3.7 3.75 3.8 3.85 3.9 3.95 4

Photon energy (eV)

10%

4%

2%

ITO


)

(b)

3.5 3.55 3.6 3.65 3.7 3.75 3.8 3.85

Photon energy (eV)

10%

4%

2%

ITO


)

(a)

140

The absorption coefficient (α) of the ITO and Ag-doped ITO thin films was calculated by using

the formula as below [440]:

(

) (5.2)

where t is the film thickness and T is the transmittance value.

Variation of the absorption coefficient for as deposited and annealed thin films with respect to

wavelength in the range of (350-800) nm is presented in Figure 5.10. From Figure 5.10, it is seen

that the absorption edge for Ag-doped ITO films shifts to longer wavelength and the thin films

show higher absorption coefficient compared to pure ITO samples. This phenomenon suggests

that the increase of Ag doping level enhances the absorption in the UV – Deep-UV region. Thus,

as noticed previously in the optical band gap calculations, the as deposited and annealed Ag-

doped ITO thin films show larger absorption coefficient, especially, those prepared with 4 % and

10 at.% Ag concentrations.

The performance of our ITO and Ag-doped ITO thin films as transparent conductive materials

was further evaluated by means of figure of merit (Φ). The figure of merit is related to the sheet

resistance and the transmittance of the synthesized transparent conductive thin films, which also

indicates the optimum cooperation of these two values in the performance of the film. The figure

of merit was defined as T10

/Rs by Haacke [441], and/or T/Rs by Fraser and Cook [442], where Rs

is the sheet resistance and T is the transmittance of the film. The higher the figure of merit value,

the better the performance of the transparent conductive device. In this study, the ‘T’ value was

selected at a wavelength of 550 nm for all the samples, the results are presented in Table 5.2. The

highest values of Φ for as deposited and annealed thin films were found to be 16×10-3

and

56×10-3

for 4 at.% Ag and ITO thin films respectively. These results are comparable with those

reported previously [355, 443].

141

350 400 450 500 550 600 650 700 750 800

Ab

sorp

tio

n c

oef

fici

ent

(α)

Wavelength (nm)

10%

4%

2%

ITO

350 400 450 500 550 600 650 700 750 800

Ab

sorp

tio

n c

oef

fici

ent

(α)

Wavelength (nm)

10%

4%

2%

ITO

(a)

(b)

5.4.5. Determination of Optical Constants

The optical constants of thin film materials such as refractive index (n) and extinction coefficient

(k) are very important aspects for achieving good performance in optoelectronic devices. For

instance, the refractive index of a material is believed to be a measure of its transparency to

incident radiation within a certain area of light spectra. The assessment of the refractive index is

notably very important in integrated optoelectronic devices [444]. The extinction coefficient k of

Figure 5.10 Variation the absorption coefficient of ITO and Ag-doped ITO thin

films with different Ag concentration, (a): as deposited and (b): annealed

142

ITO and Ag-doped ITO thin films was calculated from (Eq. 2.19), while the refractive index n

was calculated from the following formulae:

(

) √

( ) (5.3)

where R is reflectance and k is the extinction coefficient.

Figure 5.11 shows the variation of refractive index of as deposited and annealed ITO and Ag-

doped ITO thin films. It is seen in Figure 5.11 that the average value of refractive index for Ag-

doped ITO samples is higher than that for pure ITO. This could be due to increasing the carrier

concentration in the Ag-doped ITO thin films along with increasing the Ag concentration.

Increasing carrier concentration at high Ag doping level leads to the formation of Ag clusters

along with defects, which hinders the optical transparency. Chiou and Tsai [445] reported similar

results from antireflective sputtered ITO thin films on glass substrates. Also, Cao et.al, [446]

concluded that the increase of the doping level results in the enhancement of the average value of

n for Ag-doped ITO thin films. The average value of n decreased for annealed ITO and Ag-

doped ITO thin films. However, all the as deposited and annealed samples have large average

refractive index indicating that these types of coatings may become the ideal replacement for the

common antireflection coating.

The extinction coefficient of the as prepared and annealed ITO and Ag-doped ITO thin films is

presented in Figure 5.12. The extinction coefficient of all samples initially decreases in the near

ultraviolet (NUV) region. However, Ag-doped ITO samples show higher average extinction

coefficient values than that of pure ITO which is related to increasing light absorption by Ag

atoms. The extinction coefficients gradually increase in the visible range. This behaviour of Ag-

doped ITO thin films can be useful in the applications of visible light transmittance and UV light

interception. Also, the average extinction coefficient values obtained in this study are of low

order over the entire range of studied wavelength indicating the homogeneity of the samples and

smoothness of the surfaces [447].

143

1.23

1.24

1.25

1.26

1.27

350 400 450 500 550 600 650 700 750 800

Ref

ract

ive

ind

ex (

n)

Wavelength (nm)

10%

4%

2%

ITO

1.23

1.24

1.25

1.26

1.27

350 400 450 500 550 600 650 700 750 800

Ref

ract

ive

ind

ex (

n)

Wavelength (nm)

10%

4%

2%

ITO

(a)

(b)

Figure 5.11 Variation the refractive index of ITO and Ag-doped ITO thin films with


144

0.5

1

1.5

2

2.5

300 350 400 450 500 550 600 650 700 750 800

Exti

nct

ion c

oef

fici

ent

(k)

Wavelength (nm)

10%4%2%ITO

0.5

1

1.5

2

2.5

300 350 400 450 500 550 600 650 700 750 800

Exti

nct

ion c

oef

fici

ent

(k)

Wavelength (nm)

10%

4%

2%

ITO

(a)

(b)

5.4.6. Mechanical Characteristics

In this study, the thickness of fabricated ITO and Ag-doped ITO thin films is around 350 nm;

therefore, it is essential to use a suitable indentation depth to avoid any influence from the

substrate that may affect the nanoindentation measurements. It is well known that the

nanoindentation measurements should be performed on an indentation depth less than 10 at.% of

the film thickness to assure that the obtained results only reflect the mechanical properties of the

film coating. In order to obtain accurate values of both the hardness H and the elastic (Young’s)

Figure 5.12 Variation the extinction coefficient of ITO and Ag-doped ITO thin films with


145

modulus E for the ITO and Ag-doped ITO films, the nanoindentaion measurements were

collected at a maximum indenter depth of 13 nm which is less than one-tenth the thickness of the

fabricated ITO and Ag-doped ITO thin film.

Since it shows high mechanical properties, the results of the film with 6% Ag concentration have

been included in this section. Figure 5.13 (a) and (b) shows the typical load-displacement (p-h)

curves determined from the nanoindentation measurements for the as deposited and annealed

ITO/Ag-doped ITO samples, respectively. These curves reflect the deformation history of the

synthesized thin films throughout penetration of the Berkovich indenter load with continuous

contact stiffness measurements mode. It was reported by Jian and Lee that establishment of

cracking in films underneath the nanoindentor resulted in a distinct discontinuity in the loading

part of the loading displacement curve [394]. In this study, the experimental nanoindentation

results presented in Figure 5.13 (a) and (b) confirmed that continuous loading curves were

obtained for all the films, indicating that no cracking occurred during indentation for all the

samples being studied.

Both H and E were evaluated from the measurements results of indentation obtained during the

loading and unloading process. Maximum load, maximum depth and the stiffness (S) can be

determined from the load-displacement (p-h) curve. Hardness is defined as a measure of how

well the surface of a solid specimen can withstands deformation under an applied force.

Hardness and Young’s modulus of as deposited and annealed ITO and Ag-doped ITO thin films

were calculated based on (Eq. 2.33), (Eq. 2.34) and (Eq. 2.35).

The hardness and Young’s modulus values of as deposited as well as annealed ITO and Ag-

doped ITO thin films are presented in Figure 5.14. From Figure 5.14a, several as deposited thin

films (i.e., 0 at.%, 2 at.% and 4 at.% Ag-doped ITO thin films) show similar hardness and

Young’s modulus values, around 5.3 GPa and 129 GPa, respectively. In contrast, the as

deposited thin films with 6 at.% and 10 at.% Ag show higher hardness and Young’s modulus

values, which were calculated to be 5.9 GPa, 5.7 GPa and 133 GPa, 134 GPa, respectively. The

high values of H and E for the films prepared with 6 at.% and 10 at.% Ag concentration could be

attributed to metallic Ag cluster formation. Therefore, it may be naturally postulated that

increased Ag concentration within the ITO matrix frame above a certain level would help to

enhance both the H and E values.

146

50

75

100

125

150

175

200

225

0 3 6 9 12 15

Ind

enta

tio

n l

oad

(μ

m)


ITO2%4%6%10%

50

75

100

125

150

175

200

225

0 3 6 9 12 15

Ind

enta

tio

n l

oad

(μ

m)


ITO2%4%6%10%

(a)

(b)

Figure 5.13 Typical load-displacement curves of ITO and Ag-doped ITO thin films with


Figure 5.14b presents the variations of both the hardness and Young’s modulus of the annealed

ITO and Ag-doped ITO thin films. By applying annealing on the prepared samples at 500 °C, the

H and E values are significantly improved, with the 6 at.% Ag sample reaching the maximum

values of 6.8 GPa and 146 GPa, respectively. It is clear, from Table 5.3 that there is

improvement in the mechanical properties for Ag doped films over Ti-doped ITO films, which

147

were reported in our previous study [448]. It has been proposed that, the main factors governing

mechanical characteristics of nanostructure materials include: the chemical and physical nature,

and the structural composition [449]. The surface morphology of the thin films (as seen in

previous morphological and EDS analysis section) seems also to be linked to the mechanical

characteristics, which is reflected in the fact that the denser and smoother the surface is, the

higher the mechanical parameters H and E. Also, the as-deposited samples show lower degree of

crystallinity as proved by XRD measurements. While annealed ITO and Ag-doped ITO thin

films show improvement in the crystalline structure, which positively affects the mechanical

properties of the samples. As mentioned previously, the film with higher Ag concentration shows

improved mechanical properties, which could be related to the formation of smaller crystallite

size compared to other thin films (as confirmed in section 5.4.1). Variations in H and E were also

reported by Yen and co-authors [396] for ZnO thin films fabricated on silicon substrates via

atomic layer deposition and then post annealed at different temperatures. In their study, the

increase of the crystallite size of ZnO thin films along with increasing annealing temperature

results in the reduction in the mechanical properties.

Table 5.3 Mechanical properties for as deposited and annealed ITO and Ag-doped ITO thin films

T°C Ag%

Hardness

(GPa)

Young’s

modulus (GPa)

Wear resistance

(H/E 10-2

)

As deposited 0 5.3 128 4.1

2 5.3 130 4.0

4 5.4 129 4.1

6 5.9 133 4.4

10 5.7 134 4.2

Annealed at 500°C 0 6 134 4.4

2 6 136 4.4

4 6.4 141 4.5

6 6.8 146 4.6

10 6.7 148 4.5

148

5

5.2

5.4

5.6

5.8

6

127

128

129

130

131

132

133

134

135

0 2 4 6 8 10 12

Har

dnes

s (G

Pa)

Yo

ung's

mo

dulu

s (G

Pa)

Ag concentration (at.%)

Young's modulus

Hardness (H)

5.9

6.1

6.3

6.5

6.7

6.9

128

132

136

140

144

148

152

0 2 4 6 8 10 12

Har

dnes

s (G

Pa)

Yo

ung's

mo

dulu

s (G

Pa)

Ag concentration (at.%)

Young's modulus

Hardness (H)

(a)

(b)

Figure 5.14 Variation the hardness and Young’s modulus of ITO and Ag-doped ITO thin films

with different Ag concentration, (a): as deposited and (b): annealed

149

The dependence of material hardness on the grain size may be described by the Hall-Petch

formula [450]:

( ) +

⁄ (5.4)

where H0 denotes the lattice friction stress, KH-P is the Hall-Petch constant and D is the grain size.

Figure 5.15 (a and b) presents the variation of hardness with D-1/2

of as deposited and annealed

ITO/Ag-doped ITO thin films prepared at various Ag concentrations. Although this analysis does

not show strong correlation between hardness and particle size, it facilitates the calculation of

other parameters. Two sets of different values, i.e., lattice friction stress (4.5722, 3.7285) GPa

and Hall-Petch constant (4.8135, 17.148) GPa.m1/2

for as deposited and annealed samples

respectively, were derived. The obtained average H0 value is over the reported value of intrinsic

stress for ITO (ca. 2.1 GPa) [451]. The Hall-Petch values also indicate the effectiveness of the

grain boundary in obstructing the displacement movements. In this case, the annealed samples

demonstrated a larger value, indicating that here the decrease in grain size has a stronger

influence in enhancing the hardness.

The wear resistance of the specimen can be predicted from the H and E values. The H/E value is

considered to be a significant factor for predicting wear resistance, and the larger this ratio the

higher the wear resistance will be [329]. The obtained H/E values for as deposited and annealed

ITO and Ag-doped ITO thin films were in the range of (4.0×10-2

- 4.2×10-2

) and (4.4×10-2

-

4.6×10-2

), respectively, indicating a higher level of wear resistance for the annealed films over

the as-deposited samples, and Ag-doped samples over the pure ITO thin films.

150

4.75

5

5.25

5.5

5.75

6

0.17 0.175 0.18 0.185 0.19 0.195 0.2 0.205 0.21

Har

dnes

s (G

Pa)

D-1/2 (nm-1/2)

(a)

10%

6%

4%

2% 0%

5.5

5.75

6

6.25

6.5

6.75

7

0.13 0.135 0.14 0.145 0.15 0.155 0.16 0.165 0.17 0.175

Har

dnes

s (G

Pa)

D-1/2 (nm-1/2)

(b)

2% 0%

4%

6% 10%

Figure 5.15 Variation of hardness with grain size (D-1/2

) of ITO and Ag-doped ITO thin films

with different Ag concentration, (a): as deposited and (b): annealed

To compare the load carrying capability of the samples under investigation, finite element

modelling (FEM) has been performed on as-deposited ITO, as-deposited 6 at.% Ag-doped ITO,

and annealed at 500 °C 6 at.% Ag-doped ITO. A spherical indenter of 5 m in radius loaded onto

a film of 2 m thickness, with a maximum indentation depth of 0.2-0.6 m, is modelled. Note

that this thickness, which is different from those of our experimental samples, is used here for a

general evaluation and comparison between the type of films with different Ag concentration and

151

heat treatment. The consideration here is that, under real application environments, loadings with

a dimension in the order of μm are more realistic than smaller and sharper ones. Hence, the

thickness used in the modelling work is designed to match the loading conditions.

The Young’s modulus for the films is taken from Table 5.3, while that of the substrate (glass) is

90 GPa. Comsol software was used in the modelling, and more details of the modelling can be

found in our previous work [452]. The load carrying ability of the films was assessed by the

equivalent stress, or von Mises stress, which is a measure of the severity for potential damage to

the film, and can be expressed by:

[( ) ( )

( )

( )] ⁄

(5.5)

where sequiv represents the equivalent stress, sxx, syy, szz, sxy, syz, szx are the corresponding stress

tensor components, respectively.

The FEM modelling result of von Mises stress distribution in a 2 m as-deposited ITO film

under a spherical indentation of 0.4 m depth along with the displacement along the axial

symmetry line from the surface of the film to the interface between the film and the substrate

under a loading of 58 mN for pure ITO and 6 at.% Ag-doped ITO thin films are presented in

Figure 5.16.

Figure 5.16 (a) shows the modelling image of as-deposited ITO thin film. At an indentation

depth 0.4 μm, the maximum of the von Mises stress is found at the interface between the film

and the substrate, immediately below the centre of the indenter, with the value being 11 GPa.

Such a load resulted in a vertical displacement in the film close to the indenter. Figure 5.16 (b)

shows a comparison of displacement in two different films under study, top: ITO with 6 at.% Ag

and bottom: pure ITO. The displacements are plotted along the centre (axial symmetric) line,

from the surface of the film to the interface between the film and the substrate. When the

indentation loading is kept at 58 mN, the z-displacement was reduced by 10%, within the Ag

doped and annealed film. This reduced the possibility of delamination at the interface between

the film and the substrate.

152

In summary, our experimental results show that, the doping of 6 at.% Ag into ITO film resulted

in an increase of 4% and 11% in E and H, respectively. After the films were annealed at 500 °C,

further increases of 9% and 13% respectively in E and H were observed. Taking both changes

into account, the improvement is 14% and 28% for E and H, respectively. Combining Ag doping

and annealing also helps enhancing the effectiveness of the grain boundary in obstructing the

displacement movements. At the same loading smaller deformation will be induced in the Ag-

doped film, hence this film has higher load carrying capability. Wear resistance can also be

enhanced by doping of Ag, as well as by annealing at 500 °C.

153

(a)

(b)

Figure 5.16 (a): FEM modelling result of von Mises stress distribution of as-deposited ITO film

and (b): a comparison of displacement in two different films under study, top: ITO with 6 at.%

Ag film and bottom: pure ITO film

154

5.5. Conclusions

ITO and Ag-doped ITO thin films were successfully fabricated onto glass substrates via a sol-gel

spin coating technique. The effects of Ag concentrations (e.g., 2, 4, 6 and 10 at.% in this study)

on the structural, surface morphology, mechanical and optoelectronic characteristics of ITO and

Ag-doped ITO thin films were studied by means of XRD, FESEM, nanoindentation, UV-VIS

spectroscopy and Hall Effect measurements. Introducing Ag atoms to the ITO matrix did not

strongly alter the ITO structure. However, Ag phase appears at (2θ ≈ 38.2°) for Ag-doped ITO

thin films. The films of Ag-doped ITO showed larger particle size than that of ITO which are

further enlarged after annealing at 500 °C. Variations in H and E values of ITO and Ag-doped

ITO thin films were observed as a function of Ag concentration. The hardness for as deposited

and annealed thin films were within the range of (5.3 – 5.7) GPa and (6.0 – 6.7) GPa

respectively. While the Young’s modulus values for as deposited and annealed samples were in

the range of (128 – 134) GPa and (134 – 148) GPa respectively. The variation in Ag

concentration exerts a positive effect on both the hardness and Young’s modulus values. A

combination of the highest H and E (6.7 GPa and 148 GPa, respectively) were achieved in the

sample with highest Ag concentration (i.e., 10 at.% Ag) and annealed at 500 °C. Evaluated by

means of comparison, doping of 6% Ag into ITO film resulted in an increase of 4% and 11% in

E and H, respectively, while annealing the films at 500 °C further increases E and H 9% and

13% respectively. Taking both changes into account, the improvement is 14% and 28% for E and

H, respectively. Wear resistance can also be improved by both doping of Ag and thermal

treatment (i.e., annealing at 500 C). It has also been shown that combining Ag doping and

annealing helps enhance the effectiveness of the grain boundary in obstructing the displacement

movements. FEM modelling results indicate that at the same loading, smaller deformation will

be induced in the Ag-doped film; hence this film has higher load carrying capability.

Furthermore, improved wear resistance over pure ITO thin films was also observed for the

samples being studied, indicated by the higher H/E values.

FESEM images show the formation of uniform, dense, smooth and homogeneous surfaces with

a grain structure indicating the nanocrystalline features, and the Ag-doped ITO thin films

prepared at different Ag contents have larger crystallite sizes compared to that of pure ITO. The

electrical resistivity of all samples decreased after applying annealing temperature, and the

155

lowest values were assigned to be 2.7×10-4

Ωcm and 2.4×10-4

Ωcm for pure ITO and 4 at.%

Ag-doped ITO thin films, respectively. The maximum optical transparency in the visible region was

found to be 92% for both ITO and 2 at.% Ag-doped ITO thin films, while, the highest optical

band gap energy of 3.76 eV was obtained for the film of 2 at.% Ag content. The optical constants

such as refractive index and extinction coefficient of the prepared samples were extracted from

the transmittance data. The refractive index values for ITO and Ag-doped ITO thin films indicate

that they can be used as an antireflection coating in optoelectronic applications. Moreover, Ag

doping dramatically enhanced the extinction coefficient of ITO samples in the absorption UV

region. With an optimised doping and annealing process, the results from this work are expected

to facilitate the engineering design of customizable Ag-doped ITO thin films for various

industrial applications.

156

Spin-coating ITO Sol

SWCNTs Sol

ITO particles

SWCNTs conducting baths

e

6. CHAPTER 6: Novel Approach for Fabricating Transparent

and Conducting SWCNTs/ITO Thin Films for Optoelectronic

Applications

Paper IV

Hatem Taha, Zhong-Tao Jiang, Chun-Yang Yin, David J. Henry, Xiaoli Zhao, Geoffrey Trotter,

Amun Amri, A Novel Approach for Fabricating Transparent and Conducting SWCNTs/ITO

Thin Films for Optoelectronic Applications. J. Phys. Chem. C 2018, 122, 3014-3027

157

6.1. Abstract

Single walled carbon nanotubes (SWCNTs) incorporated in indium tin oxide (ITO) was

developed to fabricate transparent conductive thin films via a sol-gel spin coating technique. The

fabricated thin films were annealed at 350 C. The effects of incorporating SWCNTs and

varying film thickness on crystal structure were systematically investigated by Raman shift,

surface elemental compositions, surface topography and roughness, optoelectronic characteristics

and mechanical properties. X-ray diffraction (XRD) results confirmed the body centered cubic

structure of indium oxide polycrystalline phase, indicating that the structural properties of the

ITO films were not significantly altered by incorporating CNTs. The presence of CNTs in the

ITO matrix was confirmed by analyses of Raman spectroscopy, x-ray photoelectron

spectroscopy (XPS) and energy dispersive x-ray spectroscopy (EDX). FESEM images revealed

the formation of SWCNTs/ITO nanoparticles, and the average crystallite size increased along

with increasing film thickness. Electrical characteristics also improved as the film thickness

increased. The lowest electrical resistivity (4.6×10-4

Ωcm), as well as the highest carrier

concentration (3.3×1020

cm-3

) and carrier mobility (41 cm2/V s) were achieved from the 320 nm

thick film. However, the optical transparency decreased from 91% to 87.5% as the film thickness

increased from 150 to 320 nm. The hardness and Young’s modulus of the prepared samples

improved, with the increase of SWCNTs doping level, and achieved the maximum values of 28

GPa and 306 GPa, respectively.

158

6.2. Introduction

Over recent years, the demand for transparent conductive materials (TCMs) has been

continuously growing, mainly driven by the significant increase in the volume of components

used in optoelectronic devices. However, strict requirements need to be met before a new

material is qualified for a wide range of optoelectronic applications, particularly, high quality

TCMs should have very high UV/Vis optical transparency with extremely low electrical

resistivity [153, 182]. Transparent conductive oxides (TCOs) are the most common type of

TCMs due to their unique combination of optical transparency and electrical conductivity.

Indium oxide (In2O3) and indium tin oxide (ITO) based TCOs still represent the major TCM in

numerous optoelectronic applications including touch screens, liquid crystal displays, solar cells,

light emitting diodes, functional glasses and sensors [183, 336, 338, 381, 453]. However, one of

the major drawbacks of ITO coatings is their cost due to the scarcity of indium concentration and

fabrication efficiency [454].

Another shortcoming of ITO films is their brittleness which can lead to the formation of micro-

cracks under an applied strain which results in increased electrical resistivity. Incorporating a

hard conductive material in the ITO matrix can reduce the brittleness of the coating [455, 456].

Carbon-based materials (graphene, carbon nanotubes), metal grids and conducting polymers

have good electrical characteristics and can be formed into pure transparent conducting thin

films [457-460]. However, in pure form they have several limitations. Graphene has high charge

carrier mobility (μ) and low electrical resistivity, but it should be completely reduced from

graphene oxide to exhibit suitable electrical characteristics [461]. Metal grids have the

disadvantage of sensitivity of their surfaces to photon scattering and oxidation [462].

Alternatively, the electrical instability to thermal and chemical strain is the main disadvantage of

conducting polymers [463]. Despite relatively low transparency over the visible region, the

electrical, optical, mechanical and chemical characteristics of CNTs make them a favourable

applicant for TCMs [464]. It is known that the electrical conductivity is directly correlated to

charge carrier concentration and mobility [370, 465] and CNTs are considered to have a higher

carrier mobility than ITO [466]. Also, CNTs exhibit electrical properties higher than typical

metals such as copper and aluminium [467] as well as a high Young’s modulus of approximately

1-2 TPa [468]. So far, CNTs have mostly been used in multilayer structures and/or combined

159

with conductive polymers such as PEDOT:PSS [469-471]. Although efforts have been made to

increase the carrier concentration for ITO thin films by using dopant atoms, this can potentially

exert a negative effect on the conductivity, since these dopant atoms may act as scattering points.

As an alternative, enhancing the carrier mobility of ITO films by introducing CNTs could be

beneficial for improving the electrical conductivity. Inclusion of CNTs into an ITO film may

also improve the mechanical properties of the film.

In order to fully exploit the advantages of incorporating the reinforced materials into the ITO

coatings, a suitable technique needs to be developed. Numerous techniques are employed for

fabricating ITO based TCOs such as, chemical vapour deposition, RF and DC sputtering, pulsed

laser deposition, electron beam evaporation, molecular beam epitaxy and sol-gel method [193,

243, 259, 375-379]. However, most of these techniques are not easily adapted to incorporate

intact nanotubes into an ITO coating. The advantages of using a sol-gel process for fabricating

TCMs are (1) normal ambient wet-chemistry lab conditions, (2) less processing time, (3) lower

cost and (4) high yields [63, 64, 347, 348]. Therefore, in this study, a sol-gel method combined

with spin coating was employed to incorporate CNTs in an ITO matrix. The main objective of

this work is to enhance the characteristics of ITO films through its combination with SWCNTs

which could also potentially reduce the consumption of costly indium and enhance the

mechanical and surface morphology characteristics of ITO thin films.

6.3. Experimental Section

6.3.1. Raw Materials

Hydrated indium nitrate (In(NO3)3.5H2O, Alfa Aesar, 99.9%), tin chloride dihydrate

(SnCl2.2H2O, Chem-supply, 98%), graphene oxide (GO) solution, single walled carbon

nanotubes (SWCNTs) Sigma Aldrich, carbon > 90%, carbon + SWCNTs ≥ 70%, nitric acid

(HNO3) and sulphuric acid (H2SO4) (chem-supply 98%), absolute ethanol (EtOH, Merck,

99.5%), and milliQ water, were used as supplied.

160

6.3.2. SWCNTs/ITO Sol Preparation and Samples Deposition

In(NO3)3.5H2O and SnCl2.2H2O were dissolved separately in absolute ethanol and refluxed for 1

hour at 85 °C. The two solutions were mixed and refluxed for another 1 hour at 85 °C resulting

in (0.2 M) ITO solution. To be suitable for the incorporation process, SWCNTs were first ultra-

sonicated in a mixture of concentrated nitric acid and sulphuric acid (HNO3: H2SO4= 3:1) for 30

minutes. This acidic treatment process is important to enhance the formation of carboxylic acid

functional groups and break down the nodules in agglomerate SWCNTs, rendering the SWCNTs

hydrophilic and prevents re-bundling when mixed with the ITO solution. Also, it has been

reported that the nitric acid treatment not only reduces the CNT-CNT junction’s resistance but

also facilitates the removal of amorphous carbon and other carbonaceous impurities in these

junctions. All these eventually result in improving the tunnelling contacts between CNTs [216,

472, 473]. The SWCNTs were then washed with milliQ water and centrifuged 5 times to remove

any traces of anions. The resultant functionalized SWCNTs were then dried for 24 hours at 60 °C

in a vacuumoven prior to incorporation with the ITO sol. 0.25 wt% SWCNTs was added to 2 ml

of GO and sonicated for 2 hours, then mixed with ITO sol and stirred for 1 hour. The resultant

solution was aged for 24 hours at room temperature prior to deposition. Soda-lime glass

substrates (25×25 mm) were cleaned by detergent and washed with milliQ water. Then the

substrates were subjected to ultrasonication in ethanol for 15 minutes, and milliQ water washing

was repeated. Finally, the substrates were dried under vacuum in a conventional drying oven at

100 °C for 60 minutes. A Polos spin coater was used for depositing the SWCNT/ITO sol onto

the glass substrates. The spin coating parameters are described below: (1) dispensing the solution

on to the substrate at 300 rpm for 20 seconds, (2) spreading the solution at 2000 rpm for 30

seconds, and (3) finally drying the solution at 3500 rpm for 30 seconds. Subsequently, the

sample was dried at 100 °C for 15 minutes on a hot plate. The coating and calcination steps were

repeated until the desired film thickness was achieved. All samples were then annealed at 350 °C

for 2 hours in an air atmosphere. SWCNTs/ITO coatings with different films thicknesses (i.e.

150, 210, 250 and 320) ± 5 nm were synthesized in this study. In order to estimate the errors of

the measured quantities 4 samples were prepared for each thickness. A thicker film around 370 ±

5 nm was fabricated, but it exhibited a significant drop in the optical transparency (~ 82%) while

its electrical properties were very close to those of the 320 nm film thickness. Thus, the results of

the thicker film (i.e. 370 nm) were not considered and presented in this investigation.

161


The crystallographic structures of the SWCNTs/ITO thin films were examined via X-ray

diffraction (XRD) analysis using a GBC EMMA diffractometer with Cu-Kα emission line

(=0.154 nm). The diffraction angle 2 = 20 - 70° was scanned at 2 °/min with a step increment

of 0.02. The analysis was carried out with beam acceleration of 35 kV/28 mA. Information on

elemental compositions, binding energies and chemical bonding states for the outmost 5 nm of

the surfaces were obtained using X-ray photoelectron spectroscopy (XPS) technique. The XPS

analysis was performed using a Kratos axis ultra-X-ray photoelectron spectrometer operating at

15 kV and 10 mA with an Al-Kα monochromatic radiation source (hv = 1486.6 eV). The

specimens were mounted onto a steel sample holder using carbon tape and analysed under

ultrahigh vacuum (2.9 × 10-9

torr). The XPS data for both survey and high resolution scans were

collected without surface etching. Typical high resolution XPS scans of SWCNTs/ITO thin films

displayed In3d, Sn3d, Ti2p, O1s and C1s energy regions. Curve fitting of the high-resolution

scans were carried out using CASA XPS software (version 2.3.1.5) which affords information

about the presence of elemental compositions and chemical bonding states for the analysed

samples. Surface topographic images of the fabricated thin films were obtained via a Bruker

Dimension (Santa Barbara, California, USA) fast scan atomic force microscope (AFM). The

samples were mounted on adhesive tape prior to AFM scans and the images were recorded in

Tapping Mode using a standard Tapping Mode probe (type NCHV, resonant frequency 300 kHz,

spring constant 40N/m.

Raman analysis was conducted using a Nicolet 6700 Fourier transform infrared (FTIR)

spectrophotometer equipped with NXR FT-Raman module. Raman spectra were collected using

a Helium-Neon (He-Ne) monochromatic laser with 1064 nm excitation wavelength and 0.204 W

operation power, CaF2 beam splitter, InGaAs detector with 90° detection angle, optical velocity

of 0.3165, gain of 1, aperture of 150, and 16 scans with resolution of 8 cm-1

in the range of 0-

4000 cm-1

. Field emission scanning electron microscopy (FESEM) (Zeiss Neon 40EsB) was

used to investigate the surface morphology and the EDX analysis of SWCNT/ITO provided the

distributions of elemental concentration. The samples were first assembled onto a steel holder

using carbon tape and then coated with platinum (3nm thick) to decrease the charging effects

before FESEM imaging. The electrical properties of prepared SWCNTs/ITO thin films were

162

evaluated by a 4-point probe method using (Dasol Eng. FPP-HS8 system). Hall-effect

measurements were carried by a system of four Au deposited probe contacts on the coated

samples (10×10 mm). In this measurement the applied magnetic field was 0.9 Tesla via ECOPIA

HMS-2000 system combined with Polytec Bipolar Operation Power supplies (BPO). The optical

properties were measured within the wavelength range of 250 - 800 nm via a UV-VIS Perkin

Elmer spectrometer. The mechanical characteristics of the synthesized samples were performed

using a nanoindentation workstation (Ultra-Micro Indentation system 2000, CSIRO, Sydney,

Australia), equipped with a diamond Berkovich indenter. The area function of the indenter head

was calibrated by a standard fused silica specimen. The measurements were collected with

maximum loading of 200 nN and maximum penetration depth less than one-tenth of the coating

thickness. In order to attain better resolution, the number of measuring points was increased to

15 during loading and 20 during unloading.


6.4.1. Structural Properties

Figure 6.1 shows the XRD patterns of prepared SWCNTs/ITO coatings with different film

thickness. The XRD results revealed the formation of a body centred cubic bixbyite structure of

indium oxide (In2O3) with space group of Ia3 according to (JCPDS 06-0416) for all

SWCNTs/ITO thin films. No effect from the SWCNTs was observed on the orientation direction

and peak position of the ITO matrix. However, two small peaks where observed at 26.5° and

43.5° which are related to the reflection planes of (002) and (101) respectively of CNTs. It has

been reported that the (002) and (101) reflection planes of CNTs are normally featured as weak

and broadened peaks with low intensity [474]. The first peak at 26.5° indicates the formation of a

hexagonal-like graphene structure, while the second peak at 43.5° is due to in-plane reflection.

The preferred orientations for the SWCNTs/ITO thin films are identified as (222), (400), (411),

(332), (440) and (622) planes similar to those for In2O3, as well as (002) and (101) of CNTs. It is

interesting to notice that both the (332) orientation with a reflection line featured at 43° for ITO

and the (101) orientation with a reflection line featured at 43.5° for SWCNTs overlapped and

formed a weak and broadened peak around 43.4°. Also, it is clear from Figure 6.1 that as the

film thickness increased the preferred orientation plane of (222) and the shape and peak intensity

improved. This phenomenon indicates an enhancement of the degree of crystallinity of the thin

163

25 30 35 40 45 50 55 60 65

Inte

nsi

ty (

a.u.)

2θ (Degree)

(2

22)

(

400

)

(

440

)

* (

101

)

*(0

02

)

(

622

)

ITO

*SWCNT

320 nm

250 nm

210 nm

150 nm

films as their thickness increased, which could be a result of an increasing number of SWCNTs

that accumulate along the preferred orientation plane (222).

The crystallite sizes (D) for the SWCNTs/ITO thin films were calculated based on Scherrer

equation (Eq. 2.26) from the values of full width at half maximum (FWHM) of the preferred

orientation peak (222).

Figure 6.2 displays the variation of the crystallite size with respect to the film thickness. It is

clear that the crystallite size of the SWCNTs/ITO thin films increased with increasing film

thickness, due to the improvement in the crystal quality. The maximum crystallite size of 43 nm

was obtained for the thickest film of 320 nm compared with the minimum size of 23 nm for the

film of 150 nm thickness (Table 6.3). The lattice constant (a) value of the SWCNTs/ITO thin

films (10.13 – 10.17) were calculated which is slightly larger than that reported so far for un-

doped indium oxide of 10.118 Å [475]. This variation in the lattice parameters can be related to

increasing the number of oxygen vacancies along with the incorporation of SWCNTs into the

film. These effects will result in stress being developed near the film surface [360], which may

Figure 6.1 XRD pattern of SWCNT/ITO thin films prepared at various thicknesses and

annealed at 350 °C

164

be due to the thermal expansion coefficient mismatch between the film and the substrate [361,

362].

6.4.2. Elemental Compositions and Surface Chemical Bonding States

Figure 6.3 displays the XPS survey scan results for the prepared and annealed SWCNTs/ITO

thin films, at various films thicknesses, in the binding energy range of (200 - 600) eV. The

expected elements with their photoelectron peaks for In 3d, Sn 3d, O 1s and C 1s were confirmed

by wide XPS spectra. The observed results show that the atomic compositions of SWCNTs/ITO

samples were altered with the increase in film thickness. The atomic percentages of the main

elements (In and Sn) as well as O were increased as the film thickness increased; while, the

carbon percentage was decreased. The increasing oxygen ratio with increasing film thickness

indicates that surface oxidation occurred during the calcination process, following each

deposition step. Table 6.1 summarizes the atomic compositions of the as prepared and post

20

25

30

35

40

45

50

100 150 200 250 300 350

Gra

in s

ize

(nm

)

Thickness (nm)

Figure 6.2 Effect of film thickness on the grain crystallite size of

SWCNTs/ITO thin films

165

200250300350400450500550600

Inte

nsi

ty (

a.u)

Binding energy (eV)

C

In

Sn O

150 nm

320 nm

250 nm

210 nm

annealed SWCNTs/ITO thin films as a function of film thickness. Note that in all of the

discussions, the analysed XPS results were calibrated with respect to the binding energy of C1s

from C-H bonding state at 284.5 eV to recover any charge shift.

Table 6.1 Elemental compositions of SWCNTs/ITO thin films annealed at 350 °C

Figure 6.3 XPS wide scans of SWCNTs/ITO thin films prepared at various thicknesses and

annealed at 350 °C

t(nm)

(± 5nm) Atomic percentage of the elements %

In Sn O C

150 12.1 ± 0.1 1.4 ± 0.1 41.4 ± 0.1 45.1 ± 0.1

210 11.6 ± 0.1 1.3 ± 0.1 43.5 ± 0.1 43.6 ± 0.1

250 16.0 ± 0.2 1.8 ± 0.1 51.0 ± 0.1 31.2 ± 0.1

320 17.3 ± 0.3 1.9 ± 0.1 51.9 ± 0.1 28.9 ± 0.1

166

The surface chemical bonding states of the prepared and post annealed SWCNTs/ITO thin films

were investigated by de-convoluting the high resolution In 3d, Sn 3d, O 1s and C 1s peaks to

evaluate the contributions of metal oxides and/or compounds to the film materials (Figure 6.4).

Figure 6.4a shows the fitting curves of high resolution XPS spectra for In 3d5/2 photoelectron

lines. Two bonding states allocated at 443.99 and 444.69 eV were identified. The lowest binding

energy related to In° bonding state, specifically In-In bonds, while the highest binding energy is

attributed to In3+

bonding state from In2O3 [387, 388]. The fitting curves of high resolution XPS

spectra for photoelectron peaks of Sn 3d5/2 photoelectron peaks are presented in Figure 6.4b. In

the fitting curves, two sub peaks are assigned around 485.99 and 486.54 eV. The first peak at

485.99 eV can be attributed to Sn4+

in SnO2 while the second peak at 486.54 eV is linked to Sn2+

in SnO [389]. Fan and Goodenough [148] reported that the Sn3d peak for Sn4+

from SnO2 has a

binding energy around 0.5 eV lower than that for Sn2+

from SnO. The SnO2 feature is

thermodynamically more stable than the SnO feature and is predicted to be the major phase

between the Sn components in the ITO based surface coatings. The de-convoluted curves for O

1s core level exhibit two different peaks located at 529.64 and 531.34 eV (Figure 6.4c). The first

feature around 529.64 eV is contributed to In-O bonds from In2O3, and the second feature at

531.34 corresponds to Sn-O bonds from both SnO and SnO2 [391].

The chemical bonding of CNTs is comprised entirely of sp2 bonds, similar to those of graphite

and these bonds provide CNTs with their unique mechanical properties. The de-convoluted

curves of high resolution XPS spectra for C 1s photoelectron lines are shown in Figure 6.4d. The

C1s photoelectron peaks can be split up into three sub-peaks allocated at the binding energy

range of 284.5 eV-288.4 eV. The first peak with the lowest binding energy around 284.5 eV has

a high contribution of 66.8% and can be assigned to pure sp2 hybridized graphitic carbon atoms

of (C-C and C=C bonds), and carbon atoms bounded to hydrogen (C-H bonds).

The second peak originated at the binding energy 285.4 eV with a contribution of 17% is

attributed to hybridized sp3 carbon atoms (C-C bonds) which indicates that defects have been

introduced to SWCNTs surfaces. While, the third peak allocated at 288.4 eV with the lowest

contribution of 16.2% corresponds to carbon atoms bounded to oxygen atoms either by single

bonds (C-O) or double bonds (C=O) which are caused by high temperature oxidation of

SWCNTs [476, 477].

167

6.4.3. Raman Studies

As noted above in the XRD results, the SWCNTs/ITO has a cubic bixbyite structure with Ia3

space group similar to In2O3. The structure consists of two types of cations where 8 In3+

ions

with side symmetry are located at b-sites and 24 In3+

ions with point symmetry at d-sites.

Furthermore, 48 oxygen atoms have general C1 site symmetry. Therefore, the following phonon

vibrational modes are expected:

4Ag + 4Eg + 14Tg + 5Au + 5Eu + 16Tu

Figure 6.4 High resolution scans of (a)- In3d, (b)- Sn3d, (c)- O1s and (d)-C1s of SWCNTs/ITO thin films

527528529530531532533534

CP

S/I

nte

nsi

ty (

a.u

)

Binding energy (eV)

Sn-O, C-O

531.34 eV

In-O

529.64 eV

(c)

441442443444445446447448

CP

S/I

nte

nsi

ty (

a.u)

Binding energy (eV)

In-In

443.99 eV

In3+ in In2O3

444.69 eV

(a)

483484485486487488489

CP

S/I

nte

nsi

ty (

a.u)

Binding energy (eV)

Sn2+ from SnO

486.88 eV

Sn4+ in SnO2

485.99 eV

(b)

281282283284285286287288289290291292

CP

S/I

nte

nsi

ty (

a.u)

Binding energy (eV)

Sp2C-C, C=C, C-H

284.5 eV

Sp3 C-C

285.4 eV

C=O, C-O

288.4 eV

(d)

168

The phonon vibrational modes with Ag, Eg and Tg symmetry are known to be Raman active but

infrared inactive. The Tu vibrational mode is infrared active and Raman inactive, while the others

(Au and Eu) are inactive for both Raman and infrared. Figure 6.5 displays the Raman spectra of

sol-gel derived SWCNTs/ITO thin film. The peaks seen at 109.8, 269.7, 312.5, 368, 433.3,

558.9, 631.6 and 703.5 cm-1

belong to the cubic structure of In2O3 [68, 386]. The small peaks

featured at 171.7 and 239.7 cm-1

belong to Sn-O vibrational modes within the ITO structure

[385]. The peaks seen at 1292.4 and 1594.6 cm-1

are the D-band and G-band for SWCNTs. The

D-band at 1292.4 cm-1

is attributed to the defect in the SWCNTs, which could result in single

and/or double phonon vibrational modes for Raman scattering processes. While, the G-band at

1594.6 cm-1

is related to a carbon atom vibrational mode along the SWCNT axis with a

frequency very sensitive to the transition of charge carriers from dopants to SWCNTs [478].

Also, the G-band is characteristic of sp2 graphitic material, which is a signature of the existence

of SWCNTs in the ITO matrix [479].

6.4.4. Surface Morphology and Topography

The FESEM images of the prepared and annealed SWCNTs/ITO thin films at various

thicknesses are presented in Figure 6.6. It is evident from Figure 6.6 that all the SWCNTs/ITO

thin films have dense and homogeneous surfaces, with grain features demonstrating the

Figure 6.5 Raman spectra of SWCNTs/ITO thin film

0 200 400 600 800 1000 1200 1400 1600

Inte

nsi

ty (

a.u)

Raman shifts (cm-1)

31

2.5

46

8

G-b

and

63

1.6

55

8.9

D-b

and

70

3.5

171

.7

23

9.7

26

9.7

10

9.8

43

3.3

169

nanocrystalline structure. No cluster of SWCNTs was identified in the FESEM images,

indicating that they are well dispersed in the ITO films. However, the surface roughness of the

coatings was affected by SWCNTs incorporation. It is also seen from Figure 6.6 that by

incorporating SWCNTs in the ITO matrix, it reduces the surface micro-cracks and improves the

surface roughness especially for the films of 150 nm and 210 nm thickness which could be due

to elasticity and one-dimensional nature of SWCNTs. Nevertheless, the films of higher

thicknesses (i.e., 250 and 320 nm) demonstrate the formation of some micro-cracks. This is

possibly related to increasing the film thickness along with repeating the film drying (or thermal

treatment) during the fabrication process. Furthermore, higher film thickness led to a greater

grain size, which is consistent with the XRD results (section 6.4.1).

150 nm

320 nm 250 nm

210 nm

Figure 6.6 FESEM images of SWCNTs/ITO thin films prepared at various

thicknesses and annealed at 350 °C

170

EDX analysis confirms the presence of the main elements (i.e., indium, tin, oxygen as well as

carbon) in the thin films (Figure 6.7). The carbon ratios were observed to be higher in thicker

films, which suggest higher concentration distribution of carbon within these films. However,

metal to oxygen and carbon ratios could not be established conclusively since the EDX analysis

was focused on selected areas of the samples, and the presence of some other elements related to

glass substrate in the EDX spectra. Interestingly, while the In and Sn atomic percentages appear

to be close to values obtained from XPS measurements, carbon percentage increased along with

increasing film thickness. This is in contrast with the results obtained from XPS analysis since

the XPS measurements were performed on a maximum depth of ~10 nm from the sample

surface, in which measurements could be affected by the existence of carbon from the external

environment and/or the oxidation process of the top surface layer of the sample.

AFM scans were used to complement the FESEM imaging which provided further information

about the thin films’ roughness. Figure 6.8 shows the AFM images of the fabricated samples.

150 nm 210 nm

320 nm 250 nm

Figure 6.7 Elemental distributions of SWCNTs/ITO thin films (annealed at 350 °C) by EDX

171

Analysis of the images indicates that these samples possess smooth surfaces with low average

root mean square roughness values of approximately 4 nm. This root mean square (RMS) value

is lower than a previously reported study especially for doped ITO and ITO modified CNTs

transparent films [469]. Additionally, Figure 6.8 shows that the surfaces of thicker

SWCNTs/ITO films have rougher characteristic (with higher RMS values), but they also possess

a relatively homogeneous morphology compared to the thinner films due to increasing

concentration of SWCNTs in the film network. This homogeneous morphology may heighten the

intimate contact of the grains/crystallites through decreasing the density of the grain boundaries

within the ITO framework [480]. This might also be attributed to the improvement in the films

quality through enhancing nucleation, coalescence and continuous films growth with increasing

film’s thickness. This is evident in the XRD and FESEM results where thicker film exhibits more

prominent crystalline structure and larger grain size. This is consistent with the established

notion that changing the grain size and clusters significantly affects the surface roughness of the

films.

210 nm RMS: 3.49 nm 150 nm RMS: 3.36 nm

320 nm RMS: 4.65 nm 250 nm RMS: 4.08 nm

Figure 6.8 AFM images of SWCNTs/ITO thin films annealed at 350 °C

172


In this work, the roles of both high carrier concentration of ITO (~ 1020

/cm3) and high carrier

mobility for CNTs (~ 100 cm2 Vs

-1) on the electrical properties was considered. It is stated that

the electrical conductivity of transparent conductive materials depends on both carrier

concentration (N), carrier mobility (μ) and electron charge (e) based on the relation of σ = Neμ.

We assumed that increased film thickness results in more SWCNTs which should increase the

level of carrier mobility in the film material. Therefore, the effect of incorporating SWCNTs on

the electrical properties of ITO thin films was studied in terms of the film thickness. Figure 6.9

summarizes the dependence of electrical resistivity and conductivity, carrier concentration and

Hall carrier mobility of the SWCNTs/ITO thin films on film thickness. From Figure 6.9a, the

electrical resistivity decreased with increasing film thickness, reaching the lowest value of

4.6×10-4

Ωcm for the film of 320 ± 5 nm. This value for electrical resistivity is lower than that

reported previously by Golobostanfard and co-authors (4.1×10-3

Ωcm) for a dip coated

SWCNTs/ITO thin film prepared with 0.28% of SWCNTs.[481] This could be related to the

better uniformity and homogeneity of these thin films. Incorporation of SWCNTs in the ITO

matrix not only decreased micro-crack formation and enhanced the elasticity of ITO, but also

may have provided conducting bridges in the ITO matrix for the free charge carriers if their

concentration was sufficient enough to create these low resistance conducting bridges. This is

consistent with the results obtained by Ji et al. [222] for the conductivity enhancement of

PEDOT:PSS after incorporated with solfonated CNTs. The authors proposed that the highly

conductive sulfonated CNTs may provide direct charge transport templates and form bridges or

networks to connect the distinct PEDOT islands in the composites, resulting in a great

improvement in the conductivity.

It is known that the electronic characteristics of SWCNTs based transparent conductive system

are strongly dependent on the number of nanotubes in a system. As such, it may be assumed that

the large amount of SWCNTs in the films can be obtained by increasing the thin film thickness.

Increasing the film’s thickness has also improved the degree of crystallinity and increased the

grain size. Therefore, the mean free path of the charge carriers improved, which minimizes

charge scattering and hence reduces the electrical resistivity. It has previously been reported that

the reduction of electrical resistivity of TCO thin films is linked to an increase in the carrier

173

concentration (i.e., dopants and oxygen vacancies) [342, 364, 372]. However, this phenomenon

can be limited, especially for films synthesized by sol-gel method, since the drying process

following each deposition step affects the oxygen vacancies and leads to the formation of micro-

cracks for a thick film, which, in turn, generally increases the electrical resistivity [482].

However, SWCNTs could reduce the elimination of oxygen vacancies in the film material due to

their strong reduction ability and hence decrease the electrical resistivity. Indeed, the SWCNTs

have a higher work function (5.0 eV) [483] compared with ITO (ca. 4.3 - 4.7 eV) which may

provide a greater carrier injection into the ITO matrix due to the difference in the work functions

[484]. All these features enhance the carrier concentration in thicker SWCNTs/ITO thin film. It

has also been reported by Ho et al. [485] that the work function of ITO thin films improved from

4.68 to 5.25 eV by increasing V+4 doping concentration, and this resulting work function was

stable over a long period of time, and demonstrates potential to offer good reliability and long-

term stability in industrial applications. Figure 6.9b presents the variation of the free carrier

concentration and Hall mobility of SWCNTs/ITO thin films for different film thickness. The

carrier concentration increased from 1.9×1020

to 3.3×1020

cm-3

when the film thickness increased

from 150 ± 5 to 320 ± 5 nm. Likewise, the Hall mobility also increased from 30 to 41 cm2/Vs

with increasing film thickness. These results reflect the advantages of combining the high carrier

concentration of ITO and the high carrier mobility of SWCNTs in a thin film.

Table 6.2 Electrical properties of SWCNTs/ITO, pure ITO and Ti-doped ITO thin films

Coating t (nm)

(± 5 nm)

ρ×10-4

(Ωcm) σ (S/cm) N×1020

(cm-3

) μ (cm/Vs)

150 11.0 ± 0.4 922 ± 70 1.9 ± 0.1 30 ± 1

SWCNTs/ITO 210 8.6 ± 0.3 1198 ± 62 2.1 ± 0.2 35 ± 1

250 6.3 ± 0.2 1480 ± 81 2.5 ± 0.1 37 ± 1

320 4.6 ± 0.2 2157 ± 76 3.3 ± 0.1 41 ± 1

Pure ITO 350 3.6 2320 5.8 25

Ti-doped ITO 350 3.78 2592 6.0 27

174

Grain boundaries in the film structure can act as trapping centres for free charge carriers thereby

negatively affecting both the carrier mobility and carrier concentration [486]. Also, these

trapping centres may generate a potential barrier which hinders the drift of the free carriers and

hence decreases the electrical conductivity [487]. The enhancement in the electrical properties of

our SWCNTs/ITO thin films with increasing thickness can be linked to the improvement in the

crystalline quality (increasing the grain size and decreasing the grain boundaries) as seen

previously in the XRD and FESEM results. This effect reduces the scattering of the free carrier

due to decrease in grain boundaries. The density of grain boundaries in thinner films is higher

than those in thicker films. Therefore, with increasing SWCNTs/ITO film thickness, the number

of SWCNTs that segregate in the grain boundary regions will increase, thereby, forming

effective conducting paths in the SWCNTs/ITO matrix as discussed previously (see Figure 6.10).

Golobostanfard and co-authors reported the same tendency of forming low resistance transport

bridges in their ITO-CNT thin film network [488]. The electrical properties of thin

SWCNTs/ITO films due to film thickness are presented in Table 6.2.

175

1.5

2

2.5

3

3.5

4

0

5

10

15

20

25

30

35

40

45

100 150 200 250 300 350C

arri

er c

on

cen

trat

ion×

10

20 (

cm-3

)

Mo

bil

ity (

cm/V

s)

Thickness (nm)

Mobility

carrier concentration

(b)

0

500

1000

1500

2000

2500

0

2

4

6

8

10

12

14

100 150 200 250 300 350

Co

nd

uct

ivit

y (

S/c

m)

Res

isti

vit

y×

10

-4 (

Ω.c

m)

Thickness (nm)

Resistivity

Conductivity

(a)

Figure 6.9 Variations of (a) electrical resistivity and conductivity, (b) mobility and

carrier concentration of SWCNTs/ITO thin films with film thickness

176


The transmittance spectra of the synthesized and annealed SWCNTs/ITO thin films are

compared in Figure 6.11. It can be seen that the films exhibit a transmittance around 91% for the

film of 150 ± 5 nm in the visible region (~550 nm). However, the SWCNTs/ITO thin films show

a higher transmittance toward the near infra-red (NIR) region than in the visible region due to

higher transparency of SWCNTs in this region. SWCNTs also exhibit Van Hove singularities in

the density of states (i.e., S1 ~ 0.68 eV, S2 ~ 1.2 eV and M1 ~ 1.8 eV) resulting in chirality

(metallic to semi-conductive ratio) dependent emission and absorption. Furthermore, the

transmittance of the prepared thin films slightly decreased with increasing film thickness,

reaching a value of 87.5% at 550 nm for the film of 320 ± 5 nm. This can be explained by the

fact that, increasing the film thickness results in an increase in both the free carrier concentration

(section 6.4.5) and the number of SWCNTs that distribute in the film matrix. It must be realized

that CNTs in general, are very strong electromagnetic (EM) radiation absorbers in the visible

ITO particles

SWCNTs conducting paths

SW

CN

Ts IT

O

e

e

Figure 6.10 Schematic diagram of free carrier injection from ITO network to

SWCNTs and carrier transfer in SWCNTs/ITO thin film

177

region of the electromagnetic spectrum [489]. As a result, more light is absorbed, and hence the

optical transparency decreased [343, 490].

The absorption coefficient (α) of the SWCNTs/ITO thin films can be computed from (Eq. 5.2).

The plot for absorption coefficient versus wavelength is presented in Figure 6.12. It is found that

there is a shift in the UV absorption edge towards higher wavelengths with increasing film

thickness.

60

65

70

75

80

85

90

95

100

300 400 500 600 700 800

Tra

nsm

itta

nce

%

Wavelength (nm)

320 nm

250 nm

210 nm

150 nm

350 400 450 500 550 600 650 700 750 800

Ab

sorp

tio

n c

oef

fici

ent

(α)

Wavelength (nm)

320 nm

250 nm

210 nm

150 nm

Figure 6.11 Optical transmittance spectra of SWCNT/ITO thin films

annealed at 350 °C

Figure 6.12 Absorption coefficients of SWCNT/ITO thin films

annealed at 350 °C

178

The band gap energy (Eg) for the prepared SWCNTs/ITO thin films was obtained from the

transmittance data using (Eq. 3.1).

Figure 6.13 presents the Tauc plot of SWCNTs/ITO thin films for all the four different film

thickness studied. The optical band gap (Eg) value of SWCNTs/ITO thin films was obtained by

extrapolating the linear sections of the plotted curves to the x-axis [491]. It is clear from Figure

6.13 that the band gap energy of SWCNTs/ITO thin films decreased from 3.70 eV to the lowest

value of 3.66 eV, with increasing film thickness from 150 to 320 nm. Indeed, there is a shift in

the absorption edge towards the long wavelength side as the film thickness increases, resulting

from band gap narrowing due to the high optical absorption of SWCNTs in the visible region as

discussed previously, and explained by a Burstein-Moss shift [337]. This behaviour is in good

agreement with the electrical properties presented in the previous section. The electrical and

optical characteristics in this investigation are comparable with those from pure and doped ITO

thin films fabricated under the same conditions in our previous investigations [430]. Table 6.3

summarizes and compares the structural and optical properties of SWCNTs/ITO thin films (this

study) along with the properties of pure ITO and Ti-doped ITO thin films (previous study).

Table 6.3 Structural, and optical properties of SWCNTs/ITO, pure ITO and Ti-doped ITO thin

films

Coating t (nm)

(± 5 nm)

D (nm) α (Å) T (%) Eg (eV)

150 23 ± 1 10.13 ± 0.01 91 ± 0.5 3.70 ± 0.03

SWCNTs/ITO 210 31 ± 1 10.15 ± 0.01 90 ± 0.2 3.69 ± 0.01

250 35 ± 1 10.16 ± 0.01 89 ± 0.5 3.68 ± 0.02

320 43 ± 1 10.17 ± 0.01 88 ± 0.2 3.66 ± 0.02

Pure ITO 350 31 10.13 89.0 3.50

Ti-doped ITO 350 38 10.14 88.9 3.54

179

6.4.7. Mechanical Characteristics

The thickness of SWCNTs/ITO thin films fabricated in this work is varied within the range of

150 - 320 nm; therefore, it is crucial to select a suitable range of indentation displacement to

avoid any influence from the substrate that may affect the nanoindentation measurements. It is

well known that the nanoindentation measurements should be performed on an indentation depth

less than 10% of the thin film thickness to ensure that the obtained results are only related to the

film coating. In order to determine both the hardness H and the elastic (Young’s) modulus E

values for SWCNTs/ITO thin films, the nanoindentaion measurements were performed at a

maximum indenter depth of 13 nm which is less than one-tenth of the thickness of the thinnest

film.

Both the hardness and elastic modulus can be determined from the measurements of indentation

load and displacement for both loading and unloading process. Maximum load, maximum

displacement and stiffness (S) can be calculated from a typical load-displacement (p-h) curve.

Hardness is a measure of the resistance from a solid surface to deformation under an applied

3.5 3.55 3.6 3.65 3.7 3.75 3.8 3.85 3.9

Photon energy (eV)

320 nm

250 nm

210 nm

150 nm


)

( 𝛼 𝑣) ( 𝑐𝑚

𝑒𝑉)

Figure 6.13 Tauc plot of SWCNT/ITO thin films annealed at 350 °C

180

force. Hardness and Young’s modulus can be determined from (Eq. 2.33), (Eq. 2.34) and (Eq.

2.35).

The hardness and Young’s modulus values of the prepared and annealed SWCNTs/ITO thin

films are shown in Figure 6.14. It is expected that along with increasing the film thickness, the

number of SWCNTs in the film material is also increases. As noticed from Figure 6.14, the level

of resistance to deformation and the Young’s modulus of our sol-gel derived SWCNTs/ITO thin

film coatings increase along with rising thin film thickness, and the film of the highest thickness

(i.e., 320 nm) exhibits the highest resistance to deformation and Young’s modulus ca. to be 28

GPa and 306 GPa, respectively. The obtained hardness and Young’s modulus values for the

samples in the present study are higher than those obtained from our previous study of Ti-doped

ITO thin films [448]. The high values of hardness and Young’s modulus of our SWCNTs/ITO

thin films can be ascribed to uniformly incorporating SWCNTs in the ITO matrix. Winarto et al.

reported a similar effect from inclusion of CNTs in an alumina matrix [492]. An and co-worker

showed in their study on CNTs-alumina-iron samples that the hardness of the fabricated samples

was increased by 33% with the increase in the CNTs ratio compared to the sample with 0%

CNTs [493]. It has been reported that CNTs have hardness and Young’s modulus values of 55

GPa and 1-2 TPa respectively [494]. This high strength of CNTs is attributed to the covalent sp2

bonds formed between the individual carbon atoms (C-C bonds). The sp2 bonds are among the

strongest chemical bonds, resulting in a unique structure which is electrically stable, chemically

inert and mechanically robust. Therefore, we propose that enhancing the mechanical

characteristics of our SWCNTs/ITO thin films is attributed to the high contribution of sp2 bonds

(66.8% as seen in XPS analysis). The improved mechanical properties may also arise from the

improvement of the crystal quality for thicker thin films.

The wear resistance of a solid material coupled with elastic limit describe how this material will

withstand deformation under an applied force and recover its initial state without permanent

deformation once the external loading is removed. The wear resistance can be calculated from

the values of hardness and young’s modulus using the relation H/E. The larger this ratio the

higher the wear resistance will be. The SWCNTs/ITO thin films in this study exhibited an

increase in the wear resistance, which correlates to the increasing of H/E values, with increasing

film thickness. The obtained H/E values, in the range of 8.7×10-2

– 9.3×10-2

, indicate a higher

181

level of wear resistance than that for pure ITO thin films [448]. The mechanical properties of the

SWCNTs/ITO thin films in this study are superior in comparison with our previously published

results from Ti-doped ITO thin films see Table 6.4.

21

23

25

27

29

31

33

220

230

240

250

260

270

280

290

300

310

320

100 150 200 250 300 350 400

Har

dnes

s (G

Pa)

Yo

ung's

mo

dulu

s (G

Pa)

Thickness (nm)

Young's modulus

Hardness

Coating t (nm)

(± 5 nm)

Hardness, H

(GPa)

Young’s

modulus, E

(GPa)

Wear resistance

(H/E 10-2

)

150 22.4 ± 0.6 254.4 ± 11 8.8

SWCNTs/ITO 210 24.2 ± 0.6 269.2 ± 8 9.0

250 25.7 ± 0.9 278.6 ± 9 9.2

320 28.0 ± 1.2 306.0 ± 12 9.3

Ti doped ITO 350 6.6 ± 1.2 143 ± 8 4.6

Figure 6.14 Hardness and Young’s modulus of SWCNTs/ITO thin

films annealed at 350 °C

Table 6.4 Mechanical properties of SWCNTs/ITO thin films annealed at 350 °C and Ti-

doped ITO thin film annealed at 400°C

182

6.5. Conclusions

In this study, a thin film transparent conductive oxide with dispersed single-walled carbon

nanotubes (SWCNTs/ITO) was successfully produced via a low cost and efficient sol-gel spin

coating technique onto soda-lime glass substrates. The effects of incorporating SWCNTs into the

ITO matrix and variation of film thickness (i.e., 150, 210, 250 and 320 nm) were investigated by

FESEM and EDX measurements, AFM imaging, elemental compositions, Raman shifts,

morphological, electrical, optical, and nanoindentation measurements. Adding the SWCNTs into

the ITO matrix did not alter the crystalline structure, as shown by the lattice constant and

preferred orientation directions. The FESEM images show that the SWCNTs/ITO thin film

surface possesses dense and homogenous features. While micro-cracks were absent in the

thinnest 150 nm film, thicker films exhibited some cracks on their surfaces. The EDX and XPS

results confirmed the presence of all the main elements in the film coatings, while the AFM

imaging shows that the surfaces of thicker SWCNTs/ITO films show rougher but homogeneous

morphology compared to thinner films. The Raman spectra showed the expected peaks for the

In2O3 structure as well as the D and G bands for SWCNTs. With the increase of the film

thickness from 150 to 320 nm, the resistivity decreased from 11.2 × 10-4

to 4.6 × 10-4

Ω cm,

while the carrier concentration and Hall mobility increased, reaching their highest values of 3.3 ×

1020

cm-3

and 41 cm/V s respectively. The maximum of both optical transmittance, in the visible

region, and band gap energy were found to be 91% and 3.7 eV, respectively for the 150 nm film.

As the film thickness increased to 320 nm, the minimum values for visible optical transparency

and band gap energy dropped to 87.5% and 3.64 eV, respectively. The hardness of the different

SWCNTs/ITO films were in the range of 22.4 - 28.0 GPa and the Young’s modulus values were

in the range of 254.4 – 306.0 GPa. The film thickness and the annealing temperature exerted a

positive effect on both the hardness and Young’s modulus values. A combination of the highest

H and E were achieved in the sample with highest thickness and annealed at 350 °C. All the

synthesized thin films appeared to possess very high wear resistance. Combining the high order

of carrier concentration for ITO and the high order of both carrier mobility and mechanical

characteristics of SWCNTs, the results from this study are expected to help facilitate the

engineering design of customizable SWCNTs/ITO thin films for various industrial applications.

183

7. CHAPTER 7: ITO-Based Bi-Layer and Tri-Layer Thin Film

Coatings

7.1. Abstract

In this investigation, ITO-based bi-layer and tri-layer thin film coatings (~ 130 nm) were

synthesized via a sol-gel spin-coating process and annealed at 500 C. Thin layers of Au, Au-

NPs, Ag-NPs and AgO were inserted underneath ITO films to form bi-layer thin film systems

and/or encapsulated between two thin ITO layers to form tri-layer thin film systems. Thin film

architectures included Au/ITO (AuI), Au-NPs/ITO ((Au)nI), Ag-NPs/ITO ((Ag)nI) and AgO/ITO

((AgO)I) for bi-layer coatings and ITO/Au/ITO (IAuI), ITO/Au-NPs/ITO (I(Au)nI), ITO/Ag-

NPs/ITO (I(Ag)nI) and ITO/AgO/ITO (I(AgO)I) for tri-layer coatings. The effects of

incorporating these layers with ITO thin films were investigated by X-ray diffraction, X-ray

photoelectron spectroscopy (XPS), field emission scanning electron microscopy (FESEM), UV-

Vis spectroscopy, four-point probes and Hall-Effect. XRD results confirmed the presence of a

body-centred cubic structure of indium oxide. The existence of main elements (In, Sn and O) in

the film surface materials were detected by XPS scans. FESEM images of all fabricated films

revealed formation of dense surfaces with grain-like morphologies indicating the formation of a

polycrystalline structure of ITO. Optical studies on the Ag-NPs and Au-NPs colloidal solutions

resulted in absorption peaks featured at wavelengths 405 and 531 nm, indicating the formation of

10 – 14 nm and 48 nm Ag and Au nanoparticles, respectively. The highest optical transparency

and band gap energy were found to be ~ 91.5% and 3.75 eV for both (AgO)I and (I(AgO)I) thin

films. The lowest electrical resistivity of (1.2×10-4

Ωcm), along with the highest carrier


cm-3

) and mobility (40 cm2/V s) were obtained from the IAuI thin film.

An improvement in the power conversion efficiency (PCE) from 3.8 to 4.9% was achieved in an

organic solar cell by replacing the conventional pure ITO electrode with the (I(AgO)I) electrode.

184

7.2. Introduction

Recently, transparent conductive materials (TCMs) exhibiting favourable optoelectronic

characteristics of high optical transparency and good electrical conductivity have attracted

considerable research interest due to their potential applications as flat panel displays, touch

screens, solar cells, smart windows, sensors and storage devices [495-497]. As part of these

TCMs, transparent conductive oxides (TCOs) have become the most common TCMs used as

electrodes in optoelectronic devices. Single thin films of metal oxides such as indium oxide

(In2O3), zinc oxide (ZnO), tin oxide (SnO2), aluminium doped zinc oxide (AZO) and indium tin

oxide (ITO) are widely used as transparent conductive materials [77, 110, 498]. In2O3 and ITO

based TCO thin films have been considered the prime candidates among TCOs due to their

numerous favourable characteristics including good thermal and chemical stability, high film-

substrate adhesion and good etching properties [153, 336]. Nevertheless, the scarcity of indium

along with its expensiveness influence the replacement of ITO by other materials having

comparable optoelectronic characteristics, or the reduction of indium consumption in the thin

films production. The typical thickness of ITO thin films used as transparent electrodes lies in

the range of 150 - 700 nm. However, the lower thickness of 150 nm has major drawbacks related

to the brittleness and weakness of this material resulting in increased electrical resistivity [499].

Growing industrial demands for the next generation of optoelectronic devices inspire current

researchers to discover more novel materials and structures having higher optical transparency

and lower electrical resistivity.

Oxide/metal/oxide (OMO) multilayer structures have received much attention as potential

alternatives along with graphene [500], carbon nanotubes [211], metal oxide based nanowires

[501] and metal nanowires [502], to replace single ITO coatings in a variety optoelectronic

devices, owing to their good mechanical properties as well as high electrical conductivity and

optical transparency in the visible region [503, 504]. Similar to single In2O3 and ITO-based TCO

thin films, ITO/metal/ITO multilayers systems have also been proposed in optoelectronic

devices. Different metals such as Au, Ag, Cu, Pt, and Ni were adopted to fabricate a thin layer

encapsulated between two dielectric thin films in OMO hybrid systems [505]. Conventional ITO

thin films are usually fabricated at a temperature higher than 300 °C to obtain good film

crystalline quality with high electrical conductivity and optical transparency. For flexible

185

applications of ITO thin films, high annealing temperature should be avoided due to the lower

thermal resistance the flexible substrates possess. Therefore, OMO systems fabricated at room

temperature are of interest for these applications. Kim and co-researchers [175] studied the

effects of Ag film thickness on the optoelectronic properties of ITO/Ag/ITO coatings. They

reported that the ITO/Ag/ITO (43 nm/16 nm/43 nm) film exhibited a sheet resistance and

transparency of 8.9 Ω/sq and 79.4%, respectively. ITO/Ag/ITO thin films deposited on PET

substrates exhibited a sheet resistance of 6.7Ω/sq and optical transparency of 83% [506]. The

effect of annealing temperature (200 °C) and radio frequency (RF) power of deposited

ITO/Ag/ITO thin film were investigated by Choi and co-researchers [507]. They produced a film

with sheet resistance and transparency of 4 Ω/sq and 90%, respectively. Yang and co-researchers

[168] studied the effects of Au layer thickness on the structural and optoelectronic characteristics

of Ga-doped ZnO (GZO)/Au/(GZO) thin films. The Au layer thickness was varied in the range

from 3 to 12 nm. A GZO/Au/GZO thin film with Au interlayer thickness of 9 nm exhibited the

highest transparency of 81.9% and the lowest resistivity of 6.63 × 10-5

Ω cm, along with

relatively high figure of merit of 15.4 × 10-3

Ω-1

. Most of the OMO systems were synthesized via

a sputtering technique. An interlayer film ˂ 20 nm is the key aspect for obtaining high

performance in OMO thin films to avoid strongly reflecting incident radiation in the visible and

near-infrared regions. High reflection by the metal layer, which results in a substantial reduction

in the optical transparency of the multi-layer thin films, is an intrinsic characteristic of the metal.

In this investigation, a simple and low cost sol-gel spin-coating process was used to synthesize

bi-layer and tri-layer ITO-based thin films. Silver (-oxide and –nanoparticles) and gold (-film

and –nanoparticles) layers were combined with ITO thin films in these bi-layer and tri-layer

coatings to improve the optical transparency while maintaining the electrical properties of such

coatings. To the best of the author’s knowledge, sol-gel spin-coated ITO-based bi-layer and tri-

layer thin films combined with metal, metal oxide and metal nanoparticles layers have not been

previously reported.

186

7.3. Experimental Details

7.3.1. Sol Preparation

Sol-gel derived ITO, silver oxide, silver nanoparticle and gold nanoparticle thin films along with

sputtered gold coatings were synthesized. To prepare these thin films, the raw materials used

were hydrated indium nitrate (In(NO3)3.5H2O, purity 99.9%, Alfa Aesar), tin chloride dihydrate

(SnCl2.2H2O, purity 98%, Chem-supply), Silver nitrate (AgNO3, Chem-supply, 99.5%),

hydrogen tetrachloroaurate (III) trihydrate (HAuCl4.3H2O) Sigma Aldrich, 99.99%,

polyvinylpyrrolidone (PVP, M = 40000g/mol), sodium borohydride (NaBH4, Sigma Aldrich,

99.7%), tri-sodium citrate dihydrate (Na3C6H5O7.2H2O) Sigma Aldrich, 99.97%, pure gold target

(purity 99.9%), absolute ethanol and milli-Q water. All precursors were used as received. An

ITO solution with a concentration of 0.1M was used in this study. To prepare the ITO solution,

the required amounts of In(NO3)3.5H2O and (SnCl2.2H2O) were dissolved separately in absolute

ethanol and stirred for 1 hour. The resultant solutions were then mixed and refluxed at 85 °C for

1 hour. A 0.2M Ag nanoparticle solution was prepared by dissolving the required amount of

AgNO3 in 10 ml of milliQ water. A large excess of NaBH4 solution is needed to reduce the ionic

silver and stabilize the formation of Ag-NPs. A 2mM sodium borohydride solution was prepared

by dissolving a required amount of NaBH4 in 30 ml milli-Q water and stirred until the chemical

was completely dissolved. The resultant sodium borohydride solution was chilled in an ice bath

and the silver nitrate solution was added drop-wise. The mixture was stirred vigorously on a

magnetic stirrer plate until the solution turned light yellow after adding all of the silver nitrate

solution. Then, the stirring was stopped and the stir bar removed. The resultant clear yellow

silver solution is stable at room temperature. Silver oxide (AgO) sol with concentration 0.1M

was prepared by dissolving the required amounts of AgNO3 and PVP in 10 ml of absolute

ethanol and stirred vigorously for 60 minutes at 85 °C on a hot magnetic stir plate. To prepare

Au nanoparticle solution, 20 ml of Na3C6H5O7.2H2O (0.5 wt%) was added to 20 ml of an

aqueous solution of HAuCl4.3H2O and PVP (1mM). The solution turned dark grey after less than

30 minutes indicating the formation of Au-NPs solution.

187

7.3.2. Substrate Cleaning

Microscope glass substrates (Soda-lime silicate substrates with 25×25 mm dimensions) were

used in this study. The substrates were first cleaned with detergent and rinsed in milli-Q water.

Then, the substrates were sonicated in absolute ethanol at 60 °C for 10 min and washed

vigorously with milli-Q water. After that, the substrates were sonicated in milli-Q water at 60 °C

for 10 min followed by drying in high purity nitrogen stream. All glass substrates were dried in a

conventional vacuum drying oven at 100 °C for 30 min prior to coating process.

7.3.3. Thin Films Depositions

The resultant solutions were deposited onto glass substrates in the form of bi-layer and tri-layer

systems using a Polos spin-coater system. These coatings were: Au/ITO (AuI), Au-NPs/ITO

((Au)nI), Ag-NPs/ITO ((Ag)nI), and AgO/ITO ((AgO)I), and ITO/Au/ITO (IAuI), ITO/Au-

NPs/ITO (I(Au)nI), ITO/Ag-NPs/ITO (I(Ag)nI) and ITO/AgO/ITO (I(AgO)I) for bi-layer and tri-

layer thin films, respectively. The spin-coating procedure for thin film deposition was: (1)

dispensing the solution at 300 rpm for 15 seconds, (2) spreading the solution at 2500 rpm for 20

seconds, (3) drying the solution at 4000 rpm for 20 seconds, and (4) heat treatment on a hot plate

at 150 °C for 10 minutes. To fabricate the bi-layer thin films, single layers of Ag-NPs, Au-NPs

and AgO solutions were deposited separately onto the glass substrates, followed by drying on a

hot plate and annealing at 300 °C for 1h. Au layer was also sputtered onto glass substrates and

annealed at 300 °C for 1h. ITO thin films were then deposited sequentially four times onto the

surfaces of Au, Ag-NPs, Au-NPs and AgO layers and annealed at 300 °C. In the case of tri-layer

thin films, ITO thin films were first deposited as bottom-layers onto the glass substrates

following a similar deposition described above prior to depositing the metal based mid layer and

top ITO layers. The resultant thin films were then annealed at 500 °C for 2 h.

7.3.4. Fabrication of Organic Solar Cells (OSCs)

Two thin films with the highest optoelectronic performance in this study were adopted as

electrodes combined with a low band gap photoactive polymers to fabricate organic solar cells

(OSCs). A photoactive layer of 100 nm thickness was prepared using 25 mg/mL of both

poly(3hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butric acid methyl ester (PCBM) dissolved

separately in 1,2-dichlorobenzene and stirred at 50 °C under nitrogen atmosphere in dark glove

188

box for 8h. The resultant solutions were then mixed and further stirred for 4h prior to deposition.

A buffer layer of poly(3,4-ethylenedioxythiophene):poly-(styrenesulfonate) (PEDOT:PSS) (~50

nm thick) was prepared by diluting a solution of PEDOT:PSS with isopropyl alcohol at a ratio

being 1:2. This buffer layer which acts as a hole transport layer was spin-coated on the fabricated

ITO, AgOI and IAgOI electrodes (3000 rpm for 30s) followed by drying at a temperature of 100

°C for 10 min in a conventional vacuum oven. The solution of P3HT and PCBM was spin-coated

(1500 rpm for 40s) on the top of the PEDOT:PSS layer and stored to dry in the dark glove box at

room temperature for 24h. A layer of Al (100 nm) thick used as a cathode was evaporated on the

photoactive layer using a mask of an active area of (0.4 cm2).

7.3.5. Characterization Techniques

The crystallographic structure of the synthesized thin films were examined by X-ray diffraction

(XRD) analysis via a GBC EMMA diffractometer with CuKα radiation (=0.154nm). All the

XRD measurements were carried out at a diffraction angle (2) in the range of 25° - 70° with a

scanning rate of 2°/min and 2 step size of 0.02. The phase identification of the thin film was

confirmed by comparing observed diffraction peaks with those of the standard database.

Information on elemental compositions and chemical bonding states for the outmost 5 nm of the

synthesized thin film surfaces were obtained using a Kratos axis ultra-X-ray photoelectron

spectrometer (XPS) operating at 15 kV and 10 mA with an Al-Kα monochromatic radiation

source (hv = 1486.6 eV). The specimens were mounted onto sample holders using carbon tape

and analysed under ultrahigh vacuum (2.9 × 10-9

torr). The XPS data for both survey and high

resolution scans were collected without surface etching. Information about the presence of

elemental compositions and chemical bonding states for the analysed specimens were obtained

by de-convoluting the high-resolution scans of the In3d, Sn3d and O1s photoelectron lines using

CASA XPS software (version 2.3.1.5). In order to compensate for any charge shifts, the XPS

energy scale was calibrated by the C1s (C-H) line at 284.60 eV (bonding energy).

The surface topography was obtained using a Philips XL 20 SEM equipped with electron

dispersive X-ray (EDX) analysis column and a Zeiss Neon 40EsB field emission scanning

electron microscopy (FESEM). Prepared thin films were first mounted on the substrate holders

using double-sided carbon tape followed by coating with (3nm thick) platinum to minimize the

charging effects prior to FESEM imaging. The optical properties were determined by means of

189

transmittance spectra using UV-Vis Perkin Elmer spectrometer in a wavelength range of 250 -

800 nm. A 4-point probe (Dasol Eng. FPP-HS8) was used to observe the electrical properties

(i.e., resistivity and sheet resistance) of the prepared thin films and the Hall-effect measurements

were performed on specimens of (10×10 mm) via ECOPIA HMS-2000 system at 0.9 Tesla of

magnetic field and Polytec Bipolar Operation Power supplies (BPO). Au contacts were thermally

evaporated on the coated thin films in a system of four probes. The current density-voltage (J-V)

characteristics of the fabricated OSCs were measured via a Keithly 2400 source meter

measurements unit under simulated AM 1.5G illumination with an irradiance of 100 mW cm-2

.


7.4.1. Characterizations of Au-NPs and Ag-NPs Colloidal Solutions

The chemical reactions for reducing silver nitrate by sodium borohydride to produce a colloidal

solution of Ag-NPs, and hydrogen tetrachloroaurate (III) trihydrate by tri-sodium citrate

dehydrate to produce a colloidal solution of Au-NPs are as the follows:

(7.1)

(7.2)

The reduction of ionic silver by sodium borohydride and the growth of the silver particles follow

three distinct stages. The early stage of reduction occurs rapidly upon mixing of the reactant

solutions, and the reaction results in the formation of small particles (~ 2 – 3 nm). In the second

stage as the reaction proceeds, the resultant particles enlarge, reaching sizes of (~ 8 – 20 nm)

within a time range of (20 – 60 min). In the last stage of the reaction, the majority of borohydride

is consumed, resulting in the loss of the stabilizing BH4- and the solution switching from a

reducing to an oxidizing environment [508]. The changes in the values of surface potential of the

Ag particles and adsorbed borohydride enhance the aggregation of the Ag particles. Also, the

adsorbed borohydride plays a key role in stabilising the growing Ag nanoparticles by offering a

particle surface charge.

190

In contrast to the reduction of ionic silver, it has been reported that the preliminary reaction of

the reduction of HAuCl4 by Na3C6H5O7.2H2O creates large clusters (~ 100 – 200 nm) which

shrink in size as the reaction continues until a final cluster size of (~ 15 – 20 nm) is achieved

[509]. Due to the absorption and scattering properties of gold nanoparticles, the shrinking

process of the particles is continued along with a change in the colloidal solution colour from

black to red. This phenomenon could be attributed to competitive adsorption between AuCl4-

ions and citrate ions which reflects different surface charges. Also, initially in the reaction,

AuCl4-

binds and exhibits a low surface potential, resulting in weakly attractive inter-particle

forces. Accordingly, small particles combine reversibly. With the progression of the reaction,

AuCl4- is consumed and the citrate binds to the resultant particle surfaces. Hence, the particle

charge increases, and the pair potential becomes repulsive. Clusters then collapse to produce the

final particle size distribution [510].

The distinctive colours of the prepared colloidal solutions of Ag-NPs and Au-NPs are attributed

to plasmon absorbance, where the incident light produces oscillations in conduction electrons at

the surface of the metal nanoparticles and the electromagnetic radiation is absorbed [511, 512].

The prepared colloidal solutions were first characterized by means of their absorbance spectra

via the UV-Vis spectrophotometer. Figure 7.1 shows the absorbance spectra of both Ag-NPs and

Au-NPs. From Figure 7.1, the absorption peak of Ag-NPs colloidal solution was observed at 405

nm while the absorption peak of Au-NPs colloidal solution was detected at 531 nm. The

wavelength of the plasmon absorption maximum of the colloidal solution can be used to estimate

the particle size of metal nanoparticle. It is well-known that the position and shape of the

plasmon absorption maximum of metal nanoparticles is strongly dependent on the surface

adsorbed species, particle size and dielectric medium [513]. It has previously been reported a

plasmon absorption maximum for a silver nanoparticle colloidal solution at 405 nm corresponds

to a particle size of (10 – 14) nm, while for a gold nanoparticle colloidal solution an absorption

maximum around 531 nm corresponds to a particle size of 48 nm [511].

191

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

350 450 550 650 750 850 950 1050

Ab

sorb

ance

(a.

u)

Wavelength (nm)

Ag-NPs

Au-NPs

7.4.2. XRD Analysis of The Thin Films

The XRD patterns of annealed bi-layer (AuI), ((Au)nI), ((Ag)nI) and ((AgO)I) and tri-layer

(IAuI), (I(Au)nI), (I(Ag)nI) and (I(AgO)I) thin films are presented in Figure 7.2 (a, b). All the

thin films show pattern which are consistent with the growth of polycrystalline body centred

cubic bixbyite structures similar to In2O3, with a space group of Ia3/cubic and lattice parameter

of 10.118 Å (JCPDS card 06-0416). The main peaks for the synthesized thin films are linked to

(222), (400), (411), (332), (440) and (622) reflection orientations similar to In2O3. There is no

impure phase related to tin complexes in the ITO matrix for the bi-layer thin films, which

confirms the investigations that the polycrystalline structure of thick top layer of ITO for this

system can act as a network with a cubic structure and the tin atoms are well integrated.

However, both (I(Ag)nI) and (I(AgO)I) thin films show the existence of (110) reflection line

featured at 2θ ~ 26.6° which reflects the presence of an SnO2 phase (Figure 7.2b). This new

phase can possibly be attributed to the coalescence and agglomeration of Sn atoms at the grain

boundaries of the ITO structure. It is also noticed that the (IAuI) thin film shows a small peak

featured around 33.3° indicating the formation of an AuInO2 crystalline structure phase. This

could be explained based on the fact that, some of Au ions possibly diffused throughout the ITO

Figure 7.1 Absorption spectra of Ag-NPs and Au-NPs colloidal solutions

192

matrix, and some of them substituted indium atoms on their sites or scattered and/or

agglomerated at the grain boundaries of the ITO system. However, no Au and/or Ag lines were

observed in the bi-layer thin films (Figure 7.2a) which might be attributed to the growth of

thicker ITO films compared to the tri-layer systems. Both the (AuI) and (IAuI) thin films exhibit

the best crystalline quality among the thin films due to uniformity, continuity and enhanced

crystallinity of the Au layers. Annealing temperature also has a potential effect on improving the

crystalline quality of the fabricated thin films by providing sufficient energy to the particles to

accumulate and grow uniformly along the preferred orientations. Compared to the peak positions

of the standard card (JCPDS card 06-0416) of indium oxide, all the synthesized thin films

exhibited a shift toward lower 2θ values indicating crystalline distortion has taken place.

The Debye-Scherrer formula (eq. 2.26) was used to calculate the grain sizes (D) of the

synthesized thin films using the full width at half maximum (FWHM) of the main peak (222) of

the ITO profile. In addition, the lattice parameter (a) and its corresponding inter-planar spacing

(d-spacing) for fabricating thin films were calculated using the standard Bravias lattice for a

cubic system (Eq. 2.27) and Bragg’s law (Eq. 2.25), respectively.

Table 7.1 presents the grain size and lattice constant values of the fabricated bi-layer and tri-

layers thin films. As estimated from the XRD profiles of the synthesized thin films, all the thin

films have larger lattice constants than In2O3 as shown by the shift in the main peak (i.e., (222))

position toward smaller 2θ (JCPDS of (10.118 Å)). The average value of the grain size of the tri-

layer thin films (~ 29 nm) was found to be smaller in relation to the average value for bi-layer

thin films (~ 31 nm), but both are larger than the value of pure ITO reported in our previous

work [430]. The (IAuI) thin film shows the highest values of both D and a assigned to be 33 nm

and 10.16 Å, respectively. This could possibly relate to the substitution of In atoms by Sn and Au

atoms having different atomic radii. On the other hand, the annealing process can not only

enhance the substitution of In by other atoms but also provide the particles of ITO, Au and Ag

with sufficient energy to accumulate and grow uniformly along the preferred orientation of

(222). Both these effects result in increasing the grain sizes along with reducing the level of the

grain boundaries, thereby improving the crystalline structure of the synthesized thin films. The

variation in the lattice constants could also be due to the induced stress near the film surface

through the inter-diffusion of bottom- or mid- layer atoms to the ITO matrix, and thermal

193

expansion misfit between the glass substrates and the thin films [514]. The Au and Ag atoms

may substitute either In and/or O atoms. Since both atoms have larger radii than In and O atoms,

the increase of lattice space and bond length eventually enlarge the lattice constant.

25 30 35 40 45 50 55 60 65

Inte

nsi

ty (

a.u.)

2θ (degree)

AuI

(Ag)nI

(Au)nI

(AgO)I(2

22

)

(40

0)

(44

0)

(41

1)

(33

2)

(62

2)

25 30 35 40 45 50 55 60 65

Inte

nsi

ty (

a.u.)

2θ (degree)

IAuI

I(Ag)nI

I(Au)nI

I(AgO)I

(22

2)

(40

0)

(44

0)

(33

2) (62

2)

*

# (

11

0)

(a)

(b)

# SnO2

* AuInO

Figure 7.2 XRD patterns of (a) bi-layer and (b) tri-layer thin films annealed at 500 °C

194

Table 7.1 Structural parameters of the fabricated bi-layer and tri-layer thin films

7.4.3. Surface Morphology

A comparison of the morphological properties of bi-layer and tri-layer thin films annealed at 500

°C was investigated using the FESEM images and the results are shown in Figure 7.3. All the

fabricated bi-layer and tri-layer thin films appear as dense surfaces with grain-like morphologies

indicating the formation of a polycrystalline structure of ITO. Since the whole thickness for bi-

layer and tri-layer coatings is (~ 130 nm) as proved by cross sectional image, we may assume

that each ITO layer in tri-layer system is (~ 60 nm), while that for bi-layer one it is (~ 120 nm).

Thin films deposited on Au and AgO layers show larger grains along with significant

improvement in the morphology of the top ITO layer compared with other thin films, due to the

high degree of crystallinity that the former films possess. These results are consistent with those

observed from the XRD measurements (section 3.2). Although other thin films grown on (Au)n

and (Ag)n layers show granular morphology surfaces, few micro-cracks, grain boundaries and

pinholes were observed between the granules of the top ITO thin films. This possibly indicates

that both the Au-NPs and Ag-NPs coatings covered the substrates (for bi-layer systems) and

bottom ITO films (for tri-layer systems) incompletely as seen in Figure 7.4. These undesirable

morphologies seem to be an inherent property when ITO is deposited onto a discontinuous layer.

Cross-section imaging was performed on two of the ITO thin films with encapsulating Au and

An-NPs layers and the results are seen in Figure 7.3c. Both Au and Au-NP layers possess

thicknesses of ~ 10 nm. Thus, we may assume that AgO and Ag-NPs insertions possess similar

thicknesses since they are all fabricated with similar deposition conditions. It is clearly seen that

the Au layer covered the bottom ITO film completely, while Au-NP layer exhibited a distinct

System Coatings D (nm) α (Å) d-spacing ( Å )

AuI 32.45 ± 0.55 10.15 ± 0.01 2.931

Bi-layer (Ag)nI 29.32 ± 0.60 10.12 ± 0.01 2.921

(Au)nI 30.50 ± 0.56 10.12 ± 0.01 2.921

(AgO)I 32.66 ± 0.58 10.14 ± 0.01 2.925

IAuI 33.40 ± 0.47 10.16 ± 0.01 2.934

Tri-layer I(Ag)nI 27.71 ± 0.51 10.12 ± 0.01 2.921

I(Au)nI 25.56 ± 0.50 10.14 ± 0.01 2.925

I(AgO)I 30.42 ± 0.52 10.14 ± 0.01 2.925

195

ITO

ITO

Au

Glass

Resin

ITO

ITO

Resin

Glass

Au-NPs

(c1) (c2)

(b4) (b3) (b2) (b1)

(a1) (a2) (a3) (a4)

I(AgO)I (I(Au)nI)

AuI

(I(Ag)nI)

(AgO)I ((Au)nI) ((Ag)nI)

IAuI

island-like structure. The discontinuity and granularity of Au-NP and Ag-NP coatings explain

the morphological differences between these thin films and other thin films grown on Au and

AgO layers. The formation of island like-structures in the Au-NPs and Ag-NPs coatings for bi-

layer and tri-layer thin films restricted the lateral expansion of the top ITO granules due to the

large numbers of voids and grain boundaries these layers possess. Consequently, there was a

delay in the coalescence of neighbouring ITO particles, resulting in discontinuous film

formation. However, this is not so significant in our bi-layer thin films due to the thicker ITO

films compared to tri-layer thin films as noted in Figure 7.4 (b1 – b4).

Figure 7.3 Top-view images of bi-layer (a1 – a4) and tri-layer (b1 – b4) thin films. Cross-sectional

images for (c1) - IAuI and (c2) - I(Au)nI thin films

196

7.4.4. Atomic Compositions and Surface Chemical Bonding States

The elemental compositions of the synthesized bi-layer and tri-layer thin films were obtained via

XPS survey scans and the results are presented in Table 7.2. All the synthesized thin films show

the presence of the photoelectron peaks for In 3d, Sn 3d, O 1s, and C 1s in the binding energy

range of 200 - 600 eV, while no peaks related to Au and/or Ag. This was mainly because the

XPS scans were performed on a maximum depth of ~ 10 nm from the top surfaces of the thin

film. It is noticed from Table 7.2 that the atomic percentage of O 1s photoelectron line is higher

than the other photoelectron lines due to the formation of a thick oxide layer owing to both the

wet chemical process and annealing temperature. Also, the ratio of the atomic percentages of In

to Sn obtained from XPS measurements is consistent with the mixed ratio (In:Sn = 90:10 at.%)

for preparing the ITO solution.

Table 7.2 Variation of the atomic percentage of the main elements for bi-layer and tri-layer thin films

Coating Atomic percentage of the elements % In Sn O C

Bi-layer 16.12 ± 0.11 1.73 ± 0.07 51.16 ± 0.7 30.99 ± 0.58

Tri-layer 13.51 ± 0.15 1.46 ± 0.05 53.13 ± 0.65 31.90 ± 0.55

ITO

Au-NPs Ag-NPs AgO Au

Figure 7.4 Schematic diagram shows the differences in the formation of the mid-layer thin film

coatings

197

Since all fabricated bi-layer and tri-layer thin films exhibit similar photoelectron lines along with

their corresponding binding energies, the surface chemical bonding states of one thin film (i.e.,

IAuI) coating were investigated by de-convoluting the high resolution In 3d, Sn 3d and O 1s

peaks to evaluate the contributions of metal oxides and/or compounds to the film materials

(Figure 7.5).

The fitting curves of high resolution XPS spectra for In 3d5/2 photoelectron line are seen in

Figure 7.5a. Two bonding states with their corresponding binding energies assigned at 444.08

and 444.78 eV were identified and they are attributed to In° bonding state, specifically In-In

bonds, and In3+

bonding state from In2O3, respectively [387, 388]. Figure 7.5b shows the fitting

curves of high resolution XPS spectra of Sn 3d5/2 photoelectron peaks. Two sub peaks are

identified in the fitting curves around 486.38 and 487.45 eV. The first peak with the lowest

binding energy of 486.38 eV can be linked to Sn4+

in SnO2 while the second peak with the

highest binding energy of 487.45 eV is attributed to Sn2+

in SnO. It has previously been reported

that the Sn 3d peak for Sn4+

from SnO2 has a binding energy around 0.5 eV lower than that for

Sn2+

from SnO. Also, the SnO2 phase is thermodynamically more stable than the SnO phase and

is expected to be the major phase between these Sn components in the ITO-based surface

coatings [148, 389]. The de-convoluted fitting curves for O 1s core level show two different

peaks allocated at 529.70 and 531.20 eV, which correspond to In-O bonds from In2O3 and Sn-O

bonds from both SnO and SnO2, respectively (Figure 7.5c) [387]. Oxygen vacancies were found

to be a key aspect in enhancing the electrical properties of In2O3-based thin film coatings through

contributing free carriers to the In2O3 matrix. A theoretical study carried out by Agoston and co-

authors [392] using density function theory (DFT) and first principle calculations, confirmed that

the oxygen vacancies are shallow in the In2O3 matrix and its derivative materials, which facilitate

the production of free electrons (which are the major source of n-type conductivity in In2O3-

based thin film coatings) in the conduction band.

198


The electrical characteristics of bi-layer and tri-layer thin films represented by electrical

resistivity, conductivity, carrier concentration and mobility are shown in Table 7.3. The well-

structured morphology of both (IAuI) and (I(AgO)I) thin films fully accounted for the fact that

the values of electrical resistivity of these tri-layer thin films are much lower compared with their

relative thin films. The (IAuI) thin film shows the lowest electrical resistivity along with highest

carrier concentration and mobility assigned to be 1.2 × 10-4

Ωcm, 11.4 × 1020

cm-3

and 40

cm2/Vs, respectively, while the (I(AgO)I) thin film displays electrical resistivity, carrier

concentration and mobility of 1.6 × 10-4

Ωcm, 9.4 × 1020

cm-3

and 38 cm2/Vs, respectively. The

resistivity for (IAuI) thin film is lower than that obtained in our previous report for 350 nm ITO

and Ti-doped ITO thin films [430]. The superior electrical characteristics of the (IAuI) thin film

are possibly attributed to the uniformity and continuity of the metal Au mid-layer which

441442443444445446447448

CP

S/I

nte

nsi

ty (

a.u)

Binding energy (eV) 484485486487488489490

CP

S/I

nte

nsi

ty (

a.u)

Binding energy (eV)

528529530531532533534535

CP

S/I

nte

nsi

ty (

a.u

)

Binding energy (eV)

In-In

444.08 eV

In3+ in In2O3

444.78 eV

Sn4+ in SnO2

486.38 eV

Sn2+ from SnO

487.45 eV

Sn-O

531.34 eV

In-O

529.64 eV

(a)

(c)

(b)

Figure 7.5 High resolution scans of (a)- In 3d, (b)- Sn 3d and (c)- O 1s of IAuI thin film

199

restricted the scattering of the free carriers at the interface between this mid-layer and the top

ITO layer, thereby enhance the carrier mobility. The inter-diffusion process of Au atoms to the

top ITO layer may also have increased the carrier concentration of the thin film. In general,

metals have high free carrier concentration of the order of 1022

- 1023

cm-3

, and Au was also

found to have a high work function (ca. 5.1-5.47 eV), which makes it a good electron injector.

Therefore, it is reasonable for the (IAuI) thin film to have a high carrier concentration due to

efficient carrier injection from the Au layer to ITO matrix. Carrier scattering might be less

significant at the interface between the Au mid-layer and ITO layer due to a decrease in the

contact area between these layers when the Au layer formed a continuous film with larger

particles owing to the fast coalescence of its atoms. Annealing also enhances both the diffusion

process and the substitution of In atoms by Au and Ag atoms resulting in improved level of

carrier concentration. (I(Au)nI) and (I(Ag)nI) thin films show significantly higher electrical

resistivity assigned to be 11.4 × 10-4

Ωcm and 8.7 × 10-4

Ωcm, respectively. This could be

related to the formation of distinct Au-NP and Ag-NP mid-layer coatings with agglomeration of

these nanoparticles in the form of island like-structure as discussed previously. Moreover, the

scattering of the free carriers at the interfaces between Au-NP or Ag-NP mid-layer coatings and

ITO films, as well as the grain boundaries noticed in the top ITO layers of the (I(Au)nI), (I(Ag)nI)

and (I(AgO)I) thin films, could be the main reasons that the mobility and carrier concentration of

these thin films were less than that of the (IAuI) thin film. In the case of bi-layer thin films, the

ITO film grown onto an Au layer exhibits the lowest electrical resistivity and the highest carrier

concentration at 3.1 × 10-4

Ωcm, 6.8 × 1020

cm-3

,respectively, while its carrier mobility (29

cm2/Vs) is similar to that of ITO grown onto an AgO layer. It might be surmised that the

electrical properties of the bi-layer coatings mainly depend on the ITO thin films rather than the

bottom layers. The formation of quite thick ITO coatings for the bi-layer thin films compared to

thin top ITO coatings for tri-layer thin films results in better surface morphologies. This shows

that the differences in the electrical properties of bi-layer thin films are not as high as for the tri-

layer thin films. For instance, the ITO coatings grown on Ag- and Au- nanoparticles layers show

only a slight decrease in the electrical properties in comparison with those for tri-layer ones

grown on similar Ag- and Au- nanoparticle mid-layers. This is because thicker ITO coatings for

the bi-layer thin films compensate for the shortcomings of discontinuity of Ag- and Au-

nanoparticle layers through early coalescence and agglomeration of ITO nanoparticles.

200

Table 7.3 3 Electric properties of fabricated bi-layer and tri-layer thin films


Despite their high degree of crystallinity, (AuI) and (IAuI) films exhibit lower optical

transparency than ITO and Ag-doped ITO thin films reported in our previous investigation [514]

due to the reflectance losses that the metal layer introduces in the visible region. Figure 7.6 (a, b)

presents the transmittance spectra of the fabricated bi-layer and tri-layer thin films. (AuI) and

(IAuI) thin films have optical transparency of 85% and 82.5% respectively. In comparison, both

((AgO)I) and (I(AgO)I) thin films show high transmittance (~ 91.5%) owing to the oxidation of

Ag, which results in tuning the refractive index of these films. A similar effect has been reported

through increasing the oxygen to argon partial pressure level in a sputtering system [515, 516].

Despite the (AuI) and (IAuI) thin films having similar thicknesses, their optical transmittances

were different. The high transparency of the (AuI) thin film can be possibly related to decrease

light scattering. The bi-layer (AuI) thin film consists of two media with different refractive

indices. For the tri-layer (I(Au)I) thin film, light scattering will occur from the interfaces between

ITO and the Au layer. It is interesting to notice that both the (AuI) and (IAuI) thin films exhibit

their maximum transparencies around 500 nm, while the other thin films exhibit their maximum

transparency at a wavelength around 550 nm. However, the (IAuI) thin film shows a second

maximum transmittance (83.5%) at a wavelength around 750 nm. This high transparency at

longer wavelength can enhance the optoelectronic properties of the thin film especially for

System Coatings ρ×10-4

(Ωcm) σ (S/cm) n×1020

(cm-3

) μ (cm/Vs)

AuI 3.1 ± 0.39 3155.66 ± 140 6.8 ± 0.31 29.4 ± 0.93

Bi-layer (Ag)nI 5.0 ±0.25 1856.12 ± 120 5.8 ± 0.27 20.3 ± 0.70

(Au)nI 7.5 ±0.24 1270.10 ± 95 4.4 ± 0.28 18.5 ± 0.77

(AgO)I 3.8 ±0.24 2598.72 ± 144 5.6 ± 0.30 29.96 ± 0.85

IAuI 1.2 ± 0.15 7296.55 ± 190 11.4 ± 0.28 40.29 ± 0.82

Tri-layer I(Ag)nI 8.7 ± 0.31 1125.21 ± 105 3.0 ± 0.24 24.41 ± 0.68

I(Au)nI 11.4 ± 0.28 874.77 ± 0.91 2.6 ± 0.27 21.33 ± 0.75

I(AgO)I 1.6 ± 0.20 5720.28 ± 200 9.4 ± 0.33 38.48 ± 0.92

201

producing a high level of photocurrent at this wavelength. Although the films grown on Au-NP

and Ag-NP layers show high transmittances (~ 88%), they are still lower than those for ((AgO)I)

and (I(AgO)I) thin films. This is possibly attributed to the discontinuity of Au-NP and Ag-NP

layers that act as scattering centres for incident radiation.

The transmittance data were used to calculate the absorption coefficient and the optical band

gap energy of the synthesized bi-layer and tri-layer thin films. The absorption coefficient (α) was

calculated using (Eq. 5.2).

Variations of the absorption coefficient for bi-layer and tri-layer thin films in the range of 350-

800 nm are presented in Figure 7.7. It is seen that the AuI and IAuI thin films show the highest

average absorption coefficient among other fabricated thin films. The IAuI thin film exhibits a

shift in the absorption edge to longer wavelength, while the absorption edge of AuI thin film

shifts to shorter wavelength.

The absorption coefficient for the direct allowed transition can be described as a function of

photon energy using (Eq. 3.1). The optical band gap energies of the synthesized thin films were

obtained by extrapolating the linear parts of the plotted curves to the x-axis.

202

40

50

60

70

80

90

100

300 350 400 450 500 550 600 650 700 750 800

AuI

(Ag)nI

(Au)nI

(AgO)I

Glass

(a)

40

50

60

70

80

90

100

300 350 400 450 500 550 600 650 700 750 800

IAuI

I(Ag)nI

I(Au)nI

I(AgO)I

Glass

(b)

Figure 7.6 Transmittance spectra of (a)- bi-layer and (b)- tri-layer thin films

203

Figure 7.8 (a, b) shows the variations of the band gap energies of the bi-layer and tri-layer thin

films. The band gap energy between the top of the valence band and the lowest empty state in the

conduction band was found to be proportional to the variations in the grain size, the internal

stress and/or the free carrier concentration (N) [517-519], and it can be increased along with

increasing N due to filling of the lowest energy levels in the conduction band with a broadening

proportional to N2/3

(Burstein–Moss relation) (Eq. 3.2) [370]:

0.5

1

1.5

2

2.5

3

3.5

4

300 350 400 450 500 550 600 650 700 750 800

Ab

sorp

tio

n c

oef

fici

ent

(α)

Wavelength (nm)

AuI

(Ag)nI

(Au)nI

(AgO)I

0.5

1

1.5

2

2.5

3

3.5

4

300 350 400 450 500 550 600 650 700 750 800

Ab

sorp

tio

n c

oef

fici

ent

(α)

Wavelength (nm)

IAuI

I(Ag)nI

I(Au)nI

I(AgO)I

(b)

(a)

Figure 7.7 Absorption coefficient of (a)- bi-layer and (b)- tri-layer thin films

204

Despite the high N values, the optical band gap energies of (AuI) and (IAuI) thin films were

estimated to be low compared to the other thin films in this study. This could be mainly related

to the reflectance losses of the Au layer and the introduction of some defects that create localized

states in the Eɡ [179]. The Eɡ for the (AuI) and (IAuI) thin films were 3.73 eV and 3.5 eV,

respectively, due to high optical transparency in the visible region, and the shift of the absorption

edge towards shorter wavelength for the AuI thin film. The highest optical band gap energy (3.75

eV) was observed for both AgOI and IAgOI thin films. This high value of Eɡ is mainly attributed

to high optical transparency owing to the decrease in the light scattering, improved crystalline

quality, enlarged crystallite size and the release of internal stress. Other fabricated bi-layer and

tri-layer thin films on Au and Ag nanoparticle layers exhibit energy band gaps within the range

of (3.65 – 3.72) eV.

The performance of the synthesized bi-layer and tri-layer thin films as transparent conductive

coatings was further appraised through determination of the figure of merit (FOM) of these thin

films. The FOM was pertinent to both the sheet resistance (Rsh) and transmittance (T) (at a

wavelength of 550 nm) of the thin film, and can be expressed by the following relation:

⁄ . A comparison of the FOM of the fabricated and annealed bi-layer and tri-layer thin

films is presented in Table 7.4. It is known that a higher value of FOM suggests better

performance for a transparent conducting thin film. The highest values of FOM were calculated

to be 31 × 10-3

and 13 × 10-3

Ω-1

for (I(AgO)I) and (AgO)I thin films, respectively. These high

values can be mainly attributed to high optical transparency of the (I(AgO)I) and (AgO)I films.

Table 7.4 Optical, sheet resistance and FOM values of synthesized thin films

System Coating T (%) Eg (eV) Rsh (Ω/sq) Φ×10-3

(Ω-1

)

AuI 85.0 ± 0.7 3.73 ± 0.01 23.80 ± 2.1 7.3 ± 0.2

Bi-layer (Ag)nI 87.5 ± 0.4 3.65 ± 0.02 38.45 ± 1.8 6.4 ± 0.2

(Au)nI 88.0 ± 0.2 3.70 ± 0.01 57.70 ± 2.2 4.8 ± 0.2

(AgO)I 91.5 ± 0.2 3.75 ± 0.01 29.23 ± 2.4 13.0 ± 0.1

IAuI 82.5 ± 0.5 3.51 ± 0.02 9.23 ± 1.2 9.0 ± 0.2

Tri-layer I(Ag)nI 88.5 ± 0.2 3.72 ± 0.01 66.92 ± 3.1 4.1 ± 0.2

I(Au)nI 87.5 ± 0.3 3.65 ± 0.03 87.70 ± 2.7 2.8 ± 0.1

I(AgO)I 91.5 ± 0.3 3.75 ± 0.01 12.30 ± 0.9 31.0 ± 0.4

205

3.3 3.35 3.4 3.45 3.5 3.55 3.6 3.65 3.7 3.75 3.8 3.85 3.9 3.95 4

Photon energy (eV)

AuI

(Ag)nI

(Au)nI

(AgO)I


)

3.3 3.35 3.4 3.45 3.5 3.55 3.6 3.65 3.7 3.75 3.8 3.85 3.9 3.95 4

Photon energy (eV)

IAuI

I(Ag)nI

I(Au)nI

I(AgO)I


)

(b)

(a)

The optical constants including refractive index (n) and extinction coefficient (k) of the thin film

are considered key aspects for achieving good performance in optoelectronic applications. The

refractive index of a thin film is a measure of its transparency to incident radiation within a

specific region of studied wavelength. The valuation of the refractive index is very important in

Figure 7.8 Variations of the band gap energy of (a)- bi-layer and (b)- tri-layer thin films

206

integrated optoelectronic devices. The n and k values of the bi-layer and tri-layer thin films were

calculated based on (Eq. 5.3) and (Eq. 2.19), respectively.

Figure 7.9 (a, b) shows the variation of the refractive indices of the fabricated bi-layer and tri-

layer thin films. It can be seen in Figure 7.9 that the refractive indices of all thin films decreased

with increasing the wavelength, and the average values of n for both AuI and IAuI thin films are

higher than that for other thin films fabricated in this study. This could be attributed to the high

carrier concentration and reflectivity that the AuI and IAuI thin films possess. Increasing carrier

concentration might result in the formation of some light scattering centres, which hinders the

optical transparency.

As seen in Figure 7.10 (a, b), the extinction coefficients of all the fabricated bi-layer and tri-layer

thin films initially decrease in the near ultraviolet (NUV) region and then gradually increased in

the visible region. The AuI and IAuI thin films exhibit the highest average extinction coefficient

values in the visible region compared with the other thin film coatings, which could be also

related to increased reflection of light by Au layer.

207

1.5

1.75

2

2.25

2.5

2.75

3

350 400 450 500 550 600 650 700 750 800

Ref

ract

ive

ind

ex (

n)

Wavelength (nm)

AuI

(Ag)nI

(Au)nI

(AgO)I

1.5

1.75

2

2.25

2.5

2.75

3

350 400 450 500 550 600 650 700 750 800

Ref

ract

ive

ind

ex (

n)

Wavelength (nm)

IAuI

I(Ag)nI

I(Au)nI

I(AgO)I

(b)

(a)

Figure 7.9 Refractive index spectra of (a)- bi-layer and (b)- tri-layer thin films

208

0

0.05

0.1

0.15

0.2

300 400 500 600 700 800

Exti

nct

ion c

oef

fici

ent

(k)

Wavelength (nm)

AuI

(Ag)nI

(Au)nI

(AgO)I

0

0.05

0.1

0.15

0.2

300 400 500 600 700 800

Exti

nct

ion c

oef

fici

ent

(k)

Wavelength (nm)

IAuI

I(Ag)nI

I(Au)nI

I(AgO)I

(b)

(a)

Figure 7.10 Extinction coefficient spectra of (a)- bi-layer and (b)- tri-layer thin films

209

7.4.7. Characteristics of OSCs

The favourable performance of AgOI and IAgOI identified these thin films for further

investigation in organic solar cells (OSCs) using these thin films as electrodes combined with a

low band gap photoactive polymers. The OSCs configuration:

glass\electrode\PEDOT:PSS\P3HT:PCBM\Al is shown in Figure 7.11a. Characteristics of the

OSCs fabricated with AgOI and IAgOI electrodes were compared with OSC fabricated with an

ITO electrode reported in our previous study [430]. The power conversion efficiency (PCE) of

the OSC is defined by the following formula:

(7.3)

where Voc, Jsc and Pi are the open circuit voltage, short-circuit current density and the input

power of the solar irradiance, respectively. FF is the fill factor which is defined as:

(7.4)

where Jm and Vm are the maximum current density and maximum voltage, respectively.

A comparison of the (J-V) characteristics of the fabricated OSCs based on ITO, AgOI and IAgOI

electrodes showed that the short-circuit current density (Jsc) and fill factor (FF) values of the

solar cell with IAgOI electrode were observably higher than those of AgOI and ITO electrodes

as seen in Figure 7.11b and Table 7.5. Since the configuration of the fabricated OSCs is

identical, all the OSCs showed similar open circuit voltage (Voc). The higher value of Jsc and FF

of the OSC fabricated with IAgOI electrode could be associated with the higher incident-photon

to current conversion efficiency, owing to the higher optical transparency of the IAgOI electrode

in the wavelength range of 400 – 800 nm, and the lower sheet resistance compared to AgOI and

ITO electrodes. Series resistance (Rs) of an electrode has no effect on its Voc value, yet it

decreases the Jsc value of the electrode [20]. The power conversion efficiency (PCE) of the OSC

fabricated with IAgOI electrode is greater than those OSCs fabricated with ITO and AgOI

electrodes, despite similar optical transparency. This high PCE value observed in the OSC

fabricated with IAgOI electrode is the result of the increase in the photocurrent production due to

both low Rsh value, and high optical transparency in the wavelengths between 400 and 800 nm.

210

(a)

-14

-12

-10

-8

-6

-4

-2

0

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

Cu

rren

t d

ensi

ty (

mA

cm-2

)

Voltage (V)

I(AgO)I

(AgO)I

ITO

(b)

Table 7.5 Photovoltaic performance parameters of fabricated OSCs

7.5. Conclusions

In the present study, thin (AuI), (Au)nI, (Ag)nI, (AgO)I, (IAuI), (I(Au)nI), (I(Ag)nI) and (I(AgO)I)

film coatings were successfully fabricated via a low cost, efficient and environmentally friendly

sol-gel spin-coating method onto soda-lime glass substrates, and annealed at 500 C. The effects

of inserting thin Au, Au-NP, Ag-NP and AgO layers in bi-layer and tri-layer ITO-based thin

films were investigated by means of structural, morphological, elemental composition, optical

and electrical characteristics. XRD results confirmed the formation of a body-centered cubic

structure of polycrystalline indium oxide. XPS scans show the existence of the main elements

(In, Sn and O) in the film materials, and the contributions of the element compounds were

estimated by de-convoluting the high resolution In 3d, Sn 3d and O 1s photoelectron lines via a

Gaussian distribution. FESEM images revealed the development of dense surfaces with grain-

like morphologies indicating the formation of polycrystalline features of ITO. Optical studies

Electrode Jsc (mA cm-2

) Voc (V) FF (%) Rsh (Ω cm2) Rs (Ω cm

2) PCE (%)

IAgOI 12.5 0.64 61.25 565.68 7.4 4.9

AgOI 11.53 0.64 57.70 731.74 10.1 4.2

ITO 10.63 0.64 56.10 829.31 14.5 3.8

Figure 7.11 (a)- Device architecture and (b)- J-V characteristic curves of the fabricated and compared OSCs in

this investigation

211

revealed absorption peaks at wavelengths of 405 nm and 531 nm for Ag-NP and Au-NP colloidal

solutions, respectively, which were attributed to the formation of 10 – 14 and 48 nm Au and Ag

particles, respectively. The maximum transmittance in the visible region and band gap energy

were found to be ~ 91.5% and 3.75 eV for both (AgO)I and (I(AgO)I) thin films. The IAuI thin

film showed the lowest electrical resistivity of (1.2×10-4

Ωcm), along with the highest carrier


cm-3

) and mobility (40 cm2/V s). These results are better than those

obtained from 350 nm pure ITO and doped-ITO films An improvement in the power conversion

efficiency (PCE) from 3.8 to 4.9% is achieved in an organic solar cell by replacing the

conventional pure ITO electrode with an (I(AgO)I) electrode.

212

8. CHAPTER 8: Conclusions and Recommendations

8.1. Conclusions

An efficient, low-cost and environmentally friendly sol-gel spin-coating method was adopted to

synthesize ITO-based transparent and conducting thin films on soda-lime glass substrates. An

alternative to high energy and vacuum techniques, high crystalline quality sol-gel derived ITO-

based thin films combined with 3d transition metals including Ti and Ag, SWCNTs and metal

nanoparticles were successfully prepared and characterized for their optoelectronic properties by

means of XRD, XPS, FESEM, EDX, AFM, Raman analysis, UV-Vis, four point probes, Hall-

Effect, and nanoindentation measurements. The structural, morphological, optoelectronic and

mechanical properties were investigated for a range of variables including Ti (2 and 4 at.%) and

Ag concentrations (2, 4, 6 and 10 at.%) for 350 nm Ti- and Ag- doped ITO film thickness,

respectively, film thickness (150, 210, 250 and 320 nm) for SWCNTs (0.25 wt%) incorporated

ITO thin films. Three different ITO-based film geometries (single-, bi- and tri- layers) were

synthesized in this study. Single ITO coatings include ITO, Ti- and Ag- doped ITO and CNT

incorporated ITO. While, bi-layer and tri-layer ITO stacks consisted of: Au/ITO (AuI), Au-

NPs/ITO (Au)nI, Ag-NPs/ITO (Ag)nI and AgO/ITO (AgO)I for bi-layer coatings and

ITO/Au/ITO (IAuI), ITO/Au-NPs/ITO (I(Au)nI), ITO/Ag-NPs/ITO (I(Ag)nI) and ITO/AgO/ITO

(I(AgO)I) for tri-layer coatings. All the synthesized ITO-based thin films were fabricated using a

pre-ITO solution of (In:Sn = 90:10 at.%), and post annealed in the range (350 – 600) °C.

XRD results confirmed that all the fabricated ITO-based thin films exhibited the presence of a

body-centered cubic bixbyite structure similar to In2O3 with a space group of Ia3. The crystalline

quality of the fabricated thin films was improved along with enlarging grain sizes and lattice

constants by Ti, Ag and CNTs additives, annealing temperature and depositing ITO layers on

continuous Au and AgO layers. A Thin ITO film prepared with 4 at.% Ti and annealed at 500 °C

exhibited the largest crystallite size of 80 nm compared to other doped ITO-based thin films with

crystallite size in the range of (50 – 65) nm. XRD results also showed the presence of Ag and C

phases for Ag-doped ITO and SWCNTs/ITO thin films, respectively, which were also confirmed

by EDX and Raman analysis. EDX results showed the presence of Ag and CNTs in ITO film

materials, while Raman analysis performed on SWCNT/ITO thin films revealed the existence of

both D- and G-bands for carbon. The D-band seen at 1292.4 cm-1

is related to the defect in the

213

SWCNTs, while the G-band seen at 1594.6 cm-1

is characteristic of sp2 graphitic material, which

is attributed to a carbon atom vibrational mode along the SWCNT axis. XPS results of

SWCNTs/ITO thin films are consistent with Raman analysis. By de-convoluting the C1s

photoelectron lines, a high contribution of 66.8% observed at the lowest binding energy around

284.5 eV was assigned to pure sp2 hybridized graphitic carbon atoms of (C-C and C=C bonds),

and carbon atoms bounded to hydrogen (C-H bonds). These sp2 bonds provide CNTs with their

unique mechanical properties.

The surface morphology of the synthesized ITO-based thin films was characterized by FESEM

imaging. All the thin films showed dense and homogeneous surfaces with grain-like structures

demonstrating the nanocrystalline structure. Incorporation of Ti, Ag and CNTs in ITO film

materials resulted in enhancing the surface morphologies of these thin films along with enlarging

their grains. These findings are consistent with the results obtained from the XRD results.

Annealing temperature up to 500 °C enhanced the morphologies of the synthesized thin film.

However, further increase in the annealing temperature up to 600 °C resulted in the formation of

some cracks on the surfaces of ITO and Ti-doped ITO thin films. Also, thicker films (i.e., 250

and 320 nm) for SWCNTs/ITO coatings showed the formation of some cracks on their surfaces

compared to thinner films of the same coating. These cracks significantly affected the surface

topography of thicker SWCNTs/ITO films that displayed RMS values of 4.08 and 4.65 nm for

250 and 320 nm, respectively, compared to 3.36 and 3.49 nm for thinner films (150 and 210 nm,

respectively) as proven by AFM scan results. FESEM images of bi-layer and tri-layer ITO-based

thin film coatings revealed that the films deposited on continuously formed Au and AgO layers

are dense, smooth and homogeneous, while those deposited on Au- and Ag- nanoparticles layers

displayed a few micro-cracks, grain boundaries and pinholes due to discontinuous island-like

structures these coatings possess. These continuous and distinct Au and Au-NPs layers were

confirmed by cross-sectional FESEM images performed on ITO/Au ITO and ITO/Au-NPs/ITO

thin films, respectively.

It is believed that improving the crystalline quality and surface morphology of ITO-based thin

films resulted in enhancing their optoelectronic properties. The highest optical transparency

(~92%) was for ITO, 4 at.% Ti-doped ITO and 2 at.% Ag-doped ITO thin films. This high

optical transparency resulted from the broadening of the optical band gap energy calculated to be

214

(3.74, 3.77 and 3.75 eV) for the former thin films, respectively. Since the optical constants

including absorption coefficient, complex refractive index and extinction coefficient are very

important parameters for achieving good performance in optoelectronic devices, these

parameters were also calculated in this work for different synthesized thin films. It is noted that

increasing the Ag level in Ag-doped ITO thin films, and the film thickness of SWCNTs/ITO thin

films resulted in a shift in the absorption edge to longer wavelength regions and the thin films

showed higher absorption coefficient values compared to pure ITO films. Similarly, increasing

the Ag level in Ag-doped ITO thin films resulted in increasing the average values of both

refractive index and extinction coefficient. In relation to bi-layer and tri-layer ITO based thin

films, both Au/ITO and ITO/Au/ITO stacks revealed the highest values of both refractive index

and extinction coefficient compared to other stacked thin films.

Incorporating Ag, Ti and CNTs in ITO matrix resulted in improving the electrical and

mechanical properties of ITO thin films. ITO thin films prepared with 4 at.% Ti concentration

and annealed at 500 °C showed an improvement in the electrical resistivity of 1.6×10-4

Ωcm

along with carrier concentration and carrier mobility of 8.6×1020

cm-3

and 49 cm2/Vs,

respectively, compared with pure ITO film with resistivity, carrier concentration and mobility of

2.2×10-4

Ωcm, 7.0 ×1020

cm-3

and 39 cm2/Vs, respectively. Similarly, the ITO film prepared

with 4 at.% Ag concentration showed improved electrical properties among other Ag-doped ITO

thin films. Additionally, increasing the thickness of SWCNTs/ITO thin films resulted in

improving their electrical properties and slightly decreasing the optical transparencies. The

thickest film (i.e., 320 nm) annealed at 350 °C exhibited electrical resistivity, carrier

concentration and mobility of 4.6×10-4

Ωcm, 3.3 ×1020

cm-3

and 41 cm2/Vs, respectively.

However, the lowest electrical resistivity achieved in this study was 1.2 × 10-4

Ωcm, related to

ITO/Au/ITO along with carrier concentration and mobility of 11.4 × 1020

cm-3

and 40 cm2/Vs,

respectively.

Improvement in the mechanical properties of the thin films was attributed to crystalline quality,

chemical and electronic states of the thin films. The contributions of Ti, Ag and CNT in

improving the mechanical characteristics of the synthesized ITO-based thin films were estimated

from the nanoindentation measurements. ITO thin films prepared with the highest levels of both

Ti and Ag and annealed at 500 °C revealed a hardness and Young’s modulus in the range (6.7 -

215

6.8) and (143 – 148) GPa, respectively. Due to the high contribution of sp2 bonds (66.8%)

obtained from the XPS analysis, significant improvement in the mechanical properties was noted

by incorporating robust SWCNTs in ITO matrix, with a hardness and Young’s modulus of 28

and 306 GPa, respectively, compared to 6.6 and 143 GPa for ITO thin film. These distinctive

mechanical properties of SWCNT/ITO films resulted in a slight drop in their optical

transparencies. However, these values of optical transparency are still acceptable in

optoelectronic applications.

The performance of ITO-based thin film coatings was examined through fabricating OSCs with

ITO, AgO\ITO and ITO\AgO\ITO electrodes. The resultant OSCS have the configuration:

glass\electrode\PEDOT:PSS\P3HT:PCBM\Al. The power conversion efficiencies of 4.9%, 4.2%

and 3.8% were obtained for ITO\AgO\ITO, AgO\ITO and ITO thin film coatings, respectively.

Increasing demands for the next generations of optoelectronic devices inspires researchers in the

field of TCMs to develop new low-cost materials or use simple and low-cost techniques to

fabricate these kinds of thin films. The sol-gel spin-coating process adopted in this research work

has proven itself to compete with other, more expensive and more complicated methods in

synthesizing high performance ITO-based transparent and conductive films.

Table 8.1 A comparison of the best achieved results of ITO-based TCOs thin films fabricated in

this study

Coating ρ ×10-4

(Ω.cm) n ×1020

(cm-3

) T% Hardness

ITO 2.2 7.0 90.5 6

Ti-doped ITO 1.6 8.6 92 6.8

Ag-doped ITO 2.4 6.8 92 6.8

SWCNTs/ITO 4.6 3.3 88 28.0

Au/ITO 3.1 6.8 80 -

AgO /ITO 3.8 5.6 85 -

ITO/Au/ITO 1.2 11.4 80 -

ITO/AgO/ITO 1.6 9.4 88 -

216

8.2. Recommendations

The scientific works embodied in this thesis have led to an intriguing opportunity for further

investigations in improving the optoelectronic and mechanical properties of ITO-based thin film

coatings. The next paragraphs discuss a few suggestions for potential future works.

Since the ITO properties have been improved by doping with Ti and Ag in this dissertation and

annealing, co-doping of ITO thin films can be an efficient new strategy to further improve its

optoelectronic characteristics. Investigation of alternative elements with suitable concentrations

to dope ITO thin films without altering their crystallite structure will be beneficial for this

purpose.

Mechanical properties are very important aspects in determining favorable performance from

ITO-based thin films, especially for flexible applications. The surface of an ITO film is brittle,

weak, and seems to be susceptible to delamination and cracking under an applied strain. In this

study, CNTs incorporated with ITO showed good mechanical characteristics despite the

reduction of the optical transparency. Therefore, developing new materials and/or elements to

improve the mechanical properties of a brittle ITO while maintaining its optoelectronic

properties, is important to commercialize ITO for the next generation of flexible optoelectronic

devices.

Stacked thin films including ITO/metal/ITO have been investigated previously. However, a

metal oxide layer encapsulated between two ITO layers adopted in this research study exhibited

improved optical properties compared to pure metal layer. Further investigation is still needed to

replace a metal layer which has high reflectivity with other metal oxide layers. Another

suggestion is to fabricate ITO with other TCO thin films such as ZnO, SnO2, TiO2, etc.

Graphene and metal nanowires including Ag-, Cu-, and Ni- nanowires are of great interest in the

field of transparent conducting materials. But, they still have some drawbacks related to high

absorption by graphene in the visible region, and the oxidation issue of metal nanowires.

Combining ITO thin films with graphene and metal nanowires can overcome their shortcomings.

217

References

[1] S.-H. Choa, C.-K. Cho, W.-J. Hwang, K.T. Eun, H.-K. Kim, Mechanical integrity of flexible

InZnO/Ag/InZnO multilayer electrodes grown by continuous roll-to-roll sputtering, Sol Energ

Mater Sol Cells, 95 (2011) 3442-3449.

[2] J. Xue, S. Uchida, B.P. Rand, S.R. Forrest, Asymmetric tandem organic photovoltaic cells

with hybrid planar-mixed molecular heterojunctions, Appl Phys Lett, 85 (2004) 5757-5759.

[3] M. Bender, W. Seelig, C. Daube, H. Frankenberger, B. Ocker, J. Stollenwerk, Dependence of

oxygen flow on optical and electrical properties of DC-magnetron sputtered ITO films, Thin

Solid Films, 326 (1998) 72-77.

[4] M. Bender, N. Katsarakis, E. Gagaoudakis, E. Hourdakis, E. Douloufakis, V. Cimalla, G.

Kiriakidis, Dependence of the photoreduction and oxidation behavior of indium oxide films on

substrate temperature and film thickness, Journal of Applied Physics, 90 (2001) 5382-5387.

[5] F. Kurdesau, G. Khripunov, A. Da Cunha, M. Kaelin, A. Tiwari, Comparative study of ITO

layers deposited by DC and RF magnetron sputtering at room temperature, J Non Cryst Solids,

352 (2006) 1466-1470.

[6] T. Sugahara, Y. Hirose, S. Cong, H. Koga, J. Jiu, M. Nogi, S. Nagao, K. Suganuma, M.

Menon, Sol-Gel-Derived High-Performance Stacked Transparent Conductive Oxide Thin Films,

Journal of the American Ceramic Society, 97 (2014) 3238-3243.

[7] Y.-H. Kim, J.-S. Heo, T.-H. Kim, S. Park, M.-H. Yoon, J. Kim, M.S. Oh, G.-R. Yi, Y.-Y.

Noh, S.K. Park, Flexible metal-oxide devices made by room-temperature photochemical

activation of sol-gel films, Nature, 489 (2012) 128-132.

[8] P. Zhang, E. Jacques, R. Rogel, N. Coulon, O. Bonnaud, Quasi-vertical multi-tooth thin film

transistors based on low-temperature technology (T⩽ 600° C), Solid-State Electronics, 79 (2013)

26-30.

[9] M.G. Kim, M.G. Kanatzidis, A. Facchetti, T.J. Marks, Low-temperature fabrication of high-

performance metal oxide thin-film electronics via combustion processing, Nat Mater, 10 (2011)

382-388.

[10] H. Hagendorfer, K. Lienau, S. Nishiwaki, C.M. Fella, L. Kranz, A.R. Uhl, D. Jaeger, L.

Luo, C. Gretener, S. Buecheler, Highly transparent and conductive ZnO: Al thin films from a

low temperature aqueous solution approach, Advanced Materials, 26 (2014) 632-636.

[11] M. Duta, M. Anastasescu, J.M. Calderon-Moreno, L. Predoana, S. Preda, M. Nicolescu, H.

Stroescu, V. Bratan, I. Dascalu, E. Aperathitis, M. Modreanu, M. Zaharescu, M. Gartner, Sol–gel

versus sputtering indium tin oxide films as transparent conducting oxide materials, Journal of

Materials Science: Materials in Electronics, 27 (2016) 4913-4922.

[12] C.J. Brinker, A.J. Hurd, Fundamentals of sol-gel dip-coating, Journal de Physique III, 4

(1994) 1231-1242.

[13] M. Morales‐Masis, S. De Wolf, R. Woods‐Robinson, J.W. Ager, C. Ballif, Transparent

Electrodes for Efficient Optoelectronics, Advanced Electronic Materials, 3 (2017).

[14] K. Chopra, S. Major, D. Pandya, Transparent conductors—a status review, Thin solid films,

102 (1983) 1-46.

[15] J.L. Vossen, RF Sputtered Transparent Conductors System In2O3-SnO2, Rca Review, 32

(1971) 289.

218

[16] C. Guillén, J. Herrero, TCO/metal/TCO structures for energy and flexible electronics, Thin

Solid Films, 520 (2011) 1-17.

[17] B. Deng, P.-C. Hsu, G. Chen, B. Chandrashekar, L. Liao, Z. Ayitimuda, J. Wu, Y. Guo, L.

Lin, Y. Zhou, Roll-to-roll encapsulation of metal nanowires between graphene and plastic

substrate for high-performance flexible transparent electrodes, Nano Lett., 15 (2015) 4206-4213.

[18] H. Kim, S.-H. Hong, Y.C. Park, J. Lee, C.-H. Jeon, J. Kim, Rapid thermal-treated

transparent electrode for photodiode applications, Mater Lett, 115 (2014) 45-48.

[19] V. Brinzari, M. Ivanov, B.K. Cho, M. Kamei, G. Korotcenkov, Photoconductivity in In2O3

nanoscale thin films: Interrelation with chemisorbed-type conductometric response towards

oxygen, Sensors and Actuators B: Chemical, 148 (2010) 427-438.

[20] D.-Y. Cho, Y.-H. Shin, H.-K. Kim, Y.-J. Noh, S.-I. Na, K.-B. Chung, Roll-to-roll sputtered

Si-doped In2O3/Ag/Si-doped In2O3 multilayer as flexible and transparent anodes for flexible

organic solar cells, Journal of Vacuum Science & Technology A: Vacuum, Surfaces, and Films,

33 (2015) 021501.

[21] J.H. Choi, S.H. Kang, H.S. Oh, T.H. Yu, I.S. Sohn, Design and characterization of Ga-

doped indium tin oxide films for pixel electrode in liquid crystal display, Thin Solid Films, 527

(2013) 141-146.

[22] Y. Lin, X. Zhang, S. Bai, A. Hu, Photo-reduction of metallic ions doped in patterned

polymer films for the fabrication of plasmonic photonic crystals, J. Mater. Chem. C, 3 (2015)

6046-6052.

[23] R.B. Fair, Digital microfluidics: is a true lab-on-a-chip possible?, Microfluidics and

Nanofluidics, 3 (2007) 245-281.

[24] J. Cai, L. Qi, Recent advances in antireflective surfaces based on nanostructure arrays,

Materials Horizons, 2 (2015) 37-53.

[25] F. Wong, M. Fung, S. Tong, C. Lee, S. Lee, Flexible organic light-emitting device based on

magnetron sputtered indium-tin-oxide on plastic substrate, Thin Solid Films, 466 (2004) 225-

230.

[26] S.C. Dixon, D.O. Scanlon, C.J. Carmalt, I.P. Parkin, n-Type doped transparent conducting

binary oxides: an overview, J. Mater. Chem. C, 4 (2016) 6946-6961.

[27] T. Minami, H. Nanto, S. Takata, Highly conductive and transparent zinc oxide films

prepared by rf magnetron sputtering under an applied external magnetic field, Appl Phys Lett, 41

(1982) 958-960.

[28] P. Manoj, B. Joseph, V. Vaidyan, D.S.D. Amma, Preparation and characterization of

indium-doped tin oxide thin films, Ceramics international, 33 (2007) 273-278.

[29] P. Psuja, D. Hreniak, W. Strek, Synthesis and characterization of indium-tin oxide

nanostructures, in: Journal of Physics: Conference Series, IOP Publishing, 2009, pp. 012033.

[30] T.J. Coutts, T.O. Mason, J. Perkins, D.S. Ginley, Transparent conducting oxides: status and

opportunities in basic research, in: Photovoltaics for the 21st Century: Proceedings of the

International Symposium, The Electrochemical Society: Philadelphia, PA, USA, 1999, pp. 274-

286.

[31] S.K. Tripathy, B.P. Hota, P. Rajeswari, Study of optical characteristics of tin oxide thin film

prepared by sol–gel method, Bulletin of Materials Science, 36 (2013) 1231-1237.

[32] M. Reidinger, M. Rydzek, C. Scherdel, M. Arduini-Schuster, J. Manara, Low-emitting

transparent coatings based on tin doped indiumoxide applied via a sol–gel routine, Thin Solid

Films, 517 (2009) 3096-3099.

219

[33] M. Mekhnache, A. Drici, L.S. Hamideche, H. Benzarouk, A. Amara, L. Cattin, J. Bernede,

M. Guerioune, Properties of ZnO thin films deposited on (glass, ITO and ZnO: Al) substrates,

Superlattices Microstruct, 49 (2011) 510-518.

[34] P.D. King, T.D. Veal, Conductivity in transparent oxide semiconductors, J Phys Condens

Matter, 23 (2011) 334214.

[35] A. Stadler, Transparent conducting oxides—an up-to-date overview, Materials, 5 (2012)

661-683.

[36] Y. Shigesato, Y. Hayashi, T. Haranoh, Doping mechanisms of tin‐doped indium oxide

films, Appl Phys Lett, 61 (1992) 73-75.

[37] J. Tauc, Optical properties and electronic structure of amorphous Ge and Si, Materials

Research Bulletin, 3 (1968) 37-46.

[38] N. Yamamoto, H. Makino, S. Osone, A. Ujihara, T. Ito, H. Hokari, T. Maruyama, T.

Yamamoto, Development of Ga-doped ZnO transparent electrodes for liquid crystal display

panels, Thin Solid Films, 520 (2012) 4131-4138.

[39] T. Ratcheva, M. Nanova, L. Kinova, I. Penev, A study of the composition of In2O3 (Te)

films prepared by the spraying method, Thin solid films, 202 (1991) 243-247.

[40] H.K. Kim, C.C. Li, P.J. Barrios, Erbium‐doped indium oxide films prepared by radio

frequency sputtering, Journal of Vacuum Science & Technology A: Vacuum, Surfaces, and

Films, 12 (1994) 3152-3156.

[41] H.K. Kim, C.C. Li, G. Nykolak, P.C. Becker, Photoluminescence and electrical properties

of erbium‐doped indium oxide films prepared by rf sputtering, Journal of Applied Physics, 76

(1994) 8209-8211.

[42] G. Campet, S. Han, S. Wen, M. Shastry, B. Chaminade, E. Marquestaut, J. Portier, P.

Dordor, Correlations between the thermoelectric power and Hall effect of Sn or Ge doped In 2 O

3 semiconductors, Materials Science and Engineering: B, 22 (1994) 274-278.

[43] S. Wen, G. Campet, J. Portier, G. Couturier, J. Goodenough, Correlations between the

electronic properties of doped indium oxide ceramics and the nature of the doping element,

Materials Science and Engineering: B, 14 (1992) 115-119.

[44] T. Maruyama, T. Tago, Germanium‐and silicon‐doped indium‐oxide thin films prepared by

radio‐frequency magnetron sputtering, Appl Phys Lett, 64 (1994) 1395-1397.

[45] B. Mayer, Highly conductive and transparent films of tin and fluorine doped indium oxide

produced by APCVD, Thin Solid Films, 221 (1992) 166-182.

[46] T. Maruyama, T. Nakai, Fluorine‐doped indium oxide thin films prepared by chemical

vapor deposition, Journal of applied physics, 71 (1992) 2915-2917.

[47] S. Mirzapour, S. Rozati, M. Takwale, B. Marathe, V. Bhide, Influence of different process

parameters on physical properties of fluorine dopes indium oxide thin films, Journal of materials

science, 29 (1994) 700-707.

[48] R. Nomura, K. Konishi, H. Matsuda, Sulfur‐Doped Indium Oxide Thin Films as a New

Transparent and Conductive Layer Prepared by OMCVD Process Using Butylindium Thiolate,

Journal of the Electrochemical Society, 138 (1991) 631-632.

In2O3 thin films prepared by dc magnetron sputtering, Thin Solid Films, 290 (1996) 1-5.

[50] H. Ohta, W.S. Seo, K. Koumoto, Thermoelectric properties of homologous compounds in

the ZnO–In2O3 system, Journal of the American Ceramic Society, 79 (1996) 2193-2196.

220

[51] J.M. Phillips, R. Cava, G. Thomas, S. Carter, J. Kwo, T. Siegrist, J. Krajewski, J. Marshall,

W. Peck Jr, D. Rapkine, Zinc‐indium‐oxide: a high conductivity transparent conducting oxide,

Appl Phys Lett, 67 (1995) 2246-2248.

[52] M.M.-H. Jafan, M.-R. Zamani-Meymian, R. Rahimi, M. Rabbani, The effect of solvents and

the thickness on structural, optical and electrical properties of ITO thin films prepared by a sol–

gel spin-coating process, Journal of Nanostructure in Chemistry, 4 (2014) 89.

[53] C. Jin, I.-K. You, H.-K. Kim, Effect of rapid thermal annealing on the properties of spin-

coated In–Zn–Sn–O films, Curr. Appl. Phys., 13 (2013) S177-S181.

[54] L. Kerkache, A. Layadi, A. Mosser, Effect of oxygen partial pressure on the structural and

optical properties of dc sputtered ITO thin films, journal of Alloys and Compounds, 485 (2009)

46-50.

[55] S. Shanthi, C. Subramanian, P. Ramasamy, Growth and characterization of antimony doped

tin oxide thin films, J Cryst Growth, 197 (1999) 858-864.

[56] M. Mizuhashi, Electrical properties of vacuum-deposited indium oxide and indium tin oxide

films, Thin Solid Films, 70 (1980) 91-100.

[57] I. Rauf, J. Yuan, Effects of microstructure on the optical properties of tin-doped indium

oxide thin films studied by electron energy loss spectroscopy, Mater Lett, 25 (1995) 217-222.

[58] I. Rauf, Structure and properties of tin‐doped indium oxide thin films prepared by reactive

electron‐beam evaporation with a zone‐confining arrangement, Journal of applied physics, 79

(1996) 4057-4065.

[59] J.G. Na, Y.R. Cho, Y.H. Kim, T.D. Lee, S.J. Park, Effects of Annealing Temperature on

Microstructure and Electrical and Optical Properties of Radio‐Frequency‐Sputtered Tin‐Doped

Indium Oxide Films, Journal of the American Ceramic Society, 72 (1989) 698-701.

[60] C.B. Cao, L. Xiao, X.P. Song, Z.Q. Sun, Influence of annealing and Ag doping on structural

and optical properties of indium tin oxide thin films, J. Vac. Sci. Technol. A Vac. Surf. Films, 28

(2010) 48-53.

[61] B.C. Zhao, B. Xia, H.W. Ho, Z.C. Fan, L. Wang, Anomalous Hall effect in Cu and Fe

codoped In2O3 and ITO thin films, Physica B: Condensed Matter, 404 (2009) 2117-2121.

[62] J.W. Ok, D.J. Kwak, S.H. Kim, Y.M. Sung, Conductive and transparency characteristics of

titanium-doped indium-tin oxide (InSnO2:Ti) films deposited by radio frequency magnetron

sputtering, Vacuum, 110 (2014) 196-201.

[63] S. Mohammadi, H. Abdizadeh, M.R. Golobostanfard, Opto-electronic properties of

molybdenum doped indium tin oxide nanostructured thin films prepared via sol–gel spin coating,

Ceramics International, 39 (2013) 6953-6961.

[64] M. Mirzaee, A. Dolati, Effect of Cr doping on the structural, morphological, optical and

electrical properties of indium tin oxide films, Appl Phys A, 118 (2014) 953-960.

[65] S.M. Chung, J.H. Shin, W.-S. Cheong, C.-S. Hwang, K.I. Cho, Y.J. Kim, Characteristics of

Ti-doped ITO films grown by DC magnetron sputtering, Ceramics International, 38 (2012)

S617-S621.

[66] R. Sarhaddi, N. Shahtahmasebi, M. Rezaee Rokn-Abadi, M.M. Bagheri-Mohagheghi, Effect

of post-annealing temperature on nano-structure and energy band gap of indium tin oxide (ITO)

nano-particles synthesized by polymerizing–complexing sol–gel method, Physica E: Low-

dimensional Systems and Nanostructures, 43 (2010) 452-457.

[67] J. Lee, D.-G. Lim, W. Song, J. Yi, Influence of annealing temperature and atmosphere on

the properties of ITO films deposited using a powdery target, Journal of Korean Physical

Society, 51 (2007) 1143.

221

[68] R.N. Chauhan, R.S. Anand, J. Kumar, Structural, electrical and optical properties of radio

frequency sputtered indium tin oxide thin films modified by annealing in silicon oil and vacuum,

Thin Solid Films, 556 (2014) 253-259.

[69] D. Kalhor, R. Zahiri, S. Ketabi, A. Ebrahimzad, The effect of Ag intermediate layer on

crystalline, optical and electrical properties of nano-structured thin film, Indian Journal of

Physics, 84 (2010) 539-546.

[70] Z. Xu, P. Chen, Z. Wu, F. Xu, G. Yang, B. Liu, C. Tan, L. Zhang, R. Zhang, Y. Zheng,

Influence of thermal annealing on electrical and optical properties of indium tin oxide thin films,

Mater Sci Semicond Process, 26 (2014) 588-592.

[71] T.-H. Chen, T.-Y. Liao, Influence of annealing temperature on the characteristics of Ti-

codoped GZO thin solid film, Journal of Nanomaterials, 2013 (2013) 76.

[72] Y. Shigesato, S. Takaki, T. Haranoh, Electrical and structural properties of low resistivity

tin‐doped indium oxide films, Journal of Applied Physics, 71 (1992) 3356-3364.

[73] K. Ellmer, Magnetron sputtering of transparent conductive zinc oxide: relation between the

sputtering parameters and the electronic properties, Journal of Physics D: Applied Physics, 33

(2000) R17.

[74] M. Bender, J. Trube, J. Stollenwerk, Deposition of transparent and conducting indium-tin-

oxide films by the rf-superimposed DC sputtering technology, Thin Solid Films, 354 (1999) 100-

105.

[75] M. Lorenz, E. Kaidashev, H. Von Wenckstern, V. Riede, C. Bundesmann, D. Spemann, G.

Benndorf, H. Hochmuth, A. Rahm, H.-C. Semmelhack, Optical and electrical properties of

epitaxial (Mg, Cd) x Zn 1− x O, ZnO, and ZnO:(Ga, Al) thin films on c-plane sapphire grown by

pulsed laser deposition, Solid-State Electronics, 47 (2003) 2205-2209.

[76] M. Kamei, H. Enomoto, I. Yasui, Origin of the crystalline orientation dependence of the

electrical properties in tin-doped indium oxide films, Thin Solid Films, 392 (2001) 265-268.

[77] N. Wang, Y. Yang, J. Chen, N. Xu, G. Yang, One-Dimensional Zn-Doped In2O3− SnO2

Superlattice Nanostructures, The Journal of Physical Chemistry C, 114 (2010) 2909-2912.

[78] D.S. Bhachu, M.R. Waugh, K. Zeissler, W.R. Branford, I.P. Parkin, Textured Fluorine‐Doped Tin Dioxide Films formed by Chemical Vapour Deposition, Chemistry-A European

Journal, 17 (2011) 11613-11621.

[79] R. Mientus, K. Ellmer, Structural, electrical and optical properties of SnO2-x: F-layers

deposited by DC-reactive magnetron-sputtering from a metallic target in Ar–O2/CF4 mixtures,

Surface and Coatings Technology, 98 (1998) 1267-1271.

[80] M. White, O. Bierwagen, M. Tsai, J. Speck, Electron transport properties of antimony doped

Sn O 2 single crystalline thin films grown by plasma-assisted molecular beam epitaxy, Journal of

Applied Physics, 106 (2009) 093704.

[81] L. Cao, L. Zhu, J. Jiang, R. Zhao, Z. Ye, B. Zhao, Highly transparent and conducting

fluorine-doped ZnO thin films prepared by pulsed laser deposition, Sol Energ Mater Sol Cells,

95 (2011) 894-898.

[82] J. Hu, R.G. Gordon, Textured fluorine-doped ZnO films by atmospheric pressure chemical

vapor deposition and their use in amorphous silicon solar cells, Solar cells, 30 (1991) 437-450.

[83] V. Assunção, E. Fortunato, A. Marques, A. Gonçalves, I. Ferreira, H. Aguas, R. Martins,

New challenges on gallium-doped zinc oxide films prepared by rf magnetron sputtering, Thin

Solid Films, 442 (2003) 102-106.

222

[84] R. Ajimsha, A.K. Das, P. Misra, M. Joshi, L. Kukreja, R. Kumar, T. Sharma, S. Oak,

Observation of low resistivity and high mobility in Ga doped ZnO thin films grown by buffer

assisted pulsed laser deposition, Journal of Alloys and Compounds, 638 (2015) 55-58.

[85] H. Agura, A. Suzuki, T. Matsushita, T. Aoki, M. Okuda, Low resistivity transparent

conducting Al-doped ZnO films prepared by pulsed laser deposition, Thin solid films, 445

(2003) 263-267.

[86] T. Minami, H. Nanto, S. Takata, Highly conductive and transparent aluminum doped zinc

oxide thin films prepared by RF magnetron sputtering, Japanese Journal of Applied Physics, 23

(1984) L280.

[87] T. Shirahata, T. Kawaharamura, S. Fujita, H. Orita, Transparent conductive zinc-oxide-

based films grown at low temperature by mist chemical vapor deposition, Thin Solid Films, 597

(2015) 30-38.

[88] L.-H. Wong, Y.-S. Lai, Characterization of boron-doped ZnO thin films prepared by

magnetron sputtering with (100− x) ZnO–xB 2 O 3 ceramic targets, Thin Solid Films, 583

(2015) 205-211.

[89] H. Hosono, Exploring electro-active functionality of transparent oxide materials, Japanese

Journal of Applied Physics, 52 (2013) 090001.

[90] H. Hosono, M. Yasukawa, H. Kawazoe, Novel oxide amorphous semiconductors:

transparent conducting amorphous oxides, J Non Cryst Solids, 203 (1996) 334-344.

[91] J.E. Medvedeva, Combining Optical Transparency with Electrical Conductivity: Challenges

and Prospects, Transparent Electronics: From Synthesis to Applications, (2010) 29.

[92] A.M. Ganose, D.O. Scanlon, Band gap and work function tailoring of SnO 2 for improved

transparent conducting ability in photovoltaics, J. Mater. Chem. C, 4 (2016) 1467-1475.

[93] K. Hayashi, S. Matsuishi, T. Kamiya, M. Hirano, H. Hosono, Light-induced conversion of

an insulating refractory oxide into a persistent electronic conductor, Nature, 419 (2002) 462.

[94] S. Matsuishi, Y. Toda, M. Miyakawa, K. Hayashi, T. Kamiya, M. Hirano, I. Tanaka, H.

Hosono, High-density electron anions in a nanoporous single crystal:[Ca24Al28O64] 4+(4e-),

Science, 301 (2003) 626-629.

[95] A. Dawar, J. Joshi, Semiconducting transparent thin films: their properties and applications,

Journal of Materials Science, 19 (1984) 1-23.

[96] H. Kawazoe, K. Ueda, Transparent conducting oxides based on the spinel structure, Journal

of the American Ceramic Society, 82 (1999) 3330-3336.

[97] B. Ingram, G. Gonzalez, D. Kammler, M. Bertoni, T. Mason, Chemical and structural

factors governing transparent conductivity in oxides, Journal of Electroceramics, 13 (2004) 167-

175.

[98] A. Freeman, K. Poeppelmeier, T. Mason, R. Chang, T. Marks, Chemical and thin-film

strategies for new transparent conducting oxides, MRS bulletin, 25 (2000) 45-51.

[99] J.E. Medvedeva, C.L. Hettiarachchi, Tuning the properties of complex transparent

conducting oxides: Role of crystal symmetry, chemical composition, and carrier generation,

Physical Review B, 81 (2010) 125116.

[100] J.E. Medvedeva, E.N. Teasley, M.D. Hoffman, Electronic band structure and carrier

effective mass in calcium aluminates, Physical Review B, 76 (2007) 155107.

[101] H. Hiramatsu, H. Ohta, W.-S. Seo, K. Koumoto, Thermoelectric Properties of (ZnO)

5In2O3Thin Films Prepared by rf Sputtering Method, Journal of the Japan Society of Powder

and Powder Metallurgy, 44 (1997) 44-49.

223

[102] L. Zhang, Y. Zhou, L. Guo, W. Zhao, A. Barnes, H.-T. Zhang, C. Eaton, Y. Zheng, M.

Brahlek, H.F. Haneef, Correlated metals as transparent conductors, Nature materials, 15 (2016)

204-210.

[103] Y. Meng, X.-l. Yang, H.-x. Chen, J. Shen, Y.-m. Jiang, Z.-j. Zhang, Z.-y. Hua, A new

transparent conductive thin film In 2 O 3: Mo, Thin Solid Films, 394 (2001) 218-222.

[104] C.G. Granqvist, A. Hultåker, Transparent and conducting ITO films: new developments

and applications, Thin Solid Films, 411 (2002) 1-5.

[105] C.A. Hoel, T.O. Mason, J.-F. Gaillard, K.R. Poeppelmeier, Transparent conducting oxides

in the ZnO-In2O3-SnO2 system, Chem. Mater., 22 (2010) 3569-3579.

[106] Q. Wan, E.N. Dattoli, W. Lu, Transparent metallic Sb-doped Sn O 2 nanowires, Appl Phys

Lett, 90 (2007) 222107.

[107] T. Yang, J. Zhao, X. Li, X. Gao, C. Xue, Y. Wu, R. Tai, Preparation and characterization

of p-type transparent conducting SnO thin films, Mater Lett, 139 (2015) 39-41.

[108] P. Kadam, C. Agashe, S. Mahamuni, Al-doped ZnO nanocrystals, Journal of Applied

Physics, 104 (2008) 103501.

[109] O. Tari, A. Aronne, M.L. Addonizio, S. Daliento, E. Fanelli, P. Pernice, Sol–gel synthesis

of ZnO transparent and conductive films: A critical approach, Sol Energ Mater Sol Cells, 105

(2012) 179-186.

[110] D. Guo, K. Sato, S. Hibino, T. Takeuchi, H. Bessho, K. Kato, Low-temperature

preparation of transparent conductive Al-doped ZnO thin films by a novel sol–gel method,

Journal of materials science, 49 (2014) 4722-4734.

[111] M. Yamaga, H. Tsuzuki, S. Takano, E.G. Villora, K. Shimamura, Electron-spin resonance

of transparent conductive oxide β-Ga 2 O 3, J Non Cryst Solids, 358 (2012) 2458-2461.

[112] J. Zhang, C. Xia, Q. Deng, W. Xu, H. Shi, F. Wu, J. Xu, Growth and characterization of

new transparent conductive oxides single crystals β-Ga 2 O 3: Sn, Journal of Physics and

Chemistry of Solids, 67 (2006) 1656-1659.

[113] A. Dakhel, Transparent conducting properties of samarium-doped CdO, Journal of Alloys

and Compounds, 475 (2009) 51-54.

[114] R. Chandiramouli, B. Jeyaprakash, Review of CdO thin films, Solid State Sciences, 16

(2013) 102-110.

[115] T. Hitosugi, A. Ueda, S. Nakao, N. Yamada, Y. Furubayashi, Y. Hirose, T. Shimada, T.

Hasegawa, Fabrication of highly conductive Ti 1− x Nb x O 2 polycrystalline films on glass

substrates via crystallization of amorphous phase grown by pulsed laser deposition, Appl Phys

Lett, 90 (2007) 212106.

[116] N. Yamada, T. Shibata, K. Taira, Y. Hirose, S. Nakao, N.L.H. Hoang, T. Hitosugi, T.

Shimada, T. Sasaki, T. Hasegawa, Enhanced carrier transport in uniaxially (001)-oriented

anatase Ti0. 94Nb0. 06O2 films grown on nanosheet seed layers, Applied physics express, 4

(2011) 045801.

[117] K. Sieradzka, J. Domaradzki, E. Prociow, M. Mazur, M. Lapinski, Properties of

nanocrystalline TiO2: V thin films as a transparent semiconducting oxides, Acta Phys. Pol. A,

116 (2009) 33-35.

[118] W. Bin-Bin, P. Feng-Ming, Y. Yu-E, Annealing effect of pulsed laser deposited

transparent conductive Ta-doped titanium oxide films, Chin. Phys. Lett., 28 (2011) 118102.

[119] T. Ushiro, D. Tsuji, A. Fukushima, T. Moriga, I. Nakabayashi, K. Murayama, K.

Tominaga, Structural variation of thin films deposited from Zn 3 In 2 O 6 target by RF-

sputtering, Materials research bulletin, 36 (2001) 1075-1082.

224

[120] N. Nadaud, N. Lequeux, M. Nanot, J. Jove, T. Roisnel, Structural studies of tin-doped

indium oxide (ITO) and In4Sn3O12, Journal of Solid State Chemistry, 135 (1998) 140-148.

[121] G. Ma, R. Zou, L. Jiang, Z. Zhang, Y. Xue, L. Yu, G. Song, W. Li, J. Hu, Phase-controlled

synthesis and gas-sensing properties of zinc stannate (ZnSnO 3 and Zn 2 SnO 4) faceted solid

and hollow microcrystals, Crystengcomm, 14 (2012) 2172-2179.

[122] J. Ko, I. Kim, D. Kim, K. Lee, T. Lee, B. Cheong, W. Kim, Transparent and conducting

Zn-Sn-O thin films prepared by combinatorial approach, Applied surface science, 253 (2007)

7398-7403.

[123] J.M. Phillips, J. Kwo, G. Thomas, S. Carter, R. Cava, S. Hou, J. Krajewski, J. Marshall, W.

Peck, D. Rapkine, Transparent conducting thin films of GaInO3, Appl Phys Lett, 65 (1994) 115-

117.

[124] F. Koffyberg, F. Benko, Cd2SnO4, CdIn2O4, and Cd2GeO4 as anodes for the

photoelectrolysis of water, Appl Phys Lett, 37 (1980) 320-322.

[125] H. Mizoguchi, P.M. Woodward, Electronic structure studies of main group oxides

possessing edge-sharing octahedra: implications for the design of transparent conducting oxides,

Chem. Mater., 16 (2004) 5233-5248.

[126] T. Potlog, P. Dumitriu, M. Dobromir, A. Manole, D. Luca, Nb-doped TiO2 thin films for

photovoltaic applications, Materials & Design, 85 (2015) 558-563.

[127] Y. Ogo, H. Hiramatsu, K. Nomura, H. Yanagi, T. Kamiya, M. Hirano, H. Hosono, p-

channel thin-film transistor using p-type oxide semiconductor, SnO, Appl Phys Lett, 93 (2008)

032113.

[128] G. Hautier, A. Miglio, G. Ceder, G.M. Rignanese, X. Gonze, Identification and design

principles of low hole effective mass p-type transparent conducting oxides, Nat Commun, 4

(2013) 2292.

[129] R. Nagarajan, A. Draeseke, A. Sleight, J. Tate, p-type conductivity in CuCr 1− x Mg x O 2

films and powders, Journal of Applied Physics, 89 (2001) 8022-8025.

[130] W. Huang, R. Nechache, S. Li, M. Chaker, F. Rosei, Electrical and Optical Properties of

Transparent Conducting p‐Type SrTiO3 Thin Films, Journal of the American Ceramic Society,

99 (2016) 226-233.

[131] K.H. Zhang, Y. Du, A. Papadogianni, O. Bierwagen, S. Sallis, L.F. Piper, M.E. Bowden,

V. Shutthanandan, P.V. Sushko, S.A. Chambers, Perovskite Sr‐Doped LaCrO3 as a New p‐Type

Transparent Conducting Oxide, Advanced Materials, 27 (2015) 5191-5195.

[132] P.F. Ndione, Y. Shi, V. Stevanovic, S. Lany, A. Zakutayev, P.A. Parilla, J.D. Perkins, J.J.

Berry, D.S. Ginley, M.F. Toney, Control of the electrical properties in spinel oxides by

manipulating the cation disorder, Adv. Funct. Mater., 24 (2014) 610-618.

[133] H.-Y. Chen, H.-C. Su, C.-H. Chen, K.-L. Liu, C.-M. Tsai, S.-J. Yen, T.-R. Yew, Indium-

doped molybdenum oxide as a new p-type transparent conductive oxide, Journal of Materials

Chemistry, 21 (2011) 5745-5752.

[134] S. Narushima, H. Mizoguchi, K.-i. Shimizu, K. Ueda, H. Ohta, M. Hirano, T. Kamiya, H.

Hosono, A p‐Type Amorphous Oxide Semiconductor and Room Temperature Fabrication of

Amorphous Oxide p–n Heterojunction Diodes, Advanced Materials, 15 (2003) 1409-1413.

[135] C. Yang, M. Kneiβ, M. Lorenz, M. Grundmann, Room-temperature synthesized copper

iodide thin film as degenerate p-type transparent conductor with a boosted figure of merit,

Proceedings of the National Academy of Sciences, 113 (2016) 12929-12933.

[136] M. Grundmann, Karl Bädeker (1877–1914) and the discovery of transparent conductive

materials, physica status solidi (a), 212 (2015) 1409-1426.

225

[137] H. Yanagi, J. Tate, S. Park, C.-H. Park, D.A. Keszler, p-type conductivity in wide-band-

gap BaCuQF (Q= S, Se), Appl Phys Lett, 82 (2003) 2814-2816.

[138] M.-L. Liu, F.-Q. Huang, L.-D. Chen, Y.-M. Wang, Y.-H. Wang, G.-F. Li, Q. Zhang, p-

type transparent conductor: Zn-doped Cu Al S 2, Appl Phys Lett, 90 (2007) 072109.

[139] X. Xu, J. Bullock, L.T. Schelhas, E.Z. Stutz, J.J. Fonseca, M. Hettick, V.L. Pool, K.F. Tai,

M.F. Toney, X. Fang, Chemical bath deposition of p-type transparent, highly conducting (CuS)

x:(ZnS) 1–x nanocomposite thin films and fabrication of Si heterojunction solar cells, Nano

Lett., 16 (2016) 1925-1932.

[140] H. Peng, A. Zakutayev, S. Lany, T.R. Paudel, M. d'Avezac, P.F. Ndione, J.D. Perkins, D.S.

Ginley, A.R. Nagaraja, N.H. Perry, Li‐Doped Cr2MnO4: A New p‐Type Transparent

Conducting Oxide by Computational Materials Design, Adv. Funct. Mater., 23 (2013) 5267-

5276.

[141] J.B. Varley, A. Miglio, V.-A. Ha, M.J. van Setten, G.-M. Rignanese, G. Hautier, High-

Throughput Design of Non-oxide p-Type Transparent Conducting Materials: Data Mining,

Search Strategy, and Identification of Boron Phosphide, Chem. Mater., 29 (2017) 2568-2573.

[142] R. Weiher, R. Ley, Optical properties of indium oxide, Journal of Applied Physics, 37

(1966) 299-302.

[143] D.S. Ginley, J.D. Perkins, Transparent conductors, in: Handbook of transparent

conductors, Springer, 2011, pp. 1-25.

[144] J. De Wit, Electrical properties of In 2 O 3, Journal of solid state chemistry, 8 (1973) 142-

149.

[145] Z. Qiao, Fabrication and study of ito thin films prepared by magnetron sputtering, in,

Universität Duisburg-Essen, Fakultät für Physik, 2003.

[146] H. Odaka, Y. Shigesato, T. Murakami, S. Iwata, Electronic structure analyses of Sn-doped

In2O3, Japanese Journal of Applied Physics, 40 (2001) 3231.

[147] M. Batzill, U. Diebold, The surface and materials science of tin oxide, Progress in surface

science, 79 (2005) 47-154.

[148] J.C. Fan, J.B. Goodenough, X‐ray photoemission spectroscopy studies of Sn‐doped

indium‐oxide films, Journal of Applied Physics, 48 (1977) 3524-3531.

[149] D. Raoufi, A. Taherniya, The effect of substrate temperature on the microstructural,

electrical and optical properties of Sn-doped indium oxide thin films, The European Physical

Journal Applied Physics, 70 (2015) 30302.

[150] A.P. Amalathas, M.M. Alkaisi, Effects of film thickness and sputtering power on

properties of ITO thin films deposited by RF magnetron sputtering without oxygen, Journal of


[151] J. Kim, J.-K. Park, Y. Baik, W. Kim, J. Jeong, T.-Y. Seong, Comparative study on the

thickness-dependent properties of ITO and GZO thin films grown on glass and PET substrates,

Journal of the Korean Physical Society, 61 (2012) 1467-1470.

[152] A. Eshaghi, A. Graeli, Optical and electrical properties of indium tin oxide (ITO)

nanostructured thin films deposited on polycarbonate substrates “thickness effect”, Optik-

International Journal for Light and Electron Optics, 125 (2014) 1478-1481.

[153] D.S. Bhachu, D.O. Scanlon, G. Sankar, T.D. Veal, R.G. Egdell, G. Cibin, A.J. Dent, C.E.

Knapp, C.J. Carmalt, I.P. Parkin, Origin of high mobility in molybdenum-doped indium oxide,

Chem. Mater., 27 (2015) 2788-2796.

[154] S. Najwa, A. Shuhaimi, N. Ameera, K. Hakim, M. Sobri, M. Mazwan, M. Mamat, Y.

Yusnizam, V. Ganesh, M. Rusop, The effect of sputtering pressure on structural, optical and

226

electrical properties of indium tin oxide nanocolumns prepared by radio frequency (RF)

magnetron sputtering, Superlattices Microstruct, 72 (2014) 140-147.

[155] S.Q. Hussain, W.-K. Oh, S. Ahn, A.H.T. Le, S. Kim, S. Iftiquar, S. Velumani, Y. Lee, J.

Yi, Highly transparent RF magnetron-sputtered indium tin oxide films for a-Si: H/c-Si

heterojunction solar cells amorphous/crystalline silicon, Mater Sci Semicond Process, 24 (2014)

225-230.

[156] J.M. Gaskell, D.W. Sheel, Deposition of indium tin oxide by atmospheric pressure

chemical vapour deposition, Thin Solid Films, 520 (2012) 4110-4113.

[157] C. Viespe, I. Nicolae, C. Sima, C. Grigoriu, R. Medianu, ITO thin films deposited by

advanced pulsed laser deposition, Thin Solid Films, 515 (2007) 8771-8775.

[158] A. Amaral, P. Brogueira, C.N. de Carvalho, G. Lavareda, Early stage growth structure of

indium tin oxide thin films deposited by reactive thermal evaporation, Surface and Coatings

Technology, 125 (2000) 151-156.

[159] H.R. Fallah, M. Ghasemi, A. Hassanzadeh, H. Steki, The effect of annealing on structural,

electrical and optical properties of nanostructured ITO films prepared by e-beam evaporation,

Materials Research Bulletin, 42 (2007) 487-496.

[160] S. Rozati, T. Ganj, Transparent conductive Sn-doped indium oxide thin films deposited by

spray pyrolysis technique, Renewable Energy, 29 (2004) 1671-1676.

[161] S. Ng, F. Yam, K. Beh, S. Tneh, Z. Hassan, Improved conductivity of indium-tin-oxide

film through the introduction of intermediate layer, Superlattices Microstruct, 97 (2016) 202-

211.

[162] A.D. Acharya, B. Sarwan, R. Panda, S.B. Shrivastava, V. Ganesan, Tuning of TCO

properties of ZnO by silver addition, Superlattices Microstruct, 67 (2014) 97-109.

[163] J.H. Kim, H. Lee, J.-Y. Na, S.-K. Kim, Y.-Z. Yoo, T.-Y. Seong, Optimization of

transmittance and resistance of indium gallium zinc oxide/Ag/indium gallium zinc oxide

multilayer electrodes for photovoltaic devices, Curr. Appl. Phys., 15 (2015) 452-455.

[164] B.Y. Oh, G.S. Heo, Effects of the Ag layer embedded in NIZO layers as transparent

conducting electrodes for liquid crystal displays, Trans. Electr. Electron. Mater., 17 (2016) 33-

36.

[165] C.-H. Cheng, J.-M. Ting, Transparent conducting GZO, Pt/GZO, and GZO/Pt/GZO thin


[166] X. Ding, J. Yan, T. Li, L. Zhang, Transparent conductive ITO/Cu/ITO films prepared on

flexible substrates at room temperature, Applied Surface Science, 258 (2012) 3082-3085.

[167] N. Meshram, C. Loka, K.R. Park, K.-S. Lee, Enhanced transmittance of ITO/Ag (Cr)/ITO

(IAI) multi-layered thin films by high temperature annealing, Mater Lett, 145 (2015) 120-124.

[168] H. Yang, S. Shin, J. Park, G. Ham, J. Oh, H. Jeon, Effect of Au interlayer thickness on the

structural, electrical, and optical properties of GZO/Au/GZO multilayers, Curr. Appl. Phys., 14

(2014) 1331-1334.

[169] M.-C. Choi, Y. Kim, C.-S. Ha, Polymers for flexible displays: From material selection to

device applications, Progress in Polymer Science, 33 (2008) 581-630.

[170] C.J. Brabec, Organic photovoltaics: technology and market, Sol Energ Mater Sol Cells, 83

(2004) 273-292.

[171] C. Brabec, U. Scherf, V. Dyakonov, Organic photovoltaics: materials, device physics, and

manufacturing technologies, John Wiley & Sons, 2011.

[172] S. Logothetidis, Flexible organic electronic devices: materials, process and applications,

Materials Science and Engineering: B, 152 (2008) 96-104.

227

[173] T.H. Kim, B.H. Choi, J.S. Park, S.M. Lee, Y.S. Lee, L.S. Park, Transparent conductive

ITO/Ag/ITO multilayer films prepared by low temperature process and physical properties, Mol.

Cryst. Liq. Cryst., 520 (2010) 209/[485]-214/[490].

[174] Y.R. Lee, E.M. Kim, J.P. Oh, B.Y. Oh, D.C. Shin, G.S. Heo, Transparent and conductive

titanium indium zinc oxide/Ag/titanium indium zinc oxide multilayer films deposited by radio

frequency magnetron co-sputtering, Thin Solid Films, 558 (2014) 31-36.

[175] T.H. Kim, C.H. Kim, S.K. Kim, Y.S. Lee, L.S. Park, High quality transparent conductive

ITO/Ag/ITO multilayer films deposited on glass substrate at room temperature, Mol. Cryst. Liq.

Cryst., 532 (2010) 112/[528]-118/[534].

[176] J.H. Kim, T.W. Kang, S.N. Kwon, S.I. Na, Y.Z. Yoo, H.S. Im, T.Y. Seong, Transparent

Conductive ITO/Ag/ITO Electrode Deposited at Room Temperature for Organic Solar Cells, J

Electron Mater, 46 (2017) 306-311.

[177] J. Yun, W. Wang, T.S. Bae, Y.H. Park, Y.-C. Kang, D.-H. Kim, S. Lee, G.-H. Lee, M.

Song, J.-W. Kang, Preparation of flexible organic solar cells with highly conductive and

transparent metal-oxide multilayer electrodes based on silver oxide, ACS applied materials &

interfaces, 5 (2013) 9933-9941.

[178] H.J. Kim, K.W. Seo, Y.H. Kim, J. Choi, H.K. Kim, Direct laser patterning of transparent

ITO-Ag-ITO multilayer anodes for organic solar cells, Applied Surface Science, 328 (2015) 215-

221.

[179] M.G. Varnamkhasti, H.R. Fallah, M. Mostajaboddavati, A. Hassanzadeh, Influence of Ag

thickness on electrical, optical and structural properties of nanocrystalline MoO 3/Ag/ITO

multilayer for optoelectronic applications, Vacuum, 86 (2012) 1318-1322.

[180] M.G. Varnamkhasti, E. Shahriari, Design and fabrication of nanometric TiO 2/Ag/TiO

2/Ag/TiO 2 transparent conductive electrode for inverted organic photovoltaic cells application,

Superlattices Microstruct, 69 (2014) 231-238.

[181] N. Pramod, S. Pandey, P. Sahay, Structural, optical and methanol sensing properties of

sprayed In 2 O 3 nanoparticle thin films, Ceramics International, 38 (2012) 4151-4158.

[182] J. Lee, M.A. Petruska, S. Sun, Surface Modification and Assembly of Transparent Indium

Tin Oxide Nanocrystals for Enhanced Conductivity, The Journal of Physical Chemistry C, 118

(2014) 12017-12021.

[183] H.D. Li, C.S. Hsu, F.M. Zhan, Y.C. Chao, Efficiency enhancement in polymer light-

emitting diodes via embedded indium-tin-oxide nanorods, ACS Appl. Mater. Interfaces, 7 (2015)

7462-7465.

[184] J. Gao, R. Chen, D. Li, L. Jiang, J. Ye, X. Ma, X. Chen, Q. Xiong, H. Sun, T. Wu, UV

light emitting transparent conducting tin-doped indium oxide (ITO) nanowires, Nanotechnology,

22 (2011) 195706.

[185] E.J. van den Ham, K. Elen, G. Bonneux, G. Maino, P.H.L. Notten, M.K. Van Bael, A.

Hardy, 3D indium tin oxide electrodes by ultrasonic spray deposition for current collection

applications, Journal of Power Sources, 348 (2017) 130-137.

[186] M.T. Kesim, C. Durucan, Indium tin oxide thin films elaborated by sol-gel routes: The

effect of oxalic acid addition on optoelectronic properties, Thin Solid Films, 545 (2013) 56-63.

[187] M. Misra, D.K. Hwang, Y.C. Kim, J.M. Myoung, T.I. Lee, Eco-friendly method of

fabricating indium-tin-oxide thin films using pure aqueous sol-gel, Ceramics International, 44

(2018) 2927-2933.

228

[188] Y. Ren, X. Zhou, Q. Wang, G. Zhao, Novel preparation of ITO nanocrystalline films with

plasmon electrochromic properties by the sol-gel method using benzoylacetone as a chemical

modifier, Ceramics International, 44 (2018) 3394-3399.

[189] O. Tuna, Y. Selamet, G. Aygun, L. Ozyuzer, High quality ITO thin films grown by dc and

RF sputtering without oxygen, Journal of Physics D: Applied Physics, 43 (2010) 055402.

[190] R. Balasundaraprabhu, E. Monakhov, N. Muthukumarasamy, O. Nilsen, B. Svensson,

Effect of heat treatment on ITO film properties and ITO/p-Si interface, Materials Chemistry and

Physics, 114 (2009) 425-429.

[191] A. Pokaipisit, M. Horprathum, P. Limsuwan, Vacuum and air annealing effects on

properties of Indium Tin Oxide films prepared by ion-assisted electron beam evaporation,

Japanese Journal of Applied Physics, 47 (2008) 4692.

[192] Z. Chen, W. Li, R. Li, Y. Zhang, G. Xu, H. Cheng, Fabrication of highly transparent and

conductive indium-tin oxide thin films with a high figure of merit via solution processing,

Langmuir, 29 (2013) 13836-13842.

[193] L. Kőrösi, S. Papp, S. Beke, B. Pécz, R. Horváth, P. Petrik, E. Agócs, I. Dékány, Highly

transparent ITO thin films on photosensitive glass: sol–gel synthesis, structure, morphology and

optical properties, Applied Physics A, 107 (2012) 385-392.

[194] J.A. Jeong, H.K. Kim, H.W. Koo, T.W. Kim, Transmission electron microscopy study of

degradation in transparent indium tin oxide/Ag/indium tin oxide multilayer films, Appl Phys

Lett, 103 (2013).

[195] H. Cho, Y.-H. Yun, Characterization of indium tin oxide (ITO) thin films prepared by a

sol–gel spin coating process, Ceramics International, 37 (2011) 615-619.

[196] M. Mirzaee, A. Dolati, Effects of tin valence on microstructure, optical, and electrical

properties of ITO thin films prepared by sol–gel method, J Sol Gel Sci Technol, 75 (2015) 582-

592.

[197] B. Pujilaksono, U. Klement, L. Nyborg, U. Jelvestam, S. Hill, D. Burgard, X-ray

photoelectron spectroscopy studies of indium tin oxide nanocrystalline powder, Materials

Characterization, 54 (2005) 1-7.

[198] M. Yamaguchi, A. Ide-Ektessabi, H. Nomura, N. Yasui, Characteristics of indium tin

oxide thin films prepared using electron beam evaporation, Thin solid films, 447 (2004) 115-118.

[199] I.J. Panneerdoss, S.J. Jeyakumar, S. Ramalingam, M. Jothibas, Characterization of

prepared In2O3 thin films: The FT-IR, FT-Raman, UV–Visible investigation and optical

analysis, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, 147 (2015) 1-

13.

[200] A.K. Gupta, D. Porwal, A. Dey, N. Sridhara, A.K. Mukhopadhyay, A.K. Sharma, H.C.

Barshilia, Evaluation of elasto-plastic properties of ITO film using combined nanoindentation

and finite element approach, Ceramics International, 42 (2016) 1225-1233.

[201] S.J. Kim, K. Choi, B. Lee, Y. Kim, B.H. Hong, Materials for flexible, stretchable

electronics: graphene and 2D materials, Annual Review of Materials Research, 45 (2015) 63-84.

[202] J.K. Wassei, R.B. Kaner, Graphene, a promising transparent conductor, Materials today,

13 (2010) 52-59.

[203] A.K. Geim, Graphene: status and prospects, Science, 324 (2009) 1530-1534.

[204] K.S. Novoselov, V. Fal, L. Colombo, P. Gellert, M. Schwab, K. Kim, A roadmap for

graphene, nature, 490 (2012) 192-200.

[205] F. Bonaccorso, Z. Sun, T. Hasan, A. Ferrari, Graphene photonics and optoelectronics,

Nature photonics, 4 (2010) 611-622.

229

[206] A. Kuruvila, P.R. Kidambi, J. Kling, J.B. Wagner, J. Robertson, S. Hofmann, J. Meyer,

Organic light emitting diodes with environmentally and thermally stable doped graphene

electrodes, J. Mater. Chem. C, 2 (2014) 6940-6945.

[207] W. Liu, J. Kang, K. Banerjee, Characterization of FeCl 3 intercalation doped CVD few-

layer graphene, IEEE Electron Device Letters, 37 (2016) 1246-1249.

[208] L. Gomez De Arco, Y. Zhang, C.W. Schlenker, K. Ryu, M.E. Thompson, C. Zhou,

Continuous, highly flexible, and transparent graphene films by chemical vapor deposition for

organic photovoltaics, ACS Nano, 4 (2010) 2865-2873.

[209] W.S. Koh, C.H. Gan, W.K. Phua, Y.A. Akimov, P. Bai, The potential of graphene as an

ITO replacement in organic solar cells: An optical perspective, IEEE Journal of Selected Topics

in Quantum Electronics, 20 (2014) 36-42.

[210] N. Saran, K. Parikh, D.-S. Suh, E. Muñoz, H. Kolla, S.K. Manohar, Fabrication and

characterization of thin films of single-walled carbon nanotube bundles on flexible plastic

substrates, Journal of the American Chemical Society, 126 (2004) 4462-4463.

[211] L. Liu, S. Yellinek, N. Tal, R. Toledano, A. Donval, D. Yadlovker, D. Mandler,

Electrochemical co-deposition of sol-gel/carbon nanotube composite thin films for antireflection

and non-linear optics, J. Mater. Chem. C, 3 (2015) 1099-1105.

[212] L. Mo, J. Ran, L. Yang, Y. Fang, Q. Zhai, L. Li, Flexible transparent conductive films

combining flexographic printed silver grids with CNT coating, Nanotechnology, 27 (2016).

[213] Y.H. Hwang, M. Han, J. Lim, G.S. Lee, D. Jung, Spin-capable carbon nanotube sheet as a

substitute for TCO in transparent electronics and displays, Fullerenes Nanotubes Carbon

Nanostruct., 24 (2016) 305-312.

[214] D. Jung, K.H. Lee, D. Kim, D. Burk, L.J. Overzet, G.S. Lee, Highly conductive flexible

multi-walled carbon nanotube sheet films for transparent touch screen, Japanese Journal of

Applied Physics, 52 (2013) 03BC03.

[215] K. Wang, M. Li, Y.-N. Liu, Y. Gu, Q. Li, Z. Zhang, Effect of acidification conditions on

the properties of carbon nanotube fibers, Applied Surface Science, 292 (2014) 469-474.

[216] H.-Z. Geng, K.K. Kim, K.P. So, Y.S. Lee, Y. Chang, Y.H. Lee, Effect of acid treatment on

carbon nanotube-based flexible transparent conducting films, Journal of the American Chemical

Society, 129 (2007) 7758-7759.

[217] H.J. Lee, J.H. Hwang, K.B. Choi, S.-G. Jung, K.N. Kim, Y.S. Shim, C.H. Park, Y.W. Park,

B.-K. Ju, Effective indium-doped zinc oxide buffer layer on silver nanowires for electrically

highly stable, flexible, transparent, and conductive composite electrodes, ACS applied materials

& interfaces, 5 (2013) 10397-10403.

[218] J.T. Hu, W.J. Mei, K.L. Ye, Q.Q. Wei, S. Hu, Transparent conductive PVP/AgNWs films

for flexible organic light emitting diodes by spraying method, Optoelectron. Lett., 12 (2016)

203-207.

[219] M. Vosgueritchian, D.J. Lipomi, Z. Bao, Highly conductive and transparent PEDOT: PSS

films with a fluorosurfactant for stretchable and flexible transparent electrodes, Adv. Funct.

Mater., 22 (2012) 421-428.

[220] Q. Xu, T. Song, W. Cui, Y. Liu, W. Xu, S.-T. Lee, B. Sun, Solution-processed highly

conductive PEDOT: PSS/AgNW/GO transparent film for efficient organic-Si hybrid solar cells,

ACS applied materials & interfaces, 7 (2015) 3272-3279.

[221] D. Alemu, H.-Y. Wei, K.-C. Ho, C.-W. Chu, Highly conductive PEDOT: PSS electrode by

simple film treatment with methanol for ITO-free polymer solar cells, Energy & environmental

science, 5 (2012) 9662-9671.

230

[222] T. Ji, L. Tan, X. Hu, Y. Dai, Y. Chen, A comprehensive study of sulfonated carbon

materials as conductive composites for polymer solar cells, Physical Chemistry Chemical

Physics, 17 (2015) 4137-4145.

[223] Q. Zhou, S. Chen, M. Zhang, L. Wang, Y. Li, G. Shi, Solution‐Processed Graphene

Composite Films as Freestanding Platinum‐Free Counter Electrodes for Bendable Dye Sensitized

Solar Cells, Chinese Journal of Chemistry, 34 (2016) 59-66.

[224] I. Cruz-Cruz, M. Reyes-Reyes, M.A. Aguilar-Frutis, A. Rodriguez, R. López-Sandoval,

Study of the effect of DMSO concentration on the thickness of the PSS insulating barrier in

PEDOT: PSS thin films, Synth Met, 160 (2010) 1501-1506.

[225] M. Zhang, W. Yuan, B. Yao, C. Li, G. Shi, Solution-processed PEDOT: PSS/graphene

composites as the electrocatalyst for oxygen reduction reaction, ACS applied materials &

interfaces, 6 (2014) 3587-3593.

[226] Z. Zhao, Q. Wu, F. Xia, X. Chen, Y. Liu, W. Zhang, J. Zhu, S. Dai, S. Yang, Improving

the conductivity of PEDOT: PSS hole transport layer in polymer solar cells via copper (II)

bromide salt doping, ACS applied materials & interfaces, 7 (2015) 1439-1448.

[227] G. Gu, V. Bulović, P. Burrows, S. Forrest, M. Thompson, Transparent organic light

emitting devices, Appl Phys Lett, 68 (1996) 2606-2608.

[228] X. Zhou, M. Pfeiffer, J. Huang, J. Blochwitz-Nimoth, D. Qin, A. Werner, J. Drechsel, B.

Maennig, K. Leo, Low-voltage inverted transparent vacuum deposited organic light-emitting

diodes using electrical doping, Appl Phys Lett, 81 (2002) 922-924.

[229] H.-S. Seo, Transparent display apparatus, in, Google Patents, 2012.

[230] H. Li, L.K. Schirra, J. Shim, H. Cheun, B. Kippelen, O.L. Monti, J.-L. Bredas, Zinc oxide

as a model transparent conducting oxide: A theoretical and experimental study of the impact of

hydroxylation, vacancies, interstitials, and extrinsic doping on the electronic properties of the

polar ZnO (0002) surface, Chem. Mater., 24 (2012) 3044-3055.

[231] V. Bhosle, J. Prater, F. Yang, D. Burk, S. Forrest, J. Narayan, Gallium-doped zinc oxide

films as transparent electrodes for organic solar cell applications, Journal of Applied Physics,

102 (2007) 023501.

[232] M. Morales‐Masis, F. Dauzou, Q. Jeangros, A. Dabirian, H. Lifka, R. Gierth, M. Ruske, D.

Moet, A. Hessler‐Wyser, C. Ballif, An Indium‐Free Anode for Large‐Area Flexible OLEDs:

Defect‐Free Transparent Conductive Zinc Tin Oxide, Adv. Funct. Mater., 26 (2016) 384-392.

[233] L.-S. Hung, J. Madathil, Radiation damage and transmission enhancement in surface-

emitting organic light-emitting diodes, Thin Solid Films, 410 (2002) 101-106.

[234] J. Ma, X. Hao, S. Huang, J. Huang, Y. Yang, H. Ma, Comparison of the electrical and

optical properties for SnO 2: Sb films deposited on polyimide and glass substrates, Applied

Surface Science, 214 (2003) 208-213.

[235] K. Hayashi, S. Matsuishi, T. Kamiya, M. Hirano, H. Hosono, Light-induced conversion of

an insulating refractory oxide into a persistent electronic conductor, Nature, 419 (2002) 462-465.

[236] R.F. Pierret, Semiconductor Device Fundamentals, ISBN: 0-201-54393-1, 1996, by

Addison, in, Wesley Publishing Company, Inc. PNPN Devices.

[237] S.M. Sze, K.K. Ng, Physics of semiconductor devices, John wiley & sons, 2006.

[238] D. Neamen, Semiconductor physics and devices, McGraw-Hill, Inc., 2002.

[239] R. Enderlein, N.J. Horing, Fundamentals of semiconductor physics and devices, World

Scientific, 1997.

[240] P.Y. Yu, M. Cardona, Fundamentals of semiconductors: physics and materials properties,

Springer, 2010.

231

[241] S.M. Sze, Semiconductor devices: physics and technology, John Wiley & Sons, 2008.

[242] G.J. Exarhos, X.-D. Zhou, Discovery-based design of transparent conducting oxide films,

Thin solid films, 515 (2007) 7025-7052.

[243] R. Latz, K. Michael, M. Scherer, High conducting large area indium tin oxide electrodes

for displays prepared by dc magnetron sputtering, Japanese Journal of Applied Physics, 30

(1991) L149-L151.

[244] N.-W. Pu, W.-S. Liu, H.-M. Cheng, H.-C. Hu, W.-T. Hsieh, H.-W. Yu, S.-C. Liang,

Investigation of the Optoelectronic Properties of Ti-doped Indium Tin Oxide Thin Film,

Materials, 8 (2015) 6471-6481.

[245] R.B.H. Tahar, T. Ban, Y. Ohya, Y. Takahashi, Tin doped indium oxide thin films:

Electrical properties, Journal of Applied Physics, 83 (1998) 2631-2645.

[246] H. Kim, C. Gilmore, A. Pique, J. Horwitz, H. Mattoussi, H. Murata, Z. Kafafi, D. Chrisey,

Electrical, optical, and structural properties of indium–tin–oxide thin films for organic light-

emitting devices, Journal of Applied Physics, 86 (1999) 6451-6461.

[247] Y. Wu, C. Maree, R. Haglund Jr, J. Hamilton, M.M. Paliza, M. Huang, L. Feldman, R.

Weller, Resistivity and oxygen content of indium tin oxide films deposited at room temperature

by pulsed-laser ablation, Journal of applied physics, 86 (1999) 991-994.

[248] R.B.H. Tahar, T. Ban, Y. Ohya, Y. Takahashi, Optical, structural, and electrical properties

of indium oxide thin films prepared by the sol-gel method, Journal of applied physics, 82 (1997)

865-870.

[249] G. Frank, H. Köstlin, Electrical properties and defect model of tin-doped indium oxide

layers, Applied Physics A, 27 (1982) 197-206.

[250] H. Köstlin, R. Jost, W. Lems, Optical and electrical properties of doped In2O3 films,

physica status solidi (a), 29 (1975) 87-93.

[251] R. Weiher, Electrical properties of single crystals of indium oxide, Journal of Applied

Physics, 33 (1962) 2834-2839.

[252] R. Groth, Untersuchungen an halbleitenden Indiumoxydschichten, physica status solidi (b),

14 (1966) 69-75.

[253] C.H.L. Weijtens, Indium tin oxide for solid-state image sensors, (1990).

[254] A. Kulkarni, S. Knickerbocker, Estimation and verification of the electrical properties of

indium tin oxide based on the energy band diagram, Journal of Vacuum Science & Technology

A: Vacuum, Surfaces, and Films, 14 (1996) 1709-1713.

[255] K. Ellmer, R. Mientus, Carrier transport in polycrystalline transparent conductive oxides:

A comparative study of zinc oxide and indium oxide, Thin solid films, 516 (2008) 4620-4627.

[256] J. Manifacier, M. De Murcia, J. Fillard, E. Vicario, Optical and electrical properties of SnO

2 thin films in relation to their stoichiometric deviation and their crystalline structure, Thin Solid

Films, 41 (1977) 127-135.

[257] J.C. Manifacier, L. Szepessy, J.F. Bresse, M. Perotin, R. Stuck, In2O3 : (Sn) and SnO2 :

(F) films - application to solar energy conversion part II - Electrical and optical properties,

Materials Research Bulletin, 14 (1979) 163-175.

[258] L.-j. Meng, M. Dos Santos, Properties of indium tin oxide films prepared by rf reactive

magnetron sputtering at different substrate temperature, Thin Solid Films, 322 (1998) 56-62.

[259] H. Ohta, M. Orita, M. Hirano, H. Tanji, H. Kawazoe, H. Hosono, Highly electrically

conductive indium–tin–oxide thin films epitaxially grown on yttria-stabilized zirconia (100) by

pulsed-laser deposition, Appl Phys Lett, 76 (2000) 2740-2742.

232

[260] U. Betz, M.K. Olsson, J. Marthy, F. Atamny, Thin films engineering of indium tin oxide:

large area flat panel displays application, Surface and Coatings Technology, 200 (2006) 5751-

5759.

[261] M. Rydzek, M. Reidinger, M. Arduini-Schuster, J. Manara, Comparative study of sol-gel

derived tin-doped indium- and aluminum-doped zinc-oxide coatings for electrical conducting

and low-emitting surfaces, Prog Org Coatings, 70 (2011) 369-375.

[262] F. Fang, Y. Zhang, X. Wu, Q. Shao, Z. Xie, Electrical and optical properties of nitrogen

doped SnO2 thin films deposited on flexible substrates by magnetron sputtering, Materials

Research Bulletin, 68 (2015) 240-244.

[263] S. Mohri, Y. Hirose, S. Nakao, N. Yamada, T. Shimada, T. Hasegawa, Transparent

conductivity of fluorine-doped anatase TiO2 epitaxial thin films, Journal of Applied Physics, 111

(2012) 093528.

[264] W. Knoll, R.C. Advincula, Functional Polymer Films, 2 Volume Set, John Wiley & Sons,

2013.

[265] J. Gao, K. Kempa, M. Giersig, E.M. Akinoglu, B. Han, R. Li, Physics of transparent

conductors, Adv Phys, 65 (2016) 553-617.

[266] C. Brinker, A. Hurd, G. Frye, P. Schunk, C. Ashley, Sol-gel thin film formation, Journal of

the Ceramic Society of Japan, 99 (1991) 862-877.

[267] D.P. Partlow, T.W. O’Keeffe, Thirty-seven layer optical filter from polymerized solgel

solutions, Applied optics, 29 (1990) 1526-1529.

[268] J.-J. Priotton, I.M. Thomas, Optical coatings prepared from colloidal media, Thin Solid

Films, 175 (1989) 173-178.

[269] J.D. Mackenzie, D.R. Ulrich, Ultrastructure processing of advanced ceramics, Wiley New

York, 1988.

[270] T. Bein, K. Brown, G.C. Frye, C.J. Brinker, Molecular sieve sensors for selective detection

at the nanogram level, Journal of the American Chemical Society, 111 (1989) 7640-7641.

[271] P. Baudry, A. Rodrigues, M.A. Aegerter, L. Bulhoes, Dip-

transparent counter-electrode for transmissive electrochromic devices, J Non Cryst Solids, 121

(1990) 319-322.

[272] J. Livage, J. Lemerle, Transition metal oxide gels and colloids, Annual Review of

Materials Science, 12 (1982) 103-122.

[273] B. Fabes, G. Berry, Infiltration of glass flaws by alkoxide coatings, J Non Cryst Solids,

121 (1990) 357-364.

[274] B. Fabes, W. Doyle, B.J. Zelinski, L. Silverman, D. Uhlmann, Strengthening of silica glass

by gel-derived coatings, J Non Cryst Solids, 82 (1986) 349-355.

[275] S. Kramer, G. Kordas, J. McMillan, G. Hilton, D. Van Harligen, Highly oriented

superconducting thin films derived from the sol‐gel process, Appl Phys Lett, 53 (1988) 156-158.

[276] W. Warren, P. Lenahan, C. Brinker, G. Shaffer, C. Ashley, S. Reed, Better Ceramics

Through Chemistry IV, in: Eds. BJJ Zelinski, CJ Brinker, DE Clark, and DR Ulrich, Mat. Res.

Soc. Symp. Proc, 1990, pp. 413-419.

[277] K.D. Budd, S. Dey, D. Payne, SOL-GEL PROCESSING OF PbTiO//3, PbZrO//3, PZT,

AND PLZT THIN FILMS, in: British Ceramic Proceedings, Inst of Ceramics, 1985.

[278] J. Fukushima, K. Kodaira, T. Matsushita, Preparation of ferroelectric PZT films by thermal

decomposition of organometallic compounds, Journal of Materials Science, 19 (1984) 595-598.

[279] G. Giusti, Deposition and characterisation of functional ITO thin films, in, University of

Birmingham, 2011.

233

[280] R.C. Mehrotra, A. Singh, Recent trends in metal alkoxide chemistry, Progress in Inorganic

Chemistry, 46 (1997) 239-454.

[281] M. Niederberger, Nonaqueous sol–gel routes to metal oxide nanoparticles, Accounts of

chemical research, 40 (2007) 793-800.

[282] M. Niederberger, G. Garnweitner, Organic reaction pathways in the nonaqueous synthesis

of metal oxide nanoparticles, Chemistry–A European Journal, 12 (2006) 7282-7302.

[283] N. Pinna, M. Niederberger, Surfactant‐free nonaqueous synthesis of metal oxide

nanostructures, Angewandte Chemie International Edition, 47 (2008) 5292-5304.

[284] H. Boysen, M. Lerch, A. Stys, A. Senyshyn, Structure and oxygen mobility in mayenite

(Ca12Al14O33): a high-temperature neutron powder diffraction study, Acta Crystallographica

Section B: Structural Science, 63 (2007) 675-682.

[285] A. Larson, R. Von Dreele, General Structure Analysis System 86-748 Los Alamos

National Laboratory: Los Alamos, NM Toby BH 2001 J, Appl. Crystallogr, 34 (2000) 210.

[286] L. Znaidi, Sol–gel-deposited ZnO thin films: a review, Materials Science and Engineering:

B, 174 (2010) 18-30.

[287] C.J. Brinker, G. Frye, A. Hurd, C. Ashley, Fundamentals of sol-gel dip coating, Thin solid

films, 201 (1991) 97-108.

[288] C.J. Brinker, G.W. Scherer, Sol-gel science: the physics and chemistry of sol-gel

processing, Academic press, 2013.

[289] J. Livage, M. Henry, C. Sanchez, Sol-gel chemistry of transition metal oxides, Progress in

solid state chemistry, 18 (1988) 259-341.

[290] T.K. Boström, E. Wäckelgård, G. Westin, Anti-reflection coatings for solution-chemically

derived nickel—alumina solar absorbers, Sol Energ Mater Sol Cells, 84 (2004) 183-191.

[291] J. Vince, A.Š. Vuk, U.O. Krašovec, B. Orel, M. Köhl, M. Heck, Solar absorber coatings

based on CoCuMnO x spinels prepared via the sol–gel process: structural and optical properties,

Sol Energ Mater Sol Cells, 79 (2003) 313-330.

[292] A.G. Emslie, F.T. Bonner, L.G. Peck, Flow of a viscous liquid on a rotating disk, Journal

of Applied Physics, 29 (1958) 858-862.

[293] D. Meyerhofer, Characteristics of resist films produced by spinning, Journal of Applied

Physics, 49 (1978) 3993-3997.

[294] S. Middleman, An introduction to fluid dynamics: principles of analysis and design, John

Wiley & Sons Incorporated, 1998.

[295] D. Bornside, C. Macosko, L.-E. Scriven, On the modeling of spin coating, Journal of

imaging technology, 13 (1987) 122-130.

[296] N. Sahu, B. Parija, S. Panigrahi, Fundamental understanding and modeling of spin coating

process: A review, Indian Journal of Physics, 83 (2009) 493-502.

[297] G.A. Luurtsema, Spin coating for rectangular substrates, in, UNIVERSITY OF

CALIFORNIA, BERKELEY, 1997.

[298] F. Givens, W. Daughton, On the uniformity of thin films: a new technique applied to

polyimides, Journal of The Electrochemical Society, 126 (1979) 269-272.

[299] W. Daughton, F. Givens, An Investigation of the Thickness Variation of Spun‐on Thin

Films Commonly Associated with the Semiconductor Industry, Journal of The Electrochemical

Society, 129 (1982) 173-179.

[300] D. Bornside, C. Macosko, L. Scriven, Spin coating: One‐dimensional model, Journal of

Applied Physics, 66 (1989) 5185-5193.

234

[301] B. Chen, Investigation of the solvent‐evaporation effect on spin coating of thin films,

Polymer Engineering & Science, 23 (1983) 399-403.

[302] R. Yonkoski, D. Soane, Model for spin coating in microelectronic applications, Journal of

applied physics, 72 (1992) 725-740.

[303] L. Peurrung, D. Graves, Film thickness profiles over topography in spin coating, Journal of

the electrochemical society, 138 (1991) 2115-2124.

[304] L.M. Peurrung, D.B. Graves, Spin coating over topography, IEEE transactions on

semiconductor manufacturing, 6 (1993) 72-76.

[305] C. Lawrence, The mechanics of spin coating of polymer films, The Physics of fluids, 31

(1988) 2786-2795.

[306] B. Washo, Rheology and modeling of the spin coating process, IBM Journal of Research

and Development, 21 (1977) 190-198.

[307] W.W. Flack, D.S. Soong, A.T. Bell, D.W. Hess, A mathematical model for spin coating of

polymer resists, Journal of Applied Physics, 56 (1984) 1199-1206.

[308] S.A. Jenekhe, Effects of solvent mass transfer on flow of polymer solutions on a flat

rotating disk, Industrial & engineering chemistry fundamentals, 23 (1984) 425-432.

[309] B.D. Cullity, J.W. Weymouth, Elements of X-ray Diffraction, American Journal of

Physics, 25 (1957) 394-395.

[310] C. Kittel, D.F. Holcomb, Introduction to solid state physics, American Journal of Physics,

35 (1967) 547-548.

[311] B.B. He, Two-Dimensional X-Ray Diffraction, John Wiley and Sons, 2009.

[312] B.B. He, Two-dimensional X-ray diffraction, John Wiley & Sons, 2011.

[313] D. Briggs, J.T. Grant, Surface analysis by Auger and X-ray photoelectron spectroscopy,

IM publications, 2003.

[314] J.F. Watts, An introduction to surface analysis by electron spectroscopy, Oxford University

Press, 1990.

[315] K. Laajalehto, I. Kartio, E. Suoninen, XPS and SR-XPS techniques applied to sulphide

mineral surfaces, International Journal of Mineral Processing, 51 (1997) 163-170.

[316] R.F. Egerton, Physical principles of electron microscopy, Springer, 2005.

[317] H. Schatten, J.B. Pawley, Biological low-voltage scanning electron microscopy, Springer,

2008.

[318] G. Binnig, C.F. Quate, C. Gerber, Atomic force microscope, Physical review letters, 56

(1986) 930.

[319] K. Nakamoto, Applications in inorganic chemistry, Infrared and Raman Spectra of

Inorganic and Coordination Compounds: Part A: Theory and Applications in Inorganic

Chemistry, Sixth Edition, (2008) 149-354.

[320] G. Turrell, J. Corset, Raman microscopy: developments and applications, Academic Press,

1996.

[321] J.R. Ferraro, Introductory raman spectroscopy, Academic press, 2003.

[322] R. Baddour-Hadjean, J.P. Pereira-Ramos, Raman microspectrometry applied to the study

of electrode materials for lithium batteries, Chem. Rev., 110 (2010) 1278-1319.

[323] F. Wooten, Optical properties of solids, Academic press, 2013.

[324] M. Fox, Optical properties of solids, in, AAPT, 2002.

[325] D.L. Smith, Thin-film deposition: principles and practice, McGraw-Hill New York etc,

1995.

[326] W.Y. Lam, Electrical and optical properties of indium tin oxide, (2014).

235

[327] H. Fukuyama, H. Ebisawa, Y. Wada, Theory of Hall Effect. I Nearly Free Electron,

Progress of Theoretical Physics, 42 (1969) 494-511.

[328] A.C. Fischer-Cripps, Factors Affecting Nanoindentation Test Data, Springer, 2000.

[329] A. Leyland, A. Matthews, On the significance of the H/E ratio in wear control: A

nanocomposite coating approach to optimised tribological behaviour, Wear, 246 (2000) 1-11.

[330] S. Rajagopalan, R. Vaidyanathan, Nano-and microscale mechanical characterization using

instrumented indentation, JOM Journal of the Minerals, Metals and Materials Society, 54 (2002)

45-48.

[331] W.C. Oliver, G.M. Pharr, An improved technique for determining hardness and elastic

modulus using load and displacement sensing indentation experiments, J Mater Res, 7 (1992)

1564-1583.

[332] W.C. Oliver, G.M. Pharr, Measurement of hardness and elastic modulus by instrumented

indentation: Advances in understanding and refinements to methodology, J Mater Res, 19 (2004)

3-20.

[333] M. Bauch, T. Dimopoulos, Design of ultrathin metal-based transparent electrodes

including the impact of interface roughness, Materials & Design, 104 (2016) 37-42.

[334] A.N. Fouda, E.S.M. Duraia, Self-assembled graphene oxide on a photo-catalytic active

transparent conducting oxide, Materials & Design, 90 (2016) 284-290.

[335] R. Mart nez-Morillas, R. Ram rez, J. Sánchez-Marcos, E. Fonda, A. de Andrés, C. Prieto,

Huge Photoresistance in Transparent and Conductive Indium Titanium Oxide Films Prepared by

Electron Beam–Physical Vapor Deposition, ACS applied materials & interfaces, 6 (2014) 1781-

1787.

[336] M.J. Kim, T.G. Kim, Fabrication of Metal-Deposited Indium Tin Oxides: Its Applications

to 385 nm Light-Emitting Diodes, ACS Appl. Mater. Interfaces, 8 (2016) 5453-5457.

[337] L.G. Bloor, J. Manzi, R. Binions, I.P. Parkin, D. Pugh, A. Afonja, C.S. Blackman, S.

Sathasivam, C.J. Carmalt, Tantalum and titanium doped In2O3 thin films by aerosol-assisted

chemical vapor deposition and their gas sensing properties, Chem. Mater., 24 (2012) 2864-2871.

[338] Z. Chen, W. Li, R. Li, Y. Zhang, G. Xu, H. Cheng, Fabrication of highly transparent and

conductive indium–tin oxide thin films with a high figure of merit via solution processing,

Langmuir, 29 (2013) 13836-13842.

[339] A.A. Aal, S.A. Mahmoud, A.K. Aboul-Gheit, Sol–gel and thermally evaporated

nanostructured thin ZnO films for photocatalytic degradation of trichlorophenol, Nanoscale

research letters, 4 (2009) 627-634.

[340] J.-H. Heo, K.-Y. Jung, D.-J. Kwak, D.-K. Lee, Y.-M. Sung, Fabrication of Titanium-doped

Indium oxide films for dye-sensitized solar cell application using reactive RF magnetron sputter

method, Plasma Science, IEEE Transactions on, 37 (2009) 1586-1592.

[341] J. Yan, Y.G. Jeong, Highly elastic and transparent multiwalled carbon

nanotube/polydimethylsiloxane bilayer films as electric heating materials, Materials & Design,

86 (2015) 72-79.

[342] S. Elhalawaty, K. Sivaramakrishnan, N.D. Theodore, T.L. Alford, The effect of sputtering

pressure on electrical, optical and structure properties of indium tin oxide on glass, Thin Solid

Films, 518 (2010) 3326-3331.

[343] A. Eshaghi, A. Graeli, Optical and electrical properties of indium tin oxide (ITO)

nanostructured thin films deposited on polycarbonate substrates “thickness effect”, Optik -

International Journal for Light and Electron Optics, 125 (2014) 1478-1481.

236

[344] M.A. Flores-Mendoza, R. Castanedo-Perez, G. Torres-Delgado, J. Márquez Marín, O.

Zelaya-Angel, Influence of the annealing temperature on the properties of undoped indium oxide

thin films obtained by the sol–gel method, Thin Solid Films, 517 (2008) 681-685.

[345] C. Goebbert, R. Nonninger, M.A. Aegerter, H. Schmidt, Wet chemical deposition of ATO

and ITO coatings using crystalline nanoparticles redispersable in solutions, Thin Solid Films,

351 (1999) 79-84.

[346] C.-C. Kuo, C.-C. Liu, Y.-F. Jeng, C.-C. Lin, Y.-Y. Liou, J.-L. He, Thickness dependence

of optoelectrical properties of Mo-doped in 2 O 3 films deposited on polyethersulfone substrates

by ion-beam-assisted evaporation, Journal of Nanomaterials, 2010 (2010) 72.

[347] T.O.L. Sunde, E. Garskaite, B. Otter, H.E. Fossheim, R. Saeterli, R. Holmestad, M.-A.

Einarsrud, T. Grande, Transparent and conducting ITO thin films by spin coating of an aqueous

precursor solution, Journal of Materials Chemistry, 22 (2012) 15740-15749.

[348] L. Kőrösi, S. Papp, I. Dékány, Preparation of transparent conductive indium tin oxide thin

films from nanocrystalline indium tin hydroxide by dip-coating method, Thin Solid Films, 519

(2011) 3113-3118.

[349] S. Calnan, A.N. Tiwari, High mobility transparent conducting oxides for thin film solar

cells, Thin Solid Films, 518 (2010) 1839-1849.

[350] D.J. Kwak, B.H. Moon, D.K. Lee, C.S. Park, Y.M. Sung, Comparison of transparent

conductive indium tin oxide, titanium-doped indium oxide, and fluorine-doped tin oxide films

for dye-sensitized solar cell application, J. Electr. Eng. Technol., 6 (2011) 684-687.

[351] M. Marezio, Refinement of the crystal structure of In2O3 at two wavelengths, Acta

Crystallographica, 20 (1966) 723-728.

[352] D.W. Greve, Field effect devices and applications: devices for portable, low-power, and

imaging systems, Prentice-Hall, Inc., 1998.

[353] S.-Y. Lien, Characterization and optimization of ITO thin films for application in

heterojunction silicon solar cells, Thin Solid Films, 518 (2010) S10-S13.

[354] C.H. Yang, S.C. Lee, T.C. Lin, W.Y. Zhuang, Opto-electronic properties of titanium-

doped indium-tin-oxide films deposited by RF magnetron sputtering at room temperature, Mater

Sci Eng B Solid State Adv Technol, 134 (2006) 68-75.

[355] W.S. Liu, H.M. Cheng, H.C. Hu, Y.T. Li, S.D. Huang, H.W. Yu, N.W. Pu, S.C. Liang,

Indium tin oxide with titanium doping for transparent conductive film application on CIGS solar

cells, Applied Surface Science, 354 (2015) 31-35.

[356] K. Suzuki, N. Hashimoto, T. Oyama, J. Shimizu, Y. Akao, H. Kojima, Large scale and low

resistance ITO films formed at high deposition rates, Thin Solid Films, 226 (1993) 104-109.

[357] C.V. Kumar, A. Mansingh, Effect of target‐substrate distance on the growth and properties

of rf‐sputtered indium tin oxide films, Journal of Applied Physics, 65 (1989) 1270-1280.

[358] M.E. Warwick, C.W. Dunnill, R. Binions, Multifunctional Nanocomposite Thin Films by

Aerosol‐Assisted CVD, Chemical Vapor Deposition, 16 (2010) 220-224.

[359] A. Bourlange, D. Payne, R. Palgrave, H. Zhang, J. Foord, R. Egdell, R. Jacobs, T.D. Veal,

P. King, C.F. McConville, The influence of Sn doping on the growth of In 2 O 3 on Y-stabilized

ZrO 2 (100) by oxygen plasma assisted molecular beam epitaxy, Journal of Applied Physics, 106

(2009) 013703.

[360] L. Kerkache, A. Layadi, E. Dogheche, D. Remiens, Physical properties of RF sputtered

ITO thin films and annealing effect, Journal of Physics D: Applied Physics, 39 (2005) 184.

237

[361] J. Ye, K. Nakamura, Quantitative structure analyses of YBa2Cu3O7-δ thin films:

Determination of oxygen content from x-ray-diffraction patterns, Physical Review B, 48 (1993)

7554-7564.

[362] M. Gulen, G. Yildirim, S. Bal, A. Varilci, I. Belenli, M. Oz, Role of annealing temperature

on microstructural and electro-optical properties of ITO films produced by sputtering, Journal of


[363] Y. Abe, N. Ishiyama, Titanium-doped indium oxide films prepared by dc magnetron

sputtering using ceramic target, Journal of materials science, 41 (2006) 7580-7584.

[364] H. Han, D. Adams, J.W. Mayer, T.L. Alford, Characterization of the physical and

electrical properties of Indium tin oxide on polyethylene napthalate, Journal of Applied Physics,

98 (2005) 083705.

[365] A. Al-Kahlout, S. Heusing, T. Mueller, N. Aldahoudi, M. Quilitz, P. De Oliveira, Novel

conductive characteristics of ITO: Ti films deposited by spin coating from colloidal precursor, J

Sol Gel Sci Technol, 59 (2011) 532-538.

[366] S.H. Paeng, M.W. Park, Y.M. Sung, Transparent conductive characteristics of Ti:ITO

films deposited by RF magnetron sputtering at low substrate temperature, Surface and Coatings

Technology, 205 (2010) S210-S215.

[367] M. Girtan, G. Rusu, G. Rusu, S. Gurlui, Influence of oxidation conditions on the properties

of indium oxide thin films, Applied surface science, 162 (2000) 492-498.

[368] Z.T. Jiang, K. Ohshimo, M. Aoyama, T. Yamaguchi, A study of Cr-Al oxides for single-

layer halftone phase-shifting masks for deep-ultraviolet region photolithography, Jpn J Appl

Phys Part 1 Regul Pap Short Note Rev Pap, 37 (1998) 4008-4013.

[369] P. Wolff, Theory of the band structure of very degenerate semiconductors, Physical

Review, 126 (1962) 405.

[370] S. Parthiban, V. Gokulakrishnan, E. Elangovan, G. Gonçalves, K. Ramamurthi, E.

Fortunato, R. Martins, High mobility and visible-near infrared transparent titanium doped indium

oxide thin films produced by spray pyrolysis, Thin Solid Films, 524 (2012) 268-271.

[371] W.-S. Liu, W.-K. Chen, K.-P. Hsueh, Transparent conductive Ga-doped MgxZn 1− xO

films with high optical transmittance prepared by radio frequency magnetron sputtering, Journal

of Alloys and Compounds, 552 (2013) 255-263.

[372] K.L. Chopra, S. Major, D.K. Pandya, Transparent conductors—A status review, Thin Solid

Films, 102 (1983) 1-46.

[373] A. Subrahmanyam, A. Rajakumar, M. Rakibuddin, T.P. Ramesh, M.R. Kiran, D. Shankari,

K. Chandrasekhar, Efficacy of titanium doped-indium tin oxide (Ti/TiO 2–ITO) films in rapid

oxygen generation under photocatalysis and their suitability for bio-medical application, Physical

Chemistry Chemical Physics, 16 (2014) 24790-24799.

[374] C. Lee, Y. Ko, Y. Kim, Device Performances of Organic Light-Emitting Diodes with

Indium Tin Oxide, Gallium Zinc Oxide, and Indium Zinc Tin Oxide Anodes Deposited at Room

Temperature, J. Nanosci. Nanotechnol., 13 (2013) 8011-8015.

[375] S.-Y. Lien, A. Nautiyal, S.J. Lee, Optoelectronic Properties of Indium–Tin Oxide Films

Deposited on Flexible and Transparent Poly (dimethylsiloxane) Substrate, Japanese Journal of

Applied Physics, 52 (2013) 115801.

[376] S. Kuznetsova, L. Borilo, Obtaining sol-gel by means of indium oxide thin films with

added tin on glass substrates, Glass and Ceramics, 70 (2014) 429-433.

[377] J. George, C. Menon, Electrical and optical properties of electron beam evaporated ITO

thin films, Surface and Coatings Technology, 132 (2000) 45-48.

238

[378] T. Maruyama, K. Fukui, Indium‐tin oxide thin films prepared by chemical vapor

deposition, Journal of applied physics, 70 (1991) 3848-3851.

[379] C. O'Dwyer, M. Szachowicz, G. Visimberga, V. Lavayen, S. Newcomb, C.S. Torres,

Bottom-up growth of fully transparent contact layers of indium tin oxide nanowires for light-

emitting devices, Nature nanotechnology, 4 (2009) 239-244.

[380] A. Amri, Z.-T. Jiang, N. Wyatt, C.-Y. Yin, N. Mondinos, T. Pryor, M.M. Rahman, Optical

properties and thermal durability of copper cobalt oxide thin film coatings with integrated silica

antireflection layer, Ceramics International, 40 (2014) 16569-16575.

[381] S.H. Chuang, C.S. Tsung, C.H. Chen, S.L. Ou, R.H. Horng, C.Y. Lin, D.S. Wuu,

Transparent conductive oxide films embedded with plasmonic nanostructure for light-emitting

diode applications, ACS Appl. Mater. Interfaces, 7 (2015) 2546-2553.

[382] J.v. Deelen, H. Rendering, M. Theelen, Z. Vroon, P. Poodt, A. Hovestad, Highly improved

transparent conductors by combination of TCOs and metallic grids, in: 35th IEEE Photovoltaic

Specialists Conference, PVSC 2010, 20-25 June, 2010, Honolulu, HI, USA. Conference code:

82805, 992-994, 2010.

[383] J. Chen, S.J. Bull, Assessment of the toughness of thin coatings using nanoindentation

under displacement control, Thin Solid Films, 494 (2006) 1-7.

[384] Z.-T.J. Hatem Taha, David J. Henry, Amun Amri, Chun-Yang Yin,, M.M. Rahman,

Improving the optoelectronic properties of titanium doped indium tin.pdf, Semiconductor

Science and Technology, (2017).

[385] O.M. Berengue, A.D. Rodrigues, C.J. Dalmaschio, A.J.C. Lanfredi, E.R. Leite, A.J.

Chiquito, Structural characterization of indium oxide nanostructures: a Raman analysis, Journal

of Physics D: Applied Physics, 43 (2010) 045401.

[386] C.Y. Wang, Y. Dai, J. Pezoldt, B. Lu, T. Kups, V. Cimalla, O. Ambacher, Phase

stabilization and phonon properties of single crystalline rhombohedral indium oxide, Crystal

growth and Design, 8 (2008) 1257-1260.

[387] A. Nelson, H. Aharoni, X‐ray photoelectron spectroscopy investigation of ion beam

sputtered indium tin oxide films as a function of oxygen pressure during deposition, Journal of

Vacuum Science & Technology A, 5 (1987) 231-233.

[388] W.-F. Wu, B.-S. Chiou, S.-T. Hsieh, Effect of sputtering power on the structural and

optical properties of RF magnetron sputtered ITO films, Semiconductor science and technology,

9 (1994) 1242.

[389] W.-F. Wu, B.-S. Chiou, Effect of oxygen concentration in the sputtering ambient on the

microstructure, electrical and optical properties of radio-frequency magnetron-sputtered indium

tin oxide films, Semiconductor science and technology, 11 (1996) 196.

[390] F. Werfel, O. Brümmer, Corundum Structure Oxides Studied by XPS, Phys Scr, 28 (1983)

92-96.

[391] C. Sleigh, A.P. Pijpers, A. Jaspers, B. Coussens, R.J. Meier, On the determination of

atomic charge via ESCA including application to organometallics, J Electron Spectrosc Relat

Phenom, 77 (1996) 41-57.

[392] P. Ágoston, P. Erhart, A. Klein, K. Albe, Geometry, electronic structure and

thermodynamic stability of intrinsic point defects in indium oxide, J Phys Condens Matter, 21

(2009).

[393] B.H. Freeland, J.J. Habeeb, D.G. Tuck, Coordination compounds of indium. Part XXXIII.

X-Ray photoelectron spectroscopy of neutral and anionic indium halide species, Canadian

Journal of Chemistry, 55 (1977) 1527-1532.

239

[394] S.R. Jian, Y.H. Lee, Nanoindentation-induced interfacial fracture of ZnO thin films

deposited on Si(1 1 1) substrates by atomic layer deposition, Journal of Alloys and Compounds,

587 (2014) 313-317.

[395] K. Zeng, F. Zhu, J. Hu, L. Shen, K. Zhang, H. Gong, Investigation of mechanical

properties of transparent conducting oxide thin films, Thin Solid Films, 443 (2003) 60-65.

[396] C.Y. Yen, S.R. Jian, G.J. Chen, C.M. Lin, H.Y. Lee, W.C. Ke, Y.Y. Liao, P.F. Yang, C.T.

Wang, Y.S. Lai, J.S.C. Jang, J.Y. Juang, Influence of annealing temperature on the structural,

optical and mechanical properties of ALD-derived ZnO thin films, Applied Surface Science, 257

(2011) 7900-7905.

[397] N. Biswas, P. Ghosh, S. Sarkar, D. Moitra, P.K. Biswas, S. Jana, A.K. Mukhopadhyay,

Nanomechanical properties of dip coated indium tin oxide films on glass, Thin Solid Films, 579

(2015) 21-29.

[398] A. Barna, P. Barna, G. Radnoczi, F. Reicha, L. Toth, Formation of aluminium thin films in

the presence of oxygen and nickel, Physica status solidi (a), 55 (1979) 427-435.

[399] I. Petrov, P. Barna, L. Hultman, J. Greene, Microstructural evolution during film growth,

Journal of Vacuum Science & Technology A, 21 (2003) S117-S128.

[400] M.M. Rahman, Z.-T. Jiang, Z.-f. Zhou, Z. Xie, C.Y. Yin, H. Kabir, M.M. Haque, A. Amri,

N. Mondinos, M. Altarawneh, Effects of annealing temperatures on the morphological,

mechanical, surface chemical bonding, and solar selectivity properties of sputtered TiAlSiN thin

films, Journal of Alloys and Compounds, 671 (2016) 254-266.

[401] D. Kenfaui, G. Bonnefont, D. Chateigner, G. Fantozzi, M. Gomina, J.G. Noudem, Ca 3 Co

4 O 9 ceramics consolidated by SPS process: optimisation of mechanical and thermoelectric

properties, Materials Research Bulletin, 45 (2010) 1240-1249.

[402] D. Kenfaui, D. Chateigner, M. Gomina, J.G. Noudem, Texture, mechanical and

thermoelectric properties of Ca3Co4O9 ceramics, Journal of Alloys and Compounds, 490 (2010)

472-479.

[403] T.O.L. Sunde, M.A. Einarsrud, T. Grande, Optimisation of chemical solution deposition of

indium tin oxide thin films, Thin Solid Films, 573 (2014) 48-55.

[404] M. Singh, J. Jiu, T. Sugahara, K. Suganuma, Thin-film copper indium gallium selenide

solar cell based on low-temperature all-printing process, ACS applied materials & interfaces, 6

(2014) 16297-16303.

[405] J.-I. Kim, K.H. Ji, M. Jang, H. Yang, R. Choi, J.K. Jeong, Ti-doped indium tin oxide thin

films for transparent field-effect transistors: Control of charge-carrier density and crystalline

structure, ACS applied materials & interfaces, 3 (2011) 2522-2528.

[406] P. Mazzolini, P. Gondoni, V. Russo, D. Chrastina, C.S. Casari, A.L. Bassi, Tuning of

electrical and optical properties of highly conducting and transparent Ta doped TiO2

polycrystalline films, J. Phys. Chem. C, 119 (2015) 6988-6997.

[407] J.J.L. Hmar, T. Majumder, J.N. Roy, S.P. Mondal, Flexible, transparent, high dielectric

and photoconductive thin films using ZnO nanosheets-multi-walled carbon nanotube-polymer

nanocomposites, Journal of Alloys and Compounds, 651 (2015) 82-90.

[408] L. José Andrés, M. Fe Menéndez, D. Gómez, A. Luisa Martínez, N. Bristow, J. Paul

Kettle, A. Menéndez, B. Ruiz, Rapid synthesis of ultra-long silver nanowires for tailor-made

transparent conductive electrodes: proof of concept in organic solar cells, Nanotechnology, 26

(2015) 1-9.

[409] H.J. Van De Wiel, Y. Galagan, T.J. Van Lammeren, J.F.J. De Riet, J. Gilot, M.G.M.

Nagelkerke, R.H.C.A.T. Lelieveld, S. Shanmugam, A. Pagudala, D. Hui, W.A. Groen, Roll-to-

240

roll embedded conductive structures integrated into organic photovoltaic devices,

Nanotechnology, 24 (2013).

[410] M. Chakaroun, A. El Amrani, B. Lucas, B. Ratier, M. Aldissi, Organic optoelectronic

devices-flexibility versus performance, The European Physical Journal Applied Physics, 51

(2010) 33206.

[411] S.M. Lee, B.H. Choi, J.S. Park, T.H. Kim, H.K. Bae, L.S. Park, Effect of ITO/Ag/ITO

Multilayer Electrode Deposited onto Glass and PET Substrate on the Performance of Organic

Light-Emitting Diodes, Mol. Cryst. Liq. Cryst., 530 (2010) 110/[266]-115/[271].

[412] Y.-S. Park, K.-H. Choi, H.-K. Kim, Room temperature flexible and transparent

ITO/Ag/ITO electrode grown on flexile PES substrate by continuous roll-to-roll sputtering for

flexible organic photovoltaics, Journal of Physics D: Applied Physics, 42 (2009) 235109.

[413] S. Ishii, T. Duy Dao, K. Chen, T. Nagao, Transparent oxides forming

conductor/insulator/conductor heterojunctions for photodetection, Nanotechnology, 26 (2015).

[414] Y. Ye, L. Song, X. Song, T. Zhang, Influence of sputtering parameters on the electrical

property of indium tin oxide film used for microwave absorbing, Journal of Alloys and

Compounds, 581 (2013) 133-138.

[415] F. Selzer, N. Weiß, D. Kneppe, L. Bormann, C. Sachse, N. Gaponik, A. Eychmüller, K.

Leo, L. Müller-Meskamp, A spray-coating process for highly conductive silver nanowire

networks as the transparent top-electrode for small molecule organic photovoltaics, Nanoscale, 7

(2015) 2777-2783.

[416] P.-C. Chen, G. Shen, H. Chen, Y.-g. Ha, C. Wu, S. Sukcharoenchoke, Y. Fu, J. Liu, A.

Facchetti, T.J. Marks, High-performance single-crystalline arsenic-doped indium oxide

nanowires for transparent thin-film transistors and active matrix organic light-emitting diode

displays, ACS Nano, 3 (2009) 3383-3390.

[417] T. Ishida, H. Kobayashi, Y. Nakato, Structures and properties of electron‐beam‐evaporated

indium tin oxide films as studied by X‐ray photoelectron spectroscopy and work‐function

measurements, Journal of applied physics, 73 (1993) 4344-4350.

[418] S. Herodotou, R.E. Treharne, K. Durose, G.J. Tatlock, R.J. Potter, The Effects of Zr

Doping on the Optical, Electrical and Microstructural Properties of Thin ZnO Films Deposited

by Atomic Layer Deposition, Materials, 8 (2015) 7230-7240.

[419] Q. Liu, F. Jin, J. Dai, B. Li, L. Geng, J. Liu, Effect of thickness on the electrical and optical

properties of epitaxial (La 0.07 Ba 0.93) SnO 3 thin films, Superlattices Microstruct, 96 (2016)

205-211.

[420] L. Kőrösi, A. Scarpellini, P. Petrik, S. Papp, I. Dékány, Sol–gel synthesis of nanostructured

indium tin oxide with controlled morphology and porosity, Applied Surface Science, 320 (2014)

725-731.

[421] A.J. Forman, Z. Chen, P. Chakthranont, T.F. Jaramillo, High Surface Area Transparent

Conducting Oxide Electrodes with a Customizable Device Architecture, Chem. Mater., 26

(2014) 958-964.

[422] J.S. Allhusen, J.C. Conboy, Preparation and Characterization of Conductive and

Transparent Ruthenium Dioxide Sol–Gel Films, ACS applied materials & interfaces, 5 (2013)

11683-11691.

[423] S. Wen, G. Campet, The textural effect of Cu doping and the electronic effect of Ti, Zr and

Ge dopings upon the physical properties of In2O3 and Sn-doped In2O3 ceramics, Active and

passive electronic components, 15 (1993) 79-87.

241

[424] A. Chaoumead, B.-H. Joo, D.-J. Kwak, Y.-M. Sung, Structural and electrical properties of

sputtering power and gas pressure on Ti-dope In 2 O 3 transparent conductive films by RF

magnetron sputtering, Applied Surface Science, 275 (2013) 227-232.

[425] D.-S. Liu, C.-S. Sheu, C.-T. Lee, C.-H. Lin, Thermal stability of indium tin oxide thin

films co-sputtered with zinc oxide, Thin Solid Films, 516 (2008) 3196-3203.

[426] C. Guillén, J. Herrero, ITO/metal/ITO multilayer structures based on Ag and Cu metal

films for high-performance transparent electrodes, Sol Energ Mater Sol Cells, 92 (2008) 938-

941.

[427] D. Sahu, S.-Y. Lin, J.-L. Huang, ZnO/Ag/ZnO multilayer films for the application of a

very low resistance transparent electrode, Applied Surface Science, 252 (2006) 7509-7514.

[428] H. Han, N.D. Theodore, T.L. Alford, Improved conductivity and mechanism of carrier

transport in zinc oxide with embedded silver layer, Journal of Applied Physics, 103 (2008).

[429] M.M. Menamparambath, K. Yang, H.H. Kim, O.S. Bae, M.S. Jeong, J.Y. Choi, S. Baik,

Reduced haze of transparent conductive films by smaller diameter silver nanowires,

Nanotechnology, 27 (2016) 465706.

[430] H. Taha, Z.-T. Jiang, D.J. Henry, A. Amri, C.-Y. Yin, M.M. Rahman, Improving the

optoelectronic properties of titanium-doped indium tin oxide thin films, Semiconductor Science

and Technology, 32 (2017) 065011.

[431] M. Mirzaee, A. Dolati, Effect of content silver and heat treatment temperature on

morphological, optical, and electrical properties of ITO films by sol–gel technique, Journal of

Nanoparticle Research, 16 (2014) 1-11.

[432] H.J. Kim, U. Kim, T.H. Kim, J. Kim, H.M. Kim, B.-G. Jeon, W.-J. Lee, H.S. Mun, K.T.

Hong, J. Yu, Physical properties of transparent perovskite oxides (Ba, La) SnO 3 with high

electrical mobility at room temperature, Physical Review B, 86 (2012) 165205.

[433] B. Hadjarab, A. Bouguelia, M. Trari, Optical and transport properties of lanthanum-doped

stannate BaSnO3, Journal of Physics D: Applied Physics, 40 (2007) 5833.

[434] E.-K. Jeong, I.-S. Kim, D.-H. Kim, S.-Y. Choi, Effect of deposition and annealing

temperature on structural, electrical and optical properties of Ag doped ZnO thin films, Korean

Journal of Materials Research, 18 (2008) 84-91.

[435] A. Hultåker, K. Järrendahl, J. Lu, C.G. Granqvist, G.A. Niklasson, Electrical and optical

properties of sputter deposited tin doped indium oxide thin films with silver additive, 3rd

International Conference on Coating on Glass (ICCG), 392 (2001) 305-310.

[436] Y.S. Jung, Y.W. Choi, H.C. Lee, D.W. Lee, Effects of thermal treatment on the electrical

and optical properties of silver-based indium tin oxide/metal/indium tin oxide structures, Thin

Solid Films, 440 (2003) 278-284.

[437] A. Subrahmanyam, U.K. Barik, Electrical and optical properties of reactive dc magnetron

sputtered silver-doped indium oxide thin films: Role of oxygen, Appl Phys A, 84 (2006) 221-

225.

[438] A. Arunachalam, S. Dhanapandian, M. Rajasekaran, Morphology controllable flower like

nanostructures of Ag doped ZnO thin films and its application as photovoltaic material, J Anal

Appl Pyrolysis, (2016).

[439] S. Ilican, Y. Caglar, M. Caglar, F. Yakuphanoglu, Structural, optical and electrical

properties of F-doped ZnO nanorod semiconductor thin films deposited by sol-gel process,

Applied Surface Science, 255 (2008) 2353-2359.

[440] C.F. Klingshirn, A. Waag, A. Hoffmann, J. Geurts, Zinc oxide: From fundamental

properties towards novel applications, Springer Science & Business Media, 2010.

242

[441] G. Haacke, New figure of merit for transparent conductors, Journal of Applied Physics, 47

(1976) 4086-4089.

[442] D. Fraser, H. Cook, Highly Conductive, Transparent Films of Sputtered In2− x Sn x O 3−

y, Journal of the Electrochemical Society, 119 (1972) 1368-1374.

[443] A.H. Ali, A. Shuhaimi, Z. Hassan, Structural, optical and electrical characterization of

ITO, ITO/Ag and ITO/Ni transparent conductive electrodes, Applied Surface Science, 288

(2014) 599-603.

[444] M.M. Khan, M.W. Khan, M. Alhoshan, M. AlSalhi, A. Aldwayyan, Influences of Co

doping on the structural and optical properties of ZnO nanostructured, Applied Physics A, 100

(2010) 45-51.

[445] B.-S. Chiou, J.-H. Tsai, Antireflective coating for ITO films deposited on glass substrate,

Journal of Materials Science: Materials in Electronics, 10 (1999) 491-495.

[446] C. Cao, A. Zhou, S. Mu, G. Zhang, X. Song, Z. Sun, Compositional analysis and optical

properties of Ag-doped ITO films, Mater Sci Eng B Solid State Adv Technol, 176 (2011) 1430-

1434.

[447] A. Akkari, C. Guasch, M. Castagne, N. Kamoun-Turki, Optical study of zinc blend SnS

and cubic In2S3: Al thin films prepared by chemical bath deposition, Journal of materials

science, 46 (2011) 6285-6292.

[448] H. Taha, D.J. Henry, C.Y. Yin, A. Amri, X. Zhao, S. Bahri, C. Le Minh, N.N. Ha, M.M.

Rahman, Z.-T. Jiang, Probing the effects of thermal treatment on the electronic structure and

mechanical properties of Ti-doped ITO thin films, Journal of Alloys and Compounds, (2017).

[449] F. Mammeri, E. Le Bourhis, L. Rozes, C. Sanchez, Mechanical properties of hybrid

organic–inorganic materials, Journal of materials chemistry, 15 (2005) 3787-3811.

[450] S. Zhang, D. Sun, Y. Fu, H. Du, Recent advances of superhard nanocomposite coatings: a

review, Surface and Coatings Technology, 167 (2003) 113-119.

[451] T.-C. Li, R.-C. Chang, Improving the performance of ITO thin films by coating PEDOT:

PSS, International Journal of Precision Engineering and Manufacturing-Green Technology, 1

(2014) 329-334.

[452] X. Zhao, Z. Xie, P. Munroe, Nanoindentation of hard multilayer coatings: Finite element

modelling, Materials Science and Engineering A, 528 (2011) 1111-1116.

[453] K. Chongsri, J. Kanoksinwuttipong, W. Techitdheera, W. Pecharapa, Physical properties

of Ti-doped ITO nanoparticles synthesized by co-precipitation method, in: IEEE International

Nanoelectronics Conference, INEC 2014, Institute of Electrical and Electronics Engineers Inc.,

2014.

[454] E. Binetti, Z. El Koura, N. Bazzanella, G. Carotenuto, A. Miotello, Synthesis of

mesoporous ITO/TiO2 electrodes for optoelectronics, Mater Lett, 139 (2015) 355-358.

[455] M.A. Green, Estimates of Te and In prices from direct mining of known ores, Progress in

Photovoltaics: Research and Applications, 17 (2009) 347-359.

[456] D.R. Cairns, R.P. Witte II, D.K. Sparacin, S.M. Sachsman, D.C. Paine, G.P. Crawford, R.

Newton, Strain-dependent electrical resistance of tin-doped indium oxide on polymer substrates,

Appl Phys Lett, 76 (2000) 1425-1427.

[457] X. Li, G. Zhang, X. Bai, X. Sun, X. Wang, E. Wang, H. Dai, Highly conducting graphene

sheets and Langmuir–Blodgett films, Nature nanotechnology, 3 (2008) 538-542.

[458] Z. Wu, Z. Chen, X. Du, J.M. Logan, J. Sippel, M. Nikolou, K. Kamaras, J.R. Reynolds,

D.B. Tanner, A.F. Hebard, Transparent, conductive carbon nanotube films, Science, 305 (2004)

1273-1276.

243

[459] H. Wu, L. Hu, M.W. Rowell, D. Kong, J.J. Cha, J.R. McDonough, J. Zhu, Y. Yang, M.D.

McGehee, Y. Cui, Electrospun metal nanofiber webs as high-performance transparent electrode,

Nano Lett., 10 (2010) 4242-4248.

[460] X. Crispin, F. Jakobsson, A. Crispin, P. Grim, P. Andersson, A. Volodin, C. Van

Haesendonck, M. Van der Auweraer, W.R. Salaneck, M. Berggren, The origin of the high

conductivity of poly (3, 4-ethylenedioxythiophene)-poly (styrenesulfonate)(PEDOT-PSS) plastic

electrodes, Chem. Mater., 18 (2006) 4354-4360.

[461] X. Wang, L. Zhi, K. Müllen, Transparent, conductive graphene electrodes for dye-

sensitized solar cells, Nano Lett., 8 (2008) 323-327.

[462] D. Ghosh, T. Chen, V. Pruneri, High figure-of-merit ultrathin metal transparent electrodes

incorporating a conductive grid, Appl Phys Lett, 96 (2010) 041109.

[463] A.A. Argun, A. Cirpan, J.R. Reynolds, The First Truly All‐Polymer Electrochromic

Devices, Advanced Materials, 15 (2003) 1338-1341.

[464] Z. Li, H.R. Kandel, E. Dervishi, V. Saini, Y. Xu, A.R. Biris, D. Lupu, G.J. Salamo, A.S.

Biris, Comparative study on different carbon nanotube materials in terms of transparent

conductive coatings, Langmuir, 24 (2008) 2655-2662.

[465] Y. Meng, X.-l. Yang, H.-x. Chen, J. Shen, Y.-m. Jiang, Z.-j. Zhang, Z.-y. Hua,

Molybdenum-doped indium oxide transparent conductive thin films, Journal of Vacuum Science

& Technology A, 20 (2002) 288-290.

[466] L. Hu, D.S. Hecht, G. Gruner, Infrared transparent carbon nanotube thin films, Appl Phys

Lett, 94 (2009) 81103.

[467] S. Hong, S. Myung, Nanotube Electronics: A flexible approach to mobility, Nature

Nanotechnology, 2 (2007) 207.

[468] M.J. Treacy, T. Ebbesen, J. Gibson, Exceptionally high Young's modulus observed for

individual carbon nanotubes, Nature, 381 (1996) 678.

[469] J. Li, L. Hu, J. Liu, L. Wang, T.J. Marks, G. Grüner, Indium tin oxide modified transparent

nanotube thin films as effective anodes for flexible organic light-emitting diodes, Appl Phys

Lett, 93 (2008) 083306.

[470] D. Antiohos, G. Folkes, P. Sherrell, S. Ashraf, G.G. Wallace, P. Aitchison, A.T. Harris, J.

Chen, A.I. Minett, Compositional effects of PEDOT-PSS/single walled carbon nanotube films on

supercapacitor device performance, Journal of Materials Chemistry, 21 (2011) 15987-15994.

[471] J. Li, J.-C. Liu, C.-J. Gao, On the mechanism of conductivity enhancement in PEDOT/PSS

film doped with multi-walled carbon nanotubes, Journal of polymer research, 17 (2010) 713-718.

[472] H. Hu, B. Zhao, M.E. Itkis, R.C. Haddon, Nitric Acid Purification of Single-Walled

Carbon Nanotubes, Journal of Physical Chemistry B, 107 (2003) 13838-13842.

[473] H. Jeong, J.Y. Park, Local Electrical Investigations of Nitric Acid Treatment Effects on

Carbon Nanotube Networks, J. Phys. Chem. C, 119 (2015) 9665-9668.

[474] P. Chen, X. Wu, X. Sun, J. Lin, W. Ji, K. Tan, Electronic structure and optical limiting

behavior of carbon nanotubes, Physical review letters, 82 (1999) 2548.

[475] O. Warschkow, D.E. Ellis, G.B. González, T.O. Mason, Defect Cluster Aggregation and

Nonreducibility in Tin‐Doped Indium Oxide, Journal of the American Ceramic Society, 86

(2003) 1707-1711.

[476] S. Maldonado, S. Morin, K.J. Stevenson, Structure, composition, and chemical reactivity

of carbon nanotubes by selective nitrogen doping, Carbon, 44 (2006) 1429-1437.

244

[477] S.W. Lee, B.S. Kim, S. Chen, Y. Shao-Horn, P.T. Hammond, Layer-by-layer assembly of

all carbon nanotube ultrathin films for electrochemical applications, Journal of the American

Chemical Society, 131 (2009) 671-679.

[478] M.S. Dresselhaus, G. Dresselhaus, R. Saito, A. Jorio, Raman spectroscopy of carbon

nanotubes, Physics reports, 409 (2005) 47-99.

[479] L. Berguiga, J. Bellessa, F. Vocanson, E. Bernstein, J. Plenet, Carbon nanotube silica glass

composites in thin films by the sol–gel technique, Optical Materials, 28 (2006) 167-171.

[480] X. Hu, L. Chen, Y. Zhang, Q. Hu, J. Yang, Y. Chen, Large-scale flexible and highly

conductive carbon transparent electrodes via roll-to-roll process and its high performance lab-

scale indium tin oxide-free polymer solar cells, Chem. Mater., 26 (2014) 6293-6302.

[481] M.R. Golobostanfard, H. Abdizadeh, S. Mohammadi, M.A. Baghchesara, Carbon

nanotube/indium tin oxide hybrid transparent conductive film: Effect of nanotube diameter, Sol

Energ Mater Sol Cells, 132 (2015) 418-424.

[482] M. Gross, A. Winnacker, P.J. Wellmann, Electrical, optical and morphological properties

of nanoparticle indium–tin–oxide layers, Thin Solid Films, 515 (2007) 8567-8572.

[483] M. Itkis, S. Niyogi, M. Meng, M. Hamon, H. Hu, R. Haddon, Spectroscopic study of the

Fermi level electronic structure of single-walled carbon nanotubes, Nano Lett., 2 (2002) 155-

159.

[484] J. Kim, B. Lägel, E. Moons, N. Johansson, I. Baikie, W.R. Salaneck, R. Friend, F. Cacialli,

Kelvin probe and ultraviolet photoemission measurements of indium tin oxide work function: a

comparison, Synth Met, 111 (2000) 311-314.

[485] J.-J. Ho, C.-Y. Chen, R.Y. Hsiao, O.L. Ho, The Work Function Improvement on Indium−

Tin− Oxide Epitaxial Layers by Doping Treatment for Organic Light-Emitting Device

Applications, The Journal of Physical Chemistry C, 111 (2007) 8372-8376.

[486] H.F. Wardenga, M.V. Frischbier, M. Morales-Masis, A. Klein, In Situ Hall Effect

Monitoring of Vacuum Annealing of In2O3: H Thin Films, Materials, 8 (2015) 561-574.

[487] S. Hamrit, K. Djessas, N. Brihi, B. Viallet, K. Medjnoun, S.E. Grillo, The effect of

thickness on the physico-chemical properties of nanostructured ZnO:Al TCO thin films

deposited on flexible PEN substrates by RF-magnetron sputtering from a nanopowder target,

Ceramics International, 42 (2016) 16212-16219.

[488] M.R. Golobostanfard, S. Mohammadi, H. Abdizadeh, M.A. Baghchesara, Incorporating

carbon nanotubes in sol-gel synthesized indium tin oxide transparent conductive films,

Langmuir, 30 (2014) 11785-11791.

[489] S. Kumar, B.A. Cola, R. Jackson, S. Graham, A review of carbon nanotube ensembles as

flexible electronics and advanced packaging materials, Journal of Electronic Packaging, 133

(2011) 020906.

[490] F. Wang, M.Z. Wu, Y.Y. Wang, Y.M. Yu, X.M. Wu, L.J. Zhuge, Influence of thickness

and annealing temperature on the electrical, optical and structural properties of AZO thin films,

Vacuum, 89 (2013) 127-131.

[491] Z.-T. Jiang, K. Ohshimo, M. Aoyama, T. Yamaguchi, A study of Cr–Al oxides for single-

layer halftone phase-shifting masks for deep-ultraviolet region photolithography, Japanese

journal of applied physics, 37 (1998) 4008-4013.

[492] W. Winarto, D. Priadi, N. Sofyan, A. Wicaksono, Wear Resistance and Surface Hardness

of Carbon Nanotube Reinforced Alumina Matrix Nanocomposite by Cold Sprayed Process,

Procedia Engineering, 170 (2017) 108-112.

245

[493] J.-W. An, D.-H. You, D.-S. Lim, Tribological properties of hot-pressed alumina–CNT

composites, Wear, 255 (2003) 677-681.

[494] D.-Y. Khang, J. Xiao, C. Kocabas, S. MacLaren, T. Banks, H. Jiang, Y.Y. Huang, J.A.

Rogers, Molecular scale buckling mechanics in individual aligned single-wall carbon nanotubes

on elastomeric substrates, Nano Lett., 8 (2008) 124-130.

[495] S.H. Cho, R. Pandey, C.H. Wie, Y.J. Lee, J.W. Lim, D.H. Park, J.S. Seok, Y.H. Jang, K.K.

Kim, D.K. Hwang, D.J. Byun, W.K. Choi, Highly transparent ZTO/Ag/ZTO multilayer electrode

deposited by inline sputtering process for organic photovoltaic cells, Phys. Status Solidi A Appl.

Mater. Sci., 211 (2014) 1860-1867.

[496] M. Ghasemi Varnamkhasti, E. Shahriari, Design and fabrication of nanometric

TiO2/Ag/TiO 2/Ag/TiO2 transparent conductive electrode for inverted organic photovoltaic cells

application, Superlattices Microstruct, 69 (2014) 231-238.

[497] M. Girtan, R. Mallet, On the electrical properties of transparent electrodes, Proc. Rom.

Acad. Ser. A Math. Phys. Tech. Sci. Inf. Sci., 15 (2014) 146-150.

[498] D.S. Ginley, C. Bright, Transparent conducting oxides, Mrs Bulletin, 25 (2000) 15-18.

[499] M. Girtan, Comparison of ITO/metal/ITO and ZnO/metal/ZnO characteristics as

transparent electrodes for third generation solar cells, Sol Energ Mater Sol Cells, 100 (2012)

153-161.

[500] H. Sun, G. Ge, J. Zhu, H. Yan, Y. Lu, Y. Wu, J. Wan, M. Han, Y. Luo, High electrical

conductivity of graphene-based transparent conductive films with silver nanocomposites, RSC

Adv., 5 (2015) 108044-108049.

[501] H.K. Yu, W.J. Dong, G.H. Jung, J.-L. Lee, Three-dimensional nanobranched indium–tin-

oxide anode for organic solar cells, ACS Nano, 5 (2011) 8026-8032.

[502] S. Ye, A.R. Rathmell, Z. Chen, I.E. Stewart, B.J. Wiley, Metal nanowire networks: the

next generation of transparent conductors, Advanced Materials, 26 (2014) 6670-6687.

[503] K. Sivaramakrishnan, T.L. Alford, Metallic conductivity and the role of copper in

ZnO/Cu/ZnO thin films for flexible electronics, Appl Phys Lett, 94 (2009).

[504] C. Guillen, J. Herrero, Transparent electrodes based on metal and metal oxide stacked

layers grown at room temperature on polymer substrate, physica status solidi (a), 207 (2010)

1563-1567.

[505] R. Pandey, C.H. Wie, X. Lin, J.W. Lim, K.K. Kim, D.K. Hwang, W.K. Choi, Fluorine

doped zinc tin oxide multilayer transparent conducting Oxides for organic photovoltaic's Cells,

Sol Energ Mater Sol Cells, 134 (2014) 5-14.

[506] T.H. Kim, B.H. Choi, J.S. Park, S.M. Lee, Y.S. Lee, L.S. Park, Transparent conductive

ITO/Ag/ITO multilayer films prepared by low temperature process and physical properties, Mol.

Cryst. Liq. Cryst., 520 (2010) 209-214.

[507] K. Choi, J. Kim, Y. Lee, H. Kim, ITO/Ag/ITO multilayer films for the application of a

very low resistance transparent electrode, Thin solid films, 341 (1999) 152-155.

[508] D.L. Van Hyning, W.G. Klemperer, C.F. Zukoski, Silver nanoparticle formation:

predictions and verification of the aggregative growth model, Langmuir, 17 (2001) 3128-3135.

[509] S. Biggs, M. Chow, C.F. Zukoski, F. Grieser, The role of colloidal stability in the

formation of gold sols, in, Elsevier, 1993.

[510] S. Biggs, P. Mulvaney, C. Zukoski, F. Grieser, Study of anion adsorption at the gold-

aqueous solution interface by atomic force microscopy, Journal of the American Chemical

Society, 116 (1994) 9150-9157.

246

[511] S. Link, M.A. El-Sayed, Size and temperature dependence of the plasmon absorption of

colloidal gold nanoparticles, The Journal of Physical Chemistry B, 103 (1999) 4212-4217.

[512] D.L. Van Hyning, C.F. Zukoski, Formation mechanisms and aggregation behavior of

borohydride reduced silver particles, Langmuir, 14 (1998) 7034-7046.

[513] P.V. Kamat, M. Flumiani, G.V. Hartland, Picosecond dynamics of silver nanoclusters.

Photoejection of electrons and fragmentation, The Journal of Physical Chemistry B, 102 (1998)

3123-3128.

[514] H. Taha, Z.-T. Jiang, D.J. Henry, A. Amri, C.-Y. Yin, A.B. Alias, X. Zhao, Improved

mechanical properties of sol-gel derived ITO thin films via Ag doping, Materials Today

Communications, (2018).

[515] Y.S. Jung, Spectroscopic ellipsometry studies on the optical constants of indium tin oxide

films deposited under various sputtering conditions, Thin Solid Films, 467 (2004) 36-42.

[516] S. Goldsmith, E. Çetinörgü, R. Boxman, Modeling the optical properties of tin oxide thin


[517] M. Suchea, S. Christoulakis, N. Katsarakis, T. Kitsopoulos, G. Kiriakidis, Comparative

study of zinc oxide and aluminum doped zinc oxide transparent thin films grown by direct

current magnetron sputtering, Thin Solid Films, 515 (2007) 6562-6566.

[518] R. Marotti, P. Giorgi, G. Machado, E. Dalchiele, Crystallite size dependence of band gap

energy for electrodeposited ZnO grown at different temperatures, Sol Energ Mater Sol Cells, 90

(2006) 2356-2361.

[519] H. He, F. Zhuge, Z. Ye, L. Zhu, F. Wang, B. Zhao, J. Huang, Strain and its effect on

optical properties of Al-N codoped ZnO films, Journal of Applied Physics, 99 (2006) 023503.

I hereby declare that the work presented in this ...€¦ · To conclude, sol-gel spin-coating...

Documents

Transcript of I hereby declare that the work presented in this ...€¦ · To conclude, sol-gel spin-coating...