Abstract
Titanium dioxide is a wide bandgap semiconductor widely used in optoelectronics. Despite its potential application in photovoltaics, the overall efficiency of fabricated solar cells using TiO2 is usually low due to losses through electron-hole recombination. Introduction of defects such as the use of dopants in the TiO2 structure has been used to overcome some of these limitations. However, some of the dopants used still acts as sites of recombination for the generated electron- hole pair. In this study, pure TiO2 and TiO2:Ge composite thin films of varying Ge concentration (5%, 10%,and 15%) were deposited on conducting glass substrates by radio frequency sputtering technique at ambient temperature of 23 ℃ -25 ℃ under optimum sputtering power of 200 W and argon flow rate of 35 sccm. Post annealing heat treatment was done on the films at 450 ℃ in air, argon and nitrogen atmosphere respectively, in an attempt to tune the structural, optical and electrical properties of TiO2:Ge films. The effect of annealing ambient on structural, optical and electrical properties for the films was later analyzed. The films annealed in nitrogen and argon appeared dark brown indicating an increase in oxygen vacancies while those annealed in air were utterly transparent. The X-ray diffraction patterns of both pure TiO2 and TiO2:Ge films showed that the films were composed of anatase and rutile phases irrespective of the annealing atmosphere with crystallite sizes ranging between 19-21 nm. Scanning Electron Microscope images of the films showed crack- free structures that had good adherence to the substrates, with the films annealed in nitrogen showing larger crystals compared to those annealed in air and argon. This is an indication of improved crystallinity. It was observed that, increase in Ge content in TiO2 matrix decreased both the optical properties and electrical properties. TiO2:Ge ratio of 85:15 recorded the lowest transmittance average of 70% in wavelengths 400-700 nm. The bandgap decreased from 3.64 eV to 3.57 eV while the electrical resistivity decreased from 1110 2cm to 2.2410 2cm . On the other hand, TiO2:Ge thin films annealed in nitrogen recorded the best optical and electrical properties with the bandgap and resistivity averaging about 3.55 eV and 5.2310 2cm respectively. Generally, good films were obtained at optimal condition of 10% Ge concentration in TiO2 matrix and annealed in N2. It was therefore recommended that TiO2:Ge thin films with 10% Ge concentration and annealed in nitrogen atmosphere could be considered as a potential photoanode in Dye Sensitized Solar Cells applications.
Table of Contents
DECLARATION ii
DEDICATION iii
ACKNOWLEDGEMENT iv
Abstract v
List of Figures ix
List of Tables xii
List of Abbreviation xiii
List of Symbols xiv
CHAPTER ONE
INTRODUCTION
1.1 Background 1
1.2 Statement of the problem 4
1.3 Objectives 5
1.3.1 Main Objective 5
1.3.2 Specific objectives 5
1.4 Significance and Justification of the Study 5
CHAPTER TWO
LITERATURE REVIEW
2.1 Introduction 7
2.2 Deposition Techniques for TiO2 Thin Films 7
2.3 Improvement of TiO2 Properties for Photovoltaic and Photocatalytic Application 8
2.3.1 Dye sensitization 8
2.3.2 Doping of TiO2 9
2.3.3 Effects of Optimization of Deposition Parameters on Properties of TiO2 10
2.3.4 Annealing TiO2 in different Ambient/Atmospheres 11
2.4 Germanium 13
2.5 Titanium dioxide: Germanium (TiO2:Ge) nanocomposite 13
CHAPTER THREE
THEORETICAL BACKGROUND
3.1 Introduction 16
3.2 Titanium Dioxide Semiconductor 16
3.3 Optical Characterization 17
3.3.1 Transmittance and Reflectance 17
3.3.2 Absorption 19
3.3.3 The Energy Bandgap 19
3.4 SCOUT Software 20
3.4.1 Drude Model 21
3.4.2 OJL Model 21
3.4.3 KIM Oscillator Model 22
3.5 Deposition Techniques of Thin Films 23
3.5.1 Sputtering Technique 23
3.5.2 Reactive DC Magnetron Sputtering 24
3.5.3 RF Magnetron Sputtering Technique 24
3.6 Thin Film Formation Process 25
3.6.1 Film Growth Models 25
CHAPTER FOUR
MATERIAL AND METHODS
4.1 Introduction 27
4.2 Acquisition of Materials and Purity Levels 27
4.3 Substrate Cleaning 27
4.4 Deposition of TiO2:Ge Thin Films 28
4.5 Optimization of Argon Flow Rate and Sputtering Power 30
4.6 Optical Characterization 31
4.7 Electrical Characterization 31
4.8 Morphological and Structural Characterization 33
4.9 Raman Spectroscopy 33
4.10 Elemental Analysis-EDXRF 34
CHAPTER FIVE
RESULTS AND DISCUSSION
5.1 Introduction 36
5.2 Raman Spectroscopy 36
5.3 Energy Dispersive X-Ray Fluorescence for TiO2:Ge Thin films 37
5.4 Effect of Annealing and Sputtering Power on Optical Properties for TiO2:Ge Thin Films 38
5.4.1 Transmittance 39
5.4.2 Absorbance 43
5.4.3 Optical Energy Bandgap 44
5.4.4 Refractive Index ( nf ) 47
5.4.5 Extinction Coefficient (kλ) 50
5.5 Influence of Sputtering Power on Electrical Resistivity for TiO2:Ge Thin Films 52
5.6 Influence of Argon Flow Rate on Optical Properties for TiO2:Ge Thin Films 54
5.6.1 Transmittance 54
5.6.2 Energy Bandgap 57
5.6.3 Refractive Index 60
5.7 Effect of Argon Flow Rate on Electrical Resistivity for TiO2: Ge Thin Films 61
5.8 Elemental Analysis for Pure TiO2 and TiO2: Ge Thin Films 64
5.8.1 Energy Dispersive X-ray Fluorescence (EDXRF) for Pure TiO2 and TiO2: Ge Thin Films 64
5.9 Structural Analysis of TiO2:Ge Thin Films 65
5.10 Morphological Analysis for TiO2: Ge Thin Films Annealed in Different Atmospheres 70
5.11 Effect of Ge Concentration and Annealing Ambient on Optical Properties for TiO2: Ge thin Films 72
5.11.1 Transmittance 72
5.11.2 Energy Bandgap 76
5.11.3 Refractive Index 81
5.12 Effect of Ge Concentration in TiO2 and Annealing Atmosphere on Electrical Resistivity of TiO2: Ge Thin Films 82
CHAPTER SIX
CONCLUSION AND RECOMMENDATION
6.1 Conclusion 85
6.2 Recommendation for Further work 86
References 87
List of Figures
Figure 3. 1 :Crystalline Phases of Titanium dioxide 17
Figure 3. 2: Transmission and reflection of a unidirectional beam of light 18
Figure 3. 3: Direct and Indirect Bandgaps 20
Figure 3. 4: The fit parameters of OJL interband transition 22
Figure 3. 5: Film formation process 25
Figure 3. 6: The three growth modes of thin films 26
Figure 4. 1: The standard substrate cleaning procedure 28
Figure 4. 2: Auto 306 sputtering chamber during deposition 29
Figure 4. 3: The tube furnace set up for annealing samples 31
Figure 4. 4: The four point probe system 32
Figure 4. 5: X-ray Fluorescent set up 35
Figure 5. 1: Raman Spectra for annealed TiO2:Ge (95:5) thin films deposited at various sputtering powers 37
Figure 5. 2: EDXRF energy spectrum of TiO2:Ge (95:5) thin films 38
Figure 5. 3:Optical transmittance for as-deposited (a) and annealed (b) TiO2:Ge (95:5) thin films deposited at different sputtering powers 40
Figure 5. 4: SCOUT fitting for transmittance spectra for annealed TiO2:Ge (95:5) thin films deposited with sputtering power of 150W (44.3 nm): Inset is SCOUT fitting transmittance graph for annealed TiO2:Ge films deposited at 75W (10.8 nm) 41
Figure 5. 5: Variation of film thickness (a) and sputtering yield (b) with of sputtering power for TiO2:Ge (95:5) films 43
Figure 5. 6: Variation of absorbance with sputtering power for TiO2:Ge (95:5) films 44
Figure 5. 7: The energy bandgaps for the annealed TiO2:Ge (95:5) films deposited at 75W and 220W (a), 120W (b) and 150W (c) 45
Figure 5. 8: Energy bandgap versus sputtering power 47
Figure 5. 9: Refractive index versus wavelength for TiO2:Ge (95:5) films deposited at various sputtering powers 48
Figure 5. 10: Refractive index versus wavelength for as-deposited and annealed TiO2:Ge (95:5) films deposited at 150W 49
Figure 5. 11: Extinction Coefficient versus wavelength for annealed TiO2:Ge (95:5) films deposited at various sputtering powers 50
Figure 5. 12: Extinction coefficient against wavelength for as-deposited and annealed TiO2:Ge (95:5) film deposited at 150W 51
Figure 5. 13: Resistivity for the as-deposited and annealed TiO2:Ge (95:5) films deposited at different sputtering powers 53
Figure 5. 14: Transmittance spectra of TiO2:Ge (95:5) films with variation in argon flow rate for as-deposited (a) and annealed samples (b) 55
Figure 5. 15: Variation of thickness (a) and sputtering yield (b) with of argon flow rates (30-60 sccm) for TiO2:Ge (95:5) thin film 57
Figure 5. 16: The energy bandgap of as-deposited TiO2:Ge (95:5) thin films deposited at 30sccm (a), 35 sccm (b), 40 sccm (c) and 60 sccm(d) 58
Figure 5. 17: Energy bandgap variation for annealed TiO2:Ge (95:5) films deposited at 30 sccm (a), 35 sccm (b), 40 sccm (c), and 60 sccm (d) 59
Figure 5. 18: Refractive index versus wavelength for TiO2:Ge (95:5) thin films at different argon flow rates 61
Figure 5. 19: Variation of resistivity for as-deposited and annealed TiO2:Ge (95:5) thin films with argon flow rate 63
Figure 5. 20: EDXRF spectra for pure TiO2 (a) and TiO2:Ge (95:5) (b) thin films. The other nanocomposites films with Ge have a similar spectrum an in (b) above 65
Figure 5. 21: XRD patterns for pure TiO2 and TiO2:Ge films annealed in (a) N2, (b) Ar and (c) Air 67
Figure 5. 22: Determination of FWHM for TiO2:Ge (90:10) thin films using 380 (004) peak 69
Figure 5. 23: The SEM micrographs images for Pure TiO2 (a1, b1 and c1) and TiO2:Ge (85:15 ) a2,b2 and c2 annealed in Nitrogen (a1 and a2), Argon (b1 and b2) and Air (c1and c2) 71
Figure 5. 24: Optical transmittance for TiO2:Ge thin films for (a) as-prepared (b) annealed in Ar,
(c) annealed N2 and (d) annealed in Air with different concentration of the Ge (from 0-15 %) 73
Figure 5. 25: Optical transmittance for pure TiO2 (a), TiO2:Ge (95:5) (b), TiO2:Ge (90:10) (c), and TiO2:Ge (85:15) (d) films annealed in different atmosphere (Air, Ar, and N2) at 450 0C 75
Figure 5. 26: Bandgap variation with Ge Concentration for TiO2:Ge 85:15 and 90:10 (a), 95:5 (b) and pure TiO (c) annealed in Argon 77
Figure 5. 27: Bandgap variation with Ge concentration for TiO2:Ge films 85:15 (a), 90:10 (b), 95:5 (c) and pure TiO2 (d) annealed in Air 78
Figure 5. 28: Bandgap variation with Ge concentration for TiO2:Ge films 85:15 (a), 90:10 (b), 95:5 (c) and pure TiO2 (d) annealed in Nitrogen 79
Figure 5. 29: Energy bandgap versus different ratios of TiO2:Ge thin films annealed in N2, Ar and Air 81
Figure 5. 30: Dependence of Refractive Index on annealing atmosphere of pure TiO2 thin films 82
Figure 5. 31: Resistivity versus Ge concentration in TiO2 for different annealing atmosphere 84
List of Tables
Table 4. 1: Deposition parameters for TiO2:Ge thin films 30
Table 5. 1: Thickness and yield rate for as-deposited and annealed TiO2:Ge films deposited at different powers 42
Table 5. 2: Resistivity and conductivity for as-deposited and annealed TiO2:Ge thin films deposited at different powers 52
Table 5. 3: Thickness and sputtering yield variation with argon flow rate for TiO2:Ge films 56
Table 5. 4: The energy bandgap for the as-deposited and annealed TiO2:Ge films deposited at various argon flow rates (30 sccm-60 sccm) 60
Table 5. 5: Resistivity and conductivity for as-deposited and annealed TiO2:Ge films deposited at various argon flow rate 62
Table 5. 6: Different percentages of TiO2:Ge films annealed in air, Ar and N2 and their contents in PPM 64
Table 5. 7 : Crystallite sizes for the 38 o (004) peak of TiO2:Ge films using Scherrer’s formula 69 Table 5. 8: The dislocation defect density for TiO2:Ge films as 70
Table 5. 9: Particle size of pure TiO2 and TiO2:Ge (85:15) annealed in N2, Ar and air 72
Table 5. 10 : Variation of thickness with Ge concentration in TiO2:Ge films annealed in air, Ar, and N2 gases. 76
Table 5. 11 : Variation of the bandgap values with Ge concentration for films annealed in air, nitrogen, and argon atmosphere 80
Table 5. 12: Resistivity variation for TiO2:Ge annealed in a different atmosphere 83
List of Abbreviation
CB Conduction Band
CVD Chemical Vapour Deposition
DSSC Dye sensitized solar cell
DC Direct Current
EDXRF Energy Dispersive X-Ray Fluorescence
eV Electron Volts
FTO Fluorine doped Tin Oxide
FWHM Full Width at Half Maximum
IR Infra-red
OJL O’Leary-Johnson
PLD Pulse Laser Deposition
PPM Parts Per Million
PVD Physical Vapour Deposition
PV Photovoltaic
RF Radio Frequency
SEM Scanning Electron Microscope
Toe Tons of oil equivalent
TiO2 Titanium dioxide
UV Ultra-Violet
VB Valence Band
XRD X-Ray Diffraction
List of Symbols
α Absorption coefficient
Ar Argon
p f Average film density
pm Bulk density
I Current
q Charge
D Crystallite size
Damping constant
m Effective mass carrier
E g Energy band gap
E Electric field
Electron mobility
k Extinction coefficient
d Film Thickness
β Full Width at Half Maximum
F Force
B Magnetic field
n Refractive index
nTiO :Ge2 Refractive index of TiO2:Ge films
nTiO2 Refractive index of dense TiO2 anatase Phase
TO Resonance position
p Oscillator Strength
O2- Oxygen vacancy
ɛ0 Permittivity of free space
h ν Photon energy
ρ Resistivity
K Shape factor
Theater
λ Wavelength
CHAPTER ONE
INTRODUCTION
1.1 Background
Thin films were first made by Busen and grove in 1852 using chemical reactions (Shinen et al., 2018). In 1857, Faraday was able to obtain a thin film by thermal evaporation (Eckortova, 1977). Recently, the theoretical and experimental study of thin films has generated a lot of scientific and technological interest because of the unique electronic and optical properties and exhibition of new quantum phenomenon. The knowledge gained from research done in the area of thin films has been applied successfully for technological development in many countries. Thin films are widely used in our current technology and their applications are likely to broaden in future. Through this advancement in technology, there has been tremendous change in a lot of sectors such as in communication, transportation and industrialization just to mention a few (Tiginyanu et al., 2016).
Optoelectronics, which is the study of the interaction of light with electronic devices, is one among the technological advancements that have been widely studied. Optoelectronics device advancement has led to tremendous development activities accompanied by dynamism. Some of the optoelectronic devices that have a wide application in the current day to day life include; Light Emitting Diodes (LEDs), solar cells, optical fibers, photodiodes and laser diodes which have brought a lot of advancement in the storage and production of data, fiber optic communication, laser pointers, CD/DVD disc reading and laser printing and scanning (Brennan, 1999).
Solar cell is one of the optoelectronic devices that has been widely studied through thin film technology. Thin films solar cells (TFSCs) are the second-generation type of solar cells that have very thin layer of thickness (few nanometers). These solar cells are relatively economical compared to the conventional silicon technology. Examples of second generation TFSCs include cadmium Telluride (CdTe), amorphous silicon (a-Si) and Copper Indium Gallium Deselenide (CIGS) (Kosyachenko, 2011). Other thin film technologies that have recently emerged are the third-generation cells including perovskite, DSSCs, nanocrystal solar cells among others (Green, 2002). Research into new materials in form of thin films has been intensified in order to fabricate simple and affordable solar cells that have the capability to generate electricity for use with improved efficiency.
Metal oxides such as TiO2, CuO, and ZnO reveal good mechanical and chemical properties and hardly show deterioration (Youssef et al., 2018). Being one of the essential wide bandgap semiconductors, TiO2 has been subjected to extensive research for decades. This is because of its attractive properties and its wide application in optoelectronic (Usha et al., 2014). Intensive and extensive research has been done on TiO2 semiconductor but still new approaches are coming up to improve its properties and synthesize quality films for better performance.
Titanium dioxide semiconductor has attractive physical, optical, chemical, and electronic properties including; high photocatalytic activity, excellent transmission in the visible light, high dielectric constant, high refractive index, non toxicity, low cost and good thermal and chemical stability (DeLoach et al., 1999). These characteristics majorly depend on the crystallinity and microstructure of TiO2. Titanium dioxide is characterized as an n-type semiconductor because of its oxygen deficiency in its structure.
Titanium dioxide thin films have been fabricated for several applications. It has been used extensively as a photoelectrode in DSSC applications (Kang et al., 2009; Tan and Wu, 2006; Okuya et al., 2002). TiO2 has also be applied as an antireflection coating, as a UV filter in cosmetics, as a photocatalyst, gas sensor as well as anti-corrosion protective coating. It can be used in ceramics, in transistors, as an a node material for Li-ion batteries and as a biocompatible component of bone implants among others (Carmichael et al., 2013; Carp et al., 2004; Kirner et al., 1990) One major limitation of titanium dioxide semiconductor in its photovoltaic application is the recombination of photogenerated charge carriers (Devi and Kumar, 2011). This may be associated with the impurities, defects or surface imperfections of the crystal structure. Generally, recombination reduces the efficiency of the solar cell fabricated (Choi et al., 1994a). In addition, being a wide bandgap semiconductor, TiO2 can only be activated upon irradiation with rays from the UV region (≤ 380 nm). Ultraviolet light accounts for < 10% of the solar spectrum. This limits the practical efficiency for solar application and therefore need for broadening the spectral response to the visible range.
Research into various ways of improving the photovoltaic performance of TiO2 is underway. Among the various strategies adopted include increasing the porosity in the TiO2 structure. This reduces the rate of recombination at the grain boundaries. The incorporation of other additional component such as metals and metal ions in titanium dioxide structure has proved to be among the best techniques used to control its properties for various applications. There are two ways of doping elements in TiO2, substitution and interstitial doping (Aourag et al., 1993). In interstitial doping, the dopant fits in the empty space between the TiO2 lattices while in substitution doping, either oxygen or titanium are replaced within the lattice (Lynch et al., 2015). Dopants increase TiO2 conductivity by providing free carriers hence reducing the resistivity. In addition, dopants also introduce defects levels in the crystal symmetry which increases the absorption of visible light. Transition metals have proved to enhance the properties of TiO2 semiconductor. However, these metals still act as sites of recombination of the photogenerated electron-hole pair, results in thermal instability in TiO2 structure and cause photocorrosion of the material. This lowers the general quantum performance of the fabricated device (Zeng et al., 2007).
Apart from doping, other approaches to introduce defects in the TiO2 structure are mediation through incorporation of Ti3+ and oxygen vacancies by reduction (Zheng et al., 2013). Most researchers employ the use of hydrogen reducing environment and annealing films at elevated temperatures to produce defective TiO2 structure (Naldoni et al., 2012; Chen et al., 2011). However, defective TiO2 nanoparticles can also be achieved by annealing amorphous TiO2 nanoparticles in an oxygen deficient environment such as in a vacuum, argon or nitrogen environment (Tian et al., 2015).
Recent research demonstrated that formation of TiO2 composites nanostructures could also improve the properties of the semiconductor. Metals and non- metals nanoparticles are often introduced in TiO2 matrix to increase the separation of photo-induced charge carriers during photocatalytic processes in areas of photocatalysis and solar energy conversion. Although many elements have been proposed for this purpose, carbon and its allotropes remain to be the primary choice due to their large surface area which yield an increase in adsorbed pollutants and also exhibit a large electron storage capacity which may accept electrons excited by photons thus retarding the recombination process (Woan et al., 2009). So far, composite film of graphine/TiO2 has been used as a photoanode in the fabrication of DSSC and yielded a solar cell with improved efficiency (Tsai et al., 2011). Other composites such as TiO2/carbon nanotubes TiO2/mesoporous carbon have produced solar cells with improved efficiencies (Yin et al., 2013; Jang et al., 2004).
Germanium element, being among the carbon family, has also shown great potential when used as a composite with TiO2 in fabrication of DSSC. Chatterjee (2008a), studied TiO2:Ge nanocomposite as a photovoltaic material. From his findings, it was suggested that TiO2:Ge composite is a promising material for fabrication of DSSC. Titanium: Germaniun (TiO2:Ge) composite has not been widely studied and therefore in this work, TiO2:Ge thin films were deposited using rf magnetron sputtering and thereafter, the amorphous TiO2:Ge thin films were annealed in nitrogen, argon and air atmospheres to tailor their properties. The structural, electrical and optical properties of different ratios of TiO2:Ge thin films annealed in different atmospheres were then investigated under optimum sputtering power and argon flow rate.
1.2 Statement of the problem
Titanium dioxide semiconductor has been applied in many areas including optoelectronics. Despite its potential application in photovoltaic, the fast recombination of the photogenerated electron-hole pair both on the surface and inside its lattice reduces its practical use. In addition, being a wide bandgap semiconductor, TiO2 can only be activated upon irradiation with rays from UV region. Ultra-violet light accounts for less than 10% of the solar spectrum and therefore, this limits its practical efficiency for solar application and hence the need to broaden the spectral response to the visible region. Doping TiO2 with other elements have proved to be effective in countering the two shortcomings mentioned above, but still some dopants especially the transition metals act as sites of recombination of the photogenerated charge carriers, some cause thermal instability in the TiO2 structure and others results in photocorrosion of the material. Formations of TiO2 composites as well as annealing the films in oxygen deficient environment are among new approaches that are being used to improve the properties of TiO2 semiconductor. Not much attention has been given to Ge semiconductor, as a composite of TiO2 and therefore, there is need to study TiO2:Ge composite and explore its properties. Additionally, the potential of annealing these films in different atmospheres in order to tune their properties further has not been exploited. This work, therefore, focuses on the electrical, structural and optical properties of rf magnetron sputtered TiO2:Ge thin films annealed in air, argon and nitrogen for photovoltaic application.
1.3 Objectives
1.3.1 Main Objective
This study aims at investigating the effect of annealing ambient on the structural, optical and electrical properties of TiO2:Ge thin films for photovoltaic application.
1.3.2 Specific objectives
i. To evaluate the influence of annealing ambient on structural and morphological properties of TiO2:Ge thin films.
ii. To study the effect of annealing atmospheres on the optical and electrical and properties of TiO2:Ge thin films.
iii. To evaluate the effect of Ge concentration on structural, optical and electrical properties of TiO2:Ge thin films.
1.4 Significance and Justification of the Study
Titanium dioxide semiconductor and its attractive characteristics have been the focus of many studies, due to its wide applications in photocatalytic and photovoltaic devices. Despite its potential application, the fast recombination of the photo-induced charge carriers on the surface and inside its lattice limits its practical use. Introducing germanium nanodots in TiO2 matrix could result in an attractive semiconductor with improved properties that can be applied as an antireflection coating in devices like LED, smart windows and also as a photoanode layer in DSSC. Germanium exhibits a good electron transfer due to its narrow bandgap and this widens its use in solar cells and in other optoelectronic devices. Also, germanium has a remarkable absorption in the infrared region and also has a low sintering temperature which increases the inter-particle contact leading to improved electron transfer. However, germanium as the material has not been widely exploited and little has been reported about it. Additionally, rf magnetron sputtering was the most preferred method of depositing TiO2:Ge thin films because of its ability to prevent charge from building up on the target and also its ability to yield high deposition at a relatively low pressure.
This study therefore provides more information on the properties of TiO2:Ge thin films annealed in air, argon and nitrogen atmospheres. Radio frequency magnetron sputtering technique was used in the deposition of the films and was tailored to obtain the desired optical, structural and electrical properties of the films for photovoltaic application.
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