ABSTRACT
CdS films have received a lot of attention attributed to their high transparency, high absorption coefficient, high electron affinity, and outstanding photoconductive property. So far, the best performance of thin film solar cells has been achieved using ultra-thin polycrystalline CdS as a buffer layer. While CdS thin film is the best choice for use as a window/buffer layer, it still experiences optical losses in the low-wavelength region of the solar spectrum due to its low bandgap (2.42 eV). Previous research has shown that nickel widens the bandgap of some semiconductors such as ZnO, CdS, and Sb2S3. Therefore, nickel doping of CdS could be a feasible way of widening its bandgap. The current study is intended to enhance the optical properties of CdS, hence the performance of the solar cells and optoelectronic devices by doping with nickel during film growth to widen the bandgap of CdS. CdS:Ni thin films were synthesized using chemical bath deposition (CBD) with different concentrations of Cd2+ and Ni2+ (15 wt%, 25 wt%, 35 wt%, and 45 wt%). The films were prepared from an aqueous solution of 0.1M cadmium chloride, 1M thiourea, 0.05M nickel (II) chloride, 1M triethanolamine (complexing agent), and 35 wt% ammonia solution (pH regulator). The pH of the reaction bath was ≈11. The samples were annealed in air at varying temperatures (150 – 450 ℃). The influence of nickel concentration and annealing temperatures on structural and optical properties of CdS thin films was studied. The incorporation of nickel into the CdS structure was recognized using X-ray diffraction (XRD). Optical properties, reflectance, and transmittance, in the range 200 nm-1500 nm were determined by UV-VIS-NIR spectrophotometer. The generated data were used to calculate other optical and solid-state properties like extinction coefficient (k), bandgap (Eg), absorption coefficient (α), refractive index (n), and Urbach energy (EU). The films were found to be polycrystalline and exhibited a mixed-phase structure (cubic and hexagonal structures). It was further observed that the diffraction peaks shifted slightly to the lower angle with increasing Ni2+ concentration. This could be as a result of compressional micro-stress in the CdS lattice, due to the difference in ionic radii of Cd2+ ion and Ni2+ ion. The transmittance of the films was found to increase with an increase in both dopant concentration and annealing temperature. This may be as a result of the improvement in the crystallinity of the films with increasing dopant concentration and annealing temperature. The bandgap of as-prepared CdS:Ni was observed to widen as the nickel concentration was increased. This could be due to donor electrons occupying the states at the bottom of the conduction band blocking thus the low energy transitions (Burstein- Moss effect). On annealing the films, their bandgaps decreased when the temperature was raised upto 250 ℃ and then increased with further annealing at 350 and 450 ℃. The decrease in the energy bandgap could be attributed to the increase in the grain size leading to denser films with lower bandgaps. The increase in the energy bandgap of films annealed at 350 and 450 ℃ could be attributed to the phase transition from cubic (zinc-blend) to hexagonal (wurtzite) structure. The absorption coefficients for all CdS:Ni thin films were found to be greater than 104 cm-1 in the visible region (380 nm to 780 nm) and near-infra red (780 and 2500 nm) regions which confirmed that the films have a direct optical energy gap. CdS:Ni thin films with 25 wt % and annealed at 250 ℃ were found to be the most appropriate films for use in solar cell as window/buffer layers as they recorded the highest transmittance with minimum optical properties (of Urbach energy of 0.16 and the lowest extinction coefficient) and had negative charges as the majority charge carriers. To realize higher conversion efficiencies in thin-film solar cells using CdS as window/buffer, we recommend further studies on film thickness and composition of the CdS:Ni thin films by varying the deposition time, pH of the solution, the concentration of the reagents, and temperature of the reaction bath.
TABLE OF CONTENTS
DECLARATION ii
ABSTRACT iii
CONTENT v
LIST OF TABLES ix
LIST OF FIGURES xi
ACKNOWLEDGEMENTS xiv
DEDICATION xv
CHEMICAL SYMBOLS xvi
ABBREVIATIONS xviii
CHAPTER 1
INTRODUCTION
1.0 Background
1.1.1 Why thin-film materials? 3
1.1.2 Materials for buffer/window layers and how they affect the performance of thin film solar cells 4
1.2 Statement of the Problem 6
1.3 Justification and Significance of the Study 6
1.4 Aim and Objectives 7
1.4.1 Aim 7
1.4.2 Objectives 7
CHAPTER 2
LITERATURE REVIEW
2.1 Doping of Semiconductor Materials 8
2.2 Doping of Cadmium Sulphide 9
2.3 Nickel Doping of CdS Thin Films 11
CHAPTER 3
THEORETICAL FRAMEWORK
3.1 Introduction: Cadmium Sulphide 15
3.1.1 Chemical Bath Deposition 16
3.1.2 Principle of chemical bath deposition (CBD) technique 18
3.1.3 Thin film deposition mechanisms in chemical bath deposition (CBD) technique 18
3.2 Chemical Bath Deposition of Ni-doped CdS Thin Films 20
3.2.1 The simple ion-by-ion mechanism 20
3.2.2 The simple cluster (hydroxide) mechanism 21
3.2.3 The complex decomposition ion-by-ion mechanism 21
3.2.4 The complex decomposition cluster mechanism 21
3.3 Factors Influencing Chemical Bath Deposition Process 22
3.3.1 The pH of a solution 22
3.3.2 Temperature of the reaction bath 22
3.3.3 Nature and concentration of reactants and complexing agent 22
3.3.4 Concentration of complexing agent 23
3.3.5 Spacing of the substrates 23
3.3.6 Duration of the reaction 23
3.3.7 Nature of substrates 24
3.4. Optical Characterization of Thin Films 24
3.4.1 Reflectance (R), transmittance (T), and absorbance (α) 24
3.4.2 Absorption coefficient 24
3.4.3 Bandgap 25
3.4.4 The extinction coefficient (k ) 25
3.4.5 Refractive index 26
3.4.6 Urbach Energy 26
3.5 Hall Effect 27
3.6 X-ray Diffraction Analysis 29
3.7 Energy Dispersive X-ray Fluorescence spectrometer (EDXRF) 31
CHAPTER 4
EXPERIMENTAL PROCEDURES
4.0 Introduction 33
4.1 Substrate Cleaning and Process of Film deposition 33
4.1.1 Substrate cleaning 33
4.1.2 Chemical bath deposition of Ni-doped CdS thin films 33
4.2 Measurement of Thin Film Thickness 35
4.3 Annealing of the Deposited Thin Film 36
4.4 Optical Characterization of Thin Films 36
4.4.1 Reflectance and transmittance measurements 36
(i) Bandgap 36
(ii) The extinction coefficient (k ) 36
(iii) Refractive index 37
(iv) Urbach Energy 37
4.5 Polarity of charge carriers 37
4.6 Structural analysis 38
4.7 Elemental analysis 39
CHAPTER 5
RESULTS AND DISCUSSIONS
5.0 Introduction 40
5.1 Compositional Analysis 40
5.2 Structural Analysis 43
5.2.1 Influence of Ni2+ concentration on CdS thin films 43
5.2.2 Influence of post-deposition annealing treatments on CdS:Ni thin films 45
5.2.3 Crystallite size 50
5.3 Optical Analysis 52
5.3.1 Transmittance and reflectance 52
5.3.2 Optical bandgap 58
5.3.3 Absorption coefficients 65
5.3.4 Refractive index (n) and Extinction coefficient/absorption index (α) 67
5.4 Urbach Energy 71
5.4.1 Influence of Ni2+ concentrations on Urbach energy of Ni-doped CdS films 71
5.4.2 Influence of annealing temperatures on Urbach energy of Ni-doped CdS thin films 73
5.5 Polarity of Charge Carriers 80
CHAPTER 6
6.0 CONCLUSION AND SUGGESTIONS FOR FURTHER WORK
6.1 Conclusions 82
6.2 Suggestions for Further Work 84
REFERENCES 85
APPENDIX: CRYSTALLITE SIZE OF UNDOPED AND CdS:Ni THIN FILMS PREPARED BY CHEMICAL BATH DEPOSITION AND ANNEALED AT VARIOUS TEMPERATURES 94
LIST OF TABLES
Table 4. 1: Amounts of reagents used in the preparation of Ni-doped CdS thin films 35
Table 4. 2: Experimental conditions used for the measurement of the samples 39
Table 5. 1: Calculated crystallite grain size of the as-prepared 25 wt% Ni-doped thin films 50
Table 5. 2: Calculated crystallite grain size of the 25wt% Ni-doped thin films annealed 250 ℃ 51
Table 5.3: Calculated crystallite grain size of 25 wt% Ni-doped CdS thin films annealed at 350 ℃ 51
Table 5. 4: Calculated crystallite grain size of 25wt% Ni-doped CdS thin films annealed at 450 ℃ 52
Table 5. 5: Summary of Bandgap values for undoped and Ni-doped CdS thin films annealed at various temperatures 59
Table 5. 6: Bandgaps and Urbach energies of undoped CdS and Ni-doped CdS thin film 73
Table 5. 7: Summary of bandgap and urbach energies of pure CdS and Ni-doped CdS thin films annealed at various temperatures: (a) Pure CdS, (b) 15 wt% Ni-doped CdS, (c) 25 wt% Ni-doped CdS (d) 45 wt% Ni-doped CdS 78
Table 6. 1: Crystallite size of the as-grown undoped CdS thin films 94
Table 6. 2: Crystallite grain size of the as-prepared 35 wt% Ni-doped thin films 94
Table 6. 3: Crystallite grain size of the as-prepared 45 wt% Ni-doped thin films 95
Table 6. 4: Crystallite grain size of undoped CdS thin films annealed at 250 ℃ 95
Table 6. 5: Crystallite grain size of 15 wt% Ni-doped thin films annealed at 250 ℃ 96
Table 6. 6: Crystallite grain size of 35 wt% Ni-doped thin films annealed at 250 ℃ 96
Table 6. 7: Crystallite grain size of 45 wt% Ni-doped thin films annealed at 250 ℃ 97
Table 6. 8: Crystallite grain size of undoped CdS thin films annealed at 350 ℃ 97
Table 6. 9: Crystallite grain size of the 15 wt% Ni-doped thin films annealed at 350 ℃ 98
Table 6. 10: Crystallite grain size of 35 wt% Ni-doped thin films annealed at 350 ℃ 98
Table 6. 11: Crystallite grain size of the undoped CdS thin films annealed at 450 ℃ 99
Table 6. 12: Crystallite grain size of 15wt% Ni-doped thin films annealed at 450 ℃ 99
LIST OF FIGURES
Figure 3. 1: Cubic structure of CdS (Source: chem.libretexts.org) 15
Figure 3. 2: Hexagonal structure of CdS (Source: chem.libretexts.org) 16
Figure 3. 3: Chemical bath deposition set up 18
Figure 3.4: Hall probe configuration for magnetic field measurement (Source: http://hyperphysics.phy-astr.gsu.edu) 27
Figure 3. 5: Hall Effect configuration (Source: (Popovic, 2003) 28
Figure 3. 6: Interaction of incident rays with a material producing constructive interference 30
Figure 3. 7: Schematic of a typical Energy Dispersive X-ray Fluorescence (EDXRF) spectrometer 31
Figure 4. 1: Set up for chemical bath deposition 34
Figure 4. 2: Hall Effect set up 38
Figure 5. 1: EDXRF spectra of the substrate used to deposit the samples for Elemental analysis 40
Figure 5. 2: EDXRF spectra of as-grown undoped and Ni-doped CdS thin films with concentrations of nickel from 0 wt% to 45 wt% 41
Figure 5. 3: X-ray diffraction pattern of as-grown Ni-doped CdS thin films obtained at various nickel concentrations 43
Figure 5. 4: GIXRD pattern of as-grown Ni-doped CdS thin films showing peak (111) slightly shifting to the lower angle with increasing Ni2+ concentration 44
Figure 5. 5: X-ray diffraction pattern of 25wt% Ni-doped CdS obtained at various annealing temperatures 46
Figure 5. 6: X-ray diffraction pattern of Ni-doped CdS thin films annealed at 250 ℃ 47
Figure 5. 7: X-ray diffraction pattern of undoped and Ni-doped CdS annealed at 350 ℃. 48
Figure 5. 8: X-ray diffraction pattern of Ni-doped CdS annealed at 450 ℃ 49
Figure 5. 9: Transmittance and reflectance spectra of as-grown undoped and Ni-doped CdS thin films synthesized by chemical bath deposition 53
Figure 5. 10: Transmittance and reflectance spectra of 25 wt% Ni-doped CdS thin films prepared using chemical bath deposition and annealed at various temperatures 54
Figure 5. 11: Transmittance and reflectance spectra of undoped and Ni-doped CdS thin films prepared using chemical bath deposition and annealed at 150 ℃ 55
Figure 5. 12: Transmittance and reflectance spectra of Ni-doped CdS thin films prepared using chemical bath deposition and annealed at 250 ℃ 56
Figure 5. 13: Transmittance and reflectance spectra of Ni-doped CdS thin films prepared using chemical bath deposition and annealed at 350 ℃. 57
Figure 5. 14: Transmittance and reflectance spectra of Ni-doped CdS thin films prepared using chemical bath deposition and annealed at 450 ℃ 58
Figure 5. 15: Graphs of (αhυ)2 against hυ for Ni-doped CdS thin films with varying nickel concentration (wt%) annealed at various temperatures. (a) As-deposited Ni-doped CdS films (b) Ni-doped CdS films annealed at 150 ℃ (c) Ni-doped CdS films annealed at 250 ℃ (d) Ni-doped CdS films annealed at 350 ℃ 60
Figure 5. 16: Graphs of (αhυ)2 against hυ for Ni-doped CdS thin films with varying nickel concentration (wt%) annealed 450 ℃ 61
Figure 5. 17: Illustration of Burstein-Moss effect (Source: Wikipedia.org) 62
Figure 5. 18: Bandgap energy dependence on the thermal annealing temperature of nickel doped cadmium sulphide (CdS:Ni) 63
Figure 5. 19: Comparative bar graph showing the band-gap energy of nickel doped CdS thin films prepared using chemical bath deposition and annealed at varied temperature (150 ℃ – 450 ℃).64
Figure 5. 20: Absorption coefficients of undoped and Ni-doped CdS thin films annealed at various annealing temperatures (a) Ni-doped CdS thin films annealed at 250 ℃ (b) Ni-doped CdS thin films annealed at 350 ℃ (c) Ni-doped CdS thin films annealed at 150 ℃ (d) Ni-doped CdS thin films annealed at 450 ℃ (e) As-grown Ni-doped CdS thin films (f) 25 wt% Ni-doped CdS thin films annealed at various temperature 66
Figure 5. 21: Plots of refractive index Ni-doped CdS thin films annealed at various temperatures (a) As-prepared Ni-doped CdS thin films (b) Ni-doped CdS thin films annealed at 150 ℃ (b) Ni- doped CdS thin films annealed at 150 ℃ (c) Ni-doped CdS thin films annealed at 250 ℃ (d) Ni- doped CdS thin films annealed at 350 ℃ (e) Ni-doped CdS thin films annealed at 450 ℃ (f) 25 wt% Ni-doped CdS thin films annealed at various temperatures 68
Figure 5. 22: Plots of extinction coefficient versus wavelengths of Ni-doped CdS thin films annealed at various temperatures: (a) As-prepared Ni-doped CdS thin films (b) Ni-doped CdS thin films annealed at 150 ℃ (b) Ni-doped CdS thin films annealed at 150 ℃ (c) Ni-doped CdS thin films annealed at 250 ℃ (d) Ni-doped CdS thin films annealed at 350 ℃ (e) Ni-doped CdS thin films annealed at 450 ℃ (f) 25 wt% Ni-doped CdS thin films annealed at various temperatures 70
Figure 5. 23: Urbach energy of as-prepared Ni-doped: (a) undoped CdS (b) CdS with 15 Ni wt% (c) CdS with 25 Ni wt% (d) CdS with 45 Ni wt% 72
Figure 5. 24: Urbach energy of Ni-doped CdS thin films annealed at 150oC: (a) undoped CdS (b) CdS with 15 Ni wt% (b) CdS with 25 Ni wt% (c) CdS with 45 Ni wt% 74
Figure 5. 25: Urbach energy of Ni-doped CdS thin films annealed at 250oC: (a) undoped CdS (b) CdS with 15 Ni wt% (b) CdS with 25 Ni wt% (c) CdS with 45 Ni wt% 75
Figure 5. 26: Urbach energy of pure Ni-doped CdS thin films annealed at 350 ℃: (a) undoped CdS (b) CdS with 15 Ni wt% (c) CdS with 25 Ni wt% (d) CdS with 45 Ni wt% 76
Figure 5. 27: Urbach energy of Ni-doped CdS thin films annealed at 450 ℃: (a) undoped CdS (b) CdS with 15 Ni wt% (c) CdS with 25 Ni wt% (d) CdS with 45 Ni wt% 77
Figure 5. 28: Comparative bar chart showing Urbach energy of nickel doped CdS thin films prepared using chemical bath deposition and annealed at varied temperature (150 ℃ – 450 ℃).79
Figure 5. 29: Hall Effect measurements for Ni-doped CdS thin films: (a) undoped CdS (b) CdS with 25 Ni wt% (c) CdS with 15 Ni wt% (d) CdS with 45 Ni wt% 80
CHEMICAL SYMBOLS
Al Aluminium
Cd Cadmium
CdCl2.21/2H2O Hydrated Cadmium Chloride
CdO Cadmium Oxide
Cd(OH)2 Cadmium Hydroxide
CdS Cadmium Sulphide
CdTe Cadmium Telluride
CH4N2S Thiourea
CIGS Copper Indium Gallium Selenide
Co Cobalt
CO Carbon Monoxide
CO2 Carbon Dioxide
Cr Chromium
Cu Copper
F:SnO2 Fluorine-doped Tin Oxide
Fe Iron
Ga Gallium
H Hydrogen
In2S3 Indium Sulphide
In Indium
Li Lithium
Mn Manganese
Mn+ Metal salt
NH3 Ammonia solution
Ni Nickel
Ni3S2 Nickel sulphide
Ni3S4 Nickel sulphide
NiCl2.6H2O Hydrated Nickel (II) Chloride
O Oxygen
OH- Hydroxide
P2O5 Diphosphorus Pentoxide
Se Selenium
S Sulphur
SO2 Sulphur dioxide
Zn Zinc
ZnS Zinc Sulphide
ZnSe Zinc Selenide
Xm- Chalcogenide ion
ABBREVIATIONS
α Absorption coefficient
β Line broadening in radians
βT Band tailing parameter
θB Braggs angle
λ Wavelength
μ Carrier mobility
ρ Resistivity
σop Optical conductivity
ρ Resistivity
υ Drift velocity
Å Angstrom (10−10 m)
A Constant
Al Ligand species
Abs Absorption
B Magnetic field
CAGR Compound Annual Growth Rate
CBD Chemical Bath Deposition
Dt Thickness of the film
DMS Diluted Magnetic Semiconductors
Dp Average crystallite size
d Interplane spacing
Fm Magnetic force
Dp Average Crystallite size,
Fe Electric force
Ee Electron charge
E Electric field
Eg Bandgap
EF Fermi energy
EU Band energy
EDXRF Energy Dispersive X-ray Fluorescence
FETs Spin Field Effect Transistors
FWHM Full Width at Half Maximum
GDP Gross Domestic Product
GIXRD Grazing incidence x-ray diffraction
I Current
IRENA International Renewable Energy Agency
ISE Institute for Solar Energy Systems
J Current density
JCPDS Joint Committee on Powder Diffraction Standards
K Constant
k Extinction co-efficient
KeV Kilo electron Volts
Ki Instability constant
Ksp Solubility product
kWp Kilowatt peak
l length
LED’s Light Emitting Diodes
Log Logarithm
m Constants
n Refractive index
nc Concentration
NEA Nuclear Energy Resource
ns Sheet concentration
oC Degrees Celsius
PL Photoluminescence
PPC Persistence Photo Conductivity
PV Photovoltaic
q Charge
R Reflectance
RB Magnetoresistance
RH Hall coefficient
Rs Sheet resistance
RX9 Graphite crystal
t Time
T Transmittance
TCO Transparent Conducting Oxide
TEA Triethanolamine
TM Transition Metals
UV Ultra Violet
VIS-NIR Visible-Near Infra-Red
wt% Weight percentage
XRD X-Ray diffraction
VH Hall voltage
VOC Open circuit voltage
CHAPTER 1
INTRODUCTION
1.0 Background
Production and the supply of clean, safe, and sustainable energy is one of the critical challenges facing current civilization (Balzani and Armaroli, 2010). The rapid population and economic growth have significantly increased the energy demand, making it one of the essential resources for humankind’s existence. Currently, fossil fuels constitute over 80% of the total energy consumed by humankind (World energy outlook, 2018). Exploitation and combustion of fossil fuel to meet our energy demand has been very convenient but has caused harm to the environment and human health through the emission of greenhouse gases (e.g. CO2, CO, SO2, and P2O5) and toxic waste, climate change, ozone depletion, water and land degradation due to oil spills and coal- ash spills and air pollution (Casper, 2010). Heat-trapping properties of carbon dioxide released into the atmosphere contributes to global warming resulting in extreme weather such as heavy rain, searing and prolonged heat waves, intense hurricanes, and recurring droughts (Bradford, 2006; Balzani and Armaroli, 2010). Black carbon particles are also a significant contributor to global climate change, as it strongly absorbs sunlight and has a heating effect on the atmosphere. Black carbon particles remain in the atmosphere for only a few weeks, therefore reducing emissions would instantly lower the rate of global warming. There is a need to explore new sources of energy that are environmentally friendly, inexhaustible, and available worldwide (Bradford, 2006; Balzani and Armaroli, 2010).
With the uneven global distribution of fossil-fuel resources on earth (McGlade and Ekins, 2015), renewable energy technologies promise to slow our dependence on the expensive, scarce, and environmentally unfriendly fossil fuels. Renewable energy is a potential source of clean energy for the reason that they preserve natural resources; their operating costs are low, low carbon emissions, and are sustainable (IRENA, 2012). Lately, there has been an increase in the usage of renewable energy because of the high cost of fossil fuels and concerns over greenhouse gases and toxic waste.
Solar energy is a renewable source of energy with high potentials for meeting global energy consumption. It is an inexhaustible source of energy at any reasonable period for human civilization. Besides, it is abundant even in a situation of doubling or tripling present energy demand (Balzani and Armaroli, 2010). Photovoltaics are used to produce electricity by directly transforming light energy from the sun using the properties of suitable semiconducting materials. Solar power is also very convenient in remote areas in both terrestrial and extra-terrestrial, it is unlimited, cheap, clean, renewable, and well distributed in every part of the world. Energy from the sun reaching the surface of the earth surpasses our annual energy needs, given that one hour of sunlight could provide our annual needs for one year (Fyfe et al., 1993). The benefits of solar energy over other renewable sources of energy include environment-friendliness; universal and versatile; low-cost maintenance; and a short energy payback period (Ayieko et al., 2013).
Photovoltaic (PV) Systems are categorized into three generations, namely: first, second, and third- generation Photovoltaics Systems (IRENA, 2012). First-generation PV systems use the wafer- based crystalline silicon technology and are fully commercialized. Second-generation PV systems are based on thin-film technologies while third-generation PV systems are still under demonstration and have not been widely commercialized, e.g., intermediate-level cells (impurity PV and intermediate band solar cells), concentrator systems, thin-film tandems, a-Si tandems, Si nanostructure tandems, organic PV Systems, multi-colour (tandem) cells, group III-V tandems and hot electron -carrier cells (Conibeer, 2007).
Over 80% of commercially available solar cells are based on silicon technology which needs high purity silicon material. The best laboratory solar cell efficiencies for mono-crystalline and multi- crystalline silicon wafer-based technologies are 26.7% and 22.3%, respectively (Green et al., 2018; Fraunhofer ISE: Photovoltaics Report, 2019). The thickness of the absorber layer for a crystalline solar cell is around 300 μm. About 10 kilograms to 15 kilograms of silicon is required to produce 1 kWp (Shah et al., 1995). Production of silicon solar cells requires the use of high purity raw material which makes it expensive to produce. Unlike silicon solar cells which require about 200μm - 300μm thick active layer, an active layer of a few micrometers (μm) is necessary to produce thin-film solar cells (Chopra et al., 2004). Thin-film solar cells promise thinner, less expensive, and more flexible technology compared to silicon solar systems. Lately, efforts have been made to develop and research more on thin-film solar cells because it is easy to manufacture.
Recently, thin-film solar cell laboratory conversion efficiencies have progressively improved promising reduced manufacturing costs compared to silicon technology (Green et al., 2018).
Over the last 15 years, the global Compound Annual Growth Rate (CAGR) of photovoltaic installations was above 24%. The annual new solar PV system installations were 29.5 GW in 2012 and 99.8 GW in 2017 (Jäger-Waldau, 2018). The increase in new PV system installations during this period is attributed to a global decrease in the cost of PV systems and also as a result of shifting to large-scale utility systems. The market share of thin-film technologies in 2017 was about 5% of the total annual production (Fraunhofer ISE: Photovoltaics Report, 2019). The highest lab efficiency in thin-film technology is 22.9% for CIGS (CuIn1‐x GaxSe2) and 21% for CdTe solar cells (Green et al., 2018). In the last ten years, CdTe module solar energy conversion efficiency increased from 9% to 16% (Fraunhofer ISE: Photovoltaics Report, 2019).
1.1.1 Why thin-film materials?
Thin film semiconductors can be “grown” on a substrate through “the random nucleation and growth processes of individually condensing or reacting atomic, ionic or molecular species” (Chopra et al., 2004). Deposition parameters strongly influence the structural and physical properties of these materials. The different microstructure of films, i.e., epitaxial growth, nanocrystalline, amorphous, and extremely oriented films, can be obtained by using various deposition methods and substrates while varying the deposition parameters. The substrate can be rigid, flexible, insulator, or metal with varying shapes, sizes, areas, and structures (Chopra et al., 2004).
There exist a variety of deposition methods for thin films. These include; electrophoresis, chemical bath deposition, screen printing, sputtering, chemical vapour deposition, resistive evaporation, spray pyrolysis, laser ablation, ion-exchange reactions, and glow discharge decomposition. Thin films can be fabricated using a single method or a combination of methods. Doping of materials can also modify the characteristics of thin films in desired and controlled ways. Desired profile in the growing films can be obtained by controlling the doping profile (Chopra and Das, 1983). Different types of electronic junctions are possible too for thin film materials. Lattice constants and graded bandgap can be engineered to develop designer solar cells.
1.1.2 Materials for buffer/window layers and how they affect the performance of thin film solar cells
Window and buffers layers play a fundamental role in thin-film solar cells. Buffer and window layers minimize recombination losses while assisting in achieving large band-bending. The buffer layer also guards the junction against the chemical reactions and damage while enhancing band alignment of the cell and creating a wide depletion region with the absorber layer (Contreras et al., 2002). Therefore, for high-efficiency thin-film solar cells, the bandgaps of the window and buffer layers must be wide, and their thicknesses must be as small as possible to retain low series resistance.
Solar cells made with optimized and appropriate buffer layers produce better optical (lattice) matches with the absorber and thereby reduce recombination at the interface (Siebentritt, 2004). High-efficiency thin-film solar cells with large short circuit current and open-circuit voltage can be produced by optimizing buffer layers (Von Roedern and Bauer, 1999). Buffer layers in heterojunction solar cells play a role in adjusting the interface charge, to ensure the position of the Fermi level at the interface above the bandgap centre of the absorber (Siebentritt, 2004).
The CdS (direct bandgap ≈ 2.42 eV) thin film is an excellent heterojunction solar cell partner commonly used as a window/buffer material in thin-film CdTe and Cu(In,Ga)Se2 polycrystalline photovoltaic devices (Mustafa et al., 2012) due to symmetric emission spectrum, narrow and tunable bandgap, and broad, continuous excitation spectrum (Bhambhani and Alvi, 2016). CdS films have received a lot of attention because of their high transparency, high absorption coefficient, easy ohmic contact, outstanding photoconductive property, and high electron affinity. Thin-film CdS is more photochemically stable than silicon nanoparticles (Bhambhani and Alvi, 2016). Because of these exceptional optical properties, CdS is mostly used as a window or buffer layer in solar cells, photo-detector, photoconductors, photo-resistors, green lasers, electrical driven laser, light-emitting diodes, address decoder, and sensors (Mustafa et al., 2012; Bhambhani and Alvi, 2016).
Although several wide-bandgap Cd-free buffer layers materials have been explored for buffer layer, for example, Zn(O,S) (Thankalekshmi and Rastogi, 2012), Zn(OH,S) ZnO (Ennaoui et al., 1998), (Ennaoui et al., 2003; Hubert et al., 2008), ZnS (Nakada and Mizutani, 2002; Nakada et
al., 2003), ZnSe (Hariskos et al., 2005) and In2S3 (Huang et al., 2001; Bayon and Herrero, 2001) as an alternative to CdS, the best performance is attained using ultra-thin polycrystalline CdS (Alam et al., 2014).
Band offsets at the buffer/absorber interface mostly influence the functioning of a solar cell. These band offsets are greatly influenced by the deposition method because of the intermixing and chemical reactions at the interface (Nakada and Mizutani, 2002). A positive conduction band offset at the buffer/absorber interface helps to reduce interface recombination (Siebentritt, 2004). And so, there is a need to improve the conduction band alignment and to explore the influence of the buffer–absorber interface on the functioning of a solar cell.
Cadmium and sulphur atoms favourably influence the surface or near-surface chemistry of the absorber material. Sulphur is thought to passivate surface defects while the cadmium atoms diffuse into the near-surface region of the absorber film aiding in defect passivation and/or n-type doping (Elango, 2012).
Chemical Bath deposited CdS is commonly used for efficient solar cell technologies (Ballipinar and Rastogi, 2017). It is fine-grained and resistive. When used as a buffer layer, the chemical bath deposited CdS forms a high quality junction with the absorber at the same time allowing the maximum amount of light to the junction. CdS large bandgap allows transmission of longer wavelength sunlight into the absorber layer, enhancing the efficiency of the photovoltaic device. CdS buffer layer is very compatible with the crystal lattice of the absorber layer and has favourable conduction band alignment (Alam et al., 2014). Efficiencies of up to 22.3% laboratory efficiency have been reported on Cu(In,Ga)Se2 solar cells with CdS as a buffer layer (Jackson et al., 2011).
Chemical bath deposited CdS films show high resistance in the range 105Ω cm-107Ω cm (Ristova, 1998). This high resistivity needs to be reduced to at most 10Ω cm while preserving the films’ good photosensitivity. Several ways which have been used to try to minimise CdS’ dark resistivity include the annealing (in nitrogen/hydrogen atmosphere and rapid thermal annealing) and the doping of CdS with metal impurities during (in-situ doping) or after the deposition process (Khallaf et al., 2008).
Doping of CdS with impurities enables us to alter its properties to achieve desired properties, making them multifunctional both as a window and buffer layer for thin-film solar cells (Bhambhani and Alvi, 2016). Doping alters its carrier concentrations, transmittance, morphology, bandgap, magnetic properties, the density of states, electrical conductivity, etc. The dopant ions and dissimilarity typically determine the outcome of doping of these semiconductors in ionic radii with the host atoms (Elango, 2012). The dopant’s ionic radius determines whether the dopant ion could replace the desired ion in the lattice structure. The Ni2+ ion is one of the efficient dopants among the various transition metal ions suitable for altering the optical, electrical, and magnetic properties of multiple semiconductors. It is possible to incorporate Ni2+ ions into the CdS structure because of its small ionic radius (0.069 nm) and high electronegativity (1.91 Pauling) likened to that of Cd2+ ions whose ionic radius and electronegativity are 0.097 nm and 1.91 Pauling, respectively (Gellings and Bouwmeester, 1997).
In this work, we study the influence of nickel concentration and annealing temperatures on the optical and structural properties of CdS thin films prepared using the chemical bath deposition method for solar cell and optoelectronic applications.
1.2 Statement of the Problem
Studies made on CdS thin film prepared using various deposition methods have shown that the desired quality of films can be obtained by incorporating dopant impurities during film growth depending on the film applications. The structural, optical, electrical, and magnetic properties of CdS can be tailored to achieve desired characteristics suitable for various applications by doping with various amounts of dopant material. CdS is a suitable material for applications as a window and buffer layer in a thin-film solar cell structure. However, for these layers to play their roles well, their bandgap must be suitably wide enough to increase the amount of light reaching the absorber layer to enhance cell efficiency. CdS thin films experience optical losses in the low- wavelength region of the solar spectrum due to their low bandgap (2.42eV). In this study, we hope to enhance the optical properties of CdS and by extension, the performance of the solar cells and optoelectronic devices by doping with nickel to widen the bandgap of CdS.
1.3 Justification and Significance of the Study
Several approaches for the deposition of high-quality CdS thin films geared towards improving the conversion efficiency of these devices have been investigated over the years. But currently, the idea of doping the buffer/window layer has not been widely explored. Doping is a promising method for tailoring the structural, optical, and electrical properties of CdS. Previous research has shown that nickel widens the bandgap of some semiconductors such as ZnO (Thakur et al., 2013); CdS (Elango et al., 2012); and Sb2S3 (Nwofe and Agbo, 2017; Mushtaq et al., 2016). Therefore, nickel doping of CdS could be a feasible way of widening its (CdS’) bandgap; hence, improving the performance of the solar cell and optoelectronic devices. Incorporation of dopant impurities can be done during film growth or after deposition, and in our study, we will introduce the dopant impurity (nickel) during film growth using chemical bath deposition. There is relatively little research on the effect of nickel doping on CdS. The few studies available focus on the preparation of CdS:Ni nanoparticles (Bhambhani and Alvi, 2016); (Rao et al., 2011) while those that have prepared CdS:Ni thin films have used other deposition method including; sol-gel spin coating technique (Chtouki et al., 2017) and spray pyrolysis (Rmili et al., 2013). This study is expected to provide information on the effect of nickel doping of CdS thin films prepared using a simple and cheap method of deposition, namely chemical bath deposition.
1.4 Aim and Objectives
1.4.1 Aim
This study aims to synthesize and characterize Ni-doped CdS thin films by chemical bath deposition technique for use in thin-film solar cells as a window/buffer layer and also for applications in electronic and optoelectronic devices.
1.4.2 Objectives
The objectives of this study are to:
1. To analyze the effect of concentration of the nickel dopant on the structural and optical properties of Ni:CdS thin films prepared using a chemical bath deposition method.
2. To investigate the effect of annealing temperature on the structural and optical properties of Ni-doped CdS thin films prepared using a chemical bath deposition method.
3. To study electrical properties (hall effect measurements) of doped Ni:CdS thin films
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