ABSTRACT
The deposition of undoped and Y-doped CuSe, CdSe and CoSe with different dopant concentration (0.01 – 0.04mol%) and varying substrate temperature (140oC – 200oC) have been successfully carried out using spray pyrolysis technique. The effect of the yttrium doping and substrate temperature variation on the optical, electrical and structural properties of all the deposited samples were reported in this research work. For the undoped and Y-doped CuSe, the optical studies revealed an energy bandgap ranging from 3.0eV to 2.5eV and introduction of Y dopant was observed to narrow the energy bandgap of pure CuSe. The electrical studies result supported the fact that the thin films are semiconductors. The XRD pattern reveals that the deposited films exhibited a polycrystalline cubic structure with preferred orientation along (200) and improved crystallinity with yttrium doping. For variation in substrate temperature, the optical analysis showed that the samples have energy band gap value ranging from 1.44 eV to 1.8 eV. Increase in substrate temperature was found to decrease the conductivity thereby increasing the resistivity and thickness. The XRD pattern revealed a polycrystalline cubic structure with most preferred orientation along (111) plane for all the films irrespective of the substrate temperature. YCuSe deposited at 140oC was seen to exhibit the best crystallinity. In summary, we conclude that substrate temperature of 140oC is the optimum substrate temperature to grow YCuSe. For the undoped and Y-doped CdSe, the influence of Y-incorporation and variation (0.01, 0.02, 0.03 and 0.04mol%) were clearly seen. The energy bandgap was observed to decrease from 1.71eV for undoped CdSe to 1.41eV for Y-doped. Increase in Y-concentration from 0 to 0.04m0l%, led to a decrease in the resistivity of CuSe with decreasing film thickness and increasing conductivity value. The XRD patterns reveal that all the deposited films are polycrystalline in nature having hexagonal structure with the preferred orientation and highest intensity along the (100) plane. Introduction of yttrium dopant was observed to improve the crystallinity of CdSe. The general result shows that YCdSe films are good materials for the production of photovoltaic cells. For variation in substrate temperature, YCdSe deposited at 180oC exhibited better optical property when compared to other films with an energy band gap value of 1.50eV. The electrical studies, shows that increase in substrate temperature of the thin material decreases the resistivity with increasing thickness and increasing conductivity which reveals that the films are semiconducting. The XRD analysis revealed a polycrystalline hexagonal structure with most preferred orientation along (100) plane for all the films irrespective of the substrate temperature.Y/CdSe deposited at 160oC gave the best crystallinity. For the undoped and Y-doped CoSe, the optical analysis gave moderate (> 50%) transmittance with energy bandgaps ranging from 1.25eV to 1.81eV. Incorporation of yttrium dopant to the CoSe films was observed to enhance the optical features and the electrical studies reveals that the films are semiconducting materials. The XRD result revealed a polycrystalline cubic structure and introduction of higher concentration of yttrium dopant on the films enhanced the crystallinity. For variation in substrate temperature, the optical properties were found to vary with substrate temperature despite the fact that the variation was not totally linear as the film deposited at substrate temperature of 160oC deviated from the linearity in all the optical properties. The energy band gap of the deposited samples ranges from 1.25 eV – 1.75eV. The electrical analysis revealed that the substrate temperature and the film thickness varies directly with conductivity and inversely with resistivity which is one of the features of a typical semiconductor. The XRD result shows that the films are cubic polycrystalline in nature andY/CoSe grown at substrate temperature of 180oC gave the most excellent crystalline quality and a preferential orientation along (111) direction. From our findings we observe that these new materials can be used for photovoltaic purposes.
TABLE OF
CONTENTS
Title Page i
Declaration ii
Certification iii
Dedication iv
Acknowledgement v
Table of Contents vi
List of Tables xi
List of Figures xii
Abstract xviii
CHAPTER 1: INTRODUCTION
1.1 Background
of Study 1
1.2 Aims 7
1.3 Objectives 7
1.4 Scope of the Study 8
1.5 The
Significance of the Study 8
CHAPTER 2: LITERATURE REVIEW
2.1 Solar
Energy 10
2.2 Semiconductors
(SCs) 11
2.3 Types
Of Semiconductors (SCs) 11
2.3.1 Intrinsic semiconductor 12
2.3.2 Extrinsic semiconductor 13
2.3.2.1
n-type 13
2.3.2.2 p-type 14
2.4 Types of Solar Energy Conversion
Technologies 16
2.5 Thin Film Technology 17
2.6 Copper Selenide (CuSe) - Properties and
Applications 18
2.6.1 Review
of related works on undoped and doped CuSe Thin Film 20
2.7 Cadmium Selenide (CdSe) – Properties and
Applications 22
2.7.1 Review
of related works on undoped and doped CdSe Thin Film 23
2.8 Cobalt Selenide (CoSe) - Properties and
Applications 26
2.8.1 Review
of related works on undoped and doped CoSe thin film 27
2.9 Thin Film Deposition Techniques 29
2.9.1
Chemical deposition techniques 30
2.9.1.1
Chemical vapor deposition (CVD) 31
2.9.1.2 Sol-Gel 32
2.9.1.3 Spray pyrolysis
32
2.9.1.4
Chemical bath deposition (CBD) 34
2.9.1.5
Successive ionic layer adsorption and reaction (SILAR) method 36
2.9.1.6
Electrodeposition 36
2.9.2 Physical
Deposition 38
2.9.2.1 Sputtering 38
2.9.2.2 Pulsed laser deposition 39
2.9.2.3 Thermal
evaporation 40
2.9.2.4 Electron
beam evaporation
41
2.10 Description
of Material Characterization Techniques 41
2.10.1 Ultra
violet visible infra-red spectroscopy (UV-VIS) 41
2.10.2 Scanning
electron microscopy 43
2.10.3 Energy dispersive x-ray spectroscopy (EDS or
EDX) 44
2.10.4 X-ray diffraction (XRD) 45
2.10.5 Four
point probe
46
2.11 Literature
Gap 48
CHAPTER 3: MATERIALS AND METHODS
3.1 Method 49
3.2 Essential
Components of Spray Pyrolysis Technique 50
3.3 Substrate
Cleaning Procedure 50
3.4 Preparation
of Solutions 50
3.5 Experimental
Procedure 51
3.6 Spray Process/Procedure 52
3.7 Characterization
of Fabricated Films 55
3.7.1 Optical
and solid state characterization of fabricated films 55
3.7.1.1 Absorbance
(A) 55
3.7.1.2 Transmittance
(T) and reflectance (R) 56
3.7.1.3 Optical
absorption coefficient (α) 56
3.7.1.4 Energy
band gap (Eg) 57
3.7.1.5 Extinction
coefficient (k) and Refractive index (n) 57
3.7.1.6 Dielectric
constant (ε) 58
3.7.1.7 Optical
conductivity (σ0) 58
3.7.2 Electrical properties of the fabricated
films 59
3.7.3 X-ray diffraction (XRD) analysis 59
3.7.4 Surface morphological and compositional
analysis of the fabricated films 60
CHAPTER 4: RESULTS AND DISCUSSION
4.1 Optical
and Solid State Analysis of CuSe and Copper Selenide/Yttrium at Different Dopant Concentrations 61
4.1.1 Absorbance 61
4.1.2 Transmittance 62
4.1.3 Reflectance 62
4.1.4 Absorption coefficient (α) 63
4.1.5 Energy
band gap (Eg) 63
4.1.6 Extinction
coefficient (k) 64
4.1.7 Refractive index (n) 64
4.1.8 Dielectric constant (real and imaginary) 65
4.1.9 Optical
conductivity (σ) 65
4.2 Optical and Solid State Analysis of
Yttrium Doped Copper Selenide
(Y/CuSe) Thin Films Grown at
Different Substrate Temperatures. 72
4.2.1 Absorbance 72
4.2.2 Transmittance 72
4.2.3 Reflectance 73
4.2.4 Absorption coefficient (α) 73
4.2.5 Energy
band gap (Eg) 74
4.2.6 Extinction
coefficient (k) 74
4.2.7 Refractive index (n) 74
4.2.8 Dielectric constant (Real and imaginary) 75
4.2.9 Optical
conductivity (σ) 75
4.3 Optical and Solid State Analysis of CdSe
and Cadmium Selenide/Yttrium
at Different Dopant Concentrations 81
4.3.1 Absorbance 81
4.3.2 Transmittance 81
4.3.3 Reflectance 82
4.3.4 Absorption coefficient (α) 82
4.3.5 Energy band gap (Eg) 82
4.3.6 Extinction
coefficient (k) 83
4.3.7 Refractive index (n) 83
4.3.8 Dielectric constant (Real and imaginary) 84
4.3.9 Optical conductivity (σ) 84
4.4 Optical and Solid State Analysis of
Yttrium Doped Cadmium Selenide
(Y/CdSe) Thin Films Grown At
Different Substrate Temperatures 90
4.4.1 Absorbance 90
4.4.2 Transmittance 90
4.4.3 Reflectance 91
4.4.4 Absorption coefficient (α) 91
4.4.5 Energy
band gap (Eg) 92
4.4.6 Extinction
coefficient (k) 92
4.4.7 Refractive index (n) 92
4.4.8 Dielectric constant (real and imaginary) 93
4.4.9 Optical
conductivity (σ) 93
4.5 Optical
and Solid State Analysis of CoSe and Cobalt Selenide/Yttrium at
Different Dopant Concentrations 99
4.5.1 Absorbance 99
4.5.2 Transmittance 99
4.5.3 Reflectance 100
4.5.4 Absorption coefficient (α) 100
4.5.5 Energy
band gap (Eg) 100
4.5.6 Extinction
coefficient (k) 101
4.5.7 Refractive index (n) 101
4.5.8 Dielectric constant (Real and imaginary) 102
4.5.9 Optical
conductivity (σ) 102
4.6 Optical and Solid State Analysis of
Yttrium Doped Cobalt Selenide
(Y/CoSe) Thin Films Grown At
Different Substrate Temperatures 108
4.6.1 Absorbance 108
4.6.2 Transmittance 109
4.6.3 Reflectance 109
4.6.4 Absorption coefficient (α) 109
4.6.5 Energy
band gap (Eg) 110
4.6.6 Extinction
coefficient (k) 110
4.6.7 Refractive index (n) 110
4.6.8 Dielectric constant (real and imaginary) 111
4.6.9 Optical
conductivity (σ) 112
4.7 Electrical
Characterization Results 118
4.7.1 Electrical properties of CuSe and CuSe/Y
(with different dopant
concentrations) 118
4.7.2 Electrical properties of CuSe and CuSe/Y (at
different substrate
temperatures) 118
4.7.3 Electrical properties of CdSe and CdSe/Y
(with different dopant
concentrations) 120
4.7.4 Electrical properties of CdSe and CdSe/Y (at
different substrate
temperatures) 120
4.7.5 Electrical properties of CoSe and CoSe/Y
(with different dopant
concentrations) 122
4.7.6 Electrical
properties of CoSe and CoSe/Y (at different substrate
temperatures) 122
4.8 X-Ray
Diffraction (Xrd) Results 127
4.8.1 X-ray diffraction (XRD) result of CuSe and yttrium-doped CuSe at
different molar percentage dopant
concentrations 127
4.8.2 X-ray diffraction (XRD) result of CuSe and yttrium-doped CuSe at
different substrate temperatures 127
4.8.3 X-ray diffraction (XRD) result of CdSe and yttrium-doped CdSe at
different molar percentage dopant concentrations
130
4.8.4 X-ray diffraction (XRD) result of CdSe and yttrium-doped CdSe at
different substrate temperatures 130
4.8.5 X-ray diffraction (XRD) result of CoSe and yttrium-doped CoSe at
different molar percentage dopant concentrations 133
4.8.6 X-ray diffraction (XRD) result of CoSe and yttrium-doped CoSe at
different substrate temperatures 135
4.9 Surface Morphological (SEM) and
Compositional Analysis (EDX)
Result 140
4.9.1 SEM and EDX
result CuSe and Y doped CuSe at different molar
percentage dopant concentrations 140
4.9.2 SEM and EDX
result CuSe and Y doped CuSe at different substrate
temperatures 143
4.9.3 SEM and EDX
result CdSe and Y doped CdSe at different molar
percentage dopant concentrations 145
4.9.4 SEM and EDX
result CdSe and Y doped CdSe at different substrate
Temperatures 148
4.9.5 SEM and EDX
result CoSe and Y doped CoSe at different molar
percentage dopant concentrations 150
4.9.6 SEM and EDX
result CoSe and Y doped CoSe at different substrate
temperatures 153
CHAPTER 5: CONCLUSION AND RECOMMENDATION
5.1 Conclusion 155
5.2 Recommendations 158
5.3 Contribution
to knowledge 158
REFERENCES
LIST OF TABLES
2.1: Some
basic properties of Copper Selenide (CuSe) 20
2.2: Some
Basic Properties of Cadmium Selenide (CdSe) 23
2.3: Some
Basic Properties of Cobalt Selenide (CoSe) 27
3.1a: Deposition parameters of CuSe and YCuSe at varied dopant
concentrations 53
3.1b: Deposition parameters of CdSe and YCdSe at varied dopant
concentrations 53
3.1c: Deposition parameters of CoSe and YCoSe at varied dopant
concentrations 53
3.2a: Deposition parameters of CuSe and YCuSe at varied substrate
temperatures 54
3.2b: Deposition parameters of CdSe and YCdSe at varied substrate
temperatures 54
3.2c: Deposition parameters of CuSe and YCuSe at varied substrate
temperatures 54
4.1: Electrical
properties of CuSe and CuSe/Y (at different dopant
concentrations) 119
4.2: Electrical
properties of CuSe and CuSe/Y (at different substrate
temperatures) 119
4.3: Electrical
properties of CdSe and CdSe/Y (at different dopant
concentrations) 121
4.4: Electrical
properties of CdSe and CdSe/Y (at different substrate
temperatures) 121
4.5: Electrical
properties of CoSe and CoSe/Y (at different dopant
concentrations) 123
4.6: Electrical
properties of CoSe and CoSe/Y (at different substrate
temperatures)
123
4.7: Structural
values for the CuSe and Yttrium-doped CuSe at different
molar
percent dopant concentrations 129
4.8: Structural
values for the CuSe and Yttrium-doped CuSe at different
substrate
temperatures 129
4.9: Structural
values for the CdSe and Yttrium-doped CdSe at different
molar
percent dopant concentrations 132
4.10: Structural
values for the CdSe and Yttrium-doped CdSe at different
substrate
temperatures
132
4.11: Structural
values for the CoSe and Yttrium-doped CoSe at different
molar
percent dopant concentrations 134
4.12: Structural
values for the CoSe and Yttrium-doped CoSe at different
substrate temperatures 136
LIST OF FIGURES
2.1: Schematic
energy bands of (a) a conductor, (b) an insulator, and
(c) a semiconductor 12
2.2: Dopant
energy levels in N-type and p-type semiconductors 14
2.3(a): Schematic Diagram of a p-n
junction. 15
2.3(b):
Energy band diagram of a p-n
junction 15
2.4: Schematic
representation of photovoltaic effect 17
2.5: Layered
structure of hexagonal CuSe 19
2.6: The structures of CdSe
Nanocrystals 23
2.7: The structure of CoSe
Nanocrystals 27
2.8: Classification
and types of thin film deposition method 30
2.9: Schematic diagram of CVD
technique 31
2.10:
Schematic diagram of spray
pyrolysis 33
2.11:
Schematic diagram of
chemical bath deposition (CBD) 35
2.12: Schematic of SILAR method 36
2.13: Typical
set-up of three electrode system of electrodeposited
technique
37
2.14. Typical
RF sputtering system 39
2.15: Thermal
evaporation technique set-up 40
2.16:
Basic block diagram for
construction a spectrophotometer 42
2.17: Pictorial diagram of a
Spectrophotometer 43
2.18: Pictorial
and Skeletal image of a scanning electron microscopy (SEM) Machine 44
2.19: XRD machine 45
2.20: X-ray
diffraction beam through a parallel section of crystal lattice 46
2.21: Schematic diagram of a four-point
probe 47
3.1 Schematic diagram of the spray pyrolysis machine 50
4.1: Absorbance
spectra of CuSe and Y-doped CuSe at different
dopant
concentrations 67
4.2: %Transmittance
spectra of CuSe and Y-doped CuSe at different
dopant
concentrations 67
4.3: %Reflectance
spectra of CuSe and Y-doped CuSe at different
dopant
concentrations 68
4.4: Absorption
coefficient against Photon energy of CuSe and
Y-doped
CuSe at different dopant concentrations 68
4.5: Plot
of (αhv)2 against hv for CuSe and yttrium doped CuSe thin
materials
at different molar concentrations 69
4.6: Graph
of Extinction coefficient against photon energy of CuSe
and
Y-doped CuSe at different dopant concentrations 69
4.7: Graph of refractive index against photon
energy of CuSe and
Y-doped
CuSe at different dopant concentrations
70
4.8: Plot
of real dielectric constant verves photon energy of CuSe and
Y-doped
CuSe at different dopant concentrations 70
4.9: Plot of imaginary dielectric constant verves
photon energy of
CuSe
and Y-doped CuSe at different dopant concentrations 71
4.10: Plot
of optical conductivity verves photon energy of CuSe and
Y-doped
CuSe at different dopant concentrations 71
4.11: Absorbance
spectral of Y/CuSe thin materials deposited at
different
substrate temperatures 76
4.12: Transmittance
spectral of Y/CuSe thin materials deposited at
different
substrate temperatures 76
4.13: Reflectance
spectral of Y/CuSe thin materials deposited at
different
substrate temperatures 77
4.14: Plot
of Absorption coefficient against photon energy of Y/CuSe
thin
materials deposited at different substrate temperatures 77
4.15: Plot
of (αhv) 2 against hv of Y/CuSe thin materials deposited at
different
substrate temperatures 78
4.16: Relationship
between extinction coefficient and photon energy of
Y/CuSe
thin materials deposited at different substrate temperatures 78
4.17: Relationship
between extinction coefficient and photon energy of
Y/CuSe thin materials deposited at different substrate
temperatures 79
4.18: Plot
of real dielectric constant against hv for Y/CuSe thin materials
deposited at different substrate temperatures 79
4.19: Plot
of imaginary dielectric constant against hv for Y/CuSe thin
materials
deposited at different substrate temperatures 80
4.20: Plot
of optical conductivity against hv for Y/CuSe thin materials
deposited at different substrate temperatures 80
4.21: Graph
of absorbance vs wavelength for CdSe and Y-doped CdSe at
different
concentrations 85
4.22: Graph
of transmittance vs wavelength for CdSe and Y-doped CdSe
at
different dopant concentrations 85
4.23: Graph
of reflectance vs wavelength for CdSe and Y-doped CdSe
at
different dopant concentrations 86
4.24: Graph
of Absorption coefficient vs hv for CdSe and Y-doped
CdSe
at different dopant concentrations 86
4.25: Plots
of (αhv)2 against hv for CdSe and Y-doped CdSe at different
dopant concentrations 87
4.26: Graph
of extinction coefficient against hv for
CdSe and Y-doped
CdSe
at different dopant concentrations 87
4.27: graph
of refractive index against hv for
CdSe and Y-doped CdSe at
different dopant concentrations 88
4.28: Graph
of real dielectric constant against hv for
CdSe and Y-doped
CdSe
at different dopant concentrations 88
4.29: Graph
of imaginargy dielectric constant against hv
for CdSe and
Y-doped
CdSe at different dopant concentrations 89
4.30: Graph
of optical conductivity against hv for
CdSe and Y-doped
CdSe
at different dopant concentrations 89
4.31: Absorbance
spectral of Y/CdSe thin materials deposited at
different
substrate temperatures 94
4.32: Transmittance
spectral of Y/CdSe thin materials deposited at
different
substrate temperatures 94
4.33: Reflectance
spectral of Y/CdSe thin materials deposited at different
substrate temperatures 95
4.34: Absorption
coefficient of Y/CdSe thin materials deposited at
different
substrate temperatures 95
4.35: Plots
of (αhv)2 against hv
for Y-doped CdSe thin material at
different
substrate temperatures 96
4.36: variation
of extinction coefficient with photon energy for Y/CdSe
thin
materials deposited at different substrate temperatures 96
4.37: variation
of refractive index with photon energy for Y/CdSe thin
materials
deposited at different substrate temperatures 97
4.38: Plots
of real dielectric constant against photon energy for Y-doped
CdSe
grown at different substrate temperatures 97
4.39: Plots
of imaginary dielectric constant against photon energy for
Y-doped CdSe grown at different substrate temperatures 98
4.40: Plots
of optical conductivity against photon energy for Y-doped
CdSe
grown at different substrate temperatures 98
4.41: Absorbance
spectral of CoSe and Y-doped CoSe at different
dopant concentrations 103
4.42: Transmittance
spectral of CoSe and Y-doped CoSe at different
dopant concentrations 103
4.43: Reflectance
spectral of CoSe and Y-doped CoSe at different dopant
concentrations 104
4.44: Plot
of Absorption coefficient against hv for
CoSe and Y-doped
CoSe
at different dopant concentrations 104
4.45: Plot
of (αhv)2 against hv for
CoSe and Y-doped CoSe at different
dopant
concentrations 105
4.46: Plot
of Extinction coefficient against hv for
CoSe and Y-doped
CoSe
at different dopant concentrations 105
4.47: Plot
of refractive index against hv for
CoSe and Y-doped CoSe at
different dopant concentrations 106
4.48: Real
dielectric constant against hv for
CoSe and Y-doped CoSe at
different
dopant concentrations 106
4.49: Imaginary
dielectric constant against hv for
CoSe and Y-doped
CoSe
at different dopant concentrations 107
4.50: Optical
conductivity against hv for CoSe and
Y-doped CoSe at
different
dopant concentrations 107
4.51: Absorbance
spectral of Y/CoSe thin materials deposited at
different
substrate temperatures 113
4.52: Transmittance
spectral of Y/CoSe thin materials deposited at
different
substrate temperatures 113
4.53: Reflectance
spectral of Y/CoSe thin materials deposited at different
substrate temperatures 114
4.54: Absorption
coefficient spectral of Y/CoSe thin materials deposited
at
different substrate temperatures 114
4.55: Plot
of (αhv)2 against photon energy for Y/CoSe thin materials
deposited
at different substrate temperatures 115
4.56: Plot
of extinction coefficient against photon energy for Y/CoSe
thin
materials deposited at different substrate temperatures 115
4.47: Plot
of refractive index against photon energy for Y/CoSe thin
materials
deposited at different substrate temperatures
116
4.58: Plot
of real dielectric constant against photon energy for Y/CoSe
thin
materials deposited at different substrate temperatures 116
4.59: Plot
of imaginary dielectric constant against photon energy for
Y/CoSe thin materials deposited at different substrate
temperatures 117
4.60: Plot
of optical conductivity against photon energy for Y/CoSe thin
materials deposited at different substrate
temperatures 117
4.61: A
bar chart comparison of resistivity and conductivity of CuSe
and
CuSeY (for different dopant concentrations) with film thickness 124
4.62: A
bar chart comparison of resistivity and conductivity of CuSeY
(at different substrate temperatures) with film
thickness. 124
4.63: A
bar chart comparison of resistivity and conductivity of CdSe
and CdSeY (for different dopant concentrations) with film
thickness 125
4.64: A
bar chart comparison of resistivity and conductivity of CdSeY
(at different substrate temperatures) with film
thickness. 125
4.65: A
bar chart comparison of resistivity and conductivity of CoSe and
CoSeY (for different dopant concentrations) with film
thickness 126
4.66: A
bar chart comparison of resistivity and conductivity of CoSeY
(at different substrate temperatures) with film
thickness 126
4.67: XRD
pattern of CuSe and Yttrium-doped CuSe at different molar
percent dopant concentrations 137
4.68: XRD
pattern of CuSe and Yttrium-doped CuSe at different
substrate
temperatures 137
4.69: XRD
pattern of CdSe and Yttrium-doped CdSe at different molar
percent dopant concentrations 138
4.70: XRD
pattern of CdSe and Yttrium-doped CdSe at different
substrate
temperatures 138
4.71: XRD
pattern of CoSe and Yttrium-doped CoSe at different molar
percent dopant concentrations 139
4.72: XRD
pattern of CoSe and Yttrium-doped CoSe at different
substrate
temperatures 139
4.73: SEM
result of (a) undoped (b) 0.01mol% and(c) 0.04mol%
yttrium doped CuSe thin materials 141
4.74: EDX spectrum of (a) undoped and (b) Y doped CuSe thin materials 142
4.75: SEM
result of (a) CuSe and (b) Y:CuSe at substrate temperaturesof
140oC 144
4.76: EDX
spectrum of (a) CuSe and (b) Y:CuSe at substrate temperature
of
140oC 144
4.77: SEM
microstructure of (a)undoped CdSe; Y-doped CdSe
(b)
0.01mol% and (c) 0.04mol%) 146
4.78: EDX
spectrum of (a) undoped CdSe (b) 0.01mol%Y-doped
CdSe
and (c) 0.04mol% Y-doped CdSe thin materials 147
4.79: SEM
images of (a) pure and (b) Y doped CdSe at substrate
temperature of 140oC 149
4.80: EDX
spectrum of (a) pure and (b) Y doped CdSe at substrate
temperature
of 140oC 149
4.81: SEM
microstructure of (a)undoped CoSe; Y-doped CoSe (b)
0.01mol%
and (c) 0.04mol%) 151
4.82: EDX
spectrum of (a) undoped CoSe (b) 0.01mol%Y-doped CoSe
and
(c) 0.04mol% Y-doped CoSe thin materials 152
4.83: Pictorial
SEM images of (a) pure and (b) Y doped CoSe
at
substrate temperature of 140oC 154
4.84: EDX
spectrum of (a) pure and (b) Y doped CoSe at substrate
temperature
of 140oC 154
CHAPTER 1
INTRODUCTION
1.1 BACKGROUND OF STUDY
The role of energy in our lives and the world
at large can never be over emphasized due to the fact that energy designs the
footing of human existence and there is barely any activity that is free of
energy (Onwuemeka and Ekpunobi, 2018). We have basically two major types of
energy; renewable and non-renewable energy. Non-renewable sources of energy are
the major sources of energy globally and due to its negative effect on the
world climate, research is ongoing to find alternative renewable energy
resources that are environmentally friendly. Despite the fact that we have various energy
sources within the dynamic forces of nature such as the sun, biomass, wind,
tides and waves; the sun is the most important source of renewable energy
available today. It
is crucial in the whole procedure of growth, evolution, being of all living
things and it also plays a rudimentary part in the human well-being and
socio-economic development of a country. Due to rapid increase in world human
population and industrial uprising, the overall energy requirements have
tremendously increased within the years which has concocted troubles of demand
and supply. If this mounting world energy request is to be achieved with fossil
fuels, they will not be accessible for energy generation in few years. More so,
the use of fossil fuels dispenses several toxins that deteriorate air quality
and have a negative effect on human well-being (Khaled, 2014; Onwuemeka et al., 2017). Two of the foremost
imperative of these toxins are nitrogen oxides (NOx) and sulphur
dioxide (SO2); which comes primarily from flaming coal and also from
diesel fuel, whereas NOx comes from flaming all sorts of fossil
powers. They mainspring different environmental concerns, for instance,
corrosive rain and ground-level ozone creations. Considering the various
disadvantages attached to the use of fossil fuel, it is an important urgency of
today’s world to focus on renewable energy sources, which solar energy has a
special place in, because it can be utilized in terrestrial and even space
application. It is regularly recharged and accordingly reasonable to fulfil the
request and conserve our limited natural resources for upcoming generations
(Sumathi et al., 2015).
The sun is a serious and vital source of energy
and solar radiation is a type of one of the intense energy radiated by the sun,
particularly electromagnetic (EM) energy (Sumathi et al., 2015). Approximately a large fraction of it is in the
visible path of the EM spectrum, the other half is mostly in the close infrared
section, with a few in the ultraviolet segment of the spectrum. The best
technological approach for harvesting and converting solar radiation into
electricity is solar photovoltaic system. The photovoltaic effect
occurs when photons of light excite electrons into a higher state of energy,
allowing them to act as carriers of charge for an electric current. If the incident
photon’s energy is equal to or greater than the energy required to move an
electron from the
valence band to
the conduction band,
then the incident photon contributes to the solar cell’s
output (Khaled, 2014).
Semiconductor
materials are normally small bandgap insulators. The interplay of
semiconductors with photon is of conclusive significance for optoelectronic and
photonic devices as well as for the characterization of semiconductor
properties. When photon strikes a semiconductor, transmission, reflection and
absorption are weighed (Dakin and Brown, 2006). The reaction of the
semiconductor generally relies on the wavelength or photon energy of the light
and various methods add to the dielectric function. For the photon energy to be
absorbed efficiently it must exceed the energy band-gap of the semiconductor
material in question (Calister, 1997). Two major variables have driven the expanding
consideration gotten by semiconductor nanostructures within the last decade: to
begin with, they are appealing from a scientific perspective, since they make
available a medium to fabricate artificial potentials for carriers, holes and
electron in semiconductors, at length scales equipotential to or lesser than
the de Broglie wavelength. Hence, quantum confinement effects turn out to be
not only essential, but also designable to a high extent. Numerous concepts
that already existed only as simple theoretical models can presently be
practically achieved in semiconductor nanostructures, so that their properties
can be examined. The other important variable is that quantum mechanics gets to
be pertinent not only in framework of academic enthusiasm, but moreover to
framework of practical effect. Utilizing confinement effects, modern device
concepts tend to be attainable which get extra degrees of flexibility in design
(Ibach and Luth, 2009; Grundmann, 2016)
In recent times, transition-metal
chalcogenide semiconductors have been assiduously used for advanced
energy-related applications because of their high activity toward water
electro-catalysis (Masud et al., 2016).
They are widely-known for their appealing electronic and magnetic properties.
More importantly, the metal-chalcogen ratio in these transition metal
chalcogenides can be varied over a wide range which can lead to subtle
variation of their electronic properties thereby offering opportunities to tune
such properties. In addition, the ratio of metal chalcogen in these
transition-metal chalcogenides can be varied over a wide range resulting in
subtle variation in their electronic properties thereby offering opportunities to
tune such properties. Added to that, the transition-metal selenides possess
outstanding electro-chemical activity when compared to transition metal oxides
reason being that the electro-negativity of oxygen is higher than that of
selenium. Hence, replacing selenium for oxygen may result in a more pliable
micro-structure. Furthermore, transition-metal selenides displays much improved
electrical conductivity when compared with metal sulfides/oxides (Zhang et al., 2019). Thus, transition-metal
selenides such as CuSe, MgSe, CdSe, MnSe and CoSe are expected to exhibit good
optical and electro-chemical properties for use as an absorber
layer in solar energy collection and electrode-active materials
respectively.
Copper selenide (CuSe) is a semiconducting
p-type material because of copper vacancies with an energy band gap value
ranging from 2.3 to 2.1 eV (Ezenwa et
al., 2013). It belongs to III-VI group which has been tremendously used in
optical filters, super-ionic conductors, photo-electrochemical cells,
thermo-electric converters etc (Li et
al., 2010; Thanikaikarasan et al.,
2020). More so, its commodious optical and electrical properties makes it a
good material suitable for photovoltaic application and photo-detectors (Yadva,
2014). There are number of phases for Copper Selenide with stoichiometry and
non-stoichiometry properties such as CuSe, Cu5Se4, Cu2Se,
Cu7Se4, CuSe2, a-Cu2Se Cu3Se2
and Cu2-xSe (Lakshmi et al., 2001; Li et al., 2010; Yadav, 2014; Khusayfan and Khanfa, 2019). They are
seen to be crystallized in various forms such as monoclinic, cubic, tetragonal,
orthorhombic and hexagonal structure (Thanikaikarasan et al., 2020). Several physical and chemical techniques have been
employed to grow these phases of stable CuSe through thermal evaporation (Liew et al., 2009), Sputtering (Li et al., 2010), Spray pyrolysis (Yadva,
2014), chemical bath deposition (Lakshmi et
al., 2001), and modified CBD (Pathan et
al., 2003) etc.
Cadmium-selenide (CdSe) is a II-VI group
compound semiconductor material, crystallizing in either the zinc blende or
wurtzite structure (Fujita et al., 2003;
Deshpande et al., 2013). Its
excellent performance and suitable properties such as band gap, high absorption
coefficient and high photosensitivity make it useful in optoelectronic and
electronic applications, such as nano-sensors, laser diodes, high-efficiency
solar cells, γ-ray detectors and devices for biomedical imaging (Califano et al., 2004; Meulenberg et al., 2004; Sivasankar, 2017). CdSe has a direct
band gap and high absorption coefficient close to the band edge, making it
suitable for use in thin-film devices; it is mainly interesting for its
applications in hybrid solar systems (Pathak et al., 2019). CdSe thin materials have been synthesized by various
film deposition techniques, such as spray pyrolysis (Yadav et al., 2010), SILAR (Xiaolei,
2015), electro deposition (Choudhary and Chauchan, 2017), Electron beam
evaporation (Rani et al., 2016), chemical bath deposition (CBD) (Zhao et al., 2013, Deshpande et al., 2013) etc. To increase the
performance of CdSe nanocrystalline thin films, many researcher used the dopant
impurities and made thin film of CdSe:Fe (Singh et al., 2009, Dangi
and Dhar , 2016), CdSe:Cr (Sivasankar , 2017) , CdSe:Zn (Aplop and Johan 2014), CdSe:Cu (Singh et al., 2008; Vidhya et al., 2013), CdSe:Ni (Kumar et
al., 2012) and CdSe:Mn (Kwak
and Sung 2007; Kwshwaha, 2017) etc.
Cobalt Selenide (CoSe) is amidst the II-VI semiconductors
with direct band-gap (Chikwenze et al.,
2017). It is also a significant P-type multifunctional semiconductor material
and as a member of the transition metal selenides it shows promising properties
such as high optical absorption coefficient of the order of 105 cm-1
band-gap of approximately 1.5 eV and a good electrical conductivity (Agbo et al., 2016). It has massive extensive
and potential application possibilities in the area of photo-catalysis, super-capacitor
and solar cells (Agbo and Nwofe, 2015; Zhang
et al., 2019). They are also attractive because of the earth abundance of
Co. Cobalt selenides comprises of two stable forms (CoSe2, CoSe) and
two viable compounds (Co3Se4, Co2Se) at room
temperature. Variety of methods such as CBD (Pramanik et al., 1987; Agbo et al., 2016;
Govindasamya et al., 2016),
Electrodeopistion (Zhang et al., 2019;
Ikhioya et al., 2020), Spray
pyrolysis (Kim et al., 2018) etc have
been used to synthesized CoSe nano-materials.
More
recently, several efforts have been made to enhance the efficiency of
semiconductors via
doping, supporting and coupling of these compounds. Concerning this, rare earth-doped
nano-particles have received a lot of consideration given their massive photo-catalytic
activity in the degradation of organic contaminants due to the suppression of
electron–hole recombination, large content of oxygen vacancies, and strong
absorption of hydroxide ions on the surface of the catalyst (Khataee
et al., 2014). Because of their low diffusivity they are also known to
perform useful functions such as lowering and stabilizing the dissipation
factor in dielectric materials (Kim et
al., 2011).
One of the promising dopants in the family of
rare earth materials is Yttrium (Y3+). This is due to the fact that
it improves remnant polarization, leakage current and fatigue endurance (Hatta et al., 2016). Yttrium can also act as a
donor or acceptor ion. Another significant effect of yttrium doping is the
change in the electrical conductivity of the doped material with respect to its
doping site.
Numerous methods have been investigated for
thin film production with attention on cost and reliability. These methods can
be classified as chemical and physical methods. The physical methods include,
reactive/non-reactive sputtering techniques, oxidation of an evaporated metal
film, molecular beam epitaxy and laser ablation. The chemical methods are
divided into solution techniques (e.g. spray pyrolysis) and gas phase
deposition (e.g., chemical vapor deposition (CVD) according to Seyed, (2014).
The chemical methods have been studied extensively for the growth of thin
films. The present study uses a spray-pyrolysis technique (SPT) which has since
been used in glass industry and in solar-cell fabrication to deposit
electrically conducting electrodes. Through this technique, dense and porous
oxide films, ceramic coatings and powders can be prepared. It is relatively a
cost-effective and very simple method. Materials obtained by SPT find a wide
range of applications in optoelectronic devices, anti-reflective coatings,
sensors, etc (George, 1992; Dedova, 2007). The spray-pyrolysis technique has
several notable advantages, including the ability to change the properties of
the film by changing the composition of the starting material (addition of
dopants and modification of the film microstructure) and lowering of production
cost when large-scale production is required (Abhijit, 2014).
In this study, we look for novel
semiconductor materials that have good promising potentials in converting solar
radiation into a useful form of energy, and these semiconductors include;
Yttrium Copper Selenide (YCuSe), Yttrium Cadmium Selenide (YCdSe) and Yttrium
Cobalt Selenide (YCoSe,) semiconductor thin films
1.2 AIM
The
aim of this research is grow and characterize Yttrium doped chalcoginide
semiconductors for solar energy purposes using spray pyrolysis technique.
1.3 OBJECTIVES
The objectives of this study are as follows:
1. To Grow and characterize ternary transition metal
chalcogenides of Yttrium Copper Selenide (YCuSe), Yttrium Cadmium Selenide
(YCdSe) and Yttrium Cobalt Selenide (YCoSe)
semiconductor thin films which have not been grown before using spray
pyrolysis deposition technique.
2. To determine and study the optical and solid
state properties of these novel thin films
3. To study the electrical properties of these
novel thin films
4. To Study the structural properties of these
novel thin films
5. To study the morphology and ascertain the
elemental composition of these novel thin films
6. To optimize these novel thin materials in
order to identify which thin film is suitable for solar cells, anti-reflection
coating and photovoltaic application.
1.4 SCOPE OF THE
STUDY
The
scope is to deposit ternary
transition metal chalcogenides of Yttrium Copper Selenide (YCuSe), Yttrium
Cadmium Selenide (YCdSe) and Yttrium Cobalt Selenide (YCoSe) semiconductor
films on a soda lime glass substrate using spray pyrolysis. Also
to study the optical, electrical , structural, morphological and compositional
analysis of these novel thin films using UV-visible Spectrophotometer, four
point probe, X-ray diffractometer and scanning electron microscopy respectively
and to find the possible applications of these novel thin materials.
1.5 THE SIGNIFICANCE OF THE STUDY
Semiconductor
materials are the most cost-effective and high-efficiency devices used in solar
energy conversions. Hence, it is expected that doping Copper Selenide, Cadmium
Selenide and Cobalt Selenide with Yttrium will produce optimum material with a
good conversion efficiency of solar radiation into electricity. Doping Copper
Selenide, Cadmium Selenide and Cobalt Selenide with Yttrium will also produce
new set of semiconductor materials that will have capacity to improve on existing
functions of semiconductor and photovoltaic devices. The results that will be
obtained from this study will aid manufacturers on the various choice of
materials to be considered for manufacturing semiconductor and photovoltaic
devices. More so, for the academic world and the general public, this study
will add to the existing literature on the growth and characterization of
ternary transition-metal chalcogenide thin materials.
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