In this research an n-type CdxZn1-xS and p-type PbS thin films were optimised for solar cell applications employing chemical bath deposition technique. The thin films were prepared using thiourea and nitrates of cadmium, zinc and lead. Deposition of optimised CdxZn1-xS was done by CBD at 820 C and in alkaline conditions while that of PbS was done at room temperature and both films at normal atmospheric pressure utilizing aqueous conditions. This study concentrated on optimising optical and electrical characterization of the films. Optical constant suitable for photovoltaic applications were sort for and for this purpose a UV VIS IR spectrophotometer 3700 DUV was utilised while the electrical properties were investigated using a four point probe connected to a Keithley 2400 source meter interfaced with computer. The optical band gap of the as deposited CdxZn1-xS films varied from 2.47eV (x =0.6) to 2.72 eV (x =1.0), and transmittance above 79% in the VIS - NIR region for the concentration range of x = 0.6 to 1.0, that is, the band gap increased with increasing Zn concentration of the alloy and Cd06Zn0.4S sample showed the widest band gap. It was obtained that the presence of zinc increased optical band gap. The average extinction coefficients for the as deposited CdxZn1-xS samples were very low revealing that they absorb very little radiation hence a good window layer material. As measured by the four point probe connected to a Keithley 2400 source meter, electrical resistivity increased with increase in Zn in the bath in CdxZn1-xS and a resistivity range of 9.5×101 – 1.22× 102 Ω-cm was obtained. These properties are appropriate for window layers used for photovoltaic cell applications. PbS thin films had a band gap of 0.89 eV and a transmittance of below 55% appropriate for absorber layers of photovoltaic cells and a resistivity range of 6.78 × 103 to 1.26 × 104 Ω-cm. The fabricated photovoltaic cell had a short circuit current, Isc = 0.031 A, open voltage, Voc = 0.37V, efficiency, η = 0.9% and a fill factor, FF = 0.66 implying that the two materials are appropriate for photovoltaic applications especially in the VIS and IR light spectrum.
Table of Contents
CHAPTER ONE
INTRODUCTION
1.1 Background of the study
1.2 Problem statement for the study
1.3 Objectives
1.3.1 Main objective
1.3.2 Specific objectives
1.4 Rationale of the study
1.5 Structure of thesis
CHAPTER TWO
LITERATURE REVIEW
2.1 Survey on Cadmium Sulphide (CdS) thin films
2.1.1 Electrical properties of CdS thin films
2.1.2 Optical properties of CdS thin films
2.2 Survey on lead sulphide (PbS) thin films
2.3 CdS-PbS based photovoltaic Cells
CHAPTER THREE
THEORETICAL CONSIDERATIONS
3.1 Semiconductor thin films
3.2 Energy bands in solids
3.3 Energy band structure in semiconductors
3.3.1 Intrinsic semiconductors
3.3.2 Extrinsic Semiconductors
3.4 Semiconductor transport carriers
3.5 Optical phenomena in thin films
3.5.1 Photoconductivity in thin films
3.6 Thin film applications
3.6.1 The p-n junction
3.6.2 Photovoltaic cells
3.6.2.2 Photovoltaic cell operation
3.7 Strengths and limitation of thin films for photovoltaic cells
3.8 Thin film deposition techniques
3.8.1 Chemical Deposition Techniques
CHAPTER FOUR
MATERIALS AND METHODS
4.1 Materials for film deposition
4.1.1 Chemicals for deposition of Cadmium Zinc Sulphide and Lead Sulphide thin films
4.1.2 Cleaning of the substrates
4.3 Experimental procedures
4.3.1 Deposition of cadmium zinc sulphide (CdxZn1-xS) thin films
4.3.2 Deposition of lead sulphide (PbS) thin films
4.4 Characterization procedures
4.4.1 Measurement of electrical properties
4.4.1.1 Sheet resistivity
4.4.2 Measurement of optical properties
4.4.2.1 Energy band gap (Eg)
4.4.2.2 Refractive index (n)
4.4.2.3 Absorbance (α)
4.4.2.4 Extinction coefficient (k)
4.4.2.5 Transmittance (T)
4.5 Photovoltaic cell fabrication
4.6 Photovoltaic cell characterisation
CHAPTER FIVE
RESULTS AND DISCUSSIONS
5.1 Optical properties of the thin films
5.1.1 Optical properties of cadmium zinc sulphide (CdxZn1-xS) thin films
5.1.2 Optical properties of lead sulphide (PbS) thin films.
5.2 Electrical properties of the thin films
5.2.1 Electrical properties of CdxZn1-xS thin films
5.2.2 Electrical properties of PbS thin films
5.3 Photovoltaic properties of CdxZn1-xS / PbS cell
CHAPTER SIX
CONCLUSIONS AND RECOMMENDATIONS
6.1 Conclusions
6.2 Recommendations
Research Goals and Core Themes
The research aims to optimize n-type CdxZn1-xS and p-type PbS thin films using chemical bath deposition (CBD) techniques to fabricate efficient, cost-effective photovoltaic cells, specifically targeting improvements in optical and electrical performance for solar energy harvesting.
- Optimization of chemical bath deposition (CBD) parameters for CdxZn1-xS and PbS thin films.
- Comprehensive optical characterization, including band gap, refractive index, and transmittance analysis.
- Electrical characterization, focusing on sheet resistivity and conductivity measurements.
- Fabrication and performance evaluation of a heterojunction photovoltaic cell using the optimized thin film layers.
Excerpt from the Book
3.6.1.2 A p-n hetero-junction
This is a junction formed between any two semiconductors having different energy band gaps. If the conductivity type is the same in any of these two semiconductors, then it is called an isotype hetero-junction while in an anisotype the conductivity type is different in the two semiconductors (Keszler, 2007). The use of a hetero-junction (HJ) with a large band-gap window material and a small band-gap absorber material is a means of minimizing surface recombination losses that might otherwise dominate in direct band-gap materials (Abdullah, 2007). Thin film technology uses HJ to expand semiconductor material possibilities for solar and photovoltaic cell applications enormously. Hetero-face photovoltaic cells [where a p-n homo-junction is interfaced with a lattice matched material of larger band gap] have achieved extremely high solar efficiencies e.g. in CdS/CdTe (Kengo et al., 2006). The carrier transport properties of HJs are generally dominated by phenomena in the interface of p-n region. The current transport in the depletion layer is usually attributed to recombination, tunnelling, or a combination of both involving energy levels near the interface. The requirements for the formation of a good quality hetero-junction are (Kengo et al., 2006):
(i) the lattice constant of the two materials should be nearly equal,
(ii) the electron affinities should be compatible, and
(iii) their thermal expansion coefficients should be close.
If there is a mismatch of lattice constants and thermal expansion coefficients then interfacial dislocations at the hetero-junction interface occur which gives rise to interface states that act as trapping centres.
Summary of Chapters
CHAPTER ONE: Provides an introduction into the background of the research, the rationale of the study, and identifies the material choices and research objectives.
CHAPTER TWO: Reviews literature on the optical and electrical properties of CdS, PbS, and doped thin films, identifying specific research gaps.
CHAPTER THREE: Discusses theoretical considerations regarding semiconductor thin film performance, including band structures, carrier transport, and p-n junction physics.
CHAPTER FOUR: Details the materials and methodology used, including substrate cleaning, chemical bath deposition, and characterization techniques.
CHAPTER FIVE: Presents and discusses the experimental results, including the optical and electrical properties of the deposited films and the photovoltaic cell characteristics.
CHAPTER SIX: Offers final conclusions drawn from the study and provides recommendations for future research and industrial adaptation.
Keywords
Chemical bath deposition, CBD, Cadmium Zinc Sulphide, CdxZn1-xS, Lead Sulphide, PbS, Photovoltaic cells, Thin films, Band gap, Electrical resistivity, Heterojunction, Solar energy, Optical characterization, Semiconductor, Semiconductor transport carriers
Frequently Asked Questions
What is the core focus of this research?
The research focuses on the optimization of CdxZn1-xS and PbS thin films for photovoltaic cell applications, utilizing the cost-effective chemical bath deposition (CBD) technique to enhance optical and electrical properties.
What are the primary thematic areas covered?
The work covers semiconductor physics, thin film deposition models, optical properties analysis (transmittance, band gaps, absorbance), electrical characterization, and the design of heterojunction photovoltaic cells.
What is the primary goal of this study?
The main objective is to deposit, optically and electrically characterize, and ultimately integrate CdxZn1-xS and PbS thin films into a functional photovoltaic cell to improve solar energy conversion efficiency.
Which scientific method is employed?
The study primarily employs Chemical Bath Deposition (CBD) for film synthesis, combined with four-point probe methods for electrical resistivity and UV-VIS NIR spectrophotometry for optical analysis.
What is examined in the main section?
The main sections investigate the synthesis of ternary and binary semiconductor thin films, the influence of doping concentration and deposition conditions on material properties, and the systematic evaluation of the resulting photovoltaic cell performance.
Which keywords best describe this research?
Key terms include Chemical bath deposition, CdxZn1-xS, PbS, Photovoltaic cells, Thin films, Band gap, Electrical resistivity, and Heterojunction.
How does zinc doping affect CdS thin films?
Zinc doping converts pure CdS into a ternary CdxZn1-xS alloy, which systematically increases the optical band gap and improves transparency, making it a more effective window layer for photovoltaic cells.
What is the function of the PbS layer in this study?
PbS is utilized as an absorber layer in the heterojunction cell structure because of its narrow band gap (0.88 eV), which allows for the absorption of infrared radiation, complementing the wide-band-gap window layer.
Why are the fabricated thin films considered "thin"?
They are termed thin films because they are built up as nanometer-scale layers on solid substrates through controlled condensation of atomic or molecular species, leading to unique physical properties distinct from bulk materials.
- Quote paper
- Cliff Orori Mosiori (Author), 2011, Electrical and optical characterization of CdxZn1-xS and PbS thin films for photovoltaic applications, Munich, GRIN Verlag, https://www.grin.com/document/278908
