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About this book

This book will guide Photovoltaics researchers in a new way of thinking about harvesting light energy from all wavelengths of the solar spectrum. It closes the gap between general solar cells books and photovoltaics journal articles, by focusing on the latest developments in our understanding of solid-state device physics. The material presented is experimental and based on II-VI thin-film materials, mainly CdTe-based solar cells. The authors describe the use of new device design, based on multilayer graded bandgap configuration, using CdTe-based solar cells. The authors also explain how the photo-generated currents can be enhanced using multi-step charge carrier production. The possibility of fabricating these devices using low-cost and scalable electroplating is demonstrated. The value of electroplating for large area electronic devices such as PV solar panels, display devices and nano-technology devices are also demonstrated. By enabling new understanding of the engineering of electroplated semiconductor materials and providing an overview of the semiconductor physics and technology, this practical book is ideal to guide researchers, engineers, and manufacturers on future solar cell device designs and fabrications.Discusses in detail the processes of growths, treatments, solar cell device fabrication and solid state physics, improving readers’ understanding of fundamental solid state physics;
Enables future improvements in CdTe-based device efficiency;
Explains the significance of defects in deposited semiconductor materials and interfaces that affect the material properties and resulting device performance.

Table of Contents

Frontmatter

Chapter 1. Introduction to Photovoltaics

This chapter covers introductory topics providing a broad overview of the different aspects of energy and energy resources with a focus on solar energy and photovoltaic technology. A consideration of the energy distribution of the solar spectrum, photovoltaic solar energy conversion techniques and the operating configuration of photovoltaic solar cells is also provided.
A. A. Ojo, W. M. Cranton, I. M. Dharmadasa

Chapter 2. Photovoltaic Solar Cells: Materials, Concepts and Devices

This chapter describes the characteristic structural and electrical properties of solid-state materials with emphasis on semiconductors, surfaces and interfaces, junctions, charge carrier transport mechanisms, electrical contacts and devices. An overview of semiconductor growth techniques is also included in this chapter for readers to familiarise with some of the terminologies that describe semiconductor/semiconductor (SS), metal/semiconductor (MS) or metal/insulator/semiconductor (MIS) structures. This chapter also includes a description of the concept of bandgap grading and next-generation solar cells.
A. A. Ojo, W. M. Cranton, I. M. Dharmadasa

Chapter 3. Techniques Utilised in Materials Growth and Materials and Device Characterisation

Further to the semiconductor material and electronic properties discussed in Chap. 2, the evaluation of semiconductor materials can be examined for structural, morphological, compositional, optical and electronic properties to facilitate research towards optimisation. This chapter describes the physics and the basic functionality of X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM), energy-dispersive X-ray (EDX) spectroscopy, ultraviolet-visible (UV-Vis) spectroscopy and photoelectrochemical (PEC) cell measurement equipment. Current-voltage (I-V) and capacitance-voltage (C-V) techniques were utilised for the evaluation to facilitate research towards solar cell device performance in order to understand the factors affecting the performance of device materials.
A. A. Ojo, W. M. Cranton, I. M. Dharmadasa

Chapter 4. ZnS Deposition and Characterisation

Zinc sulphide (ZnS) layers have been used as buffer layers in the solar cells described in this book, and this chapter provides an insight into the electrodeposition of ZnS layers. Electrodeposition of zinc sulphide (ZnS) was achieved from an electrolytic bath containing zinc sulphate monohydrate (ZnSO4 ·H2O) and ammonium thiosulphate ((NH4)2S2O3) in a two-electrode electroplating configuration. Cyclic voltammetric studies show that ZnS layers can be electroplated between 1350 and 1550 mV. The grown layers were characterised for their structural, optical, morphological and electronic properties using X-ray diffraction (XRD) and Raman spectroscopy, UV-visible spectrophotometry, scanning electron microscopy (SEM), photoelectrochemical (PEC) cell and DC conductivity measurements, respectively. The structural analyses show that crystalline ZnS can be deposited within a narrow cathodic deposition range between 1420 and 1430 mV. UV-visible spectrophotometry shows that the bandgap of both as-deposited and heat-treated ZnS films is in the range of ~(3.70 and 3.90) eV. The SEM shows small grains in the ZnS layer and the full coverage of the underlying substrate by the film. PEC results show that the electroplated ZnS layers grown below 1425 mV are p-type and above 1425 mV are n-type under both as-deposited and heat-treated condition. DC conductivity shows that the highest resistivity is at the inversion growth voltage (V i) for the ZnS layers.
A. A. Ojo, W. M. Cranton, I. M. Dharmadasa

Chapter 5. CdS Deposition and Characterisation

This chapter provides an insight to the electrodeposition of CdS layers from electrolytic bath without precipitation. CdS layers used in thin-film solar cells as window layers and other electronic devices are usually grown by wet chemical methods using CdCl2 as the cadmium source and either Na2S2O3, NH4S2O3 or NH2CSNH2 as sulphur sources. Obviously, one of the sulphur precursors should produce more suitable CdS layers required to yield the highest performing devices. This can only be achieved by comprehensive experimental work on growth and characterisation of CdS layers from the above-mentioned sulphur sources. This chapter presents the results observed on CdS layers grown by electrodepositing using two-electrode configuration and thiourea as the sulphur precursor. X-ray diffraction (XRD), Raman spectroscopy, optical absorption, scanning electron microscopy (SEM), energy-dispersive X-ray analysis (EDX) and photoelectrochemical (PEC) cell methods have been used to characterise the material properties. In order to test and study the electronic device quality of the layers, ohmic and rectifying contacts were fabricated on the electroplated layers. Schottky barriers formed on the layers were also compared with previously reported work on chemical bath deposited CBD-CdS layers and bulk single crystals of CdS. Comparatively, Schottky diodes fabricated on electroplated CdS layers using two-electrode system and thiourea precursor exhibit excellent electronic properties suitable for electronic devices such as thin-film solar panels and large area display devices.
A. A. Ojo, W. M. Cranton, I. M. Dharmadasa

Chapter 6. CdTe Deposition and Characterisation

The main absorber material used in solar cells described in this book is CdTe. This chapter provides an insight to the electrodeposition of cadmium telluride (CdTe) layers. CdTe thin films were electrodeposited on glass/fluorine-doped tin oxide (FTO) using a two-electrode system from an acidic and aqueous solution containing 1.5 M Cd(NO3)2·4H2O and 0.002 M TeO2 at pH = 2.00 ± 0.02. The grown layers were characterised for their structural, optical, morphological, compositional and electronic properties using X-ray diffraction (XRD) and Raman spectroscopy, UV-visible spectrophotometry, scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), photoelectrochemical (PEC) cell and DC conductivity measurements, respectively. The XRD study reveals that the ED-CdTe layers are polycrystalline in nature with preferential peak orientation along the (111) plane. The highest crystallinity was observed when grown at 1370 mV cathodic potential with reference to graphite electrode. Optical absorption reveals that the bandgap of the as-deposited, heat-treated and CdCl2-treated CdTe layers falls within the range 1.48–1.50 eV. The SEM micrographs show a uniform coverage of the underlying glass/FTO substrate. The EDX shows the effect of growth voltage on the atomic composition of CdTe layers with 1:1 atomic ratio of the deposited CdTe layers grown at 1370 mV. The PEC cell measurements reveal that both p- and n-type CdTe layers can be electroplated. The effect of F, Cl, I and Ga as dopants to CdTe baths was evaluated and documented.
A. A. Ojo, W. M. Cranton, I. M. Dharmadasa

Chapter 7. Solar Cell Fabrication and Characterisation

This chapter provides an insight into next-generation graded bandgap photovoltaic device fabrication. All-electrodeposited devices were fabricated using n-p, n-n-p, n-n + large Schottky barrier (SB) and n-n-n + SB architecture using ZnS, CdS and CdTe thin layers. The fabricated devices were evaluated using both current-voltage (I-V) and capacitance-voltage (C-V) techniques. The inclusion of Ga into the regular CdCl2 post-growth treatment and the effect of pH were also explored with the improved result as compared to the regular CdCl2 PGT. Based on all experimental findings as explored within the limit of this work, the most promising of the configurations examined are the glass/FTO/n-CdS/n-CdTe/p-CdTe/Au with thicknesses of 120 nm (n-CdS), 1200 nm (n-CdTe), 30 nm (p-CdTe) and 100 nm (Au). The highest conversion efficiencies observed for two separate batches were 15.3 and 18.4%. The devices with the 18.4% efficiency showed some instability and therefore require further investigation. The glass/FTO/n-ZnS/n-CdS/n-CdTe/Au configuration with thicknesses of 50 nm (n-ZnS), 65 nm (n-CdS), 1200 nm (n-CdTe) and 100 nm (Au) also shows promising results with the highest efficiency achieved being 14.1% owing to bandgap grading strengths.
A. A. Ojo, W. M. Cranton, I. M. Dharmadasa

Chapter 8. Conclusions, Challenges Encountered and Future Work

The sun offers mankind virtually unlimited energy potential with photovoltaics being one of the energy-harnessing technologies. New understanding of semiconductor material issues, processing steps, graded bandgap device architectures and device physics paves the way to achieving high-energy conversion efficiency. Although the photovoltaic market is dominated by Si-based solar cells, CdTe-based solar cells provide competitiveness based on its economic viability and conversion efficiency. The use of electroplating as the semiconductor deposition technique further strengthens the exploration of science behind these devices and economic competitiveness of CdTe-based solar cells as demonstrated in this book. Graded bandgap solar cells provide a promising path to produce low-cost and high-efficiency, next-generation solar cell development.
A. A. Ojo, W. M. Cranton, I. M. Dharmadasa

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