The Physics of Solar Cells: Photons In, Electrons Out

Description

This book provides a comprehensive introduction to the physics of the photovoltaic cell. It is suitable for undergraduates, graduate students, and researchers new to the field. It covers: basic physics of semiconductors in photovoltaic devices; physical models of solar cell operation; characteristics and design of common types of solar cell; and approaches to increasing solar cell efficiency. The text explains the terms and concepts of solar cell device physics and shows the reader how to formulate and solve relevant physical problems. Exercises and worked solutions are included. Preface Chapter 1 Introduction 1.1. Photons In, Electrons Out: The Photovoltaic Effect 1.2. Brief History of the Solar Cell 1.3. Photovoltaic Cells and Power Generation 1.3.1. Photovoltaic cells, modules and systems 1.3.2. Some important definitions 1.4. Characteristics of the Photovoltaic Cell: A Summary 1.4.1. Photocurrent and quantum efficiency 1.4.2. Dark current and open circuit voltage 1.4.3. Efficiency 1.4.4. Parasitic resistances 1.4.5. Non-ideal diode behaviour 1.5. Summary References Chapter 2 Photons In, Electrons Out: Basic Principles of PV 2.1. Introduction 2.2. The Solar Resource 2.3. Types of Solar Energy Converter 2.4. Detailed Balance 2.4.1. In equilibrium 2.4.2. Under illumination 2.5. Work Available from a Photovoltaic Device 2.5.1. Photocurrent 2.5.2. Dark current 2.5.3. Limiting efficiency 2.5.4. Effect of band gap 2.5.5. Effect of spectrum on efficiency 2.6. Requirements for the Ideal Photoconverter 2.7. Summary References Chapter 3 Electrons and Holes in Semiconductors 3.1. Introduction 3.2. Basic Concepts 3.2.1. Bonds and bands in crystals 3.2.2. Electrons, holes and conductivity 3.3. Electron States in Semiconductors 3.3.1. Band structure 3.3.2. Conduction band 3.3.3. Valence band 3.3.4. Direct and indirect band gaps 3.3.5. Density of states 3.3.6. Electron distribution function 3.3.7. Electron and hole currents 3.4. Semiconductor in Equilibrium 3.4.1. Fermi Dirac statistics 3.4.2. Electron and hole densities in equilibrium 3.4.3. Boltzmann approximation 3.4.4. Electron and hole currents in equilibrium 3.5. Impurities and Doping 3.5.1. Intrinsic semiconductors 3.5.2. n type doping 3.5.3. p type doping 3.5.4. Effects of heavy doping 3.5.5. Imperfect and amorphous crystals 3.6. Semiconductor under Bias 3.6.1. Quasi thermal equilibrium 3.6.2. Electron and hole densities under bias 3.6.3. Current densities under bias 3.7. Drift and Diffusion 3.7.1. Current equations in terms of drift and diffusion 3.7.2. Validity of the drift-diffusion equations 3.7.3. Current equations for non-crystalline solids 3.8. Summary Chapter 4 Generation and Recombination 4.1. Introduction: Semiconductor Transport Equations 4.2. Generation and Recombination 4.3. Quantum Mechanical Description of Transition Rates 4.3.1. Fermi's Golden Rule 4.3.2. Optical processes in a two level system 4.4. Photogeneration 4.4.1. Photogeneration rate 4.4.2. Thermalisation 4.4.3. Microscopic description of absorption 4.4.4. Direct gap semiconductors 4.4.5. Indirect gap semiconductors 4.4.6. Other types of behaviour 4.4.7. Examples and data 4.5. Recombination 4.5.1. Types of recombination 4.5.2. Radiative recombination 4.5.3. Simplified expressions for radiative recombination 4.5.4. Auger recombination 4.5.5. Shockley Read Hall recombination 4.5.6. Surface and grain boundary recombination 4.5.7. Traps versus recombination centres 4.6. Formulation of the Transport Problem 4.6.1. Comments on the transport problem 4.6.2. Transport equations in a crystal 4.7. Summary References Chapter 5 Junctions 5.1. Introduction 5.2. Origin of Photovoltaic Action 5.3. Work Function and Types of Junction 5.4. Metal-Semiconductor Junction 5.4.1. Establishing a field 5.4.2. Behaviour in the light 5.4.3. Behaviour in the dark 5.4.4. Ohmic contacts 5.4.5. Limitations of the Schottky barrier junction 5.5. Semiconductor—Semiconductor Junctions 5.5.1. p–n junction 5.5.2. p–i–n junction 5.5.3. p–n heterojunction 5.6. Electrochemical Junction 5.7. Junctions in Organic Materials 5.8. Surface and Interface States 5.8.1. Surface states on free surfaces 5.8.2. Effect of interface states on junctions 5.9. Summary References Chapter 6 Analysis of the p–n Junction 6.1. Introduction 6.2. The p–n Junction 6.2.1. Formation of p–n junction 6.2.2. Outline of approach 6.3. Depletion Approximation 6.3.1. Calculation of depletion width 6.4. Calculation of Carrier and Current Densities 6.4.1. Currents and carrier densities in the neutral regions 6.4.2. Currents and carrier densities in the space charge region 6.4.3. Total current density 6.5. General Solution for J(V) 6.6. p–n Junction in the Dark 6.6.1. At equilibrium 6.6.2. Under applied bias 6.7. p–n Junction under Illumination 6.7.1. Short circuit 6.7.2. Photocurrent and QE in special cases 6.7.3. p–n junction as a photovoltaic cell 6.8. Effects on p–n Junction Characteristics 6.8.1. Effects of parasitic resistances 6.8.2. Effect of irradiation 6.8.3. Effect of temperature 6.8.4. Other device structures 6.8.5. Validity of the approximations 6.9. Summary References Chapter 7 Monocrystalline Solar Cells 7.1. Introduction: Principles of Cell Design 7.2. Material and Design Issues 7.2.1. Material dependent factors 7.2.2. Design factors 7.2.3. General design features of p–n 7.3. Silicon Material Properties 7.3.1. Band structure and optical absorption 7.3.2. Doping 7.3.3. Recombination 7.3.4. Carrier transport 7.4. Silicon Solar Cell Design 7.4.1. Basic silicon solar cell 7.4.2. Cell fabrication 7.4.3. Optimisation of silicon solar cell design 7.4.4. Strategies to enhance absorption 7.4.5. Strategies to reduce surface recombination 7.4.6. Strategies to reduce series resistance 7.4.7. Evolution of silicon solar cell design 7.4.8. Future directions in silicon cell design 7.4.9. Alternatives to silicon 7.5. III—V Semiconductor Material Properties 7.5.1. III—V semiconductor band structure and optical absorption 7.5.2. Gallium arsenide 7.5.3. Doping 7.5.4. Recombination 7.5.5. Carrier transport 7.5.6. Reflectivity 7.6. GaAs Solar Cell Design 7.6.1. Basic GaAs solar cell 7.6.2. Optimisation of GaAs solar cell design 7.6.3. Strategies to reduce front surface recombination 7.6.4. Strategies to reduce series resistance 7.6.5. Strategies to reduce substrate cost 7.7. Summary References Chapter 8 Thin Film Solar Cells 8.1. Introduction 8.2. Thin Film Photovoltaic Materials 8.2.1. Requirements for suitable materials 8.3. Amorphous Silicon 8.3.1. Materials properties 8.3.2. Defects in amorphous material 8.3.3. Absorption 8.3.4. Doping 8.3.5. Transport 8.3.6. Stability 8.3.7. Related alloys 8.4. Amorphous Silicon Solar Cell Design 8.4.1. Amorphous silicon p–i–n structures 8.4.2. p–i–n solar cell device physics 8.4.3. Fabrication of a-Si solar cells 8.4.4. Strategies to improve a-Si cell performance 8.5. Defects in Poly crystalline Thin Film Materials 8.5.1. Grain boundaries 8.5.2. Effects of grain boundaries on transport 8.5.3. Depletion approximation model for grain boundary 8.5.4. Majority carrier transport 8.5.5. Effect of illumination 8.5.6. Minority carrier transport 8.5.7. Effects of grain boundary recombination on solar cell performance 8.6. CuInSe 2 Thin Film Solar Cells 8.6.1. Materials properties 8.6.2. Heterojunctions in thin film solar cell design 8.6.3. CuInGaSe2 solar cell design 8.7. CdTe Thin Film Solar Cells 8.7.1. Materials properties 8.7.2. CdTe solar cell design 8.8. Thin Film Silicon Solar Cells 8.8.1. Materials properties 8.8.2. Microcrystalline silicon solar cell design 8.9. Summary References Chapter 9 Managing Light 9.1. Introduction 9.2. Photon Flux: A Review and Overview of Light Management 9.2.1. Routes to higher photon flux 9.3. Minimising Reflection 9.3.1. Optical properties of semiconductors 9.3.2. Antireflection coatings 9.4. Concentration 9.4.1. Limits to concentration 9.4.2. Practical concentrators 9.5. Effects of Concentration on Device Physics 9.5.1. Low injection 9.5.2. High injection 9.5.3. Limits to efficiency under concentration 9.5.4. Temperature 9.5.5. Series resistance 9.5.6. Concentrator cell design 9.5.7. Concentrator cell materials 9.6. Light Confinement 9.6.1. Light paths and ray tracing 9.6.2. Mirrors 9.6.3. Randomising surfaces 9.6.4. Textured surfaces 9.6.5. Practical schemes 9.6.6. Light confining structures: restricted acceptance areas and external cavities 9.6.7. Effects of light trapping on device physics 9.7. Photon Recycling 9.7.1. Theory of photon recycling 9.7.2. Practical schemes 9.8. Summary References Chapter 10 Over the Limit: Strategies for High Efficiency 10.1. Introduction 10.2. How Much is Out There? Thermodynamic Limits to Efficiency 10.3. Detailed Balance Limit to Efficiency, Reviewed 10.4. Multiple Band Gaps 10.5. Tandem Cells 10.5.1. Principles of tandem cells 10.5.2. Analysis 10.5.3. Practical tandem systems 10.6. Intermediate Band and Multiple Band Cells 10.6.1. Principles of intermediate and multiple band cells 10.6.2. Conditions 10.6.3. Practical strategies 10.7. Increasing the Work Per Photon using ‘Hot’ Carriers 10.7.1. Principles of cooling and ‘hot’ carriers 10.7.2. Analysis of the hot carrier solar cell 10.7.3. Practical strategies 10.8. Impact Ionisation Solar Cells 10.8.1. Analysis of impact ionisation solar cell 10.9. Summary References Exercises Solutions to the Exercises Index