PHYSICSOF SOLARCELLS. Jenny Nelson. Imperial College, UK. Imperial College Press ICP

Transcription

1 im- PHYSICSOF SOLARCELLS Jenny Nelson Imperial College, UK ICP Imperial College Press

2 Contents Preface v Chapter 1 Introduction Photons In, Electrons Out: The Photovoltaic Effect Brief History of the Solar Cell Photovoltaic Cells and Power Generation Photovoltaic cells, modules and Systems Some important definitions Characteristics of the Photovoltaic Cell: A Summary Photocurrent and quantum efficiency Dark current and open circuit voltage Efficiency Parasitic resistances Non-ideal diode behaviour Summary 15 References 16 Chapter 2 Photons In, Electrons Out: Basic Principles of PV Introduction The Solar Resource Types of Solar Energy Converter Detailed Balance In equilibrium Under illumination Work Available from a Photovoltaic Device Photocurrent 28

3 viii Contents Darkcurrent Limiting efficiency Effect of band gap Effect of spectrum on efficiency Requirements for the Ideal Photoconverter Summary 38 References 39 Chapter 3 Electrons and Holes in Semiconductors Introduction Basic Concepts Bonds and bands in crystals Electrons, holes and conductivity Electron States in Semiconductors Band structure Conduction band Valence band Direct and indirect band gaps Density of states Electron distribution function Electron and hole currents Semiconductor in Equilibrium Fermi Dirac statistics Electron and hole densities in equilibrium Boltzmann approximation Electron and hole currents in equilibrium Impurities and Doping Intrinsic semiconductors n type doping p type doping Effects of heavy doping Imperfect and amorphous crystals Semiconductor under Bias Quasi thermal equilibrium Electron and hole densities under bias Current densities under bias Drift and Diffusion Current equations in terms of drift and diffusion Validity of the drift-diffusion equations 75

4 Contents ix Current equations for non-crystalline solids Summary 77 Chapter 4 Generation and Recombination Introduction: Semiconductor Transport Equations Generation and Recombination Quantum Mechanical Description of Transition Rates Fermi's Golden Rule Optical processes in a two level System Photogeneration Photogeneration rate Thermalisation Microscopic description of absorption Direct gap semiconductors Indirect gap semiconductors Other types of behaviour Examples and data Recombination Types of recombination Radiative recombination Simplified expressions for radiative recombination Auger recombination Shockley Read Hall recombination Surface and grain boundary recombination Traps versus recombination centres Formulation of the Transport Problem Comments on the transport problem Transport equations in a crystal Summary 115 References 117 Chapter 5 Junctions Introduction Origin of Photovoltaic Action Work Function and Types of Junction Metal-Semiconductor Junction Establishing a field Behaviour in the light Behaviour in the dark 127

5 x Contents Ohmic contacts Limitations of the Schottky barrier junction Semiconductor-Semiconductor Junctions p-n junction p-i-n junction p-n heterojunction Electrochemical Junction Junctions in Organic Materials Surface and Interface States Surface states on free surfaces Effect of interface states on junctions Summary 143 References 144 Chapter 6 Analysis of the p n Junction Introduction The p-n Junction Formation of p-n junction Outline of approach Depletion Approximation Calculation of depletion width Calculation of Carrier^and Current Densities Currents and carrier densities in the neutral regions Currents and carrier densities in the space charge region Total current density General Solution for J(V) p-n Junction in the Dark At equilibrium Under applied bias p-n Junction under Illumination Short circuit Photocurrent and QE in Special cases p-n junction as a photovoltaic cell Effects on p-n Junction Characteristics Effects of parasitic resistances Effect of irradiation Effect of temperature 173

6 Contents xi 6.8:4. Other device structures Validity of the approximations Summary. 175 References 176 Chapter 7 Monocrystalline Solar Cells Introduction: Principles of Cell Design Material and Design Issues Material dependent factors Design factors General design features of p-n junction cells Silicon Material Properties Band structure and optical absorption Doping Recombination Carrier transport Silicon Solar Cell Design Basic Silicon solar cell Cell fabrication Optimisation of Silicon solar cell design Strategies to enhance absorption Strategies to reduce surface recombination Strategies to reduce series resistance Evolution of Silicon solar cell design Future directions in Silicon cell design Alternatives to Silicon III-V Semiconductor Material Properties III-V semiconductor band structure and optical absorption Gallium arsenide Doping Recombination Carrier transport Reflectivity GaAs Solar Cell Design Basic GaAs solar cell Optimisation of GaAs solar cell design Strategies to reduce front surface recombination Strategies to reduce series resistance 207

7 xii Contents Strategies to reduce Substrate cost Summary 208 References 210 Chapter 8 Thin Film Solar Cells Introduction Thin Film Photovoltaic Materials Requirements for suitable materials Amorphous Silicon Materials properties Defects in amorphous material Absorption Doping Transport Stability Related alloys Amorphous Silicon Solar Cell Design Amorphous Silicon p-i-n structures p-i-n solar cell device physics Fabrication of a-si solar cells Strategies to improve a-si cell Performance Defects in Polycrystalline Thin Film Materials Grain boundaries Effects of grain boundaries on transport Depletion approximation model for grain boundary Majority carrier transport Effect of illumination Minority carrier transport Effects of grain boundary recombination on solar cell Performance ^_ / CuInSe 2 Thin Film Solar Cells Materials properties Heterojunctions in thin film solar cell design CuInGaSe 2 solar cell design CdTe Thin Film Solar Cells Materials properties CdTe solar cell design Thin Film Silicon Solar Cells 248

8 Contents xiii Materials properties Microcrystalline Silicon solar cell design Summary References 251 Chapter 9 Managing Light Introduction Photon Flux: A Review and Overview of Light Management Routes to higher photon flux Minimising Reflection Optical properties of semiconductors Antireflection coatings Concentration Limits to concentration Practical concentrators Effects of Concentration on Device Physics Low injection High injection Limits to efficiency under concentration Temperature Series resistance Concentrator cell design Concentrator cell materials Light Confinement Light paths and ray tracing Mirrors Randomising surfaces Textured surfaces Practical schemes Light confining structures: restricted acceptance areas and external cavities Effects of light trapping on device physics Photon Recycling Theory of photon recycling Practical schemes Summary 286 References 288

9 xiv Contents Chapter 10 Over the Limit: Strategies for High Efficiency Introduction How Much is Out There? Thermodynamic Limits to Efficiency Detailed Balance Limit to Efficiency, Reviewed Multiple Band Gaps Tandem Cells Principles of tandem cells Analysis Practical tandem Systems Intermediate Band and Multiple Band Cells Principles of intermediate and multiple band cells Conditions Practical strategies Increasing the Work Per Photon using 'Hot' Carriers Principles of cooling and 'hot' carriers Analysis of the hot carrier solar cell Practical strategies Impact Ionisation Solar Cells Analysis of impact ionisation solar cell Summary 323 References 324 Exercises 327 Solutions to the Exercises 337 Index, 355