Principles of Solar Cells, LEDs and Diodes E-Book

Adrian Kitai

• Herausgeber: John Wiley & Sons
• Kategorie: Wissenschaft und neue Technologien
• Sprache: Englisch

Seitenzahl: 509

Veröffentlichungsjahr: 2011

Beschreibung

This textbook introduces the physical concepts required for a comprehensive understanding of p-n junction devices, light emitting diodes and solar cells. Semiconductor devices have made a major impact on the way we work and live. Today semiconductor p-n junction diode devices are experiencing substantial growth: solar cells are used on an unprecedented scale in the renewable energy industry; and light emitting diodes (LEDs) are revolutionizing energy efficient lighting. These two emerging industries based on p-n junctions make a significant contribution to the reduction in fossil fuel consumption. This book covers the two most important applications of semiconductor diodes - solar cells and LEDs - together with quantitative coverage of the physics of the p-n junction. The reader will gain a thorough understanding of p-n junctions as the text begins with semiconductor and junction device fundamentals and extends to the practical implementation of semiconductors in both photovoltaic and LED devices. Treatment of a range of important semiconductor materials and device structures is also presented in a readable manner. Topics are divided into the following six chapters: * Semiconductor Physics * The PN Junction Diode * Photon Emission and Absorption * The Solar Cell * Light Emitting Diodes * Organic Semiconductors, OLEDs and Solar Cells Containing student problems at the end of each chapter and worked example problems throughout, this textbook is intended for senior level undergraduate students doing courses in electrical engineering, physics and materials science. Researchers working on solar cells and LED devices, and those in the electronics industry would also benefit from the background information the book provides.

Leseprobe

Contents

Cover

Title Page

Copyright

Dedication

Introduction

Acknowledgements

Chapter 1: Semiconductor Physics

1.1 Introduction

1.2 The Band Theory of Solids

1.3 The Kronig–Penney Model

1.4 The Bragg Model

1.5 Effective Mass

1.6 Number of States in a Band

1.7 Band Filling

1.8 Fermi Energy and Holes

1.9 Carrier Concentration

1.10 Semiconductor Materials

1.11 Semiconductor Band Diagrams

1.12 Direct Gap and Indirect Gap Semiconductors

1.13 Extrinsic Semiconductors

1.14 Carrier Transport in Semiconductors

1.15 Equilibrium and Non-Equilibrium Dynamics

1.16 Carrier Diffusion and the Einstein Relation

1.17 Quasi-Fermi Energies

1.18 The Diffusion Equation

1.19 Traps and Carrier Lifetimes

1.20 Alloy Semiconductors

1.21 Summary

Suggestions for Further Reading

Problems

Chapter 2: The PN Junction Diode

2.1 Introduction

2.2 Diode Current

2.3 Contact Potential

2.4 The Depletion Approximation

2.5 The Diode Equation

2.6 Reverse Breakdown and the Zener Diode

2.7 Tunnel Diodes

2.8 Generation/Recombination Currents

2.9 Ohmic Contacts, Schottky Barriers and Schottky Diodes

2.10 Heterojunctions

2.11 Alternating Current (AC) and Transient Behaviour

2.12 Summary

Suggestions for Further Reading

Problems

Chapter 3: Photon Emission and Absorption

3.1 Introduction to Luminescence and Absorption

3.2 Physics of Light Emission

3.3 Simple Harmonic Radiator

3.4 Quantum Description

3.5 The Exciton

3.6 Two-Electron Atoms

3.7 Molecular Excitons

3.8 Band-to-Band Transitions

3.9 Photometric Units

3.10 Summary

Suggestions for Further Reading

Problems

Chapter 4: The Solar Cell

4.1 Introduction

4.2 Light Absorption

4.3 Solar Radiation

4.4 Solar Cell Design and Analysis

4.5 Thin Solar Cells

4.6 Solar Cell Generation as a Function of Depth

4.7 Solar Cell Efficiency

4.8 Silicon Solar Cell Technology: Wafer Preparation

4.9 Silicon Solar Cell Technology: Solar Cell Finishing

4.10 Silicon Solar Cell Technology: Advanced Production Methods

4.11 Thin Film Solar Cells: Amorphous Silicon

4.12 Telluride/Selenide/Sulphide Thin-Film Solar Cells

4.13 High-Efficiency Multijunction Solar Cells

4.14 Concentrating Solar Systems

4.15 Summary

Suggestions for Further Reading

Problems

Chapter 5: Light Emitting Diodes

5.1 Introduction

5.2 LED Operation and Device Structures

5.3 Emission Spectrum

5.4 Non-Radiative Recombination

5.5 Optical Outcoupling

5.6 GaAs LEDs

5.7 GaAs1−xPx LEDs

5.8 Double Heterojunction AlxGa1−xAs LEDs

5.9 AlGaInP LEDs

5.10 Ga1−xInxN LEDs

5.11 LED Structures for Enhanced Outcoupling and Power Output

5.12 Summary

Suggestions for Further Reading

Problems

Chapter 6: Organic Semiconductors, OLEDs and Solar Cells

6.1 Introduction to Organic Electronics

6.2 Conjugated Systems

6.3 Polymer OLEDs

6.4 Small-Molecule OLEDs

6.5 Anode Materials

6.6 Cathode Materials

6.7 Hole Injection Layer

6.8 Electron Injection Layer

6.9 Hole Transport Layer

6.10 Electron Transport Layer

6.11 Light Emitting Material Processes

6.12 Host Materials

6.13 Fluorescent Dopants

6.14 Phosphorescent Dopants

6.15 Organic Solar Cells

6.16 Organic Solar Cell Materials

6.17 Summary

Suggestions for Further Reading

Problems

Appendix 1: Physical Constants

Appendix 2: Properties of Semiconductor Materials

Appendix 3: The Boltzmann Distribution Function

The Boltzmann Distribution Function

Index

This edition first published 2011

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Library of Congress Cataloging-in-Publication Data

Kitai, Adrian, 1957–

Principles of solar cells, LEDs, and diodes : the role of the PN junction / Adrian Kitai. p. cm. Includes bibliographical references and index. ISBN 978-1-4443-1834-0 (hardback) – ISBN 978-1-4443-1833-3 (paper) 1. Diodes, Semiconductor. 2. Light emitting diodes. 3. Semiconductors–Junctions. 4. Solar cells. I. Title. TK7871.86.K48 2011 621.3815′2–dc22 2011010956

A catalogue record for this book is available from the British Library.

HB ISBN: 9781444318340 PB ISBN: 9781444318333 ePDF ISBN: 9781119974550 oBook ISBN: 9781119974543 ePub ISBN: 9781119975236 eMobi ISBN: 9781119975243

Dedicated to my wife Tomoko

Introduction

Semiconductor devices have revolutionized the way we work and live. Transistors are thought of as one of the most important developments of the twentieth century and they have given rise to the computer age as well as to compact, reliable electronics found in everything from televisions to cell phones.

An even more fundamental semiconductor device exists, however. It is the semiconductor diode or p-n junction diode. Diodes had been developed before the transistor and were used for rectification whereby alternating current can be converted to direct current by employing the unidirectional property of diodes: current normally only flows efficiently in one direction through a diode, and current flow is blocked in the opposite direction. This property of diodes is exploited in power supplies as well as in many other circuits such as those found in radios and limiters. Since an understanding of diodes is required to explain the principles of transistors, diodes are frequently presented as a stepping stone to the transistor.

In the twenty-first century, however, two new major industries are undergoing very rapid developments based directly on the p-n junction diode. Photovoltaic (PV) solar cells and light emitting diodes (LEDs) are both p-n junctions that are designed and optimized to either absorb or emit light. In both cases, an energy conversion process between photons and electrons occurs within a p-n junction.

The consequences of this development constitute a revolution in two major industrial sectors:

1. Energy production has relied on hydrocarbons and nuclear power, and although these will continue to be important, the direct conversion of solar radiation into useful power is the key to a long-term, sustainable energy supply. Ninety-seven percent of all renewable energy on earth is in the form of solar radiation. The beginning of the twenty-first century has seen the abrupt growth of a global solar photovoltaic industry in conjunction with the involvement of governments worldwide, and unprecedented growth in PV production and deployment is now underway. The worldwide consumption of silicon semiconductor material for the entire microelectronics industry is being overtaken by its use for solar cells alone.

2. Electric lighting was achieved by the incandescent lamp in the early twentieth century. In the second half of the twentieth century a significant movement to fluorescent lamps and discharge lamps in which a gas is excited into a plasma that radiates energy more efficiently than a tungsten filament occurred. The twenty-first century will give rise to the virtual displacement of both incandescent lamps and fluorescent lamps by LED lamps. In preparation for this revolution, the world's major lighting companies have acquired LED research, development and production capabilities. LEDs have already achieved lower power consumption and longer life in small, rugged packages than either incandescent or fluorescent lamps. They also eliminate heavy metals such as mercury used in fluorescent lamps.

The purpose of this book, therefore, is to introduce the physical concepts required for a thorough understanding of p-n junctions starting with semiconductor fundamentals and extending this to the practical implementation of semiconductors in both PV and LED devices. The treatment of a range of important semiconductor materials and device structures is also presented.

The book is aimed at senior undergraduate levels (years three and four). An introductory background in quantum mechanics is assumed, together with general knowledge of junior mathematics, physics and chemistry; however, no background in electronic materials is required. As such this book is designed to be relevant to all engineering students with an interest in semiconductor devices and not specifically to electrical or engineering physics/engineering science students only. This is intentional since solar cells and LEDs involve a wide range of engineering disciplines and should not be regarded as belonging to only one branch of engineering.

In Chapter 1, the physics of solid state electronic materials is covered in detail starting from the basic behaviour of electrons in crystals. The quantitative treatment of electrons and holes in energy bands is presented along with the important concepts of excess carriers that become significant once semiconductor devices are either connected to sources of power or illuminated by light. A series of semiconductor materials and their important properties is also reviewed. The behaviour of semiconductor surfaces and trapping concepts are also introduced since they play an important role in solar cell and LED device performance.

In Chapter 2, the basic physics and important models of a p-n junction device are presented. The approach taken is to present the diode as a semiconductor device that can be understood from the band theory covered in Chapter 1. Various types of diode behaviour, including tunnelling, metal-semiconductor contacts and heterojunctions, are presented as well as reverse breakdown behaviour.

Chapter 3 introduces the theory of photon emission and absorption, a topic that books on semiconductor devices frequently pay less attention to. The standard description that a photon is created when an electron and a hole recombine, or a photon is absorbed when an electron and a hole are generated, is not adequate for a deeper understanding of photon emission and absorption processes. In this chapter the physics of photon creation is explained with a minimum of mathematical complexity, and these concepts are much better understood by following radiation theory and describing the oscillating dipole both classically and using simple quantum mechanics. A section of Chapter 3 describes the exciton relevant to inorganic semiconductors as well as the molecular exciton for organic semiconductors. In addition lineshapes predicted for direct-gap semiconductors are derived. Finally the subject of photometric units introduces the concepts of luminance and colour coordinates that are essential to a discussion of organic and inorganic light emitting diodes.

Chapter 4 covers inorganic solar cells. The concepts regarding the p-n junction introduced in Chapter 2 are further developed to include illumination of the p-n junction and the simplest possible modelling is used to illustrate the behaviour of a solar cell. Then a more realistic solar cell structure and model are presented along with the attendant surface recombination and absorption issues that must be understood in practical solar cells. A series of solar cell technologies are reviewed starting with bulk single and multicrystalline silicon solar cell technology. Amorphous silicon materials and device concepts are presented. Solar cells made using other semiconductors such as CdTe are introduced followed by multijunction solar cells using layered, lattice-matched III-V semiconductor stacks.

Chapter 5 on inorganic LEDs considers the basic LED structure and its operating principles. The measured lineshape of III-V LEDs is compared with the predictions of Chapter 3. LEDs must be engineered to maximize radiative recombination, and energy loss mechanisms are discussed. The series of developments that marked the evolution of current, high-efficiency LED devices is presented starting from the semiconductors and growth techniques of the 1960s, and following trends in succeeding decades that brought better materials and semiconductor growth methods to the LED industry. The double heterojunction is introduced and the resulting energy well is analysed on the basis of the maximum current density that can be accommodated before it becomes saturated. LED optical outcoupling, which must also be maximized to achieve overall efficiency, is modelled and strategies to optimize outcoupling are discussed. Finally the concept of spectral down-conversion using phosphor materials and the white LED are introduced.

Chapter 6 introduces new concepts required for an understanding of organic semiconductors in general, in which conjugated molecular bonding gives rise to π bands and HOMO and LUMO levels in organic semiconductors. The organic LED is introduced by starting with the simplest single active layer polymer-based LED followed by successively more complex small-molecule LED structures. The roles of the various layers, including electrodes and carrier injection and transport layers, are discussed and the relevant candidate molecular materials are described. Concepts from Chapter 3, including the molecular exciton and singlet and triplet states are used to explain efficiency limitations in the light generation layer of small-molecule OLEDs. In addition the opportunity to use phosphorescent host-guest light emitting layers to improve device efficiency is explained. The organic solar cell is introduced and the concepts of exciton generation and exciton dissociation are described in the context of the heterojunction and the bulk heterojunction. The interest in the use of fullerenes and other related nanostructured materials is explained for the bulk heterojunction.

All the chapters are followed by problem sets that are designed to facilitate familiarity with the concepts and a better understanding of the topics introduced in the chapter. In many cases the problems are quantitative and require calculations; however, a number of more conceptual problems are presented and are designed to give the reader experience in using the Internet or library resources to look up further information. These problems are of particular relevance in Chapters 4, 5 and 6, in which developments in solar cells and LEDs are best understood by referring to the recent literature once the basic concepts are understood.

Adrian Kitai

Acknowledgements

I would like to acknowledge the many people who helped with this book, including Ayse Turak for her advice, students Huaxiang Shen, Bo Li and Alexander Subotich, McMaster University staff Laura Honda, Ginny Riddell, Janet Delsey and Regina Bendig, Wiley staff Rebecca Stubbs, Emma Strickland, Amie Marshall, Mohan Tamilmaran, Robert Hine, Sarah Tilley and John Peacock and Project Manager Shalini Sharma, Production Head Kamal Kishore, Manish Gupta of Aptara. I owe a special debt of gratitude to my wife Tomoko for her steady encouragement, her patience and her considerable help in obtaining the permissions for figures.

1

Semiconductor Physics

1.1 Introduction

1.2 The Band Theory of Solids

1.3 The Kronig–Penney Model

1.4 The Bragg Model

1.5 Effective Mass

1.6 Number of States in a Band

1.7 Band Filling

1.8 Fermi Energy and Holes

1.9 Carrier Concentration

1.10 Semiconductor Materials

1.11 Semiconductor Band Diagrams

1.12 Direct Gap and Indirect Gap Semiconductors

1.13 Extrinsic Semiconductors

1.14 Carrier Transport in Semiconductors

1.15 Equilibrium and Non-Equilibrium Dynamics

1.16 Carrier Diffusion and the Einstein Relation

1.17 Quasi-Fermi Energies

1.18 The Diffusion Equation

1.19 Traps and Carrier Lifetimes

1.20 Alloy Semiconductors

1.21 Summary

Suggestions for Further Reading

Problems

Objectives

1. Understand semiconductor band theory and its relevance to semiconductor devices.

2. Obtain a qualitative understanding of how bands depend on semiconductor materials.

3. Introduce the concept of the Fermi energy.

4. Introduce the concept of the mobile hole in semiconductors.

5. Derive the number of mobile electrons and holes in semiconductor bands.

6. Obtain expressions for the conductivity of semiconductor material based on the electron and hole concentrations and mobilities.

7. Introduce the concepts of doped semiconductors and the resulting electrical characteristics.

8. Understand the concept of excess, non-equilibrium carriers generated by either illumination or by current flow due to an external power supply.

9. Introduce the physics of traps and carrier recombination and generation.

10. Introduce alloy semiconductors and the distinction between direct gap and indirect gap semiconductors.

1.1 Introduction

A fundamental understanding of electron behaviour in crystalline solids is available using the band theory of solids. This theory explains a number of fundamental attributes of electrons in solids including:

i. concentrations of charge carriers in semiconductors;

ii. electrical conductivity in metals and semiconductors;

iii. optical properties such as absorption and photoluminescence;

iv. properties associated with junctions and surfaces of semiconductors and metals.

The aim of this chapter is to present the theory of the band model, and then to exploit it to describe the important electronic properties of semiconductors. This is essential for a proper understanding of p-n junction devices, which constitute both the photovoltaic (PV) solar cell and the light-emitting diode (LED).

1.2 The Band Theory of Solids

There are several ways of explaining the existence of energy bands in crystalline solids. The simplest picture is to consider a single atom with its set of discrete energy levels for its electrons. The electrons occupy quantum states with quantum numbers n, l, m and s denoting the energy level, orbital and spin state of the electrons. Now if a number N of identical atoms are brought together in very close proximity as in a crystal, there is some degree of spatial overlap of the outer electron orbitals. This means that there is a chance that any pair of these outer electrons from adjacent atoms could trade places. The Pauli exclusion principle, however, requires that each electron occupy a unique energy state. Satisfying the Pauli exclusion principle becomes an issue because electrons that trade places effectively occupy new, spatially extended energy states. The two electrons apparently occupy the same spatially extended energy state.

In fact, since outer electrons from all adjacent atoms may trade places, outer electrons from all the atoms may effectively trade places with each other and therefore a set of outermost electrons from the atoms all appear to share a spatially extended energy state that extends through the entire crystal. The Pauli exclusion principle can only be satisfied if these electrons occupy a set of , spatially extended energy states. This leads to a set of slightly different energy levels for the electrons that all originated from the same atomic orbital. We say that the atomic orbital splits into an containing a set of electron states having a set of closely spaced energy levels. Additional energy bands will exist if there is some degree of spatial overlap of the atomic electrons in lower-lying atomic orbitals. This results in a set of energy bands in the crystal. Electrons in the lowest-lying atomic orbitals will remain virtually unaltered since there is virtually no spatial overlap of these electrons in the crystal.