List of Symbols and Terms / ix

Preface / xv

Chapter 1 Introduction to Solar Cells / 1

1.1 Philosophy of the Text / 1

1.2 Renewable Energy and Photovoltaics Background / 2

1.3 What is a Solar Cell? / 4

1.4 Solar Cell Modules / 10

Chapter 2 Absorbing Solar Energy / 15

2.1 Air Mass and the Solar Spectrum / 15

2.2 Optical Properties of Solar-Cell Materials / 19

2.2.1 Absorptivity / 19

2.2.2 Absorption coefficient / 20

2.2.3 Solar cell bandgap / 23

2.2.4 Antireflection coatings / 26

2.2.5 Thickness determination / 29

2.2.6 Predicting absorptivity / 30

2.3 Photoluminescence / 32

Chapter 3 Solar Cell Equations / 37

3.1 PV Device Characteristics / 37

3.2 Quantum Efficiency for Current Collection / 41

3.3 Lifetime, Diffusion Length, and Electron Concentrations / 44

3.4 The Transport Equation and Current Extraction / 46

Chapter 4 Photoelectrochemistry / 51

4.1 Basic Photoelectrochemistry / 51

4.2 The Dye-sensitized Nanocrystalline Solar Cell / 51

Chapter 5 Generalized Model of a Solar Converter / 57

5.1 General Thermodynamics of Light Conversion / 57

5.2 Detailed Balance: The Generalized Planck Equation / 59

5.3 The Luminescent Output / 65

5.4 The Relationship Between Voltage and Luminescence Efficiency / 66

5.5 Current-Voltage Characteristics and Luminescence / 67

Chapter 6 Concentrators of Light / 71

6.1 The Thermodynamic Limits of Light Concentrators / 71

6.2 Geometrical Optics / 71

6.2.1 General theory and sine brightness law / 71

6.2.2 Examples of ideal geometric concentrators / 73

6.3 Active Concentrators (Luminescent Systems) / 77

6.3.1 Theory of luminescent systems / 77

6.4 Light Enhancement in the Environment / 78

Chapter 7 Economics of Photovoltaic Cells and Systems / 81

7.1 The Basics of PV Economics / 81

7.2 Estimated Solar Module Cost / 82

7.3 Economics of Photovoltaic Systems / 83

7.4 Economics of Solar Energy in the World Economy / 85

7.5 Conclusions and Further Study / 90

Appendix / 91

A Basic Solar Energy Library for the Optical Specialist / 93

Index / 97

Preface

With concerns about worldwide environmental security, global warming, and climate change due to emissions of CO2 from the burning of fossil fuels, it is desirable to have a wide range of energy technologies in a nation's portfolio. These technologies can be used in domestic markets, or exported to other nations, helping them to "leapfrog" to a cleaner, and less carbon intensive, energy path. Far from being an altruistic act, these energy technologies are lucrative businesses that will grow stronger in the global economy of the 21st century. According to U.S. DOE EIA, NREL U.S. PV Industry Technology Roadmap 1999 Workshop and Strategies Unlimited, photovoltaics (or PV) is a billion dollar a year industry and is expected to grow at a rate of 15-20% per year over the next few decades. Solar cells have already proven themselves a viable option as a nonpolluting renewable energy source in many applications. It is advantageous to optical engineers to have at least a basic knowledge of how these devices function, and of the important parameters that control their operation. This text is designed to be an overview for those in the fields of optics and optical engineering, as well as those who are interested in energy policy, economics, and the requirements for efficient photo-to-electric energy conversion.

Greg P. Smestad
April 2002