c Notice This publication was prepared under a contract to the United States Government. Neither the United States nor the United States Department of Energy, nor any of their employees, nor any of their contractors, subcontractors, or their employees, makes any warranty, expressed or implied, or assumes any legal liability or responsibility for the accuracy, completeness or usefulness of any information, apparatus, product or process disclosed, or represents that its use would not infringe privately owned rights. Printed in the United States of America Available in print from: Superintendent of Documents U.S. Government Printing Office Washington, DC 20402 Available in microfiche from: National Technical Information Service U.S. Department of Commerce 5285 Port Royal Road Springfield, VA 22161 Stock Number: SERIISP-290-1448 Information in this publication is current as of September 1981 Basic Photovoltaic Principles and Me1hods SER I/SP-290-1448 Solar Information Module 6213 Published February 1982 This book presents a nonmathematical explanation of the theory and design of PV solar cells and systems. It is written to address several audiences: engineers and scientists who desire an introduction to the field of photovoltaics, students interested in PV science and technology, and end users who require a greater understanding of theory to supplement their applications. The book is effectively sectioned into two main blocks: Chapters 2-5 cover the basic elements of photovoltaics-the individual electricity-producing cell. The reader is told why PV cells work, and how they are made. There is also a chapter on advanced types of silicon cells. Chapters 6-8 cover the designs of systems constructed from individual cells-including possible constructions for putting cells together and the equipment needed for a practioal producer of electrical energy. In addition, Chapter 9 deals with PV's future. Chapter 1 is a general introduction to the field. The authors of this document are Paul Hersch and Kenneth Zweibel. They would like to thank their colleagues at the Solar Energy Research Institute's Solar Electric Conversion Division who reviewed the manuscript for tech­ nical accuracy: Richard Bird, Kathryn Chewey, Satyen Deb, Keith Emery, Kay Firor, Steve Hogan, Larry Kazmerski, Jack Stone, Thomas Surek, and • Simon Tsuo. Gary Cook and Richard Piekarski of the Technical Information Office, who designed the document, were also helpful readers. Graphic Directions of Boulder, Colorado, was responsible for the text's figures, often with valuable improvements. Ray David was the cover artist. Vincent Rice of the Photovoltaics Program Office at DOE was supportive through­ out, giving impetus to the project. ollshed by Technical Information Office .Iar Energy Research Institute 1617 Cole Boulevard, Golden, Colorado 80401 erated for the U.S. Department of Energy by the Midwest Research Institute Contents Page Chapter 1. Introduction The Sun . 5 The Nature of Light Energy . 5 Sunlight Reaching Earth . 6 Photovoltaics-A History . 7 Bibliography . 8 Chapter 2.The Photovoltaic (PV) Effect Highlights .................................................................................. 9 An Atomic Description of Silicon. .............................................................. 9 The Effect of Light on Silicon .................................................................. 9 The Potential Barrier. ...................................................................... .. 10 The Function of the Barrier ................................................................ .. 10 Forming the Barrier ...................................................................... .. 11 The Potential Barrier in Action. ............................................................ .. 14 The Electric Current ....................................................................... .. 15 Bibliography. ............................................................................. .. 15 Chapter 3. Physical Aspects of SolarCell Efficiency Highlights. .......... .. .................................................................... .. 17 Reflection. ............................................................................... .. 17 Light with Too Little or Too Much Energy " 17 Recombination of Electron-Hole Pairs. ........................................................ .. 19 Direct Recombination 19 Indirect Recombination " 20 Resistance " 20 Self-Shading " 21 Performance Degradation at Nonoptimal Temperatures " 21 High-Temperature Losses " 21 Low-Temperature Losses " 22 Bibliography. ............................................................................. .. 22 Chapter 4.TheTypical Single-Crystal Sificon SolarCell Highlights. ............................................................................... .. 23 Making the Base Layer 23 Making Single-Crystal Silicon. ............................................................. .. 24 Making Wafers ............................................................................ 26 Forming the pn Junction " 26 Antireflective Coatings and Electrical Contacts " 27 Bibliography. ............................................................................. .. 28 2 Basic Photovoltaic Principles and Methods Page Chapter 5. Advances in Single·Crystal Silicon SolarCells Highlights ................................................................................ .. 29 New Fabrication. .......................................................................... .. 29 Edge-Defined Film-Fed Growth (EFG) ; ...................................... .. 29 Dendritic Web Growth. ................................................................... .. 30 Ribbon-to-Ribbon (RTR) Growth. ........................................................... .. 30 Innovative Cell Designs. .................................................................... .. 31 Back-Surface Fields (BSF) and Other Minority Carrier Mirrors (MCM) . .. 31 Schottky Barrier Cells. .................................................................... .. 32 Inversion Layer Cells. .................................................................... .. 34 Cells for Concentrated Sunlight. .................................................. .. ....... .. 34 Advances in Component Technology. ....................................................... .. 35 Bibliography. ............................................................................. .. 37 Chapter 6. SolarArrays Highlights ......................................... .. ...................................... .. 39 PV Building Blocks 39 Boosting Voltage and Amperage. ........................................................... .. 39 Design Requirements for Connecting Components 40 The Physical Connection. ...................................................... .. ............ .. 41 Placing the Cells. .......................................................................... .. 41 Array Support. ............................................................................ .. 42 Module Covers ............................................................................ .. 43 Module Cooling ........................................................................... .. 43 Hybrid Designs , ...................... .. 44 Brayton Cycle Electricity Production. ....................................................... .. 44 Thermoelectric Generators ................................................................ .. 44 Fitting the Pieces .... ...................................................................... .. 45 Bibliography. ............................................................................. .. 45 Chapter 7. SolarArray Constructions Highlights. ............................................................................... .. 47 Intercepting Sunlight. ...................................................................... .. 47 Arrays with Relectors ..................................................................... .. 47 Arrays that Follow the Sun 48 Controlling Intensity. ...................................................................... .. 49 Imaging Optics. ........................................................................... .. 49 Mirrors. ............................................................................... ... 49 Lenses. ................................................................................ .. 51 Tracking Devices .......................................................................... .. 52 Steering Mechanisms. .................................................................... .. 53 Tracking Device Controls 54 Optimizing the Use of the Spectrum. .......................................................... .. 54 Splitting the Spectrum. ................................................................... .. 54 Converting the Spectrum to a Single Color 55 Bibliography. ............................................................................. .. 55 Chapter 8. PVSupport Equipment Highlights .............................................................. .. ................ .. 57 PV vs Conventional Electricity. .............................................................. .. 57 Storing PV's Electricity. .................................................................... .. 58 Batteries. ............................................................................... .. 60 Fuel Cells ,.. .. 60 Contents 3 Page Power Conditioning Equipment. ............................................................. .