Acknowledgements Authors: Gary Cook, Lynn Billman, and Rick Adcock Typography, Design, and Artwork: Susan Sczepanski Cover Design: Susan Sczepanski and Ray David Editing: Paula Pitchford, James Jones, and Barbara Glenn Technical Advisors and Reviewers: Michael Thomas, Sandia National Laboratories and Ken Zweibel, Solar Energy Research Institute. This publication was produced under the Solar Technical Information Program at the Solar Energy Research In­ stitute (SERI) for the U.S. Department of Energy (DOE). We give special thanks to Vincent Rice and Robert Annan of the DOE for their support of the project and for their advice and review. Photovoltaic FUNDAMENTALS Contents Introduction . 1 1 An Overview of Progress . 2 A Growing Market .. .. 4 A Promising Future . ........... 6 2 The Photovoltaic Effect . ... ... ... 8 Energy from the Sun . .8 An Atomic Description of Silicon ..... 10 Light Absorption: Creating Charge Carriers 12 Forming the Electric Field . 12 The Electric Field in Action: Driving the Charge Carriers . 15 Energy Band Gaps . ...... 16 3 Solar Cells . ....... 18 Single-Crystal Silicon Cells . 18 Beyond Single-Crystal Silicon-A Wide Range of Materials .. 24 Semicrystalline and Polycrystalline Silicon Cells . 25 Thin-Film Solar Cells . ..... 27 Gallium Arsenide Solar Cells 36 Multijunction Devices 39 4 Modules, Arrays, and Systems . .. .. .... 44 Flat-Plate Collectors ........... 45 Concentrator Collectors so Balance of Systems . 56 For Further Reading . ... ... 62 ii Introduction p hotovoltaic (PY) sys- • They are reliable and process of the technology, terns convert sunlight long-lived. the photovoltaic effect. into electricity. Once an • They use solid-state Next, we consider how exotic technology used technology and are scientists and engineers almost exclusively on easily mass-produced have harnessed this satellites in space, photo- and installed. process to generate voltaics has come down to Photovoltaic devices electricity in silicon solar Earth to find rapidly ex- can be made from many cells, thin-film devices, panding energy markets. different materials in many and high-efficiency cells. Many thousands of PY sys- different designs. The We then look at how these terns have been installed diversity of PV materials devices are incorporated around the globe. For cer- and their different charac- into modules, arrays, and tain applications, such as teristics and potentials power-producing systems. remote communications, demonstrate the richness We have written and PY systems provide the of this growing technology. designed this book so that most cost-effective source This booklet describes the reader may approach of electric power possible. how PV devices and the subject on three dif- Several important char- systems work. It also ferent levels. First, for the acteristics of PV systems describes the specific person who is in a hurry make them a desirable materials and devices that or needs a very cursory source of power: are most widely used com- overview, in the margins • They rely on sunlight. mercially as of 1990 and of each page we generalize • They generate electricity those that have the the important points of with little impact on the brightest prospects. Stu- that page. Second, for a environment. dents, engineers, scientists, somewhat deeper under- • They have no moving and others needing an standing, we have pro- parts to wear out. introduction to basic PY vided ample illustrations, • They are modular, technology, and manufac- photographs, and cap- which means they can turers and consumers who tions. And third, for a be matched to a need want more information thorough introduction to for power at any scale. about PV systems should the subject, the reader can • They ca n be used as find this booklet helpful. resort to reading the text. independent power We begin with an over- sources, or in combina- view and then explain the tion with other sources. rudimentary physical Chapter 1 An Overview of Progress rench physicist exposed to light. The effect sunlight energy that a • The photovoltaic FEdmond Becquerel was first studied in solids, cell converts to electrical effect was first described the photo­ such as selenium, by energy.) Selenium was discovered in voltaic effect in 1839, but Heinrich Hertz in the quickly adopted in the 1839. it remained a curiosity of 1870s. Soon afterward, emerging field of photog­ • Bell Laboratories science for the next three­ selenium photovoltaic raphy for use in light­ scientists quarters of a century. (PV) cells were converting measuring devices. developed the first Becquerel found that light to electricity at 1% to Major steps toward viable PV cells in certain materials would 2% efficiency. (The conver­ commercializing PY were 1954. produce smaJJ amounts sion efficiency of a PV taken in the 1940s and of electric current when eel I is the proportion of early 1950s when the Czochralski process for producing highly pure crystalline silicon was developed. In 1954, scientists at Bell Labora­ tories depended on the Czochralski process to develop the first crystal­ line silicon photovoltaic (or solar) cell, which had an efficiency of 4%. Although a few at­ tempts were made in the 1950s to use silicon cells in commercial products, it was the new space pro­ gram that gave the tech­ nology its first major application. In 1958, the U.S. Vanguard space satel­ lite carried a small array of This photovoltaic module was developed in 1966 by TRW for use on a space mission. PV cells to power its radio. The cells worked so well that PV technology has 2 Shortly after the development of silicon solar cells, ads like this touted photovoltaics as a new and promising source of electricity. But the development of affordable PV cells for use on Earth had to wait for the start of the federal/industry R&D program, some 20 years later. (Poster reproduction courtesy of Bell Laboratories and ARCO Solar News.) been part of the space pro­ mid-1970s, rising energy and production. Often, II 1 / • In .the 1950s, the gram ever since. Today, costs, sparked by a world industry and the federal space program solar cells power virtually oil crisis, renewed interest government work · ushered in PV's all satellites, including in making PV technology together, sharing the cost ".\.•:: : . ~ )irsf application. ·· those used for communica­ more affordable. Since of PV research and · · ~9!afcells power tion s, defense, and scien­ then, the federal govern­ development (R&D). yirtually all of tific research. The U.S. ment, industry, and Much of this effort has · t~~a,y ' s satellites ~ space shuttle fleet uses PV research organizations gone into the development arrays to generate much have invested hundreds of crystalline silicon, the • .Despite advances, -~· .. of its electrical power. of millions of do llars in material Bell's scientists J.n a ~e " early ,1970s The computer industry, research, development, used to make the first PV'was still too especially transistor semi­ expensive for conductor technology, also terrestrial use. contributed to th e develop­ ment of PV cells. Transis­ ~f~energy crisesJtrL .; tors and PV cells are made · ofthei.191os sparked a m.ajor from similar materials 1 and operate on the basis effor! by the ~'g o·vefo;, of similar physica l ~ .. ment! and mechanisms. Advances in ~i i ~.~u~try..'t~ . m~~,~ transistor research have Lft"" :.'more affordable: ... provided a steady flow I'• I •• : •. ~'.,·~~ of new information about PV cell technology. Today, however, this technology transfer process often works in reverse, as ad van­ ces in PV research and development are some­ times adopted by the semi­ conductor industry. Despite these advances, photovoltaics in 1970 was Photovoltaic cells were first used in space to power a 5-mW backup transmitter on the Vanguard 1 in 1958. Today, solar cells power most still too expensive for most satellites; even the U.S. shuttle fleet uses PV to generate much of its terrestrial uses. In the electrical power. (Rendition courtesy of Lockheed.) 3 practical cells. As a result, 5% to 15% of sunlight into Shipments of PV modules crystalline silicon devices electricity. They are highly have risen steadily over have become more and reliable, and they last 20 the last few years. In the more efficient, reliable, years or longer. The cost of last three years, world­ and durable. Industry and PY-generated electricity wide module shipments government have also ex­ has dropped 15- to 20-fold, have grown by 60% to plored a number of other and PV modules now cost nearly 47 million watts promising materials, such around $6 per watt (W) (MW). Revenues for the as noncrystalline (amor­ and produce electricity for PV industry could be phous) silicon, polycrystal­ as little as 25cz: to 30ct per greater than $800 million line cadmium telluride kilowatt-hour (kWh). in 1991. Japan, European and copper indium di­ nations, China, India, and selenide, and other single­ A Growing other countries have either crystal materials like Market established new PV pro­ gallium arsenide. Price reductions have grams or expanded exist­ Today's commercial PV helped to spur a growing ing ones. The number of -'~'""'! systems can convert from market for photovoltaics. organizations involved in 100 ~ 50 "O a.Ql -;;; a. 00 80 :c 40 00 (/) <O~ ~ 0 "' <? :iE < 60 -;;; 30 Ql "' ~ :; !/) "O Ql 0 .g 40 E 20 a. Ql 2 (ij ~ "O 0 20 '?- 10 :iE Jg 0 1975 1978 1981 1984 1987 1990 1975 1978 1981 1984 1987 1990 Since the United States started its terrestrial PV program in the mid-1970s, the cost of PV modules has dropped from more than $100/W to less than $6/W, and module shipments have risen from nearly zero to approximately 47 MW. 4 Photovoltaic cells power millions of small consumer products such as calculators, radios, watches, car window defrosters, and - as shown here - walk lights. manufacturing and R&D · grew from less than a r~~Mu!tori~ qt ,-¢9~ ·, - < suriler'.; p~oducts, .