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 (/)

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, . -; dozen in the early 1970s to ""suciiias.. ca1cliiators ·.1 more than 200 in 1990. ( and portable ~1 The market is active in rvs, ;, -~ are powered by ' . three principal areas: con­ ' .small PV systems. sumer products, stand­ I ' alone systems, and utility · •• Mid-size, stand- applications. Millions of al<;me PV systems small PV systems (produc­ are effective power ing from a few milliwatts, sources, especially or a few thousandths of a for remote watt, to a few watts) cur­ applications. rently power watches, cal­ culators, radios, portable TVs, walk lights, and a variety of other consumer goods. These systems are typically made with thin­ film amorphous silicon material. The largest of the photovoltaic markets is for mid-size, stand-alone sys­ tems, producing from a few watts of power to a few thousand watts. These are most often used to pro­ vide electricity in remote locations not served by a utility grid. Stand-alone Mid-size PV systems (from a few watts to a few thousand watts) PY systems are used in provide power for remote applications all over the world, including automated applications water pumping. refrigeration , highway lighting, and village power. such as highway lighting, Above, photovoltaics power a water-level-monitoring station on the Laramie River in Central Wyoming. navigational buoys,

5 lighthouses, microwave • Tens of thousands repeater stations, and of homes and weather stations. These cabins worldwide stand-alone PY systems rely on PV systems have proven to be reliable, for most or all of maintenance-free, and their electrical cost-effective power needs. sources. Also, tens of • Electric utilities are thousands of homes and experimenting with cabins worldwide now large PV generating rely on PV systems for stations. most or all of their electri­ cal needs. Solar electricity • As the cost of provides power for water photovoltaics pumps, for refrigerators continues to that store vaccines and decrease, through drugs, and for communica­ the avenues of tions. PY systems have As the cost of photovoltaics continues to decrease, t~e technology R&D and mass enormous potential to fur­ becomes increasingly competitive for utilities. Above 1s a 2 - M_~ production, the single-axis-tracking photovoltaic system at Sacramento _Mun~c1pal nish electricity to towns Utility District's Rancho Seco facility in Sacramento, Cahforn1a. technology will and villages throughout become increas­ the world that are not now A Promising through research, develop­ ingly competitive connected to a utility grid. ment, and technology for large-scale py is generally not yet Future transfer, a path that both To expand markets, PV production of competitive with most industry and the federal costs need to be reduced electricity for conventional sources for government are further. There are two utilities. utility-size applications. pursuing. basic ways to reduce these However, several large PV The federal govern­ costs. The first is through systems (producing from ment's R&D program is mass production, an a hundred thousand to managed by the Depart­ avenue that the PY several million watts) do ment of Energy (DOE). industry will follow as provide supplemental The goals of the program it continues to mature power to electric utilities are twofold. ln the short and grow. The second is in the United States and a term, through the few other countries.

6 mid-1990s, the objective is to reduce the cost of PV electricity to about 12¢/kWh, which is the retail cost of electricity in many parts of the United States. The objective for the longer term, past the tum of the century, is to cut the cost of PV electricity in half again Mass production and advances in PV technology are crucial for to about 61t/kWh, which PV systems to become competitive with conventional methods of would make PV costs as generating electricity for utilities. low as those of most con­ ventional sources of photovoltaics in all ap­ already supplying a grow­ generated electricity. plications, from remote ing portion of America's Achieving these goals stand-alone systems to demand for electricity. should help spur the utility power plants. Photovoltaic systems market growth for Photovoltaic systems are are also increasingly becoming an option for the developing world, as the demand for electricity grows to serve the needs 750 of burgeoning national economies and a human ~ population that will m~ 500 Cl double in the next century. Q) :2 250

0

1988 1992 1996 2000

The world market for photovoltaic systems could increase dramatically by the end of this century.

7 Chapter2 The Photovoltaic Effect

he photovoltaic effect a photon is transferred to 11 • lnthe·PVeffed; ·j Energy from Tis the basic physical an electron in an atom of f: sunlight energy process through which a the semiconductor device. the Sun generates;free -· PV cell converts sunlight With its newfound energy, The sun's energy is vital electronsin·a into electricity. Sunlight is the electron is able to es­ to life on Earth. It deter­ , semiconductor' composed of photons­ cape from its normal posi­ mines the Earth's surface ' device to produce packets of solar energy. tion associated with a temperature, and supplies 1 electricity. These photons contain dif­ single atom in the semicon­ virtually all the energy that drives natural global • The sun supplies ferent amounts of energy ductor to become part of systems and cycles. Al­ all the energy that that correspond to the the current in an electrical though some other stars drives natural different wavelengths of circuit. Special electrical are enormous sources of global systems the solar spectrum. When properties of the PV cell­ and cycles. photons strike a PV cell, a built-in electric field­ energy in the form of they may be reflected or provide the voltage x-rays and radio signals, • Each wavelength in absorbed, or they may needed to drive the cur­ our sun releases 95% of the solar spectrum pass right through. The ab­ rent through an external its energy as visible light. corresponds to a sorbed photons generate load. Visible light represents frequency and an_ electricity. The energy of only a fraction of the total energy; the sho.rter radiation spectrum; in­ the wavelength_, frared and ultraviolet rays the higher the. are also significant parts frequency. and the of the solar spectrum. greater the energy. Each portion of the solar spectrum is as­ ·~·-=1 sociated with a different level of energy. Within the visible portion of the n-Type spectrum, red is at the semiconductor I,I Electrical energy low-energy end and violet ~Type 1,1 semiconductor is at the high-energy end (having half again as muc11 energy as red light). In the typical PV cell, photon energy frees electrical charge carriers, Jn the invisible portions which become part of the current in an electrical circuit. A built-in of the spectrum, photons electrical field provides the voltage needed to drive the current through an external load. in the ultraviolet region,

8 Visible ~ Wavelength (m) I I 10-9 10-a 10 7 10-6 10·5 1 o-4 ;a

Ultraviolet Infrared

101 1 1010 101s 1014 1013 101 2 Frequency (Hz) I I I 103 102 101 1~0 I 1Q·2 Photon energy (eV) I I '----v--' Solar radiation

The sun emits virtually all of its radiation energy in a spectrum of wavelengths that range from about 2x1Q·7 to 4x1 Q-6 m. The great majority of this energy is in the visible region. Each wavelength corresponds to a frequency and an energy; the shorter the x-rays and ultraviolet wavelength, the higher the frequency and the greater the energy rays. Yet, the amount of • An average of (expressed in eV, or electron volts; an eV is the energy an electron 1367 W of solar acquires when it passes through a potential of 1 Vin a vacuum). sunshine energy that hits the surface of the Earth energy strikes which cause the skin to Earth receives a tiny frac­ every minute is greater each square meter tan, have more energy tion of this energy; still, than the total amount of of the Earth's outer than those in the visible an average of 1367 W energy that the world's atmosphere. region. Photons in the reaches each square meter human population con­ • Although the 2 infrared region, which (m ) of the outer edge of sumes in a year. atmosphere we feel as heat, have less the Earth's atmosphere. When sunlight reaches absorbs and energy than the photons The atmosphere absorbs Earth, it is distributed un­ reflects this in the visible region. and reflects some of this evenly in different regions. radiation, a vast The movement of light radiation, including most Not surprisingly, the areas amount still from one location to reaches the another can best be Earth's surface. described as though it were a wave, and different • The amount of types of radiation are char­ sunlight striking acterized by their individ­ the Earth varies ual wavelengths. These by region, season, wavelengths-the distance time of day, climate, from the peak of one wave and measure of air to the peak of the next- pollution. i ndicate radiation with dif­ Diffuse ferent amounts of energy; the longer the wavelength, the less the energy. Red BA G06333 15 light, for example, has a longer wavelength and thus has less energy than violet light. Each second, the sun releases an enormous The Earth's atmosphere and cloud cover absorb, reflect, and scatter amount of radiant energy some of the solar radiation entering the atmosphere. Nonetheless, enormous amounts of direct and diffuse sunshine energy reach the into the solar sys tem. The Earth's surface and can be used to produce photovoltaic electricity.

9 Although the quantity of solar radiation striking the Earth varies by region, season, time of day, climate, and air pollution, the yearly amount of energy striking almost any part of the Earth is vast. Shown is the yearly average radiation on a horizontal surface across the continental United States. Units are in kWh/m2.

kWh/m2 • Less than 3.32 0 4.43-4.99 0 3.32-3.88 0 4.99 -5.54 0 3.88 - 4.43 •Greater than 5.54

near the equator receive device or solar cell efficien- gives us a basic under- • The amount of more solar radiation than cy. It is defined as the standing of how it works electricity pro­ anywhere else on Earth. amount of electricity in all devices. duced by a PV Sunlight varies with the produced divided by the All matter is composed device depends seasons, as the rotational sunlight energy striking of atoms. Atoms, in turn, on the incident axis of the Earth shifts to the PV device. Scientists are composed of positively sunlight and the lengthen and shorten days have concentrated their charged protons, negative- device efficiency. as the seasons change. The R&D efforts over the last ly charged electrons, and • Knowing how the amount of solar energy several years on improv- neutral neutrons. The PV effect works in falling per square meter ing the efficiency of solar protons and neutrons, crystalline silicon on Yuma, Arizona, in June, cells to make them more which are of approximate- helps us under­ for example, is typically competitive with conven- ly equal size, comprise the stand how it works about nine times greater tional power-generation close-packed central in all devices. than that falling on technologies. nucleus of the atom, where Caribou, Maine, in almost all of the mass of • All matter is December. The quantity An Atomic composed of of sunlight reaching any Description of atoms. region is also affected by the time of day, the climate Silicon • Positive protons (especially the cloud cover, We use crystalline and neutral whkh scatters the sun's silicon to explain the neutrons comprise rays), and the air pollution photovoltaic effect for the nucleus of the several reasons. First, crys- in that region. These atom. talline silicon was the semi- climatic factors all affect conductor material used in the amount of solar energy PV that is available to PY the earliest successful device. Second, crystalline systems. silicon is still the most All matter is composed of atoms. The amount of energy Positive protons and neutral widely used PV material. produced by a PV device neutrons, making up the bulk of And third, although other the weight of an atom, are tightly depends not only on avail- PV materials and designs packed in the nucleus. Negative able solar energy but on electrons revolve around the exploit the PV effect in nucleus at different distances, how well the device, or depending on their energy level. solar cell, converts sun- slightly different ways, light to useful electrical knowing how the effect works in crystaJline silicon energy. This is called the

10 electrons farthest from the an important role in the nucleus interact with those photovoltaic effect. of neighboring atoms to Large numbers of determine the way solid silicon atoms, through structures are formed. their valence electrons, The silicon atom has can bond together to form 14 electrons. Their natural a crystal. In a crystalline orbital arrangement al­ solid, each silicon atom lows the outer four of normally shares one of its these to be given to, ac­ four valence electrons in As depicted in this simplified diagram, silicon has 14 electrons. cepted from, or shared a covalent bond with each The four electrons that orbit the with other atoms. These of four neighboring silicon nucleus in the outermost, or outer four electrons, called atoms. The solid, then, valence, energy level are given to, accepted from, or shared with valence electrons, play consists of basic units of other atoms.

the atom is located. The much lighter electrons orbit the nucleus at very high velocities. Although the atom is built from op­ positely charged particles, its overall charge is neutral because it contains an equal number of positive protons and negative . electrons. ------r' --- The electrons orbit the nucleus at different distanc­ es, depending on their ··bf·-. ------·--·------energy level; an electron of lesser energy orbits close to the nucleus, while one In the basic unit of a crystalline silicon solid, a silicon atom shares of greater energy orbits each of its four valence electrons with each of four neighboring farther away. The atoms.

11 Light of sufficient energy can Incoming dislodge a negative electron from light its bond in the crystal, creating a positive hole (a bond missing an electron). These negative and positive charges, which move freely for a time about the crystal lattice, are the constituents of Light-generated electricity. free electron ./ ~'""""'.....-

five silicon atoms: the Meanwhile, the atom in a crystalline silicon PY • Light of sufficient original atom plus the four left behind by the freed cell. The most common energy can dis­ other atoms with which it electron contains a net technique is to slightly lodge an electron shares its valence electrons. positive charge in the form modify the structure of from its bond in the The solid silicon crystal of the generated hole. This the silicon crystal. This crystal, creating a is composed of a regular positive hole can move al­ technique, known as hole. series of units of five most as freely about the "doping," introduces an atom of another element • These negative and silicon a toms. This regular, crysta I lattice as a free positive charges fixed arrangement of electron in the conduction (called the "dopant") into (free electrons silicon atoms is known as band, as electrons from the silicon crystal to alter and holes) are the the crystal lattice. neighboring atoms switch its electrical properties. constituents of partners. These light- The dopant has either electricity. Lighf generated charges, both three or five valence Absorption: positive and negative, are electrons, as opposed to • PV cells contain an Creating the c~n~tituents of silicon's four. electric field that electr1c1ty. Phosphorus atoms, forces free negative Charge Carriers . which have five valence and positive char­ When a photon of sutfi- Forming the electrons, are used for ges in opposite ctent energy strikes a Electric Field doping n-type silicon (so directions, driving valence electron, it may Photovoltaic cells con- called because of the an electric current. impart enough energy to tain an electric field that is presence of free negative free it from its connection charges or electrons). A • To form the electric created when semiconduc­ to the atom. This leaves a phosphorus atom occupies field, the silicon tors with different electri- space in the crystal struc­ the same place in the crystal is doped cal characteristics come ture where an electron crystal lattice that was introducing into contact. The electric (by once resided (and occupied formerly by the elements of field drives positive and bonded), called a "hole." silicon atom it replaced. another element) negative charges in op­ The electron is now free to Four of its valence to alter the posite directions. The travel about the crystal lat­ electrons take over the crystal's electrical movement of charge car­ tice. The electron is now bonding responsibilities properties. riers (through an external a part of the conduction circuit) is what defines of the four silicon valence band, so called because electricity. electrons that they these free electrons are the There are several ways replaced. But the fifth means by which the crys­ to form the electric field valence electron remains tal conducts electricity. free, without bonding

12 Substituting a phosphorus atom (with five valence electrons) for a silicon atom in a silicon crystal leaves an extra, unbonded electron that is relatively free to move around the crystal. Normal bond

responsibilities. This un­ atoms to diffuse into the altered to have the ~i:7 > ·-~ ...- -, bonded valence electron silicon. The temperature is opposite electrical • Do1nn91.i~-e..t~J!}'~~@J . behaves like a permanent then lowered so that the properties. Boron, which V!!!tl! R!J.~.~p. ,;b.9..-. ~LtS f.. I• member of the crystal's rate of diffusion drops to has three valence ad_~s.~~~tr~~-·; 4r_.i;: ~ --~ conduction band. zero. Other methods of in­ electrons, is used for !!9p.·g~g J ~ .1~--.- Gtri.V.~~~J ' ttie:crystal, produc:. When numerous phos­ troducing phosphorus into doping p-type (positive­ -"S'"-,; -'>,.• ·;.6~~-_..,... -~ · ..... ~.:,...r,. phorus atoms are sub­ silicon include gaseous type) silicon. Boron is ~ i ?,~~~ ""!¥.f~ ·· riiJteria,l _;,1 , stituted for silicon in a diffusion, a liquid dopant introduced during silicon iJ D?Pi6.~-; the ,· cryst~.F . ' crystal, many free, spray-on process, and a processing, where silicon ~~ vv1tt1 boron - leav_~~ - conduction-band electrons technique in which phos­ is purified for use in PV ~. holes:(bonds · -

available. The phorus ions are driven devices (see Chapter 3). 1 become j: -· rnJs~ihg el~ctrons ; most common method of precisely into the surface When a boron atom as­ ~ - which act as posi- substitution is to coat the of the silicon. sumes a position in the :~ tive charges) in.the top of a layer of silicon This n-type silicon can­ crystal lattice formerly oc­ ' · crystal, producing with phosphorus and not form the electric field cupied by a silicon atom, p-type material. then heat the surface. This by itself; it is also neces­ there is a bond missing an I allows the phosphorus sary to have some silicon electron-in other words, •, • In p ..type material; an extra hole. Jn p-type holes, which .·are material, there are many more numerous Boron atom more positive charges than free electons, (holes) than free electrons. are the majority Holes are much more charge carriers. __ .., numerous than free --­ I ·• In n-type material, electrons in a p-type free electrons, · l Hole material and are therefore I which are more I called the majority charge numerous than I carriers. The few electrons I 1 • _holes, are the in the conduction band majority charge A of p-type material are --1------1::, I ;... ~.aJ _r~ers ; ~ I ....,. referred to as minority ~: '.:_..- ·.:_i-, -1, ...... ,. ••..... Wllil-.:r. '"1------charge carriers. In n-type ~-~-ol.· ..ll••- material, electrons are the majority carriers, and Substituting a boron atom (three valence electrons) for a silicon atom holes are the minority in a silicon crystal leaves a hole (a bond missing an electron) that is relatively free to move around the crystal. carriers. Both p-type and

13 n-Side p-Side Although both materials are electrically neutral, Extra electrons Extra holes n-type silicon has excess electrons and p-type silicon has excess holes.

(I) (I) 0 0

n-type silicon are by them­ process. The majority from the phosphorus fills • The majority selves electrically neutral; charge carriers are more the hole across the junc­ carriers respond that is, each material con­ energetic and more mobile tion in the boron atom. physically to an tains an equal number of than the minority carriers. Holes then overpopulate electric field. negatively charged They are therefore able to the immediate vicinity of • When n- and p-type electrons and positively move from where they are the interface on the n-type material come in charged protons. highly concentrated across side; electrons overpopu­ contact, an electric The majority charge car­ the junction to a lower con­ late the p-type side. This field forms at the riers, however, have excess centration. This is called overabundance is true junction (known as energy that is not bound diffusion. In addition, they only in the immediate the p-n junction). up in valence bonding are attracted (electrically) vicinity of the junction, with neighboring atoms. by the opposite charge however. The bulk of the • Once the materials This higher energy allows of the majority carriers n-type silicon is still popu­ are in contact, the them to traverse the crys­ across the junction. In the lated with negative char­ majority carriers tal lattice. The majority immediate area of the junc­ ges; holes remain the diffuse across the carriers-electrons in tion, the "extra" electron majority charge carriers junction. n-type and holes in p-type silicon-are the ones that •This creates (in the physically respond to an n-Side p-Side immediate vicinity electric field. Electrons are of the junction) attracted to and holes are excess electrons repelled by an electric on the p-side and field. excess holes on Where n-type and then-side. p-type silicon come into contact, an electric field forms at the junction I (referred to as the p-n interface, or p-n junction). Like floodwaters breaking through a dam, some majority charge carriers on each side rush over to the When n- and p-type silicon come into contact, electrons move from other side. There are two the n-side to the p-side. This causes a positive charge to build on the n-side of the interface (or p-n junction) and a negative charge to form forces at work in this on the other side.

14 n-Side p-Side

The buildup of excess positive and negative charges on either side of the junction creates an electric field across the interface; the strength of this field depends on the amount of dopant in the silicon. At equilibrium, the electric field repels any additional crossover of holes from the p-side or electrons from then-side.

in the bulk of the p-type its random course, they carriers, on the other hand, silicon. would recombine in about are repelled by this same At equilibrium, when a millionth of a second electric field . all the charge carriers and contribute nothing to By acting this way, the have settled down again, an electrical current. But field sorts out the a net charge concentration PY cells are so constructed photogenerated electrons exists on each side of the that minority carriers have and holes, pushing new junction. This overpopula­ a good chance of reaching electrons to one side of the tion of opposite charges the electric field before barrier and new holes to creates an electric field recombining. the other. This sorting-out across the interface. The When a minority carrier process is what gives the strength of the electric (on either side of the junc­ push to the charge carriers force field depends upon tion) comes close enough in an electrical circuit. the amount of dopant in to feel the force of the Without the electric field, the silicon- the more electric field, it is attracted charge carriers generated dopant we have, the to the interface; if the ca r­ by the absorption of light greater will be the dif­ rier has sufficient energy, would go nowhere except ference in electrical proper­ it is propelled over to back into the lattice. ties on each side and the the other side. Majority greater the strength of the built-in electric field. n-Side p-Side The Electric Field in Action: Driving the Charge Carriers When a photon of light energy is absorbed by a silicon atom, an electron­ e~-...... hole pair is created, and both the electron and the When sunlight striking a cell creates charge carriers, the electric field hole begin moving pushes new electrons to one side of the junction and new holes to through the material. If the other. This sorting-out process is what drives the charge carriers nature were left to take in an electric circuit.

15 Sunlight

Antireflection coating Transparent adhesive iltll Cover glass Current--

Attaching an external circuit allows the electrons to flow from the n-layer through the circuit and back to the p-layer where the electrons recombine with the holes to repeat the process.

Attaching an external atomic structures of the entire spectrum of sun­ circujt allows the electrons same material. light, from infrared to to flow from the n-layer For crystalline silicon, ultraviolet, covers a range through the circuit and the band-gap energy is 1.1 of about 0.5 eV to about back to the p-layer, where electron-volts (eV). An 2.9 eV. Red light has an the electrons combine with electron-volt is equal to energy of about 1.7 eV; the holes to repeat the the energy an electron blue light has an energy of process. If there is no exter­ acquires when it passes about 2.7 eV. nal circuit, the charge car­ through a potential of One key to obtairung an riers collect at the ends of 1 volt in a vacuum. Other efficient PV cell is to con­ the cell. This buildup con­ PV cell materials have vert as much sunlight into tinues until equilibrium band-gap energies ranging electricity as possible. voltage (called the open­ from 1 to 3.3 eV. The Choosing the best absorb­ circuit voltage) is reached. energy of individual ing material is a very im­ photons in light is also portant step, because the Energy Band measured in eV. Photons band-gap energy of the Gaps with different energies material determines When photons of sun­ correspond to distinct how much of the sun's light strike a PV cell, only wavelengths of light. The spectrum can be absorbed. the photons with a certain level of energy are able to ... free electrons from their Free i:i electrons Conduction 8 band atomic bonds to produce "~ an electric current. This . \ • • • • • level of energy, known as • • • • • • • the band-gap energy, is the Energy band gap amount of energy required to move an outer-shell 0 0 0 o o o 0 o 0 o 00 0 electron from the valence Valence band (or level) to the con­ band Holel duction band (or level). It is different for each Photons with energies greater than the semiconductor material's material and for different band-gap energy are required to move electrons from the valence band to the conduction band and produce a current; crystalline silicon's band-gap energy is 1. 1 eV.

16 Light energy

Different PV materials have different characteristic energy band gaps. Photons with energy greater than the band gap may be absorbed to create electron-hole pairs. Photons with energy less than the band gap pass through the material or create heat.

Sil icon, for example, re­ influences the strength of quires photons to have an the electric field, which energy of at least 1.1 eY determines the maximum to be absorbed and create voltage the cell can pairs of charge carriers. produce. The higher the Photons with less energy band-gap energy of the either pass right through material, the higher the the silicon or are absorbed open-circuit voltage. as heat. Photons with too The power from an much energy are absorbed electrical device such as a and contribute free charge PY cell is equal to the carriers, but they also heat product of the voltage (V) up the cell. About 55% of and the the current (I). the energy of sunlight can­ Low-band-gap cells have not be used by most PY high current but low volt­ cells because this energy age; high-band-gap cells either is below the band have high voltage and low gap or carries excess current. A compromise is energy. Researchers are necessary in the design working on advanced cell of PY cells. Cells made of designs that can reduce materials with band gaps those losses. between 1 e Y and 1.8 e V Materials with lower can be used efficiently in band-gap energies can PY devices. exploit a broader range of the sun's spectrum of energies, creating greater numbers of charge carriers (greater current). We might conclude that material with the lowest band gap would thus make the best PY cell. But it isn't quite that simple. The band-gap energy also

17 Chapter3 Solar Cells

he cell is the smallest Single-Crystal into the silicon that even­ • Single-crystal T practical element that tually becomes the basic silicon is presently harnesses the photovoltaic Silicon Cells material of a silicon solar the most popular effect to generate elec­ Silicon is the second cell. option for commer­ tricity. Although many most abundant element in cial cells. semiconductor materials the Earth's crust (oxygen Processing Silicon is the most abundant). To begin the process, we • Silicon, the second are available, single­ Silicon occurs most often first reduce quartzite in­ most abundant ele­ crystal silicon is presently in nature as silicon dioxide dustrially to metallurgical­ ment in the Earth's the most popular option (silica, Si0 ) and as sili­ grade polycrystalline crust, occurs for commercial cells. In 2 cates (compounds contain­ silicon (polysilicon), which naturally as silicon this chapter we first ex­ ing silicon, oxygen, metals, is 98% to 99% pure silicon. dioxide in the form plore how single-crystal and maybe hydrogen). Photovoltaic cells require of sand or quartzite. silicon solar cells are made. We then take a look Sand and quartz are two polysilicon that is even • High-grade at several other important of its most common forms. purer, however, so further quartzite is PY materials: polycrystal­ Sand is generally too im­ processing is required. processed into line silicon, amorphous pure to be processed into At present, one of the highly pure silicon silicon, copper indium silicon, but high-grade highest-purity polysilicons for PV cells. diselenide, cadmium deposits of quartzite can is that used in semiconduc­ telluride, and gallium be almost 99% pure silica. tor devices, referred to as • To begin the arsenide. This silica is processed semiconductor-grade process, we first reduce quartzite to metallurgical-grade silicon, which is 98% to 99% pure silicon.

Quartzite grains Metallurgical-grade silicon Semiconductor-grade silicon >90%Si 98%-99% >99.9999% Si Heat & reduction Purification

To get the pure silicon required for solar cells, grains of quartzite, which are 90% silicon, are heated and reduced to produce metallurgical-grade silicon, which is 98% to 99% pure. This material is then refined to produce semiconductor-grade silicon, a product that is at least 99.9999% pure silicon.

18 The most widely used technique for making single-crystal silicon is the Czochralski process. in which a seed of single· crystal silicon contacts the Seed top of molten silicon. As the seed is slowly raised, atoms of the silicon melt solidify in th e pattern of the seed and extend the single-crystal structure. Molten silicon Ingot

n Preparing '· · We'then refine Single-Crystal metallurgical-grade Silicon ;~. silicon to produce . To change it into the ~ ~emiconductor- polysilicon. (Photovoltaic the metallurgical-grade single-crystal state, we . g~a_de silicon, research has actually pol ysilicon is converted first melt the polysilicon. ~· .· which is 99.9999% produced a more pure to a family of chemicals Then it reforms very slow­ ;: pure silicon. grade of polysilicon, but ca lled chlorosilanes. The ly in contact with a single­ ·•-' To convert this it is not yet in widespread chlorosilanes are purified crystal "seed." The highly pure silicon use.) in a process not wilike polysilicon adapts to the :into single-crystal To produce metallurgical­ petroleum refining and pattern of the single­ · .Wf# generally use grade polysilicon, we heat then introduced into crystal seed as it cools and · the·Czochralski the quartzite and add a a chemical-reaction solidifies very gradually; '. (Cz) orthe float controlled amow1t of car­ sequence that yields a new ingot of single­ ·zone (FZ) method. bon. This allows the semiconductor-grade crystal silicon is said to be oxygen to be removed as polysilicon. "grown" out of the molten • •ll'l the Cz process, a carbon dioxide. Additional There are some promis­ polysilicon. Several seed crystal is processes are applied to ing new methods for im­ specific processes can be dipped into a remove more impurities. proving the economics of used to accomplish this. crucible of molten The result is a block or producing high-purity, The most established and silicon and slowly ingot of a medium-gray, semiconductor-grade dependable means are the withdrawn, pulling metallic-looking substance silicon. The most success­ Czochralski (Cz) method out a cylindrical that is metallurgical-grade ful has been a silane-based and the floating-zone (FZ) single crystal. polysilicon. process that yields excep­ technique. It is more expensive to tionally consistent, high­ In the Cz process, a seed convert this metallurgical­ quality polysilicon that is crystal is dipped in to a grade material to semi­ actually more pure than crucible of molten silicon conductor-grade poly­ semiconductor-grade and withdrawn slowly, silicon than it is to polysilicon. pulling a cylindrical single produce the lower grade. crystal as the silicon crys­ Among other processes, tallizes on the seed. The FZ process produces purer crystals, because they are not contaminated

19 by the crucible as Cz crys­ tals are. In the FZ process, a silicon rod is set atop a seed crystal a11d lowered through an electromag­ netic coil. The coil' s mag­ BA G-0633321 netic field induces an electric field in the rod, After the cylindrical ingot is grown it is sawed into thin wafers for heating and melting the further processing into PV cells. The sawing step wastes about 20% of the silicon as sawdust, or kerf. interface between the rod and the seed. Single- crystal silicon forms at the A group of new single­ interface, growing upward crystal-producing proces­ as the coils are slowly ses, however, generally Polysilicon ~ raised. In this floating- called shaped-ribbon rod * zone technique, the molten growth, could reduce ;a silicon is unsupported, processing costs by form­ Movable maintaining itself through ing polysilicon directly heater -~-co il surface tension. into thin, usable wafers The next step is the of single-crystal silicon. same for both Cz and FZ These methods involve ingots. The cylindrical forming thin crystalline Single-crystal ingot single-crystal ingot is sheets directly, thereby sawed into thin wafers avoiding the sawing step for further processing into required of cylindrical PV cells. The sawing step ingots. wastes 20% of valuable Growing flat crystals in silicon as sawdust, known a dendritic web has also as kerf. generated much interest. In the floating-zone technique, Although single-crystal This technology involves a silicon rod is set atop a seed silicon technology is well withdrawing single-crystal, crystal. Movable heating coils wirelike (dendritic) seeds melt the interface between the developed, the Cz, FZ, and seed and the silicon. Single­ ingot-casting processes are from molten silicon. As crystal material solidifies at the complex and expensive. the wirelike dendrites are interface and moves upward as the heating coils are slowly raised.

20 The dendritic web process uses twin, single-crystal dendritic seeds that are slowly drawn from molten silicon. As the dendrites are withdrawn, a web of single-crystal silicon forms between them, solidifying as it rises from the melt. (Half the film is shown.)

withdrawn, a web of junction that enables a of the light that strikes it. single-crystal silicon forms current to be produced; a So the top surface through between them, solidifying silicon base layer (usually which light passes must as it rises from the melt. p-type), doped oppositely be treated to minimize The dendritic-web process to the collector; and a back­ reflection. produces high-quality contact electrode. Treating the top surface cells that are more efficient with a thin layer of silicon than those from any other Adding monoxide (SiO) greatly shaped-growth process. Anti reflection reduces reflection. A single Coatings layer of SiO reduces the Producing a Silicon is a shiny gray surface reflection to about Crystalline­ material. Left untreated, 10%. Adding an additional Silicon Cell the surface of the PV cell layer of another substance After the single-crystal can act like a mirror, can reduce reflection to silicon wafer has been reflecting more than 30% less than 4%; this is manufactured, the PV cell may be constructed. Al­ though manufacturers use several different designs, there are some elements common to all PV cells. A typical single-crystal silicon PV cell consists of several layers: a conduct­ ing grid on the top surface; an antireflective coating n·Silicon or treated surface layer; a thin layer of silicon (usually n-type) about one millionth of a meter (a micrometer or micron) thick, which is called the collector; a very narrow A typical single-crystal silicon cell consists of a top-surface grid, an electric field region at the antireflection coating, n-silicon, p-silicon, and a back metal contact.

21 referred to as a double­ surface will have a 96% redirects them down into • Another way to layer antireflection coating. chance of being absorbed the cell. reduce reflection is Another way to reduce by a textured surface. The highest-efficiency to texture the top reflection is to texture the Texturing has become cells typically use a well­ surface of a cell. top surface of a cell. Tex­ routine on high-quality designed, double-layer an­ • Combining textur­ turing causes reflected solar cells. Chemical etch­ tireflection coating and a ing with antireflec­ light to strike a second sur­ ing creates texturing on textured surface. This com­ tion coating can face before it can escape, the cell's surface. It makes bination can lower reflec­ lower reflection to increasing the probability a pattern of cones and tion to less than 2% of the less than 2% of the that the light will be ab­ pyramids, which capture incoming light. incoming light. sorbed. For example, light light rays that might other­ with an 80% probability of wise be deflected a way Attaching • Electrical contacts being absorbed by a flat from the PV cell, and Electrical Contacts attached to a PV Electrical contacts at­ cell collect the tached to a PV cell allow photogenerated it to become part of an charge carriers and electrical circuit. The back allow the cell to be­ contact of a cell (away come part of an from the sun) is relatively electrical circuit. simple and usually consists of a layer of aluminum or molybdenum metal. The front contact (facing the ~--'I.- Reflected light strikes another sun) is more complicated. pyramid When placed in sunlight, the cell generates current (flowing electrons) all over its surface. Attaching con­ tacts just at the edges of a cell would not be adequate .______, because of the excessive Texturing, which produces small pyramidal structures on a cell's top electrical resistance of the surface, increases the probability that light will be absorbed into the top layer in this configura­ cell. Light that may otherwise be reflected will strike another surface, giving it another chance to be absorbed. tion. The contacts must be

22 Contacts on the top surface of a typical cell are designed as grids, with many thin, conductive fingers spreading to every part of the cell's surface.

made across the entire deposit metallic vapors back of the cell would also surface to collect the most on a cell through a mask need antireflection coat­ current. Large contact or paint them on via a ings and grid contacts. areas on the top of the cell, screen-printing method. Researchers are examining however, shade the active For the highest quality alternative grid designs, parts of the solar cell and (and greatest cost), including innovative ways reduce the cell's conver­ photolithography is the of placing them only on sion efficiency. preferred method. This is the cell's back surface. Therefore, in designing the transfer of an image They a re also working on electrical contacts, we via photography, as in improvements for trans­ must balance electrical modern printing. New cell parent conducting grid resistance losses against designs, such as those for materials and on sp ecial shading effects. The usual concentrated light, may grids and covers for approach is to design top­ require that light enter the concentrator cells (see surface contacts as grids, cell from the back as weU Chapter 4). with many thin, conduc­ as the front, so that the tive fingers spreading to every part of the cell's surface. The fin gers of the grid must be thick enough to conduct well (with low resistan ce), but thin enough to block a minimum of incoming light. Such a grid keeps resistance losses sufficient­ ly low, while shading only Sihcon wafer about 3% to 5% of the (placed in contact surface. with mask) Grids can be expensive to fabricate and can affect Grids are often deposited on the top surface of a cell through a mask that defines the grid pattern. dependability. To make top-surface grids, we ca n

23 Beyond Characteristics of Commercially Ready and Single-Crystal Nearly Ready Solar Cell Material Designs Electronic Malarial Band Silicon-A Properties Absorptivity Gap Cost Wide Range of Single-Junction Designs Single-crystal silicon HI LO MED HI Materials Polycrystalline silicon MED LO MED M ED Silicon is not the only Amorphous silicon LO HI MED LO material that will respond Polycrystalline thin films (CIS, CdTe) MED HI MED to HI LO to sunlight by generating Single-crystal thin films electron-hole pairs of (GaAs, lnP) HI HI HI HI charge carriers. Many Multljunctlon Designs LO to HI HI HI LO to HI semiconductors have HI - High cost or excellent c:hal1ldel istics. MED • MedillTl cost or fair lo good characteristics. similar properties. Further­ LO . Low oost or relatively poor characteristics. more, any two dissin1ilar semiconductor materials • The range of wave­ older silicon cell designs will form a junction and lengths in the sun's required about 300 microns an electric field at their in­ spectrum that can be of material to absorb all terface that could provide absorbed and used the sunlight possible, and the voltage to drive a solar (the band gap) newer silicon cell designs cell. There are a lot of • The expected cost, be­ still require about 100 choices to be made in cause of the amount and microns of material. making a PV device, both type of material used Silicon's band gap of 1.1 in material properties and and the complexity of eV is workable but not device designs. The most the manufacturing ideal. Solar cells with the important design param­ process. highest theoretical efficien­ eters to consider are Single-crystal silicon cies employ materials with • The material: s electronic has a high degree of crys­ band gaps of 1.4to1.5 eV. properties (especially its tallinity, which makes a The wide variety of PV degree of crystallinity) high sunlight-to-electricity materials available makes • The amount of light that conversion efficiency rela­ trade-offs possible among can be absorbed in a tively easy to attain. lts ab­ design parameters. Out of given thickness of sorptivity is relatively low, the hundreds of different material (its absorptivity) however, which is why materials and cell designs

24 Solid single-crystal material Grains packed randomly

(a) Single-crystal silicon (b) Semicrystalline silicon

Single-crystal material (a) is structurally uniform; there are no disturbances in the orderly arrangement of atoms. Semicrystalline material (b) is made up of several crystals or grains. At the interfaces of the grains, known as grain boundaries, the atomic order is disrupted. This provides an area where electrons and holes are more likely to recombine.

causes a loss of voltage that have been studied, the interface between two • Semicrystalline several have become com­ from the cell. The net crystals? At this "grain silicon is made up effect of grain boundaries mercially significant or boundary," the atomic of several relatively is to reduce the power show great promise for fu­ order is disrupted. Free large crystals, or output of a solar cell. ture commercial success. electrons and holes are grains. much more likely to Researchers have found Semicrystalline recombine at grain boun­ several ways to minimize • The interface and daries than within a single the problems caused by between grains, crystal. The same is true, grain boundaries: adjust­ or grain boundary, Polycrystalline by the way, at other kinds ing growth conditions disrupts the atomic Silicon Cells of interfaces-crystal/ through treatments such order and provides The term "single­ metal grid; crystal/ top as annealing (heating and a place for free crystal" means that all the surface, etc. Recombina­ then cooling) the semicon­ electrons and holes atoms in the active PY cell tion looks like a loss of ductor material so that to recombine. are part of a single crystal­ current or a short circuit grains are columnar and line structure, and there to the external electrical as large as possible; design­ • Because of recom­ are no disturbances in the circuit. The grain boun­ ing cells so that the charge bination, grain boundaries reduce orderly arrangements of daries look like an internal carriers are generated atoms. "Semicrystalline," electrical resistance to the within or very dose to the the power output as used here, describes a solar cell circuit, which built-in electric field; and of a cell. PY cell in which the active • Effects of grain portion of the cell is made boundaries can be up of several relatively minimized through large crystals, called grains, Heating and then cooling semicrystalline silicon can special design, each about a square cen­ change randomly oriented heating and then timeter or so in area. grains (a) into larger, columnar ones (b), which cooling the alleviates problems with material, or filling Semicrystalline recombination at grain Junction the broken bonds. Silicon boundaries. Having several large crystals in a cell introduces a problem. Charge carriers can move around relative­ ly freely within one crys­ Junction tal, but what happens at

25 The most popular method for making commercial semicrystalline silicon is casting, in which molten silicon is poured directly into a mold and allowed to solidify into an ingot.

Crystal

filling broken bonds at that can be cut and sliced •Generally, semi­ Semicrystalline Silicon grain edges with elements 1990 World Module into square cells to fit crystalline silicon such as hydrogen or Shipments more compactly into a PV cells are not as oxygen, which is called module (see Chapter 4 on efficient as single­ passivating the grain "packing efficiency"). crystal silicon cells, boundaries. but they cost less. Al though these Polycrystalline • Semicrystalline methods are effective in Silicon silicon cells increasing the output of Even very small crystals captured 33% of the semicrystalline (as well as of silicon make successful PV market in 1990, polycrystalline) solar cells, solar cells. At least one compared to 35% the processes that occur at With 15.3 million watts of company is working on for single-crystal the atomic level are not modules shipped in 1990, developing solar cells semicrystalline silicon captured silicon. well understood. 32.9% of the world PV market. made from thin polycrys­ Semicrystalline silicon talline silicon. In these • The most popular cells cannot reach quite as popular commercial cells, single-crystal grains method for making high efficiencies as single­ methods involve a casting are only about 40to100 semicrystalline crystal silicon. Lower process in whkh molten microns in diameter, about cells is casting, manufacturing costs, silicon is directly cast into the thickness of a human in which molten however, have made a mold and allowed to hair. Crystals in semicrys­ silicon is poured semicrysta1line silicon solidify into an ingot. talline silicon, on the other into a mold and very popular. In 1990, Generally the mold is hand, are 1,000 times allowed to solidify. cells from semicrystalline square, producing an ingot larger. • Polycrystalline silicon captured almost silicon is comprised 33% of the world PV of grains that are market, compared with Sem1crystalltne s11tcon grains on the order of about 35% for single­ 1 /1 OOOth the size crystal silicon. of grains in semi­ Semicrystalline silicon crystalline silicon. can be produced in a variety of ways. The most

The grains in polycrystalline silicon (a) are on the order of 1000 times smaller than grains in semicrystalline silicon (b).

26 Thin-Film tially depositing thin deposited on various low­ layers of the required cost substrates. These can Solar Cells materials. Several different be glass or plastic in vir­ One of the scientific dis­ deposition techniques are tually any shape-even coveries of the computer available, all of them flexible plastic sheets. semiconductor industry potentially cheaper than While single-crystal that has shown great the ingot-growth tech­ cells have to be individual­ potential for the PV in­ niques required for ly interconnected into a dustry is thin-film tech­ crystalline silicon. These module, thin-film devices nology. Thin films are deposition processes can can be made monolithical­ exceedingly fine layers easily be scaled up so that ly (as a single unit). Layer of semiconductors placed the same technique used upon layer is deposited on top of each other. to make a 2-inch x 2-inch sequentially on a glass Thin-film cells can be laboratory cell can be used superstrate, from the an­ made from a variety of to make a 2-foot x 5-foot tireflection coating and materials. Today, the most module. The layers can be conducting oxide, to the widely used commercial thin-film cells are made from amorphous silicon. Two other materials that are on the verge of com­ mercialization, showing great promise for low­ cost production, are polycrystalline copper indium diselenide and cadmium telluride. Thin-film devices re­ quire very little material and have the added ad­ vantage of being easy to manufacture. Rather than by growing, slicing, and Thin-film modules can be deposited on a variety of low-cost treating a crystalline ingot, substrates, including glass and flexible plastic sheets. we make them by sequen-

27 Thin-film modules are built as a single unit: layer upon layer is. deposited sequentially on a glass superstrate, fr.om the antireflection coating and conducting oxide, to the semiconductor material, to the back electrical contacts. Individual cells are . formed by scribing with a laser.

Heater I 11Lase r scribe • Individual cells are ~ ce l l strips formed by scribing Screen pnnt back contacts with a laser. ! • Instead of metal grids to collect the ~ current, thin-film devices typically semiconductor material, Amorphous fully modifying its com­ use oxides that to the back electrical con­ Silicon position, we can use amor­ are transparent tacts. Individual cells are phous silicon in PV cells. Amorphous solids, like and that conduct formed by scoring each Today, amorphous silicon common glass, are materials electricity very well. layer with a laser beam. is commonly used for in which the atoms are not Unlike most single­ solar-powered consumer • Amorphous silicon arranged in any particular crystal cells, the typical devices that have low (a-Si) was first order. They do not form thin-film device does not power requirements. classified as an crystalline structures at all, use a metal grid for the Amorphous silicon insulator. and contain large numbers electrical contact. Instead, modules captured a little of structural and bonding • But by modifying it uses a thin layer of a more than 31 % of the defects. its composition transparent conducting world's solar cell market It wasn't until 1974 that slightly, a-Si oxide. These oxides, such in 1990. researchers began to real­ becomes suitable as tin oxide, indium tin In recent years, the ize that amorphous silicon for use in PV oxide, and zinc oxide, are efficiency of amorphous could be used in PV devices. devices. highly transparent and silicon py devices has Before then, the electrical conduct electricity very steadily increased; labora­ • In 1990, in fact, properties of the mat~rial well. They collect the cur­ tory cells with efficiencies a-Si captured more were classified as an m­ rent effectively from the greater than 13% were than 31 % of the sulator, and not at all like top of the cell, and losses reported in 1989. Ot~er world PV market. those of crystalline silicon due to resistance are new thin-film matenals or similar semiconductor minimal. A separate have reported much materials. antireflection coating may greater efficienci~s-such This perception changed be used to top off the as gallium arsemde cells, with the discovery that, by device, or the transparent with efficiencies of more properly controlling ~e . conducting oxide may than 24% in unconcentrated conditions under which it serve this function as well. light. But amorphous is deposited and by care-

28 be produced at a lower capability for long-range Amorphous Silicon • A-Si absorbs light 1990 World Module temperature and deposited order. This lack of order Shipments 40 times more on low-cost substrates. results in a high degree of efficiently than Amorphous silicon does defects such as dangling does single-crystal not have the structural bonds, where at oms are silicon and can uniformity of crystalline missing a neighbor to be deposited on silicon, nor even of which they can bond. low-cost substrates. polycrystalline silicon. These defects provide There is a short-range places for electrons and • A-Si has a high order in the sense that holes to recombine. Or­ degree of structural most silicon atoms tend to dinaril y, such a material and bonding With 14.7 million watts of bond with four other would be unacceptable for defects that provide modules shipped in 1990, silicon atoms at distances because amorphous silicon captured electronic devices places for electrons 31.6% of the world PV market. and angles nearly the the defects limit the flow and holes to same as in a crystal. But of current, in much the recombine, limiting silicon solar cells, like this uniformity does not same way that grain boun­ current flow. polycrystalline thin-film translate well among daries interfere with the devices (see page 32), are units, resulting in small flow of current in polycrys­ uniquely designed for low deviations that destroy the talline material. cost. Properties of Amorphous Silicon Amorphous silicon ab­ sorbs solar radiation 40 Dangling times more efficiently than bond does single-crystal silicon. Hydrogen As a result, a film only atom about 1 micron thick ca n absorb 90% of the usable solar energy; this is one of the most important factors affecting its potential for low cost. Other principal Amorphous silicon has no long-range order. Dangling bonds can economic advantages are arise to form centers for electrons and holes to recombine, effects that amorphous silicon can that may be neutralized somewhat with hydrogen.

29 But if amorphous undoped intrinsic (mid­ charge carriers are • Adding a small silicon is deposited in such dle) layer; and a very thin generated in the intrinsic amount of hydrogen a way that it contains (0.02-micron) n+ bottom layer, the poor charge neutralizes some hydrogen (only a 5% to layer. The top layer is mobility in the doped of the defects and 10% concentration is re­ made so thin and relative­ p+ and n+ layers is less enhances the quired), then the hydrogen ly transparent that most important. mobility of electrons atoms combine chemically incident light passes right Amorphous silicon has and holes. with some of the dangling through it, to generate a band-gap energy of • Because of the high bonds and remove them. electron-hole pairs in the about 1.7 eV, which is number of defects, Removing the dangling undoped intrinsic layer. greater than crystalline devices made of bonds results in a small The top p+ and the bottom silicon's band-gap energy a-Si cannot use a but extremely important n+ layers induce an of 1.1 eV. A PV cell's out­ p-n structure. enhancement of the electric field across the put voltage is directly re­ freedom of movement, or entire intrinsic, or middle, lated to the size of its band • A-Si devices use a mobility, of electrons and region in a manner similar gap, so PV cells made of p-i-n structure in holes in amorphous silicon. to the induction bf an amorphous silicon have which an undoped Even with this remedy, electric field in the p-n higher output voltages or intrinsic (i) layer electrons and holes are junction of a crystallme than cells made of crystal­ is sandwiched much less mobile in silicon PV cell. Since the line silicon. The higher between a p-layer hydrogenated amorphous and an n-layer. silicon than in crystalline Light •The p- and n-layers silicon. Doping makes induce an electric thjs already low mobility field across the even worse. Therefore, intrinsic layer, the design of amorphous where charge silicon PV cells is quite carriers are different from the p-n generated. junction design used in crystalline silicon PY cells. Hydrogenated amor­ phous silicon cells are designed to have an ultra­ thin (0.008-micron) and The typical amorphous silicon cell employs a p-i-n design, in which highly doped p+ top layer; an undoped or intrinsic layer is sandwiched between a p-layer and a thicker (0.5- to 1-m icron) an n-layer.

30 S1H. PHJ S1H4 Plasma S1H4 B2H6 Substrate

Shutter .,...,i!! :!: 0 .,<

Radio • frequency

i Vacuum ~ i ~

Most commercial amorphous silicon modules are made with glow-discharge deposition. Doped and intrinsic amorphous silicon layers are deposited in sequence as the module moves through consecutive chambers. Doping is done in different chambers by adding diborane or phosphine to the silane. output voJtage compen­ and hydrogen gas is ferent electrode stations in sates for the fact that lower­ passed between a pair of separate chambers and •With a 1.7 eV band energy photons (with electrodes whose polarity depositing the appropriate gap, a-Si cells have energies beJow 1.7 eV) is reversed at high frequen­ materials, one at a time. higher output are wt absorbed by cy. This reversal of the volt­ Moving the modules voltages than an1orphous silicon. age induces an oscillation through different cham­ crystalline silicon Typically, the construc­ of energetic electrons be­ bers minimizes the oppor­ cells, but are less tion of a hydrogenated tween the electrodes. The tunity for contamination efficient and produce amorphous silicon p-i-n electrons collide with the from previous steps. This less power. cell begins with the deposi­ silane, breaking it down process uses little energy tion of a p+ layer onto a into SiH 3 and hydrogen. and is potentially able to • Most a-Si devices textured transparent con­ Since SiH3 is chemically produce modules a few are made by glow­ ducting tin-oxide film (the a radical, and hence square feet in area, on a discharge deposi­ top electrode). This is fol­ unstabl e, it adheres to a variety of substrates, and tion, in which a lowed by the intrinsic (i) substrate on one of the in an assortment of shapes. high frequency is layer, and a thin n+ layer. electrodes to gain stability. Stability Problems used to break ·d()Wn . The cell is then given a Hydrogen is then released and Solutions ,: · silane gas (SiH4) to reflective coating on the from the substrate, leaving After amorphous s ! deposit hydrogen ~ bottom, usually ilicon of a film of amorphous ated modules are first exposed •:.• silicon. aluminum or silver. A silicon with about 10% to light, their conversion textured transparent con­ hydrogen. Doping can be e', After:a-Si devices. i efficiency decreases by ..... : ..,_ . . .. ·.· ' . '"";,,~""''" ducting oxide substrate accomplished by adding · ···: afe.first expose~ · t_o 10% to 20%. Thereafter, 1 and a reflector help to trap diborane (B H ) or phos­ I ig ht thei(efficiency 2 6 their performance is rela­ the maximum possible phine (PH ) gas to the d~creases by 10%. : 3 tively steady. Researchers 1 amount of light in the silane. .Hf?O%. . . PV cell. have long studied this Most commercial amor­ t" - light-induced instability. I I .; · . . Glow-discharge deposi­ phous silicon PV modules 1 .... ~· Several explanations have . ~-.: ' IUJ -t"C~ '. tion has been the method are produced by this ~ ilL.:• .J - - _B;A - • been offered, including used to make the most effi­ method. The successive tiny microvoids in the cient amorphous silicon layers of the module are amorphous silicon PV cells. In the most com­ formed by moving the sub­ material. Microvoids are mon form of this method, strate sequentially to dif- atomic-level gaps in the a stream of silane (SiH ) 4 structure several angstroms

31 Grid structure -

Antireflechon c oa ti ng ----~

two semiconductor ~- Junction formed between -1;;:::~==~~~;~~~;~~~~=~~ ~ conduction types 0§ mate ri als of opposite Subslrate------';;~===~~? ~~=t:i~~====;;;~d ~ Oh mic contact Bottom p type "Absorber Top n-type "Window" layer formed by second layer formed by fi rst semiconductor matenal semiconductor matenal

Polycrystalline thin film cells have a heterojunction structure, in which the top layer is made of different semiconductor material than the bottom semiconductor layer. The top layer, usually n-type, is a window that allows almost all of the light through to the absorbing layer, usually p-type.

in diameter (an angstrom only the high-energy end • This degradation Polycrystalline is one-ten billionth of a of the spectrum. It must be can be partly Thin-Film meter). Other causes thin enough, have a wide neutralized through proposed include oxygen Solar Cells enough band gap (2.8 e V annealing. or carbon impurities that Polycrystalline thin-film or more), and have a low cells comprise many tiny • Polycrystalline thin are in the cells and ordi­ enough absorptivity to crystalline grains of semi­ film cells comprise nary stresses in the system let all the available light conductor materials. many tiny crystal­ that break silicon-silicon through the interface The materials used in line grains of semi­ bonds in the region of the (heterojunction) to the polycrstalline thin-film conductor material. imperfections. Researchers absorbing layer. The ab­ have found that devices cells have properties that sorbing layer under the • These devices use suffering from degrada­ are different from those of window, usually doped a heterojunction tion can recover their silicon. For these devices, p-type, has to have a high structure with the effectiveness if they are it has proven to be better absorptivity for high cur­ junction formed be­ annealed at 150°C for a to create the electric field rent and a suitable band tween two different few minutes. Annealing is with an interface between gap to provide a good volt­ semiconductor somewhat effective even two different semiconduc­ age. lt is typically 1 to 2 materials. at the normal operating tor materials. This type of microns thick. interface is called a hetero­ • The top layer is a tern pera tu res of PV Copper Indium modules, about 50° to junction ("hetero" because high-band-gap it is formed from two Diselenide "window" that 80°C. This is called self­ Copper indium di­ annealing, and it is being different materials, in allows the light selenide (CulnSe , or CIS) looked at by manufac­ comparison to the "homo­ 2 through to the junction" formed by two has an extremely high ab­ bottom layer, which turers of amorphous sorptivity that allows 99% silicon PV cells. doped layers of the same absorbs photons material, such as the one of the available incident and generates in silicon solar cells). light to be absorbed in charge carriers. The typical polycrystal­ the first micron of the line thin film has a very material. But that is not • Copper indium the only reason that cop­ diselenide (CIS) is thin (less than 0.1 micron) per indium diselenide is a very stable PV layer on top called the "window" layer. The attractive for PV devices. material with very It also has shown very high absorptivity. window layer's role is to absorb light energy from good stability in outdoor

32 tests, an important criter­ is evaporation. Small Another technique used ion for commercialization. amounts of each of the successfully to deposit Recent outdoor tests show elements are electrically copper indium diselenide no degradation in this heated to a point where on a substrate is spray material after many hours. the atoms vaporize. They pyrolysis. In this method, The most common then condense on a cooled solutions of the salts of the material for the window substrate to form a CIS necessary elements are layer in CIS devices is layer. sprayed onto a hot sub­ cadmium sulfide (CdS); Another common strate. They react under sometimes zinc is added method is sputtering. the elevated temperatures to improve the transparen­ High-energy ions bom­ to form the required CIS cy. Adding small amounts bard the surface, driving layer, while the solvent of gallium to the absorbing off atoms of the target evaporates. CIS layer boosts the band material. These then con­ Electrodeposition can gap of CIS (from its nor­ dense on a substrate to also be used to form the mal 1.0 eV), which form a thin layer. CIS layers in the same way improves the voltage and therefore the efficiency of the device. Deposited thin film \ The layers of materials \Substrate in CIS cells can be made I ( by several different process­ Ar atoms es that were developed in the computer-related thin­ I film industry. AU of these \ H>So b methods are well estab­ lished commercially in various industries. The CIS layer itself consists of three different elements­ copper, indium, and In the sputtering method selenium. One of for making CIS modules, high-energy atoms the most of a noble gas like argon (Ar) strike two targets, one of copper (Cu) popular preparation and one of indium (In) . This sends beams of Cu and In ions toward a substrate. A gas containing selenium method (hydrogen selenide, H2Se) s for the CIS layer interacts with the copper and indium ions to form CulnSe2 on the substrate.

33 CdS(ln) (CdZn)S CulnSei Base Sputtering of an entire CIS depos111on depos111on co·depos1tion metalhzauon Vacuum module can be done by chamber chamber chamber chamber interlock using an in-line system chamber such as this, where the module is moved through the chambers sequentially 00 0 to deposit the base metallization and the absorber and window layers.

gold is plated onto jewelry. selenium. This approach is Cadmium Telluride Passing electricity through considered the most likely The other prominent a solution containing ions to lead to commercial CIS polycrystalli11e thin-film of the required elements products. material is cadmium causes them to be Laboratory cells of CIS telluride (CdTe). With a deposited out of solution recently exceeded 14% ef­ nearly ideal band gap of onto an electrode, which ficiency. One-square-foot 1.44 eV, cadmium telluride acts as the substrate. submodules achieved an also has a very high ab­ • iTtie ottier; CIS layers can also be efficiency above 11 %, sorptivity. Although cad­ than st ~rominent P-Ol~­ made by using one of generating more mium telluride is mo crY,stalline material these methods to deposit 10 W of power. A 1-foot x often used in PV devices is cadmium only the copper and the in­ 4-foot module reached 9% without being alloyed, it telluride (Cd:re), dium. This is followed by efficiency, generating is easily alloyed with zinc, whicti nas a nearly a treatment with hydrogen 36 W. At least one major mercury, and some other ideal band gap selenide gas (called company expects to com­ elements to vary its pro­ (1.44 eV) for PV. selenization) to add the mercialize CIS soon. perties. Cadmium telluride films can be manufactured • Like CIS, CdTe has , by the same low-cost tech­ high absorptivity, ~ AirorN2 toproduce ~ niques used to make CIS is very stable, and_ ~ atomized spray ~ films, especially electro­ ,can be manufac­ deposition and spraying. tured with several ~ Compounds 1n water ;;, CIS, the best CdTe or alcohol solutions Like of the same low­ cells employ a heterojunc­ cost techniques. tion interface, with cad­ mium sulfide acting as a thin window layer. Tin ~\\ .----Substrate oxide is used as a trans­ parent conducting oxide r-1 ~-_JMIJBll+lli ~::::::r--- Heat er and an antireflection coating. Though CIS can easily Spray pyrolysis has the potential to be a low-cost deposition method for polycrystalline thin-film devices. For CIS cells, solutions containing be prepared p-type, cad­ reacting species of copper, indium, and selenium are sprayed onto a mium telluride is not so where they react to form a CIS layer. heated substrate well behaved. P-type CdTe films tend to be highly

34 layer is instead allowed One company using the to be intrinsic (natural, more conventional p-type Cathode Anode neither p-type nor n-type). CdTe cell structure has Then, a layer of p-type also developed new CdTe zinc telluride (ZnTe) is contacts that appear to be sandwiched between the stable if properly cadmium telluride and the protected from water back electrical contact. vapor and oxygen. To Even though then-type achieve this, the firm is CdTe cadmium sulfide and the developing a hermetically p-type zinc telluride are sealed module.

Cd" separated, they still form By 1989 the highest ef­ HTe()z an electrical field that ficiencies measured for extends right through laboratory CdTe cells were Like spray pyrolysis, electro­ the intrinsic cadmium greater than 12%, and for deposition carries a history of telluride. Cells employing modules, greater than 7%. industrial application and promises low-cost fabrication for this design have been Some companies plan to polycrystalline thin-film devices. tested for more than 3,000 deliver commercial To make cadmium telluride devices a current is passed hours and have shown no modules of CdTe soon. through an electrolyte containing degradation. cadmium and tellurium ions, causing CdTe to deposit on the cathode. Ant1reftect1on coating resistive electrically, which leads to large internal loss­ es. It has been difficult to make a stable back electri­ cal contact, because a rust­ like deterioration develops there. Two a pp roaches have been successful in cir­ cumventing this problem. Back contact

One approach is to avoid One way around contacting problems inherent in typical CdTe cells is trying to dope the CdTe to use an n-i-p structure, with an intrinsic layer of CdTe sandwiched layer p-type. The CdTe between an n-type CdS layer and a p-type Zn Te layer.

35 p • - a - a a •

Using GaAs cells, the Sunraycer built by GM and Hughes Aircraft won the transAustralian Solar Challenge race in 1987, averaging more than 41.6 mph.

unconcentrated light and 100 microns or more in • Gallium arsenide Gallium 29% under concentrated thickness.) (GaAs) cells have Arsenide light. Cells in commercial • Unlike silicon cells, been running nip production can average GaAs cells are relatively and tuck with Solar Cells as high as 20% efficient. insensitive to heat. (Cell single-crystal Gallium arsenide It was this type of cell temperatures are often silicon in the race (GaAs) is a compound that powered the record­ quite high, especially for the highest semiconductor, a mixture shattering performance of for concentrator efficiency PV of two elements, gallium the GM Sw'lra ycer in the applications.) device. (Ga) and arsenic (As). Gal­ lium is a by-product of the Solar Challenge car race • Alloys made from gal­ • GaAs has many smelting of other metals, across Australia in 1987. lium arsenide using desirable properties. notably aluminum and Their high efficiency and aluminum, phosphorus, It has an ideal band zinc, and is rare-rarer their resistance to radia­ antimony, or indium gap (1.43 eV), is than gold, in fact. Arsenic tion have also made GaAs have characteristics com­ highly absorptive, is not rare but is poison­ cells favorites for power­ plementary to those of is relatively insensi­ ous. Gallium arsenide for ing satellites and other gallium arsenide, allow­ tive to heat, can be solar cells has been de­ spacecraft. ing great flexibility in alloyed with many high-efficiency cell veloping synergisticall y Properties of materials, and is with gallium arsenide for design. radiation resistant. light-emitting diodes, Gallium Arsenide • Gallium arsenide is very lasers, and other opto­ Gallium arsenide is resistant to radiation electronic devices. quite suitable for use in damage. This, along Single-crystal silicon high-efficiency solar cells with its high efficiency, and single-crystal GaAs for several reasons: makes GaAs very cells have been running • The GaAs band gap is desirable for outer­ nip and tuck in the race to 1.43 eV, nearly ideal for space applications. the highest-efficiency PV single-junction solar In silicon cells, the p-n device. In 1989, experimen­ cells. junction is formed by treat­ tal silicon cells reached • Gallium arsenide has a ing the top of a p-type efficiencies of nearly 23% high absorptivity and re­ silicon wafer with an n-type under unconcentrated sun­ quires a cell only a few dopant. This process does light; experimental GaAs microns thick to absorb not work with gallium ar­ cells reached efficiencies sunlight. (Crystalline senide. Instead, all the ac­ of almost 26% under silicon requires a cell tive parts of a GaAs solar cell are made by growing

36 sequences of thin, single­ heated substrate wafer is single-crystal GaAs layers • GaAs devices crystal layers on a single­ exposed to gas-phase atoms grow epitaxially-that is, are made via either crystal substrate. As each of gallium and arsenic that the new atoms deposited molecular beam layer is grown, it is doped condense on the wafer on on the substrate continue epitaxy or metal in different ways to form contact and grow the thin the same crystal lattice organic chemical the p-n junction and to GaAs film. In MOCVD, structure as the substrate, vapor deposition. control other aspects of a heated substrate is with few disturbances in cell performance. exposed to gas-phase the atomic ordering. This • Each technique The GaAs layers are organic molecules contain­ controlled growth results offers precise grown via one of two ing gallium and arsenic, in a high degree of crystal­ growth control and popular techniques: which react under the high linity and in high cell results in high molecular beam epitaxy temperatures, freeing gal­ efficiency. degrees-of crystat­ (MBE) or metal-organic lium and arsenic atoms to One of the greatest ad­ linity and high cell chemical vapor deposition adhere to the substrate. Jn vantages of gallium ar­ efficiencies. (MOCVD). In MBE, a both of these techniques, senide and its alloys as PV cell materials is the wide • One of the greatest range of design options advantages of H2 cam er. AsH3 input ~ possible. A cell with a GaAs and its alloys Ill / H2 carrier. ~ is the wide range ~ trymethygallium GaAs base can have input ~ of design options - :I !'I several layers of slightly I I available. Dopant input different compositions that ..-----ln1ector tube allow a cell designer to • A designer can Reactor tube Substrate control the generation and precisely alter the Graphite susceptor collection of electrons and composition of holes. (To accomplish the each layer·of a same thing, silicon cells GaAs-based cell. have been limited to varia­ to control the tions in the level of generation and Thermocouple doping.) This degree of collection of charge. control allows cell design­ carriers. Gallium arsenide devices are often made by MOCVO, a process in ers to push efficiencies which gas-phase molecules of gallium, arsenic, and dopants are injected onto a heated substrate. (Gallium, some of the dopants. and closer and closer to alloys are contained in organic compounds.) The molecules react theoretical levels. For .;..._..•• a.I·- -- . ' .. _•.=:.I.ii under the high temperatures, freeing gallium and arsenic atoms to adhere to the substrate. example, one of the most

37 common GaAs cell struc- area and can produce amount of expansion and tures uses a very thin win- ample power under high contraction that occurs germanium, GaAs cell; the buffer layer is an oversimplification of rather than on the more ex- then absorbs many of the the situation. pensive GaAs substrates. imperfections in the crys- First, nobody makes The other is to grow GaAs tal structure. Another way square meters of single- cells on a removable GaAs is to treat cells with crystal gallium arsenide; substrate that can be hydrogen, as we treat the $10,000 cost is pro- reused to produce other semicrystalline silicon, jected from that of much cells. which lessens the effects smaller substrates. Growing good-quality of imperfections. Second, GaAs cells are GaAs crystals for solar The highest efficiencies used primarily in con- cells requires a substrate of GaAs-on-Si cells ob- centrator systems. The with a crystal structure tained in 1989 exceeded typical concentrator cell is matching that of gallium 22%. Several laboratories approximately 0.25 cm2 in arsenide and with similar are working to improve thermal properties, i.e., the our understanding of the

38 can then be peeled off and Thin film GaAs • Another way to top cell incorporated in a PV lower costs is to device; and the substrate reuse the GaAs can then be used to grow substrate to grow several more thin films. several cells - an Photovoltaic cells made I<( approach that has m from this process, which resulted in 24%­ can deliver efficiencies of eff icient cells. Plane of weakness about 24%, were used to make an experimental • Single-junction flat-plate module at a cells restrict record efficiency- greater efficiency because than 20%. their PV response is limited to a small Multijunction part of the solar Devices spectum. Today's most common PV devices use a single junction, or interface, to create an electric field. In a single-junction PV cell,

On~ way to cut the ~ost of gallium arsenide cells is to grow thin films only photons whose of single-crystal gallium arsenide on a reusable gallium arsenide sub­ energy is equal to or stra~e . The thin films can be peeled off and incorporated in a PV device and the substrate can be used to grow several more thin films. greater than the band gap of the cell material can GaAs-on-Si material photovoltaic and semicon­ kick an electron up into system, since GaAs-on-Si ductor research. the conduction band. In is becoming increasingly In another attempt to other words, the photo­ important not only for reduce costs, one U.S. com­ voltaic response of single­ solar cells but also for high­ pany is growing thin films junction cells is limjted to s peed microelectronic of single-crystal gallium ar­ the portion of the sun's components- another ex­ senide onto thick, reusable spectrum whose energy ample of the cross transfer substrates of the same is above the band gap of of knowledge between material. These thin films the absorbing material.

39 Lower-energy photons are hold great promise for use and the highest-energy • Multijunction not used. in concentrator PV sys­ photons are absorbed. devices stack two One way to get around tems (see Chapter 4). Photons not absorbed in or more cells atop this limitation is to use In a typical multijunc­ the first cell are trans­ each other to two (or more) different tion PY cell, individual mitted to the second cell, capture more of cells, with more than one single-junction cells with which absorbs the higher­ the solar spectrum. band gap and more than different energy band gaps energy portion of the • High band-gap cells one junction, to generate (Eg) are stacked on top of remaining solar radiation capture the high­ a cell voltage. These are each other. Sunlight falls while remaining energy part of the referred to as multijunc­ first on the material hav­ transparent to the lower­ spectrum and the tion cells (also called cas­ ing the largest band gap, energy photons. ln theory, low band-gap cells cade or tandem cells). any number of cells can capture the low­ Multijunction devices can be used in multijunction energy part of the achieve a higher total con­ devices. Practical ex­ spectrum. version efficiency because perience to date has they can convert more of shown that two or three • Multijunction the energy spectrum of cells make up the most devices may be light to electricity. workable multijunction able to reach Thin-film, multijunction design. efficiencies as high cells, which are used in Multijunction cells are as 20% for thin flat-plate configuration made in one of two basic films and 40% for (see Chapter 4), are near­ ways: monolithic or single-crystal. ing efficiencies of 15% or mechanically stacked. • We make multi­ more. In the future, these Monolithic multijunctions junction devices cells are expected to reach are made by sequentially either by mechani­ efficiencies of approxi­ growing all the necessary cally stacking mately 20%. Single-crystal, layers of materials for two multijunction cells, on the cells and the interconnec­ individual cells or Cell 3 (Eg3) by sequentially other hand, have already tion between the cells, one growing one cell recorded efficiencies A multijunction device stacks layer on top of another. individual single-junction cells on on top of another greater than 30% and may With mechanically stacked top of each other in descending­ multijunctions, different (monolithically). eventually reach efficien­ band-gap order. The top cell cies of 40% or more. captures the high-energy cells are made separately, Although not yet commer­ photons and passes the rest of stacked on top of one the photons on to be absorbed cially ready, these cells by lower-band-gap cells. another, and stuck

40 This monolithic multi­ Ant1 reflection junction device uses coating a top cell of gallium indium phosphide, a 'l~::J:::iail:JlW:~:liiiJ:1' Au g nd tunnel junction, and a bottom cell of gallium arsenide. Devices with Top cell this structure have achieved efficiencies greater than 27% under concentrated sunlight.

Bottom cell

- Substrate together with transparent adhesive. Each approach challenges than do stacked tion in 1990. There are has its advantages, and devices, have seen their ef­ several interesting things both are currently being ficiencies climb dramatical­ about this device. First, investigated at DOE labo­ ly. Two of these devices its design broke with the ratories and in industry. are especially worthy of traditional thinking that, High-Efficiency/ note. In 1989, the best for highest efficiency in a Ga As monolithic multijunction two-junction device, the device exceeded 27% ef­ top cell should have a high Much of today's research ficiency under uncon­ band gap of approxi­ in muJtijunction cells centra ted sunlight. mately 1.9 eV while the focuses on gallium arsenide (Efficiencies tend to go up bottom cell should have a as one (or all) of the com­ under concentrated sun­ band gap of about 1.4 eV. ponent cells. In 1988, a light because of the electri­ For this device, the band mechanically stacked cal and thermal properties gap of the top cell is 1.35 device reached 31 % ef­ of semiconductors.) The eV and that of the bottom ficiency under concen­ top cell was gallium in­ cell is about 0.75 eV. trated sunlight, which was dium phosphide, con­ Second, the cell is ideal- a new record for any type nected by a "tunnel 1 y suited for space appli­ of photovoltaic device. junction" to a bottom ceJl cations b ecause the It consisted of a GaAs cell of gallium arsenide. A tun­ resistance of indium phos­ on top of a single-crystal nel junction allows charge phide to radiation is 50% silicon cell. In 1990, a carriers to move between better than that of silicon device that used GaAs for the top cell and the bottom and 15% better than that the top cell and gallium cell through a very thin of gallium arsenide, the antimonide for the bottom insulating layer. two materials most widely cell achieved an efficiency The other monolithic used for PV power in of 34.2% under 100 suns cell, a device that uses space. This means that concentration, a record indium phosphide for the arrays using lnP / lnGaAs that still holds as of this top cell and indium gal­ cells could be lighter, writing. lium arsenide for the cheaper, more reliable, and Lately, monolithic multi­ bottom cell (lnP / InGaAs), longer lived and could junction devices, which reached 31.8% efficiency provide more power. face more difficult en­ under 50 suns concentra- gineering and fabrication

41 Third, the band gap of also experience less light­ three, individual amor­ indium gallium arsenide induced degradation. One phous silicon cells stacked can be altered by varying reason for this is that there on top of each other. The the ratio of the constituent is less degradation in very cells can all be of the same materials. This opens up thin cells with extremely material, with the same possibilities for tuning the thin intrinsic layers, such band gap. Or, to capture cell to meet special as the ones that make up a broader portion of the a pp1 ica tions. multijunction devices. sun's spectrum, the cells The devices that we Because the intrinsic layers can be made of different have mentioned here are are so thin, the electric field materials with different just the tip of the iceberg. sweeps charge carriers band gaps. Amorphous Many other combinations from these layers with silicon alloys with carbon, of gallium arsenide and its minimal recombination. germanium, nitrogen, and alloys are being explored Multijunction devices tin have all been used to for use in high-efficiency use two, and sometimes vary the band gap and multijunction devices. Amorphous Silicon Many advanced PV cell designs are used in making amorphous silicon devices, including silicon­ carbon alloy p+n cells, n-i-p cells, and stacked cells. In many ways, the multijunction design is the most important recent advance in amorphous silicon research and de­ velopment. Multijunction devices not only achieve higher efficiencies than Typical structure of a triple-junction amorphous silicon device, which single-junction ones, they stacks three amorphous silicon cells with different band gaps.

42 zno_ p- 1 - n- ZnO/

zno- e~~~~~ n - CdS p- CuGalnSe2

Glass

.• The trend in a-Si is away from single­

Structure of a multijunction device that employs an amorphous silicon submodule on top of a CIS junction and toward submodule. The two submodules are laminated together with transparent polymer material. The CIS multijunction device uses gallium to make the band gap more suitable for use with the high-band-gap amorphous silicon. v devices. • With a band gap material properties to gap of 1.0 eV. Viable by tandem with top cells of of 1.0 eV, CIS improve multijunction themselves, they can also other materials, such as cells .make a good devices. As of 1989, the be used with a higher­ CdTe and GaAs, both of bottom.cell in most advanced triple­ band-gap material in a which have band gaps of conjunction with junction amorphous multijunction device. One approximately 1.43 or 1.44 an a-Si top cell. silicon cell had reached an company is exploring the eV. In fact, recently an ex­ I efficiency greater than 13% commercial possibilities perimental multijunction •. A-Si/CIS multi- in the laboratory. of a multijunction device device that used GaAs for junction cells and Since the depositions employing amorphous the top cell and CIS for the modules have needed to make thin-film silicon (with a band gap bottom cell achieved an ·a,chieved ·efficien­ muJtijunction devices do of about 1.7 eV) on top of efficiency of 25.8%. cies exceeding not use much energy, CIS. Laboratory multijunc­ 16% and .12%, multijunction devices are tion devices have achieved .r~spectively. potentially inexpensive to 16% efficiency i .. and sub­ 1:• CIS is also a good fabricate. Making a multi­ modu les, 12% efficiency. A choice as the layered cell can be very commercial multijunction bottom cell in similar to making one module of CIS and amor­ conjunction with cell-just adding one thin phous silicon is expected ,, a top cell of either film after another. In fact, to produce about 67 W of 1~ . CdTe or GaAs. the trend in amorphous power, but it is likely to be silicon is away from single­ more expensive than the junction and toward multi­ combined cost of a panel junction devices. of each individual thin film. Copper Indium Copper indium di­ Diselenide selenide is a versatile Copper indium di­ material being explored selenide cells have a band for use as a bottom cell in

43 Chapter4 Modules, Arrays, and Systems

he PV cell is the basic interconnected for more modules or arrays and • A individual cell, unit in a PV system. power, and so on. In this condition the electricity the basic unit of a T An individual PV cell typi­ way, you can build a PV so that it may be utilized PV system, does cally produces between system to meet almost any in the application. These not produce much 1 and 2 W, hardly enough power need, no matter structures and compo­ power. power for the great how small or great. nents are referred to as the •To produce more majority of applications. Modules or arrays, by balance-of-system (BOS). power, cells are But you can increase the themselves, do not con­ In this chapter we dis­ connected together power by connecting cells stitute a PV system. You cuss collectors (modules into larger units together to form larger must also have structures and arrays) and balance­ to form modules, units called modules. on which to.put them and of-systems, respectively. which can be con­ Modules, in turn, can be point them toward the Further, we categorize the nected together connected to form even sun, and components that collectors into two general into larger units larger uni ts known as take the direct-current (de) types: flat plate and called arrays. arrays, which can be electricity produced by the concentrator. •We can refer to modules and arrays as collectors, and can categorize collectors into two types: flat plate and concentrator.

Cell Module Array

The basic photovoltaic unit or cell typically produces only a small amount of power. To produce more power, cells can be interconnected to form modules, which can be connected into arrays to produce more power, and so on. Because of this modularity, photovoltaic systems may be designed to meet any electrical requirement, no matter how large or how small.

44 Flat-plate collectors, which typically employ large numbers of cells mounted on a rigid, flat surface, can use both direct sunlight and diffuse sunlight that is reflected from clouds, the ground, and nearby objects. • A flat-plate collector contains a large of which is that U1ey must flat-plate collectors entails Flat-Plate number of cells use a large number of cells more land use, which also that are mounted Collectors and hence depend on becomes an economic Flat-plate collectors large areas of PV material, consideration. on a rigid, flat typically use large num­ to produce power com­ surface, and bers or areas of cells that parable to that produced Connecting encapsulated with are mounted on a rigid, by much smaller con­ Crystalline Cells a transparent cover. flat surface. These cells centrator collectors. Since to Produce • Flat-plate collectors· are encapsulated with a PV cells are very costly can use both direct transparent cover that Modules components of a PV sys­ sunlight and the lets in the sunlight and With crystalline cells, tem, using large areas of diffuse sunlight protects them from the such as crystalline silicon, cells could be an economic that is reflected environment. modules are made by liability Furthermore, for from the clouds Flat-plate collectors connecting individual ., sizable applications, the ground, and nearby have several advantages cells, either in series or in large relative area of objects. in comparison to con­ centrator collectors. They • Flat-plate collectors are simpler to design and do not require fabricate. They do not 0 special optics, require special optics, spe­ II II specially designed cially designed cells, or cells, or struc:tures mounting structures that that must track must track the sun pre­ (b) the sun. cisely. Plus, flat-plate collectors can use all the sunlight that strikes them - both the direct sunlight and the diffuse sunlight that is reflected from clouds, the ground, and Joining cells in series (a) builds voltage while attaching them in parallel (b) increases amperage. nearby objects. But they do have draw­ backs, the most important

45 - - - - - + - po, + --- ..__ - - - - + ~ + I

"" - - - - -~ - - - + - + ------

Bot.h voltage and amperage are increased by arranging cells in both sanes and parallel.

parallel. Joining individual One of the most delicate Producing cells in series builds volt­ areas in modules is the Thin-Film Modules attachment between the age without increasing Unlike single-crystal interconnecting wire and amperage. Conversely, and semicrystalline cells, the PY cell terminal. Titis attaching cells together in thin-film cells can be fabri­ attachment is made by parallel increases amper­ cated in relatively large soldering or welding. age without raising volt­ basic sizes. Though some age. When both effects are Neither technique has efficiency is lost, scale-up needed, as in most electric proved superior to the is fairly easy, from an other, and neither has power requirements, experimental cell (a few proved to be entirely satis­ then the PV cells can be square centimeters in size) factory at withstanding all arranged in both config­ to a submodule (about environmental pressures. urations: first the cells are 1000 square centimeters), The method used to link assembled in strings in to a commercial-sized cells to cells can be series to build voltage; module (about 4000 repeated when connecting then the strings are square centimeters). After modules to modules, grouped together in paral­ the required layers are though other procedures lel to boost amperage. deposited over the entire are available as well. The interconnections surface of the module, a among individual cells laser cu ts in or scribes the within the module are ex­ lines that define the tremely important. They separate cells. have proved to be one of Unlike single-crystal the most sensitive design Cell Cell modules, thin-film elements in the system. 8A G06lJJ modules are not plagued These interconnections by problems associated must provide good con­ Interconnections between cells with interconnecting in­ ductivity and reliability generally use a loop to prevent fatigue due to thermal and wind dividual cells. Individual for expanded periods loading stresses. cells of any size and num­ despite large variations ber are made by scoring in temperature and other the sequential layers environmental stresses (either with a laser beam that can harm the perfor­ or mechanically) as they mance of PV systems. are made so that the top

46 Me t al -~ Semiconductor layers Transparent conducting oxide Glass --../

The individual cells of a thin-film module may be scribed in any size, number, or pattern. Scoring the sequential layers of the cells as they are made connects the top electrode of one cell to the bottom electrode of another cell. linking them in series. By adjusting scoring patterns, we can define the voltage of the module.

Connecting partial shading of the Modules to module. But arcing can be prevented by properly in­ Produce Arrays sulating the area between Whether modules are modules. In addition, composed of thin-film safety switches must be cells or crystalline cells, provided to isolate cells good interconnections during repair work. between the modules are Individual crystalline critical to good perfor­ solar cells are fragile. They mance. Modules are typi­ must be protected from cally attached to each the elements and from the Thin-film modules are made other with metal wire or differently than single-crystal adverse effects of moving modules are. Typically, the meshlike ribbons that are them around during as­ semiconductor layers are usually kept as short as deposited over the entire surface sembly and maintenance. of the module (or submodule, possible. The interconnec­ They must also have sup­ depending on the size desired tions between modules port, in various module for the basic unit). and then a can be rigid or flexible, laser cuts in or scribes lines that arrangements, so they can define the separate cells. although flexible connec­ be incorporated into an tions contend better with overall array design. Al­ electrode of one cell con­ movement within the though thin-film modules nects with the bottom array caused by environ­ are not as fragi le, they too electrode of another cell, mental forces such as need to be protected and linking them in series. thermal expansion. supported. Since the voltages of cells Safety, Protection, The design and con­ in series add, the total volt­ struction of the module age for a module can easi­ and Support provide both the protec­ ly be adjusted by changing One key area of concern tion and the support re­ the scoring pattern that for PV designers is safety. quired. A typical module defines the number and A fire hazard occurs, for design may incorporate a size of individual cells. example, when the electri­ back structural layer of cal arcing between groups metal, epoxy board, or of electrically imbalanced glass to provide rigidity; cells causes sparks. This layers of laminate imbalance can be due to material, such as

47 ethylene vinyl acetate • For terrestrial or polyvinyl butyrate, to applications glass encapsulate the cell within covers are the module; and a trans­ preferred for their parent cover made of glass low cost, resistance or plastic. Although the =====#-- Cover him to environmental cover and the cell encap­ 1---Solar cell conditions, and sulant lend structural sup­ :-- - Encapsulant ability to retain port to the module, their ~----Substrate clarity. primary purpose is to ll'lll!lll~ ~~~~----- Cover film •For space protect the cells from ~ Frame applications, to wind, dust, temperature cut down on extremes, humidity, and oxygen seepage. weight, plastic is One typical module design uses a substrate of metal, glass, or plastic used to cover each Module covers must be to provide back structural support; encapsulant material to protect the cell (as opposed transparent to the portion cells; and a transparent cover of plastic or glass. to covering the of the sunlight spectrum important: if glass is quality of the semiconduc- entire module.) that activates PV cells, so washed regularly, its tor material used to make cover materials usually ability to transmit light (its the cell. To cut down on admit light in the violet to transmissivity) will remain weight, modules generally the far-infrared spectrum within 5% of what it was do not employ an integral and beyond. Cover at the time that it was cover. Rather, each cell is materials that have been manufactured. Even the encapsulated individually, used either separately or best plastics, however, will and plastic materials are in combinations include lose 25% or more of their often the first choice for glasses and both rigid and transmissivity within the encapsulation material. flexible plastics. For ter­ several years after first restrial applications, glass being put to use. Module Design is usually preferred for For applications in Considerations its low cost, resistance to space, the primary con­ For crystalline silicon environmental conditions, cerns are radiation modules, there are spatial and ability to retain its damage and weight. Mini­ considerations as well. clarity. Clarity is especially mizing radiation damage Areas in a module where depends primarily on the there are no PV cells do selection of the type and not produce electricity.

48 Square cells can be packed more densely than circular cells to produce more efficient modules.

Thus, overall module Semicrystalline square The silicon-ribbon efficiency, measured as cells can be produced with growth techniques also wattage per unit area of casting technology. Cast­ result in square PV cells the module, drops as the ing produces a square that require no sawing spaces between cells block of semicrystalline step. Though such increase. This seemingly silicon approximately 8 in. methods have been obvious cause-and-effect (20.3 cm) on a side. The researched extensively, situation is important to block is sliced into wafers, their costs are still not com­ PV system economics, each of which is then cut petitive with ingot or cast­ because it can drive up the into four square wafers ing techniques. Besides, balance-of-system costs 4 in. (10.1 cm) on a side. cells made from ribbon of a PV array (see pages material are smaller and 56 and 57). A less efficient often produce electricity module requires more less efficiently than cells land area to produce a cut from ingots or blocks. given amount of electricity. Cell sizing is another im­ To increase the packing portant consideration in efficiency (i.e., the percent­ creating modules with the age of a module's area desired overall electrical filled with active PV characteristics. The current material that can produce available from a cell grows electricity), most modules with the size of the cell, are packed with square because more sunlight Block t Sheet t cells. The most common lands on a larger cell, but process for making single­ the voltage remains con­ crysta 1cells, on the other stant. Thus, modules hand, relies on the Cz tech­ providing large voltage nique, which produces a outputs require many cylindrical ingot approx­ small cells connected in imately 5 in. (12.7 cm) in Two ways to make square cells series. diameter. The ingot is for modules are casting (a), in sliced into round wafers, which cubes are sliced and cut into square wafers, and ribbon­ which are subsequently growth (b), in which thin, trimmed to a square 4 in. rectangular sheets of silicon are cut (10.1 cm) on a side. into wafers.

49 The primary reason for inexpensive materials Concentrator using concentration is to (plastic lenses, metal Collectors decrease the area of solar housings, etc.) to capture The pe rformance of a cell material being used in a large area of solar energy PV array can be improved a system; solar cells are the and focus it on to a sma II in a number of ways. One most expensive compo­ area, where the solar cell option is to employ con­ nents of a PV system, on a resides. One measure centrating optics, which per-area basis. A con­ of the effectiveness of gather sunlight with lens­ centrator uses relatively this approach is the es, thereby increasing the intensity of sunlight strik­ ing the PV cell. A typical basic concentrator w1it consists of a lens to focus the light, a cell assembly, a mechanism that houses Fresnel lenses the lens at one end and the cell at the other, a second­ ary concentrator to reflect off-center light rays onto the cell, a mechanism to dissipate excess heat produced by concentrated sunlight, and various con­ tacts and adhesives. The cell may also use a pris­ matic cover to guide the light around the cell's metallic grid and onto the active cell material. These basic units may be com­ A typical basic concentrator unit consists of a lens to focus the light, a bined in any configuration cell assembly, a housing element, a secondary concentrator to reflect to produce the desired off-center light rays onto the cell, a mechanism to dissipate excess heat produced by concentrated sunlight, and various contacts and module. adhesives. Notice that the module depicted uses 12 cell units in a 2x6 matrix.

50 The concentration ratio, the power at which the sun is concentrated, is equal to the area of the aperture {the opening Sunlight .. through which the sunlight enters "' - in this case, the aperture is a lens) divided by the area of the Lens illuminated cell. \ I \\ I \ I Concentration \ I ratio \ I area of lens \ I area of cell \ I \ I ~ Cell concentration ratio. Ideally, concentrators is that they There are, on the other • Generally speaking, the concentration ratio is can use small individual hand, several drawbacks the greater the the area of the lens cells - an advantage to using concentrators. concentration, the divided by the area of the because it is harder to The concentrating optics greater the power, cell. (We say "ideally" be­ produce large-area, high­ they require, for example, and the less the cause there are reflection efficiency cells than it is to are si gnificantly more amount of cell area and ab sorption losses.) produce smaller-area cells. expensive than the simple needed. Thus, if we have a lens with an area of 200 cm2 focusing light on • Besides increasing a cell that h as an area of power and 2 4 cm , the concentration reducing cell area, ratio is SO. With this ratio concentrators have the cell receives SO times the additional the amount of sunlight it advantage of would get under uncon­ increasing cell centrated sunlight, which efficiency, up to means you can cut the cell a point. area by SO times (relative to flat-plate collectors) to get a desired amount of power. Besides increasing the power and reducing the size or number of cells used, concentrators have the additional advantage that cell efficiency in­ creases under con­ centrated light, up to a Concentrating systems, especially those with high point. How much the ef­ concentration, require mechanisms that track the sun. ficiency increases depends This particular system has a reinforced concrete largely on the cell design pedestal as a base, a sun sensor (in the middle of the system), and a drive mechanism that allows the array and the celJ material used. to track the sun east to west during the day and north Another advantage of and south through the seasons.

51 covers needed for flat­ tracking mechanisms but lost as well, because an • Most concentrator plate modules. Also, most also more precise controls inherent limitation of any systems must concentrators (those with than .flat-plate systems concentrator is that a por­ use expensive concentration ratios with stationary structures. tion of the sunlight fails to mechanisms to greater than 10, i.e., those Another inherent disad­ strike effective areas of the track the sun along that make the effective sun­ vantage of concentrators lenses. A line-focus con­ two axes. light energy striking the compared with flat-plate centrator (one that focuses • Concentrators cell 10 times greater than modules is that they can sunlight along a line) can cannot focus that of ordinary sunlight) use only direct sunlight. lose as much as 10% of in­ diffuse sunlight; must track the sun along Concentrators cannot focus cident direct sunlight, and they can only use two axes to be effective. diffuse sunlight, which a point-focus concentrator direct sunlight. This is because the field represents about 20% of (one that focuses sunlight of view-the angular the sunlight received in a at a point) can lose from •Some direct window where light must desert region. As a result, 10% to 20%. sunlight is lost as fall to be acted on by the concentrators produce lit­ High concentration well, because an concentrating optics­ tle output on cloudy days ratios are a particular prob­ inherent limitation generally gets smaller as and are intended for use lem, because the operating to a concentrator the concentration ratio chiefly in areas with very temperature of cells in­ is that some of the increases. Thus, higher few such days. creases when excess radia­ sunlight fails to concentration ratios mean Besides diffuse sunlight, tion is concentrated, strike effective using not only expensive some direct sunlight is creating heat. Cell efficien­ areas of the lenses. cies decrease as tempera­ tures increase, and higher • Because of high temperatures also threaten temperatures the long-term stability of generated, PY cells (as well as most concentrators semiconductor devices). must incorporate Therefore, PV cells must designs or be kept cool, either by pas­ mechanisms that sive cooling (such as metal keep the cells cool. fins to radiate heat) or by active cooling, such as Concentration can create heat, which can be deleterious to cell circulating coolants. efficiency and operation. One way to keep cells cool is to mount the cells on a heat spreader, which spreads the heat and radiates it away from the cell.

52 Concentrating Optics As of the late 1980s, the most promising lens for Sunlight PY concentrating optics applications was the Fresnel lens, which fea­ tures a miniature sawtooth I Linear design. When the "teeth" (b) run in straight rows, the lenses act as line-focusing concentrators; when the teeth are arranged in con­ centric circles, light is focused at a central point. The combination of Fres­ nel lenses, high-efficiency (a) (c) silicon PV cells, and two­ axis tracking is currently A Fresnel lens, the most common lens in concentrator systems, is a considered to be the most sheet of plastic that is flat on the upper side and that has sawtooth grooves on the bottom (a). If the grooves are etched in parallel promising concentrating ridges (b), they concentrate light along a line. If they are etched approach. in concentric circles (c), they concentrate light at a point.

Special Types of cells developed for one­ greater than 300 times), PV Cells sun applications with the expense of manufactur­ The development of no modifications. Mid­ ing PV cells with such PV cells for concentrators concentration systems high-efficiency perfor­ has been evolving in three (with a concentration of mance may make the major directions. Low­ 50 to 300 times) include entire system too costly. concentration systems most of the high-efficiency At mid-concentration (with a concentration of PY cells being developed ranges, concentrating 10 to 50 times), for ex­ today. As to high­ modules require some pediency and near-term concentra tion systems changes in cell features for use, generally employ PV (with concentrations the best performance. As

53 the intensity of the inci­ dent light increases, so does the short-circuit current of the PV circuit. With a higher current, Cells designed for concentrators usually have special grid patterns. This cell could handle concentrations up to 200 suns. (Depiction series resistance losses based on a cell used by Applied Solar Energy Corporation.) become important. Several resistance-lowering fea­ Usually, cells designed for currents without blocking tures can be incorporated concentration are thinner much of the cell's surface into high-efficiency cells, than normal cells, have a area. and they are particularly lower cell resistance, and Poi11t-co11tact PV cells, important for cells that have special grid patterns which are fashioned for will be used under high devised to carry the high use with concentrators, are concentration ratios. the most efficient silicon solar cells reported to date and have achieved efficiencies above 28% under concentration. These cells are uniquely designed to capture the maximum amount of light and to generate the greatest amount of cur­ Generation rent. A conventional solar cell has an electrical grid I"-- CollocllOO on the front and a solid electrical contact on the back. Each point-contact cell, in contrast, has tens of thousands of microscopic The point contact cell, designed by Swanson et al. at Stanford University. is a concentrator cell that incorporates several innovations, points of alternating posi­ including a textured top, a reflective back surface, silicon dioxide tive and negative contacts layers at the front and back surface, and thousands of small contacts that occupy only about 5% on the back surface to collect charge carriers. (Adapted from the Proceedings of the 18th IEEE Photovoltaic Specialists Conference of the cell's back surface. with permission from IEEE.)

54 The microgrooved cell designed by Green et al. at the University of South Wales uses very high quality silicon with a thin layer of Microgroove Light Finger silicon dioxide on the top surface. Thin grooves are cut into the oxide, over which are laid very thin fingers of a metal grid. (Adapted from the Proceedings of the 19th IEEE Photovoltaic Specialists Conference with permission from IEEE.)

Putting the point contacts thin (100 microns or less) grooves also help to mini- on the back removes any and still absorb nearly all mize reflection and to trap • Back contacts on shading or recombination of the usable light. The light inside the cell. The the point contact that might otherwise have high percentage of light ab- thin metal grid, laid across cell reduce shading occurred by putting them sorption produces a large the grooves at a 45° angle, and on the front surface. number of charge carriers, allows some of the light to recombination. Making them small sig- and the thinness of the cell be reflected from the metal • Texturing reduces nificantly reduces the con- helps the ca rriers reach the fingers and to enter a non- reflection and, tact surface area, which terminals for collection metallized region. along with the decreases recombination before they can recombine. To further enhance ef- reflective back and contact resistance. To help reduce surface ficiency, these cells are also surface, traps light And implanting so many recombination, the cell being used with prismatic inside of the cell. contacts enhances efficient also employs thin layers of covers, which help over- collection of light- silicon oxide at the front come shadowing losses • The microgroove generated charge carriers. and back surfaces. due to top contacts. These cell uses high· The front surface of the Finally, the cell uses an special cell covers are quality silicon, a cell is textured with small antireflection coating on etched in such a way that thin layer of ·sili.con pyramid shapes; this the front surface to they direct incoming sun- dioxide on the.top reduces the reflection of decrease reflection losses. light away from areas of surface, and thin incident sunlight and scat- Microgrooved passivated the PV cell covered by grooves cut into 1 ters the light into the cell emitter solar cells have a metal grid lines. Prismatic the silicon dioxide · at angles that allow the more conventional struc- covers have been shown in overlaid.with ·a 1 light to be trapped inside ture than point-contact the laboratory to improve 1 . metal ..grid. I the cell (reflecting back cells. The base material is efficiency by 20% (rela- q: ,Jhe microgroove and forth without escap- very high quality silicon; tive), and they are begin- '~-~ cell also usessa' ing) until the light is ab- a thin layer of silicon ning to appear in ! ,,.-.-, - .. c . . ' • ,~ Jlris{Tlati~ cov.e~, '· sorbed. To maximize this dioxide is applied to the commercial projects. ~'·Vf.hich ~ directs . , . ~ "light trapping," the back surface to inhibit High-efficiency multijunc- I' Jncoming sunlight:· surface is highly reflective recombination. I ., - tion PV cells, of which ,:.:_ away.\frqm grid. - and reflects the great Several special features there are many designs, . ... lines'and helps majority of the unab- enhance the efficiency of are particularly suited for · ·

55 binations available; you • A complete PV can choose materials that system consists of Balance·of·system perform well under ~tress­ (mounting structures, Load three subsystems: fu l conditions (heat, m storage, power (application) conditioning, etc.) the array, the load, particular) induced by and the balance-of· concentrated sunlight. system (BOS) . Another reason is that mul­ • The BOS consists tijunction devices, which The complete PV system consists of~~~ s~~S:~~~~~t~~:i:0 ~rray , of such things can be designed to reach the load1 , a~d ~~~c~~:=~~~~~f~~~~~~h~ PV )~ l ectr i city to be properly as mounting efficiencies of 30% or more ~~~ 1f:~ t~nth: load. Among these are mounting structures, storage devices. and power conditioners. structures, storage under normal sunlight, can reach even higher devices, and power properly applied to the efficiencies under con­ Mounting conditioners . load. This third subsystem centration. (Multijunction Structures is generally referred to as • Mounting devices are discussed Photovoltaic arrays the "balance-of -system, II structures, which more fully in Chapter 3.) have to be mounted on or the BOS. support PV arrays some sort of stable, The BOS typically and are built to Balance of durable strncture that can contains structures for withstand wind, support the array and Systems mounting the PV arrays or rain, hail, and other withstand wind, rain, hail, We may think of a com­ modules and the power­ conditions, come and other adverse condi­ plete PV system as com­ conditioning equipment in two types: tions. The mounting struc­ prising three subsystems. that massages and con­ stationary and ture can either be On one side, we have the verts the de electricity tracking. stationary or it can track PV devices (modules, to the proper form and the sun. • Stationary arrays, etc.) that convert magnitude required by an Stationary structures, structures, used sunlight into de electricity. alternating-current (ac) used with flat-plate sys­ with flat-plate On the other side, we have load. If required, the BOS tems, generally tilt the PV systems, tilt the the load, the application also includes storage array at a fixed angle that array at a fixed for which t11e PV electri­ devices, such as batteries, is determined by the angle that is largely city is intended. In be­ for storing PV-generated latitude of the site, the re­ determined by the tween, we have a third electricity during cloudy quirements of the load, latitude of the site. subsystem, a set of devices days and at night. and structures that enables and the availability of the the PV electricity to be sunshine. Stationary struc­ tures may be mounted

56 Among the choices for stationary mounting structures, rack mounting may be the most versatile. It requires relatively simple construction techniques and hardware and can be installed easily on the ground or on flat or slanted roofs.

either on the ground or on ing may offer the most which we have already ' of a building. Where space is flexibility, since it may be described above, is pri­ • StatiOna r-y at a premium, such as for a used either on the ground marily used for PV con­ structures can be mounted either on business or an urban or on roofs and can be easi- centrator systems that ._~.. . - .. residence, roof mounting 1 ydesigned and built to have a moderate to high the ground or on is generally the preferred accommodate the require­ concentration ratio. Single­ a building. option. But mounting a ments of the application. axis trackers, which are '• Of the options system on a roof may re­ For tracking structures, typically designed to track for stationary quire interfering with the there are two general the sun from east to west mounting integrity of the roof or spe­ kinds: one-axis and two­ on its daily route, are used structures (pole, cial designs that integrate axis. The two-axis type, with flat-plate systems rack, integral, the PV system with the standoff, and roof, tilt the system (and direct), rack roof) at the proper angle, mounting may and allow access for opera­ offer the most tion and maintenance. For flexibility. more remote, stand-alone systems, on the other • There are two hand, ground mounting is general kinds of generally preferable be­ tracking structures: cause it allows for greater one-axis and ease of installation, main­ two-axis. tenance, and operation. • One-axis structures There are several op­ track the sun east tions for stationary mount­ ' towest. ing structures, among which are pole, rack, in­ • Ivvo-axis structures tegral, standoff, and direct track the sun east mounting (the last three to west and north are roof mounts only)...... __ _ to south. Among these, rack mount- \ ~ A pole-mounted, single-axis tracker may be used with flat-plate systems or with concentrator systems that have low concentration ratios. These mounting structures often rely on passive tracking, which requires no control or power, to follow the sun on its daily east-to-west course.

57 and with concentrator sys­ batteries are the most periodic maintenance. tems that have low con­ common alternative. Like PY cells, batteries are centration ratios. Tracking But batteries have some direct-current devices and structures are almost al­ drawbacks. For example, are directly compatible ways mounted on the they decrease the efficien­ only with de loads. ground. cy of the PY system, since On the plus side, only 80% or so of the batteries not only store Storage Devices energy channeled into electrical energy, they One way to store PV them can be reclaimed. can also serve as a power electric power is by hook­ They also add to the conditioner. By being part ing the PV system into a expense of the PY system of the circuit into which utility grid. When more and last only between electricity from the PY PY electricity is being five and ten years. They supply flows, the battery generated than is being take up considerable floor keeps the electrical load used, it can be converted space, pose some safety more nearly constant, and to alternating current, the concerns, and require the PV array can be same frequency as that of the utility grid, and fed into the grid. Likewise, when the PV system is not providing enough electri­ city, the extra amount needed can be obtained from the grid. This strategy is generally used by large systems and by residential systems when a convenient backup system is desirable. In stand-alone systems, however, using the utility as a storage medium may not be desirable or even These Laguna Del Mar townhomes use PV to supply part of their electricity. They also use the local utility as a backup to store any possible. In such cases, extra power they may produce. An extra meter on each home monitors the electricity fed back to the utility company.

58 designed to operate closer maintenance, and can also have a mechanism to its optimum power stand up under more that prevents current from output. extreme conditions. flowing from the battery Today, there are several Most batteries must be to the array at night. One types of batteries specifi­ protected from overcharge solid-state mechanism that cally designed for PV and excessive discharge, does this is a blocking systems; the most com­ which can cause electro­ diode, which restricts the mon are lead-acid bat­ lyte loss and can even flow of electricity to one teries and nickel-cadmium damage or ruin the battery direction, blocking the batteries. Although nickel­ plates. Protection is most electricity from going into cadmium batteries cost often afforded by a charge the array and thus protect­ more than their lead-acid controller, which also ing the array from damage counterparts, they general­ maintains system voltage. and saving energy. ly last longer, require less Most charge controllers Power Conditioners Power conditioners process the electricity produced by a PV system to make it suitable for meeting the specific demands of the load. Although most of this equipment is standard stock, it is extremely important to match the capabilities of these devices with the characteristics of the load. Power conditioners may h For stand-alone systems, batteries are the most common alternative ave to to store PV power for later use. These are batteries specially • limit current and volt­ designed for photovoltaic applications; they are more durable than age to maximize power their automobile counterparts and can withstand deeper discharges. The type, size, and number of batteries used depends on the output, application.

59 • convert de power to ac • A power conditioner power, may have to maximize power • match the converted ac output, convert de electricity to a utilitys ac electrical network, to ac, match the and ac electricity to a • safeguard the utility net­ utility's network, work system and its per­ and safeguard the sonnel from possible utility network and harm during repairs. its personnel from The requirements of possible harm. power conditioners generally depend on the type of system they are integrated with and the applications of that sys­ tem. For de applications, power conditioning is often accomplished with regulators, which control output at some constant level of voltage and cur­ rent to maximize output. For ac loads, power conditioning must include an inverter that converts the direct current gener­ ated by the PV array into alternating current. Essentially, an inverter is a set of automatic switches that provide polarity rever­ Power conditioners are used in many systems to maximize power sals from the solar array. output, change the de electricity from the PV array to ac electricity, match the converted ac electricity to a utility's ac electrical network, The shape of the output and to protect the utility network system and its personnel from harm waveform is determined during repairs.

60 by the quality and cost of equipment that utilities the inverter. may require are automatic •A PV system lock-out switches or isola­ hooked to a utility Other Devices tion transformers. These network may also Most other power-related devices ensure that the PV need meters, equipment is placed in a and utility-grid systems lock-out switches, PV system because the sys­ will separate if there is a and isolation tem is hooked to a utility grid failure. If a PV system transf armers. grid. In such cases, it is im­ were allowed to send portant that meters record electricity into a utility the quantities of electricity grid that is shut down, being sold to and pur­ this would create a safety chased from the utility. hazard for utility workers. Other items of PV system

61 For Further Reading

his booklet introduces and Technical Information, • Green, M.A. (1986). T you to photovoltaic P.O. Box 62, Oak Ridge, Solar Cells: Operati11g technology. It is not meant TN 37831, or the Depart­ Principles, Teclmologtj and to be a complete and ex­ ment of Commerce's Systems Applications. haustive resource. How­ National Technical Infor­ Kensington, New South ever, if this booklet spurs mation Service, 5285 Port Wales, Australia: your interest, there are Royal Rd., Springfield, VA University of New many publications that 22161. South Wales; 274 pp. will broaden your • Fan, J.C.C. (1982). Clearly written and well knowledge of the field. "Photovoltaic Cells." illustrated, this text covers Some of these delve into Kirk-Otlz111er Encyclopedia many aspects of photo­ the theory of photovol­ of Chemical Technology. voltaics but is particularly taics, the materials that Third Edition. New strong in cell theory and make up PY devices, and York: John Wiley and cell designs. The mathe­ designs of devices and Sons, Lnc.; Vol. 17, pp. matics and physics in­ systems. Others are more 709-732. cluded here require a practical, describing This chapter provides a college background in applications and giving semitechnical overview of these subjects. guidelines on how to the theory and design of • Komp, R.J. (1984). design and install PY photovoltaic cells. It Practical Photovoltaics: systems for your own use. presents information on Electricity from Solar We list just a few of the the chemistry of the Cells. Ann Arbor, MI: major sources below. For photovoltaic effect, PV aatec Publications; more technical informa­ cells, and factors affecting 216 pp. tion, you may contact the efficiency of various This is a general intro­ DOE' s Office of Scientific types of solar cells. duction to photovoltaics,

62 written on a level that an practices for stand-alone of backgrounds, from the educated, nonscientific PV systems, including novice to the experienced reader can understand. It system-level trade-offs design engineer. also explains how solar necessary for any applica­ • Zweibel, K. (1990). Hnr- cells work and are made; tion. Instructions and 11essing Solnr Power, The covers modules, arrays, worksheets are included Photovoltnics Chnllenge, applications, and how to for system sizing, and 15 New York: Plenum size systems; and discuss­ specific examples of PV Press; 319 pp. es new developments and systems in a wide range of This well-written book the future of photovoltaics. applications are presented. gives the reader more than • Stn11d-A/011e Photovoltnic The guide is intended for just an introduction to PV Systems: A Handbook of a broad audience, from technology. It also delves Reco111111e11ded Desig11 beginners to PV into PV costs and trends of Prnct ices. (1988). professionals. the technology and into SAN087-7023. Albu­ • Strong, S.J. (1986). The politics and policy issues querque, NM: Sandia Solnr Electric House: A surrounding energy and National Laboratories; Design Mnnunl for Ho111e­ PV. 414 pp. Available from Scn/e Pliotovoltnic Power the National Technical Systems. Emmaus, PA: Information Service, Rodale Press; 288 pp. U.S. Department of A comprehensive Commerce, 5285 Port design manual for both Royal Road, Springfield, stand-alone and grid­ VA 22161. connected systems, this This comprehensive book provides information guide recommends design for people with a variety

63