Solar photovoltaic cellsfaculty.missouri.edu/.../SolarCells_PEC-homoJ_1981... · The photovoltaic effect, the direct conversion of solar en- ergy to electricity, was first documented

chemical principle1 Edited by

Charles D. Mickey Texas AaM at Gaiveston

Galveston, TX 77553

Solar Photovoltaic Cells Charles D. Mickey Texas A 8 M University at Galveston, Galveston, TX 77553

The sun is the oldest energy source known; the solar energy it nroduces is an unusual kind of energy because it is manifest in'so many forms. The daily influx ofsolar energy impels the atmosnhere and oceans, driving the winds and creating ocean curreits; it provides the irnpetils for the hydrologic cycle, the indisnensahle determinant of climate; and it is a prerequisite for photosynthesis, the conversion of sunlight into stored chemical energy. All of the foods and fuels (except nuclear) used by humankind have been derived, in some sense, from solar energy.

For centuries man has augmented the daily influx of solar energy by utilizing stocks (lf stored solar energy; first from plants for food, from animals for food and work, from trees for shelter and heat; then from the wind and falling water for oower. The introduction of coal to power the steam engine, in the 1750's, initiated a quantum jump in man's ability to use enerev in concentrated forms. The next significant increase in energy consumption began in 1859 when oil was discovered by Edwin L. Drake, near Titusville, Pennsylvania. Trans- portation was revolutionized by the new liquid fuels derived from oil and, as a result of the automobile's success, in- dustrialization took on a new dimension ( I ) . In the industri- alized world, the demand for energy from stored sources has paralleled technological development. This phenomenon has been maximized in the technologically advanced society of the United States where 6% of the earth's population consumes 35% of the world's energy diet (2).

The fuel crisis of the ~ a s t decade has focussed our attention

increased interest in the most readily available energy source of all: solar radiation.

Our Solar Resource

Produced hy atomic transmutation of the chemical ele- ments in the sun's interior, solar energy represents the world's most ahundant permanent source of energy. The sun emits radiation from its surface with an energy distribution very similar to a black-body (perfect radiator), at 6000 K. Delivered in payloads of incredible magnitude, the solar radiation which reaches the earth's upper atomsphere is received a t the rate of 19.3 kcal/min/m20r 1.363 kilowatts (kW)/m2 (3 ) . This radiant energy is partitioned between the ultraviolet, visible, and infrared wavelengths as shown in the table.

The solar radiation intercepted by the earth's surface is attenlmted hv the terrestrial atmomhere. This attenuation is caused by several factors: scattering hy molecules much smaller than the waveleneth of the incident radiation; selective ~

absorption by gases present in the atmosphere and especially by 03. 0%. H20, and C02; scattering by aerosol particles (e.g., pollen, dust, and smoke) of size comparable to or larger than

418 Journal of Chemical Education

The Forms of Extraterrestrial Solar Radiation

Type of Radiation U@m)

% of Solar Flux

Ultraviolet < 0 4 8.7 Visible 0.4-0.72 40.2 Infrared >0.72 51.1

the wavelength of incident radiation; reflection and absorption hv cloud masses; ower ring the hydrologic cycle; and photo^ synthesis.

Therefore, the solar radiation incident on the earth's surface is only one~third of the annual extraterrestrial total and nearly 70% of that falls on water. However, the 1.5 X 1017 kW.hr that impinges on the continental land masses is approximately 6000 times the total energy budget for the United States. Al- though a prodigious amount of radiant energy falls on the earth every day and is wasted, the technology needed to har- ness this unlimited source is just heing developed.

A Historical Perspective

The photovoltaic effect, the direct conversion of solar energy to electricity, was first documented (4) hy Edmond Recquerel in 1839; he demonstrated that a voltage was produced when light was absorbed by an electrode immersed in an electrolyte. W. Smith ( 5 ) demonstrated an analogus effect in solids using trigonal selenium in 1873. Smith's observation was confirmed in 1877 by W. G. Adams and R. E. Day (6), who also observed the photovoltaic effects in selenium. Nearly a renturv after its discoverv, this laboratory phenomenon was ~ ~ ~ -

considered as a potential source of electricity. In 1954, workers a t RCA and the Bell Telephone Laboratories announced the

This feature is aimed as a review of basic chemical principles andas a reappraisal of the state of the art. Comments, sugges- tlons tor topes, and contributions should be sent to the feature editor.

Charles Mickey received his BS from Trinity University in 1957. MA from St. Mary's University in 1966, and his PhD from Texas AaM University in 1973. He tmnht chemistrv at Alamo Heiqhts Senior ~ i $ School. s i n Antonio. ~ e i a s . for 13 years. He also has over seven years university experience, having taught at San Antonio Junior College and Texas A&M Unwersity. He is presently an Associate Professor of Chemistry in the Department of Marine Science at the Galveston branch of Texas A&M.

Dr. Mickey's excellence and dedication to teaching has been sighted in hjs achievement of the ACS James Bryant Conant Award in 1970 and the 1976-77 "Most Effective Teacher Award: Texas A&M University at Galveston."

construction of a silicon solar cell with an efficiency of 6% (7).

Technological progress in photovoltaics has been remark- ablv slow since its inception; this was mainlv due to the av&ahility of cheap fos& fuels (8). Solar ce&, the energy conversion devices used to convert sunlight to electricity by means of the photovoltaic effect, became more familiar in the 1960's and 1970's; providing electrical power for space vehicles. Because solar cells do not reauire an intervenine heat engine and generator, they offer a potentially economic method for solar nower generation. Since 1975 the use of terrestrial-based u

solar cells has surpassed their level of use in space programs; this reflects the r a~ id lv increasine interest in usine solar cells . "

as an alternate energy source.

The Key Photovoltaic Material The photovoltaic effect can be observed in nature in a va-

rietv of materials: however. the kev component of a photovoltaic cell is a seiniconductor. To set the stage for considering the physical process of converting sunlight directly into electricity it will he helpful to consider properties of semiconductors.

The electrical conductivities of crystalline solids vary over an immense range. Representative values range from 6 X lo7 mho m-1 for silver, an excellent conductor, to 10-l7 mho m-' for fused quartz, a good insulator. Metalloids, such as selenium and germanium, with intermediate conductivity values (-lo2 mho m-1) are called semiconductors. However, semiconductors need not involve metalloids: certain ionic salts may he ~~ ~~~~ ~~

used instead. Galliurn(II1) arsenide, arsenic(II1) selenide,"and cadmium selenide. for examnle. are candidates for use as solar . . photovoltaic cell semiconductors. The characteristics of semiconductors are best ex~lained in terms of the band theory of solids

The Band Theory of Solids Quantum theorv predicts, for an isolated atom, that elec-

trons are distributed a t disirete or quantized energy levels. Moreover the exclusion ~ r inc i .~ le . as enunciated bv Wolfgana l 'n~~li , limits the m~mlrer oirlectrons that ran populate nnv rlvrn tnercv lr\.c~l. This situation is illustrated for al~~minutn in Figure i: When atoms are packed close together, as in a crystal, so that their orbitals overlap, the exclusion principle is still operative and the energy levels must split into an array of acceptable bonding and antihonding molecular orhitals. The arrsv. called an e n e r n band. is formed hv the combina- " ~ -. tion of similar atomic orhitals from each atom in the crystal. When the number of interactine atomic orbitals is large. the energies of the resultant molecular orbitals are spreab.into hands consisting of very closely spaced energy levels. For a crystal containing N atoms (Fig. 21, there will be N levels in each energy band and as many hands as there are energy levels in each isolated atom.

Consider the energy hands in a lithium crystal. They are composed of extremely large numbers of closely spaced energy levels. Since lithium forms a body-centered cubic crystal lattice, the 2s orhital containing the lone valence electron of each atom overlaps the corresponding orhital of its eight nearest neiehbors. The result is a set of 2s molecular orbitals. interacting to form a single 2s energy hand that emhraces all of the atoms in the crystal. In a mole of lithium (6.939 g) there is a mole (6.02 X loz3) of lithium atoms and a mole of 2s atomic orhitals whose overlar, produces a mole of 2s molecular orbitals. Because the&& so many molecular orhitals their rnergiei arc. \.cry v l ~ ~ l v spaced: cc,nsequenrly. I ~ N form n c~mtimnlnl t F i ~ L'r wllccl the 2 . energy Ixtnd. 5:milarlv. the 2p stomw d n ~ a l s i r m tach :stmn conil~i~ie 10 h r m a ?p en- e;gy band that embraces each atom in the crystal lattice.

Each lithium atom has a filled 1s atomic orhital; therefore, the corresponding 1s energy band in the crystal is filled. Moreover, the 1s atomic orhitals are not near enough to

Figure 1. The energy level distribution scheme b r an aluminum atom which has 13 electrons.

numbers or atomic orbitals

tron; therefore, the lower half of the 2s energy hand is occu- nied. Inasmuch as the atomic orhitals above 2s are emvtv, the corresponding h ~ ~ l w r r n r y \ hsnd; i l l the lithium cry-tdl .Are emnty ' l 'h is r~t~tnt i~~n is ill.~;rrat~.d in Finwe3 111 lithitun the . . low-energy part of the 2p hand overlaps the high-energy part of the 2s band (Fie. 3).

A different arrangement is noticed in beryllium (Fig. 4); each beryllium atom has two dectrons in its 2s orhital. Thus, the entiie 2s energy band is filled. However, the 2p energy hand arising from overlap of the empty 2p atomic orhitals partially overlaps the filled 2s energy band. In other words, the lowest energy molecular orbitals in the 2p hand are of lower energy than the higher energy molecular orhitals in the 2s hand.

Every metal has an energy band arrangement either like lithium or beryllium. That is, either the highest occupied band is partially filled, or if it is filled, it overlaps an empty band of slightly higher energy.

Volume 58 Number 5 May 1981 419

> ....................................................................... ?

............................................................................ Condurlion nand , Valance Bond

} Filled Energy Band

Figure 3. Energy bands in metaiiic lithium.

b ?

.. Conduction Band

filled Volencm Band

Fi1l.d Energy Band

Figure 4. Energy bands in metallic beryllium.

Conductors, Semiconductors, and Insulators Using this concept of band theory, it is now possible to

characterize conductors, semiconductors, and insulators. When the highest filled energy band is widely separated from the nearest empty band, the substance is an insulator. In the filled hand. since all the delocalized molecular orbitals are w c ~ ~ p ~ e r l , [he, resi<Ivn1 elettr~m- 11) the hatd 3re IMI ~ ~ l l ~ ~ ~ v e d In the 1';1u11 e x v l u ~ ~ ~ m t)rinwt,lt r u :jl,iurlh sm.lll increinc!nt. or energy from an applied electric field; hence, they are unable to acquire kinetic energy and estahlish a current by their motion. The highest occupied hand corresponds to the ground state of the valence electrons in an atom. Ultimately, the distribution of electrons in this hand determines the optical, thermal, and electrical properties of a solid. This is similar to the concept of valence electrons, that mostly determine the atom's chemical properties. For this reason the upper occupied hand is named the oalence band. In an insulator the valence

called the conduction hand, is so great that under ordinary conditions a valence electron cannot he excited to the conduction band. For example, aluminum oxide (A1203) has an energy band gap of 10 electronvolts (eV) a t room temperature (Fig. 5a).

In conductors the highest occupied valence levels are very close to or overlap vacant energy levels in the conduction band (Fig. 5h). In other words, some electrons reside in or very near the conduction hand. Thus, an applied electrical potential can deliver sufficient energy to the conduction band electrons, excitine them to sliehtlv hirher unoccuuied enerev levels - " - -- within the same hand. These mobile electrons accept energy from an external field and conduct electric current.

materials have energyhaid structures that &e similar to insulators (Fig. 5c), except the energy gap is much narrower. For example, the semiconductor tin (or-% variety) has an energy gap of only 0.08 eV. Semiconductors have a close empty band

Figure 5. Patterns of orbital energy bands in (a) an insulator. (b) a metallic solid. and (c) a semiconductor.

statisti&-and so in the simplest cases a rise7n temperature increases the conductivity exponentially. These conductors are called thermal semiconductors. This behavior is contrary to the effect of temperature on metallic conductivity, which generally decreases as temperature is raised. In some cases the absorption of light causes electrons to jump from the valence hand to the conduction hand, leading to the phenomenon called phutoconductiuity. For example, visible light will cause this to happen for selenium (9 , 10). The electrons that are promoted by the absorption of heat and light are then free to move through the solid when a potential is applied. Moreover, the sites or "holes" left vacant in the valence hand become charge carriers. This happens because an electron near a hole can move in and fill the hole, leaving a new hole in the position it previously occupied, and this in turn can be filled by a neighboring electron, and so on. Thus conduction is attributed to both holes and electrons (charge carriers). When the conduction of current is due only to those electrons promoted from the valence hand to the conduction band, the material


Figure 6. Schematic representation of (a) intrinsic (pure) silicon. showing the four valence electrons that each atom uses for bonding in the crystal lanice: (b) substitution of a phosphorusatom for asilicanatom inme lanice. which forms stype silicon; and (c) substitution of a gallium atom for a silicon atom in the lanice, which forms ptype silicon.

is called an intrinsic semiconductor. In other words, such materials are semiconductors because of a relatively small energy gap between their valence and conduction bands.

Bridging the Gap; Impurity Semiconductors

~or&nately, the effectiveness of semiconductor materials depends not only on their dual charge carriers hut also on the way the ratio of charge carriers can he controlled rigorously during their manufacture.

crystal defects produced by introducing foreign atoms into semiconductor lattice sites disturbs the periodicity of the svstem and results in the creation of additional enerev levels within and between the allowed bands. The precisecontrol of the relative concentrations of charge carriers in the newly created levels is achieved by adding certain impurities (dopants) to highly purified intrinsic semiconductor materials, forming materials referred to as doped or extrinsic semiconductors. The choice of douant determines the dominant type of charge carrier (either holes or electrons) in the semiconductor material.

Figure 6 shows the physical arrangement of dopants in a silicon lattice. This two-dimensional simplification of the silicon crystal lattice will be used because i t is easier to draw, and understanding the concept of impurity semiconductors is not dependent on a precise geometrical representation. Figure 6a shows the normal bonds that exist in an extremely uure silicon crvstal. Each uair of lines represents one Si=Si covalent bond. All four valence-shell electrons in each silicon atom ~articiuates in the formation of covalent bonds with sp3 hybrihizatio;.

The conduction of silicon can be increased by adding atoms of an element (a process called doping) that has more valence-shell electrons than silicon. For example, silicon can he doned with small auantities of nhosuhorus or arsenic. Phos- . . phorus and arsenic both have five valence electrons and only four are needed to bond either of these atoms to silicon in its crystal. The dopant atoms simply replace silicon in some of the tetrahedral lattice sites as shown in Figure 6b. The fifth . electron resides in an impurity level that is very close to silicon's conduction hand. Since this electron is only held in position by i ~ , aulumt,ic uttra~.ricm u, the dolnnt nuclnl,, i t rat1 lit. r,n,niuttd t o o u > n d ~ ~ r ~ i < , n 11md with tar .r>. ( n e w thm that required to excite a valence electron across the bind gap. Silicon doped in this way is an n-type semiconductor, since electrons are neeative and the dominant charee carriers. " "

Alternately, silicon can he enhanced as a semiconductor by donine with atoms havine fewer valence electrons than silicon. ~ ~ .~ u such as boron, aluminum, gallium or indium, as shown in Fieure 6c. For examole. in ealliurn the number of valence shell - . ,

electrons (three) is one less than required to form four sp3 hybridized honds needed at each lattice site in the silicon crystal. This leaves the impurity levels empty and close to the valence band. Because of their proximity, electrons are easily excited from the valence handto the empty impurity levels.

Photons r"----

Figure 7. Schematic diagram of a typical homojunction photovoltaic cell. A photon striking the metal current collection grid ( I ) is reflected. Photons of ap- propriate wavelength (2) create charge carriers while longer-wavelength photons (3) pass through the solar cell.

. positive holes. Conduction occurs as electrons move into the ~osit ive holes leaving new uositive holes behind. Conse- quently, an extrinsic s&niconductor of this type, in which the douant is electron-deficient, is classified as a p-type semi- . ~~

conductor. In other words, the charge carries are positive.

Solid-state Photovoltaic Cell Operation If a semiconductor is exposed to solar radiation i t ahsorbs

photons by exciting electrons from the valence hand to the conduction band; concomitantly, a positive hole is left behind. Eventually the hole and electron will recombine, giving up the absorbed energy to the crystal lattice as heat or by emitting a photon. The recombination of the hole and electron is pre- vented in solar cells by creating an intrinsic voltage in the cell material so that the photovoltaic charge carriers are separated (11). The most efficient photoconversion devices for sepa- ratine the charee carriers are uroduced bv creatine an abruut u - discontinuity in the conductivity of the cell material. Such a discontinuitv can he contrived in several wavs in solid-state devices. A p-n junction can he produced by joining two op- uositelv doued semiconductors, so that there are excess holes . . (p-type) on one side of the junction and excess electrons (n-type) across the interface. When the semiconductor devices are made by adding small amounts of dopants to a pure material, it is called a homojunction cell. A schematic representation of a tvoical homoiunction cell is shown in Fieure 7. An . . - intrinsic voltage can also he produced by joining two dissimilar semiconductor materials (ex.. CdS and CuInSe?), creatine a . " . heterojunction, or by the union of a semiconductor and a metal (such as a silicon to palladium interface) creating a Schottky barrier junction.

Limitations on Solar Cell Efficiency A fundamental limit on the current output of solar cells is

due to the solar energy spectrum. Photons lacking the energy to promote electrons from the valence to the conduction hands (i.e., the band gap energy) cannot contribute to photovoltaic current; furthermore, the energy absorbed by electrons which exceeds the minimum band gap or threshold energy cannot be converted to electric current. Most of this excess photon enerev is dissinated verv rauidlv into vibrational excitation of thKmateriai, therebyheaiin$the cells.

Most of the solar energy reaching the Earth's surface falls


in the visihle region of the spectrum, where photon energies vary from 1.8 eV (deep red) to 3.0 eV (violet). In CdAsz about 1.0 eV is required to produce a photovoltaic electron, and in GaAs about 1.4 eV (12). Selecting a semiconductor with a hieher band EaD enerzv results in cauturina a larger fraction . . . . oirhf v w y y $,I tht IIIINC cnc.r~tic phttsrni hut hl-in:: 3 1ar:c.r rrm tmn ..I the ley. vnt,r-etli ohohma. Hv,.:Iu-~ 01 rhis vn?r<? mismatch, only 44% of the energy in the solar spectrum is available to silicon (hand gap energy = 1.1 eV) for conversion to photovoltaic electrons.- or a single semiconductor system the theoretical (ideal) photovoltaic efficiency peaks at a hand gap energy of 1.5 eV (13); however, the theoretical efficiency remains within 80% of this maximum for semiconductors with hand ean enereies between 1.0 and 2.2 eV (14). Photovoltaic " . " systems composed of multiple semiconductors of different hand eao enereies will have higher efficiencies (15.16).

~ h e p e r f o r k n c e of real cells never reaches the level indi- cated in the ideal case for several reasons. One conspicuous problem is the reflection of photons from the cell surface and from the metal contacts (Fig. 7). These problems can he minimized by using special coatings or texturizing the cell surface and by careful contact design. Diminution of efficiency also occurs when the photogenerated charge carriers fail to reach the region in the cell where they can he separated by the intrinsic voltaee. These charee carriers cannot contribute to

imperfections in the cell's crystal lattice. Imperfections in the lattice due to dopants, vacancies, and dislocations act as recombination sites and thus diminish the efficiency of the photovoltaic device.

Silicon Solar Cells Silicon solar cells have been the most extensivelv develooed

and utilized cells. This dominance is due to the availah& of silicon (second most abundant element) and the technolorn -. for producing the large, defect-free, silicon single crystals needed to fabricate solar cells. Although silicon does not have optimal physical characteristics (it has a hand gap of 1.1 eV and weak optical absorptivity), high quality single-crystal silicon is readily availahle, and p-n homojunction devices with efficiencies greater than 10% are commonplace (1 7). In 1978, Fossum and Burgess (18) reported the fabrication of solid- state silicon devices with efficiencies approaching 19%. Most efforts in the development of silicon solar cell technology are currently directed toward reducing the cost of solar-grade silicon. reducine the cost of cell fahrication, and imorovine the solar energy to electricity conversion efficient; (19).-~he choice of materials used to fabricate ohotovoltaic converters utfer-; tm(. 01 the mo>t usetul u,nyi t o minimu( cost and m,u- imizc cell tfti~icmcy. So tkr ihe m o t iiltra~tive silicon devices contrived as technological alternatives to the silicon p-n junction includes heterojunctions and Schottky barriers.

Heterojunction cells are fabricated by the deposition of a wide hand gap material such as CdS (2.6 eV), Gap (2.2 eV), or ZnS (2.6-e~I unon an n-tvoe or o-tvoe silicon suhstrate. - - , - , . ". . .. The wide band gap materials are transparent to most solar nhotnns. hence. most of the incident solar radiation is aassed into the silicon suhstrate. The use of this technique prevents current loss by charge carrier recombination in the front surface of the photovoltaic cell and provides cell efficiencies of 11-12s (19).

Schottky harriers are prepared by sequentially laminating a p - or n-type silicon suhstrate with an oxide film (<20 A) and a thin film (-100 A) of copper, aluminum, silver, gold, or chromium. The metal film, applied by vapor deposition, is transparent to solar radiation and permits the transmission of light to the silicon semiconductor. The interfacial oxide film enhances the voltaee ontnut without reducine the ohotocur- rent (20). ~ o ~ - c o s t ~ c h d t t k ~ barrier deviceslhaveAbeen fah- ricated that exhibit efficiencies ranging from 8 to 10% (21,

Cadmium SulfidelCopper(l) Sulfide Solar Cells The CdSICuaS cell provides a charge separation field by the

function of two dissimilar materials (heteroiunction). It ah-

Hence CdSICuzS solar cells are more attractive hecause of low fahrication and materials cost.

The cadmium sulfide cell is areoared hv the vauor denosi- tion of a thin layer of CdS (-25 p i ) on a substrate of piastic or glass coated with an electrical contact such as zinc. Suhse- quently, a Cu2S layer (-3000 A thick) is formed by dipping the device in a cooaer ion bath. The earlv cadmium cells de- . . graded in ;.nil~i~wn nlr 'and :it eIe\.?rt.d tr~ii~writture-: I I I ~ I I . ~ ~ ~ , .I) i lw n d t rn d ~ s : ~ n . t l ~ e cell. .art, In( rlnvtwalls + ? l ~ l 111 ~1;i.s and apparently have a useful life of decades. 1; another recent innovation, thin film cells have been fabricated which sub- stitute indium phosphide or copper indium selenide for the copper(1) sulfide in the CdSICu2S heteroiunction. Efficiencies ereater than 10% have been demonstrated in these exneri- mental cells (23).

Gallium Arsenide Cells Gallium arsenide (GaAs) has a high theoretical photovoltaic

efficiency because its excitation threshold, i.e., hand gap energy (1.4 eV), closely matches the solar energy spectrum. Consequently, its solar cells can achieve high efficiencies in intense radiation, particularly if they are laminated with GaAlAs which has the effect of minimizine surface recomhi- nation losses. Efficiencies as high as 24.5% cave been recorded for these devices operating in sunlight concentrated 180 times (24).

Prognosis Photovoltaics hold tremendous potential for supplying

significant portions of the nation's energy budget during the next century. Generating electricity in this way has obvious advantages. The energy (sunlight) to power the devices, although intermittent and diffuse, is cheap; the photovoltaic cells have nu moving parts, therefore the maintainence costs are low; and, they do not require high-temperature or high- pressure liquids for energy transport.

Since photovoltaic cells are extremely reliable, quiet, safe, and easy to operate, they are ideally suited to onsite applications. The most formidable harrier facing users of photovoltaic systems is the current high cost of the devices. Photovoltaic systems, using cells developed in the space program, are economical today for certain small, decentralized applications. These systems, however, have a potential for dramatic price reductions that would make them economical for a broader range of applications. Although the solar electric technologies are in varying stages of development, long term development is proceeding on central station solar electric power sta- tions.

In 1968, Glaser (25) proposed the use of a geostable satellite to collect solar energy and produce electricity. The electrical nower could he transformed to microwave for transmission to earth. Subsequently, the microwave would he used to eenerate electricitv. Although the oro~osal is verv ambitious. the concept deser;es further study.

Literature Cited (1) "Sok i" h & e d F"t"re:2"d Ed., ERDA, US. Government print in^ 0ffi"e.

Washington, D.C., ,977. (2) Wa1ters.E.A. and Wewerka,E.M.,d. CHEM EDUC., 52,282 (19751. (31 Kreith, F. and Kreider. 3. F., "Principles ot Solar Engineering," Hemisphere Publishing

Corporation, Washington,D.C.. 1978.p. 14. (4) Beequ~rsl, E.. C.R. Acod. Sr i , 9.661 11839). ( 5 ) Smith, W., Amer J Sci ,5,301 (1873). (6) Adams, W. G. andDsy,R. E..Proc. Roy. Soc London. Sar. A,25,113 (1877). 171 Brattian. W. H. and Garrett. C.. BdlSvstema Tech.. 34.129 119551.

2. (91 Smith. W.,Natuie, 7,303 (1873).

(101 Sfuke, J., "Selenium: (Editors: Zineaio, R. A,, and Cooper, C. W.) Van Nusfrand ReinhnldCo.. New York, 1976, p. 174.

. . 22).



Solar photovoltaic cellsfaculty.missouri.edu/.../SolarCells_PEC-homoJ_1981... · The photovoltaic effect, the direct conversion of solar en- ergy to electricity, was first documented

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