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. Transportation was revolutionized by the new liquid fuels derived from oil and, as a result of the automobile's success, industrialization took on a new dimension ( I ) . In the industrialized 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 sdecade t has focussed our attention
The Forms of Type of Radiation Ultraviolet Visible Infrared
% of Solar Flux
U@m)
8.7 40.2 51.1
0.72
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~thirdof the annual extraterrestrial total and nearly 70% of that falls on water. However, the 1.5 X 1017kW.hr that impinges on the continental land masses is approximately 6000 times the total energy budget for the United States. Although a prodigious amount of radiant energy falls on the earth every day and is wasted, the technology needed to harness 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 ~
increased interest in the most readily available energy source of all: solar radiation.
Extraterrestrial Solar Radiation
~
~
Our Solar Resource
Produced hy atomic transmutation of the chemical elements 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 19.3kcal/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
This feature is aimed as a review of basic chemical principles andas a reappraisal of the state of the art. Comments, suggestlons 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 remarkablv 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 ugeneration. Since 1975 the use of terrestrial-based solar cells has surpassed their level of use in space programs; this reflects the r a.~ i d"l vincreasine interest in usine solar cells as an alternate energy source. The Key Photovoltaic Material The photovoltaic effect can be observed in nature in a varietv 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~lainedin terms of the band theory of solids ~~
~~~~
Figure 1. The energy level distribution scheme b r an aluminum 13 electrons.
atom which has
~~
The Band Theory of Solids Quantum theorv predicts, for an isolated atom, that electrons are distributed a t disirete or quantized energy levels. Moreover the exclusion ~ r i n c i . ~ las e .enunciated bv Wolfgana l'n~~li, limits the m~mlreroirlectrons 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 combination 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 lv cc,nsequenrly. I ~ form N n rnergiei arc. \.cry v l ~ ~ spaced: c~mtimnlnlt F i L'r ~ wllccl the 2 . energy Ixtnd. 5:milarlv. the 2p stomw d n ~ a l i s r m tach :stmn conil~i~ie 10h r m a ?p ene;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
-.
numbers or atomic orbitals
tron; therefore, the lower half of the 2s energy hand is occunied. Inasmuch as the atomic orhitals above 2s are emvtv, the i l l the lithium cry-tdl .Are corresponding h ~ ~ l w r r n r hsnd; y\ e m.n t.y ' l ' h i s r ~ t ~ t n t iis~ ~ ill.~;rrat~.d n in F i n w e 3 111 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. I n 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
Figure 4. Energy
Energy Band
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 ~ [he, e r l ,resi
