Gerald Leondus Pearson, Calvin S. Fuller, and Daryl M. Chapin, all of Bell Telephone Laboratories, developed the photovoltaic cell, which produced electrical power from sunlight. The new techology made possible a host of innovations, from solar heating to solar power for inflight spacecraft.
Sunlight was first converted into electrical power in 1839 when French physicist Alexandre-Edmond Becquerel
Parabolic mirrors at a modern solar power plant.
In the early 1930’s, scientists “rediscovered” Fritts’s selenium cells. The new selenium cells, though useful for producing very small electrical currents, were still limited in their conversion efficiency. The search was on again for more efficient devices to convert sunlight to energy.
If a crystal is viewed as a periodic array of atoms, symmetry and proximity can be envisioned as causing the discrete electron energy levels to spread out and form bands in the solid. Two kinds of energy bands are formed: valence bands and conduction bands. In a pure semiconductor
When an electron moves from the valence to the conduction band, it leaves behind a vacant electron site, or hole. This hole can act as a charge carrier, as can the electrons in the conduction band, in that a valence electron from a nearby chemical bond can transfer into the hole. The hole, hence, effectively migrates throughout the crystal.
In a pure semiconductor, there are an equal number of conduction electrons and holes. When impurities are added to semiconductors, the band structure is altered, modifying the conducting properties of the material. An extrinsic semiconductor is formed. If an atom such as arsenic, with five valence electrons, is added to a semiconductor like silicon, four of the valence electrons participate in the covalent bonding; one electron remains unbonded. This excess electron is only weakly bonded to the arsenic atom; thus, its energy level is only slightly below the conduction band. Very little energy is necessary to raise this electron to the conduction band. Atoms with five valence electrons are called donor atoms, and semiconductors doped with donor atoms are called n-type semiconductors.
If a semiconductor is doped, instead, with an atom such as aluminum, with three valence electrons, bonds can be formed with only three of the neighboring atoms. An electron deficiency, or hole, remains in the fourth bond. The energy levels of such impurity atoms lie slightly above the valence band; hence, the levels are easily accessible to electrons in the valence band. When an electron jumps to this higher level, a hole is left behind in the valence band. Since such impurity atoms effectively accept electrons from the valence band, they are called acceptor atoms, and semiconductors doped with acceptor atoms are called p-type semiconductors.
When p- and n-type semiconductors are intimately contacted, the boundary region between the two materials is called a p-n junction
In the early 1950’s, three Bell Telephone Laboratories research scientists, Calvin S. Fuller, Daryl M. Chapin, and Gerald Leondus Pearson, were working on three independent research projects. A set of fortuitous circumstances brought them, and their research, together to develop a much more efficient photovoltaic cell. The photogeneration of a voltage across p-n junctions had been known since R. S. Ohl
In 1950, Fuller began investigating the surface properties of germanium. Two particular effects interested him: the surface properties of germanium as they affected the electrical behavior of the very pure crystals that were being grown, and a curious property called “thermal conversion.” The latter was, at this time, not understood. By this time, many advances had been made in solid state devices. The transistor and the p-n junction transistor had been developed. The band theory of metals was well established and also was being adapted to semiconductors. It was a puzzle, however, how a semiconducting crystal of, for example, germanium could change from an n-type conductor, which conducts negative charges (electrons), to a p-type conductor, which conducts positive charges, or vice versa.
Fuller recognized that thermal conversion was related to the way people were handling the crystals when they etched and washed them. As Fuller recollected,
If one took very great pains to make very pure water, better than conductivity type, and then looked for this thermal effect in the germanium crystal, it did not happen. . . . The crystal was so sensitive that if you went into a laboratory and grabbed the doorknob and then happened to lightly touch the crystal, it would convert; that is it would change type from n to p on subsequent heating above about 500°C.
It appeared that something in the ordinary water supply, later identified as copper, was responsible for this effect. The copper rapidly diffused into the crystal above about 500 degrees Celsius, creating acceptor sites, and hence, converting an n-type to a p-type crystal. Subsequent work by Fuller and his colleagues identified group three elements as acceptors and group four elements as donors in silicon and germanium.
Meanwhile, another research group at Bell Laboratories, headed by Chapin, was seeking a dependable alternative energy source to power communication systems in isolated areas. Chapin was convinced that a solar-powered device would be the ideal solution, but his attempts to develop a more efficient selenium cell were unsuccessful.
Pearson (director of the rectifier program at Bell Laboratories), was aware of Fuller’s investigations regarding diffusion in semiconductors. He was attempting to make surface junctions using lithium in silicon. Unfortunately, the diffusion rate of lithium was too high to produce useful diffusion junctions. In discussions with Fuller, it was suggested that Pearson try phosphorus and boron, which Fuller knew formed permanent junctions for room temperature use. The result was an efficient power rectifier 0.75 square centimeters in size, yielding 20 amps through a resistance of 0.08 ohm.
By chance, Pearson exposed one of Fuller’s doped semiconductor crystals to light. Pearson recalled that to his surprise, “I noticed that it was very light-sensitive.” He soon brought this information to the attention of Chapin, and the three scientists joined forces to develop an improved, more efficient photocell. According to Pearson, “Although at the start we weren’t going after a solar cell at all, upon this discovery, we turned it into a solar cell project.” The result was a permanent cell with an efficiency of nearly an order of magnitude better than the best selenium or copper oxide photocells in popular use.
The first of their new solar cells, with an efficiency of about 6 percent, was presented at the National Academy of Sciences meeting in Washington in May, 1954. Subsequent solar cells attained 12 to 15 percent efficiency on cells of almost a 2.5-centimeter-squared area, a respectable efficiency even today.
Though the new Bell Laboratories solar cell was greeted with a storm of enthusiasm, its inventors chose to emphasize small-scale applications. They recognized that the solar cells were still limited in their application, primarily because of their high cost.
The first application of the silicon solar cell, in 1957, was to power a telephone repeater in Americus, Georgia. An array of cells delivered 9 watts of power, which was used to charge a nickel-cadmium storage battery. Though the process worked very well, it was not cost-effective compared to conventional energy sources.
Commercial use of silicon solar cells began when they became the preferred source of electrical power for space satellites. Solar cells were used in the first orbiting satellite, Vanguard 1, launched on March 17, 1958. Its radio transmitter, powered by solar cells, operated for eight years before failing because of radiation damage. Solar cells were also used in the 1960’s, as a power source in the Telstar satellites; they continue today on all satellites for electrical power generation.
For many years, terrestrial applications of solar cells remained largely unexplored. With the increased concern regarding energy supplies and energy prices following the 1973 Organization of Petroleum Exporting Countries (OPEC) oil embargo, however, interest again focused on converting the sun’s energy to electrical power. Many people, including officials within the U.S. Department of Energy, hoped that solar electrical energy would compete with electrical power from coal and nuclear generation. The early estimates of a capital cost of one thousand dollars per kilowatt, about that for coal-fired plants, however, were much too optimistic, perhaps by a factor of ten or fifteen. As remarked by Fuller in 1986, “We never believed or intended the cells would compete with coal or nuclear plants at the present stage of development.”
Smaller-scale uses of photovoltaics are, nevertheless, increasing dramatically. Thousands of homes are powered by solar cell arrays. The majority of these installations are small and on remote dwellings by homeowners who wish to power some lights and a radio or television, but who are located in an isolated area. Some have more sophisticated systems and have connected their systems to the utility grid and engage in buy/sell arrangements with a power company.
The most noticeable boom in solar cell use, however, is undoubtedly in small devices such as handheld calculators and watches. These have been made possible to a large extent by the development of amorphous (noncrystalline) silicon solar cells. Compared to crystalline silicon, amorphous silicon absorbs light more strongly in the visible spectrum, and it has a greater energy gap between the valence and the conduction band. This results in a higher theoretical efficiency than for single-crystal silicon cells. These efficiencies, however, have not been realized. In spite of this, the simple fabrication steps and the thin films that can be utilized mean that they can be a practical and relatively inexpensive solar cell if the 7 percent efficiency observed in the laboratory can be attained commercially.
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Green, Martin A. Third Generation Photovoltaics: Advanced Solar Energy Conversion. New York: Springer, 2003. Reviews the invention and first two generations of photovoltaic technology while explaining the functioning of early twenty-first century cells. Bibliographic references and index. -
Komp, Richard J. Practical Photovoltaics. Ann Arbor, Mich.: Aatec, 1984. In spite of the title, which leads one to expect only a set of “how-to” instructions, this book has a lengthy foreword describing the history of photovoltaics and chapters devoted to the manufacture of solar cells, new technological developments, the photovoltaics industry, and the future of photovoltaics. Illustrated, with bibliography, glossary, and index. -
Merrigan, Joseph A. Sunlight to Electricity: Prospects for Solar Energy Conversion of Photovoltaics. Cambridge, Mass.: MIT Press, 1975. Somewhat dated as to “state-of-the-art” technology and demand, but useful and interesting from a historical perspective, especially since it was published at the peak of energy concern following the 1973 OPEC oil embargo. Summarizes the mid-1970’s perspective on economic considerations and business opportunities regarding photovoltaics. Extensive bibliography, indexed. -
Millman, S., ed. A History of Engineering and Science in the Bell System: Physical Sciences, 1925-1980. Murray Hill, N.J.: AT & T Bell Laboratories, 1980. A summary of the various developments at Bell Laboratories, providing a very brief description of the technological advances and the individuals involved at each stage. Fairly abbreviated (solar cells are discussed explicitly). More instructive for those with some previous understanding of semiconductors. -
Raisbeck, Gordon. “The Solar Battery.” Scientific American 193 (December, 1955): 102-110. One of the first scientifically oriented articles for the nonscientist regarding the Bell Laboratories solar cells. Interesting from a technical as well as historical perspective. -
Swan, Christopher C. Suncell: Energy, Economy, and Photovoltaics. San Francisco: Sierra Club Books, 1986. Describes a variety of photovoltaic-related issues, including an extremely simplified description of what they are, recent innovations in photovoltaics, the photovoltaic industry, and the marketplace. Appropriate for the nonscientist who is concerned not with the operation of solar cells but only with their potential use and markets. Very optimistic approach as to their potential. Bibliography, indexed.
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