The invention: Drawing their energy directly from the Sun, the first photovoltaic cells powered instruments on early space vehicles and held out hope for future uses of solar energy.
The people behind the invention:
Daryl M. Chapin (1906-1995), an American physicist Calvin S. Fuller (1902-1994), an American chemist Gerald L. Pearson (1905- ), an American physicist
Unlimited Energy Source
All the energy that the world has at its disposal ultimately comes from the Sun. Some of this solar energy was trapped millions of years ago in the form of vegetable and animal matter that became the coal, oil, and natural gas that the world relies upon for energy. Some of this fuel is used directly to heat homes and to power factories and gasoline vehicles. Much of this fossil fuel, however, is burned to produce the electricity on which modern society depends.
The amount of energy available from the Sun is difficult to imagine, but some comparisons may be helpful. During each forty-hour period, the Sun provides the earth with as much energy as the earth’s total reserves of coal, oil, and natural gas. It has been estimated that the amount of energy provided by the sun’s radiation matches the earth’s reserves of nuclear fuel every forty days. The annual solar radiation that falls on about twelve hundred square miles of land in Arizona matched the world’s estimated total annual energy requirement for 1960. Scientists have been searching for many decades for inexpensive, efficient means of converting this vast supply of solar radiation directly into electricity.
The Bell Solar Cell
Throughout its history, Bell Systems has needed to be able to transmit, modulate, and amplify electrical signals. Until the 1930′s, these tasks were accomplished by using insulators and metallic conductors. At that time, semiconductors, which have electrical properties that are between those of insulators and those of conductors, were developed. One of the most important semiconductor materials is silicon, which is one of the most common elements on the earth. Unfortunately, silicon is usually found in the form of compounds such as sand or quartz, and it must be refined and purified before it can be used in electrical circuits. This process required much initial research, and very pure silicon was not available until the early 1950′s.
Electric conduction in silicon is the result of the movement of negative charges (electrons) or positive charges (holes). One way of accomplishing this is by deliberately adding to the silicon phosphorus or arsenic atoms, which have five outer electrons. This addition creates a type of semiconductor that has excess negative charges (an n-type semiconductor). Adding boron atoms, which have three outer electrons, creates a semiconductor that has excess positive charges (a p-type semiconductor). Calvin Fuller made an important study of the formation of p-n junctions, which are the points at which p-type and n-type semiconductors meet, by using the process of diffusing impurity atoms—that is, adding atoms of materials that would increase the level of positive or negative charges, as described above. Fuller’s work stimulated interested in using the process of impurity diffusion to create cells that would turn solar energy into electricity. Fuller and Gerald Pearson made the first large-area p-n junction by using the diffusion process. Daryl Chapin, Fuller, and Pearson made a similar p-n junction very close to the surface of a silicon crystal, which was then exposed to sunlight.
The cell was constructed by first making an ingot of arsenic-doped silicon that was then cut into very thin slices. Then a very thin layer of p-type silicon was formed over the surface of the n-type wafer, providing a p-n junction close to the surface of the cell. Once the cell cooled, the p-type layer was removed from the back of the cell and lead wires were attached to the two surfaces. When light was absorbed at the p-n junction, electron-hole pairs were produced, and the electric field that was present at the junction forced the electrons to the n side and the holes to the p side.
The recombination of the electrons and holes takes place after the electrons have traveled through the external wires, where they do
Parabolic mirrors at a solar power plant. (PhotoDisc)
useful work. Chapin, Fuller, and Pearson announced in 1954 that the resulting photovoltaic cell was the most efficient (6 percent) means then available for converting sunlight into electricity.
The first experimental use of the silicon solar battery was in amplifiers for electrical telephone signals in rural areas. An array of 432 silicon cells, capable of supplying 9 watts of power in bright sunlight, was used to charge a nickel-cadmium storage battery. This, in turn, powered the amplifier for the telephone signal. The electrical energy derived from sunlight during the day was sufficient to keep the storage battery charged for continuous operation. The system was successfully tested for six months of continuous use in Americus, Georgia, in 1956. Although it was a technical success, the silicon solar cell was not ready to compete economically with conventional means of producing electrical power.
CONSEQUENCES
One of the immediate applications of the solar cell was to supply electrical energy for Telstar satellites. These cells are used extensively on all satellites to generate power. The success of the U.S. satellite program prompted serious suggestions in 1965 for the use of an orbiting power satellite. A large satellite could be placed into a synchronous orbit of the earth. It would collect sunlight, convert it to microwave radiation, and beam the energy to an Earth-based receiving station. Many technical problems must be solved, however, before this dream can become a reality.
Solar cells are used in small-scale applications such as power sources for calculators. Large-scale applications are still not economically competitive with more traditional means of generating electric power. The development of the Third World countries, however, may provide the incentive to search for less-expensive solar cells that can be used, for example, to provide energy in remote villages. As the standards of living in such areas improve, the need for electric power will grow. Solar cells may be able to provide the necessary energy while safeguarding the environment for future generations.
See also Alkaline storage battery; Fluorescent lighting; Fuel cell; Photoelectric cell; Solar thermal engine.
