Photovoltaic cell

Photovoltaic cell

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 largearea

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 arsenicdoped

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 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 ThirdWorld 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.