13 October 2009

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.
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.


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