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RESEARCH REPORT
Solar cells are objects that convert sunlight into energy. Solar
cells are also called photovoltaic cells. When light contacts the surface
of the cell, it releases electrons for power. Earth receives very little
of the sun’s energy, but even this amount is a staggering quantity. If
a tiny portion of it could be captured and converted to energy, all earth’s
energy needs would be satisfied. Four quadrillion kilowatt hours of solar
energy reach earth each day. This is more than 500,000 times more energy
than the United States‘ total electrical generating capacity. Capturing
and using all the energy is complicated by the nature of sunlight. First,
sunlight is widely dispersed, and needs large collectors to gather it.
Also, some form of storage is needed for night and cloudy days.
Solar power’s most apparent use is heating and cooling systems to collect
and disperse heat in buildings is already in wide use. The most common
type of collector is a flat plate collector-designed to collect radiation
directly striking it.
EFFICIENCY
Conventional, commercial cells convert about 15% of the light that comes
in contact with it. However, some experimental cells convert about 30%
into energy. Since the launching of the Vanguard 1, solar arrays were the
main source of power for satellites and space vehicles. On most watches
and calculators, only one cell is used. Multi-cell arrays on the other
hand are mostly used for remote operations like buoys, oil drilling platforms,
and microwave repeaters because power lines are too expensive.
The materials that solar cells are made of are silicon, cadmium sulfide,
and gallium arsenide. Thin slices of silicon are expensive, because most
of it is lost in saw cuts. Cadmium sulfide is cheap, but has low efficiency.
Gallium arsenide is very efficient but is also very expensive. Other methods
of producing solar cells are using amorphous rather than silicon.
HISTORY
During the 1880s Charles Fritts was the first person to make a selanium
solar cell. Fritts’ device had a thin, almost transparent layer of gold
on the cell and was very inefficient, converting less than 1% of sunlight
into electricity. Fritt’s solar cell was mostly forgotten by the 1960s
when the rush to make solar cells came back up. The key to making a photovoltaic
cell was to create a semiconductor that releases electrons when exposed
to radiation within a visible spectrum. During this time, researchers at
Bell Telephone Labs were developing solar cells to be used in communication.
By accident, Calvin Fuller and Deryl Chapin found the perfect semiconductor—a
hybridized crystal made of phosphorous and boron. The first solar cells
using these materials had a conversion efficiency of six percent. Later
improvements raised efficiency up to about 15 percent. In 1957, Bell telephone
used silicon solar cells to power a telephone repeater station. The experiment
was considered a success, but was still too inefficient. The first
real application of silicon cells was in 1985 when solar power was used
on the Vanguard 1, which was the second American satellite to orbit earth.
Once space exploration had proved solar power’s efficiency, photovoltaic
cells began appearing more frequently in homes. About any small appliance
can be adapted for solar power but the most successful would be watches
and calculators. These usually use a solar cell to charge a nickel-cadmium
battery so they can be used during periods of low light. In early days,
scientists dreamed of huge photovoltaic arrays that could power whole cities.
House owners that installed solar cells to their homes realized these dreams
to a small extent. The usefulness of these panels is limited by the relatively
low rate of efficiency as well by the limited amount of sunlight. The production
of solar cells is doubling, so at the year 2000, 10% of the world’s energy
was solar power.
The idea of harnessing solar energy is very old. Roman legends say that
at about 298-212 BC, a Roman warrior intensified the sun’s heat and caught
fire to the enemy’s approaching ships. At about 200 BC, the Chinese used
curved mirrors to start fires. In 19th century France, experiments with
solar powered engines concluded with a 1-horsepower solar steam engine
invented by Augustin Mouchot. Until recent times solar energy was considered
impractical, inefficient and unreliable. When the world’s resources started
declining, however, solar power’s attention grew as a possible replacement
for fossil fuel.
HOW A SOLAR CELL WORKS
A solar cell consists of one impurity to make the electrons negative and
one impurity to make the electrons positive. The area where the two impurities
meet is called the P-N junction. When the electrons pass through the P-N
Junction, the two charges make an electrical current. To let light pass
through the layers, they must be very thin.
When solar cells are made, impurities are deliberately added to make
two kinds of silicon, which are explained below:
-N type (negative) contains some atoms with an extra electron. For example,
arsenic has 5 electrons. When an arsenic atom replaces a silicon atom,
four of its electrons bond with their neighbors and the fifth electron
moves around the crystal.
-P type (positive) contains some atoms with one less electron, called
a hole. For example, gallium atoms only have three electrons. This presence
causes a hole in the crystal. It takes very little impurity to make a p-type
or n-type layer about one atom for every 1,000,000,000 (1 billion) silicon
atoms.
The materials used to make the N-type layers are phosphorus, arsenic,
or antimony. Aluminum, gallium, indium are added to the silicon to make
the P-type layer. Some atoms in the N-type layer have an extra loosely
bound electron while some atoms in the P-type layer have a hole where an
electron would be. Without sunlight, loosely bound electrons drift across
the junction area between the two layers to fill holes. The movement near
the junction causes a section of each area to become oppositely charged
from the rest.
Near the junction, the N-type layer is positive, and the P-type layer
is negative. When photons of light strike the solar panel, they pass through
the N-type layer to the P-type whose holes have just been filled. The photons
of light are then liberated then move back across the junction. Then the
free electrons are attracted to the positive charge. Adjacent atoms lower
in the P-type layer move up to fill the hole left by the freed electron.
After the hole is filled, electrons move across the junction to be collected
by electrical contacts on the surface of the solar panel. Then, an electrical
current is formed.
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BIBLIOGRAPHY
Brown, Warren. Alternative Sources of Energy. U.S.A. Chelsea
House Publishing 1994
Krieder, Jan. "solar cell". The World Book Encyclopedia 1983
Mc Grath, Kimberley. World of Invention-Photovoltaic Cell.
U.S.A. gale publishing 1999 601-602
Solar cells. November 21, 2001
<http://www3.umassed.edu/public/exhibit/enl600/assign6.htm>
Thorsen, Richard. "Photovoltaics". Encarta encyclopedia
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