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concentrations depending on the irradiance and

the spectral composition of the light. Photon

energy is inversely proportional to wavelength.

The highly energetic photons in the ultra-violet and

blue part of the spectrum are absorbed very near

the surface, while the less energetic longer wave

photons in the red and infrared are absorbed

deeper in the crystal and further from the junction

[Book 4]. Most are absorbed within a thickness

of 100 æm. The electrons and holes diffuse

through the crystal in an effort to produce an even

distribution. Some recombine after a lifetime of the

order of one millisecond, neutralizing their charges

and giving up energy in the form of heat. Others

reach the junction before their lifetime has expired.

There they are separated by the reverse field, the

electrons being accelerated towards the negative

contact and the holes towards the positive [Book

5]. If the cell is connected to a load, electrons will

be pushed from the negative contact through the

load to the positive contact, where they will

recombine with holes. This constitutes an electric

current. In crystalline silicon cells, the current

generated by radiation of a particular spectral

composition is directly proportional to the

irradiance [Book 2]. Some types of solar cell,

however, do not exhibit this linear relationship.

The silicon solar cell has many advantages such as

high reliability, photovoltaic power plants can be

put up easily and quickly, photovoltaic power

plants are quite modular and can respond to

sudden changes in solar input which occur when

clouds pass by. However there are still some

major problems with them. They still cost too

much for mass use and are relatively inefficient

with conversion efficiencies of 20% to 30%. With

time, both of these problems will be solved

through mass production and new technological

advances in semiconductors. Bibliography 1)

Green, Martin Solar Cells, Operating Principles,

Technology and System Applications. New

Jersey, Prentice-Hall, 1989. pg 104-106 2)

Hovel, Howard Solar Cells, Semiconductors and

Semimetals. New York, Academic Press, 1990.

pg 334-339 3) Newham, Michael ,"Photovoltaics,

The Sunrise Industry", Solar Energy, October 1,

1989, pp 253-256 4) Pulfrey, Donald

Photovoltaic Power Generation. Oxford, Van

Norstrand Co., 1988. pg 56-61 5) Treble,

Fredrick Generating Electricity from the Sun. New

York, Pergamon Press, 1991. pg 192-195

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