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A solar cell is a device that transforms the solar power that is incident on its surface into electric
power available for use in an external electric circuit that contains a load
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Both these effects take place
when a light photon strikes an electron from a solid material atom (be it a metal or
semiconductor)
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If the electron is expulsed from the surface of the material (when light strikes an atom from the
surface) the effect is called external, if the photon travels a certain distance into the material and
the electron is expulsed from the bulk of the material the effect is called internal photoelectric
effect
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Normally, if there is no other additional
factor to act upon the electron-hole pair, after a short time the electron will get back into its place
(it is said that the electron and hole will recombine)
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In the junction region, an internal electric field builds
up because of the junction asymmetry (different doping)
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(It is said that an electric drift
current will happen)
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Therefore this length of
the neutral parts of the semiconductor (outside the p-n junction) needs to be smaller than the
diffusion length of the charge carriers (a value characteristic to each semiconductor)
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What we said before about how a solar cell works, gives a good idea about how a solar cell is
constructed
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The interface
between the two regions (p and n) that is making up the p-n junction is engineered in such way
that a good part of the available light will be absorbed and will generate electro-hole pairs
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Early solar cells were built based on a
metal-semiconductor junction (Schottky junction) between one electrode (usually the surface
electrode) and the semiconductor, not on a p-n junction in the semiconductor bulk
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However
during the last years, because of the new materials available for making solar cells (apart Silicon)
and of new technologies that have been developed, the manufacture of a solar cell can involve
other different procedures
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It has a direct (see the explanation above) band gap of 1
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