Solar Cells

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but in
insulators and semiconductors there is an "energy gap", in which no
electron orbits can exist, between the inner valence band and outer
conduction band [Book 1]. Valence electrons help to bind together the atoms
in a solid by orbiting 2 adjacent nucleii, while conduction electrons,
being less closely bound to the nucleii, are free to move in response to an
applied voltage or electric field. The fewer conduction electrons there are,
the higher the electrical resistivity of the material.
In semiconductors, the materials from which solar sells are made, the
energy gap Eg is fairly small. Because of this, electrons in the valence
band can easily be made to jump to the conduction band by the injection of
energy, either in the form of heat or light [Book 4]. This explains why the
high resistivity of semiconductors decreases as the temperature is raised
or the material illuminated. The excitation of valence electrons to the
conduction band is best accomplished when the ...

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