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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