The Solar Cell

Solar cells are typically doped silicon devices which convert light energy directly into electricity.  They are specially designed diodes where charge flow is affected by light. Photons of light hitting the solar cell are absorbed by electrons. This promotes the electrons into the conducton band and leaves a positive ‘hole’ behind in the valence band.

The electric field across the ‘depletion layer’ causes the negative electrons to drift into the n-type material while the holes are attracted into the p-type. This is how the n-type silicon (beyond the depletion layer) becomes negative and the p-type becomes positive, and a p.d. now exists across the solar cell. The cell can generate about 0.5 – 0.6 volts which will drive a current through an external circuit.

Semi-conductors like silicon have a high refractive index. This means they are highly reflective. Less light is reflected (and more is transmitted) if the refractive indices of the two materials at the boundary are close in value. An anti-reflective coating is added to the top surface of a solar cell, this has a refractive index between that of air and silicon, so that less light is reflected at the boundary. This makes the solar cell appear blue, as it is the blue end of the spectrum which is mainly being reflected (the refractive index of the coating for blue light is still quite different from the refractive index of air). The effect is to increase the amount of light transmitted into the silicon by 30 %, and it is the energy in the light which creates the ‘electron-hole’ pairs.