The solar cell is made from a thin layer of semiconducting material. The key feature of this semiconductor layer is that it will absorb photons of radiation in the visible region of the electromagnetic spectrum. Each photon of light energy is absorbed by an electron within the solid material. In absorbing the energy, the electron acquires an electrical potential. This potential can be made available as electrical energy, as an electric current. The current is produced at a specific fixed voltage called the cell voltage. The cell voltage is a property of the semiconducting material. For silicon it is around 0.6 V.
The energy contained in light increases as the frequency increases from infrared through red to blue and ultraviolet light. However a solar cell must throw away some of these frequencies. It can only absorb light which is above a certain energy threshold, called the cell threshold. Light with energy content below that threshold will simply pass through the cell or be reflected.
Ideally, then, one might want to choose the lowest-feasible threshold in order to utilise as much of the solar spectrum as possible. There is, however, another difficulty. The cell threshold determines the cell voltage. If the threshold frequency is very low, the solar cell will provide a low-output voltage. When light is absorbed with an energy higher than the threshold, all the excess energy is simply thrown away, wasted. So setting the threshold too low wastes energy too. Thus the threshold must be set at an optimum level.
Most commercial solar cells use silicon as the semiconducting layer. Silicon does not represent the optimum solar absorber but it is relatively easy to work with, it is extremely abundant and therefore cheap, and it benefits from an accumulation of experience with the material as a result of its use in the manufacture of transistors and microchips.
Other materials are being introduced. Among the most promising are cadmium telluride and copper indium selenide. Gallium arsenide has also been used in space. Silicon, however, appears likely to form the backbone of the solar cell industry for the immediate future.
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