Semiconductor Diode that converts Light to electric current. When light strikes the exposed active surface, it knocks Electrons loose from their sites in the Crystal. Some of the electrons have sufficient energy to cross the diode Junction and pass through an external Electric Circuit. Because the current and Voltage obtained from these devices are small, they are usually connected in large
series-parallel arrays. Practical photovoltaic cells are currently about 10 to
15% efficient. Although cells constructed from indium phosphide and gallium
arsenide are, in principle, more efficient, Silicon-based cells are generally less costly. Solar Energy photovoltaic cells have long been used to provide electric Power for spacecraft. Recent developments, still in progress, have driven costs
down to the point where they are being used more and more as energy sources on
earth.