solar cell,semiconductor devised to convert light to electric current. It is a specially constructed diode, usually made of forms of crystalline silicon or of thin films (as of copper indium gallium selenide or amorphous silicon). When light strikes the exposed active surface, it knocks electrons loose from their sites in the semiconductor. Some of the electrons have sufficient energy to cross the diode junction and, having done so, cannot return to positions on the other side of the junction without passing through an external circuit. Since the current obtained from these devices is small and the voltage is low, they must be connected in large series-parallel arrays (solar panels) if useful amounts of energy are to be converted. Practical devices of this kind are about 10% to 15% efficient and for many years were most commonly used to provide electric power for spacecraft. For large-scale power conversion solar cells offer a number of practical problems; one of the most serious of these is the wide variation of output voltage and current accompanying changes in the amount of incident light; this can be compensated for on smaller scales by storing energy produced during peak periods in batteries. Reductions in the cost of producing solar panels have made them a viable alternative to electrical generators for homes and villages in remote areas, and solar panels have become more common as an alternative energy source for residences and businesses.