PHOTOVOLTAIC EFFECT

process in which two dissimilar materials in close contact act as an electric cell when struck by light or other radiant energy. Light striking such crystals as silicon or germanium, in which electrons are usually not free to move from atom to atom within the crystal, provides the energy needed to free some electrons from their bound condition. Free electrons cross the junction between two dissimilar crystals more easily in one direction than in the other, giving one side of the junction a negative charge and, therefore, a negative voltage with respect to the other side, just as one electrode of a battery has a negative voltage with respect to the other. The photovoltaic battery can continue to provide voltage and current as long as light continues to fall on the two materials. This current can be used to measure the brightness of the incident light or as a source of power in an electrical circuit, as in the modern solar battery. A solar battery is a combination of many individual photovoltaic cells. One composed of two different types of silicon crystals, when exposed to sunlight outside the Earth's atmosphere, can capture 14 percent of the incident energy and supply 170 watts per square metre (16 watts per square foot) of the contact area between the two materials. See also photoelectric effect.