Solar cells, or photovoltaic devices, use a silicon semiconductor to convert sunlight, or any visible light, into electrical energy. When sunlight falls on a junction between n-type and p-type semiconductor material, electrons are ejected from atoms. These electrons are collected to form a direct electric current (DC). Solar cells are often assembled into modules designed to produce a certain current at the design voltage. The current produced depends directly on the amount of light striking the module. Typically, simple solar cells convert around ten to fifteen percent of the incoming solar energy into useful electrical energy. This efficiency can be increased by using two or more materials sensitive to light of different frequencies in tandem, in order to utilise more of the Sun's spectrum, or by using focusing devices to concentrate the sunlight.
Photocells are common in electric eyes, radiation detectors and light meters. Many utilise the photoelectric effect to detect the presence of light or radiation at particular wavelengths. For example, a photoelectric photometer is used by astronomers to analyse the frequencies of light received from a star. Others respond to a change in light intensity by detecting a particular photocurrent, such as in an alarm circuit where an intruder cuts a beam of light falling on a photocell.
