A phenomenon in which an electrical voltage is produced in a material after exposure to light. Photons of the appropriate wavelength transfer energy to the atoms from which an electron is released. The empty space left by the electron has the properties of a positive charge and is called a hole. If we use a pair of semiconductors, one n-type with an excess of electrons and one p-type with an excess of holes, the incident photons will increase the fraction of electrons in the n-layer and the electric voltage will rise. When the electrodes are connected, such a device will provide a direct electric current. The photovoltaic effect is most often used in the design of photovoltaic cells.