Photoconductivity

Photoconductivity is an optical and electrical phenomenon in which a material becomes more electrically conductive due to the absorption of electromagnetic radiation such as visible light, ultraviolet light, infrared light, or gamma radiation.

When light is absorbed by a material such as a semiconductor, the number of free electrons and holes increases, resulting in increased electrical conductivity. To cause excitation, the light that strikes the semiconductor must have enough energy to raise electrons across the band gap, or to excite the impurities within the band gap. When a bias voltage and a load resistor are used in series with the semiconductor, a voltage drop across the load resistors can be measured when the change in electrical conductivity of the material varies the current through the circuit.

Classic examples of photoconductive materials include:

• photographic film: Kodachrome, Fujifilm, Agfachrome, Ilford, etc., based on silver sulfide and silver bromide.
• the conductive polymer polyvinylcarbazole, used extensively in photocopying (xerography);
• lead sulfide, used in infrared detection applications, such as the U.S. Sidewinder and Soviet (now Russian) Atoll heat-seeking missiles;
• selenium, employed in early television and xerography.

Molecular photoconductors include organic, inorganic, and – more rarely – coordination compounds.