Photoconductor

A photoconductor is a material whose electrical conductivity increases when it is exposed to light. The effect, called photoconductivity, occurs when photons with energy above the material’s band gap generate electron-hole pairs, increasing the density of mobile charge carriers and lowering resistivity. In some materials, illumination can also alter trap states, leading to persistent photoconductivity where elevated conductivity remains after the light is removed.

Common photoconductors include elemental selenium, cadmium sulfide (CdS), lead sulfide (PbS), and zinc oxide (ZnO). Amorphous and crystalline forms of these materials have been used in photodetectors, light sensors, and xerography. The spectral response depends on the material’s band gap and defect states, and response times typically range from microseconds to milliseconds, influenced by carrier mobility, lifetime, and trap density.

Applications span photodetection, light metering, and electrophotography. In xerography, a photoconductive surface is charged in the dark; illumination from a scene reduces the surface potential in exposed regions, forming a latent electrostatic image that is developed with toner.

Photoconductors are distinguished from photovoltaic materials, which generate current under illumination with minimal external bias. They are also used in photoconductive receivers for optical communications and spectroscopy, where light-induced conductivity changes modulate electrical signals.