photoconductors

Photoconductors are materials whose electrical conductivity increases when they are illuminated. In these materials, photons with energy above the band gap create electron–hole pairs, adding charge carriers that enhance current under an applied electric field. The degree and speed of the conductivity change depend on the material’s band structure, carrier mobilities, and the presence of traps that can capture or release charge carriers.

The underlying mechanism is photogeneration of carriers followed by transport. When light generates electron–hole pairs, they contribute to conduction if they can move before recombining. Some photoconductors exhibit persistent photoconductivity, in which conductivity remains elevated after illumination due to long-lived trapped charges. Temperature, material purity, and defects strongly influence response time, dark resistance, and long-term stability.

Common photoconductive materials include inorganic semiconductors such as cadmium sulfide (CdS) and zinc sulfide (ZnS), as well as lead sulfide (PbS) for infrared detection. Historically important examples include selenium and amorphous selenium, which were used in xerographic imaging. Organic photoconductors and emerging materials such as quantum dots also contribute to the field. Many photoconductors are used in thin-film form, with the resistance decreasing under illumination and a bias voltage driving a photocurrent.

Applications span light sensing and imaging technologies. Photoconductive cells or photoresistors (often CdS-based) serve as light sensors in exposure meters and automatic lighting controls. In xerography, photoconductive surfaces lose charge more readily in illuminated areas, enabling toner to be attracted to charged regions and creating images. Limitations include temperature dependence, performance drift, and, for some materials, toxicity concerns and environmental sensitivity.