How do photoconductive cells work as light-controlled switches?

Photoconductive cells function as light-controlled switches by utilizing the internal photoelectric effect to alter the electrical conductivity of semiconductor materials.12

In their resting state, semiconductors are relatively poor conductors because most electrons are bound within the valence band. When the material is exposed to light with photons of sufficient energy, these photons excite electrons, raising them across the band gap into the conduction band. This process creates free charge carriers—electrons and holes—that allow the material to carry an electric current.12

Because this increase in conductivity is temporary and ceases when the light source is removed, these materials act as effective switches. For example, cadmium sulfide is commonly used in automatic street lighting, where the presence or absence of visible light triggers the device to open or close an electrical circuit. By selecting semiconductors with specific bandgaps, these cells can be tuned to respond to different parts of the spectrum, including infrared or ultraviolet radiation.132