A photocathode is a photoelectrode that is used to convert light into electrons. It is made of a semiconductor material, typically cadmium sulfide (CdS), that has been coated with an electron-emitting material, such as cesium oxide (CsO). The photocathode is usually placed in an evacuated glass envelope. When light strikes the photocathode, it causes electrons to be emitted from the surface of the semiconductor. These electrons can then be used to generate electrical current.
The first photocathodes were developed in the late 19th century and were used in vacuum tubes. Today, they are used in a variety of devices, including solar cells, light-emitting diodes (LEDs), and photomultiplier tubes. Photocathodes have also been used in medical imaging devices, such as computed tomography (CT) scanners and positron emission tomography (PET) scanners.