involves the electron/hole pairs created when incident light is absorbed by the cadmium sulfide followed by these reacting with water and sulfide: Production of an electron–hole pair CdS + hν → e− + h+ Reaction of electron 2e− + 2H2O → H2 + 2OH− Reaction of hole 2h+ + S2− → S Structure and physical properties Cadmium sulfide has, like zinc sulfide, two crystal forms. The more stable hexagonal wurtzite structure (found in the mineral Greenockite) and the cubic zinc blende structure (found in the mineral Hawleyite). In both of these forms the cadmium and sulfur atoms are four coordinate. There is also a high pressure form with the NaCl rock salt structure. Cadmium sulfide is a direct band gap semiconductor (gap 2.42 eV). The proximity of its band gap to visible light wavelengths gives it a coloured appearance. As well as this obvious property other properties result: the conductivity increases when irradiated, (leading to uses as a photoresistor) when combined with a p-type semiconductor it forms the core component of a photovoltaic (solar) cell and a CdS/Cu2S solar cell was one of the first efficient cells to be reported (1954) when doped with for example Cu+ ("activator") and Al3+ ("coactivator") CdS luminesces under electron beam excitation (cathodoluminescence) and is used as phosphor both polymorphs are piezoelectric and the hexagonal is also pyroelectric electroluminescence CdS crystals can act as a gain medium in solid state laser In thin-film form, CdS can be combined with other layers for use in certain types of solar cells. CdS was also one of the first semiconductor materials to be used for thin-film transistors (TFTs). However interest in compound semiconductors for TFTs largely waned after the emergence of amorphous silicon technology in the late 1970s. Thin films of CdS can be piezoelectric and have been used as transducers which can operate at frequencies in the GHz region. Nanoribbons of CdS show a net cooling due annihilation of phonons, during anti-Stokes luminescence at ~510 nm. As a result, a maximum temperature drop of 40 and 15 K has been demonstrated when the nanoribbons are pumped with a 514 or 532 nm laser. Applications Pigment CdS is used as pigment in plastics, showing good thermal stability, light and weather fastness, chemical resistance and high opacity. As a pigment, CdS is known as cadmium yellow (CI pigment yellow 37). About 2000 tons are produced annually as of 1982,