Polycrystalline thin films like copper indium diselenide (CuInSe2) and cadmium telluride (CdTe) show great promise for PV cells. But these materials can't be simply doped to form n and p layers. Instead, layers of different materials are used to form these layers. For example, a "window" layer of cadmium sulfide or similar material is used to provide the extra electrons necessary to make it n-type. CuInSe2 can itself be made p-type, whereas CdTe benefits from a p-type layer made from a material like zinc telluride (ZnTe).
Gallium arsenide (GaAs) is similarly modified, usually with indium, phosphorous, or aluminum, to produce a wide range of n- and p-type materials.
