In Chapter 2, certain PV materials such as crystalline and amorphous silicon have been described. Other materials apart from silicon can, however, be used. We consider here their nature, properties and potential uses. There are two major families of PV materials: Solid crystalline materials, Solid thin films (on a substrate). We also cover other technologies, in particular Gratzel-type dye-sensitised solar cells, polymer solar cells, cells containing small organic molecules and photoelectric chemical sensors. These technologies, while promising in terms of cost and adapt ability, have yet to show their competitiveness against the traditional technologies. Most have stability problems and have low efficiency (<5%). This is why, despite much promise, they have only had a few recent commercial applications.ι The material most widely used in solar PV is silicon, a type ΓV semiconductor. It is tetravalent, meaning one atom of silicon can bind with four other atoms of the same type. Solar silicon is either crystalline or amorphous. In the amorphous state, it is used as a thin film with thicknesses of around 1 μm and above, deposited on a backing, while crystalline cells are solid and around 0.1-0.2 mm thick. Other semiconductors used are types III-V like gallium arsenide (rare and expensive) and thin films like CdTe (cadmium telluride) and CIS (copper indium selenide).