THIN-FILM SOLAR-CELL MATERIALS

Photovoltaic energy conversion is based upon the generation of charge carriers by absorption of light and on their local separation in semiconductor junctions. The necessary thickness of the absorbing layer has to correspond with the lengths across which photogenerated carriers are collected. With help of direct semiconductors, thicknesses of absorbers and devices can be reduced to 1 micrometer or less providing the entire spectrum of economic and energetic benefits of thin film technology. At present, primarily two classes of thin films are considered from which already at least laboratory cells with conversion efficiencies in the range of 12-15% were demonstrated: amorphous hydrogenated silicon (a-Si:H), and polycrystalline chalcogenide and chalcopyrite cells, like CdTe, or CuInSe2-based cells. However, additionally to the promising technological advantages of thin films some of their specific problems and limitations, such as metastable effects in a-Si:H, and the use of environmental hazardous metals like Cd have to be taken into account very carefully.