Third generation photovoltaics: Ultra-high conversion efficiency at low cost

Since the early days of terrestrial photovoltaics, a common perception has been that first generation' silicon wafer-based solar cells eventually woul d be replaced by a 'second generation' of lower cost thin-film technology, probably also involving a different semiconductor Historically, cadmium sul phide, amorphous silicon, copper indium diselenide, cadmium telluride and n ow thin-film polyclystalline silicon have been regarded as key thin-film ca ndidates. Any mature solar cell technology seems likely to evolve to the st age where costs are dominated by those of the constituent materials, be the y silicon wafers or glass sheet. It is argued therefore, that photovoltaics is likely to evolve, in its most mature form, to a 'third generation' of h igh-efficiency thin-film technology. By high efficiency, what is meant is e nergy conversion values double or triple the 15-20% range presently targete d, closer to the thermodynamic limit of 93%. Tandem cells are the best know n of such high-efficiency approaches, where efficiency can be increased mer ely by adding more cells of different bandgap to a cell stack, at the expen se of increased complexity and spectral sensitivity. However, a range of ot her more 'paralleled' approaches offer similar efficiency to an infinite st ack of tandem cells. These options are reviewed together with possible appr oaches for practical implementation, likely to become more feasible with th e evolution of materials technology over the next two decades. Copyright (C ) 2001 John Wiley & Sons, Ltd.