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 would be replaced by a 'second generation' of lower cost thin-film technology, probably also involving a different semiconductor Historically, cadmium sulphide, amorphous silicon, copper indium diselenide, cadmium telluride and now thin-film polyclystalline silicon have been regarded as key thin-film candidates. Any mature solar cell technology seems likely to evolve to the stage where costs are dominated by those of the constituent materials, be they 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 high-efficiency thin-film technology. By high efficiency, what is meant is energy conversion values double or triple the 15-20% range presently targeted, closer to the thermodynamic limit of 93%. Tandem cells are the best known of such high-efficiency approaches, where efficiency can be increased merely by adding more cells of different bandgap to a cell stack, at the expense of increased complexity and spectral sensitivity. However, a range of other more 'paralleled' approaches offer similar efficiency to an infinite stack of tandem cells. These options are reviewed together with possible approaches for practical implementation, likely to become more feasible with the evolution of materials technology over the next two decades. Copyright (C) 2001 John Wiley & Sons, Ltd.