Third generation photovoltaics: solar cells for 2020 and beyond

Many working in the field of photovoltaics believe that ‘first generation’ silicon wafer-based solar cells sooner or later will be replaced by a ‘second generation’ of lower cost thin-film technology, probably also involving a different semiconductor. Historically, CdS, a-Si, CuInSe2, CdTe and, more recently, thin-film Si have been regarded as key thin-film candidates. Since any mature solar cell technology is likely to evolve to the stage where costs are dominated by those of the constituent materials, be it silicon wafers or glass sheet, it is argued that photovoltaics will 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. Tandem cells provide the best-known example of such high-efficiency approaches, where efficiency can be increased merely by adding more cells of different band gap to a stack. However, a range of other better-integrated approaches are possible that offer similar efficiency to an infinite stack of such tandem cells.