Photovoltaics, the direct conversion of sunlight to electricity, is now the fastest growing technology for electricity generation. Present "first generation" products use the same silicon wafers as in microelectronics. "Second generation" thin-films, now entering the market, have the potential to greatly improve the economics by eliminating material costs. Martin Green, one of the world's foremost photovoltaic researchers, argues in this book that "second generation" photovoltaics will eventually reach its own material cost constraints, engendering a "third generation" of high performance thin-films. The book explores, self-consistently, the energy conversion potential of advanced approaches for improving photovoltaic performance and outlines possible implementation paths.