The author provides a well-readable and comprehensible presentation of the function of solar cells. The physical laws underlying the phenomena and processes are explained in detail. Starting with the properties of solar radiation and semiconductors, the conditions for converting radiant energy into electrical energy are thoroughly explained. The focus of the presentation is on the thermodynamic perspective, which facilitates a good understanding of the physical limits of achievable efficiency. The second edition has been completely revised and includes new concepts for further approaching the technically achievable efficiency to its theoretical limit.