Current Opinion in Green and Sustainable Chemistry, Volume 4, 1 April 2017,
Chalcopyrite semiconductors are used in thin film solar cells with the highest efficiencies, in particular for flexible solar cells. Recent progress has been made possible by an alkali postdeposition treatment. Other important trends are the development of tandem cells and of ultrathin solar cells. Recent progress has forwarded the understanding of off-stoichiometry and of bulk defects in these materials.