Chalcopyrite-based solar cells are very attractive for use in next-generation photovoltaic modules. They offer the highest conversion efficiency obtained with thin films, excellent stability, radiation hardness, and competitive projected production costs. CuInS2 has a number of additional potential advantages in comparison to the more widely researched selenium-containing chalcopyrites. These include significantly reduced toxicity and the fact that it can be prepared by simple, extremely fast sequential processes. Our research interest focuses on chalcopyrite-based solar cells　with higher band gaps for better adjustment to the solar spectrum; for higher open circuit voltages, reducing the resistive losses; and for top cells in tandem devices. Polycrystalline and epitaxial films of Cu(In,Ga)S2 are prepared by a number of established and novel methods. We also investigate defect structures, electrical transport, recombination paths, device performance, and new fabrication processes for chalcopyrite absorbers.