The chalcopyrite semiconductors based on CuInSe2 are key components in the absorber layer of solar cells which are amongst leading thin-film PV technologies in terms of efficiency and stability. These solar cells have been developed mostly using empirical approaches rather than scientific knowledge-based design: it worked first and was explained later. As a result the progress of the chalcopyrite-based solar cells shows a clear saturation towards not 30%, a theoretical limit for single-junction solar cell, but 21%. The problem seems to be that the materials are too complicated for an empirical design. A solution could be in studying first high structural quality model materials and then using the obtained knowledge for technology grade materials. That is how all other successful semiconductor technologies have been developed. A significant improvement in the quality of the chalcopyrites produced in Strathclyde facilitated the use of fine methods of optical and magneto-optical spectroscopy to study excitonic states and helped to determine a number of fundamental electronic properties. The next step in the development of the chalcopyrites is the substitution of rare and expensive In and Ga with Zn and Sn alternating on the cation sublattice. Transforming chalcopyrite CuInGaSe2 in Cu2ZnSnSe4 with kesterite structure. An encouraging efficiency in excess of 11% has already been reported for solar cells based on this compound. However the material might be far too complicated whereas some other compounds like Cu3BiS3 can probably offer a simpler solution.