The paper deals with material requirements, necessary to increase the efficiency/cost ratio of thin film solar cells. These component-specific requirements are described using an example of CuInS2/CuI device components. Relation between the energy gap and the lattice constant of isovalent chalcopyrite materials is analyzed. Benefits and drawbacks of the p p-i-n structure for CIS solar cells are discussed. It is shown how Fermi level pinning in the band gap or a band offset with a spike may result in domination of tunneling-enhanced interface recombination mechanism in the bucking current. It is traced in detail how pinning of the Fermi level in CuInS2 may result in scattering values for the valence band offset measured by photoemission spectroscopy at the interface with CdS, the result being strongly dependent on the excitation energy and on the doping concentration. (C) 2003 Elsevier B.V. All rights reserved.