In the present contribution we report on recent work covering Zn(S,O) buffer as heterojunction partner layer applied to pilot line low-gap Cu(In,Ga)(SSe)(2) (CIGSSe, E-g = 1.03 eV) and production scale wide-gap CuInS2 (CIS, E-g = 1.54 eV). We highlight the crucial role that the processing control of the Zn(S,O) plays for the fabrication of Cu-chalcopyrite solar cells and modules. The analytical information obtained by the correlation with state-of-the art high resolution Transmission electron microscopy, X-ray photoemission and Auger spectroscopy (XPS and XAES) as well as L-edge XAS are discussed. A large number of efficient laboratory-scale solar cells and monolithically interconnected prototype CIGSSe and CIS modules are produced. The efficiencies are comparable to the CdS base line references or even higher. The electrical, electronic properties and the emerging phenomena in Cd-free devices such as light soaking are discussed. (C) 2012 Published by Elsevier Ltd.