Recent progress in the development of high-efficiency solar cells based on Cu(In,Ga)Se-2 at our institute is reviewed. The post-deposition treatment with alkali elements (PDT) has been found to improve device quality, mostly through reduced recombination, with the effect of increasing the open-circuit voltage. At the same time, PDT is shown to improve initial growth properties of the chemical-bath-deposited buffer layer, so that this layer may be made thinner and the corresponding losses are reduced. Further optimization by replacing the non-doped ZnO resistive layer by (Zn,Mg)O enables gains in photocurrent for the ultraviolet region. A certified efficiency of 22.0% with a CdS/(Zn,Mg)O buffer/resistive layer combination is presented here. Furthermore, the effect of band gap grading with gallium in the absorber layer is explored with respect to the buffer layer properties by means of device simulation. (C) 2016 Elsevier B.V. All rights reserved.