Cu(In,Ga)Se-2 absorbers with several different ratios of Ga/(Ga + In) were deposited by varying the Ga content using co-evaporation. The effects of Ga concentration at the surface of the CIGS on the Zn(O,S)/CIGS photovoltaic performance were investigated. Considering application as the bottom cell in a monolithic tandem structure, the optimum Ga/(Ga + In) ratio of approximately 0.26 led to the best efficiency of 14.56%, and to complete carrier collection at long wavelengths. In addition, the degradation rate in efficiency was lowest at an annealing temperature of 400 degrees C. The reason for losses of other Ga/(Ga + In) ratios (0.33 and 0.36) at longer wavelengths might be due to interface recombination, possibly aided by conduction band offset with the Ga grading shape by varying the Ga concentration. The results suggested that the decrease in efficiency might be due to recombination at the interface between the Zn(O,S) and CIGS layers before/after annealing. The reverse bias effect during quantum efficiency measurements at long wavelengths was used to determine separate optical and electrical losses. (C) 2016 Elsevier Ltd. All rights reserved.