Photovoltaic CuInS2 (CIS) thin films were prepared from thermally evaporated Cu/In precursors, having various Cu/In ratios, under different sulfurization conditions. The sulfurization was carried out in a quartz tube furnace under a steady or flowing H2S/Ar gas mixture at various temperatures ranging from 450 to 550 °C. The effects of the Cu/In ratio and sulfurization condition on the grain size, surface morphology, surface roughness, microstructure and defect formation of the resulting CIS films were examined. It was found that the effects depend on the processing conditions. Particularly, we show that sub-bandgap generated defects can be inferred by photoluminescence data. Selected CIS thin films were used as absorber layers for the fabrication of thin film solar cells. The best solar cell efficiency of 6.29% was obtained from the 500 °C-sulfurized absorber layer having a Cu/In ratio of 1.8. Effect of absorber layer characteristics on the cell efficiency was also discussed. Copyright © 2013 John Wiley & Sons, Ltd.