Indium sulphide layers were deposited by thermal evaporation from compound In2S3 powder provided by different suppliers. The source material quality was systematically analysed in terms of contamination, stoichiometry, grain size, evaporation behaviour and crystallinity. It was found to vary significantly for different suppliers. The influence of source material quality and stability on In2S3 layer growth and properties were investigated by various optical, crystallographic and surface analysis methods. The findings were correlated with the performance of solar cells prepared with an In2S3 buffer layer. This includes the analysis of jV-curves and quantum efficiencies of solar cells prepared with different Cu(In,Ga)Se-2 absorbers. After a post annealing step in air, best cells reached a certified efficiency of 15.2% with remarkably high fill factor (75.6%) and open circuit voltage (677 mV). (c) 2008 Elsevier B.V. All rights reserved.