Copper performs an important role in obtaining high-performance thin-film CdTe solar cell devices. Both initial performance and performance after stress depends strongly on the total copper content at the back-contact, the Cd to Te ratio on the backside, the etching process, and the way the copper is activated. With regard to getting high open circuit voltage a small amount of Cu seems sufficient upon the right anneal treatment. However, regarding open circuit voltage degradation for stressed devices there seems to be an optimum amount of Cu. Te-enrichment does not seem to have a big impact on device stability. (C) 2011 Elsevier B.V. All rights reserved.