Thin film CdTe solar cells of the superstrate configuration have been fabricated in order to study the effect of Cu on device stability. The study focused on two distinct sets of solar cells: in one set of devices Cu was introduced during the formation of the back contact, by sputtering a small thickness of Cu onto the CdTe surface prior to the application of a graphite electrode; for the second set of devices Cu was introduced in CdS by briefly immersing the CdS films in a CuCl solution prior to the deposition of CdTe with the back contact electrode being sputtered Mo. The solar cells were light soaked under approximately AM 1.5 conditions for nearly 700 h during 4 h ON/4 h OFF cycles. Device degradation correlated well with the amount of Cu for the devices with Cu in the back contact. Cells with larger amounts of Cu exhibited larger degradation, suggesting that the amount of Cu utilized during the back contact formation must be minimized. On the other hand, a number of devices fabricated without any Cu in the back contact, but with Cu in the CdS, exhibited nearly no degradation during the light soaking process suggesting that in addition to the amount of Cu used for the fabrication of CdTe cells, the method of incorporating this element is also critical in achieving long term device stability. (C) 2007 Elsevier B.V. All rights reserved.