The ability to grow efficient CdTe/CdS solar cells in substrate configuration would not only allow for the use of non-transparent and flexible substrates but also enable a better control of junction formation. Yet, the problems of barrier formation at the back contact as well as the formation of a p–n junction with reduced recombination losses have to be solved. In this work, CdTe/CdS solar cells in substrate configuration were developed, and the results on different combinations of back contact materials are presented. The Cu content in the electrical back contact was found to be a crucial parameter for the optimal CdCl2-treatment procedure. For Cu-free cells, two activation treatments were applied, whereas Cu-containing cells were only treated once after the CdTe deposition. A recrystallization behavior of the CdTe layer upon its activation similar to superstrate configuration was found; however, no CdTe–CdS intermixing could be observed when the layers were treated consecutively. Remarkably high VOC and fill factor of 768 mV and 68.6%, respectively, were achieved using a combination of MoO3, Te, and Cu as back contact buffer layer resulting in 11.3% conversion efficiency. With a Cu-free MoO3/Te buffer material, a VOC of 733 mV, a fill factor of 62.3%, and an efficiency of 10.0% were obtained. Copyright © 2012 John Wiley & Sons, Ltd.