Phase control is critical for achieving high-performance CdTe cells when Cu,Te is used as a back-contact for CdTe cells. CqTe phases are mainly controlled by the Cu/Te ratio, and they can also be affected by post-heat-trcatment temperature. Although Cu2Te has the highest conductivity, it is unstable and provides more Cu diffusion into the CdS and CdTe films. Cu diffusion into the Us causes '' cross-over '', and Cu diffusion into the CdTe film creates Cu-related defects that lower photogenerated carrier lifetime and result in voltage-dependent collection. A '' recontact '' experiment clearly indicated that the mechanism giving rise to '' roll-over '' is the formation of Cu-related oxides, rather than the loss of Cu on the back-contact. (C) 2007 Elsevier B.V. All rights reserved.