Response of CdS/CdTe devices to Te exposure of back contact

Theoretical predictions of thin-film CdS/CdTe photovoltaic (PV) devices have suggested performance may be improved by reducing recombination due to Te-vacancy (V_Te) or Te-interstitial (Te_i) defects. Although formation of these intrinsic defects is likely influenced by CdTe deposition parameters, it also may be coupled to formation of beneficial cadmium vacancy (V_Cd) defects. If this is true, reducing potential effects of V_Te or Te_i may be difficult without also reducing the density of V_Cd. In contrast, post-deposition processes can sometimes afford a greater degree of defect control. Here we explore a post-deposition process that appears to influence the Te-related defects in polycrystalline CdTe. Specifically, we have exposed the CdTe surface to Te prior to ZnTe:Cu/Ti contact-interface formation with the goal of reducing V_Te but without significantly reducing V_Cd. Initial results show that when this modified contact is used on a CdCl₂-treated CdS/CdTe device, significantly poorer device performance results. This suggests two things: First, the amount of free-Te available during contact formation (either from chemical etching or CuTe or ZnTe deposition) may be a more important parameter to device performance than previously appreciated. Second, if processes have been used to reduce the effect of V_Te (e.g., oxygen and chlorine additions to the CdTe), adding even a small amount of Te may produce detrimental defects.