Pathways toward higher performance CdS/CdTe devices: Te exposure of CdTe surface before ZnTe:Cu/Ti contacting

Many studies of thin-film CdS/CdTe photovoltaic devices have suggested that performance may be improved by reducing recombination due to Te-vacancy (V _Te), Te antisite (Te_Cd), or Te-interstitial (Te _i) defects. Although formation of these intrinsic defects is likely influenced by CdTe deposition parameters, it may be also coupled to the formation of beneficial cadmium vacancy (V_Cd) defects. In this study, we expose the CdTe surface to Te vapor prior to ZnTe:Cu/Ti contact-interface formation with the goal of reducing V_Te without significantly reducing V_Cd. Initial results show that when this modified contact is used on a CdCl₂-treated CdS/CdTe device, poorer device performance results. This suggests two things: First, the amount of free-Te available during contact formation (either from chemical etching or Cu_xTe 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), adding even a small amount of Te may produce detrimental defects.