Evolution of CdS/CdTe device performance during Cu diffusion

Cu diffusion from a ZnTe:Cu/Ti back contact onto CdS/CdTe thin-film solar cells is studied. We find if Cu diffusion is insufficient, the entire CdTe layer is depleted. However, if Cu diffusion is excessive, the depletion width can become too narrow to provide optimum current collection. This analysis suggests that most contact processes used for CdS/CdTe devices are optimized (often unknowingly) to result in a depletion width that extends just far enough into the CdTe to yield the highest possible field in the region where light absorption occurs. Analysis of the samples with very high Cu concentration also suggests that Cu doping of CdS may affect carrier collection from the CdS.