EFFECTS OF Cu DIFFUSION FROM ZnTe:Cu/Ti CONTACTS ON CARRIER LIFETIME OF CdS/CdTe THIN FILM SOLAR CELLS

We study the performance of CdS/CdTe thin film PV devices processed with a ZnTe:Cu/Ti contact to investigate how carrier lifetime (tau) in the CdTe layer is affected by Cu diffusion from the contact. Time-resolved photoluminescence (TRPL) measurements show that T decreases slightly as the contacting temperature increases in the temperature regime that produces "insufficient" Cu concentration in CdTe (similar to room temperature to similar to 250 degrees C). However, T increases significantly once the contact temperature is in the range that yields "optimum" Cu concentration and high-performance devices (similar to 280 to similar to 320 degrees C). At higher substrate temperatures (>similar to 300 degrees C), T drops precipitously, consistent with a region previously identified as producing "excessive" Cu concentration - and poor device performance. The observed T increase within the "optimum temperature" range not only suggests why high-performance devices may form at these contact temperatures using many different contact processes (including paste contacts), but may provide a significant clue as to why Cu-contact formation processes impart a broad latitude in other process parameters.