CdTe contacts for CdTe/CdS solar cells: effect of Cu thickness, surface preparation and recontacting on device performance and stability

Device performance of thin film CdTe/CdS solar cells having different methods for fabricating the primary back contact are presented. Wet and dry methods for forming the primary contact (Cu2Te) were evaluated with Cu layers from 0 to 15 nm. Extensive analysis of J-V curves is presented, including effects of temperature, intensity and accelerated stress. A procedure for recontacting stress-degraded cells allowed separation of contact and junction degradation modes. The junction recombination is shown to be a Shockley-Read-Hall mechanism. Stress increases the recombination current density J0 by 2-3 orders of magnitude, resulting in a loss in Voc of 100-200 mV which is not restored with recontacting. Rollover is eliminated by recontacting the device while fill factor is partially restored with recontacting. For devices with a Cu layer, no significant differences in illuminated solar cell performance between the wet and dry process were observed before or after stress, but there were large differences in the dark J-V related to a blocking contact. To first order, unstressed devices without Cu contact layers behave similar to stressed devices with Cu; lower Voc, higher resistance, and appearance of a blocking contact.