Application of ZnO1−xSx as window layer in cadmium telluride solar cells

CdTe solar cells with ZnS window layer deposited by ultrasonic spray pyrolysis (USP) were grown. The current density of such solar cells reached 24.7 mA/cm² without anti reflection coating (Voc 594 mV, FF 64.2%, η 9.4%). For CdTe solar cells with CdS and ZnS window layers we quantified the Jsc loss mechanisms in detail. In order to tune the conduction band alignment, and increase the Voc, ZnO_1-xS_x grown by atomic layer deposition (ALD) was used. The band alignment in these devices was estimated and interpreted with the help of 1D simulation. According to the estimated band alignment the optimum sulfur content of the ZnO_1-xS_x compound seems to be around x=0.6 which was confirmed experimentally. The minimum absorber bandgap in final devices was found to be reduced in ZnO_1-xS_x/CdTe devices, which indicates interdiffusion of elements from the window layer into the absorber. To reduce the impact of thereby possibly formed pinholes a high resistive transparent layer was introduced between TCO and window layer. This layer could improve the performance of the device with the thinnest ZnO_1-xS_xbut decreased the performance of the others. In combination with ALD grown ZnO_1-xS_x the TCO material Fluorine doped tin oxide lead to higher efficiency than aluminum doped zinc oxide.