Surface treatment of the window layer in CdS/CdTe solar cells

Abstract

CdS films are used as window layers in CdS/CdTe solar cells, and an annealing process is required to promote grain growth and recrystallization in them. However, when annealing is performed in an air atmosphere, oxides such as CdO and CdSO4 form on parts of the layer's surface, which reduces the shunt resistance and the fill factor and results in poor device performance. In this study, we annealed the CdS film in a CdCl2 atmosphere instead of air. Then, CdCl2 annealing and air annealing were compared by investigating the structural and the electrical properties of the CdS window layers. Our results reveal that the CdCl2 annealing protects the CdS film from oxidation and provides good recrystallization conditions. We also found that this CdCl2 annealing enhance the device's performance. In particular, considerably higher shunt resistance and conversion efficiency were obtained for the CdCl2-annealed device compared with the air-annealed device.