Post-deposition processing options for high-efficiency sputtered CdS/CdTe solar cells

Abstract

CdCl2 activation near 400 °C is known to be critically important for obtaining high efficiency CdS/CdTe solar cells. However, this treatment step behaves differently on high-temperature-grown CdTe than on lower-temperature-grown CdTe layers such as those grown by sputtering. On sputtered films, the post-deposition activation produces grain-boundary passivation, sulfur diffusion into CdTe, and substantial grain growth. Nevertheless, we find the CdCl2 process for sputtered films to be characterized by a single activation energy that we interpret as applying to S diffusion into CdTe. We find this activation energy to hold for CdCl2 treatments from 370 to 440 °C. The completed CdS/CdTe solar-cell structures showed somewhat poorer initial performance with activation above 420 °C, but, in this case, the cell efficiency increased after accelerated life testing at 85 °C, open-circuit biasing and one-sun illumination. With an optimized CdCl2 activation process, the use of oxygenated sputtered CdS, and low-iron soda-lime glass, cell efficiencies of 14.5% were achieved.