Effects of Cu and CdCl2 treatment on the stability of sputtered CdS/CdTe solar cells

We have studied the effects on cell stability of the amount of copper in the back contact and the dependence on CdCl₂ activation treatment for sputtered CdS/CdTe cells. Unencapsulated cells were tested in the dark and at one sun at 85°C and higher. We find that cells with > 3 nm of evaporated Cu (covered with 20 nm Au) had initially higher short-circuit current but were less stable than cells with limited Cu (1-3 nm) and that cells without intentional Cu in the back contact had neither better stability nor better initial performance. For cells with Cu in the back contact, a window of CdCl₂ treatment time was found for optimal stability. The cells without back-contact Cu were not very sensitive to the treatment time. With optimized CdCl₂ treatment and Cu thickness, cells have shown very good stability. For cells with unoptimized Cu thickness or activation treatment, the decreases in fill factor (FF) and open-circuit voltage (Voc) were significant. We believe these changes may arise from possible formation of an oxide or telluride layer involving copper at the back contact and the migration of Cu from the back contact.