Effect of CuI film thickness deposited via PVD on the performance of CdS/CdTe solar cells

Copper iodide (CuI) is a p-type semiconductor with promising applications in optoelectronic devices, particularly as a buffer layer in CdS/CdTe heterojunction solar cells. Its implementation can effectively reduce charge transport losses from the CdTe absorber layer to the back electrode, typically composed of Cu/Au. In this study, CuI thin films with thicknesses ranging from 2 to 100 nm were deposited via physical vapor deposition (PVD). The optoelectronic, morphological, and structural properties of these films were systematically analyzed both with and without post-deposition thermal treatment. Based on these analyses, each CuI film, 6, 20, 40, 60, and 100 nm thick (as grown), was integrated as a buffer layer in four CdS/CdTe solar cells. The obtained data had a low standard deviation, showing low dispersion and good reproducibility. Among these, the 20 nm CuI buffer layer exhibited the best performance, surpassing the reference solar cell without a buffer layer. The optimized devices exhibited a 44% increase in short-circuit current density and a 29% improvement in power conversion efficiency on average, despite a 3% decrease in open-circuit voltage compared to the control cell. Since copper diffusion is the primary issue that reduces the lifetime of CdS/CdTe solar cells, it is expected that the lifetime will increase as the metallic copper back contact is replaced with CuI. These findings highlight the potential of CuI as an effective buffer layer to improve the performance of CdS/CdTe solar cells.