Application of Sputtered ZnO1-xSx Buffer Layers for Cu(In,Ga)Se2 Solar Cells

Abstract

A CdS buffer layer is commonly used in high efficiency Cu(In,Ga)Se2 (CIGS) solar cells. The CdS layer is typically prepared by chemical bath deposition. From the viewpoint of the environmental impact of Cd and the need for in-line processing for mass production, there is a need to find an alternative to wet-chemically deposited CdS. In this study, ZnO1-xSx thin films were prepared by radio frequency co-sputtering using ZnO and ZnS targets, which should allow for high controllability of the compositional ratios of O and S. The ZnO1-xSx thin film with an S content x of 0.18 showed a band gap energy (Eg) of 2.9 eV. The performance of a CIGS solar cell with a ZnO0.82S0.18 buffer layer was compared with that with a CdS buffer layer which had an Eg of 2.5 eV. The CIGS solar cell with the ZnO0.82S0.18 buffer layer yielded an efficiency approaching that of the cell with the CdS buffer layer. This is due to a higher short-circuit current density due to both a suitable conduction band offset of the ZnO1-xSx/CIGS interface and a decrease in absorption loss compared to the CIGS solar cell with the CdS buffer layer. These results indicate that, similar to CdS, ZnO1-xSx can be very useful as a buffer layer in CIGS solar cells.