Application of Sputtered ZnO1-xSxBuffer Layers for Cu(In,Ga)Se2Solar Cells

A CdS buffer layer is commonly used in high efficiency Cu(In,Ga)Se 2 (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, ZnO 1- x S x 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 ZnO 1- x S x thin film with an S content x of 0.18 showed a band gap energy ( E g ) of 2.9 eV. The performance of a CIGS solar cell with a ZnO 0.82 S 0.18 buffer layer was compared with that with a CdS buffer layer which had an E g of 2.5 eV. The CIGS solar cell with the ZnO 0.82 S 0.18 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 ZnO 1- x S x /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, ZnO 1- x S x can be very useful as a buffer layer in CIGS solar cells.