Application of Sputtered ZnO1-xSx Buffer Layers for Cu(In,Ga)Se2 Solar 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, 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 (E-g) 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 E-g 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. (C) 2011 The Japan Society of Applied Physics