Photoemission studies of the ZnO/CdS interface

The highest conversion efficiencies of solar cells based on Cu(In,Ga)Se2 have been achieved using multilayer CdS/ZnO front contacts. The formation of the heterojunction interface between polycrystalline ZnO and CdS has been studied with x-ray and ultraviolet photoemission spectroscopy. The valence band offset between ZnO and CdS has been determined to be 1.2 eV. No chemical reactions at the interface between ZnO and CdS have been detected up to 200 °C. In order to obtain a standard reference for the band discontinuities the valence band offsets of ZnO and CdS relative to Ge have been measured.