Band alignment studies of ZnO-ZnS-CIS interfaces

With respect to thin film solar cells based on CuInS2 and ZnO ZnS is a promising alternative to CdS as buffer layer material. A MOMBE process based on diethylzinc and water is investigated on CuInS2(112) thin films. A ZnS buffer layer grows epitaxially using the surplus of sulfur in the eventually present CuSx layer. A ZnO film grows on top of the.8 nm thick ZnS layer with its own lattice parameters in (0001) direction. Step-by-step analysis of this ZnO-ZnS-CIS interface yielded a band alignment favorable for photovoltaic applications. We present in-situ preparation and step-by-step investigation on ZnO-ZnS-CIS interfaces with varying CIS substrates. CuInS2(001) samples were prepared on GaAs(100) by gas source MBE (GSMBE) using TBDS as sulfur precursor. Sulfurization of CuIn films prepared by MBE and sputtering yielded polycrystalline CuInS2 samples of 100 nm up to 2.5 μm thickness. The samples were investigated by XPS, UPS and LEED. We discuss the interfaces with respect to the derived band alignments and their morphology. Furthermore we compare the results to those of analogous experiments on CuInSe2(112).