Photoelectron Spectroscopy, and Photovoltaic Device Study of Cu2ZnSnSe4 and ZnOxS1-x Buffer Layer Interface

Recent research has enabled Cu₂ZnSnSe₄ (CZTSe) to reach efficiencies close to 10% in photovoltaic devices with CdS as the junction partner and over 12% when the CZTSe is alloyed with sulfur. Little work, however, has been reported on the potential for wide band gap, Cd-free buffer layers in these devices. Reported here are photoelectron spectroscopy measurements (XPS/UPS) of the band energy positions between CZTSe and zinc oxysulfide (ZnOS) with sputter depth profiling. Measurements indicate the formation of a large conduction band offset (CBO) of 1.2 eV with chemical-bath deposition (CBD) of ZnOS on CZTSe (E_g = 0.96 eV). However, Ar ion sputter depth profiling is shown to produce compositional changes of the ZnOS thin film resulting in an apparent increase of the valence band maximum (VBM) for the buffer layer. With this in mind, the valence band edge energy offsets (VBO) are calculated and used to study solar cells made with the configuration glass/Mo/CZTSe/ZnOS/i-ZnO/Al:ZnO/Ni/Al. Variation of the deposition time of the ZnOS buffer layer during the CBD process has led to device efficiencies above 5%. For the thinnest ZnOS buffer layers, the short-circuit current matches that of devices with CdS buffer layers, but suffers from loss of open-circuit voltage. Interpretation of the solar cell measurements are aided by SCAPS thin-film device modeling.