Electronic structures of Cu₂ZnSnSe₄surface and CdS/Cu₂ZnSnSe₄heterointerface

Abstract

We studied the electronic structures of the Cu2ZnSnSe4 (CZTSe) surface and CdS/CZTSe heterointerface using X-ray photoemission spectroscopy (XPS), ultraviolet photoemission spectroscopy (UPS), and inversed photoemission spectroscopy (IPES) systems. These measurement systems are connected to the CdS deposition chamber via a transport chamber under ultrahigh vacuum. We revealed that the conduction band offset (CBO) and valence band offset (VBO) are +0.56 and +0.89 eV, respectively, at the CdS/CZTSe heterointerface. A positive CBO value, referred to as a "spike" structure, indicates that the position of the conduction band of CdS becomes higher than that of the absorber layer. Despite such a large spike structure in the conduction band at the interface, a conversion efficiency of 8.7% was obtained for our CdS/CZTSe heterojunction solar cells. Moreover, we found that the Fermi level at the CZTSe surface is located near the center of the bandgap and that the hole deficiency near the CZTSe surface is stronger than that inside the bulk CZTSe. We also found that Fermi level pinning did not occur at the CZTSe surface or CdS/CZTSe heterointerface by XPS.