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.

Changes of the electronic structure of the Cu2ZnSn(SxSe1−x)4 [CZTSSe] films and the band alignment at the interfaces between CdS buffer and the CZTSSe in conjunction with the anion‐mixing ratio x = 0–1 have been investigated using in situ X‐ray, ultraviolet photoemission spectroscopy (XPS, UPS), and inverse photoemission spectroscopy (IPES). Changes of the UPS and IPES spectra in conjunction with x have revealed that the electronic structure of the CZTSSe surface is characterized with the preferential rise of conduction band minimum (CBM) in conjunction with the increase of x. As x increases, interface induced band bending decreases from 0.5 to 0.6 at the CdS/CZTSe (x = 0) interface to 0.1–0.2 at the CdS/CZTS (x = 1) one. And the downward shift of CBM due to the deposition of the CdS layer is enhanced as x increases. These changes result in the monotonous decrease of conduction band offset (CBO) in conjunction with the increase of x: CBO at the x = 0 and 1 interfaces are +0.5 and −0.14 to −0.15 eV, respectively. The values of CBO are consistent with the device properties; occasional emergence of double junction like current–voltage characteristics in the CdS/CZTSe‐based cells, serious voltage‐loss in the CdS/CZTS ones, and the highest performance achieved in the CdS/CZTSSe ones.

We determined that the conduction band offset (CBO) and the valence band offset (VBO) at the CdS/ Cu2ZnSnSe4 (CZTSe) heterointerface are +0.56 and +0.89eV, respectively, by using X-ray photoemission spectroscopy (XPS), ultraviolet photoemission spectroscopy (UPS) and inversed photoemission spectroscopy (IPES). A positive CBO value, so-called “spike” structure, means that the position of conduction band becomes higher than that of absorber layer. The evaluated CBO of +0.56 eV suggests that the conduction band alignment at CdS/CZTSe interface is enough to become an electron barrier. Despite such a large spike structure in the conduction band at the interface, a conversion efficiency of 8.7 % could be obtained for the CdS/CZTSe heterojunction solar cells.