Electronic structure of Cu2ZnSn(SxSe1−x)4 surface and CdS/Cu2ZnSn(SxSe1−x)4 interface

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.