Characterization of electronic structure of Cu2ZnSn(S Se1−)4 absorber layer and CdS/Cu2ZnSn(S Se1−)4 interfaces by in-situ photoemission and inverse photoemission spectroscopies

Abstract The dependences of electronic structure of CZTS x Se 1 − x (CZTSSe) layers synthesized by sulfurization and/or selenization of the vacuum-deposited metal precursors on the anion mixing ratio x = S/(S + Se) have been studied by in-situ ultraviolet and X-ray photoemission spectroscopies (UPS, XPS) and inverse photoemission spectroscopy (IPES). The band alignment at interfaces between the CdS buffer by the sequential evaporation and the CZTSSe ( x = 0.28 and 1.0) has also been investigated by the in-situ measurements of these spectroscopies. The UPS/IPES results of the CZTSSe surfaces have revealed linear expansion of band gap energy E g with an increase of x : E g (CZTSe; x = 0) = 0.9–1.0 eV and E g (CZTS; x = 1) = 1.5–1.6 eV. This expansion mainly originates in the rise of conduction band minimum CBM: CBM (CZTSe; x = 0) = 0.45–0.50 eV and CBM (CZTS; x = 1) = 0.95–1.05 eV. The in-situ measurements of the interface electronic structure have revealed that the CdS/CZTSSe ( x = 0.28) interface has a so-called “type I” band alignment with a conduction band offset CBO about + 0.2 eV which is favorable to high cell performance. A negative CBO was distinguished for the CdS/CZTS ( x = 1.0) interface, and the observed change in the band alignment with the anion mixing ratio was consistent with that of the variation in cell-performances.