Characterization of electronic structure of Cu2ZnSn(SxSe1-x)4 absorber layer and CdS/Cu2ZnSn(SxSe1-x)4 interfaces by in-situ photoemission and inverse photoemission spectroscopies

The dependences of electronic structure of CZTS(x)Se(1-x) (CZTSSe) layers synthesized by sulfurization ancllor 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 US buffer by the sequential evaporation and the CZTSSe (x=028 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=028) 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, (C) 2014 Elsevier B.V. All rights reserved