For the Cu2ZnSnS4 (CZTS) photovoltaic (PV) cells with a common structure of the window-layer (WL) buffer-layer (BL) absorbing-layer (AL), the band offset and band slope were measured by hard x-ray photoelectron spectroscopy. In view of the efficient transport of photo-excited free-electrons from CZTS-ALs to WLs via CdS-BLs, the conversion efficiency of the PV cells would be significantly affected by the band offset at BLs ALs interface and the band slope in BLs. For the CZTS PV cells with a

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... tively high conversion efficiency of 9.4 , the conduction band offset at CdS-BLs CZTS-ALs interface was estimated to be 0.1±0.1 eV with a cliff-type, and the band slope in CdS-BLs was estimated to be 0.5±0.1 eV (downward slope to WLs) in the case of ZnO : Ga-WL. Absence of CB barrier at BLs ALs interface and larger downward band slope in BLs to WLs are essential for higher conversion efficiency of the CZTS PV cells.