Measurements of Energy Band Positions Using Hard X-ray Photoelectron Spectroscopy Aimed at Achieving High Conversion Efficiency Cu₂ZnSnS₄ Photovoltaic Cells

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 relatively 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.