Optical characterization of bandedge electronic structure and defect states in Cu₂ZnSnS₄

The bandedge electronic structure including the optical bandgap,band-tail states,and deep/shallow donor and acceptor levels in Cu2ZnSnS4 semiconductor was analyzed by absorption,photocurrent and photoluminescence spectroscopy,and the theoretical reports. It is revealed that the Sn-Zn-related defect in Cu2ZnSnS4 with abundant defect states is one of the key factors affecting the band-edge electronic structure. High concentration of the neutral defect cluster [2Cu(Zn) + Sn-Zn] can narrow the band gap substantially,while the partially-passivated(ionic)defect cluster[Cu-Zn + Sn-Zn]is the main deep donor defect. A large number of band-tail states are responsible for the obvious red-shift of the bandedge-related photoluminescence transition energy. These detrimental defects related to Sn Zn can be effectively suppressed by properly reducing the Sn content in the copper-poor and zinc-rich growth condition,which also avoids the narrowing of the optical bandgap of the Cu2ZnSnS4 absorption layer.