Alignment of energy levels at the ZnS/Cu(In,Ga)Se2 interface

Abstract

Further understanding of the electronic structure at the ZnS/Cu(In,Ga)Se₂ interface is necessary to enhance the electron injection across the interface in Cu(In,Ga)Se₂ solar cells. The valence band structure and shallow core levels were investigated by ultraviolet photoelectron spectroscopy depth profile analysis with He II line excitation. ZnS film was grown by a chemical bath deposition on a Cu(In,Ga)Se₂ absorber deposited by the co-evaporation of Cu, In, Ga, and Se elemental sources. The discontinuity of 2.0 eV in the valence band edge at the ZnS/Cu(In_0.7Ga_0.3)Se₂ interface was directly determined. This type of valence band offset yields a spike conduction band alignment of 0.25 eV. The positions of the VBM and the Zn 3d core-level emission of the buffer underwent the substantial shifts of 0.36 eV and 0.64 eV to a lower binding energy levels during the etching process. The shifts are ascribed to the contribution of the band bending in the ZnS buffer layer and its graded chemical composition. This study is the first to determine the small conduction band offset at the interface formed by the chemical bath deposited ZnS layer and the Cu(In_0.7Ga_0.3)Se₂ absorber. Our results also provide information toward the design optimization of the optoelectronic properties of the ZnS/Cu(In_0.7Ga_0.3)Se₂ interface. To enhance the electron injection from Cu(In_0.7Ga_0.3)Se₂ absorber to ZnS layer further lowering of the energy barrier is required. For this purpose, the bandgap of ZnS should be reduced by controlling the crystal structure and composition or its Fermi level should be upward shifted by appropriate doping.