Electronic structure and defect states in bismuth and antimony sulphides identified by energy-resolved electrochemical impedance spectroscopy

One of the reasons chalcogenide-based photovoltaic solar cells (SC) do not yet meet the expected high-power conversion efficiencies is a lack of understanding of their electronic structure, and particularly the nature of the point defects in the absorber materials. We show that the density of states of the characteristic features of the electronic structure, such as band edges and energy distribution of defects, can be obtained experimentally by energy-resolved electrochemical impedance spectroscopy (ER-EIS) in a technically simple and quick way. The ER-EIS data correlate well with theoretical density functional theory calculations. The ER-EIS reveals that Bi₂S_3, has only shallow defects near the conduction band minimum (CBM). In Sb₂S₃, ER-EIS also shows deep defect states, which can be the cause of the low electrical conductivity of Sb₂S₃ and lower than theoretically possible power conversion efficiency of Sb₂S₃-based SC. A dominant sulphur vacancy defect was identified in Bi- and Sb-chalcogenides. In the (Sb_xBi_(1−x))₂S₃ ternary alloy series, a gradual transformation of CBM and defect states in the band gap was observed. Notably, a 1:9 ratio of Bi:Sb cations already transforms the deep sulphur defects into shallow ones while keeping the band edges similar to those of the pristine Sb₂S₃. It can provide a novel strategy for healing the deep defect states in Sb₂S₃, a crucial step for boosting solar cell performance.