Defect States in Bi- and Sb- Chalcogenides Revealed by Combination of Experimental Techniques and DFT Modelling

Understanding the nature of the defects, namely point defects, their formation mechanism and the contribution to the properties is essential for device performance improvement. Shallow-level defects with thermal energy of about k_BT from conduction band minimum (CBM) or valence band maximum (VBM) play the major role in the carrier concentrations and conductivity type determination of the material. In contrast, deep-level defect whose activation energy is much higher than k_BT from CBM or VBM is detrimental to photogenerated carrier lifetime and transport (carrier mobility and diffusion length). These defects result in the trap-assisted Shockley–Read–Hall recombination (dominant non-radiation recombination) in solar cells, which is the primary cause of open-circuit voltage loss (Lian et al., 2021).<br/>In the center of our interest were lone-pair ns² cation chalcogenides such as Bi₂S₃and Sb₂S₃, and their (Sb/Bi)₂S₃ alloys (with 0, 10, 33, 50, 67, 90, 100 at% Sb₂S₃ content). Here we present the chalcogenides’ density of states (DOS) obtained experimentally by energy-resolved electrochemical impedance spectroscopy (ER-EIS). The information from DOS includes (but is not limited to) VBM, CBM, and the energy distribution of defect states - all obtained in several minutes with a simple setup and application technique. The DOS showed good agreement with previously reported DFT calculations, as well as experimental characterization of the chalcogenides using UPS, UV-VIS, and Hall effect measurements. Namely (i) VBM position is at ~ 5 eV; (ii) optical bandgap is of 1.3 eV and 1.7 eV for Bi₂S₃ and Sb₂S₃, respectively; (iii) the major charge carrier density in Bi₂S₃ is of the order of 10¹⁸ cm^-1.<br/>Importantly, DOS from ER-EIS directly provides information on the energy distribution of the defect states in the Bi₂S₃,Sb₂S₃ and their alloys. In Bi₂S₃only shallow defects at CBM were detected. They might be responsible for the well-known n-type conductivity of Bi₂S₃(Glatz et al., 1963). In Sb₂S₃, besides shallow defects, ER-EIS also revealed midgap states. It corroborates the PL results, in which a significant decrease in the intensity of radiative recombination in Sb₂S₃ compared to Bi₂S₃, which happens due to deep defects, was detected.<br/>In the series of (Sb/Bi)₂S₃ alloys, a gradual transformation of the intraband gap states was observed, namely with increasing Sb:Bi ratio the pronounced DOS band of shallow defects at CBM disappeared, while a distinct peak of midgap states appeared. A comparison between the defect energy distribution and the microstructure was made by comparing the experimental and DFT DOS.<br/>In summary, by combining <b>ER-EIS, PL, and DFT calculations it was possible to reveal the energy distribution of the defect states and their nature in Bi₂S_3,Sb₂S₃</b> <b>and their alloys</b>, which is crucial for their further application in photovoltaics.