Physics Research Seminar: Dr. Daria Miliaieva from Tallinn University of Technology (TTU)

Welcome to join us for a research seminar by Dr. Daria Miliaieva from Tallinn University of Technology!

Dr. Daria Miliaieva is a material scientist who works on the border of chemistry and physics, theory and experiment. Currently she is interested in the experimental way of electronic structure disclosure, specifically the electronic density of states (DOS). During her Marie Skłodowska-Curie Postdoctoral Fellowship at Tallinn University of Technology (June 2023 – May 2025), she has been studying energy-resolved electrochemical impedance spectroscopy (ER-EIS).

Host: Ondrej Krejci/CEST

Title: Defect states in Bi- and Sb- Chalcogenides Revealed by Combination of Experimental Techniques and DFT Modelling

Abstract: 

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 energy of about k_BT from conduction band minimum (CBM) or valence band maximum (VBM) play a major role in the carrier concentrations and conductivity type determination of the material. In contrast, deep-level defects whose activation energy is much higher than k_BT from VBM/CBM are detrimental to photogenerated carrier lifetime and transport (carrier mobility and diffusion length). These defects result in 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).

In the centre of our interest were lone-pair ns² cation chalcogenides such as Bi₂S₃ and Sb₂S₃, and their (Sb_xBi_(1-x))₂S₃ alloys (x = 0, 0.1, 0.33, 0.50, 0.67, 0.90, 1 of Sb content). In the talk, I will present the chalcogenides’ density of states (DOS) obtained experimentally by energy-resolved electrochemical impedance spectroscopy (ER-EIS). Noteworthy, ER-EIS proved itself on perovskites (Brunova et al., 2021) and organic polymers (Nadazdy et al., 2014). 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. The DOS showed good agreement with previously reported DFT calculations, as well as experimental characterisation of the chalcogenides using UPS and UV-VIS spectroscopies. 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. 

Importantly, DOS from ER-EIS directly provides information on the energy distribution of the defect states in the Bi₂S_3, Sb₂S₃ and their alloys. In Bi₂S₃ only 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. In the series of (Sb_xBi_(1-x))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.By comparing the experimental and DFT DOS the dominant defect was identified.

In summary, I will show how by combining ER-EIS results and DFT calculations it is possible to 1) interpret the electronic structure of the materials, 2)reveal the nature of their defect states and 3) passivate the midgap defects which is crucial for photovoltaic applications of the materials and beyond.