Many-Body Effects on the Electronic and Optical Properties of Lead-Free KNbO3-x Q x (x=0, 1, 2; Q = S, Se) Oxychalcogenide Perovskites

Oxychalcogenide perovskites are a novel class of materials that exhibit a distinct electronic structure, combining covalent and ionic bonding with a prominent excitonic effect. We investigate the many-body effects on the quasiparticle band structures and optical absorption spectra of oxychalcogenide perovskite KNbO(3-x)Q(x) (x = 0, 1, 2; Q = S, Se) compounds using the GW approximation and the Bethe-Salpeter equation (BSE) within Green's function formulation of many-body perturbation theory. By performing the evGW(0) + BSE calculations, we successfully reproduced the experimental excitonic absorption spectrum of KNbO3. In KNbO(3-x)Q(x) (x = 1, 2) compounds, strong electron-electron interaction induces notable modifications in the band topology, particularly in the KNbO2S compound, transforming it from a direct to an indirect band gap semiconductor. The exciton binding energy (E-B) increases with a higher doping concentration, ranging from 0.21 to 0.32 eV. The bound exciton is thermodynamically stable, and at a given concentration of Q, the excitonic lifetime increases from S to Se due to the reinforcement of covalent bonding. The influence of the Franck-Condon shift on E-B is examined using the effective mass approximation, revealing that electron redistribution predominantly affects Coulomb attraction, while ionic screening has a minimal contribution. Furthermore, continuum Fr & ouml;hlich polaron is studied using the Landau-Pekar model combined with Feynman's path integral approach and found that the bound exciton state is more stable than the charge-separated polaronic state in these compounds. The maximum photoconversion efficiency is calculated using the spectroscopic limited maximum efficiency (SLME) technique, exhibiting the efficiency of 32(18) and 12(8)% in KNbO2Se(S) and KNbOSe2(S-2) compounds, respectively. This comprehensive study sheds light on the many-body effects in oxychalcogenide perovskites and provides valuable insights into the potential applications of KNbO(3-x)Q(x) compounds as photoactive materials.

Automated summary

In this study, the many-body effects on the exciton binding energy, electronic structure, and optical absorption spectra of oxychalcogenide perovskite materials were investigated using the many-body perturbation theory. The results showed that increasing doping concentration can enhance the exciton binding energy and photoconversion efficiency, while electron-electron interaction can significantly modify the band topology. This study provides valuable insights into the many-body effects in oxychalcogenide perovskite materials and their potential applications as photoactive materials.