Many-Body Effects on the Electronic and Optical Properties of Lead-Free KNbO3–xQx (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 KNbO3–xQx (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 evGW0 + BSE calculations, we successfully reproduced the experimental excitonic absorption spectrum of KNbO3. In KNbO3–xQx (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 (EB) 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 EB is examined using the effective mass approximation, revealing that electron redistribution predominantly affects Coulomb attraction, while ionic screening has a minimal contribution. Furthermore, continuum Fröhlich 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(S2) compounds, respectively. This comprehensive study sheds light on the many-body effects in oxychalcogenide perovskites and provides valuable insights into the potential applications of KNbO3–xQx compounds as photoactive materials.