Electronic structure, excitonic optical response, and defect stability of RbTlCl3 from first-principles

Abstract

We present a many-body ab initio study of the electronic structure, optical properties, excitonic effects, and defect stability of RbTlCl3. Density functional theory and many-body methods are used to investigate the properties with and without spin-orbit coupling (SOC). The indirect band gaps obtained using PBE-GGA are 0.94 and 0.95 eV with and without SOC, respectively. Optical spectra are calculated using the independent-particle approximation (IPA), IPA with scissor correction (IQPA), random-phase approximation (RPA) with local-field effects (LFEs), and the Bethe-Salpeter equation (BSE). The maximum imaginary dielectric function is approximately 7 at 2 eV within IQPA and decreases to 5.7 when LFEs are included. BSE calculations yield a peak value of approximately 10.8 at 1.6 eV, revealing strong excitonic effects. Four prominent bright bound excitons exhibit binding energies of 315-365 meV. The main bright exciton is localized in reciprocal space and delocalized in real space. The spectroscopic limited maximum efficiency is estimated to be 15.4% for a 0.5-micrometer-thick film. The absence of imaginary phonon frequencies and the existence of a thermodynamically stable region indicate dynamical and thermodynamic stability. Chemical-potential analysis identifies the chlorine vacancy as the most stable neutral defect under chlorine-poor conditions and the thallium vacancy under chlorine-rich conditions. These findings establish RbTlCl3 as a promising theoretical candidate for photovoltaic applications and provide insights into excitonic effects in inorganic perovskites.

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