Quasiparticle Band Structure and Phonon-Induced Band Gap Renormalization of the Lead-Free Halide Double Perovskite Cs₂InAgCl₆

The lead-free halide double perovskite Cs 2 InAgCl 6 was recently designed in silico and subsequently synthesized in the lab. This perovskite is a wide-gap semiconductor with a direct band gap and exhibits extraordinary photoluminescence in the visible range upon Na doping. The light emission properties of Cs 2 InAgCl 6 have successfully been exploited to fabricate stable single-emitter-based white-light LEDs with near unity quantum efficiency. An intriguing puzzle in the photophysics of this compound is that the onset of optical absorption is around 3 eV, but the luminescence peak is found around 2 eV. As a first step toward elucidating this mismatch and clarifying the atomic-scale mechanisms underpinning the observed luminescence, here, we report a detailed investigation of the quasiparticle band structure of Cs 2 InAgCl 6 as well as the phonon-induced renormalization of the band structure. We perform calculations of bang gaps and effective masses using the GW method, and we calculate the phonon-induced band structure renormalization using the special displacement method. We find that GW calculations are rather sensitive to the functional used in the density functional theory calculations and that self-consistency on the eigenvalues is necessary to achieve quantitative agreement with experiments. Our most accurate band gap at room temperature is in the range of 3.1–3.2 eV and includes a phonon-induced gap renormalization of 0.2 eV. By computing the phonon-induced mass enhancement, we find that the electron carriers are in the weak polaronic coupling regime, while hole carriers are in the intermediate coupling regime as a result of the localized and directional nature of the Ag e g 4d states at the valence band top.