Temperature dependence of exciton binding energies in halide perovskites: theory and experiment

Electron-phonon interactions in halide perovskites lead to a temperature-dependent increase of the effective mass. Previous theories of finite-temperature excitons based on the solution of the Bethe-Salpeter equation (BSE) for electrons and holes renormalized via the Fan-Migdal self-energy suggest that this increase of the effective mass with temperature causes an increase in the exciton binding energy. However, this is at odds with experiment, which shows a reduction of the exciton binding energy with increasing temperature. Here we show that the temperature dependence of the exciton binding energy is dominated by the renormalization of the electron-hole interaction, as described by a phonon correction to the BSE kernel [1], which is equivalent to a phonon exchange exciton-phonon self-energy. We demonstrate that inclusion of this term in the BSE is necessary to predict the temperature dependence of CsPbCl3 and CsPbBr3 exciton binding energies, and quantitative agreement with experiment is achieved through accounting for the interplay of the phonon exchange and Fan-Migdal self-energies with thermal expansion.



[1] Alvertis, Haber, Li, Coveney, Louie, Filip, Neaton PNAS 121, e2403434121 (2024)



This work is supported by the Theory of Materials Program and Center for Computational Study of Excited-State Phenomena in Energy Materials (C2SEPEM) at Berkeley Lab, supported by Basic Energy Sciences within the Office of Science in the US Department of Energy. Computational resources provided by NERSC.