First-principles Study of Self-trapped Exciton Formation in Double Perovskites

Broadband white light emission in perovskites has attracted a lot of interest recently for its application in solid-state lighting. While the exact mechanism is still debated, the formation of self-trapped excitons (STE) in these materials may explain the observed experimental broadband emission, as well as the Stokes-shift and high photoluminescence quantum yield (PLQY).

Here, we implement a method to calculate STEs from first-principles calculations by solving a Dyson like equation for exciton-polarons and STEs, within many-body perturbation theory. The self-energy terms in the Dyson like equation are calculated using the phonon energies, exciton energies, and exciton-phonon matrix elements obtained from BSE and Density-Functional Perturbation Theory (DFPT) calculations. We then apply the method to investigate STEs in Cs2AgInCl6, a lead-free double-perovskite. Our calculations are used to identify the phonon modes that couple strongly with excitons, and the electron and hole states that change significantly in the STE formation. The STE energies and lifetimes calculations are used to explain the experimental Stokes-shift and short lifetimes in the experimental PL spectra. Comparing the exciton-polaron dispersion calculations and the STE energies, we estimate the energy reduction due to lattice distortion for the STE formation. Finally, since STE calculations are computationally intensive, we develop a method for using equivariant graph neural networks for calculating STEs in large systems. This method paves the way for studying STE formation in other structures and gaining insights from first-principles calculations for designing efficient stable STE emitters.