First-principles Study of Self-trapped Exciton Formation in Double Perovskites using Excited State Forces*

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). Despite many experimental results suggesting STE based emission, a clear understanding of the STE formation, stability, and emission mechanism is still unclear.

Here, we develop and implement a method to calculate STEs from first-principles calculations, by relaxing the structure using excited state forces obtained from the Bethe Salpeter equation (BSE) within many-body perturbation theory. We then apply the method to investigate STEs in Cs2AgInCl6, a lead-free double-perovskite. The excited state forces are calculated using the exciton wavefunction and BSE Hamiltonian from GW-BSE calculations, and electron-phonon matrix elements from Density-Functional Perturbation Theory (DFPT). Subsequent relaxation in Cs2AgInCl6 reveals the phonon modes that couple strongly with the excitons and drive the STE formation. We also find the energy barrier that traps and stabilizes the STE. Our method paves the way for studying STE formation in other structures and gaining insights from first-principles calculations for designing efficient stable STE emitters.