Ultrafast Dynamics of Self-Trapped Excitons in Cs₂AgInCl₆:Al³⁺ Double Perovskite Nanocrystals

It is a huge challenge to increase the photoluminescence (PL) of lead-free halide perovskites, and understanding the mechanism behind exciton dynamics can provide a valuable solution. Herein, we achieved enhanced broad-band emission at ambient conditions in Cs₂AgInCl₆ by tuning self-trapped excitons (STEs) through Al³⁺ doping. Cryogenic measurements showed an inhomogeneous nature of STE emission due to the presence of defect states and is subject to thermal quenching. An increased Huang–Rhys factor (S-factor) resulted in better electron–phonon coupling and high-density STE states post Al³⁺ doping. Femtosecond transient absorption (fs-TA) results provided insights into the distribution dynamics of excitons, which occurs through gradient energy levels from free excitons (FE) to STEs, where each STE state potentially possesses higher quantized energy states. Overall, this study aims to comprehend the origins of self-trapping and decay of STEs in Cs₂AgInCl₆:Al³⁺ and emphasizes the potential of compositional engineering to mitigate self-trapping in this material.