Thermodynamic Equilibrium between Free Excitons and Self‐Trapped Excitons in Efficient Quasi‐Nanocrystalline Green Perovskite Light‐Emitting Diodes

Abstract Metal halide perovskites show promise for light‐emitting diodes (LEDs) owing to their facile manufacture and excellent optoelectronic performance, including high color purity and high carrier mobility, while the photophysical processes remain partially obscure. Herein, by introducing 18‐crown‐6 and tBO‐Br as dual passivation agents, quasi‐2D phase perovskites are suppressed through controlled nucleation and crystal growth, yielding high‐quality quasi‐nanocrystalline films with improved crystallinity and isotropic orientation. Synergistic passivation reduces trap‐assisted non‐radiative recombination, achieving a photoluminescence quantum yield of 93.5% at 520 nm and enhanced carrier lifetimes. Notably, dual emission with a shoulder peak at 535 nm and directional energy transfer with long‐lived delayed component at low temperature reveal dynamic equilibrium between free excitons and self‐trapped excitons (STEs) governed by strong exciton‐phonon coupling. Finally, benefiting from suppressed STEs formation and better energy alignment, green perovskite LEDs treated with dual additives achieve a high external quantum efficiency of 29.5%, high luminance of 59232 cd m −2 , and a commendable operating half‐lifetime of 10.81 hours at an initial brightness of 1000 cd m −2 while maintaining low efficiency roll‐off. The observation of the dual emission and long‐lived delayed components, coupled with insights into STEs, provides a profound and comprehensive understanding of the fundamental carrier behavior in perovskite emitters.