Unraveling Size‐Dependent Ion‐Migration for Stable Mixed‐Halide Perovskite Light‐Emitting Diodes

Abstract Mixed‐halide perovskites show tunable emission wavelength across the visible‐light range, with optimum control of the light color. However, color stability remains limited due to the notorious halide segregation under illumination or an electric field. Here, a versatile path toward high‐quality mixed‐halide perovskites with high emission properties and resistance to halide segregation is presented. Through systematic in and ex situ characterizations, key features for this advancement are proposed: a slowed and controllable crystallization process can promote achievement of halide homogeneity, which in turn ensures thermodynamic stability; meanwhile, downsizing perovskite nanoparticle to nanometer‐scale dimensions can enhance their resistance to external stimuli, strengthening the phase stability. Leveraging this strategy, devices are developed based on CsPbCl_1.5Br_1.5 perovskite that achieves a champion external quantum efficiency (EQE) of 9.8% at 464 nm, making it one of the most efficient deep‐blue mixed‐halide perovskite light‐emitting diodes (PeLEDs) to date. Particularly, the device demonstrates excellent spectral stability, maintaining a constant emission profile and position for over 60 min of continuous operation. The versatility of this approach with CsPbBr_1.5I_1.5 PeLEDs is further showcased, achieving an impressive EQE of 12.7% at 576 nm.