Suppressing Interlayer Ion Migration in CsPbX3 Nanocrystal Films for Realizing Efficient and Stable Electroluminescence

Abstract Mixed‐halide perovskite light‐emitting diodes (PeLEDs) face the critical challenge of field‐dependent phase separation. Discrete colloidal CsPbX 3 nanocrystals anchored with ligands are promising to suppress phase separation, yet it remains a mystery how ion migration proceeds when integrated into LEDs as emissive films. Specifically, the influence of ion migration inside a single nanocrystal or across the nanocrystals along the electric field on the performance of PeLEDs needs to be decoupled. Here, a low‐temperature‐assisted transfer‐printing method is developed to construct a model PeLED containing a clear CsPbBr 3 ‐CsPbI 3 nanocrystal film interface for tracing the ion migration between perovskite nanocrystal films along the direction of electric fields. The comprehensive study demonstrates that halogen ions crossing the nanocrystal film interface lead to severe phase separation and poor device stability, rather than the horizontal intra‐layer diffusion. The monolayer CsPbX 3 nanocrystal film prevents the field‐dependent phase separation caused by interlayer ion migration, significantly improving electroluminescent stability, including spectrum and lifetime. The optimized structure achieves a high external quantum efficiency of 26.9% and a remarkably improved operational half‐lifetime of 61.2 h at an initial luminance of 100 cd m −2 in pure‐red PeLEDs based on mixed‐halide CsPb(I x /Br 1‐x ) 3 , more than 300 times longer than the control device using multilayer nanocrystals.