Tailoring CsPbBr ₃ /Cs ₄ PbBr ₆ Evolution via Controlled Moisture Treatment for High‐Performance LEDs

ABSTRACT Metal halide perovskites are promising materials for next‐generation light‐emitting diodes (LEDs) owing to their excellent optoelectronic properties and low‐cost solution processability. The efficiency and operational lifetime of perovskite LEDs (PeLEDs) are strongly governed by the crystal quality of perovskite emitters. Among the various factors influencing perovskite crystallization, the surrounding solvent vapors, particularly ambient moisture, can significantly modify both the initial‐stage nucleation and the subsequent post‐deposition structural evolution. However, current moisture treatments are highly engineering‐driven, and the fundamental mechanisms underlying the impacts of moisture on perovskite remain unclear. This work systematically investigates how moisture treatment with controllable humidity modifies the structural and optoelectronic properties of CsPbBr ₃ films. Moisture induces the rapid formation of Cs ₄ PbBr ₆ and promotes continuous crystal growth of CsPbBr ₃ , resulting in alterations to the film composition, defect landscape, and grain‐boundary characteristics that strongly affect carrier recombination and ion migration behaviors. Consequently, we achieve a champion device efficiency exceeding 20% after short‐term moisture exposure, and a half‐lifetime of 220 min at an initial luminance of 10 000 cd m ⁻² under prolonged moisture treatment. This work provides mechanistic insight into moisture‐driven film evolution and demonstrates that the moisture treatment offers an effective strategy to enhance the performance of PeLEDs.