Sequential Crystallization Decouples Cs₄PbBr₆ and CsPbBr₃ Phases Enabling Efficient Perovskite Light-Emitting Diodes

Cesium lead bromide has emerged as a leading candidate for high-performance perovskite light-emitting diodes (PeLEDs) in next-generation optoelectronics. However, the formation of the 0D Cs₄PbBr₆ phase often accompanies CsPbBr₃ crystallization and is difficult to regulate, hampering the efficiency of LED devices in scalable production. In particular, 0D and 3D perovskite phases often crystallize as disordered structures, leading to poor crystalline quality and a high density of defects in the perovskite films. To address this challenge, we employed an additive-mediated sequential crystallization (AMSC) strategy using 3-carboxyphenylphosphonic acid (3-CPPA) to modulate the crystallization kinetics of the 0D and 3D phases. Through strong interactions between the additive and perovskite precursors, the initial CsPbBr₃ formation is effectively slowed, allowing for temporally separated crystallization of Cs₄PbBr₆ and CsPbBr₃ phases. This separated crystallization process significantly enhances the film quality of CsPbBr₃ phase. Remarkably, the AMSC strategy drives the Cs₄PbBr₆ phase toward the film surface, thereby forming a favorable surface morphology and effectively improving the light outcoupling efficiency of PeLEDs. By leveraging this synergic strategy, we achieve an ultrahigh photoluminescence quantum yield (PLQY) reaching nearly 100% and realize an efficient external quantum efficiency (EQE) of 35.3% in green PeLEDs, underscoring the critical role of phase crystallization engineering in optimizing perovskite emitters.