Homogeneous Phase Distribution in Q‐2D Perovskites via Co‐Assembly of Spacer Cations for Efficient Light‐Emitting Diodes

Quasi-2D (Q-2D) perovskites are promising candidates to apply in light-emitting diodes (LEDs). However, delicate control on crystallization kinetics is needed to suppress severe phase segregation. Here, the crystallization kinetics of Q-2D perovskites are investigated via in situ absorbance spectroscopy and for the first time find the multiphase distribution is governed by the arrangement, rather than diffusion, of spacer cations at the nucleation stage, which associate with its assembling ability determined by molecular configuration. A "co-assembly" strategy is conceived by combining co-cations with different configuration characteristics, where bulky cations disturb the assembling between slender cations and lead-bromide sheet, contributing to homogeneous emitting phase with effective passivation. Correspondingly, in the phenylethylammonium (PEA⁺ )-based Q-2D perovskites ( = 3), homogeneous phase distribution is achieved by incorporating co-cation triphenylmethaneammonium (TPMA⁺ ), the branching terminals of which suppress cations assembling into low-n phases and afford adequate cations as passivating ligands. Therefore, the champion external quantum efficiency of the LED device reaches 23.9%, which is among the highest performance of green Q-2D perovskite LEDs. This work reveals that the arrangement of spacer cations determines the crystallization kinetics in Q-2D perovskites, providing further guidance on the molecular design and phase modulation of Q-2D perovskites.