Exploring Coupling‐Derived A′‐Site Cations for Crystallization Delay and Defect Passivation in Quasi‐2D Perovskite Light‐Emitting Diodes

Abstract Despite quasi‐2D perovskite light‐emitting diodes (quasi‐2D PeLEDs) having shown great potential in the fields of light emission and display, the uncontrollable crystallization processes and substantial defects of perovskite emissive layer still limit their further development. Herein, we reported the design and in‐situ synthesis of A′‐site cations with oxygen–phosphorus–nitrogen–carbon (O─P─N─C) frameworks in perovskite precursor solutions by incorporating diphenylphosphinic chloride (DPCl), a bifunctional molecule containing both P─Cl and P═O functional groups. The P─Cl groups undergo nucleophilic substitution reactions with the ammonium terminals of formamidinium (FA + ) and phenethylammonium (PEA + ) cations, yielding large A′‐site cations that regulate crystallization kinetics during spin‐coating. Concurrently, P═O groups coordinate with undercoordinated Pb 2+ ions at grain boundaries, passivating defects. This dual‐functional mechanism synergistically optimized crystallization dynamic and suppressed non‐radiative recombination, resulting in compact, low‐defect perovskite films. Owing to their synergistic enhancement on device efficiency, the quasi‐2D PeLEDs modified with DPCl exhibited a champion external quantum efficiency (EQE) of 26.07%. This study provides a new strategy for tailoring perovskite materials through A′‐site engineering, offering significant insights into the development of high‐performance PeLEDs.