Cation Engineering Enables Sequentially Ordered Growth of 3D and 2D Perovskites for High‐Efficiency Perovskite Light‐Emitting Diodes

Abstract 2D/3D perovskites, where 3D perovskite emitting cores are shelled by 2D or quasi‐2D perovskites, show great promise in defect reduction for efficient perovskite light‐emitting diodes (Pero‐LEDs). However, rapid crystallization often leads to simultaneous formation and random distribution of 3D and 2D perovskites, impeding energy transfer and increasing non‐radiative recombination. Herein, to address this issue, we develop a Double Cation strategy via the synergistic effect of 3‐(trifluoromethyl) phenethylamine cation (CF 3 ‐PEA + ) and phenethylamine cation (PEA + ), which, compared to the commonly used single cation PEA + , offers a more controlled crystallization process in the formation of 2D/3D perovskites. This improvement is attributed to the hydrogen and ionic bonds between F atoms and perovskite components, which significantly slows down the crystallization rate, enabling sequential growth of 3D and 2D perovskites. Moreover, CF 3 ‐PEA + accumulates at the upper surface of the perovskite film, altering the surface characteristics from weak n‐type to heavy n‐type, thereby constructing an energy‐level gradient. This strategy effectively suppresses non‐radiative recombination caused by disordered energy transfer and defects, achieving a near‐unity photoluminescence quantum yield of 95.3%. When incorporated into devices, this approach brings in high‐efficiency Pero‐LEDs with a maximum external quantum efficiency (EQE) of 28.2%, along with excellent reproducibility.