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

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 (CF₃-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, CF₃-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.