Tailoring the Microstructure and Electronic Properties of CsPbBr ₃ Perovskite Films: Toward High‐Efficiency Light‐Emitting Diodes

3D perovskites, offering higher thermal and humidity stability than organic-inorganic hybrid counterparts, have attracted significant attention for their promising application in high-efficiency and high-brightness light-emitting diodes (LEDs). However, the rapid crystallization kinetics of 3D perovskites often leads to a high surface defects density, severely impairing radiative recombination efficiency. Herein, (2-phenylethyl) phosphonic acid (PEPA) is employed as an in-situ additive to regulate the crystallization dynamics of CsPbBr₃ films, resulting in dense, smooth perovskite layers with substantially reduced grain sizes. The phosphonic acid functional groups in PEPA strongly interact with perovskite precursor species, increasing nucleation density and suppressing uncontrolled grain growth, yielding films withnarrower and uniform grain size distribution. Moreover, PEPA molecules form novel Pb─O─P covalent bonds on the nanocrystal surfaces, effectively replacing Pb ion vacancies and reducing defect density while enhancing the charge transport properties inherent to perovskites. Furthermore, hydroxyl groups within PEPA establish hydrogen bonds with adjacent halide ions, providing additional defects passivation. Consequently, the maximum external quantum efficiency of perovskite LEDs (PeLEDs) is enhanced from 14.2% to 24.1%, achieving a high luminance of over 260,000 cd m^-2 and an operational half-life of 0.85 h at an initial luminance of 18,564 cd m^-2.