Metal halide perovskites with quasi-2D crystal structures have shown excellent electroluminescent properties due to the inherently confined charge diffusion and efficient radiative recombination. But quasi-2D perovskite films can exhibit complex phase characteristics that need to be tailored for achieving high-performance light-emitting diodes (LEDs). Here, we report a unique quasi-2D perovskite thin film structure featuring a 3D perovskite bottom sublayer underneath a mixed 2D-3D perovskite composite upper sublayer, as imaged by low-dose scanning transmission electron microscopy. We demonstrate that the incorporation of a potassium bromide additive can trigger the self-assembly of multiphase perovskite grains toward this bilayer microstructure, probably due to its ability to create heterogeneous nucleation templates for the crystallization of 3D perovskite grains on the precursor-substrate bottom interface. The external quantum efficiency of quasi-2D perovskite LEDs is significantly improved by this bilayer film microstructure. By probing the carrier dynamics using transient absorption spectroscopy, we attribute the LED performance enhancement to the accelerated carrier transfer and recombination across the bilayer film microstructure.