Mn²⁺-doped lead halide perovskites either in 3D or 2D have been extensively explored due to their rich energy-transfer behaviors. While their application on LED is still lagging behind in comparison to non-doped 3D perovskite due to inferior film-formation and low luminescent efficiency. Here we report an in-situ-formed Mn²⁺ doped 2D perovskite nanocrystal (NCs) film by introducing quaternary phosphonium salt during the crystallizing process. The as-formed film shows improved luminescent efficiency with emission peaked at 600 nm and photoluminescence quantum yield (PLQY) of as high as 73.37%, which is about 1.3 times higher than that of pristine film. Further characterizations confirm the enhanced confinement effect from smaller size particle is responsible for the improved luminescent efficiency. The perovskite LEDs based on pristine and phosphonium passivated thin films were fabricated, and a great improvement in the external quantum efficiency of these LEDs (from 0.0017% to 0.12%) is observed due to the improved morphology and enhanced luminescent efficiency.