Charge carrier confinement in nanocrystals is an effective route to enhance luminescence efficiency of metal halide perovskites. However, difficulty in handling surface ligands of colloidal perovskite nanocrystals and the defective surface of the processed nanocrystals limit radiative efficiency and the stability of nanocrystalline films. We report a novel approach to fabricate highly luminescent nanocrystalline polymorphic formamidinium lead tri-iodide (FAPbI(3)) films. The cubic perovskite nanocrystals are formed in situ by kinetic controlled solid-state phase conversion of a hexagonal polymorph. The perovskite nanocrystal surface is benign owing to the remnant polymorph forming a type I hetero-junction with the nanocrystal surface, facilitating accelerated radiative recombination with suppressed trapping in the nanocrystals to which ultrafast carrier funneling occurs. This polymorphic energy funnel renders photoluminescence intensity enhanced by a factor of 58 compared to that of conventional bulk-FAPbI(3) film. A proof-of-concept light-emitting diode device exhibited superior efficiency and stability in comparison to devices based on the conventional bulk-FAPbI(3) film.