Double perovskites, particularly Cs2AgBiCl6, have attracted substantial attention as promising alternatives to lead-halide perovskites because they are environmentally friendly. However, the integration of these double perovskites into light-emitting diodes (LEDs) remains challenging because of their poor performance, which is primarily attributed to a limited understanding of the correlation between crystallization and defect formation. In this study, deep-trap defects caused by B-site antisites and vacancies are introduced into the double perovskite formamidinium (FA)1.5Cs0.5AgBi(Cl0.75Br0.25)6 lattice during crystallization. Remarkably, valeric acid suppresses antisites and facilitates the formation of distinct nanocrystals with a structurally ordered double perovskite. Poly(ethylene oxide) encapsulates the perovskites in situ and passivates surface defects by modifying the grain boundaries. In this manner, the coadditive strategy boosts the performance of the double-perovskite-based deep blue LED with a record external quantum efficiency of 1.8% and peak luminance of 280 Cd/m2. This study significantly advances our understanding of double perovskites and provides new perspectives for future investigations.