AbstractInorganic CsPbI₃ perovskite with high chemical stability is attractive for efficient deep‐red perovskite light‐emitting diodes (PeLEDs) with high color purity. Compared to PeLEDs based on ex‐situ‐synthesized CsPbI₃ nanocrystals/quantum dots suffering from low conductivity and efficiency droop under high current densities, in situ deposited 3D CsPbI₃ films from precursor solutions can maintain high conductivity but show high trap density. Here, it is demonstrated that introducing diammonium iodide can increase the size of colloids in the precursor solution, retard the phase‐transition rate, and passivate trap states of the in‐situ‐formed cuboid crystallites. The PeLED based on the one‐step‐formed 3D CsPbI₃ cuboid crystallite films shows a peak external quantum efficiency (EQE) value up to 15.03% because of the high conductivity and reduced trap states. Furthermore, this one‐step method also has a wide processing window, which is attractive for flow‐line production of large‐area PeLED modules. The fabrication of a 9 cm² PeLED that exhibits a peak EQE of 10.30% is successfully demonstrated.