High‐Quality γ‐CsPbI3 Cuboid Crystallites Grown by Amorphous‐Complex Conversion for Efficient and Bright Deep‐Red Light‐Emitting Diodes

Abstract Inorganic CsPbI 3 perovskite has great potential to fabricate deep‐red light‐emitting diodes (LEDs) owing to excellent color purity, thermal stability, and carrier mobility. However, fabricating high‐quality CsPbI 3 thin films for high‐performance LEDs remains challenging. While it is demonstrated that hydrogen bonds between organic ammonium cations and molecule additives can lead to high‐quality organic–inorganic hybrid perovskite films, this method cannot be applied to CsPbI 3 thin films due to the absence of hydrogen bonds between Cs + ions and additives. In this study, an amorphous complex‐seed‐crystal growth process to fabricate high‐quality γ ‐CsPbI 3 cuboid crystallite thin films, is reported. During the initial annealing, the metastable intermediates formed crystal nucleate seeds and transformed into amorphous complex intermediates, maintaining stability at an elevated temperature for perovskite crystallization. With prolonged annealing, the amorphous complex around the nucleate seeds gradually converts to γ ‐CsPbI 3 perovskite, enabling us to fabricate CsPbI 3 thin films with high crystallinity and orientation. The resulting deep‐red LEDs exhibit a peak external quantum efficiency (EQE) of 16.5% and a high maximum luminance of 3158 cd m −2 . Moreover, the devices show significantly suppressed efficiency roll‐off, maintaining an EQE of 14.8% even under a high current density of 500 mA cm −2 .