Delicate Management of Alkaline‐Substrate‐Induced Interfacial Reactions Enables High‐Efficiency and Stable Deep‐Red CsPbI ₃ Perovskite Light‐Emitting Diodes

ABSTRACT All‐inorganic CsPbI 3 perovskite holds great promise for high‐performance deep‐red light‐emitting diodes (LEDs), yet its hardly controllable phase transition and crystallization readily induce abundant defects in the resultant thin‐film emitters. The development of alkaline substrate‐assisted CsPbI 3 modulation has enabled significant performance improvements in associated perovskite LEDs (PeLEDs). However, a thorough understanding and effective management of the buried interfacial reactions remains elusive. Here, we develop a magnesium (Mg)‐doped alkaline zinc hydroxide (Zn(OH) 2 ) substrate with delicately regulated surface properties and systematically investigate the crystallization and degradation of CsPbI 3 emissive layers deposited on top. We reveal that the Mg doping is effective in reducing oxygen vacancies and surface hydroxyls, which directly attenuates the substrate basicity. The Mg‐doped Zn(OH) 2 substrate suppresses the alkaline‐interface‐induced organic deprotonation and alleviates the rapid transition process from intermediate phases to CsPbI 3 perovskite, enabling high‐quality emitters with reduced defects. More importantly, this facile interface engineering substantially mitigates the detrimental interfacial degradation and structural collapse of perovskite emitters under continuous electrical and thermal stresses. Consequently, we obtain deep‐red PeLEDs with a peak external quantum efficiency of 23.62% and an exceptional operational half‐lifetime of 376 h at 20 mA cm −2 , representing one of the best‐performing devices utilizing bulk CsPbI 3 emitters.