In Situ Interface Reaction Enables Efficient Deep‐Blue Perovskite Light‐Emitting Diodes

The development of blue perovskite light‐emitting diodes (PeLEDs) is critical for advancing next‐generation display technologies. However, the fabrication of high‐quality mixed‐halide blue perovskites remains challenging due to their intrinsic vulnerability to high defect densities, ion migration, and inefficient charge transport. To address this, we introduce a rapid in situ interface reaction at the buried interface between the perovskite layer and the underlying poly(3,4‐ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) film. This engineered interface reaction represents a novel method for preparing high‐quality deep‐blue CsPb(Br/Cl) 3 films. By incorporating lithium salicylate (SAL) as a reaction initiator into the perovskite precursor, a proton exchange is triggered between PEDOT:PSS and SAL, resulting in the formation of a multifunctional PSS‐Li interfacial layer. This layer modulates perovskite nucleation and growth, producing compact, uniform, and small‐grained deep‐blue perovskite films with reduced trap densities and enhanced quantum confinement. Combined with optimized charge dynamics, the resulting spectrally stable deep‐blue PeLEDs achieve a record external quantum efficiency of 16.3% at 468 nm. This approach facilitates the successful integration of uniform and high‐clarity active‐matrix displays on thin‐film transistor circuit substrates.