Eliminating the Adverse Impact of Composition Modulation in Perovskite Light‐Emitting Diodes toward Ultra‐High Brightness and Stability

Abstract Excess ammonium halides as composition additives are widely employed in perovskite light‐emitting diodes (PeLEDs), aiming to achieve high performance by controlling crystallinity and passivating defects. However, an in‐depth understanding of whether excess organoammonium components affect the film physical/electrical properties and the resultant device instability is still lacking. Here, the trade‐off between the performance and stability in high‐efficiency formamidinium lead iodide (FAPbI 3 )‐based PeLEDs with excess ammonium halides is pointed, and the underlying mechanism is explored. Systematic experimental and theoretical studies reveal that excess halide salt‐induced ion‐doping largely alters the PeLEDs properties (e.g., carrier injection, field‐dependent ion‐drifting, defect physics, and phase stability). A surface clean assisted cross‐linking strategy is demonstrated to eliminate the adverse impact of composition modulation and boost the operational stability without sacrificing the efficiency, achieving a high efficiency of 23.6%, a high radiance of 964 W sr −1 m −2 (The highest value for FAPbI 3 based PeLEDs), and a prolong lifetime of 106.1 h at large direct current density (100 mA cm −2 ), concurrently. The findings uncovered an important link between excess halide salts and the device performance, providing a guideline for rational design of stable, bright, and high efficiency PeLEDs.