Stable and Scalable Large‐Area InP Quantum Dot Light‐Emitting Diodes with a Convenient NH ₄ Cl Surface Treatment

InP quantum dot light‐emitting diodes (QLEDs) require defect‐controlled electron transport layer (ETL) to ensure efficiency and operational stability. However, intrinsic oxygen vacancies in ZnMgO ETL act as trap states, causing exciton quenching and compromising operational stability through unpredictable positive aging effects. Here, a simple postdeposition ammonium chloride treatment (NH 4 Cl) is introduced to passivate ZnMgO nanoparticles (NPs) via formation of Zn─Cl and Mg─Cl bonds, thereby suppressing oxygen vacancies and significantly reducing in large surface defects ( R pv decreased from 37.88 to 18.44 nm). InP QLEDs employing Cl‐passivated ZnMgO (Cl@ZnMgO) achieved comparable peak luminance of 7,274 cd/m 2 at reduced operating voltage (7.5 V vs. 9 V) and have operating lifetimes exceeding 2,500 h. Critically, after 48 h of aging, devices with Cl@ZnMgO NPs maintained stable luminance within 15% variation, whereas pristine ZnMgO devices exhibited rapid luminance increase followed by sudden failure. Additionally, large‐area fabrication (2.5 × 2.5 cm) demonstrated bright, uniform emission. This convenient postdeposition treatment effectively suppresses positive aging and enables reliable, scalable InP‐based QLEDs for next‐generation display technologies.