Trade‐Off Between Efficiency and Stability of CsPbBr 3 Perovskite Quantum Dot‐Based Light‐Emitting Diodes by Optimized Passivation Ligands for Br/Pb

Although significant progress has been made in improving the external quantum efficiencies (EQEs) of perovskite quantum dot (QD) light‐emitting diodes (QLEDs), understanding the degradation mechanism and enhancing stability remain a challenge. Herein, increasing the content of Br‐based passivation ligands is shown to enhance the EQE up to 16.1% by reducing the defects of CsPbBr 3 QDs in a Br‐rich environment. However, the operational lifetimes of perovskite QLEDs gradually decrease with the increase of halide content, owing to the intensified ion migration under continuous electric field confirmed by the current behavior of QLEDs and time‐of‐flight secondary‐ion mass spectrometry. Furthermore, a thorough analysis of the relationship between electricity and luminance of QLEDs suggests that a small amount of residue oleic acid ligands could weaken ion migration. Accordingly, a halide‐ and acid‐hybrid (HAH) co‐passivation strategy is proposed to optimize the content of Br‐ and acid‐based ligands, and achieve a maximum EQE of 18.6% and an operational lifetime ( T 50 , extrapolated) of 213 h for CsPbBr 3 QLEDs. This approach for passivating QDs combines the high efficiency of Br‐based ligands with the improved stability of acid‐based ligands. The study elucidates the correlation between ligands and device performance, highlighting the significance of two or even multiple ligands for efficient and stable perovskite QLEDs.