Quaternary phosphonium bromide passivation for high-performance perovskite quantum dot light-emitting diodes

Surface ligands are crucial for perovskite quantum dot (PQD) optoelectronics. However, long alkyl chains limit charge transport, while short chains destabilize their soft lattice, presenting a stability-mobility dilemma. Moreover, surface-bound protonated primary amines are susceptible to deprotonation, accelerating performance decay and structural collapse. In this work, we introduced tetrabutylphosphonium bromide (TBPB) as a surface passivation ligand to overcome these limitations. TBPB combines minimal molecular polarity-enabling a short chain for superior charge transport-with a fully coordinated phosphonium center that resists deprotonation and passivates halogen vacancies. This yields quantum dots with near-unity PLQY and robust stability, leading to LEDs achieving a maximum external quantum efficiency (EQE) of 24.28% and luminance of 122,786 cd m^-2, far surpassing control devices. Further shortening the ligand chain increases EQE to 27.89% but at the cost of severe luminance loss, underscoring the delicate balance in ligand engineering.