Ultra-Low Efficiency Roll-Off High Color Purity Blue Perovskite Quantum Dot LEDs with Exceeding 20% Efficiency

Blue perovskite quantum dot light-emitting diodes (QLEDs) are highly attractive for new generation high-definition displays, but their practical deployment is hindered by severe efficiency roll-off at high brightness (>4000 cd m^-²) while maintaining high color purity (CIEy < 0.1). This roll-off arises mainly from non-radiative losses induced by exposed dangling bonds, strong inter-dot coupling, and low permittivity. To conquer this challenge, we develop a multifunctional molecule passivation strategy utilizing 1-ethyl-3-methylimidazolium hexafluorophosphate (EMIMPF₆). The [PF₆]^- anions coordinate with lead dangling bonds, cesium sites, and weaken coupling, while [EMIM]⁺ cations suppress bromine-related defects and inhibit Auger recombination, collectively reducing exciton quenching pathways. This treatment increases the photoluminescence quantum yield of QD films from 78% to 92% and enables spectrally high color purity blue emission at 472 nm (CIEy = 0.091). Crucially, the resulting devices deliver a record-high external quantum efficiency (EQE) exceeding 20% at 6441 cd m^-² and maintain 18.47% EQE at 9587 cd m^-² with nearly eliminated roll-off, representing the best performance to date for blue perovskite QLEDs (CIEy < 0.1). Moreover, the operational lifetime improves by an order of magnitude compared with previous reports.