Synergistic dual-site passivation via ionic liquid engineering for efficient and stable deep-red CsPbI₃ QLEDs

Dual-site passivation via [VEIm]DCA: DCA − heals Pb 2+ defects, [VEIm] + stabilizes iodides. • Synergy triples PLQY (56%), aligns energy levels, and boosts mobility tenfold. • Deep-red QLEDs: 14.2% EQE at 684 nm and 2.8-fold lifetime gain. • Intramolecular ionic synergy: a new design principle for stable perovskites. Achieving high-efficiency and operationally stable deep-red perovskite quantum dot light-emitting diodes (QLEDs) is crucial for advanced optoelectronics but remains challenging, primarily due to the inherent trade-off between defect passivation and charge transport. Here, we propose a novel dual-site ligand engineering strategy using the ionic liquid 1-vinyl-3-ethylimidazolium dicyanamide ([VEIm]DCA) for CsPbI₃ quantum dots. This approach harnesses the intrinsic synergy within a single molecule: the DCA − anion heals iodine vacancies via coordination to under-coordinated Pb 2+ , while the [VEIm] + cation reinforces the lattice through a C–H···I hydrogen-bonding network. This synergistic action effectively passivates defects (tripling the film PLQY to 56%), optimizes energy level alignment, and enhances charge carrier mobility by over an order of magnitude. Consequently, the resulting deep-red QLEDs achieve a peak external quantum efficiency of 14.2% at 684 nm with a low turn-on voltage of 2.2 V. Notably, operational stability is substantially enhanced, with a T₅₀ lifetime of 64 min at 100 cd m −2 under ambient conditions—a ~ 2.8-fold improvement. This work establishes the synergistic anion-cation pair within a single ionic liquid as a highly efficient and streamlined design principle for harmonizing performance and stability in perovskite optoelectronics.