Multifunctional-Ligand Enabled Stable CsPbI₃ Quantum Dots for Highly Efficient Pure-Red Light-Emitting Diodes

High-performance pure-red perovskite light-emitting diodes (PeLEDs) are promising candidates for optoelectronic applications due to their remarkable photophysical properties. However, the unstable surface and insulating long-chain capping ligands of perovskite quantum dots (QDs) pose challenges to their commercialization by compromising stability and performance. Here, a simple but effective approach was demonstrated to synthesize highly stable red CsPbI₃ QDs by introducing a multifunctional molecule, phenformin hydrochloride (PhenHCl), as an additive ligand. While the biguanide functional group in PhenHCl formed multiple hydrogen-bond interactions with the lead halide octahedron, the excess Cl^- anions compensated for the iodine vacancies and eliminated trap states in the CsPbI₃ QDs. The synergistic effect of the biguanide functional group and halogen compensation significantly passivated surface defects of the red CsPbI₃ QDs, yielding a photoluminescence (PL) quantum yield of 98.6% and excellent ambient stability with 90% PL intensity retention over 80 days. The resulting pure-red PeLEDs based on the PhenHCl-treated CsPbI₃ QDs were demonstrated to show a remarkable enhancement in the electroluminescence performance at around 649 nm, with an external quantum efficiency of 13.38%, and a maximum luminance of 2159 cd m^-2. Our findings in this work provide an avenue by modulating the surface chemistry of CsPbI₃ QDs to enhance the performance of pure-red PeLEDs.