Multifunctional Biguanide Additive Stabilizes Chloride-Rich Quasi-2D Perovskites for Efficient and Stable Pure-Blue LEDs.

Quasi-2D perovskites are emerging as promising materials for light-emitting applications due to their pronounced quantum confinement effects. Blue perovskite light-emitting diodes (PeLEDs) remain fundamentally challenging yet critically demanded for display applications. Current strategies employing quasi-2D perovskites face inherent trade-offs: 1) increased spacer cation content enhances quantum confinement for blueshift but deteriorates charge transport through insulating organic layers; 2) multiphase quantum well formation broadens emission spectra (fwhm >25 nm), compromising color purity; 3) chloride incorporation for bandgap widening induces deep-level traps and accelerates halide segregation under operational voltage. Herein, we address these intertwined challenges through multifunctional additive engineering using phenylbiguanide (PBG). The conjugated molecular structure with dual -NH/═NH groups enables 3-fold functionality: First, strong Pb-PBG coordination effectively passivates uncoordinated halide vacancies, suppressing nonradiative recombination and achieving a high photoluminescence quantum yield (PLQY) of 76.6%. Second, hydrogen-bonding networks between PBG and [PbX] frameworks immobilize halide ions, inhibiting electric-field-driven Cl/Br phase segregation. Third, PBG modulates crystallization kinetics to produce a narrow quantum well distribution for narrow emission (fwhm = 21 nm) at 472 nm and efficient Förster resonance energy transfer. The synergistic effects yield pure-blue PeLEDs with an impressive EQE of 9.32% at 472 nm, with stable emission and a 10-fold enhancement of lifetime compared to the pristine device without PBG. This work offers a promising approach to the development of high-performance blue PeLEDs using quasi-2D perovskites.