Cross-relaxation engineering in Sb3+-sensitized Cs2NaLuCl6:Er3+ double perovskites enabling a record 45.3% EQE in NIR-II luminescence
Lead-free halide double perovskites have attracted significant attention owing to their eco-friendliness, structural tunability, and self-trapped exciton emission. Nevertheless, achieving efficient and stable near-infrared-II (NIR-II) luminescence, especially in materials incorporating lanthanide ions, remains a considerable challenge in photonics research. Herein, we report a notable advance in the design and synthesis of Sb3+-sensitized Cs2NaLuCl6:Er3+ double perovskite single crystals, which exhibit an unprecedented external quantum efficiency of 45.3% for emission at 1542 nm. Sb3+ acts as a broadband ultraviolet absorber and transfers energy to Er3+ via self-trapped exciton emission. Moreover, at high concentrations of Er3+, Er3+-Er3+ cross relaxation (2H11/2 + 4I15/2 → 4I9/2 + 4I13/2) selectively populates the NIR-emitting 4I13/2 state, suppressing competitive visible emission pathways. This synergistic host-sensitizer-activator design strategy, supported by density functional theory calculations, addresses long-standing efficiency limitations and opens new avenues for high-performance NIR-II emitters in bioimaging, night vision, and optical communications.