• • Record external quantum efficiency (EQE) of 45.3% at 1542 nm emission in Sb3+-sensitized Cs2NaLuCl6:Er3+ double perovskite single crystals, surpassing typical NIR-II PLQYs below 20% in prior systems, enabling high-sensitivity bioimaging and low-loss optical communication.
• • Sb3+ sensitization provides broadband ultraviolet absorption via spin-allowed s-p transitions, effectively harvesting UV photons and transferring energy to Er3+ through self-trapped exciton emission, overcoming the weak absorption of parity-forbidden 4f-4f transitions.
• • High Er3+ doping activates cross-relaxation (2H11/2 + 4I15/2 → 4I9/2 + 4I13/2), which selectively populates the NIR-emitting 4I13/2 state and suppresses competitive visible emission, a mechanism directly evidenced by temperature-dependent spectroscopy and lifetime decay analyses.
• • Density functional theory calculations reveal that Sb3+ incorporation introduces in-gap states, reduces local symmetry, and enhances charge localization, facilitating efficient energy transfer and charge redistribution, providing a general design principle for high-efficiency lanthanide-based NIR-II emitters.