Broadband and Enhanced Near‐Infrared Luminescence from Bandgap Engineered Single‐Sensitizer Sb ³⁺ ‐Activated Cs ₂ NaTmCl ₆ Double Perovskites

Thulium ions (Tm 3+ ) are employed to expand the luminescent properties of metal halide perovskites due to their rich array of long‐wavelength near‐infrared (NIR) luminescent energy levels. However, Tm 3+ still exhibits weak emission in double perovskites (DPs) owing to the parity‐forbidden nature of its f–f transitions, leading to poor absorption. In this study, the rare‐earth‐based DP Cs 2 NaTmCl 6 was successfully synthesized via a facile method. This material exhibits self‐sensitized NIR‐I and NIR‐II emission under its characteristic excitation. Upon introducing Sb 3+ ions, which possess strong absorption in the UV region, the NIR photoluminescence external quantum efficiency (EQE) of Cs 2 NaTmCl 6 :3% Sb 3+ reaches 21.6%. This enhancement is attributed to the presence of the 1 G 4 energy level of Tm 3+ , which acts as a bridge to efficiently transfer energy from the high‐energy states of Sb 3+ to the NIR‐emitting states of Tm 3+ . Both the pristine and Sb 3+ ‐doped Cs 2 NaTmCl 6 demonstrate excellent thermal quenching resistance. Through a combined experimental and theoretical approach, we demonstrate that the incorporation of Sb 3+ ions reduces the effective optical bandgap of the host matrix by providing additional absorption channels. Leveraging the efficient NIR luminescence, we designed an NIR night‐vision illumination and imaging system. Furthermore, exploiting the emergence of visible luminescence and the distinct NIR emission intensities before and after Sb 3+ doping, we developed a dual‐mode optical anti‐counterfeiting label. These findings provide novel design insights and inspiration for achieving NIR luminescence in rare‐earth‐based DPs.