Enhancement of NIR‐II Emission of Er 3+ by Doping Fe 3+ in Double Perovskites: Multimode Luminescence for Versatile Optoelectronic Applications

Abstract Erbium ions are commonly used to extend the photoelectric properties of metal halide perovskites from visible to near‐infrared range. However, achieving high‐efficiency multimode luminescence in a single system is difficult due to the weak absorption associated with forbidden 4 f ‐4 f transitions. In this study, a unique strategy is proposed to adjust multimode luminescence and enhance the second near‐infrared region (NIR‐II) emission in Cs 2 NaBiCl 6 by incorporating Fe 3+ ions. The as‐prepared material demonstrates reversible thermochromism, driven by strong electron‐phonon coupling effect, and exhibits tunable luminescence that can be adjusted by altering excitation energy and temperature. Notably, benefitting from the charge transfer transition of Fe 3+ ‐Cl − along with the influence of Fe 3+ doping on the geometrical and electronic structures, the blue‐excitable (450 nm) NIR‐II emission around 1541 nm from Er 3+ is realized for the first time, achieving an intensity 16.7 times higher and a maximum photoluminescence quantum yield (PLQY) of 22.5 %. This enhancement enables innovative applications such as two‐dimensional information encryption by the multi‐channel cooperative responses and improved NIR imaging. The study highlights the potential of Fe 3+ doping in optimizing absorption and multimode luminescence in perovskites, opening avenues for advanced applications in blue‐excitable NIR light emitting diodes (LEDs), thermometer, anti‐counterfeiting, and NIR imaging.