Enhancement of NIR‐II Emission of Er3+ by Doping Fe3+ in Double Perovskites: Multimode Luminescence for Versatile Optoelectronic Applications

Erbium ions are commonly used to extend the photoelectric properties of metal halide perovskites from visible to near‐infrared (NIR) range. However, achieving high‐efficiency multi‐mode luminescence in a single system is difficult due to the weak absorption associated with forbidden 4f‐4f transitions. In this study, a unique strategy is proposed to adjust multi‐mode luminescence and enhance NIR‐II emission in Cs2NaBiCl6 by incorporating Fe3+ 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 Fe3+‐Cl‐ along with the influence of Fe3+ doping on the geometrical and electronic structures, the blue‐excitable (450 nm) NIR‐II emission around 1541 nm from Er3+ is realized for the first time, achieving an intensity 16.7 times higher and a maximum photoluminescence quantum yield 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 Fe3+ doping in optimizing absorption and multi‐mode luminescence in perovskites, opening avenues for advanced applications in blue‐excitable NIR light emitting diodes, thermometer, anti‐counterfeiting, and NIR imaging.