Enhancement of NIR‐II Emission of Er ³⁺ by Doping Fe ³⁺ 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 range. However, achieving high-efficiency multimode 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 multimode luminescence and enhance the second near-infrared region (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 (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 Fe³⁺ 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.