Ultra‐Efficient, Broadband‐Excitable NIR Emission in Lead‐Free Cs ₂ NaYbCl ₆ via Cl → Yb Charge Transfer and Cr ³⁺ Sensitization

Abstract Ytterbium ions (Yb ³⁺ ) are commonly employed to extend the luminescent properties of metal halide perovskites due to their characteristic narrow‐band near‐infrared (NIR) emission. However, Yb ³⁺ still suffers from weak emission in lead‐free halide double perovskite systems owing to the parity‐forbidden 4f‐4f transitions with intrinsically weak absorption. In this study, undoped Cs ₂ NaYbCl ₆ single crystals demonstrate an exceptional NIR photoluminescence quantum yield (PLQY) of 43.6%. Through strategic incorporation of 5% transition metal Cr ³⁺ ions, the system achieves an ultra‐broad excitation spectrum spanning UV, visible, and NIR regions while preserving the characteristic narrow‐band Yb ³⁺ emission. Theoretical investigations encompassing band structure analysis, Bader charge calculations, and electron localization function (ELF) reveal that the highly localized [YbCl ₆ ] ³⁻ octahedra facilitate efficient NIR emission through Cl ⁻ →Yb ³⁺ charge transfer (CT) transitions. Cr ³⁺ doping introduces impurity levels, disrupts the Cl ⁻ →Yb ³⁺ CT process, and induces sublattice distortion, thereby serving as sensitization channels for intrinsic Yb ³⁺ emission. Leveraging the broadened excitation spectrum, a NIR solid‐state lighting system excitable by UV/visible/NIR illumination is engineered. These findings provide novel design principles for photo‐sensitization processes and application scenarios in lanthanide‐based perovskites and coordination compounds.