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.