Unveiling Local Electronic Structure of Lanthanide‐Doped Cs2NaInCl6 Double Perovskites for Realizing Efficient Near‐Infrared Luminescence

Lanthanide ion (Ln3+)‐doped halide double perovskites (DPs) have evoked tremendous interest due to their unique optical properties. However, Ln3+ ions in these DPs still suffer from weak emissions due to their parity‐forbidden 4f–4f electronic transitions. Herein, the local electronic structure of Ln3+‐doped Cs2NaInCl6 DPs is unveiled. Benefiting from the localized electrons of [YbCl6]3− octahedron in Cs2NaInCl6 DPs, an efficient strategy of Cl−‐Yb3+ charge transfer sensitization is proposed to obtain intense near‐infrared (NIR) luminescence of Ln3+. NIR photoluminescence (PL) quantum yield (QY) up to 39.4% of Yb3+ in Cs2NaInCl6 is achieved, which is more than three orders of magnitude higher than that (0.1%) in the well‐established Cs2AgInCl6 via conventional self‐trapped excitons sensitization. Density functional theory calculation and Bader charge analysis indicate that the [YbCl6]3− octahedron is strongly localized in Cs2NaInCl6:Yb3+, which facilitates the Cl−‐Yb3+ charge transfer process. The Cl−‐Yb3+ charge transfer sensitization mechanism in Cs2NaInCl6:Yb3+ is further verified by temperature‐dependent steady‐state and transient PL spectra. Furthermore, efficient NIR emission of Er3+ with the NIR PLQY of 7.9% via the Cl−‐Yb3+ charge transfer sensitization is realized. These findings provide fundamental insights into the optical manipulation of Ln3+‐doped halide DPs, thus laying a foundation for the future design of efficient NIR‐emitting DPs.