Unveiling Local Electronic Structure of Lanthanide‐Doped Cs(2)NaInCl(6) Double Perovskites for Realizing Efficient Near‐Infrared Luminescence

Lanthanide ion (Ln(3+))‐doped halide double perovskites (DPs) have evoked tremendous interest due to their unique optical properties. However, Ln(3+) ions in these DPs still suffer from weak emissions due to their parity‐forbidden 4f–4f electronic transitions. Herein, the local electronic structure of Ln(3+)‐doped Cs(2)NaInCl(6) DPs is unveiled. Benefiting from the localized electrons of [YbCl(6)](3−) octahedron in Cs(2)NaInCl(6) DPs, an efficient strategy of Cl(−)‐Yb(3+) charge transfer sensitization is proposed to obtain intense near‐infrared (NIR) luminescence of Ln(3+). NIR photoluminescence (PL) quantum yield (QY) up to 39.4% of Yb(3+) in Cs(2)NaInCl(6) is achieved, which is more than three orders of magnitude higher than that (0.1%) in the well‐established Cs(2)AgInCl(6) via conventional self‐trapped excitons sensitization. Density functional theory calculation and Bader charge analysis indicate that the [YbCl(6)](3−) octahedron is strongly localized in Cs(2)NaInCl(6):Yb(3+), which facilitates the Cl(−)‐Yb(3+) charge transfer process. The Cl(−)‐Yb(3+) charge transfer sensitization mechanism in Cs(2)NaInCl(6):Yb(3+) is further verified by temperature‐dependent steady‐state and transient PL spectra. Furthermore, efficient NIR emission of Er(3+) with the NIR PLQY of 7.9% via the Cl(−)‐Yb(3+) charge transfer sensitization is realized. These findings provide fundamental insights into the optical manipulation of Ln(3+)‐doped halide DPs, thus laying a foundation for the future design of efficient NIR‐emitting DPs.