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.