Unveiling Local Electronic Structure of Lanthanide-Doped Cs₂ NaInCl₆ Double Perovskites for Realizing Efficient...

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 Cs₂ NaInCl₆ DPs is unveiled. Benefiting from the localized electrons of [YbCl₆ ]^3- octahedron in Cs₂ NaInCl₆ 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 Cs₂ NaInCl₆ is achieved, which is more than three orders of magnitude higher than that (0.1%) in the well-established Cs₂ AgInCl₆ via conventional self-trapped excitons sensitization. Density functional theory calculation and Bader charge analysis indicate that the [YbCl₆ ]^3- octahedron is strongly localized in Cs₂ NaInCl₆ :Yb³⁺ , which facilitates the Cl^- -Yb³⁺ charge transfer process. The Cl^- -Yb³⁺ charge transfer sensitization mechanism in Cs₂ NaInCl₆ :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.