Unveiling Local Electronic Structure of Lanthanide-Doped Cs2NaInCl6 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 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 Ln(3+). 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 Ln(3+)-doped halide DPs, thus laying a foundation for the future design of efficient NIR-emitting DPs.