Boosting Near‐Infrared Luminescence of Lanthanide in Cs2AgBiCl6 Double Perovskites via Breakdown of the Local Site Symmetry

Abstract Currently, lanthanide (Ln 3+ )‐doped near‐infrared (NIR)‐emitting double perovskites (DPs) suffer from low photoluminescence quantum yield (PLQY). Herein, we develop a new class of NIR‐emitting DPs based on Ln 3+ ‐doped Cs 2 (Na/Ag)BiCl 6 . Benefiting from the Na + ‐induced breakdown of local site symmetry in the Cs 2 AgBiCl 6 DPs, effective NIR emissions of Ln 3+ are realized through Bi 3+ sensitization. Specifically, 7.3‐fold and 362.9‐fold enhanced NIR emissions of Yb 3+ and Er 3+ are achieved in Cs 2 Ag 0.2 Na 0.8 BiCl 6 DPs relative to those in Na‐free Cs 2 AgBiCl 6 counterparts, respectively. The optimal absolute NIR PLQYs for Yb 3+ and Er 3+ in Cs 2 Ag 0.2 Na 0.8 BiCl 6 DPs are determined to be 19.0 % and 4.3 %, respectively. Raman spectroscopy and first‐principles density functional theory calculations verify the sublattice distortion in Cs 2 (Na/Ag)BiCl 6 DPs via Na + doping. These findings provide fundamental insights into the design of efficient NIR‐emitting Ln 3+ ‐doped DPs for versatile optoelectronic applications.