Boosting near-infrared luminescence of lanthanide in Cs${}_2$AgBiCl${}_6$ double perovskites via breakdown of the local site symmetry

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. (© 2022 Wiley‐VCH GmbH)