Preparation And Near-infrared Luminescent Properties Of Lanthanide Ions Doped Cs₂AgBiCl₆ Double Perovskites

All-inorganic halide double perovskites have shown great prospects for applications in solar cells and light-emitting diodes（LED）due to their high photoluminescence quantum yield（PLQY）,facile synthesis methods,and flexibility of compositions.Although significant advances have been accomplished,their studies were mainly restricted to the visible spectral region.As such,intensive efforts have been devoted to expanding the spectral region of all-inorganic halide double perovskite materials towards the near-infrared（NIR）region,which allows for extended applications.Due to the NIR luminescence properties of some lanthanide ions(Ln³⁺),doping Ln³⁺into inorganic halide double perovskites has been proposed to tune the optical properties.Ln³⁺ions tend to be doped in sites with coordination number of 6,while double perovskites have the octahedral coordination environment（coordination number=6）,which makes all-inorganic halide double perovskite materials become ideal matrix for Ln³⁺doping.Cs₂AgBiCl₆DPs,as a kind of promising environmentally-friendly alternatives to Pb-halide perovskites,have been rarely explored for their NIR luminescence due to their indirect bandgap nature.Herein,we report the local structure engineering of Ln³⁺-doped Cs₂AgBiCl₆ double perovskites via Na⁺doping to achieve efficient NIR emission of Yb³⁺and Er³⁺.The optical properties of Ln³⁺-doped Cs₂（Na/Ag）Bi Cl₆ phosphors were systematically studied by steady-state transient fluorescence spectra to determine the optimal Na⁺content.Excited-state dynamics and the energy transfer process in the optimal Ln³⁺-doped Cs₂Ag_0.2Na_0.8Bi Cl₆ phosphors were comprehensively surveyed by means of temperature-dependent steady-state and transient photoluminescent spectroscopy.Through Raman spectroscopy analysis and first-principles density functional theory（DFT）calculations have confirmed that Na/Ag alloying caused the change of Bi-Cl bond length,which leads to the local symmetry breaking of Bi³⁺in[Bi Cl₆]^3-octahedron.Compared with the Na-free Cs₂AgBiCl₆ counterparts,the NIR emission of Yb³⁺and Er³⁺can be boosted by7.3-fold and 362.9-fold in Cs₂Ag_0.2Na_0.8Bi Cl₆DPs,with the optimal PLQYs of 19.0%and4.3%,respectively.These Ln³⁺-doped NIR-emitting Cs₂Ag_0.2Na_0.8Bi Cl₆ DPs were then employed for 365-nm UV-converted NIR LEDs,which indicated their potential applications as high-performance optoelectronic devices.These findings provide a new insight for the design of efficient NIR luminescent Ln³⁺doped all-inorganic halide double perovskites materials,which may accelerate the development of new near-infrared optoelectronic devices.