Shedding Light on Host-to-Yb³⁺ Energy Transfer in Cs₂AgBiBr₆:Yb³⁺ (nano)crystals

The optical properties of Cs₂AgBiBr₆ double perovskite nanocrystals have attracted considerable attention as lead-free alternatives to lead halide perovskites. A promising strategy to create additional flexibility in the emission color is doping lanthanide ions into Cs₂AgBiBr₆. Incorporating Yb³⁺ in the lattice has been shown to give rise to near-infrared (NIR) emission, but the energy transfer mechanism remained unclear. Here, we report on the luminescence and sensitization mechanism of Yb³⁺ in Cs₂AgBiBr₆ nano- and microcrystals. We observe that the incorporation of Yb³⁺ in the host lattice does not strongly affect the broadband red emission of the Cs₂AgBiBr₆ host but does give rise to an additional and characteristic ∼1000 nm NIR line emission from Yb³⁺. Temperature-dependent and time-resolved photoluminescence studies of undoped and Yb-doped Cs₂AgBiBr₆ reveal that the energy transfer does not take place through the red emissive state of the Cs₂AgBiBr₆ host. Instead, there is a competition between relaxation to the red-emitting state and trapping of the photoexcited charge carriers on Yb³⁺. Trapping on Yb³⁺ subsequently results in a charge transfer state that relaxes to the ²F_5/2 excited state of Yb³⁺, followed by NIR narrow line f-f emission to the ²F_7/2 ground state.