Shedding Light on Host-to-Yb<SUP>3+</SUP> Energy Transfer in Cs₂AgBiBr₆:Yb<SUP>3+</SUP> (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 2F5/2 excited state of Yb3+, followed by NIR narrow line f–f emission to the 2F7/2 ground state.