Shedding Light on Host-to-Yb Energy Transfer in CsAgBiBr:Yb (nano)crystals.

The optical properties of CsAgBiBr 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 CsAgBiBr. Incorporating Yb 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 Yb in CsAgBiBr nano- and microcrystals. We observe that the incorporation of Yb in the host lattice does not strongly affect the broadband red emission of the CsAgBiBr host but does give rise to an additional and characteristic ∼1000 nm NIR line emission from Yb. Temperature-dependent and time-resolved photoluminescence studies of undoped and Yb-doped CsAgBiBr reveal that the energy transfer does not take place through the red emissive state of the CsAgBiBr host. Instead, there is a competition between relaxation to the red-emitting state and trapping of the photoexcited charge carriers on Yb. Trapping on Yb subsequently results in a charge transfer state that relaxes to the F excited state of Yb, followed by NIR narrow line f-f emission to the F ground state.