Shedding Light on Host-to-Yb 3+ Energy Transfer in Cs 2 AgBiBr 6 :Yb 3+ (nano)crystals.

The optical properties of Cs 2 AgBiBr 6 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 Cs 2 AgBiBr 6 . Incorporating Yb 3+ 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 3+ in Cs 2 AgBiBr 6 nano- and microcrystals. We observe that the incorporation of Yb 3+ in the host lattice does not strongly affect the broadband red emission of the Cs 2 AgBiBr 6 host but does give rise to an additional and characteristic ∼1000 nm NIR line emission from Yb 3+ . Temperature-dependent and time-resolved photoluminescence studies of undoped and Yb-doped Cs 2 AgBiBr 6 reveal that the energy transfer does not take place through the red emissive state of the Cs 2 AgBiBr 6 host. Instead, there is a competition between relaxation to the red-emitting state and trapping of the photoexcited charge carriers on Yb 3+ . Trapping on Yb 3+ subsequently results in a charge transfer state that relaxes to the 2 F 5/2 excited state of Yb 3+ , followed by NIR narrow line f-f emission to the 2 F 7/2 ground state.