Abstract
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.