Anion Exchange and the Quantum-Cutting Energy Threshold in Ytterbium-Doped CsPb(Cl1–xBrx)3 Perovskite Nanocrystals

Colloidal halide perovskite nanocrystals of CsPbCl₃ doped with Yb³⁺ have demonstrated remarkably high sensitized photoluminescence quantum yields (PLQYs), approaching 200%, attributed to a picosecond quantum-cutting process in which one photon absorbed by the nanocrystal generates two photons emitted by the Yb³⁺ dopants. This quantum-cutting process is thought to involve a charge-neutral defect cluster within the nanocrystal's internal volume. We demonstrate that Yb³⁺-doped CsPbCl₃ nanocrystals can be converted postsynthetically to Yb³⁺-doped CsPb(Cl_{1- x}Br _x)₃ nanocrystals without compromising the desired high PLQYs. Nanocrystal energy gaps can be tuned continuously from E_g ≈ 3.06 eV (405 nm) in CsPbCl₃ down to E_g ≈ 2.53 eV (∼490 nm) in CsPb(Cl_0.25Br_0.75)₃ while retaining a constant PLQY above 100%. Reducing E_g further causes a rapid drop in PLQY, interpreted as reflecting an energy threshold for quantum cutting at approximately twice the energy of the Yb³⁺²F_7/2 → ²F_5/2 absorption threshold. These data demonstrate that very high quantum-cutting energy efficiencies can be achieved in Yb³⁺-doped CsPb(Cl_{1- x}Br _x)₃ nanocrystals, offering the possibility to circumvent thermalization losses in conventional solar technologies. The presence of water during anion exchange is found to have a deleterious effect on the Yb³⁺ PLQYs but does not affect the nanocrystal shapes or morphologies, or even reduce the excitonic PLQYs of analogous undoped CsPb(Cl_{1- x}Br _x)₃ nanocrystals. These results provide valuable information relevant to the development and application of these unique materials for spectral-shifting solar energy conversion technologies.