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

Colloidal halide perovskite nanocrystals of CsPbCl 3 doped with Yb 3+ 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 Yb 3+ dopants. This quantum-cutting process is thought to involve a charge-neutral defect cluster within the nanocrystal’s internal volume. We demonstrate that Yb 3+ -doped CsPbCl 3 nanocrystals can be converted postsynthetically to Yb 3+ -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 CsPbCl 3 down to E g ≈ 2.53 eV (∼490 nm) in CsPb(Cl 0.25 Br 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 Yb 3+ 2 F 7/2 → 2 F 5/2 absorption threshold. These data demonstrate that very high quantum-cutting energy efficiencies can be achieved in Yb 3+ -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 Yb 3+ 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.