Tyler J. Milstein

Ytterbium-doped all-inorganic lead-halide perovskites (Yb³⁺:CsPb(Cl_1-xBr_x)₃) show broadband absorption and exceptionally high near-infrared photoluminescence quantum yields, providing opportunities for solar spectral shaping to improve photovoltaic power conversion efficiencies. Here, we report that Yb³⁺:CsPb(Cl_1-xBr_x)₃ NCs also show extremely strong negative thermal quenching of the Yb³⁺ luminescence, with intensities at room temperature >100 times those at 5 K for some compositions. Analysis of this temperature dependence as a function of x shows that it stems from thermally activated quantum cutting related to the temperature dependence of the spectral overlap between the PL of the perovskite (donor) and the simultaneous-pair absorption of two Yb³⁺ ions (acceptor). In the Yb³⁺:CsPbBr₃ limit, this spectral overlap goes to zero at 5 K, such that only single-Yb³⁺ sensitization requiring massive phonon emission occurs. At room temperature, Yb³⁺ PL in this composition is enhanced ∼135-fold by thermally activated quantum cutting, highlighting the extreme efficiency of quantum cutting relative to single-Yb³⁺ sensitization. These results advance the fundamental mechanistic understanding of quantum cutting in doped perovskites, with potential ramifications for solar and photonics technologies.

CsPb­(Cl1–xBrx)3 (0 ≤ x ≤ 1) nanocrystals and thin films doped with a series of trivalent rare-earth ions (RE3+ = Y3+, La3+, Ce3+, Gd3+, Er3+, Lu3+) have been prepared and studied using variable-temperature and time-resolved photoluminescence spectroscopies. We demonstrate that aliovalent (trivalent) doping of this type universally generates a new and often-emissive defect state ca. 50 meV inside the perovskite band gap, independent of the specific RE3+ dopant identity or of the perovskite form (nanocrystals vs thin films). Chloride-to-bromide anion exchange is used to demonstrate that this near-band-edge photoluminescence shifts with changing band-gap energy to remain just below the excitonic luminescence for all compositions of CsPb­(Cl1–xBrx)3 (0 ≤ x ≤ 1). Computations show that this shift stems from the effect of the changing lattice dielectric constants on a shallow defect-bound exciton. Microscopic descriptions of this dopant-induced near-band-edge state and its relation to quantum cutting in Yb3+-doped CsPb­(Cl1–xBrx)3 are discussed.

Inorganic lead-halide perovskite nanocrystals (NCs) are an exciting class of luminescent materials with high defect tolerance and broad spectral tunability, but such NCs are vulnerable to degradation under ambient conditions. Here, we report a class of modular zwitterion-functionalized isopropyl methacrylate polymers designed to stabilize a wide variety of perovskite NCs of different compositions, while also enabling processing in green solvents. Specifically, we report polymers in which the zwitterion spacing is tuned to accommodate the different lattice parameters of CsPb(Cl1-xBrx)3 and CsPbI3 NCs, and we report partially fluorinated polymers prepared to accommodate the needs of infrared-emitting NCs. We show that as-synthesized CsPbBr3, CsPbI3, and Yb3+:CsPbCl3 NCs are easily transferred into these zwitterionic polymers via a simple ligand-exchange procedure. These NC/polymer composites were then cast into thin films that showed substantially improved photoluminescence (PL) and stability compared with more conventional NC/polymer films. Specifically, CsPbBr3 and CsPbI3 NCs in films of their appropriately designed polymers had PL quantum yields of ~90% and ~80%, respectively. PL quantum yields decreased under continuous illumination, but self-healed completely after dark storage. We also found that all the NC compositions studied here maintain their PL quantum yields in NC/polymer composite films even after 1 year of ambient storage. These encouraging results demonstrate the utility of such modular zwitterion-functionalized polymers for hosting specific perovskite NCs, potentially opening avenues for robust new photonic applications of this important class of NCs.

Quantum-cutting Yb³⁺:CsPb(Cl_1−xBr_x)₃ nanocrystals mitigate thermalization and reabsorption losses in a new monolithic bilayer luminescent solar concentrator device architecture.

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. Here, we demonstrate that Yb3+-doped CsPbCl3 nanocrystals can be converted post-synthetically to Yb3+-doped CsPb(Cl1-xBrx)3 nanocrystals without compromising the desired high PLQYs. Nanocrystal energy gaps can be tuned continuously from Eg ~ 3.06 eV (405 nm) in CsPbCl3 down to Eg ~ 2.53 eV (~490 nm) in CsPb(Cl0.25Br0.75)3 while retaining a constant PLQY above 100%. Reducing Eg further causes a rapid drop in PLQY, interpreted as reflecting an energy threshold for quantum cutting at approximately twice the energy of the Yb3+ 2F7/2 → 2F5/2 absorption threshold. These data demonstrate that very high quantum-cutting energy efficiencies can be achieved in Yb3+-doped CsPb(Cl1-xBrx)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, morphologies, or even reduce the excitonic PLQYs of analogous undoped CsPb(Cl1-xBrx)3 nanocrystals. These results provide valuable information relevant to development and application of these unique materials for spectral-shifting solar energy conversion technologies.