Photophysical Properties of Quantum-Cutting Ytterbium(III)-Doped Perovskite Nanocrystals

Abstract

Ytterbium(III)-doped all-inorganic lead-halide perovskites (Yb3+:CsPb(Cl1-xBrx)3) have recently been discovered to display highly efficient quantum cutting, in which the energy from each ultraviolet photon absorbed by the perovskite is split and emitted as pairs of near-infrared photons by Yb3+ dopants. Experimental photoluminescence quantum yields of nearly 200% have been reported for nanocrystals and thin films. Combining such high photoluminescence quantum yields and strong, broadband absorption of the perovskites, these materials drew extraordinary research interest for their applications in solar energy conversion technologies. This work explores the fundamental origins of quantum cutting in Yb3+:CsPb(Cl1-xBrx)3 nanocrystals and the unique structural and photophysical properties induced by Yb3+ doping. Quantum-cutting energy transfer follows two consecutive steps: (i) sub-picosecond depopulation from the perovskite host to a Yb3+-induced defect state and (ii) energy transfer from the same defect state to two Yb3+ ions simultaneously on the order of nanoseconds. The observation of this second energy transfer step, shown by a rise time of Yb3+ photoluminescence, confirms that quantum-cutting occurs via an intermediate state, which is induced by Yb3+ doping. Anion alloying via anion exchange of these materials allows tuning of the perovskite bandgap, which provides new insights to Yb3+ sensitization. Quantum cutting remains exothermic until the perovskite bandgap reaches a thermodynamic threshold for energy conservation (Yb3+:CsPb(Cl1-xBrx)3, x ~ 0.66), at which the quantum-cutting energy-transfer rate becomes increasingly temperature dependent. This temperature dependent behavior in Yb3+:CsPb(Cl1-xBrx)3 (0.66 < x < 1.00) is accompanied by strong negative thermal quenching of the Yb3+ luminescence. As a result, Yb3+:CsPbBr3 nanocrystals show single-Yb3+ sensitization at cryogenic temperature and thermally-activated quantum cutting near room temperature. Finally, Yb3+ coordination environments in CsPbCl3 and CsPbBr3 lattices, spatial distribution of Yb3+ dopants during anion-exchange reaction, and diverse Yb3+ speciation are discussed in detail. These results enhance the fundamental understanding of this unique quantum cutter with potential ramifications in solar technologies.