Recent advances in the ytterbium doping of CsPbX (X = Cl or Cl/Br) nanocrystals have presented exciting new opportunities for their application as downconverters in solar-energy-conversion technologies. Here, we describe a hot-injection synthesis of Yb:CsPbCl nanocrystals that reproducibly yields sensitized YbF → F luminescence with near-infrared photoluminescence quantum yields (PLQYs) well over 100% and almost no excitonic luminescence. Near-infrared PLQYs of 170% have been measured. Through a combination of synthesis, variable-temperature photoluminescence spectroscopy, and transient-absorption and time-resolved photoluminescence spectroscopies, we show that the formation of shallow Yb-induced defects play a critical role in facilitating a picosecond nonradiative energy-transfer process that de-excites the photoexcited nanocrystal and simultaneously excites two Yb dopant ions, i.e., quantum cutting. Energy transfer is very efficient at all temperatures between 5 K and room temperature but only grows more efficient as the temperature is elevated in this range. Our results provide insights into the microscopic mechanism behind the extremely efficient sensitization of Yb luminescence in CsPbX nanocrystals, with ramifications for future applications of high-efficiency spectral-conversion nanomaterials in solar technologies.
A two-step
solution-deposition method for preparing ytterbium-doped
(Yb3+) CsPb(Cl1–x
Br
x
)3 perovskite thin films is described.
Yb3+-doped CsPb(Cl1–x
Br
x
)3 films are made that
exhibit intense near-infrared photoluminescence with extremely high
quantum yields reaching over 190%, stemming from efficient quantum
cutting that generates two emitted near-infrared photons for each
absorbed visible photon. The near-infrared Yb3+
f–f photoluminescence is largely
independent of the anion content (x) in CsPb(Cl1–x
Br
x
)3 films with energy gaps above the quantum-cutting threshold
of twice the Yb3+
f–f transition energy, but it decreases abruptly when the perovskite
energy gap becomes too small to generate two Yb3+ excitations.
Excitation power dependence measurements show facile saturation of
the Yb3+ luminescence intensity, identifying a major challenge
for future solar applications of these materials.
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(Cl1–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(Cl0.25Br0.75)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+
2F7/2 → 2F5/2 absorption threshold. These data demonstrate that
very high quantum-cutting energy efficiencies can be achieved in Yb3+-doped CsPb(Cl1–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(Cl1–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.
Quantum-cutting Yb3+:CsPb(Cl1−xBrx)3 nanocrystals mitigate thermalization and reabsorption losses in a new monolithic bilayer luminescent solar concentrator device architecture.
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