Tuning the Quantum Dot (QD)/Mediator Interface for Optimal Efficiency of QD-Sensitized Near-Infrared-to-Visible Photon Upconversion Systems

Lead
sulfide (PbS) quantum dots (QDs) have shown promising performance
as a sensitizer in infrared-to-visible photon upconversion systems.
To investigate the key design rules, we compare three PbS-sensitized
upconversion systems using three mediator molecules with the same
tetracene triplet acceptor at different distances from the QD. Using
transient absorption spectroscopy, we directly measure the triplet
energy-transfer rates and efficiencies from the QD to the mediator
and from the mediator to the emitter. With increasing distance between
the mediator and PbS QD, the efficiency of the first triplet energy
transfer from the QD to the mediator decreases because of a decrease
in the rate of this triplet energy-transfer step, while the efficiency
of the second triplet energy transfer from the mediator to the emitter
increases because of a reduction in the QD-induced mediator triplet
state decay. The latter effect is a result of the slow rate constant
of the second triplet energy-transfer process, which is 3 orders of
magnitude slower than the diffusion-limited value. The combined results
lead to a net decrease of the steady-state upconversion quantum yield
with distance, which could be predicted by our kinetic model. Our
result shows that the QD/mediator interface affects both the first
and second triplet energy transfer processes in the photon upconversion
system, and the QD/mediator distance has an opposite effect on the
efficiencies of the first and second triplet energy transfer. These
findings provide important insight for the further rational improvement
of the overall efficiency of QD-based upconversion systems.