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Mark Wilson
University of Toronto

Abstract

The ability to efficiently convert low-intensity light between the visible and infrared would be an enabling technology—particularly for applications such as 3rd-generation photovoltaics, biological imaging, and sensitized silicon focal plane array detectors with cost-effective response in the short-wave infrared (SWIR; λ:1‒3µm). Accordingly, we are advancing our new approach that combines two excitonic materials—conjugated organic molecules and colloidal quantum dots—to achieve broadband, non-coherent photon up-conversion from the SWIR to the visible. 

To achieve upconversion, we synthesize lead sulfide nanocrystals that can absorb infrared photons, and fabricate devices where these excitations sensitize the spin-triplet excited state of a nearby organic semiconductor (e.g. rubrene). In the molecules, strong exchange-splitting allows the excitonic energy to be combined via triplet fusion to create visible light. To-date, in solid state devices, we have observed upconversion from light at the threshold of the SWIR (λ=1.1μm→612nm),1 and have achieved upconversion efficiencies of 7±1% with λ=808 nm excitation by using ligand engineering to optimize Dexter-mediated nanocrystal→molecule energy transfer.2

However, although colloidal quantum dots are attractive SWIR sensitizers— their optical gap can be tuned during synthesis, and their photoexcitations are functionally spin-mixed at room temperature—there is much more to be done. For instance, while transient spectroscopy shows that our SWIR upconversion approach could be maximally efficient with only 1/10th the intensity of natural sunlight1, poor exciton transport in the nanocrystal films—due in part to energetic disorder resulting from size-dispersity and quantum confinement—presently hinders light gathering and hampers low-intensity performance. Here, our recent work has uncovered that a pre-nucleation cluster intermediate has historically frustrated efforts to synthesis low-dispersity ensembles of small (⌀<4 nm) PbS nanocrystals, and showed that process additives can enable one-step growth and yield markedly narrower heterogeneous linewidths.3 Further, our recent experiments with a novel molecular dimer in solution suggest that asymmetric coupling between triplet-pair spin states can allow the overall-quintet state to play a beneficial role in the kinetic scheme of triplet fusion, ultimately lifting the expected spin-statistical efficiency limit from 25% to 66%,4 and offering a route to all-in-one free-floating upconverting fluorophores for imaging.

References

1. Wu, M.; Congreve, D. N.; Wilson, M. W. B.; Jean, J.; Geva, N.; Welborn, M.; Van Voorhis, T.; Bulović, V.; Bawendi, M. G.; Baldo, M. A. Solid-State Infrared-to-Visible Upconversion Sensitized by Colloidal Nanocrystals. Nature Photonics 2016, 10 (1), 31–34.
    
2. Nienhaus, L.; Wu, M.; Geva, N.; Shepherd, J. J.; Wilson, M. W. B.; Bulović, V.; Van Voorhis, T.; Baldo, M. A.; Bawendi, M. G. Speed Limit for Triplet-Exciton Transfer in Solid-State PbS Nanocrystal-Sensitized Photon Upconversion. ACS Nano 2017, 11 (8), 7848–7857.
    
3. Green, P. B.; Narayanan, P.; Li, Z.; Sohn, P.; Imperiale, C. J.; Wilson, M. W. B. Controlling Intermediates in the Synthesis of PbS Nanocrystals. Submitted 2020.
    
4. Imperiale, C. J.; Green, P. B.; Miller, E. G.; Damrauer, N. H.; Wilson, M. W. B. Triplet-Fusion Upconversion Using a Rigid Tetracene Homodimer. The Journal of Physical Chemistry Letters 2019, 7463–7469.