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Solid-state organic/nanocrystal films for infrared↔visible photon conversion via excitonics

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The ability to efficiently interconvert low-intensity light between the visible and infrared would be an enabling technology—particularly for applications such as 3rd-generation photovoltaics, biological imaging, and cost-effective cameras in the short-wave infrared (SWIR; λ:1‒3μm). Here, we present a novel approach to this problem using multi-excitonic interactions in nanostructured materials. Specifically, we show that two excitonic materials—organic semiconductors and colloidal nanocrystals—can be combined to create passive thin-film devices that achieve broadband, non-coherent down- or up-conversion between the SWIR and the visible.

For photon downconversion, we exploit the phenomenon of singlet exciton fission—the process in organic molecules with large exchange splittings (Etriplet ≤ ½Esinglet) whereby the photogenerated spin-singlet exciton is efficiently (e.g. ηtetracene > 153±5%) converted into a correlated pair of spin-triplet excitons on independent chromophores. We then demonstrate direct energy transfer of these dark excitons from tetracene to emissive colloidal PbS nanocrystals, thereby successfully harnessing molecular triplet excitons in the SWIR . Further, we use steady-state excitation spectra to show that the transfer efficiency is ≥90±13%, while transient photoluminescence and magnetic field-dependent studies show that the mechanism is a Dexter hopping process. [1] To achieve upconversion, we synthetically tune the bandgap of PbS nanocrystals to absorb SWIR photons (λ>1 μm). Photoexcitations then undergo Dexter transfer to sensitize the triplet state in an organic semiconductor (rubrene). Higher-energy (λ612 nm) singlet excitons are generated via diffusion-mediated triplet-triplet annihilation, and the emissive yield is boosted using a molecular dye (DBP) in a guest:host configuration. We achieve an upconversion efficiency of 1.2±0.2% with λ=808 nm excitation at 12 W/cm2. However, transient spectroscopy indicates that there remain opportunities to further improve performance—notably the synthetic challenge to modify the nanocrystal surface to accelerate triplet transfer (τ500 ns) and enhance exciton transport without creating competitive non-radiative channels. [2]

Our hybrid approach to achieve non-coherent upconversion may prove broadly applicable in solar and SWIR -detection applications, where effective molecular phosphors are lacking—indeed, quantum dots are ideal SWIR sensitizers, as their excitonic are functionally spin-mixed at room temperature, and both the optical gap and ionization energy can be tuned via colloidal synthesis.

1. Thompson†, Wilson†, Congreve†, et al. (Bawendi, Baldo), Nature Materials, 13:1039–1043 (2014) See also: Tabachnyk, Ehrler, et al. (Friend, Rao), Nature Materials, 13:1033‒1038 (2014) 2. Wu†, Congreve†, Wilson†, et al. (Bulović, Bawendi, Baldo) Nature Photonics, 10:31‒34 (2016) See also: Huang, et al. (Tang, Bardeen) Nano Letters, 15:5552‒5557 (2015)

This talk is part of the Optoelectronics Group series.

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