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SUMMARY:Solid-state organic/nanocrystal films for infrared↔visible photo
 n conversion via excitonics - Professor Mark Wilson\, University of Toront
 o
DTSTART:20160603T133000Z
DTEND:20160603T143000Z
UID:TALK66073@talks.cam.ac.uk
CONTACT:Stuart Higgins
DESCRIPTION:The ability to efficiently interconvert low-intensity light be
 tween the visible and infrared would be an enabling technology—particula
 rly for applications such as 3rd-generation photovoltaics\, biological ima
 ging\, and cost-effective cameras in the short-wave infrared (SWIR\; λ:1
 ‒3μm). Here\, we present a novel approach to this problem using multi-e
 xcitonic interactions in nanostructured materials. Specifically\, we show 
 that two excitonic materials—organic semiconductors and colloidal nanocr
 ystals—can be combined to create passive thin-film devices that achieve 
 broadband\, non-coherent down- or up-conversion between the SWIR and the v
 isible.\n\nFor photon downconversion\, we exploit the phenomenon of single
 t exciton fission—the process in organic molecules with large exchange s
 plittings (Etriplet ≤ ½Esinglet) whereby the photogenerated spin-single
 t exciton is efficiently (e.g. ηtetracene > 153±5%) converted into a cor
 related 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 m
 olecular triplet excitons in the SWIR. Further\, we use steady-state excit
 ation spectra to show that the transfer efficiency is ≥90±13%\, while t
 ransient photoluminescence and magnetic field-dependent studies show that 
 the mechanism is a Dexter hopping process. [1]\nTo achieve upconversion\, 
 we synthetically tune the bandgap of PbS nanocrystals to absorb SWIR photo
 ns (λ>1 μm). Photoexcitations then undergo Dexter transfer to sensitize 
 the triplet state in an organic semiconductor (rubrene). Higher-energy (λ
 ~612 nm) singlet excitons are generated via diffusion-mediated triplet-tri
 plet annihilation\, and the emissive yield is boosted using a molecular dy
 e (DBP) in a guest:host configuration. We achieve an upconversion efficien
 cy 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 s
 urface to accelerate triplet transfer (τ~500 ns) and enhance exciton tran
 sport without creating competitive non-radiative channels. [2]\n\nOur hybr
 id approach to achieve non-coherent upconversion may prove broadly applica
 ble in solar and SWIR-detection applications\, where effective molecular p
 hosphors are lacking—indeed\, quantum dots are ideal SWIR sensitizers\, 
 as their excitonic are functionally spin-mixed at room temperature\, and b
 oth the optical gap and ionization energy can be tuned via colloidal synth
 esis.\n\n1. Thompson†\, Wilson†\, Congreve†\, et al. (Bawendi\, Bald
 o)\, Nature Materials\, 13:1039–1043 (2014) See also: Tabachnyk\, Ehrler
 \, et al. (Friend\, Rao)\, Nature Materials\, 13:1033‒1038 (2014)\n2. Wu
 †\, Congreve†\, Wilson†\, et al. (Bulović\, Bawendi\, Baldo) Nature
  Photonics\, 10:31‒34 (2016) See also: Huang\, et al. (Tang\, Bardeen) N
 ano Letters\, 15:5552‒5557 (2015)
LOCATION:Kapitza Building Seminar Room\, Cavendish Laboratory\, Department
  of Physics
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