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Center for Excitonics Seminar Series
Tuesday, Dec. 13, 2011
3:00 PM - 4:00 PM
EECS CONFERENCE ROOM: 34-401A
"Singlet Exciton Fission in Polyacenes: Photophysics and Photovoltaic
Applications"
Mark Wilson, Optoelectronics Group, University of Cambridge
abstract
The development of novel technologies for harvesting solar energy is a major
contemporary research effort in the physical sciences. However, the
efficiency of any single-bandgap photovoltaic device under solar irradiation
has a fundamental limit because sub-bandgap photons are not absorbed and the
excess energy of super-bandgap photons is wasted as heat. An attractive
method to circumvent this limit is to sensitize a 'red-absorbing' solar cell
with a 'blue-absorbing' material which generates multiple electron-hole
pairs. This is possible in some organic semiconductors via 'singlet
fission', where a spin-singlet bound electron-hole pair (exciton) 'splits'
to form two triplet excitons, each with roughly half of the singlet energy.
Although singlet fission has been historically observed in molecular
crystals, we recently used transient absorption spectroscopy to demonstrate
that it occurs rapidly (~70 fs) and efficiently (>85%) in easily-fabricated
evaporated films of pentacene and that fission-generated triplet excitons
undergo long-range diffusion (>40nm) and are dissociated at a pentacene/C60
heterointerface. These results are consistent with reported
photon-to-electron quantum efficiencies that exceed 100% and have led us to
fabricate a proof-of-concept photovoltaic device where an evaporated
pentacene film absorbs visible light and generates pairs of triplets via
fission, while a second layer of inorganic colloidal quantum dots generates
charge from transferred triplets as well as directly-absorbed infrared
photons. Further transient absorption measurements address the mechanism of
exciton fission, as questions remain as to whether singlet fission is also
rapid and efficient larger-bandgap acenes and whether fission is mediated by
a 'dark' (one-photon inaccessible) multiexcitonic state.
Bio
Hailing from Port Colborne, Canada, Mark received a B.A. (History) (2008),
and a B.Sc. (2006) and M.Sc. (2008) in Engineering Physics from Queen's
University, Kingston. Working with Prof. James Fraser, his thesis concerned
the ultrafast dynamics of photoluminescence from individual, air-suspended,
single-walled carbon nanotubes. He is presently working towards a Ph.D. in
Physics under the supervision of Sir Richard Friend at the University of
Cambridge's Cavendish Laboratory.
The Center for Excitonics is an Energy Frontier Research Center funded by
the U.S. Department of Energy, Office of Science and Office of Basic Energy
Sciences
Light refreshments will be served
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