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Center for Excitonics
Seminar Series Announcement
Tuesday, April 5, 2011
3:00 PM
RLE Conference Room: 36-428
Charge separation by photoexcitation in semicrystalline polymeric
semiconductors: An intrinsic or extrinsic mechanism?
Carlos Silva, Université de Montréal
Abstract:
Understanding charge generation by light absorption in polymeric
semiconductors is of profound scientific importance due to the vigorous
drive to develop organic solar cells. Confusion prevails with respect to the
intrinsic charge photogeneration mechanism in neat (undoped) semicrystalline
films. Numerous publications report charge photogeneration yields (the
number of electron-hole pairs produced per absorbed photon) up to 30% on
sub-picosecond timescales in neat regioregular poly(3-hexylthiophene) films.
This is difficult to reconcile with the accepted picture that Frenkel
excitons are the primary photoexcitations. Their binding energy is much
higher than the lattice thermal energy at room temperature, such that direct
charge generation ought to be improbable. Considering this, two fundamental
questions arise: (i) what is the mechanism of direct charge photogeneration
in semicrystalline polymer semiconductors? (ii) What is the role of
solid-state microstructure in defining it? Here, we combine transient
photoluminescence and absorption probes and find that charge photogeneration
at 10 K occurs continuously over sub-nanosecond timescales, and not by a
diffusion-limited exciton dissociation at defect sites. Rather, we conclude
that it is an extrinsic process that occurs efficiently by dissociation of
excitons localised at interfaces between crystalline and non-crystalline
domains, and is driven by interfacial energetic disorder.
Bio:
Carlos Silva, Canada Research Chair in Organic Semiconductor Materials, is
Associate Professor of Physics at the Université de Montréal, having joined
the department in 2005. He is the 2010 laureate of the Herzberg Medal of the
Canadian Association of Physicists. He has extensive expertise in ultrafast
optical probes of electronic dynamics in organic semiconductors with
applications in optoelectronics. He obtained a PhD in chemical physics from
the University of Minnesota in 1998 and was a Postdoctoral Research
Associate in the Cavendish Laboratory of the University of Cambridge from
1998 to 2001. In 2001 he obtained an Advanced Research Fellowship from the
UK Engineering and Physical Sciences Research Council, which he undertook at
the Cavendish Laboratory. The central theme of his research programme
concerns the understanding of electronic dynamics in organic semiconductors
using transient photoluminescence and absorption spectroscopies.
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