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<div align="center"><font face="Times New Roman"
color="#000000"><b>Seminar on</b></font></div>
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<div align="center"><font face="Times New Roman"
color="#000000"><b>Modern Optics and Spectroscopy</b></font></div>
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<div align="center"><font face="Arial" color="#000000"><b>Pathways to
more efficient Third-Generation Photovoltaics</b></font></div>
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<div align="center"><font face="Times New Roman"
color="#000000"><b>John Asbury</b>, Penn State University</font></div>
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<div align="center"><font face="Times New Roman"
color="#000000">Tuesday, November 3, 2009</font></div>
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<div align="center"><font face="Times New Roman" color="#000000">12:00
- 1:00 p.m.</font></div>
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<div align="center"><font face="Times New Roman"
color="#000000">Organic solar cells are promising candidates for
inexpensive third generation photovoltaics for large area applications
because they can be processed from solution using role-to-role
technology. The efficiencies of current organic solar cells are
limited by partial overlap with the solar spectrum and sub-optimal
open-circuit voltage characteristics. Efforts to extend the
absorption spectrum of organic solar cells into the near-infrared have
produced many promising low band-gap polymers, but enhancements in
device efficiency have been incremental. New understanding of
the photophysics of these low band-gap polymers suggests that the
efficiency of the corresponding devices is limited by incomplete
charge separation at electron donor/acceptor interfaces. To
elucidate the origin of this limitation, we undertook a study of the
dynamics of charge separation in a photovoltaic polymer blend
consisting of the conjugated polymer, CN-MEH-PPV, blended with the
electron accepting functionalized fullerene, PCBM, using ultrafast
vibrational spectroscopy. We take advantage of a solvatochromic
shift of the vibrational frequency of the carbonyl (C=O) stretch of
PCBM to directly measure the rate of escape of electrons from their
Coulombically bound radical pairs. Our findings demonstrate that
the rate of free carrier formation is temperature independent
indicating that excess vibrational energy resulting from the electron
transfer reaction plays an important role in mediating charge
separation. These observations suggest that efforts to develop
new low band-gap polymers for organic solar cells should target
electron donor and acceptor pairs capable of advantageously
redistributing excess vibrational energy to efficiently separate
charge with minimal donor-acceptor energy level offsets.</font></div>
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<div align="center"><font face="Times New Roman" color="#000000">Grier
Room, MIT Bldg 34-401</font></div>
<div align="center"><font face="Times New Roman"
color="#000000">Refreshments served after the lecture</font></div>
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