[MOS] November 3, 2009

[Zina Queen]

Seminar on

Modern Optics and Spectroscopy

Pathways to more efficient Third-Generation Photovoltaics

John Asbury, Penn State University

Tuesday, November 3, 2009

12:00 - 1:00 p.m.

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.


Grier Room, MIT Bldg 34-401
Refreshments served after the lecture
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