AVS 57th International Symposium & Exhibition
    Energy Frontiers Topical Conference Monday Sessions
       Session EN-MoA

Invited Paper EN-MoA1
Beyond the Adiabatic Limit: Charge Separation in Organic Photovoltaic Materials

Monday, October 18, 2010, 2:00 pm, Room Mesilla

Session: Excitonic and Third Generation Solar Cells
Presenter: J.B. Asbury, Penn State University
Authors: R.D. Pensack, Penn State University
J.B. Asbury, Penn State University
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The dynamics of charge separation in photovoltaic polymer blends following photoinduced electron transfer from the conjugated polymer, CN-MEH-PPV, to the electron accepting functionalized fullerene, PCBM, are observed with ultrafast vibrational spectroscopy. The investigators 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 charge transfer (CT) excitons at donor/acceptor interfaces on ultrafast time scales. The data reveal that the rate of dissociation of CT excitons is temperature independent from 200 to 350 K indicating that excess energy in hot CT excitons plays an important role in mediating charge separation. These observations suggest that conceptual and theoretical descriptions properly taking into account the strong coupling of electronic and nuclear degrees of freedom in organic semiconductors are essential to understand the mechanism of charge separation in organic photovoltaic materials. From a practical stand-point, efforts to develop new low band-gap polymers for organic solar cells should target electron donor and acceptor pairs capable of advantageously redistributing excess energy in hot CT excitons to enable efficient charge separation with minimal donor-acceptor energy level offsets.