Dresden 2009 – wissenschaftliches Programm

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SYOP: Symposium Organic Photovoltaics: From Single Molecules to Devices

SYOP 5: Organic Photovoltaics III

SYOP 5.9: Vortrag

Freitag, 27. März 2009, 12:30–12:45, ZEU 222

Excitons in Conjugated Polymers - Do we need a Paradigma Change — •Wichard J. D. Beenken — Technische Universität Ilmenau, Institut für Physik, FG Theoretische Physik I, Germany

We have previously shown that both, polymer conformation and dynamics are crucial for the exciton transport in conjugated polymers. Thereby we found that the usual Förster-type hopping transfer model – even if one applies the line-dipole approximation – fall short in two respects: (i) It is still unclear of what kind the site the excitons are transferred between is. We found that the simple model of spectroscopic units defined as segments of the polymer chains separated by structural defects breaking the π-conjugation is only justified for chemical defects like hydrogenated double bonds, or extreme gauche (90^∘) torsions between the monomers. Both defects are far too rare in a well prepared conjugated polymer to explain the mean spectroscopic unit length of typically 6-7 monomers. In the meanwhile, also the concept of dynamical formation of the spectroscopic units, we had previously suggested, has turned out to fail. (ii) For the ultra-fast anisotropy decay observed in films of polythiophenes, the Förster-rates are far too low. Therefore, the mechanisms resulting in initial turns of the transition-dipoles have to be different from the excitation energy transfer valid on longer time-scales. In the present talk several possible ways out of these problems will be pinpointed. It will turn out that these considerations may lead to simplified concepts for the design of polymer-based organic solar cell, with possibly higher efficiencies and longer life-times than the common polymer-fullerene blends.