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978-3-8439-1368-3, Reihe Physik
Sarah Maria Falke
Primary Processes of the Light-to-Current Conversion in Organic Photovoltaic Materials
161 Seiten, Dissertation Carl von Ossietzky Universität Oldenburg (2013), Hardcover, A5
In organic photovoltaic devices conversion of light into electrical energy happens on the femtosecond timescale and is thought to involve the incoherent jump of an electron from the optical absorber to the electron acceptor. Here we investigate the primary process of the light-to-current conversion in both a supramolecular triad, taken as a prototypical elementary component for an organic solar cell, and a technologically more relevant system: a polymer:fullerene blend. Combined results obtained with high time resolution femtosecond spectroscopy and first-principles quantum dynamics simulations provide compelling evidence that the driving mechanism of the photoinduced intramolecular charge transfer in a reduced supramolecular carotene-porphyrin-fullerene triad is a correlated wavelike motion of electrons and nuclei on a timescale of few tens of femtoseconds.
We furthermore highlight the fundamental role of dynamical geometric rearrangement, in particular in the donor/acceptor linker region, to stabilize the charge accumulation on the acceptor. Ultrafast pump-probe spectroscopic studies of blends of the conjugated polymer P3HT and the fullerene derivative PCBM demonstrate coherent vibrational motion of the fullerene upon impulsive photoexcitation of the polymer. This novel observation, which is absent in control experiments with the individual blend components, implies that the initial excitation of an electron-hole pair in the polymer moiety is either followed by an extremely rapid interfacial charge transfer to the fullerene moiety or a strongly vibrationally coupled electron transfer mechanism with concerted motion of strongly coupled electrons and ions drives the primary intermolecular charge separation (analogously to the results obtained for the reduced supramolecular photovoltaic system). Our results show that the common static approach based on molecular level alignment and reaction-rate theories is not sufficient to describe the microscopic mechanism underlying the primary photoinduced charge separation dynamics of the light-to-current conversion in organic photovoltaic materials.
Instead a coherent dynamical picture emerges as the correct physical framework for the splitting of the photogenerated electron-hole pair.