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Correlated ultrafast exciton relaxation in conducting polymers

Recent Research from Professor David Blank and his research group.

Poly-3-hexylthiophene, or P3HT, is one of the most commonly used organic conductors in opto-electronic research. This material is often employed as both a light harvester and hole conductor in the development of new photovoltaic devices. Using ultrafast spectroscopy, graduate student Nathan Wells has been investigating the dynamics that follow light absorption in these molecules. After a photon creates a bound electron-hole pair, called an exciton, both nuclear reorganization and migration to segments of longer conjugation lengths lower the energy of the exciton. One of the interesting parts of this process is that a significant portion of the exciton cooling takes place in less than 100 femtoseconds. Using 2-color 3-photon echo spectroscopy (2C3PES), Nathan has demonstrated that transfer from the initial excitation energy is coherent and correlated to specific energies in the fluorescence band, i.e. the energy does not simply run downhill. This is shown in the graphic below, where the signals are directly related to coherent transfer of energy from the excitation at 2.64 eV to four different energies in the fluorescence band. Note that within the time resolution of the experiment (~30 femtoseconds), coherent transfer has already taken place at 2.30 eV and 2.05 eV, but it is initially zero and builds in over time at 2.18 eV and 1.91 eV. These measurements demonstrate that the initial rapid localization of the exciton is spectrally correlated, and cooling then continues with nuclear motions such as displaced torsions that allow the exciton to sample the rest of the fluorescence band. Understanding the eletronic-nuclear coupling in these organic materials can provide a foundation for understanding how to design a better photovoltaic.

This research was supported by the University of Minnesota Initiative for Renewable Energy and the Environment (IREE) and the David and Lucile Packard Foundation.

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