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Author: Naumann, Marco × Type: Text ×

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    New charge-transfer states in blends of ZnPC with F8ZnPC
    (College Park, ML : American Institute of Physics, 2021) Graf, Lukas; Ortstein, Katrin; Doctor, Louis P.; Naumann, Marco; Beyer, Jan; Heitmann, Johannes; Leo, Karl; Knupfer, Martin
    With the aim of pushing the knowledge and understanding on mixed films of organic semiconductors forward, blends of ZnPC and F8ZnPC in different ratios are manufactured. The films have a polycrystalline structure, as indicated by electron diffraction profiles and infrared-spectroscopy. Photoluminescence data show completely different spectra for the blends, compared to the pure materials, which can be ascribed to the suppressing of excimer formation and the appearance of a new charge-transfer excitation between the two different molecules in the blends. This new excitation can also be seen in optical absorption. Momentum dependent measurements of the electronic excitations by electron energy-loss spectroscopy confirm the localized character of the new charge-transfer excitation in the blends. Our experimental data help understand the important issue of donor/acceptor coupling in organic semiconductors.
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    Optical Anisotropy and Momentum-Dependent Excitons in Dibenzopentacene Single Crystals
    (Washington, DC : ACS Publications, 2022) Graf, Lukas; Liu, Fupin; Naumann, Marco; Roth, Friedrich; Debnath, Bipasha; Büchner, Bernd; Krupskaya, Yulia; Popov, Alexey A.; Knupfer, Martin
    High-quality single crystals of the organic semiconductor (1,2;8,9)-dibenzopentacene were grown via physical vapor transport. The crystal structure─unknown before─was determined by single-crystal X-ray diffraction; polarization-dependent optical absorption measurements display a large anisotropy in the ac plane of the crystals. The overall Davydov splitting is ∼110 meV, which is slightly lower than that in the close relative pentacene (120 meV). Momentum-dependent electron energy-loss spectroscopy measurements show a clear exciton dispersion of the Davydov components. An analysis of the dispersion using a simple 1D model indicates smaller electron- and hole-transfer integrals in dibenzopentacene as compared to pentacene. The spectral weight distribution of the excitation spectra is strongly momentum-dependent and demonstrates a strong momentum-dependent admixture of Frenkel excitons, charge-transfer excitons, and vibrational modes.