Ab-initio study on the exciton binding energies in organic semiconductors

Full text: In the innovative field of organic opto-electronics research a variety of organic semiconductors ranging from polymers to organic molecular crystals consisting of small π-conjugated molecules are tested for their applicability. From the experimental side, this is an elaborate task, which can be supported by using computer simulations in combination. In this work, a theoretical approach to the investigation of the photophysics of such materials is presented. The optical properties including excitonic effects of organic semiconductors is studied within density functional theory. In particular, the imaginary part of the frequency-dependent dielectric tensor has been calculated for several organic molecular crystals and polymers, which comprise prime examples for the versatile class of organic semiconductors. The electron-hole interaction has been included by solving the Bethe-Salpeter equation for the two-particle Green's function. This approach allows for the evaluation of the exciton binding energies in such materials, which are of major interest concerning their technological application in organic optoelectronic devices. The results are compared to experimental data and surveyed together with previously reported theoretical findings. In this context, first the influence of the three dimensional crystal environment versus one dimensional chains is addressed for the polymers. Second, the dependence of the exciton binding energy on the molecular size is studied by comparing the results obtained for the oligomers to those found for the polymers. As last aspect, the influence of the intermolecular interaction on the exciton binding energies is investigated by applying hydrostatic pressure. (author)