AB-INITIO STUDY ON THE EXCITON BINDING ENERGIES IN ORGANIC SEMICONDUCTORS

Abstract

In this work, the optical properties including excitonic effects of organic semiconductors investigated within an ab-initio framework are reviewed. In particular, the imaginary part of the frequency-dependent dielectric tensor has been calculated for several organic molecular crystals consisting of short molecules 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 opto-electronic devices. The results presented herein are compared to experimental data and surveyed together with previously reported theoretical findings, where particular emphasis is placed on analyzing the sensitivity of different approximations applied in the theoretical approaches. 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.