Dresden 2014 – wissenschaftliches Programm
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CPP: Fachverband Chemische Physik und Polymerphysik
CPP 19: Poster Session 1 (joint session with BP)
CPP 19.34: Poster
Dienstag, 1. April 2014, 09:30–13:00, P1
Electronic excitations in push-pull oligomers and their complexes with fullerene — •Björn Baumeier1, Michael Rohlfing2, and Denis Andrienko1 — 1Max-Planck-Institut für Polymerforschung, Ackermannweg 10, 55128 Mainz — 2Institut für Festkörpertheorie, Westfälische Wilhelms-Universität Münster, Wilhelm-Klemm-Str. 10, 48149 Münster
We present a comparative study of excited states in push-pull oligomers of PCPDTBT and PSBTBT and prototypical complexes with a C60 acceptor using GW-BSE. We analyze oligomers up to a length of 5 nm and find that, for both materials, the absorption energy saturates for structures larger than two repeat units due to the localized nature of the excitation. In the bimolecular complexes with C60, the Frenkel to CT transition is exothermic and we observe a strong influence of the acceptor’s position and orientation on the CT energy. Their binding energy is still of the order of 2 eV resulting from the lack of an explicit molecular environment. Embedding the the donor-acceptor complex into a polarizable lattice leads to an energetic stabilization of the CT exciton by about 0.5 eV, while its binding energy is reduced to about 0.3 eV. We also identify a CT state with a more delocalized hole at higher energy, which opens another potential pathway for charge separation. For both polymers, the absorption and the driving force to form intermediate CT excitations are largely similar. These results confirm that the higher power conversion efficiency observed for solar cells using PSBTBT as donor material is rather a result of molecular packing than of the electronic structure of the polymer.