Interplay Between Microscopic Structure and Intermolecular Charge-Transfer Processes in Polymer–Fullerene Bulk Heterojunctions
We provide an overview of the development and application of nonconventional techniques that allowed us to probe the molecular excited states in conjugated polymers with respect to their molecular orientation and the nanomechanical properties of the topmost surface layer. To probe triplet and charge-transfer excited states and their fate, we applied the optically detected magnetic resonance technique extended for angular resolution. The surface morphology and three-dimensional depth profiles of the topmost surface layer were obtained with multi-set point intermittent contact mode atomic force microscopy. These studies were performed on model systems such as poly(3-hexylthiophene) as well as on the novel, low-bandgap copolymer poly[(4,8-bis-(2-ethylhexyloxy)-benzo(1,2-b:4,5-b9)dithiophene)-2,6-diyl-alt-(4-(2-ethylhexyl)-3-fluorothieno[3,4-b]thiophene-)-2-carboxylate-2-6-diyl)].
KeywordsAFM Atomic force microscopy Bulk heterojunction Charge-transfer states CTS Depth profiling EPR Nanomechanical properties ODMR OPV Organic photovoltaics Polymer surfaces Triplet excitons Volume imaging
The reported work was completed in close collaboration with S. Väth from Julius-Maximilians Universität Würzburg, E.-C. Spitzner and C. Riesch from Technische Universität Chemnitz, and R. Lohwasser and M. Thelakkat from Universität Bayreuth. We gratefully acknowledge their contributions. This work was supported by the Deutsche Forschungsgemeinschaft within the SPP 1355 (DY18/6-1,2,3 and MA1744/3-1,2,3). R.M. acknowledges further funding by the Volkswagen Foundation and the Deutsche Forschungsgemeinschaft.
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