Controlling Electronic Transitions in Fullerene van der Waals Aggregates via Supra-Molecular Assembly
in: ACS Applied Materials & Interfaces (2016)
Morphologies crucially determine the optoelectronic properties of organic semiconductors. Therefore, hierarchical and supra-molecular approaches have been developed for targeted design of supramolecular ensembles of organic semi-conducting molecules and performance improvement of e.g. organic solar cells (OSCs), organic light emitting diodes (OLEDs), and organic field-effect transistors (OFETs). We demonstrated how the photonic properties of fullerenes change with formation of van der Waals aggregates. We identified supra-molecular structures with broadly tunable absorption in the visible spectral range and demonstrated how to form aggregates with targeted visible(vis)-absorption. To control supra-molecular structure formation we functionalized the C60-backbone with polar (bis-polyethylene glycol malonate -MPEG) tails, thus yielding an amphiphilic fullerene derivative that self-assembles at interfaces. Aggregates of systematically tuned size were obtained from concentrating MPEGC60 in stearic acid matrices, while different supra-molecular geometries were provoked via different thin film preparation methods, namely spin casting and Langmuir-Blodgett (LB) deposition from an air-water interface. We demonstrated that differences in molecular orientation in LB films (C2v type point group aggregates) and spin casting (stochastic aggregates) lead to huge changes in electronic absorption spectra due to symmetry and orientation reasons. These differences in the supra-molecular structures, causing the different photonic properties of spin-cast and LB films could be identified by means of quantum chemical calculations. Employing supramolecular assembly we propounded that molecular symmetry in fullerene aggregates is extremely important in controlling vis-absorption to harvest photons efficiently, when mixed with a donor molecule thus improving active layer design and performance of OSCs.