Prof. Dr. Wolfgang Brütting
University of Augsburg
Lehrstuhl für Experimentalphysik IV

Prof. Dr. Mark E. Thompson
University of Southern California, Los Angeles
Department of Chemistry

In contrast to inorganic semiconductors, organic molecular materials have additional degrees of freedom related to their non-isotropic shape. Most importantly, the relative molecular orientation with respect to interfaces in opto-electronic devices has been identified as an important parameter for device operation. Not only does the charge carrier mobility depend on the orbital overlap of neighbouring molecules and thus on their mutual orientation, but also are processes like light absorption as well as electron transfer and recombination at interfaces dependent on molecular orientation and intermolecular coupling strength. By combining the complimentary expertise of both groups on these topics, we expect to gain a deeper understanding of these processes being relevant particularly in photovoltaic cells.

Final report:

Within the BaCaTeC funded collaboration between the groups at Augsburg University and the University of Southern California we studied organic solar cells based on low-molecular weight materials. Using two prototypical perylene derivatives – one with horizontal and the other with vertical alignment of their transition dipole moments with respect to the substrate – we could clearly demonstrate the influence of molecular orientation on the photocurrent [1,2]. As a result, the power conversion efficiency of solar cells could be increased from 2% to almost 6%. Additionally, we introduced metal-dipyrrin complexes exhibiting intra-molecular symmetry breaking charge transfer as a new class of materials for improved open-circuit voltage [3].

Publications:

1. Amorphous vs crystalline exciton blocking layers at the anode interface in planar and planar-mixed heterojunction organic solar cells, S. Grob, M. Gruber, A. N. Bartynski, U. Hörmann, T. Linderl, M. E. Thompson, W. Brütting, Applied Physics Letters 104 (2014) 213304; http://dx.doi.org/10.1063/1.4879839
2. Solvent vapor annealing on perylene-based organic solar cells, S. Grob, A. N. Bartynski, A. Opitz, M. Gruber, F. Grassl, T. Linderl, U. Hörmann, C. Lorch, E. Moons, F. Schreiber, M. E. Thompson, W. Brütting, Journal of Materials Chemistry A 3 (2015) 15700-15709; http://dx.doi.org/10.1039/c5ta02806j
3. Symmetry breaking charge transfer in a Zinc chlorodipyrrin acceptor for high open circuit voltage organic photovoltaics, A. N. Bartynski, M. Gruber, S. Das, S. Rangan, S. Mollinger, C. Trinh, S. E. Bradforth, K. Vandewal, A. Salleo, R. A. Bartynski, W. Brütting, M. E. Thompson, Journal of the American Chemical Society 137 (2015) 5397-5405; http://dx.doi.org/10.1021/jacs.5b00146