Liping Huang, Department of Chemical and Biomolecular Engineering and Department of Physics, North Carolina State University, Raleigh, NC 27606, Dario Rocca, Department of Chemistry, University of California, Davis, Davis, CA 95616, Stefano Baroni, SISSA and CNR-INFM DEMOCRITOS National Simulation Center, Trieste, Italy, Keith E. Gubbins, Chemical and Biomolecular Engineering and Center for High Performance Simulation (CHiPS), North Carolina State University, Raleigh, NC 27695-7905, and Marco Buongiorno Nardelli, Department of Physics, North Carolina State University, Raleigh, NC 27695.
Organic semiconductors are promising candidates for making low-cost solar cells because of their easiness to process. However, they usually have limited response to the solar spectrum and very low charge carriers mobility. We carried out first-principles calculations at the density functional theory (DFT) and time-dependent density functional theory (TDDFT) levels to understand the effect of chain length, functional groups and molecular packing on the optical absorption spectrum and charge carrier motility of acenes. Our calculations demonstrate that these properties not only depend on the molecular identity but also on the molecular packing. By designing the interaction between metal substrates and the first layer of acene molecules, they can be packed in a cofacial (face to face) fashion instead of the conventional herringbone (face to edge) arrangement. Acenes packed in the cofacial way show improved response to the solar spectrum and high charge carriers mobility. This could open the door to a new route of materials design for cost-effective organic solar cells.