Design of Organic Semiconductors from Molecular Electrostatics

Progress in the field of organic electronics depends on the synthesis of new pi-conjugated molecules to further improve the performance of, for example, organic light-emitting diodes, organic photovoltaic cells, and organic field-effect transistors. However, the interrelation between the properties of isolated molecules on one hand and close-packed thin films on the other hand is far from trivial. Here, we review recent progress in the understanding of electrostatic phenomena, which originate in the collective action of the anisotropic charge distribution in typical conjugated molecules. Both the pi-electron systems and polar end-group substitutions exposed at the surface of a molecular or polymeric film are seen to form dipole layers, which critically impact the device-relevant ionization energy and electron affinity of that film. After briefly revisiting electrostatic fundamentals and critically assessing related experimental methods, the energies of the frontier electronic states in organic thin films are shown to depend appreciably on the orientation of the constituent molecules with respect to device-relevant interfaces. For films of preferentially "standing" or "edge-on" molecules, this opens the possibility for electronic-structure engineering with intramolecular polar bonds. On the basis of these findings, additional insights into the working principles of organic electronic devices are provided and valuable guidelines for the synthesis of improved organic semiconductors are derived.