Invited Paper SS2-TuM1
Relating Aromatic Molecule Structure to Film Structure/Property Relationships

Tuesday, October 19, 2010, 8:00 am, Room Santa Ana

The ease with which small-molecule organic semiconductors can be functionalized allows a single chromophore framework to be tuned for use in a myriad of electronic applications. Careful selection of substituents allows tuning of both solubility and crystal packing, allowing optimum structures for both film morphology and charge transport to be dialed in by careful structure-property studies. Additional substituents on the chromophore can be added to improve stability, shift phase transitions, or change the dominant carrier type for the material. Using 4, 5 and 6 fused-ringed acenes and heteroacenes as the chromophore, our straightforward functionalization approach has created organic materials for use in high-performance organic transistors and organic solar cells. The crystal-packing arrangements for these two types of devices are dramatically different. This talk will examine how materials with two-dimensional pi-stacking arrangements yield high-performance transistors, and how subtle tuning of the substituents can further improve performance and alter solubility. In the optimum case, hole mobility as high as 5 cm2 / Vs was observed from a dip-cast film. Structure-property relationships in organic transistors are also explored in high-quality single crystals, showing how changes in crystalline order changes the intrinsic carrier properties of a homologous series of materials. In the case of bulk heterojunction organic solar cells, substitution of the acene chromophore with small electron-withdrawing groups yielded effective acceptors in blends with polythiophene donors. In this case, materials with strong pi-stacking interactions yielded the poorest-performing solar cells. In contrast, materials with weak, 1-dimensional pi-stacking interactions yielded the best performance, with power conversion efficiencies greater than 1.5% in these fullerene-free blends.