Transistor Performance and Film Structure of Hexabenzocoronene Derivatives

Hexabenzocoronene (HBC) derivatives that are designed to self-assemble into lamellar aggregates were synthesized. The derivatives were deposited as an active layer in an organic field-effect transistor (OFET) using vacuum sublimation. The dihexyl and tetrahexyl derivatives (2H-HBC, 4H-HBC) increased the field-effect mobilities and on/off ratios by a factor of 10 or more compared to unsubstituted HBC and hexahexyl-hexabenzocoronene (6H-HBC). The crystal and thin film structures were determined by powder x-ray diffraction and grazing incidence X-ray diffraction (GIXD). The data indicate that 2H-HBC and 4H-HBC self-assemble into lamellar aggregates. 2H-HBC forms layers of aromatic cores that are sandwiched by the layers of hexyl groups, which is a preferable crystal structure for carrier transport. The good OFET performance could be explained by the self-assembly in lamellar aggregates of 2H-HBC and 4H-HBC, in contrast to self-assembly in the columnar aggregate of 6H-HBC and the low self-assembling properties of unsubstituted HBC.