Heteroatom substitution of oligothienoacenes: From good p-type semiconductors to good ambipolar semiconductors for organic field-effect transistors

Density functional theory calculations were carried out to investigate the effect of oligomer length, halogen substitution, and heteroatom substitution on the organic field-effect transistor (OFET) performance of a series of oligothienoacenes (1-5 for oligothienoacene with thiophene units' number from two to six). Compounds 1-5 are revealed to act only as p-type semiconductors due to their very high electron injection barrier relative to the work function potential of Au source-drain electrodes. Heteroatom substitution of the thiophene sulfur atom in particular with boron in the fused-ring thiophene oligomer 5 was revealed to elevate the HOMO energy level and lower the LUMO energy level and therefore lower both the hole and electron injection barriers. However, halogen substitution cannot effectively improve the electron injection barrier, but significantly increased the reorganization energy, therefore leading to decreased transfer mobility. The appropriate ionization potential and electron affinity, balanced charge injection barrier for both hole and electron relative to the work function potential of Au source-drain electrodes, low hole and electron reorganization energy, and good intrinsic transfer mobility for both hole and electron of both the boron-substituted hexathienoacenes 5BH and 5BH-2F-a make these two compounds good potential semiconductors for ambipolar OFET devices, with calculated intrinsic charge-transfer mobilities achieving 3.74 and 5.07 cm² V^-1 s^-1 for hole and 4.77 and 5.76 cm² V^-1 s^-1 for electron, respectively. The high intrinsic mobilities of 5BH and 5BH-2F-a are rationalized in terms of their frontier orbitals, molecular structure variation upon oxidation and reduction, and electron coupling between two neighboring molecules. All the results indicate that heteroatom substitution of sulfur atoms in oligothienoacenes is a rational way toward good ambipolar OFET semiconducting materials.