Tuning the ambipolar charge transport properties of N-heteropentacenes by their frontier molecular orbital energy levels

A combined experimental and theoretical study was carried out to investigate the ambipolar charge transport properties of a series of N-heteropentacenes in organic field-effect transistors (OFETs). Introduction of nitrogen atoms in the core and halogen atoms around the periphery of the pentacene framework can efficiently tune the highest occupied molecular orbitals (HOMOs) of the N-heteropentacenes from -5.18 eV to -5.53 eV and the lowest unoccupied molecular orbitals (LUMOs) from -3.08 eV to -3.69 eV. By lowering their HOMO and LUMO energy levels with respect to the Fermi level of the gold electrode, the transistors of these molecules exhibited a transition from hole-dominant bipolar, to balanced ambipolar, and to electron-dominant bipolar transport characteristics. Meanwhile, with the lowering of the frontier molecular orbital energy levels, the transistors also exhibited a decrease of the electron threshold voltage and an increase of the hole threshold voltage. Charge carrier mobility calculations based on Marcus theory and first principle molecular dynamics were conducted to simulate the carrier transport dynamics. The comparison between experimental and theoretical results revealed that for the given device structure, the ratio of electron and hole mobilities of the ambipolar OFETs was strongly affected by the charge injection barrier. This result provides useful guidelines for future molecular design of ambipolar OFETs.