Dicyanodistyrylbenzene-Based Copolymers for Ambipolar Organic Field-Effect Transistors with Well-Balanced Hole and Electron Mobilities

We report three types of dicyanodistyrylbenzene (DCS)-based copolymers (PBDT-DCS, PT-DCS, and PNDI-DCS), which present highly balanced ambipolar charge transport characteristics in organic field-effect transistors (OFETs). The introduction of the DCS moiety in a polymer backbone not only lowers the lowest unoccupied molecular orbital (LUMO) level but also increases the crystalline ordering via interchain dipole-dipole interactions. As a result, the LUMO levels for PBDT-DCS, PT-DCS, and PNDI-DCS were decreased to -3.76, -4.00, and -3.99 eV, respectively, which is beneficial for efficient electron injection from Au electrode for improving ambipolar charge transport. The determined hole/electron mobilities of the OFETs were 0.064/0.014, 0.492/0.181, and 0.420/0.447 cm²/(V s) for PBDT-DCS, PT-DCS, and PNDI-DCS, respectively, after thermal annealing at 250 °C. By incorporating the electron-deficient naphthalene diimide (NDI) unit in the copolymers, the n-channel transport was enhanced, with decreasing frontier molecular orbitals with enhanced electron injection and impeded hole injection from the Au electrode. Therefore, PNDI-DCS provided completely symmetric output curves in the positive and negative drain voltage regions with almost equivalent hole and electron mobilities. Benefiting from the balanced ambipolar feature of the PNDI-DCS OFETs, a complementary inverter was successfully fabricated.