Naphthalene diimide-based polymer semiconductors: Synthesis, structure-property correlations, and n-channel and ambipolar field-effect transistors

A series of nine alternating donor-acceptor copolymer semiconductors based on naphthalene diimide (NDI) acceptor and seven different thiophene moieties with varied electron-donating strength and conformations has been synthesized, characterized, and used in n-channel and ambipolar organic field-effect transistors (OFETs). The NDI copolymers had moderate to high molecular weights, and most of them exhibited moderate crystallinity in thin films and fibers. The LUMO energy levels of the NDI copolymers, at -3.9 to -3.8 eV, were constant as the donor moiety was varied. However, the HOMO energy levels could be tuned over a wide range from -5.3 eV in P8 to -5.9 eV in P1 and P3. As semiconductors in n-channel OFETs with gold source/drain electrodes, the NDI copolymers exhibited good electron transport with maximum electron mobility of 0.07 cm ²/(V s) in P5. Although head-to-head (HH) linkage induced backbone torsion, polymer P4 showed substantial electron mobility of 0.012 cm ²/(V s) in bottom-gate/top-contact device geometry. Some of the copolymers with high-lying HOMO levels (P7 and P8) exhibited ambipolar charge transport in OFETs with high electron mobilities (0.006-0.02 cm ²/(V s)) and significant hole mobilities (>10 ^-3 cm ²/(V s)). Varying the device geometry from top-contact to bottom-contact leads to the appearance or enhancement of hole transport in P4, P6, P7, and P8. Copolymers with smaller alkyl side chains on the imide group of NDI have enhanced carrier mobilities than those with bulkier alkyl side chains. These results show underlying structure-property relationships in NDI-based copolymer semiconductors while demonstrating their promise in n-channel and ambipolar transistors.