New It-conjugated copolymers based on naphtho[2,3-b:6,7-b']dithiophene-4,5,9,10-diimide (NDTI) combined with thiophene, thienothiophene, or dithienothiophene units are synthesized and used in field-effect transistors (FETs) and organic solar cells (OSCs). The low-lying lowest unoccupied molecular orbital (LUMO) and high-lying highest occupied molecular orbital (HOMO) levels of the polymers contribute to reducing injection barriers for both electrons and holes, resulting in ambipolar operation of FET devices. The charge mobilities were strongly affected by the molecular orientation of the copolymers, and the highest electron mobility of 0.26 cm(2)/(V s) was observed for the copolymer with thienothiophene unit with edge-on orientation. On the other hand, OSCs with PTB7 as the electron donor polymer and the copolymers as the acceptor showed a broad photoresponse extending to the near-IR region, and the highest power conversion efficiency of over 3.5% was obtained for the copolymer with dithienothiophene unit that showed the favorable face-on orientation in the neat thin film, though the effect of the molecular orientations in OSCs was not as clear as in OFETs owing to the lower crystallinity of the mixed films.