Naphthalene diimide copolymers are attractive n-type materials due to their high electron affinities, high electron mobilities, and exceptional stability. Herein, we report a series of NDI-fused-thiophene based copolymers with each copolymer differing in the number of fused thiophenes in the donor monomer. Increasing the number of fused-thiophene moieties within an NDI-copolymer backbone is shown to not only enable tuning of the electronic structure but also improve charge mobilities within the active layer of organic field-effect transistors. Electron mobilities and on/off ratios as high as 0.012 cm(2) V-1 s(-1) and I-on/I-off > 10(5) were measured from n-channel thin-film transistors fabricated using NDI-xfTh copolymers. Bulk heterojunction solar cell devices were also fabricated from the NDI-xfTh copolymer series in blends with poly(3-hexylthiophene) (P3HT) with PNDI-4fTh-based devices yielding the largest J(sc) (0.57 mA cm(-2)) and fill factor (55%) in addition to the highest measured PCE for this series (0.13%). (c) 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 4061-4069