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Reported here is a characterizationof a series of NDI–thiophenecopolymers with one, two, three, and four thiophene units synthesizedusing Stille polycondensation of dibromo-naphthalene diimide and thetrimethylstannylthiophene monomers. The effect of extension of thethiophene donor group is studied in terms of structure-charge transportcorrelation. The influence of side chains located on the thiopheneunits of copolymers with two and four thiophene units per monomeris also investigated. Charge transport of both signs is studied experimentallyin field-effect transistors. Microstructural data obtained by near-edgeX-ray absorption fine structure (NEXAFS) and grazing incidence wide-angleX-ray scattering (GIWAXS) is supported by AFM topography scans. Ultravioletphotoelectron spectroscopy (UPS) and UV–vis spectroscopy dataare employed in the measurement of energy levels, and changes withannealing temperature are also discussed. Most of the polymers reachexcellent electron and hole mobility with one copolymer (NDI-T4) exhibitingan especially balanced ambipolar charge transport of 0.03 cm2V–1s–1. An odd–eveneffect in hole mobility is observed with higher values for polymerswith an even number of thiophene units. The reported findings indicatethat the final charge transport properties are a result of the interplayof many factors, including crystallinity, planarity and linearityof chain, spacing between acceptor units and packing of solubilizing branched sidechains. [ABSTRACT FROM AUTHOR]