Reported here is a characterization of a series of NDI-thiophene copolymers with one, two, three, and four thiophene units synthesized using Stille polycondensation of dibromo-naphthalene diimide and the trimethylstannylthiophene monomers. The effect of extension of the thiophene donor group is studied in terms of structure-charge transport correlation. The influence of side chains located on the thiophene units of copolymers with two and four thiophene units per monomer is also investigated. Charge transport of both signs is studied experimentally in field-effect transistors. Microstructural data obtained by near-edge X-ray absorption fine structure (NEXAFS) and grazing incidence wide-angle X-ray scattering (GIWAXS) is supported by AFM topography scans. Ultraviolet photoelectron spectroscopy (UPS) and UV-vis spectroscopy data are employed in the measurement of energy levels, and changes with annealing temperature are also discussed. Most of the polymers reach excellent electron and hole mobility with one copolymer (NDI-T4) exhibiting an especially balanced ambipolar charge transport of 0.03 cm² V^-1 s^-1. An odd-even effect in hole mobility is observed with higher values for polymers with an even number of thiophene units. The reported findings indicate that the final charge transport properties are a result of the interplay of many factors, including crystallinity, planarity and linearity of chain, spacing between acceptor units and packing of solubilizing branched side chains.