Improving pi-conjugated polymer electron transport and injection efficiency are important to realize high performance n-channel organic field-effect transistors (OFETs). This paper reports a series of naphthalene diimide (NDI-) based n-type pi-conjugated polymers, NDI-T-1FP-T, NDI-T-2FP-T, and NDI-T-4FP-T, with varioulis amounts of fluorine atoms (1F, 2F, and 4F) in their backbones. We found notable differences in energy levels, ground and excited state dipole moments (Delta mu(ge)), exciton lifetimes, dihedral angles, and charge transport properties by varying the fluorine content: Delta mu(ge) and charge transport properties reduced with increasing fluorine content. Electron mobility of optimized top gate bottom contact OFETs using the n-type pi-conjugated polymers were 0.35, 0.18, and 0.16 cm(2) V-1 s(-1), respectively. OFET performances were further improved further by low concentration polyethylenimine (PEI) doping, significantly improving field-effect mobility to (maximum) 0.51, 0.34, and 0.26 cm(2) V-1 s(-1), respectively. Doped NDI-T-4FP-T with 4F in particular showed unipolar n-channel behavior by depletion of the hole current.