This report examines the influence of narrow band gap polymer end groups (bromine or thiophene) on the performance of field-effect transistors prepared using these polymers. The conjugated polymer chains that were capped with thiophene units enhanced the intermolecular packing structure and decreased device hysteresis by removing charge traps. The presence of the end-capping groups increased the hole mobility by a factor of 2 or 4, depending on the molecular weight of the polymer, in the bottom contact field-effect transistors.