Two regioregular indacenodithieno[3,2-b]thiophene and fluoro-benzothiadiazole-based donor–acceptor (D–A) type copolymers with different conjugated side chains of hexyl-phenyl or hexyl-thienyl are designed and synthesized to study the effects of the side chain on performance of organic field-effect transistors (OFETs). The effect of the different conjugated side chains on the energy levels, optoelectronic properties, film morphology, and transistors performance as well as the solvent effect on the performance of both D–A copolymers are thoroughly studied. Top-gate/bottom-contact OFETs with a hexyl-thienyl substituted polymer show higher hole mobility up to 0.211 cm2 V−1 s−1 than 0.086 cm2 V−1 s−1 observed for the polymer with hexyl-phenyl side chains. In addition, the hexyl-thienyl substituted side chain polymer shows relatively better stability under bias stress compared to hexyl-phenyl conjugated side chain. This result sheds light on the impact of conjugated side chain engineering on the performance and stability of OFETs with D–A copolymers.