Here, an Isoindigo (IID) comprised with Naphthodithiophene (NDT)-based low band-gap donor–acceptor copolymer semiconductor is designed, synthesized, and investigated. The newly reported polymer provides good solubility in a range of halogenated and non-halogenated solvents due to the attachment of long branched alkyl chains on the Isoindigo core. After optimizing the performance of organic field effect transistors (OFETs) through the use of high-boiling-point chlorinated solvents, A similar approach is followed to realize transistors without the need of using halogenated solvents as a green approach. Well-working transistors are shown with mobility values in a similar range as obtained for OFETs with films prepared from halogenated solvents, while significantly reducing the environmental impact. Finally, all-solution-processed, flexible OFETs are prepared without the use of halogenated solvents, which is an important step in the view of potential large area production of flexible electronics.