Many types of electron-rich monomers have been copolymerized with NDI for non-fullerene acceptors. The resulting NDIbased polymers usually possess extended p-delocalization andstronger p–p stacking, consequently better electron transport relative to PDIs [57]. NDIs and thiophene containing copolymersare the most investigated NDI-based polymer acceptors [62,73],in which NDI–bithiophene copolymers (Activink N2200 (P5))and their derivatives have been the most successful series owingto the high electron affinity and strong light absorption.