Optically active narrow-bandgap polymers ((R,R)- and (S,S)-PTB5) consisting of alternating n-octyl thieno3,4-bthiophene-2-carboxylate and benzo1,2-b:4,5-b′dithiophene units bearing optically pure (R)- and (S)-2-ethylhexyl pendants, respectively, were synthesized for the first time. (R,R)- and (S,S)-PTB5 films showed apparent circular dichroism in their absorption regions of the polymer backbone due to the formation of a chirally ordered superstructure induced by the chirality of the branched alkyl pendants. Inverted-type bulk heterojunction polymer solar cells (PSCs) were fabricated using (R,R)- or (S,S)-PTB5 as electron donors and 6,6-phenyl-C61-butyric acid methyl ester as an electron acceptor. The photovoltaic properties of the PSCs were compared with those of the corresponding PSC containing optically inactive PTB5 bearing racemic 2-ethylhexyl pendants. Display omitted •Development of novel thienothiophene–benzodithiophene-based donor polymers.•Chirality control of 2-ethylhexyl pendants in donor polymers.•Application of optically active donor polymers to bulk heterojunction polymer solar cells.