A wide-bandgap polymer PBDTNS-TPD based on alkylthio-naphthyl-substituted benzo[l,2-b:4,5-b] dithiophene (BDT) units was designed and synthesized. Benefiting from the prolonged conjugation of the conjugated side groups on BDT units, the PBDTNS-TPD exhibited better light-harvesting properties and charge transport properties than that of alkylthio-phenyl-substituted counterpart PBDTBS-TPD. The PBDTNS-TPD:PC71BM solar cell devices yield a power conversion efficiency (PCE) of 9.09%, which exhibits an increase of 29% in comparison with that of PBDTBS-TPD-based PSCs (PCE 7.02%). Moreover, the PBDTNS-TPD:ITIC based PSCs also showed a higher PCE of 8.19% compared to the PBDTBS-TPD:ITIC based PSCs (PCE 5.93%). This work demonstrates that the replacement of the alkylthio-phenyl moiety with alkylthio-naphthyl unit on BDT is a useful method to design high-performance wide-bandgap polymers for fullerene-based and fullerene-free PSCs. (C) 2018 Elsevier Ltd. All rights reserved.