a b s t r a c tA class of low band-gap two-dimensional conjugated polymers of PBDTT-FQ, PBDTT-TQ, PBDTT-BTQ and PBDTT-TTQ was designed and synthesized, which contains the same di(alkylthiophene)-substituted benzo [1,2-b:4,5-b 0 ]dithiophene (BDTT) and 6,7-difluoroquinoxaline (Q) units, as well as various conjugated spacers of furan, thiophene, bithiophene and thieno [3,2-b]thiophene in the main chain. Significant effect of the varied spacers between the BDTT and Q units on the thermal, optical, electrochemical and photovoltaic properties was investigated and observed for these two-dimensional copolymers in the polymer solar cells. The maximum power conversion efficiency of 5.9% with a short circuit current of 13.7 mA/cm 2 and a fill factor of 0.56 was obtained for the PBDTT-TQ with thiophene spacer in the bulk hetero-junction PSCs using [6,6]-phenyl-C 71 -butyric acid methyl ester as acceptor.