Two low band gap D-A copolymers 5,6-bis(2-ethylhexyl)naphtha[2,1-b:3,4-b]dithiophene-2,9-diyl] donor and without (P1) and with (P2) fluorine substituted 6,7-bis(9,9-didodecyl-9H-fluoren-2-yl)[1,2,5]thiadiazolo[3,4-g]quinoxaline were synthesized and their optical and electrochemical properties were investigated. These copolymers were employed as donor along with PC71BM as acceptor for the fabrication of solution processed bulk heterojunction polymer solar cells. After the optimization of the active layer, that is, different donor-to-acceptor weight ratios and concentration of DIO solvent additive, the resultant polymer solar cells showed overall PCE of 5.44% and 8.14% for P1 and P2, respectively, with low voltage loss of 0.40 eV. The higher value of power conversion efficiency of P2 based polymer solar cells as compared to P1 counterparts, my be related to the deeper highest occupied molecular orbital energy level of P2, higher hole mobility, balanced charge transport, suppressed bimolecular recombination and small - stacking distance and higher crystallinity of P2:PC71BM active layer. Although the dipole moment of P2 is slightly lower than P1, the enhanced values of power conversion efficiency of P2 may be related to improve stacking. (c) 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018, 56, 1297-1307