Three novel copolymers (PTTAFBT-C0, PTTAFBT-C1 and PTTAFBT-C2) based on thieno[3,2-b]thiophene and 5-alkoxy-6-fluorobenzo-[c][1,2,5]thiadiazole (AFBT) bearing branched alkoxy chains with varied branching positions are synthesized. The influences of the bifurcation positions on intermolecular stacking, charge mobility, film morphology and photovoltaic performance are systematically investigated. 2D-GIWAXS analyses of the optimized polymer:PC71BM blend films exhibit the crystallinity increases from PTTAFBT-C0 to PTTAFBT-C1 and PTTAFBT-C2 is more prone to form the edge-on orientation than PTTAFBT-C1, which result in a dramatic difference in film morphologies. TEM image of PTTAFBT-C1:PC71BM blend films exhibits a suitable morphology with favorable interpenetrating networks, which is in favor of high performance. The best PCE of 5.67% with a device configuration of ITO/PEDOT: PSS/PTTAFBT-C1:PC71BM/LiF/Al under AM 1.5G solar radiation (100 mW cm(-2)) is achieved. These results indicate that higher PCE can be obtained by adjusting the bifurcation points of the branched side chains away from the polymer backbone. (C) 2016 Elsevier B.V. All rights reserved.