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Bulk heterojunction(BHJ)polymer solar cells(PSCs)are regarded as one of the most promising alternatives to inorganic-based solar cells.Recent advances in PSCs performance have resulted from compressing the bandgap to enhance the short-circuit current(JSC)while lowering the highest occupied molecular orbital to increase the open-circuit voltage(VOC),consequently enhance the power conversion efficiency(PCE).However,PCEs of PSCs are still constrained by low JSC,small VOC and low fill factor(FF).Here,we report single-junction PSCs with high J SC,large VOC and high FF by combining complementary materials design,synthesis,and device engineering strategies.A two-dimensional donor-acceptor conjugated copolymer,poly[[2,6-4,8-di(5-ethylhexylthienyl)benzo[1,2-b;3,3-b]dithiophene][3-fluoro-2[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]](PTB7-DT)was developed and utilized in BHJ PSCs.Introduction of 5-alkylthiophene-2-yl unit as a conjugated side chain lowers the bandgap and facilities the π-π stacking,and improves the charge carrier mobility of PTB7-DT.These results are consistent with the observation from first-principle calculations.Consequently,the resulting polymer exhibits considerably better photovoltaic performance with PCE of 10.12%from single-junction PSCs.