Recent advances in polymer solar cell (PSC) performance have resulted from compressing the bandgap to enhance the short-circuit current while lowering the highest occupied molecular orbital to increase the open-circuit voltage. Nevertheless, PSC power conversion efficiencies are still constrained by low fill factors, typically below 70%. Here, we report PSCs with exceptionally high fill factors by combining complementary materials design, synthesis, processing and device engineering strategies. The donor polymers, PTPD3T and PBTI3T, when incorporated into inverted bulk-heterojunction PSCs with a PC71BM acceptor, result in PSCs with fill factors of 76-80%. The enhanced performance is attributed to highly ordered, closely packed and properly oriented active-layer microstructures with optimal horizontal phase separation and vertical phase gradation. The result is efficient charge extraction and suppressed bulk and interfacial bimolecular recombination. The high fill factors yield power conversion efficiencies of up to 8.7% from polymers with suboptimal bandgaps, suggesting that efficiencies above 10% should be realizable by bandgap modification.