Most　of　the　narrow　band-gap　polymers　exhibit　low　open-circuit　voltage　(V-oc)　which　limits　the　further　improvement　of　photovoltaic　performance.　How　to　improve　the　V-oc　through　fine　tuning　of　the　chemical　structure　is　a　significant　project　that　worth　exploring.　As　a　big　and　strong　electron　accepting　unit,　fluorinated-quinoxaline　based　copolymers　always　show　a　narrow　band　gap　and　broad　absorption　which　makes　it　a　good　template　to　study　the　changes　of　V-oc　through　structure　tuning.　In　this　work,　the　effect　of　the　subtle　chemical　structure　modulation　of　polymer　side　chains　and　main　chain　on　the　V-oc　and　photovoltaic　performance　was　investigated.　The　V-oc　was　adjusted　effectively,　and　the　power　conversion　efficiency　(PCE)　also　was　improved　simultaneously.　Although　the　PCE　was　not　high　enough　as　those　high-performance　polymers　reported　at　present,　the　result　indicates　that　fine　tuning　of　chemical　structure　is　an　effective　way　to　improve　V-oc　and　photovoltaic　performance　for　narrow　band-gap　polymers.　(C)　2016　Wiley　Periodicals,　Inc.