We　find　that　the　external　quantum　efficiency　of　photovoltaic　diodes　based　on　finely　mixed　blends　of　poly-9,9＇-dioctylfluorene-co-bis-N,N＇-(4-butylphenyl)-bis-N,N＇-phenyl-l,4-phenylenediamine　(PFB)　and　poly-9,9＇-　dioctylfluorene-co-benzothiadiazole　(F8BT)　depends　strongly　on　the　blend　ratio.　The　peak　external　quantum　efficiency　is　optimum　for　a　PFB:　F8BT　ratio　of　3:1.　The　difference　of　peak　efficiency　for　this　composition　and　a　1:1　ratio　is　significantly　higher　than　the　reported　yield　of　charge-transfer　excitons.　From　a　surface　topography　analysis,　we　believe　that　charge　percolation　plays　a　crucial　role　in　photocurrent　efficiency　in　PFB:　F8BT　diodes.　Furthermore,　we　present　a　qualitative　model　for　different　charge　percolation　pathways　in　diodes　of　different　blend　ratios.　(C)　2013　Elsevier　B.　V.　All　rights　reserved.