A series of donor-rich semi-random copolymers with various benzodithiophene (donor unit)/fluorinated-thienothiophene (acceptor unit) ratios - D2A, D3A, D5A, D7A - were designed and synthesized for use in polymer solar cells (PSCs). The polymer D1A was also synthesized for comparison. The optical and electrochemical properties of these polymers vary with the number of donor units in their polymer backbones. The device containing the polymer D5A exhibits a best power conversion efficiency (PCE) of 5.52% when the active layer thickness is 340 nm, a value that is 80% of its maximum PCE, whereas the best PCE of the D1A-based device is reduced by 32% when the active layer thickness is 310 nm. A large area (6,270 mm(2)) D5A-based device prepared with the slot die-coating method was found to exhibit a best PCE of 6.54%, which is significantly better than that of the equivalent D1A-based large area device (4.03%). (C) 2014 Elsevier B.V. All rights reserved.