A family of diketopyrrolopyrrole (DPP)-incorporated P3HT based semi-random copolymers was synthesized and their optical, electronic and photovoltaic properties were investigated. For the first time, the influence of acceptor content on semi-random copolymers was explored in the broad range of 10-40% acceptor. A mixture of DPP acceptor units with different side chains (ethylhexyl and decyltetradecyl) was incorporated into each copolymer to improve solubility and film quality. Increased DPP content in the polymer backbone resulted in broadened absorption between 350 and 900 nm, resulting in a monotonic decrease in optical band gap (E-g) of the polymers from 1.49 to 1.37 eV. Highest occupied molecular orbital (HOMO) energy levels showed an increase from 10% DPP to 20-30% DPP, while decreasing for 40% DPP. V-oc values followed a consistent trend with HOMO energy levels. Semi-random copolymers showed significantly improved photovoltaic properties compared with P3HT. Bulk heterojunction solar cells fabricated from the semi-random copolymers blended with PC61BM exhibited high short-circuit current densities (J(sc)) up to 10.29 mA/cm(2) and efficiencies up to 4.43%. A new method of methanol treatment was developed and applied to the semi-random copolymers resulting in high fill factors approaching 0.70 under ambient conditions. (C) 2017 Wiley Periodicals, Inc.