Two D-A alternating pi-conjugated copolymers bearing diketopyrrolopyrrole (DPP) and sextetthiophene with different molecular weights were synthesized to investigate their physical properties and performance in thin-film transistors and photovoltaic cells. The solubility and molecular weight (MW) of these copolymers with a highly rigid backbone were improved by controlling the feeding ratio of the monomer in the polymerization mixture. These polymers exhibited high crystallinity with a predominant edge-on chain orientation on the substrate. The higher-MW P(DPP-6T)-2 copolymer displayed a higher hole mobility of 3.26 cm(2) V-1 s(-1) than the low-MW P(DPP-6T)-1 copolymer. Bulk heterojunction photovoltaic cells made of a blend of P(DPP-6T)-2 and (6,6) ephenyl C-71-butyric acid methyl ester displayed good device performance with a power conversion efficiency of 4.68%. (C) 2015 Elsevier Ltd. All rights reserved.