Three new low bandgap conjugated random copolymers, containing two acceptors diketopyrrolopyrrole (DPP) and benzothiadiazole (BTD) linked by thiophene donors, were designed and synthesized using Pd-catalyzed Stille-coupling methods. The ratio between DPP and BTD was varied from N = 3:7 to 1:1 to 7:3 in the polymer backbones. Thin-film device measurements indicate that these polymers exhibit different trends in field-effect mobilities and photovoltaic properties owing to adjustable nanoscale film morphologies as well as solid-state molecular packing. The hole mobilities reach 0.05, 0.17, and 0.40 cm(2) V-1 s(-1) for N = 3:7, 1:1, and 7:3 copolymers while bulk heterojuction solar cells fabricated by using N = 3:7, 1:1, and 7:3 copolymers as donor and PC60BM as acceptor show power conversion efficiency of 2.4%, 1.3%, and 0.5%. This work sets up a good example of effectively tuning the crystallinity of thin-film device through easily varying the composition ratios.