Diketopyrrolopyrrole–Thiophene–Benzothiadiazole Random Copolymers: An Effective Strategy To Adjust Thin-Film Crystallinity for Transistor and Photovoltaic Properties

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 <i>N</i> = 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 cm² V^–1 s^–1 for <i>N</i> = 3:7, 1:1, and 7:3 copolymers while bulk heterojuction solar cells fabricated by using <i>N</i> = 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