Two new two-dimensional conjugated copolymers that contain diketopyrrolopyrrole and thiophene with different conjugation lengths as side chains, called PDPPMTD and PDPPBTD, were designed and synthesized for use in polymer solar cells (PSCs). The resulting copolymers in the thin film state displayed broad absorption in the visible range with an absorption edge at over 1000 nm, and both had relatively low-lying HOMO levels, at -5.20 and -5.18 eV for PDPPMTD and PDPPBTD, respectively. The power conversion efficiency (PCE) of the PSC that was based on PDPPBTD/PC61BM (w/w = 1:2) reached 4.10 % with a Jsc of 14.5 mA/cm2, a Voc of 0.59 V and an FF of 48%, while PDPPMTD/PC61BM (w/w = 1:2) had a PCE of 2.96% with a Jsc of 12.6 mA/cm2, a Voc of 0.60 V, and an FF of 39%. These results indicate that subtle tuning of the chemical structure can significantly influence Jsc and FF. (c) 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015, 53, 2878-2889