We demonstrated the effective structural control of the conjugation length and various donor moieties using thiophene and selenophene derivatives in a regular terpolymer system and compared the properties with two copolymers of PDPP-SS and PDPP-TT. The three different terpolymers are composed of diketopyrrolopyrrole (DPP)-based polymers such as PDPP-TTS, PDPP-TSS, and PDPP-TTSS. The optical and electronic properties of these regular terpolymers can be precisely controlled by varying the conjugation length of the repeating unit and the composition ratio of the thiophene and selenophene moieties. It was found from the grazing incidence X-ray diffraction that PDPP-TTS and PDPP-TSS with different lengths of donor moieties exhibited mixed chain orientations in face-on and edge-on manners on the substrate. However, PDPP-TTSS showed edge-on chain orientation which is similar to the copolymers of PDPP-TT and PDPP-SS, and much improved charge-carrier mobility. Moreover, the enhanced power conversion efficiencies (PCEs) of polymer solar cells made of PDPP-TSS and PDPP-TTSS were observed to be 4.18 and 4.76%, respectively, owing to the higher composition of thiophene, which could be compensated by the properties of thiophene. As a result, the PCEs of regular terpolymers were markedly enhanced by inserting the thiophene moiety into the donor part of the DPP-based terpolymer, PDPP-SS. (C) 2016 Elsevier Ltd. All rights reserved.