Comparative Studies on the Relations between Composition Ratio and Charge Transport of Diketopyrrolopyrrole-Based Random Copolymers

Abstract

Random copolymerization strategy is introduced to increase the solubility of donor–acceptor copolymers so that they can be processed with environmentally benign, halogen-free solvents. Traditionally, it has been believed that the random copolymer with a lower crystalline order should have a significantly lower charge carrier mobility. This report shows that random copolymerization between two highly planar, charge-delocalized repeating units can significantly enhance the solubility while almost preserving the high charge carrier mobility. A comparative study was conducted for a series of random copolymers consisting of diketopyrrolopyrrole–thienothiophene (DPP-TT) and DPP–selenophene–vinylene–selenophene (DPP-SVS) with composition ratios of 10:0, 9:1, 7:3, 5:5, 3:7, 1:9, and 0:10 by using the crystalline order determined from grazing-incident X-ray diffraction measurements and charge carrier mobility determined from field effect transistor measurements. The results showed that the copolymer tends to have a lower degree of intermolecular ordering as the DPP-TT:DPP-SVS composition ratio approaches 5:5 because of the increased segmental randomness in the polymer chain. Nevertheless, the FET mobility did not follow the tendency of crystalline order; instead, it simply followed the order of increasing content of DPP-SVS in the copolymer. This result implies that we can use the random copolymerization strategy to increase the solubility of donor–acceptor copolymers if the two constituent repeating units are structurally planar and the electrons are fully delocalized.