Dithienothiophene–diketopyrrolopyrrole-containing copolymers with alkyl side-chain and their application to polymer solar cells

Abstract

We synthesized a series of dithieno[3,2-b:2′,3′-d]thiophene and diketopyrrolopyrrole (DPP) based alternating copolymers, PDTTDPPs, with one of three branched alkyl side-chains (2-ethylhexyl, 2-butyloctyl, or 2-decyltetradecyl chains) tethered to the DPP moiety. These branched alkyl side-chains of various sizes were anchored to the DPP moiety owing to the relatively facile synthesis of DPP-based building blocks. We investigated the characteristics of the new PDTTDPP-based conjugated polymers bearing different alkyl chain substituents and the effect of the side chain on the device performance. A PDTTDPP-DT-based thin film transistor (TFT) exhibited the highest mobility of 6.88×10−2cm2V−1s−1, whereas the mobilities of TFTs made with PDTTDPP-EH and PDTTDPP-BO polymers were 6.13×10−3 and 7.65×10−3cm2V−1s−1, respectively. A polymer solar cell based on PDTTDPP-DT:[6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) (1:1.5w/w) showed the highest power conversion efficiency of 3.44%.