Significant Improvement of Semiconducting Performance of the Diketopyrrolopyrrole–Quaterthiophene Conjugated Polymer through Side-Chain Engineering via Hydrogen-Bonding

Three diketopyrrolopyrrole (DPP)–quaterthiophene conjugated polymers, <b>pDPP4T-1</b>, <b>pDPP4T-2</b>, and <b>pDPP4T-3</b>, in which the molar ratios of the urea-containing alkyl chains vs branching alkyl chains are 1:30, 1:20, and 1:10, respectively, were prepared and investigated. In comparison with <b>pDPP4T</b> without urea groups in the alkyl side chains and <b>pDPP4T-A</b>, <b> pDPP4T-B</b>, and <b>pDPP4T-C</b> containing both linear and branched alkyl chains, thin films of <b>pDPP4T-1</b>, <b>pDPP4T-2</b>, and <b>pDPP4T-3</b> exhibit higher hole mobilities; thin-film mobility increases in the order <b>pDPP4T-1</b> < <b>pDPP4T-2</b> < <b>pDPP4T-3</b>, and hole mobility of a thin film of <b>pDPP4T-3</b> can reach 13.1 cm<sup>2</sup> V<sup>–1</sup> s<sup>–1</sup> after thermal annealing at just 100 °C. The incorporation of urea groups in the alkyl side chains also has an interesting effect on the photovoltaic performances of DPP–quaterthiophene conjugated polymers after blending with PC<sub>71</sub>BM. Blended thin films of <b>pDPP4T-1</b>:PC<sub>71</sub>BM, <b>pDPP4T-2</b>:PC<sub>71</sub>BM, and <b>pDPP4T-3</b>:PC<sub>71</sub>BM exhibit higher power conversion efficiencies (PCEs) than <b>pDPP4T</b>:PC<sub>71</sub>BM, <b>pDPP4T-A</b>:PC<sub>71</sub>BM, <b>pDPP4T-B</b>:PC<sub>71</sub>BM, and <b>pDPP4T-C</b>:PC<sub>71</sub>BM. The PCE of <b>pDPP4T-1</b>:PC<sub>71</sub>BM reaches 6.8%. Thin films of <b>pDPP4T-1</b>, <b>pDPP4T-2</b>, and <b>pDPP4T-3</b> and corresponding thin films with PC<sub>71</sub>BM were characterized with AFM, GIXRD, and STEM. The results reveal that the lamellar packing order of the alkyl chains is obviously enhanced for thin films of <b>pDPP4T-1</b>, <b>pDPP4T-2</b>, and <b>pDPP4T-3</b>; after thermal annealing, slight inter-chain π–π stacking emerges for <b>pDPP4T-2</b> and <b>pDPP4T-3</b>. Blends of <b>pDPP4T-1</b>, <b>pDPP4T-2</b>, and <b>pDPP4T-3</b> with PC<sub>71</sub>BM show a more pronounced micro-phase separation. These observations suggest that the presence of urea groups may further facilitate the assemblies of these conjugated polymers into nanofibers and ordered aggregation of PC<sub>71</sub>BM.