Synergistically Improved\nCrystallinity and Molecular\nDoping Ability of Polythiophene-Diketopyrropyrrole Derivatives by <i>m</i>‑Trifluoromethylbenzene Containing Side Chains for\nImproved Thermoelectric Materials

<i>m</i>-Trifluoromethylbenzene (FB) groups\nhave been\nwidely employed in various fields; however, no studies have reported\nthe use of FB in side chains to enhance the carrier mobility and molecular\ndoping of conjugated polymers. In this study, based on density functional\ntheory (DFT) calculations, we discovered that FB groups can effectively\nbind to [FeCl<sub>4</sub>]<sup>−</sup>, the counterion of the\np-type dopant FeCl<sub>3</sub>, thereby increasing doping ability.\nConsequently, FB groups were incorporated into the side chains of\nthiophene-diketopyrrolopyrrole-based donor–acceptor (D-A)-conjugated\npolymers, and a series of random conjugated polymers were synthesized\n(denoted as PDPPFB-<i>x</i>, where <i>x</i> represents\nthe molar ratio of the FB side chain). The findings revealed that\nan appropriate number of FB groups can decrease the π–π\nstacking distances, enhance the films’ crystallinity, and consequently\nimprove the charge transfer ability. Furthermore, after doping with\nFeCl<sub>3</sub>, the UV–vis–NIR spectra indicated that\nthe doping efficiency was augmented by increasing the molar fraction\nof the FB side chain. Among these polymers, PDPPFB-10 exhibited the\nhighest conductivity and power factor, which were 2.0 and 1.5 times\nhigher than those of PDPPFB-0, respectively. These results illustrated\na straightforward molecular design strategy for enhancing the crystallinity\nand conductivity of conjugated polymers, thereby expanding the way\nto optimize their thermoelectric performance.