| Summary: | 3-(2-Octyldodecyl)thieno[3,2-<i>b</i>]thiophen was successfully synthesized as a new π-bridge with a long branched side alkyl chain. Two donor-π-bridge-acceptor type copolymers were designed and synthesized by combining this π-bridge structure, a fluorinated benzothiadiazole acceptor unit, and a thiophene or thienothiophene donor unit, (<b>PT-ODTTBT</b> or <b>PTT-ODTTBT</b> respectively) through Stille polymerization. Inverted OPV devices with a structure of ITO/ZnO/polymer:PC<sub>71</sub>BM/MoO<sub>3</sub>/Ag were fabricated by spin-coating in ambient atmosphere or N<sub>2</sub> within a glovebox to evaluate the photovoltaic performance of the synthesized polymers (effective active area: 0.09 cm<sup>2</sup>). The <b>PTT-ODTTBT</b>:PC<sub>71</sub>BM-based structure exhibited the highest organic photovoltaic (OPV) device performance, with a maximum power conversion efficiency (PCE) of 7.05 (6.88 ± 0.12)%, a high short-circuit current (<i>J</i><sub>sc</sub>) of 13.96 mA/cm<sup>2</sup>, and a fill factor (<i>FF</i>) of 66.94 (66.47 ± 0.63)%; whereas the <b>PT-ODTTBT</b>:PC<sub>71</sub>BM-based device achieved overall lower device performance. According to GIWAXS analysis, both neat and blend films of <b>PTT-ODTTBT</b> exhibited well-organized lamellar stacking, leading to a higher charge carrier mobility than that of <b>PT-ODTTBT</b>. Compared to <b>PT-ODTTBT</b> containing a thiophene donor unit, <b>PTT-ODTTBT</b> containing a thienothiophene donor unit exhibited higher crystallinity, preferential face-on orientation, and a bicontinuous interpenetrating network in the film, which are responsible for the improved OPV performance in terms of high <i>J</i><sub>sc</sub>, <i>FF</i>, and PCE.
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