Fluorination as an effective tool to increase the photovoltaic performance of indacenodithiophene-alt-quinoxaline based wide-bandgap copolymers

• Two novel D–A-type polymers of PIDT-DTPQx and PIDT-DTFPQx were synthesized and developed.
• The two polymers show a wide optical bandgaps (Egopt < 1.8 eV) and low HOMO levels.
• The hole PIDT-DTFPQx-based devices presented a high hole mobility of 1.02 × 10−3 cm2 V−1 s−1.
• The PIDT-DTFPQx-based devices exhibited a PCE of 5.78% with a Jsc of 10.84 mA cm−2 and FF of 61.7%.

To investigate the effect of the fluoride phenyl side-chains into quinoxaline (PQx) unit on the photovoltaic performances of polymers, we demonstrated the synthesis and characterization of two novel wide-bandgap (WBG) copolymers, PIDT-DTPQx and PIDT-DTFPQx, in which indacenodithiophene (IDT), 2,3-diphenylquinoxaline (PQx) (and/or 2,3-bis(4-fluorophenyl)quinoxaline (FPQx)) and thiophene (T) were used as the donor (D) unit, acceptor (A) unit and π-bridge, respectively. Compared to the non-fluorine substituted PIDT-DTPQx, fluorine substituted PIDT-DTFPQx presents a deep HOMO energy level and a high hole mobility. Obviously, improved the Voc, Jsc, and FF simultaneously, giving rise to overall efficiencies in the PIDT-DTFPQx/PC71BM-based PSCs. A highest PCE of 5.78% was obtained with a Voc of 0.86 V, Jsc of 10.84 mA cm−2 and FF of 61.7% in the PIDT-DTFPQx/PC71BM-based PSCs, while PIDT-DTPQx based devices also demonstrated a PCE of 5.11%, under the illumination of AM 1.5G (100 mW cm−2). Note that these PCE values were achieved for PSCs without any extra treatments. Furthermore, these optimal devices have a film thickness of about 175 nm for the polymer/PC71BM-based active layers. The results provide that introduction of the fluorine atom into quinoxaline unit by side-chain engineering is one of the effective strategies to construct the promising polymer donor materials for future application of large-area polymer solar cells.

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