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

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 V_oc, J_sc, 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 V_oc of 0.86 V, J_sc 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.