Performances of two side-chain modified medium-bandgap alternating polymers with main-chain twisted non-fullerene acceptor

Two alternating polymers (P3 and P4) based on side-chain modified benzo[1,2-b:4,5-b']dithiophene as the electron-donating unit and fluorinated benzothiadiazole derivatives as the electron-withdrawing segment were adopted as donor materials to blend with a main-chain twisted non-fullerene acceptor (i-IEICO-4 F) for bulk heterojunction polymer solar cells (PSCs). The effects of the two types of side-chain substitutions on the UV-Vis absorptive characteristics, frontier molecular orbitals, and photovoltaic performances of the polymers were systematically probed. Both two polymers have medium-bandgaps, 1.75 eV for P3 and 1.71 eV for P4, respectively, guaranteeing the complementary absorption with i-IEICO-4 F. Among these polymers, the device of polymer P3, which has 2,3-octylthienyl side-chain, shows enhanced dielectric constant, suppressed trap-assisted recombination, promoted hole mobility with more balanced carrier mobility, and suitable phase separation, thus successfully achieving an efficiency of 10.67%. While the alkoxylphenyl substituted polymer P4 has slightly deepened molecular energy level and bathochromic-shifted absorption spectrum and achieved enhanced open circuit voltage, improved electron mobility, and inhibitive bimolecular recombination in the device, thus acquiring a slightly reduced efficiency of 10.19%. This work not only elaborates the applicability of benzothiadiazole-based medium bandgap polymer donors with main-chain twisted acceptors but also reveals the importance of the side-chain modification of the polymer donors on the performances of the fabricated PSCs.