A Trialkylsilylthienyl Chain-Substituted Small-Molecule Acceptor with Higher LUMO Level and Reduced Band Gap for Over 16% Efficiency Fullerene-Free Ternary Solar Cells

The ternary approach using a smaller band gap acceptor as the near-infrared (NIR) absorber to increase the short-circuit current density ( J sc ) usually decreases the open-circuit voltage ( V oc ). In this contribution, we report a small-molecule acceptor, IN-4F, which has a reduced band gap and a higher LUMO level than IT-4F, hence enabling the concurrent increase in the J sc and V oc when using IT-4F as the acceptor guest of the host binary of PM6:IT-4F. IN-4F was judiciously designed by fusing benzodithiophene (BDT) and thieno[2′,3′:4,5]thieno to make a larger π-system so as to upshift the LUMO level and reduce the optical band gap and, meanwhile, by substituting the BDT-4,8 positions with trialkylsilylthiophene chains to downshift the HOMO level to match the deep HOMO of PM6. Again, the structural similarity between IN-4F and IT-4F makes the nanoscaled homogeneous fine film morphology and the π–π stacking patterns both well kept; hence, the fill factor (FF) is well maintained. The IN-4F-based binary solar cell shows 13.1% efficiency, and its ternary solar cell blended with IT-4F supplies 14.9% efficiency. Again, the use of IN-4F as the guest acceptor of the PM6:Y6 system enables the increase in V oc due to its higher LUMO level, the increase in J sc because of the increase in charge mobilities, and the maintenance of FF, affording 16.3% efficiency. This work demonstrates that the π-system extending and the trialkylsilylthiophene chain substitution can be an effective strategy to synthesize a nonfullerene acceptor guest to realize a ternary material system, which enables to increase V oc from its entanglement with J sc (an issue of the current material approach).