High fill factor organic solar cells with increased dielectric constant and molecular packing density

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<h2>Summary</h2> The fill factor (FF) of organic solar cells (OSCs), a critically important photovoltaic parameter, is still sub-optimal, often less than 0.8. To further reduce the FF gaps with regard to the Shockley-Queisser upper limit, we present a study unveiling the impacts of dielectric properties on obtaining high FFs and photovoltaic efficiencies in OSCs. By increasing the dielectric constant (ε) of non-fullerene acceptors (NFAs) and bulk heterojunction (BHJ) films, afforded by the increase in molecular packing density (MPD) of NFAs in solid states, we show that the FF penalties related to non-geminate charge recombination in OSCs can be appreciably mitigated. In particular, as exemplified by an NFA model system L8-BO tailored with branched side chains, the enlargements of MPD and ε allow achieving an impressive FF of 0.815 with an efficiency of 18.23% in OSCs, featuring an impressively low bimolecular recombination and reduced sensitivity of FF on the BHJ film thickness.