Realizing an Unprecedented Fill Factor of 82.2% in Ternary Organic Solar Cells via Co‐Crystallization of Non‐Fullerene Acceptors

Abstract Ternary strategy is demonstrated as an efficient approach to achieve high short‐circuit current and open‐circuit voltage to boost the performance of organic solar cells (OSCs), however, the realization of high fill‐factor (FF) in ternary OSCs has been rare. In this study, three thiophene terminated non‐fullerene acceptors (NFAs) with methyl or chlorine substitutions on their end‐groups are designed and synthesized, and further incorporated into the state‐of‐the‐art PM6:L8‐BO system to construct ternary OSCs. Subtle changes in their chemical structures significantly modify the molecular packings of these thiophene terminated NFAs. While BTP‐ThMe and BTP‐ThCl have limited forms of dimer, versatile molecular dimers, including “Z” shaped D‐D, “S” shaped A‐A, and “F” shaped A‐D packings exist in BTP‐ThMeCl, which lead to the formation of compact 3D honey‐comb network and this is analogous to the host acceptor L8‐BO. This synergetic molecular packing between BTP‐ThMeCl and L8‐BO contributes to maintain the 3D charge transport network in the ternary system via the formation of NFA co‐crystals at the molecular level, and consequently realizing a maximum power conversion efficiency of 19.1% with a superior FF of 82.2%, which is the highest FF reported so far for OSCs.