Spiro Linkage as an Alternative Strategy for Promising Nonfullerene Acceptors in Organic Solar Cells

<jats:p>This work focuses on developing diketopyrrolopyrrole (DPP)‐based small molecular nonfullerene acceptors for bulk heterojunction (BHJ) organic solar cells. The materials, <jats:bold>SF‐DPP</jats:bold>s, have an X‐shaped geometry arising from four DPP units attached to a spirobifluorene (SF) center. The spiro‐dimer of DPP‐fluorene‐DPP is highly twisted, which suppresses strong intermolecular aggregation. Branched 2‐ethylhexyl (EH), linear <jats:italic>n</jats:italic>‐octyl (C8), and <jats:italic>n</jats:italic>‐dodecyl (C12) alkyl sides are chosen as substituents to functionalize the <jats:italic>N</jats:italic>,<jats:italic>N</jats:italic>‐positions of the DPP moiety to tune molecular interactions. <jats:bold>SF‐DPPEH</jats:bold>, the best candidate in <jats:bold>SF‐DPP</jats:bold>s family, when blended with poly(3‐hexylthiophene) (P3HT) showed a moderate crystallinity and gives a <jats:italic>J</jats:italic><jats:sub>sc</jats:sub> of 6.96 mA cm<jats:sup>−2</jats:sup>, <jats:italic>V</jats:italic><jats:sub>oc</jats:sub> of 1.10 V, a fill factor of 47.5%, and a power conversion efficiency of 3.63%. However, <jats:bold>SF‐DPPC8</jats:bold> and <jats:bold>SF‐DPPC12</jats:bold> exhibit lower crystallinity in their BHJ blends, which is responsible for their reduced <jats:italic>J</jats:italic><jats:sub>sc</jats:sub>. Coupling DPP units with SF using an acetylene bridge yields <jats:bold>SF‐A‐DPP</jats:bold> molecules. Such a small modification leads to drastically different morphological features and far inferior device performance. These observations demonstrate a solid structure–property relationship by topology control and material design. This work offers a new molecular design approach to develop efficient small molecule nonfullerene acceptors.</jats:p>