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

This work focuses on developing diketopyrrolopyrrole (DPP)-based small molecular nonfullerene acceptors for bulk heterojunction (BHJ) organic solar cells. The materials, SF-DPPs, 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 n-octyl (C8), and n-dodecyl (C12) alkyl sides are chosen as substituents to functionalize the N, N-positions of the DPP moiety to tune molecular interactions. SF-DPPEH, the best candidate in SF-DPPs family, when blended with poly(3-hexylthiophene) (P3HT) showed a moderate crystallinity and gives a J(sc) of 6.96 mA cm(-2), V-oc of 1.10 V, a fill factor of 47.5%, and a power conversion efficiency of 3.63%. However, SF-DPPC8 and SF-DPPC12 exhibit lower crystallinity in their BHJ blends, which is responsible for their reduced J(sc). Coupling DPP units with SF using an acetylene bridge yields SF-A-DPP 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.