Noncovalent Interactions Induced by Fluorination of the Central Core Improve the Photovoltaic Performance of A-D-A′-D-A-Type Nonfused Ring Acceptors

Nonfused ring acceptors (NFRAs) have blazed a trail in achieving high-efficiency organic solar cells (OSCs) from low-cost materials due to their simple synthesis. In this work, two A-D-A'-D-A-type NFRAs, comprising benzotriazole or ditluorinated benzotriazole as the electron-deficient core, namely, BTz-HD and ffBTz-HD, were synthesized via direct arylation coupling reaction. The influence of fluorination of the central core on molecular packing and the photovoltaic performance of the nonfused acceptors were investigated by analyzing the single-crystal structures of two model compounds BTz-2T (fluorine free) and ffBTz-2T (fluorinated). Compared with BTz-2T, ffBTz-2T exhibits a more planar molecular skeleton and forms a slip-stack stacking with pi-pi stacking distances of 3.58 and 3.67 angstrom owing to the existence of F center dot center dot center dot S, S center dot center dot center dot H, and H center dot center dot center dot F noncovalent interactions. These characteristics favor the ordered and compact stacking of ffBTz-HD in the solid state, which facilitated charge transport and inhibited charge recombination in solar cells. These merits endowed the ffBTz-HD-based OSC with a higher short-circuit current density and fill factor than the BTz-HD-based OSC. As a result, a higher power conversion efficiency of 10.56% has been achieved by ffBTz-HD. The structure-property relationship unraveled in this study is beneficial to the development of more efficient NFRAs for application in OSCs.