End-cap Group Engineering of a Small Molecule Non-Fullerene Acceptor: The Influence of Benzothiophene Dioxide

In this study, a sulfonyl-containing end-capping moiety, benzothiophene dioxide, was selected to prepare the nonfullerene acceptor ITBC. ITBC has an acceptor–donor–acceptor (A-D-A) structure, with indacenodithieno[3,2-b]thiophene (IDTT) as the electron-rich core moiety. The strong electron-withdrawing sulfonyl acceptor units leads to extended UV–vis absorption into the near-IR region and relatively low frontier molecular orbital energy levels (LUMO/HOMO: −4.13 eV/–5.61 eV) with a narrow bandgap of 1.48 eV. These values compare favorably to the well-studied small molecule acceptor 2-(3-oxo-2,3-dihydroinden-1-ylidene)malononitrile end-capped indacenodithieno[3,2-b]thiophene (ITIC). A power conversion efficiency of 4.17% was achieved by fabricating organic solar cells with the fluorinated conjugated polymer FTAZ as the donor and ITBC as the acceptor. These results indicate that benzothiophene dioxide is a novel electron-withdrawing end-capping unit for ITBC, and can be used as an electron acceptor for organic solar cells.