Nonfullerene Small Molecular Acceptors with a Three-Dimensional (3D) Structure for Organic Solar Cells

Two spirobifluorene (SF)-functioned 3D nonfullerene electron acceptors—SF-OR and SF-ORCN—that use rhodanine and 2-(1,1-dicyanomethylene)rhodanine as the terminal units, respectively, were designed and synthesized. These new acceptor materials show reversible electrochemical reduction and a high optical absorption coefficient, which are critical in device operation. Electronic and structural characterizations reveal that the inclusion of cyano group on rhodanine improve the electron accepting ability at the sacrifice of structure order, since the conjugated backbone becomes less planar, which prohibits cofacial stacking. SF-OR and SF-ORCN show good photovoltaic performances when paired with a poly(3-hexylthiophene) (P3HT) donor, and the optimized devices give power conversion efficiencies of 4.66% and 4.48%, respectively. These values are higher than that of fullerene acceptor-based control devices (3.55%), which are also among the best in small molecular nonfullerene acceptor organic solar cells based on P3HT.