Fused perylenediimide dimer as nonfullerene acceptor for high-performance organic solar cells

Four perylenediimide (PDI) dimers UFPDI, FPDI, UFPDI-T and FPDI-T are synthesized and used as nonfullerene acceptors for the fabrication of organic solar cells with the medium-bandgap polymer PBDB-T as the donor material. These four acceptors with three-dimensional molecular structures display good solubilities in the processing solvent, which is helpful to form nanoscale separation when blended with the polymer donor. The PDI subunits in the unfused PDI dimers UFPDI and UFPDI-T are twisted and the two acceptors display a broad featureless absorption in the visible region. The fused PDI dimers FPDI and FPDI-T display an X-shaped geometry with large dihedral angles of approximately 40° and their PDI subunits are planar. The fused PDI dimers with the X-shaped molecular conformation is helpful for suppressing the formation of large sized aggregations and the planar PDI subunits are in favor of forming closely π-π stacking to ensure reasonable electron mobilities. Moreover, UFPDI-T and FPDI-T with fused thiophene at outer bay positions possess elevated LUMO energy levels, which can enhance the open circuit voltage (Voc) of devices. FPDI-T based devices display much higher and more balanced hole/electron mobilities. A high power conversion efficiency (PCE) of 7.5% is achieved for FPDI-T based devices, which is much higher than those of UFPDI (4.19%), FPDI (2.95%) and UFPDI-T (4.61%) based ones. Our results demonstrate that simultaneously inner and outer annulation of PDI dimers is a feasible strategy to design high performance PDI-based acceptors.