Tweaking the Molecular Geometry of a Tetraperylenediimide Acceptor.

In the current contribution, partially flattening the spatially extended molecular scaffold has been employed to improve the device performance of a PDI-based small-molecule acceptor, because the less twisted yet not completely planar molecular geometry is anticipated to enhance the molecular packing ability and thereby attain a more suitable balance between the carrier transporting ability and phase domain size. Hence, a small-molecule acceptor BF-PDI comprising four α-substituted PDI units attached around a 9,9'-bifluorenylidene (BF) central moiety is designed and studied in polymer solar cells. The BF group is deemed a ring-fused analogue of tetraphenylethylene (TPE). Presumably due to the more favaourable molecular packing resulting from the more planar geometry and better conjugation of the BF unit, BF-PDI displays superior charge transport ability and a higher PCE of 8.05% than TPE-PDI. GIWAXD and RSoXS confirm the more compact and ordered packing as well as smaller domain sizes in the P3TEA:BF-PDI blend. By comparing the device data and morphology features of the two PDI acceptors, it is demonstrated that fine-tuning the 3D structure and molecular packing of small-molecular acceptor is an effective approach to optimizing the PSC performance.