Tweaking the Molecular Geometry of a Tetraperylenediimide Acceptor

Partial flattening of the spatially extended molecular scaffold has been employed as an effective tactic to improve the device performance of a perylenediimide (PDI)-based small-molecule acceptor because the less twisted yet not completely planar molecular geometry is anticipated to improve the molecular packing and thereby attain a more suitable balance between the carrier transport ability and phase domain size. 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 the tetraphenylethylene (TPE) unit. Due to the less twisted and better conjugated BF skeleton, BF–PDI displays more delocalized lowest unoccupied molecular orbital. By virtue of both the electronic and steric effects, BF–PDI is suggested to bring about superior intermolecular stacking and donor–acceptor phase separation morphology in blend films. Indeed, the experimental results show that BF–PDI displays improved charge transport ability and a higher power-conversion efficiency of 8.05% than that of TPE–PDI. Grazing-incidence wide-angle X-ray diffraction and resonant soft X-ray scattering confirm the more compact and ordered molecular packing as well as smaller domain sizes in the P3TEA/BF–PDI blend