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.