Abstract
Perylene diimide (PDI) has attracted widespread interest as an inexpensive electron acceptor for photovoltaic applications; however, overcrystallization in the bulk heterojunction typically leads to low device performance. Recent work has addressed this issue by forming bay-linked PDI dimers and oligomers, where the steric bulk of adjacent PDI units forces the molecule to adopt a nonplanar structure. This disrupts the molecular packing and limits domain sizes in the bulk heterojunction. Unfortunately, the introduction of electron-donating/-withdrawing groups in the bay region is also the best way to fine-tune the frontier molecular orbitals (FMOs) of PDI, which is highly desirable from a device optimization standpoint. This competition for the bay region has made it difficult for PDI to keep pace with other non-fullerene acceptors. Here, we report the synthesis of regioisomerically pure 1,7-dicyanoperylene diimide and its dimerization through an imide linkage. We show that this is an effective strategy to tune the energies of the FMOs while simultaneously suppressing overcrystallization in the bulk heterojunction. The resulting acceptor has a low LUMO energy of -4.2 eV and is capable of accepting photogenerated electrons from donor polymers with high electron affinities, even when conventional acceptors such as PDI, PC(71)BM, and ITIC cannot.