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, PC71BM, and ITIC cannot.
Highlights
The field of organic photovoltaics (OPVs) has long-promised to produce colorful, flexible, solar cells with unique form factors and low embodied energies;[1,2] it is only recently that the power conversion efficiencies (PCEs) of OPVs have started to become competitive with other technologies
OPV performance significantly improved as the chemical structures of non-fullerene acceptors (NFAs) were tailored to circumvent these issues
Our results show that the nitrile-substitution results in a pronounced drop in energy of their lowest unoccupied molecular orbital (ELUMO), while the dimerization results in smaller domain sizes in bulk heterojunction blends
Summary
The field of organic photovoltaics (OPVs) has long-promised to produce colorful, flexible, solar cells with unique form factors and low embodied energies;[1,2] it is only recently that the power conversion efficiencies (PCEs) of OPVs have started to become competitive with other technologies. Fine-tuning of both chemical structure and device architecture has led to single-junction devices with PCEs of over 15%3,4 and tandem cells with PCEs as high as 17.3%.5 This progress shows that OPVs are able to exceed the performance limits[6] previously thought to restrict their utility. Modern OPVs typically use a bulk heterojunction architecture, which is a partially phase-separated blend of an electron donor and acceptor. These two materials need to be carefully matched in terms of their optoelectronic and physicochemical properties. Examples of NFAs had a tendency to overcrystallize and the energy of their lowest unoccupied molecular orbital (ELUMO) was often too high to drive the splitting of strongly bound Frenkel excitons.[10,11] OPV performance significantly improved as the chemical structures of NFAs were tailored to circumvent these issues. NFAs have since become the mainstay of OPV research.[1,12−15]
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