Green Solvent-Processible N-H-Functionalized Perylene Diimide Materials for Scalable Organic Photovoltaics.

The growing demand for organic electronic devices warrants further development of the scalability and green solvent processibility of π-conjugated materials. Perylene diimide (PDI)-based materials have shown impressive performance as interlayers for electronic devices due to a low ELUMO energy and high charge mobility in films. The next step in the development of these materials is the transition toward scalable production and the fabrication of devices under ambient conditions. Here, we develop a green synthetic methodology to prepare a series of PDI-based electronically active materials (X2-5), which can be slot-die-coated into uniform thin films from green solvents in air. Compounds X2-5 comprised a monomeric PDI core with a functional cyclic secondary amine appended to the bay region. Bromine or cyano moieties are incorporated into the molecular scaffold to systematically tune optoelectronic properties. The utility of these materials is demonstrated by slot-die coating them from ethanol to serve as cathode interlayers in prototype air-processed conventional organic photovoltaics. Using a PM6:Y6 active layer, device power conversion efficiencies reached 10%, among the best reported under these conditions.

Regioisomerically Pure 1,7-Dicyanoperylene Diimide Dimer for Charge Extraction from Donors with High Electron Affinities.

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.

Watching Paint Dry: Operando Solvent Vapor Annealing of Organic Solar Cells.

The commercialization of organic solar cell (OSC) technology will require highly reproducible techniques for controlling the morphology of bulk heterojunction blends. Variable-pressure solvent vapor annealing (VP-SVA) is one method for postprocessing organic solar cells with high precision; it can prevent the overannealing of cells that plagues conventional SVA processes. To gain insight into the dynamics of the VP-SVA process, we carried out operando measurements on OSCs with correlated in situ grazing-incidence wide-angle X-ray scattering (GIWAXS) measurements. We show that the partial pressure of solvent vapor controls the length scale of film reordering, with optimal restructuring taking place below the saturation vapor pressure of the solvent. The experiments reveal how the film crystallinity, domain sizes, and percolation pathways evolve over the course of the VP-SVA process and how subtle differences in these morphological parameters differentiate good OSCs from champion cells.