A Fused-Ring Electron Acceptor with Phthalimide-Based Ending Groups for Efficient Ternary Organic Solar Cells

The ternary strategy has been widely applied and recognized to be a valid strategy to enhance the organic photovoltaics' (OPVs) performance. Here, a new fused-ring electron acceptor, BTP-PIO, is designed and synthesized, whose ending groups were replaced by a phthalimide-based group (2-butylcyclopenta[f]isoindole-1,3,5,7(2H,6H)-tetraone) from traditional 2-(3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile. The phthalimide-based ending groups endow BTP-PIO with the highest lowest unoccupied molecular orbital (LUMO) level and wider band gap than those of Y6. The ternary device based on PM6:Y6 with BTP-PIO as a guest electron acceptor achieved an elevated open-circuit voltage (VOC) of 0.848 V, a short-circuit current density (JSC) of 27.31 mA cm-2, and a fill factor (FF) of 73.9%, generating a remarkable power conversion efficiency (PCE) of 17.10%, which is superior to the PM6:Y6 binary device of 16.08%. The ternary device exhibited improved charge transfer, suppressed carrier recombination, and lower energy loss. BTP-PIO exhibited a good miscibility with Y6, and an alloy phase between BTP-PIO and Y6 was formed in the ternary bulk heterojunction, leading to better phase separation and molecular packing. This research reveals that ending group modification of Y6 derivatives is a feasible way to produce highly efficient ternary devices.

Small-Molecule Electron Transport Layer with Siloxane-Functionalized Side Chains for Nonfullerene Organic Solar Cells

Active layer materials with silicone side chains have been broadly reported to have excellent long-term stability in harsh environments. However, the application of conjugated materials with silicone side chains in electron transport layers (ETLs) has rarely been reported. In this research, we synthesized for the first time a siloxane-modified perylene-diimide derivative (PDI-OSi) consisting of a side-chain substituent of siloxane and a conjugated group of perylene-diimide (PDI). The inserted siloxane functional groups not only can strengthen the light transmittance of PDI-OSi but also can remarkably expand its solubility and improve the film-forming ability and air stability of the material. Second, introducing siloxane-containing side chains can dramatically lower the work function and interfacial barrier of the electrode, thereby achieving a favorable ohmic contact. In addition, the moderate surface energy of siloxane functional groups makes PDI-OSi hydrophobic, which is conducive to forming excellent miscibility with hydrophobic active layers to promote charge transfer. When PDI-OSi is used as an ETL in organic solar cells (OSCs), operative exciton dissociation and more favorable surface morphology enable OSCs to realize a power conversion efficiency (PCE) of 13.99%. These results indicate that side-chain engineering with siloxane pendants is a facile strategy for constructing efficient OSCs.

Relationship between the Molecular Geometry and the Radiative Efficiency in Naphthyl-Based Bis-Ortho-Carboranyl Luminophores

The efficiency of intramolecular charge transfer (ICT)-based emission on π-aromatic-group-appended closo-ortho-carboranyl luminophores is known to be affected by structural fluctuations and molecular geometry, but investigation of this relationship has been in progress to date. In this study, four naphthyl-based bis-o-carboranyl compounds, in which hydrogen (15CH and 26CH) or trimethysilyl groups (15CS and 26CS) were appended at the o-carborane cage, were synthesized and fully characterized. All the compounds barely displayed an emissive trace in solution at 298 K; however, 15CH and 26CH distinctly exhibited a dual emissive pattern in rigid states (in solution at 77 K and in films), attributed to locally excited (LE) and ICT-based emission, while 15CS and 26CS showed strong ICT-based greenish emission. Intriguingly, the molecular structures of the four compounds, analyzed by single X-ray crystallography, showed that the C-C bond axis of the o-carborane cage in the trimethysilyl group-appended compounds 15CS and 26CS were more orthogonal to the plane of the appended naphthyl group than those in 15CH and 26CH. These features indicate that 15CS and 26CS present an efficient ICT transition based on strong exo-π-interaction, resulting in a higher quantum efficiency (Φ_em) for ICT-based radiative decay than those of 15CH and 26CH. Moreover, the 26CS structure revealed most orthogonal geometry, resulting in the highest Φ_em and lowest k_nr values for the ICT-based emission. Consequently, all the findings verified that efficient ICT-based radiative decay of aromatic group-appended o-carboranyl luminophores could be achieved by the formation of a specific geometry between the o-carborane cage and the aromatic plane.