Three Isomeric Non-Fullerene Acceptors Comprising a Mono-Brominated End-Group for Efficient Organic Solar Cells

Abstract

Non-fullerene acceptors (NFAs) carrying a 1,1-dicyanomethylene-3-indanone (IC) end-group are the most powerful ones to boost the power conversion efficiency of organic solar cells (OSCs). However, the well-known Knoevenagel condensation of the mono-halogenated IC end-group will result in an NFA isomeric effect, a chemical issue that needs to be addressed. Herein, facile preparations and separations of three well-defined mono-brominated isomers BTzIC-2Br-δ, BTzIC-2Br-γ, and BTzIC-2Br-δγ via column chromatography with a well-chosen mixing solvent were demonstrated for Knoevenagel condensation, and their structures were verified by NMR spectra and matrix-assisted laser desorption ionization-time of flight mass spectrometry (MALDI-TOF) mass spectra. It is the first time that an asymmetric isomer BTzIC-2Br-δγ is reported, and the regioisomeric effect on optoelectronic properties can be investigated based on all three isomers. Moreover, the single-crystal structure was successfully achieved for the symmetric molecule BTzIC-2Br-γ. With benzodithiophene (BDT)-free PFBT4T-T20 as an easily accessible and low-cost polymer donor, the three isomers could show differentiated device performances, with a power conversion efficiency order of BTzIC-2Br-γ (16.00%) > BTzIC-2Br-δγ (15.81%) > BTzIC-2Br-δ (15.29%). The best efficiency of 16.00% achieved with BTzIC-2Br-γ is among the highest ones for binary OSCs based on the low-cost BDT-free donors. The facile and complete synthesis of isomeric NFAs with mono-halogenated IC end-groups would promote the elucidation of the structure–property relationship.

Effect of Number and Position of Chlorine Atoms on the Photovoltaic Performance of Asymmetric Nonfullerene Acceptors

“…In addition, the selection and optimization of the end group are also critical to improve the photovoltaic performance of materials. − Typically, halogenated 1,1-dicyanomethylene-3-indanone (IC) end group occupies more than half of NFAs in the high-performance OSCs system. − The introduction of halogen atoms (including fluorine, chlorine, and bromine) can enhance the electron-withdrawing ability of end groups, resulting in a narrower band gap. Additionally, the halogen atoms in the end groups can notably tune the molecular packing and morphology, which are driven by intramolecular and intermolecular noncovalent interaction.…”

Isomerization strategy on a non-fullerene guest acceptor for stable organic solar cells with over 19% efficiency

“…21,22 Currently, the isomerization of Y6-derivatives has become an effective way to improve the performance of binary OSCs. [23][24][25] In particular, almost all the Y6-derivatives have been developed by employing isomeric endcapping groups or side chains, [26][27][28][29][30] but the isomerization of the central electron-deficient core in Y6-derivatives has never been investigated or reported. In addition, the low glass transition temperatures (T g ) and high diffusion properties of the Y6-derivatives lead to unstable morphologies in blend films.…”

Dithienobenzoselenadiazole-Based Polymer Donors with Tuned Side Chains for Efficient Polymer Solar Cells

“…Polymer solar cells (PSCs) have attracted extensive attention thanks to their versatile properties, including solution-processing methods, flexibility, lightweight, and easily tunable structures. − In recent years, the speedy development of non-fullerene acceptors (NFAs) − and polymer donors − has greatly improved the photovoltaic performance of PSCs, and power conversion efficiencies (PCEs) of PSCs have exceeded 18%. − Among them, design of high-performance polymer donors, which should be paired with highly developed narrow-bandgap NFAs, is a key to further improve the photovoltaic performance of PSCs.…”