Two cyanopyridinone-capped 9,9′-bifluorenylidene derivatives as non-fullerene acceptors for organic photovoltaic cells

Two new cyanopyridinone-capped 9,9 '-bifluorenylidene derivatives, 5,5',5,5'-(([9,9'-bifluorenylidene]-3,3',6,6'-tetrayltetrakis(4-hexylthiophene-5,2-diyl))tetrakis(methanylylidene)) tetrakis(1-butyl-4-methyl-2,6-dioxo-1,2,5,6-tetrahydropyridine-3-carbonitrile) (A1) and 5,5',5,5'-(([9,9'-bifluorenylidene]-2,2',7,7'-tetrayltetrakis(4-octylthiophene-5,2-diyl))tetrakis (methanylylidene))tetrakis(1-(2-ethylhexyl)-4-methyl-2,6-dioxo-1,2,5,6-tetrahydropyridine-3-carbonitrile) (A2), were synthesized as non-fullerene small molecular acceptors for organic solar cells. The compounds exhibited excellent solubility in a variety of common organic solvents such as dichloromethane, toluene and o-dichlorobenzene due to possessing different length alkyl chains on the thiophene-bridged groups and the cyanopyridinone units. The introduction of the strong electron-withdrawing cyanopyridinone groups as the capping groups can improve the intramolecular charge transfer (ICT) and thus broaden the absorption spectra of the molecules. By changing the connection points of the capping groups in the 9,9'-bifluorenylidene core, the photophysical and electrochemical properties of the compounds can be easily adjusted. The photovoltaic performance of A1 and A2 as acceptors was investigated by fabricating solar cells with the following structure: ITO/PEDOT:PSS/PBDB-T:A1(or A2)/PDTNO/Al. The results showed that power conversion efficiencies (PCEs) of 1.01% and 1.98% were obtained using A1 and A2, respectively, as the acceptors without any treatment. After treating the active layers with solvent vapor annealing, their PCEs were increased to 1.28% and 2.94%, respectively.

Automated summary

Two new cyanopyridinone-capped 9,9'-bifluorenylidene derivatives were synthesized as non-fullerene acceptors for organic solar cells, exhibiting excellent solubility and improved intramolecular charge transfer. By changing the connections points of the capping groups, the photophysical and electrochemical properties of the compounds were easily adjusted. The power conversion efficiencies of the solar cells using these acceptors were improved after treating the active layers with solvent vapor annealing.