Nonfullerene acceptor isomer with mono-fluorine end-substitution enables oligothiophene-based terpolymer donor with 17.82% efficiency

In the field of non-fullerene organic solar cells (OSCs), most of the promising non-fullerene acceptors (NFAs) are based on bi-fluorinated 1,1-dicyanomethylene-3-indanone (IC) while NFAs with mono-fluorinated IC have drawn less attenton because of low device performances. Herein, two NFA isomers BTzC4IC-2F-delta gamma and BTzC4IC-2F-delta, with benzotriazole-fused core and mono-fluorine end-substituted IC, were facilely synthesized and separated via direct column chromatography method. Based on the asymmetric and symmetric isomers, the isomeric effect for the mono-fluorination on IC were investigated, indicating BTzC4IC-2F-delta gamma possessing upper-lying energy levels while BTzC4IC-2F-delta showing red-shifted absorption, stronger intramolecular interaction, and higher electron mobility. Oligothiophene-based terpolymer PFBT4T-T20 with low-cost potential was selected as the donor, and the BTzC4IC-2F-delta based active layer afforded longer carrier lifetime and shorter charge extraction time as well as more intense and wider photoresponses to achieve external quantum efficiency of 64 % at 900 nm. The BTzC4IC-2F-delta based OSCs showed slightly lower open-circuit voltage but obviously enhanced short-circuit current density and fill factor, finally achieving a higher power conversion efficiency of 17.82 % than that of 16.91 % for the BTzC4IC-2F-delta gamma based device. The best efficiency of 17.82 % achieved with BTzC4IC-2F-delta is the highest one among binary OSCs with oligothiophene-based polymers as the donor and also the highest one among OSCs based on mono-fluorine end-substituted NFAs.