Oligo(ethylene oxide) chains in fluorene bridge units of perylenediimide dimers as an efficient strategy for improving the photovoltaic performance in organic solar cells

Perylenediimides (PDIs) are among the most promising non-fullerene electron-acceptors for applications in organic photovoltaics (PV), providing a large scope for structural modifications. PDI-based dimers have shown the highest performances in PV devices owing to their characteristic twisted conformations that reduce their tendency to form large aggregates which are detrimental for PV operation. In this work, two fluorene-bridged perylenediimide (PDI) dimers with oligo(ethylene oxide) or alkyl chains anchored to the C9 position of the fluorene unit were synthesized and investigated in solution-processed organic bulk heterojunction (BHJ) photovoltaic cells as electron acceptors. The PDI dimer substituted with oligo(ethylene oxide) chains resulted in improved power conversion efficiencies (in 20%-53%) in solution processed bulk heterojunction (BHJ) cells with two different polymer donors, PTB7 and PffBT4T-20D. Nevertheless, the replacement of alkyl chains by oligo(ethylene oxide) chains did not significantly affect the geometric characteristics of the PDI dimers and the optical and electrochemical properties were only marginally modified. Increased exciton dissociation and enhanced charge transport derived from a more densely packed pi-pi stacking in the solid state caused by ethylene oxide groups are pointed out as possible causes for the improved PV performance. The influence of 1,8-diiodooctane as solvent additive in the blend films was also investigated and allowed to further increase the efficiencies of the cells with PTB7. Overall, the results show that the simple replacement of alkyl chains by oligo (ethylene oxide) chains in PDI dimers is an efficient way to improve the PV performance without compromising the optoelectronic properties of the PDI acceptor.