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

Farinhas, J. ; Molina, D. ; Olcina, A. ; Costa, C. ; Alcácer, L. ; Fernadez-Lázaro, F. ; Sastre-Santos, Á. ; Charas, A.

Dyes and Pigments Vol. 161, Nº -, pp. 188 - 196, February, 2019.

ISSN (print): 0143-7208
ISSN (online): 1873-3743

Scimago Journal Ranking: 0,83 (in 2019)

Digital Object Identifier: 10.1016/j.dyepig.2018.09.051

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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-2OD. 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 π-π 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.