The continued improvement in efficiency for organic photovoltaics (OPVs) is due in part to the development of new conjugated polymeric materials. In this presentation, I will first discuss a new design motif to incorporate fluorine substituents into conjugated polymers using the common thiophene linkers which connect the donor and acceptor moieties of the polymer, rather than fluorinating directly on the acceptor moiety. This new design motif will allow for materials, previously with no fluorination location, to enjoy the benefits attributed to fluorination. Specifically, we introduce a difluorinated thiophene (dFT) linker into the BnDT-TAZ system to create a new polymer, PBnDT-dFT-HTAZ. Compared with the nonfluorinated counterpart (PBnDT-HTAZ), a 50% improvement in the efficiency is seen when using the dFT linker.<br/><br/>Next, of the high efficiency electron acceptors reported, the vast majority utilize derivatives of 2-(3-oxo-2,3-dihydroinden-1-ylidene)malononitrile (INCN) as the acceptor moiety. It has been postulated that the high electron mobility exhibited by FREA molecules with INCN end groups is a result of close π-π stacking between the neighboring planar INCN groups, forming an effective charge transport pathway. To explore this as a design rationale for electron acceptors, we synthesized a new fused-ring electron acceptor, IDTCF, which has methyl substituents out of plane to the conjugated acceptor backbone. These methyl groups hinder packing and expand the π-π stacking distance by ~ 1 Å, but this change doesn’t affect the optical or electrochemical properties of the individual acceptor molecule. Our results show that intermolecular interactions (especially π-π stacking between end groups) play a crucial role in performance of FREAs. We demonstrated that the planarity of the acceptor unit is of paramount importance as even minor deviations in end group distance are enough to disrupt crystallinity and cripple device performance.

Molecular engineering conjugated polymers and non-fullerene acceptors for organic photovoltaics: accomplishments and lessons

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.