We choose the high-performance nonfullerene acceptor ITIC-Th as an example, and incorporate electron-donating methoxy and electron-withdrawing F groups onto the terminal group 1,1-dicyanomethylene-3-indanone (IC) to construct a small library of four fused-ring electron acceptors. With this series, we systematically investigate the effects of the substituents on the end-groups on the electronic properties, charge transport, film morphology, and photovoltaic properties of the ITIC-Th series. The electron-withdrawing ability increases from methoxylated to unsubstituted, fluorinated, and difluorinated IC, leading to a downshift of energy levels and a redshift of absorption spectra. Optimized organic solar cells based on the ITIC-Th series show power conversion efficiencies ranging from 8.88% to 12.1%.
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The crucial role of end group planarity for fused-ring electron acceptors in organic solar cells
Newly developed fused-ring electron acceptors (FREAs) have proven to be an effective class of materials for extending the absorption window and boosting the efficiency of organic photovoltaics (OPVs). While numerous acceptors have been developed, there is surprisingly little structural diversity among high performance FREAs in literature. 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 between molecules. 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 have little impact on the optical or electrochemical properties of the individual FREA molecule. The extra steric hindrance from the out of plane methyl substituents restricts packing and results in large amounts of geminate recombination, thus degrading the device performance. 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.
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- Award ID(s):
- 1639429
- NSF-PAR ID:
- 10106844
- Date Published:
- Journal Name:
- Materials Chemistry Frontiers
- Volume:
- 3
- Issue:
- 8
- ISSN:
- 2052-1537
- Page Range / eLocation ID:
- 1642 to 1652
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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null (Ed.)Non-fullerene acceptors (NFAs) are highly promising materials for organic photovoltaics (OPVs). Exciton diffusion in NFAs is crucial to their photovoltaic performance, but is not yet well understood. Here we systematically examine exciton diffusion in a fused-ring electron acceptor (IDIC) based on a first-principles framework. We discover that low-energy excitons in disordered IDIC are charge-separated with electrons and holes residing on neighboring molecules, yielding long exciton lifetimes. With low energetic disorder, high exciton density of states (DOS) and long lifetimes, the disordered IDIC is predicted to exhibit large exciton diffusion lengths and high quantum efficiency. The temperature and energy dependences of exciton diffusion are explored and the manner in which various materials properties (exciton energy, DOS, energetic disorder, and phonon frequency) conspire to influence exciton diffusion is elucidated. Finally, we show that dilation could be an effective strategy to increase the exciton diffusion length in IDIC.more » « less
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Abstract The crystal structures of the charge‐transfer (CT) cocrystals formed by the π‐electron acceptor 1,3,4,5,7,8‐hexafluoro‐11,11,12,12‐tetracyanonaphtho‐2,6‐quinodimethane (F6TNAP) with the planar π‐electron‐donor molecules triphenylene (TP), benzo[
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Abstract Organic photovoltaic power conversion efficiencies exceeding 14% can largely be attributed to the development of nonfullerene acceptors (NFAs). Many of these molecules are structural derivatives of IDTBR and ITIC, two common NFAs. By modifying the chemical structure of the acceptor, the optical absorption, energy levels, and bulk heterojunction morphology can be tuned. However, the effect of structural modifications on NFA charge transport properties has not yet been fully explored. In this work, the relationship between chemical structure, molecular packing, and charge transport, as measured in organic thin‐film transistors (OTFTs), is investigated for two high performance NFAs, namely O‐IDTBR and ITIC, along with their structural derivatives EH‐IDTBR and ITIC‐Th. O‐IDTBR exhibits a higher n‐type saturation field effect mobility of 0.12 cm2V−1s−1compared with the other acceptors investigated. This can be attributed to the linear side chains of O‐IDTBR which direct an interdigitated columnar packing motif. The study provides insight into the transport properties and molecular packing of NFAs, thereby contributing to understanding the relationship between chemical structure, material properties, and device performance for these materials. The high electron mobility achieved by O‐IDTBR also suggests its applications can be extended to use as an n‐type semiconductor in OTFTs.
- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1039/C9QM00314B
