Effect of Acceptor Unit Length and Planarity on the Optoelectronic Properties of Isoindigo-Thiophene Donor-Acceptor Polymers

Abstract
Conjugated polymers with a donor-acceptor (DA) structural motif have found extensive use in a wide variety of optoelectronic devices; however, despite their ubiquity in the literature, the vast majority of these materials are simple alternating copolymers - one electron donor alternates with one electron acceptor in the polymer backbone. As a result, the impact of composition (e.g., donor/acceptor ratio) and structure (e.g., alternating, block, or random) on the optoelectronic properties of these copolymers remains poorly understood. In this work, the number of acceptor units in alternating DA copolymers is systematically increased. Two dimers of the common electron acceptor isoindigo are synthesized, one with free rotation between the subunits and one with enforced coplanarity. The two dimers are then used to synthesize donor-acceptor-acceptor (DAA) copolymers with either thiophene or terthiophene comonomers. These DAA polymers feature two electron acceptors in their repeat unit, and their optoelectronic properties are compared to those of the analogous DA polymers. It is shown that increasing the number of acceptor units causes a decrease in the LUMO energy of the resulting polymer; this effect is enhanced by enforcing coplanarity between acceptor units via ring fusion. All six polymers were tested in both organic photovoltaics (OPVs) and organic thin film transistors (OTFTs). While the DA polymers performed better in OPVs, the DAA polymers displayed more balanced charge carrier mobilities in OTFTs.

Citation
Randell, N. M., Radford, C. L., Yang, J., Quinn, J., Hou, D., Li, Y., & Kelly, T. L. (2018). Effect of Acceptor Unit Length and Planarity on the Optoelectronic Properties of Isoindigo–Thiophene Donor–Acceptor Polymers. Chemistry of Materials, 30(14), 4864–4873. doi:10.1021/acs.chemmater.8b02535

Acknowledgements
The Natural Science and Engineering Research Council of Canada (NSERC, RGPIN-2017-03732) and the University of Saskatchewan are acknowledged for financial support. T.L.K. is a Canada Research Chair in Photovoltaics. This research was undertaken in part, thanks to funding from the Canada Research Chairs Program. N.M.R. and C.L.R. thank the NSERC for scholarship funding. The GIWAXS experiments described in this paper were performed at the Canadian Light Source, which is supported by the Canada Foundation for Innovation, NSERC, the University of Saskatchewan, the Government of Saskatchewan, Western Economic Diversification Canada, the National Research Council Canada, and the Canadian Institutes of Health Research. Technical support from HXMA beamline scientist Dr. Chang-Yong Kim is gratefully acknowledged. Federico Cruciani and Prof. Pierre Beaujuge (KAUST) are gratefully acknowledged for the SEC analysis.

Publisher
American Chemical Society (ACS)

Journal
CHEMISTRY OF MATERIALS

DOI
10.1021/acs.chemmater.8b02535

Additional Links
https://pubs.acs.org/doi/10.1021/acs.chemmater.8b02535

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