Thermally Induced Formation of HF4TCNQ- in F4TCNQ-Doped Regioregular P3HT

Abstract
The prototypical system for understanding doping in solution-processed organic electronics has been poly(3-hexylthiophene) (P3HT) p-doped with 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ). Multiple charge transfer states, defined by the fraction of electron transfer to F4TCNQ, are known to coexist and are dependent on polymer molecular weight, crystallinity, and processing. Less well understood is the loss of conductivity after thermal annealing of these materials. Specifically, in thermoelectrics, F4TCNQ-doped regioregular (rr) P3HT exhibits significant conductivity losses at temperatures lower than other thiophene-based polymers. Through detailed spectroscopic investigation of progressively heated P3HT films co-processed with F4TCNQ, we demonstrate that this diminished conductivity is due to formation of the non-chromophoric, weak dopant HF4TCNQ-. This species is likely formed through hydrogen abstraction from the alpha aliphatic carbon of the hexyl chain at the 3-position of thiophene rings of rr-P3HT. This reaction is eliminated for polymers with ethylene glycol-containing side chains, which retain conductivity at higher operating temperatures. In total, these results provide a critical materials design guideline for organic electronics.

Citation
Watts, K. E., Neelamraju, B., Moser, M., McCulloch, I., Ratcliff, E. L., & Pemberton, J. E. (2020). Thermally Induced Formation of HF4TCNQ- in F4TCNQ-Doped Regioregular P3HT. The Journal of Physical Chemistry Letters. doi:10.1021/acs.jpclett.0c01673

Acknowledgements
The authors acknowledge support of this research by the National Science Foundation under award DMR-1608289. K.E.W. acknowledges financial support through an ARCS Foundation Scholarship and an ACS Division of Analytical Chemistry Fellowship sponsored by Eli Lilly and Company. M.M and I.M. acknowledge generous funding from KAUST for financial support. The authors also acknowledge very useful discussions with Professor Richard S. Glass regarding mechanistic aspects of this work. Use of the Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory is supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Contract No. DE-AC02-76SF00515.

Publisher
American Chemical Society (ACS)

Journal
The Journal of Physical Chemistry Letters

DOI
10.1021/acs.jpclett.0c01673

Additional Links
https://pubs.acs.org/doi/10.1021/acs.jpclett.0c01673

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