Mixed Conduction in an N-Type Organic Semiconductor in the Absence of Hydrophilic Side-Chains

Abstract
Organic electrochemical transistors (OECTs) are the building blocks of biosensors, neuromorphic devices, and complementary circuits. One rule in the materials design for OECTs is the inclusion of a hydrophilic component in the chemical structure to enable ion transport in the film. Here, it is shown that the ladder-type, side-chain free polymer poly(benzimidazobenzophenanthroline) (BBL) performs significantly better in OECTs than the donor–acceptor type copolymer bearing hydrophilic ethylene glycol side chains (P-90). A combination of electrochemical techniques reveals that BBL exhibits a more efficient ion-to-electron coupling and higher OECT mobility than P-90. In situ atomic force microscopy scans evidence that BBL, which swells negligibly in electrolytes, undergoes a drastic and permanent change in morphology upon electrochemical doping. In contrast, P-90 substantially swells when immersed in electrolytes and shows moderate morphology changes induced by dopant ions. Ex situ grazing incidence wide-angle X-ray scattering suggests that the particular packing of BBL crystallites is minimally affected after doping, in contrast to P-90. BBL's ability to show exceptional mixed transport is due to the crystallites’ connectivity, which resists water uptake. This side chain-free route for the design of mixed conductors could bring the n-type OECT performance closer to the bar set by their p-type counterparts.

Citation
Surgailis, J., Savva, A., Druet, V., Paulsen, B. D., Wu, R., Hamidi-Sakr, A., … Inal, S. (2021). Mixed Conduction in an N-Type Organic Semiconductor in the Absence of Hydrophilic Side-Chains. Advanced Functional Materials, 2010165. doi:10.1002/adfm.202010165

Acknowledgements
The research reported in this publication was supported by funding from KAUST, Office of Sponsored Research (OSR), under award number OSR-2016-CRG5-3003, URF/1/4073-01 and OSR-2018-CRG7-3709. J. S. thanks Dr. Yi Zhang for the TEM image of P-90. B.D.P., R.W., and J.R. gratefully acknowledge support from the National Science Foundation Grant No. NSF DMR-1751308. This research used resources of the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357. The authors would like to thank Joseph Strzalka and Qingteng Zhang for beam line assistance.

Publisher
Wiley

Journal
Advanced Functional Materials

DOI
10.1002/adfm.202010165

Additional Links
https://onlinelibrary.wiley.com/doi/10.1002/adfm.202010165

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