The Influence of Alkyl Spacers and Molecular Weight on the Charge Transport and Storage Properties of Oxy-Bithiophene-Based Conjugated Polymers

Hang Yu^a, Adam Marks^b^,^c, Sachetan M. Tuladhar^a, Iona Anderson^a, Nicholas Siemons^a, Sophia Bidinger^d, Scott T. Keene^d, Tyler J. Quill^c, Ruiheng Wu^e, Olivia Gough^a, Guanchen Wu^a, Flurin Eisner^a, George Malliaras^d, Piers Barnes^a, Iain McCulloch^b, Jenny Nelson^a

^aDepartment of Physics, Imperial College London, London SW7 2AZ, UK

^bDepartment of Chemistry, University of Oxford, Oxford, UK

^cDepartment of Materials Science and Engineering, Stanford University, Stanford, CA, USA;, United States

^dElectrical Engineering Division, Department of Engineering, University of Cambridge, UK

^eDepartment of Chemistry, Northwestern University, Evanston, USA, Sheridan Road, 2145, Evanston, United States

Materials for Sustainable Development Conference (MATSUS)
Proceedings of MATSUS Fall 2023 Conference (MATSUSFall23)

#OMIECs - Fundamentals of mixed ionic-electronic transport in polymers

Torremolinos, Spain, 2023 October 16th - 20th

Organizer: Simone Fabiano

Poster, Hang Yu, 186
Publication date: 18th July 2023

Conjugated polymers (CPs) with polar side chains are capable of simultaneous electronic and ionic charge transport, making them interesting materials for applications in bioelectronics, electrochromics, electrocatalysis and energy storage. Their electronic and ionic conduction properties can be tuned individually through modifications of the backbones and side chains. [1] Among various side-chain design strategies, the incorporation of alkyl spacers between the polymer backbone and polar groups on the side chain has been found to be an effective one to improve charge-carrier mobilities in mixed conducting CPs. [2] Besides, the influence of molecular weight (MW) on the electronic properties of the polymers is rarely studied in mixed conducting CPs. [3] In this work, we designed two oxy-bithiophene-based CP series, each featuring progressively varying alkyl spacer lengths and two distinct MW distributions. We used operando characterisation methods to investigate the (spectro)electrochemical and swelling properties of the polymer thin films, and their performance as organic field-effect transistors (OFETs) and organic electrochemical transistors (OECTs). Surprisingly, we find that alkyl spacers can negatively impact the electronic charge storage and transport properties of the polymers in an electrochemical device, and that high MW can amplify such a negative effect. To explain the phenomena, we use molecular dynamics (MD) simulation to explore the impact of polarity of the solvating medium on the polymer conformation. We show that, in certain media, the presence of alkyl spacers can cause the polymer to twist. Our finding provides a deeper understanding of the relationship between the side chain polarity, MW and electrolyte or solvent in influencing polymer performance, which underscores the importance of considering polymer interactions with their environment when designing new mixed conducting polymers.

References:

[1] Szumska, A. A.; Maria, I. P.; Flagg, L. Q.; et al. Reversible Electrochemical Charging of n-Type Conjugated Polymer Electrodes in Aqueous Electrolytes. J. Am. Chem. Soc. 2021, 143, 36, 14795–14805.

[2] Maria, I. P.; Paulsen, B. D.; Savva, A.; et al. The Effect of Alkyl Spacers on the Mixed Ionic-Electronic Conduction Properties of N-Type Polymers. Adv. Func. Mater. 2021, 31, 14, 2008718.

[3] Tropp, J.; Meli, D.; Wu, R.; et al. Revealing the Impact of Molecular Weight on Mixed Conduction in Glycolated Polythiophenes through Electrolyte Choice. ACS Materials Lett. 2023, 5, 5, 1367–1375.

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