Nicholas Siemons1
Imperial College London1
Semiconducting polymers bearing hydrophilic glycol side chains can transport aqueous electrolytes through their bulk, allowing for the simultaneous movement of electronic and ionic charges during electrochemical doping (therefore called organic mixed ionic-electronic conductors, or OMIECs). Their mixed charge transport properties, chemical tuneability and easy synthetic scalability make them attractive materials for bioelectronic devices such as transistors [1] and charge storage devices [2]. While hydrophobic alkylated polythiophenes have demonstrated favourable charge transport properties [3] and the ability to operate stably in devices such as field-effect transistors, upon glycolation (making them into OMIECs) some of these favourable properties are deteriorated. For example, heavily glycolated polythiophenes have shown to excessively swell when electrochemically doped in the presence of an aqueous electrolyte [4], damaging their operational stability and charge transport properties. We explore how amphiphilic chemical design strategies can be leveraged to design OMIECs that retain their operational stability and high charge transport properties. We explore two strategies – 1) the random copolymerisation of alkylated and glycolated repeat units together and 2) using homopolymers with both alkyl and glycol moieties on their side chains. In the first case we find that a small level of alkylation leads to a large increase in the electrochemical cycling stability of the OMIEC, while leaving the electrochemical doping characteristics broadly unchanged. When using homopolymers with mixed alkyl/glycol side chains, we show how the size of the alkyl group on the side chain can control the polymer backbone conformation, impacting its electronic transport properties. Through a joint experimental/theoretical approach we identify the underlying microstructural mechanism behind both the inhibited swelling in alkyl/glycol copolymers and the effect on the backbone morphology in amphiphilic homopolymers. We identify the driving forces causing them to adopt their particular microstructure and draw parallels between the formation if micelles and vesicles in surfactants. Identifying these mechanisms and their driving forces in OMIEC systems will aid in the design of future polymers which optimise the transport of both ionic and electronic charges, while having high operational stability and charge transport properties.<br/><br/>[1] Giovannitti, Alexander, et al. "Controlling the mode of operation of organic transistors through side-chain engineering." <i>Proceedings of the National Academy of Sciences</i> 113.43 (2016): 12017-12022.<br/>[2] Moia, Davide, et al. "Design and evaluation of conjugated polymers with polar side chains as electrode materials for electrochemical energy storage in aqueous electrolytes." <i>Energy & Environmental Science</i> 12.4 (2019): 1349-1357.<br/>[3] McCullough, Richard D., et al. "Design, synthesis, and control of conducting polymer architectures: structurally homogeneous poly (3-alkylthiophenes)." <i>The Journal of Organic Chemistry</i> 58.4 (1993): 904-912.<br/>[4] Gladisch, Johannes, et al. "Reversible electronic solid–gel switching of a conjugated polymer." <i>Advanced Science</i> 7.2 (2020): 1901144