Collapse or Enhance—Understanding the Impact of Amphiphilic Side Chains on Charge Transport and Storage in Mixed Conducting Polymers

Conjugated polymers (CPs) with polar side chains can conduct electronic and ionic charges simultaneously, making them promising for applications in bioelectronics, electrochromics, electrocatalysis and energy storage. Recent studies showed that using amphiphilic side chains — particularly by adding alkyl spacers between the CP backbones and polar side-chain moieties — can improve electronic charge carrier mobility, reduce swelling and enhance stability, without compromising the ion transport.¹ However, the influence of amphiphilic side chains on the conformation of CP backbones, thin-film morphology and their subsequent impact on electronic properties remains unclear.² In this work, we designed two oxy-bithiophene-based CP series, each featuring amphiphilic side chains with progressively extended alkyl moieties. These moieties were strategically positioned either next to the backbone (alkyl spacer) or at the outer ends of the side chains (alkyl tips). Using operando characterisation methods, we investigated the (spectro)electrochemical and swelling properties of the polymer thin films, and their performance in organic field-effect transistors (OFETs) and organic electrochemical transistors (OECTs). Contrary to previous reports, we found that whilst both strategies reduce swelling during charging, alkyl spacers negatively affect electronic charge transport and storage of the polymers in electrochemical devices. In contrast, alkyl tips improve both of these properties. To explain the phenomena, we used molecular dynamics (MD) simulation to explore the impact of solvent polarity on polymer conformation. We show that, in certain media, alkyl spacers induce twisting of the polymer backbone, disrupting its planarity, whereas alkyl tips facilitate polymer interchain connections without affecting the planar backbone conformation. Our findings provide a deeper understanding of the relationship between the side chain polarity and solvation environment in influencing polymer performance, underscoring the importance of considering polymer-environment interaction when designing new mixed conducting polymers.

References
1. Maria, I. P. et al. The Effect of Alkyl Spacers on the Mixed Ionic-Electronic Conduction Properties of n-Type Polymers. Adv. Funct. Mater. 31, 2008718 (2021).
2. Ohayon, D. et al. Influence of Side Chains on the n-Type Organic Electrochemical Transistor Performance. ACS Appl. Mater. Interfaces 13, 4253–4266 (2021).