A Low-Swelling Polymeric Mixed Conductor Operating in Aqueous Electrolytes

Organic mixed conductors find use in batteries, bioelectronics technologies, neuromorphic computing, and sensing. While great progress has been achieved, polymer-based mixed conductors frequently experience significant volumetric changes during ion uptake/rejection, i.e., during doping/de-doping and charging/discharging. Although ion dynamics may be enhanced in expanded networks, these volumetric changes can have undesirable consequences, e.g., negatively affecting hole/electron conduction and severely shortening device lifetime. Here, the authors present a new material poly[3-(6-hydroxy)hexylthiophene] (P3HHT) that is able to transport ions and electrons/holes, as tested in electrochemical absorption spectroscopy and organic electrochemical transistors, and that exhibits low swelling, attributed to the hydroxylated alkyl side-chain functionalization. P3HHT displays a thickness change upon passive swelling of only +2.5%, compared to +90% observed for the ubiquitous poly(3,4-ethylenedioxythiophene):polystyrene sulfonate, and +10 to +15% for polymers such as poly(2-(3,3′-bis(2-(2-(2-methoxyethoxy)ethoxy)ethoxy)-[2,2′-bithiophen]-5-yl)thieno[3,2-b]thiophene) (p[g2T-TT]). Applying a bias pulse during swelling, this discrepancy becomes even more pronounced, with the thickness of P3HHT films changing by <10% while that of p(g2T-TT) structures increases by +75 to +80%. Importantly, the initial P3HHT film thickness is essentially restored after de-doping while p(g2T-TT) remains substantially swollen. The authors, thus, expand the materials-design toolbox for the creation of low-swelling soft mixed conductors with tailored properties and applications in bioelectronics and beyond.