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Polymer semiconductors have become increasingly popular in electrochemical transistors because of their high transconductance, simple fabrication for flexible devices, and compatibility with aqueous environments. These materials form highly nanostructured films, yet to date there are few studies investigating the interplay between ionic transport and nanoscale morphological properties. In this work, we show that in situ electrochemical strain microscopy (ESM) in aqueous electrolytes can directly probe local variations in polymer devices by measuring the sub-nanometer volumetric swelling in the film upon ion diffusion. These data indicate that areas of lower elastic modulus are correlated with higher ion permeability and thus greater volumetric response, which we attribute to the polymer being more amorphous and less densely packed in these regions. Indeed, this response is also sensitive to the anion present in the electrolyte, with the anion size affecting both the magnitude in ESM as well as having a strong effect on mobility. These data suggest that balancing the high hole mobility of crystalline materials with the ionic mobility in more amorphous materials can result in better performing organic electrochemical transistors across a wider range of electrolytes. Following this approach, we show evidence that anisotropic polymer structures underneath an active ionic transport layer can provide enhanced transconductance over conventional single-component materials by balancing a highly crystalline polymer with an amorphous ionic transport layer. These data show that in situ scanning probe microscopy techniques can provide meaningful pathways for improving rational design of organic electrochemical transistors.
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