Although p-type organic mixed ionic electronic conductors (OMIECs) are susceptible to oxidation, it has not yet been considered as to whether oxygen could behave as an uncontrolled p-dopant. Here, oxygen dissolved in solvents is shown to be behave as a p-dopant, that fills traps to enable more effective electrochemical doping in OMIECs and organic electrochemical transistors (OECTs). Yet the presence of oxygen is also known to jeopardize OECT stability. A two-step strategy is introduced to solve this contradictory problem, where first the solvent is degassed, and second the OMIEC is doped in a controlled manner using a chemical dopant. This strategy increases on-off ratio, tunes the threshold voltage, and enhances the transconductance, mobility and the µC* product, while having a remarkable impact on both p-type and n-type OECT stability. This simple solution-processing technique is easily implemented, low-cost, and highly effective in an oxygen-rich environment. The data herein suggests that combining chemical doping with solvent degassing could be a broadly applicable technique to improve essential criteria needed to realize organic bioelectronics and more complex OMIEC circuitry
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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