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The addition of polyethylene oxide (PEO) to organic mixed ionic-electronic conductors, such as PEDOT:PSS, is commonly done to increase biocompatibility, hydrophilicity, or stretchability. In particular, when blended with PEDOT:PSS, PEO has been shown to change the microstructure of the thin films. In this study, we were interested in the effect of PEO on mixed ionic-electronic transport properties, particularly when the films are immersed in aqueous solutions such as for organic electrochemical transistors (OECTs). PEO is water soluble and therefore could easily leached out of the films when immersed in water or an aqueous electrolyte. We studied PEO of two different molecular weights (MW), 400 Da (low MW) and 20,000 Da (high MW), in PEDOT:PSS:PEO blends at 5 wt% PEO in solution, and crosslinked with GOPS. We monitored the composition of the spin-coated films by X-ray photoelectron spectroscopy (XPS) over a period of 4 hours of immersion in water. We found that most, if not all low MW PEO is removed from the films after 2 hours of immersion, while for high MW PEO, nearly 50% was still present in the films with a stable concentration after 1-2 h. In both cases, we noted the removal of excess PSS in the films, which is common for PEDOT:PSS immersed in water. The removal of PEO and PSS following water immersion was also confirmed by a significant reduction in film thickness, as measured by profilometry and atomic force microscopy. We then performed transconductance measurements on OECT devices to extract the mobility-capacitance product, [µC^*], of our PEDOT:PSS formulations. In all cases, without PEO, or with low or high MW PEO, the transconductance increased significantly after immersion in water, consistent with the removal of insulating material. What was more surprising, however, was a much higher [µC^*] when low MW PEO was used as an additive and then removed by soaking in water. The [µC^*] values obtained, which have yet to be validated externally, are higher than those obtained when doping with ethylene glycol and close to that of previously reported crystalline PEDOT:PSS obtained by sulfuric acid treatment. We are still exploring the exact reasons for this increase, but it seems to stem from both an increase in the charge mobility and volumetric capacitance (up to 129±22 F cm⁻³). GIWAXS measurements did not indicate a change in film crystallinity, but spectroelectrochemistry revealed a higher charge density which is consistent again with the removal of PSS and PEO from the films. This study provides an avenue towards improving the performance of PEDOT:PSS electrodes without using sulfuric acid. We also hope to encourage others to study the fate of polymeric additives and small molecules in devices operating under aqueous conditions.

Co-Author(s): Chun-Yuan Lo (University of Delaware), Lucas Flagg (National Institute of Standards and Technology)

Laure Kayser

University of Delaware

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