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Progress in bionic sensing and bio-electronic interfacing requires a thorough understanding of electronic and ionic transport in functional bio-compatible materials. The well-known polymer mixture poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) is central to many optoelectronic applications which necessitate flexible, transparent biocompatible materials. While PEDOT:PSS is traditionally studied for its hole transport properties and resistive H2O response, research interest has recently turned to ionic transport in the context of bio-electronic interfacing and sensing. Cations Na+ and K+ are often present in PEDOT:PSS as byproducts from industrial production. Uptake of water into PEDOT:PSS disrupts hydrogen bonding that maintains rigidity of the PSS matrix, causing film swelling and hydronium production due to water interaction at the hydrophilic SO3- moiety. As water uptake increases, mobility of Na+, K+, H3O+ and other ions may become hindered due to the formation of electrical double layers and hydration shells around ions. At the same time, water-induced swelling of the PSS matrix increases the distance between adjacent conductive PEDOT domains, which reduces electronic mobility. Despite its widespread use as a hole transport material in a variety of organic optoelectronic devices, the effect of water on electronic and ionic transport in PEDOT:PSS remains unclear. To probe both electronic and ionic response during water uptake, we perform ultra-wide range dielectric spectroscopy from sub-Hz to optical frequencies while changing humidity conditions in a controlled environment. We correlate the frequency-dependent measurements with those of a PEDOT:PSS-coated quartz crystal microbalance (QCM) to estimate the mass of adsorbed H2O in the film. We show that the presence of water has an effect on electronic and ionic mobility in the film and both electronic and ionic transport play a role in defining the optoelectornic properties of PEDOT:PSS under a wide range of humidities.
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