Monitoring the Swelling Behavior of PEDOT:PSS Electrodes under High Humidity Conditions

Polymer electrodes made of poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) are used in many applications but are also sensitive to humidity. We study humidity-induced changes of PEDOT:PSS electrodes as monitored with in situ time-of-flight neutron reflectivity (TOF-NR) measurements under high humidity conditions. The influence of the solvent additive Zonyl and a post-treatment of PEDOT:PSS films with ethylene glycol (EG) serving as electrodes are analyzed with respect to the swelling ratio and water uptake. Depending on the applied PEDOT:PSS treatment, PEDOT and PSS enrichment layers are clearly identified with TOF-NR at the substrate-polymer and polymer-air interface, respectively. The additive Zonyl reduces the water uptake and limits film swelling. EG post-treatment further increases hydrophobicity and thereby water incorporation into the PEDOT:PSS film is strongly suppressed. The characteristic time constants and effective interaction parameters extracted from the kinetic NR data show that additive and post-treatment reduce the sensitivity of the PEDOT:PSS electrodes to humidity.