New Insights on Electro-Optical Response of Poly(3,4-ethylenedioxythiophene):Poly(styrenesulfonate) Film to Humidity

Understanding the relative humidity (RH) response of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) is critical for improving the stability of organic electronic devices and developing selective sensors. In this work, combined gravimetric sensing, nanoscale surface probing, and mesoscale optoelectronic characterization are used to directly compare the RH dependence of electrical and optical conductivities and unfold connections between the rate of water adsorption and changes in functional properties of PEDOT:PSS film. We report three distinct regimes where changes in electrical conductivity, optical conductivity, and optical bandgap are correlated with the mass of adsorbed water. At low (RH < 25%) and high (RH > 60%) humidity levels, dramatic changes in electrical, optical, and structural properties occur, while changes are insignificant in mid-RH (25 < RH < 60%) conditions. We associate the three regimes with water adsorption at hydrophilic moieties at low RH, diffusion and swelling throughout the film at mid-RH, and saturation of the film by water at high RH. Optical film thickness increased by 150% as RH was increased from 9 to 80%. Low frequency (1 kHz) impedance increased by ∼100%, and film capacitance increased by ∼30% as RH increased from 9 to 80% due to an increase in the film dielectric constant. Changes in electrical and optical conductivities concomitantly decrease across the full range of RH tested.