Dopant passivation by adsorbed water monomers causes high humidity sensitivity in PEDOT: PSS thin films at ppm-level humidity

Thin layers of hole conducing polymer poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) are widely utilized in the fabrication of organic light emitting devices. More recently, the material has been considered as a platform for humidity sensor fabrication. This is interesting as the conducive component of this composite, PEDOT, is hydrophobic. Moreover, charge transport mechanism in PEDOT:PSS is not yet fully established and understanding the cause of sensitivity to humidity may shed further light on the obscure charge conduction in this conducive polymer composite. Here, we investigate the humidity sensitivity in PEDOT:PSS thin films in a wide relative humidity (RH) range and report the existence of a distinctly higher humidity sensitivity in the RH < 1% range. The observed sensitivity is electronic in nature, and is described by considering the hydrogen bonding of the airborne water monomers to the surface-exposed sulfonate groups on the hydrophilic PSS chain, which, in turn, passivates the doping process involving hydrogen ion transfer to the PEDOT chain from those sulfonate groups. This dopant passivation mechanism reduces the charge carrier population in the layer and increases its impedance. The results are anticipated to pave the way for the fabrication of a printable organic humidity sensor with an unprecedented wide dynamic range spanning to sub-ppm humidity levels.