Direct current powered humidity sensor based on a polymer composite with humidity sensitive electronic conduction

Abstract

Solid-state humidity sensors are generally AC powered as they sense the fluctuations in the dielectric polarization and/or ionic conductance of the water layer formed by Kelvin condensation on the effective surface of their hydrophilic sensing elements. DC-powered, printable, and bendable hygrometers, however, are demanded for battery-driven wearable electronics. Here, we demonstrate that the electronic (as opposed to ionic) conduction in the polymer composite poly(3,4-ethylenedioxythiophene–poly(styrenesulfonate) (PEDOT:PSS) thin films constantly decreases with increasing relative humidity (RH) in the surrounding atmosphere and use this property for the fabrication of a DC-powered solid state hygrometer. PEDOT, the conductive component of this composite, is hydrophobic, and the hydrogen bonding of the airborne H2O to the surface available sulfonic acid groups of the non-conductive component, PSS, is verified as the main cause of sensitivity. This hydrogen bonding deters the doping action involving H+ transfer from the sulfonic acid groups to the thiophene groups on the PEDOT chain and decreases the bulk conductivity of the sensing element. As predicted by this model, the electrical resistance measurements carried out by applying DC and AC electric fields lead to different RH dependences; the former demonstrates the positive RH dependence in the full RH range, while the latter results in strongly nonlinear RH sensitivity crossing zero around RH 60%. Our model assists better understanding of the charge conduction mechanisms in PEDOT:PSS. The described sensing mechanism is anticipated to initiate research on many two-component organic composites for the fabrication of varieties of gas and humidity sensors.