Humidity Sensing Properties of Transparent Sputter-Coated Indium–Tin Oxide and Printed Polymer Structures

Abstract

The humidity sensing properties of sputter-coated indium-tin oxide (ITO) and printed dielectric structures were tested for samples with sheet resistances ranging from 10 to 50 Q/sq. ITO/Polymer composite sensors were fabricated to form a parallel-plate capacitive-based humidity sensor that could detect relative humidity within a tested range of 5%-95%. The sensors were most stable and gave a linear response between 5% and 75% relative humidity. The capacitive sensors were characterized, using a range of techniques, to establish their capability and performance as humidity monitors. Response time of the humidity sensors was measured to be an average of 31.5 s and the recovery time was measured at an average of 31 s in the capacitive mode. Complex impedance spectroscopy was used to determine the mechanism of action for the sensors, which was found to be both the diffusion of water molecules into the dielectric layer and an increase of ionic conductivity within the dielectric layer. Stability of the humidity sensors was tested at three different humidity levels over seven days and sensors were found to be stable or follow a predictable change for this time span.