Printed, Fully Metal Oxide, Capacitive Humidity Sensors Using Conductive Indium Tin Oxide Inks

In this research, fully metal oxide, capacitive humidity sensors for printed electronic applications have been designed and fabricated through the development of conductive indium tin oxide and dielectric aluminum oxide inks for the screen-printing process. Sensors were printed in a parallel plate configuration in 4 and 9 cm² conductive plate areas. Typically, commercially available discrete humidity sensors have a sensitivity in the range of 0.2–0.5 pF/RH%, whereas the printed humidity sensors presented in this paper have sensitivities of 0.85–7.76 pF/RH% depending on the sensor size, allowing for customizable properties. Response times were measured using a weighted average and found to be 21.4 s on average and recovery times were 4.8 s on average. The sensing performance was highly linear (R² > 0.97) for sensors of all sizes across the measured humidity range of 5–95%. Impedance spectroscopy was used to determine the sensing mechanism, and the mechanism was simulated and matched with experimental data. The sensing mechanism analysis shows that the sensing is primarily dictated by alumina at a lower relative humidity. The ITO contributes through increasing the ionic conductivity at a higher relative humidity, contributing to the high linearity of the sensor.