The emerging concerns about environment protection, food storage requirements, medical and chemicals storage etc. have led to widespread research in humidity sensors. Modern hygrometers exploit changes in inherent material properties such as resistance or capacitance of the sensing materials upon exposure to humidity and predict ambient humidity based on these variations. In this context, the possibility of a resistive humidity sensor based on conducting polymer polyaniline (PANI) doped with a novel organic dopant, sulfosuccinic acid (SSA), has been explored in our laboratory. The PANI-SSA-based resistive sensor exhibited considerable humidity sensing with sensitivity of 1.7%/%RH, limit of detection (LOD) of 8%RH, and linearity covering a wide range of relative humidity (RH) from 30% to 80%. This presentation will primarily emphasize on the origin of the sensing response in terms of activation energy of electron hopping. Later on, novel device structures incorporating capacitive humidity sensing will be discussed.

The first part of this talk will cover the detailed analysis of resistive humidity sensor using temperature- and RH-dependent conductivity study. The following part of the talk will include two all-organic device structures for humidity sensing: one is based on metal-insulator-metal (MIM) capacitor and the other is based on metal-insulator-semiconductor (MIS) capacitor using PANI-SSA as a humidity-sensitive conducting layer in both the structures. A noticeable increase in capacitance upon exposure to humidity in MIM based device structure resulted in sensitivity of 1.9%/ %RH, LOD of 0.591% and linearity covering 0% to 90%RH. In MIS capacitor-based devices, the entire capacitance-voltage (C-V) characteristics was observed to shift upon exposure to humidity. In addition, a change in relative C-V characteristics with respect to humidity level in the moderate accumulation regime can offer another degree of freedom in humidity measurement. The variation in capacitance in step with humidity was attributed to conducting polymer-assisted dipole alignment leading to a modification in dielectric property of the insulator in the device stack.