Highly Sensitive and Full Range Detectable Humidity Sensor using PEDOT:PSS, Methyl Red and Graphene Oxide Materials

Abstract

Single transducer with humidity sensing materials has limitations in both range and sensitivity, which cannot be used to detect the full range of humidity with consistent sensitivity. To enlarge range and improve sensitivity in the all range relative humidity (RH), we propose a highly sensitive and full range detectable humidity sensor based on multiple inter-digital transducer (IDT) electrodes connected in series with poly(3,4-ethylenedioxythiophene) doped poly (styrene sulfonate) anions (PEDOT: PSS), C15H15N3O2 (Methyl Red), and graphene oxide (GO) thin films as the active sensing materials. The humidity sensor with single active material has a limit in the detecting ranges, where the GO, PEDOT: PSS, and Methyl Red materials have sensing responses of 0 to 78% RH, 30 to 75% RH, and 25 to 100% RH, respectively. However, a humidity sensor using combined three active materials can respond to much wider range of RH with high sensitivity, where the IDTs and the active regions were prepared using ink-jet printing and spin coating, respectively. This proposed sensor can detect a full range of 0% RH to 100% RH. The response and recovery times are 1 sec and 3.5 sec, respectively. Our single sensing device using multiple IDTs connected different active materials in series can overcome the limitations of single transducer based sensor for the high performance sensor applications.

Fabrication and Characterization of Humidity Sensors Based on Graphene Oxide–PEDOT:PSS Composites on a Flexible Substrate

“…It can also be noticed that the increase in PEDOT:PSS involved lower losses due to the increase in equivalent Rp (less shelf-discharge current) [45]. On the other hand, the capacitance increased as RH increased in all cases, which indicated that the water molecules adsorbed within the sensitive layer induced an increase in resulting dielectric permittivity [13]. Moreover, this increase also depended on the content of PEDOT:PSS, as seen in Figures 6b,d, and f. While the capacitance of the only GO-based sensors did not present significant changes for high values of RH (>75%), as already reported in previous works [13], the presence of PEDOT:PSS led to a considerable increase of the capacitance in this region.…”

“…It can also be noticed that the increase in PEDOT:PSS involved lower losses due to the increase in equivalent R p (less shelf-discharge current) [45]. On the other hand, the capacitance increased as RH increased in all cases, which indicated that the water molecules adsorbed within the sensitive layer induced an increase in resulting dielectric permittivity [13]. Moreover, this increase also depended on the content of PEDOT:PSS, as seen in Figure 6b,d,f.…”

“…On the other hand, the capacitance increased as RH increased in all cases, which indicated that the water molecules adsorbed within the sensitive layer induced an increase in resulting dielectric permittivity [13]. Moreover, this increase also depended on the content of PEDOT:PSS, as seen in Figures 6b,d, and f. While the capacitance of the only GO-based sensors did not present significant changes for high values of RH (>75%), as already reported in previous works [13], the presence of PEDOT:PSS led to a considerable increase of the capacitance in this region. This effect was a consequence of the water molecules absorbed by the PSS, which formed a water meniscus layer once the saturation point was achieved [42].…”

“…Once the absorption of water molecules by the PSS chains saturated, the impedance decreased drastically due to the physisorption of the water molecules, which facilitated the ionic current and provides better current flow paths within the PEDOT structure [43]. This occurred up to ~70% RH, where the impedance modulus started decreasing more softly, since this point is defined as a maximum detection limit of the PEDOT [13]. Furthermore, a lower concentration of PEDOT:PSS led to a softer transition between these three different states, as can be observed in Figure 5.…”

“…Once we demonstrated that the PEDOT:PSS was capable of modifying the impedance response of the sensors, we studied how these changes were translated into variations on its equivalent electrical circuit. An IDE planar capacitor, such as the ones presented in this work, can be modeled as an R p ||Z c association, where R p is the equivalent parallel resistance between electrodes (resistance of both electrode-sensitive layer interface and sensitive layer itself) and Z c is its reactance [13,38,44]. Following this model, we extracted the equivalent parallel resistance and capacitance as a function of the relative humidity at different frequencies.…”