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. 2020 Jan 29;11(2):148.
doi: 10.3390/mi11020148.

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

Francisco J Romero  1   2 , Almudena Rivadeneyra  1   2 , Markus Becherer  3 , Diego P Morales  2   4 , Noel Rodríguez  1   2
Affiliations

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

Francisco J Romero et al. Micromachines (Basel). .

Abstract

In this paper, we present a simple, fast, and cost-effective method for the large-scale fabrication of high-sensitivity humidity sensors on flexible substrates. These sensors consist of a micro screen-printed capacitive structure upon which a sensitive layer is deposited. We studied two different structures and three different sensing materials by modifying the concentration of poly(3,4-ethylenedioxythiophene)/polystyrene sulfonate (PEDOT:PSS) in a graphene oxide (GO) solution. The results show that the aggregation of the PEDOT:PSS to the GO can modify its electrical properties, boosting the performance of the capacitive sensors in terms of both resistive losses and sensitivity to relative humidity (RH) changes. Thus, in an area less than 30 mm2, the GO/PEDOT:PSS-based sensors can achieve a sensitivity much higher (1.22 nF/%RH at 1 kHz) than other similar sensors presented in the literature which, together with their good thermal stability, time response, and performance over bending, demonstrates that the manufacturing approach described in this work paves the way for the mass production of flexible humidity sensors in an inexpensive way.

Keywords: PEDOT:PSS; flexible electronics; graphene oxide; humidity; screen-printing; sensor.

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Conflict of interest statement

The authors declare no conflicts of interest.

Figures

Figure 1
Figure 1
Schematic representation of the fabrication process of the relative humidity (RH) sensors. (a) Flexible transparent substrate (thickness: 160 μm). (b) Capacitive interdigitated electrode (IDE) structure screen-printed on the substrate. (c) Dimensions of the interdigitally arranged electrodes (W: width, i: interspacing, L: length, S: spacing). (d) Sensitive layer drop-casted on top of the IDE structure.
Figure 2
Figure 2
Optical microscope images: (a) screen-printed layout 1 (scale bar: 200 μm); (b) screen-printed layout 2 (scale bar: 200 μm); (c) Ag-based conductive ink screen-printed on the flexible substrate (scale bar: 100 μm); (d) graphene oxide (GO) sensitive layer (scale bar: 50 μm); (e) GO/poly(3,4-ethylenedioxythiophene)/polystyrene sulfonate (PEDOT:PSS) sensitive layer at 10% concentration (scale bar: 50 μm); (f) GO/PEDOT:PSS sensitive layer at 20% concentration (scale bar: 50 μm). SEM images: (g) GO sensitive layer (scale bar: 20 μm); (h) GO/PEDOT:PSS sensitive layer at 10% concentration (scale bar: 20 μm); (i) GO/PEDOT:PSS sensitive layer at 20% concentration (scale bar: 20 μm).
Figure 3
Figure 3
Fourier-transform infrared (FTIR) characterizations: (a) GO; (b) GO/PEDOT:PSS (10%); (c) GO/PEDOT:PSS (20%).
Figure 4
Figure 4
Absolute value of the impedance as a function of the frequency for the different values of RH. The left column presents the results obtained for layout 1 using the three different sensitive layers: (a) GO; (c) GO/PEDOT:PSS (10%); (e) GO/PEDOT:PSS (20%). Likewise, the right column presents the result of layout 2 for the same sensitive layers: (b) GO; (d) GO/PEDOT:PSS (10%); (f) GO/PEDOT:PSS (20%).
Figure 5
Figure 5
Absolute value of the impedance as a function of the relative humidity at a frequency of 100 Hz for both layout 1 (a) and layout 2 (b) using GO and the hybrid GO/PEDOT:PSS composites as sensitive layers.
Figure 6
Figure 6
Equivalent parallel resistance and capacitance for layout 1 (W = 200 µm, i = 200 µm) at different frequencies using GO and the hybrid GO/PEDOT:PSS composites as sensitive layers; being (a,b) the results obtained for the GO layer, while (c,d) and (e,f) are the results associated to the GO/PEDOT:PSS (10%) and GO/PEDOT:PSS (20%) layers, respectively.
Figure 7
Figure 7
Effect of the thermal drift in the capacitance for the sensor with layout 2 (W = 115 µm, i = 225 µm) at different frequencies for the GO (a), GO/PEDOT:PSS (10%) (b), and GO/PEDOT:PSS (20%) (c) sensitive layers.
Figure 8
Figure 8
Transient response of the GO/PEDOT:PSS (10%) sensitive layer (W = 115 µm, i = 225 µm). (a) Values of temperature and RH obtained from the sensor incorporated in the climate chamber over time. (b) Capacitance response of the sensor at two different frequencies over time.
Figure 9
Figure 9
Change in resistance R with respect to the initial resistance R0 for an increasing number of bending cycles. Inset diagram depicts the definition of bending radius (r).
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