Comparison of organic thermistors based on PEDOT:PSS and PEDOT:tos thin films under various thermal and humidity conditions

Flexible organic thermistors based on poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) [PEDOT:PSS] and PEDOT:Fe3+(O−tosylate)3 [PEDOT:tos, tosylate ≡ p-toluenesulfonate] thin films were prepared and their structural and surface properties and the humidity and temperature dependences of their resistance were studied. The surface wettability evaluated by goniometry revealed a considerably higher contact angle (70°) on the PEDOT:tos thin film surface, indicating its hydrophobic nature. In contrast, a large number of polar SO3 groups are present on the surface of the PEDOT:PSS thin film, proving its overall wettability. The results obtained from atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS) together with electrokinetic analysis indicate expressly different surface charges (and chemistry) for the two films tested. The absence of sulfonic acid in the PEDOT:tos thermistor markedly enhances its humidity stability in the range of 20-90% RH, as confirmed during the humidity and temperature tests. Moreover, exposure of both thermistors to step-wise change in temperature showed that the PEDOT:tos thermistor features a notably higher change in resistance ΔR/R0 than the PEDOT:PSS thermistor. These observations suggest that structures based on PEDOT:Fe3+(O−tosylate)3 are promising candidates for the low-cost construction of flexible printed organic thermistors.