The effect of humidity on the electrical conductivity of mesoporous polythiophene

Commercial samples of bromine terminated polythiophene powder have been compacted to a mesoporous state in order to measure their electrical properties at different relative humidities by impedance spectroscopy, in this case the complex admittance versus frequency. The sample powders were compacted between the electrodes of a co-axial tube capacitor arrangement and connected to a standard impedance bridge with a frequency range of 20–1 MHz. The low humidity conductivity shows a dc limit at about 4 × 10−6 S/m. When the conductivity measurements were recalculated by subtracting the low humidity response, assumed dry, from the medium or high humidity response a peak was seen at values ranging from 20 to 60 kHz with a sharp fall off at about 100 kHz. Plots of the change in susceptibility did not show peaks, but the excess response vanished at a lower frequency than seen for the conductivity. The log–log plots of the data showed good low frequency fits to a straight line, implying power law dependencies similar to those seen in ionic dielectric materials. I conclude that in humid air there is no significant conduction through the adsorbed water layer itself, but that the presence of surface water molecules affects the polythiophene conduction that occurs through the overlap of adjacent π-bonds. In this model the presence of surface water enhances the π-bond overlap, but has a strong frequency dependence.