NO₂ Exposure Effect on Mixed Conductivity and Seebeck Coefficient in Thiophene Polymers

Alkylthiophene polymers with unique and differing terminally functionalized side chains were investigated by using a vapor analyte/dopant, nitrogen dioxide (NO2), and the thermoelectric properties were examined. We observed that through NO2 absorption on or in the sample that induced charge carrier generation and transport, the conductivities of regioregular poly(3-hexylthiophene-2,5-diyl) (P3HT), poly[3-(ethyl-4-butoanote)thiophene-2,5-diyl] (PT-COOR), poly[3-(3-caboxypropyl)thiophene-2,5-diyl] (PT-COOH), and poly(3-hydroxymethylthiophene) (PT-OH) were increased. Meanwhile, Seebeck coefficients (S) of PT-COOH, PT-COOR, and PT-OH displayed a declining trend with increasing NO2 ppm-level exposure. The lack of S dependence on doping level for P3HT is due to its low conductivity (σ). The correlations of thermoelectric properties of the four polymers were investigated; PT-COOR, PT-COOH, and PT-OH showed good linearity in the logarithmic plots of S and power factor against σ at various doping levels, even with possible contributions of proton transport in PT-COOH and PT-OH. In light of growing interest in conducting polymers being utilized as gas sensors, this method provides as a convenient tool to study the physics associated behind the thermoelectric properties governing the sensing response and mechanism.