Temperature sensing behavior of poly(3,4-ethylenedioxythiophene) thin film

Temperature sensing behavior of poly(3,4-ethylenedioxythiophene) (PEDOT) thin film was investigated at the temperature range of -20 C to 60 C. PEDOT thin films were fabricated by in situ polymerization, where 3,4- ethylenedioxythiophene (EDOT) and ferric p-toluene sulfonate (FTS) were used as a monomer and an oxidant, respectively. The monomer and the oxidant solutions in 1-butanol were prepared separately and mixed just before in situ polymerization. The mixed solution was cast on a substrate and polymerization was carried out at 70 C for 30 min, followed by washing with methanol and drying under vacuum. Resistance change of PEDOT film with temperature was monitored using two probe method. It was observed that resistance of PEDOT film monotonically decreased with increasing temperature, confirming a typical temperature dependence of the electrical resistance of semiconductors with negative temperature coefficient (NTC). However, resistance of PEDOT thin film was not stable with repeated heating and cooling. Resistance at the same temperature increased with repeated heating and cooling cycles. We found that stability of PEDOT thin film could be significantly improved by heat treatment of PEDOT film at 150 C for 1 h and sealing the PEDOT thin film. Temperature-dependent resistance of the stabilized PEDOT thin film could be fitted to the Steinhart-Hart equation from -20 C to 60 C range even after repeated cycles, suggesting its potential application as a flexible thin film thermistor.