Dual-Mode Polymer-Based Temperature Sensor by Dedoping of Electrochemically Doped, Conjugated Polymer Thin Films

Polymer temperature sensors are important for applications in food packaging, air conditioning, wearable devices, and biomedicine. However, the sensing range of these sensors is narrow, and the mode of sensing is restricted to either optical or electrical, which limits their implementation in practice. Here, dual-mode polymer-based temperature sensors are demonstrated with a wide sensing range based on a sensing mechanism that utilizes electrochemically doped (oxidized) regiorandom poly(3-hexylthiophene) (RRa-P3HT). When subjected to temperature, the electrochemically doped RRa-P3HT thin films dedope, resulting in a visible color change from blue (the doped state) to yellow (the dedoped state; similar in color to the pristine film). Energy-dispersive X-ray spectroscopy (EDS) and electron paramagnetic resonance (EPR) spectra show decreases in dopant concentrations with increases in the temperature to which the doped films were subjected, indicating a gradual thermal dedoping in the temperature range from 30 to 80 °C. Visible-wavelength absorption spectra of doped films subjected to increasing temperatures depict both doped and dedoped peaks. The ratio of intensities of dedoped to doped peaks exhibits a linear trend between 30 and 75 °C that can be exploited for optical-mode thermal sensing. This temperature-sensing range is the widest of any polymer-based temperature sensor reported to date. A unique aspect of this thermal sensor is that the thermally induced transition between doped and dedoped states for RRa-P3HT films can be translated into an electrical signal as doped films are electrically conducting. Two-point probe current measurements show an exponential decrease in the current with increasing in temperature.