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

Abstract

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