Air-Stable Semiconducting Ionic Polymers for Thermoelectric Applications

Thermoelectric (TE) devices that interconvert heat and electricity are the promising tools to harvest waste heat and solve energy problems. The current TE market is dominated by inorganic materials such as Bi 2 Te 3 due to their excellent TE performance; however, the issues of inorganics that involve material toxicity, rigidity, heavy weight and energy-intensive processing techniques are limiting the wide applications of TE devices. Choosing organic materials that are light weight, mechanically flexible, abundant and require low-temperature processing, as in our research focus, is paving the path to sustainability and opening up opportunities for a variety of applications such as electronic wearables and sensors. In our study, we aim to design and develop sensors that can detect high electric-field utilizing air-stable ionic polymer TE systems, a novel type of organic semiconductors, via a facile, solution-processing technique. We selected two n-type chloride-based ionic polymers, MADQUAT and PDADMAC as our starting materials because they have relatively low yet reliable ionic conductivities and intrinsically high Seebeck Coefficients under dry condition. The key challenge was that chloride-based polymers were extremely unstable due to their sensitivity to air/moisture and polymer relaxation. We adopted a solution-processable anion exchange technique and successfully created new systems with the same cationic polymer chain but various anions such as tetrafluoroborate (BF 4 - ) and hexafluorophosphate (PF 6 - ), that are more hydrophobic, increasing the system stability. Currently, we are investigating our stable ionic polymer TE performance by applying high voltages (500V-2000V) laterally across the samples through a pair of electrodes thermally evaporated on top. The change in TE properties after the high E-field application will help us probe further into the ionic movement of the polymeric systems thermodynamically and kinetically and understand what determines the p/n-type behavior. The thermoelectric sensor for high E-field detection can be incorporated into devices that can detect planetary objects in the outer space as well as microscale biological systems.