Thermoelectric devices are capable of converting low-value waste heat energy into higher value electricity in a silent, direct manner and without the need for moving parts. As such, they present themselves as promising, environmentally-friendly energy conversion modules. Polymer-based thermoelectric devices are of particular interest due to their ability to be fabricated using low-cost, large-scale methodologies (e.g., roll-to-roll coating) and their compatibility with flexible, mechanically-robust substrates. As such, conducting polymers have been studied extensively for their use in these types of energy conversion devices. Previously, π-conjugated polymers have dominated the research focus due to the high degree of electronic delocalization associated with their molecular structure; however many challenges regarding synthetic routes and the control of nanoscale structure continue to prevent their viability in widespread applications. To this end, we will discuss an emerging class of non-conjugated, electronically-active macromolecules, radical polymers, which have shown immense potential to transport charge despite being completely amorphous. These redox-active macromolecules have shown great promised in electrolyte-supported applications (e.g., flexible batteries). However, quantifying the ability of these non-conjugated macromolecules to conduct charge in the solid state has not been as well-studied. Here, a model radical polymer, poly(2,2,6,6-tetramethylpiperidinyloxy methacrylate) (PTMA), was synthesized using the RAFT controlled polymerization mechanism, which produced polymers with readily-tunable molecular weights and narrow molecular weight distributions. Additionally, we have measured the space-charge limited hole and electron mobility values of PTMA; furthermore, we have evaluated the effect of temperature on the transport ability of PTMA thin films. We find that the hole mobility (µh ~10-4 cm2 V-1 s-1) and conductivity (σ ~10-5 S cm-1) values of these radical polymers are of the same order of many common conjugated polymers [e.g., poly(3-hexylthiophene) (P3HT)]. Furthermore, because the polymer backbone is non-conjugated, these macromolecules are extremely transparent. This combination of a well-controlled synthetic methodology with high electronic performance allow these radical polymers to be of great utility when they are incorporated into the hole-transporting (p-type) leg of flexible thermoelectric devices.
Bryan Boudouris earned a bachelor’s degree in chemical engineering from the University of Illinois at Urbana-Champaign and a PhD in chemical engineering from the University of Minnesota. Before joining the faculty at Purdue, he was a postdoctoral fellow at the University of California, Berkeley.
His research interests include design of optoelectronically active polymers, functional block copolymer self-assembly, polymer-based electronics and solar cells.
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