(561c) Optimizing Solid State Conductivity in Radical Polymers
Highly conjugated macromolecules have risen to the fore in a number of organic electronic applications (e.g., organic photovoltaics, organic field-effect transistors) due to their potential for low-cost processing and compatibility with flexible substrates. In particular, poly(ethylene dioxythiophene) doped with poly(styrene sulfonate) (PEDOT:PSS) has proven to be one of the top conducting polymers that also has a relatively high degree of transparency in the visible spectrum. As such, PEDOT:PSS is utilized in a wide range of high-performance devices; however, many important synthetic, physical, and mechanical properties of this macromolecule are not optimized. Therefore, there exists a need to develop novel conducting polymers that have well-defined synthetic, thermal, and optical properties that can be tuned readily through simple alterations to the chemistry of the macromolecules.
Here, it is demonstrated that radical polymers (i.e., where a pendant stable radical group is present on each repeat unit of the polymer chain) can be synthesized in manners traditionally associated with insulating polymers composed of aliphatic carbon backbones. Specifically, for the first time, we demonstrate the ability to synthesize a model radical polymer, poly(2,2,6,6-tetramethylpiperidinyloxy-4-yl methacrylate) (PTMA), through the use of the controlled reversible addition-fragmentation chain transfer (RAFT) polymerization mechanism. This leads to conducting polymers with tunable molecular weights, narrow molecular weight distributions, accessible thermla transitions, and controllable end group functionalities. Importantly, the chemical processing conditions used during the synthesis of PTMA affect the final transport properties of PTMA thin films greatly. We will highlight the appropriate protocols to utilize in order to optimize the transport of charge in PTMA thin films. And while neat films of PTMA have lower solid state conductivity values relative to PEDOT:PSS, we show that with the addition of the appropriate dopants (i.e., in a manner akin to doping PEDOT with PSS) the conductivities of doped PTMA films increas by a factors of 100 to 1000. Furthermore, due to the lack of conjugation in this radical polymer, PTMA has a higher transparency in the visible spectrum than PEDOT:PSS. As such, PTMA presents itself as a useful and versatile alternative in solid state organic electronic applications previously dominated by π-conjugated materials.