(Invited) Solid-State Charge Transport in Redox-Active Radical Polymers
Authors: Boudouris Bryan W 
Published: February 2020

Elucidating the nanostructural and charge transport behavior of redox-active polymers has led to the implementation of these critical macromolecules in a number of energy conversion and energy storage applications. To date, however, the majority of the work associated with charge-conducting polymers has focused on systems where a large degree of π-conjugation exists along the macromolecular backbone. Here, we move away from this archetype through the design, characterization, and utilization of an emerging class of materials, radical polymers. In this context, we define radical polymers to be macromolecules that consist of non-conjugated backbones and contain stable radical sites on the pendant groups of each repeat unit. These stable radical sites are capable of undergoing oxidation-reduction reactions in a rather facile manner if there is a large population of the radical sites in relatively close (i.e., ~1 nm) proximity. In turn, this ability allows for charge to be transported in the solid state and in a straightforward manner despite the fact that these completely amorphous materials have very localized charge sites. Because the charge transfer sites are independent of the macromolecular architecture, this allows for myriad potential designs of radical polymers. Furthermore, because of this electronic decoupling between the pendant groups and the macromolecular backbone, the Singularly Occupied Molecular Orbital (SOMO) charge transport level of the radical group can be predicted easily. This information can be used in a straightforward manner to match energy and charge transfer levels of other electronically-active inorganic and organic materials.
As such, radical polymers are beginning to emerge as useful materials in a number of hybrid and organic electronic applications. Here, we quantify the solid-state electrical conductivity and hole mobility of a model radical polymer, poly(2,2,6,6-tetramethylpiperidinyloxy methacrylate) (PTMA), for the first time. In particular, we establish that the solid-state conductivity (~10-3 S m-1) and the space charge-limited hole mobility (~10-4 cm2 V-1 s-1) are on par with common conjugated polymer semiconductors [e.g., poly(3-hexylthiophene) (P3HT)]. Furthermore, these charge transport properties are found to be highly dependent on the exact chemical nature of the stable radical pendant groups. This is confirmed using a variety of spectroscopic techniques that are coupled to electronic characterization measurements. Conversely, due to the localized nature of the charge transfer groups and the fact that these materials are well into the glassy state at room temperature, charge transport is found to be relatively temperature-independent, in contrast to many conjugated organic semiconductors. By developing these fundamental structure-property-performance relationships in radical polymers, we have been able to demonstrate their high performance ability in applications where transparent conducting polymer thin films are required [e.g., high efficiency organic photovoltaic (OPV) devices]. In particular, we demonstrate that insertion of the redox-active polymer at the organic semiconductor/metal electrode interface improves charge extraction and device lifetime of inverted polymer solar cells based on a blend of P3HT and a soluble fullerene derivative relative to devices that lack this redox-active polymer interlayer. Finally, we demonstrate that the readily-tuned synthesis of radical polymers affords the opportunity to generate well-defined block polymers in a straightforward manner. We demonstrate that these materials self-assemble easily on the ~30 nm scale, and that they form ordered thin film nanostructures. This opens the possibility of creating nanostructured electron conducting-ion conducting pathways using a single material. As such, radical polymer-based block polymers could be of immense promise in future solid-state flexible battery devices as well. Therefore, we will demonstrate the ability to use carefully-designed redox-active radical polymer chemistries and nanostructures to impact important energy conversion and energy storage devices in a positive manner.