Novel Spectroscopic Characterization Reveals Design Guidelines for High-Performing Redox-Active Polymers

Synthetic efforts have brought forward a library of materials that combine ionic transport with electronic transport to achieve high performance in electrochemical devices that operate in aqueous electrolytes. The most promising materials are redox-active conjugated polymers with polar (hydrophilic) side chains. While significant gains have been made in both materials performance and understanding of the complex interplay between ionic and electronic motion, the fundamental understanding of why some materials achieve higher performances in electrochemical devices remains elusive. We demonstrate how an electrochemical charge modulation absorption technique, combined with exciton absorption modeling and differential electronic carrier mobility measurements can shed light on understanding the charging behavior of redox-active conjugated polymers. This suite of techniques allows us to track the relative populations of charge carriers in aggregate and amorphous regions, the mobility of these carriers, and the relative conjugation length where these carriers reside. We utilize a model system of thiophene homopolymers with different side chain lengths as a case study to highlight how subtle changes to microstructure and electronic landscape dictate electrochemical device performance [1,2]. Our characterization reveals that the ability to inject electronic carriers with high mobilities into structurally ordered aggregates at low charge carrier densities leads to higher performance. Furthermore, we extended characterization to other state-of-the-art redox-active conjugated polymers, showing that injection of electronic and ionic charge carriers into highly ordered aggregates appears to be a general requirement to achieve high performance in electrochemical transistors. We emphasize that our characterization allows structural order in redox-active conjugated polymers to be linked directly with electronic carrier properties and that application of our characterization serves as a sensitive probe to understand redox-active conjugated polymers.<br/><br/>[1] Moia, D.; Giovannitti, A.; Szumska, A. A.; Maria, I. P.; Rezasoltani, E.; Sachs, M.; Schnurr, M.; Barnes, P. R. F.; McCulloch, I.; Nelson, J., Design and evaluation of conjugated polymers with polar side chains as electrode materials for electrochemical energy storage in aqueous electrolytes. Energy & Environmental Science 2019, 12 (4), 1349-1357.<br/><br/>[2] Moser, M.; Gladisch, J.; Ghosh, S.; Hidalgo, T. C.; Ponder, J. F.; Sheelamanthula, R.; Thiburce, Q.; Gasparini, N.; Wadsworth, A.; Salleo, A.; Inal, S.; Berggren, M.; Zozoulenko, I.; Stavrinidou, E.; McCulloch, I., Controlling Electrochemically Induced Volume Changes in Conjugated Polymers by Chemical Design: from Theory to Devices. Advanced Functional Materials 2021, 31 (26), 2100723.