Ultrafast near-infrared spectroscopy of an electrochemically doped polythiophene film

Organic mixed ionic-electronic conducting polymers continue to emerge as promising next-generation materials for electrochemical applications ranging from bioelectronics to energy storage. However, we lack a clear understanding of how electrostatic and nanostructural changes in the polymer, which evolve during electrochemical device operation, influence charge and ion conductivity. In this work, we applied ultrafast near-infrared pump-probe spectroscopy, which is sensitive to the local nanostructure and electronic environment of charge carriers (polarons), to an electrochemically doped poly(3-hexylthiophene-2,5-diyl) [P3HT] model polymer system. The impact of electrolyte infiltration on carrier trapping was tested by comparing its photoexcited polaron dynamics to those measured in a chemically doped P3HT film lacking electrolyte and containing the same polaron mole fraction (~15%) and ClO4 − counterions. The transient absorption spectra revealed the presence of Coulombically free and trapped (ion-bound) polarons in both the electrochemically and chemically doped P3HT films, with a greater relative number of free polarons in the latter. However, the ion-bound polarons were less deeply trapped on average in the electrochemically doped film, suggesting that charge trapping was suppressed due to electrostatic screening by the electrolyte. This study highlights how fundamental knowledge gained from past chemically doped polymer studies cannot directly be applied to understand conductivity in mixed conducting polymers, encouraging future spectroscopic studies of charge trapping behavior in their electrochemically active states.