Operando X-Ray Fluorescence for Direct Ion Composition and Mobility Quantification in Organic Mixed Ionic/Electronic Conductors

Organic mixed ionic-electronic conductors (OMIECs) represent a class of organic materials, mostly conjugated polymers, characterized by their ability to transport both electronic and ionic charges. Understanding the composition and mobility of ions within OMIECs is of paramount importance, as it directly influences the performance and fundamental mechanisms of OMIEC-based devices. Typically, such assessments have relied on indirect methods, often based on assumptions that may not always be accurate. Ex situ X-ray fluorescence (XRF) has been employed to quantify ion composition in OMIECs under different doping states, but potential errors could arise due to the need for sample washing to remove residual electrolyte.<br/>Instead, operando XRF emerges as a powerful and direct tool for investigating dynamic ion composition and transport in OMIECs during electrochemical operations. Herein, operando XRF was harnessed to probe ion transport in a model OMIEC material, poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS). The findings from this investigation are highly enlightening: the initial electrochemical cycle showed a slow electrowetting and cation-proton exchange. Subsequent cycles demonstrated minimal cation fluctuation, around 5%, implying a rapid, stable response with small, consecutive steps of ion migration. The calculated effective ion mobility displayed thickness-dependent behavior, underscoring the significance of interfacial ion transport pathways with higher effective mobile ion density. This decoupling of bulk and interfacial effects on ion mobility enhances our comprehension of ion transport in both conventional and vertical organic electrochemical transistors (OECTs). Furthermore, the correlation between ion mobilities, domain boundaries, and mobile ion density suggests that ions within OMIECs migrate in a highly complex way. These findings promise to advance our understanding of ion transport in OMIECs and related devices, offering valuable insights that can guide molecular design, material processing, and charge/ion migration modeling. Ultimately, this research can contribute to enhanced device performance and faster response times. Importantly, this methodology is not limited to OMIECs; it can be effectively applied to investigate complex ion transport mechanisms in various mixed conductors, spanning diverse fields such as batteries and solar cells, holding the potential to revolutionize the design and optimization of a wide range of technologies.