X-Ray Absorption Spectroscopy—A Probe for Electroactive Polymer-Charge Interactions

Semiconducting polymers have sparked interest in electrochemical processes such as catalysis and energy storage due to their strong charge transport properties and ionic conductivity. Despite the importance of polymer-electrolyte interactions in device performance, there is a lack of understanding of the fundamental interphasial interactions that occur between the polymer and electrolyte. In this study, we use X-ray absorption spectroscopy (XAS) and X-ray scattering to study the local atomic electronic and chemical structure of two polymers. We use N2200, a well-studied polymer¹ due to its high crystallinity and charge carrier mobility changes, which has monomeric units consisting of an naphthalene diimide (NDI) with alkyl sidechains coupled with a bithiophene unit, and it’s derivative, P90, in which 90% of monomers have glycolated sidechains and 10% have alkyl sidechains, that has recently been introduced for use in aqueous applications.²<br/>N2200 and P90 are electrochemically active and have two redox events correlating to the formation of polarons (-1 charge) and bipolarons (-2 charge) during charging that are accompanied by the absorption of counter-cations from the surrounding electrolyte for charge neutralization. We will introduce and discuss results from an <i>operando</i> study in which the polymer charge is held potentiostatically, while measuring its XAS spectra and compare this to <i>ex-situ</i> charging XAS results. Using simulated XAS to assign electron excitation spectra to experimental XAS spectra, we identify polaron/bipolaron localization and how ions incorporate into the film to understand polymer-electrolyte interfacial interactions. Our results show how the atomic environments of polymer change as a function of charge, and we discuss the impact of polymer choice as electrode materials on device performance and operation.<br/>References:<br/>(1) <i>Adv. Mater.</i> <b>2010</b>, <i>22</i> (39), 4359–4363. https://doi.org/10.1002/adma.201001202.<br/>(2) <i>Chem. Mater.</i> <b>2018</b>, <i>30</i> (9), 2945–2953. https://doi.org/10.1021/acs.chemmater.8b00321.