Operando X-Ray Scattering of 3-Terminal Electrochemical Devices Based on Organic Mixed Ionic/Electronic Conductors

Organic mixed ionic/electronic conductors (OMIECs) have shown promise in a wide range of electrochemical devices including organic electrochemical transistors (OECTs), polymer batteries, and even artificial synapses for neuromorphic computing. Synchrotron X-ray scattering techniques have been widely employed to probe the structure of OMIECs, but structural transformations during operation (oxidation/reduction) of these materials are not well-characterized. This is largely because although device operation occurs in contact with an electrolyte, structural characterization of these material is almost always performed in a “dry” state to avoid complications from excessive electrolyte scattering. This constraint limits the ability to probe the structural dynamics of this diverse class of materials, hampering the development of the structure-property relations which enable future materials design. Recent progress in operando characterization of these materials has enabled potential-dependent grazing incidence x-ray scattering to be performed while suppressing stray scattering from the electrolyte, revealing dynamic changes during charging¹. Despite this promising proof-of-concept, measurement of device-relevant materials properties such as charge density, conductivity, and mobility remain elusive in existing in-situ/operando x-ray scattering experiments. To overcome this, we use a 3-terminal electrochemical cell based on an insulating ceramic which maintains the benefits of earlier work while also enabling a wide range of relevant electronic/electrochemical measurements to be performed. The improved electrochemical stability of this cell enables quantification of charge injected into the organic semiconductor, permitting concurrent electronic and structural characterization of materials. We reveal dynamic structural transformations in ionic liquid gated OMIEC devices which closely track with changes in charge carrier density in the film. We then highlight how a suite of electronic measurements such as transfer curves, rise times, and neuromorphic state retention can be easily performed during x-ray scattering experiments. The combination of high-fidelity structural characterization with simultaneous determination of relevant electronic properties (e.g. carrier density and mobility) enables a richer understanding of the structure-property relations in these materials.<br/><br/>1. Paulsen, B. D. <i>et al.</i> Electrochemistry of Thin Films with In Situ/Operando Grazing Incidence X-Ray Scattering: Bypassing Electrolyte Scattering for High Fidelity Time Resolved Studies. <i>Small</i> <b>17</b>, 2103213 (2021).