Advanced in situ and operando characterization techniques are required for the continued development and advancement of organic mixed ionic-electronic conductors (OMIECs). The key benefit of OMIECs is that ionic-electronic coupling allows the modulation of nearly all materials properties through the electrochemical control of charge density. Included is the ability to electrochemically control electronic conductivity, color, surface energy, volumetric swelling, moduli, miscibility, catalytic activity, thermal conductivity, emissivity, and more. This enables numerous applications that are inaccessible with traditional electronic materials. However, the massively tunable ionic and electronic charge density means that OMIECs do not possess a single discrete microstructure but a range of ion (and solvent) swollen structures with widely varying composition that depends on environment and applied electrochemical potential. This greatly limits the insight gained from traditional dry ex situ characterization that probe states that can have little in common with the structure, composition, and environment of OMIECs in functioning devices and applications. Therefore, in situ and operando characterization in device/application relevant conditions is crucial for understanding the complex structure-property relationships that ultimately dictate OMIEC performance. 

Synchrotron X-rays present a powerful tool for in situ/operando characterization. Their unparalleled brilliance and coherence allow for time resolved measurements of microstructure, composition, and dynamics using grazing incidence wide angle X-ray scattering (GIWAXS), X-ray fluorescence (XRF), and X-ray photon correlation spectroscopy (XPCS), respectively. Amongst other insights, in situ GIWAXS reveals the structural sources of enhanced mixed conducting properties, operando XRF reveals complicated interfacial dominated proton and metal ion transport, and XPCS reveals long time scale domain coarsening. This helps explain the underlying phenomena that dictate OMIEC structure-property relationships, device performance, and materials stability. The limits of these synchrotron measurements and their potential for future improvement will be considered. Complementary in situ/operando techniques will be addressed, as well as their multi-modal possibilities. Overall, in situ synchrotron techniques are just beginning to unravel the structural complexity of OMIECs, yet already they provide concrete directions for the rational design and processing of improved OMIEC materials.