Revealing Detailed Morphology of Semiconducting Polymers with Simulation-Guided Resonant Soft X-Ray Scattering and Spectroscopy

Semiconducting conjugated polymers have gained widespread use in organic electronic devices such as organic electrochemical transistors, organic light emitting diodes, and organic photovoltaics. A unique feature of conjugated polymer systems is strong directionality of their optoelectronic properties and ionic transport with respect to molecular orientation. Both ionic and electric charge transport is anisotropic in most conjugated polymer systems depending on a combination of molecular orientation and packing motif. This charge transport directionality leads a strong linking between morphology and device performance. Despite the importance of morphology, it remains challenging to probe wholistically. While crystalline regions can be probed with the common technique of grazing incidence wide-angle X-ray scattering (GIWAXS), morphology characterization of amorphous regions is more elusive. This is problematic because the amorphous regions are essential in device performance and as potential sites for dopant ions. In my work I demonstrate the power of using resonant X-ray scattering and spectroscopy to probe the entire polymer film of the high-performance conjugated polymer PM6, including features in the amorphous regions. Angle-resolved near edge X-ray absorption fine structure (NEXAFS) is used to extract molecular tilt at the top surface and in the bulk and polarized resonant soft X-ray scattering (p-RSoXS) is forward simulated to reveal the hierarchical structure of polymer fibrils. Tender resonant X-ray scattering is further used to verify proposed PM6 unit cells, and all resonant X-ray findings are consistent with and expand upon GIWAXS and atomic force microscopy (AFM) results. My work demonstrates the use of these emerging characterization tools and data analysis procedures to answer essential questions on polymer morphology and its connection to organic electronic device performance.