Low-Dose Electron Total Scattering Analysis Resolves Non-Crystalline Phase Separation in Polymer Semiconductors and Device Multilayers.

Nanoscale phase separation in polymer semiconductor blends significantly influences their mechanical, optical, and transport properties, and uncontrolled phase separation ultimately contributes to the long-term degradation of devices. Recent advances in electron microscopy have enabled imaging and diffraction-based analysis of polymer components, but these approaches are typically limited to blends with components exhibiting sharp differences in crystallinity or molecular structure. Here, we employ low-dose scanning electron diffraction to characterize phase-separated domains of components with nearly identical molecular structure, namely poly(9,9-di-n-octylfluorenyl-2,7-diyl) (F8) and poly(9,9-dioctylfluorene-alt-benzothiadiazole) (F8BT). For semicrystalline blends, we demonstrate phase identification and crystallographic texture analysis. In fully amorphous systems with partial phase separation, we highlight the limitations of electron pair distribution function (ePDF) analysis. Instead, we exploit differences in angle-dependent scattering, coupled with calculated intramolecular scattering intensities, to reliably map distinct amorphous phases. Finally, we showcase this suite of techniques for characterizing a model device cross-section, prepared by cryogenic focused ion beam milling. These workflows decouple phase separation and crystallization processes in F8:F8BT blends, provide corroborating insights into F8 crystalline and amorphous intermolecular π - π stacking, and support the direct visualization of non-crystalline organic multilayer interfaces in cross-section needed for failure analysis in organic optoelectronics.