Visualizing the multi-level assembly structures of conjugated molecular systems with chain-length dependent behavior

It remains challenging to understand the structural evolution of conjugated polymers from single chains to solvated aggregates and film microstructures, although it underpins the performance of optoelectrical devices fabricated via the mainstream solution processing method. With correlative imaging of cryogenic electron microscopy and liquid-phase electron microscopy, we observed the morphological evolution of multi-level assembly of a model system of isoindigo-based conjugated polymer. In confirming the ensemble measurements in solution (UV-vis) and solid state (grazing-incidence wide-angle X-ray scattering), we further unraveled the hidden molecular assembly pathways, the mesoscale network formation, and their unorthodox chain dependence. Oligomers and short polymer chains showed rigid conformations forming discrete aggregates in solution, which further grew to form a highly ordered film that exhibits poor charge transport performance. In contrast, long polymer chains exhibited flexible chain conformations, creating interlinked aggregates networks in solution, which directly imprinted into films, forming interconnective solid-state microstructure with excellent electrical performance. Our work reports direct visualization methods of the structural evolution of polymer aggregates in solution and systemically studies the chain-length dependent aggregation and charge transport of conjugated polymers. Visualizing multi-level assembly structures of conjugated polymers provides a deep understanding of the inheritance of assemblies from solution-state aggregation to solid-state microstructures, accelerating the optimization of solution processing and device fabrication.