Probing Solid-State Conformation of Polythiophenes using Transmission SANS

The molecular packing of conjugated polymers (CPs) affects aggregation and crystallization behavior, significantly altering their self-assembled structures and optoelectronic properties. Due to the growing shift towards soft, flexible, and miniaturized electronic devices, organic electronic devices are so thin (e.g., tens of nanometers for the active layer) that their properties differ from those of bulk materials (e.g., films with micron-scale thickness). While morphology studies (e.g., conformation or crystalline structure) are typically conducted in the bulk and/or solution states, distinct insights into the composition, structure, and molecular packing of thin film interfaces are needed to capture the behavior of sub-100 nm films. Common solid-state characterization techniques like X-ray scattering mainly reveal insights into crystalline domains, while the importance of charge delocalization along disordered regions remains understudied. The lack of bulk/solid-state conformations has left fundamental gaps in our understanding of the relationship between solution-processed chain conformation/rigidity and the optical and electronic properties (e.g., electronic coupling, charge transport) in solid-state devices. To address this, we focused on polymer chain conformations in the solid state using small-angle neutron scattering (SANS) for systematically engineered polythiophenes. Our findings show that molecular chains in the melt have similar dimensions to fully dissolved chains. This aligns with previous theoretical predictions, as well as prior experimental observations for flexible polymers. Further research into the conformations of CPs in thin film states, essential for enhancing the performance of organic electronic devices, is discussed.