CPOS Seminar: Dopant Distributions in Semicrystalline Conjugated Polymers from Resonant Scattering

Speaker: Phong Nguyen, PhD, Department of Chemical Engineering, UC Santa Barbara

The distribution of dopant counterions within semicrystalline conductive polymers, such as poly(3-hexylthiophene-2,5-diyl) (P3HT), plays a pivotal role in understanding the mechanisms of charge conduction. These polymers feature a combination of amorphous domains, which enable ion transport, and ordered domains that facilitate high electronic mobility, alongside the need for local charge neutrality between electronic charge carriers and dopant counterions. This complex structure prompts questions about the role of dopant distributions in shaping the energetic landscape for charge conduction. Traditional methods for concurrently probing these domains have been limited. Our study presents a comprehensive model of P3HT morphology and assesses how dopant distribution affects resonant scattering. Crucially, we investigate the role of dopant chemical structure by incorporating isotropic trifluoromethanesulfonimide (TFSI-) and planar, conjugated 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ•-), examining how these distinct dopant counterions modulate the scattering profiles of doped P3HT films. Our findings reveal that the inherent scattering anisotropy of P3HT, driven by density differences between crystalline and amorphous regions, is significantly affected by dopant placement. This dopant-driven modulation of scattering anisotropy, combined with dopant absorbance, offers a straightforward method to map dopant distribution across both domains, revealing a preference for crystallite incorporation. Moreover, by leveraging the anisotropic refractive indices of F4TCNQ•- at the N and F K-edges, we show that its conjugated planes align perpendicularly to that of the P3HT backbone. This approach underscores the capabilities of polarized resonant soft X-ray scattering in identifying orientation, structural, and compositional distributions within doped conjugated polymers and we introduce a workflow to model and interpret resonant scattering that is broadly extendable to other soft matter systems.