Resonant Soft X‑ray\nScattering Reveals the\nDistribution of Dopants in Semicrystalline Conjugated Polymers

The distribution of counterions and dopants within electrically\ndoped semicrystalline conjugated polymers, such as poly(3-hexylthiophene-2,5-diyl)\n(P3HT), plays a pivotal role in charge transport. The distribution\nof counterions in doped films of P3HT with controlled crystallinity\nwas examined using polarized resonant soft X-ray scattering (P-RSoXS).\nThe changes in scattering of doped P3HT films containing trifluoromethanesulfonimide\n(TFSI<sup>–</sup>) and 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane\n(F<sub>4</sub>TCNQ<sup>•–</sup>) as counterions to the\ncharge carriers revealed distinct differences in their nanostructure.\nThe scattering anisotropy of P-RSoXS from doped blends of P3HT was\nexamined as a function of the soft X-ray absorption edge and found\nto vary systematically with the composition of crystalline and amorphous\ndomains and by the identity of the counterion. A computational methodology\nwas developed and used to simulate the soft X-ray scattering as a\nfunction of morphology and molecular orientation of the counterions.\nModeling of the P-RSoXS at N and F K-edges was consistent with a structure\nwhere the conjugated plane of F<sub>4</sub>TCNQ<sup>•–</sup> aligns perpendicularly to that of the P3HT backbone in ordered domains.\nIn contrast, TFSI<sup>–</sup> was distributed more uniformly\nbetween domains with no significant molecular alignment. The approach\ndeveloped here demonstrates the capabilities of P-RSoXS in identifying\norientation, structural, and compositional distributions within doped\nconjugated polymers using a computational workflow that is broadly\nextendable to other soft matter systems.