Iron(III) Dopant Counterions Affect the Charge-Transport Properties of Poly(Thiophene) and Poly(Dialkoxythiophene) Derivatives.

This study investigates the charge-transport properties of poly(3-hexylthiophene-2,5-diyl) (P3HT) and poly(ProDOT--biEDOT) (PE) films doped with a set of iron(III)-based dopants and as a function of dopant concentration. X-ray photoelectron spectroscopy measurements show that doping P3HT with 12 mM iron(III) solutions leads to similar extents of oxidation, independent of the dopant anion; however, the electrical conductivities and Seebeck coefficients vary significantly (5 S cm and + 82 μV K with tosylate and 56 S cm and +31 μV K with perchlorate). In contrast, PE thermoelectric transport properties vary less with respect to the iron(III) anion chemistry, which is attributed to PE having a lower onset of oxidation than P3HT. Consequentially, PE doped with 12 mM iron(III) perchlorate obtained an electrical conductivity of 315 S cm and a Seebeck coefficient of + 7 μV K. Modeling these thermoelectric properties with the semilocalized transport (SLoT) model suggests that tosylate-doped P3HT remains mostly in the localized transport regime, attributed to more disorder in the microstructure. In contrast perchlorate-doped P3HT and PE films exhibited thermally deactivated electrical conductivities and metal-like transport at high doping levels over limited temperature ranges. Finally, the SLoT model suggests that PE has the potential to be more electrically conductive than P3HT due to PE's ability to achieve higher extents of oxidation and larger shifts in the reduced Fermi energy levels.