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- alt -biEDOT) (PE ₂ ) 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 ^-1 and + 82 μV K ^-1 with tosylate and 56 S cm ^-1 and +31 μV K ^-1 with perchlorate). In contrast, PE ₂ thermoelectric transport properties vary less with respect to the iron(III) anion chemistry, which is attributed to PE ₂ having a lower onset of oxidation than P3HT. Consequentially, PE ₂ doped with 12 mM iron(III) perchlorate obtained an electrical conductivity of 315 S cm ^-1 and a Seebeck coefficient of + 7 μV K ^-1 . 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 ₂ 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 ₂ has the potential to be more electrically conductive than P3HT due to PE ₂ 's ability to achieve higher extents of oxidation and larger shifts in the reduced Fermi energy levels.