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) (PE2) 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, PE2 thermoelectric
transport properties vary less with respect to the iron­(III) anion
chemistry, which is attributed to PE2 having a lower onset
of oxidation than P3HT. Consequentially, PE2 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 PE2 films exhibited thermally deactivated electrical
conductivities and metal-like transport at high doping levels over
limited temperature ranges. Finally, the SLoT model suggests that
PE2 has the potential to be more electrically conductive
than P3HT due to PE2’s ability to achieve higher
extents of oxidation and larger shifts in the reduced Fermi energy
levels.