Rationalizing the enhancement of the thermoelectric properties of PEDOT:PSS by secondary doping

The enhancement of the thermoelectric (TE) properties of poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) through secondary doping is well-known. In particular, the increase in the power factor (PF) following the addition of the strong polar solvent dimethyl sulfoxide (DMSO) in an aqueous solution is largely reported. However, systematic and comprehensive characterization of the morphological, compositional, and energetic evolution of the system with the addition of the secondary dopant is still missing. Here, the rationalization of the PF enhancement of PEDOT:PSS thin films with the addition of DMSO is presented. The system evolution is followed with morphological, by AFM and grazing incident wide angle x-ray scattering (GIWAXS), compositional, by x-ray photoelectron spectroscopy (XPS), and energetic, by ultraviolet photoelectron spectroscopy (UPS), measurements as varying the secondary dopant percentage from 0 to 50 vol. %. UPS measurements reveal no changes in the work function, substantiating the measured constant values of Seebeck coefficient (S) as varying the percentage of DMSO. On the other hand, XPS analysis confirms that the PSS to PEDOT ratio decreases with the addition of DMSO. However, the compositional changes do not influence the molecular packing and the thin film crystallinity, as reported by GIWAXS measurements. This comprehensive analysis of the system evolution makes possible to safely relate the increase in the electrical conductivity (sigma) as a function of the secondary dopant to the sole enhancement of the charge carrier mobility, due to both morphological and chemical composition changes.