Seebeck Coefficients of Poly(3,4-ethylenedioxythiophene):Poly(styrene sulfonate) Correlated with Oxidation Levels

Oxidation states of poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) were controlled by the electrode potential, and its thermoelectric properties were studied elaborately as a function of the oxidation level measured by potential-step chronocoulometry. With an increase in the oxidation level from 0.07 to 21%, electrical conductivities (σ) were increased from 0.37 × 10–3 to 1.1 S cm–1, accompanying a slight decrease in the Seebeck coefficient (S) from 79 to 28 μV K–1. Addition of ethylene glycol or dimethyl sulfoxide to PEDOT:PSS enhanced σ values but led to a decrease of S. Both σ and S values of the solvent-added PEDOT:PSS films became sensitive to the oxidation level, and double logarithmic plots of S and the oxidation level were found to fit a straight line with a slope of −0.25.