Lorenz number of conducting PEDOT:PSS*

The electronic thermal conductivity is related to the electrical conductivity through the Wiedemann-Franz law (WFL), which predicts that the ratio of the electronic thermal conductivity to the electrical conductivity is proportional to the absolute temperature. The WFL has been validated for various materials; however, deviations may arise under certain circumstances, in which the relaxation times for the electrical and thermal processes are not identical. In this work, we investigate the Lorenz number, the proportionality factor in the WFL, of conjugated polymers. We prepare samples made of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) with tunable electrical conductivity. The in-plane electrical resistivity is characterized with setups of both 4-point probe and Van der Pauw configurations. To determine the thermal conductivity along the same direction as that for the electrical resistivity, we measure the through-plane thermal conductivity of the cross section of PEDOT:PSS using time-domain thermoreflectance. The effects of anisotropy and inhomogeneity on the thermal conductivity of PEDOT:PSS are also examined.