Exploring the implications of the Lorentz number approaches one in organic thermoelectrics

Abstract

Organic thermoelectrics offer an alternative to their inorganic counterpart in the pursuit for high ZT materials. While a few PEDOT-related organics has shown significant promise, there is nonetheless a lack of general understanding on the transport mechanism with respect to the carriers and the phonons. This work examined the implication of the Lorentz number approaching one in de-doped PEDOT–PSS (Poly (3,4-ethylene dioxythiophene): Poly(styenesulphonate)) and compared to related properties found in oxidized PEDOT:Tos (poly(3,4-ethylenedioxythiophene-tosylate)). In our analysis, we assumed that the carriers were tightly-coupled to the phonons and their transport properties were basically metallic. Using data reported in the open literature, we computed the Lorentz numbers of the two and showed that there were major differences. For instance, the Lorentz number of oxidized PEDOT:Tos was larger than that of a metal until the organic turned metallic while the Lorentz number of de-doped PEDOT:PSS which persistently was smaller than one. Potentially, the latter would imply a drop in the phonon relaxation time. We attributed the decrease to the formation of a porous structure which reduced the effectiveness in heat transfer.