Thermal Conductivity and ZT in Disordered Organic Thermoelectrics

For decades, continuous attempts have been made to improve the figure of merit (ZT) of thermoelectrics. The theory behind the Seebeck effect itself is well researched, but the problem with ZT is related to materials properties that offset one another. This work analyzed the link between the site energy distributions and thermal conductivity of oxidized poly(3,4-ethylenedioxythiophene-tosylate) (PEDOT:Tos), which was reported to be a good organic thermoelectric. To understand how heat flow was affected by disorder in PEDOT:Tos and the associated electron-phonon interactions, we computed the values of the thermal conductivity kappa and ZT using materials parameters extracted from the open literature. By varying the values of the parameters separately, we were able to identify their individual influence on kappa and ZT. Our results suggest that ZT is most sensitive to changes in sigma, the bandwidth of the density of states (DOS) of the transport sites, and less so to changes in n (eff), the effective carrier density. Our simulations also suggested that ZT could become exceptionally large (approaching a value of similar to 20) if sigma were lowered to 1 meV to 2 meV. This would be a tremendous approach to increase ZT in oxidized PEDOT:Tos.