Charge mobility and ZT in conducting organic thermoelectric

Abstract

Charge dynamic in disordered organic semiconductors is often described in terms of transport sites and trap states. The process can be refined to include site energy distribution and hopping energetic. In turn, these will affect transport properties such as the carrier mobility and the thermal conductivity. As is well known, ZT in thermoelectric also depends on these parameters. This work attempts to analyze the relationship between site energy distribution and hopping mobility in oxidized Poly(3,4-ethylenedioxythiophene-tosylate) (PEDOT.Tos) (an organic thermoelectric known to have both high carrier density and low thermal conductivity). To understand the charge transport in oxidized PEDOT.Tos, we examined the thermoelectric data reported in the literature and used them to evaluate site parameters such as the escape frequency, the localization length and the width of the DOS (density of states). These results were used to compute the carrier mobility using the gaussian disorder model and the correlated disorder model We then used the computed parameters to assess the optimal value of ZT in PEDOT.Tos. Our simulations suggested that major improvement in ZT is achieved by increasing the localization length and to a lesser extent by reducing the bandwidth of the DOS. For the somewhat narrow bandwidth found in PEDOT.Tos (<kT), we are of the opinion that the transport site energy is strongly correlated.