Thermoelectric Power and ZT in Conducting Organic Semiconductor

Abstract

A recent report on poly(3,4-ethylenedioxythiophene-tosylate) (PEDOT.Tos) suggested that the thermoelectric figure of merit (ZT) could be enhanced when the percentage oxidation was chemically altered. This invokes the question of whether the carrier density or the mobility was modified. In this work, we analyzed data reported by Bibnova et al. (Nat. Mater. 10, 429, 2011) and extracted the transport parameters using three-dimensional (3D) and two-dimensional (2D) models. Our results indicate that the increase in the power factor (S2σ) was due primarily to upward extension in the range of thermoelectric power. A changeover from lattice scattering to ionized impurity scattering in PEDOT.Tos allowed the equation governing the thermoelectric power to be valid at higher carrier densities, resulting in an increase in the power factor. ZT was also enhanced in PEDOT.Tos due to the low intrinsic thermal conductivity (~0.37 W/m K). The peak value of ZT (~0.3) was found close to the regime where the semiconductor turned “metallic,” beyond which ZT would decrease. We are of the opinion that charge-to-charge scattering (which normally would lower the power factor in highly doped semiconductors) remain subdued in PEDOT.Tos due potentially to electronic screening and a lack of long-range order. We used the reported data to compute the carrier density and mobility assuming ionized impurity scattering and found the peak power factor to occur for carrier density of ~1 × 1026 m−3 and mobility of ~5 × 10−4 m2/V s.