Molecular Dependence of the Large Seebeck Effect in \tau-type Organic Conductors

TL;DR

This study investigates the molecular dependence of the large Seebeck effect in tau-type organic conductors, combining experimental measurements with first-principles calculations to understand how band structure influences thermoelectric properties.

Contribution

The paper introduces a combined experimental and theoretical approach to analyze the Seebeck effect in tau-type organic conductors, highlighting the role of band structure modifications via molecular design.

Findings

The Seebeck coefficient peaks at different temperatures for the two materials.

Band structure analysis links the Seebeck behavior to narrow band gaps.

Molecular modifications effectively tune thermoelectric properties.

Abstract

We study the Seebeck effect in the $\tau$-type organic conductors, $\tau$-(EDO-$S$,$S$-DMEDT-TTF)$_{2}$(AuBr$_{2}$)$_{1+y}$ and $\tau$-(P-$S$,$S$-DMEDT-TTF)$_{2}$(AuBr$_{2}$)$_{1+y}$, where EDO-$S$,$S$-DMEDT-TTF and P-$S$,$S$-DMEDT-TTF are abbreviated as OOSS and NNSS, respectively, both experimentally and theoretically. Theoretically in particular, we perform first-principles band calculation for the two materials and construct a two-orbital model, on the basis of which we calculate the Seebeck coefficient. We show that the calculated temperature dependence of the Seebeck coefficient $S$ is semi-quantitatively consistent with the experimental observation. In both materials, the absolute value of the Seebeck coefficient is maximum at a certain temperature, and this temperature is lower for NNSS than for OOSS. From a band structure viewpoint, we find that this can be traced back to the narrowness of the band gap between the upper and the lower pudding-mold type bands. On the other hand, the Seebeck coefficient of NNSS in the low temperature regime steeply increases with increasing temperature, which is due to the narrowness of the upper band. These differences in thermoelectric properties demonstrate the effectiveness of controlling the band structure through molecular modification.