Optimal thermoelectric figure of merit of a molecular junction

Abstract

We show that a molecular junction can give large values of the thermoelectric figure of merit ZT , and so could be used as a solid state energy conversion device that operates close to the Carnot efficiency. The mechanism is similar to the Mahan-Sofo model for bulk thermoelectrics -the Lorenz number goes to zero violating the Wiedemann-Franz law while the thermopower remains non-zero. The molecular state through which charge is transported must be weakly coupled to the leads, and the energy level of the state must be of order kBT away from the Fermi energy of the leads. In practice, the figure of merit is limited by the phonon thermal conductance; we show that the largest possible ZT ∼ (G ph th ) −1/2 , whereG ph th is the phonon thermal conductance divided by the thermal conductance quantum.

Thermoelectric properties of a weakly coupled quantum dot: enhanced thermoelectric efficiency

“…Deviations from the Wiedemann-Franz law in zero dimensional structures were predicted also in [19,20] without, however, relating these to the possibility of an enhancement of ZT. Recently, an interesting prediction for a significant increase of ZT in a single-level molecule was made by Murphy et al [16]. In Ref.…”

“…Humphrey and Linke [13] provided a thermodynamic explanation why the optimum density of states for enhanced ZT is a delta function. More recent theoretical studies [14][15][16][17][18] point out the possibility of significantly improving the thermoelectric efficiency in small quantum dots and molecules.…”

“…In Ref. [16] it was found that the energy parameter that controls ZT is the electron-electron repulsion. Here we show that for a multi-level system in the Coulomb blockade regime ZT is controlled by the level spacing and, consequently, the spatial confinement.…”

“…Although, the phonon contribution to the thermal conductance could washout the variation of the total thermal conductance as a function of E F , ZT will still vary periodically with E F following the structure of S 2 GT shown in Figs.2 and 5a. Moreover, the maximization of S 2 GT could be utilized for the prediction of an optimal value for κ ph that maximizes ZT [16].…”

Scattering theory of thermocurrent in quantum dots and molecules

“…In the expression of thermoelectric figure of merit which characterizes the efficiency of energy conversion at small temperature gradients,the denominator is the sum of electron and phonon thermal conductances [37,38]. We remark that within the adopted model the phonon thermal conductance through the junction is zero.…”