Optimal thermoelectric figure of merit of a molecular junction

TL;DR

This paper demonstrates that molecular junctions can achieve high thermoelectric efficiency close to Carnot limits by tuning electronic properties, with the figure of merit limited by phonon thermal conductance.

Contribution

It introduces a mechanism for high thermoelectric figure of merit in molecular junctions, analogous to the Mahan-Sofo model, highlighting conditions for optimal energy conversion.

Findings

Large $ZT$ values possible in molecular junctions.

Optimal $ZT$ limited by phonon thermal conductance.

Conditions require weak coupling and energy level tuning.

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 $k_B T$ 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 $ZT\sim(\tilde{G}_{th}^{ph})^{-1/2}$, where $\tilde{G}_{th}^{ph}$ is the phonon thermal conductance divided by the thermal conductance quantum.