Interplay between electron–electron and electron–vibration interactions on the thermoelectric properties of molecular junctions

The linear and non-linear thermoelectric properties of molecular junctions are theoretically studied close to room temperature within a model including electron–electron and electron–vibration interactions on the molecule. A non-equilibrium adiabatic approach is devised to include a strong Coulomb repulsion and applied to the self-consistent calculation of electron and phonon transport properties of massive molecules, such as fullerenes, within the Coulomb blockade regime. We show that the phonon thermal conductance is quite sensitive to strong electron–electron interactions within the intermediate electron–vibration coupling regime. Furthermore, the electron–vibration interaction enhances both phonon and electron thermal conductance, and it reduces not only the charge conductance, but also the thermopower. The effect of the strong electron–electron interactions provides a peculiar double-peak structure to the thermopower versus charge conductance curve. Finally, within the regime of weak to intermediate electron–vibration and vibration–lead phonon coupling, the peak values of the thermoelectric figure of merit are slightly less than unity, and the maximal efficiency of the junction can reach values slightly less than half of the Carnot limit for large temperature differences between the leads. Introduction and background. Recently, the possibility of controlling materials at the nanoscale has been exploited to optimize the thermoelectric efficiency, so that the conversion between heat and electricity and vice versa could be improved. In particular, the emerging field of molecular thermoelectrics has attracted the attention of many research groups showing that molecular devices can be efficient for the heat-electricity conversion since both vibrational and electronic degrees of freedom can contribute to tailor thermoelectric properties. Main results. In this paper, thermoelectric properties in molecular junctions are discussed presenting a new theoretical approach and focusing on the interplay between electron-electron and electron-vibration interactions active on the molecule within both linear and non-linear response regime. We have shown that, within the regime of weak to intermediate electron-vibration and vibration-lead phonon coupling, the peak values of the thermoelectric figure of merit can be made, for an experimentally accessible regime, close to unity, and the maximum efficiency of the junction close to half of the Carnot limit. Wider implications. This study allows us to assess the possibility to use molecular junctions in actual thermoelectric devices provided that intra- and inter-molecular couplings are tuned to make these molecular devices optimal thermoelectric converters.