Abstract
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.
Highlights
The direct conversion of temperature differences to electric voltage and vice versa take place in solid state systems
We have studied the thermoelectric properties of a molecular junction with electron-electron and electron-vibration interactions within the linear response regime focusing on a self-consistent calculation of the phonon thermal conductance GpKh close to room temperature
For ǫ ≃ −U/2, GpKh is poorly influenced by the electron-vibration effects even if EP is not small getting a value close to the asymptotic one. From this analysis emerges that the complex enhancement of the phonon thermal conductance GpKh as a function of the electron-electron and electron-vibration interactions can be mostly ascribed to the properties of additional electron-vibration induced damping rate γλ(x) discussed in the previous section
Summary
The direct conversion of temperature differences to electric voltage and vice versa take place in solid state systems. In devices with large molecules or carbon nanotube quantum dots[43], a nonequilibrium adiabatic approach has been introduced for spinless electrons exploiting the low energy of the relevant vibrational degrees of freedom[44,45,46,47,48] This method is semiclassical for the vibrational dynamics, but it is valid for arbitrary strength of electron-vibration coupling. We have studied the thermoelectric properties of a molecular junction with electron-electron and electron-vibration interactions within the linear response regime focusing on a self-consistent calculation of the phonon thermal conductance GpKh close to room temperature. For realistic parameters of the model, the peak values of ZT are still of the order of unity This effect is ascribed to the magnitude of the phonon thermal conductance which can be smaller than the electronic counterpart in a large range of gate voltages. The paper is closed by Appendix A, where the comparison between different treatments of the large Coulomb repulsion is made within the Coulomb blockade regime
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