Efficiency of thermoelectric energy conversion in biphenyl-dithiol junctions: Effect of electron-phonon interactions

Abstract

The electron-phonon interaction is the dominant mechanism of inelastic scattering in molecular junctions. Here we report on its effect on the thermoelectric properties of single-molecule devices. Using density functional theory and the nonequilibrium Green's function formalism we calculate the thermoelectric figure of merit for a biphenyl-dithiol molecule between two Al electrodes under an applied gate voltage. We find that the effect of electron-phonon coupling on the thermoelectric characteristics strongly varies with the molecular geometry. Two molecular configurations characterized by the torsion angles between the two phenyl rings of ${30}^{\ifmmode^\circ\else\textdegree\fi{}}$ and ${90}^{\ifmmode^\circ\else\textdegree\fi{}}$ exhibit significantly different responses to the inelastic scattering. We also use molecular dynamics calculations to investigate the torsional stability of the biphenyl-dithiol molecule and the phonon thermal transport in the junction.