Large enhancement in thermoelectric efficiency of quantum dot junctions due to increase of level degeneracy

It is theoretically demonstrated that the figure of merit (ZT) of quantum dot (QD) junctions can be significantly enhanced when the degree of degeneracy of the energy levels involved in electron transport is increased. The theory is based on the the Green-function approach in the Coulomb blockade regime by including all correlation functions resulting from electron-electron interactions associated with the degenerate levels (L). We found that electrical conductance (Ge) as well as electron thermal conductance (κe) are highly dependent on the level degeneracy (L), whereas the Seebeck coefficient (S) is not. Therefore, the large enhancement of ZT is mainly attributed to the increase of Ge when the phonon thermal conductance (κph) dominates the heat transport of the QD junction system. In the serially coupled double-QD case, we also obtain a large enhancement of ZT arising from higher L. Unlike Ge and κe, S is found almost independent on electron interdot hopping strength.