Why do layered structures of TE materials give higher values of thermo-electric efficiency in comparison to doped materials, even though the band energy is more dense (near Fermi level) in doped materials?
In fact, the layered structure gives almost discrete energy levels, not continuous, and not as dense as in doped materials.
The main factor is that in the interfaces the thermal conductivity is much lower than in the bulk materials. Therefore the figure of merit is higher.
Formation of the layered structures leads to enhanced scattering of phonons at sublattice interfaces that are responsible for thermal conductivity (e.g. (SrO)(SrTiO3) large drop of k, while σ and S similar to the polycrystaline)
The Main reason is 'electrical conductivity and thermal conductivity are decoupled, can independently be tuned'. Which is why, a layered structured material can have very low thermal conductivity and high electrical conductivity at the same time.
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