Interfacial phases are the key to enhancing thermoelectric figure of merit

Thermoelectric (TE) performance of complex nanostructured materials is strongly influenced by the morphology and transport properties of the grain boundaries, or interfacial regions connecting together individual grains. However, experimental and theoretical efforts probing such characteristics have been very limited so far. Here, we utilized the finite element method based calculations to evaluate both interfacial and effective properties of TE polycrystals, including their figure of merit ZT. Two popular nanocrystalline (hole-doped) chalcogenide TE systems were used as examples: BiSbTe with ZT∼1.4 and SnSe with ZT∼3.0. Our analysis reveals that, in order to achieve the measured ZT values, the (averaged) transport properties of the grain-boundary regions in both of these systems must simultaneously exhibit significantly higher electrical conductivity and lower thermal conductivity in comparison with those of the grain interiors. This trend is weaker in the lower-ZT BiSbTe compound, but very apparent in SnSe. We hope that, in addition to providing tools for the evaluation and design of TE materials, this investigation may inspire more in-depth studies of such “magical” interfaces, including the determination of their structure on the atomistic level.