Imprints of interfaces in thermoelectric materials

Contemporary thermoelectric literature is rife with material structure-related terminologies like interfaces and grain boundaries, signaling the significance of these structures. Interfaces decide the characteristics of electronic and thermal transport and mechanical properties of polycrystalline and nano thermoelectric (TE) materials. Understanding the relationship between grain boundaries/interphase boundaries and property connections in materials is a key component of material design with desired characteristics and performance. It is now widely recognized that the microstructure of materials is intimately connected to their bulk properties. Accordingly, microstructure control and interface manipulation have emerged as critical topics in the field of materials science and engineering, particularly in thermoelectrics. This paper narrates recent breakthroughs in high-performance TE material design from the standpoints of interface structure and grain boundary manipulation. First, it provides a glimpse of strategies for thermal conductivity reduction through nano and microstructure control, embedded nanoinclusions, grain size reduction, and all-scale hierarchical architectures. It then deliberates on electron and phonon transport decoupling via coherent interfaces, matrix/precipitate electronic band alignment, and charge carrier filtering effects. It proceeds to review the recent results on TE properties of materials prepared with aforementioned strategies emphasizing Bi2(Te,Se)3 and (Bi,Sb)2Te3, SnSe, SnTe, Cu2Se, skutterudides, PbTe-based compounds, GeTe, polymer TE composites, and other materials. At the end, possible strategies for further enhancing zT are addressed.