Probing the path for achieving a broad temperature plateau of the figure of merit in thermoelectric nanocomposite materials.

The efficiency of a thermoelectric device depends directly on the average figure of merit (zT) of the material. A high average zT requires a broad temperature plateau with a high zT, but state-of-the-art thermoelectric materials display a peaked zT over a narrow temperature range due to a strong temperature dependence of transport properties. In this work, using Boltzmann transport theory, we systematically investigate the underlying physics and propose a strategy for attaining a broad temperature plateau of zT through proper engineering of the interfacial barrier height in PbTe nanocomposite material. The optimized barrier height (U ) not only enhances the zT but also maintains its high value over a wide temperature range [T :T ]. It has been found that for p = 2.8 × 10 cm, the U is 0.112 eV at which zT varies between 1.9-2.14 over a wide temperature range of 550-850 K, resulting in a high average zT of 2.02 in comparison to a bulk value of 1.22. Also, for p = 5 × 10 cm, U is 0.102 eV at which zT varies between 1.046-1.435 for a temperature range of 300-600K, resulting in a high average zT of 1.27 over a bulk value of 0.844. The above results show that the range [T :T ] depends on carrier concentration which, in turn, determines the position of the Fermi level (E ) and Fermi window at T and T . To obtain a broad temperature plateau of zT, the findings show that at T, E should lie inside the band and zT should show strong variation with barrier height, whereas at T , E should lie in the band gap and zT should have little variation with barrier height. This trend allows us to choose U which synergistically optimizes the transport properties at T with T to give a broad temperature plateau of zT. This work proposes a new advantage of interfacial scattering which enhances the average zT and also provides necessary guidelines to experimentalists for synthesizing a highly efficient thermoelectric device.