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 (UconstantzT) not only enhances the zT but also maintains its high value over a wide temperature range [Tmin:Tmax]. It has been found that for p = 2.8 × 1020 cm−3, the UconstantzT 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 × 1019 cm−3, UconstantzT 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 [Tmin:Tmax] depends on carrier concentration which, in turn, determines the position of the Fermi level (Ef) and Fermi window at Tmin and Tmax. To obtain a broad temperature plateau of zT, the findings show that at Tmin, Ef should lie inside the band and zT should show strong variation with barrier height, whereas at Tmax, Ef should lie in the band gap and zT should have little variation with barrier height. This trend allows us to choose UconstantzT which synergistically optimizes the transport properties at Tmin with Tmax 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.
Enhancement of thermoelectric properties at room temperature has been recently demonstrated by spark plasma sintered PbTe nanocubes as compared to other PbTe nanostructures as well as Bulk material. The Seebeck coefficient has been reported to be 400 µV/K which is much higher than the bulk. Moreover, a moderate electrical conductivity ∼ 8000 S/m at room temperature results in considerable higher value of power factor S2σ ∼ 1.28 x 10-3 Wm-1K-2. The enhanced thermoelectric properties have been conjectured to be present due to energy filtering effects at numerous interfaces introduced by nanostructuring. We study how the interfacial scattering affects the power factor by performing theoretical modelling based on Boltzmann Transport Equation (BTE). We also investigate in detail, the role of various electronic parameters such as size, shape, mobility and effective mass etc., on interfacial scattering to optimize its effect on power factor.
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