ABSTRACT
We have theoretically investigated the underlying physics of observed high electrical conductivity (σ), simultaneous increase of σ and Seebeck coefficient (S) with temperature, and large power factors (PFs) in nominally undoped SnSe nanoflakes sintered at different temperatures, reported recently in Mandavaet al(2022Nanotechnology33155710). Given the fact that S and σ show unusual temperature trends and that the undoped SnSe samples are highly porous and disordered, the conventional Boltzmann theory does not appear to be an appropriate model to describe their transport properties. We have, instead, used a strong disorder model based on percolation theory where charge and energy transport take place through hopping between localized states to understand these observations. Our model is able to explain the observed temperature dependence of σ and S with temperature. Large σ can be explained by a high density of localized states and a large hopping rate. The sample sintered at a higher temperature has lower disorder (σDOS) and higher hopping rate (1/τ0). We findσDOS= 0.151 eV and 1/τ0= 0.143 × 1015s-1for sample sintered at 673 K andσDOS= 0.044 eV and 1/τ0= 2.023 × 1015s-1for sample sintered at 703 K. These values are comparable to the reported values of transition frequencies, confirming that the dominant charge transport mechanism in these SnSe nanoflakes is hopping transport. Finally, we suggest that hopping transport via localized states can result in enhanced thermoelectric properties in disordered polycrystalline materials.
ABSTRACT
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 constantzT) 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 U constantzT 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.
