Complex electronic band structures, with multiple valleys or bands at the same or similar energies can be beneficial for thermoelectric performance, but the advantages can be offset by inter--valley and inter--band scattering. In this paper, we demonstrate how first--principles band structures coupled with recently developed techniques for rigorous simulation of electron--phonon scattering provide the capabilities to realistically assess the benefits and trade--offs associated with these materials. We illustrate the approach using n--type silicon as a model material and show that intervalley scattering is strong. This example shows that the convergence of valleys and bands can improve thermoelectric performance, but the magnitude of the improvement depends sensitively on the relative strengths of intra--and inter--valley electron scattering. Because anisotropy of the band structure also plays an important role, a measure of the benefit of band anisotropy in the presence of strong intervalley scattering is presented.

To increase the performance of thermoelectric materials, the electronic parameters in the figure of merit must be improved. In this paper, we use full, numerical band structures and solve the Boltzmann equation in the relaxation time approximation using energy--dependent scattering times informed by first principles simulations. By varying the strength of the electron--phonon coupling or the lattice thermal conductivity, we compute the thermoelectric figure of merit, zT, vs. a generalized thermoelectric quality factor. More than a dozen different complex electronic structures are examined. Surprisingly, we find that at a given quality factor, none provides a better figure of merit than that of a material with a simple, parabolic band and acoustic deformation potential scattering. A qualitative argument for this unexpected finding is presented. This apparent universal behavior suggests that even for complex electronic band structures, the thermoelectric figure of merit depends solely on the ratio of electrical to thermal conductivity; the Seebeck coefficient and Lorenz number need not be considered. This observation should simplify the search for promising new materials, but if exceptions to this behavior can be identified, new paths for increasing thermoelectric material performance will open up.

View Article Online Metal oxides and nitrides are widely used in many applications as a result of their high mechanical, chemical, and electrical properties. In high temperature thermoelectric applications, oxides and nitrides exhibit high thermopower and thermal stability. Moreover, most oxides and nitrides are consisted of earth abundant elements, which are non-toxic, cost effective and easy for large-scale synthesis. In this article, we reviewed the recent advances of metal oxides and nitrides and their applications in thermoelectrics. The materials that are examined include both p-type semiconductors (e.g. Na CoO , Ca Co O , GaN) and n-type semiconductors (e.g. ZnO-based, SrTiO , InGaN, InN). x 2 3 4 9 3 This study is focused on the temperature dependent thermoelectric transport properties of oxides and nitrides aiming for reaching a high power factor.

Using first principles calculations, the use of strain to adjust electronic transport and the resultant thermoelectric (TE) properties is discussed using 2H phase CuAlO2 as a test case. Transparent oxide materials, such as CuAlO2, a p--type transparent conducting oxide (TCO), have recently been studied for high temperature thermoelectric power generators and coolers for waste heat. Given TCO materials with relative ease of fabrication, low cost of materials, and non-toxicity, the ability to tailor them to specific temperature ranges, power needs, and size requirements, through the use of strain opens an interesting avenue. We find that strain can have a significant effect on these properties, in some cases detrimental and in others beneficial, including the potential for n--type power factors larger than the highest p--type case. The physical reasons for this behavior are explained in the terms of the thermoelectric transport distribution and the Landauer distribution of modes. I. IntroductionThermoelectric (TE) devices and materials hold great promise for broad use in solid--state energy generation and solid--state cooling. However, as robust and reliable as these devices are, they have been limited by low conversion efficiencies since their inception 1-5 . The past three decades have witnessed the thermoelectric material figure of merit, zT, improved from under one to over two 5 . These gains have been primarily driven by a reduction in the lattice thermal conductivity of materials and devices through the use of nano--structuring 6-12 and the development of novel materials that have an inherently low thermal conductivity due to large discrepancies in the masses of their constituent elements. These advances, however, have not translated into working devices 13 . As we approach the lower limit of the lattice thermal conductivity for common and even complex TE materials at room temperature and above, the variety of avenues capable of moving the field of thermoelectrics forward are being narrowed, therefore ideas that have the potential to advance the field need to be explored carefully. In this paper we look at an alternate route forward, given materials with relative ease of fabrication, low cost, and non--toxicity, the ability to tailor them to specific

Transparent oxide materials, such as CuAlO2, a p--type transparent conducting oxide (TCO), have recently been studied for high temperature thermoelectric power generators and coolers for waste heat. TCO materials are generally low cost and non--toxic. The potential to engineer them through strain and nano--structuring are two promising avenues toward continuously tuning the electronic and thermal properties to achieve high zT values and low $cost/kW--hr devices. In this work, the strain--dependent lattice thermal conductivity of 2H CuAlO2 is computed by solving the phonon Boltzmann transport equation with interatomic force constants extracted from first--principles calculations. While the average bulk thermal conductivity is around 32 W/(K--m) at room temperature, it drops to between 5--15 W/(K--m) for typical experimental grain sizes from 3nm to 30nm at room temperature. We find that strain can offer both an increase as well as a decrease in the thermal conductivity as expected, however the overall inclusion of small grain sizes dictates the potential for low thermal conductivity in this material.