arXiv subjects

Xufeng Wang

Publications and source records attributed to Xufeng Wang.

9 recordsLinked to original sources

arXiv2020

LanTraP: A code for calculating thermoelectric transport properties with the Landauer formalism

A code for calculating the semi-classical thermoelectric and electronic transport properties is described. It uses the Landauer transport theory, which is equivalent to the Boltzmann theory, by introducing a central quantity-the distribution of modes. Its usage enables the so-called band-counting algorithm that can speed up the calculation and offers the potential to rapidly screen DFT band structures. Good agreements are found when comparing the results obtained using band-counting and established Fourier-based interpolation methods.

cond-mat.mtrl-sci↗

arXiv2019

Notes on Fermi-Dirac Integrals

Fermi-Dirac integrals appear frequently in semiconductor problems, so a basic understanding of their properties is essential. The purpose of these notes is to collect in one place, some basic information about Fermi-Dirac integrals and their properties. We also present Matlab scripts that calculate Fermi-Dirac integrals (the "script F" defined by Dingle (1957)) in three different ways. The codes are available in Appendix and at the following website: "Notes on Fermi-Dirac Integrals (4th Edition)" by Raseong Kim, Xufeng Wang, and Mark Lundstrom at http://nanohub.org/resources/5475 In the 4th edition, we also provide a new table-based Matlab script (download available at https://github.com/wang159/FDIntegral_Table) that is less likely to give large errors in a wide range of input while still much faster than the rigorous numerical integration.

cond-mat.mes-hall↗

arXiv2019

Limitations of zT as a Figure of Merit for Nanostructured Thermoelectric Materials

A numerical study of thermoelectric nanocomposites is presented. Thermoelectric properties as a function of average grain size or nanoparticle density are examined by simulating the measurements as they would be done experimentally. In accordance with previous theory and experimental results, we find that the Seebeck coefficient, power factor and figure of merit, zT, can be increased by nanostructuring when energy barriers exist around the grain boundaries or embedded nanoparticles. When we simulate the performance of a thermoelectric cooler with the same material, however, we find that the maximum temperature difference is much less than expected from the given zT. The same nanocomposite without electrically active grain boundaries has a lower measured zT but a higher maximum cooling temperature. The physical reason for these results is explained. The results illustrate the limitations of zT as a figure of merit for nanocomposites with electrically active grain boundaries and suggest that the ability to enhance the electrical performance of thermoelectric materials by nanostructuring is limited.

cond-mat.mtrl-sci↗

arXiv2018

Universal Behavior of the Thermoelectric Figure of Merit, zT, vs. Quality Factor

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.

cond-mat.mtrl-sci↗

arXiv2017

On the Calculation of Lorenz Numbers for Complex Thermoelectric Materials

A first-principles informed approach to the calculation of Lorenz numbers for complex thermoelectric materials is presented and discussed. Example calculations illustrate the importance of using accurate band structures and energy-dependent scattering times. Results obtained by assuming that the scattering rate follows the density-of-states show that in the non-degenerate limit, Lorenz numbers below the commonly assumed lower limit of 2(kB/q)^2 can occur. The physical cause of low Lorenz numbers is explained by the shape of the transport distribution. The numerical and physical issues that need to be addressed in order to produce accurate calculations of the Lorenz number are identified. The results of this study provide a general method that should contribute to the interpretation of measurements of total thermal conductivity and to the search for materials with low Lorenz numbers, which may provide improved thermoelectric figures of merit, zT.

cond-mat.mtrl-sci↗

arXiv2017

Thermoelectric Band Engineering: The Role of Carrier Scattering

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.

cond-mat.mtrl-sci↗

arXiv2016

Role of photon recycling in perovskite solar cells

Nearly perfect photon recycling helped GaAs cells achieve the highest efficiency ever reported for a solar cell. Recent reports of photon recycling in perovskite solar cells suggest that, once optimized, it may as well achieve GaAs-like performance. In this paper, we show that GaAs and perovskite cells recycle photons in different ways. First, although bare-perovskite has been shown to have lifetimes (~1us) in the radiative limit, non-radiative recombination at the transport layers restricts the solar cell operation far below the "photon-recycling" regime. GaAs cells have no such limitation. Second, even if the transport layers were optically and electrically perfect, the poor mobility of the perovskite layer would still restrict the optimum thickness ~1um. Thus, a very high quality mirror (reflectivity >96%) is required to utilize photon-recycling. The mirror reflectivity restriction was far more relaxed for the thicker (~2-3um) GaAs cells. Therefore, a nontrivial co-optimization of device geometry, mirror reflectivity, and material choice is necessary for achieving highest theoretical efficiency anticipated for perovskite cells.

cond-mat.mes-hall↗

arXiv2016

Thermal Transport at the Nanoscale - A Fourier's Law vs. Phonon Boltzmann Equation Study

Steady-state thermal transport in nanostructures with dimensions comparable to the phonon mean-free-path is examined. Both the case of contacts at different temperatures with no internal heat generation and contacts at the same temperature with internal heat generation are considered. Fourier's Law results are compared to finite volume method solutions of the phonon Boltzmann equation in the gray approximation. When the boundary conditions are properly specified, results obtained using Fourier's Law without modifying the bulk thermal conductivity are in essentially exact quantitative agreement with the phonon Boltzmann equation in the ballistic and diffusive limits. The errors between these two limits are examined in this paper. For the four cases examined, the error in the apparent thermal conductivity as deduced from a correct application of Fourier's Law is less than 6%. We also find that the Fourier's Law results presented here are nearly identical to those obtained from a widely-used ballistic-diffusive approach, but analytically much simpler. Although limited to steady-state conditions with spatial variations in one dimension and to a gray model of phonon transport, the results show that Fourier's Law can be used for linear transport from the diffusive to the ballistic limit. The results also contribute to an understanding of how heat transport at the nanoscale can be understood in terms of the conceptual framework that has been established for electron transport at the nanoscale.

cond-mat.mes-hall↗

arXiv2012

Fundamentals of PV Efficiency: Limits for Light Absorption

A simple thermodynamic argument related to a (weakly absorbing) finite dielectric slab illuminated by sunlight- originally suggested by Yablonovich- leads to the conclusion that the absorption in a dielectric can at best be increased by a factor 4n2. Therefore, the absorption in these materials is always imperfect; the Shockley-Queisser limit can be achieved only asymptotically. In this paper, we make the connection between the degradation in efficiency and the Yablonovich limit explicit and re-derive the 4n2 limit by intuitive geometrical arguments based on Snell's law and elementary rules of probability. Remarkably, the re-derivation suggests strategies of breaking the traditional limit and improving PV efficiency by enhanced light absorption.

physics.gen-ph↗