Byungki Ryu

Byungki Ryu
Korea Electrotechnology Research Institute-KERI

Ph.D in Physics
Thermoelectrics: Materials Design, Defect, Transport, Device Physics, Phenomena, Equations, Efficiency Theory

About

97
Publications
10,331
Reads
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1,450
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Introduction
Byungki Ryu is currently working at KERI, Korea. He majored first principles electronic structure calculations for defects in semiconductors, insulators, and their interfaces. He has been extending the research area toward thermoelectrics: materials and device design/fabrication for waste heat recovery. He is also interested in thermoelectric differential equation and behavior of temperature solution.
Additional affiliations
March 2011 - December 2013
Samsung Advanced Institute of Technology
Position
  • Research Staff Member
Education
March 2005 - February 2011
March 2000 - February 2005
Korea Advanced Institute of Science and Technology
Field of study
  • Physics, Mathematics

Publications

Publications (97)
Article
Full-text available
We find that O-vacancy (Vo) acts as a hole trap and play a role in negative bias illumination stress instability in amorphous In-Ga-Zn-O thin film transistors. Photo-excited holes drifted toward the channel/dielectric interface due to small potential barriers and can be captured by Vo in the dielectrics. While Vo(+2) defects are very stable at room...
Article
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We find that Ag-interstitial ($Ag_I$) acts as an electron donor and plays an important role in Ag-excess doped polycrystalline PbTe thermoelectric materials. When Ag is heavily doped in PbTe, the neutral (Ag-Ag) dimer defect is formed at the Pb-site and the environment becomes Pb-rich/Te-poor condition. Then the positively ionized Ag interstitial (...
Article
Full-text available
The thermoelectric figure of merit ZT, which is defined using electrical conductivity, Seebeck coefficient, thermal conductivity, and absolute temperature T, has been widely used as a simple estimator of the conversion efficiency of a thermoelectric heat engine. When material properties are constant or slowly varying with T, a higher ZT ensures a h...
Article
Full-text available
For over half a century, the development of thermoelectric materials has based on the dimensionless figure of merit zT, assuming that the efficiency is mainly determined by this single parameter. Here, we show that the thermoelectric conversion efficiency is determined by three independent parameters, zgen, τ, and β, which we call the three thermoe...
Article
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Herein, the effect of AgSbTe2 microstructure on the thermoelectric conversion efficiency of an AgSbTe2-based alloy was studied. The as-sintered sample exhibited material separation into a multiphase mixture of cubic Sb-rich AgSbTe2 (Ag21Sb28Te51), monoclinic Ag2Te with an inhomogeneous distribution, and an Sb-rich zone. The samples were subjected t...
Article
Full-text available
A thermoelectric generator converts a temperature difference into electrical energy. Its energy conversion efficiency is determined by the steady-state temperature distribution inside the generator. By assuming the thermoelectric material in the generator has a temperature-independent Seebeck coefficient and the generator is one-dimensional, we sho...
Article
Because the properties of thermoelectric (TE) materials are strongly dependent on temperature and differ considerably, segmented TE legs composed of multiple stacked TE materials have been investigated to provide efficient operation of TE devices. However, owing to the inherent nonlinearity that limits the application of conventional optimization a...
Article
Reaching a sufficiently high carrier concentration is crucial for thermoelectric material optimization in developing Mg2X (X = Si, Ge, and Sn)-based thermoelectric generators (TEGs). While n-type Mg2(Si,Sn) has excellent thermoelectric properties, p-type shows suboptimal thermoelectric performance because of insufficient carrier concentration, in p...
Article
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The growing industrial demand for flexible optoelectric devices has led to intensive researches on highly flexible transparent electrode materials such as graphene, reduced graphene-oxide (r-GO), Ag-nanowire, and 2D metal oxides. However, except Ag-nanowire, transparent electrode materials having optoelectric properties comparable to that of indium...
Article
Full-text available
The status of metrology for the characterization of thermoelectric generator modules (TEM) is investigated in this work by an international round robin (RR) test including twelve laboratories from nine countries on three continents. Measurements have been performed with three samples of a Bi2Te3-based commercial TEM type, which has prevailed over t...
Article
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Previous calculations have demonstrated that Te vacancies are energetically the major defects in PbTe. However, the Pb interstitials are also important because experiments have shown that the volume of Pb-rich PbTe increases at a higher Pb content. In this study, density functional theory calculations were used to investigate the defect properties...
Article
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Thermoelectric power generation is a promising candidate for automobile energy harvesting technologies because it is eco-friendly and durable owing to direct power conversion from automobile waste heat. Because Bi−Te systems are well-known thermoelectric materials, research on (Bi2)m(Bi2Te3)n homologous series can aid the development of efficient t...
Article
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Thermoelectric generators are a reliable and environmentally friendly source of electrical energy. A crucial step for their development is the maximization of their efficiency. The efficiency of a TEG is inversely related to its electrical contact resistance, which it is therefore essential to minimize. In this paper, we investigate the contacting...
Article
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Thermoelectric properties are frequently manipulated by introducing point defects into a matrix. However, these properties often change in unfavorable directions owing to the spontaneous formation of vacancies at high temperatures. Although it is crucial to maintain high thermoelectric performance over a broad temperature range, the suppression of...
Article
Copper iodide (CuI) has gained attention as a highly conductive p-type transparent material. Here, we investigate the phase stability of I-rich cubic CuI phases with Cu-vacancy defects by performing hybrid-density functional theory calculations. In the Cu-rich equilibrium condition, the nearly stoichiometric phase is the ground state with quenched...
Preprint
Full-text available
A thermoelectric generator converts a temperature difference into electrical energy. Its energy conversion efficiency is determined by the steady-state temperature distribution inside the generator. By assuming the thermoelectric material in the generator has a temperature-independent Seebeck coefficient and the generator is one-dimensional, we sho...
Article
By employing a mean-field homogenization theory to an anisotropic thermoelectric composite involving spherical fillers, we obtain analytical predictions of the effective Seebeck coefficient, effective electrical conductivity, and effective thermal conductivity in the presence of interfacial electrical and thermal resistances. First, we validate the...
Article
In this study, the effect of Ag defects on the lattice and thermoelectric properties of Ag-doped BixSb2−xTe3 (BST) alloys are systematically investigated. Using density-functional calculations, we reveal that Ag impurity defects occupying interstitial sites in the BST affect the formation of intrinsic defects in the host materials. Ag interstitial...
Article
In developing thermoelectric generators, optimizing interfaces between thermoelectric materials and contacting electrodes is a crucial step. Among the tested electrodes for Mg2(Si,Sn)-based generators, Ag shows good adhesion, controllable interfaces and low electrical contact resistances. However, it induces unexpected changes in the Seebeck coeffi...
Article
In developing thermoelectric generators, optimizing interfaces between thermoelectric materials and contacting electrodes is a crucial step. Among the tested electrodes for Mg2(Si,Sn)-based generators, Ag shows good adhesion, controllable interfaces and low electrical contact resistances. However, it induces unexpected changes in the Seebeck coeffi...
Article
Full-text available
We perform hybrid-density functional calculations to investigate the charged defect formation energy of native point defects in Mg 2 Si, Mg 2 Sn, and their solid solutions. The band gap correction by hybrid-density functional is found to be critical to determine the charged defect density in these materials. For Mg 2 Si, Mg interstitials are domina...
Preprint
In this work, we calculate the defect properties of low-symmetry Pb interstitials in PbTe using first-principles density-functional theory calculations. We break the symmetry imposed on on-centered interstitial defects and show that the lowest ground state of Pb interstitial defects is off-centered along the [111] directions. Due to the four multi-...
Article
Full-text available
The thermoelectric properties (TEPs), consisting of Seebeck coefficient, electrical resistivity and thermal conductivity, are infinite-dimensional vectors because they depend on temperature. Accordingly, a projection of them into a finite-dimensional space is inevitable for use in computers. In this paper, as a dimension reduction method, we valida...
Preprint
Full-text available
We perform hybrid-density functional calculations to investigate the charged defect formation energy of native point defects in $Mg_2 Si$, $Mg_2 Sn$, and their solid solutions. The band gap correction by hybrid-density functional is found to be critical to determine the charged defect density in these materials. For $Mg_2 Si$, $Mg$ interstitials ar...
Preprint
The thermoelectric figure of merit ZT, which is defined using electrical conductivity, Seebeck coefficient, thermal conductivity, and absolute temperature T, has been widely used as a simple estimator of the conversion efficiency of a thermoelectric heat engine. When material properties are constant or slowly varying with T, a higher ZT ensures a h...
Article
We obtained the analytical expression for the effective thermoelectric properties and dimensionless figure of merit of a composite with interfacial electrical and thermal resistances using a micromechanics-based homogenisation. For the first time, we derived the Eshelby tensor for a spherical inclusion as a function of the interfacial resistances a...
Preprint
Full-text available
In this Letter, we show thermoelectric conversion efficiency is \emph{exactly} determined by \emph{three} independent material parameters $Z_{\rm gen}$, $\tau$, and $\beta$. Each parameter is a figure of merit hence improving $\tau$ or $\beta$ is an additional way to increase the efficiency. The $Z_{\rm gen}$ generalizes the traditional figure of m...
Article
We perform hybrid functional and quasi-particle band structure calculations with spin-orbit interaction to investigate the band structures of Mg2Si, Mg2Ge, and Mg2Sn. For all Mg2X materials, where X = Si, Ge, and Sn, the characteristics of band edge states, i.e., band and valley degeneracies, and orbital characters, are found to be conserved, indep...
Article
There have been significant progresses on the enhancement of thermoelectric (TE) performance in p-type PbTe compounds while the TE performances of n-type compounds do not compatible with the p-type ones. For optimizing high performance, an effective Fermi level tuning is critical. Here, we found that the extrinsic doping gives rise to complex defec...
Article
Full-text available
A correction to this article has been published and is linked from the HTML and PDF versions of this paper. The error has not been fixed in the paper.
Preprint
Full-text available
We perform hybrid functional and quasi-particle band structure calculations with spin-orbit interaction to investigate the band structures of Mg2Si, Mg2Ge, and Mg2Sn. For all Mg2X materials, where X = Si, Ge, and Sn, the characteristics of band edge states, i.e., band and valley degeneracies, and orbital characters, are found to be conserved, indep...
Article
In general, Bi2Te3 and related alloys show the best thermoelectric performance at near-room temperature region below about 423 K, which is in some cases not high enough for waste heat recovery. In order to raise the operation temperature on the basis of Bi2Te3 technology, S element was introduced into Bi2Te3 to synthesize a ternary tetradymite comp...
Preprint
Full-text available
We obtained the analytic expression for the effective thermoelectric properties and dimensionless figure of merit ZT of a composite with interfacial electrical and thermal resistances using a micromechanics-based homogenization. For the first time, we derived the Eshelby tensor for a spherical inclusion as a function of the interfacial resistances...
Preprint
Full-text available
Thermal energy can be directly converted to electrical energy as a result of thermoelectric effects. Because this conversion realises clean energy technology, such as waste heat recovery and energy harvesting, substantial efforts have been made to search for thermoelectric materials. Under the belief that the material figure of merit $zT$ represent...
Article
2 dimensional structures can potentially lead to not only modulation of electron transport but also the variations of optical property. Protonic ruthenium oxide, a two-dimensional atomic sheet material, has been synthesized, and its optoelectric properties have been investigated. The results indicate that protonic ruthenium oxide is an excellent ca...
Article
Full-text available
We obtained an analytical solution for the effective thermal conductivity of composites composed of orthotropic matrices and spherical inhomogeneities with interfacial thermal resistance using a micromechanics-based homogenization. We derived the closed form of a modified Eshelby tensor as a function of the interfacial thermal resistance. We then p...
Article
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Many elements have been used as dopants to enhance the thermoelectric performance of Bi2Te3-related materials. Among them, Ag's effect on thermoelectric properties, where Ag acts as a donor or acceptor, remains unclear. To elucidate the role of Ag in n-type Bi2Te3 based compounds, Ag was added to n-type (Bi0.9Sb0.1)2(Te0.85Se0.15)3. As the amount o...
Article
Full-text available
Correction for ‘Defect chemistry and enhancement of thermoelectric performance in Ag-doped Sn 1+δ−x Ag x Te’ by Min Ho Lee et al. , J. Mater. Chem. A , 2017, 5 , 2235–2242.
Article
It has been in difficulty in forming well-distributed nano- and meso-sized inclusions in a Bi2Te3-based matrix and thereby realizing no degradation of carrier mobility at interfaces between matrix and inclusions for high thermoelectric performances. Herein, we successfully synthesize multi-structured thermoelectric Bi0.4Sb1.6Te3 materials with Fe-r...
Article
Full-text available
We report the enhanced thermoelectric properties of Ce-doped AgSbTe2 (AgSb1−xCexTe2) compounds. As the Ce contents increased, the proportion of heterophase Ag2Te in the AgSbTe2 gradually decreased, along with the size of the crystals. The electrical resistivity and Seebeck coefficient were dramatically affected by Ce doping and the lattice thermal...
Article
In order to achieve enhancements in thermoelectric efficiency, microstructures that can form numerous interfaces have been investigated intensively for controlling the transport of charge carriers and heat-carrying phonons. In this paper, we report the heterogeneous microstructure of Mg2Si0.6Sn0.4 thermoelectric materials synthesized by a simple B2...
Article
First-principles approach is demonstrated to calculate the work function of Bi2Te3. The reference potential and the vacuum energy levels are extracted from the Bi2Te3 (0001) surface structure using the reference potential method based on the density functional theory. The one-shot GOWO many-body perturbation theory is used to place the bulk band ed...
Article
High pressure offers an intriguing avenue for the change in physical and chemical properties of condensed matters such as superconductivity, structural phase transition, and catalytic activity. However, it is hard to achieve the high-pressure phases of elements and compounds in an ambient condition by chemical pressure due to the limitation in bond...
Article
The Bi2Te3-related binary compounds (Bi2Te3, Sb2Te3, Bi2Se3, and Sb2Se3) and ternary alloys [(Bi1-xSbx)2Te3 (BST, 0 ≤ x ≤ 1) and Bi2(Te1-ySey)3 (BTSe, 0 ≤ y ≤ 1)] are known as the high performance room temperature thermoelectric materials. Here, for the first time, we systematically study the thermoelectric transport properties of BST and BTSe allo...
Article
We investigated the thermoelectric properties and electronic band structure calculation of Sn1-xAgxTe and Sn1.03-xAgxTe (x = 1, 3, 5, 7 mol%) compounds. From the formation energy calculation, the Sn vacancy by Ag doping gives rise to the increase of carrier density as well as the decrease of lattice parameters. There are several possibilities of po...
Article
Bi2Te2.7Se0.3 bulk materials containing x vol% MgO nanoparticles (average particle size ≈ 100 nm, x = 0, 0.5, 1.0, 1.5) were synthesized by using high-energy ball milling and plasma- activated sintering (PAS) without any special process for nanoparticle dispersion. A microstructure investigation using a scanning electron microscope (SEM) confirmed...
Article
The low energy band structure near the band gap determines the electrical performance of thermoelectric materials. Here, by using the hybrid-density functional theory (hybrid-DFT) calculations, we calculate the low energy band structure of Bi2Te3, Sb2Te3, Bi2Se3 and Sb2Se3 in tetradymite phase. We find that the band structure characteristics are ve...
Article
Full-text available
This review examines computational simulations of thermoelectric properties, such as electrical conductivity, Seebeck coefficient, and thermal conductivity. With increasing computing power and the development of several efficient simulation codes for electronic structure and transport properties calculations, we can evaluate all the thermoelectric...
Article
Miniature thermoelectric modules (TEMs) are required for micro power generation as well as local cooling, and they should have small size and high performance. However, conventional bulk TEMs generally have in-plane dimensions of a few centimeters, and empty space between the legs for electrical isolation makes efficient miniaturization difficult....
Article
We found that Na is a good source to develop twin structures in the Bi-Te system, such as Ag as noted in a previous study. The twin boundaries had a considerable influence on reductions of the lattice thermal conductivity due to phonon scattering by the nano-ordered layers and on reductions of the electrical resistivity owing to the defects generat...
Article
The crystal chemistry of complex structured pavonite homologue Cux+yBi5-yS8 (1.2 ≤ x ≤ 1.4, 0.4 ≤ y ≤ 0.55) compounds with various crystallographic atomic sites were investigated in the context of thermoelectric properties. We clarified the origins of electronic and thermal transport properties for Cux+yBi5-yS8 compounds by the change of compositio...
Article
Density functional calculations are performed to study the band structures of Bi2Te3-related binary (Bi2Te3, Sb2Te3, Bi2Se3, and Sb2Se3) and Sb/Se-doped ternary compounds [(Bi1−x Sbx )2Te3 and Bi2(Te1−y Sey )3]. The band gap was found to be increased by Sb doping and to be monotonically increased by Se doping. In ternary compounds, the change in th...
Article
Ag(Sb1−x Lax )Te2 compounds (x = 0, 0.01, 0.02, 0.03, and 0.05) were fabricated to study the effect of La-doping on the thermoelectric properties by using a spark-plasma-sintering process. All specimens were nearly single-phase AgSbTe2 with a small amount of Ag2Te. The La doping in AgSbTe2 resulted in a reduction in the electrical and the thermal c...
Article
Doping with foreign atom has been shown to be an effective way to enhance the dimensionless figure of merit ZT of Bi2Te3-based thermoelectric raw materials. Herein, we report that doping with Nb is effective in enhancing the Seebeck coefficient of n-type Cu0.008Bi2Te2.7Se0.3 polycrystalline bulks. Considering compensation of the Seebeck coefficient...
Article
Bi2Te3 thermoelectric thin films were deposited on the flexible polyimide substrates by RF magnetron co-sputtering of a Bi and a Te targets. The influence of the substrate temperature and RF power on the microstructure, chemical composition, and the thermoelectric properties of the sputtered films was investigated by using scanning electron microsc...
Article
We herein report the significantly improved thermoelectric performance of n-type Bi2Te2.7Se0.3 polycrystalline bulks through band structure engineering achieved by Au-doping. The Seebeck coefficient can be increased for both Bi2Te2.7Se0.3 and Cu-intercalated Bi2Te2.7Se0.3 bulks by doping Au on the Bi site, either due to the addition of the resonant...
Article
Full-text available
Density functional calculations have performed to study the band structures of $Bi_2Te_3$-related binary ($Bi_2Te_3$, $Sb_2Te_3$, $Bi_2Se_3$, and $Sb_2Se_3$) and $Sb$/$Se$ doped ternary compounds [$(Bi_{1-x}Sb_x)_2Te_3$ and $Bi_2(Te_{1-y}Se_y)_3$]. It is found that the band gap can be increased by $Sb$ doping and it is monotonically increased by $S...
Article
The electronic structures and Seebeck coefficients of Bi2Te3, Sb2Te3, and (Bi0.25Sb0.75)2Te3 were investigated using a first-principles density-functional-theory approach. The band structures and band gaps of the compounds were obtained and compared with previous experimental results and calculations from the literature to support the accuracy of t...
Article
Full-text available
We revisit the derivation of a nonlocal problem modelling temperature distribution due to power generation using thermoelectric effect. The problem has nonlocal coefficients in reaction and convection terms rather than diffusion term, which makes the problem more interesting. In this paper, we prove that the nonlocal problem has a unique decreasing...
Article
Electronic structures and X-ray absorption spectra of rutile TiO2were obtained with the firstprinciples calculation. Density of states, charge density and differential charge density were obtained, from which charge transfer characteristics between Ti and 0 was well confirmed. Direct and indirect band gap of 1.99 and 2.03 eV were evaluated, respect...
Article
A study was conducted to investigate origin of high photoconductive gain in fully transparent heterojunction thin-film transistor (TFT) nanocrystalline oxide image sensors and interconnects. The origin of ultra-high quantum efficiencies in a all invisible imaging array with photosensors was described based on nanocrystalline oxide heterojunction TF...
Article
The Bi-doped Mg2Si powder was fabricated with solid state reaction method and consolidated with hot pressing method and then its thermoelectric properties were investigated. The n-type transport properties were measured in all samples and temperature dependence of the electrical properties shows a behavior of degenerate semiconductors for Bi-doped...