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Thermoelectric devices convert heat directly into electricity without any moving parts. Increasing the efficiency of thermoelectric devices is important for their widespread use. The efficiency of a thermoelectric material can be abstracted from the overall efficiency of a thermoelectric device through the thermoelectric figure of merit, ZT. ZT is a dimensionless quantity composed of several materials properties, specifically: the electronic conductivity, the Seebeck coefficient, the electronic thermal conductivity, and the lattice thermal conductivity. Improving the efficiency of a thermoelectric material can be done by increasing the electronic conductivity or Seebeck coefficient, or by reducing the lattice thermal conductivity of a material.

Our group uses various state-of-the-art computational tools such as density functional theory calculations, Monte Carlo simulations, molecular dynamics simulations, and high-throughput database analysis to study (i) the thermodynamic aspects of the processing-structure relationships of thermoelectric materials, (ii) the physics underlying the structure-property relationships of these materials, and (iii) various mechanisms for improving the thermoelectric properties by altering the structure of these materials. We have established very strong partnership with experimental researchers and made important contributions in the thermoelectric field. Our work is funded by DOE and previously by the Revolutionary Materials for Solid State Energy Conversion (RMSSEC), an Energy Frontier Research Center (EFRC).

Selected Publications

Soft Phonon Modes from Off-center Ge atoms Lead to Ultralow Thermal Conductivity and Superior Thermoelectric Performance in n-type PbSe-GeSe (Published in EES)

Historically PbSe has underperformed PbTe in thermoelectric efficiency and has been regarded as an inferior relative to its telluride congener. However, the fifty-fold greater natural abundance of Se relative to Te makes PbSe appealing as a thermoelectric material. We report that the n-type GeSe-alloyed PbSe system achieves a peak figure of merit, ZT, of ~1.54 at 773 K and maintains ZT values above 1.2 over a broad temperature range from 623 K to 923 K. The highest performing composition is Sb-doped PbSe-12%GeSe, which exhibits an ultralow lattice thermal conductivity of ~0.36 W/m/K at 573 K, close to the limit of amorphous PbSe. Theoretical studies reveal that the alloyed Ge2+ atoms prefer to stay at off-center lattice positions, inducing low frequency modes. The Ge atoms also cause the unexpected behavior where the next nearest atom neighbors (6 Pb atoms) oscillate at lower frequencies than in pure PbSe leading to a large reduction of the Debye temperature and acoustic phonon velocity. The Pb0.9955Sb0.0045Se-12%GeSe system also shows Ge-rich precipitates and many aligned dislocations within its microstructure which also contribute to phonon scattering. The resultant average ZT (ZTavg), a broad measure of the materials potential for functional thermoelectric modules, is 1.06 from 400 K to 800 K, the highest among all previously reported n- and p-type PbSe. This value matches or exceeds even those of the best n-type PbTe-based thermoelectric materials.

Further details: Energy Environ. Sci.,DOI: 10.1039/C8EE01755G (2018)

Weak Electron Phonon Coupling and Deep Level Impurity for High Thermoelectric Performance Pb1−xGaxTe (Published in Adv. Energ. Mater.)

High ZT of 1.34 at 766 K and a record high average ZT above 1 in the temperature range of 300‐864 K are attained in n‐type PbTe by engineering the temperature‐dependent carrier concentration and weakening electron–phonon coupling upon Ga doping. The experimental studies and first principles band structure calculations show that doping with Ga introduces a shallow level impurity contributing extrinsic carriers and imparts a deeper impurity level that ionizes at higher temperatures. This adjusts the carrier concentration closer to the temperature‐dependent optimum and thus maximizes the power factor in a wide temperature range. The maximum power factor of 35 µW cm−1K−2 is achieved for the Pb0.98Ga0.02Te compound, and is maintained over 20 µWcm−1K−2 from 300 to 767 K. Band structure calculations and X‐ray photoelectron spectroscopy corroborate the amphoteric role of Ga in PbTe as the origin of shallow and deep levels. Additionally, Ga doping weakens the electron–phonon coupling, leading to high carrier mobilities in excess of 1200 cm2V−1s−1. Enhanced point defect phonon scattering yields a reduced lattice thermal conductivity. This work provides a new avenue, beyond the conventional shallow level doping, for further improving the average ZT in thermoelectric materials.

Further details: Advanced Energy Materials 8, 1800659, (2018)

High Thermoelectric Performance in SnTe–AgSbTe2 Alloys from Lattice Softening, Giant Phonon–Vacancy Scattering, and Valence Band Convergence (Published in ACS Energy Letters)

We report on the underlying mechanism that enables the SnTe–AgSbTe2 system to exhibit superior thermoelectric figure of merit (ZT) compared to its parent compound SnTe. We show that AgSbTe2 alloying has a profound impact on the band structure of SnTe by converging the energies of its light and heavy valence bands, leading to significantly enhanced Seebeck coefficients. We have also unraveled a significant connection between alloying and defect stability in this system, wherein the Sn vacancy concentration increases significantly when Ag and Sb are alloyed on the Sn site. The increased Sn vacancy concentration dramatically reduces the lattice thermal conductivity through both lattice softening and phonon–vacancy scattering to ∼0.4 W m–1 K–1 at 800 K. Consequently, a ZT value of 1.2 at 800 K for AgSn5SbTe7 can be achieved by doping I on Te sites. This represents a 300% improvement over pristine SnTe, outperforming many reported SnTe-based thermoelectric materials.

Further details: ACS Energy Lett.,3 (3), pp 705–712, (2018)

Quaternary Pavonites A1+xSn2-xBi5+xS10 (A+= Li+, Na+): Site Occupancy Disorder Defines Electronic Structure (Published in Inorganic Chemistry)

The field of mineralogy represents an area of untapped potential for the synthetic chemist, as there are numerous structure types that can be utilized to form analogues of mineral structures with useful optoelectronic properties. In this work, we describe the synthesis and characterization of two novel quaternary sulfides A1+xSn2–xBi5+xS10 (A = Li+, Na+). Though not natural minerals themselves, both compounds adopt the pavonite structure, which crystallizes in the C2/m space group and consists of two connected, alternating defect rock-salt slabs of varying thicknesses to create a three-dimensional lattice. Despite their commonalities in structure, their crystallography is noticeably different, as both structures have a heavy degree of site occupancy disorder that affects the actual positions of the atoms. The differences in site occupancy alter their electronic structures, with band gap values of 0.31(2) eV and 0.07(2) eV for the lithium and sodium analogues, respectively. LiSn2Bi5S10 exhibits ultralow thermal conductivity of 0.62 W m–1 K–1 at 723 K, and this result is corroborated by phonon dispersion calculations. This structure type is a promising host candidate for future thermoelectric materials investigation, as these materials have narrow band gaps and intrinsically low thermal conductivities.

Further details: Inorganic chemistry, 57 (4), 2260-2268, (2018)

Computational Design of High-Efficiency Two-Phase Heuslers (Published in Chem. Mater.)

We have developed a high-throughput screening strategy to predict promising candidates for nanostructuring systems based on two Heusler phases. Our search includes all two-phase systems involving full, half, and inverse Heusler in the Open Quantum Materials Database, in total a search space of ∼1011 possible combinations of two Heusler compounds. To reduce this space, our screening approach starts with a set of known thermoelectrics as matrix phases and screens for all second phase compounds that are stable and form a two-phase equilibrium with the matrix. We compute mixing energies for the resulting combinations of a matrix and a nanostructured phase, find systems that have a moderately large positive mixing energy, and hence show an appropriate balance between tendency for nanostructuring and solubility of the second phase. Our screening approach gives 31 pairs, two of which have been explored experimentally (thus validating our screening strategy) and 29 of which represent new predictions of systems awaiting experimental synthesis.

Further details: Chemistry of Materials, DOI: 10.1021/acs.chemmater.7b03379

Computational Prediction of Promising Thermoelectric Oxide (Published in Chem. Mater.)

Transition metal oxides are a promising class of thermoelectric materials that can operate at high temperature due to their chemical and thermal stability. However, the high lattice thermal conductivity, poor electrical conductivity, and low thermopower have significantly impeded their applications to date. Using first-principles calculations, we predict a known oxide Bi2PdO4 to be a highly efficient hole-doped TE material with low lattice thermal conductivity and high power factor. These properties are due to (i) the strong anharmonicity stemming from Bi3+ 6s2 lone pair electrons (leading to low lattice thermal conductivity) and (ii) the flat-and-dispersive valence band structure with high band degeneracy originating from the localized Pd2+ dz2 orbitals in the stacked square planar ligand field (leading to a large power factor). Our results highlight the possibility of oxides as potential TE materials and also afford a novel strategy of designing TE materials by synthesizing compounds which combine a lone pair active cation with a d 8 cation in a stacked square planar ligand field.

Further details: Chemistry of Materials,29, 2529-2534, (2017).

Double Rattling in AgBi3S5 Leads to High ZT (Published in J. Am. Chem. Soc.)

We report electron-doped AgBi3S5 as a new high performance nontoxic thermoelectric material. This compound features exceptionally low lattice thermal conductivities of 0.5-0.3 W/m/K in the temperature range of 300-800 K, which is ascribed to its unusual vibrational properties: “double rattling” phonon modes associated with Ag and Bi atoms. Chlorine doping at anion sites acts as an efficient electron donor, significantly enhancing the electrical properties of AgBi3S5.

Further details: Journal of The American Chemical Society 139, 6467-6473, (2017).

Concerted Rattling in CsAg5Te3 Leads to High ZT (Published in Angew. Chem.)

A new p-type thermoelectric material, CsAg5Te3, is presented that exhibits ultralow lattice thermal conductivity (ca. 0.18 W/m/K) and a high figure of merit of about 1.5 at 727 K. The lattice thermal conductivity is the lowest among state-of-the-art thermoelectrics; it is attributed to a previously unrecognized phonon scattering mechanism that involves the concerted rattling of a group of Ag ions that strongly raises the Grgneisen parameters of the material.

Further details: Angew. Chem. Int. Ed. 55, 11431, (2016).

Non-equilibrium Processing Leads to Record High ZT in PbTe (Published in Nat. Commun.)

Using non-equilibrium processing we show that hole-doped samples of PbTe can be heavily alloyed with SrTe well beyond its thermodynamic solubility limit of <1 mol%. The much higher levels of Sr alloyed into the PbTe matrix widen the bandgap and create convergence of the two valence bands of PbTe, greatly boosting the power factors with maximal values over 30 muW/cm/K2 . Exceeding the 5 mol% solubility limit leads to endotaxial SrTe nanostructures which produce extremely low lattice thermal conductivity of 0.5 W/m//K but preserve high hole mobilities because of the matrix/precipitate valence band alignment. The best composition is hole-doped PbTe–8%SrTe.

Further details: Nature Communications 7, 12167, (2016)

Ultralow Thermal Conductivity in Full Heuslers (Published in Phys. Rev. Lett. as a Cover Story)

Semiconducting half and, to a lesser extent, full Heusler compounds are promising thermoelectric materials due to their compelling electronic properties with large power factors. However, intrinsically high thermal conductivity resulting in a limited thermoelectric efficiency has so far impeded their widespread use in practical applications. Here, we report the computational discovery of a class of hitherto unknown stable semiconducting full Heusler compounds with ten valence electrons (X2YZ) through high-throughput ab initio screening. These new compounds exhibit ultralow lattice thermal conductivity close to the theoretical minimum due to strong anharmonic rattling of the heavy noble metals, while preserving high power factors, thus resulting in excellent phonon-glass electron-crystal materials.

Further details: Physical Review Letters, 117, 046602, (2016)

Ultrahigh ZT Plateau in Hole Doped SnSe (Published in Science)

The thermoelectric efficiency is determined by the device dimensionless figure of merit ZTdev, and optimizing this efficiency requires maximizing ZT values over a broad temperature range. Here, we report a record high ZT dev ∼1.34, with ZT ranging from 0.7 to 2.0 at 300 to 773 kelvin, realized in hole-doped tin selenide (SnSe) crystals. The exceptional performance arises from the ultrahigh power factor, which comes from a high electrical conductivity and a strongly enhanced Seebeck coefficient enabled by the contribution of multiple electronic valence bands present in SnSe. SnSe is a robust thermoelectric candidate for energy conversion applications in the low and moderate temperature range.

Further details: Science 351, 141-144, (2016).

Valence Band Modification for High ZT of SnTe with MnTe (Published in J. Am. Chem. Soc.)

We demonstrate a high solubility limit of >9 mol% for MnTe alloying in SnTe. The electrical conductivity of SnTe decreases gradually while the Seebeck coefficient increases remarkably with increasing MnTe content, leading to enhanced power factors. The room-temperature Seebeck coefficients of Mn-doped SnTe are significantly higher than those predicted by theoretical Pisarenko plots for pure SnTe, indicating a modified band structure. The first-principles electronic structure calculations based on density functional theory suggests that the band convergence between the two valence bands. The high doping fraction of Mn in SnTe also creates stronger point defect scattering, which when combined with ubiquitous endotaxial MnTe nanostructures when the solubility of Mn is exceeded scatters a wide spectrum of phonons for a low lattice thermal conductivity of 0.9 W/m/K at 800 K. The synergistic role that Mn plays in regulating the electron and phonon transport of SnTe yields a high thermoelectric figure of merit of 1.3 at 900 K.

Further details: Journal of The American Chemical Society 137, 11507-11516, (2015).

New Record High ZT in SnSe Single Crystal (Published in Nature)

Here we report an unprecedented ZT of 2.6 at 923 K, realized in SnSe single crystals measured along the b axis of the room-temperature orthorhombic unit cell. We attribute the remarkably high ZT along the b axis to the intrinsically ultralow lattice thermal conductivity in SnSe. The layered structure of SnSe derives from a distorted rock-salt structure, and features anomalously high Grüneisen parameters, which reflect the anharmonic and anisotropic bonding. We attribute the exceptionally low lattice thermal conductivity (0.23 W/m/K at 973 K) in SnSe to the anharmonicity. These findings highlight alternative strategies to nanostructuring for achieving high thermoelectric performance.

Further details: Nature 508, 373-377, (2014).

Selected Topics

Nanostructured Thermoelectrics

Coherent and Incoherent Phase Stability of IV-VI Rocksalt Thermoelectrics

Nanostructures formed by phase separation can improve the ZT of lead chalcogenide semiconductor alloys, with coherent nanostructures giving larger improvements than incoherent nanostructures. However, large coherency strains in these alloys drastically alter the thermodynamics of phase stability. We use density functional theory calculations to investigate the coherent and incoherent phase stability of the IV–VI rocksalt semiconductor alloy systems Pb(S,Te), Pb(Te,Se), Pb(Se,S), (Pb,Sn)Te, (Sn,Ge)Te, and (Ge,Pb)Te. We find that strain energy dominates the thermodynamics in these systems: strain both drives incoherent phase separation and prevents coherent phase separation.

Valence Band Alignment

High Thermoelectric Performance via Hierarchical Compositionally Alloyed Nanostructures

Previous efforts to enhance thermoelectric performance have primarily focused on reduction in lattice thermal conductivity caused by broad-based phonon scattering across multiple length scales. Herein, we demonstrate a design strategy which provides for simultaneous improvement of electrical and thermal properties of p-type PbSe and leads to ZT ∼ 1.6 at 923 K, the highest ever reported for a tellurium-free chalcogenide. Our strategy goes beyond the recent ideas of reducing thermal conductivity by adding two key new theory-guided concepts in engineering, both electronic structure and band alignment across nanostructure−matrix interface. Utilizing density functional theory for calculations of valence band energy levels of nanoscale precipitates of CdS, CdSe, ZnS, and ZnSe, we infer favorable valence band alignments between PbSe and compositionally alloyed nanostructures of CdS1−xSex/ZnS1−xSex. Then by alloying Cd on the cation sublattice of PbSe,  we tailor the electronic structure of its two valence bands (lighthole L and heavy hole Σ) to move closer in energy, thereby enabling the enhancement of the Seebeck coefficients and the power

Intrinsically Low Thermal Conductivity

 Anomalously low lattice thermal conductivity in thermoelectric Cu-Sb-Se ternary semiconductors

Many methodologies have been developed to improve ZT, for example enhancing Seebeck coefficients by introducing quantum confinement effects and electron energy filtering, obtaining a high thermoelectric power factor by producing unusual electron density of states effects, achieving a low lattice thermal conductivity in phonon-glass electron crystal compounds and creating nanostructured materials. Although nanostructured materials reduce thermal conductivity by enhancing phonon scattering, they also can scatter electrons, which decreases the electrical conductivity as well. A solution is to seek materials with ordered crystal structures having low thermal conductivity due to strong lattice anharmonicity, such as the ternary semiconductors Cu3SbSe3. The strong lattice anharmonicity in Cu3SbSe3 arise from the electrostatic repulsion between the lone s2 pair at Sb sites and the bonding charge in Sb-Se bonds. Using our first-principles determined longitudinal and transverse acoustic mode Gruneisen parameters, zone-boundary frequencies, and phonon group velocities, we calculate the lattice thermal conductivity using the Debye-Callaway model. The theoretical thermal conductivity is good agreement with the experimental measurements. This work has been selected as the scientific highlight topic for EFRC (the Energy Frontier Research Center).

Further details: Y. Zhang, E. Skoug, J. Cain, V. Ozolins, D. Morelli and C. Wolverton,
Phys. Rev. B 2012 85, 054306

A full list of thermoelectric publications can be found on Prof. Wolverton's Google Scholar.

Researchers Studying this Topic

Shiqiang Hao

Eric Isaacs

Jonathan Pfluger

Xia Hua