Session R21: Thermoelectric Materials

Computational Investigation of Ternary Na-V-VI2 Chalcogenides and Their Thermoelectric Properties

Ternary chalcogenides have been of recent investigation for applications such as solar cells and thermoelectrics. We have computed the structural, energetic, electronic, optical, and thermoelectric properties of nine ternary Na-V-VI₂ chalcogenides, NaAX₂, where A represents As, Sb, Bi and where X represents S, Se, Te, using first principles methods based on density functional theory and beyond. Optimized lattice parameters have been computed using the generalized gradient approximation (GGA). Phonon density of states computed at zero-temperature shows that only four of the nine compounds, NaAsS₂, NaAsTe₂, NaSbS₂, and NaSbSe₂, are dynamically stable. Our computations for these structures show that their electronic and optical properties are highly anisotropic. In addition, thermoelectric properties such as Seebeck coefficient (S) and power factor were computed using Boltzmann statistics. The compounds are predicted to have promising thermoelectric properties at 300 K, indicating that these materials are desirable for thermoelectric applications. Particularly, NaAsTe₂ is predicted to have S = 425 μV/K. Experimental verification is suggested.   

Enhanced thermoelectric efficiency in Bi2Te3 nanoplates and SnSe2 films.

We present a facile method to improve the thermoelectric efficiency of CVD grown 2D Bi₂Te₃ nanoplates through remediation of unintentional surface doping. The as-grown flakes of Bi₂Te₃ exposed to ambient conditions exhibit relatively small thermopowers. The high surface-to-volume ratio of these thin nanoplates makes them especially sensitive to surface doping, which is a common problem among nanomaterials in general. After surface passivation by deposition of 30nm of Al₂O₃ using ALD, the Seebeck coefficient of these flakes increases by a factor of 5X (from -34 to -169 µV/K). Here, the surface passivation can prevent the degradation of the thermoelectric properties caused by gas adsorption and surface oxidation processes, thus increasing the Bi₂Te₃ Seebeck coefficient.¹

We also report cross-plane thermoelectric measurements of SnSe and SnSe₂ films grown by the modulated element reactant (MER) approach. By performing post-growth annealing at a fixed Se partial pressure, a transition from SnSe-to-SnSe₂ is induced which results in a 16-fold increase (from -38.6 to -631µV/K) in the cross-plane Seebeck coefficient.²

1. Jihan Chen, et. al., Applied Physics Letters 113, 083904 (2018)
2. Jihan Chen, et. al.,Nano Letters, 18.11, 6876 (2018)   

High Thermoelectric Figure of Merit via Tunable Valley Convergence Coupled Low Thermal Conductivity in AIIBIVC2V Chalcopyrites

Developing high performance thermoelectrics require designing strategy to obtain excellent electronic but poor phononic transport properties. Tetragonal chalcopyrites are of significant attention due to complex electronic structures affecting the transport properties. Conflicting requirements of achieving high Seebeck coefficient and electrical conductivity are simultaneously reached, by tuning crystal and electronic structures by isoelectronic substitution, leading to unprecedented enhancement in electronic transport properties of A^IIB^IVC₂^V (II = Be, Mg, Zn, and Cd; IV = Si, Ge, and Sn; and V = P and As). Existing multiple valleys in conduction bands get converged by substitution of group IV dopants to offer enhanced powerfactors for n-type carriers. These substitutions improve convergence of valence bands having a direct correlation with tetragonal distortion (η) of these chalcopyrites. For small distortion in system (η~1), complete convergence of bands is achieved, enhancing the p-type powerfactor. Excellent electronic transport and low thermal conductivity results in maximum ZT of 1.67 in CdGeAs₂ for n-type doping. The approach developed to enhance the thermoelectric efficiency can be useful to design new thermoelectric materials.
J. Phys. Chem. C 2018, 122, 29150−29157   

Thermoelectric properties of n-type PbTe driven near ferroelectric phase transition by strain

We recently showed that soft transverse optical (TO) phonons play the key role in high thermoelectric (TE) figure of merit (ZT) of PbTe: they strongly suppress its lattice thermal conductivity[1], but do not degrade its electronic transport properties[2]. In this work, using first principles calculations, we investigate how driving PbTe closer to the soft mode phase transition (PT) via strain affects the TE properties and ZT of n-type PbTe. We find that the lattice thermal conductivity decreases significantly when PbTe approaches the PT, which leads to considerable ZT enhancement. However, if PbTe is driven very close to the PT, the originally negligible electron-TO phonon scattering becomes the strongest scattering channel, due to an increased TO phonon amplitude and the electron-TO phonon scattering phase space. Such increased scattering strength rapidly degrades electrical transport and ZT very near the PT. We show how tuning the proximity to soft mode PT can increase the TE performance of PbTe and other materials with soft phonons that interact weakly with the electronic states relevant for transport.
[1] R. M. Murphy et al. Phys. Rev. B 93, 104304
[2] J. Cao et al. Phys. Rev. B 98, 205202   

Thermoelectric transport properties in p-type PbTe from first principles

A high valley degeneracy of electronic bands in a material leads to its large efficiency of conversion between thermal and electrical energy, quantified by the thermoelectric figure of merit (ZT). Large ZT enhancements due to the alignement of the valence band maxima at L and Σ (or “valley convergence”) have been demonstrated in p-type PbTe and related materials [1], by tuning temperature and the doping concentration. However, it is still unclear why this strategy of improving ZT works well since the scattering processes between L and Σ valleys can deteriorate thermoelectric properties. Here, we introduce a first principle method to model electron-phonon scattering mechanisms in p-type PbTe [2]. The calculated room temperature thermoelectric parameters are in very good agreement with available experiments. Our model also allows accounting for the temperature dependence of the electronic band structure in thermoelectric transport calculations, which enables us to quantify the effect of L and Σ valley convergence at ~620 K [3] on the ZT values of p-type PbTe.

[1] Y. Pei et al, Nature 473, 66 (2011)
[2] J. Cao et al, Phys. Rev. B 98, 205202 (2018)
[3] J. D. Querales-Flores et al, Phys. Rev. Mater. 3, 055405 (2019)
  

Lessons from a thermoelectric transport trend study on half-Heusler alloys

The thermoelectric properties of the 4-9-15 (Ti,Zr,Hf)(Co,Rh,Ir)(As,Sb,Bi) and 4-10-14 (Ti,Zr,Hf)(Ni,Pd,Pt)(Ge,Sn,Pb) series
have been studied theoretically [J. App. Phys. 126, 145102 (2019)]. The electronic transport properties were calculated with density functional theory (DFT) and Boltzmann transport equations (BTE) at the hybrid functional level utilizing a recently developed k.p-based interpolation scheme [J. App. Phys 123, 205703 (2018)]. Phonon transport properties were calculated with the temperature-dependent effective potential (TDEP) method, including alloy and grain-boundary scattering.
Our trend study provides a number of key lessons:
1) Electronic band structures show much variation and there is, in particular, the potential for n-type Half Heusler with higher ZT.
2) The predicted thermoelectric properties are quite sensitive to the choice of exchange-correlation potential.
3) The thermal conductivity of a pure material is a poor indicator of the final thermal conductivity once other scattering mechanisms are included.
4) Sub-lattice alloying is generally more effective on the Z site than on the X site.
These findings provide valuable insights for more realistic high-thruput assessments of thermoelectric properties.   

Solving the Thermoelectric Trade-Off Problem with Metallic Carbon Nanotubes

Semiconductors are generally considered far superior to metals as thermoelectric materials because of their much larger Seebeck coefficients S. However, a maximum value of S in a semiconductor is normally accompanied by a minuscule electrical conductivity σ, and hence, the thermoelectric power factor P = S²σ remains small. An attempt to increase σ by increasing the Fermi energy (E_F), on the other hand, decreases S. This trade-off between S and σ is a well-known dilemma in developing high-performance thermoelectric devices based on semiconductors. Here, we show using metallic carbon nanotubes (CNTs) with tunable E_F solves this long-standing problem, demonstrating higher thermoelectric performance than semiconducting CNTs. We studied the E_F dependence of S, σ, and P in a series of CNT films with systematically varied metallic CNT contents. In metallic CNTs, both S and σ monotonically increased with E_F, continuously boosting P with increasing E_F. Particularly, in an aligned metallic CNT film, the maximum P was ~5 times larger than that in the high-purity (>99%) semiconducting CNT film. We attribute these superior thermoelectric properties of metallic CNTs to the simultaneously enhanced S and σ of one-dimensional conduction electrons near the first van Hove singularity.   

When Band Convergence is Not Beneficial for Thermoelectricity

Band convergence is known to generally benefit thermoelectric performance for its capability to increase carrier concentration for given Fermi level, i.e., increase conductivity for given Seebeck coefficient. With explicit treatment of electron-phonon scattering, we show that this is not necessarily the case and the degree of attainability of the said benefit depends on the dominant scattering mechanism and the manner in which bands converge. Multi-band convergence at a single k-point under deformation scattering is less beneficial (if at all) than multi-pocket convergence under polar-optical scattering at distant k-points. In the former case, one band gains while the other band loses phase space, and the increasingly disparate pocket lifetimes and mobilities can lower the Seebeck coefficient and render higher power factor inaccessible. In the latter case, the convergence preserves the scattering behavior, thereby successfully leading to higher power factor. We establish these by performing state-of-the-art first-principles studies on CaMg2Sb2-CaZn2Sb2 Zintl alloy and full-Heusler Sr2SbAu.   

Potential barrier/well engineering for improving the power factor in nanostructured thermoelectric materials

Energy filtering is one of the most successful ways to improve the Seebeck coefficient in nanostructured materials and superlattices. Despite the fact that nanostructuring emerges as the most promising method to reduce thermal conductivity and improve ZT, to-date, energy filtering has not been widely utilized because it did not achieve sufficient improvements in the thermoelectric power factor. In this work, we present a theoretical analysis of a novel concept for efficient design of the potential well/barrier region such that very large power factor improvements are achieved. The concept proposed combines aspects of: i) energy filtering from potential barriers with thermionically emitted carriers, ii) highly degenerate doping conditions but with non-uniform distribution of dopants, and iii) reduced energy relaxation of carriers after they passed the barriers and propagate into the wells. We employ simple analytical models, but verify the main design ‘ingredients’ with more advanced Non-Equilibrium Green’s Function (NEGF) quantum transport simulations and semiclassical Monte Carlo simulations.   

Tuning Thermoelectricity in Molecular Junctions via Quantum Interference

Studies of molecular junctions, created by trapping a single molecule or multiple molecules between metallic electrodes, not only provide fundamental knowledge of charge transport at the atomic scale, but also offer unique opportunities in developing molecule-based devices for energy conversion. It has been theoretically proposed that quantum interference effects can be employed to obtain impressive thermoelectric performance in molecular junctions. Toward this goal, we took advantage of the destructive interferences which arise in conjugated molecules to allow high-energy electrons to pass while blocking low-energy electrons. Specifically, we investigated this effect in oligo(phenylene ethynylene) (OPE) derivatives with a para-connected central phenyl ring (para-OPE3) and meta-connected central ring (meta-OPE3). Experiments on both single molecules and monolayers reveal a two-fold increase in thermopower in meta-OPE3 (∼20 μV/K) junctions compared with para-OPE3 (∼10 μV/K) junctions, which agrees with our initio modeling. Our results illustrate how enhancements in thermopower can be achieved in molecular junctions via quantum interferences.   

Including the finite temperature atomistic evolution into thermoelectric theory

Thermoelectric (TE) materials have enjoyed much attention because of their promise for energy savings and targeted low-power cooling. Bulk monochalcogenides (SnSe) are some of the most effective TE materials [1,2]. Phonon anharmonicity and softening modes play an important role in improved TE performance [3], but so far, theoretical approaches to computing TE properties do not account for such softening explicitly. To address this shortcoming, we use molecular dynamics data for two-dimensional SnSe as inputs for the electronic structure and phonon dispersions at finite temperature. We capture an increase in phonon velocity at the Γ-point and enhanced hole conductivity at the structural transition temperature, accounting for the change in structure and anharmonicity in predicting TE performance explicitly. These ideas enhance our understanding of TE materials, and the procedure is applicable to bulk systems as well.

[1] L.-D. Zhao et al., Nature 508, 373 (2014).
[2] C. W. Li et al., Nature Physics 11, 1063 (2015).
[3] J. P. Heremans, Nature Physics 11, 990 (2015).   

Copper occupation and dynamics in Cu rich tetrahedrite; an NMR study

Thermoelectric materials can generate electricity from heat. Recently tetrahedrite with the chemical structure of Cu₁₂Sb₄S₁₃ emerged as the most promising thermoelectric material for mid-temperature applications. Here we report ⁶³Cu NMR measurements for the Cu-rich phase of Cu_12+xSb₄S₁₃ (x ≈ 2) and compared to Cu₁₂Sb₄S₁₃. Based on NMR, T₁ and T₂ measurements, the results demonstrate Cu-ion hopping below room temperature with an activation energy of ∼150 meV for the Cu-rich phase, consistent with superionic behavior. The NMR results also demonstrate the effects of Cu-ion mobility in the Cu₁₂Sb₄S₁₃ phase, but with a larger activation barrier. We identify a small difference in NMR Knight shift for the metallic phase of Cu₁₂Sb₄S₁₃, compared to the Cu-rich phase, and when compared to DFT calculations the results indicate a mix of hyperfine contributions to the metallic shift.   

Heat is Work, and Work is Heat: a first-principles approach to find improved thermoelectrics

Thermoelectric materials have the potential to dramatically improve the energy efficiency of many devices and industrial processes by converting waste heat into electricity. An ideal thermoelectric material has high electrical conductivity and low thermal conductivity, requirements which are often in conflict. Simulating thermoelectric properties is extremely challenging: predicting electronic properties requires first-principles material modelling, yet thermal conductivity must be modelled on long lengthscales which are beyond direct first-principles modelling.

In this talk I will introduce the fundamental physics of thermoelectrics and how they may be modelled accurately, using ab initio quantum mechanical simulations coupled with classical models. I will also show how this combination of modelling techniques is being used to explain the behaviour of current materials as well as predicting entirely new thermoelectrics, paving the way for a new generation of cheap, efficient thermoelectric devices.