Symposium EP13—Thermoelectrics---Materials, Methods and Devices

Thermoelectric solid-state energy conversion technology can provide a reliable and clean way to generate operative electricity from waste heat, which is a promising strategy for addressing energy conservation and management. Thus, there is a broad based push from the science and technology community to develop highly efficient thermoelectric materials as a possible route to address the worldwide power generation from heat. Today there is a variety of effective strategies to improve the properties of these narrow gap semiconductors such as achieving extremely low thermal conductivity and raising the power factors. The so-called nanostructuring and mesoscale approach has led to a new era of investigation for bulk thermoelectrics. Currently lead chalcogenides incorporating second phases hold the record in figure of merit for high temperature power generation applications. Nanostructures enable effective phonon scattering of a significant portion of the phonon spectrum while mesostructures tend to scatter phonons with long mean free paths remain. By combining all relevant length-scales in a hierarchical fashion, from atomic-scale disorder and nanoscale endotaxial precipitates to mesoscale phonon scattering a large enhancement in the thermoelectric performance of bulk materials can be achieved. Progress on device and module assembly is excellent and modules with conversion efficiency of ~11% for a delta T of 590 K have been demonstrated using nanostructured PbTe-based materials. Interestingly, nanostructuring is not necessary to obtain record high thermoelectric performance. Several systems based on PbTe and PbSe will be described that lack nanostructuring but feature mesoscale structuring and point defects, which can also achieve very low thermal conductivity. Comparisons with nanostructured materials will be made. Finally, SnSe is a new striking example of a single-phase two-dimensional material which has shed new light in thermal transport and chare transport properties both in p-type and n-type doping. The state of the art in the understanding of this system will be presented.

This talk will discuss our recent work to simulate electron and phonon thermoelectric properties based on the density-functional theory, in particular focusing on material systems with alloys and defects. Intrinsic electron transport properties are governed by carrier scatterings by phonons, but practical materials often involve alloys or have defects, that can significantly change the transport properties. The main challenge is how we can take these into account in first principles simulations and obtain more reasonable estimation for materials’ thermoelectric performance. In this talk, building upon our previous experience in understanding the electron transport in semiconductors (EPL, 109, 57006, 2015; PRL, 114, 115901, 2015; PNAS, 112, 14777, 2015; PRB, 95, 075206, 2017; Nat Comm, 9, 1721, 2018; PNAS, 115, 879, 2018), I will discuss how we can extend the simulation to include alloys and defects. First, I will use two alloy systems (ZrNiSn/ZrNiPb, and Si/Ge) to illustrate what are the general effects of alloying on electron transport in comparison to phonon transport. The latter material system also gives us the opportunity to utilize the phonon drag effect to enhance the thermoelectric performance. In the second part, I will discuss how local defects (e.g. dopants, interstitials) impacts the electron transport in half-Heuslers and Zintl compounds. The ability to quantify the defect scattering not only helps to explain the discrepancy between experiment and simulation results assuming only intrinsic scattering mechanisms, but also provides guidelines for selecting optimal dopants for thermoelectric materials. This work is partially supported by S³TEC, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences (Award No. DE-SC0001299/DE-FG02-09ER46577), and partially by DARPA MATRIX program (Grant No. HR0011-16-2-0041).

In the field of functional materials, there has been a lot of development in piezoelectrics, thermoelectrics and photovoltaics using Bayesian inference with a forward model to accurately determine material descriptors. So far, machine learning approaches have only been used to predict promising new thermoelectric compositions from density functional theory calculations, building upon open-source databases towards the discovery of high performance materials. Further, high-throughput materials screening approaches are still rudimentary, limited by the lack of universally defined material and thermoelectric transport descriptors. Finally, rapid and accurate materials characterization that can directly measure these descriptors are arduous (for example, method of four coefficients) or do not exist. In this work, we disclose a rapid and accurate way to determine the material and transport descriptors of thermoelectric performance by feeding simple single-leg power-load experimental data to a Bayesian machine learning algorithm using Boltzmann transport theory. Employing only two input parameters (temperature gradient and external load resistance) and the observed power output, the Bayesian inference algorithm is able to extract thermoelectric parameters ranging from material-layer properties (Seebeck coefficient, electrical resistivity) to transport-layer characteristics (energy-dependent scattering parameter, band gap offset, etc.) as well as extrinsic contributions such as parasitic contact resistance. In addition, systematic error from measurement can also be identified and corrected. Hence, we are able to predict band and transport descriptors that can be measured directly using experiments for the first time. While these reveal the complex dynamics of scattering in thermoelectric materials, we envision that they will also provide universal screening criteria for high performance thermoelectrics in the near future.

Fabrication of micro- thermoelectric modules is needed in order to realize on-chip integrated power generation as a solid-state thermoelectric generator (TEG) and local heat management as a Peltier cooler (TEC). The main challenges lie in the development of a feasible and cost-effective technology, which needs to be compatible with the modern semiconductor fabrication technology. In this work,¹ integrated micro- thermoelectric coolers (µ-TECs) based on the telluride compounds, with a packing density over 5500 leg pairs per cm2 and an optimized net cooling around 6 K near room temperature, have been successfully fabricated by combining conventional techniques of photolithography and electro-chemical deposition (ECD). Systematical device characterizations, including current and temperature dependent cooling performances, transient cooling response, cycling reliability, and long-term stability under constant current, have been examined and demonstrated. Particularly, the as-fabricated µ-TECs show a reliability up to 10 million cycles and cooling stability under constant current for more than 1 month under ambient environment. Further, a transient cooling response time of 1 ms outperforms that of devices reported in the literatures by at least one order of magnitude. Model simulations based on the finite-element method (FEM) and analytical calculations both show consistency with the experimental results, indicating high quality thermoelectric materials (for both n and p legs) and negligible contact resistances in the as-fabricated µ-TECs.

1. Li, G. et al. Integrated microthermoelectric coolers with rapid response time and high device reliability. Nat. Electron. 1, 555–561 (2018).

Crystal structure inversion asymmetry and strong spin-orbital coupling can induce a Rashba-type spin-split effect on the electronic band structures of materials, which has been intensively studied in diverse fields of condensed matter physics and materials science. Bulk BiTeI compound possesses a giant Rashba spin-split band dispersion with a unique Fermi surface topology evolution across the Dirac point. The dimensionality reduction in the density of states can lead to beneficial thermoelectric responses in bulk and few-layer-thick BiTeI, when the Fermi level is below the band-crossing point. In this talk I will present our recent data and analysis of the Shubnikov de Haas oscillations to distinguish electrons of the inner and outer Fermi surfaces. We also studied topological phase transitions and changes in the electrical transport properties by applying uniaxial strain on micro-size samples. Our data suggest that from bulk to few-layers, BiTeI offers an interesting platform for exploring new degrees of control for quantum materials.

In comparison with thermoelectric alloys, oxide semiconductors, which are thermally and chemically stable in air at high temperature, are regarded as the promising candidates for high-temperature thermoelectric applications in waste-heat recovery and power generation, such as solar energy harvesting through solar concentrators. However, the ZT values of the oxide ceramics still remain rather low in comparison with their alloy counterparts due to the medium electrical conductivity and high thermal conductivity. In this talk, we focus mainly on our works on layered BiCuSeO, Bi₂O₂Se and Ca-Co-O ceramics. The special layered structure can effectively scatter the phonons, resulting in an obvious reduction of thermal conductivity. Further by engineering the band structure and nanostructures, the thermoelectric properties can be significantly improved, and a high performance oxyselenide BiCuSeO ceramic with ZT>1.5 at 823 K was developed.

Antiferromagnetic MnTe:Li has shown a benevolent strong spin effect on thermoelectric properties across a broad range of temperature leading to zT>1 at 900K [1,2]. Strong spin contribution to the heat capacity and spin disorder scattering effect on carrier mobility is observed near the Neel temperature, T_N=307K. However, the spin contribution to the thermopower continues above T_N and extends up to 900K. Spin contribution to those properties below T_N has been attributed to the magnon-drag effect [3], while spin effects on thermopower above T_N are associated to the paramagnon-drag originated from the mid or short-range magnetic ordering [2]. Neutron inelastic spectroscopy and specific heat data showed an agreement to that theory to some extent while some competing theories such as the spin-fluctuations and spin entropy can also explain thermopower enhancement in magnetic materials [1,4]. To investigate the validity of those theories, we synthesized MnTe with different dopants to modify the magnetic properties and study their effects on the thermoelectric characteristics. In particular, Cr-doping was introduced to the MnTe system to break the antiferromagnetic ordering intentionally. MnTe doped with 5% and 14% Cr showed ferromagnetic properties with Curie temperature of ~100K with the magnetic moments reducing monotonically at above 100K. The specific heat, however, still showed the strong contribution from the magnon heat capacity near 300K despite lack of obvious magnetic phase transition at or near that temperature. The presence of ferromagnetic ordering in MnTe:Cr observed in the magnetic measurement is attributed to the spin-polarized hole-mediated magnetic interaction, while magnons observed in specific heat measurement can be associated to the presence of remnant short-range antiferromagnetic ordering. The breaking of the antiferromagnetic ordering suppresses the magnon and paramagnon contributions to the thermopower in MnTe:Cr. Cr-doping overall exhibited detrimental effects on the thermoelectric behavior of MnTe due to the modifications in magnon hole interactions. We will discuss the results obtained on thermoelectric properties and compare them with those of Li-doped MnTe.
[1] Vashaee, D., et al., Spin-Cal. IX, June 2018
[2] Yuanhua, Z., et al., Am. Phys. Soc., March 2018
[3] Wasscher, J. D., Phys. Lett. 8, 302-304,1964
[4] Gratz, E., Physica B 237-238, 1997

The demand for energy efficiency was motivated many researchers to seek for novel methods capable of enhancing the conversion efficiency of heat to electricity. Most of the recently published methods for thermoelectric (TE) enhancement are mainly attributed to the reduction of the lattice thermal conductivity, with a limited focus on efficiency enhancement due to an improved electronic optimization. This is attributed mainly to the fact that the electronic properties are correlated and opposing each other upon increasing the carrier concentration.
PbTe alloys considered among the most efficient TE compositions for temperatures of up to ~450°C. In the current research, BiTe- and Cu-co-doping of PbTe is reported while incorporation of Cu electron donors into PbTe–BiTe vacancies results in an effective compensation of the original high holes’ concentration of PbTe toward the optimal TE range.
Cu-doped (PbTe)_1-x(BiTe)_x alloys with different x values were examined, expecting to yield improved TE performance due to an enhanced electronic doping mechanism.
Compare to PbTe, SnTe is receiving an extensive attention due to its eco-friendly nature. Intrinsic SnTe has inherent Sn vacancies and a high hole concentration leading to degraded ZT. However, with the effective electronic approaches, related to co-doping of Bi and another effective donor-type impurity, significant improvements can be achieved.

Thermal spin transport can be used as a new, independent variable that alleviates the counter-indicated nature of the parameters that enter ZT. Fermi-Dirac statistics impose an inverse relation between the thermopower and the carrier concentration, and thus, the electrical conductivity. However, this condition does not exist for bosons, like magnons, whose magnonic thermopower is the entropy per particle [1]. For example, magnon drag can boost the thermopower of ferromagnetic metals [2]. In this talk, we will first outline how magnon drag also increases the thermopower of the antiferromagnetic (AFM) semiconductor MnTe doped with the aliovalent acceptor Li by about a factor of 3 near the ordering temperature T_N=307 K. Surprisingly, the boost in thermopower persists in the paramagnetic (PM) regime up to above 900 K. This enables optimally doped MnTe to reach ZT values in excess of 1 at T>850 K. We ascribe the increase in thermopower to the drag (transfer of linear momentum) of electrons by the local thermal fluctuations of the magnetization in the paramagnetic state, paramagnons. Neutron scattering identifies the magnon density of states in the AFM regime. An excess density of states remains visible up to the maximum temperature of the measurements, 450 K. From the energy width of this line, a paramagnons lifetime of 3 x 10^-14s is derived. The spin-spin correlation length in the PM state is then further derived to be about 2.5 nm from this and the momentum linewidth. The energy scattering time of electrons derived from the mobility is 2-to-5 x 10^-15s, about 10 times smaller than the paramagnons’ lifetime. The de Broglie wavelength and the effective Bohr radius of the electrons are both shorter than the spin-spin correlation length. Consequently, from the electron point of view, paramagnons behave like magnons in the ordered state and contribute a drag thermopower that is the main mechanism behind the high ZT. This is the first example of a spin-based effect that is much larger than a charge-based effect, and leads to a technologically significant thermoelectric performance. If this observation can be verified in other classes of paramagnetic semiconductors, of which there are many, it could open a new direction for thermoelectrics research.
The work is supported by then ARO MURI “Materials with extraordinary spin/heat coupling, grant W911NF-14-1-0016
[1] K. Vandaele et al., Mater. Today Phys.1 39-49 (2017)
[2] S. J. Watzman et al., Phys. Rev. B 94, 144407 (2016)

Chiral fermions exist in the expanding class of quantum materials that display some unique transport properties, including negative longitudinal magnetoresistance via chiral magnetic effect, anomalous Hall effect, and anomalous Nernst effect that may be exploited for thermomagnetic cooling. Among the contributing factors to these unique properties are linear electronic band dispersion, chirality, and Berry curvature in chiral fermion systems. Interesting, many chiral materials were discovered in the compounds that are normally investigated for their superior thermoelectric performance. In this presentation, I will present our studies attempting to capture the fundamental difference in seemingly similar behavior in the transport properties of chiral materials and trivial thermoelectric materials. I will also discuss how to experimentally identify which mechanism is the result of chiral fermions.

Dopability is important for thermoelectrics as it represents the ability to control carrier concentration of a semiconductor material. However, general understanding of factors governing dopability is largely missing which makes dopability predictions very difficult. To address this problem we aim to develop a predictive model that allows rapid computational assessment of the dopability of materials. As a first step, we have build a training set of accurate point defect calculations obtained using our robust automative computational framework to calculate point defect energetics using modern defect theory. Automation tool has thus enabled us to build a large dataset of first-principles defect calculations on a diverse set of thermoelectric-relevant materials. As a second step, we use this training set to develop dopability prediction model that can effectively correlate set of easily accessible bulk-derived properties to defect formation energies and carrier concentrations. Our analysis focuses on structural as well as compositional properties, bulk formation enthalpy, chemical potentials, and details of the electronic structure such as band dispersion, band gap, absolute band edge position, etc. Ultimately, this procedure allows us to parameterize dominant bulk properties that will be important in achieving reliable dopability predictions.

The design rules for active cooling systems of electronic devices or batteries are quite different from those for Peltier refrigeration. Peltier modules made from tetradymite materials [1] are used commercially for refrigeration, pumping heat opposite the natural direction, but have yet to compete in active cooling where the heat flux from a hot source to a cold sink is the parameter that must be optimized. In particular, a high thermal conductivity is beneficial in the materials used for active cooling, since it allows a higher heat load to be carried away from the device to be cooled, whereas it is obviously detrimental to the ZT of a Peltier element used in refrigeration mode. The real objective of active cooling is to maximize the effective total thermal conductance, due to both the Fourier and the Peltier heat fluxes, rather than the ZT, which determines the minimum reachable cold temperature in a refrigeration application. Optimal materials for maximum active cooling are those with high thermoelectric power factor and high thermal conductivity [2].

Metals with large thermopower due to magnon drag offer such a prospect. A cooling module is constructed from several possible n and p-type metals with high thermoelectric power factor. It is connected so as to pump heat from a variable-power heater into a heat sink. The thermal conductance is first characterized against the temperature gradient in the absence of a Peltier current (passive cooling mode). It is then characterized again in the presence of an electrical current that maximizes the Peltier heat flux (active mode). The ratio of the thermal conductance of the cooler in active and passive mode is shown as a function of temperature drop and background temperature. The dynamic response is further analyzed and compared to the time constant predicted by thermal resistance and capacitance.

1. Heremans et al., Nature Review Materials, 2 17049 (2017)
2. Zebarjadi, M. Electronic Cooling Using Thermoelectric Devices. Appl. Phys. Lett. 106, 203506 (2015); doi:10.1063/1.4921457

To devise strategies for improving the thermeoelctric performance of materials, it is essential to understand the coupled charge and thermal transport mechanisms. In heavily doped semiconductors for example we often expect ionized impurity scattering to dominate electrical transport especially when mobility increasing with temperature is observed. However, the inadequacy of this description in thermoelectric materials such as the new high-performance n-type Mg₃Sb₂, becomes apparent when trying to consistently explain various experimental observations like the enhanced mobilities in larger grain samples and sharp crossovers to metal-like mobilities that decrease with temperature. The underlying cause of such complications is largely associated with the conventional Mathiessen’s rule that interprets or models all of the charge carrier scattering as homogeneous events. The inhomogeneous nature of materials, such as that caused by grain boundaries, must be taken into account to rethink engineering strategies and further improve thermoelectric materials.
Prevailing models for thermal transport treat interfaces and grain boundaries as structureless even though at the atomic scale they are better described as arrays of linear defects of various types. Allowing for this inherent structure, several fundamental characteristics of heat transport arise, such as diffraction conditions when heat carrying phonons scatter off the periodic, linear defect arrays that should be present in grain boundaries. Furthermore, a dimensionality crossover is observed in diffusive heat transport where phonons with a wavelength longer than the linear defect spacing see the interface simply as a structureless planar defect, and phonons with see the interface as a collection of independently scattering linear defects.

[1] J. J. Kuo, G. J. Snyder “Grain boundary dominated charge transport in Mg3Sb2-based compounds” Energy & Env. Sci. 11,429 (2018)
[2] R. Hanus, G. J. Snyder “Phonon diffraction and dimensionality crossover in phonon interface scattering” Communications Physics(2018)

Layered (2D) materials exhibit a variety of extraordinary properties, and recent focus has included topological insulators, electrode materials, monolayers, hetero structures – and thermoelectrics. The physical properties such as band gap or thermal and electrical conductivity are related to the detailed structural characteristics as well as the specific chemical bonding both within the covalent layers and across the van der Waal gap. It is generally assumed that layered materials exhibit anisotropic properties, but the properties are rarely discussed in direct relation to the specific chemical bonding characteristics of the solid.
Using advanced crystallographic analysis including charge density modelling as well as ab initio theoretical calculations, we have studied the crystal structures and chemical bonding of a range of important layered thermoelectric materials including Cu₂Se [1], Mg₃Sb₂ [2], SnS₂ [3], TiS₂ [4] and SnSe [5]

[1] a) K. J. Dalgaard et al., Adv. Theory Simul. 2018, 1, 1800068, b) E. Eikeland et al., IUCr-J 2017, 4, 467-485
[2] J. Zhang et al., Nature Commun. 2016, 7, 10892; Nature Comm 2017, 8, 13901; Chem. Mater. 2017, 29, 5371-5383; Adv. Ener. Mat. 2018, 8, 1702776; Nature Comm. 2018, in press.
[3] M. Filsø et al., Dalton Trans. 2016, 45, 3798 – 3805
[4] H. Kasai et al., Nature Materials 2018, 17, 249-252
[5] M. Sist et al., Acta Crystallogr. Sect. B. 2016, 72, 310–316

Until Recently only chalcogen (S, Se, Te) anion site dopants had been considered tested for n-type Mg₃Sb₂ type materials. Herein is showed how n-Type conduction in a Mg₃Sb_1.5Bi_0.5 system is achieved with La-doping at cation sites with a peak zT > 1. La-doped samples exhibit much higher doping efficiency and dopability compared to other chalcogen-doped samples, which allows for greater tunability of the electronic properties. La-doping also significantly improves the thermal stability of n-type Mg₃Sb_1.5Bi_0.5 measured via a long-term Seebeck and Hall carrier concentration measurements. This increased stability in La doped material suggests new pathways to engineer around the degradation of electronic properties witnessed in this and other thermoelectric materials.

Considering the cost of tellurium, it is technologically important to find alternatives to the well-known Bi₂Te₃-based tetradymite family of thermoelectric materials for cooling applications near room temperature, and power generation applications below 300 ^oC. The Mg₃Sb₂ family of materials is a promising candidate [1], specifically Mg_3.3Sb_1.49Bi_0.5. We investigate here the effect of Mn doping of this material by the method of the four coefficients [2]. In this method, the four transport coefficients, resistivity, and Hall, Seebeck and Nernst effects are measured, and used to calculate the charge carrier concentration, mobility, density of states effective mass m*, and scattering exponent λ (defined by the energy (E) dependence of the relaxation time, τ∝E^(-λ) ). We then further study the effects of making the samples porous. Thus resonance or Kondo effects are separated from the effects of changes in scattering mechanisms.
[1] J. Zhang, et al. Nature communications 8 (2017).
[2] H. Jin and J.P. Heremans, Phys. Rev. Mater. (accepted, 2018)

Thermoelectric materials should have a high electrical conductivity and a low thermal conductivity simultaneously to realize high thermoelectric figure-of-merit (Z=α²σ/κ). However, the intimate coupling between electrical conductivity and thermal conductivity renders the enhancement of thermoelectric performance difficult. Nano-structuring of thermoelectric materials has been a major strategy to improve the thermoelectric performance because the phonon scattering at nano-grain boundaries effectively reduces the thermal conductivity without significant loss of electronic carrier.
Here, we propose a novel strategy with atomic layer deposition (ALD) for nano-structuring of Bi₂Te_3-xSe_x thermoelectric element to achieve a low thermal conductivity. The ALD technique is known to allow a precise thickness control at a sub-nm scale and have excellent conformality even on complex shaped substrate. Conformal ZnO layers were grown on the very fine Bi₂Te₃-_xSe_x powders. The Bi₂Te₃-_xSe_x /ZnO core-shell structured powders were sintered by a spark plasma sintering process and a hot-extrusion process. The ZnO-coated Bi₂Te₃-_xSe_x thermoelectric element shows small grains compared to the uncoated Bi₂Te₃-_xSe_x . The thermal conductivity is significantly decreased by the small grains and the thin ZnO layer at the grain boundaries. Consequently, the figure-of-merit of the n-type Bi₂Te₃-_xSe_x is improved. Furthermore, the mechanical strength of the materials is enhanced by the ZnO ALD coating. Therefore, we believe that the nano-structuring of the Bi₂Te₃-_xSe_x with ALD contributes to enhance thermoelectric performance of Bi₂Te₃-related materials.

Nanostructuring in some materials can reduce the thermal conductivity more than cutting the thermoelectric power factor leading to zT improvement. With the quest to make and test the nanostructured forms of a variety of the known good thermoelectric materials, efficient material synthesis methods become highly desired. The conventional method of synthesis is based on a multi-step process involving ball milling, melt-quenching, and sintering, which requires a significant amount of time and energy. We introduce an alternatively faster approach based on electromagnetic field induced reaction and sintering using microwave (MW) radiation that can produce nanostructured bulk compounds with excellent properties compared to the conventional methods. This is a two-step process involving the uniform mixing of the elements and followed by MW radiation. The synthesis happens at a temperature lower than the melting point with the energy heating only the material (and not the entire furnace), which lowers the overall power requirement. MW radiation has been shown to enhance diffusion rates[i] and results in an accelerated solid-state reaction. Compared to other rapid alloying methods, such as self-propagating high-temperature synthesis, there is no limit to the type of compounds that can be synthesized. Furthermore, the consolidation happens simultaneously during the reaction and does not require additional densification steps.
Achieving crystallite sizes less than 20 nm is often difficult in bulk structures using conventional sintering methods such as hot pressing and spark plasma sintering. The main obstacle is that during the sintering, the grains grow, and nanoscale features broaden or diminish. Reducing the sintering time does not help either, as the material would not be adequately sintered resulting in low carrier mobility. Therefore, the sintered materials often have average crystallites larger than 20-30 nm in size at best. Interestingly, MW radiation has also shown decrystallization effects which further reduces the requirement of other nanostructuring steps. MW synthesis can generate structures with sub-10nm crystallites appropriate to achieve high zTs in some materials.[ii]
This study has focused on the synthesis of nanostructured Bi-Sb-Te-Se alloys. The elemental powders were mixed in the required stoichiometry and sintered under MW radiation. No other source of heating was used. Materials used for this study were Bi-Sb-Te, Bi-Te and Sb-Te alloys, which are known for their thermoelectric properties near room temperature. The elemental powders were mixed for 15 minutes, cold-pressed, loaded into a die, and sintered in a MW cavity. Samples were consolidated at different temperatures ranging from 325 ^oC to 425 ^oC in steps of 25 ^oC. The x-ray diffraction (XRD) analysis of the mixed powder before MW sintering showed diffraction lines of the elemental materials with no sign of solid-state reaction after the short mixing time. However, the XRD analysis of the microwave processed samples showed a complete solid-state reaction with no trace of the elements. It was observed that the alloy entirely formed at 325 ^oC. Differential Scanning Calorimetry analysis was performed on these samples which further confirmed the formation of the alloy after the MW sintering. We will discuss the processing steps of MW sintering, material characteristics, and the thermoelectric properties of the synthesized materials.


[i] Whittaker, A. G. (2005). Diffusion in microwave-heated ceramics. Chemistry of Materials, 17(13), 3426-3432.

[ii] Nozariasbmarz, A., Dsouza, K., Vashaee, D. (2018). Field induced decrystallization of silicon: Evidence of a microwave non-thermal effect. Applied Physics Letters, 112(9), 093103.

Modern electron microscopy, especially combined with the aberration correction and monochrometization technologies, has changed the way we visualize materials across a wide range of length scales. Nevertheless, revealing thermal transport and phonon scattering mechanisms in materials with electrons still poses a major challenge. In this talk, I will first describe the electron microscopy method we developed to understand the role of lattice disorder on phonon scattering by accurately measuring local static displacement and thermal vibration via simultaneous acquisition of multiple annular-dark-field images at different electron-scattering-angles. Layered cobaltates, which exhibits unusually large thermoelectric power, will be used as an example. The system consists of a soft-rigid layered crystal structure with incommensurate displacive-modulation and local atomic vibrations. Through structure determination and refinement using electron microscopy and quantum field theory we show the specific combination of highly crystalline and strongly distorted layers is essential for enabling large electronic-conductivity and low thermoconductivity that is associated with efficient phonon scattering. We further demonstrate besides the well-known Umklapp, Rayleigh and displacement phonon scattering mechanisms, additional resonance and anharmonic scattering play an extremely important role in layered materials. As for future opportunities, I will give an overview on the RF-gun-based 2.8MeV ultrafast electron microscope we recently developed at BNL that has achieved 120fs resolution with 10⁶ electrons per pulse. Brief examples will be given on how we probe the time-resolved interactions between charge, orbital and lattice and separate the roles of various phonon modes as well as thermal transport in function materials using ultrafast pump-probe approach.

The author would like to thank the members of the Electron Microscopy and Nanostructure Group at BNL. the work at BNL is supported by US DOE, Basic Energy Sciences, Materials Sciences and Engineering Division under Contract No. DE-SC0012704.

Topology, a mathematical concept, became recently a hot topic in condensed matter physics and materials science. The topology of the electronic structure of a material determines the electronic, thermal and magnetic properties of solids. All known materials can be reclassified through the lens of topology [1]. Beside of Weyl and Dirac fermions, new fermions can be identified via linear and quadratic 3-, 6- and 8- band crossings stabilized by space group symmetries [2]. Weyl semimetals, a new class of topological phases were found in NbP, NbAs. TaP, MoP and WP₂. [3-13]. In NbP nano wires we have observed the chiral and a mixed gravitational anomaly [9, 10]. Additionally, NbP [10] and WP₂ [11] show evidence for a hydrodynamic flow of electrons, which violated strongly the Wiemann Franz law. MoP and WP₂ show exceptional transport properties such as high conductivity (better than copper), high mobilties and a large magneto-resistance [6, 7]. In WP2 a transition from a hydrodynamic electron fluid below 15 K into a conventional metallic state at higher temperatures is ovebserved in thermal and magnetoelectric transport experiments [11]. The hydrodynamic regime is characterized by a viscosity-induced dependence of the electrical resistivity on the square of the channel width that coincides with as strong violation of the Wiedemann-Franz law. Single crystalline NbP shows an exceptional high Nernst-effect with a strong magnetic field depedence [12, 13]. Even polycrystalline, spark plasma sintered samples of NbP show still a large Nernst thermopower value of ~90 µV/K and power factor of ~35×10-4 Wm^-1K^-2 at 9 Tesla. Also in magnetic samples, such as Co₂MnAl and Co₃Sn₂S₂, we can design giant Nernst effects due to an strongly enhanced Berry curvature [14,15]. In general, the concept of topology might enable us to design more energy efficient materials for thermoelectric applications and beyond.

[1] B Bradlyn et al., Nature 547 298, (2017) and M. G. Vergniory, et al. preprint arXiv:1808.01163
[2] B. Bradlyn, et al., Science 353, aaf5037A (2016).
[3] C. Shekhar, et al., Nature Physics 11, 645 (2015)
[4] Z. K. Liu, et al., Nature Materials 15, 27 (2016)
[5] L. Yang, et al., Nature Physics 11, 728 (2015)
[6] C. Shekhar, et al. preprint arXiv:1703.03736
[7] N. Kumar, et al. Nature Communications 8, 1642 (2017)
[8] A. C. Niemann et al., Sci. Rep. 7, 43394 (2017)
[9] J. Gooth et al. Nature Communications 9, 4093 (2018)
[10] J Gooth et al., Nature 547 324, (2017)
[11] S. J. Watzman,et al.,Phys. Rev. B 97, 161404 (2017)
[12] U. Stockert, et al., Journal Physics: Condensed. Matter 29, 325701 (2017)
[13] Chenguang Fu, et al., Energy & Environmental Science 11, 2813 (2018)
[14] Enke Liu, et al., Nature Physics online, preprint arXiv:1712.06722
[15] Satya N. Guin, et al., preprint arXiv:1806.06753

Phonon Liquid-Electron Crystal (PLEC) thermoelectric materials based on superionic conductors have attracted a number of research interest for their ultralow lattice thermal conductivity . It is proposed that the mobile ions are liquid-like, which either gives rise to a reduction of specific heat from the solid limit to the liquid limit , or the vanish of mean potential for transverse waves. However, it is debated that if the timescale of the mobile ions is fast enough to interrupt phonon and the low is instead attributed to anharmonicity. In this talk, we will report the study of superionic conductor AgCrSe₂ from the low temperature order phase to the high temperature superionic phase by in-situ scanning transmission electron microscopy (STEM) and ab-initio molecular dynamics simulation. We discovered that the mobile Ag⁺ ions in the “liquid-like” superionic phase are not randomly distributed in between neighboring CrSe₂ layers as in liquids, but on average still tend to dwell at lattice sites as in a positional disorder solid. The preferential occupation of mobile ions at lattice sites indicates that the diffusing ions are weakly bounded to a potential and thus they cannot move freely as in liquids. According to our theoretical calculations, it is revealed that the characteristic timescale for Ag⁺ diffusion is relatively slower than phonon vibrations. The calculated probability distribution of Ag atoms also demonstrates the same preferential occupation at lattice sites feature as observed experimentally. Therefore, the ion diffusion and vibrations are decoupled in a slow diffusion-fast vibrations way and we suggest that diffusion is not able to strongly interrupt phonon vibrations. Instead, phonon vibrations are significantly affected by the cation disorder, just as in phonon glass thermoelectric materials. The ultralow of AgCrSe₂ at its superionic phase can be explained by disorder according to the lattice gas model or Cahill’s model for disordered solids.

Metal phthalocyanine compounds are considered as the potential thermoelectric materials because of their good heat and chemical stability. But the poor electrical transport properties greatly impede their application.
Chemical doping is an efficient method to optimize the electrical transport properties of organic semiconductors. In this work, we systematically studied the effect of iodine doping on the thermoelectric properties of copper phthalocyanine (CuPc). A series of CuPcI_x (x=0, 0.5, 0.75, 1.0, 1.25, 1.5, 1.6 and 1.75) were prepared by heating in an iodine ambient at 400 K. The microstructure and charge carrier transport process of the CuPcI_x were characterized by XRD, SEM, XPS, c-AFM and PPMS. The results suggested that CuPc could be successfully doped by iodine and formed two phases CuPc and CuPcI. The amount of the two phases could be detected by the XRD refinement and ion chromatography. The electrical conductivity of iodine doped CuPc samples significantly increased in comparison to the pristine sample, which was due to formation of the three dimension percolation path of the conductive CuPcI. Moreover, it was noticed that the electrical conductivity and Seebeck coefficient of these iodine samples were decoupled, which was ascribed to the low electron energy filtration effect introduced by the CuPc/CuPcI interface. As a result, the highest electrical conductivity of iodine doped CuPc was up to 914 Sm^-1, about 11 orders of magnitude of the undoped sample, and the maximum seebeck coefficient of 65 μV K⁻¹ was obtained. Finally the optimum ZT value reached 0.038 at 300 K.

Half-Heusler are good thermoelectric materials because they have not only decent thermoelectric figure-of-merits but also many other properties such as mechanical strength, thermal stability, etc. MCoSb and MNiSn are the traditional p- and n-type half-Heuslers, where M is Ti, Zr, Hf, respectively. Recently we found that ZrCoBi and TaFeSb are even better p-type half-Heuslers with ZT above 1.5 at 700 °C. In this talk, I will discuss details regarding these two new materials.

Using first-principles DFT we systematically investigate the thermodynamic stability and off-stoichiometry in nominally 19-electron half-Heusler (hH) compounds. We demonstrate unambiguously that considering a cation deficiency towards the off-stoichiometric valence balanced, VEC = 18 composition is necessary for explaining the stability of all previously reported nominal valence electron count (VEC) = 19 compounds. This is understandable in terms of an energy benefit from valence balance considering the valence of each atom using Zintl chemistry that offsets the energy penalty of forming defects in nearly all cases. Thus, we propose a valence balanced rule to understand the ground state stability of half-Heuslers irrespective of stoichiometry and nominal electron count (8, 18 or 19). Using this generalized rule we (a) predict 16 previously unreported nominal 19-electron XYZ half-Heuslers, (b) rationalize the reports of giant off-stoichiometries in compounds such as Ti_(1-x)NiSb which has been known for over 50 years and (c) discover a new class of low thermal conductivity quaternary half-Heuslers. Of the new compounds predicted here, new ternary (Ti_(1-x)PtSb) and quaternary compounds were synthesized and the half-Heusler phase confirmed through X-ray studies. Thermoelectric performance of typical defect-free half-Heuslers based on equiatomic XYZ stoichiometry are limited by their intrinsically high thermal conductivity. Flexibility in stoichiometry of the half-Heusler systems to attain a stable valence balanced composition by accommodating large defect concentrations opens up new dimensions of low thermal conductivity defective half-Heuslers.

The high demand on cost-effective renewable power generation has led to intensified research interest in thermoelectric (TE) materials. Filled skutterudite alloys have attracted the greatest interest due to their suitable band gap, high carrier mobility, low-cost and benign constituent elements. In order to further lower the thermal conductivity and improve figure of merit, multi-fillers, e.g. Yb, Eu, La, Ba, Na etc., have been considered to maximize the phonon resonant scattering resulting from varying vibrational frequencies of fillers. However, the correlated behavior of electrical conductivity, Seebeck coefficient and thermal conductivity has limited the gain in figure of merit (ZT). Bulk heterojunction structure in organic solar cells offers a good mechanism for optimized electron transport. In the present study, mixed alloy comprising of (Ba, Yb, Ca)_0.3Co₄Sb₁₂ and In_0.4Co₄Sb₁₂ was obtained by high energy ball milling and characterized for TE performance. High ZT values of 1.3 at 700 K have been achieved. The mixture of two different compounds induces an isotype heterojunction structure, which results in enhanced carrier mobility. A DFT calculation on the mixed materials band structure is conducted to understand the formation of heterojunction. Nano-grains achieved by high energy ball milling enhances the phonon scattering on mesoscale level and decreases thermal conductivity due to the decrease of thermal diffusivity and heat capacity. Quantum effect on electron transport based on both theoretical calculation and Hall measurement have been discussed.

One of the fundamental challenge in developing high-performance thermoelectric materials has been to achieve low lattice thermal conductivity (κ_L). The exploration of new materials with intrinsically low κ_L along with a microscopic understanding of the underlying correlations among bonding, lattice dynamics and phonon transport is fundamentally important towards designing promising thermoelectric materials. InTe [i.e. In⁺In³⁺Te₂], a mixed valent compound, exhibit an ultralow κ_L, which manifests an intrinsic bonding asymmetry with coexistent covalent and ionic substructures.¹ The phonon dispersion of InTe exhibits, in addition to low-energy flat branches, weak instabilities associated with the rattling vibrations of In⁺ atoms along the columnar ionic substructure. These weakly unstable phonons originate from the 5s² lone pairs of adjacent In⁺ atoms and are strongly anharmonic, which scatter the heat-carrying acoustic phonons through phonon-phonon interactions. Similarly, a Zintl compound, TlInTe₂, also exhibit ultralow κ_L due to low energy ratting modes of weakly bound Tl.² Soft phonon modes and optical-acoustic phonon coupling cause an ultralow lattice thermal conductivity in the room-temperature hexagonal phase of AgCuTe, while the dynamic disorder of Ag/Cu cations leads to reduced phonon frequencies and mean free paths in the high-temperature rocksalt phase. A high thermoelectric figure of merit (zT) of 1.6 is achieved in the p-type AgCuTe at ~670 K.³ Recently, we have shown that the localized vibrations of Bi bilayer leading to ultralow lattice thermal conductivity and high thermoelectric performance in weak topological insulator n-type BiSe near room temperature.⁴


1. M. K. Jana, K. Pal, U. V. Waghmare, and K. Biswas, Angew. Chem Int. Ed, 2016, 55, 7792.
2. M. K. Jana, K. Pal, A. Warankar, P. Mandal, Waghmare, U. V. and Biswas, K. J. Am. Chem. Soc., 2017. 139, 4350.
3. S. Roychowdhury, M. K. Jana, J. Pan, S. N. Guin, D. Sanyal, U. V. Waghmare, and K. Biswas, Angew. Chem Int. Ed, 2018, 57, 4043.
4. M. Samanta, K. Pal. P. Pal, U. V. Waghmare, and K. Biswas, J. Am. Chem. Soc., 2018, 140, 5866

n-type Ge-chalcogenides with promising thermoelectric performance is urgently needed to match their p-type counterparts. However, their realization remains elusive due to intrinsic Ge vacancies which make them p-type semiconductors. GeSe crystallizes into a layered orthorhombic structure similar to the SnSe at ambient conditions. High symmetry cubic phase of GeSe is predicted to be stabilized either by applying external pressure of 7 GPa or by enhancing the entropy via increasing the temperature to 920 K. Herein, we report the stabilization of n-type cubic phase of GeSe at ambient conditions by alloying with AgBiSe₂ (30-50 mol %), which enhances the entropy through solid solution mixing. The interplay of positive and negative chemical pressure anomalously changes the band gap of GeSe with increasing the AgBiSe₂ concentration. The band gap of n-type cubic (GeSe)_1-x(AgBiSe₂)_x posses a value in the 0.3-0.4 eV range, which is significantly lower than orthorhombic GeSe (1.1 eV). Cubic (GeSe)_1-x(AgBiSe₂)_x exhibits an ultralow lattice thermal conductivity (κ_L ~0.43 Wm^-1K^-1) in the 300-723 K temperature range. The low κ_L is attributed to significant phonon scattering by entropy driven enhanced solid solution point defects.¹

Reference:
1. S. Roychowdhury, T. Ghosh, R. Arora, U. V. Waghmare and K. Biswas, n-type Cubic GeSe Stabilized by Entropy Driven Alloying of AgBiSe₂ Leads to Ultralow Thermal Conductivity and Promising Thermoelectric Performance, Angew. Chem. Int. Ed. 2018, DOI: 10.1002/anie.201809841.

Polymorphism in Bi₂Se₃ allows it to be tuned for unique electrical, thermal and optical properties. The commonly reported rhombohedral structure exhibits semi-metallic properties corresponding to a band gap of 0.32 eV and has been widely studied for thermoelectric applications and as topological insulators. The alternative orthorhombic structure is more semiconducting and has been reported to have a band gap close to 1.2 eV. The opportunity to fabricate a mixture of these orthorhombic and rhombohedral structures provides a chance for materials engineering with the aim of optimizing its electrical and thermal properties. Here we report the room temperature Seebeck coefficient and electrical resistance of mixed phase n-type Bi₂Se₃ films. Bi₂Se₃ films with Bi:Se atomic fraction of 38:62 was prepared by electrodeposition using an acidic bath. The XRD pattern of the electrodeposited Bi₂Se₃ films confirmed the existence of a mixed phase structure where the orthorhombic phase was found to be improved upon emergence and subsequent coverage of the film surface with large crystals, as seen from the surface morphology for these films. The n-type conductivity of Bi₂Se₃ was confirmed from XPS and Mott-Schottky analysis. A maximum room temperature Seebeck coefficient of -229.3 μV^-1K^-1 was observed upon characterizing the films having varying thicknesses. The cross-plane resistance was found to improve from increasing film thickness of 0.24 μm to 1.82 μm suggesting an optimum charge transport through these films at 1.84 μm thickness. The measured Seebeck coefficient values are consistent with calculated values based on first principles calculations.

The pseudo-layered GeTe-rich Sb₂Te₃(GeTe)_n (GST), with an unique kind of Te-Te gaps in the unit cell, is found to have promising thermoelectric performance. In this talk, we will report an ultrahigh figure of merit ZT value ~2.4 at 773 K and average ZT ~1.5 between 323K to 773K for the p-type pseudo-layered Sb₂Te₃(GeTe)₁₇ sample along the parallel direction by synergistically optimizing its electrical and thermal properties via vacancy engineering. The microstructural origin of thermoelectric property enhancement was studied by aberration-corrected transmission electron microscopy. The results by in situ characterizations revealed that upon annealing Ge vacancy gaps in quenched samples tend to migrate and recombine into long-range gaps. The recombination of Ge gaps would lead to an overall reduction of carrier concentration which took an optimizing to the power factor, reaching a high value plateau of ~45 μW cm^-1 K^-2 from 623 K to 773 K. Moreover, the abrupt decrease of carrier concentration resulted in a sharp reduction of the electrical thermal conductivity κ_e and the formed long range Van der Waals gaps contribute to the reduction of lattice thermal conductivity κ_l, approaching a low total thermal conductivity ~1.4 W m^-1 K^-1. The stable high performance offered by Ge vacancies engineering provided a promising approach in the future research in GST materials.

Thermoelectric materials enable direct conversion between electricity and thermal energy. Therefore, most of previous studies on thermoelectric materials focus on the electrical and thermal properties. Herein we will explore the interesting optical properties of thermoelectric materials. First, we demonstrate the promising ultra-broadband phonon-enhanced photodetectors based on photothermoelectric effect in SrTiO₃. Second, the visible edge plasmons is revealed by photoemission electron microscopy in Bi₂Te₃ nanocrystals. Finally, using near-field optical microscopy, we observe the electronic heterogeneity in Bi₂Se₃ and Sb₂Te₃ nanocrystals.

Precise control of carrier density is essential to synthesize high-performance thermoelectric materials. Doping by impurities is often frustrated in n-type Bi₂Te₃ alloys by incomplete activation, bipolar doping, the formation of secondary phases, and prevailing intrinsic point defects such as vacancies. This weakens the reproducibility of synthesis processes and reduces the long-term reliability of material's performance, hence aging. Here, we explore an impurity-free doping technique to synthesize n-type bismuth tellurium selenides, combining a cold deformation and a hot extrusion. The cold deformation enables controlling the electron density in the range of ~10¹⁹ cm^-3 via the formation of intrinsic point defects, and the hot extrusion allows texturing the microstructure to enhance the electrical conductivity, hence a large power factor of >5×10^-3 W-m^-1-K^-2. We confirm that our process is very reproducible, and the properties of the samples are stable without aging even after thermal stresses. Using this method, we can decouple the relationship between bandgap, carrier density, and composition to improve the high temperature thermoelectric property. Moreover, we demonstrate the fabrication of high-performance thermoelectric materials from low-graded, raw materials by modifying the degree of the mechanical deformation to reach an optimum carrier density. Our work provides a promising approach to synthesizing n-type thermoelectric materials in the reproducible and adaptable way.

Portland cement production is one of the most energy-intensive industries. The excess input energy is dissipated into the atmosphere as waste heat. A significant amount (35%) of the thermal energy consumed in the cement production process is lost to the environment as waste heat. Furthermore, cement production is one of the major contributors of CO₂. Consequently, the development of innovative technology for energy conservation in the cement industry is urgently needed. The large amount of energy from exhaust gases could potentially be used for waste heat energy recovery to generate other forms of energy. Thermoelectric power generators (TEG) could be used for such recovering of waste heat.
In this work, the performance assessment of an exhaust thermoelectric generator for cement sector application is investigated with emphasis on the heat exchanger and the optimization of its geometric characteristics. A pin-fin array was considered into the exhaust duct to form the heat absorber, which could increase the heat-exchange surface area and time. This is expected to increase the temperature on the hot side of the thermoelectric module, leading to higher energy conversion and efficiency of the device. Moreover, the influence on the efficiency of the height, length and spacing of the fins is analysed for the fixed width and length of the TEG. The influence of geometric parameters of the fins on the TEG efficiency was carried out using a standard orthogonal array of the Taguchi method. An L27 orthogonal array with four controlling factors at three levels was employed in this design of experiments method. The proposed design, through the Taguchi method, leads to a significant increase of output power and increased energy recovery of the thermoelectric generator.

Due to the increasing environmental awareness and rising cost of energy, there is a growing demand to develop renewable energy systems that reduce our reliance on fossil fuels [1]. Thermoelectric generator is a promising technology that cleanly converts temperature difference to electricity on the basis of Seebeck effect. Therefore, to maximize power-generation efficiency of thermoelectric generator, except the figure of merit, ZT, should be as high as possible, the temperature differential between the hot and cold sides also should be as large as possible [2]. In this work, we utilize evaporative cooling technology as opposed to the traditional heat sink to cooling the thermoelectric generator cold side. By combining thermodynamics and heat & mass transfer theory, the numerical model of thermoelectric generator with film water evaporative cooling is established in this study. And the numerical model has been constructed to consider temperature-dependent thermoelectric material properties, heat loss due to radiation and conduction, and Thomson effect. Based on the numerical model, the performances of a typical thermoelectric generator are simulated. Here, the thermoelectric generator hot side temperature is fixed into 50-150 Celsius degree, the thermoelectric generator cold side is covered by thin film water. The ambient temperature is 25 Celsius degree, the relative humidity of the air is set from 25 to 75%. The results show that when fixed the relative humidity, the higher hot side temperature, the higher output power and efficiency; when fixed the hot side temperature, the lower relative humidity, the higher output power and efficiency. Therefore, the maximum power output of 0.272 W and the maximum efficiency of 2.25% are available when the temperature of the hot side is 150 Celsius degree and the relative humidity is 25%. The theoretical model and calculation method may be applied to the prediction and optimization study of thermoelectric generator with evaporative cooling.


[1] H. Wei, G. Zhang, X. Zhang, J. Ji, G. Li, X. Zhao, "Recent development and application of thermoelectric generator and cooler," Applied Energy, 143, 1 (2015).
[2] L.E. Bell, “Cooling, Heating, Generating Power, and Recovering Waste Heat with Thermoelectric Systems,” Science, 321, 1457 (2008).

Magnesium Silicides materials have attracted the attention of many research groups as promising thermoelectric materials for power generators in the intermediate temperature range, due to their very good thermoelectric properties, low density, non-toxicity and low cost. However, scalable synthesis is a critical issue for this type of materials due to their large difference in melting points and high vapor pressure and reactivity of magnesium. Ball milling has been applied on thermoelectric materials since as advantageous technique for industrial scale applications since it is easily applicable and can produce powders of different chemical compositions in a short time.
In this work, Bi-doped Mg₂Si_0.6Sn_0.4 material was prepared by mechanical alloying combined with hot press sintering in an effort to increase the mass capabilities of the synthetic route compared to the typical solid state reaction. Our recent results on different alloying conditions and doping concentrations and their effect on the structural analysis and thermoelectric and mechanical properties will be presented.

There is a recent interest in semiconducting superlattice films because their low dimensionality can increase the thermal power and phonon scattering at the interface in superlattice films. However, experimental studies in all cross-plane TE properties, including thermal conductivity, Seebeck coefficient, and electrical conductivity, has not been performed from these semiconducting superlattice films, because of substantial difficulties in the direct measurement of the Seebeck coefficient and electrical conductivity. Unlike the conventional measurement method, we present technique using a structure of sandwiched superlattice films between two embedded heaters as heating source, and electrodes with two Cu plates, which directly enables the investigation of the Seebeck coefficient and electrical conductivity across the AO/ZnO superlattice films, prepared by atomic layer deposition (ALD) method. Used in combination with the promising cross-plane four-point-probe 3-ω method, our measurements and analysis demonstrate all cross-plane TE properties of AO/ZnO superlattice films in the temperature range from 80 to 500 K. Our experimental methodology and the obtaining results represent a significant advancement in the understating of phonon and electrical transports in nanostructured materials, especially in semiconducting superlattice films in various temperature ranges.

Metal dichalcogenide thin films such as bismuth telluride and molybdenum disulfide were successfully deposited on different substrates using radio-frequency magnetron sputtering technique. The structural, morphological, and thermoelectric transport properties of bismuth telluride and molybdenum disulfide thin films have been investigated systematically to develop a high-efficient thermal energy harvesting thermoelectric device. The magnitude of the Seebeck coefficient of bismuth telluride thin films decreases with increase in film thickness. Bismuth telluride grown at 350^oC displays a maximum Seebeck coefficient of -126 μv/K at 435 K. The performance of the device shows a very strong dependence on the different growth temperature of the film. The power factor increases from 0.009 W/mK² at Room temperature to 0.014 W/mK² at 350 K. Molybdenum disulfide films show the positive Seebeck coefficient values and their Seebeck coefficient increases with film thickness. The AFM images of bismuth telluride thin films display a root-mean-square (rms) roughness of 32.3 nanometer and molybdenum disulfide thin films show an rms roughness of 6.99 nanometer when both films were deposited at 350^oC. The open-circuit voltage of 130 mV shown by the pn-junction thermoelectric generator (TEG) device at 393 K. We have demonstrated a highly efficient pn-junction TEG device for waste heat recovery applications.

The thermoelectric dream is one in which thermoelectric materials can be used for energy generation and harvesting in a commercial sense at high efficiency and low cost. Efficiency is hard to achieve for most thermoelectric as electrical and thermal conductivity are closely related and correlated. The efficiency of any thermoelectric device is given by a quantity called the figure of merit ZT. For thermoelectric devices to be viable alternative energy source they should achieve a ZT greater than 2. Here we report on a thermoelectric device based on Bi₂Te₃ / WS₂ superlattice layer structure created through radio frequency (RF) magnetron sputtering deposition. Quantum confinement in these low dimensional layers creates a high density of carriers near the Fermi level thereby enhancing the electrical conductivity and ZT. The ZT is further increased by these layers through the scattering of short wavelength phonons by picking layer thicknesses less than the phonon free mean path. This makes the structure a good electrical conductor while limiting the thermal conductivity thus beginning to decouple these two material properties.

Experimental determination of material quantities such as the charge carrier density of states effective mass and dominant charge carrier scattering mechanism can provide important insight into the performance of thermoelectric materials, and plays a central role in contemporary thermoelectric materials research. In this context, measurement of the Nernst coefficient, which is sensitive to the scattering mechanism, provides useful additional information that complements the traditional measurement of electrical resistivity, Seebeck coefficient, and Hall coefficient. Here we describe a custom measurement system capable of measuring all four of these galvanomagnetic and thermomagnetic transport coefficients as a function of temperature from 80 K to 400 K, in magnetic fields from – 1 T to 1 T, on both bulk and thin films samples. In addition to data on the system’s performance, a comparative analysis for the accuracy and precision of different approaches to measuring the above transport coefficients will be presented.

Polymeric conjugated materials are very promising for developing future soft material-based semiconductors, conductors, electronic and optoelectronic devices due to their inherent advantages such as lightweight, flexible shape, low-cost, ease of processability, ease of scalability, etc. There are numerous ways to tune material properties via post-processing treatments. One way to enhance device performance is through the process of thermal annealing. Annealing allows for polymer thin films to self-assemble into a lower energy conformation which typically leads to better morphology and higher charge mobility. What is yet to be fully understood is how film morphology is related to the Seebeck coefficient of a material. In this study, we observed the relationship between the Seebeck coefficient, various annealing times and temperatures, and doping concentration with doped P3HT thin films. Our results can provide insight into how can develop high performance organic thermoelectric materials post-processing techniques.

A remarkably high thermoelectric figure of merit ZT of ~2.6 at 923 K was recently reported in p-type SnSe single crystals along the crystallographic b-axis. Afterward, realizing comparable thermoelectric performance in the polycrystalline counterparts have been intensively investigated. However, polycrystalline SnSe-based materials unexpectedly exhibit much higher thermal conductivity than the single crystals, resulting in far poorer thermoelectric performance in the former. In this presentation, we hypothesize that surface oxidation would be the main origin of their high thermal conductivity. To solve this issue, we introduce an oxygen-removal process under mildly reductive atmosphere. As a result, we are able to achieve the ultralow lattice thermal conductivity and thereby markedly enhanced ZT in polycrystalline SnSe-based materials, which are comparable to those in their single crystals.

The conventional forming processes for thermoelectric bulk materials, such as hot press, cold press, spark plasma sintering, plasma activated sintering and others, require thermoelectric alloyed powders as raw materials. While the conventional synthetic methods of thermoelectric alloyed materials require very long time of combination reaction due to slow heating and cooling processes. It typically needs a heat preservation of several hours to achieve uniformity of reaction using lower energy density of heat source. Consequently, the balance between performance and fabrication costs plays a vital role in thermoelectric application.
We report a rapid and scalable synthetic method of thermoelectric materials using selective laser melting (SLM) of powder bed with a thickness of 2mm. The mixed powders of thermoelectric element granule in stoichiometric ratio are used as starting materials. Alloy was then formed using SLM method with very high energy density. Sb₂Te₃ bulk with the merit number ZT at 0.4 was SLM-synthesized. This is comparable with that of the Sb₂Te₃ bulk obtained using conventional melting technology. Bi_0.5Sb_1.5Te₃ bulk was also SLM-synthesized with its ZT at 0.64. The SLM-synthetic time of thermoelectric materials processed is less than 30 min, compared to tens of hours for conventional melting process. SLM-synthesized method therefore opens up a novel way for rapid, scalable and low-cost manufacturing thermoelectric materials.

Hybrid halide perovskites have recently received exponential interest due to their extraordinary photovoltaic performances.^[1] Apart from solar cells, multitude of other applications such as photodetectors, lasers, LEDs, and memory devices have been already demonstrated underpinning the versatile nature of halide perovskites.^[2] Despite the unprecedented performance of hybrid perovskites, their other crucial physical properties such as thermal transports have received limited attention. The majority of the works on thermal transport of hybrid perovskites are accomplished using single crystals or polycrystalline pellets with limited information on seebeck coefficient.^[3] Furthermore, in-plane thermal conductivity and seebeck coefficient of CH₃NH₃PbI₃ films remains largely unexplored. Thermal transport behaviors strongly correlate with material structure and its meticulous understanding is imperative for thermal management of efficient and high performance devices. In this work, we systematically investigate the thermal conductivity and thermopower of sequential vapor deposited CH₃NH₃PbI₃ films. An ultralow in-plane thermal conductivity of 0.3 Wm^-1K^-1 at room temperature was recorded for CH₃NH₃PbI₃ using a chip-based 3ω method. Temperature-dependent thermal conductivity measurement of a series of CH₃NH₃PbI₃ films with different degree of methylammonium treatment reveal that the thermal conductivity value is governed by PbI₆ octahedron framework. Furthermore, n- and p-type CH₃NH₃PbI₃ films were achieved by compositional tuning of the precursors (CH₃NH₃I and PbI₂) resulting in high negative and positive thermopower. The effect of self-doping and defects on the thermopower along with the implications of the present work on future thermoelectrics based on hybrid perovskites will be discussed.

[1] M. Gratzel, Nat. Mater. 2014, 13, 838.
[2] Zhao et. al., Chem. Soc. Rev.2016, 45, 655
[3] Pisoni et. al., J. Phys. Chem. Lett. 2014, 5, 2488.

Phase change memory (PCM) is a non-volatile memory that uses crystalline (set) and amorphous (reset) states to store data. PCM device operation is heavily impacted by thermoelectric effects, resulting in polarity dependent thermal profiles. Understanding thermoelectrics in PCM devices is critically important for cell design to improve endurance, set and reset current requirements and cell-to-cell variability.

We use a finite-element PCM model^1–5 which captures stochastic nucleation, growth, and grain boundary melting⁶ to demonstrate thermoelectric effects^7,8 in a cell with a “double mushroom” geometry. The model includes Peltier heating at material interfaces and Thomson heating within materials using experimentally measured Seebeck coefficients⁹ for fcc and amorphous GST (Ge₂Sb₂Te₅), TiN, and SiO₂. The double mushroom cell consists of two phase change layers sandwiching an insulating layer which contains a conductive filament. The switching region of the device is in the hearth of the geometry, thermally isolated from the top and bottom contacts by GST layers. This geometry is symmetric and serves as a suitable platform to demonstrate the impact of thermoelectric effects on cell behavior.

Our simulation results show that it is possible to amorphize a single mushroom with lower reset currents (above or below the filament depending on polarity) or two mushrooms both below and above the filament with higher reset currents. Our analysis includes the polarity dependence of both set and reset operations, resistance ratios and the necessary time to achieve these operations. Additionally, grain boundary amorphization drastically reduces reset and set times in nanocrystalline GST (down to ~1 ns) as the mushrooms do not need to be completely amorphized to achieve good memory windows⁶.The crystal grains embedded in the amorphous regions eliminate the need for nucleation, allowing for very fast set but increasing cell-to-cell and cycle-to-cycle variability.

¹ Z. Woods, S. Jake, C. Adam, A. L’Hacene, and A. Gokirmak, IEEE Trans. Electron Devices 64, 4472 (2017).
² Z. Woods and A. Gokirmak, IEEE Trans. Electron Devices 64, 4466 (2017).
³ J. Scoggin, R.S. Khan, H. Silva, and A. Gokirmak, Appl. Phys. Lett. 112, 193502 (2018).
⁴ S. Muneer, J. Scoggin, F. Dirisaglik, L. Adnane, A. Cywar, G. Bakan, K. Cil, C. Lam, H. Silva, and A. Gokirmak, AIP Adv. 8, 065314 (2018).
⁵ A. Faraclas, N. Williams, A. Gokirmak, and H. Silva, IEEE Electron Device Lett. 32, 1737 (2011).
⁶ J. Scoggin, Z. Woods, H. Silva, and A. Gokirmak, Modeling Heterogeneous Melting in Phase Change Memory Devices (2018).
⁷ A. Faraclas, G. Bakan, L. Adnane, F. Dirisaglik, N.E. Williams, A. Gokirmak, and H. Silva, IEEE Trans. Electron Devices 61, 372 (2014).
⁸ G. Bakan, N. Khan, H. Silva, and A. Gokirmak, Sci. Rep. 3, 2724 (2013).
⁹ L. Adnane, F. Dirisaglik, A. Cywar, K. Cil, Y. Zhu, C. Lam, A.F.M. Anwar, A. Gokirmak, and H. Silva, J. Appl. Phys. 122, 125104 (2017).

Thermoelectric conversion devices directly convert thermal energy into electricity. In recent years, these devices have received significant attention for their potential to convert waste heat into electricity. Metallic materials like Bi₂Te_e with a ZT (figure of merit) value around 1 and a high thermoelectric conversion efficiency, and have been target of research since the 1950s. Despite these efforts, thermoelectric devices are yet to be utilized on a large scale due to their low thermoelectric conversion efficiency and high cost.
Hybrid materials utilizing the properties of both organic and inorganic materials have been the subject of many recent studies. Organic- inorganic perovskites have been reported to have low thermal conductivity and high Seebeck coefficients and are an appeling candidate for use in thermoelectric materials [1],[2]. Various structure of organic-inorganic prepared perovskite. We measured thermal conductivity (k) of these crystals using the steady-state method. We prepared various structures of 1D and 2D single crystals of hybrid perovskites by turning the preparation conditions and organic molecules inserted between the perovskite layer [3].
It was found that a 1D crystal with methyl pyridine showed k of 0.14 (W/mK) while that with pyridine-thiol showed k of 3.3(W/mK).In my presentation, the relationship between the crystal structure and the thermoelectric parameters, and determine the combination that optimizes the thermoelectric conversion efficiency.

References

[1] A. Pisoni, J. Jacimovic, O. S. Barisic, M. Spina, R. Gaal, L. Forro, E. Horvath J. Phys. Chem. Lett. 2014, 5, 2488.
[2] J. Xu, S. Ramakrishana, A. Q. Yan, X. Li, X. Wang, T. Ye, J. Mater. Chem. C 2017, 5, 1255.
[3] T.M.H. Duong, S. Nobusue, H. Tada, APEX 2018 in press

Thermoelectric power generation is a reliable and environmentally friendly technology to convert waste heat into a useful form of energy, which does not involve any moving parts or toxic gas emissions. However, practical application of this technology needs the exploration and development of materials which can be designed, cost-effective and mass produced. Si-based composite materials have been extensively studied for thermoelectric applications because they are cheap, abundant, non-toxic and environmentally friendly. In literature, thermoelectric properties of Si/NiSi₂, Si/β-FeSi₂ and Si/CoSi₂ composites are studied [1] [2].

Si/α-FeSi₂ composite has not been studied widely due to metallic nature and the structural transition of α-FeSi₂ at 1210 K. However, it is shown in the literature that α-FeSi₂ is stable even near room temperatures at Si-rich conditions [3]. Hence, the composite samples of Si/α-FeSi₂ and Si/β-FeSi₂ have been synthesized by controlling Fe incorporation during mechanical milling. Thermoelectric properties of the samples with α- and β- interfaces have been studied. Si/α-FeSi₂shows higher electrical conductivity and low thermal conductivity, due to the coherency with Si in ‘c’ direction and presence of ‘Fe’ vacancies. This intrinsic ‘Fe’ vacancies act as phonon scattering centers to lower the thermal conductivity. Increased electrical conduction and the reduction in thermal conductivity enhanced the thermoelectric performance of Si/α-FeSi₂ composite. Further, Si/α-FeSi₂ composites with varying ‘Fe’ vacancy concentration and chemical dopings can be employed to improve the thermoelectric performance.

Flexible, lightweight, solution-processible thermoelectric (TE) generators with high room- or low-temperature performance are much desirable in energy harvesting for wearable microelectronics, active microclimate controlling systems and waste heat utilization.[1-3] However, the thermoeletric generators (TEGs) are facing decades of transformation. Up-to-date, there has been a shortage of thermoelectric materials that can be used for such applications. Hooking up these rigid semiconductors to flexible device technology won’t be straightforward.[1-5] To address the problems of the limitation of flexible TE materials, we develop a superior room-temperature performance of Bi₂Te₃/PEDOT:PSS inorganic-organic thermoelectric composites, which are suitable for printed three-dimensionally flexible thermoelectric generators.[5] For the materials, the Seebeck coefficient of the optimized TE composite is 273.3 µV/K at room temperature, reaching that of pure Bi₂Te₃, representing the highest value for Bi_xTe_y/PEDOT:PSS composites reported so far. The corresponding power factor and ZT is 473.5 µW/m×K²and 0.4, respectively. Futhermore, the flexible TEGs were fabricated by three-dimensionally 3D printing on aramid paper and PDMS substrates, respectively. The paper-based thermoelectric generator can produce a higher output power of 30.8 mW, specific power density of 12.3 mWg^-1and areal power density of 20.5 mW/cm²working at under 70 K temperature difference. More importantly, the variation in electric resistance of the PDMS-based flexible TEG is negligible even after 100,000 bending cycles up to 2.5 cm^-1curvature, demonstrating the excellent flexibility and durability of the resultant flexible thermoelectric generator. Therefore, these solution-processible, soft and flexible inorgnic-organic composites offer unique advantages that are not available to their rigid counterparts. The methods and findings in this work pave a way to produce high-performance flexible thermoelectric generators for wearable electronic systems, waste heat harvesting and active microclimate control systems.

Acknowledgement
The work has been partially supported by Research Grants Council of Hong Kong SAR Government (grant no: BBA3) and The Hong Kong Polytechnic University.

References:
[1] Zeng, W.; Tao, X.M.; Lin, S.P.; Lee, C.; Shi, D.L.; Lam, K.H.; Huang, B.L.; Wang, Q.M.; Zhao, Y.; Nano Energy 2018.54,163-174.
[2] Zhang, L.S.; Lin, S.P.; Hua, T.; Huang, B.L.; Liu, S.R.; Tao, X.M.; Adv Energy Mater, 20188(5), 1700524.
[3] Zeng, W.; Shu, L.; Li, Q.; Chen, S.; Wang, F.; Tao, X. M. Adv Mater 2014, 26, 5310-5336.
[4] Zeng, W.; Tao, X. M.; Chen, S.; Shang, S. M.; Chan, H. L. W.; Choy, S. H. Energ Environ Sci 2013, 6, 2631-2638.
[5] Lin, S.P.; Tao, X.M.; Lam, K.H.; Huang, B.L.; Zhang, L.S.; Yeung, K.W.; Zeng, W.; Xu, J.T.; Shi, D.L.; Liu, J.; Liu, S.; Yang, B.;2019, submitted.

Thermoelectric materials show promise in converting waste heat energy in combustion processes to useful energy like electricity. The current low operational efficiencies of thermoelectric materials create difficulties in implementation of these materials for real time applications. Current methodologies such as grain boundary scattering, substitutional effects, band structure engineering, etc., have been utilized to improve the thermoelectric performance, yet have not attained the desired level. An innovative technique is to incorporate these methodologies to semiconducting magnetic thermoelectric materials and extract additional voltage using spin-Seebeck effect, which yields a higher Seebeck coefficient by combining the contribution from conventional charge and spin currents and thus resulting in a higher ZT. Co₃O₄ is an interesting material with spinel configuration that can allow substitutions with transition metal cations having divalent and trivalent oxidation states. First-principles based density functional theory (DFT) calculations were performed on Zn⁺² and Ni⁺² substituted Cobalt spinel oxides. The Zn⁺² substitution in Co⁺² sites resulted in normal spinel structures with no resultant spin polarization, while substituting Cobalt cations with Zn⁺² and Ni⁺² resulted in a partially inverse spinel configuration with a net resultant spin polarization varying from 0 μB to 4 μB depending on the amount of substitutions. Varying these substitutions alters the spin current due to magnetization fluctuations in the configurational space. DFT calculations executed in quantum espresso were performed on a 56 atom unit cell to find band gap, lattice parameter, effective mass of conduction electrons and spin polarization of each configuration formed with various substitutions of Zn⁺² and Ni⁺² cations. These fundamental properties were used to perform conventional charge and spin transport calculations by combining non-equilibrium Green’s function approach with spin transport theory to produce a spin transport model. The formalism developed in this research may also be applied to other interesting thermoelectric materials with semiconducting and magnetic properties.

Thermoelectric power generators are composed by n-type and p-type thermoelectric couples that are connected electrically in series and thermally in parallel and capable of converting heat directly into electricity.[1] The efficiency (h) of thermoelectric power generation technology can be expressed as
h=(T_H-T_C)[sqrt(1+zT)-1]/T_H/[sqrt(1+zT)+T_C/T_H]
with T_H = hot side temperature, T_C = cold side temperature and zT = σa²T/k is the materials’ dimensionless figure-of-merit, where σ, a, k, T refer to the electrical conductivity, Seebeck coefficient, thermal conductivity and the absolute temperature, respectively.[2] Decent performance requires both high electrical transport and low thermal conduction. In addition, thermal stability, machinability, cost and non-toxicity also constitute major issues in thermoelectrics development.

Considering around 50% of the energy gets lost during the industrial production process and to harvest and convert it into electricity, we focus on materials with potential applications at intermediate-high temperatures.[3] Higher manganese silicides (MnSi_γ) and iron disilicide (β-FeSi₂) are selected as candidates for the p- and n-type legs since both are made of environmentally-friendly and abundant elements. Here we will show the fabrication of thermoelectric devices from silicides with striking thermal stability and attempt to improve the efficiency via reducing contact resistance.

References
[1] Snyder, G. J.; Toberer, E. S., Complex Thermoelectric Materials. Nat. Mater. 2008, 7, 105-114.
[2] Rowe, D. M., Thermoelectrics Handbook: Macro to Nano; CRC Press, Taylor & Francis Group: Boca Raton, FL, USA, 2006
[3] Mori, T., Novel Principles and Nanostructuring Methods for Enhanced Thermoelectrics. Small 2017, 13, 1702013.

Recently, wearable electronic devices are utilized for sensor, heath monitoring and smart gear, etc. However, self-powered supply modules are required for sustainable and battery-free wearable electronics. Therefore, the demand for self-powered supplies for these devices is urgent, and energy-harvesting modules represent a promising method to achieve self-powered electronic systems. Source of renewable energies such as sunlight, wind or human being from human motion and body heat are being studied for the self-power technology. Among these, the human body can be stable and sustained natural heat energy source and temperature difference (ΔT) can be driven between the body heat and ambient atmosphere. Therefore, Thermoelectric (TE) technology, in which energy is harvested from waste body heat, is one of the most suitable candidate technology to operate wearable electronic device. Wearable thermoelectric generators (WTEG) is fast becoming a new platform for powering wireless and portable devices. However, we realized that body heat has a limit to obtaining a high temperature difference. In previous our study, we obtained a novel cross-plane type of wearable solar TEG (W-STEG) having not only high flexibility but also high performance by integrating solar absorbers on PI substrates. 500nm thick of five-period Ti/MgF2 super-lattice thin film absorbs 96.3% of sunlight in visible light spectrum and ΔT between the hot and cold sides is up to 20.9 °C. The TE legs are prepared by integrating printable BiTe-based TE ink by dispersing mechanically alloy BiTe-based powder and sintering Sb₂Te₃ and Te in a high viscosity glycerol solvent. In this study, furthermore, we simulated the heat transfer mechanism from the W-STEG module hot side and we proposed the optimal module design to maximize the temperature difference between the hot side and the cold side. We maximize temperature difference between hot side and cold side integrating PDMS cover on solar absorber to block heat convection transfer and attaching flexible copper foam with large surface area which acted as a heat sink. As a result, when exposed to Sunlight, temperature difference between the hot and cold sides was 33.4 °C. The open circuit voltage of the TEG comprising 10 p-n leg pairs is 74.4 mV and the output power is 11.4 μW.

Emergence of phase change memory as a viable non-volatile memory technology intensified the interest in high temperature non-equilibrium electro-thermal phenomena at small scales. Phase change memory devices typically operate under large thermal gradients (up to ~50 K/nm) and electrical stresses (up to ~10⁷ A/cm²). The phase change materials transition between the solid and the liquid phases. The general behavior of the phase change devices can be captured by electro-thermal models using temperature-dependent equilibrium values for thermal conductivities, electrical conductivities and Seebeck coefficients^1,2. However, as we have observed in our self-heating experiments of silicon microwires, the non-equilibrium processes at the melt-solid interfaces can significantly alter the thermal profiles, hence melting and crystallization dynamics^3,4. Large number of free electrons and holes are expected to diffuse into the solid material at the liquid-solid interfaces. This injection drastically increases electrical and thermal conductivity due to increased carrier concentrations. Thermal conductivity across the interface is further increased due to heat released by the recombining carriers in the solid. This generation (G) - transport (T) – recombination (R) process (GTR) can be more significant than the electronic convective heat flow by the carriers across the interface. In presence of an electric field at the interfaces, GTR process introduces an asymmetry since minority carriers recombine to release heat if they are drifted into the solid. In a self-heated microwire, this leads to an asymmetric thermal profile that continuously shifts in the direction of the minority carrier flow until a steady state is reached ^3–7.

References
¹ Z. Woods and A. Gokirmak, IEEE Trans. Electron Devices 64, 4466 (2017).
² Z. Woods, J. Scoggin, A. Cywar, L. Adnane, and A. Gokirmak, IEEE Trans. Electron Devices 64, 4472 (2017).
³ G. Bakan, N. Khan, H. Silva, and A. Gokirmak, Sci. Rep. 3, 2724 (2013).
⁴ A. Gokirmak and H. Silva, Crystallization and Thermoelectric Transport in Semiconductor Micro- and Nanostructures Under Extreme Conditions (United States, 2017).
⁵ S. Muneer, H. Silva, and A. Gokirmak, in Mat. Res. Soc. Spring Meet. (Phoenix, 2017).
⁶ S. Muneer, G. Bakan, H. Silva, and A. Gokirmak, in Int. Semicond. Device Res. Symp. (Bethesda, 2016).
⁷ S. Muneer, A. Gokirmak, and H. Silva, in Mat. Res. Soc. Fall Meet. (Boston, 2014).

There is an increasing interest in high-throughput computational screenings of thermoelectric (TE) materials. A representative example is that valley degeneracy and band effective mass have been demonstrated to be relatively easy to calculate, yet strong descriptors for good electronic properties [1,2].
In this study, we have predicted a high TE performance of p-type Ba₈Cu₆Ge₄₀ based on these descriptors, and experimentally confirmed the prediction. Density functional theory calculations for Ba₈Cu₆Ge₄₀ have found that the stoichiometric compound is a naturally p-doped semiconductor with 6-fold degenerated valleys with light band effective masses. It has also been indicated that introduction of Ge substitutions on the Cu site is effective in controlling the chemical potential.
Motivated by these calculation results, a series of Ba_8+δCu_6-xGe_40+x samples with different Ge substitution amount x were fabricated by arc-melting and spark plasma sintering, and the transport properties were characterized. It has been observed that the carrier type is effectively controlled with x. The p-type sample with x=0.3 shows a high power factor of ~1 mW/mK², and the thermal conductivity is found to be as low as ~1 W/mK. The resulting TE figure of merit ZT is 0.26 near room temperature, and the peak ZT is 0.62 at 615 K.
These ZT values are, although somewhat inferior compared to the record of single-crystalline clathrates (ZT~1.5 at around 500 K [3]), one of the highest among polycrystalline samples (ZT~0.5 at the same temperature range [4]). Therefore, this study further emphasizes the strength of the computational screening in search of high-performance TE materials.

[1] Y. Pei et al., Nature 473, 66 (2011).
[2] H. Tamaki, H. K. Sato and T. Kanno, Adv. Mater. 28, 10182 (2016).
[3] Y. Saiga et al., J. Alloys Compd. 537, 303 (2012).
[4] H. Zhang et al., Inorg. Chem. 50, 1250 (2011).

BiCuTeO is one of the potential thermoelectric materials owing to its low thermal conductivity and high carrier concentration. However, the thermoelectric performance of BiCuTeO is still below average and has a large room for improvement. In this study, we manipulated the nominal oxygen content in BiCuTeO and synthesized BiCuTeO_x (x=0.94–1.06) bulks by a solid-state reaction and pelletized them by the cold-press method. The power factor was enhanced via the nominal oxygen deficiency due to the increased Seebeck coefficient. In addition, the thermal conductivity was reduced due to the decrease in lattice thermal conductivity owing to the small grain size that was generated by the deficiency in the nominal oxygen content. Consequently, the ZT value enhanced to around 11% at 523 K for the stoichiometric BiCuTeO_0.94 compared to BiCuTeO. Thus, optimal manipulation of oxygen in BiCuTeO can enhance the thermoelectric performance. This study can be applied to developing the oxides with high thermoelectric performances.

A thermoelectric figure of merit ZT has been mainly improved by reducing thermal conductivity, resulting record high performance in several important thermoelectric (TE) systems. Their thermal conductivity rapidly approaches to the amorphous limit. In contrast, power factor (PF) does not have the theoretical upper bound, and it directly determines the output power of TE modules. However, enhancing PF is highly challenging, especially for Bi₂Te₃-based compounds that are representative TE materials near ambient temperature. Here we report ultrahigh power factor and carrier mobility in new n-type Bi2Te3-based materials, which are stabilized under excess Cu and Te conditions. The enhanced power factor is attributed to markedly high carrier mobility and electrical conductivity, which increase with the higher extent of Cu.

Harvesting waste heat from power plants and other sources has proven to be a great challenge. Though thermoelectric technology has the capability to directly convert waste heat energy to electricity, the strong association of electrons and phonons in thermoelectric materials is greatly hampering the field. This research studied the electron-phonon interactions in spin-polarized materials formed by substitutions of Ni⁺² in Co₃O₄ spinel matrix. The preferential substitution of Ni⁺² in the octahedral sites changes to tetrahedral sites with the increasing concentration of Ni⁺² resulting in the transition from inverse spinel to a partially inverse spinel. This change in the preferential substitution also accompanied a sudden change in spin-polarization, band gap, effective mass and lattice parameter. Thorough ab-initio studies were performed on a 56 atom unit cell of three configurations: inverse spinel {Co⁺²_1-x Co⁺³_x}_(tet){ Ni⁺²_x Co⁺³_2-x}_(oct)O₄ (0<x<0.5), normal spinel {Co⁺²_1-x Ni⁺²_x}_(tet) {Co⁺³₂}_(oct)O₄ (0<x<1) and partially inverse spinel {Co⁺²_1-x-y Ni⁺²_yCo⁺³_x}_(tet){ Ni⁺²_x Co⁺³_2-x}_(oct)O₄ (0<x<0.5; 0<y<0.5), to demonstrate the transition of fully-inverse spinel oxides to partially inverse spinel oxides. These interactions were tuned by substitutions to attain optimal thermoelectric performance while extracting additional voltage associated to the spin of electrons in the spin-polarized materials. When the spin-polarization of the material is maximized, the spin thermoelectric module takes advantage of the spin Seebeck effect to generate extra voltage even when the electronic and thermal conductivities are not at their respective optimum levels. In an apt spin thermoelectric material configuration, the contributions of conventional and spin thermoelectric modules were combined to yield a higher operational thermoelectric efficiency.

The thermoelectric method of directly converting thermal into electrical energy is attracting increased attention for such applications and thus enhancing its efficiency is of great importance.
The performance of thermoelectric devices is assessed by the dimensionless figure of merit ZT of the material, defined as ZT =α²σT/k, where α, σ, k and T are the Seebeck coefficient, the electrical and thermal conductivities, and the absolute temperature, respectively. The thermal conductivity is a combination of thermal conductivity via electrons, κ_e, and via phonons, κ_l. The main difficulty in improvement of the efficiency of a thermoelectric device is due to the complex relation between σ, α and k. Improving the performance of thermoelectric materials is usually done either by improving the power factor, α²σ, or by applying phonon scattering methods in order to lower the thermal conductivity.
The most efficient n-type thermoelectric materials for temperatures up to 300°C are currently Bi₂Te_xSe_3-x based. By optimizing the preparation process while considering the inherent crystallographic anisotropy, the efficiency of such materials can be further improved. In this study the Bi₂Te_2.4Se_0.6 composition was optimized by CHI₃ doping, preferred alignment of the crystallographic orientation, and lattice thermal conductivity minimization. The synthesis route included rocking furnace melting, ball-milling or melt spinning, and hot pressing with optimal parameters for enhancement of the thermoelectric figure of merit, ZT, at temperatures higher than 200^oC, commonly applied in low temperature power generation applications
The transport properties in the directions parallel and perpendicular to the pressing direction were examined. In the direction perpendicular to the pressing axis, a maximal ZT of ~0.9 was obtained at ~175^oC and at ~210 ^oC, which is as far as we know among the highest ever reported for n-type Bi₂Te_xSe_3-x based alloys.

Solid-state thermoelectric technology uses electrons or holes as the working fluid for heat pumping and power generation and offers the prospect for novel thermal-to-electrical energy conversion technology that could lead to significant energy savings by generating electricity from waste industrial heat. The key to the development of advanced TE technologies is to find highly efficient TE materials. Recently, several novel concepts have been proposed to enhance the efficiency of TE materials and laboratory results suggest that high zT values can be realized in several families of bulk materials. In this presentation, we show the study on the thermoelectric properties of liquid-like materials. We will show these materials possess interesting thermoelectric properties with ultralow thermal conductivity and good thermoelectric figure of merit. The physical mechanisms behind these abnormal thermoelectric properties will also be discussed. Finally, the stability of these compounds will also be presented and discussed.

With today's challenging worldwide energy needs, efficient waste heat recovery via thermoelectric energy conversion is very attractive. Nonetheless, the choice of a thermoelectric generation rather than another method of producing electrical energy is driven by a number of factors. Cost, conversion efficiency, reliability, type and temperature of heat source and heat sink, weight, size, and other variables need to be carefully considered. However, the unique and remarkable properties of thermoelectric generators must also be pointed out. Indeed, the absence of moving parts, the long life, and the flexible design are noteworthy advantages. However, bottleneck for economic viability, is the conversion efficiency. From a technological standpoint, this will be, in part, achieved with the development of new, more efficient materials. Given how easy this technology can be incorporated in such a wide range of waste energy streams, from automobile exhaust gas to industrial waste heat associated with manufacturing processes like forming and melting, the benefits at stake are enormous.
In this presentation, the results and outcome of a 3-year project aiming at discovering new TE materials will be discussed, shown and critically analyzed.

Among the emerging promising thermoelectric materials is AgBiSe₂. This class of materials shows an intrinsically low lattice thermal conductivity stemming from a strong anharmonicity of the Bi³⁺ lone pair.¹ AgBiSe₂ crystallizes in a trigonal structure at room temperature. At 460 K the structure changes into a rhombohedral structure and at 580 K to a cubic rock salt structure.² At the same time, AgSbSe₂ crystallizes in a rock salt cubic structure without any phase transitions.

In this work, we show the influence of the vacancy and Sb substitution on the structure and thermoelectric properties in Ag_1-xBiSe₂ and AgBi_1-xSb_xSe₂.^3,4 Synchrotron Bragg and pair distribution function analyses show the existence of site-disorder between Bi and Ag at room temperature, induced by the phase transitions during cooling. While the site-disorder leads to enhanced point defect scattering, the vacancy doping shows that the site-disorder can be mitigated. Furthermore, we will show how the substitution using Sb influences the phase transition temperature, the local structure and with it the thermal transport. These studies show the direct effect of defects and the local structure on the transport in thermoelectric materials.


(1) Zeier, W. G.; Zevalkink, A.; Gibbs, Z. M.; Hautier, G.; Kanatzidis, M. G.; Snyder, G. J. Thinking Like a Chemist: Intuition in Thermoelectric Materials. Angew. Chemie - Int. Ed. 2016, 55, 6826–6841.
(2) Guin, S. N.; Srihari, V.; Biswas, K. Promising Thermoelectric Performance in N-Type AgBiSe₂: Effect of Aliovalent Anion Doping. J Mater. Chem. A 2015, 3, 648–655.
(3) Böcher, F.; Culver, S. P.; Peilstöcker, J.; Weldert, K. S.; Zeier, W. G. Vacancy and Anti-Site Disorder Scattering in AgBiSe₂ Thermoelectrics. Dalt. Trans. 2017, 46, 3906–3914.
(4) Peilstöcker, J.; Culver, S. P.; Ohno, S.; Zeier, W.G.. Influence of the Local Structure on the Thermoelectric Properties of AgBi_1-XSb_xSe₂. submitted.

Prior ultralow thermal conductivity materials are not suitable for thermoelectric applications due to the limited electronic transport in the materials. Here, we present a new class of ultralow thermal conductivity materials with substantial electronic heat transport. Our samples are graphene/metal heterostructures of transferred graphene and ultrathin metal films (Pd, Au and Ni) deposited by either thermal evaporation or rf magnetron sputtering. For the evaporated samples, we achieve an ultralow thermal conductivity of 0.06 W m^-1 K^-1, with phonons as the dominant heat carriers. The ultralow thermal conductivity is due to a huge disparity in phonon energy in graphene and metals. Interestingly, for the sputtered samples, we find that about 50 % of heat is carried by electrons, even though thermal conductivity is only about 0.1 W m^-1 K^-1. We attribute the electronic contribution to transmission of electrons across atomic pinholes in graphene. With the ultralow thermal conductivity and substantial electronic transport, the new materials could be explored for thermoelectric applications.

The identification of effective approaches to significantly increase the power factor in thermoelectric materials could lead to substantially improved thermoelectric performance. In this context, modulation doping, in which the pathways for transport of charge are spatially separated from the dopant ions, has witnessed a rebirth in interest. Enhanced power factors have recently been reported in multiphase bulk nanoparticle composites, attributed to improved carrier mobilities without degradation of the Seebeck coefficient. While attractive from a device perspective, such materials can be challenging to model and experimentally characterize in order to understand the detailed charge transfer and transport processes, due to a distribution of grain sizes with random relative crystallographic orientations, and ill-defined microstructures. In contrast, precisely layered composites with well-defined layer thicknesses and compositions are ideal systems for the fundamental study of such phenomena. Here we use simple single band and composite models to predict the thermoelectric properties of modulation-doped layered composites. In particular, we have modeled the charge transfer and transport processes in layered composites comprised of doped and undoped layers, including the effects of ionized impurity and deformation potential (acoustic phonon) scattering. The potential for thermoelectric power factor enhancement in such structures will be discussed.

Nowadays, it seems to be clear for a great majority of people that energy needs to be generated, conserved, and recycled in better ways. We need to prevent as much as possible to waste it, since every joule saved means less fossil fuel burnt. Because of that harvesting waste energy are becoming a popular among the scientific community. And, one of the most promising approaches is nano-engineering thermoelectric materials to produce devices. Thermoelectrics are a class of materials able to convert wasted heat energy into electricity. By controlling nano-structuration properly, their efficiency increase. And, since the devices have no moving parts, they are extremely reliable.
In this talk, different approaches to nanostructure different materials will be shown and how those approaches help to increase the final efficiency of the material and the final device. I will show three recent examples: selenides of Ag and Cu done with our recent development Pulsed Hybrid Reactive Magnetron Sputtering (PHRMS),^1,2 large area antidote SiGe nanomeshes,³ and a three-dimensional interconnected Bi₂Te₃ nanowires array.^4,5

References:
JA Perez-Taborda, O Caballero-Calero, L Vera-Londono, F Briones, M. Martin-Gonzalez “High Thermoelectric zT in n-Type Silver Selenide films at Room Temperature” Advanced Energy Materials 2018, 8 (8), 1702024.
JA Perez-Taborda, L Vera, O Caballero-Calero, EO Lopez, JJ Romero, F Briones, M. Martin-Gonzalez “Pulsed Hybrid Reactive Magnetron Sputtering for High zT Cu₂Se Thermoelectric Films” Advanced Materials Technologies 2018, 2 (7), 1700012.
JA Perez-Taborda, MM Rojo, J Maiz, N Neophytou, M Martin-Gonzalez “Ultra-low thermal conductivities in large-area Si-Ge nanomeshes for thermoelectric applications” 2016, Scientific Reports 6, 32778.
A Ruiz-Clavijo, Y Tsurimaki, O Caballero-Calero, G Ni, G Chen, S.V. Boriskina, M. Martín-González, “Engineering a full gamut of structural colors in all-dielectric mesoporous network metamaterials” ACS Photonics, 2018, 5 (6), pp 2120–2128
A Ruiz-Clavijo, O Caballero-Calero, M Martín-González “Three-Dimensional Bi₂Te₃ Networks of Interconnected Nanowires: Synthesis and Optimization” Nanomaterials, 2018, 8 (5), 345

Chalcogenides are a class of inorganic functional solid state compounds, some of which have been widely used as important thermoelectric materials. The properties of a compound are intrinsically determined by its crystallographic structures. Therefore, the design, construction, and structure-property relationships are important. This talk introduces some recent results of our group. For example, in a new quaternary Cs₂Ge₃In₆Q₁₄, we found that the size effect of the sub-structural units causes the symmetry change from R-3m to P-3m1. The phase matching property is associated to the dipole moments and orientations of the structural building units in CsMSn₂Se_6. And the very low thermal conductivity (0.39–0.28 W m^-1 K^-1) of compound Ba₅Cu₈In₂S₁₂ is due to the complicated structure constructed by Cu atoms with diverse local coordination. With the introduction of Cs₂S₈ fcc substructure into the Cu₂S structure, the phase transition temperature increases from 370 K to 823 K, and ZT value increases by about 175% at 875 K.

Bismuth-telluride-based solid solutions are the unique thermoelectric materials near room temperature. For n-type polycrystalline counterparts, maximum figure of merit zTs are often less than 1.0 due to the rapid increased carrier concentration and much degraded carrier mobility during the pulverization of ingots. Herein, we report a novel procedure, combining liquid-phase sintering and hot deformation, to synergistically optimize the carrier concentration, enhance the carrier mobility and reduce the lattice thermal conductivity in n-type polycrystalline bismuth-telluride-based alloys. The improved power factor of 3.6×10^-3 Wm^-1K^-2 and the reduced lattice thermal conductivity of 0.45 Wm^-1K^-1, contribute to a high zT above 1.0 at 400 K for the n-type Bi₂Te_2.7Se_0.3 alloys. These results demonstrate the efficacy of the new procedure that can also be applied to optimize other thermoelectric materials.

P-type Bi_xSb_2-xTe₃ is one of the most mature and widely used thermoelectric materials, so its performance enhancement is important but challenging. Recently, tremendous amounts of studies have revealed the validity of new materials processing is beneficial to create nanostructures that lead to property enhancement. In this work, we reported a synthesis process to fabricate Bi_xSb_2-xTe₃ (x = 0.3, 0.35, 0.4, 0.45, 0.5, 0.55) with excess Te with stoichiometric ratio of 0.2 using the method of mechanical alloying combined with spark plasma sintering. We circulated (heating and cooling) the temperatures between 693 K and 743 K (covering the melting point of Te around 723 K) for the sintering procedure. As a result, the conductivity and the power factor values were greatly improved. The enhanced figure of merit of ZT, ~ 1.41, was obtained in the x = 0.3 sample at 373 K. This work shows that the circulating liquid phase sintering is an effective mean to enhance the thermoelectric properties of Bi_xSb_2-xTe₃.

SnSe has been discovered as a surprising new material with exceptional thermoelectric properties such as ultralow thermal conductivity that results in a record-high thermoelectric figure of merit ZT ~2.5-2.7 at around 773 - 923 K. Such properties, however, have been only observed in well prepared and properly handled single crystal samples. Polycrystalline SnSe samples show absurdly higher apparent thermal conductivity and much lower ZT values than single crystals. The former is strikingly contrary to the general understanding that polycrystalline samples tend to show lower lattice thermal conductivity than single crystals due to additional phonon scattering at grain boundaries. In this presentation, we reveal that their high thermal conductivity is attributed to surface tin oxides. To address this problem, we introduce facile oxide-removing strategies. These lead to exceptionally low lattice thermal conductivity in polycrystalline hole-doped SnSe samples, even lower than that of single crystals, and boost the ZT to ~2.5 to 3.0 at 773 K.

Thermoelectric materials directly convert heat into electricity and vice versa.

A large variety of thermoelectric materials' classes including, intermetallic compounds (e.g. half-Heuslers), silicides and chalcogenides (e.g. Bi₂Te₃, PbTe and GeTe), have been investigated as candidates for practical applications due to high thermoelectric figure of merit, ZT, values at different temperature ranges.

In the recent years, many of the advances of improving the ZT and thereby enhancing the heat to electricity conversion efficiency, had been focused on generation of nano-features, effectively scattering phonons, for minimization of the lattice thermal conductivity. Yet, such nano-domains are in many cases unstable under the high temperatures operating conditions, tending to grow into the micro-meter scale, while degrading the ZT.

Moreover, the effect of grains- an/or domains boundaries and their high temperature variations on the electronic properties is rarely considered.

In the current presentation, the govern temperature dependent physical metallurgical effects on the ZT, will be described in details, with some innovative approaches for mitigating these adverse effects, by means of thermodynamically driven phase separation and grain boundary Zener pinning effects. In addition, the effect of these and other metallurgical factors (e.g. vacancies and dislocations) on the electronic thermoelectric transport properties will be presented.

A focus on the related activities in the Department of Materials Engineering at Ben-Gurion University of the Negev will be given.

We describe our efforts to discover new thermoelectric materials compositions through a combination of theory and data-driven approaches. In particular, we focus on two efforts. The first is a new model of carrier transport called AMSET that allows one to model the temperature-dependent Seebeck coefficient and mobility from first-principles at reasonable computational cost. This is achieved by adapting classical scattering equations previously developed for 1D parabolic band semiconductors to complex density functional theory band structures. We will explain the theory behind AMSET as well as comparison against experiment and constant relaxation time approaches typically used in the field. Next, we will describe our data-driven approach to suggest new thermoelectrics compositions. This effort leverages a corpus of over 3.3 million abstracts of materials science journal articles that we have collected, and for which we have used natural language processing techniques to extract the correlation between a material's composition and its likelihood to be a thermoelectric based on automatically "reading" the prior literature. We demonstrate that this technique can potentially predict new thermoelectric materials years in advance of their first report in the scientific literature and explain the manner in which this machine learning model arrives at predictions. For both methods, we present comparisons to the experimental data, both previously tabulated and conducted with our collaborators for new materials.

Exploiting the fascinating properties of materials near soft mode phase transitions is an emerging concept in the quest to increase thermoelectric efficiency [1]. Soft phonons may lead to low lattice thermal conductivity, while preserving high electronic conductivity. Here we investigate the unusual electronic transport properties of n-type PbTe, which is a classic thermoelectric material that exists near a soft optical mode phase transition. Our first principles calculations show that longitudinal optical phonon scattering dominates electronic transport, while acoustic phonon scattering is relatively weak [2,3]. We find that scattering due to soft transverse optical phonons is by far the weakest scattering mechanism, due to the symmetry-forbidden scattering between the conduction band minima and the zone center soft modes [3]. Soft phonons thus play the key role in the high thermoelectric figure of merit of n-type PbTe: they do not degrade its electronic transport properties although they strongly suppress the lattice thermal conductivity [1].

[1] R. M. Murphy, E. D. Murray, S. Fahy, and I. Savic, Phys. Rev. B 93, 104304 (2016)
[2] A. R. Murphy, F. Murphy-Armando, S. Fahy, and I. Savic, Phys. Rev. B 98, 085201 (2018)
[3] J. Cao, J. D. Querales-Flores, A. R. Murphy, S. Fahy, and I. Savic, arXiv:1809.03799

This work was supported by Science Foundation Ireland under Investigators Programme No. 15/IA/3160.

Domain walls in ferroelectric oxides can have significantly different properties than their bulk counterparts [1]. This represents a new avenue for the manipulation of material properties for specific purposes. GeTe is a ferroelectric material that is also one of the best performing thermoelectrics, combining beneficial electronic properties with low lattice thermal conductivity [2] and mechanical stability [3]. In this work, we have performed first principles calculations to understand how domain walls affect structural and electronic properties of GeTe. We have identified five different types of domain walls. We find that strong strain-order parameter coupling is present at all of them, which is beneficial for the lattice thermal conductivity reduction. We also show that some of the domain walls are conducting along their planes and insulating out-of-plane. The n- and p-type conduction of these domain walls, discovered in our calculations, presents an opportunity for tuning the electronic transport properties of GeTe for thermoelectric applications.

[1] D. Meier et al, Nat. Mat. 11, 284 (2012)
[2] Z. Liu et al, Proc. Nat. Acad. Sci. USA 115, 5322 (2018)
[3] D. Dangic, A. R. Murphy, E. D. Murray, S. Fahy, and I. Savic, Phys. Rev. B 97, 224106 (2018)

This work is supported by Science Foundation Ireland PI Award 15/IA/3160.

Typical 18-electron half-Heusler (HH) compounds, ZrNiSn and NbFeSb, have been identified as promising high temperature thermoelectric materials. NbCoSb with nominal 19 valence electrons, which is supposed to be metallic, has recently been reported to also exhibit thermoelectric properties of a heavily doped n-type semiconductor. In this talk we experimentally demonstrate that the nominal 19-electron NbCoSb is actually the composite of 18-electron Nb_0.8+dCoSb (0 ≤ d < 0.05) and impurity phases. Single phase Nb_0.8+dCoSb with intrinsic Nb vacancies, following the 18-electron rule, possesses improved thermoelectric performance, and the slight change in the content of Nb vacancies has a profound effect on the thermoelectric properties. The carrier concentration can be controlled by varying the Nb deficiency, and the optimization of the thermoelectric properties can be realized within the narrow pure phase region. Benefiting from the elimination of impurity phases and the optimization of carrier concentration, thermoelectric performance is remarkably enhanced by ~100%. The similar phenomenon has also been observed in some of other defective 19-electron HH compounds. This work opens a new avenue for searching for nominal 19-electron half-Heusler compounds with intrinsic vacancies as promising thermoelectric materials.

Half-Heusler thermoelectric materials have attracted much attention due to their high thermal stability, mechanical strength, power factor (PF) and figure-of-merit (ZT) at high temperatures. In order to translate the material performance into good module performance, electrical and thermal contact resistances must be minimized. Low electrical contact resistance between electrode and thermoelectric material is critical for device performance. Poor contacts can cause Joule heating which can significantly degrade the power output. Ideally, the contact resistivity should be smaller than 1 µΩ.cm². In practice, many material systems are far from achieving this resistivity value. In this study, we demonstrate a half-Heusler thermoelectric module with excellent contact resistance and thereby excellent module performance. The contact resistance was measured using a scanning probe measurement system, and specific resistivity at each interface was calculated. A very low contact resistivity of ~1 µΩ cm² was achieved resulting in power output close to theoretically predicted value. This device fabrication technique provides pathway for high temperature power generation technology in waste heat recovery.

Many new thermoelectric materials including half-Heusler alloys, PbTe and SnSe based compounds, skutterudites, clathrates, to name a few, are known to possess rich and complex bandstructures, with highly anisotropic bands belonging to multiple valleys having manifold degeneracies in both conduction and valence bands. These materials often exhibit very low thermal conductivities, which can be further reduced using nanostructuring techniques to reach down to, or even below the amorphous limit. More recently, however, significant efforts are also being undertaken to take advantage of the rich bandstructure features to achieve power factor improvements as well. These include identifying the best alloying strategies, introducing resonant states in the bands, aligning of the multiple bands at the same energy, etc. Large efforts are also placed by the computational materials science community into bandstructure calculations through involved DFT codes and high-throughput material screening using data from a variety of databases.
Less progress, however, is achieved in combining such involved calculations with accurate transport models, which take into account different scattering mechanisms including their energy dependences, the details of intra/inter valley scattering, details of anisotropy in the bandstructure, electronic relaxation details, etc. Lack of these considerations leads to wrong performance estimations, inaccurate comparisons, and sub-optimal design directions. At the moment, most calculations, despite the use of accurate bandstructures, are performed using constant relaxation time approximation due to the very challenging computational complexities of accurate scattering treatment.

Here, we describe the development of an advanced simulator that uses an arbitrary material bandstructure and computes the thermoelectric coefficients beyond the typically assumed constant relaxation time approximation. Our code includes all major scattering mechanisms (e.g. phonons, impurities, alloying) encountered in the materials of interest. The code uses the Fermi’s Golden Rule in the energy dependent relaxation time approximation, computed by numerical considerations of all bandstructure states.
Using this, we then report on an investigation of a few typical thermoelectric materials and explore the differences in the performance predictions in cases where the various details of the scattering mechanisms as described above are considered in the calculations, versus if they are omitted using simplified methods. We indeed show that depending on the scattering physics considered, the power factor can vary substantially and materials performance rankings can be altered. More specifically, we show that in the efforts towards improving the power factor of materials through band alignment, different optimized bandstructure conclusions are reached if one uses the constant relaxation time approximation, versus scattering rates that depend on the density of intra-valley states, versus scattering rates that depend on the total density of states. In the case of half-Heusler compounds, for example, we show that the differences in the performance estimation of band aligned structures can vary significantly depending on the scattering mechanism. We show that this variation depends on the effective masses of the aligned bands of the material as well.
Finally, we discuss the numerical issues of these simulations and approximations that could speed up the computation significantly without compromising the accuracy.

Organic/inorganic thermoelectric nanocomposites (TENCs) have seized great attention since they integrate the advantages of inorganic (high electrical conductivity) and organic (low thermal conductivity and mechanical flexibility) parts. Major barriers obstructing the development of this field are the lack of n-type TE materials and the absence of precise control of the morphology directly relevant to TE performance. To resolve these two issues, here we provide a feasible strategy to improve the blend film morphology of n-type TENCs consisting of organic polymer and inorganic metallic nanofiller.

As proof-of-concept, we utilized the magnetic characteristics of cobalt nanowires (Co NWs) to direct the assembly of metallic nanowires in an insulating polyvinylidene fluoride (PVDF) matrix through solution fabrication method. The resulting highly oriented n-type TENCs exhibit significantly increased electrical conductivity (s) in comparison to randomly packed nanocomposites. It is interesting to note that the s and Seebeck coefficient (S) of these TENCs are decoupled, which is consistent with the n-type metallic behavior of Co. Both s and S simultaneously increase as a function of the Co NWs content. As a result, the maximum power factor (PF) of 523 μW m⁻¹ K⁻² is obtained at 320 K with 45 vol% Co NWs under magnetic alignment, which is among the highest PFs achieved for n-type TENCs. Importantly, these TENCs are highly bendable and thus hold great potential for flexible TE modules. By pairing these n-type TENCs with p-type PEDOT:PSS thin films, we constructed bendable and planar TE generators that produce a maximum output voltage and power of 26.4 mV and 5.2 μW, respectively, when temperature gradient reaches 50 K.

Furthermore, we employed n-type conducting polymer N2200 to replace insulating PVDF in the above TENCs and aimed to understand the favorable contributions of organic semiconductor to the charge carrier transport process. Distinct from Co NWs/PVDF counterparts, Co NWs/N2200 TENCs exhibit the intimately coupled s and S and the highest PF up to 288 μW m⁻¹ K⁻² is acquired with 31 vol% Co NWs. Despite extra conductive paths are anticipated to create in Co NWs/N2200 TENCs relative to PVDF based analogues, the s is undermined by the unsatisfactory porous microstructures in thin films, presumably caused by the rigid molecular chains of N2200 compared to flexible PVDF. To enhance the interconnectivity between Co NWs and N2200, a small amount of PEI-treated n-type single-walled carbon nanotubes (SWCNTs) were added to the optimal binary sample. The s of ternary TENCs increases and then drops with the addition of SWCNTs, yielding an optimal PF as high as 384 μW m⁻¹ K⁻² at 380 K at the content of 3 wt% SWCNTs.

Reference
[1] Y. Chen, M. He, J. Tang, G. C. Bazan, Z. Liang, Adv. Electron. Mater. 2018, 4, 1800200.

2D materials have attarcted a great deal of interest due to their electrical and thermal properties. In this work, electrical and thermal properties of semi-metallic ZrTe2 and TiSe2 have been studied using density functional theory calculations. The effect of several exchangecorrelation potentials on the band structure and thermoelectric properties has been investigated. It was found that tensile strain will open a gap and cause a corresponding non-linear change in electrical transport properties of these materials. Finally, phonons and thermal conductivity of ZrTe2 were calculated, and its thermoelectric properties assessed.

Thermoelectric effects for Heat-to-electricity conversion are fascinating phenomena with many potential technological applications from waste heat energy harvesting, IR camera, e-skin to interactive buildings. We give an overview of those phenomena in electronic or ionic conducting polymers.
In the first part, we summarize our finding on the electronic thermoelectric properties of the p-type polymer called poly(3,4-ethylenedioxythiophene) (PEDOT) [1]. PEDOT thermoelectric aerogels are presented with their applications in dual pressure and temperature sensors [2]. We then focus on the recent advances made in the lab regarding n-type conducting polymers [3].
In the second part, we explore the ionic thermoelectric effects in both mixed ionic electronic polymer conductors [4]. This phenomenon is used to create materials orthogonal sensitive to humidity, temperature and pressure.
In a third part, we investigate on ionic polymer conductors. Giant Seebeck effects are found with coefficients that reache 10 mV/K. This effect enables charging a supercapacitor for energy harvesting of intermittent heat sources; but also switching a transistor, thus creating a smart pixel for high temperature sensitivity [5]. The concept of non-aqueous ionic thermoelectric polymers is further explored to obtain both negative and positive ionic Seebeck coefficient and build the first ionic thermopiles.
[1] Nature Materials 10, 429 (2011), Nature Materials 13, 190 (2014), Adv. Mater. 27, 2101-6 (2015); JACS 134, 16456 (2012), Adv. Elect. Mater 4, 1700496 (2018).
[2] Adv. Funct. Mater. 27, 1703549 (2017).
[3] Adv. Mater. 30, 1801898 (2018); Adv. Mater. 28, 10764, (2016)
[4]. Adv. Ener. Mater. 5, 1500044, (2015); Adv. Funct. Mater, 26, 6288 (2016).
[5] Adv. Elect. Mater. 3, 1700013 (2017); Energy Environ. Sci., 2016,9, 1450; Nature Comm., 8, 14214 (2017).

As smart devices have become an integral part of our lives, there is a growing demand for the development of self-charging devices that can simultaneously generate and store energy. Among them, thermoelectric generator (TEG) are noiseless and can be miniaturized easily. Conducting polymers are the appropriate choice for wearable thermoelectric devices among TEGs, due to their flexibility, eco-friendliness, lightweight, etc. Poly (3,4-ethylene dioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) has been widely studied owing to its highest electrical conductivity (> 10³ Scm^-1) with low thermal conductivity (< 1 Wm^-1K^-1). However, the thermoelectric efficiency of PEDOT:PSS is still low due to the low electrical conductivity and Seebeck coefficient, compared to the inorganic materials. Here we present a strategy to enhance the electrical conductivity and to decrease the thermal conductivity simultaneously, in order to achieve the high thermoelectric efficiency of PEDOT:PSS. We successfully fabricated PEDOT:PSS nanotubes with an average diameter of 200 nm via infiltration in AAO template without any synthesis process. From the transmission electron microscopy (TEM) and Raman spectra, the infiltration method using the capillary phenomenon forms a PEDOT:PSS nanotube with a wall thickness of 10 nm and induces a stretching in the PEDOT:PSS C_α=C_β bond with a linear alignment. We found the increase in electrical conductivity due to stretching of PEDOT chains as well as the decrease in thermal conductivity through phonon scattering at the inner and outer interfaces of the nanotube.
At first, the PEDOT:PSS solution was dropped on to the AAO template and PEDOT:PSS solution was infiltrated into AAO template for 24 hr at room temperature. Then the sample was baked at 120^oC to remove residual solvent followed by Ethylene glycol treatment for 120 min for improving the electrical conductivity of PEDOT:PSS nanotubes. The samples were then rinsed and then baked at 120^oC for 10 min. The AAO barrier layer was removed by immersing the template in NaOH followed by vacuum filtering. The PEDOT:PSS nanotubes were then collected from NaOH solution. We believe that the fabrication process of PEDOT:PSS nanotubes using a facile infiltration method not only enhances the thermoelectric efficiency but also helps us in understanding the thermoelectric behavior at lower dimensions.

Micro energy harvesters are becoming more and more relevant as energy resources for distributed low power electronics and sensors networks. In this context, micro thermoelectric generators (µTEG) possess many advantages, as they can work in the dark and require limited maintenance. Ideally, such µTEGs should be cost effective and based on abundant materials. Organic conjugated materials have been studied to address such requites, as they can enable lightweight, flexible and cost competitive µTEGs produced through mass scaled printing techniques. Here we report on a new organic, flexible µTEG where doped p-type and n-type organic semiconductors are inkjet printed to form a micro module composed of 128 thermocouples on plastic. In particular, we will present an architecture specifically devised to improve the thermal coupling and simplify the fabrication process. Our successful realization of a printed organic µTEG and the complete thermoelectric characterization allow to envisage future efficient devices delivering µW/cm² at low average temperature and low temperature differences.

Organic thermoelectrics based on conjugated polymers are emerging as promising candidate for green energy conversion. Building such devices require both p-type (hole-transporting) and n-type (electron-transporting) materials. Although p-type polymers can efficiently be doped to exceptionally high electrical conductivity (σ) values (> 1000 S/cm), the development of n-type polymer lags far behind due to their low conductivity (typically < 0.01 S/cm). For example, it has been shown that the naphthalenediimide–bithiophene copolymer P(NDI2OD-T2), a high electron mobility polymer extensively investigated as active layers in OFET/OSC devices, can be doped to a conductivity of only 0.003 S/cm regardless of the dopant and processing conditions.
Recently, we demonstrated that the twisted donor-acceptor structure of P(NDI2OD-T2) can lead to strong charge carrier intrachain localization on the acceptor moiety, thus resulting in low electron conductivity. On the other hand, backbone planarization can be beneficial for charge carrier delocalization, enhancing the charge transport and the extent of the doping process. To this end, we replaced the bithiophene (T2) unit in P(NDI2OD-T2) with the more electron-deficient bithiazole (Tz2) unit, resulting in the analogous P(NDI2OD-Tz2). Hereby, the polymer donor-acceptor character can be reduced and the lower intrachain steric helps planarize the polymer backbone and thus enhancing intermolecular π-π stacking interactions, with consequently a much higher electrical conductivity (up to ~0.1 S/cm) and enhanced thermoelectric response. These results demonstrate how significantly the polymer backbone structure affects the polaron delocalization and the resulting n-doped conductivity, setting molecular-design guidelines for the next generation of conjugated polymers.

The ability of thermoelectric materials to convert waste heat into useful electrical energy is essential for increasing the efficiency of small-scale power supply technology and for active thermal management applications, with market projections for thermoelectrics amount to US$1 billion by 2024 if suitable materials can be developed. Due to low thermal conductivities (κ), organic semiconductors (OSCs) have incredible prospects for next-generation, flexible, printable thermoelectric devices with niche applications to wearable consumer product, and sensors that can be operated by temperature differentials from body heat. Analysis of the electronic structure of organic semiconductors indicates that large ZT values are possible by doping. This pushes electrical conductivities to the near-metallic regime (semimetal) where σS² can be maximized without compromising κ. Yet polymeric-based semiconductors generally suffer from doping-induced disorder that impedes thermoelectric performance. To compete with their inorganic counterparts, characterization of the structural, electronic, and charge transport attributes of these organic semiconductor materials with respect to processing parameters and doping mechanism plays an important role for device engineering. Control of this doping mechanism and its effect on conductivity is one of the key aspects to understanding organic thermo-electric devices.
We use the P3HT-F4TCNQ as our OSC-dopant molecule system to study these properties. It has been shown in literature that this system either forms an integral charge transfer state (ICT) or a partial charge transfer state (CPX) based on processing techniques, with ICT being desirable for improving σS². However, using high resolution FTIR we show that these states exist simultaneously and that their existence is co-related to the local density of states of the semiconductor matrix. Using these new insights, we turn to evaluate the phase diagrams of the P3HT-F4TCNQ system to further understand their thermo-electric properties and its relationship to the charge transfer states.

As the concept of practical and wearable thermoelectric generator (TEG) begins to surface, requirements such as flexibility, being lightweight and mass producibility have become important. While almost all of the waste heat is emitted in the out-of-plane direction, most reported flexible TEGs operate in the in-plane direction. Herein, we have rationally designed a bracelet-type TEG structure where the carbon nanotube (CNT) ink is printed directly on a flexible cable and the device is operated in the out-of-plane direction of heat source. While there are some reports on the printing of TE inks on flat surfaces, it is the first report to produce a flexible TEG by printing the TE ink on a curved surface. The flexible TEG based on 60 pairs of n- and p-doped CNT ink printed onto a cable shows the maximum power output of 0.2 and 1.95 μW at temperature differences of 10 and 30 K, respectively, which is one of the highest output powers for flexible TEGs based on CNT inks. In addition, the mechanical durability of the TEG shows rare change of the circuit resistance after 3500 bending cycles. The easy installment of the bracelet-type TEG on heat sources with various shapes and its ability to harvest waste heat in the out-of-plane direction of heat source has great potential as flexible /or wearable power conversion devices.

The discovery of organic-inorganic hybrid perovskite can lead to the next-generation thermoelectric energy conversion platforms due to ultra-low thermal conductivity and large Seebeck coefficient [1-3]. The conversion efficiency of thermoelectric material is quantified by figure of merit ZT (=σS²T/κ: σ is electrical conductivity, S is Seebeck coefficient and κ is thermal conductivity). CH₃NH₃SnI₃ (MASnI₃) single crystals have intrinsic high value of Seebeck coefficient due to charge-carrier mobility and ultra-low thermal conductivity due to the rotational motion of cations in the lattice [4] still it has only focused for optical energy conversion [5]. However, optimization of electrical conductivity is still challenging which can lead to a landmark for thermoelectric application of this material. In this study, we fabricate MASnI₃ thermoelectric thin films which are superior for printed electronics. It is important to note that for fabricating a thermoelectric energy harvesting module both n-type and p-type of MASnI₃ thin films are required. Keeping this in mind, we tune charge-carrier type (p- and n-) in MASnI₃ thin films.

Thin films are fabricated by spin coating technique using a yellow solution of MASnI₃. The yellow solution is prepared by MAI, SnI₂, and SnI₄ precursors. During the fabrication process, we optimized growth parameters such as baking temperature, baking time, solution mixing time and spin coating speed. The as-grown MASnI₃ thin films have shown p-type behavior and the tuning of charge carrier to n-type is performed by optimizing molar concentration ratio of precursor’s material. MASnI₃ thin films were thoroughly characterized using several state of the art techniques including XRD and SEM. Temperature dependent Seebeck coefficient, electrical resistivity and thermal conductivity measurements were performed. MASnI₃ thin films baked at 100°C shows the best result with Seebeck coefficient about 70 μV/K and electrical conductivity of about 50 S/cm near room temperature. The detailed results and analysis will be shown during the presentation. In future, our plan is to optimize the carrier concentration to enhance the value of ZT.

References:
1- A. Kojima et. al. J. Am. Chem. Soc., 131, 6050, 2009.
2- J. Zhang et. al., Energy Lett, 1, 535, 2016.
3- Y. Takahashi et. al., Dalton Trans. 40, 5563, 2011.
4- W. Lee et. al., PNAS, 114, 33, 8693, 2017.
5- Best Research Cell, Efficiencies: http://www.nrel.gov/pv/ assets/images/efficiency-chart.png

There is a vital need to develop technologies to dynamically harvest energy from surroundings to power IoT applications [1]. In regards to utilizing ubiquitous thermal energy like body heat, thermoelectric thin films can be promising [2]. Recently, we have utilized semiconductor fabrication processes to fabricate an organic π-type thermoelectric module outputting over 250 mV with 80^oC [3]. The high contact resistance and relatively low thermoelectric performance of the organic materials used, remains a large issue to solve. I will mainly present efforts on developing inorganic thermoelectric thin films, which can be applied to wearable applications via flexible substrates, for example. We have also discovered that utilizing magnetism can be a way to enhance thermoelectric properties, e.g. magnetic semiconductors [4]. There is a possibility to compatibly integrate such thermoelectrics into magnetic devices and sensors to self-power themselves. Development of measurements to accurately evaluate the thermal conductivity of thin films and interfacial thermal resistance are also vital. We have recently developed a novel TEM in-situ thermal probe, which utilizes nanothermocouples with scanning heat input under STEM, to be able construct nanoscale heat maps which visualize the heat pathways in nanocomposite materials and in-plane of thin films, for example [5]. Focused picosecond thermoreflectance with advanced analytic methods to enable unambiguous, accurate measurements and evaluation of interfacial thermal resistance will also be presented. CREST, CFSN, WPI-MANA project collaborators are acknowledged.
References
[1] T. Mori and S. Priya, MRS Bulletin, 43, 176-180 (2018).
[2] I. Petsagkourakis, K. Tybrandt, X. Crispin, I. Ohkubo, N. Satoh, and T. Mori, “Thermoelectric Materials and Applications for Energy Harvesting Power Generation”, Sci. Tech. Adv. Mater., in press. doi: 10.1080/14686996.2018.1530938
[3] N. Satoh, M. Otsuka, T. Ohki, A. Ohi, Y. Sakurai, Y. Yamashita, and T. Mori, Sci. Tech. Adv. Mater., 19:1, 517-525 (2018).
[4] T. Mori et al., Small, 13, 1702013 (2017), Appl. Phys. Express, 6, 043001 (2013), Angew. Chem. Int. Ed., 54, 12909 (2015), J. Mater. Chem. A, 5, 7545 (2017), Materials Today Phys., 3, 85-92 (2017).
[5] N. Kawamoto, Y. Kakefuda, I. Yamada, J. Yuan, K. Hasegawa, K. Kimoto, T. Hara, M. Mitome, Y. Bando, T. Mori and D. Golberg, Nano Energy, 52, 323 (2018).

Solution processing of inorganic semiconductor precursors and its subsequent thermal decomposition offers a simple, inexpensive, large area fabrication route to inorganic semiconductor thin films. However, the presence of inherent strong covalent bonds renders these inorganic semiconductors insoluble. Hydrazine as an exception is known to form soluble metal chalcogenide precursors¹ but its high toxicity, explosive and carcinogenic nature inhibits its widespread application. Diamine-dithiol solvent mixture was discovered recently to dissolve a variety of metal chalcogenides.² But this solvent system poses problems of introducing sulfur contamination in metal selenides and tellurides, the presence of free tellurium in metal tellurides, and immediate precipitation of PbTe precursors.

We propose a new approach to prepare metal chalcogenide precursors by mixing elemental metal with diphenyl dichalcogenide in a variety of solvents.³ This approach simultaneously avoids hazardous solvents, eliminates unwanted sulfur and free tellurium impurities, and yields lead selenide and telluride precursors that have been problematic via other soluble precursor syntheses. We characterize the thermal decomposition temperature of the precursors using thermogravimetric analysis and confirm the formation of phase-pure metal chalcogenides using powder X-ray diffraction. We also characterize the chemical structure of a PbSe precursor made in this manner due to its potential thermoelectric applications. We identify the PbSe precursor to be lead(II) phenylselenolate by utilizing a combination of techniques including nuclear magnetic resonance spectroscopy, mass spectrometry, elemental analysis and single crystal X-ray diffraction analysis.

We further prepare PbSe_xTe_1-x thin films by decomposing a mixture of lead selenide and lead telluride precursors and present their room temperature Seebeck coefficient and electrical conductivity. In addition, we introduce Na dopants by including Na₂S into the precursor mixture and systematically study its influence on the thermoelectric properties of PbSe_xTe_1-x thin films. The film stoichiometry and electronic state of elements were characterized by Rutherford backscattering spectroscopy and X-ray photoelectron spectroscopy. We also use secondary-ion mass spectroscopy depth profiling to confirm the presence of Na-dopants in our thin films. The room temperature measurements seem to be promising with undoped PbSe_xTe_1-x films having Seebeck coefficient of 480 μV K^-1 and electrical conductivity of 180 S m^-1. Since PbSe_xTe_1-x is best suited for high-temperature applications, we also present our ongoing efforts to measure the thermoelectric properties of these samples at high temperature.

References:
1. Yuan, M. & Mitzi, D. B. Solvent properties of hydrazine in the preparation of metal chalcogenide bulk materials and films. Dalt. Trans. 6078–6088 (2009). doi:10.1039/b900617f
2. McCarthy, C. L. & Brutchey, R. L. Solution processing of chalcogenide materials using thiol-amine ‘alkahest’ solvent systems. Chem. Commun. 53, 4888–4902 (2017).
3. Wang, Z., Ma, Y., Vartak, P. B. & Wang, R. Y. Precursors for PbTe, PbSe, SnTe, and SnSe synthesized using diphenyl dichalcogenides. Chem. Commun. 54, 9055–9058 (2018).

Additive printing is a promising method to fabricate flexible and low-cost thermoelectric devices using high-performance colloidal nanocrystals. However, most of the TE nanoparticles requires sintering at elevated temperatures, which poses a major challenge to print and sinter the TE nanoparticles on low-temperature flexible substrates.

In this work, we demonstrate for the first time the aerosol jet printing combined with intense pulsed light (IPL) sintering to transform colloidal TE nanoparticles into high-performance and flexible thermoelectric films. This paves the way for roll-to-roll fabrication of printed TE generators with high power density on a large variety of low temperature substrates. The conventional thermal sintering process requires long heating time, and is limited to substrates that can survive high temperatures. The IPL sintering is an ultrafast thermal exposure sintering method using a xenon flash lamp to deliver a high-intensity and short-duration (< 2 ms) pulsed light to the printed nanoparticles. By controlling the pulse intensity and duration, the IPL sintering can effectively sinter the printed nanoparticles without overheating or damaging low-temperature substrates.

We printed n-type bismuth telluride based TE films on flexible substrates such as kapton and PET and sintered the films by IPL. The sintering parameters were optimized in order to achieve the highest electrical conductivity and maximum thermoelectric power factor. The electrical conductivity of the flexible films is dramatically improved from non-conductive to 2.7×10⁴ S/m, which results in a very high power factor of 726 μW/mK² at room temperature. The aerosol jet printing and IPL sintering process is fully roll-to-roll compatible, thus providing a highly-scalable and low-cost manufacturing method to fabricate flexible TE devices for broad energy harvesting and cooling applications.

The conversion efficiency of thermoelectric material is quantified by figure of merit ZT (=σS²T/κ) where σ is electrical conductivity, S is Seebeck coefficient and κ is thermal conductivity. CsSnI₃ perovskite can lead to the next-generation energy conversion platforms due to its intrinsic ultra-low thermal conductivity and large Seebeck coefficient. However, enhancement of electrical conductivity is still required.

In this work, we study the growth parameters and thermoelectric properties of coated CsSnI₃ perovskite thin film. Our focus is to develop these thin films by cost-effective wet printing process for large area application. Due to shapeability thin films are superior to the corresponding bulks or single crystals. Here, we fabricate CsSnI₃ perovskite thin film by spin coating process. The base solution is prepared by CsI and SnI₂ precursors. We optimized the effects of mixing time, baking temperature and baking time to evaluate and enhance thermoelectric performance of CsSnI₃ thin films. CsSnI₃ thin films were structural and chemical characterized using several state of the art techniques including XRD and SEM. We measured thermoelectric properties near room temperature and found that growth parameters influence the crystal grain size further the value of figure of merit. CsSnI₃ thin films grown at solvent mixing time about 2 h, heating temperature about 130°C and heating time 5 min show the best thermoelectric performance as Seebeck coefficient 100 μV/K, electrical conductivity130 S/cm, thermal conductivity 0.4 W/m.K, and ZT about 0.1 near room temperature

Further, we fabricated uni-leg thermoelectric modules on a glass substrate comprised by the best grown CsSnI₃ thin films. Number of legs varies from 3 to 5 in our study. The maximum output power is about 0.8 nW for 5 legs (25 mm x 3 mm x 500 nm) module for temperature difference of about 3°C. Even though output power is low but these results will open a new pathway to thermoelectric modules for flexible electronics. In future, our plan is to optimize the module design and transfer it to flexible and large area substrate.

Solid state thermoelectric coolers (TEC) made from nanostructured materials have received attention for Peltier cooling of localized hotspots in microprocessors (1, 2). Research in past efforts has focused on steady-state operation and cooling. Here, we discuss a concept for pulsed-mode transient heat spreading using planar thin-film TECs. Using detailed computational modelling, we show that it is possible to achieve a local cooling of ~10⁰C for a high heat flux of 1000 W/cm² by optimizing the TEC material, geometry and operating parameters. We experimentally investigate ultra-thin films of (BiSb)₂(SeTe)₃ system of compounds as a viable candidate for such operation. Typical room temperature electrical resistivity of these samples is around 300 μΩ-cm (3). Excellent electrical contacts can be made, as well, with typical values ranging between 10-8 and 10-9 W-cm2. We report measurements of the thermoelectric properties of few quint-layer Bi₂Se₃ on sapphire substrate to provide insight into alteration of properties at few-atomic layer scales. This work explores new ideas for TEC operation in handling transient temperature jumps in electronics.
1) Chowdhury, Ihtesham, et al. Nature nanotechnology 4.4 (2009): 235.
2) Fukutani, Kazuhiko, and Ali Shakouri. IEEE Transactions on Components and Packaging Technologies 29.4 (2006): 750-757.
3) D. Flottoto et al., Nano Letters, Articles ASAP, (2018).

Tin selenide (SnSe) has emerged as a very promising thermoelectric material due to its record high figure of merit ZT for both n- and p-type crystals in mediate temperature range. In this talk, I will present our recent works regarding its electronic and thermal properties based on first-principles calculations, in combination with the Boltzman transport theory and experimental measurements. The origins of strongly anisotropic thermal and electrical behaviors in this layered lattice structure will be discussed, which give rise to the highest optimal ZT values along the a axis (out-of plane direction) in the n-type SnSe, whereas along b axis in p-type materials. In addition, I will discuss the effects of the intrinsic defects and dopants on its thermoelectric characteristics. Our calculations show reasonable agreements with the experimental observations and provide some guidance for optimizing the thermoelectric performance.

There has recently been an increasing interest in self-powered wearable health and environmental monitoring systems. The Advanced Self-Powered Systems of Integrated Sensors and Technologies (ASSIST) at North Carolina State University, sponsored by National Science Foundation, is a prime example of the thrust for these systems. Thermoelectric generators (TEG) can continuously harness the body heat and produce electricity, as such they are the promising candidates for making battery-less wearable systems. Several challenges need to be addressed before TEGs can power such wearables. Particularly, the thermoelectric material properties must be optimized so that the material is most efficient near the body or room temperature. Bi-Sb-Te-Se nanocomposite alloys are known as the state-of-the-art thermoelectric materials for near room temperature applications; however, the existing materials have their peak zT in the temperature range of 100 ^oC to 150 ^oC with smaller values near the body temperature. These nanocomposites often also suffer from lack of reproducibility due to the difficulties in controlling the Te vacancy during the material synthesis. Metallization of these materials for making the TEGs poses other requirements such as durability to survive the regular usage. Another essential element imposed by the low-temperature differential on the body is related to making thin and long TE legs and low fill factor devices. The integration in textile or wearable bands also dictates certain design restrictions to make reliable devices for body heat harvesting. In this study, we will describe our methods to address some of these issues starting from the material synthesis, device design, and TEG fabrication to integration in wearable testbeds. An extensive investigation was performed on doping optimization and reproducibility of the nanocomposite bismuth telluride based alloys especially n-type materials which are more sensitive to growth process parameters. The effects of Cu doping for synthesizing a reproducible n-type material with high zT near room temperature was investigated. The peak zT of 1.2 at room temperature was achieved. Reproducibility of the material was confirmed through tight statistical analysis. For p-type material, Bi-Sb-Te nanocomposites were synthesized with peak zT of 1.4 near room temperature. Furthermore, physical and chemical processing methods were tested for surface preparation before metallization. Electroplating and sputtering of metal contacts on nanocomposites were tested and compared. A process flow was designed and optimized for dicing thin and long TE legs using a CNC wire saw. Finally, devices were bonded using a custom-made bonder and customized TEG header appropriate for wearable applications. The characteristics of the developed wearable TEGs will be discussed and evaluated for body heat harvesting applications.

With ~60% of all generated energy wasted as heat[1], energy harvesting is an important avenue of study to help provide for our worlds increased power demand in a sustainable way. Thermoelectrics (TEs) offer the unique opportunity to recover this previously wasted energy, and improve performance, via a direct conversion of heat to electricity in a solid state device. The key limitation of TEs is their poor power conversion efficiency, characterised by the dimensionless figure of merit, ZT. This value depends on both the electron and phonon transport characteristics, which themselves are heavily interdependent. For optimal TE behavior the electronic conductivity should be high whilst the thermal conductivity is kept to a minimum. However, in the majority of cases an increase in electrical conductivity is accompanied by an increase in thermal conductivity due to the Wiedemann-Franz Law[2], which links the thermal conductivity of electrons to the charge carrier density. It is this interdependence which has effectively limited the maximum ZT achievable to its current value of ~3[3]. Therefore, the ideal technique for raising ZT further would break this interdependence and optimise these characteristics independently. With this in mind, we investigate interfacial patterning as a method for controlling the transport properties of phonons. This method utilises the fundamental difference in electron and phonon wavelengths to selectively scatter phonons and hence reduce thermal conductivity, whilst having a minimal effect on electronic transport. Using density functional theory, we demonstrate the effectiveness of interfacial patterning on Si/Ge structures. We investigate three differently patterned interfaces, noting how subtle changes in the patterning can have dramatic effects on properties such as the effective mass and the lattice thermal conductivity. This patterning provides a technique to enhance ZT and a framework for producing more viable devices from any heterostructure-based TE materials.

References
1. Perspectives on thermoelectrics: from fundamentals to device applications, Energy & Environmental Science, 2012, 5, 5147
2. Ueber die Wärme-Leitungsfähigkeit der Metalle, Annalen der Physik, 1853, 165, 8, 497
3. Advances in thermoelectric materials research: Looking back and moving forward, Science, 2017, 357, 6358, 9997

While electrodeposited antimony telluride thin films with silver demonstrated promising thermoelectric properties, their thermal conductivity and the silver-content-dependence remain unknown. Here we report the thermal conductivity of Ag_3.9Sb_33.6Te_62.5 and AgSbTe₂ thin films with controlled annealing and temperature conditions and demonstrate the impact of silver content on thermal transport. By annealing the as-deposited samples at 160 °C, the room-temperature thermal conductivity of Ag_3.9Sb_33.6Te_62.5 and AgSbTe₂ films increases from 0.24 to 1.59 Wm^-1K^-1 and from 0.17 to 0.56 Wm^-1K^-1, respectively. When the pre-annealing temperature is below 110 °C, the thermal conductivities of the films are lower compared with the Sb₃₇Te₆₃ films (undoped) treated by identical annealing conditions. The thermal conductivity increases are attributed to the crystal growth, which are supported by the estimated grain size based on X-ray diffraction analysis. The thermal conductivity reduction of silver-doped films compared with the undoped films is attributed to the inhibited nucleation and crystallization of Sb₂Te₃ especially for films annealed below 110 °C, supported by the transient thermal conductivity measurement at 94 °C which reveals the crystallization activation energy to be 1.14 eV and 1.16 eV for Ag_3.9Sb_33.6Te_62.5 and AgSbTe₂ films, respectively. By increasing the pre-annealing temperature, we observe competing effects between the amorphous and crystalline phases in the silver-doped samples, confirmed by the temperature-dependent thermal conductivity measurement and the semi-empirical models combining different phonon scattering mechanisms. Based on previously reported electrical data, we estimate the room-temperature thermoelectric figure of merit of Ag_3.9Sb_33.6Te_62.5 and AgSbTe₂ to be 0.95 ± 0.35 and 0.71 ± 0.07. These results improve our understanding of the role of metal doping in thermal transport, which can guide optimal designs of chalcogenide materials for room-temperature thermoelectric applications.

Silicon-germanium (SiGe) thin films are low cost and low toxicity candidates for high temperature thermoelectric applications. Thin films of p-type boron doped amorphous SiGe are deposited on quartz substrates via Plasma Enhanced Chemical Vapor Deposition (PECVD). This plasma uses RF powers of 1-10 W with pressures of 220 - 400 mTorr. The substrate is heated to 250°C during the film growth, which improves the quality of the film by providing the atoms with sufficient mobility to find a more stable and lower defect configuration. Boron is used as the dopant with concentrations ranging from 0.5% to 15%. By 15 minutes deposition, thin films with thicknesses of 250 - 400 nm are produced. Post deposition, polycrystalline structure is obtained by thermal annealing of the samples at 650°C for 10 hours. Raman spectroscopy shows three peaks for Ge-Ge, Si-Ge, and Si-Si bonds in both amorphous and polycrystalline films with higher intensities for the latter. This confirms higher crystallinity of the polycrystalline films compared to the initial amorphous samples. The thermoelectric figure-of-merit (ZT) is calculated by the conductivity and thermopower measurements at room temperature. The dark conductivity and Seebeck coefficient are measured in a vacuum chamber and the thermal conductivity is obtained by the ultrafast laser pump-probe technique. The effect of varying doping level, RF plasma power, and Ge concentration on the power factor of thin films is studied. The characterization is performed at two points in the process, pre- and post-annealing. The power factor is highly dependent on the doping concentration in both cases with a peak at the mid-range doping concentration. This is mainly due the balance of the carrier density and carrier mobility in doped samples. Results also show a minimum of three orders of magnitude improvement of the thermoelectric figure-of-merit after annealing in all conditions.

This work is supported by the Minnesota Environment and Natural Resources Trust Fund (ML 2016, Chp. 186, Sec. 2, Subd. 07b).