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Germanium Chalcogenide Thermoelectrics: Electronic Structure Modulation and Low Lattice Thermal Conductivity
Abstract
Thermoelectric materials can convert untapped heat to electricity and expected to have an important role in future energy utilization. IV-VI metal chalcogenides are the most promising candidates for mid-temperature thermoelectric power generation. Among them, PbTe and their alloys have been proven to be the superior thermoelectric materials. Unfortunately, the toxicity of lead (Pb) prevents the application of lead chalcogenides and demands search for lead-free high-performance solids. This perspective discusses about the recent progress on thermoelectric property studies on germanium chalcogenides (GeTe, GeSe and GeS) for mid-temperature power generation. Here, we have discussed the crystal structure, chemical bonding and phonon dispersion of germanium chalcogenides to understand the underlying lattice dynamics and low lattice thermal conductivity from a chemistry perception. We have also discussed about the uniqueness of the electronic structure of GeTe and GeSe, which plays important role in tailoring thermoelectric properties. Additionally, the implications of the recent state-of-art strategies such as resonant level formation, valence band convergence, slight symmetry breaking of the crystal and electronic structures, point defect and nanostructure induced phonon scattering on the high thermoelectric performance of the germanium chalcogenides are discussed in details. In conclusion, we highlight some of the innovative ideas for discovery and design of new thermoelectric compositions. Finally, we point out the major challenges and opportunities in this field. All the strategies discussed in this perspective not only make germanium chalcogenides as a promising candidate for future thermoelectric applications but also serve as a guide to enhance the thermoelectric performance of other materials.
... At 364 K, the S value increases to ∼917 µVK −1 from ∼665 µVK −1 and during orthorhombic to hexagonal (β-α) phase transition, p-to n-type conduction is noted along with the change in thermopower, which is ∆S = 1757 µVK −1 . Further, at 367 K, thermopower decreases till −840 µVK −1 from 917 µVK −1 and then with an increase in temperature, it returns back to 30 µVK −1 and at 385 K, it become p-type conduction [125,126]. Similarly, IV-VI metal chalcogenides are mid-temperature thermoelectric materials. Among them germanium chalcogenides (GeX, X = Te, Se, S) are recently discussed for power generation in mid-temperature range [126]. ...
... Similarly, IV-VI metal chalcogenides are mid-temperature thermoelectric materials. Among them germanium chalcogenides (GeX, X = Te, Se, S) are recently discussed for power generation in mid-temperature range [126]. ...
The continuous depletion of fossil fuels and the increasing demand for eco-friendly and sustainable energy sources have prompted researchers to look for alternative energy sources. The loss of thermal energy in heat engines (100-350 ºC), coal-based thermal plants (150-700 ºC), heated water pumping in the geothermal process (150-700 ºC), and burning of petrol in the automobiles (150-250 ºC) in form of untapped waste-heat can be directly and/or reversibly converted into usable electricity by means of charge carriers (electrons or holes) as moving fluids using thermoelectric (TE) technology, which works based on typical Seebeck effect. The enhancement in TE conversion efficiency has been a key challenge because of the coupled relation between thermal and electrical transport of charge carriers in a given material. In this review, we have deliberated the physical concepts governing the materials to device performance as well as key challenges for enhancing the TE performance. Moreover, the role of crystal structure in the form of chemical bonding, crystal symmetry, order-disorder and phase transition on charge carrier transport in the material has been explored. Further, this review has also emphasized some insights on various approaches employed recently to improve the TE performance, such as, (i). carrier engineering via band engineering, low dimensional effects, and energy filtering effects and (ii). Phonon engineering via doping/alloying, nano-structuring, embedding secondary phases in the matrix and microstructural engineering. Wehave also briefed the importance of magnetic elements on thermoelectric properties of the selected materials and spin Seebeck effect. Furthermore, the design and fabrication of TE modules and their major challenges are also discussed. As, thermoelectric figure of merit, zT does not have any theoretical limitation, an ideal high performance thermoelectric device should consist of low-cost, eco-friendly, efficient, n- or p-type materials that operate at wide- temperature range and similar coefficients of thermal expansion, suitable contact materials, less electrical/ thermal losses and constant source of thermal energy. Overall, this review provides the recent physical concepts adopted and fabrication procedures of TE materials and device so as to improve the fundamental understanding and to develop a promising TE device.
... Transition metal chalcogenides have been synthesized over the past years due to their vast application in various fields. Many researchers have published on the synthesis of metal chalcogenides due to their high usage in multiple applications such as Thermoelectric, [12] magnetic semiconductors, [13,14] photovoltaics, [15] and sensors. [16,17] Out of these metal chalcogenides family, metal tellurides are of special interest due to their interesting application in thermoelectric, [18] chemotherapy, [19] optoelectronic devices, [20] etc. ...
Copper telluride nanoparticles are classified as the family of metal chalcogenides and were synthesized using a simple solvothermal method using a new source of telluride, ditelluride diphenyl. The characterized techniques revealed that the atomic composition of copper tellurides is Cu2Te and particles are spherical. This study is greatly examined by different designed experiments for the proof of the intrinsic peroxidase‐like activity of copper telluride nanoparticles and its effectiveness in a variety of similar structures. Cu2Te nanoparticles can convert peroxidase substrates (TMB and OPD) to colored products in the presence of H2O2. In addition, the as‐prepared nanoparticles have the potential to oxidize DPPH and ABTS substrates. Michaelis–Menten constant KM value for TMB and H2O2 was found to be 12 mM and 0.60 mM for H2O2 and the Vmax value of the Cu2Te was found to be TMB and H2O2 was 2.50×10⁻⁷ and 0.82×10⁻⁸ MS⁻¹. These data are found to be almost comparable with natural peroxidase HRP and other artificial enzymes reported in the literature. The natural peroxidase‐like activities of Cu2Te nanoparticles originated from the catalytic decomposition of H2O2 into ⋅OH radical, which was confirmed from the ESR spin‐trapping technique and using a fluorescence probe, terephthalic acid.
... In the quest of enhanced ferroelectric properties, intersections of domain walls are expected to show highly polarized regions with complex polarization textures resulting from a strong relaxation of the lattice. Among ferroelectrics, GeTe has witnessed a sustained boom [9][10][11][12][13][14]. As a thermoelectrics, it has recently been demonstrated a record figure of merit (zT ∼ 2.4) at 330 • C for the ferroelectric GeTe phase [15]. ...
Ferroelectric germanium telluride is under active consideration for spintronic and thermoelectric applications. The control of the ferroelectric domain walls is a key issue to optimize the electronic and thermal properties of GeTe thin films. Domain walls properties are usually driven by the mechanical and electrostatic compatibility conditions of twin domains. However, in dense ferroelectric domain structures these compatibility conditions are hardly fulfilled everywhere. In particular intersection of domains may result in complex lattice relaxations and polarization textures. In this study, we have fabricated GeTe thin films on silicon substrate and elucidated the intersections of a-type domains using 3D reciprocal space maps, scanning tunneling microscopy, and second-harmonic microscopy. We demonstrate the presence of complex structural reorganizations that manifest by the formation of charged domain walls, large lattice rotations, and enhanced stretching of the rhombohedral lattice.
... Significant effort has gone into trying to accurately calculate its properties [144,192,[226][227][228][229][230], both in order to optimize its performance, but also in the hopes that understanding its fundamental properties will allow for predictions of additional high-performance thermoelectric materials. In that vein, germanium selenide (GeSe) is an obvious isostructural material that has been the focus of only a handful of experimental [231,232] and theoretical [145,[233][234][235] studies. The crystal structure of GeSe (SnSe) is shown in Fig. 3, along with the basic electronic and phononic band structures calculated using DFT. ...
Thermoelectrics are a promising class of materials for renewable energy owing to their capability to generate electricity from waste heat, with their performance being governed by a competition between charge and thermal transport. A detailed understanding of energy transport at the nanoscale is thus of paramount importance for developing efficient thermoelectrics. Here, we provide a comprehensive overview of the methodologies adopted for the computational design and optimization of thermoelectric materials from first-principles calculations. First, we introduce density-functional theory, the fundamental tool to describe the electronic and vibrational properties of solids. Next, we review charge and thermal transport in the semiclassical framework of the Boltzmann transport equation, with a particular emphasis on the various scattering mechanisms between phonons, electrons, and impurities. Finally, we illustrate how these approaches can be deployed in determining the figure of merit of tin and germanium selenides, an emerging family of layered thermoelectrics that exhibits a promising figure of merit. Overall, this review article offers practical guidelines to achieve an accurate assessment of the thermoelectric properties of materials by means of computer simulations.
... zT increases with an increase in temperature for all the samples. A maximum zT of 1.09 is obtained for Ge 1.01 Te at 673 K, showing that vacancy engineering in GeTe itself improves its TE performance [57]. Further, zT increases to 1.21 at 723 K in Ti-doped samples, consistent with the results of previous studies [30]; however, a significant increase in zT is observed in the Ti-Bi codoped sample. ...
This study shows a method of enhancing the thermoelectric properties of GeTe-based materials through Ti and Bi codoping on cation sites along with self-doping of Ge via simultaneous optimization of electronic (via crystal field engineering and precise Fermi level optimization) and thermal (via point-defect scattering) transport properties. Pristine GeTe has high carrier concentration n due to intrinsic Ge vacancies, a low Seebeck coefficient α, and high thermal conductivity κ. The Ge vacancy optimization and crystal field engineering result in an enhanced α via excess Ge and Ti doping, which is further improved by band structure engineering through Bi doping. As a result of improved α and the optimized Fermi level (carrier concentration), an enhanced power factor α2σ is obtained for Ti-Bi codoped Ge1.01Te. These experimental results are also evidenced by theoretical calculations of band structure and thermoelectric parameters using density functional theory and boltztrap calculations. A significant reduction in the phonon thermal conductivity κph from ∼3.5 to ∼1.06 W m−1 K−1 at 300 K for Ti-Bi codoping in GeTe is attributed to point-defect scattering due to mass and strain field fluctuations. This decrease in κph is in line with the Debye-Callaway model. Also, the phonon dispersion calculations show a decreasing group velocity in Ti-Bi co-doped GeTe, supporting the obtained reduced κph. The strategies used in the present study significantly increase the effective mass, optimize the carrier concentration, and decrease phonon thermal conductivity while achieving an impressive maximum zT value of 1.75 at 773 K and an average zT of 1.03 for Ge0.91Ti0.02Bi0.08Te over a temperature range of 300–773 K.
... 6,[18][19][20] Furthermore, alloying of different chalcogenides has also been pursued to control the physical properties of this material system. 10,21,22 In theory, group-IV monochalcogenides are expected to have various polymorphs owing to the versatile bonding configurations in this family of materials. [23][24][25][26][27][28] Despite the theories, there has been a lack of experiments involving various polymorphic configurations for GeS and GeTe in which only one type of polymorph is known to exist at room temperature. ...
Group-IV monochalcogenides have recently shown great potential for their thermoelectric, ferroelectric, and other intriguing properties. The electrical properties of group-IV monochalcogenides exhibit a strong dependence on the chalcogen type. For example, GeTe exhibits high doping concentration, whereas S/Se-based chalcogenides are semiconductors with sizable bandgaps. Here, we investigate the electrical and thermoelectric properties of gamma-GeSe, a recently identified polymorph of GeSe. gamma-GeSe exhibits high electrical conductivity (~106 S/m) and a relatively low Seebeck coefficient (9.4 uV/K at room temperature) owing to its high p-doping level (5x1021 cm-3), which is in stark contrast to other known GeSe polymorphs. Elemental analysis and first-principles calculations confirm that the abundant formation of Ge vacancies leads to the high p-doping concentration. The magnetoresistance measurements also reveal weak-antilocalization because of spin-orbit coupling in the crystal. Our results demonstrate that gamma-GeSe is a unique polymorph in which the modified local bonding configuration leads to substantially different physical properties.
... Significant effort has gone into trying to accurately calculate its properties [133,181,[215][216][217][218][219], both in order to optimize its performance, but also in the hopes that understanding its fundamental properties will allow for predictions of additional high-performance thermoelectric materials. In that vein, germanium selenide (GeSe) is an obvious isostructural material that has been the focus of only a handful of experimental [220,221] and theoretical [134,[222][223][224] studies. The crystal structure of GeSe (SnSe) is shown in Fig. 3 calculating thermoelectric performance. ...
Thermoelectrics are a promising class of materials for renewable energy owing to their capability to generate electricity from waste heat, with their performance being governed by a competition between charge and thermal transport. A detailed understanding of energy transport at the nanoscale is thus of paramount importance for developing efficient thermoelectrics. Here, we provide a comprehensive overview of the methodologies adopted for the computational design and optimization of thermoelectric materials from first-principles calculations. First, we introduce density-functional theory, the fundamental tool to describe the electronic and vibrational properties of solids. Next, we review charge and thermal transport in the semiclassical framework of the Boltzmann transport equation, with a particular emphasis on the various scattering mechanisms between phonons, electrons, and impurities. Finally, we illustrate how these approaches can be deployed in determining the figure of merit of tin and germanium selenides, an emerging family of layered thermoelectrics that exhibits a promising figure of merit. Overall, this review article offers practical guidelines to achieve an accurate assessment of the thermoelectric properties of materials by means of computer simulations.
... A maximum zT of 1.09 is obtained for Ge 1.01 Te at 673 K, depicting that vacancy engineering in GeTe itself improves its TE performance. [56] Further, zT increases to 1.21 at 723 K in Ti doped samples consistent with the previous studies; [30] however a significant increase in zT is observed in Ti-Bi co-doped sample. It is attributed to the simultaneous optimization of band structure, carrier concentration, and phonon thermal conductivity. ...
This study shows a method of enhancing the thermoelectric properties of GeTe-based materials by Ti and Bi co-doping on cation sites along with self-doping with Ge via simultaneous optimization of electronic (via crystal field engineering, and precise Fermi level optimization) and thermal (via point-defect scattering) transport properties. The pristine GeTe possesses high carrier concentration ($n$) due to intrinsic Ge vacancies, low Seebeck coefficient ($\alpha$), and high thermal conductivity ($\kappa$). The Ge vacancy optimization and crystal field engineering results in an enhanced $\alpha$ via excess Ge and Ti doping, which is further improved by band structure engineering through Bi doping. As a result of improved $\alpha$ and optimized Fermi level (carrier concentration), an enhanced power factor ($\alpha^2\sigma$) is obtained for Ti--Bi co-doped Ge$_{1.01}$Te. These experimental results are also evidenced by theoretical calculations of band structure, and thermoelectric parameters using density functional theory and Boltztrap calculations, respectively. A significant reduction in the phonon thermal conductivity ($\kappa_{ph}$) from $\sim$ 3.5 W.m$^{-1}$.K$^{-1}$ to $\sim$ 1.06 W.m$^{-1}$.K$^{-1}$ at 300\,K for Ti--Bi co-doping in GeTe, attributed to point-defect scattering due to mass and strain field fluctuation, in line with the Debye-Callaway model. The phonon dispersion calculations show a decreasing group velocity in Ti--Bi co-doped GeTe, supporting the obtained reduced $\kappa_{ph}$. The strategies used in the present study can significantly increase the effective mass, optimize the carrier concentration, and decrease phonon thermal conductivity while achieving an impressive maximum zT value of 1.75 at 773\,K and average zT (zT$_{av}$) of 1.03 for Ge$_{0.91}$Ti$_{0.02}$Bi$_{0.08}$Te over a temperature range of 300-773\,K.
... GeTe, one of analogs of high-performance PbTe and SnSe, has been extensively studied in recent years. GeTe polycrystalline materials do not contain toxic elements compared with PbTe and has higher average thermoelectric figure of merit compared to SnSe, leading to a great application potential [48,53,62,69]. Pristine GeTe fabricated by traditional melting plus sintering method possesses a lot of intrinsic Ge vacancies, which not only leads to the hole concentration close to 10 21 cm − 3 (which exceeds the optimal carrier concentration n opt of ~ 10 20 cm − 3 ) [70,71], but also severely reduces carrier mobility. ...
Numerous intrinsic Ge vacancies in thermoelectric GeTe not only lead to overhigh carrier concentration but also seriously deteriorate carrier mobility, which shackles its thermoelectric performance. The efficient strategy and the related underlying mechanism in suppressing intrinsic Ge vacancy, however, are rarely researched yet. Herein, we demonstrated that lattice strain could be employed to regulate the defects concentration and then optimize electrical transport performance. Theoretically, the calculated results showed that lattice strain could efficiently raise the formation energy of Ge vacancies, weakening the carrier scattering and improving the carrier mobility. Calculated band structure revealed that Sb doping and Ge vacancy introduction could promote band convergence and thus efficiently decouple the electrical transport parameters. Experimentally, lattice strain was constructed through high-energy ball milling combined with spark plasma sintering to reduce the concentration of Ge vacancy and relaxation time of phonons, leading to high carrier mobility and low lattice thermal conductivity. Additionally, we carefully modulated the nominal content of Ge, and then a high ZT of 2.0 at 723 K in Ge0.90Sb0.08Te alloy was obtained. This work highlights that the lattice strain can be utilized to simultaneously optimize thermal and electrical transport properties.
... Heavy metal-based chalcogenides where metals are mainly Pb, Sn, Ge, Bi, Sb, Tl and In are extensively studied for thermoelectric application due to their low thermal conductivity and high electrical mobility. [34][35][36] Understanding the crystal structure as well as the nature of chemical bonding is required to comprehend lattice dynamics and other associated aspects. ...
Metal Chalcogenides (MCs) are unique class of compounds which have vast compositional variety as well as diverse crystal structures. The presence of diverse crystal structures along with the remarkable physical and chemical properties make them emerging candidates for various applications such as optoelectronic devices, lithium-ion batteries, water purification, non-linear optics, superconductivity, thermoelectrics, and many more. In this book chapter, we have discussed lucidly the crystal structure of various MCs and its correlation with different intriguing properties. Recent progress of various synthesis techniques for the preparation of different crystals ranging from single crystal, polycrystal to nanocrystals of several MCs are discussed here. Moreover, we have majorly focused on the latest advancements of MCs in the field of superconductivity, topological quantum materials, thermoelectrics, non-linear optics and water purification. MC based thermoelectric materials with high thermoelectric efficiency and topological properties have been attracted wide attention to the scientific community due to their nontrivial electronic surface states. We have also explored some MCs in view of their non-linear optical properties and several non-toxic metal sulfides for their use in water purification. These structural and physical properties discussed in this chapter should serve as a general guide to rationally design and predict materials for various fields of applications.
... Further, to reduce the κ L , the following strategies such as nanostructuring [10], hierarchical architecturing [11] and nano inclusions [12] were successfully implemented. The high-thermoelectric figure of merit mainly depends on two factors as follows, high Seebeck coefficient from a multiband electronic structure and energy filtering effect, low thermal conductivity through nanostructuring [13]. ...
Tin selenide (SnSe), an important thermoelectric material in the IV-VI chalcogenide family, has attracted significant attention for Thermoelectric power generation in the mid to high temperature (600 K – 900 K) region. Herein, we report extremely low thermal conductivity and improved thermoelectric performance of polycrystalline SnSe through Sb substitution. Polycrystalline Sn1−xSbxSe (x = 0.01 to 0.05) was synthesized through a hydrothermal method followed through cold pressing technique. The existence of lone pair electrons induced the high lattice anharmonicity in Sb substituted SnSe, which reduced the thermal conductivity from enhanced the phonon scattering. Increasing Sb concentration drastically decreases the thermal conductivity to 0.23 W/mK at 640 K. Moreover, Sb substitution simultaneously enhanced the Seebeck coefficient 400 µV/K and the electrical conductivity value of 660 S/m at 550 K for X=0.05 sample. It is observed that the aliovalent Sb substitution in the polycrystalline SnSe matrix exhibits an extreme reduction in thermal conductivity, which leads to high thermoelectric performance.
... Metal-based semi-conducting chalcogenide thin films have showed considerable promise in the production of optoelectronic devices, solar cells, thermoelectric, and sensor applications in recent years [1][2][3][4]. The physical, chemical, and optoelectronic features of sandwiched metal-based chalcogenide-related materials have been best revealed lately, including excellent transparency, carrier mobility, superior pliability, and high specific surface area. ...
The current research explores the outcome of europium doping on the structure, morphology, electrical conductivity and optoelectronic characteristics of SnS2 thin films deposited on glass substrates by nebulizer spray technique. X-ray diffraction analysis substantiates the presence of hexagonal structure in both pure and Eu-doped SnS2 thin films with the highly preferred orientation diffracted from the plane (002). It shows the intensity of the predominant peak is highest for 2% Eu-doped SnS2 thin film. From the XRD data, the crystallite size was found to be highest for 2% SnS2:Eu, and same sample showed a less dislocation density of 0.97 × 10¹⁵ lines/m² and microstrain of 0.081 × 10–3/lines² m⁴. The crystallite size first increases with increasing doping concentration of Eu (0–2%), then decreases for higher concentrations. The SEM and AFM micro images reveal the agglomeration of grains at higher Eu concentration. The compositional analysis through EDAX studies supports the presence of Eu, Sn and S. The SnS2 optical band gap value is found to vary from 2.70 to 2.91 eV as the Eu doping is increased from 2 to 6%. All the SnS2 thin film samples manifest a n-type conductivity as authenticated from Hall studies and a low resistivity of 4.34 × 10⁻¹ cm with an elevated carrier concentration of 5.43 × 10¹⁷ cm⁻³, respectively, was observed for SnS2:Eu (2 wt%). The same sample established a higher responsivity (41.64 × 10⁻³ AW⁻¹), competent external quantum efficiency (97.24%), and a better detectivity (40.88 × 10⁸ Jones). Hence, the 2% Eu-doped SnS2 film is recognized to be best suited for the fabrication of high-speed optoelectronic devices. This paper also discusses a putative mechanism for photo-detector performance under air and UV radiation.
... Till now, many efforts have been devoted to the Bi 2 Te 3 [7] for near-room-temperature refrigeration and half-Heusler, [8] skutterudites, [9] group IV-VI chalcogenides [10] for power generation at elevated temperatures. The semiconducting group IV-VI chalcogenides, especially GeTe, SnTe, and PbTe, show excellent thermoelectric performance in the midtemperature range (500-800 K). [11,12] Compared with PbTe, the toxic-elements-free GeTe-based thermoelectric materials are environmentally friendly. Also, GeTe shows a higher thermoelectric performance than that of SnTe because of the favorable band structure. ...
It is known that p-type GeTe-based materials show excellent thermoelectric performance due to the favorable electronic band structure. However, n-type doping in GeTe is of challenge owing to the native Ge vacancies and high hole concentration of about 10 ²¹ cm ⁻³ . In the present work, the formation energy of cation vacancies of GeTe is increased through alloying PbSe, and further Bi-doping enables the change of carrier conduction from p-type to n-type. As a result, the n-type thermoelectric performance is obtained in GeTe-based materials. A peak zT of 0.34 at 525 K is obtained for (Ge 0.6 Pb 0.4 ) 0.88 Bi 0.12 Te 0.6 Se 0.4 . These results highlight the realization of n-type doping in GeTe and pave the way for further optimization of the thermoelectric performance of n-type GeTe.
... In contrast to GeS, GeTe is widely used as a base material for phase-change materials 11,12 and thermoelectric materials. 13,14 In phase-change materials used as media or memory, a long and short pulsed light or electric current causes a phase-change between crystalline and amorphous, and information is recorded using the difference in reflectivity or electrical resistivity. However, the recent discovery of GeCu 2 Te 3 with higher performance, lower amorphization temperature, and higher crystallization temperature has resulted in revised physical properties of GeTe-based phase-change materials. ...
Hydrogen defects sometimes form shallow impurity levels in semiconductors, and it is an important topic for semiconductor research to investigate their details. One of the experimental methods to determine the state of hydrogen is the muon spin rotation (μSR) experiment. By observing formation of a pseudo-hydrogen atom, called muonium, it is possible to investigate the hydrogen defect levels. In a previous theoretical study, the pinning levels were calculated for various materials as a reference for hydrogen defect levels, and these levels were universally distributed near the hydrogen electrode potential. Based on the prediction, μSR experiments were performed for germanium sulfide (GeS) and germanium telluride (GeTe), where the hydrogen electrode potential is located in the bandgap for GeS, but not for GeTe. As a result, the μSR spectra showed that the muonium forms in GeS, while it does not in GeTe. In GeS, 58% of the muons formed muoniums. The activation energy was obtained as ΔE=26.2±6.9 meV. The hyperfine coupling frequency was ωc(2π)−1=1.95±0.17 GHz, and the Bohr radius of muonium was 1.3 times larger than that in vacuum. These properties indicated that the identified muonium does not form a typical impurity level that affects the electrical properties.
In thermoelectrics, the manipulation of crystal symmetry is instrumental in optimizing the electrical and thermal transport parameters. Within this context, the present study explored the largely overlooked high-symmetry cubic GeSe, which presented larger band degeneracy than its widely studied medium-symmetry rhombohedral counterpart. We have successfully stabilized cubic GeSe at ambient conditions through co-alloying with AgSnTe2 and Bi. The incorporation of AgSnTe2 initiates the transition of GeSe from a low-symmetry orthorhombic to a medium-symmetry rhombohedral phase, culminating in a high-symmetry cubic structure, underpinned by variation in chemical bonding mechanisms. Notwithstanding this, the persistence of Ag2Te precipitates impedes the total elimination of the residual orthorhombic phase due to the disparate chemical bonding mechanism between Ag2Te and GeSe. Introducing Bi into the rhombohedral-dominated (GeSe)0.7(AgSnTe2)0.3 matrix leads to the dissolution of Ag2Te precipitates, elimination of the residual orthorhombic phase, and the subsequent stabilization of the exclusive cubic phase. Compared to its orthorhombic counterpart, the cubic GeSe exhibits diminished bandgap and Ge vacancy formation energy, amplified band degeneracy, reduced sound velocity, intensified lattice anharmonicity and multiple phonon scattering centres, engendering elevated carrier concentration and density-of-states effective mass, alongside restrained lattice thermal conductivity. Consequently, a peak zT of 0.46 at 573 K is attained for cubic (Ge0.7Bi0.3Se)0.7(AgSnTe2)0.3, signifying a ninefold increase relative to the initial orthorhombic GeSe. These results illuminate the critical role of crystal symmetry manipulation in advancing the thermoelectric performance.
In this work, the experimental evidence of glass‐like phonon dynamics and thermal conductivity in a nanocomposite made of GeTe and amorphous carbon is reported, which is of interest for microelectronics, and specifically phase change memories. It is shown that, the total thermal conductivity is reduced by a factor of three at room temperature with respect to pure GeTe, due to the reduction of both electronic and phononic contributions. This latter, similarly to glasses, is small and weakly increasing with temperature between 100 and 300 K, indicating a mostly diffusive thermal transport and reaching a value of 0.86(7) Wm⁻¹K⁻¹ at room temperature. A thorough investigation of the nanocomposite's phonon dynamics reveals the appearance of an excess intensity in the low energy vibrational density of states, reminiscent of the Boson peak in glasses. These features can be understood in terms of an enhanced phonon scattering at the interfaces, due to the presence of elastic heterogeneities, at wavelengths in the 2–20 nm range. The findings confirm recent simulation results on crystalline/amorphous nanocomposites and open new perspectives in phonon and thermal engineering through the direct manipulation of elastic heterogeneities.
Driven by the burgeoning demand for high performance eco-friendly thermoelectric materials in the mid-temperature range (573–773 K), we herein focus on GeTe based alloys that exhibit a high thermoelectric figure-of-merit >2.0 owing to their promising band structure with a conversion efficiency of ∼13.3% for the segmented module and ∼14% for the single leg device. However, the drawback is the phase transition at ∼700 K, which is inevitable, and the irony is that practical application of thermoelectrics has no space for phase transition, as it can be detrimental owing to the unexpected change resulting in deterioration of the devices under operation, thus limiting their mass-market applications either as power generators or as refrigerators. Nevertheless, a comprehensive review is needed for finding ways to cease or abase the phase transformation in the operating temperature regime. In this regard, the most recently developed GeTe-based thermoelectric materials are reviewed coupled with the blooming paradigm, i.e., entropy engineering. The review article thus summarizes the concept of entropy engineering owing to its contribution to boosting thermoelectric performance by increasing the configurational entropy of the system and cites several case studies. Towards the end, future scope and directions were proposed to use entropy tailored GeTe based alloys for the development of high efficiency power generators.
Group IV metal chalcogenides (group IV MCs) have garnered significant attention from scientific fraternity owing
to their distinct structural features and interesting electronic properties that could be exploited for diverse applications
including energy conversion and storage, optoelectronic devices and sensors. However, to make group
IV MCs commercially viable, it is imperative to evolve efficient, cost effective and scalable methods for their
synthesis. Single source molecular precursor (SSP) mediated synthesis of group IV MCs is one such route which
enjoys tremendous advantages over conventional solid state or dual precursor routes. Most importantly, SSP
serve as a viable tool to afford phase pure group IV MCs with high reproducibility and excellent control over size
and morphology in presence of suitable capping agents. This timely review provides a comprehensive overview
of SSPs employed for accessing group IV MCs nanomaterials as well as thin films. Effects of various ligands on
SSP performance have been rationalized. Additionally, precursors which can afford selective synthesis of
different compositions or phase of group IV MCs by the choice of solvent, temperature and mode of decomposition
have been critically assessed. Strength, limitations and opportunities associated with SSP approach are
critically evaluated to provide directions for the development of new SSPs. Furthermore, the role of capping
agents and fundamentals of viable synthetic strategies in general, for materials synthesis and deposition of thin
film have been summarised in this account. Finally, the conclusion and future prospects of SSPs have also been
included in this review. It is expected that this review will provide library of precursors and optimized conditions
to synthesize group IV MCs and further catch the attention of researchers to explore the SSP mediated route in
making size- and shape-tunable nanostructures with improved functionalities.
Several affordable and pollution-free technologies have drawn a lot of attention because of the pressure of our energy needs and environmental problems; among these, thermoelectric technology has made enormous advances. It has been known that thermoelectric materials are efficient in transforming waste heat energy into electricity. The efficiency of thermoelectric materials is typically assessed using the ZT value, ZT = S²T/ρκ. Several methods have been highlighted in the literature for improving thermoelectric figure of merit. This review stands out for its particular emphasis on cutting-edge techniques that are leading to a new era of thermoelectric innovation, including doping, co-doping, alloying, nanostructuring, and nanocompositing. Our focus is on mid-temperature range thermoelectric materials, which operate between 500 and 900 K and have enormous potential for high-efficiency thermoelectricity and waste heat recovery due to their inherent thermal and electrical properties. This review provides a foundational understanding of thermoelectric concepts as well as obstacles to improving the figure of merit and the various classes of mid-temperature range thermoelectric materials, including their structure and thermoelectric characteristics are discussed in brief. Additionally, it also discusses different methods described in the various literature regarding enhancing performance as well as recent advancements made in this area and this article emphasizes the relevance and importance of these developments in the context of urgent global energy challenges and highlights the crucial role that mid-temperature range thermoelectric materials will play in determining the future landscape of sustainable energy sources. To satisfy the practical demand, scientific research in the field of thermoelectricity still needs to be intensified, for this mid-temperature range, Chalcogenide-based thermoelectric materials play a very important role in the future.
Graphical abstract
Copper selenide nanostructures have attracted growing attention due to their potential application in cost effective and sustainable solar cells and photocatalysis. This account describes the facile conversion of new one...
GeTe has recently attracted wide attentions as a promising mid-temperature thermoelectric candidate, but the poor performance severely limits its practical application. Herein, we demonstrate a significantly improved thermoelectric properties of...
GeTe-based materials have attracted significant attention as high-efficiency thermoelectric materials for mid-temperature applications. However, GeTe thin-film materials with thermoelectric performances comparable to that of their bulk counterparts have not yet been reported, because of their unsatisfactory electrical and thermal properties caused by their poor crystal quality and high carrier concentration. Herein, a series of Sb-doped GeTe films and devices with remarkable thermoelectric performances are presented. These films are prepared through magnetron sputtering deposition at 553 K and exhibit a unique microstructure that consists of coarse- and fine-sized grains with high crystallization quality. The fine grains enhance the scattering associated with phonon transport and the coarse grains provide electron transport channels, which can suppress the thermal conductivity without obviously sacrificing the electrical conductivity. Moreover, Sb doping can effectively optimize the carrier concentration and increase the carrier effective mass, while introducing point defects and stacking faults to further scatter the phonon transport and decrease the thermal conductivity. Consequently, a peak power factor of 22.37 μW cm−1 K−2 is obtained at 703 K and a maximum thermoelectric figure of merit of 1.53 is achieved at 673 K, which are substantially larger than the values reported in the existing literature. A flexible thermoelectric generator is designed and fabricated using Sb-doped GeTe films deposited on polyimide and achieves a maximum output power density of 2.22 × 103 W m−2 for a temperature difference of 300 K.
Thermoelectric technology enables the direct interconversion between heat and electricity. SnSe has received increasing interest as a new promising thermoelectric compound due to its exceptionally high performance reported in crystals. SnSe possesses intrinsic low thermal conductivity as a congenital advantage for thermoelectric, but high thermoelectric performance can be hardly achieved due to the difficulty to realize efficient doping to raise its low carrier concentration to an optimal level. In this work, it is found that a series of rare earth elements are effective dopants for SnSe, which can greatly improve the electrical transport properties of p‐type polycrystalline SnSe. In particular, the remarkable enhancement in electrical conductivity and power factor is achieved by Na/Er co‐doping at 873 K. The lattice thermal conductivity is reduced due to the presence of abundant defects (dislocations, stacking faults, and twin boundaries). Consequently, a peak thermoelectric figure of merit ZT (2.1) as well as a high average ZT (0.77) are achieved in polycrystalline SnSe.
Group IV monochalcogenides have recently shown great potential for their thermoelectric, ferroelectric, and other intriguing properties. The electrical properties of group IV monochalcogenides exhibit a strong dependence on the chalcogen type. For example, GeTe exhibits high doping concentration, whereas S/Se-based chalcogenides are semiconductors with sizable bandgaps. Here, we investigate the electrical and thermoelectric properties of γ-GeSe, a recently identified polymorph of GeSe. γ-GeSe exhibits high electrical conductivity (∼106 S/m) and a relatively low Seebeck coefficient (9.4 μV/K at room temperature) owing to its high p-doping level (5 × 1021 cm-3), which is in stark contrast to other known GeSe polymorphs. Elemental analysis and first-principles calculations confirm that the abundant formation of Ge vacancies leads to the high p-doping concentration. The magnetoresistance measurements also reveal weak antilocalization because of spin-orbit coupling in the crystal. Our results demonstrate that γ-GeSe is a unique polymorph in which the modified local bonding configuration leads to substantially different physical properties.
The emerged strategy of manipulating the rhombohedral crystal structure provides another new degree of freedom for optimizing the thermoelectric properties of GeTe-based compounds. However, the concept is difficult to be effectively measured and often depends on heavy doping that scatters carriers severely. Herein, we synergistically manipulate lattice distortion and vacancy concentration to promote the excellent electrical transport of GeTe-Cu2Te alloys and quantify the interaxial angle-dependent density of state effective mass. Distinct from the conventional electronic coupling effect, about 2% substitution of Zr4+ significantly increases the interaxial angle, thereby enhancing the band convergence effect and improving the Seebeck coefficient. In addition, Ge-compensation attenuates the mobility deterioration, leading to improved power factor over the whole temperature range, especially exceeding ∼22 μW cm-1 K-2 at 300 K. Furthermore, the Debye-Callaway model elucidates low lattice thermal conductivity due to strong phonon scattering from Zr/Ge substitutional defects. As a result, the highest figure of merit zT of ∼1.6 (at 650 K) and average zTave of ∼0.9 (300-750 K) are obtained in (Ge1.01Zr0.02Te)0.985(Cu2Te)0.015. This work demonstrates the effective band modulation of Zr on GeTe-based materials, indicating that the modification of the interaxial angle is a deep pathway to improve thermoelectrics.
Germanium telluride (GeTe) is one of the most fascinating inorganic compounds in thermoelectrics due to its intriguing chemical bonding, crystal and electronic structure. However, thermoelectric performance of pristine GeTe is...
Low-dimensional group IV-VI metal chalcogenide-based semiconductors hold great promise for opto-electronic device applications owing to their diverse crystalline phases and intriguing properties related to thermoelectric and ferroelectric effects. Herein, we demonstrate a universal chemical vapor deposition (CVD) growth method to synthesize stable germanium chalcogenide-based (GeS, GeS2, GeSe, GeSe2) nanosheets, which increases the library of the p-type semiconductor. The phase transition between different crystalline polytypes can be deterministically controlled by hydrogen concentration in the reaction chamber. Structural characterization and synthesis experiments identify the behavior, where the higher hydrogen concentration promotes the transiton from germanium dichalcogenides to germanium monochalcogenides. The angle-polarized and temperature-dependent Raman spectra demonstrate the strong interlayer coupling and lattice orientation. Based on the optimized growth scheme and systematic comparison of electrical properties, GeSe nanosheet photodetectors were demonstrated, which exhibit superior device performance on SiO2/Si and HfO2/Si substrate with a high photoresponsivity up to 104 A W-1, fast response time less than 15 ms, and high mobility of 3.2 cm2 V-1 s-1, which is comparable to the mechanically exfoliated crystals. Our results manifest the hydrogen-mediated deposition strategy as a facile control knob to engineer crystalline phases of germanium chalcogenides for high performance optoelectronic devices.
The good co‐existence of midgap state and valence band degeneracy is realized in Bi‐alloyed GeTe through the In‐Cd codoping to play different but complementary roles in the valence band structure modification. In doping induces midgap state and results in a considerably improved Seebeck coefficient near room temperature, while Cd doping significantly increases the Seebeck coefficient in the mid‐high temperature region by promoting the valence band convergence. The synergistic effects obviously increase the density of state effective mass from 1.39 to 2.65 m0, and the corresponding carrier mobility still reaches 34.3 cm² V⁻¹ s⁻¹ at room temperature. Moreover, the Bi‐In‐Cd co‐alloying introduces various phonon scattering centers including nanoprecipitates and strain field fluctuations and suppresses the lattice thermal conductivity to a rather low value of 0.56 W m⁻¹ K⁻¹ at 600 K. As a result, the Ge0.89Bi0.06In0.01Cd0.04Te sample obtains excellent thermoelectric properties of zTmax ≈2.12 at 650 K and zTavg ≈1.43 between 300 and 773 K. This study illustrates that the thermoelectric performance of GeTe can be optimized in a wide temperature range through the synergy of midgap state and valence band convergence.
GeTe is among the most fascinating inorganic compounds for thermoelectric (TE) conversion of waste heat into electricity. However, TE performance in its ambient rhombohedral phase is strongly impeded by natural...
Thermoelectric materials have aroused wide attention because of the capability to directly convert heat into electricity. AgSbSe2 is a structural analogue of PbTe but does not contain toxic element Pb or expensive element Te. Besides, it possesses both high Seebeck coefficient and inherently low thermal conductivity, making AgSbSe2 a competitive candidate for mid‐temperature thermoelectric power generation. This review summarizes the most recent updates of AgSbSe2 ‐based thermoelectric compounds. It starts with a general introduction of the crystal and electronic band structures of pristine AgSbSe2 with particular emphasis on the debate about whether cations arrangement is ordered or disordered. We then discuss why AgSbSe2 displays glass‐like heat conduction despite its crystalline nature. Subsequently, the strategies of boosting the electrical properties of the titled compound are elaborated. Finally, we point out the challenges and outlooks toward the future development of AgSbSe2 ‐based thermoelectric materials.
The current study uses the nebulized spray pyrolysis (NSP) approach to study the effect on crystalline, morphological, electrical conductivity, and photo detector properties of Bi2S3 thin films through Fe doping produced on amorphous glass substrates. The orthorhombic structure of Bi2S3 thin films for the pristine and Fe doped Bi2S3 thin films was confirmed by X-ray diffraction studies. The XRD data of the samples were used to calculate crystallite size value and maximum crystallite size value of 38 nm was observed for 2% Fe-doped Bi2S3 film. FESEM image of 2% Fe-doped film shows the distribution of uniform grains. With the increase in concentration of the Fe dopants from 0 to 5%, the energy gap value changes from 2.2 to 2.47 eV. The 2% Fe-doped Bi2S3 film possesses highest responsivity (9.60×10⁻² AW⁻¹), external quantum efficiency (22.4%), and detectivity (1.34×10¹⁰ Jones) properties suggests that the sample might be better suited for the application of the photo detectors.
From temperature dependence of thermoelectric power and dc conductivity measurements [1] on bulk amorphous GexSe1-x samples (x = 10, 20, 30, 40) a dimensionless structural parameter ‘A-r/k’ was computed from intercept of thermoelectric power versus reciprocal temperature (1/T) plots. Based on ‘A-r/k’ and the slope ‘M’ of thermoelectric power versus logarithm of resistivity (ln ρ) plots, it was found that as Se content decreases, the bulk amorphous samples exhibit a transition from chalcogenides to tetrahedrally bonded amorphous semiconductors. There is a widespread agreement that tetrahedrally coordinated amorphous semiconductors crystallise at much higher temperature then the lone pair chalcogenide glasses. Therefore a decrease in Se content is considered as increase in the hardness of the samples.
Thermoelectric (TE) materials have attracted tremendous research interests over the past few decades, due to their application in power generation technology from waste heat, almost without producing any pollution in...
Four isomorphic P2 chalcogenide clusters named [Sn11In9Cu6S44]·11(H+DBU) (1) (DBU = 1,8-diazabicyclo[5.4.0] undec-7-ene), [Sn10In10Cu6Se44]·6(H22+DMAPA)·2(DMAPA)·9EG (2) (DMAPA = 3-dimethylaminopropylamine, EG = ethylene glycol), [Sn10In10Cu6S40O4]·6[H22+PMDETA]·10EG (3) (PMDETA = pentamethyldiethylenetriamine), [Sn10Ga10Cu6S40O4]·6(H22+DMAPA)·7EG (4) have been isolated via organotin precursor and mixed-metal strategy. These clusters exhibit excellent solubility in organic solvents. The continuous-regulation of optical band and optical limiting performance have been realized through precise controlled substituting engineering of cationic and anionic elements.
SnSe doped a different way
Heat can be converted into electricity by thermoelectric materials. Such materials are promising for use in solid-state cooling devices. A challenge for developing efficient thermoelectric materials is to ensure high electrical but low thermal conductivity. Chang et al. found that bromine doping of tin selenide (SnSe) does just this by maintaining low thermal conductivity in the out-of-plane direction of this layered material. The result is a promising n-type thermoelectric material with electrons as the charge carriers—an important step for developing thermoelectric devices from SnSe.
Science , this issue p. 778
The extrinsic routes to manipulating phonon transport, for instance, through multiple defects of hierarchical length scales are proven effective in suppressing the lattice thermal conductivity (κ_L), but their usefulness primarily relies on the selective scattering of phonons over charge-carriers. Alternatively, crystalline solids innately exhibiting a low κ_L can constitute an attractive paradigm capable of offering the long-sought approach for decoupling electron and phonon transport to realize potential candidates for thermoelectric (TE) energy-conversion. In this perspective, we discuss the correlations between chemical bonding and lattice dynamics in specific materials, and the ensuing characteristics underpinning an intrinsically low κ_L therein viz. lattice anharmonicity, resonant bonding, intrinsic rattling, part-liquid states and order-disorder transitions. The knowledge of these aspects should guide the discovery and design of new low-κ_L solids with potential TE applications.
GeTe with rhombohedral-to-cubic phase transition is a promising lead-free thermoelectric candidate. Herein, theoretical studies reveal that cubic GeTe has superior thermoelectric behavior, which is linked to (1) the two valence bands to enhance the electronic transport coefficients and (2) stronger enharmonic phonon–phonon interactions to ensure a lower intrinsic thermal conductivity. Experimentally, based on Ge1−xSbxTe with optimized carrier concentration, a record-high figure-of-merit of 2.3 is achieved via further doping with In, which induces the distortion of the density of states near the Fermi level. Moreover, Sb and In codoping reduces the phase-transition temperature to extend the better thermoelectric behavior of cubic GeTe to low temperature. Additionally, electronic microscopy characterization demonstrates grain boundaries, a high-density of stacking faults, and nanoscale precipitates, which together with the inevitable point defects result in a dramatically decreased thermal conductivity. The fundamental investigation and experimental demonstration provide an important direction for the development of high-performance Pb-free thermoelectric materials.
In order to locate the optimal carrier concentrations for peaking the thermoelectric performance in p-type group IV monotellurides, existing efforts focus on aliovalent doping, either to increase (in PbTe) or to decrease (in SnTe and GeTe) the hole concentration. The limited solubility of aliovalent dopants usually introduces insufficient phonon scattering for thermoelectric performance maximization. With a decrease in the size of cation, the concentration of holes, induced by cation vacancies in intrinsic compounds, increases rapidly from ≈1018 cm−3 in PbTe to ≈1020 cm−3 in SnTe and then to ≈1021 cm−3 in GeTe. This motivates a strategy here for reducing the carrier concentration in GeTe, by increasing the mean size of cations and vice-versa decreasing the average size of anions through isovalent substitutions for increased formation energy of cation vacancy. A combination of the simultaneously resulting strong phonon scattering due to the high solubility of isovalent impurities, an ultrahigh thermoelectric figure of merit, zT of 2.2 is achieved in GeTe–PbSe alloys. This corresponds to a 300% enhancement in average zT as compared to pristine GeTe. This work not only demonstrates GeTe as a promising thermoelectric material but also paves the way for enhancing the thermoelectric performance in similar materials.
Waste heat sources are generally diffused and provide a range of temperatures rather than a particular temperature. Thus, thermoelectric waste heat to electricity conversion requires high average thermoelectric figure of merit (ZTavg) of materials over the entire working temperature along with high peak thermoelectric figure of merit (ZTmax). Herein, we report an ultrahigh ZTavg of 1.4 for (GeTe)80(AgSbSe2)20 [TAGSSe-80] in the temperature range of 300-700K, which is one of the highest value measured among the state-of-art Pb-free thermoelectric materials. Moreover, TAGSSe-80 exhibits high ZTmax of 1.9 at 660 K. High thermoelectric performance of TAGSSe-x is attributed to extremely low lattice thermal conductivity ( 0.4 W/mK), which arises due to extensive phonon scattering by hierarchical nano/meso-structures in the TAGSSe-x matrix. Additionally, TAGSSe-80 exhibits higher Vickers microhardness value of 209 kgf/mm2 compared to the other the state-of-art metal chalcogenides.
PbTe and SnTe in their p-type forms have long been considered high-performance thermoelectrics, and both of them largely rely on two valence bands (the first band at L point and the second one along the Σ line) participating in the transport properties. This work focuses on the thermoelectric transport properties inherent to p-type GeTe, a member of the group IV monotellurides that is relatively less studied. Approximately 50 GeTe samples have been synthesized with different carrier concentrations spanning from 1 to 20 × 10²⁰ cm⁻³, enabling an insightful understanding of the electronic transport and a full carrier concentration optimization for the thermoelectric performance. When all of these three monotellurides (PbTe, SnTe and GeTe) are fully optimized in their p-type forms, GeTe shows the highest thermoelectric figure of merit (zT up to 1.8). This is due to its superior electronic performance, originating from the highly degenerated Σ band at the band edge in the low-temperature rhombohedral phase and the smallest effective masses for both the L and Σ bands in the high-temperature cubic phase. The high thermoelectric performance of GeTe that is induced by its unique electronic structure not only provides a reference substance for understanding existing research on GeTe but also opens new possibilities for the further improvement of the thermoelectric performance of this material.
Two-dimensional materials have significant potential for the development of new devices. Here we report the electronic and structural properties of β-GeSe, a new polymorph of GeSe, with a unique crystal structure that displays strong two-dimensional structural features. β-GeSe is made at high pressure and temperature and is stable under ambient conditions. We compare it to its structural and electronic relatives α-GeSe and black phosphorus. The new β form of GeSe displays a boat configuration for its Ge-Se six-ring, while the previously known α form, and black phosphorus, display the more common chair configuration for their six-rings. Electronic structure calculations indicate that β-GeSe is semiconducting, with an approximate bulk band gap of Δ ≈ 0.5 eV, and, in its monolayer form, Δ ≈ 0.9 eV. These values fall between those of α-GeSe and black phosphorus, making β-GeSe a promising candidate for future applications. The resistivity of β-GeSe measured in-plane is on the order of ρ ≈ 1 Ωcm, while being essentially temperature independent, possibly due to defect-level dominated conductivity.
Resonant levels are promising for high-performance single-phase thermoelectric materials. Recently, phase-change materials have attracted much attention for energy conversion applications. As the energetic position of resonant levels could be temperature dependent, searching for dopants in phase-change materials, which can introduce resonant levels in both low and high temperature phases, remains challenging. In this study, possible distortions of the electronic density of states due to group IIIA elements (Ga, In, Tl) in GeTe are theoretically investigated. Resonant levels induced by indium dopants in both rhombohedral and cubic phase GeTe have been demonstrated. The experimental Seebeck coefficients of InxGe1−xTe exhibit a large enhancement compared with those observed for other prior dopants. Indium dopants reduce the defect concentrations in GeTe, and thus, they lower the carrier concentrations and suppress the electronic component of the total thermal conductivity. The enhanced Seebeck coefficient, together with the suppressed thermal conductivity, leads to a reasonably high ZT of 1.3 at a temperature near 355 °C in In0.02Ge0.98Te. The corresponding average ZT is enhanced by ~70% across the entire temperature range of the rhombohedral and cubic phases. These observations indicate that indium-doped GeTe is a promising base material for achieving an even higher thermoelectric performance.
The recently reported superior thermoelectric performance of SnSe, motivates the current work on the thermoelectric properties of polycrystalline GeSe, an analogue compound with the same crystal structure. Due to the extremely low carrier concentration in intrinsic GeSe, various dopants are utilized to substitute either Ge or Se for increasing the carrier concentration and therefore for optimizing the thermoelectric power factor. It is shown that Ag-substitution on Ge site is the most effective, which enables a hole concentration up to ∼10¹⁸ cm⁻³. A further isovalent substitution by Pb and Sn leads to an effective reduction in the lattice thermal conductivity. A peak figure of merit, zT of ∼0.2 at 700 K can be achieved in Ag0.01Ge0.79Sn0.2Se, a composition with the highest carrier concentration. The transport properties can be well described by a single parabolic band model with a dominant carrier scattering by acoustic phonons at high temperatures (>500 K). This further enables a prediction on the maximal zT of ∼0.6 at 700 K and the corresponding carrier concentration of ∼5×10¹⁹ cm⁻³.
The broad-based implementation of thermoelectric materials in converting heat to electricity hinges on the achievement of high conversion efficiency. Here we demonstrate a thermoelectric figure of merit ZT of 2.5 at 923 K by the cumulative integration of several performance-enhancing concepts in a single material system. Using non-equilibrium processing we show that hole-doped samples of PbTe can be heavily alloyed with SrTe well beyond its thermodynamic solubility limit of <1 mol%. The much higher levels of Sr alloyed into the PbTe matrix widen the bandgap and create convergence of the two valence bands of PbTe, greatly boosting the power factors with maximal values over 30 μW cm⁻¹ K⁻². Exceeding the 5 mol% solubility limit leads to endotaxial SrTe nanostructures which produce extremely low lattice thermal conductivity of 0.5 W m⁻¹ K⁻¹ but preserve high hole mobilities because of the matrix/precipitate valence band alignment. The best composition is hole-doped PbTe-8%SrTe.
We report direct observation of local ferroelectric ordering above room temperature in rocksalt SnTe, which is a topological crystalline insulator and a good thermoelectric material. Although SnTe is known to stabilize in a ferroelectric ground state (rhombohedral phase) below ∼100 K, at high temperatures it is not expected to show any ferroelectric ordering forbidden by its globally centro-symmetric crystal structure (Fm-3m). Here, we show that SnTe exhibits local ferroelectric ordering that is robust above room temperature through direct imaging of ferroelectric domains by piezoresponse force microscopy and measurement of local polarization switching using switching spectroscopy. Using first-principles theoretical analysis, we show how the local ferroelectricity arises from soft bonding and competing phonon instabilities at intermediate wavelengths, which induce local Sn-off centering in the otherwise cetrosymmetric SnTe crystal structure. The results make SnTe an important member of the family of new multi-functional materials namely the ferroelectric-thermoelectrics.
Thermoelectric technology, harvesting electric power directly from heat, is a promising environmentally-friendly means of
energy savings and power generation. The thermoelectric efficiency is determined by the device dimensionless figure of merit
ZTdev, and optimizing this efficiency requires maximizing ZT values over a broad temperature range. Herein, we report a record high ZTdev ∼1.34, with ZT ranging from 0.7 to 2.0 at 300-773K, realized in hole doped SnSe crystals. The exceptional performance arises from the ultra-high
power factor, which comes from a high electrical conductivity and a strongly enhanced Seebeck coefficient enabled by the contribution
of multiple electronic valence bands present in SnSe. SnSe is a robust thermoelectric candidate for energy conversion applications
in the low and moderate temperature range.
A promising thermoelectric figure of merit, zT, of ∼1.3 at 725 K was obtained in high quality crystalline ingots of Ge1−xBixTe. The substitution of Bi³⁺ in a Ge²⁺ sublattice of GeTe significantly reduces the excess hole concentration due to the aliovalent donor dopant nature of Bi³⁺. Reduction in carrier density optimizes electrical conductivity, and subsequently enhances the Seebeck coefficient in Ge1−xBixTe. More importantly, a low lattice thermal conductivity of ∼1.1 W m⁻¹ K⁻¹ for Ge0.90Bi0.10Te was achieved, which is due to the collective phonon scattering from meso-structured grain boundaries, nano-structured precipitates, nano-scale defect layers, and solid solution point defects. We have obtained a reasonably high mechanical stability for the Ge1−xBixTe samples. The measured Vickers microhardness value of the high performance sample is ∼165 HV, which is comparatively higher than that of state-of-the-art thermoelectric materials, such as PbTe, Bi2Te3, and Cu2Se.
In the past several years, metal sulfides have been the subject of extensive research as promising thermoelectric materials with high potential in future commercial applications due to their low cost, low toxicity, and abundance. This review summarizes recent developments and progress in the research of metal sulfides, particularly for binary metal sulfides such as Bi2S3, Cu2−xS, and PbS. Methods for improving the thermoelectric properties of these binary sulfides are emphasized, and promising strategies are suggested to further enhance the thermoelectric figure of merit of these materials.
High thermoelectric conversion efficiencies can be achieved by making use of materials with, as high as possible, figure of merit, ZT, values. Moreover, even higher performance is possible with appropriate geometrical optimization including the use of functionally graded materials (FGM) technology. Here, an advanced n-type functionally graded thermoelectric material based on a phase-separated (PbSn0.05Te)0.92(PbS)0.08 matrix is reported. For assessment of the thermoelectric potential of this material, combined with the previously reported p-type Ge0.87Pb0.13Te showing a remarkable dimensionless figure of merit of 2.2, a finite-element thermoelectric model is developed. The results predict, for the investigated thermoelectric couple, a very impressive thermoelectric efficiency of 14%, which is more than 20% higher than previously reported values for operating under cold and hot junction temperatures of 50 °C and 500 °C, respectively. Validation of the model prediction is done by a thermoelectric couple fabricated according to the model's geometrical optimization conditions, showing a good agreement to the theoretically calculated results, hence approaching a higher technology readiness level.
The recent surge of interest in phase change materials GeTe, Ge2Sb2Te5, and related compounds motivated us to revisit the structural phase transition in GeTe in more details than was done before. Rhombohedral-to-cubic ferroelectric phase transition in GeTe has been studied by high resolution neutron powder diffraction on a spallation neutron source. We determined the temperature dependence of the structural parameters in a wide temperature range extending from 309 to 973 K. Results of our studies clearly show an anomalous volume contraction of 0.6% at the phase transition from the rhombohedral to cubic phase. In order to better understand the phase transition and the associated anomalous volume decrease in GeTe we have performed phonon calculations based on the density functional theory. Results of the present investigations are also discussed with respect to the experimental data obtained for single crystals of GeTe.
We report a high ZT of 2.0 at 823 K for 2% Na-doped PbTe with 6% MgTe with excellent thermal stability. We attribute the high thermoelectric performance to a synergistic combination of enhanced power factor, reduction of the lattice thermal conductivity and simultaneous suppression of bipolar thermal conductivity. MgTe inclusion in PbTe owns triple functions: the Mg alloying within the solubility limit in PbTe modifies the valence band structure by pushing the two valence bands (L and Σ bands) closer in energy, thereby facilitating charge carrier injection. When the solubility limit of Mg is exceeded, ubiquitous endotaxial nanostructures form, which when coupled with mesoscale microstructuring results in a very low (lattice) thermal conductivity through all-scaled length phonon scattering. Meanwhile, most significantly, the Mg alloying enlarges the energy gap of conduction band (C band) and light valence band (L band), thereby suppresses the bipolar thermal conductivity through an increase in band gap.
p-Type PbTe is an outstanding high temperature thermoelectric material with zT of 2 at high temperatures due to its complex band structure which leads to high valley degeneracy. Lead-free SnTe has a similar electronic band structure, which suggests that it may also be a good thermoelectric material. However, stoichiometric SnTe is a strongly p-type semiconductor with a carrier concentration of about 1 × 10(20) cm(-3), which corresponds to a minimum Seebeck coefficient and zT. While in the case of p-PbTe (and n-type La3Te4) one would normally achieve higher zT by using high carrier density in order to populate the secondary band with higher valley degeneracy, SnTe behaves differently. It has a very light, upper valence band which is shown in this work to provide higher zT than doping towards the heavier second band. Therefore, decreasing the hole concentration to maximize the performance of the light band results in higher zT than doping into the high degeneracy heavy band. Here we tune the electrical transport properties of SnTe by decreasing the carrier concentration with iodine doping, and increasing the carrier concentration with Gd doping or by making the samples Te deficient. A peak zT value of 0.6 at 700 K was obtained for SnTe0.985I0.015 which optimizes the light, upper valence band, which is about 50% higher than the other peak zT value of 0.4 for GdzSn1-zTe and SnTe1+y which utilize the high valley degeneracy secondary valence band.
Thermoelectric research on germanium telluride (GeTe) has been mainly focused on the enhancement of its performance in the high-temperature cubic phase since the 1960s. Recently in Joule, Pei and co-workers achieved an unprecedented thermoelectric figure of merit in rhombohedral-phase GeTe by exploiting slight symmetry breaking in the structure, which simultaneously improved the electronic properties and reduced the lattice thermal conductivity.
Significance
Phase-transition behavior in thermoelectric materials is detrimental for their application in thermoelectric devices. Here we designed, and experimentally realized the high thermoelectric performance of cubic GeTe-based material by suppressing the phase transition from a cubic to a rhombohedral structure to below room temperature through a simple Bi and Mn codoping on the Ge site. Bi doping reduced the hole concentration while Mn alloying largely suppressed the phase-transition temperature and also induced modification of the valence bands. Our work provides the basis for studying phase transitions in other thermoelectric materials to optimize these materials for applications.
Lead chalcogenides and their alloys belong to the heart of thermoelectrics due to their large thermoelectric figure of merit (zT). However, recent research shows the limitation in the applicability of lead (Pb)-based materials due to their toxicity and inspired to avail non-toxic analogue of lead chalcogenides. Tin chalcogenides have been predicted to be promising for this purpose for their unique electronic structure and phonon dispersion. Here, we have discussed the journey of tin chalcogenides in the field of thermoelectrics and topological materials with the main emphasis towards the bonding, crystal structures, electronic band structures, phonon dispersion and thermoelectric properties. Thermal transport properties of tin chalcogenides have been explained based on lattice dynamics, where resonant bonding and local structural distortion play an important role to create lattice anharmonicity, thereby low lattice thermal conductivity. Since thermoelectric and topological materials, especially topological insulator and topological crystalline insulator, share similar material's features, like narrow band gap, heavy constituent elements and significant spin-orbit coupling, we have discussed the thermoelectric properties of the several topological tin chalcogenides from a chemist perspective. This feature article in a way serves as a useful reference for researchers who strive both to improve the properties of tin chalcogenides and advance the field of thermoelectricity and topological material.
Rhombohedral GeSe is a promising p-type thermoelectric material with multivalley band structure. However, its figure of merit ZT, especially average ZT is still relatively low compared with the state-of-art thermoelectric materials. Here, we show that alloying with AgSbTe2 can synergistically optimize the electronic and thermal transport properties of GeSe. On one hand, alloying can tune the crystal field and promote the band convergence between the lower light valence band and higher heavy valence band. The rising light valence band maximum increases both the density of state effective mass and carrier mobility, leading to a significantly improved power factor. On the other hand, the phonon scattering is also enhanced by alloying effect, resulting in a low lattice thermal conductivity of 0.7 W/mK at 754 K. A peak ZT of ≈1.0 at 754 K was achieved in GeSeAg0.2Sb0.2Te0.4 and more importantly, the ZTavg (0.65) between 301 K and 754 K was improved by more than 56% compared to GeSeAg0.2Sb0.2Se0.4 (ZTavg=0.41).
Thermoelectric materials can be used in direct conversion of heat to electricity and vice versa. The past decade has witnessed the rapid growth of thermoelectric research, targeting high thermoelectric performance either via reduction in the lattice thermal conductivity or via enhancement of the power factor. In this review, we firstly summarize the recent advances in bulk thermoelectric materials with reduced lattice thermal conductivity by nano-microstructure control and also newly discovered materials with intrinsically low lattice thermal conductivity. We then discuss ways to enhance the electron transport abilities for achieving higher power factor by both novel and traditional methods. Finally, we highlight the recent development in single-crystal thermoelectric materials. These strategies are successful in synergistically manipulating the thermal conductivity and electron transport properties, which have significantly advanced thermoelectric performance on materials. For device applications on these high-performance materials, new opportunities may arise though stability, electrode contacts, mechanical properties, and other problems need to be solved in the near future.
High-symmetry thermoelectric materials usually have the advantage of very high band degeneracy, while low-symmetry thermoelectrics have the advantage of very low lattice thermal conductivity. If the symmetry breaking of band degeneracy is small, both effects may be realized simultaneously. Here we demonstrate this principle in rhombohedral GeTe alloys, having a slightly reduced symmetry from its cubic structure, to realize a record figure of merit (zT ∼ 2.4) at 600 K. This is enabled by the control of rhombohedral distortion in crystal structure for engineering the split low-symmetry bands to be converged and the resultant compositional complexity for simultaneously reducing the lattice thermal conductivity. Device ZT as high as 1.3 in the rhombohedral phase and 1.5 over the entire working temperature range of GeTe alloys make this material the most efficient thermoelectric to date. This work paves the way for exploring low-symmetry materials as efficient thermoelectrics.
For several decades, thermoelectric advancements have largely relied on the reduction of lattice thermal conductivity (κL). According to the Boltzmann transport theory of phonons, κL mainly depends on the specific heat, the velocity, and the scattering of phonons. Intensifying the scattering rate of phonons is the focus for reducing the lattice thermal conductivity. Effective scattering sources include 0D point defects, 1D dislocations, and 2D interfaces, each of which has a particular range of frequencies where phonon scattering is most effective. Because acoustic phonons are generally the main contributors to κL due to their much higher velocities compared to optical phonons, many low-κL thermoelectrics rely on crystal structure complexity leading to a small fraction of acoustic phonons and/or weak chemical bonds enabling an overall low phonon propagation velocity. While these thermal strategies are successful for advancing thermoelectrics, the principles used can be integrated with approaches such as band engineering to improve the electronic properties, which can promote this energy technology from niche applications into the mainstream.
In this study, a series of Ge1-xMnxTe (x=0-0.21) compounds were prepared by melting-quenching-annealing process combined with Spark Plasma Sintering (SPS). The effect of alloying MnTe into GeTe on the structure and thermoelectric properties of Ge1-xMnxTe is profound. With increasing content of MnTe, the structure of the Ge1-xMnxTe compounds gradually changes from rhombohedral to cubic, and the known R3m to Fm-3m phase transition temperature of GeTe moves from 700 K closer to room temperature. First-principles density functional theory calculations show that alloying MnTe into GeTe decreases the energy difference between the light and heavy valence bands in both the R3m and the Fm-3m structures, enhancing a multi-band character of the valence band edge that increases the hole carrier effective mass. The effect of this band convergence is a significant enhancement in the carrier effective mass from 1.44 m0 (GeTe) to 6.15 m0 (Ge0.85Mn0.15Te). In addition, alloying with MnTe decreases the phonon relaxation time by enhancing alloy scattering, and reduces the phonon velocity, and increases Ge vacancies all of which result in an ultralow lattice thermal conductivity of 0.13 Wm-1K-1 at 823 K. Subsequent doping of the Ge0.9Mn0.1Te compositions with Sb lowers the typical very high hole carrier concentration and brings it closer to its optimal value enhancing the power factor, which combined with the ultralow thermal conductivity yield a maximum ZT value of 1.61 at 823 K (for Ge0.86Mn0.10Sb0.04Te). The average ZT value of the compound over the temperature range 400 K-800 K is 1.09, making it the best GeTe-based thermoelectric material.
Complementary and beneficial effects of Sb and Bi co-doping in GeTe are shown to generate high thermoelectric figure of merit, zT, of 1.8 at 725 K in Ge1-x-yBixSbyTe samples. Sb and Bi co-doping in GeTe facilitates the valence band convergence enhancing the Seebeck coefficient as supported by density functional theoretical (DFT) calculations. Further, Sb and Bi co-doping in GeTe releases the rhombohedral strain and increases its tendency to be cubic in structure, which ultimately enhances the valence band degeneracy. At the same time, Bi forms nano-precipitates of size ~5-20 nm in GeTe matrix and Sb doping increases solid solution point defects greatly, which altogether scatter low-to mid wavelength phonons and result in reduced lattice thermal conductivity down to 0.5 W/mK in the 300-750 K range.
GeSe is a IV-VI semiconductor, like the excellent thermoelectric materials PbTe and SnSe. Orthorhombic GeSe has been predicted theoretically to have good thermoelectric performance but is difficult to dope experimentally. Like PbTe, rhombohedral GeTe has a multivalley band structure, which is ideal for thermoelectrics and also promotes the formation of Ge vacancies to provide enough carriers for electrical transport. Herein, we investigate the thermoelectric properties of GeSe alloyed with AgSbSe2, which stabilizes a new rhombohedral structure with higher symmetry that leads to a multivalley Fermi surface and a dramatic increase in carrier concentration. The zT of GeAg0.2Sb0.2Se1.4 reaches 0.86 at 710 K, which is 18 times higher than that of pristine GeSe and over four times higher than doped orthorhombic GeSe. Our results open a new avenue towards developing novel thermoelectric materials via crystal phase engineering using a strategy of entropy stabilization of high symmetry alloys.
GeSe is a IV-VI semiconductor, like the excellent thermoelectric materials PbTe and SnSe. Orthorhombic GeSe has been predicted theoretically to have good thermoelectric performance but is difficult to dope experimentally. Like PbTe, rhombohedral GeTe has a multivalley band structure, which is ideal for thermoelectrics and also promotes the formation of Ge vacancies to provide enough carriers for electrical transport. Herein, we investigate the thermoelectric properties of GeSe alloyed with AgSbSe2, which stabilizes a new rhombohedral structure with higher symmetry that leads to a multivalley Fermi surface and a dramatic increase in carrier concentration. The zT of GeAg0.2Sb0.2Se1.4 reaches 0.86 at 710 K, which is 18 times higher than that of pristine GeSe and over four times higher than doped orthorhombic GeSe. Our results open a new avenue towards developing novel thermoelectric materials via crystal phase engineering using a strategy of entropy stabilization of high symmetry alloys.
Phonon-glass electron-crystal (PGEC) behaviour is realised in La0.5Na0.5Ti1–xNbxO3 thermoelectric oxides. The vibrational disorder imposed by the presence of both La³⁺ and Na⁺ cations on the A site of the ABO3 perovskite oxide La0.5Na0.5TiO3 produces a phonon-glass with a thermal conductivity, κ, 80% lower than that of SrTiO3 at room temperature. Unlike other state-of-the-art thermoelectric oxides, where there is strong coupling of κ to the electronic power factor, the electronic transport of these materials can be optimised independently of the thermal transport through cation substitution at the octahedral B site. The low κ of the phonon-glass parent is retained across the La0.5Na0.5Ti1–xNbxO3 series without disrupting the electronic conductivity, affording PGEC behaviour in oxides.
GeTe and its derivatives constituting Pb-free elements are well known as potential thermoelectric materials for the last five decades which offer paramount technological importance. The main constrain in the way of optimizing thermoelectric performance of GeTe is the high lattice thermal conductivity (κlat). Herein, we demonstrate low κlat (~0.7 W/mK) and significantly high thermoelectric figure of merit (ZT = 2.1 at 630 K) in Sb doped pseudoternary (GeTe)1-2x(GeSe)x(GeS)x system by two step strategies. (GeTe)1-2x(GeSe)x(GeS)x system provides an excellent podium to investigate competition between entropy driven solid solution and enthalpy driven phase separation. In first step, small concentration of Se and S were substituted simultaneously in the position of Te in GeTe to reduce the κlat by phonon scattering due to mass fluctuations and point defects. When the Se/S concentration increases significantly the system deviates from solid solution, and phase separation of the GeS1-xSex (5-20 μm) precipitates in the GeTe1-xSex matrix occur, which does not participate in phonon scattering. In second stage, κlat of the optimized sample is further reduced to 0.7 W/mK by Sb alloying and spark plasma sintering (SPS), which introduces additional phonon scattering centers such as excess solid solution point defects and grain boundaries. The low κlat in Sb doped (GeTe)1-2x(GeSe)x(GeS)x is attributed to phonon scattering by entropically driven solid solution point defects rather than conventional endotaxial nanostructuring. As a consequence SPS processed Ge0.9Sb0.1Te0.9Se0.05S0.05 sample exhibits a remarkably high ZT of 2.1 at 630 K, which is reproducible and stable over temperature cycles. Moreover, Sb doped (GeTe)1-2x(GeSe)x(GeS)x exhibits significantly higher Vickers micro-hardness (mechanical stability) compared to that of pristine GeTe.
We show how tuning the proximity to the soft optical mode phase transition via chemical composition affects the lattice thermal conductivity κ of Pb1−xGexTe alloys. Using first-principles virtual-crystal simulations, we find that the anharmonic contribution to κ is minimized at the phase transition due to the maximized acoustic-optical anharmonic interaction. Mass disorder significantly lowers and flattens the dip in the anharmonic κ over a wide composition range, thus shifting the κ minimum away from the phase transition. The total κ and its anharmonic contribution vary continuously between the rocksalt and rhombohedral phases as expected for the second-order phase transition. The actual phase and its strength of resonant bonding play a less prominent role in reducing the κ of Pb1−xGexTe alloys than the proximity to the phase transition and the atomic mass. Our results show that alloys with soft optical mode transitions are promising materials for achieving low thermal conductivity and possibly high thermoelectric efficiency.
GeTe-based alloys have been intensively considered as p-type thermoelectrics for about 50 years, yet existing literature barely discussed the thermoelectric properties of pristine GeTe at high temperatures (300~800 K). This work firstly backs to a fundamental understanding on the thermoelectric transport properties inherent to p-type GeTe, based on more than 50 samples synthesized with expected carrier concentrations ranging from 1 to 20×1020 cm-3. A thermoelectric figure of merit zT as high as ~1.7 is found inherent to this compound when it is optimally doped with a Hall carrier concentration of 2.2±10% ×1020 cm-3, offering a reference substance to expose the origins for the high zT in historical GeTe-based alloys. Guided by the above knowledge, further alloying Te with Se in samples with an optimal carrier concentration, enables a reduction on the lattice thermal conductivity by ~40%, and eventually leads to a further enhancement on zT (up to 2.0) by ~20%. This work demonstrates not only GeTe as an inherently high performance thermoelectric matrix compound, but also its availability for further improvements by additional strategies.
There has been a renaissance of interest in exploring highly efficient thermoelectric materials as a possible route to address the worldwide energy generation, utilization, and management. This review describes the recent advances in designing high-performance bulk thermoelectric materials. We begin with the fundamental stratagem of achieving the greatest thermoelectric figure of merit ZT of a given material by carrier concentration engineering, including Fermi level regulation and optimum carrier density stabilization. We proceed to discuss ways of maximizing ZT at a constant doping level, such as increase of band degeneracy (crystal structure symmetry, band convergence), enhancement of band effective mass (resonant levels, band flattening), improvement of carrier mobility (modulation doping, texturing), and decrease of lattice thermal conductivity (synergistic alloying, second-phase nanostructuring, mesostructuring, and all-length-scale hierarchical architectures). We then highlight the decoupling of the electron and phonon transport through coherent interface, matrix/precipitate electronic bands alignment, and compositionally alloyed nanostructures. Finally, recent discoveries of new compounds with intrinsically low thermal conductivity are summarized, where SnSe, BiCuSeO, MgAgSb, complex copper and bismuth chalcogenides, pnicogen-group chalcogenides with lone-pair electrons, and tetrahedrites are given particular emphasis. Future possible strategies for further enhancing ZT are considered at the end of this review.
Thermoelectric materials have received recent attention due to their ability to convert waste heat to electrical energy directly and reversibly. Inorganic materials, especially Bi2Te3, PbTe and Si–Ge based alloys, have been investigated in the temperature range of 300–1000 K, among which PbTe based materials have been extensively studied, and reported to be the leading thermoelectric materials for mid-temperature power generation. However, environmental concern limits their large scale production due to the toxic nature of Pb. As an alternative, GeTe-rich alloys such as TAGS (GeTe–AgSbTe2) have been largely investigated since the 1960s. Most recently, some of the new materials in the GeTe family have been introduced such as Ge0.87Pb0.13Te, the homologous series of Sb2Te3(GeTe)n and Ge0.9Sb0.1Te, and are reported to exhibit high thermoelectric performance, inherently formed nano and microstructure modulations, and high thermal and mechanical stability. These collective enhanced properties of GeTe-rich alloys have generated great interest in investigating further new GeTe based alloys for intermediate temperature thermoelectric applications. In order to provide the fundamental understanding, technological insights, and to further promote the GeTe based alloys, we hereby present a review on (i) the crystal structure, nano/microstructure, phase transition, electronic structure, and thermoelectric properties of GeTe, (ii) correlation of compositional and microstructure modulations and thermoelectric properties of doped GeTe, TAGS based alloys, Ge–Pb–Te materials, and Ge–Sb–Te materials, (iii) mechanical properties, (iv) past and present devices based on GeTe materials and (v) future directions.
Recent findings about ultrahigh thermoelectric performance in SnSe single crystals have stimulated the related researches on this simple binary compound, which are most devoted to its polycrystalline counterparts with a focus on electrical property enhancement by effective doping. This work systematically investigated the thermoelectric proper-ties of polycrystalline SnSe doped with three alkali metals (Li, Na and K). It is found that Na has the best doping effi-ciency, leading to an increase in hole concentration from 3.2×1017 to 4.4×1019 cm-3 at room temperature, accompanied with a drop in Seebeck coefficient from 480 to 142 μV/K. An equivalent single parabolic band model was found ade-quate to capture the variation tendency of Seebeck coefficient with doping levels within a wide range. A mixed scattering of carriers by acoustic phonons and grain boundaries is suitable for numerically understanding the temperature-dependence of carrier mobility. A maximum ZT of ~0.8 was achieved in 1% Na- or K-doped SnSe at 800 K. Possible strategies to improve the mobility and ZT of polycrystals were also proposed.
The coupled transport properties required to create an efficient thermoelectric material necessitates a thorough understanding of the relationship between the chemistry and physics in a solid. We approach thermoelectric material design using the chemical intuition provided by molecular orbital diagrams, tight binding theory, and a classic understanding of bond strength. Concepts such as electronegativity, band width, orbital overlap, bond energy, and bond length are used to explain trends in electronic properties such as the magnitude and temperature dependence of band gap, carrier effective mass, and band degeneracy and convergence. The lattice thermal conductivity is discussed in relation to the crystal structure and bond strength, with emphasis on the importance of bond length. We provide an overview of how symmetry and bonding strength affect electron and phonon transport in solids, and how altering these properties may be used in strategies to improve thermoelectric performance.
Thermoelectric materials enable direct conversion between thermal and electrical energy and provide a viable route for power generation and electric refrigeration. In this paper, we use first-principles based methods to predict a very high figure of merit (ZT) performance in hole doped GeSe crystals along the crystallographic b-axis, with maximum ZT ranging from 0.8 at 300 K to 2.5 at 800 K. This extremely high thermoelectric performance is due to a threefold synergy of properties in this material: (1) the exceptionally low lattice thermal conductivity in GeSe due to anharmonicity of vibrational modes, (2) the increased electrical conductivity due to hole doping and increased carrier concentration, and (3) an enhanced Seebeck coefficient via a multiband effect induced by hole doping. The predicted ZT results of hole-doped GeSe are higher than that of hole doped SnSe, which we have recently reported as having experimentally observed record-breaking thermoelectric efficiency. The overall ZT of hole doped GeSe crystals outperforms all current state-of-the-art thermoelectric materials, and this work provides an urgent computational materials prediction that is in need of experimental testing.
High thermoelectric figure of merit, zT, of ∼1.85 at 725 K along with significant cyclable temperature stability was achieved in Pb-free p-type Ge1-xSbxTe samples through simultaneous enhancement in Seebeck coefficient and reduction of thermal conductivity. Sb doping in GeTe decreases the carrier concentration due to the donor dopant nature of Sb and enhances the valence band degeneracy by increasing the cubic nature of the sample, which collectively boost Seebeck coefficient in the temperature range of 300-773 K. Significant thermal conductivity reduction was achieved due to collective phonon scattering from various meso-structured domain variants, twin and inversion boundaries, nanostructured defect layers, and solid solution point defects. The high performance Ge0.9Sb0.1Te sample shows mechanical stability (Vickers microhardness) of ∼206 Hv, which is significantly higher compared to other popular thermoelectric materials such as Bi2Te3, PbTe, PbSe, Cu2Se, and TAGS.
Heterostructures that consist of a germanium antimony telluride matrix and cobalt germanide precipitates can be obtained by straightforward solid-state synthesis including simple annealing and quenching procedures. The microscale precipitates are homogeneously distributed in a matrix with pronounced "herringbone-like" nanostructure associated with very low thermal conductivities. In comparison to the corresponding pure tellurides, the ZT values of heterostructured materials are remarkably higher. This is mostly due to an increase of the Seebeck coefficient with only little impact on the electrical conductivity. In addition, the phononic part of the thermal conductivity is significantly reduced in some of the materials. As a result, ZT values of ca. 1.9 at 450 °C are achieved. Temperature-dependent changes of the thermoelectric properties are well-understood and correlate with complex phase transitions of the telluride matrix. However, the high ZT values are retained in multiple measurement cycles.
The recent surge of interest in phase-change materials GeTe, Ge2Sb2Te5, and related compounds motivated us to revisit the structural phase transition in GeTe in more detail than was done before. The rhombohedral-to-cubic ferroelectric phase transition in GeTe has been studied using high-resolution neutron powder diffraction on a spallation neutron source. We determined the temperature dependence of the structural parameters in a wide temperature range extending from 309 to 973 K. The results of our studies clearly show an anomalous volume contraction of 0.6% at the phase transition from the rhombohedral-to-cubic phase. In order to better understand the phase transition and the associated anomalous volume decrease in GeTe, we have performed phonon calculations based on the density functional theory. Results of the present investigations are also discussed with respect to the experimental data obtained for single crystals of GeTe.
SnTe, a lead-free rock-salt analogue of PbTe, having valence band structure similar to PbTe, recently has attracted attention for thermoelectric heat to electricity generation. However, pristine SnTe is a poor thermoelectric material because of very high hole concentration resulting from intrinsic Sn vacancies, which give rise to low Seebeck coefficient and high electrical thermal conductivity. In this report, we show that SnTe can be optimized to be a high performance thermoelectric material for power generation by controlling the hole concentration and significantly improving the Seebeck coefficient. Mg (2−10 mol %) alloying in SnTe modulates its electronic band structure by increasing the band gap of SnTe and results in decrease in the energy separation between its light and heavy hole valence bands. Thus, solid solution alloying with Mg enhances the contribution of the heavy hole valence band, leading to significant improvement in the Seebeck coefficient in Mg alloyed SnTe, which in turn results in remarkable enhancement in power factor. Maximum thermoelectric figure of merit, ZT, of ∼1.2 is achieved at 860 K in the high quality crystalline ingot of p-type Sn 0.94 Mg 0.09 Te.
The thermoelectric properties of the Ge1−x
Mnx
Te compounds were investigated in the temperature range from 300 K to 773 K. The crystal structure of the compound was gradually changed with Mn, changing from a rhombohedral to a cubic-like cell. The Seebeck coefficient and the electrical resistivity were increased with Mn. From the Hall coefficient measurement, the reduction of the carrier concentration was confirmed and was responsible for the change of the electrical properties. The thermal conductivity was also reduced with Mn. The maximum dimensionless figure of merit, ZT, was obtained for x = 0.05 composition, where the value was ZT = 1.3 at 773 K. The evolution of the crystal structure with Mn attributed to the change of the thermoelectric properties. The Mn-doped compound which has a more cubic phase than a rhombohedral exhibited superior thermoelectric properties to the pure rhombohedral phase.
This review discusses recent developments and current research in high performance bulk thermoelectric materials, comprising nanostructuring, mesostructuring, band alignment, band engineering and synergistically defining key strategies for boosting the thermoelectric performance. To date, the dramatic enhancements in the figure of merit achieved in bulk thermoelectric materials have come either from the reduction in lattice thermal conductivity or improvement in power factors, or both of them. Here, we summarize these relationships between very large reduction of the lattice thermal conductivity with all-scale hierarchical architecturing, large enhanced Seebeck coefficients with intra-matrix electronic structure engineering, and control of the carrier mobility with matrix/inclusion band alignment, which enhance the power factor and reduce the lattice thermal conductivity. The new concept of hierarchical compositionally alloyed nanostructures to achieve these effects is presented. Systems based on PbTe, PbSe and PbS in which spectacular advances have been demonstrated are given particular emphasis. A discussion of future possible strategies is aimed at enhancing the thermoelectric figure of merit of these materials.
Methods for enhancement of the direct thermal to electrical energy conversion efficiency, upon development of advanced thermoelectric materials, are constantly investigated mainly for efficient implementation of thermoelectric devices in automotive vehicles, for converting the waste heat generated in such engines into useful electrical power and thereby reduction of the fuel consumption and CO2 emission levels. It was recently shown that GeTe based compounds and specifically GeTe-PbTe rich alloys are efficient p-type thermoelectric compositions. In the current research, Bi2Te3 doping and PbTe alloying effects in GexPb1-xTe alloys, subjected to phase separation reactions, were investigated for identifying the phase separation potential for enhancement of the thermoelectric properties beyond a pure alloying effect. All of the investigated compositions exhibit maximal dimensionless figure of merit, ZT, values beyond 1, with the extraordinary value of 2.1 found for the 5% Bi2Te3 doped-Ge0.87Pb0.13Te composition, considered as among the highest ever reported.
An investigation into the thermoelectric properties of semiconductor materials to uncover the feasibility of power-generating efficiencies at temperatures from 25 to 700 C. Discussion is centered on general considerations of efficiency factors leading to materials selection and some preliminary results in the evaluation of compound semiconductors and their solid-solution alloys.
We demonstrate the potential of metallurgical controlling of the phase separation reaction, by means of spark plasma sintering consolidation and subsequently controlled heat treatments sequence, for enhancement the thermoelectric properties of the p-type Ge0.87Pb0.13Te composition. Very high ZTs of up to ∼2, attributed to the nucleation of sub-micron phase separation domains and to comparable sized twinning and dislocation networks features, were observed. Based on the experimentally measured transport properties, combined with the previously reported phase separated n-type (Pb0.95Sn0.05Te)0.92(PbS)0.08 composition, a maximal efficiency value of ∼11.5% was theoretically calculated. These ZT and efficiency values are among the highest reported for single composition non-segmented bulk material legs.
As a lead-free material, GeTe has drawn a growing attention in thermoelectrics, and the figure of merit ZT close to unity was previously obtained via traditional doping/alloying, largely owing to the hole carrier concentration tuning. In this report, we show that a remarkably high ZT of ~1.9 can be achieved at 773K in Ge0.87Pb0.13Te upon the introduction of 3mol% Bi2Te3. Bismuth Telluride promotes the solubility of PbTe in the GeTe matrix thus leading to a significantly reduced thermal conductivity. At the same time, it enhances the thermopower by activating a much higher fraction of charge transport from the highly degenerate Σ valence band, as evidenced by Density Functional Theory calculations. These mechanisms are incorporated and discussed in a 3-band (L+Σ+C) model, and found to well explain the experimental results. The detailed microstructure (including rhombohedral twin structures) analysis in Ge0.87Pb0.13Te+3mol%Bi2Te3 is carried out using transmission electron microscopy (TEM) and crystallographic group theory. The complex microstructure explains the reduced lattice thermal conductivity, and electrical conductivity as well.
Te/Sb/Ge/Ag (TAGS) materials with rather high concentrations of cation vacancies exhibit improved thermoelectric properties as compared to corresponding conventional TAGS (with constant Ag/Sb ratio of 1) due to a significant reduction of the lattice thermal conductivity. There are different vacancy ordering possibilities depending on the vacancy concentration and the history of heat treatment of the samples. In contrast to the average α-GeTe-type structure of TAGS materials with cation vacancy concentrations <∼3%, quenched compounds like Ge0.53Ag0.13Sb0.27□0.07Te1 and Ge0.61Ag0.11Sb0.22□0.06Te1 exhibit "parquet-like" multidomain nanostructures with finite intersecting vacancy layers. These are perpendicular to the pseudocubic ⟨111⟩ directions but not equidistantly spaced, comparable to the nanostructures of compounds (GeTe)nSb2Te3. Upon heating, the nanostructures transform into long-periodically ordered trigonal phases with parallel van der Waals gaps. These phases are slightly affected by stacking disorder but distinctly different from the α-GeTe-type structure reported for conventional TAGS materials. Deviations from this structure type are evident only from HRTEM images along certain directions or very weak intensities in diffraction patterns. At temperatures above ∼400 °C, a rock-salt-type high-temperature phase with statistically disordered cation vacancies is formed. Upon cooling, the long-periodically trigonal phases are reformed at the same temperature. Quenched nanostructured Ge0.53Ag0.13Sb0.27□0.07Te1 and Ge0.61Ag0.11Sb0.22□0.06Te1 exhibit ZT values as high as 1.3 and 0.8, respectively, at 160 °C, which is far below the phase transition temperatures. After heat treatment, i.e., without pronounced nanostructure and when only reversible phase transitions occur, the ZT values of Ge0.53Ag0.13Sb0.27□0.07Te1 and Ge0.61Ag0.11Sb0.22□0.06Te1 with extended van der Waals gaps amount to 1.6 at 360 °C and 1.4 at 410 °C, respectively, which is at the top end of the range of high-performance TAGS materials.
Inelastic neutron scattering experiments on powder samples of GeTe together with density functional theory investigations of the phonon dynamics in the low- and high-temperature phases of GeTe crystal are reported. The dispersion of phonons in the high-temperature rocksalt phase show soft branches with the lowest one at the high-symmetry point. The structural phase transition in GeTe is reconsidered and shown to be driven by the con- densation of exactly three components of the triply degenerate optical transverse soft-phonon mode at the Brillouin zone center. The mechanism proposed allows us to explain the formation of structural distortions in the low-temperature ferroelectric phase of GeTe revealed by various experiments. A displacive nature of the phase change in crystalline GeTe is supported by the results of the present theoretical studies.