Compositional Fluctuations Locked by Athermal Transformation Yielding High Thermoelectric Performance in GeTe

Reducing Domain Density Enhances Conversion Efficiency in GeTe

Incorporating dilute doping and controlled synthesis provides a means to modulate the microstructure, defect density, and transport properties. Transmission electron microscopy (TEM) and geometric phase analysis (GPA) have revealed that hot-pressing can increase defect density, which redistributes strain and helps prevent unwanted Ge precipitates formation. An alloy of GeTe with a minute amount of indium added has shown remarkable TE properties compared to its undoped counterpart. Specifically, it achieves a maximum figure-of-merit zT of 1.3 at 683 K and an exceptional TE conversion efficiency of 2.83% at a hot-side temperature of 723 K. Significant zT and conversion efficiency improvements are mainly due to domain density engineering facilitated by an effective hot-pressing technique applied to lightly doped GeTe. The In-GeTe alloy exhibits superior TE properties and demonstrates notable stability under significant thermal gradients, highlighting its promise for use in mid-temperature TE energy generation systems.

Cation Modulation in AgSbTe2 Realizes Carrier Optimization, Defect Engineering, and a 7% Single-Leg Thermoelectric Efficiency

AgSbTe2 plays a pivotal role in mid-temperature thermoelectric generators (TEGs). Leveraging the seminal advances in cation manipulation within AgSbTe2, this study demonstrates an enhanced TE power factor (PF = S2σ) of 1.5 mWm-1 K-2 and a peak zT of 1.5 at 583 K in an off-stoichiometric Ag1.04Sb0.96Te2 crystal. The introduction of Ge in place of Ag leads to an increased nH as evidenced by the detection of trace Ge4+ through XPS analysis. Further chemical state analysis reveals the simultaneous presence of Ag+, Sb3+, and Ge4+, elucidating the effect of cation modulations. TEM characterizations validate the presence of superlattice structure, and the linear defects discerned within the AgSbTe2 matrix. Consequently, the lattice thermal conductivity κL is substantially reduced in the Ag1.02Ge0.02Sb0.96Te2 crystal, yielding a peak zT of 1.77 at 623 K. This notable advancement is attributed to the counterbalance achieved between the enhanced PF and the reduced κL, facilitated by cation modulation. Additionally, a single-leg TE device incorporating Ag1.02Ge0.02Sb0.96Te2 demonstrates a conversion efficiency of 7% across a temperature gradient (ΔT) of 350 K. This study corroborates the efficacy of cation modulation through thermodynamic approaches and establishes a relationship between transport properties and the presence of defects.

Tuning Interstitials in Fully Dense β-Zn4Sb3 Doubles Single-Leg Thermoelectric Efficiency

Zinc antimonides, particularly the β-Zn4Sb3 compound, act as prototypes in the early phases of thermoelectric generator (TEG) development. However, their potential applications are constrained by structural instability at elevated temperatures. In this study, introducing a low concentration of aluminum (Al) achieves a highly stable Al-Zn4Sb3, exhibiting an improved peak zT value compared to undoped Zn4Sb3. Notably, a single-leg device utilizing a fully dense Al0.01Zn3.99Sb3 demonstrates an impressive conversion efficiency (η) of 3% even at a temperature difference (ΔT) of 225 K. This result represents an approximately 200% increase compared with the pristine one. The combination of dilute cationic doping and phase diagram engineering solidifies the potential of Zn4Sb3 as an efficient and sustainable green energy device.

Ultralow Lattice Thermal Conductivity and High Thermoelectric Performance in Ge1-x-yBixCayTe with Ultrafine Ferroelectric Domain Structure

GeTe and its derivatives emerging as a promising lead-free thermoelectric candidate have received extensive attention. Here, a new route was proposed that the minimization of κ_L in GeTe through considerable enhancement of acoustic phonon scattering by introducing ultrafine ferroelectric domain structure. We found that Bi and Ca dopants induce strong atomic strain disturbance in the GeTe matrix because of large differences in atom radius with host elements, leading to the formation of ultrafine ferroelectric domain structure. Furthermore, large strain field and mass fluctuation induced by Bi and Ca codoping result in further reduced κ_L by effectively shortening the phonon relaxation time. The co-existence of ultrafine ferroelectric domain structure, large strain field, and mass fluctuation contribute to an ultralow lattice thermal conductivity of 0.48 W m^-1 K^-1 at 823 K. Bi and Ca codoping significantly enhances the Seebeck coefficient and power factor through reducing the energy offset between light and heavy valence bands of GeTe. The modified band structure boosts the power factor up to 47 μW cm^-1 K^-2 in Ge_0.85Bi_0.09Ca_0.06Te. Ultimately, a high ZT of ∼2.2 can be attained. This work demonstrates a new design paradigm for developing high-performance thermoelectric materials.

A Review of Key Properties of Thermoelectric Composites of Polymers and Inorganic Materials

This review focusses on the development of thermoelectric composites made of oxide or conventional inorganic materials, and polymers, with specific emphasis on those containing oxides. Discussion of the current state-of-the-art thermoelectric materials, including the individual constituent materials, i.e., conventional materials, oxides and polymers, is firstly presented to provide the reader with a comparison of the top-performing thermoelectric materials. Then, individual materials used in the inorganic/polymer composites are discussed to provide a comparison of the performance of the composites themselves. Finally, the addition of carbon-based compounds is discussed as a route to improving the thermoelectric performance. For each topic discussed, key thermoelectric properties are tabulated and comparative figures are presented for a wide array of materials.

Synergistically Optimized Carrier and Phonon Transport Properties in Bi-Cu2S Coalloyed GeTe

GeTe is an emerging lead-free thermoelectric material, but its excessive carrier concentration and high thermal conductivity severely restrict the enhancement of thermoelectric properties. In this study, the synergistically optimized thermoelectric properties of p-type GeTe through Bi-Cu₂S coalloying are reported. It can be found that the donor behavior of Bi and the substitution-interstitial defect pairs of Cu⁺ ions effectively reduce the hole concentration to an optimal level with carrier mobility less affected. At the same time, Bi-Cu₂S coalloying induces many phonon scattering centers involving stacking faults, nanoprecipitations, grain boundaries and tetrahedral dislocations and suppresses the lattice thermal conductivity to 0.64 W m^-1 K^-1. Consequently, all effects synergistically yield a peak ZT of 1.9 at 770 K with a theoretical conversion efficiency of 14.5% (300-770 K) in the (Ge_0.94Bi_0.06Te)_0.988(Cu₂S)_0.012 sample, which is very promising for mid-low temperature range waste heat harvest.

Aliovalent Dilute Doping and Nano-Moiré Fringe Advance the Structural Stability and Thermoelectric Performance in β-Zn4Sb3

Thermoelectric (TE) generators have come a long way since the first commercial apparatus launched in the 1950s. Since then, the β-Zn₄Sb₃ has manifested its potential as a cost-effective and environmentally friendly TE generator compared with the tellurium-bearing TE materials. Although the β-Zn₄Sb₃ features an intrinsically low thermal conductivity κ, it suffers from a long-lasting structural instability issue arising from the highly mobile zinc ions. Herein, the dilute Ga dopant gives rise to the aliovalent substitution, lowers the mobile zinc ions, and optimizes the hole carrier concentration n _H simultaneously. Meanwhile, the formation of nano-moiré fringes suggests the modulated distribution of point defect that results from soluble Ga in a β-Zn₄Sb₃ lattice, which elicits an ultralow lattice thermal conductivity κ _L = 0.2 W m^-1 K^-1 in a (Zn_0.992Ga_0.008)₄Sb₃ alloy. Hence, a fully dense β-Zn₄Sb₃ incorporated with the dilute Ga doping reveals superior structural stability with a peak zT > 1.4 at 623 K. In this work, the aliovalent dilute doping coupled with phase diagram engineering optimizes the fluxes of moving electrons and charged ions, which stabilizes the single-phase β-Zn₄Sb₃ while boosting the TE performance at the mid-temperature region. The synergistic strategies endow the ionic crystals with a thermodynamic route, which opens up a new category for high-performance and thermal robust TE alloys.

Upcycling Silicon Photovoltaic Waste into Thermoelectrics

Two decades after the rapid expansion of photovoltaics, the number of solar panels reaching end-of-life is increasing. While precious metals such as silver and copper are usually recycled, silicon, which makes up the bulk of a solar cells, goes to landfills. This is due to the defect- and impurity-sensitive nature in most silicon-based technologies, rendering it uneconomical to purify waste silicon. Thermoelectrics represents a rare class of material in which defects and impurities can be engineered to enhance the performance. This is because of the majority-carrier nature, making it defect- and impurity-tolerant. Here, the upcycling of silicon from photovoltaic (PV) waste into thermoelectrics is enabled. This is done by doping 1% Ge and 4% P, which results in a figure of merit (zT) of 0.45 at 873 K, the highest among silicon-based thermoelectrics. The work represents an important piece of the puzzle in realizing a circular economy for photovoltaics and electronic waste.

Thermoelectric Coolers: Progress, Challenges, and Opportunities

Owing to the free of noise, mechanical component, working fluid, and chemical reaction, thermoelectric cooling is regarded as a suitable solution to address the greenhouse emission for the broad cooling scenarios. Here, the significant progress of state-of-the-art thermoelectric coolers is comprehensively summarized and the related aspects of materials, fundamental design, heat sinks, and structures, are overviewed. Particularly, the usage of thermoelectric coolers in smart city, greenhouse, and personal and chip thermal management is highlighted. In the end, current challenges and future opportunities for further improvement of designs, performance, and applications of thermoelectric coolers are pointed out.

Achieving High-Performance Ge0.92 Bi0.08 Te Thermoelectrics via LaB6 -Alloying-Induced Band Engineering and Multi-Scale Structure Manipulation

In this work, a LaB₆ -alloying strategy is reported to effectively boost the figure-of-merit (ZT) of Ge_0.92 Bi_0.08 Te-based alloys up to ≈2.2 at 723 K, attributed to a synergy of La-dopant induced band structuring and structural manipulation. Density-function-theory calculations reveal that La dopant enlarges the bandgap and converges the energy offset between the sub-valence bands in cubic-structured GeTe, leading to a significantly increased effective mass, which gives rise to a high Seebeck coefficient of ≈263 µV K^-1 and in turn a superior power factor of ≈43 µW cm^-1 K^-2 at 723 K. Besides, comprehensive electron microscopy characterizations reveal that the multi-scale phonon scattering centers, including a high density of planar defects, Boron nanoparticles in tandem with enhanced boundaries, dispersive Ge nanoprecipitates in the matrix, and massive point defects, contribute to a low lattice thermal conductivity of ≈0.67 W m^-1 K^-1 at 723 K. Furthermore, a high microhardness of ≈194 H_v is witnessed in the as-designed Ge_0.92 Bi_0.08 Te(LaB₆ )_0.04 alloy, derived from the multi-defect-induced strengthening. This work provides a strategy for developing high-performance and mechanical robust middle-temperature thermoelectric materials for practical thermoelectric applications.

Comparative Study of Thermoelectric Properties of Sb2Si2Te6 and Bi2Si2Te6

Charge carrier transport and corresponding thermoelectric properties are often affected by several parameters, necessitating a thorough comparative study for a profound understanding of the detailed conduction mechanism. Here, as a model system, we compare the electronic transport properties of two layered semiconductors, Sb₂Si₂Te₆ and Bi₂Si₂Te₆. Both materials have similar grain sizes and morphologies, yet their conduction characteristics are significantly different. We found that phase boundary scattering can be one of the main factors for Bi₂Si₂Te₆ to experience significant charge carrier scattering, whereas Sb₂Si₂Te₆ is relatively unaffected by the phenomenon. Furthermore, extensive point defect scattering in Sb₂Si₂Te₆ significantly reduces its lattice thermal conductivity and results in high zT values across a broad temperature range. These findings provide novel insights into electron transport within these materials and should lead to strategies for further improving their thermoelectric performance.

Giant Thermal Transport Tuning at a Metal/Ferroelectric Interface

Interfacial thermal transport plays a prominent role in the thermal management of nanoscale objects and is of fundamental importance for basic research and nanodevices. At metal/insulator interfaces, a configuration commonly found in electronic devices, heat transport strongly depends upon the effective energy transfer from thermalized electrons in the metal to the phonons in the insulator. However, the mechanism of interfacial electron-phonon coupling and thermal transport at metal/insulator interfaces is not well understood. Here, the observation of a substantial enhancement of the interfacial thermal resistance and the important role of surface charges at the metal/ferroelectric interface in an Al/BiFeO₃ membrane are reported. By applying uniaxial strain, the interfacial thermal resistance can be varied substantially (up to an order of magnitude), which is attributed to the renormalized interfacial electron-phonon coupling caused by the charge redistribution at the interface due to the polarization rotation. These results imply that surface charges at a metal/insulator interface can substantially enhance the interfacial electron-phonon-mediated thermal coupling, providing a new route to optimize the thermal transport performance in next-generation nanodevices, power electronics, and thermal logic devices.

(GeTe)1-x(AgSnSe2)x: Strong Atomic Disorder-Induced High Thermoelectric Performance near the Ioffe-Regel Limit

In thermoelectrics, the material's performance stems from a delicate tradeoff between atomic order and disorder. Generally, dopants and thus atomic disorder are indispensable for optimizing the carrier concentration and scatter short-wavelength heat-carrying phonons. However, the strong disorder has been perceived as detrimental to the semiconductor's electrical conductivity owing to the deteriorated carrier mobility. Here, we report the sustainable role of strong atomic disorder in suppressing the detrimental phase transition and enhancing the thermoelectric performance in GeTe. We found that AgSnSe₂ and Sb co-alloying eliminates the unfavorable phase transition due to the high configurational entropy and achieve the cubic Ge_1-x-ySb_yTe_1-x(AgSnSe₂)_x solid solutions with cationic and anionic site disorder. Though AgSnSe₂ substitution drives the carrier mean free path toward the Ioffe-Regel limit and minimizes the carrier mobility, the increased carrier concentration could render a decent electrical conductivity, affording enough phase room for further performance optimization. Given the lowermost carrier mean free path, further Sb alloying on Ge sites was implemented to progressively optimize the carrier concentration and enhance the density-of-state effective mass, thereby substantially enhancing the Seebeck coefficient. In addition, the high density of nanoscale strain clusters induced by strong atomic disorders significantly restrains the lattice thermal conductivity. As a result, a state-of-the-art zT ≈ 1.54 at 773 K was attained in cubic Ge_0.58Sb_0.22Te_0.8(AgSnSe₂)_0.2. These results demonstrate that the strong atomic disorder at the high entropy scale is a previously underheeded but promising approach in thermoelectric material research, especially for the numerous low carrier mobility materials.

Dramatically enhanced Seebeck coefficient in GeMnTe2-NaBiTe2 alloys by tuning the Spin's thermodynamic entropy

The emerging material GeMnTe₂ provides a rare example to study the spin degree of freedom in thermoelectric transport, as it exhibits an anomalous Seebeck coefficient driven by the spin's thermodynamic entropy. This work presents an unconventional strategy to optimize the thermoelectric performance of GeMnTe₂ by manipulating the spin degree of freedom. NaBiTe₂ is alloyed into GeMnTe₂ to disorder the spin orientation under finite temperature, and the obtained Seebeck coefficient is confirmed to be dramatically enhanced by more than 150%. The measurements of XRD and magnetic susceptibility indicate that the increased Seebeck coefficient is due to the increase of the spin's thermodynamic entropy. Finally, the maximum ZT of 1.06 at 820 K is obtained in Ge_0.8Na_0.1Bi_0.1MnTe₂. This work enriches the physical picture of spin degree of freedom in thermoelectric materials.