Optimized Strategies for Advancing n-Type PbTe Thermoelectrics: A Review

Advancing flexible thermoelectrics for integrated electronics

With the increasing demand for energy and the climate challenges caused by the consumption of traditional fuels, there is an urgent need to accelerate the adoption of green and sustainable energy conversion and storage technologies. The integration of flexible thermoelectrics with other various energy conversion technologies plays a crucial role, enabling the conversion of multiple forms of energy such as temperature differentials, solar energy, mechanical force, and humidity into electricity. The development of these technologies lays the foundation for sustainable power solutions and promotes research progress in energy conversion. Given the complexity and rapid development of this field, this review provides a detailed overview of the progress of multifunctional integrated energy conversion and storage technologies based on thermoelectric conversion. The focus is on improving material performance, optimizing the design of integrated device structures, and achieving device flexibility to expand their application scenarios, particularly the integration and multi-functionalization of wearable energy conversion technologies. Additionally, we discuss the current development bottlenecks and future directions to facilitate the continuous advancement of this field.

Space-Confined Growth of Ultrathin P-Type GeTe Nanosheets for Broadband Photodetectors

As p-type phase-change degenerate semiconductors, crystalline and amorphous germanium telluride (GeTe) exhibit metallic and semiconducting properties, respectively. However, the massive structural defects and strong interface scattering in amorphous GeTe films significantly reduce their performance. In this work, two-dimensional (2D) p-type GeTe nanosheets are synthesized via a specially designed space-confined chemical vapor deposition (CVD) method, with the thickness of the GeTe nanosheets reduced to 1.9 nm. The space-confined CVD method improves the crystallinity of ultrathin GeTe by lowering the partial pressure of the reactant gas, resulting in GeTe nanosheets with excellent p-type semiconductor properties, such as a satisfactory on/off ratio of 105. Temperature-dependent electrical measurements demonstrate that variable-range hopping and optical-phonon-assisted hopping mechanisms dominate transport behavior at low and high temperatures, respectively. GeTe devices exhibit significantly high responsivity (6589 and 2.2 A W-1 at 633 and 980 nm, respectively) and detectivity (1.67 × 1011 and 1.3 × 108 Jones at 633 and 980 nm, respectively), making them feasible for broadband photodetectors in the visible to near-infrared range. Furthermore, the fabricated GeTe/WS2 diode exhibits a rectification ratio of 103 at zero gate voltage. These satisfactory p-type semiconductor properties demonstrate that ultrathin GeTe exhibits enormous potential for applications in optoelectronic interconnection circuits.

Effective Nonstoichiometric Strategy Combined Post-annealing Process for Boosting Thermoelectric Properties in n-Type PbTe

The intrinsic low electrical properties have hindered the enhancement of thermoelectric performance for n-type PbTe over a long period of time, primarily due to the generation of intrinsic Pb vacancies and other defects. In this work, PbTe samples with nonstoichiometric excess Pb atoms were successfully prepared by a melting reaction followed by spark plasma sintering. First, the introduction of precisely controlled excess Pb atoms has effectively eliminated the typical p-n transition phenomenon in PbTe systems by suppressing the generation of Pb vacancies. Further, the vacuum annealing process employed in nonstoichiometric samples increases the carrier mobility significantly because of the improved crystallinity and the lowered holes. Thus, the Hall mobility was optimized from 754.3 to 1215.9 cm2 V-1 s-1, while the power factor was ultimately elevated from 3087.8 to 4565.7 μW m-1 K-2 for the Pb1.03Te sample at 323 K. Benefited from the enhanced electrical transport properties near room temperature, an average zT ∼ 1.03 ranging from 323 to 723 K was achieved, demonstrating an outstanding performance in n-type nondoped PbTe. This work provides guidance for optimizing the thermoelectric performance of n-type PbTe and relevant telluride by reducing vacancies and other defects.

Improved Thermoelectric Performance of Sb2Te3 Nanosheets by Coating Pt Particles in Wide Medium-Temperature Zone

The p-type Sb2Te3 alloy, a binary compound belonging to the V2VI3-based materials, has been widely used as a commercial material in the room-temperature zone. However, its low thermoelectric performance hinders its application in the low-medium temperature range. In this study, we prepared Sb2Te3 nanosheets coated with nanometer-sized Pt particles using a combination of solvothermal and photo-reduction methods. Our findings demonstrate that despite the adverse effects on certain properties, the addition of Pt particles to Sb2Te3 significantly improves the thermoelectric properties, primarily due to the enhanced electronic conductivity. The optimal ZT value reached 1.67 at 573 K for Sb2Te3 coated with 0.2 wt% Pt particles, and it remained above 1.0 within the temperature range of 333-573 K. These values represent a 47% and 49% increase, respectively, compared to the pure Sb2Te3 matrix. This enhancement in thermoelectric performance can be attributed to the presence of Pt metal particles, which effectively enhance carrier and phonon transport properties. Additionally, we conducted a Density Functional Theory (DFT) study to gain further insights into the underlying mechanisms. The results revealed that Sb2Te3 doped with Pt exhibited a doping level in the band structure, and a sharp rise in the Density of States (DOS) was observed. This sharp rise can be attributed to the presence of Pt atoms, which lead to enhanced electronic conductivity. In conclusion, our findings demonstrate that the incorporation of nanometer-sized Pt particles effectively improves the carrier and phonon transport properties of the Sb2Te3 alloy. This makes it a promising candidate for medium-temperature thermoelectric applications, as evidenced by the significant enhancement in thermoelectric performance achieved in this study.

Lead Vacancy Promotes Sodium Solubility to Achieve Ultra-High zT in Only Ternary Pb1- x Nax Te

Chemical doping of sodium is an indispensable means to optimize thermoelectric properties of PbTe materials, while a bottleneck is that an aliovalent atom doping leads to spontaneous intrinsic defects in the PbTe matrix, resulting in low dopant solubility. Therefore, it is urgent to improve the doping efficiency of Na for maximizing optimization. Here, an amazing new insight that the intentionally introduced Pb vacancies can promote Na solubility in ternary Pb_{1- x} Na_x Te is reported. Experimental analysis and theoretical calculations provide new insights into the inherent mechanism of the enhancement of Na solubility. The Pb vacancies and the resultant more dissolved Na not only synergistically optimize the carrier concentration and further facilitate the band convergence, but also induce a large number of dense dislocations in the grains. Consequently, benefiting from the self-enhancement of Seebeck coefficient and the minimization of lattice thermal conductivity, an 18% growth is obtained for the figure of merit zT in vacancy-containing Pb_0.95 Na_0.04 Te sample, reaching maximum zT_max  ≈ 2.0 at 823 K, which achieves an ultra-high performance in only Na-doped ternary Pb_{1- x} Na_x Te materials. The strategy utilized here provides a novel route to optimize PbTe materials and represents an important step forward in manipulating thermoelectrics to improve dopant solubility.

Copper-Based Diamond-like Thermoelectric Compounds: Looking Back and Stepping Forward

The research on thermoelectric (TE) materials has a long history. Holding the advantages of high elemental abundance, lead-free and easily tunable transport properties, copper-based diamond-like (CBDL) thermoelectric compounds have attracted extensive attention from the thermoelectric community. The CBDL compounds contain a large number of representative candidates for thermoelectric applications, such as CuInGa₂, Cu₂GeSe₃, Cu₃SbSe₄, Cu₁₂SbSe₁₃, etc. In this study, the structure characteristics and TE performances of typical CBDLs were briefly summarized. Several common synthesis technologies and effective strategies to improve the thermoelectric performances of CBDL compounds were introduced. In addition, the latest developments in thermoelectric devices based on CBDL compounds were discussed. Further developments and prospects for exploring high-performance copper-based diamond-like thermoelectric materials and devices were also presented at the end.

High-Conductivity Chalcogenide Glasses in Ag-Ga2Te3-SnTe Systems and Their Suitability as Thermoelectric Materials

A novel high-conductivity Ag_x[(Ga₂Te₃)₃₄(SnTe)₆₆]_100-x tellurium-based glassy system was fabricated via melt spinning with the glass formation area in the range of x = 0-15 mol %. A bulk Ag₁₀[(Ga₂Te₃)₃₄(SnTe)₆₆]₉₀ glass (A10) was obtained via spark plasma sintering at 450 K using a 5 min dwell time and 400 MPa pressure. The fabricated A10 glass exhibited higher room-temperature conductivity (σ_300 K = 46 S m^-1), larger glass transition temperature (T_g = 482 K), and ultralower thermal conductivity (∼0.19 W m^-1 K^-1) compared to those of previously reported Cu-Ge-Te, Cu-As-Te, Cu-Ge-As-Te, and Cu-As-Se-Te glassy systems with the approximate doping concentrations of 5-20%, demonstrating that this distinctive Ag-Ga₂Te₃-SnTe system is interesting materials for thermoelectric applications. The high-conductivity Ag-Ga₂Te₃-SnTe glassy system will extend investigations into similar glassy semiconductors and also can be used for preparing glass ceramics with potential applications in other fields.

Large-Scale Colloidal Synthesis of Chalcogenides for Thermoelectric Applications

A simple and effective preparation of solution-processed chalcogenide thermoelectric materials is described. First, PbTe, PbSe, and SnSe were prepared by gram-scale colloidal synthesis relying on the reaction between metal acetates and diphenyl dichalcogenides in hexadecylamine solvent. The resultant phase-pure chalcogenides consist of highly crystalline and defect-free particles with distinct cubic-, tetrapod-, and rod-like morphologies. The powdered PbTe, PbSe, and SnSe products were subjected to densification by spark plasma sintering (SPS), affording dense pellets of the respective chalcogenides. Scanning electron microscopy shows that the SPS-derived pellets exhibit fine nano-/micro-structures dictated by the original morphology of the key constituting particles, while the powder X-ray diffraction and electron microscopy analyses confirm that the SPS-derived pellets are phase-pure materials, preserving the structure of the colloidal synthesis products. The resultant solution-processed PbTe, PbSe, and SnSe exhibit low thermal conductivity, which might be due to the enhanced phonon scattering developed over fine microstructures. For undoped n-type PbTe and p-type SnSe samples, an expected moderate thermoelectric performance is achieved. In contrast, an outstanding figure-of-merit of 0.73 at 673 K was achieved for undoped n-type PbSe outperforming, the majority of the optimized PbSe-based thermoelectric materials. Overall, our findings facilitate the design of efficient solution-processed chalcogenide thermoelectrics.

Research Progress of Interfacial Design between Thermoelectric Materials and Electrode Materials

Intensive efforts have been conducted to realize the reliable interfacial joining of thermoelectric materials and electrode materials with low interfacial contact resistance, which is an essential step to make thermoelectric materials into thermoelectric devices for industrial application. In this review, the roles of structural integrity, interdiffusion, and contact resistance in long-term reliabilities of thermoelectric modules are outlined first. Then interfacial reactions of near-room-temperature Bi₂Te₃-based thermoelectric materials and various electrode materials are reviewed comprehensively. We also summarized the joining behavior of the mid-temperature PbTe-based thermoelectric materials and commonly used electrode materials. Subsequently, for other thermoelectric materials systems, i.e., SiGe, CoSb₃, and Mg₃Sb₂, previous attempts to join with some electrode materials are also recapitulated. Finally, some future prospects to further improve the joint reliability in thermoelectric device manufacturing are proposed. We believe that this review will provide guidance for preparing thermoelectric devices and optimizing thermoelectric device design.

Enhanced Thermoelectric Performance of n-Type PbTe via Carrier Concentration Optimization over a Broad Temperature Range

The optimal carrier concentration of thermoelectric materials increases with increasing temperature. However, conventional aliovalent doping usually provides an approximately constant carrier concentration over the whole temperature range, which can only match the optimal carrier concentration in a narrow temperature range. In this work, n-type indium and aluminum codoped PbTe were prepared with high-pressure synthesis, followed by spark plasma sintering. While Al doping can provide a roughly constant carrier concentration with varying temperatures, In doping can trap electrons at low temperatures and release them at high temperatures, thus optimizing the carrier concentration over a broad temperature range. As a result, both electrical transport properties and thermal conductivity are optimized, and a significantly enhanced thermoelectric performance is achieved in InxAl0.02Pb0.98Te. The optimal In0.008Al0.02Pb0.98Te shows a peak ZT of 1.3 and an average ZT of 1, with a decent conversion efficiency of 14%. Current work demonstrates that optimizing carrier concentration with varying temperatures is effective to enhance the thermoelectric performance of n-type PbTe.

Relevance of Solidification Kinetics for Enhanced Thermoelectric Performance in Al-Doped Higher Manganese Silicides

The solidification kinetics of an alloy from its liquid state forms an underlying basis for microstructural engineering, wherein the state of thermodynamic equilibrium associated with the melt-grown crystal and the quenched amorphous solid denotes the two limits for crystallinity in the alloy synthesis. In this study, we report the implication of the crystalline state on the thermal and electrical transport properties of partially substituted Mn(Si_1-xAl_x)_γ by comparing the single crystals melt-grown by the Bridgman method, and polycrystals synthesized from melt spinning (MS) and subsequent spark plasma sintering (SPS). The rapidly solidified alloys exhibited nanocrystalline microstructures in MS ribbons, while melt-grown single crystals displayed characteristics evolution of MnSi striations with limited solubility of Al. It was observed that Al as a p-type dopant enhances the carrier concentration and electrical conductivity, while nanocrystallinity in MS + SPS polycrystals and secondary phases in monocrystals were effective in enhancing the phonon scattering. Maximum zT values of ∼0.54 (±0.05) at 823 K and 0.75 (±0.05) at 873 K were attained for the single crystal (directed perpendicular to the c-axis) and melt-spun polycrystals (along the in-plane direction), respectively. These results present the efficacy of aliovalent Al substitution and demonstrate the critical role of the solidification kinetics in optimizing the carrier concentration and enhancing the phonon scattering in higher manganese silicide crystals for thermoelectric applications.

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.

Review on Wearable Thermoelectric Generators: From Devices to Applications

Wearable thermoelectric generators (WTEGs) can incessantly convert body heat into electricity to power electronics. However, the low efficiency of thermoelectric materials, tiny terminal temperature difference, rigidity, and neglecting optimization of lateral heat transfer preclude WTEGs from broad utilization. In this review, we aim to comprehensively summarize the state-of-the-art strategies for the realization of flexibility and high normalized power density in thermoelectric generators by establishing the links among materials, TE performance, and advanced design of WTEGs (structure, heatsinks, thermal regulation, thermal analysis, etc.) based on inorganic bulk TE materials. Each section starts with a concise summary of its fundamentals and carefully selected examples. In the end, we point out the controversies, challenges, and outlooks toward the future development of wearable thermoelectric devices and potential applications. Overall, this review will serve to help materials scientists, electronic engineers, particularly students and young researchers, in selecting suitable thermoelectric devices and potential applications.

Superconductivity in In-doped AgSnBiTe3 with possible band inversion

We investigated the chemical pressure effects on structural and electronic properties of SnTe-based material using partial substitution of Sn by Ag_0.5Bi_0.5, which results in lattice shrinkage. For Sn_1-2x(AgBi)_xTe, single-phase polycrystalline samples were obtained with a wide range of x. On the basis of band calculations, we confirmed that the Sn_1-2x(AgBi)_xTe system is basically possessing band inversion and topologically preserved electronic states. To explore new superconducting phases related to the topological electronic states, we investigated the In-doping effects on structural and superconducting properties for x = 0.33 (AgSnBiTe₃). For (AgSnBi)_(1-y)/3In_yTe, single-phase polycrystalline samples were obtained for y = 0-0.5 by high-pressure synthesis. Superconductivity was observed for y = 0.2-0.5. For y = 0.4, the transition temperature estimated from zero-resistivity state was 2.4 K, and the specific heat investigation confirmed the emergence of bulk superconductivity. Because the presence of band inversion was theoretically predicted, and the parameters obtained from specific heat analyses were comparable to In-doped SnTe, we expect that the (AgSnBi)_(1-y)/3In_yTe and other (Ag, In, Sn, Bi)Te phases are candidate systems for studying topological superconductivity.

Enhancing Thermoelectrics for Small-Bandwidth n-Type PbTe-MnTe Alloys via Balancing Compromise

Small-bandwidth n-type PbTe-MnTe alloys effectively modify the valley shape, while it also inevitably aggravates the deterioration of carrier mobility as nonpolar phonons dominate the scattering. It is found that a trace amount of Cu doping can alleviate the compromises among thermoelectric parameters, thereby significantly optimizing the electrical-transport performance near room temperature of n-type PbTe-MnTe alloys. The single-Kane model reveals that the physical origin of performance improvement lies in the carrier mobility enhancement and self-optimization of carrier concentration. The Debye-Callaway model further quantifies the contribution of copper defect scattering to the lattice thermal conductivity. Notably, the high thermoelectric quality factor obtained in this work rationalizes their superior properties and reveals immense potential for achieving higher zT. Herein, an extremely high zT of ∼0.52 at room temperature and a maximum zTmax of ∼1.2 at 823 K are achieved in 0.3% Cu-intercalated n-type PbTe-MnTe. The mechanism in balancing compromise elaborated in principle contributes to an improvement of thermoelectric properties of the n-type PbTe alloys in a broad temperature range.

Zn-Induced Defect Complexity for the High Thermoelectric Performance of n-Type PbTe Compounds

Although defect engineering is the core strategy to improve thermoelectric properties, there are limited methods to effectively modulate the designed defects. Herein, we demonstrate that a high ZT value of 1.36 at 775 K and a high average ZT value of 0.99 in the temperature range from 300 to 825 K are realized in Zn-containing PbTe by designing complex defects. By combining first-principles calculations and experiments, we show that Zn atoms occupy both Pb sites and interstitial sites in PbTe and couple with each other. The contraction stress induced via substitutional Zn on Pb sites alleviates the swelling stress by Zn atoms occupying the interstitial sites and promotes the solubility of interstitial Zn atoms in the structure of PbTe. The stabilization of Zn impurity as a complex defect extends the region of PbTe phase stability toward Pb_0.995Zn_0.02Te, while the solid solution region in the other direction of the ternary phase diagram is much smaller. The evolution of defects in PbTe was further explicitly corroborated by aberration-corrected scanning transmission electron microscopy (Cs-corrected STEM) and positron annihilation measurement. The Zn atoms compensate the Pb vacancies (V_Pb) and Zn interstitials (Zn_i) significantly improve the electron concentration, producing a high carrier mobility of 1467.7 cm² V^-1 s^-1 for the Pb_0.995Zn_0.02Te sample. A high power factor of 4.11 mW m^-1 K^-2 is achieved for the Pb_0.995Zn_0.02Te sample at 306 K. This work provides new insights into understanding the nature and evolution of the defects in n-type PbTe as well as improving the electronic and thermal transport properties toward higher thermoelectric performance.

Hydrostatic Pressure Tuning of Thermal Conductivity for PbTe and PbSe Considering Pressure-Induced Phase Transitions

Flexibly modulating thermal conductivity is of great significance to improve the application potential of materials. PbTe and PbSe are promising thermoelectric materials with pressure-induced phase transitions. Herein, the lattice thermal conductivities of PbTe and PbSe are investigated as a function of hydrostatic pressure by first-principles calculations. The thermal conductivities of both PbTe and PbSe in NaCl phase and Pnma phase exhibit complex pressure-dependence, which is mainly ascribed to the nonmonotonic variation of a phonon lifetime. In addition, the thermal transport properties of the Pnma phase behave anisotropically. The thermal conductivity of Pnma-PbTe is reduced below 1.1 W/m·K along the c-axis direction at 7-12 GPa. The mean free path for 50% cumulative thermal conductivity increases from 7 nm for NaCl-PbSe at 0 GPa to 47 nm for the Pnma-PbSe in the a-axis direction at 7 GPa, making it convenient for further thermal conductivity reduction by nanostructuring. The thermal conductivities of Pnma-PbTe in the c-axis direction and Pnma-PbSe in the a-axis direction are extremely low and hypersensitive to the nanostructure, showing important potential in thermoelectric applications. This work provides a comprehensive understanding of phonon behaviors to tune the thermal conductivity of PbTe and PbSe by hydrostatic pressure.