Enhancement of Thermoelectric Performance of n‐Type PbSe by Cr Doping with Optimized Carrier Concentration

Ti, V, Cr, Nb, and Mo are found to be effective at increasing the Seebeck coefficient and power factor of n‐type PbSe at temperatures below 600 K. It is found that the higher Seebeck coefficients and power factors are due to higher Hall mobility ≈1000 cm² V^?1s^?1 at lower carrier concentration. A larger average ZT value (relevant for applications) can be obtained by an optimization of carrier concentration to ≈10¹⁸–10¹⁹ cm^?3. Even though the highest room temperature power factor ≈3.3 × 10^?3 W m^?1 K^?2 is found in 1 at% Mo‐doped PbSe, the highest ZT is achieved in Cr‐doped PbSe. Combined with the lower thermal conductivity, ZT is improved to ≈0.4 at room temperature and peak ZTs of ≈1.0 are observed at ≈573 K for Pb_0.9925Cr_0.0075Se and ≈673 K for Pb_0.995Cr_0.005Se. The calculated device efficiency of Pb_0.995Cr_0.005Se is as high as ≈12.5% with cold side 300 K and hot side 873 K, higher than those of all the n‐type PbSe materials reported in the literature.     

Synergistically Optimizing Electrical and Thermal Transport Properties of BiCuSeO via a Dual‐Doping Approach

The layered oxyselenide BiCuSeO system is known as one of the high‐performance thermoelectric materials with intrinsically low thermal conductivity. By employing atomic, nano‐ to mesoscale structural optimizations, low thermal conductivity coupled with enhanced electrical transport properties can be readily achieved. Upon partial substitution of Bi³⁺ by Ca²⁺ and Pb²⁺, the thermal conductivity can be reduced to as low as 0.5 W m^?1 K^?1 at 873 K through dual‐atomic point‐defect scattering, while a high power factor of ≈1 × 10^?3 W cm^?1 K^?2 is realized over a broad temperature range from 300 to 873 K. The synergistically optimized power factor and intrinsically low thermal conductivity result in a high ZT value of ≈1.5 at 873 K for Bi_0.88Ca_0.06Pb_0.06CuSeO, a promising candidate for high‐temperature thermoelectric applications. It is envisioned that the all‐scale structural optimization is critical for optimizing the thermoelectricity of quaternary compounds.     

Studies on Thermoelectric Properties of n‐type Polycrystalline SnSe1‐xSx by Iodine Doping

Iodine‐doped n‐type SnSe polycrystalline by melting and hot pressing is prepared. The prepared material is anisotropic with a peak ZT of ≈0.8 at about 773 K measured along the hot pressing direction. This is the first report on thermoelectric properties of n‐type Sn chalcogenide alloys. With increasing content of iodine, the carrier concentration changed from 2.3 × 10¹⁷ cm^?3 (p‐type) to 5.0 × 10¹⁵ cm^?3 (n‐type) then to 2.0 × 10¹⁷ cm^?3 (n‐type). The decent ZT is mainly attributed to the intrinsically low thermal conductivity due to the high anharmonicity of the chemical bonds like those in p‐type SnSe. By alloying with 10 at% SnS, even lower thermal conductivity and an enhanced Seebeck coefficient were achieved, leading to an increased ZT of ≈1.0 at about 773 K measured also along the hot pressing direction.     

Amphoteric Indium Enables Carrier Engineering to Enhance the Power Factor and Thermoelectric Performance in n‐Type AgnPb100InnTe100+2n (LIST)

The Ag and In co‐doped PbTe, Ag_nPb₁₀₀In_nTe_100+2n (LIST), exhibits n‐type behavior and features unique inherent electronic levels that induce self‐tuning carrier density. Results show that In is amphoteric in the LIST, forming both In³⁺ and In¹⁺ centers. Through unique interplay of valence fluctuations in the In centers and conduction band filling, the electron carrier density can be increased from ≈3.1 × 10¹⁸ cm^?3 at 323 K to ≈2.4 × 10¹⁹ cm^?3 at 820 K, leading to large power factors peaking at ≈16.0 µWcm^?1 K^?2 at 873 K. The lone pair of electrons from In⁺ can be thermally continuously promoted into the conduction band forming In³⁺, consistent with the amphoteric character of In. Moreover, with rising temperature, the Fermi level shifts into the conduction band, which enlarges the optical band gap based on the Moss–Burstein effect, and reduces bipolar diffusion and thermal conductivity. Adding extra Ag in LIST improves the electrical transport properties and meanwhile lowers the lattice thermal conductivity to ≈0.40 Wm^?1 K^?1. The addition of Ag creates spindle‐shaped Ag₂Te nanoprecipitates and atomic‐scale interstitials that scatter a broader set of phonons. As a result, a maximum ZT value ≈1.5 at 873 K is achieved in Ag₆Pb₁₀₀InTe₁₀₂ (LIST).     

Integrating Band Structure Engineering with All‐Scale Hierarchical Structuring for High Thermoelectric Performance in PbTe System

PbTe_1?x Se_x ‐2%Na‐y%SrTe system is investigated and a high maximum ZT of 2.3 at 923 K for PbTe_0.85Se_0.15‐2%Na‐4%SrTe is reported. This is achieved by performing electronic band structures modifications as well as all‐scale hierarchical structuring and combining the two effects. It is found that high ZTs in PbTe_0.85Se_0.15‐2%Na‐4%SrTe are possible at all temperature from 300 to 873 K with an average ZT_ave of 1.23. The high performance in PbTe_1?x Se_x ‐2%Na‐y%SrTe can be achieved by either choosing PbTe‐2Na‐4SrTe or PbTe_0.85Se_0.15‐2Na as a matrix. At room temperature the carrier mobility shows negligible variations as SrTe fraction is increased, however the lattice thermal conductivity is significantly reduced from ≈1.1 to ≈0.82 W m^?1 K^?1 when 5.0% SrTe is added, correspondingly, the lattice thermal conductivity at 923 K decreases from ≈0.59 to ≈0.43 W m?¹ K^?1. The power factor maxima of PbTe_1?x Se_x ‐2Na‐4SrTe shift systematically to higher temperature with rising Se fractions due to bands divergence. The maximum power factors reach ≈27, ≈30, ≈31 μW cm^?1 K^?2 for the x = 0, 0.05, and 0.15 samples peak at 473, 573, and 623 K, respectively. The results indicate that ZT can be increased by synergistic integration of band structure engineering and all‐scale hierarchical architectures.     

Lithium Doping to Enhance Thermoelectric Performance of MgAgSb with Weak Electron–Phonon Coupling

Despite the unfavorable band structure with twofold degeneracy at the valence band maximum, MgAgSb is still an excellent p‐type thermoelectric material for applications near room temperature. The intrinsically weak electron–phonon coupling, reflected by the low deformation potential E_def ≈ 6.3 eV, plays a crucial role in the relatively high power factor of MgAgSb. More importantly, Li is successfully doped into Mg site to tune the carrier concentration, leading to the resistivity reduction by a factor of 3 and a consequent increase in power factor by ≈30% at 300 K. Low lattice thermal conductivity can be simultaneously achieved by all‐scale hierarchical phonon scattering architecture including high density of dislocations and nanoscale stacking faults, nanoinclusions, and multiscale grain boundaries. Collectively, much higher average power factor ≈25 μW cm^?1 K^?2 with a high average ZT ≈ 1.1 from 300 to 548 K is achieved for 0.01 Li doping, which would result in a high output power density ≈1.56 W cm^?2 and leg efficiency ≈9.2% by calculations assuming cold‐side temperature T_c = 323 K, hot‐side temperature T_h = 548 K, and leg length = 2 mm.     

High Thermoelectric Performance in PbSe–NaSbSe2 Alloys from Valence Band Convergence and Low Thermal Conductivity

PbSe is an attractive thermoelectric material due to its favorable electronic structure, high melting point, and lower cost compared to PbTe. Herein, the hitherto unexplored alloys of PbSe with NaSbSe₂ (NaPb_mSbSe_m+2) are described and the most promising p‐type PbSe‐based thermoelectrics are found among them. Surprisingly, it is observed that below 500 K, NaPb_mSbSe_m+2 exhibits unorthodox semiconducting‐like electrical conductivity, despite possessing degenerate carrier densities of ≈10²⁰ cm^?3. It is shown that the peculiar behavior derives from carrier scattering by the grain boundaries. It is further demonstrated that the high solubility of NaSbSe₂ in PbSe augments both the thermoelectric properties while maintaining a rock salt structure. Namely, density functional theory calculations and photoemission spectroscopy demonstrate that introduction of NaSbSe₂ lowers the energy separation between the L‐ and Σ‐valence bands and enhances the power factors under 700 K. The crystallographic disorder of Na⁺, Pb²⁺, and Sb³⁺ moreover provides exceptionally strong point defect phonon scattering yielding low lattice thermal conductivities of 1–0.55 W m^‐1 K^‐1 between 400 and 873 K without nanostructures. As a consequence, NaPb₁₀SbSe₁₂ achieves maximum ZT ≈1.4 near 900 K when optimally doped. More importantly, NaPb₁₀SbSe₁₂ maintains high ZT across a broad temperature range, giving an estimated record ZT_avg of ≈0.64 between 400 and 873 K, a significant improvement over existing p‐type PbSe thermoelectrics.     

Realizing High Thermoelectric Performance in n‐Type Highly Distorted Sb‐Doped SnSe Microplates via Tuning High Electron Concentration and Inducing Intensive Crystal Defects

In this study, a record high figure of merit (ZT) of ≈1.1 at 773 K is reported in n‐type highly distorted Sb‐doped SnSe microplates via a facile solvothermal method. The pellets sintered from the Sb‐doped SnSe microplates show a high power factor of ≈2.4 µW cm^?1 K^?2 and an ultralow thermal conductivity of ≈0.17 W m^?1 K^?1 at 773 K, leading a record high ZT. Such a high power factor is attributed to a high electron concentration of 3.94 × 10¹⁹ cm^?3 via Sb‐enabled electron doping, and the ultralow thermal conductivity derives from the enhanced phonon scattering at intensive crystal defects, including severe lattice distortions, dislocations, and lattice bent, observed by detailed structural characterizations. This study fills in the gaps of fundamental doping mechanisms of Sb in SnSe system, and provides a new perspective to achieve high thermoelectric performance in n‐type polycrystalline SnSe.     

Thermoelectric Property Studies on Cu‐Doped n‐type CuxBi2Te2.7Se0.3 Nanocomposites

Combining high energy ball‐milling and hot‐pressing, significant enhancements of the thermoelectric figure‐of‐merit (ZT) have been reported for p‐type Bi_0.4Sb_1.6Te₃ nanocomposites. However, applying the same technique to n‐type Bi₂Te_2.7Se_0.3 showed no improvement on ZT values, due to the anisotropic nature of the thermoelectric properties of n‐type Bi₂Te_2.7Se_0.3. Even though texturing was effective in improving peak ZT of Bi₂Te_2.7Se_0.3 from 0.85 to 1.04, reproducibility from batch to batch remains unsatisfactory. Here, we show that good reproducibility can be achieved by introducing an optimal concentration of 0.01 copper (Cu) per Bi₂Te_2.7Se_0.3 to make Cu_0.01Bi₂Te_2.7Se_0.3 samples. A peak ZT value of 0.99 was achieved in Cu_0.01Bi₂Te_2.7Se_0.3 samples without texturing. With texturing by re‐pressing, the peak ZT was increased to 1.06. Aging in air for over 5 months did not deteriorate but further improved the peak ZT to 1.10. The mechanism by which copper improves the reproducibility, enhances the carrier mobility, and reduces the lattice thermal conductivity is also discussed.     

A Dual Role by Incorporation of Magnesium in YbZn2Sb2 Zintl Phase for Enhanced Thermoelectric Performance

1‐2‐2‐type Zintl phase compounds have promising thermoelectric properties because of their complex crystal structures and multiple valence‐band structures. In this work, a series of single phase (Yb_0.9Mg_0.1)Mg_xZn_2?xSb₂ (x = 0, 0.2, 0.4, 0.6, 0.8, and 1) compounds are prepared by alloying YbZn₂Sb₂ with 10 at% MgZn₂Sb₂ and different amounts of YbMg₂Sb₂. The incorporation of Mg at the Yb site, as well as at the Zn site, not only leads to an effective orbital alignment confirmed by the dramatically enhanced density of states effective mass and Seebeck coefficients, but also increases the point defect scattering, contributing to a low lattice thermal conductivity ≈0.54 W m^?1 K^?1 at 773 K. Combined with the optimization of the carrier concentration by Ag doping at the Zn site, a highest ZT value ≈1.5 at 773 K is achieved in (Yb_0.9Mg_0.1)Mg_0.8Zn_1.2Ag_0.002Sb₂, which is higher than that of all the previously reported 1‐2‐2‐type Zintl phase compounds.     

n‐Type SnSe2 Oriented‐Nanoplate‐Based Pellets for High Thermoelectric Performance

It is reported that electron doped n‐type SnSe₂ nanoplates show promising thermoelectric performance at medium temperatures. After simultaneous introduction of Se deficiency and Cl doping, the Fermi level of SnSe₂ shifts toward the conduction band, resulting in two orders of magnitude increase in carrier concentration and a transition to degenerate transport behavior. In addition, all‐scale hierarchical phonon scattering centers, such as point defects, nanograin boundaries, stacking faults, and the layered nanostructures, cooperate to produce very low lattice thermal conductivity. As a result, an enhanced in‐plane thermoelectric figure of merit ZT_max of 0.63 is achieved for a 1.5 at% Cl doped SnSe_1.95 pellet at 673 K, which is much higher than the corresponding in‐plane ZT of pure SnSe₂ (0.08).     

Enhancement in Thermoelectric Figure of Merit in Nanostructured Bi2Te3 with Semimetal Nanoinclusions

The effect of Bi (semimetal) nanoinclusions in nanostructured Bi₂Te₃ matrices is investigated. Bismuth nanoparticles synthesized by a low temperature solvothermal method are incorporated into Bi₂Te₃ matrix phases, synthesized by planetary ball milling. High density pellets of the Bi nanoparticle/Bi₂Te₃ nanocomposites are created by hot pressing the powders at 200 °C and 100 MPa. The effect of different volume fractions (0–7%) of Bi semimetal nanoparticles on the Seebeck coefficient, electrical conductivity, thermal conductivity and carrier concentration is reported. Our results show that the incorporation of semimetal nanoparticles results in a reduction in the lattice thermal conductivity in all the samples. A significant enhancement in power factor is observed for Bi nanoparticle volume fraction of 5% and 7%. We show that it is possible to reduce the lattice thermal conductivity and increase the power factor resulting in an increase in figure of merit by a factor of 2 (from ZT = 0.2 to 0.4). Seebeck coefficient and electrical conductivity as a function of carrier concentration data are consistent with the electron filtering effect, where low‐energy electrons are preferentially scattered by the barrier potentials set up at the semimetal nanoparticle/semiconductor interfaces.     

Engineering the Thermoelectric Transport in Half‐Heusler Materials through a Bottom‐Up Nanostructure Synthesis

Half‐Heusler (HH) alloys are among the best promising thermoelectric (TE) materials applicable for the middle‐to‐high temperature power generation. Despite of the large thermoelectric power factor and decent figure‐of‐merit ZT (≈1), their broad applications and enhancement on TE performance are limited by the high intrinsic lattice thermal conductivity (κ_L) due to insufficiencies of phonon scattering mechanisms, and the fewer powerful strategies associated with the microstructural engineering for HH materials. This study reports a bottom‐up nanostructure synthesis approach for these HH materials based on the displacement reaction between metal chlorides/bromides and magnesium (or lithium), followed by vacuum‐assisted spark plasma sintering process. The samples are featured with dense dislocation arrays at the grain boundaries, leading to a minimum κ_L of ≈1 W m^?1 K^?1 at 900 K and one of the highest ZT (≈1) and predicted η (≈11%) for n‐type Hf_0.25Zr_0.75NiSn_0.97Sb_0.03. Further manipulation on the dislocation defects at the grain boundaries of p‐type Nb_0.8Ti_0.2FeSb leads to enhanced maximum power factor of 47 × 10^?4 W m^?1 K^?2 and the predicted η of ≈7.5%. Moreover, vanadium substitution in FeNb_0.56V_0.24Ti_0.2Sb significantly promotes the η to ≈11%. This strategy can be extended to a broad range of advanced alloys and compounds for improved properties.     

Progressive Regulation of Electrical and Thermal Transport Properties to High‐Performance CuInTe2 Thermoelectric Materials

p‐type CuInTe₂ thermoelectric (TE) materials are of great interest for applications in the middle temperature range because of their environmentally benign chemical component and stable phase under operating temperatures. In order to enhance their TE performance to compete with the Pb based TE materials, a progressive regulation of electrical and thermal transport properties has been employed in this work. Anion P and Sb substitution is used to tune the electrical transport properties of CuInTe₂ for the first time, leading to a sharp enhancement in power factor due to the reduction of electrical resistivity by acceptor doping and the increase of the Seebeck coefficient resulted from the improvement of density of states. Concurrently, In₂O₃ nanoinclusions are introduced through an in situ oxidation between CuInTe₂ and ZnO additives, rendering a great reduction in the thermal conductivity of CuInTe₂ by the extra phonon scattering. Then, by integrating the anion substitution and nanoinclusions, a high power factor of 1445 μW m^?1 K^?2 and enhanced ZT of 1.61 at 823 K are achieved in the CuInTe₂ based TE material. This implies that the synergistic regulation of electrical and thermal transport properties by anion substitution and in situ nanostructure is a very effective approach to improve the TE performance of CuInTe₂ compounds.     

High Thermoelectric Performance in p‐type Polycrystalline Cd‐doped SnSe Achieved by a Combination of Cation Vacancies and Localized Lattice Engineering

Herein, a high figure of merit (ZT) of ≈1.7 at 823 K is reported in p‐type polycrystalline Cd‐doped SnSe by combining cation vacancies and localized‐lattice engineering. It is observed that the introduction of Cd atoms in SnSe lattice induce Sn vacancies, which act as p‐type dopants. A combination of facile solvothermal synthesis and fast spark plasma sintering technique boosts the Sn vacancy to a high level of ≈2.9%, which results in an optimum hole concentration of ≈2.6 × 10¹⁹ cm^?3 and an improved power factor of ≈6.9 µW cm^?1 K^?2. Simultaneously, a low thermal conductivity of ≈0.33 W m^?1 K^?1 is achieved by effective phonon scattering at localized crystal imperfections, as observed by detailed structural characterizations. Density functional theory calculations reveal that the role of Cd atoms in the SnSe lattice is to reduce the formation energy of Sn vacancies, which in turn lower the Fermi level down into the valence bands, generating holes. This work explores the fundamental Cd‐doping mechanisms at the nanoscale in a SnSe matrix and demonstrates vacancy and localized‐lattice engineering as an effective approach to boosting thermoelectric performance. The work provides an avenue in achieving high‐performance thermoelectric properties of materials.     

High‐Efficiency Thermoelectric Power Generation Enabled by Homogeneous Incorporation of MXene in (Bi,Sb)2Te3 Matrix

The (Bi,Sb)₂Te₃ (BST) compounds have long been considered as the benchmark of thermoelectric (TE) materials near room temperature especially for refrigeration. However, their unsatisfactory TE performances in wide‐temperature range severely restrict the large‐scale applications for power generation. Here, using a self‐assembly protocol to deliver a homogeneous dispersion of 2D inclusion in matrix, the first evidence is shown that incorporation of MXene (Ti₃C₂T_x) into BST can simultaneously achieve the improved power factor and greatly reduced thermal conductivity. The oxygen‐terminated Ti₃C₂T_x with proper work function leads to highly increased electrical conductivity via hole injection and retained Seebeck coefficient due to the energy barrier scattering. Meanwhile, the alignment of Ti₃C₂T_x with the layered structure significantly suppresses the phonon transport, resulting in higher interfacial thermal resistance. Accordingly, a peak ZT of up to 1.3 and an average ZT value of 1.23 from 300 to 475 K are realized for the 1 vol% Ti₃C₂T_x/BST composite. Combined with the high‐performance composite and rational device design, a record‐high thermoelectric conversion efficiency of up to 7.8% is obtained under a temperature gradient of 237 K. These findings provide a robust and scalable protocol to incorporate MXene as a versatile 2D inclusion for improving the overall performance of TE materials toward high energy‐conversion efficiency.     

Weak Electron Phonon Coupling and Deep Level Impurity for High Thermoelectric Performance Pb1−xGaxTe

High ZT of 1.34 at 766 K and a record high average ZT above 1 in the temperature range of 300‐864 K are attained in n‐type PbTe by engineering the temperature‐dependent carrier concentration and weakening electron–phonon coupling upon Ga doping. The experimental studies and first principles band structure calculations show that doping with Ga introduces a shallow level impurity contributing extrinsic carriers and imparts a deeper impurity level that ionizes at higher temperatures. This adjusts the carrier concentration closer to the temperature‐dependent optimum and thus maximizes the power factor in a wide temperature range. The maximum power factor of 35 µW cm⁻¹ K⁻² is achieved for the Pb_0.98Ga_0.02Te compound, and is maintained over 20 µWcm⁻¹ K⁻² from 300 to 767 K. Band structure calculations and X‐ray photoelectron spectroscopy corroborate the amphoteric role of Ga in PbTe as the origin of shallow and deep levels. Additionally, Ga doping weakens the electron–phonon coupling, leading to high carrier mobilities in excess of 1200 cm² V⁻¹ s⁻¹. Enhanced point defect phonon scattering yields a reduced lattice thermal conductivity. This work provides a new avenue, beyond the conventional shallow level doping, for further improving the average ZT in thermoelectric materials.     

Subtle Roles of Sb and S in Regulating the Thermoelectric Properties of N‐Type PbTe to High Performance

A high ZT (thermoelectric figure of merit) of ≈1.4 at 900 K for n‐type PbTe is reported, through modifying its electrical and thermal properties by incorporating Sb and S, respectively. Sb is confirmed to be an amphoteric dopant in PbTe, filling Te vacancies at low doping levels (<1%), exceeding which it enters into Pb sites. It is found that Sb‐doped PbTe exhibits much higher carrier mobility than similar Bi‐doped materials, and accordingly, delivers higher power factors and superior ZT . The enhanced electronic transport is attributed to the elimination of Te vacancies, which appear to strongly scatter n‐type charge carriers. Building on this result, the ZT of Pb_0.9875Sb_0.0125Te is further enhanced by alloying S into the Te sublattice. The introduction of S opens the bandgap of PbTe, which suppresses bipolar conduction while simultaneously increasing the electron concentration and electrical conductivity. Furthermore, it introduces point defects and induces second phase nanostructuring, which lowers the lattice thermal conductivity to ≈0.5 W m^?1 K^?1 at 900 K, making this material a robust candidate for high‐temperature (500–900 K) thermoelectric applications. It is anticipated that the insights provided here will be an important addition to the growing arsenal of strategies for optimizing the performance of thermoelectric materials.     

Weak Electron–Phonon Coupling and Enhanced Thermoelectric Performance in n-type PbTe–Cu2Se via Dynamic Phase Conversion

This study investigates Ga-doped n-type PbTe thermoelectric materials and the dynamic phase conversion process of the second phases via Cu₂Se alloying. Introducing Cu₂Se enhances its electrical transport properties while reducing its lattice thermal conductivity (κ_lat) via weak electron–phonon coupling. Cu₂Te and CuGa(Te/Se)₂ (tetragonal phase) nanocrystals precipitate during the alloying process, resulting in Te vacancies and interstitial Cu in the PbTe matrix. At room temperature, Te vacancies and interstitial Cu atoms serve as n-type dopants, increasing the carrier concentration and electrical conductivity from ≈1.18 × 10¹⁹ cm⁻³ and ≈1870 S cm⁻¹ to ≈2.26 × 10¹⁹ cm⁻³ and ≈3029 S cm⁻¹, respectively. With increasing temperature, the sample exhibits a dynamic change in Cu₂Te content and the generation of a new phase of CuGa(Te/Se)₂ (cubic phase), strengthening the phonon scattering and obtaining an ultralow κ_lat. Pb_0.975Ga_0.025Te-3%CuSe exhibits a maximum figure of merit of ≈1.63 at 823 K, making it promising for intermediate-temperature device applications.     

High Thermoelectric Performance in Polycrystalline SnSe Via Dual‐Doping with Ag/Na and Nanostructuring With Ag8SnSe6

Single crystalline SnSe is one of the most intriguing new thermoelectric materials but the thermoelectric performance of polycrystalline SnSe seems to lag significantly compared to that of a single crystal. Here an effective strategy for enhancing the thermoelectric performance of p‐type polycrystalline SnSe by Ag/Na dual‐doping and Ag₈SnSe₆ (STSe) nanoprecipitates is reported. The Ag/Na dual‐doping leads to a two orders of magnitude increase in carrier concentration and a convergence of valence bands (VBM₁ and VBM₅), which in turn results in sharp enhancement of electrical conductivities and high Seebeck coefficients in the Ag/Na dual‐doped samples. Additionally, the SnSe matrix becomes nanostructured with dispersed nanoprecipitates of the compound Ag₈SnSe₆, which further strengthens the scattering of phonons. Specifically, ≈20% reduction in the already ultralow lattice thermal conductivity is realized for the Sn_0.99Na_0.01Se–STSe sample at 773 K compared to the thermal conductivity of pure SnSe. Consequently, a peak thermoelectric figure of merit ZT of 1.33 at 773 K with a high average ZT (ZT_ave) value of 0.91 (423–823 K) is achieved for the Sn_0.99Na_0.01Se–STSe sample.