Figure Of ZT Research Articles

Impact of the nanostructuring on the thermal and thermoelectric properties of α-SrSi2

α-SrSi2 is a promising material for thermoelectric application around room temperature, especially if we can decrease the lattice contribution to the thermal conductivity. We report in this article the effect of the nanostructuring and the effect of the purity on the thermal and thermoelectric properties. Combining ball milling and spark plasma sintering we show that it is possible to obtain nanostructured pellets with grain size about 200 nm which permit to decrease the lattice components to the thermal conductivity close to that expected to highly disordered or amorphous materials. The results obtained in this study also show that the figure of merit ZT is improved after nanostructuring to a value of 0.20 around room temperature. The effect of the Sr purity on the thermoelectric properties is also presented.

Vacancy Suppression Induced Synergetic Optimization of Thermoelectric Performance in Sb-Doped GeTe Evidenced by Positron Annihilation Spectroscopy.

Synergetic optimization of the electrical and thermal transport performance of GeTe has been achieved through Sb doping in this work, resulting in a high thermoelectric figure of merit ZT of 2.2 at 723 K. Positron annihilation measurements provided clear evidence that Sb doping in GeTe can effectively suppress the Ge vacancies, and the decrease of vacancy concentration coincides well with the change of hole carrier concentration after Sb doping. The decreased scattering by hole carriers and vacancies causes notable increase in carrier mobility. Despite this, the density of states effective mass is not enhanced by Sb doping, a maximum power factor of 4562 μW m-1 K-2 at 723 K is obtained for Ge0.94Sb0.06Te with an optimized carrier concentration of ∼3.65 × 1020 cm-3. Meanwhile, the electronic thermal conductivity κe is reduced because of the decreased electrical conductivity σ with the increase of the Sb doping amount. In addition, the lattice thermal conductivity κL is also suppressed due to multiple phonon scattering mechanism, such as the large mass and strain fluctuations by the substitution of Sb for Ge atoms, and also the unique microstructure including grain boundary, nano-pore, and dislocation in the samples. In conclusion, a maximum ZT of 2.2 is gained at 723 K, which contributes to preferable TE property for GeTe-based materials.

Structure and thermoelectric properties of half-Heusler-like TiFeCuxSb alloys

Previous research on the ternary Ti-Fe-Sb system has revealed that stoichiometric TiFeSb cannot exist as a stable compound, whereas a single-phase TiFe1.33Sb alloy with the half-Heusler-like structure has been synthesized by adding excessive Fe. In this work, we report that TiFeSb can also be stabilized by filling additional Cu to the vacant 4d site of the half-Heusler lattice. Our experiments indicate that the TiFeCuxSb (x = 0–0.25) samples exhibit a p-type conduction with extremely high carrier concentration ((0.5 –2.5) × 1022 cm−3)), while these samples attain very large Seebeck coefficients, over 100 μV/K in the whole measured temperature range for the samples with x = 0.15–0.25. In addition, a logarithmic divergence of the temperature-dependent specific heat capacity (CP/T) is observed at low temperatures, implying the strange-metal behavior of TiFeCuxSb samples. The partial filling of the vacant 4d site results in significantly reduced lattice thermal conductivity, leading to the low total thermal conductivity of 2.8 W·m−1·K−1 at 823 K for the TiFeCu0.20Sb sample. Consequently, a dimensionless figure of merit zT of 0.54 at 923 K is realized for TiFeCu0.20Sb, demonstrating that promising thermoelectric materials with intriguing physical properties can be discovered in the composition gap of half- and full-Heusler alloys.

Large Improvement of Thermoelectric Performance by Magnetism in Co-Based Full-Heusler Alloys.

Full-Heusler alloys (fHAs) exhibit high mechanical strength with earth-abundant elements, but their metallic properties tend to display small electron diffusion thermopower, limiting potential applications as excellent thermoelectric (TE) materials. Here, it is demonstrated that the Co-based fHAs Co2 XAl (X = Ti, V, Nb) exhibit relatively high thermoelectric performance due to spin and charge coupling. Thermopower contributions from different magnetic mechanisms, including spin fluctuation and magnon drag are extracted. A significant contribution to thermopower from magnetism compared to that from electron diffusion is demonstrated. In Co2 TiAl, the contribution to thermopower from spin fluctuation is higher than that from electron diffusion, resulting in an increment of 280µWm-1 K-2 in the power factor value. Interestingly, the thermopower contribution from magnon drag can reach up to -47µVK-1 , which is over 2400% larger than the electron diffusion thermopower. The power factor of Co2 TiAl can reach 4000µWm-1 K-2 which is comparable to that of conventional semiconducting TE materials. Moreover, the corresponding figure of merit zT can reach ≈0.1 at room temperature, which is significantly larger than that of traditional metallic materials. The work shows a promising unconventional way to create and optimize TE materials by introducing magnetism.

External magnetic field effects on the thermoelectric and thermodynamic properties of doped monolayer S-graphene: A theoretical study

This study explores the electronic, thermoelectric, and thermal properties of S-Graphene, a unique graphene allotrope with two Dirac cones and zero band gap, in the presence of magnetic field and doping. A full tight-binding model and Green function approach are utilized to study the effects of robustness, magnetic field, and doping on the thermoelectric properties of S-Graphene, including the heat capacity C(T), magnetic susceptibility χ(T), thermoelectric figure of merit ZT(T), and Seedbeck coefficients S(T) and the results are compared with those for T-Graphene. To including the robustness effects, four S1–S4 structures with different hopping parameters are selected. The results demonstrate that the temperature dependent of thermal properties of S-Graphene are strongly affected by the external parameters such as external field, chemical potential and Robustness effects. The C(T) exhibits a Schottky anomaly whose position is dependent on the magnetic field and doping parameters. Pure cases demonstrate minimum values for C(T) and χ(T), while electron/hole doping leads to increase their intensity due to enhanced density concentration of charge carriers. Additionally, the behavior of ZT(T) and S(T) depends on the structure type of S-Graphene and displays intriguing features in different temperature regions. The magnetic field significantly affects the number, position and intensity of peaks in S(μ). These findings provide important insights into the electronic, thermoelectric, and thermal properties of S-Graphene and offer possibilities for future device applications.

Attaining High Figure of Merit in the N-Type Bi2Te2.7Se0.3-Ag2Te Composite System via Comprehensive Regulation of Its Thermoelectric Properties.

n-Type Bi2Te2.7Se0.3 (BTS) is the state-of-the-art thermoelectric material near room temperature. However, the figure of merit ZT of commercial BTS ingots is still limited and further improvement is imperative for their wide applications. Here, the results show that through dispersion of the Ag2Te nanophase in BTS, one can not only elevate its power factor (PF) by as high as 14% (at 300 K) but also reduce its thermal conductivity κtot to as small as ∼29% (at 300 K). Experimental evidences show that the improved PF comes from both increased electron mobility via inhibited Te vacancies and enhanced thermopower due to energy filtering effect, while the reduction of κtot originates from the drop of both electronic thermal conductivity largely owing to the reduced number of vacancy VTe·· and intensified phonon scattering chiefly from the dispersed Ag2Te nanophase. Consequently, the largest ZTmax = 1.31 (at 350 K) and average ZTave = 1.16 (300-500 K) are achieved for the Bi2Te2.7Se0.3-0.3 wt % Ag2Te composite sample, leading to a projected conversion efficiency η = 8.3% (300-500 K). The present results demonstrate that incorporation of nanophase Ag2Te is an effective approach to boosting the thermoelectric performance of BTS.

Strategies to enhance polycrystal SnSe thermoelectrics: Structure control offers a novel direction

Thermoelectric materials can directly convert between thermal energy and electrical energy, and their efficiency is determined by the average dimensionless figure of merit ZT. The crystal SnSe has been discovered with unique transport features and ultra-high ZT values. Considering the high production cost, long processing time, and poor mechanical properties of crystal SnSe, it is important to achieve mass production and commercial applications in polycrystal SnSe with good processability and low cost. This perspective summarizes the strategies to improve the thermoelectric properties of polycrystal SnSe, including composition control to tune the carrier concentration and modify the band structure, and process control to texture the grains and remove oxides, which all mainly focused on enhancing the peak ZT values of polycrystal SnSe. Recently, a novel structure control strategy was implemented to modulate the crystal structure of SnSe. By alloying proper ternary compounds, the cubic-structure polycrystal SnSe was obtained with completely different thermoelectric transports and high-ranged overall ZT values. This structure control strategy offers an effective way to enhance the wide-range thermoelectric performance of polycrystal SnSe and provides a new research idea for the development of highly efficient thermoelectric materials, especially for those with low-dimensional structures.

Electronic transport computation in thermoelectric materials: from ab initio scattering rates to nanostructures

Over the last two decades a plethora of new thermoelectric materials, their alloys, and their nanostructures were synfthesized. The ZT figure of merit, which quantifies the thermoelectric efficiency of these materials increased from values of unity to values consistently beyond two across material families. At the same time, the ability to identify and optimize such materials, has stressed the need for advanced numerical tools for computing electronic transport in materials with arbitrary bandstructure complexity, multiple scattering mechanisms, and a large degree of nanostructuring. Many computational methods have been developed, the majority of which utilize the Boltzmann transport equation (BTE) formalism, spanning from fully ab initio to empirical treatment, with varying degree of computational expense and accuracy. In this paper we describe a suitable computational process that we have recently developed specifically for thermoelectric materials. The method consists of three independent software packages that we have developed and: (1) begins from ab initio calculation of the electron–phonon scattering rates, (2) to then be used within a Boltzmann transport simulator, and (3) calculated quantities from the BTE are then passed on to a Monte Carlo simulator to examine electronic transport in highly nanostructured material configurations. The method we describe is computationally significantly advantageous compared to current fully ab initio and existing Monte Carlo methods, but with a similar degree of accuracy, thus making it truly enabling in understanding and assessing thermoelectric transport in complex band, nanostructured materials.

Flexible Ag-S-Te System with Promising Room-Temperature Thermoelectric Performance.

Silver chalcogenides demonstrate great potential as flexible thermoelectric materials due to their excellent ductility and tunable electrical and thermal transport properties. In this work, we report that the amorphous/crystalline phase ratio and thermoelectric properties of the Ag2SxTe1-x (x = 0.55-0.75) samples can be modified by altering the S content. The room-temperature power factor of the Ag2S0.55Te0.45 sample is 4.9 μW cm-1 K-2, and a higher power factor can be achieved by decreasing the carrier concentration as predicted by the single parabolic band model. The addition of a small amount of excessive Te into Ag2S0.55Te0.45 (Ag2S0.55Te0.45+y) not only enhances the power factor by decreasing the carrier concentration but also reduces the total thermal conductivity due to decreased electronic thermal conductivity. Owing to the effectively optimized carrier concentration, the thermoelectric power factor and dimensionless figure of merit zT of the sample with y = 0.007 reaches, respectively, 6.2 μW cm-1 K-2 and 0.39, while the excellent plastic deformability is well maintained, demonstrating its promising potential as a flexible thermoelectric material at room temperature.

Full thermoelectric characterization of a single molecule

Molecules are predicted to be chemically tunable towards high thermoelectric efficiencies and they could outperform existing materials in the field of energy conversion. However, their capabilities at the more technologically relevant temperature of 300 K are yet to be demonstrated. A possible reason could be the lack of a comprehensive technique able to measure the thermal and (thermo)electrical properties, including the role of phonon conduction. Here, by combining the break junction technique with a suspended heat-flux sensor, we measured the total thermal and electrical conductance of a single molecule, at room temperature, together with its Seebeck coefficient. We used this method to extract the figure of merit zT of a tailor-made oligo(phenyleneethynylene)-9,10-anthracenyl molecule with dihydrobenzo[b]thiophene anchoring groups (DHBT-OPE3-An), bridged between gold electrodes. The result is in excellent agreement with predictions from density functional theory and molecular dynamics. This work represents the first measurement, within the same setup, of experimental zT of a single molecule at room temperature and opens new opportunities for the screening of several possible molecules in the light of future thermoelectric applications. The protocol is verified using SAc-OPE3, for which individual measurements for its transport properties exist in the literature.

Ab initio predictions of thermoelectric, mechanical, and phonon characteristics of FeTiSe half-Heusler compound

In order to meet the increasing global energy demand, half-Heusler (H–H) alloys are found to be a cheap and efficient choice for energy generation applications. The electrical, structural, thermoelectric, dynamical, and mechanical properties of the novel FeTiSe half-Heusler alloy were predicted using the density functional theory (DFT). The equilibrium lattice constant was obtained as 5.62 Å. The electronic band-structure (B–S) and projected-density of states (P-DOS) were evaluated, and the band gap was obtained as 0.813 eV. Furthermore, applying the density functional perturbation theory (DFPT), we predicted the alloy's dynamical stability. Our findings from elastic constants revealed that the alloy is stiff, brittle, elastically anisotropic, and mechanically stable. For the n-type and p-type compositions, the Seebeck coefficients were found to be −273.53 and 190.13 μV/K at 1300 K, and figure of merit zT = 1.02 and 0.46 and zT = 2.92 and 2.8 at 300 and 1300 K, respectively. Thus, FeTiSe exhibits promising thermoelectric characteristics.

Structure and thermoelectric property evolution of TiFe1.1−xNixSb with the filling of Ni

TiFeSb is a multiphase material with the half-Heusler structure as the main phase. In this work, we discover that TiFeSb can be stabilized by filling Ni to the vacant 4d site of the half-Heusler structure, and a single-phase TiFeNi0.1Sb alloy is obtained. To improve the thermoelectric properties, TiFe1.1−xNixSb (x = 0.1–1.0) samples with more valence electrons are designed and synthesized. Structural analysis reveals that the samples with x = 0–0.3 are single-phase alloys with the half-Heusler-like structure, the samples with x = 0.4–0.7 are two-phase materials with the double half-Heusler as the main phase, and the samples with x = 0.8–1.0 are single-phase alloys with the full-Heusler-like structure. Meanwhile, both promising p-type (x = 0–0.3) and n-type (x = 0.8–1.0) electrical properties are realized in TiFe1.1−xNixSb, which can be well explained by the Slater-Pauling rule. In combination with greatly reduced lattice thermal conductivity owing to the disordered crystal structure, the p-type TiFe0.8Ni0.3Sb and n-type TiFe0.3Ni0.8Sb achieve dimensionless thermoelectric figure of merits zT about 0.6 and 0.4 at 973 K, respectively. We also demonstrate that the thermoelectric properties of these samples can be further optimized by conventional doping or alloying methods. By replacing Ni (or Fe) with Cu, the carrier concentration of p-type TiFe0.8Ni0.2Cu0.1Sb is decreased, and the zT value reaches 0.8 at 973 K, which is the record of vacancy-filled Heusler alloys.

Phase formation and thermoelectric properties of FeSe2–CoSe2 system

The phase formation and thermoelectric transport properties of FeSe2–CoSe2 systems were investigated by synthesizing a series of (Fe1–xCox)Se2 polycrystalline alloys (x = 0, 0.25, 0.5, 0.75, and 1). It was observed that as x increased from 0 to 0.5, lattice parameters increased while the orthorhombic structure of FeSe2 was retained. The bipolar conduction behavior of FeSe2 (x = 0) changed to n-type conduction at all measured temperatures. The electrical conductivity is greatly increased with x, and the largely enhanced power factor of 1.37 mW/mK2 at 600 K is seen for Fe0·5Co0·5Se2 (x = 0.5) compared to 0.37 mW/mK2 for FeSe2. As x is further increased to 0.75 (Fe0·25Co0·75Se2), the cubic phase of CoSe2 started to form and no further enhancement of power factor is seen. Lattice thermal conductivity is gradually decreased to 3.18 and 1.79 W/mK Fe0·75Co0·25Se2 (x = 0.25) and Fe0·5Co0·5Se2 (x = 0.5) at 600 K, compared to 4.36 W/mK for x = 0 (FeSe2). Consequently, the thermoelectric figure of merit zT at 600 K for Fe0·75Co0·25Se2 (x = 0.25) and Fe0·5Co0·5Se2 (x = 0.5) is greatly enhanced to 0.14 and 0.16, respectively, compared to 0.05 for x = 0 (FeSe2).

Structure Evolution and Bonding Inhomogeneity toward High Thermoelectric Performance in Cu2CoSnS4-xSex Materials.

Lightweight diamond-like structure (DLS) materials are excellent candidates for thermoelectric (TE) applications due to their low costs, eco-friendly nature, and property stability. The main obstacles restricting the energy-conversion performance by the lightweight DLS materials are high lattice thermal conductivity and relatively low carrier mobility. By investigating the anion substitution effect on the structural, microstructural, electronic, and thermal properties of Cu2CoSnS4-xSex, we show that the simultaneous enhancement of the crystal symmetry and bonding inhomogeneity engineering are effective approaches to enhance the TE performance in lightweight DLS materials. Particularly, the increase of x in Cu2CoSnS4-xSex makes the DLS structure with the ideal tetrahedral bond angles of 109.5° favorable, leading to better crystal symmetry and higher carrier mobility in samples with higher selenium content. In turn, the phonon transport in the investigated DLS materials is strongly disturbed due to the bonding inhomogeneity between anions and three sorts of cations inducing large lattice anharmonicity. The increase of Se content in Cu2CoSnS4-xSex only intensified this effect resulting in a lower lattice component of the thermal conductivity (κL) for Se-rich samples. As a result of the enhanced power factor S2ρ-1 and the low κL, the dimensionless thermoelectric figure of merit ZT achieves a high value of 0.75 for Cu2CoSnSe4 DLS material. This work demonstrates that crystal symmetry and bonding inhomogeneity play an important role in the transport properties of DLS materials and provide a path for the development of new perspective materials for TE energy conversion.

Enhanced thermoelectric properties of SWCNT by new nitrogen chains doping

Single walled carbon nanotubes (SWCNT) are attractive materials for thermoelectric (TE) devices because of their unique one-dimensional structure and provide us for developing a flexible TE devices due to their mechanical properties. In this paper, the TE properties of pristine small SWCNT (3,3) and three new nitrogen (N) doping models namely CN, C2N2 (doping by two N-chains) and C4N2 (doping by three N-chains) are calculated using the density functional theory (DFT) combined with Boltzmann's semi-classical transport theory. Dynamical and energetic stability of the compounds are confirmed from the phonon density of states curve and formation energy calculations. The values of Seebeck coefficient (S), electrical conductivity (σ), thermal conductivity (κ), power factor (PF) and figure of merit (ZT) are reported at different temperature and discussed in detail. The results show that N-doping significantly improves the TE properties of the pristine nanotube and show a reasonable agreement with previous experiments and ab-initio calculations. It was found that the C2N2 configuration presents the largest Seebeck coefficient of 70.53 μV/K and the figure of merit ZT of 0.25 compared to other doping models. N-chains doping is therefore a new suitable way to improve the TE properties of small SWCNT in order to making them a good candidate for flexible TE devices.

Selective Scatterings of Phonons and Electrons in Defective Half-Heusler Nb1- δ CoSb for the Figure of Merit zT > 1.

The recently developed defective 19-electron half-Heusler (HH) compounds, represented by Nb1- δ CoSb, possess massive intrinsic vacancies at the cation site and thus intrinsically low lattice thermal conductivity that is desirable for thermoelectric (TE) applications. Yet the TE performance of defective HHs with a maximum figure of merit (zT) <1.0 is still inferior to that of the conventional 18-electron ones. Here, a peak zT exceeding unity is obtained at 1123 K for both Nb0.7 Ta0.13 CoSb and Nb0.6 Ta0.23 CoSb, a benchmark value for defective 19-electron HHs. The improved zT results from the achievement of selective scatterings of phonons and electrons in defective Nb0.83 CoSb, using lanthanide contraction as a design factor to select alloying elements that can strongly impede the phonon propagation but weakly disturb the periodic potential. Despite the massive vacancies induced strong point defect scattering of phonons in Nb0.83 CoSb, Ta alloying is still found effective in suppressing lattice thermal conductivity while maintaining the carrier mobility almost unchanged. In comparison, V alloying significantly deteriorates the carrier transport and thus the TE performance. These results enlarge the category of high-performance HH TE materials beyond the conventional 18-electron ones and highlight the effectiveness of selective scatterings of phonons and electrons in developing TE materials even with massive vacancies.

Re‐Doped p‐Type Thermoelectric SnSe Polycrystals with Enhanced Power Factor and High ZT &gt; 2

AbstractThermoelectric 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.

Thermal Conductivity for p–(Bi, Sb)2Te3 Films of Topological Insulators

In this study, we investigated the temperature dependencies of the total, crystal lattice, and electronic thermal conductivities in films of topological insulators p–Bi0.5Sb1.5Te3 and p–Bi2Te3 formed by discrete and thermal evaporation methods. The largest decrease in the lattice thermal conductivity because of the scattering of long-wavelength phonons on the grain interfaces was observed in the films of the solid-solution p–Bi0.5Sb1.5Te3 deposited by discrete evaporation on the amorphous substrates of polyimide without thermal treatment. It was shown that in the p–Bi0.5Sb1.5Te3 films with low thermal conductivity, the energy dependence of the relaxation time is enhanced, which is specific to the topological insulators. The electronic thermal conductivity was determined by taking into account the effective scattering parameter in the relaxation time approximation versus energy in the Lorentz number calculations. A correlation was established between the thermal conductivity and the peculiarities of the morphology of the interlayer surface (0001) in the studied films. Additionally, the total κ and the lattice κL thermal conductivities decrease, while the number of grains and the roughness of the surface (0001) increase in unannealed films compared to annealed ones. It was demonstrated that increasing the thermoelectric figure of merit ZT in the p–Bi0.5Sb1.5Te3 films formed by discrete evaporation on a polyimide substrate is determined by an increase in the effective scattering parameter in topological insulators due to enhancement in the energy dependence of the relaxation time.

Bytizite Cu3SbSe3: Solid-State Synthesis and Thermoelectric Performance

Bytizite (Cu3SbSe3 ) has attracted interest as a promising thermoelectric material because of its ultralow thermal conductivity; however, there are few experimental studies. This study investigated the optimal processing conditions for the synthesis of Cu3SbSe3 using mechanical alloying (MA) and hot pressing (HP). The MA powder exhibited an orthorhombic Cu3SbSe3 phase, which remained even after HP. However, secondary phases of permingeatite (Cu3SbSe4) and berzelianite (Cu1.78Se) were also identified in the X-ray diffraction patterns. Thermal analysis revealed that the MA powder and HP compacts exhibited a large endothermic peak near 727 K, which corresponds to the melting point of Cu3SbSe3 . Dense compacts with a relative density higher than 99% were obtained at HP temperatures above 573 K. Microstructural and elemental analyses confirmed the presence of the secondary phase Cu3 SbSe4 in the matrix of Cu3SbSe3 . However, the Cu1.78Se phase could not be observed. All specimens exhibited an electrical conductivity of (0.66–1.06) × 10 3 Sm-1, a Seebeck coefficient of 324–376 µVK-1, and a power factor of 0.09–0.11 mWm-1K-2 at 623 K. The thermal conductivity was lower than 0.7 Wm-1K-1 in the measured temperature range, mainly due to the phonon scattering caused by the lone-pair electrons of Sb. A dip in thermal conductivity was observed at 423 K, which was possibly caused by the order-disorder transition of bytizite. The dimensionless figure of merit ZT increased with increasing temperature, and the maximum &lt;i&gt;ZT&lt;/i&gt; was 0.16 at 623 K.

Thermoelectric Properties of PbS Doped with Bi2S3 and Cu2S Prepared by Hydrothermal Synthesis and Spark Plasma Sintering

Hierarchical PbS powders doped with different contents of Bi2S3 and Cu2S were synthesized using the hydrothermal method. Subsequently, the powders were subjected to spark plasma sintering (SPS) for consolidation into bulk ceramics. X-ray photoelectron spectroscopy results showed that Bi2S3 and Cu2S were doped into PbS successfully. The effect of doping with different Bi2S3 and Cu2S contents on thermoelectric performance was investigated systematically. The results showed that pure PbS was an n-type semiconductor, and Bi2S3 doping or Bi2S3-Cu2S co-doping could decrease the thermal conductivity of PbS effectively. PbS doped with 1% Bi2S3 exhibited a moderate Seebeck coefficient, high electric conductivity, and low thermal conductivity simultaneously, thus attaining a maximum figure of merit ZT of 0.55 at 773 K. PbS doped with 1% Bi2S3-1% Cu2S exhibited an enhanced power factor and reduced thermal conductivity at an elevated temperature; the maximum ZT value obtained at 773 K was 0.83, which is more than twice that of pure PbS at 758 K (0.29), as a result.