High Thermoelectric Performance of n-type BiTeSe-Based Composites Incorporated with Both Inorganic and Organic Nanoinclusions.

N-type Bi2Te2.7Se0.3 (BTS) alloy has relatively low thermoelectric performance as compared to its p-type counterpart, which restricts its widespread applications. Herein, we designed and prepared a novel composite system, which consists of an n-type BTS matrix incorporated with both inorganic and organic nanoinclusions. The results indicate that the thermopower of the composite samples can be enhanced by more than 19% upon incorporating inorganic nanophase AgBi3S5 (ABS) due to the energy-dependent carrier scattering, which ensures a high power factor. On the other hand, further incorporation of organic nanophase polypyrrole (PPy) can drastically reduce its lattice thermal conductivity owing to the strong scattering of mid- and low-frequency phonons at these nanoinclusions. As a result, high figures of merit ZTmax = 1.3 at 348 K and ZTave = 1.17 (300-500 K) are achieved with improved mechanical properties in BTS-based composites incorporated with 1.5 wt % ABS and 0.5 wt % PPy, demonstrating that the incorporation of both inorganic and organic nanoinclusions is an effective way to improve its thermoelectric performance.

Simultaneous Enhancement of the Power Factor and Phonon Blocking in Nb-Doped WSe2.

Transition-metal dichalcogenide WSe2 is a potentially good thermoelectric (TE) material due to its high thermopower (S). However, the low electrical conductivity (σ), power factor (PF), and relatively large lattice thermal conductivity (κL) of pristine WSe2 degenerate its TE performance. Here, we show that through proper substitution of Nb for W in WSe2, its PF can be increased by ∼10 times, reaching 5.44 µW cm-1 K-2 (at 850 K); simultaneously, κL lowers from 1.70 to 0.80 W m-1 K-1. Experiments reveal that the increase of PF originates from both increased hole concentration due to the replacement of W4+ by Nb3+ and elevated thermopower (S) caused by the enhanced density of states effective mass, while the reduced κL comes mainly from phonon scattering at point defects NbW. As a result, a record high figure of merit ZTmax ∼0.42 is achieved at 850 K for the doped sample W0.95Nb0.05Se2, which is ∼13 times larger than that of pristine WSe2, demonstrating that Nb doping at the W site is an effective approach to improve the TE performance of WSe2.

Theoretical Study of Intrinsic and Extrinsic Point Defects and Their Effects on Thermoelectric Properties of Cu2SnSe3.

Current understanding of the intrinsic point defects and potential extrinsic dopants in p-type Cu2SnSe3 is limited, which hinders further improvement of its thermoelectric performance. Here, we show that the dominant intrinsic defects in Cu2SnSe3 are CuSn and VCu under different chemical conditions, respectively. The presence of VCu will damage the hole conduction network and reduce hole mobility. Besides, we find that the substitution of Al, Ga, In, Cd, Zn, Fe, and Mn for Sn can inhibit the formation of VCu; introducing CuSn, FeSn, MnSn, and NiCu defects can significantly enhance electronic density of states near the Fermi level due to the contribution of 3d orbitals. Therefore, increasing the Cu content and/or introducing the above beneficial dopants appropriately are expected to cause enhancement of carrier mobility and/or thermopower of Cu2SnSe3. Furthermore, introducing AgCu, AlSn, ZnSn, GeSn, and MnSn defects can induce large mass and strain field fluctuations, lowering lattice thermal conductivity remarkably. Present results not only deepen one's insights into point defects in Cu2SnSe3 but also provide us with a guide to improve its thermoelectric properties.

Achieving higher thermoelectric performance of n-type PbTe by adjusting the band structure and enhanced phonon scattering.

Due to the very distinct electronic band structure and lower power factor, the exploration of n-type PbTe as thermoelectric materials has historically fallen behind that of p-type PbTe. In this work, n-type Pb0.97Sb0.03Te-based composites incorporated with Cu12Sb4S13 nanoparticles are synthesized and investigated. Sb doping is utilized to modify its carrier concentration in order to obtain n-type PbTe materials with a high power factor. Then, the incorporation of Cu12Sb4S13 nanoparticles can generate semi-coherent nanophase CuTe, and simultaneously optimize the thermal and electrical properties due to remarkable energy filtering effects and interface scattering in the higher temperature range. Eventually, a peak figure of merit ZT ≈ 1.58 was obtained at 773 K for the sample Pb0.97Sb0.03Te + 1.5 wt% Cu12Sb4S13, indicating that the incorporation of Cu12Sb4S13 in Pb0.97Sb0.03Te is an effective approach to improve its thermoelectric performance.

Boosting Thermoelectric Performance of Cu2SnSe3 via Comprehensive Band Structure Regulation and Intensified Phonon Scattering by Multidimensional Defects.

As an eco-friendly thermoelectric material, Cu2SnSe3 has recently drawn much attention. However, its high electrical resistivity ρ and low thermopower S prohibit its thermoelectric performance. Herein, we show that a widened band gap and the increased density of states are achieved via S alloying, resulting in 1.6 times enhancement of S (from 170 to 277 µV/K). Moreover, doping In at the Sn site can cause a 19-fold decrease of ρ and a 2.2 times enhancement of S (at room temperature) due to both multivalence bands' participation in electrical transport and the further enhancement of the density of states effective mass, which allows a sharp increase in the power factor. As a result, PF = 9.3 µW cm-1 K-2 was achieved at ∼800 K for the Cu2Sn0.82In0.18Se2.7S0.3 sample. Besides, as large as 44% reduction of lattice thermal conductivity is obtained via intensified phonon scattering by In-doping-induced formation of multidimensional defects, such as Sn vacancies, dislocations, twin boundaries, and CuInSe2 nanoprecipitates. Consequently, a record high figure of merit of ZT = 1.51 at 858 K is acquired for Cu2Sn0.82In0.18Se2.7S0.3, which is 4.7-fold larger than that of pristine Cu2SnSe3.

Improved Thermoelectric Performance of Cu12Sb4S13 through Gd-Substitution Induced Enhancement of Electronic Density of States and Phonon Scattering.

Cu12Sb4S13 has aroused great interest because of its earth-abundant constituents and intrinsic low thermal conductivity. However, the applications of Cu12Sb4S13 are hindered by its poor thermoelectric performance. Herein, it is shown that Gd substitution not only causes a significant increase in both electrical conductivity σ and thermopower S but also leads to dramatic drop in lattice thermal conductivity κL. Consequently, large ZT reaches 0.94 at 749 K for Cu11.7Gd0.3Sb4S13, which is ∼41% higher than the ZT value of undoped sample. Rietveld refinements of XRD results show that accompanying inhibition of impurity phase Cu3SbS4, the number of Cu vacancies increases substantially with substituted content x (x ≤ 0.3), which leads to reduced κL owing to intensive phonon scattering by the point defects and increased σ arising from the charged defects (VCu'). Crucially, synchrotron radiation photoelectron spectroscopy reveals substantial increment of electronic density of states at Fermi level upon Gd substitution, which is proven, by our first-principle calculations, to originate from contribution of Gd 4f orbit, resulting in enhancement of S. Our study provides us with a new path to enhance thermoelectric performance of Cu12Sb4S13.

Improving the thermoelectric performance of Cu2SnSe3via regulating micro- and electronic structures.

As a p-type thermoelectric material, Cu2SnSe3 (CSS) has recently drawn much attention, with its constituents being abundant and free of toxic elements. However, the low electrical conductivity σ and thermopower S of CSS prohibit its thermoelectric performance. Here, we show that through mechanical milling, a 14 times increase in σ, around a 2-fold rise in S and a 40% reduction in the lattice thermal conductivity κL (at 300 K) can be achieved, amazingly. Microstructural analysis combined with first-principles calculations reveal that the increased σ originates from the generated Sn vacancies , Se dangling bonds and the reconstructed Cu-Sn-terminated acceptor-like surface states; while the enhanced S comes mainly from the enhanced density of states effective mass caused by the Sn vacancies. In addition, the generated Sn vacancies and the in situ formed SnO2 nanoparticles give rise to strong phonon scattering, leading to the reduced κL. As a result, a maximum ZTm = 0.9 at 848 K is obtained for the CSS specimen milled for 2 h, which is ∼3 times larger than that of CSS milled for 0.5 h.

Achieving High Thermoelectric Performance in p-Type BST/PbSe Nanocomposites through the Scattering Engineering Strategy.

To achieve high thermoelectric conversion efficiency in Bi0.4Sb1.6Te3 (BST) alloy is vital for its applications in low-grade energy harvesting. Here, we show that 56% increase in the power factor (PF) (from 16 to 25 µW cm-1 K-2) and 32% reduction of lattice thermal conductivity κL (from 0.56 to 0.38 W m-1 K-1) as well as an approximately four-fold decrease in bipolar-effect contribution κb (from 0.48 to 0.12 W m-1 K-1) can be achieved at 512 K through the incorporation of 0.2 vol % PbSe nanoparticles in the BST matrix. Analyses indicate that the remarkable increase in PF for the composite samples can be mainly attributed to strong electron scattering at the large interface barriers, inhibiting effectively the electron contribution to the total thermopower at elevated temperatures, while the large drop of κL and κb originates from enhanced phonon scattering by PbSe nanoinclusions as well as phase boundaries (among BST and PbSe nanophase) and suppression of electron transport, respectively. As a result, a maximum figure of merit (ZT) of 1.56 (at 400 K) and an average ZT (ZTave) of 1.44 in the temperature range of 300-512 K are reached. Correspondingly, a record projected conversion efficiency η = 11% is achieved at the cold side 300 K and hot side 512 K in the BST-based composite incorporated with 0.2 vol % PbSe nanoinclusions.

Ultralow Thermal Conductivity and High Thermoelectric Performance of N-type Bi2Te2.7Se0.3-Based Composites Incorporated with GaAs Nanoinclusions.

Bi2Te2.7Se0.3 (BTS) is known to be the unique n-type commercial thermoelectric (TE) alloy used at room temperatures, but its figure of merit (ZT) is relatively low, and it is vital to improve its ZT for its wide applications. Here, we show that incorporation of an appropriate amount of GaAs nanoparticles in BTS not only causes the large enhancement of Seebeck coefficients because of energy-dependent carrier scattering, but also gives rise to drastic reduction of lattice thermal conductivity κL. Specifically, ultralow κL ∼ 0.27W m-1 K-1 (at 300 K) is achieved for the composite sample incorporated with a 0.3 wt % GaAs nanophase, which is proved to originate mainly from the intensified phonon scattering by the GaAs nanoinclusions and interfaces between the GaAs and BTS matrix. As a result, a maximum ZT = 1.19 (∼372 K) and an average ZTave = 1.01 (at T = 300-550 K) are reached in the composite sample with 0.3 wt % GaAs nanoinclusions, which are respectively ∼78% and ∼82% larger than those of the BTS matrix in this study, demonstrating that incorporation of the GaAs nanophase is an effective way to improve TE performance of BTS.

Improved Figure of Merit of Cu2SnSe3 via Band Structure Modification and Energy-Dependent Carrier Scattering.

As an ecofriendly thermoelectric material with intrinsic low thermal conductivity, ternary diamond-like Cu2SnSe3 (CSS) has attracted much attention. Nevertheless, its figure of merit, ZT, is limited by its small thermopower (S) and power factor (PF). Here, we show that an increase in thermopower by 63% and a carrier-mobility rise of 81% at 300 K can be simultaneously achieved through 5% substitution of Fe for Sn due to both enhancement of electronic density of states and degeneracy of multiple valence band maxima, which lead to high PF = 10.3 µW·cm-1·K-2 at 823 K for Fe-doped CSS (CSFS). Besides, an ultrahigh PF of 14.8 µW·cm-1·K-2 (at 773 K) and 45% reduction of lattice thermal conductivity (at 823 K) are realized for CSFS-based composites with 0.125 wt % of MgO nanoinclusions, owing to further enhancement of S via energy-dependent scattering and strong phonon scattering by the embedded nanoparticles. Consequently, a maximum ZT = 1 at 823 K is reached for the CSFS/f MgO composite samples with f = 0.125 wt %, which is around 2.5 times larger than that of the CSS compound.