New Insight on Tuning Electrical Transport Properties via Chalcogen Doping in n‐type Mg3Sb2‐Based Thermoelectric Materials

n‐type Mg₃Sb_1.5Bi_0.5 has recently been discovered to be a promising thermoelectric material, yet the effective n‐type dopants are mainly limited to the chalcogens. This may be attributed to the limited chemical insight into the effects from different n‐type dopants. By comparing the effects of different chalcogen dopants Q (Q = S, Se, and Te) on thermoelectric properties, it is found that the chalcogen dopants Q become more efficient with decreasing electronegativity difference between Q and Mg, which is mainly due to the increasing carrier concentration and mobility. Using density functional theory calculations, it is shown that the improving carrier concentration originates from the increasing doping limit induced by the stabilizing extrinsic defect. Moreover, the increasing electron mobility with decreasing electronegativity difference between Q and Mg is attributed to the smaller effective mass resulting from the enhancing chemical bond covalency, which is supported by the decreasing theoretical density of states. According to the above trends, a simple guiding principle based on electronegativity is proposed to shed new light on n‐type doping in Zintl antimonides.     

Large Voltage Generation of Flexible Thermoelectric Nanocrystal Thin Films by Finger Contact

This paper demonstrates that thermal energy radiated from a human finger can be converted efficiently into electricity by a nanocrystal (NC) thin film that substantially suppresses thermal conduction, but still allows electric conduction. The converting efficiencies of the chalcogenide NC thin films with dimensions 40 µm × 20 µm × 20 nm, prepared on flexible substrates by a solution process, are maximized by adjusting the NC size. A Seebeck coefficient of S = 1829 µV K^?1, and a dimensionless thermoelectric figure‐of‐merit, ZT = 0.68 are achieved at ambient temperature for p‐ and n‐type NC thin films, respectively. A thermoelectric array consisting of p‐ and n‐type NC thin films generates a voltage of 645 mV for a temperature gradient of 10 K. Furthermore, the donut‐shaped pn array can generate a voltage of 170 mV from the heat supplied by an individual's finger.     

Fine‐Grained and Nanostructured AgPbmSbTem+2 Alloys with High Thermoelectric Figure of Merit at Medium Temperature

AgPb_mSbTe_m+2 (abbreviated as LAST) has received tremendous attention as a promising thermoelectric material at medium temperature. It can be synthesized by a simple process combining mechanical alloying (MA) and spark plasma sintering (SPS). This work reveals that the thermoelectric figure of merit (ZT value) of LAST can be increased by 50%, benefiting from enhanced electrical conductivity and thermopower due to refined grains and from nanostructuring realized by repeating the milling and SPS processes. This modified process and further compositional optimization enables ZT values of the LAST alloys up to 1.54 at 723 K. This supports the potential of the LAST alloy as a promising medium‐temperature thermoelectric material and reveals the validity of ZT enhancement by a simple microstructural refining and nanostructuring method.     

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.     

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.     

Achieving zT > 2 in p‐Type AgSbTe2−xSex Alloys via Exploring the Extra Light Valence Band and Introducing Dense Stacking Faults

Through simultaneously enhancing the power factor by engineering the extra light band and enhancing phonon scatterings by introducing a high density of stacking faults, a record figure‐of‐merit over 2.0 is achieved in p‐type AgSbTe_2?xSe_x alloys. Density functional theory calculations confirm the presence of the light valence band with large degeneracy in AgSbTe₂, and that alloying with Se decreases the energy offset between the light valence band and the valence band maximum. Therefore, a significantly enhanced power factor is realized in p‐type AgSbTe_2?xSe_x alloys. In addition, transmission electron microscopy studies indicate the appearance of stacking faults and grain boundaries, which together with grain boundaries and point defects significantly strengthen phonon scatterings, leading to an ultralow thermal conductivity. The synergetic strategy of simultaneously enhancing power factor and strengthening phonon scattering developed in this study opens up a robust pathway to tailor thermoelectric performance.     

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).     

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.     

Heterogeneous Distribution of Sodium for High Thermoelectric Performance of p‐type Multiphase Lead‐Chalcogenides

Despite the effectiveness of sodium as a p‐type dopant for lead chalcogenides, its solubility is shown to be very limited in these hosts. Here, a high thermoelectric efficiency of ≈2 over a wide temperature range is reported in multiphase quaternary (PbTe)_0.65(PbS)_0.25(PbSe)_0.1 compounds that are doped with sodium at concentrations greater than the solubility limits of the matrix. Although these compounds present room temperature thermoelectric efficiencies similar to sodium doped PbTe, a dramatically enhanced Hall carrier mobility at temperatures above 600 K for heavily doped compounds results in significantly enhanced thermoelectric efficiencies at elevated temperatures. This is achieved through the composition modulation doping mechanism resulting from heterogeneous distribution of the sodium dopant between precipitates and the matrix at elevated temperatures. These results can lead to further advances in designing high performance multiphase thermoelectric materials with intrinsically heterogeneous dopant distributions.