Tuning Of Carrier Concentration Research Articles

Exploring Thermoelectric Transport Properties and Band Parameters of n-Type Bi2-xSbxTe3 Compounds Using the Single Parabolic Band Model

The n-type Bi2-xSbxTe3 compounds have been of great interest due to its potential to achieve a high thermoelectric performance, comparable to that of p-type Bi2-xSbxTe3. However, a comprehensive understanding on the thermoelectric properties remains lacking. Here, we investigate the thermoelectric transport properties and band characteristics of n-type Bi2-xSbxTe3 (x = 0.1 – 1.1) based on experimental and theoretical considerations. We find that the higher power factor at lower Sb content results from the optimized balance between the density of state effective mass and nondegenerate mobility. Additionally, a higher carrier concentration at lower x suppresses bipolar conduction, thereby reducing thermal conductivity at elevated temperatures. Consequently, the highest zT of ~ 0.5 is observed at 450 K for x = 0.1 and, according to the single parabolic band model, it could be further improved by ~70 % through carrier concentration tuning.

High thermoelectric performance in n-type bismuth sulfide by carrier concentration tuning and dense nanodomains

Bi2S3 is regarded as one of the most promising thermoelectric materials because of its abundance, cost-effectiveness and environmentally friendly characteristics. This work delves into the impact of interstitial Cu on the composition, microstructure, and thermoelectric properties of the Cl-doped Bi2S3 bulks. The introduction of interstitial Cu as donors generates free electrons without compromising electron mobility. The excessive Cu induces the precipitation of Cu/Bi-rich phase and the Cu ions dynamically diffuse from these precipitates into the Bi2S3 lattice with increasing temperature. This effect significantly enhances electrical conductivity and power factor across a broad temperature range. Intriguingly, dense nanodomains are successfully constructed in the Bi2S3 sample. This unique defect, coupled with interstitial Cu, nanodomains, precipitates, and dislocations, establishes the hierarchical structures, leading to a low lattice thermal conductivity of 0.34 W m−1 K−1. Notably, the sample incorporating 4 at% Cu achieves an exceptional peak ZT value of 0.67 at 673 K.

Synergetic effect of Bi and Al co-doping in GeTe-based thermoelectric materials leading to optimized carrier concentration tuning and high ZT

Improving the performance of thermoelectric (TE) materials that can convert waste heat into electricity is becoming increasingly important. Accordingly, the dimensionless figure of merit, ZT needs to be increased. Al, a group-III element, has not been widely used as a dopant in GeTe-based thermoelectric materials because it acts as a p-type dopant and thus further increases the hole carrier concentration of GeTe, which has already a high intrinsic hole concentration due to inherent Ge vacancies. Therefore, Al as a dopant in GeTe has been considered to have detrimental effects on the resulting thermoelectric performance. In this study, we aim to enhance the suitability of Al doping in GeTe-based materials through synergistically co-doping it with Bi. In this case, co-doping Al and Bi optimizes the carrier concentration and induces a high Seebeck coefficient from an additional increased effective mass and low total thermal conductivity arising from the numerous phonons scattering centers and decreased electronic contribution to heat transport. Consequently, a maximum ZT of ∼1.95 at 673 K with an increased average ZT is achieved. These results indicate that the p-type dopant Al, co-doped with Bi in GeTe-based thermoelectric materials is one of the very effective dopants which can eventually improve the overall TE performance of these materials for useful waste heat recovery application and cooling.

Far-infrared transparent conductors

The long-standing challenge in designing far-infrared transparent conductors (FIRTC) is the combination of high plasma absorption edge (λp) and high conductivity (σ). These competing requirements are commonly met by tuning carrier concentration or/and effective carrier mass in a metal oxide/oxonate with low optical dielectric constant (εopt = 2–7). However, despite the high σ, the transparent band is limited to mid-infrared (λp < 5 μm). In this paper, we break the trade-off between high σ and λp by increasing the “so-called constant” εopt that has been neglected, and successfully develop the material family of FIRTC with εopt > 15 and λp > 15 μm. These FIRTC crystals are mainly octahedrally-coordinated heavy-metal chalcogenides and their solid solutions with shallow-level defects. Their high εopt relies on the formation of electron-deficiency multicenter bonds resulting in the great electron-polarization effect. The new FIRTC enables us to develop the first “continuous film” type far-infrared electromagnetic shielder that is unattainable using traditional materials. Therefore, this study may inaugurate a new era in far-infrared optoelectronics.

Impact of resonant state formation and band convergence in In and Sr co-doped SnTe thermoelectric material evaluated via the single parabolic band model

SnTe is a promising thermoelectric material for mid-temperature waste heat recovery. Recently, a high thermoelectric performance (zT) is reported in In and Sr co-doped Sn1−3xInxSr2xTe (x = 0.0 – 0.03). A high zT of 1.075 is achieved when x = 0.02 at 823 K. The In dopant is known to engineer the valence band of the SnTe by forming a resonant state near 300 K. The Sr dopant converges the two valence bands of the SnTe. However, when both In and Sr are doped in SnTe while keeping their atomic ratio constant, their effects on band engineering with increasing are not known. Here, the effects of resonant state formation and band convergence on electronic transport properties of Sn1−3xInxSr2xTe (x = 0.0 – 0.03) are investigated from the estimated temperature-dependent electronic band parameters. Both the resonant state formation and band convergence become most significant at the x = 0.02. According to the single parabolic band model, the zT of the x = 0.02 can be further improved to ∼1.834 (approximately 70 % zT improvement) upon carrier concentration tuning.

Strengthened phonon scattering and band convergence synergistically realize the high-performance SnTe thermoelectric

By combining the effects of defect engineering, band convergence, and carrier concentration tuning, we successfully suppress the lattice thermal conductivity to ultra-low 0.35 W m−1 K−1 and realize one of the highest ZT values of 1.75 at 833 K.

Tuning of carrier concentration and superconductivity in high-entropy-alloy-type metal telluride (AgSnPbBi)(1-)/4In Te

High-entropy-alloy-type (HEA-type) compound superconductors have been drawing much attention as a new class of exotic superconductors with local structural inhomogeneity. NaCl-type (Ag,In,Sn,Pb,Bi)Te is a typical HEA-type superconductor, but the carrier doping mechanism had been unclear. In this study, we synthesized (Ag,In,Sn,Pb,Bi)Te with various In concentration using high-pressure synthesis: the studied system is (AgSnPbBi)(1-x)/4InxTe (x = 0-0.4). Single-phase samples were obtained for x = 0-0.3. A semiconductor-like temperature dependence of resistivity was observed for x = 0, while superconductivity appeared for the In-doped samples. The highest transition temperature (Tc) was 3.0 K for x = 0.3. The Seebeck coefficient decreases with increase of x, which suggests that In3+ generates electron carriers in (AgSnPbBi)(1-x)/4InxTe. Tuning of carrier concentration and superconducting properties of (Ag,In,Sn,Pb,Bi)Te would be useful for further investigation of exotic superconductivity in the HEA-type compound.

Reduction of Hf via Hf/Zr Substitution in Mechanically Alloyed (Hf,Ti)CoSb Half-Heusler Solid Solutions

(Hf,Zr,Ti)Co(Sb,Sn) Solid solutions were prepared by mechanical-alloying followed by hot-press method as an attempt to reduce Hf concentration and therefore the material’s cost without negatively affecting the thermoelectric performance. To this end, two different methods were applied: (a) Hf substitution with its lighter and cheaper homologue Zr; and (b) fine tuning of carrier concentration by the substitution of Sb with Sn. The isoelectronic substitution of Hf with Zr was investigated in Hf0.6-xZrxTi0.4CoSb0.8Sn0.2 solid solutions and resulted in lower power factors and ZTs. However, the low thermal conductivity of Hf0.4Zr0.2Ti0.4CoSb0.8Sn0.2 contributed in achieving a relatively good ZT~0.67 at 970 K. The effect of charge carrier concentration was investigated by preparing Hf0.4Zr0.2Ti0.4CoSb1-ySny (y = 0.15–0.25) compounds. Hf0.4Zr0.2Ti0.4CoSb0.83Sn0.17 composition prepared by six hours milling reached the highest ZT of 0.77 at 960 K.

Thermoelectric Performance of n-Type Magnetic Element Doped Bi2S3.

Thermoelectric technology offers great potential for converting waste heat into electrical energy and is an emission-free technique for solid-state cooling. Conventional high-performance thermoelectric materials such as Bi2Te3 and PbTe use rare or toxic elements. Sulfur is an inexpensive and nontoxic alternative to tellurium. However, achieving high efficiencies with Bi2S3 is challenging due to its high electrical resistivity that reduces its power factor. Here, we report Bi2S3 codoped with Cr and Cl to enhance its thermoelectric properties. An enhanced conductivity was achieved due to an increase in the carrier concentration by the substitution of S with Cl. High values of the Seebeck coefficients were obtained despite high carrier concentrations; this is attributed to an increase in the effective mass, resulting from the magnetic drag introduced by the magnetic Cr dopant. A peak power factor of 566 μW m–1 K–2 was obtained for a cast sample of Bi2–x/3Crx/3S3–xClx with x = 0.01 at 320 K, as high as the highest values reported in the literature for sintered samples. These results support the success of codoping thermoelectric materials with isovalent magnetic and carrier concentration tuning elements to enhance the thermoelectric properties of eco-friendly materials.

Remarkable electron and phonon transports in low-cost SnS: A new promising thermoelectric material

As a deceptively simple material, SnS has received extensive attention in the thermoelectric community recently due to its fascinating electron and phonon transport features, making it a very promising thermoelectric candidate. In this review, we first discuss the fundamental characteristics of SnS, including crystal structures, electronic and phononic band structures, and physical or chemical properties. Meanwhile, the approaches of improving thermoelectric performance are summarized, including simultaneous carrier concentration tuning and carrier mobility improvement, multiple valence bands transport and their synergetic optimization, and anharmonicity and phonon “softening” behavior. We also compare the difference in electrical and thermal transport properties between crystals and polycrystalline SnS. Then, theoretical calculations to predict the maximum ZT in SnS system are also established for potential performance enhancement. Finally, some future possible strategies are proposed to aim at further promoting the figure of merit of SnS. The exploration and research on this new emerged material can contribute the thermoelectrics toward practical applications to meet market demands of low-cost, high-effectiveness, and environmental compatibility.

Disorder-induced Anderson-like localization for bidimensional thermoelectrics optimization

Disorder-induced Anderson-like localization for bidimensional thermoelectrics optimization

Disorder-Induced Anderson-Like Localization for Bidimensional Thermoelectrics Optimization

Thermoelectric materials, which can generate electricity from waste heat or be used as solid-state Peltier coolers, couldplay an important role in global sustainable energy. However, improving thermoelectric efficiency has proven difficult, largelydue to the complex interdependence of electronic properties of solids. Early work by Ioffe has developed into the standardthermoelectric optimization paradigm of tuning the electronic carrier concentration in semiconductors. Although the localizationtheory of electrons by Anderson and Mott has developed in parallel, its potential for thermoelectrics optimization has notbeen explored. Here, we show that electron localization induced by structural disorder also provides an effective optimizationstrategy for thermoelectric materials. By using a transport model that includes the relevant physics of localization it is shownthat the maximum thermoelectric figure-of-merit can be increased ~20% by tuning both carrier concentration and disorder.This approach is demonstrated in two model Ge-Sb-Te material systems and confirms that the peak figure-of-merit occursin slightly disordered materials. Particularly for highly degenerate semiconductors this bidimensional optimization strategyprovides a new methodology to attain high thermoelectric performance.

Calculated Electric Transport Properties of Thermoelectric Semiconductors Under Different Carrier Scattering Mechanisms

The widespread application of thermoelectric devices in cooling and waste heat recovery systems will be achieved when materials achieve high thermoelectric performance. However, improving thermoelectric performance is not straightforward because the Seebeck coefficient and electrical conductivity of the materials have opposite trends with varying carrier concentration. Here, we demonstrate that carrier scattering mechanism engineering can improve the power factor, which is the Seebeck coefficient squared multiplied by electrical conductivity, by significantly improving the electrical conductivity with a decreased Seebeck coefficient. The effect of engineering the carrier scattering mechanism was evaluated by comparing the band parameters (density-of-states effective mass, non-degenerate mobility) of Te-doped and Te, transition metal co-doped &lt;i&gt;n&lt;/i&gt;-type Mg2Sb3 fitted via the single parabolic band model under different carrier scattering mechanisms. Previously, it was reported that co-doping transition metal with Te only changed the carrier scattering mechanism from ionized impurity scattering to mixed scattering between ionized impurities and acoustic phonons, compared to Te-doped samples. The approximately three times enhancement in the power factor of Te, transition metal co-doped samples reported in the literature have all been attributed to a change in the scattering mechanism. However, here it is demonstrated that Te, transition metal co-doping also increased the density-of-states effective mass. Here, the impact of the scattering mechanism change on the electric transport properties of &lt;i&gt;n&lt;/i&gt;-type Mg2Sb3 without an effective mass increase was studied. Even without the effective mass increase, carrier scattering mechanism engineering improved the power factor, and its effect was maximized by appropriate carrier concentration tuning.

Understanding Cu incorporation in the Cu2xHg2−xGeTe4 structure using resonant x-ray diffraction

The ability to control carrier concentration based on the extent of Cu solubility in the ${\mathrm{Cu}}_{2x}{\mathrm{Hg}}_{2\ensuremath{-}x}{\mathrm{GeTe}}_{4}$ alloy compound (where $0\ensuremath{\le}x\ensuremath{\le}1$) makes ${\mathrm{Cu}}_{2x}{\mathrm{Hg}}_{2\ensuremath{-}x}{\mathrm{GeTe}}_{4}$ an interesting case study in the field of thermoelectrics. While Cu clearly plays a role in this process, it is unknown exactly how Cu incorporates into the ${\mathrm{Cu}}_{2x}{\mathrm{Hg}}_{2\ensuremath{-}x}{\mathrm{GeTe}}_{4}$ crystal structure and how this affects the carrier concentration. In this work, we use a combination of resonant energy x-ray diffraction (REXD) experiments and density functional theory (DFT) calculations to elucidate the nature of Cu incorporation into the ${\mathrm{Cu}}_{2x}{\mathrm{Hg}}_{2\ensuremath{-}x}{\mathrm{GeTe}}_{4}$ structure. REXD across the ${\mathrm{Cu}}_{k}$ edge facilitates the characterization of Cu incorporation in the ${\mathrm{Cu}}_{2x}{\mathrm{Hg}}_{2\ensuremath{-}x}{\mathrm{GeTe}}_{4}$ alloy and enables direct quantification of antisite defects. We find that Cu substitutes for Hg at a 2:1 ratio, wherein Cu annihilates a vacancy and swaps with a Hg atom. DFT calculations confirm this result and further indicate that the incorporation of Cu occurs preferentially on one of the $z=1/4$ or $z=3/4$ planes before filling the other plane. Furthermore, the amount of ${\mathrm{Cu}}_{\mathrm{Hg}}$ antisite defects quantified by REXD was found to be directly proportional to the experimentally measured hole concentration, indicating that the ${\mathrm{Cu}}_{\mathrm{Hg}}$ defects are the driving force for tuning carrier concentration in the ${\mathrm{Cu}}_{2x}{\mathrm{Hg}}_{2\ensuremath{-}x}{\mathrm{GeTe}}_{4}$ alloy. The link uncovered here between crystal structure, or more specifically antisite defects, and carrier concentration can be extended to similar cation-disordered material systems and will aid the development of improved thermoelectric and other functional materials through defect engineering.

High thermoelectric performance of p-type half-Heusler (Hf,Ti)Co(Sb,Sn) solid solutions fabricated by mechanical alloying

Mechanical alloying synthesis was applied on the thermoelectric half-Heusler materials in order to explore the potential of such scalable synthesis method in this family of compounds. (Hf,Ti)Co(Sb,Sn) solid solutions were prepared by mechanical alloying followed by hot pressing and their thermoelectric properties were studied in the temperature range of 300–980 K. After successful synthesis, two different methods were applied to enhance the thermoelectric performance of p-type (Hf,Ti)Co(Sb,Sn) materials: (a) adjustment of the Ti/Hf ratio and (b) fine tuning of carrier concentration by the substitution of Sb with Sn. The isoelectronic substitution of Ti with Hf led to an important reduction in the lattice thermal conductivity and a high ZT of 0.84 at 980 K for Hf0.6Ti0.4CoSb0.8Sn0.2. The effect of charge carrier concentration was investigated by preparing Hf0.6Ti0.4CoSb1−ySny (0.15, 0.17, 0.20, 0.23, 0.25) compounds, using different mechanical alloying durations. The Hf0.6Ti0.4CoSb0.83Sn0.17 composition, prepared by 4 h ball-milling, reached an impressive ZT ~1.1 at 973 K.

Mg-pressure-controlled annealing for tuning Mg content and thermoelectric properties of Mg2–δ(Si0.5Sn0.5)1–xSbx

Control of the Mg content is essential for tuning the carrier concentration of Mg2Si-based thermoelectric materials. In this study, we achieved precise control of the Mg content in Mg2–δ(Si0.5Sn0.5)1–xSbx by Mg-pressure-controlled annealing via solid-gas equilibrium. The range of possible Mg deficiency δ, which is estimated under assumption that each Sb donates one electron and Mg vacancy compensates them, depends on the Sb content x. At x = 0.10, the widest range Δδ = 0.012 (0.037 ≤ δ ≤ 0.049) is obtained. Owing to Mg non-stoichiometry, Mg-rich Mg2–δ(Si0.5Sn0.5)0.90Sb0.10 (δ = 0.037) shows a roughly 18 times higher carrier concentration (n = 3.7 × 1020 cm−3) than the Mg-poor composition (δ = 0.049 and n = 0.2 × 1020 cm−3). The Mg content of Mg2–δ(Si0.5Sn0.5)1–xSbx shifts the Fermi level within the rigid band approximation, resulting in a consistent relationship between the Seebeck coefficient and carrier concentration in both experiment and DFT calculation. Mg-pressure-controlled annealing enables carrier concentration tuning for zT maximization and yields a zTmax of 1.0 at 773 K for Mg2–δ(Si0.5Sn0.5)0.90Sb0.10 with an intermediate Mg content (δ = 0.042 and n = 2.2 × 1020 cm−3 under PMg = 2 Pa) that is larger than those for Mg-rich and Mg-poor compositions (zTmax = 0.9 and 0.6 for δ = 0.037 and 0.049, respectively).

Effect of impurity elements on the structural stability and electronic state in tin iodide perovskite

Effect of impurity elements on the structural stability and electronic state in the tin iodide perovskite, which is useful for tuning carrier concentration to improve the thermoelectric performance, have been investigated using first-principles calculations. MASnI3 (MA ​= ​CH3NH3+) and CsSnI3 are considered as mother materials, and an impurity injection from three types of oxides—Al2O3, TiO2, and Y2O3— acting as scaffolds are considered. Three cations, Al3+, Ti3+, and Y3+, occupy the A or B sites, and one anion O2− occupies an I site. From the estimated formation energy Eform, Al and Ti impurities cause structural instability. In contrast, a Y impurity placed at a B site decrease Eform to −0.05 ​eV/atom and renders the perovskite structure more stable, while a Y impurity at an A site causes instability in the structure. For the calculated structural stability, the bonding analysis clearly explains the difference between the aforementioned sites. The Y–I covalent bonding around E ​= ​−4 ​eV is relatively strong when a Y is placed at a B site. Covalent bonding formed by the impurity elements has a finite contribution to the structural stability, even in the ionic crystal like an iodide perovskite.

Interplay between positive magnetoresistance and thermoelectric properties by tuning carrier concentration in Sb2−xSnxTe3 (x ⩽ 0.05) crystals

We report transport properties of Sb2−xSnxTe3 (x ⩽ 0.05) single crystals, where the tuning of the charge carrier densities via Sn doping significantly improves the magnetoresistance (MR) and the thermoelectric (TE) properties. The MR increases significantly at 2 K for x = 0.01, which reduces considerably with further Sn doping at x = 0.05. The MR results below 90 K for x = 0.01 satisfy the semi-classical two-band approach of Kohler’s rule, which does not satisfy the MR results for x = 0.05. At 300 K, the Seebeck coefficient (S) is positive and small for x = 0.01 and it changes the sign with the further increase in Sn doping at x = 0.05 at 300 K. The value of |S| is considerable with an enhancement of the thermoelectric power factor compared to the value for x = 0.01 at 300 K. Analysis of the Hall conductivity indicates the interplay between carrier mobilities and densities, leading to the tuning of MR and TE properties with Sn doping. These results demonstrate that the p-type Sb2Te3 becomes n-type with a suitable doping engineering.

Size-Controlled Au–Cu2Se Core–Shell Nanoparticles and Their Thermoelectric Properties

One promising approach to improving thermoelectric energy conversion is to use nanostructured interfaces that enhance Seebeck coefficient while reducing thermal conductivity. Here, we synthesized Au-Cu2Se core-shell nanoparticles with different shell thicknesses by controlling the precursor concentration in solution. The Au-Cu2Se core-shell nanoparticles are about 37-53 nm in size, and the cores of the nanostructures are composed of Au nanoparticles with sizes of ∼11 nm. The effect of shell thickness on the thermoelectric properties of core-shell nanocomposites is investigated after sintering the core-shell nanoparticles into pellets using the spark plasma sintering (SPS) technique. The power factor was optimized by the synergetic effect of the improvement of Seebeck coefficient by energy filtering in the Au/Cu2Se interface and the effective tuning of carrier concentration by Ohmic contact in the interface. The lattice thermal conductivity of core-shell nanocomposites is reduced by coherent phonon scattering, which is caused by the wavelike interference of phonons due to the phase shift in the core-shell interface. The highest ZT value of 0.61 is obtained at 723 K for Au-Cu2Se core-shell nanocomposite with a shell thickness of 21 nm, which is higher than that of pure Cu2Se nanocomposite or a mixture of Au and Cu2Se particles.

Off-stoichiometry effect on thermoelectric properties of the new p-type sulfides compounds Cu2CoGeS4

An in-depth analysis of Cu2+xCo1-xGeS4 (CCGS) has been performed to illustrate its potential as a new p-type compound for thermoelectric (TE) applications with suitable thermal stability. An intrinsic semiconductor behavior is reported with attractive TE properties, characterized by a large Seebeck coefficient and intrinsic low thermal conductivity. Band structures and elastic properties calculations sustained the experimental observations and revealed the dominant role of Cu/Co ratio in electrical transport properties. A promising TE performance is achieved through a carrier concentration tuning by Cu/Co ratio control, with a zT = 0.37 at 725 K resulting from combined large PF enhancement and a reduced κ.