Optimum Carrier Concentration Research Articles

Trade-offs encountered in traditional mobility enhancement techniques applied to contact-controlled thin film transistors

Various mobility enhancement strategies are used at the material, process, and design geometry levels, to improve the performance of conventional thin film transistors (TFTs). These include the optimization of contact barrier height, semiconductor carrier concentration, post-annealing, and channel length optimization. In this study, the effects of these strategies on the performance metrics of contact-controlled amorphous Indium Gallium Zinc oxide (a-IGZO) TFTs are analyzed. Using a low barrier contact metal and a higher channel concentration can enhance the mobility of these TFTs by aiding the high field mode of operation with barrier lowering effect, but this leads to higher values of saturation coefficient, channel length sensitivity, and saturation voltage, with a lower value of output impedance. For a device working in the low-field mode, though the process of annealing improves the mobility characteristics, it deteriorates contact-controlled performance. Channel length upscaling also negatively affects all the contact-controlled performance metrics except output impedance. Thus, this study brings out the deteriorative effect of conventional mobility enhancement strategies on the performance metrics of a contact-controlled device, by unraveling the underlying semiconductor transport physics and the interplay between contact resistance and channel resistance.

Unlocking the mechanical, thermodynamic and thermoelectric properties of NaSbS2: A DFT scheme.

The present study focuses on the ground state mechanical, acoustic, thermodynamic and electronic transport properties of NaSbS2 polymorphs using the density functional theory (DFT) and semi-classical Boltzmann transport theory. The mechanical stability of the polymorphs is affirmed by the calculated elastic tensor. The calculated elastic properties asserted that all the polymorphs exhibit soft, brittle, anisotropic nature containing dominant covalent bonding. The 2D polar graphs are used to describe the anisotropic characteristic of the elastic parameters. The estimated value of Young's modulus and lattice thermal conductivity suggested that the polymorphs could be suitable for thermal barrier coating. Heat capacity, melting temperature, thermal conductivities, Grüneisen parameter, and thermal expansion coefficient of the polymorphs have also been studied to demonstrate thermodynamic behavior. The predicted lower values of lattice thermal conductivity declared that NaSbS2 polymorphs exhibit excellent electrical conductivity and transport properties. The estimated Seebeck coefficient (S), power factor (PF) and figure of merit (ZT) suggested that n-type triclinic and monoclinic, as well as p-type trigonal NaSbS2, are better for thermoelectric applications. The optimal carrier concentration for monoclinic structure is 1021cm-3 for T<750K, while it becomes 1020cm-3 for T>750K. It is also found that the optimal carrier concentration of the trigonal is 1021cm-3, whereas it is 1020cm-3 for triclinic structures. Therefore, it can be stated that NaSbS2 polymorphs possess excellent thermoelectric features, making them a promising choice for thermoelectric (TE) applications.

Advances in Mg3Sb2 thermoelectric materials and devices.

Thermoelectric technology offers a green-viable and carbon-neutral solution for energy problems by directly converting waste heat to electricity. For years, Bi2Te3-based compounds have been the main choice materials for commercial thermoelectric devices. However, Bi2Te3 comprises scarce and toxic tellurium (Te) elements, which might limit its large-scale application. Recently, Mg3Sb2 compounds have drawn increasing attention as an alternative to Bi2Te3 thermoelectrics due to their excellent thermoelectric performance. Enabled by effective strategies such as optimizing carrier concentration, introducing point defects, and manipulating carrier scattering mechanisms, Mg3Sb2 compounds have realized an improved thermoelectric performance. In this review, optimizing strategies for both Mg3Sb2-based thermoelectric materials and devices are discussed. Moreover, the flexibility and plasticity of Bi-alloyed Mg3Sb2 mainly stemming from the dense dislocations are outlined. The above strategies summarized here for enhancing Mg3Sb2 thermoelectrics are believed to be applicable to many other thermoelectrics.

Solvothermally optimizing Ag2Te/Ag2S composites with high thermoelectric performance and plasticity.

Silver-based fast ionic conductors show promising potential in thermoelectric applications. Among these, Ag2S offers unique high plasticity but low electrical conductivity, whereas Ag2Te exhibits high intrinsic electrical conductivity yet faces limitations due to high thermal conductivity and poor plasticity. Developing a composite thermoelectric material that combines the benefits of both is therefore essential. Here, this study reports the successful synthesis of Ag2Te/Ag2S composites via a facile and low-cost solvothermal method. By finely adjusting the composition of Ag2S and Ag2Te to obtain the optimized carrier concentration and the enhanced mobility, the figure of merit ZT of Ag2Te/Ag2S composites reached ∼0.42 at 373 K and ∼0.38 at 298 K, both surpassing those of pure Ag2S and Ag2Te. This increase in ZT also benefits from lattice defects created by the solvothermally synthesized biphasic composition, effectively scattering phonons of various wavelengths and reducing thermal conductivity compared to pure Ag2Te. Additionally, the plasticity of the Ag2Te/Ag2S composites improved considerably over pure Ag2Te, achieving a bending strain of ∼2.5% (versus ∼1.2% for intrinsic Ag2Te). This study can fill a critical gap in the research on composite silver-based fast ionic conductors synthesized via wet chemical methods and provide valuable guidance for future exploration.

Optimization of Thermoelectric Performance in p-Type SnSe Crystals Through Localized Lattice Distortions and Band Convergence.

Crystalline thermoelectric materials, especially SnSe crystals, have emerged as promising candidates for power generation and electronic cooling. In this study, significant enhancement in ZT is achieved through the combined effects of lattice distortions and band convergence in multiple electronic valence bands. Density functional theory (DFT) calculations demonstrate that cation vacancies together with Pb substitutional doping promote the band convergence and increase the density of states (DOS) near the Fermi surface of SnSe, leading to a notable increase in the Seebeck coefficient (S). The complex defects formed by Sn vacancies and Pb doping not only boost the Seebeck coefficient but also optimize carrier concentration (nH) and enhance electrical conductivity (σ), resulting in a higher power factor (PF). Furthermore, the localized lattice distortions induced by these defects increase phonon scattering, significantly reducing lattice thermal conductivity (κlat) to as low as 0.29Wm-1K-1at 773K in Sn0.92Pb0.03Se. Consequently, these synergistic effects on phonon and electron transport contribute to a high ZT of 1.8. This study provides a framework for rational design of high-performance thermoelectric materials based on first-principles insights and experimental validation.

Simultaneous Suppression of Phonon Transport and Carrier Concentration for Efficient Rhombohedral GeTe Thermoelectric.

Superior electronic performance due to the highly degenerated Σ valence band (Nv∼12) makes rhombohedral GeTe a promising low-temperature (<600 K) thermoelectric candidate. Minimizing lattice thermal conductivity (κL) is an essential route for enhancing thermoelectric performance, but the temperature-dependent κL, corelated to T-1, makes its reduction difficult at low temperature. In this work, a room-temperature κL of ≈0.55Wm-1-K-1, the lowest ever reported in GeTe-based thermoelectric, is realized in (Ge1- ySbyTe)1- x(Cu8GeSe6)x, primarily due to strong phonon scattering induced by point defects and precipitates. Simultaneously, Cu8GeSe6-alloying effectively suppresses the precipitation of Ge, enabling the optimization of carrier concentration with the additional help of aliovalent Sb doping. As a result, an extraordinary peak zT of up to 2.3 and an average zTavg. of ≈1.2 within 300-625 K are achieved, leading to a conversion efficiency of ≈9% at a temperature difference of 282 K. This work robustly demonstrates its potential as a promising component in thermoelectric generator utilizing low-grade waste heat.

Constructing high-performance bulk thermoelectric composites by incorporating uniformly dispersed fullerene sub-nanoclusters

Sub-nanomaterials possess unprecedented size-dependent properties compared to conventional nanomaterials, which endow them with great potential in catalysis, biomedicine, sensors, and so on. However, their applications in thermoelectrics are unknown due to poor thermal stability and low yields. Herein, we construct a series of thermoelectric composites by incorporating highly thermally stable and commercial fullerene sub-nanoclusters (C60 or C70). We find that sub-nanoclusters as the second phase can conduce to optimized carrier concentration through charge transfer at interfaces while the carrier mobility is significantly enhanced due to atom orbital hybridization and size-dependent electrical scattering mechanism. Furthermore, the ultra-low thermal conductivity of C60 due to its distorted chemical bonding and sub-nanometer pore, and the interfacial thermal resistance greatly suppress the phonon transport. Consequently, the 0.15 mol.% C60/Bi0.4Sb1.6Te3 realizes an ultra-high ZT of ∼1.6 at 373 K, an excellent thermoelectric conversion efficiency of ∼7.4 %, and a huge cooling performance of ∼73 K. This work demonstrates the application of sub-nanomaterials in thermoelectrics and may shed light on other fields such as electronic devices, thermal management, and fullerene chemistry.

Stepwise Optimization of Thermoelectric Performance in n‐Type SnS

AbstractTin sulfide (SnS) has been developed as an earth‐abundant and eco‐friendly compound that exhibits promising thermoelectric (TE) performance. While significant achievements are achieved in p‐type SnS, the development of its n‐type counterpart remains at a nascent stage, making it rather essential for developing n‐type SnS to ensure good compatibility in SnS‐based TE devices. In this study, Br is selected as the aliovalent dopant for realizing n‐type transport in SnS. Subsequently, isoelectronic Se alloying and vacancy compensation are utilized to further promote the TE performance for n‐type SnS. Se alloying can effectively narrow the bandgap of SnS for enhancing carrier concentration and diminishing thermal conductivity by strain and mass field fluctuations, while the excess Sn further compensates intrinsic Sn vacancies, thus optimizing carrier concentration and mobility simultaneously. These results are well verified by microstructure characterization and defect formation energy calculations. Consequently, the maximum ZT value of ≈0.7 and quality factor B of ≈1.02 at 823 K can be obtained after stepwise optimization, which is currently the highest value for n‐type SnS thermoelectrics. This study presents a significant progress for n‐type SnS and lays an important foundation for constructing all‐SnS‐based TE devices.

Broad-Temperature Thermoelectric Figure of Merit Enhancement in Unconventional n-Type Bi2Te2.3Se0.7 Alloys.

Bi2Te2.7Se0.3-based alloys are conventional n-type thermoelectric materials for solid-state cooling and heat harvest near room temperature; high thermoelectric performance over a wide temperature range and superior mechanical properties are essential for their use in practical thermoelectric devices. In this work, we demonstrated that decent thermoelectric performance can also be realized in an unconventional composite with a nominal composition of Bi2Te2.3Se0.7 since the emergence of a Bi2Te2Se phase with Se ordered occupation could induce an enlargement of the electronic band gap. Follow-up Cu/Na codoping could generate a dynamic optimization of carrier concentration, significantly broadening the temperature range of high thermoelectric performance. Further B incorporation and annealing treatment resulted in obvious grain refinement and stacking fault structures, which help pushing the ultimate maximal figure of merit up to ∼1.3 at 423 K with an average value of ∼1.2 at 300-573 K. This work might provide insights for further research on bismuth tellurides and other thermoelectric materials.

Achieving enhanced power factor using dual substitution at Cu and S sites in tetrahedrites thermoelectric materials Cu12Sb4S13

Thermoelectric (TE) materials-based devices are valuable for translating heat into electricity, but the cost is largely driven by the use of purified metals required for efficient TE performance. This study focuses on tetrahedrite materials, which are cost-effective due to their abundance in copper ore mining waste and status as one of the most widespread sulfosalts on Earth. The research investigates the effects of co-doping and iso-valent doping on the TE properties of p-type tetrahedrite samples, specifically Cu10Mn2Sb4S11Se2 (Sample A) and Cu10MnZnSb₄S11Se2 (Sample B). These samples were synthesized using hot pressing at 773 K and compared with the existing pristine compound (Cu10Mn2Sb4S13). Doping with Mn and Zn (1:1) at the Cu site was found to optimize carrier concentration and enhance phonon scattering, leading to an improved power factor. Additionally, the Se substitution at the S site also introduced band degeneracy, further increasing the power factor. Further, sample B exhibited the highest Seebeck coefficient (∼300 μV/K) at 650 K, in comparison to Sample A (∼225 μV/K) and the pristine compound (∼250 μV/K). On the other hand, Sample A demonstrated significantly higher electrical conductivity (∼0.76 × 10⁴ S/m at 650 K), ∿ three times greater than that of the other prepared samples. Notably, the power factor of Sample A (∼0.389 mW/mK2 at 650 K) was more than twice that of Sample B (∼0.141 mW/mK2) and the pristine compound (∼0.160 mW/mK2). These findings suggest that dual-site substitution at the Cu and S positions presents a promising strategy to augment the TE performance of tetrahedrite compounds.

Achieving High Thermoelectric Performance in Bi2(Te, Se)3 Thin Film with (00l)-Oriented by Incorporating Tellurization and Selenization Processes.

Flexible thermoelectric (TE) generators have received great attention as a sustainable and reliable option to convert heat from the human body and other ambient sources into electricity. This study provides a synthesis route that involves thermally induced diffusion to introduce Te and Se into Bi, fabricating an n-type Bi-Te-Se flexible thin film on a flexible substrate. This specific synthesis alters the crystal orientation (00l) of the thin film, improving in-plane electrical transportation and optimizing carrier concentration. Consequently, BixTeySe0.42 enhanced both the Seebeck coefficient and electrical conductivity, achieving a power factor of 17.1 μW cm-1 K-2 at room temperature. The TE device assembled with p-type Sb2Te3 exhibited exceptional flexibility with only a 26.2% change in resistance after 1000 times of bending at a radius of about 6 mm. The resistance change was further reduced to 7.5% after the application of a vinyl laurate coating. The fabricated TE device generated an ultrahigh output power of 792 nW with a temperature difference of 30 K.

Boosting the Thermoelectric Properties of Ge0.94Sb0.06Te via Trojan Doping for High Output Power.

GeTe stands as a promising lead-free medium-temperature thermoelectric material that has garnered considerable attention in recent years. Suppressing carrier concentration by aliovalent doping in GeTe-based thermoelectrics is the most common optimization strategy due to the intrinsically high Ge vacancy concentration. However, it inevitably results in a significant deterioration of carrier mobility, which limits further improvement of the zT value. Thus, an effective Trojan doping strategy via CuScTe2 alloying is utilized to optimize carrier concentration without intensifying charge carrier scattering by increasing the solubility of Sc in the GeTe system. Because of the high doping efficiency of the Trojan doping strategy, optimized hole concentration and high mobility are obtained. Furthermore, CuScTe2 alloying leads to band convergence in GeTe, increasing the effective mass m* in (Ge0.84Sb0.06Te0.9)(CuScTe2)0.05 and thus significantly improving the Seebeck coefficient throughout the measured temperature range. Meanwhile, the achievement of the ultralow lattice thermal conductivity (κL ∼ 0.34 W m-1 K-1) at 623 K is attributed to dense point defects with mass/strain-field fluctuations. Ultimately, the (Ge0.84Sb0.06Te0.9)(CuScTe2)0.05 sample exhibits a desirable thermoelectric performance of zTmax ∼ 1.81 at 623 K and zTave ∼ 1.01 between 300 and 723 K. This study showcases an effective doping strategy for enhancing the thermoelectric properties of GeTe-based materials by decoupling phonon and carrier scattering.

Multiple defect states engineering towards high thermoelectric performance in GeTe-based materials

GeTe is a promising material for thermoelectric (TE) applications. However, its low Seebeck coefficient and high thermal conductivity in the mid-temperature range of 298–550 K have hindered its widespread use in TE devices. In this work, we show that a combination of band engineering, Fermi level tuning, and miscibility gap exploration can significantly improve the energy conversion performance of GeTe. Bi provides the optimized carrier concentration and induces band convergence, while Ga possibly forms a resonant state. The synergistic effects of Bi and Ga significantly improve the Seebeck coefficient from 38 µVK−1 for undoped GeTe to over 120 µVK−1 for Bi and Ga-containing materials at 298 K and provide a high power factor over a wide temperature range. Phonon scattering is strengthened by optimizing the microstructure through Pb-induced spinodal decomposition and enhanced point defect scattering due to increased dopant solubility, resulting in a very low lattice thermal conductivity of about 0.5 Wm-1K−1 at 600 K for the triple-doped samples. The maximum thermoelectric figure of merit ZT was significantly increased to 2.1 at 600 K for the Ge0.87Pb0.05Bi0.06Ga0.02Te sample due to the improved power factor and reduced lattice thermal conductivity. Additionally, the average figure of merit ZTave for this sample achieved a very high value of 1.4 for Ge0.87Pb0.05Bi0.06Ga0.02Te at a temperature gradient of 475 K (Tc = 298 K). The developed material shows great potential for use in energy conversion devices, with an estimated energy conversion efficiency exceeding 17 %.

Optimizing carrier concentration for enhanced thermoelectric performance in AgSbS2 monolayer

Optimizing carrier concentration for enhanced thermoelectric performance in AgSbS2 monolayer

Enhanced thermoelectric performance in p-type AgBiSe2 through carrier concentration optimization and valence band modification

Enhanced thermoelectric performance in p-type AgBiSe2 through carrier concentration optimization and valence band modification

The Doping Strategies for Modulation of Transport Properties in Thermoelectric Materials

AbstractDoping is a key method employed in semiconductor technology for tuning the carrier‐density‐dependent electrical properties. In thermoelectric materials, which are typically heavily doped semiconductors, more impact factors are needed to be considered due to the heavy dose doping compared to normal semiconductors beyond carrier concentration optimization, such as band structure modification, carrier scattering mechanism and even thermal transport property. In this review, the doping effect determined by the intrinsic band features and vice versa the influence of dopants on band structure is first illustrated; and then introduce the approaches to overcoming the dilemma in effective doping caused by the temperature‐dependent optimal carrier concentration. Second, the strategies including rational vacancy design, proper selection of dopants and lattice purification toward high efficient doping and weakened carrier scattering strength are reviewed. Third, the recent discovery of the effect of dopant on thermal transport is highlighted covering the dopant‐induced local lattice distortion and exceptional strong electron‐phonon coupling. Finally, a perspective is given for the doping strategies to further boost the performance of thermoelectric materials.

Vacancy Manipulation by Ordered Mesoporous Induced Optimal Carrier Concentration and Low Lattice Thermal Conductivity in BixSb2- xTe3 Yielding Superior Thermoelectric Performance.

For BixSb2- xTe3 (BST) in thermoelectric field, the element ratio is easily influenced by the chemical environment, deviating from the stoichiometric ratio and giving rise to various intrinsic defects. In P-type polycrystalline BST, SbTe and BiTe are the primary forms of defects. Defect engineering is a crucial strategy for optimizing the electrical transport performance of Bi2Te3-based materials, but achieving synchronous improvement of thermal performance is challenging. In this study, mesoporous SiO2 is utilized to successfully mitigate the adverse impacts of vacancy defects, resulting in an enhancement of the electrical transport performance and a pronounced reduction in thermal conductivity. Crystal and the microstructure of the continuous modulation contribute to the effective phonon-electronic decoupling. Ultimately, the peak zT of Bi0.4Sb1.6Te3/0.8 wt% SiO2 (with a pore size of 4nm) nanocomposites reaches as high as 1.5 at 348 K, and a thermoelectric conversion efficiency of 6.6% is achieved at ΔT = 222.7 K. These results present exciting possibilities for the realization of defect regulation in porous materials and hold reference significance for other material systems.

Electrical and thermal transport properties of Cr2Se3-Cr2Te3 solid-solution alloy system and estimation of optimal thermoelectric properties

Cr2Se3 is a layered narrow-bandgap semiconductor with a low lattice thermal conductivity of ∼1 W/mK, which can be considered as a potential thermoelectric material. A solid-solution alloy system with Cr2Te3 can be designed to manipulate the electrical and thermal transport properties. In this study, a series of Cr2Se3-Cr2Te3 solid-solution alloys (Cr2(Se1-xTex)3, x = 0, 0.33, 0.5, 0.67, 0.83, and 1) were synthesized, and their electrical and thermal transport properties were investigated. Regarding the electrical transport properties, the power factor (∼0.35 mW/mK2) of Cr2Se3 gradually and significantly decreased as the Te content (x) increased owing to the significant decrease in the Seebeck coefficient that resulted from the large increase in the carrier concentration. Regarding the thermal transport properties, the total thermal conductivity increased with x as a result of the large increase in the electrical conductivity, despite the continuous decrease in the lattice thermal conductivity. Consequently, the thermoelectric figure of merit (zT ∼ 0.20) of Cr2Se3 gradually and significantly decreased to 0.009 for Cr2Te3 at 700 K; thus, no enhancement of zT was observed from the experimental results of simple solid-solution alloying. On the other hand, the thermoelectric quality factors of the solid-solution compositions for x = 0.33–0.83 were found to be enhanced compared to that of the Cr2Se3 sample, implying that zT could be further enhanced by optimizing the carrier concentration. Indeed, enhanced zT values were estimated based on a single parabolic band model for the solid-solution compositions with x = 0.33–0.83 if the carrier concentration was optimized to ∼1019 cm−3, which was found to be owing to the increased nondegenerate and weighted mobility (inferring weaker carrier–phonon interaction). Therefore, it is suggested that the solid-solution alloying approach could provide a feasible strategy to enhance the thermoelectric performance by reducing carrier-phonon interaction when further combined with carrier concentration optimization.

High Defect Tolerance in Heavy‐Band Thermoelectrics

AbstractThe insensitivity of thermoelectric (TE) performance to defects and impurities induced by off‐stoichiometry or low‐purity raw elements is crucial for the low‐cost mass production of highly efficient TE materials. Here it is demonstrated that heavy‐band TE materials with high optimized carrier concentration can exhibit a high tolerance against the off‐stoichiometry in their composition. Using the heavy‐band half‐Heusler compounds as examples, it is found that for ZrNiSn with optimal carrier concentration, the off‐stoichiometry of the nominal composition in between ±5% only results in a fluctuation of its peak zT value less than 10%. A composition‐zT phase diagram is thus established, guiding the exploration of a wide nominal composition region with peak zT values exceeding 1.0. The high defect tolerance observed in heavy‐band TE materials enables the achievement of high TE performance using low‐purity raw elements. As demonstrated in several systems, including n‐type ZrNi(Sn, Sb), (Zr, Nb)CoSb, and p‐type ZrCo(Sb, Sn), comparable zT values are attained when utilizing low‐purity raw elements (≈99.5%), instead of the high‐purity ones (99.95–99.999%), leading to substantial cost saving. These findings highlight the importance of understanding defect tolerance in TE materials, which could offer an additional advantage for their practical applications.

Enhanced thermoelectric performance of Cr-doped ZnSb through lattice thermal conductivity reduction below the phonon glass limit

Zinc antimonide (ZnSb) is a promising thermoelectric material due to its economic viability, elemental abundance, and superior phase stability compared to other Zn-Sb compounds. However, its widespread application is hindered by a low figure-of-merit (zT). Extensive research has explored doping, alloying, and nanostructuring to improve zT. This study investigates the impact of Cr doping in CryZn1-ySb (y = 0.0 – 0.03). Cr doping with y = 0.01 significantly reduces lattice thermal conductivity below the phonon glass limit. Debye-Callaway model calculations suggest a combined effect of point defects and decreased grain size caused by Cr incorporation. This reduction translates to enhanced thermoelectric performance, with the y = 0.01 sample exhibiting a 70 % improvement in zT at 673 K, reaching ∼0.67. Exceeding the Cr substitution limit leads to the formation of a detrimental secondary CrSb2 phase, increasing thermal conductivity. The Single Parabolic Band model calculations predict further zT enhancement in y = 0.01 sample to ∼0.2 at 300 K through optimized carrier concentration (307 % improvement in zT). These findings demonstrate the potential for significant zT improvement in ZnSb via defect engineering and carrier concentration tuning.