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Half‐Heusler compounds are promising materials for thermoelectric applications due to their high zT at elevated temperatures. However, their intrinsic high thermal conductivity limits their efficiency. Doping with Hf or Zr can improve the zT of these materials. Recently, a high zT of 1.5 at 1200 K achieved in p‐type Nb1‐xHfxFeSb has attracted much attention. While the effect of doping Hf in thermal conductivity is studied thoroughly, the effect of Hf doping on band parameters is not fully evaluated. This study investigates the effect of Hf and Zr doping on the electronic band parameters and thermoelectric properties of NbFeSb using the Single Parabolic Band model. The results show that Hf doping increases the weighted mobility of the samples, while Zr doping has no significant effect. Hf doping with x = 0.14 is predicted to improve the zT of NbFeSb by 35% at 300 K (0.19 → 0.26). These results show the intricate effects of Hf and Zr doping on the electronic and thermal properties of NbFeSb. [ABSTRACT FROM AUTHOR]

Control of bipolar conduction is essential to improve the high-temperature thermoelectric performance of materials for power generation applications. Recently, Hf(Te1−xSex)2 alloys have gained much attention due to their potential use in thermoelectric power generation. Increasing the Se alloying content significantly increases the band gap while decreasing its carrier concentration. These two factors affect bipolar conduction substantially. In addition, the weighted mobility ratio is estimated from the experimental electronic transport properties of Hf(Te1−xSex)2 alloys (x = 0.0, 0.025, 0.25, 0.5, 1.0) by using the Two-Band model. From the bipolar thermal conductivity also calculated using the Two-Band model, we find that it peaks near x = 0.5. The initial bipolar conductivity increase of x < 0.5 is mostly due to the decrease in the weighted mobility ratio and carrier concentration with increasing x. For x > 0.5, the drop in the bipolar conductivity can be understood with significant band gap enlargement. [ABSTRACT FROM AUTHOR]

Zintl compounds are promising thermoelectric materials for power generation as their electronic and thermal transport properties can be simultaneously engineered with anion/cation alloying. Recently, a peak thermoelectric figure‐of‐merit, zT, of 1.4 was achieved in a (Yb0.9Mg0.1)Cd1.2Mg0.4Zn0.4Sb2 Zintl phase at 700 K. Although the effects of alloying Zn in lattice thermal conductivity had been studied thoroughly, how the Zn alloying affects its electronic transport properties has not yet been fully investigated. This study evaluates how the Zn alloying at Cd sites alters the band parameters of (Yb0.9Mg0.1)Cd1.6−xMg0.4ZnxSb2 (x=0‐0.6) using the Single Parabolic Band model at 700 K. The Zn alloying increased the density‐of‐states effective mass (md*) from 0.87 to 0.97 m0. Among Zn‐alloyed samples, the md* of the x=0.4 sample was the lowest (0.93 m0). The Zn alloying decreased the non‐degenerate mobility (μ0) from 71 to 57 cm2 s−1 V−1. Regardless of Zn alloying content, the μ0 of the Zn‐alloyed samples were similar (∼57 cm2 s−1 V−1). Consequently, the x=0.4 with the highest zT exhibited the lowest weighted mobility (μW). The lowest μW represents the lowest theoretical electronic transport properties among other x. The highest zT at x=0.4 despite the lowest μW was explained with a significant lattice thermal conductivity reduction achieved with Zn alloying with x=0.4, which outweighed the deteriorated electronic transport properties also due to the alloying. [ABSTRACT FROM AUTHOR]

Re-based chalcogenides have been studied in various fields such as strain engineering, photodetection, spintronics, and electromechanics, as well as in piezoelectric and photonic devices. In this study, the electrical, thermal, and thermoelectric transport properties of two representative Re-based chalcogenides, ReSe2 and Re2Te5, are investigated systematically. Furthermore, their electronic band dispersions are calculated using density functional theory and compared with the phenomenological data. The maximum power factor values for the ReSe2 and Re2Te5 were measured 0.0066 and 0.11 mW/mK2 at 880 K, respectively. Thermal conductivity of layered ReSe2 at room temperature was between 1.93 and 8.73 W/mK according to the measuring direction. For Re2Te5 with a complex orthorhombic crystal structure, the thermal conductivity was quite low in the range between 0.62 and 1.23 W/mK at room temperature. As a result, the maximum z T values of ReSe2 were quite low as 0.0016 at 880 K due to very low power factor and high thermal conductivity. Meanwhile, the relatively high z T of 0.145 in Re2Te5 is obtained at 880 K, which is originated from the acceptable power factor value and the low thermal conductivity. [ABSTRACT FROM AUTHOR]

Hybrid organic/inorganic composites have recently attracted intensive interests as a promising candidate for flexible thermoelectric (TE) devices using inherently soft polymers as well as for increasing the degree of freedom to control TE properties. Experimentally, however, enhanced TE performance in hybrid composites has not been commonly observed, primarily due to inhomogeneous mixing between the inorganic and organic components which leads to limited electrical conduction in the less conductive component and consequently a low power factor in the composites compared to their single-component counterparts. In this study, we investigated the effects of different surfactants on the uniformity of mixing and the TE behaviors of the hybrid composites consisting of Bi0.5Sb1.5Te3 (BST) and poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS). We found that compared to dimethyl sulfoxide, which is the most widely used surfactant, Triton X-100 (TX-100) can lead to homogenous dispersion of BST in PEDOT:PSS. By systematically studying the effects of the surfactant concentration, we can attribute the better mixing capability of TX-100 to its non-ionic property, which results in homogenous mixing with a lower critical micelle concentration. Consequently, we observed simultaneous increase in electrical conductivity and Seebeck coefficient in the BST/PEDOT:PSS composites with the TX-100 surfactant. [ABSTRACT FROM AUTHOR]

Transition metal dichalcogenides have being actively studied on the account of their large density‐of‐state effective mass and low lattice thermal conductivity. Herein, thermoelectric and electrical transport properties of MoSe2–MoTe2 system are investigated. A series of Mo(Se1−xTex)2 (x = 0,.25,.5,.75, and 1) polycrystalline samples was synthesized by conventional solid‐state reaction. Single hexagonal phase was identified for each sample without secondary phase, confirming the formation of complete solid solutions between MoSe2 and MoTe2. The electrical conductivity and magnitude of the Seebeck coefficient increased simultaneously with increasing Te content x at high temperatures, with the power factor following the same trend. As results, the MoTe2 sample exhibited the maximum power factor of.014 mW/(m K2) at 823 K. The lattice thermal conductivities of the alloyed samples (x =.25,.5, and.75) significantly decreased compared to MoSe2 and MoTe2 owing to point defect phonon scattering via anion substitution, which is verified by the Debye–Callaway model. A maximum zT of.0044 was obtained for MoTe2 sample at 773 K; however, the calculated quality factor B values of the samples with x =.75 and 1 at 773 K were almost identical, suggesting that a further enhancement of zT can be achieved by optimizing an nH of the Mo(Se0.25Te0.75)2 sample. [ABSTRACT FROM AUTHOR]

Transition metal dichalcogenides have being actively studied on the account of their large density‐of‐state effective mass and low lattice thermal conductivity. Herein, thermoelectric and electrical transport properties of MoSe2–MoTe2 system are investigated. A series of Mo(Se1−xTex)2 (x = 0,.25,.5,.75, and 1) polycrystalline samples was synthesized by conventional solid‐state reaction. Single hexagonal phase was identified for each sample without secondary phase, confirming the formation of complete solid solutions between MoSe2 and MoTe2. The electrical conductivity and magnitude of the Seebeck coefficient increased simultaneously with increasing Te content x at high temperatures, with the power factor following the same trend. As results, the MoTe2 sample exhibited the maximum power factor of.014 mW/(m K2) at 823 K. The lattice thermal conductivities of the alloyed samples (x =.25,.5, and.75) significantly decreased compared to MoSe2 and MoTe2 owing to point defect phonon scattering via anion substitution, which is verified by the Debye–Callaway model. A maximum zT of.0044 was obtained for MoTe2 sample at 773 K; however, the calculated quality factor B values of the samples with x =.75 and 1 at 773 K were almost identical, suggesting that a further enhancement of zT can be achieved by optimizing an nH of the Mo(Se0.25Te0.75)2 sample. [ABSTRACT FROM AUTHOR]

Band engineering is an effective strategy to improve the electronic transport properties of semiconductors. In thermoelectric materials research, density‐of‐states effective mass is an undoubted key factor in verifying the band engineering effect and establishing a strategy for enhancing thermoelectric performance. However, estimation of the effective mass is demanding or inaccurate depending on the methods taken. A simple equation is proposed, valid for all degeneracy: Log10 (md*T/300) = (2/3) Log10 (n) − (2/3) [20.3 − (0.00508 × |S|) + (1.58 × 0.967|S|)] that utilizes experimentally determined Seebeck coefficient (S) and carrier concentration (n) to determine the effective mass (md*) at a temperature (T). This straightforward equation, which gives an accurate analysis of the band modulation in terms of md*, is indispensable in designing thermoelectric materials of maximized performance. [ABSTRACT FROM AUTHOR]

Summary: Excess Cu has been reported as an effective way to enhance the thermoelectric performance of n‐type Bi2Te3‐based alloys as well as to secure the reproducibility of their electronic properties. However, the effect of Cu doping into Bi2Te3 lattice is also known to be complex since Cu can occupy either interlayer or cation/anion sites, depending on conditions. Herein, Cu doping behavior in a binary Bi2Te3 prepared by a conventional melt‐solidification process was demonstrated, and corresponding changes in electronic and thermal transport properties were investigated. We found that the mechanism behind electronic transport properties improvements was different depending on Cu doping behavior: (a) power factor enhancement (especially at elevated temperatures) at low Cu concentrations (x ≤ 0.008 in CuxBi2Te3) is mainly due to interlayer intercalation of Cu, which optimizes electron concentration and increases the effective mass and (b) power factor enhancement at higher Cu concentrations (0.012 ≤ x ≤ 0.02 in CuxBi2Te3) is due to the substituted Cu at Bi‐site, which increases the weighted mobility ratio, as well as intercalated Cu. Enhanced room temperature zT ~ 0.68 and average zT ~ 0.53 were obtained in Cu0.02Bi2Te3 due to synergetic effect of intensified point defect (intercalated and substituted Cu) phonon scattering. [ABSTRACT FROM AUTHOR]

[Display omitted] • We tuned thermoelectric PF using nondestructive modification of carrier concentration and conductivity by photoirradiation. • Surface-sensitive reaction in combination with optical gating altered thermoelectric properties of ZnO. • Depletion of charge carriers by the surface-sensitive oxygen adsorption/desorption tuned the thermoelectric PF. We have studied the complex relationship between the thermoelectric properties of Zinc Oxide (ZnO) thin films and their interaction with various atmospheric conditions and ultraviolet (UV) illumination. We focused on the dynamic changes in the Seebeck coefficient (SC) and conductivity of ZnO under varied environments, which are crucial for tuning the thermoelectric power factor (PF). ZnO films showed pronounced variations in their carrier concentration and mobility when exposed to UV light in air and argon atmospheres, which can be attributed to the interplay of oxygen adsorption and desorption on the ZnO surface. The profound effect of the oxygen interaction, evidenced in a depletion layer wider than the ZnO film itself, significantly changed the electron concentration, and affected the surface conductivity and thus, the thermoelectric PF of ZnO. The experiments revealed that the SC decreased with increasing carrier concentration and mobility under UV illumination, while the thermoelectric PF increased in both atmospheres, providing pivotal insights into the nondestructive optimization of the thermoelectric properties of ZnO through controlled surface-sensitive reactions and optical gating. [ABSTRACT FROM AUTHOR]

The formation of secondary phases of NiTe2 and NbTe2 in p-type Bi0.5Sb1.5Te3 thermoelectric alloys was investigated through in situ phase separation by using the melt spinning process. Adding stoichiometric Ni, Nb, and Te in a solid-state synthesis process of Bi0.5Sb1.5Te3, followed by rapid solidification by melt spinning, successfully segregated NiTe2 and NbTe2 in the Bi0.5Sb1.5Te3 matrix. Since heterointerfaces of Bi0.5Sb1.5Te3 with NiTe2 and NbTe2 form potential barriers of 0.26 and 0.08 eV, respectively, a low energy carrier filtering effect can be expected; higher Seebeck coefficients and power factors were achieved for Bi0.5Sb1.5Te3(NiTe2)0.01 (250 μV/K and 3.15 mW/mK2), compared to those of Bi0.5Sb1.5Te3 (240 μV/K and 2.69 mW/mK2). However, there was no power factor increase for NbTe2 segregated samples. The decrease in thermal conductivity was seen due to the possible additional phonon scattering by the phase segregations. Consequently, zT at room temperature was enhanced to 0.98 and 0.94 for Bi0.5Sb1.5Te3(NiTe2)0.01 and Bi0.5Sb1.5Te3(NbTe2)0.01, respectively, compared to 0.79 for Bi0.5Sb1.5Te3. The carrier filtering effect induced by NiTe2 segregations with an interface potential barrier of 0.26 eV effectively increased the Seebeck coefficient and power factor, thus improving the zT of p-type Bi0.5Sb1.5Te3, while the interface potential barrier of 0.08 eV of NbTe2 segregation appeared to be too small to induce an effective carrier filtering effect. [ABSTRACT FROM AUTHOR]

Herein we report a significantly reduced lattice thermal conductivity of Sb-doped Hf0.35Zr0.35Ti0.3NiSn half-Heusler alloys with sub-micron grains (grain size of ~300 nm). Polycrystalline bulks of Hf0.35Zr0.35Ti0.3NiSn1−xSbx (x = 0.01, 0.02, 0.03) with a complete single half-Heusler phase are prepared using temperature-regulated melt spinning and subsequent spark plasma sintering without a long annealing process. In these submicron-grained bulks, a very low lattice thermal conductivity value of ~2.4 W m−1 K−1 is obtained at 300 K due to the intensified phonon scatterings by highly dense grain boundaries and point-defects (Zr and Ti substituted at Hf-sites). A maximum thermoelectric figure of merit, zT, of 0.5 at 800 K is obtained in Hf0.35Zr0.35Ti0.3NiSn0.99Sb0.01. [ABSTRACT FROM AUTHOR]

Polycrystalline bulks of Hf-doped Cu0.01Bi2Te2.7Se0.3 are prepared via a conventional melt-solidification process and subsequent spark plasma sintering technology, and their thermoelectric performances are evaluated. To elucidate the effect of Hf-doping on the thermoelectric properties of n-type Cu0.01Bi2Te2.7Se0.3, electronic and thermal transport parameters are estimated from the measured data. An enlarged density-of-states effective mass (from ~0.92 m0 to ~1.24 m0) is obtained due to the band modification, and the power factor is improved by Hf-doping benefitting from the increase in carrier concentration while retaining carrier mobility. Additionally, lattice thermal conductivity is reduced due to the intensified point defect phonon scattering that originated from the mass difference between Bi and Hf. Resultantly, a peak thermoelectric figure of merit zT of 0.83 is obtained at 320 K for Cu0.01Bi1.925Hf0.075Te2.7Se0.3, which is a ~12% enhancement compared to that of the pristine Cu0.01Bi2Te2.7Se0.3. [ABSTRACT FROM AUTHOR]

Incorporation of a foreign element is considered as a promising approach to enhance the performance of thermoelectric materials since this can either improve the power factor by a band structure modification or reduce the thermal conductivity by a phonon scattering strengthening. We fabricated the polycrystalline bulk samples of Cu-incorporated Bi0.5Sb1.5Te3 by melt spinning and spark plasma sintering, and evaluated the electronic and thermal transport properties. From the phase analysis and thermoelectric properties measurement, we found that most of the added excess Cu atoms were substituted at a Sb-site and a small amount of Cu was intercalated at the van der Waals gap between quintuple layers. By the formation of two different point defects (substituted Cu and intercalated Cu), the thermoelectric power factor was enhanced because of the increased density of states effective mass, and simultaneously reduced thermal conductivity originated from the intensified phonon scattering and suppressed bipolar contribution. Maximum thermoelectric figure of merit zT of 1.13 was obtained at 400 K. [ABSTRACT FROM AUTHOR]

One means of enhancing the performance of thermoelectric materials is to generate secondary nanoprecipitates of metallic or semiconducting properties in a thermoelectric matrix, to form proper band bending and, in turn, to induce a low-energy carrier filtering effect. However, forming nanocomposites is challenging, and proper band bending relationships with secondary phases are largely unknown. Herein, we investigate the in situ phase segregation behavior during melt spinning with various metal elements, including Ti, V, Nb, Mo, W, Ni, Pd, and Cu, in p-type Bi0.5Sb1.5Te3 (BST) thermoelectric alloys. The results showed that various metal chalcogenides were formed, which were related to the added metal elements as secondary phases. The electrical conductivity, Seebeck coefficient, and thermal conductivity of the BST composite with various secondary phases were measured and compared with those of pristine BST alloys. Possible band alignments with the secondary phases are introduced, which could be utilized for further investigation of a possible carrier filtering effect when forming nanocomposites. [ABSTRACT FROM AUTHOR]

• Bipolar contribution to thermoelectric properties in Cu 0.008 Bi 2 (Te 1- x Se x) 3 was investigated by tuning Te/Se ratio. • Weighted mobility and bipolar thermal conductivity was calculated using two-band model. • Weighted mobility for hole carriers gradually decreased with increasing Se content. • Bipolar thermal conductivity was remarkably reduced from 0.4 to 0.06 W m−1 K−1 in Cu 0.008 Bi 2 Te 2.1 Se 0.9. • Peak zT was enhanced to 0.92 in Cu 0.008 Bi 2 Te 2.1 Se 0.9 at 440 K. Bipolar conduction in Bi-Te-based alloys is a critical barrier to high conversion efficiency in thermoelectric power generation applications. Herein, we investigate the effect of Te/Se ratio control on bipolar conduction in n -type Cu 0.008 Bi 2 (Te,Se) 3. Based on the two-band model and Callaway model, we found that electronic and thermal transports of minority carriers (holes) were gradually decreased with an increase in Se content. The high-temperature power factor of Se-rich Cu 0.008 Bi 2 Te 2.1 Se 0.9 was higher in value compared to reference Cu 0.008 Bi 2 Te 2.7 Se 0.3 due to the weighted mobility ratio increase, and its bipolar thermal conductivity was significantly reduced simultaneously. As a result, a peak figure of merit (zT) of 0.92 was obtained at 440 K in Cu 0.008 Bi 2 Te 2.1 Se 0.9. [ABSTRACT FROM AUTHOR]

Re-based chalcogenides have been studied in various fields such as strain engineering, photodetection, spintronics, and electromechanics, as well as in piezoelectric and photonic devices. In this study, the electrical, thermal, and thermoelectric transport properties of two representative Re-based chalcogenides, ReSe2 and Re2Te5, are investigated systematically. Furthermore, their electronic band dispersions are calculated using density functional theory and compared with the phenomenological data. The maximum power factor values for the ReSe2 and Re2Te5 were measured 0.0066 and 0.11 mW/mK2 at 880 K, respectively. Thermal conductivity of layered ReSe2 at room temperature was between 1.93 and 8.73 W/mK according to the measuring direction. For Re2Te5 with a complex orthorhombic crystal structure, the thermal conductivity was quite low in the range between 0.62 and 1.23 W/mK at room temperature. As a result, the maximum z T values of ReSe2 were quite low as 0.0016 at 880 K due to very low power factor and high thermal conductivity. Meanwhile, the relatively high z T of 0.145 in Re2Te5 is obtained at 880 K, which is originated from the acceptable power factor value and the low thermal conductivity. [ABSTRACT FROM AUTHOR]

Compositional tuning is one of the important approaches to enhance the electronic and thermal transport properties of thermoelectric materials since it can generate point defects as well as control the phase evolution behavior. Herein, we investigated the Ti addition effect on the grain growth during melt spinning and thermoelectric transport properties of Hf0.5Zr0.5NiSn0.98Sb0.02 half-Heusler compound. The characteristic grain size of melt-spun ribbons was reduced by Ti addition, and very low lattice thermal conductivity lower than 0.27 W m−1 K−1 was obtained within the whole measured temperature range (300–800 K) due to the intensified point defect (substituted Ti) and grain boundary (reduced grain size) phonon scattering. Due to this synergetic effect on the thermal transport properties, a maximum thermoelectric figure of merit, zT, of 0.47 was obtained at 800 K in (Hf0.5Zr0.5)0.8Ti0.2NiSn0.98Sb0.02. [ABSTRACT FROM AUTHOR]

A significant obstacle to obtaining enhanced thermoelectric performance (defined by a thermoelectric figure of merit, zT) in commercial p -type Bi-Sb-Te alloys is bipolar transport originating from their intrinsic narrow-band-gap semiconducting characteristics. Cation-site doping is commonly used to suppress the bipolar conduction. However, zT enhancement occurs often only at elevated temperatures since the electronic thermal conductivity mainly increases at low temperatures due to the increase of hole concentration. Herein, the substitution of isovalent S ions in the anion Te-site of Bi-Sb-Te is explored to obtain a high zT over a wide temperature range by simultaneously increasing the density-of-states effective mass and weighted mobility. The zT of Bi 0.49 Cu 0.01 Sb 1.5 Te 3 is enhanced by ~10 % for all measured temperatures, and the average zT increases beyond 1.0 between 300 and 520 K, benefitting from the synergetic control of band structure and deformation potential via S substitution. Image, graphical abstract [ABSTRACT FROM AUTHOR]

Summary: Excess Cu has been reported as an effective way to enhance the thermoelectric performance of n‐type Bi2Te3‐based alloys as well as to secure the reproducibility of their electronic properties. However, the effect of Cu doping into Bi2Te3 lattice is also known to be complex since Cu can occupy either interlayer or cation/anion sites, depending on conditions. Herein, Cu doping behavior in a binary Bi2Te3 prepared by a conventional melt‐solidification process was demonstrated, and corresponding changes in electronic and thermal transport properties were investigated. We found that the mechanism behind electronic transport properties improvements was different depending on Cu doping behavior: (a) power factor enhancement (especially at elevated temperatures) at low Cu concentrations (x ≤ 0.008 in CuxBi2Te3) is mainly due to interlayer intercalation of Cu, which optimizes electron concentration and increases the effective mass and (b) power factor enhancement at higher Cu concentrations (0.012 ≤ x ≤ 0.02 in CuxBi2Te3) is due to the substituted Cu at Bi‐site, which increases the weighted mobility ratio, as well as intercalated Cu. Enhanced room temperature zT ~ 0.68 and average zT ~ 0.53 were obtained in Cu0.02Bi2Te3 due to synergetic effect of intensified point defect (intercalated and substituted Cu) phonon scattering. [ABSTRACT FROM AUTHOR]

Doping is known as an effective way to modify both electrical and thermal transport properties of thermoelectric alloys to enhance their energy conversion efficiency. In this project, we report the effect of Pd doping on the electrical and thermal properties of n-type Cu0.008Bi2Te2.7Se0.3 alloys. Pd doping was found to increase the electrical conductivity along with the electron carrier concentration. As a result, the effective mass and power factors also increased upon the Pd doping. While the bipolar thermal conductivity was reduced with the Pd doping due to the increased carrier concentration, the contribution of Pd to point defect phonon scattering on the lattice thermal conductivity was found to be very small. Consequently, Pd doping resulted in an enhanced thermoelectric figure of merit, zT, at a high temperature, due to the enhanced power factor and the reduced bipolar thermal conductivity. [ABSTRACT FROM AUTHOR]

Cu doping has been known to enhance thermoelectric performance of p -type (Bi,Sb) 2 Te 3 alloys by suppressing bipolar conduction at higher temperatures. However, detailed mechanisms are not provided yet. Herein, we investigate the unchallenged role of Cu doping in (Bi,Sb) 2 Te 3 alloys by means of Bi-rich compositions of Bi 0.6 Sb 1.4 Te 3 and Bi 0.7 Sb 1.3 Te 3 , which were chosen since they exhibit much inferior thermoelectric performance owing to severe bipolar conduction. It was found that Cu doping increases non-degenerate mobility and density-of-states effective mass of valence band as well as hole concentration. Small amount of Cu doping enhanced maximum thermoelectric figure-of-merit of Bi 0.6 Sb 1.4 Te 3 composition from 0.25 to 0.98. Image, graphical abstract [ABSTRACT FROM AUTHOR]

Cr 2 Se 3 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 Cr 2 Te 3 can be designed to manipulate the electrical and thermal transport properties. In this study, a series of Cr 2 Se 3 -Cr 2 Te 3 solid-solution alloys (Cr 2 (Se 1- x Te x) 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 Cr 2 Se 3 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 Cr 2 Se 3 gradually and significantly decreased to 0.009 for Cr 2 Te 3 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 Cr 2 Se 3 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. • A series of Cr 2 (Se 1- x Te x) 3 alloys (x = 0, 0.33, 0.5, 0.67, 0.83, and 1) was investigated. • Solid solution alloy is formed for the Cr 2 (Se 1- x Te x) 3 compositions. • Cr 2 (Se 1- x Te x) 3 showed the degraded thermoelectric properties with x increasing. • However, thermoelectric quality factors for x = 0.33–0.83 were found to be enhanced. • Enhanced thermoelectric properties are analyzed when carrier concentration optimized further. [ABSTRACT FROM AUTHOR]

To meet the growing demand for thermoelectric devices operating in intermediate temperature ranges, it is essential to develop high-performance materials with superior thermoelectric properties and robust mechanical strength. In this study, we systematically optimized carrier concentration by introducing acceptor impurities into ZnSb materials. Our results demonstrate that doping Cu into the Zn site effectively modulates hole carrier concentration, leading to a substantial enhancement in electrical conductivity and a remarkable improvement in power factor (107 %). Consequently, we achieved a high peak ZT of 1.04 at 600 K and an average ZT ave value of 0.63 within the temperature range of 300–600 K. This yielded a calculated efficiency of η max = 7 % at ΔT = 300 K, for the Zn 0.99 Cu 0.01 Sb sample, which is 134 % higher than that of the pristine ZnSb sample (η max = 2.98 %). Moreover, the superior hardness and fracture toughness (K IC) of ZnSb samples compared to other state-of-the-art thermoelectric materials make them highly desirable for real-time applications. [Display omitted] • Synthesized a series of Zn 1-x Cu x Sb (x = 0, 0.01, 0.03, and 0.1) samples. • Cu-doping at the Zn site modulates the electronic properties, enhancing the power factor. • Achieved a peak ZT of 1.04 at 600 K and η max of 7 % in the optimal Zn 0.99 Cu 0.01 Sb. • Obtained improved fracture toughness (K IC) by inhibiting crack propagation. [ABSTRACT FROM AUTHOR]

Sb 2 Te 3 -based alloys exhibit decent thermoelectric transport properties in the mid-temperature range above 550 K. However, the study on Sb 2 Te 3 alloys has been limited in simple doping approach. Herein, evolution in the thermoelectric transport properties associated with the phase formation of solid solution alloys of Sb 2 (Te 1− x Se x) 3 (x = 0, 0.25, 0.33, 0.5, 0.75, and 1.0) compositions is investigated to widen the strategy to solid solution alloying. A single phase of the Sb 2 Te 3 rhombohedral structure formed from x = 0 to 0.5, whereas mixed phases with an orthorhombic structure of Sb 2 Se 3 was observed at x = 0.75. The electrical conductivity weighted mobility were gradually decreased as Se content increases to x = 0.75. Consequently, the power factor was decreased gradually and significantly to 0.076 mW/mK2 for x = 0.75 (Sb 2 (Te 0·25 Se 0.75) 3) compared with 2.6 mW/mK2 of the pristine sample. The total thermal conductivity of 2.0 W/mK for the Sb 2 Te 3 was significantly reduced gradually to 0.64 W/mK for x = 0.75 owing to simultaneous decrease in electrical and lattice thermal conductivities. The reduction in lattice thermal conductivity is mainly owing to the addition point defect scattering caused by the Se addition. Nevertheless, thermoelectric figure of merit zT of Sb 2 Te 3 is gradually decreased from 0.64 to 0.11 for x = 0.75 at 650 K due to the degradation in electrical transport properties. Furthermore, the theoretical zT was estimated for all samples with varying carrier concentration based on single parabolic band model. [ABSTRACT FROM AUTHOR]

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 Cr y Zn 1- y Sb (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 CrSb 2 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. • Cr doping into p -type ZnSb improves the zT until the formation of a CrSb 2 phase. • Cr doping increases effective mass while suppressing deformation potential. • Cr doping suppresses lattice thermal conductivity down to the amorphous limit. • Cr doping with optimized carrier concentration can improve the 300 K zT by 300 %. [ABSTRACT FROM AUTHOR]

While extensive research has explored Cu doping in n -type Bi 2 (Te,Se) 3 for its beneficial effects on reproducibility and mobility, its impact on p -type (Bi,Sb) 2 Te 3 remains incompletely understood. Recently, Saglik et al. demonstrated Cu 2.9 Te 2 incorporation into Bi 0.5 Sb 1.5 Te 3 as a novel approach for simultaneous Cu doping and intercalation, surpassing prior studies focused solely on Cu doping at Bi/Sb sites. The influence of additional Cu intercalation on both electronic band parameters (density-of-states effective mass, deformation potential, and weighted mobility) and phonon scattering by point defects has yet to be investigated. Here, we employ the Effective Mass model to comparatively assess the impact of Cu intercalation on these band parameters relative to single Cu doping. Furthermore, the Callaway-von Baeyer and Debye-Callaway models are employed to evaluate the effect of Cu intercalation in scattering phonons. Our findings reveal that additional Cu intercalation effectively suppresses the lattice thermal conductivity of Bi 0.5 Sb 1.5 Te 3 to the amorphous limit, offering the potential to improve the figure-of-merit (zT) to ∼2.0 near 420 K with optimized carrier concentration. This approach highlights Cu intercalation as a readily applicable and powerful tool for maximizing the thermoelectric performance of Bi 2 Te 3 -based materials. • Incorporating Cu 2.9 Te 2 into p -type (Bi,Sb) 2 Te 3 makes Cu intercalation possible. • Cu intercalation increases effective mass more effectively than Cu doping. • Cu intercalation suppresses lattice thermal conductivity down to the amorphous limit. • Cu intercalation with optimized carrier concentration can improve the zT to ∼2.0. [ABSTRACT FROM AUTHOR]

Metal chalcogenide alloys are considered as electronic materials owing to their adjustable electrical properties either by doping or alloying. In this study, the electrical and thermal properties of the NiS 2 –NiSe 2 system are investigated by synthesizing Ni(S 1- x Se x) 2 (x = 0, 0.25, 0.5, 0.75, and 1.0) compositions. All samples exhibit pyrite-type cubic structures and form complete solid solution alloys. It is found that the NiS 2 –NiSe 2 system exhibits a wide spectrum of electrical characteristics; NiS 2 exhibits semiconducting conduction with low electrical conductivity σ of 7.8 S/cm and a carrier concentration of ∼1019 cm−3 at room temperature, whereas NiSe 2 exhibits metallic conduction with high σ of 11,600 S/cm and ∼1021 cm−3 carriers. As x of Ni(S 1- x Se x) 2 increased, the σ is gradually increased. On the other hand, the magnitude of Seebeck coefficient S is gradually decreased with x at 700 K. Thus, the optimized power factor (S 2∙ σ) of 0.10 mW/mK2 at 700 K is achieved for Ni(S 0.75 Se 0.25) 2 composition. Lattice thermal conductivities of the solid solution samples (x = 0.25, 0.5 and 0.75) range from 2.0 to 3.8 W/mK at 300 K, representing a reduction compared to those of NiS 2 and NiSe 2 (5.5 and 6.1 W/mK, respectively). Consequently, an enhanced thermoelectric performance was achieved in Ni(S 0.75 Se 0.25) 2 benefiting from an optimized solid solution alloying, with a maximum thermoelectric figure of merit of 0.020 at 700 K. [Display omitted] • A series of Ni(S 1- x Se x) 2 (x = 0, 0.25, 0.5, 0.75, and 1.0) was investigated. • NiS 2 and NiSe 2 exhibits semiconducting and metallic conduction, respectively. • The optimized power factor of 0.10 mW/mK2 is achieved for Ni(S 0.75 Se 0.25) 2. • Lattice thermal conductivities of the solid solution samples were reduced. • Optimized zT of 0.02 was obtained for Ni(S 0.75 Se 0.25) 2. [ABSTRACT FROM AUTHOR]

Atomic-scale defects are essential for improving thermoelectric (TE) performance of most state-of-the-art materials by simultaneously tuning the electronic and thermal properties. However, because the plural atomic-scale defects are generally inherent and disordered in nanostructured TE materials, their complexity and ambiguity on determining TE performance remain a challenge to be solved. Furthermore, the thermal stability of atomic-scale defects in nanostructured TE materials has not been studied much so far. Herein, we report that the atomic-scale defect disorders are indispensable for high TE performance of nanostructured Ti 1–x Hf x NiSn 1–y Sb y half-Heusler alloys, but gradually degraded at over 773 K, deteriorating the TE performance. It is found from the thermal annealing of nanostructured Ti 0.5 Hf 0.5 NiSn 0.98 Sb 0.02 alloys that the annihilation of Ti,Hf/Sn antisite defects primarily reduces atomic-scale defect disorders and largely contributes to the increase of lattice thermal conductivity. Moreover, it is verified that the Ni interstitial defects mainly dominate the electronic transport properties, leading to the enhancement of power factor. Direct atomic structure observations clearly demonstrate the inherent Ni interstitial defects and the thermal vulnerability of Ti,Hf/Sn antisite defects. These results provide an important guide for the application of half-Heusler alloys with highly disordered atomic-scale defects. Image 1 [ABSTRACT FROM AUTHOR]

Herein, we report the enhanced thermoelectric performance of Cu incorporated Bi 2 Te 2.7 Se 0.3 prepared by melt spinning. The electronic transport properties were significantly improved by Cu incorporation due to the synergetic effect of carrier tuning, band modification, and electron-phonon transport control. The steady dimensionless thermoelectric figure of merit higher than 0.90 was achieved in the wide range of temperature range of 350–450 K for Cu 0.008 Bi 2 Te 2.7 Se 0.3 , which is ~30% enhancement in comparison with pristine Bi 2 Te 2.7 Se 0.3. This result can lead to high thermoelectric power generation efficiency 4.2% at Δ T = 180 K. Unlabelled Image [ABSTRACT FROM AUTHOR]

Bi 2 Te 3 -based alloys are studied as most suitable thermoelectric alloys at room temperature. However, the lower zT values of Se-doped n -type Bi 2 Te 3- x Se x than those of p -type Bi x Sb 2- x Te 3 still hinder provision of high device performance. In this study, in effort to search for n -type Bi 2 Te 3 -based alloys other than Se-doped Bi 2 Te 3 , electrical, thermal, and thermoelectric transport properties of Bi 2- x Co x Te 3 (x = 0, 0.03, 0.06, 0.09, and 0.12) alloys were examined systematically. By Co doping, power factor of Bi 2 Te 3 alloys was increased by 65% to 2.89 mW/mK2 at 300 K. The enhanced electric transport properties were analyzed by experimental phenomenological parameters deduced from two-band model. It was also found that Co doping reduced bipolar and lattice thermal conductivities. Consequently, zT at 300 K was improved to 0.48 for Bi 1.91 Co 0.09 Te 3 (x = 0.09) by 37% compared to that of 0.35 for Bi 2 Te 3. The maximum zT of 0.62 was obtained for Bi 1.91 Co 0.09 Te 3 at 400 K. The two-band model predicts that the z T of Co-doped Bi 2 Te 3 can be further improved by optimizing carrier concentration. [ABSTRACT FROM AUTHOR]

Bi-Te based materials have been used for near-room-temperature thermoelectric applications. However, their properties dramatically decrease at high temperatures (over 100 °C), limiting their use in power generation. In this study, we investigated the enhanced thermoelectric properties of Bi-Te based materials by Cu doping and employing the melt-spinning (MS) process that can be utilized especially at elevated temperatures. By changing the doping amount, we could modulate the temperature dependence of thermoelectric properties, where the maximum ZT temperature could be shifted from room temperature to 450 K. The highest ZT value, 1.34, was achieved at 400 K for 2% Cu-doped Bi 0.5 Sb 1.5 Te 3 , which is due to the enhancement in power factor and reduction in lattice thermal conductivity. The average ZT value between room temperature and 530 K was 1.17 for 2% Cu-doped Bi 0.5 Sb 1.5 Te 3 , which is 46% higher than that of pristine Bi 0.5 Sb 1.5 Te 3 . Consequently, the synergetic effect of MS process and Cu incorporation can be a promising method to widen the application of Bi-Te based thermoelectric materials for mid-temperature power generation. [ABSTRACT FROM AUTHOR]

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 Sn 1−3 x In x Sr 2 x Te (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 Sn 1−3 x In x Sr 2 x Te (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. • Resonant state formation and band convergence are induced in In and Sr co-doped SnTe. • There are band convergences due to temperature and doping in the co-doped SnTe. • Weighted mobility is a good indicator of band convergence. • Theoretical maximum zT of In and Sr co-doped SnTe is as high as ∼1.8 at 823 K. [ABSTRACT FROM AUTHOR]

Ti 2 FeNiSb 2 is a promising double half-Heusler thermoelectric compound with intrinsically low thermal conductivity due to low phonon group velocity. Since it is introduced in 2019, many efforts have been focused on further reducing its thermal conductivity via doping. However, the effects of doping on its electronic transport properties have been neglected. Here, we investigate the effects of doping Co and Bi at the Fe- and Sb-sites, respectively, in Ti 2 FeNiSb 2 for the first time. Changes in band parameters due to atomic site-targeted doping are estimated by the Single Parabolic Band model. Bypassing the trade-off relation between the Seebeck coefficient and electrical conductivity is observed when doping Co at Fe-sites. The physics behind the bypass is explained in terms of temperature-dependent reduced chemical potential and non-degenerate mobility. As a result, peak figure of merit zT values of ∼0.69 in Ti 2 Fe 0.9 Co 0.1 NiSb 2 is achieved, which is approximately six times higher than that of the pristine Ti 2 FeNiSb 2. The thermoelectric performance of Ti 2 FeNiSb 2 can be improved by selective band engineering to bypass the Seebeck coefficient-electrical conductivity trade-off relation from atomic-site targeted doping. • Atomic site-targeted doping can fine-tune band parameters of double half-Heusler. • Doping at different atomic sites has a different impact on the phonon scattering. • Doping Co at Fe-sites of Ti 2 FeNiSb 2 improves zT of pristine Ti 2 FeNiSb 2 by six times. [ABSTRACT FROM AUTHOR]

• Cu-doped Bi 2 Te 2.7 Se 0.3 was first synthesized by using Cu nanoparticle precursor. • The power factor is enhanced by the substitutional Cu doping in quintuple layers. • Substantial reduction of electronic thermal conductivity originates from Cu doping. • Average zT value at 300–500 K for 2% Cu–Bi 2 Te 2.7 Se 0.3 increased to 0.79. • The power generation efficiency can be enhanced by 25% from the pristine sample. Bi-Te-based materials have been used for room-temperature thermoelectric applications. However, n -type Bi 2 (Te,Se) 3 thermoelectric alloys show a limited conversion efficiency, as compared to their p -type counterparts, thus hindering further widespread room-temperature applications. In this study, we investigated the enhanced thermoelectric properties of n -type Bi 2 (Te,Se) 3 materials by the addition of Cu nanoparticles via a conventional high-energy ball milling process. The electrical and thermal transport properties were modulated by changing the amount of Cu nanoparticles. The power factor was enhanced by controlling the carrier, and the total thermal conductivity was reduced mainly due to the reduction in electronic thermal conductivity. Thus, dimensionless thermoelectric figure of merit (zT) at room temperature was enhanced for the Cu-added samples, and the highest zT value of 0.85 at 375 K was achieved in 2% Cu-doped Bi 2 Te 2.7 Se 0.3. The average zT (zT avg) value between room temperature and 500 K was 0.79 for the 2% Cu-doped Bi 2 Te 2.7 Se 0.3 , which was 20% higher than that of the pristine Bi 2 Te 2.7 Se 0.3 , whereas a zT higher than 0.80 was sustained from room temperature to ~450 K. These results can lead to a high thermoelectric power generation efficiency of 7.6% at Δ T = 200 K. [ABSTRACT FROM AUTHOR]