P-type Thermoelectric Materials Research Articles

Modulated Fermi Level and Relaxed Lattice Strain Leading to Enhanced Thermoelectric Properties in AgSbSe2

Cubic-phase AgSbSe2 has been known as a decent p-type thermoelectric material, due to its intrinsically low lattice thermal conductivity; nevertheless, the difficulty in modulating its electrical transport performance impeded its further competition with other state-of-the-art mid-temperature thermoelectrics. In this work, we investigated the effects of Pb doping and Sb vacancies on modulating the thermoelectric properties of AgSbSe2. Pb doping was found to be able to obviously increase the hole concentration, leading to a descending of Fermi level well into the multiple valence bands, thus leading to an enhanced band degeneracy; subsequent Sb vacancies could play the role of relaxing the local lattice strains due to PbSb substitution, resulting in a weakened charge carrier scattering and enhanced weighted mobility. Consequently, a peak power factor PFmax of 7.5 μW cm–1 K–2 at 523 K was achieved in the composition of AgSb0.935Pb0.06Se2, which is 40% higher than that of pristine AgSbSe2. Furthermore, Pb doping and Sb vacancies induce enhanced point defect scattering of high-frequency heat-carrying phonons, yielding a reduced lattice thermal conductivity and a remarkably enhanced maximal ZTmax value of 1.02 at 723 K in the sample AgSb0.935Pb0.06Se2.

High-performance bulk Bi0.4Sb1.6Te3.0 thermoelectrics prepared from nanocrystal precursor synthesized via chemical precipitation

Bi0.4Sb1.6Te3.0 is a known p-type thermoelectric material with high thermoelectric performance near room temperature. In this study, bulk Bi0.4Sb1.6Te3.0 materials are synthesized via spark plasma sintering using single nanometer-sized nanocrystals as precursors, which are prepared in the solution phase by chemical precipitation from metallic ions. The obtained bulk material consists of submicron-to-micrometer-sized disk-shaped grains with isotropic distribution. Additionally, structural distortion such as strain and/or defects can be directly introduced into grains upon sintering, presumably due to the incomplete coalescence of the small nanocrystal precursors to form larger grains during the sintering process. The resultant bulk material exhibits high electrical conductivity and considerably low lattice thermal conductivity, due to the utilization of impurity-free nanocrystal precursors and local lattice distortions included inside grains acting as scattering nodes of phonons, respectively. The maximum dimensionless figure of merit (ZT) is achieved to be 1.42 ​at 355 ​K, and, therefore, this work provides effective methodology to fabricate high-performance chalcogenide-based thermoelectric materials.

Improving thermoelectric performance of half-Heusler Ti0.2Hf0.8CoSb0.8Sn0.2 compounds via the introduction of excessive Ga and Co-deficiencies

(Ti/Zr/Hf)CoSb0.8Sn0.2 compounds are promising p-type thermoelectric materials. In this work, excessive Ga and Co deficiencies were introduced into Ti0.2Hf0.8CoSb0.8Sn0.2 compounds. The microstructure, electrical, and thermal transport properties were investigated in the temperature range of 300 K < T < 1000 K. The excessive Ga formed nanoinclusions with Ti and Sn at the grain boundaries. The Co deficiencies lowered the thermal conductivity and simultaneously improved the electrical conductivity. As a result, the power factor was enhanced to 3.2 mW/m K2, and the maximum Z.T. of ∼0.93 at 988 K was obtained in the Ti0.2Hf0.8Co0.99Sb0.8Sn0.2–0.01Ga sample, which is an increase of ∼48% compared to that of the pristine Ti0.2Hf0.8CoSb0.8Sn0.2 sample.

Thermoelectric properties of hole-doped CuRhO2 thin films

Design and realization of high-efficiency p-type thermoelectric materials with excellent performance are the demand for integrated thermoelectric components. Compared with single crystal bulk materials, thermoelectric thin films are more suitable for the miniaturization of thermoelectric devices. Here, c-axis oriented CuRh1−xMgxO2 (x = 0, 0.05, and 0.1) thin films were prepared and the thermoelectric properties are reported. The power factor of a p-type 10% Mg-doped CuRhO2 thin film shows a large value of 535.7 μW K–2 m-1 at 300 K. The results suggest that the hole-doped CuRhO2 thin films can be regarded as potential p-type thermoelectric oxide and will pave an avenue to develop Rh-based thermoelectric thin films.

Discovery of YbNiSb-Based Half-Heusler Alloys as Promising Thermoelectric Materials

Half-Heusler materials are promising candidates for high-temperature power generation and have relatively high lattice thermal conductivity compared to other thermoelectric material systems. In this work, we report novel p-type YbNiSb-based half-Heusler alloys with a low lattice thermal conductivity (∼3.6 W m–1 K–1 at 340 K) that resulted from their large Grüneisen parameter, low sound speed, and low Debye temperature. All YbNiSb-based alloys exhibit a high carrier mobility of 30–50 cm2 V–1 s–1 at room temperature because of their relatively small effective mass. Importantly, the structural analysis reveals that Yb-rich Yb1.3Ni0.9Sb0.8 exhibits Yb/Ni and Yb/Sb substitution, indicating a wide homogeneity region of the YbNiSb phase experimentally. The adjustable Yb and Ni contents in YbNiSb-based alloys can modify the band structure around the Fermi level and significantly affect electrical transport properties. Additionally, by doping Ta at Yb sites, the carrier concentration and lattice thermal conductivity of these alloys can be manipulated. Consequently, a peak zT value of 0.45 at 823 K was achieved for Yb0.95Ta0.05NiSb. Our work demonstrates that YbNiSb-based alloys are promising p-type thermoelectric materials and suggests the possibility of exploring novel thermoelectric alloys in rare-earth nickel pnictides via tuning their composition and crystal structure.

Probing Optoelectronic and Thermoelectric Properties of Lead-Free Perovskite SnTiO3: HSE06 and Boltzmann Transport Calculations

In order to develop a useful material for the optoelectronic sector with a variety of uses in thermoelectric and optical properties at a reasonable price, we researched SnTiO3, a Pb-free and Sn-based perovskite. We used the most recent density functional theory (DFT) methods, such as the gradient approximation (GGA) approach and the screened hybrid functional (HSE06). The calculated electronic structure yields to an indirect band gap of 2.204 eV along with two different K-points such as (X-Γ) using HSE06. The accomplished optical properties have been examined by dispersion, absorption, reflection, optical conductivity, and loss function against photon energy. The thermoelectric properties and electronic fitness function (EFF) were predicted DFT along with the Boltzmann transport theory. The Seebeck coefficient (S) and related thermoelectric properties such as electronic/thermal conductivity and the Hall coefficient were calculated as a function of chemical potential and carrier density (electrons and holes concentration) for room temperature. It was established that the temperature increases the Seebeck coefficient (S) at every hole carrier concentration. SnTiO3 has good EFF at 300, 500, and 800 K as well. The discovered EFF suggests that this material’s thermoelectric performance rises with temperature and can also be improved through doping. These findings demonstrated the potential of SnTiO3 as an n-type or p-type thermoelectric material depending on the doping.

Electrochemical Reduction and Preparation of Cu-Se Thermoelectric Thin Film in Solutions with PEG.

Investigation of Cu(II) and Se(IV) electrochemical reduction processes in solutions with poly(ethylene glycol) (PEG) provides important theoretical guidance for the preparation of Cu-Se alloy films with stronger thermoelectric properties. The results reveal that PEG adsorbing on the electrode surface does not affect the electrochemical reduction mechanism of Cu(II), Se(IV), and Cu(II)-Se(IV), but inhibits the electrochemical reduction rates. The surface morphology and composition change with a negative shift in the deposition potentials. The Cu-Se alloy film, which was prepared at 0.04 V, was α-Cu2Se as-deposited and P-type thermoelectric material after annealing. The highest thermoelectric properties were as follows: Seebeck coefficient (α) was +106 μV·K−1 and 1.89 times of Cu-Se alloy film electrodeposited in Cu(II)-Se(IV) binary solution without PEG; resistivity (ρ) was 2.12 × 10−3 Ω·cm, and the calculated power factor (PF) was 5.3 μW·cm−1K−2 and 4.07 times that without PEG.

Thermoelectric power factor of MnSb2X4 (X = S, Se) spinel chalcogenides – A DFT study

Recently, spinel chalcogenides have been found to be promising class of materials in thermoelectric power generation. In this line, the complete understanding about electronic and thermoelectric properties of MnSb2S4 and MnSb2Se4 is crucial. With the help of first-principles calculations, its interesting characteristics are explored, including bonding nature, ground state magnetic ordering, mechanical stability, etc. A systematic study on these materials revealed their antiferromagnetic and semiconducting behaviour. The Seebeck coefficient as a function of carrier concentration has its maximum at 600 and 681 μV K−1 (T = 300 K) for hole doped MnSb2S4 and MnSb2Se4 respectively. Besides the coupled behaviour of Seebeck coefficient and electrical conductivity, an optimal power factor of 1.82 and 0.96 mW m−1 K−2 is attained at 300 K for MnSb2S4 and MnSb2Se4 respectively. This study suggests that both the spinels can be potential p-type thermoelectric materials for near room temperature power generation applications and enables the path for their experimental investigations.

The Thermo-Mechanical Response of GeTe under Compression.

Thermoelectric generators (TEGs) are devices capable of transforming heat energy into electricity and vice versa. Although TEGs are known and have been in use for around five decades, they are implemented in only a limited range of applications, mainly extraterrestrial applications. This is due to their low technical readiness level (TRL) for widespread use, which is only at levels of 3–5 approaching laboratory prototypes. One of the most setbacks in reaching higher TRL is the lack of understanding of the mechanical and thermo-mechanical properties of TE materials. Out of ~105,000 entries about TE materials only ~100 entries deal with mechanical properties, while only 3 deal with thermo-mechanical properties. GeTe-based alloys with varying other elements, forming efficient p-type thermoelectric materials in the 200 ÷ 500 °C temperature range, have been intensively researched since the 1960s and have been successfully applied in practical TEGs. Yet, their temperature-dependent mechanical properties were never reported, preventing the fulfillment of their potential in a wide variety of practical applications. The combined effects of temperature and mechanical compression of GeTe were explored in the current research by implementing novel quantitative crystallographic methods to statistically describe dislocation activity and modification of the micro-texture as inflecting by the testing conditions. It is suggested, through utilizing these methods, that the combined effect of compression and temperature leads to the dissolving of twin boundaries, which increases dislocation mobility and results in a brittle-to-ductile transition at ~0.45 of the homologous temperature.

Enhancing thermoelectric and mechanical properties of p-type Cu3SbSe4-based materials via embedding nanoscale Sb2Se3

Cu3SbSe4, a promising p-type thermoelectric (TE) material consisting of earth-abundant, nontoxic, cost-efficient and eco-friendly constituent elements. However, its low TE performance is relatively poor because of the low carrier concentration and mobility. Herein, we synergistically optimize the electrical and thermal transport properties of Cu3SbSe4 via embedding nanoscale Sb2Se3. The Sb2Se3 nanophase markedly increases the carrier concentration and carrier effective mass simultaneously, resulting in a higher power factor of ∼1100 μW m−1 K−2. Surprisingly, Sb2Se3 nanophase induces high-density phase boundaries and dislocations, which significantly depress the lattice thermal conductivity. The multiple effects of incorporation of Sb2Se3 boost Cu3SbSe4 to an outstanding ZT value of ∼0.86 at 673 K for Cu3SbSe4 +1.5 mol% Sb2Se3 composite, which is ∼1.70 times as high as that of pristine Cu3SbSe4. It is highly remarkable that Cu3SbSe4 with nanoscale Sb2Se3 is mechanically robust, showing the Vickers hardness (Hv) and fracture toughness (KIC) of ∼2.54 GPa and ∼0.73 ± 0.02 MPa m½, which are ∼20% and ∼12% higher than those of pristine Cu3SbSe4, respectively. This work provides guidance for enhancing the comprehensive TE and mechanical properties of the copper-based chalcogenides by embedding nanophases.

Flexible thermoelectrics based on ductile semiconductors.

Flexible thermoelectrics provide a different solution for developing portable and sustainable flexible power supplies. The discovery of silver sulfide-based ductile semiconductors has driven a shift in the potential for flexible thermoelectrics, but the lack of good p-type ductile thermoelectric materials has restricted the reality of fabricating conventional cross-plane π-shaped flexible devices. We report a series of high-performance p-type ductile thermoelectric materials based on the composition-performance phase diagram in AgCu(Se,S,Te) pseudoternary solid solutions, with high figure-of-merit values (0.45 at 300 kelvin and 0.68 at 340 kelvin) compared with other flexible thermoelectric materials. We further demonstrate thin and flexible π-shaped devices with a maximum normalized power density that reaches 30 μW cm-2 K-2. This output is promising for the use of flexible thermoelectrics in wearable electronics.

Thermal transport and thermoelectric properties of alkali-metal telluride Na2Te from first-principles study

The excellent thermoelectric performance of alkali-metal telluride Na2Te has been systematically studied from the electronic and phonon transport properties within the density functional theory combined with the Boltzmann transport theory framework. The phonon dispersion without any imaginary mode demonstrates that the Na2Te compound is dynamically stable. The calculated lattice thermal conductivity is as small as 3.01 W m−1 K−1 at room temperature, suggesting that Na2Te can serve as a promising thermoelectric material. The related anharmonic effect parameters, phonon lifetime and Grüneisen parameter, are discussed to further explain the lattice thermal conductivity. Also, the electronic transport parameters, such as Seebeck coefficient S, electrical conductivity σ, and electronic thermal conductivity κe, are investigated to determine the thermoelectric performance by utilizing the semi-classical Boltzmann transport theory. The final figure of merit ZT can be evaluated based on the lattice thermal conductivity and the corresponding electronic transport coefficients. The maximum ZT of 1.53 can be achieved at a carrier concentration n = 3.08 × 1020 cm−3 for p-type Na2Te at T = 900 K, greatly larger than that of the n-type Na2Te (0.86), suggesting that the Na2Te can be a potential p-type thermoelectric material.

Regulating thermoelectric properties of Eu0.5Ca0.5Zn2Sb2 through Mg dopant

CaAl2Si2 structure Zintl compounds are promising thermoelectric materials and have been extensively studied recently. In this study, Eu0.5Ca0.5Zn2-xMgxSb2 (0 ≤ x ≤ 0.3) compounds have been prepared by ball milling and spark plasma sintering. Results show that for Eu0.5Ca0.5Zn2Sb2, ZT has been improved about 20% compared with previously reported thermoelectric properties through the ball milling and hot-pressing preparation process. Moreover, the thermoelectric performance of Eu0.5Ca0.5Zn2Sb2 can be further increased through Mg substitution. Mg replacing Zn has a strong impact on phonon scattering, resulting in low thermal conductivity. A highest ZT about 1.05 is obtained in the Eu0.5Ca0.5Zn1.8Mg0.2Sb2 sample at 773 K. The obtained high thermoelectric performance in our work is comparable with other p-type Zintl thermoelectric materials, and this work provides a reference for the improvement of properties of other CaAl2Si2 structure Zintl thermoelectric materials.

Optimizing the thermoelectric transmission of monolayer HfSe2 by strain engineering

The demand for clean energy is driving enhancements in the efficiency of conversion of thermoelectric materials. The electronic and phononic properties of thermoelectric materials can be adjusted through straining to improve their figure of merit. In this work, the authors systematically examine the effects of biaxial strain on the phononic, electronic, and thermoelectric properties of monolayer 1T-HfSe2 by using first-principles calculations combined with semi-classical Boltzmann transport theory. The results show that the monolayer HfSe2 is a good n-type thermoelectric material. The tensile strain caused the valence band to converge and increased the Seebeck coefficient to induce the transition of HfSe2 from n-type to p-type thermoelectric material. The thermal conductivity κl of the lattice decreased from 5.54 Wm-1κ−1 to 1.16 Wm-1κ−1 at 300 K owing to stronger phonon scattering and lower velocity of the phonon group as the tensile strain increased the bond distance and weakened the bonds. Due to improvements in electrical and thermal transport, ZTmax significantly increased from 0.16 to 2.41 for the p-type material, and from 0.53 to 1.91 for the n-type material. This shows that strain engineering can improve electrical transport and reduce thermal conductivity to improve thermoelectric performance.

Mixed Anion Semiconductor In8S2.82Te6.18(Te2)3.

The new heteroanionic compound In8S2.82Te6.18(Te2)3 crystallizes in the monoclinic space group C2/c with lattice parameters a = 14.2940(6) Å, b = 14.3092(4) Å, c = 14.1552(6) Å, and β = 90.845(3)°. The three-dimensional (3D) framework of In8S2.82Te6.18(Te2)3 is composed of a complex 3D network of corner-connected InQ4 tetrahedra with three Te22- dumbbell dimers per formula unit. The optical bandgap is 1.12(2) eV and the work function is 5.15(5) eV. First-principles electronic structure calculations using density functional theory (DFT) indicate that this material has potential as a p-type thermoelectric material as it is a narrow bandgap semiconductor, incorporates several heavy elements, and has multiple overlapping bands near the valence band maximum.

Improving electrical and thermal properties synchronously via introducing CsPbBr3 QDs into higher manganese silicides

Higher manganese silicide (HMS) is a P-type medium temperature thermoelectric (TE) material, which has attracted widespread attention over the past few decades due to its remarkable mechanical properties, excellent chemical and thermal stability, as well as the non-toxicity, abundance and competitive price. The peak power factor (PF) of HMS is as high as ∼1.50 × 10−3 W m−1 K−2 because of its intrinsic high electrical conductivity and Seebeck coefficient. However, the thermal conductivity of HMS is also high, resulting in relatively low zT values. Introducing nano-dispersion in the matrix is one of the most effective methods to enhance the TE properties via reducing the lattice thermal conductivity significantly without drastic changes on the other parameters. In this study, CsPbBr3 QDs with uniform size were synthesized and introduced into HMS bulks. The PF (at 823 K) was enhanced to 1.71 × 10−3 W m−1 K−2, which is improved 14.0% approximately compared with that of pure HMS owing to the combined effect of element doping and energy filtering. The lattice thermal conductivity (at 823 K) decreased from 2.56 W m−1 K−1 to 1.99 W m−1 K−1 synchronously (∼22.0%) due to the intensive phonon scattering caused by Cs doping, and the embedding of Pb riched CsPbBr3 QDs and Pb QDs. A maximum zT value of 0.57 (823 K) is achieved in CsPbBr3 QDs/HMS composites, which is 36.0% higher than that of pure HMS. Predictably, for other TE materials, it is also feasible to improve the TE properties via introducing metastable quantum dots.

High-performance lead-free cubic GeTe-based thermoelectric alloy

The ferroelectric phase transition of GeTe thermoelectric materials caused by the 4s2 lone-pair electrons of Ge2+ leads to the sudden change of thermal expansion coefficient, which severely restrains its practical applications. Herein, we demonstrate a successful way to suppress the ferroelectric phase-transition temperature down to below room temperature in GeTe. Our approach involves LiSbTe2 alloying, where the charge distribution is manipulated to reduce the influence of the stereochemical activity of the cation lone-pair electrons on the crystal structure. LiSbTe2 alloying can also promote multi-band convergence and enhance phonon scattering. Consequently, a superior ZTave of 1.38 within 300—773 K is achieved in cubic (GeTe)0.70(LiSbTe2)0.30 alloys, which is significantly higher than the reported results for room temperature or near-room temperature cubic GeTe systems so far. Moreover, the calculated conversion efficiency of 17% is also the highest among all nontoxic p-type polycrystalline thermoelectric materials. This work takes a solid step forward in the application of GeTe.

Thermoelectric and magnetic properties of (Fe,Co)2TiSn Heusler compounds

The substitution of Co for Fe in the Fe2TiSn Heusler compound resulted in a change in the magnetic properties. Although the Fe2TiSn and Fe2-xCoTiSn (x = 0.5–1.0) Heusler compounds were paramagnetic, the Fe2-xCoTiSn (x = 1.5–2.0) Heusler compounds were found to be ferromagnetic. The Fe2TiSn Heusler compound is a p-type thermoelectric material that has a positive Seebeck coefficient of 30 μV/K at room temperature. On the other hand, Co-substituted (Fe,Co)2TiSn Heusler compounds prepared in the present study were shown to be n-type thermoelectric materials, regardless of the Co content. The (Fe,Co)2TiSn Heusler compounds exhibited relatively low electrical resistivity values, ranging from 1.71 μΩm for the Co2TiSn compound to 11.5 μΩm for the Fe1.5Co0.5TiSn compound. It was found that the power factor of the Co2TiSn Heusler compound (647 μW/mK2 at room temperature) was more than double that of the Fe2TiSn Heusler compound.

Monolayer SnI2: An Excellent p-Type Thermoelectric Material with Ultralow Lattice Thermal Conductivity.

Using density functional theory and semiclassical Boltzmann transport equation, the lattice thermal conductivity and electronic transport performance of monolayer SnI2 were systematically investigated. The results show that its room temperature lattice thermal conductivities along the zigzag and armchair directions are as low as 0.33 and 0.19 W/mK, respectively. This is attributed to the strong anharmonicity, softened acoustic modes, and weak bonding interactions. Such values of the lattice thermal conductivity are lower than those of other famous two-dimensional thermoelectric materials such as MoO3, SnSe, and KAgSe. The two quasi-degenerate band valleys for the valence band maximum make it a p-type thermoelectric material. Due to its ultralow lattice thermal conductivities, coupled with an ultrahigh Seebeck coefficient, monolayer SnI2 possesses an ultrahigh figure of merits at 800 K, approaching 4.01 and 3.34 along the armchair and zigzag directions, respectively. The results indicate that monolayer SnI2 is a promising low-dimensional thermoelectric system, and would stimulate further theoretical and experimental investigations of metal halides as thermoelectric materials.

Carrier transport model and novel design for micro thermoelectric generator with enhanced performance

The carrier transport processes in thermoelectric materials are significantly correlated with the performance of thermoelectric devices (TEs), which is modeled in this study considering the drift-diffusion equation with thermoelectric contributions for detailing the effects of carrier transport on the yielded current density. A micro thermoelectric generator (μTEG) with boron and arsenic being respectively doped into Si0.7Ge0.3 for p-type and n-type thermoelectric materials is employed as the study case by the three-dimensional numerical simulation to reveal the effects of doping concentrations on the thermoelectric performance. Based on the simulation results a novel μTEG design that incorporates PN junctions to extract minority carriers to reduce the recombination rates of electrons and holes is proposed. The performances for conventional and the present novel μTEGs at different temperature differences are compared. The results show that there is an optimal doping concentration for the conventional μTEG to peak the output power and the thermoelectric conversion efficiency. In particular, the novel μTEG reveals a significant improvement of thermoelectric efficiency by approximately 20% as the hot-end temperature is above 900 K. The transport of minority carriers by the PN junctions demonstrates the expected effects through the analysis on the vector distribution of minority carrier current density.