Functional Graded Germanium–Lead Chalcogenide‐Based Thermoelectric Module for Renewable Energy Applications

High thermoelectric conversion efficiencies can be achieved by making use of materials with, as high as possible, figure of merit, ZT, values. Moreover, even higher performance is possible with appropriate geometrical optimization including the use of functionally graded materials (FGM) technology. Here, an advanced n‐type functionally graded thermoelectric material based on a phase‐separated (PbSn_0.05Te)_0.92(PbS)_0.08 matrix is reported. For assessment of the thermoelectric potential of this material, combined with the previously reported p‐type Ge_0.87Pb_0.13Te showing a remarkable dimensionless figure of merit of 2.2, a finite‐element thermoelectric model is developed. The results predict, for the investigated thermoelectric couple, a very impressive thermoelectric efficiency of 14%, which is more than 20% higher than previously reported values for operating under cold and hot junction temperatures of 50 °C and 500 °C, respectively. Validation of the model prediction is done by a thermoelectric couple fabricated according to the model's geometrical optimization conditions, showing a good agreement to the theoretically calculated results, hence approaching a higher technology readiness level.     

Increase in the Figure of Merit by Cd‐Substitution in Sn1–xPbxTe and Effect of Pb/Sn Ratio on Thermoelectric Properties

The effects of Cd‐doping on the thermoelectric properties of Sn_1–xPb_xTe are investigated and compared to the properties of the corresponding Sn_1–xPb_xTe solid solutions. The addition of Cd results in a reduction in the carrier concentration and changes in the physical properties, as well as in the conduction type of Sn_1–xPb_xTe. A significant increase in the power factor accompanied by a reduction in the thermal conductivity result in a higher figure of merit (ZT) for (Sn_1–xPb_x)_0.97Cd_0.03Te than that of undoped Sn_1–xPb_xTe. The maximum ZT (～0.7) values are observed for p‐type material with x = 0.36 at 560 K. Much higher values (ZT ～ 1.2 at 560 K for x = 0.73) are obtained on n‐type samples.     

Enhancement of Thermoelectric Performance of n‐Type PbSe by Cr Doping with Optimized Carrier Concentration

Ti, V, Cr, Nb, and Mo are found to be effective at increasing the Seebeck coefficient and power factor of n‐type PbSe at temperatures below 600 K. It is found that the higher Seebeck coefficients and power factors are due to higher Hall mobility ≈1000 cm² V^?1s^?1 at lower carrier concentration. A larger average ZT value (relevant for applications) can be obtained by an optimization of carrier concentration to ≈10¹⁸–10¹⁹ cm^?3. Even though the highest room temperature power factor ≈3.3 × 10^?3 W m^?1 K^?2 is found in 1 at% Mo‐doped PbSe, the highest ZT is achieved in Cr‐doped PbSe. Combined with the lower thermal conductivity, ZT is improved to ≈0.4 at room temperature and peak ZTs of ≈1.0 are observed at ≈573 K for Pb_0.9925Cr_0.0075Se and ≈673 K for Pb_0.995Cr_0.005Se. The calculated device efficiency of Pb_0.995Cr_0.005Se is as high as ≈12.5% with cold side 300 K and hot side 873 K, higher than those of all the n‐type PbSe materials reported in the literature.     

Skutterudite Unicouple Characterization for Energy Harvesting Applications

Skutterudites are promising thermoelectric materials because of their high figure of merit, ZT, and good thermomechanical properties. This work reports the effective figure of merit, ZT_eff, and the efficiency of skutterudite legs and a unicouple working under a large temperature difference. The p‐ and n‐type legs are fabricated with electrodes sintered directly to the skutterudite during a hot pressing process. CoSi₂ is used as the electrode for the n‐type skutterudite (Yb_0.35Co₄Sb₁₂) and Co₂Si for the p‐type skutterudite (NdFe_3.5Co_0.5Sb₁₂). A technique is developed to measure the ZT_eff of individual legs and the efficiency of a unicouple. An ZT_eff of 0.74 is determined for the n‐type legs operating between 52 and 595 °C, and an ZT_eff of 0.51 for the p‐type legs operating between 77 and 600 °C. The efficiency of the p–n unicouple is determined to be 9.1% operating between ～70 and 550 °C.     

Subtle Roles of Sb and S in Regulating the Thermoelectric Properties of N‐Type PbTe to High Performance

A high ZT (thermoelectric figure of merit) of ≈1.4 at 900 K for n‐type PbTe is reported, through modifying its electrical and thermal properties by incorporating Sb and S, respectively. Sb is confirmed to be an amphoteric dopant in PbTe, filling Te vacancies at low doping levels (<1%), exceeding which it enters into Pb sites. It is found that Sb‐doped PbTe exhibits much higher carrier mobility than similar Bi‐doped materials, and accordingly, delivers higher power factors and superior ZT . The enhanced electronic transport is attributed to the elimination of Te vacancies, which appear to strongly scatter n‐type charge carriers. Building on this result, the ZT of Pb_0.9875Sb_0.0125Te is further enhanced by alloying S into the Te sublattice. The introduction of S opens the bandgap of PbTe, which suppresses bipolar conduction while simultaneously increasing the electron concentration and electrical conductivity. Furthermore, it introduces point defects and induces second phase nanostructuring, which lowers the lattice thermal conductivity to ≈0.5 W m^?1 K^?1 at 900 K, making this material a robust candidate for high‐temperature (500–900 K) thermoelectric applications. It is anticipated that the insights provided here will be an important addition to the growing arsenal of strategies for optimizing the performance of thermoelectric materials.     

Enhancement of Thermoelectric Figure of Merit by the Insertion of MgTe Nanostructures in p‐type PbTe Doped with Na2Te

The thermoelectric properties of crystalline melt‐grown ingots of p‐type PbTe–xMgTe (x = 1–3 mol%) doped with Na₂Te (1–2 mol%) were investigated over the temperature range of 300 K to 810 K. While the powder X‐ray diffraction patterns show that all samples crystallize in the NaCl‐type structure with no MgTe or other phases present, transmission electron microscopy reveals ubiquitous MgTe nanoprecipitates in the PbTe. The very small amounts of MgTe in PbTe have only a small effect on the electrical transport properties of the system, while they have a large effect on thermal transport significantly reducing the lattice thermal conductivity. A ZT of 1.6 at 780 K is achieved for the PbTe containing 2% MgTe doped with 2% Na₂Te.     

Enhancement in Thermoelectric Figure‐Of‐Merit of an N‐Type Half‐Heusler Compound by the Nanocomposite Approach

An enhancement in the dimensionless thermoelectric figure‐of‐merit (ZT) of an n‐type half‐Heusler material is reported using a nanocomposite approach. A peak ZT value of 1.0 was achieved at 600 °C–700 °C, which is about 25% higher than the previously reported highest value. The samples were made by ball‐milling ingots of composition Hf_0.75Zr_0.25NiSn_0.99Sb_0.01 into nanopowders and hot‐pressing the powders into dense bulk samples. The ingots were formed by arc‐melting the elements. The ZT enhancement mainly comes from reduction of thermal conductivity due to increased phonon scattering at grain boundaries and crystal defects, and optimization of antimony doping.     

Weak Electron Phonon Coupling and Deep Level Impurity for High Thermoelectric Performance Pb1−xGaxTe

High ZT of 1.34 at 766 K and a record high average ZT above 1 in the temperature range of 300‐864 K are attained in n‐type PbTe by engineering the temperature‐dependent carrier concentration and weakening electron–phonon coupling upon Ga doping. The experimental studies and first principles band structure calculations show that doping with Ga introduces a shallow level impurity contributing extrinsic carriers and imparts a deeper impurity level that ionizes at higher temperatures. This adjusts the carrier concentration closer to the temperature‐dependent optimum and thus maximizes the power factor in a wide temperature range. The maximum power factor of 35 µW cm⁻¹ K⁻² is achieved for the Pb_0.98Ga_0.02Te compound, and is maintained over 20 µWcm⁻¹ K⁻² from 300 to 767 K. Band structure calculations and X‐ray photoelectron spectroscopy corroborate the amphoteric role of Ga in PbTe as the origin of shallow and deep levels. Additionally, Ga doping weakens the electron–phonon coupling, leading to high carrier mobilities in excess of 1200 cm² V⁻¹ s⁻¹. Enhanced point defect phonon scattering yields a reduced lattice thermal conductivity. This work provides a new avenue, beyond the conventional shallow level doping, for further improving the average ZT in thermoelectric materials.     

Effect of Hf Concentration on Thermoelectric Properties of Nanostructured N‐Type Half‐Heusler Materials HfxZr1–xNiSn0.99Sb0.01

Half‐Heusler n‐type thermoelectric materials MNiSn (M = Hf, Zr) have been shown to exhibit peak thermoelectric dimensionless figure‐of‐merit (ZT) of ～1.0 at 600–700 °C with a composition of Hf_0.75Zr_0.25NiSn_0.99Sb_0.01. This work demonstrates that it is possible to achieve the same ZT by reducing the concentration of the most expensive component Hf to one third of the previously reported best composition, i.e., Hf_0.25Zr_0.75NiSn_0.99Sb_0.01, which corresponds to an overall 50% reduction on material cost. The samples are prepared by ball milling the arc melted ingot and hot pressing the finely ground powders. The reduction of Hf concentration is crucial for such materials to be used in large‐scale waste heat recovery.     

Thermoelectric Property Studies on Cu‐Doped n‐type CuxBi2Te2.7Se0.3 Nanocomposites

Combining high energy ball‐milling and hot‐pressing, significant enhancements of the thermoelectric figure‐of‐merit (ZT) have been reported for p‐type Bi_0.4Sb_1.6Te₃ nanocomposites. However, applying the same technique to n‐type Bi₂Te_2.7Se_0.3 showed no improvement on ZT values, due to the anisotropic nature of the thermoelectric properties of n‐type Bi₂Te_2.7Se_0.3. Even though texturing was effective in improving peak ZT of Bi₂Te_2.7Se_0.3 from 0.85 to 1.04, reproducibility from batch to batch remains unsatisfactory. Here, we show that good reproducibility can be achieved by introducing an optimal concentration of 0.01 copper (Cu) per Bi₂Te_2.7Se_0.3 to make Cu_0.01Bi₂Te_2.7Se_0.3 samples. A peak ZT value of 0.99 was achieved in Cu_0.01Bi₂Te_2.7Se_0.3 samples without texturing. With texturing by re‐pressing, the peak ZT was increased to 1.06. Aging in air for over 5 months did not deteriorate but further improved the peak ZT to 1.10. The mechanism by which copper improves the reproducibility, enhances the carrier mobility, and reduces the lattice thermal conductivity is also discussed.     

High Thermoelectric Performance in PbSe–NaSbSe2 Alloys from Valence Band Convergence and Low Thermal Conductivity

PbSe is an attractive thermoelectric material due to its favorable electronic structure, high melting point, and lower cost compared to PbTe. Herein, the hitherto unexplored alloys of PbSe with NaSbSe₂ (NaPb_mSbSe_m+2) are described and the most promising p‐type PbSe‐based thermoelectrics are found among them. Surprisingly, it is observed that below 500 K, NaPb_mSbSe_m+2 exhibits unorthodox semiconducting‐like electrical conductivity, despite possessing degenerate carrier densities of ≈10²⁰ cm^?3. It is shown that the peculiar behavior derives from carrier scattering by the grain boundaries. It is further demonstrated that the high solubility of NaSbSe₂ in PbSe augments both the thermoelectric properties while maintaining a rock salt structure. Namely, density functional theory calculations and photoemission spectroscopy demonstrate that introduction of NaSbSe₂ lowers the energy separation between the L‐ and Σ‐valence bands and enhances the power factors under 700 K. The crystallographic disorder of Na⁺, Pb²⁺, and Sb³⁺ moreover provides exceptionally strong point defect phonon scattering yielding low lattice thermal conductivities of 1–0.55 W m^‐1 K^‐1 between 400 and 873 K without nanostructures. As a consequence, NaPb₁₀SbSe₁₂ achieves maximum ZT ≈1.4 near 900 K when optimally doped. More importantly, NaPb₁₀SbSe₁₂ maintains high ZT across a broad temperature range, giving an estimated record ZT_avg of ≈0.64 between 400 and 873 K, a significant improvement over existing p‐type PbSe thermoelectrics.     

Synergistical Enhancement of Thermoelectric Properties in n‐Type Bi2O2Se by Carrier Engineering and Hierarchical Microstructure

Oxygen‐containing compounds are promising thermoelectric (TE) materials for their chemical and thermal stability. As compared with the high‐performance p‐type counterparts (e.g., ZT ≈1.5 for BiCuSeO), the enhancement of the TE performance of n‐type oxygen‐containing materials remains challenging due to their mediocre electrical conductivity and high thermal conductivity. Here, n‐type layered Bi₂O₂Se is reported as a potential TE material, of which the thermal conductivity and electrical transport properties can be effectively tuned via carrier engineering and hierarchical microstructure. By selective modification of insulating [Bi₂O₂]²⁺ layers with Ta dopant, carrier concentration can be increased by four orders of magnitude (from 10¹⁵ to 10¹⁹ cm^?3) while relatively high carrier mobility can be maintained, thus greatly enhancing the power factors (≈451.5 µW K^?2 m^?1). Meanwhile, the hierarchical microstructure can be induced by Ta doping, and the phonon scattering can be strengthened by atomic point defects, nanodots of 5–10 nm and grains of sub‐micrometer level, which progressively suppresses the lattice thermal conductivity. Accordingly, the ZT value of Bi_1.90Ta_0.10O₂Se reaches 0.36 at 773 K, a ≈350% improvement in comparison with that of the pristine Bi₂O₂Se. The average ZT value of 0.30 from 500 to 823 K is outstanding among n‐type oxygen‐containing TE materials. This work provides a desirable way for enhancing the ZT values in oxygen‐containing compounds.     

High Thermoelectric Efficiency of n‐type PbS

PbS shares several features with the other lead chalcogenides PbX (X: Te, Se), which are good thermoelectric materials. PbS has a potential advantage in that it is quite earth abundant and inexpensive. In this work we tune the transport properties in n‐type, single‐phase polycrystalline PbS_1‐xCl_x (x ≤ 0.008) with different carrier densities. Lead chloride provides a nearly 100% efficient doping control up to 1.2 × 10²⁰ cm^?3. The maximum zT achieved at 850 K is 0.7 with a predicted zT ～ 1 at 1000 K. This is about twice as high as what was previously reported (～0.4) for binary PbS. Compared with the other lead chalcogenides the higher effective mass and higher lattice thermal conductivity makes binary PbS an inferior thermoelectric material. However this study also predicts greater potential of zT improvement in PbS by material engineering such as alloying or nanostructuring compared to PbSe or PbTe. Considering their abundance and low cost, PbS based materials are quite competitive among the lead chalcogenides for thermoelectric applications.     

Three‐Stage Inter‐Orthorhombic Evolution and High Thermoelectric Performance in Ag‐Doped Nanolaminar SnSe Polycrystals

The ultrahigh thermoelectric performance of SnSe‐based single crystals has attracted considerable interest in their polycrystalline counterparts. However, the temperature‐dependent structural transition in SnSe‐based thermoelectric materials and its relationship with their thermoelectric performance are not fully investigated and understood. In this work, nanolaminar SnSe polycrystals are prepared and characterized in situ using neutron and synchrotron powder diffraction measurements at various temperatures. Rietveld refinement results indicate that there is a complete inter‐orthorhombic evolution from Pnma to Cmcm by a series of layer slips and stretches along the a‐ and b‐axes over a 200 K temperature range. This phase transition leads to drastic enhancement of the carrier concentration and phonon scattering above 600 K. Moreover, the unique nanolaminar structure effectively enhances the carrier mobility of SnSe. Their grain and layer boundaries further improve the phonon scattering. These favorable factors result in a high ZT of 1.0 at 773 K for pristine SnSe polycrystals. The thermoelectric performances of polycrystalline SnSe are further improved by p‐type and n‐type dopants (i.e., doped with Ag and SnCl₂, respectively), and new records of ZT are achieved in Ag_0.015Sn_0.985Se (ZT of 1.3 at 773 K) and SnSe_0.985Cl_0.015 (ZT of 1.1 at 773 K) polycrystals.     

Amphoteric Indium Enables Carrier Engineering to Enhance the Power Factor and Thermoelectric Performance in n‐Type AgnPb100InnTe100+2n (LIST)

The Ag and In co‐doped PbTe, Ag_nPb₁₀₀In_nTe_100+2n (LIST), exhibits n‐type behavior and features unique inherent electronic levels that induce self‐tuning carrier density. Results show that In is amphoteric in the LIST, forming both In³⁺ and In¹⁺ centers. Through unique interplay of valence fluctuations in the In centers and conduction band filling, the electron carrier density can be increased from ≈3.1 × 10¹⁸ cm^?3 at 323 K to ≈2.4 × 10¹⁹ cm^?3 at 820 K, leading to large power factors peaking at ≈16.0 µWcm^?1 K^?2 at 873 K. The lone pair of electrons from In⁺ can be thermally continuously promoted into the conduction band forming In³⁺, consistent with the amphoteric character of In. Moreover, with rising temperature, the Fermi level shifts into the conduction band, which enlarges the optical band gap based on the Moss–Burstein effect, and reduces bipolar diffusion and thermal conductivity. Adding extra Ag in LIST improves the electrical transport properties and meanwhile lowers the lattice thermal conductivity to ≈0.40 Wm^?1 K^?1. The addition of Ag creates spindle‐shaped Ag₂Te nanoprecipitates and atomic‐scale interstitials that scatter a broader set of phonons. As a result, a maximum ZT value ≈1.5 at 873 K is achieved in Ag₆Pb₁₀₀InTe₁₀₂ (LIST).     

Coordination Engineering of Single‐Crystal Precursor for Phase Control in Ruddlesden–Popper Perovskite Solar Cells

2D Ruddlesden–Popper perovskites (RPPs) have recently drawn significant attention because of their structural variability that can be used to tailor optoelectronic properties and improve the stability of derived photovoltaic devices. However, charge separation and transport in 2D perovskite solar cells (PSCs) suffer from quantum well barriers formed during the processing of perovskites. It is extremely difficult to manage phase distributions in 2D perovskites made from the stoichiometric mixtures of precursor solutions. Herein, a generally applicable guideline is demonstrated for precisely controlling phase purity and arrangement in RPP films. By visually presenting the critical colloidal formation of the single‐crystal precursor solution, coordination engineering is conducted with a rationally selected cosolvent to tune the colloidal properties. In nonpolar cosolvent media, the derived colloidal template enables RPP crystals to preferentially grow along the vertically ordered alignment with a narrow phase variation around a target value, resulting in efficient charge transport and extraction. As a result, a record‐high power conversion efficiency (PCE) of 14.68% is demonstrated for a (TEA)₂(MA)₂Pb₃I₁₀ (n = 3) photovoltaic device with negligible hysteresis. Remarkably, superior stability is achieved with 93% retainment of the initial efficiency after 500 h of unencapsulated operation in ambient air conditions.     

A Dual Role by Incorporation of Magnesium in YbZn2Sb2 Zintl Phase for Enhanced Thermoelectric Performance

1‐2‐2‐type Zintl phase compounds have promising thermoelectric properties because of their complex crystal structures and multiple valence‐band structures. In this work, a series of single phase (Yb_0.9Mg_0.1)Mg_xZn_2?xSb₂ (x = 0, 0.2, 0.4, 0.6, 0.8, and 1) compounds are prepared by alloying YbZn₂Sb₂ with 10 at% MgZn₂Sb₂ and different amounts of YbMg₂Sb₂. The incorporation of Mg at the Yb site, as well as at the Zn site, not only leads to an effective orbital alignment confirmed by the dramatically enhanced density of states effective mass and Seebeck coefficients, but also increases the point defect scattering, contributing to a low lattice thermal conductivity ≈0.54 W m^?1 K^?1 at 773 K. Combined with the optimization of the carrier concentration by Ag doping at the Zn site, a highest ZT value ≈1.5 at 773 K is achieved in (Yb_0.9Mg_0.1)Mg_0.8Zn_1.2Ag_0.002Sb₂, which is higher than that of all the previously reported 1‐2‐2‐type Zintl phase compounds.     

n‐Type SnSe2 Oriented‐Nanoplate‐Based Pellets for High Thermoelectric Performance

It is reported that electron doped n‐type SnSe₂ nanoplates show promising thermoelectric performance at medium temperatures. After simultaneous introduction of Se deficiency and Cl doping, the Fermi level of SnSe₂ shifts toward the conduction band, resulting in two orders of magnitude increase in carrier concentration and a transition to degenerate transport behavior. In addition, all‐scale hierarchical phonon scattering centers, such as point defects, nanograin boundaries, stacking faults, and the layered nanostructures, cooperate to produce very low lattice thermal conductivity. As a result, an enhanced in‐plane thermoelectric figure of merit ZT_max of 0.63 is achieved for a 1.5 at% Cl doped SnSe_1.95 pellet at 673 K, which is much higher than the corresponding in‐plane ZT of pure SnSe₂ (0.08).     

Toward High‐Thermoelectric‐Performance Large‐Size Nanostructured BiSbTe Alloys via Optimization of Sintering‐Temperature Distribution

High thermoelectric performance of mechanically robust p‐type Bi₂Te₃‐based materials prepared by melt spinning (MS) combined with plasma‐activated sintering (PAS) method can be obtained with small, laboratory grown samples. However, large‐size samples are required for commercial applications. Here, large‐size p‐type Bi₂Te₃‐based ingots with 30, 40, and 60 mm in diameter are produced by MS‐PAS, and the influence of temperature distribution during the sintering process on the composition and thermoelectric properties is systematically studied for the first time. Room‐temperature scanning Seebeck Microprobe results show that the large‐size ingot is inhomogeneous, induced by ellipsoidal‐shape‐distributed temperature field during the sintering process, which is verified by finite‐element analysis. Although some temperature differences are unavoidable in the sintering process, homogeneity and mechanical properties of ingots can be improved by appropriately extending the sintering time and design of graphite die. Samples cut from ingots attain the peak ZT value of 1.15 at 373 K, about 17% enhancement over commercial zone‐melted samples. Moreover, the compressive and bending strengths are improved by several times as well. It is important to ascertain that large‐size p‐type Bi₂Te₃‐based thermoelectric materials with high thermoelectric performance can be fabricated by MS‐PAS.     

Enantiomeric separation of type I and type II pyrethroid insecticides with different chiral stationary phases by reversed‐phase high‐performance liquid chromatography

The enantiomeric separation of type I (bifenthrin, BF) and type II (lambda‐cyhalothrin, LCT) pyrethroid insecticides on Lux Cellulose‐1, Lux Cellulose‐3, and Chiralpak IC chiral columns was investigated by reversed‐phase high‐performance liquid chromatography. Methanol/water or acetonitrile/water was used as mobile phase at a flow rate of 0.8 mL/min. The effects of chiral stationary phase, mobile phase composition, column temperature, and thermodynamic parameters on enantiomer separation were carefully studied. Bifenthrin got a partial separation on Lux Cellulose‐1 column and baseline separation on Lux Cellulose‐3 column, while LCT enantiomers could be completely separated on both Lux Cellulose‐1 and Lux Cellulose‐3 columns. Chiralpak IC provided no separation ability for both BF and LCT. Retention factor (k) and selectivity factor (α) decreased with the column temperature increasing from 10°C to 40°C for both BF and LCT enantiomers. Thermodynamic parameters including ?H and ?S were also calculated, and the maximum R_s were not always obtained at lowest temperature. Furthermore, the quantitative analysis methods for BF and LCT enantiomers in soil and water were also established. Such results provide a new approach for pyrethroid separation under reversed‐phase condition and contribute to environmental risk assessment of pyrethroids at enantiomer level.