Zn-Induced Defect Complexity for the High Thermoelectric Performance of n-Type PbTe Compounds

High Carrier Mobility and Promising Thermoelectric Module Performance of N-Type PbSe Crystals

The scarcity of Te hampers the widespread use of Bi2Te3-based thermoelectric modules. Here, the thermoelectric module potential of PbSe is investigated by improving its carrier mobility. Initially, large PbSe crystals are grown with the temperature gradient method to mitigate grain boundary effects on carrier transport. Subsequently, light doping with <1mole‰ halogens (Cl/Br/I) increases room-temperature carrier mobility to ~1600 cm2 V-1 s-1, achieved by reducing carrier concentration compared to traditional heavy doping. Crystal growth design and light doping enhance carrier mobility without affecting effective mass, resulting in a high power factor ~40 µW cm-1 K-2 in PbSe-Cl/Br/I crystals at 300 K. Additionally, Cl/Br/I doping reduces thermal conductivity and bipolar diffusion, leading to significantly lower thermal conductivity at high temperature. Enhanced carrier mobility and suppressed bipolar effect boost ZT values across the entire temperature range in n-type PbSe-Cl/Br/I crystals. Specifically, ZT values of PbSe-Br crystal reach ~0.6 at 300 K, ~1.2 at 773 K, and the average ZT (ZTave) reaches ~1.0 at 300-773 K. Ultimately, ~5.8% power generation efficiency in a PbSe single leg with a maximum temperature cooling difference of 40 K with 7-pair modules is achieved. These results indicate the potential for cost-effective and high-performance thermoelectric cooling modules based on PbSe.

High Ductility and Excellent Thermoelectric Performance in Te-Stabilized Cubic Ag2TexS1-x Solid Solutions

The stabilization at low temperatures of the Ag2S cubic phase could afford the design of high-performance thermoelectric materials with excellent mechanical behavior, enabling them to withstand prolonged vibrations and thermal stress. In this work, we show that the Ag2TexS1-x solid solutions, with Te content within the optimal range 0.20 ≤ x ≤ 0.30, maintain a stable cubic phase across a wide temperature range from 300 to 773 K, thus avoiding the detrimental phase transition from monoclinic to cubic phase observed in Ag2S. Notably, the Ag2TexS1-x (0.20 ≤ x ≤ 0.30) samples showed no fractures during bending tests and displayed superior ductility at room temperature compared to Ag2S, which fractured at a strain of 6.6%. Specifically, the Ag2Te0.20S0.80 sample demonstrated a bending average yield strength of 46.52 MPa at 673 K, significantly higher than that of Ag2S, whose bending average yield strength dropped from 80.15 MPa at 300 K to 12.66 MPa at 673 K. Furthermore, the thermoelectric performance of the Ag2TexS1-x (0.20 ≤ x ≤ 0.30) samples surpassed that of both InSe and pure Ag2S, with the Ag2Te0.30S0.70 sample achieving the highest ZT value of 0.59 at 723 K. These results indicate substantial potential for practical applications due to enhanced durability and thermoelectric performance.

Interstitials in Thermoelectrics

Defect structure is pivotal in advancing thermoelectric performance with interstitials being widely recognized for their remarkable roles in optimizing both phonon and electron transport properties. Diverse interstitial atoms are identified in previous works according to their distinct roles and can be classified into rattling interstitial, decoupling interstitial, interlayer interstitial, dynamic interstitial, and liquid interstitial. Specifically, rattling interstitial can cause phonon resonance in cage compound to scatter phonon transport; decoupling interstitial can contribute to phonon blocking and electron transport due to their significantly different mean free paths; interlayer interstitial can facilitate out-of-layer electron transport in layered compounds; dynamic interstitial can tune temperature-dependent carrier density and optimize electrical transport properties at wide temperatures; liquid interstitial could improve the carrier mobility at homogeneous dispersion state. All of these interstitials have positive impact on thermoelectric performance by adjusting transport parameters. This perspective therefore intends to provide a thorough overview of advances in interstitial strategy and highlight their significance for optimizing thermoelectric parameters. Finally, the profound potential for extending interstitial strategy to various other thermoelectric systems is discussed and some future directions in thermoelectric material are also outlined.

A Recipe for a Great Meal: A Benchtop Route from Elemental Se to Superior Thermoelectric β-Ag2Se

The low-temperature modification of β-Ag2Se has proven to be useful as a near-room-temperature thermoelectric material. Over the past years, research has been devoted to interstitial, vacancy, and substitutional doping into the parent β-Ag2Se structure, aiming at tuning the material's charge and heat transport properties to enhance thermoelectric performance. The transformation of β-Ag2Se into α-Ag2Se at ∼134 °C and the low solubility of dopants are the main obstacles for the doping approach. Herein, we report a facile, safe, scalable, and cost-effective benchtop approach to successfully produce metal-doped β-Ag2Se. The doped materials display a remarkable enhancement of thermoelectric performance with a record-high peak zT of 1.30 at 120 °C and an average zT of ∼1.15 in the 25-120 °C range for 0.2 at. % Zn-doped Ag2Se. The enhancement in zT is attributed to point defects created by Zn doping into Ag vacancies/interstitials, which enhances the scattering of phonons and tunes the charge carrier properties, leading to the significant suppression of thermal conductivity. The simplicity of the synthetic method developed herein and the high performance of the final products provide an avenue to produce high-quality Ag2Se-based thermoelectric materials.

Enhancement in Thermoelectric Performance in Ti-doped Yb0.4Co4Sb12 Skutterudites via Carrier Optimization and Phonon Anharmonicity

Yb0.4Co4Sb12, being a well-studied system, has shown notably high thermoelectric performance due to the Yb filler atom-driven large concentration of charge carriers and lower value of thermal conductivity. In this work, the thermoelectric performance of YbzCo4-xTixSb12 (where z = 0, x = 0 and z = 0.4, x = 0, 0.04, and 0.08) upon Ti doping prepared by the melt-quenched-annealing followed by spark plasma sintering (SPS) has been studied in the temperature range of 300-700 K. Addition of Yb and doping of donor Ti at the Co site simultaneously increase the electrical conductivity to 1453.5 S/cm at 300 K, which ultimately boosts the power factor as high as ∼4.3 mW/(m·K2) at 675 K in Yb0.4Co3.96Ti0.04Sb12. Adversely, a significant reduction in thermal conductivity is obtained from ∼7.69 W/(m·K) (Co4Sb12) to ∼3.50 W/(m·K) (Yb0.4Co3.96Ti0.04Sb12) at ∼300 K. As a result, the maximum zT is achieved as ∼0.85 at 623 K with high hardness of 584 HV for the composition of Yb0.4Co3.96Ti0.04Sb12, which demonstrates it to be an efficient material suitable for intermediate temperature thermoelectric applications.

Fine Tuning of Defects Enables High Carrier Mobility and Enhanced Thermoelectric Performance of n-Type PbTe

High carrier mobility is critical to improving thermoelectric performance over a broad temperature range. However, traditional doping inevitably deteriorates carrier mobility. Herein, we develop a strategy for fine tuning of defects to improve carrier mobility. To begin, n-type PbTe is created by compensating for the intrinsic Pb vacancy in bare PbTe. Excess Pb²⁺ reduces vacancy scattering, resulting in a high carrier mobility of ∼3400 cm² V^-1 s^-1. Then, excess Ag is introduced to compensate for the remaining intrinsic Pb vacancies. We find that excess Ag exhibits a dynamic doping process with increasing temperatures, increasing both the carrier concentration and carrier mobility throughout a wide temperature range; specifically, an ultrahigh carrier mobility ∼7300 cm² V^-1 s^-1 is obtained for Pb_1.01Te + 0.002Ag at 300 K. Moreover, the dynamic doping-induced high carrier concentration suppresses the bipolar thermal conductivity at high temperatures. The final step is using iodine to optimize the carrier concentration to ∼10¹⁹ cm^-3. Ultimately, a maximum ZT value of ∼1.5 and a large average ZT _ave value of ∼1.0 at 300-773 K are obtained for Pb_1.01Te_0.998I_0.002 + 0.002Ag. These findings demonstrate that fine tuning of defects with <0.5% impurities can remarkably enhance carrier mobility and improve thermoelectric performance.