Optimizing the average power factor of p-type (Na, Ag) co-doped polycrystalline SnSe

Enhanced Room-Temperature Thermoelectric Performance of 2D-SnSe Alloys via Electric-Current-Assisted Sintering

Single-crystalline tin-selenide (SnSe) has emerged as a high-performance and eco-friendly alternative to the lead-chalcogens often used in mid-temperature thermoelectric (TE) generators. At high temperature >800 K, the phase transition from Pnma to Cmcm causes a significant rise in the TE figure-of-merit (zT) curve. Conversely, the SnSe TE requires a booster at low temperatures, which allows broader applicability from a device perspective. Herein, a synergy of Cu alloy and Ag-coating is realized through a sequential multi-step synthesis, designed to combine different metal deposition effects. Single-crystalline (Cu2Se)x(SnSe)1−x alloys grown by the Bridgman method were then coated with a thin Ag layer by radio frequency (RF) sputtering, and the interlayer epitaxial film was observed via electric-current assisted sintering (ECAS). Consequently, the thin Ag-coating improves the electrical conductivity (σ) and reduces the thermal conductivity (κ) for (Cu2Se)0.005(SnSe)0.995+Ag alloy, increasing the zT curve at close to room temperature (373 K). The incorporation of multistep addition by ECAS enables tuning of the overall solubility of the alloy, which opens a new avenue to optimize TE performance in anisotropic 2D materials.

Achieving High Thermoelectric Performance of Eco-Friendly SnTe-Based Materials by Selective Alloying and Defect Modulation

Recently, rock-salt lead-free chalcogenide SnTe-based thermoelectric (TE) materials have been considered an alternative to PbTe because of the nontoxic properties of Sn as compared to Pb. However, high carrier concentration that originated from intrinsic Sn vacancies and relatively high thermal conductivity of pristine SnTe lead to poor TE efficiency, which makes room for improving its TE properties. In this study, we present that the Na incorporation into the SnTe matrix is helpful for modifying the electronic band structure, optimization of carrier concentration, introducing dislocations, and kink planes; benefiting from these synergistic effects obviates the disadvantages of SnTe and makes a significant improvement in TE performance. We reveal that Na favorably impacts the structure of electronic bands by valence, conduction band engineering, leading to a nice enhancement in the Seebeck coefficient, which exhibits the highest power factor value of 37.93 μWcm^-1 K^-2 at 898 K, representing the best result for the SnTe material system. Moreover, a broader phonon spectrum is introduced by new phonon-scattering centers, scattered by dislocations and kink planes which suppressed lattice thermal conductivity to 0.57 Wm^-1 K^-1 at 898 K, which is much lower than that of pristine SnTe. Ultimately, a maximum ZT of 1.26 at 898 K is achieved in the Sn_1.03Te + 3% Na sample, which is 97% higher than that of the pristine SnTe, suggesting that SnTe-based materials are a robust candidate for TE applications specifically, an ideal alternative of lead chalcogenides for TE power generation at high temperatures.

Investigating the key role of carrier transport mechanism in SnSe nanoflakes with enhanced thermoelectric power factor

A novel SnSe nanoflake system is explored for its thermoelectric properties from both experiments andab initiostudy. The nanoflakes of the low temperature phase of SnSe (Pnma) are synthesized employing a fast and efficient refluxing method followed by spark plasma sintering at two different temperatures. We report an enhanced power factor (12-67μW mK^-2in the temperature range 300-600 K) in our p-type samples. We find that the prime reason for a high PF in our samples is a significantly improved electrical conductivity (1050-2180 S m^-1in the temperature range 300-600 K). From ourab initioband structure calculations accompanied with the models of temperature and surface dependent carrier scattering mechanisms, we reveal that an enhanced electrical conductivity is due to the reduced carrier-phonon scattering in our samples. The transport calculations are performed using the Boltzmann transport equation within relaxation time approximation. With our combined experimental and theoretical study, we demonstrate that the thermoelectric properties of p-type Pnma-SnSe could be improved by tuning the carrier scattering mechanisms with a control over the spark plasma sintering temperature.

Enhancing Thermoelectric Performance of AgSbTe2-Based Compounds via Microstructure Modulation Combining with Entropy Engineering

Modulation of the microstructure and configurational entropy tuning are the core stratagem for improving thermoelectric performance. However, the correlation of evolution among the preparation methods, chemical composition, structural defects, configurational entropy, and thermoelectric properties is still unclear. Herein, two series of AgSbTe₂-based compounds were synthesized by an equilibrium melting-slow-cooling method and a nonequilibrium melting-quenching-spark plasma sintering (SPS) method, respectively. The equilibrium method results in coarse grains with a size of >300 μm in the samples and a lower defect concentration, leading to higher carrier mobility of 10.66 cm² V^-1 s^-1 for (Ag₂Te)_0.41(Sb₂Te₃)_0.59 compared to the sample synthesized by nonequilibrium preparation of 1.83 cm² V^-1 s^-1. Moreover, tuning the chemical composition of nonstoichiometric AgSbTe₂ effectively improves the configurational entropy and creates a large number of cation vacancies, which evolve into dense dislocations in the samples. Owing to all of these in conjunction with the strong inharmonic vibration of lattice, an ultralow thermal conductivity of 0.51 W m^-1 K^-1 at room temperature is achieved for the (Ag₂Te)_0.42(Sb₂Te₃)_0.58 sample synthesized by the equilibrium preparation method. Due to the enhanced carrier mobility, optimized carrier concentration, and low thermal conductivity, the (Ag₂Te)_0.42(Sb₂Te₃)_0.58 sample synthesized by the equilibrium preparation method possesses the highest ZT of 1.04 at 500 K, more than 60% higher than 0.64 at 500 K of the same composition synthesized by nonequilibrium preparation.

Enhancement of the power factor of SnSe by adjusting the crystal and energy band structures

A high power factor 686 μW m−1 K−2 at 773 K for the SnSe sample origins from temperature dependence of energy valley degeneration and m*DOSvia Ab initio molecular dynamics (AIMD) simulations on basis of the lattice contraction model.

Tin-selenide as a futuristic material: properties and applications

SnSe/SnSe2 is a promising versatile material with applications in various fields like solar cells, photodetectors, memory devices, lithium and sodium-ion batteries, gas sensing, photocatalysis, supercapacitors, topological insulators, resistive switching devices due to its optimal band gap. In this review, all possible applications of SnSe/SnSe2 have been summarized. Some of the basic properties, as well as synthesis techniques have also been outlined. This review will help the researcher to understand the properties and possible applications of tin selenide-based materials. Thus, this will help in advancing the field of tin selenide-based materials for next generation technology.

Boosting Thermoelectric Properties of AgBi3(SeyS1-y)5 Solid Solution via Entropy Engineering

AgBi₃S₅ is an environmentally friendly n-type thermoelectric material composed of earth-abundant and nontoxic elements. It has a complex monoclinic structure with distorted NaCl-type fragments, which provide its intrinsically low thermal conductivity. However, poor electrical properties limit its overall performance. Configurational entropy engineering is an effective method to enhance thermoelectric properties. With the increase of configurational entropy, phonon point defect scattering is amplified, yielding lower lattice thermal conductivity, while the structure symmetry can also be improved, which leads to the enhanced electrical transport property. In this study, we combine carrier modulation and entropy engineering, utilizing melting-annealing and spark plasma sintering, to synthesize a series of AgBi₃(Se_yS_1-y)_5.08 bulks. Se substitution effectively increases the configurational entropy and thus dramatically decreases the thermal conductivity. Moreover, anion deficiency modulation effectively optimizes the carrier concentration and the electrical transport properties. Due to a power factor of 2.7 μW/(cm·K²) and a low thermal conductivity of 0.45 W/(m·K) at 723 K, the AgBi₃(Se_0.9S_0.1)_5.08 sample possesses the highest ZT of 0.42 at 723 K, nearly double the value of AgBi₃S_5.08 or pristine AgBi₃S₅. Our work demonstrates that apart from carrier optimization, entropy engineering opens a new avenue for enhancing the thermoelectric properties of a given material.

High-Performance Thermoelectric SnSe: Aqueous Synthesis, Innovations, and Challenges

Tin selenide (SnSe) is one of the most promising candidates to realize environmentally friendly, cost-effective, and high-performance thermoelectrics, derived from its outstanding electrical transport properties by appropriate bandgaps and intrinsic low lattice thermal conductivity from its anharmonic layered structure. Advanced aqueous synthesis possesses various unique advantages including convenient morphology control, exceptional high doping solubility, and distinctive vacancy engineering. Considering that there is an urgent demand for a comprehensive survey on the aqueous synthesis technique applied to thermoelectric SnSe, herein, a thorough overview of aqueous synthesis, characterization, and thermoelectric performance in SnSe is provided. New insights into the aqueous synthesis-based strategies for improving the performance are provided, including vacancy synergy, crystallization design, solubility breakthrough, and local lattice imperfection engineering, and an attempt to build the inherent links between the aqueous synthesis-induced structural characteristics and the excellent thermoelectric performance is presented. Furthermore, the significant advantages and potentials of an aqueous synthesis route for fabricating SnSe-based 2D thermoelectric generators, including nanorods, nanobelts, and nanosheets, are also discussed. Finally, the controversy, strategy, and outlook toward future enhancement of SnSe-based thermoelectric materials are also provided. This Review guides the design of thermoelectric SnSe with high performance and provides new perspectives as a reference for other thermoelectric systems.

High Thermoelectric Performance of Co-Doped P-Type Polycrystalline SnSe via Optimizing Electrical Transport Properties

This work systematically investigated the thermoelectric properties of p-type Na and M (M = K, Li, Ag) codoped polycrystalline SnSe. It is found that the electrical properties of polycrystalline SnSe can be improved significantly for (Na, Ag) codoped samples, contributed by the enhanced carrier concentration. Specifically, a carrier concentration of 6.23 × 10¹⁹ cm^-3 was obtained in Sn_0.98Na_0.016Ag_0.004Se sample at 335 K, an increase of 18% compared with that of the Na single-doped sample (5.22 × 10¹⁹ cm^-3). The power factor reached ∼0.73 mW m^-1 K^-2 for the Sn_0.98Na_0.016Ag_0.004Se sample at 785 K, enhanced by ∼26% compared with Na single-doped one. In addition, Sn-rich and Ag-rich particles/areas observed in the matrix of Sn_0.98Na_0.016Ag_0.004Se contribute to the reduction of lattice thermal conductivity from 0.61 W m^-1 K^-1 for Sn_0.98Ag_0.02Se to 0.47 W m^-1 K^-1 at 785 K. The combination of simultaneously enhanced power factor and depressed thermal conductivity leads to a maximum ZT ≈ 1.2 at 785 K and a high average ZT ≈ 0.74 at 335-785 K for Sn_0.98Na_0.016Ag_0.004Se, and generating a high theoretical conversion efficiency of ∼11%. These illuminating discoveries could provide routes to enhance the thermoelectric performance in p-type polycrystalline SnSe.