Thermoelectric Flexible Silver Selenide Films: Compositional and Length Optimization

Advancing Ag2Se thin-film thermoelectrics via selenization-driven anisotropy control

The debate over the optimal orientation of Ag2Se thin films and its influence on thermoelectric performance remains ongoing. Here, we report a wet-chemical selenization-based anisotropy optimization technique to control the in-plane orientation of the Ag2Se thin film, steering it away from (002) nearly parallel planes that hinder charge carrier mobility. This approach enables us to achieve an impressive power factor of 30.8 μW cm-1 K-2 at 343 K. The as-fabricated Ag2Se thin film demonstrates remarkable durability, retaining over 90% of its power factor after six months of air exposure, and outstanding flexibility, with performance variation staying within 5% after 2000 bending cycles at a 5 mm radius. These attributes are attributed to the controlled film thickness, crystallinity, and strong adhesion to the polyimide substrate. Additionally, the as-assembled slotted thermoelectric device delivers an output power of 0.58 μW and a competitive power density of 807 μW cm-2 at a temperature difference of 20 K, alongside a high normalized power density of 1.8 μW cm-2 K-2, highlighting its potential for practical application. This study provides valuable insights into the design of high-performance, highly flexible thermoelectric thin films for real-world applications.

A Rapid Methodology to Obtain Silver Selenide thin Films with Highly Enhanced Thermoelectric Performance

Metal chalcogenides have been extensively studied for thermoelectric applications. Among other metal chalcogenides, silver selenide (Ag2Se) is considered one of the most promising n-type semiconducting materials for thermoelectric applications due to its low band gap value, Seebeck coefficient, and superior power factor (PF) rendered at room temperature. However, one of the main drawbacks of using Ag2Se as a thermoelectric material on a large scale is the time-consuming physical methods to obtain them, and the need for high vacuum synthesis conditions as well as high-cost. On the contrary, the electrodeposition route offers a fast, low-cost, reliable, eco-friendly, and reproducible synthesis methodology to obtain β-Ag2Se thin films, avoiding the use of high vacuum, which is especially important for scaling up to industrial processing levels. In this study, a facile and rapid strategy is reported to obtain β-Ag2Se thin films with controlled thickness using an electrodeposition technique. As-electrodeposited β-Ag2Se film with a thickness of 740 nm delivered a PF = 5.59 µW cm-1 K2, while an 880 nm β-Ag2Se film annealed at 210 °C exhibits a higher PF = 11.69 µW cm-1 K2. The results demonstrate a rapid preparation of high purity β-Ag2Se thin film with superior thermoelectric performance, provides potential opportunities in the development of efficient thermoelectric devices.

Robust bendable thermoelectric generators enabled by elasticity strengthening

Using body heat for instance, thermoelectric generators have promising applications for driving wearable electronics continuously but remain a challenge in terms of recoverable flexibility, as known highly-performing thermoelectrics are usually inorganics showing rigidity. It is conceptualized in this work a large elastic strain ensuring both a largely-curved recoverable bending and a full recoverability in thermoelectric performance after enormous bendings. This leads the current work to focus on a microstructure engineering approach for strengthening the elasticity of Ag2Se, in which dense dislocations and refined grain induced by a multi-pass hot-rolling technique enable a significant enhancement in elasticity. The resultant hot-rolled elastic thin thermoelectric generators realize a record bendability, for at least 1,000,000 times at a tiny bending radius of 3 mm with an extraordinary power density. Such a bendability is applicable to the most curved surfaces of a human body, suggesting a promising strategy for powerful wearable thermoelectrics of all inorganics.

Deviceization of high-performance and flexible Ag2Se films for electronic skin and servo rotation angle control

Ag2Se shows significant potential for near-room-temperature thermoelectric applications, but its performance and device design are still evolving. In this work, we design a novel flexible Ag2Se thin-film-based thermoelectric device with optimized electrode materials and structure, achieving a high output power density of over 65 W m-2 and a normalized power density up to 3.68 μW cm-2 K-2 at a temperature difference of 42 K. By fine-tuning vapor selenization time, we strengthen the (013) orientation and carrier mobility of Ag2Se films, reducing excessive Ag interstitials and achieving a power factor of over 29 μW cm-1 K-2 at 393 K. A protective layer boosts flexibility of the thin film, retaining 90% performance after 1000 bends at 60°. Coupled with p-type Sb2Te3 thin films and rational simulations, the device shows rapid human motion response and precise servo motor control, highlighting the potential of high-performance Ag2Se thin films in advanced applications.

Stretchable and Skin-Conformal Thermoelectric Generator with Highly Flexible and Plastically Bendable Silver Selenide Films

Among inorganic thermoelectric materials, flexible thermoelectric materials have attracted considerable attention. In this study, highly flexible and plastically bendable silver selenide films with excellent thermoelectric performance at room temperature are presented. The flexibility of the freestanding silver selenide films was significantly improved through a simple annealing treatment. The highly flexible silver selenide films with a thickness of 26.0 μm displayed outstanding n-type thermoelectric performance, achieving an in-plane zT value of 0.38 at room temperature. Because silver selenide films are plastically bendable with a bending radius of less than 1 mm, they can be shaped into various forms. To achieve stretchability and skin-conformality in the thermoelectric generator, S-shaped silver selenide strips were used as an n-type thermoelectric element. Effective harvesting of electricity from heat of the human body was successfully demonstrated.

A Strategy for Fabricating Ultra-Flexible Thermoelectric Films Using Ag2Se-Based Ink

Flexible thermoelectric materials have drawn significant attention from researchers due to their potential applications in wearable electronics and the Internet of Things. Despite many reports on these materials, it remains a significant challenge to develop cost-effective methods for large-scale, patterned fabrication of materials that exhibit both excellent thermoelectric performance and remarkable flexibility. In this study, we have developed an Ag2Se-based ink with excellent printability that can be used to fabricate flexible thermoelectric films by screen printing and low-temperature sintering. The printed films exhibit a Seebeck coefficient of -161 μV/K and a power factor of 3250.9 μW/m·K2 at 400 K. Moreover, the films demonstrate remarkable flexibility, showing minimal changes in resistance after being bent 5000 times at a radius of 5 mm. Overall, this research offers a new opportunity for the large-scale patterned production of flexible thermoelectric films.

Enhanced Thermoelectric Performance in Flexible Sulfur-Alloyed Ag2Se Thin Films

Flexible thermoelectric generators can directly convert thermal energy harvested from the human body into electricity. The Ag2Se flexible film, a promising material for wearable thermoelectric generators, normally demonstrates an inferior electrical transport property due to its weakened in-plane mobility. In this study, the in-plane electrical transport properties of flexible Ag2Se films were optimized by alloying with additional sulfur. This optimization is achieved by leveraging the differences in elemental electronegativity and the preferred orientation of the Ag2Se films. The sulfur-alloyed Ag2Se thin film, with a nominal ratio of 3 atom %, can reach a maximum mobility of 1150 cm-2 V-1 s-1 at 300 K. So, the optimized room-temperature power factor increases to 1935 μW m-1 K-2. Furthermore, the Ag2Se film alloyed with 3 atom % sulfur exhibits excellent flexibility even after 1000 bending cycles with a radius of 5 mm, characterized by a relative resistance increment of less than 3%. In addition, the corresponding π-type flexible thermoelectric generator possesses a maximum power density of 51 W m-2 at a temperature difference of 50 K.

Significant Enhancement of Thermoelectric Performance in Bi0.5 Sb1.5 Te3 Thin Film via Ferroelectric Polarization Engineering

The Bi0.5 Sb1.5 Te3 (BST) thin film shows great promise in harvesting low-grade heat energy due to its excellent thermoelectric performance at room temperature. In order to further enhance its thermoelectric performance, specifically the power factor and output power, new approaches are highly desirable beyond the common "composition-structure-performance" paradigm. This study introduces ferroelectric polarization engineering as a novel strategy to achieve these goals. A Pb(Zr0.52 Ti0.48 )O3 /Bi0.5 Sb1.5 Te3 (PZT/BST) hybrid film is fabricated via magnetron sputtering. Density functional theory calculations demonstrate PZT polarization's influence on charge redistribution and interlayer charge transfer at the PZT/BST interface, facilitating adjustable carrier transport behavior and power factor of the BST film. As a result, a 26.7% enhancement of the power factor, from unpolarized 12.0 to 15.2 µW cm-1 K-2 , is reached by 2 kV out-of-plane downward polarization of PZT. Furthermore, a five-leg generator constructed using this PZT/BST hybrid film exhibits a maximum output power density of 13.06 W m-2 at ΔT = 39 K, which is 20.8% higher than that of the unpolarized one (10.81 W m-2 ). The research presents a new approach to enhance thermoelectric thin films' power factor and output performance by introducing ferroelectric polarization engineering.

Flexible power generators by Ag2Se thin films with record-high thermoelectric performance

Exploring new near-room-temperature thermoelectric materials is significant for replacing current high-cost Bi2Te3. This study highlights the potential of Ag2Se for wearable thermoelectric electronics, addressing the trade-off between performance and flexibility. A record-high ZT of 1.27 at 363 K is achieved in Ag2Se-based thin films with 3.2 at.% Te doping on Se sites, realized by a new concept of doping-induced orientation engineering. We reveal that Te-doping enhances film uniformity and (00l)-orientation and in turn carrier mobility by reducing the (00l) formation energy, confirmed by solid computational and experimental evidence. The doping simultaneously widens the bandgap, resulting in improved Seebeck coefficients and high power factors, and introduces TeSe point defects to effectively reduce the lattice thermal conductivity. A protective organic-polymer-based composite layer enhances film flexibility, and a rationally designed flexible thermoelectric device achieves an output power density of 1.5 mW cm-2 for wearable power generation under a 20 K temperature difference.

Modulation of the morphotropic phase boundary for high-performance ductile thermoelectric materials

The flexible thermoelectric technique, which can convert heat from the human body to electricity via the Seebeck effect, is expected to provide a peerless solution for the power supply of wearables. The recent discovery of ductile semiconductors has opened a new avenue for flexible thermoelectric technology, but their power factor and figure-of-merit values are still much lower than those of classic thermoelectric materials. Herein, we demonstrate the presence of morphotropic phase boundary in Ag2Se-Ag2S pseudobinary compounds. The morphotropic phase boundary can be freely tuned by adjusting the material thermal treatment processes. High-performance ductile thermoelectric materials with excellent power factor (22 μWcm-1 K-2) and figure-of-merit (0.61) values are realized near the morphotropic phase boundary at 300 K. These materials perform better than all existing ductile inorganic semiconductors and organic materials. Furthermore, the in-plane flexible thermoelectric device based on these high-performance thermoelectric materials demonstrates a normalized maximum power density reaching 0.26 Wm-1 under a temperature gradient of 20 K, which is at least two orders of magnitude higher than those of flexible organic thermoelectric devices. This work can greatly accelerate the development of flexible thermoelectric technology.

Hybrid Data-Driven Discovery of High-Performance Silver Selenide-Based Thermoelectric Composites

Optimizing material compositions often enhances thermoelectric performances. However, the large selection of possible base elements and dopants results in a vast composition design space that is too large to systematically search using solely domain knowledge. To address this challenge, a hybrid data-driven strategy that integrates Bayesian optimization (BO) and Gaussian process regression (GPR) is proposed to optimize the composition of five elements (Ag, Se, S, Cu, and Te) in AgSe-based thermoelectric materials. Data is collected from the literature to provide prior knowledge for the initial GPR model, which is updated by actively collected experimental data during the iteration between BO and experiments. Within seven iterations, the optimized AgSe-based materials prepared using a simple high-throughput ink mixing and blade coating method deliver a high power factor of 2100 µW m-1 K-2 , which is a 75% improvement from the baseline composite (nominal composition of Ag2 Se1 ). The success of this study provides opportunities to generalize the demonstrated active machine learning technique to accelerate the development and optimization of a wide range of material systems with reduced experimental trials.

High-Performance Ag2Se Film by a Microwave-Assisted Synthesis Method for Flexible Thermoelectric Generators

Flexible Ag2Se thermoelectric (TE) films are promising for wearable applications near room temperature (RT). Herein, a Ag2Se film on a nylon membrane with high TE performance was fabricated by a facile method. First, Ag2Se powders were prepared by a microwave-assisted synthesis method using Ag nanowires as a template. Second, the Ag2Se powders were deposited onto nylon via vacuum filtration followed by hot pressing. Through modulating the Ag/Se molar ratio for synthesizing the Ag2Se powders, an optimized Ag2Se film demonstrates a high power factor of 1577.1 μW m-1 K-2 and good flexibility at RT. The flexibility of the Ag2Se film is mainly attributed to the flexible nylon membrane. In addition, a six-leg flexible TE generator (f-TEG) fabricated with the optimized Ag2Se film exhibits a maximum power density of 18.4 W m-2 at a temperature difference of 29 K near RT. This work provides a new solution to prepare high-TE-performance flexible Ag2Se films for f-TEGs.

Substrate-Free Thermoelectric 25 μm-Thick Ag2Se Films with High Flexibility and In-Plane zT of 0.5 at Room Temperature

Thermoelectric inorganic films are flexible when sufficiently thin. By removing the substrate, that is, making them free-standing, the flexibility of thermoelectric films can be enhanced to the utmost extent. However, studies on the flexibility of free-standing thermoelectric inorganic films have not yet been reported. Herein, the high thermoelectric performance and flexibility of free-standing thermoelectric Ag₂Se films are reported. Free-standing Ag₂Se films with a thickness of 25.0 ± 3.9 μm exhibited an in-plane zT of 0.514 ± 0.060 at room temperature. These films exhibited superior flexibility compared to Ag₂Se films constrained on a substrate. The flexibility of the Ag₂Se films was systematically investigated in terms of bending strain, bending radius, thickness, and elastic modulus. Using free-standing Ag₂Se films, a substrate-free, flexible thermoelectric generator was fabricated. The energy-harvesting capacity of the thermoelectric generator was also demonstrated.

Thermoelectric Silver-Based Chalcogenides

Heat is abundantly available from various sources including solar irradiation, geothermal energy, industrial processes, automobile exhausts, and from the human body and other living beings. However, these heat sources are often overlooked despite their abundance, and their potential applications remain underdeveloped. In recent years, important progress has been made in the development of high-performance thermoelectric materials, which have been extensively studied at medium and high temperatures, but less so at near room temperature. Silver-based chalcogenides have gained much attention as near room temperature thermoelectric materials, and they are anticipated to catalyze tremendous growth in energy harvesting for advancing internet of things appliances, self-powered wearable medical systems, and self-powered wearable intelligent devices. This review encompasses the recent advancements of thermoelectric silver-based chalcogenides including binary and multinary compounds, as well as their hybrids and composites. Emphasis is placed on strategic approaches which improve the value of the figure of merit for better thermoelectric performance at near room temperature via engineering material size, shape, composition, bandgap, etc. This review also describes the potential of thermoelectric materials for applications including self-powering wearable devices created by different approaches. Lastly, the underlying challenges and perspectives on the future development of thermoelectric materials are discussed.

Microstructurally Tailored Thin β-Ag2 Se Films toward Commercial Flexible Thermoelectrics

Aiming to overcome both the structural and commercial limitations of flexible thermoelectric power generators, an efficient room-temperature aqueous selenization reaction that can be completed in air within less than 1 min, to directly fabricate thin β-Ag₂ Se films consisting of perfectly crystalline and large columnar grains with both in-plane randomness and out-of-plane [201] preferred orientation, is designed. A high power factor (PF) of 2590 ± 414 µW m^-1 K^-2 and a figure-of-merit (zT) of 1.2 ± 0.42 are obtained from a sample with a thickness of ≈1 µm. The maximum output power density of the best 4-leg thermoelectric generator sample reach 27.6 ± 1.95 and 124 ± 8.78 W m^-2 at room temperature with 30 and 60 K temperature differences, respectively, which may be useful in future flexible thermoelectric devices.

Simultaneous Realization of Flexibility and Ultrahigh Normalized Power Density in a Heatsink-Free Thermoelectric Generator via Fine Thermal Regulation

Wearable thermoelectric generators (w-TEGs) can incessantly convert body heat into electricity to power electronics. However, the low efficiency of thermoelectric materials, tiny terminal temperature difference, rigidity, and negligence of lateral heat transfer preclude broad utilization of w-TEGs. In this work, we employ finite element simulation to find the key factors for simultaneous realization of flexibility and ultrahigh normalized power density. Using melamine foam with an ultralow thermal conductivity (0.03 W/m K) as the encapsulation material, a novel lightweight π-type w-TEG with no heatsink and excellent stretchability, comfortability, processability, and cost efficiency has been fabricated. At an ambient temperature of 24 °C, the maximum power density of the w-TEG reached 7 μW/cm² (sitting) and 29 μW/cm² (walking). Under suitable heat exchange conditions (heatsink with 1 m/s air velocity), 32 pairs of w-TEGs can generate 66 mV voltage and 60 μW/cm² power density. The output performance of our TEG is remarkably superior to that of previously reported w-TEGs. Besides, the practicality of our w-TEG was showcased by successfully driving a quartz watch at room temperature.

Some Thermoelectric Phenomena in Copper Chalcogenides Replaced by Lithium and Sodium Alkaline Metals

This review presents thermoelectric phenomena in copper chalcogenides substituted with sodium and lithium alkali metals. The results for other modern thermoelectric materials are presented for comparison. The results of the study of the crystal structure and phase transitions in the ternary systems Na-Cu-S and Li-Cu-S are presented. The main synthesis methods of nanocrystalline copper chalcogenides and its alloys are presented, as well as electrical, thermodynamic, thermal, and thermoelectric properties and practical application. The features of mixed electron–ionic conductors are discussed. In particular, in semiconductor superionic copper chalcogenides, the presence of a “liquid-like phase” inside a “solid” lattice interferes with the normal propagation of phonons; therefore, superionic copper chalcogenides have low lattice thermal conductivity, and this is a favorable factor for the formation of high thermoelectric efficiency in them.

Screen-Printed Flexible Thermoelectric Device Based on Hybrid Silver Selenide/PVP Composite Films

In recent years, the preparation of flexible thermoelectric generators by screen printing has attracted wide attention due to easy processing and high-volume production. In this work, we propose an n-type Ag2Se/polymer polyvinylpyrrolidone (PVP) film based on screen printing and investigate the effect of PVP on thermoelectric performance by varying the ratio of PVP. When the content ratio of Ag2Se to PVP is 30:1, i.e., PI30, the fabricated PI30 film has the best thermoelectric property. The maximum power factor (PF) of the PI30 is 4.3 μW·m-1·K-2, and conductivity reaches 81% of its initial value at 1500 bending cycles. Then, the film thermoelectric generator (F-TEG) fabricated by PI30 is tested for practical application; the output voltage and the maximum output power are 21.6 mV and 233.3 nW at the temperature difference of 40 K, respectively. This work demonstrates that the use of PVP combined with screen printing to prepare F-TEG is a simple and rapid method, which provides an efficient preparation solution for the development of environmentally friendly and wearable flexible thermoelectric devices.

Superionic phase transition in individual silver selenide nanowires

Silver selenide (Ag₂Se) is a promising material for applications as a solid-state electrolyte, with a superionic phase transition at 133 °C. Here, we studied the temperature dependent transport properties of single Ag₂Se nanowires in a transistor geometry, which allowed us to determine charge carrier type, concentration, and mobility below and above the superionic phase transition temperature. We found the majority charge carriers to be n-type in the temperature range of 30-150 °C. Across the superionic phase-transition, we observed a sudden increase in conductivity by about 30%, which was accompanied by an increase in charge carrier density by about 200% and a decrease in mobility by about 45%. Interestingly, the size dependent shift of the transition temperatures to below 100 °C in our wires is much more pronounced than for nanocrystals of comparable size. This surprising and potentially useful effect could be caused by changes in crystal structure arising from the synthesis process.