Ultraflexible and High-Thermoelectric-Performance Sulfur-Doped Ag2Se Film on Nylon for Power Generators

Indium-Doping Advances High-Performance Flexible Ag2Se Thin Films

Enhancing the thermoelectric performance of Ag2Se thin films via physical vapor deposition remains challenging. In this study, a precursor doping strategy is introduced to fabricate In-doped Ag2Se thin films. In substitutional doping at the Ag cation sites increases the charge density distribution of Ag2Se, improving electrical conductivity, while maintaining a high Seebeck coefficient and relatively low thermal conductivity. This approach yields a competitive room-temperature power factor of ≈26.3 µW cm-1 K-2 and a ZT value approaching 1. The films, supported by a polyimide substrate and optimized for thickness, exhibit uniform composition and excellent flexibility, retaining over 90% of their initial electrical conductivity after 500 bending cycles with a 5 mm bending radius. Additionally, a five-leg flexible thermoelectric device constructed from these films achieves a power density of up to 630.6 µW cm-2 under a temperature difference of 18 K, corresponding to a normalized power density of nearly 2 µW cm-2 K-2, highlighting its potential for practical applications.

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.

Unraveling the Synergistic Role of Kinks in Zig-Zag Ag2Se Nanorod Arrays for High Room-Temperature zT and Improved Mechanical Properties: Experimental and First-Principles Studies

Flexible thermoelectric materials are usually fabricated by incorporating conducting or organic polymers; however, it remains a formidable task to achieve high thermoelectric properties comparable to those of their inorganic counterparts. Here, we present a high zT value of 1.29 ± 0.31 at room temperature in the hierarchical zig-zag Ag2Se nanorod arrays fabricated using the glancing angle deposition (GLAD) technique followed by a facile selenization process. The high zT value at 300 K is ascribed to the ultrahigh power factor of 3101 ± 252 μW/m-K2 and the reduced thermal conductivity of 0.72 ± 0.01 W/mK. Based on ab initio computational and experimental evidence, we reveal that kinked Ag2Se nanorod arrays consisting of rough interfaces modulate the lattice thermal conductivity up to 48.5% at room temperature. The modulation results from interchanging of phonon modes at kink points and enhanced scattering from a large number of rough interfaces. Further, benefiting from kinked hierarchy, a notable improvement in the mechanical performance is observed for zig-zag Ag2Se nanorods which is confirmed by nanoindentation measurements. The synergic improvement in thermoelectric and mechanical performance not only unravels a paradigm to harness thermoelectric heat but also offers deeper insights into tuning the mechanical properties of inorganic thermoelectric materials.

Flexible Ag2Se Film with Enhanced Thermoelectric Performance

Flexible thermoelectric devices offer huge potential for wearable electronics due to their ability to generate green energy by using low-grade heat. However, achieving both high thermoelectric performance and flexibility simultaneously remains a challenge for these devices. Here, we present a simple and cost-effective method for fabricating a high-performance flexible inorganic-organic thermoelectric film by depositing Ag2Se on a porous nylon membrane. The resulting Ag2Se/nylon film exhibits an ultrahigh power factor of approximately 3.4 mW/mK2 at 300 K, the highest value reported for flexible thermoelectric films to date. The Ag2Se/nylon film also demonstrates remarkable flexibility, confirming its potential application in wearable electronics. A thermoelectric module constructed using eight strips of the processed Ag2Se/nylon film generates an impressive output voltage of 65 mV and an output power of 30 μW at a 30 K temperature difference. Additionally, the fabricated module maintains strong flexibility, producing more than 2 mV when wrapped around a human arm with a temperature difference of 1.6 K. This work highlights the significant potential of inorganic-organic films for flexible thermoelectric applications in wearable and implantable devices.

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.

High-Performance Printed Ag2Se/PI Flexible Thermoelectric Film for Powering Wearable Electronics

We report the magnificent thermoelectric properties of the n-type Ag2Se film printed onto a flexible polyimide (PI) substrate. The orthorhombic β-Ag2Se phase of the processed Ag2Se film is confirmed from the X-ray diffractogram. Remarkably, the resulting Ag2Se/PI film exhibits outstanding thermoelectric properties, boasting maximum power factors of 1.4 and 2.1 mW/mK2 at 300 and 405 K, respectively. Furthermore, the flexibility of the Ag2Se/PI film remains intact even after undergoing 1500 bending cycles with no degradation observed in its thermoelectric performance. To demonstrate the practical application of our findings, a flexible thermoelectric prototype is constructed using the fabricated Ag2Se/PI films, which can harvest an impressive output voltage of 52 mV across a temperature difference of 53 K. Additionally, the prototype generates a maximum power output of 7.2 μW with a 40 K temperature difference and can produce 13 mV output voltage when subjected to around a 10 K temperature gradient when the cold side temperature is maintained at 308 K. Moreover, leveraging body heat with just a 1 K temperature variance between the body and the surrounding environment, the prototype could yield an impressive voltage output of 1.6 mV, marking the highest reported voltage output from human body heat to date. Our study not only introduces a cost-effective method for producing high-performance flexible thermoelectric films but also highlights their potential applications in wearable and implantable electronics.

Synergistic Dual Doping of Sulfur and Copper for Improved Thermoelectric Properties of Silver Selenide Nanomaterials

Research on flexible thermoelectric (TE) materials has typically focused on conducting polymers and conducting polymer-based composites. However, achieving TE properties comparable in magnitude to those exhibited by their inorganic counterparts remains a formidable challenge. This study focuses on the synthesis of silver selenide (Ag2Se) nanomaterials using solvothermal methods and demonstrates a significant enhancement in their TE properties through the synergistic dual doping of sulfur and copper. Flexible TE thin films demonstrating excellent flexibility are successfully fabricated using vacuum filtration and hot-pressing techniques. The resulting thin films also exhibited outstanding TE performance, with a high Seebeck coefficient (S = -138.5 µV K-1) and electrical conductivity (σ = 1.19 × 105 S m-1). The record power factor of 2296.8 µW m-1 K-2 at room temperature is primarily attributed to enhanced carrier transport and interfacial energy filtration effects in the composite material. Capitalizing on these excellent TE properties, the maximum power output of flexible TE devices reached 1.13 µW with a temperature difference of 28.6 K. This study demonstrates the potential of Ag2Se-based TE materials for flexible and efficient energy-harvesting applications.

Nanoengineering Approach toward High Power Factor Ag2Se/Se Composite Films for Flexible Thermoelectric Generators

Herein, a flexible Ag₂Se/Se composite film with a high power factor has been fabricated on a nylon membrane. The film has a high density and contains well-crystallized Ag₂Se grains and embedded Se nanoinclusions, which exhibits not only excellent flexibility but also a comparably large room-temperature power factor and Seebeck coefficient of up to 2023 μW m^-1 K^-2 and -155 μV K^-1, respectively. The high Seebeck coefficient is ascribed to the energy-filtering effect as caused by the Se/Ag₂Se heterointerface. The assembled flexible thermoelectric generator (4-leg) exhibits a maximum output power of 1135 nW and a power density of up to 16.4 W m^-2 when the applied temperature difference is 30 K. This work offers a feasible method to design high-performance and low-cost flexible thermoelectric generators used for wearable electronics.

Ag2Se Nanorod Arrays with Ultrahigh Room Temperature Thermoelectric Performance and Superior Mechanical Properties

Ag₂Se is an intriguing material for room-temperature energy harvesting. Herein, we report the fabrication of Ag₂Se nanorod arrays by glancing angle deposition technique (GLAD) followed by simple selenization in a two-zone furnace. Ag₂Se planar films of different thickness were also prepared. The unique tilted Ag₂Se nanorod arrays show excellent zT = 1.14 ± 0.09 and a power factor of 3229.21 ± 149.01 μW/m-K², respectively, at 300 K. The superior thermoelectric performance of Ag₂Se nanorod arrays compared to planar Ag₂Se films could be ascribed to the unique nanocolumnar architecture that not only facilitates efficient electron transport but also significantly scatters phonons at the interfaces. Furthermore, the nanoindentation measurements were performed to explore mechanical properties of the as-prepared films. The Ag₂Se nanorod arrays showed hardness values of 116.51 ± 4.25 MPa and elastic modulus of 10,966.01 ± 529.61 MPa, which are lowered by 51.8 and 45.6%, compared to Ag₂Se films, respectively. The synergetic dependence between the tilt structure and thermoelectric properties accompanied with the simultaneous improvement in mechanical properties opens a new avenue for the practical applications of Ag₂Se in next-generation flexible thermoelectric devices.

High Thermoelectric Performance and Ultrahigh Flexibility Ag2S1-xSex film on a Nylon Membrane

Flexible thermoelectric (TE) generators have recently attracted increasing attention as they have the potential to power wearable devices using the temperature difference between the human body and the environment. Ag₂S is recently reported to have plasticity near room temperature; however, it has very low electrical conductivity, leading to its poor TE property. Here, to improve the TE property, different amounts of Se (Se/Ag₂S molar ratios being 0.4, 0.5, and 0.6) solid solution-substituted Ag₂S films on a nylon membrane are prepared by combing wet-chemical synthesis, vacuum filtration, and hot-pressing. The film (Se/Ag₂S molar ratio = 0.6) exhibits a better TE performance with a power factor of 477.4 ± 15.20 μW m^-1 K^-2 at room temperature, which is comparable to that of bulk Ag₂S_1-xSe_x. In addition, the film possesses excellent flexibility (only ∼5.4% decrease in electrical conductivity after 2000 times bending along a rod with a radius of 4 mm). The power density of a 6-leg TE generator assembled with the film is 6.6 W/m² under a temperature difference of 28.8 K. This work provides a facile new route to Ag₂S-based TE films with low cost, high TE performance, and ultrahigh flexibility.

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.

From Brittle to Ductile: A Scalable and Tailorable All-Inorganic Semiconductor Foil through a Rolling Process toward Flexible Thermoelectric Modules

Inorganic thermoelectric (TE) materials with outstanding capacity for energy conversion are expected to be promising eco-friendly and renewable power sources, but they are always intrinsically brittle, restricting their development in flexible TE electronics. Therefore, we have developed a facile manufacturing method of large-scale all-inorganic silver chalcogenide foils and flexible TE generators in this work. A rolling process, as an effective and facile molding technique, is applied in ductile TE materials. The figure-of-merit for flexibility of this free-standing foil is in the range of 0.02-0.13, suggesting the superior flexibility of the all-inorganic TE foils. A high TE figure-of-merit ZT of 0.47 at room temperature is reached for Ag₂S_0.45Se_0.45Te_0.1, which is one of the most promising room-temperature ZTs among flexible TE materials. A proof-of-concept flexible TE generator based on silver chalcogenide foils achieves an open-circuit voltage of 1.19 mV and an output power density of 1.8 mW/m² with a temperature difference of 2.7 °C across the TE leg, showing great potential in heat-to-electricity conversion.

Realizing a 10 °C Cooling Effect in a Flexible Thermoelectric Cooler Using a Vortex Generator

In this study, flexible thermoelectric coolers (FTECs) are used to develop an alternative personalized cooling technology to achieve a large temperature drop of 10 °C and cooling capacity of 256 W m^-2 . Such an excellent cooling performance is attributed to the innovative design of the quadra-layered Ag₂ Se/poly(3,4-ethylenedioxythiophene) polystyrene sulfonate structure in FTECs and the induced air vortices by the vortex generator attached to the hot surface of the device. The applied pulse-width modulation technique guarantees human body comfort at inconsistent ambient temperature by modulating the duty ratio of the power source, which also saves 35% of the power consumption. As a result, the as-prepared FTECs only consume 68.5 W so as to maintain a comfortable skin temperature (32 ± 0.5 °C) when the ambient temperature is at 31 °C. This technology provides a reliable and adjustable solution for personalized cooling in environments where comfortable temperatures are exceeded.

Ag–Se/Nylon Nanocomposites Grown by Template-Engaged Reaction: Microstructures, Composition, and Optical Properties

Ag–Se nanostructure films were deposited on a–Se/nylon templates by a template-engaged reaction. Firstly, amorphous selenium (a–Se) was deposited on nylon by employing the chemical bath deposition method while using H2SeO3 and Na2SO3 solutions with an increasing selenium deposition time. Then, these a–Se/nylon templates were exposed into AgNO3 solution at ambient temperature and pressure. The Ag–Se/nylon nanocomposites surface morphology, elemental and phase composition, and optical properties were monitored depending on the selenium deposition time on nylon. Scanning electron microscopy (SEM) analysis confirmed the development of a very complex surface composed of pyramidal-like sub-micron structures, agglomerates, and grid-like structures. Energy dispersive spectroscopy (EDS) proved the presence of carbon, oxygen, nitrogen, selenium, and silver. SEM/EDS cross-sectional analysis confirmed the multilayer character with different individual elemental composition in each film layer. X-ray diffraction analysis revealed a polycrystalline Ag2Se phase with or without metallic Ag. The RMS value obtained from atomic force microscopy varies from 25.82 nm to 57.04 nm. From the UV-Vis spectrophotometry, the direct optical band gaps were found to be 1.68–1.86 eV. Ag–Se/nylon composites exhibit high refractive indices in the near infrared region.

Anion Size Effect of Ionic Liquids in Tuning the Thermoelectric and Mechanical Properties of PEDOT:PSS Films through a Counterion Exchange Strategy

Poly(3,4-ethylene dioxythiophene):poly(styrenesulfonic acid) (PEDOT:PSS) thermoelectric thin films have attracted significant interest due to their solution-processable manufacturing. However, molecular-level tuning or doping is still a challenge to synergistically boost their thermoelectric performance and mechanically stretchable capabilities. In this work, we report a counterion exchange between ionic liquid bis(x-fluorosulfonyl) amide lithium (Li:nFSI, n = 1, 3, 5) with different sizes of anions and a PEDOT:PSS-induced bipolaron network, which significantly boosted the thermoelectric power factor from 0.8 to 157 μW m K^-2 at 235 °C and the maximum tensile strain from 3% to over 30%. The π-π* stacking of the PEDOT polymer chains was fine-tuned by the hydrophobic anions of nFSI^-, providing a technical route for constructing a bipolaron network and inducing the transition from hopping transport to band-like transport. Furthermore, we found that the stretchable capabilities, that is, ε_max, were connected to the gelation time of the PEDOT:PSS-Li:nFSI aqueous solution. Thus, more fluorine-containing groups resulted in longer gelation times and higher ε_max values, which significantly improved the processability of the solution-derived films.