High-Performance Ag-Se-Based n-Type Printed Thermoelectric Materials for High Power Density Folded Generators

A Waterproof Flexible Paper-Based Thermoelectric Generator for Humidity and Underwater Environments

A thermoelectric generator (TEG) is one of the important energy harvesting sources for wearable electronic devices, which converts waste heat into electrical energy without any external stimuli, such as light or mechanical motion. However, the poor flexibility of traditional TEGs (e.g., Si-based TE devices) causes the limitations in practical applications. Flexible paper substrates are becoming increasingly attractive in wearable electronic technology owing to their usability, environmental friendliness (disposable, biodegradable, and renewable materials), and foldability. The high water-absorbing quality of paper restricts its scope of application due to water failure. Therefore, we propose a high-performance flexible waterproof paper-based thermoelectric generator (WPTEG). A modification method that infiltrates TE materials into cellulose paper through vacuum filtration is used to prepare the TE modules. By connecting the TE-modified paper with Al tape, as well as a superhydrophobic layer encapsulation, the WPTEG is fabricated. The WPTEG with three P-N modules can generate an output voltage of up to 235 mV at a temperature difference of 50 K, which can provide power to portable electronic devices such as diodes, clocks, and calculators in hot water. With the waterproof property, the WPTEG paves the way for achieving multi-scenario applications in humid environments on human skin.

Fully inkjet-printed Ag2Se flexible thermoelectric devices for sustainable power generation

Flexible thermoelectric devices show great promise as sustainable power units for the exponentially increasing self-powered wearable electronics and ultra-widely distributed wireless sensor networks. While exciting proof-of-concept demonstrations have been reported, their large-scale implementation is impeded by unsatisfactory device performance and costly device fabrication techniques. Here, we develop Ag2Se-based thermoelectric films and flexible devices via inkjet printing. Large-area patterned arrays with microscale resolution are obtained in a dimensionally controlled manner by manipulating ink formulations and tuning printing parameters. Printed Ag2Se-based films exhibit (00 l)-textured feature, and an exceptional power factor (1097 μWm-1K-2 at 377 K) is obtained by engineering the film composition and microstructure. Benefiting from high-resolution device integration, fully inkjet-printed Ag2Se-based flexible devices achieve a record-high normalized power (2 µWK-2cm-2) and superior flexibility. Diverse application scenarios are offered by inkjet-printed devices, such as continuous power generation by harvesting thermal energy from the environment or human bodies. Our strategy demonstrates the potential to revolutionize the design and manufacture of multi-scale and complex flexible thermoelectric devices while reducing costs, enabling them to be integrated into emerging electronic systems as sustainable power sources.

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.

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.

Plasma-jet printing of colloidal thermoelectric Bi2Te3 nanoflakes for flexible energy harvesting

Thermoelectric generators (TEGs) convert temperature differences into electrical power and are attractive among energy harvesting devices due to their autonomous and silent operation. While thermoelectric materials have undergone substantial improvements in material properties, a reliable and cost-effective fabrication method suitable for microgravity and space applications remains a challenge, particularly as commercial space flight and extended crewed space missions increase in frequency. This paper demonstrates the use of plasma-jet printing (PJP), a gravity-independent, electromagnetic field-assisted printing technology, to deposit colloidal thermoelectric nanoflakes with engineered nanopores onto flexible substrates at room temperature. We observe substantial improvements in material adhesion and flexibility with less than 2% and 11% variation in performance after 10 000 bending cycles over 25 mm and 8 mm radii of curvature, respectively, as compared to previously reported TE films. Our printed films demonstrate electrical conductivity of 2.5 × 10³ S m^-1 and a power factor of 70 μW m^-1 K^-2 at room temperature. To our knowledge, these are the first reported values of plasma-jet printed thermoelectric nanomaterial films. This advancement in plasma jet printing significantly promotes the development of nanoengineered 2D and layered materials not only for energy harvesting but also for the development of large-scale flexible electronics and sensors for both space and commercial applications.

Stamp-Like Energy Harvester and Programmable Information Encrypted Display Based on Fully Printable Thermoelectric Devices

Thermoelectric (TE) devices exhibit considerable application potential in Internet of Things and personal health monitoring systems. However, TE self-powered devices are expensive and their fabrication process is complex. Therefore, large-scale preparation of the TE devices remains challenging. In this work, simple screen-printing technology is used to fabricate a user-friendly and high-performance paper-based TE device, which can be used in both stamp-like paper-based TE generators and infrared displays. When used as a paper-based TE generator, an output power of 940.8 µW is achieved with a temperature difference of 40 K. The programmable infrared pattern based on the TE array display could be used to realize encryption and anti-counterfeiting properties. Moreover, a visual extraction algorithm is used to develop a mobile application for processing and decoding the infrared quick response code information. These findings offer an exciting approach to using paper-based TEGs in applications such as energy harvesting devices, optical encryption, anti-counterfeiting, and dynamic infrared display.

High Seebeck Coefficient from Screen-Printed Colloidal PbSe Nanocrystals Thin Film

Thin-film thermoelectrics (TEs) with a thickness of a few microns present an attractive opportunity to power the internet of things (IoT). Here, we propose screen printing as an industry-relevant technology to fabricate TE thin films from colloidal PbSe quantum dots (QDs). Monodisperse 13 nm-sized PbSe QDs with spherical morphology were synthesized through a straightforward heating-up method. The cubic-phase PbSe QDs with homogeneous chemical composition allowed the formulation of a novel ink to fabricate 2 μm-thick thin films through robust screen printing followed by rapid annealing. A maximum Seebeck coefficient of 561 μV K^-1 was obtained at 143 °C and the highest electrical conductivity of 123 S m^-1 was reached at 197 °C. Power factor calculations resulted in a maximum value of 2.47 × 10^-5 W m^-1 K^-2 at 143 °C. To the best of our knowledge, the observed Seebeck coefficient value is the highest reported for TE thin films fabricated by screen printing. Thus, this study highlights that increased Seebeck coefficients can be obtained by using QD building blocks owing to quantum confinement.

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.

High Figure-of-Merit Telluride-Based Flexible Thermoelectric Films through Interfacial Modification via Millisecond Photonic-Curing for Fully Printed Thermoelectric Generators

The thermoelectric generator (TEG) shows great promise for energy harvesting and waste heat recovery applications. Cost barriers for this technology could be overcome by using printing technologies. However, the development of thermoelectric (TE) materials that combine printability, high-efficiency, and mechanical flexibility is a serious challenge. Here, flexible (SbBi)₂ (TeSe)₃ -based screen-printed TE films exhibiting record-high figure of merits (ZT) and power factors are reported. A high power factor of 24 µW cm^-1  K^-2 (ZT_max  ≈ 1.45) for a p-type film and a power factor of 10.5 µW cm^-1  K^-2 (ZT_max  ≈ 0.75) for an n-type film are achieved. The TE inks, comprised of p-Bi_0.5 Sb_1.5 Te₃ (BST)/n-Bi₂ Te_2.7 Se_0.3 (BT) and a Cu-Se-based inorganic binder (IB), are prepared by a one-pot synthesis process. The TE inks are printed on different substrates and sintered using photonic-curing leading to the formation of a highly conducting β-Cu_{2- δ} Se phase that connects "microsolders," the grains resulting in high-performance. Folded TEGs (f-TEGs) are fabricated using the materials. A half-millimeter thick f-TEG exhibits an open-circuit voltage (V_OC ) of 203 mV with a maximum power density (p_max ) of 5.1 W m^-2 at ∆T = 68 K. This result signifies that a few millimeters thick f-TEG could power Internet-of-Things (IoTs) devices converting low-grade heat to electricity.

Self-Sustainable Biomedical Devices Powered by RF Energy: A Review

Wearable and implantable medical devices (IMDs) have come a long way in the past few decades and have contributed to the development of many personalized health monitoring and therapeutic applications. Sustaining these devices with reliable and long-term power supply is still an ongoing challenge. This review discusses the challenges and milestones in energizing wearable and IMDs using the RF energy harvesting (RFEH) technique. The review highlights the main integrating frontend blocks such as the wearable and implantable antenna design, matching network, and rectifier topologies. The advantages and bottlenecks of adopting RFEH technology in wearable and IMDs are reviewed, along with the system elements and characteristics that enable these devices to operate in an optimized manner. The applications of RFEH in wearable and IMDs medical devices are elaborated in the final section of this review. This article summarizes the recent developments in RFEH, highlights the gaps, and explores future research opportunities.

Flexible thermoelectrics based on ductile semiconductors

Flexible thermoelectrics provide a different solution for developing portable and sustainable flexible power supplies. The discovery of silver sulfide-based ductile semiconductors has driven a shift in the potential for flexible thermoelectrics, but the lack of good p-type ductile thermoelectric materials has restricted the reality of fabricating conventional cross-plane π-shaped flexible devices. We report a series of high-performance p-type ductile thermoelectric materials based on the composition-performance phase diagram in AgCu(Se,S,Te) pseudoternary solid solutions, with high figure-of-merit values (0.45 at 300 kelvin and 0.68 at 340 kelvin) compared with other flexible thermoelectric materials. We further demonstrate thin and flexible π-shaped devices with a maximum normalized power density that reaches 30 μW cm^-2 K^-2. This output is promising for the use of flexible thermoelectrics in wearable electronics.

Printed Thermoelectrics

The looming impact of climate change and the diminishing supply of fossil fuels both highlight the need for a transition to more sustainable energy sources. While solar and wind can produce much of the energy needed, to meet all our energy demands there is a need for a diverse sustainable energy generation mix. Thermoelectrics can play a vital role in this, by harvesting otherwise wasted heat energy and converting it into useful electrical energy. While efficient thermoelectric materials have been known since the 1950s, thermoelectrics have not been utilized beyond a few niche applications. This can in part be attributed to the high cost of manufacturing and the geometrical restraints of current commercial manufacturing techniques. Printing offers a potential route to manufacture thermoelectric materials at a lower price point and allows for the fabrication of generators that are custom built to meet the waste heat source requirements. This review details the significant progress that has been made in recent years in printing of thermoelectric materials in all thermoelectric material groups and printing methods, and highlights very recent publications that show printing can now offer comparable performance to commercially manufactured thermoelectric materials.

Photonic Curing Enables Ultrarapid Processing of Highly Conducting β-Cu2-δSe Printed Thermoelectric Films in Less Than 10 ms

It has been a challenge to obtain high electrical conductivity in inorganic printed thermoelectric (TE) films due to their high interfacial resistance. In this work, we report a facile synthesis process of Cu-Se-based printable ink for screen printing. A highly conducting TE β-Cu_2-δSe phase forms in the screen-printed Cu-Se-based film through ≤10 ms sintering using photonic-curing technology, minimizing the interfacial resistance. This enables overcoming the major challenges associated with printed thermoelectrics: (a) to obtain the desired phase, (b) to attain high electrical conductivity, and (c) to obtain flexibility. Furthermore, the photonic-curing process reduces the synthesis time of the TE β-Cu_2-δSe film from several days to a few milliseconds. The sintered film exhibits a remarkably high electrical conductivity of ∼3710 S cm^-1 with a TE power factor of ∼100 μW m^-1 K^-2. The fast processing and high conductivity of the film could also be potentially useful for different printed electronics applications.

Improved thermoelectric properties of multi-walled carbon nanotubes/Ag2Se via controlling the composite ratio

MWCNTs/Ag2Se composites were synthesized via a facile hydrothermal method; higher electrical conductivity and lower thermal conductivity were simultaneously achieved compared with Ag2Se, resulting in enhanced thermoelectric performance.

Realizing High Thermoelectric Performance of Bi-Sb-Te-Based Printed Films through Grain Interface Modification by an In Situ-Grown β-Cu2-δSe Phase

It has been a substantial challenge to develop a printed thermoelectric (TE) material with a figure-of-merit ZT > 1. In this work, high ZT p-type Bi_0.5Sb_1.5Te₃-based printable TE materials have been advanced by interface modification of the TE grains with a nonstoichiometric β-Cu_2-δSe-based inorganic binder (IB) through a facile printing-sintering process. As a result, a very high TE power factor of ∼17.5 μW cm^-1 K^-2 for a p-type printed material is attained in the optimized compounds at room temperature (RT). In addition, a high ZT of ∼1.2 at RT and of ∼1.55 at 360 K is realized using thermal conductivity (κ) of a pellet made of the prepared printable material containing 10 wt % of IB. Using the same material for p-type TE legs and silver paste for n-type TE legs, a printed TE generator (print-TEG) of four thermocouples has been fabricated for demonstration. An open-circuit voltage (V_OC) of 14 mV and a maximum power output (P_max) of 1.7 μW are achieved for ΔT = 40 K for the print-TEG.

Flexible n-Type Abundant Chalcopyrite/PEDOT:PSS/Graphene Hybrid Film for Thermoelectric Device Utilizing Low-Grade Heat

Combining inorganic thermoelectric (TE) materials with conductive polymers is one promising strategy to develop flexible thermoelectric (FTE) films and devices. As most inorganic materials tried up until now in FTE composites are composed of scarce or toxic elements, and n-type FTE materials are particularly desired, we combined the abundant, inexpensive, nontoxic Zn-doped chalcopyrite (Cu_1-xZn_xFeS₂, x = 0.01, 0.02, 0.03) with a flexible electrical network constituted by poly(3,4-ethylenedioxythiophene) polystyrenesulfonate (PEDOT:PSS) and graphene for n-type FTE films. Hybrid films from the custom design of binary Cu_1-xZn_xFeS₂/PEDOT:PSS to the optimum design of ternary Cu_0.98Zn_0.02FeS₂/PEDOT:PSS/graphene are characterized. Compared with the binary film, a 4-fold enhancement in electrical conductivity was observed in the ternary film, leading to a maximum power factor of ∼ 23.7 μW m^-1 K^-2. The optimum ternary film could preserve >80% of the electrical conductivity after 2000 bending cycles, exhibiting an exceptional flexibility due to the network constructed by PEDOT:PSS and graphene. A five-leg thermoelectric prototype made of optimum films generated a voltage of 4.8 mV with a ΔT of 13 °C. Such an evolution of an inexpensive chalcopyrite-based hybrid film with outstanding flexibility exhibits the potential for cost-sensitive FTE applications.

Wearable Thermoelectric Devices Based on Three-Dimensional PEDOT:Tosylate/CuI Paper Composites

Thermoelectric composites of organic and inorganic materials exhibit significantly enhanced thermoelectric properties compared with pristine organic thermoelectrics so they might be better suited as core materials of wearable thermoelectric devices. This study describes the development of three-dimensional (3D) paper PEDOT:tosylate/CuI composites that could be shaped as 3 mm thick blocks to convert a temperature difference between their bottom and top sides into power; the majority of organic thermoelectric materials are shaped as thin strips usually on a planar substrate and convert a temperature difference between the opposite edges of the strips into power. The 3D paper PEDOT:tosylate/CuI composites can produce a power density equal to 4.8 nW/cm² (ΔΤ = 6 Κ) that is 10 times higher than that of the pristine paper PEDOT:Tos composites. The enhanced thermoelectric properties of the paper PEDOT:tosylate/CuI composites are attributed to the CuI nanocrystals entrapped inside the composite that increases the Seebeck coefficient of the composite to 225 μV K^-1; the Seebeck coefficient of paper PEDOT:Tos is 65 μV K^-1. A proof-of-concept wearable thermoelectric device that uses 36 blocks of the paper PEDOT:tosylate/CuI composites (as p-type elements) and 36 wires of monel (as n-type elements) can produce up to 4.7 μW of power at ΔΤ = 20 K. The device has a footprint of 64 cm² and can be placed directly over the skin or can be embedded into clothing.

Optimization on thermoelectric characteristics of indium tin oxide/indium oxide thin film thermocouples based on screen printing technology

In this work, indium tin oxide (ITO)/indium oxide (In₂O₃) thin film thermocouples (TFTCs) were prepared based on screen printing technology for high temperature measurement. With terpilenol as solvent, epoxy resin and polyether amine as binders and glass powders as additives, the ITO and In₂O₃ slurries were printed onto the Al₂O₃ substrate to form thermocouples. The effect on thermoelectric properties of the TFTCs with heat treatment and different contents of additives was investigated through microstructure observation and thermal cycle test. The static calibration experiment shows that the annealed TFTCs with 7.5 wt. % glass powders additives have the maximum Seebeck coefficient. The thermoelectric voltage output of the TFTCs can reach 126.5 mV at 1275 °C while the temperature difference is 1160 °C and the sensitivity of the TFTCs was 109.1 µV/°C. The drift rate of the TFTCs was 8.34 °C/h at a measuring time of 20 min at 1275 °C. The TFTCs prepared via screen printing technology with excellent thermoelectric properties and thermal stability are aimed to be a viable replacement for practical applications.

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.

High-Performance Temperature Sensor by Employing Screen Printing Technology

In the present study, a high-performance n-type temperature sensor was developed by a new and facile synthesis approach, which could apply to ambient temperature applications. As impacted by the low sintering temperature of flexible polyimide substrates, a screen printing technology-based method to prepare thermoelectric materials and a low-temperature heat treatment process applying to polymer substrates were proposed and achieved. By regulating the preparation parameters of the high-performance n-type indium oxide material, the optimal proportioning method and the post-treatment process method were developed. The sensors based on thermoelectric effects exhibited a sensitivity of 162.5 μV/°C, as well as a wide range of temperature measurement from ambient temperature to 223.6 °C. Furthermore, it is expected to conduct temperature monitoring in different scenarios through a sensor prepared in masks and mechanical hands, laying a foundation for the large-scale manufacturing and widespread application of flexible electronic skin and devices.

Energy Solutions for Wearable Sensors: A Review

Wearable sensors have gained popularity over the years since they offer constant and real-time physiological information about the human body. Wearable sensors have been applied in a variety of ways in clinical settings to monitor health conditions. These technologies require energy sources to carry out their projected functionalities. In this paper, we review the main energy sources used to power wearable sensors. These energy sources include batteries, solar cells, biofuel cells, supercapacitors, thermoelectric generators, piezoelectric and triboelectric generators, and radio frequency (RF) energy harvesters. Additionally, we discuss wireless power transfer and some hybrids of the above technologies. The advantages and drawbacks of each technology are considered along with the system components and attributes that make these devices function effectively. The objective of this review is to inform researchers about the latest developments in this field and present future research opportunities.