Printed Thermoelectrics

Flexible thermoelectric generators from spray-printed PEDOT:PSS/Bi0.5Sb1.5Te3 composites

Energy harvesting technologies play a pivotal role in powering the next generation of wearable and portable devices, where thin-film thermoelectric generators (TEGs) offer a compact and flexible solution. In this study, flexible thin films of poly(3,4-ethylenedioxythiophene)polystyrene sulfonate (PEDOT:PSS)/Bi0.5Sb1.5Te3 composite on flexible polymeric substrates were initially developed using a spray printing technique. The effect of substrate temperature during printing was assessed on the microstructural and thermoelectric properties, yielding a maximum power factor at a substrate temperature of 110 °C. Additionally, the printed films demonstrated excellent flexibility and mechanical/electrical stability during 1000 cycles of bending, confirming their suitability for wearable electronic applications. Subsequently, a flexible thin-film TEG containing 40 thermoelectric legs was fabricated by spray printing of the composite ink for the first time. Finally, the electrical performance of the flexible thin-film TEG was thoroughly assessed under various temperature gradients, exhibiting maximum open circuit voltage of 52 mV at a temperature difference of 50 °C. This study establishes a foundation for the facile fabrication of flexible TEGs using organic/inorganic composite inks. Further enhancement of the thermoelectric performance can be envisaged through post-processing chemical treatments to optimize charge carrier concentration in the printed TEGs.

Touch Empowerment: Self-Sustaining e-Tattoo Thermoelectric System for Temperature Mapping

In recent advancements within sensing technology, driven by the Internet of Things (IoT), significant impacts are observed on health sector applications, notably through wearable electronics like electronic tattoos (e-tattoos). These e-tattoos, designed for direct contact with the skin, facilitate precise monitoring of vital physiological parameters, including body heat, a critical indicator for conditions such as inflammation and infection. Monitoring these indicators can be crucial for early detection of chronic conditions, steering toward proactive healthcare management. This study delves into a thermoelectric sensor e-tattoo designed for detailed skin temperature mapping. Utilizing a novel design, this sensor detects temperature variations across thermoelectric stripes, leveraging screen-printed films of p-type Bi0.35Sb1.65Te3, n-type Bi2Te2.8Se0.2, and poly(vinyl alcohol) (PVA) for enhanced thermoelectric and flexible properties. The application of a prototype printed thermoelectric device on temporary tattoo paper, a pioneering development in wearable health technology is demonstrated. This device, validated through numerical simulations, exhibits significant potential as a non-invasive tool for temperature monitoring, highlighting its value in health diagnostics and management.

Making High Thermoelectric and Superior Mechanical Performance Nb0.88Hf0.12FeSb Half-Heusler via Additive Manufacturing

Thermoelectric generators held great promise through energy harvesting from waste heat. Their practical application, however, is greatly constrained by poor raw material utilization and tedious processing in fabricating desired shapes. Herein, a state-of-the-art process is reported for 3D printing the half-Heusler (Nb0.88Hf0.12FeSb) thermoelectric material using laser powder bed fusion (LPBF). The multi-dimensional intra- and inter-granular defects created by this process greatly suppress thermal conductivity by providing numerous phonon scattering centers. The resulting LPBF-fabricated half-Heusler exhibits a high figure of merit ≈1.2 at 923 K and a single-leg maximum efficiency of ≈3.3% at a temperature difference (ΔT) of 371 K. Hafnium oxide nanoparticles generated during LPBF effectively prevent crack propagation, ensuring competent mechanical performance and reliable thermoelectric output. The findings highlight the significant potential of LPBF in driving the next industrial revolution of highly efficient and customizable thermoelectric materials.

3D Printing Elastocaloric TiNiCu Thermoelectric Shape Memory Alloys

The development of new thermoelectric conversion and cooling materials is an important means of addressing global climate and heat emissions in the future. While heavy and toxic elements like tellurium and lead are traditionally used to make thermoelectric materials with poor mechanical properties, recent decades have seen a gradual push towards greener and more sustainable alternatives. One such potential alternative material for thermoelectric and thermal management applications would be the Nitinol (TiNi) shape memory alloy, due to their superior mechanical properties. In this study, we have investigated the use of 3D melt printing techniques that can be used to achieve thermoelectric performance and efficiency of elastic memory alloys below 500 °C. The electrical and thermal properties of TiNiCu materials and their relation to morphology were investigated. All the alloys show similar effect sizes, their fatigue behavior is however different. By adjusting the composition of Ti and Ni elements and we have obtained memory alloys with high thermoelectric properties, with a 50% increase in power factor and a 100% increase in ZT values. This work demonstrates the feasibility of using novel 3D-printed SMAs for thermoelectric and cooling applications, providing an experimental basis for the future preparation of low-cost, efficient, long service lifetime and stable materials for thermal energy harvesting and management.

3D printed energy devices: generation, conversion, and storage

The energy devices for generation, conversion, and storage of electricity are widely used across diverse aspects of human life and various industry. Three-dimensional (3D) printing has emerged as a promising technology for the fabrication of energy devices due to its unique capability of manufacturing complex shapes across different length scales. 3D-printed energy devices can have intricate 3D structures for significant performance enhancement, which are otherwise impossible to achieve through conventional manufacturing methods. Furthermore, recent progress has witnessed that 3D-printed energy devices with micro-lattice structures surpass their bulk counterparts in terms of mechanical properties as well as electrical performances. While existing literature focuses mostly on specific aspects of individual printed energy devices, a brief overview collectively covering the wide landscape of energy applications is lacking. This review provides a concise summary of recent advancements of 3D-printed energy devices. We classify these devices into three functional categories; generation, conversion, and storage of energy, offering insight on the recent progress within each category. Furthermore, current challenges and future prospects associated with 3D-printed energy devices are discussed, emphasizing their potential to advance sustainable energy solutions.

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.

Evolution of Thermoelectric Generators: From Application to Hybridization

Considerable thermal energy is emitted into the environment from human activities and equipment operation in the course of daily production. Accordingly, the use of thermoelectric generators (TEGs) can attract wide interest, and it shows high potential in reducing energy waste and increasing energy recovery rates. Notably, TEGs have aroused rising attention and been significantly boosted over the past few years, as the energy crisis has worsened. The reason for their progress is that thermoelectric generators can be easily attached to the surface of a heat source, converting heat energy directly into electricity in a stable and continuous manner. In this review, applications in wearable devices, and everyday life are reviewed according to the type of structure of TEGs. Meanwhile, the latest progress of TEGs' hybridization with triboelectric nanogenerator (TENG), piezoelectric nanogenerator (PENG), and photovoltaic effect is introduced. Moreover, prospects and suggestions for subsequent research work are proposed. This review suggests that hybridization of energy harvesting, and flexible high-temperature thermoelectric generators are the future trends.

Highly Flexible and Foldable Paper-Based Thermoelectric Generator Prepared with Post-Treatment-Free PEDOT:PSS Hybrid Ink

Paper-based thermoelectric (PTE) generators have recently emerged as a green technology that can help alleviate environment pollution and the energy crisis. In this work, a PTE generator was prepared by coating a post-treatment-free thermoelectric ink consisting of poly(3,4-ethylenedioxythiophene)/polystyrene sulfonate (PEDOT:PSS) doped with 1-ethyl-3-methylimidazolium:tricyanomethanide (EMIM:TCM) onto the card paper. By tuning the molar concentration of the EMIM:TCM to 0.17 M and with hot-pressing, the PTE generator showed a decent power factor (PF) value of 6.82 μW m-1 K-2, which was higher than the values of PTE in the literature. This phenomenon could be attributed to the synergistic effect of high-performance thermoelectric ink (i.e., PF = 175 μW m-1 K-2 when deposited on glass slide) and the hot-pressing. The hot-pressing enhanced the packing density of cellulose fibers and the associated PEDOT:PSS hybrid, which enabled the formation of long-range conductive paths. In addition, the PTE had good mechanical stability, indicated by no significant change of the power factor values after cyclic folding 10,000 times. Moreover, the structure of as-prepared PTE could be easily tuned into different shapes that are promising for the preparation of flexible wearable thermoelectric generators.

Piezoelectric Drop-on-Demand Inkjet Printing with Ultra-High Droplet Velocity

Improving droplet velocity as much as possible is considered as the key to improving both printing speed and printing distance of the piezoelectric drop-on-demand inkjet printing technology. There are 3 tough and contradictory issues that need to be addressed simultaneously, namely, the actuation pressure of the piezoelectric printhead, satellite droplets, and the air resistance, which seems almost impossible to achieve with classical methods. Herein, a novel solution is introduced. By modulating the positive crosstalk effect inside and outside the printhead, self-tuning can be achieved, including self-reinforcing of the actuation pressure, self-restraining of satellite droplets, and self-weakening of the air resistance, thereby greatly improving droplet velocity. Based on these mechanisms, waveform design methods for different inks and printheads are investigated. The results demonstrate that monodisperse droplet jetting with a maximum velocity of 27.53 m/s can be achieved, reaching 3 to 5 times that of the classical method (5 to 8 m/s). Correspondingly, the printing speed and distance can be simultaneously increased by almost 10 times, demonstrating an ability of direct printing on irregular surface. Meanwhile, the compatibility of ink materials is expanded, as the Ohnesorge number and the viscosity of printable inks for the printhead used are increased from 0.36-0.72 to 0.03-1.18 and from 10-12 cp to 1-40.3 cp, respectively, even breaking the traditional limitations of the piezoelectric printing technology (Ohnesorge number of 0.1 to 1; viscosity of 1 to 25 cp). All the above provide a new perspective for improving droplet velocity and may even offer a game-changing choice for expanding the boundaries of the piezoelectric drop-on-demand inkjet printing technology.

Fully 3D Printed Tin Selenide (SnSe) Thermoelectric Generators with Alternating n-Type and p-Type Legs

Tin selenide (SnSe) has attracted much attention in the field of thermoelectrics since the discovery of the record figure of merit (zT) of 2.6 ± 0.3. While there have been many publications on p-type SnSe, to manufacture efficient SnSe thermoelectric generators, ann-type is also required. Publications on n-type SnSe, however, are limited. This paper reports a pseudo-3D-printing technique to fabricate bulk n-type SnSe elements, by utilizing Bi as a dopant. Various Bi doping levels are investigated and characterized over a wide range of temperatures and through multiple thermal cycles. Stable n-type SnSe elements are then combined with printed p-type SnSe elements to fabricate a fully printed alternating n- and p-type thermoelectric generator, which is shown to produce 145 μW at 774 K.

Rapid Printing of Pseudo-3D Printed SnSe Thermoelectric Generators Utilizing an Inorganic Binder

There has been much interest in tin selenide (SnSe) in the thermoelectric community since the discovery of the record zT in the material in 2014. Manufacturing techniques used to produce SnSe are largely energy-intensive (e.g., spark plasma sintering); however, recently, in previous work, SnSe has been shown to be produced via a low embodied energy printing technique, resulting in 3D samples with high zT values (up to 1.7). Due to the additive manufacturing technique, the manufacturing time required was substantial. In this work, 3D samples were printed using the inorganic binder sodium metasilicate and reusable molds. This facilitated a single-step printing process that substantially reduced the manufacturing time. The printed samples were thermally stable through multiple thermal cycles, and a peak zT of 0.751 at 823 K was observed with the optimum binder concentration. A proof-of-concept thermoelectric generator produced the highest power output of any reported printed Se-based TEG to date.

Recent advances in inkjet-printing technologies for flexible/wearable electronics

The rapid development of flexible/wearable electronics requires novel fabricating strategies. Among the state-of-the-art techniques, inkjet printing has aroused considerable interest due to the possibility of large-scale fabricating flexible electronic devices with good reliability, high time efficiency, a low manufacturing cost, and so on. In this review, based on the working principle, recent advances in the inkjet printing technology in the field of flexible/wearable electronics are summarized, including flexible supercapacitors, transistors, sensors, thermoelectric generators, wearable fabric, and for radio frequency identification. In addition, some current challenges and future opportunities in this area are also addressed. We hope this review article can give positive suggestions to the researchers in the area of flexible electronics.

A Critical Review of Machine Learning Techniques on Thermoelectric Materials

Thermoelectric (TE) materials can directly convert heat to electricity and vice versa and have broad application potential for solid-state power generation and refrigeration. Over the past few decades, efforts have been made to develop new TE materials with high performance. However, traditional experiments and simulations are expensive and time-consuming, limiting the development of new materials. Machine learning (ML) has been increasingly applied to study TE materials in recent years. This paper reviews the recent progress in ML-based TE material research. The application of ML in predicting and optimizing the properties of TE materials, including electrical and thermal transport properties and optimization of functional materials with targeted TE properties, is reviewed. Finally, future research directions are discussed.

Soft Fiber Electronics Based on Semiconducting Polymer

Fibers, originating from nature and mastered by human, have woven their way throughout the entire history of human civilization. Recent developments in semiconducting polymer materials have further endowed fibers and textiles with various electronic functions, which are attractive in applications such as information interfacing, personalized medicine, and clean energy. Owing to their ability to be easily integrated into daily life, soft fiber electronics based on semiconducting polymers have gained popularity recently for wearable and implantable applications. Herein, we present a review of the previous and current progress in semiconducting polymer-based fiber electronics, particularly focusing on smart-wearable and implantable areas. First, we provide a brief overview of semiconducting polymers from the viewpoint of materials based on the basic concepts and functionality requirements of different devices. Then we analyze the existing applications and associated devices such as information interfaces, healthcare and medicine, and energy conversion and storage. The working principle and performance of semiconducting polymer-based fiber devices are summarized. Furthermore, we focus on the fabrication techniques of fiber devices. Based on the continuous fabrication of one-dimensional fiber and yarn, we introduce two- and three-dimensional fabric fabricating methods. Finally, we review challenges and relevant perspectives and potential solutions to address the related problems.

High Power Factor Nb-Doped TiO2 Thermoelectric Thick Films: Toward Atomic Scale Defect Engineering of Crystallographic Shear Structures

Donor-doped TiO₂-based materials are promising thermoelectrics (TEs) due to their low cost and high stability at elevated temperatures. Herein, high-performance Nb-doped TiO₂ thick films are fabricated by facile and scalable screen-printing techniques. Enhanced TE performance has been achieved by forming high-density crystallographic shear (CS) structures. All films exhibit the same matrix rutile structure but contain different nano-sized defect structures. Typically, in films with low Nb content, high concentrations of oxygen-deficient {121} CS planes are formed, while in films with high Nb content, a high density of twin boundaries are found. Through the use of strongly reducing atmospheres, a novel Al-segregated {210} CS structure is formed in films with higher Nb content. By advanced aberration-corrected scanning transmission electron microscopy techniques, we reveal the nature of the {210} CS structure at the nano-scale. These CS structures contain abundant oxygen vacancies and are believed to enable energy-filtering effects, leading to simultaneous enhancement of both the electrical conductivity and Seebeck coefficients. The optimized films exhibit a maximum power factor of 4.3 × 10^-4 W m^-1 K^-2 at 673 K, the highest value for TiO₂-based TE films at elevated temperatures. Our modulation strategy based on microstructure modification provides a novel route for atomic-level defect engineering which should guide the development of other TE materials.

Effect of the annealing treatment on structural and transport properties of thermoelectric Smy(FexNi1-x)4Sb12thin films

The crystallographic and transport properties of thin films fabricated by pulsed laser deposition and belonging to the Sm_y(Fe_xNi_1-x)₄Sb₁₂filled skutterudite system were studied with the aim to unveil the effect exerted by temperature and duration of thermal treatments on structural and thermoelectric features. The importance of annealing treatments in Ar atmosphere up to 523 K was recognized, and the thermal treatment performed at 473 K for 3 h was selected as the most effective in improving the material properties. With respect to the corresponding bulk compositions, a significant enhancement in phase purity, as well as an increase in electrical conductivity and a drop in room temperature thermal conductivity, were observed in annealed films. The low thermal conductivity, in particular, can be deemed as deriving from the reduced dimensionality and the consequent substrate/film interfacial stress, coupled with the nanometric grain size.

Flexible Thermoelectric Reduced Graphene Oxide/Ag2S/Methyl Cellulose Composite Film Prepared by Screen Printing Process

As an organic-inorganic thermoelectric composite material, a flexible, reduced graphene oxide (rGO)/silver sulfide (Ag₂S)/methyl cellulose (MC) film was fabricated by a two-step method. Firstly, a rGO/Ag₂S composite powder was prepared by a chemical synthesis method, and then, the rGO/Ag₂S/MC composite film was prepared by a combined screen printing and annealing treatment process. The rGO and rGO/Ag₂S composite powders were evenly dispersed in the rGO/Ag₂S/MC composite films. A power factor of 115 μW m^-1 K^-2 at 520 K was acquired for the rGO/Ag₂S/MC composite film, which is ~958 times higher than the power factor at 360 K (0.12 μW m^-1 K^-2), mainly due to the significant increase in the electrical conductivity of the composite film from 0.006 S/cm to 210.18 S/cm as the test temperature raised from 360 K to 520 K. The as-prepared rGO/Ag₂S/MC composite film has a good flexibility, which shows a huge potential for the application of flexible, wearable electronics.

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.