High performance magnesium-based plastic semiconductors for flexible thermoelectrics

High-performance Ag2Se-based thermoelectrics for wearable electronics

Flexible thermoelectric materials and devices hold enormous potential for wearable electronics but are hindered by inadequate material properties and inefficient assembly techniques, leading to suboptimal performance. Herein, we developed a flexible thermoelectric film, comprising Ag2Se nanowires as the primary material, a nylon membrane as a flexible scaffold, and reduced graphene oxide as a conductive network, achieving a record-high room-temperature ZT of 1.28. Hot-pressed Ag2Se nanowires exhibited strong (013) orientation, enhancing carrier mobility and electrical conductivity. Dispersed reduced graphene oxide further boosts electrical conductivity and induces an energy-filtering effect, decoupling electrical conductivity and the Seebeck coefficient to achieve an impressive power factor of 37 μW cm-1 K-2 at 300 K. The high-intensity between Ag2Se and reduced graphene oxide interfaces enhance phonon scattering, effectively reducing thermal conductivity to below 0.9 W m-1 K-1 and enabling the high ZT value. The nylon membrane endowed the film with exceptional flexibility. A large-scale out-of-plane device with 100 pairs of thermoelectric legs, assembled from these films, delivers an ultrahigh normalized power density of >9.8 μW cm-2 K-2, outperforming all reported Ag2Se-based flexible devices. When applied to the human body, the device generated sufficient power to operate a thermo-hygrometer and a wristwatch, demonstrating its practical potential for wearable electronics.

Sandwich Engineering Advances Ductile Thermoelectrics

Flexible thermoelectrics offer the possibility of utilizing human body heat to generate electricity, enabling self-powered wearable electronics. Ductile and plastic semiconductors are promising materials for flexible thermoelectrics due to their inherent ductility and tunable electrical properties. However, balancing ductility with high thermoelectric performance remains challenging, especially for n-type materials. Here, a novel n-type ductile Ag2(S, Se)-Ag2Se sandwich-like thermoelectric film is designed with different functional layers, where the Ag2(S, Se) core layer provides ductile deformation ability and low thermal conductivity, while epitaxially grown highly oriented Ag2Se shell layers ensure superior electrical transport performance. This architecture achieves a record high figure-of-merit near room-temperature range, 0.91 at 323 K, among n-type ductile semiconductors while preserving excellent flexibility. Additionally, a flexible in-plane device fabricated from this material delivers an exceptional power density of 26.5 W m-2 at a temperature difference of 50 K, demonstrating its great application potential for wearable electronics. Importantly, such novel sandwich engineering can pave the way to alleviate the compromise between thermoelectric performance and ductility in inorganic semiconductors.

Elevating thermoelectric performance in the sub-ambient temperature range for electronic refrigeration

Solid-state thermoelectric coolers, which enable direct heat pumping by utilizing electricity, play an essential role in electronic refrigeration. Given that these devices usually cool down to the sub-ambient temperature range, their performance is critically dependent on the material properties at temperatures below 300 K. Consequently, enhancing the thermoelectric properties of materials at sub-ambient temperature is of paramount importance for advancing cooling technology. Herein, a single-crystalline Mg3Bi2-based material has been prepared and exhibits high electron mobility. As a result, thermoelectric figure-of-merit values of ∼1.05 at 300 K and ∼0.87 at 250 K (along the ab plane) have been achieved, which are superior to commercial n-type Bi2(Te, Se)3. Thermoelectric coolers (single- and double-stage devices) based on the n-type single-crystalline Mg3Bi1.497Sb0.5Te0.003 and p-type (Bi, Sb)2Te3 have been fabricated. The double-stage cooler demonstrates a remarkable maximum cooling temperature difference of ∼106.8 K at the hot-side temperature of 350 K, surpassing the performance of commercial Bi2Te3-based devices. Notably, the Mg3Bi2-based double-stage device exhibits exceptional cyclic stability, maintaining its cooling performance without any observable degradation after approximately 2,000 cycles between the input currents of 1 and 3 A. These findings show that single-crystalline Mg3Bi2 alloys hold great promise for thermoelectric cooling applications.

Realizing High Performance in Flexible Mg3Sb2- xBix Thin-Film Thermoelectrics

As advancements in Mg-based thermoelectric materials continue, increasing attention is directed toward enhancing the thermoelectric performance of Mg3Sb2 and its integration into thermoelectric devices. However, research on Mg3Sb2 thin films and their application in flexible devices remains limited, leaving ample room for improvements in fabrication techniques and thermoelectric properties. To address these gaps, this study employs magnetron sputtering combined with ex-situ annealing to dope Bi into Mg3Sb2 thin films, partially substituting Sb. This approach enhances the near-room-temperature performance and plasticity, yielding high-performance Mg3Sb2- xBix thermoelectric thin films. The films achieve a power factor of 3.77 µW cm-1 K-2 at 500 K, the highest value reported for p-type Mg3Sb2 thin films to date. Comprehensive characterization demonstrates precise thickness control, strong adhesion to various substrates, and excellent flexibility, with performance degradation of less than 12% after 1000 bending cycles at a radius of 5 mm. Additionally, a flexible thermoelectric device is constructed using p-type Mg3Sb1.1Bi0.9 and n-type Ag2Se legs, achieving an output power of 9.96 nW and a power density of 77.38 µW cm-2 under a temperature difference of 10 K. These findings underscore the potential of these devices for practical applications in wearable electronics.

Atomic-scale interface strengthening unlocks efficient and durable Mg-based thermoelectric devices

Solid-state thermoelectric technology presents a compelling solution for converting waste heat into electrical energy. However, its widespread application is hindered by long-term stability issues, particularly at the electrode-thermoelectric material interface. Here we address this challenge by constructing an atomic-scale direct bonding interface. By forming robust chemical bonds between Co and Sb atoms, we develop MgAgSb/Co thermoelectric junctions with a low interfacial resistivity (2.5 µΩ cm2), high bonding strength (60.6 MPa) and high thermal stability at 573 K. This thermally stable and ohmic contact interface enables MgAgSb-based thermoelectric modules to achieve a conversion efficiency of 10.2% at a temperature difference of 287 K and to exhibit negligible degradation over 1,440 h of thermal cycling. Our findings underscore the critical role of atomic-scale interface engineering in advancing thermoelectric semiconductor devices, enabling more efficient and durable thermoelectric modules.

Integrated Thermoelectric Generation System for Sustainable All-Day Power Supply Based on Solar Energy and Radiative Cooling

Thermoelectric generators have a promising application in the field of sustainable energy due to their ability to utilize low-grade waste heat and their high reliability. The sun radiates a large amount of energy to the earth, yet most of which is wasted. Efficient utilization of solar energy can be achieved by integrating a solar absorber, phase change material, and Fresnel lens with thermoelectric generators. In this work, multiwalled carbon nanotubes are mixed with polydimethylsiloxane as a solar absorber and achieve up to 99% absorption of sunlight. Meanwhile, the solar absorber as a radiative cooler also exhibits high emissivity in the atmospheric window, and a cooling effect of 7.3 K from the ambient temperature can be realized at night. Paraffin wax as a phase change material realizes heat storage during the day and release at night. The Fresnel lens realizes a high degree of convergence of sunlight over a large area, obtaining a higher output performance at a lower cost. The multienergy integrated and synergistic thermoelectric generation system achieves an output power density of 4.1 mW/cm2 during the day and a peak power density of 0.2 mW/cm2 during the night, which can meet the demand for ab uninterrupted power supply to electronic devices. This work realizes the efficient utilization of solar energy and provides an option for thermoelectric application.

Strategic vacancy engineering advances record-high ductile AgCu(Te, Se, S) thermoelectrics

AgCu(Te, Se, S) alloys, as one of the rare p-type plastic inorganic thermoelectrics, are receiving striking attention for their application foreground in high-performing flexible thermoelectric generators. However, strategies to enhance their thermoelectric performance while maintaining exceptional plasticity remain largely unexplored. Here, we introduce a strategic vacancy-engineering approach to address this challenge. Using computational design as a guide, we carefully tune the cation vacancy concentration to optimize hole carrier concentration, achieving impressive ZTs of ~0.62 at 300 K and ~0.83 at 343 K in (AgCu)0.998Te0.8Se0.1S0.1, ranking among the highest in this class of material. Importantly, numerous diffuse Ag-S bonds combined with amorphous phase introdeuced by vacancy engineering ensure that (AgCu)0.998Te0.8Se0.1S0.1 retains high plasticity while having high performance. A novel flexible thermoelectric device, comprising ductile p-type (AgCu)0.998Te0.8Se0.1S0.1 and n-type commercial Bi2Te3, achieves an impressive power density of ~126 μW cm-2 under 25 K temperature difference, demonstrating significant application prospects for wearable electronics.

Self-optimized contact in air-robust thermoelectric junction towards long-lasting heat harvesting

Ensuring long-term reliable contacts in thermoelectric devices is particularly challenging due to their operation under high temperatures and has been one of the large obstacles in the field for application. Typically, thermodynamically driven atomic diffusion and reactions often degrade the contacts, leading to increased contact resistivity and ultimately limiting the device's lifespan. Here, we report an unconventional self-optimized contact resistivity mechanism in the Sb/MgAgSb junction. Mg diffusion from MgAgSb to Sb does not degrade but instead optimizes its contact resistivity even after aging in air for 30 days. This unexpected automatic optimization arises from an increased carrier concentration in MgAgSb, which enhances electron tunneling across the interface, effectively reducing the contact resistivity. Leveraging the self-optimized contact in Sb/MgAgSb and stable thermoelectric performance of MgAgSb, a two-pair thermoelectric device employing 100-day air-aged Sb/MgAgSb achieves an impressive conversion efficiency of 8.1% and a rare power density of 0.41 W cm-2 under 294 K temperature gradient. These results underscore its significant potential for robust, long-term heat harvesting. The self-optimization mechanism identified in this work also offers valuable insights for designing future junctions for high-temperature applications.

Substrate-Free Inorganic-Based Films for Thermoelectric Applications

The development of highly integrated electronic components and the Internet of Things demands efficient thermal management and uninterrupted energy harvesting, which provides exciting opportunities for thermoelectric (TE) technology since it allows direct conversion between electricity and thermal energy. The improved output performance of TE devices has traditionally been driven by advancements in inorganic materials. Recently, there has been growing interest in studying substrate-free inorganic-based TE thin films because they provide improved adherence to curved surfaces and offer a more compact size compared to the corresponding rigid form of these materials. This review begins by summarizing various methods for fabricating freestanding inorganic-based TE films, including leveraging the intrinsic plasticity of certain materials, exfoliating layered-structure materials, using sacrificial substrates, and creating composites with flexible components such as polymers and carbon-based materials. A key challenge in achieving high device performance is determining how to maintain the favorable TE properties of inorganic materials. This can be addressed through strategies such as high inorganic content loading, multicomponent engineering, and interfacial structure design. The review also discusses the applications of substrate-free inorganic-based TE devices in both power generation and solid-state cooling. Finally, it outlines current challenges and proposes potential research directions to further advance the field.

Helical dislocation-driven plasticity and flexible high-performance thermoelectric generator in α-Mg3Bi2 single crystals

Inorganic plastic semiconductors play a crucial role in the realm of flexible electronics. In this study, we present a cost-effective plastic thermoelectric semimetal magnesium bismuthide (α-Mg3Bi2), exhibiting remarkable thermoelectric performance. Bulk single-crystalline α-Mg3Bi2 exhibits considerable plastic deformation at room temperature, allowing for the fabrication of intricate shapes such as the letters "SUSTECH" and a flexible chain. Transmission electron microscopy, time-of-flight neutron diffraction, and chemical bonding theoretic analyses elucidate that the plasticity of α-Mg3Bi2 stems from the helical dislocation-driven interlayer slip, small-sized Mg atoms induced weak interlayer Mg-Bi bonds, and low modulus of intralayer Mg2Bi22- networks. Moreover, we achieve a power factor value of up to 26.2 µW cm-1 K-2 along the c-axis at room temperature in an n-type α-Mg3Bi2 crystal. Our out-of-plane flexible thermoelectric generator exhibit a normalized power density of 8.1 μW cm-2 K-2 with a temperature difference of 7.3 K. This high-performance plastic thermoelectric semimetal promises to advance the field of flexible and deformable electronics.

Advances in Mg3Sb2 thermoelectric materials and devices

Thermoelectric technology offers a green-viable and carbon-neutral solution for energy problems by directly converting waste heat to electricity. For years, Bi2Te3-based compounds have been the main choice materials for commercial thermoelectric devices. However, Bi2Te3 comprises scarce and toxic tellurium (Te) elements, which might limit its large-scale application. Recently, Mg3Sb2 compounds have drawn increasing attention as an alternative to Bi2Te3 thermoelectrics due to their excellent thermoelectric performance. Enabled by effective strategies such as optimizing carrier concentration, introducing point defects, and manipulating carrier scattering mechanisms, Mg3Sb2 compounds have realized an improved thermoelectric performance. In this review, optimizing strategies for both Mg3Sb2-based thermoelectric materials and devices are discussed. Moreover, the flexibility and plasticity of Bi-alloyed Mg3Sb2 mainly stemming from the dense dislocations are outlined. The above strategies summarized here for enhancing Mg3Sb2 thermoelectrics are believed to be applicable to many other thermoelectrics.

Room-temperature exceptional plasticity in defective Bi2Te3-based bulk thermoelectric crystals

The recently discovered metal-like room-temperature plasticity in inorganic semiconductors reshapes our knowledge of the physical properties of materials, giving birth to a series of new-concept functional materials. However, current room-temperature plastic inorganic semiconductors are still very rare, and their performance is inferior to that of classic brittle semiconductors. Taking classic bismuth telluride (Bi2Te3)-based thermoelectric semiconductors as an example, we show that antisite defects can lead to high-density, diverse microstructures that substantially affect mechanical properties and thus successfully transform these bulk semiconductors from brittle to plastic, leading to a high figure of merit of up to 1.05 at 300 kelvin compared with other plastic semiconductors, similar to the best brittle semiconductors. We provide an effective strategy to plastify brittle semiconductors to display good plasticity and excellent functionality simultaneously.

Defect Engineering Advances Thermoelectric Materials

Defect engineering is an effective method for tuning the performance of thermoelectric materials and shows significant promise in advancing thermoelectric performance. Given the rapid progress in this research field, this Review summarizes recent advances in the application of defect engineering in thermoelectric materials, offering insights into how defect engineering can enhance thermoelectric performance. By manipulating the micro/nanostructure and chemical composition to introduce defects at various scales, the physical impacts of diverse types of defects on band structure, carrier and phonon transport behaviors, and the improvement of mechanical stability are comprehensively discussed. These findings provide more reliable and efficient solutions for practical applications of thermoelectric materials. Additionally, the development of relevant defect characterization techniques and theoretical models are explored to help identify the optimal types and densities of defects for a given thermoelectric material. Finally, the challenges faced in the conversion efficiency and stability of thermoelectric materials are highlighted and a look ahead to the prospects of defect engineering strategies in this field is presented.

High-Performing Flexible Mg3Bi2 Thin-Film Thermoelectrics

With the advances in bulk Mg3Bi2, there is increasing interest in pursuing whether Mg3Bi2 can be fabricated into flexible thin films for wearable electronics to expand the practical applications. However, the development of fabrication processes for flexible Mg3Bi2 thin films and the effective enhancement of their thermoelectric performance remain underexplored. Here, magnetron sputtering and ex-situ annealing techniques is used to fabricate flexible Mg3Bi2 thermoelectric thin films with a power factor of up to 1.59 µW cm-1 K-2 at 60 °C, ranking as the top value among all reported n-type Mg3Bi2 thin films. Extensive characterizations show that ex-situ annealing, and optimized sputtering processes allow precise control over film thickness. These techniques ensure high adhesion of the films to various substrates, resulting in excellent flexibility, with <10% performance degradation after 500 bending cycles with a radius of 5 mm. Furthermore, for the first time, flexible thermoelectric devices are fabricated with both p-type and n-type Mg3Bi2 legs, which achieve an output power of 0.17 nW and a power density of 1.67 µW cm-2 at a very low temperature difference of 2.5 °C, highlighting the practical application potential of the device.

Interface kinetic manipulation enabling efficient and reliable Mg3Sb2 thermoelectrics

Development of efficient and reliable thermoelectric generators is vital for the sustainable utilization of energy, yet interfacial losses and failures between the thermoelectric materials and the electrodes pose a significant obstacle. Existing approaches typically rely on thermodynamic equilibrium to obtain effective interfacial barrier layers, which underestimates the critical factors of interfacial reaction and diffusion kinetics. Here, we develop a desirable barrier layer by leveraging the distinct chemical reaction activities and diffusion behaviors during sintering and operation. Titanium foil is identified as a suitable barrier layer for Mg3Sb2-based thermoelectric materials due to the creation of a highly reactive ternary MgTiSb metastable phase during sintering, which then transforms to stable binary Ti-Sb alloys during operation. Additionally, titanium foil is advantageous due to its dense structure, affordability, and ease of manufacturing. The interfacial contact resistivity reaches below 5 μΩ·cm2, resulting in a Mg3Sb2-based module efficiency of up to 11% at a temperature difference of 440 K, which exceeds that of most state-of-the-art medium-temperature thermoelectric modules. Furthermore, the robust Ti foil/Mg3(Sb,Bi)2 joints endow Mg3Sb2-based single-legs as well as modules with negligible degradation over long-term thermal cycles, thereby paving the way for efficient and sustainable waste heat recovery applications.