Showing papers on "Flexible electronics published in 2020"

A New Frontier of Printed Electronics: Flexible Hybrid Electronics.

Abstract: The performance and integration density of silicon integrated circuits (ICs) have progressed at an unprecedented pace in the past 60 years. While silicon ICs thrive at low-power high-performance computing, creating flexible and large-area electronics using silicon remains a challenge. On the other hand, flexible and printed electronics use intrinsically flexible materials and printing techniques to manufacture compliant and large-area electronics. Nonetheless, flexible electronics are not as efficient as silicon ICs for computation and signal communication. Flexible hybrid electronics (FHE) leverages the strengths of these two dissimilar technologies. It uses flexible and printed electronics where flexibility and scalability are required, i.e., for sensing and actuating, and silicon ICs for computation and communication purposes. Combining flexible electronics and silicon ICs yields a very powerful and versatile technology with a vast range of applications. Here, the fundamental building blocks of an FHE system, printed sensors and circuits, thinned silicon ICs, printed antennas, printed energy harvesting and storage modules, and printed displays, are discussed. Emerging application areas of FHE in wearable health, structural health, industrial, environmental, and agricultural sensing are reviewed. Overall, the recent progress, fabrication, application, and challenges, and an outlook, related to FHE are presented.

Organic Photodetectors for Next-Generation Wearable Electronics.

Abstract: Next-generation wearable electronics will need to be mechanically flexible and stretchable such that they can be conformally attached onto the human body. Photodetectors that are available in today's market are based on rigid inorganic crystalline materials and they have limited mechanical flexibility. In contrast, photodetectors based on organic polymers and molecules have emerged as promising alternatives due to their inherent mechanical softness, ease of processing, tunable optoelectronic properties, good light sensing performance, and biocompatibility. Here, the recent advances of organic photodetectors in terms of both optoelectronic and mechanical properties are outlined and discussed, and their application in wearable electronics including health monitoring sensors, artificial vision, and self-powering integrated devices are highlighted.

Laser Fabrication of Graphene-Based Flexible Electronics

Abstract: Recent years have witnessed the rise of graphene and its applications in various electronic devices. Specifically, featuring excellent flexibility, transparency, conductivity, and mechanical robustness, graphene has emerged as a versatile material for flexible electronics. In the past decade, facilitated by various laser processing technologies, including the laser-treatment-induced photoreduction of graphene oxides, flexible patterning, hierarchical structuring, heteroatom doping, controllable thinning, etching, and shock of graphene, along with laser-induced graphene on polyimide, graphene has found broad applications in a wide range of electronic devices, such as power generators, supercapacitors, optoelectronic devices, sensors, and actuators. Here, the recent advancements in the laser fabrication of graphene-based flexible electronic devices are comprehensively summarized. The various laser fabrication technologies that have been employed for the preparation, processing, and modification of graphene and its derivatives are reviewed. A thorough overview of typical laser-enabled flexible electronic devices that are based on various graphene sources is presented. With the rapid progress that has been made in the research on graphene preparation methodologies and laser micronanofabrication technologies, graphene-based electronics may soon undergo fast development.

Realizing Ultrahigh Mechanical Flexibility and >15% Efficiency of Flexible Organic Solar Cells via a "Welding" Flexible Transparent Electrode.

Abstract: The power conversion efficiencies (PCEs) of flexible organic solar cells (OSCs) still lag behind those of rigid devices and their mechanical stability is unable to meet the needs of flexible electronics at present due to the lack of a high-performance flexible transparent electrode (FTE). Here, a so-called "welding" concept is proposed to design an FTE with tight binding of the upper electrode and the underlying substrate. The upper electrode consisting of solution-processed Al-doped ZnO (AZO) and silver nanowire (AgNW) network is well welded by utilizing the capillary force effect and secondary growth of AZO, leading to a reduction of the AgNWs junction site resistance. Meanwhile, the poly(ethylene terephthalate) is modified by embedding the AgNWs, which are then used to link with the AgNWs in the upper hybrid electrode, thus enhancing the adhesion of the electrode to the substrate. By this welding strategy, critical bottleneck issues relating to the FTEs in terms of optoelectronic and mechanical properties are comprehensively addressed. The single-junction flexible OSCs based on this welded FTE show a high performance, achieving a record high PCE of 15.21%. In addition, the PCEs of the flexible OSCs are less influenced by the device area and display robust bending durability even under extreme test conditions.

Large-scale flexible and transparent electronics based on monolayer molybdenum disulfide field-effect transistors

Abstract: Atomically thin molybdenum disulfide (MoS2) is a promising semiconductor material for integrated flexible electronics due to its excellent mechanical, optical and electronic properties. However, the fabrication of large-scale MoS2-based flexible integrated circuits with high device density and performance remains a challenge. Here, we report the fabrication of transparent MoS2-based transistors and logic circuits on flexible substrates using four-inch wafer-scale MoS2 monolayers. Our approach uses a modified chemical vapour deposition process to grow wafer-scale monolayers with large grain sizes and gold/titanium/gold electrodes to create a contact resistance as low as 2.9 kΩ μm−1. The field-effect transistors are fabricated with a high device density (1,518 transistors per cm2) and yield (97%), and exhibit high on/off ratios (1010), current densities (~35 μA μm−1), mobilities (~55 cm2 V−1 s−1) and flexibility. We also use the approach to create various flexible integrated logic circuits: inverters, NOR gates, NAND gates, AND gates, static random access memories and five-stage ring oscillators. Wafer-scale monolayers of MoS2 can be used to create flexible transistors and circuits that exhibit on/off ratios of 1010, current densities of ~35 μA μm−1 and mobilities of ~55 cm2 V−1 s−1.

Ultrarobust Ti3C2Tx MXene-Based Soft Actuators via Bamboo-Inspired Mesoscale Assembly of Hybrid Nanostructures

Abstract: Soft actuation materials are highly desirable in flexible electronics, soft robotics, etc. However, traditional bilayered actuators usually suffer from poor mechanical properties as well as deterio...

Bio-inspired vertebral design for scalable and flexible perovskite solar cells.

Abstract: The translation of unparalleled efficiency from the lab-scale devices to practical-scale flexible modules affords a huge performance loss for flexible perovskite solar cells (PSCs). The degradation is attributed to the brittleness and discrepancy of perovskite crystal growth upon different substrates. Inspired by robust crystallization and flexible structure of vertebrae, herein, we employ a conductive and glued polymer between indium tin oxide and perovskite layers, which simultaneously facilitates oriented crystallization of perovskite and sticks the devices. With the results of experimental characterizations and theoretical simulations, this bionic interface layer accurately controls the crystallization and acts as an adhesive. The flexible PSCs achieve the power conversion efficiencies of 19.87% and 17.55% at effective areas of 1.01 cm2 and 31.20 cm2 respectively, retaining over 85% of original efficiency after 7000 narrow bending cycles with negligible angular dependence. Finally, the modules are assembled into a wearable solar-power source, enabling the upscaling of flexible electronics.

Stable and Biocompatible Carbon Nanotube Ink Mediated by Silk Protein for Printed Electronics

Abstract: Ink-based processes, which enable scalable fabrication of flexible devices based on nanomaterials, are one of the practical approaches for the production of wearable electronics. However, carbon nanotubes (CNTs), which possess great potential for flexible electronics, are facing challenges for use in inks due to their low dispersity in most solvents and suspicious cytotoxicity. Here, a stable and biocompatible CNT ink, which is stabilized by sustainable silk sericin and free from any artificial chemicals, is reported. The ink shows stability up to months, which can be attributed to the formation of sericin-CNT (SSCNT) hybrid through non-covalent interactions. It is demonstrated that the SSCNT ink can be used for fabricating versatile circuits on textile, paper, and plastic films through various techniques. As proofs of concept, electrocardiogram electrodes, breath sensors, and electrochemical sensors for monitoring human health and activity are fabricated, demonstrating the great potential of the SSCNT ink for smart wearables.

The Evolution of Flexible Electronics: From Nature, Beyond Nature, and To Nature

Abstract: The flourishing development of multifunctional flexible electronics cannot leave the beneficial role of nature, which provides continuous inspiration in their material, structural, and functional designs. During the evolution of flexible electronics, some originated from nature, some were even beyond nature, and others were implantable or biodegradable eventually to nature. Therefore, the relationship between flexible electronics and nature is undoubtedly vital since harmony between nature and technology evolution would promote the sustainable development. Herein, materials selection and functionality design for flexible electronics that are mostly inspired from nature are first introduced with certain functionality even beyond nature. Then, frontier advances on flexible electronics including the main individual components (i.e., energy (the power source) and the sensor (the electric load)) are presented from nature, beyond nature, and to nature with the aim of enlightening the harmonious relationship between the modern electronics technology and nature. Finally, critical issues in next-generation flexible electronics are discussed to provide possible solutions and new insights in prospective exploration directions.

Flexible two-dimensional indium tin oxide fabricated using a liquid metal printing technique

Abstract: Indium tin oxide (ITO) is a transparent conductor used in applications such as touch screens, smart windows and displays. A key limitation of ITO is its brittle nature, which prohibits its use in flexible electronics. The commercial deposition of high-quality ITO also currently relies on a costly vacuum manufacturing approach. Here we report the centimetre-scale synthesis of flexible two-dimensional ITO using a low-temperature liquid metal printing technique. The approach can directly deposit monolayer or bilayer ITO onto desired substrates, with the resulting bilayer samples offering a transparency above 99.3% and a sheet resistance as low as 5.4 kΩ □−1. We also show that the bilayer ITO features a stratified structure with a pronounced van der Waals spacing. To illustrate the capabilities of the technique, we develop a capacitive touch screen using centimetre-sized monolayer ITO sheets. A liquid metal printing technique can be used to create monolayer and bilayer indium tin oxide, with the bilayer samples offering a transparency above 99.3% and a sheet resistance as low as 5.4 kΩ □−1.

Wood-Based Flexible Electronics

Abstract: Next-generation electronics (e.g., substrate and conductor) need to be high performance, multifunctional, and environmentally friendly. Here, we report the creation of a fully wood-based flexible electronics circuit meeting these requirements, where the substrate, a strong, flexible and transparent wood film, is printed with a lignin-derived carbon nanofibers conductive ink. The wood film fabrication involves extensive removal of lignin and hemicellulose to tailor the nanostructure of the material followed by collapsing of the cell walls. This process preserves the original alignment of the cellulose nanofibers and promotes their binding. The film is flexible, yet strong in fiber direction with a Young's modulus and a tensile strength of 49.9 GPa and 469.9 MPa, respectively. Furthermore, a sustainable and bio-based conductive ink is formulated with lignin-derived carbon nanofibers. The bio-based ink is printed on transparent wood film, and a strain sensor application of the printed circuit is demonstrated. Combining the transparent wood film with the conductive ink produces environmental friendly and sustainable wood-based electronics for potential applications such as flexible circuits and sensors. Moreover, we envision the potential for a scalable and continuous fabrication process as well as end-of-life recyclability.

Silver nanoparticle conductive inks: synthesis, characterization, and fabrication of inkjet-printed flexible electrodes

Abstract: Flexible electronics can be developed with a low-cost and simple fabrication process while being environmentally friendly. Conductive silver inks have been the most applied material in flexible substrates. This study evaluated the performance of different conductive ink formulations using silver nanoparticles by studying the material properties, the inkjet printing process, and application based on electrical impedance spectroscopy using a buffer solution. Silver nanoparticles synthesis was carried out through chemical reduction of silver nitrate; then, seven conductive ink formulations were produced. Properties such as resistivity, viscosity, surface tension, adhesion, inkjet printability of the inks, and electrical impedance of the printed electrodes were investigated. Curing temperature directly influenced the electrical properties of the inks. The resistivity obtained varied from 3.3 × 100 to 5.6 × 10−06 Ω.cm. Viscosity ranged from 3.7 to 7.4 mPa.s, which is suitable for inkjet printing fabrication. By using a buffer solution as an analyte, the printed electrode pairs presented electrical impedance lower than 200 Ω for all the proposed designs, demonstrating the potential of the formulated inks for utilization in flexible electronic devices for biological sensing applications.

Recent progress in silver nanowire networks for flexible organic electronics

Abstract: Recently, flexible transparent electrodes (FTEs) have attracted extensive attention as an essential element for future organic electronics (OEs), i.e. solution-processable, scalable and flexible organic electronics (FOEs). Although the traditional transparent electrode indium tin oxide (ITO) has been widely used in OEs, its brittleness and high cost significantly limit its application in the next generation of devices, typically flexible electronics. Thus, many alternatives, such as graphene, carbon nanotubes, conductive polymers, metal nanowires, metal grids and electrospun metallic nanofibers, have arisen at the forefront of FTEs. Among them, silver nanowire (AgNW) networks have attracted particular attention due to their excellent electrical conductivity and high transmittance, as well as facile availability and low cost. Since many studies on AgNWs have been published, a comprehensive review highlighting the advantages of AgNWs in FOEs is highly required. In this review, the synthesis and film fabrication of AgNWs have been firstly summarized, focusing especially on the properties of conductivity and light transmittance. Next, post treatments with different approaches to improve the conductivity of AgNWs have been included. And then, characterization of FTEs has been introduced with details on key parameters for FOEs. Furthermore, AgNW-based FOEs have been summarized to demonstrate the recent progress, such as organic light-emitting diodes (OLEDs), organic solar cells (OSCs), light-emitting electrochemical cells (LECs), organic field effect transistors (OFETs), organic memory devices (OMDs), etc. Finally, perspectives for AgNWs in FOEs have been discussed and concluded as well. It is expected that AgNWs could be the focus of future FOEs compared with other alternatives in terms of their advantages of optoelectronic properties, film-formation, solution-processability and flexibility.

High-Resolution Laser-Induced Graphene. Flexible Electronics beyond the Visible Limit

Abstract: Laser-induced graphene (LIG) is a multifunctional graphene foam that is commonly direct-written with an infrared laser into a carbon-based precursor material. Here, a visible 405 nm laser is used to directly convert polyimide into LIG. This enabled the formation of LIG with a spatial resolution of ∼12 μm and a thickness of 60% reduction in LIG feature sizes reported in prior publications. This process occurs in situ in an SEM chamber, thus allowing direct observation of LIG formation. The reduced size of the LIG features enables the direct-write formation of flexible electronics that are not visible to the unaided eye. A humidity sensor is demonstrated which could detect human breath with a response time of 250 ms. With the growing interest in LIG for flexible electronics and sensors, finer features can greatly expand its utility.

Flexible Electronics: Status, Challenges and Opportunities

Abstract: The concept of flexible electronics has been around for several decades. In principle, anything thin or very long can become flexible. While cables and wiring are the prime example for flexibility, it was not until the space race that silicon wafers used for solar cells in satellites were thinned to increase their power per weight ratio, thus allowing a certain degree of warping. This concept permitted the first flexible solar cells in the 1960s (Crabb and Treble, 1967). The development of conductive polymers (Shirakawa et al., 1977), organic semiconductors, and amorphous silicon (Chittick et al., 1969; Okaniwa et al., 1983) in the following decades meant huge strides toward flexibility and processability, and thus these materials became the base for electronic devices in applications that require bending, rolling, folding, and stretching, among other properties that cannot be fulfilled by conventional electronics (Cheng and Wagner, 2009) (Figure 1). Presently there is great interest in new materials and fabrication techniques which allow for highperformance scalable electronic devices to be manufactured directly onto flexible substrates. This interest has also extended to not only flexibility but also properties like stretchability and healability which can be achieved by utilizing elastomeric substrates with strong molecular interactions (Oh et al., 2016; Kang et al., 2018). Likewise, biocompatibility and biodegradability has been achieved through polymers that do not cause adverse effect to the body and can be broken down into smaller constituent pieces after utilization (Bettinger and Bao, 2010; Irimia-Vladu et al., 2010; Liu H. et al., 2019). This new progress is now enabling devices which can conform to complex and dynamic surfaces, such as those found in biological systems and bioinspired soft robotics. These next-generation flexible electronics open up a wide range of exciting new applications such as flexible lighting and display technologies for consumer electronics, architecture, and textiles, wearables with sensors that help monitor our health and habits, implantable electronics for improved medical imaging and diagnostics, as well as extending the functionality of robots and unmanned aircraft through lightweight and conformable energy harvesting devices and sensors. While conventional electronics are very capable of these functions, flexible electronics are intended to expand the mechanical features to adhere to novel form factors through hybrid strategies, or as standalone solutions where the application does not require high computation power, intended to be highly robust to deformation, low cost, thin, or disposable. The definition of flexibility differs from application to application. From bending and rolling for easier handling of large area photovoltaics, to conforming onto irregular shapes, folding, twisting, stretching, and deforming required for devices in electronic skin, all while maintaining device performance and reliability. While early progress and many important innovations have already been achieved, the field of flexible electronics has many challenges before it becomes part of our daily life. This represents a huge opportunity for scientific research and development to rapidly and considerably advance this area (Figure 2). In this article the status, key challenges and opportunities for the field of nextgeneration flexible devices are elaborated in terms of materials, fabrication and specific applications. Edited and Reviewed by: Jhonathan Prieto Rojas, King Fahd University of Petroleum and Minerals, Saudi Arabia

V2O5 Textile Cathodes with High Capacity and Stability for Flexible Lithium‐Ion Batteries

Abstract: Textile-based energy-storage devices are highly appealing for flexible and wearable electronics. Here, a 3D textile cathode with high loading, which couples hollow multishelled structures (HoMSs) with conductive metallic fabric, is reported for high-performance flexible lithium-ion batteries. V2 O5 HoMSs prepared by sequential templating approach are used as active materials and conductive metallic fabrics are applied as current collectors and flexible substrates. Taking advantage of the desirable structure of V2 O5 HoMSs that effectively buffers the volume expansion and alleviates the stress/strain during repeated Li-insertion/extraction processes, as well as the robust flexible metallic-fabric current collector, the as-prepared fabric devices show excellent electrochemical performance and ultrahigh stability. The capacity retains a high value of 222.4 mA h g-1 at a high mass loading of 2.5 mg cm-2 even after 500 charge/discharge cycles, and no obvious performance degradation is observed after hundreds of cycles of bending and folding. These results indicate that V2 O5 HoMSs/metallic-fabric cathode electrode is promising for highly flexible lithium-ion batteries.

A Super-Stretchable Liquid Metal Foamed Elastomer for Tunable Control of Electromagnetic Waves and Thermal Transport

Abstract: It is remarkably desirable and challenging to design a stretchable conductive material with tunable electromagnetic-interference (EMI) shielding and heat transfer for applications in flexible electronics. However, the existing materials sustained a severe attenuation of performances when largely stretched. Here, a super-stretchable (800% strain) liquid metal foamed elastomer composite (LMF-EC) is reported, achieving super-high electrical (≈104 S cm-1) and thermal (17.6 W mK-1) conductivities under a large strain of 400%, which also exhibits unexpected stretching-enhanced EMI shielding effectiveness of 85 dB due to the conductive network elongation and reorientation. By varying the liquid and solid states of LMF, the stretching can enable a multifunctional reversible switch that simultaneously regulates the thermal, electrical, and electromagnetic wave transport. Novel flexible temperature control and a thermoelectric system based on LMF-EC is furthermore developed. This work is a significant step toward the development of smart electromagnetic and thermal regulator for stretchable electronics.

Conductive Wood for High-Performance Structural Electromagnetic Interference Shielding

Abstract: Electric conductors are ubiquitously used for electromagnetic shielding, flexible electronics, and energy storage, with metals and carbon-based compounds as traditional choices for these applicatio...

Tough and Stretchable Dual Ionically Cross-Linked Hydrogel with High Conductivity and Fast Recovery Property for High-Performance Flexible Sensors

Abstract: As a kind of typical soft and wet material, hydrogel has been increasingly investigated as another way to develop flexible electronics. However, the traditional hydrogel with poor strain and strength performance cannot meet the requirements for stretchable electronics; fabricating a stretchable hydrogel with balanced tensile strength, toughness, and conductivity is still a big challenge. Herein, a new type of physically cross-linked hydrogel with poly(acrylamide-co-acrylic acid)-Fe3+ and chitosan-SO42- dual ionic networks via facile free radical polymerization and soaking processes is developed to fabricate excellent high-performance flexible sensors. The abundant Fe3+ and SO42- ions in the hydrogel can not only construct tough and strong dual ionic networks but also give the hydrogel high conductivity. Consequently, the optimal hydrogel possesses high tensile strength (∼5.1 MPa), large strain capacity (∼1225%), elasticity (∼1.13 MPa), high toughness (∼32.1 MJ/m3), and high conductivity (3.04 S/m at f = 0.1M), as well as rapid self-recovery property. Furthermore, the hydrogel conductor has high stretching sensitivity with a gauge factor of 6.0 at strain of 700% and was able to detect conventional motions of the human body such as the motions of the knuckle, speaking, and swallowing, which indicates that our ionic conductive hydrogels can be used to fabricate excellent high-performance flexible sensors.