Flexible Electronics Industry Research Articles

Applications of 5G technology in flexible electronics

As an emerging field in the industry, flexible electronics not only integrates technologies in fields such as electronic circuits, materials, and flat displays, but also spans industries such as semiconductors, materials, chemicals, and printed circuit boards. Its application importance in various fields such as information, energy, healthcare, and manufacturing are increasingly prominent. The main topic of this article is the implementation of 5G technology in the flexible electronics industry. To begin with, this article presents the traits of flexible electronics. Secondly, it stated the development of 5G and introduced its characteristics. The result shows that 5G is not only an air interface technology with higher rates, larger bandwidth, and stronger capabilities, but also an intelligent network for user experience and business applications. Then, combined with 5G and flexible electronics, they are applied in three aspects: mobile antennas, intelligent equipment, and remote medicine. Finally, the current problems and future prospects in the fields of flexible electronics and 5G were summarized. Currently, it faces challenges such as signal interference and power consumption due to the unique characteristics of flexible materials. And in the future, 5G can potentially enhance the performance of flexible electronics, leading to the creation of new products and services in various industries.

Biobased Thermoset Substrate for Flexible and Sustainable Organic Photovoltaics

AbstractToward a sustainable development of the flexible electronics industry, the design and facile preparation of sustainable flexible substrates with satisfactory mechanical properties and transparency is still a challenge. Herein, taking structural advantages of biobased eugenol and vanillin, a polymerizable aromatic carbonate monomer with α,ω‐diene functionality (2E), and a robust aromatic acetal monomer with α,ω‐tetraene functionality (4E), are designed and prepared. Then, they are cross‐linked with pentaerythritol tetrakis(2‐mercaptoacetate) (4SH) via thiol‐ene click reaction to generate a series of ternary poly(thioether carbonate acetal) thermosets. It is found that the introduction of 4E allows the production of thiol‐ene thermosets with higher crosslink density, which can balance their mechanical, optical, and surface properties. In addition, the thermoset shows good resistance against various natural environmental conditions and can be decomposed into recyclable small molecules under harsher conditions (3 m NaOH, 80 °C). As a result, the optimal thermoset is well‐suited for the fabrication of flexible organic solar cells (OSCs), yielding a power conversion efficiency of 15.41%, which is superior to polyethylene terephthalate‐based OSC. The results provided significant insights for the design and preparation of sustainable polymer substrates for OSCs, even flexible electronics.

Enhanced reduction of graphene oxide via laser-dispersion coupling: Towards large-scale, low-defect graphene for crease-free heat-dissipating membranes in advanced flexible electronics

The advancement of flexible electronics demands improved components, necessitating heat dissipation membranes (HDMs) to exhibit high thermal conductivity while maintaining structural integrity and performance stability even after extensive deformation. Herein, we have devised a laser-modulated reduction technique for graphene oxide (GO), enabling the fabrication of high-quality, large-scale, low-defect graphene, which yields high-performance HDMs after orderly deposition. The work underscores the crucial role of the laser wavelength and dispersion liquid’s coupling intensity in influencing the morphology and properties of graphene. Optimal coupling effect and energy conversion are realized when a laser of 1064 nm wavelength irradiates a triethylene glycol (TEG)/N,N-Dimethylformamide (DMF) dispersion. This unique synergy generates high transient energy, which facilitates the deprotonation process and ensures a swift, comprehensive GO reduction. In contrast to conventional water-based laser reduction methods, the accelerated reaction magnifies the size of the graphene sheets by mitigating the ablation effect. After membrane construction with an ordered structure, the corresponding membrane exhibits a high thermal conductivity of 1632 W m−1 K−1, requiring only ∼1/10 of the total preparation time required by other reported methods. Remarkably, the resulting HDM demonstrates superior resilience against creasing and folding, maintaining excellent smoothness and negligible reduction in thermal conductivity after violent rubbing. The combination of exceptional flexibility and thermal conductivity in HDMs paves the way for long-term practical use in the flexible electronics industry.

Tuning the persistence lengths of main chain towards colorless and transparent polyimide with low dielectric loss and excellent general properties

Colorless transparent polyimide (CPI) film simultaneously combining with low dielectric loss at high frequency, excellent thermal, mechanical properties and folding resistance is urgently required in nowadays flexible electronics industry. Inspired by liquid-crystal elastomers containing both soft and hard segments, one rigid and flexible single block copolymerization strategy was designed by introducing the hard segments of liquid-crystal-like PI into the flexible perfluorinated CPI, where it was found that tuning the persistence lengths of hard segments of liquid-crystal-like PI can significantly improve the general properties of CPI films. With the persistence lengths of main chain of CPI films increased from 2.17 ± 0.07 Å to 16.56 ± 0.12 Å, the transmittance of CPI films at 550 nm remained above 85%, while the dielectric loss at 10 GHz of CPI films reduced from 0.00479 to 0.00407 with decreasement of 15.1% due to the increased chain rigidity limits friction work. Besides, CPI film with the persistence length of 16.56 ± 0.12 Å also exhibited well mechanical properties like thermal expansion coefficients of 24.8 ppm/K, tensile strengths of 162.5 MPa and Young's moduli of 4.88 GPa, which were 30.1%, 25.8% and 49.7% better than that of the CPI film with the persistence length of 2.17 ± 0.07 Å. Furthermore, this type of CPI films with tunable elasticity also had better folding resistance properties. Therefore, tuning the persistence lengths of main chain of CPI films can meet the application of high frequency/speed communication, flexible display and other fields at the same time.

Ultrashort‐Pulsed‐Laser Annealing of Amorphous Atomic‐Layer‐Deposited MoS2 Films

To implement 2D molybdenum disulfide (MoS2) in the flexible electronic industry, large, uniform, and crystalline films on flexible substrates are needed. Thermal atomic layer deposition (ALD) generates large‐area uniform MoS2 films at low temperatures directly on temperature‐sensitive substrates. But if the grown films are amorphous, a high‐temperature posttreatment of the whole sample, which may cause thermal degradation of the substrate or other layers, needs to be avoided. In this article, the crystallization of amorphous MoS2 layers deposited by thermal ALD processed with picosecond laser pulses (λ = 532 nm), in a “cold” annealing process, is reported. The laser fluence range varies from = 8.73 mJ cm−2 to = 18.25 mJ cm−2 with scanning speeds from = 240 mm s−1 to = 2640 mm s−1. The crystallization and the influence of the processing parameters on the film morphology are analyzed in detail by Raman spectroscopy and scanning electron microscopy. Furthermore, a transition of amorphous MoS2 by laser annealing to self‐organized patterns is demonstrated and a possible process mechanism for the ultrashort‐pulsed (usp)‐laser annealing is discussed. Finally, the usp laser annealed films are compared to thermally and continuous‐wave‐laser‐annealed samples.

Highly Flexible, Self-Bonding, Self-Healing, and Conductive Soft Pressure Sensors Based on Dicarboxylic Cellulose Nanofiber Hydrogels.

Conductive hydrogels have gained a great deal of interest in the flexible electronics industry because of their remarkable inherent properties. However, a significant challenge remains for balancing hydrogel's conductivity, self-healing, and strength properties. Herein, double network ionic hydrogels were fabricated by concurrently introducing borax into dicarboxylic cellulose nanofiber (DCNFs) and polyacrylamide (PAM) hydrogels. The incorporation of borax provided a superabsorbent feature to the PAM/DCNF hydrogels (without borax) with the equilibrium water absorption rate increased from 552 to 1800% after 42 h. The compressive strength of the prepared hydrogel was 935 kPa compared to 132 kPa for the PAM hydrogel, with high cycling stability (stable after 1000 compression cycles with 50% strain). The hydrogel pressure sensor had a very sensitive response (gauge factor = 1.36) in the strain range from 10 to 80%, which made it possible to detect mechanical motion accurately and reliably. The developed hydrogels with high-performance, environmentally friendly properties are promising for use in future artificial skin and human-machine interface applications.

Progress, challenges, and perspectives on polymer substrates for emerging flexible solar cells: A holistic panoramic review

AbstractIn pursuit of a renewable, inexpensive, sustainable, and compact energy source to replace fossil fuels, solar photovoltaic devices have become an ideal alternative to meet human needs for environmentally friendly, affordable, and portable power sources. It is due to their excellent mechanical robustness and outstanding energy conversion efficiency. Concerning the increasing demand for flexible and wearable electronic devices with standalone power sources, much attention has been paid to photovoltaics' flexibility and lightweight developments. Along with high mechanical flexibility and lightweight, flexible photovoltaic devices have the advantages of conformability, bendability, wearability, moldability, and roll‐to‐roll processing into complex shapes that can produce niche products. Emerging solar cells, among other photovoltaic technologies, have been exalted for their high conversion efficiency, low cost, and ease of production, making them a viable new‐generation photovoltaic technology. The main commercialization choice for cutting‐edge solar cells is flexible dye‐sensitized and perovskite solar cells since they can be made using a roll‐to‐roll printing technique and are appropriate for mass manufacturing. More significantly, flexible evolving solar cells may be created on ultrathin and light substrates to fulfill the demands of the developing flexible electronics industry and discover uses that are not possible with traditional photovoltaic technology. In any flexible device, the substrate is a backbone on which further materials rely. A flexible substrate reduces the installation and transportation charges, thereby reducing the system price and increasing power conversion efficiency. In this review, we comprehensively assess relevant materials suitable for making flexible photovoltaic devices. Several flexible substrate materials, including ultra‐thin glass, metal foils, and various types of polymer materials, have been considered. For conducting materials, transparent conducting oxides, metal nanowires/grids, carbon nanomaterials, and conducting polymers have also been comprehended. Progress on various flexible foils, fabrication and stability issues, current challenges, and solutions to those challenges of using conductive polymer substrate is endorsed and reviewed in detail. The originality of this holistic study lies in its ability to offer a thorough overview of recent advancements in flexible dye‐sensitized and perovskite solar cells on polymer substrates, which is conceivable and worthy as a roadmap for future research work.

The mechanism study of laser peeling of ultra-thin polyimide film from the transparent substrate

Laser peeling of ultra-thin polyimide (PI) film from transparent substrate is a critical technology in the flexible electronics industry. This paper studies the laser ablation and interface separation of the PI film to reveal the mechanism of laser peeling. In the laser ablation process, the supercritical environment is formed, and gas bubbles appear. Then the gas bubbles fuse together and become cavitation holes. These cavitation holes fuse together and produce sparse nanopillar structures that can be seen as a blister. In the interface separation process, the blister absorbs thermal energy and expands under laser irradiation. This paper proposes a discrete quasi-static method to analyze the separation of the PI film from the transparent substrate during the blister expansion. The cohesive zone model is also introduced to predict the peeling behaviors of the PI film. The results show that the blister profile is related to the laser influence, and the blister profile obtained by the theoretical model can fit well with the experiment. When the laser fluence is larger, the blister profile is steeper, and the pressure inside the blister is larger. This paper provides a new perspective for understanding the laser peeling mechanism of ultra-thin PI film from a transparent substrate.

Cellulose‐Based Soft Actuators

AbstractCompared to other commonly used materials in the field of soft actuators, cellulose offers many advantages such as low cost, sustainability, versatile applications, and abundancy. Due to the enormous and growing demand in the functional flexible electronics industry, cellulose‐based soft actuators are receiving a lot of attention and are undergoing an exciting development. In this focused review, the milestones and recent achievements of cellulose‐based actuators are presented. The actuation energy of the actuators is mainly generated through external stimuli like electricity, temperature, magnetic fields, light, or moisture. Different systems which use these stimuli and corresponding fabrication techniques for these materials, such as self‐assembly and layer by layer deposition are discussed. Further, the currently applied strategies to achieve programmable actions in soft actuators will be reviewed, which allow control over responding deformation. Finally, the pending challenges and some potential solutions in the upcoming development of cellulose‐based actuators are summarized.

Research Progress of Microtransfer Printing Technology for Flexible Electronic Integrated Manufacturing.

In recent years, with the rapid development of the flexible electronics industry, there is an urgent need for a large-area, multilayer, and high-production integrated manufacturing technology for scalable and flexible electronic products. To solve this technical demand, researchers have proposed and developed microtransfer printing technology, which picks up and prints inks in various material forms from the donor substrate to the target substrate, successfully realizing the integrated manufacturing of flexible electronic products. This review retrospects the representative research progress of microtransfer printing technology for the production of flexible electronic products and emphasizes the summary of seal materials, the basic principles of various transfer technology and fracture mechanics models, and the influence of different factors on the transfer effect. In the end, the unique functions, technical features, and related printing examples of each technology are concluded and compared, and the prospects of further research work on microtransfer printing technology is finally presented.

Preparation and application of water-based nano-silver conductive ink in paper-based 3D printing

PurposeIn flexible electronics applications, organic inks are mostly used for inkjet printing. Three-dimensional (3 D) printing technology has the advantages of low cost, high speed and good precision in modern electronic printing. The purpose of this study is to solve the high cost of traditional printing and the pollution emissions of organic ink. It is necessary to develop a water-based conductive ink that is easily degradable and can be 3 D printed. A nano-silver ink printed circuit pattern with high precision, high conductivity and good mechanical properties is a promising strategy.Design/methodology/approachThe researched nano-silver conductive ink is mainly composed of silver nanoparticles and resin. The effect of adding methyl cellulose on the ink was also explored. A simple 3 D circuit pattern was printed on photographic paper. The line width, line length, line thickness and conductivity of the printed circuit were tested. The influence of sintering temperature and sintering time on pattern resistivity was studied. The relationship between circuit pattern bending performance and electrical conductivity is analyzed.FindingsThe experimental results show that the ink has the characteristics of low silver content and good environmental protection effect. The printing feasibility of 3 D printing circuit patterns on paper substrates was confirmed. The best printing temperature is 160°C–180°C, and the best sintering time is 30 min. The circuit pattern can be folded 120°, and the cycle is folded more than 60 times. The minimum resistivity of the circuit pattern is 6.07 µΩ·cm. Methyl cellulose can control the viscosity of the ink. The mechanical properties of the pattern have been improved. The printing method of 3 D printing can significantly reduce the sintering time and temperature of the conductive ink. These findings may provide innovation for the flexible electronics industry and pave the way for alternatives to cost-effective solutions.Originality/valueIn this study, direct ink writing technology was used to print circuit patterns on paper substrates. This process is simple and convenient and can control the thickness of the ink layer. The ink material is nonpolluting to the environment. Nano-silver ink has suitable viscosity and pH value. It can meet the requirements of pneumatic 3 D printers. The method has the characteristics of simple process, fast forming, low cost and high environmental friendliness.

Precision cutting of PDMS film with UV-nanosecond laser based on heat generation-diffusion regulation

Laser precision cutting of polydimethylsiloxane (PDMS) films technology will greatly promote development of flexible electronics industry. However, mechanism and optimization method of PDMS films laser cutting is still unclear. On one hand, mechanism of ultraviolet (UV) nanosecond laser precision cutting of PDMS films with laser ablation as the core was explored based on analysis of by-product components. On the other hand, laser cutting quality optimization method of heat generation- diffusion co-regulation was proposed based on the mechanism of laser ablation. The minimum width of the groove and the heat affected zone were obtained by regulating the heat generation process at room temperature, 67.1 ± 1.7 μm and 6.8 ± 0.8 μm, respectively. In addition, the heat diffusion process can be regulated by lowering ambient temperature to further optimize the quality of cut. Using this optimization method, a spring-shape flexible electrode was prepared, and circumferential stretch limit of this spring-shape flexible electrode has been increased from 10% to 150% compared with planar PDMS film flexible electrodes.

Textile-derived freestanding Fe3O4/Porous carbon cloth composite electrode for flexible Li-ion batteries with remarkable cycling stability

The flexible substrates commonly used (carbon cloth, CNT film, graphene film) as a current collector provide negligible capacity, the complicated preparation process and high cost further limit its practicality. Herein, a flexible Fe3O4/porous carbon cloth composite electrode with controllable structure was successfully prepared by utilizing the unique dyeing technique and subsequent pyrolysis. The analysis of lithium storage contribution confirmed that the carbon cloth matrix provides 0.53 mAh cm−2 capacity at a high current density of 1 mA cm−2 because the carbon fibers were partly etched by Fe-base species. As we expected, this composite electrode delivers a high areal capacity of 1.53 mAh cm−2 at a high current density of 1 mA cm−2 after 100 cycles. The excellent performance can be ascribed to that the carbon coated Fe3O4 electrode possesses several advantages, including rendering an efficient electron/ion transport network, relieving the volume expansion of Fe3O4 nanoparticles, and enabling superior kinetics with higher capacitive contribution. More importantly, the flexible electrodes with robust nature endow the pouch-cell with superior performance and excellent flexibility. In this regard, the low-cost and large-scale production methodology as well as the promising electrochemical results will bring a new dawn for the development of flexible electronic industry.

Recent advances in printable carbon nanotube transistors for large-area active matrices

Active-matrices serve as the backplane circuitry for large-area display technologies and distributed sensors. Recently, there has been significant interest in developing flexible, additively manufactured active matrices for the burgeoning flexible electronics industry. Carbon nanotubes (CNTs) are prime candidate materials for semiconducting elements of transistors due to their solution processability, carrier mobility, and mechanical flexibility. There have been many recent accomplishments in the development of CNT inks and in their deposition via printing that enable their use in thin-film transistors (TFTs), and their appropriate performance make them suitable for use in large-area active matrices. In this review, we provide an overview of the field, with a specific focus on recent advancements in CNT sorting, ink preparation, and printing techniques. We also provide a benchmarking study of printed CNT devices presented in literature after 2017. Next, we discuss printable CNT-TFTs used for active-matrix applications. Finally, we provide a concluding perspective on the outlook and challenges for printed CNT-TFT active matrices.

Lightweight and elastic wood-derived composites for pressure sensing and electromagnetic interference shielding

Due to the rapid development of the wearable flexible electronics industry, the demand for multifunctional materials has greatly increased; however, it is still challenging to apply wearable flexible materials across different applications while ensuring suitable performance. In this study, inspired by the structure and morphology of natural wood, a multi-channel 3D cellulose scaffold (CS) was obtained by delignification. Then, carbon nanotubes (CNTs)/MXene composite nanosheets were inserted into the CS via vacuum impregnation. Finally, a hydrophobic and multifunctional wood-derived composite (CMP/CS) was prepared by wrapping with poly (dimethylsiloxane) (PDMS). The CMP/CS composite possessed a high compressive strength of 1.53 MPa and a maximum strain at fracture of 74.1%, and also exhibited excellent electromagnetic interference (EMI) shielding performance (29.3 dB), meeting the requirements of commercial EMI shielding applications. A constructed CMP/CS sensor exhibited a high gauge factor (3.94) and good cyclic stability (over 1000 cycles) and could monitor various human movements in real-time. The thin PDMS protective layer rendered the CMP/CS composite hydrophobic and improved its durability for daily applications. Thus, this smart wood-derived composite provides a reference for studying next-generation lightweight wearable electronic devices.

Research progress in laser direct writing of flexible circuit

<p indent="0mm">At present, the flexible electronics industry is in an important transitional period, and various flexible electronic devices with novel properties and enhanced functions are constantly emerging, from limited flexibility to flexible electronic devices with shape adaptability and ductility. This has greatly stimulated demand for flexible electronic devices. The development of manufacturing technologies for flexible circuits with smaller feature sizes and better performance on larger-format substrates at lower costs has attracted increasing attention. Among various technologies, laser direct writing has proven to be an efficient and flexible manufacturing method that can produce large-area flexible electronic circuits. As a nonlithographic, nonvacuum, and online processing technology, laser direct writing has received considerable attention. It can be employed in the production of circuit electrodes of various substrates, including heat-sensitive flexible substrates, and it has huge application prospects in the production of flexible electronic and energy storage devices, sensors, and wearable electronic devices. On this basis, this article summarizes the latest development in the laser direct writing of flexible circuit (LDWFC) technology for manufacturing flexible circuits. It focuses on the material types and characteristics of the conductive inks suitable for LDWFC in the production of flexible circuits. The LDWFC processing technology is introduced in detail from four aspects: laser sintering technology, laser reduction technology, laser-induced modification technology, and laser auxiliary circuit manufacturing technology. In addition, the application of LDWFC in flexible energy storage devices, tenderness sensors, and flexible displays is introduced, and the development prospects of LDWFC technology in flexible circuit manufacturing are highlighted.

Flexible and thin multifunctional waterborne polyurethane/Ag film for high-efficiency electromagnetic interference shielding, electro-thermal and strain sensing performances

With the rapid development of the flexible electronics industry, the range of multifunctional applications of materials has become extensive. But it is still a challenge to apply flexible materials to different fields while ensuring corresponding performance. Herein, a simple and available approach is addressed to fabricate conductive polymer composite films supported on Ag flakes and waterborne polyurethane (WPU) by spraying. The WPU/Ag films with uniform structure show outstanding flexibility and electromagnetic interference shielding effectiveness of 68.9 dB with thickness of only 50 μm, and could sustain a high standard even undergoing excessive mechanical deformations. Besides, the resulted films possess prominent electro-thermal effect, which can achieve a temperature of over 120 °C by applying a low voltage of 2 V, and the heat can be dissipated rapidly along with the connecting Ag network. Moreover, it shows a high gauge factor (229.2), good durability and reproductivity (over 1000 cycles), and rapid response time to strain (50 ms), which can be potentially used in the field of human movement monitoring and wearable electronic devices. These results indicate the multifunction of the WPU/Ag film, which could widen its practical applications after the corresponding treatment.

Optimization of graphene conductive ink using solvent exchange techniques for flexible electronics applications

In the development of the flexible electronics industry, creating high stability and environmentally friendly conductive inks has remained a key challenge. In this study, less-toxic solvent exchange techniques graphene conductive inks were printed on polyethylene terephthalate substrate. This study aimed to investigate the properties of graphene conductive ink prepared by solvent exchange techniques using four combinations of solvents. To this end, four combinations of solvents were used to fabricate the graphene conductive inks, namely, dimethylformamide (DMF): xylene, DMF: toluene, DMF: ethanol, and DMF: acetone. Conductive ink fabricated by DMF: ethanol solvent exchange was produced with a higher degree of stability and wettability, as well as a 5% enhancement of electrical conductivity, in comparison with other solvent exchange solutions. The findings showed that the electrical conductivity of conductive film increased with an increase of 45% printing cycles. Furthermore, it has been observed that the electrical conductivity of printed graphene film decreased at only 0.02% even over 10 times of bending cycles. Therefore, the combination of DMF: ethanol solvent exchange is the most suitable formulation for graphene conductive inks.

Hole transport free flexible perovskite solar cells with cost-effective carbon electrodes

Low temperature derived carbon electrodes are employed to fabricate low cost hole transport layer free perovskite solar cells, in which perovskite films annealed in glovebox and ambient air are used as the absorbers, respectively. Results suggest that the air annealed sample has bigger crystal grains and higher crystallinity, and the existence of a small amount of lead iodide which passivates grain boundaries contributes to a lower trap density. As a result, a maximum power conversion efficiency (PCE) of 13.07% was obtained on the air annealed device, which is higher than those of devices annealed in glovebox (11.25%). Furthermore, the stability of unencapsulated devices stored in wet (with humidity around 90% ± 5%) air atmosphere are investigated and the results prove that our devices exhibit good stability. In addition to rigid devices, flexible perovskite solar cells are also fabricated using the same procedure. The highest PCE of 11.53% is demonstrated on the champion flexible device, and 69% of its initial PCE can be maintained even after 2000 bending cycles with a bending radius of 2 mm. Our work provides a promising and simple rout for low-cost, air-stable, high-efficiency carbon perovskite solar cells for both large area production and flexible electronic devices industry.

High performance and near body temperature induced self-healing thermoplastic polyurethane based on dynamic disulfide and hydrogen bonds

It is necessary to develop polymer materials with excellent mechanical properties and can be quickly repaired under reasonable conditions. However, it is difficult to simultaneously optimize the mechanical properties and repair conditions as they have conflicting realization requirements. In this study, we prepare a thermoplastic polyurethane (I-4-P-HM) containing two kinds of alicyclic isocyanate with a tensile strength and toughness of 11.35 MPa and 64.1 MJ m−3, respectively. The mechanical properties of the cut and spliced I-4-P-HM films can be completely restored to their original values at near body temperature (40 °C) in only 2 h. In I-4-P-HM, isophorone diisocyanate (IPDI) directly connect to aromatic disulfide that enables a more efficient exchange of the aromatic disulfides, while dicyclohexylmethane 4,4′-diisocyanate (HMDI) with a symmetrical alicyclic structure endows the TPU with better mechanical properties. The two work together to make polyurethane have good mechanical properties and can be repaired quickly under mild condition. Furthermore, we carried out the scratch recovery experiments on I-4-P-HM covered with a layer of conductive silver paste, and the results indicate that this TPU has the potential to be applied in the flexible electronics industry.