Flexible Electronic Materials Research Articles

Utilizing Dip-Coating to Fabricate Gate Dielectric and Semiconductor for Thin-Film Transistors

The potential applications of advanced electronic materials in large-area, printable, and flexible electronics have generated significant interest. However, creating high-performance, low-voltage thin-film transistors (TFTs) for these applications remains difficult due to a lack of advanced gate dielectric and semiconductor materials that meet both ease-of-fabrication requirements and high electrical performance. In this study, we present high-performance gate dielectric thin-films, which were fabricated using a facile solution-based technique, and then employed to realize low operating voltage organic and metal oxide semiconductor-based thin-film transistors. The high-k oxide gate dielectrics were produced via a simple dip-coating method, resulting in the formation of thin-oxide layers. These novel oxide gate dielectrics demonstrated exceptional dielectric properties, with large capacitances (up to 430 nF/ cm2), low-level leakage current densities (< 3 × 10-8A/cm2 at 4 V), featureless morphology (rms roughness < 0.36 nm), and high transparency (> 85%). Consequently, these dip-coated gate dielectrics can be incorporated into thin-film transistors, utilizing pentacene as p-type organic semiconductors. Furthermore, by employing dip-coating, indium oxide and indium-gallium-zinc oxide can be utilized as n-type inorganic semiconductors, allowing for the fabrication of low-voltage operation and high-performance inorganic TFTs. The resulting TFTs functioned at ultralow voltages (< ± 2 V) and achieved high transistor performance (hole mobility: 0.28 cm2V-1·s-1, electron mobility: ~2.0 cm2V-1·s-1 and on/off current ratio >105).

Flexible resistive memory device based on agar

Potential applications of natural materials in environmentally friendly electronics include for information storage. In this work, natural material–agar was used to fabricate a flexible resistive memory device. Agar is one of the most widely used biomaterials for tissue engineering, medicine and other biotechnological applications. Agar is a suitable material for flexible electronics due to its good film formation, biocompatibility, low-temperature solution processability, transparency and flexibility. The flexible agar memory device described in this article exhibits an ON/OFF ratio of 103 under a bending radius of 5 mm, good bending endurance and a stable data retention time of over 104 s. Moreover, the agar could easily use a leaf as a substrate to make a fully biodegradable device. Agar, due to its exceptional flexibility, is emerging as a promising candidate for wearable and skin-compatible electronics, particularly in memory devices.

Influence of Clay on Structure, Thermal Properties and Mechanics of Poly(vinyl acetate)/Maghnite Nanocomposites.

AbstractPoly (vinyl acetate) (PVAC) based on Algerian clay maghnite was synthesized by an emulsion polymerization process in the present study. The prepared nanocomposites were analyzed by means of FTIR, UV‐visible, XRD, AFM, SEM, TGA and optical microscopy. The results obtained showed a semi‐crystalline structure of the composite system revealed by the XRD scheme. The addition of maghnite to the fillers up to 5 wt % has been shown to reduce the optical band gap energy, while the refractive index is greatly increased. The presence of maghnite in PVAC leads to an increase in surface roughness as measured by AFM analysis. The thermal stability of PVAC was significantly improved by the presence of maghnite in the polymer matrix. The mechanical test results indicate that the tensile strength of the polymer composites was improved by the increase in the concentration of clay platelets. The electrical property data show that as the frequency increases, the dielectric constant decreases and the electrical conductivity increases. The loading content of composites also influences their electrical and mechanical properties. This widens the range of applications for these materials in flexible electronics and in conductive coatings.

Ultrastrong and Tough Urushiol-Based Ionic Conductive Double Network Hydrogels as Flexible Strain Sensors.

Ionic conductive hydrogels have attracted increasing research interest in flexible electronics. However, the limited resilience and poor fatigue resistance of current ionic hydrogels significantly restrict their practical application. Herein, an urushiol-based ionic conductive double network hydrogel (PU/PVA-Li) was developed by one-pot thermal initiation polymerization assisted with freeze-thaw cycling and subsequent LiCl soaking. Such a PU/PVA-Li hydrogel comprises a primary network of covalently crosslinked polyurushiol (PU) and a secondary network formed by physically crosslinked poly(vinyl alcohol) (PVA) through crystalline regions. The obtained PU/PVA-Li hydrogel demonstrates exceptional mechanical properties, including ultrahigh strength (up to 3.4 MPa), remarkable toughness (up to 1868.6 kJ/m3), and outstanding fatigue resistance, which can be attributed to the synergistic effect of the interpenetrating network structure and dynamic physical interactions between PU and PVA chains. Moreover, the incorporation of LiCl into the hydrogels induces polymer chain contraction via ionic coordination, further enhancing their mechanical strength and resilience, which also impart exceptional ionic conductivity (2.62 mS/m) to the hydrogels. Based on these excellent characteristics of PU/PVA-Li hydrogel, a high-performance flexible strain sensor is developed, which exhibits high sensitivity, excellent stability, and reliability. This PU/PVA-Li hydrogel sensor can be effectively utilized as a wearable electronic device for monitoring various human joint movements. This PU/PVA-Li hydrogel sensor could also demonstrate its great potential in information encryption and decryption through Morse code. This work provides a facile strategy for designing versatile, ultrastrong, and tough ionic conductive hydrogels using sustainable natural extracts and biocompatible polymers. The developed hydrogels hold great potential as promising candidate materials for future flexible intelligent electronics.

Heterodimensional Structure Switching Multispectral Stealth and Multimedia Interaction Devices.

Lightweight and flexible electronic materials with high energy attenuation hold an unassailable position in electromagnetic stealth and intelligent devices. Among them, emerging heterodimensional structure draws intensive attention in the frontiers of materials, chemistry, and electronics, owing to the unique electronic, magnetic, thermal, and optical properties. Herein, an intrinsic heterodimensional structure consisting of alternating assembly of 0D magnetic clusters and 2D conductive layers is developed, and its macroscopic electromagnetic properties are flexibly designed by customizing the number of oxidative molecular layer deposition (oMLD) cycles. This unique heterodimensional structure features highly ordered spatial distribution, with an achievement of electron-dipole and magnetic-dielectric double synergies, which exhibits the high attenuation of electromagnetic energy (160) and substantial improvement of dielectric loss tangent (≈200%). It can respond to electromagnetic waves of different bands to achieve multispectral stealth, covering visible light, infrared radiation, and gigahertz wave. Importantly, two kinds of ingenious information interaction devices are constructed with heterodimensional structure. The hierarchical antennas allow precise targeting of operating bands (S- to Ku- bands) by oMLD cycles. The strain imaging device with high sensitivity opens a new horizon for visual interaction. This work provides a creative insight for developing advanced micro-nano materials and intelligent devices.

Enhanced interlayer adhesion of wetted graphene oxide by confined water bridge.

Graphene oxide (GO) sheets are widely used as building blocks in flexible electronic devices, structural materials, and energy storage technology owing to physicochemical flexibility and remarkable mechanical properties. GO exists as lamellar structures in these applications and, thus, it urges to enhance interface interaction to prevent interfacial failure. This study explores the adhesion of GO with and without intercalated water utilizing steered molecular dynamics (SMD) simulations. We find the interfacial adhesion energy (γ) depends on the synergistic effect of the types of functional groups, the degree of oxidation (c), and water content (wt). The intercalated monolayer water confined within GO flakes can improve the γ by more than 50% whereas the interlayer spacing is enlarged. The enhancement of adhesion is from the cooperative hydrogen bonding bridges between confined water and functional group on GO. Furthermore, the optimal water content wt = 20% and oxidation degree c = 20% are obtained. Our findings provide an experimentally available way to improve interlayer adhesion through molecular intercalation, which opens the possibility of high-performance laminate nanomaterial-based films for versatile applications.

Tough, stretchable dual-network liquid metal-based hydrogel toward high-performance intelligent on-off electromagnetic interference shielding, human motion detection and self-powered application

The stretchable conductive materials with adjustable electromagnetic interference (EMI) shielding performance and other multifunctional integrated applications are remarkably desirable and challenging in flexible electronics. However, the existing stretchable materials often suffer from the decay of EMI shielding performance during the stretching process, which limits their practical applications. Meanwhile, it remains a great challenge to construct materials with ordered porous frameworks as integrated shielding devices. Incorporating porous structure will generate abundant interior surfaces/interfaces, facilitating multiple reflections of incident electromagnetic waves (EMWs). Herein, we employ ultrasound method to modify liquid metal (LM) with carbon nanotubes (CNTs), and further utilize an UV in-situ polymerization method to fabricate CNT@LM/polyacrylamide/gelatin (LMCPG) dual network hydrogel elastomer. The LMCPG hydrogel elastomer possesses high strength and durability, and its tensile strength reaches 117 kPa at 1033% strain owing to the high strength of the rigid network and the beneficial deformation ability of the flexible network. Deformable CNT@LM droplets elongate randomly during stretching process, which facilitates establishing more conductive paths. The EMI shielding capacity of the stretched elastomer can be controlled, achieving an intelligent on-off behavior, which exhibits a 211% increase at 200% strain in EMI shielding effectiveness (SE) compared with the undeformed state. Moreover, the LMCPG elastomer shows a significant triboelectric performance, which can provide power supply for flexible electronic devices, achieving an uninterrupted monitoring of the human body motion by devices. Therefore, as a flexible electronic material, it can offer a facile solution to achieve intelligent multifunctional integrated EMI shielding materials.

Polypyrrole decorated with an anthraquinone dye for energy storage applications

In this study, an electrode material based on polypyrrole (PPy) conducting polymer with a versatile fabrication method has been fabricated instead of electrodes and electrolytes containing expensive metal materials for flexible and wearable electronics. An anthraquinone dye (Acid Blue 127) was used as a counter ion. Structural and morphological analyses of the electrodes, whose energy storage ability was investigated, showed that the surface morphology changes in an interesting way and increases the amount of stored charge by increasing the electrode/electrode interface interaction with increasing surface area (∼3 times). The involvement of AB 127, a textile dye, in the production of supercapacitor electrodes in wearable electronics provides new ideas that multifunctional properties of materials can be discovered and applied in many fields. The electrode material (PPy/AB-127) produced at 2 mM anthraquinone-dye concentration showed a maximum value of 606.6 F g−1 at a current density of 1.0 A g−1. When a symmetrical supercapacitor was formed, it exhibited an energy density of 22.9 W h kg-1 at a power density of 3500 W kg−1 in an aqueous electrolyte. this device retained 91.6% of its capacitance for 5000 cycles.

Amorphous Transparent Cu(S,I) Thin Films with Very High Hole Conductivity.

Amorphous transparent conductors (a-TCs) are key materials for flexible and transparent electronics but still suffer from poor p-type conductivity. By developing an amorphous Cu(S,I) material system, record high hole conductivities of 103-104 S cm-1 have been achieved in p-type a-TCs. These high conductivities are comparable with commercial n-type TCs made of indium tin oxide and are 100 times greater than any previously reported p-type a-TCs. Responsible for the high hole conduction is the overlap of large p-orbitals of I- and S2- anions, which provide a hole transport pathway insensitive to structural disorder. In addition, the bandgap of amorphous Cu(S,I) can be modulated from 2.6 to 2.9 eV by increasing the iodine content. These unique properties demonstrate that the Cu(S,I) system holds great potential as a promising p-type amorphous transparent electrode material for optoelectronics.

Organic–inorganic hybrid cathode interlayer for efficient flexible inverted organic solar modules

Organic solar cells (OSC) have great potential for flexible and wearable electronics due to their significant energy supply. However, the brittleness of inorganic electron transport layers (ETL) and their large-area production make it difficult to use them in flexible inverted OSCs. Herein, an organic–inorganic hybrid cathode interlayer of incorporating poly(4-vinylphenol) (P4VP) into the ZnO precursor solution was developed. The addition of P4VP improves the conductibility of ETL and facilitates the favorable vertical component distribution of active layer on the ZnO:P4VP substrate. Thus, the blade-coated OSC based on ZnO:P4VP performs better than the ZnO-based OSC in terms of photovoltaic performance and thickness insensitivity. The P4VP acts as an adhesive in ZnO grain boundaries and eliminates cracks in the bent ETL, leading to a significantly improved mechanical flexibility. Consequently, the ZnO:P4VP-based large-area flexible OSC achieves a power conversion efficiency of 14.05% and retains 80% of its initial efficiency after 1000 bending cycles, which is much better than that based on pristine ZnO (12.26%, 44%). Furthermore, flexible inverted organic solar modules were fabricated and achieved a considerable efficiency of 12.01%. These findings provide a general approach for using inorganic materials in flexible and wearable electronics.

Constructing high-capacitance electrochemical capacitors through the introduction of V ions into MoS2/Ni3S2 nanosheets

Designing the electrode materials with high energy density is crucial to improve the total performance of supercapacitors. In this work, we report V-doped MoS2/Ni3S2 nanosheets grown on nickel foam by a convenient hydrothermal route. Due to the large specific surface area and rapid ion migration rate, the obtained 0.07V–MoS2/Ni3S2 composites deliver a specific capacitance of 960C/g at 1 A/g. After cycling 10,000 times, they still remain at 71.65% of the initial capacity. Several asymmetric supercapacitors are assembled using the above samples as cathode materials. They achieve an energy density of 75 Wh/kg at a power density of 2700 W/kg. This work provides a feasible strategy to construct high-performance transition metal compound electrode materials in future flexible energy storage electronics.

Green Flexible Electronics: Natural Materials, Fabrication, and Applications.

The emergence of plastic electronics satisfies the increasing demand for flexible electronics. However, it has caused severe ecological problems. Flexible electronics based on natural materials are increasing to hopefully realize the "green" and eco-friendly concept. Herein, recent advances in the design and fabrication of green flexible electronics are reviewed. First, this review comprehensively introduces various natural materials and derivatives, focusing particularly on fibroin and silk, wood and paper, plants, and biomass. Second, fabrication techniques for modifying natural materials, including physical and chemical methods, are presented, after which their merits and demerits are thoroughly discussed. Green flexible electronics based on natural materials, comprising electrical wires/electrodes, antennas, thermal management devices, transistors, memristors, sensors, energy-harvesting devices, energy-storage devices, displays, actuators, electromagnetic shielding, and integration systems, are described in detail. Finally, perspectives on the existing challenges and opportunities to employ natural materials in green flexible electronics are presented.

Matryoshka-inspired continuous assembly of flexible silicon microribbons and photodetectors via selective transfer printing

Silicon microribbon (SiMR), as an intermediate structure format between silicon nanowire and silicon nanomembrane, is of great interest to serve as the functional material for flexible electronics, which represents an unusual electronic platform with widespread applications and promising future. However, the scalable production of SiMRs and/or devices on flexible substrates, in a deterministic assembly manner, still faces significant challenges. Herein, inspired by the matryoshka doll, referring to a set of dolls with decreased size and placed one inside another, we develop a universal and convenient approach to continuously and deterministically assemble flexible SiMRs and electronic devices through the selective transfer printing technique. By optimizing and repeating the selective transfer printing, continuous assembly of flexible SiMRs and devices are realized. Both theoretical calculation and simulation are performed to analyze the influence of the utilized viscoelastic stamp on the adhesive energy with SiMR, thus providing guidance for optimizing the design of stamp microstructure and enhancing the transfer efficiency of SiMRs. Finally, matryoshka-like flexible photodetectors, as an example, are fabricated by utilizing the sequentially prepared SiMRs via the proposed approach. This matryoshka-inspired assembly approach can be extended to prepare other functional semiconducting microribbons or devices in high controllability and yield, thus implying a promising future in the field of flexible electronics.

Ultrathin Crystalline Silicon Nano and Micro Membranes with High Areal Density for Low-Cost Flexible Electronics.

Ultrathin crystalline silicon is widely used as an active material for high-performance, flexible, and stretchable electronics, from simple passive and active components to complex integrated circuits, due to its excellent electrical and mechanical properties. However, in contrast to conventional silicon wafer-based devices, ultrathin crystalline silicon-based electronics require an expensive and rather complicated fabrication process. Although silicon-on-insulator (SOI) wafers are commonly used to obtain a single layer of crystalline silicon, they are costly and difficult to process. Therefore, as an alternative to SOI wafers-based thin layers, here, a simple transfer method is proposed for printing ultrathin multiple crystalline silicon sheets with thicknesses between 300nm to 13µm and high areal density (>90%) from a single mother wafer. Theoretically, the silicon nano/micro membrane can be generated until the mother wafer is completely consumed. In addition, the electronic applications of silicon membranes are successfully demonstrated through the fabrication of a flexible solar cell and flexible NMOS transistor arrays.

Cuttlefish‐Inspired Self‐Adaptive Liquid Metal Network Enabling Electromagnetic Interference Shielding and Thermal Management

AbstractWith the refinement and miniaturization of integrated electronic devices, the requirements for stable operating temperature and antielectromagnetic interference are increasingly high. However, most of the current materials necessitate to develop the intelligent self‐adaptive regulation in response to external field stimuli to gratify the stipulations of different operating environments. Herein, cuttlefish‐inspired smart liquid metal based‐liquid crystal elastomer (LGN‐LCE) dual‐functional material enabling self‐adaptive electromagnetic wave interference shielding and thermal management is presented. Liquid crystal elastomer matrix endows the LGN‐LCE dynamic self‐morphing properties under thermal activation, which leads to the liquid metal network with tunable thermal/electrical conduction. The thermal conductivity of LGN‐LCE can be increased to 10.3 W m−1 K−1 with the rise of surrounding temperature, while the electrical conductivity of LGN‐LCE can be enhanced to 4.3 × 105 S m−1. Such promoted electrical conductivity helps strengthen electromagnetic interference (EMI) shielding performance of LGN‐LCE, and the minimum EMI shielding effectiveness of LGN‐LCE can be improved from 48 to 62 dB within the X‐band. This work may not only pave a new way to rationally design a self‐adaptive EMI shielding and thermal management systems, but also show good prospects for practical applications, including thermal management materials, EMI materials, flexible electronic materials, smart materials, artificial intelligence systems, biomedical, and aerospace applications.

Radiation synthesis of rapidly self-healing, durable, and flexible poly(ionic liquid)/MXene gels with anti-freezing property for multi-functional strain sensors

As a new type of flexible electronic materials, conductive gels have attracted considerable interest because of their distinctive physical and chemical properties. In this work, the ILs with -C=C- double bond function as monomer and crosslinker, respectively; while the two-dimensional (2D) transition metal carbides/nitrides (Ti3C2Tx MXene) functioned simultaneously as a “crosslinker” and nanofiller, then a series of new poly(ionic liquid)/MXene (Cn-PIL/MXene) gels with stable covalent and non-covalent cross-linking structures based on n-alkyl-imidazolium (n = 2, 8) ionic liquid and MXene are fabricated by a one-step radiation method. The results reveal that the gel containing shorter alkyl (C2-PIL/MXene) is more suitable for flexible materials. Additionally, the C2-PIL/MXene gel exhibits remarkable stretchability (nearly 600%), good conductivity (4.9 mS cm−1), rapidly self-healing ability (within 8 min), excellent adhesion (266.9 kPa), and a fast response time (within 100 ms) due to the stable covalent bonding and abundant hydrogen bonding in the gel network. Notably, the C2-PIL/MXene gel also shows an excellent anti-freezing property (−60 °C), excellent durability (90 days at room temperature), and outstanding antibacterial activity against E. coli (99.82%) and S. aureus (99.98%). In addition, the multi-functional sensor based on the C2-PIL/MXene gel exhibits a real-time and rapid response to detect various human activities, recognize writing strokes, and realize the encrypted transmission of information, thus showing considerable potential in flexible smart wearable devices.

Novel Lignin Hydrogel Sensors with Antiswelling, Antifreezing, and Anticreep Properties

Hydrogel shows great potential as a flexible wearable electronic device. However, the practical application of hydrogel is still significantly limited due to the poor functional stability caused by swelling behavior, non-frost resistance caused by high water content, and obvious creep behavior under repeated external force. In this work, macromolecular lignin with a three-dimensional network structure, and active functional groups are introduced into the hydrogel through the esterification grafting reaction of methacryloyl chloride. The introduction of lignin effectively improves the antiswelling, antifreezing, and creep resistance of the hydrogel sensor, while maintaining the mechanical properties (elongation at break > 350%, tensile strength > 1.5 MPa) and electrical conductivity (10 S/m). Under extreme environments, the toughness, tensile strength, and elongation at break of the hydrogel remain at more than 96%. Furthermore, the antifreezing, creep resistance, and conductivity of hydrogel sensors are not significantly affected after immersion in water for a long time (72 h). This work proposes a simple strategy to improve the antiswelling, antifreezing, and anticreep properties of hydrogels, which has guiding significance for the preparation of high-performance flexible wearable electronic materials.

Pressure Sensors Painted on Flexible Cellulose Substrates from Polyaniline-Based Conductive Ink

Electrically conductive inks based on polyaniline (PANI) have been prepared using phytic acid as a cross-linking agent. The resistive response of these inks at low vacuum pressure has been reported in this article. The polyaniline phytic acid (PPA) ink is mixed with SnO2 and graphite flakes to form tin oxide-incorporated polyaniline phytic acid (PPAS) and graphite-incorporated polyaniline phytic acid (PPAG) ink systems, respectively. The inks have been used to paint on cellulose substrates to obtain flexible electronic materials. X-ray diffractometry measurements confirm that the prepared ink systems (PPA, PPAS, and PPAG) are crystalline in nature. Surface morphology and molecular structures are confirmed by scanning electron microscopy, UV–vis–NIR spectroscopy, and Fourier transform infrared spectroscopy, which reveal the formation of a 3D structure of the molecules, reasonably supported by phytic acid having six anionic phosphate groups per molecule. The mechanism governing the charge transport phenomenon has been discussed based on the electrical and optical properties of the prepared inks. The experimental results demonstrate that the flexible, use-and-throw vacuum pressure sensor based on cellulose substrates can be a promising candidate for chemical processing, gas delivery manifolds, and wearable circuitry in artificial intelligence.

Hydrogels for Flexible Electronics.

Hydrogels have emerged as promising materials for flexible electronics due to their unique properties, such as high water content, softness, and biocompatibility. In this perspective, we provide an overview of the development of hydrogels for flexible electronics, with a focus on three key aspects: mechanical properties, interfacial adhesion, and conductivity. We discuss the principles of designing high-performance hydrogels and present representative examples of their potential applications in the field of flexible electronics for healthcare. Despite significant progress, several challenges remain, including improving the antifatigue capability, enhancing interfacial adhesion, and balancing water content in wet environments. Additionally, we highlight the importance of considering the hydrogel-cell interactions and the dynamic properties of hydrogels in future research. Looking ahead, the future of hydrogels in flexible electronics is promising, with exciting opportunities on the horizon, but continued investment in research and development is necessary to overcome the remaining challenges.

Free-standing SnNb2O6@CSN film as flexible anode for high performance sodium-ion batteries

Free-standing electrodes are promising candidates for flexible rechargeable batteries, toward the application of flexible energy storage devices, due to their merits of additive-free, lightweight, and high energy density. Herein, we report a free-standing SnNb2O6@CSN flexible film with SnNb2O6 encapsulated in 3D carbon skeleton nanofibers by electrospinning and carbonization processes as flexible anode for sodium-ion batteries (SIBs). The 3D carbon skeleton nanofibers serve as ion/electron transport pathway to improve the electrochemical reaction kinetics and meanwhile alleviate the volume changes of SnNb2O6 during charge-discharge processes. The as-constructed half-cell (SnNb2O6@CSN‖Na) exhibits excellent cycling stability of 99.2 mAh/g at 0.5 A/g after 950 cycles (coulombic efficiency of ∼100%) and a high rate performance of 108.6 mAh/g at 10 A/g. In addition, the pouch cell can light up the LEDs at different bending angles (0°, 90°, 180°). This research shows a promising anode material for flexible energy storage electronics.