High-performance Flexible Electronic Devices Research Articles

Direct-Write Printed Slippery Surface for Assembling a High-Quality Graphene Structure and Its Application in Flexible Electric Actuators.

Graphene is a two-dimensional honeycomb-like nanomaterial generated by carbon atoms in sp2 hybridized orbitals to form a hexagonal lattice structure with excellent electrical, optical, and mechanical properties. The solution process method has been widely used to realize large-area patterned graphene structures for high-performance devices. In the method, graphene usually needs to be dispersed in solution, and the π-π bonding gravitational interactions between graphene sheets would lead to uncontrollable structures in solution and difficulty in obtaining high performance. In this work, a patterned graphene oxide (GO) structure with controllable thickness and layer spacing was realized by a direct-write printed slippery surface, which was used as a slippery limited template. After reducing GO into reduced graphene oxide (rGO), a flexible electric pattern with a conductivity of up to 6.425 × 103 S/m was realized. Furthermore, the patterned rGO structure was transferred on polydimethylsiloxane (PDMS), which could generate less than a 5% change in resistance after 10,000 consecutive bends, and an anisotropic expansion based on rGO and PDMS materials under electro-thermal coupling. The patterned rGO structures could meet the performance requirements of highly sensitive and complex deformation applications as flexible electric actuators. This study provides great research significance and application value for patterning high-quality graphene structures and realizing high-performance flexible electronic devices.

Stable Flexible Electronic Devices under Harsh Conditions Enabled by Double-Network Hydrogels Containing Binary Cations.

Hydrogels are increasingly used in flexible electronic devices, but the mechanical and electrochemical stabilities of hydrogel devices are often limited under specific harsh conditions. Herein, chemically/physically cross-linked double-network (DN) hydrogels containing binary cations Zn2+ and Li+ are constructed in order to address the above challenges. Double networks of chemically cross-linked polyacrylamide (PAM) and physically cross-linked κ-Carrageenan (κ-CG) are designed to account for the mechanical robustness while binary cations endow the hydrogels with excellent ionic conductivity and outstanding environmental adaptability. Excellent mechanical robustness and ionic conductivity (25 °C, 2.26 S·m-1; -25 °C, 1.54 S·m-1) have been achieved. Utilizing the DN hydrogels containing binary cations as signal-converting materials, we fabricated flexible mechanosensors. High gauge factors (resistive strain sensors, 2.4; capacitive pressure sensors, 0.82 kPa-1) and highly stable sensing ability have been achieved. Interestingly, zinc-ion hybrid supercapacitors containing the DN hydrogels containing binary cations as electrolytes have achieved an initial capacity of 52.5 mAh·g-1 at a current density of 3 A·g-1 and a capacity retention rate of 82.9% after 19,000 cycles. Proper working of the zinc-ion hybrid supercapacitors at subzero conditions and stable charge-discharge for more than 19,000 cycles at -25 °C have been demonstrated. Overall, DN hydrogels containing binary cations have provided promising materials for high-performance flexible electronic devices under harsh conditions.

An Environment-Tolerant Ion-Conducting Double-Network Composite Hydrogel for High-Performance Flexible Electronic Devices

HighlightsNovel double-network (DN) ion-conducting hydrogel (ICH) based on a poly(ionic liquid)/MXene/poly(vinyl alcohol) system (named PMP DN ICH) was synthesized using freeze–thawing and ionizing radiation technology.The PMP DN ICH possesses a multiple cross-linking mechanism and exhibits outstanding ionic conductivity (63.89 mS cm−1), excellent temperature resistance (−60–80 °C) and decent mechanical performance.The well-designed PMP DN ICH shows considerable potential in wearable sensing, energy storage, and energy harvesting.

Enhancing the cycling stability of Li-S batteries with flexible freestanding sulfur cathodes through a spider-hunting strategy

Flexible, high-capacity, and long-lasting freestanding sulfur cathodes are essential for developing advanced lithium-sulfur (Li-S) batteries in next-generation flexible electronic devices. However, the severe shuttle effect of lithium polysulfides (LiPSs) hinders their potential applications. This work introduces a flexible freestanding MX-CPNTs-CNTs@NS cathode for Li-S batteries by utilizing a spider-hunting strategy. In this strategy, carbonized polypyrrole nanotubes (CPNTs) and carbon nanotubes (CNTs) intertwine to form a 3D flexible and conductive spiderweb-like skeleton. The incorporation of MXene nanosheets as a function of spiders enables the regulation of a rich hierarchical porous structure. Additionally, the polar terminal functional groups of MXene nanosheets and the pyridinic N of CPNTs contribute to the chemical adsorption and electrocatalysis of LiPSs. The synergistic effect of physical and chemical inhibition strategies results in stable electrochemical performance, including a high specific capacity of 1203.2 mAh/g at 0.1C, and extended cycling stability of 739.6 mAh/g at 1C after 500 cycles (low decay rate of 0.036%). The excellent flexibility, specific capacity, and cycling stability make the freestanding MX-CPNTs-CNTs@NS cathode promising for high-performance flexible electronic devices.

Nano-scale charge trapping memory based on two-dimensional conjugated microporous polymer

There is a growing interest in new semiconductor nanostructures for future high-density high-performance flexible electronic devices. Two-dimensional conjugated microporous polymers (2D-CMPs) are promising candidates because of their inherent optoelectronic properties. Here, we are reporting a novel donor–acceptor type 2D-CMP based on Pyrene and Isoindigo (PI) for a potential nano-scale charge-trapping memory application. We exfoliated the PI polymer into ~ 2.5 nm thick nanoparticles (NPs) and fabricated a Metal–Insulator–Semiconductor (MIS) device with PI–NPs embedded in the insulator. Conductive AFM (cAFM) is used to examine the confinement mechanism as well as the local charge injection process, where ultrathin high-κ alumina supplied the energy barrier for confining the charge carrier transport. We have achieved a reproducible on-and-off state and a wide memory window (ΔV) of 1.5 V at a relatively small reading current. The device displays a low operation voltage (V < 1 V), with good retention (104 s), and endurance (103 cycles). Furthermore, a theoretical analysis is developed to affirm the measured charge carriers’ transport and entrapment mechanisms through and within the fabricated MIS structures. The PI–NPs act as a nanoscale floating gate in the MIS-based memory with deep trapping sites for the charged carriers. Moreover, our results demonstrate that the synthesized 2D-CMP can be promising for future low-power high-density memory applications.

Iron-doped nickel–cobalt bimetallic phosphide nanowire hybrids for solid-state supercapacitors with excellent electromagnetic interference shielding

The development of flexible and wearable electronics subjects to the limited energy density and accompanying electromagnetic pollution. With a high theoretical specific capacity, nickel–cobalt bimetallic phosphide (NiCoP) is considered to be potential cathode materials for supercapacitor. However, the pristine NiCoP fails to display excellent electrochemical performance due to its inferior rate performance and cycling stability. Herein, we design Fe doped NiCoP nanowire arrays on carbon cloth (Fe-NiCoP/CC) as the cathode for supercapacitors. The introduced Fe doping enable to increase in the electronic conductivity and enhance the adsorption of OH−, supported by the density functional theory (DFT) analysis. As a result, Fe-NiCoP/CC electrode displays a high areal capacity of 3.18 F cm−2 at 1 mA cm−2, superb rate capability (86.3 % capacity retention at 20 mA cm−2) and outstanding structure stability, superior to the NiCo/CC, FeNiCo/CC, and NiCoP/CC counterparts. Moreover, the assembled Fe-NiCoP/CC||VN/CNT/CC hybrid supercapacitor (HSC) device delivers a high energy density of 176.9 μWh cm−2 at the power density of 750 μW cm−2. More importantly, the designed electrodes and assembled HSC device exhibits excellent electromagnetic interference (EMI) shielding performance. This design concept presented in this paper can provide insights into the construction of multifunctional and high-performance flexible electronic devices.

Partially fluorinated metallic surface constructing robust, thermally conductive and electrically insulating polymer composite

Metallic fillers have shown limited potential for developing advanced polymer substrates for flexible electronics due to their electrical conductivity and poor interfacial interaction with polymers, despite their excellent thermal conductivity (λ). Here, metallic aluminum powders (APs) after surface modification were selected as thermally conductive fillers to modify polyimide (PI) films. We constructed a partially fluorinated alumina layer (PFAL) on the APs surface by employing direct fluorination of F2/N2 mixture. It was demonstrated that the new core-shell structure equipped the fluorinated APs (FAPs) with much better dispersibility and stronger interaction with polyamide acids (PAAs) and PIs bulk, in contrast to APs. Even when the loading of FAPs was up to 50 wt%, the tensile strength of PI/FAPs still remained at a high value of 87.2 MPa. Moreover, the in-plane λ (λ∥) and out-of-plane λ (λ⊥) at room temperature reached 16.83 W/(m·K) and 1.42 W/(m·K), respectively. Both theoretical simulations and experiments demonstrated that the positive effect was due to the strong interaction between the electron-deficient imide ring in PI macromolecules and the electro-negative F atom in the PFAL shell. Overall, this study provides a promising strategy for enhancing the interfacial interaction between metallic fillers and polymers for the development of high-performance flexible electronic devices.

The development of stretchable and self-repairing materials applied to electronic skin.

Flexible electronic devices play a key role in the fields of flexible batteries, electronic skins, and flexible displays, which have attracted more and more attention in the past few years. Among them, the application areas of electronic skin in new energy, artificial intelligence, and other high-tech applications are increasing. Semiconductors are an indispensable part of electronic skin components. The design of semiconductor structure not only needs to maintain good carrier mobility, but also considers extensibility and self-healing capability, which is always a challenging work. Though flexible electronic devices are important for our daily life, the research on this topic is quite rare in the past few years. In this work, the recently published work regarding to stretchable semiconductors as well as self-healing conductors are reviewed. In addition, the current shortcomings, future challenges as well as an outlook of this technology are discussed. The final goal is to outline a theoretical framework for the design of high-performance flexible electronic devices that can at the same time address their commercialization challenges.

Dewetting-Induced Hierarchical Self-Assembly of Block Copolymers Templated by Colloidal Crystals.

Recent advances in high-performance flexible electronic devices have increased the demand for more diverse and complex nanofabrication methods; high-resolution, high-efficiency, and low-cost patterning strategies for next-generation devices are therefore required. In this study, we demonstrate the formation of dewetting-induced hierarchical patterns using two self-assembled materials: block copolymers (BCPs) and colloidal crystals. The combination of the two self-assembly methods successfully generates multiscale hierarchical patterns because the length scales of the periodic colloidal crystal structures are suitable for templating the BCP patterns. Various concentric ring patterns were observed on the templated BCP films, and a free energy model of the polymer chain was applied to explain the formation of these patterns relative to the template width. Frequently occurring spiral-defective features were also examined and found to be promoted by Y-junction defects.

Optimized chemical mechanical polishing of Parylene C for high-density wiring in flexible electronics

With the rapid development of flexible electronics, an increasing number of microfabrication strategies originating from the Si-based integrated circuits field have been explored on organic materials. Parylene C, a polymer, has been widely used in the microelectromechanical systems field because of its outstanding fabrication merits, such as room-temperature processability, conformal coating, and precise thin film deposition capability with the thickness tunable from 1 nm to 100 μm. As a good dielectric material, the Parylene C is also suitable for interlayer dielectrics in flexible electronics. This study develops an optimized chemical mechanical polishing (CMP) technique of Parylene C for high-density redistribution wiring in high-performance flexible electronics. The roughness of the Parylene C surface after CMP was as low as 14.3 ± 1.5 Å. The problems of slurry pollution and mechanical failure of the Parylene film that degrade the dielectric performance of the Parylene C could be avoided by taking the optimized CMP method. The multi-material structure constructed by Parylene C, Silicon and electroplated Copper was prepared and polished using the optimized CMP process. Additionally, a flexible wiring sample has been successfully patterned by the Damascene process through the optimized CMP process. In this sample, both the distance between each wire and the wire width were as small as 5 μm. The optimized Parylene C CMP process is easy-to-realize, highly efficient, low cost, and with minor defects; it provides a promising way to achieve high-density interconnection in high-performance flexible electronic devices.

Fermi-Level Modulation of Chemical Vapor Deposition-Grown Monolayer Graphene via Nanoparticles to Macromolecular Dopants.

It is critical to modulate the Fermi level of graphene for the development of high-performance electronic and optoelectronic devices. Here, we have demonstrated the modulation of the Fermi level of chemical vapor deposition (CVD)-grown monolayer graphene (MLG) via doping with nanoparticles to macromolecules such as titanium dioxide nanoparticles (TiO2 NPs), nitric acid (HNO3), octadecyltrimethoxysilane (OTS) self-assembled monolayer (SAM), and poly(3,4-ethylene-dioxythiophene):polystyrene sulfonate (PEDOT:PSS). The electronic properties of pristine and doped graphene samples were investigated by Raman spectroscopy and electrical transport measurements. The right shifting of G and 2D peaks and reduction in 2D to G peak intensity ratio (I2D/IG) assured that the dopants induced a p-type doping effect. Upon doping, the shifting of the Dirac point towards positive voltage validates the increment of the hole concentration in graphene and thus downward shift of the Fermi level. More importantly, the combination of HNO3/TiO2 NP doping on graphene yields a substantially larger change in the Fermi level of MLG. Our study may be useful for the development of graphene-based high-performance flexible electronic devices.

Recent Advances in Natural Functional Biopolymers and Their Applications of Electronic Skins and Flexible Strain Sensors.

In order to replace nonrenewable resources and decrease electronic waste disposal, there is a rapidly rising demand for the utilization of reproducible and degradable biopolymers in flexible electronics. Natural biopolymers have many remarkable characteristics, including light weight, excellent mechanical properties, biocompatibility, non-toxicity, low cost, etc. Thanks to these superior merits, natural functional biopolymers can be designed and optimized for the development of high-performance flexible electronic devices. Herein, we provide an insightful overview of the unique structures, properties and applications of biopolymers for electronic skins (e-skins) and flexible strain sensors. The relationships between properties and sensing performances of biopolymers-based sensors are also investigated. The functional design strategies and fabrication technologies for biopolymers-based flexible sensors are proposed. Furthermore, the research progresses of biopolymers-based sensors with various functions are described in detail. Finally, we provide some useful viewpoints and future prospects of developing biopolymers-based flexible sensors.

Fully-printed flexible n-type tin oxide thin-film transistors and logic circuits

We achieved fully-printed SnO2 based n-type thin-film transistors with high electrical performance and robust mechanical flexibility, promising for constructing low-cost, high-performance flexible electronic devices and circuits.

Direct fabrication of high-resolution and high-performance flexible electronics via surface-activation-localized electroless plating

Patterning high-resolution multi-metal layers without using subtractive lithography poses a substantial challenge but is indispensable for the development of the modern electronics industry, especially flexible electronics. Herein, the general and feasible additive manufacturing approach is reported to selectively deposit high-resolution multi-metal patterns via surface-activation-localized electroless plating (SALEP) and enable the fabrication of 3-dimensional (3D) interconnections and high-performance organic thin-film transistor (OTFT) arrays for flexible electronics. The SALEP approach comprises the direct layer-by-layer deposition of multi-metal patterns interconnected by via holes, which are defined by the selective adsorption of palladium catalysts and a metallization process. The localized adsorption of the catalyst is guided by the difference in surface wettability and adsorbability caused by vacuum ultraviolet (VUV)-induced photochemical modification. The low-temperature and 3D approach enables miniaturization of flexible electrical circuits down to 1 µm in width; it also enables integration of an OTFT array on a flexible substrate. The fabricated OTFT array with multi-metal Cu/Ni/Au as contact electrodes exhibits a high hole mobility exceeding 10 cm2 V−1 s−1, demonstrating the adaptability of the SALEP approach for the low-cost, high-efficiency, and scalable fabrication of high-performance flexible electronic devices.

Low-Voltage, High-Performance Flexible Organic Field-Effect Transistors Based on Ultrathin Single-Crystal Microribbons

Organic field-effect transistors (OFETs) have acquired increasing attention because of their wide range of potential applications in electronics; nevertheless, high operating voltage and low carrier mobility are considered as major bottlenecks in their commercialization. In this work, we demonstrate low-voltage, flexible OFETs based on ultrathin single-crystal microribbons. Flexible OFETs fabricated with 2,7-dioctylbenzothieno[3,2-b]benzothiophene (C8-BTBT) based solution-processed ultrathin single-crystal microribbon as the semiconductor layer and high-k polymer, polysiloxane-poly(vinyl alcohol) composite as an insulator layer manifest a significantly low operating voltage of -4 V, and several devices showed a high mobility of >30 cm2 V-1 s-1. Besides, the carrier mobility of the fabricated devices exhibits a slight degradation in static bending condition, which can be retained by 83.3% compared with its original value under a bending radius of 9 mm. As compared to the bulk C8-BTBT single-crystal-based OFET, which showed a large crack only after 50 dynamic bending cycles, our ultrathin single-crystal-based counterpart demonstrates a much better dynamic force stability. Moreover, under a 20 mm bending radius, the mobility of the device decreased by only 11.7% even after 500 bending cycles and no further decrease was observed until 1000 bending cycles. Our findings reveal that ultrathin C8-BTBT single-crystal-based flexible OFETs are promising candidates for various high-performance flexible electronic devices.

Solution-Processed DyO x for Aging Diffusion ZnSnO Transistors and Applications in Low-Voltage-Operating Logic Circuits

Metal–oxide high- ${k}$ material is regarded as the promising building block for large area high-performance flexible electronic devices. Here, we report that dysprosium oxide (DyO x ) films were successfully integrated as dielectrics into thin-film transistors (TFTs) for the first time. Solution-driven DyO x films behave high transmittance over 85% and imply the possible application in transparent devices. Measurements based on 500 °C annealing ZnSnO/DyO x TFTs have indicated the dramatic improvement in electrical performance after natural aging diffusion for 10 days, containing the increased saturation mobility value from 0.57 to 2.5 cm $^{{2}}\cdot \text{V}^{-{1}}\cdot \text{s}^{-{1}}$ and the lifted current ON/ OFF ratio from $7.8\times 10^{{4}}$ to $2.4\times 10^{{6}}$ . It is worth mentioning that the mobility of aging samples with the lower annealing temperature of 420 °C even surpasses the aforementioned 500 °C annealing without aging TFTs. The variable-range-hopping (VRH) percolation model and coherent interface theory are used to explore electronic transport mechanism for amorphous and polycrystalline ZnSnO films, respectively. Finally, the resistor-loaded inverter has been fabricated and revealed the gain of 7.3 at the low voltage of 2.5 V, which even surpasses many In-based TFT devices. It can be concluded that the solution-based samples can proceed with 10 days aging treatment before encapsulation to obtain the stable electrical performance, which affords a simple and effective approach toward building low-temperature, low power consumption transparent electronic devices.

Conductive Hydrogels as Smart Materials for Flexible Electronic Devices.

Flexible conductive materials have gained considerable research interest in recent years because of their potential applications in flexible energy storage devices, sensors, touch panels, electronic skins, etc. With excellent flexibility, outstanding electric properties and tunable mechanical properties, conductive hydrogels as conductive materials offer plentiful insights and opportunities for flexible electronic devices. Numerous synthetic strategies have been developed to obtain various conductive hydrogels, and high-performance flexible electronic devices based on these conductive hydrogels have been realized. This review provides a comprehensive overview of conductive-hydrogel-based flexible electronics, ranging from conductive hydrogels synthesis to several important flexible devices applications, including touch panels, sensors and energy storage. Finally, we provide new future research directions and perspectives for conductive-hydrogel-based flexible and portable electronic devices.

Control of Molecular Ordering, Alignment, and Charge Transport in Solution-Processed Conjugated Polymer Thin Films.

Morphology of conjugated polymers is a critical factor that significantly affects intrinsic charge transport characteristics and in turn performance of polymer-based devices. Morphological defects including misaligned crystalline grains and grain boundaries significantly impede efficient charge hopping between transport sites, resulting in degradation of device performance. Therefore, one important challenge is to control morphology of active polymer thin-films for achieving high performance flexible electronic devices. In the past decade, significant progress has been achieved in morphology control of conjugated polymer thin-films using solution-based processing techniques. This review focuses on recent advances in processing strategies that can tune the morphologies and thus impact charge transport properties of conjugated polymer thin films. Of the available processing strategies, polymer solution treatments and film deposition techniques will be mainly highlighted. The correlation between processing conditions, active layer morphologies, and device performance will be also be discussed.

Highly conductive and flexible molybdenum oxide nanopaper for high volumetric supercapacitor electrode

Paper-like electrodes with high conductivity and flexibility hold great potential for assembling high-performance flexible electronic devices.

Stretchable composite electrode based on carbon network with interwoven structure for flexible supercapacitors

A stretchable carbon network (CNW), modified with NiCo2O4 (NCO) nanorod arrays, is designed as a composite electrode for flexible supercapacitors. Cotton textile with interwoven structure is used as starting material to produce the conductive CNW by a thermal reduction process. NCO nanorod arrays are then successfully deposited on the surface of the CNW through a hydrothermal method to obtain the CNW/NCO composite electrode. The CNW/NCO electrode exhibits favorable mechanical strength, large capacitance (1402.7 F g−1 in 6 M KOH), and good stability. The composite electrode showed almost the same electrochemical performance under stretching mode. These features make the designed material attractive for applications in high-performance flexible electronic devices.