Junction‐Free Electrospun Ag Fiber Electrodes for Flexible Organic Light‐Emitting Diodes

Fabrication of junction‐free Ag fiber electrodes for flexible organic light‐emitting diodes (OLEDs) is demonstrated. The junction‐free Ag fiber electrodes are fabricated by electrospun polymer fibers used as an etch mask and wet etching of Ag thin film. This process facilitates surface roughness control, which is important in transparent electrodes based on metal wires to prevent electrical instability of the OLEDs. The transmittance and resistance of Ag fiber electrodes can be independently adjusted by controlling spinning time and Ag deposition thickness. The Ag fiber electrode shows a transmittance of 91.8% (at 550 nm) at a sheet resistance of 22.3 Ω □^?1, leading to the highest OLED efficiency. In addition, Ag fiber electrodes exhibit excellent mechanical durability, as shown by measuring the change in resistance under repeatable mechanical bending and various bending radii. The OLEDs with Ag fiber electrodes on a flexible substrate are successfully fabricated, and the OLEDs show an enhancement of EQE (≈19%) compared to commercial indium tin oxide electrodes.     

Highly Efficient (>10%) Flexible Organic Solar Cells on PEDOT‐Free and ITO‐Free Transparent Electrodes

A novel approach to fabricate flexible organic solar cells is proposed without indium tin oxide (ITO) and poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) using junction‐free metal nanonetworks (NNs) as transparent electrodes. The metal NNs are monolithically etched using nanoscale shadow masks, and they exhibit excellent optoelectronic performance. Furthermore, the optoelectrical properties of the NNs can be controlled by both the initial metal layer thickness and NN density. Hence, with an extremely thin silver layer, the appropriate density control of the networks can lead to high transmittance and low sheet resistance. Such NNs can be utilized for thin‐film devices without planarization by conductive materials such as PEDOT:PSS. A highly efficient flexible organic solar cell with a power conversion efficiency (PCE) of 10.6% and high device yield (93.8%) is fabricated on PEDOT‐free and ITO‐free transparent electrodes. Furthermore, the flexible solar cell retains 94.3% of the initial PCE even after 3000 bending stress tests (strain: 3.13%).     

Nanofiber‐Reinforced Silver Nanowires Network as a Robust,Ultrathin, and Conformable Epidermal Electrode for Ambulatory Monitoring of Physiological Signals

Extremely soft and thin electrodes with high skin conformability have potential applications in wearable devices for personal healthcare. Here, a submicrometer thick, highly robust, and conformable nanonetwork epidermal electrode (NEE) is reported. Electrospinning of polyamide nanofibers and electrospraying of silver nanowires are simultaneously performed to form a homogeneously convoluted network in a nonwoven way. For a 125 nm thick NEE, a low sheet resistance of ≈4 Ω sq^?1 with an optical transmittance of ≈82% is achieved. Due to the nanofiber‐based scaffold that undertakes most of the stress during deformation, the electric resistance of the NEE shows very little variation; less than 1.2% after 50 000 bending cycles. The NEE can form a fully conformal contact to human skin without additional adhesives, and the NEE shows a contact impedance that is over 50% lower than what is found in commercial gel electrodes. Due to conformal contact even under deformation, the NEE proves to be a stable, robust, and comfortable approach for measuring electrocardiogram signals, especially when a subject is in motion. These features make the NEE promising for use in the ambulatory measurement of physiological signals for healthcare applications.     

Cryo‐Transferred Ultrathin and Stretchable Epidermal Electrodes

A simple cryo‐transfer method to fabricate ultrathin, stretchable, and conformal epidermal electrodes based on a combination of silver nanowires (AgNWs) network and elastomeric polymers is developed. This method can temporarily enable the soft elastomers with much higher elastic modulus and dimensional contraction through exploiting their glass‐transition behaviors. During this process, a much higher Von Mises stress can be loaded on AgNWs than usual, and the generated strong grip force can facilitate the complete transfer of AgNWs. Afterward, the thawed AgNWs and elastomer composites quickly recover to their soft state at room temperature. The obtained ultrathin and soft electrode with a thickness of 8.4 µm and transmittance of 90.8% at a sheet resistance of 13.2 Ω sq^?1 can tolerate a stretching strain of 70% and 50 000 repeated bending cycles, which meets rigorous requirements of epidermal applications. The as‐prepared epidermal electrodes are effective and comfortable for electrophysiological signal monitoring, and while showing excellent performance exceeding the commercialized gel electrodes.     

Waterproof,Ultrahigh Areal‐Capacitance,Wearable Supercapacitor Fabrics

High‐performance supercapacitors (SCs) are promising energy storage devices to meet the pressing demand for future wearable applications. Because the surface area of a human body is limited to 2 m², the key challenge in this field is how to realize a high areal capacitance for SCs, while achieving rapid charging, good capacitive retention, flexibility, and waterproofing. To address this challenge, low‐cost materials are used including multiwall carbon nanotube (MWCNT), reduced graphene oxide (RGO), and metallic textiles to fabricate composite fabric electrodes, in which MWCNT and RGO are alternatively vacuum‐filtrated directly onto Ni‐coated cotton fabrics. The composite fabric electrodes display typical electrical double layer capacitor behavior, and reach an ultrahigh areal capacitance up to 6.2 F cm^?2 at a high areal current density of 20 mA cm^?2. All‐solid‐state fabric‐type SC devices made with the composite fabric electrodes and water‐repellent treatment can reach record‐breaking performance of 2.7 F cm^?2 at 20 mA cm^?2 at the first charge–discharge cycle, 3.2 F cm^?2 after 10 000 charge–discharge cycles, zero capacitive decay after 10 000 bending tests, and 10 h continuous underwater operation. The SC devices are easy to assemble into tandem structures and integrate into garments by simple sewing.     

Reverse‐Offset Printed Ultrathin Ag Mesh for Robust Conformal Transparent Electrodes for High‐Performance Organic Photovoltaics

Mechanically durable transparent electrodes are needed in flexible optoelectronic devices to realize their long‐term stable functioning, for applications in various fields such as energy, healthcare, and soft robotics. Several promising transparent electrodes based on nanomaterials have been previously reported to replace the conventional and fragile indium‐tin oxide (ITO); however, obtaining feasible printed transparent electrodes for ultraflexible devices with a multistack structure is still a great challenge. Here, a printed ultrathin (uniform thickness of 100 nm) Ag mesh transparent electrode is demonstrated, simultaneously achieving high conductance, high transparency, and good mechanical properties. It shows a 17 Ω sq^?1 sheet resistance (R_sh) with 93.2% transmittance, which surpasses the performance of sputtered ITO electrodes and other ultrathin Ag mesh transparent electrodes. The conductance is stable after 500 cycles of 100% stretch/release deformation, with an insignificant increase (10.6%) in R_sh by adopting a buckling structure. Furthermore, organic photovoltaics (OPVs) using our Ag mesh transparent electrodes achieve a power conversion efficiency of 8.3%, which is comparable to the performance of ITO‐based OPVs.     

Roll‐to‐Roll Production of Graphene Hybrid Electrodes for High‐Efficiency,Flexible Organic Photoelectronics

Despite nearly two decades of research, the absence of ideal, flexible, and transparent electrodes has been the biggest bottleneck for realizing flexible and printable electronics via roll‐to‐roll (R2R) method. A fabrication of poly(3,4‐ethylenedioxythiophene):poly(styrene sulfonate):graphene:ethyl cellulose (PEDOT:PSS:G:EC) hybrid electrodes by R2R process, which allows for the elimination of strong acid treatment. The high‐performance flexible printable electrode includes a transmittance (T) of 78% at 550 nm and a sheet resistance of 13 Ω sq⁻¹ with excellent mechanical stability. These features arise from the PSS interacting strongly with the ethyoxyl groups from EC promoting a favorable phase separation between PEDOT and PSS chains, and the highly uniform and conductive G:EC enable rearrangement of the PEDOT chains with more expanded conformation surrounded by G:EC via the π–π interaction between G:EC and PEDOT. The hybrid electrodes are fully functional as universal electrodes for outstanding flexible electronic applications. Organic solar cells based on the hybrid electrode exhibit a high power conversion efficiency of 9.4% with good universality for active layer. Moreover, the organic light‐emitting diodes and photodetector devices hold the same level to or outperform those based on indium tin oxide flexible transparent electrodes.     

“Quasi‐freestanding” Graphene‐on‐Single Walled Carbon Nanotube Electrode for Applications in Organic Light‐emitting Diode

An air‐stable transparent conductive film with “quasi‐freestanding” graphene supported on horizontal single walled carbon nanotubes (SWCNTs) arrays is fabricated. The sheet resistance of graphene films stacked via layer‐by‐layer transfer (LBL) on quartz, and modified by 1‐Pyrenebutyric acid N‐hydroxysuccinimide ester (PBASE), is reduced from 273 Ω/sq to about 76 Ω/sq. The electrical properties are stable to heat treatment (up to 200 ºC) and ambient exposure. Organic light‐emitting diodes (OLEDs) constructed of this carbon anode (T ≈ 89.13% at 550 nm) exhibit ≈88% power efficiency of OLEDs fabricated on an ITO anode (low turn on voltage ≈3.1 eV, high luminance up to ≈29 490 cd/m², current efficiency ≈14.7 cd/A). Most importantly, the entire graphene‐on‐SWCNT hybrid electrodes can be transferred onto plastic (PET) forming a highly‐flexible OLED device, which continues to function without degradation in performance at bending angles >60°.     

Transparent Flexible Heteroepitaxy of NiO Coated AZO Nanorods Arrays on Muscovites for Enhanced Energy Storage Application

Transparent flexible energy storage devices are considered as important chains in the next‐generation, which are able to store and supply energy for electronic devices. Here, aluminum‐doped zinc oxide (AZO) nanorods (NRs) and nickel oxide (NiO)‐coated AZO NRs on muscovites are fabricated by a radio frequency (RF) magnetron sputtering deposition method. Interestingly, AZO NRs and AZO/NiO NRs are excellent electrodes for energy storage application with high optical transparency, high conductivity, large surface area, stability under compressive and tensile strain down to a bending radius of 5 mm with 1000 bending cycles. The obtained symmetric solid‐state supercapacitors based on these electrodes exhibit good performance with a large areal specific capacitance of 3.4 mF cm^?2, long cycle life 1000 times, robust mechanical properties, and high chemical stability. Furthermore, an AZO/NiO//Zn battery based on these electrodes is demonstrated, yielding a discharge capacity of 195 mAh g^?1 at a current rate of 8 A g^?1 and a discharge capacity of over 1000 cycles with coulombic efficiency to 92%. These results deliver a concept of opening a new opportunity for future applications in transparent flexible energy storage.     

Easy‐to‐clean Schichten – spezielle Anwendungen

Easy to clean surfaces – special applications Easy to clean surfaces can be made by wet‐chemical coating with subsequent heat‐treatment. Organically modified metal oxide films form the base reinforced by nano composite structures. The hydro‐ and oleophobic effect is obtained by perfluorinated organic molecule chains in the nano composite sol‐gel coatings. Application specific materials can be synthesized by the proper choice of suitable starting compounds and process parameters. The resulting coatings consist of a three‐dimensional cross‐linked inorganic part (such as a silica network) combined with an organic part. The organic material acts either as a surface modifier (example: alkyl, phenyl) or as crosslinker (example: acrylic, epoxy). The properties of such coating systems can be adjusted to obtain a wide range of glass‐ceramic or polymer‐like properties. The incorporation of nanoparticles into these materials significantly enhances the abrasion and the scratch resistance. Such coatings mainly on metal parts are used in diagnostics, analytical chemistry and medical technology.     

Embedded Fin‐Like Metal/CNT Hybrid Structures for Flexible and Transparent Conductors

In this paper, an embedded fin‐like metal‐coated carbon nanotube (Fin‐M/CNT) structure is demonstrated for flexible and transparent conductor wire applications. Embedded in a polydimethylsiloxane polymeric substrate, Fin‐M/CNT wires with a minimum width of 5 μm and a minimum pitch of 10 μm have been achieved. Direct current resistances of single Fin‐M/CNT wires, where the supporting CNT structures have been covered by Ti/Al/Au metal coatings of different thicknesses, have been measured. The high aspect ratio of the fin‐like structures not only improves the adhesion between the wires and the polymeric substrate, but also yields a low resistance at a small surface footprint. In addition, transparent Fin‐M/CNT grid lines with hexagonal patterns, with a sheet resistance of as low as 45 Ω sq^?1, have been achieved at an optical transmittance of 88%. The robustness of the Fin‐M/CNT structures has been demonstrated in bending tests up to 500 cycles and no significant changes in wire resistances are observed.     

Pressure Welding of Silver Nanowires Networks at Room Temperature as Transparent Electrodes for Efficient Organic Light‐Emitting Diodes

In this work, polymethylmethacrylate (PMMA) as a superior mediate for the pressure welding of silver nanowires (Ag NWs) networks as transparent electrodes without any thermal treatment is demonstrated. After a pressing of 200 kg cm^?2, not only the sheet resistance but also the surface roughness of the PMMA‐mediated Ag NWs networks decreases from 2.6 kΩ sq^?1 to 34.3 Ω sq^?1 and from 76.1 to 12.6 nm, respectively. On the other hand, high transparency of an average transmittance in the visible wavelengths of 93.5% together with a low haze value of 2.58% can be achieved. In terms of optoelectronic applications, the promising potential of the PMMA‐mediated pressure‐welded Ag NWs networks used as a transparent electrode in a green organic light‐emitting diode (OLED) device is also demonstrated. In comparison with the OLED based on commercial tin‐doped indium oxide electrode, the increments of power efficiency and external quantum efficiency (EQE) from 80.1 to 85.9 lm w^?1 and 19.2% to 19.9% are demonstrated. In addition, the PMMA‐mediated pressure welding succeeds in transferring Ag NWs networks to flexible polyethylene naphthalate and polyimide substrates with the sheet resistance of 42 and 91 Ω sq^?1 after 10 000 times of bending, respectively.     

Single‐Crystalline,Metallic TiC Nanowires for Highly Robust and Wide‐Temperature Electrochemical Energy Storage

Customized electrode materials with good temperature adaptability and high‐rate capability are critical to the development of wide‐temperature power sources. Herein, high‐quality TiC nanowires are uniformly grown on flexible carbon cloth as free‐standing electric‐double‐layer supercapacitor electrode. The TiC nanowires, 20–40 nm wide and 3–6 µm long, are single‐crystalline and highly conductive that is close to typical metal. Symmetric supercapacitors are constructed with ionic liquid electrolyte and TiC nanowires electrodes as wide‐temperature and long‐cycle stable power source. Ultrastable high‐rate cycling life of TiC nanowire arrays electrodes is demonstrated with capacitance retention of 96.8% at 60 °C (≈440 F g^?1), 99% at 25 °C (≈400 F g^?1), and 98% at ?25 °C (≈240 F g^?1) after 50 000 cycles at 10 A g^?1. Moreover, due to high electrical conductivity, the TiC nanowire arrays show ultrafast energy release with a fast response time constant of ≈0.7 ms. The results demonstrate the viability of metal carbide nanostructures as wide‐temperature, robust electrode materials for high‐rate and ultrastable supercapacitors.     

Multilayer Stacked Low‐Temperature‐Reduced Graphene Oxide Films: Preparation,Characterization, and Application in Polymer Memory Devices

Highly reduced graphene oxide (rGO) films are fabricated by combining reduction with smeared hydrazine at low temperature (e.g., 100 °C) and the multilayer stacking technique. The prepared rGO film, which has a lower sheet resistance (≈160–500 Ω sq⁻¹) and higher conductivity (26 S cm⁻¹) as compared to other rGO films obtained by commonly used chemical reduction methods, is fully characterized. The effective reduction can be attributed to the large “effective reduction depth” in the GO films (1.46 µm) and the high C1s/O1s ratio (8.04). By using the above approach, rGO films with a tunable thickness and sheet resistance are achieved. The obtained rGO films are used as electrodes in polymer memory devices, in a configuration of rGO/poly(3‐hexylthiophene) (P3HT):phenyl‐C61‐butyric acid methyl ester (PCBM)/Al, which exhibit an excellent write‐once‐read‐many‐times effect and a high ON/OFF current ratio of 10⁶.     

The Pine‐Needle‐Inspired Structure of Zinc Oxide Nanorods Grown on Electrospun Nanofibers for High‐Performance Flexible Supercapacitors

Flexible supercapacitors with high electrochemical performance and stability along with mechanical robustness have gained immense attraction due to the substantial advancements and rampant requirements of storage devices. To meet the exponentially growing demand of microsized energy storage device, a cost‐effective and durable supercapacitor is mandatory to realize their practical applications. Here, in this work, the fabrication route of novel electrode materials with high flexibility and charge‐storage capability is reported using the hybrid structure of 1D zinc oxide (ZnO) nanorods and conductive polyvinylidene fluoride‐tetrafluoroethylene (P(VDF‐TrFE)) electrospun nanofibers. The ZnO nanorods are conformably grown on conductive P(VDF‐TrFE) nanofibers to fabricate the light‐weighted porous electrodes for supercapacitors. The conductive nanofibers acts as a high surface area scaffold with significant electrochemical performance, while the addition of ZnO nanorods further enhances the specific capacitance by 59%. The symmetric cell with the fabricated electrodes presents high areal capacitance of 1.22 mF cm^?2 at a current density of 0.1 mA cm^?2 with a power density of more than 1600 W kg^?1. Furthermore, these electrodes show outstanding flexibility and high stability with 96% and 78% retention in specific capacitance after 1000 and 5000 cycles, respectively. The notable mechanical durability and robustness of the cell acquire both good flexibility and high performance.     

Highly Efficient Sodium Storage in Iron Oxide Nanotube Arrays Enabled by Built‐In Electric Field

High‐power sodium–ion batteries capable of charging and discharging rapidly and durably are eagerly demanded to replace current lithium–ion batteries. However, poor activity and instable cycling of common sodium anode materials represent a huge barrier for practical deployment. A smart design of ordered nanotube arrays of iron oxide (Fe₂O₃) is presented as efficient sodium anode, simply enabled by surface sulfurization. The resulted heterostructure of oxide and sulfide spontaneously develops a built‐in electric field, which reduces the activation energy and accelerates charge transport significantly. Benefiting from the synergy of ordered architecture and built‐in electric field, such arrays exhibit a large reversible capacity, a superior rate capability, and a high retention of 91% up to 200 cycles at a high rate of 5 A g^?1, outperforming most reported iron oxide electrodes. Furthermore, full cells based on the Fe₂O₃ array anode and the Na_0.67(Mn_0.67Ni_0.23Mg_0.1)O₂ cathode deliver a specific energy of 142 Wh kg^?1 at a power density of 330 W kg^?1 (based on both active electrodes), demonstrating a great potential in practical application. This material design may open a new door in engineering efficient anode based on earth‐abundant materials.     

High‐Performance Flexible Transparent Electrode with an Embedded Metal Mesh Fabricated by Cost‐Effective Solution Process

A new structure of flexible transparent electrodes is reported, featuring a metal mesh fully embedded and mechanically anchored in a flexible substrate, and a cost‐effective solution‐based fabrication strategy for this new transparent electrode. The embedded nature of the metal‐mesh electrodes provides a series of advantages, including surface smoothness that is crucial for device fabrication, mechanical stability under high bending stress, strong adhesion to the substrate with excellent flexibility, and favorable resistance against moisture, oxygen, and chemicals. The novel fabrication process replaces vacuum‐based metal deposition with an electrodeposition process and is potentially suitable for high‐throughput, large‐volume, and low‐cost production. In particular, this strategy enables fabrication of a high‐aspect‐ratio (thickness to linewidth) metal mesh, substantially improving conductivity without considerably sacrificing transparency. Various prototype flexible transparent electrodes are demonstrated with transmittance higher than 90% and sheet resistance below 1 ohm sq^?1, as well as extremely high figures of merit up to 1.5 × 10⁴, which are among the highest reported values in recent studies. Finally using our embedded metal‐mesh electrode, a flexible transparent thin‐film heater is demonstrated with a low power density requirement, rapid response time, and a low operating voltage.     

Silver‐Quantum‐Dot‐Modified MoO3 and MnO2 Paper‐Like Freestanding Films for Flexible Solid‐State Asymmetric Supercapacitors

Free‐standing paper‐like thin‐film electrodes have great potential to boost next‐generation power sources with highly flexible, ultrathin, and lightweight requirements. In this work, silver‐quantum‐dot‐ (2–5 nm) modified transition metal oxide (including MoO₃ and MnO₂) paper‐like electrodes are developed for energy storage applications. Benefitting from the ohmic contact at the interfaces between silver quantum dots and MoO₃ nanobelts (or MnO₂ nanowires) and the binder‐free nature and 0D/1D/2D nanostructured 3D network of the fabricated electrodes, substantial improvements on the electrical conductivity, efficient ionic diffusion, and areal capacitances of the hybrid nanostructure electrodes are observed. With this proposed strategy, the constructed asymmetric supercapacitors, with Ag quantum dots/MoO₃ “paper” as anode, Ag quantum dots/MnO₂ “paper” as cathode, and neutral Na₂SO₄/polyvinyl alcohol hydrogel as electrolyte, exhibit significantly enhanced energy and power densities in comparison with those of the supercapacitors without modification of Ag quantum dots on electrodes; present excellent cycling stability at different current densities and good flexibility under various bending states; offer possibilities as high‐performance power sources with low cost, high safety, and environmental friendly properties.     

Meso‐Reconstruction of Wool Keratin 3D “Molecular Springs” for Tunable Ultra‐Sensitive and Highly Recovery Strain Sensors

Wool keratin (WK) consists of a large number of α‐helices, which are just like many molecular‐scale springs. Herein, the construction of 3D WK molecular spring networks are reported by cross‐linking individual WK molecules via a Michael addition reaction. The as‐prepared springs display a superior recovery capability with unusual nonlinear elasticity, very low dissipative energy, and turntable elastic constant achieved by adjusting the chemical crosslinking density of WK networks. Owing to these unique characteristics, the 3D WK networks based flexible strain sensors reveal a high sensitivity, broad sensing ranges, and extremely long and stable performance. While normal highly sensible strain sensors, obtained by highly sophisticated surface or bulk patterning, often exhibit a relatively narrow range of measurements and limited life cycles. Such the WK mediated sensing materials have widespread applications in wearable electronics, such as detection and tracking of different human motions, and even discern voice during speaking.