A 2D Titanium Carbide MXene Flexible Electrode for High-Efficiency Light-Emitting Diodes

Although several transparent conducting materials such as carbon nanotubes, graphene, and conducting polymers have been intensively explored as flexible electrodes in optoelectronic devices, their insufficient electrical conductivity, low work function, and complicated electrode fabrication processes have limited their practical use. Herein, a 2D titanium carbide (Ti₃C₂) MXene film with transparent conducting electrode (TCE) properties, including high electrical conductivity (≈11 670 S cm⁻¹) and high work function (≈5.1 eV), which are achieved by combining a simple solution processing with modulation of surface composition, is described. A chemical neutralization strategy of a conducting-polymer hole-injection layer is used to prevent detrimental surface oxidation and resulting degradation of the electrode film. Use of the MXene electrode in an organic light-emitting diode leads to a current efficiency of ≈102.0 cd A⁻¹ and an external quantum efficiency of ≈28.5% ph/el, which agree well with the theoretical maximum values from optical simulations. The results demonstrate the strong potential of MXene as a solution-processable electrode in optoelectronic devices and provide a guideline for use of MXenes as TCEs in low-cost flexible optoelectronic devices.     

Embedded Nickel-Mesh Transparent Electrodes for Highly Efficient and Mechanically Stable Flexible Perovskite Photovoltaics: Toward a Portable Mobile Energy Source

The rapid development of Internet of Things mobile terminals has accelerated the market's demand for portable mobile power supplies and flexible wearable devices. Here, an embedded metal-mesh transparent conductive electrode (TCE) is prepared on poly(ethylene terephthalate) (PET) using a novel selective electrodeposition process combined with inverted film-processing methods. This embedded nickel (Ni)-mesh flexible TCE shows excellent photoelectric performance (sheet resistance of ≈0.2–0.5 Ω sq⁻¹ at high transmittance of ≈85–87%) and mechanical durability. The PET/Ni-mesh/polymer poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS PH1000) hybrid electrode is used as a transparent electrode for perovskite solar cells (PSCs), which exhibit excellent electric properties and remarkable environmental and mechanical stability. A power conversion efficiency of 17.3% is obtained, which is the highest efficiency for a PSC based on flexible transparent metal electrodes to date. For perovskite crystals that require harsh growth conditions, their mechanical stability and environmental stability on flexible transparent embedded metal substrates are studied and improved. The resulting flexible device retains 76% of the original efficiency after 2000 bending cycles. The results of this work provide a step improvement in flexible PSCs.     

Heterogeneous Integration of Carbon‐Nanotube–Graphene for High‐Performance,Flexible, and Transparent Photodetectors

Low‐dimensional carbon materials, such as semiconducting carbon nanotubes (CNTs), conducting graphene, and their hybrids, are of great interest as promising candidates for flexible, foldable, and transparent electronics. However, the development of highly photoresponsive, flexible, and transparent optoelectronics still remains limited due to their low absorbance and fast recombination rate of photoexcited charges, despite the considerable potential of photodetectors for future wearable and foldable devices. This work demonstrates a heterogeneous, all‐carbon photodetector composed of graphene electrodes and porphyrin‐interfaced single‐walled CNTs (SWNTs) channel, exhibiting high photoresponse, flexibility, and full transparency across the device. The porphyrin molecules generate and transfer photoexcited holes to the SWNTs even under weak white light, resulting in significant improvement of photoresponsivity from negligible to 1.6 × 10^?2 A W^?1. Simultaneously, the photodetector exhibits high flexibility allowing stable light detection under ≈50% strain (i.e., a bending radius of ≈350 µm), and retaining a sufficient transparency of ≈80% at 550 nm. Experimental demonstrations as a wearable sunlight sensor highlight the utility of the photodetector that can be conformally mounted on human skin and other curved surfaces without any mechanical and optical constraints. The heterogeneous integration of porphyrin–SWNT–graphene may provide a viable route to produce invisible, high‐performance optoelectronic systems.     

A Contact-Sliding-Triboelectrification-Driven Dynamic Optical Transmittance Modulator for Self-Powered Information Covering and Selective Visualization

Triboelectrification-enabled self-powered flexible electronic/optical systems have aroused a new surge of interest in recent years. All-in-one integration of such a system, which could significantly improve its adaptability, operability, and portability, still remains a challenge due to the absence of suitable architectures and integration schemes. Herein, a previously reported self-powered optical switch (OS) is thoroughly remolded and upgraded to a fully integrated contact-sliding-triboelectrification-driven dynamic optical transmittance modulator (OTM). The OTM is constructed with a multilayered structure, comprising a transparent triboelectrification top layer, a SiO₂-spaced polymer dispersed liquid crystal (PDLC) intermediate layer, and a flexible transparent conductive substrate. The working mechanism is that an alternating electric field can be induced once contact-sliding occurs upon the OTM, rendering the PDLC layer immediately switching its initial translucent state to an instantaneous transparent state. As such, a decent dimming range with the relative transmitted light intensity from 0.17 to 0.72 can be achieved at low mechanical thresholds of contact pressure (≈20 kPa) and sliding velocity (≈0.3 m s⁻¹). Moreover, for practical applications, demonstrations of information covering and selective visualization are successfully implemented without any extra optical elements nor external power supplies, explicitly showing great potential for the OTM in various self-powered optical interactive applications.     

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.     

Roll‐to‐Roll Production of Transparent Silver‐Nanofiber‐Network Electrodes for Flexible Electrochromic Smart Windows

Electrochromic smart windows (ECSWs) are considered as the most promising alternative to traditional dimming devices. However, the electrode technology in ECSWs remains stagnant, wherein inflexible indium tin oxide and fluorine‐doped tin oxide are the main materials being used. Although various complicated production methods, such as high‐temperature calcination and sputtering, have been reported, the mass production of flexible and transparent electrodes remains challenging. Here, a nonheated roll‐to‐roll process is developed for the continuous production of flexible, extralarge, and transparent silver nanofiber (AgNF) network electrodes. The optical and mechanical properties, as well as the electrical conductivity of these products (i.e., 12 Ω sq^?1 at 95% transmittance) are comparable with those AgNF networks produced via high‐temperature sintering. Moreover, the as‐prepared AgNF network is successfully assembled into an A4‐sized ECSW with short switching time, good coloration efficiency, and flexibility.     

Highly Flexible and Transparent Multilayer MoS2 Transistors with Graphene Electrodes

A highly flexible and transparent transistor is developed based on an exfoliated MoS₂ channel and CVD‐grown graphene source/drain electrodes. Introducing the 2D nanomaterials provides a high mechanical flexibility, optical transmittance (～74%), and current on/off ratio (>10⁴) with an average field effect mobility of ～4.7 cm² V^?1 s^?1, all of which cannot be achieved by other transistors consisting of a MoS₂ active channel/metal electrodes or graphene channel/graphene electrodes. In particular, a low Schottky barrier (～22 meV) forms at the MoS₂/graphene interface, which is comparable to the MoS₂/metal interface. The high stability in electronic performance of the devices upon bending up to ±2.2 mm in compressive and tensile modes, and the ability to recover electrical properties after degradation upon annealing, reveal the efficacy of using 2D materials for creating highly flexible and transparent devices.     

Visually Imperceptible Liquid‐Metal Circuits for Transparent,Stretchable Electronics with Direct Laser Writing

A material architecture and laser‐based microfabrication technique is introduced to produce electrically conductive films (sheet resistance = 2.95 Ω sq^?1; resistivity = 1.77 × 10^?6 Ω m) that are soft, elastic (strain limit >100%), and optically transparent. The films are composed of a grid‐like array of visually imperceptible liquid‐metal (LM) lines on a clear elastomer. Unlike previous efforts in transparent LM circuitry, the current approach enables fully imperceptible electronics that have not only high optical transmittance (>85% at 550 nm) but are also invisible under typical lighting conditions and reading distances. This unique combination of properties is enabled with a laser writing technique that results in LM grid patterns with a line width and pitch as small as 4.5 and 100 µm, respectively—yielding grid‐like wiring that has adequate conductivity for digital functionality but is also well below the threshold for visual perception. The electrical, mechanical, electromechanical, and optomechanical properties of the films are characterized and it is found that high conductivity and transparency are preserved at tensile strains of ≈100%. To demonstrate their effectiveness for emerging applications in transparent displays and sensing electronics, the material architecture is incorporated into a couple of illustrative use cases related to chemical hazard warning.     

Coupling a Sponge Metal Fibers Skeleton with In Situ Surface Engineering to Achieve Advanced Electrodes for Flexible Lithium–Sulfur Batteries

Lithium–sulfur batteries (LSBs) are regarded as promising next-generation energy storage systems, however, the uncontrollable dendrite formation and serious polysulfide shuttling severely hinder their commercial success. Herein, a powerful 3D sponge nickel (SN) skeleton plus in situ surface engineering strategy, to address these issues synergistically, is reported, and a high-performance flexible LSB device is constructed. Specifically, the rationally designed spray-quenched lithium metal on the SN matrix (solid electrolyte interface (SEI)@Li/SN), as dendrite inhibitor, combines the merits of the 3D lithiophilic SN skeleton and the in situ formed SEI layer derived from the spray-quenching process, and thereby exhibits a steady overpotential within 75 mV for 1500 h at 5 mA cm⁻²/10 mA h cm⁻². Meanwhile, in situ surface sulfurization of the SN skeleton hybridizing with the carbon/sulfur composite (SC@Ni₃S₂/SN) serves as efficient lithium polysulfide adsorbent to catalyze the overall reaction kinetics. COMSOL Multiphysics simulations and density functional theory calculations are further conducted to explore the underlying mechanisms. As a proof of concept, the well-designed SEI@Li/SN||SC@Ni₃S₂/SN full cell shows excellent electrochemical performance with a negative/positive ratio in capacity of ≈2 and capacity retention of 99.82% at 1 C under mechanical deformation. The novel design principles of these materials and electrodes successfully shed new light on the development of flexible LSBs.     

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.     

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.     

Unusually High Optical Transparency in Hexagonal Nanopatterned Graphene with Enhanced Conductivity by Chemical Doping

Graphene has received appreciable attention for its potential applications in flexible conducting film due to its exceptional optical, mechanical, and electrical properties. However increasing transmittance of graphene without sacrificing the electrical conductivity has been difficult. The fabrication of optically highly transparent (≈98%) graphene layer with a reasonable electrical conductivity is demonstrated here by nanopatterning and doping. Anodized aluminium oxide nanomask prepared by facile and simple self‐assembly technique is utilized to produce an essentially hexagonally nanopatterned graphene. The electrical resistance of the graphene increases significantly by a factor of ≈15 by removal of substantial graphene regions via nanopatterning into hexagonal array pores. However, the use of chemical doping on the nanopatterned graphene almost completely recovers the lost electrical conductivity, thus leading to a desirably much more optically transparent conductor having ≈6.9 times reduced light blockage by graphene material without much loss of electrical conductivity. It is likely that the availability of large number of edges created in the nanopatterned graphene provides ideal sites for chemical dopant attachment, leading to a significant reduction of the sheet resistance. The results indicate that the nanopatterned graphene approach can be a promising route for simultaneously tuning the optical and electrical properties of graphene to make it more light‐transmissible and suitable as a flexible transparent conductor.     

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.     

Graphene as an anti-permeation and protective layer for indium-free transparent electrodes

We show that graphene can be used as a protective layer for transparent electrodes made of materials which would otherwise deteriorate when exposed to the environment. In particular, we investigate aluminum-doped zinc oxides and ultrathin copper films capped with a one-atom graphene layer in damp heat (95% relative humidity and 95?°C) and high temperature (up to 180?°C) conditions. The results clearly indicate that a graphene layer can strongly reduce degradation of the electrodes' electrical, optical properties and surface morphology, thus preserving the functionality of the transparent electrodes. The proposed technique is particularly suitable for flexible optoelectronic devices thanks to the mechanical strength of graphene when subjected to bending.     

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.     

Comparison of ZnO:Al,ITO and carbon nanotube transparent conductive layers in flexible solar cells applications

In the paper the mechanical, optical and electrical parameters of transparent conductive layers (TCLs) made of carbon nanotubes and metal conductive oxides are explored and compared. All investigated materials are deposited on transparent, flexible polymer foils used for solar cell applications. Obtained results are compared with available parameters of rigid transparent conductive oxides (TCOs) as well as literature reports about Indium–Tin Oxide (ITO) on flexible substrates. Presented paper is a report from the preliminary stage of a new flexible solar cell construction.     

Transparent and Flexible Supercapacitors with Networked Electrodes

Transparent and flexible energy storage devices have received immense attention due to their suitability for innovative electronics and displays. However, it remains a great challenge to fabricate devices with high storage capacity and high degree of transmittance. This study describes a simple process for fabrication of supercapacitors with ≈75% of visible transparency and areal capacitance of ≈3 mF cm^?2 with high stability tested over 5000 cycles of charging and discharging. The electrodes consist of Au wire networks obtained by a simple crackle template method which are coated with MnO₂ nanostructures by electrodeposition process. Importantly, the membrane separator itself is employed as substrate to bring in the desired transparency and light weight while additionally exploiting its porous nature in enhancing the interaction of electrolyte with the active material from both sides of the substrate, thereby enhancing the storage capacity. The method opens up new ways for fabricating transparent devices.     

Graphene‐Based Integrated Photovoltaic Energy Harvesting/Storage Device

Energy scavenging has become a fundamental part of ubiquitous sensor networks. Of all the scavenging technologies, solar has the highest power density available. However, the energy source is erratic. Integrating energy conversion and storage devices is a viable route to obtain self‐powered electronic systems which have long‐term maintenance‐free operation. In this work, we demonstrate an integrated‐power‐sheet, consisting of a string of series connected organic photovoltaic cells (OPCs) and graphene supercapacitors on a single substrate, using graphene as a common platform. This results in lighter and more flexible power packs. Graphene is used in different forms and qualities for different functions. Chemical vapor deposition grown high quality graphene is used as a transparent conductor, while solution exfoliated graphene pastes are used as supercapacitor electrodes. Solution‐based coating techniques are used to deposit the separate components onto a single substrate, making the process compatible with roll‐to‐roll manufacture. Eight series connected OPCs based on poly(3‐hexylthiophene)(P3HT):phenyl‐C61‐butyric acid methyl ester (PC₆₀BM) bulk‐heterojunction cells with aluminum electrodes, resulting in a ≈5 V open‐circuit voltage, provide the energy harvesting capability. Supercapacitors based on graphene ink with ≈2.5 mF cm^?2 capacitance provide the energy storage capability. The integrated‐power‐sheet with photovoltaic (PV) energy harvesting and storage functions had a mass of 0.35 g plus the substrate.     

“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°.     

Solution-processed metal nanowire mesh transparent electrodes

Transparent conductive electrodes are important components of thin-film solar cells, light-emitting diodes, and many display technologies. Doped metal oxides are commonly used, but their optical transparency is limited for films with a low sheet resistance. Furthermore, they are prone to cracking when deposited on flexible substrates, are costly, and require a high-temperature step for the best performance. We demonstrate solution-processed transparent electrodes consisting of random meshes of metal nanowires that exhibit an optical transparency equivalent to or better than that of metal-oxide thin films for the same sheet resistance. Organic solar cells deposited on these electrodes show a performance equivalent to that of devices based on a conventional metal-oxide transparent electrode.