Reduced Graphene Oxide Micromesh Electrodes for Large Area,Flexible, Organic Photovoltaic Devices

A laser‐based patterning technique—compatible with flexible, temperature‐sensitive substrates—for the production of large area reduced graphene oxide micromesh (rGOMM) electrodes is presented. The mesh patterning can be accurately controlled in order to significantly enhance the electrode transparency, with a subsequent slight increase in the sheet resistance, and therefore improve the tradeoff between transparency and conductivity of reduced graphene oxide (rGO) layers. In particular, rGO films with an initial transparency of ≈20% are patterned, resulting in rGOMMs films with a ≈59% transmittance and a sheet resistance of ≈565 Ω sq^?1, that is significantly lower than the resistance of ≈780 Ω sq^?1, exhibited by the pristine rGO films at the same transparency. As a proof‐of‐concept application, rGOMMs are used as the transparent electrodes in flexible organic photovoltaic (OPV) devices, achieving power conversion efficiency of 3.05%, the highest ever reported for flexible OPV devices incorporating solution‐processed graphene‐based electrodes. The controllable and highly reproducible laser‐induced patterning of rGO hold enormous promise for both rigid and flexible large‐scale organic electronic devices, eliminating the lag between graphene‐based and indium–tin oxide electrodes, while providing conductivity and transparency tunability for next generation flexible electronics.     

Simultaneous Electrochemical Dual‐Electrode Exfoliation of Graphite toward Scalable Production of High‐Quality Graphene

Developing scalable methods to produce large quantities of high‐quality and solution‐processable graphene is essential to bridge the gap between laboratory study and commercial applications. Here an efficient electrochemical dual‐electrode exfoliation approach is developed, which combines simultaneous anodic and cathodic exfoliation of graphite. Newly designed sandwich‐structured graphite electrodes which are wrapped in a confined space with porous metal mesh serve as both electrodes, enabling a sufficient ionic intercalation. Mechanism studies reveal that the combination of electrochemical intercalation with subsequent thermal decomposition results in drastic expansion of graphite toward high‐efficiency production of graphene with high quality. By precisely controlling the intercalation chemistry, the two‐step approach leads to graphene with outstanding yields (85% and 48% for cathode and anode, respectively) comprising few‐layer graphene (1–3 layers, >70%), ultralow defects (I_D/I_G < 0.08), and high production rate (exceeding 25 g h^?1). Moreover, its excellent electrical conductivity (>3 × 10⁴ S m^?1) and great solution dispersibility in N‐methyl pyrrolidone (10 mg mL^?1) enable the fabrication of highly conductive (11 Ω sq^?1) and flexible graphene films by inkjet printing. This simple and efficient exfoliation approach will facilitate the development of large‐scale production of high‐quality graphene and holds great promise for its wide application.     

Flexible Organic Photovoltaic Cells with In Situ Nonthermal Photoreduction of Spin‐Coated Graphene Oxide Electrodes

The first reduction methodology, compatible with flexible, temperature‐sensitive substrates, for the production of reduced spin‐coated graphene oxide (GO) electrodes is reported. It is based on the use of a laser beam for the in situ, non‐thermal, reduction of spin‐coated GO films on flexible substrates over a large area. The photoreduction process is one‐step, facile, and is rapidly carried out at room temperature in air without affecting the integrity of the graphene lattice or the flexibility of the underlying substrate. Conductive graphene films with a sheet resistance of as low as 700 Ω sq^?1 and transmittance of 44% can be obtained, much higher than can be achieved for flexible layers reduced by chemical means. As a proof of concept of our technique, laser‐reduced GO (LrGO) films are utilized as transparent electrodes in flexible, bulk heterojunction, organic photovoltaic (OPV) devices, replacing the traditional ITO. The devices displayed a power‐conversion efficiency of 1.1%, which is the highest reported so far for OPV device incorporating reduced GO as the transparent electrode. The in situ non‐thermal photoreduction of spin‐coated GO films creates a new way to produce flexible functional graphene electrodes for a variety of electronic applications in a process that carries substantial promise for industrial implementation.     

Graphene-based flexible and wearable electronics

Graphene with an exceptional combination of electronic,optical and outstanding mechanical features has been proved to lead a completely different kind of 2-D electronics.The most exciting feature of graphene is its ultra-thin thickness,that can be conformally contacted to any kind of rough surface without losing much of its transparency and conductivity.Graphene has been explored demonstrating various prototype flexible electronic applications,however,its potentiality has been proven wherever transparent conductive electrodes (TCEs) are needed in a flexible,stretchable format.Graphene-based TCEs in flexible electronic applications showed greatly superior performance over their conventionally available competitor indium tin oxide (ITO).Moreover,enormous applications have been emerging,especially in wearable devices that can be potentially used in our daily life as well as in biomedical areas.However,the production of high-quality,defect-free large area graphene is still a challenge and the main hurdle in the commercialization of flexible and wearable products.The objective of the present review paper is to summarize the progress made so far in graphene-based flexible and wearable applications.The current developments including challenges and future perspectives are also highlighted.     

Lithium–Sulfur Battery Cable Made from Ultralight,Flexible Graphene/Carbon Nanotube/Sulfur Composite Fibers

The emergence of flexible and wearable electronic devices with shape amenability and high mobility has stimulated the development of flexible power sources to bring revolutionary changes to daily lives. The conventional rechargeable batteries with fixed geometries and sizes have limited their functionalities in wearable applications. The first‐ever graphene‐based fibrous rechargeable batteries are reported in this work. Ultralight composite fibers consisting of reduced graphene oxide/carbon nanotube filled with a large amount of sulfur (rGO/CNT/S) are prepared by a facile, one‐pot wet‐spinning method. The liquid crystalline behavior of high concentration GO sheets facilitates the alignment of rGO/CNT/S composites, enabling rational assembly into flexible and conductive fibers as lithium–sulfur battery electrodes. The ultralight fiber electrodes with scalable linear densities ranging from 0.028 to 0.13 mg cm^?1 deliver a high initial capacity of 1255 mAh g^?1 and an areal capacity of 2.49 mAh cm^?2 at C /20. A shape‐conformable cable battery prototype demonstrates a stable discharge characteristic after 30 bending cycles.     

A Universal Method of Producing Transparent Electrodes Using Wide‐Bandgap Materials

A UV light‐emitting diode (LED) is an eco‐friendly optical source with diverse applications. However, currently, the external quantum efficiency (EQE) of AlGaN‐based UV LEDs, particularly in the UV‐C band (<280 nm), is very low (<11%) mainly due to a large optical absorption via p‐GaN contact layers. A direct Ohmic contact to p‐AlGaN layers should be obtained using UV‐transparent conductive electrodes (TCEs) to solve this problem. A universal method is presented here to make such contact using electrical breakdown, with wide‐bandgap materials, to form conductive filaments (CFs), providing a current path between the TCEs and the p‐(Al)GaN layers. The contact resistance between the TCEs and the p‐GaN layers (or p‐AlGaN) is found to be on the order of 10^?5 Ω cm² (or 10^?3 Ω cm²), while optical transmittance is maintained up to 95% for AlN‐based TCEs at 250 nm. These findings could be a critical turning point delivering a breakthrough in UV LED technologies.     

Hydrophilic Nanotube Supported Graphene–Water Dispersible Carbon Superstructure with Excellent Conductivity

In this work, it is shown that the hydrophilic functionalized multiwall carbon nanotubes (MWCNs) can stabilize a large amount of pristine graphene nanosheets in pure water without the assistance of surfactants, ionic liquids, or hydrophilic polymers. Role of stabilizer is conveyed by highly hydrophilic carbon nanotubes, functionalized by dihydroxy phenyl groups, affording a stable dispersion at concentrations as high as 15 mg mL^?1. Such multidimensional (2D/1D) graphene/MWCN hybrid is found to be dispersible also in other polar organic solvents such as ethanol, isopropanol, N,N‐dimethylformamide, ethylene glycol, and their mixtures. High‐resolution transmission microscopy and atomic force microscopy (AFM) including a liquid mode AFM manifest several types of interaction including trapping of multiwalled carbon nanotubes between the graphene sheets or the modification of graphene edges. Molecular dynamic simulations show that formation of an assembly is kinetically controlled. Importantly, the hybrid can be deposited on the paper by drop casting or dispersed in water‐soluble polymers resulting in record values of electrical conductivity (sheet resistance up to R_s ≈ 25 Ω sq^?1 for free hybrid material and R_s ≈ 1300 Ω sq^?1 for a polyvinilalcohol/hybrid composite film). Thus, these novel water dispersible carbon superstructures reveal a high application potential as conductive inks for inkjet printing or as highly conductive polymers.     

Highly Conductive and Environmentally Stable Organic Transparent Electrodes Laminated with Graphene

Improving the lifetime and the operational and thermal stability of organic thin‐film materials while maintaining high conductivity and mechanical flexibility is critical for flexible electronics applications. Here, it is reported that highly conductive and environmentally stable organic transparent electrodes (TEs) can be fabricated by mechanically laminating poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) films containing dimethylsulfoxide and Zonyl fluorosurfactant (PDZ films) with a monolayer graphene barrier. The proposed lamination process allows graphene to be coated onto the PDZ films uniformly and conformally with tight interfacial binding, free of wrinkles and air gaps. The laminated films exhibit an outstanding room‐temperature hole mobility of ≈85.1 cm² V^?1 s^?1 since the graphene can serve as an effective bypass for charge carriers. The significantly improved stability of the graphene‐laminated TEs against high mechanical/thermal stress, humidity, and ultraviolet irradiation is particularly promising. Furthermore, the incorporation of the graphene barrier increases the expected lifetime of the TEs by more than two orders of magnitude.     

Ultra-Robust Flexible Electronics by Laser-Driven Polymer-Nanomaterials Integration

Polyethylene terephthalate (PET) is the most widely used polymer in the world. For the first time, the laser-driven integration of aluminum nanoparticles (Al NPs) into PET to realize a laser-induced graphene/Al NPs/polymer composite, which demonstrates excellent toughness and high electrical conductivity with the formation of aluminum carbide into the polymer is shown. The conductive structures show an impressive mechanical resistance against >10000 bending cycles, projectile impact, hammering, abrasion, and structural and chemical stability when in contact with different solvents (ethanol, water, and aqueous electrolytes). Devices including thermal heaters, carbon electrodes for energy storage, electrochemical and bending sensors show this technology's practical application for ultra-robust polymer electronics. This laser-based technology can be extended to integrating other nanomaterials and create hybrid graphene-based structures with excellent properties in a wide range of flexible electronics’ applications.     

Direct Growth of Nanocrystalline Graphene/Graphite Transparent Electrodes on Si/SiO2 for Metal‐Free Schottky Junction Photodetectors

Conventional methods to produce graphene/silicon Schottky junctions inevitably involve graphene transfer and metal deposition, which leads to the techniques being complicated, high‐cost, and environmentally unfriendly. It is possible to directly grow hybrid nanocrystalline graphene/graphite transparent electrodes from photoresist on quartz without any catalyst. Due to the source material being photoresist, nanographene/graphite patterns can easily be made on Si/SiO₂ structures to form nanographene/silicon Schottky junctions via commercial photolithography and silicon techniques. The obtained Schottky junctions exhibit excellent properties with respect to photodetection, with photovoltage responsivity of 300 V W^‐1 at a light power of 0.2 μW and photovoltage response time of less than 0.5 s. The devices also exhibit an excellent reliability with the photovoltage deviating less than 1% when cycled over 200 times.     

Mechanical integrity of hybrid indium-free electrodes for flexible devices

Maintaining electrical conductivity, optical transparency, and mechanical integrity against bending and stretching are key requirements for flexible transparent electrodes. Transparent conducting oxides (TCOs) are widely used thin film electrodes in optoelectronic devices. However, these materials are brittle and reducing film thickness to improve their mechanical integrity compromises their electrical performance. Here we combine TCO thin films with metal grids embedded in a polymer substrate to create hybrid electrodes with low sheet resistance and high resilience to bending. Amorphous zinc tin oxide (ZTO) and aluminum-doped zinc oxide (AZO) films sputtered onto polyethylene-terephthalate (PET) substrates with and without embedded metal grids are studied. The hybrid electrodes have an optical absorptance below 5% in the visible range and their electrical sheet resistance is less than 1 Ω/sq. The critical strain for tensile failure is analyzed through a combination of electrical measurements and in-situ observations of crack initiation and propagation during tensile loading. The mean critical strain for failure of the AZO/metal grid is 8.5% and that of the ZTO/metal grid is as high as 10%. The AZO and ZTO films alone present critical strain values around 0.6% and 1% respectively, demonstrating that the addition of the metal grid considerably improves the resistance onset strain of the electrodes far beyond these critical strain limits.     

Alternating‐Current MXene Polymer Light‐Emitting Diodes

MXenes (Ti₃C₂) are 2D transition‐metal carbides and carbonitrides with high conductivity and optical transparency. However, transparent MXene electrodes suitable for polymer light‐emitting diodes (PLEDs) have rarely been demonstrated. With the discovery of the excellent electrical stability of MXene under an alternating current (AC), herein, PLEDs that employ MXene electrodes and exhibit high performance under AC operation (AC MXene PLEDs) are presented. The PLED exhibits a turn‐on voltage, current efficiency, and brightness of 2.1 V, 7 cd A^?1, and 12 547 cd m^?2, respectively, when operated under AC with a frequency of 1 kHz. The results indicate that the undesirable electric breakdown associated with heat arising from the poor interface of the MXene with a hole transport layer in the direct‐current mode is efficiently suppressed by the transient injection of carriers accompanied by the alternating change of the electric polarity under the AC, giving rise to reliable light emission with a high efficiency. The solution‐processable MXene electrode can be readily fabricated on a flexible polymer substrate, allowing for the development of a mechanically flexible AC MXene PLED with a higher performance than flexible PLEDs employing solution‐processed nanomaterial‐based electrodes such as carbon nanotubes, reduced graphene oxide, and Ag nanowires.     

Highly Conductive Ordered Mesoporous Carbon Based Electrodes Decorated by 3D Graphene and 1D Silver Nanowire for Flexible Supercapacitor

Ordered mesoporous carbon (OMC) is considered one of the most promising materials for electric double layer capacitors (EDLC) given its low‐cost, high specific surface area, and easily accessed ordered pore channels. However, pristine OMC electrode suffers from poor electrical conductivity and mechanical flexibility, whose specific capacitance and cycling stability is unsatisfactory in flexible devices. In this work, OMC is coated on the surface of highly conductive three‐dimensional graphene foam, serving as both charge collector and flexible substrate. Upon further decoration with silver nanowires (Ag NWs), the novel architecture of Ag NWs/3D‐graphene foam/OMC (Ag‐GF‐OMC) exhibits exceptional electrical conductivity (up to 762 S cm^?1) and mechanical robustness. The Ag‐GF‐OMC electrodes in flexible supercapacitors reach a specific capacitance as high as 213 F g^?1, a value five‐fold higher than that of the pristine OMC electrode. Moreover, these flexible electrodes also exhibit excellent long‐term stability with >90% capacitance retention over 10 000 cycles, as well as high energy and power density (4.5 Wh kg^?1 and 5040 W kg^?1, respectively). This study provides a new procedure to enhance the device performance of OMC based supercapacitors, which is a promising candidate for the application of flexible energy storage devices.     

A Universal Aqueous Conductive Binder for Flexible Electrodes

The development of wearable electronics has led to new requirements for flexible and high-energy batteries. However, the conventional polyvinylidene fluoride binder fails in the manufacture of high-loaded and thick battery electrodes owing to its insufficient adhesiveness and electronic insulation, let alone for flexible devices. Furthermore, organic processing is expensive and not eco-friendly. Herein, the authors report a novel aqueous conductive binder made of carbon nanotubes interwoven in cellulose nanosheets, successfully satisfying the fabrication of flexible yet high-strength electrodes for universal active materials of different sizes, morphologies, and negative-to-positive working potentials, with a high mass loading of up to ≈ 90 mg cm⁻². The conductive binder has an ultrathin 2D-reticular nanosheet structure that forms continuous conductive skeletons in electrodes to segregate and warp active particles via a robust “face-to-point” bonding mode, allowing the fabricated electrodes to have remarkable flexibility and excellent mechanical integrity even under various external forces and excessive electrolyte erosion. Flexible LCO cathodes with a mass loading of > 30 mg cm⁻² as a case study exhibit high mechanical strength ( > 20 MPa) and can easily achieve an ultrahigh areal capacity of 12.1 mAh cm⁻². This cellulose-based binder system is ideal for advanced high-performance functional devices, especially for flexible and high-energy batteries.     

Trilayer Nanomesh Films with Tunable Wettability as Highly Transparent,Flexible, and Recyclable Electrodes

Metallic mesh materials are promising candidates to replace traditional transparent conductive oxides such as indium tin oxide (ITO) that is restricted by the limited indium resource and its brittle nature. The challenge of metal based transparent conductive networks is to achieve high transmittance, low sheet resistance, and small perforation size simultaneously, all of which significantly relate to device performances in optoelectronics. In this work, trilayer dielectric/metal/dielectric (D/M/D) nanomesh electrodes are reported with precisely controlled perforation size, wire width, and uniform hole distribution employing the nanosphere lithography technique. TiO₂/Au/TiO₂ nanomesh films with small hole diameter (≤700 nm) and low thickness (≤50 nm) are shown to yield high transmittance (>90%), low sheet resistance (≤70 Ω sq^?1), as well as outstanding flexural endurance and feasibility for large area patterning. Further, by tuning the surface wettability, these films are applied as easily recyclable flexible electrodes for electrochromic devices. The simple and cost‐effective fabrication of diverse D/M/D nanomesh transparent conductive films with tunable optoelectronic properties paves a way for the design and realization of specialized transparent electrodes in optoelectronics.     

Low‐Temperature‐Processed Printed Metal Oxide Transistors Based on Pure Aqueous Inks

Additive patterning of transparent conducting metal oxides at low temperatures is a critical step in realizing low‐cost transparent electronics for display technology and photovoltaics. In this work, inkjet‐printed metal oxide transistors based on pure aqueous chemistries are presented. These inks readily convert to functional thin films at lower processing temperatures (T ≤ 250 °C) relative to organic solvent‐based oxide inks, facilitating the fabrication of high‐performance transistors with both inkjet‐printed transparent electrodes of aluminum‐doped cadmium oxide (ACO) and semiconductor (InO_x ). The intrinsic fluid properties of these water‐based solutions enable the printing of fine features with coffee‐ring free line profiles and smoother line edges than those formed from organic solvent‐based inks. The influence of low‐temperature annealing on the optical, electrical, and crystallographic properties of the ACO electrodes is investigated, as well as the role of aluminum doping in improving these properties. Finally, the all‐aqueous‐printed thin film transistors (TFTs) with inkjet‐patterned semiconductor (InO_x ) and source/drain (ACO) layers are characterized, which show ideal low contact resistance (R _c < 160 Ω cm) and competitive transistor performance (µ _lin up to 19 cm² V^?1 s^?1, Subthreshold Slope (SS) ≤150 mV dec^?1) with only low‐temperature processing (T ≤ 250 °C).     

Seriography‐Guided Reduction of Graphene Oxide Biopapers for Wearable Sensory Electronics

Novel nacre‐mimic bio‐nanocomposites, such as graphene‐based laminates, are pushing the boundaries of strength and toughness as flexible engineering materials. Translating these material advances to functional flexible electronics requires methods for generating print‐scalable microcircuits (conductive elements surrounded by dielectric) into these strong, tough, lightweight bio‐nanocomposites. Here, a new paradigm for printing flexible electronics by employing facile, eco‐friendly seriography to confine the reduction of graphene oxide biopapers reinforced by silk interlayers is presented. Well‐defined, micropatterned regions on the biopaper are chemically reduced, generating a 10⁶ increase in conductivity (up to 10⁴ S m^?1). Flexible, robust graphene‐silk circuits are showcased in diverse applications such as resistive moisture sensors and capacitive proximity sensors. Unlike conductive (i.e., graphene‐ or Ag nanoparticle‐loaded) inks printed onto substrates, seriography‐guided reduction does not create mechanically weak interfaces between dissimilar materials and does not require the judicious formation of ink. The unimpaired functionality of printed‐in graphene‐silk microcircuits after thousands of punitive folding cycles and chemical attack by harsh solvents is demonstrated. This novel approach provides a low‐cost, portable solution for printing micrometer‐scale conductive features uniformly across large areas (>hundreds of cm²) in layered composites for applications including wearable health monitors, electronic skin, rollable antennas, and conformable displays.     

Large‐Area Schottky Barrier Transistors Based on Vertically Stacked Graphene–Metal Oxide Heterostructures

The fabrication of all‐transparent flexible vertical Schottky barrier (SB) transistors and logic gates based on graphene–metal oxide–metal heterostructures and ion gel gate dielectrics is demonstrated. The vertical SB transistor structure is formed by (i) vertically sandwiching a solution‐processed indium‐gallium‐zinc‐oxide (IGZO) semiconductor layer between graphene (source) and metallic (drain) electrodes and (ii) employing a separate coplanar gate electrode bridged with a vertical channel through an ion gel. The channel current is modulated by tuning the Schottky barrier height across the graphene–IGZO junction under an applied external gate bias. The ion gel gate dielectric with high specific capacitance enables modulation of the Schottky barrier height at the graphene–IGZO junction over 0.87 eV using a voltage below 2 V. The resulting vertical devices show high current densities (18.9 A cm^?2) and on–off current ratios (>10⁴) at low voltages. The simple structure of the unit transistor enables the successful fabrication of low‐power logic gates based on device assemblies, such as the NOT, NAND, and NOR gates, prepared on a flexible substrate. The facile, large‐area, and room‐temperature deposition of both semiconducting metal oxide and gate insulators integrates with transparent and flexible graphene opens up new opportunities for realizing graphene‐based future electronics.     

TiN Nanofibers: A New Material with High Conductivity and Transmittance for Transparent Conductive Electrodes

Motivated by the rising cost of tin‐doped indium oxide (ITO), the search for new transparent electrode materials to replace ITO is ongoing. TiN exhibits high electric conductivity, however, it is generally non‐transparent. Here, nanostructured TiN fiber patterns are synthesized on quartz glass and the resulting materials have a combination of high electric conductivity and optical transparency. A low sheet resistance of 15.8 Ohm sq^?1 at 84% transparency is achieved on TiN nanofiber arrayed quartz glass. The achievements show a successful integration of electric and optical properties in ceramic nanofibers and provide a method for finding new materials to replace traditional ITO‐based transparent electrodes.     

High Mobility WS2 Transistors Realized by Multilayer Graphene Electrodes and Application to High Responsivity Flexible Photodetectors

The electrical contact is one of the main issues preventing semiconducting 2D materials to fulfill their potential in electronic and optoelectronic devices. To overcome this problem, a new approach is developed here that uses chemical vapor deposition grown multilayer graphene (MLG) sheets as flexible electrodes for WS₂ field‐effect transistors. The gate‐tunable Fermi level, van der Waals interaction with the WS₂, and the high electrical conductivity of MLG significantly improve the overall performance of the devices. The carrier mobility of single‐layer WS₂ increases about a tenfold (50 cm² V^?1 s^?1 at room temperature) by replacing conventional Ti/Au metal electrodes (5 cm² V^?1 s^?1) with the MLG electrodes. Further, by replacing the conventional SiO₂ substrate with a thin (1 µm) parylene‐C flexible film as insulator, flexible WS₂ photodetectors that are able to sustain multiple bending stress tests without significant performance degradation are realized. The flexible photodetectors exhibited extraordinarily high gate‐tunable photoresponsivities, reaching values of 4500 A W^?1, and with very short (<2 ms) response time. The work of the heterostacked structure combining WS₂, graphene, and the very thin polymer film will find applications in various flexible electronics, such as wearable high‐performance optoelectronics devices.