Advances in Flexible Metallic Transparent Electrodes

Room-Temperature, Solution-Processed, Robust, Transparent, and Conductive SiOx/AgNW Nanocomposite Coating

AgNW networks show high promise as a conductive material due to excellent flexibility, low resistance, high transparency, and ease of large-scale preparation. However, the application of AgNW networks has been hindered by their inherent characteristics, such as easy oxidation degradation, chemical corrosion, and structural instability at high temperatures. In this study, a dense SiOx protective layer derived from perhydropolysilazane was introduced to fabricate a robust SiOx/AgNW nanocomposite coating through an all-solution process at room temperature. The achieved nanocomposite coating shows outstanding thermal stability up to 450 °C, resistance to ultraviolet radiation, and excellent mechanical performance by maintaining stability after 10,000 cycles of bending at a radius of 2.5 mm, 1000 cycles of peeling, and 1200 cycles of wearing. Meanwhile, the nanocomposite coating demonstrates exceptional chemical tolerance against HCl, Na2S, and organic solvents. A transparent heater based on the nanocomposite coating achieves a remarkable benchmark with a maximum temperature of 400 °C at 20 V. These features highlight the potential of the nanocomposite coating in flexible electronics, optoelectronics, touch screens, and high-performance heaters.

Enhancing the Electrical Conductivity and Long-Term Stability of PEDOT:PSS Electrodes through Sequential Treatment with Nitric Acid and Cesium Chloride

Solution-processable poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) is an important polymeric conductor used extensively in organic flexible, wearable, and stretchable optoelectronics. However, further enhancing its conductivity and long-term stability while maintaining its superb mechanical properties remains challenging. Here, a novel post-treatment approach to enhance the electrical properties and stability of sub-20-nm-thin PEDOT:PSS films processed from solution is introduced. The approach involves a sequential post-treatment with HNO3 and CsCl, resulting in a remarkable enhancement of the electrical conductivity of PEDOT:PSS films to over 5500 S cm-1, along with improved carrier mobility. The post-treated films exhibit remarkable air stability, retaining over 85% of their initial conductivity even after 270 days of storage. Various characterization techniques, including X-ray photoelectron spectroscopy, atomic force microscopy, Raman spectroscopy, Hall effect measurements, and grazing incidence wide angle X-ray scattering, coupled with density functional theory calculations, provide insights into the structural changes and interactions responsible for these improvements. To demonstrate the potential for practical applications, the ultrathin PEDOT:PSS films are connected to an inorganic light-emitting diode with a battery, showcasing their suitability as transparent electrodes. This work presents a promising approach for enhancing the electrical conductivity of PEDOT:PSS while offering a comprehensive understanding of the underlying mechanisms that can guide further advances.

One-Step Synthesis of Graphene-Covered Silver Nanowires with Enhanced Stability for Heating and Strain Sensing

Solution-processed silver nanowire (AgNW) networks have been considered as promising electrode candidates for next-generation electronic devices. However, they suffer from poor thermal and electrical stability and low mechanical properties, hindering their practical applications. In this work, graphene nanosheets are successfully introduced into AgNW via a facile one-step solvothermal process. Benefiting from increased conductive paths, the resultant AgNW/graphene films exhibit high electrical conductivity. More importantly, the interlocking NW morphology can be maintained under high temperature and applied voltage due to suppressed Ag migration, which is enabled by the introduction of graphene. This feature leads to enhanced thermal and electrical stability, making them suitable for use as transparent heaters. Furthermore, the composite films present excellent mechanical performance, and negligible resistance change is observed after 10 000 repeated bending cycles. To demonstrate their feasibility toward sensor applications, sandwiched strain sensors are designed, which can endure larger tensile strains and show higher sensitivity and repeatability compared with pure AgNW-based device. Furthermore, various hand gestures can be easily recognized by the resultant sensors based on unique combinations of sensing response. This work not only provides a low-cost method to realize large-scale synthesis of AgNW/graphene composites but also offers guidance to prepare high-performance electrodes for advanced electronics.

Advancements in stretchable organic optoelectronic devices and flexible transparent conducting electrodes: Current progress and future prospects

The rapid evolution of flexible optoelectronic devices in consumer markets, such as solar cells, photonic skins, displays, lighting, supercapacitors, and smart windows, has spurred global innovation in the design and development of Stretchable Transparent Conducting Electrode (STCE) materials. These materials, which combine the flexibility of organic materials with the functionality of optoelectronic components, have drawn a lot of attention because of their potential uses in a variety of disciplines, such as medical equipment, wearable electronics, and soft robotics. Recent advancements in material science and device design have significantly improving performance, durability, and functionality of these stretchable organic optoelectronic devices. Furthermore, flexible conducting transparent electrodes play an essential role in a wide range of flexible and transparent electronics, including touch screens, displays, and solar cells. Traditional materials like indium tin oxide (ITO) electrodes, while effective, and constrained by their fragility and high cost. Recent innovations in alternative materials, such as metal mesh, nanowires, conducting polymers and graphene have ushered in a new era of affordable, flexible, and transparent conductive electrodes. Materials like graphene, metal nanowires, metallic grids, metal meshes, and dielectric-metal-dielectric electrodes are explored as potential substitutes for fragile ITO electrodes, thanks to their excellent combination of mechanical flexibility and electrical conductivity. This abstract delves into the opportunities and challenges in the development of flexible and transparent organic optoelectronic devices and flexible conducting transparent electrodes. In this review, we explain the technological advancements of transparent and stretchable electrodes, as well as their applications in organic optoelectronic devices such as organic and perovskite solar cells, OLED, heaters, and supercapacitors. We will specifically examine the basic characteristics, optoelectronic properties, and manufacturing procedures of transparent conducting electrodes. We also discuss the key criteria for evaluating proposals for new research lines in this burgeoning sector.

Haze factor of silver nanowires in variable refractive index environment: experimental and simulation approaches

While silver nanowires (Ag NWs) have been demonstrated as a highly efficient transparent conducting material, they suffer from strong light scattering, which is quantified by a large haze factor (HF) in the optical spectrum. Here we investigate the influence of the dielectric environment on the light scattering of Ag NWs by comparing experimental measurements and simulations. In air, two peaks on the HF spectra are observed experimentally at the wavelength ofλI= 350 nm andλII= 380 nm and are attributed by simulations to the influence of the Ag NWs pentagonal shape on the localized surface plasmon resonance. The relative intensity between the two peaks is found to be dependent on whether the Ag NWs are in contact with the glass substrate or not. The HF behaviour in the near IR region seems to be dominated by Rayleigh scattering following simulations results. Dielectric environments of Ag NWs with various refractive indexes were obtained experimentally by the conformal deposition of different metal oxide coatings using atomic layer deposition, including Al-doped zinc oxide, Al2O3and SiO2coatings. The HF is found to be correlated with the refractive index environment in terms of HF peaks position, intensity and broadening. This trend of HF peaks is supported by a theoretical model to understand the optical mechanism behind this phenomenon.

Customizable Metal Micromesh Electrode Enabling Flexible Transparent Zn-Ion Hybrid Supercapacitors with High Energy Density

Emerging flexible and wearable electronic products are placing a compelling demand on lightweight transparent energy storage devices. Owing to their distinguishing features of safety, high specific energy, cycling stability, and rapid charge/discharge advantages, Zn-ion hybrid supercapacitors are a current topic of discussion. However, the trade-off for optical transmittance and energy density remains a great challenge. Here, a high-performance Zn-ion hybrid supercapacitor based on the customizable ultrathin (5 µm), ultralight (0.45 mg cm-2), and ultra-transparent (87.6%) Ni micromesh based cathode and Zn micromesh anode with the highest figure of merit (84 843) is proposed. The developed flexible transparent Zn-ion hybrid supercapacitors reveal excellent cycle stability (no decline after 20 000 cycles), high areal energy density (31.69 µWh cm-2), and high power density (512 µW cm-2). In addition, the assembled solid flexible and transparent Zn-ion hybrid supercapacitor with polyacrylamide gel electrolyte shows extraordinary mechanical properties even under extreme bending and twisting operation. Furthermore, the full device displays a high optical transmittance over 55.04% and can be conformally integrated with diverse devices as a flexible transparent power supply. The fabrication technology offers seamless compatibility with industrial manufacturing, making it an ideal model for the advancement of portable and wearable devices.

Bridge percolation: electrical connectivity of discontinued conducting slabs by metallic nanowires

The properties of nanostructured networks of conductive materials have been extensively studied under the lens of percolation theory. In this work, we introduce a novel type of local percolation phenomenon used to investigate the conduction properties of a new hybrid material that combines sparse metallic nanowire networks and fractured conducting thin films on flexible substrates. This original concept could potentially lead to the design of a novel composite transparent conducting material. Using a complementary approach including formal analytical derivations, Monte Carlo simulations and electrical circuit representation for the modelling of bridged-percolating nanowire networks, we unveil the key relations between linear crack density, nanowire length and network areal mass density that ensure electrical percolation through the hybrid. The proposed theoretical model provides key insights into the conduction mechanism associated with the original concept of bridge percolation in random nanowire networks.

Flexible Embedded Metal Meshes by Sputter-Free Crack Lithography for Transparent Electrodes and Electromagnetic Interference Shielding

A facile and novel fabrication method is demonstrated for creating flexible poly(ethylene terephthalate) (PET)-embedded silver meshes using crack lithography, reactive ion etching (RIE), and reactive silver ink. The crack width and spacing in a waterborne acrylic emulsion polymer are controlled by the thickness of the polymer and the applied stress due to heating and evaporation. Our innovative fabrication technique eliminates the need for sputtering and ensures stronger adhesion of the metal meshes to the PET substrate. Crack trench depths over 5 μm and line widths under 5 μm have been achieved. As a transparent electrode, our flexible embedded Ag meshes exhibit a visible transmission of 91.3% and sheet resistance of 0.54 Ω/sq as well as 93.7% and 1.4 Ω/sq. This performance corresponds to figures of merit (σDC/σOP) of 7500 and 4070, respectively. For transparent electromagnetic interference (EMI) shielding, the metal meshes achieve a shielding efficiency (SE) of 42 dB with 91.3% visible transmission and an EMI SE of 37.4 dB with 93.7% visible transmission. We demonstrate the highest transparent electrode performance of crack lithography approaches in the literature and the highest flexible transparent EMI shielding performance of all fabrication approaches in the literature. These metal meshes may have applications in transparent electrodes, EMI shielding, solar cells, and organic light-emitting diodes.

Diphylleia Grayi-Inspired Intelligent Temperature-Responsive Transparent Nanofiber Membranes

Nanofiber membranes (NFMs) have become attractive candidates for next-generation flexible transparent materials due to their exceptional flexibility and breathability. However, improving the transmittance of NFMs is a great challenge due to the enormous reflection and incredibly poor transmission generated by the nanofiber-air interface. In this research, we report a general strategy for the preparation of flexible temperature-responsive transparent (TRT) membranes, which achieves a rapid transformation of NFMs from opaque to highly transparent under a narrow temperature window. In this process, the phase change material eicosane is coated on the surface of the polyurethane nanofibers by electrospray technology. When the temperature rises to 37 °C, eicosane rapidly completes the phase transition and establishes the light transmission path between the nanofibers, preventing light loss from reflection at the nanofiber-air interface. The resulting TRT membrane exhibits high transmittance (> 90%), and fast response (5 s). This study achieves the first TRT transition of NFMs, offering a general strategy for building highly transparent nanofiber materials, shaping the future of next-generation intelligent temperature monitoring, anti-counterfeiting measures, and other high-performance devices.

Exploring the degradation of silver nanowire networks under thermal stress by coupling in situ X-ray diffraction and electrical resistance measurements

The thermal instability of silver nanowires (AgNWs) leads to a significant increase of the electrical resistance of AgNW networks. A better understanding of the relationship between the structural and electrical properties of AgNW networks is primordial for their efficient integration as transparent electrodes (TEs) for next-generation flexible optoelectronics. Herein, we investigate the in situ evolution of the main crystallographic parameters (i.e. integrated intensity, interplanar spacing and peak broadening) of two Ag-specific Bragg peaks, (111) and (200), during a thermal ramp up to 400 °C through in situ X-ray diffraction (XRD) measurements, coupled with in situ electrical resistance measurements on the same AgNW network. First, we assign the (111) and (200) peaks of χ-scans to each five crystallites within AgNWs using a rotation matrix model. Then, we show that the thermal transition of bare AgNW networks occurs within a temperature range of about 25 °C for the electrical properties, while the structural transition spans over 200 °C. The effect of a protective tin oxide coating (SnO2) on AgNW networks is also investigated through this original in situ coupling approach. For SnO2-coated AgNW networks, the key XRD signatures from AgNWs remain constant, since the SnO2 coating prevents Ag atomic surface diffusion, and thus morphological instability (i.e. spheroidization). Moreover, the SnO2 coating does not affect the strain of both (111) and (200) planes. The thermal expansion for bare and SnO2-coated AgNW networks appears very similar to the thermal expansion of bulk Ag. Our findings provide insights into the underlying failure mechanisms of AgNW networks subjected to thermal stress, helping researchers to develop more robust and durable TEs based on metallic nanowire networks.

Fabrication of Flexible and Transparent Metal Mesh Electrodes Using Surface Energy-Directed Assembly Process for Touch Screen Panels and Heaters

Transparent conductive electrodes (TCEs) are indispensable components of various optoelectronic devices such as displays, touch screen panels, solar cells, and smart windows. To date, the fabrication processes for metal mesh-based TCEs are either costly or having limited resolution and throughput. Here, a two-step surface energy-directed assembly (SEDA) process to efficiently fabricate high resolution silver meshes is introduced. The two-step SEDA process turns from assembly on a functionalized substrate with hydrophilic mesh patterns into assembly on a functionalized substrate with stripe patterns. During the SEDA process, a three-phase contact line pins on the hydrophilic pattern regions while recedes on the hydrophobic non-pattern regions, ensuring that the assembly process can be achieved with excellent selectivity. The necessity of using the two-step SEDA process rather than a one-step SEDA process is demonstrated by both experimental results and theoretical analysis. Utilizing the two-step SEDA process, silver meshes with a line width down to 2 µm are assembled on both rigid and flexible substrates. The thickness of the silver meshes can be tuned by varying the withdraw speed and the assembly times. The assembled silver meshes exhibit excellent optoelectronic properties (sheet resistance of 1.79 Ω/□, optical transmittance of ≈92%, and a FoM value of 2465) as well as excellent mechanical stability. The applications of the assembled silver meshes in touch screen panels and thermal heaters are demonstrated, implying the potential of using the two-step SEDA process for the fabrication of TCEs for optoelectronic applications.

Metallic Micro-Nano Network-Based Soft Transparent Electrodes: Materials, Processes, and Applications

Soft transparent electrodes (TEs) have received tremendous interest from academia and industry due to the rapid development of lightweight, transparent soft electronics. Metallic micro-nano networks (MMNNs) are a class of promising soft TEs that exhibit excellent optical and electrical properties, including low sheet resistance and high optical transmittance, as well as superior mechanical properties such as softness, robustness, and desirable stability. They are genuinely interesting alternatives to conventional conductive metal oxides, which are expensive to fabricate and have limited flexibility on soft surfaces. This review summarizes state-of-the-art research developments in MMNN-based soft TEs in terms of performance specifications, fabrication methods, and application areas. The review describes the implementation of MMNN-based soft TEs in optoelectronics, bioelectronics, tactile sensors, energy storage devices, and other applications. Finally, it presents a perspective on the technical difficulties and potential future possibilities for MMNN-based TE development.

Smooth, Chemically Altered Nucleating Platform for Abrupt Performance Enhancement of Ultrathin Cu-Layer-Based Transparent Electrodes

Rapid advances in flexible optoelectronic devices necessitate the concomitant development of high-performance, cost-efficient, and flexible transparent conductive electrodes (TCEs). This Letter reports an abrupt enhancement in the optoelectronic characteristics of ultrathin Cu-layer-based TCEs via Ar⁺-mediated modulation of the chemical and physical states of a ZnO support surface. This approach strongly regulates the growth mode for the subsequently deposited Cu layer, in addition to marked alteration to the ZnO/Cu interface states, resulting in exceptional TCE performance in the form of ZnO/Cu/ZnO TCEs. The resultant Haacke figure of merit (T¹⁰/R_s) of 0.063 Ω^-1, 53% greater than that of the unaltered, otherwise identical structure, corresponds to a record-high value for Cu-layer-based TCEs. Moreover, the enhanced TCE performance in this approach is shown to be highly sustainable under severe simultaneous loadings of electrical, thermal, and mechanical stresses.

Stretchable, Electrochemically-Stable Electrochromic Devices Based on Semi-Embedded Ag@Au Nanowire Network

Stretchable electrochromic (EC) devices that can adapt the irregular and dynamic human surfaces show promising applications in wearable display, adaptive camouflage, and visual sensation. However, challenges exist in lacking transparent conductive electrodes with both tensile and electrochemical stability to assemble the complex device structure and endure harsh electrochemical redox reactions. Herein, a wrinkled, semi-embedded Ag@Au nanowire (NW) networks are constructed on elastomer substrates to fabricate stretchable, electrochemically-stable conductive electrodes. The stretchable EC devices are then fabricated by sandwiching a viologen-based gel electrolyte between two conductive electrodes with the semi-embedded Ag@Au NW network. Because the inert Au layer inhibits the oxidation of Ag NWs, the EC device exhibits much more stable color changes between yellow and green than those with pure Ag NW networks. In addition, since the wrinkled semi-embedded structure is deformable and reversibly stretched without serious fractures, the EC devices still maintain excellent color-changing stability under 40% stretching/releasing cycles.

A simple and efficient transfer method for fabricating stretchable AgNW patterns on PDMS using carboxylated cellulose nanofibers as a sacrificial layer

Silver nanowire (AgNW) networks are one of the most promising materials of transparent electrodes in flexible applications. However, they still face challenges in fabricating AgNW transparent conductive films (TCFs) with excellent comprehensive performance on stretchable substrates. In this work, we developed an efficient and simple water-assisted method to completely transfer AgNW films from glass to polydimethylsiloxane (PDMS). Carboxylated cellulose nanofibers (CNF-C) are introduced between the AgNW network and glass as a sacrificial layer, which is dissolved in water in the transfer process, releasing the AgNW network on the PDMS. The transferred AgNW networks show an increase of sheet resistance less than 30% and a slight decrease of transmittance. The stretchable AgNW TCFs exhibited good opto-electrical performance with a figure of merit of about 200, low surface roughness, good film uniformity, long-term stability, electrical stability and mechanical performance. Two patterning approaches based on the transfer method were proposed and fine stretchable AgNW patterns with a linewidth of 200 μm were fabricated. The fabricated stretchable AgNW patterns were used in flexible wires, a film heater and sensors as a demonstration.

Sensing and Stimulation Applications of Carbon Nanomaterials in Implantable Brain-Computer Interface

Implantable brain–computer interfaces (BCIs) are crucial tools for translating basic neuroscience concepts into clinical disease diagnosis and therapy. Among the various components of the technological chain that increases the sensing and stimulation functions of implanted BCI, the interface materials play a critical role. Carbon nanomaterials, with their superior electrical, structural, chemical, and biological capabilities, have become increasingly popular in this field. They have contributed significantly to advancing BCIs by improving the sensor signal quality of electrical and chemical signals, enhancing the impedance and stability of stimulating electrodes, and precisely modulating neural function or inhibiting inflammatory responses through drug release. This comprehensive review provides an overview of carbon nanomaterials’ contributions to the field of BCI and discusses their potential applications. The topic is broadened to include the use of such materials in the field of bioelectronic interfaces, as well as the potential challenges that may arise in future implantable BCI research and development. By exploring these issues, this review aims to provide insight into the exciting developments and opportunities that lie ahead in this rapidly evolving field.

Rapid photonic curing effects of xenon flash lamp on ITO-Ag-ITO multilayer electrodes for high throughput transparent electronics

High-throughput transparent and flexible electronics are essential technologies for next-generation displays, semiconductors, and wearable bio-medical applications. However, to manufacture a high-quality transparent and flexible electrode, conventional annealing processes generally require 5 min or more at a high temperature condition of 300 °C or higher. This high thermal budget condition is not only difficult to apply to general polymer-based flexible substrates, but also results in low-throughput. Here, we report a high-quality transparent electrode produced with an extremely low thermal budget using Xe-flash lamp rapid photonic curing. Photonic curing is an extremely short time (~ μs) process, making it possible to induce an annealing effect of over 800 °C. The photonic curing effect was optimized by selecting the appropriate power density, the irradiation energy of the Xe-flash lamp, and Ag layer thickness. Rapid photonic curing produced an ITO-Ag-ITO electrode with a low sheet resistance of 6.5 ohm/sq, with a high luminous transmittance of 92.34%. The low thermal budget characteristics of the rapid photonic curing technology make it suitable for high-quality transparent electronics and high-throughput processes such as roll-to-roll.

All-atmospheric fabrication of Ag-Cu core-shell nanowire transparent electrodes with Haacke figure of merit >600 × 10-3 Ω-1

Transparent conducting electrodes (TCEs) are essential components in devices such as touch screens, smart windows, and photovoltaics. Metal nanowire networks are promising next-generation TCEs, but best-performing examples rely on expensive metal catalysts (palladium or platinum), vacuum processing, or transfer processes that cannot be scaled. This work demonstrates a metal nanowire TCE fabrication process that focuses on high performance and simple fabrication. Here we combined direct and plating metallization processes on electrospun nanowires. We first directly metallize silver nanowires using reactive silver ink. The silver catalyzes subsequent copper plating to produce Ag-Cu core-shell nanowires and eliminates nanowire junction resistances. The process allows for tunable transmission and sheet resistance properties by adjusting electrospinning and plating time. We demonstrate state-of-the-art, low-haze TCEs using an all-atmospheric process with sheet resistances of 0.33 Ω sq^-1 and visible light transmittances of 86% (including the substrate), leading to a Haacke figure of merit of 652 × 10^-3 Ω^-1. The core-shell nanowire electrode also demonstrates high chemical and bending durability.

A crack templated copper network film as a transparent conductive film and its application in organic light-emitting diode

In this paper, a highly transparent, low sheet resistance copper network film fabricated by a crack template, which made by drying an acrylic based colloidal dispersion. The fabricated copper network film shows excellent optoelectronic performances with low sheet resistance of 13.4 Ω/sq and high optical transmittance of 93% [excluding Polyethylene terephthalate (PET) substrate] at 550 nm. What’s more, the surface root mean square of the copper network film is about 4 nm, and the figure of merit is about 380. It’s comparable to that of conventional indium tin oxide thin film. The repeated bending cycle test and adhesive test results confirm the reliability of the copper network film. As a transparent conductive film, the copper network film was used as an anode to prepare organic light-emitting diode (OLED). The experiment results show that the threshold voltage of the OLED is less than 5 V and the maximum luminance is 1587 cd/m2.