Uniform self-forming metallic network as a high-performance transparent conductive electrode

Ruptured liquid metal microcapsules enabling hybridized silver nanowire networks towards high-performance deformable transparent conductors

Extensive studies have been carried out on silver nanowires (AgNWs) in view of their impressive conductivity and highly flexible one-dimensional structure. They are seen as a promising choice for producing deformable transparent conductors. Nonetheless, the widespread adoption of AgNW-based transparent conductors is hindered by critical challenges represented by the significant contact resistance at the nanowire junctions and inadequate interfacial adhesion between the nanowires and the substrate. This study presents a novel solution to tackle the aforementioned challenges by capitalizing on liquid metal microcapsules (LMMs). Upon exposure to acid vapor, the encapsulated LMMs rupture, releasing the fluid LM which then forms a metallic overlay and hybridizes with the underlying Ag network. As a result, a transparent conductive film with greatly enhanced electrical and mechanical properties was obtained. The transparent conductor displays negligible resistance variation even after undergoing chemical stability, adhesion, and bending tests, and ultrasonic treatment. This indicates its outstanding adhesion strength to the substrate and mechanical flexibility. The exceptional electrical properties and robust mechanical stability of the transparent conductor position it as an ideal choice for direct integration into flexible touch panels and wearable strain sensors, as evidenced in this study. By resolving the critical challenges in this field, the proposed strategy establishes a compelling roadmap to navigate the development of high-performance AgNW-based transparent conductors, setting a solid foundation for further advancement in the field of deformable electronics.

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.

Flexible, Transparent and Conductive Metal Mesh Films with Ultra-High FoM for Stretchable Heating and Electromagnetic Interference Shielding

Despite the growing demand for transparent conductive films in smart and wearable electronics for electromagnetic interference (EMI) shielding, achieving a flexible EMI shielding film, while maintaining a high transmittance remains a significant challenge. Herein, a flexible, transparent, and conductive copper (Cu) metal mesh film for EMI shielding is fabricated by self-forming crackle template method and electroplating technique. The Cu mesh film shows an ultra-low sheet resistance (0.18 Ω □-1), high transmittance (85.8%@550 nm), and ultra-high figure of merit (> 13,000). It also has satisfactory stretchability and mechanical stability, with a resistance increases of only 1.3% after 1,000 bending cycles. As a stretchable heater (ε > 30%), the saturation temperature of the film can reach over 110 °C within 60 s at 1.00 V applied voltage. Moreover, the metal mesh film exhibits outstanding average EMI shielding effectiveness of 40.4 dB in the X-band at the thickness of 2.5 μm. As a demonstration, it is used as a transparent window for shielding the wireless communication electromagnetic waves. Therefore, the flexible and transparent conductive Cu mesh film proposed in this work provides a promising candidate for the next-generation EMI shielding applications.

Plasmon mediated SERS and photocatalysis enhancement in Au nanoparticle decorated 2D-TiSe2

The combination of 2D materials and noble metallic nanostructure is becoming an attractive research domain for label-free, highly sensitive surface-enhanced Raman spectroscopy (SERS) applications. This study demonstrated photocatalysis degradation and SERS enhancements of organic fluorophore (Rhodamine 6G) on two-dimensional (2D) TiSe2using Raman spectroscopy. The Au nanoparticles (NPs) were decorated on TiSe2thin films by thermal annealing at variable temperatures. The selective deposition of Au NPs on the hexagonal TiSe2nanocrystals increases surface roughness, creating a larger surface area for molecule adsorption. It has been observed that the Au decoration at 250 °C on TiSe2exhibits efficient detection capabilities for R6G with the Raman intensity enhancement factors of the order of ≈105along with the significantly improved visible light-induced photodegradation efficiency. The optimized Au NP size creates large electromagnetic hot spots produced by strong plasmon coupling that assists in the charge transfer mechanism among TiSe2, Au NPs, and R6G for enhanced SERS and photocatalysis activities. It has been observed that the intensity of Raman scattering decreases as the Au NP size increases on the TiSe2material. A possible charge transfer mechanism is proposed with an energy band diagram. The simultaneous measurement of SERS and photocatalytic dye degradation in Au decorated TiSe2can be used as a sensitive technique for water pollution treatment and biodegradable organic contaminants for the environmental ecosystem.

Highly Flexible and Acid-Alkali Resistant TiN Nanomesh Transparent Electrodes for Next-Generation Optoelectronic Devices

Transparent electrodes are vital for optoelectronic devices, but their development has been constrained by the limitations of existing materials such as indium tin oxide (ITO) and newer alternatives. All face issues of robustness, flexibility, conductivity, and stability in harsh environments. Addressing this challenge, we developed a flexible, low-cost titanium nitride (TiN) nanomesh transparent electrode showcasing exceptional acid-alkali resistance. The TiN nanomesh electrode, created by depositing a TiN coating on a naturally cracked gel film substrate via a sputtering method, maintains a stable electrical performance through thousands of bending cycles. It exhibits outstanding chemical stability, resisting strong acid and alkali corrosion, which is a key hurdle for current electrodes when in contact with acidic/alkaline materials and solvents during device fabrication. This, coupled with superior light transmission and conductivity (88% at 550 nm with a sheet resistance of ∼200 Ω/sq), challenges the reliance on conventional materials. Our TiN nanomesh electrode, successfully applied in electric heaters and electrically controlled thermochromic devices, offers broad potential beyond harsh environment applications. It enables alternative possibilities for the design and fabrication of future optoelectronics for advancements in this pivotal field.

Liquid-Film Rupture for Web-like Ag Nanowires toward High-Performance Organic Schottky Barrier Transistors

Organic vertical transistors are promising device with benefits such as high operation speed, high saturation current density, and low-voltage operation owing to their short channel length. However, a short channel length leads to a high off-current, which is undesirable because it affects the on-off ratio and power consumption. This study presents a breakthrough in the development of high-performance organic Schottky barrier transistors (OSBTs) with a low off-current by utilizing a near-ideal source electrode with a web-like Ag nanowire (AgNW) morphology. This is achieved by employing a humidity- and surface-tension-mediated liquid-film rupture technique, which facilitates the formation of well-connected AgNW networks with large pores between them. Therefore, the gate electric field is effectively transmitted to the semiconductor layer. Also, the minimized surface area of the AgNWs causes complete suppression of the off-current and induces ideal saturation of the OSBT output characteristics. p- and n-type OSBTs exhibit off-currents in the picoampere range with on/off ratios exceeding 106 and 105, respectively. Furthermore, complementary inverters are prepared using an aryl azide cross-linker for patterning, with a gain of >16. This study represents a significant milestone in the development of high-performance organic vertical transistors and verifies their applicability in organic electronic circuitry.

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.

Flexible Transparent Conductive Electrodes: Unveiling Growth Mechanisms, Material Dimensions, Fabrication Methods, and Design Strategies

Flexible transparent conductive electrodes (FTCEs) constitute an indispensable component in state-of-the-art electronic devices, such as wearable flexible sensors, flexible displays, artificial skin, and biomedical devices, etc. This review paper offers a comprehensive overview of the fabrication techniques, growth modes, material dimensions, design, and their impacts on FTCEs fabrication. The growth modes, such as the "Stranski-Krastanov growth," "Frank-van der Merwe growth," and "Volmer-Weber growth" modes provide flexibility in fabricating FTCEs. Application of different materials including 0D, 1D, 2D, polymer composites, conductive oxides, and hybrid materials in FTCE fabrication, emphasizing their suitability in flexible devices are discussed. This review also delves into the design strategies of FTCEs, including microgrids, nanotroughs, nanomesh, nanowires network, and "kirigami"-inspired patterns, etc. The pros and cons associated with these materials and designs are also addressed appropriately. Considerations such as trade-offs between electrical conductivity and optical transparency or "figure of merit (FoM)," "strain engineering," "work function," and "haze" are also discussed briefly. Finally, this review outlines the challenges and opportunities in the current and future development of FTCEs for flexible electronics, including the improved trade-offs between optoelectronic parameters, novel materials development, mechanical stability, reproducibility, scalability, and durability enhancement, safety, biocompatibility, etc.

Electrical conductivity of crack-template-based transparent conductive films: A computational point of view

Crack-template-based transparent conductive films (TCFs) are promising kinds of junction-free, metallic network electrodes that can be used, e.g., for transparent electromagnetic interference shielding. Using image processing of published photos of TCFs, we have analyzed the topological and geometrical properties of such crack templates. Additionally, we analyzed the topological and geometrical properties of some computer-generated networks. We computed the electrical conductance of such networks against the number density of their cracks. Comparison of these computations with predictions of the two analytical approaches revealed the proportionality of the electrical conductance to the square root of the number density of the cracks was found, this being consistent with the theoretical predictions.

Photoelectrochemical UV Detector Based on High-Temperature Resistant ITO Nanowire Network Transparent Conductive Electrodes: Both the Response Range and Responsivity Are Improved

UV transparent conductive electrodes based on transferable ITO nanowire networks were prepared to solve the problem of low UV light utilization in conventional photoelectrochemical UV detectors. The mutually cross-linked ITO nanowire network achieved good electrical conductivity and light transmission, and the novel electrode had a transmission rate of more than 80% throughout the near-UV and visible regions. Compared to Ag nanowire electrodes with similar functionality, the chemical stability of the ITO nanowire transparent conductive electrode ensured that the device worked stably in iodine-based electrolytes. More importantly, ITO electrodes composed of oxides could withstand temperatures above 800 °C, which is extremely critical for photoelectrochemical devices. After the deposition of a TiO₂ active layer using the high-temperature method, the response range of the photoelectrochemical UV detector was extended from a peak-like response between 300-400 nm to a plateau-like response between 200-400 nm. The responsivity was significantly increased to 56.1 mA/W. The relationship between ITO nanowire properties and device performance, as well as the reasons for device performance enhancement, were intensively investigated.

Novel Mg- and Ga-doped ZnO/Li-Doped Graphene Oxide Transparent Electrode/Electron-Transporting Layer Combinations for High-Performance Thin-Film Solar Cells

Herein, a novel combination of Mg- and Ga-co-doped ZnO (MGZO)/Li-doped graphene oxide (LGO) transparent electrode (TE)/electron-transporting layer (ETL) has been applied for the first time in Cu₂ ZnSn(S,Se)₄ (CZTSSe) thin-film solar cells (TFSCs). MGZO has a wide optical spectrum with high transmittance compared to that with conventional Al-doped ZnO (AZO), enabling additional photon harvesting, and has a low electrical resistance that increases electron collection rate. These excellent optoelectronic properties significantly improved the short-circuit current density and fill factor of the TFSCs. Additionally, the solution-processable alternative LGO ETL prevented plasma-induced damage to chemical bath deposited cadmium sulfide (CdS) buffer, thereby enabling the maintenance of high-quality junctions using a thin CdS buffer layer (≈30 nm). Interfacial engineering with LGO improved the V_oc of the CZTSSe TFSCs from 466 to 502 mV. Furthermore, the tunable work function obtained through Li doping generated a more favorable band offset in CdS/LGO/MGZO interfaces, thereby, improving the electron collection. The MGZO/LGO TE/ETL combination achieved a power conversion efficiency of 10.67%, which is considerably higher than that of conventional AZO/intrinsic ZnO (8.33%).

Cracked Film Lithography with CuGaOx Buffers for Bifacial CdTe Photovoltaics

Bifacial CdTe solar cells with greater power density than the monofacial baselines are demonstrated by using a CuGaO_x rear interface buffer that passivates while reducing sheet resistance and contact resistance. Inserting CuGaO_x between the CdTe and Au increases mean power density from 18.0 ± 0.5 to 19.8 ± 0.4 mW cm^-2 for one sun front illumination. However, coupling CuGaO_x with a transparent conductive oxide leads to an electrical barrier. Instead, CuGaO_x is integrated with cracked film lithography (CFL)-patterned metal grids. CFL grid wires are spaced narrowly enough (≈10 µm) to alleviate semiconductor resistance while retaining enough passivation and transmittance for a bifacial power gain: bifacial CuGaO_x /CFL grids generate 19.1 ± 0.6 mW cm^-2 for 1 sun front + 0.08 sun rear illumination and 20.0 ± 0.6 mW cm^-2 at 1 sun front + 0.52 sun rear-the highest reported power density at field albedo conditions for a scaled polycrystalline absorber.

Engineered kirigami design of PVDF-Pt core-shell nanofiber network for flexible transparent electrode

Nanofiber networks comprising polymer-metal core-shell structures exhibit several advantages, such as high uniformities and considerable flexibilities. Additionally, the flexibility of the nanofiber network may be further enhanced by engineering the network topology. Therefore, in this study, the topologies of polyvinylidene fluoride (PVDF)-Pt core-shell nanofiber (CS NF) networks were engineered, and their performances as flexible transparent electrodes were comprehensively evaluated. Three distinct topologies of nanofiber networks were induced using circular, square, and rectangular electrode collectors. A highly uniform nanofiber network was obtained using the square electrode collector, which generated a high density of nanofiber junctions (nodes). Consequently, this nanofiber network exhibited the smallest sheet resistance [Formula: see text] and lowest optical transmittance [Formula: see text] among the three CS NF networks. In contrast, nanofiber bundles were frequently formed in the randomly aligned CS NF network prepared using the circular electrode collector, reducing the node density. As a result, it simultaneously exhibited a very small [Formula: see text] and high [Formula: see text], generating the largest percolation figure of merit [Formula: see text]. Under certain strain directions, the CS NF network with the engineered topology exhibited a significantly enhanced mechanical durability. Finally, a flexible piezoelectric pressure sensor with CS NF network electrodes was fabricated and its sensing performance was excellent.

High-Wettability Poly(dimethylsiloxane) Substrate for Ultrastable Conductive Three-Dimensional Woven Ag Nanowire Grids

Three-dimensional (3D) woven Ag nanowire (AgNW) grids have great potential for enhancing the mechanical stabilities, conductivity, and transmittance of flexible transparent electrodes (FTEs). However, it is a great challenge to control the formation of 3D woven AgNW grids on various substrates, especially the poly(dimethylsiloxane) (PDMS) substrate. This work presents a microtransfer-printing method for preparing a high-wettability poly(dimethylsiloxane) (PDMS) substrate to control the formation of 3D woven AgNW grids. The as-prepared PDMS substrate shows a high wettability performance. The surface structures of the PDMS substrate can control the sharp shrinkage of the ink membrane to give rise to a uniform liquid membrane evaporation behavior, which is the key factor for preparing a uniform 3D woven nanowire network. A thin uniform 3D woven AgNW network with a low sheet resistance of 24.3 Ω/□ and high transmittance of 92% was coated on the PDMS substrate. The networks directly coated the surface of the replicated PDMS, which simplified the peeling process and protected the networks from peeling strain and mechanical deformations. Moreover, the increment of resistance retained a small value (∼5%) when bending cycles reached 9,000. An alternating current electroluminescent (ACEL) device was prepared, and the uniform electroluminescence implies that a defect-free electrode has been fabricated. These results indicate that the as-prepared FTEs have excellent mechanical performance and great potential for flexible optoelectronic applications.

Crack-Templated Wire-Like Semitransparent Electrodes with Unique Irregular Patterns

The development of novel methods of producing transparent electrodes is important because of their ever-evolving applications and thus the additional parameters they must meet. In this work, we present a method of manufacturing semitransparent silver electrodes. This technique involves cracking the polyvinylpyrrolidone layer in the presence of a colloidal nanodispersion of zinc oxide. The resulting cracked polymer layer serves as the disposable mask for metal deposition. The whole procedure is valuable due to the fast and easy step of cracks formation caused by the elevated temperature and reduced pressure. The obtained electrodes have high transparency (82.4%) in a wide spectral range, which is only limited by the transparency of the applied substrate, and low resistivity (27.3 × 10^-7 Ωm). The presence of unique patterns suggests new ideas for the applications of such electrodes, such as coding, security, and antiplagiarism protection.

Self-assembly, alignment, and patterning of metal nanowires

Armed with the merits of one-dimensional nanostructures (flexibility, high aspect ratio, and anisotropy) and metals (high conductivity, plasmonic properties, and catalytic activity), metal nanowires (MNWs) have stood out as a new class of nanomaterials in the last two decades. They are envisaged to expedite significantly and even revolutionize a broad spectrum of applications related to display, sensing, energy, plasmonics, photonics, and catalysis. Compared with disordered MNWs, well-organized MNWs would not only enhance the intrinsic physical and chemical properties, but also create new functions and sophisticated architectures of optoelectronic devices. This paper presents a comprehensive review of assembly strategies of MNWs, including self-assembly for specific structures, alignment for anisotropic constructions, and patterning for precise configurations. The technical processes, underlying mechanisms, performance indicators, and representative applications of these strategies are described and discussed to inspire further innovation in assembly techniques and guide the fabrication of optoelectrical devices. Finally, a perspective on the critical challenges and future opportunities of MNW assembly is provided.

Cost-Effective Fabrication of Uniformly Aligned Silver Nanowire Microgrid-Based Transparent Electrodes with Higher than 99% Transmittance

Silver nanowire (Ag NW)-based transparent electrodes (TEs) are promising alternatives to indium tin oxide (ITO) for next-generation flexible optoelectronic devices. Although many different constructs of Ag NW networks and post-treatment methods have been developed for TE applications, trade-offs between optical and electrical performance still remain. Herein, aided by electrohydrodynamic (EHD) printing, we present a cost-effective strategy to fabricate aligned Ag NW microgrids in a large area with excellent uniformity, resulting in superior optoelectronic properties. Guided by the percolation theory and simulation, we demonstrated that by confining aligned Ag NWs into a microgrid arrangement, the percolation threshold can be reduced significantly and adequate electrical conducting pathways can be achieved with an optimized combination of sheet resistance and optical transparency, which surpass conventional random Ag NW networks and random aligned Ag NW networks. The resulting TEs exhibit an ultrahigh transmittance of 99.1% at a sheet resistance of 91 Ω sq^-1 with extremely low nanowire usage, an areal mass density of only 8.3 mg m^-2, and uniform spatial distribution. Based on this TE design, we demonstrated transparent heaters exhibiting rapid thermal response and superior uniformity in heat generation. Using UV-curable epoxy, highly flexible Ag NW-embedded TEs were fabricated with superior mechanical stabilities and low surface roughness of 2.6 nm. Bendable organic light-emitting diodes (OLEDs) are directly fabricated on these flexible Ag NW electrodes, with higher current efficiency (27.7 cd A^-1) than ITO devices (24.8 cd A^-1).

High-Performance Flexible Pressure Sensor with a Self-Healing Function for Tactile Feedback

High-performance flexible pressure sensors have attracted a great deal of attention, owing to its potential applications such as human activity monitoring, man-machine interaction, and robotics. However, most high-performance flexible pressure sensors are complex and costly to manufacture. These sensors cannot be repaired after external mechanical damage and lack of tactile feedback applications. Herein, a high-performance flexible pressure sensor based on MXene/polyurethane (PU)/interdigital electrodes is fabricated by using a low-cost and universal spray method. The sprayed MXene on the spinosum structure PU and other arbitrary flexible substrates (represented by polyimide and membrane filter) act as the sensitive layer and the interdigital electrodes, respectively. The sensor shows an ultrahigh sensitivity (up to 509.8 kPa^-1 ), extremely fast response speed (67.3 ms), recovery speed (44.8 ms), and good stability (10 000 cycles) due to the interaction between the sensitive layer and the interdigital electrodes. In addition, the hydrogen bond of PU endows the device with the self-healing function. The sensor can also be integrated with a circuit, which can realize tactile feedback function. This MXene-based high-performance pressure sensor, along with its designing/fabrication, is expected to be widely used in human activity detection, electronic skin, intelligent robots, and many other aspects.

Nanosphere Lithography: A Versatile Approach to Develop Transparent Conductive Films for Optoelectronic Applications

Transparent conductive films (TCFs) are irreplaceable components in most optoelectronic applications such as solar cells, organic light-emitting diodes, sensors, smart windows, and bioelectronics. The shortcomings of existing traditional transparent conductors demand the development of new material systems that are both transparent and electrically conductive, with variable functionality to meet the requirements of new generation optoelectronic devices. In this respect, TCFs with periodic or irregular nanomesh structures have recently emerged as promising candidates, which possess superior mechanical properties in comparison with conventional metal oxide TCFs. Among the methods for nanomesh TCFs fabrication, nanosphere lithography (NSL) has proven to be a versatile platform, with which a wide range of morphologically distinct nanomesh TCFs have been demonstrated. These materials are not only functionally diverse, but also have advantages in terms of device compatibility. This review provides a comprehensive description of the NSL process and its most relevant derivatives to fabricate nanomesh TCFs. The structure-property relationships of these materials are elaborated and an overview of their application in different technologies across disciplines related to optoelectronics is given. It is concluded with a perspective on current shortcomings and future directions to further advance the field.

Directly Printed Embedded Metal Mesh for Flexible Transparent Electrode via Liquid Substrate Electric-Field-Driven Jet

Flexible transparent electrodes (FTEs) with embedded metal meshes play an indispensable role in many optoelectronic devices due to their excellent mechanical stability and environmental adaptability. However, low-cost, simple, efficient, and environmental friendly integrated manufacturing of high-performance embedded metal meshes remains a huge challenge. Here, a facile and novel fabrication method is proposed for FTEs with an embedded metal mesh via liquid substrateelectric-field-driven microscale 3D printing process. This direct printing strategy avoids tedious processes and offers low-cost and high-volume production, enabling the fabrication of high-resolution, high-aspect ratio embedded metal meshes without sacrificing transparency. The final manufactured FTEs with 80 mm × 80 mm embedded metal mesh offers excellent optoelectronic performance with a sheet resistance (R_s ) of 6 Ω sq^-1 and a transmittance (T) of 85.79%. The embedded metal structure still has excellent mechanical stability and good environmental suitability under different harsh working conditions. The practical feasibility of the FTEs is successfully demonstrated with a thermally driven 4D printing structure and a resistive transparent strain sensor. This method can be used to manufacture large areas with facile, high-efficiency, low-cost, and high-performance FTEs.

Advances in Flexible Metallic Transparent Electrodes

Transparent electrodes (TEs) are pivotal components in many modern devices such as solar cells, light-emitting diodes, touch screens, wearable electronic devices, smart windows, and transparent heaters. Recently, the high demand for flexibility and low cost in TEs requires a new class of transparent conductive materials (TCMs), serving as substitutes for the conventional indium tin oxide (ITO). So far, ITO has been the most used TCM despite its brittleness and high cost. Among the different emerging alternative materials to ITO, metallic nanomaterials have received much interest due to their remarkable optical-electrical properties, low cost, ease of manufacturing, flexibility, and widespread applicability. These involve metal grids, thin oxide/metal/oxide multilayers, metal nanowire percolating networks, or nanocomposites based on metallic nanostructures. In this review, a comparison between TCMs based on metallic nanomaterials and other TCM technologies is discussed. Next, the different types of metal-based TCMs developed so far and the fabrication technologies used are presented. Then, the challenges that these TCMs face toward integration in functional devices are discussed. Finally, the various fields in which metal-based TCMs have been successfully applied, as well as emerging and potential applications, are summarized.

Mesh size control in forming an Ag/AgO nano-network structure for transparent conducting application

The variation behaviors of the morphology, transmission, and sheet resistance of the surface Ag/AgO nano-network (NNW) structures fabricated under different illumination conditions and with different Ag deposition thicknesses and thermal annealing temperatures in forming initial Ag nanoparticles (NPs) are studied. Generally, an NNW structure with a smaller mesh size or a denser branch distribution has a lower transmission and a lower sheet resistance level. Under the fabrication condition of a broader illumination spectrum, a lower thermal annealing temperature, or a thicker Ag deposition, we can obtain an NNW structure of a smaller mesh size. The mesh size of an NNW structure is basically controlled by the seed density of Brownian tree (BT) at the beginning of light illumination. A BT seed can be formed through a stronger local localized surface plasmon resonance for accelerating Ag oxidation in a certain region. Once an Ag/AgO BT seed is formed, the surrounding Ag NPs are reorganized to form the branches of a BT. Multiple BTs are connected to form a large-area NNW structure, which can serve as a transparent conductor. Under the fabrication conditions of a broader illumination spectrum, 3 nm Ag deposition, and 100 °C thermal annealing, we can implement an NNW structure to achieve ∼1.15μm in mesh size, ∼90 Ω sq^-1in sheet resistance, and 93%-77% in transmittance within the wavelength range between 370 and 700 nm.

Bio-inspired strategies for next-generation perovskite solar mobile power sources

Smart electronic devices are becoming ubiquitous due to many appealing attributes including portability, long operational time, rechargeability and compatibility with the user-desired form factor. Integration of mobile power sources (MPS) based on photovoltaic technologies with smart electronics will continue to drive improved sustainability and independence. With high efficiency, low cost, flexibility and lightweight features, halide perovskite photovoltaics have become promising candidates for MPS. Realization of these photovoltaic MPS (PV-MPS) with unconventionally extraordinary attributes requires new 'out-of-box' designs. Natural materials have provided promising designing solutions to engineer properties under a broad range of boundary conditions, ranging from molecules, proteins, cells, tissues, apparatus to systems in animals, plants, and humans optimized through billions of years of evolution. Applying bio-inspired strategies in PV-MPS could be biomolecular modification on crystallization at the atomic/meso-scale, bio-structural duplication at the device/system level and bio-mimicking at the functional level to render efficient charge delivery, energy transport/utilization, as well as stronger resistance against environmental stimuli (e.g., self-healing and self-cleaning). In this review, we discuss the bio-inspired/-mimetic structures, experimental models, and working principles, with the goal of revealing physics and bio-microstructures relevant for PV-MPS. Here the emphasis is on identifying the strategies and material designs towards improvement of the performance of emerging halide perovskite PVs and strategizing their bridge to future MPS.

Stretchable Transparent Electrode via Wettability Self-Assembly in Mechanically Induced Self-Cracking

Stretchable and transparent electrodes (STEs) are indispensable components in numerous emerging applications such as optoelectrical devices and wearable devices used in health monitoring, human-machine interaction, and artificial intelligence. However, STEs have limitations in conductivity, robustness, and transmittance owing to the exposure of the substrate and fatigue deformation of nanomaterials under strain. In this study, an STE consisting of conductive materials embedded in in situ self-cracking strain-spread channels by wettability self-assembly is fabricated. Finite element analysis is used to simulate the crevice growth using the representative unit cell network and strain deformation using a random network. The embedded conductive materials are partly protected by the strain-opening crevice channel, and network dissociation is avoided under stretching, showing a maximum strain of 125%, a transmittance of approximately 89.66% (excluding the substrate) with a square resistance of 9.8 Ω sq-1, and high stability in an environment with high temperature and moisture. The wettability self-assembly coating process is verified and expanded to several kinds of hydrophilic inks and hydrophobic coating materials. The fabricated STE can be employed as a strain sensor in motion sensing, vital sign and posture feedback, and mimicking bioelectronic spiderweb with spatial gravity induction.

Time Evolution and Spatial Hierarchy of Crack Patterns

Cracks generated due to desiccation of wet colloidal systems are ubiquitous, examples being nanomaterial films, painted walls, cemented floors, mud fields, river beds, and even giant rocks. In all such cases, crack patterns are often appreciably similar but for the length and time scales, which can be widely differing. In this work, we have examined the crack formation more closely to see if there exists some generality with regard to the length scale of parameters and the formation time. Specifically, using a commonly used colloidal dispersion and optimized conditions to form polygonal network patterns rather than isolated cracks (films of subcritical thickness), we have studied the time evolution of the pattern parameters, the area occupied by the cracks, their lengths, and the widths. As is well known, initially, a network of cracks forms, which we term as the primary generation, followed by interconnecting cracks inside the polygonal regions (secondary) and, later, cracks spreading in local regions (tertiary). We find that the area and the width increase nearly linearly with time with the change in the slope corresponding to the change in the generation. When normalized with respect to the final values, the trends obtained for different film thicknesses overlap, the only exception being the pattern containing unconnected cracks. Thus, the time evolution of cracks is shown to be predictable based on width filtering. Including the angle between cracks as further input into the recursive model, the possibility of identifying the hierarchy of crack segments is also shown. The approach may be useful in determining the age, authenticity, and details of old paintings, understanding the stress profile of geological rocks, and analyzing various natural and manmade hierarchical structures.

Cohesively Enhancing the Conductance, Mechanical Robustness, and Environmental Stability of Random Metallic Mesh Electrodes via Hot-Pressing-Induced In-Plane Configuration

Flexible transparent conductive electrode (FTCE) is highly desirable due to the fast-growing flexible optoelectronic devices. Several promising FTCEs based on metal material have been developed to replace conventional indium tin oxide (ITO). The random metal mesh is considered to be one of the competitive candidates. However, obtaining feasible random metal mesh with low sheet resistance, high transparency, good mechanical durability, and strong environmental stability is still a great challenge. Here, a random metal mesh-based FTCE with an in-plane structure, achieved by a facile hot-pressing process, is demonstrated. The hot-pressing process enables the fabrication of highly conductive FTCE with improved mechanical robustness and environmental stability. The in-plane FTCE shows a low sheet resistance of 1.63 Ω·sq^-1 with an 80.6% transmittance, low relative resistance increase (RRI) of 7.9% after 240 h 85 °C/85% RH test, and low RRI of 8.0% after 10⁵ cycles of bending test. Besides, various applications of the in-plane FTCE were demonstrated, including the flexible heater, flexible touch screen, and flexible electroluminescence. We anticipate that these results will spark interest in in-plane random metal mesh electrodes and enable the application of random metal mesh in flexible optoelectronic devices.

Patterned few nanometer-thick silver films with high optical transparency and high electrical conductivity

Transparent conductive electrodes (TCEs) are experimentally demonstrated using patterned few nanometer-thick silver films on zinc oxide-coated rigid and flexible substrates. The grid lines are completely continuous, but only 8.4 nm thick. This is the thinnest metallic grid we are aware of. Owing to the high transparency of both the grid lines and spacing, our TCE with an opening ratio (OR) as small as 36% achieves an average optical transmittance up to ∼90% in the visible regime, breaking the optical limits of both the unpatterned film counterpart and the thick grid counterpart (whose optical transmittance is determined by the OR). The small OR enables a low sheet resistance of ∼21.5 Ω sq⁻¹. The figure of merit up to ∼17 kΩ⁻¹ is superior to those of the unpatterned film counterpart, our fabricated 180 nm thick ITO, as well as most reported thick metal grid TCEs. Our ultrathin TCE, firmly attached to the substrate, is mechanically more flexible and more stable than the film counterpart and ITO. As a flexible transparent film heater, it achieves comparable or even superior heating performances with previously-reported heaters and performs well in a thermochromic test.

Patterned few nanometers thick silver films with high optical transparency, electrical conductivity, mechanical flexibility and stability.

Non-Iridescent Metal Nanomesh with Disordered Nanoapertures Fabricated by Phase Separation Lithography of Polymer Blend as Transparent Conductive Film

Metallic nanomesh, one of the emerging transparent conductive film (TCF) materials with both high electrical conductivity and optical transmittance, shows great potential to replace indium tin oxide (ITO) in optoelectronic devices. However, lithography-fabricated metallic nanomeshes suffer from an iridescence problem caused by the optical diffraction of periodic nanostructures, which has negative effects on display performance. In this work, we propose a novel approach to fabricate large-scale metallic nanomesh as TCFs on flexible polyethylene terephthalate (PET) sheets by maskless phase separation lithography of polymer blends in a low-cost and facile process. Polystyrene (PS)/polyphenylsilsequioxane (PPSQ) polymer blend was chosen as resist material for phase separation lithography due to their different etching selectivity under O₂ reactive ion etching (RIE). The PS constituent was selectively removed by O₂ RIE and the remained PPSQ nanopillars with varying sizes in random distribution were used as masks for further pattern transfer and metal deposition process. Gold (Au) nanomeshes with adjustable nanostructures were achieved after the lift-off step. Au nanomesh exhibited good optoelectronic properties (R_S = 41 Ω/sq, T = 71.9%) and non-iridescence, without angle dependence owing to the aperiodic structures of disordered apertures. The results indicate that this Au nanomesh has high potential application in high-performance and broad-viewing-angle optoelectronic devices.

Fabrication of Silver Mesh/Grid and Its Applications in Electronics

With the development of flexible electronics, researchers have endeavored to improve the characteristics of the commonly used indium tin oxide such as brittleness, poor mechanical or chemical stability, and scarcity. Currently, many alternative materials have been considered such as conductive polymers, graphene, carbon nanotubes, metallic nanoparticles (NPs), nanowires (NWs), or nanofibers. Among them, silver (Ag) mesh/grid NPs or NWs have been considered as an excellent substitute due to the good transmittance, excellent electrical conductivity, outstanding mechanical robustness, and cost competitiveness. So far, much effort has been devoted to the fabrication of Ag mesh/grid, and many methods such as printing technology, self-assembly, electrospun, hot-pressing, and atomic layer deposition have been reported. Here printing technologies include jet printing, gravure printing, screen printing, nanoimprint lithography, microcontact printing, and flexographic printing. The solution-based self-assembly usually combines with coating, template, or mask assistance. This review summarizes the characteristics of these fabrication methods for the Ag mesh/grid with its related applications in electronics. Then the prospect and challenges of the fabrication methods are discussed, and the new preparation approaches and applications of the Ag mesh/grid are highlighted, which will be of significance for the applications in electronics such as transparent conducting electrodes, organic light-emitting diode, energy harvester, strain sensor, cells, etc.

Titanium Nanopillar Arrays Functioning as Electron Transporting Layers for Efficient, Anti-Aging Perovskite Solar Cells

Electron transporting layers (ETLs), required to be optically transparent in perovskite solar cells (PSCs) having regular structures, possess a determinant effect on electron extraction and collection. Metal oxides (e.g., TiO₂ ) have overwhelmingly served as ETLs, but usually have low electron mobility (μ_e  < 10^-2  cm² V^-1 s^-1 ) not favorable for photovoltaic conversion. Here, metal oxides are replaced with metals (e.g., Ti with μ_e  ≈ 294 cm² V^-1 s^-1 ) that are sculptured via glancing angle deposition to be a close-packed nanopillar array (NaPA), which vertically protrudes on a transparent electrode to obtain sufficient optical transmission for light harvesting in perovskite. Ti NaPAs, whose rough surfaces are passivated with 5 nm thick TiO₂ (i.e., Ti NaPAs@TiO₂ ) to suppress exciton recombination, lead to the champion power conversion efficiency (PCE) of 18.89% that is superior to that of MAPbI₃ PSCs without Ti NaPAs@TiO₂ or containing TiO₂ NaPAs@TiO₂ , owing to high surface wettability, high μ_e , and relatively low work function of Ti. Furthermore, Ti NaPAs@TiO₂ effectively prevents the decomposition of MAPbI₃ to achieve long-term shelf stability whereby 50-day aging only causes 15% PCE degradation. This work paves the way toward widening the material spectrum, from semiconductors to metals, to generate a diverse range of ETLs for producing efficient optoelectronic devices with long-term shelf stability.

Flexible Transparent Crystalline-ITO/Ag Nanowire Hybrid Electrode with High Stability for Organic Optoelectronics

Metal nanowires (NWs) are promising transparent conducting electrode (TCE) materials because of their excellent optoelectrical performance, intrinsic mechanical flexibility, and large-scale processability. However, the surface roughness, thermal/chemical instability, and limited electrical conductivity associated with empty spaces between metal NWs are problems that are yet to be solved. Here, we report a highly reliable and robust composite TCE/substrate all-in-one platform that consists of crystalline indium tin oxide (c-ITO) top layer and surface-embedded metal NW (c-ITO/AgNW-GFRH) films for flexible optoelectronics. The c-ITO top layer (thickness: 10-30 nm) greatly improves the electrical performance of a AgNW-based electrode, retaining its transparency even after a high-temperature annealing process at 250 °C because of its thermally stable basal substrate (i.e., AgNW-GFRH). By introducing c-ITO thin film, we achieve an extremely smooth surface (R_rms < 1 nm), excellent optoelectrical performance, superior thermal (> 250 °C)/chemical stability (in sulfur-contained solution), and outstanding mechanical flexibility (bending radius = 1 mm). As a demonstration, we fabricate flexible organic devices (organic photovoltaic and organic light-emitting diode) on c-ITO/AgNW-GFRH films that show device performance comparable to that of references ITO/glass substrates and superior mechanical flexibility. With excellent stability and demonstrations, we expect that the c-ITO/AgNW-GFRHs can be used as flexible TCE/substrate films for future thin-film optoelectronics.

Patterning Metal Grids for GaAs Solar Cells with Cracked Film Lithography: Quantifying the Cost/Performance Tradeoff

We introduce cracked film lithography (CFL) as a way to reduce the cost of III-V photovoltaics (PV). We spin-coat nanoparticle suspensions onto GaAs thin-film device stacks. The suspensions dry in seconds, forming crack networks that we use as templates through which to electroplate the solar cells' front metal grids. For the first time, we show that heating the crack template allows it to flow and refill cracks, which decreases crack footprint and improves final grid transmittance. We demonstrate 24.7%-efficient single-junction GaAs solar cells using vacuum-free CFL grids. These devices are only 1.7% (absolute) less efficient than the baseline grids patterned by photolithography with the loss mostly resulting from the reduced transparency of the CFL pattern. Additional optimization could decrease this difference. Initial cost modeling suggests that CFL is more scalable than photolithography: In particular, CFL's lower materials and equipment costs could greatly reduce the levelized cost of electricity of III-V PV at scale, a potential step toward terrestrial deployment.

Recent progress in controlled nano/micro cracking as an alternative nano-patterning method for functional applications

Generally, cracking occurs for many reasons connected to uncertainties and to the non-uniformity resulting from intrinsic deficiencies in materials or the non-linearity of applied external (thermal, mechanical, etc.) stresses. However, recently, an increased level of effort has gone into analyzing the phenomenon of cracking and also into methods for controlling it. Sophisticated manipulation of cracking has yielded various cutting-edge technologies such as transparent conductors, mechanical sensors, microfluidics, and energy devices. In this paper, we present some of the recent progress that has been made in controlling cracking by giving an overview of the fabrication methods and working mechanisms used for various mediums. In addition, we discuss recent progress in the various applications of methods that use controlled cracking as an alternative to patterning tools.

Macroscopic Nonuniformities in Metal Grids Formed by Cracked Film Lithography Result in 19.3% Efficient Solar Cells

Cracked film lithography (CFL) is an emerging method for patterning transparent conductive metal grids. CFL can be vacuum- and Ag-free, and it forms more durable grids than nanowire approaches. In spite of CFL's promising transmittance/grid sheet resistance/wire spacing tradeoffs, previous solar cell demonstrations have had relatively low performance. This work introduces macroscopic nonuniformities in the grids to improve the short-circuit current density/fill factor tradeoff in small area Cu(In,Ga)Se₂ cells. The performance of optimized baseline grids is matched by CFL grids with microscopic openings and macroscopic patterns, culminating in a 19.3% efficient cell. Simulations show that uniform CFL grids are enhanced by patterning because it leads to better balance among shadowing, grid resistance, and transparent conductive oxide resistance losses. Thin-film module efficiency calculations are performed to highlight the performance gains that metal grids can enable by eliminating the transparent conductive oxide losses and widening monoliths. Adding the patterned CFL grids demonstrated in this work to CIGS modules is predicted to reach 0.7% higher efficiency (absolute) than screen-printed grids.

Active and Deformable Organic Electronic Devices based on Conductive Shape Memory Polyimide

Smart, deformable, and transparent electrodes are a significant part of flexible optoelectronic devices. In this work, a novel approach to making highly transparent, smooth, and conductive shape memory polyimide hybrids has been proposed. Colorless shape memory polyimide (CSMPI) with high optical transparency and high heat resistance is served as the substrate for flexible electronic devices for the first time. A hybrid (Au/Ag) metal grid electrode embedded in CSMPI (BMG/CSMPI) is first fabricated via self-cracking template and solution-coating, the advantages of which include ultrasmooth surface, superior mechanical flexibility and durability, strong surface adhesion, and excellent chemical stability due to the unique embedded hybrid structure. The resulting white polymer light emitting diodes (WPLEDs) based on BMG/CSMPI with shape memory effect are active and deformable, and are converted from 2D device into 3D devices depending on its variable stiffness characteristics. The deformed 3D devices could actively recover to the original shape upon heating. Furthermore, ultrathin and flexible 3D optoelectronic devices fabricated using shape memory polymers can promote the development of advanced optoelectronic applications in the future.

Naturally Derived Wearable Strain Sensors with Enhanced Mechanical Properties and High Sensitivity

Flexible strain sensors are of great interest for future applications in the next-generation wearable electronic devices. However, most of the existing flexible sensors are based on synthetic polymer materials with limitations in biocompatibility and biodegradability, which may lead to potential environmental pollution. Here, we propose a naturally derived wearable strain sensor based on natural-sourced materials including milk protein fabric, natural rubber, tannic, and vitamin C. The obtained sensors exhibit remarkably enhanced mechanical properties and high sensitivity contrast to currently reported natural resource-based sensors, owing to the metal-ligand interface design and the construction of an organized three-dimensional conductive network, which well fit the requirements of electronic skin. This work represents an important advance toward the fabrication of naturally derived high-performance strain sensors and expanding possibilities in the design of environmental-friendly soft actuators, artificial muscle, and other wearable electronic devices.

Fundamentals of Using Cracked Film Lithography to Pattern Transparent Conductive Metal Grids for Photovoltaics

The fundamentals of using cracked film lithography (CFL) to fabricate metal grids for transparent contacts in solar cells were studied. The underlying physics of drying-induced cracks were well-predicted by an empirical correlation relating crack spacing to capillary pressure. CFL is primarily controlled by varying the crack template thickness, which establishes a three-way tradeoff between the areal density of cracks, crack width, and spacing between cracks, which in turn determine final grid transmittance, grid sheet resistance, and the semiconductor resistance for a given solar cell. Since CFL uses a lift-off process, an additional constraint is that the metal thickness must be less than 1/3 of the crack template thickness. The transmittance/grid sheet resistance/wire spacing tradeoffs measured in this work were used to calculate solar cell performance: CFL-patterned grids should outperform screen-printed grids for narrow cells (0.5-2 cm wide) and/or cells with high semiconductor sheet resistance (≥100 Ω/sq), making CFL attractive for monolithically integrated thin-film photovoltaic modules.

Selective multi-nanosoldering for fabrication of advanced solution-processed micro/nanoscale metal grid structures

Solution-processed metal grid transparent conductors with low sheet resistance, high optical transmittance and good mechanical flexibility have great potential for use in flexible optoelectronic devices. However, there are still remaining challenges to improve optoelectrical properties and electromechanical stability of the metallic structures due to random loose packings of nanoparticles and the existence of many pores. Here we introduce a selective multi-nanosoldering method to generate robust metallic layers on the thin metal grid structures (< a thickness of 200 nm), which are generated via self-pining assisted direct inking of silver ions. The selective multi-nanosoldering leads to lowering the sheet resistance of the metal grid transparent conductors, while keeping the optical transmittance constant. Also, it reinforces the electromechanical stability of flexible metal grid transparent conductors against a small bending radius or a repeated loading. Finally, organic light-emitting diodes based on the flexible metal grid transparent conductors are demonstrated. Our approach can open a new route to enhance the functionality of metallic structures fabricated using a variety of solution-processed metal patterning methods for next-generation optoelectronic and micro/nanoelectronic applications.

Geometrical optimization for silver nanowire mesh as a flexible transparent conductive electrode

We report the effect of the geometric parameters on transparency and conductivity in a metallic nanowire mesh as a transparent electrode. Today, indium tin oxide and fluorine-doped tin oxide are used as the transparent electrode for displays and solar cells. Still, there is a definite need for their replacement due to drawbacks such as brittleness, scarcity, and adverse environmental effects. Metallic nanowire mesh is likely the best replacement option, but the main issue is how to find the optimal structure and how to get the best performance. Since the interaction of light with nanowire mesh is complicated, there is no straightforward rule with a simple analytical solution. We developed a kit based on wave optics for calculating the optical transmission of metallic nanowire mesh, which, unlike previous works, includes the interaction of light with the nanowire mesh, such as localized surface plasmon resonance (LSPR), surface plasmon polariton (SPP), and Rayleigh anomaly (RA). So, it is possible to accurately predict the effect of these phenomena and the transmission of mesh. Using the mentioned kit, we will be able to investigate the different geometrical structures of meshes to achieve optimal geometry. This kit is based on the classical Maxwell theory and empirical data and uses finite-difference time-domain for solving equations and experiential results for validation. Comparing the results by a redefined figure of merit shows that LSPR has the most significant reduction on transparency, whereas increasing the thickness (t) to width (w) ratio of the nanowire in the metallic mesh can reduce the LSPR effect and/or shifts it to the invisible region. The wire pitch (p) has no tangible impact on LSPR, but p can be chosen higher than 700 or lower than 350 nm to remove the extinction effects of the first-order RA. If p was larger than 150 nm, SPP could appear in the visible region of the spectrum. In small p, lower modes of SPP with higher intensities occur; therefore, there is an optimum value for p around 300 nm. The reduction of t and w reduces the intensity of SPP and causes it to red shift. By comparing the 900 different structures, the highest figure of merit is obtained in a p of 300 nm with a minimum w (10 nm) and maximum t (100 nm).

A Supersensitive, Multidimensional Flexible Strain Gauge Sensor Based on Ag/PDMS for Human Activities Monitoring

For more comprehensive monitoring human state of motion, it is necessary to sense multidimensional stimulus information. In this paper, we reported a supersensitive flexible sensor based on Ag/PDMS composites with sensing abilities of strain and force. The fabrication method is simple and rapid, which only need physically grinding the silver particles and mixing with liquid PDMS. The flexible sensor has excellent performances in multidimensional detection. The strain gauge factor can reach as high as 939 when it was stretched to 36%, and the minimum resolution for force detection is 0.02 N. The sensing characteristic of the sensors with different filling fraction and thickness were analyzed from the microscopic point of view. Multidimensional sensing abilities of flexible sensor have greatly expands its applications. We experimentally verified the Ag/PDMS based sensor in human body dynamic monitoring and sound detecting in real-time, which has shown great potential in motion recognition, haptic perception and soft robotics.

Recrystallized ice-templated electroless plating for fabricating flexible transparent copper meshes

Flexible transparent conductors as a replacement for indium tin oxide (ITO) have been urgently pursued due to the inherent drawbacks of ITO films. Here, we report the fabrication of flexible transparent copper meshes with recrystallized ice-crystal templates. Completely different to conventional approaches, this novel method needs neither the fabrication of mesh patterns via micro/nanofabrication technologies nor the deposition of copper through evaporation or sputtering. The linewidth and mesh size of the prepared copper meshes can be regulated, as the ice recrystallization process is controllable. Therefore, the formed copper meshes have tailorable conductivity and transparency, which are critical for optoelectronic devices. Remarkably, the electrical performance of the copper meshes is maintained even after storing for 60 days in ambient conditions or bending for 1000 cycles. This strategy is modular and can also be employed to prepare other metal meshes, such as silver meshes, offering versatile substitutes for ITO in electronic devices.

Herein, we report the fabrication of flexible copper meshes using recrystallized ice-crystal templates. The linewidth and mean size of the copper meshes can be tuned by adjusting the ice grains.

A leaf vein-like hierarchical silver grids transparent electrode towards high-performance flexible electrochromic smart windows

As essential components of numerous flexible and wearable optoelectronic devices, the flexible transparent conducting electrodes (TCEs) with sufficient optical transmittance and electric conductivity become more and more important. In this work, we fabricated a large-area flexible TCE based on leaf vein-like hierarchical metal grids (HMG) comprising of mesoscale "trunk" and microscale "branches". The self-formed branched grids made the conducting paths distributing uniformly while the laser-etching trunk grids enabled to transport the collected electrons across long-distance. The Ag HMG exhibited high optical transmittance (~81%) with low sheet resistance (1.36 Ω sq^-1), which could be simply optimized through adjusting the grids' widths, spaces, and the sizes of the TiO₂ colloidal crackle patterns. In addition, on the basis of such advanced HMG electrode, flexible electrochromic devices (ECDs) with remarkable cyclic performance were fabricated. The HMG with high transparency, conductivity, and flexibility provides a promising TCE for the next-generation flexible and wearable optoelectronic devices.

Biomimic Vein-Like Transparent Conducting Electrodes with Low Sheet Resistance and Metal Consumption

In this contribution, inspired by the excellent resource management and material transport function of leaf veins, the electrical transport function of metallized leaf veins is mimicked from the material transport function of the vein networks. By electroless copper plating on real leaf vein networks with copper thickness of only several hundred nanometre up to several micrometre, certain leaf veins can be converted to transparent conductive electrodes with an ultralow sheet resistance 100 times lower than that of state-of-the-art indium tin oxide thin films, combined with a broadband optical transmission of above 80% in the UV-VIS-IR range. Additionally, the resource efficiency of the vein-like electrode is characterized by the small amount of material needed to build up the networks and the low copper consumption during metallization. In particular, the high current density transport capability of the electrode of > 6000 A cm^-2 was demonstrated. These superior properties of the vein-like structures inspire the design of high-performance transparent conductive electrodes without using critical materials and may significantly reduce the Ag consumption down to < 10% of the current level for mass production of solar cells and will contribute greatly to the electrode for high power density concentrator solar cells, high power density Li-ion batteries, and supercapacitors.

Fabrication of a Highly Flexible and Affordable Transparent Electrode By Aligned U-Shaped Copper Nanowires Using a New Electrospinning Collector with Convenient Transferability

By making aligned and suspended copper nanowires, a high performance, transferable, and flexible transparent electrode is reported. Indium tin oxide is often used in devices such as displays, solar cells, and touchscreens that require transparent and conductive plates. Because of problems such as brittleness, high cost, and environmental effects, this material is facing rivals, the most serious of which are metallic nanowire meshes, especially copper. We developed a simple technique which uses a U-shaped collector in the electrospinning process with three advantages including the enhancement of the figure of merit (which is related to the surface resistance R _s and the transmittance T) by about five times (about T = 90% and R _s = 5 Ω/□, respectively), solving the transfer problem of the nanowire metal mesh after production, and producing aligned metal nanowires for special applications. In this work, T and R _s of aligned copper nanowires were both measured and calculated, which are consistent with each other, and also, the mentioned results were compared with the work of others.

Narrowing Desiccating Crack Patterns by an Azeotropic Solvent for the Fabrication of Nanomesh Electrodes

Desiccation of a colloidal layer produces crack patterns because of stress arising out of solvent evaporation. Associated with it is the rearrangement of particles, while adhesion to the substrate resists such movements. The nature of solvent, which is often overlooked, plays a key role in the process as it dictates evaporation and wetting properties of the colloidal film. Herein, we study the crack formation process by using a mixture of solvents, water, and isopropyl alcohol (IPA). Among the various ratios, a water/IPA mixture (15:85 by volume) close to the azeotropic composition possesses unusual evaporation and wetting properties, leading to narrower cracks with widths down to ∼162 nm, uncommon among the known crackle patterns. The dense and narrow crack patterns have been used as sacrificial templates to obtain metal meshes on transparent substrates for optoelectronic applications.

Promising Hybrid Graphene-Silver Nanowire Composite Electrode for Flexible Organic Light-Emitting Diodes

Thanks to its high transparency, high carrier mobility, and thermal conductivity, graphene is often used as transparent conductive electrode (TCE) in optoelectronic devices. However, the low carrier concentration and high resistance caused by vacancy defects, grain boundaries, and superposed folds in typical graphene films limit its application. In this study, we propose a method to increase both the conductivity and carrier concentration in single-layer graphene (SLG) by blending it with silver nanowires (AgNWs). AgNWs provide connections between grain boundaries of graphene to improve charge-carrier transport. The AgNWs in this study can reduce the resistance of SLG from 650 Ω/◻ to 27 Ω/◻ yet still maintain a transmittance of 86.7% (at 550 nm). Flexible organic light-emitting diode, with a maximum 15000 cd m⁻² luminance was successfully fabricated using such graphene and AgNWs composite transparent electrodes.

Realization of Optical Network Structures for Robust Films through Immobilization of Europium Complexes

In this study, the bio-inspired self-cracking substrate can be employed as a template to control the formation of a robust mirco-patterning film. Uniform materials with variable diameters are achieved by optimizing the egg white/water ratios and reaction temperature. The incorporated europium ions exhibit its typical f-f transitions in patterned films. Moreover, the mechanical property and excellent transmittance features have been demonstrated. The current approach can be extended to design two-dimensional nanofabrication of different materials.