Solution-Grown Silver Nanowire Ordered Arrays as Transparent Electrodes

Multilayer Graphene Stacked with Silver Nanowire Networks for Transparent Conductor

A mechanically robust flexible transparent conductor with high thermal and chemical stability was fabricated from welded silver nanowire networks (w-Ag-NWs) sandwiched between multilayer graphene (MLG) and polyimide (PI) films. By modifying the gas flow dynamics and surface chemistry of the Cu surface during graphene growth, a highly crystalline and uniform MLG film was obtained on the Cu foil, which was then directly coated on the Ag-NW networks to serve as a barrier material. It was found that the highly crystalline layers in the MLG film compensate for structural defects, thus forming a perfect barrier film to shield Ag NWs from oxidation and sulfurization. MLG/w-Ag-NW composites were then embedded into the surface of a transparent and colorless PI thin film by spin-coating. This allowed the MLG/w-Ag-NW/PI composite to retain its original structural integrity due to the intrinsic physical and chemical properties of PI, which also served effectively as a binder. In view of its unique sandwich structure and the chemical welding of the Ag NWs, the flexible substrate-cum-electrode had an average sheet resistance of ≈34 Ω/sq and a transmittance of ≈91% in the visible range, and also showed excellent stability against high-temperature annealing and sulfurization.

Biointerface Fiber Technology from Electrospinning to Inflight Printing

Building two-dimensional (2D) and three-dimensional (3D) micro- and nanofibril structures with designable patterns and functionalities will offer exciting prospects for numerous applications spanning from permeable bioelectronics to tissue engineering scaffolds. This Spotlight on Applications highlights recent technological advances in fiber printing and patterning with functional materials for biointerfacing applications. We first introduce the current state of development of micro- and nanofibers with applications in biology and medical wearables. We then describe our contributions in developing a series of fiber printing techniques that enable the patterning of functional fiber architectures in three dimensions. These fiber printing techniques expand the material library and device designs, which underpin technological capabilities from enabling fundamental studies in cell migration to customizable and ecofriendly fabrication of sensors. Finally, we provide an outlook on the strategic pathways for developing the next-generation bioelectronics and "Fiber-of-Things" (FoT) using nano/micro-fibers as architectural building blocks.

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.

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.

Nanoparticle Imprint Lithography: From Nanoscale Metrology to Printable Metallic Grids

Large scale and low-cost nanopatterning of materials is of tremendous interest for optoelectronic devices. Nanoimprint lithography has emerged in recent years as a nanofabrication strategy that is high-throughput and has a resolution comparable to that of electron-beam lithography (EBL). It is enabled by pattern replication of an EBL master into polydimethylsiloxane (PDMS), that is then used to pattern a resist for further processing, or a sol-gel that could be calcinated into a solid material. Although the sol-gel chemistry offers a wide spectrum of material compositions, metals are still difficult to achieve. This gap could be bridged by using colloidal nanoparticles as resist, but deep understanding of the key parameters is still lacking. Here, we use supported metallic nanocubes as a model resist to gain fundamental insights into nanoparticle imprinting. We uncover the major role played by the surfactant layer trapped between nanocubes and substrate, and measure its thickness with subnanometer resolution by using gap plasmon spectroscopy as a metrology platform. This enables us to quantify the van der Waals (VDW) interactions responsible for the friction opposing the nanocube motion, and we find that these are almost in quantitative agreement with the Stokes drag acting on the nanocubes during nanoimprint, that is estimated with a simplified fluid mechanics model. These results reveal that a minimum thickness of surfactant is required, acting as a spacer layer mitigating van der Waals forces between nanocubes and the substrate. In the light of these findings we propose a general method for resist preparation to achieve optimal nanoparticle mobility and show the assembly of printable Ag and Au nanocube grids, that could enable the fabrication of low-cost transparent electrodes of high material quality upon nanocube epitaxy.

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.

Transparent Conducting Films Based on Carbon Nanotubes: Rational Design toward the Theoretical Limit

Electrically conductive thin-film materials possessing high transparency are essential components for many optoelectronic devices. The advancement in the transparent conductor applications requires a replacement of indium tin oxide (ITO), one of the key materials in electronics. ITO and other transparent conductive metal oxides have several drawbacks, including poor flexibility, high refractive index and haze, limited chemical stability, and depleted raw material supply. Single-walled carbon nanotubes (SWCNTs) are a promising alternative for transparent conducting films (TCFs) because of their unique and excellent chemical and physical properties. Here, the latest achievements in the optoelectronic performance of TCFs based on SWCNTs are analyzed. Various approaches to evaluate the performance of transparent electrodes are briefly reviewed. A roadmap for further research and development of the transparent conductors using "rational design," which breaks the deadlock for obtaining the TCFs with a performance close to the theoretical limit, is also described.

Nanoscale plasmonic wires with maximal figure of merits as a superior flexible transparent conducting electrode for RGB colors

Based on incredibly increasing applications in modern optoelectronic devices, the demand for securing a superior conductive transparent electrode (TCE) candidate becomes significant and urgent. However, boosting both transmittance and conductance simultaneously is an intrinsic limitation. In this work, we present silver nanoscale plasmonic wires (Ag NPWs) to function as TCEs in the visible light region by lowering their corresponding plasma frequencies. By carefully designing geometric dimensions of the Ag NPWs, we also optimize the performance for red, green, and blue colors, respectively. The demonstrated figure of merits for RGB colors appeared respectively 443.29, 459.46, and 133.78 in simulation and 302.75, 344.11, and 348.02 in experiments. Evidently, our Ag NPWs offer much greater FoMs beyond conventional TCEs that are most frequently comprised of indium tin oxide and show further advantages of flexibility and less Moire effect for the applications of flexible and high-resolution optoelectronic devices.

Bio-acceptability of wearable sensors: a mechanistic study towards evaluating ionic leaching induced cellular inflammation

The recent need for remote health wellness monitoring has led to the extensive use of wearable sensors. Owing to their increased use, these sensors are required to exhibit both functionality and safety to the user. A major component in the fabrication of these sensors and their associated circuitry is the use of metallic/organic conductive inks. However, very less is known about the interfacial and molecular interactions of these inks with biological matter as they can result in an inflammatory reaction to the user. Significant efforts are thus needed to explore and improve the bio-acceptability of such conductive ink-based wearable sensors. The present study investigates the biocompatibility of encapsulated and non-encapsulated wearable electrochemical sensors used for sensing uric acid as a biomarker for wound healing fabricated using screen-printing technique. Ionic release of metallic ions was investigated first to understand the susceptibility of the conductive inks towards ionic leaching when in contact with a fluid. Time-lapse investigation using ICPS (inductive couple plasma spectroscopy) shows a high concentration (607.31 ppb) of leached silver (Ag⁺) ions from the non-encapsulated sensors. The cell viability data suggests a 2.5-fold improvement in the sensor biocompatibility for an encapsulated sensor. While the carbon ink shows negligible effect on cell viability, the silver ink elicits significant decrease (< 50%) in cell viability at concentrations higher than 2 mg ml^-1. The toxicity pathway of these sensors was further determined to be through the generation of reactive oxygen species resulting in over 20% apoptotic cell death. Our results show that the lower biocompatibility of the non-encapsulated sensor attributes to the higher leaching of Ag⁺ ions from the printed inks which elicits several different inflammatory pathways. This work highlights the importance biocompatibility evaluation of the material used in sensor fabrication to develop safe and sustainable sensors for long-term applications.

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.

Single-Crystal Nanostructure Arrays Forming Epitaxially through Thermomechanical Nanomolding

For nanostructures in advanced electronic and plasmonic systems, a single-crystal structure with controlled orientation is essential. However, the fabrication of such devices has remained challenging, as current nanofabrication methods often suffer from either polycrystalline growth or the difficulty of integrating single crystals with substrates in desired orientations and locations to create functional devices. Here we report a thermomechanical method for the controlled growth of single-crystal nanowire arrays, which enables the simultaneous synthesis, alignment, and patterning of nanowires. Within such diffusion-based thermomechanical nanomolding (TMNM), the substrate material diffuses into nanosized cavities under an applied pressure gradient at a molding temperature of ∼0.4 times the material's melting temperature. Vertically grown face-centered cubic (fcc) nanowires with the [110] direction in an epitaxial relationship with the (110) substrate are demonstrated. The ability to control the crystal structure through the substrate takes TMNM a major step further, potentially allowing all fcc and body-centered cubic (bcc) materials to be integrated as single crystals into devices.

Monocrystalline Methylammonium Lead Halide Perovskite Materials for Photovoltaics

Lead halide perovskite solar cells have been gaining more and more interest. In only a decade, huge research efforts from interdisciplinary communities enabled enormous scientific advances that rapidly led to energy conversion efficiency near that of record silicon solar cells, at an unprecedented pace. However, while for most materials the best solar cells were achieved with single crystals (SC), for perovskite the best cells have been so far achieved with polycrystalline (PC) thin films, despite the optoelectronic properties of perovskite SC are undoubtedly superior. Here, by taking as example monocrystalline methylammonium lead halide, the authors elaborate the literature from material synthesis and characterization to device fabrication and testing, to provide with plausible explanations for the relatively low efficiency, despite the superior optoelectronics performance. In particular, the authors focus on how solar cell performance is affected by anisotropy, crystal orientation, surface termination, interfaces, and device architecture. It is argued that, to unleash the full potential of monocrystalline perovskite, a holistic approach is needed in the design of next-generation device architecture. This would unquestionably lead to power conversion efficiency higher than those of PC perovskites and silicon solar cells, with tremendous impact on the swift deployment of renewable energy on a large scale.

Performance-Enhancing Approaches for PEDOT:PSS-Si Hybrid Solar Cells

The emerging energy crisis has focused significant worldwide attention on solar cells. Although crystalline silicon solar cells are currently widely used, their high cost limits the development of solar power generation. Consequently, hybrid solar cells are becoming increasingly important, especially organic-Si hybrid solar cells (HSCs). Organic-Si HSCs combine a mature technology and high efficiency with the low-temperature manufacturing process and tunable optoelectronic properties of organic solar cells. The organic material can be P3HT, carbon nanotubes, graphene, and PEDOT:PSS. Here we review the performance of PEDOT:PSS/Si HSCs and methods for improving their efficiency, such as PEDOT:PSS modification, optimization of the trapping effect, passivation of the silicon surface, addition of an interface layer, improvement of a back contact, and optimization of the metal top electrode. This Review should help fill the gap in this area and provide perspectives for the future development of the PEDOT:PSS/Si HSCs.

Nanocube Imprint Lithography

In recent years, imprint lithography has emerged as a promising patterning technique capable of high-speed and volume production. In this work, we report highly reproducible one-step printing of metal nanocubes. A dried film of monocrystalline silver cubes serves as the resist, and a soft polydimethylsiloxane stamp directly imprints the final pattern. The use of atomically smooth and sharp faceted nanocubes facilitates the printing of high-resolution and well-defined patterns with face-to-face alignment between adjacent cubes. It also permits digital control over the line width of patterns such as straight lines, curves, and complex junctions over an area of several square millimeters. Single-particle lattices as well as three-dimensional nanopatterns are also demonstrated with an aspect ratio up to 5 in the vertical direction. The high-fidelity nanocube patterning combined with the previously demonstrated epitaxial overgrowth can enable curved (single) crystals from solution at room temperature or highly efficient transparent conductors.

Inflight fiber printing toward array and 3D optoelectronic and sensing architectures

Scalability and device integration have been prevailing issues limiting our ability in harnessing the potential of small-diameter conducting fibers. We report inflight fiber printing (iFP), a one-step process that integrates conducting fiber production and fiber-to-circuit connection. Inorganic (silver) or organic {PEDOT:PSS [poly(3,4-ethylenedioxythiophene) polystyrene sulfonate]} fibers with 1- to 3-μm diameters are fabricated, with the fiber arrays exhibiting more than 95% transmittance (350 to 750 nm). The high surface area-to-volume ratio, permissiveness, and transparency of the fiber arrays were exploited to construct sensing and optoelectronic architectures. We show the PEDOT:PSS fibers as a cell-interfaced impedimetric sensor, a three-dimensional (3D) moisture flow sensor, and noncontact, wearable/portable respiratory sensors. The capability to design suspended fibers, networks of homo cross-junctions and hetero cross-junctions, and coupling iFP fibers with 3D-printed parts paves the way to additive manufacturing of fiber-based 3D devices with multilatitude functions and superior spatiotemporal resolution, beyond conventional film-based device architectures.

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.

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.

Heterogeneous semiconductor nanowire array for sensitive broadband photodetector by crack photolithography-based micro-/nanofluidic platforms

The in situ growth of nanowires (NWs) into nano-/microelectromechanical systems (NEMS/MEMS) by solution processing is attractive for its relative simplicity and economic value.

Flexible and Hierarchical 3D Interconnected Silver Nanowires/Cellulosic Paper-Based Thermoelectric Sheets with Superior Electrical Conductivity and Ultrahigh Thermal Dispersion Capability

To date, various electronic devices have been strategically fabricated, and simultaneous realization of high electrical conductivity, sensing property, and heat-conducting property by a simple, efficient, and accurate approach is significant but still challenging. Here, cellulosic fiber supported 3D interconnected silver nanowire (AgNW) networks with hierarchical structures are rationally designed to achieve excellent electrical conductivity and superior thermal dispersion capability. In particular, thermal annealing at the junctions enables both phonon and electron transfer as well as impedes interfacial slippage. In the current study, the AgNW/cellulosic paper with the low Ag content (1.55 wt %) exhibits a low sheet resistance of 0.51 Ω sq^-1. More importantly, the AgNW/cellulosic paper-based flexible strain sensor has been reasonably developed, which can be applied to monitor various microstructural changes and human motions with high sensitivity and robust stability (fast response/relaxation time of ∼100 ms and high stability >2000 bending-stretching cycles). The AgNW/cellulosic paper-based device also displays efficient thermal dispersion property, which offers exciting opportunities for thermal management application. Furthermore, the obtained hybrid paper exhibits superior heat dispersion capacity for thermal management devices. Overall, uniform dispersion and 3D interconnected junctions of AgNW among the fibers inside the cellulosic papers lead to the combination of high mechanical strength, highly efficient electrical conductivity, and ultrahigh heat dispersion property. The AgNW/cellulosic paper has promising potentials in the flexible and wearable sensing elements, thermal management materials, and artificial intelligence devices.

Halbach array assisted assembly of orderly aligned nickel nanowire networks as transparent conductive films

The aspect ratio and arrangement of nanowires play an important role in achieving excellent optoelectronic performance for metal nanowire-based transparent conductive films (TCFs). However, limited to the technology and material properties, studies are always focused on only one of the issues. Here, a novel strategy for manipulating the relative aspect ratio and arrangement of nickel nanowires (NiNWs) at nanoscale by Halbach array assisted assembly technology is introduced. Head-to-tail nickel nanowire chains as large as hundreds of micrometers are formed as a result of the dipole-dipole interactions of wire-wire. The arrangement of nickel nanowires can be preciously controlled by layer-by-layer deposition. Notably, the alignment create a significant improvement on the optoelectronic performance of nickel nanowire TCFs. The optimized orderly aligned NiNWs TCFs demonstrate super optoelectronic performance (90 Ω sq^-1, 86%) than disordered NiNW TCFs (200 Ω sq^-1, 80%). Moreover, NiNW-based TCFs exhibit outstanding long-term oxidation stability at 80 °C over 30 d as well as high-temperature oxidization stability even up to 300 °C, that is the most stable metal nanowire-based TCFs in air as far as we know. The low-cost, good optoelectronic performance and excellent oxidation resistance of aligned NiNWs will make them as attractive alternatives to silver nanowires for TCFs application.

Recyclable and Flexible Starch-Ag Networks and Its Application in Joint Sensor

Flexible transparent conductive electrodes are essential component for flexible optoelectronic devices and have been extensively studied in recent years, while most of the researches are focusing on the electrode itself, few topics in material green and recyclability. In this paper, we demonstrate a high-performance transparent conductive electrode (TCE), based on our previous cracking technology, combined with a green and recyclable substrate, a starch film. It not only shows low R_s (less than 1.0 Ω sq^-1), high transparency (> 82%, figure of merit ≈ 10,000), but also provides an ultra-smooth morphology and recyclability. Furthermore, a series of biosensors on human joints are demonstrated, showing great sensitivity and mechanical stability.

Synergetic effects of ligand exchange and reduction process enhancing both electrical and optical properties of Ag nanocrystals for multifunctional transparent electrodes

In this work, we introduce a low cost, room-temperature and atmospheric pressure based chemical method to produce highly transparent, conductive, and flexible nano-mesh structured electrodes using Ag nanocrystals (NCs). Sequential treatments of ligand exchange and reduction processes were developed to engineer the optoelectronic properties of Ag NC thin films. Combinatorial analysis indicates that the origin of the relatively low conductivity comes from the non-metallic compounds that are introduced during ligand exchange. The reduction process successfully removed these non-metallic compounds, yielding structurally uniform, optically more transparent, dispersive, and electrically more conductive thin films. We optimized the design of Ag NC thin film mesh structures, and achieved low sheet resistance (9.12 Ω □-1), high optical transmittance (94.7%), and the highest figure of merit (FOM) of 6.37 × 10-2. Solution processed flexible transparent heaters, touch pads, and wearable sensors are demonstrated, emphasizing the potential applications of Ag NC transparent electrodes in multifunctional sensors and devices.

Anisotropic Nanoscale Wrinkling in Solid-State Substrates

Pattern formation induced by wrinkling is a very common phenomenon exhibited in soft-matter substrates. In all these systems, wrinkles develop in the presence of compressively stressed thin films lying on compliant substrates. Here, the controlled growth of self-organized nanopatterns exploiting a wrinkling instability on a solid-state substrate is demonstrated. Soda-lime glasses are modified in the surface layers by a defocused ion beam, which triggers the formation of a compressively stressed surface layer deprived of alkali ions. When the substrate is heated up near its glass transition temperature, the wrinkling instability boosts the growth rate of the pattern by about two orders of magnitude. High-aspect-ratio anisotropic ripples bound by faceted ridges are thus formed, which represent an optimal template for guiding the growth of large-area arrays of functional nanostructures. The engineering over large square centimeter areas of quasi-1D arrays of Au nanostripe dimers endowed with tunable plasmonic response, strong optical dichroism, and high electrical conductivity is demonstrated. These peculiar functionalities allow these large-area substrates to be exploited as active metamaterials in nanophotonics, biosensing, and optoelectronics.

Stretchable Transparent Electrodes with Solution-Processed Regular Metal Mesh for an Electroluminescent Light-Emitting Film

We report stretchable metal-mesh transparent electrodes (TEs) with excellent electrical conductivity (<2 Ω/sq) and optical transparency (>80%) under up to 55% strain. The figures of merit on these electrodes, as defined as the ratio between electrical conductivity and optical conductivity, are among the highest reported for stretchable TEs under moderate stretching. Moreover, we demonstrate their application in a stretchable electroluminescent (EL) light-emitting film as top and bottom electrodes. EL lighting devices require low-resistance electrodes to unleash their potential for large-area low-power-consumption applications, in which our highly conductive and transparent stretchable TEs provide an edge on other competitor approaches. Importantly, our stretchable metal-mesh electrodes are fabricated through a vacuum-free solution-processed approach that is scalable for cost-effective mass production. We also investigate the fracture and fatigue mechanisms of stretchable metal-mesh electrodes with various mesh patterns and observe different behaviors under one-time and cyclic stretching conditions. Our solution-processed fabrication method, failure mechanism investigation, and device demonstration for metal-mesh stretchable TEs will facilitate the adoption of this promising high-performance approach in stretchable and wearable electronics applications.

High-Quality AZO/Au/AZO Sandwich Film with Ultralow Optical Loss and Resistivity for Transparent Flexible Electrodes

Transparent flexible electrodes are in ever-growing demand for modern stretchable optoelectronic devices, such as display technologies, solar cells, and smart windows. Such sandwich-film-electrodes deposited on polymer substrates are unattainable because of the low quality of the films, inducing a relatively large optical loss and resistivity as well as a difficulty in elucidating the interference behavior of light. In this article, we report a high-quality AZO/Au/AZO sandwich film with excellent optoelectronic performance, e.g., an average transmittance of about 81.7% (including the substrate contribution) over the visible range, a sheet resistance of 5 Ω/sq, and a figure-of-merit (FoM) factor of ∼55.1. These values are well ahead of those previously reported for sandwich-film-electrodes. Additionally, the interference behaviors of light modulated by the coat and metal layers have been explored with the employment of transmittance spectra and numerical simulations. In particular, a heater device based on an AZO/Au/AZO sandwich film exhibits high performance such as short response time (∼5 s) and uniform temperature field. This work provides a deep insight into the improvement of the film quality of the sandwich electrodes and the design of high-performance transparent flexible devices by the application of a flexible substrate with an atomically smooth surface.

Synthesis of Ultralong Copper Nanowires for High-Performance Flexible Transparent Conductive Electrodes: The Effects of Polyhydric Alcohols

Copper nanowires (Cu NWs) have become a promising material for flexible transparent conductive electrodes (FTCEs) owing to their outstanding transparency and conductivity properties. In this work, ultralong Cu NWs with an average length over 250 μm and a diameter of around 50 nm (aspect ratio ∼5000) were synthesized in a water/polyhydric alcohol cosolvent. The effects of polyhydric alcohols (including ethanol, ethylene glycol, and glycerol) on the aspect ratio of Cu NWs were investigated. The diameter of Cu NWs decreased with the increased number of hydroxyl groups of polyhydric alcohols. In addition, the capping ligands (oleylamine and oleic acid) and glucose also exhibit important effects on the dispersity and morphology of Cu NWs. The ultralong Cu NW-based poly(dimethylsiloxane) (PDMS) FTCEs exhibit high performance with a low sheet resistance of 92.1 Ω sq^-1 at a transmittance of 91.524%. Inspired by the stretchable ability of PDMS, wearable sensors were fabricated to detect the movement of the finger joint through the chronoamperometry method. The prepared sensors exhibit high sensitivity and a fast response time. The excellent performance of FTCEs and wearable sensors suggests that the ultralong Cu NWs have a bright future in the application of the next generation of flexible optoelectronic devices.

Emerging Novel Metal Electrodes for Photovoltaic Applications

Emerging novel metal electrodes not only serve as the collector of free charge carriers, but also function as light trapping designs in photovoltaics. As a potential alternative to commercial indium tin oxide, transparent electrodes composed of metal nanowire, metal mesh, and ultrathin metal film are intensively investigated and developed for achieving high optical transmittance and electrical conductivity. Moreover, light trapping designs via patterning of the back thick metal electrode into different nanostructures, which can deliver a considerable efficiency improvement of photovoltaic devices, contribute by the plasmon-enhanced light-mattering interactions. Therefore, here the recent works of metal-based transparent electrodes and patterned back electrodes in photovoltaics are reviewed, which may push the future development of this exciting field.

Cohesively enhanced electrical conductivity and thermal stability of silver nanowire networks by nickel ion bridge joining

A facile method for producing high-performance nickel enhanced silver nanowire (Ag NW) transparent electrodes on a flexible substrate is reported. The modified electroplating method called enhanced nickel ion bridge joining of Ag NWs, which provides a new route for improving the loose junctions in bare Ag NW networks. The sheet resistance of Ag NW electrode drops from over 2000 Ω sq⁻¹ to 9.4 Ω sq⁻¹ with excellent thermal uniformity after the electroplating process within 10 s. Nickel enhanced Ag NW transparent films are applied on flexible heaters with good thermal stability (165 °C for 2 h) and mechanical flexibility (3500 cycles under 2.5 mm bending radius) after mechanical bending process. Moreover, the mechanism of nickel growth is also confirmed that the nickel electroplating of the Ag NWs obeyed Faraday’s Laws.

Thermally induced breakup of metallic nanowires: experiment and theory

We present time-resolved transmission electron microscopy studies of the degradation of Au, Ag, Cu and Ni nanowires deposited on a heated support. The wires are grown under fully inert conditions in superfluid helium droplets and deposited onto amorphous carbon. The inherent stability of these pristine metal nanowires with diameters below 10 nm is investigated in the absence of any stabilizers, templates or solvents. The phenomenon of Rayleigh-breakup, a consequence of diffusion processes along the wire surfaces, is analysed in situ via scans over time and support temperature. Our experimental efforts are combined with simulations based on a novel model featuring a cellular automaton to emulate surface diffusion. Based on this model, correlations between the material parameters and actual breakup behaviour are studied.

Facile Synthesis of Silver Nanowires with Different Aspect Ratios and Used as High-Performance Flexible Transparent Electrodes

Silver nanowires (Ag NWs) are the promising materials to fabricate flexible transparent electrodes, aiming to replace indium tin oxide (ITO) in the next generation of flexible electronics. Herein, a feasible polyvinylpyrrolidone (PVP)-mediated polyol synthesis of Ag NWs with different aspect ratios is demonstrated and high-quality Ag NWs transparent electrodes (NTEs) are fabricated without high-temperature thermal sintering. When employing the mixture of PVP with different average molecular weight as the capping agent, the diameters of Ag NWs can be tailored and Ag NWs with different aspect ratios varying from ca. 30 to ca. 1000 are obtained. Using these as-synthesized Ag NWs, the uniform Ag NWs films are fabricated by repeated spin coating. When the aspect ratios exceed 500, the optoelectronic performance of Ag NWs films improve remarkably and match up to those of ITO films. Moreover, an optimal Ag NTEs with low sheet resistance of 11.4 Ω/sq and a high parallel transmittance of 91.6% at 550 nm are achieved when the aspect ratios reach almost 1000. In addition, the sheet resistance of Ag NWs films does not show great variation after 400 cycles of bending test, suggesting an excellent flexibility. The proposed approach to fabricate highly flexible and high-performance Ag NTEs would be useful to the development of flexible devices.

Roller-Induced Bundling of Long Silver Nanowire Networks for Strong Interfacial Adhesion, Highly Flexible, Transparent Conductive Electrodes

Silver nanowires (AgNWs) have been the most promising electrode materials for fabrication of flexible transparent touch panel, displays and many other electronics because of their excellent electrical properties, cost effectiveness, synthesis scalability, and suitability for mass production. Although a few literature reports have described the use of short Ag NWs in fabrication of randomly oriented Ag NW network-based electrode, their electrical conductivities are still far lower than that of Ag films. So far, no any literature report was able to provide any simple solution to fabrication of large-area and mass-manufactural ability to address the issues, such as, conductivity, transparency, electrical current withstand, bending stability, and interfacial adhesion. In the current work, we provide a simple solution to conquer the above-mentioned challenges, and report the development of long Ag NW bundle network electrodes on large area PET films that were coated, aligned, and bundled quickly and simply using a steel roller. Our developed AgNWs-bundle networks had superior performance in optoelectronic properties (sheet resistance 5.8 Ω sq⁻¹; optical transmittance 89% at 550 nm wavelength), electrical current withstand up to 500 mA, and bending stability over 5000 bending cycles, and strong interfacial adhesion.

The Joule heating problem in silver nanowire transparent electrodes

Silver nanowire transparent electrodes have shown considerable potential to replace conventional transparent conductive materials. However, in this report we show that Joule heating is a unique and serious problem with these electrodes. When conducting current densities encountered in organic solar cells, the average surface temperature of indium tin oxide (ITO) and silver nanowire electrodes, both with sheet resistances of 60 ohms/square, remains below 35 °C. However, in contrast to ITO, the temperature in the nanowire electrode is very non-uniform, with some localized points reaching temperatures above 250 °C. These hotspots accelerate nanowire degradation, leading to electrode failure after 5 days of continuous current flow. We show that graphene, a commonly used passivation layer for these electrodes, slows nanowire degradation and creates a more uniform surface temperature under current flow. However, the graphene does not prevent Joule heating in the nanowires and local points of high temperature ultimately shift the failure mechanism from nanowire degradation to melting of the underlying plastic substrate. In this paper, surface temperature mapping, lifetime testing under current flow, post-mortem analysis, and modelling illuminate the behaviour and failure mechanisms of nanowires under extended current flow and provide guidelines for managing Joule heating.

Nanoscale Back Contact Perovskite Solar Cell Design for Improved Tandem Efficiency

Tandem photovoltaics, combining absorber layers with two distinct band gap energies into a single device, provide a practical solution to reduce thermalization losses in solar energy conversion. Traditionally, tandem devices have been assembled using two-terminal (2-T) or four-terminal (4-T) configurations; the 2-T limits the tandem performance due to the series connection requiring current matching, while the standard 4-T configuration requires at least three transparent electrical contacts, which reduce the total collected power due to unavoidable parasitic absorption. Here, we introduce a novel architecture based on a nanoscale back-contact for a thin-film top cell in a three terminal (3-T) configuration. Using coupled optical-electrical modeling, we optimize this architecture for a planar perovskite-silicon tandem, highlighting the roles of nanoscale contacts to reduce the required perovskite electronic quality. For example, with an 18% planar silicon base cell, the 3-T back contact design can reach a 32.9% tandem efficiency with a 10 μm diffusion length perovskite material. Using the same perovskite quality, the 4-T and 2-T configurations only reach 30.2% and 24.8%, respectively. We also confirm that the same 3-T efficiency advantage applies when using 25% efficient textured silicon base cells, where the tandems reach 35.2% and 32.8% efficiency for the 3-T, and 4-T configurations, respectively. Furthermore, because our design is based on the individual subcells being back-contacted, further improvements can be readily made by optimizing the front surface, which is left free for additional antireflective coating, light trapping, surface passivation, and photoluminescence outcoupling enhancements.

Modeling the Performance Limitations and Prospects of Perovskite/Si Tandem Solar Cells under Realistic Operating Conditions

Perovskite/Si tandem solar cells have the potential to considerably out-perform conventional solar cells. Under standard test conditions, perovskite/Si tandem solar cells already outperform the Si single junction. Under realistic conditions, however, as we show, tandem solar cells made from current record cells are hardly more efficient than the Si cell alone. We model the performance of realistic perovskite/Si tandem solar cells under real-world climate conditions, by incorporating parasitic cell resistances, nonradiative recombination, and optical losses into the detailed-balance limit. We show quantitatively that when optimizing these parameters in the perovskite top cell, perovskite/Si tandem solar cells could reach efficiencies above 38% under realistic conditions, even while leaving the Si cell untouched. Despite the rapid efficiency increase of perovskite solar cells, our results emphasize the need for further material development, careful device design, and light management strategies, all necessary for highly efficient perovskite/Si tandem solar cells.

Modeling the Performance Limitations and Prospects of Perovskite/Si Tandem Solar Cells under Realistic Operating Conditions

Perovskite/Si tandem solar cells have the potential to considerably out-perform conventional solar cells. Under standard test conditions, perovskite/Si tandem solar cells already outperform the Si single junction. Under realistic conditions, however, as we show, tandem solar cells made from current record cells are hardly more efficient than the Si cell alone. We model the performance of realistic perovskite/Si tandem solar cells under real-world climate conditions, by incorporating parasitic cell resistances, nonradiative recombination, and optical losses into the detailed-balance limit. We show quantitatively that when optimizing these parameters in the perovskite top cell, perovskite/Si tandem solar cells could reach efficiencies above 38% under realistic conditions, even while leaving the Si cell untouched. Despite the rapid efficiency increase of perovskite solar cells, our results emphasize the need for further material development, careful device design, and light management strategies, all necessary for highly efficient perovskite/Si tandem solar cells.

A cracking-assisted micro-/nanofluidic fabrication platform for silver nanobelt arrays and nanosensors

Nanowires (NWs) with a high surface-to-volume ratio are advantageous for bio- or chemical sensor applications with high sensitivity, high selectivity, rapid response, and low power consumption. However, NWs are typically fabricated by combining several nanofabrication and even microfabrication processes, resulting in drawbacks such as high fabrication cost, extensive labor, and long processing time. Here, we show a novel NW fabrication platform based on "crack-photolithography" to produce a micro-/nanofluidic channel network. Solutions were loaded along the microchannel, while chemical synthesis was performed in the nanoslit-like nanochannels for fabricating silver nanobelts (AgNBs). In addition, the NW/NB fabrication platform not only made it possible to produce AgNBs in a repeatable, high-throughput, and low-cost manner but also allowed the simultaneous synthesis and alignment of AgNBs on a chip, eliminating the need for special micro- and/or nanofabrication equipment and dramatically reducing the processing time, labor, and cost. Finally, we demonstrated that the AgNBs can be used as chemical sensors, either as prepared or when integrated in a flexible substrate, to detect target analytes such as hydrogen peroxide.

Monocrystalline Nanopatterns Made by Nanocube Assembly and Epitaxy

Monocrystalline materials are essential for optoelectronic devices such as solar cells, LEDs, lasers, and transistors to reach the highest performance. Advances in synthetic chemistry now allow for high quality monocrystalline nanomaterials to be grown at low temperature in solution for many materials; however, the realization of extended structures with control over the final 3D geometry still remains elusive. Here, a new paradigm is presented, which relies on epitaxy between monocrystalline nanocube building blocks. The nanocubes are assembled in a predefined pattern and then epitaxially connected at the atomic level by chemical growth in solution, to form monocrystalline nanopatterns on arbitrary substrates. As a first demonstration, it is shown that monocrystalline silver structures obtained with such a process have optical properties and conductivity comparable to single-crystalline silver. This flexible multiscale process may ultimately enable the implementation of monocrystalline materials in optoelectronic devices, raising performance to the ultimate limit.

Flexible Transparent Conductive Films with High Performance and Reliability Using Hybrid Structures of Continuous Metal Nanofiber Networks for Flexible Optoelectronics

We report an Ag nanofiber-embedded glass-fabric reinforced hybrimer (AgNF-GFRHybrimer) composite film as a reliable and high-performance flexible transparent conducting film. The continuous AgNF network provides superior optoelectronic properties of the composite film by minimizing transmission loss and junction resistance. In addition, the excellent thermal/chemical stability and mechanical durability of the GFRHybrimer matrix provides enhanced mechanical durability and reliability of the final AgNF-GFRHybrimer composite film. To demonstrate the availability of our AgNF-GFRHybrimer composite as a transparent conducting film, we fabricated a flexible organic light-emitting diode (OLED) device on the AgNF-GFRHybrimer film; the OLED showed stable operation during a flexing.

Area-Selective Lift-Off Mechanism Based on Dual-Triggered Interfacial Adhesion Switching: Highly Facile Fabrication of Flexible Nanomesh Electrode

With the recent emergence of flexible and wearable optoelectronic devices, the achievement of sufficient bendability and stretchability of transparent and conducting electrodes (TCEs) has become an important requirement. Although metal-mesh-based structures have been investigated for TCEs because of their excellent performances, the fabrication of mesh or grid structures with a submicron line width is still complex due to the requirements of laborious lithography and pattern transfer steps. Here, we introduce an extremely facile fabrication technique for metal patterns embedded in a flexible substrate based on submicron replication and an area-selective delamination (ASD) pattern. The high-yield, area-specific lift-off process is based on the principle of solvent-assisted delamination of deposited metal thin films and a mechanical triggering effect by soft wiping or ultrasonication. Our fabrication process is very simple, convenient, and cost-effective in that it does not require any lithography/etching steps or sophisticated facilities. Moreover, their outstanding optical and electrical properties (e.g., sheet resistances of 0.43 Ω sq^-1 at 94% transmittance), which are markedly superior to those of other flexible TCEs, are demonstrated. Furthermore, there is no significant change of resistance over 1000 repeated bending cycles, with a bending radius of 5 mm, and immersion in various solvents such as salt water and organic solvents. Finally, we demonstrate high-performance transparent heaters and flexible touch panels fabricated using the nanomesh electrode, confirming the long-range electrical conduction and reliability of the electrode.

One-step fabrication of single-crystalline ZnS nanotubes with a novel hollow structure and large surface area for photodetector devices

ZnS nanotubes (NTs) were successfully prepared via a one-step thermal evaporation process without using any templates. The resulting NTs were single crystalline and structurally uniform. Based on experimental analysis, a tube-growth vapor-liquid-solid process was proposed as the growth mechanism of ZnS NTs. A metal-semiconductor-metal full-nanostructured ultraviolet (UV) photodetector with ZnS NTs as the active layer, and Ag nanowires of low resistivity and high transmissivity as electrodes, was fabricated and characterized. The ZnS NT-based device displayed a high I _on/I _off ratio of up to ∼1.56 × 10⁵ with a high response to UV incident light at low operation voltage. This work is a meaningful exploration for preparing other one-dimensional semiconductor NTs, and developing a high-performance and power-saving UV sensor.

Simple silver nanowire patterning using a DUV lamp direct write with sol–gel IZO capping

We develop a method to pattern silver nanowire (AgNW) electrode by using DUV lamp to directly write on indium–zinc-oxide capped AgNW. The patterned electrodes keep good conducting property on flexible polyimide substrate after 1000-times bending.

A Flexible and Robust Transparent Conducting Electrode Platform Using an Electroplated Silver Grid/Surface-Embedded Silver Nanowire Hybrid Structure

In this paper, we report flexible transparent conducting electrode (TCE) film using a silver grid (Ag grid)/silver nanowire (AgNW) hybrid structure (AG/NW-GFRHybrimer). The AG/NW-GFRHybrimer consists of an AgNW-embedded glass-fabric reinforced plastic film (AgNW-GFRHybrimer) and an electroplated Ag grid. The AgNW-GFRHybrimer is used as a flexible transparent substrate and a seed layer for electroplating. The Ag grid is fabricated via an all-solution-process; the grid pattern is formed using conventional photolithography, and Ag is deposited through electroplating. The AG/NW-GFRHybrimer exhibits excellent opto-electrical properties (transparency = 87%, sheet resistance = 13 Ω/□), superior thermal stability (250 °C for 720 min and 85 °C/85% RH for 100 h), and outstanding mechanical flexibility (bending radius = 1 mm for 2000 cycles). Finally, a touch-screen panel (four-wire resistive type) was fabricated using the AG/NW-GFRHybrimer to demonstrate its potential for use in actual optoelectronic applications.

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

A new structure of flexible transparent electrodes is reported, featuring a metal mesh fully embedded and mechanically anchored in a flexible substrate, and a cost-effective solution-based fabrication strategy for this new transparent electrode. The embedded nature of the metal-mesh electrodes provides a series of advantages, including surface smoothness that is crucial for device fabrication, mechanical stability under high bending stress, strong adhesion to the substrate with excellent flexibility, and favorable resistance against moisture, oxygen, and chemicals. The novel fabrication process replaces vacuum-based metal deposition with an electrodeposition process and is potentially suitable for high-throughput, large-volume, and low-cost production. In particular, this strategy enables fabrication of a high-aspect-ratio (thickness to linewidth) metal mesh, substantially improving conductivity without considerably sacrificing transparency. Various prototype flexible transparent electrodes are demonstrated with transmittance higher than 90% and sheet resistance below 1 ohm sq(-1) , as well as extremely high figures of merit up to 1.5 × 10(4) , which are among the highest reported values in recent studies. Finally using our embedded metal-mesh electrode, a flexible transparent thin-film heater is demonstrated with a low power density requirement, rapid response time, and a low operating voltage.

18.4%-Efficient Heterojunction Si Solar Cells Using Optimized ITO/Top Electrode

We optimize the thickness of a transparent conducting oxide (TCO) layer, and apply a microscale mesh-pattern metal electrode for high-efficiency a-Si/c-Si heterojunction solar cells. A solar cell equipped with the proposed microgrid metal electrode demonstrates a high short-circuit current density (JSC) of 40.1 mA/cm(2), and achieves a high efficiency of 18.4% with an open-circuit voltage (VOC) of 618 mV and a fill factor (FF) of 74.1% as result of the shortened carrier path length and the decreased electrode area of the microgrid metal electrode. Furthermore, by optimizing the process sequence for electrode formation, we are able to effectively restore the reduction in VOC that occurs during the microgrid metal electrode formation process. This work is expected to become a fundamental study that can effectively improve current loss in a-Si/c-Si heterojunction solar cells through the optimization of transparent and metal electrodes.