Effect of epiretinal electrical stimulation on the glial cells in a rabbit retinal eyecup model

Introduction

We examined how pulse train electrical stimulation of the inner surface of the rabbit retina effected the resident glial cells. We used a rabbit retinal eyecup preparation model, transparent stimulus electrodes, and optical coherence tomography (OCT). The endfeet of Müller glia processes line the inner limiting membrane (ILM).

Methods

To examine how epiretinal electrode stimulation affected the Müller glia, we labeled them post stimulation using antibodies against soluble glutamine synthetase (GS). After 5 min 50 Hz pulse train stimulation 30 μm from the surface, the retina was fixed, immunostained for Müller glia, and examined using confocal microscopic reconstruction. Stimulus pulse charge densities between 133-749 μC/cm2/ph were examined.

Results

High charge density stimulation (442-749 μC/cm2/ph) caused significant losses in the GS immunofluorescence of the Müller glia endfeet under the electrode. This loss of immunofluorescence was correlated with stimuli causing ILM detachment when measured using OCT. Müller cells show potassium conductances at rest that are blocked by barium ions. Using 30 msec 20 μA stimulus current pulses across the eyecup, the change in transretinal resistance was examined by adding barium to the Ringer. Barium caused little change in the transretinal resistance, suggesting under low charge density stimulus pulse conditions, the Müller cell radial conductance pathway for these stimulus currents was small. To examine how epiretinal electrode stimulation affected the microglia, we used lectin staining 0-4 h post stimulation. After stimulation at high charge densities 749 μC/cm2/ph, the microglia under the electrode appeared rounded, while the local microglia outside the electrode responded to the stimulated retina by process orientation inwards in a ring by 30 min post stimulation.

Discussion

Our study of glial cells in a rabbit eyecup model using transparent electrode imaging suggests that epiretinal electrical stimulation at high pulse charge densities, can injure the Müller and microglia cells lining the inner retinal surface in addition to ganglion cells.

Highly Transparent Red Organic Light-Emitting Diodes with AZO/Ag/AZO Multilayer Electrode

Free-form factor optoelectronics is becoming more important for various applications. Specifically, flexible and transparent optoelectronics offers the potential to be adopted in wearable devices in displays, solar cells, or biomedical applications. However, current transparent electrodes are limited in conductivity and flexibility. This study aims to address these challenges and explore potential solutions. For the next-generation transparent conductive electrode, Al-doped zinc oxide (AZO) and silver (AZO/Ag/AZO) deposited by in-line magnetron sputtering without thermal treatment was investigated, and this transparent electrode was used as a transparent organic light-emitting diode (OLED) anode to maximize the transparency characteristics. The experiment and simulation involved adjusting the thickness of Ag and AZO and OLED structure to enhance the transmittance and device performance. The AZO/Ag/AZO with Ag of 12 nm and AZO of 32 nm thickness achieved the results of the highest figure of merit (FOM) (Φ550 = 4.65 mΩ-1) and lowest roughness. The full structure of transparent OLED (TrOLED) with AZO/Ag/AZO anode and Mg:Ag cathode reached 64.84% transmittance at 550 nm, and 300 cd/m2 at about 4 V. The results demonstrate the feasibility of adopting flexible substrates, such as PET, without the need for thermal treatment. This research provides valuable insights into the development of transparent and flexible electronic devices.

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.

Challenge for Trade-Off Relationship between the Mechanical Property and Healing Efficiency of Self-Healable Polyimide

Polyimide is actively applied in various industrial fields because of its strong mechanical properties, owing to the interactions between the polymer chains. Fully aromatic imide structures exhibit high glass-transition temperatures due to the strong interactions between their chains, which hinder chain mobility. Therefore, preparing a material that exhibits self-healing at a low temperature of ≤100 °C and good mechanical properties is challenging. Thus, we prepared imides with four-component semiaromatic structures by adjusting the contents of 4,4'-(hexafluoroisopropylidene)diphthalic anhydride and 4,4'-(4,4'-isopropylidenediphenoxy)bis(phthalic anhydride) to yield four-component self-healable colorless polyimides (f-SH-CPIs) with novel structures, flexibilities, good mechanical properties, and low healing temperatures. The flexibilities and distances between the polymer chains, as the basis of the trade-off relationship between the mechanical properties and healing efficiency, were controlled. These materials may be used as substrates in wearable devices and multilayer insulation that may protect from space dust, cosmic rays, and satellite fragments.

Transparent Conducting Amorphous IZO Thin Films: An Approach to Improve the Transparent Electrode Quality

It is common knowledge that using different oxygen contents in the working gas during sputtering deposition results in fabrication of indium zinc oxide (IZO) films with a wide range of optoelectronic properties. It is also important that high deposition temperature is not required to achieve excellent transparent electrode quality in the IZO films. Modulation of the oxygen content in the working gas during RF sputtering of IZO ceramic targets was used to deposit IZO-based multilayers in which the ultrathin IZO unit layers with high electron mobility (μ-IZO) alternate with ones characterized by high concentration of free electrons (n-IZO). As a result of optimizing the thicknesses of each type of unit layer, low-temperature 400 nm thick IZO multilayers with excellent transparent electrode quality, indicated by the low sheet resistance (R ≤ 8 Ω/sq.) with high transmittance in the visible range (T¯ > 83%) and a very flat multilayer surface, were obtained.

Fabrication of Large-Area, Affordable Dual-Function Electrochromic Smart Windows by Using a Hybrid Electrode Coated with an Oxygen-Deficient Tungsten Oxide Ultrathin Porous Film

Electrochromic (EC) devices are not commercialized extensively owing to their high cost. The best large-area devices in the market suffer from not reaching a distinct dark-colored state. These devices appear more like a blue tinted glass. While a better performance demands the use of appropriate components, the cost-effectiveness of such components is crucial for commercialization. Specifically, the utilization of cost-effective electrodes, thin WO₃ coatings, and inexpensive electrolytes are essential for reducing the cost of EC devices. Here, we report a high-performing porous WO₃ thin film (∼130 nm) achieved by optimizing the DC sputtering process parameters. This way, an affordable dual-function EC energy-storage device was fabricated, showing 84% transmittance modulation and a high power density of 3036 mW/m², thus functioning simultaneously as a transparency switching energy-storage device. With a large-area (900 cm²) device, we have demonstrated that the need for expensive ITO electrodes and Li⁺ ion-based electrolytes can be eliminated by using a hybrid electrode (ITO/Al-mesh) and multivalent Al³⁺ ion-based electrolytes while not compromising the device performance. The findings of this study may revolutionize the EC device industry and their commercialization owing to inexpensive ingredients and scalable processing.

Dry Synthesis of Pure and Ultrathin Nanoporous Metallic Films

Nanoporous metals possess unique properties attributed to their high surface area and interconnected nanoscale ligaments. They are mostly fabricated by wet synthetic methods that are not universal to various metals and not free from impurities due to solution-based etching processes. Here, we demonstrate that the plasma treatment of metal nanoparticles formed by physical vapor deposition is a general route to form such films with many metals including the non-noble ones. The resultant nanoporous metallic films are free of impurities and possess highly curved ligaments and nanopores. The metal films are ultrathin, yet remarkably robust and very well connected, and thus are highly promising for various applications such as transparent conducting electrodes.

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.

Study on Ultrathin Silver Film Transparent Electrodes Based on Aluminum Seed Layers with Different Structures

Ag has the lowest electrical resistivity among all metals, and at the same time, the best optical properties in the visible and near-IR spectral range; it is therefore the most widely employed material for thin-metal-film-based transparent conductors. In this work, an ultra-thin transparent silver film electrode with aluminum as seed layer was prepared by a resistive thermal evaporation technique. Using a range of electrical, optical and surface morphology techniques, it can be noted that the presence of the thin layer of aluminum changes the growth kinetics (nucleation and evolution) of the thermal evaporation of Ag, leading to silver films with smooth surface morphology and high electrical conductivity, and the threshold thickness of the silver film is reduced. It is inferred that the aluminum layer showed a good infiltration effect on the ultra-thin silver film, by analyzing the transmittance spectrum, sheet resistance and surface morphology. Moreover, the average transmittance of silver film with 10 nm is 40% in the 400-2500 nm band, whereas the sheet resistance is 13 Ωsq ^-1. A series of experiments show that the introduction of Al seed layer has certain effect on improving the properties of transparent conductive silver films. Then, a new method for deposition of 1 nm Al seed layer was proposed; that is, the 1 nm aluminum infiltrated layer is divided into two or more layers, and the average transmittance of silver film with 5 nm is 60% in the 400-2500 nm band, whereas the sheet resistance does not exceed 100 Ω sq^-1.

Ultra-Smooth and Efficient NiO/Ag/NiO Transparent Electrodes for Flexible Organic Light-Emitting Devices

We demonstrate highly flexible and efficient organic light-emitting devices (OLEDs) using an ultra-smooth NAN anode. A template-stripping process was employed to create an ultra-smooth NAN anode on a photopolymer substrate. The flexible OLEDs obtained by this method maintained good electroluminescent properties and mechanical stability after bending. The efficiency of the flexible OLEDs was improved by 30.6% compared with conventional OLEDs deposited on PET/ITO substrate due to the enhanced hole injection from the ultra-smooth anode.

WSe2 as Transparent Top Gate for Infrared Near-Field Microscopy

Independent control of carrier density and out-of-plane displacement field is essential for accessing novel phenomena in two-dimensional (2D) material heterostructures. While this is achieved with independent top and bottom metallic gate electrodes in transport experiments, it remains a challenge for near-field optical studies as the top electrode interferes with the optical path. Here, we characterize the requirements for a material to be used as the top-gate electrode and demonstrate experimentally that few-layer WSe₂ can be used as a transparent, ambipolar top-gate electrode in infrared near-field microscopy. We carry out nanoimaging of plasmons in a bilayer graphene heterostructure tuning the plasmon wavelength using a trilayer WSe₂ gate, achieving a density modulation amplitude exceeding 2 × 10¹² cm^-2. The observed ambipolar gate-voltage response allows us to extract the energy gap of WSe₂, yielding a value of 1.05 eV. Our results provide an additional tuning knob to cryogenic near-field experiments on emerging phenomena in 2D materials and moiré heterostructures.

Self-Assembled 4-Aminopyridine Monolayer as a Nucleation-Inducing Layer for Transparent Silver Electrodes

The role of a self-assembled monolayer obtained by vacuum deposition of 4-aminopyridine (4-AP), a small organic molecule having amine and pyridine groups, as a metal nucleation inducer and adhesion promoter was verified, and the applicability was evaluated. 4-AP deposited to an extremely thin thickness effectively changed the substrate surface properties, increasing the nucleation density of silver (Ag) more than 3 times and eventually forming a more transparent, low-resistance Ag thin film. The optical transmittance of the Ag thin film, which was less than 60% when 4-AP was not applied, could be increased to about 77% by simply applying 4-AP, and the electrical resistance could be lowered from 37 to 14 Ω/square at the same time. Transmittance could be further improved to higher than 90% by depositing an antireflection layer for use as a transparent Ag electrode. It was also verified that 4-AP not only serves as a nucleation inducer but also contributes to improving interfacial adhesion. The Ag transparent electrode using 4-AP provided the improved performance of the organic light-emitting device due to higher transmittance, lower resistance, and surface roughness. Small organic molecules including functional groups that can be vacuum deposited, such as 4-AP, are expected to be used as surface pretreatment materials for various depositions because they can be easily patterned and can efficiently modify the surface even with extremely thin thickness.

Highly stable semitransparent multilayer graphene/LaVO3vertical-heterostructure photodetectors

A heterostructure composed of a combination of semi-metallic graphene (Gr) and high-absorption LaVO₃is ideal for high-performance translucent photodetector (PD) applications. Here, we present multilayer Gr/LaVO₃vertical-heterostructure semitransparent PDs with various layer numbers (L_n). AtL_n= 2, the PD shows the best performance with a responsivity (R) of 0.094 A W^-1and a specific detectivity (D*) of 7.385 × 10⁷cm Hz^1/2W^-1at 532 nm. Additionally, the average visible transmittance of the PD is 63%, i.e. it is semitransparent. We increased photocurrent (PC) by approximately 13%, from 0.564 to 0.635μA cm^-2by using an Al reflector on the semitransparent PD. The PC of an unencapsulated PD maintains about 86% (from 0.571 to 0.493μA cm^-2) of its initial PC value after 2000 h at 25 °C temperature/30% relative humidity, showing good stability. This behavior is superior to that of previously reported graphene-based PDs. These results show that these PDs have great potential for semitransparent optoelectronic applications.

Efficient Protection of Silver Nanowire Transparent Electrodes by All-Biorenewable Layer-by-Layer Assembled Thin Films

An efficient protection strategy for silver nanowire-based transparent electrodes (AgNW TEs) is developed to enhance their poor adhesion force on substrates and thermal, optical, chemical, and electrical stabilities. Chitin nanofibers (CNFs) and alkali lignin (AL), which possess high mechanical property, a gas/moisture barrier, and UV absorption properties, are successively assembled on AgNW TEs through layer-by-layer (LBL) assembly based on their oppositely charged surfaces. The formation of LBL-assembled CNFs and AL (CNF/AL)₁₀ bilayers, where 10 is the optimized number of bilayers, on the aldehyde-modified AgNW (Al-AgNW) TEs does not deteriorate their electrical conductivity (17.3 ± 2.1 Ω/□) and transmittance (90.1 ± 0.3% at 550 nm), and the (CNF/AL)₁₀ bilayer-coated Al-AgNW [(CNF/AL)₁₀@Al-AgNW] TEs present considerable enhancement in their adhesion force and thermal, optical, chemical, and electrical durability. In detail, their optoelectrical properties are stable over 200 cycles of the scotch peel-off test, for 10 h sonication, up to 350 °C, under UV/O₃ treatment for 100 min, in 10% HCl and 28% NH₃ for 6 and 12 h, and at an electrical potential up to 14 V, respectively. These features make (CNF/AL)₁₀@Al-AgNW TEs suitable as a durable high-performance transparent heater.

Strategy for Fabricating Ultrathin Au Film Electrodes with Ultralow Optoelectrical Losses and High Stability

A vital objective in the wetting of Au deposited on chemically heterogeneous oxides is to synthesize a completely continuous, highly crystalline, ultrathin-layered geometry with minimized electrical and optical losses. However, no effective solution has been proposed for synthesizing an ideal Au-layered structure. This study presents evidence for the effectiveness of atomic oxygen-mediated growth of such an ideal Au layer by improving Au wetting on ZnO substrates with a substantial reduction in free energy. The unexpected outcome of the atomic oxygen-mediated Au growth can be attributed to the unconventional segregation and incorporation of atomic oxygen along the outermost boundaries of Au nanostructures evolving in the clustering and layering stages. Moreover, the experimental and numerical investigations revealed the spontaneous migration of atomic oxygen from an interstitial oxygen surplus ZnO bulk to the Au-ZnO interface, as well as the segregation (float-out) of the atomic oxygen toward the top Au surfaces. Thus, the implementation of a 4-nm-thick, two-dimensional, quasi-single-crystalline Au layer with a nearly complete crystalline realignment at a mild temperature (570 K) enabled exceptional optoelectrical performance with record-low resistivity (<7.5 × 10^-8 Ω·m) and minimal optical loss (∼3.5%) at a wavelength of 700 nm.

Characterization of Silver Nanowire-Based Transparent Electrodes Obtained Using Different Drying Methods

Metal-based transparent top electrodes allow electronic devices to achieve transparency, thereby expanding their application range. Silver nanowire (AgNW)-based transparent electrodes can function as transparent top electrodes, owing to their excellent conductivity and transmittance. However, they require a high-temperature drying process, which damages the bottom functional layers. Here, we fabricated two types of AgNW-based electrodes using the following three drying methods: thermal, room-temperature, and vacuum. Thereafter, we investigated the variation in their morphological, electrical, and optical characteristics as a function of the drying method and duration. When the AgNW-exposed electrode was dried at room temperature, it exhibited a high surface roughness and low conductivity, owing to the slow solvent evaporation. However, under vacuum, it exhibited a similar electrical conductivity to that achieved by thermal drying because of the decreased solvent boiling point and fast solvent evaporation. Conversely, the AgNW-embedded electrodes exhibited similar roughness values and electrical conductivities regardless of the drying method applied. This was because the polymer shrinkage during the AgNW embedding process generated capillary force and improved the interconnectivity between the nanowires. The AgNW-based electrodes exhibited similar optical properties regardless of the drying method and electrode type. This study reveals that vacuum drying can afford transparent top electrodes without damaging functional layers.

Patterned Sandwich-Type Silver Nanowire-Based Flexible Electrode by Photolithography

Silver nanowires (AgNWs) are one of the important flexible electrode material candidates that can replace brittle indium tin oxide (ITO). In this work, we demonstrated novel patterned sandwich-type AgNW-based transparent electrodes easily prepared using the photolithography method for application in flexible devices. A cross-linked underlayer was introduced to increase the adhesion properties between a poly(ethylene terephthalate) substrate and AgNWs, and as a result, a uniform AgNW layer was easily deposited. Finally, the AgNW layer could be easily patterned by introducing a photocross-linkable upper layer without lift-off, dry transfer, and removal methods. A mixture of poly(sodium-4-styrene sulfonate) (PSS^-Na⁺) and 2,4-hexadiyne-1,6-diol (HDD), which is a component of the upper layer, exhibited good cross-linking properties as well as excellent adhesion to the AgNW layer. Through the above method, it was possible to easily fabricate a patterned electrode with smooth surface morphology. Moreover, AgNW-based patterned electrodes exhibit good optical and electrical properties (R_s = 29.8 Ω/□, T_550 nm = 94.6%), making them suitable for optoelectronic devices. Flexible polymer solar cells (PSCs) using patterned AgNW electrodes showed a high power conversion efficiency of over 10%, which is comparable to that of PSCs using rigid ITO electrodes. In addition, the high mechanical stability of AgNW-based PSCs was confirmed by bending experiments.

Optically Transparent and Highly Conductive Electrodes for Acousto-Optical Devices

The study was devoted to the creation of transparent electrodes based on highly conductive mesh structures. The analysis and reasonable choice of technological approaches to the production of such materials with a high Q factor (the ratio of transparency and electrical conductivity) were carried out. The developed manufacturing technology consists of the formation of grooves in a transparent substrate by photolithography methods, followed by reactive ion plasma etching and their metallization by chemical deposition using the silver mirror reaction. Experimental samples of a transparent electrode fabricated using this technology have a sheet resistance of about 0.1 Ω/sq with a light transmittance in the visible wavelength range of more than 60%.

Synthesis of transparent bio-electrodes for biophysiological measurements based on modified graphene oxide

The main objective of this work was to fabricate smart nanocomposite transparent conductive biophysiological electrodes based on modified graphene oxide (GO). The GO is abundant, flexible conductors that can be formulated as a transparent sheet and thereby alleviate the drawbacks of using indium tin oxide in transparent electrodes, like its scarcity, brittleness, and cost. GO was synthesized by a modified version of Hummers' method under highly acidic conditions with sulfuric acid and showed good distribution at a high temperature of 90 °C. Polyvinyl alcohol (PVA) was used as a polymer host in the composite. Glycerol (Gl) was used to increase the flexibility and conductivity through an esterification reaction. Characteristic techniques were used to detect the morphology and structure of GO fillers and their polymer composites, such as transmission electron microscopy, x-ray diffraction, scanning electron microscopy and Fourier transform infrared spectroscopy. The GO/Gl/PVA transparent nanocomposite was tested for the synthesis of electrocardiogram (ECG) and electrodermal (EDA) electrodes. The Biopac device was used to evaluate the behavior of the GO/Gl/PVA plastic transparent electrode in comparison to the GO/Gl/PVA black electrode and a commercial one. The results indicated improved efficiency of the GO/Gl/PVA ECG transparent electrode. The GO/Gl/PVA EDA electrode produced signals with higher conductivity and lower noise than the commercial electrode.

Ultra-Flexible Organic Photovoltaics with Low-Temperature Deposited IZTO on a Cyclic Polymer Substrate Having Excellent Mechanical Properties

In this study, ultra-flexible organic photovoltaics (OPVs) with a new device structure are developed as a power source for ultra-flexible wearable devices with excellent biocompatibility. To fabricate ultra-flexible OPVs with excellent mechanical properties, we develop an ultra-flexible substrate with a bilayer structure based on polymers and transparent conducting oxides. An amorphous perfluorinated polymer (cyclic transparent optical polymer, CYTOP) is introduced as an ultra-flexible substrate by a solution process. An indium zinc tin oxide (IZTO) transparent electrode possessing an amorphous structure is fabricated via pulsed DC magnetron sputtering at room temperature using a target containing 80 atom % In₂O₃-10 atom % ZnO-10 atom % SnO₂. Ultra-flexible OPVs with a one-dimensional (1D) grating pattern are fabricated on the buffer layer and photoactive layer. These OPVs exhibit an increase of 12% in power conversion efficiency (PCE) (maximum PCE: 8.52%) compared to the reference, thereby minimizing reliance on the incident angle of light. In addition, even after 1000 compression/relaxation tests with a compression strain of 33%, the PCE of the ultra-flexible OPVs is maintained up to 94.8% of its initial value.

Silver Nanowire Networks: Ways to Enhance Their Physical Properties and Stability

Silver nanowire (AgNW) networks have been intensively investigated in recent years. Thanks to their attractive physical properties in terms of optical transparency and electrical conductivity, as well as their mechanical performance, AgNW networks are promising transparent electrodes (TE) for several devices, such as solar cells, transparent heaters, touch screens or light-emitting devices. However, morphological instabilities, low adhesion to the substrate, surface roughness and ageing issues may limit their broader use and need to be tackled for a successful performance and long working lifetime. The aim of the present work is to highlight efficient strategies to optimize the physical properties of AgNW networks. In order to situate our work in relation to existing literature, we briefly reported recent studies which investigated physical properties of AgNW networks. First, we investigated the optimization of optical transparency and electrical conductivity by comparing two types of AgNWs with different morphologies, including PVP layer and AgNW dimensions. In addition, their response to thermal treatment was deeply investigated. Then, zinc oxide (ZnO) and tin oxide (SnO2) protective films deposited by Atmospheric Pressure Spatial Atomic Layer Deposition (AP-SALD) were compared for one type of AgNW. We clearly demonstrated that coating AgNW networks with these thin oxide layers is an efficient approach to enhance the morphological stability of AgNWs when subjected to thermal stress. Finally, we discussed the main future challenges linked with AgNW networks optimization processes.

Ultra-Flexible Organic Solar Cell Based on Indium-Zinc-Tin Oxide Transparent Electrode for Power Source of Wearable Devices

We have developed a novel structure of ultra-flexible organic photovoltaics (UFOPVs) for application as a power source for wearable devices with excellent biocompatibility and flexibility. Parylene was applied as an ultra-flexible substrate through chemical vapor deposition. Indium-zinc-tin oxide (IZTO) thin film was used as a transparent electrode. The sputtering target composed of 70 at.% In₂O₃-15 at.% ZnO-15 at.% SnO₂ was used. It was fabricated at room temperature, using pulsed DC magnetron sputtering, with an amorphous structure. UFOPVs, in which a 1D grating pattern was introduced into the hole-transport and photoactive layers were fabricated, showed a 13.6% improvement (maximum power conversion efficiency (PCE): 8.35%) compared to the reference device, thereby minimizing reliance on the incident angle of the light. In addition, after 1000 compression/relaxation tests with a compression strain of 33%, the PCE of the UFOPVs maintained a maximum of 93.3% of their initial value.

Unzipping Carbon Nanotube Bundles through NH-π Stacking for Enhanced Electrical and Thermal Transport

Bundling of single-walled carbon nanotubes (SWCNTs) significantly undermines their superior thermal and electrical properties. Realizing stable, homogeneous, and surfactant-free dispersion of SWCNTs in solvents and composites has long been regarded as a key challenge. Here, we report amine-containing aromatic and cyclohexane molecules, which are common chain extenders (CEs) for epoxy curing in industry, can be used to effectively disperse CNTs. We achieve single-tube-level dispersion of SWCNTs in CE solvents, as demonstrated by the strong chirality-dependent absorption and photoluminescence emission. The SWCNT-CE dispersion remains stable under ambient conditions for months. The excellent dispersibility and stability are attributed to the formation of an n-type charge-transfer complex through the NH-π interaction between the amine group of CEs and the delocalized π bond of SWCNTs, which is confirmed by the negative Seebeck coefficient of the CE-functionalized SWCNT films, the red shift of the G band in the Raman spectra, and the NH-π peak in X-ray photoelectron spectroscopy. The high dispersibility of CEs significantly improves the electrical and thermal transport of macroscale CNT assemblies. The sheet resistance of the CE-dispersed SWCNT thin films reaches 161 Ω sq^-1 at 80.8% optical transmittance after functional modification by HNO₃. Moreover, the CEs cross-link CNTs and epoxy molecules, forming a pathway for phonon transport in CNT/epoxy nanocomposites. The thermal conductivity of the CE-CNT-epoxy composite is enhanced by 1850% compared with the original epoxy, which is the highest enhancement reported to date for CNT/epoxy nanocomposites. The CE-based NH-π interaction provides a new paradigm for the effective and stable dispersion of SWCNTs in a facile and scalable process.

One-Way Continuous Deposition of Monolayer MXene Nanosheets for the Formation of Two Confronting Transparent Electrodes in Flexible Capacitive Photodetector

MXenes based on titanium carbide are promising next-generation transparent electrode materials due to their high metallic conductivity, optical transparency, mechanical flexibility, and abundant hydrophilic surface functionality. MXene electrodes offer a much wider conductive surface coverage than metal nanowires, thereby gaining popularity as flexible electrode materials in supercapacitors and energy devices. However, given that monolayer MXene nanosheets are only a few nanometers thick, meticulous surface treatments and deposition technologies are required for a practical implementation of these transparent electrodes. Unfortunately, a capacitor produced by forming high-quality transparent MXene electrodes on both sides of a film has not yet been reported. We report the successful development of a one-way continuous deposition technology to form high-quality MXene nanosheet-based transparent electrodes on both surfaces of a polymer film without large physical stresses on the MXene nanosheets. One transparent electrode was formed by transferring MXene nanosheets predeposited on a temporary glass substrate to the film surface, while the other was directly deposited on the exposed film surface. The Ti₃AlC₂ precursor (MAX) was synthesized via a spark plasma sintering crystallization, and the MXene nanosheets were prepared via a subsequent Al-selective etching and delamination. We used this material to implement a capacitive photodetector consisting of two layers of opposing transparent electrodes. The flexible photodetector was based on poly(vinyl butyral) (PVB), which was solidly bonded with MXene nanosheets to serve as a free-standing binder for the Cu-doped ZnS semiconductor particles. The fabricated device exhibited excellent mechanical stability due to the high affinity between the MXene nanosheets and PVB. Furthermore, the device exhibited an initial capacitance of 2 nF, photosensitivity of 12.5 μF/W, and rise and decay times of 0.031 and 0.751 s, respectively. All these parameters were 1 to 2 orders of magnitude higher or faster than reported capacitive photodetectors. Overall, the proposed approach resolves the core issues associated with existing metal nanowire-based electrodes, and it is a breakthrough in the development of next-generation flexible devices comprising two layers of confronting transparent electrodes.

Microscopic Deformation Modes and Impact of Network Anisotropy on the Mechanical and Electrical Performance of Five-fold Twinned Silver Nanowire Electrodes

Silver nanowire (AgNW) networks show excellent optical, electrical, and mechanical properties, which make them ideal candidates for transparent electrodes in flexible and stretchable devices. Various coating strategies and testing setups have been developed to further improve their stretchability and to evaluate their performance. Still, a comprehensive microscopic understanding of the relationship between mechanical and electrical failure is missing. In this work, the fundamental deformation modes of five-fold twinned AgNWs in anisotropic networks are studied by large-scale SEM straining tests that are directly correlated with corresponding changes in the resistance. A pronounced effect of the network anisotropy on the electrical performance is observed, which manifests itself in a one order of magnitude lower increase in resistance for networks strained perpendicular to the preferred wire orientation. Using a scale-bridging microscopy approach spanning from NW networks to single NWs to atomic-scale defects, we were able to identify three fundamental deformation modes of NWs, which together can explain this behavior: (i) correlated tensile fracture of NWs, (ii) kink formation due to compression of NWs in transverse direction, and (iii) NW bending caused by the interaction of NWs in the strained network. A key observation is the extreme deformability of AgNWs in compression. Considering HRTEM and MD simulations, this behavior can be attributed to specific defect processes in the five-fold twinned NW structure leading to the formation of NW kinks with grain boundaries combined with V-shaped surface reconstructions, both counteracting NW fracture. The detailed insights from this microscopic study can further improve fabrication and design strategies for transparent NW network electrodes.

Large-Area Tunable Red/Green/Blue Tri-Stacked Quantum Dot Light-Emitting Diode Using Sandwich-Structured Transparent Silver Nanowires Electrodes

Quantum dot light-emitting diodes (QLEDs), particularly those capable of emitting light with tunable colors, have attracted the attention of researchers for their variability in lighting and displays. So far, various color-tunable QLEDs have been developed using techniques of inkjet printing or white light combining with color filters (CFs), which however suffered from difficulties in mass production. Here, by inserting an insulating resin layer between two conductive silver nanowire (AgNW) layers, a unique AgNWs/resin/AgNWs (A/R/A) sandwich-structured electrode was developed, showing rather small sheet resistances at both sides and high transparency. The as-prepared A/R/A electrode is applicable for making a large-area transparent red QLED with an external quantum efficiency (EQE) of 11.42% and a transmittance of 72.5%. Furthermore, the A/R/A electrode can be used as intermediate connecting electrodes to stack three single-colored QLEDs, forming a novel structured R/G/B tri-stacked QLED, which enables emission not only of primary colors red, green, and blue independently with the maximum EQE of 8.22, 8.07, and 2.28%, respectively, but also arbitrary hybrid colors that cover a 107% National Television System Committee (NTSC) color triangle. Such large-area full-color-tunable tri-stacked QLED offers new perspectives for the next-generation solid-state scene lighting and full-color displays.

Pushing the conductance and transparency limit of monolayer graphene electrodes for flexible organic light-emitting diodes

Graphene has emerged as an attractive candidate for flexible transparent electrode (FTE) for a new generation of flexible optoelectronics. Despite tremendous potential and broad earlier interest, the promise of graphene FTE has been plagued by the intrinsic trade-off between electrical conductance and transparency with a figure of merit (σDC/σOp) considerably lower than that of the state-of-the-art ITO electrodes (σDC/σOp <123 for graphene vs. ∼240 for ITO). Here we report a synergistic electrical/optical modulation strategy to simultaneously boost the conductance and transparency. We show that a tetrakis(pentafluorophenyl)boric acid (HTB) coating can function as highly effective hole doping layer to increase the conductance of monolayer graphene by sevenfold and at the same time as an anti-reflective layer to boost the visible transmittance to 98.8%. Such simultaneous improvement in conductance and transparency breaks previous limit in graphene FTEs and yields an unprecedented figure of merit (σDC/σOp ∼323) that rivals the best commercial ITO electrode. Using the tailored monolayer graphene as the flexible anode, we further demonstrate high-performance green organic light-emitting diodes (OLEDs) with the maximum current, power and external quantum efficiencies (111.4 cd A-1, 124.9 lm W-1 and 29.7%) outperforming all comparable flexible OLEDs and surpassing that with standard rigid ITO by 43%. This study defines a straightforward pathway to tailor optoelectronic properties of monolayer graphene and to fully capture their potential as a generational FTE for flexible optoelectronics.

Multi-Level Analog Resistive Switching Characteristics in Tri-Layer HfO2/Al2O3/HfO2 Based Memristor on ITO Electrode

Atomic layer deposited (ALD) HfO2/Al2O3/HfO2 tri-layer resistive random access memory (RRAM) structure has been studied with a transparent indium tin oxide (ITO) transparent electrode. Highly stable and reliable multilevel conductance can be controlled by the set current compliance and reset stop voltage in bipolar resistive switching. Improved gradual resistive switching was achieved because of the interdiffusion in the HfO2/Al2O3 interface where tri-valent Al incorporates with HfO2 and produces HfAlO. The uniformity in bipolar resistive switching with Ion/Ioff ratio (>10) and excellent endurance up to >103 cycles was achieved. Multilevel conductance levels in potentiation/depression were realized with constant amplitude pulse train and increasing pulse amplitude. Thus, tri-layer structure-based RRAM can be a potential candidate for the synaptic device in neuromorphic computing.

Highly Deformable Transparent Au Film Electrodes and Their Uses in Deformable Displays

With emerging interest in foldable and stretchable displays, the need to develop transparent deformable electrode and interconnection is increasing. Even though metal films have been standard electrodes in conventional electronic devices due to their high conductivity and well-established process, they have never been used for transparent deformable electrodes. We present highly conductive transparent deformable Au film electrodes and use them to fabricate a foldable perovskite light-emitting diode (PeLED) and a biaxially stretchable alternating current electroluminescence (ACEL) display. We exhibit the formation of an ultrathin (6 nm) continuous Au film on an anisotropic conductive ultrathin film (ACUF) of amorphous carbon. The ultrathin Au film was first formed on an ACUF-coated Si wafer (4 in. scale) through metal evaporation and transferred to the polymer substrates by a simple and effective water-assisted delamination process. Then, a hybrid electrode (ACUF/ACUF/Au) was produced as the transparent deformable electrode. Complicated interconnections could be created by metal deposition through a mask. The electrical conductance of the hybrid electrode was not affected by the crack formation in the Au film during electrode folding, crumpling, and stretching. We reveal the reason why the hybrid electrode can maintain such excellent electrical stability under deformation.

Electroplated Silver-Nickel Core-Shell Nanowire Network Electrodes for Highly Efficient Perovskite Nanoparticle Light-Emitting Diodes

The low sheet resistance and high optical transparency of silver nanowires (AgNWs) make them a promising candidate for use as the flexible transparent electrode of light-emitting diodes (LEDs). In a perovskite LED (PeLED), however, the AgNW electrode can react with the overlying perovskite material by redox reactions, which limit the electroluminescence efficiency of the PeLED by causing the degradation of and generating defect states in the perovskite material. In this study, we prepared Ag-Ni core-shell NW electrodes using the solution-electroplating technique to realize highly efficient PeLEDs based on colloidal formamidinium lead bromide (FAPbBr₃) nanoparticles (NPs). Solvated Ni ions from the NiSO₄ source were deposited onto the surface of AgNW networks in three steps: (i) cathodic cleaning, (ii) adsorption of the Ni-ion complex onto the AgNW surface, and (iii) uniform electrodeposition of Ni. An ultrathin (∼3.5 nm) Ni layer was uniformly deposited onto the AgNW surface, which exhibited a sheet resistance of 16.7 Ω/sq and an optical transmittance of 90.2%. The Ag-Ni core-shell NWs not only increased the work function of the AgNW electrode, which facilitated hole injection into the emitting layer, but also suppressed the redox reaction between Ag and FAPbBr₃ NPs, which prevented the degradation of the emitting layer and the generation of defect states in it. The resulting PeLEDs based on FAPbBr₃ NPs with the Ag-Ni core-shell NWs showed high current efficiency of 44.01 cd/A, power efficiency of 35.45 lm/W, and external quantum efficiency of 9.67%.

Programmed Ultrafast Scan Welding of Cu Nanowire Networks with a Pulsed Ultraviolet Laser Beam for Transparent Conductive Electrodes and Flexible Circuits

Metal nanowires (NWs) have shown superior advances for the next-generation transparent conducting (TC) materials. Most concerns were focused on uniform conductive films; however, fabrication of a programmed circuit is still lacking. Here, we demonstrate a programmable ultrafast welding method by pulsed laser beam scanning under ambient conditions to achieve a Cu NW pattern-free TC circuit as well as various size films. High-aspect ratio Cu NWs (> 3000) are synthesized through an oleylamine-mediated solution system. Pulsed ultraviolet laser irradiation together with a programmed moving station is set up for the welding of Cu NW networks. Finite element simulations reveal that the transient heating by efficient absorption of UV light (∼ 250 nm) could remove the organic residues on the surface and realize local welding of interlaced NW junctions. With only 10 ms pulsed irradiation, high optoelectronic performance (33 ohm/sq. at 87% transmittance at 550 nm) and excellent stability of the Cu NW TC film have been achieved. The line-by-line and selected route scanning modes could rapidly make large area TC films and directly write flexible circuits. Moreover, completely transparent micron-size UV and blue LED chips are fabricated and successfully lit with bright emission. This method opens up a future way of circuit and device fabrication by direct one-step laser writing.

High-Resolution Filtration Patterning of Silver Nanowire Electrodes for Flexible and Transparent Optoelectronic Devices

Silver nanowire (AgNW) electrodes attract significant attention in flexible and transparent optoelectronic devices; however, high-resolution patterning of AgNW electrodes remains a considerable challenge. In this study, we have introduced a simple technique for high-resolution solution patterning of AgNW networks, based on simple filtration of AgNW solution on a patterned polyimide shadow mask. This solution process allows the smallest pattern size of AgNW electrodes, down to a width of 3.5 μm. In addition, we have demonstrated the potential of these patterned AgNW electrodes for applications in flexible optoelectronic devices, such as photodetectors. Specifically, for flexible and semitransparent UV photodetectors, AgNW electrodes are embedded in sputtered ZnO films to enhance the photocurrent by light scattering and trapping, which resulted in a significantly enhanced photocurrent (up to 800%) compared to devices based on AgNW electrodes mounted on top of ZnO films. In addition, our photodetector could be operated well under extremely bent conditions (bending radius of approximately 770 μm) and provide excellent durability even after 500 bending cycles.

Redox-Active Vertically Aligned Mesoporous Silica Thin Films as Transparent Surfaces for Energy Storage Applications

Organic-inorganic hybrid membranes, made of a high density of redox active moieties covalently bonded to the internal surfaces of vertically aligned mesoporous silica thin films, are relevant for applications in transparent energy storage devices. This is demonstrated here on the basis of functionalized transparent mesoporous silica thin films prepared on the indium-tin oxide electrode from the combination of an electrochemically induced self-assembly method (to generate azide-functionalized silica) and a copper-catalyzed azide-alkyne click reaction (to derivatize the material with electroactive groups). The very small thickness (105 nm) and the uniformly distributed vertical mesochannels with ultranarrow diameter (2 nm) make the hybrid film a promising substrate that not only achieves a transparency of 82% but also provides large surface area to accommodate a high density of redox active species such as ferrocene. In such rigid and insulating porous membranes, the charge transfer reactions take place through a pure electron-hopping mechanism between adjacent redox sites, which are favored by the ordered and oriented mesostructure containing large amounts of uniformly distributed ferrocene functions in the mesochannels. Their performance results from both high charge transfer rates (electron hopping) and easy mass transport (fast diffusion of counter ions). The most effective system is the ferrocene-functionalized silica film prepared from 40% organosilane, which is able to deliver a capacity of 105 C cm^-3 (1.10 mC cm^-2) at a current density of 0.4 A cm^-3 (with up to 48% capacity retention achieved at a charging time as short as 2.8 s). Such an electrode can be associated to an electrodeposited graphene anode in a solid-state battery-capacitor hybrid device, which can deliver 0.74 mC cm^-2 at a potential scan rate of 20 mV s^-1. The azide-functionalized mesoporous silica film is actually a versatile platform that can be functionalized with different redox molecules, as shown here for cobaltocenium moieties, which may broaden its application field.

Solution-Processed Polymeric Thin Film as the Transparent Electrode for Flexible Perovskite Solar Cells

In the past decade, perovskite solar cells (PSCs) were arising as a new generation of low-cost solar technology for renewable energy generation. More than 25% of power conversion efficiency (PCE) was reported from PSCs on the rigid indium tin oxide (ITO)/glass electrode. However, PSCs fabricated on flexible solution-processed transparent electrodes have still been a challenge to date. In this work, we report a solution-processed transparent polymeric thin film as the electrode for flexible solution-processed PSCs. The solution-processed polymeric thin film exhibits superior optical transparency and decent electrical conductivity. As compared with a PCE of 16.60% from PSCs on the ITO/glass substrate, PSCs on the solution-processed transparent polymeric electrode/glass substrate exhibit a PCE of 13.36% and PSCs on the solution-processed transparent polymeric thin-film/polyethylene terephthalate flexible substrate possess a PCE of 10.16%. Systematic studies demonstrate that poor electrical conductivity of the solution-processed transparent polymeric electrode and serious interfacial charge carrier recombination are responsible for low PCEs. Nevertheless, our results demonstrate that we provide a facile route to develop flexible PSCs by utilization of solution-processed polymeric thin films as the transparent electrodes.

Electrodeposited Silver Nanowire Transparent Conducting Electrodes for Thin-Film Solar Cells

Silver nanowire (AgNW) networks have demonstrated high optical and electrical properties, even better than those of indium tin oxide thin films, and are expected to be a next-generation transparent conducting electrode (TCE). Enhanced electrical and optical properties are achieved when the diameter of the AgNWs in the network is fairly small, that is, typically less than 30 nm. However, when AgNWs with such small diameters are used in the network, stability issues arise. One method to resolve the stability issues is to increase the diameter of the AgNWs, but the use of AgNWs with large diameters has the disadvantage of causing a rough surface morphology. In this work, we resolve all of the aforementioned issues with AgNW TCEs by the electrodeposition of Ag onto as-spin-coated thin AgNW TCEs. The electrodeposition of Ag offers many advantages, including the precise adjustment of the AgNW diameter and wire-to-wire welding to improve the junction conductance while minimizing the increase in protrusion height because of the overlap of AgNWs upon increasing the diameter. In addition, Ag electrodeposition on AgNW TCEs can provide higher conductance than that of as-spin-coated AgNW TCEs at the same transparency because of the reduced junction resistance, which generates a superior figure of merit. We applied the electrodeposited (ED) AgNW network to a Cu(In,Ga)Se₂ thin-film solar cell and compared the device performance to a device with a standard sputtered transparent conducting oxide (TCO). The cell fabricated by the electrodeposition method showed nearly equal performance to that of a cell with the sputtered TCO. We expect that ED AgNW networks can be used as high-performance and robust TCEs for various optoelectronic applications.

Near-Field Electrospinning for Three-Dimensional Stacked Nanoarchitectures with High Aspect Ratios

Near-field electrospinning (NFES) was developed to overcome the intrinsic instability of traditional electrospinning processes and to facilitate the controllable deposition of nanofibers under a reduced electric field. This technique offers a straightforward and versatile method for the precision patterning of two-dimensional (2D) nanofibers. However, three-dimensional (3D) stacked structures built by NFES have been limited to either micron-scale sizes or special shapes. Herein, we report on a direct-write 3D NFES technique to construct self-aligned, template-free, 3D stacked nanoarchitectures by simply adding salt to the polymer solution. Numerical simulations suggested that the electric field could be tuned to achieve self-aligned nanofibers by adjusting the conductivity of the polymer solution. This was confirmed experimentally by using poly(ethylene oxide) (PEO) solutions containing 0.1-1.0 wt% NaCl. Using 0.1 wt% NaCl, nanowalls with a maximum of 80 layers could be built with a width of 92 ± 3 nm, height of 6.6 ± 0.1 μm, and aspect ratio (height/width) of 72. We demonstrate the 3D printing of nanoskyscrapers with various designs, such as curved "nanowall arrays", nano "jungle gyms," and "nanobridges". Further, we present an application of the 3D stacked nanofiber arrays by preparing transparent and flexible polydimethylsiloxane films embedded with Ag-sputtered nanowalls as 3D nanoelectrodes. The conductivity of the nanoelectrodes can be precisely tuned by adjusting the number of 3D printed layers, without sacrificing transmittance (98.5%). The current NFES approach provides a simple, reliable route to build 3D stacked nanoarchitectures with high-aspect ratios for potential application in smart materials, energy devices, and biomedical applications.

MEA Recordings and Cell-Substrate Investigations with Plasmonic and Transparent, Tunable Holey Gold

Microelectrode arrays are widely used in different fields such as neurobiology or biomedicine to read out electrical signals from cells or biomolecules. One way to improve microelectrode applications is the development of novel electrode materials with enhanced or additional functionality. In this study, we fabricated macroelectrodes and microelectrode arrays containing gold penetrated by nanohole arrays as a conductive layer. We used this holey gold to optically excite surface plasmon polaritons, which lead to a strong increase in transparency, an effect that is further enhanced by the plasmon's interaction with cell culture medium. By varying the nanohole diameter in finite-difference time domain simulations, we demonstrate that the transmission can be increased to above 70% with its peak at a wavelength depending on the holey gold's lattice constant. Further, we demonstrate that the novel transparent microelectrode arrays are as suitable for recording cellular electrical activity as standard devices. Moreover, we prove using spectral measurements and finite-difference time domain simulations that plasmonically induced transmission peaks of holey gold red-shift upon sensing medium or cells in close vicinity (<30 nm) to the substrate. Thus, we establish plasmonic and transparent holey gold as a tunable material suitable for cellular electrical recordings and biosensing applications.

Versatile and Tunable Electrical Properties of Doped Nonoxidized Graphene Using Alkali Metal Chlorides

With the rapid development of wearable and flexible electronics, graphene has been intensively studied for the transparent, hole transport electrode layer (HTL) of field-effect transistors, light-emitting diodes, and organic photovoltaic (OPV) cells. To modulate the sheet resistance and the work function of graphene as a HTL, the surface doping is versatile while retaining high transparency. In this work, we used a chemical doping method to control the charge carrier density, band gap, and work function of graphene with minimizing the damage of the carbon network, for which metal chlorides (NaCl, KCl, and AuCl₃) were used as chemical dopants. The high-quality graphene flakes were synthesized with large lateral sizes of more than 5 μm using ternary graphite intercalation compounds. Interestingly, the AuCl₃-doped graphene flake film with a film thickness of about 20 nm showed the lowest reported sheet resistance of ∼249 Ω/sq with ∼75% transmittance. Furthermore, it could control the work function from 4.32 to 5.1 eV. The interfacial dipole complexes of metal cations with a low work function and the reactive radicals such as -OH were discussed to explain this result. For the practical application, an OPV device using the AuCl₃-doped graphene flake film as the HTL was fabricated and it demonstrated enhanced power conversion efficiency while maintaining high optical transparency in visible light.

Constructing a Novel Electroluminescent Device with High-Temperature and High-Humidity Resistance based on a Flexible Transparent Wood Film

Plastic-based electroluminescent devices generally suffer from thermal expansion owing to the high coefficient of thermal expansion (CTE) of the plastic substrate, which reduces the service lifetime of the electroluminescent device. In this study, we employed a delignified veneer synergistically reinforced with epoxy resin as a low-cost substrate for alternating current electroluminescent (ACEL) devices. In brief, the natural interconnected porous structure of wood had a good antideformation capacity to restrict the volume expansion of the epoxy resin under thermal conditions. Furthermore, the impregnation of epoxy resin dramatically improved the optical transmittance of delignified veneer. Considering its low CTE and antideformation capability, the intrinsically high-temperature and high-humidity resistance device based on transparent sliced veneer (TSV) was constructed. Remarkably, the TSV-ACEL device exhibited excellent stability and maintained good luminescence performance even at a high temperature (100 °C, 30 min; as a reference, the poly(ethylene terephthalate)-based ACEL device has stopped operating), completely submerged in water (30 min), or under high-temperature and high-humidity conditions (90 °C, relative humidity: >90%, 30 min). These results pave the way for the realization of flexible and high-temperature resistance ACEL devices.

Silver Nanowires on Carbon Nanotube Aerogel Sheets for Flexible, Transparent Electrodes

Flexible, free-standing transparent conducting electrodes (TCEs) with simultaneously tunable transmittances up to 98% and sheet resistances down to 11 Ω/sq were prepared by a facile spray-coating method of silver nanowires (AgNWs) onto dry-spun multiwall carbon nanotube (MWNT) aerogels. Counterintuitively, the transmittance of the hybrid electrodes can be increased as the mass density of AgNWs within the MWNT aerogels increases; however, the final achievable transmittance depends on the initial transparency of the MWNT aerogels. Simultaneously, a strong decrease in sheet resistance is obtained when AgNWs form a percolated network along the MWNT aerogel. Additionally, anisotropic reduction in sheet resistance and polarized transmittance of AgNW/MWNT aerogels is achieved with this method. The final AgNW/MWNT hybrid TCEs transmittance and sheet resistance can be fine-tuned by spray-coating mechanisms or by choosing initial MWNT aerogel density. Thus, a wide range of AgNW/MWNT hybrid TCEs with optimized optoelectronic properties can be achieved depending of the requirements needed. Finally, the free-standing AgNW/MWNT hybrid TCEs can be laminated onto a wide range of substrates without the need of a bonding aid.

High Brightness Organic Light-Emitting Diodes with Capillary-Welded Hybrid Diameter Silver Nanowire/Graphene Layers as Electrodes

The development of silver nanowire electrodes is always limited due to some disadvantages, such as roughness, oxidative properties, and other disadvantages. In this research, a capillary-welded silver nanowire/graphene composite film was used as an electrode for organic light-emitting diode (OLED) devices. As an encapsulation layer, graphene reduced the surface roughness and the oxidation probability of silver nanowires. The composite electrode showed an excellent transmittance of 91.5% with low sheet resistant of 26.4 ohm/sq. The devices with the silver nanowire/graphene composite electrode emitted green electroluminescence at 516 nm, and the turn-on voltage was about 3.8 V. The maximum brightness was 50810 cd/cm², which is higher than the indium tin oxide-based (ITO-based) devices with the same configuration. Finally, it was proved that the silver nanowire/graphene composite electrodes possessed better heat dissipation than the ITO-based ones under energization. In summary, it means that this novel silver nanowires/graphene electrode has great potential in OLED device applications.

Efficient Ni/Au Mesh Transparent Electrodes for ITO-Free Planar Perovskite Solar Cells

Indium thin oxide (ITO)-free planar perovskite solar cells (PSCs) were fabricated at a low temperature (150 °C) in this work based on the transparent electrode of photolithography processed nickel/gold (Ni/Au) mesh and the high conductivity polymer, PH1000. Ultrathin Au was introduced to increase the conductivity of metal mesh, and the optimal hexagonal Ni (30 nm)/Au (10 nm) mesh (line width of 5 μm) shows a transmittance close to 80% in the visible light region and a sheet resistance lower than 16.9 Ω/sq. The conductive polymer PH1000 not only smooths the raised surface of the metal mesh but also enhances the charge collection ability of metal mesh. The fabricated PSCs have the typical planar structure (glass/Ni-Au mesh/PH1000/PEDOT:PSS/MA_yFA_1−yPbI_xCl_3−x/PCBM/BCP/Ag) and the champion PSC (0.09 cm²) obtains a power conversion efficiency (PCE) of 13.88%, negligible current hysteresis, steady current density and PCE outputs, and good process repeatability. Its photovoltaic performance and stability are comparable to the reference PSC based on the ITO electrodes (PCE = 15.70%), which demonstrates that the Ni/Au mesh transparent electrodes are a promising ITO alternative to fabricate efficient PSCs. The relatively lower performance of Ni/Au based PSC results from the relatively slower charge extraction and stronger charge recombination than the ITO based PSC. Further, we tried to fabricate the large area (1 cm²) device and achieve a PCE over 6% with negligible hysteresis and steady current density and PCE outputs. The improvements of perovskite film quality and interface modification should be an effective approach to further enhance the device performance of Ni/Au based PSCs, and the Ni/Au mesh electrode may find wider applications in PSCs and flexible devices.

Tailoring Electro/Optical Properties of Transparent Boron-Doped Carbon Nanowalls Grown on Quartz

Carbon nanowalls (CNWs) have attracted much attention for numerous applications in electrical devices because of their peculiar structural characteristics. However, it is possible to set synthesis parameters to vary the electrical and optical properties of such CNWs. In this paper, we demonstrate the direct growth of highly transparent boron-doped nanowalls (B-CNWs) on optical grade fused quartz. The effect of growth temperature and boron doping on the behavior of boron-doped carbon nanowalls grown on quartz was studied in particular. Temperature and boron inclusion doping level allow for direct tuning of CNW morphology. It is possible to operate with both parameters to obtain a transparent and conductive film; however, boron doping is a preferred factor to maintain the transparency in the visible region, while a higher growth temperature is more effective to improve conductance. Light transmittance and electrical conductivity are mainly influenced by growth temperature and then by boron doping. Tailoring B-CNWs has important implications for potential applications of such electrically conductive transparent electrodes designed for energy conversion and storage devices.

Nitrogen-Mediated Growth of Silver Nanocrystals to Form UltraThin, High-Purity Silver-Film Electrodes with Broad band Transparency for Solar Cells

Controlling the shape and crystallography of nanocrystals during the early growth stages of a noble metal layer is important because of its correlation with the final layer morphology and optoelectrical features, but this task is unattainable in vapor deposition processes dominated by artificially uncontrollable thermodynamic free energies. We report on experimental evidence for the controllable evolution of Ag nanocrystals as induced by the addition of nitrogen, presumed to be nonresidual in the Ag lattice given its strong float-out behavior. This atypical formation of energetically stable Ag nanocrystals with significantly improved wetting abilities on a chemically heterogeneous substrate promotes the development of an atomically flat, ultrathin, high-purity Ag layer with a thickness of only 5 nm. This facilitates the fabrication of Ag thin-film electrodes exhibiting highly enhanced optical transparency over a broad spectral range in the visible and near-infrared spectral range. An Ag thin-film electrode with a ZnO/Ag/ZnO configuration exhibits an average transmittance of about 95% in the spectral range of 400-800 nm with a maximum transmittance of over 98% at 580 nm, which is comparable with the best transparency values so far reported for transparent electrodes. This degree of optical transparency provides an excellent chance to improve the photon absorption of photovoltaic devices employing an Ag thin film as their window electrode. This is clearly confirmed by the superior performance of a flexible organic solar cell with a power conversion efficiency of 8.0%, which is far superior to that of the same solar cell using a conventional amorphous indium tin oxide electrode (6.4%).

Selectively Patterned Regrowth of Bilayer Graphene for Self-Integrated Electronics by Sequential Chemical Vapor Deposition

There is a critical demand for the highly qualified synthesis of graphene with precisely controlled thickness over a large coverage area. Selective growth can be considered as one method of preparing a vertically stacked graphene, but it usually requires elaborately alloyed substrates for chemical vapor deposition (CVD). Here, we report on a newly developed synthesis strategy for a selectively patterned grown graphene sheet in a spatially defined multithickness scale, exhibiting single- and bilayer graphene produced by a conventional CVD process. In particular, a sequential CVD growth technique on a single Cu substrate was used to produce highly ordered and alternatively patterned single- and bilayer graphene, maintaining its continuous configuration in a simplified and scalable manner. Our regrowth process did not require multiple transfer procedures or an alloying catalytic substrate to satisfy the properties of graphene associated with the needs for various applications. We also investigated the most valid mechanisms for our regrowth CVD process, which suggests that it is useful for the cost-effective synthetic approach into a built-in heterostructured single- and bilayer graphene. Finally, we demonstrated the possible accesses of transparent flexible electrodes and monolithically self-integrated all-graphene-based thin-film transistors to fully utilize regrown graphene.

Aqueous Synthesis, Degradation, and Encapsulation of Copper Nanowires for Transparent Electrodes

Copper nanowires (CuNWs) have increasingly become subjected to academic and industrial research, which is attributed to their good performance as a transparent electrode (TE) material that competes with the one of indium tin oxide (ITO). Recently, an environmentally friendly and aqueous synthesis of CuNWs was demonstrated, without the use of hydrazine that is known for its unfavorable properties. In this work, we extend the current knowledge for the aqueous synthesis of CuNWs by studying their up-scaling potential. This potential is an important aspect for the commercialization and further development of CuNW-based devices. Due to the scalability and homogeneity of the deposition process, spray coating was selected to produce films with a low sheet resistance of 7.6 Ω/sq. and an optical transmittance of 77%, at a wavelength of 550 nm. Further, we present a comprehensive investigation of the degradation of CuNWs when subjected to different environmental stresses such as the exposure to ambient air, elevated temperatures, high electrical currents, moisture or ultraviolet (UV) light. For the oxidation process, a model is derived to describe the dependence of the breakdown time with the temperature and the initial resistance. Finally, polymer coatings made of polydimethylsiloxane (PDMS) and polymethylmethacrylate (PMMA), as well as oxide coatings composed of electron beam evaporated silicon dioxide (SiO₂) and aluminum oxide (Al₂O₃) are tested to hinder the oxidation of the CuNW films under current flow.

Highly Durable and Flexible Transparent Electrode for Flexible Optoelectronic Applications

A highly-durable, highly-flexible transparent electrode (FTE) is developed by applying a composite made of a thin metal grid and a doped conducting polymer onto a colorless polyimide-coated NOA63 substrate. The proposed FTE exhibits a transparency of 90.7% at 550 nm including the substrate and a sheet resistance of 30.3 Ω/sq and can withstand both moderately high-temperature annealing (∼180 °C) and acidic solution (70 °C, pH 0.3) processes without performance degradation. The fabricated FTE yielded good mechanical stability under 10 000 cycles of bending deformations at a bending radius less than 1 mm without degradation of electrical conductivity. The high durability of the proposed FTE allows for the fabrication of flexible energy harvesting devices requiring harsh conditions, such as highly flexible perovskite solar cells (FPSCs) with a steady-state power conversion efficiency (PCE) of 12.7%. Notably, 93% of the original PCE is maintained after 2000 bending cycles at an extremely small bending radius of 1.5 mm. The FPSCs installed on curved surfaces of commercial devices drive them under various environments. The applicability of the proposed FTE is further confirmed via the fabrication of a flexible perovskite light-emitting diode. The proposed FTE demonstrates great potential for applications in the field of flexible optoelectronic devices.

Pressure Welding of Silver Nanowires Networks at Room Temperature as Transparent Electrodes for Efficient Organic Light-Emitting Diodes

In this work, polymethylmethacrylate (PMMA) as a superior mediate for the pressure welding of silver nanowires (Ag NWs) networks as transparent electrodes without any thermal treatment is demonstrated. After a pressing of 200 kg cm^-2 , not only the sheet resistance but also the surface roughness of the PMMA-mediated Ag NWs networks decreases from 2.6 kΩ sq^-1 to 34.3 Ω sq^-1 and from 76.1 to 12.6 nm, respectively. On the other hand, high transparency of an average transmittance in the visible wavelengths of 93.5% together with a low haze value of 2.58% can be achieved. In terms of optoelectronic applications, the promising potential of the PMMA-mediated pressure-welded Ag NWs networks used as a transparent electrode in a green organic light-emitting diode (OLED) device is also demonstrated. In comparison with the OLED based on commercial tin-doped indium oxide electrode, the increments of power efficiency and external quantum efficiency (EQE) from 80.1 to 85.9 lm w^-1 and 19.2% to 19.9% are demonstrated. In addition, the PMMA-mediated pressure welding succeeds in transferring Ag NWs networks to flexible polyethylene naphthalate and polyimide substrates with the sheet resistance of 42 and 91 Ω sq^-1 after 10 000 times of bending, respectively.

Selective Electroless Metallization of Micro- and Nanopatterns via Poly(dopamine) Modification and Palladium Nanoparticle Catalysis for Flexible and Stretchable Electronic Applications

The authors report a new patterned electroless metallization process for creating micro- and nanoscale metallic structures on polymeric substrates, which are essential for emerging flexible and stretchable optical and electronic applications. This novel process features a selective adsorption of catalytic Pd nanoparticles (PdNPs) on a lithographically masked poly(dopamine) (PDA) interlayer in situ polymerized on the substrates. The moisture-resistant PDA layer has excellent stability under a harsh electroless plating bath, which enables electroless metallization on versatile substrate materials regardless of their hydrophobicity, and significantly strengthens the attachment of electroless plated metallic structures on the polymeric substrates. Prototype devices fabricated using this PDA-assisted electroless metallization patterning exhibit superior mechanical stability under high bending and stretching stress. The lithographic patterning of the PDA spatially confines the adsorption of PdNPs and reduces defects due to random adsorption of catalytic particles on the undesired area. The high resolution of the lithographic patterning enables the demonstration of a copper micrograting pattern with a linewidth down to 2 μm and a silver plasmonic nanodisk array with a 500 nm pitch. A copper mesh is also fabricated using our new patterned electroless metallization process and functions as flexible transparent electrodes with >80% visible transmittance and <1 Ω sq^-1 sheet resistance. Moreover, flexible and stretchable dynamic electroluminescent displays and functional flexible printed circuits are demonstrated to show the promising capability of our fabrication process in versatile flexible and stretchable electronic devices.

Biodegradable Transparent Substrate Based on Edible Starch-Chitosan Embedded with Nature-Inspired Three-Dimensionally Interconnected Conductive Nanocomposites for Wearable Green Electronics

Electronic waste (E-waste) contain large environmental contaminants such as toxic heavy metals and hazardous chemicals. These contaminants would migrate into drinking water or food chains and pose a serious threat to environment and human health. Biodegradable green electronics has great potential to address the issue of E-waste. Here, we report on a novel biodegradable and flexible transparent electrode, integrating three-dimensionally (3D) interconnected conductive nanocomposites into edible starch-chitosan-based substrates. Starch and chitosan are extracted from abundant and inexpensive potato and crab shells, respectively. Nacre-inspired interface designs are introduced to construct a 3D interconnected single wall carbon nanotube (SCNT)-pristine graphene (PG)-conductive polymer network architecture. The inorganic one-dimensional SCNT and two-dimensional PG sheets are tightly cross-linked together at the junction interface by long organic conductive poly(3,4-ethylenedioxythiophene) (PEDOT) chains. The formation of a 3D continuous SCNT-PG-PEDOT conductive network leads to not only a low sheet resistance but also a superior flexibility. The flexible transparent electrode possesses an excellent optoelectronic performance: typically, a sheet resistance of 46 Ω/sq with a transmittance of 83.5% at a typical wavelength of 550 nm. The sheet resistance of the electrode slightly increased less than 3% even after hundreds of bending cycles. The lightweight flexible and biocompatible transparent electrode could conform to skin topography or any other arbitrary surface naturally. The edible starch-chitosan substrate-based transparent electrodes could be biodegraded in lysozyme solution rapidly at room temperature without producing any toxic residues. SCNT-PG-PEDOT can be recycled via a membrane process for further fabrication of conductive and reinforcement composites. This high-performance biodegradable transparent electrode is a promising material for next-generation wearable green optoelectronics, transient electronics, and edible electronics.