A transparent electrode based on a metal nanotrough network

Flexible transparent electrodes (FTEs) with high stability and scalability are in high demand for the extremely widespread applications in flexible optoelectronic devices. Traditionally, thin films of indium thin oxide (ITO) served the role of FTEs, but film brittleness and scarcity of materials limit its further application. This review provides a summary of recent advances in emerging transparent electrodes and related flexible devices (e.g., touch panels, organic light-emitting diodes, sensors, supercapacitors, and solar cells). Mainly focusing on the FTEs based on carbon nanomaterials (e.g., carbon nanotubes and graphene) and metal materials (e.g., metal grid and metal nanowires), we discuss the fabrication techniques, the performance improvement, and the representative applications of these highly transparent and flexible electrodes. Finally, the challenges and prospects of flexible transparent electrodes will be summarized.

Sputtered transparent electrodes for optoelectronic devices: Induced damage and mitigation strategies

Transparent electrodes are commonly used in various applications, such as solar cells, touch screens, smart windows, wearable electronic devices, and rollable flexible displays. Currently, indium tin oxide (ITO) is widely used as a transparent electrode material. However, ITO is not suitable for next-generation transparent electrodes that require flexibility; therefore, alternative nanomaterials, such as carbon nanotubes, conductive polymers, and metal nanowires, are being studied. However, these nanomaterials have poor mechanical strength and limited substrate availability. In this study, we developed a high-performance transparent electrode web film fabrication process based on conductive nanofibers, in which metal nanofibers are semiembedded in polydimethylsiloxane (PDMS). The mechanical strength of the conductive nanofibers was improved through the PDMS coating on the entire surface of the film, and the semiembedded structure of the nanofibers was realized using the reactive ion etching (RIE) process. In this study, we confirmed through transparency/conductivity analysis and bending, cycle, and taping tests that the transparent electrode fabricated using our approach has excellent mechanical strength and conductivity. Finally, the transparent electrode fabricated using our method can be widely applied as a next-generation transparent electrode because the process is easy and simple and requires inexpensive equipment and materials.

The development of new nanomaterial like carbon nanotubes, graphene and nanowires brings new possibilities to existing technologies. With the emergence of transparent electrode based on these materials, we could imagine new type of sensors, which combines transparency and flexibility. Silver nanowire transparent electrodes are currently transforming this industry. Touch screen sensors, OLED and solar panel can now be fabricated on flexible plastic sheets with low cost printing techniques. In the same way, this new type of electrode could bring electrochemical sensor toward new transparent and flexible sensing application for life science. In our study, we fabricate and characterize transparent electrode based in metal nanowires for electrochemical detection. By changing the length, diameter and amount of deposited nanowires, we can vary their transparency (up to 90% transparent), sheet resistance (up to 0.1 ohm/square) and effective surface area. Compared to conventional evaporated plain electrodes, printed nanowire electrodes can achieve more than 80% transparency (in visible range) with the same electrochemical effective surface area. Using these electrodes, we can fabricate new sensors with high transparency (> 90%), flexibility and high sensitivity to oxygen and hydrogen peroxide. The detection of these two components is predominant in healthcare and environmental monitoring. The combination of those properties could allow us to use this type of sensors as opto-electrochemical sensors for monitoring living cells and also for glucose detection on humans. Transparent nanowire electrodes can also be a real alternative to existing indium tin oxide (ITO) transparent electrodes for other applications. Figure 1.Pictures of electrochemical sensors based on metal nanowires on PET transparent substrate. WE and REF electrodes are made from metal nanowire have 80% transparency, CE electrode are made from gold. Figure 2. SEM image of silver nanowire electrode and the size distribution of nanowires. Figure 3. Voltammetry response of the nanowire based electrochemical sensor to hydrogen peroxide in phosphate buffer (pH=7) Figure 4. Dose-response curve to hydrogen peroxide using the nanowire-based sensor in phosphate buffer (pH=7) Figure 1

Optoelectronic devices are advancing from existing rigid configurations to deformable configurations. These developing devices need transparent electrodes (TEs) having high mechanical deformability while preserving the high electrical conductivity and optical transparency. In agreement with these requirements, vacuum-fabricated conventional TEs based on transparent conducting oxides (TCOs) are receiving difficulties due to its low abundance, film brittleness, and low optical transmittance. Novel solution-processed TE materials including regular metal meshes, metal nanowire (NW) grids, carbon materials, and conducting polymers have been studied and confirmed their capabilities to address the limitations of the TCO-based TEs. This chapter presents a comprehensive review of the latest advances of these vacuum-free TEs, comprising the electrode material classes, the optical, electrical, mechanical and surface feature properties of the soft TEs, and the vacuum-free practices for their fabrication.

In recent years, extensive progress has been made in the development of primary and applied aspects of fabrication of transparent conductive electrodes especially in flexible, stretchable, low-cost, and lightweight electrode materials for the enhancement of energy generation devices. Fabrication of high-performance transparent conductive flexible plastic is necessary for industrial-scale manufacturing with an extensive range of applications. Transparent electrodes (TE’s) are optically transparent to visible light and are electrically conductive. These qualities are important for many renewable energy conversion processes. As transparent electrodes, they are widely used in industry, especially in optoelectronic devices. TE’s are essential components for touch panels, organic photovoltaic (OPV) cells, liquid crystal displays (LCDs), and organic light-emitting diodes (OLEDs). Functionalized nanomaterials having the characteristic properties such as flexible, stretchable, and lightweight-based TE’s are promising substitutes for commonly used indium tin oxide (ITO)-based TE’s for future flexible optoelectronic devices. This chapter broadly summarizes recent developments in the fabrication, properties, modification, patterning, and integration of functionalized nanomaterials for the applications of optoelectronic devices. Their challenges and potential applications, such as in touch panels, optoelectronic devices, liquid crystal displays, and photovoltaic cells, are discussed in detail. Despite many challenges, nanomaterials such as carbon nanotube and graphene TE’s have exhibited various applications in optoelectronic devices and some commercially available products such as touch panels of smartphones. An account of recent developments in the fabrication, performance, and significant opportunities for the industrially used transparent conductive electrode, followed by a brief introduction, is provided.

For decades, transparent conductive electrodes (TCEs) have been widely used in various applications owing to their high optical transmittance and excellent electrical conductivity. Despite indium tin oxide (ITO) being the most commonly used TCE nowadays, developing the potential substitution materials of ITO is necessary due to the (1) high cost of indium and (2) the brittleness of ITO film, which makes ITO film difficult to use in flexible substrates. In recent years, the intensive development of nanotechnology leads the growth of nanostructured TCEs because of their high surface area, enhanced active sites, and shortened diffusion distances. This chapter starts at briefly introducing the principles and requirements of TCEs, followed by reviewing and comparing the synthetic methodologies and physical properties of various nanostructured TCEs such as transparent conductive oxides (TCOs), single-walled carbon nanotubes (SWCNTs), and metallic nanowires. The applications based on those TCEs, such as photovoltaic devices, light-emitting diodes (LED), touch panels, smart windows, and transparent heaters, are also discussed.

There has been lately a growing interest into flexible, efficient and low-cost transparent electrodes which can be integrated for many applications. This includes several applications related to energy technologies (photovoltaics, lighting, supercapacitor, electrochromism, etc.) or displays (touch screens, transparent heaters, etc.) as well as Internet of Things (IoT) linked with renewable energy and autonomous devices. This associated industrial demand for low-cost and flexible industrial devices is rapidly increasing, creating a need for a new generation of transparent electrodes (TEs). Indium tin oxide has so far dominated the field of TE, but indium’s scarcity and brittleness have prompted a search into alternatives. Metallic nanowire (MNW) networks appear to be one of the most promising emerging TEs. Randomly deposited MNW networks, for instance, can present sheet resistance values below 10 Ω/sq., optical transparency of 90% and high mechanical stability under bending tests. AgNW or CuNW networks are destined to address a large variety of emerging applications. The main properties of MNW networks, their stability and their integration in energy devices are discussed in this contribution.

Transparent electrodes are essential components for optoelectronic devices, such as touch panels, organic light-emitting diodes, and solar cells. Indium tin oxide (ITO) is widely used as transparent electrode in optoelectronic devices. ITO has high transparency and low resistance but contains expensive rare elements, and ITO-based devices have poor mechanical flexibility. Therefore, alternative transparent electrodes with excellent opto-electrical performance and mechanical flexibility will be greatly demanded. Here, organics are introduced into dielectric–metal–dielectric structures to construct the transparent electrodes on rigid and flexible substrates. We show that organic-metal-organic (OMO) electrodes have excellent opto-electrical properties (sheet resistance of below 10 Ω sq−1 at 85% transmission), mechanical flexibility, thermal and environmental stabilities. The OMO-based polymer photovoltaic cells show performance comparable to that of devices based on ITO electrodes. This OMO multilayer structure can therefore be used to produce transparent electrodes suitable for use in a wide range of optoelectronic devices.

Two-dimensional (2D) networks consisting of one-dimensional (1D) wires, such as carbon nanotubes, metal nanowires, and graphene nanoribbons, are promising candidates for next-generation flexible transparent conducting electrodes in devices such as organic light-emitting diodes (OLEDs), solar cells, touch screens, smart windows, transparent heaters, and liquid crystal displays. Nanotube networks also have broad application potential in flexible electronics, such as thin film transistors, wearables, electronic skin, and internet of things (IoT) sensors.The electrical conductivity of carbon nanotube networks is governed by percolation, which deals with the formation of long-range connectivity in random networks. As a result, Monte Carlo simulations need to be employed in order to compute the electrical properties of these networks [1,2]. Understanding the impact of voids, which could be present due to lack of control in the deposition process or introduced intentionally, on the percolation conductivity of carbon nanotube networks is critical for applications such as transparent conductive electrodes, thin film transistors, sensing, and hardware security.In this work, we generate two-dimensional square carbon nanotube networks with square voids located at the center of the nanotube network. We define the relative void size as the ratio of the length of the side of the void to the length of the side of the nanotube network. We first study the impact of voids on networks consisting of randomly oriented and straight nanotubes. We compute the percolation probability in these networks as a function of nanotube density for different relative void sizes ranging from 0 (no void) to 0.8. Assuming a Gaussian percolation probability density function (PDF), we find that both the mean and standard deviation of the PDF increase with increasing relative void size. We then compute the relative conductivity change as a function of nanotube density for different relative void sizes and find that it increases approximately linearly with relative void size.According to percolation theory, the conductivity of a nanotube network has a power-law dependence on nanotube density. Next, we extract the local power-law critical exponent as a function of nanotube density for different relative void sizes. We find that the critical exponent approaches 2 at high density for all relative void sizes, in agreement with previous observations for junction-resistance dominated networks without voids [1,3].Furthermore, we generate curvy carbon nanotubes using third order Bezier curves characterized by the curviness angle and aligned nanotubes using an orientation characterized by the alignment angle [1,2]. Using the same procedure as randomly oriented and straight nanotubes, we then investigate the impact of voids on the electronic properties of networks consisting of curvy and aligned nanotubes, including percolation probability, mean and standard deviation of the PDF, relative conductivity change, and nanotube density critical exponent.Our results demonstrate the impact voids have on the percolation conductivity of two-dimensional networks consisting of one-dimensional wires such as carbon nanotubes. These results also show that Monte Carlo simulations are an essential predictive tool for providing insights into the electronic properties of nanotube and nanowire networks, which are promising candidates for a wide range of applications such as flexible transparent conductors, thin film transistors, and resistive switching memory.

ABSTRACTThe recent advent of unprecedented wearable applications engendered the need for stretchable electronics, which can be realized by making the individual components stretchable. The transparent conducting electrode is one of the most important components of optoelectronic devices. Therefore, developing transparent electrodes in a stretchable form is essential for the implementation of stretchable electronics. In this paper, the recent efforts in the development of stretchable and transparent electrodes, particularly those using nanomaterials such as metal nanowires, metal nanofibers, and carbon nanotubes are introduced.

The development of new transparent electrodes (TEs) has been thoroughly investigated by both academics and companies during the last decade. TEs are widely used in many functional devices such as solar cells, touchscreens, transparent heaters, and displays. This chapter presents the key challenges related to the integration of AgNW-based TEs. It shows how these new TEs have been integrated into various flexible displays. Displays taking advantage of metallic nanowire TEs and based on several technologies are presented. Emphasis is placed on touch screens, OLEDs, polymer dispersed liquid crystals, and thermochromic devices. Metallic nanowires appear as very promising building nanoblocks for the fabrication of flexible TEs for application in flexible displays. The performances of TEs depend on various parameters, and in particular on the now well-controlled dimensions of nanowires and the techniques used for deposition.

Recently, two-dimensional (2D) nanomaterials have received huge attention because of their attractiveness for use in many electronic and optoelectronic devices. Graphene is the two-dimensional basic building block for carbon allotropes of any dimensionality, such as graphite, nanotubes and fullerenes. As we know, transparent electrodes are an important component in many modern electronic devices such as touch screen, liquid crystal display (LCD), light-emitting diode (LED) and solar cells. In addition, all of electronic appliances are growing in demand too much fast due to the rapid industrialization and growing human population. Right now, this role has been well used by doped metal oxide materials; most common are tin doped indium oxide (ITO) and fluorine doped tin oxide (FTO). In recent years many other transparent conducting materials (TCM) have also been developed such as carbon nanotubes (CNTs), graphene, metal nanowires and nanoparticles. Among the all these TCM, graphene has received greater attention due to advantages over other materials because of its very high electrical conductivity, optical transparency and flexibility. The flexibility of graphene-based devices goes beyond conventional transistor circuits and includes flexible and transparent electronics, optoelectronics, sensors, electromechanical systems, and energy technologies. This review article will explore the production of graphene by different methods, properties of graphene and also analyze the application in transparent conducting electronic devices.

Transparent conductive electrodes are one of the essential components for organic optoelectronic devices, including photovoltaic cells and light-emitting diodes. Indium-tin oxide (ITO) is the most common transparent electrode in these devices due to its excellent optical and electrical properties. However, the manufacturing of ITO film requires precious raw materials and expensive processes, which limits their compatibility with mass production of large-area, low-cost devices. The optical/electrical properties of ITO are strongly dependent on the deposition processes and treatment conditions, whereas its brittleness and the potential damage to underlying films during deposition also present challenges for its use in flexible devices. Recently, several other transparent conductive materials, which have various degrees of success relative to commercial applications have been developed to address these issues. Starting from the basic properties of ITO and the effect of various ITO surface modification methods, here we review four different groups of materials, doped metal oxides, thin metals, conducting polymers, and nanomaterials (including carbon nanotubes, graphene, and metal nanowires), that have been reported as transparent electrodes in organic optoelectronic materials. Particular emphasis is given to their optical/electrical and other material properties, deposition techniques, and applications in organic optoelectronic devices.

Transparent electrodes, indespensible in displays and solar cells, are currently dominated by indium tin oxide (ITO) films although the high price of indium, brittleness of films, and high vacuum deposition are limiting their applications. Recently, solution-processed networks of nanostructures such as carbon nanotubes (CNTs), graphene, and silver nanowires have attracted great attention as replacements. A low junction resistance between nanostructures is important for decreasing the sheet resistance. However, the junction resistances between CNTs and boundry resistances between graphene nanostructures are too high. The aspect ratios of silver nanowires are limited to ∼100, and silver is relatively expensive. Here, we show high-performance transparent electrodes with copper nanofiber networks by a low-cost and scalable electrospinning process. Copper nanofibers have ultrahigh aspect ratios of up to 100000 and fused crossing points with ultralow junction resistances, which result in high transmitance at low sheet resistance, e.g., 90% at 50 Ω/sq. The copper nanofiber networks also show great flexibility and stretchabilty. Organic solar cells using copper nanowire networks as transparent electrodes have a power efficiency of 3.0%, comparable to devices made with ITO electrodes.