Ambient Processed rGO/Ti3CNTx MXene Thin Film with High Oxidation Stability, Photosensitivity, and Self-Cleaning Potential

Solution-based processing offers advantages for producing thin films due to scalability, low cost, simplicity, and benignity to the environment. Here, we develop conductive and photoactivated self-cleaning reduced graphene oxide (rGO)/Ti3CNTx MXene thin films via spin coating under ambient conditions. The addition of a thin rGO layer on top of Ti3CNTx resulted in up to 45-fold improvement in the environmental stability of the film compared to the bare Ti3CNTx film. The optimized rGO/Ti3CNTx thin film exhibits an optical transmittance of 74% in the visible region of the spectrum and a sheet resistance of 19 kΩ/sq. The rGO/Ti3CNTx films show high rhodamine B discoloration activity upon light irradiation. Under UV irradiation, the electrically conductive MXene in combination with in situ formed semiconducting titanium oxide induces photogenerated charge carriers, which could potentially be used in photocatalysis. On the other hand, due to film transparency, white light irradiation can bleach the adsorbed dye via photolysis. This study opens the door for using MXene thin films as multifunctional coatings with conductive and potentially self-cleaning properties.

Versatile Applications of Silver Nanowire-Based Electrodes and Their Impacts

Indium tin oxide (ITO) is currently the most widely used material for transparent electrodes; however, it has several drawbacks, including high cost, brittleness, and environmental concerns. Silver nanowires (AgNWs) are promising alternatives to ITO as materials for transparent electrodes owing to their high electrical conductivity, transparency in the visible range of wavelengths, and flexibility. AgNWs are effective for various electronic device applications, such as touch panels, biosensors, and solar cells. However, the high synthesis cost of AgNWs and their poor stability to external chemical and mechanical damages are significant challenges that need to be addressed. In this review paper, we discuss the current state of research on AgNW transparent electrodes, including their synthesis, properties, and potential applications.

Analysis of the Effect of Graphene, Metal, and Metal Oxide Transparent Electrodes on the Performance of Organic Optoelectronic Devices

Transparent electrodes (TEs) are important components in organic optoelectronic devices. ITO is the mostly applied TE material, which is costly and inferior in mechanical performance, and could not satisfy the versatile need for the next generation of transparent optoelectronic devices. Recently, many new TE materials emerged to try to overcome the deficiency of ITO, including graphene, ultrathin metal, and oxide-metal-oxide structure. By finely control of the fabrication techniques, the main properties of conductivity, transmittance, and mechanical stability, have been studied in the literatures, and their applicability in the potential optoelectronic devices has been reported. Herein, in this work, we summarized the recent progress of the TE materials applied in optoelectronic devices by focusing on the fabrication, properties, such as Graphene, ultra-thin metal film, and metal oxide and performance. The advantages and insufficiencies of these materials as TEs have been summarized and the future development aspects have been pointed out to guide the design and fabrication TE materials in the next generation of transparent optoelectronic devices.

Conformal Encapsulation of Silver Nanowire Transparent Electrodes by Nanosized Reduced Graphene Oxide Leading to Improved All-Round Stability

Solution-processed silver nanowire (AgNW) networks are promising as next-generation transparent conductive electrodes due to their excellent optoelectronic properties, mechanical flexibility, as well as low material and processing costs. However, AgNWs are prone to thermally induced fragmentation and chemical degradation, necessitating a conformal protective coating typically achieved by low-throughput methods such as sputtering or atomic layer deposition. Herein, we report a facile all-solution-based approach to synthesize a conformally coated AgNW network by nanosized reduced graphene oxide R(nGO). In this method, probe ultrasonication is used to obtain nanosized GO, which is coated on AgNWs by a layer-by-layer approach and then chemically treated to form R(nGO)/AgNW. We show that our transparent electrode has excellent transmittance (85-92%) and sheet resistance (17.5 Ω/sq), combined with outstanding thermal and electrothermal stability, thanks to the conformal nature of the R(nGO) film, and demonstrate its use as a transparent heater with a high maximum temperature. This, in conjunction with improved long-term chemical and mechanical bending stability of R(nGO)/AgNW, indicates that our newly developed process represents an effective and low-cost strategy to improve the overall operational resilience of metal nanowire-based transparent conductive electrodes.

A Review on the Deformation Behavior of Silver Nanowire Networks under Many Bending Cycles

Silver nanowire networks are attractive for flexible transparent electrodes due to their excellent optical transparency and electrical conductivity. Their mechanical reliability under bending is an important feature for the adoption of silver nanowire transparent electrodes for flexible electronics. Therefore, various studies have been conducted to understand the deformation behavior of silver nanowire networks, which are different from those of bulk silver or silver thin films. The focus of this review is to elucidate the deformation mechanism of silver nanowire networks under high cycles of bending and to present ways to improve the mechanical reliability of silver nanowire transparent electrodes.

Failing Forward: Stability of Transparent Electrodes Based on Metal Nanowire Networks

Metal nanowire (MNW)-based transparent electrode technologies have significantly matured over the last decade to become a prominent low-cost alternative to indium tin oxide (ITO). Beyond reaching the same level of performance as ITO, MNW networks offer additional advantages including flexibility and low materials cost. To facilitate adoption of MNW networks as a replacement to ITO, they must overcome their inherent stability issues while maintaining their properties and cost-effectiveness. Herein, the fundamental failure mechanisms of MNW networks are discussed in detail. Recent strategies to computationally model MNWs from the nano- to macroscale and suggest future work to capture dynamic failure to unravel mechanisms that account for convolution of the failure modes are highlighted. Strategies to characterize MNW network failure in situ and postmortem are also discussed. In addition, recent work about improving the stability of MNW networks via encapsulation is discussed. Lastly, a perspective is given on how to frame the requirements of MNW-encapsulant hybrids with reference to their target applications, namely: solar cells, transparent film heaters, sensors, and displays. A cost analysis to comment on the feasibility of implementing MNW hybrids is provided, and critical areas to focus on for future work on MNW networks are suggested.

Double-Sided Graphene Oxide Encapsulated Silver Nanowire Transparent Electrode with Improved Chemical and Electrical Stability

Owing to their high conductivity, transparency, flexibility, and compatibility with solution processes, silver nanowire (AgNW) networks have been widely explored as a promising alternative to indium tin oxide (ITO). However, their susceptibility to corrosion and thermal instability still remain limiting factors for widespread adoption in a range of devices including solar cells, transparent heaters, and light-emitting diodes. In this study, we report a scalable and economically viable process involving electrophoretic deposition (EPD) to fabricate a highly stable hybrid transparent electrode with a sandwich-like structure, where a AgNW network is covered by graphene oxide (GO) films on both sides. The newly developed all solution process allows the conductive transparent film to be transferred to an arbitrary surface after deposition and demonstrates excellent sheet resistance (15 Ω/sq) and tunable transmittance (70-87% at 550 nm). Unlike bare AgNW networks, the hybrid electrode retains its original conductivity under long-term storage at up to 80% relative humidity. This chemical resilience is explained by the absence of silver corrosion products for the AgNW encapsulated by GO as indicated by X-ray photoelectron spectroscopy. In situ voltage ramping and resistance measurements up to 20 V indicate a novel stabilization mechanism enabled by the presence of GO which delays the failure onset and prevents abrupt divergence of the resistance to the megaohm range experienced by bare AgNW networks. The double-sided nature of the GO coating offers combined stability and performance to the AgNW network, which adds unique versatility of our electrodes to be used toward applications that require a wide range of thermal and chemical stabilities.

Silver Nanowire Networks: Mechano-Electric Properties and Applications

With increasing technological demand for portable electronic and photovoltaic devices, it has become critical to ensure the electrical and mechano-electric reliability of electrodes in such devices. However, the limited flexibility and high processing costs of traditional electrodes based on indium tin oxide undermine their application in flexible devices. Among various alternative materials for flexible electrodes, such as metallic/carbon nanowires or meshes, silver nanowire (Ag NW) networks are regarded as promising candidates owing to their excellent electrical, optical, and mechano-electric properties. In this context, there have been tremendous studies on the physico-chemical and mechano-electric properties of Ag NW networks. At the same time, it has been a crucial job to maximize the device performance (or their mechano-electric performance) by reconciliation of various properties. This review discusses the properties and device applications of Ag NW networks under dynamic motion by focusing on notable findings and cases in the recent literature. Initially, we introduce the fabrication (deposition process) of Ag NW network-based electrodes from solution-based coating processes (drop casting, spray coating, spin coating, etc.) to commercial processes (slot-die and roll-to-roll coating). We also discuss the electrical/optical properties of Ag NW networks, which are governed by percolation, and their electrical contacts. Second, the mechano-electric properties of Ag NW networks are reviewed by describing individual and combined properties of NW networks with dynamic motion under cyclic loading. The improved mechano-electric properties of Ag NW network-based flexible electrodes are also discussed by presenting various approaches, including post-treatment and hybridization. Third, various Ag NW-based flexible devices (electronic and optoelectronic devices) are introduced by discussing their operation principles, performance, and challenges. Finally, we offer remarks on the challenges facing the current studies and discuss the direction of research in this field, as well as forthcoming issues to be overcome to achieve integration into commercial devices.

Emerging Novel Metal Electrodes for Photovoltaic Applications

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

Novel Flexible Transparent Conductive Films with Enhanced Chemical and Electromechanical Sustainability: TiO2 Nanosheet-Ag Nanowire Hybrid

Flexible transparent conductive films (TCFs) of TiO₂ nanosheet (TiO₂ NS) and silver nanowire (Ag NW) network hybrid were prepared through a simple and scalable solution-based process. The as-formed TiO₂ NS-Ag NW hybrid TCF shows a high optical transmittance (TT: 97% (90.2% including plastic substrate)) and low sheet resistance (R_s: 40 Ω/sq). In addition, the TiO₂ NS-Ag NW hybrid TCF exhibits a long-time chemical/aging and electromechanical stability. As for the chemical/aging stability, the hybrid TCF of Ag NW and TiO₂ NS reveals a retained initial conductivity (ΔR_s/R_s < 1%) under ambient oxidant gas over a month, superior to that of bare Ag NW (ΔR_s/R_s > 4000%) or RuO₂ NS-Ag NW hybrid (ΔR_s/R_s > 200%). As corroborated by the density functional theory simulation, the superb chemical stability of TiO₂ NS-Ag NW hybrid is attributable to the unique role of TiO₂ NS as a barrier, which prevents Ag NW's chemical corrosion via the attenuated adsorption of sulfidation molecules (H₂S) on TiO₂ NS. With respect to the electromechanical stability, in contrast to Ag NWs (ΔR/R₀ ∼ 152.9%), our hybrid TCF shows a limited increment of fractional resistivity (ΔR/R₀ ∼ 14.4%) after 200 000 cycles of the 1R bending test (strain: 6.7%) owing to mechanically welded Ag NW networks by TiO₂ NS. Overall, our unique hybrid of TiO₂ NS and Ag NW exhibits excellent electrical/optical properties and reliable chemical/electromechanical stabilities.

A graphene mesh as a hybrid electrode for foldable devices

A graphene mesh with arrays of micro-holes was fabricated on a polymer substrate using photolithography for use as an electrode in flexible devices. The optimal mesh structure with high optical transmittance and electrical conductivity was designed using a finite element method, in which the conductivity of the mesh was simulated as a function of structure, size, and periodicity of the hole array. The sheet resistance of the graphene mesh was lowered to that of a graphene monolayer by chemical doping and found to be 330 Ω Sq^-1 at 98.5% transparency. The figure of merit of the doped graphene mesh was calculated to be 106 at 98% transmittance, a value that has not yet been reported for any conventional transparent electrode material. Due to strong bonding between the polymer and substrate, the hybrid electrode composed of a silver nanowire (AgNW)/graphene mesh coated with an over-coating layer exhibited more stable electrical characteristics during mechanical fatigue deformation compared to a hybrid film composed of a AgNW/graphene sheet. The AgNW/graphene sheet underwent breakdown at less than 20 000 cycles in cyclic bending tests with 6.5% strain, but the AgNW/graphene mesh showed a 38% increase in resistance at 20 000 cycles and no breakdown even at 100 000 cycles. Therefore, in this study, we propose a hybrid structure composed of a AgNW/graphene mesh, which is optically and mechanically superior to AgNW/graphene sheets, and therefore suitable for application as a transparent electrode in foldable devices with long-term stability.

Highly Flexible and Transparent Ag Nanowire Electrode Encapsulated with Ultra-Thin Al2O3: Thermal, Ambient, and Mechanical Stabilities

There is an increasing demand in the flexible electronics industry for highly robust flexible/transparent conductors that can withstand high temperatures and corrosive environments. In this work, outstanding thermal and ambient stability is demonstrated for a highly transparent Ag nanowire electrode with a low electrical resistivity, by encapsulating it with an ultra-thin Al₂O₃ film (around 5.3 nm) via low-temperature (100 °C) atomic layer deposition. The Al₂O₃-encapsulated Ag nanowire (Al₂O₃/Ag) electrodes are stable even after annealing at 380 °C for 100 min and maintain their electrical and optical properties. The Al₂O₃ encapsulation layer also effectively blocks the permeation of H₂O molecules and thereby enhances the ambient stability to greater than 1,080 h in an atmosphere with a relative humidity of 85% at 85 °C. Results from the cyclic bending test of up to 500,000 cycles (under an effective strain of 2.5%) confirm that the Al₂O₃/Ag nanowire electrode has a superior mechanical reliability to that of the conventional indium tin oxide film electrode. Moreover, the Al₂O₃ encapsulation significantly improves the mechanical durability of the Ag nanowire electrode, as confirmed by performing wiping tests using isopropyl alcohol.

Neutral plane control by using polymer/graphene flake composites for flexible displays

PMMA/graphene composites with varying graphene content can control the modulus and thus the neutral plane of flexible displays while enhancing barrier properties.