Silver Nanowire-Based Transparent Electrodes for V2O5 Thin Films with Electrochromic Properties

The development of electrochromic systems, known for the modulation of their optical properties under an applied voltage, depends on the replacement of the state-of-the-art ITO (In2O3:Sn) transparent electrode (TE) as well as the improvement of electrochromic films. This study presents an innovative ITO-free electrochromic film architecture utilizing oxide-coated silver nanowire (AgNW) networks as a TE and V2O5 as an electrochromic oxide layer. The TE was prepared by simple spray deposition of AgNWs that allowed for tuning different densities of the network and hence the resistance and transparency of the film. The conformal oxide coating (SnO2 or ZnO) on AgNWs was deposited by atmospheric-pressure spatial atomic layer deposition, an open-air fast and scalable process yielding a highly stable electrode. V2O5 thin films were then deposited by radio frequency magnetron sputtering on the AgNW-based TE. Independent of the oxide's nature, a 20 nm protective layer thickness was insufficient to prevent the deterioration of the AgNW network during V2O5 deposition. On the contrary, crystalline V2O5 films were grown on 30 nm thick ZnO or SnO2-coated AgNWs, exhibiting a typical orange color. Electrochromic characterization demonstrated that only V2O5 films deposited on 30 nm thick SnO2-coated AgNW showed characteristic oxidation-reduction peaks in the Li+-based liquid electrolyte associated with a reversible orange-to-blue color switch for at least 500 cycles. The electrochromic key properties of AgNW/SnO2 (30 nm)/V2O5 films are discussed in terms of structural and morphological changes due to the AgNW network and the nature and thickness of the two protective oxide coatings.

Facile Transfer of a Transparent Silver Nanowire Pattern to a Soft Substrate Using Graphene Oxide as a Double-Sided Adhesion-Tuning Layer

Silver nanowires (AgNWs) have been employed in various optoelectronic devices as transparent electrodes. However, it remains a great challenge to facilely pattern silver nanowires to realize desirable soft skin devices. Here, we develop an intact transfer method via a double-layered adhesion regulator of graphene oxide (GO) enabling complete transfer of a silver nanowire pattern from a tough substrate onto soft polydimethylsiloxane (PDMS) and flexible polyethylene (PE). We achieve positive and negative patterns simultaneously when selectively transferring silver nanowire patterns. The resulting patterned AgNW electrodes have uniform conductivity and long-term stability. The underlying mechanism of the clean transfer is thoroughly investigated via transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). GO plays a role in reducing the adhesion of AgNW to the donor tough substrate and enhancing adhesion of AgNW to the target soft substrate simultaneously. Finally, we demonstrate the utility of the patterned electrodes as transparent sensors detecting body motion. This work offers an effective solution to the challenging patterning problem of silver nanowires on a hydrophobic soft substrate, which is compatible with the soft component in emerging smart skin or wearable electronics.

Room-temperature nanojoining of silver nanowires by graphene oxide for highly conductive flexible transparent electrodes

Flexible transparent electrodes for touch panels, solar cells, and wearable electronics are in great demand in recent years, and the silver nanowire (AgNW) flexible transparent electrode (FTE) is among the top candidates due to its excellent light transmittance and flexibility and the highest conductivity of silver among all metals. However, the conductivity of an AgNWs network has long been limited by the large contact resistance. Here we show a room-temperature solution process to tackle the challenge by nanojoining AgNWs with two-dimensional graphene oxide (GO). The conductivity of the AgNWs network is improved 18 times due to the enhanced junctions between AgNWs by the coated GOs, and the AgNW-GO FTE exhibits a low sheet resistance of 23 Ohm sq^-1and 88% light transmittance. It is stable under high temperature and current and their flexibility is intact after 1000 cycles of bending. Measurements of a bifunctional electrochromic device shows the high performance of the AgNW-GO FTE as a FTE.

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.

Exploring Molecular and Electronic Property Predictions of Reduced Graphene Oxide Nanoflakes via Density Functional Theory

In this research, we perform a theoretical interpretation of molecular and electronic properties of reduced graphene oxide (rGO) nanoflakes through the density functional theory. Here, two pristine graphene nanoflake systems were passivated by hydrogen atoms at their edges, armchair (C58H20) and zigzag (C54H20); besides, we implemented 12 rGO systems with a range of low oxide coverage (1, 3, and 4%). Computational calculations were carried out employing the functional hybrid B3LYP and the basis 6-31G(d, p) and 6-311G(d, p) levels of theory. We brought the proposed molecular structures to a stable minimum. We determined the global reactivity descriptors through chemical potential, hardness, softness, and index of electrophilicity. Besides, the maps of electrostatic potential were generated. We found that the hydroxyl and epoxy functional groups dope the graphene molecule in p-type and n-type forms, respectively. In addition, we could attribute the increases of the oxide coverage and the chemical potential to the softness of the molecule. These results suggest that structures with this type of doping can help in developing advanced electronics of sensors and devices.

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.

Fabrication, Mechanisms, and Properties of High-Performance Flexible Transparent Conductive Gas-Barrier Films Based on Ag Nanowires and Atomic Layer Deposition

Thin films of Ag nanowires (NWs) offer many advantages as transparent electrodes for flexible electronics, but their applications are hindered by issues including poor stability/durability of Ag NWs, high processing temperatures, heterogeneity of surfaces, and lack of gas-barrier function. This study reports novel mechanisms through which a conductive Hf:ZnO (HZO) film by atomic layer deposition (ALD) can be integrated with a sprayed Ag NWs film to address the issues of Ag NWs. First, the ALD surface reactions can induce fusing of the Ag NWs into a connected network without the need for a thermal sintering process. Second, the ALD process can in situ functionalize the Ag NWs to yield defect-free (in terms of blocking gas permeation) coverage of the ALD HZO over the entire nanowire surfaces, which also enhances the ALD-induced fusing of Ag NWs. The composite HZO/Ag NWs films exhibit low sheet resistance (15 Ω sq^-1), low water vapor transmission rate (WVTR) (5.1 × 10^-6 g m^-2 day^-1), high optical transmission (92%), excellent flexibility (12.5 mm bending radius), high stability/durability (against an extensive set of degradation modes and photolithographic patterning processes), and low processing temperature (90 °C) and can be used in perovskite solar cells to obtain high power conversion efficiency (14.46%).

Improved Stability of Metal Nanowires via Electron Beam Irradiation Induced Surface Passivation

Suppressing the corrosion of nanoscaled metal materials is a critical issue for various devices. Herein, we demonstrate the electron beam irradiation can be a simple and efficient method to realize silver/copper nanowires protection by transforming the original organic capping agents into dense carbonaceous shells. Single nanowire tests prove the significant stability improvement from 4 days to 20 days for silver nanowire and from 20 h to at least 1 week for copper nanowire. The comprehensive advantages such as solution/pollution-free and continuous process with high precision offer this method substantial potential applications in bottom-up assembled electronic and optoelectronic devices.

Metal-to-insulator transition near room temperature in graphene oxide and graphene oxide + TiO2 thin films

Thin films of graphene oxide and a composite of graphene oxide with titanium oxide were prepared via an alternative chemical route based on Hummer's method. Metal-to-Insulator Transition (MIT) were observed for GO (at 280 K) and for GO + TiO₂ (at 260 K).