High performing AgNW transparent conducting electrodes with a sheet resistance of 2.5 Ω Sq-1 based upon a roll-to-roll compatible post-processing technique

Solution Processed Semi-Transparent Organic Solar Cells Over 50% Visible Transmittance Enabled by Silver Nanowire Electrode with Sandwich Structure

Photovoltaic windows with easy installation for the power supply of household appliances have long been a desire of energy researchers. However, due to the lack of top electrodes that offer both high transparency and low sheet resistance, the development of high-transparency photovoltaic windows for indoor lighting scenarios has lagged significantly behind photovoltaic windows where privacy issues are involved. Addressing this issue, this work develops a solution-processable transparent top electrode using sandwich structure silver nanowires, realizing high transparency in semi-transparent organic solar cells. The wettability and conducting properties of the electrode are improved by a modified hole-transport layer named HP. The semi-transparent solar cell exhibits good see-through properties at a high average visible transmittance of 50.8%, with power conversion efficiency of 7.34%, and light utilization efficiency of 3.73%, which is the highest without optical modulations. Moreover, flexible devices based on the above-mentioned architecture also show excellent mechanical tolerance compared with Ag electrode counterparts, which retains 94.5% of their original efficiency after 1500 bending cycles. This work provides a valuable approach for fabricating solution-processed high transparency organic solar cells, which is essential in future applications in building integrated photovoltaics.

Stretchable conductors for stretchable field-effect transistors and functional circuits

Stretchable electronics have received intense attention due to their broad application prospects in many areas, and can withstand large deformations and form close contact with curved surfaces. Stretchable conductors are vital components of stretchable electronic devices used in wearables, soft robots, and human-machine interactions. Recent advances in stretchable conductors have motivated basic scientific and technological research efforts. Here, we outline and analyse the development of stretchable conductors in transistors and circuits, and examine advances in materials, device engineering, and preparation technologies. We divide the existing approaches to constructing stretchable transistors with stretchable conductors into the following two types: geometric engineering and intrinsic stretchability engineering. Finally, we consider the challenges and outlook in this field for delivering stretchable electronics.

Flexible, Transparent, and Hazy Composite Cellulosic Film with Interconnected Silver Nanowire Networks for EMI Shielding and Joule Heating

An optical transparent and hazy film with admirable flexibility, electromagnetic interference (EMI) shielding, and Joule heating performance meeting the requirements of optoelectronic devices is significantly desirable. Herein, a cellulose paper was infiltrated by epoxy resin to fabricate a transparent cellulose paper (TCP) with high transparency, optical haze, and favorable flexibility, owing to effective light scattering and mechanical enhancement of the cellulose network. Moreover, a highly connected silver nanowire (AgNW) network was constructed on the TCP substrate by the spray-coating method and appropriate thermal annealing technique to realize high electrical conductivity and favorable optical transmittance of the composite film at the same time, followed by coating of a polydimethylsiloxane (PDMS) layer for protection of the AgNW network. The obtained PDMS/AgNWs/TCP composite film features considerable optical transmittance (up to 86.8%) and haze (up to 97.7%), while satisfactory EMI shielding effectiveness (SE) (up to 39.1 dB, 8.2-12.4 GHz) as well as strong mechanical strength (higher than 41 MPa) were achieved. The coated PDMS layer prevented the AgNW network from falling off and ensured the long-term stability of the PDMS/AgNWs/TCP composite film under deformations. In addition, the multifunctional PDMS/AgNWs/TCP composite film also exhibited excellent Joule heating performance with low supplied voltages, rapid response, and sufficient stability. This work demonstrates a novel pathway to improve the performance of multifunctional transparent composite films for future advanced optoelectronic devices.

Uniform Silver Nanowire Patterned Electrode on Robust PEN Substrate Using Poly(2-hydroxyethyl methacrylate) Underlayer

Silver nanowire (AgNW) electrodes are among the most essential flexible transparent electrodes (FTEs) emerging as promising alternatives to brittle indium tin oxide (ITO) electrodes. The polymer comprising the plastic substrate to which the AgNWs are applied must also satisfy the mechanical requirements of the final device and withstand the device processing conditions. However, AgNW-based FTEs have some limitations, such as poor adhesion to coated plastic substrates, surface roughness, and difficulty in patterning. This study demonstrates a new strategy for creating AgNW-based patterned flexible poly(ethylene 2,6-naphthalate) (PEN)-based electrodes with appreciable optical and electrical properties. Introducing poly(2-hydroxyethyl methacrylate) on the PEN substrate enhanced the adhesion between the substrate and AgNWs and improved the dispersibility of the AgNWs. Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) and a small amount of 2,4-hexadiyne-1,6-diol as a photosensitizer were coated onto the AgNW layer to improve the surface roughness and achieve an effective electrode pattern. By varying the AgNW concentration, we could tune the density and thickness of the AgNWs to optimize the sheet resistance and transmittance. Optimized AgNWs with a sheet resistance of 22.6 Ω/□ and transmittance of 92.3% at 550 nm were achieved. A polymer solar cell (PSC) was fabricated to evaluate the characteristics of the device employing the flexible electrodes. This PSC showed not only a high power conversion efficiency of 11.20%, similar to that of ITO-based devices, but also excellent mechanical stability, which is difficult to achieve in ITO-based flexible devices.

Advances in Flexible Metallic Transparent Electrodes

Transparent electrodes (TEs) are pivotal components in many modern devices such as solar cells, light-emitting diodes, touch screens, wearable electronic devices, smart windows, and transparent heaters. Recently, the high demand for flexibility and low cost in TEs requires a new class of transparent conductive materials (TCMs), serving as substitutes for the conventional indium tin oxide (ITO). So far, ITO has been the most used TCM despite its brittleness and high cost. Among the different emerging alternative materials to ITO, metallic nanomaterials have received much interest due to their remarkable optical-electrical properties, low cost, ease of manufacturing, flexibility, and widespread applicability. These involve metal grids, thin oxide/metal/oxide multilayers, metal nanowire percolating networks, or nanocomposites based on metallic nanostructures. In this review, a comparison between TCMs based on metallic nanomaterials and other TCM technologies is discussed. Next, the different types of metal-based TCMs developed so far and the fabrication technologies used are presented. Then, the challenges that these TCMs face toward integration in functional devices are discussed. Finally, the various fields in which metal-based TCMs have been successfully applied, as well as emerging and potential applications, are summarized.

Hydroprinting Technology to Transfer Ultrathin, Transparent, and Double-Sided Conductive Nanomembranes for Multiscale 3D Conformal Electronics

Transparent multiscale 3D conformal electronics using hydroprinting with polyvinyl alcohol (PVA) as a sacrificial layer to transfer networks of silver nanowires (AgNWs) without a carrier layer is developed. However, AgNWs are known to disperse on water surfaces during the transfer process. Therefore, a functional film is developed by simultaneously welding and embedding AgNWs in the PVA through a simple one-step thermal pressing, demonstrating that ultrathin, transparent, and double-sided conductive/patterned nanomembranes with welded AgNWs can float on water without dispersion. The nanomembrane with an excellent figure of merit of 1200, a low sheet resistance of 16.2 Ω sq^-1 , and a high transmittance of 98.17% achieves conformal contact with excellent step surface coverage of complex macro- and microstructures because of its nanoscale thickness (54.39 nm) and numerous deformable micro- and nanopores. Furthermore, the double-sided conductive nanomembranes facilitate wiring and layer-by-layer assembly, regardless of the transfer direction of the surface. As a proof-of-concept demonstration, a nanomembrane-based aneurysm sensor is developed. Its high transparency enables coil embolization, and the sensor can measure the pushing force of the coil within an aneurysm in an endovascular simulator. Moreover, this newly developed hydroprinting technology provides a new method for the fabrication of transparent multiscale 3D conformal electronics.

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.

Plasmon-Enhanced Photoresponse of Self-Powered Si Nanoholes Photodetector by Metal Nanowires

In this work, we report the development of self-powered photodetectors that integrate silicon nanoholes (SiNHs) and four different types of metal nanowires (AgNWs, AuNWs, NiNWs, PtNWs) applied on the SiNHs’ surface using the solution processing method. The effectiveness of the proposed architectures is evidenced through extensive experimental and simulation analysis. The AgNWs/SiNHs device showed the highest photo-to-dark current ratio of 2.1 × 10⁻⁴, responsivity of 30 mA/W and detectivity of 2 × 10¹¹ Jones along with the lowest noise equivalent power (NEP) parameter of 2.4 × 10⁻¹² WHz^−1/2 in the blue light region. Compared to the bare SiNHs device, the AuNWs/SiNHs device had significantly enhanced responsivity up to 15 mA/W, especially in the red and near-infrared spectral region. Intensity-modulated photovoltage spectroscopy (IMVS) measurements revealed that the AgNWs/SiNHs device generated the longest charge carrier lifetime at 470 nm, whereas the AuNWs/SiNHs showed the slowest recombination rate at 627 nm. Furthermore, numerical simulation confirmed the local field enhancement effects at the MeNWs and SiNHs interface. The study demonstrates a cost-efficient and scalable strategy to combine the superior light harvesting properties of SiNHs with the plasmonic absorption of metallic nanowires (MeNWs) towards enhanced sensitivity and spectral-selective photodetection induced by the local surface plasmon resonance effects.

Improved thermal/chemical stability of flexible Ag NWs/Chitosan composite electrode integrated by chemical welding and sequential protection for better PV performance

An integration strategy of chemical welding and subsequent protection was demonstrated to address silver nanowires (Ag NWs)-based issues. Preferentially, a halogenated salt of NaCl solution was used to stimulate the junction welding thus to reduce the junction resistance, by virtue of the autocatalytic redox of Ag atoms with halogen ions and dissolved oxygen molecules. Subsequently, chitosan, possessing the biocompatible, degradable, environmentally friendly non-toxic features, was embedded to protect Ag NWs. With these two steps, the composite electrode consisting Ag NWs and chitosan reaches a lowest sheet resistance of ∼8 Ω, with a transmittance over 80% at 550 nm, along with high thermal and chemical stabilities, accompanying with excellent flexibility. Besides, it also prompts a synergistic improvement when pioneered in Cu(In, Ga)Se₂(CIGS) device as a transparent conductive electrode. It yields a power conversion efficiency of 6.6%, with 32% improvement relative to that bare Ag NWs, and 85% of the conventional one. Our findings present a new strategy for addressing instable/inefficient Ag NWs-based devices, driving their rapid development and its practical applications.

Inkjet Printing of Flexible Transparent Conductive Films with Silver Nanowires Ink

The inkjet printing process is a promising electronic printing technique for large-scale, printed, flexible and stretchable electronics because of features such as its high manufacturing speed, environmental friendliness, simple process, low cost, accurate positioning, and so on. As the base material of printed conductive patterns, conductive ink is the foundation of the development of printed electronics technology, and directly affects the performance and the quality of electronic products. In this paper, conductive ink with silver nanowires (AgNWs) was prepared, with AgNWs of lengths of 2–5 µm and diameters of 20 nm or so, isopropyl alcohol and ethylene glycol as the mixed solvents, and modified polysilane as the wetting agent. We discussed the relationship between the formula of the AgNWs ink and the surface tension, viscosity, contact angle between ink droplet and poly(ethylene) terephthalate (PET) surface, as well as the film-forming properties of the ink. Further, we analyzed the effects of the number of printed layers and the ink concentration of the AgNWs on the microstructures, photoelectric properties and accuracy of the printed patterns, as well as the change in the sheet resistance of the film during different bending cycles. The experimental results show that flexible transparent conductive patterns with a light transmittance of 550 nm of 83.1–88.4% and a sheet resistance of 34.0 Ω∙sq⁻¹–78.3 nm∙sq⁻¹ can be obtained by using AgNWs ink of 0.38 mg∙mL⁻¹ to 0.57 mg∙mL⁻¹, a poly (ethylene terephthalate) (PET) substrate temperature of 40 °C, a nozzle temperature of 35 °C, and heat treated at 60 °C for 10 min. These performances indicate the excellent potential of the inkjet printing of AgNWs networks for developing flexible transparent conductive film.

Laser shock pressing of silver nanowires on flexible substrates to fabricate highly uniform transparent conductive electrode films

Large surface roughness, wire-to-wire junction resistance, and poor adhesion strength of percolated silver nanowire films on polymer substrates are critical issues responsible for low shunt resistance, electron concentration, and thermal damage, resulting in the occurrence of dark spots and damage to flexible electronic devices. Therefore, the fabrication of transparent conductive electrode (TCE) thin films with high surface smoothness and enhanced film properties without the aforementioned problems is essential. Herein, we propose an innovative method to mechanically join silver nanowires on heat-sensitive polymer substrates using a laser-induced shock pressure wave generated by laser ablation of a sacrificial layer. The physical joining mechanism and film properties, that is, sheet resistance, transmittance, adhesion strength, and flexibility, were experimentally analyzed. When a high laser shock pressure was applied to the silver nanowires, plastic deformation occurred; thus, a sintered network film was fabricated through solid-state atomic diffusion at the nanowire junctions. Under optimal process conditions, the sintered films showed high resistance to the adhesion tape test (R/R ₀ = 1.15), a significantly reduced surface roughness less than 6 nm, and comparable electrical conductivity (8 ± 2 [Formula: see text]) and visible transmittance (84% ± 3%) to typical joining methods. Consequently, this work demonstrates that the laser-induced shock pressing technique has strong potential for the production of TCE metal films on heat-sensitive flexible substrates with film properties superior to those of films produced by conventional methods.

Interfacial control and design of conductive nanomaterials for transparent nanocomposite electrodes

A few critical issues in preparing transparent conductive electrodes (TCEs) based on solution-processable conductive nanomaterials are their low electrical conductivity and the unfavorable trade-off between electrical conductivity and optical transparency, which are closely related to the organic ligands bound to the nanomaterial surface. In particular, bulky/insulating organic ligands bound to the surface of conductive nanomaterials unavoidably act as high contact resistance sites at the interfaces between neighboring nanomaterials, which adversely affects the charge transfer kinetics of the resultant TCEs. This article reviews the latest research status of various interfacial control approaches for solution-processable TCEs. We describe how these approaches can be effectively applied to conductive nanomaterials and how interface-controlled conductive nanomaterials can be employed to improve the electrical and/or electrochemical performance of various transparent nanocomposite electrodes, including TCEs, energy storage electrodes, and electrochromic electrodes. Last, we provide perspectives on the development direction for next-generation transparent nanocomposite electrodes and breakthroughs for significantly mitigating the complex trade-off between their electrical/electrochemical performance and optical transparency.

Rapid synthesis of ultra-long silver nanowires for high performance transparent electrodes

By using 1,2-propanediol instead of the classic polyol solvent, ethylene glycol, ultra-long silver nanowires are obtained in only 1 h. These nanowires lead to transparent electrodes with a sheet resistance of 5 Ohms per sq at a transparency of 94%, one of the highest figures of merit for nanowire electrodes ever reported.

A review of electronic skin: soft electronics and sensors for human health

This article reviews several categories of electronic skins (e-skins) for monitoring signals involved in human health. It covers advanced candidate materials, compositions, structures, and integrate strategies of e-skin, focusing on stretchable and wearable electronics. In addition, this article further discusses the potential applications and expected development of e-skins. It is possible to provide a new generation of sensors which are able to introduce artificial intelligence to the clinic and daily healthcare.

Optoelectronic and Electrothermal Properties of Transparent Conductive Silver Nanowires Films

Silver nanowires (AgNWs) show promise for fabricating flexible transparent conductors owing to their excellent conductivity, high transparency, and good mechanical properties. Here, we present the fabrication of transparent films composed of AgNWs with diameters of 20-30 nm and lengths of 25-30 μm on polyethylene terephthalate substrates and glass slides substrates using the Meyer rod method. We systematically investigated the films' optoelectronic and electrothermal properties. The morphology remained intact when heated at 25-150 °C and the AgNWs film showed high conductivity (17.6-14.3 Ω∙sq-1), excellent transmittance (93.9-91.8%) and low surface roughness values (11.2-14.7 nm). When used as a heater, the transparent AgNW conductive film showed rapid heating at low input voltages owing to a uniform heat distribution across the whole substrate surface. Additionally, the conductivity of the film decreased with increasing bending cycle numbers; however, the film still exhibited a good conductivity and heating performances after repeated bending.

New Insights into Flexible Transparent Conductive Silver Nanowires Films

Flexible transparent conductive films (FTCFs) composed of silver nanowires (AgNWs) have become an important research direction because of their potential in flexible electronic devices. The optoelectronic properties of FTCFs composed of AgNWs of different lengths were evaluated in this study. AgNWs, with an average diameter of about 25 nm and length of 15.49-3.92 μm were obtained by a sonication-induced scission process. AgNW-FTCFs were prepared on polyethylene terephthalate substrates using a Meyer bar and then dried in the ambient environment. The sheet resistance, non-uniformity factor of the sheet resistance, the root mean square roughness, and haze of the FTCFs increased as the length of AgNWs decreased. The transmittance of the films increased slightly as the length of AgNWs increased. AgNWs with a length of 15.49 μm provided an AgNW-FTCF with excellent properties including haze of 0.95%, transmittance of 93.42%, and sheet resistance of 80.15 Ω∙sq-1, without any additional post-treatment of the film. This work investigating the dependence of the optoelectronic properties of AgNW-FTCFs on AgNW length provides design guidelines for development of AgNW-FTCFs.