Indium-free, highly transparent, flexible Cu2O/Cu/Cu2O mesh electrodes for flexible touch screen panels

Effect of sputtering power on the physical properties of amorphous SiO2-doped InZnO transparent conductive oxide

In order to control the optical and electrical properties of the transparent conductive oxide, the radio frequency (RF) sputtering power was changed from 30 to 40, 50, and 60 W. To optimize the power condition of the SiInZnO (SIZO) layer, we changed the sputtering power from 30 to 60 W, systematically. The chemical properties of the SIZO layer were analyzed using X-ray photoelectron spectroscopy (XPS). XPS proved that this change is dominant in thickness. In order to fabricate the SIZO transparent conducting oxide (TCO) with the optimized power of 50 W, the transmittance of 99.1% at 550 nm and the figure of merit of 12.4×10-3 Ω -1 were obtained.

Fabrication of Flexible and Transparent Metal Mesh Electrodes Using Surface Energy-Directed Assembly Process for Touch Screen Panels and Heaters

Transparent conductive electrodes (TCEs) are indispensable components of various optoelectronic devices such as displays, touch screen panels, solar cells, and smart windows. To date, the fabrication processes for metal mesh-based TCEs are either costly or having limited resolution and throughput. Here, a two-step surface energy-directed assembly (SEDA) process to efficiently fabricate high resolution silver meshes is introduced. The two-step SEDA process turns from assembly on a functionalized substrate with hydrophilic mesh patterns into assembly on a functionalized substrate with stripe patterns. During the SEDA process, a three-phase contact line pins on the hydrophilic pattern regions while recedes on the hydrophobic non-pattern regions, ensuring that the assembly process can be achieved with excellent selectivity. The necessity of using the two-step SEDA process rather than a one-step SEDA process is demonstrated by both experimental results and theoretical analysis. Utilizing the two-step SEDA process, silver meshes with a line width down to 2 µm are assembled on both rigid and flexible substrates. The thickness of the silver meshes can be tuned by varying the withdraw speed and the assembly times. The assembled silver meshes exhibit excellent optoelectronic properties (sheet resistance of 1.79 Ω/□, optical transmittance of ≈92%, and a FoM value of 2465) as well as excellent mechanical stability. The applications of the assembled silver meshes in touch screen panels and thermal heaters are demonstrated, implying the potential of using the two-step SEDA process for the fabrication of TCEs for optoelectronic applications.

Positive and Negative Changes in the Electrical Conductance Related to Hybrid Filler Distribution Gradient in Composite Flexible Thermoelectric Films Subjected to Bending

P-type multiwalled carbon nanotubes (MWCNTs), as well as heterostructures fabricated by direct deposition of inorganic thermoelectric materials as antimony and bismuth chalcogenides on MWCNT networks are known as perspective materials for application in flexible thermoelectric polymer-based composites. In this work, the electrical response of three types of Sb₂Te₃-MWCNT heterostructures-based flexible films-free standing on a flexible substrate, encapsulated in polydimethylsiloxane (PDMS), and mixed in polyvinyl alcohol (PVA) is studied in comparison with the flexible films prepared by the same methods using bare MWCNTs. The electrical conductance of these films when each side of it was subsequently subjected to compressive and tensile stress during the film bending down to a 3 mm radius is investigated in relation to the distribution gradient of Sb₂Te₃-MWCNT heterostructures or bare MWCNTs within the film. It is found that all investigated Sb₂Te₃-MWCNT films exhibit a reversible increase in the conductance in response to the compressive stress of the film side with the highest filler concentration and its decrease in response to the tensile stress. In contrast, free-standing and encapsulated bare MWCNT networks with uniform distribution of nanotubes showed a decrease in the conductance irrelevant to the bending direction. In turn, the samples with the gradient distribution of the MWCNTs, prepared by mixing the MWCNTs with PVA, revealed behavior that is similar to the Sb₂Te₃-MWCNT heterostructures-based films. The analysis of the processes impacting the changes in the conductance of the Sb₂Te₃-MWCNT heterostructures and bare MWCNTs is performed. The proposed in this work bending method can be applied for the control of the uniformity of distribution of components in heterostructures and fillers in polymer-based composites.

Comparative Study of Aluminum-Doped Zinc Oxide, Gallium-Doped Zinc Oxide and Indium-Doped Tin Oxide Thin Films Deposited by Radio Frequency Magnetron Sputtering

A timely replacement of the rather expensive indium-doped tin oxide with aluminum-doped zinc oxide is hindered by the poor uniformity of electronic properties when deposited by magnetron sputtering. Recent results demonstrated the ability to improve the uniformity and to decrease the resistivity of aluminum-doped zinc oxide thin films by decreasing the energy of the oxygen-negative ions assisting in thin film growth by using a tuning electrode. In this context, a comparative study was designed to elucidate if the same phenomenology holds for gallium-doped zinc oxide and indium-doped tin oxide as well. The metal oxide thin films have been deposited in the same setup for similar discharge parameters, and their properties were measured with high spatial resolution and correlated with the erosion track on the target's surface. Furthermore, the films were also subject to post annealing and degradation tests by wet etching. While the tuning electrode was able to reduce the self-bias for all three materials, only the doped zinc oxide films exhibited properties correlating with the erosion track.

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.

Highly flexible Ag nanowire network covered by a graphene oxide nanosheet for high-performance flexible electronics and anti-bacterial applications

We investigated a flexible and transparent conductive electrode (FTCE) based on Ag nanowires (AgNWs) and a graphene oxide (GO) nanosheet and fabricated through a simple and cost-effective spray coating method. The AgNWs/GO hybrid FTCE was optimized by adjusting the nozzle-to-substrate distance, spray speed, compressor pressure, and volume of the GO solution. The optimal AgNWs/GO hybrid FTCE has a high transmittance of 88% at a wavelength of 550 nm and a low sheet resistance of 20 Ohm/square. We demonstrate the presence of the GO nanosheet on the AgNWs through Raman spectroscopy. Using scanning electron microscopy and atomic force microscopy, we confirmed that the nanosheet acted as a conducting bridge between AgNWs and improved the surface morphology and roughness of the electrode. Effective coverage by the GO sheet improved the conductivity of the AgNWs electrode Effective coverage of the GO sheet improved conductivity of the AgNWs electrode with minimum degradation of optical and mechanical properties. Flexible thin film heater (TFH) and electroluminescent (EL) devices fabricated on AgNWs/GO hybrid FTCEs showed better performance than devices on bare AgNWs electrodes due to lower sheet resistance and uniform conductivity. In addition, an AgNWs/GO electrode layer on a facial mask acts as a self-heating and antibacterial coating. A facial mask with an AgNWs/GO electrode showed a bacteriostatic reduction rate of 99.7 against Staphylococcus aureus and Klebsiella pneumonia.

Flexible touch sensor fabricated by double-sided nanoimprint lithography metal transfer

A double-sided nanoimprint lithography metal transfer method has been developed to fabricate a flexible capacitive touch sensor. The electrodes of this sensor are aligned and overlapped to each other and consist of a diamond aluminum mesh, which achieved a transmittance of 94% and anisotropic surface resistivity. The maximum capacitance change of the touch sensor unit is up to 41.8% when fullly touched. A 3 × 3 sensor array was tested to prove good touch detection function and the potential for large-scale applications.

Transparent heater with meshed amorphous oxide/metal/amorphous oxide for electric vehicle applications

For electric vehicle application, one of the problems to be solved is defrosting or defogging a windshield or a side mirror without gas-fired heaters. In this paper, we report on a high performance of transparent heater with meshed amorphous-SiInZnO (SIZO)/ Ag/ amorphous-SiInZnO (SIZO) (SAS) for pure electric vehicles. We have adopted amorphous oxide materials like SIZO since SIZO is well known amorphous oxide materials showing high transparency and smooth surface roughness. With the mesh processing technology, a transparent electrode with high transmittance of 91% and low sheet resistance of 13.8 Ω/ϒ was implemented. When a 10 V supply voltage is applied to transparent heater, the transparent heater on glass substrate was heated up to 130^oC in just 5 seconds and then reached to 250^oC after tens of seconds due to the low sheet resistance. In addition, the SAS transparent meshed heater (TMH) showed high stability under cycling test and long time working stability test.

Transparent Film-Type Vibrotactile Actuator Array and Its Haptic Rendering Using Beat Phenomenon

The most important thing in a thin and soft haptic module with an electroactive polymer actuator array is to increase its vibrotactile amplitude and to create a variety of vibrotactile sensations. In this paper, we introduce a thin film-type electroactive polymer actuator array capable of stimulating two types of human mechanoreceptors simultaneously, and we present a haptic rendering method that maximizes the actuators’ vibrational force without improving the array’s haptic performance. The increase in vibrational amplitude of the soft electroactive polymer actuator array is achieved by creating a beat vibration, which is an interference pattern of two vibrations with slightly different frequencies. The textures of a target object are translated into haptic stimuli using the proposed method. We conducted qualitative and quantitative experiments to evaluate the performance of the proposed rendering method. The results showed that this method not only amplifies the vibration’s amplitude but also haptically simulates various objects’ surfaces.

Moiré-Free Imperceptible and Flexible Random Metal Grid Electrodes with Large Figure-of-Merit by Photonic Sintering Control of Copper Nanoparticles

Flexible micro/nano metal grid transparent conductors emerged as an alternative to the fragile/rigid indium tin oxide electrode. They are usually fabricated by the combination of the conventional photolithography and the vacuum deposition of regular metal grid patterns, however, seriously suffer from moiré and starburst problems induced by periodic regular pattern structures. In this paper, we demonstrated flexible and imperceptible random copper microconductors with an extremely high figure-of-merit (∼2000) by the thermal conduction layer-assisted photonic sintering of copper nanoparticles without damages in the plastic substrate. This process can be easily applied to complicated structures and surfaces including a random pattern which is imperceptible and free of interferences. As a proof-of-concept, a transparent windshield defogger in a car was demonstrated with a Cu transparent random conductor at an extreme and reversible fogging state.

High-performance flexible transparent nanomesh electrodes

A cost-effective process for producing high-performance Ag-paste-based flexible transparent nanomesh electrodes (FTNEs) was developed by optimizing their linewidth, pitch, and height. These nanomesh electrodes, with a linewidth of several hundred nanometers and a pitch of 10-200 μm on a PET substrate, achieved wide ranges of transmittance (83.1%-98.8%) and sheet resistance (1.2-30.9 Ω/sq) and a figure of merit (992-1619) superior to those of indium tin oxide and silver nanowire (AgNW) electrodes. Our evaluation of their flexibility (testing up to 50 000 cycles) and their electromagnetic interference shielding effectiveness verifies the applicability of these FTNEs to various flexible optoelectronic devices.

Highly Conductive Flexible Metal-Ceramic Nanolaminate Electrode for High-Performance Soft Electronics

Realization of flexible electronics is an attractive challenge because of its great potential in many applications. However, the design of flexible and highly conductive metal electrodes has been a bottleneck for the fabrication of flexible devices because bulk metals are easily fractured when subjected to elongation or compression. Here, we demonstrate metal-ceramic nanolaminates as electrodes for flexible electronic devices. Insertion of ceramic layers, each with a thickness of a few nanometers, into an otherwise metal electrode significantly improved its strength and bending stability and only slightly reduced its electrical conductivity. Finally, we demonstrated that a touch screen panel fabricated with metal-ceramic nanolaminate electrodes was stable to 200 000 cycles of folding to a bending radius of 3 mm.

Universal Testing Apparatus Implementing Various Repetitive Mechanical Deformations to Evaluate the Reliability of Flexible Electronic Devices

A requirement of flexible electronic devices is that they maintain their electrical performance during and after repetitive mechanical deformation. Accordingly, in this study, a universal test apparatus is developed for in-situ electrical conductivity measurements for flexible electrodes that are capable of applying various mechanical deformations such as bending, twisting, shearing, sliding, stretching, and complex modes consisting of two simultaneous deformations. A novel method of deforming the specimen in an arc to induce uniform bending stress in single and alternating directions is also proposed with a mathematically derived control method. As an example of the arc bending method, the changes in the resistance of the printed radio frequency identification (RFID) tag antennas were measured by applying repetitive inner bending, outer bending, and alternating inner-outer bending. After 5000 cycles, the increases in resistance of the specimens that were subjected to inner or outer bending only were under 30%; however, specimens that were subjected to alternating inner-outer bending showed an increase of 135% in resistance. It is critical that the reliability of flexible electronic devices under various mechanical deformations be determined before they can be commercialized. The proposed testing apparatus can readily provide various deformations that will be useful to inform the design of device shapes and structures to accommodate deformations during use.

Solution-Assembled Ordered Grids Constructed with Silver Nanowires as Transparent Conductive Electrodes

The transparent conductive electrodes (TCEs) composed of silver nanowires (Ag NWs) have shown promising applications recently. In this study, we propose a solution-assembled process to obtain the pattern controllable and uniform-ordered Ag NW grid TCEs by combining with the lithographic technique. The transmittance of Ag NW grid TCEs is controlled by the pattern of grids, but its sheet resistance can be tuned by the diameter of Ag NWs in the grids. As the pattern of grids is fixed, conductive property will improve with the decline of the diameter of Ag NWs. This is a new and efficient strategy to resolve the trade-off between optical transmittance and conductive properties of the random metal nanowire networks for optoelectronic devices.

Roll-to-roll sputtered and patterned Cu2-x O/Cu/Cu2-x O multilayer grid electrode for flexible smart windows

We fabricated cost-effective Cu2-x O/Cu/Cu2-x O multilayer grid electrodes using roll-to-roll (RTR) sputtering and patterning processes for use as transparent and flexible electrodes in flexible smart windows. To optimize the patterned Cu2-x O/Cu/Cu2-x O multilayer grid, the electrical and optical properties of the Cu2-x O/Cu/Cu2-x O multilayer grid electrodes were investigated as a function of grid width and pitch, which directly influence the filling factor of the grid. At the optimized grid width of 16 and pitch of 600 μm, the Cu2-x O/Cu/Cu2-x O multilayer grid had a sheet resistance of 7.17 Ohm per square and an optical transmittance of 87.6%. In addition, the mechanical properties of the optimized Cu2-x O/Cu/Cu2-x O multilayer grid electrode was compared to those of brittle ITO electrodes to demonstrate its outstanding flexibility. To show the potential of the Cu2-x O/Cu/Cu2-x O multilayer grid for smart windows, we fabricated a flexible and transparent thin film heater (TFH) and a flexible electrochromic (EC) device, which are key components of smart windows. The low saturation voltage of the Cu2-x O/Cu/Cu2-x O grid-based TFH and the fast on-off performance of the Cu2-x O/Cu/Cu2-x O grid-based EC device indicates that the RTR-processed Cu2-x O/Cu/Cu2-x O multilayer grid is promising as a low-cost and large-area flexible transparent electrode for high-performance smart windows.

Microbubble-Triggered Spontaneous Separation of Transparent Thin Films from Substrates Using Evaporable Core-Shell Nanocapsules

The spontaneous separation of a polymer thin film from a substrate is an innovative technology that will enable material recycling and reduce manufacturing cost in the film industry, and this can be applied in a wide range of applications, from optical films to wearable devices. Here, we present an unprecedented spontaneous strategy for separating transparent polymer films from substrates on the basis of microbubble generation using nanocapsules containing an evaporable material. The core-shell nanocapsules are prepared from poly(methyl methacrylate)-polyethyleneimine nanoparticles via the encapsulation of methylcyclohexane (MCH). A spherical nanostructure with a vaporizable core is obtained, with the heat-triggered gas release ability leading to the formation of microbubbles. Our separation method applied to transparent polymer films doped with a small amount of the nanocapsules encapsulating evaporable MCH enables spontaneous detachment of thin films from substrates via vacuum-assisted rapid vaporization of MCH over a short separation time, and clear detachment of the film is achieved with no deterioration of the inherent optical transparency and adhesive property compared to a pristine film.

Metal Nanowire-Coated Metal Woven Mesh for High-Performance Stretchable Transparent Electrodes

This work presents a new template-assisted fabrication method to obtain stretchable metal grids for high-performance stretchable transparent conducting electrodes (TCEs). Readily accessible metal woven mesh (MWM) is used as a template to make the fabrication process simple, cost-effective, reproducible, and potentially scalable by combining it with silver nanowire (AgNW) coating and elastomer filling processes. Stretchable TCEs are made with the AgNW-coated MWM and show remarkable optoelectronic performance with a sheet resistance of ∼3.2 Ω/sq and optical transmittance of >80%, large maximum stretchability of 40%, and electrical and mechanical robustness even under repeated stretching and bending deformations (1000 cycles). The device is demonstrated in a highly flexible touch screen panel that can operate well even in a bent state.

Controllable Formation of (004)-Orientated Nb:TiO2 for High-Performance Transparent Conductive Oxide Thin Films with Tunable Near-Infrared Transmittance

A niobium-doped titanium dioxide (Nb:TiO₂, NTO) film is a promising candidate material for indium-free transparent conductive oxide (TCO) films. It is challenging and interesting to control (004)-oriented growth to decrease resistivity. In this work, NTO films with different fractions of preferential (004) orientation (η₍₀₀₄₎) were controllably prepared by direct current sputtering. Notably, the direction of local-ordering of ions-packing could be adjusted by slightly changing the angle between the sputtering source and the glass substrate, which is identified as a key factor to determine the growth direction of a columnar crystal as well as the η₍₀₀₄₎ of films. Hall effect measurements indicate that NTO films with the highest η₍₀₀₄₎ present the lowest resistivity (6.4 × 10^-4 Ω cm), which originates from super-high carrier concentration (2.9 × 10²¹ cm^-3) and mobility (3.4 cm² V^-1 s^-1). The corresponding low sheet resistance (10.3 Ω sq^-1) makes it a potential material for commercial TCO films. We also observe that films with higher η₍₀₀₄₎ show lower transmittance in the near-infrared region.

Flexibility and non-destructive conductivity measurements of Ag nanowire based transparent conductive films via terahertz time domain spectroscopy

Highly stable and flexible transparent electrodes are fabricated based on silver nanowires (AgNWs) on both polyethylene-terephthalate (PET) and polyimide (PI) substrates. Terahertz time domain spectroscopy (THz-TDS) was utilized to probe AgNW films while bended with a radius 5 mm to discover conductivity of bended films which was further analyzed through Drude-Smith model. AgNW films experience little degradation in conductivity (<3%) before, after, and during 1000 bending cycles. Highly stable AgNW flexible electrodes have broad applications in flexible optoelectronic and electronic devices. THz-TDS is an effective technique to investigate the electrical properties of the bended and flattened conducting films in a nondestructive manner.

An antireflection transparent conductor with ultralow optical loss (<2 %) and electrical resistance (<6 Ω sq-1)

Transparent conductors are essential in many optoelectronic devices, such as displays, smart windows, light-emitting diodes and solar cells. Here we demonstrate a transparent conductor with optical loss of ∼1.6%, that is, even lower than that of single-layer graphene (2.3%), and transmission higher than 98% over the visible wavelength range. This was possible by an optimized antireflection design consisting in applying Al-doped ZnO and TiO₂ layers with precise thicknesses to a highly conductive Ag ultrathin film. The proposed multilayer structure also possesses a low electrical resistance (5.75 Ω sq⁻¹), a figure of merit four times larger than that of indium tin oxide, the most widely used transparent conductor today, and, contrary to it, is mechanically flexible and room temperature deposited. To assess the application potentials, transparent shielding of radiofrequency and microwave interference signals with ∼30 dB attenuation up to 18 GHz was achieved.

Transparent conductors are fundamental for optoelectronics. Using the transfer matrix method to optimise a multistructure of anti-reflection coatings containing an ultrathin metal film, Maniyara et al. achieve the highest transmittance of an antireflection transparent conductor combined with low resistance.

Organic Optoelectronic Materials: Mechanisms and Applications

Organic (opto)electronic materials have received considerable attention due to their applications in thin-film-transistors, light-emitting diodes, solar cells, sensors, photorefractive devices, and many others. The technological promises include low cost of these materials and the possibility of their room-temperature deposition from solution on large-area and/or flexible substrates. The article reviews the current understanding of the physical mechanisms that determine the (opto)electronic properties of high-performance organic materials. The focus of the review is on photoinduced processes and on electronic properties important for optoelectronic applications relying on charge carrier photogeneration. Additionally, it highlights the capabilities of various experimental techniques for characterization of these materials, summarizes top-of-the-line device performance, and outlines recent trends in the further development of the field. The properties of materials based both on small molecules and on conjugated polymers are considered, and their applications in organic solar cells, photodetectors, and photorefractive devices are discussed.

Roll-to-roll slot-die coating of 400 mm wide, flexible, transparent Ag nanowire films for flexible touch screen panels

We report fabrication of large area Ag nanowire (NW) film coated using a continuous roll-to-roll (RTR) slot die coater as a viable alternative to conventional ITO electrodes for cost-effective and large-area flexible touch screen panels (TSPs). By controlling the flow rate of shear-thinning Ag NW ink in the slot die, we fabricated Ag NW percolating network films with different sheet resistances (30–70 Ohm/square), optical transmittance values (89–90%), and haze (0.5–1%) percentages. Outer/inner bending, twisting, and rolling tests as well as dynamic fatigue tests demonstrated that the mechanical flexibility of the slot-die coated Ag NW films was superior to that of conventional ITO films. Using diamond-shape patterned Ag NW layer electrodes (50 Ohm/square, 90% optical transmittance), we fabricated 12-inch flexible film-film type and rigid glass-film-film type TSPs. Successful operation of flexible TSPs with Ag NW electrodes indicates that slot-die-coated large-area Ag NW films are promising low cost, high performance, and flexible transparent electrodes for cost-effective large-area flexible TSPs and can be substituted for ITO films, which have high sheet resistance and are brittle.