High diopter spectacle using a flexible Fresnel lens with a combination of grooves.

In this study, the Fresnel lens was investigated as a potential candidate for vision correction in patients with myopia. A few previous studies have suggested this idea; however, Fresnel lenses are limited by their aesthetics and quality. Therefore, we designed a combination of Fresnel lens grooves with a constant height and pitch of 13 µm and 0.1 mm, respectively, to overcome the limitations caused by ultra-precision machining with a tool nose radius of 30 µm. A thin replicated Fresnel lens with a power of -5 diopter was procured and applied directly as spectacles that are unattached to the normal lens. The optical performance and image quality of the Fresnel lens were compared with those of a conventional lens possessing the same power in both near and far vision. These results extend the applicability for the use of Fresnel lenses as vision-correcting ophthalmological lenses and imaging systems.

Plasmonic Color Printing via Bottom-Up Laser-Induced Photomodification Process.

Plasmonic color printing has received significant attention owing to its advantages such as nonfading and nontoxic color expression, without necessitating the use of chemical dyes. Recently, color generation from laser-induced plasmonic nanostructures has been extensively explored because of its simplicity, cost-effectiveness, and large-scale processability. However, these methods usually utilize a top-down method that causes unexpected background colors. Here, we proposed a novel method of plasmonic color printing via a bottom-up type laser-induced photomodification process. In the proposed method, selective silver nanoparticles (Ag NPs) structure could be fabricated on a transparent substrate through a unique organometallic solution-based laser patterning process. A set of color palettes was formed on the basis of different processing parameters such as laser fluence, scanning speed, and baking time. This color change was verified by finite-difference time-domain (FDTD) simulations via monitoring the spectral peak shift of the localized surface plasmon resonance (LSPR) at Ag NPs. It was also confirmed that the colors can be fabricated at a relatively high scanning speed (≥10 mm/s) on a large substrate (>300 mm2). Since semitransparent color images can be patterned on various transparent substrates, this process will broaden the application range of laser-induced plasmonic color generation.

Solution-processable electrode-material embedding in dynamically inscribed nanopatterns (SPEEDIN) for continuous fabrication of durable flexible devices.

A facile and scalable lithography-free fabrication technique, named solution-processable electrode-material embedding in dynamically inscribed nanopatterns (SPEEDIN), is developed to produce highly durable electronics. SPEEDIN uniquely utilizes a single continuous flow-line manufacturing process comprised of dynamic nanoinscribing and metal nanoparticle solution coating with selective embedding. Nano- and/or micro-trenches are inscribed into arbitrary polymers, and then an Ag nanoparticle solution is dispersed, soft-baked, doctor-bladed, and hard-baked to embed Ag micro- and nanowire structures into the trenches. Compared to lithographically embossed metal structures, the embedded SPEEDIN architectures can achieve higher durability with comparable optical and electrical properties and are robust and power-efficient even under extreme stresses such as scratching and bending. As one tangible application of SPEEDIN, we demonstrate a flexible metal electrode that can operate at 5 V at temperatures up to 300 °C even under the influence of harsh external stimuli. SPEEDIN can be applied to the scalable fabrication of diverse flexible devices that are reliable for heavy-duty operation in harsh environments involving high temperatures, mechanical deformations, and chemical hazards.

Imaging performance of an ultra-precision machining-based Fresnel lens in ophthalmic devices.

This study aims to resolve the trade-off between the constraints and capabilities of ultra-precision machining to achieve ophthalmic Fresnel lenses. A general Fresnel lens pattern has a narrow variable pitch and curved grooves. However, we obviate the limitations of the tool nose radius constraint and the long tool path via ultra-precision machining of the modified Fresnel lens, ensuring a constant pitch of 0.1 mm and varying the height of straight grooves from 0 to 11 µm. Photorealistic raytracing visualization and MTF simulation verified the compatibility of the lens pattern with human perception sensitivity. Copper-coated mold was fabricated using a diamond tool with a tool nose radius of 5 µm. The replicated flexible Fresnel lens demonstrated a relative MTF imaging performance of 89.1% and was attached to the goggles for the qualitative assessment. The proposed Fresnel lens design and fabrication approach can be extended to applications in the visual and infrared ranges as well.

Flashlight-Induced Strong Self-Adhesive Surface on a Nanowire-Impregnated Transparent Conductive Film.

The flashlight annealing process has been widely used in the field of flexible and printed electronics because it can instantly induce chemical and structural modifications over a large area on an electronic functional layer in a subsecond time range. In this study, for the first time, we explored a straightforward method to develop strong self-adhesion on a metal nanowire-based flexible and transparent conductive film via flashlight irradiation. Nanowire interlocking, for strong mechanical bonding at the interface between the nanowires and polyamide film, was achieved by simple hot pressing. Then, by irradiating the nanowire-impregnated film with a flashlight, several events such as interdiffusion and melting of surface polymers could be induced along with morphological changes leading to an increase in the film surface area. As a result, the surface of the fabricated film exhibited strong interfacial interactions while forming intimate contact with the heterogeneous surfaces of other objects, thereby becoming strongly self-adhesive. This readily achievable, self-attachable, flexible, and transparent electrode allowed the self-interconnection of a light-emitting diode chip, and it was also compatible for various applications, such as defogging windows and transparent organic light-emitting diodes.

Control of thermal deformation with photonic sintering of ultrathin nanowire transparent electrodes.

Development of electronic devices on ultrathin flexible plastic substrates is of great value in terms of portability, cost reduction, and mechanical flexibility. However, because thin plastic substrates with low heat capacity can be more easily damaged by thermal energy, their use is limited. Highly flexible nanowire (NW) transparent conductive electrodes on ultrathin (∼10 µm) low cost polyethylene terephthalate (PET) substrates are fabricated. The control of intense pulsed light (IPL) irradiation process parameters to induce NW welding for maximum conductivity and minimal thermal damage of the PET substrate is explored. For this purpose, trends in temperature variation of NW thin films irradiated by IPL under various operating conditions are numerically analyzed using commercial software. Simulations indicate that irradiating light operated at a higher voltage and for a shorter time, and use of multiple pulses of low frequency can reduce thermal deformation of the PET substrate. Furthermore, we experimentally confirm that NW transparent electrodes can be successfully fabricated with less thermal deformation of the ultrathin plastic substrate when light is irradiated under well-controlled conditions derived from the simulation. The highly flexible NW transparent conducting electrode exhibits excellent mechanical flexibility to withstand severe deformation and can be successfully implemented in flexible organic light-emitting diodes (OLEDs).

Facile Fabrication of Highly Conductive, Ultrasmooth, and Flexible Silver Nanowire Electrode for Organic Optoelectronic Devices.

So far, one of the fundamental limitations of silver nanowires (Ag NWs) is the high contact resistance among their junctions. Moreover, a rough surface due to its random arrangement is inevitable to electrical short when the nanowire-based electronics is driving. To improve the contact resistance, we suggest that the particle shape nanocrystals are intentionally reduced at the junctions by a localized joule-heat reduction approach from the silver ions. Via localized reductions, the reduced nanoparticles effectively weld the junction's areas, resulting in a 19% decrease in sheet resistance to 9.9 Ω sq-1. Besides, the nanowires are embedded into a polyamide film with gentle hot pressing. Consequently, the roughness was considerably dropped so that it was successful to demonstrate organic light-emitting diodes (OLEDs) with nanowires, which was beneficial to be laminated with OLEDs under the low temperature. The experimental results show that the Ag NW-embedded films reach 10.9 Ω sq-1 of the sheet resistance at 92% transmittance and the roughness was only 1.92 nm.

Facile and Scalable Fabrication of Flexible Reattachable Ionomer Nanopatterns by Continuous Multidimensional Nanoinscribing and Low-temperature Roll Imprinting.

We develop a facile route to the scalable fabrication of flexible reattachable ionomer nanopatterns (RAINs) by continuous nanoinscribing and low-temperature roll imprinting, which are repeatedly attachable to and detachable from arbitrarily shaped surfaces. First, by sequentially performing continuous nanoinscribing over a polymer substrate along the multiple directions, we readily create the multidimensional nanopattern, which otherwise demands complex nanofabrication. After its transfer to an elastomer pad for use as a soft nanoimprinting stamp, we then conduct a low-temperature roll imprinting of the ionomer film to fabricate a flexible and highly transparent RAIN. Reversible loosening of ionic units in the ionomer material at the mild temperature as low as ∼60-70 °C enables the faithful nanopatterning over thermosensitive organic compounds and fragile materials under a slight pressure. The excellent adhesion purely emerging from ionic interactions uniquely realizes the conformal attachability and clean detachability of RAINs for universal targets in ambient conditions, particularly beneficial for individual wearable and mobile devices requiring the user-specific "on/off" of the nanopattern-driven functionalities. As one vivid example, we demonstrate that a single light-emitting device can be switched from the focused pointer to the widespread flashlight depending on the RAIN application upon user's purpose.

Defect-Free, Highly Uniform Washable Transparent Electrodes Induced by Selective Light Irradiation.

A simple route to fabricate defect-free Ag-nanoparticle-carbon-nanotube composite-based high-resolution mesh flexible transparent conducting electrodes (FTCEs) is explored. In the selective photonic sintering-based patterning process, a highly soft rubber or thin plastic substrate is utilized to achieve close and uniform contact between the composite layer and photomask, with which uniform light irradiation can be obtained with diminished light diffraction. This well-controlled process results in developing a fine and uniform mesh pattern (≈12 µm). The mesh patternability is confirmed to be dependent on heat distribution in the selectively light-irradiated film and the pattern design for FTCE could be adopted for more precise patterns with desired performance. Moreover, using a very thin substrate could allow the mesh to be positioned closer to the strain-free neutral mechanical plane. Due to strong interfacial adhesion between the mesh pattern and substrate, the mesh FTCE could tolerate severe mechanical deformation without performance degradation. It is demonstrated that a transparent heater with fine mesh patterns on thin substrate can maintain stability after 100 repeated washing test cycles in which a variety of stress situations occurring in combination. The presented highly durable FTCE and simple fabrication processes may be widely adoptable for various flexible, large-area, and wearable optoelectronic devices.

A photonic sintering derived Ag flake/nanoparticle-based highly sensitive stretchable strain sensor for human motion monitoring.

Recently, the demand for stretchable strain sensors used for detecting human motion is rapidly increasing. This paper proposes high-performance strain sensors based on Ag flake/Ag nanocrystal (NC) hybrid materials incorporated into a polydimethylsiloxane (PDMS) elastomer. The addition of Ag NCs into an Ag flake network enhances the electrical conductivity and sensitivity of the strain sensors. The intense localized heating of Ag flakes/NCs is induced by intense pulsed light (IPL) irradiation, to achieve efficient sintering of the Ag NCs within a second, without damaging the PDMS matrix. This leads to significant improvement in the sensor sensitivity. Our strain sensors are highly stretchable (maximum strain = 80%) and sensitive (gauge factor = 7.1) with high mechanical stability over 10 000 stretching cycles under 50% strain. For practical demonstration, the fabrication of a smart glove for detecting the motions of fingers and a sports band for measuring the applied arm strength is also presented. This study provides an effective method for fabricating elastomer-based high-performance stretchable electronics.

Multi-layer transfer and lamination (MTL) process assisted by a high-pressure air jet for highly efficient solution-processed polymer light emitting diodes.

This research reports fabrication of highly efficient polymer light-emitting diodes (PLEDs) by a thermally activated multi-layer transfer and lamination (MTL) process. As the first stage of the fabrication process, multiple layers consisting of a light-emitting polymer, electron transport/electron injection layer and metal cathode were sequentially deposited onto a hydrophobic low-surface energy self-assembled monolayer (SAM)-coated glass substrate. Subsequently, a very rapid delamination of PLEDs multi-layers from the SAM-coated glass was achieved by applying a high-pressure air jet. The custom air jet system ensures the excellent multi-film quality and also prevents the occurrence of common buckling and cracks. In addition, the adhesiveness of polyamide (PA) onto polyethylene terephthalate (PET), which serves as a protective barrier from atmospheric water and oxygen, is thermally activated by heating PA above its glass transition temperature. Finally, once delaminated from the PA/PET, the multi-layer structure (light-emitting polymer/electron transport layer/electron injection layer/metal cathode) was successfully laminated onto the target glass (hole transport layer/transparent anode) with a soft roller under mild pressure for the realization of the PLED device. The maximum brightness of the device fabricated by a thermally activated lamination process was around 13 120 cd m-2 at 8.4 V, whereas the maximum current efficiency and the power efficiency were 5.2 cd A-1 and 4.0 lm W-1, respectively.

Rapid and conformal coating of polymer resins by airbrushing for continuous and high-speed roll-to-roll nanopatterning: parametric quality controls and extended applications.

We present a facile and scalable coating method based on controlled airbrushing, which is suitable for conformal resin coating in continuous roll-to-roll (R2R) nanoimprint lithography (NIL) process. By controlling the concentration of UV-curable polymeric resin with mixing the volatile solvent and its airbrushing time, the coated resin film thickness can be readily tuned. After R2R NIL using a flexible nanoscale line pattern (nanograting) mold is conducted upon the airbrushed resin film, a large-area uniform nanograting pattern is fabricated with controlled residual layer thickness (RLT) based on the initial film thickness. We investigate the faithful airbrushing condition that can reliably create the uniform thin films as well as various nanopatterns with controlled morphologies. Using more diluted resin and shorter airbrushing time can reduce the RLTs favourably for many applications, yet is apt to induce the nanoscale pores and discontinued lines. We also discuss how to further improve the quality and scalability of resin airbrushing and its potential applications particularly requiring high-speed and conformal coating on highly topographic and flexible surfaces.

Organic photovoltaic cells: from performance improvement to manufacturing processes.

Organic photovoltaics (OPVs) have been pursued as a next generation power source due to their light weight, thin, flexible, and simple fabrication advantages. Improvements in OPV efficiency have attracted great attention in the past decade. Because the functional layers in OPVs can be dissolved in common solvents, they can be manufactured by eco-friendly and scalable printing or coating technologies. In this review article, the focus is on recent efforts to control nanomorphologies of photoactive layer and discussion of various solution-processed charge transport and extraction materials, to maximize the performance of OPV cells. Next, recent works on printing and coating technologies for OPVs to realize solution processing are reviewed. The review concludes with a discussion of recent advances in the development of non-traditional lamination and transfer method towards highly efficient and fully solution-processed OPV.

Optical Properties of Hybrid Inorganic/Organic Thin Film Encapsulation Layers for Flexible Top-Emission Organic Light-Emitting Diodes.

Inorganic/organic hybrid thin film encapsulation layers consist of a thin Al2O3 layer together with polymer material. We have investigated optical properties of thin film encapsulation layers for top-emission flexible organic light-emitting diodes. The transmittance of hybrid thin film encapsulation layers and the electroluminescent spectrum of organic light-emitting diodes that were passivated by hybrid organic/inorganic thin film encapsulation layers were also examined as a function of the thickness of inorganic Al203 and monomer layers. The number of interference peaks, their intensity, and their positions in the visible range can be controlled by varying the thickness of inorganic Al2O3 layer. On the other hand, changing the thickness of monomer layer had a negligible effect on the optical properties. We also verified that there is a trade-off between transparency in the visible range and the permeation of water vapor in hybrid thin film encapsulation layers. As the number of dyads decreased, optical transparency improved while the water vapor permeation barrier was degraded. Our study suggests that, in top-emission organic light-emitting diodes, the thickness of each thin film encapsulation layer, in particular that of the inorganic layer, and the number of dyads should be controlled for highly efficient top-emission flexible organic light-emitting diodes.

Roll-to-roll cohesive, coated, flexible, high-efficiency polymer light-emitting diodes utilizing ITO-free polymer anodes.

This paper reports solution-processed, high-efficiency polymer light-emitting diodes fabricated by a new type of roll-to-roll coating method under ambient air conditions. A noble roll-to-roll cohesive coating system utilizes only natural gravity and the surface tension of the solution to flow out from the capillary to the surface of the substrate. Because this mechanism uses a minimally cohesive solution, the roll-to-roll cohesive coating can effectively realize an ultra-thin film thickness for the electron injection layer. In addition, the roll-to-roll cohesive coating enables the fabrication of a thicker polymer anode film more than 250 nm at one time by modification of the surface energy and without wasting the solution. It is observed that the standard sheet resistance deviation of the polymer anode is only 2.32 Ω/□ over 50 000 bending cycles. The standard sheet resistance deviation of the polymer anode in the different bending angles (0 to 180°) is 0.313 Ω/□, but the case of the ITO-PET is 104.93 Ω/□. The average surface roughness of the polymer anode measured by atomic force microscopy is only 1.06 nm. Because the surface of the polymer anode has a better quality, the leakage current of the polymer light-emitting diodes (PLEDs) using the polymer anode is much lower than that using the ITO-PET substrate. The luminous power efficiency of the two devices is 4.13 lm/W for the polymer anode and 3.21 lm/W for the ITO-PET. Consequently, the PLEDs made by using the polymer anode exhibited 28% enhanced performance because the polymer anode represents not only a higher transparency than the ITO-PET in the wavelength of 560 nm but also greatly reduced roughness. The optimized the maximum current efficiency and power efficiency of the device show around 6.1 cd/A and 5.1 lm/W, respectively, which is comparable to the case of using the ITO-glass.

Fabrication and characterization of WO3/Ag/WO3 multilayer transparent anode with solution-processed WO3 for polymer light-emitting diodes.

The dielectric/metal/dielectric multilayer is suitable for a transparent electrode because of its high-optical and high-electrical properties; however, it is fabricated by an expensive and inefficient multistep vacuum process. We present a WO3/Ag/WO3 (WAW) multilayer transparent anode with solution-processed WO3 for polymer light-emitting diodes (PLEDs). This WAW multilayer not only has high transmittance and low resistance but also can be easily and rapidly fabricated. We devised a novel method to deposit a thin WO3 layer by a solution process in an air environment. A tungstic acid solution was prepared from an aqueous solution of Na2WO4 and then converted to WO3 nanoparticles (NPs) by a thermal treatment. Thin WO3 NP layers form WAW multilayer with a thermal-evaporated Ag layer, and they improve the transmittance of the WAW multilayer because of its high transmittance and refractive index. Moreover, the surface of the WO3 layer is homogeneous and flat with low roughness because of the WO3 NP generation from the tungstic acid solution without aggregation. We performed optical simulation and experiments, and the optimized WAW multilayer had a high transmittance of 85% with a sheet resistance of 4 Ω/sq. Finally, PLEDs based on the WAW multilayer anode achieved a maximum luminance of 35,550 cd/m2 at 8 V, and this result implies that the solution-processed WAW multilayer is appropriate for use as a transparent anode in PLEDs.