Ultrafast Photoinduced Interconnection of Metal-Polymer Composites for Fabrication of Transparent and Stretchable Electronic Skins.

The high interest sparked by the foldable smartphones recently released on the market is gradually shifting to the next generation of flexible electronic devices, such as electronic skins in the form of stretchable thin films. To develop such devices, good mechanical flexibility of all components (including the substrate, electrode, and encapsulant) is critical. Various technologies have been developed to enhance the flexibility of these components; however, progress in developing interconnection methods for flexible and stretchable devices has been limited. Here, we developed an ultrafast photoinduced interconnection method that does not require any adhesive or surface treatment. This method is based on heating metal nanostructures using intense pulsed light (IPL) and the reversible cross-linking of polymers. First, we synthesized a stretchable, transparent, and free-standing polymer substrate that can be reversibly cross-linked, and then Ag nanowire (AgNW) networks were formed on its surface. This electrode was irradiated with IPL, which locally heated the AgNWs, followed by decomposition of the polymer via the retro-Diels-Alder reaction and recross-linking. Independently fabricated AgNW/polymer films were layered and irradiated three times with IPL to form a bonded sample with excellent joint quality and no increase in electrical resistance compared to a single electrode. Furthermore, the interconnected electrodes were stretchable and optically transparent. Even when more than 200% strain was applied in a peel test, no breakage at the joint was observed. This allowed us to successfully produce a stretchable, transparent, and bending-insensitive pressure sensor for various applications such as motion detectors or pressure sensor arrays.

Pressure-Sensitive Adhesive with Controllable Adhesion for Fabrication of Ultrathin Soft Devices.

As the interest in foldable smartphones recently launched onto the market shifts toward the next generation of flexible electronics, the development of ultrathin devices is gaining considerable attention. The strain formed on the surfaces of film-based devices approximates the film thickness divided by twice the radius of curvature; therefore, the use of an ultrathin substrate is the key for the development of next generation foldable devices. However, the stiffness of ultrathin films is extremely low; thus, it cannot be easily used directly as a substrate for device fabrication. Therefore, these films generally undergo device manufacturing processes while being attached to a rigid substrate such as glass and are peeled from the rigid substrate after the process is finished. Thus, the initial adhesion of the adhesive used to fix the film to the temporary substrate should be strong, and after the process is completed, the adhesion must be lessened to enable soft peeling. In this study, we succeeded in developing a novel pressure-sensitive adhesive (PSA) whose adhesive strength can be severely reduced by water treatment. Accordingly, considering that amphiphilic oligomers promote water absorption through hydrogen bonding to water, amphiphilic oligomers were mixed with an acrylic polymer to prepare the water-responsive PSA (wr-PSA). The adhesion strength of the wr-PSA in the early stage, which reached 382(±22) N/m, dramatically dropped to 9(±2) N/m after a water immersion test. Using the wr-PSA, a 1.4 µm-thick polyethylene terephthalate film coated with Ag nanowires was softly peeled off from the glass after being immersed in warm water. In addition, the adhesion reduced by the immersion in water was recovered again when the water absorbed by the adhesive was dried. This implies that the developed adhesive can be reusable.

Effects of Intense Pulsed Light (IPL) Rapid Annealing and Back-Channel Passivation on Solution-Processed In-Ga-Zn-O Thin Film Transistors Array.

We report on the effects of the intense pulsed light (IPL) rapid annealing process and back-channel passivation on the solution-processed In-Ga-Zn-O (IGZO) thin film transistors (TFTs) array. To improve the electrical properties, stability and uniformity of IGZO TFTs, the oxide channel layers were treated by IPL at atmospheric ambient and passivated by photo-sensitive polyimide (PSPI). When we treated the IGZO channel layer by the IPL rapid annealing process, saturation field effect mobility and subthreshold swing (S.S.) were improved. And, to protect the back-channel of oxide channel layers from oxygen and water molecules, we passivated TFT devices with photo-sensitive polyimide. The IGZO TFTs on glass substrate treated by IPL rapid annealing without PSPI passivation showed the field effect mobility (µFE) of 1.54 cm2/Vs and subthreshold swing (S.S.) of 0.708 V/decade. The PSPI-passivated IGZO TFTs showed higher µFE of 2.17 cm2/Vs than that of device without passivation process and improved S.S. of 0.225 V/decade. By using a simple and fast intense pulsed light treatment with an appropriate back-channel passivation layer, we could improve the electrical characteristics and hysteresis of IGZO-TFTs. We also showed the improved uniformity of electrical characteristics for IGZO TFT devices in the area of 10 × 40 mm2. Since this IPL rapid annealing process could be performed at a low temperature, it can be applied to flexible electronics on plastic substrates in the near future.

Self-Integratable, Healable, and Stretchable Electroluminescent Device Fabricated via Dynamic Urea Bonds Equipped in Polyurethane.

Reversible bonding between polymer chains has been used primarily to induce self-healing of damaged polymers. Inspired by the dynamic nature of such bonding, we have developed a polyurethane equipped with dynamic urea bonds (PEDUB) that has high strength sufficient to make it be freestanding and have a healing capability and self-bonding property. This allowed subsequent heterogeneous multicomponent device integration of electrodes/substrate and light-emitting pixels into a light-emitting device. We first used the PEDUB to individually fabricate a highly stretchable electrode containing Ag nanowires and stretchable composites with ZnS-based particles. They were successfully assembled into a stretchable, waterproof electroluminescent (EL) device even under mild conditions (60 °C for 10 min) owing to the reversible exchange of urea bonds and low glass transition temperature of PEDUB. The assembled device with an AC-driven EL architecture retained excellent EL characteristics even after stretching, submersion in water, and cutting owing to the robust solid-state bonding interfaces induced by the dynamic urea bonds. Consequently, various shapes of the illuminating elastomer and an illuminated picture were realized for the first time using the mosaic-like assembly method. This first demonstration of multicomponent assembly paves the way for future stretchable multifunctional devices.

Fabrication of a Bending-Insensitive In-Plane Strain Sensor from a Reversible Cross-Linker-Functionalized Silicone Polymer.

A reversibly cross-linkable and transparent polymer featuring stretchability and thermal healability is prepared by introducing Diels-Alder (DA)-reactive moieties into polydimethylsiloxane (PDMS), namely, a healable PDMS (h-PDMS). Inspired by the fact that retro-DA reactions occur even at low temperatures (albeit at a low rate), we maximize the effectiveness of small reactant products, demonstrating that self-healing and self-integration realized by 1-3 min exposure of cured h-PDMS to methyl ethyl ketone (MEK) vapor is more efficient than that achieved by direct sample heating at high temperatures. This technology is first used to uniformly transfer Ag nanowires (Ag NWs) formed on a temporary substrate to the h-PDMS surface, and further MEK vapor treatment allows the transferred NWs to be impregnated below the h-PDMS surface to afford an in-plane strain sensor. Most importantly, the developed method is used to perfectly integrate two identical Ag NW/h-PDMS films and thus place NWs on a neutral plane. Consequently, because of the unique structure in which a percolated network of AgNWs is formed on the interface where the two identical h-PDMS films are chemically integrated, the fabricated sensor is transparent, self-healable, stretchable, and insensitive to bending but sensitively responds to in-plane strain induced by lateral deformation.

Fabrication and Characterization of a Capacitive Photodetector Comprising a ZnS/Cu Particle/Poly(vinyl butyral) Composite.

Most photodetectors developed to date essentially measure photocurrents induced by the generation and separation of electron-hole pairs in semiconductors during irradiation. Although the above light detection method is well established, highly sensitive, and applicable to a broad range of semiconductor materials, it requires the presence of a stable and direct contact between the semiconductor and the electrode for accurate photocurrent measurements. In turn, this prerequisite necessitates the use of various costly processes for device fabrication (e.g., photolithography and vacuum deposition of semiconductors/metals) and complicates the development of flexible devices. Herein, inspired by the fact that the dielectric properties of certain materials can be changed by light irradiation, we dispersed ZnS/Cu semiconducting particles in poly(vinyl butyral) to prepare a free-standing composite film and formed two layers of Ag nanowire electrodes on both sides of the cured composite to fabricate a photodetector of a completely new type. The developed device exhibited a capacitance very sensitive to irradiation with light of a specific wavelength and additionally featured the advantages of simple structure/operation mechanism, mechanical flexibility, and transparency, not showing any signs of performance deterioration even after severe damage.

Crack-induced Ag nanowire networks for transparent, stretchable, and highly sensitive strain sensors.

Crack-based strain sensor systems have been known for its high sensitivity, but suffer from the small fracture strain of the thin metal films employed in the sensor which results in its negligible stretchability. Herein, we fabricated a transparent (>90% at 550 nm wavelength), stretchable (up to 100%), and sensitive (gauge factor (GF) of 30 at 100% strain) strain gauge by depositing an encapsulated crack-induced Ag nanowire (AgNW) network on a hydroxylated poly(dimethylsiloxane) (PDMS) film. Stretching the encapsulated AgNWs/PDMS resulted in the formation of a percolation network of nanowire ligaments with abundant percolation paths. The encapsulating polymer was designed to adhere strongly to both the AgNW and PDMS. The improved adhesion ensured the resistance of the crack-induced network of AgNWs varied reversibly, stably, and sensitively when stretched and released, at strains of up to 100%. The developed sensor successfully detected human motions when applied to the skin.

Fabric Active Transducer Stimulated by Water Motion for Self-Powered Wearable Device.

The recent trend of energy-harvesting devices is an adoption of fabric materials with flexible and stretchable according to the increase of wearable electronics. But it is a difficult process to form a core structure of dielectric layer or electrode on fabric materials. In particular, a fabric-based energy-harvesting device in contact with water has not been studied, though there are many challenging issues including insulation and water absorption in a harsh environment. So we propose an effective method to obtain an electrical energy from the water contact using our new fabric energy harvesting device. Our water motion active transducer (WMAT) is designed to obtain electrical energy from the variable capacitance through the movement and contact of water droplet. In this paper, we succeeded in generating an electrical energy with peak to peak power of 280 µW using a 30 µL of water droplet with the fabric WMAT device of 70 mm × 50 mm dimension. Furthermore, we specially carried out spray-coating and transfer processes instead of the conventional spin-coating process on fabric materials to overcome the limitation of its uneven morphology and porous and deformable assembly.

Photoenhanced Patterning of Metal Nanowire Networks for Fabrication of Ultraflexible Transparent Devices.

Network structures of metal nanowires are a promising candidate for producing a wide range of flexible electronic devices, but only if they can be suitably patterned and retained on various materials. Here we present a new approach to the patterning of metal nanowires by employing intense-pulsed-light (IPL) irradiation to reduce the process to just two steps: irradiation and the subsequent removal of nonirradiated nanowires. This ultrasimple method eliminates the need to employ chemical reagents for etching or improving the adhesion of nanowires, and is compatible with Ag nanowires (AgNWs), Cu nanowires (CuNWs), and most transparent polymers. Furthermore, it is not reliant on additional processes, such as coating, heating, developing, and etching to make a patterned nanowire structure. Using this simple method, ultraflexible and transparent devices such as touch sensor, heater and light emitting diode with an exceptionally high mechanical stability have been successfully fabricated. This new method is expected to be directly applicable to the fabrication of a wide range of high-performance, low-cost, biocompatible, and wearable devices.

Ultra-thin and smooth transparent electrode for flexible and leakage-free organic light-emitting diodes.

A smooth, ultra-flexible, and transparent electrode was developed from silver nanowires (AgNWs) embedded in a colorless polyimide (cPI) by utilizing an inverted film-processing method. The resulting AgNW-cPI composite electrode had a transparency of >80%, a low sheet resistance of 8 Ω/□, and ultra-smooth surfaces comparable to glass. Leveraging the robust mechanical properties and flexibility of cPI, the thickness of the composite film was reduced to less than 10 µm, which is conducive to extreme flexibility. This film exhibited mechanical durability, for both outward and inward bending tests, up to a bending radius of 30 µm, while maintaining its electrical performance under cyclic bending (bending radius: 500 µm) for 100,000 iterations. Phosphorescent, blue organic light-emitting diodes (OLEDs) were fabricated using these composites as bottom electrodes (anodes). Hole-injection was poor, because AgNWs were largely buried beneath the composite's surface. Thus, we used a simple plasma treatment to remove the thin cPI layer overlaying the nanowires without introducing other conductive materials. As a result, we were able to finely control the flexible OLEDs' electroluminescent properties using the enlarged conductive pathways. The fabricated flexible devices showed only slight performance reductions of <3% even after repeated foldings with a 30 µm bending radius.

Investigation of colloidal PbS quantum dot-based solar cells with near infrared emission.

Colloidal quantum dots (QD)-based solar cells with near infrared (NIR) emission have been investigated. Lead sulfide (PbS) QDs, which have narrow band-gap and maximize the absorption of NIR spectrum, were chosen as active materials for efficient solar cells. The inverted structure of indium tin oxide/titanium dioxide/PbS QDs/poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate)/silver (ITO/TiO2/PbS QDs/ PEDOT: PSS/Ag) was applied for favorable electron and hole seperation from the PbS QD. Through the ligand exchange by 1,2-Ethanedithiol (EDT), the interparticle distance of the PbS QDs in thin film became closer and the performance of the PbS QD-based solar cells was improved. Our PbS QD-based inverted solar cells showed open circuit voltages (V(oc)) of 0.33 V, short circuit current density (J(sc)) of 10.89 mA/cm2, fill factor (FF) of 30%, and power conversion efficiency (PCE) of 1.11%. In our PbS QD-based multifunctional solar cell, the NIR light emission intensity was simply detected with photodiode system, which implies the potential of multi-functional diode device for various applications.

Graded synthetic approach for the fabrication of nanocrystal quantum dots for enhanced carrier injection in light-emitting diodes.

A synthetic approach for fabricating compositionally graded multishell nanocrystal quantum dots (NQDs) from cubic CdSe is presented. The structural shapes with each shell formation were examined using scanning transmission electron microscopy (STEM) coupled with energy dispersive x-ray spectroscopy and electron energy loss spectroscopy (EELS). The optical properties probed via time-resolved spectroscopy further revealed detailed carrier behavior at the core/graded shell interface. A highly luminescing stable optical device was built using our graded multishell NQDs.

Investigation of space charge distribution of low-density polyethylene/GO-GNF (graphene oxide from graphite nanofiber) nanocomposite for HVDC application.

This paper reported a research on space charge distribution in low-density polyethylene (LDPE) nanocomposites with different types of graphene and graphene oxide (GO) at low filler content (0.05 wt%) under high DC electric field. Effect of addition of graphene oxide or graphene, its dispersion in LDPE polymer matrix on the ability to suppress space charge generation will be investigated and compared with MgO/LDPE nanocomposite at the same filler concentration. At an applied electric field of 80 kV/mm, a positive packet-like charge was observed in both neat LDPE, MgO/LDPE, and graphene/LDPE nanocomposites, whereas only little homogenous space charge was observed in GO/LDPE nanocomposites, especially with GO synthesized from graphite nano fiber (GNF) which is only -100 nm in diameter. Our research also suggests that dispersion of graphene oxide particles on the polymer matrix plays a significant role to the performance of nanocomposites on suppressing packet-like space charge. From these results, it is expected that nano-sized GO synthesized from GNF can be a promising filler material to LDPE composite for HVDC applications.