Transparent Omni-Directional Stretchable Circuit Lines Made by a Junction-Free Grid of Expandable Au Lines.

Although various stretchable optoelectronic devices have been reported, omni-directionally stretchable transparent circuit lines have been a great challenge. Cracks are engineered and fabricated to be highly conductive patterned metal circuit lines in which gold (Au) grids are embedded. Au is deposited selectively in the cracks to form a grid without any junction between the grid lines. Since each grid line is expandable under stretching, the circuit lines are stretchable in all the directions. This study shows that a thin coating of aluminum on the oxide surface enables precise control of the cracks (crack density, crack depth) in the oxide layer. High optical transparency and high stretchability can be achieved simultaneously by controlling the grid density in the circuit line. Light-emitting diodes are integrated directly on the circuit lines and stable operation is demonstrated under 100% stretching.

Hydrogen-doped viscoplastic liquid metal microparticles for stretchable printed metal lines.

Conductive and stretchable electrodes that can be printed directly on a stretchable substrate have drawn extensive attention for wearable electronics and electronic skins. Printable inks that contain liquid metal are strong candidates for these applications, but the insulating oxide skin that forms around the liquid metal particles limits their conductivity. This study reveals that hydrogen doping introduced by ultrasonication in the presence of aliphatic polymers makes the oxide skin highly conductive and deformable. X-ray photoelectron spectroscopy and atom probe tomography confirmed the hydrogen doping, and first-principles calculations were used to rationalize the obtained conductivity. The printed circuit lines show a metallic conductivity (25,000 S cm-1), excellent electromechanical decoupling at a 500% uniaxial stretching, mechanical resistance to scratches and long-term stability in wide ranges of temperature and humidity. The self-passivation of the printed lines allows the direct printing of three-dimensional circuit lines and double-layer planar coils that are used as stretchable inductive strain sensors.

Highly Deformable Transparent Au Film Electrodes and Their Uses in Deformable Displays.

With emerging interest in foldable and stretchable displays, the need to develop transparent deformable electrode and interconnection is increasing. Even though metal films have been standard electrodes in conventional electronic devices due to their high conductivity and well-established process, they have never been used for transparent deformable electrodes. We present highly conductive transparent deformable Au film electrodes and use them to fabricate a foldable perovskite light-emitting diode (PeLED) and a biaxially stretchable alternating current electroluminescence (ACEL) display. We exhibit the formation of an ultrathin (6 nm) continuous Au film on an anisotropic conductive ultrathin film (ACUF) of amorphous carbon. The ultrathin Au film was first formed on an ACUF-coated Si wafer (4 in. scale) through metal evaporation and transferred to the polymer substrates by a simple and effective water-assisted delamination process. Then, a hybrid electrode (ACUF/ACUF/Au) was produced as the transparent deformable electrode. Complicated interconnections could be created by metal deposition through a mask. The electrical conductance of the hybrid electrode was not affected by the crack formation in the Au film during electrode folding, crumpling, and stretching. We reveal the reason why the hybrid electrode can maintain such excellent electrical stability under deformation.

Electro-Photoluminescence Color Change for Deformable Visual Encryption.

Although structural coloring and photoluminescence (PL) have been investigated for radiation-responsive color change, electroluminescence (EL) has not been used for the radiation-responsive system. An electro-photoluminescence (EPL) color change is presented here. The phosphors in the alternating current electroluminescence (ACEL) act simultaneously as electro-luminophores and photo-luminophores. The EPL chromaticity is systematically investigated depending on the ACEL frequency and UV intensity. It is found that the PL variation depending on UV intensity is the mechanism of the EPL color change. It is revealed that EL and PL can be controlled independently in the low electric field so that the EPL chromaticity can be adjusted by a linear combination of the EL color and the PL color. The EPL color-changing device is used as a deformable visual encryption system and a soft skin for a soft robotic rover, imitating the concealment and signaling functions in nature.

Liquid Metal Covered with Thermoplastic Conductive Composites for High Electrical Stability and Negligible Electromechanical Coupling at Large Strains.

Stretchable electrode is an essential part of soft electronic devices. Practical stretchable electrodes must meet the following requirements: metallic conductivity and no resistance change in various situations such as repeated large deformation, toxic environment, and large temperature change. This study suggests a simple electrode design that meets all of these requirements simultaneously. The electrode consists of a liquid metal (LM) mesh pattern that is sandwiched between a thermoplastic block copolymer (BCP) film and a BCP/Ag flake composite film with a microfibril network structure on its surface. The electrode has a high conductivity (1.2 × 104 S/cm) and is stretchable up to 600% uniaxial strain (ε). Its resistance remains unchanged during repeated stretching cycles at ε = 300% (ΔR < 0.04 Ω) as well as under simultaneous situation of large deformation (ε = 400%) and large temperature change (20-70 °C). The electrode is anticorrosive in an acidic solution owing to the hydrophobic BCP layer that protects the LM from being etched. This study shows the connection of two separate electrodes and complete healing of scratched electrodes by finger pressing. In addition, it demonstrates the fabrication of superstretchable electroluminescence display as an example of potential uses of the electrode.

Boosting up the electrical performance of low-grade PEDOT:PSS by optimizing non-ionic surfactants.

Although PEDOT:PSS has already been applied to various electronic devices, commercialized PEDOT:PSS products having high conductivity are expensive, which is a considerable burden on device manufacturing. In this study, we optimize non-ionic surfactants mixed in a PEDOT:PSS solution to upgrade a low-grade product of low conductivity to the level of a high-grade product of high conductivity. This study systematically investigates the phase diagram, morphology, conductivity, and mechanical stability of the PEDOT:PSS films according to the hydrophilicity of non-ionic surfactants. This study reveals that the conductivity of the PEDOT:PSS film varies greatly depending on the chemical structure of the surfactant and its weight fraction in the thin film. Under the optimum conditions (chemical structure and weight fraction) of the surfactant, the conductivity of the low value product could be improved to the conductivity level of the high value product. The electrical properties of the films were excellently stable even under the extreme cyclic bending tests at a bending radius of 1.5 mm. The low-grade and high grade products showed the same electrical performance when they were used in the Ag nanowires/PEDOT:PSS hybrid transparent electrodes. The results are expected to be applied immediately not only in the laboratory but also in various industrial fields.