Chul Jong Han

An effective energy harvesting method from a natural water motion active transducer

We demonstrated a new water motion active transducer (WMAT) without any external bias-voltage sources or additional processes, which critically limit the use of conventional passive capacitive transducers that convert mechanical motion into electric energy. From a simple structure, we successfully turned on an LED using various kinds of natural water motion. The WMAT, which has wide applicability, has good potential to be a candidate for generating sustainable electric energy.

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.

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.

A pressure-induced bending sensitive capacitor based on an elastomer-free, extremely thin transparent conductor

An elastomer-free, extremely thin (less than 2 μm), and highly transparent (higher than 95%) pressure–sensitive capacitor is successfully fabricated using a combination of polyvinyl butyral (PVB) and a percolated network of silver nanowires (AgNWs). The strong affinity of PVB to the polyvinyl pyrrolidone (PVP) layer, which capped the AgNWs, stabilizes the sensor mechanically to an extent that it can resist 100000 cycles of bending under a curvature radius of 200 μm. The strong hydrogen bonding interactions and entanglement of PVB chains make the cross-linked PVB freestanding and very strong, facilitating the reduction of the thickness to less than 2 μm. The capacitance is formed on an AgNW tandem compound electrode pattern by the fringing effect, which increases with an increase in the pressure-induced bending applied to the surface of the sensor. The sensitivity is three times higher than that of an elastomeric pressure sensor with the same sensor design. The use of the AgNW/PVB sensor as a wearable acupressure sensor and a transparent motion detector is successfully demonstrated.

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.

Inverted InP quantum dot light-emitting diodes using low-temperature solution-processed metal–oxide as an electron transport layer

The present work shows the inverted InP quantum dot light-emitting diodes (QD-LEDs) with inorganic metal oxide layers. In the inverted structure of ITO/ZnO/InP QDs/CBP/MoO3/Al, a sol?gel derived ZnO film was used as an electron transport layer (ETL) and MoO3 was used as a hole injection layer (HIL). In contrary to high annealing temperature (>200 ?C) for conventional ZnO films, low temperature annealing (?150 ?C) was performed for sol?gel derived ZnO film. The performance of the inverted QD-LEDs was efficiently improved by optimization of the annealing time and temperature of ZnO ETL. The current efficiency was significantly improved about 215% by lowering annealing temperature of ZnO ETL.

Study of ethanolamine surface treatment on the metal-oxide electron transport layer in inverted InP quantum dot light-emitting diodes

The present work shows the effect of ethanolamine surface treatment on inverted InP quantum dot light-emitting diodes (QD-LEDs) with inorganic metal oxide layers. In the inverted structure of ITO/ZnO/InP QDs/CBP/MoO3/Al, a sol-gel derived ZnO film was used as an electron transport layer (ETL) and MoO3 was used as a hole injection layer (HIL). First, ethanolamine was treated as a surface modifier on top of the ZnO electron transport layer. The optical performance of the QD-LED device was improved by the ethanolamine surface treatment. Second, low temperature annealing (<200°C) was performed on the ZnO sol-gel electron transport layer, followed by an investigation of the effect of the ZnO annealing temperature. The efficiency of the inverted QD-LEDs was significantly enhanced (more than 3-fold) by optimization of the ZnO annealing temperature.

P-107: Colloidal Quantum Dot LED Transparent Display on Flexible Substrate

We report Quantum Dot light-emitting diode(QD-LED) designed on transparent and flexible PEN substrate, which shows saturated red electroluminescence spectra with full-width half-maximum(FWHM) as narrow as 40nm and transmittance of 60% at 550nm. In order to make flexible and transparent device, we fabricated a set of devices through combination of various cathodes materials and substrates.