Byung Doo Chin

Foldable Graphene Electronic Circuits Based on Paper Substrates

Graphene electronic circuits are prepared on paper substrates by using graphene nanoplates and applied to foldable paper-based electronics. The graphene circuits show a small change in conductance under various folding angles and maintain an electronic path on paper substrates after repetition of folding and unfolding. Foldable paper-based applications with graphene circuits exhibit excellent folding stability.

White emission from polymer/quantum dot ternary nanocomposites by incomplete energy transfer

White light emission was obtained from a light-emitting diode prepared from polymer/quantum dot nanocomposites consisting of poly(9,-dihexylfluorene-2,7-divinylene- m-phenylenevinylene-stat-p-phenylenevinylene) (PDHFPPV) and two kinds of CdSe nanoparticles with different particle size. Blue emission from the polymer, green emission from the ?nm CdSe and red emission from the ?nm CdSe, which is triggered by partial excitation energy transfer from the polymer, jointly contribute to white emission of the organic?inorganic hybrid device. Also, the blue-emitting matrix polymer makes the device preparation process simpler due to its high processability. By controlling the blend ratio, we could obtain a pure white colour from the hybrid device.

Carrier trapping and efficient recombination of electrophosphorescent device with stepwise doping profile

We have presented a physical concept for enhancing efficiency and lifetime of doped electrophosphorescent organic light-emitting devices. In order to provide a control parameter for higher device performance, a stepwise doping concentration profile at the emission layer was prepared. A more than 30% improvement of power efficiency was obtained for green electrophosphorescent device with a higher doping ratio at the emission layer-hole transport layer interface. We explained the carrier trapping and transport mechanism with direct recombination of an exciton in an iridium-based dopant system. When compared to green device, phosphorescent red devices showed a more significant charge trapping effect at low doping concentration, which is responsible for shifting the recombination zone far from the emission layer-hole transport layer interface. Therefore, charge trapping by doping control in an emission layer could be utilized for a charge-balancing technique for the confinement of a triplet exciton.

Electrospun polyacrylonitrile-based carbon nanofibers and their hydrogen storages

Electrospun polyacrylonitrile (PAN) nanofibers were carbonized with or without iron (III) acetylacetonate to induce catalytic graphitization within the range of 900–1,500 °C, resulting in ultrafine carbon fibers with a diameter of about 90–300 nm. Their structural properties and morphologies were investigated. The carbon nanofibers (CNF) prepared without a catalyst showed amorphous structures and very low surface areas of 22–31 m2/g. The carbonization in the presence of the catalyst produced graphite nanofibers (GNF). The hydrogen storage capacities of these CNF and GNF materials were evaluated through the gravimetric method using magnetic suspension balance (MSB) at room temperature and 100 bar. The CNFs showed hydrogen storage capacities which increased in the range of 0.16-0.50 wt% with increasing carbonization temperature. The hydrogen storage capacities of the GNFs with low surface areas of 60-253 m2/g were 0.14-1.01 wt%. Micropore and mesopore, as calculated using the nitrogen gas adsorption-desorption isotherms, were not the effective pore for hydrogen storage.

Rheology and microstructures of electrorheological fluids containing both dispersed particles and liquid drops in a continuous phase

The effects of dispersed drops in the electrorheological (ER) behavior of a polyaniline particle suspension were considered. Oil-in-oil emulsions, which differ in the electrical conductivity and dielectric constant, were employed for the liquid biphase. The yield stress behavior of ER suspension under steady shear and electric field was examined. Only when the dispersed drops with a higher conductivity formed the dispersed liquid phases in the presence of dispersed polyaniline particles was a synergistic effect in the yield stress observed, giving a better ER performance and reduced current density. A direct microscopic observation demonstrated that such a synergistic effect is due to the unique microstructures in a complex composed of particles and deformed emulsion drops. Not only the magnitude but also the dependence of the yield stresses on the electric field strength were strongly affected by the change in relative composition of particulate and liquid drop phases. The associated mechanism of ER resp...

Vertical orientation of copper phthalocyanine in organic solar cells using a small molecular weight organic templating layer

Ultrathin film material templating layers that force the morphology of subsequently grown electrically active thin films have been found to increase the performance of small molecule organic photovoltaic (OPV) cells. Here, we show that the electron-transporting material, hexaazatriphenylene-hexacarbonitrile (HAT-CN) can be used as a templating material that forces the copper phthalocyanine (CuPc) donor molecule to assume a vertical-standing morphology when deposited onto its surface on an indium tin oxide (ITO) electrode. For a device with HAT-CN as the templating buffer layer, the fill factor and short circuit current of CuPc:C60 OPVs were both significantly increased compared with cells lacking the HAT-CN template. This is explained by the reduction of the series resistance due to the improved crystallinity of CuPc grown onto the ITO surface.

Improved blue light-emitting polymeric device by the tuning of drift mobility and charge balance

We have prepared blue polymer-small molecule hybrid electroluminescence devices with improved efficiency and lower driving voltage by the statistical design method. Analysis of time-of-flight measurement shows that amorphous small molecule hole-transporter blended with a blue light-emitting polymer increases the field-dependent hole mobility, with transition from nondispersive to dispersive transport induced by the charge-trapping effect. Moreover, at the electroluminescent devices with different electron injection/transport layer (LiF/Al, LiF/Ca/Al, and Alq3/LiF/Al), efficiency was further increased. We have analyzed that carrier mobility of a multilayered device can also be controlled by the change of electron injection and transport layers. We find that structural design and matching overall charge balance is an essential factor to improve both the operating voltage and efficiency of existing blue polymer devices.

Effects of cathode thickness and thermal treatment on the design of balanced blue light-emitting polymer device

The interface between layered conjugated polymer and electrode is a most important factor to improve the performance and lifetime of polymeric light-emitting devices (PLEDs). In this work, a blue PLED with improved stability was achieved by the combination of optimized cathode structure as well as thermal treatment of light-emitting polymer (LEP). Experimental evidence of the initial luminance “settling in” stage was found to be dependent upon the cathode structure, while the long-term slope of luminance as a function of elapsed time is governed by the annealing conditions. Our study revealed the importance of extrinsic design of device for the improvement of PLED stability. Experimental data shows that a blue PLED annealed at 170°C and 6nm LiF at LiF∕Ca∕Al cathode retained the best lifetime, which can be explained by the improved polymer–metal interface and LEP’s charge mobility.

High efficiency AMOLED using hybrid of small molecule and polymer materials patterned by laser transfer

Abstract Laser‐Induced Thermal Imaging (LITI) is a laser addressed patterning process and has unique advantages such as high‐resolution patterning with over all position accuracy of the imaged stripes of within 2.5 micrometer and scalability to large‐size mother glass. This accuracy is accomplished by real‐time error correction and a high‐resolution stage control system that includes laser interferometers. Here the new concept of hybrid system that complement the merits of small molecule and polymer to be used as an OLED; our system can realize easy processing of light emitting polymers and high luminance efficiency of small molecules. LITI process enables the stripes to be patlerned with excellent thickness uniformity and multi‐stacking of various functional layers without having to use any type of fine metal shadow mask. In this study, we report a full‐color hybrid OLED using the multi‐layered structure consisting of small molecules and polymers.

Two-Dimensional Micropatterns via Crystal Growth of Na2CO3 for Fabrication of Transparent Electrodes

The simple and versatile method to generate two-dimensional micropatterns by controlling precisely crystallization of sodium carbonate (Na2CO3) was investigated. Dense clusters of dendrites of salt crystals were homogeneously formed in a large area with an aqueous solution of Na2CO3 during evaporation of water. The dimensions and morphologies of dendritic salt crystals were tuned by changing the growth conditions such as salt concentration, relative humidity, and temperature. Then, 2D micropatterns of salt crystals were directly used as a mask for the deposition of a silver (Ag) layer to fabricate transparent electrodes. After salt crystals were completely dissolved in water, the network of an electrically conductive Ag layer, whose patterns were reversely produced from salt crystals, was generated on glass substrates. In addition, salt crystals were used as a master to prepare a replica mold of poly(dimethylsiloxane) (PDMS) for utilizing the imprinting technique. By imprinting a flexible PDMS mold with Ag inks, Ag micropatterns that were perfectly identical to dendrites of salt crystals were transferred to the other substrate.