Hyonkwang Choi

Graphene counter electrodes for dye-sensitized solar cells prepared by electrophoretic deposition

The graphene counter electrodes (GCEs) for dye-sensitized solar cells (DSSCs) prepared by electrophoretic deposition (EPD) are reported. Thermogravimetric analysis (TGA) was carried out to find the optimum annealing temperatures. It was found that charge transfer resistance (Rct1) related to the interface between electrolyte and counter electrode was significantly reduced by proper annealing. DSSC with GCE annealed at 600 °C demonstrated by the best conversion efficiency of 5.69% under AM 1.5 and 1 sun condition. Once GCE prepared by EPD is fully developed, it could be utilized for a low-cost, high throughput process for DSSCs.

Fabrication and characterization of carbon-based counter electrodes prepared by electrophoretic deposition for dye-sensitized solar cells

Three different carbon-based counter electrodes are investigated in light of catalytic activities such as electrochemical frequencies and interface impedances. We fabricated carbon-based counter electrodes of dye-sensitized solar cells [DSSCs] using graphene, single-walled carbon nanotubes [SWNTs], and graphene-SWNT composites by electrophoretic deposition method. We observed the optical and electrochemical properties of the carbon-based counter electrodes. The DSSC with the graphene-deposited counter electrode demonstrated the best conversion efficiency of 5.87% under AM 1.5 and 1 sun condition. It could be utilized for a low-cost and high-throughput process for DSSCs.

Carbon nanotube-graphene composite for ionic polymer actuators

In this paper, we develop a new ionic polymer‐metal composite (IPMC) by replacing a typical platinum or gold electrode with a multi-walled carbon nanotube (MWNT)‐graphene based electrode. A solvent of MWNT and graphene is formed on both sides of the ionic polymer membranes as electrodes by means of spray coating and baking. Then, the ionic liquid process is performed for actuating in air. The four kinds of IPMC samples with different MWNT‐graphene ratios are fabricated with the same solid Nafion film. Experimental results show that the IPMC with a pure MWNT based electrode exhibits higher displacement compared to the conventional IPMC with a platinum electrode. Also, the increment of the ratio of graphene to the MWNT‐graphene electrode decreases the resultant displacement but increases the fundamental natural frequency of the polymer actuator. (Some figures may appear in colour only in the online journal)

Effects of the electrical conductivity and orientation of silicon substrate on the synthesis of multi-walled carbon nanotubes by thermal chemical vapor deposition

We studied the effects of the electrical conductivity and orientation of silicon substrate on both catalytic Fe thin film and the structure and morphology of multi-walled carbon nanotube (MWNT) grown by low-pressure chemical vapor deposition. Both p-type Si(100) and Si(111) substrates with three different doping concentrations (high, low, undoped) were used to evaluate the formation of catalytic nanoparticles and the growth of MWNTs. The morphology of catalytic nanoparticles such as size and density was characterized by field-emission scanning electron microscopy, Cs-corrected energy-filtered transmission electron microscopy, and X-ray photoelectron spectroscopy. Structural characteristics of MWNTs grown on different combinations of silicon substrate orientation and electrical conductivities (σ) were also systematically analyzed. Based on the experimental results, growth modes of MWNTs could be controlled by choosing an appropriate combination of σ and orientation of Si substrates.

Precise control of chemical vapor deposition graphene layer thickness using NixCu1−x alloys

We investigated a simple but effective method to precisely control the desired number of graphene layers on the NixCu1−x alloy substrates by thermal chemical vapor deposition. Our method could be utilized to precisely control the number of graphene layers without altering growth conditions such as growth temperature and the cooling rate.

Structural, optical, and electrical properties of ZnO thin films deposited by sol-gel dip-coating process at low temperature

Sol-gel dip-coating was used to prepare ZnO thin films with relaxed residual stress by lowering the deposition temperature from room temperature (25°C) to −25°C. The effect of deposition temperature on the structural, optical, and electrical properties of the films was characterized using scanning electron microscopy (SEM), Raman spectroscopy, photoluminescence (PL), ultraviolet-visible (UV-Vis) spectroscopy and reflectance accessory, and the van der Pauw method. All the thin films were deposited successfully onto quartz substrates and exhibited fibrous root morphology. At low temperature, the deposition rate was higher than at room temperature (RT) because of enhanced viscosity of the films. Further, lowering the deposition temperature affected the structural, optical, and electrical properties of the ZnO thin films. The surface morphology, residual stress, PL properties, and optical transmittance and reflectance of the films were measured, and this information was used to determine the absorption coefficient, optical band gap, Urbach energy, refractive index, refractive index at infinite wavelength, extinction coefficient, single-oscillator energy, dispersion energy, average oscillator wavelength, moments M−1 and M−3, dielectric constant, optical conductivity, and electrical resistivity of the ZnO thin films.

Effect of SiO 2 Layer of Si Substrate on the Growth of Multiwall-Carbon Nanotubes

Multi-walled carbon nanotubes (MWNTs) were synthesized on different substrates (bare Si and /Si substrate) to investigate dye-sensitized solar cell (DSSC) applications as counter electrode materials. The synthesis of MWNTs samples used identical conditions of a Fe catalyst created by thermal chemical vapor deposition at . It was found that the diameter of the MWNTs on the Si substrate sample is approximately larger than that of a /Si substrate sample. Moreover, MWNTs on a Si substrate sample were well-crystallized in terms of their Raman spectrum. In addition, the MWNTs on Si substrate sample show an enhanced redox reaction, as observed through a smaller interface resistance and faster reaction rates in the EIS spectrum. The results show that DSSCs with a MWNT counter electrode on a bare Si substrate sample demonstrate energy conversion efficiency in excess of 1.4 %.

Synthesis and characterization of Cu(In, Ga)Se-2 absorption/scattering layers by electrodeposition method

We propose a Cu(In, Ga)Se2 double absorption layer structure with both absorption and scattering effects. We synthesized CIGS absorption layers by electro-deposition using different electrical potentials. Structural properties were characterized by x-ray diffraction and field-emission scanning electron microscopy. Optical properties were measured using UV/VIS spectrometry. Grain size and surface roughness were controlled by manipulation of the electrical potential. From these results, the currently typical, single CIGS absorption layer, was replaced with two CIGS layers: a top CIGS layer of smaller grain size and a bottom CIGS layer of larger grain size. We found that the larger-grain CIGS layer provides better scattering effect, so that the CIGS double layer structure provides enhanced light absorption performance in visible light. By applying this new CIGS double layer structure, it is expected that conversion efficiency will be increased.

Design and Growth of InAs Multi-Quantum Dots and InGaAs Multi-Quantum Wells for Tandem Solar Cell

The InAs multi-quantum dots (MQDs) solar cell and InGaAs multi-quantum wells (MQWs) solar cell to cover 1.1 eV and 1.3 eV were designed by 1D poisson, respectively. The MQDs and MQWs of 5, 10, 15 layers were grown by molecular beam epitaxy. The photo luminescence results showed that the 5 period stacked MQDs have the highest intensity at around 1.1 eV with 57.6 meV full width at half maximum (FWHM). Also we can observe 10 period stacked MQWs peak position which has highest intensity at 1.31 eV with 12.37 meV FWHM. The density and size of QDs were observed by reflection high energy electron diffraction pattern and atomic force microscope. Futhermore, AlGaAs/GaAs sandwiched tunnel junctions were modified according to the width of GaAs layer on p-type GaAs substrates. The structures with GaAs width of 30 nm and 50 nm have backward diode characteristics. In contrast, tunnel diode characteristics were observed in the 20 nm of that of sample.

OP02.04: The association between maternal prepregnancy obesity and congenital malformations in offspring: Short oral presentation abstracts

Obese women had an increased risk of overall congenital malformations (aORs 1.04, 95% CI 1.02-1.06) after adjustment for confounding factors compared with control. For specific organs, obese women had an increased risk for congenital malformations in the circulatory system (aORs 1.19, 95% CI 1.13-1.24), the respiratory system (aORs 1.15, 95% CI 1.00-1.32), cleft lip and cleft palate (aORs 1.20, 95% CI 1.00-1.45), the digestive system (aORs 1.05, 95% CI 1.01-1.09), genital organs (aORs 1.10, 95% CI 1.031.18), and the urinary system (aORs 1.14, 95% CI 1.04-1.26) compared to control. Table 2. Multiple logistic regression analysis with congenital malformations by maternal obesity