Sookhyun Hwang

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.

Enhanced Electrical Conductance of ZnO Nanowire FET by Nondestructive Surface Cleaning

Electrical characteristics of zinc oxide nanowire (ZNW) FETs are investigated by nondestructive surface cleaning, ultraviolet irradiation treatment at high temperature and vacuum. UV-light-stimulated oxygen desorption from the active channel improves the device performance of ZNW-FETs. Results show that charge transport in single ZNW strongly depends on its surface environmental conditions and can be explained by formation of depletion layer at the surface by various surface states present on it. The nondestructive surface cleaning removes these absorbed surface states from the nanowire and the current values increase upto ~ 7 muA from ~ 0.4 muA at a bias voltage of 3 V. ZNW-FETs fabricated in this study exhibit mobility of ~ 28 cm2/Vmiddots and a high I ON ne I OFF ratio of ~ 106.

Unusually High Optical Transparency in Hexagonal Nanopatterned Graphene with Enhanced Conductivity by Chemical Doping

Graphene has received appreciable attention for its potential applications in flexible conducting film due to its exceptional optical, mechanical, and electrical properties. However increasing transmittance of graphene without sacrificing the electrical conductivity has been difficult. The fabrication of optically highly transparent (≈98%) graphene layer with a reasonable electrical conductivity is demonstrated here by nanopatterning and doping. Anodized aluminium oxide nanomask prepared by facile and simple self-assembly technique is utilized to produce an essentially hexagonally nanopatterned graphene. The electrical resistance of the graphene increases significantly by a factor of ≈15 by removal of substantial graphene regions via nanopatterning into hexagonal array pores. However, the use of chemical doping on the nanopatterned graphene almost completely recovers the lost electrical conductivity, thus leading to a desirably much more optically transparent conductor having ≈6.9 times reduced light blockage by graphene material without much loss of electrical conductivity. It is likely that the availability of large number of edges created in the nanopatterned graphene provides ideal sites for chemical dopant attachment, leading to a significant reduction of the sheet resistance. The results indicate that the nanopatterned graphene approach can be a promising route for simultaneously tuning the optical and electrical properties of graphene to make it more light-transmissible and suitable as a flexible transparent conductor.

Carbon Nanomaterials: A Review

The present chapter explored the advancement of research in carbon nanomaterials (graphene and carbon nanotubes), in the areas of synthesis, properties and applications including electronics, field emission, biological and energy applications. The reported properties and applications of these carbon nanomaterials have opened up new opportunities for the future devices and materials. The knowledge presented here should lead to a better understanding of the key factors that can influence the future research directions.

Characteristics of Gallium-Doped Zinc Oxide Thin-Film Transistors Fabricated at Room Temperature Using Radio Frequency Magnetron Sputtering Method

In this paper, we present bottom-gate-type Ga-doped zinc oxide (GZO) thin-film transistors (TFTs) using a certain conventional SiO2 gate insulator by applying a radio-frequency (RF) magnetron sputtering method at room temperature. A low gate leakage current was achieved using this conventional SiO2 gate insulator instead of new gate oxide materials. The root mean square (RMS) value of the GZO film surface was found to be 1.65 nm, and the transmittance was higher than 75% in the visible region. The GZO TFTs operated in a depletion mode with a threshold voltage of -3.4 V. A mobility of 0.023 cm2/(Vs), an on/off ratio of 2×103, and a gate voltage swing of 3.3 V/decade were obtained. We successfully demonstrated that the TFT of depletion-mode type can be fabricated using a GZO film that has good surface uniformity, transparency, and electrical characteristics.

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 %.

The Structural-Dependent Characteristics of Rashba Spin Transports in In 0.5 Ga 0.5 As/In 0.5 Al 0.5 As Heterojunctions

The growth and characterization of narrow-gap inverted high electron mobility transistor structures, developed as a candidate material for spin-injection devices, are presented in this study. We have grown samples possessing surface channels of different thicknesses (30 nm and 60 nm) both with and without a thin 3 nm cap layer by using molecular beam epitaxy. We then investigated the in-plane transport properties as well as the Rashba spin-orbit coupling constant of the two-dimensional electron gas confined at the heterojunction interface.

Effect of Electrochemical Properties and Optical Transmittance of Carbon Nanotubes Counter Electrodes on the Energy Conversion Efficiency of Dye-sensitized Solar Cells

In this work, electrochemical characteristics and optical transmittance of carbon nanotubes (CNTs) counter electrodes which had different amount of CNTs in CNTs slurries were analyzed. Two-step heat treatment processes were applied to achieve well-fabricated CNTs electrode. Three sets of CNTs electrodes and dye-sensitized solar cells (DSSCs) with CNTs counter electrodes were prepared. As the amount of CNTs increased, sheet resistance of CNTs electrode decreased. CNTs electrode with low sheet resistance had low electrochemical impedance and fast redox reaction. On the other hand, in case of CNTs counter electrode with low density of CNTs, performance of the dye-sensitized solar cell was improved due to its high optical transmittance. We found that the transmittance of CNTs counter electrode influence the performance of dye-sensitized solar cells.