Kang-Pil Kim

Work Function Modification of Indium–Tin Oxide by Surface Plasma Treatments Using Different Gases

We report on the effects of surface treatment with N2, O2, and N2O plasmas on the work function of indium–tin oxide (ITO). UV photoelectron spectroscopy (UPS) showed that the work function on the ITO surface treated with N2O plasma increased more than that on the samples treated with N2 or O2 plasma. X-ray photoelectron spectroscopy (XPS) showed that the intensity of the O–O bonding peak at 532.3 eV markedly increased owing to the adsorption of O- ions on the ITO surface from breaking bonds in N2O gas by the plasma. The dipole layer formed by O- ions on the ITO surface increases the work function of ITO. Accordingly, N2O plasma treatment leads to a reduction of the potential barrier between the Fermi level of ITO and the highest occupied molecular orbital (HOMO) level of an organic layer when ITO is used as an anode for organic light-emitting devices (OLEDs) and related devices. Therefore, N2O plasma treatment enhances the hole-injection properties from the ITO thin film to the organic layer.

The Effects of Conditions for Polymerization Induced Phase Separation Processes on the Electro-optic Characteristics of Polymer Dispersed Liquid Crystals

The electro-optic properties of PDLC films were investigated by process conditions such as PDLC injection temperature, UV curing temperature and UV power. The mixture of liquid crystal (TL205) and the prepolymer substituted 1,6-hexanediol diacrylate (HDDA) for trimethylolpropane triacrylate (TMPTA), and the crosslinker in PN393 was used for PDLC formulation. The contrast ratio and driving voltage of PDLC films were mainly affected by UV curing temperature, rather than by the injection temperature. The film that was prepared at a relatively low UV process temperature revealed a good contrast ratio at a low driving voltage.

Effect of TiO2 Nanoparticle Modification on Ultraviolet Photodetection Properties of Al-Doped ZnO Nanowire Network

This study was conducted in order to observe the changes in the ultraviolet (UV) photodetection characteristic when TiO2 nanoparticles are modified on the surface of an aluminum-doped zinc oxide (AZO) nanowire in an AZO nanowire network with enhanced conductivity. According to the experiment results, the UV photosensitivity under a bias of 5 V was 52-fold in a bare AZO network, and it increased to 147-fold in the case when TiO2 nanoparticles were modified. The UV reset time decreased from 9 to 5 s. The AZO nanowire onto which TiO2 nanoparticles were adsorbed showed an approximately 2-fold faster response time and an approximately 3-fold higher UV photosensitivity than the existing bare AZO nanowire. The faster UV photoresponse time of the AZO nanowire network with adsorbed TiO2 nanoparticles is because the adsorbed TiO2 nanoparticles serve as recombination sites for electrons excited by UV illumination. In addition, with enhanced UV photosensitivity, adsorbed TiO2 nanoparticles serve as electron donation sites that provide additional electrons to the AZO nanowire when UV light is turned on.

Formation of Macropore and Three-Dimensional Nanorod Array in p-Type Silicon

We carried out a study on the change in pore wall thickness depending on the current density in p-type silicon. We attempted the formation of a uniform macropore or nanorod array with a high aspect ratio in p-type silicon by electrochemical etching through the optimization of the hydrogen fluoride (HF)/organic electrolyte composition and the design of the mask pattern. The electrochemical etching of p-type silicon in the HF: dimethylsulfoxide (DMSO): deionized (DI) water = 1:5:5 electrolyte can control the velocity of a reaction between an electrolyte and a hole necessary for the electrochemical etching of silicon through the mixing of the protic property of DI water and the aprotic property of DMSO. In this study, we fabricated a p-type silicon nanorod array of three-dimensional structures with an approximately 350 nm diameter from macroporous Si by applying two-step currents (40 mA, 200 s + 38 mA, 1600 s) to a 1.8 cm2 circular area using an optimized HF: DMSO: DI water = 1:5:5 electrolyte composition.

Preferential (100)-oriented CH3NH3PbI3 perovskite film formation by flash drying and elucidation of formation mechanism

Most previous studies of perovskite films have explored the use of highly (110)-oriented perovskite films, even though films having the (100) orientation exhibit more desirable characteristics. In this study, we examined a simple method for growing (100)-oriented perovskite films for solar cells and elucidated their growth mechanism. Oriented perovskite grains with a lateral size of as much as 20 μm and a very flat surface morphology could be obtained. It was found that the amount of thermal energy delivered during annealing and the amount of residual solvent remaining after spin coating play critical roles in determining the growth orientation of the perovskite film. It was suggested that the formation mechanism of the preferentially (100)-oriented grains is most likely the classical hetero-nucleation and selective growth process in the solution system, even though DMSO was included in the solvent. The results of this study will aid in the optimization of preferentially (100)-oriented CH3NH3PbI3 perovskite film, which will be useful for the study of the effect of crystal orientation and the properties of perovskite optoelectronic devices, such as LEDs, sensors, transistors, and photovoltaic cells.

Effect of hot-pressing on an electrospun TiO2 electrode for dye-sensitized solar cells

A hot-pressing process is shown to enhance the adhesion of TiO2 nanofibers electrospun onto fluorine-doped tin oxide substrates for use in dye-sensitized solar cells. By using the resulting electrochemical impedance spectroscopic characteristics, the application of 7, 14, and 21 MPa of pressure was found to result in calculated lifetimes of 7.48, 15.13, and 13.74 ms, respectively. From the results of a thermogravimetric analysis, a stepped heat treatment was developed for calcination of the hot-pressed TiO2 electrodes. Hot-pressing at 14 MPa and 100 °C yielded the dye-sensitized solar cell with the highest energy conversion efficiency.

Mesoporous TiO2 hierarchical structures: preparation and efficacy in solar cells

We investigated an electrospray-based method to manufacture photoelectrodes for dye-sensitized solar cells (DSSCs). TiO2 doughnut-, spherical-, and disk-shaped particles with a large surface area, high crystallinity, uniform nanostructure, and good light scattering properties were fabricated via a simple electrospray method. The control of the morphology of the nanostructured particles prepared by electrospraying a dispersion of nanoparticles was investigated experimentally; the results are qualitatively explained on the basis of the available theory. The solvent in the droplet, droplet size, surface tension, process temperature, and process humidity are crucial to the morphology of the resulting particles. The effect of the particle morphology on the performance of DSSCs is demonstrated. Compared to the DSSCs with conventional photoelectrodes (with a power conversion efficiency of 8.4%), the DSSCs based on doughnut-, spherical-, and disk-shaped particle photoelectrodes yielded higher power conversion efficiencies of 8.8%, 9.3%, and 10.4%, respectively. The DSSC utilizing the disk-type photoelectrode showed the best performance under AM1.5 global illumination through a photo-mask at an illumination power of 100 mW cm−2. This is because the generated TiO2 disks provide a large surface area and exhibit excellent light scattering capabilities, thus resulting in a low total internal resistance and long electron lifetime.

Fabrication of Sb2S3 Hybrid Solar Cells Based on Embedded Photoelectrodes of Ag Nanowires-Au Nanoparticles Composite

The Ag nanowire (NW) + Au nanoparticle (NP)-embedded TiO2 photoelectrodes were adopted for conventional planar TiO2-based Sb2S3 hybrid solar cells to improve the cell efficiency. Compared to conventional planar TiO2-based Sb2S3 hybrid solar cells, the Ag NW + Au NP/TiO2-based Sb2S3 hybrid solar cells exhibited an improvement of approximately 40% in the cell efficiency due to the significant increase in both Jsc and Voc. These enhanced Jsc and Voc were attributed to the increased surface area, charge-collection efficiency, and light absorption by embedding the Ag NWs + Au NPs composite. The Ag NW + Au NP/TiO2-based Sb2S3 hybrid solar cells showed the highest efficiency of 2.17%, demonstrating that the Ag NW + Au NP-embedded TiO2 photoelectrode was a suitable photoelectrode structure to improve the power conversion efficiency in the Sb2S3 hybrid solar cells.

Effect of Perovskite Overlayers on TiO2 Electrodes in Perovskite-Sensitized Solar Cells.

In this paper, we have studied the effect of the thickness of a CH3NH3PbI3 perovskite overlayer on mesoporous TiO2 electrodes in perovskite solar cells. The overlayers were prepared by spin coating PbI2 films on the electrodes, which were subsequently exposed to a CH3NH3I/2-propanol solution. We controlled the thickness of the perovskite overlayer by changing the PbI2 solution concentration. The thicknesses of the overlayers spin-coated from 0.5, 0.75, 0.9, and 1 M PbI2 solutions were approximately 179, 262, 316, and 341 nm, respectively. Perovskite solar cells with an approximately 316-nm-thick overlayer showed the highest efficiency of 9.11%. We conclude that optimization of the perovskite overlayer thickness in the solar cell structure is necessary to improve the cell efficiency.

Ion beam lithography using membrane masks

In this paper, we demonstrate both by simulation and experiment that, by using membrane masks, sub-100 nm patterns can be generated with practical mask to wafer distances (∼10 μm). We discuss the straggling problem and membrane mask preparation, and present ion beam lithography results.