Eun-ho Choi

Enhancement of light-extraction efficiency of organic light-emitting diodes using silica nanoparticles embedded in TiO 2 matrices

We investigate two types of internal light-extraction layer structures for organic light-emitting diodes (OLEDs) that consist of silica nanoparticles (NPs) embedded in high-refractive-index TiO₂ matrices. The composite of silica NPs and TiO₂ matrices was coated on the glass substrate and fabricated with and without a SiO₂ planarization layer. An increase in the optical out-coupling efficiency by a factor of 2.0 was obtained at a high luminance of 3,000 cd/m² from OLEDs containing the silica NPs embedded in TiO₂ matrices between glass substrates and Zn-doped In₂O₃ (IZO) electrodes after additional planarization processes. This is consistent with the analytical result using the finite-difference time-domain (FDTD) method. Randomly distributed silica NPs acting as scattering centers could reduce the optical loss when extracting light. By using additional planarization processes with a PECVD-derived SiO₂ layer, one can assure that smoother surfaces provide higher out-coupling efficiency, which attain 100% and 97% enhancements in power (lm/W) and current (cd/A) efficiencies, respectively.

Fabrication of polarization-dependent reflective metamaterial by focused ion beam milling.

By focused ion beam milling, we fabricated near-IR reflective metamaterials consisting of nano-aperture arrays. Optimum parameters of ion beam current and accelerating voltage in the fabrication process are obtained. Nano-apertures constituting reflective metamaterial are successfully milled, and possess a reflective resonance in the near-IR spectral range. With a double-split-ring resonator structure for the nano-aperture, the intensity reflection at resonance is rendered polarization dependent. It is found that the point group symmetry of the nano-aperture array determines the amount of anisotropy in the intensity reflection. Finite-difference time-domain simulation was adopted to identify details of nano-aperture metastructures transferred from nano-aperture patterns by the focused ion beam milling.

Effects of Oxygen Working Pressure on Structural and Optical Properties of TiO2 Films Grown on Glass Substrate

TiO2 thin films were grown on glasses using a pulsed laser deposition (PLD) system. In order to fully clarify the correlation between their properties and the working pressure of oxygen at which they are grown, a wide range of pressures from 10 to 75 mTorr was investigated. The optical gap of TiO2 films gradually increased from 3.3 to 3.41 eV with increasing working pressure up to 25 mTorr but became notably large at 50 mTorr and continued to increase significantly with further pressure increase. X-ray diffraction (XRD) results showed that the films grew preferentially along the (101) orientation. (101) peak intensity increased as working pressure increased up to 25 mTorr. However, the peak intensity began to decrease and eventually disappeared with further increase in working pressure, indicating structural changes from crystalline to disorderlike phases. The TiO2 films showed high crytallinity with large grains at 25 mTorr but showed a columnar structure with small grains at 75 mTorr. The growth mode of TiO2 thin films was observed to change from lateral to islandlike growth in response to the increase in working pressure. The correlation between oxygen pressure and film properties is discussed in terms of structural and compositional changes.

Temperature Dependence Of Closed Mode Q-Factor In Terahertz Metamaterial Superlattice

Terahertz metamaterial superlattice is fabricated with double-split ring resonators oriented alternately. By cooling down to cryogenic temperature 4K, changes in Q-factor of closed mode resonance originating from coherent coupling in metamaterial superlattice is investigated.