Young-woo Song

Enhanced light extraction efficiency from organic light emitting diodes by insertion of a two-dimensional photonic crystal structure

We studied the characteristics of organic light emitting diode (OLED) devices containing two-dimensional (2D) SiO2∕SiNx photonic crystal (PC) layers. The finite-difference time-domain (FDTD) method was employed for the design and analysis of the PC OLED. Based on the design parameters derived from the FDTD calculations, a 2D PC layer was introduced on the glass substrate of a typical OLED structure by two-step irradiated hologram lithography and reactive ion etching. Experiments showed that incorporation of the PC layer improved the light extraction efficiency by over 50% compared to the conventional OLED, without noticeable degradation in electrical characteristics, under typical operating conditions. This improvement originates from the liberation of the photons trapped in the high-index guiding layers.

Planarized SiNx/spin-on-glass photonic crystal organic light-emitting diodes

The light extraction characteristics of low-index spin-on-glass (SOG)-assisted, planarized photonic crystal organic light-emitting diodes (PC OLEDs) are reported. The light extraction efficiencies of planarized two-dimensional (2D) SiNx∕SOG PC OLEDs (type II) and 2D SiNx∕SOG PC OLEDs with an additional high-index SiNx layer (type III) are significantly better under typical operating conditions than those of the first generation of 2D SiO2∕SiNx PC OLEDs (type I). The enhancements in the extraction efficiencies of type-II and type-III PC OLEDs are about 63% and 85%, respectively, with respect to those of conventional OLEDs with indium tin oxide layers of identical thicknesses. These improvements in extraction efficiencies are attributed not only to the liberation of the photons trapped in the high-index guiding layer but also to a reduction in the surface plasmon contribution.

Far-field radiation of photonic crystal organic light-emitting diode

Utilizing the near- to far-field transformation based on the 3-D finite difference time domain (FDTD) method and Fourier transformation, the far-field profile of a photonic crystal organic light emitting diode is studied to understand the viewing angle dependence. The measured far-field profiles agree well with those of the simulation. The enhancement of the extraction efficiency in excess of 60% is observed for the optimized photonic crystal pattern.

Weak-microcavity organic light-emitting diodes with improved light out-coupling

We propose and demonstrate weak-microcavity organic light-emitting diode (OLED) displays with improved light-extraction and viewing-angle characteristics. A single pair of low- and high-index layers is inserted between indium tin oxide (ITO) and a glass substrate. The electroluminescent (EL) efficiencies of discrete red, green, and blue weak-microcavity OLEDs are enhanced by 56%, 107%, and 26%, respectively, with improved color purity. Moreover, full-color passive-matrix bottom-emitting OLED displays are fabricated by employing low-index layers of two thicknesses. As a display, the EL efficiency of white color was 27% higher than that of a conventional OLED display.

Optically bifacial thin-film wire-grid polarizers with nano-patterns of a graded metal-dielectric composite layer

We report on the concept of a thin film wire-grid polarizer (WGP) with optically dual characteristics by introducing a nano-patterned graded metal-dielectric composite-material layer. The Ti-SiO(2) composite layer with a depth profile of a gradually-varied composition ratio shows an absorptive feature due to the elimination of an optical interface between a metal and a glass substrate, while the metal side of the WGP gives a reflective character. The unprecedented optically-bifacial thin-film WGP with the 144 nm-period straight-line patterns of a 100 nm-thick Ti-SiO(2) composite layer and a 185 nm-thick Al layer shows the exceptionally low reflectance below 15 % from the absorptive side and the high polarization extinction ratio (PER) of over 500 at 550 nm, which is acceptable for use as various display applications such as AMOLEDs and LCDs.

Weak-microcavity organic light-emitting diodes with improved light-extraction and wide viewing-angle

We propose and demonstrate weak-microcavity organic light-emitting diode (OLED) displays that deliver both a high light-extraction efficiency and wide viewing-angle characteristics. A single pair of low- and high-index layers is inserted between indium tin oxide (ITO) and a glass substrate. The electroluminescent (EL) efficiencies of discrete red, green, and blue weak-microcavity OLEDs (WMOLEDs) are enhanced by 56%, 107%, and 26%, respectively with minimal changes viewing angle and EL spectra characteristics. The color purity is also improved for all three colors. Moreover, we fabricated full-color 128×160 passive-matrix bottom-emitting WMOLED displays to prove their manufacturability. This design is realized by simple one-step 20-nm etching of the low-index layer of red/green subpixels. The EL efficiency of white color in the WMOLED display is 27% higher than that of a conventional OLED display.