Yoon-Chang Kim

A high-extraction-efficiency nanopatterned organic light-emitting diode

To improve light extraction from organic light-emitting diodes (OLEDs), we introduced a photonic crystal pattern into the glass substrate of an OLED. The periodic modulation converts the guided waves in the high-refractive-index indium-tin-oxide/organic layers into external leaky waves. We used the finite-difference time-domain method to optimize the structural parameters of the photonic crystal pattern and to analyze the microcavity effect by the metallic cathode of the OLED. With the use of an optimized photonic crystal pattern, an increase of over 80% in the extraction efficiency of the OLED is expected theoretically. An increase in the extraction efficiency of over 50% was achieved experimentally, without detriment to the crucial electrical properties of the OLED.

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.

Nanohole-templated organic light-emitting diodes fabricated using laser-interfering lithography: moth-eye lighting

We describe the architecture, fabrication, and electro-optical characteristics of a two-dimensional (2D), periodic, highly ordered array of subwavelength scale organic light-emitting diodes (OLEDs). A 2D nanohole array template was introduced onto a patterned ITO glass substrate by two-step irradiated hologram lithography and reactive ion etching, and then a 2D nanohole OLED array was prepared by following typical OLED fabrication procedures. Our analysis of the electro-optical characteristics of this device showed that shrinking the OLEDs to sub-wavelength scale has only a minimal effect on their optical properties. We also used the Bragg scattering effect to confirm the compounding of the millions of ~220 nm OLED light sources to form 2D periodic nanohole emission by comparing the angular dependence of the emission spectrum of the OLED array with that of a conventional OLED.

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.

Effect of corrugated substrates on light extraction efficiency and the mechanism of growth in pulsed laser deposited Y2O3:Eu3+ thin-film phosphors

The luminance level of thin-film Y2O3:Eu3+ phosphors deposited by a pulsed laser deposition techniques are not candidates for field emission display because of their low extraction efficiency. A square lattice nanorod-type two-dimensional SiO2 photonic crystal layer (PCL) was built on a quartz glass substrate in an attempt to improve the efficiency of light extraction. The pitch of the PCL was varied, such that 500, 550, and 600nm pitches were produced while maintaining the depth of the PCL fixed at 200nm. The integrated photoluminescence efficiency of an Y2O2:Eu3+ thin-film phosphor deposited on the PCL was improved by 4.8 times and well matched with the calculated result based on the finite-difference time-domain method. The PCL played a significant role not only in enhancing the extraction efficiency but also aiding in the control of the film growth.

The variation of the enhanced photoluminescence efficiency of Y 2 O 3 :Eu 3+ films with the thickness to the photonic crystal layer

This study examined the effects of the thickness of Y(2)O(3):Eu(3+) phosphor films on quartz substrates coated with two-dimensional (2D) SiO(2) square-lattice nanorod photonic crystal layers (PCL) at identical heights on their extraction and absorption efficiency. The photoluminescence (PL) efficiency enhancement ratio decreased exponentially with increasing Y(2)O(3):Eu(3+) film thickness. The 2D PCL-assisted Y(2)O(3):Eu(3+) film with a thickness (t) = 400 nm showed enhancement in the upward and downward PL emission by factors of 6.2 and 8.6, respectively, with respect to those of a conventional flat film. This observation was attributed to diffraction scattering of the excitation and emission light.

Design of fine phosphor system for the improvement in the luminescent properties of the phosphor layer in the plasma display panel: Theoretical and experimental analysis

Improvement in the luminescent properties of plasma display panels via phosphor size control was theoretically and experimentally investigated. From theoretical analyses of photon extraction and plasma efficiency, fine phosphor system was designed, which was compared with experimental data. The denser microstructure of finer phosphor-based layer promoted photon extraction efficiency by higher reflectivity. Also, the finer phosphor increased vacuum-ultraviolet discharge space and corresponding plasma efficiency via decrease in layer thickness. Based on the results, the phosphor size control improved the panel efficiency by the synergistic effect of improvements in photon extraction and plasma efficiency.

Optical Properties of Y2O3 : Eu3 + Thin-Film Phosphors Coated with 2D SiN x ∕ Air Photonic Crystal Layers

We determined the effects of varying the array lattice parameters of the nanorods of a two-dimensional (2D) SiN x photonic crystal (PC) layer on the efficiency of extraction of photoluminescence (PL) of the Y 2 O 3 :Eu 3+ thin films, in which the interface between the phosphor layer and the air was modified with a 2D SiN x PC pattern. The PL extraction was improved by a factor of over 3.7 compared to that of a conventional Y 2 O 3 :Eu 3+ thin film by adding a PC array with a lattice constant of 600 nm, a height of 200 nm, and a filling ratio of 0.27. This improvement arises from the liberation of the photons trapped in the high-index guiding layers of the Y 2 O 3 :Eu 3+ thin films.

Design and Optical Properties of ZnS:Mn Thin-Film Electroluminescent Devices on 2D SiO2 Corrugated Photonic Crystal Substrates

We studied the characteristics of ZnS:Mn thin-film electroluminescence (TFEL) display devices in which the glass substrate had been modified with a two-dimensional (2D) SiO 2 corrugated photonic crystal (PC) pattern. The finite-difference time-domain (FDTD) method was used to predict the behavior of 2D corrugated PC ZnS:Mn TFEL devices with varying PC structural parameters. Based on the optimal parameters derived from the FDTD calculations, a 2D SiO 2 corrugated PC layer was introduced onto the glass substrate of a typical ZnS:Mn TFEL structure by two-step irradiated hologram lithography and reactive ion etching. Experiments conducted under typical operating conditions showed that incorporation of the 2D corrugated structure improved the light extraction luminance at 40 V above the threshold voltage by over 3.1-fold compared to the conventional ZnS:Mn TFEL device, without noticeable degradation of the electrical characteristics. This improvement originates from the liberation of the photons trapped in the high-index guiding layers.