Jin Yeong Kim

Extracting optical modes of organic light-emitting diodes using quasi-periodic WO 3 nanoislands

Quasi-periodic WO(3) nanoislands are introduced to extract two optical modes in organic light-emitting diodes. The nano-scaled and size-tunable WO(3) islands were fabricated by use of wet-etching with self-aggregated Ag mask. The improvement of light extraction efficiency originates to the recovery of light losses which are surface plasmon mode and waveguide mode. As a result, external quantum efficiency and power efficiency were increased. No changes in emission spectrum and CIE color coordinates with WO(3) nanoislands at various observation angles are desirable if this device is to be utilized in optical system. Furthermore, cost-effective fabrication makes it possible to adopt this system in large area fabrication.

Improvement in Outcoupling Efficiency and Image Blur of Organic Light-Emitting Diodes by Using Imprinted Microlens Arrays

We demonstrated a new approach to fabricate microlens arrays attached to an organic light-emitting diode (OLED) to improve the outcoupling efficiency. Polydimethylsiloxane (PDMS) microlens arrays were fabricated by imprint lithography. Two kinds of photoresists were used to make a mold to imprint the microlens array. Using this simple and low-cost fabrication process, a maximum of 43% luminance enhancement was acquired with the green OLED. When attaching the microlens array to a green OLED, the efficacy and power efficiency were increased by 42% and 69% at 900 , respectively. The external quantum efficiency of the OLED with the microlens array was increased by 59% at 7.5 due to luminance enhancement at a large viewing angle without spectral change. Additionally, the image blur problem is quantitatively analyzed by defining “blur width.” The image blur problem induced by the microlens array is reduced by adopting the microlens array with lower height while the luminance enhancement by the microlens array remained.

Surface plasmon-waveguide hybrid polymer light-emitting devices using hexagonal Ag dots.

We investigated surface plasmon-waveguide hybrid resonances for enhancement of light emission in polymer light-emitting diodes (PLEDs). Hybrid waveguide-plasmon resonances in the visible range for waveguide mode and near IR range for surface plasmons were observed by incorporation of hexagonal Ag dot arrays. Considerable overlap between the emission wavelength of the PLEDs and the waveguide mode by an Ag dot array with a lattice constant of 500 nm was observed. Because of enhanced light extraction by Bragg scattering of waveguide modes, photoluminescence (PL) and electroluminescence (EL) were increased by 70% and 50%, respectively.

Analysis of Out-Coupling Mechanism in Organic Light-Emitting Diodes

We investigated the quantitative mechanism of out-coupling enhancement in nanostructure-embedded organic light-emitting diodes (OLEDs). A numerical calculation was performed for a 2-D configuration with nanostructures, and power dissipation via various optical modes was achieved. The extraction of light loss from the surface plasmon mode was three times higher than that from the waveguide mode in nanostructure-embedded OLEDs. Thus, we conclude that out-coupling enhancement originates from the recovery of light losses related to the surface plasmon mode. Furthermore, we simulated the relation between out-coupling enhancement and the geometry of the nanostructure.

Analysis and structure optimization of nanostructure-embedded organic light-emitting diodes

In the authors’ previous work, a quasi-periodic nanostructure was introduced to extract the light loss in organic light-emitting diodes (OLEDs). Two optical losses – waveguide loss and surface plasmon loss – were recovered via Bragg scattering. In this work, the dependency of the grating period for efficient light extraction was analyzed. Optical calculation was performed based on the transfer matrix, with various thicknesses of the transparent electrode and organic layers. The electrical profiles of each optical mode in the OLEDs were also plotted, and the optimized design of OLEDs was investigated.

Simulation of Surface Plasmon Coupled Conjugate Polymer for Polymer Light-Emitting Diodes

Since 1977, conjugated polymers have received attention as materials for display devices with a low-cost solution process, but the low efficiency of these materials has been considered as a drawback which should be overcome. Nowadays metal nanoparticles are inserted on the display devices cathode to overcome the low efficiency of the materials through the enhanced coupling between the Localized surface plasmon resonance (LSPR) and exciton in emitting material . In our previous work, conjugated polymer with an imprinted regular Ag-dot-array structure showed a 2.7-fold improvement of integrated photoluminescence (PL) intensity , but the result was not optimized. Therefore, in this study, we calculated the Ag-dot-array absorbance-peak shift in detail using finite-difference time-domain (FDTD) simulation and found the absorbance peak location which maximized photoluminescence (PL) intensity, depending on various Ag dot condition. The resulting information was applied to the previous structure . Thus, we reduced the trial and error of finding the optimized absorbance peak location and the imprint processing costs. The most important parameter of the Ag-dot-array absorbance peak was the lattice constant. Furthermore, we proved the indium tin oxide (ITO) waveguide effect in our structure using FDTD.