Joohyun Hwang

Multilayered graphene anode for blue phosphorescent organic light emitting diodes

In this work, we report on blue organic light emitting devices (OLEDs), which have multilayered graphene as its anode. Our graphene films have been grown catalytically and transferred to the support. The fabricated blue OLEDs with graphene anode showed outstanding external quantum efficiency of 15.6% and power efficiency of 24.1 lm/W at 1000 cd/m2. Weak oxygen plasma treatments on graphene film surfaces improved the injection property between the anode and hole injection layer.

The Optical Effects of Capping Layers on the Performance of Transparent Organic Light-Emitting Diodes

In transparent organic light-emitting diodes (TOLEDs), the asymmetry in the optical paths causes difference between the bottom and top emitting lights, both in emissions and spectral distributions. Capping layers (CLs) can be used as an optical functional to enhance the emissions and adjust the spectral distributions. Here, we report on the optical effects of an organic CL, 1,1-bis[(di-4-tolylamino)phenyl]cyclohexane (TAPC), on the characteristics of TOLEDs Both simulated and experimental results are presented. We demonstrate the possibility of improving the total emission and achieving spectral matching of bottom and top emissions by varying the CL thickness. The optical effects of CLs have been interpreted from interference perspectives. Finally, we have presented a guideline that is practically useful in designing high-performance TOLEDs with CLs.

Random nanostructure scattering layer for suppression of microcavity effect and light extraction in OLEDs.

In this study, we investigated the effect of a random nanostructure scattering layer (RSL) on the microcavity and light extraction in organic light emitting diodes (OLEDs). In the case of the conventional OLED, the optical properties change with the thickness of the hole transporting layer (HTL) because of the presence of a microcavity. However, OLEDs equipped with the an RSL showed similar values of external quantum efficiency and luminous efficacy regardless of the HTL thickness. These phenomena can be understood by the scattering effect because of the RSL, which suppresses the microcavity effect and extracts the light confined in the device. Moreover, OLEDs with the RSL led to reduced spectrum and color changes with the viewing angle.

A new method for monitoring an OLED panel for lighting by sensing the wave-guided light

In this work, we report on a new monitoring method for an organic light-emitting diode (OLED) panel for lighting by optical sensing of the wave-guided light in the substrate. Using microlens array films, the wave-guided light was extracted into the edge or back side of the panel to be monitored by a photodiode. The luminance of the extracted light was measured as linearly proportional to the front light. Thus, by converting the extracted light into photo voltage, monitoring the luminance change occurring in the OLED is possible. Based on the results and concepts, we have proposed a photodiode-equipped driving circuit which can generate compensated driving current for uniform luminance of OLED panels.

Improved Device Performances in Phosphorescent Organic Light-Emitting Diodes by Microcavity Effects

In order to improve the external quantum efficiency of organic light-emitting diodes (OLEDs), we purpose an anode which has a structure of indium tin oxide (ITO)/Ag/ITO, in which the Ag layer has a function of micro-cavity inducing reflector. In order to maximize the microcavity effect the thicknesses of the hole transport layers of blue and red phosphorescent OLEDs (PhOLEDs) were deduced using simulations. By the use of our optically designed anode, it was possible to achieve approximately 50% improvements in the external quantum efficiency.

Large area organic light emitting diodes with multilayered graphene anodes

In this work, we demonstrate fully uniform blue fluorescence graphene anode OLEDs, which have an emission area of 10×7 mm2. Catalytically grown multilayered graphene films have been used as the anode material. In order to compensate the current drop, which is due to the graphene’s electrical resistance, we have furnished metal bus lines on the support. Processing and optical issues involved in graphene anode OLED fabrications are presented. The fabricated OLEDs with graphene anode showed comparable performances to that of ITO anode OLEDs. Our works shows that metal bus furnished graphene anode can be extended into large area OLED lighting applications in which flexibility and transparency is required.

Yellowing effects of TiO 2 /Epoxy nano composite layer on organic light emitting diodes with internal light extraction structure

We have developed the highly efficient OLEDs using the embossed glass substrate and the low yellowing TiO2/Epoxy nano composite layer. The nano composite can be applied to a scattering layer and a planarization layer on an embossed glass. The TiO2/Epoxy nano composite was effective for increasing the light extraction efficiency of OLEDs, however the yellowing degraded the luminous efficacy seriously. The luminous efficacy of the OLED with the nano composite layer on the embossed glass was 21.0 lm/W at 1000 cd/m2 and it increased by 36% compared to the conventional OLED.