Jaehyun Moon

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.

FDTD analysis of the light extraction efficiency of OLEDs with a random scattering layer

The light extraction efficiency of OLEDs with a nano-sized random scattering layer (RSL-OLEDs) was analyzed using the Finite Difference Time Domain (FDTD) method. In contrast to periodic diffraction patterns, the presence of an RSL suppresses the spectral shift with respect to the viewing angle. For FDTD simulation of RSL-OLEDs, a planar light source with a certain spatial and temporal coherence was incorporated, and the light extraction efficiency with respect to the fill factor of the RSL and the absorption coefficient of the material was investigated. The design results were compared to the experimental results of the RSL-OLEDs in order to confirm the usefulness of FDTD in predicting experimental results. According to our FDTD simulations, the light confined within the ITO-organic waveguide was quickly absorbed, and the absorption coefficients of ITO and RSL materials should be reduced in order to obtain significant improvement in the external quantum efficiency (EQE). When the extinction coefficient of ITO was 0.01, the EQE in the RSL-OLED was simulated to be enhanced by a factor of 1.8.

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.

Oxide-silicon-oxide buffer structure for ultralow temperature polycrystalline silicon thin-film transistor on plastic substrate

A novel oxide-silicon-oxide buffer structure to prevent damage to a plastic substrate in an ultralow temperature (<120/spl deg/C) polycrystalline silicon thin-film transistor (ULTPS TFT) process is presented. Specifically, an amorphous silicon film was inserted as an absorption layer into buffer oxide films. The maximum endurable laser energy was increased from 200 to 800 mJ/cm/sup 2/. The fabricated ULTPS nMOS TFT showed a performance with mobility of 30 cm/sup 2//Vs.

White transparent organic light-emitting diodes with high top and bottom color rendering indices

Reported in this work is the fabrication of white transparent organic light-emitting diodes (TOLEDs) with high color rendering indices (CRIs). An architecture in which the green and red emission layers are sandwiched between two blue emissions layers was used. By tuning the thicknesses of the green and red emission layers, CRI and power efficiency values of 90/20.5 and 87/6.8 lm/W were achieved in the bottom and top emissions, respectively. The study results suggest an effective engineering approach for realizing high CRI white TOLED lighting sources.

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.

Luminescence Enhancement by Ga3+ Codoping of BaTiO3:Pr3+ Phosphors

The effects of dopant Pr and codopant Ga on the luminescence of BaTiO3 phosphors have been investigated. The maximum luminescence intensity of red emission was observed at a Pr concentration of 0.011 mol %, which is a much lower concentration than those in previous reports on the luminescence characteristics of phosphors. Ga codoping into BaTiO3:Pr3+ phosphors results in a marked increase in luminescence. This result has been interpreted from the viewpoint of symmetry, which makes the occurrence of band-to-band transition probable.

A Light Scattering Layer for Internal Light Extraction of Organic Light-Emitting Diodes Based on Silver Nanowires

We propose and fabricate a random light scattering layer for light extraction in organic light-emitting diodes (OLEDs) with silver nanodots, which were obtained by melting silver nanowires. The OLED with the light scattering layer as an internal light extraction structure was enhanced by 49.1% for the integrated external quantum efficiency (EQE). When a wrinkle structure is simultaneously used for an external light extraction structure, the total enhancement of the integrated EQE was 65.3%. The EQE is maximized to 65.3% at a current level of 2.0 mA/cm(2). By applying an internal light scattering layer and wrinkle structure to an OLED, the variance in the emission spectra was negligible over a broad viewing angle. Power mode analyses with finite difference time domain (FDTD) simulations revealed that the use of a scattering layer effectively reduced the waveguiding mode while introducing non-negligible absorption. Our method offers an effective yet simple approach to achieve both efficiency enhancement and spectral stability for a wide range of OLED applications.

Fabrication and Characterization of Semiconducting ZnO Nanofibers for CO Sensing

We used an electrospinning method to produce ZnO nanofibers for CO sensing. ZnO nanofibers were electrospun from a solution containing poly(4-vinyl phenol) (PVP) and Zn acetate. The calcination process of the ZnO/PVP composite nanofibers brought forth polycrystalline wurtzite ZnO nanofibers with diameters of 30-70 nm. Regarding the applications of the ZnO nanofibers, their CO gas-sensing responses were evaluated in a concentration range of 10-25 ppm. The resistances of the ZnO nanofibers were observed to decrease upon exposure to CO. This is attributed to the reducing nature of CO, which reacts with adsorbate O - to release an extra electron.

Optical Effects of Graphene Electrodes on Organic Light-Emitting Diodes

We performed optical simulations and experiments to investigate the internal optics of graphene anode organic light-emitting diodes (OLEDs). The efficiencies, emission distribution, and spectral characteristics of four-layered graphene anode OLEDs were extracted and compared to those of ITO anode OLEDs. Unlike the case of the ITO anode OLED, the efficiencies and emission distributions of graphene anode OLEDs showed a weak dependency on the thickness of the organic layers. Furthermore, marginal changes in the emission spectra were observed. These results were ascribed to the negligible presence of a microcavity effect in the graphene anode OLED.