Jin Woo Huh

Evaluation of gas permeation barrier properties using electrical measurements of calcium degradation

In this work, we developed a thin calcium degradation method introducing sensitive electrical resistance monitoring. We have demonstrated structural models of the inorganic/organic thin films to evaluate barrier properties against water and oxygen permeation. The time-dependent transmission curve of a multibarrier coated on both sides of the polyethersulfone substrate had a linear slope which was measured as 5.17 x 10(-3) gm(2) day at 20 degrees C and 60% relative humidity. This system can measure an accurate permeation rate with a high sensitivity in the measurable range of 10(1)-10(-6) gm(2) day. In addition, the test structure devised is applicable to various fabrication techniques for passivation layers with durability and ultralow permeability for flexible organic light emitting diodes.

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.

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.

Characteristics of organic light-emitting diodes with conducting polymer anodes on plastic substrates

The fabrication of conducting polymer films by a screen-printing method and characterization of an organic light-emitting diode (OLED) implemented using these films as an anode on plastic substrates are reported. Organic transparent electrode materials containing poly(3,4-ethylenedioxythiophene) (PEDOT) doped with poly(styrenesulfonic acid) (PSS) (PEDOT:PSS) were used after the modification by a unique nanoparticle binder design. The electro-optical properties as well as mechanical stabilities of these films were measured. The OLED performances when employing these films were comparable to that of OLEDs using indium tin oxide (ITO) despite their relatively poor conductivity. An external quantum efficiency of the OLED using this anode is ∼3.5%, which is about twice as that of OLEDs using ITO. These results show that the organic transparent electrode of a conducting polymer film patterned by the screen-printing method is a potential candidate for an electrode of the flexible OLED.

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.

Carbon nanotube and conducting polymer dual-layered films fabricated by microcontact printing

We report carbon nanotube/conducting polymer dual-layered film (CPDF) electrodes fabricated by microcontact printing for flexible transparent electrodes of organic thin film transistors (OTFTs). The CPDFs show ∼1000 Ω/sq surface resistivity and ∼93% transmittance at an extremely low loading of single-walled carbon nanotubes, and can be self-aligned with a precision of 20 μm. The CPDFs are applied as the source and drain electrodes in OTFTs without any supplementary alignment process, which leads to a mobility and a current on/off ratio of approximately 0.02 cm2 V−1 s−1 and ∼104, respectively.

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.

Highly efficient tris(8-hydroxyquinoline) aluminum-based organic light-emitting diodes utilized by balanced energy transfer with cosensitizing fluorescent dyes

We report on the development of highly efficient organic light-emitting diodes (OLEDs) utilized by balancing the energy transfer between multiple dopants, that is, multiple emissions from the multiple dopants were realized by balanced distributed energy transfer. From the cosensitizing fluorescent OLEDs, the peak external quantum efficiency (EQE) of 4.8% at 130 cd/m2 is demonstrated, which realized theoretical limits of ∼5.0% and means that nearly 100% of the singlet excitons are radiative. Also, the optimized device accompanying thickness-modulated electron transport layer for the enhanced light outcoupling demonstrated the highly improved peak EQE and current efficiency of 6.7%, and 23.4 cd/A.

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.