Sang Hun Choi

The effect of electrode heat sink in organic-electronic devices

Most of organic devices showed poor thermal stability and short lifetime due to Joule heating by current injection during operation. To increase the lifetime of the devices, thermal management must be considered. We demonstrated the polymer light-emitting diodes with thermally conductive substrate and Al/Cu double cathode to enhance the thermal stability of the device. Also, we proposed the correlation between lifetime (Δt) and device heat sink (ΔT). The heat sink of all organic devices is required to enhance device durability.

Transparent organic light-emitting diodes using resonant tunneling double barrier structures

A semitransparent cathode of indium tin oxide (ITO)/Ag/ITO was developed as a resonant tunneling double barrier structure for transparent organic light-emitting diodes. A weak negative differential resistance was observed in devices using a 100nm thick ITO/Ag/ITO layer as a cathode in combination with a thin LiF∕Al layer. The current injection of devices was dominated by resonant tunneling, which induced no luminance at low voltage. This was achieved by employing an e-beam evaporated ITO/Ag/ITO cathode due to the double quantum barriers of ITO and the quantum well of Ag. The authors also applied the multilayer cathode to small molecule devices, which showed the same resonant tunneling currents.

Work function increase of indium-tin-oxide surfaces by atmospheric air plasma treatment with steady-state airflow

Atmospheric air-plasma treatment of indium–tin–oxide (ITO) surfaces has been investigated as an alternative to a conventional oxygen (O2) vacuum plasma process. For this study, we devised an atmospheric air barrier plasma system having a dimension of 1000×600mm2 and successfully verified a possibility to ignite and maintain an atmospheric pressure discharge only in the ambient air. In particular, we used the steady-state airflow to generate more atomic oxygen radicals as oxygen gas during the vacuum plasma process and to prevent redeposition of the removed or transformed impurities onto the indium–tin–oxide substrate. The x-ray photoemission spectroscopy examination indicated that the adoption of the atmospheric-air plasma treatment reduced the surface content of carbon from 22.1% down to 8.5% and increased that of oxygen from 43% up to 57%. According to the photoelectron spectrometer (AC-1, RIKKEN) result, we obtained a work function of 5.11eV for the treated ITO surfaces after 1min treatment time, which...

Highly efficient transparent organic light-emitting diodes by ion beam assisted deposition-prepared indium tin oxide cathode

The authors have investigated the effects of indium tin oxide (ITO) deposited by ion beam assisted e-beam evaporation on the performance of polymer light-emitting diodes. ITO was evaporated as a cathode onto a thin Mg:Ag layer by an e-beam process, and its performance as a transparent cathode was subsequently compared to that of Mg:Ag and sputtering-prepared ITO. Polymer devices’ luminance and efficiency were improved by more than ten times by ion beam assisted deposition (IBAD)-prepared ITO deposition, with little observable damage to the organic layer. Implementation of the IBAD process resulted in the reduction of the interfacial energy barrier which induced band bending. Furthermore, outcoupling with ITO resulted in enhanced luminance.

Effects of aluminum cathodes prepared by ion-beam-assisted deposition in organic light-emitting devices

We have fabricated highly stable organic electroluminescent devices based on spin-coated soluble phenyl-substituted poly-p-phenylene-vinylene (Ph-PPVs) thin films. The electrical properties of aluminum cathode, prepared by ion-beam-assisted deposition, on Ph-PPV have been investigated and compared to those prepared by thermal evaporation. Although energetic particles of Al assisted by Ar+ ion may damage the organic material, I–V–L characteristics are improved by evaporating a thin Al buffer layer prior to ion-beam-assisted deposition. In addition, a dense Al cathode inhibits the permeation of H2O and O2 into Ph-PPV film by suppressing pinhole defects, and thus retards dark spot growth. This may be deduced from highly packed structure of Al cathode with smaller contact resistance between Al and Ph-PPV. The lifetime of an organic light-emitting device has been extended by dense Al film through an ion-beam-assisted deposition process.

Improved stability of organic light-emitting diode with aluminum cathodes prepared by ion beam assisted deposition

Abstract We have fabricated highly stable organic electroluminescent devices based on spin-coated poly-p-phenylene-vynylene (PPV) thin films. The electrical properties of aluminum cathode, prepared by ion beam assisted deposition, on PPV have been investigated and compared to those by thermal evaporation. Although energetic particles of Al assisted by ArC ion may damage the organic material, I–V–L characteristics are improved by applying thin Al buffer layer. In addition, a dense Al cathode inhibits the permeation of H2O and O2 into PPV film through pinhole defects, and thus retards dark spot growth. It may be deduced from highly packed structure of Al cathode with an increase in the contact area between Al and PPV that reduce the contact resistance. In conclusion, the lifetime of organic light-emitting device (OLED) has been extended effectively by dense Al film through ion beam assisted deposition process.

Relationship between optical properties and microstructure of CeO2–SiO2 composite thin films

CeO2–SiO2 composite thin films were prepared by e-beam evaporation and ion beam assisted deposition using an End-Hall ion source. The refractive index of composite thin films exhibited a maximum value at 20%–35%SiO2 fraction, indicating the high packing density. Optical analysis revealed that the transmittance and reflectance spectra of composite films were consistent with the results of the refractive index. The results from x-ray diffractometry, atomic force microscopy and scanning electron microscopy measurements showed that composite thin films containing 20%–35%SiO2 concentration had a dense and smooth amorphous surface, compared to the roughened granular structure of the pure SiO2 and CeO2 thin films. The composite thin films with 20%–35%SiO2 concentration exhibited a higher resistance to water absorption than the CeO2 thin films in spite of the highest refractive index.

Plasma treatment and its prospective application to polymer light-emitting diodes fabricated by ink-jet printing method

The influence of CF4 plasma treatment of indium-tin-oxide (ITO) and polyimide (PI) on the patterning of ink-jet printed polymer is presented. Not much difference between the as-received ITO and PI surface energies was found, but a significant difference in surface energies between ITO and PI after CF4 plasma treatment was noted. It is expected that precise patterning can be achieved by using the difference in surface energies between the inside of the pixel and its surroundings. Also the effects of CF4 plasma treatment of ITO have been studied on the performance of polymer light-emitting diodes (PLEDs). X-ray photoelectron spectroscopy revealed that CF4 plasma treatment led to a decrease in the surface content of carbon contaminants and an increase in the surface content of fluorine, which in turn enhance the performance of PLEDs.

Fabrication of Organic Electroluminescence Device with Ion Beam-Assisted Deposition of Ultrathin Lithium Fluoride as a Hole Injection Layer

An ultrathin lithium fluoride (LiF) hole injection layer was deposited on an indium-tin-oxide (ITO) anode by an ion-beam-assisted deposition (IBAD) technique to fabricate an organic electroluminescence device. The device with the LiF layer deposited by this method has higher external quantum efficiency than a device with a LiF deposited by conventional thermal evaporation. Moreover, the hole-injection ability of the device prepared by IBAD is better than that of the device produced by a conventional method such as thermal evaporation. It is found that the surface of the LiF layer prepared by IBAD has high surface coverage on an ITO anode because the LiF layer prepared has high adatom mobility due to the ion beam energy.

Investigation into the effects of aluminum cathode modification and ion-beam-induced damage in organic light-emitting devices

We report a fabrication of organic light-emitting diodes (OLEDs) based on soluble phenyl-substituted poly-p-phenylene-vinylene (Ph-PPVs) thin films with aluminum cathode prepared by ion-beam-assisted deposition (IBAD). The electrical properties of the aluminum cathode, prepared by IBAD, on Ph-PPV have been investigated and compared to those by thermal evaporation. Energetic particles of Al assisted by an Ar+ ion may damage the organic material generating undesirable leakage current even though a thin Al buffer layer is applied to avoid Ar+-ion-induced damages. Substantial improvements of passivation characteristics were observed in IBAD device because the dense Al cathode inhibits the permeation of H2O and O2 into Ph-PPV film through pinhole defects, and thus retards dark spot growth. These results may be deduced from the highly packed structure that has small contact resistance between Al and Ph-PPV in ion-beam-assisted aluminum devices.