Won Hoe Koo

Polarization Conversion in Surface-Plasmon-Coupled Emission from Organic Light-Emitting Diodes Using Spontaneously Formed Buckles

A surface plasmon (SP) polariton is an electromagnetic wave propagating along the interface between a dielectric and a metal, and its electromagnetic fi eld exponentially decays into the surrounding media. Because the wavevector of the SP mode is larger than that of a photon of the same frequency in vacuum, the SP mode on a fl at surface is nonradiative and its energy dissipates as heat in the metal layer. Organic light-emitting diodes (OLEDs) consisting of organic layers and metallic electrodes inevitably accompany the SP mode and the power loss to the SP mode signifi cantly limits the device effi ciency, particularly in small-moleculebased OLEDs. [ 1 , 2 ] Although wavelength-scale periodic gratings have been introduced in OLED structures to convert the dissipated energy to the SP mode into useful light, reported structures are effective only at a specifi c wavelength and angle, satisfying the Bragg diffraction condition. [ 1–6 ] In our previous study, we demonstrated that quasiperiodic buckling structures with broad distribution and directional randomness can effectively enhance the light-extraction effi ciency without introducing spectral changes and directionality by outcoupling the waveguide modes. [ 7 ] In that study, however, we could not differentiate the outcoupling of transverse electric (TE) mode from that of the SP mode (transverse magnetic (TM) mode) by buckles because of the broad periodicity of the buckling structure and the similar propagation vectors of the TE and SP modes. In this study, we report that a buckling structure is remarkably effective at outcoupling the SP mode over all emission wavelengths and angles through an OLED structure with a thin indium–tin oxide (ITO) layer, by which any other waveguide modes are suppressed and only the SP mode is excited. Interestingly, we found that the diffraction of the SP mode by buckles produces TE-polarized light in addition to TM-polarized light, which indicates polarization conversion from TM (SP) to TE mode.

Ultraviolet-enduring performance of flexible pentacene TFTs with SnO2 encapsulation films

We report on the fabrication of flexible pentacene thin-film transistors (TFTs) encapsulated with an ultraviolet (UV)-protecting SnO 2 thin-film that has been prepared by ion-beam-assisted deposition (IBAD). We deposited thermally evaporated SnO 2 on a pentacene TFT as a buffer layer prior to the IBAD process. When a UV light of 254 nm wavelength was continuously illuminated onto our encapsulated device for periods of 5, 30, 60, and 120 min under a vacuum of 1 X 10 -6 Torr, its field mobility gradually degraded from 0.34 to 0.22 cm 2 /V s in 2 h while the other device without encapsulation rapidly degraded (its mobility was reduced to 0.021 cm 2 /V s during the same period). Our X-ray diffraction data indicates that the UV-induced-degradation of device characteristics is directly correlated to the degradation of pentacene crystallinity against UV radiation. We conclude that our IBAD SnO 2 encapsulation is a promising way to protect pentacene TFTs.

Passivation effects on the stability of pentacene thin-film transistors with SnO2 prepared by ion-beam-assisted deposition

The long-term stability of pentacene thin-film transistors (TFTs) encapsulated with a transparent SnO2 thin-film prepared by ion-beam-assisted deposition (IBAD) was investigated. When a buffer layer of 100-nm SnO2 film had been thermally deposited to reduce ion-induced damage prior to the IBAD process, our encapsulated organic thin-film transistors (OTFTs) showed somewhat degraded field-effect mobility of 0.5cm2∕V that was initially 0.62cm2∕Vs, while the OTFTs without a buffer layer showed a 60% reduction in field-effect mobility after the IBAD process. However, surprisingly, the mobility was sustained up to one month and then gradually degraded down to 0.35cm2∕Vs, which was still three times higher than that of the OTFT without any encapsulation layer after 100 days in air. The encapsulated OTFTs also exhibited superior on/off current ratio of over 105 to that of the unprotected devices (∼104), which was reduced from ∼106 before aging. Therefore, the enhanced long-term stability of our encapsulated OTFTs...

Spontaneously Buckled Microlens for Improving Outcoupled Organic Electroluminescence

We fabricate a buckling structure by thermally depositing aluminum on a polymer surface and cooling it to room temperature, and utilize the structure as a microlens aggregate for organic light emitting diodes. The aspect ratio of the lens is increased by a multiple deposition process. We demonstrate that the characteristics of the outcoupled emission of this device are improved from various perspectives such as efficiency, emission angle, and reduced spectral change. These improved characteristics originate from randomized double-curvatured lens structure with full surface coverage.

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.

Encapsulation of Organic Field-Effect Transistors with Highly Polarizable Transparent Amorphous Oxide

To enhance the stability of pentacene field-effect transistors, a highly polarizable amorphous oxide is used as a gas barrier due to the strong chemical interaction between permeating polar water molecules and an oxide. For a more reliable barrier function, in addition to chemical effects, a dense amorphous microstructure of a barrier is also preferred and achieved by ion-beam-assisted deposition (IBAD) at room temperature in this study. Our OTFTs encapsulated with IBAD SnO2 showed a degraded field-effect mobility of 0.5 cm2/(V s) after encapsulation. However, the field-effect mobility was surprisingly sustained up to one month and then gradually degraded down to 0.35 cm2/(V s) after 100 days in air, which was still three times higher than that of an OTFT without an encapsulation layer. The encapsulated OTFTs also exhibited a superior on/off current ratio of more than 105 to that of the unprotected device (~104), which was reduced from ~106 before aging. The effects of encapsulation on the electrical properties of OTFTs are discussed in terms of the physical and chemical properties of barrier films.

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.

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.

Simultaneous Extraction of Indium Tin Oxide/Organic and Substrate Waveguide Modes from Buckled Organic Light Emitting Diodes

We demonstrate that the devices with buckling as a grating can effectively extract the substrate mode as well as the indium tin oxide/organic mode even without any hemisphere lens or microlens array. This is achieved by multiple reflections of the substrate mode with the randomly buckled cathode layer through appropriate device dimensions.

Alignment Properties of Liquid Crystal Molecules with Negative Dielectric Anisotropy on Hydrogenated Silicon Carbide Films

We study the alignment properties of liquid crystals (LCs) with a negative dielectric anisotropy on a hydrogenated silicon carbide (SiC:H) film, which is an alternative alignment material. SiC:H layers align LC molecules with a negative dielectric anisotropy via ion beam (IB) irradiation and control the pretilt angle in a range from 90 to 75° depending on the change of IB irradiation angle. Also, when they are exposed to high temperatures for a long time, they show robust properties without degradation. We conclude that although the SiC:H alignment layers are not sensitive to IB irradiation compared with the SiC layers, they show a potential as alternative LC alignment layers for IB irradiation method.