Keunsoo Lee

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.

Effect of plasma treatment on the gas sensor with single-walled carbon nanotube paste.

The effect of plasma treatment on the gas sensing properties of screen-printed single-walled carbon nanotube (SWCNT) pastes is reported. The gas sensors, using SWCNT pastes as a sensing material, were fabricated by photolithography and screen printing. The SWCNT pastes were deposited between interdigitated electrodes on heater membrane by screen printing. In order to functionalize the pastes, they were plasma treated using several gases which produce defects caused by reactive ion etching. The Ar and O(2) plasma-treated SWCNT pastes exhibited a large response to NO(2) exposure and the fluorinated gas, such as CF(4) and SF(6), plasma-treated SWCNT pastes exhibited a large response to NH(3) exposure.

Simultaneously enhanced device efficiency, stabilized chromaticity of organic light emitting diodes with lambertian emission characteristic by random convex lenses

An optical functional film applicable to various lighting devices is demonstrated in this study. The phase separation of two immiscible polymers in a common solvent was used to fabricate the film. In this paper, a self-organized lens-like structure is realized in this manner with optical OLED functional film. For an OLED, there are a few optical drawbacks, including light confinement or viewing angle distortion. By applying the optical film to an OLED, the angular spectra distortion resulting from the designed organic stack which produced the highest efficiency was successfully stabilized, simultaneously enhancing the efficiency of the OLED. We prove the effect of the film on the efficiency of OLEDs through an optical simulation. With the capability to overcome the main drawbacks of OLEDs, we contend that the proposed film can be applied to various lighting devices.

Outcoupling Efficiency Analysis of OLEDs Fabricated on a Wrinkled Substrate

Enhancement of outcoupling efficiency of OLEDs by using a buckled or wrinkled substrate has been recently reported by several groups, while an optical analysis for understanding the coupling between various modes in OLED devices has not been conducted. For various wrinkle shape, FDTD simulation was conducted to find the external quantum efficiency improved by the coupling into a radiation mode, and the power fraction of various modes existing in the OLED device was examined. Consequently, we found that the coupling between TE mode and surface plasmon mode was critically affecting the fraction of outcoupled mode. For confirming the simulation results with an experiment, the wrinkled OLED device was fabricated by incorporating a novel UV curable liquid prepolymer material. The experimental result corresponded well to the prediction from FDTD analysis.

Spontaneously Formed Wrinkled Substrates for Stretchable Electronics Using Intrinsically Rigid Materials

The preparation of a stretchable substrate was carried out by blending poly (ethylene glycol)-block-poly (propylene glycol)-block-poly(ethylene glycol) and polydimethylsiloxane (PDMS) to enhance the stretchability and the surface energy of the elastic film. The deposition of a thin parylene film on this modified PDMS led to the formation of a spontaneously wrinkled surface, resulting in a highly stretchable substrate. Rigid Au conductors were deposited on the resultant wrinkles substrates, and could be stretched up to 20% strain. In addition, stretchable pentacene thin-film transistors were successfully fabricated on the wrinkled substrate without the formation of stiff islands.

Field emission characteristics of patterned carbon nanotubes on metal substrate

Multi-walled carbon nanotubes were fabricated KOVAR substrates using electrophoretic deposition (EPD). The surface treatments of the substrate were performed by palladium coating process which is an activation procedure for electroless plating. Through the surface treatments, the roughness of the surface increased and cathodes which have high roughness factor showed better field emission characteristics compared to non-treated ones.

Effect of surface treatment of substrate on field emission characteristics of carbon nanotubes

The field emission characteristics of patterned carbon nanotubes whose average diameter is 16nm cathodes on substrates with surface treatment were investigated. The surface treatment of the substrate was performed by nickel electroless plating. Carbon nanotubes were patterned on the surface treated substrate with radius of 200μm through conventional photolithography process. It was revealed that the different surface morphologies of the substrates have influence on the field emission characteristics in this study. Through the surface treatment, the roughness of the surface increased and cathode with a high roughness factor showed better field emission characteristics compared to non-treated ones.