Mu-Hyun Kim

Carrier trapping and efficient recombination of electrophosphorescent device with stepwise doping profile

We have presented a physical concept for enhancing efficiency and lifetime of doped electrophosphorescent organic light-emitting devices. In order to provide a control parameter for higher device performance, a stepwise doping concentration profile at the emission layer was prepared. A more than 30% improvement of power efficiency was obtained for green electrophosphorescent device with a higher doping ratio at the emission layer-hole transport layer interface. We explained the carrier trapping and transport mechanism with direct recombination of an exciton in an iridium-based dopant system. When compared to green device, phosphorescent red devices showed a more significant charge trapping effect at low doping concentration, which is responsible for shifting the recombination zone far from the emission layer-hole transport layer interface. Therefore, charge trapping by doping control in an emission layer could be utilized for a charge-balancing technique for the confinement of a triplet exciton.

Formation and optical properties of InAs/GaAs quantum dots for applications as infrared photodetectors operating at room temperature

The formation and optical properties of lateral quantum-dot infrared photodetector (QDIP) structures utilizing uniform-sized InAs QDs obtained by controlling the intensity of the reflection high-energy diffraction pattern and the As/In flux ratio were investigated. The thickness of the pseudomorphic, strained InAs wetting layer increased slightly with decreasing As/In flux ratio. The characteristic peaks corresponding to the interband and the intersubband transitions were observed, respectively, in the photoluminescence and Fourier transform infrared spectra. The absorption spectrum showed an absorption peak corresponding to the intersubband transition at around 10 μm. These results indicate that InAs/GaAs QDs obtained by controlling growth conditions hold promise for potential applications in QDIP devices operating at room temperature.

Ultra-thin fluoropolymer buffer layer as an anode stabilizer of organic light emitting devices

We have investigated the effect of thin fluoro-acrylic polymer as an anode stabilizer on the lifetime of an organic light emitting device (OLED). Surface chemical properties of commercial fluoropolymer, FC-722 (Fluorad™ of 3M), on indium–tin oxide (ITO) were characterized by x-ray photoemission spectroscopy. An OLED with 1 nm thick fluoropolymeric film showed identical brightness and efficiency behaviour and improved operational stability compared with the reference device with UV-O3 treated ITO. The improvement in the lifetime was accompanied by the suppression of the voltage increase at the initial stage of constant-current driving, which can be attributed to the action of the FC-722 layer by smoothing the ITO surface. Fluoropolymer coating, therefore, improves the lifetime of the small molecular OLED by the simple and reliable anode-stabilizing process.

ZnSSe epilayers with extremely low defect density by the growth-temperature optimization

In order to realize ZnSSe/GaAs heteroepitaxial layers with lower defect density, we have introduced molecular beam epitaxy growth by a three-step temperature correction which takes account of the cell radiation and surface emissivity effects superposed on the Fabry - Perot interference oscillation in the pyrometric temperature. The optimum correction rates of growth temperature are determined by the analysis of double-crystal x-ray rocking curves, and an empirical profile for the control temperature as a function of growth time is proposed for the optimum ZnSSe growth. While the etch pit density is in normal molecular beam epitaxy growth, an etch pit density of , which is the lowest value known so far, is realized in ZnSSe/GaAs(100) epilayers grown by using the linearly corrected profile.

Current development status and issues of AMOLED toward an invaluable display

Organic Light-Emitting Device (OLED) has been attracted much interests to realize a flat panel display owing to its promising characteristics. Active matrix OLED can make it possible to make big displays with the good image qualities and create new values and concepts, such as a flexible display, transparent display, and so on. However, in spite of the big potential of the AMOLED, it is still needed to improve lifetime, efficiency and productivity for large size displays. In this paper, we present current development status and issues of the AMOLED toward an ultimate champion among flat panel displays.