Yi-Yeol Lyu

Silicon-Cored Anthracene Derivatives as Host Materials for Highly Efficient Blue Organic Light-Emitting Devices**

Blue host materials for organic light-emitting diodes (OLEDs) based on silicon-cored (tetraphenylsilane) anthracene derivatives are synthesized. These compounds, with a non-coplanar molecular structure, have high glass-transition temperatures and good amorphous-film-forming capabilities. When doped with a blue-fluorescent dopant, blue emission with high color purity and high efficiency, up to 7.5 cd A(-1) and 6.3%, is achieved.

Highly efficient red electrophosphorescence from a solution-processed zwitterionic cyclometalated iridium(III) complex

A mononuclear red-emitting zwitterionic iridium(III) complex, bis(1-(phenyl)isoquinolinato) iridium(III) (2,2′-bipyridine-3-ol-3′-olate), was synthesized and used as a triplet emitter doped in a polymeric host in solution-processed electrophosphorescent light-emitting diodes. We achieved a high luminous effieciency (LE) of 12.62cd∕A with fast response time in the device using the dopant unlike in the device using cationic irridium(III) phosphorescent materials. A hole transporting polymeric interlayer between the anode and the emitting layer improved the LE and decreased the exciton quenching at high electric fields.

New carbazole-based host material for low-voltage and highly efficient red phosphorescent organic light-emitting diodes

A new carbazole-based host material for red emitters, BBTC, was designed, synthesized and characterized with the phosphorescent organic light-emitting diodes (PHOLEDs). With the molecular design strategy of maintaining the large triplet energy and a good hole transporting ability of carbazole while increasing the morphological and electrochemical stability, the C3 and C6 positions of carbazole are blocked with biphenyl groups and the C9 position is terminated with a terphenyl group. Red PHOLEDs employing a conventional dopant material, bis(1-phenylisoquinoline)(acetylacetonate)iridium(III) ((piq)2Ir(acac)), in the emissive layer showed nearly 100% internal quantum efficiency (corresponding to the external quantum efficiency of 19.3%) with reduced efficiency roll-off. We attribute these results to the good electron–hole balance resulting from the good hole mobility of BBTC and low hole injection barrier from the hole transport layer to BBTC host in the emission layer. In addition, owing to its good hole transporting property, it can be utilized as a hole transport layer in organic light-emitting devices enabling low voltage operation of the devices.

Trap-level-engineered common red layer for fabricating red, green, and blue subpixels of full-color organic light-emitting diode displays.

We report a novel strategy to reduce one fine metal mask (FMM) step in a full-color organic light-emitting diode (OLED) display by introducing a common red layer (CRL) which replaces a hole transporting layer (HTL) with the same thickness of a red phosphorescent dye-doped layer. Because the dopant in the HTL acts as a hole trap, careful trap-level engineering is required for achieving efficient green and blue emission from the emitting layer while minimizing the red emission from the CRL. We investigated the characteristics of OLEDs depending on hole trap levels of the CRL with five different organic HTLs, and demonstrated efficient red, green and blue (RGB) emitting devices using the CRL. The electroluminescence spectrum of the devices with the CRL is nearly identical with those of the devices without the CRL. These results open up the possibility of simplified fabrication of practical full-color OLED displays with the reduced FMM steps, resulting in lower manufacturing cost.

Effects of hybrid hardener on properties of a composite bipolar plate for polymer electrolyte membrane fuel cells

AbstractThe effects of a hybrid hardener on the mechanical and electrical properties and corrosion resistance of conductive graphite based epoxy composites for the bipolar plate of polymer electrolyte membrane fuel cells (PEMFC) is investigated. Diaminodiphenylsulfone (DDS) is used as an additional hardener in order to enhance the physicochemical properties of graphite-based bisphenol type epoxy-cresol novolac phenol resin composites. The characteristics of the graphite composites, based on epoxy resin respectively with and without DDS, are compared in order to shed light on the curing behaviors of the epoxy/graphite composites. The tensile strength and conductivity of the epoxy/graphite composites with DDS are much higher than those without DDS. From the results of the scanning electron microscope and optical microscope analyses, the morphology of the epoxy/graphite composite with DDS was evaluated and is shown to be more compact than that without DDS. This is attributed to the strong crosslinking effect of DDS. This epoxy/graphite composite can be successfully molded as a real bipolar plate, with fairly good physicochemical properties through a compression molding process.

Tunable Phosphorescent Emission through Energy Transfer within Multilayer Thin Films Based on a Carbazole-Based Host and Ir(III)-Complex Guest System

A new method to tune both phosphorescence emission intensity and spectroscopic colors based on the alternatively structured host/guest multilayer thin films prepared by the spin-assisted LbL deposition is presented. Their emission characteristics are demonstrated with pairs of positively charged Ir-PEI complexes as guests and negatively charged CBZ-PAA as hosts. The phosphorescent emission of Ir-PEI complexes is enhanced by the energy transfer from the host to the guest and, additionally, this energy transfer can be finely tuned by the insertion of spacer layers between the phosphorescent donor and acceptor layers to vary the emission intensity as well as to render different emission colors.

Effect of UV Pretreatment on the Nanopore Formation within Organosilicate Thin Films

We have investigated the low-temperature cure process to realize nanoporous organosilicate thin films at temperature below 150°C by adding a small amount of photoacid generator (PAG) followed by UV irradiation. The Gemini surfactant, which decomposes in the temperature range from 170 to 420°C, was used as a pore-generating material (porogen) for organosilicate matrix. The UV pretreatment in the presence of PAG lowers the condensation temperature of poly(methyl silsesquioxane) matrix and leads to the fast matrix vitrification enabling the addition of increased amount of porogens. Because the full vitrification of the matrix (150°C) by UV pretreatment in the presence of PAG below the decomposition temperature of porogens (170°C) prevents the pore collapse, the porosity up to 35.5% was achieved with an average pore size of 3.4 nm, as measured from X-ray reflectivity as well as ellipsometric porosimetry. It is shown that both dielectric constant and refractive index continue to decrease to 2.0 and 1.26, respectively. The present experimental system demonstrates that porogens with low degradation temperature can be successfully incorporated to realize nanoporous films without pore collapse. Consequently, this process can widen the choice of porogens to prepare nanoporous films.

Improvement of Device Efficiency by Cross-Linkable Interlayer in Blue Polymer Light-Emitting Diodes

We report that high efficiency blue polymer light-emitting diodes obtained by adding a thin interlayer between a poly(3,4-ethylene dioxythiophene)-poly(styrene sulfonic acid) (PEDT:PSS) hole transporter and emissive polymer. The one of OLED hole transporters, N,N′-bis(4-methylphenyl)-N,N′-diphenyl-1,4′-phenylenediamine (PDA), was modified with cross-linkable trichlorosilyl groups and used for the interlayer material. The devices, with configuration of indium tin oxide (ITO)/PEDT:PSS (65 nm)/interlayer (10–20 nm)/emitting polymer layer (70 nm)/BaF2 (2 nm)/Ca (50 nm)/Al (300 nm), were fabricated by spin coating and thermal evaporation. In this device structure, the cross-linkable interlayer is more adherent and mechanically robust as well as impervious to spin coating of next emitting polymer layer and shows a better stability. In addition, the devices with the interlayer exhibit a higher luminescence and current efficiency than those in devices without the interlayer.