Byoungchoo Park

THERMAL AND OPTICAL STABILITIES OF PHOTOISOMERIZABLE POLYIMIDE LAYERS FOR NEMATIC LIQUID CRYSTAL ALIGNMENTS

We have investigated the thermal and optical stabilities of the photoalignment layers for nematic liquid crystals (LCs). For the photoalignment layers, three kinds of photoisomerizable polymer systems were studied; a polyamic acid doped with azobenzene molecules, a side-chain-substituted polyamic acid with azobenzene units, and a main-chain-substituted polyamic acid with azobenzene units. Photoinduced anisotropy was produced by illumination with linearly polarized near-UV light from a mercury lamp, and evaluated by measuring polarized UV absorption spectra and optical birefringence. The nematic LC-alignment properties were investigated using the photoalignment layers processed under various thermal and optical conditions. It was observed that the main-chain polyimide system exhibited good unidirectional LC alignment and excellent thermal (300°C for 1 h) and optical stabilities. Moreover, we suggest a new procedure, in which the photoillumination process is carried out before the thermal imidization process, to achieve a stable photoalignment layer.

PHOTONIC QUANTUM RING

We report a quantum ringlike toroidal cavity naturally formed in a vertical-cavity-like active microdisk plane due to Rayleighs band of whispering gallery modes. The

Enhanced out-coupling factor of microcavity organic light-emitting devices with irregular microlens array

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Advantages of Highly Ordered Polymer-Dyes for Lasing in Chiral Nematic Liquid Crystals

-dependent redshift and a square-law property of microampere-range threshold currents down to

Collective Molecular Motion during V-Shaped Switching in a Smectic Liquid Crystal.

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Photovoltaic characteristics of polymer solar cells fabricated by pre-metered coating process

are consistent with a photonic quantum wire view, due to whispering gallery mode-induced dimensional reduction.

Organic light-emitting devices fabricated using a premetered coating process

We studied microcavity organic light-emitting devices with a microlens system. A microcavity for organic light-emitting devices (OLED) was fabricated by stacks of SiO(2) and SiN(x) layers and a metal cathode together with the microlens array. Electroluminescence of the devices showed that color variation under the viewing angle due to the microcavity is suppressed remarkably by microlens arrays, which makes the use of devices acceptable in many applications. It was also demonstrated that the external out-coupling factor of the devise increases by a factor of ~1.8 with wide viewing angles compared to conventional OLEDs.

Holographically generated twisted nematic liquid crystal gratings

We have studied lasing in dye-doped chiral nematic liquid crystals (N*LCs). To demonstrate the advantages of using a polymer dye, that is highly aligned along the local director of N*LCs, over commercial small molecule dyes such as 4-(dicyanomethylene)-2-methyl-6-(4-dimethylaminostryl)-4H-pyran (DCM), comparative studies of the fluorescence spectra, lasing conditions and order parameters have been made using polymer-dye doped N*LC and DCM-doped N*LC cells. Right- and left-circularly polarized fluorescence spectra for both dyes were accurately simulated by taking account of their density of modes and order parameters. These results indicate that the greater alignment afforded by the polymer dye in N*LCs provides ideal conditions for lasing.

Solution processable single layer organic light-emitting devices with a single small molecular ionic iridium compound

The molecular motion during V-shaped switching in a homogeneously aligned smectic C*-like liquid crystal (LC) cell has been investigated by measuring the effective optical anisotropy Δneff, apparent tilt angle θapp, switching current, and second-harmonic generation and comparing them with the simulated results based on two extreme models, i.e., random model and collective model, where molecules switch randomly and collectively, respectively. The comparison revealed that the collective switching motion of LC molecules is more reasonable than the random motion. Moreover, it was also confirmed that the observed infrared absorption anisotropy of the phenyl stretching mode due to LC molecular distributions strongly supports the collective model. From these results, it was demonstrated that LC molecules do not switch randomly but all the LC molecules rotate collectively on a cone under the driving field.

Ferroelectricity in the Lyotropic Cholesteric Phase of Poly L-Glutamate

We present the results of a study of flat and uniform poly(3-hexylthiophene):methanofullerene bulk-heterojunction photovoltaic (PV) layers that were produced by a simple pre-metered horizontal-dipping process for the fabrication of polymer solar cells (PSCs). It is shown that this process can produce high quality and thin films by utilizing the downstream meniscus of the solution, which can be controlled by adjusting experimental parameters of the gap height and the carrying speed. It is also shown that the produced PV film exhibits high power conversion efficiency of ca. 4.2% with a large active area. It was demonstrated that this pre-metered process for solution coating may be promising for achieving highly efficient, reliable, and large-area PSCs.