Jun-Ho Yum

Y 3Al5 O 12 : Ce0.05 Phosphor Coatings on Gallium Nitride for White Light Emitting Diodes

White light was obtained by mixing blue light from the emission of a gallium nitride (GaN) chip and yellow light from the fluorescence of a Y 3 Al 5 O 12 :Ce 0.05 yellow phosphor. A uniform coating and an optimized thickness of yellow phosphor layer on a GaN chip were necessary for achieving an efficient white light emitting diode. The phosphor particles were coated on a GaN chip or indium tin oxide by several methods including the slurry method, the settling method, and electrophoretic deposition (EPD). The properties of the phosphor layers prepared by these methods were examined using scanning electron microscope and photoluminescence. The chromaticity of white light was dependent upon the thickness of the phosphor layer. The properties of the phosphor layer prepared by EPD such as packing density, thickness, and uniformity could be more easily controlled than those by the slurry and settling methods. Further high packing density of the EPD could compensate for the typical thick phosphor layer, allowing the thin layer to be fabricated. To overcome the weak adhesion strength of phosphor particles by the EPD, an aqueous solution including poly(vinyl alcohol) + ammonium dichromate was coated on the phosphor layer and cured by exposure to ultraviolet light.

Improved performance of a dye-sensitized solar cell using a TiO2/ZnO/Eosin Y electrode

TiO2/ZnO/Eosin Y structure films were prepared by a one-step cathodic electrodeposition method and used as a photoanode in a dye-sensitized solar cell (DSSC). Using this TiO2/ZnO/Eosin Y electrode in DSSC, the degradation of the cell with time was reduced and ISC, VOC and fill factor values were increased. The use of a thin ZnO layer, permitted the formation of an energy barrier at the electrode/electrolyte interface, thus reducing recombination rate and improving cell performance. In addition, the adsorbed dye molecules prepared by one-step cathodic electrodeposition with ZnO were very stable compared with that prepared by conventional immersing method, as evidenced by UV/vis absorption spectroscopy measurements.

Comparison of Y3Al5O12:Ce0.05 phosphor coating methods for white light emitting diode on gallium nitride

White light was obtained by mixing blue light from the emission of a GaN chip and a yellow light by the fluorescence of a Y3Al5O12:Ce0.05 yellow phosphor. A uniform coating of yellow phosphor on a GaN chip and an optimized thickness of a phosphor layer were necessary for achieving efficient white LED. Several methods for coating yellow phosphor particles such as the slurry method, the settling method, the electrophoretic deposition (EPD), and the modified EPD were examined for preparing the phosphor layer. The properties of the phosphor layer prepared by these methods were examined using SEM, XRD, and photoluminescence. The intensity of white light and the harmony between blue light and yellow light were dependent on the thickness of the phosphor layer. The properties of the phosphor layer made by the EPD such as packing density, thickness and uniformity could be more easily controlled than the slurry and settling methods. Further weak adhesion strength of phosphor particles by the EPD could be overcome via the use of a UV curable PVA+ADC layer on the phosphor layer in the EPD.

Full Color Screen by EPD Combined with Photolithography for Flat Panel Displays

Electrophoretic deposition (EPD) was employed for applying a phosphor coating for use in the fabrication of a full color screen consisting of red (R), green (G), and blue (B) phosphors. Patterned phosphor layers with a high packing density and uniformity can be fabricated by EPD combined with photolithography. For this process, R, G, and B phosphors were deposited on a patterned indium tin oxide glass by means of a commercial positive photoresist based on novolac resin and naphthoquinone diazide. The phosphor can be deposited on the patterned conductive substrate, but not on the surface of the photoresist film, because the photoresist has insulating properties. Conventional photolithographic conditions could not be used for this deposition because the commercial photoresist was soluble in the alcoholic solvent, which was used during the phosphor deposition. Instead, an insoluble photoresist was obtained by optimizing the baking temperature. Patterned color screens were studied using scanning electron microscopy and photoluminescence spectroscopy. The adhesion strength of the patterned color screens was substantially improved by the use of a coating of aqueous solution including polyvinyl alcohol and ammonium dichromate, followed by curing under UV light.

The Effect of Ar/O2 Ratio on Electrochromic Response Time of Ni Oxides Grown Using an RF Sputtering System

The effect of Ar:O2 ratio on the electrochromic properties and the response time of NiO grown by RF sputtering were investigated by in situ transmittance measurements with continuous potential cycling and pulse potential cycling. The transmittance difference, coloration efficiency, memory effect, and cycling stability were all found to be independent of the Ar:O2 ratio. However, the transmittance of the as-deposited NiO as well as the response time were significantly affected. This may be attributed to the excess of oxygen occupied interstitial sites in the sputtered NiO that could result in the generation of Ni3+ ions and interference with proton intercalation/deintercalation.

Adhesion Improvement of Phosphor Layer by Combining Electrophoretic Deposition and UV Curing

Electrophoretic deposition of a phosphor layer was examined in order to improve the adhesion strength of phosphor particles using (Y 0.964 Eu 0.036 ) 2 O 3 . A considerable level of improvement in adhesion strength between phosphor particles (and substrate) was obtained when an ultraviolet (UV) curing process was used instead of water inclusion and/or postannealing. An aqueous solution of polyvinyl alcohol and ammonium dichromate was coated by spraying onto the electrophoretically deposited phosphor layer followed by curing under UV light for cross-linking. Based upon photoluminescence and scanning electron microscope studies, little optical and structural changes in the phosphor layer were found as a result of optimized amount of the curing agents and UV exposure.

CdSe Quantum Dots Sensitized TiO 2 Electrodes for Photovoltaic Cells

The electronic properties of quantum dots can be tuned by changing the size of particles without any change in their chemical composition. CdSe quantum dots, the sizes of which were controlled by changing the concentrations of Cd and Se precursors, were adsorbed on photoelectrodes and used as sensitizers for photovoltaic cells. For applications of CdSe quantum dot as sensitizers, films on conducting glass were employed in a sandwich-type cell that incorporated a platinum-coated conductive glass and an electrolyte consisting of an redox. The fill factor (FF) and efficiency for energy conversion () of the photovoltaic cell was 62 % and 0.32 %, respectively.

Fabrication of full color phosphor screen by electrophoretic deposition combined with photolithography

Electrophoretic deposition was employed to apply a phosphor coating, for use in the fabrication of a full color screen consisting of red (R), green (G), and blue (B) phosphors. Patterned phosphor layers can be fabricated by the electrophoretic deposition, combined with photolithography. For this process, R,G, and B phosphors were deposited on a patterned ITO glass by means of photoresist. The phosphor could be deposited, not on the photoresist surface, but on the patterned conductive substrate, since the photoresist has insulating properties. Conventional photolithographic conditions cannot be used for electrophoretic deposition due to the fact that the photoresist is soluble in the alcoholic solvent, which is used after baking. A suspension of the insoluble photoresist was obtained by optimizing the baking temperature. Patterned color screens, prepared by electrophoretic deposition combined with photolithography were studied using SEM and photo luminescence spectroscopy.