Kimiaki Nakamura

ZnS:TbOF Thin-Film Green Electroluminescent Panels Fabricated by Two-Target Sputtering

We developed a two-target rf-magnetron sputtering system to reproducibly fabricate bright ZnS:TbOF thin-film green electroluminescent panels. ZnS and Tb are sputtered from independent targets, and the substrate moves between these targets. This results in good controllability of both ZnS thickness and Tb doping concentration. To improve luminance, we have developed a TbOF target to form efficient TbOF complex centers, and used an Ar/He mixture to avoid sputtering damage to the ZnS host. We made a prototype 640 × 200 thin-film green electroluminescent panel to demonstrate large-area capability of this system. We also made a multicolor thin-film electroluminescence panel by arranging ZnS:TbOF and ZnS:Mn active layer segments laterally.

Wide‐viewing‐angle full‐color TFT‐LCDs

— Conventional TFT TN-LCDs display full-color images only within a narrow vertical viewing angle. Viewed obliquely, the display reverses or brightens. We studied these problems by calculating the light propagation through TN cells and concluded that these reversal phenomena are unavoidable in TN-LCDs. To widen the viewing angle, we propose a film-laid double-twisted-nematic display (FDTN), consisting of a conventional TN panel, a light-compensation panel, and two retardation films stacked together. The problem of black images appearing purple is solved using a clockwise-twisted nematic panel as an optical compensator. The viewing angle of the FDTN-LCD is about 1.5 times that of the conventional positive TN-LCD. The contrast ratio normal to the panel exceeds 100.

A full-color TFT-LCD with a polymer-dispersed structure

— A direct-view TFT-LCD with a wide viewing angle, which uses a polymer-dispersed liquid-crystal panel and a pair of polarizers, is proposed. The device has a vertical viewing-angle range of 120°, with a contrast ratio over 5:1 without display-image reversal. This is about three times the twisted-nematic displays viewing-angle range.

Direct-view polymer-dispersed LCD with crossed Nicols and uniaxial film

— By adding a uniaxial retardation film and using a chlorinated LC mixture, the performance of polymer-dispersed LCDs with crossed Nicols (PDN-LCDs) was improved. A wide viewing angle of more than 60° (contrast ratio >5) for all azimuth directions was obtained. In the horizontal and vertical viewing azimuth, the contrast ratio was over 30 even at an inclination viewing angle of 60°.

Reflector Parameters for High Reflectance and High Contrast Ratio with White-Taylor Reflective Display

We have calculated the appropriate reflector parameters for high white-state reflectivity. First, we studied the reflector with and without a refractive layer. With a refractive layer on the reflector, the appropriate average reflector slope is smaller than that without, because the incident light refracts at the air interface and the incident angle to the reflector is small. Next, we investigated the appropriate reflector for a White-Taylor display. We found that the optimum shape of the reflector for reflective display depends on the type of light source, i.e., a diffused light source or a point light source. The optimum average slope of the reflector (K) for the White-Taylor display is between 7° and 10°. With this value of K, we can achieve both high reflectivity and a high contrast ratio with both a diffused light source and a point light source.

A Full-Color Direct-View TFT-LCD with a Polymer-Dispersed Structure.

We propose a direct-view TFT-LCD with a wide viewing-angle which uses a polymer-dispersed liquid-crystal panel and a pair of polarizers. We call this device a polymer-dispersed LCD with crossed Nicols (PDN). The PDN allows panels to be made with low scattering levels.This suggests the use of fluorinated liquid crystals (F-LC) that show a high voltage-holding ratio in TN-LCDs. We found that F-LCs show good solubility with resins having low solubility parameters. A voltage-holding ratio of over 98% is shown, which is almost equal to the value for TN-LCD. The vertical viewing range exceeds ±60°, with a contrast ratio of over 5 and no image reversal. The range is almost three times the TN-LCD value. Molecular alignment is symmetrical and the variation of overall retardation is suppressed. The TFT-LCDs fabricated show a high contrast ratio and no imag reversal at any viewing angle.