Showing papers by "Hoi Sing Kwok published in 2007"
TL;DR: In this article, nonemissive tetraphenylethene (TPE) 1 and diphenylated derivative 2 were induced to emit intensely by aggregate formation and they were turned on at ∼2.9 and ∼5V and emitted blue lights with maximum luminance of ∼1800 and ∼11000cd∕m2, respectively.
Abstract: Nonemissive tetraphenylethene (TPE) 1 and its diphenylated derivative 2 were induced to emit intensely by aggregate formation. Crystalline aggregates of the dyes emitted bluer lights than their amorphous counterparts. The emissions of the TPE dyes could be switched off and on continuously and reversibly by wetting and dewetting with solvent vapors, respectively, manifesting their ability to optically sense volatile organic compounds. The light-emitting diodes fabricated from 1 and 2 were turned on at ∼2.9 and ∼5V and emitted blue lights with maximum luminance of ∼1800 and ∼11000cd∕m2, respectively.
470 citations
TL;DR: 1,2-Diphenyl-3,4-bis(dipenylmethylene)-1-cyclobutene can be induced to emit efficiently by aggregate formation, with the crystalline aggregates emitting brighter, bluer lights than their amorphous counterparts.
250 citations
TL;DR: In this article, the synthesis and characterization of two new phosphorescent cationic iridium(III) cyclometalated diimine complexes with formula [Ir(L) 2 (N-N)] + (PF 6 - ) (HL=(9,9-diethyl-7-pyridinylfluoren-2-yl)diphenylamine); N-N = 4,4'-dimethyl-2,2'-bipyridine (1), 4,7-dimethyl -1,10-phenanthroline (
Abstract: The synthesis and characterization of two new phosphorescent cationic iridium(III) cyclometalated diimine complexes with formula [Ir(L) 2 (N-N)] + (PF 6 - ) (HL=(9,9-diethyl-7-pyridinylfluoren-2-yl)diphenylamine); N-N = 4,4'-dimethyl-2,2'-bipyridine (1), 4,7-dimethyl-1,10-phenanthroline (2)) are reported. Both complexes are coordinated by cyclometalated ligands consisting of hole-transporting diphenylamino (DPA)- and fluorene-based 2-phenylpyridine moieties. Structural information on these heteroleptic complexes has been obtained by using an X-ray diffraction study of complex 2. Complexes 1 and 2 are morphologically and thermally stable ionic solids and are good yellow phosphors at room temperature with relatively short lifetimes in both solution and solid phases. These robust iridium complexes can be thermally vacuum-sublimed and used as phosphorescent dyes for the fabrication of high-efficiency organic light-emitting diodes (OLEDs). These devices doped with 5 wt % 1 can produce efficient electrophosphorescence with a maximum brightness of up to 15 610 cd m -2 and a peak external quantum efficiency of ca. 7 % photons per electron that corresponds to a luminance efficiency of ca. 20 cd A -1 and a power efficiency of ca. 19 lm W -1 . These results show that charged iridium(III) materials are useful alternative electrophosphors for use in evaporated devices in order to realize highly efficient doped OLEDs.
152 citations
TL;DR: In this paper, a method to generate any liquid crystal pretilt angle is proposed and demonstrated based on deep ultraviolet photoalignment of a layer consisting of nanostructured domains of horizontal and vertical alignment polyimides.
Abstract: A method to generate any liquid crystal pretilt angle is proposed and demonstrated. This method is based on deep ultraviolet photoalignment of a layer consisting of nanostructured domains of horizontal and vertical alignment polyimides. Competition between the two types of domains produces the controlled pretilt angle. The obtained anchoring energy is reasonably large.
103 citations
TL;DR: In this paper, an efficient pure orange emitting organic light-emitting devices (OLEDs) based on a multi-component orange phosphorescent platinum complexes (Pt(L)(acac) were fabricated and characterized by spectroscopic and X-ray crystallographic methods.
73 citations
TL;DR: In this article, a new optical rewritable liquid-crystal alignment (ORW) display was developed to create a display and to demonstrate its maturity and potential, which has no electrodes and use polarizers as substrates.
Abstract: — A new optical rewritable (ORW) liquid-crystal-alignment technology has been developed to create a display and to demonstrate its maturity and potential. ORW displays have no electrodes and use polarizers as substrates. The display requires no photolithography on plastic. Its simple construction secures durability and low cost for mass production. The on-screen information is optically changed in a writing unit that consists of an LCD mask and an exposure source that is based on LEDs, low power, and low cost in comparison with Hg lamps or lasers. A high contrast image can be easily written, viewed, and rewritten through a polarizer, while the multi-stable gray-level image requires zero power to maintain the image. Reconfigurable LC alignment using ORW technology best suits plastic-card displays as well as for LC photonics and various one-mask processes of patterned LC-alignment applications.
57 citations
TL;DR: In this paper, a novel design methodology for LED-array-based projection displays has been developed by combining etendue limitation, system intensity, and efficiency requirements, a novel parameter space is proposed.
Abstract: A novel design methodology for LED-array-based projection displays has been developed. By combining etendue limitation, system intensity, and efficiency requirements, a novel parameter space is proposed. Using this parameter space, LED lens-array and compound parabolic concentrator (CPC)-array illumination systems have been designed. A 1000-lm LED light source is built. Based on these lens-array and CPC-array illuminators, several LED-based liquid crystal on silicon (LCOS) projection systems are suggested. Among them, a one-panel LCOS projection system is proposed and tested. The method discussed here should be useful in the design of LED-array illumination systems for projectors in general.
46 citations
TL;DR: In this article, a top-emitting white organic light emitting device (WOLED) using Al as the reflective bottom anode and LiF/Ca/Au as the semitransparent top cathode, has been fabricated, which showed a maximum external quantum efficiency of 91% and a maximum power efficiency of 128 lm/W.
Abstract: High-performance top-emitting white organic light-emitting device (WOLED), using Al as the reflective bottom anode and LiF/Ca/Au as the semitransparent top cathode, has been fabricated The WOLED consisted of three emissive layers with red, green, blue phosphorescent emitters, respectively Its microcavity structure was optimized by varying the thicknesses of the organic functional layers and the dielectric capping layer of the semitransparent top cathode using an optical model The WOLED showed a maximum external quantum efficiency of 91% and a maximum power efficiency of 128 lm/W As well, broadband emission spectra with very weak angle dependence were achieved The emission in normal direction has a Commission Internationale de L'Eclairage coordinates of (x,y)=(036,043) The characteristics of this top-emitting WOLED are competitive with those of bottom-emitting devices
33 citations
TL;DR: In this paper, high brightness and efficient white stacked organic light-emitting diodes have been fabricated by connecting individual blue and red emissive units with the anode-cathode layer (ACL) consisting of LiF (1nm)/Ca (25nm)/Ag (15nm).
33 citations
TL;DR: In this article, the performance of a C545T-doped Alq3 TOLED with Al and MoO3 can achieve a maximum current efficiency of 22 cd/A at 20 mA/cm2 at low brightness and 8.9 lm/W at 1000 cd/m2.
Abstract: Al- and Ag-based top-emitting organic light-emitting diodes (TOLED) are investigated. Both MoO3 and V2O5 have been used as hole-injection layer (HIL). The performance of the devices is significantly improved using the metal oxides as HIL. A C545T-doped Alq3 TOLED with Al and MoO3 can achieve a maximum current efficiency of 22 cd/A at 20 mA/cm2. The power efficiency is 20 lm/W at a low brightness and about 8.9 lm/W at 1000 cd/m2. For the Ag-based TOLED using V2O5 as HIL, very low operating voltages are obtained. For instance, 1000 cd/m2 can be obtained at a voltage of 4.7 V with a power efficiency of about 10 lm/W. From the analysis of the current–voltage characteristics of the single hole transport layer devices, it is believed that the hole injection from the metal anodes was greatly enhanced because of the lowering of the injection barrier induced by the metal oxides. The interface dipole theory was applied to the metal-metal oxide interface to explain the experimental observations.
30 citations
TL;DR: In this paper, the effect of annealing conditions on the performance of polycrystalline silicon thin-film transistors was systematically investigated, and the performance was dominated by the solution-based metal-induced crystallization (SMIC) process.
Abstract: The authors studied electrical characteristics of p-channel polycrystalline silicon thin-film transistors (TFT) fabricated by solution-based metal-induced crystallization (SMIC). In particular, the effect of annealing conditions was systematically investigated. The current-voltage characteristics show that the performance of TFTs was dominated by the SMIC process rather than the annealing temperature up to 630 degC. All the devices exhibited good uniformity after annealing. The effects of gate stress and self-heating stress were also studied. The device performance was not affected by both stresses for all devices, p-channel SMIC poly-Si TFT has high performance, good uniformity, and satisfactory reliability with a process temperature < 600 degC
TL;DR: In this article, an extremely low exposure dose (less than 50 mJ) is needed to induce an excellent planar alignment of PLC, which can be easily patterned using minimal numbers of masks and exposure steps.
Abstract: We demonstrate the excellent photoalignment capability of sulfuric azodyes for polymerizable liquid crystals (PLC) whose anisotropic films are extensively studied as basic elements of many passive optical devices (e.g., retardation films, polarizers, and color filters). These dyes exhibit a high affinity to various substrates and insolubility in PLC and organic solvents commonly used for their dilution. An extremely low exposure dose (less than 50 mJ) is needed to induce an excellent planar alignment of PLC. This alignment can be easily patterned using minimal numbers of masks and exposure steps. No sign of alignment deterioration is observed after photopolymerization. These imply that new photoalignment materials can be effectively used for the industrial production of optical films based on PLC. Particularly, they suit the continuous manufacturing process for such films.
TL;DR: In this paper, anodic alumina and pillar titania structures have been fabricated and used for the vertical alignment of nematic liquid crystals (LCs) for a pore entrance of more than 20 nm in width.
Abstract: Porous anodic alumina and pillar titania structures have been fabricated and used for the vertical alignment of nematic liquid crystals (LCs). Reliable LC penetration into pores was achieved for a pore entrance of more than 20 nm in width. The LC penetration into pillar structures has no restrictions owing to the absence of closed pore spaces. The aspect ratios of titania pillars ranged from 2 up 6, the maximum of which was limited by the mechanical stability of the fabricated pillars. The approaches developed are useful in fabricating LC projection displays.
01 May 2007
TL;DR: In this paper, the authors present and compare two single-panel LCOS projectors using LED lamps and show that the 0.59″ sequential-color LCOS microdisplay with integrated frame buffers can deliver 10.5 lm by a 4W red, green and blue LED.
Abstract: We present and compare two single-panel LCOS projectors using LED lamps. The 0.59″ sequential-color LCOS microdisplay with integrated frame buffers can deliver 10.5 lm by a 4W red, green and blue LED. The 0.59″ spatial-color LCOS microdisplay with white sub-pixels can deliver 14 lm by a 6W white LED.
TL;DR: In this article, a solution-based metal-induced crystallized (S-MIC) poly silicon (poly-Si) thin films with different domain sizes were post-annealed at 600, 700, 800, and 900 °C for 2 h each in this order.
Abstract: Solution-based metal-induced crystallized (S-MIC) poly silicon (poly-Si) thin films with different domain sizes were post-annealed at 600, 700, 800, and 900 °C for 2 h each in this order. Such annealing has increased the Hall mobility of the boron doped (p-type) and phosphorous doped (n-type) S-MIC poly-Si films at a rate of ∼0.03 cm2/V s/°C and >0.07 cm2/V s/°C, respectively. However, the carrier concentration in n-type S-MIC poly-Si decreased after each further post annealing treatment. We believe it is due to the residual nickels in poly-Si, which might induce additional dopant segregation besides the effect of grain boundaries. The performance of p-type S-MIC poly-Si TFT was greatly enhanced by post furnace annealing.
TL;DR: Nanoporous titania was obtained by anodic oxidation of magnetron deposited aluminum-titanium alloy films with subsequent alumina etching as discussed by the authors, which has the merit of low temperature operation.
Abstract: Nanoporous titania was obtained by anodic oxidation of magnetron deposited aluminum–titanium alloy films with subsequent alumina etching. This method has the merit of low temperature operation. The porosity of the titania film obtained can be controlled by the titanium concentration in the alloy. These nanoporous titania layers consist of vertical pores. They can provide anisotropic light scattering in a fabricated electrochromic cell. The vertical pore structure with high layer porosity yields a diffuse reflectance in the electrochromic cell of about 60%.
09 Feb 2007
TL;DR: In this article, a review of photo-alignment in liquid crystal (LC) devices is presented, and the advantages and drawbacks of various photo-aligning materials are analyzed from the point of view of practical applications.
Abstract: In this original review we briefly consider the novel azo-dye photo-aligning technology: history and the
perspectives for future applications in liquid crystal (LC) devices. The review describes the following items.
The brief introduction to the history of photoalignment and the basic classes of the photoaligning materials:
photosensitive polymers, azodyes and monolayers will follow with an introduction to the physical mechanisms of
the photo-aligning and photo-patterning technology. The advantages and drawbacks of various photo-aligning
materials are analyzed from the point of view of practical applications. The detail description of the diffusion
photo-aligning in azo-dye materials is provided. The characteristics of azo-dye photoaligning LC layers are
compared with those ones prepared by polyimide rubbing method.
The characterization of LC-surface interaction, such as pretilt angle and azimuthal anchoring energy is
discussed. The newly developed materials should have a controllable pretilt angle and anchoring energy, thus
enabling to develop a new generation of the LC devices: with low voltage, fast response and wide viewing angles.
The problem of image sticking can be considerably reduced due to the high anchoring energy of azo-dye
materials. Promising results, obtained for voltage holding Ratio (VHR) and residual DC voltage (RDC) in azodye
photo-aligning materials are also shown. This implies that the azo-dyes can be applied as aligning layers in
active matrix liquid crystal displays (AM-LCDs).
The possibility to use the photo-aligning layers for new types of liquid crystal displays such as FLCD, VAN-LCD,
p-BTN LCD, optical rewritable memory, microdisplays, and TN-LCD on plastic substrates is
demonstrated. The photoaligning of liquid crystal polymers (LCP) and the new classes of devices based on them
(optical retarders and compensators) is discussed. Special types of 3D LC alignment, LC alignment inside thin
micro tubes, and grating surface were concerned.
New superthin photo-aligned polarizers based on azo-dye layers were demonstrated. The polarizers are based
both on photo-aligned lyotropic LC as well as pure azo-dye layers. The polarizers can be patterned and put inside
LC display cell to serve as internal polarizers. Both color and neutral internal polarizers can be fabricated with the
thickness 0.3-0.7 mm. The electo-optic response of TN-LCD with internal polarizers is practically the same as in
case of usual external polarizers. New applications in transflective and 3D displays are envisaged.
Patent•
31 Dec 2007TL;DR: In this article, a liquid crystal material treated with a chiral dopant, which is aligned between two substrates with conductive layer on each substrate, is used for displaying videos with high contrast using fast transient response of liquid crystal materials.
Abstract: Methods and systems for displaying videos with high contrast using fast transient response of liquid crystal materials are disclosed. The system comprises a liquid crystal material treated with a chiral dopant, which is aligned between two substrates with conductive layer on each substrate. The system can be operated in an active or passive matrix mode display. The active matrix display can be a thin film transistor (TFT) or MOS transistor, whereas no transistors are used for the passive matrix mode display. A full color display, with high contrast, can be achieved by illuminating the transient liquid crystal material with a pulsed backlight.
01 May 2007
TL;DR: In this paper, a transflective liquid crystal display (LCD) with single cell gap consisting optically compensated bend (OCB) and low twist nematic modes has been studied.
Abstract: Transflective liquid crystal display (LCD) with single cell gap consisting optically compensated bend (OCB) and low twist nematic modes has been studied. Since no double cell gap structure is included in the configuration, the fabrication process is easy and only one more step UV exposure is needed during photoalignment process to produce domains. Based on this new configuration, a transflective LCD with high brightness and high contrast could be obtained.
01 May 2007
TL;DR: In this article, metal induced polycrystalline silicon (poly-Si) films composing of continuous zonal domain (CZD) in exactly same width have been obtained through pre-defined crystalline nucleation lines (CNL) on a nano-layer of silicon dioxide.
Abstract: Metal induced polycrystalline silicon (poly-Si) films composing of continuous zonal domain (CZD) in exactly same width have been obtained through pre-defined crystalline nucleation lines (CNL) on a nano-layer of silicon dioxide. Employing this technology, the impact of glass substrate shrinking on subsequent alignment process is determined. Additionally, the crystallization process is strictly controllable and the annealing time is shorter than one hour. P-channel thin film transistors (TFTs) built on CZD poly-Si exhibit high performance and high uniformity.
01 May 2007
TL;DR: The stability of hysteresis-free passively addressed FLC display with an inherent memorized gray scale is governed generally by relationships between dispersion and polar parts of FLC anchoring energy with solid substrates as mentioned in this paper.
Abstract: The steadiness of hysteresis-free passively addressed FLC display with an inherent memorized gray scale is governed generally by relationships between dispersion and polar parts of FLC anchoring energy with solid substrates.
TL;DR: In this paper, the authors obtained polycrystalline silicon (poly-Si) films with Solution-based Metal Induced Crystallization (S-MIC) of amorphous silicon (α-Si).
Abstract: Dish-like and wadding-like domain polycrystalline silicon (poly-Si) films were obtained with Solution-based Metal Induced Crystallization (S-MIC) of amorphous silicon (α-Si). The hall mobility of poly-Si was much higher in dish-like domain than in wadding-like domain. The field effect mobility of dish-like domain poly-Si TFTs and wadding-like poly-Si FTFs were 70–80 cm2/Vs and 40–50 cm2/Vs, respectively. P-type TFTs exhibited excellent reliability.
TL;DR: In this article, a twisted nematic (TN) liquid crystal display (LCD) was optimized for transflective liquid crystal displays (TFT) for high contrast ratio, wide viewing angles and fast response time.
Abstract: Twisted nematic (TN) liquid crystal display (LCD) is optimized for transflective liquid crystal displays This new configuration does not separate the pixel into two sub-pixels of transmissive and reflective mode Each pixel is partially transmissive and reflective with the help of a semi transparent mirror The electro-optical characteristics in the transmissive mode and reflective mode match each other, which indicate that the transmissive and reflective functions can be controlled using the same voltage in a transflective LCD TN LCD configuration exhibits a high contrast ratio, wide viewing angles, fast response time, and suitable for high quality transflective thin film transistor (TFT) LCDs
01 May 2007
TL;DR: In this article, the design and construction of retardation films with any desired dispersion properties by using conventional dispersive uniaxial retardation film has been described and demonstrated.
Abstract: We disclose here the design and construction of retardation films with any desired dispersion properties by using conventional dispersive uniaxial retardation films. In particular we demonstrate the design of retardation films which have constant retardation over the entire visible spectrum. They also have very large viewing angles.
01 May 2007
TL;DR: In this paper, a fast liquid crystal display mode is demonstrated, which is based on a stressed splay twist deformation and can be driven in passive matrix or active matrix manners.
Abstract: fast liquid crystal display mode is demonstrated. This liquid crystal mode is based on a stressed splay twist deformation and can be driven in passive matrix or active matrix manners. This new display mode can be used to achieve full color display effect, in conjunction with a pulsed red-green-blue backlight unit.
01 May 2007
TL;DR: In this paper, a twisted Nematic (TN) liquid crystal display is optimized for transflective liquid crystal displays, which does not need to separate the pixel into two sub-pixels of transmissive and reflective mode.
Abstract: Twisted Nematic (TN) liquid crystal display is optimized for transflective liquid crystal displays. This new configuration does not need to separate the pixel into two sub-pixels of transmissive and reflective mode. Each pixel is partially transmissive and reflective with the help of a semi transparent mirror. The electro-optical characteristics in the transmissive mode and reflective mode match each other, which indicates that the transmissive and reflective function could be controlled using the same voltage in a transflective LCD. This TN LCD configuration has a good contrast, viewing angle, response time and color coordination. Also it is very easy to fabricate and suitable for high quality transflective TFT-LCDs.