Showing papers by "Hoi Sing Kwok published in 2005"
TL;DR: The optical band gap of ZnO thin films deposited on fused quartz by metal-organic chemical-vapor deposition was studied in this article, where X-ray diffraction measurements showed that samples deposited at low temperatures (<450°C) consisted of amorphous and crystalline phases.
Abstract: The optical band gap of ZnO thin films deposited on fused quartz by metal-organic chemical-vapor deposition was studied. The optical band gap of as-grown ZnO blueshifted from 3.13to4.06eV as the growth temperature decreased from 500to200°C. After annealing, the optical band gap shifted back to the single-crystal value. All the ZnO thin films studied show strong band-edge photoluminescence. X-ray diffraction measurements showed that samples deposited at low temperatures (<450°C) consisted of amorphous and crystalline phases. The redshift of the optical band gap back to the original position after annealing was strong evidence that the blueshift was due to an amorphous phase. The unshifted photoluminescence spectra indicated that the luminescence was due to the crystalline phase of ZnO, which was in the form of nanocrystals embedded in the amorphous phase.
662 citations
TL;DR: It is revealed that 1(3,4) was a sensitive chemosensor capable of optically discriminating nitroaromatic regioisomers of p-, o-, and m-nitroanilines.
Abstract: We synthesized a group of silole regioisomers 1x,y, whose photoluminescence varied dramatically with its regiostructure. By internally hindering the intramolecular rotation, we succeeded in creating a novel silole (13,4) that is strongly luminescent in solutions and whose fluorescence quantum yield in acetone is as high as 83%. We revealed that 13,4 was a sensitive chemosensor capable of optically discriminating nitroaromatic regioisomers of p-, o-, and m-nitroanilines. Against general belief, crystal formation of 12,4 blue-shifted its emission color and boosted its emission efficiency. The light-emitting diode based on the crystal of 12,4 emitted a strong blue light (464 nm) in a high current efficiency (5.86 cd/A).
342 citations
TL;DR: A novel class of luminescent cyclometalated gold(III) alkynyl complexes has been demonstrated to possess EL properties and has been employed in the roles of electrophosphorescent emitters or dopants of organic light-emitting diodes (OLEDs) with high brightness and efficiency.
243 citations
TL;DR: In this article, a theoretical model based on electromagnetic wave propagation and geometric ray tracing is developed to simulate the enhancement effects and optimize the structure parameters of the lens pattern of organic light-emitting devices (OLEDs).
Abstract: Microlens arrays are introduced on glass substrates to improve the out-coupling efficiency of organic light-emitting devices (OLEDs). The microlenses suppress waveguiding loss in the substrate. A theoretical model, based on electromagnetic wave propagation and geometric ray tracing, is developed to simulate the enhancement effects and optimize the structure parameters of the lens pattern. A simple soft-lithography approach is employed to fabricate the microlens array on glass substrates. With the use of an optimized lens pattern, an increase of over 85% in the coupling efficiency of the OLED is expected theoretically. An increase of 70% in the coupling efficiency is achieved experimentally, without detrimental effect to the electrical performance of the OLED.
111 citations
TL;DR: In this paper, azo-dyes-based photo alignment method was proposed to provide a controllable pretilt angle and strong anchoring energy of the liquid crystal cell, as well as high thermal and UV stability.
Abstract: Liquid crystal photo‐alignment using azo‐dyes is reviewed. This alignment method is very different from previously reported ones, such as photo‐crosslinking, photo degradation and photo‐isomerization. It will be shown that this photo‐aligning method can provide a controllable pretilt angle and strong anchoring energy of the liquid crystal cell, as well as high thermal and UV stability. The application of this method to the alignment and fabrication of various types of liquid crystal displays is also discussed.
93 citations
TL;DR: Zhu et al. as discussed by the authors showed that by depositing an ultrathin vanadium pentoxide (V2O5) layer on the p-Si anode, the performance of the OLED can be greatly improved.
Abstract: Recently, polycrystalline silicon (p-Si) has been demonstrated to be an efficient anode for organic light-emitting diode (OLED) [X. L. Zhu, J. X. Sun, H. J. Peng, Z. G. Meng, M. Wong, and H. S. Kwok, Appl. Phys. Lett. 87, 083504 (2005)]. In this letter, we show that, by depositing an ultrathin vanadium pentoxide (V2O5) layer on the p-Si anode, the performance of the OLED can be greatly improved. Detailed x-ray photoelectron spectroscopy study shows that strong band bending occurs at the p-Si∕V2O5 interface, leading to much stronger hole injection. This modified p-Si anode can be integrated with the active p-Si layer of thin-film transistors in active-matrix OLED displays.
76 citations
TL;DR: In this paper, effective intermediate electrode layers comprising of LiF(1nm) ∕Ca(25nm)∕Ag(15nm) or LiF∕Al∕Au(15 nm) were studied in stacked organic light-emitting devices (OLEDs).
Abstract: Effective intermediate electrode layers comprising of LiF(1nm)∕Ca(25nm)∕Ag(15nm) or LiF(1nm)∕Al(3nm)∕Au(15nm) were studied in stacked organic light-emitting devices (OLEDs). Stacked OLEDs with two identical emissive units consisting of NPB∕Alq3: C545T/BCP exhibited superior luminous efficiency-current density characteristics over conventional single-unit devices. At 20mA∕cm2, the luminous efficiency of the stacked OLEDs using the intermediate layers of LiF∕Ca∕Ag and LiF∕Al∕Au were about 19.6cd∕A and 17.5cd∕A, respectively, almost doubling that of the corresponding control devices, as expected.
70 citations
TL;DR: Appending carbazolyl groups to a hexaphenylsilole core yielded thermally and morphologically stablecarbazolylsiloles; the silole carrying two carbazolel peripheral groups showed photovoltaic activity.
66 citations
TL;DR: In this article, a semitransparent silver layer was used as an anode for organic light-emitting devices (OLEDs) to enhance hole injection into the hole-transport layer.
Abstract: A semitransparent silver layer is investigated as the anode for organic light-emitting devices (OLEDs). By pretreating the silver layer in a CF4 plasma, hole injection into the hole-transport layer is greatly enhanced. A bottom-emitting OLED using the modified, semitransparent silver anode, demonstrates improved current density-voltage characteristics and a 20% higher external quantum efficiency, compared to a conventional OLED using indium tin oxide as an anode. The superior optical characteristics are attributed to a higher outcoupling efficiency in the microcavity structure.
50 citations
TL;DR: In this paper, an optical switch structure which combines a planar lightwave circuit and liquid crystal is proposed and experimentally demonstrated, which is realized by inducing total internal reflection through controlling the refractive index of the liquid crystal.
Abstract: An optical switch structure which combines a planar lightwave circuit and liquid crystal is proposed and experimentally demonstrated. The switching is realized by inducing total internal reflection through controlling the refractive index of the liquid crystal. The principle and the design of the integrated optical switch are described. Experimental results suggest that anchoring of the liquid crystal molecules on nonplanar structures is very crucial in affecting the switching performance.
49 citations
TL;DR: In this paper, the photo-induced orientational order in various photo-aligning materials and in azo-dye layers in particular is discussed. And the application of this method to the alignment and fabrication of various types of LC displays is also discussed.
Abstract: Liquid crystal (LC) photo-alignment using azo-dyes is reviewed. This alignment method is very different from previously reported ones, which are due to mechanisms such as photo-crosslinking, photo degradation, and photo-isomerization. We present the basic physical mechanisms of the photo-induced orientational order in various photo-aligning materials and in azo-dye layers in particular. This method is based on rotational diffusion in a potential created by the light field as well as intermolecular forces. It will be shown that this photo-aligning method can provide a controllable pretilt angle and strong anchoring energy of the LC cell, as well as having high thermal and ultraviolet (UV) stability. The application of this method to the alignment and fabrication of various types of LC displays is also discussed.
TL;DR: The present finding provides useful guidelines for designing inhomogeneous alignment surfaces for variable LC pretilt control as the area fraction of the homeotropic region increases from 0 to 1.
Abstract: We consider the pretilt alignment of a nematic liquid crystal (LC) on inhomogeneous surface patterns comprising patches of homeotropic alignment domains in a matrix favoring homogeneous alignment, or vice versa. We found that the resultant LC pretilt generally increases continuously from the homogeneous limit to the homeotropic limit as the area fraction of the homeotropic region increases from 0 to 1. For any given homeotropic area fraction, the variations are qualitatively different depending on how the distance between adjacent patches compares to the extrapolation lengths of the anchoring domains. Our results agree with those previously found in stripe patterns. The present finding provides useful guidelines for designing inhomogeneous alignment surfaces for variable LC pretilt control.
TL;DR: This work used this method to investigate the polar anchoring properties of photoaligning material before and after illumination and for LC structures with a high pretilt angle.
Abstract: We propose a simple method for the determination of liquid-crystal (LC) polar anchoring energy by electrical measurements. The basic idea of this method is a two-channel scheme for capacitance measurements. The first channel uses one cell with a planar LC cell, while the second a LC cell with vertical alignment. One of the LC cells can have a high pretilt angle. The proposed method allows investigating anchoring properties of both planar and vertical aligned LC materials. Simultaneous measurements of the two cells compensate all volume effects in LC bulk and provide a good opportunity to study directly the LC-surface interaction. The method can be applied for LC cells, which do not have uniform azimuthal orientation. We used this method to investigate the polar anchoring properties of photoaligning material before and after illumination and for LC structures with a high pretilt angle.
TL;DR: A group of new poly(1-phenyl-1-undecyne)s with different mesogenic and chromophoric pendant groups or side chains were successfully synthesized and the structural variations were found to greatly affect the mesomorphic and luminescent properties of the polymers as discussed by the authors.
Abstract: A group of new poly(1-phenyl-1-undecyne)s with different mesogenic and chromophoric pendant groups or side chains were successfully synthesized and the structural variations were found to greatly affect the mesomorphic and luminescent properties of the polymers. The 1-phenyl-1-undecyne monomers (C6H5)C⋮C(CH2)9OCOR with R = C6H4−C6H10−C5H11 (1), C6H4−OCO−C6H4−OC6H13 (2), and C6H4−C⋮C−C6H4−OC7H15 (3) were prepared by simple esterification and/or coupling reactions. The polymerizations of 1−3 were effected by WCl6−Ph4Sn in toluene at 60−80 °C, giving polymers with high molecular weights in good isolation yields. The structures and properties of the polymers were characterized and evaluated by GPC, IR, NMR, TGA, DSC, POM, XRD, UV, and PL analyses. The polymerizations of 1 and 2 yield linear poly(1-phenyl-1-alkyne)s P1 and P2, respectively, while that of 3 gives a nonlinear macromolecule with its poly(1-phenyl-1-alkyne) main chain cross-linked by the oligo(diphenylacetylene) side chains resulted from the parti...
TL;DR: A three-step photoexposure method was developed to solve problems with alignment disturbances on reflective silicon panels in this article, and the azimuthal and polar anchoring energies of photoalignment layers produced by this method were high and comparable with that of rubbed polyimide layers.
Abstract: We applied photoalignment technology to liquid-crystal-on-silicon microdisplays and characterized their performance. A three-step photoexposure method was developed to solve problems with alignment disturbances on reflective silicon panels. The azimuthal and polar anchoring energies of photoalignment layers produced by this method were high and comparable with that of rubbed polyimide layers. We assembled both the rubbed and photoaligned microdisplays on a MTN 90° configuration for characterization and comparison. These two kinds of silicon microdisplays had good and comparable characteristics in terms of contrast, response time, voltage holding ratio and residual DC charge. The rubbed microdisplays were more uniform, which we attributed to a more mature fabrication technology, while the photoaligned microdisplays had far fewer defects due to the noncontact nature of the photoalignment technology.
TL;DR: In this article, a 3D optical modeling of small color pixels in color filter liquid-crystal-on-silicon (CF-LCOS) microdisplays is presented.
Abstract: We have developed a three-dimensional (3D) optical modeling of small color pixels in color filter liquid-crystal-on-silicon (CF-LCOS) microdisplays. The 3D optical modeling includes a LC electromechanical analysis of color LC cells, a calculation of optical reflectance using the extended Jones matrix, and a standard RGB (sRGB) representation of the optical reflectance in the pixel array. The simulated optical reflectance agreed well with the experiments. With this 3D optical analysis as a tool, the relation of the lateral color fringing field with pixel size and thickness of color filter were studied. Minimizations of the fringing field in the CF-LCOS microdisplays were obtained by pixel arrangement, rubbing direction, and LC mode. The color purity of the CF-LCOS microdisplays could attain 63% National Television System Committee (NTSC) level for a typical pixel size of 15μm. With an optimized LC mode, the color purity could still be maintained at 54% NTSC level even when the pixel size was reduced to 9μm...
TL;DR: In this article, the alignment properties of the azo dye photo-alignment material SD-1/SDA-2 on plastic substrates are investigated, and a reflective 6-digit flexible passive matrix driven TN-LCD for smart card applications showing excellent electro-optical properties is demonstrated.
Abstract: The alignment properties of the azo dye photo-alignment material SD-1/SDA-2 on plastic substrates are investigated. Excellent alignment with high anchoring energy can be achieved with a polarized UV dose less than 1.0 J/cm2. A reflective 6-digit flexible passive matrix driven TN-LCD for smart card applications showing excellent electro-optical properties is demonstrated.
TL;DR: In this paper, the application of boron-doped polycrystalline silicon (p-Si) as a semi-transparent anode in making different color OLEDs is presented.
Abstract: Polycrystalline silicon (p-Si) is a good material for the construction of thin-film transistors (TFT). It is used for fabricating active-matrix organic light-emitting diode (AMOLED) displays. In this letter, we propose and demonstrate the application of boron-doped p-Si as a semi-transparent anode in making different color OLEDs. Without removing the ultrathin native oxide on the p-Si surface and employing p-doped hole transport layer to enhance holes injection, these OLEDs show comparable or even better performance to conventional OLEDs which use ITO as anodes. The present technique has the advantage of less masking steps in making AMOLED.
01 May 2005
TL;DR: In this article, a nano-structured alignment layer is proposed and demonstrated to produce pretilt angles of any value from 0° to 90°, which is robust, stable, reliable and highly manufacturable.
Abstract: We propose and demonstrate a nano-structured alignment layer which is capable of producing pretilt angles of any value from 0° to 90°. This alignment layer is robust, stable, reliable and highly manufacturable. We utilized these alignment layers to obtain ∼45° pretilt angles to make a pi-cell which is in the bend alignment with no bias voltage. This No-bias bend (NBB) cell has a total response time of <2ms. We have also applied this method to fabricate a bistable bend-splay display successfully.
01 May 2005
TL;DR: In this article, a single-panel WXGA color projector based on color filter microdisplay technology was demonstrated, where the color filters are aligned on a separate glass plate and aligned to pixels on a TFT plate.
Abstract: a color liquid-crystal-on-silicon microdisplay that integrates color filters on silicon for colors. Fabrication parameters, color purity and light stability of this breed of microdisplays were characterized. A single-panel WXGA color projector based on this color filter microdisplay technology was demonstrated. 1. Introductionn-silicon (LCOS) microdisplay is an advanced display that integrates silicon very large-scale integration (VLSI) circuits with liquid crystal displays (LCD). The LCOS microdisplay has very high resolution and yet maintains a large aperture ratio or optical efficiency. Ancillary VLSI circuits such as display drivers or digital signal processors can also be integrated into the display for the goal of system on a chip. The LCOS microdisplay is monochrome and requires a color technique to produce colors. Conventional three-panel designs, which use three panels for three primary colors, are expensive and very difficult in manufacturing (1, 2). Another method that uses time sequential color has limitations in bandwidth and response time of the display (3, 4). In short, the LCOS microdisplay is promising, but lacks of a simple color method to make it prosperous. In this paper, we present a simple color method that integrates color filters on LCOS microdisplays. Color filters are used in LCD for colors. The color filters are coated on a separate glass plate and aligned to pixels on a thin-film-transistor (TFT) plate. These color dots have a typical size of 100µm by 300µm and a thickness of 1.5µm. The alignment accuracy of these two plates is typically 10µm. To apply this color filter technology to LCOS microdisplays, several modifications have to be done. Firstly, the size of color dots shall be reduced to less than 10µm for matching fine pixels on silicon. Secondly, the thickness of color filters shall be reduced from 1.5 to 0.75µm, since the light transverse through the color filters twice in reflective LCOS microdisplays. Thirdly, these micro color filters shall be coated directly on silicon to achieve sub-micron alignment accuracy. Finally and most importantly, a very flat surface of this micro-color-filter array should be obtained for good liquid crystal alignment.
Patent•
12 Apr 2005TL;DR: In this paper, the authors disclosed a polyacetylene having a repeat structure of the formula where spa is a spacer group and mes is a mesogenic substituent, and the structure was shown to be stable.
Abstract: There is disclosed a liquid crystalline polyacetylene having a repeat structure of the formula where spa is a spacer group and mes is a mesogenic substituent.
01 May 2005
TL;DR: In this article, a color liquid-crystal-on-silicon (LCOS) microdisplay that integrated color filters on silicon was presented, where a polarizing beam splitter was used to direct a polarized light into the display and back to a projection lens.
Abstract: a three-dimensional optical analysis of fringing effect in small color pixels. In addition to the simulation of optical reflectance in visible spectrum, we further expressed the reflectance in spatial color. Color leakage was easily located and it agreed well with the experiments. 1. Introduction developed a novel but practical color liquid-crystal-on- silicon (LCOS) microdisplay that integrated color filters on silicon as illustrated in Figure 1. The projection optics is greatly simplified since color is already available on the display. What it needs is just a polarizing beam splitter (PBS) to direct a polarized light into the display and back to a projection lens. In order to have high resolution for this spatial color microdisplay, pixels have to be very small. This is not a problem for silicon panels, which were fabricated by sub-micron very large-scale integration (VLSI) processes. But the lateral field effect among small color pixels could be severe (1-3), and degrade contrast and color of this microdisplay.
Patent•
07 Nov 2005TL;DR: In this article, a method and apparatus for driving bistable cholesteric liquid crystal displays is described, in which all the pixels are initially driven to the P state and selected pixels for the display are then driven to FC state to provide the desired message.
Abstract: This invention provides a method and apparatus for driving bistable cholesteric liquid crystal displays. The method and apparatus provided a display in which all the pixels are initially driven to the P state. Selected pixels for the display are then driven to the FC state to provide the desired message. The state of the pixels is then maintained for a viewing period prior to any resetting of the display to the P state. The switching from the P state to the FC state allows fast addressing times and lower voltages to be used in driving the display.
TL;DR: In this paper, a photoaligned twisted nematic liquid crystal display (TN-LCD) cell is fabricated by one-step illumination with oblique nonpolarized UV light of an empty cell with azo-dye layers coated on indium tin oxide (ITO) substrates.
Abstract: A novel photoaligned twisted nematic liquid crystal display (TN-LCD) cell is fabricated by one-step illumination with oblique nonpolarized UV light of an empty cell with azo-dye layers coated on indium tin oxide (ITO) substrates. If the incident angle is sufficiently large (>75°), the p-polarization of the light is more pronounced for the bottom azo-dye layer in comparison with that for the upper one. Thus, the photo-alignment of the azo-dye layer on the bottom substrate is perpendicular to that of the upper one, and LC directors on the top and bottom substrates also become perpendicular. The method provides a new simple way of manufacturing photoaligned TN-LCDs.
Patent•
07 Sep 2005TL;DR: In this paper, an organic light emitting diode consisting of multiple organic layers, disposed between a transparent conducting anode and metallic cathode, was provided with a metal fluoride layer to enhance the overall performance of the device.
Abstract: An organic light emitting diode consisting of multiple organic layers, disposed between a transparent conducting anode and metallic cathode. The anode is provided with a metal fluoride layer to enhance the overall performance of the device, including higher power efficiency, lower voltage threshold and improved device operation stability.
01 May 2005
TL;DR: In this article, a bend and splay display based on dual frequency liquid crystal has been demonstrated using two regular electrodes, and a convenient multiplex driving scheme for a BBS matrix display has been proposed.
Abstract: A bistable liquid crystal display based on the bend and splay configurations has been demonstrated using dual frequency liquid crystal. Unlike the previous three-electrode version, only two regular electrodes are needed in the present case. We also propose and demonstrate a convenient multiplex driving scheme for a BBS matrix display.
01 May 2005
TL;DR: Inverted top-emitting organic light emitting devices (ITOLEDs) employing thin film of vanadium pentoxide (V2O5) capped with vacuum deposited semitransparent silver layer as top anode were studied in this paper.
Abstract: Inverted top-emitting organic light-emitting devices (ITOLEDs) employing thin film of vanadium pentoxide (V2O5) capped with vacuum deposited semitransparent silver layer as top anode were studied. The devices consisted of C-545T doped in Alq3 as emitting layers exhibited a maximum external current efficiency of ~11cd/A and a turn on voltage of ~6.4V.We attribute the good performance of the devices to the high work function of V2O5 which provides efficient hole injection into the devices.