Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
1999-05-14
2004-02-17
Chowdhury, Tarifur R. (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S123000
Reexamination Certificate
active
06693694
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to display devices for use in display systems for displaying image employing electro-optic light modulators.
Display systems employing electro-optic light modulating display devices are particularly suitable for displaying color information in the form of continuously updated image information signals arranged in successive frames representing full color frames, such as color video information, in which each frame is composed of component color sub-frames.
These systems employ one or more electro-optic light modulating display devices comprised of a row-and-column matrix array of pixels, for modulating light in accordance with the image information signals during successive frame periods. The signal information is applied to the pixel rows of the array a line at a time during each frame period.
Such display systems typically employ three such display devices, one for each of the primary color components of the color display signal. A scrolling color projection display system is also known in which color bars are repetitively scrolled across a single electro-optic light modulator device to produce a color display. See, for example, commonly assigned U.S. Pat. No. 5,532,763, incorporated herein by reference.
Active-matrix liquid crystal display (AMLCD) devices on silicon, are being considered for use as electro-optic light modulators for color displays. Such devices operating in the reflective mode are particularly suitable for high resolution displays, due to the fact that the matrix structure of row and column electrodes, switches and storage capacitors can all be integrated on the silicon substrate below the reflective pixel electrodes, thus enabling the high pixel density required for such high resolution displays.
In a typical construction, an array of electrodes of a reflective metal such as aluminum are deposited on the silicon substrate. These electrodes define the individual pixels of the array. Next, an orienting layer is formed on the array of reflective pixel electrodes. This layer is formed of a material capable of orienting the molecules of a liquid crystal material in a preferred direction, such as a polyimide, after which the layer is rubbed to produce a direction of orientation corresponding to the rubbing direction. An opposing transparent substrate, typically of glass, carries an opposing, transparent electrode, typically of indium-tin oxide (ITO), and a second orienting layer, which may also be a rubbed polyimide. These opposing structures define a cell within which the liquid crystal material is contained.
In order to avoid degradation of the liquid crystal material, and to maintain a symmetric electrical behavior of the LCD during driving, such LCDs are driven with AC signals. That is, the polarity of successive information signals for each pixel alternates between positive and negative.
Unfortunately, the asymmetrical physical structure of the reflective LCD device leads to a large electrical asymmetry during driving, which manifests itself in the form of a DC offset superimposed on the AC drive voltage in the LC. This DC off-set produces noticeable flicker in a continuously updated display image, such as a video image, particularly when the array is driven by the frame inversion method, in which all of the pixels of the array are driven to the same polarity in one frame, and then to the opposite polarity in the next frame.
The use of other drive inversion schemes, such as row, column or pixel inversion, can reduce the perceived flicker in the display image. In row and column inversion, alternate rows or columns are driven to opposite polarities in each frame, while in pixel inversion, some other pattern of inversion is employed on the pixel level, such as a checkerboard pattern. However, none of these methods will eliminate flicker unless the electrical asymmetry is very small. In addition, these driving schemes may cause artifacts such as contrast loss or the appearance of vertical or horizontal stripes in moving objects.
Another way to suppress flicker is to adjust the drive voltage to compensate for the DC off-set. However, such an active compensation scheme is difficult to implement, since the off-set voltage tends to be non-uniform over the pixel array, and also tends to vary with time.
Another way to suppress flicker is to drive the display device at a higher frame rate, as is commonly done in the case of CRTs used in computer monitors. However, driving at a higher frame rate increases the cost and complexity of the driving circuitry. Moreover, driving a color sequential system at a higher frame rate requires either a faster switching LC or else reduces brightness of the display due to the increased overhead needed to compensate for the LC response.
In U.S. Pat. No. 5,764,324, a flicker-free reflective LCD cell is said to be provided in which the work function of the reflective electrode is made approximately equal to that of the transparent electrode by covering the reflective layer with a layer of the transparent electrode material. In addition, a layer of a dielectric material may be disposed between the reflective layer and the transparent layer of the reflective electrode.
In Japanese patent abstract 08184824, a flicker-free reflective LCD cell is said to be provided by forming an insulating film on the liquid crystal side of the transparent electrode. The insulating film is said to act similarly to the reflective electrode, thus providing electrical symmetry between both electrodes.
In Japanese patent abstract 63-77016, a flicker-free LCD cell is said to be provided by equalizing the electric resistance of the orientation layers.
OBJECTS AND SUMMARY OF THE INVENTION
It is an object of the invention to reduce or substantially eliminate the DC offset which occurs in electro-optic light modulator devices during AC driving, without adding additional electrically conducting layers, altering the orientation layers, or changing the drive conditions.
In accordance with the invention, it has been discovered that the DC offset which is present during AC driving of an electro-optic display device is due to an electro-chemical interaction between the electrodes and the liquid crystal orientation layers which are in contact with the electrodes. In any such display device in which there are differences in electro-chemical interaction on opposite sides of the LC cell, leading to an internal DC voltage, a DC offset is likely to occur, with greater differences leading to greater offsets.
Such differences are likely to be greatest in reflective display devices having a reflective electrode on one side of the LC cell, and a transmissive electrode on the other side. As used herein, the term “reflective” is meant to include partially reflecting, semitransparent electrodes, such as are used in transflective display devices.
In accordance with the invention, the DC offset occurring during AC driving of an electro-optic display device caused by an internal DC voltage in the display cell is reduced or substantially eliminated by forming intermediate barrier layers (eg., electrically insulating layers) between the liquid crystal orienting layers and adjacent layers (eg., electrode layers), to prevent the electro-chemical interaction which would otherwise occur between these layers.
In accordance with the invention, there is provided an electro-optic display device comprising an electro-optic material between two supporting substrates, each of the supporting substrates having an orienting layer in contact with the electro-optic material, and at least one layer of another material which is electro-chemically interactive with the orienting layer, wherein the improvement comprises barrier layers formed between the orientation layers and the electro-chemically interactive layers.
The electro-chemically interactive layers are typically of electrically conducting material, while the barrier layers are typically of electrically insulating material, either organic or inorganic, but preferably chosen from the inorganic oxides o
Janssen Peter J.
Konovalov Victor Alexeevich
Melnik George A.
Muravski Anatoli Alexeevich
Yakovenko Sergei Yevgenyevich
Chowdhury Tarifur R.
Koninklijke Philips Electronics , N.V.
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