Patent Publication Number: US-2003227588-A1

Title: Reflective liquid crystal display

Description:
FIELD OF THE INVENTION  
       [0001] The present invention relates to a liquid crystal display, and more particularly to a reflective liquid crystal display.  
       BACKGROUND OF THE INVENTION  
       [0002] Liquid crystal displays (LCDs) are widely used in portable televisions, laptop personal computers, notebooks, electronic watches, calculators, mobile phones and office automation devices due to their advantages of small size, light weight, low driving voltage, low power consumption and good portability. LCDs are generally divided into three types: penetrative LCDs, reflective LCDs, and transflective LCDs. A penetrative LCD requires backlight, while a reflective LCD reflects environmental light. So far, reflective LCDs are relatively popular.  
       [0003] Referring to FIG. 1( a ), a typical reflective LCD comprises a reflective pixel electrode layer  10 , a liquid crystal layer  12  and a transparent electrode layer  13 . The liquid crystal layer  12  contains a plurality of liquid crystal molecules and is sandwiched between the pixel electrode layer  10  and the transparent electrode layer  13 . The liquid crystal molecules are aligned according to a driving voltage applied between the pixel electrodes  10  and the transparent electrode  13 , thereby controlling light passing through the liquid crystal layer  12 .  
       [0004] In order to acquire high reflectivity, the pixel electrodes  10  are made of metallic materials such as aluminum (Al), silver (Ag) and aluminum-neodymium (Al—Nd) alloy. Due to the metallic materials, the surfaces of the pixel electrodes  10  are readily corroded at the interface in contact with the liquid crystal molecules. Therefore, the reflectivity of the pixel electrodes  10  will be degraded for an extended period of time, and the brightness and color of the liquid crystal display are deteriorated.  
       [0005] For a purpose of improving the image quality, a liquid crystal display described in U.S. Pat. No. 5,926,240 employs a dielectric layer to overcome the above problem. The structure of such liquid crystal display is shown in FIG. 1( b ). Referring to FIG. 1( b ), a dielectric layer  11  is formed on the pixel electrodes  10 , isolating the pixel electrodes from liquid crystal molecules. The dielectric layer  11  is usually made of silicon nitride and has a thickness in a range from 80 nm to 170 nm. The dielectric layer  11  serves as a passivation layer for protecting the pixel electrodes from corrosion. As known, corrosion results in the increasing roughness of the pixel electrode surface and thus renders reduced reflectivity. Furthermore, when the refractive index and the thickness of the dielectric layer  11  are well controlled, an optimal reflectivity of the pixel electrode is obtained accordingly. It is suggested that the refractive index be in a range from 1.6 to 1.9, and the thickness be from 80 nm to 170 nm.  
       [0006] Unfortunately, the overall capacitance of the liquid crystal display might be increased due to the presence of the dielectric layer  11  so as to consume the driving voltage applied between the pixel electrodes  10  and the transparent electrode  13 . Since the effective voltage for driving the liquid crystal molecules in the liquid crystal layer  12  is reduced, it is required to raise the externally applied voltage from the driving circuit. Therefore, the liquid crystal display has relatively high power consumption.  
       SUMMARY OF THE INVENTION  
       [0007] It is an object of the present invention to provide a reflective liquid crystal display capable of operating at a relatively small driving voltage compared to the prior art with the dielectric layer as a passivation layer.  
       [0008] It is another object of the present invention is to provide a reflective liquid crystal display having improved reflectivity of the pixel electrodes without adversely affecting driving voltage.  
       [0009] In accordance with an aspect of the present invention, there is provided a liquid crystal display. The liquid crystal display comprises a reflective pixel electrode, a transparent electrode, a liquid crystal layer and a transparent conductive layer. The transparent electrode cooperates with the reflective pixel electrode to provide therethrough a driving voltage. The liquid crystal layer comprises a plurality of liquid crystal molecules and sandwiched between the reflective pixel electrode and the transparent electrode to align in a predetermined manner in response to the driving voltage. The transparent conductive layer disposed between the reflective pixel electrode and the liquid crystal layer.  
       [0010] Generally, the reflective pixel electrode is made of a material selected from a group consisting of aluminum (Al), silver (Ag) and aluminum-neodymium (Al—Nd) alloy.  
       [0011] Preferably, the transparent conductive layer has a refractive index of at least 1.95. More preferably, the transparent conductive layer has a refractive index in a range from 1.95 to 2.2.  
       [0012] In an embodiment, the transparent conductive layer has a thickness in a range from 80 nm to 170 nm. And the transparent conductive layer is made of a material selected from a group consisting of indium tin oxide (ITO), indium zinc oxide (IZO) and conductive polymers.  
       [0013] In accordance with another aspect of the present invention, there is provided a liquid crystal display comprises a reflective pixel electrode, a transparent electrode, a liquid crystal layer and a protective layer. The liquid crystal layer comprises a plurality of liquid crystal molecules and sandwiched between the reflective pixel electrode and the transparent electrode, the liquid crystal molecules being controlled by a driving voltage applied between the reflective pixel electrode and the transparent electrode. The protective layer is disposed between the reflective pixel electrode and the liquid crystal layer for protecting the reflective pixel electrode from corrosion. Specially, the protective layer is transparent and conductive and has a refractive index of at least 1.95.  
       [0014] In accordance with another aspect of the present invention, there is provided a use of a transparent conductive layer for use in a liquid crystal display for protecting a pixel electrode layer from corrosion by a corrosive material.  
       [0015] In an embodiment, the corrosive material is liquid crystal molecule, and the transparent conductive layer is disposed between the pixel electrode layer and the liquid crystal molecule.  
       [0016] The above objects and advantages of the present invention will become more readily apparent to those ordinarily skilled in the art after reviewing the following detailed description and accompanying drawings, in which: 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0017]FIG. 1( a ) is a cross-sectional view illustrating a conventional liquid crystal display;  
     [0018]FIG. 1( b ) is a cross-sectional view illustrating another conventional liquid crystal display; and  
     [0019]FIG. 2 is a cross-sectional view illustrating a liquid crystal display according to a preferred embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
     [0020] Referring to FIG. 2, the liquid crystal display of the present invention comprises reflective pixel electrodes  20 , a transparent conductive layer  21 , liquid crystal layer  22 , and a transparent electrode  23 .  
     [0021] For high performance, the pixel electrodes  20  are preferably made of a highly reflective metallic material such as aluminum (Al), silver (Ag) or aluminum-neodymium (Al—Nd) alloy. The liquid crystal layer  22  is sandwiched between the reflective pixel electrode  20  and the transparent electrode  23 . The liquid crystal molecules are aligned in response to a driving voltage applied between the reflective pixel electrodes  20  and the transparent electrode  23 , thereby controlling the brightness or darkness of the liquid crystal display. Since the operation principles relating to the liquid crystal molecules, the reflective pixel electrodes and the transparent electrode are well known in the art, it need not be further described in details herein.  
     [0022] In accordance with the present invention, the transparent conductive layer  21  is disposed between the reflective pixel electrode  20  and the liquid crystal layer  22  as a passivation layer for protecting the pixel electrodes from corrosion by the liquid crystal molecules. Therefore, the reflectivity of the pixel electrodes will not be adversely affected by the roughened surface of the pixel electrode layer resulting from corrosion. The transparent conductive layer  21  has a thickness in a range from 80 nm to 170 nm and is preferably selected from indium tin oxide (ITO), indium zinc oxide (IZO) and any other suitable conductive polymer. Due to the conductive feature of the transparent conductive layer, the driving voltage will not be undesirably consumed by the passivation layer as in the prior art. Furthermore, the transparent conductive layer  21  preferably has a refractive index of at least 1.95 that is higher than the refractive index of the silicon nitride dielectric layer, which is about 1.6˜1.9. For example, ITO has a refractive index of 1.95 to 2.2. For such a high refractive index, the reflectivity of the pixel electrode  21  is even improved.  
     [0023] While the invention has been described in terms of what is presently considered to be the most practical and preferred embodiments, it is to be understood that the invention needs not be limited to the disclosed embodiment. On the contrary, it is intended to cover various modifications and similar arrangements included within the spirit and scope of the appended claims which are to be accorded with the broadest interpretation so as to encompass all such modifications and similar structures.