Abstract:
A flat panel display device including a first substrate formed of a first material; a second substrate spaced apart from the first substrate and formed of a second material; and a plurality of pixelated emissive devices interposed between the first and second substrates. The second material having a relatively higher light transmittance property than the first material, and the second substrate transmitting light for the plurality of pixelated emissive devices.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to a flat panel display device, and more particularly to a reflective liquid crystal display device.  
         BACKGROUND OF THE INVENTION  
         [0002]    As information technologies rapidly develop, display devices are developed in accordance with the pace of the technology development. The display devices process and display a great deal of information. A cathode ray tube (CRT) has served as a mainstream of the display device area. However, to meet the needs of the current development, a flat panel display device having small size, light weight, and low power consumption is an important subject of research.  
           [0003]    The flat panel display (FPD) devices can be divided depending on whether a light source is used or not. Emissive display devices, which display the images by the luminescence, include the plasma display panel (PDP), the field emission display (FED) and the electroluminiscence display (ELD). On the other hand, non-emissive display devices, which display the images by using the exterior light sources, are represented by the liquid crystal display (LCD) that is widely used due to a high resolution a color display and a high quality of images.  
           [0004]    Generally, typical LCD devices include an upper and a lower substrate with liquid crystal molecules interposed therebetween. The upper and lower substrates are generally referred to as a color filter and array substrates respectively. The upper and lower substrates respectively include electrodes disposed on opposing surfaces of the upper and lower substrates. Since typical LCD devices can not emit the light, the light source is needed. Therefore, the backlight is disposed at the rear side of the liquid crystal panel and images can be displayed by adjusting the transmittance of the incident light from the backlight corresponding to the liquid crystal alignment.  
           [0005]    An electric field is generated by applying a voltage to the electrodes made of transparent conductive materials, thereby driving the liquid crystal molecules per each pixel to display images depending on the light transmittance. Both substrates of the LCD devices are transparent because they have to transmit the light of the backlight. Glass substrates are mainly used as the substrates of the LCD devices and transmit about 90% to 95% of the incident light.  
           [0006]    This type of LCD device is called a transmission type LCD. Since the transmission type LCD devices use an artificial rear light source, they can display bright images under a dark environment. However, the power consumption of the transmission type LCD devices is high owing to the backlight.  
           [0007]    To reduce power consumption, reflection type LCD devices are suggested. The power consumption of the reflection type LCD devices is lower than that of transmission type LCD devices because they reflect the exterior natural or artificial light (the ambient light) and then use the reflected light as a light source. Therefore, the electrode of the lower substrate is made good reflective materials and the electrode of the upper substrate is made of transparent materials.  
           [0008]    Since the price of LCD devices is high, there are many attempts to reduce the fabrication cost during the development of the materials or a decrease in the number of fabrication steps. Furthermore, the low cost is also an important object in the field of the emissive display devices, for example, ELD, FED and PDP.  
         BRIEF SUMMARY OF THE INVENTION  
         [0009]    Accordingly, the present invention is directed to the reflective liquid crystal display device that substantially obviates one or more of the problems due to limitations and disadvantages of the related art.  
           [0010]    An object of the present invention is to provide an flat panel display (FPD) device, which can reduce the fabrication cost.  
           [0011]    To achieve these and other advantages and in accordance with the purpose of the present invention, as emobidied and broadly described, a flat panel display device includes a first substrate formed of a first material; a second substrate spaced apart from the first substrate and formed of a second material having a relatively higher light transmittance property than the first material; and a plurality of pixelated emissive devices interposed between the first and second substances.  
           [0012]    In another aspect, a reflective liquid crystal display device includes a first substrate formed of a first material; a second substrate spaced apart from the first substrate and for transmitting light into and out of the liquid crystal display device, the second substrate formed of a second material having a relatively higher light transmittance property than the first material; at least one pixel electrode on the first substrate for reflecting incident light towards the second substrate; and a liquid crystal layer interposed between the pixel electrode and the second substrate.  
           [0013]    In a further aspect, an emissive display device includes a first substrate formed of a first material; a second substrate spaced apart from the first substrate and formed of a second material having a relatively higher light transmittance property than the first material; and a light emitting layer disposed between the first substrate and the second substrate and including phosphor.  
       
    
    
       [0014]    Other features and characteristics of the present invention; methods, operation, and functions of the related elements of the structure; combination of parts; and economies of manufacture will become apparent from the following detailed description of the preferred embodiments and accompanying drawings, all of which form a part of this specification.  
       BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]    The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, wherein like reference numerals designate corresponding parts in the various drawings and wherein;  
         [0016]    [0016]FIG. 1 is a plane view of a reflective liquid crystal display device according to a preferred embodiment of the present invention;  
         [0017]    FIG. 2  is a cross-sectional view of a reflective liquid crystal display device according to a preferred embodiment of the present invention taken along line II-II of FIG. 1;  
         [0018]    [0018]FIG. 3 is a cross-sectional view of an electroluminiscence display according to a second embodiment of the invention;  
         [0019]    [0019]FIG. 4 is a cross-sectional view of a field emission display according to a third embodiment of the invention; and  
         [0020]    [0020]FIG. 5 is a cross-sectional view of a plasma display panel according to the fourth embodiment of the invention. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0021]    [0021]FIG. 1 is a plane view of a reflective liquid crystal display device according to a preferred embodiment of the present invention and FIG. 2 is a cross-sectional view of a reflective liquid crystal display device according to a preferred embodiment of the present invention taken along line II-II of FIG. 1.  
         [0022]    As shown, a gate electrode  121  is patterned on a lower first substrate  110  and connected with a gate line  120 . A gate insulator  130  is formed on the entire surface of the first substrate  110 , except for the area of the first substrate  110  covered by the gate electrode  121 . An active layer  141  of amorphous silicon is patterned on the gate insulator  130 . The ohmic contact layers  151  and  152  are formed from doped amorphous silicon and are patterned on active layer  141 . A source electrode  161  and drain electrode  162  are patterned on the ohmic contact layers  151  and  152 , active layer  141  and gate insulator  130 . The source and drain electrodes  161  and  162  constitute the thin film transistor with the gate electrode  121 . The source electrode  161  is connected with the data line  160 , which defines the pixel region  190  by perpendicularly crossing the gate line  120 . A passivation layer  170  covers the source and drain electrodes  161  and  162 , and a contact hole  171  is patterned in the passivation layer  170  over the drain electrode  162 . A metallic pixel electrode  181  is patterned on the passivation layer  170  of the pixel region and connected with the drain electrode  162  through the contact hole  171 .  
         [0023]    The pixel electrode  181  functions as a reflective film and reflects the incident light. Materials of low resistance and high reflectance, for example, aluminum or aluminum alloy are adequate for the pixel electrode  181 . In other embodiments, the reflective film can be formed by other layers, for example, the source and drain electrodes  161  and  162 .  
         [0024]    A second substrate  210  is spaced apart from the first substrate  110 , and a black matrix  220  is patterned on the inner surface of the second substrate  210  at the position corresponding to the thin film transistor and non-pixel region of the first substrate  110 . Red, green and blue color filters  230  are patterned and overlap with the black matrix  220 . A common electrode  240  generates the electric field with the pixel electrode  181  and is formed on the color filters  230 .  
         [0025]    A liquid crystal layer  250  is interposed between the first and second substrates  110 ,  210 .  
         [0026]    The dashed arrow  280  of FIG. 2 depicts a propagation route of the light, which is subsequently transmitted through the second substrate  210  and the liquid crystal layer  250 , reflected at the pixel electrode  181  and then emitted through the liquid crystal layer  250  and the second substrate  210 .  
         [0027]    In the reflective LCD devices, the light passes the second substrate  210  twice and does not pass the first substrate  110 . Therefore, though the transparent substrate of high transmittance is used for the second substrate  210 , the adoption of the substrate that is not transparent or has relatively lower transmittance than the second substrate  210  for the first substrate  110  has little effect on the display quality.  
         [0028]    In the present invention, the fabrication cost of the LCD device can be reduced by using a first substrate whose transmittance is relatively lower than the transmittance of the second substrate. For example, glass or plastic substrates can be used for the first substrate.  
         [0029]    In another aspect, of the invention, the use of a first substrate with a relatively lower transmittance than the second substrate may be applied to other emissive display devices such as ELD, FED and PDP, which do not need the light source. For example, the backlight of the LCD devices and the light on the emissive display devices transmits only through the upper substrate.  
         [0030]    [0030]FIG. 3 is a cross-sectional view of an electroluminiscence display according to a second embodiment of the invention. Upper electrode  360 , lower electrode  320 , upper insulator  350 , lower insulator  330 , and a thin film phosphor layer  340  are disposed between first and second substrates  310  and  370 . The thin film phosphor layer  340  emits light when a high voltage is applied to the upper and lower electrodes  360  and  320 . To display the images, the second substrate  370  and the upper electrode  360  are transparent. However, since the first substrate  310  is not used for displaying images, the first substrate  310  need not be transparent and can have a relatively low transmittance. For example, the lower substrate  310  may be composed of the same materials of the first substrate  110 , as discussed above.  
         [0031]    [0031]FIG. 4 is a cross-sectional view of a field emission display according to a third embodiment of the invention. Cathodes  420 , insulators  430 , gates  440  and tips  450  are formed on a first substrate  410 , and anodes  480  and phosphors  470  are formed under a second substrate  490 . The interspaces between the first and second substrates  410  and  490  are evacuated and form vacuum  460 . Electrons are emitted from the tip  450  to vacuum  460  by the applied high voltage between the tip  450  and the gate  440  and accelerated by the voltage of the anode  480 . The accelerated electrons collide with the phosphor  470  and then the phosphor  470  emits the light. Here, since only the light emitted to the second substrate  490  is used for displaying images, the first substrate  410  need not be transparent and can have a relatively low transmittance. The first substrate  410 , for example, may be composed of the same materials as the first substrate  110 .  
         [0032]    [0032]FIG. 5 is a cross-sectional view of a plasma display panel according to a fourth embodiment of the invention. Upper and lower electrodes  550  and  520  are patterned on second and first substrates  560  and  510 , respectively. An intermediate glass sheet  530  having small holes is interposed between the first and second substrates  510  and  560  and then the small holes are filled with phosphor  540 . The phosphor  540  is discharged by the applied voltage between the upper and lower electrodes  550  and  520 , and emits the light. Same as the above examples of the emissive display devices, since only the light emitted to the second substrate  560  is used for displaying images, the first substrate  510  need not be transparent and can have a relatively low transmittance. Further, the first substrate  510  may be composed, for example, of the same materials discussed above for the first substrate  110 .  
         [0033]    Consequently, in the case of the emissive display devices, though the substrate of high transmittance is used for the upper substrate, a material that is not transparent or has lower transmittance than the upper substrate can be used for the lower substrate.  
         [0034]    The preferred embodiment of the invention being thus described, it will be obvious to those skilled in the art that various modifications and variation may be made in the invention without departing from the spirit or scope of the invention. Further, all such modifications are intended to be within the scope of the following claims.