Patent Document

BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The invention relates to an electroluminescent display device, particularly to an electroluminescent display device having color filter layers.  
           [0003]    2. Description of the Related Art  
           [0004]    An organic electroluminescent (hereafter, referred to as EL) element is a self-emission element. An organic EL display device using the organic EL element is receiving an attention as a new display device substituted for a CRT or an LCD.  
           [0005]    [0005]FIG. 5 is a schematic cross-sectional view showing a pixel of a full-color organic EL display device of the conventional art. A numeral  200  designates a glass substrate, a numeral  201  designates an organic EL element driving TFT (thin film transistor) formed on the glass substrate  200 , and a numeral  202  designates a first planarization insulating film. A numeral  203  designates an anode layer made of ITO (indium tin oxide) which is connected with the TFT  201  and extends over the first planarization insulating film  202 , and a numeral  204  designates a second planarization insulating film formed so as to cover an end portion of the anode layer  203 . A numeral  205  designates R (red), G (green), and B (blue) organic EL layers formed on the anode layer  203 , and a numeral  206  designates a cathode layer formed on the organic EL layer  205 .  
           [0006]    A glass substrate  207  covers the cathode layer  206 . The glass substrate  207  and the glass substrate  200  are attached at their edges, and the R, G, and B organic EL layers  205  are enclosed therein. Here, the R, G, and B organic EL layers  205  are respectively formed by selectively performing vapor-deposition of each of R, G, and B organic EL materials by using a metal mask.  
           [0007]    On the other hand, as a method of realizing a full-color organic EL display device without using the above R, G, and B organic EL layers  205 , a method of using color filter layers has been proposed. In this case, a combination of a white organic EL layer and color filter layers is employed.  
           [0008]    [0008]FIG. 6 is a cross-sectional view of such a full-color organic EL display device. An insulating layer  301  as a substrate is formed on the glass substrate  300 , and an R color filter layer  302 , a G color filter layer  303 , and a B color filter layer  304  are formed on the insulating film  301 . Each of these color filter layers transmit light having a predetermined wavelength corresponding to each of the R, G, and B colors, which is irradiated from the white organic EL layer. Although not shown, an organic EL element driving TFT is formed under these color filter layers  302 ,  303 , and  304  in a manner similar to the TFT  201  of FIG. 5.  
           [0009]    A first planarization insulating film  305  is formed on these color filter layers  302 ,  303 , and  304 . Anode layers  306 ,  307 , and  308  are formed on the first planarization insulating film  305 , corresponding to each of the R, G, and B colors. A second planarization insulating film  309  is formed so as to cover end portions of the anode layers  306 ,  307 , and  308 , and a white organic EL layer  310  and a cathode layer  311  are laminated thereon in this order. Furthermore, a glass substrate  312  covers the cathode layer  311 , and the glass substrate  312  and the glass substrate  300  are attached at their edges, so that the white organic EL layer  310  is enclosed therein.  
           [0010]    Here, the reason to provide the second planarization insulating film  309  is that the distance between the anode layers  306 ,  307 , and  308  and the cathode layer  311  becomes small without the second planarization insulating film  309  and may cause a short circuit. The second planarization insulating film  309  is formed with openings except above the end portions of the anode layers  306 ,  307 , and  308 , and the white organic EL layer  310  is in contact with the anode layers  306 ,  307 , and  308  exposed in the openings.  
           [0011]    The organic EL display device of this type is described in a Japanese Patent Application Publication No. Hei 11-251059.  
           [0012]    However, the organic EL display device employing the described structure of the white organic EL layer and the color filter layers has following problems. Firstly, since the first planarization insulating film  305  is formed on the R color filter layer  302 , the G color filter layer  303 , and the B color filter layer  304 , a manufacturing cost increases accordingly. This can be solved by eliminating the first planarization insulating film  305  by using the color filter layers as the first planarization insulating film  305 . In this case, the adjacent color filter layers need to overlap each other for planarization and for increasing an aperture ratio. However, since the step height at the overlapping portion of the color filter layers becomes large, a display defect may occur by a cut in the organic EL layer, moisture absorption at the step portion, and so on.  
           [0013]    Secondly, as the first planarization insulating film  305 , organic resin such as acrylic resin having a thickness of 2 to 3 micrometers must be used for planarization. However, since the organic resin has high moisture absorbency, it can have an adverse effect on the organic EL layer which is sensitive to moisture, causing a display defect.  
         SUMMARY OF THE INVENTION  
         [0014]    The invention provides a color electroluminescent display device that includes a plurality of color pixels and a plurality of color filter layers provided for the color pixels on an insulating substrate. Each of the color filter layers allows a transmission of light of a color of a corresponding color pixel. The display device also includes an anode layer formed on each of the color filter layers, a white electroluminescent layer formed on the anode layers, and a cathode layer formed on the white electroluminescent layer. End portions of the color filter layers are tapered, and the tapered end portions of adjacent color filter layers overlap each other. A thin planarization insulating film may be formed directly on the color filter layers in addition to another planarization insulating film that covers end portions of the anode layers. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]    [0015]FIG. 1 is a cross-sectional view of an organic EL display device of an embodiment of the invention.  
         [0016]    [0016]FIGS. 2A, 2B,  2 C, and  2 D are cross-sectional views of processing steps of color filter layers of the display device of FIG. 1.  
         [0017]    [0017]FIG. 3 is a cross-sectional view of a modified organic EL display device of the embodiment of the invention.  
         [0018]    [0018]FIG. 4 is an equivalent circuit diagram of the organic EL display device of the embodiment of the invention.  
         [0019]    [0019]FIG. 5 is a cross-sectional view of a conventional organic EL display device.  
         [0020]    [0020]FIG. 6 is a cross-sectional view of another conventional organic EL display device. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0021]    An embodiment of the invention will be described with reference to the drawings. FIG. 1 is a cross-sectional view showing a pixel of an organic EL display device of the embodiment of the invention. In an actual organic EL display device, a plurality of the pixels is arranged in a matrix.  
         [0022]    An insulating film  2  made of SiO 2  as a substrate is formed on a glass substrate  1 . An R color filter layer  3 , a G color filter layer  4 , and a B color filter layer  5  are formed adjacent each other on the insulating film  2 . Each of these color filter layers transmits light having a predetermined wavelength corresponding to each of R, G, and B colors, which is irradiated from a white organic EL layer  10 . Although not shown, an organic EL element driving TFT and a pixel selecting TFT are formed under these color filter layers.  
         [0023]    The R color filter layer  3 , the G color filter layer  4 , and the B color filter layer  5  also serve as a first planarization insulating film, such as the one  202  in FIG. 5. End portions of the color filter layers are overlapped for planarization. The end portions of the color filter layers are formed in a tapered shape so as to reduce a step height H 2  at an overlapping portion. For example, the both end portions of the R color filter layer  3  are formed in a tapered shape, and one of the end portions of the G color filter layer  4  is formed to cover one of the end portions of the R color filter layer  3 . Furthermore, the both end portions of the B color filter layer  5  are formed to respectively cover the end portion of the R color filer layer  3  and the end portion of the G color filter layer  4 .  
         [0024]    A conventional planarization insulating film is not formed on these color filter layers, but anode layers  6 ,  7 , and  8  are formed directly on the R color filter layer  3 , the G color filter layer  4 , and the B color filter layer  5 , respectively. Furthermore, a second planarization insulating film  9  is formed to cover end portions of the anode layers  6 ,  7 , and  8 , and a white organic EL layer  10  and a cathode layer  11  are laminated thereon in this order. A glass substrate  30  covers the cathode layer  11 , and the glass substrate  30  and the glass substrate  1  are attached at their edges to enclose the white organic EL layer  10  therein.  
         [0025]    The reason to provide the second planarization insulating film  9  is the same as the conventional art, that is, the distance between the anode layers  6 ,  7 , and  8  and the cathode layer  11  becomes small without the second planarization insulating film  9  so that a short circuit can occur between the anode layers  6 ,  7 , and  8  and the cathode layer  11 . Openings are formed in the second planarization insulating film  9  except above the end portions of the anode layers  6 ,  7 , and  8 . The white organic EL layer  10  is formed on the anode layers  6 ,  7 , and  8  exposed in the openings, being in contact therewith.  
         [0026]    A forming method of the color filter layers will be described with reference to FIGS. 2A,  2 B,  2 C, and  2 D. Here, a forming method of the R color filter layer  3  and the G color filter layer  4  will be described. As shown in FIG. 2A, the R color filter material layer  3   a  made of a negative photoresist containing a predetermined pigment is coated on the whole surface of the insulating film  2  serving as a substrate formed on the glass substrate  1 . Then, the R color filter material layer  3   a  is exposed to light through predetermined masks  12 . When the R color filter material layer  3   a  undergoes next development treatment, as shown in FIG. 2B, a portion of the R color filter material layer  3   a  which is exposed to light remains to form the R color filter layer  3 . The R color filter layer  3  is formed by this exposure and development process, having tapered portions at its ends. This is because that the R color filter material layer  3   a  receives light beyond the area corresponding to the opening of the mask  12  with an intensity that is smaller than that of the central portion of the mask and is gradually decreasing.  
         [0027]    Next, as shown in FIG. 2C, a G color filter material layer  4   a  made of a negative photoresist containing a predetermined pigment is coated on the whole surface. The G color filter material layer  4   a  is exposed to light through predetermined masks  13 . When the G color filter material layer  4   a  undergoes next development treatment, as shown in FIG. 2D, a portion of the G color filter material layer  4   a  which is exposed to light remains to form the G color filter layer  4 . By positioning the masks  13  as shown in FIG. 2D, the end portion of the G color filter layer  4  overlaps the end portion of the R color filter layer  3 .  
         [0028]    The end portion of the R color filter layer  3  is formed in a tapered shape. The end portion of the G color filter layer  4  has a tapered shape and becomes gradually thinner toward its end. Therefore, a step height H 2  of an overlapping portion of the G color filter layer  4  and the R color filter layer  3  is reduced. The forming method of the B color filter layer  5  is the same as this.  
         [0029]    Here, the less the step height H 2  of the overlapping portion of the R, G, and B color filter layers is, the better the display performs. However, for preventing a cut in the white organic EL layer  9  formed above the R, G, and B color filter layers, which can be caused by the step height H 2 , when a film thickness of the white organic EL layer  9  is H 1 , it is preferable that H 1  is larger than H 2 . In this embodiment, both end portions of the B color filter layer  5  are formed to cover the end portions of the adjacent R color filter layer  3  and G color filter layer  4 , respectively.  
         [0030]    For minimizing the step height H 2  of the overlapping portion of the color filter layers, the color filter layers are preferably formed in a decreasing order of thickness. For example, when the thicknesses of the R color filter layer  3 , the G color filter layer  4 , and the B color filter layer  5  are T 1 , T 2 , and T 3 , respectively, it is preferable that T 1  is lager than T 2  and T 2  is larger than T 3 . In this case, the R color filter layer  3 , the G color filter layer  4 , and the B color filter layer  5  are formed in this order.  
         [0031]    Accordingly, in this embodiment, the R color filter layer  3 , the G color filter layer  4 , and the B color filter layer  5  serve as the first planarization insulating film. However, as shown in FIG. 3, the first planarization insulating film  20  can be further formed on these color filter layers. This first planarization insulating film  20  can be formed thinner than the conventional art since the planarization is already realized to some extent by the R color filter layer  3 , the G color filter layer  4 , and the B color filter layer  5 . A preferable film thickness is between 200 nm and 300 nm.  
         [0032]    Furthermore, since the first planarization insulating film  20  is thin, the first planarization insulating film  20  can be formed of an inorganic insulating film having low absorbency by a PCVD (plasma-activated chemical vapor deposition) method. It is preferable to employ a silicon oxide film, a TEOS film, or a silicon nitride film as the inorganic insulating film.  
         [0033]    Next, an equivalent circuit of the described organic EL display device and its operation will be described. FIG. 4 is an equivalent circuit diagram of the organic EL display device, showing a pixel formed in a periphery of a gate signal line  50  at an n-th row and a drain signal line  60  at an m-th column.  
         [0034]    The gate signal line  50  for supplying a gate signal Gn and the drain signal line  60  for supplying a drain signal, that is, a video signal Dm cross each other. An organic EL element  120 , a TFT  100  for driving the organic EL element  120 , and a TFT  110  for selecting a pixel are formed in a periphery of an intersection of the both signal lines  50  and  60 .  
         [0035]    A drive source  105  is connected with a drain  100 d of the organic EL element driving TFT  100 , and supplies a positive drive voltage PVdd. A source  100   s  is connected with an anode  121  of the organic EL element  120 .  
         [0036]    A gate  110   g  of the selecting TFT  110  for selecting a pixel is connected with the gate signal line  50  and supplied with a gate signal Gn. A drain  110   d  is connected with the drain signal line  60  and supplied with the video signal Dm. The source  110   s  of the selecting TFT  110  is connected with the gate  100   g  of the driving TFT  100 . Here, the gate signal Gn is outputted from a gate driver circuit (not shown). The video signal Dm is outputted from a drain driver circuit (not shown).  
         [0037]    The organic EL element is made of the anode  121 , a cathode  122 , and an emissive layer  123  formed between the anode  121  and the cathode  122 . The cathode  122  is connected with a common source  140  for supplying a negative common voltage CV.  
         [0038]    Furthermore, the gate  100   g  of the driving TFT  100  is connected with a storage capacitor  130 . That is, one electrode of the storage capacitor  130  is connected with the gate  100   g,  and another electrode thereof is connected with the storage capacitor electrode  131 . The storage capacitor  130  is provided for storing the video signal of the pixel for one field period by storing electric charge corresponding to the video signal Dm.  
         [0039]    An operation of the EL display device having the described structure will be described as follows. When the gate signal Gn becomes high level for one horizontal period, the selecting TFT  110  turns on. Then, the video signal Dm is applied from the drain signal line  60  to the gate  100   g  of the driving TFT  100  through the selecting TFT  110 . In response to the video signal Dm supplied to the gate  100   g,  conductance of the driving TFT  100  changes. The drive electric current corresponding to the conductance is supplied from the drive source  105  to the organic EL element  120  through the driving TFT  100 . Accordingly, luminance of the organic EL element  120  is controlled.  
         [0040]    Although colors of the color pixels and the color filter layers are R (red), G (green), and B (blue) in this embodiment, the colors may be yellow or magenta. Furthermore, the “white EL” is mainly white, but may be reddish or bluish.

Technology Category: 5