Patent Publication Number: US-2003222576-A1

Title: Full color organic light-emitting display device

Description:
BACKGROUND OF THE INVENTION  
       [0001] 1. Field of the Invention  
       [0002] The present invention relates to a flat-panel display device, and more particularly, to an organic light-emitting display (OLED) device suitable for full color display.  
       [0003] 2. Description of Related Art  
       [0004] There are two common technologies known in the prior full-color OLEDs, i.e., three primary color light emitting technology which uses an organic light-emitting material of three primary colors (i.e., red, green and blue) to separately serve as organic electroluminescent pixels, and color filtering technology which adopts only a white organic light-emitting material in combination with red, blue and green color filters to produce various colors.  
       [0005] The conventional three primary color light-emitting technology is shown FIG. 1, which is a schematic view of the conventional three primary color based technology. The three primary color based technology forms a plurality of anodes  102  over a transparent substrate  100 , and then a red organic electroluminescent layer  110 , a green organic electroluminescent layer  120  and a blue organic electroluminescent layer  130  are respectively formed on the anodes  102  by evaporation. A cathode  104  is subsequently formed on the organic electroluminescent layers  110 ,  120  and  130  by evaporation, and appropriate treatments are made. The three primary color based technology employs the characteristic of active luminescence inherent in the OLED to produce different colors. Hence, no additional color-tuning filter element is necessary. However, the technology of manufacturing the separate pixels emitting light of three primary colors is complex and involves many difficulties. For manufacturing a large-sized and full-colored OLED panel with high resolutions, an excellent and complex evaporation processing is required, especially for the mass production.  
       [0006] In addition, a fine small-molecule red-light emitting organic material is indispensable. A reliable source of supply of such fine material is currently limited, however. Meanwhile, the luminescent efficiency of the light-emitting material of each of the three primary colors appears different. To ameliorate the uniformity of image display, a drive circuit to this end will be very complex. Also, in an attempt to harmonize the image display, difficulties in structural integration of films and driving of a circuit occur.  
       [0007] The conventional technology of color change medium (CCM) is shown in FIG. 2, which is a schematic view of the conventional technology of CCM. The CCM based technology forms a printed color CCM  210  including red CCM  211  and green CCM  212  over a transparent substrate  200 . Then blue organic electroluminescent layers  220  are formed over the color CCM  210 . As the currents is applied for driving the blue organic electroluminescent layer  220 , the light from the blue organic electroluminescent layer  220  will excite the red CCM  211  and green CCM  212  from the printed color CCM  210  and further converts into a full color display. However, the luminescent efficiency of the CCM technology is poor and an appropriate red CCM material is not available. Therefore, the scope of application of the CCM based technology is limited, and not suitable for mass production.  
       [0008] However, another new full color organic light-emitting device has been proposed. This new full color organic light-emitting device utilizes an organic electroluminescent layer emitting ultraviolet light first. The ultraviolet light then radiates and excites another organic electroluminescent layers for converting ultraviolet radiation into the light of red, green and blue colors located at predetermined positions for producing a full-color effect. However, since the degradation effect on the organic electroluminescent layer of red, green and blue caused by ultraviolet is very strong, the lifetime of this full color organic light-emitting device is short. Besides, it is found that the ultraviolet light is greatly absorbed by other elements of this full color organic light-emitting device. Hence, the efficiency of the light conversion is low, too. Moreover, since some of the ultraviolet light leaks out from the active area of the OLED panel, it is harmful to human eyes, especially for a long time watching.  
       [0009] Therefore, there is a need for the commercial market to provide a new full color technology to avoid the above-mentioned processing problems. Also, the features of close uniformity of luminescent efficiency and high resolution are achieved, and can be applicable to large-sized full color OELD devices.  
       SUMMARY OF THE INVENTION  
       [0010] Accordingly, it is a primary object of the present invention is to provide a full color OLED device to harmonize the luminescent efficiency of the pixels of the colors, reduce difference in luminescent efficiency between various colors, improve luminescent efficiency, enhance color resolution, and be applicable to large-sized displays.  
       [0011] A further object of the present invention is to provide a process for fabricating full color OLED devices to simplify the fabrication process without application of red organic electroluminescent layer, and enhance the purity of white light.  
       [0012] To attain the afore-mentioned objectives, an OLED device according to the present invention comprises a substrate; a first electrode layer (cathode) mounted on one side of said substrate; a second electrode layer (anode) sandwiched between said substrate and said first electrode layer (cathode); at least one organic electroluminescent layer sandwiched between said first electrode layer (cathode) and said second electrode layer (anode); a color conversion layer of fluorescent powder sandwiched between said substrate and said second electrode layer (anode); and at least one filter layer sandwiched between said color conversion layer of fluorescent powder and said substrate; wherein said color conversion layer of fluorescent powder converts the light emitted by excitation of said organic electroluminescent layer through an electric current into white combination light.  
       [0013] A process for fabricating OLED devices according to the present invention comprises the following steps: forming at least one filter layer over a substrate; forming a color conversion layer of fluorescent powder over said filter layer; forming a second electrode layer (anode) over said color conversion layer of fluorescent powder; forming at least one organic electroluminescent layer over said second electrode layer (anode); and forming a first electrode layer (cathode) over the organic electroluminescent layer; wherein said color conversion layer of fluorescent powder converts the light emitted by excitation of said organic electroluminescent layer through an electric current into white combination light.  
       [0014] Other objects, advantages, and novel features of the invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0015]FIG. 1 is a schematic view showing the conventional three primary color based technology of the full color OLEDs;  
     [0016]FIG. 2 is a schematic view showing the conventional technology of CCM of the full color OLEDs; and  
     [0017]FIG. 3 is schematic view of an OLED device according to a preferred embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
     [0018] The material of the substrate of the OLED device according to the present invention is selected from transparent material, and preferably is soda-lime glass, borophosphosciliate glass, plastic or silicon wafer. The first electrode can be made of any conventional material. Preferably, the first electrode is made of InSnO 3 , SnO 2 , In 2 O 3  with doped ZnO, CdSnO or Sb. The second electrode can be made of any conventional material. Preferably, the second electrode is made of MgAg, Al, diamond, quasi-diamond or Ca. The OLED device according to the present invention can be either a single-layered or multi-layered structure of an organic electroluminescent material. The multi-layered organic electroluminescent structure may selectively include a hole injection layer, a hole transport layer, a light-emission layer, an electron transport layer or an electron injection layer. A dark-color frame of a light-absorption matrix for shading light may be selectively provided between the filters or the filter and the substrate to lessen the luminescent interference caused by ambient lights and increase sharpness of display image at the edge of the pixels.  
     [0019] The color conversion layer of the present invention is a thin film composed of a fluorescent powder and a binding polymer to convert light emitted from the organic electroluminescent layer by excitation through an electric current into white combination light and produce colors by means of the filter. The color conversion layer is constructed by a fluorescent powder capable of absorbing ultraviolet or blue light in a short wavelength. The fluorescent powder is formed by wet coating process or dry deposition. The fluorescent powder is preferably selected to be endurable to highly intensive light and capable of combining with a blue organic light-emitting component to result in white light. Basically, the fluorescent powder is used based on the wavelength of light emitted from the organic electroluminescent layer. If the organic electroluminescent layer emits ultraviolet light, the fluorescent powder used for the color conversion layer is preferably a composition capable of converting the emitted ultraviolet light into red, green or blue. If the organic electroluminescent layer emits blue light, the fluorescent powder used for the color conversion layer is preferably a composition capable of converting the emitted blue light into red or green. Preferably, the binding polymer is transparent epoxy, polyimide, urea resin, silicone or transparent inorganic adhesive. The transparent inorganic adhesive preferably is SiO 2  or TiO 2 . It is preferable for the transparent epoxy to be used in the wet coating processing while the transparent adhesive is preferably used in the dry deposition processing. The mixture ratio of the fluorescent powder and the binding polymer is adjustable in accordance with the luminescent efficiency of various colors to balance the luminescent efficiency. The distribution of the fluorescent powder in the color conversion layer can be controlled by arranging the structure of the color conversion layer as well as the temperature, viscosity, crystal structure and grain size used for forming the fluorescent powder. The OLED device can further comprise an overcoating layer sandwiched between the second electrode (anode) and the filter layer, wherein the second electrode layer (anode) and the substrate sandwich the overcoating layer to protect the color conversion layer. The material of the overcoating layer of the present invention is not specifically defined, and preferably is transparent epoxy, polyimide, urea resin, silicone or transparent inorganic adhesive. The transparent inorganic adhesive preferably is SiO 2  or TiO 2 . It is preferable for the transparent epoxy to be used in the wet coating processing while the transparent adhesive is preferably used in the dry deposition processing.  
     [0020] The fluorescent powder used in the present invention is not specifically defined, and preferably is yttrium aluminum oxide (YAG) fluorescent powder with doped rare-earth element. Because part of the yttrium will be substituted by rare-earth element in the crystal lattice, Y 2.9 R 0.1 Al 5 O 12  (R is rare-earth element) is formed. YAG is a transparent material of thermal stability, and capable of emitting light of different colors after doping different rare-earth elements; for example, Tb is doped into Y 3 Al 5 O 12  for emitting green light, and Ce for emitting yellow light.  
     [0021] The color conversion layer can be prepared by either wet processing or dry processing. A wet processing technique is to weigh the required quantity of the fluorescent powder to be blended directly, and then appropriate solvent and epoxy are added to the fluorescent powder, whereafter the powder, solvent and epoxy are mixed together. Another wet processing technique is to mix the fluorescent powder with solvent at the atomic scale by the sol-gel process or co-precipitation method, and then, mix them with epoxy. Thereafter, the mixture is applied to the overcoating layer or light-emitting panel by spin coating or printing, and then, is baked to remove the solvent and water.  
     [0022] An overcoating layer is coated or deposited as necessary to form a white-light color conversion layer with protection. This is a technique for forming a single white-light color conversion layer microscopically blended at the atomic scale to overcome the deteriorations in luminescent uniformity and luminescent efficiency of the prior art.  
     [0023] The dry processing technique is to weigh the required quantity of the fluorescent powder to be blended directly or mixed with a solvent at the atomic scale by the sol-gel process or co-precipitation method, and then, the mixture is blended with transparent adhesive such as SiO 2  or TiO 2 . It is necessary to consider the deposition rate for different color fluorescent powders in deposition. Further, an overcoating layer can be selectively formed to cover the color conversion structure.  
     [0024] It is preferable that the OLED according to the present invention be formed as a display panel having an array of a plurality of pixels emitting red, green and blue light to display an image, and also, the OLED according to the present invention can be a monochromatic display panel having an array of pixels emitting light, if so desired. The OLED panel fabricated according to the present invention can be applied to any environment or apparatus for displaying images, graphics, characters and text, and preferably, to the display panel of televisions, computers, printers, monitors, vehicles, to the displays of signal machines, communication apparatus, telephones, lamp equipment, headlights, interactive electronic books, micro-displays, fishing devices, personal digital assistants (PDA), game means, airplane equipment and head mounted displays.  
     [0025] Embodiment 1  
     [0026] OLED  
     [0027]FIG. 3 is a schematic view of the OLED device according to a preferred embodiment of the present invention. The OLED device of the present invention is passive matrix type, comprising a substrate  300 , a cathode (a first electrode layer)  342 , an anode (a second electrode layer)  344 , a blue organic electroluminescent layer  340 , a color conversion layer of fluorescent powder  330 , an overcoating layer  320 , a plurality of filters  310 , a dark-color frame of a light-absorption matrix  312 , and a cathode passivation  360 . In the process of the OLED device of the present invention, the dark-color frame of the light-absorption matrix  312  and a plurality of filters  310  are first formed over the substrate  300 . Hence, each of the filters  310  corresponds to a pixel. The pixel as herein referred is constructed by the cathode (first electrode layer)  342 , the anode (second electrode layer)  344  and the organic electroluminescent layer  340 . Although the organic electroluminescent layer is of a single-layer structure as illustrated hereinto, it can also be of a multi-layer structure. The dark-color frame of light-absorption matrix  312  is a black light-shading shadow mask used to shade diffusion light at the edge of the pixels. The dark-color frame of the light-absorption matrix  312  surrounds the edge of the pixels to define the size of the pixels. The overcoating layer is formed over the black frame of the light-absorption matrix  312  and the filter  310  to protect the black light-absorption shadow mask and the filter. In the present preferred embodiment, an overcoating layer  320  is formed over the black light-absorption shadow mask  312  and the filter  310 . A color conversion layer of fluorescent powder  330  is formed over the overcoating layer  320 . The color conversion layer of fluorescent powder  330  is a thin film consisting of a fluorescent powder and a binding polymer to convert the light emitted by excitation of the organic electroluminescent layer through an electric current into white combination light. A transparent indium tin oxide (ITO) layer  344  in the form of stripes is provided over the color conversion layer of fluorescent powder  330 . The stripe-shaped ITO layers  344  can be separated by isolation bodies of a photoresist parallel to each other to isolate a cathode substance formed among the pixels. The organic electroluminescent layer  340  is formed over the ITO layer by evaporation or sputtering to emit in a certain wavelength range. In this preferred embodiment, the electroluminescent layer  340  emits light in a wavelength range of blue after excitation by an electric current.  
     [0028] The fabrication of the OLED device of the preferred embodiment is to prepare for a fluorescent powder for forming a layer of fluorescent powder over the substrate at the beginning. The fluorescent powder is prepared by co-precipitation with triethylamine oxalate. The process for preparation of the fluorescent powder is briefly described below.  
     EXAMPLE 1  
     Fluorescent Powder Production  
     [0029] YAG fluorescent powder is produced by co-precipitation with triethylamine oxalate. R(NO 3 ) 3  (wherein R is La, Ce, Pr, Sm, Tb, Ho, Tm or Yb), Y(NO 3 ) 3  and Al(NO 3 ) 3  are mixed under stoichiometric ratio, and sufficiently dissolved in 25 ml of deionized water. Thereafter, 15 ml of triethylamine and 10 ml of oxalic acid (1.2 moles) are added to the above-mentioned mixture and processed at a pH of approximately 10.22, thereby obtaining white precipitated gels in the solution. Subsequently, the liquid mixture is agitated for several minutes, and then purified with a filtering process by air-extraction. After filtering, the white precipitated gels are baked in an oven for about twelve hours, and then, the baked white precipitated gels are taken out. Thereafter, they are placed in a furnace. Initially, the furnace is maintained at a temperature of 300° C. for an hour, and then, the temperature is increased to 500° C. and is maintained for another one hour, and finally, the temperature is increased to 1000° C. and maintained for another 24 hours. After cooling, the fluorescent powder with doped rare-earth element is obtained. As a result, the fluorescent powder is characterized by having a short period of residual fluorescence for about 120 nano-seconds, and is therefore applicable to be described as a component requiring fast response time.  
     [0030] The composition of the fluorescent powder prepared by this example in accordance with different light-emitting sources is shown in Table 1.  
                   TABLE 1                       Wavelength of light-emitting source   Composition of fluorescent powder                  470 nm (Blue light)   YAG : Ce 3+ (Yellow)       420-473 nm (Blue light/Ultraviolet   YBO 3  : Ce 3+ ,Tb 3+ (Green)/       light)   SrGa 2 S 4  : Eu 2+ (Blue)/           Y 2 O 2 S:Eu 3+ , Bi 3+ (Red)       370 nm (Ultraviolet light   Ca 8  Mg (SiO 4 ) 4  Cl 2  : Eu 2+ ,           Mn 2+ (Green) 20-50% /           Y 2 O 3  : EU 3+ ,Bi 3+ (Red)40-80% /           Ca 5 (PO 4 )  3 Cl:Eu 2+ (Blue) or           BaMg 2 Al 16 O 27 :Eu 2+ (Blue) 5-25%       460 nm (Blue light)   SrGa2O4 : Eu2+ (Green) /           CaS : Eu (Red)                  
 
     [0031] The white-light color conversion layer is prepared subsequent to completion of the preparation of the fluorescent powder layer. The process for preparation of the white-light color conversion layer is briefly described below.  
     EXAMPLE 2  
     Color Conversion Layer Production—Wet Process  
     [0032] The proportion of the fluorescent material is dosed in accordance with the principle of balancing the different luminescent efficiencies of the fluorescent material caused by three primary colors with reference to predetermined wavelength (e.g. blue light wavelength) of the emitted light in the spectrum. Then, the fluorescent materials are blended with epoxy resin at atomic scale through sol-gel process.  
     EXAMPLE 3  
     Color Conversion Layer Production—Dry Process  
     [0033] In the dry process, the quantities of the fluorescent material and transparent medium are weighed, and then, sufficiently blended to form a target. The target may be alternatively formed by the sol-gel process or co-precipitation method. A planed fluorescence color conversion layer is formed on the panel of organic light-emitting device by evaporation, sputtering or ion-beam deposition, wherein the proportion of the fluorescent material is dosed in accordance with the principle of balancing the difference in deposition rate of different fluorescent materials, to convert the light emitted at a shorter wavelength (e.g. blue light) in the spectrum into light of a longer wavelength in the spectrum (e.g. red light).  
     [0034] After completing the preparation for the white-light color conversion layer, a layer of color filters  310  is formed by printing in order of red, green and blue matrixes. An overcoating  320  is optionally coated onto the layer of color filters  310  by deposition. Thereafter, a white-light fluorescent powder is coated onto the filter layer  310  or the overcoating  320  by wet spin-coating. Then, the white-light fluorescent powder layer is baked to remove the solvent and water, and a white-light color conversion layer of fluorescent powder is completed after depositing. After the formation of the color conversion layer of fluorescent powder  330  over the filter layer  310 , an anode layer  344  (transparent electrode of ITO material) is formed over the color conversion layer of the fluorescent powder  330  by sputtering. The anode layer  344  is patterned by photolithography to form a plurality of transparent electrodes in form parallel stripes over the substrate. After sufficient rinsing of the substrate with patterned transparent electrodes, a photoresist layer  350  of a uniform thickness is formed over the substrate by spin-coating the composition of positive chemical amplification photoresist. Thereafter, the substrate coated with the positive chemical amplification photoresist composition is pre-baked in an oven. Then, the substrate is exposed in a development machine by means of stripe-patterned shadow masks. Further, the substrate is subjected to post-exposure baking treatment, and simultaneously, the surface of the photoresist layer is treated under the atmosphere full of tetramethyl ammonium hydroxide. After development, a plurality of transparent ITO electrodes in the form of parallel stripes which run perpendicular to the parallel stripe-shaped photoresist layer are formed over the substrate. The cross-section of the photoresist layer in form of parallel stripes shows a top of a reverse trapezoid with a thickness of 0.8 μm. The line width of the stripe-shape photoresist layer is 0.18 μm. Then, an organic electroluminescent layer  340  is formed over the anode layer  344  by evaporation. The formation of the organic electroluminescent layer  340  is made by using the stripe-shaped isolation body layer having a top of a reverse trapezoid as shadow masks, to form a layer of CuPc (copper phthalocyanine) with a thickness of 250 angstroms in the gaps between the parallel shadow masks by vacuum evaporation, and subsequently forming a layer of NPB(4,4′-bis[N-(1-naphthyl)-N-phenyl-amino]-biphenyl) with a thickness of 500 angstroms by evaporation, and then forming a layer of BA-1 (Bis (2-methyl-8-quinolinolato) aluminum(III)-μ-oxo-bis (2-methyl-8-quinolinolato) aluminum (III)) with a thickness of 500 angstroms by evaporation, and further forming a layer of LiF with a thickness of 15 angstroms. A cathode layer of aluminum  342  is formed over the organic electroluminescent layer  340  by evaporation. Similarly, the formation of the cathode layer  342  is made by vacuum evaporation to have a thickness of 1000 angstroms. Thus, an OLED component is formed. Finally, a passivation layer such as aromatic polyimide, parylene or teflon copolymer is deposited over the cathode layer  342 .  
     [0035] The device is turned on by providing an electric current through the cathode  342  and the anode  344 , and the organic electroluminescent layer  340  is driven to emit blue light. The blue light sheds light on the color conversion layer of fluorescent powder  330  and is converted into white combination light. The white combination light produces different color information by filtering through the color filter  310 .  
     [0036] Embodiment 2  
     [0037] Except for the use of the dry processing to form the white-light color conversion layer of the fluorescent powder and sputtering of the target material to form the white-light color conversion layer of fluorescent powder over the filter layer, the other processing steps are the same as those of Embodiment 1. After forming the white-light fluorescent powder, a white-light color conversion layer is formed over the filter layer by sputtering the composition, and simultaneously, the white-light color conversion structure is completed by depositing another passivation layer.  
     [0038] It is clear from the above description that the present invention is the first to disclose a technique for forming a single white light color conversion layer microscopically blended at the atomic level to overcome the deteriorations in luminescent uniformity and luminescent efficiency of the prior art. The present invention provides a component structure of high brightness and good uniformity for full color applications of the displays. Another advantage of the present invention is to dispense with the selective deposition of three primary colors so that the resolution of the upcoming display is no longer limited to the precision of the shadow mask. Also, the present invention increases yields of fabrication, and is very suitable for use in large-sized panels. Further, the present invention adopts the mature techniques of fluorescent material and color filtering for use in the field of OLED, and thus, can accelerate the commercial availability of the full color OLEDs.  
     [0039] In addition, the organic electroluminescent component of the present invention uses blue light to excite the fluorescent powder for emission without causing the specific orientation on spectrum. Hence, the present invention can provide uniform radiation of a wide bandwidth in the spectrum, and is particularly suitable for being a light source used for a scanner or display.  
     [0040] Although the present invention has been explained in relation to its preferred embodiment, it is to be understood that many other possible modifications and variations can be made without departing from the spirit and scope of the invention as hereinafter claimed.