Abstract:
A light-emitting device may include a light-emitting element, a microresonator, and a color filter. The light-emitting element may include first and second electrode and an emissive layer provided between the first and second electrodes, and may emit light of a predetermined color when a voltage is applied between the first and second electrodes to allow a current to flow in the emissive layer. The microresonator may repetitively reflect light of a predetermined color emitted from the emissive layer within an interval having an optical length corresponding to the predetermined color, and thereby intensifying and selecting the light of the predetermined color. The color filter may pass the light intensified and selected by the microresonator and further limiting to light having a wavelength of the predetermined color.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a continuation application under 35 U.S.C. §120 of U.S. application Ser. No. 10/953,667 filed on Sep. 29, 2004, priority to which is claimed herein and the contents of which are incorporated herein by reference. U.S. application Ser. No. 10/953,667 claims priority to Japanese Application No. 2003-342665, filed on Sep. 30, 2003, and Japanese Application No. 2004-275673, filed on Sep. 22, 2004, priority to both of which is claimed herein, and the contents of both of which are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an organic EL element which comprises a transparent electrode, an organic emissive layer disposed over the transparent electrode, and a counter electrode disposed over the organic emissive layer, and emits light when a voltage is applied between the transparent electrode and the counter electrode. 
     2. Description of the Related Art 
     In recent years, organic electroluminescence (hereinafter referred to as “EL”) displays have gained attention as one type of flat display which would replace liquid crystal displays in the coming generation. In a display panel of an organic EL display (hereinafter referred to as “organic EL panel”), the color of light emitted from each pixel may be determined depending on the emissive material used in the organic emissive layer of each pixel. By allowing the pixels to emit light of different colors using different emissive materials, RGB indication can be achieved. 
     However, when employing this method, the panel manufacturing process becomes difficult and complex because measures must be effected to compensate for differences in emissive efficiency of the emissive materials for different colors, and steps for applying different emissive materials to corresponding pixels must be carried out separately. 
     In order to achieve full color indication, other methods for determining pixel colors are proposed. In such methods, light of a single color alone is initially emitted, and color filters or color conversion layers are employed to obtain light of other colors. However, according to these methods, it is difficult to achieve sufficient emissive efficiency for each color. 
     Another alternative method using microcavities is disclosed in the following document: Takahiro NAKAYAMA and Atsushi KADOTA, “Element Incorporating Optical Resonator Structure, Third Meeting (1993)”, in “From the Basics to the Frontiers in the Research of Organic EL Materials and Devices”, Dec. 16 and 17, 1993, Tokyo University Sanjo Conference Hall, Japan Society of Applied Physics, Organic Molecular Electronics and Bioelectronics Division, JSAP Catalog Number AP93 2376, p. 135-143. According to this method, a microcavity which functions as a microresonator is provided in each pixel to extract light having a specific wavelength. Using this microresonator, light having a specific wavelength can be selectively intensified. 
     However, an EL element configured with a conventional microresonator disadvantageously has high dependency on viewing angle, such that unintended colors are detected when the element is viewed at an angle. Further, in order to select light having a specific wavelength using a conventional microresonator, the optical length of the microcavity must be set precisely, resulting in difficulties in the manufacturing process. 
     SUMMARY OF THE INVENTION 
     According to the present invention, a microresonator (microcavity) is provided in a portion of a pixel having an organic EL element. Light ejected through a semi-transmissive film is thereby limited to a specific wavelength while the specific wavelength is intensified. The light obtained after the wavelength selection by the microresonator is subsequently passed through a color filter so as to further restrict the wavelength. According to this arrangement, viewing angle dependency of a displayed color can be eliminated significantly. Furthermore, the required precision concerning the thickness of the EL element portion constituting the microresonator can be reduced, facilitating the panel manufacturing process. 
     In place of the color filter, a color conversion layer may alternatively be employed to convert light of a specific color into light of another color. When using color conversion layers, microresonators in the respective pixels can be configured to intensify light of one specific color alone. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view showing a configuration of a pixel portion of an organic EL element according to the present invention. 
         FIG. 2  shows an example configuration of organic EL elements for the respective colors of R, G, and B, according to the present invention. 
         FIG. 3  shows another configuration of organic EL elements for the respective colors of R, G, and B, according to the present invention. 
         FIG. 4  shows a configuration of an organic EL element which emits white light. 
         FIG. 5  shows an example configuration for the respective colors of R, G, and B using white-emitting organic EL elements, according to the present invention. 
         FIG. 6  is a diagram showing an example spectrum of a white-emitting organic EL element. 
         FIG. 7  shows an example configuration of a white-emitting organic EL element having a top-emission structure. 
         FIGS. 8-11  are schematic diagrams showing example pixel configurations of an organic EL panel. 
         FIG. 12  is a cross-sectional view showing a configuration of a pixel portion of an organic EL element using a color conversion layer, according to the present invention. 
         FIG. 13  is a schematic diagram showing an example RGB pixel configuration using color conversion layers, according to the present invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Preferred embodiments of the present invention will next be described referring to the drawings. 
       FIG. 1  is a cross-sectional view showing a configuration of a light-emitting region and a drive TFT (thin film transistor) within one pixel. It should be noted that each pixel actually includes a plurality of TFTs. The drive TFT is the TFT which controls a current supplied from a power line to an organic EL element within the pixel. On a glass substrate  30 , a buffer layer  11  composed of a lamination of an SiN layer and an SiO 2  layer is formed over the entire surface. Further on top, an active layer  22  made of polysilicon is disposed in predetermined areas (where TFTs are to be created). 
     Covering the active layer  22  and the buffer layer  11 , a gate insulation film  13  is formed over the entire surface. The gate insulation film  13  may be formed by laminating an SiO 2  layer and an SiN layer. On top of the gate insulation film  13  at a position above a channel region  22   c , a gate electrode  24  composed of chromium or the like is arranged. Subsequently, impurities are doped into the active layer  22  while using the gate electrode  24  as a mask. As a result of this process, in the active layer  22 , the channel region  22   c  without impurities is provided in the central portion under the gate electrode  24 , while a source region  22   s  and a drain region  22   d  doped with impurities are formed on both sides of the channel region  22   c.    
     Next, covering the gate insulation film  13  and the gate electrode  24 , an interlayer insulation film  15  is formed over the entire surface. Contact holes are then created in the interlayer insulation film  15  at positions corresponding to the source region  22   s  and the drain region  22   d  located under the interlayer insulation film  15 . Subsequently, a source electrode  53  and a drain electrode  26  are provided through these contact holes and on the upper surface of the interlayer insulation film  15 , so as to connect with the source region  22   s  and the drain region  22   d , respectively. It should be noted that the source electrode  53  is connected to a power line (not shown). While the drive TFT formed as described above is a p-channel TFT in this example, the drive TFT may alternatively be constituted as an n-channel TFT. 
     Covering the interlayer insulation film  15 , source electrode  53 , and drain electrode  26 , a film  71  of SiN or the like is formed over the entire surface. A color filter  70  is next formed on top of the SiN film  71  at a position corresponding to the light-emitting region in each pixel. 
     Covering the SiN film  71  and the color filter  70 , a planarization film  17  is provided over the entire surface. On top of the planarization film  17  at the position of the light-emitting region, a semi-transmissive film  69  composed of a thin film of Ag or the like is formed. A transparent electrode  61  which serves as an anode is then disposed on the semi-transmissive film  69 . At a position above the drain electrode  26 , a contact hole is created through the planarization film  17 . The drain electrode  26  and the transparent electrode  61  are connected via this contact hole. 
     While an organic film such as acrylic resin is typically used to form the interlayer insulation film  15  and planarization film  17 , it is also possible to employ TEOS or an inorganic film. A metal such as aluminum may be favorably used to create the source electrode  53  and drain electrode  26 . For the transparent electrode  61 , ITO is typically employed. 
     The transparent electrode  61  normally has a substantially rectangular overall shape with a contacting portion protruding laterally and downward through the contact hole for connection with the drain electrode  26 . As can be seen in  FIG. 1 , the semi-transmissive film  69  is formed slightly smaller than the anode  61 . 
     An organic layer  65  and a counter electrode  66  are arranged on top of the transparent electrode  61 . The organic layer  65  comprises a hole transport layer  62  formed over the entire surface, an organic emissive layer  63  formed slightly larger than the light-emitting region, and an electron transport layer  64  formed over the entire surface. The counter electrode  66 , which serves as a cathode, is made of metal such as aluminum, and is formed over the entire surface. 
     A planarization film  67  is provided at a position on the upper surface of the peripheral portion of the transparent electrode  61  and underneath the hole transport layer  62 . The planarization film  67  limits the portion in which the hole transport layer  62  directly contacts the transparent electrode  61 , thereby defining the light-emitting region in each pixel. It should be noted that, while an organic film such as acrylic resin is typically used for the planarization film  67 , it is also possible to employ TEOS or an inorganic film. 
     The hole transport layer  62 , the organic emissive layer  63 , and the electron transport layer  64  are composed of materials that are conventionally used in an organic EL element. The color of emitted light is determined depending on the material (usually the dopant) of the organic emissive layer  63 . For example, the hole transport layer  62  may be composed of NPB, the organic emissive layer  63  for emitting red light may be composed of TBADN+DCJTB, the organic emissive layer  63  for emitting green light may be composed of Alq 3 +CFDMQA, the organic emissive layer  63  for emitting blue light may be composed of TBADN+NPB, and the electron transport layer  64  may be composed of Alq 3 . 
     In the above-described arrangement, when the drive TFT is turned on by a voltage set in the gate electrode  24 , current from the power line flows from the transparent electrode  61  to the counter electrode  66 . This current causes light emission in the organic emissive layer  63 . The emitted light passes through the transparent electrode  61 , planarization film  17 , interlayer insulation film  15 , gate insulation film  13 , and glass substrate  30 , to be ejected downward in  FIG. 1 . 
     In the present embodiment, a semi-transmissive film  69  composed of a thin film of silver (Ag) or the like is provided on the underside of the transparent electrode  61  at the position of the light-emitting region. Accordingly, light generated in the organic emissive layer  63  is reflected by the semi-transmissive film  69 . Because the counter electrode  66  functions as a reflective layer, the light is repetitively reflected between the semi-transmissive film  69  and the counter electrode  66 . 
     The interval structure between the semi-transmissive film  69  and the counter electrode  66  is configured such that this interval optically functions as a microresonator for a specific color. In other words, the optical length of the interval is set to a value obtained by multiplying the wavelength of a desired color by an integer or a reciprocal of an integer (such as ½, 1, and 2). For example, the values of refractive index for the materials constituting each layer in the interval may be approximately as follows: 1.9 for ITO constituting the transparent electrode  61 ; 1.46 for SiO 2  constituting the gate insulation film  13 ; 2.0 for SiN also used for the gate insulation film  13 ; and 1.7 for an organic layer including the organic emissive layer  63 . By multiplying the physical thickness of each layer between the semi-transmissive film  69  and the counter electrode  66  by a corresponding refractive index, and then summing the calculated values, the optical thickness of the interval can be obtained. In the present embodiment, this optical thickness is set to a value relative to the wavelength of light to be extracted. With this arrangement, the interval between the semi-transmissive film  69  and the counter electrode  66  functions as a microresonator, and enables efficient extraction of light having a desired wavelength. More specifically, light emitted from the organic emissive layer  63  is repetitively reflected between the semi-transmissive film  69  and the counter electrode  66 , and as a result, light components having a specific wavelength are selectively passed through the semi-transmissive film  69 . By further repeating such reflection within the microresonator, the probability that light having the specific frequency will be ejected can be increased, resulting in enhanced efficiency. 
     According to the present embodiment, the color filter  70  is arranged in a layer between the interlayer insulation film  15  and the planarization film  17 . The color filter  70  may be composed of a material such as a photosensitive resin or polymer having a pigment mixed therein, similar to color filters used in a liquid crystal display and a CCD camera. 
     The color filter  70  serves to selectively pass the ejected light so as to limit the wavelength of the obtained light, thereby enabling reliable control of the obtained color. When the microresonator limits light passing through the semi-transmissive film  69  as described above, it may be considered that the color filter  70  is not a fundamental requirement. However, the microresonator basically regulates only the wavelength of light that is incident from a direction perpendicular to the surface of the semi-transmissive film  69 . Accordingly, the wavelength of light ejected from the microresonator is highly dependent on the viewing direction, such that different colors are likely to be detected when the panel is viewed at an angle. By providing the color filter  70  as in the present embodiment to pass the ejected light through the color filter  70 , the obtained light would unfailingly have a specific wavelength. In this manner, the viewing angle dependency of the panel can be substantially eliminated. 
     The position of the color filter  70  is not limited to the top of the interlayer insulation film  15 . Alternatively, the color filter  70  may be formed on the upper surface or the underside of the glass substrate  30 . A light-shielding film is often provided on the upper surface of the glass substrate  30  in order to prevent external light from irradiating on the drive TFT. In such a case, the color filter  70  may be formed in the same layer as the light-shielding film to simplify the manufacturing process. 
       FIG. 2  shows a configuration of pixel portions constituting the microresonators in R, G, and B pixels. In this example, resonance frequency is varied among the pixels of the respective colors of RGB by changing the thickness of the hole transport layer  62  in the decreasing order of R, G, and B. Thickness changes are made in the hole transport layer  62  because it is considered that a change in thickness of the hole transport layer  62  would have the least influence on the function compared to when such a change is made in the other layers. 
     By employing different emissive materials in the organic emissive layer  63 , each pixel is designed to emit light of one color among R, G, and B. In each pixel, the optical length from the upper surface of the semi-transmissive film  69  to the underside of the cathode  66  is configured in accordance with the wavelength of the emitted color. Accordingly, in each pixel, light of the emitted color is intensified by the microresonator, thereby achieving an increase in emissive efficiency. 
     Further, because color filters  70  are provided, even if the optical length of a microresonator is slightly deviated from the predetermined value, the resulting minor variances in the wavelength of the ejected light do not cause any problems. Accordingly, control of the thickness of each layer constituting the microresonator can be facilitated. 
     When changing the thickness of the hole transport layer  62  for each color as in the present embodiment, it is preferable to form the hole transport layer  62  only in the necessary portion (display area) in each pixel, similarly to the organic emissive layer  63 . Alternatively, it may be effective to change the thickness of the transparent electrode  61 . 
       FIG. 3  diagrammatically shows three pixels of R, G, and B. As can be seen, the semi-transmissive film  69  is provided for the pixel of one color alone, while no semi-transmissive film is provided for the pixels of other colors. This arrangement is employed because the interval between the semi-transmissive film  69  and the counter electrode  66  is configured to form a microresonator for the one color alone (red R in the present example). In the pixel for the one color, light of this color is intensified and passed through the semi-transmissive film  69 . In the pixels for the other colors, emitted light is ejected downward without further processing. Further, corresponding RGB color filters  70 R,  70 G, and  70 B are provided for the respective pixels. 
     While light emission of the three colors of RGB can be achieved using different organic materials, each organic material has a different emissive efficiency (amount of light emission/current) By employing a microresonator for a pixel of the color having the lowest emissive efficiency so as to intensify the emitted light, a more uniform light emission can be accomplished, such that the life of organic EL elements can be equalized among different colors. Moreover, because the microresonator is formed for one color alone, the thickness of each layer constituting the microresonator can be set easily. 
     Because the microresonator and the color filters  70  are provided in the present embodiment, the color of light emitted by each pixel can be white. In order to achieve emission of white light, the organic emissive layer  63  may be constituted with a two-layer structure including a blue emissive layer  63   b  and an orange emissive layer  63   o , as shown in  FIG. 4 . According to this arrangement, holes and electrons combine in regions near the border between the two emissive layers  63   b  and  63   o , thereby generating both blue light and orange light. The light of the two colors in combination are emitted as white light. The orange organic emissive layer  63   o  may be composed of materials such as NPB+DBzR. 
     Subsequently, in the present embodiment, light of a specific color among the emitted white light is intensified and selected using a microresonator, and further selected by a color filter  70  to be ejected. 
     When employing a white organic emissive layer  63  as described above, the organic emissive layer  63  can be formed over the entire surface, without the need to separately perform the emissive layer forming process for the pixels of different colors. The organic emissive material can be simply deposited without using masks. When adopting this configuration, it is preferable to control the thickness of the transparent electrode  61  in order to adjust the optical length of the microresonator. In this manner, all layers disposed above the transparent electrode  61  can be formed over the entire surface without using masks, further facilitating the panel manufacturing process. 
     The present embodiment is more specifically illustrated in  FIG. 5  showing the respective pixels of R, G, and B. As can be seen, the distance from the underside of the transparent electrode  61  to the underside of the cathode  66  is identical among all of the pixels. This distance is configured to have an optical length which selects and intensifies light of one color (green G, for example). In the pixel for this one color, the semi-transmissive film  69  is disposed beneath the transparent electrode  61 . In the pixels of other colors (red R and blue B, for example), no semi-transmissive film is provided. 
     According to this arrangement, in the G pixel, the microresonator extracts a specific color (green) from among the emitted white light as described above, and the extracted light is passed and ejected through a green color filter  70 . In the R and B pixels, the white light emitted from the organic emissive layer  63  is simply passed through the color filters  70  to be ejected as light of predetermined colors (red and blue, respectively). 
     In this embodiment, the only difference among the pixels is whether or not the semi-transmissive film  69  is provided. Further, the optical length can be set easily, and the panel manufacturing process can be very much simplified. Moreover, light for one color can be intensified using the microresonator. When white light obtained by emission of two colors is used, one color among the three primary colors tends to have lower intensity compared to the other two colors. By employing the microresonator for the low-intensity color, a favorable color display can be achieved. For example, when light emission is executed by two emissive layers of blue and orange, the intensity of green light becomes lower than the other colors, as shown in  FIG. 6 . In order to equalize intensity, the semi-transmissive film  69  is provided for the green pixel so as to configure the microresonator to intensify the green light. In this manner, effective color display can be accomplished. 
     While the above-described embodiments refer to a bottom emission type panel in which light is ejected via the glass substrate  30 , an EL panel according to the present invention may alternatively be configured as top emission type in which light is ejected via the cathode. 
       FIG. 7  shows a configuration of a pixel portion of a top emission type panel. In this example, a transparent cathode  90  composed of ITO is employed as the cathode. Further, a semi-transmissive film  91  is disposed on the underside of the transparent cathode  90 . 
     Furthermore, a metal reflective layer  93  is formed under the transparent electrode  61 . The interval structure between the surface of the metal reflective layer  93  and the semi-transmissive film  91  functions as the microresonator. 
     In this embodiment, the color filter  70  is provided on the underside of a sealing substrate  95 . It should be noted that the sealing substrate  95  connects to the substrate  30  at its peripheral portion alone, and serves to seal the upper space of the substrate  30  having components such as the organic EL element formed thereon. The top emission structure shown in  FIG. 7  can be employed in any of the above-described configurations according to the present invention. 
     While the TFTs in the above embodiments are described as top gate type TFTs, bottom gate type TFTs may alternatively be used. 
       FIGS. 8-11  diagrammatically illustrate example configurations of the present invention. To simplify explanation, only the characteristic structures are shown in these drawings. 
     In  FIG. 8 , three types of organic emissive layers, namely, red emissive layer (red EL), green emissive layer (green EL), and blue emissive layer (blue EL) are employed. Further, corresponding to the red EL alone, a red color filter (red CF) is arranged. In this example, the color filter is provided for the color (red, in this case) having the highest dependency on viewing angle. It should be noted that it is also possible to provide a color filter for any of the other colors alone. 
       FIG. 9  shows an example in which color filters are arranged for all of the three colors. In this example, a white organic emissive layer (white EL) is provided as the organic emissive layer on the entire surface. Each of the pixels for the respective colors is provided with a microresonator along with a color filter of the corresponding color. 
     In  FIG. 10 , color filters are arranged for all of the three colors, while a red emissive layer (red EL), green emissive layer (green EL), and blue emissive layer (blue EL) are employed as the organic emissive layer. Each of the pixels for the respective colors is provided with a microresonator along with a color filter of the corresponding color. 
     In  FIG. 11 , an additional white pixel having a transparent electrode is provided in addition to the configuration of  FIG. 9 . By adding a white (W) pixel to RGB pixels as shown, a bright screen display can be easily achieved. 
       FIG. 12  shows the configuration of  FIG. 1  in which the color filter  70  is replaced with a color conversion layer  80 . An example of this color conversion layer  80  is described in Japanese Patent Laid-Open Publication No. 2003-187975. Using this color conversion layer  80 , a specific color can be converted into another specific color. For example, light emitted by a blue emissive layer may be converted into red and green light. In this case, a single blue emissive layer  63  alone is formed as the organic emissive layer over the entire surface. Further, red and green pixels are provided with color conversion layers  80  for converting the emitted blue light into red and green light, respectively. Each of the RGB pixels can be realized in this manner. 
       FIG. 13  shows an example configuration of three pixels of RGB. It should be noted that  FIG. 13  is a schematic diagram in which the TFT structure and the structure connecting the TFT and the transparent electrode  61  are not shown. 
     A color conversion layer  80 R for converting blue light into red light is arranged below the transparent electrode  61  in a red pixel, while a color conversion layer  80 G for converting blue light into green light is arranged below the transparent electrode  61  in a green pixel. A blue pixel does not include any color conversion layer. 
     The hole transport layer  62 , blue organic emissive layer  63   b , electron transport layer  64 , and counter electrode  66  are formed extensively over the entire surface so as to serve commonly for all pixels. 
     Microresonators are formed in the respective pixels by the layers between the semi-transmissive film  69  and the counter electrode  66 . In the present example, because the microresonators only need to intensify blue light, the interval between the semi-transmissive film  69  and the counter electrode  66  can be made identical in all of the pixels. 
     According to this arrangement, all of the hole transport layer  62 , organic emissive layer  63  ( 63   b ), and electron transport layer can be formed extensively over the entire substrate (commonly for all pixels), thereby simplifying the manufacturing process.