Patent Publication Number: US-2007120464-A1

Title: Display

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
      This is a Continuation Application of PCT Application No. PCT/JP2005/018223, filed Sep. 26, 2005, which was published under PCT Article 21(2) in English.  
      This application is based upon and claims the benefit of priority from prior Japanese Patent Applications No. 2004-282679, filed Sep. 28, 2004; and No. 2005-002898, filed Jan. 7, 2005, the entire contents of both of which are incorporated herein by reference.  
     BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to a display.  
      2. Description of the Related Art  
      Since organic EL displays are of self-emission type, they have a wide viewing angle and a high repose speed. In addition, they do not require a backlight, and therefore, low-profile and lightweight are possible. For these reasons, the organic EL displays are attracting attention as a display which substitutes the liquid crystal display.  
      An organic EL element, which is the main part of the organic EL displays, includes a light-transmitting front electrode, a light-reflecting or light-transmitting back electrode facing the front electrode, and an organic layer interposed between the electrodes and containing a light-emitting layer. The organic EL element is a charge-injection type light-emitting element which emits light when an electric current flows through the organic layer. The light emitted by the organic EL element travels as natural light with no directivity to the outside of the display through a glass substrate, for example.  
      The organic EL display includes a multilayer film formed on a substrate. The light emitted by the light-emitting layer causes multiple-beam interference in the multilayer film. Thus, luminous efficiency of the display and color purity of the light emitted by the display depends on a structure of the multilayer film.  
      Jpn. Pat. Appln. KOKAI Publication No. 11-288786 discloses an organic EL element employing an optical resonator, i.e., a micro-cavity structure. In this organic EL element, an organic layer including a light-emitting layer is sandwiched between interfaces, each of which has a high reflectance. In the micro-cavity structure, of the light beams emitted by the light-emitting layer, light having a resonant wavelength is enhanced, and light having any other wavelength is attenuated. Therefore, when the micro-cavity structure is employed for the organic EL element of the organic EL display, the luminous efficiency of the display and the color purity of the light emitted by the display can be significantly improved.  
      However, the inventors have found in the course of achieving the present invention that, in the case where the micro-cavity structure is employed for a display, the following problem can occur. That is, the display employing the micro-cavity structure emits light having high directivity. Thus, the brightness of a display image significantly changes in response to an observation angle. In addition, an optical length of the micro-cavity structure concerning the light traveling in an oblique direction is different from an optical length of the micro-cavity structure concerning the light traveling in the direction of the normal line to the micro-cavity structure. Therefore, when the display employs the micro-cavity structure, chromaticity of a display image changes in response to the observation angle. That is, when the micro-cavity structure is employed for the display, there is a possibility that a display quality is significantly lowered.  
     BRIEF SUMMARY OF THE INVENTION  
      An object of the present invention to improve a display quality of a display employing a micro-cavity structure.  
      According to an aspect of the present invention, there is provided a display comprising an insulating substrate, a sealing member facing the insulating substrate, pixels interposed between the insulating substrate and the sealing member and each comprising a microcavity structure, wherein the microcavity structure comprises a reflecting layer, a half mirror layer facing the reflecting layer, and a light source interposed between the reflecting layer and the half mirror layer, and a diffusion layer facing the half mirror layer. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING  
       FIG. 1  is a sectional view schematically showing a display according to an embodiment of the present invention;  
       FIG. 2  is a partial cross section of the display shown in  FIG. 1 ;  
       FIG. 3  is a cross section schematically showing an example of a structure which can be employed for the display of  FIGS. 2 and 3 ;  
      FIGS.  4  to  7  are sectional views each schematically showing an example of a diffusion layer which can be used for the display of  FIGS. 1 and 2 ;  
       FIG. 8  is a graph showing an example of emission spectra of a display which omits a diffusion layer from the structure of  FIGS. 1 and 2 ;  
       FIG. 9  is a graph showing an example of emission spectra of the display shown in  FIGS. 1 and 2 ;  
       FIG. 10  is a graph showing an example of a relationship between an observation angle and display luminance;  
       FIG. 11  is a sectional view schematically showing a display according to a modified example;  
       FIGS. 12 and 13  are sectional views each schematically showing another example of a structure which can be employed for the display of  FIGS. 1 and 2 ;  
       FIG. 14  is a sectional view schematically showing a display according to another modified example;  
       FIG. 15  is a sectional view schematically showing an example of a structure which can be employed for the display of  FIG. 14 ;  
       FIG. 16  is a sectional view schematically showing an example of a structure which can be employed for a part of pixels of the display shown in  FIGS. 1 and 2 ;  
       FIG. 17  is a sectional view schematically showing an example of a structure which can be employed for another part of the pixels of the display shown in  FIGS. 1 and 2 ; and  
       FIG. 18  is a graph showing an example of a relationship between thickness of an optical adjustment layer and order of interference in the case where a resin layer having a refractive index of 1.5 is used as the optical adjustment layer. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      An embodiment of the present invention will be described below in detail with reference to the accompanying drawings. The same reference numerals denote the same or similar constituent elements throughout the drawings, and a repetitive description thereof will be omitted.  
       FIG. 1  is a sectional view schematically showing a display according to an embodiment of the present invention.  FIG. 2  is a partial cross section of the display shown in  FIG. 1 .  FIG. 3  is a cross section schematically showing an example of a structure which can be employed for the display of  FIGS. 2 and 3 . In  FIGS. 1 and 2 , the display is illustrated such that its display surface, that is, the front surface or the light-emitting surface, faces upwardly and the back surface faces downwardly.  
      The display  1  shown in  FIGS. 1 and 2  is a top emission organic EL color display employing an active matrix driving method. The organic EL display  1  includes an array substrate  2  and a sealing member  3 .  
      The sealing member  3  is a glass substrate in this example, and its surface facing the array substrate  2  has a recessed shape, for example. The array substrate  2  and the sealing substrate are joined together at peripheries thereof by means of, for example, adhesive or frit seal so as to form an enclosed space therebetween. The enclosed space is gastight and may be filled with an inert gas such as nitrogen gas or be evacuated.  
      A sealing technique of filling the enclosed space between the array substrate  2  and the sealing member  3  with a solid such as a resin may be utilized. Alternatively, as the sealing member  3 , a film sealing technique in which an organic material layer, an inorganic material layer, or a laminate of the organic material layer and the inorganic material layer is used instead of the glass substrate may be utilized, for example.  
      The organic EL display  1  may further include a polarizer  4  on an outermost surface on a front side of the display. The polarizer is useful in preventing the display surface from reflecting extraneous light.  
      The array substrate  2  includes an insulating substrate  10  such as a glass substrate.  
      On the insulating substrate  10 , pixels are arranged in a matrix form. Each pixel includes a pixel circuit and an organic EL element  40 .  
      The pixel circuit includes, for example, a drive transistor (not shown) and an output control switch  20  connected in series with the organic EL element  40  between a pair of power supply terminals, and pixel switch (not shown). A gate of the drive transistor is connected via the pixel switch to a video signal line (not shown) laid correspondently with a column of the pixels. The drive transistor outputs a current, whose magnitude corresponds to a video signal supplied from the video signal line, to the organic EL element  40  via the output control switch  20 . A gate of the pixel switch is connected to a scan signal line (not shown) laid correspondently with a row of the pixels. A switching operation of the pixel switch is controlled by a scan signal supplied from the scan signal line. Note that other structures can be employed for the pixels.  
      On the insulating substrate  10 , as an undercoat layer  12 , for example, an SiN X  layer and an SiO X  layer are formed in this order. A semiconductor layer  13  such as a polysilicon layer in which a channel, source and drain are formed, a gate insulator  14  which can be formed with use of, for example, TEOS (tetraethyl orthosilicate), and a gate electrode  15  made of, for example, MoW, are arranged in this order on the undercoat layer  12 , and these layers form a top gate-type thin film transistor (referred to as a TFT hereinafter). In this example, the TFTs are used as the pixel switch ST, output control switch and drive transistor. Further, on the gate insulator  14 , scan signal lines which can be formed in the same step as that for the gate electrode  15  are arranged.  
      An interlayer insulating film  17  made of, for example, SiO X  which is deposited by a plasma CVD method, covers the gate insulator  14  and gate electrode  15 . Source and drain electrodes  16  are arranged on the interlayer insulating film  17 , and they are buried in a passivation film  18  made of, for example, SiN X . The source and drain electrodes  16  have, for example, a three-layer structure of Mo/Al/Mo, and electrically connected to the source and drain of the TFT via contact holes formed in the interlayer insulating film  17 . Further, on the interlayer insulating film  17 , video signal lines (not shown) which can be formed in the same step as that for the source and drain electrodes  16  are arranged.  
      A flattening layer  19  is formed on the passivation film  18 . On the flattening layer  19 , first electrodes  41  with light-reflection property are arranged spaced apart from one another. Each first electrode  41  is connected to a drain electrode  16  via through-holes formed in the passivation film  18  and the flattening layer  19 .  
      The first electrode  41  is an anode in this example. As a material of the first electrode  41 , for example, Al, Ag, Au and Cr can be used.  
      A partition insulating layer  50  is placed on the flattening layer  19 . In the partition insulating layer  50 , through-holes are formed at positions corresponding to the first electrodes  41 . The partition insulating layer  50  is an organic insulating layer, for example, and can be formed by using a photolithography technique.  
      An active layer (or an organic layer)  42  including a light-emitting layer  420  is placed on each first electrode  41  which is exposed to a space in the through-hole of the partition insulating layer  50 .  
      The light-emitting layer  420  is a thin film containing a luminescent organic compound which can generate, for example, red, green or blue light. The active layer  42  can further include a layer other than the light-emitting layer  420 . For example, the active layer  42  can further include a hole transporting layer  422 , a hole blocking layer  423 , an electron transporting layer, an electron injection layer  425 , buffer layer  426 , etc. Materials of the layers other than the light-emitting layer  420  may be inorganic material or organic material.  
      The partition insulating layer  50  and the active layer  42  are covered with a second electrode  43  with light-transmission property. In this example, the second electrode  43  is a cathode which is continuously formed and common to all pixels. The second electrode  43  is electrically connected to an electrode wiring (not shown), the electrode wiring being formed on the layer on which the video signal lines are formed, via contact holes (not shown) formed in the passivation film  18 , the flattening layer  19 , and the partition insulating layer  50 . Each organic EL element  40  includes the first electrode  41 , active layer  42  and second electrode  43 .  
      A diffusion layer  60  is placed on the second electrode. A variety of structures can be employed for the diffusion layer  60 .  
       FIGS. 4 and 7  are sectional views each schematically showing an example of a diffusion layer which can be used in the display of  FIGS. 1 and 2 .  
      The diffusion layer  60  shown in  FIG. 4  is a light-transmitting layer having a main surface which is provided with randomly arranged recesses and/or protrusions. The diffusion layer  60  decreases observation direction dependencies of the brightness and chromaticity of a display image. In addition, the diffusion layer  60  increases the luminous energy of the light which travels from an inside of the display  1  to its outside by its light scattering effect. That is, the diffusion layer  60  improves an outcoupling efficiency.  
      In the example of  FIG. 2 , the diffusion layer  60  shown in  FIG. 4  is, for example, a resin sheet or a resin film which can be handled by itself. In this case, the diffusion layer  60  is fixed on a second electrode  43  by means of an adhesive layer  61 , for example. The thickness of the adhesive layer  61  is 20 μm or more in general. Thus, even if irregularities occur on a surface of the second electrode  43 , a gap is prevented from being generated between the adhesive layer  61  and the second electrode  43 .  
      The diffusion layer  60  shown in  FIG. 5  includes light-transmitting particles  62  placed on the second electrode  43 . The light-transmitting particles  62  are formed by coating transparent particles  62   a  with an adhesive  62   b.  The adhesive  62   b  bonds the transparent particles  62   a  together and bonds the transparent particles  62   a  to the second electrode  43 . The diffusion layer  60  shown in  FIG. 5  can be formed by distributing the light-transmitting particles  62  over the second electrode  43  by wet or dry process. The diffusion layer  60  shown in  FIG. 6  can be formed by distributing the transparent particles  62   a  over the adhesive layer  61  by wet or dry process. The diffusion layer  60  shown in  FIGS. 5 and 6  makes it possible to improve the outcoupling efficiency by light-scattering.  
      The diffusion layer  60  shown in  FIG. 7  is a light-scattering layer which includes a light-transmitting resin  63  and particles  64  dispersed therein. The particles  64  are different in optical property such as refractive index from the light-transmitting resin  63 . The diffusion layer  60  can be formed, for example, by coating the second electrode  43  with a coating solution which contains the particles  64  and a material for the light-transmitting resin  63  and curing the obtained coating film. Note that the material for the light-transmitting resin  63  is the one which can be cured at a temperature equal to or lower than the glass transition temperature of the organic layer  42 .  
      In the diffusion layer  60  of FIGS.  5  to  7 , a material higher in refractive index than a waveguide layer such as TiO 2  or ZrO 2  may be used for the light-transmitting particles  62   a  and the particles  64 . In this case, higher outcoupling efficiency can be achieved as compared with the case where a resin having a refractive index of about 1.5 is used.  
      In the display  1 , the organic EL element  40  forms at least a part of a micro-cavity structure MC. The micro-cavity structure MC includes a reflection layer RF and a half mirror layer HM facing each other and a light source LS interposed between these layers. In this example, the reflection layer RF is the first electrode  41 . The half mirror layer HM is, for example, a buffer layer  426  made of MgAg. The light source LS has a layered structure including a hole injection layer  421 , a hole transporting layer  422 , an emitting layer  420 , a hole blocking layer  423 , and an electron injection layer  425 .  
      The reflection layer RF is a layer which has light-reflection property, and typically, is a metal thin film. The half mirror layer HM is a layer which has light-transmission property and light-reflection property. The half mirror layer HM has a higher transmittance as compared with the reflection layer RF. The reflection layer RF has a higher reflectance as compared with the half mirror layer HM. For example, the reflectance of the reflection layer RF is 30% or more, and the reflectance of the half mirror layer HM is 15% or more.  
      The micro-cavity structure MC enhances the light whose wavelength A satisfies the relationship represented by the following equation (1). On the other hand, the light whose wavelength λ satisfies the relationship represented by the following equation (2) is attenuated. In the equations, L is an optical length between the reflection layer RF and the half mirror layer HM; Φ 1  is a phase shift of light caused by being reflected on the half mirror layer HM; Φ 2  is a phase shift of light caused by being reflected on the reflection layer RF; and m is an integer.  
                   2   ⁢   L     λ     +       Φ   1       2   ⁢   π       +       Φ   2       2   ⁢   π         =   m           (   1   )                     2   ⁢   L     λ     +       Φ   1       2   ⁢   π       +       Φ   2       2   ⁢   π         =         2   ⁢   m     +   1     2             (   2   )             
 
      As is evident from equations (1) and (2), the luminance and color purity of an image observed in a specific direction can be improved by employing the micro-cavity structure MC. In the equations, however, the optical length L is a function of the observation angle θ. Specifically, the optical length L is proportional to 1/cos θ.  
      Therefore, when a minimum value L 0  of the optical length L is increased, directivity is improved. That is, a slight shift of the observation angle θ greatly influences luminance.  
      On the other hand, if the minimum value L 0  of the optical length L is reduced, the shift of the observation angle θ less influences the luminance. However, in this case, it becomes difficult to equalize the minimum value L 0  of the optical length L between pixels having their different emitting colors. That is, there is a possibility that a display structure or its manufacturing process becomes complicated.  
      In this display  1 , the diffusion layer  60  is placed on the front side of the micro-cavity structure MC. Therefore, as described below, it is possible to prevent brightness of a display image from significantly changing according to the observation angle or chromaticity of the display image from significantly changing according to the observation angle. That is, according to the present embodiment, a high display quality can be achieved.  
       FIG. 8  is a graph showing an example of emission spectra of a display which omits a diffusion layer from the structure of  FIGS. 1 and 2 .  FIG. 9  is a graph showing an example of emission spectra of the display shown in  FIGS. 1 and 2 . Here, the structure of  FIG. 6  has been employed for the diffusion layer  60 .  
      In  FIGS. 8 and 9 , the abscissa indicates a wavelength, and the ordinate indicates emitting intensity of the display  1 . Curves A 1  and A 2  indicate emission spectra of the display  1  in the case where a display surface has been observed in the direction of the normal line (observation angle θ=0°). Curves B 1  and B 2  indicate emission spectra of the display  1  in the case where the display surface has been observed in the direction crossing the direction of the normal line at an angle of 60° (observation angle θ=60°).  
      As shown in  FIG. 8 , in the display omitting the diffusion layer  60 , a peak wavelength is changed by about 100 nm as the observation angle θ is changed by 60°. In contrast, in the display including the diffusion layer  60 , as shown in  FIG. 9 , even if the observation angle θ is changed by 60°, the peak wavelength is not changed. In this manner, in the display including the micro-cavity structure MC, by placing the diffusion layer  60  on the front side of the micro-cavity structure MC, the observation direction dependency of the chromaticity can be reduced.  
       FIG. 10  is a graph showing an example of a relationship between an observation angle and display luminance. In the figure, the abscissa indicates a direction parallel to a display surface, and the ordinate indicates a direction vertical to the display surface. A curve C indicates luminance of a display omitting the diffusion layer  60  from the structure of  FIGS. 1 and 2 . A curve D indicates luminance of the display of  FIGS. 1 and 2 . A distance from an origin to a certain point on the curve C or D corresponds to luminance in the case where a display surface is seen in a direction parallel to a straight line passing through that point and the origin. An angle made by this straight line and the vertical line is defined as an observation angle θ.  
      As is evident from the curve C, in the display omitting the diffusion layer  60 , if the observation angle θ is slightly shifted from 0°, the luminance is significantly lowered. On the other hand, in the display  1  including the diffusion layer  60 , as is evident from the curve D, even if the observation angle θ is significantly shifted from 0°, the luminance is slightly lowered. That is, the display  1  including the diffusion layer  60  has smaller observation direction dependency of luminance as compared with the display omitting the diffusion layer  60 .  
      Note that even in the case where the minimum value Lo of the optical length L is small, a slight shift of the minimum value L 0  significantly influences luminance and chromaticity. That is, in the case where the micro-cavity structure has been employed, it has been necessary to control the film thickness of each layer with high precision. In addition, if the minimum value L 0  of the optical length L is reduced, a short circuit between electrodes caused by dust adhesion is likely to occur.  
      In the present embodiment, in achieving the above effect, there is no need for reducing the minimum value Lo of the optical length L or there is no need for controlling the film thickness of each layer with high precision. Therefore, according to the present embodiment, a manufacturing process is facilitated and the yield can be improved.  
      Various modifications can be made in the display  1 .  
       FIG. 11  is a sectional view schematically showing a display according to a modified example. The display  1  of  FIG. 11  has a structure which is substantially identical to that of the display  1  of  FIGS. 1 and 2  except that the diffusion layer  60  is placed between the sealing member  3  and the polarizer  4  instead of attaching the diffusion layer  60  onto the second electrode  43 . Thus, a position of the diffusion layer  60  is not limited in particular as long as the layer is positioned on the front side of the micro-cavity structure MC.  
      Various modifications can occur in the micro-cavity structure MC of the display  1  shown in  FIGS. 1 and 2 .  
       FIGS. 12 and 13  are sectional views each schematically showing another example of a structure which can be employed for the display of  FIGS. 1 and 2 .  
      In the micro-cavity structure MC of  FIG. 12 , the first electrode  41  is a light transmission electrode. In addition, the reflection layer RF is placed on the back side of the first electrode  41 . With exception to the above, the micro-cavity structure of  FIG. 12  has a structure which is substantially identical to that of the micro-cavity structure MC of  FIG. 3 . Note that the micro-cavity structure MC further includes an electron transporting layer  424  between the hole blocking layer  423  and the electron injection layer  425 .  
      In the micro-cavity structure MC of  FIG. 12 , an ITO (indium tin oxide) can be used as a material of the first electrode  41 , for example. In addition, for example, Al, Al alloy, Ag, Ag alloy, Au, and Cu or the like can be used as a material of the reflection layer RF.  
      In the micro-cavity structure MC of  FIG. 13 , the buffer layer  426  does not serve as a half mirror layer HM, and a half mirror layer HM is placed on the second electrode  43 . With exception to the above, the micro-cavity structure MC of  FIG. 13  has a structure which is substantially identical to that of the micro-cavity structure MC of  FIG. 12 .  
      In the micro-cavity structure MC of  FIG. 13 , for example, an organic layer doped with alkaline metal and/or alkaline earth metal can be used as the buffer layer  426 . In addition, a multilayer film made of dielectrics or a metal thin film can be used as the half mirror layer HM, for example.  
      In the case where a metal thin film is used as the half mirror layer HM, in general, a high reflectance can be obtained in a wide wavelength range. On the other hand, in the case where a multilayer film made of dielectrics is used as the half mirror layer HM, in general, a high reflectance can be obtained only in a narrow wavelength range. However, in general, in the case where a multilayer film is used, higher transmittance can be obtained.  
      While  FIGS. 1 and 11  have shown a top emission display, the present invention can also be applied to a bottom emission display.  
       FIG. 14  is a sectional view schematically showing a display according to another modified example.  FIG. 15  is a sectional view schematically showing an example of a structure which can be employed for the display of  FIG. 14 . In  FIG. 14 , the display is illustrated such that its display surface, that is, the front surface or the light-emitting surface, faces downwardly and the back surface faces upwardly.  
      In the display  1  of  FIG. 14 , unlike the display  1  of  FIG. 1 , the diffusion layer  60  and the polarizer  4  are sequentially arranged on an outer surface of the array substrate  2 . The display  1  of  FIG. 14  has a structure which is substantially identical to that of the display  1  of  FIGS. 1 and 2  except that a structure of  FIG. 15  has been employed in addition to the above structure.  
      In the structure of  FIG. 15 , the buffer layer  426  is omitted from the structure of  FIG. 3 , and an electron transporting layer  424  is further placed between the hole blocking layer  423  and the electron injection layer  425 . In the structure of  FIG. 15 , the first electrode  41  is a light transmission electrode, the second electrode  43  is a reflection layer RF, and a half mirror layer HM is further placed on a front side of the first electrode  41 .  
      Al and/or MgAg can be used as a material of the second electrode  43 , for example. A multilayer film made of dielectrics or metal thin film can be used as the half mirror layer HM, for example.  
      As has been described with reference to  FIGS. 1 and 11 , the display  1  may be of top emission type. Alternatively, as has been described with reference to  FIG. 14 , the display  1  may be of bottom emission type.  
      As shown in  FIG. 13 , the entirety of an organic EL element  40  may be sandwiched between the reflection layer RF and the half mirror layer HM. That is, the organic EL element  40  may be a part of a micro-cavity structure MC. Alternatively, the organic EL element  40  itself may be the micro-cavity structure MC. Alternatively, as shown in  FIGS. 2, 11 , and  15 , only a part of the organic EL element  40  may be sandwiched between the reflection layer RF and the half mirror layer HM. That is, a part of the organic EL element  40  may be a part of the micro-cavity structure MC.  
      The organic EL element  40  may emit white light. In this case, for example, a color image can be displayed by using a color filter.  
      The diffusion layer  60  may be placed only at positions which correspond to pixels having a specific emitting color, and may not be placed at positions which correspond to pixels having another emitting color. In this case, for example, in pixels which do not face the diffusion layer  60 , an optical length L is set such that a wavelength λ meets the relationship represented by the above equation (1). By doing this, even if the wavelength does not meet the above relationship (1) in the pixels which face the diffusion layer  60 , a sufficient color balance can be achieved.  
      The following construction may be employed for the above described display  1 .  
       FIG. 16  is a sectional view schematically showing an example of a structure which can be employed for a part of pixels of the display shown in  FIGS. 1 and 2 .  FIG. 17  is a sectional view schematically showing an example of a structure which can be employed for another part of the pixels of the display shown in  FIGS. 1 and 2 .  
      A structure of  FIG. 16  is similar to that of  FIG. 12  except that an optical adjustment layer  70  is further included between the reflection layer RF and the first electrode  41 . A structure of  FIG. 17  is similar to that of  FIG. 12 .  
      As is evident from the equations (1) and (2) above, the luminance in the case where a screen has been observed in the direction of the normal line depends on the wavelength λ and the optical length L. Thus, in general, in order to obtain the effect of the micro-cavity structure on improving light emission efficiency and color purity in all the pixels differing in emitting color from each other, the optical length L should be appropriately set for each emitting color.  
      However, in general, a layered structure of the organic EL element  40  is determined in consideration of electron-hole injection balance, degradation of luminance, etc. Thus, it may be difficult to achieve an optimal optical length L.  
      In such a case, for example, the structure of  FIG. 16  may be employed for pixels having a certain emitting color, and the structure of  FIG. 17  is employed for pixels having another emitting color. The pixel employing the structure of  FIG. 16  includes the optical adjustment layer  70 , and thus, is different in the optical length L from the pixel employing the structure of  FIG. 17 . In the pixel employing the structure of  FIG. 16 , the optical length L can be optimized depending on the optical characteristics and thickness of the optical adjustment layer  70 . Moreover, the optical adjustment layer  70  is placed between the anode  41  and the reflection layer RF, and thus, does not influence the electron-hole injection balance, degradation of luminance, etc.  
      Therefore, when the structure of  FIG. 16  is employed for pixels having a certain emitting color and the structure of  FIG. 17  is employed for pixels having another emitting color, the luminance or the like in the case where a screen has been observed in the direction of the normal line can be optimized without influencing the electron-hole injection balance, degradation of luminance, etc. That is, it becomes possible to achieve more excellent display quality.  
       FIG. 18  is a graph showing an example of a relationship between thickness of an optical adjustment layer and order of interference in the case where a resin layer having a refractive index of 1.5 is used as the optical adjustment layer. In the figure, the abscissa indicates the thickness of an optical adjustment layer  70  and the ordinate indicates an order of interference which is caused by the light traveling in the micro-cavity structure MC in the direction normal to a film surface. In addition, in the figure, reference symbols B, G, and R indicate data obtained by carrying out simulation for pixels whose emitting colors are blue (λ=480 nm), green (λ=530 nm), and red (λ=630 nm), respectively. In the simulation, it is presumed that the organic EL elements  40  whose emitting colors are blue, green, and red have the same structure except that a material for the emitting layer  420  is different.  
      According to the data shown in  FIG. 18 , in the pixels whose emitting colors are blue and green, when the thickness of the optical adjustment layer  70  is set to, for example, about 100 nm, an order of interference is obtained as a value close to an integer (about 2). On the other hand, in the pixels whose emitting color is red, when the optical adjustment layer  70  is not placed, an order of interference is obtained as a value close to an integer (about 1). That is, when an optical adjustment layer  70  having thickness of about 100 nm is placed only in the pixels whose emitting colors are blue and green, high front brightness can be obtained in respective pixels of different emission colors.  
      In this embodiment, the optical adjustment layer  70  is placed between the anode  41  and the reflection layer RF only in the pixels of specific emission color. The above described effect can also be obtained when another structure is employed. For example, it is possible to employ a structure in which the optical adjustment layer  70  is placed between the anode  41  and the reflection layer RF in all the pixels and the optical thickness of the optical adjustment layer  70  is different between the pixels whose emitting colors are different from each other. For example, in an example shown in  FIG. 18 , the optical adjustment layer  70  having thickness of about 100 nm may be placed in the pixels whose emitting colors are blue and green, and the optical adjustment layer  70  having thickness of  180  nm may be placed in the pixels whose emitting color is red.  
      Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and representative embodiments shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.