Patent Application
20120256562
1. Field of the Invention
The present invention relates to a display apparatus including organic electroluminescence (EL) elements, and more particularly, to a full-color display apparatus in which each one pixel includes a plurality of sub-pixels for emitting light with different colors.
2. Description of the Related Art
In recent years, organic light emitting elements that perform self luminescence at a low driving voltage of about several volts have attracted attention. By virtue of outstanding features, such as surface-emitting property, light weight, and high visibility, an organic EL element is proceeding toward practical utilization as a light emitting element for use in a flat panel display, a lighting apparatus, a head mount display, or a print head light source of an electrophotographic printer.
An organic EL element has a structure in which a light emitting layer formed of an organic material and a plurality of other functionally separated layers formed of organic materials are sandwiched between an anode and a cathode. At least an electrode on a light emitting side is transparent. Because of this layered structure, when light travels in a direction at an angle more than a critical angle of interfaces determined by the refractive index of the light emitting layer, the medium of the light emitting side, and the refractive index of air in which the light is finally emitted, the light is totally reflected and is confined as propagation light in the element. The propagation light is absorbed by an organic compound layer and a metal electrode in the element, and is not extracted outside. This reduces the light extraction efficiency.
To extract propagation light outside for enhancement of the light extraction efficiency, various methods have been proposed, which change the light traveling direction by forming a fine uneven structure or a lens structure on a light emitting surface so as to disturb a total reflection condition. Japanese Patent Laid-Open No. 2004-296429 proposes a method that is particularly effective in enhancing the light extraction efficiency. In this method, a transparent layer having a refractive index higher than or equal to that of a light emitting layer is provided in contact with a light emitting side of a transparent electrode, and a region for disturbing light reflection and scattering angles is provided on a light emitting side of or in the transparent layer.
In this method, according to the traditional Snell's law, propagation light in the light emitting layer, which makes up about 80% of light emitted from the light emitting layer, is brought into the high-refractive-index transparent layer having a refractive index higher than that of the light emitting layer, where the propagation light is converted into propagation light in the transparent layer. The propagation light can be extracted outside by the region provided on the surface of or in the transparent layer to disturb the light reflection and scattering angles.
However, this method for propagating light in the high-refractive-index transparent layer has problems unique to application to a display apparatus such as a display. When light is led into the high-refractive-index transparent layer and is finally extracted into air by the region for disturbing the light reflection and scattering angles, the light includes light traveling at an angle larger than or equal to the critical angle. This traveling light is normally subjected to total reflection. Therefore, it is recognized, because of parallax resulting from the thickness of the high-refractive-index transparent layer, that the light is emitted from a position different from an actual light emitting point. This causes a problem of bleeding of a display image. To address this problem, Japanese Patent Laid-Open No. 2005-322490 proposes a method in which the thickness of a substrate in which light propagates, not a high-refractive-index transparent layer, is limited to be lower than or equal to a given ratio of the pixel size.
Further, when the light led into the high-refractive-index transparent layer enters the disturbing region that disturbs the light reflection and scattering angles, it is not always extracted into air at one time. Even when the traveling direction is changed by the disturbing region, the light traveling at the angle larger than or equal to the critical angle of the interface between the high-refractive-index transparent layer and air is totally reflected again, and propagates in the high-refractive-index transparent layer. As a result, the light laterally propagates in the high-refractive-index transparent layer, and is eventually emitted into air from a position distant from the light emitting point where the total reflection condition is disturbed. Hence, a problem of bleeding of a display image also occurs. In particular, since the amount of light of a high-angle component increases as the refractive index of the transparent layer increases, the number of entries in the disturbing region is reduced, and the distance by which the light is laterally guided until it is extracted into air is increased.
The present invention reduces bleeding of a display image by efficiently extracting, to the outside, propagation light that propagates in a transparent layer having a refractive index higher than that of an organic compound layer in a display apparatus using an organic EL element.
A display apparatus according to an aspect of the present invention includes a plurality of pixels each including a plurality of sub-pixels configured to emit light with different colors. Each of the sub-pixels has an organic EL element including a first electrode, a second electrode, and an organic compound layer including a light emitting layer provided between the first electrode and the second electrode. A transparent layer having a refractive index higher than a refractive index of the organic compound layer is provided on a light emitting side of the organic EL element. A light extraction structure is provided on a light emitting side of the transparent layer. The light extraction structure is provided on the sub-pixels in the pixels and is not provided on a region between two adjacent pixels.
According to the present invention, it is possible to provide a display apparatus that reduces bleeding of a display image while enhancing light extraction efficiency.
Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings.
An organic EL element includes, on a first electrode, several organic compound layers including a light emitting layer having a light emission region, and a second electrode. The organic EL element emits light by utilizing energy produced when a hole and an electron injected in the organic compound layers are recombined by applying voltage between the first electrode and the second electrode. One of the first electrode and the second electrode is a reflection electrode, and the other is a transparent electrode. One of the first electrode and the second electrode is an anode, and the other is a cathode. In the display apparatus of the present invention, the reflection electrode is provided as the first electrode on a support substrate, and light is extracted from the transparent electrode. In the display apparatus of the present invention, a high-refractive-index transparent layer having a refractive index higher than that of the organic compound layers is provided adjacent to the transparent electrode so as to effectively extract the light emitted in the organic EL element to the outside. Further, a light extraction structure is provided adjacent to the high-refractive-index transparent layer so as to extract the light. With this structure, light from the light emitting layer reaches the light extraction structure without being totally reflected, and is effectively extracted outside.
In the present invention, a region where the light extraction structure is provided and a region where the light extraction structure is not provided are separately provided to suppress bleeding of a display image. This suppresses bleeding of a display image resulting from color mixture in an interpixel region.
An embodiment of the present invention will be described below.
In the display apparatus of the embodiment, light emission regions of the sub-pixels 1, 2, and 3 are determined by the area of an electrode patterned on a below-described support substrate. In this case, the display apparatus has a cross-sectional structure schematically illustrated in
In the structure of
The sub-pixels 1, 2, and 3 are formed by organic EL elements that emit light having different luminescent colors. In
In the example of
By adjusting the film thickness between the light emitting position of the light emitting layer 26 and a reflective surface of the reflection electrode 22 in the organic compound layer 16, a radiation distribution in the light emitting layer 26 can be controlled. As the display apparatus, by setting the thicknesses of the organic compound layers so that the luminance particularly on the front side becomes high, the luminescent color is controlled by optical interference and light is more efficiently emitted toward the front side. More specifically, by setting the optical distance from the light emitting position of the light emitting layer 26 to an interface between the transparent electrode 23 and the reflection electrode 22 to be n/4 (n=1, 3, 5, . . . ) of the emission wavelength, the luminance in the frontward direction from the light emitting layer 26 toward the light emitting side can be increased further. To enhance the light extraction efficiency, it is preferable that n=1.
For a higher light extraction efficiency, the reflectance of the reflection electrode 22 is preferably as high as possible. For example, as the material of the reflection electrode 22, silver (Ag) is preferable to aluminum (Al). To further increase the reflectance, layers having different refractive indexes may be stacked like a dielectric multilayer film mirror.
In the example of
The transparent electrode 20 serving as the second electrode may be replaced with a translucent electrode. In this case, the reflectance of the second electrode increases, and the property of an optical resonator is developed. However, a high-angle radiation light component is produced from the light emitting layer 26, although the amount thereof is small. Hence, the increase in light extraction efficiency is less than in the transparent electrode 20, but the translucent electrode is also effective. The second electrode does not always need to be transparent.
The high-refractive-index transparent layer 10 may be used as a barrier layer against entry of gas such as vapor and oxygen. To function as the barrier layer, although depending on the material to be used, the high-refractive-index transparent layer 10 preferably has a thickness of about several micrometers, more preferably, within the range of 0.5 to 6.0 μm. The preferred thickness does not need to be specified because it depends on the size of the light extraction structures 7. If the thickness of the high-refractive-index transparent layer 10 is more than 6.0 μm, light easily propagates for a long distance through the high-refractive-index transparent layer 10, and is easily extracted from the light extraction structures 7 in the next pixel 4. To enhance the light extraction efficiency, the thickness of the high-refractive-index transparent layer 10 is more preferably within the range of 0.5 to 1.0 μm.
Although the refractive index of the organic compound layers 16, 17, and 18 varies according to the material, it is about 1.6 to 2.0 in a blue light emission region, about 1.5 to 1.9 in a green light emission region, and about 1.5 to 1.8 in a red light emission region. Therefore, it is satisfactory as long as the refractive index of the high-refractive-index transparent layer 10 is at least higher than the refractive index of the organic compound layers 16, 17, and 18 used in the organic EL elements in the blue, green, and red light emission regions. More preferably, the refractive index of the high-refractive-index transparent layer 10 is higher than or equal to 2.0.
For example, the high-refractive-index transparent layer 10 is formed of titanium oxide, zirconium oxide, or zinc oxide. However, it is difficult to process these materials. In the present invention, the high-refractive-index transparent layer 10 is preferably formed by a silicon nitride film (SiNx). The elemental composition and elemental composition ratio of the silicon nitride film (SiNx) are not particularly limited, and the silicon nitride film may be formed of a mixture of nitrogen and silicon serving as major components and other elements. As a film deposition process for obtaining a silicon nitride film, chemical vapor deposition (CVD) is used. While the optical constant of the silicon nitride film varies according to film deposition conditions such as the substrate temperature and the film deposition speed, in the present invention, it is satisfactory as long as the silicon nitride film is formed by a transparent layer having a refractive index higher than that of the organic compound layers 16, 17, and 18. The light transmittance of the high-refractive-index transparent layer 10 is preferably 85% or more, and more preferably 90% or more.
In the present invention, preferably, the light extraction structures 7 are preferably formed by directly working the high-refractive-index transparent layer 10 so that there is no difference in refractive index between the high-refractive-index transparent layer 10 and the light extraction structures 7.
The light extraction structures 7 are not limited to lens-shaped objects having a lens structure illustrated in
These light extraction structures 7 are preferably arranged in the pixels 4 to extract as much light emitted 360° in the plane as possible. For example, when the bottom shape is circular, the light extraction structures 7 are preferably arranged in a close-packed hexagonal manner, as illustrated in
The arrangement pattern of the light extraction structures 7 may be uniform over the entire plane. Further, the shape of the light extraction structures 7 may be different between the sub-pixels 1, 2, and 3 and the inter-sub-pixel regions, as in light extraction structures 7a and 7b illustrated in
While a production method for the light extraction structures 7 is not particularly limited, for example, after a resist pattern is formed on a film of SiNx or the like by photolithography, desired structures may be formed by dry etching. Alternatively, after a desired mold pattern is transferred on a SiNx film by nanoimprinting, the SiNx film may be subjected to dry etching.
When the size of the sub-pixels 1, 2, and 3 is several tens of micrometers square, the light extraction structures 7 preferably have a size or width of the order of microns. This is because, when a high-angle component of light emitted into the high-refractive-index transparent layer 10 enters the light extraction structure 7, it is not always extracted at one time, but may be extracted from the second or third light extraction structure 7. Further, the light may be extracted after impinging the second or third light extraction structure 7 and changing its angle because of reflection caused at the interface between the light extraction structure 7, and air or a low-refractive-index layer. Therefore, to achieve a higher light extraction efficiency, it is preferable to form a sufficient number and size of light extraction structures 7 in correspondence to the area of the sub-pixels 1, 2, and 3. More preferably, the light extraction structures 7 are formed not only on the sub-pixels 1, 2, and 3, but also in the regions between two adjacent sub-pixels in the pixels 4 (inter-sub-pixel regions 5 in
In order for the light extraction structures 7 to sufficiently contribute to enhancement of the light extraction efficiency, the light extraction structures 7 are preferably arranged tightly. More preferably, as illustrated in the
1.0≦B/A≦1.2 (1)
where A represents the diameter of the bottoms of the light extraction structures 7 (
When the light extraction structures 7 are more tightly arranged, the opportunity for light reaching the high-refractive-index transparent layer 10 to exit outside through the light extraction structures 7 increases. For example, since light from a specific point is emitted 360°, when there is a gap between two adjacent light extraction structures 7, light emitted at an angle corresponding to the gap is not extracted, and is extracted after entering the next light extraction structure 7.
When the light extraction structures 7 are provided on the inter-sub-pixel regions 5, light emitted from the sub-pixels adjacent to the inter-sub-pixel regions 5 enters the inter-sub-pixel regions 5 and are then extracted therefrom. However, color mixture caused by the light extraction structures 7 in the pixels 4, for example, color mixture of blue, green and red is additive color mixture of gradation-controlled colors. Hence, this does not have any influence on the control for obtaining a desired chromaticity. If anything, the light extraction efficiency is enhanced because light propagating to the adjacent sub-pixels can be extracted.
In contrast, in the light extraction structures 7 provided on the interpixel regions 6, luminescent colors of light emitted from the sub-pixels differently gradation-controlled are mixed. For example, a mixed color between a red sub-pixel 3 and a blue sub-pixel 1, which are included in different pixels 4 and are adjacent to each other with the interpixel region 6 being disposed therebetween, does not correspond to luminescent colors to be extracted by gradation control of the sub-pixels, but the mixed color is extracted as an unintended color of additive mixture.
Here, a MacAdam ellipse will be considered as an example. Green is less sensitive to a chromaticity shift than red and blue, and blue is highly sensitive to a chromaticity shift. Accordingly, bleeding of a display image will be described by taking, as an example, a blue sub-pixel 1 that emits light having a blue luminescent color in the structure of
In a normal display apparatus, the apertures of the pixels are typically arranged at a regular pitch. Since the aperture ratio is no less than about 50%, the interpixel regions 6 occupy a wide area. Therefore, color mixture occurring in the blue region forms a region having a different chromaticity, and this causes bleeding of a display image. To prevent such bleeding, the light extraction structures are not provided on the interpixel regions 6, as illustrated in
The width of the interpixel regions 6, that is, the distance between the adjacent pixels 4 needs to be determined in consideration of a light propagation distance. The light propagation distance refers to the distance between a light emitting point and a point where the light intensity at the light emitting point is reduced by half. The light propagation distance relates to the thickness and absorptance of the high-refractive-index transparent layer 10 and the luminescent color. In the present invention, the bleeding prevention effect of the regions, where the light extraction structures are not provided, is not specified by the luminescent color for the above-described reason. However, the width of the interpixel regions 6 is at least more than the diameter of the bottom faces of the light extraction structures 7 or the length of the longest diagonal line in the polygonal bottom faces. Further, the width of the interpixel regions 6 is preferably more than or equal to the light propagation distance.
In the present invention, the width of the interpixel regions 6 corresponds to the length of the interpixel regions 6 in the X-direction and Y-direction when the pixels and the sub-pixels are arranged linearly in the X-direction and Y-direction, as illustrated in
The shape of the apertures of the sub-pixels 1, 2, and 3 is not limited to a rectangular shape, and may be a circular shape illustrated in
Circuits and lines for driving the display apparatus of the present invention and the arrangement and properties of TFTs to be used are not particularly specified, and may be appropriately designed to obtain the required performance.
In the display apparatus of the present invention, the light extraction structures serve to extract, to the outside, light confined in the elements, and the light extraction structures may be further hermetically covered with sealing glasses such as glass caps or glass plates. On the sealing glasses, color filters for improving chromaticity or circularly polarizing plates for reducing reflection of outside light may be provided.
Specific examples of the present invention will be described below.
A display apparatus was produced as Example 1, in which organic EL elements had a cross-sectional structure of
In Example 1, first, a TFT driving circuit (not illustrated) formed of low-temperature polysilicon was formed on a glass substrate, and a planarizing film (not illustrated) formed of acrylic resin was formed thereon to obtain a support substrate 9. Next, an Ag alloy film having a thickness of about 150 nm was formed as a reflection electrode 22 on the support substrate 9 by sputtering. The reflection electrode 22 formed of an Ag alloy is a high reflection film having a spectral reflectivity of 80% or more in a visible light wavelength range (λ=380 to 780 nm). Further, an indium tin oxide (ITO) film was formed as a transparent electrode 23 by sputtering. After that, polyimide resin was spin-coated as partitions 15, and apertures were formed in desired sub-pixels by photolithography.
Subsequently, organic compound layers were sequentially formed and stacked by vacuum deposition. In each of the sub-pixels 1, 2, and 3 in the display apparatus of Example 1, the thickness of a hole transport layer 25 was changed so that the optical film thickness from a light emitting layer 26 to the reflection electrode 22 corresponded to ¾ of the wavelength of light with the luminescent color. As a luminescent dopant in the light emitting layer 26, a fluorescent material was used for blue, and a phosphorescent material was used for green and red. The phosphorescent material can be effective in enhancing internal quantum efficiency. The refractive index of the layer having the highest refractive index in the organic compound layers of the sub-pixels was 1.86 in the blue sub-pixels, 1.80 in the green sub-pixels, and 1.78 in the red sub-pixels.
Next, an indium zinc oxide (IZO) film was formed as a transparent electrode 20 by sputtering. After that, a silicon nitride (SiN) film having a thickness of 4 μm was formed by CVD. The refractive index of the SiN film was 1.89 at 450 nm (blue region), 1.88 at 520 nm (green region), and 1.86 at 620 nm (red region). Hence, in all the sub-pixels, the refractive index of the SiN film was higher than that of the organic compound layers. The surface of the SiN film was reformed by spin-coating hexamethyldisilazane, and AZ1500 was spin-coated as a photoresist, thereby obtaining a film having a thickness of about 2.5 μm. The film was exposed by a mask aligner MPA600FA with a photomask that does not have a pattern on interpixel regions 6 and has dots of 5 μm in diameter on pixels, as illustrated in
The width of the interpixel regions 6 (regions where light extraction structures were not provided) was 11.0 μm in the X-direction and 12.3 μm in the Y-direction. Therefore, in the interpixel regions 6, there was no portion narrower than the diameter of the bottoms of the microlenses. The ratio (B/A) between the diameter (A) of the bottoms of the microlenses and the pitch (B) of the microlenses was 1.4.
To check the degree of bleeding in the display apparatus produced in the above-described manner, a human image was displayed against a background of the blue sky, and luminescent color at an outline of a white portion, such as skin, was checked. At the outline of the displayed human image obtained by Example 1, a change in luminescent color due to bleeding was not found.
In Example 1, the light extraction efficiency was about 40%. The light emission intensity was increased at all the viewing angles.
A display apparatus was produced as Comparative Example 1 by a production process similar to that adopted in Example 1. The display apparatus of Comparative Example 1 was similar in structure to the display apparatus of Example 1 except that light extraction structures are also provided on interpixel regions 6. The degree of bleeding in the obtained display apparatus was checked in a manner similar to that adopted in Example 1. A change in luminescent color resulting from bleeding was found at an outline of a displayed human image, and bleeding of a blue-violet color was found at the outline. The light extraction efficiency was about 41% similarly to Example 1, and the luminance was increased at all the viewing angles.
A display apparatus was produced as Example 2 by a production method similar to that adopted in Example 1. The display apparatus of Example 2 was similar in structure to the display apparatus of Example 1 except that the lens pitch was reduced to 6 μm. The width of interpixel regions 6 (regions where light extraction structures were not provided) was 13.0 μm in the X-direction and 12.3 μm in the Y-direction. In the interpixel regions 6 (regions where light extraction structures were not provided), there was no portion narrower than the diameter of bottoms of microlenses. The ratio (B/A) between the diameter (A) of the bottoms of the microlenses and the pitch (B) of the microlenses was 1.2.
The degree of bleeding in the obtained organic EL display apparatus was checked in a manner similar to that adopted in Example 1. A change in luminescent color resulting from bleeding was not found at an outline of a displayed human image. The light extraction efficiency was about 44%, and the luminance was increased at all the viewing angles.
A display apparatus was produced as Example 3 by a production method similar to that adopted in Example 1 except that photography was performed by gradation exposure using a gray-tone mask as a photomask. The display apparatus of Example 3 was similar in structure to the display apparatus of Example 1 except that the lens pitch was 5 μm and microlenses having a diameter of 5 μm were tightly arranged. The length of interpixel regions 6 (regions where light extraction structures were not provided) was 15.0 μm in the X-direction and 15.7 μm in the Y-direction. In the interpixel regions 6 (regions where light extraction structures were not provided), there was no portion narrower than the diameter of bottoms of the microlenses. The ratio (B/A) between the diameter (A) of the bottoms of the microlenses and the pitch (B) of the microlenses was 1.0.
The degree of bleeding in the obtained display apparatus was checked in a manner similar to that adopted in Example 1. A change in luminescent color resulting from bleeding was not found at an outline of a displayed human image. The light extraction efficiency was about 47%, and the luminance was increased at all the viewing angles.
While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions.
This application claims the benefit of Japanese Patent Application No. 2011-085397 filed Apr. 7, 2011, which is hereby incorporated by reference herein in its entirety.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2011-085397 | Apr 2011 | JP | national |
