JP2012002896A - Display device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a display device in which light extraction efficiency is improved, and luminance unevenness and flicker are decreased.SOLUTION: The display device is composed of a plurality of pixels, and each pixel in the plurality of pixels includes a light emission layer, a diffraction grating having an in-plane refraction index distribution, and an intermediate layer arranged between the light emission layer and the diffraction grating and composed of at least one or more layers. The intermediate layer includes an in-plane concavo-convex structure having an optical path length difference equal to or more than prescribed times of the wavelength of light emitted by the light emission layer, in order to uniformize the extraction rate of the light extracted to the outside.

Description

  The present invention relates to a display device, and more particularly to a display device that uses a diffraction grating to increase light extraction efficiency.

In recent years, self-luminous display devices such as an electron-emitting device display, an organic EL display, and an LED display using a fluorescent substance have been developed, and the screen size is rapidly increasing.
An electron-emitting device type display has a configuration in which electrons emitted from an electron source are used as an excitation source, a light-emitting layer made of a phosphor or the like is excited and emitted, and light is extracted outside.
The organic EL display / LED display has a configuration in which a current is injected as an excitation source into a light emitting layer to emit light and light is extracted outside.
In any configuration, the light emitted from the light emitting layer takes out the light to the outside through the front plate or the electrode.
However, since a difference in refractive index occurs at the interface of the light emitting layer, the front plate, the electrode, etc., light having an emission angle greater than the critical angle among the light emitted from the light emitting layer causes total reflection. For this reason, light cannot be extracted to the outside, the light extraction efficiency is lowered, and the luminance of the display device is lowered.
Conventionally, as a technique for improving the light extraction efficiency, for example, in Patent Document 1, a diffraction grating (refractive index distribution layer) is formed on the front surface side or the back surface side of the light emitting layer to diffract the light from the light emitting layer and externally. An organic electroluminescence element that extracts light is proposed.

Japanese Patent No. 02991183

However, the conventional example of Patent Document 1 has the following problems.
That is, in the above conventional example, it is difficult to fabricate the light emitting layer and the diffraction grating directly adjacent to each other, and the light emission characteristics of the light emitting layer are deteriorated. In order to avoid such deterioration of the light emission characteristics, it is necessary to form an intermediate layer between the light emitting layer and the diffraction grating.
However, as the screen of the display device becomes larger, it is difficult to produce the intermediate layer with a uniform thickness over the entire surface of the display device (all pixels). Since the light extraction efficiency is very sensitive to the film thickness of the intermediate layer, if the film thickness of the intermediate layer varies from pixel to pixel of the display device, the light extraction efficiency varies from pixel to pixel. As a result, luminance unevenness or flickering of the display device occurs, and the visual characteristics of the display device are deteriorated.

  In view of the above problems, an object of the present invention is to provide a display device that can improve light extraction efficiency and can reduce luminance unevenness and flicker.

The display device of the present invention includes a plurality of pixels, and each pixel in the plurality of pixels is
A display device comprising: a light emitting layer; a diffraction grating having an in-plane refractive index distribution; and an intermediate layer formed between the light emitting layer and the diffraction grating and including at least one layer. ,
The intermediate layer includes a concavo-convex structure having an optical path length difference equal to or greater than a predetermined multiple of the wavelength of the light in the plane of the intermediate layer in order to average the rate at which the light emitted from the light emitting layer is extracted to the outside. It is characterized by.

  According to the present invention, it is possible to realize a display device that can improve the light extraction efficiency and can reduce luminance unevenness and flicker.

1 is a schematic cross-sectional view of a display device of Example 1. FIG. 3 is a graph showing the relationship between the normalized optical path length and light extraction efficiency in Example 1. 1 is a schematic cross-sectional view of a display device of Example 1. FIG. 3 is a graph showing the relationship between the optical path length and the light extraction efficiency in Example 1. 3 is a graph showing the relationship between the optical path length and the light extraction efficiency in Example 1. 1 is a schematic cross-sectional view of a display device of Example 1. FIG. FIG. 6 is a schematic cross-sectional view of a display device of Example 2. FIG. 10 is a diagram illustrating a manufacturing process of the display device according to the third embodiment.

The form for implementing the display apparatus in this invention is demonstrated with reference to drawings by the following Examples.
In all the drawings for explaining the embodiments, parts having the same function are given the same reference numerals, and repeated explanation thereof is omitted.

[Example 1]
As Example 1, a configuration example of a display device to which the present invention is applied will be described with reference to FIG.
FIG. 1 shows a schematic cross-sectional view of a single pixel of a field emission device type display formed of a plurality of pixels.
The display device according to the present embodiment is configured to irradiate the light emitting layer 103 made of a phosphor with the electrons 102 emitted from the electron source 101 and emit light of a color corresponding to each pixel.
The emitted light is diffracted by the diffraction grating 105 through the intermediate layer 104 and extracted to the outside.
However, the diffraction grating 105 has a structure having an in-plane refractive index distribution. Further, the intermediate layer has an uneven structure having an optical path length difference equal to or greater than a predetermined multiple of the light wavelength in the plane.
Specifically, the intermediate layer 104 has a concavo-convex structure having an optical path length difference equal to or greater than K times the wavelength in the plane when the value of K is obtained by the following equation (1).
However, n _m is the average refractive index of the intermediate layer, n _L is the refractive index of the light-emitting layer.

Here, the rate at which the light emitted from the light emitting layer 103 is extracted to the outside (light extraction efficiency) increases or decreases in a cycle of K times the wavelength according to the optical path length of the intermediate layer.
Therefore, when the intermediate layer 104 is provided with an uneven structure having an optical path length difference of K times or more of the wavelength in the pixel, the light extraction efficiency changes for each region in the pixel, but the entire light extraction in a single pixel is performed. The efficiency is a constant value obtained by averaging the efficiency of each region in the pixel.
At this time, even if the film thickness of the intermediate layer fluctuates between pixels due to a manufacturing error or the like, each pixel has a constant light extraction efficiency value that is averaged. The efficiency is all the same constant value.
As described above, by providing the uneven structure in the intermediate layer, the sensitivity of the light extraction efficiency with respect to the film thickness of the intermediate layer can be reduced. As a result, a change in light extraction efficiency due to fluctuations in film thickness within or between pixels can be reduced, and luminance unevenness or flickering of the display device can be reduced.
However, the intermediate layer is not limited to a single layer, and may be composed of a plurality of layers, and an uneven shape may be produced for each layer.
Further, the uneven shape is a shape different from the shape of the diffraction grating, and may be periodic or aperiodic.

Here, the reason why the light extraction efficiency η periodically changes according to the optical path length (film thickness) of the intermediate layer 104 will be described in detail.
First, the light extraction efficiency η varies according to the intermediate layer 104 as shown by the following equation (2).

Here, α is the phase difference between the reflected light at the intermediate layer, and η ₀ and Δη are the average value of the light extraction efficiency and the amount of change in the light extraction efficiency, respectively. η ₀ and Δη have different values depending on the refractive index and configuration of the light emitting layer 103, the diffraction grating 105, and the like.
Further, the light extraction efficiency η is a ratio at which light isotropically emitted from the light emitting layer 103 is extracted to the outside (air) of the front plate 106.
FIG. 2 shows the relationship between the phase difference α between the reflected light at the intermediate layer and the light extraction efficiency.

At this time, the reason why a region having a high light extraction efficiency and a region having a low light extraction efficiency will be described. When the intermediate layer 104 is formed between the light-emitting layer 103 and the diffraction grating 105, light emitted from the light-emitting layer at an angle larger than the critical angle is multiple-reflected at the intermediate layer or the interface of the light-emitting layer, and is reflected in the intermediate layer or the light-emitting layer. Be trapped.
The confined light is coupled to the diffraction grating and extracted to the outside while propagating through the intermediate layer and the light emitting layer (FIG. 3A).
When the phases of the components that are multiple-reflected at the interface of the intermediate layer match and strengthen each other (α = (4m + 1) π / 2 m is an integer of 0 or more), the light emitted from the light emitting layer is strongly confined in the intermediate layer ( FIG. 3 (b)).
The light confined in the intermediate layer has little loss and the coupling efficiency with the diffraction grating is high, so that the light extraction efficiency is high (region 1 in FIG. 2).
On the other hand, when the phases of the components that undergo multiple reflection at the interface of the intermediate layer weaken each other, the confinement of light in the intermediate layer becomes weak, and the light emitted from the light emitting layer is mainly confined in the light emitting layer (FIG. 3C). .
The light confined in the light emitting layer is reabsorbed by the phosphor in the light emitting layer, is emitted from the back surface, or reaches the absorbing material at the pixel end and is absorbed. Since part of the emitted light is lost, the light extraction efficiency is reduced (region 2 in FIG. 2).

Then, the phase difference between the reflected light in the intermediate layer α, using the optical path length n _{m d} of the intermediate layer is expressed by the following equation (3).

Where _nm is the average refractive index of the intermediate layer, d is the film thickness of the intermediate layer, and λ ₀ is the wavelength in vacuum.
The average refractive index of the intermediate layer is a value obtained by dividing the optical path length of the intermediate layer by the total film thickness of the intermediate layer. Θ _m is the angle of light propagating through the intermediate layer, and satisfies the following equation (4) from Snell's law.

Here, n _L is a refractive index of the light emitting layer, and θ _L is an angle of light propagating through the light emitting layer, and is 80 ° to 90 °.
That is, light having θ _L of 80 ° to 90 ° contributes to increase / decrease in light extraction efficiency. This is because the amount of light emitted from the light emitting layer increases as the solid angle increases. The initial phase (α ₀ ) is generated by the coupling between the evanescent wave generated in the intermediate layer and the diffraction grating, and is caused by the penetration length of the evanescent wave.
From the above formulas (1), (2), (3), and (4), the light extraction efficiency η varies with the period Λ shown in the following formula (5).

When the difference in the optical path length n _md of the intermediate layer is greater than or equal to K times the wavelength, the light extraction efficiency has an increase or decrease of one period or more.
At this time, the light extraction efficiency of the entire pixel is obtained as an average value η ₀ of the light extraction efficiency obtained by averaging the efficiency of each region.
Even if the optical path length of the intermediate layer fluctuates, the average value η ₀ of the light extraction efficiency does not change.
For this reason, even if a change in film thickness occurs due to a manufacturing error between pixels, the light extraction efficiency of each pixel becomes the average value η ₀ of the light extraction efficiency. As a result, luminance unevenness and flicker can be reduced over the entire display screen.

Next, numerical examples will be described.
The display device includes a light emitting layer 103 having a width of 100 μm and a length of 250 μm in one pixel, and the light emitting layer 103 includes a layer containing a phosphor that emits light with a refractive index of 1.7 and a center wavelength of 550 nm.
The diffraction grating 105 is formed of Al ₂ O ₃ having a refractive index of 1.8 and a film thickness of 950 nm with holes having a diameter of 1450 nm in a triangular lattice pattern with a period of 2300 nm.
The intermediate layer is formed of at least one layer, and the intermediate layer 104 in this numerical example is composed of three layers of a transparent electrode, a high dielectric layer, and a low dielectric layer.
The two or more different dielectric layers constitute an uneven structure having an optical path length difference of K times or more.
The transparent electrode is made of ITO having a refractive index of 1.8 and a film thickness of 300 nm, the high dielectric layer is made of TiO ₂ having a refractive index of 2.2, and the low dielectric layer is made of SiO ₂ having a refractive index of 1.46. To do.
In addition, the high dielectric layer and the low dielectric layer are in contact with each other in a wavy shape with a period of 10 μm. At this time, the difference (optical path length difference) between the shortest optical path length and the longest optical path length in the plane of the intermediate layer is set to 1 μm.
Therefore, the optical path length difference of the intermediate layer is 1.8 times the wavelength, which is larger than K (= 1.5) in Expression (4).
At this time, the light extraction efficiency of the light emitted from the light emitting layer 103 periodically changes as shown in FIG. 5 according to the optical path length of the intermediate layer.
In this embodiment, the intermediate layer has a wave-like structure and has a change of one cycle or more in the pixel, so that an average value η0 = 57.5% is obtained from the entire pixel.
At this time, the sensitivity of the light extraction efficiency with respect to the film thickness of the intermediate layer can be reduced, and a display device with reduced luminance unevenness and flicker can be obtained.

In this embodiment, the average refractive index of the intermediate layer 104 is set to be larger than the effective refractive index of the diffraction grating 105, but it is not necessarily required to be larger than the effective refractive index of the diffraction grating.
However, when the average refractive index of the intermediate layer is larger than the effective refractive index of the diffraction grating, the light emitted from the light emitting layer is strongly confined in the intermediate layer.
Since the light confined in the intermediate layer has little loss and high coupling efficiency with the diffraction grating, the average refractive index of the intermediate layer is preferably larger than the effective refractive index of the diffraction grating.
However, the effective refractive index of the diffraction grating is a value obtained by summing up the products of the filling rate and the refractive index for each material constituting the diffraction grating.
In this embodiment, the optical path length difference of the intermediate layer is formed by only the two high dielectric layers and the low dielectric layer. However, the intermediate layer may not necessarily be formed by the two dielectric layers. However, it is preferable to form two or more layers because the optical path length of the intermediate layer can be changed in the plane and the intermediate layer can be flattened.
In addition, it is preferable to form only the dielectric layer because the number of manufacturing processes is small and the manufacturing can be easily performed.

In this embodiment, the intermediate layer 104 includes an electrode made of ITO. However, the intermediate layer does not necessarily require an electrode layer, and an electrode may be formed between the front plate and the diffraction grating. The diffraction grating may be formed of electrodes.
However, it is desirable to form an electrode near the light emitting layer because the internal quantum efficiency of the light emitting layer is improved.
In the present embodiment, the concavo-convex structure is formed in a periodic wavy shape. However, as long as the intermediate layer has an optical path length difference, a non-periodic structure as shown in FIG. 6 may be used.
In addition, when the concavo-convex structure is a periodic structure, it is desirable that the period is a half or less of the pixel size.
This is because an error in overall light extraction efficiency is smaller when a structure having a plurality of optical path length differences in a single pixel is used. More preferably, the period may be set to ¼ or less of the pixel size.
The period of the periodic structure is preferably 5 μm or more. This is because light emitted from the light emitting layer propagates through the intermediate layer. In order to propagate sufficiently stably, it is desirable that the structure period is 5 μm or more.
In this embodiment, the intermediate layer is formed of SiO ₂ , TiO ₂ , or ITO, which are inorganic materials. However, the intermediate layer is not necessarily formed of only an inorganic material, and may be formed using an organic material. However, it is desirable to form with an inorganic material because the structure is strong against heat generated in the light emitting layer and high durability is obtained.

Next, numerical examples where only the emission wavelength of the light emitting layer 103 is different will be described.
When the wavelengths of light emitted from the light emitting layer are 450 nm and 650 nm, the optical path length of the intermediate layer and the light extraction efficiency are as shown in FIGS.
At this time, the optical path length difference of the intermediate layer is 2.2 times and 1.5 times of the wavelength, respectively, and is larger than K (= 1.5) in the equation (4), and light of one period or more is extracted into the pixel. Has a change in efficiency. Therefore, averaged values η0 = 56.0% and 54.5% are obtained from the entire pixels.
As described above, the sensitivity of the light extraction efficiency with respect to the film thickness of the intermediate layer can be reduced, and a display device with reduced luminance unevenness and flicker can be obtained.

[Example 2]
As Example 2, a configuration example of a display device having a different form from Example 1 will be described with reference to FIG.
FIG. 7 shows a schematic cross-sectional view of a single pixel of an organic EL display formed of a plurality of pixels.
The display device of this embodiment is configured to excite the light emitting layer 203 by emitting a potential difference between the electrode 201 and the transparent electrode 214 and injecting a current to emit light of a color corresponding to each pixel. .
A part of the emitted light is diffracted by the diffraction grating 205 through the transparent electrode 214 and the dielectric layer 224 which are the intermediate layer 204 and extracted to the outside. The diffraction grating 205 may have any configuration as long as it is a diffraction grating such as a periodic diffraction grating, a highly quasi-photonic crystal having high symmetry, and fine particles arranged non-periodically.
The dielectric layer 224 is a film in which randomly arranged silica particles are filled with a TiO ₂ film, and the film thickness is 2.5 μm. At this time, when the emission wavelength of the light emitting layer 203 is 550 nm, a configuration with an optical path length difference of 1.0 μm or more can be obtained by the region of the dielectric layer 224.

Therefore, it becomes larger than K (= 1.5) of Formula (4). Therefore, in order to obtain a structure having a change in light extraction efficiency for one period or more in the pixel, averaged values are obtained from the entire single pixel.
In addition, even if the film thickness varies between pixels, the average value of each pixel does not change. Therefore, a light extraction efficiency between pixels becomes a constant value, and a display device that reduces luminance unevenness and flickering is provided. Obtainable.
In this embodiment, the difference in the optical path length of the intermediate layer is formed by the difference in refractive index between the fine particle sphere and the background medium that are randomly dispersed. At this time, there is no etching process and it can be manufactured at low cost.

[Example 3]
As Example 3, a manufacturing process of Example 1 or Example 2 will be described with reference to FIG.
In the manufacturing process of this embodiment, first, the material 1 is laminated in order to form the diffraction grating 305 on the glass substrate 106 (FIG. 8A).
Subsequently, a resist film is deposited or sputtered, and a predetermined position is exposed to form a resist mask 307 (FIG. 8B).
Thereafter, the material 1 is etched to a predetermined depth by an etching method such as RIE, and the resist mask 307 is removed by ashing or the like (FIG. 8C).
Next, the material 2 made of a dielectric or the like is embedded in the holes formed in the material 1 (FIG. 8D).
Subsequently, in order to form an intermediate layer having an uneven structure, the material 3 is etched or dispersed in the uneven structure (FIG. 8E).
Thereafter, the material 3 may be deformed by a reflow method or the like as necessary (FIG. 8 (f)), or the material 4 is deposited and formed into a wave shape by sputtering or vapor deposition with good straightness.
Next, the material 3 or the material 5 having a refractive index different from that of the material 4 is laminated, and the intermediate layer is flattened.
At this time, if necessary, a planarization process such as a CMP method may be performed. Subsequently, an electrode 314 such as ITO is deposited or sputtered as necessary to form the intermediate layer 304 (FIG. 8G).
Thereafter, a light emitting layer 303 is formed (FIG. 8H), and an electrode or a rear plate is produced as necessary to obtain a display device.

1, 2, 3, 4, 5: Material 101: Electron source 102: Electrons 103, 203, 303: Light emitting layers 104, 204, 304: Intermediate layers 105, 205, 305: Diffraction grating 106: Front plate 307: Resist mask

Claims (10)

It is composed of a plurality of pixels, and each pixel in the plurality of pixels is
A display device comprising: a light emitting layer; a diffraction grating having an in-plane refractive index distribution; and an intermediate layer formed between the light emitting layer and the diffraction grating and including at least one layer. ,
The intermediate layer includes a concavo-convex structure having an optical path length difference equal to or greater than a predetermined multiple of the wavelength of the light in the plane of the intermediate layer in order to average the rate at which the light emitted from the light emitting layer is extracted to the outside. A display device. 2. The display according to claim 1, wherein the optical path length difference of the intermediate layer has an optical path length difference equal to or greater than K times the wavelength of the light when the value of K is obtained by the following expression (1). apparatus.

However,
n _m : Average refractive index of the intermediate layer n _L : Refractive index of the light emitting layer   The display device according to claim 1, wherein an average refractive index of the intermediate layer is larger than an effective refractive index of a diffraction grating having a refractive index distribution in the plane.   The said intermediate | middle layer has two or more different dielectric layers, and the uneven structure which has the optical path length difference more than said K times is comprised by this dielectric material layer. The display device according to any one of the above.   The display device according to claim 1, wherein the intermediate layer includes a layer including an electrode.   The display device according to claim 1, wherein the uneven structure of the intermediate layer is configured by a periodic uneven structure.   The display device according to claim 6, wherein a period of the concavo-convex structure is a period of half or less of a pixel size.   The display device according to claim 6, wherein the uneven structure has a period of 5 μm or more.   The display device according to any one of claims 1 to 5, wherein the uneven structure of the intermediate layer is formed of an aperiodic uneven structure.   The display device according to claim 1, wherein the intermediate layer is made of an inorganic material.

Images