JP2001357979A - El element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an EL element with excellent contrast characteristics and display brightness characteristics. SOLUTION: A polarizing plate 16, a quarter wavelength plate 14, a selective reflection layer 12, a light emission layer 22, and a reflection electrode 24 are successively laminated. The selective reflection layer is formed by polymerizing a cholesteric liquid crystal layer and reflects selectively the first circular polarization component of incident light. The light from outside, penetrating through the polarizing plate and the quarter wave length plate, is reflected by the reflection electrode after passing through the selective reflection layer, and absorbed at the polarizing plate after passing through the selective reflection layer again. Out of the light from the light emission layer, one of the circular polarization component passes through the selective reflection layer and the polarizing plate and is emitted to the observing side, and the other circular polarization component is reflected by the selective reflection layer and reflected further by the reflection electrode in the direction of observation side, and passes through the selective reflection layer and the polarizing plate, and emitted to the observation side.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electroluminescence (EL) device, and more particularly to a high-contrast EL device having a circularly polarizing plate.

[0002]

2. Description of the Related Art In recent years, since organic light-emitting materials have reached the practical range, EL elements have been receiving attention as low power consumption, wide viewing angles, thin and lightweight display elements. Generally, according to a conventional EL element, a light emitting layer is provided on a reflective electrode. This is because the light emitted from the light-emitting layer toward the reflective electrode is reflected by the reflective electrode and emitted to the opposite side, so that the point-emitted light is emitted to the observer as much as possible.

However, in such a configuration, as the reflectivity of the reflective electrode is higher, incident external light is reflected by the reflective electrode in a state in which the light emitting layer does not emit light, that is, when a black display is desired. Therefore, there is a problem that the black level of the black display deteriorates and the contrast characteristic is deteriorated.

As a method of avoiding this problem, a method of lowering the reflectance of the reflective electrode or providing a polarizing plate and a quarter-wave plate to absorb only external light and a reflected component has been considered. The latter method raises the cost by the following principle. That is, the incident external light becomes circularly polarized light by transmitting through the polarizing plate and the quarter-wave plate. Even if the circularly polarized light is reflected by the interface between the glass substrate and the reflective electrode, the phase of the circularly polarized light is shifted by 180 degrees due to the reflection. It becomes linearly polarized light and is absorbed by the polarizing plate. Therefore, regardless of the reflectivity of the reflective electrode, the same effect as when the external light is not reflected by the reflective electrode can be obtained.

The latter method is effective not only for reflection at the reflection electrode but also for reflection at the interface of the glass substrate and reflection at the wiring electrode, and is provided with a polarizing plate and a quarter-wave plate. The contrast characteristics are significantly improved as compared with the case without the contrast.

However, when a polarizing plate is provided, at least 50% of the light emitted from the light emitting layer is absorbed by the polarizing plate, so that there is a problem that the display luminance itself is reduced. For example, when a polarizing plate currently in practical use is used, the display luminance is reduced by about 56% as compared with a case where a polarizing plate and a quarter-wave plate are not provided.

[0007]

As described above, in the conventional EL device, when the polarizing plate and the quarter-wave plate are not provided, the contrast characteristics are extremely low, and particularly in a bright environment.
In other words, there has been a great problem in use under conditions of strong external light. Further, when the polarizing plate and the quarter-wave plate are provided, the contrast characteristic is remarkably improved.
There is a problem that the display luminance is reduced by about 6%.

The present invention has been made in view of the above problems, and has as its object to provide an EL device having excellent contrast characteristics and display luminance characteristics.

[0009]

To achieve the above object, an EL device according to the present invention comprises a polarizing plate, a quarter-wave plate provided behind the polarizing plate, and a quarter-wave plate. A light-emitting layer provided behind, a reflective electrode provided behind the light-emitting layer, and a cholesteric liquid crystal layer provided between the quarter-wave plate and the light-emitting layer, and formed by polymerizing the cholesteric liquid crystal layer. And a selective reflection layer that selectively reflects the first circularly polarized light component of the incident light.

According to the EL device of the present invention,
The light emitting layer and the reflective electrode are formed on a transparent substrate, the light emitting layer is located between the transparent substrate and the reflective electrode,
The selective reflection layer is provided between the light emitting layer and the transparent substrate. According to another EL device of the present invention, the light emitting layer and the reflective electrode are formed on a transparent substrate, the reflective electrode is provided between the transparent substrate and the light emitting layer, and the light emitting layer is , Provided between the reflective electrode and the selective reflection layer.

In the EL element according to the present invention, the selective reflection layer is formed by laminating a plurality of cholesteric liquid crystal layers having different pitches from each other, or a single layer in which the pitch continuously changes in the layer thickness direction. The cholesteric liquid crystal layer is characterized by reflecting circularly polarized light in the same direction as the helical direction of the cholesteric layer with respect to light at wavelengths in the entire visible light range.

According to the EL device configured as described above, external light incident through the polarizing plate and the quarter-wave plate becomes circularly polarized light, and is transmitted as it is without being reflected by the selective reflection layer. Then, the external light transmitted through the selective reflection layer enters the reflection electrode, where it is reflected to the observation side. At that time, since the phase of the external light is shifted by 180 degrees, the external light passes through the selective reflection layer and further passes through the quarter-wave plate, and then becomes linearly polarized light orthogonal to that at the time of incidence of the polarizing plate, and is absorbed by this polarizing plate. . This makes it possible to absorb external light reflected by the reflective electrode and prevent the light from being emitted to the outside, and as a result, it is possible to improve the contrast characteristics.

The light emitted from the light emitting layer is non-polarized light and is composed of both left circularly polarized light and right circularly polarized light.
Among them, one circularly polarized light component is transmitted through the selective reflection layer without being reflected by the selective reflection layer, is converted into linearly polarized light of an azimuth that passes through the polarizing plate by a quarter-wave plate, and is transmitted through the polarizing plate. The other circularly polarized light component is selectively reflected by the selective reflection layer and is incident on the reflection electrode, and is again reflected by the reflection electrode with a phase shifted by 180 degrees to become a circular polarization component of the opposite direction. And is emitted to the observation side.

Therefore, according to the EL device, the contrast characteristics can be greatly improved as compared with the conventional device, and most of the light emitted from the light emitting layer can be emitted to the observation side. Display luminance twice or more as compared with the related art can be obtained.

[0015]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, an EL device according to an embodiment of the present invention will be described in detail with reference to the drawings. First, the basic configuration of the EL element according to the present embodiment will be described.

As shown in FIG. 1, the EL device includes a substantially rectangular transparent substrate 10 made of, for example, glass, and a transparent cholesteric liquid crystal layer formed by polymerizing a cholesteric liquid crystal on the observation side of the transparent substrate. A selective reflection layer 12, a quarter-wave plate 14, and a polarizing plate 16 are sequentially provided. On the back side of the transparent substrate 10, a plurality of wirings 20 are formed in a matrix, and a light emitting layer 22 and a reflective electrode 24 are sequentially formed for each pixel.

The polarizing plate 16 and the quarter-wave plate 14 have, for example, optical axes arranged so that incident external light (non-polarized light) is left circularly polarized light, and the incident external light is left circularly polarized light. The molecules of the cholesteric liquid crystal layer constituting the selective reflection layer 12 have a counterclockwise spiral structure when viewed from the light emitting layer 22 side. As a result, the selective reflection layer 12 is
Of the incident light that reaches the main surface on the second side, it reflects only the left-handed circularly polarized light component, transmits the right-handed circularly polarized light component that is opposite to the reflected component, and transmits the incident light that reaches the opposite main surface. Of these, it has a function of reflecting only the left circularly polarized light component and transmitting the right circularly polarized light component.

According to the EL device having the above structure, the polarizing plate 16
External light incident through the polarizer 16 and the quarter-wave plate 14 is converted into left-handed circularly polarized light. The left-handed circularly polarized light is transmitted as it is without being reflected by the selective reflection layer 12. Therefore, the incident external light is incident on the reflective electrode 24,
Here, it is reflected to the observation side. Since the phase of the external light reflected by the reflective electrode 24 is shifted by 180 degrees, the external light passes through the selective reflection layer 12 and further transmits through the quarter-wave plate 14,
Since the light becomes linearly polarized light orthogonal to that when the light enters the polarizing plate 16, it is absorbed by the polarizing plate 16. Therefore, at the time of black display of the EL element, external light reflected by the reflective electrode 24 can be absorbed to prevent emission to the outside, and as a result, the contrast characteristics can be improved.

On the other hand, the light emitted from the light emitting layer 22 is non-polarized light, and is composed of both left circularly polarized light and right circularly polarized light. The right-handed circularly polarized light component is transmitted through the selective reflection layer without being reflected by the selective reflection layer 12, is converted into linearly polarized light having an azimuth that passes through the polarizing plate 16 by the quarter-wave plate 14, and is transmitted through the polarizing plate. I do. On the other hand, the left circularly polarized light component is selectively reflected by the selective reflection layer 12 and is incident on the reflection electrode 24,
The light is again reflected by the reflection electrode with the phase shifted by 180 degrees, becomes a right-handed circularly polarized light component, and finally passes through the selective reflection layer 12 and the polarizing plate and is emitted to the observation side.

As described above, according to the present EL device, the contrast characteristics can be greatly improved as compared with the conventional EL device, and most of the light emitted from the light emitting layer 22 is emitted to the observation side. Is possible, and a display luminance twice or more as compared with the related art can be obtained. Further, the selective reflection layer 12 is formed by laminating a plurality of cholesteric liquid crystal layers each having a different helical pitch, or is constituted by one or more cholesteric liquid crystal layers in which the helical pitch changes continuously in the layer thickness direction. The function and effect described above can be obtained irrespective of the emission color by employing a configuration in which a circularly polarized light line segment in the same direction as the spiral direction is reflected with respect to light having a wavelength in the entire light range.

Next, a specific example of the EL device configured as described above will be described. Here, an EL element was manufactured using an organic EL light emitting material by the same manufacturing method and material as the conventional one. In this case, as the selective reflection layer 12, the spiral pitch is 300, 3 on the transparent substrate 10 made of glass.
40, 390, 450, 520, 590 nm, Δ
A plurality of cholesteric liquid crystal layers having n of 0.1 and an average refractive index n of 1.6 were laminated so as to be in a planar orientation.

The thickness of each cholesteric liquid crystal layer was about 10 times the helical pitch. Further, each cholesteric liquid crystal layer has a length in which the helical pitch is effectively coincident with various visible light wavelengths, and of light having a wavelength of np value obtained by multiplying the helical pitch by the average refractive index, the twist direction of the liquid crystal molecules. It reflects circularly polarized light of the same direction, that is, left circularly polarized light. The reflectance of each cholesteric liquid crystal layer depends on the film thickness, and becomes 100% when the film thickness is about 10 times the helical pitch. Therefore, each cholesteric liquid crystal layer reflects light of a band width obtained by multiplying Δn and the helical pitch, centering on light having a wavelength of np value obtained by multiplying the helical pitch by the average refractive index among the left circularly polarized lights.

As described above, six types of cholesteric liquid crystal layers having different helical pitches were formed to have a film thickness such that the reflectance of left-handed circularly polarized light was 100% in each layer, and each layer was multiplied by Δn and the helical pitch. By obtaining the reflection of the bandwidth, it is possible to obtain the selective reflection layer 12 that selectively reflects the left-handed circularly polarized light with respect to the wavelength in almost the entire visible light range. The cholesteric liquid crystal polymer layer thus formed was cured by ultraviolet curing, thermal polymerization, or the like so that it could be handled as a film or film.

The selective reflection layer is not limited to a structure in which a plurality of cholesteric liquid crystal layers having different helical pitches are stacked.
It may be composed of a single cholesteric liquid crystal layer in which the helical pitch changes continuously along the layer thickness direction. In this case, the same operation and effect as described above can be obtained. The same optical effect can be obtained by forming a liquid crystal layer using two or more substrates without using a liquid crystal polymer.

On the selective reflection layer 12 formed as described above, the optical axis is 1 counterclockwise with respect to the longitudinal direction of the EL element.
A retardation plate made of ARTON resin having a retardation value of 140 nm is attached so as to form an angle of 25 °.
A 270 nm retardation plate made of ARTON resin is stuck so as to form an angle of
The wave plate 14 was formed. Thereafter, the polarizing plate 16 was bonded so that the optical axis was at an angle of 45 ° with respect to the longitudinal direction of the EL element.

When such a retardation value is 140n
By attaching a 270 nm retardation plate and a polarizing plate in the above-described angle configuration, the two retardation plates function as a quarter-wave plate 14 with respect to the wavelength in the entire visible light range, and the polarizing plate 16 Function as a left circularly polarizing plate.

In the EL device having the above-described structure, the effects of preventing reflection of external light and emitting most of emitted light can be obtained. FIG. 2 shows an EL element (curve A) according to the present embodiment, an EL including only a polarizing plate.
The results of comparing the contrast characteristics of the element (curve B) and the EL element without the polarizing plate and the selective reflection layer (curve C) at various illuminances are shown.

As can be seen from this figure, when comparing the EL element without the polarizing plate (curve C) with the EL element with the polarizing plate (curves A and B), it is clear that the illuminance is higher when the polarizing plate is provided. High contrast characteristics can be obtained.
This is due to the effect that the external light reflected by the reflective electrode is absorbed by the polarizing plate, as described above.

When two EL elements (curves A and B) provided with a polarizing plate are compared with each other, as shown in this embodiment,
It can be seen that the EL element having the selective reflection layer can obtain higher contrast characteristics. This is because the display luminance of the EL element according to the present embodiment is about twice that of the EL element having no selective reflection layer.

From the above, according to the present embodiment,
EL with excellent contrast characteristics and high display brightness
An element can be obtained. The present invention is not limited to the above-described embodiment, but can be variously modified within the scope of the present invention. For example, although an organic EL light emitting element has been described as an example, the present invention is not limited to the type of light emitting layer, and it goes without saying that the same effect can be obtained even when an inorganic EL light emitting layer is used.

In the above-described embodiment, the EL element may have no transparent substrate between the selective reflection layer and the light emitting layer. In this case, parallax caused by the transparent substrate can be eliminated.

In an EL device having a light emitting layer and a reflective electrode formed on a transparent substrate, when the light emitting layer is located between the transparent substrate and the reflective electrode, as shown in FIG. 3, the light emitting layer comprises a cholesteric liquid crystal layer. The selective reflection layer 12 includes the light emitting layer 2
2 and the transparent substrate 10. Similarly, when the reflection electrode 24 is located between the transparent substrate 10 and the light emitting layer 22, as shown in FIG. 4, the light emitting layer 22 may be provided between the reflection electrode and the selective reflection layer. According to such a configuration, the light emitted from the light emitting layer 22 is reflected by the selective reflection layer 12, then further reflected by the reflective electrode, converted into light that passes through the selective reflection layer again, and finally polarized. The light is emitted to the observation side through the plate, and at this time, the emitted light can be emitted without generating parallax.

[0033]

As described above in detail, according to the present invention, by providing the selective reflection layer and the polarizing plate for selectively reflecting the first circularly polarized light component of the incident light, both the contrast characteristic and the display luminance can be improved. An excellent EL element can be provided.

[Brief description of the drawings]

FIG. 1 is a diagram schematically showing the structure and operation of an EL element according to an embodiment of the present invention.

FIG. 2 is a diagram showing an example of illuminance dependence of contrast characteristics of an EL element of the present invention and a conventional EL element.

FIG. 3 is a diagram schematically showing the structure and operation of an EL element according to another embodiment of the present invention.

FIG. 4 is a diagram schematically showing the structure and operation of an EL element according to still another embodiment of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 10 ... Transparent substrate 12 ... Selective reflection layer 14 ... 1/4 wavelength plate 16 ... Polarizing plate 22 ... Light emitting layer 24 ... Reflection electrode

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁷ Identification code FI Theme coat ゛ (Reference) H05B 33/12 H05B 33/12 Z 33/14 33/14 A F term (Reference) 2H049 BA02 BA03 BA05 BA07 BA42 BB03 BB61 BC21 3K007 AB02 BA06 CB01 CC01 DA01 DB03 EB00 5C094 AA06 AA10 BA27 DA13 EB02 ED11 ED14 ED20 FA02 5G435 AA02 AA03 AA04 BB05 BB16 DD12 FF03 FF05 FF14

Claims (5)

[Claims] 1. A polarizing plate, a quarter-wave plate provided behind the polarizing plate, a light-emitting layer provided behind the quarter-wave plate, and a light-emitting layer provided behind the light-emitting layer. A reflection electrode provided, and is provided between the １ ／ wavelength plate and the light emitting layer, and is formed by polymerizing a cholesteric liquid crystal layer;
And a selective reflection layer that selectively reflects the first circularly polarized light component of the incident light. 2. The light-emitting layer and the reflection electrode are formed on a transparent substrate, the light-emitting layer is located between the transparent substrate and the reflection electrode, and the selective reflection layer is formed between the light-emitting layer and the transparent substrate. 2. The EL device according to claim 1, wherein the EL device is provided between them. 3. The light emitting layer and the reflective electrode are formed on a transparent substrate, the reflective electrode is provided between the transparent substrate and the light emitting layer, and the light emitting layer is formed by the reflective electrode and the selective reflective layer. The EL device according to claim 1, wherein the EL device is provided between the EL devices.
element. 4. The selective reflection layer is formed by laminating a plurality of cholesteric liquid crystal layers each having a different pitch from each other. 4. The EL according to claim 1, wherein the EL is reflected.
element. 5. The selective reflection layer is formed of a single cholesteric liquid crystal layer whose pitch continuously changes in the layer thickness direction, and has a helical direction of the cholesteric liquid crystal layer with respect to light having a wavelength in the entire visible light range. 4. E according to any one of claims 1 to 3, characterized in that it reflects circularly polarized light in the same direction.
L element.