Organic electroluminescence cell, planar light source and display device Number:7,109,651 from the United States Patent and Trademark Office (PTO) owispatent An organic electroluminescence cell including at least one organic layer and a pair of electrodes, the organic layer including a light-emitting layer and sandwiched between the pair of electrodes, the pair of electrodes including a reflective electrode and a transparent electrode, the organic electroluminescence cell formed to satisfy the expression: B.sub.0<B.sub..theta. in which B.sub.0 is a frontal luminance value of luminescence radiated from a light extraction surface to an observer, and B.sub..theta. is a luminance value of the luminescence at an angle of from 50.degree. to 70.degree., wherein a reflection/refraction angle disturbance region is provided so that the angle of reflection/refraction of the luminescence is disturbed while the luminescence is output from the light-emitting layer to the observer side through the transparent electrode.<H electroluminescence, display, Organic, electro, 7109651.html, Patent, pto, 7109651

Title: Organic electroluminescence cell, planar light source and display device

Abstract: An organic electroluminescence cell including at least one organic layer and a pair of electrodes, the organic layer including a light-emitting layer and sandwiched between the pair of electrodes, the pair of electrodes including a reflective electrode and a transparent electrode, the organic electroluminescence cell formed to satisfy the expression: B.sub.0<B.sub..theta. in which B.sub.0 is a frontal luminance value of luminescence radiated from a light extraction surface to an observer, and B.sub..theta. is a luminance value of the luminescence at an angle of from 50.degree. to 70.degree., wherein a reflection/refraction angle disturbance region is provided so that the angle of reflection/refraction of the luminescence is disturbed while the luminescence is output from the light-emitting layer to the observer side through the transparent electrode.

Patent Number: 7,109,651 Issued on 09/19/2006 to Nakamura, et al.

Inventors: Nakamura; Toshitaka (Ibaraki, JP), Miyatake; Minoru (Ibaraki, JP), Nakano; Shusaku (Ibaraki, JP)
Assignee: Nitto Denko Corporation (Osaka, JP)

Appl. No.: 10/721,269

Filed: November 26, 2003

Foreign Application Priority Data


Nov 26, 2002 [JP]			P2002-341821
Jan 06, 2003 [JP]			P2003-000080
Mar 07, 2003 [JP]			P2003-062554
Jul 28, 2003 [JP]			P2003-280576

Current U.S. Class: 313/504 ; 313/110

Current International Class: H01J 1/62 (20060101); H01J 5/16 (20060101)

Field of Search: 313/504,506,509,110-112 428/690

References Cited [Referenced By]

U.S. Patent Documents


6091384	July 2000	Kubota et al.
6476550	November 2002	Oda et al.
6507379	January 2003	Yokoyama et al.
6607277	August 2003	Yokoyama et al.
6617784	September 2003	Abe et al.
6630684	October 2003	Lee et al.
6724140	April 2004	Araki
6828042	December 2004	Imanishi
6831409	December 2004	Yamada
2003/0122481	July 2003	Song et al.
2004/0012980	January 2004	Sugiura et al.
2004/0145303	July 2004	Yamada et al.
2004/0212296	October 2004	Nakamura et al.
2004/0263045	December 2004	Smith et al.
2005/0099113	May 2005	Yamada

Foreign Patent Documents


63-314795	Dec., 1988	JP
4-268505	Sep., 1992	JP
5-3081	Jan., 1993	JP
6-151061	May., 1994	JP
6-347617	Dec., 1994	JP
8-271892	Oct., 1996	JP
9-506984	Jul., 1997	JP
9-507308	Jul., 1997	JP
10-321371	Dec., 1998	JP
11-214162	Aug., 1999	JP
11-214163	Aug., 1999	JP
11-231132	Aug., 1999	JP
11-283751	Oct., 1999	JP
11-316376	Nov., 1999	JP
2000-182774	Jun., 2000	JP
2001-203074	Jul., 2001	JP
2001-244080	Sep., 2001	JP
2001-311826	Nov., 2001	JP
2001-313178	Nov., 2001	JP
2001-356027	Dec., 2001	JP

Other References

Takuya Ogawa, et al. (IEICE Trans Electron) vol. E-85-C, No. 6, p. 1239, 2002. cited by other .
M.H. Lu, et al. (J. Appl. Phys., vol. 91, No. 2, p. 595, 2002). cited by other .
J. McElvain, et al. (J. Appl. Phys. vol. 80, No.10, p. 6002, 1996). cited by other .
Asuka et al. (Appl. Phy. Lett., vol. 78, p. 3343, 2001). cited by other .
Matsumura, et al. (Appl. Phy. Lett., vol. 79, p. 4491, 2001. cited by othe- r .
Matsumoto [OPTRONICS] No. 2, p. 136, 2003. cited by other.

Primary Examiner: Guharay; Karabi
Assistant Examiner: Dong; Dalei
Attorney, Agent or Firm: Sughrue Mion, PLLC

Claims

What is claimed is:

1. An organic electroluminescence cell comprising: at least one organic layer; and a pair of electrodes serving as an anode and a cathode respectively; said organic layer including a light-emitting layer and being sandwiched between said pair of electrodes, at least one of said pair of electrodes being provided as a transparent electrode, said organic electroluminescence cell being formed to satisfy the expression B.sub.0<B.sub..theta. in which B.sub.0 is a frontal luminance value of luminescence radiated from a light extraction surface, and B.sub..theta. is a luminance value of said luminescence at an angle of from 50.degree. to 70.degree.; and wherein, said organic electroluminescence cell further comprises a reflection/refraction angle disturbance region being provided substantially without interposition of any air layer so that said transparent electrode is between said light-emitting layer and said reflection/refraction angle disturbance region, and so that the angle of reflection/refraction of said luminescence is disturbed while said luminescence is output from said light-emitting layer through said transparent electrode wherein, one of said anode and said cathode is a transparent electrode and the other is a reflective electrode; and said organic electroluminescence cell satisfies the expression (0.3/n).lamda.<d<(0.5/n).lamda. in which d (nm) is a distance between an approximate center portion of a hole-electron recombination light-emitting region and said reflective electrode, .lamda. (nm) is a peak wavelength of a fluorescence spectrum of a material used in said light-emitting layer, and n is a refractive index of said organic layer between said light-emitting layer and said reflective electrode.

2. An organic electroluminescence cell according to claim 1, wherein said reflection/refraction angle disturbance region is constituted by a light-diffusing site which contains a transparent material, and a transparent or opaque material different in refractive index from said transparent material and dispersed/distributed in said transparent material.

3. An organic electroluminescence cell according to claim 2, further comprising a reflection type polarizing element provided on a light emission side viewed from said reflection/refraction angle disturbance region.

4. An organic electroluminescence cell according to claim 3, wherein said reflection type polarizing element is a reflection type circular polarizing element made of a cholesteric liquid crystal layer.

5. An organic electroluminescence cell according to claim 3, wherein said reflection type polarizing element is a reflection type linear polarizing element made of a multilayer laminate of at least two materials different in refractive index.

6. An organic electroluminescence cell according to claim 3, further comprising an optically compensating layer which has no anisotropy in in-plane refractive index and in which a refractive index in a direction of thickness is higher than said in-plane refractive index.

7. An organic electroluminescence cell according to claim 1, wherein said reflection/refraction angle disturbance region is constituted by a lens structure.

8. An organic electroluminescence cell according to claim 1, wherein said reflection/refraction angle disturbance region is constituted by a protruded and grooved face.

9. A planar light source having an organic electroluminescence cell defined in any one of claims 1, 2, 7, and 8.

10. A display device having a planar light source defined in claim 9.

11. An organic electroluminescence cell according to claim 1, wherein said reflection/refraction angle disturbance region is constituted by a polarizing/scattering site which contains a light-transmissive resin, and micro domains different in birefringence characteristic from said light-transmissive resin and dispersed/distributed in said light-transmissive resin.

12. An organic electroluminescence cell according to claim 11, wherein said micro domains in said polarizing/scattering site are made of one member selected from the group consisting of a liquid crystal material, a vitrified material with a liquid crystal phase supercooled and solidified, and a material with a liquid crystal phase of polymerizable liquid crystal crosslinked and fixed by an energy beam.

13. An organic electroluminescence cell according to claim 11, wherein said polarizing/scattering site contains a light-transmissive resin, and micro domains which are made of a liquid crystal polymer having a glass transition temperature of not lower than 50.degree. C. to exhibit a nematic liquid crystal phase at a lower temperature than the glass transition temperature of said light-transmissive resin and which are dispersed in said light-transmissive resin.

14. An organic electroluminescence cell according to claim 11, wherein: said polarizing/scattering site exhibits refractive index differences .DELTA.n.sub.1, .DELTA.n.sub.2 and .DELTA.n.sub.3 between said micro domains and the other portions in directions of respective optical axes of said micro domains; and the refractive index difference .DELTA.n.sub.1 in an axial direction (.DELTA.n.sub.1 direction) as the highest one of the refractive index differences .DELTA.n.sub.1, .DELTA.n.sub.2 and .DELTA.n.sub.3 is in a range of from 0.03 to 0.5 whereas each of the refractive index differences .DELTA.n.sub.2 and .DELTA.n.sub.3 in two axial directions (.DELTA.n.sub.2 direction and .DELTA.n.sub.3 direction) perpendicular to the .DELTA.n.sub.1 direction is not larger than 0.03.

15. A polarizing-type planar light source having an organic electroluminescence cell defined in any one of claims 3, 4, 5, 6, 11, 12, 13, 14.

16. A display device having a polarizing-type planar light source defined in claim 15.

Description

The present application is based on Japanese Patent Applications Nos. 2002-341821, 2003-000080, 2003-062554 and 2003-280576, which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an organic electroluminescence cell excellent in light-emitting efficiency and especially excellent in efficiency of extracting luminescence to the outside, a (polarizing-type) planar light source of high efficiency using the organic electroluminescence cell, and a (liquid crystal) display device having the planar light source.

2. Description of the Related Art

An electroluminescence cell or a light-emitting diode in which a light-emitting layer is provided between electrodes to obtain luminescence electrically has been researched and developed actively not only for application to a display device but also for application to various types of light sources such as a flat illuminator, a light source for optical fiber, a backlight unit for liquid crystal display, a backlight unit for liquid crystal projector, etc.

Particularly, an organic electroluminescence cell has attracted public attention in recent years because it is excellent in light-emitting efficiency, low-voltage drive, lightweight and low cost. A primary concern with the purpose of application to these light sources is enhancement in light-emitting efficiency. Improvement in cell structure, material, drive method, production method, etc. has been examined to obtain light-emitting efficiency equivalent to that of a fluorescent lamp.

In an inter-solid luminescent element such as an organic electroluminescence cell in which luminescence is extracted from a light-emitting layer per se, however, light generated at a angle not lower than a critical angle decided on the basis of the refractive index of the light-emitting layer and the refractive index of an output medium is totally reflected and confined in the inside, so that the light is lost as guided light.

According to calculation based on classical law of refraction (Snell's law), light-extracting efficiency .eta. in taking out generated light to the outside can be given by the approximate expression .eta.=1/(2n.sup.2) in which n is the refractive index of the light-emitting layer. Assuming that the refractive index of the light-emitting layer is 1.7, then 80% or more of the light is lost as guided light, that is, as a loss in a side face direction of the cell because .eta. is nearly equal to 17%.

In the organic electroluminescence cell, excitons contributing to luminescence are only singlet excitons among excitons generated by recombination of electrons and holes injected from the electrodes. The probability that singlet excitons will be generated is 1/4. Even in the case where only such a thing is taken into consideration, the efficiency is not higher than 5%, that is, the efficiency is very low.

As a method for improving the light-emitting efficiency of the light-emitting layer per se, development of a luminescent material (Unexamined Japanese Patent Publication No. 2001-313178) for generating light also from phosphorescence due to triplet excitons has advanced in recent years, so that the possibility that quantum efficiency will be improved remarkably has been found.

Even if quantum efficiency were improved, light-emitting efficiency would be reduced in accordance with light-extracting efficiency multiplied by the quantum efficiency. In other words, if light-extracting efficiency can be improved, there is room for remarkable improvement in light-emitting efficiency according to the synergy between the quantum efficiency and the light-extracting efficiency.

As described above, in order to extract guided light to the outside, a region for disturbing an angle of reflection/refraction need to be formed between the light-emitting layer and an emergence surface to destroy Snell's law to thereby change an angle of transmission of light originally totally reflected as guided light or beam-condensing characteristic need to be given to luminescence per se. It is however not easy to form such a region that outputs all guided light to the outside. Therefore, a proposal for taking out guided light as much as possible has been made.

For example, as methods for improving light-extracting efficiency, there have been proposed a method in which beam-condensing characteristic is given to a substrate per se to improve light-extracting efficiency (Unexamined Japanese Patent Publication No. Sho. 63-314795), a method in which a light-emitting layer is made of discotic liquid crystal to improve frontal directivity of generated light per se (Unexamined Japanese Patent Publication No. Hei. 10-321371) and a method in which a stereostructure, an inclined surface, a diffraction grating, etc. are formed in the cell per se (Unexamined Japanese Patent Publication No. Hei. 11-214162, Unexamined Japanese Patent Publication No. Hei. 11-214163 and Unexamined Japanese Patent Publication No. Hei. 11-283751). These proposals, however, have a problem on complication in structure, reduction in light-emitting efficiency of the light-emitting layer per se, etc.

As a relatively simple method, there has been also proposed a method in which a light-diffusing layer is formed to change an angle of refraction of light to thereby reduce light satisfying the condition of total-reflection.

For example, there have been proposed various methods such as a method using a diffusing plate having a transparent substrate, and particles-dispersed in the transparent substrate so as to form such a distributed index structure that the refractive index of the inside is different from the refractive index of the outside (Unexamined Japanese Patent Publication No. Hei. 6-347617), a method in which a diffusing member having a light-transmissive substrate, and a single particle layer arranged on the light-transmissive substrate (Unexamined Japanese Patent Publication No. 2001-356207) are used, and a method in which scattering particles are dispersed in the same material as that of the light-emitting layer (Unexamined Japanese Patent Publication No. Hei.6-151061). These proposals have been provided by finding features in the characteristic of scattering particles, the refractive index difference from a dispersion matrix, the dispersing form of particles, the place for formation of the scattering layer, and so on.

Incidentally, the organic electroluminescence cell uses such a principle that holes injected from the anode and electrons injected from the cathode by application of an electric field are recombined into excitons to generate luminescence from a fluorescent (or phosphorescent) substance. It is therefore necessary to perform the recombination efficiently in order to improve quantum efficiency. As this method, there is generally used a method in which the cell is formed as a laminated structure. For example, a two-layer structure having a hole transport layer and an electron transport light-emitting layer or a three-layer structure having a hole transport layer, a light-emitting layer and an electron transport layer is used as the laminated structure. There have been also made various proposals for a laminated cell formed as a double hetero structure in order to improve efficiency.

In such a laminated structure, recombination is substantially concentrated in a certain region. For example, in a two-layer type organic electroluminescence cell, as shown in FIG. 12, recombination is concentrated in an electron transport light-emitting layer side region 6 which is about 10 nm distant from an interfacial layer between a hole transport layer 4 and an electron transport light-emitting layer 5 which are sandwiched between a pair of electrodes constituted by a reflective electrode 3 and a transparent electrode 2 (as reported by Takuya, Ogawa et al, "IEICE TRANS ELECTRON" Vol. E85-C, No. 6, p. 1239, 2002).

Light generated in the light-emitting region 6 is radiated in all directions. Consequently, as shown in FIG. 13, an optical path difference is produced between light radiated toward a light-extracting surface on the transparent electrode 2 side and light radiated toward the reflective electrode 3, reflected by the reflective electrode 3 and radiated toward the light-extracting surface.

In FIG. 13, the thickness of the electron transport light-emitting layer in the organic electroluminescence cell is generally in a range of from tens of nm to a hundred and tens of nm, that is, the order of wavelength of visible light. Accordingly, light beams finally outgoing from the cell interfere with each other. The interference becomes destructive or constructive according to the distance d between the light-emitting region and the reflective electrode. Although only light radiated in a frontal direction is shown in FIG. 13, light radiated in oblique directions is also present actually. The condition of interference varies according to the angle of radiated light in addition to the distance d and the wavelength .lamda. of generated light. As a result, there may occur the case where light beams radiated in a frontal direction interfere with each other constructively but light beams radiated in a wide-angle direction interfere with each other destructively, or there may occur the case reverse to the aforementioned case. That is, luminance of generated light varies according to the viewing angle.

It is a matter of course that the intensity of light varies remarkably according to the angle as the distance d increases. Therefore, the thickness of the electron transport light-emitting layer is generally selected so that the distance d is made equal to about a quarter of the wavelength of generated light to obtain constructive interference of light in the frontal direction.

When, for example, the distance d is smaller than about 50 nm, absorption of light becomes remarkable in the reflective electrode generally made of a metal. This causes reduction in intensity of generated light and influence on intensity distribution. That is, in the organic electroluminescence cell, the distribution of radiated light varies remarkably according to the distance d between the light-emitting region and the reflective electrode, so that the guided light component varies widely according to the variation in the distribution of radiated light. Furthermore, the emission spectrum of this type cell has broad characteristic in a relatively wide wavelength range. Accordingly, variation in the wavelength range for constructive interference of light according to the distance d causes variation in peak wavelength of generated light. Furthermore, the emission spectrum varies according to the viewing angle in addition to the distance d.

To solve these problems, there has been made a proposal for selecting the film thickness to suppress a phenomenon that the color of generated light varies according to the viewing angle (see Patent Document 1). In this proposal, however, there is no description concerning guided light. It is obvious that the film thickness range selected by this proposal for suppressing the dependence of the color of generated light on the viewing angle is different from the range according to the invention which will be described later.

For the aforementioned reason, the light-extracting efficiency of the laminated organic electroluminescence cell cannot be calculated correctly on the classical assumption that about 80% of generated light is confined as guided light in the inside of the cell. That is, the guided light component varies remarkably according to the structure of the cell. For example, as reported by M. H. Lu et al. (J. Appl. Phys., Vol. 91, No. 2, p. 595, 2002), detailed research on change in the guided light component according to the structure of the cell has been made on the basis of a quantum-mechanical calculation method in consideration of a micro-cavity effect.

Accordingly, there is a possibility that the obtained effect will not be so large as estimated by the classical theory even in the case where a light-diffusing layer or the like is formed in order to destroy the condition of total reflection.

When the organic electroluminescence cell is used as a backlight unit for a liquid crystal display device, luminescence generated from the cell needs to be converted into linearly polarized light by a polarizer when used for liquid crystal display because the luminescence is natural light. As a result, absorption loss due to the polarizer is produced. There is a problem that the rate of utilization of light cannot be set to be higher than 50%. Hence, even in the case where guided light is extracted efficiently by the aforementioned method, a half or more of the guided light is absorbed to the polarizer.

As a method for solving the problem, there has been made a proposal for forming an organic electroluminescence cell layer on an oriented film to extract luminescence per se as linearly polarized light (see Patent Document 2). Although the absorption loss due to the polarizer can be reduced to half, at the most, by the aforementioned proposal, there is a possibility that the light-emitting efficiency of the cell will be lowered because of the insertion of the oriented film for orienting an organic thin film. In addition, like the related-art cell, the problem of the guided light due to total reflection cannot be solved at all by this proposal.

It has been proposed a method in which light generated in an organic electroluminescence cell is extracted through a polarizing/scattering film (see Patent Document 3). According to this proposal, light lost as guided light can be scattered so as to be extracted, and output light can be extracted as polarized light which is rich in linearly polarized light. Accordingly, the absorption loss due to the polarizer can be reduced, so that a polarizing-type planar light source of high efficiency can be provided as a light source for a liquid crystal display device.

For example, the relation between the guided light and the influence of the distance between the light-emitting region and the reflective electrode on interference has not been described yet in this proposal. It cannot be said that this proposal brings out the greatest possible effect of the light source for a liquid crystal display device.

As a method for reducing absorption loss of backlight due to a polarizer in a liquid crystal display device, there is known a method using a polarized light separating layer made of a reflection type polarizing element (see Patent Documents 4 and 5). There has been made a proposal for applying this method to an organic electroluminescence cell (see Patent Documents 6 and 7).

No proposal has ever been made for bringing out the greatest light-emitting efficiency, for example, by combination of an organic electroluminescence cell and a reflection type circular polarizing element on the assumption of detailed research on the relation between the guided light and the influence of the distance between the light-emitting region and the reflective electrode on interference in the organic electroluminescence cell. Therefore, the provision of a polarizing-type planar light source of high efficiency best adapted to a liquid crystal display device using polarized light is desired earnestly in the existing circumstances.

[Patent Document 1]

Unexamined Japanese Patent Publication No. Hei. 5-3081 (pages 2 to 4)

[Patent Document 2]

Unexamined Japanese Patent Publication No. Hei. 11-316376 (pages 2 to 5)

[Patent Document 3]

Unexamined Japanese Patent Publication No. 2001-203074 (pages 2 to 6)

[Patent Document 4]

Unexamined Japanese Patent Publication No. Hei. 4-268505 (pages 2 to 6)

[Patent Document 5]

Unexamined Japanese Patent Publication No. Hei. 8-271892 (pages 2 to 5)

[Patent Document 6]

Unexamined Japanese Patent Publication No. 2001-244080 (pages 2 to 4)

[Patent Document 7]

Unexamined Japanese Patent Publication No. 2001-311826 (pages 2 and 3)

SUMMARY OF THE INVENTION

Under such circumstances, an object of the invention is to provide an organic electroluminescence cell which is so excellent in light-extracting efficiency that loss light confined as guided light in the inside of the organic electroluminescence cell in the related art can be extracted efficiently. Another object of the invention is to provide an organic electroluminescence cell of high efficiency in which: the loss light can be extracted as polarized light efficiently when the cell is used as a backlight unit for a liquid crystal display device; and absorption due to a polarizer can be minimized. A further object of the invention is to provide a planar light source or a polarizing-type planar light source of high efficiency using the organic electroluminescence cell, and a display device such as a liquid crystal display device having the planar light source or the polarizing-type planar light source.

The inventors have made examination earnestly to solve the problem. As a result, the following knowledge has been obtained. The knowledge will be described with reference to FIG. 9. FIG. 9 is a schematic view showing the case where light generated in the light-emitting region 6 of the two-layer type organic electroluminescence cell depicted in FIG. 12 is extracted from the cell. Although only light radiated toward an upper semispherical surface is shown in FIG. 9, light radiated toward the reflective electrode 3 is not shown in FIG. 9 but actually present.

In FIG. 9, the critical angle decided on the basis of the difference between the refractive index of a support substrate (glass substrate) 1 and the refractive index of an air layer is about 40 degrees. That is, light having an angle greater than 40 degrees is totally reflected by the glass/air interface so as to be confined as guided light in the inside of the cell. Although about 45% of light calculated by 40 degrees per 90 degrees appears to be extracted from the cell in FIG. 9, actually generated light is radiated in all directions. For this reason, solid angles are decided so that the intensity of a light component becomes higher as the angle of the light component becomes wider. For this reason, the light-emitting efficiency calculated according to the classical theory is not higher than 20%.

Actually, the organic electroluminescence cell causes an effect of interference of light. For the interference of light, the structure of the cell is usually decided so that light beams radiated in a frontal direction so as to be able to be extracted from the cell interfere with each other: constructively. In this case, guided light beams interfere with each other destructively rather than constructively. Accordingly, even in the case where a region for disturbing an angle of reflection/refraction is provided in the structure of the cell, a large luminance-improving effect cannot be expected.

On the other hand, the inventors have tried forming a region for disturbing an angle of reflection/refraction in addition to amplifying guided light having a large part of light intensity distributed therein by deciding the structure of the cell intentionally to obtain destructive interference of light radiated in the frontal direction and constructive interference of the wide-angle light component usually confined as guided light in the inside of the cell. As a result, it has been found that light-emitting efficiency is improved remarkably compared with the related-art method. That is, it has been found that an organic electroluminescence cell obtained when a region for disturbing an angle of reflection/refraction is provided in a cell structure which is formed without the region so as to exhibit low light-emitting efficiency is higher in efficiency than that obtained when the region is provided in the related-art cell structure.

It has been also found that a polarizing-type planar light source and a liquid crystal display device which are so high in efficiency that absorption due to a polarizer can be minimized are obtained by using a reflection type polarizing element for reducing light absorbed to the polarizer when the organic electroluminescence cell configured as described above is applied to a backlight unit for a liquid crystal display device.

The inventors have further found that guided light confined in the inside of the cell can be extracted as polarized light efficiently without necessity of use of the reflection type polarizing element particularly when a polarizing/scattering site containing a light-transmissive resin, and micro domains different in birefringence characteristic from the light-transmissive resin and dispersed/distributed into the light-transmissive resin is provided as the region for disturbing the angle of reflection/refraction. That is, in the organic electroluminescence cell shown in FIG. 9, the state in which light generated in the light-emitting region 6 is extracted from the cell is examined in the condition that a polarizing/scattering site containing a light-transmissive resin, and micro domains different in birefringence characteristic from the light-transmissive resin and dispersed/distributed into the light-transmissive resin is formed on a light-extracting surface side on the support substrate (glass substrate) 1, that is, on a side opposite to the transparent electrode 2, substantially without interposition of any air layer.

Among generated light, light present in the upper hemisphere of FIG. 9 passes through the transparent electrode and the glass substrate and is made incident on the polarizing/scattering site. Light present in the lower sphere of FIG. 9 is reflected by the cathode and then also made incident on the polarizing/scattering site. In this process, generated light can be made incident on the polarizing/scattering site without influence of total reflection because there is no air layer (refractive index=1) low in refractive index (Incidentally, depending on the refractive indices of the transparent electrode and the glass substrate, part of light may be totally reflected). As shown in FIG. 9, in the general cell, light having an angle not smaller than the critical angle decided on the basis of the difference between the refractive index of the glass substrate and the refractive index of the air layer is totally reflected so as to be lost as guided light, so that only about 20%, at the most, of the generated light can be extracted from the cell.

Large part of the generated light incident on the polarizing/scattering site without influence of total reflection is totally reflected because of the refractive index difference between the polarizing/scattering site and the air interface and propagated through the polarizing/scattering site. In the propagated light, a linearly polarized light component having a plane of vibration parallel to an axial direction (.DELTA.n.sub.1 direction) exhibiting the maximum refractive index difference (.DELTA.n.sub.1) between the micro domains and other regions in the polarizing/scattering site is selectively intensively scattered. As a result, the angle of part of the propagated light becomes smaller than the angle of total reflection, so that part of the propagated light emerges from the cell to the outside (air). Incidentally, if there is no micro domain so that selective polarized scattering does not occur, about 80% of the generated light judged from solid angles is confined in the inside of the cell while total reflection is repeated. The confined light can emerge from the cell to the outside only when the condition of total reflection is destroyed by scattering in the interface between each micro domain and the light-transmissive resin. Accordingly, the light-extracting efficiency can be controlled optionally on the basis of the size of each micro domain and the degree of presence of the micro domains.

On the other hand, light beams scattered at large angles at the time of scattering in the .DELTA.n.sub.1 direction, light beams satisfying the condition of the .DELTA.n.sub.1 direction but not scattered and light beams having other directions of vibration than the .DELTA.n.sub.1 direction are confined in the polarizing/scattering site and propagated through the polarizing/scattering site while total reflection is repeated. These confined light beams wait for a change to emerge from the polarizing/scattering site by satisfying the condition of the .DELTA.n.sub.1 direction in such a manner that the polarized state is eliminated by birefringence retardation or the like. By the repetition of the aforementioned operation, linearly polarized light having a predetermined plane of vibration can emerge from the polarizing/scattering site efficiently. That is, light origin ally confined as guided light can be finally extracted as a linearly polarized light component. Hence, according to this method, generated light can be extracted, as polarized light rich in its linearly polarized light component, from the cell efficiently without provision of any special light output means such as microlenses or reflecting dots. In addition, the direction of vibration of linearly polarized light can be changed optionally by the angle at which the polarizing/scattering site is set. Accordingly, when the organic electroluminescence cell is used as a backlight unit for a liquid crystal display device, electric power consumption can be reduced.

As described above, it has been found that light-emitting efficiency can be improved compared with the related art method when the polarizing/scattering site is integrally formed without interposition of any air layer in addition to amplifying wide-angle light having a large part of light intensity distributed therein in accordance with solid angles by deciding the structure of the cell intentionally to obtain destructive interference of light radiated in the frontal direction and constructive interference of the wide-angle light component usually confined as guided light in the inside of the cell. That is, when such a cell structure that will make light-emitting efficiency low in ordinary use of the cell is combined with the polarizing/scattering site, generated light can be efficiently extracted from the cell and as polarized light rich in its linearly polarized light component. When the organic electroluminescence cell finally obtained is used as a backlight unit for a liquid crystal display device, light-emitting efficiency can be improved remarkably.

The biggest weakness of the organic electroluminescence cell is that the cell deteriorates on contact with a small amount of moisture or oxygen. For this reason, there is a problem that dark spots are produced with fine defects as start points, to say nothing of reaction in light-emitting efficiency. Such dark spots have been described in detail as reported by J. McElvain et al. (J. Appl. Phys., Vol. 80, No. 10, p. 6002, 1996). Although the cell is generally perfectly sealed in order to avoid this problem, it is still difficult to prevent the production of dark spots perfectly. The dark spots remarkably reduce the external appearance and visibility of a planar light source or a display device.

When the polarizing/scattering site is formed in the aforementioned manner, there can be however obtained such a vary excellent effect that lowering of visibility caused by the production of dark spots becomes almost inconspicuous even if the dark spots were more or less produced, because light finally extracted toward the observer side is light emerging from the cell after repeatedly scattered in the polarizing/scattering site.

The invention is accomplished on the basis of the aforementioned knowledge.

That is, the invention provides an organic electroluminescence cell (hereinafter merely referred to as organic EL cell) including at least one organic layer, and a pair of electrodes serving as an anode and a cathode respectively, the organic layer having a light-emitting layer and being sandwiched between the pair of electrodes, at least one of the pair of electrodes being provided as a transparent electrode, the organic EL cell being formed to satisfy the expression (1); B.sub.0<B.sub..theta. in which B.sub.0 is a frontal luminance value of luminescence radiated from a light extraction surface to an observer, and B.sub..theta. is a luminance value of the luminescence at an angle of from 50.degree. to 70.degree., wherein a reflection/refraction angle disturbance region is provided substantially without interposition of any air layer so that the angle of reflection/refraction of the luminescence is disturbed while the luminescence is output from the light-emitting layer to the observer side through the transparent electrode.

That is, the organic electroluminescence cell according to the present invention comprising:

at least one organic layer;

and a pair of electrodes serving as an anode and a cathode respectively;

the organic layer including a light-emitting layer and being sandwiched between the pair of electrodes, at least one of the pair of electrodes being provided as a transparent electrode, the organic electroluminescence cell being formed to satisfy the expression (1): B.sub.0<B.sub..theta. in which to is a frontal luminance-value of luminescence radiated from a light extraction surface to an observer, and B.sub..theta. is a luminance value of the luminescence at an angle of from 50.degree. to 70.degree.; and

a reflection/refraction angle disturbance region being provided substantially without interposition of any air layer so that the angle of reflection/refraction of the luminescence is disturbed while the luminescence is output from the light-emitting layer to the observer through the transparent electrode.

In an especially preferred mode of the organic EL cell configured according to the invention, one of the anode and the cathode is a transparent electrode and the other is a reflective electrode; and the organic EL cell satisfies the expression (2): (0.3/n).lamda.<d<(0.5/n) in which d (nm) is a distance between an approximate center portion of a hole-electron recombination light-emitting, region and the reflective electrode, .lamda. (nm) is a peak wavelength of a fluorescence spectrum of a material used in the light-emitting layer, and n is a refractive index of the organic layer between the light-emitting layer and the reflective electrode.

Preferably, in the organic EL cell configured according to the invention, the reflection/refraction angle disturbance region may be constituted by a light-diffusing site which contains a transparent material, and a transparent or opaque material different in refractive index from the transparent material and dispersed/distributed in the transparent material, or the reflection/refraction angle disturbance region may be constituted by a lens structure or by a protruded and grooved face.

The invention also provides a planar light source including an organic EL cell having any one of the aforementioned configurations or a display device including the planar light source.

Preferably, the organic EL cell configured according to the invention further includes a reflection type polarizing element provided on the observer side viewed from the reflection/refraction angle disturbance region. Particularly, in the organic EL cell, the reflection type polarizing element maybe a reflection type circular polarizing element made of a cholesteric liquid crystal layer or the reflection type polarizing element may be a reflection type linear polarizing element made of a multilayer laminate of at least two materials different in refractive index. Preferably, the organic EL cell configured according to the invention further includes an optically compensating layer which has no anisotropy in in-plane refractive index and in which a refractive index in a direction of thickness is higher than the in-plane refractive index.

The invention also provides a polarizing-type planar light source including an organic EL cell having any one of the aforementioned configurations or a display device such as a liquid crystal display device including the polarizing-type planar light source.

Preferably, in the organic EL cell configured according to the invention, the reflection/refraction angle disturbance region is constituted by a polarizing/scattering site which contains a light-transmissive resin, and micro domains different in birefringence characteristic from the light-transmissive resin and dispersed/distributed in the light-transmissive resin. Particularly, in the organic EL cell, the micro domains in the polarizing/scattering site maybe made of one member selected from the group consisting of a liquid crystal material, a vitrified material with a liquid crystal phase supercooled and solidified, and a material with a liquid crystal phase of polymerizable liquid crystal crosslinked and fixed by an energy beam or the polarizing/scattering site may contain a light-transmissive resin, and micro domains which are made of a liquid crystal polymer having a glass transition temperature of not lower than 50.degree. C. to exhibit a nematic liquid crystal phase at a lower temperature than the glass transition temperature of the light-transmissive resin and which are dispersed in the light-transmissive resin. Preferably, in the organic EL cell configured according to the invention, the polarizing/scattering site exhibits refractive index differences .DELTA.n.sub.1, .DELTA.n.sub.2 and .DELTA.n.sub.3 between the micro domains and the other portions in directions of respective optical axes of the micro domains; and the refractive index difference .DELTA.n.sub.1 in an axial direction (.DELTA.n.sub.1 direction) as the highest one of the refractive index differences .DELTA.n.sub.1, .DELTA.n.sub.2 and .DELTA.n.sub.3 is in a range of from 0.03 to 0.5 whereas each of the refractive index differences .DELTA.n.sub.2 and .DELTA.n.sub.3 in two axial directions (.DELTA.n.sub.2 direction and .DELTA.n.sub.3 direction) perpendicular to the .DELTA.n.sub.1 direction is not larger than 0.03.

The invention also provides a polarizing-type planar light source including an organic EL cell having any one of the aforementioned configurations or a display device such as a liquid crystal display device including the polarizing-type planar light source.

According to the invention, light-emitting efficiency is low before formation of a reflection/refraction angle disturbance region but the reflection/refraction angle disturbance region is formed to amplify a guided light component origin ally confined in the inside of the cell to thereby disturb the angle of reflection/refraction of the guided light component to thereby extract the guided light component. Accordingly, the intensity of light finally extracted can be increased, so that an organic EL cell of high efficiency can be provided.

Further, a polarizing/scattering site is formed as the reflection/refraction angle disturbance region to make it possible not only to increase the intensity of light emerging from the cell but also to extract the light as polarized light rich in its linearly polarized light component. Accordingly, the organic EL cell can be provided as a cell that exhibits very high efficiency when the cell is applied to a polarizing-type planar light source used in a liquid crystal display device.

A reflection type polarizing element may be further set, in addition to formation of a light-diffusing layer, a lens structure or the like, as the reflection/refraction angle disturbance region. In the case, a half or more of the guided light component can be extracted as polarized light. The organic EL cell can be provided as a cell that exhibits high efficiency when the cell is applied to a polarizing-type planar light source used in a liquid crystal display device.

For this reason, in accordance with the invention, the life of the cell can be prolonged because electric power consumption can be reduced greatly to thereby reduce the current flowing in the cell. In addition, the light-extracting efficiency of the organic EL cell can be improved. Accordingly, if internal quantum efficiency of the organic EL cell is improved with the advance of improvement in organic EL material, electrode material, etc., light-emitting efficiency will be improved according to the synergy between the light-extracting efficiency and the internal quantum efficiency to thereby make the effect of the invention higher.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1 is a sectional view showing a first embodiment of an organic electroluminescence cell according to the invention;

FIG. 2 is a characteristic graph showing basic configurations of organic electroluminescence cells according to the invention and the related art (before a region for disturbing the angle of reflection/refraction is formed on each basic configuration);

FIG. 3 is a sectional view showing a second embodiment of the organic electroluminescence cell according to the invention;

FIG. 4 is a sectional view showing a third embodiment of the organic electroluminescence cell according to the invention;

FIG. 5 is a sectional view showing a fourth embodiment of the organic electroluminescence cell according to the invention;

FIG. 6 is a sectional view showing a fifth embodiment of the organic electroluminescence cell according to the invention;

FIG. 7 is a sectional view showing a sixth embodiment of the organic electroluminescence cell according to the invention;

FIG. 8 is a sectional view showing an example of a liquid crystal display device using the organic electroluminescence dell according to the invention;

FIG. 9 is a view for explaining the principle of the organic electroluminescence cell according to the invention;

FIG. 10 is a view for explaining the characteristic of the organic electroluminescence cell obtained in Example 1-1;

FIG. 11 is a characteristic graph showing luminance of the organic electroluminescence cell obtained in each of Examples 3-1 and 3-3;

FIG. 12 is an explanatory view showing a light-emitting region of the organic electroluminescence cell; and

FIG. 13 is an explanatory view showing luminance of the organic electroluminescence cell.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of the invention will be described below with reference to the drawings.

FIG. 1 shows an example of a two-layer type organic EL cell according to the invention. The cell is basically configured so that a transparent electrode 2, a hole transport layer 4, an electron transport light-emitting layer 5 and a reflective electrode 3 are laminated successively on a support substrate 1.

That is, the cell is configured so that an organic layer composed of a hole transport layer 4 and an electron transport light-emitting layer 5 as described above is sandwiched between a pair of electrodes constituted by a transparent electrode 2 and a reflective electrode 3. When the cell operates, hole-electron recombination occurs in a region on the electron transport light-emitting layer side about 10 nm distant from an interfacial layer between the hole transport layer 4 and the electrode transport light-emitting layer 5. As a result, luminescence is generated while concentrated in a light-emitting region 6 shown in FIG. 1.

Incidentally, another organic EL cell than the two-layer type organic EL cell, for example, a three-layer type organic EL cell having a hole transport layer, a light-emitting layer and an electron transport layer operates as follows. When a voltage is applied between electrodes (i.e., an anode and a cathode), holes injected from the anode and electrons injected from the cathode are moved in carrier transport layers respectively and recombined in the light-emitting layer to produce excitons. As a result, luminescence is generated in the same manner as in the two-layer type organic EL cell.

The invention is also basically configured so that light beams generated particularly in a frontal direction interfere with each other destructively but guided light beams confined in the inside of the cell interfere with each other constructively. This feature will be described with reference to FIG. 2.

FIG. 2 is a characteristic graph in the case where the angular distribution of luminance of the organic EL cell having only the basic configuration (i.e., before provision of a reflection/refraction angle disturbance region which will be described later) is measured at intervals of 10 degrees in an angle range of from 0 degrees to 80 degrees viewed from the frontal direction. In FIG. 2, the curve a represents an organic EL cell according to the invention, and the curve b represents an organic EL cell according to the related art.

Incidentally, in the basic configuration, the transparent electrode 2 is 100 nm thick, the hole transport layer 4 is 50 nm thick, and the electron transport light-emitting layer 5 is 140 nm thick (according to the invention) or 60 nm thick (according to the related art). The measurement is made in the condition that a voltage is applied between the electrodes so that the same current flows in each of the cell according to the invention and the cell according to the related art.

As is obvious from FIG. 2, the cell according to the related art exhibits a preferred perfect diffuse type luminance distribution in which the frontal luminance value, that is, the luminance value in a direction of 0 degrees from the frontal direction is high and in which, moreover, the luminance value is kept approximately constant on a relatively wide angle range. On the contrary, the cell according to the invention exhibits characteristic in which the frontal luminance value is low and in which, moreover, luminance increases as the angle increases. That is, the cell according to the invention is configured so that dependence of the luminance on the angle satisfies the expression: B.sub.0<B.sub..theta. (1) in which B.sub.0 is the frontal luminance value, and B.sub..theta. is the luminance value in an angle range of from 50.degree. to 70.degree..

Although this example has been described on the case where the relation given by the expression (1) is achieved on the basis of variation in thickness of the electron transport light-emitting layer 5, the relation can be achieved optionally it the materials, thicknesses, etc. of the light-emitting layer 5--containing organic layer and the pair of electrodes are selected suitably.

In a more preferred embodiment of the invention, the cell may be configured to satisfy the expression: (0.3/n).lamda.<d<(0.5/n).lamda. (2) in which d is the distance between an approximate center portion of the hole-electron recombination light-emitting region 6 and the reflective electrode 3, .lamda. is the peak wavelength of a fluorescence spectrum of a material used in the light-emitting layer (i.e., the electron transport light-emitting layer 5 in this case) and n is the refractive index of the organic layer (i.e., the electron transport light-emitting layer 5 in this case) between the light-emitting layer and the reflective electrode 3.

When, for example, the electron transport light-emitting layer 5 in the aforementioned example generates green fluorescence with a peak wavelength of 540 nm and has a refractive index of 1.65, the distance d is preferably selected to be in a range of from 98.2 nm to 163.6 nm.

In addition to the basic configuration satisfying the expression (1) or preferably the expression (2), the invention is characterized in that a region 7 for disturbing the angle of reflection/refraction of light is further provided between the light-emitting layer and an output medium on the observer side.

That is, when light origin ally confined as guided light enters the region 7, the angle of transmission of the light is changed. Part of the light having the angle of transmission changed to a smaller angle than the angle of total reflection emerges from the cell. The other part of the light finally emerges from the cell while repeatedly reflected and scattered in the cell. As a result, an aimed high light-emitting efficiency can be achieved.

In order to satisfy the expression (1) or (2), it may be necessary to make the thickness of the organic layer somewhat larger than the thickness generally used in the organic layer. In this case, because the film thickness becomes larger, electrical short-circuiting caused by defects, fine bumps in the electrodes, etc. can be reduced to bring about an advantage of improvement in yield but the resistance of the cell increases in accordance with increase in film thickness to bring about a problem of increase in operating voltage. That is, though efficiency (cd/A) per unit current may be improved, there is the possibility that the amount (lm/W) of luminous flux per unit electric power will be equal to or lower than that in the related art according to circumstances.

For example, this problem can be avoided by a method disclosed in Unexamined Japanese Patent Publication No. 2000-182774 and references cited therein. That is, in the disclosed method, the electric resistance of the organic layer is reduced by means of mixing a metal organic complex compound containing alkali metal ions, alkali earth metal ions, rare earth metal ions or the like with an organic EL material so that the organic layer can. be thickened while increase in operating voltage is suppressed. There is no limit to a dopant material represented by a metal complex added to the mixture layer and to a mixing method used. Any suitable method can be used. Because the effect of the invention can be obtained by such a method without increase in operating voltage, power efficiency as well as current efficiency can be improved.

Although the expression (2) is applied to the case where luminescence generated in the organic EL cell is monochromatic light, the effect of the invention can be obtained sufficiently also in the case where the organic EL cell is a white cell using EL generated from a plurality of luminous materials such as red, green, blue, etc. It is a matter of course that luminous materials may be combined to generate light of white obtained by mixing blue and yellow, and that the invention can be used for generating light of any other color than white by color mixing. When, for example, three colors of