Patent Grant
8129712
The entire disclosure of Japanese Patent Application No. 2005-53894 filed Feb. 28, 2005 including specification, claims, drawings, and abstract is incorporated herein by reference.
1. Field of the Invention
The present invention relates to an organic EL panel including plural pixels each having an organic EL element, and more particularly to an organic EL panel having, in each pixel, a micro-resonator (i.e., microcavity) capable of enhancing the light of a specific wavelength.
2. Description of the Related Art
A flat panel display (FPD) is a prospective display device having a thin and compact structure. Some liquid crystal displays, which are a type of FPD, are practically used in various devices. Furthermore, there are light emitting devices (e.g., display devices or light sources) including self-luminous type electro-luminescence elements (hereinafter, referred to as “EL elements”). In particular, organic EL display devices (i.e., organic EL panels) can produce a variety of luminescent colors depending on organic compound materials contained in the organic EL elements.
The organic EL display devices are the self-luminous type and accordingly different from liquid crystal displays or other display devices that control the transmissivity of the light emitted from a back light. Thus, the organic EL display devices have excellent efficiency in the use of light. Namely, the organic EL display devices can realize highly effective and bright emission of the light.
However, the organic EL element includes an organic layer that tends to deteriorate depending on conditions of use. In particular, to increase the brightness of the light, the injection current to the organic layer can be increased. In such a case, the organic layer deteriorates significantly.
As discussed in Japanese Patent Application Laid-open NO. hei06-275381, or in “Element Employing an Optical Resonator Structure” by Takahiro NAKAYAMA and Atsushi SUMIDA, the Japan Society of Applied physics, Molecular Electronics and Bioelectronics division, 1993, third meeting, p 135-p 143, it is known that an EL display device employing a micro-resonator (i.e., microcavity) can enhance the light of a specific wavelength.
However, the viewing angle dependence of a display color is not negligible when the microcavity is used, because an optical length of the microcavity when seen from the vertical direction is different from an optical length seen from an oblique direction. Accordingly, the wavelength of the enhanced light varies depending on the viewing angle.
It is therefore necessary to reduce or eliminate the viewing angle dependence caused by the microcavity.
According to the present invention, one pixel includes a region where no microcavity structure is present or includes a region where the thickness of a semi-transmissive film is differentiated, in order to improve (i.e., reduce or eliminate) the viewing angle dependence. Furthermore, the cavity length varies depending on the color (resonant wavelength). When the cavity length is long, the viewing angle dependence is large. Namely, the viewing angle dependence is different in each color.
Hence, in the present invention, the microcavity effect of enhancing the light of a specific wavelength can be changed or adjusted in accordance with the color of the light emitted from a pixel. The viewing angle dependence can be effectively improved.
The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate an embodiment of the invention and, together with the description, serve to explain the principles of the invention, in which:
Embodiments of the present invention will be described below with reference to the drawings.
A semiconductor layer 12, formed at a predetermined place of a glass substrate 10, can constitute a TFT, electrodes, or wiring. According to the example shown in
An interlayer insulating film 20, formed on the gate electrode 18 and the gate insulating film 16, entirely covers the upper surface of the glass substrate 10. A source electrode 22, extending across the interlayer insulating film 20, contacts the source region of the drive TFT 14. A first flattening film 24 covers the source electrode 22 and the interlayer insulating film 20. A transparent electrode 26, formed on the first flattening film 24, has a size corresponding to a display area of the pixel. The transparent electrode 26 is connected to the drain region of the drive TFT 14 via a contact hole.
A second flattening film 28 conceals the peripheral portion of the transparent electrode 26. A portion above the transparent electrode 26 constitutes an organic EL element 30.
The organic EL element 30 has a multilayered structure including an organic layer (luminous element layer) 34, positioned between the transparent electrode 26 and a counter electrode 32. The organic layer 34 contains organic compounds, such as organic luminous materials. According to this arrangement, injection of holes from an anode and injection of electrons from a cathode are available for the organic layer 34. The injected holes and electrons can recombine with each other in the organic layer 34 while releasing the recombination energy. The recombination energy can put organic luminous materials into the excited state. The organic layer 34 generates the light when the excited materials return to the ground state.
The transparent electrode 26 can be constructed from a conductive metal-oxide material, such as ITO (Indium Tin Oxide) or IZO (Indium Zinc Oxide). The counter electrode 32, made of Al or its alloy, can function as an upper reflection film. A lower reflection film 36, partly provided beneath the transparent electrode 26, can constitute a micro-resonator (i.e., microcavity) structure together with the upper reflection film (i.e., the counter electrode 32). More specifically, the lower reflection film 36 occupies a part of the entire surface of a light emitting region. The light generated from the organic layer 34 can advance straight in a downward direction in a region where the lower reflection film 36 is not provided.
According to the example shown in
Furthermore, the lower reflection film 36 can be constituted by multilayered dielectric films consisting of two types of dielectric members which are different from each other in refractivity and are alternately laminated. The optical film thickness of each dielectric film layer is, for example, ¼ of a target reflection wavelength or its integer multiple.
The organic layer 34, including a light emitting layer containing at least organic luminous molecules, can be configured into a single layer, double layers, triple layers, quadruple layers, or any other multilayered structure depending on the material (s).
According to the example shown in
The microcavity structure of the present embodiment is in an internal space sandwiched by the transparent electrode 26 and the counter electrode 32 which oppose each other via the organic layer 34, namely in an interlayer space between the lower reflection film 36 located beneath the transparent electrode 26 and the upper reflection film (i.e., the counter electrode 32). An optical length (optical distance) L of the microcavity structure can be expressed by the formula L=ρnidi - - - (1), i.e., by the sum of products of a refractivity ni and a thickness di of respective layers existing between the lower reflection film 36 and the counter electrode 32 (i.e., the upper reflection film). The relationship between the optical length L and a light emitting wavelength λ can be set as shown by a later-described formula (3), to emit the light having a selectively enhanced wavelength λ.
As described above, enhancing the light of a specific wavelength by the use of a microcavity structure must solve the problems of the viewing angle dependence. Hence, according to the present embodiment, each pixel has a region where the lower reflection film 36 is not present in the light emitting region. The light generated from the organic layer 34 can penetrate the region where the lower reflection film 36 is not present.
The light penetrating the region where the lower reflection film 36 does not exist has a relatively wide peak, because the light is not subjected to the microcavity effect of enhancing the light of a specific wavelength. In other words, the viewing angle dependence can be reduced. As a result, the present embodiment can bring about the effects of adequately enhancing the light of a specific wavelength emitted from a pixel while improving the viewing angle dependence.
In general, a full-color organic EL panel includes three types of pixels, i.e., R, G, and B light emitting pixels. The R, G, and B light emitting pixels have predetermined optical lengths Lr, Lg, and Lb corresponding to R, G, and B wavelengths λr, λg, and λb. According to this example, the lower reflection film 36 and the counter electrode 32 are made of metallic materials. The phase shift, occurring when the light reflects on these films, can be expressed by φ in the later-described formula (3).
The present embodiment is characterized in that an area ratio of a region where the lower reflection film 36 is present and another region where the lower reflection film 36 is not present is differentiated depending on the color of the light emitted from each pixel (i.e., depending on the difference of RGB colors of each pixel). More specifically, as schematically shown in
Accordingly, as described in the present embodiment, setting the area percentage of the lower reflection film 36 relative to the entire light emitting region to be smallest in the R light emitting pixel, medium in the G light emitting pixel, and largest in the B light emitting pixel can bring about substantially the same effects for each of the R, G, and B light emitting pixels in enhancing the light of a specific wavelength as well as in improving (i.e., reducing or eliminating) the viewing angle dependence.
In the example shown in
The characteristics of the microcavity structure can be determined according to the distance (i.e., cavity length) between the upper reflection film and the lower reflection film. Accordingly, in the present embodiment, the organic layer 34 having a partly changed thickness can provide two regions in the light emitting layer which are differentiated in cavity length. Thus, the wavelength of the light enhanced by the microcavity structure can be locally differentiated.
D=Σdi (2).
The MC2720 EL element (indicated by a line of -x-x-x-) has a resonant wavelength of 570 nm at the viewing angle 0° as a chromaticity shown at a lower right portion (x=0.39, y=0.58). The resonant wavelength shifts toward the short wavelength side when the viewing angle increases (namely, shifts in the direction advancing to the upper left in the drawing). Furthermore, the MC2720 EL element has a resonant wavelength of 520 nm at the viewing angle 60° as the green light having higher color purity at an upper left portion (x=0.21, y=0.71) in the drawing.
On the other hand, the MC3800 EL element (indicated by a line of -▴-▴-▴-) has a resonant wavelength of 510 nm at the viewing angle 0° (i.e., at the front side of the microcavity structure) as the green light having higher color purity at an upper left portion (x=0.19, y=0.69). The resonant wavelength shifts towards the short wavelength side when the viewing angle increases (namely, shifts in the direction advancing to the lower right), and reaches the lower right portion (y=0.37, y=0.55) at the viewing angle 60°.
The light emission spectrum obtained from the organic luminous materials used in the element has a peak at green and has smaller blue components. Even when the resonant wavelength shifts in accordance with increasing viewing angle, the luminous intensity of the element decreases because their resonant wavelength components are small in the original light emission spectrum. Thus, the semi-transmissive green light is obtained.
In the case of Reference EL element (indicated by a line of -□-□-□-), the shift range is approximately 0.30 to approximately 0.33 in the value of x and approximately 0.60 to approximately 0.63 in the value of y. Accordingly, the viewing angle dependence is relatively small.
In the case of MC2720+MC3800 EL element (i.e., the present embodiment), the viewing angle dependence becomes smaller compared with the cases using only one type of cavity condition. More specifically, the MC3800A EL element can realize high color purity at the viewing angle 0° and the MC2700A EL element can realize high color purity at the viewing angle 60°. Thus, the color purity can be maintained at a higher level at any viewing angle.
Accordingly, the element including plural regions mutually differentiated in the cavity length has the chromaticity shifting in the small range of x=approximately 0.27 to approximately 0.29 and y=approximately 0.63 to approximately 0.65, which is positioned at an upper left portion compared with the reference EL element. The color shift is small, and the color purity can be improved.
As described above, according to the arrangement of the present embodiment, one pixel can include two types of microcavity structures having the cavity lengths of 2720 A and 3800 A (i.e., mutually differentiated in the thickness by a difference of approximately 1100 A). The viewing angle dependence with respect to the green color light can be greatly improved (i.e., reduced).
The resonance conditions and the viewing angle dependence of the resonant wavelength can be expressed by the following formula.
In the formula, L represents an above-described optical length L (=Σnidi), φ is a constant representing a phase shift caused by the reflection on the metallic material, m is an integer, and θ represents an angle (i.e., viewing angle).
The abscissa of
As apparent from
In this manner, the waveform having a peak value at 600 nm at the viewing angle 0° decreases by 60 nm when shifting to the viewing angle of 60° and accordingly becomes a waveform having a peak value at 540 nm.
Accordingly, the interpolation for the green light of 550 nm can be performed based on the combination with the cavity length D of approximately 600 nm to approximately 700 nm (i.e., the cavity length D required to resonate the wavelength of approximately 600 nm to approximately 700 nm). The stepwise difference is approximately 1000 A (100 nm) to approximately 1300 A (130 nm), when m=1, from the above-described formula.
MC2720 and MC3800 have resonant wavelengths of 570 nm (m=1) and 510 nm (m=2), respectively, with respect to the light of 0°. Namely, according to the above-described example, the viewing angle dependence is small. To provide the microcavity structure for the green light, one pixel has two cavity lengths for obtaining the resonant peaks of 510 nm and 570 nm at the viewing angle of 0°.
Furthermore, the interpolation for the red light of 630 nm can be performed based on the combination with the cavity length D of approximately 700 nm to approximately 800 nm. The stepwise difference is approximately 1600 A, when m=1.
Furthermore, the interpolation for the blue light of 450 nm can be performed based on the combination with the cavity length D of approximately 480 nm to approximately 5800 nm. The stepwise difference is approximately 1000 A, when m=1.
Although the stepwise difference varies depending on a selected value of m, it is desirable to set the stepwise difference of 2000 A or less, considering the coverage (drape) of the stepwise difference. More specifically, one pixel can include a first region (e.g., the above-described example MC3800A) and a second region (e.g., the above-described example MC2700A) which are differentiated in cavity length. In this case, the cavity length D of the first region and the cavity length D of the second region should be determined considering their target resonant wavelengths, original light emission spectrum, or filter characteristics if a color filter is combined. A preferable difference of the cavity length (i.e., the stepwise difference) is 200 nm (2000 A) or less.
With the above-described settings, for example, even in the above-described case where the thickness of the transparent electrode is partly changed to form the first and second regions, it becomes possible to prevent the stepwise difference formed in the transparent electrode from cutting an upper layer of the electrode.
For example, when the thickness of the electron transporting layer is increased, the drive voltage will increase. To suppress the increase in the voltage, the thickness of the electron transporting layer is usually set to be 3000 A or less. For the purpose of preventing the electron transporting layer from being cut by the stepwise difference, a desirable stepwise difference is 200 nm or less.
As described above, setting plural cavity lengths D in one pixel region for obtaining a particular color can improve the viewing angle dependence and increase the color purity. In this case, the plural cavity lengths D can be set in the following manner.
For example, the cavity length D in the first region can be set to a predetermined thickness that can equalize the target resonant wavelength with a peak wavelength at the viewing angle 0°. The cavity length D (required for compensation) in the second region can be set to another thickness that can equalize the target resonant wavelength with a peak wavelength at another viewing angle (e.g., 60°). One of the selected cavity lengths D can be longer or shorter than the other.
Furthermore, according to the above-described embodiment, one pixel includes two types of regions differentiated in the cavity length. However, the differentiated regions are not limited to only two types and accordingly one pixel can include three or more types of differentiated regions.
In the present embodiment, when the organic layer constituting a compensating microcavity is thin, an area ratio of the thick region of the organic layer 34 to the thin region constituting the compensating microcavity can be differentiated for each color (RGB) of the light emitted from the pixel.
More specifically, as schematically shown in
The above settings are based on the fact that the viewing angle dependence becomes larger at long wavelengths, as confirmed by the characteristics of the microcavity structure that enhances a specific wavelength.
In this manner, setting the area percentage of the thick region of the organic layer 34 to be smallest in the R light emitting pixel, medium in the G light emitting pixel, and largest in the B light emitting pixel can bring about substantially the same effects for each of R, G, and B light emitting pixels in enhancing the light of a specific wavelength as well as in improving the viewing angle dependence. Furthermore, the thickness of the transparent electrode 26 can be changed instead of changing the thickness of the organic layer 34.
More specifically, a thick region of the lower reflection film 36 can be constituted by a larger number of dielectric film layers while a thin region of the lower reflection film 36 can be constituted by a smaller number of dielectric film layers. According to the example shown in
The characteristics of the microcavity structure can be determined according to the distance (i.e., cavity length) between the upper reflection film and the lower reflection film. When the thickness of the lower reflection film 36 is partly changed as described in the present embodiment, the microcavity effect of enhancing the light of a specific wavelength is large in the thick region.
Accordingly, in the present embodiment, one pixel includes a region where the microcavity effect of enhancing the light of a specific wavelength is large and another region where the microcavity effect of enhancing the light of a specific wavelength is small. Thus, the present embodiment can adequately enhance the light of a specific wavelength and improve the viewing angle dependence.
Similar to the above-described embodiment, in the present embodiment, the area ratio of the thick region and the thin region of the lower reflection film 36 can be differentiated depending on the color (RGB) of the light emitted from the pixel. As schematically shown in
According to the embodiment shown in
According to the embodiment shown in
According to the embodiment shown in
In the embodiments shown in
can be changed or adjusted for each of the R, G, and B light emitting pixels.
The differences in respective colors caused by the microcavity structure can be reduced, and the viewing angle dependence can be effectively improved. The area ratio can be set as shown in
While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all modifications, equivalent structures and functions.
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