JP2007141790A - Display device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To improve angle of field dependency in a display device using an organic EL layer, especially, with respect to control of display color. <P>SOLUTION: The thickness of a flat layer 34 located on a wiring layer 32 is made 1.5 μm or more. Thereby, interference by reflection by the wiring layer 32 of the light emitted from the organic EL element 40 is prevented, and angle of field dependency can be improved. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a display device using an organic EL layer, and in particular, to display color control.

  Conventionally, a display device using an organic EL element is known. In this organic EL element, a current flows through an organic EL layer between electrodes, and light emission occurs according to the flowing current.

  As light emitting materials for organic EL, those emitting red light, blue light, and green light are known. Therefore, full-color display can be performed by separately painting RGB.

  Here, in the case of the RGB separate coloring method, since the materials of the organic EL layers of the respective colors are different, usually a vapor deposition process for each of the RGB colors is necessary, and for this purpose, separate masks are used. As the number of formation steps increases, the yield tends to deteriorate. Therefore, it has also been proposed to form a white light emitting layer common to all pixels and form each pixel of RGB by a color filter (Patent Document 1). With this configuration, it is possible to simplify the formation of a relatively difficult organic EL layer and improve the yield.

JP 2004-127602 A

  Here, in any of RGB display, white light emission + color filter type display devices, various layers exist in the light emission path from the organic EL element. In an active matrix display device in which a current to each organic EL element is controlled by a thin film transistor (TFT) provided for each pixel, light emitted from the organic EL element is emitted to the outside by a layer forming the TFT. Exists in the route to. Then, various reflections occur in the TFT layer, which interferes with light from the organic EL element, resulting in a problem that viewing angle dependency is increased. Therefore, there is a problem that it is desired to solve such a problem.

  The present invention is a display device including display pixels arranged in a matrix, and includes a TFT layer including a thin film transistor, a planarizing layer formed on the TFT layer, and an organic layer formed on the planarizing film. And the thickness of the planarizing layer is sufficiently thick so that the influence of interference of reflected light in the TFT layer with light emission in the organic EL layer can be substantially reduced.

  Further, it is preferable that the organic EL layer is a white light emitting layer, and that the flattening film includes an RGB color filter layer to enable RGB display.

  The thickness of the planarizing layer is preferably 1.5 μm or more.

  Further, it is preferable that the white color temperature adjusted at the front is set so that Δuv moves within the range of ± 0.02 from the front by tilting the viewing angle from the front.

  In addition, it is preferable that a light emission path (an optical path length from the anode to the cathode) is set so that blue light is enhanced by interference with respect to the light emitted from the organic EL layer.

  Thus, according to the present invention, the planarizing film is made sufficiently thick to prevent the enhancement of visible light having a specific wavelength due to interference of the TFT layer. Thereby, the viewing angle dependency in the display can be suppressed. In addition, by setting the color temperature to move in a lower direction as the viewing angle is tilted from the front, the change in color sensed by human eyes is reduced, and the viewing angle dependency is improved.

  Hereinafter, light-emitting elements according to embodiments of the present invention will be described with reference to the drawings.

  FIG. 1 is a schematic diagram illustrating a cross-sectional configuration of a light-emitting element. In the drawing, only one light emitting element is taken out and described, but a light emitting element and pixel circuits for driving the light emitting element are arranged in a matrix to constitute a display device. A layer that can be formed in common for all pixels, such as a glass substrate, a light emitting layer, and a cathode, is formed in common for all pixels.

  A TFT (thin film transistor) / wiring layer 32 including a pixel circuit and various wirings is formed on the glass substrate 30. The pixel circuit is, for example, a switching TFT that controls the taking in of the data signal from the data line in accordance with a control signal from the gate line, a storage capacitor that stores the data voltage captured by the switching TFT, and a data voltage that is stored in the storage capacitor. Drive TFTs for supplying the drive current from the power supply line to the EL element. There are many proposals for the pixel circuit, and various modifications such as including a threshold compensation circuit for the drive TFT are possible.

  On the TFT / wiring layer 32, a planarizing layer 34 made of an acrylic resin or the like is formed. An organic EL element 40 is formed on the planarization layer 34. The organic EL element 40 includes an anode 10, a red light emitting layer 16, a blue light emitting layer 18, and a cathode 24. In addition, as the red color developing layer 16, a layer that emits orange light is also suitable.

  Here, a specific configuration example of the organic EL element 40 will be described with reference to FIG. On the anode 10 made of a transparent conductor, a hole transport layer 14 is provided via a hole injection layer 12. In this example, IZO (Indium Zinc Oxide) is used for the anode 10, but ITO (Indium Tin Oxide) or the like is also used. In this example, CFx is used for the hole injection layer 12 and NPB (N, N′-di (naphthalen-1-yl) -N, N′-diphenyl-benzidine) is used as the host for the hole transport layer 14. Is used.

  On the hole transport layer 14, an orange light emitting layer 16 and a blue light emitting layer 18 are sequentially formed. The orange light emitting layer 16 uses NPB which is a triarylamine derivative or a triphenylamine derivative as a host, tertiary-butyl substituted dinaphthylanthracene (TBADN) as a dopant 1, and 5,12-bis ( 4- (6-Methylbenzothiazol-2-yl) phenyl) -6,11-diphenylnaphthacene (DBzR) has been used. The blue light-emitting layer 18 uses tertiary-butyl-substituted dinaphthylanthracene (TBADN) as a host, NPB as dopant 1, and 1,4,7,10-tetra-tert-butylperylene (TBP) as dopant 2. ing.

  A first electron transport layer 20 and a second electron transport layer 22 are provided on the blue light emitting layer 18, and a cathode 24 is provided thereon.

  The first electron transport layer 20 is made of tris (8-hydroxyquinolinato) aluminum (Alq), and the second electron transport layer 22 is made of a phenanthroline derivative. The cathode 24 is made of aluminum (Al) provided with LiF on the surface.

  As described above, the organic EL element 40 of the present embodiment includes the orange light emitting layer 16 and the blue light emitting layer 18 between the electrodes of the anode 10 and the cathode 24, and light emission occurs in both the light emitting layers 16 and 18. As a result, white light is emitted. Accordingly, in the vicinity of the interface between the light emitting layers 16 and 18, recombination occurs with holes supplied from the anode 10 and electrons supplied from the cathode 24, and light emission occurs in both the light emitting layers 16 and 18. Injected from the glass substrate 30. In practice, an RGB filter is provided for each pixel in order to perform full-color display. In the case of the WRGB type, a pixel that emits white light without a color filter is also provided.

  Here, in the present embodiment, the two light emitting layers 16 and 18 emit light. For this purpose, light emission occurs near the interface between the two light-emitting layers 16 and 18, and the boundary between the light-emitting layers 16 and 18 becomes the light-emitting interface. This is an indispensable condition for causing light emission in both the light emitting layers 16 and 18. The light emitted in the vicinity of the interface may be emitted as it is, or may be reflected by the cathode 24. That is, the cathode 24 is made of aluminum, and light emitted from the light emitting layers 16 and 18 cannot be transmitted therethrough but is reflected.

  Accordingly, the light emitted from the organic EL element 40 is a combination of the light directly coming from the interface between the light emitting layers 16 and 18 and the light reflected by the cathode 24, and interference occurs between them.

  In the present embodiment, by reducing the distance from the interface to the surface (reflection surface) of the cathode 24, the directly emitted light interferes with the light reflected by the reflective layer, thereby preventing the visible light from decreasing. .

  That is, the optical distance between the interface between the orange light emitting layer 16 and the blue light emitting layer 18 in the organic EL element 40 and the surface of the cathode 24 is set to 100 nm or less. Thereby, a decrease in visible light of 400 nm to 800 nm can be suppressed. The optical distance between the interface between the orange light-emitting layer 16 and the blue light-emitting layer 18 and the surface of the cathode 24 can be substantially reduced as long as the attenuation of visible light, which is a problem in the display visually recognized by the observer, can be eliminated. The optical length may be ¼ or less of the minimum wavelength of visible light. Furthermore, an optical length slightly longer than an optical length that is ¼ of the minimum wavelength of visible light may be an optical length in which attenuation due to interference occurs at a blue wavelength in a region close to ultraviolet light.

  In addition, the refractive index of an organic layer is about 1.6-1.9, and it is good to determine the thickness of each layer according to an actual refractive index.

  Further, there is a blue light emitting layer 18 or the like between this interface and the cathode 24, and it is preferable to set these distances to about 50 to 60 nm.

  In the present embodiment, the planarization layer 34 is thickened. For example, the thickness of the planarizing layer 34 is 1 μm or more, particularly 1.5 μm. As described above, when the thickness of the planarizing layer 34 is increased, various paths are secured for light passing in an oblique direction, and a sharp interference peak is hardly generated. Therefore, by increasing the thickness of the flattening layer 34, it is possible to reduce the influence of reflection on the underlying TFT / wiring layer 32, and to suppress the occurrence of a peak of visible light having a specific wavelength due to interference here. Thereby, the change in the color due to the change in the viewing angle can be reduced. That is, when a specific wavelength is strengthened due to interference, the specific wavelength varies depending on the optical path length, and thus has a large viewing angle dependency. Therefore, by making the planarizing film sufficiently thick to reduce the influence of interference, the viewing angle dependency of the display can be reduced.

  When color filters are arranged either above or below the flattening film (normally, the color filter is covered with the flattening film), the total thickness of the color filter and the flattening film is 1.5 μm as described above. It is good to do. For example, in the case of an RGBW display device having a white light-emitting layer, a color filter is provided in the RGB pixel and not in the W pixel. The color filter is usually formed as a lower layer of the planarizing film.

  The thickness of the planarization film is 1.5 μm or more. For the purpose of improving the viewing angle dependency, the thicker the planarization film, the better. On the other hand, if the planarizing film is made thicker, the material cost is increased, and the attenuation of light here is also increased. Therefore, the thinner one is desirable, and the thickness is preferably 5 μm or less, more preferably 3 μm or less.

  Moreover, the refractive index of ITO and IZO which comprise the anode 10 is about 1.8-2.1. On the other hand, the planarization layer 34 formed under the anode 10 is usually formed of an acrylic resin or the like as described above, and its refractive index is about 1.5 to 1.6. The difference in refractive index is relatively large, and reflection tends to occur at this interface. Therefore, the light reflected at the interface between the anode 10 and the planarization layer 34 is reflected by the cathode 24 and interferes with the light emitted directly from the interface.

  In the present embodiment, the distance (optical length) from the interface between the anode 10 and the planarization layer 34 to the surface of the cathode 24 is set so that blue light is enhanced by the interference at this time. That is, the optical length from the interface where the reflection occurs to the cathode 24 serving as the reflective layer is set so that the peak of the interference waveform exists at the blue wavelength.

For example, the thickness is set as follows.
(A) In the case of the WRGB system, the distance from the lower surface of the anode to the lower surface of the cathode is set to 50 to 600 nm. (1) Anode (IZO) 160 nm, organic layer 210 nm, 200 nm, (3) anode (IZO) 20 nm, organic layer 70 nm, and so on. Thereby, the peak of interference is set to blue, and the change in the viewing angle of blue is increased.
(B) In the case of the RGB (white light emitting layer + color filter) method, the distance from the lower surface of the anode to the lower surface of the cathode is set to 50 to 600 nm. (1) Anode (IZO) 160 nm, Organic layer 230 nm, (2) Anode (IZO) 30 nm, organic layer 210 nm, (3) Anode (IZO) 20 nm, organic layer 80 nm. As a result, the peak of interference is set to blue, and blue has the largest change in viewing angle.
(C) In the case of the RGB (RGB light emission) method, the distance from the lower surface of the anode to the lower surface of the cathode is set as follows in each color pixel of RGB.
(R) 120, 260, 400 nm, (G) 130, 270, 410 nm, (B) 110, 250, 390 nm. As a result, the blue pixel has a blue interference peak, but the red and green interference peaks do not match.

  Thus, in this embodiment, blue light is intensified by interference. Here, since the interference is affected by the optical path length, the viewing angle dependency is large. Therefore, when blue is strengthened by interference as described above, blue is relatively weakened by the viewing angle. That is, as shown in FIG. 3, blue is most reduced depending on the viewing angle. FIG. 3 shows characteristics when a color filter is provided for the white light emitting layer of (B) described above and pixels of each color of RGB are formed.

  Then, as the viewing angle is shifted from the front in an oblique direction, the blue color is weakened, so that the change in Δuv on the color temperature coordinate becomes very small as shown in FIG. 4 indicates the color temperature when viewed from the front, and the black triangle indicates the chromaticity at a viewing angle of 70 °. Thus, according to the present embodiment, when the viewing angle changes, the chromaticity changes approximately on Δuv = 0. Thereby, the color change of white becomes small, and the color tone change by a viewing angle is relieved. Also, by setting Δuv within ± 0.02, it becomes difficult for humans to recognize color changes due to viewing angles, and the viewing angle dependency of display can be reduced.

  In the above example, the anode and the organic layer are optical path lengths to be interfered with, but depending on the arrangement of the color filters, the color filters are also included in the optical path lengths. Further, when there is no planarization layer, the TFT layer is also included in the optical path length.

  As described above, according to the present embodiment, the emission light from the front surface is set to an interference condition that enhances the blue color. As a result, when viewed from an oblique direction, blue is weakened by interference, and the color temperature changes in a direction in which the color temperature decreases, so that a change in color with respect to a change in viewing angle can be reduced.

  FIG. 5 shows a configuration of an organic EL element 40 according to another embodiment. In this example, a single white light emitting layer 50 is employed in place of the red light emitting layer 16 and the blue light emitting layer 18.

  The white light emitting layer 50 includes, for example, tertiary-butyl-substituted dinaphthylanthracene (TBADN) as a host, 1,4,7,10-tetra-tert-butylperylene (TBP) as a blue dopant, and a red dopant. 5,12-bis (4- (6-methylbenzothiazol-2-yl) phenyl) -6,11-hr phenylnaphthacene DBzR) is used. When such a white light emitting layer 50 is used, it becomes a light emitting interface in which light emission occurs at the interface between the white light emitting layer 50 and the hole transport layer 14.

It is a schematic diagram which shows the cross-sectional structure of a light emitting element. It is a schematic diagram which shows the cross-sectional structure of an organic EL element part. It is a figure which shows the relationship between a viewing angle and the brightness | luminance of RGB confirmation. It is a figure which shows the color temperature change by a viewing angle change. It is a schematic diagram which shows the cross-sectional structure of the other structural example of an organic EL element part.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 10 Anode, 12 Hole injection layer, 14 Hole transport layer, 16 Red light emitting layer, 18 Blue light emitting layer, 20 1st electron transport layer, 22 2nd electron transport layer, 24 Cathode, 30 Glass substrate, 32 Wiring layer, 34 planarization layer, 40 organic EL element, 50 white light emitting layer.

Claims (5)

A display device including display pixels arranged in a matrix,
A TFT layer including a thin film transistor;
A planarization layer formed on the TFT layer;
An organic EL layer formed on the planarizing film;
Including
A display device characterized in that the thickness of the planarizing layer is made sufficiently thick to substantially reduce the influence of interference of reflected light in the TFT layer with light emission in the organic EL layer. The display device according to claim 1,
The organic EL layer is a white light emitting layer,
A display device, wherein the flattening film includes an RGB color filter layer to enable RGB display. The display device according to claim 1 or 2,
The flattening layer has a thickness of 1.5 μm or more. The display device according to any one of claims 1 to 3,
A display device, wherein a white color temperature adjusted in front is set to move within a range where Δuv is within ± 0.02 from the front when the viewing angle is tilted from the front. The display device according to claim 4,
A light emitting path is set for light emitted from the organic EL layer so that blue light is enhanced by interference.

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