The present application claims priority from Japanese application JP2014-117668 filed on Jun. 6, 2014, the content of which is hereby incorporated by reference into this application.
1. Field of the Invention
The present invention relates to an organic electroluminescent (EL) display device.
2. Description of the Related Art
In recent years, image display devices including a self-luminous element called an organic light-emitting diode, hereinafter simply “organic EL display devices”, have been in practical use. Such an organic EL display device with a self-luminous element is excellent in visibility and response speed compared with liquid crystal display devices known in the art. Additionally, the organic EL display device eliminates the need for an auxiliary lighting device, such as a backlight, and thus can be made thinner.
JP 2006-049906 A discloses an organic light-emitting device that includes a cathode, an electron transport layer, an electroluminescent layer, a hole transport layer, an electron-accepting layer, and an anode in this order, and further includes an anode-capping layer between the electron-accepting layer and the anode.
For the organic EL display device, O2 plasma treatment is applied to its anode electrode, which is formed of indium tin oxide (ITO) or the like in an organic EL element in each pixel, to remove organic matter stacked on the anode electrode and lower the drive voltage of the organic EL element. The O2 plasma treatment, however, may cause decomposition of the materials for the circuit substrate depending on conditions and thus affect the ionization potential of the anode electrodes to result in unevenness in the hole-injecting properties of the anode electrodes. Other factors than this may also cause unevenness in the hole-injecting properties of the anode electrodes. The unevenness in the hole-injecting properties may reduce the device efficiency and increase the drive voltage, thus resulting in a shorter device life.
In view of the above circumstances, it is an object of the present invention to provide an organic EL display device that has a higher device efficiency and a longer device life even when there is unevenness in the hole-injecting properties of its anode electrodes.
An organic EL display device according to an aspect of the present invention includes an anode electrode made of a conductive material, a cathode electrode made of a conductive material, an anode-side electron injection layer that is an electron injection layer on the anode electrode and between the anode electrode and the cathode electrode, and an anode-side charge generation layer that is a charge generation layer on the anode-side electron injection layer.
The organic EL display device according to the aspect may further include an anode-side hole injection layer that is a hole injection layer on the anode-side charge generation layer, an anode-side hole transport layer that is a hole transport layer on the anode-side hole injection layer, a light-emitting portion that is on the anode-side hole transport layer and includes at least one light-emitting layer made of an organic light-emitting material, a cathode-side electron transport layer that is an electron transport layer on the light-emitting portion, and a cathode-side electron injection layer that is an electron injection layer between the cathode-side electron transport layer and the cathode electrode.
In the organic EL display device according to the aspect, the light-emitting portion may include a cathode-side light-emitting layer made of an organic light-emitting material, an anode-side light-emitting layer that is made of an organic light-emitting material and is closer to the anode electrode than the cathode-side light-emitting layer, a tandem electron transport layer that is an electron transport layer on the anode-side light-emitting layer, a tandem electron injection layer that is an electron injection layer on the tandem electron transport layer, a tandem charge generation layer that is a charge generation layer on the tandem electron injection layer, a tandem hole injection layer that is a hole injection layer on the tandem charge generation layer, and a tandem hole transport layer that is a hole transport layer between the tandem hole injection layer and the cathode-side light-emitting layer.
Embodiments of the present invention are described below with reference to the accompanying drawings. The disclosure herein is merely an example, and appropriate modifications coming within the spirit of the present invention, which are easily conceived by those skilled in the art, are intended to be included within the scope of the invention as a matter of course. The accompanying drawings schematically illustrate widths, thicknesses, shapes, or other characteristics of each part for clarity of illustration, compared to actual configurations. However, such a schematic illustration is merely an example and not intended to limit the present invention. In the present specification and drawings, some elements identical or similar to those shown previously are denoted by the same reference signs as the previously shown elements, and thus repetitive detailed descriptions of them may be omitted as appropriate.
The electron injection layer is preferably a layer formed from a mixture of a high-mobility material, such as biphasic calcium phosphate (BCP), Tris-(8-hydroxyquinoline) aluminum (Alq3), an oxadiazole (polybutadiene:PBD) based material, or a triazole-based material, and an alkali metal, such as Li, Mg, Ca, or Cs. The charge generation layer is preferably formed of an electron acceptor material, such as HAT-CN(6) (1,4,5,8,9,12-hexaazatriphenylene-hexacarbonitrile). The hole injection layer can be formed of, for example, any of HAT-CN(6), CuPc, and PEDOT:PSS. The hole transport layer can be formed of, for example, N,N′-Di-[(1-naphthyl)-N,N′-diphenyl]-1,1′-biphenyl)-4,4′-diamine (NPB).
The electron transport layer can be formed by co-deposition of Alq3 and 8-hydroxy-quinolinato-lithium (Liq). Here, for example, Li may be substituted for Liq. The materials used for each of the above layers are not limited to the materials shown here, and any material that those skilled in the art use for each layer may be used.
In this embodiment, the anode-side electron injection layer 311 is formed between the anode electrode 285 and the anode-side charge generation layer 312. This structure makes hole injection from the anode electrode 285 to the anode-side electron injection layer 311 less likely to occur. In addition, the anode-side charge generation layer 312 generates holes for light emission and provides them to the anode-side hole transport layer 314. Thus, the organic EL element 340 can be driven unaffected by the hole-injecting properties of the anode electrode 285. Electrons generated in the anode-side charge generation layer 312 move to the anode electrode 285 through the anode-side electron injection layer 311. Thus, the amount of holes can be controlled regardless of the surface treatment condition of the anode electrode 285, and the device efficiency and the life of the organic EL element 340 can be increased. The materials for the anode-side electron injection layer 311 and the cathode-side electron injection layer 332 may be the same or different. The structure, above the anode-side electron injection layer 311, from the anode-side hole injection layer 313 to the cathode-side electron injection layer 332 is not particularly limited, and may be stacked differently only if the anode-side electron injection layer 311, which makes the hole injection less likely to occur, is formed on the anode electrode 285 and a charge generation layer, which generates electric charges, is formed on the anode-side electron injection layer 311.
Such a structure enables the material for the anode electrode 285 to be a metal that has low hole-injecting properties, that is, a low work function. Whereas the work function of the ITO commonly used for the anode electrode 285 is about 4.26 eV, it is modified to be about 5.0 to 5.5 eV for practical use, for example, by O2 plasma treatment. However, if the hole-injecting properties of the anode electrode 285 can be lowered, a low work function metal, such as Al with a work function of 4.28 eV or Ag with a work function of 4.26 eV, can be used. Use of such a metal has a cost advantage over use of the ITO and can also improve the flatness of the anode electrode 285 to reduce leakage. Moreover, its non-transparency can eliminate the need for a fine adjustment of the thickness of the anode electrode 285 when optical interference is used.
The light-emitting portion 320 includes a tandem electron transport layer 323, which is an electron transport layer formed on the anode-side light-emitting layer 322, a tandem electron injection layer 324, which is an electron injection layer formed on the tandem electron transport layer 323, a tandem charge generation layer 325, which is a charge generation layer formed on the tandem electron injection layer 324, a tandem hole injection layer 326, which is a hole injection layer formed on the tandem charge generation layer 325, and a tandem hole transport layer 327, which is a hole transport layer formed between the tandem hole injection layer 326 and the cathode-side light-emitting layer 328, stacked in this order.
Even the organic layer 400, which includes the light-emitting portion 320 having such a tandem structure, can produce the same effects as the above embodiment because the organic layer 400 includes the anode-side electron injection layer 311 between the anode electrode 285 and the anode-side charge generation layer 312. The tandem stack structure between the two light-emitting layers is not limited to this structure, and may have another stack structure. Whereas the tandem structure includes two light-emitting layers, it may include three or more light-emitting layers.
While there have been described what are at present considered to be certain embodiments of the invention, it will be understood that various modifications may be made thereto, and it is intended that the appended claims cover all such modifications as fall within the true spirit and scope of the invention.
Number | Date | Country | Kind |
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2014-117668 | Jun 2014 | JP | national |