Abstract:
The present invention provides a top-emission-type organic EL display device, that is, an organic EL display device which can suppress changes of a threshold voltage and a light emitting start voltage, and the generation of brightness irregularities. The organic EL display device includes lower electrodes arranged on a main surface of an element substrate, a multi-layered organic EL layer arranged on the lower electrodes, and a light transmitting upper electrode arranged on the organic EL layer. A layer in contact with the lower electrode of the organic EL layer forms a hole injection layer constituted of a V 2 O 5  layer.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The disclosure of Japanese Patent Application No. 2007-049658 filed on 2007 Feb. 28 (yyyy/mm/dd) including the claims, the specification, the drawings and the abstract is incorporated herein by reference in its entirety. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to an organic EL display device, and more particularly to an organic EL display device which includes top-emission-type organic EL light-emitting elements. 
         [0004]    2. Description of Related Art 
         [0005]    The organic EL display device is classified into a so-called bottom-emission-type organic EL display device and a so-called top-emission-type organic EL display device. In the bottom-emission-type organic EL display device, on a main surface of an insulation substrate preferably formed of a glass substrate which constitutes a TFT substrate, organic EL elements are formed, and each organic EL element is constituted of a light emitting mechanism formed by sequentially stacking a transparent electrode (made of ITO or the like) which constitutes a first electrode or one electrode, a multi-layered organic film which emits light with applying of an electric field thereto (also referred to as an organic light emitting layer), and a metal electrode having reflection property which constitutes a second electrode or another electrode. A large number of organic EL elements are arranged on the insulation substrate in a matrix array. Another substrate or a sealing film referred to as a sealing can is provided to cover the stacked structure for shielding the above-mentioned light emitting structure from an external atmosphere. Further, for example, using the transparent electrode constituting one electrode as an anode and a metal electrode constituting another electrode as a cathode electrode, an electric field is applied between both electrodes so as to inject carriers (electrons and holes) into the organic multi-layered film thus allowing the organic multi-layered film to emit light. The emitted light is radiated to the outside from a glass substrate side. 
         [0006]    On the other hand, the top-emission-type organic EL display device is configured such that the above-mentioned one electrode is formed of a metal electrode having reflection property and the above-mentioned another electrode is formed of a transparent electrode made of ITO or the like, an electric field is applied between both electrodes to allow the light emitting layer to emit light, and the emitted light is radiated from another electrode side. In the top-emission-type organic EL display device, an area above a drive circuit formed on the insulation substrate can be also used as a light emitting area. Further, the top-emission-type organic EL display device can use a transparent plate preferably formed of a glass plate as a member corresponding to the sealing can of the bottom-emission-type organic EL display device. 
         [0007]    As exemplified in  FIG. 5 , this type of organic EL display device is configured to seal a sealing substrate  81  and an element substrate  82  using a sealing member  83 . Here,  FIG. 5  schematically shows a cross section of one example of the organic EL display device as viewed in the direction parallel to the light radiation direction. 
         [0008]    In the constitution of the organic EL display device shown in  FIG. 5 , a trench  81   a  is formed in an inner surface of the sealing substrate  81  facing the element substrate  82  in an opposed manner, and a desiccant assembly  84  is fixed in the inside of the trench  81   a . The desiccant assembly  84  is, for example, formed of a desiccant  86  made of CaO (calcium oxide), Sr (strontium) or the like and a bonding material  87  such as an adhesive agent, for example, and the desiccant assembly  84  is fixedly mounted on the sealing substrate  81  using the bonding material  87 . The desiccant assembly  84  and the bonding material  87  are transparent. 
         [0009]    On the other hand, on a main surface of the element substrate  82 , that is, on a surface of the element substrate  82  facing the sealing substrate  81  and forming TFT elements and the like not shown in the drawing thereon, a light emitting element portion  85  is arranged. The light emitting element portion  85  is constituted by sequentially stacking a lower electrode  88  formed of a metal film having reflection property, an organic multi-layered film  89  having a light emitting layer and a transparent upper electrode  90  in this order from an element substrate  82  side. 
         [0010]    In such a constitution, the desiccant assembly  84  is assembled in the element substrate  82  for preventing lowering of performance of the organic multi-layered film  89  attributed to the absorption of water. 
         [0011]    With respect to this kind of organic EL display device, patent document 1 (JP-A-2005-32618) discloses a top-emission-type organic EL light emitting element which arranges a hole injection layer being in contact with a transparent upper electrode and including inorganic materials formed of a transition-metal oxide such as a vanadium oxide. Further, patent document 2 (JP-A-9-63771) discloses a bottom-emission-type organic EL light emitting element which uses a lower electrode made of an ITO as an anode and an electrode having reflection property as an upper electrode. 
       SUMMARY OF THE INVENTION 
       [0012]    In such a top-emission-type organic EL display device, there has been proposed the constitution which uses the upper electrode as a cathode and the lower electrode as an anode, Al (aluminum) having a high reflection coefficient as the lower electrode, and stacks an ITO film or an IZO film having a high work function on the Al film. 
         [0013]    With such a constitution, however, the restriction is imposed on the separation of pixels due to low insulating property of the ITO film or the IZO film. To cope with such restriction, an organic EL layer or an upper electrode having the multi-layered stacked structure has been provided. However, during manufacturing steps of the organic EL layer or the upper electrode, foreign materials are absorbed in films including the above-mentioned ITO film or IZO film or these films are contaminated with the foreign materials thus giving rise to the fluctuation of a threshold voltage or a light emitting start voltage, the generation of brightness irregularities or the like attributed to these absorption of the foreign materials in the films or the contamination of films by the foreign materials. Accordingly, there has been a demand for ideas which can cope with such drawbacks. 
         [0014]    It is an object of the present invention to provide an organic EL display device which can overcome the above-mentioned drawbacks and can acquire a stable threshold voltage and a stable light emitting start voltage for a long period, and has an excellent light emitting property without generating brightness irregularities. 
         [0015]    To achieve the above-mentioned object, the present invention is directed to a top-emission-type organic EL display device configured as follows. A lower electrode is made of Al or Al alloy, a hole injection layer formed of a V 2 O 5  layer is stacked on the lower electrode, a multi-layered organic EL layer such as a hole transport layer is arranged on the hole injection layer formed of the V 2 O 5  layer, and a light-transmitting upper electrode constituting a cathode electrode is further stacked on the organic EL layer. 
         [0016]    By stacking the hole injection layer formed of the V 2 O 5  layer on the lower electrode in a state that the hole injection layer is in contact with the lower electrode made of Al or Al alloy and constituting the anode, the present invention can obtain following advantageous effects. 
         [0017]    (1) All of the V 2 O 5  layer, the organic layer, the electron injection layer and the upper electrode can be formed in vacuum consecutively and hence, bonding portions of the respective layers can be held in a clean state, and the number of interface ions or the like which move due to the application of voltage is small and hence, a change of a threshold value is also small. 
         [0018]    (2) A threshold voltage and a light emitting start voltage can be held stable for a long period thus providing an organic EL display device exhibiting excellent light emitting property and having a prolonged lifetime. 
         [0019]    (3) The generation of brightness irregularities can be suppressed effectively. 
         [0020]    (4) The lowering of light reflection property of the lower electrode can be suppressed effectively. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0021]      FIG. 1  is a schematic cross-sectional view for explaining the schematic structure of one embodiment of an organic EL display device according to the present invention; 
           [0022]      FIG. 2  is a schematic cross-sectional view of a light emitting element side of the organic EL display device in  FIG. 1 ; 
           [0023]      FIG. 3  is a schematic enlarged cross-sectional view of an organic EL layer; 
           [0024]      FIG. 4  is a schematic cross-sectional view for explaining another embodiment of an organic EL display device according to the present invention; and 
           [0025]      FIG. 5  is a schematic cross-sectional view for explaining schematic structure of a conventional organic EL display device. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0026]    Hereinafter, preferred embodiments of the present invention are explained in detail in conjunction with drawings showing these embodiments. 
       Embodiment 1 
       [0027]      FIG. 1  to  FIG. 3  are schematic views for explaining the schematic structure of one embodiment of an organic EL display device according to the present invention.  FIG. 1  is a cross-sectional view of the organic EL display device as viewed in the direction parallel to the light radiation direction,  FIG. 2  is a cross-sectional view of an element substrate shown in  FIG. 1 , and  FIG. 3  is an enlarged cross-sectional view of an organic EL layer. In  FIG. 1  to  FIG. 3 , numeral  1  indicates a sealing substrate, numeral  2  indicates an element substrate, numeral  3  indicates a sealing member, numeral  4  indicates a desiccant, numeral  5  indicates a light emitting element portion, numeral  51  indicates an organic EL layer, numeral  52  indicates lower electrodes having reflection property, numeral  53  indicates an upper electrode having light transmitting property, numeral  54  indicates projecting banks, numeral  6  indicates a V 2 O 5  layer, and numeral  7  indicates a sealing space. 
         [0028]    The sealing substrate  1  is, for example, formed of a glass substrate having light transmitting property. To be more specific, the sealing substrate  1  is bonded to the element substrate  2  described later by way of the sealing material  3  to define a region surrounded by both substrates  1 ,  2  and the sealing material  3 , that is, a sealing space  7 . The sealing substrate  1  is configured to hold the transparent desiccant  4  on an inner surface  1   a  thereof and to absorb moisture in the inside of the sealing space  7 . Further, the element substrate  2  bonded to the sealing substrate  1  forms the light emitting element portion  5  on a portion thereof facing the sealing substrate  1  in an opposed manner. 
         [0029]      FIG. 2  shows one example of the organic EL display device in detail. The element substrate  2  is a substrate which forms a silicon nitride SiN film  21  and a silicon oxide SiO 2  film  22  on a main surface thereof and is preferably formed of a transparent glass. The element substrate  2  constitutes a TFT substrate. Semiconductor films  23  are formed in switching element regions arranged on the silicon oxide SiO 2  film  22  by patterning. A gate insulation film  24  is formed on the semiconductor films  23  so as to cover the semiconductor films  23 . Gates  25  are formed on the gate insulation film  24  by patterning. A leveling film  26  having insulating property is formed on the gates  25  so as to cover the gates  25 . Lines  27  are constituted of various lines between switching elements (lines between switches, signal lines, drain lines) constituting drain electrodes of the switching elements. Lines  28  are shield members which are also used as lines between the switching elements (shield members also being used as the lines between the switches) constituting source electrodes. The lines  27  and the lines  28  are connected to the semiconductor films  23  via contact holes which run through the leveling film  26  and the gate insulation film  24 . An insulation film  29  is formed to cover the lines  27  between the switches and the shield members  28  which are also used as the lines between the switches. Numeral  30  indicates the TFT substrate. 
         [0030]    The lower electrodes  52 , the V 2 O 5  layer  6 , the multi-layered organic EL film  51  including the V 2 O 5  layer  6 , the upper electrode  53 , and the projecting banks  54  for separating pixels are respectively arranged on the TFT substrate  30 . 
         [0031]    First of all, the planar lower electrode  52  made of Al or Al alloy which constitutes a pixel electrode has one end  52   a  thereof connected to the shield member  28  which is also used as the line between the switches via the contact hole formed in the insulation film  29  and has another end  52   b  thereof extended to and arranged on a neighboring TFT element (not shown in the drawing) side. The lower electrode  52  constitutes a portion of the light emitting element portion  5  and functions as an anode. 
         [0032]    The projecting bank  54  is stacked to cover a portion of the lower electrode  52 . The bank  54  is, for example, made of an inorganic insulation material such as silicon oxide or silicon nitride and is arranged to cover distal end portions of one end  52   a  and another end  52   b  except for a center portion  52   c  of the lower electrode  52 . A light emitting portion defined by the banks  54  and corresponding to the center portion  52   c  of the lower electrode  52  forms the light emitting areas  8 . The light emitting areas  8  are separated from each other by the banks  54 . 
         [0033]    On the other hand, the V 2 O 5  layer  6  is arranged to cover the center portion  52   c  of the lower electrode  52  defined by the banks  54  and having a surface thereof exposed. The V 2 O 5  layer  6  is arranged in common on neighboring pixel units (not shown in the drawing) after getting over the banks  54 . 
         [0034]    The V 2 O 5  layer  6  can be formed by vapor deposition and has a thickness of 1 nm to 30 nm in practical use. The thickness is more preferably set to a value which falls within a range from 5 nm to 10 nm. When the thickness of the V 2 O 5  layer  6  is less than nm, there exists a possibility that the lower electrode does not function as the anode, while when the thickness of the V 2 O 5  layer  6  exceeds 30 nm, there exists a possibility that the reflection property and the conductivity of the V 2 O 5  layer  6  are lowered. 
         [0035]    Further, in the constitution which applies the V 2 O 5  layer  6  to the lower electrode  52  made of Al or Al alloy, a threshold voltage tends to become slightly high by an amount corresponding to difference in work function between A 1  and V 2 O 5 . However, a change of the threshold voltage with time is small and hence, the constitution eventually facilitates a control of the organic EL display device. 
         [0036]    In this embodiment, the organic EL layer  51  which covers the V 2 O 5  layer  6  and forms the hole injection layer using the V 2 O 5  layer  6 , and the light-transmitting upper electrode  53  formed of an IZO film and constituting the common electrode are stacked to each other. The upper electrode  53  functions as a cathode. 
         [0037]    Here, the formation of the V 2 O 5  layer  6 , the organic EL layer  51  and the upper electrode  53  can be performed in vacuum consecutively without exposing these layers to atmosphere. 
         [0038]    Due to the consecutive formation of these layers, not to mention the avoidance of adhesion of foreign materials, since the interface is not contaminated, the elevation of a light-emitting start voltage can be obviated thus contributing to the prolongation of lifetime. 
         [0039]    One example of the organic EL layer  51  which adopts V 2 O 5  layer  6  as the hole injection layer is shown in detail in  FIG. 3 . In the organic EL layer  51  shown in  FIG. 3 , the V 2 O 5  layer  6  is arranged in contact with the lower electrode  52  as the hole injection layer, and a hole transport layer  51   a , a light emitting layer  51   b , an electron transport layer  51   c , and an electron injection layer  51   d  are respectively stacked on the V 2 O 5  layer  6  sequentially, and the upper electrode  53  which constitutes the common electrode is formed as an uppermost layer. 
         [0040]    In the above-mentioned constitution, the upper electrode  53  functions as a cathode having light transmitting property, while the lower electrode  52  of the pixel electrode functions as an anode having reflection property. 
         [0041]    Although the upper electrode  53  functions as the cathode having light transmitting property, the upper electrode  53  may be made of other transparent conductive material in place of the above-mentioned IZO. Further, the upper electrode  53  may preferably be made of a material having low light reflectance for suppressing reflection of light radiated from the light emitting layer. 
         [0042]    On the other hand, the lower electrode  52 , for enhancing properties thereof, may be formed using Al alloy such as Al/Nd alloy or Al/Si alloy, for example, in stead of using Al in a single form. Further, the lower electrode  52  may be formed using other metal having high reflection property. 
         [0043]    The light emitting layer  51   b  may be formed using a material which emits light of desired color when a predetermined voltage is applied between the transparent upper electrode  53  constituting the cathode and the lower electrode  52  constituting the anode. 
         [0044]    To explain materials of the light emitting layers  51   b , for example, the light emitting layer may adopt a material formed by dispersing DCM-1 (4-(dicyanomethylene)-2-methyl-6-(p-dimethylamino-styryl-4H-pyran) in Alq3 (tris(8-quinolinolate)aluminum) for emitting red light, the light emitting layer may adopt Alq3, Bebq, or Alq3 doped with quinacridone for emitting green light, for example, and the light emitting layer may adopt DPVBi(4,4′-bis(2,2-diphenylvinyl)biphenyl), a material formed of DPVBi(4,4′-bis(2,2-diphenyl vinyl)biphenyl) and BCzVBi (4,4′-bis(2-carbazole vinylene)biphenyl) or a material doped with di-styryl arylene derivative as a host and di-styryl amine derivative as a guest for emitting blue light, for example. 
         [0045]    Further, in the respective light emitting layers  51   b , the hole transport layer  51   a  may be formed using α-NPD (N,N-di(α-naphthyl)-N,N-diphenyl 1,1′-biphenyl-4,4′-diamine), or triphenyl diamine derivative TPD (N,N′-bis(3-methyl phenyl) 1,1′-biphenyl-4,4′-diamine). The electron transport layer  51   c  may be formed using Alq3. Further, polymer materials may be used for forming the respective light emitting layers in place of the above-mentioned low-molecular materials. 
         [0046]    In the organic EL element with the organic EL layer  51  having such a constitution, a DC power source is connected to the lower electrode  52  constituting the anode and the upper electrode  53  constituting the cathode and, when a DC voltage is applied between both electrodes, holes injected from the lower electrode  52  and electrons injected from the upper electrode  53  respectively arrive at the light emitting layer, and the recoupling of electrons and holes is generated thus generating the emission of light having a predetermined wavelength. 
       Embodiment 2 
       [0047]      FIG. 4  is a schematic cross-sectional view of light emitting element side for explaining the schematic structure of another embodiment of the organic EL display device according to the present invention, wherein parts identical with the parts in the above-mentioned drawings are given the same symbols. The embodiment 2 shown in  FIG. 4  is characterized by the constitution which defines an organic EL layer  51  including a V 2 O 5  layer  6  for every pixel unit using banks  54 . Other constitutions are equal to the corresponding constitutions shown in  FIG. 1  to  FIG. 3 .