Abstract:
Disclosed is an organic electroluminescent element including: a substrate; and a positive electrode, an organic light-emitting layer, and a laminated negative electrode of two or more layers including a lower negative electrode and an upper negative electrode having different specific resistances from each other, the positive electrode, organic light-emitting layer, and laminated negative electrode sequentially provided on the substrate from the substrate side, and in the organic electroluminescent element, the lower negative electrode and the upper negative electrode include any one of ITO, IZO, GZO, and AZO as the main constituent, and the lower negative electrode is provided closer to the organic light-emitting layer than the upper negative electrode, and the lower negative electrode has a specific resistance higher than the specific resistance of the upper negative electrode adjacent to the lower negative electrode.

Description:
BACKGROUND 
       [0001]    1. Technical Field 
         [0002]    This application claims the priority of Japanese Patent Application No. 2009-140194 filed in Japan on Jun. 11, 2009, the contents of which are hereby incorporated by reference. 
         [0003]    The present invention relates to an organic electroluminescent element and a method for manufacturing the same. 
         [0004]    2. Background Art 
         [0005]    Conventional organic electroluminescent elements include a top-emission type element for extracting light from a top-side negative electrode, as shown in Japanese Patent Laid-Open Publication No. 2005-44799 (Patent Document 1).  FIG. 7  is a cross-sectional view schematically illustrating the structure of a conventional organic electroluminescent element disclosed in Japanese Patent Laid-Open Publication No. 2005-44799 (Patent Document 1). 
         [0006]    In  FIG. 7 , the conventional organic electroluminescent element has, on a base material  101 , a stacked structure of a positive electrode  102 , an organic light-emitting layer  103 , an electron injecting layer  104 , a conductive protective layer  105 , and a negative electrode  106 . In this organic electroluminescent element, voltage is applied between the positive electrode  102  and the negative electrode  106  to inject holes from the positive electrode  106  into the organic light-emitting layer  103 , and injects electrons from the negative electrode  106  through the conductive protective layer  105  and the electron injecting layer  104  into the organic light-emitting layer  103 . The two types of carriers are moved through the organic light-emitting layer  103 , and recombined in the organic light-emitting layer  103  to generate excitons, and produce luminescence. 
       PRIOR ART DOCUMENTS 
     Patent Documents 
     Patent Document 1: Japanese Patent Laid-Open Publication No. 2005-44799  
     Patent Document 2: Japanese Patent Laid-Open Publication No. 2004-200141  
     Patent Document 3: Japanese Patent Laid-Open Publication No. 10-294182  
     SUMMARY OF THE INVENTION 
       [0007]    However, the conventional structure described above has a problem of inability to achieve a sufficiently high luminous efficiency, when the specific resistance of a transparent electrode such as ITO, used for the negative electrode, is excessively low. When the specific resistance of the deposited film is excessively low in the deposition of the negative electrode, deposition damage to the organic light-emitting layer  103  is produced to cause a decrease in luminous efficiency. 
         [0008]    An object of the present invention is to provide, mainly in view of the negative electrode structure of a top-emission type organic electroluminescent element, an organic electroluminescent element which achieves a high luminous efficiency, and a method for manufacturing the same. 
         [0009]    An organic electroluminescent element according to the present invention includes: a substrate; and a positive electrode, an organic light-emitting layer, and a laminated negative electrode of two or more layers including a lower negative electrode and an upper negative electrode having different specific resistances from each other, the positive electrode, organic light-emitting layer, and laminated negative electrode sequentially provided on the substrate from the substrate side, wherein the lower negative electrode and of the upper negative electrode include any one of ITO, IZO, GZO, and AZO as a main constituent, and the lower negative electrode is provided closer to the organic light-emitting layer than the upper negative electrode, and the lower negative electrode has a specific resistance higher than the specific resistance of the upper negative electrode adjacent to the lower negative electrode. 
         [0010]    This configuration can achieve a high luminous efficiency without adding a new material layer to a material layer of transparent conductive film such as ITO, mainly in terms of the negative electrode structure of a top-emission type organic electroluminescent element. 
         [0011]    As described above, the organic electroluminescent element according to the present invention can achieve a high luminous efficiency without adding a new material layer to a material layer of transparent conductive film such as ITO, mainly in terms of the negative electrode structure of a top-emission type organic electroluminescent element. Furthermore, the method for manufacturing an electroluminescent element according to the present invention can manufacture an organic electroluminescent element which has a high luminous efficiency, without preparing any other new material in addition to materials for use in a conventional element structure. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    The present invention will become readily understood from the following description of preferred embodiments thereof made with reference to the accompanying drawings, in which like parts are designated by like reference numeral and in which: 
           [0013]      FIG. 1  is a cross-sectional view schematically illustrating the structure of an organic electroluminescent element according to the first embodiment of the present invention; 
           [0014]      FIG. 2  is a schematic view illustrating the structure of a sample used in a quantum efficiency measurement for an organic electroluminescent element according to the first embodiment of the present invention; 
           [0015]      FIG. 3  is a schematic view illustrating the configuration of a measurement system used in a quantum efficiency measurement for an organic electroluminescent element according to the first embodiment of the present invention; 
           [0016]      FIG. 4  is a graph showing the result of a quantum efficiency measurement for an organic electroluminescent element according to the first embodiment of the present invention, with the horizontal axis for the specific resistance of a laminated negative electrode and the vertical axis for a normalized internal quantum efficiency; 
           [0017]      FIG. 5  is a diagram showing the relationship between an oxygen flow rate and the specific resistance of a negative electrode in the case of deposition according to the first embodiment of the present invention; 
           [0018]      FIG. 6  is a cross-sectional view schematically illustrating the structure of an organic electroluminescent element according to the second embodiment of the present invention; and 
           [0019]      FIG. 7  is a cross-sectional view schematically illustrating the structure of a conventional organic electroluminescent element. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0020]    Organic electroluminescent elements according to embodiments of the present invention will be described below with reference to the accompanying drawings. It is to be noted that substantially the same members are denoted by the same reference numerals in the drawings. 
       First Embodiment 
       [0021]      FIG. 1  is a cross-sectional view schematically illustrating the structure of an organic electroluminescent element according to the first embodiment of the present invention. This organic electroluminescent element has, on a base material  101 , a stacked structure of a positive electrode  102 , an organic light-emitting layer  103 , an electron injecting layer  104 , a laminated negative electrode (lower negative electrode)  206   a,  and a laminated negative electrode (upper negative electrode)  206   b.  In this organic electroluminescent element, voltage is applied between the positive electrode  102  and the laminated negative electrode (lower negative electrode)  206   a  and laminated negative electrode (upper negative electrode)  206   b  to inject holes from the positive electrode  102  into the organic light-emitting layer  103 , and to inject electrons from the laminated negative electrode (upper negative electrode)  206   b  through the electron injecting layer  104  into the organic light-emitting layer  103 . The two types of carriers are moved as excitons through the organic light-emitting layer  103 , and recombined in the organic light-emitting layer  103  to produce luminescence. 
         [0022]    In  FIG. 1 , the laminated negative electrode (lower negative electrode)  206   a  and the laminated negative electrode (upper negative electrode)  206   b  constitute a negative electrode of ITO films stacked as transparent conductive films, which is composed of two layers having different specific resistances from each other. The laminated negative electrode (lower negative electrode)  206   a  is deposited as having a specific resistance higher than the specific resistance of laminated negative electrode (upper negative electrode)  206   b,  and the laminated negative electrode (upper negative electrode)  206   b  is stacked on the laminated negative electrode (lower negative electrode)  206   a  to form a negative electrode of two-layer structure. In this the first embodiment, the laminated negative electrode (lower negative electrode)  206   a  has a specific resistance of 3×10 −3  Ω·cm and a film thickness of 10 nm, whereas the laminated negative electrode (upper negative electrode)  206   b  has a specific resistance of 3×10 −4  Ω·cm and a film thickness of 90 nm. 
         [0023]    In this configuration, the laminated negative electrode (lower negative electrode)  206   a  having higher specific resistance causes no deposition damage to the organic light-emitting layer  103  during the deposition. Furthermore, during the deposition of the laminated negative electrode (upper negative electrode)  206   b  having lower specific resistance, the laminated negative electrode (lower negative electrode)  206   a  serves as a barrier layer against deposition damage to the organic light-emitting layer  103 . As a result, even while improving the luminous efficiency, the laminated negative electrode (upper negative electrode)  206   b  having lower specific resistance suppresses the voltage drop in a horizontal direction, thereby making it possible to keep the driving voltage low. 
         [0024]    As described above, according to this configuration, the laminated negative electrode (lower negative electrode)  206   a  composed of an ITO film is stacked on the laminated negative electrode (upper negative electrode)  206   b  composed of another ITO film which is different only in resistance value, thereby allowing the driving voltage to be kept low due to the lower specific resistance of the laminated negative electrode (upper negative electrode)  206   b,  even while improving the luminous efficiency. 
         [0025]    A method for manufacturing the organic electroluminescent element shown in  FIG. 1  will be described below. 
         [0026]    a) The positive electrode  102  is obtained through deposition of a metal having a high reflectivity by a method such as sputtering. The metal having a high reflectivity can be selected from metals such as aluminum, molybdenum, and silver, and alloys thereof. The positive electrode  102  is subjected, after the deposition, to patterning by methods such as lithography and etching. 
         [0027]    b) Next, the organic light-emitting layer  103  is typically composed of a hole transporting layer, a hole injecting layer, a light-emitting layer, etc. Deposition method for the respective layers constituting the organic light-emitting layer  103  may be selected from vapor deposition, spin coating, ink jet and the like. These deposition method is selected depending on the types, etc. of the layers constituting the organic light-emitting layer  103 . In any of these methods, patterning is required, and in many cases, etching or the like is not carried out. 
         [0028]    c) Further, alkaline-earth metals and salts thereof, and mixtures of alkaline-earth metals with organic matters can be used for the electron injecting layer  104 . In addition, the electron injecting layer  104  can be deposited by a vapor deposition method. 
         [0029]    d) A transparent conductive film or a semi-transparent conductive film is used for the laminated negative electrode (lower negative electrode)  206   a  and the laminated negative electrode (upper negative electrode)  206   b  in the case of a top-emission type element. As the material, a conductive oxide is used such as ITO, IZO, AZO, or GZO. The deposition method can include sputtering, ion plating, vapor deposition and the like. Furthermore, for example, depositing the laminated negative electrode (lower negative electrode)  206   a  and the laminated negative electrode (upper negative electrode)  206   b  by varying the composition between the laminated negative electrode (lower negative electrode)  206   a  and the laminated negative electrode (upper negative electrode)  206   b  each other, such that the laminated negative electrode (lower negative electrode)  206   a  and the laminated negative electrode (upper negative electrode)  206   b  have different specific resistances from each other. Alternatively, the inclusion of an additive for changing specific resistance can make the respective specific resistances of the laminated negative electrode (lower negative electrode)  206   a  and the laminated negative electrode (upper negative electrode)  206   b  which are different from each other. In addition, the specific resistances can be changed by changing the deposition conditions. For example, the oxygen flow rate changed during deposition as described later can change the content of oxygen contained in each laminated negative electrode, and thus change the specific resistances. 
         [0030]    e) Furthermore, the organic light-emitting layer  103  and the electron injecting layer  104  are often unstable to oxygen and water in the atmosphere. Therefore, can sealing may be carried out in such a way that the organic electroluminescent element is covered with glass and subjected to sealing with a resin, or film sealing may be carried out in such a way that the organic electroluminescent element is coated with a passivation film such as SiN, SiON, or SiO 2 . Thus, the ingress of oxygen and water from the atmosphere can be prevented to prevent the organic electroluminescent element from being deteriorated. 
         [0031]    Next, the advantageous effects of the first embodiment will be described with reference to  FIGS. 2 ,  3 , and  4 .  FIG. 2  is a cross-sectional view schematically illustrating a test structure for examining the effect of the specific resistance value of the laminated negative electrode (lower negative electrode)  206   a  on the PL quantum efficiency in connection with  FIG. 1 . 
         [0032]    It is to be noted that the PL quantum efficiency (which may be referred to as “PL luminous efficiency”) refers to the ratio of emitted light energy to injected light energy. 
         [0033]      FIG. 3  is a schematic view illustrating a measurement system for measuring the change in PL quantum efficiency in the case of changing the specific resistance of the laminated negative electrode (lower negative electrode)  206   a  in  FIG. 2 . In  FIG. 3 , light of waves guided from a light source  301  via an optical cable  302  is turned into monochromatic light with a desired wavelength in a wavelength-tunable monochromator  303 , and then guided by an optical cable  304  into an integrating sphere  305 . The sample  306  shown in  FIG. 2  is placed in the integrating sphere  305 , the organic light-emitting layer of the sample  306  is irradiated with the monochromatic light to produce PL luminescence, and the PL luminescence is measured by a detector  308 , and subjected to counting by a personal computer  309 , The PL quantum efficiency is calculated by obtaining the ratio of the number of photons of the PL luminescence to the number of photons of incident light falling on the integrating sphere  305 . It is necessary to cancel light absorption at the negative electrode, because the incident light passes through the negative electrode to reach the organic light-emitting layer. Thus, the PL quantum efficiency is calculated in such a way that the number of photons absorbed by the negative electrode is subtracted from the number of incident photons. Furthermore, the value of the obtained internal quantum efficiency is normalized so that a sample with no negative electrode deposited has a PL quantum efficiency of 100%. 
         [0034]      FIG. 4  shows the results obtained in the way described above, with the horizontal axis for the specific resistance of the laminated negative electrode (lower negative electrode)  206   a  and the vertical axis for a normalized PL quantum efficiency. Referring to  FIG. 4 , it is determined that the PL quantum efficiency is increased as the specific resistance of the negative electrode is increased. This is believed to be because the deposition of the ITO film having higher specific resistance reduces deposition damage to the organic light-emitting layer.  FIG. 4  shows that the effect of improvement in PL quantum efficiency is produced at 1×10 −3  Ω·cm or more, and the specific resistance of the laminated negative electrode (lower negative electrode)  206   a  is thus desirably adjusted to this value or more, and in particular, 5×10 −3  Ω·cm or more preferably, at which the effect is remarkable in  FIG. 4 . 
         [0035]    In order not to increase the driving voltage significantly, the thickness of the laminated negative electrode (lower negative electrode)  206   a  is adjusted in the range of 100 nm to 1 nm with respect to the preferable specific resistance 1×10 −3  to 1×10 −1  Ω·cm mentioned above, thereby allowing the resistance value in the thickness direction to be kept constant. In this case, when the thickness is excessively thinner than 1 nm, there is a possibility that the in-plane uniformity in deposition may be degraded excessively. Thus, it is considered necessary that the thickness is 1 nm or more. 
         [0036]    While the specific resistance of the laminated negative electrode (upper negative electrode)  206   b  is preferably small as much as possible, the specific resistance of ITO as the most frequently used transparent conductive film has a limit on the order of 1×10 −4  Ω·cm. Furthermore, in order to perform a conductive function in the in-plane direction of the film as a negative electrode, the sheet resistance measured for the laminated negative electrode (upper negative electrode)  206   b  itself desirably meets 100 Ω/□ or less. 
         [0037]      FIG. 5  is a diagram showing the variation in the specific resistance of an ITO film deposited for a thickness of 100 nm, while changing the oxygen introduction amount in the case of sputtering deposition of an ITO film for the laminated negative electrode. From  FIG. 5 , it is determined that the adjustment of the oxygen flow rate during the deposition makes it possible to adjust the specific resistance. Further, when the oxygen flow rate is further decreased, the specific resistance shows a minimal value, and when the further decreased oxygen flow rate is further decreased, the specific resistance is increased reversely, while the transparency is decreased to be opaque. 
         [0038]    It is to be noted that while the case of two layers has been described as the laminated negative electrode in this the first embodiment, multiple layers of three or more layers may be adopted. For example, as long as, among the three or more layers, the laminated negative layer closest to the organic light-emitting layer has specific resistance higher than the specific resistance of the second closest layer, it is believed that at least the effect of improvement in luminous efficiency is produced. 
         [0039]    In addition, while the case of using ITO as the negative electrode material has been described in this first embodiment, the negative electrode material is not limited to ITO. The specific resistance is important for producing the effect of improvement in luminous efficiency, it is thus believed that similar effects are also produced in the case of IZO, GZO, AZO, etc. in addition to ITO, as the negative electrode material. 
         [0040]    In addition, while the typical structure has been described as the structure of the organic electroluminescent element in this first embodiment, it is obvious that a similar effect is produced as long as a transparent conductive film is mainly used as the negative electrode in the structure, and for example, a structure including no electron injecting layer may be adopted, or a conductive protective layer (barrier layer) may be located on the electron injecting layer. 
       Second Embodiment 
       [0041]      FIG. 6  illustrates the structure of an organic electroluminescent element according to the second embodiment of the present invention. In  FIG. 6 , the same reference numerals are used for the same components as those in  FIG. 1 , and descriptions of the components will be omitted. Referring to  FIG. 6 , a negative electrode  207  is composed of an ITO film, which has a section at which the specific resistance continuously varies along a direction perpendicular to the surface of the negative electrode. Thus, the specific resistance on the electron injecting layer  104  side of the negative electrode  207  is higher; on the other hand, the specific resistance on the top of the negative electrode  207  is lower. While the element structure described in the first embodiment has the negative electrode composed of separate multiple layers, the multiple layers having different specific resistance each other, a similar effect can be produced by continuously varying the specific resistance of the negative electrode along the direction perpendicular to the surface of the negative electrode as in this second embodiment. 
         [0042]    Methods for manufacturing the negative electrode in the organic electroluminescent element according to this second embodiment include sputtering deposition of ITO. In this case, the oxygen flow rate can be increased to form an ITO film having higher specific resistance in the initial stage of the deposition, and the oxygen flow rate can be decreased gradually to form an ITO film having lower specific resistance on the upper side. In this regard, for how to vary the specific resistance, it is not always necessary to vary the relationship between the depth and the specific resistance in a linear manner over all of the region. For example, the specific resistance is preferably decreased as much as possible in the region other than the region for improving the luminous efficiency, because the driving voltage is not increased. 
         [0043]    In addition, it is considered preferable from the result in  FIG. 4  that the specific resistance on the lower side (the electron injecting layer  104  side) of the negative electrode  207  has a value within the range of 1×10 −3  to 1×10 −1  Ω·cm, and in particular, 5×10 −3  or more, for the formation of an internal electric field for keeping out electrons. While the specific resistance is preferably low as much as possible on the upper side of the negative electrode  207 , ITO as the most frequently used transparent conductive film has a limit on the order of 1×10 −4  Ω·cm. Furthermore, in order to perform a function as the negative electrode, the sheet resistance measured from the upper side desirably meets 100 Ω/□ or less. In addition, it is considered preferable that the thickness from the lower side with a specific resistance of 1×10 −3  Ω·cm or more through the gradually decreased specific resistance down to 1×10 −3  Ω·cm or less is 100 nm or less because the excessively increased thickness causes an increase in driving voltage, and it is considered necessary that the thickness is 1 nm or more because the thickness excessively thinner than 1 nm fails to produce an effect as a film. 
         [0044]    While the case of using an ITO film as the negative electrode material has been also described as an example in this second embodiment, it comes near to stating the obvious that similar effects are also produced in the case of using other conductive materials for transparent oxide films, such as IZO, AZO, and GZO, as the negative electrode material. 
         [0045]    In addition, on the subject of the present invention, while the typical structures have been described as the structure of the organic electroluminescent element in each of the embodiments, it is obvious that a similar effect is produced as long as a transparent conductive film is mainly used as the negative electrode in the structure. For example, a structure including no electron injecting layer may be adopted, or a conductive protective layer (barrier layer) may be provided on the electron injecting layer. 
         [0046]    The organic electroluminescent element according to the present invention is a top-emission type element which has a high luminous efficiency and a low driving voltage while using a transparent conductive film such as ITO. Therefore, the present invention can be applied to uses of two-sided light extraction structures such as organic electroluminescent elements. 
       EXPLANATION OF REFERENCE NUMERALS 
       [0000]    
       
           101  substrate 
           102  positive electrode 
           103  organic light-emitting layer 
           104  electron injecting layer 
           105  conductive protective film 
           106  negative electrode 
           206   a  laminated negative electrode (lower negative electrode) 
           206   b  laminated negative electrode (upper negative electrode) 
           207  negative electrode 
           301  light source 
           302  optical cable 
           303  wavelength-tunable monochromator 
           304  optical cable 
           305  integrating sphere 
           306  sample 
           307  optical cable 
           308  detector 
           309  personal computer