Abstract:
A method of manufacturing an organic EL element includes applying a reverse bias to an organic EL element with an inverse layered structure until a current density of the organic EL element at an impressed voltage of  10  V reaches 0.075 mA/cm 2  or more.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is based upon and claims the benefit of priority from prior Japanese Patent Application No. 2004-320935, filed Nov. 4, 2004, the entire contents of which are incorporated herein by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to a method of manufacturing an organic electroluminescence (EL) element.  
         [0004]     2. Description of the Related Art  
         [0005]     The structures of organic EL elements can be classified into a normal layered structure and inverse layered structure. In the normal layered structure, an anode, an organic layer including an emitting layer, and a cathode are stacked in this order on a substrate. In the inverse layered structure, these components are stacked in the reverse order. In these organic EL elements, a transparent conductive oxide such as ITO (indium tin oxide) is normally used as the material of the anode, and a metal or alloy is normally used as the material of the cathode.  
         [0006]     The structures of the organic EL elements can also be classified into a bottom emission structure and top emission structure. In the bottom emission structure, light emitted by the emitting layer is extracted from the substrate side. In the top emission structure, light emitted by the emitting layer is extracted from the opposite side. Presently, many organic EL elements use the normal layered structure regardless of whether they have the bottom emission structure or top emission structure.  
         [0007]     Jpn. Pat. Appln. KOKAI Publication No. 4-14794 describes aging which is performed to stabilize the emission of an organic EL element. In this aging, a forward voltage, a reverse voltage, or a voltage having a waveform obtained by combining these voltages is applied to an organic EL element.  
       BRIEF SUMMARY OF THE INVENTION  
       [0008]     According to a first aspect of the present invention, there is provided a method of manufacturing an organic EL element, comprising applying a reverse bias to an organic EL element with an inverse layered structure until a current density of the organic EL element at an impressed voltage of 10 V reaches 0.075 mA/cm 2  or more.  
         [0009]     According to a second aspect of the present invention, there is provided a method of manufacturing an organic EL element, comprising applying a reverse bias to an organic EL element with an inverse layered structure until a current density of the organic EL element at an impressed voltage of 10 V reaches or exceed three times as high as that of the organic EL element before application of the reverse bias.  
         [0010]     According to a third aspect of the present invention, there is provided a method of manufacturing an organic EL element, comprising applying a reverse voltage to an organic EL element with an inverse layered structure while heating the organic EL element by a heat source disposed outside the organic EL element.  
         [0011]     “An organic EL element having an inverse layered structure” means an organic EL element having a structure in which a cathode, emitting layer, and anode are stacked in this order on an underlayer. Also, “a reverse bias is applied to the organic EL element” means that a voltage by which the electric potential of the anode of the organic EL element becomes lower than that of its cathode is applied between the anode and cathode. 
     
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING  
       [0012]      FIG. 1  is a sectional view schematically showing an example of an organic EL element which can be manufactured by a method according to an embodiment of the present invention;  
         [0013]      FIG. 2  is a sectional view schematically showing a test element;  
         [0014]      FIG. 3  is a graph showing the current-voltage characteristics of the test element shown in  FIG. 2 ;  
         [0015]      FIG. 4  is a graph showing the relationship between duration of aging and forward characteristic when aging was performed at room temperature by setting the current density at 30 mA/cm 2 ;  
         [0016]      FIG. 5  is a graph showing the relationship between duration of aging and reverse characteristic when aging was performed at room temperature by setting the current density at 30 mA/cm 2 ;  
         [0017]      FIG. 6  is a graph showing the relationship between duration of aging and forward characteristic when aging was performed at room temperature by setting the current density at 20 mA/cm 2 ;  
         [0018]      FIG. 7  is a graph showing the relationship between duration of aging and reverse characteristic when aging was performed at room temperature by setting the current density at 20 mA/cm 2 ;  
         [0019]      FIG. 8  is a graph showing the relationship between duration of aging and forward characteristic when aging was performed at room temperature by setting the current density at 10 mA/cm 2 ;  
         [0020]      FIG. 9  is a graph showing the relationship between duration of aging and reverse characteristic when aging was performed at room temperature by setting the current density at 10 mA/cm 2 ;  
         [0021]      FIG. 10  is a graph showing the relationship between duration of aging and forward characteristic when aging was performed at room temperature by setting the current density at 5 mA/cm 2 ;  
         [0022]      FIG. 11  is a graph showing the relationship between duration of aging and reverse characteristic when aging was performed at room temperature by setting the current density at 5 mA/cm 2 ;  
         [0023]      FIG. 12  is a graph showing the relationship between duration of aging and forward characteristic when aging was performed at room temperature by setting the current density at 30 mA/cm 2 ; and  
         [0024]      FIG. 13  is a graph showing the relationship between duration of aging and forward characteristic when aging was performed by setting the current density at 30 mA/cm 2  while the test element was heated to 60° C. by using a heater. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0025]     An embodiment of the present invention will be described in detail below with reference to the accompanying drawing. Note that the same reference numerals denote constituent elements which achieve the same or similar functions in the drawing, and a repetitive explanation thereof will be omitted.  
         [0026]      FIG. 1  is a sectional view schematically showing an example of an organic EL element which can be manufactured by a method according to the embodiment of the present invention.  
         [0027]     An organic EL element  1  has an inverse layered structure, and is formed on a glass substrate or on an underlayer  2  such as an insulating layer formed on the glass substrate. The organic EL element  1  includes a cathode  11 , electron injection layer  12 , electron transporting layer  13 , emitting layer  14 , hole transporting layer  15 , hole injection layer  16 , and anode  17 .  
         [0028]     The cathode  11  reflects light and functions as a reflecting layer. As the material of the cathode  11 , it is possible to use, e.g., a metal such as aluminum or silver, or an alloy such as an aluminum alloy.  
         [0029]     The electron injection layer  12  is an inorganic layer formed between the cathode  11  and emitting layer  14  and in contact with the cathode  11 . The material of the electron injection layer  12  is typically a material having electron affinity intermediate between the work function of the cathode  11  and the electron affinity of the emitting layer  14 . As the material of the electron injection layer  12 , it is possible to use, e.g., an alkali metal compound such as lithium fluoride, or an alkaline-earth compound. The thickness of the electron injection layer  12  is, e.g., about 0.5 to 3 nm.  
         [0030]     The electron transporting layer  13  is an organic layer interposed between the electron injection layer  12  and emitting layer  14 . The material of the electron transporting layer  13  is typically a material having electron affinity intermediate between the electron affinity of the electron injection layer  12  and that of the emitting layer  14 . As the material of the electron transporting layer  13 , it is possible to use, e.g., an organometallic complex such as tris(8-hydroxyquinolate)aluminum (Alq 3 ) which is a coordinated complex of aluminum having three oxines as ligands. The thickness of the electron transporting layer  13  is, e.g., about 5 to 50 nm.  
         [0031]     The emitting layer  14  is an organic layer which emits light by recombination of electron and hole. The emitting layer  14  may also function as the electron transporting layer  13 . As the material of the emitting layer  14 , it is possible to use, e.g., a material obtained by adding a dopant as a luminescent center to a host material. The thickness of the emitting layer  14  is, e.g., about 5 to 50 nm.  
         [0032]     The hole transporting layer  15  is interposed between the emitting layer  14  and hole injection layer  16 . The material of the hole transporting layer  15  is typically a material having ionization energy intermediate between the ionization energy of the emitting layer  14  and that of the hole injection layer  16 . As the material of the hole transporting layer  15 , it is possible to use, e.g., an organic material such as 4,4′-bis[N-(1-naphthyl)-N-phenyl-amino]biphenyl (αNPD). The thickness of the hole transporting layer  15  is, e.g., about 5 to 200 nm.  
         [0033]     The hole injection layer  16  is interposed between the emitting layer  14  and anode  17 , and in contact with the anode  17 . The material of the hole injection layer  17  is typically a material having ionization energy intermediate between the ionization energy of the emitting layer  14  and the work function of the anode  17 . As the material of the hole injection layer  16 , copper phthalocyanine (CuPc) or the like can be used. The thickness of the hole injection layer  16  is, e.g., about 3 to 10 nm.  
         [0034]     The anode  17  transmits light. As the material of the anode  17 , it is possible to use, e.g., a transparent conductive oxide such as ITO.  
         [0035]     In this embodiment, the organic EL element  1  can be manufactured by, e.g., the following method. First, the organic EL element  1  is obtained by sequentially forming the cathode  11 , electron injection layer  12 , electron transporting layer  13 , emitting layer  14 , hole transporting layer  15 , hole injection layer  16 , and anode  17  on the underlayer  2 . These layers can be formed by using, e.g., vacuum evaporation or sputtering.  
         [0036]     Then, aging is performed. That is, a reverse bias is applied between the cathode  11  and anode  17  in an inert gas ambient such as a nitrogen ambient. This makes the injection efficiency of the organic EL element  1  higher than that before aging. Consequently, light having the same luminance as that before aging can be emitted by a driving voltage lower than that before aging.  
         [0037]     As will be described in detail later, the present inventors consider that this aging changes the structure between the cathode  11  and emitting layer  14 , and as a consequence injection of electrons into the emitting layer  14  readily occurs. In particular, when the organic EL element  1  includes the electron injection layer  12  between the cathode  11  and emitting layer  14 , the injection efficiency of electrons can be significantly increased by the aging. That is, in the organic EL element  1  in which the cathode  11 , electron injection layer  12 , emitting layer  14 , and anode  17  are formed in this order, the aging changes the structure between the cathode  11  and emitting layer  14 , and this change presumably greatly increases the injection efficiency of electrons.  
         [0038]     The aging described above is performed under conditions by which, e.g., the injection efficiency of the organic EL element  1  becomes three times or more the value before the aging.  
         [0039]     The reverse bias applied to the organic EL element  1  is, e.g., about 5 V to about 30 V. If the reverse bias is low, the injection efficiency of the organic EL element  1  cannot be increased within a short time period. If the reverse bias is high, the organic EL element  1  may be broken.  
         [0040]     The duration of the aging is, e.g., about 30 min to about 24 hrs. If the aging duration is short, the injection efficiency of the organic EL element  1  cannot be well increased. If the aging duration is long, no high productivity can be realized.  
         [0041]     The aging may also be performed while the organic EL element  1  is heated by a heat source positioned outside the organic EL element  1 . For example, the aging may also be performed while the organic EL element  1  is heated to about 40° C. to 80° C., desirably, about 60° C. In this manner, the injection efficiency of the organic EL element  1  can be increased within a shorter aging duration or by a lower reverse bias.  
         [0042]     The organic EL element  1  manufactured by the above method can be used as an emitting element of various apparatuses. For example, the organic EL element  1  can be used as a display element of an organic EL display.  
         [0043]     The organic EL display can use one of a bottom emission structure in which light emitted by the organic EL element is extracted from the underlayer side, and a top emission structure in which this light is extracted from the opposite side. In the top emission organic EL display, even when a thin-film transistor (TFT), interconnection, and the like are formed in a position where they overlap the organic EL element in the direction of thickness, they do not intercept light from the organic EL element, unlike in the bottom emission organic EL display. Accordingly, the top emission structure can realize the same luminance as the bottom emission structure by a lower current density.  
         [0044]     It is, therefore, possible to realize both low power consumption and a long life when the inverse layered structure is used as the organic EL element in the top emission organic EL display and the above-mentioned aging is performed.  
         [0045]     Examples of the present invention will be described below.  
         [0000]     (Test 1)  
         [0046]      FIG. 2  is a sectional view schematically showing a test element. A test element  10  has a structure in which a 100-nm thick Al layer  11   a , 1-nm thick LiF layer  12   a , 200-nm thick Alq 3  layer  13 , 1-nm thick LiF layer  12   b , and 100-nm thick Al layer  11   b  are stacked in this order on a glass substrate  2 . Note that these layers were formed by vacuum evaporation.  
         [0047]     In this example, the current-voltage characteristics of the test element  10  shown in  FIG. 2  were measured first. The results are shown in  FIG. 3 .  
         [0048]      FIG. 3  is a graph showing the current-voltage characteristics of the test element shown in  FIG. 2 . Referring to  FIG. 3 , the abscissa indicates the voltage applied between the Al layers  11   a  and  11   b , and the ordinate indicates the current density. Also, data represented by “Initial (forward)” was obtained when the voltage was applied such that the electric potential of the Al layer  11   a  was lower than that of the Al layer  11   b , and data represented by “Initial (reverse)” was obtained when the voltage was applied such that the electric potential of the Al layer  11   a  was higher than that of the Al layer  11   b . Note that in this example, the Al layers  11   a  and  11   b  are regarded as a cathode and anode, respectively, the current-voltage characteristic when the voltage was so applied that the electric potential of the Al layer  11   a  was lower than that of the Al layer  11   b  will be called “forward characteristic”, and the current-voltage characteristic when the voltage was so applied that the electric potential of the Al layer  11   a  was higher than that of the Al layer  11   b  will be called “reverse characteristic”.  
         [0049]     As described above, the test element  10  has a layered structure symmetrical with respect to the direction of thickness. Originally, therefore, the forward and reverse characteristics of the test element  10  ought to be equal to each other. In effect, however, as shown in  FIG. 3 , the forward characteristic was much inferior to the reverse characteristic.  
         [0050]     In the test element  10 , an electric current flows by using electrons as carriers. Also, the electron affinity of Alq 3  is smaller than the work function of Al, and the electron affinity of LiF is intermediate between the work function of Al and the electron affinity of Alq 3 . Therefore, the difference between the forward and reverse characteristics described above indicates that the electron injection efficiency on the side of the Al layer  11   a  is lower than that on the side of the Al layer  11   b.    
         [0051]     From the foregoing, the emission efficiency of an organic EL element having the inverse layered structure is lower than that of an organic EL element having the normal layered structure because the electron injection efficiency of the former is lower than that of the latter. Accordingly, a high emission efficiency can be realized even by an organic EL element having the inverse layered structure if the electron injection efficiency can be increased.  
         [0000]     (Test 2)  
         [0052]     The test element  10  was then aged at room temperature (25° C.), and the forward and reverse characteristics after aging were measured. In this aging, a DC voltage was applied between the Al layers  11   a  and  11   b  so that the electric potential of the former was lower than that of the latter, and the current density was maintained at 30 mA/cm 2  from the start to the end of aging. The results are shown in  FIGS. 4 and 5 .  
         [0053]      FIG. 4  is a graph showing the relationship between the aging duration and forward characteristic when aging was performed at room temperature by setting the current density at 30 mA/cm 2 .  FIG. 5  is a graph showing the relationship between the aging duration and reverse characteristic when aging was performed at room temperature by setting the current density at 30 mA/cm 2 .  
         [0054]     Referring to  FIGS. 4 and 5 , the abscissa indicates the voltage, and the ordinate indicates the current density. Also, data represented by “Initial” was obtained before aging, and data represented by “17 h”, “38 h”, “83 h”, “222 h”, “520 h”, “750 h”, and “1160 h” was obtained after aging when the aging durations were 17, 38, 83, 222, 520, 750, and 1,160 hrs, respectively.  
         [0055]     As shown in  FIG. 5 , the reverse characteristic scarcely changed before and after aging. In contrast, as shown in  FIG. 4 , the forward characteristic improved by aging. That is, aging made it possible to improve the electron injection efficiency on the side of the Al layer  11   a.    
         [0056]     Also, as shown in  FIG. 4 , the longer the aging duration, the larger the effect of improving the forward characteristic by aging. However, when the aging duration prolonged, the change in forward characteristic with respect to the aging duration decreased.  
         [0000]     (Test 3)  
         [0057]     Then, a test element  10  identical to that used in test 1 was used to check the influence of the magnitude of an electric current flowing between the Al layers  11   a  and  11   b  during a period from the start to the end of aging upon the current-voltage characteristic after aging. The results are shown in FIGS.  6  to  11 .  
         [0058]      FIG. 6  is a graph showing the relationship between the aging duration and forward characteristic when aging was performed at room temperature by setting the current density at 20 mA/cm 2 .  FIG. 7  is a graph showing the relationship between the aging duration and reverse characteristic when aging was performed at room temperature by setting the current density at 20 mA/cm 2 .  FIG. 8  is a graph showing the relationship between the aging duration and forward characteristic when aging was performed at room temperature by setting the current density at 10 mA/cm 2 .  FIG. 9  is a graph showing the relationship between the aging duration and reverse characteristic when aging was performed at room temperature by setting the current density at 10 mA/cm 2 .  FIG. 10  is a graph showing the relationship between the aging duration and forward characteristic when aging was performed at room temperature by setting the current density at 5 mA/cm 2 .  FIG. 11  is a graph showing the relationship between the aging duration and reverse characteristic when aging was performed at room temperature by setting the current density at 5 mA/cm 2 .  
         [0059]     Referring to FIGS.  6  to  11 , the abscissa indicates the voltage, and the ordinate indicates the current density. Also, data represented by “Initial” was obtained before aging, and data represented by “17 h”, “38 h”, “83 h”, “222 h”, “520 h”, “750 h”, and “1160 h” was obtained after aging when the aging durations were 17, 38, 83, 222, 520, 750, and 1,160 hrs, respectively.  
         [0060]     As shown in  FIGS. 5, 7 ,  9 , and  11 , the reverse characteristic scarcely changed before and after aging regardless of the magnitude of the electric current supplied between the Al layers  11   a  and  11   b  during the period from the start to the end of aging. In contrast, as shown in  FIGS. 4, 6 ,  8 , and  10 , the larger the electric current described above, the shorter the time period which is necessary to achieve a certain effect of improving the characteristic.  
         [0000]     (Test 4)  
         [0061]     A test element  10  identical to that used in test 1 was used to check the influence of the temperature during the aging period upon the forward characteristic after aging. The results are shown in  FIGS. 12 and 13 .  
         [0062]      FIG. 12  is a graph showing the relationship between the aging duration and forward characteristic when aging was performed at room temperature by setting the current density at 30 mA/cm 2 .  FIG. 13  is a graph showing the relationship between the aging duration and forward characteristic when aging was performed by setting the current density at 30 mA/cm 2  while the test element  10  was heated to 60° C. by using a heater.  
         [0063]     Referring to  FIGS. 12 and 13 , the abscissa indicates the voltage, and the ordinate indicates the current density. Also, data represented by “Initial” was obtained before aging, and data represented by “17 h”, “20 h”, “38 h”, “45 h”, “83 h”, “90 h”, “164 h”, and “222 h” was obtained after aging when the aging durations were 17, 20, 38, 45, 83, 90, 164, and 222 hrs, respectively.  
         [0064]     As can be seen by comparing  FIGS. 12 and 13 , a time period necessary to achieve a certain effect of improving the characteristic is shorter in the case where aging was performed while heating the test element  10  than in the case where aging was performed without heating the test element  10 .  
         [0065]     Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and representative embodiments shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.