Abstract:
An organic electroluminescence (EL) device capable of digital driving, and a method for manufacturing the same are disclosed herein. The organic EL device comprises a substrate, an anode formed on the substrate, an organic EL layer formed on the anode and constructed as a multilayer structure including a hole injection layer and a hole transport layer, a cathode formed on the organic EL layer, and an interface deterioration preventing layer formed between the anode and the organic EL layer.

Description:
[0001]     This application claims the benefit of Korean Patent Application No. 10-2004-0040425, filed on, Jun. 3, 2004, which is hereby incorporated by reference as if fully set forth herein.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to an organic electroluminescent device, and more particularly, to an organic electroluminescent device capable of digital driving, and a method for manufacturing the same.  
         [0004]     2. Discussion of the Related Art  
         [0005]     Generally, an organic electroluminescent device (referred to hereinafter simply as an “organic EL device”) is operated in such a manner that, when electric charges are injected into an organic film formed between an electron injection electrode (anode) and a hole injection electrode (cathode), electrons and holes are recombined and then extinguished, thereby generating light. Such an organic EL device has been evaluated as a next generation display device having characteristics of low driving voltage and low power consumption.  
         [0006]     Construction of a conventional organic EL device and a method for manufacturing the same will be described with reference to the accompanying drawings.  
         [0007]      FIG. 1  illustrates a conventional organic EL device.  
         [0008]     As shown in  FIG. 1 , the organic EL device has an anode  102  formed on a transparent substrate  101 . A material for the anode  102  generally comprises indium tin oxide (ITO), and the anode  102  is surface-treated using O 2  plasma, UVO, and the like after the substrate  101  is coated with ITO. When impurities on the surface of the anode  102  are removed by surface treatment as described above, interface properties between the anode and a hole injection layer are enhanced, thereby allowing easy injection of holes.  
         [0009]     Then, the hole injection layer (which will also be referred to “HIL”)  103  is formed on the anode  102 . As for the HIL  103 , copper phthalocyanine (CuPc) is generally coated to a thickness of about 10˜30 nm on the anode  102 .  
         [0010]     A hole transport layer (HTL)  104  is formed on the HIL  103 . As the HLT  104 , N,N′-diphenyl-N,N′-bis(3-methylphenyl)-(1,1′biphenylyl)-4,4′-diamine (TPD) or 4,4′-bis[N-(1-naphthyl)-N-phenyl-amino]biphenyl (NPD) is formed to a thickness of about 30˜60 nm on the HIL  103 .  
         [0011]     An organic emitting layer  105  is formed on the HTL  104 . At this time, dopants can be added, if necessary. In the case of a green light emitting device, tris(8-hydroxy-quinolate)aluminum (Alq3) as the organic emitting layer  105  is typically deposited to a thickness of about 30˜60 nm, and coumine derivative (C545T) or quinacridone (Qd) so the dopant is used. In the case of a red light emitting device, Alq3 is used as the organic emitting layer  105 , and DCM, DCJT, DCJTB or the like is used as the dopant. In the case of a blue light emitting device, DPVBi is generally used as the organic emitting layer  105  without any dopant.  
         [0012]     An electron transport layer (which will also be referred to as “ETL”)  106 , and an electron injecting layer (which will also be referred to as “EIL”)  107  are sequentially formed on the organic emitting layer  105 . In the case of the green light emitting device, since Alq3 has good electron transportation capabilities, it is not necessary to form the ETL  106  and the EIL  107  thereon.  
         [0013]     For the electron injecting layer  107 , LiF or Li20 is thinly deposited to a thickness of about 5 Å, otherwise alkali metal or alkaline earth metal such as Li, Ca, Mg, Sm and the like is deposited to a thickness of about 200 Å thereby allowing easy injection of electrons.  
         [0014]     For a cathode  108 , Al is coated to a thickness of about 1,000 Å on the electron injecting layer  107 , and a closing plate (not shown) containing a hygroscopic agent is bonded to the cathode  108  using an ultraviolet curable bonding agent thereby protecting the organic EL device from moisture or O 2  in the atmosphere.  
         [0015]     The organic EL device constructed as described above may suffer great changes in life span and efficiency according to materials, surface treatment conditions for the anode, and stacked structures of the organic EL device.  
         [0016]     Generally, it is important for the organic EL device to have an extended life span and a stabilized interface between the layers for stable current injection.  
         [0017]     However, in the organic EL device, the interfaces between inorganic layers and organic layers cause a deterioration of the device.  
         [0018]     In the organic EL device, the interfaces between the inorganic layers and the organic layers include an interface between the anode and the hole injection layer, and an interface between the electron transport layer and the electron injecting layer (or the cathode). Especially, the interface between the anode and the hole injection layer is the most influential upon deterioration of the device.  
         [0019]     Thus, according to the prior art, in order to solve this problem, the HIL is deposited on the anode after the anode formed of ITO is surface-treated using O 2  plasma or UVO such that the impurities are removed from the surface of the anode  102 .  
         [0020]     As a result, the anode is enhanced in adhesion to the hole injection layer, so that life span of the device is extended, and current is stably injected therefrom.  
         [0021]     However, with such a method, there is a limitation in extending the life span of the device.  
         [0022]      FIG. 2  is a graph depicting results of a test of constant current acceleration for a green light emitting device having a surface treated anode, and  FIG. 3  is a graph depicting results of a test of constant voltage acceleration for the green light emitting device having the surface treated anode.  
         [0023]     As shown in  FIG. 2 , when the green light emitting device is driven in a constant current mode, a voltage applied to the device is increased. As a result, the device suffers reduction in brightness of 50% compared to an initial brightness. For the green light emitting device, the voltage is increased by approximately 2 V.  
         [0024]     Additionally, as shown in  FIG. 3 , when the green light emitting device is driven in a constant voltage mode, current applied to the device is increased along with deterioration of the device. As a result, the brightness of the green light emitting device is sharply decreased 10 times of less of the brightness of the case where the device is driven in the constant current mode.  
         [0025]     As such, in the organic EL device, the voltage increase upon constant current driving and the current decrease upon constant voltage driving are mostly caused by deterioration in the interfaces between the organic materials and the inorganic materials rather than deterioration of the organic material itself. In particular, among the interfaces between the organic materials and the inorganic materials, the interface between the anode formed of the organic material and the hole injection layer is the most influential upon deterioration of the device.  
       SUMMARY OF THE INVENTION  
       [0026]     Accordingly, the present invention is directed to an organic electroluminescent device that substantially obviates one or more problems due to limitation and disadvantages of the related art.  
         [0027]     An object of the present invention is to provide an organic electroluminescent device, which reduces deterioration created in an interface between an anode and an organic EL layer, thereby ensuring implementation of digital driving, extended life span, stabilized light emitting characteristics and minimized increase of voltage, and a method for manufacturing the same.  
         [0028]     Additional advantages, objects, and features of the invention will be set forth in part in the description which follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from practice of the invention. The objectives and other advantages of the invention may be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.  
         [0029]     To achieve these objects and other advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, an organic electroluminescence (EL) device comprises: a substrate; an anode formed on the substrate; an organic EL layer formed on the anode and constructed as a multilayer structure comprising a hole injection layer and a hole transport layer; a cathode formed on the organic EL layer; and an interface deterioration preventing layer formed between the anode and the organic EL layer.  
         [0030]     The interface deterioration preventing layer may be formed either between the anode and the hole injection layer or between the anode and the hole transport layer.  
         [0031]     The interface deterioration preventing layer may consist of a mixture of an organic material and an inorganic material, and the organic material may be an aromatic amine compound having hole transmission properties.  
         [0032]     The aromatic amine compound may have a chemical structure of either  
                         
 
         [0033]     In the chemical structure, n may be an integer selected from 1 to 4, and Ar 1 , Ar 2  and Ar 3  may be either substituted or non-substituted aromatic groups, respectively.  
         [0034]     Ar 1 , Ar 2  and Ar 3  may be ones selected from the group consisting of phenyl, naphthyl, biphenylyl, biphenylylelnyl, phenanthrenyl, fluorenyl, terphenylyl and anthracenyl, and a substituent may be one selected from methyl, ethyl, propyl, t-buthyl, methoxy, ethoxy, propoxy, dimethylamine, diethylamine, phenyl, fluorine, chlorine, and bromine.  
         [0035]     The inorganic material may be one selected from a halide or an oxide of 1A, 2A, 3A and 4A-groups of the periodic table. The halide may be one selected from LiF, NaF, KF, RbF, CsF, FrF, MgF 2 , CaF 2 , NaCl, CaCl 2 , LiCl, KCl, RbCl, CsCl, FrCl, MgCl 2 . The oxide may be one selected from Li 2 O, Na 2 O, K 2 O, BeO, MgO, CaO, B 2 O 3 , Al 2 O 3  and SiO 2 .  
         [0036]     In another aspect of the present invention, a method for manufacturing an organic electroluminescence (EL) device comprises the steps of: forming an anode on a substrate; forming an interface deterioration preventing layer consisting of a mixture of an organic material and an inorganic material on the anode; forming an organic EL layer comprising a hole injection layer on the interface deterioration preventing layer; and forming a cathode on the organic EL layer.  
         [0037]     The interface deterioration preventing layer may be formed by repetitiously stacking organic and inorganic layers in the sequence of either organic layer/inorganic layer or inorganic layer/organic layer.  
         [0038]     Each organic layer and inorganic layer may have a thickness of 0.1˜10 nm, and the interface deterioration preventing layer may have a total thickness of 0.1˜100 nm.  
         [0039]     The interface deterioration preventing layer may be formed by depositing the organic material and the inorganic material at the same time such that the organic material is mixed with the inorganic material. Alternatively, the interface deterioration preventing layer may be formed by stacking the organic and inorganic layers in the sequence of either organic layer/inorganic layer or inorganic layer/organic layer such that a mixing ratio of the organic material and the inorganic material is linearly changed at an interface between the organic layer and the inorganic layer.  
         [0040]     It is to be understood that both the foregoing general description and the following detailed description of the present invention are exemplary and explanatory and are intended to provide further explanation of the invention as claimed. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0041]     The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this application, illustrate embodiment(s) of the invention and together with the description serve to explain the principle of the invention. In the drawings:  
         [0042]      FIG. 1  illustrates a conventional organic EL device;  
         [0043]      FIG. 2  is a graph depicting results of a test of constant current acceleration for a conventional green light emitting device;  
         [0044]      FIG. 3  is a graph depicting results of a test of constant voltage acceleration for the conventional green light emitting device;  
         [0045]      FIG. 4  illustrates an organic EL device in accordance with the present invention;  
         [0046]      FIG. 5  illustrates one example of the organic EL device shown in  FIG. 4 ;  
         [0047]      FIG. 6  illustrates organic material for an interface deterioration preventing layer of the organic EL device in accordance with the present invention;  
         [0048]      FIG. 7  is a graph depict results of a test of constant current acceleration for the organic EL device in accordance with the present invention; and  
         [0049]      FIG. 8  is a graph depicting results of a test of constant voltage acceleration for the organic EL device in accordance with the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0050]     Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts.  
         [0051]     According to the invention, an organic EL device has an interface deterioration preventing layer consisting of a mixture of an organic material and an inorganic material formed between an anode comprising ITO as the organic material and a hole injection layer consisting of the inorganic material, so that deterioration and thermal stress of the device is relieved, and so that the device is suppressed in voltage increase upon constant current driving while minimizing current decrease upon constant voltage driving.  
         [0052]     Additionally, according to the present invention, an amount of holes injected into the organic EL device can be controlled, and a balance of electric charge between the holes and electrons can be optimized by controlling an amount of the inorganic material added to the interface deterioration preventing layer and a thickness of the interface deterioration preventing layer, thereby maximizing light efficiency of the device.  
         [0053]      FIG. 4  illustrates an organic EL device in accordance with the invention, and  FIG. 5  illustrates one inventive example of the organic EL device of  FIG. 4 .  
         [0054]     As shown in  FIGS. 4 and 5 , an anode  202  is formed on a transparent substrate  201 .  
         [0055]     The anode  202  comprises indium tin oxide (ITO)  
         [0056]     Surface treatment is performed on the anode  202  formed on the transparent substrate  201  using O 2  plasma, UVO and the like in order to remove impurities from the surface of the anode  202 .  
         [0057]     When the impurities are removed from the surface of the anode  202 , properties of an interface between the anode  202  and a hole injection layer described below are enhanced, thereby allowing easy injection of holes.  
         [0058]     Then, an interface deterioration preventing layer (IDPL)  203  is formed on the anode  202 .  
         [0059]     Here, the interface deterioration preventing layer  203  consists of two components, that is, an organic material and an inorganic material.  
         [0060]     The organic material can be one selected from aromatic amine compounds having hole transmission properties.  
         [0061]     The aromatic amine compounds have a chemical structure of either  
                         
 
         [0062]     In the chemical structure, n is an integer selected from 1 to 4, and Ar 1 , Ar 2  and Ar 3  are either substituted or non-substituted aromatic groups, respectively.  
         [0063]     Examples of Ar 1 , Ar 2  and Ar 3  include phenyl, naphthyl, biphenylyl, biphenylylelnyl, phenanthrenyl, fluorenyl, terphenylyl and anthracenyl, and when the substituted aromatic groups are used, a substituent may be one selected from methyl, ethyl, propyl, t-buthyl, methoxy, ethoxy, propoxy, dimethylamine, diethylamine, phenyl fluorine. chlorine, bromine, and the like.  
         [0064]     According to the invention, the organic material has the formula, in particular, as shown in  FIG. 6  while satisfying the chemical structure as described above.  
         [0065]     The inorganic material of the interface deterioration preventing layer  203  is one selected from a halide or an oxide of 1A, 2A, 3A and 4A-groups of the periodic table. The halide includes LiF, NaF, KF, RbF, CsF, FrF, MgF 2 , CaF 2 , NaCl, CaCl 2 , LiCl, KCl, RbCl, CsCl, FrCl, MgCl 2  and the like. The oxide includes Li 2 O, Na 2 O, K 2 O, BeO, MgO, CaO, B 2 O 3 , Al 2 O 3  and SiO 2 , and the like.  
         [0066]     The interface deterioration preventing layer  203  is formed by mixing the organic and inorganic materials appropriately selected from the materials as described above.  
         [0067]     As for a method of forming the interface deterioration preventing layer  203 , there are three methods of providing the organic and inorganic materials as follows.  
         [0068]     As a first method, the organic material is deposited to a thickness of about 0.5 nm on the anode  202 , and the inorganic material is then deposited to a thickness of about 0.1 nm on the organic material in a layer-by-layer manner by a vacuum deposition process. In this method, it should be noted that the thickness of the deposited material is proposed as an example, and that the organic material and the inorganic material may have a thickness in the range of 0.1˜10 nm. At this time, the interface deterioration preventing layer  203  has a total thickness of 100 nm or less.  
         [0069]     As a second method, the organic and inorganic materials are mixed by co-depositing the organic and inorganic materials. At this time, a mixing ratio of these materials is set as the following relationship.  
         [0070]     If organic material:Inorganic material=X:Y, 1≦X≦100 and Y=1, or X=1 and 1≦Y≦100.  
         [0071]     The ratio of the organic material to the inorganic material is selected so as to satisfy the above relationship, and the total thickness of the interface deterioration preventing layer  203  is 100 nm or less.  
         [0072]     As a third method, when mixing the organic and inorganic material, the mixing ratio of the organic material and the inorganic material is changed according to a location in the device.  
         [0073]     Specifically, When X′=inorganic material/(organic material+inorganic material) and Y′=organic material/(organic material+inorganic material), X′ is 1 and Y′ is 0 at an interface between the anode  202  and the interface deterioration preventing layer  203 , whereas X′ is 0 and Y′ is 1 at an interface between an bole transport layer  204  and the interface deterioration preventing layer  203 . At this time, the values of X′ and Y′ are linearly changed at both interfaces, respectively. In this method, the total thickness of the interface deterioration preventing layer  203  is 100 nm or less.  
         [0074]     As such, the interface deterioration preventing layer  203  is formed by appropriately providing the organic material and the inorganic material as described above, so that thermal stress between the anode and the interface deterioration preventing layer  203  is relieved, and mobility of the holes is reduced, thereby optimizing balance of electric charge between holes and electrons resulting in enhancement of light emitting efficiency of the organic EL device.  
         [0075]     Next, an organic EL layer is formed on the interface deterioration preventing layer  203 .  
         [0076]     The organic EL layer has a multilayer structure in which a hole injection layer  204 , a hole transport layer  205 , a light emitting layer  206 , an electron transport layer (ETL)  207 , and an electron injection layer (ETL)  208  are sequentially stacked. In some cases, other layers may be further added thereto or some of the layers may be removed therefrom.  
         [0077]     Then, a cathode  209  is formed on the organic EL layer, thereby completing the organic EL device.  
         [0078]     For an inventive example, as shown in  FIG. 5 , the transparent substrate  201  was formed of a glass, the anode  202  was formed of ITO, and the interface deterioration preventing layer  203  was formed of NPD as an organic material and MgF 2  as an inorganic material.  
         [0079]     As a method for providing the organic and inorganic materials, the second method described above was used for forming the interface deterioration preventing layer  203  such that the organic material was mixed with the inorganic material in the ratio of NPD:MgF 2 =5:1 in terms of weight ratio, and the interface deterioration preventing layer  203  had a total thickness of about 10 nm.  
         [0080]     The hole injection layer (HIL)  204  for hole injection and the hole transport layer (HTL)  205  for hole transmission were formed to a thickness of about 50 nm by depositing NPD (4,4′-bis[N-(1-naphthyl)-N-phenyl-amino]biphenyl) on the interface deterioration preventing layer  203 .  
         [0081]     Then, the light emitting layer  206  was formed on the HTL  205 , and in the case of a green light emitting device, it was formed to a thickness of about 25 nm by doping about 1% coumine derivative (C545T) to Alq3 (8-hydroxyquinoline aluminum).  
         [0082]     The electron transport layer (ETL)  207  was formed to a thickness of about 35 nm by depositing Alq3 (8-hydroxyquinoline aluminum), and the electron injection layer (ETL)  208  was formed to a thickness of about 0.5 nm by depositing LiF. The cathode  209  was formed to a thickness of about 200 nm by depositing Al.  
         [0083]     Characteristics of the device of the invention manufactured as described above will be described as follows in comparison to a comparative example.  
         [0084]     As for the comparative example, an organic EL device was manufactured, which has the same construction as that of the inventive example shown in  FIG. 5 , except that the comparative example does not have an interface deterioration preventing layer (IDPD) and has an HIL (or HTL) consisting of NPD.  
         [0085]      FIG. 7  is a graph depicting results of a test of constant current acceleration for the organic EL device of the invention, and  FIG. 8  is a graph depicting results of a test of constant voltage acceleration for the organic EL device of the invention.  
         [0086]     As shown in  FIG. 7 , the comparative example and the inventive example comprising the IDPD were tested for constant current acceleration at a current density of 50 mA/cm 2 .  
         [0087]     As can be seen from  FIG. 7 , the device comprising the IDPL is remarkably suppressed in change of voltage during an operation time, and the life span of the device is about twice the life span of the comparative example since the IDPL suppresses interface deterioration.  
         [0088]     Additionally, as shown in  FIG. 8 , it can be seen that, when the device comprising the IDPD is driven by a predetermined voltage, the life span of the device is about ten times the life span of the comparative example.  
         [0089]     The following Table 1 shows the light emitting efficiency, the life span, and the voltage variation of the inventive example and the comparative example according to the test of constant current acceleration.  
                                                 TABLE 1                                   Inventive example   Comparative example           (IDPL)   (NPD)                                    Efficiency (cd/A)   ˜15       ˜12           (@5000nits)       Life span (hrs)   &gt;1200   hrs   ˜600   hrs       (@50 mA/cm2       ΔV   &lt;0.2   V   ˜2   V       @50 mA/cm2                  
 
         [0090]     As apparent from the above description, the organic EL device of the invention has an interface deterioration preventing layer consisting of a mixture of an organic material and an inorganic material inserted between an anode comprising ITO as the organic material, and HIL comprising the inorganic material, so that deterioration and thermal stress of the device is relieved, and so that the device is suppressed in voltage increase upon constant current driving while minimizing current decrease upon constant voltage driving.  
         [0091]     Additionally, an amount of holes injected into the organic EL device can be controlled, and a balance of electric charge between the holes and electrons can be optimized by controlling the amount of the inorganic material added to the interface deterioration preventing layer and the thickness of the interface deterioration preventing layer, thereby maximizing light efficiency of the device.  
         [0092]     It will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the spirit or scope of the inventions. Thus, it is intended that the present invention covers the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.