Patent Publication Number: US-2007108894-A1

Title: Organic electroluminescent device

Description:
TECHNICAL FIELD  
      The invention relates to an organic electroluminescent (EL) device.  
     BACKGROUND ART  
      In general, conventional organic electroluminescent (EL) devices have the following device structure:  
      (1) an anode/a hole-injecting layer/a hole-transporting layer/an emitting layer/an electron-transporting layer/a cathode,  
      (2) an anode/a hole-transporting layer/an emitting layer/an electron-transporting layer/a cathode, or  
      (3) an anode/a hole-injecting layer/a hole-transporting layer/an emitting layer/an electron-transporting layer/an electron-injecting layer/a cathode.  
       FIG. 5  shows the energy level of each of the constituting members in the organic EL devices ( 1 ).  
      As illustrated in  FIG. 5 , holes are injected from an anode  10  to a hole-injecting layer  20 , and further the holes are injected from the hole-injecting layer  20  to a hole-transporting layer  30 . The holes transported in the hole-injecting layer  20  and the hole-transporting layer  30  are finally injected to an emitting layer  40 . On the other hand, electrons are injected from a cathode  60  to an electron-transporting layer  50 , and further injected to the emitting layer  40 . In the emitting layer  40 , the holes are recombined with the electrons to emit light. An energy barrier E and another energy barrier E are present between the anode  10  and the hole-injecting layer  20  and between the hole-injecting layer  20  and the hole-transporting layer  30 , respectively. The holes need to go over the energy barriers E, so that a voltage loss is generated. When electrons are injected from the emitting layer  40  to the hole-transporting layer  30 , the electrons stay in the hole-transporting layer  30 , for instance, for the following reasons: the hole-transporting layer  30  has a low electron-transporting capability, and the electron barrier is present in the interface between the hole-transporting layer  30  and the hole-injecting layer  20 . Consequently, the hole-transporting layer  30  is deteriorated.  
      For example, an arylamine compound is conventionally used for the hole-injecting layer or the hole-transporting layer. However, the layer made of the arylamine compound is remarkably low in electron-transporting capability, and further the arylamine compound itself has no durability at the time of reduction (that is, electron injection). Consequently, in the case of aiming to extend the lifetime of an EL device, deterioration in the arylamine compound becomes a problem. Moreover, there arises a problem wherein voltage loss is caused by a high resistance of the arylamine compound layer and the applied voltage becomes high.  
      Thus, a technique has been discovered wherein an oxidizer or an acceptor is mixed into a hole-injecting layer in an amount of 20% or less by weight, thereby making the resistance of the hole-injecting layer low.  FIG. 6  shows the motion of electrons and holes when an acceptor is added to a hole-injecting layer. The hole-injecting layer  20  is made of a hole-injecting molecule A 1  and the acceptor molecule B, and a hole-transporting layer  30  is made of a hole-transporting molecule A 2 . Holes injected from an anode (not illustrated) are transported to an emitting layer 40 by action of the hole-injecting molecule A 1  in the hole-injecting layer  20  and the hole-transporting molecule A 2  in the hole-transporting layer  30 . The acceptor molecule B withdraws an electron from the hole-injecting molecule A 1  and further generates a hole. The holes generated herein are also transported to the emitting layer  40  by action of the hole-injecting molecule A 1  and the hole-transporting molecule A 2 . However, in the hole-injecting layer  20 , the acceptor molecule Bs are not adjacent to each other, so that electrons are not transported.  
      Known techniques for mixing an oxidizer or an acceptor into such a hole-injecting layer are for example, techniques of mixing into polyaniline a low molecular weight compound or polymer having a sulfonic acid group as an oxidizer (See, Non-patent Document 1 and 2, and Patent Document 1). Known are also techniques of vapor-depositing a hole-injecting material and an oxidizer simultaneously to make the resistance of a hole-injecting layer low (See, Patent Document 2 and Non-patent Document 3).  
      [Patent Document 1] Japanese Patent Application Laid-open (JP-A) No. 2005-108828  
      [Patent Document 2] JP-A No. 11-251067  
      [Non-patent Document 1] Nature, Vol. 357, 477-479, 1992  
      [Non-patent Document 2] Applied Physics Letters, Vol. 64, 1245-1247, 1994  
      [Non-patent Document 3] Jpn. J. Appl. Phys., Vol.41, 358, 2002  
      When these techniques are used, the applied voltage can be made low by a drop in the resistance. However, when electrons are injected to the hole-injecting layer, problems such that compounds therein are deteriorated are identified. Furthermore, there is a problem that the oxidizer aggregates so as to change with time. Accordingly, a long-lifetime device cannot be necessarily obtained.  
      In order to raise the capability of hole injection from the anode to the hole-injecting layer, it is necessary to reduce the injection barrier, which is decided by a difference between the work function of the anode and the ionization potential of the hole-injecting layer, as much as possible. Accordingly, a material higher in work function than ITO, which has been hitherto used as an anode, has been desired; however, a high-work-function material satisfying practical performances has not yet been discovered.  
      An object of the invention is to provide an organic EL device which has a long lifetime and requires only a low voltage even if various anode materials having a lower work function than ITO are used.  
     DISCLOSURE OF THE INVENTION  
      According to the invention, the following organic EL device can be provided. 
      1. An organic electroluminescent device comprising:    

      an emitting layer between an anode and a cathode,  
      an acceptor-containing layer, and  
      a hole-transporting layer,  
      the acceptor-containing layer and the hole-transporting layer being disposed between the anode and the emitting layer in this order from anode side. 
      2. An organic electroluminescent device according to 1, wherein the acceptor-containing layer comprises an organic compound having an electron-withdrawing substituent.     3. An organic electroluminescent device according to 2, wherein the organic compound having an electron-withdrawing substituent is a quinoid derivative.     4. An organic electroluminescent device according to any one of claims  1  to  3 , wherein the acceptor-containing layer contacts the hole-transporting layer.     5. An organic electroluminescent device according to any one of 1 to 3, wherein a buffer layer is interposed between the acceptor-containing layer and the hole-transporting layer.     6. An organic electroluminescent device according to 5, wherein the buffer layer is a doped layer.     7. An organic electroluminescent device according to 6, wherein the doped layer is an N-doped layer and/or a P-doped layer.     8. An organic electroluminescent device according to 5, wherein the buffer layer is a semiconductive oxide layer.     9. An organic electroluminescent device according to any one of 1 to 8, wherein an electron-donating compound is added to the acceptor-containing layer in a concentration lower than the concentration of an acceptor.     10. An organic electroluminescent device according to any one of 1 to 9, wherein the reduction potential of an acceptor is higher than the reduction potential of tetracyanoquinodimethane.     11. An organic electroluminescent device according to any one of 1 to 10, wherein electrons are transported from the buffer layer or a contacting surface between the acceptor-containing layer and the hole-transporting layer into the acceptor-containing layer toward the anode, and    

      holes are transported from the buffer layer or the contacting surface between the acceptor-containing layer and the hole-transporting layer into the hole-transporting layer toward the emitting layer.  
      According to the invention, an organic EL device which has a long lifetime and requires only a low driving voltage can be provided. The material used in its anode can be appropriately selected from various materials ranging from metals having a low work function to ITO independently of the ionization potential of the organic material.  
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a sectional view illustrating a first embodiment of the organic EL device according to the invention.  
       FIG. 2  is a view for explaining the motion of electrons and holes in an acceptor-containing layer, a hole-transporting layer and an emitting layer in the organic EL device in  FIG. 1 .  
       FIG. 3  is a diagram showing the energy level of each of the constituting members in the organic EL device in  FIG. 1 .  
       FIG. 4  is a sectional view illustrating a second embodiment of the organic EL device according to the invention.  
       FIG. 5  is a diagram showing the energy level of each constituting member in a conventional organic EL device.  
       FIG. 6  is a view for explaining the motion of electrons and holes when an acceptor is added to a hole-injecting layer in the-conventional organic EL device. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      The organic EL device of the invention includes an emitting layer interposed between an anode and a cathode, an acceptor-containing layer and a hole-transporting layer, and the acceptor-containing layer and the hole-transporting layer are disposed between the anode and the emitting layer in this order from the anode.  FIG. 1  illustrates a device structure of a first embodiment of the organic EL device according to the invention.  
      As illustrated in  FIG. 1 , an organic EL device  1  has a structure wherein an anode  10 , an acceptor-containing layer  70 , a hole-transporting layer  30 , an emitting layer  40 , an electron-transporting layer  50 , and a cathode  60  are stacked in this order.  
      In this device, the acceptor contained in the acceptor-containing layer  70  withdraws electrons from a contacting surface present between the layer  70  and the hole-transporting layer  30 , and simultaneously holes are generated. The acceptor-containing layer  70  has electron-transportability, and thus the electrons are transported from this contacting surface in the direction toward the anode  10  into the acceptor-containing layer  70 . Furthermore, the holes are transported from the contacting surface in the direction toward the emitting layer  40  into the hole-transporting layer  30 . On the other hand, electrons are injected from the cathode  60  to the electron-transporting layer  50 , and further injected to the emitting layer  40 . In the emitting layer  40 , the holes are recombined with the electrons to emit light.  
      When electrons are injected from the emitting layer  40  to the hole-transporting layer  30 , the electrons in the hole-transporting layer  30  flow into the acceptor-containing layer  70  so as to suppress the deterioration of the hole-transporting layer  30 .  
      With reference to  FIG. 2 , the motion of electrons and holes in the acceptor-containing layer, the hole-transporting layer and emitting layer is described. The acceptor-containing layer  70  is made of an acceptor molecule B, and the hole-transporting layer  30  is made of a hole-transporting molecule A. Based on the acceptor molecule B, holes and electrons are generated in or near the interface between the acceptor-containing layer  70  and the hole-transporting layer  30 . The holes are shifted in the hole-transporting layer  30  by the hole-transporting molecule A to be injected into the emitting layer  40 . On the other hand, the electrons are shifted in the direction toward the anode (not illustrated) in the acceptor-containing layer  70 .  
      As described above, in conventional devices, their hole-injecting layer has no electron transportability; therefore, injected electrons are present in the hole-transporting layer or the hole-injecting layer until the electrons are recombined with holes, and thus, cause deterioration of these layers.  
      However, even if electrons are injected from the emitting layer to the hole-transporting layer in the invention, the hole-transporting layer can be prevented from being deteriorated since the electrons flow from the electron-transportable acceptor-containing layer to the anode.  
      Further  FIG. 3  shows the energy level of each of the constituting members in the organic EL device shown in  FIG. 1 .  
      Since the acceptor-containing layer  70  has a high ionization potential as illustrated in this figure, no energy barrier is present between the layer  70  and the hole-transporting layer  30 .  
      In the present embodiment, therefore, it is unnecessary that holes go over the energy barrier E illustrated in  FIG. 5 , as in conventional devices. Thus, the voltage applied thereto is decreased. In other words, according to the device structure of the organic EL device of the embodiment, the device neither suffer any voltage loss in the energy barrier against hole injection which is present between the hole-injecting layer and the hole-transporting layer nor any voltage loss in the energy barrier against hole injection which is present between the anode and the hole-injecting layer. Accordingly, the voltage necessary for the device can be made low.  
      The acceptor used in the embodiment will be described later.  
      The device structure of the organic EL device of the invention is not limited to the structure illustrated in  FIG. 1 . For example, an electron-injecting layer is disposed, or the emitting layer can be made into a stacked body composed of two or more layers emitting different colors.  
      The hole-transporting layer can be made into a stacked body composed of two or more layers. For example, first and second hole-transporting layers which are made of different compounds can be disposed between the acceptor-containing layer and the emitting layer.  
      A second embodiment of the organic EL device will be described hereinafter.  
       FIG. 4  is a sectional view illustrating the second embodiment of the organic EL device according to the invention.  
      This embodiment is different from the first embodiment in that a buffer layer  80  is disposed between the acceptor-containing layer  70  and the hole-transporting layer  30 .  
      The buffer layer is a layer which generates an electric charge by itself, or a layer in which an electric charge is present in itself. Specific examples thereof include a doped layer, an electroconductive or semiconductive inorganic compound layer, an alkali metal layer, a metal halide layer, a metal complex layer, any combination thereof, and a combination of a metal complex layer and an A 1  thin layer reactive therewith. The buffer layer is preferably a doped layer or a semiconductive inorganic compound layer.  
      According to the device structure illustrated in  FIG. 4 , electrons are transported in the direction toward the anode from the buffer layer, which is present between the acceptor-containing layer and the hole-transporting layer, into the acceptor-containing layer while holes are transported in the direction toward the emitting layer from the buffer layer, which is present between the acceptor-containing layer and the hole-transporting layer, into the hole-transporting layer.  
      Since the carriers (electrons or holes) contributing to electric conduction are present in the buffer layer, the energy which the acceptor-containing layer requires for the withdrawing of the electrons is small. Thus, the voltage necessary for the device can be made lower.  
      In the case that the buffer layer is a doped layer, the doped layer is preferably an electron-transportable compound layer to which a reducing agent is added (an N doped layer), a hole-transportable compound layer to which an oxidizer or an acceptor (easily-reducible organic compound) which will be detailed later is added (a P doped layer), or a stacked body composed of an N doped layer and a P doped layer. The added amount of the oxidizer or the reducing agent is usually 20% or less by weight.  
      As the reducing agent, there is preferably used an alkali metal, an alkaline earth metal, a rare earth metal, an alkali metal complex, an alkaline earth metal complex, a rare earth metal complex, an alkali metal halide, an alkaline earth metal halide, a rare earth metal halide or the like.  
      As the oxidizer, a Lewis acid, an acceptor which will be detailed later or the like is preferably used. Preferred examples of the Lewis acid include iron chloride, antimony chloride, and transition metal oxides such as vanadium oxide and molybdenum oxide.  
      As the electron transporting compound, known compounds may be used, but a metal complex of 8-hydroxyquinoline or a derivative thereof may preferably be used.  
      Specific examples of the above-mentioned metal complex of 8-hydroxyquinoline or derivative include metal chelate oxynoid compounds (for example, Alq) containing a chelate of oxine (generally, 8-quinolinol or 8-hydroxyquinoline).  
      As the electron transporting compound, an oxadiazole derivative may preferably be used. Examples of the oxadiazole derivative include electron-transferring compounds represented by the following general formulas:  
                 
 
 wherein Ar 5′ , Ar 6′ , Ar 7′ , Ar 9′ , Ar 10′  and Ar 13′  each represent a substituted or unsubstituted aryl group and may be the same as or different from each other, and Ar 8′ , Ar 11′  and Ar 12′  represent substituted or unsubstituted arylene groups and may be the same as or different from each other. 
 
      Examples of the aryl group include phenyl, biphenyl, anthranyl, perylenyl, and pyrenyl groups. Examples of the arylene group include phenylene, naphthylene, biphenylene, anthranylene, perylenylene, and pyrenylene groups. Examples of the substituent include alkyl groups with 1 to 10 carbon atoms, alkoxy groups with 1 to 10 carbon atoms, and a cyano group. The electron transferring compounds are preferably ones having capability of forming a thin film.  
      Specific examples of the electron transferring compounds include the following:  
                 
 
      Nitrogen-containing heterocyclic derivatives represented by the following formula:  
                 
 
 wherein A 3′  to A 5′  are each a nitrogen atom or a carbon atom. 
 
      R is an aryl group which has 6 to 60 carbon atoms and may have a substituent, a heteroaryl group which has 3 to 60 carbon atoms and may have a substituent, an alkyl group which has 1 to 20 carbon atoms, a haloalkyl group which has 1 to 20 carbon atoms, or an alkoxy group which has 1 to 20 carbon atoms; n is an integer of 0 to 5 and when n is an integer of 2 or more, Rs may be the same as or different from each other.  
      Adjacent Rs may be bonded to each other to form a substituted or unsubstituted carbocyclic aliphatic ring or a substituted or unsubstituted carbocyclic aromatic ring.  
      Ar 14  is an aryl group which has 6 to 60 carbon atoms and may have a substituent, or a heteroaryl group which has 3 to 60 carbon atoms and may have a substituent.  
      Ar 15  is a hydrogen atom, an alkyl group which has 1 to 20 carbon atoms, a haloalkyl group which has 1 to20 carbon atoms, an alkoxy group which has 1 to 20 carbon atoms, an aryl group which has 6 to 60 carbon atoms and may have a substituent, or a heteroaryl group which has 3 to 60 carbon atoms and may have a substituent.  
      Provided that either one of Ar 14  and Ar 15  is a condensed cyclic group which has 10 to 60 carbon atoms and may have a substituent or a condensed heterocyclic group which has 3 to 60 carbon atoms and may have a substituent.  
      L 1  and L 2 are each a single bond, a condensed cyclic group which has 6 to 60 carbon atoms and may have a substituent, a condensed heterocyclic group which has 3 to 60 carbon atoms and may have a substituent, or a fluorenylene group which may have a substituent.  
      Nitrogen-containing heterocyclic derivatives represented by the following formula: 
 
HAr-L 3 -Ar 16 -Ar 17  
 
 wherein HAr is a nitrogen-containing heterocyclic ring, with 3 to 40 carbon atoms which may have a substituent; 
 
      L 3  is a single bond, an arylane group with 6 to 60 carbon atoms which may have a substituent, a heteroarylane group with 3 to 60 carbon atoms which may have a substituent or a fluorenylene group which may have a substituent;  
      Ar 16  is a bivalent aromatic hydrocarbon group with 6 to 60 carbon atoms which may have a substituent; and  
      Ar 17  is an aryl group with 6 to 60 carbon atoms which may have a substituent or a heteroaryl group with 3 to 60 carbon atoms which may have a substituent.  
      An electroluminescent device using a silacyclopentadiene derivative represented by the following formula, disclosed in JP-A-09-087616:  
                 
 
 wherein Q 1  and Q 2  are each a saturated or unsaturated hydrocarbon group with 1 to 6 carbon atoms, an alkoxy group, an alkenyloxy group, an alkynyloxy group, a hydroxyl group, a substituted or unsubstituted aryl group, or a substituted or unsubstituted hetero ring, or Q 1  and Q 2  are bonded to each other to form a saturated or unsaturated ring; R 11  to R 14  are each a hydrogen atom, a halogen atom, a substituted or unsubstituted alkyl group with 1 to 6 carbon atoms, an alkoxy group, an aryloxy group, a perfluoroalkyl group, a perfluoroalkoxy group, an amino group, an alkylcarbonyl group, an arylcarbonyl group, an alkoxycarbonyl group, an aryloxycarbonyl group, an azo group, an alkylcarbonyloxy group, an arylcarbonyloxy group, an alkoxycarbonyloxy group, an aryloxycarbonyloxy group, a sulfinyl group, a sulfonyl group, a sulfanyl group, a silyl group, a carbamoil group, an aryl group, a heterocyclic group, an alkenyl group, an alkynyl group, a nitro group, a formyl group, a nitroso group, a formyloxy group, an isocyano group, a cyanate group, an isocyanate group, a thiocyanate group, an isothiocyanate group or a cyano group, or adjacent groups of R 11  to R 14  may be joined to form a substituted or unsubstituted condensed ring. 
 
      Silacyclopentadiene derivative represented by the following formula, disclosed in JP-A-09-194487: 
                 
 
 wherein Q 3  and Q 4  are each a substituted or unsubstituted hydrocarbon group with 1 to 6 carbon atoms, an alkoxy group, an alkenyloxy group, an alkynyloxy group, a substituted or unsubstituted aryl group, or a substituted or unsubstituted heterocyclic group, or Q 3  or Q 4  are bonded to each other to form a substituted or unsubstituted ring; R 15  to R 18  are each a hydrogen atom, a halogen atom, a substituted or unsubstituted alkyl group with 1 to 6 carbon atoms, an alkoxy group, an aryloxy group, a perfluoroalkyl group, a perfluoroalkoxy group, an amino group, an alkylcarbonyl group, an arylcarbonyl group, an alkoxycarbonyl group, an aryloxycarbonyl group, an azo group, an alkylcarbonyloxy group, an arylcarbonyloxy group, an alkoxycarbonyloxy group, an aryloxycarbonyloxy group, a sulfinyl group, a sulfonyl group, a sulfanyl group, a silyl group, a carbamoil group, an aryl group, a heterocyclic group, an alkenyl group, an alkynyl group, a nitro group, a formyl group, a nitroso group, a formyloxy group, an isocyano group, a cyanate group, an isocyanate group, a thiocyanate group, an isothiocyanate group, a cyano group, or a substituted or unsubstituted condensed ring structure formed by adjacent substituents of R 15  to R 18 : however, in the case where R 15  and R 18  are each a phenyl group, Q 3  and Q 4  are neither an alkyl group nor a phenyl group; in the case where R 15  and R 18  are each a thienyl group, Q 3 , Q 4 , R 16  and R 17  do not form the structure where Q 3  and Q 4  are a monovalent hydrocarbon group, and at the same time R 16  and R 17  are an alkyl group, an aryl group, an alkenyl group, or an aliphatic group with a cycle formed by R 16  and R 17  bonded; in the case where R 15  and R 18  are a silyl group, R 16 , R 17 , Q 3  and Q 4  are each neither a monovalent hydrocarbon group with 1 to 6 carbon atoms nor a hydrogen atom; and in the case where R 15  and R 16 are bonded to form a condensed structure with a benzene ring, Q 3  and Q 4  are neither an alkyl group nor a phenyl group. 
 
      Borane derivatives represented by the following formula, disclosed in JP-Al-2000-040586:  
                 
 
 wherein R 19  to R 26  and Q 8  are each a hydrogen atom, a saturated or unsaturated hydrocarbon group, an aromatic group, a heterocyclic group, a substituted amino group, a substituted boryl group, an alkoxy group or an aryloxy group; Q 5 , Q 6  and Q 7  are each a saturated or unsaturated hydrocarbon group, an aromatic group, a heterocyclic group, a substituted amino group, an alkoxy group or an aryloxy group; the substituents of Q 7 and Q 8  may be bonded to each other to form condensed rings; r is an integer of 1 to 3, and Q 7 s may be different from each other when r is 2 or more; provided that excluded are the compounds where r is 1, Q 5 , Q 6  and R 20  are each a methyl group and R 26  is a hydrogen atom or a substituted boryl group, and the compounds where r is 3 and Q 7  is a methyl group. 
 
      Compounds represented by the following formula, disclosed in JP-A-10-088121: 
                 
 
 wherein Q 9  and Q 10  are independently a ligand represented by the following formula; and L 4  is a halogen atom, a substituted or unsubstituted alkyl group, a substituted or unsubstituted cycloalkyl group, a substituted or unsubstituted aryl group, a substituted or unsubstituted heterocyclic group, —OR 27  wherein R 27  is a hydrogen atom, a substituted or unsubstituted alkyl group, a substituted or unsubstituted cycloalkyl group, a substituted or unsubstituted aryl group, a substituted or unsubstituted heterocyclic group, or —O—Ga-Q 11  (Q 12 ) wherein Q 11  and Q 12  are the same ligands as Q 9  and Q 10 .  
                 
 
 wherein rings A 4 and A 5  are each a 6-membered aryl ring structure which may have a substituent, and are condensed to each other. 
 
      The metal complexes have the strong nature of an n-type semiconductor and large ability of injecting electrons. Further the energy generated at the time of forming a complex is small so that a metal is then strongly bonded to ligands in the complex formed and the fluorescent quantum efficiency becomes large as the emitting material.  
      Specific examples of the rings A 4  and A 5  which form the ligands of the above formula include halogen atoms such as chlorine, bromine, iodine and fluorine; substituted or unsubstituted alkyl groups such as methyl, ethyl, propyl, butyl, sec-butyl, tert-butyl, pentyl, hexyl, heptyl, octyl, stearyl and trichloromethyl; substituted or unsubstituted aryl groups such as phenyl, naphthyl, 3-methylphenyl, 3-methoxyphenyl, 3-fluorophenyl, 3-trichloromethylphenyl, 3-trifluoromethylphenyl and 3-nitrophenyl; substituted or unsubstituted alkoxy groups such as methoxy, n-butoxy, tert-butoxy, trichloromethoxy, trifluoroethoxy, pentafluoropropoxy, 2,2,3,3-tetrafluoropropoxy, 1,1,1,3,3,3-hexafluoro-2-propoxy and 6-(perfluoroethyl)hexyloxy; substituted or unsubstituted aryloxy groups such as phenoxy, p-nitrophenoxy, p-tert-butylphenoxy, 3-fluorophenoxy, pentafluorophenyl and 3-trifluoromethylphenoxy; substituted or unsubstituted alkylthio groups such as methythio, ethylthio, tert-butylthio, hexylthio, octylthio and trifruoromethyltio; substituted or unsubstituted arylthio groups such as phenylthio, p-nitrophenylthio, p-tert-butylphenylthio, 3-fluorophenylthio, pentafluorophenylthio and 3-trifluoromethylphenylthio; a cyano group; a nitro group, an amino group; mono or di-substituted amino groups such as methylamino, dimethylamino, ethylamino, diethylamino, dipropylamino, dibutylamino and diphenylamino; acylamino groups such as bis(acetoxymethyl)amino, bis(acetoxyethyl)amino, bis(acetoxypropyl)amino and bis(acetoxybutyl)amino; a hydroxy group; a siloxy group; an acyl group; carbamoyl groups such as methylcarbamoyl, dimethylcarbamoyl, ethylcarbamoyl, diethylcarbamoyl, propylcarbamoyl, butylcarbamoyl and phenylcarbamoyl; a carboxylic group; a sulfonic acid group; an imido group; cycloalkyl groups such as cyclopentyl and cyclohexyl; aryl groups such as phenyl, naphthyl, biphenyl, anthranyl, phenanthryl, fluorenyl and pyrenyl; and heterocyclic groups such as pyridinyl, pyrazinyl, pyrimidinyl, pryidazinyl, triazinyl, indolinyl, quinolinyl, acridinyl, pyrrolidinyl, dioxanyl, piperidinyl, morpholidinyl, piperazinyl, triathinyl, carbazolyl, furanyl, thiophenyl, oxazolyl, oxadiazolyl, benzooxazolyl, thiazolyl, thiadiazolyl, benzothiazolyl, triazolyl, imidazolyl, benzoimidazolyl and puranyl. Moreover the above-mentioned substituents may be bonded to each other to form a six-membered aryl or heterocyclic ring.  
      As the hole transporting compound, known compounds may be used.  
      Specific examples thereof include triazole derivatives (see U.S. Pat. No. 3,112,197 and others), oxadiazole derivatives (see U.S. Pat. No. 3,189,447 and others), imidazole derivatives (see JP-B-37-16096 and others), polyarylalkane derivatives (see U.S. Pat. Nos. 3,615,402, 3,820,989 and 3,542,544, JP-B-45-555 and 51-10983, JP-A-51-93224, 55-17105, 56-4148, 55-108667, 55-156953and56-36656, and others), pyrozoline derivatives and pyrozolone derivatives (see U.S. Pat. Nos. 3,180,729 and 4,278,746, JP-A-55-88064, 55-88065, 49-105537, 55-51086, 56-80051, 56-88141, 57-45545, 54-112637 and 55-74546, and others), phenylene diamine derivatives (see U.S. Pat. No. 3,615,404, JP-B-51-10105, 46-3712 and 47-25336, JP-A-54-53435, 54-110536 and 54-119925, and others), arylamine derivatives (see U.S. Pat. Nos. 3,567,450, 3,180,703, 3,240,597, 3,658,520, 4,232,103, 4,175,961 and 4,012,376, JP-B-49-35702 and 39-27577, JP-A-55-144250, 56-119132 and 56-22437, DE1,110,518, and others), amino-substituted chalcone derivatives (see U.S. Pat. No. 3,526,501, and others), oxazole derivatives (ones disclosed in U.S. Pat. No. 3,257,203, and others), styrylanthracene derivatives (see JP-A-56-46234, and others), fluorenone derivatives (JP-A-54-110837, and others), hydrazone derivatives (see U.S. Pat. No. 3,717,462, JP-A-54-59143, 55-52063, 55-52064, 55-46760, 55-85495, 57-11350, 57-148749 and 2-311591, and others), stilbene derivatives (see JP-A-61-210363, 61-228451, 61-14642, 61-72255, 62-47646, 62-36674, 62-10652, 62-30255, 60-93455, 60-94462, 60-174749 and 60-175052, and others), silazane derivatives (U.S. Pat. No. 4,950,950), polysilanes (JP-A-2-204996), aniline copolymers (JP-A-2-282263), and electroconductive macromolecular oligomers (in particular thiophene oligomers) disclosed in JP-A-1-211399.  
      The formation of the N-doped layer makes it possible that the acceptor-containing layer withdraws a larger number of electrons to make the driving voltage of the organic EL device lower.  
      The formation of the P-doped layer makes it possible that a larger number of holes are sent to the hole-transporting layer to make the driving voltage of the organic EL device lower.  
      The formation of the stacked body composed of an N-doped layer and a P-doped layer makes it possible that the acceptor-containing layer withdraws a larger number of electrons and further a larger number of holes are sent to the hole-transporting layer to make the driving voltage of the organic EL device lower still.  
      In the case that the buffer layer is a semiconductive inorganic compound layer, the semiconductive inorganic compound layer is preferably made of a transition metal oxide. Specific examples of the transition metal oxide include NbO, LaO, NdO, SmO, EuO x , MoO 3 , MoO 2 , ReO 2 , ReO 3 , OSO 2 , IrO 2 , and PtO 2 . Preferred are LiTi 2 O 4 , LiV 2 O 4 , Er x NbO 3 , LaTiO 3 , SrVO 3 , CaCrO 3 , Sr x CrO 3 , A x MoO 3 , and AV 2 O 5  wherein A=K, Cs, Rb, Sr, Na, Li or Ca.  
      The same advantageous effects as produced by the doped layer can also be expected by the formation of the semiconductive inorganic compound layer.  
      The following will describe the acceptor.  
      The acceptor is an easily-reducible organic compound.  
      The easiness of the reduction of a compound can be measured based on the reduction potential thereof. In the invention, the acceptor is preferably a compound having a reduction potential of −0.8 V or more, and is more preferably a compound having a higher reduction potential than that (approximately 0 V) of tetracyanoquinodimethane (TCNQ). The reduction potential is a reduction potential measured by use of a saturated calomel electrode (SCE) as a reference electrode.  
      The easily-reducible organic compound is preferably an organic compound having an electron-withdrawing substituent. Specific examples thereof include quinoide derivatives, arylborane derivatives, and imide derivatives. Examples of the quinoide derivatives include quinodimethane derivatives, thiopyrandioxide derivatives, and quinone derivatives.  
      Preferred examples of the quinoide derivatives include compounds represented by the following formulae (I) to (III):  
                 
 
      In the formula (I), R 1   1  R 2   1  R 3  and R 4  are each a hydrogen or halogen atom, or an alkyl, alkoxy, nitro, cyano, fluoroalkyl, alkoxycarbonyl or aryl group. Preferred is a hydrogen or halogen atom, or a cyano or trifluoromethyl group. Some of the carbon atoms which constitute R 1  and R 2 , or R 3  and R 4  may be bonded to each other so that R 1  and R 2 , or R 3  and R 4  may form a saturated or unsaturated 5-membered or 6-membered ring. In this case, the ring may contain a nitrogen atom.  
      R 5  and R 6  are each an electron-withdrawing group, and examples thereof include an oxygen atom, and dicyanomethylene, dicyanocarbonylmethylene, cyanoimino, cyanoalkoxycarbonylmethylene, dialkoxycarbonylmethylene, dicarbonylmethylene and cyanocarbonylmethylene groups, which are represented by the following formulae (Q1) to (Q9) or the like. R 5  and R 6 each maybe a cyclic electron-withdrawing group.  
                 
                 
 
 wherein R 25 , R 26  R 27 and R 28  are each an alkyl, fluoroalkyl, or aryl group, some of the carbon atoms which constitute R 27  and R 28  may be bonded to each other so that R 27  and R 28  may form a saturated or unsaturated 5-membered or 6-membered ring, and in this case the ring may contain a nitrogen, oxygen, sulfur, selenium or tellurium atom. 
 
      Preferred is a dicyanomethylene, dicyanocarbonylmethylene or cyanoimino group.  
      In the formula (II), R 11  and R 12  are electron-withdrawing groups equal to R 5  and R 6  in the formula (I), and R 7 , R 8 , R 9 , R 10 , R 13 , R 14 , R 15  and R 16  are equal to R 1  to R 4  in the formula (I). Some of the carbon atoms which constitute these groups may be bonded to each other so that these groups may form a saturated or unsaturated 5-membered or 6-membered ring, and in this case the ring may contain a nitrogen, oxygen, sulfur, selenium or tellurium atom.  
      In the formula (III), R 17  and R 18  are electron-withdrawing groups equal to R 5  and R 6  in the formula (I), and R 19 , R 20 , R 21 , R 22 , R 23 , and R 24  are equal to R 1  to R 4  in the formula (I). Some of the carbon atoms which constitute these groups may be bonded to each other so that these groups may form a saturated or unsaturated 5-membered or 6-membered ring, and in this case the ring may contain a nitrogen, oxygen, sulfur, selenium or tellurium atom.  
      As the thiopyrandioxide compounds, a compound represented by the following formula (IV) is also preferably used.  
                 
 
      In the formula, R 29  represents an electron-withdrawing group, and is equal to R 5  in the formula (I).  
      R 31  and R 32  are each a hydrogen atom, or an alkyl or aryl group. Preferred is a hydrogen atom or an aryl group.  
      R 30  and R 33  are each a hydrogen or halogen atom, or a fluoroalkyl or alkoxycarbonyl group. Preferred is a hydrogen or halogen atom, or a fluoroalkyl group. Some of the carbon atoms which constitute R 30  and R 31 , or R 32  and R 33  may be bonded to each other so that R 30  and R 31  or R 32  and R 33  may form a condensed, unsaturated 6-membered ring.  
      Furthermore, in the formulae (I) to (IV), the electron withdrawing groups may each be a substituent (x) or (y) represented by the following formula:  
                 
 
      In the formula, Ar 1  and Ar 2 are each a substituted or unsubstituted heterocyclic group, or substituted or unsubstituted aryloxycarbonyl or aldehyde, and are each preferably pyridine, pyrazine, or quinoxaline. Ar 1 and Ar 2  may be bonded to each other to form a 5-membered or 6-membered cyclic structure.  
      Preferred examples of the arylborane derivatives include compounds having at least one fluorine as a substituent positioned on the aryl. Particularly preferred is tris β-(pentafluoronaphthyl)borane (PNB).  
      Preferred examples of the imide derivatives include naphthalene tetracarboxylic acid diimide compounds and pyromellitic acid diimide compounds.  
      In the invention, an acceptor is mixed into the acceptor-containing layer so that the layer can transport electrons. The acceptor content is preferably more than 20% by weight of the total of the layer. In order to set the electron mobility of the acceptor-containing layer to be more than 10 −5 , the acceptor content is more preferably 40% or more by weight, even more preferably 50% or-more by weight.  
      The acceptor-containing layer has electron-transportability, and this matter means that the layer is relatively electron-transportable rather than hole-transportable.  
      In order to confirm the electron-transportability of the acceptor-containing layer, various methods may be used. For example, the transportability can be confirmed by any one of the following methods (1) to (3): 
      (1) a method of sandwiching a thin film having a thickness of 2 to 10 μm and the same composition as the acceptor-containing layer between electrodes, then causing optical pumping therein by a laser ray from the cathode side thereof, and then measuring the transient optical current (the time-of-flight or TOF method),     (2) a method of sandwiching a thin film having the same composition-as the acceptor-containing layer between electrodes, the cathode thereof being an electron-injecting electrode made of Mg:Ag, Al/LiF or the like, applying a step-form voltage thereto, and then measuring the electron mobility by determining the shape of the transient current, and     (3) a method of sandwiching a thin film having the same composition as the acceptor-containing layer between electrodes wherein only electron injection is caused (for example, Al or Al/LiF), and then measuring the value of the current.    

      An electron-donating compound may be added to the acceptor-containing layer. The added amount of the electron-donating compound is preferably smaller than the concentration of the acceptor. For example, the added amount thereof is from 1 to 20% by weight of the total of the layer.  
      The addition of the electron-donating compound makes it possible to improve the electron-conductivity of the acceptor-containing layer to make the driving voltage of the organic EL device lower and prevent the hole-transporting layer from being deteriorated.  
      Examples of the electron-donating compound include inorganic materials such as alkali metals, alkaline earth metals, rare earth elements, Al, Ag, Cu and In; and organic materials such as anilines, phenylenediamines, benzidines (such as N,N,N′,N′-tetraphenylbenzidine, N,N′-bis-(3-methylphenyl)-N,N′-bis-(phenyl)-benzidine, and N,N′-di(naphthalene-1-yl)-N,N′-diphenyl-benzidine), compounds having as their skeleton an aromatic tertiary amine such as triphenylamines (such as triphenylamine, 4,4′,4″-tris(N,N-diphenyl-amino)-triphenylamine, 4,4′,4″-tris(N-3-methylphenyl-N-phenyl-amino)-triphenylamine, and 4,4′,4″-tris(N-(1-naphthyl)-N-phenyl-amino)-triphenylamine), and triphenyldiamines (such as N,N′-di-(4-methyl-phenyl)-N,N′-diphenyl-1,4-phenylenediamine), condensed polycyclic compounds (these condensed polycyclic compounds may have a substituent) such as pyrene, perylene, anthracene, tetracene and pentacene, and TTF (tetrathiafluvalene).  
     EXAMPLES  
     Example 1  
      &lt;Reduction Potential of a Material Used in an Acceptor-Containing Layer&gt; 
      A compound A described below was selected as a material for forming an acceptor-containing layer. In a cyclic voltammetric measurement thereof wherein a saturated calomel electrode (SCE) was used as a reference electrode, the reduction potential thereof was 0.71 V.  
                 
 
 &lt;Confirmation of Electron-Transportability&gt;
 
      Al as an electrode material and the compound A as a material for forming an acceptor-containing layer were each mounted on a molybdenum heating boat of a vacuum deposition device. A 150 nm thickness Al layer, a 200 nm thickness layer made of the compound A, and a 150 nm thickness Al layer were formed on a 0.7 mm thickness glass substrate in this order.  
      A voltage of 1 V was applied across the resultant two Al electrodes, and the value of the current flowing therein was measured. The value was 251 mA/cm 2 .  
      (Fabrication of Organic EL Device)  
      An ITO film was formed on a 0.7 mm thick glass substrate by using a sputtering method to a thickness of 130 nm. The substrate was subjected to ultrasonic cleaning in isopropyl alcohol for 5 minutes, and cleaned with ultraviolet ozone for 30 minutes. Then the substrate with the ITO electrode was mounted on a substrate holder in a vacuum vapor deposition apparatus.  
      The compound A as a material for an acceptor-containing layer, HT1 as a first hole-transporting material, HT2 as a second hole-transporting material, BH as a host material for an emitting layer, BD as a blue emitting material, Alq 3  as an electron-transporting material, LiF as an electron-injecting material and Al as a cathode material were mounted on respective molybdenum heating boats in advance. Moreover, MoO 3  was mounted as a semiconductive oxide material.  
                 
 
      First, a compound A film which functioned as the acceptor-containing layer was formed to a thickness of 10 nm. After forming the acceptor-containing layer, a HT2 film which functioned as the second hole-transporting layer was formed to a thickness of 50 nm. Subsequently, a HT 1  film which functioned as the first hole-transporting layer was formed to a thickness of 20 nm. After forming the HT1 film, a compound BH and compound BD were co-deposited to a thickness of 40 nm at a ratio of 40:2 as the emitting layer. An Alq 3  film was formed on the above film to a thickness of 20 nm as the electron-transporting layer. Then a LiF film was deposited to a thickness of 1 nm as the electron-injecting layer, and an Al film which functioned as the cathode was formed on the above film to a thickness of 150 nm to obtain an organic EL device.  
     Comparative Example 1  
      An organic EL device was fabricated in the same manner as in. Example 1 except that no acceptor-containing layer film was formed, and the thickness of the HT2 film as the second hole-transporting layer was 60 nm.  
     Example 2  
      An organic EL device was fabricated in the same manner as in Example 1 except that after forming the acceptor-containing layer, a P-doped layer was formed to a thickness of 50 nm at a ratio of the second hole-transporting material HT2 to compound A of 100:5, and the HT1 film as the first hole-transporting layer was then formed to a thickness of 20 nm.  
     Example 3  
      An organic EL device was fabricated in the same manner as in Example 1 except that after forming the acceptor-containing layer, a molybdenum oxide MoO 3  film was formed to a thickness of 5 nm.  
     Example 4  
      An organic EL device was fabricated in the same manner as in Example 1 except that an Al film was formed on the ITO film to a thickness of 5 nm as an anode, and the thickness of the HT2 film was 15 nm.  
                                                           TABLE 1                                           Semi-       Second   First       Electron-                       Acceptor-   conductive       hole-   hole-       trans-   Electron-               containing   oxide   P-doped   transporting   transporting   Emitting   porting   injecting           Anode   layer   layer   layer   layer   layer   layer   layer   layer   Cathode                                                                                Example 1   ITO   Compound A   —   —   HT2   HT1   BH/BD   Alq 3     LiF   Al       Comparative   ITO   —   —   —   HT2   HT1   BH/BD   Alq 3     LiF   Al       Example 1       Example 2   ITO   Compound A   —   HT2/   —   HT1   BH/BD   Alq 3     LiF   Al                       Compound A       Example 3   ITO   Compound A   MoO 3     —   HT2   HT1   BH/BD   Alq 3     LiF   Al       Example 4   ITO/Al   Compound A   —   —   HT2   HT1   BH/BD   Alq 3     LiF   Al                  
 
 (Evaluation of Organic EL Device) 
 
      The following evaluations were conducted for the organic EL devices obtained in Example 1, Comparative Example 1, Example 2, Example 3 and Example 4. The results are shown in Table 2. 
      (1) Voltage (unit:V) at the time of applying a current between ITO and Al such that a current density was 10 mA/cm 2  was measured.    

      (2) EL spectrum at the time of applying voltage at a current density of 10 MA/cm 2  was measured with a spectroradiometer CS1000A (manufactured by Konica Minolta Holdings, Inc.), and the chromaticity and luminous efficiency (unit:cd/A) were calculated.  
                                   TABLE 2                                   Voltage           Luminous efficiency           (v)   CIE x   CIE y   (cd/A)                                                        Example 1   6.3   0.15   0.17   6.1       Comparative   6.9   0.15   0.17   6.3       Example 1       Example 2   5.8   0.15   0.18   6.6       Example 3   6.2   0.15   0.17   6.0       Example 4   6.0   0.12   0.18   7.9                  
 
      The above table confirmed that the voltages were lowered, and equal or more luminous efficiencies were shown in Examples 1 to 4 compared with Comparative Example 1.  
      Moreover, as shown in Example 4, even an electrode having a work function smaller than 4.8 eV such as Al electrode (work function of 4.1 eV) can emit light with a similar luminance at a lower voltage compared to heretofore. Considering that conventionally emissions could be observed only at a voltage exceeding 10V, the invention shows a great effect.  
      The above Examples describe the bottom emission structure, but it is apparent that the invention can also be applied to the top emission structure. The invention can especially have a structure of an organic medium/light transparent cathode including a reflective metal layer/acceptor-containing layer/emitting layer. For the reflective metal, Al, Ag, Ni, No, W, Ta, Ti, Cr and alloys thereof are often used. However, even when using metal or alloy layers having a work function smaller than 4.8 eV, light emission at a low voltage is enabled. Conventionally in the case where a reflective metal directly contacts a hole-transporting layer or hole-injecting layer, the voltage substantially increases.  
     INDUSTRIAL APPLICABILITY  
      The organic EL device of the invention can be used as organic EL materials in various colors including blue. The device can be used in fields such as various display apparatuses, display, back light, light source, signs, signboard and interior; and especially suitable as a display device for color display.