Organic electroluminescent device

An object of the invention is to achieve an organic EL device which has an extended life, weather resistance, high stability, and high efficiency, and is inexpensive as well. This object is accomplished by the provision of an organic EL device comprising a substrate, a pair of a hole injecting electrode and a cathode formed on the substrate, and an organic layer located between these electrodes and taking part in at least a light emission function, wherein between the organic layer and the cathode there is provided an inorganic insulating electron injecting and transporting layer comprising a first component comprising at least one oxide selected from the group consisting of lithium oxide, rubidium oxide, potassium oxide, sodium oxide and cesium oxide, a second component comprising at least one oxide selected from the group consisting of strontium oxide, magnesium oxide and calcium oxide, and a third component comprising silicon oxide and/or germanium oxide.

BACKGROUND OF THE INVENTION
 1. Technical Art
 The present invention relates generally to an organic EL
 (electroluminescent) device, and more specifically to an inorganic/organic
 junction structure used for a device comprising an organic compound thin
 film which emits light at an applied electric field.
 2. Background Art
 In general, an organic EL device is basically built up of an ITO or other
 transparent electrode formed on a glass substrate, an organic amine base
 hole transporting layer laminated on the transparent electrode, an organic
 light emitting layer formed of a material having electronic conductivity
 and giving out strong light emission, for instance, an Alq.sup.3 material,
 and an electrode provided on the organic light emitting layer and formed
 of a material having a low work function, for instance, an MgAg material.
 As reported so far in the art, the device has a structure wherein one or
 plural organic compound layers are interleaved between a hole injecting
 electrode and an electron injecting electrode. The organic compound layer
 has a double- or triple-layer structure.
 Examples of the double-layer structure are a structure wherein a hole
 transporting layer and a light emitting layer are formed between the hole
 injecting electrode and the electron injecting electrode, and a structure
 wherein a light emitting layer and an electron transporting layer are
 formed between the hole injecting electrode and the electron injecting
 electrode. In an exemplary triple-layer structure, a hole transporting
 layer, a light emitting layer and an electron transporting layer are
 provided between the hole injecting electrode and the electron injecting
 electrode. A single-layer structure wherein a single layer has all
 functions, too, is reported in conjunction with a polymer or mixture
 system.
 Typical structures of the organic EL device are shown in FIGS. 3 and 4.
 In FIG. 3, a hole transporting layer 14 and a light emitting layer 15, each
 made of an organic compound, are formed between a hole injecting electrode
 12 provided on a substrate 11 and an electron injecting electrode 13. In
 this case, the light emitting layer 15 also functions as an electron
 transporting layer.
 In FIG. 4, a hole transporting layer 14, a light emitting layer 15 and an
 electron transporting layer 16, each made of an organic compound, are
 formed between a hole injecting electrode 12 provided on a substrate 11
 and an electron injecting electrode 13.
 A problem common to these organic EL devices is reliability. In principle,
 an organic EL device comprises a hole injecting electrode and an electron
 injecting electrode and requires an organic layer for efficient injection
 and transportation of holes and electrons from between these electrodes.
 However, these materials are sensitive to damages during device
 fabrication, and offer a problem in conjunction with an affinity for
 electrodes. For the electron injecting electrode for the injection of
 electrons, it is required to use a metal having a low work function. For
 this reason, there is no choice but to use MgAg, AlLi, etc. for materials.
 However, these materials are susceptible to oxidation and lack stability,
 and so are a grave factor responsible for a reduction in the service life
 of the organic EL device and a reliability problem. A further problem is
 that the deterioration of an organic thin film is much severer than that
 of an LED or LD.
 Most organic materials are relatively expensive. Otherwise stated, there is
 a great merit in providing low-cost organic EL device products by
 substituting a partial constitution film with an inexpensive inorganic
 material.
 Furthermore, the development of a device having higher light emission
 efficiency, lower driving voltage and lesser power consumption than ever
 before is strongly desired.
 To provide a solution to such problems, methods of taking advantage of
 merits of both an organic material and an inorganic material have been
 envisaged. That is, an organic/inorganic semiconductor junction structure
 wherein an organic hole transporting layer is substituted by an inorganic
 p-type semiconductor has been contemplated. Such contemplation has been
 investigated in Japanese Patent No. 2636341, and JP-A's 2-139893, 2-207488
 and 6-119973. However, it is still impossible to obtain an organic EL
 device superior to prior art organic ELs in terms of emission performance
 and basic device reliability.
 SUMMARY OF THE INVENTION
 An object of the invention is to achieve an organic EL device which has an
 ever-longer service life, weather resistance, high stability and high
 efficiency, and is inexpensive as well.
 The above object is achieved by the inventions defined below.
 (1) An organic EL device comprising a substrate, a pair of a hole injecting
 electrode and a cathode formed on said substrate, and an organic layer
 located between these electrodes and taking part in at least a light
 emission function, wherein:
 between said organic layer and said cathode there is provided an inorganic
 insulating electron injecting and transporting layer comprising:
 a first component comprising at least one oxide selected from the group
 consisting of lithium oxide, rubidium oxide, potassium oxide, sodium oxide
 and cesium oxide,
 a second component comprising at least one oxide selected from the group
 consisting of strontium oxide, magnesium oxide and calcium oxide, and
 a third component comprising silicon oxide and/or germanium oxide.
 (2) The organic EL device according to (1), wherein said cathode is formed
 of one or two or more metal elements selected from the group consisting of
 Al, Ag, In, Ti, Cu, Au, Mo, W, Pt, Pd, and Ni.
 (3) The organic EL device according to (1), wherein in said inorganic
 insulating electron injecting and transporting layer, the ratio of each
 constituent with respect to all components is:
 5 to 95 mol % for said first component,
 5 to 95 mol % for said second component, and
 5 to 95 mol % for said third component.
 (4) The organic EL device according to (1), wherein said inorganic
 insulating electron injecting and transporting layer has a thickness of
 0.1 to 2 nm.

DETAILED EXPLANATION OF THE PREFERRED EMBODIMENTS
 The organic EL device of the invention comprises a substrate, a pair of a
 hole injecting electrode and a cathode formed on said substrate, and an
 organic layer located between these electrodes and taking part in at least
 a light emission function, wherein between said organic layer and said
 cathode there is provided an inorganic insulating electron injecting and
 transporting layer comprising a first component comprising at least one
 oxide selected from the group consisting of lithium oxide, rubidium oxide,
 potassium oxide, sodium oxide and cesium oxide, a second component
 comprising at least one oxide selected from the group consisting of
 strontium oxide, magnesium oxide and calcium oxide, and a third component
 comprising silicon oxide and/or germanium oxide.
 By constructing the inorganic insulating electron injecting and
 transporting layer of the first, second, and third (stabilizer)
 components, it is possible to use a metal electrode having relatively high
 stability and satisfactory conductivity with no need of providing any
 special electrode having an electron injection function. This makes it
 possible to improve the electron injection and transportation efficiency
 of the inorganic insulating electron injecting and transporting layer and,
 hence, extend the service life of the device.
 The inorganic insulating electron injecting and transporting layer
 comprises as the first component one or two or more oxides of lithium
 oxide (Li.sub.2 O), rubidium oxide (Rb.sub.2 O), potassium oxide (K.sub.2
 O), sodium oxide (Na.sub.2 O), and cesium oxide (Cs.sub.2 O). These oxides
 may be used alone or in combination of two or more. When two or more such
 oxides are used, they may be mixed together at any desired ratio. of these
 oxides, lithium oxide (Li.sub.2 O) is most preferred. Next is rubidium
 oxide (Rb.sub.2 O), then potassium oxide (K.sub.2 O), and finally sodium
 oxide (Na.sub.2 O). When these oxides are used in a mixture form, it is
 preferable that lithium oxide and rubidium oxide account for at least 40
 mol %, and especially at least 50 mol %, of the mixture.
 The inorganic insulating electron injecting and transporting layer
 comprises as the second component one or two or more oxides of strontium
 oxide (SrO), magnesium oxide (MgO), and calcium oxide (CaO). These oxides
 may be used singly or in combination of two or more. When two or more such
 oxides are used, they may be mixed together at any desired ratio. Of these
 oxides, strontium oxide is most preferred. Next is magnesium oxide, and
 finally calcium oxide. When these oxides are used in a mixture form, it is
 preferable that strontium oxide accounts for at least 40 mol % of the
 mixture.
 The inorganic insulating electron injecting and transporting layer
 comprises as the third component (stabilizer) silicon oxide (SiO.sub.2)
 and/or germanium oxide (GeO.sub.2). These oxides may be used singly or in
 a mixture form at any desired mixing ratio.
 Each of the above oxides is usually present with its stoichiometric
 composition. However, the oxide may deviate somewhat from the
 stoichiometric composition.
 In the inorganic insulting electron injecting and transporting layer
 according to the invention, the ratio of each constituent to all
 components is:
 5 to 95 mol %, preferably 50 to 90 mol % for the first component,
 5 to 95 mol %, preferably 50 to 90 mol % for the second component, and
 0.5 to 20 mol %, preferably 5 to 10 mol % for the third component, as
 calculated on an SrO, MgO, CaO, Li.sub.2 O, Rb.sub.2 O, K.sub.2 O,
 Na.sub.2 O, Cs.sub.2 O, SiO.sub.2, and GeO.sub.hd 2 basis.
 The inorganic insulating electron injecting and transporting layer should
 have a thickness of preferably 0.1 to 2 nm, and more preferably 0.3 to 0.8
 nm.
 The inorganic insulating electron injecting and transporting layer may be
 fabricated by various physical or chemical thin-film formation processes
 such as a sputtering process, and an EB evaporation process, with the
 sputtering process being preferred.
 When the inorganic insulating electron injecting and transporting layer is
 formed by the sputtering process, it is preferable that the pressure of
 the sputtering gas during sputtering is in the range of 0.1 to 1 Pa. For
 the sputtering gas, inert gases used with an ordinary sputtering system,
 for instance, Ar, Ne, Xe, and Kr may be used. If required, N.sub.2 may be
 used. Use may then be made of a sputtering atmosphere comprising a mixture
 of the sputtering gas with about 1 to 99% Of O.sub.2. A single- or
 multi-stage sputtering process may be performed using the aforesaid oxide
 or oxides for a target or targets. It is here to be noted that the target
 is usually a mixed target comprising the above main component, a
 subordinate component and an additive. In this case, the obtained film
 composition has a oxygen content substantially equivalent to, or slightly
 lower than, that of the target.
 For the sputtering process, an RF sputtering process using an RF power
 source, a DC sputtering process, etc. may be used, with the RF sputtering
 process being most preferred. Power for a sputtering system is preferably
 in the range of 0.1 to 10 W/cm.sup.2 for RF sputtering, and the film
 formation rate is preferably in the range of 0.1 to 50 nm/min., and
 especially 1 to 10 nm/min.
 When there is a fear that the organic layer, etc. may be ashed and so
 damaged during the lamination of the inorganic insulating electron
 injecting layer, it is preferable to divide the inorganic insulating
 electron injecting layer to two layers before lamination. That is, the
 first layer is laminated thin with no addition of oxygen thereto, and the
 second layer is then laminated thick with the addition of oxygen thereto.
 In this case, the first layer with no oxygen added thereto should have a
 thickness that is about 1/5 to 4/5 of the total thickness. At this time,
 the oxygen-deficient layer formed with no oxygen added thereto should
 preferably an oxygen content of the order of 60 to 90%. The oxidized layer
 formed with the addition of oxygen thereto is usually present with the
 stoichiometric composition for an ordinary oxide. However, the oxide layer
 may have a composition deviating somewhat from the stoichiometric
 composition. Accordingly, the difference in oxygen content between the
 oxygen-deficient layer and the oxidized layer should preferably be at
 least 10%, and especially at least 20%. Alternatively, the oxygen content
 may change continuously in the above range.
 During film formation, the temperature of the substrate is about room
 temperature (25.degree. C.) to 150.degree. C.
 A cathode is located on the inorganic insulating electron injecting and
 transporting layer (that faces away from the light emitting layer). For
 the cathode, an ordinary metal rather than a special metal may be used
 because it is not required to have electron injection capability with a
 low work function. Especially in view of conductivity and ease of
 handling, it is preferable to use one or two metal elements selected from
 the group of Al, Ag, In, Ti, Cu, Au, Mo, W, Pt, Pd, and Ni, and especially
 Al, and Ag.
 When the organic EL device of the invention is fabricated in combination
 with the foregoing inorganic insulating electron injecting and
 transporting layer, it is preferable to use the above metal element or
 elements for the cathode. If required, however, it is acceptable to use
 the following metals. For instance, mention may be made of pure metal
 elements such as K, Li, Na, Mg, La, Ce, Ca, Sr, Ba, Sn, Zn, and Zr, and a
 binary or ternary alloy system containing these metals and serving as a
 stability improver, for instance, Ag.multidot.Mg (Ag: 0.1 to 50 at %),
 Al.multidot.Li (Li: 0.01 to 14 at %), In.multidot.Mg (Mg: 50 to 80 at %),
 and Al.multidot.Ca (Ca: 0.01 to 20 at %).
 The cathode thin film should preferably have at least a certain thickness
 enough to impart electrons to the inorganic insulating electron injecting
 and transporting layer or a thickness of at least 50 nm, and preferably at
 least 100 nm. Although there is no particular upper limit to the cathode
 thickness, the cathode may usually have a thickness of the order of 50 to
 500 nm.
 The hole injecting electrode should preferably be primarily composed of a
 material that can inject holes in the hole injecting layer with high
 efficiency and has a work function of 4.5 eV to 5.5 eV, for instance, any
 one of tin-doped indium oxide (ITO), zinc-doped indium oxide (IZO), indium
 oxide (In.sub.2 O.sub.3), tin oxide (SnO.sub.2), and zinc oxide (ZnO). It
 is here to be noted that these oxides may deviate somewhat from their
 stoichiometric compositions. Regarding ITO, it is desired that the mixing
 ratio of SnO.sub.2 with respect to In.sub.2 O.sub.3 be in the range of 1
 to 20 wt %, and especially 5 to 12 wt %. Regarding IZO, the mixing ratio
 of ZnO with respect to In.sub.2 O.sub.3 is usually or the order of 12 to
 32 wt %.
 The hole injecting electrode may further contain silicon oxide (SiO.sub.2)
 for work function control. The content of silicon oxide (SiO.sub.2) should
 preferably be of the order of 0.5 to 10% in terms of the molar ratio of
 SiO.sub.2 to ITO. The incorporation of SiO.sub.2 contributes to an
 increase in the work function of ITO.
 The electrode on the side out of which light is taken should preferably
 have a light transmittance of at least 50%, especially at least 60%, more
 especially at least 80%, and even more especially at least 90% with
 respect to light emitted at an emission wavelength of usually 400 to 700
 nm. With decreasing transmittance, the light emitted from the light
 emitting layer attenuates, and so it is difficult to obtain the luminance
 needed for a light emitting device. In some cases, the light transmittance
 may be intentionally reduced as by increasing the contrast ratio for
 better viewability, etc.
 The electrode should preferably have a thickness of 50 to 500 nm, and
 especially 50 to 300 nm. Although there is no particular upper limit to
 the electrode thickness, too thick an electrode gives rise to concerns
 such as a transmittance drop, and defoliation. Too thin an electrode fails
 to obtain sufficient effect, and offers a problem in conjunction with film
 thickness, etc. during device fabrication.
 As typically shown in FIG. 1, one embodiment of the organic EL device of
 the invention comprises, in order from a substrate 1, a hole injecting
 electrode 2, a hole injecting and transporting layer 4, a light emitting
 layer 5, an inorganic insulating electron injecting and transporting layer
 6 and a cathode 3 which are laminated together on the substrate 1. Another
 embodiment of the organic EL device of the invention is reversed in
 multilayer arrangement to the first embodiment of the invention (the hole
 injecting electrode is opposite to the substrate side); it comprises, in
 order from a substrate 1, a cathode 3, an inorganic insulating electron
 injecting and transporting layer 6, a light emitting layer 5, a hole
 injecting and transporting layer 4 and a hole injecting electrode 2 which
 are laminated together on the substrate 1, as shown in FIG. 2. The
 reversed multilayer arrangement can prevent damage to the organic layer
 due to ashing during the formation of the inorganic insulating electron
 injecting layer. In this case, it is preferable to divide the inorganic
 insulating electron injecting and transporting layer to two layers as
 mentioned above. These embodiments may be properly chosen depending on
 display fabrication processes, etc. In FIGS. 1 and 2, a driving power
 source E is connected between the hole injecting electrode 2 and the
 cathode 3. The term "light emitting layer" is here understood to refer to
 a light emitting layer in a broad sense and so encompass a hole injecting
 and transporting layer, a light emitting layer in a narrow sense, a hole
 transporting layer, etc.
 In the above device, a plurality of light emitting layers may be cascaded
 one upon another for color tone control of emitted light, and multicolor
 light emission.
 The light emitting layer is made up by lamination of one or two or more
 organic compound thin films taking part in at least the light emission
 function.
 The light emitting layer has functions of injecting holes and electrons,
 transporting them, and recombining holes and electrons to create excitons.
 For the light emitting layer, it is preferable to use a relatively
 electronically neutral compound, so that the electrons and holes can be
 easily injected and transported in a well-balanced state.
 If required, the light emitting layer may comprise in addition to the light
 emitting layer in a narrow sense a hole injecting and transporting layer,
 an electron transporting layer, etc.
 The hole injecting and transporting layer has functions of facilitating
 injection of holes from the hole injecting electrode, providing stable
 transportation of holes and blocking electrons. The electron injecting and
 transporting layer, provided as occasion demands, has functions of
 facilitating injection of electrons from the inorganic insulating electron
 injecting and transporting layer, providing stable transportation of
 electrons and blocking holes. These layers are effective for increasing
 the number of holes and electrons injected into the light emitting layer
 and confining holes and electrons therein for optimizing the recombination
 region to improve light emission efficiency. Usually, an organic electron
 transporting layer is not provided.
 No particular limitation is imposed on the thickness of the light emitting
 layer, the thickness of the hole injecting and transporting layer, and the
 thickness of the electron transporting layer. However, these layers should
 preferably a thickness of the order of usually 5 to 500 nm, and especially
 10 to 300 nm although varying depending on formation processes.
 The thickness of the hole injecting and transporting layer, and the
 electron transporting layer are approximately equal to, or range from
 about 1/10 times to about 10 times, the thickness of the light emitting
 layer although it depends on the design of the recombination/light
 emitting region. When the hole injecting and transporting layer is
 separated into an injecting layer and a transporting layer, it is
 preferable that the injecting layer is at least 1 nm thick and the
 transporting layer is at least 1 nm thick. The upper limit to thickness is
 usually about 500 nm for the injecting layer and about 500 nm for the
 transporting layer. The same film thickness is also true of the case where
 two injecting and transporting layers are provided.
 In the organic EL device according to the invention, the light emitting
 layer contains a fluorescent material that is a compound capable of
 emitting light. The fluorescent material used herein, for instance, may be
 at least one compound selected from compounds such as those disclosed in
 JP-A 63-264692, e.g., quinacridone, rubrene, and styryl dyes. Use may also
 be made of quinoline derivatives such as metal complex dyes containing
 8-quinolinol or its derivative as ligands, for instance,
 tris(8-quinolinolato)aluminum, tetraphenylbutadiene, anthracene, perylene,
 coronene, and 12-phthaloperinone derivatives. Use may further be made of
 phenylanthracene derivatives disclosed in JP-A 8-12600 (Japanese Patent
 Application No. 6-110569) and tetraarylethene derivatives disclosed in
 JP-A 8-12969 (Japanese Patent Application No. 6-114456).
 Preferably, the fluorescent compound is used in combination with a host
 substance capable of emitting light by itself; that is, it is preferable
 that the fluorescent compound is used as a dopant. In such a case, the
 content of the fluorescent compound in the light emitting layer is in the
 range of preferably 0.01 to 10% by weight, and especially 0.1 to 5% by
 weight. By using the fluorescent compound in combination with the host
 substance, it is possible to vary the wavelength performance of light
 emission of the host substance, thereby making light emission possible on
 a longer wavelength side and, hence, improving the light emission
 efficiency and stability of the device.
 Quinolinolato complexes, and aluminum complexes containing 8-quinolinol or
 its derivatives as ligands are preferred for the host substance. Such
 aluminum complexes are typically disclosed in JP-A's 63-264692, 3-255190,
 5-70733, 5-258859, 6-215874, etc.
 Exemplary aluminum complexes include tris(8-quinolinolato)aluminum,
 bis(8-quinolinolato)magnesium, bis(benzo{f}-8-quinolinolato)zinc,
 bis(2-methyl-8-quinolinolato)aluminum oxide, tris(8-quinolinolato)indium,
 tris(5-methyl-8-quinolinolato)aluminum, 8-quinolinolatolithium,
 tris(5-chloro-8-quinolinolato)gallium,
 bis(5-chloro-8-quinolinolato)calcium,
 5,7-dichloro-8-quinolinolato-aluminum,
 tris(5,7-dibromo-8-hydroxyquinolinolato)aluminum, and
 poly[zinc(II)-bis(8-hydroxy-5-quinolinyl)methane].
 Use may also be made of aluminum complexes containing other ligands in
 addition to 8-quinolinol or its derivatives, for instance,
 bis(2-methyl-8-quinolinolato)(phenolato) aluminum (III),
 bis(2-methyl-8quinolinolato)(o-cresolato) aluminum (III),
 bis(2-methyl8-quinolinolato)(m-cresolato) aluminum (III),
 bis(2-methyl-8-quinolinolato)(p-cresolato) aluminum (III),
 bis(2-methyl-8-quinolinolato)(o-phenyl-phenolato)aluminum (III),
 bis(2-methyl-8-quinolinolato)(mphenylphenolato)aluminum (III),
 bis(2-methyl-8-quinolinolato) (p-phenylphenolato)aluminum (III),
 bis(2-methyl-8-quinolinolato) (2,3-dimethylphenolato)aluminum (III),
 bis(2-methyl-8-quinolinolato)(2,6-dimethylphenolato)aluminum (III),
 bis(2-methyl-8-quinolinolato)(3,4-dimethylphenolato)aluminum (III),
 bis(2-methyl-8-quinolinolato)(3,5-dimethylphenolato) aluminum (III),
 bis(2-methyl-8-quinolinolato) (3,5-di-tertbutylphenolato)aluminum (III),
 bis(2-methyl-8-quinolinolato) (2,6-diphenylphenolato)aluminum (III),
 bis(2-methyl-8-quinolinolato)(2,4,6-triphenylphenolato)aluminum (III),
 bis(2-methyl-8-quinolinolato)(2,3,6-trimethylphenolato) aluminum (III),
 bis(2-methyl-8-quinolinolato) (2,3,5,6-tetramethylphenolato)aluminum
 (III), bis(2-methyl-8-quinolinolato)(1-naphtholato)aluminum (III),
 bis(2-methyl8-quinolinolato)(2-naphtholato)aluminum (III),
 bis(2,4-dimethyl-8-quinolinolato)(o-phenylphenolato)aluminum (III),
 bis(2,4-dimethyl-8-quinolinolato)(pphenylphenolato)aluminum (III),
 bis(2,4-dimethyl-8-quinolinolato)(m-phenylphenolato) aluminum (III),
 bis(2,4-dimethyl-8-quinolinolato) (3,5-dimethylphenolato)aluminum (III),
 bis(2,4-dimethyl-8-quinolinolato) (3,5-di-tertbutylphenolato)aluminum
 (III), bis(2-methyl-4-ethyl-8-quinolinolato)(p-cresolato)aluminum (III),
 bis(2-methyl-4-methoxy-8-quinolinolato)(p-phenyl-phenolato)aluminum (III),
 bis(2-methyl-5-cyano-8-quinolinolato)(ocresolato)aluminum (III), and
 bis(2-methyl-6-trifluoromethyl-8-quinolinolato) (2-naphtholato)aluminum
 (III).
 Besides, use may be made of bis(2-methyl-8-quinolinol-ato)aluminum
 (III)-.mu.-oxo-bis(2-methyl-8-quinolinolato) aluminum (III),
 bis(2,4-dimethyl-8-quinolinolato)aluminum
 (III)-.mu.-oxo-bis(2,4-dimethyl-8-quinolinolato)aluminum (III),
 bis(4-ethyl-2-methyl-8-quinolinolato)aluminum
 (III)-.mu.-oxo-bis(4-ethyl-2-methyl-8-quinolinolato)aluminum (III),
 bis(2-methyl-4-methoxyquinolinolato)aluminum (III)-.mu.-oxo-bis
 (2-methyl-4-methoxyquinolinolato)aluminum (III),
 bis(5-cyano-2-methyl8-quinolinolato)aluminum
 (III)-.mu.-oxo-bis(5-cyano-2-methyl8-quinolinolato)aluminum (III),
 bis(2-methyl-5-trifluoromethyl-8-quinolinolato)aluminum
 (III)-.mu.-oxo-bis(2-methyl-5-trifluoromethyl-8-quinolinolato)aluminum
 (III), etc.
 Other preferable host substances include phenylanthracene derivatives
 disclosed in JP-A 8-12600 (Japanese Patent Application No. 6-110569),
 tetraarylethene derivatives disclosed in JP-A 8-12969 (Japanese Patent
 Application No. 6-114456), etc.
 In the practice of the invention, the light emitting layer may also serve
 as an electron transporting layer. In this case, it is preferable to use a
 fluorescent material, e.g., tris(8-quinolinolato)aluminum or the like,
 which may be provided by evaporation.
 If necessary or preferably, the light emitting layer is formed of a mixed
 layer of at least one compound capable of injecting and transporting holes
 with at least one compound capable of injecting and transporting
 electrons. Preferably in this case, a dopant is incorporated in the mixed
 layer. The content of the dopant compound in the mixed layer is in the
 range of preferably 0.01 to 20% by weight, and especially 0.1 to 15% by
 weight.
 In the mixed layer with a hopping conduction path available for carriers,
 each carrier migrates in the polarly prevailing substance, so making the
 injection of carriers having an opposite polarity unlikely to occur. This
 leads to an increase in the service life of the device due to less damage
 to the organic compound. By incorporating the aforesaid dopant in such a
 mixed layer, it is possible to vary the wavelength performance of light
 emission that the mixed layer itself possesses, thereby shifting the
 wavelength of light emission to a longer wavelength side and improving the
 intensity of light emission, and the stability of the device as well.
 The compound capable of injecting and transporting holes and the compound
 capable of injecting and transporting electrons, both used to form the
 mixed layer, may be selected from compounds for the injection and
 transportation of holes and compounds for the injection and transportation
 of electrons, as will be described later. Especially for the compounds for
 the injection and transportation of holes, it is preferable to use amine
 derivatives having strong fluorescence, for instance, hole transporting
 materials such as triphenyldiamine derivatives, styrylamine derivatives,
 and amine derivatives having an aromatic fused ring.
 For the compounds capable of injecting and transporting electrons, it is
 preferable to use metal complexes containing quinoline derivatives,
 especially 8-quinolinol or its derivatives as ligands, in particular,
 tris(8-quinolinolato) aluminum (Alq.sup.3). It is also preferable to use
 the aforesaid phenylanthracene derivatives, and tetraarylethene
 derivatives.
 For the compounds for the injection and transportation of holes, it is
 preferable to use amine derivatives having strong fluorescence, for
 instance, hole transporting materials such as triphenyldiamine
 derivatives, styrylamine derivatives, and amine derivatives having an
 aromatic fused ring.
 In this case, the ratio of mixing the compound capable of injecting and
 transporting holes with respect to the compound capable of injecting and
 transporting electrons is determined while the carrier mobility and
 carrier density are taken into consideration. In general, however, it is
 preferred that the weight ratio between the compound capable of injecting
 and transporting holes and the compound capable of injecting and
 transporting electrons is of the order of 1/99 to 99/1, particularly 10/90
 to 90/10, and more particularly 20/80 to 80/20.
 The thickness of the mixed layer should preferably be equal to or larger
 than the thickness of a single molecular layer, and less than the
 thickness of the organic compound layer. More specifically, the mixed
 layer has a thickness of preferably 10 to 150 nm, and more preferably 50
 to 100 nm.
 Preferably, the mixed layer is formed by co-evaporation where the selected
 compounds are evaporated from different evaporation sources. When the
 compounds to be mixed have identical or slightly different vapor pressures
 (evaporation temperatures), however, they may have previously been mixed
 together in the same evaporation boat for the subsequent evaporation.
 Preferably, the compounds are uniformly mixed together in the mixed layer.
 However, the compounds in an archipelagic form may be present in the mixed
 layer. The light emitting layer may generally be formed at a given
 thickness by the evaporation of the organic fluorescent substance or
 coating a dispersion of the organic fluorescent substance in a resin
 binder.
 For the compounds for the injection and transportation of holes, use may be
 made of various organic compounds as disclosed in JP-A's 63-295695,
 2-191694, 3-792, 5-234681, 5-239455, 5-299174, 7-126225, 7-126226 and
 8-100172 and EP 0650955A1. Examples are tetraarylbenzidine compounds
 (triaryldiamine or triphenyldiamine (TPD)), aromatic tertiary amines,
 hydrazone derivatives, carbazole derivatives, triazole derivatives,
 imidazole derivatives, oxadiazole derivatives having an amino group, and
 polythiophenes. The compounds may be used singly or in combination of two
 or more. Where two or more such compounds are used, they may be stacked as
 separate layers, or otherwise mixed.
 For the electron transporting layer which is provided if necessary, there
 may be used quinoline derivatives such as organic metal complexes
 containing 8-quinolinol or its derivatives as ligands, for instance,
 tris(8-quinolinolato)aluminum (Alq.sup.3), oxadiazole derivatives,
 perylene derivatives, pyridine derivatives, pyrimidine derivatives,
 quinoxaline derivative, diphenylquinone derivatives, and nitro-substituted
 fluorene derivatives. The electron transporting layer may also serve as a
 light emitting layer. In this case, it is preferable to use
 tris(8-quinolilato)aluminum, etc. The electron transporting layer may be
 formed as by deposition by evaporation, as is the case with the light
 emitting layer. The electron transporting layer formed of the organic
 material may be provided depending on device structure, and other
 conditions, although this layer is usually unnecessary.
 Preferably, the hole injecting and transporting layer, the light emitting
 layer, and the electron transporting layer composed of an organic material
 are formed by a vacuum evaporation process because a uniform thin film can
 then be obtained. With the vacuum evaporation process, it is thus possible
 to obtain a uniform thin film in an amorphous state or with a grain size
 of up to 0.2 .mu.m. A grain size of greater than 0.2 .mu.m results in
 non-uniform light emission. To avoid this, it is required to make the
 driving voltage of the device high. However, this in turn gives rise to
 some considerable drop of hole injection efficiency.
 No particular limitation is imposed on conditions for vacuum evaporation.
 However, the vacuum evaporation should preferably be carried out at a
 degree of vacuum of up to 10.sup.-4 Pa and a deposition rate of about 0.01
 to 1 nm/sec. Also, the layers should preferably be continuously formed in
 vacuum, partly because the deposition of impurities on the interface
 between adjacent layers is avoidable resulting in the achievement of high
 performance, and partly because the driving voltage of the device can be
 lowered with elimination of dark spots or no growth of dark spots.
 When the layers, each containing a plurality of compounds, are formed by
 the vacuum evaporation process, it is preferable that co-evaporation is
 carried out while each boat with the compounds charged therein is placed
 under temperature control.
 Preferably, the device is sealed up by means of a sealing sheet, etc. for
 the purpose of preventing oxidation of the organic layers and electrodes
 in the device. To prevent penetration of moisture, the sealing sheet is
 bonded to the device using an adhesive resin layer to seal up the device.
 An inert gas such as Ar, He, and N.sub.2 is preferably used as a sealing
 gas. Then, the sealing gas should preferably have a moisture content of up
 to 100 ppm, especially up to 10 ppm, and more especially up to 1 ppm.
 Although there is no particular lower limit to the moisture content, the
 sealing gas should usually have a moisture content of about 0.1 ppm.
 The sealing sheet, preferably in a flat sheet form, may be made of
 transparent or translucent materials such as glasses, quartz, and resins,
 among which the glasses are preferred. For such a glass material, alkali
 glass is preferable from a cost standpoint. Other preferable glass
 materials, for instance, include soda lime glass, lead alkali glass,
 borosilicate glass, aluminosilicate glass, and silica glass. In
 particular, a soda glass material subjected to no surface treatment is
 inexpensive and so is preferable. A metal sheet, a plastic sheet, etc.,
 too, may be used in place of the sealing glass sheet.
 For height control, a spacer is used to keep the sealing sheet at a height
 as desired. The spacer material may be resin beads, silica beads, glass
 beads, glass fibers, etc., with the glass beads being most preferred. The
 spacer is usually in a particulate form having a uniform particle size. In
 the invention, however, a spacer of any desired shape may be used provided
 that it can function well. The spacer size should preferably be 1 to 20
 .mu.m, especially 1 to 10 .mu.m, and more especially 2 to 8 .mu.m as
 calculated on a circle diameter basis. A spacer having such a diameter
 should preferably have a particle length of up to about 100 .mu.m.
 Although there is no particular lower limit to the particle size, the
 particle size should usually be equal to or larger than the diameter.
 The spacer may or may not be used when a recess is provided in the sealing
 sheet. When the spacer is used, its size is preferably within the
 aforesaid range, and more preferably within the range of 2 to 8 .mu.m.
 The spacer may have been incorporated in the sealing adhesive agent or may
 be incorporated in the sealing adhesive agent at the time of bonding. The
 content of the spacer in the sealing adhesive agent should be preferably
 0.01 to 30 wt %, and more preferably 0.1 to 5 wt %.
 For the adhesive agent, it is preferable to use a cation curing epoxy resin
 of the ultraviolet curing type, although an adhesive agent of the type
 that ensures stable adhesion strength and good airtightness may be used.
 For the substrate on which an organic EL structure is formed according to
 the invention, a noncrystalline substrate such as a glass or quartz
 substrate, and a crystalline substrate such as an Si, GaAs, ZnSe, ZnS, GaP
 or InP substrate may be used. The crystalline substrate may also be
 provided with a crystalline or noncrystalline buffer layer or a metal
 buffer layer. For a metal substrate, Mo, Al, Pt, Ir, Au, Pd, and other
 metal substrates may be used. However, it is preferable to use a glass
 substrate. The substrate is usually located on the side out of which light
 is taken, and so it should preferably be transparent to light as in the
 above electrodes.
 In the invention, a number of devices may be arranged on a plane. A color
 display may be constructed by changing the colors of light emitted from
 the devices arranged on the plane.
 The substrate may be provided with a color filter film, fluorescent
 material-containing color conversion film or dielectric reflecting film
 for controlling the color of light emission.
 For the color filter film, a color filter employed with liquid crystal
 display devices, etc. may be used. However, it is preferable to control
 the properties of the color filter in conformity to the light emitted from
 the organic EL device, thereby optimizing the efficiency of taking out
 light emission and color purity.
 By using a color filter capable of cutting off extraneous light of such
 short wavelength as absorbed by the EL device material or the fluorescent
 conversion layer, it is possible to improve the light resistance of the
 device and the contrast of what is displayed on the device.
 Instead of the color filter, an optical thin film such as a dielectric
 multilayer film may be used.
 The fluorescent color conversion film absorbs light emitted from an EL
 device and gives out light from the phosphors contained therein for the
 color conversion of light emission, and is composed of three components, a
 binder, a fluorescent material and a light absorbing material.
 In the practice of the invention, it is basically preferable to use a
 fluorescent material having high fluorescent quantum efficiency, and
 especially a fluorescent material having strong absorption in an EL light
 emission wavelength region. Laser dyes are suitable for the practice of
 the invention. To this end, for instance, it is preferable to use
 rohodamine compounds, perylene compounds, cyanine compounds,
 phthalocyanine compounds (including subphthalocyanine compounds, etc.),
 naphthaloimide compounds, fused cyclic hydrocarbon compounds, fused
 heterocyclic compounds, styryl compounds, and coumarin compounds.
 For the binder, it is basically preferable to make an appropriate selection
 from materials that do not extinguish fluorescence. It is particularly
 preferable to use a material that can be finely patterned by
 photolithography, printing or the like. It is also preferable to use a
 material that is not damaged during ITO or IZO film formation.
 The light absorbing material is used when light is not fully absorbed by
 the fluorescent material, and so may be dispensed with, if not required.
 For the light absorbing material, it is preferable to make a selection
 from materials that do not extinguish fluorescence.
 The organic EL device of the invention is generally of the DC drive type
 while it may be of the AC or pulse drive type. The applied voltage is
 generally of the order of 2 to 30 volts.
 EXAMPLE
 The present invention is explained more specifically with reference to some
 examples.
 Example 1
 A 7059 substrate (made by Corning) used as a glass substrate was scrubbed
 with neutral detergent. Then, the substrate was fixed to a substrate
 holder in a sputtering system, where an ITO hole injecting electrode layer
 was formed by a DC magnetron sputtering process using an ITO oxide target.
 The substrate with the ITO film formed on it was ultrasonically washed with
 neutral detergent, acetone, and ethanol, and then pulled up from boiling
 ethanol, followed by drying. The substrate was subsequently cleaned on its
 surface with UV/O.sub.3. Then, the substrate was fixed to the substrate
 holder in a vacuum evaporation system, which was evacuated to a vacuum of
 1.times.10.sup.-4 Pa or lower.
 Then, polythiophene was deposited by evaporation at a deposition rate of
 0.1 nm/sec. to a thickness of 10 nm thereby forming a hole injecting
 layer, and TPD was deposited by evaporation at a deposition rate of 0.1
 nm/sec. to a thickness of 20 nm thereby forming a hole transporting layer.
 With the vacuum kept, tris(8-quinolinolato)aluminum (Alq.sup.3) and rubrene
 were deposited by evaporation at an overall deposition rate of 0.2 nm/sec.
 to a thickness of 40 nm thereby forming a light emitting layer. Alq.sup.3
 was doped with 5 vol % of rubrene.
 With the vacuum still maintained, this EL structure was transferred to a
 sputtering system where a 0.8-nm thick inorganic insulating electron
 injecting and transporting layer was formed using a target obtained by
 mixing together the target materials strontium oxide (SrO), lithium oxide
 (Li.sub.2 O) and silicon oxide (SiO.sub.2) in such a manner that SrO was
 80 mol % with respect to all components, Li.sub.2 O was 10 mol % with
 respect to all components and SiO.sub.2 was 10 mol % with all components.
 Referring to film formation conditions at this time, the substrate
 temperature was 25.degree. C., the sputtering gas was Ar, the film
 formation rate was 1 nm/min., the operating pressure was 0.5 Pa, and the
 input power was 5 W/cm.sup.2. In this case, the inorganic insulating
 electron injecting and transporting layer was first formed to a thickness
 of 0.4 nm while 100% Ar was fed at 100 SCCM, and the inorganic insulating
 electron injecting and transporting layer was then formed to a thickness
 of 0.4 nm while Ar and O.sub.2 at 1:1 were fed at 100 SCCM.
 With the vacuum still kept, Al was deposited by evporation to a thickness
 of 200 nm, thereby forming a cathode.
 Finally, the EL structure was sealed up by glass to obtain an organic EL
 device (forward lamination). A comparative sample was obtained as in
 Example 1 with the exception that instead of the inorganic insulating
 electron injecting and transporting layer, an electron injecting and
 transporting layer was formed by deposition by evaporation of
 tris(8-quinolinolato)aluminum (Alq.sup.3) at a deposition rate of 0.2
 nm/sec. to a thickness of 30 nm, and an electron injecting electrode was
 formed by deposition by evaporation of MgAg (at a weight ratio of 10:1) at
 a deposition rate of 0.2 nm/sec. to a thickness of 200 nm, while the
 vacuum was still kept.
 When an electric field was applied to the obtained organic EL device in the
 air, it showed diode performance. When the device was biased with ITO on a
 positive side and Al on a negative side, the current increased with
 increasing voltage, and distinct light emission was observed from the
 sealing sheet side in an ordinary room. Even upon repetitive emission
 operations, no luminance decrease was found.
 Each sample was subjected to accelerated testing while a constant current
 of 100 mA/cm.sup.2 was applied thereto, thereby examining its emission
 luminance and life performance. The inventive sample showed an about 10%
 emission luminance improvement over the comparative sample constructed in
 quite the same manner as mentioned above with the exception that an
 electron injecting and transporting layer was formed of an organic
 material as in the prior art. The luminance half-life of the comparative
 sample was 100 hours or shorter whereas that of the inventive sample was
 200 hours or longer.
 Example 2
 Experiments were carried out as in Example 1 with the exception that the
 main component, subordinate component and stabilizer in the inorganic
 insulating electron injecting and transporting layer were changed from SrO
 to MgO, CaO or an oxide mixture thereof, from Li.sub.2 O to K.sub.2 O,
 Rb.sub.2 O, K.sub.2 O, Na.sub.2 O, Cs.sub.2 O or an oxide mixture thereof,
 and from SiO.sub.2 to GeO.sub.2 or an SiO.sub.2 and GeO.sub.2 oxide
 mixture. The obtained results were much the same as Example 1. The same
 was also true of when the cathode constituting material was changed from
 Al to Ag, In, Ti, Cu, Au, Mo, W, Pt, Pd, Ni or an alloy thereof.
 EFFECT OF THE INVENTION
 According to the invention as explained above, it is possible to achieve an
 organic EL device which has an extended life, weather resistance, high
 stability, and high efficiency, and is inexpensive as well.