Light-emitting element and light-emitting device using the same

The present invention provides a light-emitting element that includes a pair of electrodes, and an organic compound that has a glass-transition temperature of 150° C. or more, preferably 160° C. or more and 300° C. or less, and a metal oxide that are provided between the pair of electrodes, or includes a pair of electrodes, and a compound having a spiro ring and a triphenylamine skeleton and a metal oxide that are provided between the pair of electrodes. It is a feature that the compound has a spiro ring and a triphenylamine skeleton is a benzidine derivative represented by a general formula (1) In the formula, R1 is hydrogen or an alkyl group having 1 to 4 carbon atoms.

TECHNICAL FIELD

The present invention relates to the structure of a light-emitting element including an organic compound and an inorganic compound, and further relates to a light-emitting device that has the light-emitting element.

BACKGROUND ART

Many of light-emitting elements that are used for displays have a structure in which a layer including a luminescent material is sandwiched between a pair of electrodes, and luminescence is produced when an exciton formed by recombining an electron injected from one of the electrodes and a hole injected from the other electrode returns to the ground state.

Regarding these light-emitting elements, studies for improving a luminous efficiency and stability and preventing increase in driving voltage have been conducted.

For example, Patent Document 1 discloses a highly durable organic thin-film light-emitting element that is able to keep light-emitting performance. According to Patent Document 1, it is disclosed that a lower driving voltage for the light-emitting element is achieved by using a metal oxide that has a higher work function, such as a molybdenum oxide, for an anode.

In addition, crystallization of a material constituting a light-emitting element is cited as a cause of deterioration of the light-emitting element. Therefore, a material that is not likely to be crystallized is desired, and for example, Patent Document 2 disclosed a heat-resistant organic material that has a higher glass-transition temperature.

DISCLOSURE OF INVENTION

This development has been continued since the power consumption of a light-emitting device can be made lower by achieving a lower driving voltage. In particular, when a light-emitting element is incorporated in a mobile light-emitting device, great importance is placed on achievement of lower power consumption.

Therefore, the way disclosed in Patent Document 1 alone is said to be insufficient, and technologic development for achieving a much lower driving voltage has been needed.

Further, for mass production of light-emitting devices, a high-yield structure for a light-emitting element and the production process thereof have been desired.

Consequently, it is an object of the present invention to provide a light-emitting element achieving a lower driving voltage and provide a light-emitting element that can be produced at a high yield. Further, it is an object of the present invention to provide a light-emitting element including a material that has excellent heat resistance, in particular, that is not likely to be crystallized and is likely to be kept amorphous.

Further, it is an object of the present invention to provide a light-emitting device that has this light-emitting element.

In view of the objects described above, the present invention has a feature of a light-emitting element that has a layer including an organic compound that has a glass-transition temperature of 150° C. or more, preferably 160° C. or more and 300° C. or less, and an inorganic compound. In addition, the organic compound according to the present invention has a feature of a melting point that is 180° C. or more and 400° C. or less.

Further, the present invention has a feature of a light-emitting element including an organic compound and an inorganic compound, and has a feature of using a compound having a spiro ring and a triphenylamine skeleton for the organic compound. The inventors have found out that it is preferable to use a benzidine derivative represented by a general formula (1) as the compound having a spiro ring and a triphenylamine skeleton. More specifically, it has been determined that it is preferable to use a benzidine derivative represented by a structure formula (2) as the compound having a spiro ring and a triphenylamine skeleton.

Specific structure according to the present invention will be described below. In addition, a case of using a metal oxide for the inorganic compound will be exemplified.

A light-emitting element according to the present invention includes a pair of electrodes and a plurality of layers provided between the pair of electrodes, where any one of the plurality of layers includes an organic compound that has a glass-transition temperature of 150° C. or more, preferably 160° C. or more and 300° C. or less, and a metal oxide.

The organic compound according to the present invention can be obtained by a coupling reaction of N,N′-diphenylbenzidine with 2-bromo-spiro-9,9′-bifluorene or 2-bromo-2′,7′-dialkyl-spiro-9,9′-bifluorene.

In addition, the organic compound according to the present invention can be obtained by a coupling reaction of N,N′-diphenylbenzidine with 2-bromo-spiro-9,9′-bifluorene.

In addition, the present invention has a feature that a layer including the organic compound that has a glass-transition temperature of 150° C. or more, preferably 160° C. or more and 300° C. or less, and the inorganic compound is used as a layer that generates holes. Further, the light-emitting element according to the present invention can include the organic compound that has a glass-transition temperature of 150° C. or more, preferably 160° C. or more and 300° C. or less, as a hole transporting material.

In addition, the present invention has a feature that the organic compound has a melting point of 180° C. or more and 400° C. or less.

Besides, the organic compound is a compound having a spiro ring and a triphenylamine skeleton in the present invention, and a light-emitting element according to the present invention includes a pair of electrodes, and a compound having a spiro ring and a triphenylamine skeleton and a metal oxide that are provided between the pair of electrodes. This organic compound has a melting point of 180° C. or more and 400° C. or less, and has a glass-transition temperature of 90° C. or more, preferably 150° C. or more, more preferably 160° C. or more and 300° C. or less.

In the present invention, it is a feature that the organic compound is a benzidine derivative represented by the general formula (1). Furthermore, a light-emitting element according to the present invention includes a pair of electrodes, and a benzidine derivative represented by the general formula (1) and a metal oxide that are provided between the pair of electrodes.

(In the Formula, R1is Hydrogen or an Alkyl Group Having 1 to 4 Carbon Atoms.)

In the present invention, it is a feature that the organic compound is a benzidine derivative represented by the structure formula (2). Furthermore, a light-emitting element according to the present invention includes a pair of electrodes, and a benzidine derivative represented by the structure formula (2) and a metal oxide that are provided between the pair of electrodes. The benzidine derivative according to the present invention has a glass-transition temperature that meets 150° C. or more, preferably 160° C. or more and 300° C. or less. It is a feature that this benzidine derivative is used as a hole transporting material.

In the present embodiment, a material that is used for the inorganic compound may be a metal nitride or a metal oxynitride besides a metal oxide.

For example, when the inorganic compound described above is used for a layer that functions as an electron accepting material, an oxide of a transition metal belonging to any one of Group 4 to 12 of the periodic table can be used as a specific material. Among others, an oxide of a transition metal belonging to any one of Groups 4 to 8 of the periodic table often has a higher electron accepting property, and a vanadium oxide, a molybdenum oxide, a niobium oxide, a rhenium oxide, a tungsten oxide, a ruthenium oxide, a titanium oxide, a chromium oxide, a zirconium oxide, a hafnium oxide, and a tantalum oxide are particularly preferable.

When the inorganic compound described above is used for a layer that functions as an electron donating material, a metal that is used for the inorganic compound is a material selected from alkali metals and alkali-earth metals, specifically such as lithium (Li), calcium (Ca), sodium (Na), potassium (K), and magnesium (Mg). Specific inorganic compounds include oxides of the alkali metals, oxides of the alkali-earth metals, nitrides of the alkali metals, and nitrides of the alkali-earth metals, specifically, a lithium oxide (Li2O), a calcium oxide (CaO), a sodium oxide (Na2O), a potassium oxide (K2O), and a magnesium oxide (MgO), and further include lithium fluoride (LiF), cesium fluoride (CsF), and calcium fluoride (CaF2).

It is to be noted that “including the organic compound and the inorganic compound” includes a layer in which the organic compound and the inorganic compound are mixed and a layer in which the organic compound and the inorganic compound are laminated in the present invention.

Further, the present invention has a feature of a light-emitting device that has the light-emitting element described above. A specific light-emitting device according to the present invention includes a semiconductor film including an impurity region, a first electrode connected to the impurity region, a second electrode provided to be opposed to the first electrode, and a first layer, a second layer, and a third layer provided in order between the first electrode and the second electrode, where any one of the first to third layers includes an organic compound that has a glass-transition temperature of 150° C. or more, preferably 160° C. or more and 300° C. or less, and a metal oxide. In addition, another light-emitting device according to the present invention includes a semiconductor film including an impurity region, a first electrode connected to the impurity region, a second electrode provided to be opposed to the first electrode, and a first layer, a second layer, and a third layer provided in order between the first electrode and the second electrode, where any one of the first to third layers includes a compound having a spiro ring and a triphenylamine skeleton and a metal oxide.

According to the present invention, a light-emitting element that is not likely to be crystallized can be obtained and a light-emitting element achieving a lower driving voltage can be obtained. In addition, even when the light-emitting element according to the present invention is made thicker, the driving voltage is not increased. Accordingly, the light-emitting element can be formed to be thicker, and can be thus produced at a favorable yield. Further, an electrode and a light-emitting layer can be kept further away from each other depending on which layer is made thicker, and quenching of luminescence can be thus prevented.

Further, the light-emitting element according to the present invention has high thermal stability and high heat resistance, which is preferable. Accordingly, the light-emitting element is less deteriorated with time.

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiments of the present invention will be described below with reference to the accompanying drawings. However, the present invention may be embodied in a lot of different forms, and it is to be easily understood that various changes and modifications will be apparent to those skilled in the art unless such changes and modifications depart from the scope of the present invention. Therefore, the present invention is not to be construed with limitation to what is described in the embodiments. It is to be noted that the same reference numeral denotes the same portion or a portion that has the same function in the all drawings for describing the embodiments, and repeated description of the portion will be omitted.

In the present embodiment, an example of a light-emitting element using a layer including an inorganic compound and an organic compound that has a glass-transition temperature of 150° C. or more, preferably 160° C. or more and 300° C. or less, will be described with reference toFIG. 1, where a benzidine derivative is used as the organic compound. It is to be noted that benzidine derivative according to the present invention is a compound having a spiro ring and a triphenylamine skeleton.

As shown inFIG. 1, the light-emitting element according to the present invention has a first electrode101and a second electrode102that are opposed to each other, and has a first layer111, a second layer112, and a third layer113that are stacked in this order from the first electrode101side. When a voltage is applied to this light-emitting element so that the potential of the first electrode101is higher than the potential of the second electrode102, a hole is injected from the first layer111into the second layer112, and an electron is injected from the third layer113into the second layer112. The hole and the electron are recombined to excite a luminescent material. Then, luminescence is produced when the excited luminescent material returns to the ground state.

Next, the first to third layers111to113, the first electrode101, and the second electrode102will be described.

The first layer111is a layer that generates holes. This function can be achieved by using, for example, a layer including a hole transporting material and a material that exhibits an electron accepting property to the hole transporting material. In addition, it is preferable that the material that exhibits an electron accepting property to the hole transporting material be included so that the molar ratio of the material to the hole transporting material is 0.5 to 2 (=the material that exhibits an electron accepting property to the hole transporting material/the hole transporting material).

The hole transporting material is a material in which electrons are transported more then electrodes, and for example, organic compounds, aromatic amine compounds such as 4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: α-NPD), 4,4′-bis[N-(3-methylphenyl)-N-phenylamino]biphenyl (abbreviation: TPD), 4,4′,4″-tris(N,N-diphenylamino)triphenylamine (abbreviation: TDATA), 4, 4′,4″-tris[N-(3-methylphenyl)-N-phenylamino]-triphenylamine (abbreviation: MTDATA), 4,4′-bis[N-{4-(N,N-di-m-tolylamino)phenyl}-N-phenylamino]biphenyl (abbreviation: DNTPD), and phthalocyanine compounds such as phthalocyanine (abbreviation: H2Pc) and copper phthalocyanine (abbreviation: CuPc), can be used as the hole transporting material. It is to be noted that the hole transporting material is not to be considered limited to these.

In addition, an oxide of a transition metal belonging to any one of Group 4 to 12 of the periodic table can be used as the material that exhibits an electron accepting property to the hole transporting material. Among others, an oxide of a transition metal belonging to any one of Groups 4 to 8 of the periodic table often has a higher electron accepting property, and a vanadium oxide, a molybdenum oxide, a niobium oxide, a rhenium oxide, a tungsten oxide, a ruthenium oxide, a titanium oxide, a chromium oxide, a zirconium oxide, a hafnium oxide, and a tantalum oxide are particularly preferable. Besides the oxides, nitrides and oxynitrides of the metals mentioned above may be used. It is to be noted that the material that exhibits am electron accepting property to the hole transporting material is not to be considered limited to these.

When the first layer111is formed by using a layer in which the hole transporting material, which is composed of an organic material, and the material that exhibits an electron accepting property to the hole transporting material, which is composed of the inorganic material mentioned above, are mixed, the conductivity thereof gets higher. Therefore, it is preferable to form the first layer111in this way. When the conductivity is higher, the first layer111can be made thicker, and the yield of manufacturing can be thus improved. Further, the first electrode101and the second layer112can be kept further away from each other by making the first layer111thicker. Accordingly, quenching of luminescence due to a metal can be prevented.

Crystallization of the organic compound that is used for the first layer111can be suppressed by using this layer in which the organic material and the inorganic material are mixed, and the first layer111can be thus formed to be thicker without increase in resistance. Therefore, even when there is irregularity due to dust, contamination, and the like on a substrate, the irregularity has almost no influence since the first layer111is made thicker. Accordingly, defects such as a short circuit between the first electrode101and the second electrode102due to irregularity can be prevented.

As will be described in further detail below in the example, it is determined that the first layer111is not likely to be deteriorated with time when a layer in which a benzidine derivative that has a glass-transition temperature of 150° C. or more, preferably 160° C. or more and 300° C. or less, and a molybdenum oxide that is an inorganic compound are mixed is used for the first layer111. In short, this structure makes it possible to provide a light-emitting element that is excellent in stability.

Further, the first layer111may include another organic compound. As the organic compound, rubrene and the like can be cited. The reliability can be improved by the addition of rubrene.

In addition to this, the first layer111may be a layer composed of a metal oxide such as a molybdenum oxide, a vanadium oxide, a ruthenium oxide, a cobalt oxide, and a copper oxide.

This first layer111can be formed by evaporation. When a layer including a plurality of mixed compounds is used as the first layer111, co-evaporation can be used. The co-evaporation includes co-evaporation by resistance-heating evaporation, co-evaporation by electron-beam evaporation, and co-evaporation by resistance-heating evaporation and electron-beam evaporation, and in addition, there are methods such as deposition by resistance-heating evaporation and sputtering and deposition by electron-beam, evaporation and sputtering. The first layer111can be formed by combining the same type of methods or different types of methods. In addition, the example described above shows a layer including two kinds of materials. However, when three or more kinds of materials are included, the first layer111can be formed also in the same way by combining the same type of methods or different types of methods as described above.

Next, the second layer112that is a layer including a light-emitting layer will be described. The layer including the light-emitting layer may be a single layer composed of only the light-emitting layer or a multilayer. To cite a case, a specific multilayer includes a light-emitting layer and additionally a plurality of layers selected from electron transporting layers and hole transporting layers. InFIG. 1, a multilayer case in which the second layer112includes a light-emitting layer123, an electron transporting layer124, and a hole transporting layer122is shown.

A benzidine derivative that has a glass-transition temperature of 150° C. or more, preferably 160° C. or more and 300° C. or less, according to the present invention, which is represented by a general formula (1) or any one of structure formulas (2) to (4), can be used for the hole transporting layer112.

Since the benzidine derivative according to the present invention has high heat resistance, the hole transporting layer122that is less likely to change in characteristics due to heat can be formed by using the benzidine derivative according to the present invention as a hole transporting material. Further, since the benzidine derivative according to the present invention is not likely to be crystallized, the hole transporting layer122that is not likely to be crystallized can be formed by using the benzidine derivative according to the present invention as a hole transporting material.

It is to be noted that the hole transporting layer112may be a layer formed by combining two or more layers each including the benzidine derivative according to the present invention, which is represented by the general formula (1) or any one of the structure formulas (2) to (4).

Further, it is preferable that a layer in which the benzidine derivative that has a glass-transition temperature of 150° C. or more, preferably 160° C. or more and 300° C. or less, according to the present invention, and an inorganic compound are mixed be used for the hole transporting layer122. Crystallization of the benzidine derivative can be further suppressed by mixing the inorganic compound, and the layer can be thus formed to be thicker without increase in resistance. Therefore, even when there is irregularity due to dust, contamination, and the like, on a substrate, the irregularity has almost no influence due to the hole transporting layer122being thicker. Accordingly, defects such as a short circuit between the first electrode101and the second electrode102due to irregularity can be prevented.

Metal oxides, metal nitrides, and metal oxynitrides can be used for this inorganic compound. For example, an oxide of a transition metal belonging to any one of Group 4 to 12 of the periodic table can be used as a metal oxide. Among others, an oxide of a transition metal belonging to any one of Groups 4 to 8 of the periodic table often has a higher electron accepting property, and a vanadium oxide, a molybdenum oxide, a niobium oxide, a rhenium oxide, a tungsten oxide, a ruthenium oxide, a titanium oxide, a chromium oxide, a zirconium oxide, a hafnium oxide, and a tantalum oxide are particularly preferable.

Next, the light-emitting layer123will be described. It is preferable that the light-emitting layer123be a layer including a luminescent material dispersed in a material that has a larger energy gap than the luminescent material. However, the light-emitting layer123is not to be considered limited to this. It is to be noted that the energy gap indicates the energy gap between the LUMO level and the HOMO level. In addition, a material that provides a favorable luminous efficiency and is capable of producing luminescence of a desired emission wavelength may be used for the luminescent material.

In addition, for the material that is used for dispersing the luminescent material, for example, anthracene derivatives such as anthracene derivatives such as 9,10-di(2-naphthyl)-2-tert-butylanthracene (abbreviation: t-BuDNA), carbazole derivatives such as 4,4′-bis(N-carbazolyl)-biphenyl (abbreviation: CBP), and metal complexes such as bis[2-(2-hydroxyphenyl)-pyridinato]zinc (abbreviation: Znpp2) and bis[2-(2-hydroxyphenyl)-benzoxazolato]zinc (abbreviation: ZnBOX) can be used. However, the material that is used for dispersing the luminescent material is not limited to these materials. When the luminescent material is dispersed as described above, concentration quenching of luminescence from the luminescent material can be prevented.

In order to produce white or whitish light emission from this light-emitting layer123, for example, a structure of TPD (aromatic diamine), 3-(4-tert-butylphenyl)-4-phenyl-5-(4-biphenylyl)-1,2,4-triazole (abbreviation: TAZ), tris(8-quinolinolato) aluminum (abbreviation: Alq3), Alq3doped with NileRed that is a red luminescent dye, and Alq3that are laminated by evaporation or the like in this order from the first electrode101side can be used.

In addition, a structure of NPB, NPB doped with perylene, bis(2-methyl-8-quinolinolato)-4-phenylphenolato-aluminum (abbreviation: BAlq) doped with DCM1, BAlq, and Alq3that are laminated by evaporation or the like in this order from the first electrode101side can be used.

In addition, white or whitish light emission can be obtained by dispersing 30 wt % 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (abbreviation: PBD) as an electron transporting agent in poly(N-vinylcarbasole) (abbreviation: PVK) and dispersing appropriate amounts of four kinds of dyes (TPB, coumarin 6, DCM 1, and NileRed).

Moreover, white or whitish light emission can be also obtained by forming the light-emitting layer123using a laminated structure with the use of materials that produce luminescence in relation of complementary colors to each other, for example, a first and second layers using luminescent materials for red and blue-green, respectively.

In the case of emitting white or whitish light as described above, full-color display can be performed with the use of a color filter or a color conversion layer. It is to be noted that mono-color display can be performed in the case of using a monochromatic color filter or a monochromatic color conversion layer.

Further, materials for the light-emitting layer123can be appropriately selected besides the light-emitting elements described above, which provide white or whitish light emission. For example, the light-emitting layer123may be formed by using respective luminescent materials for red (R), green (G), and blue (B).

For example, when red or reddish luminescence is desired to be obtained, 4-dicyanomethylene-2-isopropyl-6-[2-(1,1,7,7-tetramethyljulolidine-9-yl)ethenyl]-4H-pyran (abbreviation: DCJTI), 4-dicyanomethylene-2-methyl-6-[2-(1,1,7,7-tetramethyljulolidine-9-yl)ethenyl]-4H-pyran (abbreviation: DCJT), 4-dicyanomethylene-2-tert-butyl-6-[2-(1,1,7,7-tetramethyljulolidine-9-yl)ethenyl]-4H-pyran (abbreviation: DCJTB), periflanthene, and 2,5-dicyano-1,4-bis-[2-(10-methoxy-1,1,7,7-tetramethyljulolidine-9-yl)ethenyl]-benzene, bis[2,3-bis(4-fluorophenyl)quinoxalinato]iridium (acetylacetonato) (abbreviation: Ir(Fdpq)2(acac)), and the like can be used for the light-emitting layer123. However, the material for obtaining red or reddish luminescence is not limited to these materials, and a material that produces luminescence with an emission spectrum peak from 600 nm to 700 nm can be used.

When green or greenish luminescence is desired to be obtained, N,N′-dimethylquinacridone (abbreviation: DMQd), coumarin 6, coumarin 545T, tris(8-quinolinolato) aluminum (abbreviation: Alq3), and the like can be used for the light-emitting layer123. However, the material for obtaining green or greenish luminescence is not limited to these materials, and a material that produces luminescence with an emission spectrum peak from 500 nm to 600 nm can be used.

In addition, when blue or bluish luminescence is desired to be obtained, 9,10-bis(2-naphthyl)-tert-butylanthracene (abbreviation: t-BuDNA), 9,9′-bianthryl, 9,10-diphenylanthracene (abbreviation: DPA), 9,10-bis(2-naphthyl)anthracene (abbreviation: DNA), bis(2-methyl-8-quinolinolato)-4-phenylphenolato-gallium (abbreviation: BGaq), bis(2-methyl-8-quinolinolato)-4-phenylphenolato-aluminum (abbreviation: BAlq), and the like can be used for the light-emitting layer123. However, the material for obtaining blue or bluish luminescence is not limited to these materials, and a material that produces luminescence with an emission spectrum peak from 400 nm to 500 nm can be used.

Also, when the light-emitting layer123is formed to include respective luminescent materials for red (R), green (G), and blue (B), the peak of each emission spectrum and the like may be adjusted by providing a color filter or a color conversion layer. A color filter or a color conversion layer may be formed on the side from which light emission is extracted outside, or can be provided on any of the substrate side where a thin film transistor is formed and the opposed substrate side.

Next, the electron transporting layer124will be described. The electron transporting layer124is a layer that has a function of transporting electrons injected from the second electrode102to the light-emitting layer123. By providing the electron transporting layer124in this way to keep the second electrode102and the light-emitting layer123further away from each other, quenching of luminescence due to a metal can be prevented.

It is to be noted that the electron transporting layer124is not particularly limited, and can be formed by using a metal complex having a quinoxaline skeleton or a benzoquinoline skeleton, such as tris(8-quinolinolato) aluminum (abbreviation: Alq3), tris(4-methyl-8-quinolinolato) aluminum (abbreviation: Almq3), bis(10-hydroxybenzo[h]-quinolinato) beryllium (abbreviation: BeBq2), and bis(2-methyl-8-quinolinolato)-4-phenylphenolato-aluminum (abbreviation: BAlq). In addition, the electron transporting layer124may be formed by using a metal complex having an oxazole-based or thiazole-based ligand, such as bis[2-(2-hydroxyphenyl)-benzoxazolato]zinc (abbreviation: Zn(BOX)2) and bis[2-(2-hydroxyphenyl)-benzothiazolato]zinc (abbreviation: Zn(BTZ)2). Moreover, the electron transporting layer124may be formed by using 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (abbreviation: PBD), 1,3-bis[5-(p-tert-butylphenyl)-1,3,4-oxadiazole-2-yl]benzene (abbreviation: OXD-7), 3-(4-tert-butylphenyl)-4-phenyl-5-(4-biphenylyl)-1,2,4-triazole (abbreviation: TAZ), 3-(4-tert-butylphenyl)-4-(4-ethylphenyl)-5-(4-biphenylyl)-1,2,4-triazole (abbreviation: p-EtTAZ), bathophenanthroline (abbreviation: BPhen), bathocuproin (abbreviation: BCP), or the like.

It is preferable that the electron transporting layer124be formed with the use of a material in which the hole mobility is higher than the electron mobility. Further, it is more preferable that the electron transporting layer124be formed with the use of a material that has an electron mobility of 10−6cm2/Vs or more. In addition, the electron transporting layer124may have a laminated structure formed by combining two or more layers each including the material described above.

Further, it is preferable that a layer in which the organic compound described above and an inorganic compound are mixed is used for the electron transporting layer124. Crystallization of the organic compound can be further suppressed by mixing the inorganic compound, and the layer can be thus formed to be thicker without increase in resistance. Therefore, even when there is irregularity due to dust, contamination, and the like, on a substrate, the irregularity has almost no influence due to the electron transporting layer124being thicker. Accordingly, defects such as a short circuit between the first electrode101and the second electrode102due to irregularity can be prevented.

Metal oxides, metal nitrides, and metal oxynitrides can be used for this inorganic compound. For example, the metal oxides include a lithium oxide (Li2O), a calcium oxide (CaO), a sodium oxide (Na2O), a potassium oxide (K2O), a magnesium oxide (MgO), and further include a lithium fluoride (LiF), a cesium fluoride (CsF), and calcium fluoride (CaF2).

This second layer112can be manufactured by evaporation. When a mixed layer is formed for the second layer112, co-evaporation can be used. The co-evaporation includes co-evaporation by resistance-heating evaporation, co-evaporation by electron-beam evaporation, and co-evaporation by resistance-heating evaporation and electron-beam evaporation, and in addition, there are methods such as deposition by resistance-heating evaporation and sputtering and deposition by electron-beam evaporation and sputtering. The first layer111can be formed by combining the same type of methods or different types of methods. In addition, the example described above shows a layer including two kinds of materials. However, when three or more kinds of materials are included, the first layer111can be formed also in the same way by combining the same type of methods or different types of methods as described above.

Next, the third layer113that is layer that generates electrons will be described. As this third layer113, for example, a layer including an electron transporting material and a material that exhibits an electron donating property to the electron transporting material can be cited.

It is to be noted that the electron transporting material is a material in which more electrons are transported than holes, and for example, metal complexes such as tris (8-quinolinolato) aluminum (abbreviation: Alq3), tris(4-methyl-8-quinolinolato) aluminum (abbreviation: Almq3), bis(10-hydroxybenzo[h]-quinolinato)beryllium (abbreviation: BeBq2), bis(2-methyl-8-quinolinolato)-4-phenylphenolato-aluminum (abbreviation: BAlq), bis[2-(2-hydroxyphenyl)-benzoxazolato]zinc (abbreviation: Zn(BOX)2), and bis[2-(2-hydroxyphenyl)-benzothiazolato]zinc (abbreviation: Zn(BTZ)2), and further, 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (abbreviation: PBD), 1,3-bis[5-(p-tert-butylphenyl)-1,3,4-oxadiazole-2-yl]benzene (abbreviation: OXD-7), 3-(4-tert-butylphenyl)-4-phenyl-5-(4-biphenylyl)-1,2,4-triazole (abbreviation: TAZ), 3-(4-tert-butylphenyl)-4-(4-ethylphenyl)-5-(4-biphenylyl)-1,2,4-triazole (abbreviation: p-EtTAZ), bathophenanthroline (abbreviation: BPhen), bathocuproin (abbreviation: BCP), and 4,4-bis(5-methyl-benzoxazol-2-yl)stilbene (abbreviation: BzOs) can be used for the electron transporting material. In addition, the third layer113can be formed with the use of an n-type semiconductor. However, the electron transporting material is not limited to these.

In addition, for the material that exhibits an electron donating property to the electron transporting material, a substance selected from alkali metals and alkali-earth metals, specifically such as lithium (Li), calcium (Ca), sodium (Na), potassium (K), and magnesium (Mg), can be used. Further, specific materials include oxides of the alkali metals, oxides of the alkali-earth metals, nitrides of the alkali metals, and nitrides of the alkali-earth metals, specifically, a lithium oxide (Li2O), a calcium oxide (CaO), a sodium oxide (Na2O), a potassium oxide (K2O), and a magnesium oxide (MgO), and further include lithium fluoride (LiF), cesium fluoride (CsF), and calcium fluoride (CaF2). However, the material that exhibits an electron donating property to the electron transporting material is not limited to these. It is to be noted that it is preferable that the material that exhibits an electron donating property to the electron transporting material be included so that the molar ratio of material that exhibits an electron donating property to the electron transporting material to the electron transporting material is 0.5 or more and 2 or less (=the material that exhibits an electron donating property to the electron transporting material/the electron transporting material).

Alternatively, the third layer113may be a layer composed of a material such as zinc oxide, zinc sulfide, zinc selenide, tin oxide, or titanium oxide.

The third layer113is preferably formed by using a layer in which the above-mentioned organic compound and an inorganic compound are mixed. Accordingly, the conductivity of the third layer113can be made higher. When the conductivity is higher, the third layer113can be made thicker, and the yield of manufacturing can be thus improved. Further, the light-emitting layer123and the second electrode102can be kept further away from each other by making the third layer113thicker, and quenching of luminescence can be thus prevented.

Further, crystallization of the organic compound that is used for the third layer113can be suppressed by using the layer in which the organic material and the inorganic material are mixed, and the third layer113can be thus formed to be thicker without increase in resistance. Therefore, even when there is irregularity due to dust, contamination, and the like on a substrate, the irregularity has almost no influence since the first layer111is made thicker. Accordingly, defects such as a short circuit between the first electrode101and the second electrode102due to irregularity can be prevented.

This third layer113can be manufactured by evaporation. When a mixed layer is formed for the third layer113, co-evaporation can be used. The co-evaporation includes co-evaporation by resistance-heating evaporation, co-evaporation by electron-beam evaporation, and co-evaporation by resistance-heating evaporation and electronic-beam evaporation, and in addition, there are methods such as deposition by resistance-heating evaporation and sputtering and deposition by electron-beam evaporation and sputtering. The first layer111can be formed by combining the same type of methods or different types of methods. In addition, the example described above shows a layer including two kinds of materials. However, when three or more kinds of materials are included, the first layer111can be formed also in the same way by combining the same type of methods or different types of methods as described above.

In the light-emitting element described above, the difference between the electron affinity of the electron transporting material included in the third layer113and the electron affinity of the material included in the layer in contact with the third layer113among the layers included in the second layer112is preferably 2 eV or less, more preferably 1.5 eV or less. Alternatively, when the third layer113is composed of an n-type semiconductor, the difference between the work function of the n-type semiconductor and the electron affinity of the material included in the layer in contact with the third layer113among the layers included in the second layer112is preferably 2 eV or less, more preferably 1.5 eV or less. By joining the second layer112and the third layer113as described above, electrons can be injected more easily from the third layer113to the second layer112.

It is to be noted that the present invention has a feature of a light-emitting element including an organic compound as typified by an organic compound a benzidine derivative that has a glass-transition temperature of 150° C. or more, preferably 160° C. or more and 300° C. or less and an inorganic compound between a pair of electrodes, and is not to be considered limited to the structure of the light-emitting element shown inFIG. 1. For example, there may be a case where the electron transporting layer124is not provided although the structure provided with the electron transporting layer124formed in contact with the third layer113is shown. Accordingly, the light-emitting layer123in contact with the third layer113is provided. In this case, a material for dispersing a luminescent material is preferably used for the light-emitting layer123. Also, it may well be that the electron transporting layer124is not provided.

In addition, a material that is capable of producing luminescence without being dispersed, such as Alq3, can be used for the light-emitting layer123. Since the material such as Alq3is a luminescent material that has a favorable carrier transporting property, a layer composed of only Alq3can function as a light-emitting layer without dispersing Alq3. In this case, the light-emitting layer123corresponds to a luminescent material itself.

These first to third layers111to113can be formed by the same method, and can be therefore formed continuously without being exposed to the air. Impurity mixing into an interface and the like can be reduced by forming the first to third layers111to113continuously without being exposed to the air in this way.

Next, the electrodes will be described. Each of the first electrode101and the second electrode102are formed by using a conductive material. Further, the electrode provided on the side from which light from the light-emitting layer is extracted outside needs to have a light-transmitting property in addition to conductivity. The light-transmitting property can be obtained also by forming a quite thin film composed of a material that has no light-transmitting property.

As a material for the first electrode101, light-transmitting materials such as indium tin oxide (ITO), indium tin oxide containing silicon oxide (hereinafter, referred to as ITSO), and indium oxide containing zinc oxide, and in addition, metal materials such as gold (Au), platinum (Pt), nickel (Ni), tungsten (W), chromium (Cr), molybdenum (Mo), iron (Fe), cobalt (Co), copper (Cu), and palladium (Pd) can be used in addition to aluminum (Al). Further, the first electrode101can be formed, for example, by sputtering or evaporation. However, the material for the first electrode101is not limited to these.

When the above-mentioned material that has no light-transmitting property is used and the first electrode101needs to have a light-transmitting property, a thin film composed of the material may be formed.

In addition, a single-layer of the metal material mentioned above or a lamination layer can be used for the first electrode101. Therefore, when a lamination layer is used for the first electrode101, it is also possible to use a structure of forming a thin film of the material mentioned above and laminating a light-transmitting material thereon. Of course, the first electrode101may be formed with the use of the thin material as a single layer. In order to prevent the resistance from increasing by forming the first electrode101to be thin, an auxiliary wiring can also be provided. Further, the use of a lamination layer can prevent the resistance from increasing.

Further, as a material for the second electrode102, light-transmitting materials such as indium tin oxide (ITO), indium tin oxide containing silicon oxide (ITSO), and indium oxide containing zinc oxide, and metal materials such as gold (Au), platinum (Pt), nickel (Ni), tungsten (W), chromium (Cr), molybdenum (Mo), iron (Fe), cobalt (Co), copper (Cu), and palladium (Pd) can be used. However, the material for the first electrode101is not limited to these.

When the above-mentioned material that has no light-transmitting property is used and the second electrode102needs to have a light-transmitting property, a thin film composed of the material may be formed.

In addition, a single layer of the metal material mentioned above or a lamination layer can be used for the second electrode102. Therefore, when a lamination layer is used for the second electrode102, it is also possible to use a structure of forming a thin film of the material mentioned above and laminating a light-transmitting material thereon. Of course, the second electrode102may be formed with the use of the thin material as a single layer. In order to prevent the resistance from increasing by forming the second electrode102to be thin, an auxiliary wiring can also be provided. Further, the use of a lamination layer can prevent the resistance from increasing.

It is to be noted that the first electrode101or the second electrode103can be an anode or a cathode depending on a voltage that is applied to the light-emitting element. In the case of an anode, a material that has a larger work function (a work function of 4.0 eV or more) is used. Alternatively, in the case of a cathode, a material that has a smaller work function (a work function of 3.8 eV or less) is used.

The first electrode101or the second electrode102can be formed by sputtering, evaporation, or the like. In the case of using evaporation, the first electrode101, the first to third layers111to113, and the second electrode102can be formed continuously without being exposed to the air. Impurity mixing into an interface and the like can be reduced by forming the light-emitting element continuously without being exposed to the air in this way.

As described above, according to the present invention, a light-emitting element that is less likely to change in characteristics by change in characteristics of a hole transporting layer due to heat can be obtained by forming a layer that generates holes with the use of a layer including an organic compound as typified by a benzidine derivative that has a glass-transition temperature of 150° C. or more, preferably 160° C. or more and 300° C. or less and an inorganic compound.

Further, by forming a layer that generates holes with the use of a layer including an organic compound as typified by a benzidine derivative that has a glass-transition temperature of 150° C. or more, preferably 160° C. or more and 300° C. or less and an inorganic compound, a light-emitting element that is less likely to be deteriorated by crystallization of the layer can be obtained.

Accordingly, a light-emitting element achieving a lower driving voltage can be obtained. Further, even when the light-emitting element according to the present invention is made thicker, the driving voltage is not increased. Accordingly, the light-emitting element can be formed to be thicker, and can be thus produced at a favorable yield. Further, a light-emitting layer can be kept further away from a first electrode or a second electrode by making a layer thicker, and quenching of luminescence can be thus prevented.

As described above, the present embodiment is described with the use of a benzidine derivative as an organic compound that has a glass-transition temperature of 150° C. or more, preferably 160° C. or more and 300° C. or less. However, the present invention provides a light-emitting element that includes a layer in which an organic compound and a metal oxide are mixed, and is not limited to the present embodiment as long as an effect of reduced deterioration with time is achieved.

An example of a light-emitting element using a layer including a benzidine derivative according to the present invention, which is a compound having a spiro ring and a triphenylamine skeleton, and an inorganic compound will be described with reference toFIG. 18.

As shown inFIG. 18, the light-emitting element according to the present invention has a first electrode101and a second electrode102that are opposed to each other, and has a first layer111, a second layer112, and a third layer113that are stacked in this order from the first electrode101side. When a voltage is applied to this light-emitting element so that the potential of the first electrode101is higher than the potential of the second electrode102, a hole is injected from the first layer111into the second layer112, and an electron is injected from the third layer113into the second layer112. The hole and the electron are recombined to excite a luminescent material. Then, luminescence is produced when the excited luminescent material returns to the ground state.

Next, the first to third layers111to113, the first electrode101, and the second electrode102will be described.

The first layer111is a layer that generates holes. This function can be achieved by using, for example, a layer including a hole transporting material and a material that exhibits an electron accepting property to the hole transporting material. In addition, it is preferable that the material that exhibits an electron accepting property to the hole transporting material be included so that the molar ratio of the material to the hole transporting material (=the material that exhibits an electron accepting property to the hole transporting material/the hole transporting material) is 0.5 or more and 2 or less.

The hole transporting material is a material in which electrons are transported more then electrodes, and for example, organic compounds, aromatic amine compounds such as 4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: α-NPD), 4,4′-bis[N-(3-methylphenyl)-N-phenylamino]biphenyl (abbreviation: TPD), 4,4′,4″-tris(N,N-diphenylamino)triphenylamine (abbreviation: TDATA), 4, 4′,4″-tris[N-(3-methylphenyl)-N-phenylamino]-triphenylamine (abbreviation: MTDATA), 4,4′-bis[N-{4-(N,N-di-m-tolylamino)phenyl}-N-phenylamino]biphenyl (abbreviation: DNTPD), and phthalocyanine compounds such as phthalocyanine (abbreviation: H2Pc) and copper phthalocyanine (abbreviation: CuPc), can be used as the hole transporting material. It is to be noted that the hole transporting material is not to be considered limited to these.

In addition, an oxide of a transition metal belonging to any one of Group 4 to 12 of the periodic table can be used as the material that exhibits an electron accepting property to the hole transporting material. Among others, an oxide of a transition metal belonging to any one of Groups 4 to 8 of the periodic table often has a higher electron accepting property, and a vanadium oxide, a molybdenum oxide, a niobium oxide, a rhenium oxide, a tungsten oxide, a ruthenium oxide, a titanium oxide, a chromium oxide, a zirconium oxide, a hafnium oxide, and a tantalum oxide are particularly preferable. Besides the oxides, nitrides and oxynitrides of the metals mentioned above may be used. It is to be noted that the material that exhibits am electron accepting property to the hole transporting material is not to be considered limited to these.

When the first layer111is formed by using a layer in which the hole transporting material, which is composed of an organic material, and the material that exhibits an electron transporting material to the hole transporting material, which is composed the inorganic material mentioned above, are mixed, the conductivity thereof gets higher. Therefore, it is preferable to form the first layer111in this way. When the conductivity is higher, the first layer111can be made thicker, and the yield of manufacturing can be thus improved. Further, the first electrode101and the second layer112can be kept further away from each other by making the first layer111thicker. Accordingly, quenching of luminescence due to a metal can be prevented.

Crystallization of the organic compound that is used for the first layer111can be suppressed by using this layer in which the organic material and the inorganic material are mixed, and the first layer111can be thus formed to be thicker without increase in resistance. Therefore, even when there is irregularity due to dust, contamination, and the like on a substrate, the irregularity has almost no influence since the first layer111is made thicker. Accordingly, defects such as a short circuit between the first electrode101and the second electrode102due to irregularity can be prevented.

As will be described in further detail below in the example, it is determined that the first layer111is not likely to be deteriorated with time when a layer in which a benzidine derivative and a molybdenum oxide that is an inorganic compound are mixed is used for the first layer111. In short, this structure makes it possible to provide a light-emitting element that is excellent in stability.

Further, the first layer111may include another organic compound. As the organic compound, rubrene and the like can be cited. The reliability can be improved by the addition of rubrene.

In addition to this, the first layer111may be a layer composed of a metal oxide such as a molybdenum oxide, a vanadium oxide, a ruthenium oxide, a cobalt oxide, and a copper oxide.

This first layer111can be formed by evaporation. When a layer including a plurality of mixed compounds is used as the first layer111, co-evaporation can be used. The co-evaporation includes co-evaporation by resistance-heating evaporation, co-evaporation by electron-beam evaporation, and co-evaporation by resistance-heating evaporation and electron-beam evaporation, and in addition, there are methods such as deposition by resistance-heating evaporation and sputtering and deposition by electron-beam evaporation and sputtering. The first layer111can be formed by combining the same type of methods or different types of methods. In addition, the example described above shows a layer including two kinds of materials. However, when three or more kinds of materials are included, the first layer111can be formed also in the same way by combining the same type of methods or different types of methods as described above.

Next, the second layer112that is a layer including a light-emitting layer will be described. The layer including the light-emitting layer may be a single layer composed of only the light-emitting layer or a multilayer. To cite a case, a specific multilayer includes a light-emitting layer and additionally a plurality of layers selected from electron transporting layers and hole transporting layers. InFIG. 1, a multilayer case in which the second layer112includes a light-emitting layer123, an electron transporting layer124, and a hole transporting layer124is shown.

A benzidine derivative according to the present invention, which is represented by a general formula (1) or any one of structure formulas (2) to (4), can be used for the hole transporting layer112.

Since the benzidine derivative according to the present invention has high heat resistance, the hole transporting layer122that is less likely to change in characteristics due to heat can be formed by using the benzidine derivative according to the present invention as a hole transporting material. Further, since the benzidine derivative according to the present invention is not likely to be crystallized, the hole transporting layer122that is not likely to be crystallized can be formed by using the benzidine derivative according to the present invention as a hole transporting material.

It is to be noted that the hole transporting layer112may be a multilayer formed by combining two or more layers each including the benzidine derivative according to the present invention, which is represented by the general formula (1) or any one of the structure formulas (2) to (4).

Further, it is preferable that a layer in which the benzidine derivative according to the present invention and an inorganic compound are mixed be used for the hole transporting layer122. Crystallization of the benzidine derivative can be further suppressed by mixing the inorganic compound, and the layer can be thus formed to be thicker without increase in resistance. Therefore, even when there is irregularity due to dust, contamination and the like, on a substrate, the irregularity has almost no influence since the hole transporting layer122is made thicker. Accordingly, defects such as a short circuit between the first electrode101and the second electrode102due to irregularity can be prevented.

Metal oxides, metal nitrides, and metal oxynitrides can be used for this inorganic compound. For example, an oxide of a transition metal belonging to any one of Group 4 to 12 of the periodic table can be used as a metal oxide. Among others, an oxide of a transition metal belonging to any one of Groups 4 to 8 of the periodic table often has a higher electron accepting property, and a vanadium oxide, a molybdenum oxide, a niobium oxide, a rhenium oxide, a tungsten oxide, a ruthenium oxide, a titanium oxide, a chromium oxide, a zirconium oxide, a hafnium oxide, and a tantalum oxide are particularly preferable.

Next, the light-emitting layer123will be described. It is preferable that the light-emitting layer123be a layer including a luminescent material dispersed in a material that has a larger energy gap than the luminescent material. However, the light-emitting layer123is not to be considered limited to this. It is to be noted that the energy gap indicates the energy gap between the LUMO level and the HOMO level. In addition, a material that provides a favorable luminous efficiency and is capable of producing luminescence of a desired emission wavelength may be used for the luminescent material.

In addition, for the material that is used for dispersing the luminescent material, for example, anthracene derivatives such as anthracene derivatives such as 9,10-di(2-naphthyl)-2-tert-butylanthracene (abbreviation: t-BuDNA), carbazole derivatives such as 4,4′-bis(N-carbazolyl)-biphenyl (abbreviation: CBP), and metal complexes such as bis[2-(2-hydroxyphenyl)-pyridinato]zinc (abbreviation: Znpp2) and bis[2-(2-hydroxyphenyl)-benzoxazolato]zinc (abbreviation: ZnBOX) can be used. However, the material that is used for dispersing the luminescent material is not limited to these materials. When the luminescent material is dispersed as described above, concentration quenching of luminescence from the luminescent material can be prevented.

In order to produce white or whitish light emission from this light-emitting layer123, for example, a structure of TPD (aromatic diamine), 3-(4-tert-butylphenyl)-4-phenyl-5-(4-biphenylyl)-1,2,4-triazole (abbreviation: TAZ), tris(8-quinolinolato) aluminum (abbreviation: Alq3), Alq3doped with NileRed that is a red luminescent dye, and Alq3that are laminated by evaporation or the like in this order from the first electrode101side can be used.

In addition, a structure of NPB, NPB doped with perylene, bis(2-methyl-8-quinolinolato)-4-phenylphenolato-aluminum (abbreviation: BAlq) doped with DCM1, BAlq, and Alq3that are laminated by evaporation or the like in this order from the first electrode101side can be used.

In addition, white or whitish light emission can be obtained by dispersing 30 wt % 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (abbreviation: PBD) as an electron transporting agent in poly(N-vinylcarbasole) (abbreviation: PVK) and dispersing appropriate amounts of four kinds of dyes (TPB, coumarin 6, DCM 1, and NileRed).

Moreover, white or whitish light emission can be also obtained by forming the light-emitting layer123using a laminated structure with the use of materials that produce luminescence in relation of complementary colors to each other, for example, a first and second layers using luminescent materials for red and blue-green, respectively.

In the case of emitting white or whitish light as described above, full-color display can be performed with the use of a color filter or a color conversion layer. It is to be noted that mono-color display can be performed in the case of using a monochromatic color filter or a monochromatic color conversion layer.

Further, materials for the light-emitting layer123can be appropriately selected besides the light-emitting elements described above, which provide white or whitish light emission. For example, the light-emitting layer123may be formed by using respective luminescent materials for red (R), green (G), and blue (B).

For example, when red or reddish luminescence is desired to be obtained, 4-dicyanomethylene-2-isopropyl-6-[2-(1,1,7,7-tetramethyljulolidine-9-yl)ethenyl]-4H-pyran (abbreviation: DCJTI), 4-dicyanomethylene-2-methyl-6-[2-(1,1,7,7-tetramethyljulolidine-9-yl)ethenyl]-4H-pyran (abbreviation: DCJT), 4-dicyanomethylene-2-tert-butyl-6-[2-(1,1,7,7-tetramethyljulolidine-9-yl)ethenyl]-4H-pyran (abbreviation: DCJTB), periflanthene, and 2,5-dicyano-1,4-bis-[2-(10-methoxy-1,1,7,7-tetramethyljulolidine-9-yl)ethenyl]-benzene, bis[2,3-bis(4-fluorophenyl)quinoxalinato]iridium(acetylacetonato) (abbreviation: Ir(Fdpq)2(acac)), and the like can be used for the light-emitting layer123. However, the material for obtaining red or reddish luminescence is not limited to these materials, and a material that produces luminescence with an emission spectrum peak from 600 nm to 700 nm can be used.

When green or greenish luminescence is desired to be obtained, N,N′-dimethylquinacridone (abbreviation: DMQd), coumarin 6, coumarin 545T, tris(8-quinolinolato) aluminum (abbreviation: Alq3), and the like can be used for the light-emitting layer123. However, the material for obtaining green or greenish luminescence is not limited to these materials, and a material that produces luminescence with an emission spectrum peak from 500 nm to 600 nm can be used.

In addition, when blue or bluish luminescence is desired to be obtained, 9,10-bis(2-naphthyl)-tert-butylanthracene (abbreviation: t-BuDNA), 9,9′-bianthryl, 9,10-diphenylanthracene (abbreviation: DPA), 9,10-bis(2-naphthyl)anthracene (abbreviation: DNA), bis(2-methyl-8-quinolinolato)-4-phenylphenolato-gallium (abbreviation: BGaq), bis(2-methyl-8-quinolinolato)-4-phenylphenolato-aluminum (abbreviation: BAlq), and the like can be used for the light-emitting layer123. However, the material for obtaining blue or bluish luminescence is not limited to these materials, and a material that produces luminescence with an emission spectrum peak from 400 nm to 500 nm can be used.

Also, when the light-emitting layer123is formed to include respective luminescent materials for red (R), green (G), and blue (B), the peak of each emission spectra and the like may be adjusted by providing a color filter or a color conversion layer. A color filter or a color conversion layer may be formed on the side from which light emission is extracted outside, or can be provided on any of the substrate side where a thin film transistor is formed and the opposed substrate side.

Next, the electron transporting layer124will be described. The electron transporting layer124is a layer that has a function of transporting electrons injected from the second electrode102to the light-emitting layer123. By providing the electron transporting layer124in this way to keep the second electrode102and the light-emitting layer123further away from each other, quenching of luminescence due to a metal can be prevented.

It is to be noted that the electron transporting layer124is not particularly limited, and can be formed by using a metal complex having a quinoxaline skeleton or a benzoquinoline skeleton, such as tris(8-quinolinolato) aluminum (abbreviation: Alq3), tris(4-methyl-8-quinolinolato) aluminum (abbreviation: Almq3), bis(10-hydroxybenzo[h]-quinolinato) beryllium (abbreviation: BeBq2), and bis(2-methyl-8-quinolinolato)-4-phenylphenolato-aluminum (abbreviation: BAlq). In addition, the electron transporting layer124may be formed by using a metal complex having an oxazole-based or thiazole-based ligand, such as bis[2-(2-hydroxyphenyl)-benzoxazolato]zinc (abbreviation: Zn(BOX)2) and bis[2-(2-hydroxyphenyl)-benzothiazolato]zinc (abbreviation: Zn(BTZ)2). Moreover, the electron transporting layer124may be formed by using 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (abbreviation: PBD), 1,3-bis[5-(p-tert-butylphenyl)-1,3,4-oxadiazole-2-yl]benzene (abbreviation: OXD-7), 3-(4-tert-butylphenyl)-4-phenyl-5-(4-biphenylyl)-1,2,4-triazole (abbreviation: TAZ), 3-(4-tert-butylphenyl)-4-(4-ethylphenyl)-5-(4-biphenylyl)-1,2,4-triazole (abbreviation: p-EtTAZ), bathophenanthroline (abbreviation: BPhen), bathocuproin (abbreviation: BCP), or the like.

It is preferable that the electron transporting layer124be formed with the use of a material in which the hole mobility is higher than the electron mobility. Further, it is more preferable that the electron transporting layer124be formed with the use of a material that has an electron mobility of 10−6cm2/Vs or more. In addition, the electron transporting layer124may have a laminated structure formed by combining two or more layers each including the material described above.

Further, it is preferable that a layer in which the organic compound described above and an inorganic compound are mixed is used for the electron transporting layer124. Crystallization of the organic compound can be further suppressed by mixing the inorganic compound, and the layer can be thus formed to be thicker without increase in resistance. Therefore, even when there is irregularity due to dust, contamination, and the like on a substrate, the irregularity has almost no influence by to the electron transporting layer124is made thicker. Accordingly, defects such as a short circuit between the first electrode101and the second electrode102due to irregularity can be prevented.

Metal oxides, metal nitrides, and metal oxynitrides can be used for this inorganic compound. For example, the metal oxides include a lithium oxide (Li2O), a calcium oxide (CaO), a sodium oxide (Na2O), a potassium oxide (K2O), a magnesium oxide (MgO), and further include a lithium fluoride (LiF), a cesium fluoride (CsF), and calcium fluoride (CaF2).

This second layer112can be manufactured by evaporation. When a mixed layer is formed for the second layer112, co-evaporation can be used. The co-evaporation includes co-evaporation by resistance-heating evaporation, co-evaporation by electron-beam evaporation, and co-evaporation by resistance-heating evaporation and electron-beam evaporation, and in addition, there are methods such as deposition by resistance-heating evaporation and sputtering and deposition by electron-beam evaporation and sputtering. The first layer111can be formed by combining the same type of methods or different types of methods. In addition, the example described above shows a layer including two kinds of materials. However, when three or more kinds of materials are included, the first layer111can be formed also in the same way by combining the same type of methods or different types of methods as described above.

Next, the third layer113that is layer that generates electrons will be described. As this third layer113, for example, a layer including an electron transporting material and a material that exhibits an electron donating property to the electron transporting material can be cited.

It is to be noted that the electron transporting material is a material in which more electrons are transported than holes, and for example, metal complexes such as tris (8-quinolinolato) aluminum (abbreviation: Alq3), tris(4-methyl-8-quinolinolato) aluminum (abbreviation: Almq3), bis(10-hydroxybenzo[h]-quinolinato)beryllium (abbreviation: BeBq2), bis(2-methyl-8-quinolinolato)-4-phenylphenolato-aluminum (abbreviation: BAlq), bis[2-(2-hydroxyphenyl)-benzoxazolato]zinc (abbreviation: Zn(BOX)2), and bis[2-(2-hydroxyphenyl)-benzothiazolato]zinc (abbreviation: Zn(BTZ)2), and further, 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (abbreviation: PBD), 1,3-bis[5-(p-tert-butylphenyl)-1,3,4-oxadiazole-2-yl]benzene (abbreviation: OXD-7), 3-(4-tert-butylphenyl)-4-phenyl-5-(4-biphenylyl)-1,2,4-triazole (abbreviation: TAZ), 3-(4-tert-butylphenyl)-4-(4-ethylphenyl)-5-(4-biphenylyl)-1,2,4-triazole (abbreviation: p-EtTAZ), bathophenanthroline (abbreviation: BPhen), bathocuproin (abbreviation: BCP), and 4,4-bis(5-methyl-benzoxazol-2-yl)stilbene (abbreviation: BzOs) can be used for the electron transporting material. In addition, the third layer113can be formed with the use of an n-type semiconductor. However, the electron transporting material is not limited to these.

In addition, for the material that exhibits an electron donating property to the electron transporting material, a substance selected from alkali metals and alkali-earth metals, specifically such as lithium (Li), calcium (Ca), sodium (Na), potassium (K), and magnesium (Mg), can be used. Further, specific materials include oxides of the alkali metals, oxides of the alkali-earth metals, nitrides of the alkali metals, and nitrides of the alkali-earth metals, specifically, a lithium oxide (Li2O), a calcium oxide (CaO), a sodium oxide (Na2O), a potassium oxide (K2O), and a magnesium oxide (MgO), and further include lithium fluoride (LiF), cesium fluoride (CsF), and calcium fluoride (CaF2). However, the material that exhibits an electron donating property to the electron transporting material is not limited to these. It is to be noted that it is preferable that the material that exhibits an electron donating property to the electron transporting material be included so that the molar ratio of material that exhibits an electron donating property to the electron transporting material to the electron transporting material is 0.5 or more and 2 or less (=the material that exhibits an electron donating property to the electron transporting material/the electron transporting material).

Alternatively, the third-layer113may be a layer composed of a material such as zinc oxide, zinc sulfide, zinc selenide, tin oxide, or titanium oxide.

The third layer113is preferably formed by using a layer in which the above-mentioned organic compound and an inorganic compound are mixed. Accordingly, the conductivity of the third layer113can be made higher. When the conductivity is higher, the third layer113can be made thicker, and the yield of manufacturing can be thus improved. Further, the light-emitting layer123and the second electrode102can be kept further away from each other by making the third layer113thicker, and quenching of luminescence can be thus prevented.

Further, crystallization of the organic compound that is used for the third layer113can be suppressed by using the layer in which the organic material and the inorganic material are mixed, and the third layer113can be thus formed to be thicker without increase in resistance. Therefore, even when there is irregularity due to dust, contamination, and the like on a substrate, the irregularity has almost no influence since the third layer113is made thicker. Accordingly, defects such as a short circuit between the first electrode101and the second electrode102due to irregularity can be prevented.

This third layer113can be manufactured by evaporation. When a mixed layer is formed for the third layer113, co-evaporation can be used. The co-evaporation includes co-evaporation by resistance-heating evaporation, co-evaporation by electron-beam evaporation, and co-evaporation by resistance-heating evaporation and electron-beam evaporation, and in addition, there are methods such as deposition by resistance-heating evaporation and sputtering and deposition by electron-beam evaporation and sputtering. The first layer111can be formed by combining the same type of methods or different types of methods. In addition, the example described above shows a layer including two kinds of materials. However, when three or more kinds of materials are included, the first layer111can be formed also in the same way by combining the same type of methods or different types of methods as described above.

In the light-emitting element described above, the difference between the electron affinity of the electron transporting material included in the third layer113and the electron affinity of the material included in the layer in contact with the third layer113among the layers included in the second layer112is preferably 2 eV or less, more preferably 1.5 eV or less. Alternatively, when the third layer113is composed of an n-type semiconductor, the difference between the work function of the n-type semiconductor and the electron affinity of the material included in the layer in contact with the third layer113among the layers included in the second layer112is preferably 2 eV or less, more preferably 1.5 eV or less. By joining the second layer112and the third layer113as described above, electrons can be injected more easily from the third layer113to the second layer112.

It is to be noted that the present invention has a feature of a light-emitting element including an organic compound as typified by a benzidine derivative and an inorganic compound between a pair of electrodes, and is not to be considered limited to the structure of the light-emitting element shown inFIG. 18. For example, there may be a case where the electron transporting layer124is not provided although the structure provided with the electron transporting layer124formed in contact with the third layer113is shown. Accordingly, the light-emitting layer123in contact with the third layer113is provided. In this case, a material for dispersing a luminescent material is preferably used for the light-emitting layer123. Also, it may well be that the electron transporting layer124is not provided.

In addition, a material that is capable of producing luminescence without being dispersed, such as Alq3, can be used for the light-emitting layer123. Since the material such as Alq3is a luminescent material that has a favorable carrier transporting property, a layer composed of only Alq3can function as a light-emitting layer without dispersing Alq3. In this case, the light-emitting layer123corresponds to a luminescent material itself.

These first to third layers111to113can be formed by the same method, and can be therefore formed continuously without being exposed to the air. Impurity mixing into an interface and the like can be reduced by forming the first to third layers111to113continuously without being exposed to the air in this way.

Next, the electrodes will be described. Each of the first electrode101and the second electrode102are formed by using a conductive material. Further, the electrode provided on the side from which light from the light-emitting layer is extracted outside needs to have a light-transmitting property in addition to conductivity. The light-transmitting property can be obtained also by forming a quite thin film composed of a material that has no light-transmitting property.

As a material for the first electrode101, light-transmitting materials such as indium tin oxide (ITO), indium tin oxide containing silicon oxide (ITSO), and indium oxide containing zinc oxide, and in addition, metal materials such as gold (Au), platinum (Pt), nickel (Ni), tungsten (W), chromium (Cr), molybdenum (Mo), iron (Fe), cobalt (Co), copper (Cu), and palladium (Pd) can be used in addition to aluminum (Al). Further, the first electrode101can be formed, for example, by sputtering or evaporation. However, the material for the first electrode101is not limited to these.

When the above-mentioned material that has no light-transmitting property is used and the first electrode101needs to have a light-transmitting property, a thin film composed of the material may be formed.

In addition, a single layer of the metal material mentioned above or a lamination layer can be used for the first electrode101. Therefore, when a lamination layer is used for the first electrode101, it is also possible to use a structure of forming a thin film of the material mentioned above and laminating a light-transmitting material thereon. Of course, the first electrode101may be formed with the use of the thin material as a single layer. In order to prevent the resistance from increasing by forming the first electrode101to be thin, an auxiliary wiring can also be provided. Further, the use of a lamination layer can prevent the resistance from increasing.

Further, as a material for the second electrode102, light-transmitting materials such as indium tin oxide (ITO), indium tin oxide containing silicon oxide (hereinafter, also referred to as ITSO), and indium oxide containing zinc oxide, and metal materials such as gold (Au), platinum (Pt), nickel (Ni), tungsten (W), chromium (Cr), molybdenum (Mo), iron (Fe), cobalt (Co), copper (Cu), and palladium (Pd) can be used. However, the material for the first electrode101is not limited to these.

When the above-mentioned material that has no light-transmitting property is used and the second electrode102needs to have a light-transmitting property, a thin film composed of the material may be formed.

In addition, a single layer of the metal material mentioned above or a lamination layer can be used for the second electrode102. Therefore, when a lamination layer is used for the second electrode102, it is also possible to use a structure of forming a thin film of the material mentioned above and laminating a light-transmitting material thereon. Of course, the second electrode102may be formed with the use of the thin material as a single layer. In order to prevent the resistance from increasing by forming the second electrode102to be thin, an auxiliary wiring can also be provided. Further, the use of a lamination layer can prevent the resistance from increasing.

It is to be noted that the first electrode101or the second electrode103can be an anode or a cathode depending on a voltage that is applied to the light-emitting element. In the case of an anode, a material that has a larger work function (a work function of 4.0 eV or more) is used. Alternatively, in the case of a cathode, a material that has a smaller work function (a work function of 3.8 eV or less) is used.

The first electrode101or the second electrode102can be formed by sputtering, evaporation, or the like. In the case of using evaporation, the first electrode101, the first to third layers111to113, and the second electrode102can be formed continuously without being exposed to the air. Impurity mixing into an interface and the like can be reduced by forming the light-emitting element continuously without being exposed to the air in this way.

As described above, according to the present invention, a light-emitting element that is less likely to change in characteristics by change in characteristics of a hole transporting layer due to heat can be obtained by forming a layer that generates holes with the use of a layer including an organic compound as typified by a benzidine derivative and an inorganic compound. Further, by forming a layer that generates holes with the use of a layer including an organic compound as typified by a benzidine derivative and an inorganic compound, a light-emitting element that is less likely to be deteriorate by crystallization of the layer can be obtained.

Accordingly, a light-emitting element achieving a lower driving voltage can be obtained. Further, even when the light-emitting element according to the present invention is made thicker, the driving voltage is not increased. Accordingly, the light-emitting element can be formed to be thicker, and can be thus produced at a favorable yield. Further, a light-emitting layer can be kept further away from a first electrode or a second electrode by making a layer thicker, and quenching of luminescence can be thus prevented.

In the present embodiment, the structure of a light-emitting element that is different from the embodiments-described above will be described.

As shown inFIG. 2, the light-emitting element shown in the present embodiment has a first electrode101and a second electrode102that are opposed to each other, and has a first layer111, a second layer112, a third layer113, and a fourth layer128that are stacked in this order from the first electrode101side, where it is a feature that the fourth layer128is provided. The fourth layer128can be formed by using the same material as the first layer111, and the other structure is the same as the embodiment described above. Therefore, description of the structure other than the fourth layer128is omitted.

When the fourth layer128is provided in this way, damage during forming the second electrode102can be further reduced.

Further, a layer in which an organic compound and an inorganic compound are mixed is preferably used for the fourth layer128. Metal oxides, metal nitrides, and metal oxynitrides can be used for this inorganic compound. For example, an oxide of a transition metal belonging to any one of Group 4 to 12 of the periodic table can be used as a metal oxide. Among others, an oxide of a transition metal belonging to any one of Groups 4 to 8 of the periodic table often has a higher electron accepting property, and a vanadium oxide, a molybdenum oxide, a niobium oxide, a rhenium oxide, a tungsten oxide, a ruthenium oxide, a titanium oxide, a chromium oxide, a zirconium oxide, a hafnium oxide, and a tantalum oxide are particularly preferable.

When the organic compound and the inorganic compound are mixed in this way, the driving voltage can be kept lower even when the fourth layer128is made thicker.

In the present embodiment, a benzidine derivative that is an organic compound that has a glass-transition temperature of 150° C. or more, preferably 160° C. or more and 300° C. or less, and has a spiro ring and a triphenylamine skeleton will be described with reference to a general formula.

An aspect of the present invention is a benzidine derivative represented by a general formula (1).

In the general formula (1), R1is hydrogen or an alkyl group having 1 to 4 carbon atoms.

A benzidine derivative according to the present invention is a compound that is obtained by a coupling reaction of N,N′-diphenylbenzidine with 2-bromo-spiro-9,9′-bifluorene or 2-bromo-2′,7′-dialkyl-spiro-9,9′-bifluorene.

A light-emitting element according to the present invention is a light-emitting element that has a layer including an organic compound that is a compound that is obtained by a coupling reaction of N,N′-diphenylbenzidine with 2-bromo-spiro-9,9′-bifluorene.

The benzidine derivative according to the present invention has a feature that the glass-transition temperature meets 150° C. or more, preferably 160° C. or more and 300° C. or less, and a feature that the melting point meets 180° C. or more and 400° C. or less.

Since the benzidine derivative according to the present invention has excellent heat resistance, a light-emitting element that is less likely to change in characteristics due to heat can be obtained by using the benzidine derivative according to the present invention. Further, since the benzidine derivative according to the present invention can be easily kept amorphous, a light-emitting element that is less likely to be deteriorated due to crystallization can be obtained by using the benzidine derivative according to the present invention.

In the present embodiment, an organic compound as typified by a benzidine derivative that has a glass-transition temperature of 150° C. or more, preferably 160° C. or more and 300° C. or less, and has a spiro ring and a triphenylamine skeleton will be described with reference to structure formulas (2) to (5).

The benzidine derivatives represented by structural formulas (2) to (5) have high glass-transition temperatures of 150° C. or more, preferably 160° C. or more and 300° C. or less and have excellent heat resistance. Further, the benzidine derivatives represented by structural formulas (2) to (5) are not likely to be crystallized. Moreover, the benzidine derivatives represented by structural formulae (2) to (5) have high melting temperature of 180° C. or more and 400° C. or less.

Although the synthesis method of the benzidine derivative according to the present invention is not particularly limited, the benzidine derivative can be synthesized by a coupling reaction of N,N′-diphenylbenzidine with 2-bromo-spiro-9,9′-bifluorene or 2-bromo-2′,7′-dialkyl-spiro-9,9′-bifluorene as represented by a synthesis scheme (a-1).

In the synthesis scheme (a-1), R10is hydrogen or an alkyl group having 1 to 4 carbon atoms. It is preferable here that an alkyl group having 3 or 4 carbon atoms be preferably in a state of branching.

The thus described benzidine derivative according to the present invention can be used as a material for forming a hole transporting layer, that is, a hole transporting material.

The benzidine derivative according to the present invention has a feature that the glass-transition temperature meets 150° C. or more, preferably 160° C. or more and 300° C. or less, and a feature that the melting point meets 180° C. or more and 400° C. or less.

Since the benzidine derivative according to the present invention has excellent heat resistance, a light-emitting element that is less likely to change in characteristics due to heat can be obtained by using the benzidine derivative according to the present invention. Further, since the benzidine derivative according to the present invention can be easily kept amorphous, a light-emitting element that is less likely to be deteriorated due to crystallization can be obtained by using the benzidine derivative according to the present invention.

A light-emitting element according to the present invention can be applied to a pixel portion of a light-emitting device that has a display function and a lighting portion of a light-emitting device that has a lighting function. Further, since the light-emitting element according to the present invention is less likely to change in characteristics due to heat and less likely to be deteriorated due to crystallization, a light-emitting device that has fewer image defects or fewer defects of irradiated light due to deterioration of the light-emitting element can be obtained by using the light-emitting element according to the present invention. Now, in the present embodiment, the cross-sectional structure of a pixel portion including the light-emitting element will be described.

FIGS. 3A to 3Cshow cross-sectional views in which a p-channel thin film transistor (TFT) is used as a semiconductor element that controls supply of current to a light-emitting element, and a case in which a first electrode of the light-emitting element functions as an anode and a second electrode thereof functions as a cathode will be exemplified.

FIG. 3Ashows a cross-sectional view of a pixel where a TFT601is a p-channel transistor and light emitted from a light-emitting element603is extracted from a first electrode101side (in the direction of a dashed arrow). InFIG. 3A, the TFT601provided over a substrate600has a semiconductor film, a gate electrode that is provided over the semiconductor film with an insulating film interposed therebetween, and a wiring connected to an impurity region formed in the semiconductor film. Further, a wiring of the TFT601is electrically connected to the first electrode101in the light-emitting element603.

The TFT601is covered with an interlayer insulating film608, and a partition609with an opening is formed on the interlayer insulating film608. In the opening of the partition609, the first electrode101is partly exposed. The first electrode101, an electroluminescent layer605, and a second electrode102are stacked in this order in the opening. It is to be noted that the electroluminescent layer605in the present embodiment indicates the first layer111to the third layer113, and additionally the fourth layer128of in the embodiment described above; namely, the electroluminescent layer605indicates the layers between the first electrode101and the second electrode102.

The interlayer insulating film608can be formed by using an organic resin film, an inorganic insulating film, or an insulating film including a Si—O—Si bond formed by a siloxane material as a starting material (hereinafter, referred to as a “siloxane insulating film”). It is to be noted that siloxane has a framework structure formed by a bond of silicon (Si) and oxygen (O), in which an organic group containing at least hydrogen (such as an alkyl group or aromatic hydrocarbon) is used as a substituent. Alternatively, a fluoro group may be used as the substituent. Further, an organic group containing at least hydrogen and a fluoro group may also be used as substituents. For the interlayer insulating film608, a so-called low dielectric constant material (low-k material) may also be used. The interlayer insulating film608may be either a single layer or a lamination layer.

The partition609can be formed by using an organic resin film, an inorganic insulating film, or a siloxane insulating film. For example, organic resin films such as acrylic, polyimide, and polyamide, and inorganic insulating films such as silicon oxide, silicon nitride oxide, and the like can be used. When a photosensitive organic resin film is used for the partition609, the opening formed in the organic resin film can be formed so that a side wall of the opening has a slope with a continuous curvature, with the result that the first electrode101, the electroluminescent layer605, and the second electrode102can be prevented from being disconnected to each other. In addition, the partition609may be either a single layer or a lamination layer. Further, since the electroluminescent layer605can be made thicker when the light-emitting element according to the present invention is used, a short circuit between the first electrode101and a second electrode102can be prevented.

InFIG. 3A, in order to extract light emission to the first electrode101side (in the direction of the dashed arrow), the first electrode101is formed by using a light-transmitting material or formed to have a film thickness through which light is transmitted. In addition to the light-transmitting material mentioned above, for example, a single-layer film including one or more of TiN, ZrN, Ti, W, Ni, Pt, Cr, Ag, Al and the like, a lamination layer of a titanium nitride film and a film including aluminum as its main component, and a three-layer structure of a titanium nitride film, a film including aluminum as its main component, and a titanium nitride film can be used for the first electrode101. However, when a material other than the light-transmitting material is used, the first electrode101is formed to have a film thickness (preferably, approximately 5 to 30 nm) through which light is transmitted. Further, since the TFT601is a p-channel transistor, the first electrode101is formed by using a material that is suitable for being used as an anode.

The second electrode102is formed by using a material that reflects or blocks light or formed to have a film thickness that blocks light. Further, the second electrode102is formed by using a material that is suitable for being used as a cathode; namely, can be formed by using a metal, alloy, or electrically conductive compound that has a smaller work function, or a mixture thereof. Specifically, an alkali metal such as Li and Cs, an alkali-earth metal such as Mg, Ca and Sr, an alloy including the metal (Mg:Ag, Al:Li, Mg:In, or the like), a compound of the metal (calcium fluoride or calcium nitride), or a rare-earth metal such as Yb and Er can be used.

The electroluminescent layer605is composed of a signal layer or a plurality of layers. Although the embodiments described above show the figures (refer toFIGS. 1,2,18) in which the interface between the layers is clear, it is not always necessary to be clear. The materials forming the respective layers may be partly mixed to make the interface unclear.

In the case of this pixel shown inFIG. 3A, light emitted from the light-emitting element603can be extracted from the first electrode101side as indicated by the dashed arrow.

Then,FIG. 3Bshows a cross-sectional view of a pixel where a TFT601is a p-channel transistor and light emitted from a light-emitting element603is extracted from a second electrode102side (in the direction of a dashed arrow). InFIG. 3B, the structures of the TFT601, an electroluminescent layer605, an interlayer insulating film608, a partition609, and the like that are provided over a substrate600are the same as inFIG. 3A. Therefore, description thereof will be omitted.

InFIG. 3B, in order to extract light emission to the second electrode102side (in the direction of the dashed arrow), the first electrode101is formed by using a material that reflects or blocks light or formed to have a film thickness that blocks light. Further, since the TFT601is a p-channel transistor, the first electrode101is formed by using a material that is suitable for being used as an anode. For example, a single-layer film including one or more of TiN, ZrN, Ti, W, Ni, Pt, Cr, Ag, Al and the like, a lamination layer of a titanium nitride film and a film including aluminum as its main component, and a three-layer structure of a titanium nitride film, a film including aluminum as its main component, and a titanium nitride film can be used for the first electrode101.

In addition the first electrode101and a wiring that is connected to the TFT601can be formed by using the same material. Therefore, a process for forming the first electrode can be cut.

The second electrode102is formed by using a light-transmitting material or formed to have a film thickness through which light is transmitted. Further, the second electrode102is formed by using a material that is suitable for being used as a cathode; namely, can be formed by using a metal, alloy, or electrically conductive compound that has a smaller work function, or a mixture thereof. Specifically, an alkali metal such as Li and Cs, an alkali-earth metal such as Mg, Ca and Sr, an alloy including the metal (Mg:Ag, Al:Li, Mg:In, or the like), a compound of the metal (calcium fluoride or calcium nitride), or a rare-earth metal such as Yb and Er can be used. When this material through which light is not transmitted is used for the second electrode102, the second electrode102is formed to have a film thickness (preferably, approximately 5 to 30 nm) through which light is transmitted. It is to be noted that other light-transmitting conductive oxide materials such as indium tin oxide (ITO), zinc oxide (ZnO), indium zinc oxide (IZO), and zinc oxide doped with gallium (GZO) can also be used. Alternatively, indium tin oxide containing and silicon oxide (ITSO) or ITSO (indium tin oxide containing silicon oxide) further mixed with zinc oxide (ZnO) may be used.

In the case of the pixel shown inFIG. 3B, light emitted from the light-emitting element603can be extracted from the second electrode102side as indicated by the dashed arrow.

Then,FIG. 3Cshows a cross-sectional view of a pixel where a TFT601is a p-channel transistor and light emitted from the light-emitting element603is extracted from a first electrode101side and the second electrode102side (in the directions of dashed arrows). InFIG. 3C, the structures of the TFT601, an electroluminescent layer605, an interlayer insulating film608, a partition609, and the like that are provided over a substrate600are the same as inFIG. 3A. Therefore, description thereof will be omitted.

InFIG. 3C, in order to extract light emission to the first electrode101side and the second electrode102side (in the directions of the dashed arrows), each of the first electrode101and the second electrode102is formed by using a material that reflects or blocks light or formed to have a film thickness that blocks light (>>>a light-transmitting material or formed to have a film thickness through which light is transmitted?); namely, the first electrode101can be formed in the same way as the first electrode101shown inFIG. 3A, and the second electrode102can be formed in the same way as the second electrode102shown inFIG. 3B.

In the case of the pixel shown inFIG. 3C, light emitted from the light-emitting element603can be extracted from the first electrode101side and the second electrode102side as indicated by the dashed arrows.

It is to be noted that the pixel structure according to the present invention is not limited to these. For example, an n-type TFT can be used for the semiconductor element that controls current to the light-emitting element603. In this case, it is preferable that the first electrode101and the second electrode102function as a cathode and an anode, respectively.

Further, the connecting structure of the first electrode101and the wiring of the TFT601is not limited toFIGS. 3A to 3C. For example, unlike the connecting structure shown inFIGS. 3A and 3C, the wiring of the TFT601may be formed after forming the first electrode101.

The present embodiment can be freely combined with the embodiments described above.

In the present embodiment, a pixel circuit that is used for the display device described above will be described. In addition, a case of display on a digital gray scale will be exemplified.

FIG. 4Ashows an example of an equivalent circuit diagram of a pixel350, which includes a signal line614, a power supply line615, a scan line616, and at an intersecting portion thereof, an light-emitting element603, transistors610and611that serve as TFTs, and a capacitor612. For the light-emitting element603, the structure shown in the embodiment described above is used. A video signal is input to the signal line614by a signal line driving circuit. The transistor610is able to control supply of the video signal to a gate of the transistor601in accordance with a selection signal that is input to the scan line616. The transistor601is a driving transistor that is able to control supply of current to the light-emitting element603in accordance with the potential of the video signal. The capacitor612is able to hold a voltage between the gate and source of the transistor601. It is to be noted that, although the capacitor612is shown inFIG. 4A, it is not necessary that the capacitor612be provided when the gate capacitance of the transistor601or another parasitic capacitance is enough.

FIG. 4Bis an equivalent circuit diagram of a pixel351where a transistor618and a scan line619are additionally provided to the pixel350shown inFIG. 4A. Transistor618makes it possible to make the potentials of the gate and source of the transistor611equal to each other so that a state in which no current flows into the light-emitting element603can be forcibly made. Therefore, the length of a sub-frame period can be made shorter than a period for inputting video signals into all pixels. Further, depending on the driving method, a state in which no current flows into the light-emitting element603can be forcibly made even in a pixel shown inFIG. 4A.

FIG. 4Cis an equivalent circuit diagram of a pixel352where a transistor625and a wiring626are additionally provided to the pixel351shown inFIG. 4B. The gate of the transistor625has a potential fixed by the wiring626. In addition, the transistors601and625are connected in series between the power supply line615and the light-emitting element603. Therefore, inFIG. 4C, the transistor625is able to control the amount of current supplied to the light-emitting element603whereas the transistor601is able to control whether or not the current is supplied to the light-emitting element603.

It is to be noted that the pixel circuit according to the present invention is not limited to the structures shown in the present embodiment, and an analog gray scale can be used besides a digital gray scale. In addition, the present embodiment can be freely combined with the embodiments described above.

In the present embodiment, the structure of a light-emitting device that has the light-emitting element described above will be described.

FIG. 5shows a panel in which driving circuits of a signal line driving circuit302and a scan line driving circuit303are provided a around a pixel portion300. It is to be noted that the panel indicates a state in which the pixel portion100, the signal line driving circuit302, and the scan line driving circuit303are provided over the same substrate, and also includes a state in which a FPC (flexible printed circuit) is connected thereto.

The pixel portion300has a plurality of pixels355, and the light-emitting element described above is provided in each pixel. A semiconductor element (corresponding to the TFT601inFIGS. 4A to 4C) for controlling supply of current to the light-emitting element is connected to the light-emitting element. The cross section of the pixel including the light-emitting element is as shown in the embodiment described above. Further, the pixel355may have any of the equivalent circuits shown in the embodiment described above. It is to be noted that the pixel portion according to the present invention is not limited to this structure and may have a passive structure.

Two scan line driving circuit are provided with the pixel portion300interposed therebetween, and are a first scan line driving circuit303aand a second scan line driving circuit303b. It is to be noted a single scan line driving circuit or three or more scan line driving circuits may be provided. The first scan line driving circuit303aand the second scan line driving circuit303bhave circuits that serve as shift registers311aand311b, level shifters312aand312b, and buffers313aand313b, respectively. Signals such as first and second gate start pulses (G1SP and G2SP), first and second gate clock signals (G1CK and G2CK), and signals obtained by inverting the clock signals (G1CKB and G2CKB) are input to the shift resisters311aand311b, respectively.

Scan lines (G1to Gn) are connected to the first and second scan line driving circuit303aand303b, from which signals are supplied to each pixel355provided in the pixel portion300. A semiconductor element (corresponding to the transistor610inFIGS. 4A to 4C) for selecting each pixel is connected to each scan line, and is selected when an image signal is written in the pixel.

Further, the signal line driving circuit302has circuits that serve as a shift register321, a first latch322, a second latch323, a level shifter324, and a buffer325. Signals such as a start pulse (SSP), data (DATA) such as a video signal, and signals such as a latch (LAT) signal are input to the shift register321, the first latch322, and the second latch323, respectively. Signal lines (S1to Sm) are connected to the signal line driving circuit303aand303b, from which image signals are supplied to each pixel355provided in the pixel portion300.

These signal line driving circuit302, scan line driving circuit303, and pixel portion300can be formed by using semiconductor elements provided over the same substrate, for example, can be formed by using thin film transistors provided over a glass substrate.

Next, the cross-sectional structure of the panel shown inFIG. 5will be described with reference toFIGS. 6A and 6B.

FIG. 6Ashows enlarged views of cross sections of the first scan line driving circuit303a, second scan line driving circuit303b, and pixel portion300provided over a substrate600. In the pixel portion300, a TFT601that controls supply of current to a light-emitting element603and a capacitor612are provided. It is to be noted that the capacitor612can function with a structure of an insulator provided between a pair of conductors, and a conductor composed of the same layer as a gate electrode, an insulator composed of an interlayer insulating film608, a gate insulating film, and a conductor composed of the same layer as a wiring of the TFT constitute the capacitor612in the present embodiment. However, the capacitor612is not to be considered limited to this structure.

It is to be noted that the structures of the light-emitting element603and the TFT601can be combined with the structures shown in Embodiment 5.

The first and second scan line driving circuits303aand303bhave CMOS circuits630aand630bcomposed of thin film transistors, respectively. Since the buffers313aand313bof the respective scan line driving circuits are simpler as compared to the other circuits, the CMOS circuits630aand630bcan be applied. Of course, the CMOS circuits630aand630bmay be applied to the other circuits.

In addition, although not shown inFIG. 6A, the CMOS circuits630aand630bcan be applied to the circuits of the signal line driving circuit302.

Further, an opposed substrate650is attached with a sealing material651. Known materials such as an epoxy resin can be used for the sealing material651. InFIG. 6A, the sealing material651is provided to cover a portion of the scan line driving circuit. Accordingly, the frame of the panel can be narrower.

As a result of the attachment, a space is formed between the substrate600and the opposed substrate650. Since the light-emitting element603is deteriorated due to air and water, the space is preferably filled with an inert gas such as nitrogen, or may be filled with a resin or the like. Further, in order to keep the gap, a spacer may be disposed. The spacer may have a function as a drying agent.

Outside the sealing material651, a connecting terminal652for inputting signals to the first and second scan line driving circuits303aand303bis provided. The connecting terminal652is connected to an FPC through an anisotropic conductive film.

Although the opposed substrate650is attached with the sealing material651inFIG. 6A, a resin that is used for filling the space can be used for the attachment.

Further,FIG. 6Bshows a cross-sectional view for the case of providing a black matrix655and a color filter656on an opposed substrate650. The color filter656is provided to be located above at least a light-emitting element603.

The black matrix655is provided to be located above wirings such as the scan lines (G1to Gn), signal lines (S1to Sm), or power supply lines (V1to Vm). Further, the black matrix655may be provided over the first and second scan line driving circuits303aand303b. Since the other structure is the same as the structure shown inFIG. 6A, description there of is omitted.

Although a case of using the color filter656is shown as an example inFIG. 6B, a color conversion layer may be further combined.

In addition, although the color filter656is provided on the opposed substrate650side inFIG. 6B, a color filter may be provided on the substrate600side. For example, a color filter may be formed on the back side of the substrate600or over a second electrode102.

Similarly, it is not always necessary to provide the black matrix655on the opposed substrate650side, and the black matrix655may be provided on the substrate600side. Alternatively, a black pigment mixed in an interlayer insulating film608can be function as a black matrix.

It is to be noted that a structure of a lower-concentration impurity region overlapped with a gate electrode in a thin film transistor, a so-called GOLD (Gate Overlapped LDD) structure is shown as an example inFIGS. 6A and 6B. In addition, a structure of a lower-concentration impurity region that is not overlapped with a gate electrode, a so-called LDD (Lightly Doped Drain) structure may be used.

Although the panel that has the scan line driving circuit and signal line driving circuit provided together with the pixel portion300on the same substrate is shown as an example in the present embodiment, the present invention is note limited to this. For example, each driving circuit may be formed by using an IC chip. In this case, the IC chip can be mounted on a substrate by a COG method or a TAB method.

The present embodiment can be freely combined with the embodiments described above.

Electronic devices provided with a light-emitting device according to the present invention include a television set (also, simply referred to as a TV or a television receiver), a digital camera, a digital video camera, a cellular phone unit (also, simply referred to as a mobile-phone unit or a cellular phone), a mobile information terminal such as PDA, a portable game machine, a monitor for a computer, a computer, a sound reproduction device such as a in-car audio system, an image reproduction device provided with a recording medium such as a home-use game machine, and the like. Specific examples thereof will be described with reference toFIGS. 7A to 7F.

A mobile terminal device shown inFIG. 7Aincludes a main body9201, a display portion9202, and the like. A light-emitting device according to the present invention can be applied to the display portion9202. Accordingly, it is possible to provide a mobile terminal device achieving lower power consumption without increasing the driving voltage even when a light-emitting element is made thicker. Further, the yield of the mobile terminal device is improved since the light-emitting element can be made thicker.

A digital video camera shown inFIG. 7Bincludes a display portion9701, a display portion9702, and the like. A light-emitting device according to the present invention can be applied to the display portion9701. Accordingly, it is possible to provide a digital video camera achieving lower power consumption without increasing the driving voltage even when a light-emitting element is made thicker. Further, the yield of the digital video camera is improved since the light-emitting element can be made thicker.

A cellular phone shown inFIG. 7Cincludes a main body9101, a display portion9102, and the like. A light-emitting device according to the present invention can be applied to the display portion9102. Accordingly, it is possible to provide a cellular phone achieving lower power consumption without increasing the driving voltage even when a light-emitting element is made thicker. Further, the yield of the cellular phone is improved since the light-emitting element can be made thicker.

A portable television set shown inFIG. 7Dincludes a main body9301, a display portion9302, and the like. A light-emitting device according to the present invention can be applied to the display portion9302. Accordingly, it is possible to provide a portable television set achieving lower power consumption without increasing the driving voltage even when a light-emitting element is made thicker. Further, the yield of the portable television set is improved since the light-emitting element can be made thicker. In addition, the light-emitting device according to the present invention can be applied to various types of television sets such as a small-sized television incorporated in a mobile terminal such as a cellular phone handset, a medium-sized television that is portable, and a large-sized television (for example, 40 inches in size or more).

A portable computer shown inFIG. 7Eincludes a main body9401, a display portion9402, and the like. A light-emitting device according to the present invention can be applied to the display portion9402. Accordingly, it is possible to provide a portable computer achieving lower power consumption without increasing the driving voltage even when a light-emitting element is made thicker. Further, the yield of the portable computer is improved since the light-emitting element can be made thicker.

A television set shown inFIG. 7Fincludes a main body9501, a display portion9502, and the like. A light-emitting device according to the present invention can applied to the display portion9502. Accordingly, it is possible to provide a television set achieving lower power consumption without increasing the driving voltage even when a light-emitting element is made thicker. Further, the yield of the television set is improved since the light-emitting element can be made thicker.

As described above, according to the present invention, it is possible to provide an electronic device achieving lower power consumption without increasing the driving voltage even when a light-emitting element is made thicker.

EXAMPLES

In the present example, a light-emitting element manufactured by using a layer in which an organic compound and an inorganic compound are mixed for a layer that generates holes will be described with reference toFIG. 8.

Indium tin oxide containing silicon was deposited over a glass substrate700by sputtering to form a first electrode701so as to have a thickness of 110 nm.

Then, DNTPD, a molybdenum oxide, and rubrene were deposited over the first electrode701by evaporation in vacuum to form a layer711composed of the DNTPD, the molybdenum oxide, and the rubrene so as to have a thickness of 120 nm. The mass ratio of the DNTPD, the molybdenum oxide, and the rubrene was made to be 1:0.5:0.02.

Then, NPB was deposited over the layer711composed of the DNTPD, the molybdenum oxide, and the rubrene by evaporation in vacuum to form a second layer722composed of the NPB so as to have a thickness of 10 nm.

Then, Alq3and coumarin 6 were deposited over the layer722composed of the NPB by co-evaporation in vacuum to form a layer723including the Alq3and the coumarin 6. It is to be noted that the coumarin 6 was made to be included at 0.01 percent by mass to the Alq3. Accordingly, the coumarin 6 was dispersed in the Alq3. Further, the thickness of the layer723was made to be 37.5 nm.

Then, Alq3was deposited over the layer723including the Alq3and the coumarin 6 by evaporation in vacuum to form a layer724composed of the Alq3so as to have a thickness of 37.5 nm.

Then, lithium fluoride was deposited over the layer724composed of the Alq3by evaporation in vacuum to form a layer713composed of the lithium fluoride so as to have a thickness of 1 nm.

Then, aluminum was deposited over the layer713compose of the lithium fluoride by evaporation in vacuum to form a second electrode702so as to have a thickness of 200 nm.

When a voltage is applied to the first electrode701and the second electrode702in the thus manufactures light-emitting element to pass an electric current, the coumarin 6 produces luminescence.

In this case, the first electrode701serves as an anode whereas the second electrode702serves as a cathode. Further, the layer711composed of the DNTPD, the layer722composed of the NPB, the layer723including the Alq3and the coumarin 6, the layer724composed of the Alq3, and the layer713compose of the lithium fluoride serve as a layer that generates holes, a hole transporting layer, a light-emitting layer, an electron transporting layer, and a layer that generates electrons, respectively.

FIG. 9shows current density-luminance characteristics for the case of keeping the light-emitting element according to the present example at 105° C.FIG. 10shows voltage-luminance characteristics thereof.FIG. 11shows luminance-current efficiency characteristics thereof. InFIG. 9, the horizontal axis indicates current density whereas the vertical axis indicates luminance. InFIG. 10, the horizontal axis indicates voltage whereas the vertical axis indicates luminance. InFIG. 11, the horizontal axis indicates luminance whereas the vertical axis indicates current efficiency. It is to be noted that the respective characteristics were measured in the initial condition (Sample 1), after a lapse of 30 hours (Sample 2), after a lapse of 154 hours (Sample 3), and after a lapse of 462 hours (Sample 4).

Based on these results, deterioration (normalized evaluation) with elapsed time on applying 7 V is shown (refer toFIG. 12). In addition,FIG. 12shows a result of the sample according to the present invention (Sample A) together with a result of a comparative sample (Sample B) using a layer composed of DNTPD and rubrene for the layer711that generates holes.

It is determined fromFIG. 12that Sample A is less deteriorated with time than Sample B. Accordingly, a stable light-emitting element can be obtained by using a layer in which a molybdenum oxide is mixed for a layer that generates holes.

As described above, an effect of reduced deterioration with time can be achieved when DNTPD is used for the organic compound. Since the glass-transition temperature of DNTPD is 94° C., the glass-transition temperature according to the present invention is 80° C. or more, preferably 90° C. or more and 300° C. or less, considering the glass-transition temperature of DNTPD.

Synthesis Example

A synthesis method of the benzidine derivative represented by the structure formula (2) will be described.

A synthesis method of 2-bromo-spiro-9,9′-bifluorene will be described.

In a 100 mL three neck flask, 1.26 g (0.052 mol) of magnesium was put, vacuum was formed in the system, and stirring on heating was carried out for 30 minutes activate magnesium. After cooling to room temperature, a nitrogen gas stream was formed in the system. Then, 5 ml of diethyl ether and a few drops of dibromoethane were added, and 11.65 g of 2-bromobiphenyl (0.050 mol) dissolved in 15 ml of diethyl ether was slowly dropped. After dropping, the reaction was refluxed for 3 hours to provide a Grignard reagent. In a 200 mL three neck flask, 11.7 g of 2-bromofluorenone (0.045 mol) and 40 ml of diethyl ether were put. The synthesized Grignard reagent was slowly dropped into this reaction solution. After the dropping, the solution was refluxed for 2 hours and stirred at room temperature overnight (15 hours). After reaction, the reaction solution was washed twice with a saturated ammonium chloride solution, and the water layer was extracted twice with ethyl acetate, mixed with the organic layer, and washed twice with a saturated salt solution. After drying with magnesium sulfate, suction filtration and condensation were performed to obtain 18.76 g of solid 9-(2-biphenylyl)-2-bromo-9-fluorenol at a yield of 90%.

Next, in a 200 mL three neck flask, 18.76 g (0.045 mol) of the synthesized 9-(2-biphenylyl)-2-bromo-9-fluorenol and 100 mL of glacial acetic acid were, and a few drops of concentrated hydrochloric acid were put and refluxed for 2 hours. After reaction, precipitation was collected by suction filtration, and washed with saturated sodium hydrogencarbonate and water on filtrating. An obtained brown solid was recrystallized with ethanol to obtain 10.24 g of a light-brown powdery solid at a yield of 57%. Nuclear magnetic resonance analysis (NMR) confirmed that this light-brown powdery solid is 2-bromo-spiro-9,9′-bifluorene. The1H-NMR is as follows.

Here is the1H-NMR of this compound.

Further, here is a synthesis scheme (b-1) of the synthesis method described above.

A synthesis method of N,N′-bis(spiro-9,9′-bifluorene-2-yl)-N,N′-diphenylbenzidine (abbreviation: BSPB) will be described.

In a 100 ml three neck flask, 1.00 g (0.0030 mol) of N,N′-diphenylbenzidine, 2.49 g (0.0062 mol) of 2-bromo-spiro-9,9′-bifluorene synthesized by the synthesis method of Step 1, 170 mg (0.30 mmol) of bis(dibenzylideneacetone) palladium(0), and 1.08 g (0.011 mol) of sodium tert-butoxide were put. After a forming nitrogen gas stream in the system, 20 mL of dehydrated toluene and 0.6 mL of a 10 wt % hexane solution of tri-(tert-butyl)phosphine were added, and stirring was carried out at 80° C. for 6 hours. After reaction, the reaction solution was cooled to room temperature, water was then added thereto, and a precipitated solid was collected by suction filtration and washed with dichloromethane. An obtained white solid was purified by alumina column chromatography (chloroform) and recrystallized with dichloromethane to obtain a white powdery solid (2.66 g) at a yield of 93%.

The obtained white powdery solid was analyzed by nuclear magnetic resonance analysis (NMR) to obtain the following result, which was able to confirm that the white powdery solid is the benzidine derivative represented by the structure formula (2).FIG. 19shows a chart of the1H-NMR.

Here is a synthesis scheme (b-2) of the synthesis method described above. As described above, the compound according to the present invention can be synthesized by a coupling reaction of N,N′-diphenylbenzidine and 2-bromo-spiro-9,9′-bifluorene.

Further, the glass-transition temperature, crystallization temperature, and melting point of the obtained compound were obtained by a differential scanning calorimeter (DSC) manufactured by PerkinElmer, Inc. under model number Pyrisl DSC. The measurement by the DSC here was carried out in accordance with the following procedure. First, a sample (obtained compound) was heated to 450° C. at a heating rate of 40° C./min and then cooled at a cooling rate of 40° C./min to make the sample in a glass state. Then, the sample in the glass state sample was heated at a heating rate of 10° C./min. Hence, the measurement result shown inFIG. 17was obtained. InFIG. 17, the horizontal axis indicates temperature (° C.) whereas the vertical axis indicates heat flow (the upward direction indicates endotherm) (mW). From the measurement result, it is determined that the glass-transition temperature and crystallization temperature of the obtained compound are 172° C. and 268° C., respectively. Further, it is determined that the melting point is 323° C. or more and 324° C. or less from the intersection of a tangential line at 312° C. with a tangential line at a temperature of 327° C. or more and 328° C. or less. Accordingly, it is determined that the glass-transition temperature of the BSPB synthesized in the present example meets the range of 150° C. or more, preferably 160° C. or more and 300° C. or less, and that the BSPB has a melting point in the range of 180° C. or more and 400° C. or less.

The obtained compound (4.74 g) was sublimed and purified under the condition of 14 Pa and 350° C. for 24 hours to collect 3.49 g compound at a collection rate of 74%.

As described above, the obtained compound has a high glass-transition temperature of 172° C. and has favorable heat resistance. Further, the peak that indicates crystallization of the obtained compound is broad inFIG. 17, and it is thus determined that the obtained compound is not likely to be crystallized.

A light-emitting element manufactured by using a compound (hereinafter, simply referred to as BSPB) that has a glass-transition temperature of 172° C., obtained by the synthesis described in Step 2 of the example described above, will be described with reference toFIG. 8.

Indium tin oxide containing silicon was deposited over a glass substrate700by sputtering to form a first electrode701so as to have a thickness of 110 nm.

Then, BSPB, a molybdenum oxide, and rubrene were deposited over the first electrode701by evaporation in vacuum to form a layer711composed of the BSPB, the molybdenum oxide, and the rubrene so as to have a thickness of 120 nm. The mass ratio of the BSPB, the molybdenum oxide, and the rubrene was made to be 2:0.75:0.04.

Then, NPB was deposited over the layer711composed of the BSPB, the molybdenum oxide, and the rubrene by evaporation in vacuum to form a second layer722composed of the NPB so as to have a thickness of 10 nm.

Then, Alq3and coumarin 6 were deposited over the layer722composed of the NPB by co-evaporation in vacuum to form a layer723including the Alq3and the coumarin 6. It is to be noted that the coumarin 6 was made to be included at 0.01 percent by mass to the Alq3. Accordingly, the coumarin 6 was dispersed in the Alq3. Further, the thickness of the layer723was made to be 37.5 nm.

Then, Alq3was deposited over the layer723including the Alq3and the coumarin 6 by evaporation in vacuum to form a layer724composed of the Alq3so as to have a thickness of 37.5 nm.

Then, lithium fluoride was deposited over the layer724composed of the Alq3by evaporation in vacuum to form a layer713composed of the lithium fluoride so as to have a thickness of 1 nm.

Then, aluminum was deposited over the layer713composed of the lithium fluoride by evaporation in vacuum to form a second electrode702so as to have a thickness of 200 nm.

When a voltage is applied to the first electrode701and the second electrode702in the thus manufactured light-emitting element to pass an electric current, the coumarin 6 produces luminescence.

In this case, the first-electrode701serves as an anode whereas the second electrode702serves as a cathode. Further, the layer711composed of the BSPB, the layer722composed of the NPB, the layer723including the Alq3and the coumarin 6, the layer724composed of the Alq3, and the layer713composed of the lithium fluoride serve as a layer that generates holes, a hole transporting layer, a light-emitting layer, an electron transporting layer, and a layer that generates electrons, respectively.

FIG. 13shows current density-luminance characteristics for the case of keeping the light-emitting element according to the present example at 105° C.FIG. 14shows voltage-luminance characteristics thereof.FIG. 15shows luminance-current efficiency characteristics thereof. InFIG. 13, the horizontal axis indicates current density whereas the vertical axis indicates luminance. InFIG. 14, the horizontal axis indicates voltage whereas the vertical axis indicates luminance. InFIG. 15, the horizontal axis indicates luminance whereas the vertical axis indicates current efficiency. It is to be noted that the respective characteristics were measured in the initial condition (Sample 5) and after a lapse of 30 hours (Sample 6).

Based on these results, deterioration (normalized evaluation) with elapsed time on applying 7 V is shown (refer toFIG. 16). In addition,FIG. 16shows a result of the sample according to the present invention (Sample C) together with a result of a comparative sample (Sample D) using a layer composed of BSPB and rubrene for the layer711that generates holes.

It is determined fromFIG. 16that Sample C is less deteriorated with time than Sample D. Accordingly, a stable light-emitting element can be obtained by using a layer in which a molybdenum oxide is mixed for a layer that generates holes.

Further, it is determined that Sample C is less deteriorated with time than Sample A. As described above, a thermally stable and highly heat-resistant light-emitting element can be obtained by using a layer in which BSPB that has a higher glass-transition temperature and a higher melting point and a molybdenum oxide are mixed. Further, even when this layer is made thicker, a light-emitting element without increase in driving voltage can be obtained.

In the present example, the measurement result of voltage-current characteristics of a light-emitting element including a layer in which an organic compound and an inorganic compound are mixed, and the result of a constant-current driving test for the light-emitting element are shown.

FIG. 20shows voltage-luminance (cd/m2) characteristics of Samples 8 and 9. It is determined that the voltage for obtaining the same luminance, that is, the driving voltage, is lower in the case of Sample 9 including the molybdenum oxide.

Next,FIG. 21shows the result of a constant-current driving test (initial luminance: 3000 cd/m2) for Samples 8 and 9. It is determined that Sample 9 including the molybdenum oxide deteriorates at a slower rate and has higher reliability.