Light-emitting element

Abstract

Light-emitting element comprising: a first electrode; a first light-emitting layer over the first electrode, the first light-emitting layer comprising a first compound and a host material; a second light-emitting layer over the first light-emitting layer, the second light-emitting layer comprising a second compound and the host material; and a second electrode above the second light-emitting layer, where a peak on the side of the longest wavelength of a function ε(λ)λ 4 the second compound overlaps with an emission spectrum of the first compound, where λ represents a wavelength, where ε(λ) represents a molar absorption coefficient at wavelength λ, and wherein an emission peak emitted from the second compound is at a longer wavelength in an emission spectrum of the light-emitting element than an emission peak emitted from the first compound.

Description

Technical area

[0001] The present invention relates to a light-emitting element in which an organic compound is used as a light-emitting substance. State of the art

[0002] In recent years, light-emitting elements that utilize electroluminescence (EL) have been extensively researched and developed. The basic structure of such a light-emitting element consists of a layer containing a light-emitting substance sandwiched between a pair of electrodes. By applying a voltage to this element, light emission from the light-emitting substance can be achieved.

[0003] Such light-emitting elements are self-luminous elements and have the advantages over liquid crystal displays, for example, that the pixels are highly visible and that no backlight is required. Therefore, such light-emitting elements are considered suitable for flat-panel display elements. Displays that incorporate such light-emitting elements are also very advantageous because they can be thin and lightweight. Furthermore, a very fast response time is one of the features of such elements.

[0004] Because the light-emitting layers of such light-emitting elements can be formed in film form, they enable planar light emission. Therefore, large-area elements can be easily fabricated. This is a feature that is difficult to achieve with point light sources, such as incandescent lamps and LEDs, or linear light sources, such as fluorescent lamps. Consequently, these light-emitting elements also have great potential as planar light sources for lighting and the like.

[0005] In the case of an organic EL element, in which a layer containing an organic compound used as a light-emitting substance is sandwiched between a pair of electrodes, applying a voltage between the pair of electrodes results in the injection of electrons from a cathode and holes from an anode into the layer containing the organic compound with a light-emitting property, causing a current to flow. Through recombination of the injected electrons and holes, the organic compound with a light-emitting property is excited to emit light.

[0006] It should be noted that the excited states formed by an organic compound include a singlet excited state and a triplet excited state, and that luminescence from the singlet excited state (S*) is referred to as fluorescence, while luminescence from the triplet excited state (T*) is referred to as phosphorescence. Furthermore, it is considered that the statistical generation ratio of these in the light-emitting element is as follows: S*:T* = 1:3.

[0007] For a compound that emits light from the excited singlet state (hereinafter referred to as a fluorescent compound), light emission from the excited triplet state (phosphorescence) is generally not observed at room temperature, while only light emission from the excited singlet state (fluorescence) is observed. Therefore, the internal quantum efficiency (the ratio of generated photons to injected carriers) of a light-emitting element using a fluorescent compound is considered to have a theoretical limit of 25% based on the ratio S*:T* = 1:3.

[0008] In contrast, in a compound that emits light from the excited triplet state (hereinafter referred to as a phosphorescent compound), light emission from the excited triplet state (phosphorescence) is observed. Furthermore, since intersystem crossing (i.e., a transition from a singlet excited state to a triplet excited state) easily occurs in a phosphorescent compound, the internal quantum efficiency can theoretically be increased to 100%. This means that higher emission efficiency can be achieved than when using a fluorescent compound. As a result, light-emitting elements using phosphorescent compounds are currently being actively developed to obtain highly efficient light-emitting elements.

[0009] A white light-emitting element disclosed in JP 2004-522276 A includes a light-emitting region containing multiple types of light-emitting dopants that emit phosphorescence. Light-emitting elements are known from JP 2010- 34 484 A, JP 2007- 250 296 A, US 2010 / 0 052 527 A1 and US 2011 / 0 210 316 A1. Disclosure of the invention

[0010] Although an internal quantum efficiency of 100% is theoretically possible for a phosphorescent compound, such high efficiency is difficult to achieve without optimizing the element structure or combining it with another material. Especially for a light-emitting element containing multiple types of phosphorescent compounds with different bands (different emission colors) as light-emitting dopants, it is difficult to achieve highly efficient light emission without not only considering energy transfer but also optimizing the energy transfer efficiency. Indeed, even if all the light-emitting dopants of a light-emitting element in JP 2004-522276 A are phosphorescent compounds, the external quantum efficiency is approximately 3% to 4%.It is therefore assumed that even if the light extraction efficiency is taken into account, the internal quantum efficiency is 20% or lower, which is low for a phosphorescent light-emitting element.

[0011] In a multicolor light-emitting element that uses dopants with different emission colors (e.g., an element that emits white light by combining blue, green, and red), in addition to improving emission efficiency, it is also necessary to maintain a good balance between the light emissions of the dopants with different emission colors. It is not easy to maintain a balance between the light emissions of the dopants and achieve high emission efficiency at the same time.

[0012] In view of the above description, an object of one embodiment of the present invention is to provide a light-emitting element that uses multiple types of light-emitting dopants and has high emission efficiency. Another object of one embodiment of the present invention is to provide a light-emitting device, a display device, an electronic device, and a lighting device, each having reduced power consumption by using the above light-emitting element.

[0013] It is only necessary that at least one of the above objects be achieved in the present invention.

[0014] In one embodiment of the present invention, attention is paid to the Förster mechanism, which is one of the mechanisms of energy transfer between molecules. Efficient energy transfer through the Förster mechanism is achieved using a combination of molecules, which combination enables an overlap between a peak of an emission spectrum of the molecule that releases energy and a peak having a local maximum on the longest wavelength side of a characteristic curve obtained by multiplying an absorption spectrum of the molecule that receives energy by the fourth power of a wavelength. Here, one of the features of the above energy transfer is that the energy transfer is not a general energy transfer from a host to a dopant, but an energy transfer from a dopant to a dopant.The light-emitting element according to an embodiment of the present invention can be obtained by using such a combination of dopants between which energy can be transferred in a highly efficient manner and by designing an element structure such that dopant molecules are appropriately separated from each other.

[0015] That is, one embodiment of the present invention is a light-emitting element including the following between a pair of electrodes: a first light-emitting layer in which a first phosphorescent compound is dispersed in a first host material; a second light-emitting layer in which a second phosphorescent compound is dispersed in a second host material; and a third light-emitting layer in which a third phosphorescent compound is dispersed in a third host material. The first phosphorescent compound emits blue light. The second phosphorescent compound has a local maximum value A on the longest wavelength side of a function ε(λ)λ in a range from 440 nm to 520 nm. 4The second phosphorescent compound emits light with a wavelength longer than the wavelength of the blue light emitted by the first phosphorescent compound. The third phosphorescent compound exhibits a local maximum value B on the longest wavelength side of the function ε(λ)λ in a range from 520 nm to 600 nm. 4 The third phosphorescent compound emits light with a wavelength longer than the wavelength of the light emitted by the second phosphorescent compound. The first through third light-emitting layers are stacked in this order. Note that ε(λ) represents a molar absorption coefficient of each of the phosphorescent compounds and is a function of a wavelength λ.

[0016] Another embodiment of the present invention is a light-emitting element including the following between a pair of electrodes: a first light-emitting layer in which a first phosphorescent compound is dispersed in a first host material; a second light-emitting layer in which a second phosphorescent compound is dispersed in a second host material; and a third light-emitting layer in which a third phosphorescent compound is dispersed in a third host material. The first phosphorescent compound emits blue light. The second phosphorescent compound has a local maximum value A on the longest wavelength side of a function ε(λ)λ in a range from 440 nm to 520 nm. 4The second phosphorescent compound exhibits a peak wavelength of phosphorescence in a range from 520 nm to 600 nm. The third phosphorescent compound exhibits a local maximum value B in a range from 520 nm to 600 nm on the side of the longest wavelength of the function ε(λ)λ 4 The third phosphorescent compound emits light with a wavelength longer than the wavelength of the second phosphorescent compound. The first through third light-emitting layers are stacked in this order. Note that ε(λ) represents a molar absorption coefficient of each of the phosphorescent compounds and is a function of a wavelength λ.

[0017] Another embodiment of the present invention is a light-emitting element having the above structure, in which the local maximum value B is larger than the local maximum value A.

[0018] Yet another embodiment of the present invention is a light-emitting element having the above structure, wherein the first light-emitting layer has an electron-transport property, and the second light-emitting layer and the third light-emitting layer each have a hole-transport property.

[0019] An even further embodiment of the present invention is a light-emitting element having the above structure, wherein the first host material has an electron transport property and the second host material and the third host material each have a hole transport property.

[0020] An even further embodiment of the present invention is a light-emitting element having the above structure, wherein the first light-emitting layer has a hole transport property and the second light-emitting layer and the third light-emitting layer each have an electron transport property.

[0021] An even further embodiment of the present invention is a light-emitting element having the above structure, wherein the first host material has a hole transport property and the second host material and the third host material each have an electron transport property.

[0022] An even further embodiment of the present invention is a light-emitting element having the above structure, in which the first to third light-emitting layers are stacked in this order and are in contact with each other.

[0023] An even further embodiment of the present invention is a light-emitting element having the above structure, wherein the second light-emitting layer has a thickness of greater than or equal to 5 nm and less than or equal to 20 nm, preferably greater than or equal to 5 nm and less than or equal to 10 nm.

[0024] An even further embodiment of the present invention is a light-emitting device, a light-emitting display device, an electronic device, and a lighting device, each including a light-emitting element having the above structure.

[0025] Note that the category of "light-emitting device" in this specification includes an image display device using a light-emitting element. Furthermore, the category of light-emitting device in this specification includes a module in which a light-emitting element is provided with a connecting member such as an anisotropic conductive film or a TCP (Tape Carrier Package); a module in which the tip of the TCP is provided with a printed circuit board; and a module in which an IC (integrated circuit) is directly mounted on a light-emitting element by a COG (Chip-on-Glass) method. Furthermore, the category includes light-emitting devices used in lighting devices or the like.

[0026] One embodiment of the present invention provides a light-emitting element with high emission efficiency. Using the light-emitting element, another embodiment of the present invention provides a light-emitting device, a light-emitting display device, an electronic device, and a lighting device, each having reduced power consumption. Short description of the drawings Fig. 1A to Fig. 1C are conceptual representations of light-emitting elements. Fig. Figure 2 shows energy transfer in light-emitting layers. Fig. 3A and Fig. 3B illustrate Förster energy transfer from a blue phosphorescent compound. Fig. 4A and Fig. 4B illustrate Förster energy transfer from a blue phosphorescent compound. Fig. Figure 5 illustrates Förster energy transfer from a blue phosphorescent compound. Fig. 6A and Fig. 6B are conceptual illustrations of an active matrix light-emitting device. Fig. 7A and Fig. 7B are conceptual illustrations of a passive matrix light-emitting device. Fig. 8A and Fig. 8B are conceptual illustrations of the structures of an active matrix light-emitting device. Fig. Figure 9 is a conceptual diagram of an active matrix light-emitting device. Fig. 10A and Fig. 10B are conceptual illustrations of a lighting device. Fig. 11A, Fig. 11B1, Fig. 11B2, Fig. 11C and Fig. 11D each show an electronic device. Fig. 12 shows an electronic device. Fig. 13 shows a lighting device. Fig. 14 shows a lighting device and a display device. Fig. 15 shows display devices and lighting devices installed in a car. Fig. 16A to Fig. 16C show an electronic device. Fig. 17 is a graph showing current density-luminance characteristics of a light-emitting element 1 and a light-emitting element 2. Fig. 18 is a graph showing luminance-current efficiency characteristics of a light-emitting element 1 and a light-emitting element 2. Fig. 19 is a graph showing voltage-luminance characteristics of a light-emitting element 1 and a light-emitting element 2. Fig. 20 is a graph illustrating luminance-chromaticity characteristics of a light-emitting element 1 and a light-emitting element 2. Fig. 21 is a graph illustrating luminance-power efficiency characteristics of a light-emitting element 1 and a light-emitting element 2. Fig. 22 is a graph illustrating luminance-external quantum efficiency characteristics of a light-emitting element 1 and a light-emitting element 2. Fig. 23 shows emission spectra of a light-emitting element 1 and a light-emitting element 2. Fig. Figure 24 illustrates Förster energy transfer from a blue phosphorescent compound. Fig. 25 is a graph showing current density-luminance characteristics of a light-emitting element 3. Fig. 26 is a graph illustrating luminance-current efficiency characteristics of a light-emitting element 3. Fig. 27 is a graph showing voltage-luminance characteristics of a light-emitting element 3. Fig. 28 is a graph showing luminance-chromaticity characteristics of a light-emitting element 3. Fig. 29 is a graph illustrating luminance-power efficiency characteristics of a light-emitting element 3. Fig. 30 is a graph illustrating luminance-external quantum efficiency characteristics of a light-emitting element 3. Fig. 31 shows an emission spectrum of a light-emitting element 3. Fig. 32 is a graph showing current density-luminance characteristics of a light-emitting element 4. Fig. 33 is a graph illustrating luminance-current efficiency characteristics of a light-emitting element 4. Fig. 34 is a graph showing voltage-luminance characteristics of a light-emitting element 4. Fig. 35 is a graph illustrating luminance-chromaticity characteristics of a light-emitting element 4. Fig. 36 is a graph illustrating luminance-power efficiency characteristics of a light-emitting element 4. Fig. 37 is a graph illustrating luminance-external quantum efficiency characteristics of a light-emitting element 4. Fig. 38 shows an emission spectrum of a light-emitting element 4. Fig. 39 is a graph illustrating time-normalized luminance characteristics of a light-emitting element 4. Best mode for carrying out the invention

[0027] Embodiments of the present invention will be described in detail below with reference to the drawings. It should be noted that the present invention is not limited to the following description, and it will be readily understood by those skilled in the art that various changes and modifications can be made without departing from the spirit of the present invention. (Embodiment 1)

[0028] An operating principle of a light-emitting element according to an embodiment of the present invention will be described. The point of the present invention is that a first phosphorescent compound that emits blue light (particularly, a phosphorescent compound having an emission peak at 440 nm to 520 nm or a phosphorescent compound that emits light in a color of a color gamut where the CIE tristimulus values ​​(x, y) are 0.12 ≤ x ≤ 0.25 and 0.05 ≤ y ≤ 0.5) and second and third phosphorescent compounds that emit light (e.g., green light or red light) with wavelengths longer than the wavelength of the blue light emitted by the first phosphorescent compound are used, and that the first to third phosphorescent compounds are all efficiently caused to emit light, thereby obtaining a multicolor light-emitting element with a high efficiency.

[0029] As a general method for obtaining a multicolor light-emitting element containing a phosphorescent compound, one can mention a method in which several types of phosphorescent compounds exhibiting different emission colors are dispersed in a certain host material at an appropriate ratio. However, in such a method, the phosphorescent compound emitting light with the longest wavelength readily emits light, so it is extremely difficult to design and control a structure (especially the concentration of the phosphorescent compounds in the host material) to achieve polychrome light.

[0030] Another technique for obtaining a multi-color light-emitting element is a so-called tandem structure, in which light-emitting elements having different emission colors are stacked in series. For example, a blue light-emitting element, a green light-emitting element, and a red light-emitting element are stacked in series and emitted simultaneously, making polychrome light (white light in this case) easy to achieve. The element structure can be designed and controlled relatively easily because the blue light-emitting element, the green light-emitting element, and the red light-emitting element can be optimized separately. However, stacking the three elements involves an increase in the number of layers and complicates manufacturing. In addition, increasing the drive voltage, i.e.a loss of performance, can be caused when a problem of electrical contact occurs at connecting sections between the elements (so-called interlayers).

[0031] In contrast, in the light-emitting element according to one embodiment of the present invention, the following are arranged between the pair of electrodes: the first light-emitting layer in which the first phosphorescent compound emitting blue light is dispersed in the first host material; the second light-emitting layer in which the second phosphorescent compound emitting light with a wavelength longer than that of the light emitted by the first phosphorescent compound is dispersed in the second host material; and the third light-emitting layer in which the third phosphorescent compound emitting light with a wavelength longer than that of the light emitted by the second phosphorescent compound is dispersed in the third host material. The first to third light-emitting layers are stacked in this order.Unlike the case of a tandem structure, the first to third light-emitting layers can be arranged in contact with each other.

[0032] An element structure of the above-described light-emitting element according to an embodiment of the present invention is schematically shown in Fig. 1A to Fig. 1C. In Fig. 1C, a first electrode 101, a second electrode 102, and an EL layer 103 are shown. The EL layer 103 includes at least one light-emitting layer 113, and further layers may be provided as needed. Fig. In the structure shown in Figure 1C, it is assumed that a hole injection layer 111, a hole transport layer 112, an electron transport layer 114, and an electron injection layer 115 are provided. Note that the first electrode 101 is assumed to serve as an anode and the second electrode 102 is assumed to serve as a cathode.

[0033] Fig. 1A and Fig. 1B are each an enlarged view of the light-emitting layer 113 in the light-emitting element. Both Fig. 1A as well as in Fig. 1B, a first light-emitting layer 113B, a second light-emitting layer 113G, a third light-emitting layer 113R, that is, the light-emitting layer 113 which is a combination of the three layers, a first phosphorescent compound 113Bd, a second phosphorescent compound 113Gd, a third phosphorescent compound 113Rd, a first host material 113Bh, a second host material 113Gh, a third host material 113Rh, and a recombination region 113ex are shown. Fig. Fig. 1A is a schematic diagram illustrating the case where the first light-emitting layer 113B is arranged on the cathode side, and Fig. Figure 1B is a schematic diagram illustrating the case where the first light-emitting layer 113B is disposed on the anode side. In each case, the phosphorescent compounds (the first to third phosphorescent compounds) are dispersed in the host materials, so that the phosphorescent compounds are separated from each other by the host materials. Note that the first to third host materials may be the same or different from each other.

[0034] In this case, energy transfer through electron exchange interaction (so-called Dexter mechanism) between the phosphors is suppressed. In other words, a phenomenon in which, after the first phosphorescent compound 113Bd is excited, the excitation energy is transferred to the second phosphorescent compound 113Gd or the third phosphorescent compound 113Rd through the Dexter mechanism can be prevented. In addition, a phenomenon in which, after the second phosphorescent compound 113Gd is excited, the excitation energy is transferred to the third phosphorescent compound 113Rd through the Dexter mechanism can also be prevented. In this way, a phenomenon in which the third phosphorescent compound 113Rd, which emits light with the longest wavelength, mainly emits light can be suppressed.Note that even when an exciton is directly generated in the third light-emitting layer 113R, the third phosphorescent compound 113Rd mainly emits light. Therefore, it is preferable that the carrier recombination region 113ex is located in the first light-emitting layer 113B or near the interface between the first light-emitting layer 113B and the second light-emitting layer 113G (that is, the first phosphorescent compound 113Bd is mainly excited).

[0035] It should be noted that if the energy transfer from the first phosphorescent compound 113Bd were completely suppressed, on the contrary, no light emission from the third phosphorescent compound 113Rd could be achieved. Therefore, in one embodiment of the present invention, an element is designed such that excitation energy from the first phosphorescent compound 113Bd, which emits blue light, is partially transferred to the second phosphorescent compound 113Gd, and excitation energy from the second phosphorescent compound 113Gd is partially transferred to the third phosphorescent compound 113Rd. Such energy transfer between separate molecules can occur through dipole-dipole interaction (Förster mechanism).

[0036] The Förster mechanism will now be described. The molecule that donates excitation energy and the molecule that receives excitation energy are referred to as the energy donor and the energy acceptor, respectively. This means that in one embodiment of the present invention, both the energy donor and the energy acceptor are phosphorescent compounds and are separated from each other by the host materials.

[0037] In the Förster mechanism, direct intermolecular contact is not necessary for energy transfer. Energy transfer occurs through a resonance phenomenon of dipole vibration between an energy donor and an energy acceptor. The resonance phenomenon of dipole vibration causes the energy donor to donate energy to the energy acceptor; consequently, the energy donor transitions from an excited state to a ground state, and the energy acceptor in the ground state becomes excited. The rate constant k Fthe energy transfer through the Förster mechanism is represented by a formula (1). [Formula 1] kF=9000c4K2ϕln10128π5n4NτR6∫F(v)ε(v)v4dv

[0038] In formula (1), v represents a frequency, F(v) represents a normalized emission spectrum of an energy donor (a fluorescence spectrum upon energy transfer from an excited singlet state, and a phosphorescence spectrum upon energy transfer from an excited triplet state), ε(v) represents a molar absorption coefficient of an energy acceptor, N represents the Avogadro number, n represents a refractive index of a medium, R represents an intermolecular distance between the energy donor and the energy acceptor, τ represents a measured lifetime of an excited state (fluorescence lifetime or phosphorescence lifetime), c represents the speed of light, ϕ represents a luminescence quantum yield (a fluorescence quantum yield upon energy transfer from an excited singlet state, and a phosphorescence quantum yield upon energy transfer from an excited triplet state), and K 2represents a coefficient (0 to 4) for the orientation of a transition dipole moment between the energy donor and the energy acceptor. It should be noted that in the case of random orientation K 2 = 2 / 3 is fulfilled.

[0039] As suggested by formula (1), the following can be stated as necessary conditions for energy transfer by the Förster mechanism (Förster energy transfer): 1. the energy donor and the energy acceptor are not too far apart (in terms of the distance R); 2. the energy donor emits light (in terms of the luminescence quantum yield ϕ); and 3. an emission spectrum of the energy donor overlaps an absorption spectrum of the energy acceptor (in terms of the integral term).

[0040] As already shown by Fig. 1A to Fig. 1C, here the phosphorescent compounds (the first to third phosphorescent compounds) are dispersed in the respective host materials and separated from each other by the host materials. The distance R is therefore at least the length of a molecule or longer (i.e., 1 nm or more). Therefore, the excitation energy generated in the first phosphorescent compound is not all transferred to the second or third phosphorescent compound by the Förster mechanism. Meanwhile, the Förster energy transfer can occur across the distance R when R is less than or equal to approximately 10 nm to 20 nm. This means that, for example, when the thickness of the second light-emitting layer 113G in Fig. 1A and Fig. 1B is set to 20 nm or less, energy can be partially transferred so that the first phosphorescent compound 113Bd, the second phosphorescent compound 113Gd, and the third phosphorescent compound 113Rd can all be made to emit light.

[0041] Fig. Figure 2 schematically illustrates the Förster energy transfer between the phosphorescent compounds in the light-emitting element according to an embodiment of the present invention, which includes the following: the first phosphorescent compound 113Bd, which emits blue light; the second phosphorescent compound 113Gd, which emits light (e.g., green light) with a wavelength longer than that of the light emitted by the first phosphorescent compound; and the third phosphorescent compound 113Rd, which emits light (e.g., red light) with a wavelength longer than that of the light emitted by the second phosphorescent compound 113Gd. Fig. 2 shows a structure in which the first light-emitting layer 113B, the second light-emitting layer 113G, and the third light-emitting layer 113R are stacked between an electrode 10 and an electrode 11. Note that one of the electrodes 10 and 11 serves as the anode, and the other serves as the cathode. As shown in Fig. 2, an excited singlet state formed in the first phosphorescent compound 113Bd (S B ), first by intersystem crossing into an excited triplet state (T B ). In other words, an exciton in the first light-emitting layer 113B is basically converted into T B offset.

[0042] The energy of the exciton in T B -state can then be partially mapped to the excited triplet state of the second phosphorescent compound 113Gd (T G), whereby a certain amount of energy is converted into blue light emission. This results from the fact that the first phosphorescent compound 113Bd exhibits a light-emitting property (has a high phosphorescence quantum yield ϕ) and that direct absorption, corresponding to the electron transition from a singlet ground state to an excited triplet state, is observed in the second phosphorescent compound 113Gd (an absorption spectrum of an excited triplet state exists). If these conditions are met, the triplet-triplet Förster energy transfer of T B on T G possible. Furthermore, energy transfer from T B to an excited singlet state of the third phosphorescent compound 113Rd (S R) can take place as long as the conditions for Förster energy transfer are met, although the contribution is small. In the case where the third phosphorescent compound 113Rd is a red-emitting material, this energy transfer occurs readily, as described later. Through intersystem crossing, S R into an excited triplet state of the third phosphorescent compound 113Rd (T R ) to contribute to the emission of the third phosphorescent compound 113Rd. Note that since the energy donor in the Förster mechanism (here, the first phosphorescent compound 113Bd) must have a light-emitting property, the phosphorescence quantum yield of the first phosphorescent compound 113Bd is preferably 0.1 or more.

[0043] It should be noted that in many cases an excited singlet state of the second phosphorescent compound 113Gd (S G) has a higher energy than the excited triplet state of the first phosphorescent compound 113Bd (T B ) and therefore in many cases does not contribute much to the energy transfer described above. For this reason, its description is omitted here.

[0044] The energy of an exciton in triplet T G -state of the second phosphorescent compound 113Gd can further be partially attributed to the excited triplet state of the third phosphorescent compound 113Rd (T R), whereby a certain amount of energy is converted into light emission (e.g., green light emission). This results from the fact that the second phosphorescent compound 113Gd exhibits a light-emitting property (has a high phosphorescence quantum yield ϕ) and that direct absorption, corresponding to the electron transition from a singlet ground state to an excited triplet state, is observed in the third phosphorescent compound 113Rd (an absorption spectrum of an excited triplet state exists). If these conditions are met, the triplet-triplet Förster energy transfer of T G on T Rpossible. It should be noted that since the energy donor in the Förster mechanism (here, the second phosphorescent compound 113Gd) must have a light-emitting property, the phosphorescence quantum yield of the second phosphorescent compound 113Gd is preferably 0.1 or more.

[0045] The T R The energy resulting from such energy transfer is converted into light emission from the third phosphorescent compound 113Rd (e.g., red light emission). In this way, light emission from any of the first to third phosphorescent compounds can be achieved.

[0046] Note that it is preferable that the absorption spectra of the first to third host materials not be in the blue wavelength range so that the above Förster energy transfer can occur efficiently between the phosphorescent compounds serving as dopants without transferring energy to the host materials. Specifically, an absorption edge of the absorption spectrum is preferably located at 440 nm or less. In this way, energy is transferred directly between dopants without being transferred via the host material (especially the second or third host material), so that the formation of an extra energy transfer path is suppressed and high emission efficiency can be achieved.

[0047] In addition, the first host material preferably has a triplet excitation energy higher than that of the first phosphorescent compound in order not to quench the blue light-emitting first phosphorescent compound.

[0048] As described above, a basic concept of one embodiment of the present invention is an element structure in which the first phosphorescent compound emitting light with the shortest wavelength is mainly excited, and the first to third phosphorescent compounds are separated from each other using the host materials and the stacked layer structure. In such an element structure, since a part of energy is transferred by the Förster mechanism over a certain distance (20 nm or less), the excitation energy of the blue-light-emitting first phosphorescent compound is partially transferred to the second phosphorescent compound, and moreover, the excitation energy of the second phosphorescent compound is partially transferred to the third phosphorescent compound. As a result, light emission from each of the first to third phosphorescent compounds can be achieved.

[0049] Here, in one embodiment of the present invention, it is more important that the materials and the element structure are determined in consideration of the above energy transfer.

[0050] To effect Förster energy transfer, the energy donor must have a high luminescence quantum yield ϕ. Regarding the luminescence quantum yield, there is no problem in one embodiment of the present invention because a phosphorescent compound (particularly, a light-emitting compound with a phosphorescence quantum yield of 0.1 or more) is used. An important point is that the integral term of formula (1) is set large; that is, an emission spectrum F(v) of the energy donor is set to appropriately overlap the molar absorption coefficient ε(v) of the energy acceptor.

[0051] In general, it is assumed that the emission spectrum F(v) of the energy donor simply needs to overlap a wavelength range where the molar absorption coefficient ε(v) of the energy acceptor is large (i.e., the product of F(v) times ε(v) needs to be large). However, this does not always hold for the Förster mechanism, since the integral term in formula (1) is inversely proportional to the fourth power of the frequency v and depends on the wavelength.

[0052] For ease of understanding, formula (1) is converted here. Since v = c / λ, where λ represents a wavelength of light, formula (1) can be converted into formula (2). [Formula 2] kF=9000c4K2ϕln10128π5n4NτR6∫F(λ)ε(λ)λ4dλ

[0053] In other words, it can be found that the longer the wavelength λ, the larger the integral term. In simpler terms, it is suggested that energy transfer occurs more easily on the longer wavelength side. That is, this does not simply mean that F(λ) must overlap the wavelength range in which the molar absorption coefficient ε(λ) is large. It is necessary that F(λ) overlap a range in which ε(λ)λ 4 is large, overlaps.

[0054] Therefore, in order to increase the efficiency of energy transfer from the first phosphorescent compound 113Bd emitting blue light (particularly a phosphorescent compound having an emission peak at 440 nm to 520 nm), the light-emitting element according to an embodiment of the present invention uses a phosphorescent compound having a local maximum value A on the longest wavelength side of the function ε(λ)λ in a range of 440 nm to 520 nm. 4and emits light having a wavelength longer than that of the light emitted by the first phosphorescent compound 113Bd (in particular, a phosphorescent compound having an emission peak at 520 nm to 600 nm) is used as the second phosphorescent compound 113Gd. Furthermore, in order to increase the efficiency of energy transfer from the second phosphorescent compound 113Gd, a phosphorescent compound having a local maximum value B on the longest wavelength side of the function ε(λ)λ in a range of 520 nm to 600 nm is used. 4and emits light having a wavelength longer than that of the light emitted by the second phosphorescent compound 113Gd than the third phosphorescent compound 113Rd. It should be noted that the use of the phosphorescent compounds that emit light in the above manner enables lighting devices to provide light with a high color rendering property and enables displays to efficiently emit light with good color values.

[0055] To better understand the structures of such phosphorescent compounds (especially the local maximum values ​​A and B), concrete examples are given in the following description. As an example, a case is described in which a compound (1) shown below (tris{2-[5-(2-methylphenyl)-4-(2,6-dimethylphenyl)-4H-1,2,4-triazol-3-yl-ĸN2]phenyl-κC}iridium(III) (abbreviation: Ir(mpptz-dmp)3)) is used as the first phosphorescent compound 113Bd that emits blue light; a compound (2) shown below ((acetylacetonato)bis(6-tert-butyl-4-phenylpyrimidinato)iridium(III) (abbreviation: Ir(tBuppm)2(acac))) is used as the second phosphorescent compound 113Gd, which emits light (green light) having a wavelength longer than that of the light emitted by the first phosphorescent compound 113Bd;and a compound (3) shown below (bis(2,3,5-triphenylpyrazinato)(dipivaloylmethanato)iridium(III) (abbreviation: Ir(tppr)2(dpm))) is used as the third phosphorescent compound 113Rd, which emits light (red light) having a wavelength longer than that of the light emitted by the second phosphorescent compound 113Gd. ;

[0056] Fig. 3A shows a molar absorption coefficient ε(λ) and ε(λ)λ 4 of compound (2), which is the second phosphorescent compound. The molar absorption coefficient ε(λ) becomes smaller on the longer wavelength side, but ε(λ)λ 4 has the local maximum value A at about 490 nm (which corresponds to the triplet MLCT absorption band of compound (2)). As can be understood from this example, under the influence of the term λ 4 , ε(λ)λ 4the second phosphorescent compound exhibits the local maximum value A in the absorption band (triplet MLCT absorption band) which lies on the side of the longest wavelength.

[0057] Fig. Figure 3B shows a photoluminescence (PL) spectrum F(λ) of compound (1) and ε(λ)λ 4 of compound (2). Compound (1) is the first phosphorescent compound and emits blue light with emission peaks at approximately 475 nm and 505 nm. Near the local maximum value A of ε(λ)λ 4 of the second phosphorescent compound, the PL spectrum F(λ) of the first phosphorescent compound shows a large overlap with ε(λ)λ 4 and energy transfer from the first phosphorescent compound to the second phosphorescent compound occurs through the Förster mechanism. Note that in this case, the energy transfer is triplet-triplet Förster energy transfer (T B -TG -Energy transfer in Fig. 2), since the local maximum value A corresponds to the triplet MLCT absorption band.

[0058] Fig. 4A shows a molar absorption coefficient ε(λ) and ε(λ)λ 4 of compound (3), which is the third phosphorescent compound. The molar absorption coefficient ε(λ) becomes smaller on the side of a longer wavelength, but ε(λ)λ 4 has the local maximum value B at about 550 nm (which corresponds to the triplet MLCT absorption band of compound (3)). As can be understood from this example, under the influence of the term λ 4 , ε(λ)λ 4 the third phosphorescent compound has the local maximum value B in the absorption band (triplet MLCT absorption band), which lies on the side of the longest wavelengths.

[0059] Fig. Figure 4B shows a photoluminescence (PL) spectrum F(λ) of compound (2) and ε(λ)λ 4of compound (3). Compound (2) is the second phosphorescent compound and emits green light with an emission peak at about 545 nm. Near the local maximum value B of ε(λ)λ 4 of the third phosphorescent compound, the PL spectrum F(λ) of the second phosphorescent compound shows a large overlap with ε(λ)λ 4 and energy transfer from the second phosphorescent compound to the third phosphorescent compound occurs through the Förster mechanism. Note that in this case, the energy transfer is triplet-triplet Förster energy transfer (T G -T R -Energy transfer in Fig. 2), since the local maximum value B corresponds to the triplet MLCT absorption band.

[0060] It should be noted that, for the above reason, it is preferable that the absorption spectra of the second and third phosphorescent compounds exhibit direct absorption corresponding to the electron transition from a singlet ground state to a triplet excited state (e.g., triplet MLCT absorption) on the longest wavelength side. Such a structure leads to a high efficiency of triplet-triplet energy transfer in Fig. 2.

[0061] Fig. Figure 5 shows a PL spectrum of compound (3), which is the third phosphorescent compound, where a combination of Fig. 3B and Fig. 4B. It can be found that energy can be transferred stepwise first from compound (1) to compound (2) by examining the overlap between the PL spectrum of compound (1) and ε(λ)λ 4of the compound (2) (near the local maximum value A), and then the energy can be transferred from the compound (2) to the compound (3) by using the overlap between the PL spectrum of the compound (2) and ε(λ)λ 4 of compound (3) (near the local maximum value B). It should be noted that a direct energy transfer from compound (1), which is the first phosphorescent compound, to compound (3), which is the third phosphorescent compound, is also possible. The reason for this is that, as can be seen from Fig. 5 can understand the PL spectrum F(λ) of the compound (1) and ε(λ)λ 4 of the compound (3) on the side of a wavelength shorter than that of the triplet MLCT absorption band (near the local maximum value B) of the compound (3), overlap each other, indicating the presence of triplet-singlet Förster energy transfer (T B -S R-Energy transfer in Fig. 2) suggests.

[0062] One from Fig. 3A, Fig. 3B, Fig. 4A, Fig. 4B and Fig. 5 is that due to the influence of λ 4 the third phosphorescent compound receives energy more easily (becomes an energy acceptor) than the second phosphorescent compound. The second phosphorescent compound and the third phosphorescent compound have a substantially equal molar absorption coefficient ε(λ) of the triplet MLCT absorption band on the longest wavelength side, which is approximately 5000 [M -1 cm -1 ] As can be seen from Fig. 5, the local maximum value B is nevertheless about 1.6 times as large as the local maximum value A, if the local maximum value A and the local maximum value B of ε(λ)λ 4 be compared with each other. This leads to the influence of the term λ4 back, and it is suggested that ε(λ)λ 4 tends to become large for a compound that has an absorption band on the longer wavelength side. This shows that the third phosphorescent compound receives energy more easily than the second phosphorescent compound.

[0063] In view of the above description, attention is drawn to an element structure (which is Fig. 1A to Fig. 1C), in which the first to third light-emitting layers are stacked in this order, and the recombination region of carriers is located in the first light-emitting layer or near the interface between the first light-emitting layer and the second light-emitting layer (that is, the first phosphorescent compound is mainly excited). In such an element structure, the third light-emitting layer containing the third phosphorescent compound is located farther from the recombination region of carriers than the second light-emitting layer containing the second phosphorescent compound.In this way, the third phosphorescent compound, which readily receives energy, is placed far away from the recombination region, and the second phosphorescent compound, which does not receive energy relatively easily, is placed close to the recombination region, thereby achieving a good balance of light emissions from the first to third phosphorescent compounds. As a result, a light-emitting element with high emission efficiency and a good spectral balance can be obtained.

[0064] It should be noted that in order to obtain the above-described recombination region, the first light-emitting layer preferably has an electron transport property, and the second light-emitting layer and the third light-emitting layer preferably have hole transport properties ( Fig. 1A). Specifically, for example, a material having an electron-transport property can be used as the first host material, and a material having a hole-transport property can be used as the second host material and the third host material.

[0065] It should be noted that in a further embodiment for obtaining the above-described recombination region, the first light-emitting layer preferably has a hole transport property and the second light-emitting layer and the third light-emitting layer preferably have electron transport properties ( Fig. 1B). Specifically, for example, a material with a hole-transporting property can be used as the first host material, and a material with an electron-transporting property can be used as the second host material and the third host material.

[0066] In order to enable both the second light-emitting layer and the third light-emitting layer to emit light, it is additionally preferable that the thickness of the second light-emitting layer be set to greater than or equal to 5 nm and less than or equal to 20 nm, taking into account the Förster energy transfer distance R. It is more preferable that the thickness be set to greater than or equal to 5 nm and less than or equal to 10 nm. (Embodiment 2)

[0067] In this embodiment, a detailed example of the structure of the light-emitting element described in Embodiment 1 will be explained with reference to Fig. 1A to Fig. 1C described below.

[0068] A light-emitting element of this embodiment includes an EL layer comprising a plurality of layers between a pair of electrodes. In this embodiment, the light-emitting element includes the first electrode 101, the second electrode 102, and the EL layer 103 disposed between the first electrode 101 and the second electrode 102. Note that the description of this embodiment assumes that the first electrode 101 serves as an anode and that the second electrode 102 serves as a cathode. In other words, light emission can be observed when a voltage is applied between the first electrode 101 and the second electrode 102 such that the potential of the first electrode 101 is higher than that of the second electrode 102.

[0069] Since the first electrode 101 serves as an anode, the first electrode 101 is preferably formed using one of metals, alloys, electrically conductive compounds with a high work function (particularly, a work function of 4.0 eV or more), mixtures thereof, and the like. As concrete examples, indium oxide-tin oxide (ITO), indium oxide-tin oxide containing silicon or silicon oxide, indium oxide-zinc oxide, indium oxide containing tungsten oxide and zinc oxide (IWZO), and the like can be cited. Films of these electrically conductive metal oxides are usually formed by a sputtering method; however, they may also be formed using a sol-gel method or the like. In one example of the manufacturing method, indium oxide-zinc oxide is deposited by a sputtering method using a target prepared by adding 1 wt% to 20 wt% of indium oxide.-% zinc oxide to indium oxide. Furthermore, an indium oxide film containing tungsten oxide and zinc oxide (IWZO) can be formed by a sputtering method using a target in which 0.5 wt% to 5 wt% of tungsten oxide and 0.1 wt% to 1 wt% of zinc oxide are added to indium oxide. In addition, gold (Au), platinum (Pt), nickel (Ni), tungsten (W), chromium (Cr), molybdenum (Mo), iron (Fe), cobalt (Co), copper (Cu), palladium (Pd), nitrides of metal materials (e.g., titanium nitride), and the like can be specified. Graphene can also be used. Note that when a composite material described later is used for a layer in contact with the first electrode 101 in the EL layer 103, an electrode material can be selected regardless of its work function.

[0070] There is no particular limitation on the stacked layer structure of the EL layer 103, as long as the EL layer includes the light-emitting layer 113 having a structure similar to that described in Embodiment 1. For example, the EL layer 103 may be formed by appropriately combining a hole injection layer, a hole transport layer, the light-emitting layer, an electron transport layer, an electron injection layer, a carrier blocking layer, an intermediate layer, and the like. In this embodiment, the EL layer 103 has a structure in which the hole injection layer 111, the hole transport layer 112, the light-emitting layer 113, the electron transport layer 114, and the electron injection layer 115 are stacked in this order above the first electrode 101. Specific materials included in the layers will be mentioned below.

[0071] The hole-injection layer 111 is a layer containing a substance with a high hole-injection property. Molybdenum oxide, vanadium oxide, ruthenium oxide, tungsten oxide, manganese oxide, or the like can be used. Alternatively, the hole-injection layer 111 can be formed using a phthalocyanine-based compound such as phthalocyanine (abbreviation: H2Pc) or copper phthalocyanine (abbreviation: CuPc), an aromatic amine compound such as 4,4'-bis[N-(4-diphenylaminophenyl)-N-phenylamino]biphenyl (abbreviation: DPAB) or N,N'-bis{4-[bis(3-methylphenyl)amino]phenyl}-N,N'-diphenyl-(1,1'-biphenyl)-4,4'-diamine (abbreviation: DNTPD), a high-molecular compound such as ethylenediamine (EDI). B. poly(ethylenedioxythiophene) / poly(styrenesulfonic acid) (PEDOT / PSS), or the like.

[0072] Alternatively, a composite material in which a substance with a hole-transporting property contains a substance with an accepting property can be used for the hole-injection layer 111. Note that the use of such a substance with a hole-transporting property containing a substance with an accepting property allows a material for forming an electrode to be selected regardless of its work function. In other words, in addition to a material with a high work function, a material with a low work function can also be used for the first electrode 101. As the substance with an accepting property, 7,7,8,8-tetracyano-2,3,5,6-tetrafluoroquinodimethane (abbreviation: F4-TCNQ), chloranil, and the like can be cited. Further, transition metal oxides can be cited. Oxides of metals belonging to Group 4 to Group 8 of the Periodic Table can be cited.Vanadium oxide, niobium oxide, tantalum oxide, chromium oxide, molybdenum oxide, tungsten oxide, manganese oxide, and rhenium oxide are particularly preferred because of their high electron-accepting properties. Among these, molybdenum oxide is particularly preferred because it is stable in air, has low hygroscopicity, and is easy to handle.

[0073] As a substance with a hole-transporting property used for the composite material, one of various organic compounds, such as aromatic amine compounds, carbazole derivatives, aromatic hydrocarbons, and high-molecular compounds (e.g., oligomers, dendrimers, or polymers), can be used. It should be noted that the organic compound for the composite material is preferably an organic compound with a high hole-transporting property. In particular, a substance with a hole mobility of 10 -6 cm 2 / Vs or more. Specific organic compounds that can be used as substances with a hole-transporting property in the composite material are listed below.

[0074] Examples of the aromatic amine compounds are N,N-di(p-tolyl)-N,N'-diphenyl-p-phenylenediamine (abbreviation: DTDPPA), 4,4'-bis[N-(4-diphenylaminophenyl)-N-phenylamino]biphenyl (abbreviation: DPAB), N,N-bis{4-[bis(3-methylphenyl)amino]phenyl}-N,N'-diphenyl-(1,1'-biphenyl)-4,4'-diamine (abbreviation: DNTPD), 1,3,5-tris[N-(4-diphenylaminophenyl)-N-phenylamino]benzene (abbreviation: DPA3B) and the like.

[0075] Specific examples of the carbazole derivatives that can be used for the composite material are 3-[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole (abbreviation: PCzPCA1), 3,6-bis[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole (abbreviation: PCzPCA2), 3-[N-(1-naphthyl)-N-(9-phenylcarbazol-3-yl)amino]-9-phenylcarbazole (abbreviation: PCzPCN1) and the like.

[0076] Other examples of the carbazole derivatives that can be used for the composite material are 4,4'-di(N-carbazolyl)biphenyl (abbreviation: CBP), 1,3,5-tris[4-(N-carbazolyl)phenyl]benzene (abbreviation: TCPB), 9-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazole (abbreviation: CzPA), 1,4-bis[4-(N-carbazolyl)phenyl]-2,3,5,6-tetraphenylbenzene and the like.

[0077] Examples of the aromatic hydrocarbons that can be used for the composite material are 2-tert-butyl-9,10-di(2-naphthyl)anthracene (abbreviation: t-BuDNA), 2-tert-butyl-9,10-di(1-naphthyl)anthracene, 9,10-bis(3,5-diphenylphenyl)anthracene (abbreviation: DPPA), 2-tert-butyl-9,10-bis(4-phenylphenyl)anthracene (abbreviation: t-BuDBA), 9,10-di(2-naphthyl)anthracene (abbreviation: DNA), 9,10-diphenylanthracene (abbreviation: DPAnth), 2-tert-butylanthracene (abbreviation: t-BuAnth), 9,10-Bis(4-methyl-1-naphthyl)anthracene (abbreviation: DMNA), 2-tert-butyl-9,10-bis[2-(1-naphthyl)phenyl]anthracene, 9,10-bis[2-(1-naphthyl)phenyl]anthracene, 2,3,6,7-tetramethyl-9, 10-di(1-naphthyl)anthracene, 2,3,6,7-tetramethyl-9,10-di(2-naphthyl)anthracene, 9,9'-bianthryl, 10,10'-diphenyl-9,9'-bianthryl, 10,10'-bis(2-phenylphenyl)-9,9'-bianthryl, 10,10'-bis[(2,3,4,5,6-pentaphenyl)phenyl]-9,9'-bianthryl, anthracene, tetracene, rubrene, perylene, 2,5,8,11-tetra(tert-butyl)perylene and the like.Pentacene, coronene, or the like can also be used. The aromatic hydrocarbon that has a hole mobility greater than or equal to 1 × 10 -6 cm 2 / Vs and has 14 to 42 carbon atoms is particularly preferred.

[0078] It should be noted that the aromatic hydrocarbons that can be used for the composite material may have a vinyl backbone. Examples of the aromatic hydrocarbon with a vinyl group are 4,4'-bis(2,2-diphenylvinyl)biphenyl (abbreviation: DPVBi), 9,10-bis[4-(2,2-diphenylvinyl)phenyl]anthracene (abbreviation: DPVPA), and the like.

[0079] A high molecular weight compound such as poly(N-vinylcarbazole) (abbreviation: PVK), poly(4-vinyltriphenylamine) (abbreviation: PVTPA), poly[N-(4-{N'-[4-(4-diphenylamino)phenyl]phenyl-N'-phenylamino}phenyl)methacrylamide] (abbreviation: PTPDMA) or poly[N,N'-bis(4-butylphenyl)-N,N'-bis(phenyl)benzidine] (abbreviation: poly-TPD) can also be used.

[0080] By providing a hole injection layer, a high hole transport property can be achieved, which enables a light-emitting element to have a low driving voltage.

[0081] The hole-transport layer 112 is a layer containing a substance with a hole-transport property. Examples of the substance with a hole-transport property are aromatic amine compounds, such as ethylenediamine dimethylamine (EDI). B. 4,4'-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: NPB), N,N'-bis(3-methylphenyl)-N,N'-diphenyl-[1,1'-biphenyl]-4,4'-diamine (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-(spiro-9,9'-bifluoren-2-yl)-N-phenylamino]biphenyl (abbreviation: BSPB), 4-phenyl-4'-(9-phenylfluoren-9-yl)triphenylamine (abbreviation: BPAFLP) and the like. The substances listed here have high hole transport properties and are mainly those that have a hole mobility of 10 -6 cm 2 / Vs or more. An organic compound exemplified as the substance having a hole-transporting property in the composite material described above can also be used for the hole-transporting layer 112. A high-molecular compound such as poly(N-vinylcarbazole) (abbreviation: PVK) or poly(4-vinyltriphenylamine) (abbreviation: PVTPA) can also be used. Note that the layer containing a substance having a hole-transporting property is not limited to a single layer and may be a stack of two or more layers each containing one of the above substances.

[0082] The light-emitting layer 113 is a layer containing a light-emitting substance. Since the light-emitting layer 113 has a structure similar to that described in Embodiment 1, the light-emitting element of this embodiment can have very high emission efficiency. Regarding the structure and materials of the light-emitting layer 113, reference is made to Embodiment 1.

[0083] There is no particular limitation on a material that can be used as the light-emitting substance or emission center substance in the light-emitting layer 113. The following can be given as examples of the above light-emitting substance or emission center substance.

[0084] A compound that emits blue light is preferably used as the first phosphorescent compound, and for example, a phosphorescent compound with an emission peak at 440 nm to 520 nm can be selected. The following are specific examples: an organometallic iridium complex having a 4H-triazole skeleton, such as [a compound with a 4H-triazole skeleton]. B. Tris{2-[5-(2-methylphenyl)-4-(2,6-dimethylphenyl)-4H-1,2,4-triazol-3-yl-κN2]phenyl-κC}iridium(III) (abbreviation: Ir(mpptz-dmp)3), Tris(5-methyl-3,4-diphenyl-4H-1,2,4-triazolato)iridium(III) (abbreviation: Ir(Mptz)3) or Tris[4-(3-biphenyl)-5-isopropyl-3-phenyl-4H-1,2,4-triazolato]iridium(III) (abbreviation: Ir(iPrptz-3b)3); an organometallic iridium complex with a 1H-triazole framework, such asTris[3-methyl-1-(2-methylphenyl)-5-phenyl-1H-1,2,4-triazolato]iridium(III) (abbreviation: Ir(Mptz1-mp)3) or tris(1-methyl-5-phenyl-3-propyl-1H-1,2,4-triazolato)iridium(III) (abbreviation: Ir(Prptz1-Me)3); an organometallic iridium complex with an imidazole framework, such as fac-tris[1-(2,6-diisopropylphenyl)-2-phenyl-1H-imidazole]iridium(III) (abbreviation: Ir(iPrpmi)3) or tris[3-(2,6-dimethylphenyl)-7-methylimidazo[1,2-f]phenanthridinato]iridium(III) (abbreviation: Ir(dmpimpt-Me)3); and an organometallic iridium complex in which a phenylpyridine derivative having an electron-withdrawing group is a ligand, such as bis[2-(4',6'-difluorophenyl)pyridinato-N,C. 2' ]iridium(III)tetrakis(1-pyrazolyl)borate (abbreviation: Flr6), bis[2-(4',6'-difluorophenyl)pyridinato-N,C 2' ]iridium(III)picolinate (abbreviation: Flrpic), bis{2-[3',5'-bis(trifluoromethyl)phenyl]pyridinato-N,C 2'}iridium(III)picolinate (abbreviation: Ir(CF3ppy)2(pic)) or bis[2-(4',6'-difluorophenyl)pyridinato-N,C2'[Iridium(III) acetylacetonate (abbreviation: Flr(acac)). Among the above compounds, an organometallic iridium complex having a polyazole framework, such as a 4H-triazole framework, a 1H-triazole framework, or an imidazole framework, exhibits a high hole-trapping property. Therefore, in the case where the first light-emitting layer in the light-emitting element according to one embodiment of the present invention has an electron-transport property (particularly when the first host material is an electron-transport material), it is preferable to use an organometallic iridium complex having a polyazole framework as the first phosphorescent compound. In this case, a recombination region of carriers can be controlled to be located in the first light-emitting layer or near the interface between the first light-emitting layer and the second light-emitting layer.It should be noted that an organometallic iridium complex with a 4H-triazole framework has excellent reliability and emission efficiency and is therefore particularly preferred.

[0085] Any compound can be selected as the second phosphorescent compound as long as the compound emits light with a wavelength longer than that emitted by the first phosphorescent compound. For example, it is preferable to select a phosphorescent compound with an emission peak at 520 nm to 600 nm. The following are specific examples: an organometallic iridium complex having a pyrimidine skeleton, such asTris(4-methyl-6-phenylpyrimidinato)iridium(III) (abbreviation: Ir(mppm)3), Tris(4-t-butyl-6-phenylpyrimidinato)iridium(III) (abbreviation: Ir(tBuppm)3), (Acetylacetonato)bis(6-methyl-4-phenylpyrimidinato)iridium(III) (abbreviation: Ir(mppm)2(acac)), (Acetylacetonato)bis(6-tert-butyl-4-phenylpyrimidinato)iridium(III) (abbreviation: Ir(tBuppm)2(acac)), (Acetylacetonato)bis[6-(2-norbornyl)-4-phenylpyrimidinato]iridium(III) (abbreviation: Ir(nbppm)2(acac)), Bis{2-[5-methyl-6-(2-methylphenyl)-4-pyrimidinyl-κN3]phenyl-κC}(2,4-pentanedionato-κ. 2O,O')iridium(III) (abbreviation: Ir(mpmppm)2(acac)) or (acetylacetonato)bis(4,6-diphenylpyrimidinato)iridium(III) (abbreviation: Ir(dppm)2(acac)); an organometallic iridium complex with a pyrazine skeleton, such as (acetylacetonato)bis(3,5-dimethyl-2-phenylpyrazinato)iridium(III) (abbreviation: Ir(mppr-Me)2(acac)) or (acetylacetonato)bis(5-isopropyl-3-methyl-2-phenylpyrazinato)iridium(III) (abbreviation: Ir(mppr-iPr)2(acac)); an organometallic iridium complex with a pyridine skeleton, such as tris(2-phenylpyridinato-N,C 2' )iridium(III) (abbreviation: Ir(ppy)3), bis(2-phenylpyridinato-N,C 2' )iridium(III)acetylacetonate (abbreviation: Ir(ppy)2(acac)), bis(benzo[h]quinolinato)iridium(III)acetylacetonate (abbreviation: Ir(bzq)2(acac)), tris(benzo[h]quinolinato)iridium(III) (abbreviation: Ir(bzq)3), tris(2-phenylquinolinato-N,C 2' )iridium(III) (abbreviation: Ir(pq)3) or bis(2-phenylquinolinato-N,C 2')iridium(III) acetylacetonate (abbreviation: Ir(pq)2(acac)); and a rare earth metal complex such as tris(acetylacetonato)(monophenanthroline)terbium(III) (abbreviation: Tb(acac)3(Phen)). Among the above compounds, an organometallic iridium complex having a diazine skeleton, such as a pyrimidine skeleton or a pyrazine skeleton, has a low hole-trapping property and a high electron-trapping property. Therefore, in the case where the second light-emitting layer in the light-emitting element according to one embodiment of the present invention has a hole-transporting property (particularly when the second host material is a hole-transporting material), it is preferable to use an organometallic iridium complex having a diazine skeleton as the second phosphorescent compound.In this case, a carrier recombination region can be controlled to be located in the first light-emitting layer or near the interface between the first light-emitting layer and the second light-emitting layer. Note that an organometallic iridium complex with a pyrimidine framework exhibits significantly high reliability and emission efficiency and is therefore particularly preferred.

[0086] Any compound can be selected as the third phosphorescent compound as long as the compound emits light with a wavelength longer than that emitted by the second phosphorescent compound. For example, it is preferable to select a phosphorescent compound that emits red light with an emission peak at 600 nm to 700 nm. The following are specific examples: an organometallic iridium complex having a pyrimidine skeleton, such as iridium nitride. B. Bis[4,6-bis(3-methylphenyl)pyrimidinato](diisobutylylmethano)iridium(III) (abbreviation: Ir(5mdppm)2(dibm)), Bis[4,6-bis(3-methylphenyl)pyrimidinato](dipivaloylmethanato)iridium(III) (abbreviation: Ir(5mdppm)2(dpm)) or Bis[4,6-di(naphthalen-1-yl)pyrimidinato](dipivaloylmethanato)iridium(III) (abbreviation: Ir(d1npm)2(dpm)); an organometallic iridium complex with a pyrazine framework, such as(Acetylacetonato)bis(2,3,5-triphenylpyrazinato)iridium(III) (abbreviation: Ir(tppr)2(acac)), bis(2,3,5-triphenylpyrazinato)(dipivaloylmethanato)iridium(III) (abbreviation: Ir(tppr)2(dpm)) or (acetylacetonato)bis[2,3-bis(4-fluorophenyl)quinoxalinato]iridium(III) (abbreviation: Ir(Fdpq)2(acac)); an organometallic iridium complex with a pyridine framework, such as tris(1-phenylisoquinolinato-N,C 2' )iridium(III) (abbreviation: Ir(piq)3) or bis(1-phenylisoquinolinato-N,C 2')iridium(III) acetylacetonate (abbreviation: Ir(piq)2(acac)); a platinum complex such as 2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphyrin platinum(II) (abbreviation: PtOEP); and a rare earth metal complex such as tris(1,3-diphenyl-1,3-propanedionato)(monophenanthroline)europium(III) (abbreviation: Eu(DBM)3(Phen)) or tris[1-(2-thenoyl)-3,3,3-trifluoroacetonato](monophenanthroline)europium(III) (abbreviation: Eu(TTA)3(Phen)). Among the above materials, an organometallic iridium complex with a diazine framework, such as B. a pyrimidine framework or a pyrazine framework, have a low hole capture property and a high electron capture property.Therefore, in the case where the third light-emitting layer in the light-emitting element according to one embodiment of the present invention has a hole-transport property (particularly when the third host material is a hole-transport material), it is preferable that an organometallic iridium complex having a diazine skeleton is used as the third phosphorescent compound. In this case, a recombination region of carriers can be controlled to be located in the first light-emitting layer or near the interface between the first light-emitting layer and the second light-emitting layer. Note that an organometallic iridium complex having a pyrimidine skeleton has significantly high reliability and emission efficiency and is therefore particularly preferable.Furthermore, since an organometallic iridium complex having a pyrazine skeleton can emit red light with good color values, by using the organometallic iridium complex in a white light-emitting element according to an embodiment of the present invention, a color rendering property of the white light-emitting element is improved.

[0087] It is also possible to select a first phosphorescent material, a second phosphorescent material, and a third phosphorescent material having the relationship described in Embodiment 1 from known phosphorescent materials other than the above phosphorescent compounds.

[0088] There is no particular limitation on the materials that can be used as the first to third host materials; various charge carrier transport materials can be selected and appropriately combined such that the Fig. 1A to Fig. 1C is obtained. As previously described, it is preferred that a host material with an electron transport property and a host material with a hole transport property are combined.

[0089] The following are examples of the host material with an electron transport property: a metal complex, such as bis(10-hydroxybenzo[h]quinolinato)beryllium(II) (abbreviation: BeBq2), bis(2-methyl-8-quinolinolato)(4-phenylphenolato)aluminum(III) (abbreviation: BAlq), bis(8-quinolinolato)zinc(II) (abbreviation: Znq), bis[2-(2-benzoxazolyl)phenolato]zinc(II) (abbreviation: ZnPBO), or bis[2-(2-benzothiazolyl)phenolato]zinc(II) (abbreviation: ZnBTZ); a heterocyclic compound with a polyazole backbone, such as2-(4-Biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (abbreviation: PBD), 3-(4-biphenylyl)-4-phenyl-5-(4-tert-butylphenyl)-1,2,4-triazole (abbreviation: TAZ), 1,3-bis[5-(p-tert-butylphenyl)-1,3,4-oxadiazol-2-yl]benzene (abbreviation: OXD-7), 9-[4-(5-phenyl-1,3,4-oxadiazol-2-yl)phenyl]-9H-carbazole (abbreviation: CO11), 2,2',2"-(1,3,5-benzenetriyl)tris(1-phenyl-1H-benzimidazole) (abbreviation: TPBI) or 2-[3-(dibenzothiophen-4-yl)phenyl]-1-phenyl-1 H-benzimidazole (abbreviation: mDBTBIm-II); a heterocyclic compound with a diazine skeleton, such as2-[3-(Dibenzothiophen-4-yl)phenyl]dibenzo[f,h]quinoxaline (abbreviation: 2mDBTPDBq-II), 2-[3'-(Dibenzothiophen-4-yl)biphenyl-3-yl]dibenzo[f,h]quinoxaline (abbreviation: 2mDBTBPDBq-II), 2-[3'-(9H-carbazol-9-yl)biphenyl-3-yl]dibenzo[f,h]quinoxaline (abbreviation: 2mCzBPDBq), 4,6-bis[3-(phenanthren-9-yl)phenyl]pyrimidine (abbreviation: 4,6mPnP2Pm) or 4,6-bis[3-(4-dibenzothienyl)phenyl]pyrimidine (abbreviation: 4,6mDBTP2Pm-II); and a heterocyclic compound with a pyridine skeleton, such as 3,5-bis[3-(9H-carbazol-9-yl)phenyl]pyridine (abbreviation: 35DCzPPy) or 1,3,5-tri[3-(3-pyridyl)phenyl]benzene (abbreviation: TmPyPB). Among the above materials, a heterocyclic compound with a diazine skeleton and a heterocyclic compound with a pyridine skeleton exhibit high reliability and are therefore preferred.In particular, a heterocyclic compound with a diazine (pyrimidine or pyrazine) framework exhibits a high electron transport property, which contributes to a reduction in the driving voltage.

[0090] The following are examples of the host material with a hole transport property: a compound with an aromatic amine backbone, such as4,4'-Bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: NPB), N,N'-Bis(3-methylphenyl)-N,N'-diphenyl-[1,1'-biphenyl]-4,4'-diamine (abbreviation: TPD), 4,4'-Bis[N-(spiro-9,9'-bifluoren-2-yl)-N-phenylamino]biphenyl (abbreviation: BSPB), 4-Phenyl-4'-(9-phenylfluoren-9-yl)triphenylamine (abbreviation: BPAFLP), 4-Phenyl-3'-(9-phenylfluoren-9-yl)triphenylamine (abbreviation: mBPAFLP), 4-Phenyl-4'-(9-phenyl-9H-carbazol-3-yl)triphenylamine (abbreviation: PCBA1BP), 4,4'-Diphenyl-4"-(9-phenyl-9H-carbazol-3-yl)triphenylamine (abbreviation: PCBBi1BP), 4-(1-naphthyl)-4'-(9-phenyl-9H-carbazol-3-yl)triphenylamine (abbreviation: PCBANB), 4,4'-Di(1-naphthyl)-4"-(9-phenyl-9H-carbazol-3-yl)triphenylamine (abbreviation: PCBNBB), 9,9-Dimethyl-N-phenyl-N-[4-(9-phenyl-9H-carbazol-3-yl)phenyl]fluoren-2-amine (abbreviation: PCBAF) or N-phenyl-N-[4-(9-phenyl-9H-carbazol-3-yl)phenyl]spiro-9,9'-bifluoren-2-amine (abbreviation: PCBASF); a compound with a carbazole skeleton, such as1,3-bis(N-carbazolyl)benzene (abbreviation: mCP), 4,4'-di(N-carbazolyl)biphenyl (abbreviation: CBP), 3,6-bis(3,5-diphenylphenyl)-9-phenylcarbazole (abbreviation: CzTP) or 3,3'-bis(9-phenyl-9H-carbazole) (abbreviation: PCCP); a compound with a thiophene skeleton, such as B. 4,4',4"-(benzene-1,3,5-triyl)tri(dibenzothiophene) (abbreviation: DBT3P-II), 2,8-diphenyl-4-[4-(9-phenyl-9H-fluoren-9-yl)phenyl]dibenzothiophene (abbreviation: DBTFLP-III) or 4-[4-(9-phenyl-9H-fluoren-9-yl)phenyl]-6-phenyldibenzothiophene (abbreviation: DBTFLP-IV); and a compound with a furan skeleton, such as 4,4',4"-(benzene-1,3,5-triyl)tri(dibenzofuran) (abbreviation: DBF3P-II) or 4-{3-[3-(9-phenyl-9H-fluoren-9-yl)phenyl]phenyl}dibenzofuran (abbreviation: mmDBFFLBi-II).Among the above materials, a compound having an aromatic amine skeleton and a compound having a carbazole skeleton are preferred because these compounds are highly reliable and have high hole transport properties, which contributes to a reduction in the driving voltage.

[0091] Host materials can be selected from known substances as well as from the above-mentioned host materials. It should be noted that host materials are preferably selected from substances whose triplet level (energy gap between a ground state and an excited triplet state) is higher than that of the phosphorescent compound. It is preferable that these host materials do not exhibit an absorption spectrum in the blue wavelength range. Specifically, an absorption edge of the absorption spectrum is preferably located at 440 nm or less.

[0092] For forming the light-emitting layer 113 having the above-described structure, co-evaporation by a vacuum evaporation method may be used, or alternatively, an ink-jet method, a spin coating method, a dip coating method, or the like using a mixed solution may also be used.

[0093] The electron-transport layer 114 is a layer containing a substance having an electron-transport property. For example, a layer containing a metal complex having a quinoline skeleton or a benzoquinoline skeleton, such as tris(8-quinolinolato)aluminum (abbreviation: Alq), tris(4-methyl-8-quinolinolato)aluminum (abbreviation: Almq3), bis(10-hydroxybenzo[h]quinolinato)beryllium (abbreviation: BeBq2), or bis(2-methyl-8-quinolinolato)(4-phenylphenolato)aluminum (abbreviation: BAlq), or the like, can be used. Alternatively, a metal complex containing an oxazole-based or thiazole-based ligand, such as tris(8-quinolinolato)aluminum (abbreviation: Alq2), may also be used. B. Bis[2-(2-hydroxyphenyl)benzoxazolato]zinc (abbreviation: Zn(BOX)2) or Bis[2-(2-hydroxyphenyl)benzothiazolato]zinc (abbreviation: Zn(BTZ)2), or the like.In addition to metal complexes, 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (abbreviation: PBD), 1,3-bis[5-(p-tert-butylphenyl)-1,3,4-oxadiazol-2-yl]benzene (abbreviation: OXD-7), 3-(4-biphenylyl)-4-phenyl-5-(4-tert-butylphenyl)-1,2,4-triazole (abbreviation: TAZ), bathophenanthroline (abbreviation: BPhen), bathocuproine (abbreviation: BCP), and the like can also be used. The substances listed here exhibit high electron transport properties and are mainly those with an electron mobility of 10. -6 cm 2 / Vs or more. It should be noted that any of the host materials described above having electron transport properties can be used for the electron transport layer 114.

[0094] Furthermore, the electron transport layer 114 is not limited to a single layer and may be a stack of two or more layers, each containing one of the above substances.

[0095] A layer that controls the transport of electron carriers can be arranged between the electron-transport layer and the light-emitting layer. This layer is formed by adding a small amount of a substance with high electron-capturing properties to a material with high electron-transport properties as described above. This layer is capable of regulating the carrier balance by suppressing the transport of electron carriers. Such a structure is very effective in preventing problems (such as a reduction in the lifetime of the element) that arise when electrons are conducted through the light-emitting layer.

[0096] Additionally, the electron injection layer 115 may be disposed in contact with the second electrode 102 between the electron transport layer 114 and the second electrode 102. For the electron injection layer 115, an alkali metal, an alkaline earth metal, or a compound thereof, such as lithium fluoride (LiF), cesium fluoride (CsF), or calcium fluoride (CaF2), may be used. For example, a layer formed using a substance having an electron transport property and containing an alkali metal, an alkaline earth metal, or a compound thereof may be used. Note that a layer formed using a substance having an electron transport property and containing an alkali metal or an alkaline earth metal is preferably used as the electron injection layer 115, in which case electron injection from the second electrode 102 occurs efficiently.

[0097] For the second electrode 102, any of metals, alloys, electrically conductive compounds, and mixtures thereof having a low work function (particularly, a work function of 3.8 eV or less), or the like, can be used. Specific examples of such a cathode material are elements belonging to groups 1 and 2 of the periodic table, such as alkali metals (e.g., lithium (Li) and cesium (Cs)), magnesium (Mg), calcium (Ca), and strontium (Sr), alloys thereof (e.g., MgAg and AlLi), rare earth metals such as europium (Eu) and ytterbium (Yb), alloys thereof, and the like. However, when the electron injection layer is disposed between the second electrode 102 and the electron transport layer, any of various conductive materials, such asAl, Ag, ITO, or indium oxide-tin oxide containing silicon or silicon oxide can be used regardless of the work function. Films of these electrically conductive materials can be formed by a sputtering method, an inkjet method, a spin-coating method, or the like.

[0098] Furthermore, any of various methods can be used to form the EL layer 103, regardless of whether it is a dry process or a wet process. For example, a vacuum evaporation method, an inkjet method, a spin-coating method, or the like can be used. Different manufacturing methods can be used for the electrodes or the layers.

[0099] Additionally, the electrode can be formed by a wet process using a sol-gel method or by a wet process using a paste of a metal material. Alternatively, the electrode can be formed by a dry process such as a sputtering method or a vacuum evaporation method.

[0100] In the light-emitting element having the above-described structure, a current flows due to a potential difference between the first electrode 101 and the second electrode 102, and holes and electrons recombine in the light-emitting layer 113 containing a substance with a high light-emitting property, thus emitting light. That is, a light-emitting region is formed in the light-emitting layer 113.

[0101] Light emission is extracted by the first electrode 101 and / or the second electrode 102. Therefore, the first electrode 101 and / or the second electrode 102 are / is light-transmitting electrodes. In the case where only the first electrode 101 is a light-transmitting electrode, the light emission is extracted by the first electrode 101. In the case where only the second electrode 102 is a light-transmitting electrode, the light emission is extracted by the second electrode 102. In the case where both the first electrode 101 and the second electrode 102 are light-transmitting electrodes, the light emission is extracted by the first electrode 101 and the second electrode 102.

[0102] The structure of the layers disposed between the first electrode 101 and the second electrode 102 is not limited to the structure described above. Preferably, a light-emitting region where holes and electrons recombine is placed away from the first electrode 101 and the second electrode 102, so that quenching due to the proximity of the light-emitting region and a metal used for electrodes and charge injection layers can be prevented.

[0103] Moreover, in order to suppress energy transfer from an exciton generated in the light-emitting layer, it is preferable that the hole transport layer and the electron transport layer in contact with the light-emitting layer 113, particularly a carrier transport layer in contact with a side closer to the light-emitting region in the light-emitting layer 113, is formed using a substance having a larger band gap than the light-emitting substance of the light-emitting layer or the emission center substance contained in the light-emitting layer.

[0104] A light-emitting element of this embodiment is preferably formed over a substrate made of glass, plastic, or the like. As a method for stacking layers over the substrate, layers may be stacked one on top of the other from the first electrode 101 side or one on top of the other from the second electrode 102 side. Although a single light-emitting element may be formed over a substrate in a light-emitting device, a plurality of light-emitting elements may be formed over a substrate. When a plurality of light-emitting elements are formed over a substrate as described above, a lighting device in which the elements are separated from each other or a passive matrix light-emitting device can be manufactured.A light-emitting element can be formed over an electrode electrically connected to, for example, a thin-film transistor (TFT) formed over a substrate made of glass, plastic, or the like, so that an active matrix light-emitting device in which the TFT controls the operation of the light-emitting element can be manufactured. Note that there is no particular limitation on the structure of the TFT, which may be a staggered TFT or an inverted-staggered TFT. In addition, the crystallinity of a semiconductor used for the TFT is also not particularly limited; an amorphous semiconductor or a crystalline semiconductor may be used. In addition, a driving circuit formed on a TFT substrate may be formed with an n-type TFT and a p-type TFT, or with either an n-type TFT or a p-type TFT.

[0105] It should be noted that this embodiment may be appropriately combined with any of the other embodiments. (Embodiment 3)

[0106] In this embodiment, a light-emitting device using the light-emitting element described in Embodiments 1 and 2 will be described.

[0107] In this embodiment, the light-emitting device using the light-emitting element described in Embodiments 1 and 2 is described by Fig. 6A and Fig. 6B. It should be noted that Fig. 6A is a plan view of the light emitting device and that Fig. 6B is a cross-sectional view along lines AB and CD in Fig. 6A. This light-emitting device includes a driving circuit section (source line driving circuit) 601, a pixel section 602, and a driving circuit section (gate line driving circuit) 603, which control the light emission of the light-emitting element and are shown with dotted lines. Furthermore, reference numeral 604 denotes a sealing substrate; 605, a desiccant; 605, a sealing material; and 607, a space surrounded by the sealing material 605.

[0108] Reference numeral 608 denotes a line for transmitting signals input to the source line driver circuit 601 and the gate line driver circuit 603, and for receiving signals such as a video signal, a clock signal, a start signal, and a reset signal from an FPC (flexible printed circuit) 609 serving as an external input terminal. Although only the FPC is illustrated here, a printed wiring board (PWB) may be attached to the FPC. The light-emitting device in the present specification includes within its category not only the light-emitting device itself but also the light-emitting device provided with the FPC or the PWB.

[0109] Below, a cross-sectional structure is presented based on Fig. 6B. The driver circuit section and the pixel section are formed over an element substrate 610; the source line driver circuit 601, which is a driver circuit section, and one of the pixels in the pixel section 602 are shown here.

[0110] As the source line driver circuit 601, a CMOS circuit combining an n-channel TFT 623 and a p-channel TFT 624 is formed. Additionally, the driver circuit may be formed with any of various circuits, such as a CMOS circuit, a PMOS circuit, or an NMOS circuit. In this embodiment, although a driver-integrated type in which the driver circuit is formed over the substrate is shown, the driver circuit is not necessarily formed over the substrate, and the driver circuit may be formed outside the substrate, not over it.

[0111] The pixel section 602 includes a plurality of pixels, each including a switching TFT 611, a current-controlling TFT 612, and a first electrode 613 electrically connected to a drain terminal of the current-controlling TFT 612. Note that an insulator 614, for which a positive photosensitive acrylic resin film is used here, is formed to cover an end portion of the first electrode 613.

[0112] To improve coverage, the insulator 614 is formed with a curved surface with a curvature at its upper or lower end portion. For example, in the case where a positive photosensitive acrylic is used as the material for the insulator 614, only the upper end portion of the insulator 614 preferably has a curved surface with a radius of curvature (0.2 μm to 3 μm). Either a negative photosensitive resin or a positive photosensitive resin can be used as the insulator 614.

[0113] An EL layer 616 and a second electrode 617 are formed over the first electrode 613. Here, as the material for the first electrode 613 serving as an anode, a material with a high work function is preferably used. For example, a single-layer film such as an ITO film, an indium tin oxide film containing silicon, an indium oxide film containing 2 wt% to 20 wt% zinc oxide, a titanium nitride film, a chromium film, a tungsten film, a Zn film, a Pt film, or the like, a stack of a titanium nitride film and a film containing aluminum as a main component, a three-layer stack of a titanium nitride film, a film containing aluminum as a main component, and a titanium nitride film, or the like can be used. The stacked-layer structure enables low conduction resistance, good ohmic contact, and function as an anode.

[0114] In addition, the EL layer 616 is formed by one of various methods, such as an evaporation method using an evaporation mask, an inkjet method, and a spin coating method. The EL layer 616 has a structure similar to that described in Embodiments 1 and 2. Furthermore, any low-molecular compound or high-molecular compound (including oligomers and dendrimers) can be used as another material contained in the EL layer 616.

[0115] As the material for the second electrode 617 formed above the EL layer 616 and serving as a cathode, a material with a low work function (e.g., Al, Mg, Li, Ca, or an alloy or compound thereof, such as MgAg, Mgln, or AlLi) is preferably used. In the case where light generated in the EL layer 616 is transmitted through the second electrode 617, a stack of a thin metal film and a transparent conductive film (e.g., ITO, indium oxide containing 2 wt% to 20 wt% zinc oxide, indium tin oxide containing silicon, or zinc oxide (ZnO)) is preferably used for the second electrode 617.

[0116] Note that the light-emitting element is formed with the first electrode 613, the EL layer 616, and the second electrode 617. The light-emitting element is the light-emitting element described in Embodiments 1 and 2. In the light-emitting device of this embodiment, the pixel section including a plurality of light-emitting elements may include both the light-emitting element described in Embodiments 1 and 2 and a light-emitting element having a different structure.

[0117] Furthermore, the sealing substrate 604 is fixed to the element substrate 610 with the sealing material 605, so that a light-emitting element 618 is arranged in the space 607 surrounded by the element substrate 610, the sealing substrate 604, and the sealing material 605. The space 607 may be filled with a filler, and it may be filled with an inert gas (such as nitrogen or argon) or with the sealing material 605. It is preferable that the sealing substrate be provided with a recessed portion, and that the desiccant 625 be arranged in the recessed portion, in which case deterioration due to humidity can be suppressed.

[0118] An epoxy-based resin or a glass frit is preferably used for the sealing material 605. It is preferable that such a material be as moisture-resistant or oxygen-resistant as possible. A glass substrate, a quartz substrate, or a plastic substrate made of fiberglass-reinforced plastic (FRP), poly(vinyl fluoride) (PVF), polyester, acrylic, or the like can be used as the sealing substrate 604.

[0119] As described above, the light-emitting device using the light-emitting element described in Embodiments 1 and 2 can be obtained.

[0120] The light-emitting device of this embodiment is manufactured using the light-emitting element described in Embodiments 1 and 2 and can therefore have advantageous properties. In particular, since the light-emitting element described in Embodiments 1 and 2 has high emission efficiency, the light-emitting device can have reduced power consumption. Furthermore, the light-emitting device can be operated at a low voltage because the light-emitting element has a low drive voltage.

[0121] Although, as described above, an active matrix light-emitting device is described in this embodiment, a passive matrix light-emitting device can also be manufactured. Fig. 7A and Fig. 7B show a passive matrix light-emitting device fabricated using the present invention. Fig. 7A is a perspective view of the light-emitting device, and Fig. 7B is a cross-sectional view along the line XY in Fig. 7A. In Fig. 7A and Fig. 7B, an EL layer 955 is disposed above a substrate 951 between an electrode 952 and an electrode 956. An end portion of the electrode 952 is covered with an insulating layer 953. In addition, a separation layer 954 is disposed over the insulating layer 953. The sidewalls of the separation layer 954 are slanted so that the distance between both sidewalls gradually decreases toward the surface of the substrate. In other words, a cross-section along the short side direction of the separation layer 954 is trapezoidal, and the lower side (a side lying in the direction equal to an in-plane direction of the insulating layer 953 and in contact with the insulating layer 953) is shorter than the upper side (a side lying in the direction equal to the in-plane direction of the insulating layer 953 and not in contact with the insulating layer 953).The separation layer 954 thus disposed can prevent defects in the light-emitting element due to static electricity or the like. The passive matrix light-emitting device can also operate with low power consumption by incorporating the light-emitting element described in Embodiments 1 and 2, which can operate at a low voltage. Furthermore, the light-emitting device can have high reliability by incorporating the light-emitting element described in Embodiments 1 and 2.

[0122] For full-color display, a color layer or a color conversion layer may be further provided in a light path through which light is guided from the light-emitting element to the outside of the light-emitting device. An example of a light-emitting device in which full-color display is achieved using a color layer and the like is shown in Fig. 8A and Fig. 8B. In Fig. 8A, a substrate 1001, a base insulating film 1002, a gate insulating film 1003, gate electrodes 1006, 1007, and 1008, a first interlayer insulating film 1020, a second interlayer insulating film 1021, a peripheral portion 1042, a pixel portion 1040, a driving circuit portion 1041, first electrodes 1024W, 1024R, 1024G, and 1024B of light-emitting elements, a partition wall 1025, a layer 1028 containing an organic compound, a second electrode 1029 of the light-emitting elements, a sealing substrate 1031, and a sealing agent 1032 are shown. Furthermore, color layers (a red color layer 1034R, a green color layer 1034G, and a blue color layer 1034B) are arranged on a transparent base material 1033. In addition, a black layer (a black matrix) 1035 may also be provided.The transparent base material 1033, provided with the color layers and the black layer, is placed and fixed to the substrate 1001. Note that the color layers and the black layer are covered with a covering layer 1036. In this embodiment, light emitted from some of the light-emitting layers is not transmitted through any color layer, while light emitted from the other light-emitting layers is transmitted through the color layers. Since the light not transmitted through any color layer is white, and the light transmitted through any of the color layers is red, blue, or green, an image can be displayed using pixels for the four colors.

[0123] The light-emitting device described above is a light-emitting device having a structure in which light is extracted from the side of the substrate 1001 on which the TFTs are formed (a bottom emission structure), but it may also be a light-emitting device having a structure in which light is extracted from the side of the sealing substrate 1031 (a top emission structure). Fig. 9 is a cross-sectional view of a light-emitting device having an up-emission structure. In this case, a substrate that does not transmit light can be used as the substrate 1001. The process up to the step of forming a connection electrode, via which the TFT and the anode of the light-emitting element are connected, is performed in a similar manner to the manufacturing process of the light-emitting device having a down-emission structure. A third interlayer insulating film 1037 is then formed to cover an electrode 1022. The third interlayer insulating film 1037 may have a planarization function. The third interlayer insulating film 1037 may be formed using a material similar to that of the second interlayer insulating film, or alternatively, using another known material.

[0124] The first electrodes 1024W, 1024R, 1024G, and 1024B of the light-emitting elements each serve as an anode, but can also serve as a cathode. Furthermore, in the case of a light-emitting device with an upward emission structure, as shown in Fig. 9, the first electrodes are preferably reflective electrodes. The organic compound-containing layer 1028 is formed to have a structure similar to that described in Embodiments 1 and 2, and to have a structure capable of emitting white light. As a structure capable of emitting white light, in the case where two EL layers are used, the following structures can be given: a structure in which blue light is obtained from a light-emitting layer of one of the EL layers and orange light is obtained from a light-emitting layer of the other EL layer; a structure in which blue light is obtained from a light-emitting layer of one of the EL layers, and red light and green light are obtained from a light-emitting layer of the other EL layer; and the like.Furthermore, in the case where three EL layers are used, red light, green light, and blue light are obtained from the respective light-emitting layers, so that a light-emitting element that emits white light can be obtained. Needless to say, the structure that emits white light is not limited to this, as long as the structure described in Embodiments 1 and 2 is used.

[0125] The color layers are each provided in a light path through which light is guided from the light-emitting element to the outside of the light-emitting device. In the case of the light-emitting device having a downward emission structure as shown in Fig. As shown in Figure 8A, the color layers 1034R, 1034G, and 1034B may be disposed on the transparent base material 1033 and then attached to the substrate 1001. The color layers may be disposed between the gate insulating film 1003 and the first interlayer insulating film 1020, as shown in Fig. 8B. In the case of a structure with upward emission, as shown in Fig. As shown in FIG. 9, sealing may be performed using the sealing substrate 1031 on which the color layers (the red color layer 1034R, the green color layer 1034G, and the blue color layer 1034B) are disposed. The sealing substrate 1031 may be provided with the black layer (the black matrix) 1035 sandwiched between pixels. The color layers (the red color layer 1034R, the green color layer 1034G, and the blue color layer 1034B) and the black layer (the black matrix) 1035 may be covered with the covering layer 1036. Note that a light-transmitting substrate is used as the sealing substrate 1031.

[0126] When a voltage is applied between the pair of electrodes of the organic light-emitting element obtained in this way, a white light-emitting region 1044W can be obtained. In addition, a red light-emitting region 1044R, a blue light-emitting region 1044B, and a green light-emitting region 1044G can be obtained using the color layers. The light-emitting device of this embodiment includes the light-emitting element described in Embodiments 1 and 2. Consequently, a low-power light-emitting device can be obtained.

[0127] In addition, an example is shown here in which full-color display is performed using four colors, namely red, green, blue, and white; however, there is no particular limitation, and full-color display can also be performed using three colors, namely red, green, and blue.

[0128] This embodiment can be freely combined with one of the other embodiments. (Embodiment 4)

[0129] In this embodiment, an example in which the light-emitting element described in Embodiments 1 and 2 is used for a lighting device will be explained with reference to Fig. 10A and Fig. 10B described. Fig. 10B is a plan view of the lighting device, and Fig. 10A is a cross-sectional view taken along the line ef in Fig. 10B.

[0130] In the lighting device of this embodiment, a first electrode 401 is formed over a substrate 400, which is a support and has a light-transmitting property. The first electrode 401 corresponds to the first electrode 101 of Embodiment 3.

[0131] An auxiliary electrode 402 is arranged above the first electrode 401. Since light emission is extracted through the first electrode 401 side in the example given in this embodiment, the first electrode 401 is formed using a material having a light-transmitting property. The auxiliary electrode 402 is provided to compensate for the low conductivity of the material having a light-transmitting property and has a function of suppressing the unevenness of the luminance of a light-emitting surface due to a voltage drop caused by the high resistance of the first electrode 401. The auxiliary electrode 402 is formed using a material having at least higher conductivity than the material of the first electrode 401, and is preferably formed using a material having high conductivity, such as aluminum.It should be noted that surfaces of the auxiliary electrode 402, except for a part in contact with the first electrode 401, are preferably covered with an insulating layer. The purpose of this is to suppress light emission above the upper part of the auxiliary electrode 402 that cannot be extracted, reduce reactive current, and suppress a reduction in power efficiency. It should be noted that a pad 412 for applying a voltage to a second electrode 404 may be formed simultaneously with the formation of the auxiliary electrode 402.

[0132] An EL layer 403 is formed over the first electrode 401 and the auxiliary electrode 402. The EL layer 403 has the structure described in Embodiments 1 and 2. Regarding the structure, reference is made to the descriptions of those. Note that the EL layer 403 is preferably slightly larger than the first electrode 401 when viewed from above. In this case, the EL layer 403 can also serve as an insulating layer that suppresses a short circuit between the first electrode 401 and the second electrode 404.

[0133] The second electrode 404 is formed to cover the EL layer 403. The second electrode 404 corresponds to the second electrode 102 of Embodiment 3 and has a similar structure. In this embodiment, it is preferable that the second electrode 404 be formed using a material with a high reflectance because the light emission is extracted through the first electrode 401 side. In this embodiment, the second electrode 404 is connected to the pad 412, thereby applying a voltage.

[0134] As described above, the lighting device described in this embodiment includes a light-emitting element including the first electrode 401, the EL layer 403, and the second electrode 404 (and the auxiliary electrode 402). Since the light-emitting element is a light-emitting element with high emission efficiency, the lighting device of this embodiment can be a low-power lighting device. Furthermore, since the light-emitting element is a high-reliability light-emitting element, the lighting device of this embodiment can be a high-reliability lighting device.

[0135] The light-emitting element having the above structure is attached to a sealing substrate 407 with sealing materials 405 and 406, and sealing is performed, thereby completing the lighting device. It is possible to use either only the sealing material 405 or the sealing material 406. Additionally, the inner sealing material 406 may be mixed with a desiccant, which can adsorb moisture, thus increasing reliability.

[0136] If the pad 412, the first electrode 401, and the auxiliary electrode 402 extend partially to the outside of the sealing materials 405 and 406, the extending portions can serve as external input terminals. An IC chip 420 equipped with a converter or the like can be arranged over the external input terminals.

[0137] As described above, since the lighting device described in this embodiment includes the light-emitting element described in Embodiments 1 and 2 as an EL element, the lighting device can be a lighting device with low power consumption. Furthermore, the lighting device can be a lighting device with a low drive voltage. Furthermore, the lighting device can be a lighting device with high reliability. (Embodiment 5)

[0138] In this embodiment, examples of electronic devices each including the light-emitting element described in Embodiments 1 and 2 are described. The light-emitting element described in Embodiments 1 and 2 has high emission efficiency and reduced power consumption. As a result, the electronic devices described in this embodiment can each have a light-emitting region with reduced power consumption. Furthermore, the electronic devices can be operated at a low voltage because the light-emitting element described in Embodiments 1 and 2 has a low drive voltage.

[0139] Examples of the electronic device to which the above light-emitting element is applied include televisions (also referred to as TVs or television receivers), monitors for computers and the like, cameras such as digital cameras and digital video cameras, digital photo frames, mobile phones (also referred to as cellular phones or mobile phone devices), portable game consoles, portable information terminals, audio playback devices, large-scale gaming machines such as pinball machines, and the like. Specific examples of these electronic devices are given below.

[0140] Fig. 11A shows an example of a television set. In the television set, a display section 7103 is installed in a cabinet 7101. In addition, the cabinet 7101 is supported by a stand 7105. Images can be displayed on the display section 7103, and the light-emitting elements described in Embodiments 1 and 2 are arranged in a matrix in the light-emitting region 7103. The light-emitting elements can have high emission efficiency. Furthermore, the light-emitting elements can be driven at a low voltage. Furthermore, the light-emitting elements can have a long lifetime. Therefore, the television set including the display section 7103 formed using the light-emitting elements can be a television set with reduced power consumption. Furthermore, the television set can be a television set with a low drive voltage.In addition, the TV can be a TV with high reliability.

[0141] The television can be operated using a control switch on the housing 7101 or a separate remote control 7110. Using control buttons 7109 on the remote control 7110, the television channels and volume can be adjusted, and images displayed on the display section 7103 can be controlled. Furthermore, the remote control 7110 may include a display section 7107 for displaying data output from the remote control 7110.

[0142] It should be noted that the television is equipped with a receiver, a modem, and the like. The receiver can receive a general television broadcast. Furthermore, if the television is connected to a communications network via the modem, either wirelessly or non-wirelessly, unidirectional (from a transmitter to a receiver) or bidirectional (between a transmitter and a receiver, or between receivers) information communication can be performed.

[0143] Fig. 11B1 illustrates a computer including a main body 7201, a housing 7202, a display section 7203, a keyboard 7204, an external connection terminal 7205, a pointing device 7206, and the like. Note that this computer is constructed by using matrix-arranged light-emitting elements, similar to those described in Embodiment 2 or 3, in the display section 7203. The computer in Fig. 11B1 can also be a Fig. 11B2 shown structure. The computer in Fig. 11B2 is provided with a second display section 7210 instead of the keyboard 7204 and the pointing device 7206. The second display section 7210 is a touchscreen, and input can be performed by controlling the input display on the second display section 7210 with a finger or an associated stylus. The second display section 7210 can also display images other than the input display. The display section 7203 can also be a touchscreen. The two screens can be connected via a hinge, which prevents problems. For example, the screens can be prevented from being cracked or broken while the computer is stored or carried. The light-emitting elements can have high emission efficiency. Consequently, this computer with the display section 7203 formed using the light-emitting elements consumes less power.

[0144] Fig. Figure 11C shows a portable game console with two housings, a housing 7301 and a housing 7302, which are connected to each other via a hinge connection 7303 so that the portable game console can be opened or closed. The housing 7301 contains a display section 7304, which includes the light-emitting elements described in embodiments 1 and 2 and arranged in a matrix, and the housing 7302 contains a display section 7305. In addition, the portable game console in Fig. 11C, a speaker section 7306, a recording medium insertion portion 7307, an LED lamp 7308, an input means (an operation button 7309, a connection terminal 7310, a sensor 7311 (a sensor having a function of measuring force, displacement, position, speed, acceleration, angular velocity, rotation frequency, distance, light, liquid, magnetism, temperature, chemical substance, sound, time, hardness, electric field, current, voltage, electric power, radiation, flow rate, humidity, gradient, oscillation, odor, or infrared rays) and a microphone 7312), and the like.Of course, the structure of the portable game console is not limited to the above structure, as long as the display section including the light-emitting elements described in Embodiments 1 and 2 and arranged in a matrix is ​​used as at least one of the display sections 7304 and 7305, and the structure may include other accessories as necessary. The portable game console in . Fig. 11C has a function of reading a program or data stored in a storage medium and displaying it on the display section, and a function of sharing information with another portable game console through wireless communication. It should be noted that functions of the portable game console in Fig. 11C are not limited thereto, and the portable game console can have various functions. Since the light-emitting elements used in the display section 7304 have high emission efficiency, the portable game console having the above-described display section 7304 can be a portable game console with reduced power consumption. Since the light-emitting elements used in the display section 7304 can each be driven at a low voltage, the portable game console can also be a portable game console with a low drive voltage. Furthermore, since the light-emitting elements used in the display section 7304 each have a long service life, the portable game console can be highly reliable.

[0145] Fig. 11D shows an example of a mobile phone. The mobile phone is provided with a display section 7402 installed in a housing 7401, operation buttons 7403, an external connection terminal 7404, a speaker 7405, a microphone 7406, and the like. Note that the mobile phone 7400 includes the display section 7402, which includes the light-emitting elements described in Embodiments 1 and 2 and arranged in a matrix. The light-emitting elements can have high emission efficiency. Furthermore, the light-emitting elements can be operated at a low voltage. Moreover, the light-emitting elements can have a long lifetime. Consequently, the mobile phone including the display section 7402 formed using the light-emitting elements can be a mobile phone with reduced power consumption.Furthermore, the mobile phone can be a mobile phone with a low drive voltage. Furthermore, the mobile phone can be a mobile phone with high reliability.

[0146] When the display section 7402 of the mobile phone is in Fig. 11D is touched with a finger or the like, data can be input into the mobile phone. In this case, operations such as making a call and sending an email can be performed by touching the display section 7402 with a finger or the like.

[0147] There are mainly three screen modes of the display section 7402. The first mode is a display mode, which primarily displays an image. The second mode is an input mode, which primarily inputs information such as characters. The third mode is a display and input mode, which combines the two modes, display mode and input mode.

[0148] For example, in the case of making a call or writing an email, a character input mode primarily for inputting characters is selected for the display section 7402, so that the characters displayed on a screen can be input. In this case, it is preferable to display a keyboard or number buttons on almost the entire screen of the display section 7402.

[0149] When a detection device having a sensor for detecting an inclination, such as a gyroscope or an acceleration sensor, is provided in the mobile phone, the display on the screen of the display section 7402 can be automatically changed by determining the orientation of the mobile phone (whether the mobile phone is held horizontally or vertically, for landscape or portrait mode).

[0150] The screen modes are switched by touching the display section 7402 or by operating the operation buttons 7403 of the case 7401. The screen modes can be switched depending on the type of images displayed on the display section 7402. For example, if the signal of an image displayed on the display section is a moving image data signal, the screen mode is switched to the display mode. If the signal is a text data signal, the screen mode is switched to the input mode.

[0151] Furthermore, in the input mode, when an input by touching the display section 7402 is not performed for a certain period of time while a signal detected by an optical sensor in the display section 7402 is detected, the screen mode can be controlled to switch from the input mode to the display mode.

[0152] The display section 7402 can serve as an image sensor. For example, an image of a palm print, a fingerprint, or the like is captured by touching the display section 7402 with a hand or finger, thereby performing personal authentication. Furthermore, an image of a finger vein, a palm vein, or the like can be captured by providing a backlight or a scanning light source that emits light in the near-infrared range in the display section.

[0153] It should be noted that the structure described in this embodiment can be appropriately combined with any of the structures described in Embodiments 1 to 4.

[0154] As described above, the application range of the light-emitting device using the light-emitting element described in Embodiments 1 and 2 is so broad that this light-emitting device can be applied to electronic devices in various fields. Using the light-emitting element described in Embodiments 1 and 2, an electronic device with reduced power consumption can be obtained.

[0155] Fig. Figure 12 shows an example of a liquid crystal display device in which the light-emitting element described in Embodiments 1 and 2 is used for a backlight. The liquid crystal display device in Fig. 12 includes a package 901, a liquid crystal layer 902, a backlight unit 903, and a case 904. The liquid crystal layer 902 is connected to a driver IC 905. The light-emitting element described in Embodiments 1 and 2 is used in the backlight unit 903, to which a current is supplied via a terminal 906.

[0156] The light-emitting element described in Embodiments 1 and 2 is used for the backlight of the liquid crystal display device. Therefore, the backlight can have reduced power consumption. In addition, the use of the light-emitting element described in Embodiment 2 enables the manufacture of a surface-emission lighting device and further enables the manufacture of a surface-emission lighting device. Therefore, the backlight can be a large-area backlight, and the liquid crystal display device can also be a large-area device. In addition, the light-emitting device using the light-emitting element described in Embodiment 2 can be thinner than a conventional light-emitting device. Accordingly, the display device can also be thinner.

[0157] Fig. Figure 13 shows an example in which the light-emitting element described in Embodiments 1 and 2 is used for a table lamp, which is a lighting device. The table lamp in Fig. 13 includes a housing 2001 and a light source 2002, and the light-emitting device described in Embodiment 4 is used for the light source 2002.

[0158] Fig. 14 shows an example in which the light-emitting element described in Embodiments 1 and 2 is applied to an interior lighting device 3001 and a display device 3002. Since the light-emitting element described in Embodiments 1 and 2 has reduced power consumption, a lighting device with reduced power consumption can be obtained. Furthermore, since the light-emitting element described in Embodiments 1 and 2 can have a large area, the light-emitting element can be applied to a large-area lighting device. Furthermore, since the light-emitting element described in Embodiments 1 and 2 is thin, the light-emitting element can be applied to a lighting device with a reduced thickness.

[0159] The light-emitting element described in Embodiments 1 and 2 can also be used for a car windshield or a car dashboard. Fig. 15 illustrates an embodiment in which the light-emitting elements described in Embodiment 2 are applied to a car windshield and a car instrument panel. Displays 5000 to 5005 each include the light-emitting element described in Embodiments 1 and 2.

[0160] The display 5000 and the display 5001 are display devices provided in the car windshield, in which the light-emitting elements described in Embodiments 1 and 2 are incorporated. The light-emitting element described in Embodiments 1 and 2 can be formed into a so-called see-through display device through which the opposite side can be seen by including a first electrode and a second electrode formed as electrodes with light-transmitting properties. Such display devices through which the opposite side can be seen can even be provided in the car windshield without obstructing the view. Note that in the case where a transistor is provided for driving, a transistor with a light-transmitting property, such as a transistor with a light-transmitting property, is used.an organic transistor using an organic semiconductor material or a transistor using an oxide semiconductor is preferably used.

[0161] The display 5002 is a display device arranged in a pillar area and incorporating the light-emitting elements described in Embodiments 1 and 2. The display 5002 can compensate for the visibility obstructed by the pillar area by displaying an image captured by an imaging unit in the vehicle body. The display 5003 in the instrument panel can also compensate for the visibility obstructed by the vehicle body by displaying an image captured by an imaging unit outside the vehicle body, resulting in a reduction of blind spots and an increase in safety. By displaying an image to compensate for the area a driver cannot see, the driver can easily and conveniently confirm safety.

[0162] Displays 5004 and 5005 can provide various information, such as navigation data, a speedometer, a tachometer, a mileage reading, a fuel gauge, a gearshift indicator, and air conditioning settings. The content or layout of the display can be freely modified by a user. Note that such information can also be displayed on displays 5000 to 5003. Displays 5000 to 5005 can also be used as lighting devices.

[0163] The light-emitting element described in Embodiments 1 and 2 can exhibit high emission efficiency and low power consumption. Therefore, the load on a battery is small even when many large screens, such as displays 5000 to 5005, are provided, resulting in convenient use. For this reason, the light-emitting device and the lighting device each incorporating the light-emitting element described in Embodiments 1 and 2 can be suitably used as a light-emitting device in a vehicle and a lighting device in a vehicle.

[0164] Fig. 16A and Fig. 16B shows an example of a foldable tablet computer. Fig. 16A illustrates the tablet computer that is not closed. The tablet computer includes a case 9630, a display section 9631a, a display section 9631b, a display mode switch 9034, a power switch 9035, a power saving mode switch 9036, a clasp 9033, and an operation switch 9038. Note that in the tablet computer, the display section 9631a and / or the display section 9631b are formed using a light-emitting device including the light-emitting element described in Embodiments 1 and 2.

[0165] Part of the display section 9631a may be a touchscreen area 9632a, and data can be input when a displayed operation button 9637 is touched. Half of the display section 9631a has only a display function, and the other half has a touchscreen function; however, an embodiment of the present invention is not limited to this structure. The entire display section 9631a may have a touchscreen function. For example, a keyboard is displayed on the entire area of ​​the display section 9631a, so that the display section 9631a is used as a touchscreen. Thus, the display section 9631b can be used as a display screen.

[0166] As with the display section 9631a, a part of the display section 9631b may be a touchscreen area 9632b. When a keyboard display / hide button 9639 on the touchscreen is touched with a finger, a stylus, or the like, the keyboard may be displayed on the display section 9631b.

[0167] A touch-sensitive input may be performed on the touchscreen area 9632a and the touchscreen area 9632b at the same time.

[0168] For example, the display mode switch 9034 can switch the display between portrait mode, landscape mode, and the like, and between monochrome display and color display. The power-saving switch 9036 can control the display luminance according to the amount of external light detected by an optical sensor in the tablet computer when the tablet computer is in use. A further detection device with a sensor for detecting inclination, such as a gyroscope or an accelerometer, can be installed in the tablet computer in addition to the optical sensor.

[0169] Fig. Although FIG. 16A shows an example in which the display section 9631a and the display section 9631b have the same display area, an embodiment of the present invention is not limited to this example. The display section 9631a and the display section 9631b may have different display areas and different display qualities. For example, higher-resolution images may be displayed on one of the display sections 9631a and 9631b.

[0170] Fig. Figure 16B shows the tablet computer in the closed state. The tablet computer includes the housing 9630, a solar cell 9633, a charge and discharge control circuit 9634, a battery 9635, and a DC-DC converter 9636. As an example, Fig. 16B illustrates the charge and discharge control circuit 9634 including the battery 9635 and the DC-DC converter 9636.

[0171] Since the tablet computer is foldable, the casing 9630 can be closed when the tablet computer is not in use. As a result, the display section 9631a and the display section 9631b can be protected, thereby providing a tablet computer with high durability and high reliability for long-term use.

[0172] The tablet computer in Fig. 16A and Fig. 16B may have other functions such as a function for displaying various types of data (e.g., a still image, a moving image, and a text image), a function for displaying a calendar, a date, a time, or the like on the display section, a touch input function for processing or modifying the data displayed on the display section, and a function for controlling the processing by means of various types of software (programs).

[0173] The solar cell 9633, provided on a surface of the tablet computer, can supply power to the touchscreen, the display section, a video signal processing section, or the like. Note that the solar cell 9633 can be provided on one or both surfaces of the housing 9630 so that the battery 9635 can be charged efficiently.

[0174] The structure and operation of the charge and discharge control circuit 9634 in Fig. 16B are described with reference to a block diagram in Fig. 16C described. Fig. 16C illustrates the solar cell 9633, the battery 9635, the DC-DC converter 9636, a converter 9638, switches SW1 to SW3, and the display section 9631. The battery 9635, the DC-DC converter 9636, the converter 9638, and the switches SW1 to SW3 correspond to the charge and discharge control circuit 9634 in Fig. 16B.

[0175] First, an example of the operation in the case where power is generated by the solar cell 9633 using external light will be described. The voltage of the power generated by the solar cell is increased or decreased by the DC-DC converter 9636 to a voltage for charging the battery 9635. Then, when the power from the solar cell 9633 is used to drive the display section 9631, the switch SW1 is turned on, and the voltage of the power is increased or decreased by the converter 9638 to a voltage required by the display section 9631. When no image is displayed on the display section 9631, the switch SW1 is turned off, and the switch SW2 is turned on, so that the battery 9635 is charged.

[0176] Although the solar cell 9633 is described as an example of a power generation means, the power generation means is not particularly limited, and the battery 9635 may be charged by another power generation means, such as a piezoelectric element or a thermoelectric conversion element (Peltier element). The battery 9635 may be charged by a non-contact power transmission module that can perform charging by wirelessly (contactlessly) transmitting and receiving power, or one of the other charging means may be used in combination therewith, and the power generation means is not necessarily provided.

[0177] An embodiment of the present invention is not limited to the tablet computer with the Fig. 16A to Fig. 16C as long as the display section 9631 is included. [Example 1]

[0178] In this example, a light-emitting element 1 and a light-emitting element 2 are described, each containing the compounds (1) to (3) described in Embodiment 1 and corresponding to an embodiment of the present invention. In each of the light-emitting elements of this example, the compound (1), the compound (2), and the compound (3) described in Embodiment 1 are used as phosphorescent compounds in the first light-emitting layer 113B, the second light-emitting layer 113G, and the third light-emitting layer 113R, respectively, and therefore the relationships between the emission wavelengths (F(λ)) and ε(λ)λ 4 equal to the relationships that were determined in the embodiment 1 using Fig. 5 have been described.

[0179] The substances used in the light-emitting elements of this example are shown below.

[0180] A method for manufacturing the light-emitting elements 1 and 2 of this example will be described below. (Method for manufacturing the light-emitting element 1)

[0181] First, a film of indium tin oxide containing silicon oxide (ITSO) was formed over a glass substrate using a sputtering method, forming the first electrode 101. Its thickness was 110 nm and the electrode area was 2 mm × 2 mm. Here, the first electrode 101 is an electrode serving as the anode of the light-emitting element.

[0182] Next, as a pretreatment for fabricating the light-emitting element over the substrate, UV-ozone treatment was performed for 370 seconds after a surface of the substrate was washed with water and heating was performed at 200 °C for one hour.

[0183] The substrate was then placed in a vacuum evaporation device in which the pressure was increased to approximately 10 -4 Pa, and heated for 30 minutes in vacuum at 170 °C in a heating chamber of the vacuum evaporation apparatus, and then the substrate was cooled over a period of about 30 minutes.

[0184] Then, the substrate over which the first electrode 101 was formed was attached to a substrate holder in the vacuum evaporation apparatus such that the surface on which the first electrode 101 was formed was directed downward. The pressure in the vacuum evaporation apparatus was set to about 10 -4Pa. Next, 4,4',4"-(benzene-1,3,5-triyl)tri(dibenzothiophene) (abbreviation: DBT3P-II), represented by structural formula (i), and molybdenum(VI) oxide were co-evaporated over the first electrode 101 using an evaporation method using resistance heating, so that the hole-injection layer 111 was formed. The thickness of the hole-injection layer 111 was set to 40 nm, and the weight ratio of DBT3P-II to molybdenum oxide was set to 4:2. Note that the co-evaporation method refers to an evaporation method in which evaporation is performed from a plurality of evaporation sources simultaneously in a treatment chamber.

[0185] Next, a film of 4-phenyl-4'-(9-phenylfluoren-9-yl)triphenylamine (abbreviation: BPAFLP), represented by structural formula (ii), was formed to a thickness of 20 nm over the hole injection layer 111 to form the hole transport layer 112.

[0186] Furthermore, the light-emitting layer 113 was formed in the following manner. Over the hole transport layer 112, 2-[3-(dibenzothiophen-4-yl)phenyl]dibenzo[f,h]quinoxaline (abbreviation: 2mDBTPDBq-II), represented by structural formula (iii), 4-phenyl-4'-(9-phenyl-9H-carbazol-3-yl)triphenylamine (abbreviation: PCBA1BP), represented by structural formula (iv), and bis(2,3,5-triphenylpyrazinato)(dipivaloylmethanato)iridium(III) (abbreviation: [Ir(tppr)2(dpm)]), represented by structural formula (v) (the compound (3)), were deposited by co-evaporation to a thickness of 10 nm, with the mass ratio of 2mDBTPDBq-II to PCBA1BP and [Ir(tppr)2(dpm)] 0.5:0.5:0.05, so that the third light-emitting layer 113R was formed.Then, 2mDBTPDBq-II, PCBA1BP, and (acetylacetonato)bis(6-tert-butyl-4-phenylpyrimidinato)iridium(III) (abbreviation: [Ir(tBuppm)2(acac)]), which is represented by structural formula (vi) (the compound (2)), were deposited by co-evaporation to a thickness of 5 nm, wherein the mass ratio of 2mDBTPDBq-II to PCBA1BP and [Ir(tBuppm)2(acac)] was 0.5:0.5:0.05, so that the second light-emitting layer 113G was formed.Thereafter, 3,5-bis[3-(9H-carbazol-9-yl)phenyl]pyridine (abbreviation: 35DCzPPy), which is represented by structural formula (vii), 3,3'-bis(9-phenyl-9H-carbazole) (abbreviation: PCCP), which is represented by structural formula (viii), and tris{2-[5-(2-methylphenyl)-4-(2,6-dimethylphenyl)-4H-1,2,4-triazol-3-yl-κN2]phenyl-κC}iridium(III) (abbreviation: [Ir(mpptz-dmp)3]), which is represented by structural formula (ix) (the compound (1)), were deposited by co-evaporation to a thickness of 30 nm, with the mass ratio of 35DCzPPy to PCCP and [Ir(mpptz-dmp)3] 0.5:0.5:0.06, so that the first light-emitting layer 113B was formed.

[0187] Note that 2mDBTPDBq-II and PCBA1BP form an exciplex, and 35DCzPPy and PCCP form an exciplex. Furthermore, the second light-emitting layer 113G and the third light-emitting layer 113R exhibit hole-transport properties by containing 2mDBTPDBq-II, which has an electron-transport property, and PCBA1BP, which has a hole-transport property, in a ratio of 0.5:0.5. The first light-emitting layer 113B exhibits an electron-transport property by containing 35DCzPPy, which has an electron-transport property, and PCCP, which has a hole-transport property, in a ratio of 0.5:0.5.

[0188] The electron transport layer 114 was then formed over the light-emitting layer 113 such that a 10 nm thick film of 2-[3-(dibenzothiophen-4-yl)phenyl]-1-phenyl-1H-benzimidazole (abbreviation: mDBTBIm-II), which is represented by structural formula (x), was formed and that a 20 nm thick film of bathophenanthroline (abbreviation: BPhen), which is represented by structural formula (xi), was formed.

[0189] After forming the electron-transport layer 114, lithium fluoride (LiF) was deposited by evaporation to a thickness of 1 nm to form the electron-injection layer 115. Finally, aluminum was deposited by evaporation to a thickness of 200 nm to form the second electrode 102, which served as the cathode. In this way, the light-emitting element 1 of this example was manufactured.

[0190] It should be noted that in all the above evaporation steps, evaporation was carried out by a resistance heating method. (Method for manufacturing the light-emitting element 2)

[0191] The light-emitting element 2 was manufactured in the same process in the same structure as the light-emitting element 1, except that the second light-emitting layer 113G was formed to a thickness of 10 nm.

[0192] In a glove box containing a nitrogen atmosphere, light-emitting element 1 and light-emitting element 2 were sealed with a glass substrate to prevent exposure to air (specifically, a sealing material was applied to an outer edge of the element, and a heat treatment was performed at 80°C for 1 hour at the time of sealing). Then, the reliability of each of the light-emitting elements was measured. Note that the measurement was performed at room temperature (in an atmosphere maintained at 25°C).

[0193] Fig. 17 shows current density-luminance characteristics of the light-emitting element 1 and the light-emitting element 2, Fig. 18 shows their luminance-current efficiency characteristics, Fig. 19 shows their voltage-luminance properties, Fig. 20 shows their luminance-chromaticity properties, Fig. 21 shows their luminance-power efficiency characteristics, Fig. 22 shows their luminance-external quantum efficiency properties, and Fig. 23 shows their emission spectra.

[0194] It was found that at about 1000 cd / m 2 , which is a practical luminance, the light-emitting element 1 exhibited very advantageous properties, namely a current efficiency of 47 cd / A, an external quantum efficiency of 22%, and a power efficiency of 32 lm / W. It was also shown that the emission color was a warm white color of 2930 K and that the overall color rendering index Ra was 91.7, which demonstrated good color rendering properties. It was found that at approximately 1000 cd / m 2, which is a practical luminance, the light-emitting element 2 exhibited extremely high efficiencies, namely a current efficiency of 52 cd / A, an external quantum efficiency of 22%, and a power efficiency of 36 lm / W. In addition, Fig. 20 that the color values ​​of the light-emitting element 1 and the light-emitting element 2, each of which is an embodiment of the present invention, are less dependent on the luminance.

[0195] Carrier recombination regions in the light-emitting element 1 and the light-emitting element 2 are located near the interface between the first light-emitting layer 113B and the second light-emitting layer 113G, respectively, due to the transport properties of the light-emitting layers. Despite this fact, the third light-emitting layer 113R emitted sufficient light in both the light-emitting element 1, which included the second light-emitting layer 113G with a thickness of 5 nm, and the light-emitting element 2, which included the second light-emitting layer 113G with a thickness of 10 nm.Moreover, the spectrum clearly indicates light emitted from the light-emitting substances contained in the first light-emitting layer 113B to the third light-emitting layer 113R, which means that effective transfer of excitation energy occurred in a good balance. [Example 2]

[0196] In this example, a light-emitting element (a light-emitting element 3) having a structure different from that in Example 1 and being an embodiment of the present invention is described. In the light-emitting element 3, a compound (4) (bis{2-[5-methyl-6-(2-methylphenyl)-4-pyrimidinyl-κN3]phenyl-κC)(2,4-pentanedionato-κ 2O,O')iridium(III) (abbreviation: [Ir(mpmppm)2(acac)])), which is a phosphorescent compound exhibiting yellow emission color, was used instead of [Ir(tBuppm)2(acac)] used as the second compound in Example 1. Note that the other substances used in the light-emitting element 3 are the same as the substances used in the light-emitting element 1 and the light-emitting element 2 of Example 1.

[0197] A structural formula of compound (4) ([Ir(mpmppm)2(acac)]) is shown below. Structural formulas of the other compounds are shown in Example 1 and are therefore omitted here.

[0198] Here are in Fig. 24 the emission wavelengths F(λ) of the three types of phosphorescent compounds used in the light-emitting element 3 and the relationship between the emission wavelengths F(λ) and ε(λ)λ 4of the compound (3) and the compound (4). In the light-emitting element of this example, as shown in Fig. 24, the first light-emitting layer 113B contains compound (1) as the first phosphorescent compound that emits blue light. The second light-emitting layer 113G contains compound (4) as the second phosphorescent compound that emits light with a wavelength (an emission peak wavelength of 566 nm) longer than that of the light emitted by the first phosphorescent compound and has the local maximum value A on the longest wavelength side of the function ε(λ)λ in a range from 440 nm to 520 nm (at 512 nm). 4The third light-emitting layer 113R contains the compound (3) as the third phosphorescent compound, which emits light with a wavelength longer than that of the light emitted by the second phosphorescent compound and has, in a range from 520 nm to 600 nm (about 542 nm), the local maximum value B on the longest wavelength side of the function ε(λ)λ 4 has. Fig. 24 also shows that the local maximum value B is larger than the local maximum value A.

[0199] A method for manufacturing the light-emitting element 3 of this example will be described below. (Method for manufacturing the light-emitting element 3)

[0200] First, a film of indium tin oxide containing silicon oxide (ITSO) was formed over a glass substrate using a sputtering method, forming the first electrode 101. Its thickness was 110 nm and the electrode area was 2 mm × 2 mm. Here, the first electrode 101 is an electrode serving as the anode of the light-emitting element.

[0201] Next, as a pretreatment for fabricating the light-emitting element over the substrate, UV-ozone treatment was performed for 370 seconds after a surface of the substrate was washed with water and heating was performed at 200 °C for one hour.

[0202] The substrate was then placed in a vacuum evaporation device in which the pressure was increased to approximately 10 -4Pa, and heated for 30 minutes in vacuum at 170 °C in a heating chamber of the vacuum evaporation apparatus, and then the substrate was cooled over a period of about 30 minutes.

[0203] Then, the substrate over which the first electrode 101 was formed was attached to a substrate holder in the vacuum evaporation apparatus such that the surface on which the first electrode 101 was formed was directed downward. The pressure in the vacuum evaporation apparatus was set to about 10 -4Pa. Next, 4,4',4"-(benzene-1,3,5-triyl)tri(dibenzothiophene) (abbreviation: DBT3P-II), represented by structural formula (i), and molybdenum(VI) oxide were co-evaporated over the first electrode 101 using an evaporation method using resistance heating, so that the hole-injection layer 111 was formed. The thickness of the hole-injection layer 111 was set to 40 nm, and the weight ratio of DBT3P-II to molybdenum oxide was set to 4:2. Note that the co-evaporation method refers to an evaporation method in which evaporation is performed from a plurality of evaporation sources simultaneously in a treatment chamber.

[0204] Next, a film of 4-phenyl-4'-(9-phenylfluoren-9-yl)triphenylamine (abbreviation: BPAFLP), represented by structural formula (ii), was formed to a thickness of 20 nm over the hole injection layer 111 to form the hole transport layer 112.

[0205] Furthermore, the light-emitting layer 113 was formed in the following manner. Over the hole transport layer 112, 2-[3-(dibenzothiophen-4-yl)phenyl]dibenzo[f,h]quinoxaline (abbreviation: 2mDBTPDBq-II), which is represented by structural formula (iii), 4-phenyl-4'-(9-phenyl-9H-carbazol-3-yl)triphenylamine (abbreviation: PCBA1BP), which is represented by structural formula (iv), and bis(2,3,5-triphenylpyrazinato)(dipivaloylmethanato)iridium(III) (abbreviation: [Ir(tppr)2(dpm)]), which is represented by structural formula (v) (the compound (3)), were deposited by co-evaporation to a thickness of 20 nm, wherein the mass ratio of 2mDBTPDBq-II to PCBA1BP and [Ir(tppr)2(dpm)] 0.5:0.5:0.05, so that the third light-emitting layer 113R was formed. Then, 2mDBTPDBq-II, PCBA1BP and bis{2-[5-methyl-6-(2-methylphenyl)-4-pyrimidinyl-κN3]phenyl-κC}(2,4-pentanedionato-κ 2O,O')iridium(III) (abbreviation: [Ir(mpmppm)2(acac)]), which is represented by structural formula (xii) (the compound (4)), was deposited by co-evaporation to a thickness of 5 nm, wherein the mass ratio of 2mDBTPDBq-II to PCBA1BP and [Ir(mpmppm)2(acac)] was 0.5:0.5:0.05, so that the second light-emitting layer 113G was formed.Thereafter, 3,5-bis[3-(9H-carbazol-9-yl)phenyl]pyridine (abbreviation: 35DCzPPy), which is represented by structural formula (vii), 3,3'-bis(9-phenyl-9H-carbazole) (abbreviation: PCCP), which is represented by structural formula (viii), and tris{2-[5-(2-methylphenyl)-4-(2,6-dimethylphenyl)-4H-1,2,4-triazol-3-yl-κN2]phenyl-κC}iridium(III) (abbreviation: [Ir(mpptz-dmp)3]), which is represented by structural formula (ix) (the compound (1)), were deposited by co-evaporation to a thickness of 30 nm, with the mass ratio of 35DCzPPy to PCCP and [Ir(mpptz-dmp)3] 0.5:0.5:0.06, so that the first light-emitting layer 113B was formed.

[0206] Note that 2mDBTPDBq-II and PCBA1BP form an exciplex, and 35DCzPPy and PCCP form an exciplex. Furthermore, the second light-emitting layer 113G and the third light-emitting layer 113R exhibit hole-transport properties by containing 2mDBTPDBq-II, which has an electron-transport property, and PCBA1BP, which has a hole-transport property, in a ratio of 0.5:0.5. The first light-emitting layer 113B exhibits an electron-transport property by containing 35DCzPPy, which has an electron-transport property, and PCCP, which has a hole-transport property, in a ratio of 0.5:0.5.

[0207] The electron transport layer 114 was then formed over the light-emitting layer 113 such that a 10 nm thick film of 2-[3-(dibenzothiophen-4-yl)phenyl]-1-phenyl-1H-benzimidazole (abbreviation: mDBTBIm-II), which is represented by structural formula (x), was formed and that a 20 nm thick film of bathophenanthroline (abbreviation: BPhen), which is represented by structural formula (xi), was formed.

[0208] After forming the electron-transport layer 114, lithium fluoride (LiF) was deposited by evaporation to a thickness of 1 nm to form the electron-injection layer 115. Finally, aluminum was deposited by evaporation to a thickness of 200 nm to form the second electrode 102, which served as the cathode. In this way, the light-emitting element 3 of this example was manufactured.

[0209] It should be noted that in all the above evaporation steps, evaporation was carried out by a resistance heating method.

[0210] In a glove box containing a nitrogen atmosphere, the light-emitting element 3 was sealed with a glass substrate to prevent exposure to air (specifically, a sealing material was applied to an outer edge of the element, and at the time of sealing, a heat treatment was performed at 80°C for 1 hour). Then, the reliability of the light-emitting elements was measured. Note that the measurement was performed at room temperature (in an atmosphere maintained at 25°C).

[0211] Fig. 25 shows current density-luminance characteristics of the light-emitting element 3, Fig. 26 shows its luminance-current efficiency characteristics, Fig. 27 shows its voltage-luminance properties, Fig. 28 shows its luminance-chromaticity properties, Fig. 29 shows its luminance-power efficiency characteristics, Fig. 30 shows its luminance-external quantum efficiency properties, and Fig. 31 shows its emission spectrum.

[0212] It was found that at about 1000 cd / m 2 , which is a practical luminance, the light-emitting element 3 exhibited very advantageous properties, namely a current efficiency of 48 cd / A, an external quantum efficiency of 23%, and a power efficiency of 32 lm / W. It was also shown that the emission color was a white color of 3860 K and that the overall color rendering index Ra was 85.1, which demonstrated good color rendering properties. In addition, Fig. 20 that the color values ​​of the light-emitting element 3, which is an embodiment of the present invention, are less dependent on the luminance.

[0213] A carrier recombination region in the light-emitting element 3 is located near the interface between the first light-emitting layer 113B and the second light-emitting layer 113G, which is due to the transport properties of the light-emitting layers. Despite this fact, the third light-emitting layer 113R emitted sufficient light. Moreover, the spectrum clearly indicates light emitted from the light-emitting substances contained in the first light-emitting layer 113B to the third light-emitting layer 113R, which means that effective excitation energy transfer occurred in a good balance in the light-emitting element 3. [Example 3]

[0214] In this example, a light-emitting element 4 having a structure different from the structures in Examples 1 and 2 and being an embodiment of the present invention will be described. In the light-emitting element 4, 4,4'-di(1-naphthyl)-4"-(9-phenyl-9H-carbazol-3-yl)triphenylamine (abbreviation: PCBNBB), represented by structural formula (xiii), was used instead of BPAFLP and PCBA1BP, which were used in the light-emitting elements of Example 1; and 2-[3'-(dibenzothiophen-4-yl)biphenyl-3-yl]dibenzo[f,h]quinoxaline (abbreviation: 2mDBTBPDBq-II), represented by structural formula (xiv), was used instead of a portion of 35DCzPPy and 2mDBTPDBq-II, which were used in the light-emitting elements of Example 1. Note that the phosphorescent compounds contained in the light-emitting layers were similar to those in Example 1.The relationships between the emission wavelengths (F(λ)) and ε(λ)λ. 4 are therefore similar to the relationships described in Example 1.

[0215] The structural formulas of PCBNBB and 2mDBTBPDBq-II are shown below. The other compounds are identical to those used in Example 1, and their structural formulas are therefore omitted here. Reference should be made to Example 1.

[0216] A method for manufacturing the light-emitting element 4 of this example will be described below. (Method for manufacturing the light-emitting element 4)

[0217] First, a film of indium tin oxide containing silicon oxide (ITSO) was formed over a glass substrate using a sputtering method, forming the first electrode 101. Its thickness was 110 nm and the electrode area was 2 mm × 2 mm. Here, the first electrode 101 is an electrode serving as the anode of the light-emitting element.

[0218] Next, as a pretreatment for fabricating the light-emitting element over the substrate, UV-ozone treatment was performed for 370 seconds after a surface of the substrate was washed with water and heating was performed at 200 °C for one hour.

[0219] The substrate was then placed in a vacuum evaporation device in which the pressure was increased to approximately 10 -4Pa, and heated for 30 minutes in vacuum at 170 °C in a heating chamber of the vacuum evaporation apparatus, and then the substrate was cooled over a period of about 30 minutes.

[0220] Then, the substrate over which the first electrode 101 was formed was attached to a substrate holder in the vacuum evaporation apparatus such that the surface on which the first electrode 101 was formed was directed downward. The pressure in the vacuum evaporation apparatus was set to about 10 -4Pa. Next, 4,4',4"-(benzene-1,3,5-triyl)tri(dibenzothiophene) (abbreviation: DBT3P-II), represented by structural formula (i), and molybdenum(VI) oxide were co-evaporated over the first electrode 101 using an evaporation method using resistance heating, so that the hole-injection layer 111 was formed. The thickness of the hole-injection layer 111 was set to 40 nm, and the weight ratio of DBT3P-II to molybdenum oxide was set to 4:2. Note that the co-evaporation method refers to an evaporation method in which evaporation is performed from a plurality of evaporation sources simultaneously in a treatment chamber.

[0221] Next, a film of PCBNBB was formed to a thickness of 20 nm over the hole injection layer 111 to form the hole transport layer 112.

[0222] Furthermore, the light-emitting layer 113 was formed in the following manner. 2mDBTBPDBq-II, PCBNBB, and [Ir(tppr)2(dpm)] were co-evaporated over the hole-transport layer 112 to a thickness of 10 nm, with the mass ratio of 2mDBTBPDBq-II to PCBNBB and [Ir(tppr)2(dpm)] being 0.5:0.5:0.05, thus forming the third light-emitting layer 113R. Then, 2mDBTBPDBq-II, PCBNBB, and [Ir(tBuppm)2(acac)] were co-evaporated to a thickness of 10 nm, with the mass ratio of 2mDBTBPDBq-II to PCBNBB and [Ir(tBuppm)2(acac)] being 0.5:0.5:0.05, to form the second light-emitting layer 113G. Next, 35DCzPPy, PCCP, and [Ir(mpptz-dmp)3] were co-evaporated to a thickness of 30 nm, with the mass ratio of 35DCzPPy to PCCP and [Ir(mpptz-dmp)3] being 0.7:0.3:0.06, to form the first light-emitting layer 113B.

[0223] Note that 2mDBTBPDBq-II and PCBNBB form an exciplex, and 35DCzPPy and PCCP form an exciplex. Furthermore, the second light-emitting layer 113G and the third light-emitting layer 113R exhibit hole-transport properties by containing 2mDBTBPDBq-II, which has an electron-transport property, and PCBNBB, which has a hole-transport property, in a ratio of 0.5:0.5. The first light-emitting layer 113B exhibits an electron-transport property by containing 35DCzPPy, which has an electron-transport property, and PCCP, which has a hole-transport property, in a ratio of 0.5:0.5.

[0224] The electron transport layer 114 was then formed over the light-emitting layer 113 such that a 10 nm thick film of 35DCzPPy was formed and that a 20 nm thick film of BPhen was formed.

[0225] After forming the electron-transport layer 114, lithium fluoride (LiF) was deposited by evaporation to a thickness of 1 nm to form the electron-injection layer 115. Finally, aluminum was deposited by evaporation to a thickness of 200 nm to form the second electrode 102, which served as the cathode. In this way, the light-emitting element 4 of this example was manufactured.

[0226] It should be noted that in all the above evaporation steps, evaporation was carried out by a resistance heating method.

[0227] In a glove box containing a nitrogen atmosphere, the light-emitting element 4 was sealed with a glass substrate to prevent exposure to air (specifically, a sealing material was applied to an outer edge of the element, and at the time of sealing, a heat treatment was performed at 80°C for 1 hour). Then, the reliability of the light-emitting elements was measured. Note that the measurement was performed at room temperature (in an atmosphere maintained at 25°C).

[0228] Fig. 32 shows current density-luminance characteristics of the light-emitting element 4, Fig. 33 shows its luminance-current efficiency characteristics, Fig. 34 shows its voltage-luminance properties, Fig. 35 shows its luminance-chromaticity properties, Fig. 36 shows its luminance-power efficiency characteristics, Fig. 37 shows its luminance-external quantum efficiency properties, and Fig. 38 shows its emission spectrum.

[0229] It was found that at about 1000 cd / m 2 , which is a practical luminance, the light-emitting element 4 exhibited advantageous properties, namely a current efficiency of 39 cd / A, an external quantum efficiency of 21%, and a power efficiency of 29 lm / W. It was also shown that the light had a color of 2260 K and that the overall color rendering index Ra was 93.4, which showed good color rendering property.

[0230] A carrier recombination region in the light-emitting element 4 is located near the interface between the first light-emitting layer 113B and the second light-emitting layer 113G, which is due to the transport properties of the light-emitting layers. Despite this fact, the third light-emitting layer 113R emitted sufficient light. Moreover, the spectrum clearly indicates light emitted by the light-emitting substances contained in the first light-emitting layer 113B to the third light-emitting layer 113R, which means that effective excitation energy transfer occurred in a good balance.

[0231] A reliability test was conducted. During the reliability test, a change in luminance over the operating time was measured under the conditions that the initial luminance was 3000 cd / m 2and that the current density was constant, assuming that the initial luminance was 100%. The measurement result is shown in Fig. 39. From the graph, it was found that despite the fact that all the light-emitting layers emit phosphorescence, that is, the light-emitting element 4 is a so-called all-phosphorescent element, 65% of the initial luminance was retained even after 440 hours. This means that the light-emitting element 4, which is an embodiment of the present invention, also has high durability as an element. (Reference example 1)

[0232] A synthesis method of tris{2-[5-(2-methylphenyl)-4-(2,6-dimethylphenyl)-4H-1,2,4-triazol-3-yl-κN2]phenyl-κC}iridium(III) (abbreviation: [Ir(mpptz-dmp)3]), which is the organometallic complex used in the above embodiment, is described. A structure of [Ir(mpptz-dmp)3] (abbreviation) is shown below. (Step 1: Preparation of 3-(2-methylphenyl)-4-(2,6-dimethylphenyl)-5-phenyl-4H-1,2,4-triazole (abbreviation: Hmpptz-dmp))

[0233] First, 12.6 g (43.3 mmol) of N-[1-chloro-1-(2-methylphenyl)methylidene]-N'-[1-chloro-(1-phenyl)methylidene]hydrazine, 15.7 g (134.5 mmol) of 2,6-dimethylaniline, and 100 mL of N,N-dimethylaniline were placed in a 500 mL receiver flask and heated and stirred at 120 °C for 20 hours. After the reaction was completed for the predetermined time, this reaction solution was slowly added to 200 mL of 1N hydrochloric acid. Dichloromethane was added to this solution, and a target substance was extracted into an organic layer. The obtained organic layer was washed with water and an aqueous solution of sodium bicarbonate and dried with magnesium sulfate. The magnesium sulfate was removed by gravity filtration, and the obtained filtrate was concentrated to obtain a black liquid. This liquid was purified by silica gel column chromatography.A solvent mixture of ethyl acetate and hexane (ratio: 1:5) was used as the eluent. The recovered fraction was concentrated to obtain a white solid. This solid was recrystallized with ethyl acetate to obtain 4.5 g of a white solid of Hmpptz-dmp in a yield of 31%. A synthesis scheme of step 1 is shown below. (Step 2: Preparation of Tris{2-[5-(2-methylphenyl)-4-(2,6-dimethylphenyl)-4H-1,2,4-triazol-3-yl-κN2]phenyl-κC}iridium(III) (abbreviation: Ir(mpptz-dmp)3))

[0234] Then, 2.5 g (7.4 mmol) of Hmpptz-dmp, which was the ligand obtained in step 1, and 0.7 g (1.5 mmol) of tris(acetylacetonato)iridium(III) were placed in a high-temperature heating vessel, and degassing was performed. The mixture in the reaction vessel was heated and stirred at 250 °C for 48 hours under an argon stream. After the reaction was carried out for the predetermined time, the obtained solid was washed with dichloromethane, and an insoluble green solid was collected by suction filtration. This solid was dissolved in toluene and filtered through a stack of alumina and Celite. The obtained fraction was concentrated to obtain a green solid. This solid was recrystallized with toluene to yield 0.8 g of a green powder of [Ir(mpptz-dmp)3] (abbreviation), which is the phosphorescent organometallic iridium complex, in a yield of 45%.A synthesis scheme of step 2 is shown below.

[0235] The result of an analysis of the green powder obtained in step 2 by nuclear magnetic resonance spectroscopy ( 1 H-NMR) is given below. The result showed that the organometallic complex Ir(mpptz-dmp)3 (abbreviation) was obtained by the synthesis method.

[0236] 1 H NMR. δ(Toluene-d8): 1.82 (s, 3H), 1.90 (s, 3H), 2.64 (s, 3H), 6.56-6.62 (m, 3H), 6.67-6.75 (m, 3H), 6.82-6.88 (m, 1H), 6.91-6.97 (t, 1H), 7.00-7.12 (m, 2H), 7.63-7.67 (d, 1H). (Reference example 2)

[0237] An example of the preparation of the organometallic complex (acetylacetonato)bis(6-tert-butyl-4-phenylpyrimidinato)iridium(III) (other name: bis[2-(6-tert-butyl-4-pyrimidinyl-κN3)phenyl-κC](2,4-pentanedionato-κ 2O,O')iridium(III)) (abbreviation: [Ir(tBuppm)2(acac)]) used in the above embodiment is described. The structure of [Ir(tBuppm)2(acac)] is shown below. (Step 1: Preparation of 4-tert-butyl-6-phenylpyrimidine (abbreviation: HtBuppm)

[0238] First, 22.5 g of 4,4-dimethyl-1-phenylpentane-1,3-dione and 50 g of formamide were placed in a receiving flask equipped with a reflux tube, and the air in the flask was replaced with nitrogen. This reaction vessel was heated so that the reaction solution was refluxed for 5 hours. Afterward, this solution was poured into an aqueous sodium hydroxide solution, and an organic layer was extracted with dichloromethane. The obtained organic layer was washed with water and saturated saline solution and dried with magnesium sulfate. After drying, the solution was filtered. The solvent of this solution was distilled off, and then the obtained residue was purified by silica gel column chromatography using hexane and ethyl acetate as the eluent in a volume ratio of 10:1 to obtain a pyrimidine derivative HtBuppm (colorless oily substance, yield 14%).A synthesis scheme of step 1 is shown below. (Step 2: Preparation of DI-µ-chloro-bis[bis(6-tert-butyl-4-phenylpyrimidinato)iridium(III)] (abbreviation: [Ir(tBuppm)2Cl]2)〉

[0239] Next, 15 ml of 2-ethoxyethanol, 5 ml of water, 1.49 g of HtBuppm obtained in step 1, and 1.04 g of iridium chloride hydrate (IrCl3·H2O) were placed in a receiving flask equipped with a reflux tube, and the air in the flask was replaced with argon. Subsequently, microwave irradiation (2.45 GHz, 100 W) was carried out for 1 hour to initiate a reaction. The solvent was distilled off, and the resulting residue was filtered off with suction and washed with ethanol to obtain a binuclear complex [Ir(tBuppm)2Cl]2 (yellow-green powder, yield 73%). A synthesis scheme of step 2 is shown below. (Step 3: Preparation of (acetylacetonato)bis(6-tert-butyl-4-phenylpyrimidinato)iridium(III) (abbreviation: [Ir(tBuppm)2(acac)]))

[0240] Furthermore, 40 ml of 2-ethoxyethanol, 1.61 g of [Ir(tBuppm)2Cl]2, which was the dinuclear complex obtained in step 2, 0.36 g of acetylacetone, and 1.27 g of sodium carbonate were placed in a receiving flask equipped with a reflux tube, and the air in the flask was replaced with argon. Subsequently, microwave irradiation (2.45 GHz, 120 W) was carried out for 60 minutes to cause a reaction. The solvent was distilled off, and the obtained residue was filtered with ethanol under suction and washed with water and ethanol. This solid was dissolved in dichloromethane, and the mixture was filtered through a filter aid in which Celite (manufactured by Wako Pure Chemical Industries, Ltd., Catalog No. 531-16855), alumina, and Celite were stacked in that order.The solvent was distilled off, and the resulting solid was recrystallized with a mixed solvent of dichloromethane and hexane to yield the target compound as a yellow powder (yield 68%). A synthesis scheme for step 3 is shown below.

[0241] The result of an analysis of the yellow powder obtained in step 3 by nuclear magnetic resonance spectroscopy ( 1 H-NMR) is given below. The result showed that the organometallic complex Ir(tBuppm)2(acac) was obtained.

[0242] 1 H NMR. δ (CDCl3): 1.50 (s, 18H), 1.79 (s, 6H), 5.26 (s, 1H), 6.33 (d, 2H), 6.77 (t, 2H), 6.85 (t, 2H), 7.70 (d, 2H), 7.76 (s, 2H), 9.02 (s, 2H). Reference symbol

[0243] 10: Electrode, 11: Electrode, 101: First electrode, 102: Second electrode, 103: EL layer, 111: Hole injection layer, 112: Hole transport layer, 113: Light-emitting layer, 113B: First light-emitting layer, 113Bd: First phosphorescent compound, 113Bh: First host material, 113G: Second light-emitting layer, 113Gd: Second phosphorescent compound, 113Gh: Second host material, 113R: Third light-emitting layer, 113Rd: Third phosphorescent compound, 113Rh: Third host material, 113ex: Recombination region, 114: Electron transport layer, 115: Electron injection layer, 400: Substrate, 401: First electrode, 402: Auxiliary electrode, 403: EL layer, 404: Second electrode, 405: Sealing material, 406: Sealing material, 407: Sealing substrate, 412: Pad, 420: IC chip, 601: Driver circuit section (source line driver circuit), 602: Pixel section, 603: Driver circuit section (gate line driver circuit), 604: Sealing substrate,605: Sealing material, 607: Space, 608: Lead, 609: FPC (flexible printed circuit), 610: Element substrate, 611: Switching TFT, 612: Current-controlling TFT, 613: First electrode, 614: Insulator, 616: EL layer, 617: Second electrode, 618: Light-emitting element, 623: N-channel TFT, 624: P-channel TFT, 625: Desiccant, 901: Case, 902: Liquid crystal layer, 903: Backlight unit, 904: Case, 905: Driver IC, 906: Terminal, 951: Substrate, 952: Electrode, 953: Insulating layer, 954: Separating layer, 955: EL layer, 956: electrode, 1001: substrate, 1002: insulating base film, 1003: gate insulating film, 1006: gate electrode, 1007: gate electrode, 1008: gate electrode, 1020: first interlayer insulating film, 1021: second interlayer insulating film, 1022: electrode, 1024W: first electrode of a light-emitting element, 1024R: first electrode of a light-emitting element,1024G: first electrode of a light-emitting element, 1024B: first electrode of a light-emitting element, 1025: partition wall, 1028: layer containing an organic compound, 1029: second electrode of a light-emitting element, 1031: sealing substrate, 1032: sealant, 1033: transparent base material, 1034R: red color layer, 1034G: green color layer, 1034B: blue color layer, 1035: black layer (black matrix), 1036: cap layer, 1037: third interlayer insulating film, 1040: pixel section, 1041: driving circuit section, 1042: peripheral section, 1044W: white light-emitting region, 1044R: red light-emitting Area, 1044B: blue light emitting area, 1044G: green light emitting area, 2001: housing, 2002: light source, 3001: illumination device, 3002: display device, 5000: display, 5001: display, 5002: display, 5003: display, 5004: display, 5005: display, 7101: housing, 7103: display section,7105: Base, 7107: Display section, 7109: Operation button, 7110: Remote control, 7201: Main body, 7202: Housing, 7203: Display section, 7204: Keyboard, 7205: External connection port, 7206: Pointing device, 7210: Second display section, 7301: Housing, 7302: Housing, 7303: Articulating joint, 7304: Display section, 7305: Display section, 7306: Speaker section, 7307: Recording medium insertion section, 7308: LED lamp, 7309: Operation button, 7310: Connection port, 7311: Sensor, 7400: Mobile phone, 7401: Housing, 7402: Display section, 7403: Operation knob, 7404: external connection port, 7405: speaker, 7406: microphone, 9033: clasp, 9034: switch, 9035: power switch, 9036: switch, 9038: operation switch, 9630: housing, 9631: display section, 9631a: display section, 9631b: display section, 9632a: touch screen area, 9632b: touch screen area, 9633: solar cell, 9634: charge and discharge control circuit, 9635: battery, 9636: DC-DC converter,9637: control button, 9638: converter and 9639: knob.

Claims

[1] Light-emitting element comprising: a first electrode; a first light-emitting layer over the first electrode, the first light-emitting layer comprising a first compound and a host material; a second light-emitting layer over the first light-emitting layer, the second light-emitting layer comprising a second compound and the host material; and a second electrode above the second light-emitting layer, where a peak on the side of the longest wavelength of a function ε(λ)λ 4 the second compound overlaps with an emission spectrum of the first compound, where λ represents a wavelength, where ε(λ) represents a molar absorption coefficient at wavelength λ, and wherein an emission peak emitted from the second compound is at a longer wavelength in an emission spectrum of the light-emitting element than an emission peak emitted from the first compound. [2] Light-emitting element comprising: a first electrode; a first light-emitting layer over the first electrode, the first light-emitting layer comprising a first compound and a host material; a second light-emitting layer over the first light-emitting layer, the second light-emitting layer comprising a second compound and the host material; and a second electrode above the second light-emitting layer, where a peak on the side of the longest wavelength of a function ε(λ)λ 4 the second compound overlaps with an emission spectrum of the first compound, where λ represents a wavelength, where ε(λ) represents a molar absorption coefficient at wavelength λ, and wherein an excited singlet state of the first compound has a higher energy than an excited singlet state of the second compound. [3] A light-emitting element according to claim 1 or 2, further comprising: a first layer between the first electrode and the first light-emitting layer, wherein the first layer comprises an aromatic amine compound and a substance having an acceptor property. [4] A light-emitting element according to claim 1 or 2, further comprising: a second layer between the second light-emitting layer and the second electrode, wherein the second layer comprises a third compound and one of an alkali metal and an alkaline earth metal. [5] The light-emitting element according to claim 4, wherein a material in the second layer is an element belonging to Groups 1 or 2 of the Periodic Table. [6] The light-emitting element according to claim 1 or 2, wherein the second electrode comprises an indium oxide containing zinc oxide. [7] The light-emitting element according to claim 1 or 2, wherein the first electrode is a reflective electrode. [8] The light-emitting element according to claim 1 or 2, wherein the first light-emitting layer is in contact with the second light-emitting layer. [9] Light-emitting element comprising: a first electrode; a second electrode a first light-emitting layer between the first electrode and the second electrode, the first light-emitting layer comprising a first compound and a first host material; a second light-emitting layer between the first light-emitting layer and the second electrode, the second light-emitting layer comprising a second compound and the first host material; and a third light-emitting layer between the second light-emitting layer and the second electrode, wherein the third light-emitting layer comprises a third compound and a second host material, where a peak on the side of the longest wavelength of a function ε(λ)λ 4 the third compound overlaps with an emission spectrum of the second compound, where λ represents a wavelength, where ε(λ) represents a molar absorption coefficient at wavelength λ, and wherein an emission peak emitted from the third compound is at a longer wavelength in an emission spectrum of the light-emitting element than an emission peak emitted from the second compound. [10] The light-emitting element according to claim 9, wherein the first compound, the second compound and the third compound each have different emission colors. [11] The light-emitting element according to claim 9, wherein each of the first compound, the second compound and the third compound is a phosphorescent compound. [12] The light-emitting element according to claim 9, wherein the first light-emitting layer is in contact with the second light-emitting layer. [13] The light-emitting element according to claim 9, wherein the second light-emitting layer is in contact with the third light-emitting layer. [14] The light-emitting element according to claim 9, wherein the second compound has the emission peak in a wavelength range of 520 nm to 600 nm. [15] The light-emitting element according to claim 9, wherein the third compound has the emission peak in a wavelength range of 600 nm to 700 nm. [16] Light-emitting element comprising: a first electrode; a second electrode a first light-emitting layer between the first electrode and the second electrode, the first light-emitting layer comprising a first compound and a first host material; a second light-emitting layer between the first light-emitting layer and the second electrode, the second light-emitting layer comprising a second compound and the first host material; and a third light-emitting layer between the second light-emitting layer and the second electrode, wherein the third light-emitting layer comprises a third compound and a second host material, wherein each of the first compound, the second compound and the third compound is a phosphorescent compound, where a peak on the side of the longest wavelength of a function ε(λ)λ 4 the third compound overlaps with an emission spectrum of the second compound, where λ represents a wavelength, where ε(λ) represents a molar absorption coefficient at wavelength λ, and wherein the second light-emitting layer is in contact with the first light-emitting layer and the third light-emitting layer. [17] The light-emitting element according to claim 16, wherein the second compound has an emission peak in a wavelength range of 520 nm to 600 nm. [18] The light-emitting element according to claim 16, wherein the third compound has an emission peak in a wavelength range of 600 nm to 700 nm. [19] A light-emitting element according to any one of claims 9 or 16, wherein the second electrode comprises indium oxide containing zinc oxide.

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