Light-emitting element, display device, electronic device, and lighting device

A light-emitting element with high emission efficiency which includes fluorescent materials is provided. The light-emitting element includes a first light-emitting layer and a second light-emitting layer. The first light-emitting layer includes a first fluorescent material and a first host material, and the second light-emitting layer includes a second fluorescent material and a second host material. The second host material includes a first organic compound and a second organic compound. The first organic compound and the second organic compound form an exciplex. A singlet excited energy level of the first host material is higher than a singlet excited energy level of the first fluorescent material, and a triplet excited energy level of the first host material is lower than a triplet excited energy level of the first fluorescent material.

BACKGROUND OF THE INVENTION

1. Field of the Invention

One embodiment of the present invention relates to a light-emitting element, or a display device, an electronic device, and a lighting device each including the light-emitting element.

2. Description of the Related Art

In recent years, research and development have been extensively conducted on light-emitting elements using electroluminescence (EL). In a basic structure of such a light-emitting element, a layer containing a light-emitting substance (an EL layer) is interposed between a pair of electrodes. By applying a voltage between the pair of electrodes of this element, light emission from the light-emitting substance can be obtained.

Since the above light-emitting element is of a self-luminous type, a display device using this light-emitting element has advantages such as high visibility, no necessity of a backlight, low power consumption, and the like. Further, the display device also has advantages in that it can be formed to be thin and lightweight, and has high response speed.

In the case where the above light-emitting element is used for a display device, there are the following two methods: a method of providing subpixels in a pixel with EL layers having functions of emitting light of different colors (hereinafter referred to as a separate coloring method) and a method of providing subpixels in a pixel with, for example, a common EL layer having a function of emitting white light and color filters each having a function of transmitting light of a different color (hereinafter referred to as a white EL+color filter method, and it is noted that the emission color of the common EL layer is not limited to white).

One of the advantages of the white EL+color filter method is that the EL layer can be shared by all of the subpixels. Therefore, compared with the separate coloring method, loss of a material of the EL layer and cost of patterning for forming the EL layer can be reduced; thus, display devices can be manufactured at low cost with high productivity. Further, although it is necessary, in the separate coloring method, to provide a space between the subpixels to prevent mixture of the materials of the EL layers in the subpixels, the white EL+color filter method does not need such a space and therefore enables a high-resolution display device having higher pixel density.

The light-emitting element can emit light of a variety of colors depending on the kind of light-emitting substance included in the EL layer. A light-emitting element which can emit white light or light of color close to white with high efficiency has been particularly demanded for the application to a lighting device and a display device utilizing the white EL+color filter method. In addition, a light-emitting element having low power consumption is required.

In a light-emitting element (e.g., an organic EL element) whose EL layer contains an organic compound as a light-emitting substance and is provided between a pair of electrodes, application of a voltage between the pair of electrodes causes injection of electrons from a cathode and holes from an anode into the EL layer having a light-emitting property and thus a current flows. By recombination of the injected electrons and holes, the organic compound having a light-emitting property is brought into an excited state to provide light emission.

Note that an excited state formed by an organic compound can be a singlet excited state (S*) or a triplet excited state (T*). Light emission from the singlet excited state is referred to as fluorescence, and light emission from the triplet excited state is referred to as phosphorescence. The statistical generation ratio of the excited states in the light-emitting element is S*:T*=1:3. In other words, a light-emitting element containing a compound emitting phosphorescence has higher emission efficiency than a light-emitting element containing a compound emitting fluorescence. Therefore, light-emitting elements including phosphorescent compounds capable of converting a triplet excited state into light emission have been actively developed in recent years. For example, a white-light-emitting element including a plurality of phosphorescent compounds is disclosed in Patent Document 1.

Among light-emitting elements including phosphorescent compounds, in particular, a light-emitting element that emits blue light has not yet been put into practical use because it is difficult to develop a stable compound having a high triplet excited energy level. For this reason, the development of a more stable fluorescent compound for a light-emitting element that emits blue light or a white-light-emitting element in which blue light emission occurs has been conducted and a technique for increasing the emission efficiency of a light-emitting element including a fluorescent compound (fluorescent light-emitting element) has been searched.

As a light emission mechanism capable of converting part of a triplet excited state into light emission, triplet-triplet annihilation (TTA) is known. The term TTA refers to a process in which, when two triplet excitons approach each other, excited energy is transferred and spin angular momentum are exchanged to form a singlet exciton (Non-Patent Document 1).

As one of materials capable of partly converting the triplet excited state into light emission, a thermally activated delayed fluorescence (TADF) substance has been known. In a thermally activated delayed fluorescence substance, a singlet excited state is generated from a triplet excited state by reverse intersystem crossing, and the singlet excited state is converted into light emission.

In order to increase emission efficiency of a light-emitting element using a thermally activated delayed fluorescence substance, not only efficient generation of a singlet excited state from a triplet excited state but also efficient emission from a singlet excited state, that is, high fluorescence quantum yield is important in a thermally activated delayed fluorescence substance. It is, however, difficult to design a light-emitting material that meets these two.

Patent Document 2 discloses a method: in a light-emitting element containing a thermally activated delayed fluorescence substance and a fluorescent compound, singlet excited energy of the thermally activated delayed fluorescence substance is transferred to the fluorescent compound and light emission is obtained from the fluorescent compound.

REFERENCE

Patent Document

SUMMARY OF THE INVENTION

Since an element including a charge-generation layer between a plurality of EL layers (also referred to as a tandem element) uses light-emitting materials emitting light of different colors in different EL layers, it is suitable for forming a white-light-emitting element. However, many layers are stacked in the tandem element and thus there are problems of many manufacturing steps and high manufacturing cost.

As phosphorescent compounds having high emission efficiency, organometallic complexes whose central metal is a platinum group element (an element in groups 8 to 10 and periods 5 and 6, such as platinum and iridium) are known. However, platinum group elements are expensive because they are noble metals. Furthermore, such organometallic complexes have problems of poor sublimation properties because of having a plurality of ligands in the molecular structure and thus having high molecular weight, and high production cost because of the necessity of complicated synthesis.

What is important in increasing the emission efficiency of a light-emitting element including a fluorescent compound is to convert energy of triplet excitons, which do not contribute to light emission, into energy of singlet excitons, which have a light-emitting property. In particular, in a light-emitting element including a plurality of light-emitting materials like a white-light-emitting element, efficient conversion from energy of triplet excitons into energy of singlet excitons, which have a light-emitting property, in the plurality of light-emitting materials is important.

For example, a substance having a condensed aromatic ring skeleton, which is often used as a host material in a light-emitting element including a fluorescent compound, has a low triplet excited energy level. This leads to a problem of difficulty in obtaining white light emission in a light-emitting element that uses a plurality of fluorescent compounds because excited energy is likely to be transferred to a material having a lower singlet and triplet excited energy levels.

Among light-emitting elements that use a thermally activated delayed fluorescence substance, in particular, a light-emitting element that emits blue light is difficult to develop because it needs a stable compound having a high triplet excited energy level as in a light-emitting element that uses a blue phosphorescent compound.

In view of the above, an object of one embodiment of the present invention is to provide a light-emitting element including a fluorescent compound which has high emission efficiency. Another object of one embodiment of the present invention is to provide a light-emitting element including a plurality of fluorescent compounds which has high emission efficiency. Another object of one embodiment of the present invention is to provide a white-light-emitting element including a fluorescent compound which has high emission efficiency. Another object of one embodiment of the present invention is to provide a light-emitting element including a fluorescent compound which is easily formed in a relatively small number of steps for forming films. Another object of one embodiment of the present invention is to provide a light-emitting element with low power consumption. Another object of one embodiment of the present invention is to provide a novel light-emitting element. Another object of one embodiment of the present invention is to provide a novel light-emitting device. Another object of one embodiment of the present invention is to provide a novel display device.

Note that the description of these objects does not disturb the existence of other objects. In one embodiment of the present invention, there is no need to achieve all the objects. Other objects will be apparent from and can be derived from the description of the specification and the like.

A light-emitting element of one embodiment of the present invention includes a light-emitting layer in which an exciplex is formed and a light-emitting layer in which TTA occurs, whereby triplet excitons are converted into singlet excitons and light emission is caused from the singlet excitons. Thus, the light-emitting element can emit light from a fluorescent compound with the use of energy transfer of the singlet excitons.

One embodiment of the present invention is a light-emitting element including at least two fluorescent materials, which exhibits light emission from the fluorescent materials with a relatively small number of deposited layers.

One embodiment of the present invention is a light-emitting element including a first light-emitting layer and a second light-emitting layer, in which the first light-emitting layer includes a first fluorescent material and a first host material, the second light-emitting layer includes a second fluorescent material and a second host material, the second host material includes a first organic compound and a second organic compound, and the first organic compound and the second organic compound form an exciplex.

Another embodiment of the present invention is a light-emitting element including a first light-emitting layer and a second light-emitting layer, in which the first light-emitting layer includes a first fluorescent material and a first host material, the second light-emitting layer includes a second fluorescent material and a second host material, the second host material includes a first organic compound and a second organic compound, a singlet excited energy level of the first host material is higher than a singlet excited energy level of the first fluorescent material, a triplet excited energy level of the first host material is lower than a triplet excited energy level of the first fluorescent material, and the first organic compound and the second organic compound form an exciplex.

Another embodiment of the present invention is a light-emitting element including a first light-emitting layer and a second light-emitting layer, in which the first light-emitting layer includes a first fluorescent material and a first host material, the second light-emitting layer includes a second fluorescent material and a second host material, the second host material includes a first organic compound and a second organic compound, a triplet excited energy level of the first host material is lower than a triplet excited energy level of the first organic compound, a triplet excited energy level of the first host material is lower than a triplet excited energy level of the second organic compound, and the first organic compound and the second organic compound form an exciplex.

In each of the above-described structures, the exciplex preferably has a function of supplying excited energy to the second fluorescent material.

In each of the above-described structures, light emitted from the exciplex preferably has a region overlapping with an absorption band on a lowest energy side of the second fluorescent material.

In each of the above-described structures, 20% or more of light emitted from the exciplex is preferably a delayed fluorescence component.

In each of the above-described structures, a region in which the first light-emitting layer is in contact with the second light-emitting layer is preferably included.

In each of the above-described structures, a spectrum of emission from the first light-emitting layer preferably includes a region on a short wavelength side of a spectrum of emission from the second light-emitting layer.

In each of the above-described structures, the first fluorescent material preferably has a function of emitting light in a blue region. In each of the above-described structures, the first fluorescent material preferably has a function of emitting light having a peak in a region of longer than or equal to 400 nm and shorter than 490 nm.

In each of the above-described structures, the second light-emitting layer preferably further includes a third fluorescent material.

In each of the above-described structures, a spectrum of emission from the third fluorescent material is preferably different from a spectrum of emission from the second fluorescent material.

In each of the above-described structures, the second fluorescent material preferably has a function of emitting light in a green region, and the third fluorescent material preferably has a function of emitting light in a red region. In each of the above-described structures, the second fluorescent material preferably has a function of emitting light having an emission spectrum peak in a region of longer than or equal to 490 nm and shorter than 580 nm, and the third fluorescent material preferably has a function of emitting light having an emission spectrum peak in a region of longer than or equal to 580 nm and shorter than or equal to 680 nm.

In each of the above-described structures, the first host material is preferably an organic compound having a condensed aromatic ring skeleton. In each of the above-described structures, the first host material is preferably an organic compound having an anthracene skeleton.

In each of the above-described structures, the first fluorescent material is preferably an organic compound having a pyrene skeleton.

In each of the above-described structures, one of the first organic compound and the second organic compound preferably has a function of transporting electrons, and the other of the first organic compound and the second organic compound preferably has a function of transporting holes. In each of the above-described structures, one of the first organic compound and the second organic compound preferably has a π-electron deficient heteroaromatic ring skeleton, and the other of the first organic compound and the second organic compound preferably has a π-electron rich heteroaromatic ring skeleton or an aromatic amine skeleton.

Another embodiment of the present invention is a display device including the light-emitting element having any of the above-described structures, and at least one of a color filter and a transistor. Another embodiment of the present invention is an electronic device including the above-described display device and at least one of a housing and a touch sensor. Another embodiment of the present invention is a lighting device including the light-emitting element having any of the above-described structures, and at least one of a housing and a touch sensor. The category of one embodiment of the present invention includes not only a light-emitting device including a light-emitting element but also an electronic device including a light-emitting device. The light-emitting device in this specification refers to an image display device and a light source (e.g., a lighting device). The light-emitting device may be included in a display module in which a connector such as a flexible printed circuit (FPC) or a tape carrier package (TCP) is connected to a light-emitting device, a display module in which a printed wiring board is provided on the tip of a TCP, or a display module in which an integrated circuit (IC) is directly mounted on a light-emitting element by a chip on glass (COG) method.

With one embodiment of the present invention, a light-emitting element including a fluorescent compound which has high emission efficiency can be provided. With one embodiment of the present invention, a light-emitting element including a plurality of fluorescent compounds which has high emission efficiency can be provided. With one embodiment of the present invention, a white-light-emitting element including a fluorescent compound which has high emission efficiency can be provided. With one embodiment of the present invention, a light-emitting element which is easily formed in a relatively small number of steps for forming films can be provided. With one embodiment of the present invention, a light-emitting element with low power consumption can be provided. With one embodiment of the present invention, a novel light-emitting element can be provided. With one embodiment of the present invention, a novel light-emitting device can be provided. With one embodiment of the present invention, a novel display device can be provided.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention will be described below with reference to the drawings. However, the present invention is not limited to description to be given below, and it is to be easily understood that modes and details thereof can be variously modified without departing from the purpose and the scope of the present invention. Accordingly, the present invention should not be interpreted as being limited to the content of the embodiments below.

Note that the position, the size, the range, or the like of each structure illustrated in drawings and the like is not accurately represented in some cases for simplification. Therefore, the disclosed invention is not necessarily limited to the position, the size, the range, or the like disclosed in the drawings and the like.

Note that the ordinal numbers such as “first”, “second”, and the like in this specification and the like are used for convenience and do not denote the order of steps or the stacking order of layers. Therefore, for example, description can be made even when “first” is replaced with “second” or “third”, as appropriate. In addition, the ordinal numbers in this specification and the like are not necessarily the same as those which specify one embodiment of the present invention.

In the description of modes of the present invention in this specification and the like with reference to the drawings, the same components in different diagrams are commonly denoted by the same reference numeral in some cases.

In this specification and the like, a singlet excited state (S*) refers to a singlet state having excited energy. The lowest level of the singlet excited energy level (S1level) refers to the excited energy level of the lowest singlet excited state. A triplet excited state (T*) refers to a triplet state having excited energy. The lowest level of the triplet excited energy level (T1level) refers to the excited energy level of the lowest triplet excited state.

In this specification and the like, a fluorescent material refers to a material that emits light in the visible light region when the singlet excited state relaxes to the ground state. A phosphorescent material refers to a material that emits light in the visible light region at room temperature when the triplet excited state relaxes to the ground state. That is, a phosphorescent material refers to a material that can convert triplet excited energy into visible light.

Note that in this specification and the like, “room temperature” refers to a temperature higher than or equal to 0° C. and lower than or equal to 40° C.

In this specification and the like, a wavelength range of blue refers to a wavelength range of greater than or equal to 400 nm and less than 490 nm, and blue light has at least one peak in that range in an emission spectrum. A wavelength range of green refers to a wavelength range of greater than or equal to 490 nm and less than 580 nm, and green light has at least one peak in that range in an emission spectrum. A wavelength range of red refers to a wavelength range of greater than or equal to 580 nm and less than or equal to 680 nm, and red light has at least one peak in that range in an emission spectrum.

In this embodiment, a light-emitting element of one embodiment of the present invention will be described below with reference toFIGS. 1A and 1B,FIGS. 2A to 2C, andFIGS. 3A and 3B.

First, a structure of the light-emitting element of one embodiment of the present invention will be described with reference toFIGS. 1A and 1B.

FIG. 1Ais a schematic cross-sectional view of a light-emitting element250of one embodiment of the present invention.

The light-emitting element250includes a pair of electrodes (an and an electrode102) and an EL layer100between the pair of electrodes. The EL layer100includes at least a light-emitting layer150.

The EL layer100illustrated inFIG. 1Aincludes functional layers such as a hole-injection layer111, a hole-transport layer112, an electron-transport layer118, and an electron-injection layer119, in addition to the light-emitting layer150. The light-emitting layer150includes at least a light-emitting layer120and a light-emitting layer130.

In this embodiment, although description is given assuming that the electrode101and the electrode102of the pair of electrodes serve as an anode and a cathode, respectively, they are not limited thereto for the structure of the light-emitting element250. That is, the electrode101may be a cathode, the electrode102may be an anode, and the stacking order of the layers between the electrodes may be reversed. In other words, the hole-injection layer111, the hole-transport layer112, the light-emitting layer150, the electron-transport layer118, and the electron-injection layer119may be stacked in this order from the anode side.

The structure of the EL layer100is not limited to the structure illustrated inFIG. 1A, and a structure including at least one layer selected from the hole-injection layer111, the hole-transport layer112, the electron-transport layer118, and the electron-injection layer119may be employed. Alternatively, the EL layer100may include a functional layer which is capable of lowering a hole- or electron-injection barrier, improving a hole- or electron-transport property, inhibiting a hole- or electron-transport property, or suppressing a quenching phenomenon by an electrode, for example. Note that the functional layers may each be a single layer or stacked layers.

FIG. 1Bis a schematic cross-sectional view illustrating an example of the light-emitting layer150inFIG. 1A. The light-emitting layer120inFIG. 1Bincludes a host material121and a guest material122. The light-emitting layer130includes a host material131and a guest material132. The host material131includes an organic compound131_1and an organic compound131_2.

The guest material122and the guest material132may be light-emitting organic compounds, and the light-emitting organic compounds are preferably substances capable of emitting fluorescence (hereinafter also referred to as fluorescent materials). An example in which fluorescent materials are used as the guest materials122and132will be described below. Note that the guest materials122and132may be referred to as the fluorescent materials.

In the light-emitting element250of one embodiment of the present invention, voltage application between the pair of electrodes (the electrodes101and102) allows electrons and holes to be injected from the cathode and the anode, respectively, into the EL layer100and thus a current flows. By recombination of the injected carriers (electrons and holes), excitons are formed. The ratio of singlet excitons to triplet excitons (hereinafter referred to as exciton generation probability) which are generated by carrier (electrons and holes) recombination is approximately 1:3 according to the statistically obtained probability. Accordingly, in a light-emitting element that uses a fluorescent material, the probability of generation of singlet excitons, which contribute to light emission, is 25% and the probability of generation of triplet excitons, which do not contribute to light emission, is 75%. Therefore, converting the triplet excitons, which do not contribute to light emission, into singlet excitons, which contribute to light emission, is important in increasing the emission efficiency of the light-emitting element.

First, the light emission mechanism of the light-emitting layer120is described below.

By recombination of the electrons and holes injected from the pair of electrodes (the electrode101and the electrode102) in the light-emitting layer120, excitons are formed. Because the amount of the host material121is larger than that of the guest material122, the host material121is brought into an excited state by the exciton generation.

Note that the term “exciton” refers to a carrier (electron and hole) pair. Since excitons have energy, a material where excitons are generated is brought into an excited state.

In the case where the formed excited state of the host material121is a singlet excited state, singlet excited energy transfers from the S1level of the host material121to the S1level of the guest material122, thereby forming the singlet excited state of the guest material122.

Since the guest material122is a fluorescent material, when a singlet excited state is formed in the guest material122, the guest material122readily emits light. To obtain high emission efficiency in this case, the fluorescence quantum yield of the guest material122is preferably high. The same can apply to a case where a singlet excited state is formed by recombination of carriers in the guest material122.

Next, a case where recombination of carriers forms a triplet excited state of the host material121is described. The correlation of energy levels of the host material121and the guest material122in this case is shown inFIG. 2A. The following explains what terms and signs inFIG. 2Arepresent. Note that because it is preferable that the T1level of the host material121be lower than the T1level of the guest material122,FIG. 2Ashows this preferable case. However, the T1level of the host material121may be higher than the T1level of the guest material122.

Host (121): the host material121;

Guest (122): the guest material122(the fluorescent material);

SFH: the S1level the host material121;

TFH: the T1level of the host material121;

SFG: the S1level of the guest material122(the fluorescent material); and

TFG: the T1level of the guest material122(the fluorescent material).

When the triplet excitons formed by carrier recombination approach each other, a reaction in which one of the triplet excitons is converted into a singlet exciton having energy of the S1level of the host material121(SFH) is caused (see TTA inFIG. 2A). The singlet excited energy of the host material121is transferred from SFHto the S1level of the guest material122(SFG) having a lower energy than SFH(see Route E1inFIG. 2A), and a singlet excited state of the guest material122is formed, whereby the guest material122emits light.

Note that in the case where the density of triplet excitons in the light-emitting layer120is sufficiently high (e.g., 1×10−12cm−3or more), only the reaction of two triplet excitons approaching each other can be considered whereas deactivation of a single triplet exciton can be ignored.

In the case where a triplet excited state of the guest material122is formed by carrier recombination, the triplet excited state of the guest material122is thermally deactivated and is difficult to use for light emission. However, in the case where the T1level of the host material121(TFH) is lower than the T1level of the guest material122(TFG), the triplet excited energy of the guest material122can be transferred from the T1level of the guest material122(TFG) to the T1level of the host material121(TFH) (see Route E2inFIG. 2A) and then is utilized for TTA.

In other words, the host material121preferably has a function of converting triplet excited energy into singlet excited energy by causing TTA, so that the triplet excited energy generated in the light-emitting layer120can be partly converted into singlet excited energy by TTA in the host material121. The singlet excited energy can be transferred to the guest material122and extracted as fluorescence. In order to achieve this, the S1level of the host material121(SFH) is preferably higher than the S1level of the guest material122(SFG). In addition, the T1level of the host material121(TFH) is preferably lower than the T1level of the guest material122(TFG).

Note that particularly in the case where the T1level of the guest material122(TFG) is lower than the T1level of the host material121(TFH), the weight ratio of the guest material122to the host material121is preferably low. Specifically, the weight ratio of the guest material122to the host material121is preferably greater than 0 and less than or equal to 0.05, in which case, the probability of carrier recombination in the guest material122can be reduced. In addition, the probability of energy transfer from the T1level of the host material121(TFH) to the T1level of the guest material122(TFG) can be reduced.

Note that the host material121may be composed of a single compound or a plurality of compounds.

Next, the light emission mechanism of the light-emitting layer130is described below.

The organic compound131_1and the organic compound131_2included in the host material131in the light-emitting layer130form an exciplex.

Although it is acceptable as long as the combination of the organic compound131_1and the organic compound131_2can form an exciplex, it is preferable that one of them be a compound having a function of transporting holes (a hole-transport property) and the other be a compound having a function of transporting electrons (an electron-transport property). In that case, a donor-acceptor exciplex is formed easily; thus, efficient formation of an exciplex is possible. In the case where the combination of the organic compounds131_1and131_2is a combination of a compound having a hole-transport property and a compound having an electron-transport property, the carrier balance can be easily controlled depending on the mixture ratio. Specifically, the weight ratio of the compound having a hole-transport property to the compound having an electron-transport property is preferably within a range of 1:9 to 9:1. Since the carrier balance can be easily controlled with the structure, a carrier recombination region can also be controlled easily.

In order to efficiently form an exciplex, the combination of the host materials preferably satisfies the follows: the highest occupied molecular orbital (also referred to as HOMO) level of one of the organic compound131_1and the organic compound131_2is higher than the HOMO level of the other of the organic compounds, and the lowest unoccupied molecular orbital (also referred to as LUMO) level of the one of the organic compounds is higher than the LUMO level of the other of the organic compounds. For example, when one of the organic compounds has a hole-transport property and the other of the organic compounds has an electron-transport property, it is preferable that the HOMO level of the one of the organic compounds be higher than the HOMO level of the other of the organic compounds and the LUMO level of the one of the organic compounds be higher than the LUMO level of the other of the organic compounds. Specifically, a difference in HOMO level between the organic compounds is preferably greater than or equal to 0.05 eV, further preferably greater than or equal to 0.1 eV, and still further preferably greater than or equal to 0.2 eV. A difference in LUMO level between the organic compounds is preferably greater than or equal to 0.05 eV, further preferably greater than or equal to 0.1 eV, and still further preferably greater than or equal to 0.2 eV.

FIG. 2Bshows a correlation of energy levels of the organic compound131_1, the organic compound131_2, and the guest material132in the light-emitting layer130.

The following explains what terms and signs inFIG. 2Brepresent:

Host (131_1): the organic compound131_1;

Host (131_2): the organic compound131_2;

Guest (132): the guest material132(the fluorescent material);

SH: the S1level of the organic compound131_1(the host material);

TH: the T1level of the organic compound131_1(the host material);

SG: the S1level of the guest material132(the fluorescent material);

TG: the T1level of the guest material132(the fluorescent material);

SE: the S1level of the exciplex; and

TE: the T1level of the exciplex.

In the light-emitting element of one embodiment of the present invention, the organic compounds131_1and131_2included in the light-emitting layer130form an exciplex. The lowest singlet excited energy level of the exciplex (SE) and the lowest triplet excited energy level of the exciplex (TE) are adjacent to each other (see Route E3inFIG. 2B).

An exciplex is an excited state formed from two kinds of substances. In photoexcitation, the exciplex is formed by interaction between one substance in an excited state and the other substance in a ground state. The two kinds of substances that have formed the exciplex return to a ground state by emitting light and serve as the original two kinds of substances. In electrical excitation, when one substance is brought into an excited state, the one immediately interacts with the other substance to form an exciplex. Alternatively, one substance receives a hole and the other substance receives an electron to readily form an exciplex. In this case, any of the substances can form an exciplex without forming an excited state and; accordingly, most excitons in the light-emitting layer130can exist as exciplexes. Because the excited energy levels of the exciplex (SEand TE) are lower than the singlet excited energy level of the host materials (SH) (the organic compound131_1and the organic compound131_2) that form the exciplex, the excited state of the host material131can be formed with lower excited energy. Accordingly, the driving voltage of the light-emitting element250can be reduced.

Since the singlet excited energy level (SE) and the triplet excited energy level (TE) of the exciplex are adjacent to each other, the exciplex has a function of exhibiting thermally activated delayed fluorescence. In other words, the exciplex has a function of converting triplet excited energy to singlet excited energy by reverse intersystem crossing (upconversion) (see Route E4inFIG. 2B). Thus, the triplet excited energy generated in the light-emitting layer130is partly converted into singlet excited energy by the exciplex. In order to cause this conversion, the energy difference between the singlet excited energy level (SE) and the triplet excited energy level (TE) of the exciplex is preferably greater than 0 eV and less than or equal to 0.2 eV. Note that in order to efficiently make reverse intersystem crossing occur, the triplet excited energy level of the exciplex (TE) is preferably lower than the triplet excited energy levels of the organic compounds (the organic compound131_1and the organic compound131_2) in the host material which form the exciplex. Thus, quenching of the triplet excited energy of the exciplex due to the organic compounds is less likely to occur, which causes reverse intersystem crossing efficiently.

Furthermore, the singlet excited energy level of the exciplex (SE) is preferably higher than the singlet excited energy level of the guest material132(SG). In this way, the singlet excited energy of the formed exciplex can be transferred from the singlet excited energy level of the exciplex (SE) to the singlet excited energy level of the guest material132(SG), so that the guest material132is brought into the singlet excited state, causing light emission (see Route E5inFIG. 2B).

The proportion of a thermally activated delayed fluorescence component in the light emitted from the exciplex is preferably high. Specifically, the proportion of a thennally activated delayed fluorescence component in the light emitted from the exciplex is preferably higher than or equal to 20%, further preferably higher than or equal to 50%.

Note that since direct transition from a singlet ground state to a triplet excited state in the guest material132is forbidden, energy transfer from the singlet excited energy level of the exciplex (SE) to the triplet excited energy level of the guest material132(TG) is unlikely to be a main energy transfer process.

When transfer of the triplet excited energy from the triplet excited energy level of the exciplex (TE) to the triplet excited energy level of the guest material132(TG) occurs, the triplet excited energy is deactivated (see Route E6inFIG. 2B). Thus, it is preferable that the energy transfer of Route E6be less likely to occur because the efficiency of generating the triplet excited state of the guest material132can be decreased and thermal deactivation can be reduced. In order to make this condition, the weight ratio of the guest material132to the host material131is preferably low, specifically, preferably greater than or equal to 0.001 and less than or equal to 0.05, further preferably greater than or equal to 0.001 and less than or equal to 0.01.

Note that when the direct carrier recombination process in the guest material132is dominant, a large number of triplet excitons are generated in the light-emitting layer130, resulting in a decreased emission efficiency due to thermal deactivation. Thus, it is preferable that the probability of the energy transfer process through the exciplex formation process (Routes E4and E5inFIG. 2B) be higher than the probability of the direct carrier recombination process in the guest material132because the efficiency of generating the triplet excited state of the guest material132can be decreased and thermal deactivation can be reduced. Therefore, as described above, the weight ratio of the guest material132to the host material131is preferably low, specifically, preferably greater than or equal to 0.001 and less than or equal to 0.05, further preferably greater than or equal to 0.001 and less than or equal to 0.01.

By making all the energy transfer processes of Routes E4and E5efficiently occur in the above-described manner, both the singlet excited energy and the triplet excited energy of the host material131can be efficiently converted into the singlet excited energy of the guest material132, whereby the light-emitting element250can emit light with high emission efficiency.

The above-described processes through Routes E3, E4, and E5may be referred to as exciplex-singlet energy transfer (ExSET) or exciplex-enhanced fluorescence (ExEF) in this specification and the like. In other words, in the light-emitting layer130, excited energy is given from the exciplex to the guest material132.

When the light-emitting layer130has the above-described structure, light emission from the guest material132of the light-emitting layer130can be obtained efficiently.

Next, factors controlling the processes of intermolecular energy transfer between the host material131and the guest material132will be described. As mechanisms of the intermolecular energy transfer, two mechanisms, i.e., Forster mechanism (dipole-dipole interaction) and Dexter mechanism (electron exchange interaction), have been proposed. Although the intermolecular energy transfer process between the host material131and the guest material132is described here, the same can apply to a case where the host material131is an exciplex.

In Förster mechanism, energy transfer does not require direct contact between molecules and energy is transferred through a resonant phenomenon of dipolar oscillation between the host material131and the guest material132. By the resonant phenomenon of dipolar oscillation, the host material131provides energy to the guest material132, and thus, the host material131in an excited state is brought to a ground state and the guest material132in a ground state is brought to an excited state. Note that the rate constant kh*→gof Förster mechanism is expressed by Formula (1).

In Formula (1), v denotes a frequency, f′h(v) denotes a normalized emission spectrum of the host material131(a fluorescent spectrum in energy transfer from a singlet excited state, and a phosphorescent spectrum in energy transfer from a triplet excited state), εg(v) denotes a molar absorption coefficient of the guest material132, N denotes Avogadro's number, n denotes a refractive index of a medium, R denotes an intermolecular distance between the host material131and the guest material132, τ denotes a measured lifetime of an excited state (fluorescence lifetime or phosphorescence lifetime), c denotes the speed of light, ϕ denotes a luminescence quantum yield (a fluorescence quantum yield in energy transfer from a singlet excited state, and a phosphorescence quantum yield in energy transfer from a triplet excited state), and K2denotes a coefficient (0 to 4) of orientation of a transition dipole moment between the host material131and the guest material132. Note that K2is ⅔ in random orientation.

In Dexter mechanism, the host material131and the guest material132are close to a contact effective range where their orbitals overlap, and the host material131in an excited state and the guest material132in a ground state exchange their electrons, which leads to energy transfer. Note that the rate constant kh*→gof Dexter mechanism is expressed by Formula (2).

In Formula (2), h denotes a Planck constant, K denotes a constant having an energy dimension, v denotes a frequency, f′h(v) denotes a normalized emission spectrum of the host material131(a fluorescent spectrum in energy transfer from a singlet excited state, and a phosphorescent spectrum in energy transfer from a triplet excited state), ε′g(v) denotes a normalized absorption spectrum of the guest material132, L denotes an effective molecular radius, and R denotes an intermolecular distance between the host material131and the guest material132.

Here, the efficiency of energy transfer from the host material131to the guest material132(energy transfer efficiency ϕET) is expressed by Formula (3). In the formula, krdenotes a rate constant of a light-emission process (fluorescence in energy transfer from a singlet excited state, and phosphorescence in energy transfer from a triplet excited state) of the host material131, kndenotes a rate constant of a non-light-emission process (thermal deactivation or intersystem crossing) of the host material131, and τ denotes a measured lifetime of an excited state of the host material131.

According to Formula (3), it is found that the energy transfer efficiency ϕETcan be increased by increasing the rate constant kh*→gof energy transfer so that another competing rate constant kr+kn(=1/τ) becomes relatively small.

<<Concept for Promoting Energy Transfer>>

First, an energy transfer by Firster mechanism is considered. When τ is eliminated from Formula (1) and Formula (3), it can be said that the energy transfer efficiency ϕETis higher when the luminescence quantum yield ϕ (here, the fluorescence quantum yield because energy transfer from a singlet excited state is discussed) is higher. However, in practice, a more important factor is that the emission spectrum of the host material131(here, a fluorescent spectrum because energy transfer from a singlet excited state is discussed) largely overlaps with the absorption spectrum of the guest material132(absorption corresponding to the transition from the singlet ground state to the singlet excited state). Note that it is preferable that the molar absorption coefficient of the guest material132be also high. This means that the emission spectrum of the host material131overlaps with the absorption band of the guest material132which is on the longest wavelength side.

Next, an energy transfer by Dexter mechanism is considered. According to Formula (2), in order to increase the rate constant kh*→g, it is preferable that an emission spectrum of the host material131(here, a fluorescent spectrum because energy transfer from a singlet excited state is discussed) largely overlap with an absorption spectrum of the guest material132(absorption corresponding to transition from a singlet ground state to a singlet excited state).

The above description suggests that the energy transfer efficiency can be optimized by making the emission spectrum of the host material131overlap with the absorption band of the guest material132which is on the longest wavelength side.

In a manner similar to that of the energy transfer from the singlet excited state of the host material131to the singlet excited state of the guest material132, the energy transfer by both Förster mechanism (Formula (1)) and Dexter mechanism (Formula (2)) also occurs in the energy transfer process from the singlet excited state of the exciplex to the singlet excited state of the guest material132.

Accordingly, one embodiment of the present invention provides a light-emitting element including, as the host material131, the organic compound131_1and the organic compound131_2which are a combination for forming an exciplex which functions as an energy donor capable of efficiently transferring energy to the guest material132. The exciplex formed by the organic compound131_1and the organic compound131_2has a singlet excited energy level and a triplet excited energy level which are adjacent to each other; accordingly, transition from a triplet exciton generated in the light-emitting layer130to a singlet exciton (reverse intersystem crossing) is likely to occur. This can increase the efficiency of generating singlet excitons in the light-emitting layer130. Furthermore, in order to facilitate energy transfer from the singlet excited state of the exciplex to the singlet excited state of the guest material132having a function as an energy acceptor, it is preferable that the emission spectrum of the exciplex overlap with the absorption band of the guest material132which is on the longest wavelength side (low energy side). Thus, the efficiency of generating the singlet excited state of the guest material132can be increased.

Next, the light emission mechanism of the light-emitting layer150including the light-emitting layer120and the light-emitting layer130is described below. Note that the light emission mechanisms of the light-emitting layers120and130are each the same as those shown inFIGS. 2A and 2B.

In the case where the light-emitting layer120and the light-emitting layer130are in contact with each other as in the light-emitting element250, even when energy transfer (in particular, triplet excited energy transfer) from the exciplex formed in the light-emitting layer130to the host material121of the light-emitting layer120occurs at an interface between the light-emitting layer120and the light-emitting layer130, the triplet excited energy can be converted into light emission with the use of TTA in the light-emitting layer120.

The T1level of the host material121in the light-emitting layer120is preferably lower than the T1levels of the organic compound131_1and the organic compound131_2included in the host material131of the light-emitting layer130. In the light-emitting layer120, the S1level of the host material121is preferably higher than the S1level of the guest material122and the T1level of the host material121is preferably lower than the T1level of the guest material122.

FIG. 2Cshows a correlation of energy levels in the case where TTA is utilized in the light-emitting layer120and ExEF is utilized in the light-emitting layer130. The following explains what terms and signs inFIG. 2Crepresent:

SFH: the S1level of the host material121;

TFH: the T1level of the host material121;

SFG: the S1level of the guest material122(the fluorescent material);

TFG: the T1level of the guest material122(the fluorescent material);

SH: the S1level of the host material131(the organic compound131_1or131_2);

TH: the T1level of the host material131(the organic compound131_1or131_2);

SG: the S1level of the guest material132(the fluorescent material);

TG: the T1level of the guest material132(the fluorescent material);

SE: the S1level of the exciplex; and

TE: the T1level of the exciplex.

As shown inFIG. 2C, the exciplex exists only in an excited state; thus, exciton diffusion between the exciplexes is unlikely to occur. In addition, because the excited levels of the exciplex (SE, TE) are lower than the excited levels of the host material131(the organic compound131_1) of the light-emitting layer130(SH, TH), energy diffusion from the exciplex to the organic compound131_1does not occur. That is, excited energy of the light-emitting layer130can be maintained in the light-emitting layer130because an exciton diffusion distance of the exciplex is short in the light-emitting layer130.

In addition, even when part of the triplet excited energy of the exciplex of the light-emitting layer130diffuses into the light-emitting layer120through the interface between the light-emitting layer120and the light-emitting layer130, energy loss can be reduced because the triplet excited energy in the light-emitting layer120caused by the diffusion is converted into singlet excited energy by TTA and used for light emission. Therefore, the triplet excited energy level of the host material121is preferably lower than the triplet excited energy levels of the organic compound131_1and the organic compound131_2.

The light-emitting element250, which is a light-emitting element including fluorescent materials as light-emitting materials, can have high emission efficiency because ExEF is utilized in the light-emitting layer130and TTA is utilized in the light-emitting layer120as described above so that energy loss is reduced. Since the light-emitting element250does not include a phosphorescent material, it can be manufactured at low manufacturing cost.

As in the light-emitting element250, in the case where the light-emitting layer120and the light-emitting layer130are in contact with each other, the number of EL layers100as well as the energy loss can be reduced. Therefore, the light-emitting element250can be manufactured at low manufacturing cost.

Note that the light-emitting layer120and the light-emitting layer130are not necessarily in contact with each other. In that case, it is possible to prevent energy transfer by the Dexter mechanism (particularly triplet energy transfer) from the organic compound131_1, the organic compound131_2, or the guest material132(the fluorescent material) in an excited state which is generated in the light-emitting layer130to the host material121or the guest material122(the fluorescent material) in the light-emitting layer120. Therefore, the thickness of a layer provided between the light-emitting layer120and the light-emitting layer130may be several nanometers and is specifically preferably more than or equal to 1 nm and less than or equal to 5 nm.

The layer provided between the light-emitting layer120and the light-emitting layer130may contain a single material or a plurality of materials. The layer provided between the light-emitting layer120and the light-emitting layer130may contain both a hole-transport material and an electron-transport material. In the case of a single material, a bipolar material may be used. The bipolar material here refers to a material in which the ratio between the electron mobility and the hole mobility is 100 or less. Alternatively, in the case where the hole-transport material and the electron-transport material are used, at least one of the materials may be the same as the host material (the organic compound131_1or131_2) of the light-emitting layer130. This facilitates the manufacture of the light-emitting element and reduces the drive voltage. Furthermore, a structure with which the hole-transport material and the electron-transport material can form an exciplex may be employed, in which case exciton diffusion can be prevented effectively. Specifically, it is possible to prevent energy transfer from the host material131(the organic compound131_1) in an excited state or the guest material132(the fluorescent material) in an excited state of the light-emitting layer130to the host material121or the guest material122(the fluorescent material) in the light-emitting layer120.

In the light-emitting element250, although the light-emitting layer120and the light-emitting layer130have been described as being positioned on the hole-transport layer112side and the electron-transport layer118side, respectively, the light-emitting element of one embodiment of the present invention is not limited to this structure. For example, the light-emitting layer120and the light-emitting layer130may be positioned on the electron-transport layer118side and the hole-transport layer112side, respectively, as in a light-emitting element250aillustrated inFIG. 3A.

Note that in the light-emitting element250, a carrier (holes and electrons) recombination region is preferably distributed to some extent. Therefore, it is preferable that the light-emitting layer120or130have an appropriate degree of carrier (hole or electron)-trapping property. It is particularly preferable that the guest material132(the fluorescent material) in the light-emitting layer130have a carrier (hole or electron)-trapping property.

Note that light emitted from the light-emitting layer120preferably has an emission spectrum peak on the shorter wavelength side than light emitted from the light-emitting layer130. The luminance of a light-emitting element using an organic compound having a high triplet excited energy level tends to degrade quickly. In view of the above, emission utilizing TTA is used for light emission on the short wavelength side, so that a light-emitting element with less degradation of luminance can be provided.

Furthermore, the light-emitting layers120and130have functions of emitting light with different emission wavelengths, so that the light-emitting element can be a multicolor light-emitting element. In that case, the emission spectrum is formed by combining light having different emission peaks, and thus has at least two peaks.

The above-described structure is suitable for obtaining white light emission. When the light-emitting layers120and130emit light of complementary colors, white light emission can be obtained.

In addition, white light emission with a high color rendering property that is formed of three primary colors or four or more colors can be obtained by using a plurality of light-emitting materials emitting light with different wavelengths for one or both of the light-emitting layers120and130. In that case, the light-emitting layer120and/or the light-emitting layer130may be divided into layers and each of the divided layers may contain a different light-emitting material from the others.

For example, as illustrated inFIG. 3B, in the case where the light-emitting layer130includes a plurality of light-emitting layers that are a light-emitting layer130_1and a light-emitting layer130_2, the light-emitting layer130_1preferably includes the host material131and the guest material132, and the light-emitting layer130_2preferably includes a host material136and a guest material137. In this case, a combination of the organic compound131_1and the organic compound131_2included in the host material131preferably forms an exciplex, and a combination of an organic compound136_1and an organic compound136_2included in the host material136preferably forms an exciplex. In this way, the efficiency of generating singlet excitons can be increased in both the light-emitting layer130_1and the light-emitting layer130_2, so that energy loss in the light-emitting layer130can be reduced.

In the above-described structure, when the light-emitting layer130includes two kinds of guest materials (the guest material132and the guest material137), it is preferable that the guest material122in the light-emitting layer120emit blue light, one of the guest materials in the light-emitting layer130emit green light, and the other guest material in the light-emitting layer130emit red light. The guest material122in the light-emitting layer120preferably emits light having a peak of an emission spectrum in a wavelength region of longer than or equal to 400 nm and shorter than 490 nm, one of the guest materials in the light-emitting layer130preferably emits light having a peak of an emission spectrum in a wavelength region of longer than or equal to 490 nm and shorter than 580 nm, and the other guest material in the light-emitting layer130preferably emits light having a peak of an emission spectrum in a wavelength region of longer than or equal to 580 nm and shorter than or equal to 680 nm. In this way, the light-emitting element can emit lights of three primary colors of blue, green, and red, achieving high color rendering properties. Furthermore, the light-emitting element can be favorably used in a display device.

Note that inFIG. 3B, the organic compound131_1and the organic compound136_1may be the same compound or different compounds. It is preferable to use the same compound as the organic compound131_1and the organic compound136_1because fabrication of the light-emitting element is easy. The same can apply to the organic compound131_2and the organic compound136_2.

Furthermore, in the light-emitting layer130_1and the light-emitting layer130_2, either one of the organic compounds131_1and131_2or either one of the organic compounds136_1and136_2may be included. The organic compounds131_1and131_2and the organic compounds136_1and136_2which are used in the light-emitting layer130_1and the light-emitting layer130_2are selected as appropriate so as to obtain appropriate carrier, balance or emission color for the light-emitting element.

Next, components of a light-emitting element of one embodiment of the present invention are described in detail below.

Next, materials that can be used for the light-emitting layers120and130included in the light-emitting layer150will be described below.

In the light-emitting layer120, the host material121is present in the largest proportion by weight, and the guest material122(the fluorescent material) is dispersed in the host material121. The S1level of the host material121is preferably higher than the S1level of the guest material122(the fluorescent material) while the T1level of the host material121is preferably lower than the T1level of the guest material122(the fluorescent material).

In the light-emitting layer120, the guest material122is preferably, but not particularly limited to, an anthracene derivative, a tetracene derivative, a chrysene derivative, a phenanthrene derivative, a pyrene derivative, a perylene derivative, a stilbene derivative, an acridone derivative, a coumarin derivative, a phenoxazine derivative, a phenothiazine derivative, or the like, and for example, any of the following materials can be used.

Among them, an organic compound which emits light including a delayed fluorescence component due to triplet-triplet annihilation (TTA) is preferable as the material that can be used as the host material121in the light-emitting layer120. Specifically, a compound having a condensed aromatic ring skeleton is preferable, and a compound having an anthracene skeleton is further preferable.

The light-emitting layer120can have a structure in which two or more layers are stacked. For example, in the case where the light-emitting layer120is formed by stacking a first light-emitting layer and a second light-emitting layer in this order from the hole-transport layer side, the first light-emitting layer is formed using a substance having a hole-transport property as the host material and the second light-emitting layer is formed using a substance having an electron-transport property as the host material.

In the light-emitting layer120, the host material121may be composed of one kind of compound or a plurality of compounds. The light-emitting layer120may include a material other than the host material121and the guest material122.

In the light-emitting layer130, the host material131is present in the largest proportion by weight, and the guest material132(the fluorescent material) is dispersed in the host material131. The S1level of the host material131(the organic compound131_1and the organic compound131_2) in the light-emitting layer130is preferably higher than the S1level of the guest material132(the fluorescent material) in the light-emitting layer130. The T1level of the host material131(the organic compound131_1and the organic compound131_2) in the light-emitting layer130is preferably higher than the T1level of the guest material132(the fluorescent material) in the light-emitting layer130.

In the light-emitting layer130, any of the above-described materials that can be used as the guest material122can be used as the guest material132.

Examples of the organic compound131_1include a zinc- or aluminum-based metal complex, an oxadiazole derivative, a triazole derivative, a benzimidazole derivative, a quinoxaline derivative, a dibenzoquinoxaline derivative, a dibenzothiophene derivative, a dibenzofuran derivative, a pyrimidine derivative, a triazine derivative, a pyridine derivative, a bipyridine derivative, a phenanthroline derivative, and the like. Other examples are an aromatic amine, a carbazole derivative, and the like.

Any of the following hole-transport materials and electron-transport materials can be used.

A material having a property of transporting more holes than electrons can be used as the hole-transport material, and a material having a hole mobility of 1×10−6cm2/Vs or higher is preferable. Specifically, an aromatic amine, a carbazole derivative, an aromatic hydrocarbon, a stilbene derivative, or the like can be used. Furthermore, the hole-transport material may be a high molecular compound.

The aromatic hydrocarbon may have a vinyl skeleton. Examples of the aromatic hydrocarbon having 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.

As the electron-transport material, a material having a property of transporting more electrons than holes can be used, and a material having an electron mobility of 1×10−6cm2/Vs or higher is preferable. A π-electron deficient heteroaromatic compound such as a nitrogen-containing heteroaromatic compound, a metal complex, or the like can be used as the material which easily accepts electrons (the material having an electron-transport property). Specific examples include a metal complex having a quinoline ligand, a benzoquinoline ligand, an oxazole ligand, and a thiazole ligand. Other examples include an oxadiazole derivative, a triazole derivative, a phenanthroline derivative, a pyridine derivative, a bipyridine derivative, a pyrimidine derivative, and the like.

As the organic compound131_2, a substance which can form an exciplex together with the organic compound131_1is preferably used. Specifically, any of the hole-transport materials and the electron-transport materials described above can be used. In that case, it is preferable that the organic compound131_1, the organic compound131_2, and the guest material132(the fluorescent material) be selected such that the emission peak of the exciplex formed by the organic compound131_1and the organic compound131_2overlaps with an absorption band on the longest wavelength side (low energy side) of the guest material132(the fluorescent material). This makes it possible to provide a light-emitting element with drastically improved emission efficiency.

As the host material131(the organic compound131_1and the organic compound131_2) included in the light-emitting layer130, a material having a function of converting triplet excited energy into singlet excited energy is preferable. As the material capable of converting triplet excited energy into singlet excited energy, a thermally activated delayed fluorescent (TADF) material can be given in addition to the exciplex. Therefore, the term “exciplex” in the description can be replaced with the term “thermally activated delayed fluorescence material”. Note that the thermally activated delayed fluorescence material is a material having a small difference between the triplet excited energy level and the singlet excited energy level and a function of converting triplet excited energy into singlet excited energy by reverse intersystem crossing. Thus, the thermally activated delayed fluorescence material can up-convert a triplet excited state into a singlet excited state (i.e., reverse intersystem crossing) using a little thermal energy and efficiently exhibit light emission (fluorescence) from the singlet excited state. Thermally activated delayed fluorescence is efficiently obtained under the condition where the difference between the triplet excited energy level and the singlet excited energy level is more than 0 eV and less than or equal to 0.2 eV, preferably more than 0 eV and less than or equal to 0.1 eV.

The material that exhibits thermally activated delayed fluorescence may be a material that can form a singlet excited state by itself from a triplet excited state by reverse intersystem crossing. In the case where the thermally activated delayed fluorescence material is composed of one kind of material, any of the following materials can be used, for example.

First, a fullerene, a derivative thereof, an acridine derivative such as proflavine, eosin, and the like can be given. Furthermore, a metal-containing porphyrin, such as a porphyrin containing magnesium (Mg), zinc (Zn), cadmium (Cd), tin (Sn), platinum (Pt), indium (In), or palladium (Pd), can be given. Examples of the metal-containing porphyrin include a protoporphyrin-tin fluoride complex (SnF2(Proto IX)), a mesoporphyrin-tin fluoride complex (SnF2(Meso IX)), a hematoporphyrin-tin fluoride complex (SnF2(Hemato IX)), a coproporphyrin tetramethyl ester-tin fluoride complex (SnF2(Copro III-4Me)), an octaethylporphyrin-tin fluoride complex (SnF2(OEP)), an etioporphyrin-tin fluoride complex (SnF2(Etio I)), and an octaethylporphyrin-platinum chloride complex (PtCl2(OEP)).

As the thermally activated delayed fluorescence material composed of one kind of material, a heterocyclic compound having a π-electron rich heteroaromatic ring and a π-electron deficient heteroaromatic ring can be used. Specifically, 2-(biphenyl-4-yl)-4,6-bis(12-phenylindolo[2,3-a]carbazol-11-yl)-1,3,5-triazine (abbreviation: PIC-TRZ), 2-{4-[3-(N-phenyl-9H-carbazol-3-yl)-9H-carbazol-9-yl]phenyl}-4,6-diphenyl-1,3,5-triazine (abbreviation: PCCzPTzn), 2-[4-(1 OH-phenoxazin-10-yl)phenyl]-4,6-diphenyl-1,3,5-triazine (abbreviation: PXZ-TRZ), 3-[4-(5-phenyl-5,10-dihydrophenazine-10-yl)phenyl]-4,5-diphenyl-1,2,4-triazole (abbreviation: PPZ-3TPT), 3-(9,9-dimethyl-9H-acridin-10-yl)-9H-xanthen-9-one (abbreviation: ACRXTN), bis[4-(9,9-dimethyl-9,10-dihydroacridine)phenyl]sulfone (abbreviation: DMAC-DPS), or 10-phenyl-10H, 10′H-spiro[acridin-9,9′-anthracen]-10′-one (abbreviation: ACRSA), can be used. The heterocyclic compound is preferable because of having the π-electron rich heteroaromatic ring and the π-electron deficient heteroaromatic ring, for which the electron-transport property and the hole-transport property are high. Note that a substance in which the π-electron rich heteroaromatic ring is directly bonded to the π-electron deficient heteroaromatic ring is particularly preferable because the donor property of the π-electron rich heteroaromatic ring and the acceptor property of the π-electron deficient heteroaromatic ring are both increased and the difference between the singlet excited level and the triplet excited level becomes small.

The light-emitting layer130can have a structure in which two or more layers are stacked. For example, in the case where the light-emitting layer130is formed by stacking a first light-emitting layer and a second light-emitting layer in this order from the hole-transport layer side, the first light-emitting layer is formed using a substance having a hole-transport property as the host material and the second light-emitting layer is formed using a substance having an electron-transport property as the host material.

The light-emitting layer130may include a material other than the host material131and the guest material132.

There is no limitation on the emission colors of the light-emitting material included in the light-emitting layer120and the light-emitting material included in the light-emitting layer130, and they may be the same or different. Light emitted from the light-emitting materials is mixed and extracted out of the element; therefore, for example, in the case where their emission colors are complementary colors, the light-emitting element can emit white light. In consideration of the reliability of the light-emitting element, the emission peak wavelength of the light-emitting material included in the light-emitting layer120is preferably shorter than that of the light-emitting material included in the light-emitting layer130.

The electrode101and the electrode102have functions of injecting holes and electrons into the light-emitting layer150. The electrode101and the electrode102can be formed using a metal, an alloy, or a conductive compound, a mixture or a stack thereof, or the like. A typical example of the metal is aluminum (Al); besides, a transition metal such as silver (Ag), tungsten, chromium, molybdenum, copper, or titanium, an alkali metal such as lithium (Li) or cesium, or a Group2metal such as calcium or magnesium (Mg) can be used. As the transition metal, a rare earth metal such as ytterbium (Yb) may be used. An alloy containing any of the above metals can be used as the alloy, and MgAg and AlLi can be given as examples. Examples of the conductive compound include metal oxides such as indium tin oxide (hereinafter, referred to as ITO), indium tin oxide containing silicon or silicon oxide (ITSO), indium zinc oxide, indium oxide containing tungsten and zinc, and the like. It is also possible to use an inorganic carbon-based material such as graphene as the conductive compound. As described above, the electrode101and/or the electrode102may be formed by stacking two or more of these materials.

Light emitted from the light-emitting layer150is extracted through the electrode101and/or the electrode102. Therefore, at least one of the electrodes101and102transmits visible light. As the conductive material transmitting light, a conductive material having a visible light transmittance higher than or equal to 40% and lower than or equal to 100%, preferably higher than or equal to 60% and lower than or equal to 100%, and a resistivity lower than or equal to 1×10−2Ω·cm can be used. The electrode on the light extraction side may be formed using a conductive material having functions of transmitting light and reflecting light. As the conductive material, a conductive material having a visible light reflectivity higher than or equal to 20% and lower than or equal to 80%, preferably higher than or equal to 40% and lower than or equal to 70%, and a resistivity lower than or equal to 1×10−2Ω·cm can be used. In the case where the electrode through which light is extracted is formed using a material with low light transmittance, such as metal or alloy, the electrode101and/or the electrode102is formed to a thickness that is thin enough to transmit visible light (e.g., a thickness of 1 nm to 10 nm).

In this specification and the like, as the electrode transmitting light, a material that transmits visible light and has conductivity is used. Examples of the material include, in addition to the above-described oxide conductor layer typified by an ITO, an oxide semiconductor layer and an organic conductor layer containing an organic substance. Examples of the organic conductive layer containing an organic substance include a layer containing a composite material in which an organic compound and an electron donor (donor material) are mixed and a layer containing a composite material in which an organic compound and an electron acceptor (acceptor material) are mixed. The resistivity of the transparent conductive layer is preferably lower than or equal to 1×105Ω·cm, further preferably lower than or equal to 1×104Ω·cm.

As the method for forming the electrode101and the electrode102, a sputtering method, an evaporation method, a printing method, a coating method, a molecular beam epitaxy (MBE) method, a CVD method, a pulsed laser deposition method, an atomic layer deposition (ALD) method, or the like can be used as appropriate.

The hole-injection layer111has a function of reducing a barrier for hole injection from one of the pair of electrodes (the electrode101or the electrode102) to promote hole injection and is formed using a transition metal oxide, a phthalocyanine derivative, or an aromatic amine, for example. As the transition metal oxide, molybdenum oxide, vanadium oxide, ruthenium oxide, tungsten oxide, manganese oxide, or the like can be given. As the phthalocyanine derivative, phthalocyanine, metal phthalocyanine, or the like can be given. As the aromatic amine, a benzidine derivative, a phenylenediamine derivative, or the like can be given. It is also possible to use a high molecular compound such as polythiophene or polyaniline; a typical example thereof is poly(ethylenedioxythiophene)/poly(styrenesulfonic acid), which is self-doped polythiophene.

As the hole-injection layer111, a layer containing a composite material of a hole-transport material and a material having a property of accepting electrons from the hole-transport material can also be used. Alternatively, a stack of a layer containing a material having an electron accepting property and a layer containing a hole-transport material may also be used. In a steady state or in the presence of an electric field, electric charge can be transferred between these materials. As examples of the material having an electron-accepting property, organic acceptors such as a quinodimethane derivative, a chloranil derivative, and a hexaazatriphenylene derivative can be given. A specific example is a compound having an electron-withdrawing group (a halogen group or a cyano group), such as 7,7,8,8-tetracyano-2,3,5,6-tetrafluoroquinodimethane (abbreviation: F4-TCNQ), chloranil, or 2,3,6,7,10,11-hexacyano-1,4,5,8,9,12-hexaazatriphenylene (abbreviation: HAT-CN). Alternatively, a transition metal oxide such as an oxide of a metal from Group4to Group8can also be used. Specifically, vanadium oxide, niobium oxide, tantalum oxide, chromium oxide, molybdenum oxide, tungsten oxide, manganese oxide, rhenium oxide, or the like can be used. In particular, molybdenum oxide is preferable because it is stable in the air, has a low hygroscopic property, and is easily handled.

A material having a property of transporting more holes than electrons can be used as the hole-transport material, and a material having a hole mobility of 1×10−6cm2/Vs or higher is preferable. Specifically, any of the aromatic amine, carbazole derivative, aromatic hydrocarbon, stilbene derivative, and the like described as examples of the hole-transport material that can be used in the light-emitting layer130can be used. Furthermore, the hole-transport material may be a high molecular compound.

The hole-transport layer112is a layer containing a hole-transport material and can be formed using any of the materials given as examples of the material of the hole-injection layer111. In order that the hole-transport layer112has a function of transporting holes injected into the hole-injection layer111to the light-emitting layer150, the HOMO level of the hole-transport layer112is preferably equal or close to the HOMO level of the hole-injection layer111.

As the hole-transport material, any of the materials given as examples of the material of the hole-injection layer111can be used. As the hole-transport material, a substance having a hole mobility of 1×10−6cm2/Vs or higher is preferably used. Note that any substance other than the above substances may be used as long as the hole-transport property is higher than the electron-transport property. The layer including a substance having a high hole-transport property is not limited to a single layer, and two or more layers containing the aforementioned substances may be stacked.

The electron-transport layer118has a function of transporting, to the light-emitting layer150, electrons injected from the other of the pair of electrodes (the electrode101or the electrode102) through the electron-injection layer119. A material having a property of transporting more electrons than holes can be used as the electron-transport material, and a material having an electron mobility of 1×10−6cm2/Vs or higher is preferable. As the compound which easily accepts electrons (the material having an electron-transport property), a π-electron deficient heteroaromatic compound such as a nitrogen-containing heteroaromatic compound, a metal complex, or the like can be used, for example. Specifically, a metal complex having a quinoline ligand, a benzoquinoline ligand, an oxazole ligand, or a thiazole ligand, which are described as the electron-transport materials that can be used in the light-emitting layer130, can be given. Further, an oxadiazole derivative; a triazole derivative, a phenanthroline derivative, a pyridine derivative, a bipyridine derivative, a pyrimidine derivative, and the like can be given. A substance having an electron mobility of higher than or equal to 1×10−6cm2/Vs is preferable. Note that other than these substances, any substance that has a property of transporting more electrons than holes may be used for the electron-transport layer. The electron-transport layer118is not limited to a single layer, and may include stacked two or more layers containing the aforementioned substances.

Between the electron-transport layer118and the light-emitting layer150, a layer that controls transfer of electron carriers may be provided. This is a layer formed by addition of a small amount of a substance having a high electron-trapping property to a material having a high electron-transport property described above, and the layer is capable of adjusting carrier balance by suppressing transfer of electron carriers. Such a structure is very effective in preventing a problem (such as a reduction in element lifetime) caused when electrons pass through the light-emitting layer.

The electron-injection layer119has a function of reducing a barrier for electron injection from the electrode102to promote electron injection and can be formed using a Group1metal or a Group2metal, or an oxide, a halide, or a carbonate of any of the metals, for example. Alternatively, a composite material containing an electron-transport material (described above) and a material having a property of donating electrons to the electron-transport material can also be used. As the material having an electron-donating property, a Group1metal, a Group2metal, an oxide of any of the metals, or the like can be given. Specifically, an alkali metal, an alkaline earth metal, or a compound thereof, such as lithium fluoride (LiF), cesium fluoride (CsF), calcium fluoride (CaF2), or lithium oxide (LiOx), can be used. Alternatively, a rare earth metal compound like erbium fluoride (ErF3) can be used. Electride may also be used for the electron-injection layer119. Examples of the electride include a substance in which electrons are added at high concentration to calcium oxide-aluminum oxide. The electron-injection layer119can be formed using the substance that can be used for the electron-transport layer118.

A composite material in which an organic compound and an electron donor (donor) are mixed may also be used for the electron-injection layer119. Such a composite material is excellent in an electron-injection property and an electron-transport property because electrons are generated in the organic compound by the electron donor. In this case, the organic compound is preferably a material that is excellent in transporting the generated electrons. Specifically, the above-listed substances for forming the electron-transport layer118(e.g., the metal complexes and heteroaromatic compounds) can be used, for example. As the electron donor, a substance showing an electron-donating property with respect to the organic compound may be used. Specifically, an alkali metal, an alkaline earth metal, and a rare earth metal are preferable, and lithium, cesium, magnesium, calcium, erbium, and ytterbium are given. In addition, an alkali metal oxide or an alkaline earth metal oxide is preferable, and lithium oxide, calcium oxide, barium oxide, and the like are given. A Lewis base such as magnesium oxide can also be used. An organic compound such as tetrathiafulvalene (abbreviation: TTF) can also be used.

Note that the light-emitting layer, the hole-injection layer, the hole-transport layer, the electron-transport layer, and the electron-injection layer described above can each be formed by an evaporation method (including a vacuum evaporation method), an inkjet method, a coating method, a gravure printing method, or the like. Besides the above-mentioned materials, an inorganic compound such as a quantum dot or a high molecular compound (e.g., an oligomer, a dendrimer, or a polymer) may be used in the light-emitting layer, the hole-injection layer, the hole-transport layer, the electron-transport layer, and the electron-injection layer.

The quantum dot may be a colloidal quantum dot, an alloyed quantum dot, a core-shell quantum dot, or a core quantum dot, for example. A material containing elements belonging to Groups2and16, elements belonging to Groups13and15, elements belonging to Groups13and17, elements belonging to Groups11and17, or elements belonging to Groups14and15may be used. Alternatively, a quantum dot containing an element such as cadmium (Cd), selenium (Se), zinc (Zn), sulfur (S), phosphorus (P), indium (In), tellurium (Te), lead (Pb), gallium (Ga), arsenic (As), or aluminum (Al) may be used.

A light-emitting element in one embodiment of the present invention may be formed over a substrate of glass, plastic, or the like. As the way of stacking layers over the substrate, layers may be sequentially stacked from the electrode101side or sequentially stacked from the electrode102side.

For the substrate over which the light-emitting element of one embodiment of the present invention can be formed, glass, quartz, plastic, or the like can be used, for example. Alternatively, a flexible substrate can be used. The flexible substrate means a substrate that can be bent, such as a plastic substrate made of polycarbonate or polyarylate, for example. Alternatively, a film, an inorganic vapor deposition film, or the like can be used. Another material may be used as long as the substrate functions as a support in a manufacturing process of the light-emitting element or an optical element or as long as it has a function of protecting the light-emitting element or an optical element.

In this specification and the like, a light-emitting element can be formed using any of a variety of substrates, for example. The type of a substrate is not limited particularly. Examples of the substrate include a semiconductor substrate (e.g., a single crystal substrate or a silicon substrate), an SOI substrate, a glass substrate, a quartz substrate, a plastic substrate, a metal substrate, a stainless steel substrate, a substrate including stainless steel foil, a tungsten substrate, a substrate including tungsten foil, a flexible substrate, an attachment film, paper including a fibrous material, a base material film, and the like. As an example of a glass substrate, a barium borosilicate glass substrate, an aluminoborosilicate glass substrate, a soda lime glass substrate, and the like can be given. Examples of the flexible substrate, the attachment film, the base material film, and the like are substrates of plastics typified by polyethylene terephthalate (PET), polyethylene naphthalate (PEN), polyether sulfone (PES), and polytetrafluoroethylene (PTFE). Another example is a resin such as acrylic. Furthermore, polypropylene, polyester, polyvinyl fluoride, and polyvinyl chloride can be given as examples. Other examples are polyamide, polyimide, aramid, epoxy, an inorganic vapor deposition film, paper, and the like.

Alternatively, a flexible substrate may be used as the substrate such that the light-emitting element is provided directly on the flexible substrate. Further alternatively, a separation layer may be provided between the substrate and the light-emitting element. The separation layer can be used when part or the whole of a light-emitting element formed over the separation layer is separated from the substrate and transferred onto another substrate. In such a case, the light-emitting element can be transferred to a substrate having low heat resistance or a flexible substrate as well. For the above separation layer, a stack including inorganic films, which are a tungsten film and a silicon oxide film, or a structure in which a resin film ofpolyimide or the like is formed over a substrate can be used, for example.

In other words, after the light-emitting element is formed using a substrate, the light-emitting element may be transferred to another substrate. Example of the substrate to which the light-emitting element is transferred are, in addition to the above substrates, a cellophane substrate, a stone substrate, a wood substrate, a cloth substrate (including a natural fiber (e.g., silk, cotton, or hemp), a synthetic fiber (e.g., nylon, polyurethane, or polyester), a regenerated fiber (e.g., acetate, cupra, rayon, or regenerated polyester), and the like), a leather substrate, a rubber substrate, and the like. When such a substrate is used, a light-emitting element with high durability, high heat resistance, reduced weight, or reduced thickness can be formed.

The light-emitting element250may be formed over an electrode electrically connected to a field-effect transistor (FET), for example, that is formed over any of the above-described substrates. Accordingly, an active matrix display device in which the FET controls the driving of the light-emitting element can be manufactured.

In this embodiment, one embodiment of the present invention has been described. Other embodiments of the present invention are described in the other embodiments. One embodiment of the present invention is not limited to the above-described examples. For example, described in one embodiment of the present invention is an example in which an EL layer includes two light-emitting layers where each of the light-emitting layers includes a fluorescent material and one of the light-emitting layers further includes two organic compounds that form an exciplex; however, one embodiment of the present invention is not limited to this example. Depending on the case or situation, for example, the EL layer in one embodiment of the present invention does not necessarily include two light-emitting layers. Furthermore, either or both of the light-emitting layers may include no fluorescent material. Moreover, a structure in which the light-emitting layer does not include two organic compounds that form an exciplex may be employed. Furthermore, a structure in which the two organic compounds do not form an exciplex may be employed. An example of a case in which an exciplex has a function of supplying excited energy to the fluorescent material in one of the two light-emitting layers included in the EL layer or a case in which light emitted from the exciplex has a region overlapping with an absorption band on the lowest energy side of the fluorescent material in one of the two light-emitting layers included in the EL layer has been described in one embodiment of the present invention; however, one embodiment of the present invention is not limited to this example. Depending on the case or situation, for example, in one embodiment of the present invention, an exciplex does not necessarily have a function of supplying excited energy to the fluorescent material in a light-emitting layer included in the EL layer. Further, light emitted from the exciplex does not necessarily have a region overlapping with an absorption band on the lowest energy side of the fluorescent material.

The structure described above in this embodiment can be used in combination with any of the structures described in the other embodiments as appropriate.

In this embodiment, a light-emitting element having a structure different from that described in Embodiment 1 and light emission mechanisms of the light-emitting element are described below with reference toFIGS. 4A to 4C. InFIG. 4A, a portion having a function similar to that inFIG. 1Ais represented by the same hatch pattern as inFIG. 1Aand not especially denoted by a reference numeral in some cases. In addition, common reference numerals are used for portions having similar functions, and a detailed description of the portions is omitted in some cases.

FIG. 4Ais a schematic cross-sectional view of a light-emitting element252.

The light-emitting element252illustrated inFIG. 4Aincludes a plurality of light-emitting units (a light-emitting unit106and a light-emitting unit108inFIG. 4A) between a pair of electrodes (the electrode101and the electrode102). One light-emitting unit has the same structure as the EL layer100illustrated inFIG. 1A. That is, the light-emitting element250inFIG. 1Aincludes one light-emitting unit, while the light-emitting element252includes a plurality of light-emitting units. Note that the electrode101functions as an anode and the electrode102functions as a cathode in the following description of the light-emitting element252; however, the functions may be interchanged in the light-emitting element252.

In the light-emitting element252illustrated inFIG. 4A, the light-emitting unit106and the light-emitting unit108are stacked, and a charge-generation layer115is provided between the light-emitting unit106and the light-emitting unit108. Note that the light-emitting unit106and the light-emitting unit108may have the same structure or different structures. For example, it is preferable that the EL layer100illustrated inFIG. 1Abe used in the light-emitting unit106.

The charge-generation layer115may have either a structure in which an acceptor substance that is an electron acceptor is added to a hole-transport material or a structure in which a donor substance that is an electron donor is added to an electron-transport material. Alternatively, both of these structures may be stacked.

In the case where the charge-generation layer115contains a composite material of an organic compound and an acceptor substance, the composite material that can be used for the hole-injection layer111described in Embodiment 1 may be used for the composite material. As the organic compound, a variety of compounds such as an aromatic amine compound, a carbazole compound, an aromatic hydrocarbon, and a high molecular compound (such as an oligomer, a dendrimer, or a polymer) can be used. A substance having a hole mobility of 1×10−6cm2/Vs or higher is preferably used as the organic compound. Note that any other material may be used as long as it has a property of transporting more holes than electrons. Since the composite material of an organic compound and an acceptor substance has excellent carrier-injection and carrier-transport properties, low-voltage driving or low-current driving can be realized. Note that when a surface of a light-emitting unit on the anode side is in contact with the charge-generation layer115like the light-emitting unit108, the charge-generation layer115can also serve as a hole-injection layer or a hole-transport layer of the light-emitting unit; thus, a hole-injection layer or a hole-transport layer need not be included in the light-emitting unit.

The charge-generation layer115may have a stacked structure of a layer containing the composite material of an organic compound and an acceptor substance and a layer containing another material. For example, the charge-generation layer115may be formed using a combination of a layer containing the composite material of an organic compound and an acceptor substance with a layer containing one compound selected from among electron-donating materials and a compound having a high electron-transport property. Furthermore, the charge-generation layer115may be formed using a combination of a layer containing the composite material of an organic compound and an acceptor substance with a layer containing a transparent conductive material.

The charge-generation layer115provided between the light-emitting unit106and the light-emitting unit108may have any structure as long as electrons can be injected to the light-emitting unit on one side and holes can be injected into the light-emitting unit on the other side when a voltage is applied between the electrode101and the electrode102. For example, inFIG. 4A, the charge-generation layer115injects electrons into the light-emitting unit106and holes into the light-emitting unit108when a voltage is applied such that the potential of the electrode101is higher than that of the electrode102.

Note that in terms of light extraction efficiency, the charge-generation layer115preferably has a visible light transmittance (specifically, a visible light transmittance of higher than or equal to 40%). The charge-generation layer115functions even if it has lower conductivity than the pair of electrodes (the electrodes101and102). In the case where the conductivity of the charge-generation layer115is as high as those of the pair of electrodes, carriers generated in the charge-generation layer115flow toward the film surface direction, so that light is emitted in a region where the electrode101and the electrode102do not overlap, in some cases. To suppress such a defect, the charge-generation layer115is preferably formed using a material whose conductivity is lower than those of the pair of electrodes.

Note that forming the charge-generation layer115by using any of the above materials can suppress an increase in drive voltage caused by the stack of the light-emitting layers.

The light-emitting element having two light-emitting units has been described with reference toFIG. 4A; however, a similar structure can be applied to a light-emitting element in which three or more light-emitting units are stacked. With a plurality of light-emitting units partitioned by the charge-generation layer between a pair of electrodes as in the light-emitting element252, it is possible to provide a light-emitting element which can emit light with high luminance with the current density kept low and has a long lifetime. A light-emitting element with low power consumption can be provided.

When the structure of the EL layer100illustrated inFIG. 1Ais used for at least one of the plurality of units, a light-emitting element with high emission efficiency can be provided.

It is preferable to use the light-emitting layer150illustrated inFIGS. 1A and 1Bas the light-emitting layer140included in the light-emitting unit108. In other words, both the light-emitting unit106and the light-emitting unit108preferably include the light-emitting layer150, in which case light emitted from the light-emitting layer150can be obtained from both of the light-emitting units106and108. Thus, the light-emitting element252can include fluorescent materials as the light-emitting materials, with reduced energy loss, which is preferable.

Alternatively, it is preferable to use the light-emitting layer120illustrated inFIGS. 1A and 1Bas the light-emitting layer140included in the light-emitting unit108. In other words, the light-emitting unit106preferably includes the light-emitting layer150illustrated inFIGS. 1A and 1Band the light-emitting unit108preferably includes the light-emitting layer120illustrated inFIGS. 1A and 1B, in which case light emitted from the light-emitting layer120can be obtained from both of the light-emitting units106and108. Thus, the light-emitting element252can include fluorescent materials as the light-emitting materials, with reduced energy loss, which is preferable.

Further, it is preferable to use the light-emitting layer130illustrated inFIGS. 1A and 1Bas the light-emitting layer140included in the light-emitting unit108. In other words, the light-emitting unit106preferably includes the light-emitting layer150illustrated inFIGS. 1A and 1Band the light-emitting unit108preferably includes the light-emitting layer130illustrated inFIGS. 1A and 1B, in which case light emitted from the light-emitting layer130can be obtained from both of the light-emitting units106and108. Thus, the light-emitting element252can include fluorescent materials as the light-emitting materials, with reduced energy loss, which is preferable.

Note that in each of the above-described structures, the guest materials (fluorescent materials) used in the light-emitting unit106and the light-emitting unit108may be the same or different. In the case where the same guest material is used for the light-emitting unit106and the light-emitting unit108, the light-emitting element252can exhibit high emission luminance at a small current value, which is preferable. In the case where different guest materials are used for the light-emitting unit106and the light-emitting unit108, the light-emitting element252can exhibit multi-color light emission, which is preferable. It is particularly favorable to select the guest materials so that white light emission with high color rendering properties or light emission of at least red, green, and blue can be obtained.

It is preferable that the light-emitting layer of the light-emitting unit.108include a phosphorescent material. In other words, it is preferable that the light-emitting unit106include the light-emitting layer150illustrated inFIGS. 1A and 1Band the light-emitting unit108include the light-emitting layer140including a phosphorescent material. A structure example of the light-emitting element252in this case is described below.

In this embodiment, the light-emitting layer150included in the light-emitting unit106has a structure similar to that of the light-emitting layer150illustrated inFIGS. 1A and 1B. In other words, the light-emitting layer150included in the light-emitting unit106includes the light-emitting layer120and the light-emitting layer130. In a manner similar to that inFIG. 1B, the light-emitting layer120includes the host material121and the guest material122. Furthermore, the light-emitting layer130includes the host material131and the guest material132. The host material131includes the organic compound131_1and the organic compound131_2.

The light-emitting layer140included in the light-emitting unit108includes a host material141and a guest material142as illustrated inFIG. 4B. The host material141includes an organic compound141_1and an organic compound141_2. In the following description, the guest material142included in the light-emitting layer140is a phosphorescent material.

In the light-emitting element252, the electrode101, the electrode102, the hole-injection layers111and116, the hole-transport layers112and117, the electron-transport layers113and118, and the electron-injection layers114and119have functions similar to those of the electrode101, the electrode102, the hole-injection layers111, the hole-transport layer112, the electron-transport layer118, and the electron-injection layer119described in Embodiment 1, respectively. Therefore, a detailed description thereof is omitted in this embodiment.

<Light Emission Mechanism of First Light-Emitting Layer>

The light emission mechanism of the light-emitting layer150is similar to that of the light-emitting layer150illustrated inFIGS. 1A and 1B.

<Light Emission Mechanism of Second Light-Emitting Layer>

Next, a light emission mechanism of the light-emitting layer140is described below.

The organic compound141_1and the organic compound141_2which are included in the light-emitting layer140form an exciplex.

It is acceptable as long as the combination of the organic compound141_1and the organic compound141_2can form an exciplex in the light-emitting layer140, and it is preferred that one organic compound have a hole-transport property and the other organic compound have an electron-transport property.

FIG. 4Cillustrates the correlation of energy levels of the organic compound141_1, the organic compound141_2, and the guest material142in the light-emitting layer140. The following explains what terms and signs inFIG. 4Crepresent:

SPH: the S1level of the organic compound141_1(host material);

TPH: the T1level of the organic compound141_1(host material);

TPG: the T1level of the guest material142(phosphorescent material);

SPE: the S1level of the exciplex; and

TPE: the T1level of the exciplex.

The lowest level of the singlet excited state of the exciplex (SPE) formed by the organic compounds141_1and141_2and the lowest level of the triplet excited state of the exciplex (TPE) are close to each other (see Route C inFIG. 4C).

Both energies of SPEand TPEof the exciplex are then transferred to the lowest level of the triplet excited state of the guest material142(phosphorescent material); thus, light emission is obtained (see Route D inFIG. 4C).

The above-described processes through Route C and Route D may be referred to as exciplex-triplet energy transfer (ExTET) in this specification and the like.

When one of the organic compounds141_1and141_2receives holes and the other receives electrons, the exciplex is formed. Alternatively, when one compound is brought into an excited state, the one interacts with the other compound to form the exciplex. Therefore, most excitons in the light-emitting layer140exist as exciplexes. The band gap of the exciplex is narrower than that of each of the organic compounds141_1and141_2; therefore, an excited state can be formed with lower excitation energy. Thus, the formation of the exciplex can lower the drive voltage of the light-emitting element.

When the light-emitting layer140has the above structure, light emission from the guest material142(phosphorescent material) of the light-emitting layer140can be efficiently obtained.

Note that light emitted from the light-emitting layer150preferably has an emission peak on the shorter wavelength side than light emitted from the light-emitting layer140. Since the luminance of a light-emitting element using a phosphorescent material emitting light with a short wavelength tends to be degraded quickly, fluorescence with a short wavelength is employed so that a light-emitting element with less degradation of luminance can be provided.

Furthermore, the light-emitting layer150and the light-emitting layer140may be made to emit light with different emission wavelengths, so that the light-emitting element can be a multicolor light-emitting element. In that case, the emission spectrum of the light-emitting element is formed by combining light having different emission peaks, and thus has at least two peaks.

The above structure is also suitable for obtaining white light emission. When the light-emitting layer150and the light-emitting layer140emit light of complementary colors, white light emission can be obtained.

In addition, white light emission with a high color rendering property that is formed of three primary colors or four or more colors can be obtained by using a plurality of light-emitting materials emitting light with different wavelengths for one of the light-emitting layers150and140or both. In that case, one of the light-emitting layers150and140or both may be divided into layers and each of the divided layers may contain a light-emitting material different from the others.

<Material that can be Used in Light-Emitting Layers>

Next, materials that can be used in the light-emitting layers150and140are described below.

<<Material that can be Used in Light-Emitting Layer150>>

As a material that can be used in the light-emitting layer150, any of the materials that can be used in the light-emitting layers120and130in Embodiment 1 may be used. Thus, a light-emitting element with high emission efficiency with little energy deactivation can be fabricated.

<<Material that can be Used in Light-Emitting Layer140>>

In the light-emitting layer140, the host material141is present in the largest proportion by weight, and the guest material142(phosphorescent material) is dispersed in the host material141. The T1levels of the host materials141(organic compounds141_1and141_2) of the light-emitting layer140are preferably higher than the T1level of the guest material (the guest material122or the guest material132) of the light-emitting layer150.

Examples of the organic compound141_1include a zinc- or aluminum-based metal complex, an oxadiazole derivative, a triazole derivative, a benzimidazole derivative, a quinoxaline derivative, a dibenzoquinoxaline derivative, a dibenzothiophene derivative, a dibenzofuran derivative, a pyrimidine derivative, a triazine derivative, a pyridine derivative, a bipyridine derivative, a phenanthroline derivative, and the like. Other examples are an aromatic amine, a carbazole derivative, and the like. Specifically, any of the electron-transport materials and the hole-transport materials described in Embodiment 1 can be used.

As the organic compound1412, a substance which can form an exciplex together with the organic compound141_1is used. Specifically, any of the electron-transport materials and the hole-transport materials described in Embodiment 1 can be used. In that case, it is preferable that the organic compound141_1, the organic compound141_2, and the guest material142(phosphorescent material) be selected such that the emission peak of the exciplex formed by the organic compound141_1and the organic compound141_2overlaps with an absorption band, specifically an absorption band on the longest wavelength side, of a triplet metal to ligand charge transfer (MLCT) transition of the guest material142(phosphorescent material). This makes it possible to provide a light-emitting element with drastically improved emission efficiency. Note that in the case where a thermally activated delayed fluorescence material is used instead of the phosphorescent material, it is preferable that the absorption band on the longest wavelength side be a singlet absorption band.

As the guest material142(phosphorescent material), an iridium-, rhodium-, or platinum-based organometallic complex or metal complex can be used; in particular, an organoiridium complex such as an iridium-based ortho-metalated complex is preferable. As an ortho-metalated ligand, a 4H-triazole ligand, a 1H-triazole ligand, an imidazole ligand, a pyridine ligand, a pyrimidine ligand, a pyrazine ligand, an isoquinoline ligand, and the like can be given. As the metal complex, a platinum complex having a porphyrin ligand and the like can be given.

Examples of the substance that has an emission peak in the yellow or red wavelength range include organometallic iridium complexes having a pyrimidine skeleton, such as (diisobutyrylmethanato)bis[4,6-bis(3-methylphenyl)pyrimidinato]iridium(III) (abbreviation: Ir(5mdppm)2(dibm)), bis[4,6-bis(3-methylphenyl)pyrimidinato](dipivaloylmethanato)iridium(III) (abbreviation: Ir(5mdppm)2(dpm)), and bis[4,6-di(naphthalen-1-yl)pyrimidinato](dipivaloylmethanato)iridium(III) (abbreviation: Ir(dlnpm)2(dpm)); organometallic iridium complexes having a pyrazine skeleton, such as (acetylacetonato)bis(2,3,5-triphenylpyrazinato)iridium(II) (abbreviation: Ir(tppr)2(acac)), bis(2,3,5-triphenylpyrazinato) (dipivaloylmethanato)iridium(II) (abbreviation: Ir(tppr)2(dpm)), and (acetylacetonato)bis[2,3-bis(4-fluorophenyl)quinoxalinato]iridium(III) (abbreviation: Ir(Fdpq)2(acac)); organometallic iridium complexes having a pyridine skeleton, such as tris(1-phenylisoquinolinato-N,C2′)iridium(III) (abbreviation: Ir(piq)3) and bis(1-phenylisoquinolinato-N,C2′)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 rare earth metal complexes such as tris(1,3-diphenyl-1,3-propanedionato)(monophenanthroline)europium(III) (abbreviation: Eu(DBM)3(Phen)) and tris[1-(2-thenoyl)-3,3,3-trifluoroacetonato](monophenanthroline)europium(III) (abbreviation: Eu(TTA)3(Phen)). Among the materials given above, the organometallic iridium complexes having a pyrimidine skeleton have distinctively high reliability and emission efficiency and are thus particularly preferable. Further, the organometallic iridium complexes having a pyrazine skeleton can provide red light emission with favorable chromaticity.

As the light-emitting material included in the light-emitting layer140, any material can be used as long as the material can convert the triplet excitation energy into light emission. As an example of the material that can convert the triplet excitation energy into light emission, a thermally activated delayed fluorescence (TADF) material can be given in addition to a phosphorescent material. Therefore, it is acceptable that the “phosphorescent material” in the description is replaced with the “thermally activated delayed fluorescence material”. Note that the thermally activated delayed fluorescence material is a material having a small difference between the triplet excited energy level and the singlet excited energy level and a function of converting triplet excited energy into singlet excited energy by reverse intersystem crossing. Thus, the TADF material can up-convert a triplet excited state into a singlet excited state (i.e., reverse intersystem crossing is possible) using a little thermal energy and efficiently exhibit light emission (fluorescence) from the singlet excited state. The TADF is efficiently obtained under the condition where the difference in energy between the triplet excited energy level and the singlet excited energy level is preferably greater than 0 eV and less than or equal to 0.2 eV, further preferably greater than 0 eV and less than or equal to 0.1 eV.

The material that exhibits thermally activated delayed fluorescence may be a material that can form a singlet excited state by itself from a triplet excited state by reverse intersystem crossing or may be a combination of a plurality of materials which form an exciplex.

In the case where the thermally activated delayed fluorescence material is formed of one kind of material, any of the thermally activated delayed fluorescence materials described in Embodiment 1 can be specifically used.

In the case where the thermally activated delayed fluorescence material is used as the host material, it is preferable to use a combination of two kinds of compounds which form an exciplex. In this case, it is particularly preferable to use the above-described combination of a compound which easily accepts electrons and a compound which easily accepts holes, which form an exciplex.

There is no limitation on the emission colors of the light-emitting materials contained in the light-emitting layers150and140, and they may be the same or different. Light emitted from the light-emitting materials is mixed and extracted out of the element; therefore, for example, in the case where their emission colors are complementary colors, the light-emitting element can emit white light. In consideration of the reliability of the light-emitting element, the emission peak wavelength of the light-emitting material included in the light-emitting layer150is preferably shorter than that of the light-emitting material included in the light-emitting layer140.

Note that the light-emitting units106and108and the charge-generation layer115can be formed by an evaporation method (including a vacuum evaporation method), an ink-jet method, a coating method, gravure printing, or the like.

The structure described above in this embodiment can be used in combination with any of the structures described in the other embodiments as appropriate.

In this embodiment, examples of light-emitting elements having structures different from those described in Embodiments 1 and 2 are described below with reference toFIGS. 5A and 5B,FIGS. 6A and 6B, andFIGS. 7A to 7E.

FIGS. 5A and 5Bare cross-sectional views each illustrating a light-emitting element of one embodiment of the present invention. InFIGS. 5A and 5B, a portion having a function similar to that inFIG. 1Ais represented by the same hatch pattern as inFIG. 1Aand not especially denoted by a reference numeral in some cases. In addition, common reference numerals are used for portions having similar functions, and a detailed description of the portions is omitted in some cases.

Light-emitting elements254and256inFIGS. 5A and 5Bmay have a bottom-emission structure in which light is extracted through the substrate200or may have a top-emission structure in which light emitted from the light-emitting element is extracted in the direction opposite to the substrate200. However, one embodiment of the present invention is not limited to this structure, and a light-emitting element having a dual-emission structure in which light emitted from the light-emitting element is extracted in both top and bottom directions of the substrate200may be used.

In the case where the light-emitting elements254and256each have a bottom emission structure, the electrode101preferably has a function of transmitting light and the electrode102preferably has a function of reflecting light. Alternatively, in the case where the light-emitting elements254and256each have a top emission structure, the electrode101preferably has a function of reflecting light and the electrode102preferably has a function of transmitting light.

The light-emitting elements254and256each include the electrode101and the electrode102over the substrate200. Between the electrodes101and102, a light-emitting layer123B, a light-emitting layer123G, and a light-emitting layer123R are provided. The hole-injection layer111, the hole-transport layer112, the electron-transport layer118, and the electron-injection layer119are also provided.

The light-emitting element256includes, as part of the electrode101, a conductive layer101a, a conductive layer101bover the conductive layer101a, and a conductive layer101cunder the conductive layer101a. In other words, the light-emitting element256includes the electrode101having a structure in which the conductive layer101ais sandwiched between the conductive layer101band the conductive layer101c.

In the light-emitting element256, the conductive layer101band the conductive layer101cmay be formed of different materials or the same material. The electrode101preferably has a structure in which the conductive layer101ais sandwiched by the layers formed of the same conductive material, in which case patterning by etching can be performed easily.

In the light-emitting element256, the electrode101may include one of the conductive layer101band the conductive layer101c.

For each of the conductive layers101a,101b, and101c, which are included in the electrode101, the structure and materials of the electrode101or102described in Embodiment 1 can be used.

InFIGS. 5A and 5B, a partition wall145is provided between a region221B, a region221G, and a region221R, which are sandwiched between the electrode101and the electrode102. The partition wall145has an insulating property. The partition wall145covers end portions of the electrode101and has openings overlapping with the electrode. With the partition wall145, the electrode101provided over the substrate200in the regions can be divided into island shapes.

Note that the light-emitting layer123B and the light-emitting layer123G may overlap with each other in a region where they overlap with the partition wall145. The light-emitting layer123G and the light-emitting layer123R may overlap with each other in a region where they overlap with the partition wall145. The light-emitting layer123R and the light-emitting layer123B may overlap with each other in a region where they overlap with the partition wall145.

The partition wall145has an insulating property and is formed using an inorganic or organic material. Examples of the inorganic material include silicon oxide, silicon oxynitride, silicon nitride oxide, silicon nitride, aluminum oxide, and aluminum nitride. Examples of the organic material include photosensitive resin materials such as an acrylic resin and a polyimide resin.

The light-emitting layers123R,123G, and123B preferably contain light-emitting materials having functions of emitting light of different colors. For example, when the light-emitting layer123R contains a light-emitting material having a function of emitting red, the region221R emits red light. When the light-emitting layer123G contains a light-emitting material having a function of emitting green, the region221G emits green light. When the light-emitting layer123B contains a light-emitting material having a function of emitting blue, the region221B emits blue light. The light-emitting element254or256having such a structure is used in a pixel of a display device, whereby a full-color display device can be fabricated. The thicknesses of the light-emitting layers may be the same or different.

Any one or more of the light-emitting layers123B,123G, and123R preferably include the light-emitting layer150described in Embodiment 1, in which case a light-emitting element with high emission efficiency can be fabricated.

One or more of the light-emitting layers123B,123G, and123R may include two or more stacked layers.

When at least one light-emitting layer includes the light-emitting layer described in Embodiment 1, which has little energy loss, as described above and the light-emitting element254or256including the light-emitting layer is used in pixels in a display device, a display device with high emission efficiency can be fabricated. The display device including the light-emitting element254or256can thus have reduced power consumption.

By providing a color filter over the electrode through which light is extracted, the color purity of each of the light-emitting elements254and256can be improved. Therefore, the color purity of a display device including the light-emitting element254or256can be improved.

By providing a polarizing plate over the electrode through which light is extracted, the reflection of external light by each of the light-emitting elements254and256can be reduced. Therefore, the contrast ratio of a display device including the light-emitting element254or256can be improved.

For the other components of the light-emitting elements254and256, the components of the light-emitting element in Embodiment 1 or 2 may be referred to.

Next, structure examples different from the light-emitting elements illustrated inFIGS. 5A and 5Bwill be described below with reference toFIGS. 6A and 6B.

FIGS. 6A and 6Bare cross-sectional views of a light-emitting element of one embodiment of the present invention. InFIGS. 6A and 6B, a portion having a function similar to that inFIGS. 5A and 5Bis represented by the same hatch pattern as inFIGS. 5A and 5Band not especially denoted by a reference numeral in some cases. In addition, common reference numerals are used for portions having similar functions, and a detailed description of such portions is not repeated in some cases.

FIGS. 6A and 6Billustrate structure examples of a light-emitting element including the light-emitting layer between a pair of electrodes. A light-emitting element258illustrated inFIG. 6Ahas a top-emission structure in which light is extracted in a direction opposite to the substrate200, and a light-emitting element260illustrated inFIG. 6Bhas a bottom-emission structure in which light is extracted to the substrate200side. However, one embodiment of the present invention is not limited to these structures and may have a dual-emission structure in which light emitted from the light-emitting element is extracted in both top and bottom directions with respect to the substrate200over which the light-emitting element is formed.

The light-emitting elements258and260each include the electrode101, the electrode102, an electrode103, and an electrode104over the substrate200. The light-emitting layer150is included between the electrode101and the electrode102, between the electrode102and the electrode103, and between the electrode102and the electrode104. The hole-injection layer111, the hole-transport layer112, the electron-transport layer118, and the electron-injection layer119are also included. The light-emitting layer150includes the light-emitting layer120and the light-emitting layer130.

The electrode101includes a conductive layer101aand a conductive layer101bover and in contact with the conductive layer101a. The electrode103includes a conductive layer103aand a conductive layer103bover and in contact with the conductive layer103a. The electrode104includes a conductive layer104aand a conductive layer104bover and in contact with the conductive layer104a.

The light-emitting element258illustrated inFIG. 6Aand the light-emitting element260illustrated inFIG. 6Beach include a partition wall145between a region222B sandwiched between the electrode101and the electrode102, a region222G sandwiched between the electrode102and the electrode103, and a region222R sandwiched between the electrode102and the electrode104. The partition wall145has an insulating property. The partition wall145covers end portions of the electrodes101,103, and104and has openings overlapping with the electrodes. With the partition wall145, the electrodes provided with the substrate200in the regions can be separated into island shapes.

The light-emitting elements258and260each include a substrate220provided with an optical element224B, an optical element224G, and an optical element224R in the direction in which light emitted from the region222B, light emitted from the region222G, and light emitted from the region222R are extracted. The light emitted from each region is emitted outside the light-emitting element through each optical element. In other words, the light from the region222B, the light from the region222G, and the light from the region222R are emitted through the optical element224B, the optical element224G, and the optical element224R, respectively.

The optical elements224B,224G, and224R each have a function of selectively transmitting light of a particular color out of incident light. For example, the light emitted from the region222B through the optical element224B is blue light, the light emitted from the region222G through the optical element224G is green light, and the light emitted from the region222R through the optical element224R is red light.

For example, a coloring layer (also referred to as color filter), a band pass filter, a multilayer filter, or the like can be used for the optical elements224R,224G, and224B. Alternatively, color conversion elements can be used as the optical elements. A color conversion element is an optical element that converts incident light into light having a longer wavelength than the incident light. As the color conversion elements, quantum-dot elements can be favorably used. The usage of the quantum-dot type can increase color reproducibility of the display device.

A plurality of optical elements may also be stacked over each of the optical elements224R,224G, and224B. As another optical element, a circularly polarizing plate, an anti-reflective film, or the like can be provided, for example. A circularly polarizing plate provided on the side where light emitted from the light-emitting element of the display device is extracted can prevent a phenomenon in which light entering from the outside of the display device is reflected inside the display device and returned to the outside. An anti-reflective film can weaken external light reflected by a surface of the display device. This leads to clear observation of light emitted from the display device.

Note that inFIGS. 6A and 6B, blue light (B), green light (G), and red light (R) emitted from the regions through the optical elements are schematically illustrated by arrows of dashed lines.

A light-blocking layer223is provided between the optical elements. The light-blocking layer223has a function of blocking light emitted from the adjacent regions. Note that a structure without the light-blocking layer223may also be employed.

The light-blocking layer223has a function of reducing the reflection of external light. The light-blocking layer223has a function of preventing mixture of light emitted from an adjacent light-emitting element. As the light-blocking layer223, a metal, a resin containing black pigment, carbon black, a metal oxide, a composite oxide containing a solid solution of a plurality of metal oxides, or the like can be used.

As for the substrate200and the substrate220provided with the optical elements, Embodiment 1 can be referred to.

Light emitted from the light-emitting layer120and the light-emitting layer130resonates between a pair of electrodes (e.g., the electrode101and the electrode102). The light-emitting layer120and the light-emitting layer130are formed at such a position as to intensify the light of a desired wavelength among light to be emitted. For example, by adjusting the optical length from a reflective region of the electrode101to the light-emitting region of the light-emitting layer120and the optical length from a reflective region of the electrode102to the light-emitting region of the light-emitting layer120, the light of a desired wavelength among light emitted from the light-emitting layer120can be intensified. By adjusting the optical length from the reflective region of the electrode101to the light-emitting region of the light-emitting layer130and the optical length from the reflective region of the electrode102to the light-emitting region of the light-emitting layer130, the light of a desired wavelength among light emitted from the light-emitting layer130can be intensified. In the case of a light-emitting element in which a plurality of light-emitting layers (here, the light-emitting layers120and130) are stacked, the optical lengths of the light-emitting layers120and130are preferably optimized.

In each of the light-emitting elements258and260, by adjusting the thicknesses of the conductive layers (the conductive layer101b, the conductive layer103b, and the conductive layer104b) in each region, the light of a desired wavelength among light emitted from the light-emitting layers120and130can be increased. Note that the thickness of at least one of the hole-injection layer111and the hole-transport layer112may differ between the regions to increase the light emitted from the light-emitting layers120and130.

For example, in the case where the refractive index of the conductive material having a function of reflecting light in the electrodes101to104is lower than the refractive index of the light-emitting layer120or130, the thickness of the conductive layer101bof the electrode101is adjusted so that the optical length between the electrode101and the electrode102is mλB/2 (m is a natural number and λBis the wavelength of light intensified in the region222B). Similarly, the thickness of the conductive layer103bof the electrode103is adjusted so that the optical length between the electrode103and the electrode102is m′λG/2 (m′ is a natural number and λGis the wavelength of light intensified in the region222G). Furthermore, the thickness of the conductive layer104bof the electrode104is adjusted so that the optical length between the electrode104and the electrode102is m″λR/2 (m″ is a natural number and λRis the wavelength of light intensified in the region222R).

In the above manner, with the microcavity structure, in which the optical length between the pair of electrodes in the respective regions is adjusted, scattering and absorption of light in the vicinity of the electrodes can be suppressed, resulting in high light extraction efficiency. In the above structure, the conductive layers101b,103b, and104bpreferably have a function of transmitting light. The materials of the conductive layers101b,103b, and104bmay be the same or different. Each of the conductive layers101b,103b, and104bmay have a stacked structure of two or more layers.

Since the light-emitting element258illustrated inFIG. 6Ahas a top-emission structure, it is preferable that the conductive layer101a, the conductive layer103a, and the conductive layer104ahave a function of reflecting light. In addition, it is preferable that the electrode102have functions of transmitting light and reflecting light.

Since the light-emitting element260illustrated inFIG. 6Bhas a bottom-emission structure, it is preferable that the conductive layer101a, the conductive layer103a, and the conductive layer104ahave functions of transmitting light and reflecting light. In addition, it is preferable that the electrode102have a function of reflecting light.

In each of the light-emitting elements258and260, the conductive layers101a,103a, and104amay be formed of different materials or the same material. When the conductive layers101a,103a, and104aare formed of the same material, manufacturing cost of the light-emitting elements258and260can be reduced. Note that each of the conductive layers101a,103a, and104amay have a stacked structure including two or more layers.

The structures of the light-emitting layers120and130included in the light-emitting layer150described in Embodiment 1 are preferably used for the light-emitting layers120and130included in the light-emitting layer150in the light-emitting elements258and260. In this way, the light-emitting elements can have high emission efficiency.

Either or both of the light-emitting layers120and130may have a stacked structure of two layers. The two light-emitting layers each including two kinds of light-emitting materials (a first light-emitting material and a second light-emitting material) for emitting different colors of light enable emission of light of a plurality of colors. It is particularly preferable to select the light-emitting materials of the light-emitting layers so that white light can be obtained by combining light emissions from the light-emitting layers120and130.

Either or both of the light-emitting layers120and130may have a stacked structure of three or more layers, in which a layer not including a light-emitting material may be included.

In the above-described manner, by using the light-emitting element258or260including the light-emitting layer having the structure described in Embodiment 1, in which a fluorescent material is included and energy loss is reduced, in pixels in a display device, a display device with high emission efficiency can be fabricated. Accordingly, the display device including the light-emitting element258or260can have low power consumption.

For the other components of the light-emitting elements258and260, the components of the light-emitting element254or256or the light-emitting element in Embodiment 1 or 2 may be referred to.

Next, a method for fabricating a light-emitting element of one embodiment of the present invention is described below with reference toFIGS. 7A to 7E. Here, a method for fabricating the light-emitting element258illustrated inFIG. 6Ais described.

FIGS. 7A to 7Eare cross-sectional views illustrating a method for fabricating the light-emitting element of one embodiment of the present invention.

The method for fabricating the light-emitting element258described below includes first to sixth steps.

In the first step, the electrodes (specifically the conductive layer101aof the electrode101, the conductive layer103aof the electrode103, and the conductive layer104aof the electrode104) of the light-emitting elements are formed over the substrate200(seeFIG. 7A).

In this embodiment, a conductive layer having a function of reflecting light is formed over the substrate200and processed into a desired shape; whereby the conductive layers101a,103a, and104aare formed. As the conductive layer having a function of reflecting light, an alloy film of silver, palladium, and copper (also referred to as an Ag—Pd—Cu film and APC) is used. The conductive layers101a,103a, and104aare preferably formed through a step of processing the same conductive layer, because the manufacturing cost can be reduced.

Note that a plurality of transistors may be formed over the substrate200before the first step. The plurality of transistors may be electrically connected to the conductive layers101a,103a, and104a.

In the second step, the transparent conductive layer101bhaving a function of transmitting light is formed over the conductive layer101aof the electrode101, the transparent conductive layer103bhaving a function of transmitting light is formed over the conductive layer103aof the electrode103, and the transparent conductive layer104bhaving a function of transmitting light is formed over the conductive layer104aof the electrode104(seeFIG. 7B).

In this embodiment, the conductive layers101b,103b, and104beach having a function of transmitting light are formed over the conductive layers101a,103a, and104aeach having a function of reflecting light, respectively, whereby the electrode101, the electrode103, and the electrode104are formed. As the conductive layers101b,103b, and104b, ITSO films are used.

The conductive layers101b,103b, and104bhaving a function of transmitting light may be formed in a plurality of steps. When the conductive layers101b,103b, and104bhaving a function of transmitting light are formed in a plurality of steps, they can be formed to have thicknesses which enable microcavity structures appropriate in the respective regions.

In the third step, the partition wall145that covers end portions of the electrodes of the light-emitting element is formed (seeFIG. 7C).

The partition wall145includes an opening overlapping with the electrode. The conductive film exposed by the opening functions as the anode of the light-emitting element. As the partition wall145, a polyimide-based resin is used in this embodiment.

In the first to third steps, since there is no possibility of damaging the EL layer (a layer containing an organic compound), a variety of film formation methods and micromachining technologies can be employed. In this embodiment, a reflective conductive layer is formed by a sputtering method, a pattern is formed over the conductive layer by a lithography method, and then the conductive layer is processed into an island shape by a dry etching method or a wet etching method to form the conductive layer101aof the electrode101, the conductive layer103aof the electrode103, and the conductive layer104aof the electrode104. Then, a transparent conductive film is formed by a sputtering method, a pattern is formed over the transparent conductive film by a lithography method, and then the transparent conductive film is processed into island shapes by a wet etching method to form the electrodes101,103, and104.

In the fourth step, the hole-injection layer111, the hole-transport layer112, the light-emitting layer120, the light-emitting layer130, the electron-transport layer118, the electron-injection layer119, and the electrode102are formed (seeFIG. 7D).

The hole-injection layer111can be formed by co-evaporating a hole-transport material and a material containing an acceptor substance. Note that a co-evaporation method is an evaporation method in which a plurality of different substances are concurrently vaporized from respective different evaporation sources. The hole-transport layer112can be formed by evaporating a hole-transport material.

The light-emitting layer120can be formed by evaporating a guest material that emits light of at least one color selected from violet, blue, and blue green. As the guest material, a fluorescent organic compound can be used. The fluorescent organic compound may be evaporated alone or the fluorescent organic compound mixed with another material may be evaporated. For example, the fluorescent organic compound may be used as a guest material, and the guest material may be dispersed into a host material having higher excited energy than the guest material.

The light-emitting layer130can be formed by evaporating a guest material that emits light of at least one color selected from green, yellow green, yellow, orange, and red. As the guest material, a fluorescent organic compound can be used. The fluorescent organic compound may be evaporated alone or the fluorescent organic compound mixed with another material may be evaporated. The fluorescent organic compound may be used as a guest material, and the guest material may be dispersed into a host material having higher excited energy than the guest material. The light-emitting layer130may have a two-layer structure. In such a case, the two light-emitting layers each preferably contain a light-emitting material that emits light of a different color.

The electron-transport layer118can be formed by evaporating a substance having a high electron-transport property. The electron-injection layer119can be formed by evaporating a substance having a high electron-injection property.

The electrode102can be formed by stacking a reflective conductive film and a light-transmitting conductive film. The electrode102may have a single-layer structure or a stacked-layer structure.

Through the above-described steps, the light-emitting element including the region222B, the region222G, and the region222R over the electrode101, the electrode103, and the electrode104, respectively, are formed over the substrate200.

In the fifth step, the light-blocking layer223, the optical element224B, the optical element224G, and the optical element224R are formed over the substrate220(seeFIG. 7E).

As the light-blocking layer223, a resin fihn containing black pigment is formed in a desired region. Then, the optical element224B, the optical element224G, and the optical element224R are formed over the substrate220and the light-blocking layer223. As the optical element224B, a resin film containing blue pigment is formed in a desired region. As the optical element224G, a resin film containing green pigment is formed in a desired region. As the optical element224R, a resin film containing red pigment is formed in a desired region.

In the sixth step, the light-emitting element formed over the substrate200is attached to the light-blocking layer223, the optical element224B, the optical element224G, and the optical element224R formed over the substrate220, and sealed with a sealant (not illustrated).

The structure described above in this embodiment can be used in combination with any of the structures described in the other embodiments as appropriate.

In this embodiment, a display device of one embodiment of the present invention will be described below with reference toFIGS. 8A and 8B,FIGS. 9A and 9B,FIG. 10,FIGS. 11A and 11B,FIGS. 12A and 12B,FIG. 13, andFIGS. 14A and 14B.

<Structure Example 1 of Display Device>

FIG. 8Ais a top view illustrating a display device600andFIG. 8Bis a cross-sectional view taken along the dashed-dotted line A-B and the dashed-dotted line C-D inFIG. 8A. The display device600includes driver circuit portions (a signal line driver circuit portion601and a scan line driver circuit portion603) and a pixel portion602. Note that the signal line driver circuit portion601, the scan line driver circuit portion603, and the pixel portion602have a function of controlling light emission of a light-emitting element.

The display device600also includes an element substrate610, a sealing substrate604, a sealant605, a region607surrounded by the sealant605, a lead wiring608, and an FPC609.

Note that the lead wiring608is a wiring for transmitting signals to be input to the signal line driver circuit portion601and the scan line driver circuit portion603and for receiving a video signal, a clock signal, a start signal, a reset signal, and the like from the FPC609serving as an external input terminal. Although only the FPC609is illustrated here, the FPC609may be provided with a printed wiring board (PWB).

As the signal line driver circuit portion601, a CMOS circuit in which an n-channel transistor623and a p-channel transistor624are combined is formed. As the signal line driver circuit portion601or the scan line driver circuit portion603, various types of circuits such as a CMOS circuit, a PMOS circuit, or an NMOS circuit can be used. Although a driver in which a driver circuit portion is formed and a pixel are formed over the same surface of a substrate in the display device of this embodiment, the driver circuit portion is not necessarily formed over the substrate and can be formed outside the substrate.

The pixel portion602includes a switching transistor611, a current control transistor612, and a lower electrode613electrically connected to a drain of the current control transistor612. Note that a partition wall614is formed to cover end portions of the lower electrode613. As the partition wall614, for example, a positive type photosensitive acrylic resin film can be used.

In order to obtain favorable coverage, the partition wall614is formed to have a curved surface with curvature at its upper or lower end portion. For example, in the case of using a positive photosensitive acrylic as a material of the partition wall614, it is preferable that only the upper end portion of the partition wall614have a curved surface with curvature (the radius of the curvature being 0.2 μm to 3 μm). As the partition wall614, either a negative photosensitive resin or a positive photosensitive resin can be used.

Note that there is no particular limitation on a structure of each of the transistors (the transistors611,612,623, and624). For example, a staggered transistor can be used. In addition, there is no particular limitation on the polarity of these transistors. For these transistors, n-channel and p-channel transistors may be used, or either n-channel transistors or p-channel transistors may be used, for example. Furthermore, there is no particular limitation on the crystallinity of a semiconductor film used for these transistors. For example, an amorphous semiconductor film or a crystalline semiconductor film may be used. Examples of a semiconductor material include Group14semiconductors (e.g., a semiconductor including silicon), compound semiconductors (including oxide semiconductors), organic semiconductors, and the like. For example, it is preferable to use an oxide semiconductor that has an energy gap of 2 eV or more, preferably 2.5 eV or more and further preferably 3 eV or more, for the transistors, so that the off-state current of the transistors can be reduced. Examples of the oxide semiconductor include an In—Ga oxide and an In-M-Zn oxide (M is aluminum (Al), gallium (Ga), yttrium (Y), zirconium (Zr), lanthanum (La), cerium (Ce), tin (Sn), hafnium (Hf), or neodymium (Nd)).

An EL layer616and an upper electrode617are formed over the lower electrode613. Here, the lower electrode613functions as an anode and the upper electrode617functions as a cathode.

In addition, the EL layer616is formed by various methods such as an evaporation method with an evaporation mask, an ink-jet method, or a spin coating method. As another material included in the EL layer616, a low molecular compound or a high molecular compound (including an oligomer or a dendrimer) may be used.

Note that a light-emitting element618is formed with the lower electrode613, the EL layer616, and the upper electrode617. The light-emitting element618has any of the structures described in Embodiments 1 to 3. In the case where the pixel portion includes a plurality of light-emitting elements, the pixel portion may include both any of the light-emitting elements described in Embodiments 1 to 3 and a light-emitting element having a different structure.

When the sealing substrate604and the element substrate610are attached to each other with the sealant605, the light-emitting element618is provided in the region607surrounded by the element substrate610, the sealing substrate604, and the sealant605. The region607is filled with a filler. In some cases, the region607is filled with an inert gas (nitrogen, argon, or the like) or filled with an ultraviolet curable resin or a thermosetting resin which can be used for the sealant605. For example, a polyvinyl chloride (PVC)-based resin, an acrylic-based resin, a polyimide-based resin, an epoxy-based resin, a silicone-based resin, a polyvinyl butyral (PVB)-based resin, or an ethylene vinyl acetate (EVA)-based resin can be used. It is preferable that the sealing substrate be provided with a recessed portion and a desiccant be provided in the recessed portion, in which case deterioration due to influence of moisture can be inhibited.

An optical element621is provided below the sealing substrate604to overlap with the light-emitting element618. A light-blocking layer622is provided below the sealing substrate604. The structures of the optical element621and the light-blocking layer622can be the same as those of the optical element and the light-blocking layer in Embodiment 3, respectively.

An epoxy-based resin or glass frit is preferably used for the sealant605. It is preferable that such a material do not transmit moisture or oxygen as much as possible. As the sealing substrate604, a glass substrate, a quartz substrate, or a plastic substrate formed of fiber reinforced plastic (FRP), poly(vinyl fluoride) (PVF), polyester, acrylic, or the like can be used.

In the above-described manner, the display device including any of the light-emitting elements and the optical elements which are described in Embodiments 1 to 3 can be obtained.

<Structure Example 2 of Display Device>

Next, another example of the display device is described with reference toFIGS. 9A and 9BandFIG. 10. Note thatFIGS. 9A and 9BandFIG. 10are each a cross-sectional view of a display device of one embodiment of the present invention.

InFIG. 9A, a substrate1001, a base insulating film1002, a gate insulating film1003, gate electrodes1006,1007, and1008, a first interlayer insulating film1020, a second interlayer insulating film1021, a peripheral portion1042, a pixel portion1040, a driver circuit portion1041, lower electrodes1024R,1024G, and1024B of light-emitting elements, a partition wall1025, an EL layer1028, an upper electrode1026of the light-emitting elements, a sealing layer1029, a sealing substrate1031, a sealant1032, and the like are illustrated.

InFIG. 9A, examples of the optical elements, coloring layers (a red coloring layer1034R, a green coloring layer1034G, and a blue coloring layer1034B) are provided on a transparent base material1033. Further, a light-blocking layer1035may be provided. The transparent base material1033provided with the coloring layers and the light-blocking layer is positioned and fixed to the substrate1001. Note that the coloring layers and the light-blocking layer are covered with an overcoat layer1036. In the structure inFIG. 9A, red light, green light, and blue light transmit the coloring layers, and thus an image can be displayed with the use of pixels of three colors.

FIG. 9Billustrates an example in which, as examples of the optical elements, the coloring layers (the red coloring layer1034R, the green coloring layer1034G, and the blue coloring layer1034B) are provided between the gate insulating film1003and the first interlayer insulating film1020. As in this structure, the coloring layers may be provided between the substrate1001and the sealing substrate1031.

FIG. 10illustrates an example in which, as examples of the optical elements, the coloring layers (the red coloring layer1034R, the green coloring layer1034G, and the blue coloring layer1034B) are provided between the first interlayer insulating film1020and the second interlayer insulating film1021. As in this structure, the coloring layers may be provided between the substrate1001and the sealing substrate1031.

The above-described display device has a structure in which light is extracted from the substrate1001side where the transistors are formed (a bottom-emission structure), but may have a structure in which light is extracted from the sealing substrate1031side (a top-emission structure).

<Structure Example 3 of Display Device>

FIGS. 11A and 11Bare each an example of a cross-sectional view of a display device having a top emission structure. Note thatFIGS. 11A and 11Bare each a cross-sectional view illustrating the display device of one embodiment of the present invention, and the driver circuit portion1041, the peripheral portion1042, and the like, which are illustrated inFIGS. 9A and 9BandFIG. 10, are not illustrated therein.

In this case, as the substrate1001, a substrate that does not transmit light can be used. The process up to the step of forming a connection electrode which connects the transistor and the anode of the light-emitting element is performed in a manner similar to that of the display device having a bottom-emission structure. Then, a third interlayer insulating film1037is formed to cover an electrode1022. This insulating film may have a planarization function. The third interlayer insulating film1037can be formed using a material similar to that of the second interlayer insulating film, or can be formed using any other various materials.

The lower electrodes1024R,1024G, and1024B of the light-emitting elements each function as an anode here, but may function as a cathode. Further, in the case of a display device having a top-emission structure as illustrated inFIGS. 11A and 11B, the lower electrodes1024R,1024G, and1024B preferably have a function of reflecting light. The upper electrode1026is provided over the EL layer1028. It is preferable that the upper electrode1026have a function of reflecting light and a function of transmitting light and that a microcavity structure be used between the upper electrode1026and the lower electrodes1024R,1024G, and1024B, in which case the intensity of light having a specific wavelength is increased.

In the case of a top-emission structure as illustrated inFIG. 11A, sealing can be performed with the sealing substrate1031on which the coloring layers (the red coloring layer1034R, the green coloring layer1034G, and the blue coloring layer1034B) are provided. The sealing substrate1031may be provided with the light-blocking layer1035which is positioned between pixels. Note that a light-transmitting substrate is favorably used as the sealing substrate1031.

FIG. 11Aillustrates the structure provided with the light-emitting elements and the coloring layers for the light-emitting elements as an example; however, the structure is not limited thereto. For example, as shown inFIG. 11B, a structure including the red coloring layer1034R and the blue coloring layer1034B but not including a green coloring layer may be employed to achieve full color display with the three colors of red, green, and blue. The structure as illustrated inFIG. 11Awhere the light-emitting elements are provided with the coloring layers is effective to suppress reflection of external light. In contrast, the structure as illustrated inFIG. 11Bwhere the light-emitting elements are provided with the red coloring layer and the blue coloring layer and without the green coloring layer is effective to reduce power consumption because of small energy loss of light emitted from the green light-emitting element.

<Structure Example 4 of Display Device>

Although a display device including sub-pixels of three colors (red, green, and blue) is described above, the number of colors of sub-pixels may be four (red, green, blue, and yellow, or red, green, blue, and white).FIGS. 12A and 12B,FIG. 13, andFIGS. 14A and 14Billustrate structures of display devices each including the lower electrodes1024R,1024G,1024B, and1024Y.FIGS. 12A and 12BandFIG. 13each illustrate a display device having a structure in which light is extracted from the substrate1001side on which transistors are formed (bottom-emission structure), andFIGS. 14A and 14Beach illustrate a display device having a structure in which light is extracted from the sealing substrate1031side (top-emission structure).

FIG. 12Aillustrates an example of a display device in which optical elements (the coloring layer1034R, the coloring layer1034G, the coloring layer1034B, and a coloring layer1034Y) are provided on the transparent base material1033.FIG. 12Billustrates an example of a display device in which optical elements (the coloring layer1034R, the coloring layer1034G, the coloring layer1034B, and the coloring layer1034Y) are provided between the gate insulating film1003and the first interlayer insulating film1020.FIG. 13illustrates an example of a display device in which optical elements (the coloring layer1034R, the coloring layer1034G, the coloring layer1034B, and the coloring layer1034Y) are provided between the first interlayer insulating film1020and the second interlayer insulating film1021.

The coloring layer1034R transmits red light, the coloring layer1034G transmits green light, and the coloring layer1034B transmits blue light. The coloring layer1034Y transmits yellow light or transmits light of a plurality of colors selected from blue, green, yellow, and red. When the coloring layer1034Y can transmit light of a plurality of colors selected from blue, green, yellow, and red, light released from the coloring layer1034Y may be white light. Since the light-emitting element which transmits yellow or white light has high emission efficiency, the display device including the coloring layer1034Y can have lower power consumption.

In the top-emission display devices illustrated inFIGS. 14A and 14B, a light-emitting element including the lower electrode1024Y preferably has a microcavity structure between the lower electrode and the upper electrode1026as in the display device illustrated inFIG. 11A. In the display device illustrated inFIG. 14A, sealing can be performed with the sealing substrate1031on which the coloring layers (the red coloring layer1034R, the green coloring layer1034G, the blue coloring layer1034B, and the yellow coloring layer1034Y) are provided.

Light emitted through the microcavity and the yellow coloring layer1034Y has an emission spectrum in a yellow region. Since yellow is a color with a high luminosity factor, a light-emitting element emitting yellow light has high emission efficiency. Therefore, the display device ofFIG. 14Acan reduce power consumption.

FIG. 14Aillustrates the structure provided with the light-emitting elements and the coloring layers for the light-emitting elements as an example; however, the structure is not limited thereto. For example, as shown inFIG. 14B, a structure including the red coloring layer1034R, the green coloring layer1034G, and the blue coloring layer1034B but not including a yellow coloring layer may be employed to achieve full color display with the four colors of red, green, blue, and yellow or of red, green, blue, and white. The structure as illustrated inFIG. 14Awhere the light-emitting elements are provided with the coloring layers is effective to suppress reflection of external light. In contrast, the structure as illustrated inFIG. 14Bwhere the light-emitting elements are provided with the red coloring layer, the green coloring layer, and the blue coloring layer and without the yellow coloring layer is effective to reduce power consumption because of small energy loss of light emitted from the yellow or white light-emitting element.

The structure described in this embodiment can be combined as appropriate with any of the other structures in this embodiment and the other embodiments.

In this embodiment, a display device including a light-emitting element of one embodiment of the present invention will be described with reference toFIGS. 15A and 15B,FIGS. 16A and 16B, andFIGS. 17A and 17B.

FIG. 15Ais a block diagram illustrating the display device of one embodiment of the present invention, andFIG. 15Bis a circuit diagram illustrating a pixel circuit of the display device of one embodiment of the present invention.

<Description of Display Device>

The display device illustrated inFIG. 15Aincludes a region including pixels of display elements (the region is hereinafter referred to as a pixel portion802), a circuit portion provided outside the pixel portion802and including circuits for driving the pixels (the portion is hereinafter referred to as a driver circuit portion804), circuits having a function of protecting elements (the circuits are hereinafter referred to as protection circuits806), and a terminal portion807. Note that the protection circuits806are not necessarily provided.

A part or the whole of the driver circuit portion804is preferably formed over a substrate over which the pixel portion802is formed, in which case the number of components and the number of terminals can be reduced. When a part or the whole of the driver circuit portion804is not formed over the substrate over which the pixel portion802is formed, the part or the whole of the driver circuit portion804can be mounted by COG or tape automated bonding (TAB).

The pixel portion802includes a plurality of circuits for driving display elements arranged in X rows (X is a natural number of 2 or more) and Y columns (Y is a natural number of 2 or more) (such circuits are hereinafter referred to as pixel circuits801). The driver circuit portion804includes driver circuits such as a circuit for supplying a signal (scan signal) to select a pixel (the circuit is hereinafter referred to as a scan line driver circuit804a) and a circuit for supplying a signal (data signal) to drive a display element in a pixel (the circuit is hereinafter referred to as a signal line driver circuit804b).

The scan line driver circuit804aincludes a shift register or the like. Through the terminal portion807, the scan line driver circuit804areceives a signal for driving the shift register and outputs a signal. For example, the scan line driver circuit804areceives a start pulse signal, a clock signal, or the like and outputs a pulse signal. The scan line driver circuit804ahas a function of controlling the potentials of wirings supplied with scan signals (such wirings are hereinafter referred to as scan lines GL_1to GL_X). Note that a plurality of scan line driver circuits804amay be provided to control the scan lines GL_1to GL_X separately. Alternatively, the scan line driver circuit804ahas a function of supplying an initialization signal. Without being limited thereto, the scan line driver circuit804acan supply another signal.

The signal line driver circuit804bincludes a shift register or the like. The signal line driver circuit804breceives a signal (video signal) from which a data signal is derived, as well as a signal for driving the shift register, through the terminal portion807. The signal line driver circuit804bhas a function of generating a data signal to be written to the pixel circuit801which is based on the video signal. In addition, the signal line driver circuit804bhas a function of controlling output of a data signal in response to a pulse signal produced by input of a start pulse signal, a clock signal, or the like. Furthermore, the signal line driver circuit804bhas a function of controlling the potentials of wirings supplied with data signals (such wirings are hereinafter referred to as data lines DL_1to DL_Y). Alternatively, the signal line driver circuit804bhas a function of supplying an initialization signal. Without being limited thereto, the signal line driver circuit804bcan supply another signal.

The signal line driver circuit804bincludes a plurality of analog switches or the like, for example. The signal line driver circuit804bcan output, as the data signals, signals obtained by time-dividing the video signal by sequentially turning on the plurality of analog switches. The signal line driver circuit804bmay include a shift register or the like.

A pulse signal and a data signal are input to each of the plurality of pixel circuits801through one of the plurality of scan lines GL supplied with scan signals and one of the plurality of data lines DL supplied with data signals, respectively. Writing and holding of the data signal to and in each of the plurality of pixel circuits801are controlled by the scan line driver circuit804a. For example, to the pixel circuit801in the m-th row and the n-th column (nz is a natural number of less than or equal to X, and n is a natural number of less than or equal to Y), a pulse signal is input from the scan line driver circuit804athrough the scan line GL_nm, and a data signal is input from the signal line driver circuit804bthrough the data line DL_n in accordance with the potential of the scan line GL_m.

The protection circuit806shown inFIG. 15Ais connected to, for example, the scan line GL between the scan line driver circuit804aand the pixel circuit801. Alternatively, the protection circuit806is connected to the data line DL between the signal line driver circuit804band the pixel circuit801. Alternatively, the protection circuit806can be connected to a wiring between the scan line driver circuit804aand the terminal portion807. Alternatively, the protection circuit806can be connected to a wiring between the signal line driver circuit804band the terminal portion807. Note that the terminal portion807means a portion having terminals for inputting power, control signals, and video signals to the display device from external circuits.

The protection circuit806is a circuit that electrically connects a wiring connected to the protection circuit to another wiring when a potential out of a certain range is applied to the wiring connected to the protection circuit.

As illustrated inFIG. 15A, the protection circuits806are provided for the pixel portion802and the driver circuit portion804, so that the resistance of the display device to overcurrent generated by electrostatic discharge (ESD) or the like can be improved. Note that the configuration of the protection circuits806is not limited to that, and for example, a configuration in which the protection circuits806are connected to the scan line driver circuit804aor a configuration in which the protection circuits806are connected to the signal line driver circuit804bmay be employed. Alternatively, the protection circuits806may be configured to be connected to the terminal portion807.

InFIG. 15A, an example in which the driver circuit portion804includes the scan line driver circuit804aand the signal line driver circuit804bis shown; however, the structure is not limited thereto. For example, only the scan line driver circuit804amay be formed and a separately prepared substrate where a signal line driver circuit is formed (e.g., a driver circuit substrate formed with a single crystal semiconductor film or a polycrystalline semiconductor film) may be mounted.

<Structure Example of Pixel Circuit>

Each of the plurality of pixel circuits801inFIG. 15Acan have a structure illustrated inFIG. 15B, for example.

One of a source electrode and a drain electrode of the transistor852is electrically connected to a wiring to which a data signal is supplied (a data line DL_n). A gate electrode of the transistor852is electrically connected to a wiring to which a gate signal is supplied (a scan line GL_m).

The transistor852has a function of controlling whether to write a data signal.

One of a pair of electrodes of the capacitor862is electrically connected to a wiring to which a potential is supplied (hereinafter referred to as a potential supply line VL_a), and the other is electrically connected to the other of the source electrode and the drain electrode of the transistor852.

The capacitor862functions as a storage capacitor for storing written data.

One of a source electrode and a drain electrode of the transistor854is electrically connected to the potential supply line VL_a. Furthermore, a gate electrode of the transistor854is electrically connected to the other of the source electrode and the drain electrode of the transistor852.

One of an anode and a cathode of the light-emitting element872is electrically connected to a potential supply line VL_b, and the other is electrically connected to the other of the source electrode and the drain electrode of the transistor854.

As the light-emitting element872, any of the light-emitting elements described in Embodiments 1 to 3 can be used.

Note that a high power supply potential VDD is supplied to one of the potential supply line VL_a and the potential supply line VL_b, and a low power supply potential VSS is supplied to the other.

In the display device including the pixel circuits801inFIG. 15B, the pixel circuits801are sequentially selected row by row by the scan line driver circuit804ainFIG. 15A, for example, whereby the transistors852are turned on and a data signal is written.

When the transistors852are turned off, the pixel circuits801in which the data has been written are brought into a holding state. Furthermore, the amount of current flowing between the source electrode and the drain electrode of the transistor854is controlled in accordance with the potential of the written data signal. The light-emitting element872emits light with a luminance corresponding to the amount of flowing current. This operation is sequentially performed row by row; thus, an image is displayed.

Alternatively, the pixel circuit can have a function of compensating variation in threshold voltages or the like of a transistor.FIGS. 16A and 16BandFIGS. 17A and 17Billustrate examples of the pixel circuit.

The pixel circuit illustrated inFIG. 16Aincludes six transistors (transistors303_1to303_6), a capacitor304, and a light-emitting element305. The pixel circuit illustrated inFIG. 16Ais electrically connected to wirings301_1to301_5and wirings302_1and302_2. Note that as the transistors303_1to303_6, for example, p-channel transistors can be used.

The pixel circuit shown inFIG. 16Bhas a configuration in which a transistor303_7is added to the pixel circuit shown inFIG. 16A. The pixel circuit illustrated inFIG. 16Bis electrically connected to wirings301_6and301_7. The wirings301_5and301_6may be electrically connected to each other. Note that as the transistor303_7, for example, a p-channel transistor can be used.

The pixel circuit shown inFIG. 17Aincludes six transistors (transistors308_1to308_6), the capacitor304, and the light-emitting element305. The pixel circuit illustrated inFIG. 17Ais electrically connected to wirings306_1to306_3and wirings307_1to307_3. The wirings306_1and306_3may be electrically connected to each other. Note that as the transistors308_1to308_6, for example, p-channel transistors can be used.

The pixel circuit illustrated inFIG. 17Bincludes two transistors (transistors309_1and309_2), two capacitors (capacitors304_1and304_2), and the light-emitting element305. The pixel circuit illustrated inFIG. 17Bis electrically connected to wirings311_1to311_3and wirings312_1and312_2. With the configuration of the pixel circuit illustrated inFIG. 17B, the pixel circuit can be driven by a voltage inputting current driving method (also referred to as CVCC). Note that as the transistors309_1and3092, for example, p-channel transistors can be used.

A light-emitting element of one embodiment of the present invention can be used for an active matrix method in which an active element is included in a pixel of a display device or a passive matrix method in which an active element is not included in a pixel of a display device.

In the active matrix method, as an active element (a non-linear element), not only a transistor but also a variety of active elements (non-linear elements) can be used. For example, a metal insulator metal (MIM), a thin film diode (TFD), or the like can also be used. Since these elements can be formed with a smaller number of manufacturing steps, manufacturing cost can be reduced or yield can be improved. Alternatively, since the size of these elements is small, the aperture ratio can be improved, so that power consumption can be reduced and higher luminance can be achieved.

As a method other than the active matrix method, the passive matrix method in which an active element (a non-linear element) is not used can also be used. Since an active element (a non-linear element) is not used, the number of manufacturing steps is small, so that manufacturing cost can be reduced or yield can be improved. Alternatively, since an active element (a non-linear element) is not used, the aperture ratio can be improved, so that power consumption can be reduced or higher luminance can be achieved, for example.

The structure described in this embodiment can be combined with any of the structures described in the other embodiments as appropriate.

In this embodiment, a display device including a light-emitting element of one embodiment of the present invention and an electronic device in which the display device is provided with an input device will be described with reference toFIGS. 18A and 18B,FIGS. 19A to 19C,FIGS. 20A and 20B,FIGS. 21A and 21B, andFIG. 22.

In this embodiment, a touch panel2000including a display device and an input device will be described as an example of an electronic device. In addition, an example in which a touch sensor is included as an input device will be described.

FIGS. 18A and 18Bare perspective views of the touch panel2000. Note thatFIGS. 18A and 18Billustrate only main components of the touch panel2000for simplicity.

The touch panel2000includes a display device2501and a touch sensor2595(seeFIG. 18B). The touch panel2000also includes a substrate2510, a substrate2570, and a substrate2590. The substrate2510, the substrate2570, and the substrate2590each have flexibility. Note that one or all of the substrates2510,2570, and2590may be inflexible.

The display device2501includes a plurality of pixels over the substrate2510and a plurality of wirings2511through which signals are supplied to the pixels. The plurality of wirings2511are led to a peripheral portion of the substrate2510, and parts of the plurality of wirings2511form a terminal2519. The terminal2519is electrically connected to an FPC2509(1). The plurality of wirings2511can supply signals from a signal line driver circuit2503s(1) to the plurality of pixels.

The substrate2590includes the touch sensor2595and a plurality of wirings2598electrically connected to the touch sensor2595. The plurality of wirings2598are led to a peripheral portion of the substrate2590, and parts of the plurality of wirings2598form a terminal. The terminal is electrically connected to an FPC2509(2). Note that inFIG. 18B, electrodes, wirings, and the like of the touch sensor2595provided on the back side of the substrate2590(the side facing the substrate2510) are indicated by solid lines for clarity.

As the touch sensor2595, a capacitive touch sensor can be used. Examples of the capacitive touch sensor are a surface capacitive touch sensor and a projected capacitive touch sensor.

Examples of the projected capacitive touch sensor are a self capacitive touch sensor and a mutual capacitive touch sensor, which differ mainly in the driving method. The use of a mutual capacitive type is preferable because multiple points can be sensed simultaneously.

Note that the touch sensor2595illustrated inFIG. 18Bis an example of using a projected capacitive touch sensor.

Note that a variety of sensors that can sense proximity or touch of a sensing target such as a finger can be used as the touch sensor2595.

The projected capacitive touch sensor2595includes electrodes2591and electrodes2592. The electrodes2591are electrically connected to any of the plurality of wirings2598, and the electrodes2592are electrically connected to any of the other wirings2598.

The electrodes2592each have a shape of a plurality of quadrangles arranged in one direction with one corner of a quadrangle connected to one corner of another quadrangle as illustrated inFIGS. 18A and 18B.

The electrodes2591each have a quadrangular shape and are arranged in a direction intersecting with the direction in which the electrodes2592extend.

A wiring2594electrically connects two electrodes2591between which the electrode2592is positioned. The intersecting area of the electrode2592and the wiring2594is preferably as small as possible. Such a structure allows a reduction in the area of a region where the electrodes are not provided, reducing variation in transmittance. As a result, variation in luminance of light passing through the touch sensor2595can be reduced.

Note that the shapes of the electrodes2591and the electrodes2592are not limited thereto and can be any of a variety of shapes. For example, a structure may be employed in which the plurality of electrodes2591are arranged so that gaps between the electrodes2591are reduced as much as possible, and the electrodes2592are spaced apart from the electrodes2591with an insulating layer interposed therebetween to have regions not overlapping with the electrodes2591. In this case, it is preferable to provide, between two adjacent electrodes2592, a dummy electrode electrically insulated from these electrodes because the area of regions having different transmittances can be reduced.

<Description of Display Device>

Next, the display device2501will be described in detail with reference toFIG. 19A.FIG. 19Acorresponds to a cross-sectional view taken along dashed-dotted line X1-X2inFIG. 18B.

The display device2501includes a plurality of pixels arranged in a matrix. Each of the pixels includes a display element and a pixel circuit for driving the display element.

In the following description, an example of using a light-emitting element that emits white light as a display element will be described; however, the display element is not limited to such an element. For example, light-emitting elements that emit light of different colors may be included so that the light of different colors can be emitted from adjacent pixels.

For the substrate2510and the substrate2570, for example, a flexible material with a vapor permeability of lower than or equal to 1×10−5g·m2·day−1, preferably lower than or equal to 1×10−6g·m−2·day−1can be favorably used. Alternatively, materials whose thermal expansion coefficients are substantially equal to each other are preferably used for the substrate2510and the substrate2570. For example, the coefficients of linear expansion of the materials are preferably lower than or equal to 1×10−3/K, further preferably lower than or equal to 5×10−5/K, and still further preferably lower than or equal to 1×10−5/K.

Note that the substrate2510is a stacked body including an insulating layer2510afor preventing impurity diffusion into the light-emitting element, a flexible substrate2510b, and an adhesive layer2510cfor attaching the insulating layer2510aand the flexible substrate2510bto each other. The substrate2570is a stacked body including an insulating layer2570afor preventing impurity diffusion into the light-emitting element, a flexible substrate2570b, and an adhesive layer2570cfor attaching the insulating layer2570aand the flexible substrate2570bto each other.

For the adhesive layer2510cand the adhesive layer2570c, for example, polyester, polyolefin, polyamide (e.g., nylon, aramid), polyimide, polycarbonate, or acrylic, urethane, or epoxy can be used. Alternatively, a material that includes a resin having a siloxane bond can be used.

A sealing layer2560is provided between the substrate2510and the substrate2570. The sealing layer2560preferably has a refractive index higher than that of air. In the case where light is extracted to the sealing layer2560side as illustrated inFIG. 19A, the sealing layer2560can also serve as an optical adhesive layer.

A sealant may be formed in the peripheral portion of the sealing layer2560. With the use of the sealant, a light-emitting element2550R can be provided in a region surrounded by the substrate2510, the substrate2570, the sealing layer2560, and the sealant. Note that an inert gas (such as nitrogen and argon) may be used instead of the sealing layer2560. A drying agent may be provided in the inert gas so as to adsorb moisture or the like. An ultraviolet curable resin or a heat curable resin may be used. An epoxy-based resin or a glass frit is preferably used as the sealant. As a material used for the sealant, a material which is impermeable to moisture and oxygen is preferably used.

The display device2501includes a pixel2502R. The pixel2502R includes a light-emitting module2580R.

The pixel2502R includes the light-emitting element2550R and a transistor2502tthat can supply electric power to the light-emitting element2550R. Note that the transistor2502tfunctions as part of the pixel circuit. The light-emitting module2580R includes the light-emitting element2550R and a coloring layer2567R.

The light-emitting element2550R includes a lower electrode, an upper electrode, and an EL layer between the lower electrode and the upper electrode. As the light-emitting element2550R, any of the light-emitting elements described in Embodiments 1 to 3 can be used.

A microcavity structure may be employed between the lower electrode and the upper electrode so as to increase the intensity of light having a specific wavelength.

In the case where the sealing layer2560is provided on the light extraction side, the sealing layer2560is in contact with the light-emitting element2550R and the coloring layer2567R.

The coloring layer2567R is positioned in a region overlapping with the light-emitting element2550R. Accordingly, part of light emitted from the light-emitting element2550R passes through the coloring layer2567R and is emitted to the outside of the light-emitting module2580R as indicated by an arrow inFIG. 19A.

The display device2501includes a light-blocking layer2567BM on the light extraction side. The light-blocking layer2567BM is provided so as to surround the coloring layer2567R.

The coloring layer2567R is a coloring layer having a function of transmitting light in a particular wavelength region. For example, a color filter for transmitting light in a red wavelength region, a color filter for transmitting light in a green wavelength region, a color filter for transmitting light in a blue wavelength region, a color filter for transmitting light in a yellow wavelength region, or the like can be used. Each color filter can be formed with any of various materials by a printing method, an inkjet method, an etching method using a photolithography technique, or the like.

An insulating layer2521is provided in the display device2501. The insulating layer2521covers the transistor2502t. Note that the insulating layer2521has a function of covering unevenness caused by the pixel circuit. The insulating layer2521may have a function of suppressing impurity diffusion. This can prevent the reliability of the transistor2502tor the like from being lowered by impurity diffusion.

The light-emitting element2550R is formed over the insulating layer2521. A partition2528is provided so as to overlap with an end portion of the lower electrode of the light-emitting element2550R. Note that a spacer for controlling the distance between the substrate2510and the substrate2570may be formed over the partition2528.

A scan line driver circuit2503g(1) includes a transistor2503tand a capacitor2503c. Note that the driver circuit can be formed in the same process and over the same substrate as those of the pixel circuits.

The wirings2511through which signals can be supplied are provided over the substrate2510. The terminal2519is provided over the wirings2511. The FPC2509(1) is electrically connected to the terminal2519. The FPC2509(1) has a function of supplying a video signal, a clock signal, a start signal, a reset signal, or the like. Note that the FPC2509(1) may be provided with a PWB.

In the display device2501, transistors with any of a variety of structures can be used.FIG. 19Aillustrates an example of using bottom-gate transistors; however, the present invention is not limited to this example, and top-gate transistors may be used in the display device2501as illustrated inFIG. 19B.

In addition, there is no particular limitation on the polarity of the transistor2502tand the transistor2503t. For these transistors, n-channel and p-channel transistors may be used, or either n-channel transistors or p-channel transistors may be used, for example. Furthermore, there is no particular limitation on the crystallinity of a semiconductor film used for the transistors2502tand2503t. For example, an amorphous semiconductor film or a crystalline semiconductor film may be used. Examples of semiconductor materials include Group14semiconductors (e.g., a semiconductor including silicon), compound semiconductors (including oxide semiconductors), organic semiconductors, and the like. An oxide semiconductor that has an energy gap of 2 eV or more, preferably 2.5 eV or more, further preferably 3 eV or more is preferably used for one of the transistors2502tand2503tor both, so that the off-state current of the transistors can be reduced. Examples of the oxide semiconductors include an In—Ga oxide, an In-M-Zn oxide (Mrepresents Al, Ga, Y, Zr, La, Ce, Sn, Hf, or Nd), and the like.

<Description of Touch Sensor>

Next, the touch sensor2595will be described in detail with reference toFIG. 19C.FIG. 19Ccorresponds to a cross-sectional view taken along dashed-dotted line X3-X4inFIG. 18B.

The touch sensor2595includes the electrodes2591and the electrodes2592provided in a staggered arrangement on the substrate2590, an insulating layer2593covering the electrodes2591and the electrodes2592, and the wiring2594that electrically connects the adjacent electrodes2591to each other.

The electrodes2591and the electrodes2592are formed using a light-transmitting conductive material. As a light-transmitting conductive material, a conductive oxide such as indium oxide, indium tin oxide, indium zinc oxide, zinc oxide, or zinc oxide to which gallium is added can be used. Note that a film including graphene may be used as well. The film including graphene can be formed, for example, by reducing a film containing graphene oxide. As a reducing method, a method with application of heat or the like can be employed.

The electrodes2591and the electrodes2592may be formed by, for example, depositing a light-transmitting conductive material on the substrate2590by a sputtering method and then removing an unnecessary portion by any of various pattern forming techniques such as photolithography.

Examples of a material for the insulating layer2593are a resin such as an acrylic resin or an epoxy resin, a resin having a siloxane bond, and an inorganic insulating material such as silicon oxide, silicon oxynitride, or aluminum oxide.

Openings reaching the electrodes2591are formed in the insulating layer2593, and the wiring2594electrically connects the adjacent electrodes2591. A light-transmitting conductive material can be favorably used as the wiring2594because the aperture ratio of the touch panel can be increased. Moreover, a material with higher conductivity than the conductivities of the electrodes2591and2592can be favorably used for the wiring2594because electric resistance can be reduced.

One electrode2592extends in one direction, and a plurality of electrodes2592are provided in the form of stripes. The wiring2594intersects with the electrode2592.

Adjacent electrodes2591are provided with one electrode2592provided therebetween. The wiring2594electrically connects the adjacent electrodes2591.

Note that the plurality of electrodes2591are not necessarily arranged in the direction orthogonal to one electrode2592and may be arranged to intersect with one electrode2592at an angle of more than 0 degrees and less than 90 degrees.

The wiring2598is electrically connected to any of the electrodes2591and2592. Part of the wiring2598functions as a terminal. For the wiring2598, a metal material such as aluminum, gold, platinum, silver, nickel, titanium, tungsten, chromium, molybdenum, iron, cobalt, copper, or palladium or an alloy material containing any of these metal materials can be used.

Note that an insulating layer that covers the insulating layer2593and the wiring2594may be provided to protect the touch sensor2595.

A connection layer2599electrically connects the wiring2598to the FPC2509(2).

As the connection layer2599, any of various anisotropic conductive films (ACF), anisotropic conductive pastes (ACP), or the like can be used.

Next, the touch panel2000will be described in detail with reference toFIG. 20A.FIG. 20Acorresponds to a cross-sectional view taken along dashed-dotted line X5-X6inFIG. 18A.

In the touch panel2000illustrated inFIG. 20A, the display device2501described with reference toFIG. 19Aand the touch sensor2595described with reference toFIG. 19Care attached to each other.

The touch panel2000illustrated inFIG. 20Aincludes an adhesive layer2597and an anti-reflective layer2567pin addition to the components described with reference toFIGS. 19A and 19C.

The adhesive layer2597is provided in contact with the wiring2594. Note that the adhesive layer2597attaches the substrate2590to the substrate2570so that the touch sensor2595overlaps with the display device2501. The adhesive layer2597preferably has a light-transmitting property. A heat curable resin or an ultraviolet curable resin can be used for the adhesive layer2597. For example, an acrylic resin, a urethane-based resin, an epoxy-based resin, or a siloxane-based resin can be used.

The anti-reflective layer2567pis positioned in a region overlapping with pixels. As the anti-reflective layer2567p, a circularly polarizing plate can be used, for example.

Next, a touch panel having a structure different from that illustrated inFIG. 20Awill be described with reference toFIG. 20B.

FIG. 20Bis a cross-sectional view of a touch panel2001. The touch panel2001illustrated inFIG. 20Bdiffers from the touch panel2000illustrated inFIG. 20Ain the position of the touch sensor2595relative to the display device2501. Different parts are described in detail below, and the above description of the touch panel2000is referred to for the other similar parts.

The coloring layer2567R is positioned in a region overlapping with the light-emitting element2550R. The light-emitting element2550R illustrated inFIG. 20Bemits light to the side where the transistor2502tis provided. Accordingly, part of light emitted from the light-emitting element2550R passes through the coloring layer2567R and is emitted to the outside of the light-emitting module2580R as indicated by an arrow inFIG. 20B.

The touch sensor2595is provided on the substrate2510side of the display device2501.

The adhesive layer2597is provided between the substrate2510and the substrate2590and attaches the touch sensor2595to the display device2501.

As illustrated inFIG. 20A or 20B, light may be emitted from the light-emitting element through one or both of the substrate2510and the substrate2570.

<Description of Method for Driving Touch Panel>

Next, an example of a method for driving a touch panel will be described with reference toFIGS. 21A and 21B.

FIG. 21Ais a block diagram illustrating the structure of a mutual capacitive touch sensor.FIG. 21Aillustrates a pulse voltage output circuit2601and a current sensing circuit2602. Note that inFIG. 21A, six wirings X1to X6represent the electrodes2621to which a pulse voltage is applied, and six wirings Y1to Y6represent the electrodes2622that detect changes in current.FIG. 21Aalso illustrates capacitors2603that are each formed in a region where the electrodes2621and2622overlap with each other. Note that functional replacement between the electrodes2621and2622is possible.

The pulse voltage output circuit2601is a circuit for sequentially applying a pulse voltage to the wirings X1to X6. By application of a pulse voltage to the wirings X1to X6, an electric field is generated between the electrodes2621and2622of the capacitor2603. When the electric field between the electrodes is shielded, for example, a change occurs in the capacitor2603(mutual capacitance). The approach or contact of a sensing target can be sensed by utilizing this change.

The current sensing circuit2602is a circuit for detecting changes in current flowing through the wirings Y1to Y6that are caused by the change in mutual capacitance in the capacitor2603. No change in current value is detected in the wirings Y1to Y6when there is no approach or contact of a sensing target, whereas a decrease in current value is detected when mutual capacitance is decreased owing to the approach or contact of a sensing target. Note that an integrator circuit or the like is used for sensing of current values.

FIG. 21Bis a timing chart showing input and output waveforms in the mutual capacitive touch sensor illustrated inFIG. 21A. InFIG. 21B, sensing of a sensing target is performed in all the rows and columns in one frame period.FIG. 21Bshows a period when a sensing target is not sensed (not touched) and a period when a sensing target is sensed (touched). Sensed current values of the wirings Y1to Y6are shown as the waveforms of voltage values.

A pulse voltage is sequentially applied to the wirings X1to X6, and the waveforms of the wirings Y1to Y6change in accordance with the pulse voltage. When there is no approach or contact of a sensing target, the waveforms of the wirings Y1to Y6change in accordance with changes in the voltages of the wirings X1to X6. The current value is decreased at the point of approach or contact of a sensing target and accordingly the waveform of the voltage value changes.

By detecting a change in mutual capacitance in this manner, the approach or contact of a sensing target can be sensed.

AlthoughFIG. 21Aillustrates a passive matrix type touch sensor in which only the capacitor2603is provided at the intersection of wirings as a touch sensor, an active matrix type touch sensor including a transistor and a capacitor may be used.FIG. 22illustrates an example of a sensor circuit included in an active matrix type touch sensor.

A signal G2is input to a gate of the transistor2613. A voltage VRES is applied to one of a source and a drain of the transistor2613, and one electrode of the capacitor2603and a gate of the transistor2611are electrically connected to the other of the source and the drain of the transistor2613. One of a source and a drain of the transistor2611is electrically connmected to one of a source and a drain of the transistor2612, and a voltage VSS is applied to the other of the source and the drain of the transistor2611. A signal G1is input to a gate of the transistor2612, and a wiring ML is electrically connected to the other of the source and the drain of the transistor2612. The voltage VSS is applied to the other electrode of the capacitor2603.

Next, the operation of the sensor circuit inFIG. 22will be described. First, a potential for turning on the transistor2613is supplied as the signal G2, and a potential with respect to the voltage VRES is thus applied to the node n connected to the gate of the transistor2611. Then, a potential for turning off the transistor2613is applied as the signal G2, whereby the potential of the node n is maintained.

Then, mutual capacitance of the capacitor2603changes owing to the approach or contact of a sensing target such as a finger, and accordingly the potential of the node n is changed from VRES.

In reading operation, a potential for turning on the transistor2612is supplied as the signal G1. A current flowing through the transistor2611, that is, a current flowing through the wiring ML is changed in accordance with the potential of the node n. By sensing this current, the approach or contact of a sensing target can be sensed.

In each of the transistors2611,2612, and2613, an oxide semiconductor layer is preferably used as a semiconductor layer in which a channel region is formed. In particular, such a transistor is preferably used as the transistor2613so that the potential of the node n can be held for a long time and the frequency of operation of resupplying VRES to the node n (refresh operation) can be reduced.

The structure described in this embodiment can be combined with any of the structures described in the other embodiments as appropriate.

In this embodiment, a display module and electronic devices including a light-emitting element of one embodiment of the present invention will be described with reference toFIG. 23andFIGS. 24A to 24G.

In a display module8000inFIG. 23, a touch sensor8004connected to an FPC8003, a display device8006connected to an FPC8005, a frame8009, a printed board8010, and a battery8011are provided between an upper cover8001and a lower cover8002.

The light-emitting element of one embodiment of the present invention can be used for the display device8006, for example.

The shapes and sizes of the upper cover8001and the lower cover8002can be changed as appropriate in accordance with the sizes of the touch sensor8004and the display device8006.

The touch sensor8004can be a resistive touch sensor or a capacitive touch sensor and may be formed to overlap with the display device8006. A counter substrate (sealing substrate) of the display device8006can have a touch sensor function. A photosensor may be provided in each pixel of the display device8006so that an optical touch sensor is obtained.

The frame8009protects the display device8006and also serves as an electromagnetic shield for blocking electromagnetic waves generated by the operation of the printed board8010. The frame8009may serve as a radiator plate.

The printed board8010has a power supply circuit and a signal processing circuit for outputting a video signal and a clock signal. As a power source for supplying power to the power supply circuit, an external commercial power source or the battery8011provided separately may be used. The battery8011can be omitted in the case of using a commercial power source.

The display module8000can be additionally provided with a member such as a polarizing plate, a retardation plate, or a prism sheet.

The electronic devices illustrated inFIGS. 24A to 24Gcan have a variety of functions, for example, a function of displaying a variety of data (a still image, a moving image, a text image, and the like) on the display portion, a touch sensor function, a function of displaying a calendar, date, time, and the like, a function of controlling a process with a variety of software (programs), a wireless communication function, a function of being connected to a variety of computer networks with a wireless communication function, a function of transmitting and receiving a variety of data with a wireless communication function, a function of reading a program or data stored in a memory medium and displaying the program or data on the display portion, and the like. Note that functions that can be provided for the electronic devices illustrated inFIGS. 24A to 24Gare not limited to those described above, and the electronic devices can have a variety of functions. Although not illustrated inFIGS. 24A to 24G, the electronic devices may include a plurality of display portions. The electronic devices may have a camera or the like and a function of taking a still image, a function of taking a moving image, a function of storing the taken image in a memory medium (an external memory medium or a memory medium incorporated in the camera), a function of displaying the taken image on the display portion, or the like.

The electronic devices illustrated inFIGS. 24A to 24Gwill be described in detail below.

FIG. 24Ais a perspective view of a portable information terminal9100. The display portion9001of the portable information terminal9100is flexible. Therefore, the display portion9001can be incorporated along a bent surface of a bent housing9000. In addition, the display portion9001includes a touch sensor, and operation can be performed by touching the screen with a finger, a stylus, or the like. For example, when an icon displayed on the display portion9001is touched, an application can be started.

FIG. 24Bis a perspective view of a portable information terminal9101. The portable information terminal9101functions as, for example, one or more of a telephone set, a notebook, and an information browsing system. Specifically, the portable information terminal can be used as a smartphone. Note that the speaker9003, the connection terminal9006, the sensor9007, and the like, which are not shown inFIG. 24B, can be positioned in the portable information terminal9101as in the portable information terminal9100shown inFIG. 24A. The portable information terminal9101can display characters and image information on its plurality of surfaces. For example, three operation buttons9050(also referred to as operation icons, or simply, icons) can be displayed on one surface of the display portion9001. Furthermore, information9051indicated by dashed rectangles can be displayed on another surface of the display portion9001. Examples of the information9051include display indicating reception of an incoming email, social networking service (SNS) message, call, and the like; the title and sender of an email and SNS message; the date; the time; remaining battery; and the reception strength of an antenna. Instead of the information9051, the operation buttons9050or the like may be displayed on the position where the information9051is displayed.

FIG. 24Cis a perspective view of a portable information terminal9102. The portable information terminal9102has a function of displaying information on three or more surfaces of the display portion9001. Here, information9052, information9053, and information9054are displayed on different surfaces. For example, a user of the portable information terminal9102can see the display (here, the information9053) with the portable information terminal9102put in a breast pocket of his/her clothes. Specifically, a caller's phone number, name, or the like of an incoming call is displayed in a position that can be seen from above the portable information terminal9102. Thus, the user can see the display without taking out the portable information terminal9102from the pocket and decide whether to answer the call.

FIG. 24Dis a perspective view of a watch-type portable information terminal9200. The portable information terminal9200is capable of executing a variety of applications such as mobile phone calls, e-mailing, viewing and editing texts, music reproduction, Internet communication, and computer games. The display surface of the display portion9001is bent, and images can be displayed on the bent display surface. The portable information terminal9200can employ near field communication that is a communication method based on an existing communication standard. In that case, for example, mutual communication between the portable information terminal9200and a headset capable of wireless communication can be performed, and thus hands-free calling is possible. The portable information terminal9200includes the connection terminal9006, and data can be directly transmitted to and received from another information terminal via a connector. Power charging through the connection terminal9006is possible. Note that the charging operation may be performed by wireless power feeding without using the connection terminal9006.

FIGS. 24E, 24F, and 24Gare perspective views of a foldable portable information terminal9201.FIG. 24Eis a perspective view illustrating the portable information terminal9201that is opened.FIG. 24Fis a perspective view illustrating the portable information terminal9201that is being opened or being folded.FIG. 24Gis a perspective view illustrating the portable information terminal9201that is folded. The portable information terminal9201is highly portable when folded. When the portable information terminal9201is opened, a seamless large display region is highly browsable. The display portion9001of the portable information terminal9201is supported by three housings9000joined together by hinges9055. By folding the portable information terminal9201at a connection portion between two housings9000with the hinges9055, the portable information terminal9201can be reversibly changed in shape from an opened state to a folded state. For example, the portable information terminal9201can be bent with a radius of curvature of greater than or equal to 1 mm and less than or equal to 150 mm.

The electronic devices described in this embodiment each include the display portion for displaying some sort of data. Note that the light-emitting element of one embodiment of the present invention can also be used for an electronic device which does not have a display portion. The structure in which the display portion of the electronic device described in this embodiment is flexible and display can be performed on the bent display surface or the structure in which the display portion of the electronic device is foldable is described as an example; however, the structure is not limited thereto and a structure in which the display portion of the electronic device is not flexible and display is performed on a plane portion may be employed.

The structure described in this embodiment can be combined with any of the structures described in the other embodiments as appropriate.

In this embodiment, a light-emitting device including the light-emitting element of one embodiment of the present invention will be described with reference toFIGS. 25A to 25CandFIGS. 26A to 26D.

FIG. 25Ais a perspective view of a light-emitting device3000shown in this embodiment, andFIG. 25Bis a cross-sectional view along dashed-dotted line E-F inFIG. 25A. Note that inFIG. 25A, some components are illustrated by broken lines in order to avoid complexity of the drawing.

The light-emitting device3000illustrated inFIGS. 25A and 25Bincludes a substrate3001, a light-emitting element3005over the substrate3001, a first sealing region3007provided around the light-emitting element3005, and a second sealing region3009provided around the first sealing region3007.

Light is emitted from the light-emitting element3005through one or both of the substrate3001and a substrate3003. InFIGS. 25A and 25B, a structure in which light is emitted from the light-emitting element3005to the lower side (the substrate3001side) is illustrated.

As illustrated inFIGS. 25A and 25B, the light-emitting device3000has a double sealing structure in which the light-emitting element3005is surrlounded by the first sealing region3007and the second sealing region3009. With the double sealing structure, entry of impurities (e.g., water, oxygen, and the like) from the outside into the light-emitting element3005can be favorably suppressed. Note that it is not necessary to provide both the first sealing region3007and the second sealing region3009. For example, only the first sealing region3007may be provided.

Note that inFIG. 25B, the first sealing region3007and the second sealing region3009are each provided in contact with the substrate3001and the substrate3003. However, without limitation to such a structure, for example, one or both of the first sealing region3007and the second sealing region3009may be provided in contact with an insulating film or a conductive film provided on the substrate3001. Alternatively, one or both of the first sealing region3007and the second sealing region3009may be provided in contact with an insulating film or a conductive film provided on the substrate3003.

The substrate3001and the substrate3003can have structures similar to those of the substrate200and the substrate220described in Embodiment 3, respectively. The light-emitting element3005can have a structure similar to that of any of the light-emitting elements described in the above embodiments.

For the first sealing region3007, a material containing glass (e.g., a glass frit, a glass ribbon, and the like) can be used. For the second sealing region3009, a material containing a resin can be used. With the use of the material containing glass for the first sealing region3007, productivity and a sealing property can be improved. Moreover, with the use of the material containing a resin for the second sealing region3009, impact resistance and heat resistance can be improved. However, the materials used for the first sealing region3007and the second sealing region3009are not limited to such, and the first sealing region3007may be formed using the material containing a resin and the second sealing region3009may be formed using the material containing glass.

As the above glass frits, for example, a frit paste is applied to a substrate and is subjected to heat treatment, laser light irradiation, or the like. The frit paste contains the glass frit and a resin (also referred to as a binder) diluted by an organic solvent. Note that an absorber which absorbs light having the wavelength of laser light may be added to the glass frit. For example, an Nd:YAG laser or a semiconductor laser is preferably used as the laser. The shape of laser light may be circular or quadrangular.

As the above material containing a resin, for example, materials that include polyester, polyolefin, polyamide (e.g., nylon, aramid), polyimide, polycarbonate, an acrylic resin, urethane, an epoxy resin, or a resin having a siloxane bond can be used.

Note that in the case where the material containing glass is used for one or both of the first sealing region3007and the second sealing region3009, the material containing glass preferably has a thermal expansion coefficient close to that of the substrate3001. With the above structure, generation of a crack in the material containing glass or the substrate3001due to thermal stress can be suppressed.

For example, the following advantageous effect can be obtained in the case where the material containing glass is used for the first sealing region3007and the material containing a resin is used for the second sealing region3009.

The second sealing region3009is provided closer to an outer portion of the light-emitting device3000than the first sealing region3007is. In the light-emitting device3000, distortion due to external force or the like increases toward the outer portion. Thus, the outer portion of the light-emitting device3000where a larger amount of distortion is generated, that is, the second sealing region3009is sealed using the material containing a resin and the first sealing region3007provided on an inner side of the second sealing region3009is sealed using the material containing glass, whereby the light-emitting device3000is less likely to be damaged even when distortion due to external force or the like is generated.

Furthermore, as illustrated inFIG. 25B, a first region3011corresponds to the region surrounded by the substrate3001, the substrate3003, the first sealing region3007, and the second sealing region3009. A second region3013corresponds to the region surrounded by the substrate3001, the substrate3003, the light-emitting element3005, and the first sealing region3007.

The first region3011and the second region3013are preferably filled with, for example, an inert gas such as a rare gas or a nitrogen gas. Note that for the first region3011and the second region3013, a reduced pressure state is preferred to an atmospheric pressure state.

FIG. 25Cillustrates a modification example of the structure inFIG. 25B.FIG. 25Cis a cross-sectional view illustrating the modification example of the light-emitting device3000.

FIG. 25Cillustrates a structure in which a desiccant3018is provided in a recessed portion provided in part of the substrate3003. The other components are the same as those of the structure illustrated inFIG. 25B.

As the desiccant3018, a substance which adsorbs moisture and the like by chemical adsorption or a substance which adsorbs moisture and the like by physical adsorption can be used. Examples of the substance that can be used as the desiccant3018include alkali metal oxides, alkaline earth metal oxide (e.g., calcium oxide, barium oxide, and the like), sulfate, metal halides, perchlorate, zeolite, silica gel, and the like.

Next, modification examples of the light-emitting device3000which is illustrated inFIG. 25Bare described with reference toFIGS. 26A to 26D. Note thatFIGS. 26A to 26Dare cross-sectional views illustrating the modification examples of the light-emitting device3000illustrated inFIG. 25B.

In each of the light-emitting devices illustrated inFIGS. 26A to 26D, the second sealing region3009is not provided but only the first sealing region3007is provided. Moreover, in each of the light-emitting devices illustrated inFIGS. 26A to 26D, a region3014is provided instead of the second region3013illustrated inFIG. 25B.

For the region3014, for example, polyester, polyolefin, polyamide (e.g., nylon or aramid), polyimide, polycarbonate, an acrylic resin, urethane, or an epoxy resin can be used. Alternatively, a material including a resin having a siloxane bond can be used.

When the above-described material is used for the region3014, what is called a solid-sealing light-emitting device can be obtained.

In the light-emitting device illustrated inFIG. 26B, a substrate3015is provided on the substrate3001side of the light-emitting device illustrated inFIG. 26A.

The substrate3015has unevenness as illustrated inFIG. 26B. With a structure in which the substrate3015having unevenness is provided on the side through which light emitted from the light-emitting element3005is extracted, the efficiency of extraction of light from the light-emitting element3005can be improved. Note that instead of the structure having unevenness and illustrated inFIG. 26B, a substrate having a function as a diffusion plate may be provided.

In the light-emitting device illustrated inFIG. 26C, light is extracted through the substrate3003side, unlike in the light-emitting device illustrated inFIG. 26A, in which light is extracted through the substrate3001side.

The light-emitting device illustrated inFIG. 26Cincludes the substrate3015on the substrate3003side. The other components are the same as those of the light-emitting device illustrated inFIG. 26B.

In the light-emitting device illustrated inFIG. 26D, the substrate3003and the substrate3015included in the light-emitting device illustrated inFIG. 26Care not provided but a substrate3016is provided.

The substrate3016includes first unevenness positioned closer to the light-emitting element3005and second unevenness positioned farther from the light-emitting element3005. With the structure illustrated inFIG. 26D, the efficiency of extraction of light from the light-emitting element3005can be further improved.

Thus, the use of the structure described in this embodiment can provide a light-emitting device in which deterioration of a light-emitting element due to impurities such as moisture and oxygen is suppressed. Alternatively, with the structure described in this embodiment, a light-emitting device having high light extraction efficiency can be obtained.

In this embodiment, examples in which the light-emitting element of one embodiment of the present invention is used for various lighting devices and electronic devices will be described with reference toFIGS. 27A to 27CandFIG. 28.

An electronic device or a lighting device that has a light-emitting region with a curved surface can be obtained with the use of the light-emitting element of one embodiment of the present invention which is manufactured over a substrate having flexibility.

Furthermore, a light-emitting device to which one embodiment of the present invention is applied can also be used for lighting for motor vehicles, examples of which are lighting for a dashboard, a windshield, a ceiling, and the like.

FIG. 27Ais a perspective view illustrating one surface of a multifunction terminal3500, andFIG. 27Bis a perspective view illustrating the other surface of the multifunction terminal3500. In a housing3502of the multifunction terminal3500, a display portion3504, a camera3506, lighting3508, and the like are incorporated. The light-emitting device of one embodiment of the present invention can be used for the lighting3508.

The lighting3508that includes the light-emitting device of one embodiment of the present invention functions as a planar light source. Thus, unlike a point light source typified by an LED, the lighting3508can provide light emission with low directivity. When the lighting3508and the camera3506are used in combination, for example, imaging can be performed by the camera3506with the lighting3508lighting or flashing. Because the lighting3508functions as a planar light source, a photograph as if taken under natural light can be taken.

Note that the multifunction terminal3500illustrated inFIGS. 27A and 27Bcan have a variety of functions as in the electronic devices illustrated inFIGS. 24A to 24G.

The housing3502can include a speaker, a sensor (a sensor having a function of measuring force, displacement, position, speed, acceleration, angular velocity, rotational 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), a microphone, and the like. When a detection device including a sensor for detecting inclination, such as a gyroscope or an acceleration sensor, is provided inside the multifunction terminal3500, display on the screen of the display portion3504can be automatically switched by determining the orientation of the multifunction terminal3500(whether the multifunction terminal is placed horizontally or vertically for a landscape mode or a portrait mode).

The display portion3504may function as an image sensor. For example, an image of a palm print, a fingerprint, or the like is taken when the display portion3504is touched with the palm or the finger, whereby personal authentication can be performed. Furthermore, by providing a backlight or a sensing light source which emits near-infrared light in the display portion3504, an image of a finger vein, a palm vein, or the like can be taken. Note that the light-emitting device of one embodiment of the present invention may be used for the display portion3504.

FIG. 27Cis a perspective view of a security light3600. The security light3600includes lighting3608on the outside of the housing3602, and a speaker3610and the like are incorporated in the housing3602. The light-emitting device of one embodiment of the present invention can be used for the lighting3608.

The security light3600emits light when the lighting3608is gripped or held, for example. An electronic circuit that can control the manner of light emission from the security light3600may be provided in the housing3602. The electronic circuit may be a circuit that enables light emission once or intermittently a plurality of times or may be a circuit that can adjust the amount of emitted light by controlling the current value for light emission. A circuit with which a loud audible alarm is output from the speaker3610at the same time as light emission from the lighting3608may be incorporated.

The security light3600can emit light in various directions; therefore, it is possible to intimidate a thug or the like with light, or light and sound. Moreover, the security light3600may include a camera such as a digital still camera to have a photography function.

FIG. 28illustrates an example in which the light-emitting element is used for an indoor lighting device8501. Since the light-emitting element can have a larger area, a lighting device having a large area can also be formed. In addition, a lighting device8502in which a light-emitting region has a curved surface can also be formed with the use of a housing with a curved surface. A light-emitting element described in this embodiment is in the form of a thin film, which allows the housing to be designed more freely. Therefore, the lighting device can be elaborately designed in a variety of ways. Furthermore, a wall of the room may be provided with a large-sized lighting device8503. Touch sensors may be provided in the lighting devices8501,8502, and8503to control the power on/off of the lighting devices.

Moreover, when the light-emitting element is used on the surface side of a table, a lighting device8504which has a function as a table can be obtained. When the light-emitting element is used as part of other furniture, a lighting device which has a function as the furniture can be obtained.

As described above, lighting devices and electronic devices can be obtained by application of the light-emitting device of one embodiment of the present invention. Note that the light-emitting device can be used for electronic devices in a variety of fields without being limited to the lighting devices and the electronic devices described in this embodiment.

The structure described above in this embodiment can be used in combination with any of the structures described in the other embodiments as appropriate.

In this example, the measurement results of the fluorescence lifetimes of materials that can be used in the host material131of a light-emitting element of one embodiment of the present invention are described. The structures and abbreviations of compounds used in this example are shown below.

<Fabrication of Thin Film Sample>

To measure the fluorescence lifetimes of the host materials, the host materials were deposited over a quartz substrate by a vacuum evaporation method; in this way, thin film samples were fabricated. As a thin film1, 4,6-bis[3-(9H-carbazol-9-yl)phenyl]pyrimidine (abbreviation: 4,6mCzP2Pm) and N-(1,1′-biphenyl-4-yl)-N-[4-(9-phenyl-9H-carbazol-3-yl)phenyl]-9,9-dimethyl-9H-fluoren-2-amine (abbreviation: PCBBiF) were deposited by co-evaporation such that the deposited film has a weight ratio of 4,6mCzP2Pm to PCBBiF of 0.8:0.2 and a thickness of 50 nm. As a thin film2,2-[3′-(dibenzothiophen-4-yl)biphenyl-3-yl]dibenzo[f,h]quinoxaline (abbreviation: 2mDBTBPDBq-II) and PCBBiF were deposited by co-evaporation such that the deposited film has a weight ratio of 2mDBTBPDBq-II to PCBBiF of 0.8:0.2 and a thickness of 50 nm. As a thin film3, a thin film4, and a thin film5, 4,6mCzP2Pm, 2mDBTBPDBq-II, and PCBBiF, respectively, were deposited each to a thickness of 50 nm. In other words, the thin films1and2each include two compounds, and the thin films3to5each include one compound. The thin film1is a thin film in which the compounds included in the thin films3and5are mixed, and the thin film2is a thin film in which the compounds included in the thin films4and5are mixed.

The thin films1to5were sealed by fixing a sealing substrate to the quartz substrate over which the thin-film samples were deposited using a sealant for an organic EL device in a glove box under a nitrogen atmosphere. Specifically, after the sealant was applied to surround the thin films over the quartz substrate and the quartz substrate was bonded to the sealing substrate, irradiation with ultraviolet light having a wavelength of 365 nm at 6 J/cm2and heat treatment at 80° C. for one hour were performed.

<Time-Resolved Fluorescence Measurement of Thin Film Sample>

A picosecond fluorescence lifetime measurement system (manufactured by Hamamatsu Photonics K.K.) was used for the measurement. In this measurement, the lifetimes of fluorescence of the thin films were measured as follows. The thin films were irradiated with pulsed laser, and emission from the thin films which started to be attenuated after the laser irradiation underwent time-resolved measurement using a streak camera. A nitrogen gas laser with a wavelength of 337 nm was used as the pulsed laser. The thin films were irradiated with pulsed laser with a pulse width of 500 ps at a repetition rate of 10 Hz. By integrating data obtained by the repeated measurement, data with a high S/N ratio was obtained. The measurement was performed at room temperature (in an atmosphere kept at 23° C.).

Emissions from the thin films1to5in the following wavelength ranges which extend around peaks in the emission spectra were observed. The wavelength range for the thin film1was 509 nm±69 nm (440 nm or longer and 578 nm or shorter), the wavelength range for the thin film2was 514 nm±69 nm (445 nm or longer and 583 nm or shorter), the wavelength range for the thin film3was 449 nm±69 nm (380 nm or longer and 518 nm or shorter), the wavelength range for the thin film4was 444 nm±69 nm (375 nm or longer and 513 nm or shorter), and the wavelength range for the thin film5was 434 nm±69 nm (365 nm or longer and 503 nm or shorter). This shows that the thin film1emits light having a longer wavelength than light emitted from the thin films3and5, and the thin film2emits light having a longer wavelength than light emitted from the thin films2and4. In other words, the thin films1and2, which are each formed of a mixture of two compounds, emitted light having longer wavelengths than light emitted from the thin films3to5, which are each formed of a single compound. Thus, emissions from the thin films1and2are each emission from an exciplex formed by two compounds.

The results of the time-resolved fluorescence measurement are shown inFIGS. 29A and 29B. InFIGS. 29A and 29B, the vertical axis represents emission intensity normalized to the value at the time of pulse laser irradiation. The horizontal axis represents time elapsed after the stop of the pulse laser irradiation.

The attenuation curves shown inFIGS. 29A and 29Bwere fitted with Formula 4.

In Formula 4, L and t represent normalized emission intensity and elapsed time, respectively. The results obtained by fitting the attenuation curves are shown in Table 1. In addition, approximate lines using A2and a2of the thin films1to3are shown inFIGS. 29A and 29B. Fitting was performed on the thin films1and2under conditions where n=1 to 3, and fitting was performed on the thin films3to5under conditions where n=1 and 2.

From the results of fitting the attenuation curves, the proportion of an initial fluorescence component (A1) and the lifetime of the fluorescence (a1) are determined, and the proportion of a component with the shortest lifetime in the delayed fluorescence component (A2) and the lifetime of the fluorescence (a2) are determined. The thin films1and2include large proportions of the delayed fluorescence components in addition to the initial fluorescence components having lifetimes of approximately 16 ns and 60 ns, respectively. The fluorescence lifetimes of the delayed fluorescence components of the thin films1and2can be estimated at 0.19 μs and 0.28 μs, respectively, and the proportions of the delayed fluorescence components of the thin films1and2can be estimated at 53% and 12%, respectively. In contrast, in the thin films3to5, most of the emission components showed a single-exponential decay and the initial fluorescence component having short fluorescence lifetimes of approximately ten to several tens of nanoseconds was dominant. That is, the proportions of the delayed fluorescence components in the thin films3to5were lower than 1% and the delayed fluorescence was not substantially generated. Since the thin films1and2including two compounds had long fluorescence lifetimes due to delayed fluorescence, it was found that the delayed fluorescence component emitted by the exciplex was included in the emission component of the thin films1and2. Note that because part of excited light was observed in the initial emission component, the proportion of the initial fluorescence component (A1) was overestimated. However, as is seen fromFIGS. 29A and 29B, at least 10% or more delayed fluorescence component (20% or more in the thin film1) was certainly observed from the thin films1and2.

Note that an exciplex has a feature that the energy level of the singlet excited state is adjacent to that of the triplet excited state. Accordingly, the delayed fluorescence component shown by the thin films1and2are thermally activated delayed fluorescence derived from intersystem crossing and reverse intersystem crossing between the singlet excited state and the triplet excited state of the exciplex.

The large proportion that is more than 50% of the delayed fluorescence component in the thin film1indicates efficient formation of an exciplex and an efficient occurrence of intersystem crossing between the triplet excited state and the singlet excited state. Therefore, the host material in which 4,6mCzP2Pm and PCBBiF are combined can be suitably used in a light-emitting element including a fluorescent material as a guest material.

The structure described in this example can be used in appropriate combination with any of the structures described in the embodiments and the other examples.

As described in Embodiment 1, the emission efficiency of a light-emitting element can be increased by utilizing TTA. In this example, the results of the time-resolved fluorescence measurement on such light-emitting elements (the light-emitting elements1and2) are described. The schematic cross-sectional view of the light-emitting elements fabricated in this example corresponds to an element structure in which the light-emitting layer130is eliminated from the light-emitting element250illustrated inFIG. 1Aand the light-emitting layer120includes only the host material121and does not include the guest material122. The structures and abbreviations of compounds used in this example are shown below.

As the electrode101, an ITSO film was formed to a thickness of 110 nm over a substrate. Note that the area of the electrode101was 4 mm2(2 mm×2 mm).

Next, the EL layer100was formed over the electrode101. As the hole-injection layer111, 3-[4-(9-phenanthryl)-phenyl]-9-phenyl-9H-carbazole (abbreviation: PCPPn) and molybdenum oxide (MoO3) were deposited by co-evaporation such that the deposited layer has a weight ratio of PCPPn to MoO3of 1:0.5 and a thickness of 60 nm. Then, as the hole-transport layer112, PCPPn was vapor-deposited to a thickness of 30 nm.

As the electron-transport layer118, bathophenanthroline (BPhen) was vapor-deposited to a thickness of 25 nm over the light-emitting layer120. Next, as the electron-injection layer119, lithium fluoride (LiF) was vapor-deposited to a thickness of 1 nm.

As the electrode102, aluminum (Al) was deposited to a thickness of 200 nm.

Next, in a glove box containing a nitrogen atmosphere, the light-emitting element1was sealed by fixing a sealing substrate to the substrate provided with the EL layer100using a sealant for an organic EL device. Specifically, after the sealant was applied to surround the EL layer100over the substrate and the substrate was bonded to the sealing substrate, irradiation with ultraviolet light having a wavelength of 365 nm at 6 J/cm2and heat treatment at 80° C. for one hour were performed. Through the above process, the light-emitting element1was obtained.

The light-emitting element2was fabricated through the same steps as those for the light-emitting element1except for the step of forming the light-emitting layer120.

As the light-emitting layer120of the light-emitting element2, 9-(2-naphthyl)-10-[4-(1-naphthyl)phenyl]anthracene (abbreviation: BH-1) was vapor-deposited to a thickness of 25 nm.

FIG. 30shows the results of measuring fluorescent lifetimes of the fabricated light-emitting elements (the light-emitting elements1and2). Note that blue light emission emitted from the compound in the light-emitting layer120was observed by the fluorescent lifetime measurement.

A picosecond fluorescence lifetime measurement system (manufactured by Hamamatsu Photonics K.K.) was used for the measurement. In this measurement, the lifetimes of fluorescence of the light-emitting elements were measured as follows. A square wave pulse voltage was applied to the light-emitting elements, and light which started to be attenuated after the falling of the voltage underwent time-resolved measurement using a streak camera. The pulse voltage was applied at a frequency of 10 Hz. By integrating data obtained by repeated measurement, data with a high S/N ratio was obtained. The measurement was performed at room temperature (300 K) under the conditions of a pulse voltage of 3.5 V, a pulse time width of 100 μsec, a negative bias voltage of −5 V, and a measurement time of 50 μsec.FIG. 30shows the measurement results. InFIG. 30, the vertical axis represents emission intensity normalized to the value in a state where carriers are steadily injected (at the time when the pulse voltage is ON). The horizontal axis represents time elapsed after the falling of the pulse voltage.

The attenuation curves shown inFIG. 30were fitted with Formula 4. The fitting results are shown in Table 3. In addition, an approximate line using A2and a2of the light-emitting element1obtained by the fitting is shown inFIG. 30. Fitting was performed on the light-emitting elements1and2under conditions where a component except the initial fluorescence component having a short fluorescence lifetime was extracted (i.e., conditions where only a delayed fluorescence component was extracted). Fitting was performed on the light-emitting element1under conditions where n=1 and 2, and fitting was performed on the light-emitting element2under conditions where n=1.

From the results of fitting the attenuation curve, the proportion of a component with the shortest lifetime in the delayed fluorescence component (A1) and the lifetime of the fluorescence (a1) are determined. The light-emitting element1includes a large proportion of the delayed fluorescence component. The fluorescence lifetime of the delayed fluorescence component of the light-emitting element1can be estimated at 2.2 ups, and the proportion of the delayed fluorescence component of the light-emitting element1can be estimated at 9.7%. In contrast, the proportion of the delayed fluorescence component in the thin film2is as low as less than 1%.

As described later in Example 4, the singlet excited energy of cgDBCzPA used in the light-emitting element1was 2.95 eV (calculated from the absorption edge in the absorption spectrum of the cgDBCzPA evaporated film), and the triplet excited energy thereof was 1.72 eV (calculated from a phosphorescence spectrum peak at 10 K of a co-evaporated film in which Ir(ppy)3was added to cgDBCzPA as a sensitizer). Thus, cgDBCzPA has an energy difference between the S1level and the T1level of 0.5 eV or more. Therefore, the delayed fluorescence component observed from the light-emitting element1is owing to not the thermal activation but TTA, and accordingly, the light-emitting element1has higher TTA efficiency than the light-emitting element2. A light-emitting element including the compound used for the light-emitting element1can increase the singlet exciton generation probability by the use of TTA and thus has reduced energy loss.

In the fluorescence measurement described with reference toFIG. 30, possible causes of the delayed fluorescence other than the formation of a singlet exciton due to triplet-triplet annihilation (TTA) include the formation of a singlet exciton due to recombination of carriers that remain in the light-emitting element when the pulse voltage is OFF. In these measurement, however, since a negative bias voltage (−5 V) was applied, recombination of the remaining carriers was suppressed. Therefore, the delayed fluorescence component shown in the measurement results inFIG. 30was attributed to light emission due to triplet-triplet annihilation (TTA).

The structure described in this example can be used in appropriate combination with any of the structures described in the embodiments and the other examples.

In this example, examples of fabricating light-emitting elements3and4according to one embodiment of the present invention will be described. A schematic cross-sectional view of the light-emitting elements fabricated in this example is similar to that of the light-emitting element250illustrated inFIG. 1A. The detailed element structure is shown in Table 4. The structures and abbreviations of compounds used in this example are shown below. As the host materials, the compounds used in the above example were used.

As the electrode101, an ITSO film was formed to a thickness of 110 nm over a substrate. Note that the area of the electrode101was 4 mm2(2 mm×2 mm).

Next, the EL layer100was formed over the electrode101. As the hole-injection layer111, PCPPn and molybdenum oxide (MoO3) were deposited by co-evaporation such that the deposited layer has a weight ratio of PCPPn to MoO3of 1:0.5 and a thickness of 50 nm. Then, as the hole-transport layer112, PCPPn was vapor-deposited to a thickness of 10 nm.

Next, as the light-emitting layer120, cgDBCzPA and N,N′-bis(3-methylphenyl)-N,N′-bis[3-(9-phenyl-9H-fluoren-9-yl)phenyl]pyrene-1,6-dia mine (abbreviation: 1,6mMemFLPAPrn) were deposited by co-evaporation such that the deposited layer has a weight ratio of cgDBCzPA to 1,6mMemFLPAPrn of 1:0.03 and a thickness of 5 nm. In the light-emitting layer120, cgDBCzPA is the host material121and 1,6mMemFLPAPrn is the guest material122(fluorescent material).

Then, as the light-emitting layer130, 4,6mCzP2Pm, PCBBiF, and rubrene were deposited by co-evaporation such that the deposited layer has a weight ratio of 4,6mCzP2Pm to PCBBiF and rubrene of 0.8:0.2:0.01 and a thickness of 15 nm. In the light-emitting layer130, 4,6mCzP2Pm and PCBBiF are the host material131and rubrene is the guest material132(fluorescent material).

As the electron-transport layer118, 4,6mCzP2Pm and BPhen were sequentially vapor-deposited to each have a thickness of 10 nm, over the light-emitting layer130. Next, as the electron-injection layer119, lithium fluoride (LiF) was vapor-deposited to a thickness of 1 nm.

As the electrode102, aluminum (Al) was deposited to a thickness of 200 nm.

Next, in a glove box containing a nitrogen atmosphere, the light-emitting element3was sealed by fixing a sealing substrate to the substrate provided with the EL layer100using a sealant for an organic EL device. Specifically, after the sealant was applied to surround the EL layer100over the substrate and the substrate was bonded to the sealing substrate, irradiation with ultraviolet light having a wavelength of 365 nm at 6 J/cm2and heat treatment at 80° C. for one hour were performed. Through the above process, the light-emitting element3was obtained.

The light-emitting element4was fabricated through the same steps as those for the light-emitting element3except that a different host material for the light-emitting layer130and a different electron-transport material for the electron-transport layer118were used.

Then, as the light-emitting layer130of the light-emitting element4, 2mDBTBPDBq-II, PCBBiF, and rubrene were deposited by co-evaporation such that the deposited layer has a weight ratio of 2mDBTBPDBq-II to PCBBiF and rubrene of 0.8:0.2:0.01 and a thickness of 15 nm. In the light-emitting layer130, 2mDBTBPDBq-II and PCBBiF are the host material131and rubrene is the guest material132(fluorescent material).

As the electron-transport layer118, 2mDBTBPDBq-II and BPhen were sequentially vapor-deposited to each have a thickness of 10 nm, over the light-emitting layer130.

Next, emission characteristics of the fabricated light-emitting elements3and4were measured. Note that the measurement was performed at room temperature (in an atmosphere kept at 23° C.).

The emission characteristics of the light-emitting elements at a luminance around 1000 cd/m2are shown below in Table 5. The current efficiency-luminance characteristics, external quantum efficiency-luminance characteristics, and luminance-voltage characteristics of the light-emitting elements are shown inFIG. 31,FIG. 32, andFIG. 33, respectively.FIG. 34shows electroluminescence spectra at the time when a current was made to flow in the light-emitting elements at a current density of 2.5 mA/cm2.

As shown by the peaks in the electroluminescence spectra inFIG. 34, blue light emission derived from 1,6mMemFLPAPrn, which is a fluorescent material, and yellow light emission derived from rubrene were observed from the light-emitting elements3and4. Both of the fluorescent materials that emit light of different colors were able to emit light in the light-emitting elements3and4.

From the results ofFIG. 31,FIG. 32, and Table 5, the light-emitting element3was found to have a high current efficiency and a high external quantum efficiency. Note that since the probability of generating singlet excitons by recombination of carriers (holes and electrons) injected from a pair of electrodes is 25% at the maximum, when the light extraction efficiency to the outside is 20%, the external quantum efficiency is 5% at the maximum. A high external quantum efficiency over 5% was obtained from the light-emitting element3. This is because light emission derived from singlet excitons generated from triplet excitons by TTA and ExEF was obtained from the light-emitting element3, in addition to the light emission derived from singlet excitons generated by recombination of carriers (holes and electrons) injected from a pair of electrodes.

As described in Example 1, the materials (4,6mCzP2Pm and PCBBiF) used as the host material131of the light-emitting element3emit light with a large proportion of delayed fluorescence component; accordingly, reverse intersystem crossing between the triplet excited state and the singlet excited state occurs efficiently. Therefore, the light-emitting element3had a high probability of generating singlet excitons and a high emission efficiency. In other words, preferably 20% or more, further preferably 50% or more of light emitted from the exciplex is a delayed fluorescence component.

Furthermore, as described in Example 2, the compound (cgDBCzPA) used as the host material121in the light-emitting elements3and4emit light with a large proportion of delayed fluorescence component; accordingly, singlet excitons are efficiently generated from triplet excitons by TTA.

As shown inFIG. 33and Table 5, the light-emitting elements3to4are each driven at low voltages. In other words, a light-emitting element that is driven at low voltage can be manufactured by including a light-emitting layer in which TTA and ExEF are utilized. Furthermore, a light-emitting element with reduced power consumption can be manufactured.

The structure described in this example can be used in appropriate combination with any of the structures described in the embodiments and the other examples.

In this example, the results of measuring the lowest levels of the singlet excited energy level (S1levels) and the lowest levels of the triplet excited energy level (T1levels) of materials that can be used in a light-emitting element of one embodiment of the present invention will be described. The compounds used in this example are those used in the above examples. Specifically, cgDBCzPA used as the host material121of the light-emitting layer120, 1,6mMemFLPAPrn used as the guest material122of the light-emitting layer120, and 4,6mCzP2Pm, 2mDBTBPDBq-II, and PCBBiF used as the host material131of the light-emitting layer130were measured.

To measure the S1level, a thin film (having a thickness of approximately 50 nm) was formed over a quartz substrate by a vacuum evaporation method as a thin film sample, and an absorption spectrum of the thin film sample was measured. The absorption spectrum was measured with an ultraviolet-visible spectrophotometer (V-550 manufactured by JASCO Corporation). Then, the absorption spectrum of quartz was subtracted from the measured spectrum of the sample. An absorption edge was obtained from the Tauc plot assuming a direct transition, which was constructed from the data of the absorption spectrum of the thin film, and the absorption edge was used as an optical bandgap. Thus, the S1level was estimated.

To measure the T1level, phosphorescence measurement was performed. The measurement was performed with a PL microscope, LabRAM HR-PL, produced by HORIBA, Ltd., a He—Cd laser (325 nm) as excitation light, and a CCD detector, at a measurement temperature of 10 K.

Note that in a fluorescent compound, intersystem crossing is unlikely to occur and light emitted from the T1level is faint; for this reason, measurement of the T1level is sometimes difficult. Further, in the case where a compound having a high fluorescent quantum yield is used alone in a thin film sample, direct observation of phosphorescence by a low-temperature PL method is sometimes very difficult. Therefore, the T1levels of cgDBCzPA and 1,6mMemFLPAPrn were estimated by phosphorescence measurement with the use of a method using a triplet sensitizer that is described below.

A co-evaporated film in which tris(2-phenylpyridinato-N, C2′)iridium(III) (abbreviation: Ir(ppy)3) was added as a triplet sensitizer to the compound whose T1level was to be measured was formed. This film was subjected to a low-temperature PL method, and the T1level thereof was estimated from the measured phosphorescence spectrum. The co-evaporation of Ir(ppy)3and the fluorescent material increases the probability of occurrence of intersystem crossing in the measured fluorescent material. Thus, phosphorescence from the fluorescent material can be measured, which is difficult to achieve when the co-evaporation is not employed.

For the measurement, each thin film was formed over a quartz substrate to a thickness of approximately 50 nm and another quartz substrate was attached to the deposition surface in a nitrogen atmosphere.

The measured S1and T1levels are shown in Table 6.

From Table 6, it can be found that the S1level of cgDBCzPA is higher than that of 1,6mMemFLPAPrn, which is a guest material that emits blue light, and is sufficient for the host material. Moreover, the T1level of cgDBCzPA is lower than that of 1,6mMemFLPAPrn, which is a guest material, and satisfies the conditions for efficiently causing TTA.

Furthermore, the T1level of cgDBCzPA is lower than those of 4,6mCzP2Pm, 2mDBTBPDBq-II, and PCBBiF. Thus, even when the transfer of triplet excited energy from the light-emitting layer130to the light-emitting layer120occurs, the triplet excitons can be converted into singlet excitons by TTA. By using the above-described compounds as the host material, a light-emitting element with a high probability of generating singlet excitons can be manufactured.

Table 6 also shows that an energy difference between the S1level and T1level of cgDBCzPA is 0.5 eV or more. If delayed fluorescence is caused by thermally activated delayed fluorescence due to reverse intersystem crossing from the triplet excited state to the singlet excited state, an energy difference between the S1level and the T1level is preferably more than 0 eV and less than or equal to 0.2 eV in order to efficiently cause reverse intersystem crossing. Hence, the delayed fluorescence component of the host material used in the light-emitting layer120in the light-emitting elements in the above examples is attributed not to the thermally activated delayed fluorescence but to TTA. Furthermore, because the energy at the S1level is lower than twice the energy at the T1level, energy transfer from the triplet excited state to the singlet excited state by TTA is possible.

The spectrum peak of fluorescence emitted from the thin film of cgDBCzPA was at 442 nm (2.81 eV). Accordingly, a difference in energy between the peak wavelength of the fluorescence spectrum of cgDBCzPA and the peak wavelength of phosphorescence spectrum of cgDBCzPA was 0.5 eV or more. This also shows that the delayed fluorescence component of the material (cgDBCzPA) used as the host material121in the light-emitting layer120in the light-emitting elements of the above examples is attributed not to the thermally activated delayed fluorescence but to TTA. Note that the fluorescence spectra were measured with a PL-EL measurement apparatus (manufactured by Hamamatsu Photonics K.K.).

Furthermore, Table 6 shows that the energy difference between the S1level and the T1level in each of 4,6mCzP2Pm, 2mDBTBPDBq-II, and PCBBiF is more than 0.5 eV. In order to efficiently generate thermally activated delayed fluorescence due to reverse intersystem crossing from the triplet excited state to the singlet excited state, the energy difference between the S1level and the T1level is preferably more than 0 eV and less than or equal to 0.2 eV. Therefore, it is difficult for these compounds to singly generate thermally activated delayed fluorescence. Thus, as described in the above examples and Embodiment 1, it is preferable to use two compounds which are a combination that can form an exciplex as a host material. Since the energy difference between the S1level and the T1level of an exciplex is small, reverse intersystem crossing is efficiently caused in the exciplex, whereby the probability of generating singlet excitons can be increased. As a result, the emission efficiency of the light-emitting element can be increased.

The structure described in this example can be used in appropriate combination with any of the structures described in the embodiments and the other examples.

This application is based on Japanese Patent Application serial no. 2015-033617 filed with Japan Patent Office on Feb. 24, 2015, the entire contents of which are hereby incorporated by reference.