LIGHT-EMITTING ELEMENT AND LIGHT-EMITTING DEVICE

A light-emitting element includes a first electrode, a second electrode, a light-emitting layer provided between the first electrode and the second electrode and including a material having a perovskite structure, and a blocking layer provided in at least one of a position between the first electrode and the light-emitting layer or a position between the second electrode and the light-emitting layer, and configured to suppress migration of charges from the light-emitting layer.

TECHNICAL FIELD

The disclosure relates to a light-emitting element and a light-emitting device.

BACKGROUND ART

PTL 1 proposes a light-emitting element including a light-emitting layer including a metal halide perovskite, and an electron transport layer including a 1,10 phenanthroline derivative having a substituent at one or both of 2- and 9-positions of a 1,10 phenanthroline skeleton. This configuration can suppress light emission quenching due to diffusion, into the light-emitting layer, of an alkali metal or an alkali earth metal used for electron injection.

CITATION LIST

Patent Literature

SUMMARY OF INVENTION

Technical Problem

The light-emitting element disclosed in PTL 1 described above can improve luminous efficiency. However, in the light-emitting element disclosed in PTL 1, charges injected into the light-emitting layer cannot be sufficiently confined in the light-emitting layer, and thus there is a problem that a luminance lifetime is reduced.

An object of the disclosure is to provide a light-emitting element and a light-emitting device capable of improving a luminance lifetime.

Solution to Problem

A light-emitting element according to an aspect of the disclosure includes a first electrode, a second electrode, a light-emitting layer provided between the first electrode and the second electrode and including a material having a perovskite structure, and a blocking layer provided in at least one of a position between the first electrode and the light-emitting layer or a position between the second electrode and the light-emitting layer, and configured to suppress migration of charges from the light-emitting layer.

Further, a light-emitting device according to an aspect of the disclosure includes a thin film transistor, and a light-emitting element electrically connected to the thin film transistor, and including a first electrode, a second electrode, a light-emitting layer provided between the first electrode and the second electrode and including a material having a perovskite structure, and a blocking layer provided in at least one of a position between the first electrode and the light-emitting layer or a position between the second electrode and the light-emitting layer, and configured to suppress migration of charges from the light-emitting layer.

DESCRIPTION OF EMBODIMENTS

With reference to the drawings, embodiments of the disclosure will be described below. Note that, in each drawing, similar configurations are denoted by the same reference sign, and descriptions thereof are omitted.

EMBODIMENTS

A configuration of a light-emitting device100according to the embodiment will be described with reference toFIGS.1and2.FIG.1is a schematic cross-sectional view of the light-emitting device100according to the embodiment of the disclosure. InFIG.1, a direction from an array substrate2of the light-emitting device100toward a light-emitting element3may be described as an “upward direction”, and an opposite direction may be described as a “downward direction”.FIG.2is a table showing a correspondence relationship between each layer constituting the light-emitting element3included in the light-emitting device100illustrated inFIG.1, and a material forming each layer.

The light-emitting device100is a device that can be used for a display of a television, a smartphone, or the like, for example. As illustrated inFIG.1, the light-emitting device100includes the array substrate2and the light-emitting element3. The array substrate2is a glass substrate on which a thin film transistor (TFT) (not illustrated) for driving the light-emitting element3is formed. In the light-emitting device100, each layer of the light-emitting element3is layered on the array substrate2, and the TFT of the array substrate2and the light-emitting element3are electrically connected to each other.

The light-emitting element3includes an anode electrode4(first electrode), a hole injection layer5, a hole transport layer6, an electron blocking layer7, a light-emitting layer8, a hole blocking layer9, an electron transport layer10, an electron injection layer11, and a cathode electrode12(second electrode). The light-emitting element3can be formed by layering, on the array substrate2, the anode electrode4, the hole injection layer5, the hole transport layer6, the electron blocking layer7, the light-emitting layer8, the hole blocking layer9, the electron transport layer10, the electron injection layer11, and the cathode electrode12in this order from the bottom.

Anode Electrode

The anode electrode4formed on the array substrate2and is electrically connected to the TFT provided on the array substrate2. As shown inFIG.2, the anode electrode4can be formed by layering, for example, Ag functioning as a reflective layer and having high light reflectivity and a transparent conductive film of ITO functioning as a transparent electrode and having optical transparency. The anode electrode4is formed on the array substrate2by using, for example, sputtering or vapor deposition as follows.

First, a reflective layer (e.g., Ag) is layered on the array substrate2by sputtering. A thickness of the reflective layer film-formed on the array substrate2can be, for example, 100 nm. Subsequently, a transparent electrode (ITO) is continuously layered. A thickness of the transparent electrode layered herein can be, for example, 20 nm. The layered body of Ag/ITO formed in such a manner is processed into a desired pattern by, for example, photolithography to form the anode electrode4.

Note that the anode electrode4is formed of the layered body of Ag/ITO, which is not limited thereto. For example, the reflective layer may be metal including Al, Cu, Au, or the like instead of Ag. The transparent electrode may be a transparent conductive film of IZO, ZnO, AZO, BZO, or the like instead of ITO.

Hole Injection Layer and Hole Transport Layer

The hole injection layer5allows holes to be injected from the anode electrode4. The hole transport layer6transports, to the light-emitting layer8via the electron blocking layer7described later, the holes injected from the anode electrode4into the hole injection layer5. The hole injection layer5and the hole transport layer6are formed on the anode electrode4and are electrically connected to the anode electrode4.

As shown inFIG.2, the hole injection layer5is formed on the anode electrode4by co-evaporating diphenylnaphthyldiamine (NPD) and MoO3. As shown inFIG.2, the hole transport layer6is formed on the hole injection layer5by performing vapor deposition on NPD.

Specifically, first, the following pretreatment is performed before the hole injection layer5and the hole transport layer6are formed. In other words, a surface of the anode electrode4formed as described above is washed with pure water and baked at 120° C. for an hour in a circulation type oven in an N2atmosphere for dehydration. Subsequently, plasma surface treatment using a non-polymerizable gas (for example, Ar) or the like is performed on the anode electrode4.

On the anode electrode4subjected to the pretreatment in such a manner, diphenylnaphthyldiamine (NPD) being a hole transport material and MoO3being a hole injection material are co-evaporated in vacuum to form the hole injection layer5. Note that NPD and MoO3are deposited at a ratio of NPD:MoO3=1.00:0.15. A film thickness of the hole injection layer5formed at this time can be, for example, 90 nm. Subsequently, only NPD is deposited to form the hole transport layer6. A film thickness of the hole transport layer6can be, for example, 20 nm. In this way, the hole injection layer5and the hole transport layer6can be formed on the anode electrode4.

Electron Blocking Layer

The electron blocking layer7suppresses migration of electrons (charges) such that the electrons do not leak from the adjacent light-emitting layer8. The electron blocking layer7is formed on the hole transport layer6and is electrically connected to the anode electrode4. As shown inFIG.2, the electron blocking layer7can be formed of, for example, 3-[4-(9-phenanthryl)-phenyl]-9-phenyl-9H-carbazole (PCPPn).

In other words, PCPPn is vacuum vapor deposited on the hole transport layer6to form the electron blocking layer7. A film thickness of the electron blocking layer7can be, for example, 10 nm. However, the electron blocking layer7may be damaged by a solvent or the like used in a step of forming the light-emitting layer8subsequently formed on the electron blocking layer7. Thus, the electron blocking layer7may be formed thicker by 10 nm in consideration of the influence of such damage. Alternatively, the electron blocking layer7may be formed by co-evaporating PCPPn and an oxide film such as an SiOxfilm or a TiOxfilm on the hole transport layer6. Alternatively, the electron blocking layer7may be formed by co-evaporating PCPPn and a p-type semiconductor material such as MoO3or V2O5on the hole transport layer6. By forming the electron blocking layer7in such a manner, the electron blocking layer7can be an organic layer having a hole injection property.

Further, a film thickness of the electron blocking layer7may be appropriately adjusted such that an optical distance in which light emitted from the light-emitting layer8of the light-emitting element3moves has a layered structure of the light-emitting element3that satisfies a condition (1/2×λ×n (n is an odd number)) described below.

The light-emitting layer8is provided between the anode electrode4and the cathode electrode12, more specifically, between the electron blocking layer7and the hole blocking layer9. The light-emitting layer8includes a metal halide perovskite material as a material having a perovskite structure. The metal halide perovskite material may be a composite material of an organic material and an inorganic material or a material formed of an inorganic material. Examples of the metal halide perovskite material include a lead metal halide compound represented by MPbX3(M;Cs, MeNH3, X;I, Br, Cl).

The metal halide perovskite material has a narrow full width at half maximum (FWHM) of a peak wavelength of electroluminescence (EL) and can emit light having relatively deep chromaticity as compared with a phosphorescent material used in a light-emitting layer of a conventionally known OLED.

In the light-emitting element3according to the embodiment, the light-emitting layer8is formed of, for example, CsPbBr3as shown inFIG.2. In other words, an HBr solvent solution is prepared by adding PbBr2serving as a precursor of the light-emitting layer8to a solution including HBr as a solvent. Furthermore, an aqueous solution of CsBr is prepared and dripped to the HBr solvent solution described above to obtain CsPbBr3.

The obtained CsPbBr3is filtered, washed with ethanol, and vacuum-degassed at a temperature of 60° C. for 12 hours to obtain a powdery raw material. The powdered CsPbBr3and CH3NH3Br are added to a DMSO solvent and mixed, and the mixture is applied onto the electron blocking layer7. Note that a combination ratio of CsPbBr3and CH3NH3Br is adjusted to 1:1. After the application, it is dried at a temperature of 90° C. for 30 minutes in an N2atmosphere to form the light-emitting layer8. A film thickness of the light-emitting layer8can be, for example, 30 to 120 nm.

Note that the light-emitting layer8is formed by the application as described above, but the formation method is not limited thereto. Other methods may be used as long as the light-emitting layer8can be formed with an appropriate film thickness by using a metal halide perovskite material.

Hole Blocking Layer

The hole blocking layer9suppresses migration of holes (charges) such that the holes do not leak from the adjacent light-emitting layer8. The hole blocking layer9is formed on the light-emitting layer8. The hole blocking layer9is formed on the light-emitting layer8by using, for example, vapor deposition or the like as follows.

In other words, after the light-emitting layer8is formed as described above, 4,4′-bis (N-carbazolyl)-1,1′-biphenyl (CBP) and CsCO3are co-evaporated in vacuum on the light-emitting layer8to form the hole blocking layer9. A film thickness of the hole blocking layer9can be, for example, 10 nm.

The hole blocking layer9may include ZnO or TiO2instead of CsCO3described above. In other words, the hole blocking layer9may include, in addition to CBP, an n-type semiconductor material including at least one type selected from a group of CsCO3, ZnO, SiO, and TiO2.

Electron Injection Layer and Electron Transport Layer

The electron injection layer11allows electrons to be injected from the cathode electrode12. The electron transport layer10transports, to the light-emitting layer8via the hole blocking layer9, the electrons injected from the cathode electrode12into the electron injection layer11. The electron injection layer11and the hole transport layer6are formed on the hole blocking layer9and are electrically connected to the cathode electrode12. The electron injection layer11and the electron transport layer10are formed on the hole blocking layer9by using, for example, vapor deposition or the like as follows.

In other words, 4,7-diphenyl-1,10-phenanthroline (Bphen) is deposited in vacuum on the hole blocking layer9to form the electron transport layer10. A film thickness of the electron transport layer10can be, for example, 20 nm. LiF is further deposited in vacuum on the electron transport layer10formed in such a manner to form the electron injection layer11. A film thickness of the electron injection layer11can be, for example, 0.5 nm.

Cathode Electrode

The cathode electrode12is provided on the electron injection layer11and is electrically connected to the electron injection layer11, the electron transport layer10, and the hole blocking layer9. The cathode electrode12can be formed of, for example, a metal thinned to a degree having optical transparency, or a transparent material. In the light-emitting element3according to the embodiment of the disclosure, the cathode electrode12is formed of, for example, an alloy of Mg and Ag as shown inFIG.2.

In other words, MgAg that is an alloy including Mg and Ag at a ratio of 0.5:0.5 is layered on the hole blocking layer9by vacuum vapor deposition to form the cathode electrode12. Alternatively, MgAg that is an alloy including Mg and Ag at a ratio of and Ag may be layered on the hole blocking layer9by vacuum vapor deposition to form the cathode electrode12. A film thickness of the cathode electrode12can be, for example, 10 to 50 nm.

In the light-emitting device100having the configuration described above, holes (arrow h+inFIG.1) injected from the anode electrode4are transported to the light-emitting layer8via the hole injection layer5, the hole transport layer6, and the electron blocking layer7. Further, electrons (arrow e−inFIG.1) injected from the cathode electrode12are transported to the light-emitting layer8via the electron injection layer11, the electron transport layer10, and the hole blocking layer9. The hole and the electron transported to the light-emitting layer8recombine to generate an exciton. Then, the exciton returns from an excited state to a ground state, and thus light is emitted.

Note that, in the light-emitting device100according to the embodiment of the disclosure, as illustrated inFIG.1, a top-emitting configuration in which light emitted from the light-emitting layer8is extracted from an opposite side to the array substrate2(upward direction inFIG.1) is exemplified.

Energy Relationship of Light-Emitting Element

Next, with reference toFIG.3, a relationship of energy between layers constituting the light-emitting element3having the configuration described above will be described.FIG.3is an energy diagram illustrating a relationship between a lowest unoccupied molecular orbital (LUMO) and a highest occupied molecular orbital (HOMO) in each layer of the light-emitting element3according to the embodiment of the disclosure.FIG.3illustrates a state where no voltage is applied from the outside and each layer included in the light-emitting element3is isolated.

Note that, as illustrated inFIG.3, the cathode electrode12, the electron injection layer11, the electron transport layer10, the hole blocking layer9, the light-emitting layer8, the electron blocking layer7, the hole transport layer6, the hole injection layer and the anode electrode4are arranged from left to right in the drawing. In the specification, the electron injection layer11, the electron transport layer10, the hole blocking layer9, the light-emitting layer8, the electron blocking layer7, the hole transport layer6, and the hole injection layer5are denoted as EIL, ETL, HBL, EML, EBL, HTL, and HIL, respectively, in the drawing.

In the energy diagram illustrated inFIG.3, the anode electrode4, the cathode electrode12, and the electron injection layer11are represented by a work function. A lower end of each of the electron transport layer10, the hole blocking layer9, the light-emitting layer8, the electron blocking layer7, the hole transport layer6, and the hole injection layer5corresponds to the HOMO, and indicates an ionization potential of each layer based on a vacuum level20.

InFIG.3, an upper end of each of the electron transport layer10, the hole blocking layer9, the light-emitting layer8, the electron blocking layer7, the hole transport layer6, and the hole injection layer5corresponds to the LUMO, and indicates an electron affinity of each layer based on the vacuum level20. In the following description, both the ionization potential and the electron affinity are assumed to be based on the vacuum level20when the ionization potential or the electron affinity is described simply.

In the light-emitting element3according to the embodiment, as described above, the light-emitting layer8includes a material having a perovskite structure. Thus, the light-emitting layer8has a narrow full width at half maximum (FWHM) of a peak wavelength of an electroluminescence (EL) spectrum, and can emit light having deeper chromaticity than a light-emitting layer including an organic light-emitting material used in a general OLED.

In the light-emitting layer8including a material having a perovskite structure, a crystal itself of a semiconductor constituting the light-emitting layer8emits light, and thus insulating properties increase and charge transport properties decrease. Further, charges that do not consume energy in the light-emitting layer8pass through the light-emitting layer8as they are and leak from the light-emitting layer8.

Thus, when a material having a perovskite structure is used as a light-emitting material in the light-emitting layer8, it is necessary to adjust charge injection into the light-emitting layer8to achieve carrier balancing in the light-emitting layer8, and to confine charges in the light-emitting layer8to improve a recombination probability of electrons and holes.

Thus, in the light-emitting element3according to the embodiment, the electron blocking layer7and the hole blocking layer9are provided so as to sandwich the light-emitting layer8, and the electron blocking layer7and the hole blocking layer9are configured to have an effect of confining charges in the light-emitting layer8.

In other words, as illustrated inFIG.3, a value of the LUMO of the light-emitting layer8is −3.3, and a value of the LUMO of the electron blocking layer7provided adjacent to an anode electrode-side main surface of the light-emitting layer8is −2.4. In this way, a value of the LUMO of the electron blocking layer7is greater than that of the light-emitting layer8. In other words, the electron blocking layer7having an electron affinity less than that of the light-emitting layer8is provided adjacent to the anode electrode-side main surface of the light-emitting layer8. Thus, the electron blocking layer7can suppress migration, from the light-emitting layer8to the anode electrode4side, of electrons (indicated by (−) inFIG.3) injected into the light-emitting layer8.

On the other hand, a value of the HOMO of the light-emitting layer8is −5.8, and a value of the HOMO of the hole blocking layer9provided adjacent to a main surface of the light-emitting layer8on the cathode electrode12side is −6.0. In this way, a value of the HOMO of the hole blocking layer9is less than that of the light-emitting layer8. In other words, the hole blocking layer9having an ionization potential greater than that of the light-emitting layer8is provided adjacent to the cathode electrode-side main surface of the light-emitting layer8. Thus, the hole blocking layer9can suppress migration, from the light-emitting layer8to the cathode electrode12side, of holes (indicated by (+) inFIG.3) injected into the light-emitting layer8.

Thus, in the light-emitting element3according to the embodiment, the recombination probability can be improved by confining holes and electrons in the light-emitting layer8. Therefore, the light-emitting element3according to the embodiment can improve the lifetime of the light-emitting layer8.

Note that, in the above description, the configuration of the light-emitting element3including the light-emitting layer8including CsPbBr3, the electron blocking layer7including PCPPn, and the hole blocking layer9including CBP/CsCO3is described as an example. However, the light-emitting element3is not limited to this configuration as long as the recombination probability can be improved by confining electrons and holes in the light-emitting layer8as described above.

For example, when the light-emitting layer8includes Cl as a material having a perovskite structure, the electron blocking layer7may include PCPPn and a p-type semiconductor material such as MoO3or V2O5, and the hole blocking layer9may include CBP and an n-type semiconductor material such as ZnO, SiO, and TiO2.

Further, the light-emitting element3according to the embodiment described above has the configuration in which the electron blocking layer7and the hole blocking layer9are provided in the positions adjacent to the light-emitting layer8. However, the light-emitting element3does not necessarily need to include both of the electron blocking layer7and the hole blocking layer9, and may be configured to include only one of the electron blocking layer7and the hole blocking layer9as long as an effect of confining charges is obtained in the light-emitting layer8.

Viewing Angle Characteristic of Light-Emitting Device

Next, a viewing angle characteristic of the light-emitting device100according to the embodiment will be described with reference toFIG.4.FIG.4is a diagram schematically illustrating an example of a path of light emitted from the light-emitting element3included in the light-emitting device100according to the embodiment of the disclosure.

As illustrated inFIG.4, in the light-emitting element3according to the embodiment, the first electrode located in the lower layer (the anode electrode4located on the array substrate2side) is a reflective electrode, and the second electrode located in the upper layer (the cathode electrode12located on a side opposite to the array substrate2) is a transparent electrode. Then, the light-emitting element3has a top-emitting configuration in which light is extracted from a light extraction surface (not illustrated) provided above the light-emitting element3. In such a configuration, light emitted from the light-emitting layer8has a path A directly traveling toward the light extraction surface and a path B reflected by the first electrode (anode electrode4) and traveling toward the light extraction surface, and an optical distance of the path B is longer than that of the path A by a round-trip distance between the light-emitting layer8and the anode electrode4. In particular, the light-emitting element3according to the embodiment has a configuration in which the hole transport layer6and the hole injection layer5having a film thickness thicker than that of the electron transport layer10and the electron injection layer11are disposed between the light-emitting layer8and the first electrode (anode electrode4). Thus, a distance between the light-emitting layer8and the first electrode (anode electrode4) increases. When the optical distance between the light-emitting layer8and the first electrode (anode electrode4) is a half wavelength of a wavelength (λ) of the light emitted from the light-emitting layer8, the extracted light is strongly subjected to optical interference. Thus, it is necessary to optimize a film thickness of each layer included in the light-emitting element3.

Thus, the light-emitting element3according to the embodiment is configured to have a layered structure in which an optical distance from the anode electrode4to the light-emitting layer8satisfies the relationship of 1/2×λ×n (n is an odd number), where a wavelength of the light emitted from the light-emitting layer8is λ. Note that it is particularly preferable that n is 3. By adjusting a film thickness of the electron blocking layer7, the light-emitting element3can have a layered body structure having an optical distance that satisfies the relationship described above.

In this way, since the light-emitting element3has a layered structure in which an optical distance from the first electrode (cathode electrode12) to the light-emitting layer8satisfies the relationship described above, the light directly extracted from the light-emitting layer8and the light reflected by the anode electrode4and extracted are in the same phase, and the two light beams have a relationship in which they intensify each other by interference. Thus, a waveform of the peak wavelength of the EL spectrum becomes steeper and more conspicuous. In other words, only light of a desired wavelength can be emphasized. Thus, the viewing angle characteristic of the light-emitting device100can be improved.

Note that the viewing angle characteristic is a chromaticity shift, a change in luminance, or the like between when a display surface is viewed from the front of the light-emitting device100(a direction perpendicular to the display surface of the light-emitting device100) and when the display surface is viewed from a direction inclined at a certain angle from the front.

Evaluation Experiment on Viewing Angle Characteristic

Here, the light-emitting device100according to the embodiment described above and a light-emitting device according to a first comparative example including a light-emitting element including an organic light-emitting material used in a general OLED were prepared. Angle dependence related to a chromaticity shift between the two was simulated by using SETFOS manufactured by Cyber Net Inc. As a result, a graph shown inFIG.5was obtained.FIG.5is a graph showing the angle dependence related to the chromaticity shift between the light-emitting device100according to the embodiment of the disclosure and the light-emitting device according to the first comparative example. InFIG.5, the horizontal axis represents an angle from the front, and the vertical axis represents a chromaticity shift (Δx, y).

Note that the angle from the front indicates an inclination from a reference (0 degree) which is a direction perpendicular to the display surface of the light-emitting device100. The chromaticity shift (Δx, y) indicates a difference between chromaticity when the display surface is viewed from the front and chromaticity when the display surface is viewed from a position inclined from the reference. Specifically, the chromaticity shift (Δx, y) is represented by a Euclidean distance between two colors represented by color coordinates (x, y) in the CIE color system.

The light-emitting element included in the light-emitting device according to first comparative example had a layered structure similar to that of the light-emitting element3according to the first embodiment, and a tricoordinate iridium complex (Ir(ppy)3) was used as an organic light-emitting layer material forming a light-emitting layer. Note that each layer constituting the light-emitting element included in the light-emitting device according to the first comparative example has a configuration similar to that of each layer of the light-emitting element3according to the first embodiment except for the light-emitting layer.

As shown inFIG.5, for example, when the angle from the front is 40 degrees, the chromaticity shift (Δx, y) is 0.051 in the light-emitting device100according to the first embodiment, whereas the chromaticity shift (Δx, y) is 0.100 in the light-emitting device according to the first comparative example. In this way, it was found that the chromaticity shift of the light-emitting device100according to the first embodiment was reduced by half as compared with the light-emitting device according to the first comparative example.

Further, there was no significant difference in luminance between the light-emitting device100according to the first embodiment and the light-emitting device according to the first comparative example. On the other hand, the chromaticity of the light-emitting device100in the CIE color system (x, y) was (0.13, 0.81), and the chromaticity of the light-emitting device according to the first comparative example in the CIE color system (x, y) was (0.20, 0.79). From this result, it was found that the color purity of the light-emitting device100was higher than that of the light-emitting device according to the first comparative example.

First Modified Example

Next, the light-emitting device100according to a first modified example of the embodiment of the disclosure will be described with reference toFIGS.6and7.FIG.6is a schematic cross-sectional view of the light-emitting device100according to the first modified example of the embodiment of the disclosure. InFIG.6, a direction from the array substrate2of the light-emitting device100toward the light-emitting element3may be described as an “upward direction”, and an opposite direction may be described as a “downward direction”.FIG.7is a table showing a correspondence relationship between each layer constituting the light-emitting element3included in the light-emitting device100illustrated inFIG.6, and a material forming each layer.

The light-emitting device100according to the embodiment has a configuration in which the anode electrode4is disposed in the lower layer and the cathode electrode12is disposed in the upper layer. In contrast, the light-emitting device100according to the first modified example of the embodiment has a configuration in which the cathode electrode12is disposed in the lower layer and the anode electrode4is disposed in the upper layer. In other words, a layering order of the layers is reversed between the light-emitting device100according to the embodiment and the light-emitting device100according to the first modified example of the embodiment.

Further, a material constituting each layer is changed by reversing the layering order of each layer. In particular, the light-emitting device100according to the first modified example is significantly different from the light-emitting element3according to the embodiment in a point that an inorganic material is used as a material constituting the electron injection layer11.

Specifically, the light-emitting element3included in the light-emitting device100according to the first modified example of the embodiment has a configuration in which the cathode electrode12(first electrode), the electron injection layer11, the electron transport layer10, the hole blocking layer9, the light-emitting layer8, the electron blocking layer7, the hole transport layer6, the hole injection layer5, and the anode electrode4(second electrode) are layered in this order from the bottom.

The light-emitting element3according to the first modified example of the embodiment can be manufactured as follows. First, as shown inFIG.7, the cathode electrode12is formed of a layered body of Ag and ITO. In other words, Ag is layered as a reflective layer on the array substrate2by sputtering. A thickness of the reflective layer film-formed on the array substrate2can be, for example, 100 nm. Subsequently, a transparent electrode (ITO) is continuously layered. A thickness of the transparent electrode layered herein can be, for example, 20 nm. The layered body of Ag/ITO formed in such a manner is processed into a desired pattern by, for example, photolithography to form the cathode electrode12.

Next, as shown inFIG.7, the electron injection layer11is formed of amorphous Zn—Si—O (ZSO) in which ZnO is doped with SiO. A composition ratio of Si in the ZSO is a value such that a proportion of Zn in Zn+Si is in a range of 75% or more and 80% or less.

The electron injection layer11is formed by depositing a ZSO layer having a film thickness of 100 nm on the cathode electrode12by sputter deposition. Although a configuration in which ZSO is used as a material for forming the electron injection layer11has been described, for example, (CaO)12(Al2O3)7or an oxide such as BaO having a work function around −3 eV may be used as a material for forming the electron injection layer11.

Next, as shown inFIG.7, the electron transport layer10is formed by co-evaporating 1,3,5-tris (1-phenyl-1H-benzimidazol-2-yl) benzene (TPBi) and CsCO3on the electron injection layer11.

Subsequently, CBP is deposited on the electron transport layer10to form the hole blocking layer9having a film thickness of 10 nm. Note that, before the hole blocking layer9is formed, surface treatment may be performed by Ar plasma in order to activate the surface of the oxide of the electron transport layer10located below the hole blocking layer9.

Note that a film thickness of the hole blocking layer9may be appropriately adjusted such that an optical distance in which the light emitted from the light-emitting layer8of the light-emitting element3according to the first modified example moves has a layered structure that satisfies the above-described condition (1/2×λ×n (n is an odd number)).

After the hole blocking layer9is formed as described above, the light-emitting layer8is formed on the hole blocking layer9. A method for forming the light-emitting layer8is similar to that of the light-emitting layer8included in the light-emitting element3according to the embodiment described above, and thus description thereof will be omitted.

After the light-emitting layer8is formed, PCPPn and MoO3are co-evaporated in vacuum on the light-emitting layer8to form the electron blocking layer7having a film thickness of 10 nm.

After the electron blocking layer7is formed, NPD is deposited on the electron blocking layer7to form the hole transport layer6having a film thickness of 20 nm.

After the hole transport layer6is formed, MoO3is deposited on the hole transport layer6to form the hole injection layer5having a film thickness of 5 nm.

After the hole injection layer5is formed, Ag is deposited on the hole injection layer5to form the anode electrode4having a film thickness of 20 nm.

FIG.8illustrates a relationship of energy between layers constituting the light-emitting element3according to the first modified example having the configuration described above.FIG.8is an energy diagram illustrating a relationship between a lowest unoccupied molecular orbital (LUMO) and a highest occupied molecular orbital (HOMO) in each layer of the light-emitting element3according to the first modified example of the embodiment of the disclosure.FIG.8illustrates a state where no voltage is applied from the outside and each layer included in the light-emitting element3is isolated.

As illustrated inFIG.8, a value of the LUMO of the light-emitting layer8is −3.3, and a value of the LUMO of the electron blocking layer7provided adjacent to an anode electrode-side main surface of the light-emitting layer8is −2.4. In this way, a value of the LUMO of the electron blocking layer7is greater than that of the light-emitting layer8. In other words, the electron blocking layer7having an electron affinity less than that of the light-emitting layer8is provided adjacent to the anode electrode-side main surface of the light-emitting layer8. Thus, the electron blocking layer7suppresses migration from the light-emitting layer8to the anode electrode4side of electrons (indicated by (−) inFIG.8) injected into the light-emitting layer8.

On the other hand, a value of the HOMO of the light-emitting layer8is −5.8, and a value of the HOMO of the hole blocking layer9provided adjacent to a main surface of the light-emitting layer8on the cathode electrode12side is −6.0. In this way, a value of the HOMO of the hole blocking layer9is less than that of the light-emitting layer8. In other words, the hole blocking layer9having an ionization potential greater than that of the light-emitting layer8is provided adjacent to the cathode electrode-side main surface of the light-emitting layer8. Thus, the hole blocking layer9suppresses migration from the light-emitting layer8to the cathode electrode12side of holes (indicated by (+) inFIG.8) injected into the light-emitting layer8.

Thus, in the light-emitting element3according to the first modified example of the embodiment, the recombination probability can be improved by confining holes and electrons in the light-emitting layer8. Therefore, the light-emitting element3according to the first modified example of the embodiment can improve the lifetime of the light-emitting layer8.

Further, the light-emitting element3according to the first modified example of the embodiment described above has the configuration in which the electron blocking layer7and the hole blocking layer9are provided in the positions adjacent to the light-emitting layer8. However, the light-emitting element3does not necessarily need to include both of the electron blocking layer7and the hole blocking layer9, and may be configured to include only one of the electron blocking layer7and the hole blocking layer9as long as an effect of confining charges is obtained in the light-emitting layer8.

For example, in a case of a configuration in which the electron transport layer10is formed of an inorganic material such as ZSO, the electron transport layer10can function as the hole blocking layer9, and thus a configuration in which only the electron blocking layer7is provided and the hole blocking layer9is not provided may be employed.

In the light-emitting element3according to the first modified example of the embodiment, the first electrode located in the lower layer (the cathode electrode12located on the array substrate2side) is a reflective electrode, and the second electrode located in the upper layer (the anode electrode4located on a side opposite to the array substrate2) is a transparent electrode. Then, the light-emitting element3has a top-emitting configuration in which light is extracted from a light extraction surface (not illustrated) provided above the light-emitting element3.

Thus, similarly to the light-emitting element3according to the embodiment, the light-emitting element3according to the first modified example of the embodiment is configured to have a layered structure in which an optical distance from the first electrode (cathode electrode12) to the light-emitting layer8satisfies the relationship of 1/2×λ×n (n is an odd number), where a wavelength of the light emitted from the light-emitting layer8is λ. Note that it is particularly preferable that n is 3. By adjusting a film thickness of the hole blocking layer9, the light-emitting element3according to the first modified example may achieve a layered body structure having an optical distance that satisfies the relationship described above.

In this way, since the light-emitting element3according to the first modified example has a layered structure in which an optical distance from the first electrode (cathode electrode12) to the light-emitting layer8satisfies the relationship described above, the light directly extracted from the light-emitting layer8and the light reflected by the anode electrode4and extracted are in the same phase, and the two light beams have a relationship in which they intensify each other by interference. Thus, the viewing angle characteristic of the light-emitting device100can be improved.

Evaluation Experiment on Viewing Angle Characteristic

An evaluation experiment on a viewing angle characteristic is also performed on the light-emitting device100according to the first modified example of the embodiment similarly to the light-emitting device according to the embodiment. As a result, a graph shown inFIG.9was obtained.FIG.9is a graph showing angle dependence related to a chromaticity shift between the light-emitting device100according to the first modified example of the embodiment of the disclosure and the light-emitting device according to the first comparative example. InFIG.9, the horizontal axis represents an angle from the front, and the vertical axis represents a chromaticity shift (Δx, y). Note that a technique of the evaluation experiment is similar to the technique performed on the optical device100according to the embodiment, and thus description thereof will be omitted.

As shown inFIG.9, for example, when the angle from the front is 40 degrees, the chromaticity shift (Δx, y) is 0.060 in the light-emitting device100according to the first modified example, whereas the chromaticity shift (Δx, y) is 0.100 in the light-emitting device according to the first comparative example. In this way, it was found that the chromaticity shift of the light-emitting device100according to the first modified example was reduced by half as compared with the light-emitting device according to the first comparative example.

Further, there was no significant difference in luminance between the light-emitting device100according to the first modified example and the light-emitting device according to the first comparative example. On the other hand, the chromaticity of the light-emitting device100in the CIE color system (x, y) was (0.12, 0.81), and the chromaticity of the light-emitting device according to the first comparative example in the CIE color system (x, y) was (0.20, 0.79). From this result, it was found that the color purity of the light-emitting device100was higher than that of the light-emitting device according to the first comparative example.

Second Modified Example

Next, the light-emitting device100according to a second modified example of the embodiment of the disclosure will be described with reference toFIG.10.FIG.10is a table showing a correspondence relationship between each layer constituting the light-emitting element3included in the light-emitting device100according to the second modified example of the embodiment of the disclosure, and a material forming each layer.

The light-emitting element3included in the light-emitting device100according to the second modified example has a configuration substantially similar to that of the light-emitting element3included in the light-emitting device100according to the first modified example. However, the light-emitting element3according to the second modified example is different from the light-emitting element3according to the first modified example in a point that the cathode electrode12disposed in the lower layer of the light-emitting element3according to the first modified example is a reflective electrode and the anode electrode4disposed in the upper layer is a transparent electrode to form a top-emitting configuration, whereas the cathode electrode12disposed in the lower layer of the light-emitting element3according to the second modified example is a transparent electrode to form a bottom-emitting configuration.

Thus, as shown inFIG.10, the light-emitting element3according to the second modified example is different from the light-emitting element3according to the first modified example in a point that the cathode electrode12in the light-emitting element3according to the first modified example is formed of an Ag/ITO layered body, whereas the cathode electrode12in the light-emitting element3according to the second modified example is formed of ITO. In this way, in the light-emitting element3according to the second modified example, each layer is formed of a material similar to that of the light-emitting element3according to the first modified example except that a material forming the cathode electrode12is changed, and thus a method for manufacturing each layer will be omitted.

Further, since each layer of the light-emitting element3according to the second modified example is formed of a material similar to that of each layer of the light-emitting element3according to the first modified example except for the cathode electrode12as described above, an energy relationship of each layer is also similar. Thus, similarly to the light-emitting element3according to the first modified example, in the light-emitting element3according to the second modified example, the recombination probability can be improved by confining holes and electrons in the light-emitting layer8. Therefore, the light-emitting element3according to the second modified example of the embodiment can improve the lifetime of the light-emitting layer8.

Evaluation Experiment on Viewing Angle Characteristic

An evaluation experiment on a viewing angle characteristic related to a chromaticity shift is also performed on the light-emitting device100according to the second modified example of the embodiment similarly to the light-emitting device according to the embodiment. The light-emitting device100according to the second modified example, the light-emitting device according to the first comparative example described above, and a light-emitting device according to a second comparative example were prepared. The light-emitting device according to the second comparative example is obtained by changing the configuration of the light-emitting device according to the first comparative example from the top-emitting configuration to the bottom-emitting configuration. A technique of the evaluation experiment is similar to the technique performed on the optical device100according to the embodiment and the optical device100according to the first modified example of the embodiment, and thus description thereof will be omitted.

The results shown inFIG.11were obtained by this evaluation experiment.FIG.11is a graph showing angle dependence related to a chromaticity shift among the light-emitting device100according to the second modified example of the embodiment of the disclosure, the light-emitting device according to the first comparative example, and the light-emitting device according to the second comparative example. InFIG.11, the horizontal axis represents an angle from the front, and the vertical axis represents a chromaticity shift (Δx, y).

As shown inFIG.11, when the light-emitting device according to the first comparative example having a top-emitting light-emitting element was compared with the light-emitting device according to the second comparative example having a bottom-emitting light-emitting element, it was found that the light-emitting device according to the first comparative example had a greater chromaticity shift in accordance with the angle from the front. In particular, it was found that a difference in the magnitude of the chromaticity shift significantly increased as the angle from the front increased. The conceivable reason is that optical interference hardly occurs in the bottom-emitting light-emitting element unlike the top-emitting light-emitting element.

Next, the light-emitting device100according to the second modified example of the embodiment and the light-emitting device according to the second comparative example are compared. Since both of the light-emitting devices have a configuration including a bottom-emitting light-emitting element, a chromaticity shift hardly occurs even when the angle from the front becomes greater. However, it was found that the light-emitting device100according to the second modified example of the embodiment had a less chromaticity shift than that of the light-emitting device according to the second comparative example. Thus, it was found that the angle dependence related to the chromaticity shift could be further improved by forming the light-emitting layer8of the light-emitting element3by using a material having a perovskite structure.

Next, the angle dependence related to luminance between the light-emitting device100according to the second modified example of the embodiment described above and the light-emitting device according to the second comparative example was simulated by using SETFOS manufactured by Cyber Net Inc. As a result, a graph shown inFIG.12was obtained.FIG.12is a graph showing angle dependence related to luminance between the light-emitting device100according to the second modified example of the embodiment of the disclosure and the light-emitting device according to the second comparative example. InFIG.12, the horizontal axis represents an angle from the front, and the vertical axis represents a luminance ratio (%) to the front. Note that the luminance ratio to the front indicates a ratio of a luminance value when the display surface of the light-emitting device is viewed from the front to a luminance value when the display surface is viewed from a direction inclined at a certain angle from the front.

As shown inFIG.12, in both of the light-emitting device100according to the second modified example of the embodiment and the light-emitting device according to the second comparative example, it was found that a luminance value decreased as an inclination from the front increased. For example, when the angle from the front was 60 degrees, the luminance ratio of the light-emitting device100according to the second modified example of the embodiment to the front was 55%, and the luminance ratio of the light-emitting device according to the second comparative example to the front was 38%. However, it was found that the light-emitting device100according to the second modified example of the embodiment was less likely to decrease in luminance than the light-emitting device according to the second comparative example.

Further, the chromaticity of the light-emitting device100according to the second modified example of the embodiment in the CIE color system (x, y) was (0.12, 0.81), and the chromaticity of the light-emitting device according to the second comparative example in the CIE color system (x, y) was (0.27, 0.67). From this result, it was found that the color purity of the light-emitting device100according to the second modified example of the embodiment was higher than that of the light-emitting device according to the second comparative example.

As described above, it was found that the light-emitting element3according to second modified example was more improved in chromaticity shift, luminance decrease, and color purity than the light-emitting element according to the second comparative example.

Note that the elements described in the above-described embodiments and the modified examples may be appropriately combined in a range in which a contradiction does not arise.

REFERENCE SIGNS LIST