Patent ID: 12256589

DESCRIPTION OF EMBODIMENTS

First Embodiment

The following describes aspects of the present disclosure. In the following, a layer formed in a process step anterior to a process step of forming a comparative layer will be referred to as a “lower layer” or will be described using an equivalent word, and a layer formed in a process step posterior to a process step of forming a comparative layer will be referred to as an “upper layer” or will be described using an equivalent word.

Schematic Configuration of Light-Emitting Element

FIG.1is a longitudinal sectional view of an example schematic configuration of a light-emitting element10according to this embodiment cut in the direction of the normal to the light-emitting element10(that is, the light-emitting element10cut in its stacking direction).

As illustrated inFIG.1, the light-emitting element10includes an anode1, a cathode6, and an emission layer (hereinafter, referred to as an EML)4disposed between the anode1and cathode6. Disposed between the anode1and EML4is a hole transport layer (hereinafter, referred to as an “HTL”)3, which is a layer capable of transporting holes. Disposed between the anode1and HTL3is a reducing-material-containing layer (hereinafter, referred to as an “REL”)2being in contact with the anode1and HTL3. Disposed between the EML4and cathode6may or may not be an electron-transport layer (hereinafter, referred to as an “ETL”)5.

FIG.1illustrates, by way of example, the light-emitting element10having, in the order of listing, the anode1, the REL2, the HTL3, the EML4, the ETL5, and the cathode6stacked from the bottom. However, the configuration of the light-emitting element10is not limited to the foregoing, as described above.

The anode1is made of a conductive material and injects holes into a layer between the anode1and cathode6. The cathode6is made of a conductive material and injects electrons into a layer between the cathode6and anode1.

Examples of the conductive material used for the anode1include the following: metals that are used for an anode in related art, such as aluminum (Al), silver (Ag), and magnesium (Mg); alloys of these metals; inorganic oxides, such as indium tin oxide (ITO) and indium gallium zinc oxide (InGaZnOx); and conductive compounds containing these inorganic oxides doped with impurities. These conductive materials may be used alone or in combination, as appropriate, with two or more kinds.

Examples of the conductive material used for the cathode6include the following: metals that are used for a cathode in related art, such as Al, Ag, and Mg; and alloys of these metals. These conductive materials may be used alone or in combination, as appropriate, with two or more kinds. The foregoing alloys may further contain lithium (Li).

One of the anode1and cathode6that serves as a surface from which light is taken out needs to be transparent. The other one of these electrodes that is opposite to this light-taking surface may or may not be transparent. At least one of the anode1and cathode6is thus made of a light-transparency material. One of the anode1and cathode6may be made of a light-reflective material. When the light-emitting element10inFIG.1is a top-emission light-emitting element, the cathode6, which is an upper layer, is made of a light-transparency material, and the anode1, which is a lower layer, is made of a light-reflective material. When the light-emitting element10inFIG.1is a bottom-emission light-emitting element, the cathode6, which is an upper layer, is made of a light-reflective material, and the anode1, which is a lower layer, is made of a light-transparency material.

The REL2is a layer containing a reducing material that reduces the HTL3(more strictly, a hole-transporting material that constitutes the HTL3) and having the action of reduction on the HTL3. The REL2has the function of forming a defect onto a surface of the HTL3adjacent to the anode1(to be specific, a surface being in contact with the REL2) after the anode1to the HTL3are stacked sequentially. This embodiment includes introducing the REL2between the anode1and HTL3in order to introduce a defect onto the surface of the HTL3, as described above.

This embodiment describes an instance where the REL2contains, as a reducing material, a hydrogen-absorbing material, which is referred to as a hydrogen-absorbing metal or hydrogen-absorbing alloy, that can absorb and desorb hydrogen reversibly.

The hydrogen-absorbing material may be a metal or an alloy having the property of absorbing hydrogen through transformation into a solid solution, or the hydrogen-absorbing material may be a metal or an alloy having the property of absorbing hydrogen through chemical bonding.

The reducing material contains at least one of hydrogen-absorbing materials, i.e., a hydrogen-absorbing metal and a hydrogen-absorbing alloy, that absorb hydrogen through transformation into a solid solution, or through chemical bonding. This enables the surface of the HTL3being in contact with the REL2to undergo reduction using the hydrogen contained in the hydrogen-absorbing metal or hydrogen-absorbing alloy, thereby forming a defect onto the surface of the HTL3being in contact with REL2.

A suitable example of the hydrogen-absorbing material is either at least one hydrogen-absorbing metal or at least one hydrogen-absorbing alloy selected from the group consisting of the following: a metal or an alloy containing at least one element selected from the group consisting of palladium (Pd), platinum (Pt), hafnium (Hf), and tantalum (Ta); a so-called, AB2-type alloy consisting of a transition metal, such as titanium (Ti), manganese (Mn), zirconium (Zr), or nickel (Ni); a so-called, AB5-type alloy with the composition ratio between a rare earth, a rare-earth element, niobium (Nb) or zirconium (Zr), and a transition element having a catalyst effect, such as nickel (Ni), cobalt (Co), or aluminum (Al), standing at 1:5; a Ti—Fe-based alloy, which is an alloy containing titanium and iron; a V-based alloy, which is an alloy containing vanadium; a Mg-based alloy, which is an alloy containing magnesium; a Pd-based alloy, which is an alloy containing palladium; and a Ca-based alloy, which is an alloy containing calcium.

It is desirable that the foregoing reducing material (i.e., the foregoing hydrogen-absorbing material) desirably contain, in its structure, hydrogen either at a concentration ratio of 1 to 1 with respect to a base metal, or at a larger concentration ratio than the base metal. This enables the surface of the HTL3being in contact with the REL2to undergo reduction efficiently.

Depending on the kind of the reducing material, the REL2preferably has a thickness of 0.5 to 1 nm inclusive. The HTL3can undergo reduction with certainty when the REL2has a thickness equal to or greater than 0.5 nm. Moreover, forming the REL2thinly to be 1 nm or smaller thick enables only the uppermost surface of the HTL3to undergo reduction and enables hole conduction through tunneling.

When having a thickness equal to or greater than 1 nm, the REL2in the form of a flat film can be obtained. In contrast, the REL2loses flatness when having a thickness less than 1 nm. Further, a continuous film as shown inFIG.1can be obtained when the REL2has a thickness equal to or greater than 0.5 nm. The continuous film refers to a dense film having a void ratio of less than 1%. That is, the continuous film refers to a film having no void substantially. However, such a continuous film has surface asperities as thick as its thickness in some cases. In this embodiment, a film in such a state is referred to as a film with flatness lost. It is noted that the REL2having a thickness of less than 0.5 nm naturally constitutes an island form rather than a continuous film.

The HTL3may be either of a hole transport layer and a hole injection layer. The hole transport layer is a layer that transports holes from the anode1to the EML4. The hole injection layer is a layer that promotes hole injection from the anode1to the EML4. In some embodiments, the hole transport layer may serve also as a hole injection layer. As such, the light-emitting element10may include the HTL3with a hole injection layer and a hole transport layer stacked on the REL2in this order for instance. Alternatively, the light-emitting element10may include the HTL3with a hole transport layer stacked directly on the REL2.

The HTL3can be made of a known hole-transporting material consisting of a wide-gap compound, a wide-gap metal oxide, or other substances. The HTL3may contain an inorganic hole-transporting material, including, but not limited to, a metal oxide or an oxide semiconductor, a group IV semiconductor, a group II-VI compound semiconductor, and a group III-V compound semiconductor, all of which are commonly known as a hole-transporting material. Examples of the metal oxide or oxide semiconductor include, but not limited to, molybdenum trioxide (MoO3), chromium oxide (Cr2O3), nickel oxide (NiO), tungsten trioxide (WO3), indium tin oxide (ITO), indium gallium zinc oxide (InGaZnOx), gallium oxide (Ga2O3), and indium oxide (In2O3). It is noted that the classification between a metal oxide and an oxide semiconductor is not necessarily clear; it is thus safe to say that both are together referred to as “a metal oxide or an oxide semiconductor” in order to achieve a function as the HTL3. Examples of the foregoing group IV semiconductor includes, but not limited to, silicon (Si) and germanium (Ge). Examples of the foregoing group II-VI compound semiconductor include, but not limited to, zinc indium-doped oxide (IZO), zinc aluminum-doped oxide (ZAO), zinc oxide (ZnO), magnesium oxide (MgO), zinc magnesium oxide (ZnMgO), zinc sulfide (ZnS), zinc selenide (ZnSe), zinc sulfide selenide (ZnSSe), magnesium sulfide (MgS), magnesium selenide (MgSe), and magnesium sulfide selenide (MgSSe). Examples of the foregoing group III-V compound semiconductor include, but not limited to, the following: aluminum arsenide (AlAs), gallium arsenide (GaAs), indium arsenide (InAs), and aluminum gallium indium arsenide (AlGaInAs), which is a mixed crystal of these substances; aluminum nitride (AlN), gallium nitride (GaN), indium nitride (InN), aluminum gallium indium nitride (AlGaInN), which is a mixed crystal of these substances; and gallium phosphide (GaP), and aluminum gallium indium phosphide (AlGaInP). These hole-transporting materials are mere examples, and thus the hole-transporting material is not limited to only the foregoing examples. Furthermore, these hole-transporting materials may be used alone or in combination, as appropriate, with two or more kinds. In other words, the HTL3may contain at least one hole-transporting material selected from the group consisting of the foregoing example hole-transporting materials. Here, when the HTL3is an organic substance, reducing the HTL3loses the reliability of the light-emitting element10. The HTL3is hence made of an inorganic substance (i.e., an inorganic hole-transporting material) as described above, even when the light-emitting element10is an OLED for instance as described later on.

The ETL5may be either of an electron transport layer and an electron injection layer. The electron transport layer is a layer that transports electrons from the cathode6to the EML4. The electron injection layer is a layer that promotes electron injection from the cathode6to the EML4. In some embodiments, the electron transport layer may serve also as an electron injection layer, or the cathode6may serve also as an electron injection layer. The light-emitting element10may thus include, between the cathode6and EML4, the ETL5with an electron injection layer and an electron transport layer on the cathode6in this order, or with only an electron transport layer on the cathode6.

The ETL5can be made of a known electron-transporting material. The ETL5may contain, as an electron-transporting material, an inorganic electron-transporting material, including, but not limited to, zinc oxide (ZnO), magnesium oxide (MgO), magnesium zinc oxide (ZnMgO), titanium oxide (TiO2), lithium fluoride (LiF), molybdenum trioxide (MoO3), tungsten trioxide (WO3), indium gallium zinc oxide (InGaZnOx), zinc aluminum-doped oxide (ZAO), indium oxide (In2O3), gallium oxide iodide (Ga2OI3), zinc sulfide (ZnS), zinc selenide (ZnSe), and zinc telluride (ZnTe); alternatively, the ELT5may contain, as an electron-transporting material, an organic electron-transporting material, including, but not limited to, oxadiazoles, triazoles, phenanthrolines, a silole derivative, and a cyclopentadiene derivative. These electron-transporting materials may be used alone or in combination, as appropriate, with two or more kinds.

The HTL3and the ETL5may have any thickness such that the respective functions of hole transport and electron injection are exerted sufficiently. The HTL3and the ETL5can be set to have a thickness similar to the thickness of a hole transport layer and to the thickness of an electron transport layer both included in a light-emitting element that is known.

The EML4is a layer that contains a light-emitting material and emits light by rejoining of electrons transported from the cathode6and holes transported from the anode1together.

The EML4may contain, as a light-emitting material, nano-sized quantum dots (semiconductor nanoparticles) for instance. The quantum dots can be a known quantum dot. Each quantum dot may contain, for instance, at least one semiconductor material composed of at least one element selected from the group consisting of cadmium (Cd), sulfur(S), tellurium (Te), selenium (Se), zinc (Zn), indium (In), nitrogen (N), phosphorus (P), arsenic (As), antimony (Sb), aluminum (Al), gallium (Ga), lead (Pb), silicon (Si), germanium (Ge), and magnesium (Mg). The quantum dot may be a binary core, a tertiary core, a quaternary core, a core-shell, or a core-multi shell. Alternatively, the quantum dot may contain a nanoparticle doped with at least one of the foregoing elements or may have a composition-graded structure.

The diameter of the quantum dot can be set as before. The diameter of the core of the quantum dot is 1 to 30 nm for instance, and the outermost diameter of the quantum dot including the shell is 1 to 50 nm for instance. Moreover, the light-emitting element10has 1 to 20 overlapping layers of quantum dots for instance. The EML4may have any thickness that can offer electron-hole rejoining to achieve the function of light emission and can be, for instance, about 1 to 200 nm thick. It is preferable that the EML4have a thickness that is about several times greater than the outermost diameter of the quantum dot.

This embodiment is not limited to the foregoing example. The EML4may contain, as light-emitting materials, organic light-emitting materials for instance that emit respective colors of light, instead of quantum dots.

When the light-emitting element10is a QLED that contains quantum dots as light-emitting materials, as described above, a drive current between the anode1and cathode6causes hole-electron rejoining within the EML4, thus generating excitons, which then emit light (fluorescent light) in the process of transition from the conduction band level of the light of quantum dots to the valence band level of the quantum dots.

When the light-emitting element10is an OLED that contains an organic light-emitting material as a light-emitting material, a drive current between the anode1and cathode6causes hole-electron rejoining within the EML4, thus generating excitons, which then emit light in the process of transition to a ground state.

The light-emitting element10may be a light-emitting element (e.g., an inorganic light-emitting diode) other than an OLED and a QLED.

At least one (e.g., multiple) light-emitting element10may be included in a light-emitting device, such as an illumination device or a display device, to be used as the light source of the light-emitting device.

The light-emitting element10may have a substrate not shown, and the anode1may be disposed on the substrate not shown.

Method for Manufacturing Light-Emitting Element10

The following describes, by way of example, a method for manufacturing the light-emitting element10.FIG.2is a flowchart showing, step by step, process steps for manufacturing the light-emitting element10according to this embodiment.

As illustrated inFIG.1andFIG.2, the manufacture of the light-emitting element10according to this embodiment starts with Step S1, i.e., forming the anode1onto a substrate not shown.

This substrate may be, for instance, a glass substrate or a flexible substrate, such as a resin substrate. When the light-emitting element10is part of a light-emitting device, such as a display device for instance, the substrate is a substrate of the light-emitting device. The substrate may be thus, for instance, an array substrate with a plurality of thin-film transistors on. The anode1in this case is electrically connected to the thin-film transistors of the array substrate. Forming the anode1can use various methods that are known as a method of anode formation, including, but not limited to, sputtering, vacuum deposition, chemical vapor deposition (CVD), plasma CVD, and printing.

The next is Step S2, i.e., forming (stacking) the REL2onto the anode1so as to be in contact with the anode1. Forming the REL2can use sputtering or evaporation for instance.

The next is Step S3, i.e., forming (stacking) the HTL3onto the REL2so as to be in contact with the REL2. Forming the HTL3can use various methods that are known as a method of forming a hole transport layer, including, but not limited to, sputtering, nanoparticle application, and precursor application.

The next is heating the stack obtained in Step S3. The hydrogen-absorbing metal or hydrogen-absorbing alloy within the REL2emits hydrogen in response to heating. The surface of the HTL3being in contact with the REL2undergoes reduction by the action of strong hydrogen reduction. Accordingly, a defect is formed onto the surface of HTL3being in contact with the REL2. This process step is Step S4.

It is noted that the stack obtained in Step3is a stack including the anode1, REL2, and HTL3, and is a stack of, in order of listing, the anode1, REL2, and HTL3on a substrate not shown, as described above.

In Step S4, the temperature at which the foregoing stack undergoes heating needs to equal to or higher than a temperature at which hydrogen absorbed by the REL2is emitted, and this heating temperature desirably stands at equal to or higher than 150° C. and more desirably stands at equal to or higher than 180° C. Although the temperature for heating the stack needs to be, as described above, equal to or higher than a temperature at which hydrogen absorbed by the REL2is emitted, the heating temperature desirably stands at equal to or lower than 250° C. and more desirably stands at equal to or lower than 200° C. As such, this embodiment can perform the foregoing reduction at a temperature not exceeding the heat-resistance temperature of a material, such as a glass substrate, that is used in related art as a support for a light-emitting element.

Although the stack may undergo heating at any atmosphere, the stack desirably undergoes heating either in the air or under a nitrogen atmosphere in view of practical use. In this case, a surface opposite to the surface of the HTL3being in contact with the REL2has no likelihood of reduction. Here, the REL2hopefully exerts reduction action under a hydrogen atmosphere as well, but in this case, a surface facing the surface where the HTL3is in contact with the REL2is also likely to undergo reduction. It is hence desirable to avoid heating under a hydrogen atmosphere.

In this embodiment, forming the REL2that is very thin onto the interface between the HTL3and anode1, followed by stacking the anode1through the HTL3, followed by heating the stack can introduce a defect onto the surface of the HTL3in a region where the REL2is in contact with the HTL3, as described above.

The HTL3in this embodiment, which has ionic bonding, involves a deep surface defect, and pinning the work function of the anode1to the level (surface level) of this defect can achieve a work function that is conspicuously larger and more effective than the original work function of the anode1, as detailed later on. In this embodiment, such increase in effective work function can lower an injection barrier, thus improving hole injection.

Patent Literature 1 discloses a reaction production layer, which is a layer with metal ions within an organometallic-complex layer undergone reduction into a metal in a vacuum. These reaction production layer and organometallic-complex layer are disposed close to the electron injection layer, as earlier described, and can conduct electricity. Patent Literature 1 fails to disclose reducing the hole transport layer to form a surface level, pinning the work function of the anode1by using the surface level, or a structure for the pinning.

The next is Step S5, i.e., forming (stacking) the EML4onto the HTL3so as to be in contact with the HTL3. Forming the EML4can use various methods that are known as a method of forming an emission layer, including, but not limited to, evaporation, printing, ink-jetting, spin coating, casting, dipping, bar coating, blade coating, roll coating, gravure coating, flexographic printing, spray coating, photolithography, and self-assembling (e.g., a layer-by-layer self-assembly method and a self-assembled monolayer method).

The next is Step S6, i.e., forming (stacking) the ETL5onto the EML4as necessary. The next is forming (stacking) the cathode6. Forming the ETL5can use various methods that are known as a method of forming an electron transport layer. To be specific, forming the ETL5can use a method similar to the foregoing method of forming the HTL3. Forming the cathode6can use various methods that are known as a method of forming a cathode. To be specific, forming the cathode6can use a method similar to the foregoing method of forming the anode1.

The light-emitting element10can be manufactured through the foregoing process steps. It is noted that when the light-emitting element10is part of a display device for instance, the cathode6is formed as a common layer shared among all sub-pixels. Each of the anode1, REL2, HTL3, EML4, and ETL5may be provided for each sub-pixel by patterning in conformance with the shape of the sub-pixel after they are formed. Like the cathode6, the REL2, the HTL3, the EML4, and the ETL5except the anode1and EML4may be provided as a common layer shared among all the sub-pixels.

Effect

With reference toFIG.3andFIG.4, the following details an effect of the foregoing pinning.

FIG.3illustrates an energy band provided for describing a hole injection barrier Eh disposed between the HTL3and anode1in a comparative light-emitting element100, which includes no REL2.FIG.4illustrates an energy band provided for describing a hole injection barrier Eh′ disposed between the HTL3and anode1in the light-emitting element10according to this embodiment. The light-emitting element100has the same structure as the light-emitting element10with the exception that the REL2is not provided.

As illustrated inFIG.3, in the light-emitting element100, the band is bent in such a manner that the original work function W of the anode1and the Fermi level of the HTL3are equal at the joint interface between the anode1and HTL3, thus forming the hole injection barrier Eh equal to the energy difference between the original work function W of the anode1and the valence band level of the HTL3. Here, when the anode1is metal, the work function W is equal to the difference between a vacuum level and the original Fermi level Ef of the anode1at the time when the temperature is equal to absolute zero (T=0K). In other words, when the anode1is metal, the work function W of the anode1is equal to the Fermi level Ef of the anode1.

When the light-emitting element100is a QLED, a wide-gap compound or a wide-gap metal oxide is typically used for a hole-transporting material that is used for the HTL3(hole transport layer or hole injection layer), in order to confine electrons and inject holes into quantum dots having a deep ionization potential. The HTL3transports holes, and hence this compound or metal oxide is composed of a p-type compound or a p-type metal oxide.

As earlier described, these hole-transporting materials typically tend to contain p-type impurities having activation energy higher than the thermal energy of room temperature by about equal to or greater than 10 times, and the hole-transporting materials typically tend to have a great compensation effect on holes. It is hence difficult to obtain a p-type hole-transporting material having holes of high concentration. Thus, the Fermi level of the HTL3is close to the band gap middle, because these hole-transporting materials have low hole concentration though they are p-type materials. However, these materials are p-type materials, and hence the Fermi level of the HTL3is deeper than a half of the band gap.

As earlier described, the energy level of the EML4, which is an emission layer, is determined depending on the material of quantum dots (QDs) that are used for the EML4. Ionization potential is equal to valence band level; the ionization potential of the EML4of the light-emitting element100shown inFIG.3measures 5.2 eV, which is over 5 eV and is thus considerably great. Electron affinity is equal to conduction band level; the EML4of the light-emitting element100shown inFIG.3has an electron affinity of 3.2 eV. Moreover, the HTL3made of NiO for instance, as illustrated inFIG.3has an ionization potential of 5.6 eV and an electron affinity of 2.1 eV. Moreover, the ETL5made of ZnO for instance, as illustrated inFIG.3has an ionization potential of 7.0 eV and an electron affinity of 3.8 eV.

The hole injection barrier Eh, provided for hole injection from the anode1into the HTL3, is low if the work function W of the anode1is deep to a degree close to the ionization potential of the HTL3, but such an anode material has not been found. Hole injection from the anode1into the HTL3in the light-emitting element100shown inFIG.3is thus not easy.

To lower the hole injection barrier between the anode1and HTL3, the HTL3in this embodiment undergoes reduction on its surface adjacent to the anode1, thus introducing a defect (surface defect) of high density onto the surface of the HTL3adjacent to the anode1. The defect formed in such a manner forms a defect level (surface level) that is deeper than the band gap of the HTL3by a half or more, thus pinning the Fermi level of the anode1.

Without a defect on the surface of the HTL3, the band curves in such a manner that the original work function W of the anode1and the Fermi level of the HTL3are equal at the bonding interface between the anode1and HTL3, and hence the work function of the anode1does not undergo pinning. The original hole injection barrier Eh by material combination is formed in the comparative light-emitting element100, which includes no REL2.

The height of the foregoing defect level is determined depending on the bonding state of atoms constituting a host crystal. Energy necessary for defect production (i.e., for bonding release) increases along with increase in the bounding. As earlier described, the HTL3is composed of a wide-gap compound or a wide-gap metal oxide as a hole-transporting material. Such a hole-transporting material contains constituent elements ion-bonded together. Ion bonding is strong and has large bonding energy. Accordingly, for a defect of a material having ion bonding, the ion bonding between constituent elements needs to be released, and large energy is required in order to produce a defect for pinning the Fermi level of the anode1. The defect on the HTL3thus constitutes a deep defect level, as described above.

The defect level within a band gap typically has no spread at low defect density, but at high defect density, the defect level within the band gap generally has Gaussian function-like spread with respect to energy. The surface of the HTL3undergone reduction by the REL2has a defect of high density, and hence the defect level is formed, with spread, deep within the band gap.

The REL2is extremely thin, as earlier described, and hence the defects on the HTL3are locally located on the surface adjacent to the anode1. It is noted that a situation where defects are all over the HTL3in its thickness direction is undesirable, because the defects in parts of the HTL3except a part close to the surface function as a carrier trap. That is, if defects are all over the HTL3in its thickness direction, a defect level is introduced all over the HTL3in the thickness direction, thereby trapping free carriers, having conductivity, in the defect level at a portion excluding the vicinity of the surface of the HTL3, thereby increasing the resistance of the HTL3. This lowers the efficiency of hole injection.

Asperities resulting from the defects on the HTL3, which are likely to cause delamination or fine pores on the interface with the anode1, are desirably small. If asperities resulting from the defects on the HTL3are large, a carrier trap causes increase in the resistance of the HTL3, as described above, and hence the asperities are desirably small in order to prevent such resistance increase. To be specific, the asperities resulting from the defects on the HTL3desirably have, in the stacking direction, a height equal to or smaller than 1/20 (i.e., over 0 and equal to or smaller than 1/20) of the thickness of the HTL3, and the asperities more desirably have, in the stacking direction, a height equal to or smaller than 1/80 (i.e., over 0 and equal to or smaller than 1/80) of the same. To be more specific, the asperities resulting from the defects on the HTL3are desirably equal to or smaller than 4 nm (i.e., over 0 and equal to or smaller than 4 nm) for instance, and are more desirably equal to or smaller than 0.5 nm (i.e., over 0 and equal to or smaller than 0.5 nm) for instance.

In this embodiment, such defect formation as described above forms, in the HTL3, a deeper defect level than the Fermi level of the HTL3, as illustrated inFIG.4. This enables the Fermi level of the anode1to undergo pinning to the foregoing defect level deeper than the Fermi level of the HTL3. The Fermi level of the anode1is equal to the work function of the anode1. Thus, the work function of the anode1undergoes pinning to the foregoing defect level, which is deeper than the Fermi level of the HTL3, through the foregoing pinning. This embodiment can accordingly achieve a work function W′ that is conspicuously larger and more effective than the original work function W of the anode1.

This embodiment thus enables the hole injection barrier Eh to effectively lower to the hole injection barrier Eh′ having a height corresponding to the energy difference between the defect level of the HTL3and the valence band level of the HTL3, as illustrated inFIG.4. Here, the hole injection barrier Eh′ is the energy difference between the effective work function W′ of the anode1and the valence band level of the HTL3, and in this embodiment, the hole injection barrier Eh′ corresponds to a value with a defect level subtracted from a half of the band gap of the HTL3. The light-emitting element in this embodiment can lower a hole injection barrier when compared with the comparative light-emitting element100, which includes no REL2.

The HTL3that is formed through sputtering is exposed to ion impact at the time of formation, as earlier described, and thus has a defect formed on the surface slightly. In forming the HTL3through nanoparticle application, nanoparticles undergo a huge surface impact as a result of a size effect; hence, the HTL3in this case as well has a defect formed on the surface slightly. In forming the HTL3through precursor application, a chemical reaction is not completed 100%; hence, the HTL3in this case as well has a defect formed on the surface slightly. In any of these cases however, the density of a defect formed on the surface of the HTL3is not so high as to cause pinning. For this reason, even if the HTL3is formed through any of the methods, the hole injection barrier of the light-emitting element100is the same as the original hole injection barrier Eh produced by combination of a hole transport material that is used for the HTL3and of an anode material.

This embodiment can thus offer the light-emitting element10and a method for manufacturing the same that can lower the hole injection barrier between the HTL3and anode1further than a known light-emitting element including no REL2, thereby improving the efficiency of hole injection into the EML4.

Second Embodiment

This embodiment will describe a difference between this embodiment and the first embodiment. For convenience in description, components with the same functions as the components described in the first embodiment will be denoted by the same signs, and their detailed description will be omitted.

The light-emitting element10according to this embodiment and a method for manufacturing the same are the same as those in the first embodiment with the exception that the REL2contains WOx (x≥3) as a reducing material. The longitudinal sectional view of the light-emitting element10according to this embodiment is thus the same as that inFIG.1.

WOx (x≥3) is a reducing catalyst. For an oxide that is a target substance for reduction, performing reduction (i.e., providing electrons to take away oxygen) requires the target substance be supplied with electrons that exceed an energy barrier corresponding to at least oxygen's bonding, to thus release the bonding of the target substance. However, oxygen within an oxide normally has large bonding energy, and hence promoting reduction requires electron energy to be increased or requires a barrier to be lowered. A reducing catalyst, which has the action of lowering an energy barrier at its interface with a target substance for reduction without changing its state, promotes reduction.

A tungsten oxide, denoted by WOx, is known as taking some forms and can be produced in the form of a film through reactive sputtering using, for instance, a tungsten (W) target as well as oxygen (O2) gas and argon gas (Ar).

O2, which oxidizes W undergone sputtering from a target, can change form by regulating its flow ratio with respect to Ar. Ar is ionized by plasma and sputters W from a target. At this time, mutual action between physical energy exchange and charged particles causes W to undergo sputtering and to simultaneously undergo ionization. The ionized W reacts with O to take three forms: WO2, WO3, and W2O3; they are easily generated in the order of WO3, W2O3, and WO2along with increase in the density of Ar plasma.

The following describes an evaluation result about an effect of reduction of the HTL3using the foregoing compounds.

Firstly, 10 nm thick tungsten oxides in the respective forms were formed onto a nickel oxide (NiO) film, which is the HTL3, and were maintained (heated) at 200° C. for five hours, followed by observation of the surface of the NiO film. Asperities with average roughness of about 50 nm were found on the surface of the NiO film with a WO3film on. Here, the NiO film before processing had average roughness of about 0.5 nm. The condition, i.e., five-hour heating at 200° C., is for emphasizing the action of reduction on the NiO film, and in actual application to the light-emitting element10, short-time heating, i.e., heating at less than 200° C. for less than five hours, is required.

The foregoing samples were evaluated with an electron probe micro analyzer (EPMA). Accordingly, no asperities were found on the surface of the sample with a chemical shift corresponding to Ni—O and to Ni alone being observed and with a WO2or W2O3film on; in addition, the EPMA detected no Ni on the sample's surface. This result has demonstrated that only WO3among the tungsten oxides exhibits the ability of reduction on NiO.

Next, a similar experiment was conducted by regulating O2partial pressure in sputtering, followed by forming a WOx film having an oxygen ratio x of 0.25 to 3.2 (i.e., a WOx film ranging from WO0.25to WO3.2) onto NiO. Oxygen ratio exhibiting the ability of reduction on NiO was equal to or greater than three.

The foregoing demonstrates that tungsten oxides having the action of reduction on NiO are compounds including from a compound with W:O=1:3, which is the composition ratio of oxygen to tungsten, to a compound with excess of oxygen (i.e., WOx of x≥3).

This embodiment enables the surface of the HTL3being in contact with the REL2to undergo reduction by catalysis using the foregoing compounds, to thus form a defect onto the surface of the HTL3being in contact with the REL2. This embodiment can consequently achieve an effect similar to that in the first embodiment.

Third Embodiment

This embodiment will describe a difference between this embodiment and the first and second embodiments. For convenience in description, components with the same functions as the components described in the first and second embodiments will be denoted by the same signs, and their detailed description will be omitted.

The second embodiment has described an instance where WOx(x≥3), a reducing catalyst, is used as a reducing material. A reducing catalyst used in this present disclosure needs to be any substance that has such action that a substance constituting the HTL3receives electrons to be thus deprived of oxygen, or has the action of promoting a reaction by which the substance constituting the HTL3bonds with hydrogen, and that can obtain the substance constituting the HTL3in a solid state at room temperature and under a normal-pressure atmosphere. Accordingly, the REL2may contain a reducing catalyst other than WOx (x≥3) as a reducing material.

The light-emitting element10according to this embodiment and the method for manufacturing the same are the same as those in the first and second embodiments with the exception that the REL2contains, as a reducing material, a reducing catalyst other than WOx (x≥3) instead of or in addition to WOx (x≥3). The longitudinal sectional view of the light-emitting element10according to this embodiment is thus the same as that inFIG.1.

Examples of the reducing catalyst used in this embodiment, other than WOx (x≥3) include, but not limited to, titanium dioxide (TiO2), tungsten trioxide (WO3), molybdenum trioxide (MoO3), a manganese (Mn)-based catalyst, and an iron (Fe)-based catalyst. Examples of the Mn-based catalyst include, but not limited to, manganese oxide (MnOx) and a quadruple manganese perovskite oxide. An example of the foregoing MnOx is MnOx (0.9≤x≤1) having oxygen composition of 0.9≤x≤1, where x denotes a deviation from stoichiometric composition due to a Mn hole. Examples of the quadruple manganese perovskite oxide include, but not limited to, CaMn7O12and LaMn7O12. In particular, CaMn7O2and LaMn7O12has a strong ability of reduction. Furthermore, CaMn7O12, which is an oxide of Ca and Mn, can be synthesized easily in the atmosphere and under normal pressure. An example of the iron-based catalyst is a triiron-tetraoxide-based catalyst containing triiron tetraoxide (Fe3O4). Examples of the triiron-tetraoxide-based catalyst include, but not limited to, triiron tetraoxide (Fe3O4), a mixture of Fe3O4and potassium oxide (K2O), and a mixture of Fe3O4, calcium oxide (CaO) and K2O. As earlier described, Fe3O4alone functions as a reducing catalyst. The ratio of these mixtures is thus non-limiting. These reducing catalysts may be used alone or in combination, as appropriate, with two or more kinds.

The foregoing reducing catalyst may contain at least one compound selected from the group consisting of, for instance, TiO2, MoO3, MnOx (0.9≤x≤1), a quadruple manganese perovskite oxide, and a triiron-tetraoxide-based catalyst. As a matter of course, the reducing catalyst may be used in combination with WOx (x≥3) described in the second embodiment, as earlier described. That is, the reducing catalyst may contain at least one compound selected from the group consisting of WOx (x≤3), TiO2, MoO3, MnOx (0.9≤x≤1), a quadruple manganese perovskite oxide, and a triiron-tetraoxide-based catalyst.

The following describes an evaluation result about an effect of reduction of the HTL3by the use of TiO2as a reducing catalyst.

Multiple samples with a film of TiO2for instance formed on the HTL3, which was herein a nickel oxide (NiO) film, were produced by changing the thickness of the TiO2film and were maintained (heated) at 200° C. for five hours, followed by evaluation on the presence or absence of reduction of the NiO film. The reduction of the NiO film was found in TiO2having a thickness equal to or greater than 0.5 nm. In addition, a TiO2film of flat thickness was successfully obtained with TiO2having a thickness equal to or greater than 1 nm. Further, with TiO2having a thickness less than 1 nm, the flatness was lost, and with TiO2having a thickness less than 0.5 nM, TiO2was changed into an island shape rather than a continuous film.

It is noted that the condition, i.e., five-hour heating at 200° C., is for emphasizing the action of reduction on the NiO film, and in actual application to the light-emitting element10, short-time heating, i.e., heating at less than 200° C. for less than five hours, is required.

This embodiment enables the surface of the HTL3being in contact with the REL2to undergo reduction by catalysis using the foregoing compounds, to thus form a defect onto the surface of the HTL3being in contact with the REL2, as earlier described. This embodiment can consequently achieve an effect similar to that in the first and second embodiments.

Fourth Embodiment

This embodiment will describe a difference between this embodiment and the first to third embodiments. For convenience in description, components with the same functions as the components described in the first to third embodiments will be denoted by the same signs, and their detailed description will be omitted.

FIG.5is a longitudinal sectional view of an example schematic configuration of the light-emitting element10according to this embodiment cut in the direction of the normal to the light-emitting element10(that is, the light-emitting element10cut in its stacking direction).FIG.6is a sectional view of the light-emitting element10taken along line A-A inFIG.5.FIG.6corresponds to a drawing illustrating, when viewed from above (i.e., in a plan view), the REL2and HTL3of the light-emitting element10according to this embodiment cut, within the REL2, along a plane perpendicular to the staking direction.

The light-emitting element10according to this embodiment and a method for manufacturing the same are the same as those in the first to third embodiments with the exception that, as illustrated inFIG.5, a plurality of RELs2are arranged discretely in island form rather than in the form of a continuous film.

FIG.6illustrates an instance where a plurality of island-shaped RELs2are distributed uniformly in a plan view, all over the emission region of the light-emitting element10(to be more specific, all over the upper surface of the HTL3).

Herein, the emission region of the light-emitting element10is a region where light is emitted in the light-emitting element10. When an edge cover (not shown) is disposed between the anode1and cathode6so as to cover the end of the anode1, for instance, the emission region of the light-emitting element10refers to the opening of the edge cover exposing the inside of the anode1.

The RELs2can be formed in island form having a desired pattern by for instance, in Step S2, forming the REL2through sputtering or evaporation using a mask having a plurality of openings. As a matter of course, Step S2may include forming the REL2through, for instance, sputtering or evaporation, followed by patterning through photolithography to thus form an island shape having a desired pattern.

As illustrated inFIG.5andFIG.6, the HTL3is located between the island-shaped RELs2in a plan view. As illustrated inFIG.6, the anode1is provided in contact with the island-shaped RELs2and in contact with the HTL3located between the island-shaped RELs2.

The first to third embodiments have described an instance where the REL2is a continuous film, as illustrated inFIG.1. Here, when a defect level can be formed in even a part of the surface of the HTL3adjacent to the anode1, the Fermi level of the anode1can undergo pinning to the defect level. When the Fermi level of the anode1can even partly undergo pinning, the Fermi level of the anode1in whole can undergo pinning. Moreover, when the Fermi level of the anode1can even partly undergo pinning, the hole injection barrier Eh between the anode1and HTL3can be lowered even partly. As such, when a defect level can be formed in even a part of the surface of the HTL3adjacent to the anode1, the efficiency of hole injection can improve further than before.

The HTL3does not thus necessarily have to have a defect all over the surface adjacent to the anode1. The REL2does not thus necessarily have to be a continuous film. This embodiment can achieve an effect similar to that in the first to third embodiments.

A commonly used HTL material has high resistance and is thin, i.e., about several-ten nanometer thick; hence, current spread in the lateral direction (in-plane direction) of the HTL3is small, and thus current tends to flow immediately upward. The REL2between the anode1and HTL3improves hole injection efficiency at the contact portion between the REL2and anode1. However, current tends to flow immediately above the contact portion between the REL2and anode1, as described above, and is less likely to spread around the contact portion. For this reason, an emission pattern with the light-emitting element10viewed right opposite the emission region possibly does not necessarily emit light uniformly. Accordingly, making the distribution at the contact portion uniform within the emission region enables the emission pattern to be uniform. Even for a discontinuous contact portion, making the area at the contact portion highly dense enables the emission pattern to be further uniform.

Fifth Embodiment

This embodiment will describe a differences between this embodiment and the first to fourth embodiments. For convenience in description, components with the same functions as the components described in the first to fourth embodiments will be denoted by the same signs, and their detailed description will be omitted.

FIG.7is a lateral sectional view of an example schematic configuration of the light-emitting element10according to this embodiment cut in its horizontal direction. To be specific,FIG.7corresponds to a drawing illustrating, when viewed from above (i.e., in a plan view), the REL2and HTL3of the light-emitting element10according to this embodiment cut, within the REL2, along a plane perpendicular to the staking direction.FIG.7corresponds to the sectional view of the light-emitting element10taken along line A-A inFIG.5.

The light-emitting element10according to this embodiment and a method for manufacturing the same are the same as those in the first to fourth embodiments with the exception that, as illustrated inFIG.7, a plurality of island-shaped RELs2are distributed non-uniformly (irregularly) in a plan view, all over the emission region of the light-emitting element10(to be more specific, all over the upper surface of the HTL3).

As described in the fourth embodiment, when a defect level can be formed in even a part of the surface of the HTL3adjacent to the anode1, the Fermi level of the anode1can undergo pinning to the defect level. This can improve hole injection efficiency further than before.

The RELs2may be thus distributed non-uniformly in a plan view, as described above. This embodiment can achieve an effect similar to that in the first to fourth embodiments.

Sixth Embodiment

This embodiment will describe a differences between this embodiment and the first to fifth embodiments. For convenience in description, components with the same functions as the components described in the first to fifth embodiments will be denoted by the same signs, and their detailed description will be omitted.

FIG.8is a lateral sectional view of an example schematic configuration of the light-emitting element10according to this embodiment cut in its horizontal direction.FIG.8is a lateral sectional view of an example schematic configuration of the light-emitting element10according to this embodiment cut in its horizontal direction. To be specific,FIG.8corresponds to a drawing illustrating, when viewed from above (i.e., in a plan view), the REL2and HTL3of the light-emitting element10according to this embodiment cut, within the REL2, along a plane perpendicular to the staking direction.FIG.8corresponds to the sectional view of the light-emitting element10taken along line A-A inFIG.5.

The light-emitting element10according to this embodiment and a method for manufacturing the same are the same as those in the first to fifth embodiments with the exception that, as illustrated inFIG.8, a plurality of island-shaped RELs2are distributed non-uniformly (irregularly) in a plan view in the emission region of the light-emitting element10(to be more specific, the upper surface of the HTL3in the example inFIG.8) in such manner that the density of arrangement of the RELs2is higher at the perimeter of the emission region of the light-emitting element10than at the center of the emission region.

The “density of arrangement of the RELs2” indicates the density of area of how much the island-shaped RELs2are in contact with the anode1with respect to the area of the emission region of the light-emitting element10.

Hole injection efficiency can improve further than before in this case as well, for the same reason as that described in the fourth and fifth embodiments. An effect can be consequently achieved that is similar to that in the first to fifth embodiments. This embodiment enables formation of a surface defect at the perimeter of the emission region of the light-emitting element10, where an electric field tends to concentrate. The Fermi level of the anode1can consequently undergo pinning at the perimeter, where an electric field tends to concentrate, to thus improve hole injection efficiency.

In some embodiments, there may be no REL2at the center of the emission region, as illustrated inFIG.8.

Seventh Embodiment

This embodiment will describe a differences between this embodiment and the first to sixth embodiments. For convenience in description, components with the same functions as the components described in the first to sixth embodiments will be denoted by the same signs, and their detailed description will be omitted.

FIG.9is a perspective view of a schematic configuration of main components of the light-emitting element10according to this embodiment. To be more specific,FIG.9is a perspective view of the REL2of the light-emitting element10according to this embodiment viewed from above the light-emitting element10.

The light-emitting element10according to this embodiment includes, between the anode1and cathode6, an edge cover7covering the end of the anode1. The opening of the edge cover7, exposing the inside of the anode1, is an emission region10aof the light-emitting element10according to this embodiment. The light-emitting element10according to this embodiment is the same as that in the sixth embodiment with the exception that the end of the emission region10aof the light-emitting element10and a plurality of RELs2overlap.

The REL2in this embodiment as well can be formed in island form having a desired pattern by forming the REL2through sputtering or evaporation using a mask having a plurality of openings. As a matter of course, Step S2may include forming the REL2through, for instance, sputtering or evaporation, followed by patterning through photolithography to thus form an island shape having a desired pattern.

In this case as well, an effect similar to that in the sixth embodiment can be achieved for the same reason as that in the sixth embodiment.

The present disclosure is not limited to the foregoing embodiments. Various modifications can be devised within the scope of the claims. An embodiment that is obtained in combination, as appropriate, with the technical means disclosed in the respective embodiments is also included in the technical scope of the present disclosure. Furthermore, combining the technical means disclosed in the respective embodiments can form a new technical feature.