The present disclosure relates to a light-emitting element, a display device including the light-emitting element, and a light-emitting element manufacturing method.
In recent years, quantum-dot light-emitting diodes (QLEDs) as light-emitting elements, and display devices including the QLEDs, have attracted widespread attention.
However, a typical QLED currently under development includes a light-emitting layer containing: quantum dots (QDs) emitting light; and organic ligands bonding to the quantum dots. A problem of the QLED is that the organic ligands themselves are prone to deterioration and desorption from the quantum dots. As a result, the QLED is not sufficiently reliable.
Hence, for example, Patent Document 1 describes a layer containing quantum dots each including: a core; and a shell whose outermost layer contains a metal chalcogenide compound, and the quantum dots are embedded in a matrix containing metal chalcogenide. The layer is used as a light-emitting layer.
However, as described in Patent Document 1, if the layer, whose quantum dots are embedded in a matrix containing metal chalcogenide, is used as a light-emitting layer of a light-emitting element that emits light by electroluminescence (EL), the matrix containing the metal chalcogenide with low bandgap and low electrical resistance inevitably causes a leak of a current. The current leak interferes injection of carriers into the cores of the quantum dots, developing a problem of hindering improvement in light emission efficiency.
An aspect of the present disclosure is conceived in view of the above problem, and intended to provide a light-emitting element that reduces a leak of a current in the light-emitting layer and that improves reliability and light emission efficiency. The present disclosure is also intended to provide a display device including the light-emitting element and a light-emitting element manufacturing method.
In order to solve the above problem, a light-emitting element according to the present disclosure includes:
In order to solve the above problem, a display device according to the present disclosure includes:
In order to solve the above problem, a display device according to the present disclosure includes:
In order to solve the above problem, a light-emitting element manufacturing method according to the present disclosure includes:
An aspect of the present disclosure can provide a light-emitting element that reduces a leak of a current in the light-emitting layer and that improves reliability and light emission efficiency. The present disclosure is also intended to provide a display device including the light-emitting element and a light-emitting element manufacturing method.
Described below are embodiments of the present invention, with reference to
As illustrated in
This embodiment exemplifies a case where the light-emitting element 1 includes a hole transport layer 3 between the anode 2 and the light-emitting layer 4. However, this embodiment shall not be limited to such a case. For example, the light-emitting element 1 may further include a not-shown hole injection layer. Between the anode 2 and the light-emitting layer 4, the hole injection layer and the hole transport layer 3 may be provided in the stated order from toward the anode 2. Furthermore, the hole transport layer 3 may be replaced with the hole injection layer, and the hole transport layer 3 and the hole injection layer may be omitted as appropriate.
Moreover, this embodiment exemplifies a case where the light-emitting element 1 includes an electron transport layer 5 between the light-emitting layer 4 and the cathode 6. However, this embodiment shall not be limited to such a case. For example, the light-emitting element 1 may further include a not-shown electron injection layer. Between the light-emitting layer 4 and the cathode 6, the electron transport layer 5 and the electron injection layer may be provided in the stated order from toward the light-light-emitting layer 4. Furthermore, the electron transport layer 5 may be replaced with the electron injection layer, and the electron transport layer 5 and the electron injection layer may be omitted as appropriate.
As illustrated in
This embodiment exemplifies a case where the whole shell 7S is made of the metal chalcogenide compound. However, this embodiment shall not be limited to such a case. For example, the shell 7S may be made of a plurality of layers, and, among the plurality of layers, the outermost layer may contain the metal chalcogenide compound. Moreover, as will be described later, the whole shell 7S may be made of a metal chalcogenide complex. If the shell 7S is made of a plurality of layers, the outermost layer among the plurality of layers may contain the metal chalcogenide complex. For example, the shell 7S may include: an inner shell coating the core 7C and containing a semiconductor material; and an outer shell containing either a metal chalcogenide complex or a metal chalcogenide compound.
As can be seen, in this embodiment, each of the quantum dots 7 includes: the core 7; and the shell 7S coating the core 7C and having an outermost layer containing either a metal chalcogenide complex 7L or a metal chalcogenide compound. Hence, compared with quantum dots having organic ligands, namely an insulating organic substance, bonding thereto, the quantum dots 7 can improve electrical conductivity in the quantum dot layer. Such a feature can reduce the driving voltage of the light-emitting element 1. Moreover, compared with quantum dots having organic ligands, namely an organic substance, bonding thereto, the quantum dots 7 do not contain an organic substance prone to deterioration by heat and light. Such a feature can provide the light-emitting element 1 with high reliability. Furthermore, in the case of the organic ligands, each of the ligands simply bonds independently to the surface of the quantum dots, such that the organic ligands are likely to come off while the light-emitting element is driving. Thus, high reliability is not expected. However, the shell 7S contains a metal chalcogenide compound made of complexes originally bonding as ligands, and reacting and recombining together. The shell 7S is a thin film made of the metal chalcogenide compound and shaped to coat the core 7C. Such a feature keeps the shell 7S from coming off while the light-emitting element 1 is driving, thereby making it possible to provide the light-emitting element 1 with high reliability. In addition, thanks to such a feature, the light-emitting layer does not contain a ligand material acting as a barrier to injection of the charges. Hence, the light-emitting element 1 can operate on a lower voltage and achieve even higher reliability.
This embodiment exemplifies a case where the quantum dot 7 includes the shell 7S and the core 7C. The shell 7S is formed of the metal chalcogenide complex 7L bonding to the core 7C. The metal chalcogenide complex 7L is at least irradiated with an ultraviolet (UV) light or heated (treated with heat), and chemically altered to be electrically neutral and cured to form the shell 7S. However, the quantum dot 7 shall not be limited to such a case. For example, the quantum dot may include the core 7C and the metal chalcogenide complex 7L bonding to the core 7C, and the metal chalcogenide complex 7L bonding to the core 7C may be neither irradiated with an ultraviolet light nor treated with heat to be cured. In such a case, the metal chalcogenide complex 7L bonding to the core 7C serves as a shell. Moreover, the quantum dot may include: the core 7C; and a shell containing a metal chalcogenide compound coating the core 7C and the metal chalcogenide complex 7L bonding to the core 7C.
As seen in
Furthermore, as illustrated in
As described above, in the case where the shell is the metal chalcogenide complex 7L boding to the core 7C, even if the shell 7S contains any of the metal chalcogenide compound coating the core 7C or of the metal chalcogenide compound and the metal chalcogenide complex 7L, a distance between the quantum dots is shorter than a distance between quantum dots modified with typical organic ligands.
It is known that the Foerster resonance energy transfer (FRET) efficiency (E), which is one factor causing a decrease in the light emission efficiency, is expressed by the following Equation (1), where “r” is the distance between the cores of quantum dots.
In the above Equation (1), R0, which is referred to as the Forster distance, is a distance between cores whose FRET efficiency (E) is 50%.
As described above, if the distance between the quantum dots; that is, the distance “r” between the cores of the quantum dots, is short, the Foerster resonance energy transfer (FRET) efficiency (E) increases, and there is a concern that the light emission efficiency decreases because of the Foerster resonance energy transfer (FRET).
Moreover, as described above, if the shells to be used contain either the metal chalcogenide compound or the metal chalcogenide complex 7L, the cores come into contact with one another through the metal chalcogenide with low electrical resistance. This inevitably causes a leak of a current in the light-emitting layer. The current leak interferes injection of carriers into the cores of the quantum dots, developing a problem of hindering improvement in light emission efficiency.
Hence, in this embodiment, as shown in
As can be seen, the clearance, formed with the spacer particles 8 and separating the quantum dots 7 from one another, can increase a distance between the quantum dots 7; that is, the distance r between the cores of the quantum dots. In the light-emitting element 1 including the light-emitting layer, such a feature can reduce the Foerster resonance energy transfer (FRET) and a leak of a current in the light-emitting layer 4. Hence, the light-emitting element 1 can improve light emission efficiency.
The spacer particles 8 may be an organic material, an inorganic material, or an inorganic-organic hybrid material as long as the spacer particles 8 can form clearance to provide a distance between the quantum dots 7. This embodiment exemplifies a case where the spacer particles 8 are formed of CdSe. However, this embodiment shall not be limited to such a case. As will be described later, the spacer particles 8 may be formed of a core material. Note that a preferable material, characteristic, and size of the spacer particles 8, as well as a preferable amount of the spacer particles 8 to be mixed, will be described later.
The core (a first core) 7C preferably contains one or a plurality of semiconductor materials selected from the group consisting of, for example, Cd, S, Te, Se, Zn, In, N, P, As, Sb, Al, Ga, Pb, Si, Ge, Mg, and a compound of these materials.
The core 7C preferably contains one or more selected from the group consisting of, for example, CdS, CdSe, CdTe, ZnS, ZnSe, ZnTe, ZnO, HgS, HgSe, HgTe, CdSeS, CdSeTe, CdSTe, ZnSeS, ZnSeTe, ZnSTe, HgSeS, HgSeTe, HgSTe, CdZnS, CdZnSe, CdZnTe, CdHgS, CdHgSe, CdHgTe, HgZnS, HgZnSe, CdHgZnTe, CdZnSeS, CdZnSeTe, CdZnSTe, CdHgSeS, CdHgSeTe, CdHgSTe, HgZnSeS, HgZnSeTe, HgZnSTe, GaN, GaP, GaAs, GaSb, AlN, AlP, AlAs, AlSb, InN, InP, InAs, InSb, GaNP, GaNAs, GaNSb, GaPAs, GaPSb, AlNP, AlNAs, AlNSb, AlPAs, AlPSb, InNP, InNAs, InNSb, InPAs, InPSb, GaAlNP, GaAlNAs, GaAlNSb, GaAlPAs, GaAlPSb, GaInNP, GaInNAs, GaInNSb, GaInPAs, GaInPSb, InAlNP, InAlNAs, InAlNSb, InAlPAs, InAlPSb, SnS, SnSe, SnTe, PbS, PbSe, PbTe, SnSeS, SnSeTe, SnSTe, PbSeS, PbSeTe, PbSTe, SnPbS, SnPbSe, SnPbTe, SnPbSSe, SnPbSeTe, SnPbSTe, Si, Ge, SiC, and SiGe. In order for the light-emitting layer 4 containing the quantum dots 7 to emit light in different colors using such core materials, the cores may be formed of a single material with different particle sizes. For example, cores having the largest particle size may be used for a light-emitting layer that emits a red light. Cores having the smallest particle size may be used for a light-emitting layer that emits a blue light. Cores having a particle size between the particle sizes of the cores used for the light-emitting layer that emits the red light and for the light-emitting layer that emits the blue light may be used for a light-emitting layer that emits a green light. Moreover, in order for the light-emitting layer 4 containing the quantum dots 7 to emit light in different colors, the cores may be formed of different materials.
Either the metal chalcogenide complex 7L or the metal chalcogenide compound preferably contains at least one element selected from the group consisting of, for example, S, Se, and Te, and at least one element selected from the group consisting of, for example, Sn, In, Ga, and Sb. When these materials are used, and when metal chalcogenide complex ligands are at least irradiated with an ultraviolet (UV) light or heated, the metal chalcogenide complex ligands can be chemically altered to the metal chalcogenide compound that is stable and electrically neutral. Hence, the shells are successfully cured.
The metal chalcogenide compound preferably contains at least one selected from the group consisting of, for example, SnS2, SnSe2, In2Se3, In2Te3, Ga2Se3, Sb2Se3, and Sb2Te3.
The metal chalcogenide complex preferably contains at least one selected from the group consisting of, for example, Sn2Se64−, Sn2Se64−, In2Se42−, In2Te42−, Ga2Se42−, Sb2Se42−, and Sb2Te42−.
As illustrated in
Each of the plurality of spacer particles 18 includes a core (a second core) 18C.
For example, a non-patent document [Adv. Optical Mater. 2020, 8, 1902092] discloses that red light-emitting quantum dots are mixed with blue light-emitting quantum dots including cores larger in bandgap than cores included in the red light-emitting quantum dots. The document reports that the mixture can increase an average distance between the red light-emitting quantum dots, and reduce a decrease in light emission efficiency.
Thus, the core (the second core) 18C preferably contains a material larger in bandgap than the material of the core (first core) 7C.
As described above, in the case where the shell is the metal chalcogenide complex 7L boding to the core 7C, even if the shell 7S contains any of the metal chalcogenide compound coating the core 7C or of the metal chalcogenide compound and the metal chalcogenide complex 7L, a distance between the quantum dots is shorter than a distance between quantum dots modified with typical organic ligands.
Because the light-emitting layer 4′ includes the plurality of spacer particles 18, the light-emitting layer 4′ can be formed to include clearance to provide a distance at least between at least two quantum dots included in the plurality of quantum dots 7 and positioned next to each other. Moreover, the clearance is provided between at least two spacer particles included in the spacer particles 18 and positioned next to each other. Such a feature can reduce a current flowing not through the quantum dots 7 but only through the spacer particles 18.
As can be seen, the clearance, formed with the spacer particles 18 and separating the quantum dots 7 from one another, can increase a distance between the quantum dots 7; that is, the distance “r” between the cores of the quantum dots. In the light-emitting element including the light-emitting layer 4′, such a feature can reduce the Foerster resonance energy transfer (FRET) and a leak of a current in the light-emitting layer 4′. Hence, the light-emitting element can improve light emission efficiency.
The core (the second core) 18C preferably contains one or a plurality of semiconductor materials selected from the group consisting of, for example, Cd, S, Te, Se, Zn, In, N, P, As, Sb, Al, Ga, Pb, Si, Ge, Mg, and a compound of these materials.
The core 18C preferably contains one or more selected from the group consisting of, for example, CdS, CdSe, CdTe, ZnS, ZnSe, ZnTe, ZnO, HgS, HgSe, HgTe, CdSeS, CdSeTe, CdSTe, ZnSeS, ZnSeTe, ZnSTe, HgSeS, HgSeTe, HgSTe, CdZnS, CdZnSe, CdZnTe, CdHgS, CdHgSe, CdHgTe, HgZnS, HgZnSe, CdHgZnTe, CdZnSeS, CdZnSeTe, CdZnSTe, CdHgSeS, CdHgSeTe, CdHgSTe, HgZnSeS, HgZnSeTe, HgZnSTe, GaN, GaP, GaAs, GaSb, AlN, AlP, AlAs, AlSb, InN, InP, InAs, InSb, GaNP, GaNAs, GaNSb, GaPAs, GaPSb, AlNP, AlNAs, AlNSb, AlPAs, AlPSb, InNP, InNAs, InNSb, InPAs, InPSb, GaAlNP, GaAlNAs, GaAlNSb, GaAlPAs, GaAlPSb, GaInNP, GaInNAs, GaInNSb, GaInPAs, GaInPSb, InAlNP, InAlNAs, InAlNSb, InAlPAs, InAlPSb, SnS, SnSe, SnTe, PbS, PbSe, PbTe, SnSeS, SnSeTe, SnSTe, PbSeS, PbSeTe, PbSTe, SnPbS, SnPbSe, SnPbTe, SnPbSSe, SnPbSeTe, SnPbSTe, Si, Ge, SiC, and SiGe.
Different carrier transport techniques are used for a light-emitting layer containing quantum dots with organic ligands bonding thereto and the light-emitting layers 4 and 4′ containing the quantum dots 7 each including: the core 7C; and the shell 7S coating the core 7C and having an outermost layer containing either the metal chalcogenide complex 7L or the metal chalcogenide compound. There are two types of carrier movement in the light-emitting layer described below, and, in an actual light-emitting layer, these two types are combined at certain proportions to determine how to transport the carriers. The first carrier transport technique is hopping conduction. The hopping conduction exhibits equal transportation capability between the electrons and the holes, and the transportation of the electrons and the holes is not affected by the mobility of the carriers inside the quantum dots. The second carrier transport technique is band conduction. The band conduction exhibits different transportation capability between the electrons and the holes (typically, the mobility of the holes is lower than the mobility of the electrons), and the transportation of the electrons and the holes is affected by the mobility of the carriers inside the quantum dots.
In a light-emitting layer containing quantum dots with organic ligands bonding thereto, the organic ligands are insulative and the quantum dots are greatly distant from each other. Hence, in transporting the carriers, the hopping conduction is probably rate-limiting.
Meanwhile, the light-emitting layers 4 and 4′ contain the quantum dots 7 each including: the core 7C; and the shell 7S coating the core 7C and having an outermost layer containing either the metal chalcogenide complex 7L or the metal chalcogenide compound. As to the light-emitting layers 4 and 4′, in transporting the carries, the band conduction is probably rate-limiting.
For example, a non-patent document [12 Jun. 2009 VOL 324 SCIENCE] discloses that an Au particle film with organic ligands bonding thereto has an electrical conductivity of approximately up to 10−9 Scm−1, whereas, an Au particle film whose ligands are substituted with Sn2S64− ligands has an electrical conductivity of approximately up to 200 Scm−1, which is the 11th power of the electrical conductivity of the former Au particle film. This is probably because, in the Au particle film with the Sn2S64− ligands bonding thereto, the Au particles are positioned significantly closer to each other at a short distance, and the barrier formed by the organic ligands; namely, an insulator, is removed. As to the carrier transport technique of the Au particle film with the Sn2S64− bonding thereto, the resistance due to the hopping conduction is negligibly small. Thus, the band conduction is probably rate-limiting
As can be seen in the Au particle film with the Sn2S64− ligands bonding thereto, as to the carrier transportation technique of the light-emitting layers 4 and 4′ containing the quantum dots 7 each including: the core 7C; and the shell 7S coating the core 7C and having an outermost layer containing either the metal chalcogenide complex 7L or the metal chalcogenide compound, the band conduction is probably rate-limiting, and the holes are less likely to be conducted than the electrons are. As a result, the light-emitting layers 4 and 4′ exhibit an imbalance in the mobility of positive and negative carriers. Since the mobility of the electrons is higher than the mobility of the holes in a semiconductor to be typically used as a core material, the mobility of the electrons is higher than the mobility of the holes in the light-emitting layers 4 and 4′.
In such a case, recombination of the electrons and the holes is likely to occur at the interface between the light-emitting layers 4 and 4′ and the hole transport layer 3, and the electrons inevitably flow into the hole transport layer 3; that is, a current leak occurs. The leak of the current could cause a decrease in light emission efficiency. In addition, imbalance of the carriers occurs in the quantum dots, and the Auger recombination is likely to reduce light emission efficiency. In order to prevent the reduction in light-emission efficiency, the mobility of the electrons alone needs to be reduced.
Hence, in order to reduce the mobility of the electrons alone in the light-emitting layer 4′, as illustrated in
In order to most readily prevent the electrons alone from moving in terms of band level as described above, the cores 18C may be made of the same material as the material of the cores 7C, and may be formed smaller in particle size than the cores 7C. This is because a variation in which the CBM becomes shallower is larger than a variation in which the VBM becomes deeper thanks to the quantum effect caused when the quantum dots are downsized for a material to be used for light-emitting quantum dots.
Note that if the electrons alone are kept from moving in terms of band level as described above, the cores 18C and the cores 7C may be made of different materials, and the cores 18C may be formed as large in particle size as the cores 7C or larger.
The cores 18C preferably have a bandgap of 3 eV or more and 6 eV or less. The cores 18C having a bandgap in such a range are used in appropriate combination with the cores 7C, thereby making it possible to keep the cores 18C from unnecessarily emitting light.
Meanwhile, the cores 7C may have a first peak emission wavelength, and the cores 18C may have a second peak emission wavelength shorter than the first peak emission wavelength. Moreover, the second peak emission wavelength may be 480 nm or less. Thanks to such a feature, the light-emitting element allows the cores 7C and the cores 18C to emit light in different colors.
Note that, each of the spacer particles 18 may include: the core 18C; and a not-shown shell coating the core 18C and having an outermost layer containing either a metal chalcogenide complex or a metal chalcogenide compound.
Note that, the spacer particle 18 may include: the core 18C; a not-shown shell coating the core 18; and an organic ligand bonding to the shell.
There is a desirable condition for the kinds of the spacer particles 18.
As seen in
As shown in
Meanwhile, in the case where PMMA as an organic insulator is provided between the quantum dot layer (QDs) and ZnO as an n-type semiconductor, PMMA provides a distance between ZnO and the quantum dots. The distance reduces a flow of the excitons from the quantum dot layer (QDs) to ZnO and non-light-emitting recombination. As a result, the photoexcitation emission lifetime is increased. Hence, it is not preferable that ZnO and a doped semiconductor containing a large number of defect levels are found near the quantum dots.
As can be seen, the spacer particles 8 and 18 are desirably intrinsic semiconductors that are not doped and less likely to cause defects. Candidates of these intrinsic semiconductors desirably include CdSe, Cds, ZnS, ZnSe, and InP, which are used as core materials and shell materials of quantum dots.
A non-patent document [634 Nature Vol. 575, 28 Nov. 2019] reports that when a distance between the surface of a quantum dot and the core surface of an adjacent quantum dot is approximately 9 nm, the FRET efficiency (E) is 6% or less and the FRET is successfully reduced. Typically, the shell of a quantum dot has a thickness of approximately 2 to 3.5 nm. Hence, in order to reduce the FRET efficiency (E), the distance between the surfaces of adjacent quantum dots has to be left at least 2 nm or more. Hence, the spacer particles 8 and 18 preferably have an average particle size of 2 nm or more and 50 nm or less.
As illustrated in
In
When one side “a” and the height “c” of the bottom surface of the hexagonal close-packed structure illustrated in
Considered next is a case where spacer particles are mixed, and the quantum dots 7 are spaced apart from one another at sufficient distances to reduce the FRET. A quantum dot 7 has a diameter of approximately 11 nm ((a core radius of 1.94 nm+a shell thickness of 3.46 nm)×2) and a distance kept by the organic ligands and provided between the quantum dots is approximately 2 nm, with reference to the high-efficiency quantum dots (QD-3R) in Extended Data Table 3 cited in a non-patent document [634 Nature Vol 575, 28 Nov. 2019].
With reference to the above numerical values, the atomic packing factor APF is calculated where the quantum dots 7 having a diameter of 11 nm are found at the lattice points of the hexagonal close-packed structure, and a distance between the centers of the quantum dots 7 is 13 nm. The calculated atomic packing factor APF is approximately 44.9%.
Here, considered is a case where a distance between the surfaces of the quantum dots 7 which is provided by the ligands is set to 2 nm by the spacer particles. If the particle size of the spacer particles 8 and 18 is sufficiently smaller than the particle size of the quantum dots 7, it can be approximately understood that a void portion with which no quantum dots 7 are filled is tightly filled with the spacer particles 8 and 18. That is, in such a case, it can be said that the spacer particles 8 and 18 having a volume ratio of approximately 55% are required in order to leave a distance of 2 nm between the quantum dots 7 having a diameter of 11 nm (1 (a volume ratio of the light-emitting layer)−0.45 (a volume ratio of the quantum dots 7)=0.55 (a volume ratio of the spacer particles 8 and 18)).
As can be seen, the particle size of the spacer particles 8 and 18 is preferably smaller than the particle size of the quantum dots 7, and, more preferably, smaller than the particle size of the cores 7C.
Moreover, the volume ratio of the spacer particles 8 and 18 to the light-emitting layers 4 and 4′ is preferably 55% or more and 90% or less.
As illustrated in
Note that if the spacer particles 18′ are the spacer particles 8 shown in
As can be seen, because the spacer particles include the organic ligands, the light-emitting layer can be formed to include clearance to provide a further distance between the quantum dots 7.
The thickness of each of the light-emitting layers 4, 4′ and 4″ shall not be limited to a particular thickness as long as the thickness can provide room for the electrons and the holes to recombine together to emit light. For example, the thickness may be approximately 1 to 200 nm.
A material to be used for the hole transport layer 3 illustrated in
Examples of the material to be used for the hole transport layer 3 include an arylamine derivative, an anthracene derivative, a carbazole derivative, a thiophene derivative, a fluorene derivative, a distyrylbenzene derivative, and a spiro compound. Moreover, the material to be used for the hole transport layer 4 is more preferably polyvinyl carbazole (PVK) or poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(4,4′-(N-(4-sec-butylphenyl))diphenylamine)] (TFB).
In addition, the hole transport layer 3 may be formed of an inorganic semiconductor material. Examples of the inorganic semiconductor material include a metal oxide (including a metal oxide semiconductor), a nitride semiconductor, an arsenide semiconductor, a halide semiconductor, and a pseudohalide semiconductor. If the hole transport layer 3 is formed of an inorganic semiconductor material, the inorganic semiconductor material may be doped with an acceptor impurity so that the hole transport layer 3 can exhibit a significant hole transporting capability.
If the hole transport layer 3 is formed first and then the light-emitting layer 4 is formed, the hole transport layer 3 is preferably formed of a metal oxide semiconductor, and the hole transport layer 3 formed of the metal oxide semiconductor is preferably in contact with the light-emitting layer 4. Such a feature can reduce damage including heat and light and given to the hole transport layer 3 in forming the light-emitting layer 4.
A material to be used for a not-shown hole injection layer shall not be limited to a particular material as long as the material is a hole injecting material capable of stably injecting the holes into the light-emitting layer 4. Examples of the hole injecting material include an arylamine derivative, a porphyrin derivative, a phthalocyanine derivative, a carbazole derivative, and conductive polymers such as a polyaniline derivative, a polythiophene derivative, and a polyphenylene vinylene derivative. Moreover, the material to be used for the hole injection layer 3 is preferably poly(3,4-ethylenedioxythiophene)-polystyrene sulfonic acid (PEDOT-PSS).
A thickness of the not-shown hole injection layer and a thickness of the hole transport layer 3 shall not be limited to particular thicknesses as long as the thicknesses can sufficiently exhibit capabilities to inject and transport the holes.
A material to be used for the electron transport layer 5 illustrated in
Examples of the electron transporting material include oxadiazoles, triazoles, phenanthrolines, a silole derivative, a cyclopentadiene derivative, an aluminum complex, a metal oxide (including a metal oxide semiconductors), a nitride semiconductor, and an arsenide semiconductor. Specific examples of the oxadiazole derivative include (2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole) (PBD). Specific examples of the phenanthrolines include bathocuproine (BCP) and bathophenanthroline (Bphen). Specific examples of the aluminum complex include a tris(8-quinolinol) aluminum complex (Alq3) and a bis(2-methyl-8-quinolato)(p-phenylphenolate) aluminum complex (Balq).
Examples of the metal oxide as the electron transporting material include ZnO, MgZnO, TiO2, Ta2O3, SrTiO3, and MgxZn1-xO (where x is the ratio of Zn in ZnO replaced with Mg).
Furthermore, examples of the inorganic semiconductor material as the electron transporting material include a group II-VI semiconductor material and a group III-V semiconductor material. Examples of the group II-VI semiconductor material include ZnS, ZnSe, ZnTe, Cds, CdSe, CdTe, HgTe, and a mixed crystal of these materials, and examples of the group III-V semiconductor material include AlP, AlAs, AlN, AlSb, GaN, GaP, GaAs, GaSb, InP, InAs, InSb, InN, and a mixed crystal of these materials.
There is no problem as long as the semiconductor material is a native n-type semiconductor material. If necessary, the semiconductor material may contain a donor impurity.
The material to be used for the electron transport layer 5 is preferably MgxZn1-xO. Ionization potential and electron affinity of MgxZn1-xO can be adjusted when x is adjusted. Such a feature makes it possible to readily prepare an electron transport layer suitable to an emission wavelength of the QD emission layer.
Note that if the electron transport layer 5 is formed first and then the light-emitting layer 4 is formed, the electron transport layer 5 is preferably formed of a metal oxide semiconductor, and the electron transport layer 5 formed of the metal oxide semiconductor is preferably in contact with the light-emitting layer 4. Such a feature can reduce damage including heat and light and given to the electron transport layer 5 in forming the light-emitting layer 4.
A material to be used for a not-shown electron injection layer shall not be limited to a particular material as long as the material is an electron injecting material capable of stably injecting the electrons into the light-emitting layer 4. Examples of the electron injecting material include: alkali metals or alkaline earth metals such as aluminum, strontium, calcium, lithium, cesium, magnesium oxide, aluminum oxide, strontium oxide, lithium oxide, lithium fluoride, magnesium fluoride, strontium fluoride, calcium fluoride, barium fluoride, cesium fluoride, sodium polymethylmethacrylate polystyrene sulfonate; oxides of alkali metals or alkaline earth metals; fluorides of alkali metals or alkaline earth metals, and organic complexes of alkali metals.
A thickness of the electron transport layer 5 and a thickness of the electron injection layer shall not be limited to particular thicknesses as long as the thicknesses can sufficiently exhibit capabilities to transport and inject the electrons.
A material to be used for the anode 2 illustrated in
A material to be used for the cathode 6 illustrated in
Of the anode 2 and the cathode 6, the electrode provided toward a light releasing face needs to be transparent. Meanwhile, the electrode across from the light releasing face may be either transparent or non-transparent. Moreover, the anode 2 and the cathode 6 are preferably low in resistance, and typically made of a metal material: that is, a conductive material. The anode 2 and the cathode 6 may be made of an organic compound or inorganic compound.
A method for manufacturing the light-emitting element 1 illustrated in
Note that described below as an example is how to prepare the solution including the plurality of quantum dots 7 and the spacer particles 8 and 18, the plurality of quantum dots 7 each including a core 7C and a shell 7S coating the core 7C and having an outermost layer containing a metal chalcogenide complex or a metal chalcogenide compound.
First, a five-milligram metal chalcogenide ligand material such as (CH3NH3)4Sn2S6 capable of curing the shell is weighed into, for example, a cleaned screw tube. After that, the metal chalcogenide ligand material is dissolved in a mixed solution of 2 ml of dimethyl sulfoxide (DMSO) and 1 ml of ethanolamine (EA). Then, for example, a dispersion solution in which the cores 7C are dispersed in hexane or octane is put into the mixed solution in which the metal chalcogenide ligand material is melted, and stirred with a stirrer at about 400 rpm for 3 to 4 hours. The mixed solution, in which the dispersion solution and the metal chalcogenide ligand material are dissolved, separates into two layers. The mixed solution in which the metal chalcogenide ligand material is dissolved is the lower layer and the dispersion solution is the upper layer. Then, when the metal chalcogenide ligands are completely substituted, the color of the upper layer (hexane or octane) transfers to the lower layer (the mixed solution of DMSO and EA), and the upper layer becomes transparent. After that, the liquid in the upper layer is removed, and the liquid in the lower layer is washed several times with hexane. Then, the cores 7C with the metal chalcogenide ligands bonding thereto in the lower layer are taken out. Acetonitrile is added to the cores 7C so that the cores 7C are precipitated. Acetonitrile is centrifuged at 6000 rpm for 10 minutes so that the cores 7C are completely precipitated. Then, the supernatant liquid is removed, and the cores 7C with the metal chalcogenide ligands bonding thereto are redispersed in a mixed solution of dimethyl sulfoxide (DMSO) and ethanolamine (EA). Finally, the redispersion solution, in which the cores 7C with the metal chalcogenide ligands bonding thereto are redispersed, and a dispersion solution, in which the spacer particles 8 or the cores 18C to serve as the spacer particles 18 are dispersed, are mixed together at any given ratio. Hence, a solution to coat a light-emitting layer can be obtained.
If the metal chalcogenide ligands have to be bonded to the surface of the cores 18C serving as the spacer particles 18, a redispersion solution of the cores 18C with the metal chalcogenide ligands bonding thereto may be prepared by the same technique as the technique of obtaining the redispersion solution of the cores 7C with the metal chalcogenide ligands boding thereto. After that, the redispersion solution, in which the cores 7C with the metal chalcogenide ligands bonding thereto, and the redispersion solution, in which the cores 18C with the metal chalcogenide ligands thereto, are mixed together at any given ratio. Hence, a solution to coat a light-emitting layer can be obtained.
Furthermore, if spacer particles 18′, which includes the organic ligands 18L bonding to the cores 18C illustrated in
Moreover, for example, a non-patent document [12 Jun. 2009 VOL 324 SCIENCE] describes the use of hydrazine as a solvent. However, when hydrazine is used, the hydrazine inevitably remains in the quantum dot layer after film formation. Hydrazine is not preferable because strong reducibility of hydrazine corrodes quantum dots and other layers. Furthermore, since hydrazine has high hygroscopicity, a large amount of water molecules are contained in the light-emitting element depending on a process, which is not preferable from the viewpoint of long-term reliability.
In the above-described method for manufacturing the light-emitting element 1, a coating solution to be used preferably contains dimethyl sulfoxide (DMSO). In the case of dimethyl sulfoxide (DMSO), strong reducibility is not exhibited even when dimethyl sulfoxide (DMSO) remains in the quantum dots after film formation. Hence, the light-emitting element 1 can achieve long-term reliability.
Note that described below may be examples of a step of coating with the above coating solution and a step of irradiating the coating solution with an ultraviolet light, or of heating the coating solution, and forming the light-emitting layers 4, 4, and 4″ including clearance to provide a distance between at least two quantum dots 7 included in the plurality of quantum dots 7 and positioned next to each other.
The coating solution can be applied, for example, by a technique such as spin coating, dipping, or misting. After that, the applied solution is, for example, at least either irradiated with an ultraviolet light of 365 nm, or heated, to form the light-emitting layers 4, 4, and 4″. After that, if necessary, the light-emitting layers 4, 4, and 4″ are washed with a mixed solution of dimethyl sulfoxide (DMSO) and ethanolamine (EA).
As described above, if the shells are the metal chalcogenide complex 7L bonding to the cores 7C, the metal chalcogenide complex 7L does not have to be irradiated with the ultraviolet light or heated to form the shells.
In the first embodiment, the quantum dots include the cores and the shells coating the cores. Alternatively, it may also be described that the shells are provided to the surface of the cores. Moreover, a shell coats at least a portion of a core, and, preferably, the shell coats the entire core. If a cross-section of a quantum dot shows that the shell covers the core, it may be said that the shell covers the core.
Described next is a second embodiment of the present invention, with reference to
As illustrated in
Typically, one pixel of a display device includes a red subpixel, a green subpixel, and a blue subpixel. The red subpixel includes a light-emitting element including a light-emitting layer that emits a red light. The green subpixel includes a light-emitting element including a light-emitting layer that emits a green light. The blue subpixel includes a light-emitting element including a light-emitting layer that emits a blue light.
The red light-emitting element 1R illustrated in
The green light-emitting element 21G illustrated in
The blue light-emitting element 21B illustrated in
In this embodiment, the cores 7C contained in the light-emitting layer 4′ included in the red light-emitting element 1R, the cores 37C contained in the light-emitting layer 24G included in the green light-emitting element 21G, and the cores 47C contained in the light-emitting layer 24B included in the blue light-emitting element 21B are made of the same material with different particle sizes. For example, the cores 7C having the largest particle size can be used for the light-emitting layer 4′ that emits a red light. The cores 47C having the smallest particle size can be used for the light-emitting layer 24B that emits a blue light. The cores 37C having a particle size between the particle size of the cores 7C to be used for the light-emitting layer 4′ that emits the red light and the particle size of the cores 47C to be used for the light-mitting layer 24B that emits the blue light can be used for the light-emitting layer 24G that emits a green light. The cores 7C, 37C, and 47C shall not be limited to such examples, and may be formed of different materials.
This embodiment exemplifies a case where the spacer particles are contained only in the light-emitting layer 4′ of the red light-emitting element 1R. However, this embodiment shall not be limited to such a case. The spacer particles may be contained in the light-emitting layer of at least one of the red light-emitting layer, the green light-emitting layer, or the blue light-emitting layer.
Moreover,
Moreover,
Described next is a third embodiment of the present invention, with reference to
The red light-emitting element 1R illustrated in
The green light-emitting element 1G illustrated in
The blue light-emitting element 1B illustrated in
The core 18C whose bandgap is 2.4 eV is larger in particle size than the core 28C whose bandgap is 3.0 eV. The core 28C whose bandgap is 3.0 eV is larger in particle size than the core 38C whose bandgap is 3.4 eV
Note that the spacer particles 38 including the cores 38C included in the light-emitting layer 4′B of the blue light-emitting element 1B can be used for the light-emitting layer 4′ of the red light-emitting element 1R and the light-emitting layer 4′G of the green light-emitting element 1G; however, the problem is that the spacer particles 38 exhibit leakage reduction capability larger than necessary, and inevitably increase the drive voltage. This is because the charge injection barrier is higher as the bandgap of spacer particles is larger. Accordingly, transportation of the charges is difficult. Hence, as described in this embodiment, the size of the spacer particles is determined so that the particle size is greater in the order of the cores 18C, the cores 28C, and the cores 38C. Such a feature can reduce the risk that the leakage reduction capability is larger than necessity, and decrease a drive voltage.
In the step of manufacturing the display device of this embodiment, similar to the method described in the first embodiment, respective coating solutions are prepared for forming the light-emitting layer 4′ of the red light-emitting element 1R, the light-emitting layer 4′G of the green light-emitting element 1G, the light-emitting layer 4′B of the blue light-emitting element 1B. Then, first, one of the above three coating solutions is thoroughly applied by various wet film forming techniques (spin coating, bar coating, dip coating, flow coating, screen printing, and inkjet printing). After that, only a portion to form the light-emitting layer is irradiated with light and cured. Here, if necessary, the portion may be baked before and after the exposure to the light. Then, the portion is rinsed with dimethyl sulfoxide (DMSO), and a film of an unnecessary portion; that is, an uncured portion, is removed. After that, the remaining two coating solutions similarly form the light-emitting layers in different positions. Hence, the display device of the present embodiment is successfully manufactured.
The present invention shall not be limited to the embodiments described above, and can be modified in various manners within the scope of claims. The technical aspects disclosed in different embodiments are to be appropriately combined together to implement another embodiment. Such an embodiment shall be included within the technical scope of the present invention. Moreover, the technical aspects disclosed in each embodiment may be combined to achieve a new technical feature.
The present invention is applicable to a light-emitting element, a display device, and a light-emitting element manufacturing method.
Filing Document | Filing Date | Country | Kind |
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PCT/JP2020/040833 | 10/30/2020 | WO |