JP2009088276A - Light emitting element - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a light emitting element having a light emitting layer formed of a monomolecular film of quantum dots, the light emitting element being increased in luminance and light emission efficiency. <P>SOLUTION: The light emitting element 1 has at least an anode 3, a hole transporting layer 6, the light emitting element 5 formed of the monomolecular film of quantum dots 11, an electron transporting layer 7, and a cathode 4 in this order, and hole mobility of the hole transporting layer 6 is made smaller than electron mobility of the electron transporting layer 7 to allow the hole transporting layer 6 to have an electron blocking function, thereby solving the problem to be solved. In this case, an energy value of LUMO (lowest unoccupied molecular orbital) is made much smaller than an energy value of LUMO (lowest unoccupied molecular orbital) of the light emitting layer 5. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a light emitting device, and more particularly to a light emitting device having an EL light emitting layer made of a monomolecular film of quantum dots.

  An organic electroluminescence element (hereinafter also referred to as an organic EL element) is a light emitting element having a laminated structure in which an organic light emitting layer is sandwiched between an anode and a cathode, and injected from holes and cathodes injected from the anode. It is a self-luminous device that utilizes light emission caused by recombination that occurs in the light-emitting layer. The problem of such an organic EL element is to increase the lifetime of the light emitting material constituting the organic light emitting layer and to improve the light emission efficiency. Currently, research for overcoming the problem is being actively conducted.

  On the other hand, light-emitting devices using semiconductor fine particles (referred to as “quantum dots”) whose emission color is regulated by the particle size as EL light-emitting materials have been proposed (for example, Non-Patent Document 1 and Patent Document 1). See). In these documents, as a representative example of a quantum dot, a structure composed of a core made of CdSe, a ZnS shell provided around the core, and a capping compound provided further around the core is illustrated. A light emitting element using the quantum dots as a light emitting material has an advantage that it has a longer life than a light emitting element using the organic EL material.

However, as shown in FIG. 1 of Non-Patent Document 1, since the light-emitting layer of the light-emitting element proposed in the same document is a monomolecular film of quantum dots, the charges supplied from both electrodes are recombined. The excitons generated in this way reach the monomolecular film and have little opportunity to be consumed for EL emission, and there is a problem that sufficient luminance and luminous efficiency cannot be achieved. In the same document, an example in which a hole blocking layer is provided between the light emitting layer and the electron transport layer to increase the probability of recombination in the light emitting layer is proposed. It does not bring about luminous efficiency.
Seth Coe et.al., Nature, 420, 800-803 (2002) JP 2005-522005 gazette

  The present invention has been achieved as a result of studying from a viewpoint different from that of Non-Patent Document 1 in order to achieve sufficient luminance and luminous efficiency, and the object thereof is a light-emitting layer comprising a monomolecular film of quantum dots. It is an object of the present invention to provide a light emitting element having improved luminance and light emission efficiency.

  The light-emitting element of the present invention for solving the above problems is a light-emitting device having at least an anode, a hole transport layer, a light-emitting layer composed of a monomolecular film of quantum dots, an electron transport layer, and a cathode in that order. In the device, the hole mobility of the hole transport layer is smaller than the electron mobility of the electron transport layer, and the hole transport layer has an electron blocking function.

  According to this invention, the hole mobility of the hole transport layer is made smaller than the electron mobility of the electron transport layer, and the hole transport layer is configured to have an electron blocking function. The electrons that have reached the interface between the hole transport layer and the light emitting layer are recombined with holes supplied from the anode at or near the interface. As a result, the excitons generated by the recombination are consumed so that the quantum dots in the light emitting layer emit EL light, so that the light emission efficiency is improved.

  As a preferred embodiment of the light emitting device of the present invention, the LUMO (lowest empty orbit) energy value of the hole transport layer is configured to be smaller than the LUMO (lowest empty orbit) energy value of the light emitting layer.

  As a preferred embodiment of the light emitting device of the present invention, the hole transport layer includes TPD as a hole transport material, and the electron transport layer includes fullerene as an electron transport material.

  As a preferred embodiment of the light emitting device of the present invention, the light emitting layer is configured such that the quantum dots are phase-separated from a mixed liquid of a hole transporting material constituting the hole transport layer and the quantum dots. .

  According to the light emitting device of the present invention, the electrons supplied from the cathode and the holes supplied from the anode recombine at or near the interface between the hole transport layer and the light emitting layer, and thus are generated by the recombination. The excitons are consumed so that the quantum dots in the light emitting layer emit EL. As a result, luminance and luminous efficiency can be improved. Such a light emitting element can realize high luminous efficiency.

  Hereinafter, although the embodiment of the light emitting element of the present invention is described, the present invention is not limited to the following embodiment and drawings.

  FIG. 1 is a schematic cross-sectional view showing an example of a light emitting device of the present invention, and FIG. 2 is a schematic diagram for explaining the light emission principle of the light emitting device of the present invention. As shown in FIG. 1, the light emitting device 1 of the present invention includes at least an anode 3, a hole transport layer 6, a light emitting layer 5 made of a monomolecular film of quantum dots 11, an electron transport layer 7, and a cathode 3. In that order. The hole mobility of the hole transport layer 6 is smaller than the electron mobility of the electron transport layer 7, and the hole transport layer 6 is configured to have an electron blocking function.

  It should be noted that the following components constituting the light emitting element 1 are selected and a reflective layer or the like may be provided to constitute a top emission type element or a bottom emission type element. .

  Next, although the component of the light emitting element 1 of this invention is demonstrated in detail, it is not limitedly interpreted only to the following specific examples. In the following, when the expressions “upper” and “lower” are used, the upper side in the plan view of FIG. 1 means “upper”, and the lower side means “lower”.

(Base material)
The substrate 2 is provided as a base substrate for the anode 3 in the example of FIG. 1, but is not particularly limited to the example of FIG. 1, and may be provided on the upper side of the cathode 4 or provided on both of them. It may be done. The transparency of the base material 2 is arbitrarily selected depending on the light emission direction, and in the case of a bottom emission type light emitting element, the base material 2 shown in FIG. 1 needs to be transparent. The structure such as the type, shape, size, and thickness of the substrate is not particularly limited, and can be appropriately determined depending on the use of the light-emitting element 1 and the material of each layer laminated on the substrate. For example, a material made of various materials such as a metal such as Al, glass, quartz, or resin can be used. Specific examples include glass, quartz, polyethylene, polypropylene, polyethylene terephthalate, polyethylene naphthalate, polymethacrylate, polymethyl methacrylate, polymethyl acrylate, polyester, polycarbonate, and the like. In addition, the shape of the substrate 2 may be a single wafer shape or a continuous shape, and specific examples include a card shape, a film shape, a disk shape, and a chip shape.

(electrode)
The anode 3 and the cathode 4 are electrodes for supplying holes and electrons for emitting light from the quantum dots 11 which are EL light emitting materials. Normally, as shown in FIG. The cathode 4 is provided opposite to the anode 3 with at least the light emitting layer 5 and the electron transport layer 7 sandwiched between the anode.

As the anode 3, a thin film of metal, conductive oxide, conductive polymer or the like is used. Specifically, for example, a transparent conductive film such as ITO (Indium Tin Oxide), Indium Oxide, IZO (Indium Zinc Oxide), SnO ₂ , ZnO or the like, a hole injection property such as gold or chromium, and a large work function is large. Examples thereof include conductive polymers such as metals, polyaniline, polyacetylene, polyalkylthiophene derivatives, and polysilane derivatives. Such an anode 3 can be formed by a vacuum process such as vacuum deposition, sputtering, or CVD, or coating, and the film thickness thereof is preferably about 10 nm to 1000 nm, for example, although it varies depending on the material used.

  As the cathode 4, a thin film made of metal, conductive oxide, conductive polymer or the like is used. Specifically, for example, single metals such as aluminum and silver, magnesium alloys such as MgAg, aluminum alloys such as AlLi, AlCa, and AlMg, alkali metals including Li and Ca, and alloys of these alkali metals A metal having a small work function and a good electron injection property can be given. The cathode 4 is formed by a vacuum process such as vacuum deposition, sputtering, CVD, or coating, as in the case of the anode 3 described above, and the film thickness varies depending on the material used, but is about 10 nm to 1000 nm, for example. Is preferred.

(Light emitting layer)
The light emitting layer 5 is provided between the anode 3 and the cathode 4, and the holes supplied from the anode 3 recombine with the electrons supplied from the cathode 4, and the recombination is performed. The quantum dots 11 of the EL material constituting the light emitting layer 5 emit light by the excitons (excitons) generated by the above. The light emitting layer 5 is provided as a monomolecular film as shown in FIG. 2, but the monomolecular film may be in a single layer form as shown in FIG. 2 or in a laminated form of two or more layers. Good.

  Quantum dots 11 constituting the light emitting layer 5 are semiconductor fine particles whose emission color is regulated by the particle size. This quantum dot 11 is also called a nanoparticle or a nanocrystal, and a typical example thereof is a core made of CdSe, a ZnS shell provided around the core, and further provided around the core. The capping compound formed can be exemplified. The quantum dots 11 have different emission colors depending on their particle sizes. For example, the particle size for blue emission is in the range of 1.0 nm to 1.9 nm, and the particle size for green emission is 2.0 nm to 2 nm. The particle diameter of red light emission is in the range of 4.2 nm to 6.0 nm.

The quantum dot 11 will be described in further detail. The quantum dots 11 are not particularly limited as long as they are semiconductor nanometer-sized fine particles (semiconductor nanocrystals) and are light-emitting materials that produce a quantum confinement effect (quantum size effect). Specifically, II such as MgS, MgSe, MgTe, CaS, CaSe, CaTe, SrS, SrSe, SrTe, BaS, BaSe, BaTe, ZnS, ZnSe, ZnTe, CdS, CdSe, CdTe, HgS, HgSe and HgTe. Group VI semiconductor compounds, AlN, AlP, AlAs, AlSb, GaAs, GaP, GaN, GaSb, InN, InAs, InP, InSb, III-V group semiconductor compounds such as TiN, TiP, TiAs and TiSb, Si, Ge In addition to semiconductor crystals containing a group IV semiconductor such as Pb and the like, semiconductor compounds containing three or more elements such as InGaP can be given. Alternatively, a semiconductor crystal obtained by doping the semiconductor compound with a rare earth metal cation or a transition metal cation such as Eu ³⁺ , Tb ³⁺ , Ag ⁺ , or Cu ⁺ can be used.

  Among these, semiconductor crystals such as CdS, CdSe, CdTe, and InGaP are preferable from the viewpoints of ease of production, controllability of particle diameters for obtaining light emission in the visible range, and fluorescence quantum yield.

  The quantum dot 11 may be made of one kind of semiconductor compound or may be made of two or more kinds of semiconductor compounds. For example, the quantum dot 11 is made of a core made of a semiconductor compound and a semiconductor compound different from the core. You may have a core shell type structure which has a shell. The core-shell quantum dot uses a material with a higher band gap than the semiconductor compound that forms the core as the semiconductor compound that forms the core so that excitons are confined in the core, thereby improving the luminous efficiency of the quantum dot. Can be increased. Examples of the core-shell structure (core / shell) having such a bandgap relationship include CdSe / ZnS, CdSe / ZnSe, CdSe / CdS, CdTe / CdS, InP / ZnS, GaP / ZnS, Si / ZnS, Examples include InN / GaN, InP / CdSSe, InP / ZnSeTe, GaInP / ZnSe, GaInP / ZnS, Si / AlP, InP / ZnSTe, GaInP / ZnSTe, and GaInP / ZnSSe.

  The size of the quantum dots 11 may be appropriately controlled depending on the material constituting the quantum dots so that light having a desired wavelength can be obtained. As the particle size of the quantum dot decreases, the energy band gap increases. That is, as the crystal size decreases, the light emission of the quantum dots shifts to the blue side, that is, to the high energy side. Therefore, by changing the size of the quantum dots, the emission wavelength can be adjusted over the wavelength range of the spectrum in the ultraviolet region, the visible region, and the infrared region.

  Generally, the particle size (diameter) of the quantum dots 11 is in the range of 0.5 to 20 nm, and preferably in the range of 1 to 10 nm. The narrower the quantum dot size distribution, the clearer the emission color.

  Moreover, the shape of the quantum dot 11 is not specifically limited, A spherical shape, rod shape, disk shape, and other shapes may be sufficient. When the quantum dot is not spherical, the particle diameter of the quantum dot can be a value when it is assumed to be a true sphere having the same volume.

  Information such as the particle size, shape, and dispersion state of the quantum dots 11 can be obtained by a transmission electron microscope (TEM). Further, the crystal structure and particle size of the quantum dots can be known by X-ray crystal diffraction (XRD). Furthermore, the information regarding the particle diameter and surface of a quantum dot can also be obtained by a UV-Vis absorption spectrum.

  As an example of the quantum dots 11, for example, a CdSe / ZnS type core-shell structure having a basic structure of a core made of CdSe, a ZnS shell provided around the core, and a capping compound provided further around the core is provided. A thing can be illustrated preferably. In such a core-shell structure, the core is made of a semiconductor compound, the shell is made of a semiconductor compound different from the core, and the exciton is confined in the core by using a material having a higher band gap than the semiconductor compound forming the core. Works. Also, the capping compound acts as a dispersant. Specific examples of such capping compounds include TOPO (trioctylphosphine oxide), TOP (trioctylphosphine), TBP (tributylphosphine), and the like, and these materials can be dispersed in an organic solvent. .

  The light-emitting layer 5 is usually composed of a single quantum dot 11 and emits a predetermined light emission color, but may be formed using two or more types of quantum dots that emit different light emission colors at the same time.

  Although the formation method of the light emitting layer 5 is not specifically limited, For example, it can form simultaneously with formation of the below-mentioned hole transport layer 6. Specifically, for example, TPD (N, N′-bis- (3-methylphenyl) -N, N′-bis- (phenyl) -benzidine), which is a material for forming a hole transport layer, and quantum dots The hole transport layer 6 is formed by preparing a mixed solution and applying the mixed solution, and a monomolecular film composed of quantum dots 11 phase-separated from the hole transport layer 6 is formed as the light emitting layer 5. Can do. The phase separation at this time occurs because the phenyl group possessed by TPD and the alkyl group possessed by the cap material of the quantum dot 11 are not compatible with each other. If the group and the group of the quantum dot cap material are selected, the hole transport layer 6 and the light emitting layer 5 can be formed simultaneously by phase separation. The simultaneous formation of the light-emitting layer 5 and the hole transport layer 6 is extremely effective in manufacturing for such phase separation.

  Further, if the upper and lower film forming procedures in FIG. 1 are reversed, the light emitting layer 5 and the electron transport layer 7 can be simultaneously formed by the same concept. The light emitting layer 5 can also be formed by methods other than those described above, and examples thereof include an ink jet method and a micro contact print method.

  Since the obtained light emitting layer 5 is basically a monomolecular film of the quantum dots 11, the thickness T1 thereof is almost the same as the particle diameter of the used quantum dots 11. Although the particle size varies depending on the emission color of the quantum dots 11 and cannot be generally stated, it is within the range of each particle size, that is, 1 nm or more and 10 nm or less.

(Hole transport layer)
The hole transport layer 6 is usually provided on the anode 3, but may be provided via a hole injection layer (not shown). In the present invention, the hole transport layer 6 acts to transport holes supplied from the anode 3 to the light emitting layer 5 side, and is supplied from the cathode 4 and passes through the light emitting layer 5. It also serves as an electron blocking function layer (electron blocking layer) for blocking electrons. The hole mobility of the hole transport layer 6 having such a function is configured to be smaller than the electron mobility of the electron transport layer 7 described later. The material for forming the hole transport layer 6 is selected in consideration of the combination with the material for forming the electron transport layer 7 on the premise of the relative magnitude relationship of the charge mobility.

  Examples of the material for forming the hole transport layer 6 include arylamine derivatives, anthracene derivatives, carbazole derivatives, thiophene derivatives, fluorene derivatives, distyrylbenzene derivatives, and spiro compounds. In consideration of simultaneous formation with the light emitting layer 5 described above, the above-described N, N′-bis- (3-methylphenyl) -N, N′-bis- (phenyl) -benzidine (TPD) is preferably used. For example, specific examples of arylamine derivatives include bis (N- (1-naphthyl-N-phenyl) -benzidine (α-NPD), copoly [3,3′-hydroxy-tetra Phenylbenzidine / diethylene glycol] carbonate (PC-TPD-DEG), etc. Specific examples of carbazoles include polyvinyl carbazole (PVK), etc. Specific examples of thiophene derivatives include poly [(9,9-dioctylfluorenyl-2,7-diyl) -co- (bithiophene)] and the like. Specific examples of the len derivative include poly [(9,9-dioctylfluorenyl-2,7-diyl) -co- (4,4 ′-(N- (4-sec-butylphenyl)) diphenylamine)]. Specific examples of spiro compounds include poly [(9,9-dioctylfluorenyl-2,7-diyl) -alt-co- (9,9′-spiro-bifluorene). -2,7-diyl)] etc. These materials may be used alone or in combination of two or more.

  Such a hole transport layer 6 can be formed by various methods, and its thickness varies depending on the material used, but is preferably in the range of, for example, about 1 nm to 50 nm.

Among the hole transporting materials, N, N′-bis- (3-methylphenyl) -N, N′-bis- (phenyl) -benzidine (TPD) used in Examples described later is a hole transfer. The degree is about 10 ⁻³ cm ² / V · s, which can be preferably used. Then, the hole transport layer 6 is formed with this TPD, and the electron transport layer 7 is further formed by using, for example, fullerene having an electron mobility of about 10 ⁻² cm ² / V · s as a material for forming the electron transport layer 7 described later. If formed, since the hole mobility of the hole transport layer 6 is smaller than the electron mobility of the electron transport layer 7, the electrons easily pass through the electron transport layer 7 and further pass through the light emitting layer 5. It reaches the interface between the transport layer 6 and the light emitting layer 5 or near the interface. Moreover, the LUMO energy value of the hole transport layer 6 made of TPD is 2.1 eV, which is significantly smaller than the LUMO energy value of the light emitting layer 5 made of the quantum dots 11 of 4.6 eV. Since the value is large, the hole transport layer 6 functions as an electron blocking layer. For this reason, as shown in FIG. 2, the electrons that have reached the interface between the hole transport layer 6 and the light emitting layer 5 recombine with the holes supplied from the anode 3 at or near the interface. The excitons are consumed so that the quantum dots 11 in the light emitting layer 5 emit EL light.

(Electron transport layer)
The electron transport layer 7 is provided between the light emitting layer 5 and the cathode 4, but may be provided via an electron injection layer (not shown). The electron transport layer 7 acts to transport electrons supplied from the cathode 4 to the light emitting layer 5 side. The electron mobility of the electron transport layer 7 is configured to be larger than the hole mobility of the hole transport layer 6 as described above. The material for forming the electron transport layer 7 is selected in consideration of the combination with the material for forming the hole transport layer 6 on the premise of the relative magnitude relationship between the hole mobility and the electron mobility.

Examples of the material for forming the electron transport layer 7 include fullerene derivatives, metal complexes, oxadiazole derivatives, triazole derivatives, phenanthroline derivatives, and silyl compounds. Specifically, specific examples of phenanthrolines include bathocuproin and bathophenanthroline, and specific examples of metal complexes include tris (8-quinolinolato) aluminum complex (Alq ₃ ), bis (2-methyl-8). -Quinolinato) (p-phenylphenolate) aluminum complex (BAlq2) and the like. Examples of the oxadiazole derivative include (2- (4-biphenylyl) -5- (4-tert-butylphenyl) -1,3,4-oxadiazole) (PBD). Such an electron transport layer 7 is formed by a vacuum deposition method or a coating method using an electron transport layer forming coating solution containing the above-described material.

  Such an electron transport layer 7 can be formed by various methods, and the thickness thereof varies depending on the material used, but is preferably in the range of, for example, about 1 nm to 100 nm.

(Other layers)
An electron injection layer (not shown) is provided between the cathode 4 and the electron transport layer 7 as necessary, and acts so that electrons are easily injected from the cathode 4. As the material for forming the electron injection layer, aluminum, lithium fluoride, strontium, magnesium oxide, magnesium fluoride, strontium fluoride, calcium fluoride, barium fluoride, aluminum oxide, strontium oxide, calcium, polymethyl methacrylate polystyrene sulfonic acid Examples thereof include alkali metals such as sodium, lithium, cesium and cesium fluoride, halides of alkali metals, and organic complexes of alkali metals. Such an electron injection layer can be formed by various methods, and the thickness thereof varies depending on the material used, but is preferably in the range of, for example, about 0.1 nm to 200 nm.

  A hole injection layer (not shown) is provided between the anode 3 and the hole transport layer 6 as necessary, and acts so that holes (holes) are easily injected from the anode 3. As a material for forming the hole injection layer, for example, poly (3,4) ethylenedioxythiophene / polystyrene sulfonate (abbreviated as PEDOT / PSS, manufactured by Bayer, trade name: Baytron P CH8000, commercially available as an aqueous solution) is conventionally used. What is known as a hole injection layer forming material can be used. Such a hole injection layer can be formed by various methods, and the thickness thereof varies depending on the material used, but is preferably in the range of, for example, about 0.1 nm to 200 nm.

Although the reflective layer (not shown) is not an essential layer, it is a layer for efficiently extracting the light generated in the light emitting layer 5 to the outside, and is a layer provided to increase the light emission efficiency.
Since it is possible, it is provided preferably. This reflective layer may be provided independently as an independent layer, or may be provided as a resonator structure constituted by a pair of a total reflection layer and a semitransparent reflection layer. Usually, a transparent conductive film or a metal layer such as gold or chrome is preferably used for such a reflective layer.

(Energy diagram)
Next, features of the light-emitting element of the present invention will be described with reference to an energy diagram. FIG. 3 is an energy diagram showing the ionization potential of the material constituting each layer used in Examples described later. In the embodiment of the present application, since the TPD having an energy value of LUMO (lowest orbital) of 2.1 eV is used as the hole transport layer 6, the difference from the energy value (4.6 eV) of the quantum dot 11 of the light emitting layer 5 is used. Is a remarkably small value of 2.5 eV. Therefore, even if the electrons e supplied from the electron transport layer 7 to the light emitting layer 5 try to enter the hole transport layer 6, there is a large energy barrier between the light emitting layer 5 and the hole transport layer 6. The transport layer 6 functions as a block layer for electrons e. The light emitting device of the present invention can be applied by arbitrarily selecting various hole transport materials and quantum dots as long as it has such a viewpoint.

  As described above, according to the light emitting device 1 of the present invention, the electrons supplied from the cathode 4 and the holes supplied from the anode 3 are the interface between the hole transport layer 6 and the light emitting layer 5 or the vicinity of the interface. Therefore, excitons generated by the recombination are consumed so that the quantum dots 11 in the light emitting layer 5 emit EL light. As a result, luminance and luminous efficiency can be improved. Such a light emitting device 1 can realize high luminous efficiency.

  EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited to the following examples.

(Example 1)
First, a poly (3,4-ethylenedioxythiophene) -polystyrene sulfonic acid (PEDOT-PSS) thin film (thickness) is used as a hole injection layer on a glass substrate on which an ITO film having a thickness of 150 nm is formed as an anode. : 80 nm) was formed by applying a PEDOT-PSS solution in the air by a spin coating method. After the PEDOT-PSS film formation, the film was dried using a hot plate in the air in order to evaporate the water.

  Next, on the hole injection layer, N, N′-bis- (3-methylphenyl) -N, N′-bis- (phenyl) -benzidine (TPD) is used as a hole transport layer. The thin film (thickness: 40 nm) is formed, and a light emitting layer (thickness: 5. nm) comprising a monomolecular film of quantum dots (particle size: 5.6 nm, manufactured by Evident Technology) emitting red light is further formed thereon. 6 nm). The hole transport layer and the light emitting layer are prepared as a mixed solution of TPD having a phenyl group and quantum dots having an alkyl group in a cap material (prepared in a ratio of TPD: quantum dots = (1: 1)), A hole transport layer was formed by applying the mixed solution on the hole injection layer, and a monomolecular film composed of quantum dots phase-separated from the hole transport layer was simultaneously formed as a light emitting layer.

Next, a layer made of fullerene (thickness: 40 nm) as an electron transport layer was formed on the light emitting layer by a resistance heating vapor deposition method in vacuum (pressure: 1 × 10 ⁻⁴ Pa). Next, LiF (thickness: 0.5 nm) is formed as an electron injection layer on the electron transport layer, and Al (thickness: 100 nm) is further formed thereon as a cathode in a vacuum (pressure: 1). The film was formed by a resistance heating vapor deposition method at × 10 ⁻⁴ Pa).

  After forming the light emitting element in this way, the light emitting element was sealed with a non-alkali glass and a UV curable epoxy adhesive in a glove box, and the light emitting element of Example 1 was manufactured.

(Comparative Example 1)
In Example 1, as an electron transport layer, instead of fullerene, tris (8-quinolinolato) aluminum complex (Alq3) (thickness: 20 nm) was heated by resistance heating in vacuum (pressure: 1 × 10 ⁻⁴ Pa). A light emitting device of Comparative Example 1 was fabricated in the same manner as in Example 1 except that the film was formed by the above.

(Measurement of charge mobility)
The hole mobility of the hole transport layer and the electron mobility of the electron transport layer were measured by the Time Of Flight method (transient photocurrent measurement method, TOF method). This TOF method used a method of exciting a sample with an N ₂ pulse laser having a wavelength of 337 nm.

(Measurement of LUMO and HOMO)
The value of the work function measured using photoelectron spectrometer AC-1 (made by Riken Keiki) was applied to the value of the highest occupied orbit (HOMO). In the measurement, a layer formed of a material to be measured is formed as a single layer on a cleaned glass substrate with ITO (manufactured by Sanyo Vacuum Co., Ltd.), and photoelectrons are emitted from the photoelectron spectrometer AC-1. Determined by energy value. As measurement conditions, it was performed in increments of 0.05 eV with a light amount of 50 nW. On the other hand, as the lowest empty orbit (LUMO) value, the energy gap Eg was calculated from the value of the edge of the optical absorption spectrum, and the difference between Eg and the HOMO obtained as described above was determined as the LUMO value. The absorption spectrum is obtained by forming a layer formed of a material to be measured as a single layer on a cleaned quartz substrate, and calculating the difference in optical absorption between the substrate with a thin film and the reference quartz substrate by UV-3100PC (Hitachi). ).

  The energy diagram used in Example 1 is as shown in FIG. 3. The LUMO (lowest empty orbit) energy value of the hole transport layer made of TPD is 2.1 eV, and HOMO (highest occupied orbit). The energy value of was 5.4 eV. On the other hand, the energy value of LUMO (lowest empty orbit) of the light emitting layer made of quantum dots was 4.6 eV, and the energy value of HOMO (highest occupied orbit) was 6.8 eV.

(Measurement of film thickness)
Unless otherwise specified, the thickness of each layer described in the present invention is obtained by forming each layer as a single film on a cleaned glass substrate with ITO (manufactured by Sanyo Vacuum Co., Ltd.) and measuring the steps produced. Were determined. A probe microscope (manufactured by SII Nanotechnology Inc., Nanopics 1000) was used for film thickness measurement.

(Current efficiency and power efficiency of light-emitting elements)
The current efficiency and lifetime characteristics of the light-emitting elements of Example 1 and Comparative Example 1 were evaluated. Current efficiency and power efficiency were calculated by current-voltage-luminance (IV-L) measurement. The IVL measurement was performed by grounding the cathode, applying a positive DC voltage to the anode in 100 mV increments (1 sec./div.), And recording the current and luminance at each voltage. The luminance was measured using a luminance meter BM-8 manufactured by Topcon Corporation. Based on the obtained results, the light emission efficiency (cd / A) was calculated from the light emission area, current, and luminance. As a result, at 100 nit (when luminance = 100 Cd / m ² ), the light emission efficiency of the light emitting element of Example 1 was higher than that of the light emitting element of Comparative Example 1.

It is a schematic cross section which shows an example of the light emitting element of this invention. It is a schematic diagram for demonstrating the light emission principle of the light emitting element of this invention. It is an energy diagram which shows the ionization potential of the material which comprises each layer used in the Example.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Light emitting element 2 Base material 3 Anode 4 Cathode 5 Light emitting layer 6 Hole transport layer 7 Electron transport layer 11 Quantum dot

Claims (4)

A light emitting device having at least an anode, a hole transport layer, a light emitting layer composed of a monomolecular film of quantum dots, an electron transport layer, and a cathode in that order;
The hole transport layer has a hole mobility smaller than that of the electron transport layer, and the hole transport layer has an electron blocking function.   The light emitting device according to claim 1, wherein an energy value of LUMO (lowest empty orbit) of the hole transport layer is smaller than an energy value of LUMO (lowest empty orbit) of the light emitting layer.   The light emitting device according to claim 1, wherein the hole transport layer includes TPD as a hole transport material, and the electron transport layer includes fullerene as an electron transport material.   The said light emitting layer is obtained by phase-separating this quantum dot from the liquid mixture of the hole transportable material which comprises the said hole transport layer, and the said quantum dot. Light emitting element.

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