JP2009087756A - Light emitting element - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a light emitting element with a luminescent layer capable of effectively converting an exciton which is easily recombined by trapping charges (hole, electron) and is generated by its recombination to a luminous phenomenon, in the light emitting element having the luminescent layer containing the a quantum dot. <P>SOLUTION: This light emitting element 1 has at least a positive electrode 3, the luminescent layer 5 having the quantum dot 11, and a negative electrode 4 in this order, and the luminescent layer 5 contains a semiconductor particle 12 having a band gap larger than that of the quantum dot 11, and the particle diameter of the semiconductor particle 12 is larger than the particle diameter of the quantum dot 11. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a light emitting device, and more particularly to a light emitting device including a light emitting layer including 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 can be adjusted by the particle size as EL light-emitting materials have been proposed (for example, Non-Patent Document 1 and Patent Document 1). reference). In these documents, as a representative example of a quantum dot, a core-shell 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. ing. 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 quantum dot monomolecular film, the charges supplied from both electrodes are recombined. There is a problem that the exciton generated in this way reaches the monomolecular film and is consumed for EL emission, and sufficient luminance and light emission 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.

In order to solve the weak point of such a quantum dot monomolecular film, Patent Documents 2 and 3 below have a light emitting layer in which quantum dots are dispersed in a host material, and charge recombination in the light emitting layer. An example of a light emitting element that attempts to increase the probability is proposed. In this light emitting element, the generated excitons move in the light emitting layer to cause the quantum dots to emit EL. In Patent Document 4, a quantum dot serving as a light emission center and a filler composed of semiconductor fine particles having a particle diameter smaller than that of the quantum dot are mixed to form a light emitting layer region, and then a high temperature of 300 ° C. or higher. A light emitting device in which a light emitting layer is formed by melting and solidifying a fine filler has been proposed. This light-emitting layer utilizes the feature that a filler with a small particle size is easily melted, and the quantum dots are melted and solidified in the filler to improve conductivity and to improve the light emission efficiency based thereon. It is.
Seth Coe et.al., Nature, 420, 800-803 (2002) JP 2005-522005 gazette JP 2005-502176 Gazette JP-T-2007-513478 Special table 2007-520077 gazette

  However, in the conventional light emitting device, the hole mobility of the hole transport material for supplying holes to the light emitting layer is larger than the electron mobility of the electron transport material for supplying electrons to the light emitting layer. As a result, there is a problem that holes supplied from the anode penetrate the light emitting layer and there are few opportunities for recombination in the light emitting layer. Such a problem also applies to the light-emitting elements of Patent Documents 2 and 3 having a relatively thick light-emitting layer in which quantum dots are dispersed in a host material.

  In addition, the light-emitting layer formed by melting and solidifying the quantum dots in the filler is considered to have increased the chance of recombination with electrons by trapping holes to some extent. Since heat treatment and pressure treatment are added, for example, in a light-emitting element having an organic thin film or a light-emitting element using a plastic substrate, damage is easily caused by the heat treatment or pressure treatment, resulting in a decrease in luminous efficiency. There is a problem that it ends up.

  The present invention has been made to solve the above-described problems, and its object is to effectively trap and recombine charges (holes, electrons) in a light-emitting element having a light-emitting layer containing quantum dots. An object of the present invention is to provide a light emitting element including a light emitting layer that can easily convert excitons generated by the recombination into a light emitting phenomenon.

  The light-emitting element of the present invention for solving the above-described problem is a light-emitting element having at least an anode, a light-emitting layer having quantum dots, and a cathode in that order, and the light-emitting layer is more than the quantum dots. It includes semiconductor fine particles having a large band gap, and the particle size of the semiconductor fine particles is larger than the particle size of the quantum dots.

  According to the present invention, the light emitting layer has quantum dots that are emission centers and semiconductor fine particles having a larger particle diameter and a larger band gap than the quantum dots, so that holes and electrons injected from both electrodes are The excitons that are trapped and recombined by the semiconductor particles with a large particle size easily move to the low-energy quantum dots that exist around the semiconductor particles, and emit light from the quantum dots that are the emission centers. Let As a result, since the injected charge is efficiently used for the light emission phenomenon, a light emitting element capable of realizing good light emission efficiency is obtained.

  As a preferred embodiment of the light emitting device of the present invention, the semiconductor fine particles are configured such that the particle diameter thereof is 1.1 to 40 times the particle diameter of the quantum dots.

  According to this invention, since the particle size of the semiconductor fine particles is 1.1 to 40 times the particle size of the quantum dots, many quantum dots exist around the semiconductor fine particles. As a result, excitons generated in the semiconductor fine particles due to recombination can be efficiently distributed to surrounding quantum dots.

  As a preferred embodiment of the light emitting device of the present invention, the difference between the band gap of the semiconductor fine particles and the band gap of the quantum dots is 0.1 eV or more and 2 eV or less.

  According to this invention, excitons generated in the semiconductor fine particles can be easily moved to the quantum dots having low energy existing around the semiconductor fine particles.

  As a particularly preferred example of the light emitting device of the present invention, the semiconductor fine particles are zinc oxide, and the quantum dots have a CdSe / ZnS type core-shell structure.

  According to the light emitting device of the present invention, holes and electrons injected from both electrodes are trapped and recombined by semiconductor particles having a large particle diameter, and excitons generated by the recombination exist around the semiconductor particles. Since it moves easily to the quantum dot with low energy, the quantum dot which is a light emission center can be light-emitted easily. As a result, since the injected charge is efficiently used for the light emission phenomenon, a light emitting element capable of realizing good light emission efficiency is obtained.

  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. The light-emitting element 1 of the present invention has at least an anode 3, a light-emitting layer 5 having quantum dots 11, and a cathode 4 in that order. As an example, as shown in FIG. The light emitting element 1 which has the transport layer 6, the light emitting layer 5 which has the quantum dot 11, the electron injection transport layer 7, and the cathode 4 in that order can be illustrated. The light emitting layer 5 includes semiconductor fine particles 12 having a larger band gap than the quantum dots 11, and the particle size of the semiconductor fine particles 12 is configured to be larger than the particle size of the quantum dots 11.

  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. Particularly flexible plastic substrates are used as flexible substrates for white light source elements for illumination, RGB flexible panels combined with color filters, and LCD-OLED and LCD-LED backlight flexible substrates. It is preferably used as a material.

(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 to face the anode 3 with at least the light emitting layer 5 sandwiched between the anode 3 and the cathode 4.

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 in a mode sandwiched between the anode 3 and the cathode 4 and has quantum dots 11 serving as a light emission center and semiconductor fine particles 12 having a larger particle diameter and a larger band gap than the quantum dots 11. is doing.

  The light emission principle of the light emitting layer 5 constructed in this way is as follows. First, holes and electrons injected from the electrodes 3 and 4 are trapped and recombined by the semiconductor fine particles 12 having a large particle diameter, and are generated by the recombination. The excitons easily move to the low-energy quantum dots 11 present around the semiconductor fine particles 12, and the quantum dots 11 that are the emission centers emit light. In the light emitting element based on such a light emitting principle, an electron injecting and transporting layer in which the hole mobility of the hole injecting and transporting layer 6 usually provided between the anode 3 and the light emitting layer 5 is usually provided between the cathode 4 and the light emitting layer 5 is used. In the normal case where the electron mobility is higher than 7, the supply of electrons to the light emitting layer 5 is insufficient, and some of the holes penetrate the light emitting layer 5 to inject injected charges (holes, electrons). However, in the present invention, some of the holes that have hitherto penetrated the light-emitting layer 5 are trapped by the semiconductor fine particles 12 having a large particle diameter, and the trapped holes are Since the semiconductor fine particles 12 recombine with electrons, the injected charges can be efficiently recombined, and the generated excitons are distributed to a large number of quantum dots 11 existing around the semiconductor fine particles. So quantum Tsu bets luminous phenomenon can be made to efficiently carried out by, it is possible to realize an excellent emission efficiency.

  The quantum dots 11 used are semiconductor fine particles whose emission color can be adjusted 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 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 type 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. 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 by considering the relationship with the size of the semiconductor fine particles 12 depending on the material constituting the quantum dots so that light with 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 used herein, “particle size” refers to the average particle size.

  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. .

  On the other hand, the semiconductor fine particles 12 are particles having a larger particle size and a larger band gap than the quantum dots 11. The particle size of the semiconductor fine particles 12 is larger than the particle size (0.5 to 20 nm) of the quantum dots 11 and in the range of 1 nm to 100 nm, preferably in the range of 1 nm to 40 nm. When the particle size of the semiconductor fine particles 12 is the same as or smaller than the particle size of the quantum dots 11, many quantum dots 11 cannot be arranged around the semiconductor fine particles 11. If the particle size of the semiconductor fine particles 12 is too large, it may be difficult to move excitons generated in the semiconductor fine particles 12. The semiconductor fine particles 12 and the particle size are preferably selected so as to be 1.1 to 40 times the particle size of the quantum dots 11. If the semiconductor fine particles 12 and the quantum dots 11 within this range are selected, the quantum dots 11 can be arranged in a large number around the semiconductor fine particles 12, and as a result, many excitons generated in the semiconductor fine particles 12 The dots 11 can be distributed.

  The semiconductor fine particles 12 are particles having a larger band gap than the quantum dots 11. Such semiconductor fine particles 12 need to be selected in consideration of the band gap of the quantum dots 11 to be combined. The difference in the band gap is such that the difference between the band gap of the semiconductor fine particles 12 and the band gap of the quantum dots 11 is preferably 0.1 eV or more and 2 eV or less, and 0.3 eV or more and 2 eV or less. Is particularly preferred. Due to such a difference in band gap, excitons generated in the semiconductor fine particles 12 can be easily moved to the quantum dots 11 existing around the semiconductor fine particles 12, and EL emission at the quantum dots 11 is facilitated.

  Specific examples of the semiconductor fine particles 12 include, for example, MgS, MgSe, MgTe, CaS, CaSe, CaTe, SrS, SrSe, SrTe, BaS, BaSe, BaTe, ZnO, ZnS, ZnSe, ZnTe, CdS, CdSe, CdTe, HgS. II-VI group semiconductor compounds such as HgSe and HgTe, III- such as AlN, AlP, AlAs, AlSb, GaAs, GaP, GaN, GaSb, InN, InAs, InP, InSb, TiN, TiP, TiAs and TiSb In addition to a semiconductor crystal containing a group V semiconductor compound, a group IV semiconductor such as Si, Ge and Pb, a semiconductor compound containing three or more elements such as InGaP, and the like can be given. The band gap can be adjusted by doping the semiconductor fine particles 12 with a dopant as required. More specifically, ZnO, ZnS, ZnSe or the like can be preferably used.

  The light emitting layer 5 can be formed by various methods. As an example, a quantum dot 11 selected in consideration of the particle size and the band gap and the semiconductor fine particles 12 are mixed with a host material to prepare a mixed solution, and the mixed solution is placed on the hole injecting and transporting layer 6. The method of apply | coating and drying-curing can be illustrated. As the host material used at this time, a fluorescent material or a phosphorescent material which is used as a host material of a general light emitting layer can be used, and specifically, a dye material or a metal complex material can be mentioned. it can.

  Examples of the dye-based material include arylamine derivatives, anthracene derivatives, phenylanthracene derivatives, oxadiazole derivatives, oxazole derivatives, oligothiophene derivatives, carbazole derivatives, cyclopentadiene derivatives, silole derivatives, distyrylbenzene derivatives, distyrylpyrazine derivatives. , Distyrylarylene derivatives, silole derivatives, stilbene derivatives, spiro compounds, thiophene ring compounds, tetraphenylbutadiene derivatives, triazole derivatives, triphenylamine derivatives, trifumanylamine derivatives, pyrazoloquinoline derivatives, hydrazone derivatives, pyrazoline dimers, pyridine A ring compound, a fluorene derivative, a phenanthroline, a perinone derivative, a perylene derivative, and the like can be given. These dimers, trimers, oligomers, and compounds of two or more derivatives can also be used. Specifically, as the triphenylamine derivative, N, N′-bis- (3-methylphenyl) -N, N′-bis- (phenyl) -benzidine (abbreviated as TPD), 4, 4, 4 -Tris (3-methylphenylphenylamino) triphenylamine (abbreviated as MTDATA) and the like, and arylamines include bis (N- (1-naphthyl-N-phenyl) benzidine) (abbreviated as α-NPD). The oxadiazole derivatives include (2- (4-biphenylyl) -5- (4-tert-butylphenyl) -1,3,4-oxadiazole) (abbreviated as PBD) and the like. Examples of the anthracene derivative include 9,10-di-2-naphthylanthracene (abbreviated as DNA), and examples of the carbazole derivative include 4,4-N, N′-di. Carbazole - and biphenyl (abbreviated as CBP), (abbreviated as DPVBi) 1,4-bis (2,2-diphenyl vinyl) benzene. Examples of the phenanthrolines, bathocuproine and bathophenanthroline and the like. These materials may be used alone or in combination of two or more.

  Examples of the metal complex material include an aluminum quinolinol complex, a benzoquinolinol beryllium complex, a benzoxazole zinc complex, a benzothiazole zinc complex, an azomethylzinc complex, a porphyrin zinc complex, a europium complex, and mainly Al, Zn, Be, and the like. Or a metal complex having a rare earth metal such as Tb, Eu, or Dy, and having a oxadiazole, thiadiazole, phenylpyridine, phenylbenzimidazole, quinoline structure, or the like as a ligand. Specifically, tris (8-quinolinolato) aluminum complex (abbreviated as Alq3), bis (2-methyl-8-quinolinato) (p-phenylphenolate) aluminum complex (abbreviated as BAlq), tri (dibenzoylmethyl) Phenanthroline europium complex, bis (benzoquinolinolato) beryllium complex (abbreviated as BeBq), and the like can be given. These materials may be used alone or in combination of two or more.

  In addition, a medium molecular or high molecular material obtained by introducing a low molecular weight host material such as the above-described dye-based material or metal complex-based material into the molecule as a straight chain, a side chain, or a functional group can be used. Specific examples include polyparaphenylene vinylene derivatives, polythiophene derivatives, polyparaphenylene derivatives, polysilane derivatives, polyacetylene derivatives, polyvinylcarbazole, polyfluorenone derivatives, polyfluorene derivatives, polyquinoxaline derivatives, and copolymers thereof. Can do.

  In addition, you may form the light emitting layer 5 also by methods other than the above. The thickness of the light emitting layer 5 thus formed is, for example, 1 nm or more and 100 nm or less. In particular, the thickness is preferably close to the particle size of the semiconductor fine particles 12. As shown in FIG. 2, the semiconductor fine particles 12 are uniformly dispersed in the lateral direction in the obtained light emitting layer 5, and the quantum dots 11 are evenly dispersed around the semiconductor fine particles 12.

(Hole injection transport layer)
The hole injecting and transporting layer 6 is provided as necessary depending on the use of the light emitting element 1, and is usually provided on the anode 3. This “hole injection transport layer 6” may be provided as a single layer (see FIGS. 1 and 2) having both hole injection properties and hole transport properties, or a hole injection layer and a hole transport layer. It may consist of a laminated form. In any case, in the present invention, the hole injecting and transporting layer 6 acts to transport holes supplied from the anode 3 to the light emitting layer 5 side.

  As a material for forming the hole injection transport layer 6, a hole transport material can be used. For example, arylamine derivatives, anthracene derivatives, carbazole derivatives, thiophene derivatives, fluorene derivatives, distyrylbenzene derivatives, spiro compounds, and the like can be given. 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. Further, for example, poly (3,4) ethylenedioxythiophene / polystyrene sulfonate (abbreviation PEDOT / PSS, manufactured by Bayer, trade name: Baytron P CH8000, as an aqueous solution. (Commercially available) etc. These materials may be used alone or in combination of two or more.

  The hole injecting and transporting layer 6 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 50 nm.

(Electron injection transport layer)
The electron injecting and transporting layer 7 is provided as necessary depending on the use of the light emitting element 1, and is provided between the light emitting layer 5 and the cathode 4. This “electron injection transport layer 7” may be provided as a single layer (see FIGS. 1 and 2) having both electron injection properties and electron transport properties, or is formed of a laminated form of an electron injection layer and an electron transport layer. It may be. In any case, the electron injecting and transporting layer 7 functions to transport electrons supplied from the cathode 4 to the light emitting layer 5 side.

As a material for forming the electron injecting and transporting layer 7, an electron transporting material can be used. For example, a metal complex, an oxadiazole derivative, a triazole derivative, a phenanthroline derivative, a silyl compound, fullerene, and the like can be given. 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 injecting and transporting layer 7 is formed by a vacuum deposition method or a coating method using an electron injecting and transporting layer forming coating solution containing the above materials.

  Such an electron injecting and transporting layer 7 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 100 nm.

(Other layers)
An electron injection layer (not shown) is provided between the cathode 4 and the electron injection / transport layer 7 as needed to further enhance electron injection properties so that electrons can be easily injected from the cathode 4. Act on. 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 injection / transport layer 6 as needed to further increase the hole injection property. Hole) acts so as to be easily injected. 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.

  A passivation layer (not shown) is also provided as necessary, and is a layer provided so as to cover the entire element so that the formed light emitting layer 5, electron injection transport layer 7 and the like are not deteriorated by water vapor or oxygen. It is. Examples of the material for forming such a passivation layer include silicon oxide, silicon nitride, and silicon oxynitride. The thickness differs depending on the forming material, but is formed to a thickness that does not deteriorate with water vapor or oxygen.

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.

  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 injecting and transporting layer on a glass substrate on which an ITO film having a thickness of 150 nm is formed as an anode. The film was formed by applying the PEDOT-PSS solution in the air by spin coating. After the PEDOT-PSS film formation, the film was dried using a hot plate in the air in order to evaporate the water.

Next, a mixture consisting of quantum dots emitting red light (particle size: 5.6 nm, manufactured by Evident Technology, band gap: 2 eV) and semiconductor fine particles composed of ZnO (particle size: 10 nm, band gap: 3.4 eV) The solution was prepared, and the mixed solution was applied onto the hole injecting and transporting layer to form a light emitting layer having a thickness of 40 nm. Next, on the light emitting layer, tris (8-quinolinolato) aluminum complex (Alq3) (thickness: 20 nm) is deposited by resistance heating in vacuum (pressure: 1 × 10 ⁻⁴ Pa) as an electron injecting and transporting layer. Then, LiF (thickness: 0.5 nm) is further formed thereon as an electron injection layer, and Al (thickness: 100 nm) is further formed thereon in a vacuum (pressure: 1 × 10 ⁻⁴ Pa) was formed by resistance heating vapor deposition.

  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)
A light emitting device of Comparative Example 1 was fabricated in the same manner as in Example 1, except that a light emitting layer having a thickness of 40 nm was formed using only quantum dots emitting red light in the light emitting layer.

(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.

(Band gap)
For the band gap between the quantum dots and the semiconductor fine particles, the band gap Eg was calculated from the value of the optical absorption spectrum. 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). ).

(Particle size)
The particle diameters of the quantum dots and the semiconductor fine particles were evaluated by a transmission electron microscope (TEM) and X-ray crystal diffraction (XRD), and represented by an average particle diameter.

(Current efficiency and power efficiency of organic EL 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, the light emission efficiency of the light emitting element of Example 1 was higher than the light emission efficiency 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.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Light emitting element 2 Base material 3 Anode 4 Cathode 5 Light emitting layer 6 Hole injection transport layer 7 Electron injection transport layer 11 Quantum dot 12 Semiconductor fine particle

Claims (4)

A light emitting device having at least an anode, a light emitting layer having quantum dots, and a cathode in that order;
The light emitting layer includes semiconductor fine particles having a larger band gap than the quantum dots, and the particle size of the semiconductor fine particles is larger than the particle size of the quantum dots.   The light emitting element of Claim 1 whose particle size of the said semiconductor fine particle is 1.1 to 40 times the particle size of the said quantum dot.   The light emitting element of Claim 1 or 2 whose difference of the band gap of the said semiconductor fine particle and the band gap of the said quantum dot is 0.1 eV or more and 2 eV or less.   4. The light emitting device according to claim 1, wherein the semiconductor fine particles are zinc oxide, and the quantum dots have a CdSe / ZnS type core-shell structure.

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