JP2008300270A - Light emitting element - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an organic-inorganic hybrid light emitting element which has a high luminous efficiency and a long life. <P>SOLUTION: A light emitting elements 11 has a positive electrode 2, and a negative electrode 9, wherein at least a hole transport layer 3, a recombination layer 4, a light emitting layer 6 and an electron transport layer 7 are included in this order between the positive electrode 2 and the negative electrode 9. The light emitting layer 6 is comprised of semiconductor nanoparticles 5, and it emits light by the energy transfer of exciton energy generated at the recombination layer 4 to the semiconductor nanoparticles 5. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a light emitting element.

  Currently, research and development of light-emitting elements, both organic (organic light-emitting elements) and inorganic (inorganic light-emitting elements), have been actively conducted. Here, in organic light-emitting elements, organic-inorganic hybrid light-emitting elements using semiconductor fine particles made of an inorganic compound such as CdSe instead of an organic compound as a material constituting the light-emitting layer have been proposed (Patent Documents 1 to 3). reference). At the time of the proposal, a light-emitting element using a light-emitting layer made by dispersing semiconductor fine particles in a conductive polymer such as polyparaphenylene vinylene (PPV: poly (p-phenylenevinylene)) was proposed. Thereafter, a light emitting element shown in FIG. 7 has been proposed. In the light emitting device 100 of FIG. 7, an anode 2, a hole transport layer 3, a light emitting layer 6, an electron transport layer 7, an electron injection layer 8, and a cathode 9 are laminated on a substrate 1 in this order. The light emitting element of FIG. 7 is characterized in that the light emitting layer 6 is composed of a thin film of semiconductor fine particles 5. Thus, by forming the semiconductor fine particles having low charge conductivity in the form of a thin film, the carrier transportability is improved and the light emission efficiency is greatly improved.

  Here, the emission spectrum of light emitted from the semiconductor fine particles is known to be very sharp with a half-value width of 30 nm or less, and it is considered that a light-emitting element with high color purity can be realized. Therefore, since chromaticity adjustment using a cavity structure or the like is not necessary, it is expected to realize an element having excellent viewing angle characteristics and high production robustness. Further, since the semiconductor fine particles can increase the emission quantum yield up to 90%, the emission efficiency can be expected to be improved. Furthermore, since the semiconductor fine particles made of an inorganic compound are excellent in thermal stability, when used as a constituent material of the light emitting layer, the lifetime of the light emitting element can be expected to be extended.

US Patent No. 005537000 US Patent Application Publication No. 2004/0023010 JP 2004-303592 A

  As described above, although the organic-inorganic hybrid light-emitting element has high potential, the light-emitting efficiency is lower than that of the conventional organic light-emitting element and the durability characteristics are not good. One of the causes is insufficient energy confinement in the semiconductor fine particles constituting the light emitting layer. In an organic / inorganic hybrid light emitting device, it is ideal that all injected carriers are recombined in a light emitting layer composed of semiconductor fine particles and used for light emission energy. However, in the conventional configuration including the light emitting element of FIG. 7, there is a problem that carrier leakage occurs and the hole transport layer and the electron transport layer emit light or heat deactivates.

  An object of the present invention is to provide an organic-inorganic hybrid light-emitting device having high luminous efficiency and a long lifetime.

  In order to achieve the above object, a light emitting device of the present invention comprises an anode, a cathode, and at least a hole transport layer, a recombination layer, a light emitting layer, an electron transport layer between the anode and the cathode. Are included in this order, the light emitting layer is composed of semiconductor fine particles, and excitonic energy generated in the recombination layer is transferred to the semiconductor fine particles to emit light.

  ADVANTAGE OF THE INVENTION According to this invention, the organic inorganic hybrid light emitting element which is a long lifetime with high luminous efficiency can be provided.

  Hereinafter, the light emitting device of the present invention will be described in detail with reference to the drawings.

  FIG. 1 is a cross-sectional view showing a first embodiment of the light emitting device of the present invention. A light emitting element 11 in FIG. 1 includes a light emitting layer 6 composed of an anode 2, a hole transport layer 3, a recombination layer 4, and semiconductor fine particles 5, an electron transport layer 7, an electron injection layer 8, and a cathode 9 on a substrate 1. Are provided sequentially. As shown in FIG. 1, the light-emitting element of the present invention is composed of layers composed of organic materials (hole transport layer 3, recombination layer 4, electron transport layer 7, electron injection layer 8) and inorganic materials. It is an organic-inorganic hybrid light-emitting element in which the layers (light-emitting layer 6) that are configured are combined. When the light emitting element 11 is energized, the holes injected from the anode 2 and the electrons injected from the cathode 9 are recombined in the recombination layer 4. Excitation energy is generated by this recombination, and the excitation energy is transferred to the semiconductor fine particles 5 constituting the light emitting layer 6, whereby the light emitting element 11 emits light.

  FIG. 2 is a cross-sectional view showing a second embodiment of the light emitting device of the present invention. The light emitting element 12 in FIG. 2 has the same layer structure as the light emitting element 11 in FIG. However, the light emitting layer 6 includes at least two kinds of semiconductor fine particles 5 having different particle diameters. Here, the peak wavelength of light emitted from the semiconductor fine particles 5 depends on the particle size. Taking CdSe / ZnS semiconductor fine particles as an example, the relationship between the particle diameter and the peak wavelength is as shown in Table 1.

  By utilizing this relationship, the color of light emitted from the light emitting element can be appropriately controlled by controlling the particle size of the semiconductor particles or by combining semiconductor particles having different particle sizes. A light emitting element that emits white light as a whole can also be realized by combining two or more kinds of semiconductor fine particles having different particle diameters and superimposing a plurality of emission spectra.

  In addition, when making the light emitting element of this invention a white light emitting element, it is not limited to the form shown in FIG. 2 as embodiment, You may form an intervening layer as needed. FIG. 3 is a cross-sectional view showing a third embodiment of the light emitting device of the present invention. Like the light emitting element 13 in FIG. 3, the light emitting element of the present invention may be provided with the electron blocking layer 10 between the hole transport layer 3 and the recombination layer 4. 2 and 3, the light emitting layer 5 is formed by dispersing a plurality of types of semiconductor fine particles having different particle diameters. However, the present invention is not limited to this, and a configuration in which semiconductor fine particles having different particle diameters are stacked. It may be.

  The first to third embodiments described above are configuration examples of a light emitting element in which an anode is formed on a substrate, but the present invention is not limited to this embodiment. The cathode 9, the electron injection layer 8, the electron transport layer 7, the light emitting layer 6, the recombination layer 4, the hole transport layer 3, and the anode 2 may be formed in this order from the substrate side. The light emitting device of the present invention may be a bottom emission type light emitting device that extracts light emitted from the light emitting layer 6 from the substrate side, or a top emission type light emitting device that extracts from the upper electrode on the side opposite to the substrate. May be.

  The recombination layer included in the light emitting device of the present invention is a semiconductor fine particle in which holes injected from the anode and electrons injected from the cathode are recombined, and excitation energy generated by the recombination is included in the light emitting layer. To play a role in transferring energy to Since the recombination layer bears recombination of electrons and holes, light emission and deactivation in the hole transport layer and the electron transport layer can be suppressed.

  In general, the hole transport layer is weak in generating radical anions, and the electron transport layer is weak in generating radical cations. In addition, since the compounds constituting the hole transport layer and the electron transport layer are generally not capable of withstanding repeated excited states, the hole transport layer and the electron transport layer regenerate electrons and holes. It is not suitable as a layer for bonding.

  On the other hand, since the recombination layer included in the light-emitting element of the present invention contains a fluorescent organic compound or a phosphorescent organic compound that is resistant to repeated excitation states, the lifetime of the light-emitting element can be improved. Here, the recombination layer is preferably composed of an organic compound as a host and a fluorescent or phosphorescent organic compound as a guest. When the recombination layer is composed of a host and a guest, the guest plays the role of a carrier trap, so that the recombination probability is improved and the crystallization of the host is suppressed. As a material for the recombination layer, a cyclic organic compound such as a condensed ring aromatic compound and a derivative thereof, a condensed ring heterocyclic compound and a derivative thereof, which are a fluorescent material or a phosphorescent material, can be used.

The light emitting element of the present invention emits light by excitonic energy generated in the recombination layer being transferred to the semiconductor fine particles constituting the light emitting layer. As the energy transfer mechanism from the recombination layer to the semiconductor fine particles, Forster energy transfer (dipole-dipole interaction) and Dexter energy transfer (electron exchange interaction) can be considered. Since the ease of energy transfer is determined by a rate constant, it is necessary to increase the rate constant in order to cause energy transfer efficiently. In order to obtain a large rate constant, it is necessary to reduce the distance between the position where electrons and holes are recombined and the semiconductor fine particles. Specifically, how close this distance should be is 6 nm or less when considering the Forster energy transfer, and 2 nm or less when considering the Dexter energy transfer. Considering from such an energy transfer mechanism, the recombination layer included in the light emitting device of the present invention preferably has the relationship of the following formula (1).
μH> μE (1)
(In the formula, μH represents the mobility of holes, and μE represents the mobility of electrons.)

  If this relationship is established, holes are rapidly transported in the recombination layer and blocked by semiconductor fine particles having a deepest maximum occupied orbital (HOMO) energy. On the other hand, since electrons travel slowly in the recombination layer, the probability that electrons and holes recombine in the vicinity of the light emitting layer in the recombination layer is increased. That is, in the light emitting element of the present invention, the position where electrons and holes are recombined can be located in the vicinity of the semiconductor fine particles, so that energy transfer to the semiconductor fine particles can be performed efficiently.

Moreover, in order to implement | achieve the relationship of the said Formula (1), Preferably it has the relationship of following formula (2) between the host and guest which comprise a recombination layer.
ELG-ELH ≧ 0.15 eV (2)
(In the formula, ELH represents the absolute value of the lowest free orbital (LUMO) energy of the host material, and ELG represents the absolute value of the lowest free orbital (LUMO) energy of the guest material.)

  By having this relationship, the guest functions as a trap for electrons in the presence of the host. That is, the electron mobility μE of the recombination layer can be reduced. Since the guest functions as an electron trap, the guest becomes a carrier recombination center compound, and the probability of recombination of electrons and holes is improved. In the above formula (2), if the left side is less than 0.15 eV, the guest cannot sufficiently trap electrons.

In addition, the relationship between the host and the guest constituting the recombination layer preferably has the relationship of the following formula (3).
EHG-EHH> 0 eV (3)
(In the formula, EHH represents the absolute value of the host's highest occupied orbital (HOMO) energy, and EHG represents the absolute value of the guest's highest occupied orbital (HOMO) energy.)

  By having this relationship, the guest does not trap holes in the presence of the host. That is, in the recombination layer, the relationship of the above formula (1) can be realized without impairing the hole mobility μH of the recombination layer.

  By the way, the semiconductor fine particles have remarkable characteristics different from the bulk in the absorption spectrum. Specifically, the semiconductor fine particles have a wide absorption spectrum having strong absorption toward the short wavelength side. In the light emitting device of the present invention, preferably, the emission spectrum of the recombination layer overlaps with the absorption spectrum of the semiconductor fine particles. By overlapping both spectra, the rate constant of Forster energy transfer and Dexter energy transfer can be increased. That is, energy transfer from the recombination layer to the semiconductor fine particles can be performed efficiently. Therefore, the rate constant can be maximized if the emission spectrum of the recombination layer completely overlaps the absorption spectrum of the semiconductor fine particles. However, the speed constant can be increased by the overlapping area even if they do not overlap completely. By increasing the rate constant, energy transfer from the recombination layer to the semiconductor fine particles can be performed more efficiently.

  The semiconductor fine particles constituting the light emitting layer have an average particle size of 0.1 nm to 50 nm. Preferably, the average particle size is 1 nm to 20 nm. When the particle size of the semiconductor fine particles is reduced to about the Bohr radius, electro-optical characteristics that cannot be realized with a bulk semiconductor can be extracted. In the semiconductor fine particles reduced to about the Bohr radius, the three-dimensional strong quantum confinement of the electron cloud occurs, and the energy levels that existed continuously in the bulk are discretized. As a result, a very sharp emission spectrum (Gaussian type) is emitted from the semiconductor fine particles. The limit of the half-value width is determined by heat dispersion and particle size distribution. For example, CdSe (cadmium selenium) has a sharp emission spectrum with a particle size distribution of 5% and a half-value width of 30 nm or less. Therefore, in the light emitting element of the present invention, a high color purity light emitting element can be realized without the need for chromaticity adjustment by a cavity structure or the like.

  Furthermore, since the discretized band gap is inversely proportional to the particle size of the semiconductor fine particles, the peak wavelength of the emission spectrum can be controlled by the particle size of the semiconductor fine particles. For this reason, a red light emitting element, a green light emitting element, a blue light emitting element, and a white light emitting element can be produced by controlling the particle diameter of the semiconductor fine particles constituting the light emitting layer.

  The material of the semiconductor fine particle is not particularly limited, but carbon, silicon, silicon carbide (SiC), II-VI group compound semiconductor (ZnO, ZnS, ZnSe, CdO, CdS, CdSe, etc.), III-V group compound Examples thereof include semiconductors (GaN, GaP, GaAs, InN, InP, InAs, etc.).

  The semiconductor fine particles preferably have a core / shell structure. In semiconductor fine particles, it is known that energy loss that does not contribute to light emission mainly occurs at the interface between the fine particle surface and the other fine particle surface. Here, by covering the core material of the semiconductor fine particles with a shell material having a wider band gap than the core material, the wave function is localized inside the semiconductor fine particles, and depending on the surface level, lattice defect level, etc. The energy loss which arises can be suppressed. Examples of the semiconductor fine particles having a core / shell structure include cadmium selenium / zinc sulfide (CdSe / ZnS) and cadmium selenium / cadmium sulfide (CdSe / CdS).

  If necessary, the surface of the semiconductor fine particles can be modified with an organic compound ligand. By performing the surface modification, functions such as solubility in a solvent, dispersibility in the light emitting layer, and carrier injection property can be imparted. For example, in order to improve the solubility in toluene or chloroform, the surface of the semiconductor fine particles (CdSe / ZnS or the like) can be surface-modified with a specific ligand. Specific examples of the ligand include TOPO (trioctylphosphine oxide) and TOP (trioctylphosphine).

  The light emitting layer included in the light emitting device of the present invention is preferably constituted by closest packing of semiconductor fine particles with a single layer or multiple layers. Since the region where the semiconductor fine particles are not present does not contribute to light emission from the semiconductor fine particles, the luminous efficiency of the device can be improved by reducing the region where the semiconductor fine particles are filled as close as possible in the surface and the semiconductor fine particles are not present. In addition, the material formed in the gap between the semiconductor fine particles is not particularly limited, but considering the energy transfer from the recombination layer to the semiconductor fine particles, the material of the recombination layer is formed in the gap between the semiconductor fine particles. It is preferable that it is filled.

  Next, other members constituting the light emitting device of the present invention will be described.

  The anode and the cathode may be any of a transparent electrode, a reflective electrode, and a translucent electrode. The material constituting the anode and the cathode is not particularly limited, and examples thereof include oxide conductive films such as ITO and IZO, simple metals such as gold, platinum, silver, aluminum, and magnesium, and alloys that combine these. It is done. Further, the anode and the cathode may be composed of a single layer, or may be composed of a plurality of layers like the light emitting element shown in FIG.

  The hole transport layer is a layer that plays a role of taking holes from the anode and transporting the holes to the recombination layer and the light emitting layer. Examples of the low-molecular and high-molecular materials having hole injection / transport performance that constitute the hole transport layer include triphenyldiamine derivatives, oxadiazole derivatives, polyphyllyl derivatives, stilbene derivatives, poly (vinylcarbazole), poly (thiophene), In addition, although a conductive polymer is mentioned, it is not limited to these. Moreover, you may provide the positive hole injection layer comprised with a copper phthalocyanine, vanadium oxide, etc. between an anode and a positive hole transport layer as needed. Further, if necessary, an electron blocking layer having a small absolute value of the lowest unoccupied orbital (LUMO) energy may be formed between the hole transport layer and the recombination layer.

  The electron transport layer is a layer that plays a role of taking electrons from the cathode and transporting the electrons to the light emitting layer and the recombination layer. Examples of the electron transporting material constituting the electron transport layer include, but are not limited to, phenanthroline derivatives, diazafluorene derivatives, quinoline derivatives, oxadiazole derivatives, triazole derivatives, organometallic complexes, and the like. Moreover, you may form a positive hole block layer with a large absolute value of highest occupied orbital (HOMO) energy between a light emitting layer and an electron carrying layer as needed.

  As a constituent material of the electron injection layer, an alkali metal, an alkaline earth metal or a compound thereof, or a mixture obtained by doping the above-described electron transport material with an alkali metal, an alkaline earth metal or a compound thereof can be used. Examples thereof include lithium fluoride (LiF), potassium fluoride (KF), and a mixture in which an electron transport material is doped with several wt% to several tens wt% of cesium carbonate.

  As a method for forming a light emitting layer containing a plurality of types of semiconductor fine particles, a wet process such as a spin coating method, a spray method, an ink jet method, or a printing method can be used. For example, it is known that when a chloroform solution of a hole transport material and semiconductor fine particles is spin-coated, the semiconductor fine particles are deposited on the surface of the film made of the hole transport material, and a layer made of the semiconductor fine particles can be formed in a self-integrating manner. . Examples of the hole transporting material here include TPD (4,4'-bis [N- (3-methylphenyl) -N-phenylamino] -biphenyl.

  Next, the light emitting element array of the present invention will be described. The light emitting element array of the present invention is characterized in that the light emitting elements of the present invention exhibiting at least two different luminescent colors are arranged. The light-emitting element array of the present invention will be described below with reference to the drawings.

  FIG. 4 is a cross-sectional view showing an embodiment of the light-emitting element array of the present invention. The light emitting element array 30 shown in FIG. 4 is configured by arranging red light emitting elements, green light emitting elements, and blue light emitting elements in parallel.

  In the light emitting element array 30 of FIG. 4, as a red light emitting element, on a substrate 1, an anode 2, a hole transport layer 3, a recombination layer 4, a red light emitting layer 6R, an electron transport layer 7, an electron injection layer 8, and a cathode 9 are provided. Are provided sequentially.

  In the light emitting element array 30 of FIG. 4, as a green light emitting element, on a substrate 1, an anode 2, a hole transport layer 3, a recombination layer 4, a green light emitting layer 6G, an electron transport layer 7, an electron injection layer 8, and a cathode 9 are provided. Are provided sequentially.

  In the light emitting element array 30 of FIG. 4, as a blue light emitting element, on a substrate 1, an anode 2, a hole transport layer 3, a recombination layer 4, a blue light emitting layer 6B, an electron transport layer 7, an electron injection layer 8, and a cathode 9 are provided. Are provided sequentially.

  The light-emitting element array of the present invention is not limited to the embodiment shown in FIG. 4, and an electron blocking layer or a hole blocking layer may be formed as necessary. It is also possible to configure such that the light emitting element is applied separately. Further, in the light emitting element array of FIG. 4, the light emitting layer is formed of fine particles of a single size as a single layer, but the present invention is not limited to this, and the light emitting layer is formed of a plurality of layers. Alternatively, two or more kinds of semiconductor fine particles having different particle diameters may be dispersed in one layer.

  As described above, the emission spectrum of the semiconductor fine particles constituting each light emitting layer 6R, 6G, 6B can be controlled by the particle diameter. For this reason, the semiconductor fine particles corresponding to the desired color can be appropriately selected depending on the particle diameter. For example, a semiconductor fine particle (peak wavelength: 625 nm) having a particle size of 5.3 nm is used for a red light emitting element. For the green light emitting element, semiconductor fine particles (peak wavelength 545 nm) having a particle size of 2.5 nm are used. For the blue light emitting element, semiconductor fine particles having a particle diameter of 1.8 nm (peak wavelength: 480 nm) are used.

  As shown in FIG. 4, the light emitting element array of the present invention is provided with a recombination layer 4 common to all the light emitting elements. On the other hand, as described above, the semiconductor fine particles contained in the light emitting layers 6R, 6G, and 6B in each light emitting element have a feature that they have a wide absorption spectrum on the short wavelength side. Therefore, it is preferable to select a material having an emission spectrum overlapping with any of the absorption spectra of the semiconductor fine particles contained in each of the light emitting layers 6R, 6G, and 6B as the material of the recombination layer 4. By doing so, the excitation energy generated in the recombination layer is efficiently transferred to the semiconductor fine particles. However, even if the emission spectrum of the recombination layer and the absorption spectrum of the semiconductor fine particles do not completely overlap, the rate constant can be increased by the overlapping area. By increasing the rate constant, energy transfer from the recombination layer to the semiconductor fine particles can be performed more efficiently.

  In addition, a printing method or the like can be used for the separate process of the red light emitting layer 6R, the green light emitting layer 6G, and the blue light emitting layer 6B. For example, by applying semiconductor fine particles of each particle size on a mold having a recess, and pressing this mold on the recombination layer at a position corresponding to each light emitting layer 6R, 6G, 6B, A film of semiconductor particulates can be transferred onto the recombination layer.

  Hereinafter, the present invention will be described in more detail with reference to examples, but the present invention is not limited thereto.

<Example 1>
The bottom emission type light emitting element shown in FIG. 1 was manufactured by the method shown below.

  On a glass substrate (substrate 1), indium tin oxide (ITO) was formed as an anode 2 with a film thickness of 120 nm by a sputtering method. This was ultrasonically washed successively with acetone and isopropyl alcohol (IPA), boiled and washed with IPA, and then dried. Further, the surface of this substrate was washed with UV / ozone.

  Next, N, N′-α-dinaphthylbenzidine (α-NPD) represented by the following formula [1] is formed on the substrate processed by the above method by a thermal evaporation method, A hole transport layer 3 having a thickness of 40 nm was formed.

  Next, a compound represented by the following formula [2] as a host and a compound represented by the following formula [3] as a guest were co-deposited so that the weight concentration ratio was 90:10, and the film thickness was 15 nm. The recombination layer 4 was formed.

  Next, the light emitting layer 5 composed of CdSe / ZnS semiconductor fine particles having a particle diameter of 5.3 nm was formed by a printing method. Specifically, in a glove box in a nitrogen atmosphere, by applying semiconductor fine particles on a mold provided with a recess at a position corresponding to the light emitting portion, and pressing this mold on the recombination layer 4, A film of semiconductor fine particles was transferred onto the recombination layer 4. Here, when the formed film was separately observed with an atomic force microscope (AFM), it was observed that a thin film of semiconductor fine particles having an average film thickness of 10 nm was formed in a single layer or a double layer.

Next, a film of tris (8-quinolinolato) aluminum (Alq ₃ ) represented by the following formula [4] was formed by a heating vapor deposition method to form an electron transport layer 3 having a thickness of 30 nm.

  Next, lithium fluoride (LiF) was formed into a film by a heating vapor deposition method to form an electron injection layer 8 having a thickness of 0.5 nm.

  Next, aluminum was formed into a film by a heating vapor deposition method to form a reflective cathode (cathode 9) having a thickness of 150 nm. After forming up to the cathode 9, the device was sealed with a glass cap containing a desiccant in a glove box in a nitrogen atmosphere. The light emitting element was obtained by the above method.

  When the obtained light-emitting element was energized, an emission spectrum (peak wavelength: 625 nm) from the CdSe / ZnS semiconductor fine particles was observed, and it was confirmed that the element had a good display efficiency and stable display characteristics.

  Further, the highest occupied orbital (HOMO) energy and the lowest unoccupied orbital (LUMO) energy of the host and guest included in the recombination layer 4 were evaluated by the following method.

[HOMO energy]
A thin film was prepared for each of the host [2] and the guest [3] constituting the recombination layer 4 on a glass substrate, and this thin film was measured with an atmospheric photoelectron spectrometer (device name: AC-1) to thereby produce a HOMO. Energy was evaluated.

[LUMO energy]
From the ultraviolet-visible light absorption spectrum measurement (device name: U-3010), the LUMO energy was calculated together with the energy gap.

  As a result, the absolute value (EHH) of the HOMO energy of the host was 5.61 eV, and the absolute value (ELH) of the LUMO energy was 2.64 eV. On the other hand, the absolute value (EHG) of the HOMO energy of the guest was 5.85 eV, and the absolute value (ELG) of the LUMO energy was 3.06 eV. From this result, it was found that the lowest unoccupied orbit (LUMO) related to electron transport has a relationship of ELG-ELH ≧ 0.15 eV, and the guest molecule functions as an electron trap in the host. It was also found that the guest molecule does not function as a hole trap in the host because the highest occupied orbital (HOMO) related to hole transport has a relationship of EHG-EHH> 0.

Furthermore, in order to measure the carrier mobility of the recombination layer, a mobility measuring element was fabricated and evaluated. Here, the mobility measuring element was specifically manufactured by the following steps.
(1) First, a host [2] and a guest [3] were co-deposited on a glass substrate with ITO so as to have a concentration weight ratio of 90:10 to form a thin film having a thickness of 2 μm.
(2) Subsequently, an aluminum film having a thickness of 150 nm was formed by a heating vapor deposition method.

Carrier mobility was measured for this mobility measuring element using time of flight (TOF-301, manufactured by Toptel Co., Ltd.). As a result, at an electric field strength of 4 × 10 ⁵ V / cm, the electron mobility (μE) is 2 × 10 ⁻⁶ Vcm ² / Vs, and the hole mobility (μH) is 1 × 10 ⁻⁴ Vcm ² / V. Vs. Therefore, it was found that the carrier mobility in the recombination layer is higher than the electron mobility (that is, a relationship of μH> μE).

  Further, in order to measure the emission spectrum of the recombination layer, a host [2] and a guest [3] were co-evaporated on the glass substrate so as to have a concentration weight ratio of 90:10 to form a thin film. The emission spectrum of this thin film was measured. On the other hand, in order to measure the absorption spectrum of the semiconductor fine particles, a thin film of CdSe / ZnS semiconductor fine particles having a particle diameter of 5.3 nm was prepared by a printing method. The absorption spectrum of this thin film was measured. FIG. 5 is a diagram showing an emission spectrum of a thin film corresponding to the recombination layer and an absorption spectrum of the thin film corresponding to the light emitting layer in this example. In FIG. 5, LR represents an emission spectrum of a thin film corresponding to the recombination layer, and AR represents an absorption spectrum of the thin film corresponding to the light emitting layer. FIG. 5 shows that the LR of the recombination layer has a peak in the vicinity of 460 nm, and the AR of the CdSe / ZnS semiconductor fine particles having a particle size of 5.3 nm constituting the light emitting layer has a wide absorption spectrum on the short wavelength side. It was. FIG. 5 shows that LA overlaps AR.

<Comparative Example 1>
In Example 1, a light emitting device was manufactured by the same method as Example 1 except that the step of forming the recombination layer 4 was omitted. That is, as shown in FIG. 7, the light emitting layer composed of semiconductor fine particles is sandwiched between the hole transport layer and the electron transport layer.

  When the obtained light emitting element was energized, a spectrum derived from the hole transport layer or the electron transport layer was observed in addition to the emission spectrum (peak wavelength: 625 nm) from the CdSe / ZnS semiconductor fine particles having a particle size of 5.3 nm. Further, it was confirmed that the light emission intensity was greatly deteriorated in the light emission test for a long time as compared with the light emitting device of Example 1.

<Example 2>
The top emission type white light emitting element shown in FIG. 6 was manufactured by the method shown below.

  A silver alloy (AgAuSn) was formed on a glass substrate (substrate 1) by a sputtering method to form a reflective anode 21 having a thickness of 100 nm. Next, IZO was deposited by a sputtering method to form a transparent anode 22 having a thickness of 20 nm. Next, after ultrasonic cleaning sequentially with acetone and isopropyl alcohol (IPA), boiling cleaning with IPA was performed, followed by drying. Further, the surface of this substrate was washed with UV / ozone.

  Next, a hole transporting material represented by the following formula [5] was formed on the substrate treated by the above method by a heating vapor deposition method to form a hole transporting layer 3 having a thickness of 60 nm.

  Next, a hole transport (electron block) material represented by the following formula [6] was formed on the hole transport layer 3 by a heating vapor deposition method to form an electron block layer 10 having a thickness of 10 nm.

  Next, the compound represented by the following formula [2] as the host and the compound represented by the following formula [3] as the guest were co-evaporated so that the weight concentration ratio was 90:10, and the film thickness was 10 nm. A recombination layer 4 was formed.

  Next, a light emitting layer in which three types of CdSe / ZnS semiconductor fine particles having particle sizes of 5.3 nm, 2.5 nm, and 1.8 nm were dispersed was formed by a printing method. Specifically, in a glove box in a nitrogen atmosphere, a solution in which the above three types of semiconductor fine particles are mixed is applied onto a mold having a recess at a position corresponding to the light emitting section, and the mold is recombined. The film of semiconductor fine particles was transferred onto the recombination layer 4 by being pressed onto the layer 4. Here, when the formed film was separately observed with an atomic force microscope (AFM), it was observed that a film in which the above three types of semiconductor fine particles having an average film thickness of 10 nm were dispersed was formed.

Next, an electron transport material Alq _{3 represented} by the following formula [4] was deposited to form an electron transport layer 7 having a thickness of 20 nm.

Next, Alq _{3 as} an electron transport material as a host and cesium carbonate as a dopant of an alkali metal compound are co-evaporated so that the cesium concentration in the layer is 8.3% by weight, and electron injection with a film thickness of 20 nm is performed. Layer 8 was formed.

  Next, IZO was deposited by sputtering to form a transparent cathode (cathode 9) having a thickness of 60 nm. After forming up to the cathode 9, the device was sealed with a glass cap containing a desiccant in a glove box in a nitrogen atmosphere. Thus, a light emitting element was manufactured.

  When the obtained light-emitting element was energized, white light (white emission spectrum) was observed. In addition, it was found that the light-emitting element of this example had high luminous efficiency and stable display characteristics. Here, the white light specifically refers to light obtained by synthesizing all the light emitted from the CdSe / ZnS semiconductor fine particles having the respective particle sizes. Here, the relationship between the particle diameter of the used semiconductor fine particles and the emission peak wavelength is as shown in Table 2 below.

  Moreover, in order to measure the absorption spectrum of the semiconductor fine particles of each particle size, thin films of CdSe / ZnS semiconductor fine particles having particle sizes of 5.3 nm, 2.5 nm, and 1.8 nm, respectively, were formed on the glass substrate. . An absorption spectrum was measured for each of the produced semiconductor fine particle films. The results are shown in FIG. FIG. 6 is a diagram showing an emission spectrum of a thin film corresponding to the recombination layer and an absorption spectrum of a thin film corresponding to the light emitting layer in this example. In FIG. 6, LR represents an emission spectrum of a thin film corresponding to the recombination layer. AR represents an absorption spectrum of CdSe / ZnS semiconductor fine particles having a particle diameter of 5.3 nm. AG represents an absorption spectrum of CdSe / ZnS semiconductor fine particles having a particle diameter of 2.5 nm. AB represents an absorption spectrum of CdSe / ZnS semiconductor fine particles having a particle diameter of 1.8 nm. As shown in FIG. 6, the emission spectrum LR of the recombination layer has a peak around 460 nm. On the other hand, the absorption spectra AR, AG, AB of the CdSe / ZnS semiconductor fine particles having various particle diameters have a wide range of absorption spectra on the short wavelength side. For this reason, it was found that the emission spectrum LA of the recombination layer overlaps with the absorption spectra AR, AG, AB of the semiconductor fine particles.

<Comparative example 2>
In Example 2, a light emitting device was manufactured in the same manner as in Example 2 except that the step of forming the recombination layer 4 was omitted. That is, the light emitting layer 6 made of semiconductor fine particles is sandwiched between the electron block layer 10 and the electron transport layer 7.

  When the obtained light-emitting element is energized, the light emission derived from the electron transport layer 7 in addition to the light emission spectrum of light emitted from the CdSe / ZnS semiconductor fine particles of each particle size (5.3 nm, 2.5 nm, 1.8 nm). A spectrum was observed. Further, it was confirmed that the emission intensity was significantly deteriorated in the light emission test for a long time as compared with the light emitting device of Example 2.

  The organic / inorganic hybrid light-emitting device of the present invention may be used for televisions, personal digital assistants, mobile phones, monitors for digital cameras and digital video cameras, lighting, and the like.

It is sectional drawing which shows 1st embodiment in the light emitting element of this invention. It is sectional drawing which shows 2nd embodiment in the light emitting element of this invention. It is sectional drawing which shows 3rd embodiment in the light emitting element of this invention. It is sectional drawing which shows one Embodiment of the light emitting element array of this invention. It is a figure which shows the light emission spectrum of the recombination layer which comprises the light emitting element of Example 1, and the absorption spectrum of a semiconductor fine particle. It is a figure which shows the light emission spectrum of the recombination layer which comprises the light emitting element of Example 2, and the absorption spectrum of a semiconductor fine particle. It is sectional drawing which shows the conventional light emitting element.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Board | substrate 2 Anode 3 Hole transport layer 4 Recombination layer 5 Semiconductor fine particle 6 Light emitting layer 6R Red light emitting layer 6G Green light emitting layer 6B Blue light emitting layer 7 Electron transport layer 8 Electron injection layer 9 Cathode 10 Electron block layer 11, 12, 13 Light emitting element 21 Reflective anode 22 Transparent anode 30 Light emitting element array

Claims (9)

An anode and a cathode;
Between the anode and the cathode, at least a hole transport layer, a recombination layer, a light emitting layer, and an electron transport layer are included in this order,
A light-emitting element, wherein the light-emitting layer is composed of semiconductor fine particles, and excitonic energy generated in the recombination layer emits light by transferring energy to the semiconductor fine particles.   The light emitting device according to claim 1, wherein the recombination layer is composed of an organic compound as a host and a fluorescent or phosphorescent organic compound as a guest. The light-emitting element according to claim 1, wherein the recombination layer has a relationship represented by the following formula (1).
μH> μE (1)
(In the formula, μH represents the mobility of holes, and μE represents the mobility of electrons.) The light emitting device according to claim 2, wherein the host and the guest have a relationship represented by the following formula (2).
ELG-ELH ≧ 0.15 eV (2)
(In the formula, ELH represents the absolute value of the lowest empty orbit (LUMO) energy of the host, and ELG represents the absolute value of the lowest empty orbit (LUMO) energy of the guest.) Furthermore, the said host and the said guest have the relationship of following formula (3), The light emitting element of Claim 2 or 4 characterized by the above-mentioned.
EHG-EHH> 0 eV (3)
(In the formula, EHH represents the absolute value of the host's highest occupied orbital (HOMO) energy, and EHG represents the absolute value of the guest's highest occupied orbital (HOMO) energy.)   The light emitting device according to claim 1, wherein an emission spectrum of light generated from the recombination layer overlaps with an absorption spectrum of the semiconductor fine particles.   The light emitting device according to any one of claims 1 to 6, wherein the semiconductor fine particles are closely packed with a single layer or a plurality of layers.   The light emitting device according to claim 1, wherein the light emitting layer includes at least two kinds of semiconductor fine particles having different particle diameters.   9. The light emitting device according to claim 1, wherein the light emitting devices exhibit at least two different luminescent colors, and a recombination layer common to all of the light emitting devices is provided. , Light emitting element array.

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