JP2005101002A - Manufacturing method of light-emitting element - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high-efficiency triplet light-emitting element with stability which can save labor in manufacturing as compared with a conventional element, by simplifying an element structure without using an unstable material. <P>SOLUTION: In the element structure dispensing with a hole blocking layer as introduced in the conventional triplet light-emitting element, that is, the element structure in which a positive electrode 102, a hole transport layer 103, an electron transport light-emitting layer 104 made of an electron transport material and a dopant material emitting triplet light, and a negative electrode 105 laminated in turn on a base plate 101, a combination of the hole transport material and the electron transport material as well as that of the electron transport material and the dopant material is optimized. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

The present invention relates to an organic light-emitting device including an anode, an organic compound film that can emit light when an electric field is applied, and a cathode. In particular, the present invention relates to an organic light emitting device using a light emitting material that emits light from a triplet excited state.

  An organic light-emitting device is a light-emitting device that utilizes light emitted from excited molecules (excited molecules) formed by injecting electrons and holes from both electrodes into an organic compound film by applying voltage and recombining them. .

  The light emitted from the organic compound film is light that is emitted when excited molecules are formed and deactivated to the ground state. There are two main types of deactivation processes. There are cases of deactivation via singlet excited molecules (fluorescence occurs at this time) and cases of deactivation via triplet excited molecules. The deactivation process via triplet excited molecules includes a phosphorescent light emission process and a triplet-triplet annihilation process, but basically few organic materials contain a deactivation process via phosphorescence at room temperature ( Most of them are heat-inactivated and are not deactivated by light emission). For this reason, most organic compounds used in organic light-emitting elements are materials that emit fluorescence via singlet excited molecules, and many organic light-emitting elements use fluorescence.

  The organic light-emitting device using this fluorescent organic compound was reported by CWTang et al. In 1987 as a two-layer type in which an organic compound film with a total thickness of about 100 nm sandwiched between two organic compounds was sandwiched between electrodes. The structure is fundamental (see Non-Patent Document 1). Thereafter, in 1988, a three-layer structure was proposed by Adachi et al. (See Non-Patent Document 2), and at present, a multi-layer element structure using these stacked structures is adopted.

C.W.Tang and S.A.Vanslyke, "Organic electroluminescent diodes", Applied Physics Letters, Vol.51, No.12, 913-915 (1987) Chihaya ADACHI, Shozuo TOKITO, Tetsuo TSUTSUI and Shogo SAITO, "Electroluminescence in Organic Films with Three-Layered Structure", Japanese Journal of Applied Physics, Vol. 27, No. 2, L269-L271 (1988)

  Such an element having a multilayer structure has a feature of “functional separation of layers”. The functional separation of layers is not to give various functions to one kind of organic compound at the same time but to share the functions for each layer. For example, a device with a two-layer structure uses a hole transport layer that plays a role of hole transport and a light-emitting electron transport layer that plays a role of electron transport and light emission. A hole transport layer that plays a role only in hole transport, an electron transport layer that plays a role only in electron transport, and a light emitting layer that emits light between the two layers. Thus, by separating the functions of each layer, there is an advantage that the degree of freedom increases in the molecular design of the organic compound used in the organic light emitting device.

  For example, an element having a single-layer structure is required to have many characteristics that it is easy to inject electrons and holes into one layer, has a function of transporting both carriers, and has a high fluorescence quantum yield. However, when an electron transporting light emitting layer is used as in a device having a two-layer structure, an organic compound that easily injects holes into the hole transporting layer is high, and electrons are easily injected into the electron transporting light emitting layer. Each organic compound that obtains a fluorescence quantum yield may be applied, reducing the requirement for one layer and facilitating selection of materials.

  In addition, in an element having a three-layer structure, the function of electron transport and light emission can be separated by introducing a “light emission layer”. In addition, by using a fluorescent material (guest) with a high quantum yield such as a laser dye dispersed in a solid medium (host) material in the light emitting layer, the fluorescent quantum yield of the light emitting layer can be improved, and organic light emission Not only the quantum efficiency of the device is greatly improved, but also the emission wavelength can be freely controlled by selecting the fluorescent dye to be used (see Non-Patent Document 3). An element in which a pigment (guest) is dispersed in a host material in this way is called a doped element.

C.W.Tang, S.A.Vanslyke and C.H.Chen, "Electroluminescence of doped organic thin films", Journal of Applied Physics, Vol.65, 3610-3616 (1989)

  Another effective point of the multilayer structure element is a “carrier confinement effect”. For example, in the case of the two-layer structure of Non-Patent Document 1, holes are injected from the anode to the hole transport layer, and electrons are injected from the cathode to the electron transport layer, and move to the interface between the hole transport layer and the electron transport layer. Then, holes are injected into the electron transport layer because the difference in ionization potential between the hole transport layer and the electron transport layer is small, whereas electrons have a low electric affinity for the hole transport layer and the electron transport layer. Is too large to be injected into the hole transport layer, blocked by the hole transport layer, and confined in the electron transport layer. Accordingly, the density of both holes and electrons is increased in the electron transport layer, and carrier recombination is efficiently performed.

  An example of a material effective for exhibiting such a carrier confinement effect is a material having a very high ionization potential. It is difficult to inject holes into a material having a high ionization potential, and such materials are widely used as materials capable of blocking holes (hole blocking materials). For example, when a hole transport layer composed of an aromatic diamine compound reported in Non-Patent Document 1 and an electron transport layer composed of tris (8-quinolinolato) -aluminum (hereinafter referred to as “Alq”) are laminated, a voltage is applied thereto. When is applied, Alq of the electron transport layer emits light. However, by inserting a hole blocking material between the two layers of this element, holes are confined in the hole transport layer, and the hole transport layer side can also emit light.

  In this way, by introducing layers with various functions (hole transport layer, hole blocking layer, electron transport layer, electron injection layer, etc.), it becomes possible to achieve high efficiency, control the emission color, In the present organic light emitting device, a multilayer structure has been established as a basic structure.

  Under such circumstances, in 1998, SRForrest et al. Used a triplet light emitting material (in the literature, a metal complex having platinum as the central metal) that can emit light (phosphorescence) from a triplet excited state at room temperature as a guest. A doped type element (hereinafter referred to as “triplet light emitting element”) has been announced (see Non-Patent Document 4). Hereinafter, in order to distinguish from this triplet light emitting element, an element that utilizes light emission from a singlet excited state is referred to as a “singlet light emitting element”.

M.A.Baldo, D.F.O'Brien, Y.You, A.Shoustilkov, S.Silbley, M.A.Thomoson and S.R.Forrest, "Highly efficient phosphorescent emission from organic electroluminescent devices", Nature, Vol.395, 151-154 (1998)

  As described above, excited molecules generated by recombination of holes and electrons injected into an organic compound include singlet excited molecules and triplet excited molecules. In this case, singlet excited molecules and triplet excited molecules are generated at a ratio of 1: 3 due to the difference in spin multiplicity. Conventionally, triplet excited molecules have basically been thermally deactivated at room temperature, so that only singlet excited molecules have been used for light emission. For this reason, only one quarter of the generated excited molecules are used for light emission. If triplet excited molecules can be used for light emission here, light emission with high efficiency can be obtained about 3 to 4 times.

Non-Patent Document 4 uses the multilayer structure described above. That is, using an aromatic amine compound 4,4′-bis [N- (1-naphthyl) -N-phenyl-amino] -biphenyl (hereinafter referred to as “α-NPD”) as a hole transporting layer, light emission Electron transport layer using 6% dispersion of 2,3,7,8,12,13,17,18-octaethyl-21H, 23H-porphyrin-platinum (hereinafter referred to as “PtOEP”) in Alq In the device structure using Alq, the maximum value of the external quantum efficiency was 4%, and the value was 1.3% at 100 cd / m ² .

After that, in the device structure using the hole blocking layer, α-NPD is used as the hole transport layer, and PtOEP is used as 4,4 '-N, N' -dicarbazole-biphenyl (hereinafter referred to as “CBP”) as the light emitting layer. 6% dispersed using 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (hereinafter referred to as “BCP”) as the hole blocking layer and Alq as the electron transport layer, The light emission efficiency of the device is improved at 100 cd / m ² with an external quantum efficiency of 2.2% and a maximum of 5.6% (see Non-Patent Document 5).

D.F.O'Brien, M.A.Baldo, M.E.Thompson and S.R.Forrest, "Improved energy transfer in electrophosphorescent devices", Applied Physics Letters, Vol.74, No.3, 442-444 (1999)

Furthermore, a triplet light emitting device using tris (2-phenylpyridine) iridium (hereinafter referred to as “Ir (ppy) ₃ ”) as a triplet light emitting material has been reported (see Non-Patent Document 6). A highly efficient organic light-emitting device having an external quantum efficiency of 14.9% at 100 cd / m ² by optimizing the thickness of the organic compound film with the same device structure has also been reported (see Non-Patent Document 7). As a result, an element having a luminous efficiency nearly three times that of a conventional singlet light emitting element has been manufactured.

M.A.Baldo, S.Lamansky, P.E.Burrows, M.E.Thompson and S.R.Forrest, "Very high-efficiency green organic light-emitting devices based on electrophosphorescence", Applied Physics Letters, Vol. 75, No. 1, 4-6 (1999) Teruichi Watanabe, Kenji Nakamura, Shin Kawami, Yoshinori Fukuda, Taishi Tsuji, Takeo Wakimoto, Satoshi Miyaguchi, Masayuki Yahiro, Moon-Jae Yang, Tetsuo Tsutsui, "Optimization of emittimg efficiency in organic LED cells using Ir complex", Synthetic Metals, Vol .122, 203-207 (2001)

  Currently, triplet light-emitting materials using iridium or platinum as the central metal have been searched, and triplet light-emitting elements that are very efficient compared to singlet light-emitting elements have attracted attention, and research has been conducted energetically. .

  A triplet light emitting device has a much higher light emission efficiency than a singlet light emitting device, but has a life much shorter than a singlet light emitting device and lacks stability. In addition, a multi-layer structure is adopted to increase the efficiency. Therefore, there is a simple demerit that the element structure is at least a four-layer structure, and it takes time to manufacture the element.

Regarding the lifetime of the device, a hole transport layer using α-NPD, a light emitting layer using CBP as the host material and Ir (ppy) ₃ as the guest (dopant) material, and hole blocking using BCP In a device in which a layer and an electron transport layer using Alq are laminated, there is a report that the half-life is only 170 hours under the condition of an initial luminance of 500 cd / m ² (see Non-Patent Document 8). It's far from being

Tetsuo TSUTSUI, Moon-Jae YANG, Masayuki YAHIRO, Kenji NAKAMURA, Teruichi WATANABE, Taishi TSUJI, Yoshinori FUKUDA, Takeo WAKIMOTO and Satoshi MIYAGUCHI, "High Quantum Efficiency inorganic Light-Emitting Devices with Iridium-Complex as a Japanese Japanese Journal of Applied Physics, Vol.38, No.12B, L1502-L1504 (1999)

  Non-patent document 8 mentions that the stability of BCP used in the hole blocking material is low. In the triplet light emitting device, the device structure shown in Non-Patent Document 5 is a basic structure, and the hole blocking layer is used as an indispensable one. FIG. 12 shows a structure of a conventional triplet light emitting device. The element structure is such that an anode 1102 is formed on a substrate 1101, and a hole transport layer 1103, a light emitting layer 1104, a hole blocking layer 1105, an electron transport layer 1106, and a cathode 1107 are stacked thereon as an organic compound film. The confinement effect of the carrier by the hole blocking layer allows the carrier recombination to be performed efficiently, but on the other hand, the commonly used hole blocking material has the disadvantage of being less stable As a result, the service life will not be extended. In addition, CBP used as a host material is also a low-stability material, which is considered to be one of the reasons that the lifetime is not extended.

  An element having a three-layer structure that does not use a hole blocking layer has also been produced (see Non-Patent Document 9). Here, the host material is characterized in that it is used as an electron transport material instead of CBP which is said to be capable of transporting both carriers. However, the electron transport material used for the host material is BCP, 1,3-bis (N, Nt-butyl-phenyl) -1,3,4-oxazole (hereinafter referred to as “OXD7”) used as a hole blocking material. ), 3-phenyl-4- (1′-naphthyl) -5-phenyl-1,2,4-triazole (hereinafter referred to as “TAZ”), although no hole blocking layer is introduced, Are used in the device as those often used as hole blocking materials. Of course, BCP and all other materials are low-stability materials, and although they have high efficiency, they are low-stability devices.

Chihaya ADACHI, Marc A. Baldo, Stephen R. Forrest and Mark E. Thompson, "High-efficiency organic electrophosphorescent devices with tris (2-phinylpyridine) iridium doped into electron-transporting materials", Applied Physics Letters, Vol. 77, No .6, 904-906 (2000)

  In addition, a simple two-layer device structure that does not use a hole blocking material has been reported (see Non-Patent Document 10). However, although CBP is used as the host material and high luminous efficiency is obtained, it is stable. Lack of sex.

Chihaya ADACHI, Raymond KWONG, Stephen R. Forrest, "Efficient electrophosphorescence using a doped ambipolar conductive molecular organic thin film", Organic Electronics, Vol. 2, 37-43 (2001)

  Thus, although a device having high emission efficiency in a triplet light emitting device has been reported, there has not yet been reported a triplet light emitting device that is efficient and stable, and the host material used as the cause thereof is not yet reported. This is due to the instability of the hole blocking material.

  Therefore, in the present invention, the use of such an unstable material and the simplification of the device structure make the device structure highly efficient and stable, and can save time and effort in device fabrication compared to conventional devices. It is an object to provide a term light emitting element.

  In the triplet light emitting device, the hole blocking layer introduced in the conventional triplet light emitting device is not used, and the organic compound film has a stability of the hole transport layer and the triplet light emitting dopant material. This is achieved by forming a simple device structure (FIG. 1) in which a layer dispersed in a certain electron transport material is laminated. That is, an anode 102 is formed on a substrate 101, a hole transport layer 103 made of a hole transport material, an electron transporting light emitting layer 104 made of an electron transport material and a triplet light emitting dopant material, and a cathode 105 are laminated thereon. This is achieved with an element structure. Here, a region sandwiched between the anode 102 and the cathode 105 (that is, the hole transport layer 103 and the electron transport light-emitting layer 104) corresponds to the organic compound film.

  Therefore, in the present invention, in the organic light emitting device composed of an anode, an organic compound film, and a cathode, the organic compound film is in contact with a hole transport layer made of a hole transport material and the hole transport layer. And an electron transport layer made of an electron transport material, and a light emitting material that emits light from a triplet excited state is added to the electron transport layer.

  Note that a hole injection layer may be inserted between the anode 102 and the hole transport layer 103. Further, an electron injection layer may be inserted between the cathode 105 and the electron transporting light emitting layer 104. Further, both of these hole injection layer and electron injection layer may be inserted.

  By the way, in the element as described above, in order to prevent the hole transport layer 103 from emitting light, it is also important as a means for solving the problem to consider the combination of the hole transport material and the electron transport material. is there.

  Therefore, in the present invention, the energy difference between the highest occupied molecular orbital level and the lowest unoccupied molecular orbital level in the hole transport material is the highest occupied molecular orbital level and the lowest unoccupied molecular orbital level in the electron transport material. It is characterized by being larger than the energy difference.

  As another means, the absorption spectrum of the hole transport material and the emission spectrum of the electron transport material do not overlap. In this case, it is preferable that not only the spectra do not overlap, but also that the absorption spectrum of the hole transport material is located on the shorter wavelength side than the emission spectrum of the electron transport material as a positional relationship of the spectra.

  Here, in order to improve the light emission efficiency of the triplet light emitting device of the present invention as described above, the device configuration in which the triplet light emitting dopant easily traps carriers can also be achieved. As important.

  Therefore, in the present invention, the highest occupied molecular orbital level and the lowest unoccupied molecular orbital level of the light emitting material exhibiting light emission from the triplet excited state are both the highest occupied molecular orbital level and the lowest unoccupied molecular molecule in the electron transport material. It is located in the energy gap with the orbital level.

  As another means, the value of the ionization potential of the hole transport material is equal to or larger than the value of the ionization potential of the light emitting material exhibiting light emission from a triplet excited state.

  Further, as another means, the absolute value of the value indicating the lowest unoccupied molecular orbital level of the hole transport material is 0.2 eV or more smaller than the absolute value of the value indicating the lowest unoccupied molecular orbital level of the electron transporting material. It is characterized by.

  It should be noted that the element configuration combining these, that is, the value of the ionization potential of the hole transport material is the same as or larger than the value of the ionization potential of the light emitting material that emits light from the triplet excited state, and is positive. It is more preferable that the absolute value of the value indicating the lowest unoccupied molecular orbital level of the hole transport material is 0.2 eV or more smaller than the absolute value of the value indicating the lowest unoccupied molecular orbital level of the electron transporting material.

  In view of the above, 4,4 ′, 4 ″ -tris (N-carbazole) triphenylamine, 4,4′-bis [N, N-bis (3 -Methylphenyl) -amino] -diphenylmethane, 1,3,5-tris [N, N-bis (2-methylphenyl) -amino] -benzene, 1,3,5-tris [N, N-bis (3 -Methylphenyl) -amino] -benzene or 1,3,5-tris [N, N-bis (4-methylphenyl) -amino] -benzene is used.

  In addition, 2,2 ′, 2 ”-(1,3,5-benzenetriyl) tris- [1-phenyl-1H-benzoimidazole], lithium tetra (2- (2-hydroxy) Phenyl) benzoxazolatoboron, bis (2- (2-hydroxyphenyl) benzoxazolate) (triphenylsilanolato) aluminum, bis (2- (2-hydroxyphenyl) benzothiazolate) (triphenylsilanolato) aluminum , 2- (2-hydroxyphenyl) benzoxazolate lithium, (2- (2-hydroxyphenyl) benzoxazolate) -diphenylboron, tris (8-quinolinolato) -aluminum, bis (2-methyl-8-quinolinolato ) (Triphenylsilanolato) aluminum, bis (2-methyl-8-quinolinolato) (4-phenylphenolato) aluminum, lithium tetra (2-methyl-8-hydroxy-quinolinato) boron, (2-methyl-8- Quinolinolato) -It is characterized by using either diphenylboron or bis (2-methyl-8-quinolinolato) aluminum hydroxide.

  Furthermore, in the device of the present invention, it is effective to use a combination of these hole transport materials and electron transport materials.

  By implementing the present invention, it is possible to obtain a triplet light-emitting element that is as highly efficient as a conventional triplet light-emitting element with a simple element configuration. Moreover, a stable organic light-emitting element can be provided by omitting a layer using an unstable material.

  Hereinafter, embodiments of the present invention will be described in detail. Note that the organic light-emitting element may be transparent so that at least one of the anode and the cathode is transparent in order to extract light emission. In this embodiment, a transparent anode is formed on the substrate and light is extracted from the anode side. Describe the device structure. Actually, the present invention can also be applied to a structure in which a cathode is formed on a substrate to extract light from the cathode, a structure in which light is extracted from the side opposite to the substrate, and a structure in which light is extracted from both sides of the electrode.

  As described above, the present invention is characterized in that no hole blocking layer is used in the triplet light emitting device (FIG. 1). However, it does not simply mean that an element in which the hole blocking layer is removed from the conventional element structure (FIG. 12) is produced.

  First, there is a difference in the recombination region between the conventional triplet light emitting device and the two-layer device of the present invention. In the conventional triplet light emitting device, the hole recombination region is the interface between the light emitting layer and the hole blocking layer because the hole blocking layer is used. On the other hand, in the device structure proposed in the present invention, the carrier recombination region is an interface between the hole transport layer and the host electron transport material.

  For this reason, the light emission mechanism of a triplet light emitting element is important. In general, two types of light emission mechanisms are conceivable as the light emission mechanism of an element using a host-guest light emitting layer using a dopant (guest).

  The first light emission mechanism is light emission of the dopant by energy transfer from the host. In this case, first, both carriers are injected into the host to form excited molecules of the host. The energy of this excited molecule is transferred to the dopant, and the dopant is excited by the energy and emits light when deactivated. In the case of a triplet light emitting device, since the dopant is a material that emits phosphorescence via a triplet excited molecule, the emitted light is phosphorescence.

  What is important in the light emission mechanism by energy transfer is that there is a large overlap between the emission spectrum of the host material and the absorption spectrum of the dopant material. The positional relationship between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) in the host material and dopant material is not important.

  In this specification, the value of the ionization potential observed by photoelectron spectroscopy in the atmosphere is used as the value of HOMO. The absorption edge of the absorption spectrum is the energy difference between HOMO and LUMO (hereinafter, this energy difference is referred to as “energy gap value”). Therefore, the LUMO value is obtained by subtracting the energy gap value estimated by the absorption edge of the absorption spectrum from the ionization potential value measured by photoelectron spectroscopy. Here, these values (HOMO (ionization potential), LUMO, energy gap value) are actually negative values because they are based on the vacuum level, but in this specification, all values are expressed as absolute values. To do. A conceptual diagram of HOMO, LUMO, and energy gap values is shown in FIG.

  By the way, if the HOMO and LUMO energy levels of the dopant material are both within the energy gap between the HOMO and LUMO in the host material, in addition to the emission mechanism of energy transfer from the host to the dopant described above, There is also a direct recombination emission mechanism where the carriers are trapped and the carriers are recombined directly on the dopant. This is the second light emitting mechanism.

  However, when the dopant material and the host material are in such an energy level positional relationship, energy transfer is also a condition, so it is usually difficult to separate which mechanism contributes from the light emission mechanism, It is also possible that both light emission mechanisms are involved.

  First, consider a case where the triplet light emitting element emits light by an energy transfer mechanism (first light emission mechanism). In the conventional device structure, since the carrier recombination region is the interface between the light emitting layer and the hole blocking layer, in addition to the energy transfer from the host material to the dopant material, energy transfer to the hole blocking material is also conceivable. . However, since the absorption spectrum of the hole blocking material is on the very short wavelength side, the emission spectrum of the host material and the absorption spectrum of the hole blocking material reported for conventional triplet light emitting devices do not overlap. Energy transfer between the material and the hole blocking material cannot occur. That is, it can be said that the conventional triplet light emitting device has an element structure in which energy transfer does not occur from the host material to the hole blocking material.

  On the other hand, in the device structure of the present invention, the carrier recombination region is an interface between the hole transport layer containing the hole transport material and the electron transporting light emitting layer containing the host material. For this reason, in the element of the present invention, energy transfer from the host material to the hole transport material can be considered. If energy transfer occurs from the host material to the hole transport material, efficient light emission cannot be obtained.

  Therefore, the magnitude relationship between the energy gap value of the host material and the energy gap value of the hole transport material is a rough guide for energy transfer. If the energy gap value of the host material is smaller than the energy gap value of the hole transport material, it is difficult to excite the hole transport material by energy transfer from the host material. Therefore, in order to prevent energy transfer from the host material to the hole transport material, the hole transport material preferably has an energy gap value larger than that of the host material.

  FIG. 3 is an energy diagram in this case. As shown in FIG. 3, the material may be selected so that the energy gap value A of the hole transport material is larger than the energy gap value B of the host material.

  There is also a method of selecting a combination of materials that does not overlap the emission spectrum of the host material and the absorption spectrum of the hole transport material as a condition for preventing energy transfer between the host material and the hole transport material. At this time, the absorption spectrum of the hole transport material is preferably located on the shorter wavelength side than the emission spectrum of the electron transport material.

  FIG. 4 illustrates this condition. (A) shows the positional relationship of the spectrum when energy transfer occurs between the host material and the hole transport material, and (b) shows the spectral position when energy transfer does not occur between the host material and the hole transport material. Each relationship is shown. In the present invention, the positional relationship (b) is preferable.

  In addition to these conditions, if the host material is selected so that both the HOMO and LUMO energy levels of the dopant material are within the energy gap between the HOMO and LUMO of the host material, the direct recombination emission mechanism (second It is important to consider further conditions since the light emission mechanism) is also considered.

  In this case, a material having a large ionization potential value indicating HOMO of the hole transport material is suitable so that hole carriers are easily injected from the hole transport material into the dopant material. That is, the hole transport material is combined so that the ionization potential of the hole transport material becomes larger than the ionization potential of the dopant material. If the ionization potential of the hole transport material is too large, it will be difficult to inject holes from the anode into the hole transport material. In this case, it is improved by introducing a hole injection layer between the anode and the hole transport layer. Is done.

  In addition, for the electron carrier, it is considered that the dopant traps the electron carrier through the electron transporting host. If electrons not trapped by the dopant move through the electron transport layer and reach the interface with the hole transport layer, if the LUMO level of the hole transport material is not significantly different from the LUMO level of the host material, Electrons that arrive at the interface enter the hole transport layer. For this reason, electrons are not confined in the electron transport layer, and efficient recombination is not performed. In order to avoid such a situation, it is desirable that the difference in LUMO level between the hole transport material and the electron transport material that is the host material is sufficiently large to block electrons. This difference is desirably 0.2 eV or more.

  Next, a method for manufacturing the triplet light-emitting element of the present invention and a material to be used will be described more specifically.

  In the element manufacturing method of the present invention shown in FIG. 1, a hole transport material is first deposited on a substrate having an anode (ITO), and then an electron transport material (host material) and a triplet light-emitting material (dopant material) are used together. Vapor deposition is performed, and finally a cathode is formed by vapor deposition. The dopant concentration when co-evaporating the host material and the dopant material is adjusted to about 8 wt%. Finally, sealing is performed to obtain an organic light emitting device.

  Next, materials suitable for the hole injection material, hole transport material, electron transport material (host material), and triplet light emitting material (dopant material) that can be used in the device of the present invention are listed below. However, the material used for the element of the present invention is not limited to these.

As the hole injecting material, porphyrin compounds, phthalocyanine (hereinafter referred to as “H2Pc”), copper phthalocyanine (hereinafter referred to as “CuPc”), and the like are effective as organic compounds. In addition, any material that has a smaller ionization potential than the hole transport material used and has a hole transport function can also be used as the hole injection material. There is also a material obtained by chemically doping a conductive polymer compound, and examples thereof include polyethylene dioxythiophene (hereinafter referred to as “PEDOT”) doped with polystyrene sulfonic acid (hereinafter referred to as “PSS”), polyaniline, and the like. An insulating polymer compound is also effective in terms of planarization of the anode, and polyimide (hereinafter referred to as “PI”) is often used. In addition, inorganic compounds are also used. In addition to metal thin films such as gold and platinum, there are ultra thin films of aluminum oxide (hereinafter referred to as “alumina”).

  As the hole transport material, those having an energy gap value larger than the energy gap value of the electron transport material used as the host material are effective. Further, it is preferable that the ionization potential is larger than that of the light emitting material, or that the LUMO absolute value is 0.2 eV or more smaller than that of the electron transporting material.

  As a hole transport material having a large energy gap value suitable for the device of the present invention, 4,4 ′, 4 ”-tris (N-carbazole) triphenylamine (hereinafter referred to as“ TCTA ”) represented by the following structural formula (1): ), 1,3,5-tris [N, N-bis (2-methylphenyl) -amino] -benzene (hereinafter referred to as “o-MTDAB”) represented by the following structural formula (2), 1,3,5-tris [N, N-bis (3-methylphenyl) -amino] -benzene (hereinafter referred to as “m-MTDAB”) represented by the following structural formula (3), 1,3,5-tris [N, N-bis (4-methylphenyl) -amino] -benzene (hereinafter referred to as “p-MTDAB”), represented by the following structural formula (5) 4,4′-bis [N, N-bis (3-methylphenyl) -amino] -diphenylmethane (hereinafter referred to as “BPPM”).

  On the other hand, 4-4′-bis [N- (3-methylphenyl) -N-phenyl-amino] -biphenyl (hereinafter referred to as “TPD”), which is the most widely used aromatic amine compound, Α-NPD, which is a derivative thereof, has a smaller energy gap value than the compounds of structural formulas (1) to (5), and is difficult to use for the device of the present invention. Table 1 summarizes the energy gap values (measured values) of the compounds of structural formulas (1) to (5), α-NPD, and TPD.

  Next, as the electron transport material serving as a host, a material having high stability is preferable, and many metal complexes having high stability are listed. The host material must be a material having a larger energy gap value than the triplet light emitting material which is a dopant. Such a host material varies depending on the light emitting material to be used. Examples of the electron transport material that can be used as a host in the device of the present invention are shown below.

  In the present invention, a substance that can be used as a host material for a blue light-emitting material is 2,2 ', 2 "-(1,3,5-benzenetolyl) tris- [1-phenyl] represented by the following structural formula (6). -1H-benzimidazole ”(hereinafter referred to as“ TPBI ”), and the like whose emission spectrum is observed at a very short wavelength in the ultraviolet region.

In the present invention, as a host material for the green light emitting material, lithium tetra (2- (2-hydroxyphenyl) benzoxazolatoboron (hereinafter referred to as “LiB (PBO) ₄ ”) represented by the following structural formula (7) ), Bis (2- (2-hydroxyphenyl) benzoxazolate) (triphenylsilanolato) aluminum (hereinafter referred to as “SAlo”) represented by the following structural formula (8), and the following structural formula (9) Bis (2- (2-hydroxyphenyl) benzothiazolate) (triphenylsilanolato) aluminum (hereinafter referred to as “SAlt”), 2- (2-hydroxyphenyl) benzo represented by the following structural formula (10) Oxazolate lithium (hereinafter referred to as “Li (PBO)”), (2- (2-hydroxyphenyl) benzoxazolate) -diphenylboron (hereinafter referred to as “B (PBO) Ph” represented by the following structural formula (11) ₂ ”). It is also possible to use a material that can emit blue light.

In the present invention, Alq represented by the following structural formula (12), bis (2-methyl-8-quinolinolato) (triphenylsilanolato) represented by the following structural formula (13) are used as host materials for the red light emitting material. ) Aluminum (hereinafter referred to as “SAlq”), bis (2-methyl-8-quinolinolato) (4-phenylphenolato) aluminum (hereinafter referred to as “BAlq”) represented by the following structural formula (14), and the following structure Lithium tetra (2-methyl-8-hydroxy-quinolinato) boron (hereinafter referred to as “LiB (mq) ₄ ”) represented by the formula (15), represented by the following structural formula (16) (2-methyl- 8-quinolinolato) -diphenylboron (hereinafter referred to as “BmqPh”), bis (2-methyl-8-quinolinolato) aluminum hydroxide represented by the following structural formula (17) (hereinafter referred to as “Almq ₂ (OH)”) ) And the like. In addition to these, materials that can emit blue light and materials that can emit green light can also be used as host materials.

  Table 2 shows energy gap values (actually measured values) for some of the host materials described here.

Examples of the triplet light-emitting material that is a dopant include many complexes having iridium or platinum as a central metal, but any material that emits phosphorescence at room temperature may be used. PtOEP, Ir (ppy) ₃ , bis (2-phenylpyridinato-N, C ^{2 '} ) acetylacetonatoiridium (hereinafter referred to as “acacIr (ppy) ₂ ”), bis (2- (4′-tolyl) Pyridinato-N, C ^{2 ′} ) acetylacetonatoiridium (hereinafter referred to as “acacIr (tpy) ₂ ”), bis (2- (2′-benzothienyl) -pyridinato-N, C ^{3 ′} ) acetylacetonatoiridium ( (Hereinafter referred to as “acacIr (btp) ₂ ”).

  The energy gap values (actual measurement values) of the dopant materials described here are as shown in Table 3.

  The electron transport material described above can be used as the electron injection material. However, electron transport materials (such as BCP and OXD7) that are used as hole blocking materials are not suitable because of their low stability. In addition, an insulating ultrathin film such as an alkali metal halide such as lithium fluoride or an alkali metal oxide such as lithium oxide is often used. Alkali metal complexes such as lithium acetylacetonate (hereinafter referred to as “Li (acac)”) and 8-quinolinolato-lithium (hereinafter referred to as “Liq”) are also effective.

  By combining the materials having the functions as described above, and applying them to the organic light emitting device of the present invention, it is possible to save time and effort in the manufacturing process compared to the conventional triplet light emitting device, and the stability is high. Efficiently, an organic light-emitting element with the same efficiency as a conventional triplet light-emitting element can be produced.

  In this example, the organic light emitting device shown in FIG. 2 of the present invention is specifically exemplified.

  First, BPPM, which is a hole transport material, is deposited to 40 nm on a glass substrate 101 on which ITO, which is the anode 102, is formed to a thickness of about 100 nm. This is the hole transport layer 103.

After the hole transport layer is formed, co-evaporation of acacIr (tpy) ₂ that is a triplet light emitting material and TPBI that is an electron transport material (host material) to a ratio (weight ratio) of about 2:23. Do. That is, acacIr (tpy) ₂ is dispersed in TPBI at a concentration of about 8 wt%. This co-deposited film is deposited to 50 nm. This is the electron transporting light emitting layer 104.

Finally, Mg and Ag are co-deposited as the cathode 105 so that the atomic ratio is 10: 1, and the cathode is deposited to a thickness of 150 nm. Thus, a green light emitting triplet light emitting element derived from acacIr (tpy) ₂ is obtained.

  FIG. 5 is a graph of initial characteristics and an emission spectrum of this device. Even with a simple two-layer device structure, the maximum external quantum efficiency was about 10%, which showed high efficiency device characteristics.

  A device of the present invention was fabricated using a hole transport material different from that in Example 1 (however, a material satisfying the conditions of the present invention).

  First, o-MTDAB, which is a hole transport material, is deposited to 40 nm on a glass substrate 101 on which ITO, which is the anode 102, is formed to a thickness of about 100 nm. This is the hole transport layer 103.

After the hole transport layer is formed, co-evaporation of acacIr (tpy) ₂ that is a triplet light emitting material and TPBI that is an electron transport material (host material) to a ratio (weight ratio) of about 2:23. Do. That is, acacIr (tpy) ₂ is dispersed in TPBI at a concentration of about 8 wt%. This co-deposited film is deposited to 50 nm. This is the electron transporting light emitting layer 104.

Finally, Mg and Ag are co-deposited as the cathode 105 so that the atomic ratio is 10: 1, and the cathode is deposited to a thickness of 150 nm. Thus, a green light emitting triplet light emitting element derived from acacIr (tpy) ₂ is obtained.

  FIG. 6 is a graph of the initial characteristics and emission spectrum of this light emitting device. As in Example 1, a highly efficient element can be manufactured.

An organic light-emitting device according to the present invention was produced using an electron transporting material different from that in Example 1 (provided that the material satisfies the conditions of the present invention) as a host material. The production method is the same as in Examples 1 and 2, using BPPM as the hole transport material, SAlt as the electron transport material as the host, and acacIr (tpy) ₂ as the dopant. A green light-emitting triplet light-emitting element derived from acacIr (tpy) ₂ can be obtained.

  FIG. 7 shows the initial characteristics and emission spectrum of this device. As in the first and second embodiments, a highly efficient element equivalent to the conventional triplet light emitting element is obtained.

  A triplet light-emitting material different from those in Examples 1, 2, and 3 was used as a dopant, and organic light-emitting elements having different emission colors from those in Examples 1, 2, and 3 were produced. The production method is the same as in Examples 1, 2, and 3. The hole transport material is BPPM, the host electron transport material is TPBI, and the dopant is bis (2- (2 ′, 4′-difluorophenyl) pyridinato. -N, C2 ') Uses picola toridium. A triplet light emitting element emitting blue light derived from the dopant material can be obtained.

  FIG. 8 shows the initial characteristics and emission spectrum of this device. As in Examples 1, 2, and 3, a highly efficient element equivalent to a conventional triplet light emitting element is obtained.

[Comparative Example 1]
In this comparative example, an element having a structure similar to that of a conventional triplet light emitting element as shown in FIG. 12 was manufactured, and the characteristics at that time were compared with those of the element of the present invention.

  First, α-NPD, which is a hole transport material, is vapor-deposited to 40 nm on a glass substrate 1101 on which ITO, which is the anode 1102, is formed to a thickness of about 100 nm. This is the hole transport layer 1103.

After the hole transport layer is formed, co-evaporation of acacIr (tpy) ₂ that is a triplet light-emitting material and CBP that is a host material is performed at a ratio (weight ratio) of approximately 2:23. That is, acacIr (tpy) ₂ is dispersed in CBP at a concentration of about 8 wt%. This co-deposited film is deposited to 50 nm. This is the light emitting layer 1104.

  After forming the light emitting layer, 20 nm of BCP, which is a hole blocking material, is deposited to form the hole blocking layer 1105. Thereafter, Alq, which is an electron transport material, is deposited by 30 nm to form an electron transport layer 1106.

Finally, Mg and Ag are co-deposited as the cathode 1107 so that the atomic ratio is 10: 1, and the cathode is formed to a thickness of 150 nm. Thus, a green light emitting triplet light emitting element derived from acacIr (tpy) ₂ is obtained.

  FIG. 9 shows the initial characteristics and emission spectrum of this device. When compared with Examples 1, 2, and 3, it can be seen that the elements of the present invention shown in the Examples are as highly efficient as the conventional elements. It was confirmed that sufficient device characteristics were exhibited without using a hole blocking layer.

[Comparative Example 2]
In this comparative example, the characteristics of a triplet light-emitting element having a two-layer structure using a hole transport material that does not satisfy the conditions of the element of the present invention are illustrated.

The manufacturing method is the same as in the example, but a hole transport material-host material combination is used so that the energy gap value of the hole transport material to be used is smaller than that of the host material. TPD is used as the hole transport material, TPBI as the electron transport material is used as the host material, and acacIr (tpy) ₂ is used as the dopant.

FIG. 10 shows the initial characteristics and emission spectrum of this device. When TPD was used as the hole transport material, the device was very inefficient for a triplet light emitting device. As can be seen from the emission spectrum, in addition to the emission from acacIr (tpy) ₂ , a spectrum (near 400 nm) that is emission from TPD is observed. This reduces efficiency. As described above, when a material not satisfying the conditions is used, the initial characteristics of the element are poor.

[Comparative Example 3]
In this comparative example, as in comparative example 2, the characteristics of a triplet light emitting device having a two-layer structure using a hole transport material that does not meet the conditions of the device of the present invention are illustrated.

The manufacturing method is the same as in the example, but a combination of a hole transport material and a host material is used so that the energy gap value of the hole transport material to be used is smaller than that of the host material. Α-NPD is used as the hole transport material, TPBI as the electron transport material is used as the host material, and acacIr (tpy) ₂ is used as the dopant.

  FIG. 11 shows the initial characteristics and emission spectrum of this device. When α-NPD was used as the hole transport material, as in Comparative Example 2, the device was very inefficient for a triplet light emitting device. Similarly to Comparative Example 2, the emission spectrum is also a spectrum (near 440 nm) which is emission from α-NPD which is a hole transport material. This reduces efficiency. As described above, when a material that does not satisfy the conditions is used, the initial characteristics of the element are deteriorated.

The figure which shows the element structure of the double layer type triplet light emitting element in this invention. The figure which shows the energy level of HOMO-LUMO. Device energy gap diagram. The figure which shows the positional relationship of the emission spectrum of a host material, and the absorption spectrum of a hole transport material. The initial characteristic and emission spectrum of Example 1. The initial characteristic and emission spectrum of Example 2. The initial characteristic and emission spectrum of Example 3. The initial characteristic and emission spectrum of Example 4. The initial characteristic and emission spectrum of Comparative Example 1. The initial characteristic and emission spectrum of the comparative example 2. The initial characteristic and emission spectrum of the comparative example 3. The figure which shows the element structure of the conventional triplet light emitting element.

Claims (1)

Forming an anode on the substrate,
Forming a hole transport layer on the anode;
By co-evaporation of an electron transport material and a light emitting material exhibiting light emission from a triplet excited state, an electron transporting light emitting layer is formed on the hole transport layer,
A method for manufacturing a light-emitting element, comprising forming a cathode on the electron-transporting light-emitting layer.

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