JP2006108458A - Light emitting element and display unit - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide host materials so as to obtain a light emitting element and a display unit which can be both kept high in brightness for a long term. <P>SOLUTION: The light emitting element is equipped with an organic compound-containing layer pinched between a pair of electrodes and a light emitting layer composed of a host compound and a guest compound, wherein the host compound is represented by Formula (1). In Formula, R<SB>1</SB>to R<SB>8</SB>are separately selected from halogen and alkyl groups. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a light emitting device using an organic compound, and more particularly to an organic electroluminescence device (organic EL device) using a compound having a specific molecular structure as a host of a light emitting layer.

  Organic EL devices have been vigorously studied for application as light-emitting devices with high-speed response and high efficiency (Non-Patent Document 1). The basic configuration is shown in FIG.

  As shown in FIG. 1, the organic EL element is generally composed of a plurality of organic film layers sandwiched between a transparent electrode 14 on a transparent substrate 15 and a metal electrode 11.

  In FIG. 1A, the organic layer is composed of a light emitting layer 12 and a hole transport layer 13. As the transparent electrode 14, ITO or the like having a large work function is used, and good hole injection characteristics from the transparent electrode 14 to the hole transport layer 13 are given. As the metal electrode 11, a metal material having a small work function such as aluminum, magnesium, or an alloy using them is used to give a good electron injection property to the organic layer. A thickness of 50 to 200 nm is used for these electrodes.

For the light emitting layer 12, an aluminum quinolinol complex or the like having electron transport properties and light emitting characteristics (a typical example is Alq ₃ shown in Chemical Formula 1) is used. For the hole transport layer 13, a material having an electron donating property such as a biphenyldiamine derivative (a typical example is α-NPD shown in Chemical Formula 1) is used.

  The element having the above structure exhibits rectifying properties. When an electric field is applied so that the metal electrode 11 serves as a cathode and the transparent electrode 14 serves as an anode, electrons are injected from the metal electrode 11 into the light-emitting layer 12. Holes are injected. The injected holes and electrons emit light by recombination in the light emitting layer 12 due to recombination. At this time, the hole transport layer 13 serves as an electron blocking layer, and the recombination efficiency at the interface of the light emitting layer 12 / hole transport layer 13 is increased, and the light emission efficiency is increased.

  Furthermore, in FIG.1 (b), the electron carrying layer 16 is provided between the metal electrode 11 and the light emitting layer 12 of Fig.1 (a). Efficient light emission can be performed by separating light emission and electron / hole transport to achieve a more effective carrier blocking configuration. As the electron transport layer 16, for example, an oxadiazole derivative or the like can be used.

  Until now, the light emitted from the singlet exciton of the molecule at the emission center has been extracted from the light emission generally used in the organic EL element. On the other hand, an element that utilizes phosphorescence emission via a triplet exciton rather than using fluorescence emission via a singlet exciton has been studied (Non-patent Documents 2 and 3).

In these documents, the organic layer shown in FIG. 1C is mainly used in a four-layer structure. It consists of a hole transport layer 13, a light emitting layer 12, an exciton diffusion preventing layer 17 and an electron transport layer 16 from the anode side. The materials used are the carrier transport material and phosphorescent material shown in Chemical formula 1. Abbreviations for each material are as follows.
Alq _3: Aluminum - quinolinol complex α-NPD: N4, N4'- Di-naphthalen-1-yl-N4, N4'-diphenyl-biphenyl-4,4'-diamine
CBP: 4,4′-N, N′-dicarbazole-biphenyl
BCP: 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline
Ir (ppy) ₃ : Iridium-phenylpyridine complex

In both Non-Patent Document 2 and Non-Patent Document 3, high efficiency was obtained because the hole transport layer 13 was α-NPD, the electron transport layer 16 was Alq ₃ , the exciton diffusion prevention layer 17 was BCP, and the light emitting layer 12 was CBP. Is a phosphorescent material, PtOEP or Ir (ppy) ₃ , mixed at a concentration of about 6%.

  The reason why phosphorescent light-emitting materials are particularly attracting attention is that high luminous efficiency can be expected in principle. The reason is that excitons generated by carrier recombination are composed of singlet excitons and triplet excitons, and the probability is 1: 3. Until now, the organic EL element has taken out fluorescence upon transition from the singlet exciton to the ground state as luminescence, but in principle, the luminescence yield is 25% of the number of excitons generated. This was the theoretical upper limit. However, if phosphorescence from excitons generated from triplets is used, a yield of at least three times is expected in principle, and further, the intersystem crossing from triplets to singlets with high energy is performed. In light of the transition caused by the above, in principle, a light emission yield of 100%, which is 4 times, can be expected.

  In addition, there are Patent Documents 1 to 3 and Non-Patent Document 4 as documents using light emission from a triplet.

JP 11-329739 A Japanese Patent Laid-Open No. 11-256148 JP-A-8-319482 Macromol. Symp. 125,1-48 (1997) Improve energy transfer in electrophoretic device (DF O'Brien et al., Applied Physics Letters Vol 74, No3 p422 (1999)) Very high-efficiency green organic light-emitting devices basd on electrophoresis (MA Baldo et al., Applied Physics Letters Vol 75, No1p 4) Electroluminescence from triple exited states of benzophenone (Satoshi Hoshino et al., Applied Physics Letters Vol 69, No2 p224 (1986)).

  In an organic EL device in which the light-emitting layer is composed of two or more compounds of a host and a guest, crystallization of the host molecule, concentration quenching due to formation of an excited multimer between the host and guest, chromaticity change, and the like deteriorate the characteristics. It is thought to be causing.

  Moreover, in the organic EL element using phosphorescence emission, luminous efficiency and element stability are particularly problematic. Although the cause of light emission deterioration of phosphorescent light emitting devices is not clear, in general, the triplet life is three orders of magnitude longer than the singlet life, so the molecules are placed in a high energy state for a long time. It is thought that reactions, formation of excited multimers, changes in molecular microstructure, structural changes in surrounding materials, etc. may occur.

  As a host material used for an organic EL element, a highly stable compound is desired.

  In view of the above, an object of the present invention is to provide a novel host material, thereby providing a light emitting element and a display device that can emit light with high efficiency and maintain high luminance for a long period of time.

  In the light-emitting element of the present invention, a layer containing at least one organic compound is sandwiched between a pair of electrodes, and the light-emitting element includes at least a host compound and a guest compound. 1).

(R _{1 to} R ₈ are each independently a halogen atom, an alkyl group having 1 to 20 carbon atoms, wherein the carbon atom may be substituted by a silicon atom, or one or two or more non-adjacent methylene groups are -O-, -S- may be substituted, a hydrogen atom may be substituted by a halogen atom, and an ethylene group may be epoxidized}. It may combine to form a ring structure.)

  In addition, a display device of the present invention includes the above light emitting element.

  The light-emitting element of the present invention using the compound represented by the general formula (1) as a host of the light-emitting layer is an excellent element that maintains high luminance for a long period of time as well as high-efficiency light emission. In addition, the light-emitting element of the present invention is also excellent as a display element.

When the light emitting layer is composed of a carrier material having a carrier transport property and a guest, the main process leading to light emission includes the following several processes.
1. 1. Transport of electrons and holes in the light emitting layer 2. Host exciton generation 3. Excitation energy transfer between host molecules Excitation energy transfer from host to guest

  Desired energy transfer and light emission in each process occur in competition with various deactivation processes.

  Needless to say, in order to increase the luminous efficiency of the EL element, the emission quantum yield of the emission center material itself is large. However, how to efficiently transfer energy between the host and the host or between the host and the guest is also a big problem. Further, although the cause of light emission deterioration due to energization is not clear at present, it is assumed that it is related to the environmental change of the light emitting material due to at least the luminescent center material itself or its peripheral molecules.

  Therefore, the present inventors have made various studies and found that an element using the compound represented by the general formula (1) as a host of the light emitting layer emits high efficiency, maintains high luminance for a long period of time, and has a small deterioration in energization. I found it.

  As the light emitting material, a compound having a structure having π electrons is often used. Since the host of the present invention does not have π electrons in the molecule, it is considered that the intermolecular interaction between the host and guest can be weakened and the generation of exciplex between the host and guest can be suppressed. During light emission, the guest is directly excited, suppressing "2. Exciton generation of host", "3. Excitation energy transfer between host molecules", and "4. Excitation energy transfer from host to guest". It is thought that traps in the host will be difficult.

  The light-emitting element of the present invention has a light-emitting layer containing a host compound represented by the following general formula (1), and the layer containing the light-emitting layer is sandwiched between two opposing electrodes as shown in FIG. An electroluminescent element that emits light by applying a voltage between the electrodes is preferable.

(R _{1 to} R ₈ are each independently a halogen atom, an alkyl group having 1 to 20 carbon atoms, wherein the carbon atom may be substituted by a silicon atom, or one or two or more non-adjacent methylene groups are -O-, -S- may be substituted, a hydrogen atom may be substituted by a halogen atom, and an ethylene group may be epoxidized}. It may combine to form a ring structure.)

  Here, the alkyl group in the general formula (1) may be linear, branched, cyclic, or a combination thereof.

  The characteristics of the organic EL material greatly change due to crystallization and melting of the material, but the host compound used in the present invention is not involved in exciton or carrier transmission, regardless of the glass transition temperature of the host compound, It is considered that stable light emission can be obtained. Actually, high stability was shown in the current test when the host compound of the present invention was used.

  Specific examples of the host compound represented by the general formula (1) are shown below. However, the present invention is not limited to these.

  In general, fluorescent light-emitting materials easily cause concentration quenching because excitation energy moves due to the Ferrester transition, and are used at a low concentration of about 0.1 to 1%. In contrast, phosphorescent materials can be highly doped at 3 to 20% because the excitation energy moves at the Dexter transition. The guest compound used in the present invention may be any of phosphorescent materials such as benzophenone and Ir (ppy) 3, and fluorescent materials such as Alq3, DCM2 and coumarin-6. Since it does not have carrier conductivity, the carrier is considered to flow directly through the host molecule, and a phosphorescent material that can be more highly doped with the guest compound is preferable. More preferably, organometallic coordination compounds such as Ir complexes, Pt complexes, Re complexes, Cu complexes, Eu complexes, and Rh complexes that are known to emit strong phosphorescence are preferred.

  Further, when T1 of the host compound> T1 of the guest compound, the quenching of the guest does not occur and it is preferable in terms of light emission efficiency.

  Specific examples of the guest compound used in the present invention are shown below. However, the present invention is not limited to these.

  The dope amount of the guest compound is not particularly limited, but is preferably 0.1 to 50 wt%, and more preferably 1 to 20 wt%.

  The highly efficient light-emitting element of the present invention can be applied to products that require energy saving and high luminance. Application examples include light sources for display devices / illuminators and printers, backlights for liquid crystal display devices, and the like. As a display device, energy-saving, high visibility and lightweight flat panel display can be realized. In addition, as a light source of a printer, a laser light source unit of a laser beam printer that is currently widely used can be replaced with the light emitting element of the present invention. For example, an independently addressable element is arranged on an array, and a photosensitive drum An image is formed by performing desired exposure. Thereby, a device volume can be reduced significantly. With respect to the lighting device and the backlight, the energy saving effect according to the present invention can be expected. In application to a display, a method of driving using a TFT driving circuit which is an active matrix method can be considered.

  The present invention will be specifically described below with reference to examples. However, the present invention is not limited to these.

<Example 1>
In this example, an element having three organic layers as shown in FIG. 1B was used as the element structure.

100 nm ITO (transparent electrode 14) was patterned on a glass substrate (transparent substrate 15). On the ITO substrate, the following organic layer and electrode layer were continuously formed by resistance heating vacuum deposition in a vacuum chamber of 10 ⁻⁴ Pa so that the opposing electrode area was 3 mm ² .
Hole transport layer 13 (40 nm): α-NPD (manufactured by Dojin Chemical Laboratory)
Light emitting layer 12 (10 nm): [Host] Exemplified Compound No. 4 (octacyclopentyl-POSS (registered trademark), manufactured by Aldrich), [guest (5% by weight)] Ir (ppy) ₃
Electron transport layer 16 (50 nm): Bphen (manufactured by Dojin Chemical Laboratory)
Metal electrode layer (1 nm): KF
Metal electrode layer (100 nm): Al

  The current-voltage characteristics of the obtained EL element were measured with a microammeter “4140B” (manufactured by Hured Packard), and the luminance was measured with a luminance meter “BM7” (manufactured by Topcon).

As a result, the device of this example showed good rectification. Further, when a voltage of 8 V was applied, light emission of 350 cd / m ² was confirmed, and stable light emission was obtained even when energized continuously for 100 hours.

<Examples 2 and 3>
A device was prepared in the same manner as in Example 1 except that the guest compound of the light emitting layer was changed to that shown in Table 2.

The PtOEP used in Example 2 was purchased from Pure Appl. Chem. Vol. 71 no. 11 2095-2106 (1999). Further, Re (CO) ₃ (Phen) Cl used in Example 3 was synthesized with reference to Journal of American Chemical Society 101 2888-2897 (1979).

  The obtained EL device was evaluated in the same manner as in Example 1. All the elements showed good rectification. The light emission luminance is as shown in Table 2. Stable light emission was obtained even when energized continuously for 100 hours.

<Example 4>
A solution obtained by dissolving 0.2 wt% of PVK in chlorobenzene was applied by spin coating to form a hole transport layer 13 having a film thickness of 26 nm, on which chloroform: [host] Exemplified Compound No. 6 (Octamethyl-POSS (registered trademark), manufactured by Aldrich): [Guest] Cu (dtbp) (dmp) PF ₆ = 100: 1: 0.1 was applied by spin coating to emit light having a thickness of 10 nm. A device was prepared in the same manner as in Example 1 except that the layer 12 was formed.

Cu (dtbp) (dmp) PF ₆ was synthesized with reference to Journal of American Chemical Society 121 4292-4293 (1999).

  The obtained EL device was evaluated in the same manner as in Example 1. All the elements showed good rectification. The light emission luminance is as shown in Table 2. Stable light emission was obtained even when energized continuously for 100 hours.

<Examples 5 and 6>
A device was prepared in the same manner as in Example 1 except that the guest compound of the light emitting layer was changed to that shown in Table 2.

Eu (DBM) ₃ (Phen) used in Example 5 is the same as Pure Appl. Chem. Vol. 71 no. 11 2095-2106 (1999).

  The obtained EL device was evaluated in the same manner as in Example 1. All the elements showed good rectification. The light emission luminance is as shown in Table 2. Stable light emission was obtained even when energized continuously for 100 hours.

It is a figure which shows an example of the light emitting element of this invention.

Claims (6)

In a light-emitting element in which at least one layer containing an organic compound is sandwiched between a pair of electrodes, and the light-emitting layer includes at least a host compound and a guest compound, the host compound is represented by the following general formula (1). A light emitting device characterized.

(R _{1 to} R ₈ are each independently a halogen atom, an alkyl group having 1 to 20 carbon atoms, wherein the carbon atom may be substituted by a silicon atom, or one or two or more non-adjacent methylene groups are -O-, -S- may be substituted, a hydrogen atom may be substituted by a halogen atom, and an ethylene group may be epoxidized}. It may combine to form a ring structure.)   The light emitting device according to claim 1, wherein light emission from the guest compound is phosphorescence light emission and / or fluorescence light emission.   3. The light-emitting element according to claim 1, wherein a T1 level of the host compound is higher than T1 of the guest compound.   The light-emitting element according to claim 1, wherein the guest compound is an organometallic coordination compound.   5. The electroluminescent device according to claim 1, wherein the organic compound-containing layer is sandwiched between two opposing electrodes, and emits light when a voltage is applied between the electrodes. Light emitting element.   A display device comprising the light-emitting element according to claim 1.

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