JP2002373786A - Light-emitting device and display device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a light-emitting device having high initial brightness and keeping low deterioration in brightness. SOLUTION: This light-emitting device has at least one organic compound layer, and has at least one layer of an organic compound in which the content of impurities produced by cross-coupling reaction is 0.5 muss the or less.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a light emitting device using an organic compound, and more particularly, to a light emitting device having a stable efficiency by reducing impurities which can be generated by a cross coupling reaction in an organic compound layer. The present invention relates to a high organic electroluminescence element (organic EL element).

[0002]

2. Description of the Related Art Organic EL devices are being intensively studied for application as light-emitting devices having high response speed and high efficiency. The basic configuration is shown in FIGS. 1A and 1B [for example, Macromol. Symp. 125, 1 to 48 (19
97)].

As shown in FIG. 1, an organic EL device generally comprises a plurality of organic film layers on a transparent substrate 15 between a transparent electrode 14 and a metal electrode 11.

In FIG. 1A, the organic layer includes a light emitting layer 12 and a hole transport layer 13. The transparent electrode 14 is made of ITO or the like having a large work function, and has good hole injection characteristics from the transparent electrode 14 to the hole transport layer 13. 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 provide good electron injection into the organic layer. These electrodes have a thickness of 50 to 200 nm.

For the light emitting layer 12, an aluminum quinolinol complex having an electron transporting property and a light emitting property (a typical example is Alq3 shown in Chemical formula 1) is used. In addition, the hole transport layer 13
For example, 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-described structure exhibits rectifying properties. When an electric field is applied such 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 and the transparent electrode Holes are injected from 15.

The injected holes and electrons recombine in the light emitting layer 12 to generate excitons and emit light. At this time, the hole transport layer 13 functions as an electron blocking layer, and the recombination efficiency at the interface between the light emitting layer 12 and the hole transport layer 13 increases, and the luminous efficiency increases.

Further, in FIG. 1B, an electron transport layer 16 is provided between the metal electrode 11 and the light emitting layer 12 of FIG. 1A. Efficient light emission can be achieved by separating light emission from electron / hole transport to form a more effective carrier blocking structure. As the electron transport layer 16, for example, an oxadiazole derivative or the like can be used.

Heretofore, in light emission generally used in an organic EL device, fluorescence at the time of transition to a ground state from a singlet exciton of a molecule at the emission center has been extracted. On the other hand, devices that utilize phosphorescence via triplet excitons instead of using fluorescence via singlet excitons have been studied. A representative document that has been published is Reference 1: Improved energy transfer.
r in electrophosphorescent
t device (DFO'Brien et al., App.
led Physics Letters Vol
74, No3 p422 (1999)), Reference 2: Ve
ry high-efficiencygreen o
rganic light-emitting dev
icesbasd on electrophosph
oresence (MA Baldo et al., Appl.
ied Physics Letters Vol 7
5, No1 p4 (1999)).

In these documents, a four-layer structure of an organic layer shown in FIG. 1C is mainly used. It consists of a hole transport layer 13, a light emitting layer 12, an exciton diffusion preventing layer 1 from the anode side.
7. The electron transport layer 16 is formed. The materials used are
A carrier transporting material shown in Chemical formula 1 and a phosphorescent material. Abbreviations of each material are as follows. Alq3: aluminum-quinolinol complex α-NPD: N4, N4′-Di-naphthal
n-1-yl-N4, N4'-diphenyl-bi
phenyl-4,4'-diamine CBP: 4,4'-N, N'-dicarbazole
-Biphenyl BCP: 2,9-dimethyl-4,7-diph
enyl-1,10-phenanthroline PtOEP: platinum-octaethylporphyrin complex Ir (ppy) ₃ : iridium-phenylpyrimidine complex

[0011]

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The high efficiency was obtained in both References 1 and 2 because the hole transport layer 13 was α-NPD and the electron transport layer 16 was Alq.
3. BCP for the exciton diffusion preventing layer 17 and CB for the light emitting layer 12
It is composed of P as a host material mixed with PtOEP or Ir (ppy) ₃ which is a phosphorescent material 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 1
Consisting of singlet and triplet excitons, the probability of which is 1: 3
It is. Until now, the organic EL element has taken out the fluorescence when transitioning from the singlet exciton to the ground state as light emission. In principle, the emission yield is 25% of the number of excitons generated. And this was the upper limit in principle. However, if phosphorescence from an exciton generated from a triplet is used, at least a triple yield is expected in principle, and furthermore, the intersystem crossing from a singlet to a triplet, which is higher in energy, is expected. In principle, the transfer by 10 times
A luminescence yield of 0% can be expected.

Other documents that require light emission from a triplet include Japanese Patent Application Laid-Open No. H11-329739 (organic EL device and its manufacturing method) and Japanese Patent Application Laid-Open No. H11-256148 (light-emitting material and a light-emitting material using the same). Organic EL element) and JP-A-8-319482 (organic electroluminescent element).

[0015]

Even at present, the luminance degradation of a light-emitting element (organic EL element) using a layer containing an organic compound due to long-term energization is a serious problem, and further improvement is required. In particular, organic EL using phosphorescence
While high luminous efficiency is expected in the device, deterioration of energization is a serious problem.

Accordingly, an object of the present invention is to provide a light emitting element and a display device having a high initial luminance and little luminance deterioration due to durability.

[0017]

Means for Solving the Problems The present inventors have conducted intensive studies to solve the above-mentioned problems, and as a result, impurities derived from the cross-coupling reaction have a great effect on the initial luminance of the organic EL element and the decrease in luminance due to durability. This led to the completion of the present invention.

That is, in the light emitting device of the present invention, in a light emitting device having at least one organic compound layer, the content of impurities which can be generated by a cross coupling reaction is 0.5%.
It is characterized by having at least one organic compound layer in an amount of not more than 0.3% by mass, preferably not more than 0.3% by mass, more preferably not more than 0.1% by mass.

In the light-emitting device of the present invention, the organic compound layer containing the impurity comprises a light-emitting layer, a hole transport layer,
An electron transport layer, a hole injection layer, or an exciton diffusion preventing layer is preferable.

Preferably, the light emitting element is a light emitting element using phosphorescence.

It is preferable that the organic compound layer is an electroluminescent element sandwiched between a pair of electrodes and emits light by applying a voltage between the electrodes.

Further, the display device of the present invention is characterized in that the above-mentioned light-emitting element is provided as a display element.

[0023]

BEST MODE FOR CARRYING OUT THE INVENTION The light emitting device of the present invention has at least one organic compound layer, preferably an organic compound layer sandwiched between a pair of electrodes, and emits light by applying a voltage between the electrodes. This is an electroluminescent element. The layer configuration of the light emitting element is not particularly limited, and examples include a configuration as shown in FIG.

In the light-emitting device of the present invention, the content of impurities that can be generated by the cross-coupling reaction is 0.5% by mass or less, preferably 0.3% by mass or less, more preferably 0.1% by mass. It has at least one organic compound layer described below. If the content of impurities is 0.5% by mass or less,
In the case of a light-emitting element using phosphorescence, which has excellent durability, it also has excellent initial characteristics.

Here, the cross-coupling reaction in organic synthesis is a reaction that forms a covalent bond between different reactive species, and is distinguished from a homo-coupling reaction that generates a covalent bond between the same reactive species. Among the cross-coupling reactions, transition metal catalysts (Pd, Ni, Cu, Fe
Cross-coupling reaction of an organometallic compound and an organic halide using a Heck reaction, a Suzuki-coupling reaction using an organic boron compound, an amination reaction using a Pd catalyst, and a diaryl ether synthesis using a Pd catalyst Various studies have been made on reactions and the like, and this has become a promising synthetic technique for various organic compounds. These methods are also important synthesis methods for materials for organic EL devices, and examples of synthesis of known materials include the following.

[0026]

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[0027]

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[0028]

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The impurities other than the target product produced by the cross-coupling reaction include a homocoupling compound bonded between the same reactive species, a material in which a halogen atom or a metal atom of a raw material is replaced with hydrogen, and a transition material used in the reaction. Examples thereof include an organic compound containing a metal derived from a metal catalyst and an organic metal compound.

The organic compound layer containing impurities which can be generated by the cross coupling reaction may be any of a light emitting layer, a hole transport layer, an electron transport layer, a hole injection layer, and an exciton diffusion preventing layer. May be plural.

The highly efficient and highly durable light emitting device of the present invention comprises:
It can be applied to products that require energy saving and high brightness. Examples of applications include light sources for display devices and lighting devices, printers, and backlights for liquid crystal display devices.
As a display device, an energy-saving, high-visibility, light-weight flat panel display becomes possible. Further, as a light source of the printer, a laser light source unit of a laser beam printer widely used at present can be replaced with the light emitting element of the present invention. An image can be formed by arranging independently addressable elements on the array and performing desired exposure on the photosensitive drum. By using the element of the present invention, the volume of the device can be significantly reduced. For lighting devices and backlights, the energy saving effect of the present invention can be expected.

[0032]

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

<Embodiment 1> Tetrahedr shown below:
on Lett. , 1998, 39, 2367-237.
0. Α-NPD was synthesized by the method described above.

[0034]

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At the stage of the crude product, it was confirmed by high performance liquid chromatography that 2.3% by mass of biphenyl which was an impurity derived from the cross-coupling reaction was contained.
By repeating silica gel column chromatography and recrystallization, purified α-NPD in which biphenyl was not detected by high performance liquid chromatography was obtained (detection limit: 0.05% by mass). Using this α-NPD, a device was prepared in the following steps.

This device is a device having two organic layers as shown in FIG. 100 on a glass substrate (transparent substrate 15)
The ITO (transparent electrode 14) of nm was patterned so that the area of the opposing electrode was 3 mm ² . IT
The following organic layer and electrode layer were vacuum-deposited on an O substrate by resistance heating in a vacuum chamber of 10 ^-4 Pa to form a continuous film. Organic layer 1 (hole transport layer 13) (40 nm): purified α-
NPD Organic layer 2 (light emitting layer 12) (30 nm): Alq3 Metal electrode layer 1 (15 nm): AlLi alloy (Li content 1.8% by weight) Metal electrode layer 2 (100 nm): Al

An electric field was applied with the ITO side as the anode and the Al side as the cathode, and a voltage was applied so that the current value was the same for each element, and the time change of the luminance was measured. The constant current amount was 70 mA / cm ² . Since oxygen and water are problems as a cause of device deterioration, the above measurement was performed in a dry nitrogen flow after removing the device from the vacuum chamber in order to eliminate the cause. Table 1 shows the results.

<Examples 2 to 4, Comparative Examples 1 and 2>
In the organic layer 1, a device was prepared under exactly the same conditions except that biphenyl was co-evaporated at a ratio shown in Table 1 with respect to the purified α-NPD, and the time change of luminance was measured. Table 1 shows the results.

[0039]

[Table 1]

From Table 1, it can be seen that when the biphenyl content in α-NPD is 0.5% by mass or less, the durability performance of the device is particularly improved. It was found to improve.

<Embodiment 5> Tetrahedr shown below:
on Lett. , 1998, 39, 2367-237.
0. CBP was synthesized by the method described above.

[0042]

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At the stage of the crude product, it was confirmed by high performance liquid chromatography that 3.1% by mass of biphenyl which was an impurity derived from the cross-coupling reaction was contained.
By repeating silica gel column chromatography and recrystallization, purified CBP in which biphenyl was not detected by high performance liquid chromatography was obtained (detection limit: 0.05% by mass). Using this CBP, a device was prepared in the following steps.

This device is a device having three organic layers as shown in FIG. 100 on a glass substrate (transparent substrate 15)
The ITO (transparent electrode 14) of nm was patterned so that the area of the opposing electrode was 3 mm ² . IT
The following organic layer and electrode layer were vacuum-deposited on an O substrate by resistance heating in a vacuum chamber of 10 ^-4 Pa to form a continuous film. Organic layer 1 (hole transport layer 13) (40 nm): Example 1
Α-NPD organic layer 2 (light-emitting layer 12) (30 nm): purified CBP: I
r (ppy) ₃ (5% by weight based on purified CBP) Organic layer 3 (electron transport layer 16) (30 nm): Alq3 Metal electrode layer 1 (15 nm): AlLi alloy (Li content 1.8% by weight) Metal electrode Layer 2 (100 nm): Al

The time change of the luminance was measured in the same manner as in Example 1. Table 2 shows the results.

<Examples 6 to 8, Comparative Examples 3 and 4>
A device was prepared under exactly the same conditions except that biphenyl was co-evaporated at a ratio shown in Table 2 with respect to the purified CBP in the organic layer 2 of Example 2, and the time change of luminance was measured. Table 2 shows the results.

[0047]

[Table 2]

As can be seen from Table 2, the durability of the device is particularly improved when the biphenyl content in the CBP is 0.5% by mass or less, and further improved as the content becomes 0.3% by mass and 0.1% by mass. It turns out.

Further, when the initial luminances of the devices of Example 8 and Comparative Example 3 were compared, the device of Example 8 was clearly higher.

From the above results, light emission in which impurities which can be generated by the cross-coupling reaction is reduced to 0.5% by mass or less, preferably 0.3% by mass or less, more preferably 0.1% by mass or less. It has been found that the device has a small reduction in luminance due to energization and is highly efficient.

[0051]

As described above, according to the present invention, it is possible to obtain a light-emitting element and a display device which emit light with high efficiency, have excellent durability, and maintain high luminance for a long time. Further, in the case of a phosphorescent device, the initial characteristics are also improved.

[Brief description of the drawings]

FIG. 1 is a diagram illustrating an example of a light emitting element of the present invention.

[Explanation of symbols]

 REFERENCE SIGNS LIST 11 metal electrode 12 light emitting layer 13 hole transport layer 14 transparent electrode 15 transparent substrate 16 electron transport layer 17 exciton diffusion prevention layer

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Shinjiro Okada 3-30-2 Shimomaruko, Ota-ku, Tokyo Inside Canon Inc. (72) Inventor Satoshi Miura 3-30-2 Shimomaruko, Ota-ku, Tokyo Canon Inside (72) Inventor Takashi Moriyama 3-30-2 Shimomaruko, Ota-ku, Tokyo Inside Canon Inc. (72) Inventor Jun Kamiya 3-30-2 Shimomaruko, Ota-ku, Tokyo Inside Canon Inc. ( 72) Inventor Manabu Furugo 3-30-2 Shimomaruko, Ota-ku, Tokyo F-term in Canon Inc. (reference) 3K007 AB02 AB03 AB11 CA01 CB01 DA01 DB03 EB00

Claims (11)

[Claims] 1. A light-emitting element having at least one organic compound layer, wherein the light-emitting element has at least one organic compound layer in which the content of impurities that can be generated by a cross-coupling reaction is 0.5% by mass or less. element. 2. The light emitting device according to claim 1, wherein the content of the impurity is 0.3% by mass or less. 3. The light emitting device according to claim 1, wherein the content of the impurity is 0.1% by mass or less. 4. The light emitting device according to claim 1, wherein the organic compound layer containing impurities is a light emitting layer. 5. The light emitting device according to claim 1, wherein the organic compound layer containing impurities is a hole transport layer. 6. The light emitting device according to claim 1, wherein the organic compound layer containing impurities is an electron transport layer. 7. The light emitting device according to claim 1, wherein the organic compound layer containing impurities is a hole injection layer. 8. The light emitting device according to claim 1, wherein the organic compound layer containing impurities is an exciton diffusion preventing layer. 9. The light emitting device according to claim 1, wherein said light emitting device is a light emitting device using phosphorescence. 10. The electroluminescent device according to claim 1, wherein the organic compound layer is sandwiched between a pair of electrodes, and emits light by applying a voltage between the electrodes. The light-emitting element according to any one of the preceding claims. 11. A display device comprising the light-emitting element according to claim 1 as a display element.