JPH02261889A - Organic electroluminescent element - Google Patents

Abstract

PURPOSE:To obtain the subject element having high luminous intensity and luminous efficiency and capable of controlling the emission wavelength by providing a luminescent layer comprising a specified thin film of organic dyes between electrodes at least one of which transmits light. CONSTITUTION:Between two electrodes at least one of which transmits light is provided a luminescent layer which comprises a thin film of organic dyes, made of a dispersion formed by mixing a first organic dye (e.g. anthracene) with a second organic dye having the light absorption edge on the side of wavelength longer than the light absorption edge of the first organic dye (e.g. perylene, tetracene or pentacene) in an amount of 10mol% or less based on the first organic dye.

Description

DETAILED DESCRIPTION OF THE INVENTION [Object of the Invention] (Industrial Application Field) The present invention relates to an organic electroluminescent device used as a display device, a lighting device, etc.

(Prior art) In recent years, with the increase in demand for portable TVs and computers,
Development of thin and lightweight display elements, mainly flat panel displays, is progressing rapidly. Currently, the mainstream is a liquid crystal display element, but the liquid crystal display element has drawbacks such as being unsightly depending on the viewing angle as the screen becomes larger.

For this reason, there is a demand for the development of light-emitting display elements that can be expected to have excellent display functions, such as vivid colors, elegant video display, and the ability to display images even in dark places. As such light emitting display elements, plasma displays, inorganic electroluminescent elements, fluorescent display tubes, light emitting diodes, and the like are being studied. In order to realize a full color display with these elements, high brightness RGB light emission is required. However, at present, it is difficult for any of these devices to emit blue light, and a full-color display has not been realized.

Incidentally, many organic dye molecules emit fluorescence or phosphorescence in the blue region (wavelength near 4B0 ni) during their photoluminescence. From this, an organic electroluminescent device having a structure in which a light-emitting layer made of a thin organic dye film is provided between two electrodes has a high possibility of realizing a full-color display device, and is highly anticipated. However, organic electroluminescent devices have a problem in that their luminance is so low that they cannot be recognized with the naked eye.

Therefore, in order to improve the brightness of organic electroluminescent devices, the basic structure of the device is an organic dye thin film containing organic dyes or a multilayer stacked structure of organic dye thin films. Organic electroluminescent devices having structures in which these are combined in various forms have been proposed (JP-A No. 11-43G84, JP-A-61-44974, JP-A-61-44978, JP-A-81) -44981,
JP-A No. 81-44988, etc.).

In addition, it has been reported that an organic electroluminescent device with a structure in which a hole transfer layer is provided between the positive electrode and the light-emitting layer can emit high-intensity light with a low-voltage DC power source (Appl, P.
hys, Lett, 51.21 (1987), JP-A-63-49450, JP-A-83-284692,
JP-A-63-295895).

In addition, Shogo Saito and his colleagues at Bonyo University have found that an organic electroluminescent device has a structure in which a hole transfer layer is provided between the positive electrode and the light emitting layer, and an electron transfer layer is provided between the negative electrode and the light emitting layer. reported that the brightness was further improved (Jl, A
ppl, PhyS,, Alua 75 (198B), 27. L289 (198B)). RGB light emission can be obtained by using, for example, three types of dyes, anthracene (B), coronene (G), and perylene (R), as the dyes constituting the light emitting layer.

However, according to Saito et al., for example, organic dye molecules emit light efficiently due to photoexcitation only when the dye concentration is dilute, such as in a gas or solution, and it is often difficult to emit light in a solid aggregated state. He states that this is one of the reasons why it is difficult to observe light emission in organic electroluminescent devices.

From this point of view, 1. Saito et al. studied the luminescence behavior of various solid-state organic dyes upon photoexcitation and discovered phthaloperinone as an organic dye that emits light strongly even in a solid aggregated state.In organic electroluminescent devices using this in the light-emitting layer, They reported that strong light emission was obtained by applying a DC electric field. (J, J, Appl, Phys,
, 27. L713 (198g)).

On the other hand, organic electroluminescent devices have other features as described below.
There are two problems. That is, carriers are injected into the emissive layer to excite the dye molecules, and the excited dye molecules dimerize or multimerize, and such dimers or multimers emit light (called excimer emission or exciton emission). ) occurs. When dye molecules in an excited state become dimers or multimers, they become stable, and the emission wavelength shifts to a longer wavelength side than the emission wavelength from an isolated dye molecule in an excited state. Therefore, the emission wavelength of the organic electroluminescent device is 460
Even if the material of the light emitting layer is designed to exist in the blue region of rv, the actual emission wavelength may be green or red on the longer wavelength side.

(Problems to be Solved by the Invention) As described above, in an organic electroluminescent device, by providing a carrier transfer layer between a light emitting layer and an electrode, it is possible to obtain high luminance light emission with a low voltage DC power supply. It has been found that there is a However, when the organic dye molecules are in a solid aggregated state, there is a problem that light emission is difficult to occur. Further, even if light emission occurs, the light emission is mainly from dimerized or multimerized excited dye molecules, and there is a problem in that the emission wavelength shifts to the longer wavelength side.

An object of the present invention is to solve these problems and provide an organic electroluminescent device that has high luminance and can control the wavelength of the emitted light.

[Structure of the Invention] (Means and Effects for Solving the Problems) The organic electroluminescent device of the present invention is an organic electroluminescent device in which a light-emitting layer made of a thin organic dye film is provided between two electrodes, at least one of which transmits light. In the electroluminescent device, the light emitting layer includes a first
A second organic dye having a light absorption edge on the longer wavelength side than the light absorption edge of the first organic dye is added to the second organic dye.
It is characterized by comprising a thin organic dye film in which the organic dye is dispersed in a proportion of 10 mol % or less.

In the present invention, the characteristics required of the first organic dye are that holes or electrons are efficiently injected as carriers from the electrode, that the injected carriers are efficiently recombined with dye molecules, and that the carriers are The dye molecules are efficiently excited by recombination, and there is little non-radiative deactivation process from the excited state. In addition, it is easy to form a thin film and has excellent structural and chemical stability.

In the present invention, the characteristics required of the second organic dye include that it can efficiently receive and take excitation energy from the first organic dye in an excited state (high energy acceptance), and that it can efficiently emit light at a specific wavelength. There are many things that can be done.

Here, the excited state of the first organic dye has two states: a singlet state and a triplet state. Among these, it is known that in organic electroluminescent devices, it is fluorescence from excitation that mainly contributes to light emission.

Therefore, as the second organic dye, one is selected that is likely to cause singlet excitation energy transfer. The criterion for selection is the existence of an overlap between the fluorescence emission spectrum of the first organic dye and the optical absorption spectrum of the second organic dye. - For boat fishing, it is sufficient that the absorption edge wavelength of the light absorption spectrum of the second organic dye is on the longer wavelength side than the absorption edge wavelength of the light absorption spectrum of the first organic dye.

Furthermore, in organic electroluminescent devices, no contribution of phosphorescence, which is light emission from the triplet state, which is another excited state, is observed at room temperature. This is because many organic dyes suitable as the first organic dye do not exhibit phosphorescence at room temperature (although these dyes also exhibit phosphorescence at low temperatures). Therefore, it is possible to select a second organic dye that has the property of receiving and absorbing excitation energy from the excited triplet state of the first organic dye to become an excited state and emitting fluorescence or phosphorescence at room temperature.

In the present invention, the number of second organic dyes dispersed in the first organic dye is not limited to one type, but may be two or more types. for example,
The first organic dye contains a second organic dye that receives and takes excitation energy from the excited-principal state of the first organic dye, and an organic dye that receives excitation energy from the excited triplet state of the first organic dye. By dispersing the organic dye, it becomes possible to emit light efficiently. Further, by dispersing a plurality of dyes as the second organic dye in the first organic dye, multi-wavelength light emitting characteristics can be obtained, and white light emission can be obtained with high efficiency by adjusting the RGB intensities.

As shown in Table 1, the above-mentioned first and second organic dyes include (a) a condensed polycyclic aromatic dye consisting of only C and H elements, and (b) a compound other than C and H elements. , fused polycyclic aromatic dyes containing heteroatoms such as O, N, and S in their skeletons, and (C) fluorescent dyes developed for dye lasers.

benzene naphthalene anthracene phenanthrene pyrene crescent triphenylene perylene pentacene hexacene carbazole phenazine C, laser dye acridine coumarin derivative In the present invention, the second organic dye is dispersed in the first organic dye at a ratio of 10 mol % or less.

The organic electroluminescent device of the present invention may have any structure other than the light emitting layer. For example, a structure may be used in which a hole transfer layer is provided between the positive electrode and the light emitting layer, or a structure in which an electron transfer layer is further provided between the negative electrode and the light emitting layer.

Hereinafter, the organic electroluminescent device of the present invention will be explained in more detail.

The light emitting mechanism of an organic electroluminescent device can be divided into two stages. In the first stage, carriers are injected into the luminescent layer by applying a voltage to the electrodes, and the carriers recombine to bring the luminescent dye into an excited state. The second stage is a stage in which the excited state of the luminescent dye returns to the ground state. Second
The stages include a luminescent process and a non-luminescent process. this house,
The rate of light emission from the excited-isotropic state is on the order of 109 s-1 and is called fluorescence. In addition, the emission rate from the excited triad state is on the order of 103~1O° sec-1,
called phosphorescence. The non-luminescent process is due to the thermal movement of molecules, and at room temperature both singlet and triplet molecules are 107 to 108.
It is on the order of seconds-1. Therefore, although fluorescence is often observed at room temperature, phosphorescence is usually not observed.

By the way, when organic dyes are aggregated, such as in solid crystals, the excited organic dyes become excitons.
It is believed that energy can be transferred within a certain range during the lifetime of the excited state. The energy transfer range is generally 103 to 5 molecules. If non-emissive sites exist in this range due to impurities or lattice defects, organic dye molecules in an excited state will be trapped and deactivated in a non-emissive state.

As reported by Saito et al., this is the reason why fluorescence is no longer observed in a solid aggregated state, even if the dye exhibits fluorescence when the dye concentration is dilute, such as in a gas or solution.

In addition, in a solid aggregated state, molecules in an excited state multimerize with adjacent molecules (generally form dimers (excimers)).
It is known that it becomes energetically stable. This is a type of luminescent trap that involves energy transfer. As described above, dye molecules in an excited state become stable when dimerized or multimerized, and the emission wavelength shifts to a longer wavelength side than the emission wavelength from an isolated excited state dye molecule.

To summarize the above, (1) the light emission process from the excited triplet state (phosphorescence) is difficult to occur at room temperature, so the theoretical light emission efficiency decreases. ■In the process of excitation energy transfer, 103~
If there is a non-luminescent site even at a ratio of 1 in every 10' molecules, no luminescence will be observed.

■When molecules in an excited state multimerize and become stable, the emission wavelength shifts to longer wavelengths. These factors have made it difficult to realize an organic electroluminescent device.

In contrast, in the present invention, by dispersing the second organic dye in the first organic dye, these problems can be solved and luminous efficiency can be improved.

In other words, regarding (■), there are organic dyes whose phosphorescence can be observed in iDI even at room temperature, and by using this as the second organic dye, the energy of the excited triplet state of the first organic dye can be efficiently utilized. I can do it. Examples of such organic dyes include those having a carbonyl group, those in which hydrogen is replaced with deuterium, and those containing heavy elements such as halogen. All of these substituents have the effect of increasing the phosphorescence rate and decreasing the non-luminescence rate.

However, it is not appropriate to add such an organic dye at a high concentration because it causes deactivation of the excitation -1 term.

Regarding (2), by dispersing the second organic dye at a higher concentration than the non-emissive site, energy from the first organic dye in the excited state, especially the excited-multiplet state, is prevented from moving to the non-emissive site. However, energy can be transferred to the second organic dye to efficiently emit light.

The same applies to (2), and before the first organic dye in the excited state multimerizes and becomes stable, energy can be transferred to the second organic dye to efficiently emit light.

However, if the proportion of the second organic dye increases, problems ① and ③ will occur with the second organic dye itself, so it is necessary to keep this concentration to an appropriate level and keep the second organic dye in an isolated state. .

In the present invention, the ratio of the second organic dye (B) to the first organic dye (A) is 10 mol% or less, that is, B/
The reason for setting (A+B)≦0.1 is as follows.

That is, if the second organic dye is dispersed in the first organic dye and the second organic dye is excited by receiving energy from the first organic dye in an excited state as described above, It is believed that light emission from the second organic dye in an isolated excited state is obtained. According to experiments conducted by the present inventors, when the ratio of the second organic dye to the first organic dye exceeds 10 mol%, the probability that the excited second organic dye dimerizes or multimerizes increases. In this case, the emission wavelength shifts to a longer wavelength side than the emission from the isolated second organic dye. More preferably, the ratio of the second organic dye to the first organic dye is in the range of 0.1 to 1 mol%.

Such an organic electroluminescent device of the present invention has high luminous efficiency and can obtain emission wavelength characteristics from the second organic dye in an isolated excited state, making it easy to design the emission color of the device.

(Example) The present invention will be explained in detail below.

FIG. 1 shows a configuration diagram of an organic electroluminescent device according to the present invention. In FIG. 1, an ITO electrode 2 is placed above the glass substrate 1.
, a hole transfer layer (TPD) 3, a light emitting layer 4 consisting of anthracene as the first organic dye and perylene, tetracene, or pentacene as the second organic dye, an electron transfer layer (TPD)
V) 5 and Al electrode 6 are formed in sequence. Also,
A DC power supply 7 is connected between the ITO electrode 2 and the Aj7 electrode 6.

The ITO electrode 2 was formed by sputtering.

The hole transfer layer 3, the light emitting layer 4, and the electron transfer layer 5 are formed by vacuum sublimation of an organic compound, and each has a thickness of 0.5 to 11. The Al electrode 6 was formed by vacuum evaporation.

Of these, the light emitting layer 4 was formed as follows. First, a second organic dye (perylene, tetracene, or pentacene) is added to the anthracene crystal purified by sublimation.
．． The mixture was blended at a ratio of 0.01 to 1 mol %, heated to the melting point in a quartz container under flowing argon gas, and melted with stirring. Once the crystals are thoroughly mixed, they cool relatively quickly and become solid. The light-emitting layer 4 was formed by sublimating this as a raw material. The content of the second organic dye in the formed light emitting layer is determined by forming a film of only the light emitting layer components alone on a quartz substrate using raw materials with a predetermined composition, and determining the absorption spectrum of the light emitting layer by l? This was investigated by determining 1.

With the configuration shown in FIG. 1, a DC voltage was applied using the ITO electrode 2 as a positive pole and the All electrode 6 as a negative pole, and the amount of current was measured, and the emission spectrum and its intensity were also measured on the glass substrate 1 side.

As a result, 5 mA/cs at 30 V DC voltage
A current of 5000 c d/m 2 was obtained. In addition, the emission spectra are perylene and
The emission was mainly from the isolated excited doublet of tetracene or pentacene (Figure 2). In addition, for a device using perylene dispersed in anthracene as a light-emitting layer, the relationship between the amount of perylene added and the emission intensity was evaluated in the third study.
As shown in the figure. From Figure 3, the amount of perylene added is 0.1 to 1
It can be seen that the range of mol % is optimal.

For comparison, a device whose light-emitting layer was made only of anthracene and a device whose light-emitting layer was made only of perylene were prepared, and the same measurements as above were performed.

As a result, when the DC voltage is 30V, the brightness is only 10V.
It was 0 cd/m2. Regarding the emission spectrum, the device with the anthracene-emitting layer showed blue light emission, but the device with the perylene-emitting layer did not emit blue light, but instead emitted orange light from the excited state dimer.

[Effects of the Invention] As detailed above, in the organic electroluminescent device of the present invention, 10 mol% of the second organic dye is added to the first organic dye as a light emitting layer.
Since we use materials dispersed in the following proportions, we can obtain emission wavelength characteristics from the second organic dye in an isolated excited state with high luminous efficiency, and design the emission color of the device. becomes easier.

[Brief explanation of drawings]

FIG. 1 is a block diagram of an organic electroluminescent device in an example of the present invention, FIG. 2 is a diagram showing an absorption spectrum of an organic electroluminescent device in an example of the present invention, and FIG. 3 is a diagram showing an absorption spectrum of an organic electroluminescent device in an example of the present invention. FIG. 4 is a diagram showing the relationship between the amount of perylene added in anthracene and the emission intensity of an electroluminescent device, and FIG. 4 is a diagram showing the absorption spectrum of an organic electroluminescent device of a comparative example. DESCRIPTION OF SYMBOLS 1... Glass substrate, 2... ITO electrode, 3... Hole transfer layer, 4... Light emitting layer, 5... Electron transfer layer, 6...
...AI electrode, 7...DC power supply. Figure 1; Skin surface (nm Figure 2) Applicant's representative Patent attorney Takehiko Suzue Procedural amendment form 83568 No. 2, Name of the invention Organic electroluminescent device 3, Relationship with the person making the amendment Patent applicant (307) Co., Ltd. 4, Agent Toshiba, 3-7-2 Kasumigaseki, Chiyoda-ku, Tokyo, Figure 4, 7. Contents of the amendment (1) The word "multimer" on page 5, line 8 of the specification is corrected to "multimerization." (2) The word "multimer" on page 15, line 2 of the specification is corrected to "multimer." There is,
Corrected to "multimerization." (3) Delete the sentences from "■" on the 6th line of page 15 of the specification to "Decrease." on the 8th line. (4) Replace “■” on page 15, line 8 of the specification with “■”
I am corrected. (5) Replace “■” on page 15, line 11 of the specification with “■
” he corrected. (8) Delete the sentences from "that is," on page 15, line 19 of the specification to "not appropriate." on page 16, line 9. (7) Replace “■” on page 16, line 10 of the specification with “■
” he corrected. (8) Specification jii, page 16, line 16,
Correct it with "■". (9) In the first line of page 17 of the specification, replace “■,■” with “
■、■" and correct it. (10) Add the following sentence after line 14 on page 18 of the specification. TPD constituting the hole transfer layer, P constituting the electron transfer layer
Each v is represented by the following structural formula.

Claims (1)

[Claims] In an organic electroluminescent device, a light-emitting layer made of a thin organic dye film is provided between two electrodes, at least one of which transmits light. It is characterized by comprising an organic dye thin film in which a second organic dye having a light absorption edge on the longer wavelength side than the absorption edge is dispersed such that the second organic dye has a proportion of 10 mol% or less. organic electroluminescent device.