JP2002359079A - Light emitting element and display device - Google Patents

Abstract

(57) [Problem] To provide a light-emitting element which solves a problem that a light-emitting state changes or a light-emitting characteristic such as a light-emitting efficiency largely changes. SOLUTION: In an organic light-emitting device having a light-emitting layer composed of two or more components between opposing electrodes, the distribution of the lowest triplet excitation energy of the main constituent material of the light-emitting layer on the solid film is determined. The center value is outside the distribution range of the lowest triplet excitation energy of the sub-constituent material on the solid film, and is larger than the center value of the distribution of the lowest triplet excitation energy of the sub-constituent material on the solid film.

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.

[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
Are, for example, triphenyldiamine derivatives (typical examples are
A material having an electron donating property such as 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 the light emission generally used in organic EL devices, the fluorescence at the time of entering the ground state from the singlet exciton of the molecule of the emission center material has been extracted. on the other hand,
Instead of using fluorescence emission via singlet excitons,
Devices using phosphorescence emission via triplet excitons have been studied. Representative literatures that have been published include: Reference 1: Improved energy trans.
fer in electrophophoresc
ent device (DFO'Brien et al., A
applied Physics Letters Vo
l 74, No3p422 (1999)), Reference 2: V
ery high-efficiency green
organic light-emitting d
devices bass on electropho
sporence (MA Baldo et al., A
applied Physics Letters Vo
175, No1 p4 (1999)).

[0010] 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 transport material and a phosphorescent material shown below. 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]

Embedded image

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]

In the above-mentioned organic EL device using phosphorescence, there is a problem that the light emission deteriorates particularly in a current-carrying state. Although the cause of light emission deterioration of the phosphorescent light emitting element is not clear, generally, since the triplet lifetime is longer than the singlet lifetime by three orders of magnitude or more, molecules are placed in a high energy state for a long time. It is thought that reactions, formation of excited multimers, changes in the molecular microstructure, changes in the structure of surrounding substances, etc. may occur.

Further, a host material (main constituent material; CBP in the above example) constituting the light emitting layer and a guest material (sub constituent material; Ir (ppy) ₃ in the above example, which is a luminescent center material,
(PtOEP), there has been a problem in that the light emission state changes and the light emission characteristics such as the light emission efficiency greatly change.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a light emitting element and a display device which solve the above-mentioned problems, that is, a problem that a light emission state changes or a light emission characteristic such as a light emission efficiency largely changes.

[0018]

That is, the light emitting device of the present invention is an organic light emitting device having a light emitting layer composed of two or more components between opposing electrodes. The center value of the distribution of the lowest triplet excitation energy of the sub-constituent material on the solid film is out of the distribution range of the lowest triplet excitation energy of the sub-constituent material and the lowest triplet excitation energy of the sub-constituent material Is larger than the central value of the distribution on the solid film.

In the light emitting device of the present invention, the distribution range of the lowest triplet excitation energy of the sub-constituent material on the solid film is an energy distribution range in which the maximum value of the photoexcitation triplet emission intensity is 10% or less. Is preferred.

Further, it is preferable that the sub-constituting material is a phosphorescent light emitting material.

Further, the central value of the distribution of the lowest triplet excitation energy of the main constituent material on the solid film is more than the central value of the distribution of the lowest triplet excitation energy of the sub constituent material on the solid film.
It is preferably larger than 22 eV.

It is preferable that the material having a wide band gap has a higher ionization potential than the material having a narrow band gap.

Further, the display device of the present invention is characterized in that the above-mentioned light emitting element is provided on an electrode substrate using a TFT.

[0024]

BEST MODE FOR CARRYING OUT THE INVENTION When the light-emitting layer is composed of a host material having a carrier-transporting property and a guest having a phosphorescent light-emitting property, the main processes leading to phosphorescent light emission from triplet excitons are as follows. Process. 1. 1. Transport of electrons and holes in the light emitting layer 2. Exciton generation of host 3. Transfer of excitation energy between host molecules 4. Excitation energy transfer from host to guest 5. Production of guest triplet exciton Phosphorescence emission from guest triplet exciton to ground state

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

In order to increase the luminous efficiency of the EL element, it goes without saying that the luminous quantum yield of the guest material itself, which is the luminescent center material, is large. However, the host
It is also a big problem how efficient the energy transfer between the host and between the host and the guest can be performed. Also,
Although the cause of light emission deterioration due to energization is not clear at present, it is assumed that the deterioration is related to at least the guest material itself or a change in the environment of the guest material due to molecules around the guest material.

The present inventors have developed a host material (main constituent material).
By setting the relationship between the distribution of the lowest triplet excitation energy of the guest material (sub-constituent material) and the guest material (sub-constituent material) on the solid film, it was found that the change in the light emission state or the light emission characteristics can be suppressed.

The change in the light emission state here indicates a light emission state different from the light emission of the guest material itself, which is the light emission center material. For example, an exciplex is formed between the host material and the guest material. Change of the emission wavelength due to the change. The formation of the excited aggregate can occur due to various factors, but it is considered that the formation of the excited aggregate is likely to be caused by the overlap of the lowest triplet excitation energy distribution. In the present invention, such a relationship is confirmed by experiments.

More specifically, the distribution of the lowest triplet excitation energy can be obtained from the phosphorescence emission spectrum of the guest material, and the energy distribution is between "A" eV and "C" eV, and the emission is "B" eV. It shall have a peak. At this time, the points A and C take energy which makes the light emission intensity 10% of the light emission intensity at the point B. Similarly, the distribution of the lowest triplet excitation energy and the peak P are determined from the phosphorescence emission spectrum of the host material. At this time, it was found that when P>C>B> A, there was no change in the light emission state and the light emission characteristics were good.

However, the distribution of such a spectrum is greatly affected by the molecular arrangement in the film, and greatly varies depending on the deposition conditions, the degree of mixing of impurities, and the like. . When the changes in the energy and the light emitting state of the host material and the guest material were examined, they were as follows.

Here, as the guest material, (Ir (PPy) ₃ [minimum triplet excitation energy 2.61]
eV-2.28 eV, Peak = 2.41 eV], and the light emission state was examined using the host materials A to E having different minimum triplet excitation energy distributions shown in Table 1.

[0032]

[Table 1]

Here, the host materials having a change in the light emitting state were the host materials A to D. The center value of the distribution of the lowest triplet excitation energy of these host materials is lower than the upper limit of the energy distribution of the lowest triplet excited state of the guest material, and it is considered that some interaction is caused. In particular, the host materials C and D have a large change in the light emission state, causing a change in the light emission spectrum as shown in FIG.

On the other hand, as shown in FIG. 2, the host material E showed normal light emission without a change in the light emission state. This host material is considered to exhibit normal light emission because the central value of the lowest triplet excitation energy is higher than the upper limit of the energy distribution of the lowest triplet excited state of the guest material.

Here, the lowest triplet excitation energy state is measured by measuring the photoluminescence of the solid film at 77 K and obtaining the phosphorescence emission (0,0 transition peak shape) of the maximum energy. The phosphorescence emission and the fluorescence emission were distinguished by delaying the photoluminescence observation start time after the excitation light was incident. The photoluminescence is measured by applying incident light separated from the xenon lamp to the sample as excitation light, and the wavelength of the excitation light is determined by measuring the absorption wavelength by spectroscopy of the same sample. Absorption was measured using a Shimadzu UV3100s spectrophotometer.
Photoluminescence was measured using an F-4500 type spectrofluorometer.

Since such a change in the light emission state can also be caused by resonance of the energy state or transfer of electric charge, the relationship between the ionization potential Ih of the host material and the ionization potential Ig of the guest material depends on the band gap BGh ( When the minimum absorption energy in the spectrum is larger than the band gap BGg of the guest material, it is desirable that the relationship of Ih> Ig is satisfied. The band gap BGh of the host material (the minimum absorption energy in the spectrum) is larger than the band gap BGg of the guest material. Is smaller, it is desirable that the relationship of Ih <Ig be satisfied.

Examples of the guest material used in the present invention include a metal complex which emits phosphorescent light, and MLCT * (Metal-to-Metal) having the lowest excited state in a triplet state.
(Ligand charge transfer) excited state. Phosphorescence occurs when transitioning from these states to the ground state.

The specific structural formula of the phosphorescent metal complex used in the present invention is shown below. However, these are only representative examples, and the present invention is not limited thereto. Compound 1: Ir (PIQ) ₃ [minimum triplet excitation energy 2.14 eV-1.89 eV, Peak = 2.01 e
V]

[0039]

Embedded image Compound 2: Ir (T4TPy) ₃ [minimum triplet excitation energy 2.14 eV-1.93 eV, Peak = 2.03
eV]

[0040]

Embedded image Compound 3: Ir (PPy) ₃ [minimum triplet excitation energy 2.61 eV-2.28 eV, Peak = 2.41 e
V]

[0041]

Embedded image

The high-efficiency light-emitting element shown in the present invention can be applied to products requiring energy saving and high luminance. Examples of applications include light sources for display and lighting devices and printers,
A backlight of a liquid crystal display device is conceivable. As a display device, an energy-saving, high-visibility, light-weight flat panel display can be realized. 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.

For application to a display, a driving method using a TFT driving circuit of an active matrix method is considered.

Hereinafter, an example in which an active matrix substrate is used in the device of the present invention will be described with reference to FIGS.

FIG. 3 schematically shows the structure of the device provided with a driving means. A scanning signal driver, an information signal driver, and a current supply source are arranged on the panel, and are connected to a gate scanning line, an information line, and a current supply line, respectively. The pixel circuit shown in FIG. 4 is arranged at the intersection of the gate scanning line and the information line. The scanning signal driver includes gate scanning lines G1, G2, G3. . . Gn are sequentially selected, and an image signal is applied from the information signal driver in synchronization with the selection.

Next, the operation of the pixel circuit will be described. In this pixel circuit, when a selection signal is applied to a gate selection line, TFT1 is turned on, an image signal is supplied to Cadd, and the gate potential of TFT2 is determined. A current is supplied to the EL element from a current supply line according to the gate potential of the TFT 2. The gate potential of TFT2 is
Since 1 is held at Cadd until the next scan is selected, the EL element continues to flow until the next scan is performed. As a result, it is possible to always emit light for one frame period.

FIG. 5 is a schematic diagram showing a sectional structure of a TFT used in this embodiment. P- on glass substrate
A Si layer is provided, and the channel, drain, and source regions are each doped with necessary impurities. On this, a gate electrode is provided via a gate insulating film, and a drain electrode and a source electrode connected to the drain region and the source region are formed. An insulating layer and an ITO electrode as a pixel electrode are stacked thereon, and the ITO and the drain electrode are connected by a contact hole.

The present invention is not particularly limited to a switching element, and can be easily applied to a single crystal silicon substrate, an MIM element, an a-Si type, and the like.

An organic EL display panel can be obtained by sequentially laminating a multilayer or single-layer organic EL layer / cathode layer on the ITO electrode. By driving the display panel using the light emitting layer of the present invention, stable display can be performed with good image quality even for a long time display.

[0050]

EXAMPLES The present invention will be specifically described below with reference to examples.

First, the common parts of the element forming process used in this embodiment will be described.

As a device configuration, a device having four organic layers as shown in FIG. 1C was used. Glass substrate (transparent substrate 15)
A 100-nm ITO (transparent electrode 14) was patterned thereon so that the area of the opposing electrode was 3 mm ² . On the ITO substrate, the following organic layers and electrode layers were
Vacuum evaporation was performed by resistance heating in a ^-4 Pa vacuum chamber to form a continuous film. Organic layer 1 (hole transport layer 13) (40 nm): α-NP
D Organic layer 2 (light emitting layer 12) (30 nm): predetermined host:
Predetermined guest (5% by weight) Organic layer 3 (exciton diffusion preventing layer 17) (10 nm) BCP Organic layer 4 (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

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 would be the same for each element, and the time change of the luminance was measured. The constant current amount was 70 mA / cm ² . The luminance range of each device obtained at that time was 80 to 240 cd / m ² .

Since oxygen and water are problems as a cause of element deterioration, the above measurement was carried out in a dry nitrogen flow after removing the element from the vacuum chamber in order to eliminate the cause.

Example 1 Compound 1 (Ir (PIQ) ₃
[Minimum triplet excitation energy 2.14 eV-1.89 e
V, Peak = 2.01 eV]) as a guest material,
5% by weight of a host material (Ir metal complex having ligand L, IrL ₃ ) shown in Tables 2 and 3 was doped by co-evaporation to form a light emitting layer. The driving evaluations in Tables 2 and 3 were performed from the viewpoint of reproducibility of device characteristics and the viewpoint of luminous efficiency.

As a result, when the center value (peak value T1) of the lowest triplet excitation energy distribution of the host material is equal to or higher than the upper limit of 2.14 eV of the lowest triplet excitation energy range of the guest material compound 1, it is preferable. Light emission characteristics were shown. In addition, when the peak value T1 of the lowest triplet excitation energy distribution of the host material is higher than the upper limit of the lowest triplet excitation energy range of the guest material by 0.22 eV or more, the emission luminance is higher than in the other cases. (Drive evaluation ◎).
This is because the energy difference between the peak value T1 of the lowest triplet excitation energy distribution of the host material and the upper limit of the lowest triplet excitation energy range of the guest material is large, so that energy transfer from the guest material to the host material is prevented. It is thought that it was done.

[0057]

[Table 2]

[0058]

[Table 3]

Example 2 Compound 2 (Ir (T4TP
y) ₃ [minimum triplet excitation energy 2.14 eV-1.9
3 eV, Peak = 2.03 eV] as a guest material, 5% by weight of a host material (Ir metal complex having ligand L, IrL ₃ ) shown in Table 4 was doped by co-evaporation to obtain a light emitting layer. Was formed. The driving evaluation in Table 4 was performed from the viewpoint of reproducibility of element characteristics.

As a result, it is preferable that the center value (peak value T1) of the lowest triplet excitation energy distribution of the host material is equal to or higher than the upper limit of 2.14 eV of the lowest triplet excitation energy range of the compound 2 which is a guest material. Light emission characteristics were shown.

[0061]

[Table 4]

Example 3 Compound 3 (Ir (PPy) ₃
[Minimum triplet excitation energy 2.61 eV-2.28 e
V, Peak = 2.41 eV]) as the guest material,
The light emitting layer was formed by doping by 5% by weight in a host material (D-CBP) by co-evaporation. However, the distribution of the lowest triplet excitation energy of the host material was changed by changing the purification and deposition methods (the center value also changed due to the change in the distribution).

As a result, when a 98% purified product was used as D-CBP and the vapor deposition conditions were 1.7 nm / sec, the center value (peak value) of the lowest triplet excitation energy distribution was 2.64 eV, and The upper limit value of the lowest triplet excitation energy range of the guest material, Compound 3, was 2.61 eV or more, indicating good emission characteristics.

The drive evaluation in the present embodiment was performed from the viewpoint of reproducibility of element characteristics.

Example 4 Compound 1 (Ir) was used as a guest material.
(PIQ) ₃ [minimum triplet excitation energy 2.14 eV−
1.89 eV, Peak = 2.01 eV]),
Using D-CBP as the host material, a light emitting layer was formed in the same manner as in Example 3.

In this example, the ionization potential of compound 1 was 5.1 eV, and the band gap was 2.
0 eV. On the other hand, the ionization potential of D-CBP is 5.5 eV, and the band gap is 3.3 eV.
In this case, although the host material had a wider band gap than the guest material, the ionization potential of the host material was larger than that of the guest material.

The center value (peak value) of the lowest triplet excitation energy distribution of the host material is 2.64 eV,
Since the upper limit of the lowest triplet excitation energy range of compound 1, which is the guest material, is 2.14 eV or more, good emission characteristics were exhibited.

[0068]

As described above, according to the present invention, good light emission can be obtained by setting the distribution of the lowest triplet excitation energy of the host material and the guest material of the light emitting layer on the solid film to a specific relationship. Characteristics could be obtained.

[Brief description of the drawings]

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

FIG. 2 is a graph showing a change in light emission state due to distribution of the lowest triplet excitation energy of a host material and a guest material on a solid film.

FIG. 3 is a diagram illustrating an example of a pixel circuit.

FIG. 4 is a schematic diagram showing an example of a cross-sectional structure of a TFT substrate.

FIG. 5 is a diagram schematically illustrating an example of a configuration of a panel including an EL element and a driving unit.

[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 Continuing on the front page (72) Inventor Akira Tsuboyama 3-30-2 Shimomaruko, Ota-ku, Tokyo Canon Inc. (72) Inventor Takao Takiguchi 3-30-2 Shimomaruko, Ota-ku, Tokyo Canon (72) Inventor Seishi Miura 3-30-2 Shimomaruko, Ota-ku, Tokyo Canon Inc. (72) Inventor Takashi Moriyama 3-30-2 Shimomaruko, Ota-ku, Tokyo Canon Inc. ( 72) Inventor Atsushi Kamagaya 3-30-2 Shimomaruko, Ota-ku, Tokyo F-term in Canon Inc. (reference) 3K007 AB03 AB04 AB12 BA06 CA01 CB01 DA01 DB03 EB00

Claims (6)

[Claims] 1. An organic light-emitting device having a light-emitting layer composed of two or more components between opposing electrodes, wherein the lowest triplet excitation energy of a main constituent material of the light-emitting layer is distributed on a solid film. Is outside the distribution range of the lowest triplet excitation energy of the sub-constituent material on the solid film, and is larger than the central value of the distribution of the lowest triplet excitation energy of the sub-constituent material on the solid film. A light emitting element characterized by the above-mentioned. 2. A distribution range of the lowest triplet excitation energy of the sub-constituent material on the solid film is an energy distribution range in which the maximum value of the photoexcitation triplet emission intensity is 10% or less. Item 2. A light emitting device according to item 1. 3. The light emitting device according to claim 1, wherein the auxiliary constituent material is a phosphorescent light emitting material. 4. The central value of the distribution of the lowest triplet excitation energy of the main constituent material on the solid film is 0.22 from the central value of the distribution of the lowest triplet excitation energy of the sub constituent material on the solid film.
The light emitting device according to claim 1, wherein the light emitting device is larger than eV. 5. The light emitting device according to claim 1, wherein the ionization potential of the constituent material having a wide band gap is larger than the ionization potential of the constituent material having a narrow band gap. 6. The method according to claim 1, further comprising the steps of:
A display device comprising the light-emitting element according to any one of claims 1 to 4.