Organic electroluminescence device

Abstract

An organic electroluminescence device includes: a cathode; an anode; and an organic thin-film layer disposed between the cathode and the anode, the organic thin-film layer having one or more layers including an emitting layer, in which the emitting layer includes a first material represented by the following formula (1) and a second material in a form of a fluorescent dopant material.

##STR00001##

Description

CROSS-REFERENCE TO RELATED APPLICATION(S)

The

##STR00019##
In the above formula (6), Ar^b represents a single bond, or a substituted or unsubstituted aryl group having 6 to 30 ring carbon atoms.

ΔST(H)=EgS(H)−Eg_77K(H)<0.3 [eV]  (2)

Further, a difference ΔT between the energy gap Eg_77K(H) at 77K of the host material and an energy gap Eg_77K(T) at 77K of the dopant material satisfies a relationship of the following numerical formula (3).
ΔT=Eg_77K(H)−Eg_77K(D)≥0.6 [eV]  (3)
ΔST

The organic EL device emits light at a high efficiency in a high current density area by using a compound having a small energy gap (ΔST) between singlet energy EgS and triplet energy EgT as the host material. The ΔST(H) refers to ΔST of the host material.

From quantum chemical viewpoint, decrease in the energy difference (ΔST) between the singlet energy EgS and the triplet energy EgT can be achieved by a small exchange interaction therebetween. Physical details of the relationship between ΔST and the exchange interaction are exemplarily described in the following:

Literature 5: Organic EL Symposium, proceeding for the tenth meeting edited by Chihaya Adachi et al., S2-5, pp. 11-12; and

Literature 6: Organic Photochemical Reaction Theory edited by Katsumi Tokumaru, Tokyo Kagaku Dojin Co., Ltd. (1973).

Such a material can be synthesized according to molecular design based on quantum calculation. Specifically, the material is a compound in which a LUMO electron orbit and a HOMO electron orbit are localized to avoid overlapping.

Examples of the compound having a small ΔST, which is used as the host material in the exemplary embodiment, are compounds in which a donor element is bonded to an acceptor element in a molecule and ΔST is in a range of 0 eV or more and less than 0.3 eV in terms of electrochemical stability (oxidation-reduction stability).

A more preferable compound is such a compound that dipoles formed in the excited state of a molecule interact with each other to form an aggregate having a reduced exchange interaction energy. According to analysis by the inventors, the dipoles are oriented substantially in the same direction in the compound, so that ΔST can be further reduced by the interaction of the molecules. In such a case, ΔST can be extremely small in a range of 0 eV to 0.2 eV.

Aggregate

Decrease in the energy gap (ΔST) between the singlet energy EgS and the triplet energy EgT can also be achieved by aggregate formation. Herein, the aggregate does not reflect an electronic state by a single molecule, but the aggregate is provided by several molecules physically approaching each other. After the plurality of molecules approach each other, electronic states of a plurality of molecules are mixed and changed, thereby changing an energy level. A value of singlet energy is mainly decreased, thereby decreasing a value of ΔST. The decrease in the value of ΔST by the aggregate formation can also be explained by Davydov splitting model showing that two molecules approach each other to change electronic states thereof (see FIG. 2). As shown in Davydov splitting model, it is considered that change of the electronic states by two molecules different from change of an electronic state by a single molecule is brought about by two molecules physically approaching each other. A singlet state exists in two states represented by S1-m⁺ and S1-m⁻. A triplet state exists in two states represented by T1-m⁺ and T1-m⁻. Since S1-m⁻ and T1-m⁻ showing a lower energy level exist, ΔST representing a gap between S1-m⁻ and T1-m⁻ becomes smaller than that in the electronic state by a single molecule.

The Davydov splitting model is exemplarily described in the following:

Literature 7: J. Kang, et al, International Journal of Polymer Science, Volume 2010, Article ID 264781;

Literature 8: M. Kasha, et al, Pure and Applied Chemistry, Vol. 11, pp 371, 1965; and

Literature 9: S. Das, et al, J. Phys. Chem. B. vol. 103, p 209, 1999.

The inventors found usage of sublevels of a singlet state and a triplet state of a compound easily forming an aggregate in a thin film, and consequent possibility of promotion of inverse intersystem crossing by molecules and aggregates in the thin film.

For instance, a compound having a large half bandwidth of a photoluminescence spectrum is considered to easily form an aggregate in a thin film of the compound. A relationship between the half bandwidth of the photoluminescence spectrum and easy formability of the aggregate can be estimated as follows.

In a compound having a property of typically existing as a single molecule without forming an aggregate, a vibrational level is less recognized in the singlet state, so that a narrow half bandwidth of the photoluminescence spectrum is observed. For instance, CBP (4,4′-bis[9-dicarbazolyl]-2,2′-biphenyl) exhibits a property to typically exist as a single molecule, in which a half bandwidth of a photoluminescence spectrum is relatively as narrow as about 50 nm.

On the other hand, in the compound easily forming the aggregate, a plurality of molecules electronically influence each other, whereby a lot of vibrational levels exist in the singlet state. As a result, since the vibrational levels of the singlet state are often relaxed to the ground state, the half bandwidth of the photoluminescence spectrum is increased.

Such a compound easily forming the aggregate is expected to have a lot of vibrational levels even in a triplet state. Consequently, it is speculated that ΔST in relation to heat is decreased through the sublevels to promote the inverse intersystem crossing, since a lot of sublevels exist between the singlet state and the triplet state.

It should be noted that the aggregate according to the exemplary embodiment means that a single molecule forms any aggregate with another single molecule. In other words, a specific aggregate state is not shown in the exemplary embodiment. An aggregate state of an organic molecule is probably formable in various states in a thin film, which is different from an aggregate state of an inorganic molecule.

TADF Mechanism

As described above, when ΔST(H) of the organic material is small, inverse intersystem crossing from the triplet level of the host material to the singlet level thereof is easily caused by heat energy given from the outside. Herein, an energy state conversion mechanism to perform spin exchange from the triplet state of electrically excited excitons within the organic EL device to the singlet state by inverse intersystem crossing is referred to as TADF Mechanism.

In the exemplary embodiment, since the material having a small ΔST(H) is used as the host material, inverse intersystem crossing from the triplet level of the host material to the singlet level thereof is easily caused by heat energy given from the outside.

FIG. 3 shows a relationship in energy level between the host material and the dopant material in the emitting layer. In FIG. 3, S0 represents a ground state, S1_H represents a lowest singlet state of the host material, T1_H represents a lowest triplet state of the host material, S1_D represents a lowest singlet state of the dopant material, and T1_D represents a lowest triplet state of the dopant material. As shown in FIG. 3, a difference between S1_H and T1_H corresponds to ΔST(H), a difference between S1_H and S0 corresponds to EgS(H), a difference between S1_D and S0 corresponds to EgS(D), and a difference between T1_H and T1_D corresponds to ΔT. A dotted-line arrow shows energy transfer between the respective excited states in FIG. 3.

As described above, a compound having a small ΔST(H) is selected as the compound for the host material in the exemplary embodiment. This is because the material having a small ΔST(H) is considered to easily cause inverse intersystem crossing from the triplet excitons generated in the lowest triplet state T1_H to the lowest singlet state S1_H of the host material by heat energy. Due to the small ΔST(H), inverse intersystem crossing is easily caused, for instance, even around a room temperature. When the inverse intersystem crossing is thus easily caused, a ratio of energy transfer from the host material to the lowest singlet state
Specifically,
5³A*→4¹A+¹A*
It is expected that, among triplet excitons initially generated, which account for 75%, one fifth thereof (i.e., 20%) is changed to singlet excitons.

Accordingly, the amount of singlet excitons which contribute to emission is 40%, which is a value obtained by adding 15% (75%×(⅕)=15%) to 25% (the amount ratio of initially generated singlet excitons).

At this time, a ratio of luminous intensity derived from TTF (TTF ratio) relative to the total luminous intensity is 15/40, i.e., 37.5%. Thus, it is recognized that the delayed fluorescence ratio of the organic EL device according to the exemplary embodiment exceeds the theoretical upper-limit of only the TTF ratio.

Residual Strength Ratio in 1 μs

A method for relatively measuring an amount of delayed fluorescence is exemplified by a method for measuring a residual strength in 1 μs. The residual strength in 1 μs is defined as a ratio of a luminous intensity after the elapse of 1 μs after removal of a pulse voltage measured by a transitional EL method to a luminous intensity at the time of the removal of the pulse voltage. The relative amount of delayed fluorescence can be estimated based on reduction behavior of EL emission after the removal of the pulse voltage measured by the transitional EL method. The residual strength ratio in 1 μs can be obtained by reading luminous intensity at the time of 1.0 μs in the graph of FIG. 6A.

The residual strength ratio in 1 μs is preferably larger than 36.0%, more preferably 38.0% or more.

Dopant Properties

A preferable dopant in the exemplary embodiment has properties to emit fluorescence and to have a large speed constant of radiational transition. In this arrangement, singlet excitons electrically excited on the host material, singlet excitons generated by the TADF mechanism and the like are transferred to singlet excitons of the dopant material by Förster energy transfer and the dopant material immediately emits light. In other words, fluorescent emission is possible through the above energy transition before triplet excitons on the host material causes TTA, by which decrease in an efficiency in the high current area is likely to be considerably improved.

It is preferable to select a dopant material having a fluorescence lifetime of 5 ns or less, more preferably 2 ns or less as the dopant material having a large speed constant of radiational transition in the exemplary embodiment. A fluorescence quantum efficiency of the dopant material is preferably 80% or more in a solution. The fluorescence quantum efficiency can be obtained by measuring the dopant material in a range of 10⁻⁵ mol/l to 10⁻⁶ mol/l of a concentration in a toluene solution using Absolute PL Quantum Yield Measurement System C9920-02 manufactured by HAMAMATSU PHOTONICS K.K.

It is also expected by measuring an EL spectrum of the device and confirming a luminescence component of a material other than the dopant material is 1/10 or less of the luminescence component of the dopant that the dopant material has a large speed constant of radiational transition.

Relationship Between Emitting Layer and Electron Transporting Layer

When ΔST(H) of the host material is small, the energy gap between the host material and the electron transporting layer adjacent thereto is small, so that the electrons are likely to be injected into the emitting layer. As a result, carrier balance is easily obtainable to decrease roll-off.

Relationship Between Emitting Layer and Hole Transporting Layer

When an ionization potential of the hole transporting layer is represented by IP_HT, IP_HT≤5.7 eV is preferable. With this arrangement, balance between the electrons and the holes can be enhanced. The ionization potential can be obtained, for instance, by measuring the material in a form of a thin film using a photoelectron spectroscopy (AC-3: manufactured by RIKEN KEIKI Co., Ltd.).

Relationship in Singlet Energy Between Host Material and Dopant Material

In the exemplary embodiment, the dopant material is a fluorescent dopant material. A compound used as the host material and a compound used as the dopant material satisfy a relationship represented by the numerical formula 5:5:5
:EgT_D [eV]=1239.85/λedge

The tangent to the rise of the phosphorescence spectrum on the short-wavelength side was drawn as follows. While moving on a curve of the phosphorescence spectrum from the short-wavelength side to the maximum spectral value closest to the short-wavelength side among the maximum spectral values, a tangent is checked at each point on the curve toward the long-wavelength of the phosphorescence spectrum. An inclination of the tangent is increased as the curve rises (i.e., a value of the ordinate axis is increased). A tangent drawn at a point of the maximum inclination was defined as the tangent to the rise of the phosphorescence spectrum on the short-wavelength side.

The maximum with peak intensity being 10% or less of the maximum peak intensity of the spectrum is not included in the above-mentioned maximum closest to the short-wavelength side of the spectrum. The tangent drawn at a point of the maximum spectral value being closest to the short-wavelength side and having the maximum inclination is defined as a tangent to the rise of the phosphorescence spectrum on the short-wavelength side.

For phosphorescence measurement, a spectrophotofluorometer body F-4500 and optional accessories for low temperature measurement (which were manufactured by Hitachi High-Technologies Corporation) were used. The measurement instrument is not limited to this arrangement. A combination of a cooling unit, a low temperature container, an excitation light source and a light-receiving unit may be used for measurement.

(3) ΔST

ΔST was obtained as a difference between EgS and Eg_77K measured in the above (1) and (2) (see the above numerical formula (2)). The results are shown in Table 1.

A half bandwidth of photoluminescence spectrum was obtained as follows.

Each of the compounds was formed in a 100 nm-thick film on a glass substrate with a deposition apparatus to prepare a sample for fluorescence measurement.

The sample for phosphorescence measurement was irradiated with excitation light at a room temperature 300(K), so that fluorescence intensity was measured while changing a wavelength.

The photoluminescence spectrum was expressed in coordinates of which ordinate axis indicated fluorescence intensity and of which abscissa axis indicated the wavelength. For fluorescence measurement, a spectrophotofluorometer F-4500 (manufactured by Hitachi High-Technologies Corporation) was used.

The half bandwidth (unit: nm) was measured based on the photoluminescence spectrum.

The compounds H-1, H-2 and H-3 were measured with respect to the half bandwidth. The results are shown in Table 1.

TABLE 1
		EgS	Eg		Half
	Host	(Thin Film)	(77K)	ΔST	Bandwidth
	Material	[eV]	[eV]	[eV]	[nm]
	H-1	3.02	2.73	0.29	57
	H-2	2.99	2.71	0.28	67
	H-3	3.02	2.74	0.28	62
	GH-4	2.98	2.91	0.07	—
	GD-1	2.47	1.80	0.67	—
	BH-1	2.90	2.84	0.06	—
	BD-1	2.69	—	—	—

Molecular orbital views of the compounds H-1 to H-3 are respectively shown in FIGS. 9 to 11. Preparation and Evaluation of Organic EL Device

The organic EL devices were prepared in the following manner and evaluated.

Example 1

A glass substrate (size: 25 mm×75 min×1.1 mm thick, manufactured by Geomatec Co., Ltd.) having an ITO transparent electrode (anode) was ultrasonic-cleaned in isopropyl alcohol for five minutes, and then UV/ozone-cleaned for 30 minutes. A film of ITO was 77 nm thick.

After the glass substrate having the transparent electrode line was cleaned, the glass substrate was mounted on a substrate holder of a vacuum evaporation apparatus. Initially, a compound HI-1 was deposited on a surface of the glass substrate where the transparent electrode line was provided in a manner to cover the transparent electrode, thereby forming a 5-nm thick film of the compound HI-1. The HI-1 film serves as a hole injecting layer.

After the film formation of the HI-1 film, a compound HT-1 was deposited on the HI-1 film to form a 125-nm thick HT-1 film. The HT-1 film serves as a first hole transporting layer.

A compound HT-2 was deposited on the HT-1 film to form a 25-nm thick HT-2 film. The HT-2 film serves as a second hole transporting layer.

A compound H-1 (a host material) and a compound BD-1 (a fluorescent dopant material) were co-deposited on the HT-2 film to form a 25-nm thick emitting layer. The concentration of the dopant material was set at 4 mass %.

An electron transporting compound ETA was deposited on the emitting layer to form a 5-nm thick hole blocking layer.

Further, the compound ET-2 and Liq were co-deposited on the ET-1 film to form a 20-nm thick electron transporting layer. The concentration of Liq was set at 50 mass %.

Liq was deposited on the electron transporting layer to form a 1-nm thick Liq film.

A metal Al was deposited on the Liq film to form an 80-nm thick metal cathode.

A device arrangement of the organic EL device in Example 1 is schematically shown as follows.

ITO(77)/HI-1(5)/HT-1(125)/HT-2(25)/H-1:BD-1(25,4%)/ET-1(5)/ET-2:Liq(20,50%)/Liq(1)/Al(80)

Numerals in parentheses represent a film thickness (unit: nm). Numerals represented by percentage in the same parentheses represent a ratio (mass %) of an added component such as the fluorescent dopant material in the emitting layer.

Example 2

A glass substrate (size: 25 mm×75 mm×1.1 mm thick, manufactured by Geomatec Co., Ltd.) having an ITO transparent electrode (anode) was ultrasonic-cleaned in isopropyl alcohol for five minutes, and then UV/ozone-cleaned for 30 minutes. A film of ITO was 77 nm thick.

After the glass substrate having the transparent electrode line was cleaned, the glass substrate was mounted on a substrate holder of a vacuum evaporation apparatus. Initially, a compound HI-1 was deposited on a surface of the glass substrate where the transparent electrode line was provided in a manner to cover the transparent electrode, thereby forming a 5-nm thick film of the compound HI-1. The HI-1 film serves as a hole injecting layer.

After the film formation of the HI-1 film, a compound HT-1 was deposited on the HI-1 film to form a 65-nm thick HT-1 film. After the film formation of the HT-1 film, a compound HT-2 was deposited on the HT-1 film to form a 10-nm thick HT-2 film on the HT-1 film. The HT-1 film and the HT-2 film serve as a hole transporting layer.

A compound H-2 (a host material) and a compound YD-1 (a fluorescent dopant material) were co-deposited on the HT-2 film to form a 25-nm thick emitting layer. The concentration of the dopant material was set at 4 mass %.

An electron transporting compound ET-3 was deposited on the emitting layer to form a 5-nm thick hole blocking layer.

ET-2 was deposited on the hole blocking layer to form a 30-nm thick electron transporting layer.

Liq was deposited on the electron transporting layer to form a 1-nm thick LiF film.

A metal Al was deposited on the Liq film to form an 80-nm thick metal cathode.

A device arrangement of the organic EL device in Example 2 is schematically shown as follows.

ITO(77)/HI-1(5)/HT-1(65)/HT-2(10)/H-2:YD-1(25,4%)/ET-3(5)/ET-2(30)/LiF(1)/Al(80)

Numerals in parentheses represent a film thickness (unit: nm). The numerals represented by percentage in parentheses indicate a ratio (mass %) of YD-1.

Example 3

An organic EL device was manufactured in the same manner as Example 2 except that the compound H-3 was used as the host material in place of the compound H-2.

Example 4

A glass substrate (size: 25 mm×75 mm×1.1 mm thick, manufactured by Geomatec Co., Ltd.) having an ITO transparent electrode (anode) was ultrasonic-cleaned in isopropyl alcohol for five minutes, and then UV/ozone-cleaned for 30 minutes. A film of ITO was 130 nm thick.

After the glass substrate having the transparent electrode line was cleaned, the glass substrate was mounted on a substrate holder of a vacuum evaporation apparatus. Initially, a compound HI-2 was deposited on a surface of the glass substrate where the transparent electrode line was provided in a manner to cover the transparent electrode, thereby forming a 50-nm thick film of the compound HI-2. The HI-2 film serves as a hole injecting layer.

After the film formation of the HI-2 film, a compound HT-2 was deposited on the HI-2 film to form a 60-nm thick HT-2 film. The HT-2 film serves as a hole transporting layer.

The compound GH-4 (a host material) and the compound GD-1 (a fluorescent dopant material) were co-deposited on the HT-2 film to form a 30-nm thick emitting layer. The concentration of the dopant material was set at 5 mass %.

An electron transporting compound ET-4 was deposited on the emitting layer to form a 25-nm thick electron transporting layer.

LiF was deposited on the electron transporting layer to form a 1-nm thick LiF film.

A metal Al was deposited on the LiF film to form an 80-nm thick metal cathode.

Thus, the organic EL device of Example 4 was manufactured.

A device arrangement of the organic EL device in Example 4 is schematically shown as follows.

ITO(130)/HI-2(50)/HT-2(60)/GH-4:GD-1(30,5%)/ET-4(25)/LiF(1)/Al(80)

Example 5

A glass substrate (size: 25 mm×75 mm×1.1 mm thick, manufactured by Geomatec Co., Ltd.) having an ITO transparent electrode (anode) was ultrasonic-cleaned in isopropyl alcohol for five minutes, and then UV/ozone-cleaned for 30 minutes. A film of ITO was 70 nm thick.

After the glass substrate having the transparent electrode line was cleaned, the glass substrate was mounted on a substrate holder of a vacuum evaporation apparatus. Initially, a compound HI-1 was deposited on a surface of the glass substrate where the transparent electrode line was provided in a manner to cover the transparent electrode, thereby forming a 5-nm thick film of the compound HI-1. The HI-1 film serves as a hole injecting layer.

After the film formation of the HI-1 film, a compound HT-1 was deposited on the HI-1 film to form a 125-nm thick HT-1 film. After the film formation of the HT-1 film, a compound HT-2 was deposited on the HT-1 film to form a 25-nm thick HT-2 film on the HT-1 film. The HT-1 film and the HT-2 film serve as a hole transporting layer.

A compound BH-1 (a host material) and a compound BD-1 (a fluorescent dopant material) were co-deposited on the HT-2 film to form a 25-nm thick emitting layer. The concentration of the dopant material was set at 4 mass %.

An electron transporting compound ET-1 was deposited on the emitting layer to form a 5-nm thick hole blocking layer.

ET-2 and Liq were co-deposited on the hole blocking layer to form a 20-nm thick electron transporting layer. A concentration ratio between ET-2 and Liq was set at 50 mass %:50 mass %.

Liq was deposited on the electron transporting layer to form a 1-nm thick Liq film.

A metal Al was deposited on the Liq film to form an 80-nm thick metal cathode.

Thus, the organic EL device of Example 5 was manufactured.

A device arrangement of the organic EL device in Example 5 is schematically shown as follows.

ITO(70)/HI-1(5)/HT-1(125)/HT-2(25)/BH-1:BD-1(25,4%)/ET-1(5)/ET-2:Liq(20,50%)/Liq(1)/Al(80)

Evaluation of Organic EL Devices

The manufactured organic EL devices were evaluated in terms of drive voltage, luminous intensity, CIE1931 chromaticity, current efficiency L/J, power efficiency primary peak wavelength λ_p and external quantum efficiency EQE. The current density was set at 1.00 mA/cm² or 10.00 mA/cm². The results are shown in Table 2.

The manufactured organic EL devices were evaluated in terms of delayed fluorescence ratio and residual strength ratio supposing that the current density was 1.00 mA/cm². The details are shown below.

Drive Voltage

Voltage was applied between ITO and Al such that the current density was 1.00 mA/cm² or 10.00 mA/cm², where the voltage (unit: V) was measured.

CIE1931 Chromaticity

Voltage was applied on each of the organic EL devices such that the current density was 1.00 mA/cm² or 10.00 mA/cm², where CIE1931 chromaticity coordinates (x, y) were measured using a spectroradiometer CS-1000 (manufactured by Konica Minolta Holdings, Inc.).

Current Efficiency L/J and Power Efficiency η

Voltage was applied on each of the organic EL devices such that the current density was 1.00 mA/cm² or 10.00 mA/cm², where spectral radiance spectra were measured by the aforementioned spectroradiometer. Based on the obtained spectral radiance spectra, the current efficiency (unit: cd/A) and the power efficiency η (unit: lm/W) were calculated.

Main Peak Wavelength λ_p

A main peak wavelength λ_p was calculated based on the obtained spectral-radiance spectra.

External Quantum Efficiency EQE

The external quantum efficiency EQE (unit: %) was calculated based on the obtained spectral-radiance spectra, assuming that the spectra was provided under a Lambertian radiation.

Delayed Fluorescence Ratio

Voltage pulse waveform (pulse width: 500 micro second, frequency: 20 Hz, voltage: equivalent to 0.1 to 100 mA/cm²) output from a pulse generator (8114A: manufactured by Agilent Technologies) was applied. EL emission was input in a photomultiplier (R928: manufactured by Hamamatsu Photonics K.K.). The pulse voltage waveform and the EL emission were synchronized and loaded in an oscilloscope (2440: Tektronix) to obtain a transitional EL waveform. Reciprocal numbers of square root of luminous intensity were plotted, which were fitted in a linear line using a value before the elapse of 10⁻⁵ seconds calculated by the method of least squares to determine a delayed fluorescence ratio.

The transitional EL waveform where voltage of 1.00 mA/cm² was applied on the organic EL device of the Example 1 at the room temperature is shown in FIG. 12. The pulse voltage was removed at the time of about 3×10⁻⁸ seconds.

Based on the graph, where the voltage removal time was a starting point and the reciprocal numbers of the square root of luminous intensity before the elapse of 1.5×10⁻⁵ seconds after voltage removal were plotted, the delayed fluorescence ratio of the organic EL device of the Example 1 was 45.9%. This delayed fluorescence ratio exceeded the theoretical upper-limit (37.5%) of the TTF ratio.

It was read from the graph in FIG. 12 that a residual strength ratio in 1 μs was 46.2%.

For the organic EL devices according to the Examples 2 to 5, transitional EL waveforms were obtained in the same manner as in Example 1. Specifically, reciprocal numbers of square root of the luminous intensity were plotted and were fitted in a linear line using a value before the elapse of 10⁻⁵ seconds calculated by the method of least squares, which were analyzed to determine a delayed fluorescence ratio, thereby obtaining residual strength ratio in 1 μs. The transitional EL waveforms of the organic EL devices in the Examples 2, 3 and 5 are respectively shown in FIGS. 13 to 15. The transitional EL waveform of the organic EL device in the Example 4 is shown in FIG. 6A.

The delayed fluorescence ratio and residual strength ratio of the organic EL devices in the Examples 1 to 5 are shown in Table 3.

TABLE 2
		Current
	Host	Density	Voltage	Luminance	L/J	η	Chroma-	Chroma-	λp	EQE
	Material	[mA/cm2]	[V]	[cd/m2]	[cd/A]	[lm/W]	ticity x	ticity y	[nm]	[%]
Ex. 1	H-1	1.00	3.80	67.6	6.76	5.59	0.128	0.214	473	4.67
Ex. 2	H-2	1.00	3.52	147.2	14.72	13.12	0.497	0.481	572	4.94
Ex. 3	H-3	1.00	3.30	122.0	12.20	11.60	0.506	0.482	572	4.11
Ex. 4	GH-4	10.00	3.97	1585.2	15.85	12.85	0.274	0.606	520	4.59
	GH-1	1.00	3.44	174.0	17.40	15.89	0.276	0.604	522	5.04
Ex. 5	BH-1	1.00	3.81	57.6	5.76	4.75	0.130	0.197	471	4.16

TABLE 3
		Delayed	Residual
		Fluorescence	Strength
		Ratio [%]	Ratio [%]
	Ex. 1	45.9	46.2
	Ex. 2	47.5	41.2
	Ex. 3	54.1	50.1
	Ex. 4	41.0	39.8
	Ex. 5	38.7	36.3

Reference Example

Herein, the organic EL device described in Literature 3 are shown as a reference example and compared with the organic EL device of Example 1 in terms of the device arrangement.

A device arrangement of the organic EL devices in the reference example is schematically shown below in the same manner as in Example 1.

ITO(110)/NPD(40)/m-CP(10)/m-CP:PIC-TRZ(20,6%)/BP4 mPy(40)/LiF(0.8)/Al(70)

Compounds used in the reference example will be shown below.

##STR00052## ##STR00053##

The device only exhibits the maximum EQE of 5.1% in the current density area of 0.01 mA/cm² which is much lower than the current density area in a practical use. Accordingly, in a high current density area around 1 mA/cm², roll-off is generated and a luminous efficiency is reduced.

Accordingly, it is recognized that the organic EL devices of Examples 1 to 5 emitted light with a high efficiency even in the high current density area.

Claims

1. An organic electroluminescence device comprising:

a cathode;

an anode; and

an organic thin-film layer disposed between the cathode and the anode, the organic thin-film layer having one or more layers comprising an emitting layer,

the emitting layer comprising a first material represented by a formula (1) below and a second material in a form of a fluorescent dopant material,

2. The organic electroluminescence device according to claim 1, wherein B in the formula (1) is represented by one of formulae (2), (3), (4), (5) and (6) below,

3. The organic electroluminescence device according to claim 2, wherein

the formula (2) is represented by a formula (2a) below,

4. The organic electroluminescence device according to claim 2, wherein

at least one of R³¹ to R³⁸ in the formula (5) is represented by a formula (51) below,

5. The organic electroluminescence device according to claim 1, wherein

A in the formula (1) is represented by one of formulae (8), (9), (10), (11), (12), (13) and (14) below,

6. The organic electroluminescence device according to claim 1, wherein

the second material in the form of the fluorescent dopant material is represented by a formula (20) below,

7. The organic electroluminescence device according to claim 1, wherein

the organic electroluminescence device exhibits a delayed fluorescence ratio larger than 37.5%.

8. The organic electroluminescence device according to claim 1, wherein

the organic electroluminescence device exhibits a residual strength ratio larger than 36.0% after an elapse of 1 μs after voltage removal in a transitional EL measurement.

9. The organic electroluminescence device according to claim 1, wherein

a half bandwidth of a photoluminescence spectrum of the first material is 50 nm or more.

10. An organic electroluminescence device comprising:

a cathode;

an anode; and

an organic thin-film layer disposed between the cathode and the anode, the organic thin-film layer having one or more layers comprising an emitting layer,

the emitting layer comprising a first material represented by a formula (1) below and a second material in a form of a dopant material,

with a proviso that the dopant material is not a heavy metal complex,

11. An organic electroluminescence device, comprising:

a cathode;

an anode; and

an organic thin-film layer disposed between the cathode and the anode, the organic thin-film layer having one or more layers comprising an emitting layer, the emitting layer comprising a first material of formula (1e) below and a second material in a form of a fluorescent dopant material,

12. The device of claim 11, wherein the emitting layer does not comprise a heavy metal complex.

13. The device of claim 11, wherein, in B,

at least one of R³¹ to R³⁸ in the formula (5) has a structure of formula (51) below,

14. The device of claim 11, wherein the second material in the form of the fluorescent dopant material has a structure of formula (20) below,

15. The device of claim 11, exhibiting a delayed fluorescence ratio larger than 37.5%.

16. The device of claim 11, exhibiting a residual strength ratio larger than 36.0% after an elapse of 1 μs after voltage removal in a transitional EL measurement.

17. The device of claim 11, wherein a half bandwidth of a photoluminescence spectrum of the first material is 50 nm or more.

18. The device of claim 11, wherein X¹ is a linking group of the formula (43), and R^x in the formula (43) is a single bond bonded with L.

19. The device of claim 11, wherein at least one of R³¹ to R³⁸ has a structure of formula (51),

20. The device of claim 19, wherein a plurality of X¹ are linking groups of the formula (43), and at least one of R^x in the formula (43) is a single bond bonded with L.

21. The device of claim 11, wherein the fluorescent dopant material is selected from the group consisting of a bisarylamino naphthalene derivative, an aryl-substituted naphthalene derivative, a bisarylamino anthracene derivative, an aryl-substituted anthracene derivative, a bisarylamino pyrene derivative, an aryl-substituted pyrene derivative, a bisarylamino chrysene derivative, an aryl-substituted chrysene derivative, a bisarylamino fluoranthene derivative, an aryl-substituted fluoranthene derivative, an indenoperylene derivative, a pyrromethene boron complex compound, a compound having a pyrromethene skeleton or a metal complex thereof, a diketopyrrolopyrrole derivative, and a perylene derivative.

22. The device of claim 11, wherein the first material does not comprise a structure of formula (51A) in a molecule,

23. The device of claim 11, wherein the first material does not comprise a partial structure of formula (b-5) in a molecule,

24. The device of claim 23, wherein the first material does not comprise a structure of formula (51C) in a molecule,

25. The device of claim 11, wherein the first material does not comprise a structure of formula (51D) in a molecule,

26. The device of claim 11, wherein the first material does not comprise in a molecule (i) and (ii):

(i) a structure of the formula (2),

27. The device of claim 24, wherein the first material does not comprise in a molecule (i) and (ii):

(i) a structure of the formula (2),

28. The device of claim 24, wherein the first material does not comprise in a molecule a structure of the formula (2),

29. The device of claim 24, wherein the first material does not comprise in a molecule (i) and (ii):

(i) a structure of the formula (2),

30. The device of claim 14, wherein the emitting layer does not comprise a heavy metal complex.

31. The device of claim 15, wherein the emitting layer does not comprise a heavy metal complex.

32. The device of claim 16, wherein the emitting layer does not comprise a heavy metal complex.

33. The device of claim 17, wherein the emitting layer does not comprise a heavy metal complex.

34. The device of claim 26, wherein the emitting layer does not comprise a heavy metal complex.

35. The device of claim 27, wherein the emitting layer does not comprise a heavy metal complex.

36. The device of claim 28, wherein the emitting layer does not comprise a heavy metal complex.

37. The device of claim 29, wherein the emitting layer does not comprise a heavy metal complex.

38. The device of claim 11, wherein a difference ΔST(H) between a singlet energy EgS(H) of the first material and an energy gap Eg_77K(H) of the first material at 77[K] satisfies a formula (2),

ΔST(H)=EgS(H)−Eg_77K(H)<0.3 [eV]  (2).

39. The device of claim 38, wherein the emitting layer does not comprise a heavy metal complex.

40. The device of claim 11, wherein the first material is a thermally activated delayed fluorescent compound.

41. The device of claim 40, wherein the emitting layer does not comprise a heavy metal complex.

42. The device of claim 11, wherein the emitting layer is not phosphorescent.

43. The device of claim 11, wherein X¹, in the group of the formula (5), is the linking group of the formula (41).

44. The device of claim 11, wherein the X¹, in the group of the formula (5), is the linking group of the formula (42).

45. The device of claim 11, wherein the X¹, in the group of the formula (5), is the linking group of the formula (43).

46. The device of claim 11, wherein the X¹, in the group of the formula (5), is the linking group of the formula (45).

47. The device of claim 11, wherein, in two B in the formula (1e), one of the combinations of R³¹ and R³², R³² and R³³, R³³ and R³⁴, and R³⁵ and R³⁶, R³⁶ and R³⁷, and R³⁷ and R³⁸ forms a saturated or unsaturated cyclic structure.

48. The device of claim 11, wherein, in at least one of the two B in the formula (1e), R³¹ and R³² form a saturated or unsaturated cyclic structure.

49. The device of claim 11, wherein, in at least one of the two B in the formula (1e), R³² and R³³ form a saturated or unsaturated cyclic structure.

50. The device of claim 11, wherein, in at least one of the two B in the formula (1e), R³³ and R³⁴ form a saturated or unsaturated cyclic structure.

51. The device of claim 11, wherein, in at least one of the two B in the formula (1e), R³⁵ and R³⁶ form a saturated or unsaturated cyclic structure.

52. The device of claim 11, wherein, in at least one of the two B in the formula (1e), R³⁶ and R³⁷ form a saturated or unsaturated cyclic structure.

53. The device of claim 11, wherein, in at least one of the two B in the formula (1e), R³⁷ and R³⁸ form a saturated or unsaturated cyclic structure.

54. An organic electroluminescence device, comprising:

a cathode;

an anode; and

an organic thin-film layer disposed between the cathode and the anode, the organic thin-film layer having one or more layers comprising an emitting layer, the emitting layer comprising a first material of formula (1e) below and a second material in a form of a fluorescent dopant material,