CN113795938B - Organic light-emitting element, laminate, and light-emitting method - Google Patents
Organic light-emitting element, laminate, and light-emitting method Download PDFInfo
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- CN113795938B CN113795938B CN202080032406.1A CN202080032406A CN113795938B CN 113795938 B CN113795938 B CN 113795938B CN 202080032406 A CN202080032406 A CN 202080032406A CN 113795938 B CN113795938 B CN 113795938B
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Classifications
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- H—ELECTRICITY
- H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
- H10K—ORGANIC ELECTRIC SOLID-STATE DEVICES
- H10K50/00—Organic light-emitting devices
- H10K50/10—OLEDs or polymer light-emitting diodes [PLED]
- H10K50/18—Carrier blocking layers
- H10K50/181—Electron blocking layers
-
- H—ELECTRICITY
- H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
- H10K—ORGANIC ELECTRIC SOLID-STATE DEVICES
- H10K50/00—Organic light-emitting devices
- H10K50/10—OLEDs or polymer light-emitting diodes [PLED]
- H10K50/11—OLEDs or polymer light-emitting diodes [PLED] characterised by the electroluminescent [EL] layers
-
- H—ELECTRICITY
- H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
- H10K—ORGANIC ELECTRIC SOLID-STATE DEVICES
- H10K50/00—Organic light-emitting devices
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- H—ELECTRICITY
- H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
- H10K—ORGANIC ELECTRIC SOLID-STATE DEVICES
- H10K50/00—Organic light-emitting devices
- H10K50/10—OLEDs or polymer light-emitting diodes [PLED]
- H10K50/11—OLEDs or polymer light-emitting diodes [PLED] characterised by the electroluminescent [EL] layers
- H10K50/12—OLEDs or polymer light-emitting diodes [PLED] characterised by the electroluminescent [EL] layers comprising dopants
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- H—ELECTRICITY
- H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
- H10K—ORGANIC ELECTRIC SOLID-STATE DEVICES
- H10K50/00—Organic light-emitting devices
- H10K50/10—OLEDs or polymer light-emitting diodes [PLED]
- H10K50/11—OLEDs or polymer light-emitting diodes [PLED] characterised by the electroluminescent [EL] layers
- H10K50/125—OLEDs or polymer light-emitting diodes [PLED] characterised by the electroluminescent [EL] layers specially adapted for multicolour light emission, e.g. for emitting white light
- H10K50/13—OLEDs or polymer light-emitting diodes [PLED] characterised by the electroluminescent [EL] layers specially adapted for multicolour light emission, e.g. for emitting white light comprising stacked EL layers within one EL unit
- H10K50/131—OLEDs or polymer light-emitting diodes [PLED] characterised by the electroluminescent [EL] layers specially adapted for multicolour light emission, e.g. for emitting white light comprising stacked EL layers within one EL unit with spacer layers between the electroluminescent layers
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- H—ELECTRICITY
- H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
- H10K—ORGANIC ELECTRIC SOLID-STATE DEVICES
- H10K85/00—Organic materials used in the body or electrodes of devices covered by this subclass
- H10K85/30—Coordination compounds
- H10K85/351—Metal complexes comprising lanthanides or actinides, e.g. comprising europium
-
- H—ELECTRICITY
- H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
- H10K—ORGANIC ELECTRIC SOLID-STATE DEVICES
- H10K85/00—Organic materials used in the body or electrodes of devices covered by this subclass
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Abstract
An organic light-emitting element having a structure in which an exciton generation layer including a material having Δe ST of 0.3eV or less, a blocking layer, and a singlet splitting layer including a singlet splitting material are sequentially stacked has high light-emitting efficiency.
Description
Technical Field
The present invention relates to a laminate having high exciton generation efficiency and an organic light-emitting element using the laminate. The invention also relates to a light emitting method.
Background
An organic electroluminescent element (organic EL element) is an organic light-emitting element that emits light by deactivation of emission of excitons generated by current excitation of an organic light-emitting layer. Wherein in current excitation, singlet excitons and triplet excitons are generated with a probability of 25% to 75% according to the spin statistics theorem, but emission deactivation from the excited triplet state to the ground state singlet state is otherwise prohibitive. Therefore, in a general organic light emitting material, since triplet excitons are thermally deactivated before emission deactivation, only singlet excitons generated with a probability of 25% can be utilized in light emission, and thus, the improvement of light emission efficiency is limited.
Accordingly, as a result of intensive studies on light-emitting materials to break through such a limit of light-emitting efficiency, phosphorescent materials capable of utilizing triplet excitons in light emission or thermally active delayed fluorescent materials (TADF) capable of converting triplet excitons into singlet excitons and utilizing them in light emission have been developed, and singlet excitons and triplet excitons generated by current excitation can be converted into light at 100% efficiency.
However, even with the above-described phosphorescent material or thermally active delayed fluorescent material, the upper limit of the exciton generation efficiency is 100%, which determines the upper limit of the light emission efficiency of the organic EL element. On the other hand, studies on "singlet split materials" that split the singlet excitons into 2 triplet excitons after being excited into an excited singlet state have been made, and organic EL elements have been proposed in which the singlet split materials are used in the host material of the light-emitting layer so that the exciton generation efficiency exceeds 100% (see patent document 1).
Technical literature of the prior art
Patent literature
Patent document 1 International publication No. 2019/022120
Disclosure of Invention
Technical problem to be solved by the invention
As described above, an organic EL element using a singlet split material has been proposed. In this organic EL element, specifically, since the singlet excitons and the triplet excitons are generated with a probability of 25% to 75%, and then the singlet excitons are split into 2 triplet excitons, 125% of the triplet excitons are theoretically generated. It is considered that, although the exciton generation efficiency of 125% is greatly improved as compared with the case where the singlet split material is not used, this is caused by a simple structure in which only the singlet split material is mixed into the light emitting layer, and by further investigation, a larger exciton generation efficiency can be obtained.
Under such circumstances, the present inventors have conducted intensive studies with a view to developing a mechanism in which the theoretical limit value of the exciton generation efficiency is higher to significantly improve the light emission efficiency of the organic light emitting element.
Means for solving the technical problems
As a result of intensive studies, the present inventors have obtained ideas such as combining reverse intersystem crossing and singlet splitting. That is, the following novel mechanism is conceived: when the singlet excitons and the triplet excitons are generated by current excitation with a probability of 25% to 75%, the generated triplet excitons are all converted to the singlet excitons by reverse intersystem crossing, and then the singlet excitons are split, 200% of the triplet excitons can theoretically be generated. The present invention has been made in view of such a concept, and specifically has the following structure.
[1] An organic light emitting element comprising a material capable of converting from triplet excitons to singlet excitons and a singlet splitting material.
[2] The organic light-emitting element according to [1], which is provided with a mechanism for suppressing direct transfer of energy of triplet excitons in the material capable of converting from triplet excitons to singlet excitons into the singlet split material.
[3] The organic light-emitting element according to [1] or [2], wherein a difference Δe ST between a lowest excited singlet energy level (E S1) and a lowest excited triplet energy level (E T1) of the material capable of converting from triplet excitons to singlet excitons is 0.3eV or less.
[4] The organic light-emitting element according to any one of [1] to [3], which has a structure in which an exciton generation layer containing a material capable of converting from triplet excitons to singlet excitons, a blocking layer, and a singlet split layer containing a singlet split material are laminated in this order.
[5] The organic light-emitting element according to [4], wherein the singlet split layer contains a light-emitting material.
[6] The organic light-emitting element according to [5], wherein a lowest excited singlet energy level E S1 of the light-emitting material is 0.2eV or more higher than a lowest excited singlet energy level E S1 of the material capable of converting from triplet excitons to singlet excitons.
[7] The organic light-emitting element according to [5] or [6], wherein the light-emitting material is a phosphorescent material.
[8] The organic light-emitting element according to any one of [5] to [7], wherein the light-emitting material is a metal complex with a lanthanoid (lanthanoid) as a central metal.
[9] The organic light-emitting element according to any one of [4] to [8], wherein a thickness of the barrier layer is 2 to 10nm.
[10] The organic light-emitting element according to any one of [4] to [9], wherein a lowest excited singlet energy level E S1 of a material of the blocking layer is 0.2eV or more higher than a lowest excited singlet energy level E S1 of the material capable of converting from triplet excitons to singlet excitons.
[11] The organic light-emitting element according to any one of [4] to [10], wherein a lowest excited triplet level E T1 of a material of the blocking layer is 0.2eV or more higher than a lowest excited triplet level E T1 of the material capable of converting from triplet excitons to singlet excitons.
[12] The organic light-emitting element according to any one of [1] to [11], wherein the material capable of converting from triplet excitons to singlet excitons is a delayed fluorescent material.
[13] The organic light-emitting element according to any one of [4] to [12], which has a1 st singlet split layer and a2 nd singlet split layer as the singlet split layer and has a1 st barrier layer and a2 nd barrier layer as the barrier layer, and has a structure in which the 1 st singlet split layer, the 1 st barrier layer, the exciton generation layer, the 2 nd barrier layer, and the 2 nd singlet split layer are laminated in this order.
[14] The organic light-emitting element according to [13], wherein the 1 st barrier layer contains a material having a hole-transporting property, and the 2 nd barrier layer contains a material having an electron-transporting property.
[15] The organic light-emitting element according to [13] or [14], wherein the 1 st barrier layer contains a compound having an aromatic ring in which at least 1 hydrogen atom is substituted with a substituent bonded through a nitrogen atom, and the 2 nd barrier layer contains a compound having an aromatic hetero six-membered ring containing at least 1 nitrogen atom as a ring member.
[16] The organic light-emitting element according to any one of [1] to [15], comprising an anode, a cathode, and a laminate disposed between the anode and the cathode and having a structure in which a1 st singlet split layer, a1 st barrier layer, an exciton generation layer, a 2 nd barrier layer, and a 2 nd singlet split layer are laminated in this order from the anode side, the 1 st barrier layer containing a material having hole-transporting property, and the 2 nd barrier layer containing a material having electron-transporting property.
[17] The organic light-emitting element according to any one of [1] to [16], which is an organic electroluminescent element.
[18] A laminate having a structure in which an exciton generation layer containing a material capable of converting from triplet excitons to singlet excitons, a blocking layer, and a singlet splitting layer containing a singlet splitting material are laminated in this order.
[19] A method of emitting light comprising the steps of:
a material capable of converting from triplet excitons to singlet excitons is subjected to current excitation,
The energy of the singlet excitons generated by the current excitation and the reverse intersystem crossing is transferred into the singlet splitting material to generate singlet excitons of the singlet splitting material, and the singlet excitons of the singlet splitting material are split.
Effects of the invention
The theoretical limit value of the exciton generation efficiency of the organic light emitting element and the light emitting method of the present invention is extremely high, 200%. Therefore, by using the present invention, very high luminous efficiency can be achieved.
Drawings
Fig. 1 is a schematic diagram for explaining an exciton generation mechanism and a light emission mechanism of the laminate of the present invention.
Fig. 2 is a schematic diagram for explaining an exciton generation mechanism and a light emission mechanism of the laminate of the present invention in which a blocking layer is formed.
Fig. 3 is a schematic cross-sectional view showing an example of a layer structure of an organic electroluminescent element to which the laminate of the present invention is applied.
Fig. 4 is an energy level diagram of the laminate 1 produced in example 1.
Fig. 5 is a schematic diagram for explaining the exciton generation mechanism and the light emission mechanism of the laminate 1 produced in example 1.
Fig. 6 is an energy level diagram of the comparative laminate 1 produced in comparative example 1.
Fig. 7 is a schematic diagram for explaining the exciton generation mechanism and the light emission mechanism of the comparative laminate 1 produced in comparative example 1.
Fig. 8 is an energy level diagram of the comparative laminate 2 produced in comparative example 2.
Fig. 9 is a schematic diagram for explaining the exciton generation mechanism and the light emission mechanism of the comparative laminate 2 produced in comparative example 2.
Fig. 10 is an energy level diagram of the comparative laminate 3 produced in comparative example 3.
Fig. 11 shows emission spectra of the laminate 1 and the comparative laminates 1 to 3.
Fig. 12 is a light emission spectrum of the organic electroluminescent element fabricated in example 2.
Detailed Description
The following describes the content of the present invention in detail. The following description of the constituent elements may be based on the representative embodiments or specific examples of the present invention, but the present invention is not limited to such embodiments or specific examples. In the present specification, the numerical range indicated by "to" is a range including the numerical values described before and after "to" as the lower limit value and the upper limit value. The isotope type of the hydrogen atom present in the molecule of the compound used in the present invention is not particularly limited, and for example, the hydrogen atom in the molecule may be 1 H in its entirety, or 2 H (deuterium (deuterium) D) in its entirety.
In the present specification, the "fluorescent material" is a luminescent material having a higher luminescence intensity of fluorescence than luminescence intensity of phosphorescence when luminescence is observed at 20 ℃, and the "phosphorescent material" is a luminescent material having a higher luminescence intensity of phosphorescence than luminescence intensity of fluorescence when luminescence is observed at 20 ℃. The "delayed fluorescent material" is a material that emits light from an excited singlet state and that emits both fluorescence having a short emission lifetime and fluorescence having a long emission lifetime (delayed fluorescence) at 20 ℃. The usual fluorescence (not fluorescence of delayed fluorescence) is a material that emits light from an excited singlet state, and the light emission lifetime is of the order of ns, the phosphorescence is a material that emits light from a triplet state, and the light emission lifetime is usually of the order of μs or more. In the present specification, "reverse intersystem crossing" means conversion from triplet excitons to singlet excitons.
In this specification, the thickness of each layer (exciton generation layer, blocking layer, singlet split layer, other layer) constituting the laminate is measured by, for example, a stylus analysis system (profiling system).
< Organic light-emitting element >)
The organic light-emitting element of the present invention is characterized by comprising a material capable of converting from triplet excitons to singlet excitons and a singlet split material. In the organic light-emitting element of the present invention, energy transfer from a material capable of converting from triplet excitons to singlet excitons to a singlet splitting material is performed, whereby light can be emitted with good efficiency. More specifically, by transferring the energy of singlet excitons in a material capable of converting from triplet excitons to singlet excitons into a singlet splitting material to generate singlet excitons of the singlet splitting material and splitting the generated singlet excitons of the singlet splitting material, triplet excitons can be efficiently obtained and emitted. In the organic light-emitting element of the present invention, phosphorescence may be directly emitted from the singlet split material, or energy of triplet excitons in the singlet split material may be transferred to the light-emitting material to emit light from the light-emitting material.
The "material capable of converting from triplet excitons to singlet excitons" in the present invention refers to a material capable of converting triplet excitons of the material 1 to singlet excitons and utilizing singlet excitons in light emission. In the present invention, a material capable of converting from triplet excitons to singlet excitons is not preferable as it is, but a material capable of self-luminescence under conditions other than the present invention can be used in the present invention. The conversion of 1 to 1 means conversion of 1 Triplet exciton to 1 singlet exciton, for example, triplet-Triplet annihilation (Triplet annihilation) (TTA) and the like. The use of singlet excitons for light emission includes the following cases: fluorescence is emitted when the singlet excitons return to the ground state, or the energy of the singlet excitons is used in exciton generation or the like of other materials to finally induce luminescence. Therefore, as long as the material is a material capable of converting triplet excitons 1 to 1 into singlet excitons and of being used for light emission by transferring the energy of the singlet excitons into a singlet split material according to the light emission method of the present invention, the material "a material capable of converting from triplet excitons into singlet excitons" is satisfied regardless of the structure thereof.
As an example of the "material capable of converting from triplet excitons to singlet excitons", a material which is likely to cause a transition from an excited triplet state to an excited singlet state in a reverse system or a material capable of transition from a higher-order excited triplet state in a reverse system (Δe Tn) to a lowest excited triplet state in a material (for example, a material described in WO2014185408 A1) may be mentioned, since a difference Δe ST between a lowest excited triplet state energy level (E S1) and a lowest excited triplet state energy level (E T1) is small and the lowest excited triplet state energy level (E S1) is close to the lowest excited triplet state energy level (E T1). In addition, a material that actually emits delayed fluorescence or a material that induces emission of delayed fluorescence (including a so-called auxiliary dopant) is also "a material capable of converting from triplet excitons to singlet excitons". Hereinafter, a specific example of "a material capable of converting from triplet excitons to singlet excitons" will be described using "a material having Δe ST of 0.3eV or less". The following description of the constituent elements may be based on the representative embodiments or specific examples of the present invention, but the present invention is not limited to such embodiments or specific examples. Therefore, in the following description, "a material having Δe ST of 0.3eV or less" can be replaced with another "a material capable of converting from triplet excitons to singlet excitons".
In the present invention, Δe ST of "a material having Δe ST of 0.3eV or less" means a difference (E S1-ET1) between the lowest excited singlet energy level (E S1) and the lowest excited triplet energy level (E T1). Since the lowest excited singlet energy level (E S1) of the "material having Δe ST of 0.3eV or less" is close to the lowest excited triplet energy level (E T1), a reverse intersystem crossing from the excited triplet state to the excited singlet state is likely to occur. Therefore, by using "a material having Δe ST of 0.3eV or less" as a material of the exciton generation layer, singlet excitons can be efficiently generated in the layer.
It can be confirmed that it is a material that causes a reverse intersystem crossing from an excited triplet state to an excited singlet state by observing emission of delayed fluorescence. The "delayed fluorescence" is fluorescence that is emitted when returning from an excited triplet state to a ground state after generation of an intersystem crossing from the excited triplet state to the excited singlet state and is observed with a delay from fluorescence (normal fluorescence) of the excited triplet state that is directly generated.
The Δe ST of "material having Δe ST of 0.3eV or less" is preferably 0.2eV or less, more preferably 0.1eV or less, and still more preferably 0.05eV or less.
For the method of measuring Δe ST, the following description in column (method of measuring E S1、ET1、ΔEST) can be referred to.
The "singlet split material" in the present invention refers to a material in which each singlet exciton generated therein can be split into 2 triplet excitons after transition to an excited singlet state.
If the compound causes singlet cleavage, the number of triplet excitons increases as a result. Therefore, it can be confirmed that it is a singlet split material using the triplet generation efficiency Φ ISC as an index. Specifically, a solution containing the object compound to be determined at different concentrations is irradiated with pump light as excitation light, and then immediately after that, the amount of change Δabs in absorbance with respect to Probe light (Probe light) is measured. By observing a correlation in which the triplet formation efficiency Φ ISC obtained from the following formula (I) increases as the concentration of the target compound increases, it can be determined that the target compound is a singlet split material. Here, the "change in absorbance Δabs" refers to a change in absorbance with reference to absorbance ABS 0 with respect to the probe light before the pump light irradiation, and here refers to a value obtained by subtracting ABS 0 from absorbance ABS EX with respect to the probe light measured immediately after the pump light irradiation. The concentration of the target compound in the solution is selected within a concentration range in which concentration quenching is substantially suppressed. The singlet-splitting material may be confirmed by observing a phenomenon or a sign indicating an increase in the number of triplet excitons by singlet splitting, or by using a measurement method other than the above.
[ Number 1]
(Formula I)
In the formula (I), Φ ISC denotes triplet formation efficiency, I 0 denotes intensity of pump light (excitation light intensity) irradiated to the solution, Δabs denotes absorbance change amount (ABS EX-ABS0), ε denotes a molar absorption coefficient of the target compound at a pump light wavelength, ε T denotes a molar absorption coefficient of the target compound at a probe light wavelength, c denotes concentration of the target compound in the solution, and L denotes an optical path length (1 mm) of a unit used in measurement.
The organic light-emitting element of the present invention preferably has a mechanism for suppressing direct transfer of energy of triplet excitons of a material having Δe ST of 0.3eV or less into a singlet split material. The energy of the triplet exciton described herein may be the energy of the triplet exciton converted from the singlet exciton, for example, or the energy of the triplet exciton directly generated by current excitation. The mechanism for suppressing the direct transfer of the energy of the triplet exciton to the singlet splitting material is not particularly limited as long as the mechanism is not present and more energy of the triplet exciton is transferred to the singlet splitting material. A preferable example of such a mechanism is a blocking layer formed between an exciton generation layer containing a material having Δe ST of 0.3eV or less and a singlet splitting layer containing a singlet splitting material.
The "blocking layer" in the present invention is a layer which is disposed between the exciton generation layer and the singlet split layer, and which prevents the transition from "material having Δe ST of 0.3eV or less" to the tex of the excited triplet energy of the "singlet split material" contained in the singlet split layer, and which allows the transition from "material having Δe ST of 0.3eV or less" to the foster of the excited triplet energy of the "singlet split material" contained in the singlet split layer ". The barrier layer must be a layer that inhibits energy transfer from a material having a Δe ST of 0.3eV or less or a singlet split material. The structure in which the exciton generation layer, the blocking layer, and the singlet split layer are laminated in this order is referred to as a "laminate of the present invention".
The laminate of the present invention has such a structure that, when energy or carriers causing excitation are supplied from the outside to excite molecules in the exciton generation layer, singlet excitons are efficiently generated in the exciton generation layer, and the excited singlet energy thereof is transferred into the singlet splitting layer to split the singlet excitons generated therein into 2 triplet excitons. Therefore, high exciton generation efficiency can be obtained, for example, in the case where a light-emitting material is contained in the singlet split layer or in the case where the singlet split material itself emits light, high light-emitting efficiency can be obtained. In the following, a case where the singlet split layer includes a phosphorescent material will be described as an example, with reference to fig. 1 and 2, as an exciton generation mechanism and a light emission mechanism thereof. In fig. 1 and 2, S 1 represents "singlet excitons of the lowest excited singlet state", T 1 represents "triplet excitons of the lowest excited triplet state", S n represents both "singlet excitons of the lowest excited singlet state" and "singlet excitons of the higher-order excited singlet states having an order of 2 to n (n represents a natural number)". Each energy level of S 1、Sn、T1 is denoted as E S1、ESn、ET1, respectively. The energy relationship shown in fig. 1 and 2 represents an example of the energy relationship that can be used in the present invention, and S 1 may be S n,T1 or T n. That is, the laminate of the present invention should not be construed as being limited to a laminate having such an energy relationship.
In the following description, a value obtained by the following formula (II) is referred to as "exciton generation efficiency".
Formula (II)
Exciton generation efficiency (%) = (N 1/N0) ×100
In formula (II), N 0 represents the amount of excitons (initial excitons) directly generated in the exciton generation layer by excitation, and N 1 represents the amount of excitons (secondary excitons) generated in the singlet split layer by the initial excitons in the subsequent process. In the case of the conventional structure described in patent document 1, that is, in the case of a single singlet split layer or a single light emitting layer without an exciton generation layer and a blocking layer, N 0 represents the amount of excitons (initial excitons) directly generated in the singlet split layer or in the light emitting layer by excitation, and N 1 represents the sum of the amount of excitons (secondary excitons) generated in the singlet split layer or in the light emitting layer by the initial excitons and the amount of the initial excitons remaining without being used for the generation of the secondary excitons. In either structure, the secondary excitons refer to excitons generated by the initial excitons, but in the present invention, for example, singlet excitons generated in a singlet split material by receiving transfer of excitation energy from the initial excitons, triplet excitons split from the singlet excitons, and the like.
As shown in fig. 1 and 2, when carriers (h +、e-) are injected into the exciton generation layer of the laminate and recombination of carriers occurs in a material having Δe ST of 0.3eV or less, singlet excitons 1 and triplet excitons T 1 are generated with a probability of 25%:75%, and triplet excitons T 1 cause a transition from an excited triplet state to an excited singlet state in reverse intersystem crossing, thereby converting into singlet excitons 1. The energy of the singlet exciton S 1 generated in the exciton generation layer is transferred to the excited singlet energy level E Sn of the singlet split material by the foster mechanism (n represents a natural number). ). On the other hand, the laminate of the present invention has "a mechanism for suppressing direct transfer of energy of triplet excitons of a material having Δe ST of 0.3eV or less into a singlet-splitting material", and thus prevents direct transfer of energy of triplet excitons T 1 generated in the exciton generation layer into the singlet-splitting material. The transfer mechanism of the excited triplet energy is a different texel mechanism from the transfer mechanism of the excited singlet energy. Therefore, for example, as shown in fig. 2, energy transfer of triplet excitons T 1 generated in the exciton generation layer into the singlet split material is prevented by providing a blocking layer. Thus, the triplet exciton T 1 generated with a probability of 75% is maximally converted to the singlet exciton S 1 by its entire use in the reverse intersystem crossing, and its excited singlet energy is also transferred to the excited singlet energy level E Sn of the singlet splitting material. Therefore, in theory, 100% of the energies of singlet excitons 1 and triplet excitons T 1 generated in a material having Δe ST of 0.3eV or less can be transferred as the excited singlet energy to the excited singlet energy level E Sn of the singlet split material. Then, in the singlet-splitting material that received the excitation singlet energy, singlet excitons 1 are generated at an exciton generation efficiency of 100% at maximum, and each singlet exciton S 1 is split into 2 triplet excitons T 1, whereby triplet excitons T 1 are generated at an exciton generation efficiency of 200%. Phosphorescent light emission is generated by transferring the energy of the triplet exciton T 1 to the lowest excited triplet level E T1 of the phosphorescent material and performing emission deactivation. In this laminate, since the triplet exciton generation efficiency is theoretically 200%, an extremely high light emission efficiency can be obtained as compared with a case where only triplet excitons generated with a probability of 75% are used in a normal light emitting layer in phosphorescence.
Further, assuming that the above 3 layers have no exciton generation layer and blocking layer but only a singlet split layer, by injecting carriers in the singlet split layer, in the singlet split material thereof, singlet excitons and triplet excitons are generated with a probability of 25%:75%, and the singlet excitons therein are split into 2 triplet excitons. At this time, the theoretical limit value of triplet exciton generation efficiency is 25% ×2+75% =125%, and becomes a value far lower than the structure of the present invention having the exciton generation layer and the blocking layer.
As described above, the laminate of the present invention has a high theoretical limit value of exciton generation efficiency and exhibits high light emission efficiency by including all of the exciton generation layer, the blocking layer, and the singlet split layer.
Among them, in order to reliably obtain such an effect, it is important to allow transfer of the excited triplet energy from a material having Δe ST of 0.3eV or less to a singlet-splitting material, and to reliably prevent transfer of the excited triplet energy from a material having Δe ST of 0.3eV or less to a singlet-splitting material using a barrier layer.
From this viewpoint, the thickness of the barrier layer is preferably 2 to 10nm. The lower limit of the thickness of the barrier layer is more preferably 2.5nm or more, still more preferably 3nm or more, particularly preferably 5nm or more, and the upper limit of the thickness of the barrier layer is more preferably 10nm or less, still more preferably 8nm or less, particularly preferably 7nm or less. If the thickness of the blocking layer is too thin, energy of triplet excitons generated in a material having Δe ST of 0.3eV or less is transferred to an excited triplet energy level E Tn (n represents a natural number) of the singlet-splitting material or the light-emitting material by a texel mechanism. As a result, in a material having Δe ST of 0.3eV or less, the probability of occurrence of a reverse intersystem crossing from the excited triplet state (T 1) to the excited singlet state (S 1) becomes low, and thus singlet excitons cannot be sufficiently increased. In contrast, if the thickness of the blocking layer is too thick, it is difficult to cause the transfer of the foster resonance energy from the lowest excited singlet energy level E S1 of the material having Δe ST of 0.3eV or less to the excited singlet energy level E Sn of the singlet split material, and there is a possibility that the exciton multiplication mechanism based on the singlet split material may not function sufficiently.
The lowest excited singlet energy level E S1 of the material of the blocking layer or the light-emitting material is preferably 0.2eV or more, more preferably 0.3eV or more higher than the lowest excited singlet energy level E S1 of the material having Δe ST of 0.3eV or less. This suppresses energy transfer from the lowest excited singlet energy level E S1 of a material having a Δe ST of 0.3eV or less to the lowest excited singlet energy level E S1 of a material of the blocking layer or a light-emitting material, and thereby enables energy efficiency of singlet excitons generated in a material having a Δe ST of 0.3eV or less to be transferred to the excited singlet energy level E Sn of the singlet splitting material. As a result, the exciton multiplication mechanism based on the singlet split material can be effectively operated.
The lowest excited triplet level E T1 of the material of the barrier layer is preferably 0.2eV or more, more preferably 0.3eV or more, higher than the lowest excited triplet level E T1 of the material having Δe ST of 0.3eV or less. This can prevent the energy of triplet excitons generated in a material having Δe ST of 0.3eV or less from being transferred to the excited triplet level E Tn of the singlet-splitting material by energy transfer to the lowest excited triplet level E T1 of the blocking layer.
Further, it is preferable that the light-emitting peak of the compound having Δe ST of 0.3eV or less overlaps with the light absorption band of the singlet split material, and the material of the blocking layer and the light-emitting material do not exhibit light absorption in the wavelength region where they overlap. This suppresses energy transfer from the compound having Δe ST of 0.3eV or less to the barrier layer material and the light-emitting material, and efficiently generates energy transfer from the compound having Δe ST of 0.3eV or less to the singlet split material.
The exciton generation mechanism and the light emission mechanism of the laminate of the present invention are described above, but the mechanism by which the laminate of the present invention exerts its effect is not limited to such a mechanism. For example, in fig. 1 and 2, the excited state is represented by "S 1"、"T1" in a limited manner, but the excited singlet state and the excited triplet state of each material of the laminate are not limited to the lowest excited singlet state (S 1) and the lowest excited triplet state (T 1), and may be the excited singlet state S 2、S3……Sn or the excited triplet state T 2、T3……Tn higher than the above. In this case, the same mechanism can also be used to obtain high exciton generation efficiency and high light emission efficiency. In the case of using a metal complex as a light-emitting material, the target of the triplet energy transfer of triplet excitons generated by singlet cleavage may be an electron excited state energy level in a central metal of the light-emitting material (metal complex) in addition to the excited triplet energy level E T1 of the light-emitting material. In addition to the luminescent material, the luminescent material may be an organic phosphorescent material or an inorganic phosphorescent material. Also, a material capable of converting from triplet excitons to singlet excitons is used herein, and may be caused to emit light from S n instead of T n.
(Method for measuring E S1、ET1、ΔEST)
"Δe ST" in the present invention is a value obtained by calculating the minimum excited singlet energy level (E S1) and the minimum triplet energy level (E T1) by the following method and calculating from Δe ST＝ES1-ET1. In addition, in the case of a literature value, a simple study or investigation can be performed using the literature value.
(1) Minimum excited singlet energy level E S1
The compound to be measured was dissolved in toluene at a concentration of 10 -4 M or 10 -5 M to prepare a sample, and the fluorescence spectrum of the sample was measured at room temperature (300K). The fluorescence spectrum of the sample was measured at room temperature (300K). Specifically, by integrating the luminescence from the immediately incident excitation light to 100 ns after the incident excitation light, a fluorescence spectrum was obtained in which the vertical axis was the luminescence intensity and the horizontal axis was the wavelength. A tangential line is drawn with respect to the rise of the fluorescence spectrum on the short wavelength side, and the wavelength value λedge [ nm ] of the intersection point of the tangential line and the horizontal axis is obtained. The wavelength value was converted into an energy value by a conversion equation shown below, and the obtained value was set to E S1.
Conversion formula: e S1 [ eV ] = 1239.85/λedge
The measurement of the luminescence spectrum can be performed by using, for example, a fluorescence spectrophotometer (HORIBA, ltd., fluoroMax Plus).
(2) Minimum excited triplet level E T1
The same sample as that used in the measurement of the lowest excited singlet energy level E S1 was cooled to 77K, excitation light was irradiated to the sample for phosphorescence measurement, and the phosphorescence intensity was measured using a spectrophotometer. Specifically, by accumulating the luminescence after 100 milliseconds after the incidence of the excitation light, a phosphorescence spectrum is obtained in which the vertical axis is the luminescence intensity and the horizontal axis is the wavelength. A tangential line is drawn with respect to the rise of the phosphorescence spectrum on the short wavelength side, and the wavelength value λedge [ nm ] of the intersection point of the tangential line and the horizontal axis is obtained. The wavelength value was converted into an energy value by a conversion equation shown below, and the obtained value was set to E T1.
Conversion formula: e T1 [ eV ] = 1239.85/λedge
The tangent to the rise on the short wavelength side of the phosphorescence spectrum is drawn as follows. First, consider that when moving from the short wavelength side of the phosphorescence spectrum to the shortest wavelength side maximum of the spectrum maxima on the spectrum curve, the tangent line on each point on the curve on the long wavelength side is oriented. As the curve rises (i.e., as the vertical axis increases), the slope of the tangent increases. The tangential line drawn at the point where the slope value takes the maximum value is defined as the tangential line rising toward the short wavelength side of the phosphorescence spectrum.
The maximum point of the peak intensity having 10% or less of the maximum peak intensity of the spectrum does not include the maximum value on the shortest wavelength side, and a tangential line drawn at a point closest to the maximum value on the shortest wavelength side and having a maximum value of the slope is defined as a tangential line rising on the short wavelength side of the phosphorescence spectrum.
Hereinafter, materials and thicknesses of the exciton generation layer, the blocking layer, and the singlet split layer used in the laminate of the present invention and the layer structure of the laminate will be specifically described.
[ Exciton generating layer ]
The exciton generation layer contains a material having Δe ST of 0.3eV or less. The exciton generation layer may be composed of only a material having Δe ST of 0.3eV or less, or may include a material (other material) other than a material having Δe ST of 0.3eV or less, in addition to a material having Δe ST of 0.3eV or less.
(ΔE ST is 0.3eV or less)
As a material having Δe ST of 0.3eV or less, a delayed fluorescent material can be preferably used.
Examples of the delayed fluorescence material that can be used as a material having Δe ST of 0.3eV or less are given below.
[ Chemical formula 1-1]
[ Chemical formulas 1-2]
Preferable delayed fluorescent materials include 0008 to 0048 and 0095 to 0133 of WO2013/154064, 0007 to 0047 and 0073 to 0085 of WO 2013/01954, 0007 to 0033 and 0059 to 0066 of WO 2013/01955, 0008 to 0071 and 0118 to 0133 of WO2013/081088, 0009 to 0046 and 0093 to 0134 of JP-A2013-256490, and 0093 to 0134 of WO 2013/01955, The compounds contained in the general formula described in paragraphs 0008 to 0020 and 0038 to 0040 of Japanese patent application laid-open No. 2013-116975, paragraphs 0007 to 0032 and 0079 to 0084 of WO2013/133359, paragraphs 0008 to 0054 and 0101 to 0121 of WO2013/161437, paragraphs 0007 to 0041 and 0060 to 0069 of Japanese patent application laid-open No. 2014-9352, paragraphs 0008 to 0048 and 0067 to 0076 of Japanese patent application laid-open No. 2014-9224, In particular, compounds which exemplify the compounds and emit delayed fluorescence. Furthermore, japanese patent application laid-open No. 2013-253121, WO2013/133359, WO2014/034535, WO2014/115743, WO2014/122895, WO2014/126200, WO2014/136758, WO2014/133121, WO2014/136860, WO2014/196585, WO2014/189122, WO2014/168101, and the like can be preferably used, WO2015/008580, WO2014/203840, WO2015/002213, WO2015/016200, WO2015/019725, WO2015/072470, WO2015/108049, WO2015/080182, WO2015/072537, WO2015/080183, japanese patent application laid-open publication No. 2015-129240, WO2015/129714, A material which emits delayed fluorescence and emits light-emitting materials described in WO2015/129715, WO2015/133501, WO2015/136880, WO2015/137244, WO2015/137202, WO2015/137136, WO2015/146541, and WO 2015/159541. The above publication described in this paragraph is incorporated herein by reference as part of the present specification.
These Δe ST are 0.3eV or less, and 1 or 2 or more may be used alone or in combination.
(Materials other than the material having ΔE ST of 0.3eV or less)
The exciton generation layer may contain materials (other materials) other than the material having Δe ST of 0.3eV or less as necessary. As the other material, a host material can be exemplified. When the exciton generation layer contains a host material, a material having Δe ST of 0.3eV or less may be uniformly dispersed and present in the host material, or may be locally present in a partial region.
When the exciton generation layer contains other materials, the content of the material having Δe ST of 0.3eV or less is preferably 1 wt% or more, more preferably 5wt% or more, and still more preferably 10 wt% or more, with respect to the total amount of materials of the exciton generation layer.
(Thickness of exciton generation layer)
The thickness of the exciton generation layer is not particularly limited, but is preferably 1nm to 25nm, more preferably 3nm to 20nm, and even more preferably 5nm to 15nm.
[ Barrier layer ]
As described above, as the material of the barrier layer, a material having a lowest excited singlet energy level E S1 higher by 0.2eV or more than a material having a Δe ST of 0.3eV or less is preferably used, and a material having a highest excited singlet energy level E S1 by 0.3eV or more is more preferably used. The material of the barrier layer preferably has a lowest excited triplet energy level E T1 higher by 0.2eV or more, more preferably 0.3eV or more than a lowest excited triplet energy level E T1 of a material having Δe ST of 0.3eV or less.
In the case where the laminate is used in a light-emitting portion of an organic electroluminescent element, the material of the barrier layer is preferably selected in consideration of carrier transport properties. Specifically, the blocking layer disposed on the anode side of the exciton generation layer preferably contains a material having hole-transporting property, and the blocking layer disposed on the cathode side of the exciton generation layer preferably contains a material having electron-transporting property.
As a material having hole-transporting property, a compound having an aromatic ring in which at least 1 hydrogen atom is substituted with a substituent bonded through a nitrogen atom is exemplified.
The aromatic ring may be a single ring, a condensed ring formed by condensing 2 or more aromatic rings, or a connection ring formed by connecting 2 or more aromatic rings. When 2 or more aromatic rings are linked, they may be linked in a straight chain or branched chain. The number of carbon atoms of the aromatic ring is preferably 6 to 40, more preferably 6 to 22, still more preferably 6 to 18, still more preferably 6 to 14, and particularly preferably 6 to 10. Specific examples of the aromatic ring include benzene ring, naphthalene ring and biphenyl ring.
Examples of the substituent bonded through the nitrogen atom include a substituted or unsubstituted diphenylamino group and a 3-ring heteroaryl group having a structure in which phenyl groups of the substituted or unsubstituted diphenylamino group are linked to each other by a single bond or a linking group (for example, an alkylene group).
Examples of the material having electron-transporting properties include compounds having an aromatic hetero six-membered ring containing at least 1 nitrogen atom as a ring member. Among them, examples of the aromatic hetero six-membered ring include a pyridine ring, a pyrazine ring, a pyrimidine ring, a pyridazine ring, and a triazine ring.
Hereinafter, preferable examples of the compound that can be used as a material of the barrier layer are given.
[ Chemical formula 2]
The material of these barrier layers may be used alone or in combination of 1 or more than 2.
The material of the barrier layer is preferably selected in consideration of a combination with a material having Δe ST of 0.3eV or less so as to satisfy the above-described preferable energy relationship with a material having Δe ST of 0.3eV or less. Preferred combinations of the material having ΔE ST of 0.3eV or less and the material of the barrier layer include combinations of ACR-XTN and mAP, combinations of ACR-XTN and B3PyMPM, and combinations of DACT-II and mCP, combinations of DACT-II and PPF, combinations of DMAC-TRZ and mCP, and combinations of DMAC-TRZ and Bphen, which are used in examples described later.
The preferable range of the thickness of the barrier layer can be referred to as above.
[ Chemical formula 3]
[ Singlet splitting layer ]
The singlet splitting layer comprises a singlet splitting material. The singlet splitting layer may be composed of only the singlet splitting material, or may contain a material other than the singlet splitting material (other material) other than the singlet splitting material.
(Singlet split materials)
As described above, the singlet split material is a material in which each singlet exciton generated therein can be split into 2 triplet excitons after transition to excited singlet. The lowest excited singlet energy level E S1 of the singlet split material is preferably lower than the lowest excited singlet energy level E S1 of a material with Δe ST below 0.3 eV. Specifically, the lowest excited singlet energy level E (f) S1 of the singlet split material is more preferably 0.1eV or more, and still more preferably 0.2eV or more lower than the lowest excited singlet energy level E (f) S1 of a material having Δe ST of 0.3eV or less. Further, it is more preferable that the emission spectrum of a material having Δe ST of 0.3eV or less sufficiently overlaps the absorption spectrum of the singlet split material. Thus, the energy of singlet excitons generated in a material having Δe ST of 0.3eV or less can be easily transferred to the emission peak wavelength of a material having a lowest excited singlet energy level E S1.ΔEST of 0.3eV or less of the singlet split material and the absorption peak wavelength of the singlet split material, which are preferably within 100nm, more preferably within 50nm, and even more preferably within 30 nm.
Examples of the compound that can be used as the singlet-splitting material include acenes such as anthracene, tetracene, pentacene, and the like. At least 1 hydrogen atom of these acenes may be substituted with: substituted or unsubstituted aryl, alkenyl substituted with substituted or unsubstituted aryl, alkynyl substituted with substituted or unsubstituted aryl. For the description and preferred ranges and specific examples of the substituted or unsubstituted aryl group, reference can be made to the description and preferred ranges and specific examples of the substituted or unsubstituted aryl group in R 1 of the following general formula (1).
Hereinafter, examples of preferable compounds that can be used as the singlet splitting material are given.
[ Chemical formula 4]
As the singlet split material, a compound represented by the following general formula (1) can also be used.
[ Chemical formula 5]
General formula (1)
In the general formula (1), R 1 represents a substituted or unsubstituted aryl group. One of R 2 and R 3 represents a hydrogen atom and the other represents a substituted or unsubstituted aryl group. The substituted or unsubstituted aryl group represented by one of R 2 and R 3 and the substituted or unsubstituted aryl group represented by R 1 may be the same as or different from each other, but are preferably the same. The number of ring skeleton constituent atoms of the aryl group (in the case of a substituted aryl group, a portion other than a substituent) described in "substituted or unsubstituted aryl group" is preferably 6 to 26, more preferably 6 to 22, still more preferably 6 to 18. Specific examples of the aryl group include phenyl, 1-naphthyl, 2-naphthyl, 1-anthryl, 2-anthryl, 9-anthryl, 1-tetracenyl (tetracenyl), 2-tetracenyl, 5-tetracenyl, 1-pyrenyl and 2-pyrenyl.
The aryl groups that R 1～R3 may employ may be substituted or unsubstituted, but preferably at least 1 of R 1～R3 is an unsubstituted aryl group, more preferably all of the substituted or unsubstituted aryl groups that R 1～R3 may employ are unsubstituted aryl groups. The substituent in the case where the aryl group has a substituent is preferably an alkyl group or an aryl group. The alkyl group may be any of linear, branched, and cyclic. The number of carbon atoms is preferably 1 to 20, more preferably 1 to 10, and still more preferably 1 to 6. For example, methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, tert-butyl, and the like can be exemplified. As the preferable range and specific example of the aryl group, reference can be made to the preferable range and specific example of the aryl group described in the above "substituted or unsubstituted aryl group". The alkyl group or aryl group as a substituent may be further substituted, and the substituent in this case may preferably be an alkyl group or an aryl group.
The total number of carbon atoms of the substituted or unsubstituted aryl groups that R 1～R3 may employ is preferably 6 to 32, more preferably 6 to 28, still more preferably 6 to 24. Examples of the substituted aryl group that may be used for R 1～R3 include alkylphenyl (tolyl, t-butylphenyl, etc.), alkylphenyl (methylbiphenyl, t-butylbiphenyl, etc.), terphenyl, alkylterphenyl (methylterphenyl, t-butylterphenyl, etc.), phenylnaphthyl, alkylnaphthyl (methylnaphthyl, t-butylnaphthyl, etc.), phenylanthryl, naphthylanthryl, alkylanthrenyl (methylanthryl, t-butylanthryl, etc.), phenyltetracenyl, naphthyltetracenyl, alkyltetracenyl (methyltetracenyl, t-butyltetracenyl, etc.), phenylpyrenyl, naphthylpyrenyl, alkylpyrenyl (methylpyrenyl, t-butylpyrenyl, etc.).
Specific examples of the compound represented by the general formula (1) are shown below.
[ Chemical formula 6]
[ Chemical formula 7]
[ Chemical formula 8]
[ Chemical formula 9]
[ Chemical formula 10]
[ Chemical formula 11]
These singlet split materials may be used alone or in combination of 1 or more than 2.
(Materials other than singlet split materials)
The singlet split layer may contain materials other than the singlet split material (other materials) as needed. Examples of the other material include a light-emitting material and a host material.
By including the light-emitting material in the singlet-splitting layer, energy of triplet excitons generated by singlet splitting in the singlet-splitting material can be transferred to the light-emitting material by a texel mechanism, so that the light-emitting material emits light. In the laminate of the present invention, triplet excitons are generated in the singlet split layer with high exciton generation efficiency, and therefore the light emitting material can emit light with good efficiency.
As the light-emitting material, a light-emitting organic compound that receives energy from triplet excitons and can emit light by using the energy may be used, and the light-emitting material may be a metal complex, an organic compound other than a metal complex, a phosphorescent material, or a delayed fluorescent material.
The phosphorescent material as a metal complex may receive the energy of triplet excitons and transit to the lowest excited triplet energy level E T1 and emit phosphorescence with deactivation from the lowest excited triplet energy level E T1 to the ground state singlet state, and may also receive the energy of triplet excitons and transit to an electron state (electron excited state) having higher energy in the central metal and emit phosphorescence with transit from the electron excited state to the original electron state (ground state electron state). As the phosphorescent material, a material having a lowest excited triplet level E T1 lower than the lowest excited triplet level E T1 of triplet excitons generated by singlet cleavage or a material having an electron excited state in its central metal lower than the lowest excited triplet level E T1 of triplet excitons generated by singlet cleavage can be preferably used in order to easily transfer the excited triplet energy from triplet excitons generated by singlet cleavage and to enclose the excited triplet energy in the molecule. The phosphorescent material of the metal complex is preferably a metal complex having a lanthanoid element as a central metal, and more preferably a metal complex having Er as a central metal.
And, after the delayed fluorescent material receives the energy of the triplet exciton and transits to the lowest excited triplet level, transits to the lowest excited singlet level by the reverse intersystem crossing, and emits delayed fluorescence with deactivation from the lowest excited singlet level. Therefore, as a delayed fluorescent material used in a light-emitting material, a material in which the energy difference Δe ST between the lowest excited singlet energy level and the lowest excited triplet energy level is 0.3eV or less, and the lowest excited triplet energy level E T1 is lower than the lowest excited triplet energy level E T1 of triplet excitons generated by singlet cleavage in order to easily transfer the excited triplet energy from triplet excitons generated by singlet cleavage and to enclose the excited triplet energy in a molecule is preferable.
The wavelength (emission wavelength) of light emitted from the light emitting material is not particularly limited, and may be, for example, in the visible region or in the near infrared region. The laminate can be applied to a light emitting section or illumination of a display device or the like for displaying images, characters, marks, or the like by having a light emission wavelength in the visible region, and can be applied to a light source used for a near infrared sensor or biological imaging by having a light emission wavelength in the near infrared region.
Hereinafter, preferred examples of the compound that can be used as a light-emitting material are given. In the following formula, ar represents an aryl group.
[ Chemical formula 12]
The luminescent materials may be used singly or in combination of 1 or more than 2 kinds
In addition, in the case where the singlet split layer contains a light emitting material, the light emitting material may be uniformly dispersed and present in the singlet split layer or may be locally present in a part of the region.
The content of the light emitting material in the singlet split layer is preferably 0.1% by weight or more, more preferably 1% by weight or more, and preferably 50% by weight or less, more preferably 20% by weight or less, and further preferably 10% by weight or less, relative to the total amount of materials of the singlet split layer.
(Thickness of singlet split layer)
The thickness of the singlet split layer is not particularly limited, but is preferably 1nm to 20nm, more preferably 3nm to 15nm, and further preferably 5nm to 10nm.
[ Layer Structure of laminate ]
The laminate of the present invention has a structure in which an exciton generation layer, a blocking layer, and a singlet split layer are laminated in this order. The laminate of the present invention may be constituted by stacking only 3 layers in order, or may have other layers. The other layer may be disposed between the exciton generation layer and the blocking layer or between the blocking layer and the singlet split layer, or may be disposed on the opposite side of the exciton generation layer from the blocking layer or on the opposite side of the singlet split layer from the blocking layer. The other layer may be a layer selected from the group consisting of an exciton generation layer, a blocking layer, and a singlet split layer, or may be a layer other than these layers. In the case where a layer selected from the exciton generation layer, the blocking layer, and the singlet split layer is provided as another layer, the materials, composition ratios, and thicknesses of the plurality of exciton generation layers, the blocking layer, and the singlet split layer may be the same as or different from each other.
Preferable layer structures of the laminate include a 3-layer structure composed of an exciton generation layer/a blocking layer/a singlet split layer, and a 5-layer structure composed of a1 st singlet split layer/a 1 st blocking layer/an exciton generation layer/a 2 nd blocking layer/a 2 nd singlet split layer. The 1 st singlet splitting layer and the 2 nd singlet splitting layer respectively correspond to the "singlet splitting layer", and the 1 st blocking layer and the 2 nd blocking layer respectively correspond to the "blocking layer". The materials or composition ratios of the 1 st singlet splitting layer and the 2 nd singlet splitting layer, and the 1 st barrier layer and the 2 nd barrier layer may be the same or different from each other. Each layer constituting the laminate may have a single-layer structure or a multilayer structure.
As described above, the theoretical limit of the exciton generation efficiency of the laminate of the present invention is high, and when a light-emitting material is added to the singlet split layer, the light-emitting efficiency can be significantly improved as compared with the case where the singlet split layer to which the light-emitting material is added is alone. Therefore, by using the laminate of the present invention as a light-emitting portion of an organic photoluminescent element (organic PL element) or an organic electroluminescent element (organic EL element), an organic light-emitting element having high light-emitting efficiency can be provided.
The organic photoluminescent element has a structure in which at least a light emitting portion is formed on a substrate. The organic electroluminescent element has at least an anode, a cathode, and an organic layer formed between the anode and the cathode. The organic layer includes at least a light emitting portion, and may be constituted only by the light emitting portion, or may have 1 or more organic layers other than the light emitting portion. Examples of such other organic layers include a hole transport layer, a hole injection layer, an electron blocking layer, a hole blocking layer, an electron injection layer, an electron transport layer, and an exciton blocking layer. The hole transport layer may be a hole injection transport layer having a hole injection function, and the electron transport layer may be an electron injection transport layer having an electron injection function. A specific structure of the organic electroluminescent element is illustrated in fig. 3. In fig. 3, 1 denotes a substrate, 2 denotes an anode, 3 denotes a hole injection layer, 4 denotes a hole transport layer, 5 denotes a light emitting portion, 6 denotes an electron transport layer, and 7 denotes a cathode.
Hereinafter, each component and each layer of the organic electroluminescent element will be described. The description of the substrate and the light-emitting portion is also in accordance with the substrate and the light-emitting portion of the organic photoluminescent element.
(Substrate)
The organic electroluminescent element of the present invention is preferably supported by a substrate. The substrate is not particularly limited as long as it is a substrate conventionally used in an organic electroluminescent element, and for example, a substrate composed of glass, transparent plastic, quartz, silicon, or the like can be used.
(Anode)
As the anode of the organic electroluminescent element, an anode using a metal, an alloy, a conductive compound, or a mixture thereof having a large work function (4 eV or more) as an electrode material can be preferably used. Specific examples of such electrode materials include metals such as Au, conductive transparent materials such as CuI, indium Tin Oxide (ITO), snO 2, and ZnO. Further, a material which is amorphous such as IDIXO (In 2O3 -ZnO) and which can produce a transparent conductive film may be used. The anode may be formed by forming the electrode materials into a thin film by vapor deposition, sputtering, or the like, and patterning the electrode materials into a desired shape by photolithography, or may be formed through a mask of a desired shape at the time of vapor deposition or sputtering of the electrode materials in the case where the accuracy of the pattern is not required to be high (about 100 μm or more). Alternatively, when a material that can be applied such as an organic conductive compound is used, a wet film forming method such as a printing method or a coating method can be used. In the case of extracting light emission from the anode, the transmittance is preferably set to be more than 10%, and the sheet resistance as the anode is preferably hundreds Ω/≡or less. The film thickness is also selected depending on the material, and is usually in the range of 10 to 1000nm, preferably 10 to 200 nm.
(Cathode)
On the other hand, as the cathode, a cathode using a metal (referred to as an electron-injecting metal) having a small work function (4 eV or less), an alloy, a conductive compound, or a mixture thereof as an electrode material can be used. Specific examples of such electrode materials include sodium, sodium-potassium alloy, magnesium, lithium, magnesium/copper mixture, magnesium/silver mixture, magnesium/aluminum mixture, magnesium/indium mixture, aluminum/aluminum oxide (Al 2O3) mixture, indium, lithium/aluminum mixture, rare earth metal, and the like. Among them, from the viewpoints of electron injection property and durability against oxidation and the like, a mixture of an electron injection metal and a second metal which is a metal having a larger and stable work function value than that of the electron injection metal is preferable, for example, a magnesium/silver mixture, a magnesium/aluminum mixture, a magnesium/indium mixture, an aluminum/aluminum oxide (Al 2O3) mixture, a lithium/aluminum mixture, aluminum and the like. The cathode can be produced by forming these electrode materials into a thin film by vapor deposition, sputtering, or the like. The sheet resistance of the cathode is preferably several hundred Ω/≡or less, and the film thickness is usually selected in the range of 10nm to 5 μm, preferably 50 to 200 nm. In addition, it is preferable that the anode or the cathode of the organic electroluminescent element be transparent or translucent in order to transmit the emitted light, so that the emission luminance is improved.
Further, by using the conductive transparent material described in the description of the anode for the cathode, a transparent or semitransparent cathode can be produced, and by using the cathode, a transparent element having both the anode and the cathode can be produced.
(Light-emitting part)
The light-emitting portion is a layer which is formed of the laminate of the present invention, and emits light after holes and electrons injected from the anode and the cathode, respectively, are recombined in the exciton generation layer of the laminate to generate excitons. For the description of the laminate of the present invention, reference can be made to the description in the column < laminate > above. As described above, since the theoretical limit value of the exciton generation efficiency of the laminate of the present invention is high, a high light emission efficiency can be obtained by using the laminate in the light emitting section. The laminate constituting the light emitting portion of the organic electroluminescent element preferably contains a light emitting material in the singlet split layer, and more preferably has a layer structure of 1 st singlet split layer/1 st blocking layer/exciton generation layer/2 nd blocking layer/2 nd singlet split layer. The laminate is arranged such that the 1 st singlet split layer is the anode side and the 2 nd singlet split layer is the cathode side. When the laminate having such a layer structure is used in a light-emitting portion of an organic electroluminescent element, the 1 st barrier layer and the 2 nd barrier layer preferably contain a material having hole-transporting properties, and the 1 st barrier layer and the 2 nd barrier layer preferably contain a material having electron-transporting properties, respectively, on the anode side of the exciton generation layer and on the cathode side of the exciton generation layer. For preferable ranges and specific examples of the material having hole-transporting property and the material having electron-transporting property, the description in the column above (barrier layer) can be referred to.
In the organic light-emitting element and the organic electroluminescent element of the present invention, luminescence is generated from the luminescent material contained in the singlet split layer. The light emission may be phosphorescence or delayed fluorescence. In addition, some of the light emission may be light emission from the exciton generation layer material, the blocking layer material, or the singlet splitting material of the singlet splitting layer.
(Injection layer)
The injection layer is a layer provided between the electrode and the organic layer in order to reduce the driving voltage or increase the light emission luminance, and may exist between the anode and the light emitting portion or the hole transporting layer and between the cathode and the light emitting portion or the electron transporting layer. The implanted layer can be provided as desired.
(Barrier layer)
The blocking layer is a layer capable of preventing diffusion of charges (electrons or holes) and/or excitons existing in the light emitting portion to the outside of the light emitting portion. The electron blocking layer may be disposed between the light emitting portion and the hole transporting layer, and prevents electrons from passing through the light emitting portion toward the hole transporting layer. Similarly, the hole blocking layer may be disposed between the light emitting portion and the electron transport layer, and prevent holes from passing through the light emitting portion toward the electron transport layer. The blocking layer can also be used to prevent excitons from diffusing to the outside of the light emitting portion. That is, the electron blocking layer and the hole blocking layer can also function as exciton blocking layers. The electron blocking layer or the exciton blocking layer described in this specification is used in a meaning that it includes a layer having functions of an electron blocking layer and an exciton blocking layer as one layer.
(Hole blocking layer)
The hole blocking layer has a function of an electron transport layer in a broad sense. The hole blocking layer has a function of transporting electrons and preventing holes from reaching the electron transporting layer, and thus can improve the probability of recombination of electrons and holes in the light emitting portion. As a material of the hole blocking layer, a material of an electron transport layer described later can be used as needed.
(Electron blocking layer)
The electron blocking layer has a function of transporting holes in a broad sense. The electron blocking layer has a function of transporting holes and preventing electrons from reaching the hole transporting layer, and thus the probability of recombination of electrons and holes in the light emitting portion can be improved.
(Exciton blocking layer)
The exciton blocking layer is a layer for preventing excitons generated by recombination of holes and electrons in the light emitting portion from diffusing into the charge transport layer, and the excitons can be efficiently sealed in the light emitting portion by insertion of the layer, so that the light emitting efficiency of the device can be improved. The exciton blocking layer may be interposed adjacent to the light emitting portion on either one of the anode side and the cathode side, or may be interposed on both sides. That is, in the case where the exciton blocking layer is provided on the anode side, the layer may be interposed between the hole transport layer and the light emitting portion so as to be adjacent to the light emitting portion, and in the case where the layer is interposed on the cathode side, the layer may be interposed between the light emitting portion and the electron transport layer so as to be adjacent to the light emitting portion. A hole injection layer, a hole transport layer, an electron blocking layer, or the like may be provided between the anode and the exciton blocking layer adjacent to the anode side of the light emitting section, and an electron injection layer, an electron transport layer, a hole blocking layer, or the like may be provided between the cathode and the exciton blocking layer adjacent to the cathode side of the light emitting section. In the case of disposing the barrier layer, at least either one of the excited singlet energy and the excited triplet energy of the material used as the barrier layer is preferably higher than the excited singlet energy and the excited triplet energy of the light-emitting material.
(Hole transporting layer)
The hole transport layer is made of a hole transport material having a function of transporting holes, and can be provided in a single layer or a plurality of layers.
The hole transport material may be any of organic and inorganic materials having any of injection and transport of holes and blocking of electrons. Examples of the well-known hole transporting materials that can be used include triazole derivatives, oxadiazole derivatives, imidazole derivatives, carbazole derivatives, indolocarbazole derivatives, polyarylalkane derivatives, pyrazoline derivatives, pyrazolone derivatives, phenylenediamine derivatives, arylamine derivatives, amino-substituted chalcone derivatives, oxazole derivatives, styrylanthracene derivatives, fluorenone derivatives, hydrazone derivatives, stilbene derivatives, silazane derivatives, and aniline copolymers, and conductive polymer oligomers, particularly thiophene oligomers, etc., and porphyrin compounds, aromatic tertiary amine compounds, and styrylamine compounds are preferably used, and aromatic tertiary amine compounds are more preferably used.
(Electron transport layer)
The electron transport layer is made of a material having a function of transporting electrons, and can be provided in a single layer or a plurality of layers.
As the electron transporting material (which may also serve as a hole blocking material), it is sufficient to have a function of transporting electrons injected from the cathode to the light emitting portion. Examples of the electron-transporting layer that can be used include nitro-substituted fluorene derivatives, diphenylquinone derivatives, thiopyran dioxide (Thiopyran dioxide) derivatives, carbodiimides, fluorenylmethane derivatives, anthraquinone dimethane (anthraquino dimethane), anthrone derivatives, oxadiazole derivatives, and the like. Further, among the above oxadiazole derivatives, a thiadiazole derivative obtained by substituting an oxygen atom of an oxadiazole ring with a sulfur atom, or a quinoxaline derivative having a quinoxaline ring known as an electron withdrawing group can also be used as an electron transporting material. In addition, a polymer material in which these materials are incorporated into a polymer chain or a main chain of a polymer may be used.
The method for forming the layers and other layers of the laminate constituting the organic electroluminescent element is not particularly limited, and the laminate may be produced by either a dry process or a wet process.
Hereinafter, a preferable material that can be used for the organic electroluminescent element is specifically exemplified. Among them, materials that can be used in the present invention are not limitedly explained by the following exemplified compounds. In addition, even a compound illustrated as a material having a specific function can be used as a material having another function.
First, in the case where the exciton generation layer or the singlet split layer contains a host material, a preferable compound that can be used as a host material is exemplified.
[ Chemical formula 13]
[ Chemical formula 14]
[ Chemical formula 15]
[ Chemical formula 16]
[ Chemical formula 17]
Next, a preferable compound which can be used as a hole injection material is exemplified.
[ Chemical formula 18]
Next, examples of preferable compounds that can be used as a hole transport material are given.
[ Chemical formula 19]
[ Chemical formula 20-1]
[ Chemical formula 20-2]
[ Chemical formula 21]
[ Chemical formula 22]
[ Chemical formula 23]
[ Chemical formula 24]
Next, examples of preferable compounds that can be used as an electron blocking material are given.
[ Chemical formula 25]
Next, examples of preferable compounds that can be used as a hole blocking material are given.
[ Chemical formula 26]
Next, examples of preferable compounds that can be used as an electron transport material are given.
[ Chemical formula 27]
[ Chemical formula 28]
[ Chemical formula 29]
Next, a preferable compound which can be used as an electron injection material is exemplified.
[ Chemical formula 30]
Further, examples of preferable compounds are materials that can be added. For example, addition as a stabilizing material or the like can be considered.
[ Chemical formula 31]
The organic electroluminescent element manufactured by the above method emits light by applying an electric field between the anode and the cathode of the obtained element. At this time, if light emission is based on the energy of the excited triplet state, light having a wavelength corresponding to the energy level is confirmed to be phosphorescence. As long as the light is emitted by the excited singlet energy, the light of the wavelength corresponding to the energy level is recognized as fluorescence emission and delayed fluorescence emission. Further, since the fluorescence lifetime of normal fluorescence is shorter than that of delayed fluorescence, the luminescence lifetime can be distinguished from delayed fluorescence.
The organic electroluminescent element is applicable to any of individual elements, elements having an array-like structure, and structures in which an anode and a cathode are arranged in an X-Y matrix. According to the present invention, when the laminate of the present invention is used in a light-emitting portion, an organic light-emitting element having greatly improved exciton generation efficiency and light-emitting efficiency can be obtained. An organic light-emitting element such as an organic electroluminescent element using the laminate of the present invention can be further used in various applications. For example, an organic electroluminescent display device can be manufactured using the organic electroluminescent element, and in detail, reference can be made to "organic EL display" (Ohmsha, ltd.) commonly known as Ren Jingshi, daku-wave vector, and village Tian Yingxing. In addition, the organic electroluminescent element can be applied to organic electroluminescent lighting or backlight having a high demand.
Examples
The following examples are given to further illustrate the features of the present invention. The materials, processing contents, processing steps, and the like described below can be appropriately modified as long as they do not depart from the gist of the present invention. Therefore, the scope of the present invention should not be construed in a limited manner by the following examples. The absorption characteristics were evaluated by using an ultraviolet-visible spectrophotometer (manufactured by Shimadzu Corporation. Co., ltd., UV-2600), and the luminescence characteristics were evaluated by using a spectrophotometer (manufactured by HORIBA, ltd., fluoroMax Plus), a source meter (manufactured by Keithley Co., ltd.: 2400 series), an external quantum efficiency measuring device (manufactured by Hamamatsu Photonics K.K., manufactured by C9920-12), an absolute quantum yield measuring device (manufactured by Hamamatsu Photonics K.K., manufactured by C9920-02), and a small fluorescent lifetime measuring device (manufactured by Hamamatsu Photonics K.K., manufactured by C11367-21).
< Compounds used in examples >
The compounds used in this example are shown below.
[ Chemical formula 32-1]
Materials with ΔE ST of 0.3eV or less
Material of barrier layer
[ Chemical formula 32-2]
Singlet split materials
Luminescent material
Materials other than the above materials used in the organic EL element
The lowest excited singlet energy levels E S1 and the lowest excited triplet energy levels E T1 of ACR-XTN, mAP, B3PyMPM, rubrene and Er (hfa) and 4I13/2 of Er (hfa) are shown in Table 1. In Table 1, the lowest excited singlet energy level E S1 and the lowest excited triplet energy level E T1 of ACR-XTN are Nakanotani et al, the literature values described in Nat.Commun.,2014,5,4016, the lowest excited singlet energy level E S1 and the lowest excited triplet energy level E T1 of mAP are Seo et al, the literature values described in Dye pigment, 2015,254, the lowest excited singlet energy level E S1 and the lowest excited triplet energy level E T1 of B3PyMPM are Sasabe et al, the literature values described in adv.Mater,2011,21,336, the lowest excited singlet energy level E S1 and the lowest excited triplet energy level E T1 of rubrene are literature values described in Nagata et al, adv.
Further, as a result of measuring the light emission spectrum of ACR-XTN and the absorption spectra of mAP, B3PyMPM, rubrene and Er (hfa), it was confirmed that overlapping was observed between the light emission peak of ACR-XTN and the light absorption band of rubrene, and that B3PyMPM, rubrene compound and Er (hfa) did not exhibit light absorption in these overlapping wavelength regions.
Example 1 fabrication and evaluation of laminate 1 consisting of 1 st singlet split layer/1 st blocking layer/exciton generation layer/2 nd blocking layer/2 nd singlet split layer
Each thin film was laminated on a quartz substrate by vacuum evaporation at a vacuum degree of less than 10 -4 Pa.
First, rubrene and Er (hfa) were vapor-deposited on a quartz substrate from different vapor deposition sources to form a 1 st singlet split layer having a thickness of 10 nm. Wherein the concentration of Er (hfa) was set to 2.0 wt.%. Next, the maps were vapor deposited on the 1 st singlet split layer to form a 1 st blocking layer having a thickness of 2nm, and ACR-XTN was vapor deposited on the 1 st blocking layer to form an exciton generation layer having a thickness of 15 nm. Then, B3PyMPM was vapor deposited on the exciton generation layer to form a 2 nd blocking layer having a thickness of 2 nm. Next, rubrene and Er (hfa) were vapor deposited on the 2 nd barrier layer from different vapor deposition sources to form a 10nm thick 2 nd singlet split layer. At this time, the concentration of Er (hfa) was set to 2.0 wt%. Through the above steps, a laminate 1 having a 5-layer structure was obtained.
Fig. 4 shows an energy level diagram of the produced laminate 1, and fig. 5 shows an estimated light emission mechanism. In the energy level diagram shown in fig. 4, the numerical value shown on the lower side of each layer is the absolute value of the energy level of HOMO (Highest Occupied Molecular Orbital: highest occupied molecular orbital) of the layer, the numerical value shown on the upper side of each layer is the absolute value of the energy level of LUMO (Lowest Unoccupied Molecular Orbital: lowest unoccupied molecular orbital) of the layer, and the units are all "eV". The meaning of the numerical values is the same as in fig. 6, 8, and 10.
As shown in fig. 5, in the laminate 1, when ACR-XTN of the exciton generation layer is excited to generate singlet excitons 1 and triplet excitons T 1, triplet excitons T 1 undergo intersystem crossing in reverse order to convert into singlet excitons 1. The energy of singlet excitons S 1 generated in ACR-XTN is transferred to the lowest excited singlet energy level E S1 of rubrene (singlet splitting material) contained in the 1 st singlet splitting layer or the 2 nd singlet splitting layer across the 1 st barrier layer or the 2 nd barrier layer by the ford mechanism. On the other hand, the mechanism for transferring the excited triplet energy is a texel mechanism. Therefore, the 1 st blocking layer and the 2 nd blocking layer are used to prevent energy transfer from triplet exciton T 1 to rubrene. Therefore, 100% of triplet excitons T 1 generated in ACR-XTN is maximally used in reverse intersystem crossing to be converted into excited singlet S 1, and transferred to the lowest excited singlet level E S1 of rubrene contained in the 1 st and 2 nd singlet split layers. In rubrene that receives excited singlet energy, its singlet exciton S 1 splits into 2 triplet excitons T 1, and its excited triplet energy is transferred to the 4I13/2 order of Er (hfa) by the tex mechanism. Also, phosphorescence is emitted as the light is relaxed from 4I13/2 stages to 4I15/2 stages. As such, all of the energy of excitons 1、T1 generated in the exciton generation layer of the laminate 1 is transferred as excited singlet energy to the lowest excited singlet energy level E S1 of rubrene so that each singlet exciton S 1 generated therein is split into 2 triplet excitons T 1. Therefore, the theoretical limit value of the exciton generation efficiency of the laminate 1 becomes 200%.
Comparative example 1 preparation and evaluation of comparative laminate 1 composed of 1 st singlet split layer/exciton generation layer/2 nd singlet split layer
A comparative laminate 1 was produced in the same manner as in example 1, except that the 1 st barrier layer and the 2 nd barrier layer were not formed.
Fig. 6 shows an energy level diagram of the fabricated comparative laminate 1, and fig. 7 shows an estimated light emission mechanism. Wherein, ACR-XTN of FIG. 6 corresponds to the material of ΔE ST.ltoreq.0.3 eV of FIG. 7, rubrene of FIG. 6 corresponds to the singlet split material of FIG. 7, and Er (hfa) of FIG. 6 corresponds to the phosphorescent material of FIG. 7.
As shown in fig. 7, since the 1 st barrier layer and the 2 nd barrier layer are not provided in the comparative laminate 1, the energy of the singlet exciton S 1 generated in the exciton generation layer and the energy of the triplet exciton T 1 are transferred together to the singlet splitting material (rubrene) of the 1 st singlet splitting layer or the 2 nd singlet splitting layer, and the singlet exciton S 1 and the triplet exciton T 1 are generated in the singlet splitting material (rubrene). Among them, singlet exciton S 1 splits into 2 triplet excitons T 1 and uses its excited triplet energy for phosphorescence of Er (hfa), but triplet exciton T 1 directly uses its excited triplet energy for phosphorescence of Er (hfa). In the comparison of the exciton generation efficiency of the laminate 1, if the ratio of the singlet excitons S 1 and the triplet excitons T 1 in the equilibrium state of the exciton generation layer is (100-R T)：RT), the theoretical limit value of the exciton generation efficiency is lower than that of the laminate 1 (100-R T)×2+RT＝200-RT is the upper limit).
Comparative example 2 production and evaluation of comparative laminate 2 comprising 1 st light-emitting layer/exciton generation layer/2 nd light-emitting layer
Comparative laminate 2 was produced in the same manner as in example 2, except that Er (hfa) was used only for vapor deposition to form the 1 st light-emitting layer and the 2 nd light-emitting layer having a thickness of 10nm, instead of forming the 1 st singlet split layer and the 2 nd singlet split layer.
Fig. 8 shows an energy level diagram of the fabricated comparative laminate 2, and fig. 9 shows an estimated light emission mechanism. Wherein, ACR-XTN of FIG. 8 corresponds to the material of ΔE ST +.0.3 eV of FIG. 9, and Er (hfa) of FIG. 8 corresponds to the phosphorescent material of FIG. 9.
As shown in fig. 8, the comparative laminate 2 does not have the 1 st barrier layer and the 2 nd barrier layer, and the 1 st light-emitting layer and the 2 nd light-emitting layer do not include rubrene (singlet split material). And, as shown in FIG. 9, wherein the lowest excited singlet energy level E S1 of the phosphorescent material (Er (hfa)) is higher than the lowest excited singlet energy level E S1 of the material (ACR-XTN) of ΔE ST +.0.3 eV. Therefore, energy of triplet exciton T 1 among energies of excitons generated in the exciton generation layer is transferred to 4I13/2 level of phosphorescent material (Er (hfa)) and is utilized in phosphorescent light emission, but energy of singlet exciton S 1 cannot be transferred to phosphorescent material (Er (hfa)) and does not contribute to phosphorescent light emission. In the comparison of the exciton generation efficiency of the laminate 2, if the ratio of the singlet excitons S 1 and the triplet excitons T 1 in the exciton generation layer in the equilibrium state is (100-R T)：RT), R T becomes the upper limit, and the theoretical limit of the exciton generation efficiency becomes far lower than that of the laminate 1.
Comparative example 3 fabrication and evaluation of singlet split layers alone
A singlet split layer having a thickness of 20nm was formed on a quartz substrate by the same method as in example 1.
The energy level diagram of the fabricated singlet split layer is shown in fig. 10.
In the singlet split layer, there is no energy transfer from the exciton generation layer, and Er (hfa) is caused to phosphoresce only by energy of triplet excitons generated in the singlet split layer (triplet excitons split from singlet excitons directly generated by excitation of the singlet split layer and triplet excitons directly generated by excitation of the singlet split layer). Regarding the exciton generation efficiency of the comparative laminate 3, if the ratio of the singlet excitons 1 and the triplet excitons T 1 in the exciton generation layer in the equilibrium state is (100-R T)：RT), the theoretical limit value of the exciton generation efficiency becomes lower than that of the laminate 1 (100-R T)×2+RT＝200-RT is the upper limit).
Fig. 11 shows light emission spectra of the laminate 1, the comparative laminate 2, and the singlet split layer produced in comparative example 3 based on excitation light of 400 nm. In the above, FIG. 11 (a) shows an emission spectrum in the range of 450 to 700nm, and FIG. 11 (b) shows an emission spectrum in the range of 1400 to 1650 nm.
In FIG. 11 (a), the luminescence peak around 510nm is from the luminescence of ACR-XTN, and the luminescence peak around 560nm is from the luminescence of rubrene. In FIG. 11 (b), the luminescence peak around 1500nm is derived from the luminescence of Er (hfa).
As shown in fig. 11, in laminate 1 including all of Er (hfa) -containing rubrene layers (1 st singlet split layer and 2 nd singlet split layer), mAP layer (1 st blocking layer), ACR-XTN layer (exciton generation layer), and B3PyMPM layer (2 nd blocking layer), light emission from Er (hfa) exhibits higher intensity than light emission from ACR-XTN and light emission from rubrene. In contrast, the comparative laminate 1 having no barrier layer had lower intensity of luminescence from Er (hfa) than the laminate 1. This is thought to be because the excited triplet energy generated in ACR-XTN is also transferred to rubrene, and thus the exciton multiplication mechanism based on the singlet cleavage of rubrene does not function sufficiently. On the other hand, the laminate of comparative example 2 in which rubrene was not used had a high emission intensity from ACR-XTN, and the emission intensity from Er (hfa) was lower than that of laminate 1. The luminescence intensity from ACR-XTN is considered to be high because the excited singlet energy generated in ACR-XTN is not transferred to Er (hfa) but is used in the luminescence of ACR-XTN, and the luminescence intensity from Er (hfa) is considered to be low because the excited singlet energy generated in ACR-XTN does not contribute to the luminescence of Er (hfa) at all. In addition, the singlet split layer produced in comparative example 3 alone was the layer produced, and the light emission intensity from Er (hfa) was the lowest. This is because the exciton generation layer supplying singlet excitons is not provided in the singlet split layer.
From the above, it is known that the exciton amplification action by the singlet split material is effectively exerted by using the structure in which the exciton generation layer, the blocking layer, and the singlet split layer are sequentially laminated, thereby remarkably improving the light emission efficiency of phosphorescence.
Example 2 fabrication and evaluation of organic electroluminescent device having laminate composed of 1 st singlet split layer/1 st blocking layer/exciton generation layer/2 nd blocking layer/2 nd singlet split layer
Each thin film was laminated on a glass substrate on which an anode made of indium/tin oxide (ITO) having a film thickness of 100nm was formed by a vacuum deposition method at a vacuum of less than 10- 4 Pa.
First, TAPC was formed on ITO at a thickness of 50nm.
Next, a rubrene layer (1 st singlet split layer) containing 2.0 wt% of Er (hfa), a mAP layer (1 st blocking layer), an ACR-XTN layer (exciton generation layer), a B3PyMPM layer (2 nd blocking layer), and a rubrene layer (2 nd singlet split layer) containing 1.8 wt% of Er (hfa) were sequentially formed on the TAPC layer in the same manner as in example 1, thereby forming a laminate.
Next, B3PyMPM was formed on the laminate at a thickness of 55 nm. Further, a cathode was formed by forming 8-sodium quinoline (Liq) at a thickness of 0.8nm and aluminum (Al) was deposited thereon at a thickness of 80nm, and was used as an organic electroluminescent element.
Fig. 12 shows a light emission spectrum of the organic electroluminescent device. As shown in fig. 12, the emission peak around 1500nm from the light emission of Er (hfa) was observed in the organic electroluminescent element to which the laminate of the present invention was applied.
Industrial applicability
The theoretical limit value of the exciton generation efficiency of the laminate of the present invention is high. Therefore, when the laminate of the present invention is used in a light-emitting portion of an organic light-emitting element, the light-emitting efficiency can be significantly improved. Therefore, the present invention has high industrial applicability.
Symbol description
1-Substrate, 2-anode, 3-hole injection layer, 4-hole transport layer, 5-light emitting part, 6-electron transport layer, 7-cathode.
Claims (15)
1. An organic light-emitting device having a structure in which an exciton generation layer containing a material capable of converting from triplet excitons to singlet excitons, a blocking layer, and a singlet splitting layer containing a singlet splitting material are sequentially stacked,
Which has a1 st singlet split layer and a 2 nd singlet split layer as the singlet split layers and has a1 st barrier layer and a 2 nd barrier layer as the barrier layers, and has a structure in which the 1 st singlet split layer, the 1 st barrier layer, the exciton generation layer, the 2 nd barrier layer, and the 2 nd singlet split layer are sequentially laminated,
The difference DeltaE ST between the lowest excited singlet energy level (E S1) and the lowest excited triplet energy level (E T1) of the material capable of converting from triplet excitons to singlet excitons is 0.3eV or less,
The blocking layer is a layer which is disposed between the exciton generation layer and the singlet split layer, and which prevents the transfer of the triplet energy from the triplet-exciton-convertible material to the singlet-split material and allows the transfer of the triplet energy from the triplet-exciton-convertible material to the singlet-split material to the triplet-energy-convertible material.
2. The organic light-emitting element according to claim 1, wherein,
The singlet split layer comprises a luminescent material.
3. The organic light-emitting element according to claim 2, wherein,
The lowest excited singlet energy level E S1 of the light-emitting material is 0.2eV or more higher than the lowest excited singlet energy level E S1 of the material capable of converting from triplet excitons to singlet excitons.
4. An organic light-emitting element according to claim 2 or 3, wherein,
The luminescent material is a phosphorescent material.
5. An organic light-emitting element according to claim 2 or 3, wherein,
The luminescent material is a metal complex with lanthanide as a central metal.
6. The organic light-emitting element according to any one of claim 1 to 3, wherein,
The thickness of the barrier layer is 2-10 nm.
7. The organic light-emitting element according to any one of claim 1 to 3, wherein,
The lowest excited singlet energy level E S1 of the material of the blocking layer is above 0.2eV higher than the lowest excited singlet energy level E S1 of the material capable of converting from triplet excitons to singlet excitons.
8. The organic light-emitting element according to any one of claim 1 to 3, wherein,
The lowest excited triplet energy level E T1 of the material of the blocking layer is 0.2eV or more higher than the lowest excited triplet energy level E T1 of the material capable of converting from triplet excitons to singlet excitons.
9. The organic light-emitting element according to any one of claim 1 to 3, wherein,
The material capable of converting from triplet excitons to singlet excitons is a delayed fluorescent material.
10. The organic light-emitting element according to claim 1, wherein,
The 1 st barrier layer includes a material having a hole-transporting property, and the 2 nd barrier layer includes a material having an electron-transporting property.
11. The organic light-emitting element according to claim 1 or 10, wherein,
The 1 st barrier layer contains a compound having an aromatic ring in which at least 1 hydrogen atom is substituted with a substituent bonded through a nitrogen atom, and the 2 nd barrier layer contains a compound having an aromatic hetero six-membered ring containing at least 1 nitrogen atom as a ring member.
12. The organic light-emitting element according to any one of claims 1 to 3, comprising an anode, a cathode, and a laminate disposed between the anode and the cathode, the laminate having a structure in which a 1 st singlet split layer, a 1 st barrier layer, an exciton generation layer, a 2 nd barrier layer, and a 2 nd singlet split layer are laminated in this order from the anode side, the 1 st barrier layer containing a material having hole-transporting property, and the 2 nd barrier layer containing a material having electron-transporting property.
13. The organic light-emitting element according to any one of claims 1 to 3, which is an organic electroluminescent element.
14. A laminate having a structure in which an exciton generation layer containing a material capable of converting from triplet excitons to singlet excitons, a blocking layer, and a singlet splitting layer containing a singlet splitting material are laminated in this order,
Which has a1 st singlet split layer and a 2 nd singlet split layer as the singlet split layers and has a1 st barrier layer and a 2 nd barrier layer as the barrier layers, and has a structure in which the 1 st singlet split layer, the 1 st barrier layer, the exciton generation layer, the 2 nd barrier layer, and the 2 nd singlet split layer are sequentially laminated,
The difference DeltaE ST between the lowest excited singlet energy level (E S1) and the lowest excited triplet energy level (E T1) of the material capable of converting from triplet excitons to singlet excitons is 0.3eV or less,
The blocking layer is a layer which is disposed between the exciton generation layer and the singlet split layer, and which prevents the transfer of the triplet energy from the triplet-exciton-convertible material to the singlet-split material and allows the transfer of the triplet energy from the triplet-exciton-convertible material to the singlet-split material to the triplet-energy-convertible material.
15. A light-emitting method for emitting light of the laminate of claim 14, comprising the steps of:
a material capable of converting from triplet excitons to singlet excitons is subjected to current excitation,
Transferring energy of the singlet excitons generated by current excitation and reverse intersystem crossing into the singlet split material to generate singlet excitons of the singlet split material,
Singlet excitons of the singlet splitting material are split.
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