JP2006066380A - Organic electroluminescence element and organic electroluminescence display device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To obtain an organic EL element which can drive at a low voltage and has a high emission efficiency. <P>SOLUTION: The organic electroluminescence element includes an intermediate unit 30 arranged between a cathode 51 and an anode 52, a first luminescence unit 41 arranged between the cathode 51 and an intermediate unit 30, and a second luminescence unit 42 arranged between the anode 52 and the intermediate unit 30. An electron drawing-out layer and an electron injection layer adjacent to the anode side of the electron drawing-out layer are provided in the intermediate unit 30. The absolute value ¾LUMO(A)¾ of the energy level of the lowest unoccupied molecular orbital (LUMO) of the electron drawing-out layer, and the absolute value ¾HOMO(B)¾ of the energy level of the highest occupied molecular orbital (HOMO) of the adjacent layer, are in a relation of ¾HOMO(B)¾-¾LUMO(A)¾≤1.5 eV. The intermediate unit 30 supplies the holes generated by the electron drawing-out to the first luminescence unit 41 and supplies the drawn-out electron to the second luminescence unit 42 through the electron injection layer. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an organic electroluminescent element and an organic electroluminescent display device.

  Organic electroluminescent elements (organic EL elements) are being actively developed from the viewpoint of application to displays and lighting. The driving principle of the organic EL element is as follows. That is, holes and electrons are injected from the anode and the cathode, respectively, and these are transported through the organic thin film, recombined in the light emitting layer to generate an excited state, and light emission can be obtained from this excited state. In order to increase the luminous efficiency, it is necessary to inject holes and electrons efficiently and transport the organic thin film. However, since the movement of carriers in the organic EL element is limited by the energy barrier between the electrode and the organic thin film and the low carrier mobility in the organic thin film, there is a limit to improving the light emission efficiency.

  On the other hand, as another method for improving the light emission efficiency, there is a method of laminating a plurality of light emitting layers. For example, it may be possible to obtain higher luminous efficiency than a single layer by laminating an orange light-emitting layer and a blue light-emitting layer that are in a complementary color relationship so as to be in direct contact with each other. For example, when the light emission efficiency of the blue light emitting layer is 10 cd / A and the light emission efficiency of the orange light emitting layer is 8 cd / A, when these are stacked to form a white light emitting element, the light emission efficiency of 15 cd / A Is obtained.

  However, when three or more light emitting layers are laminated so as to be in direct contact with each other, improvement in light emission efficiency cannot be obtained. This is because there is a limit to the expansion of the recombination region of electrons and holes, and the recombination region does not extend over three layers.

In Non-Patent Document 1, two light emitting units are stacked via inorganic semiconductor layers such as V ₂ O ₅ and ITO, carriers are generated inside the inorganic semiconductor layer, and the carriers are supplied to the two light emitting layers. A method has been reported. This method uses carriers contained in the inorganic semiconductor layer, and a high voltage must be applied in order to generate carriers. For this reason, a drive voltage becomes high and cannot be applied to low voltage drive of a portable device or the like.

Patent Documents 1 to 4 also propose an organic EL element in which a plurality of light emitting units are stacked via a charge generation layer or the like, but it is necessary to drive at a high voltage, and high luminous efficiency can be obtained. It wasn't.
JP 2003-272860 A JP 2003-264085 A Japanese Patent Laid-Open No. 11-329748 JP 2004-39617 A 2004 Spring 51st Applied Physics Related Conference Lecture Proceedings No. 3 Page 1464 Lecture number 28p-ZQ-14 “Carrier recombination organic EL device with double insulation layer” SYNTHESIS, April, 1994, 378-380 “Improved Synthesis of 1,4,5,8,9,12-Hexaazatriphenylenehexacarboxylic Acid”

  An object of the present invention is to provide an organic EL element and an organic EL display device which can be driven at a low voltage and have high light emission efficiency in an organic EL element including at least two light emitting units.

  The organic EL device of the present invention includes a cathode, an anode, an intermediate unit disposed between the cathode and the anode, a first light emitting unit disposed between the cathode and the intermediate unit, and the anode and the intermediate unit. A second light emitting unit disposed on the intermediate unit, and the intermediate unit is provided with an electron extraction layer for extracting electrons from an adjacent layer adjacent to the cathode side, and an electron injection layer adjacent to the anode side of the electron extraction layer The absolute value of the energy level of the lowest unoccupied molecular orbital (LUMO) | LUMO (A) | of the electron extraction layer and the absolute value of the energy level of the highest occupied molecular orbital (HOMO) of the adjacent layer | HOMO (B ) | Is in a relationship of | HOMO (B) | --LUMO (A) | ≦ 1.5 eV, and the absolute value of the energy level of the lowest unoccupied molecular orbital (LUMO) of the electron injection layer | LUMO (C) Alternatively, the absolute value of the work function | WF (C) | is smaller than | LUMO (A) |, and the intermediate unit supplies holes generated by the extraction of electrons from the adjacent layer by the electron extraction layer to the first light emitting unit. In addition, the extracted electrons are supplied to the second light emitting unit through the electron injection layer.

  According to the present invention, the intermediate unit is provided between the first light emitting unit and the second light emitting unit, and the electronic extraction layer is provided in the intermediate unit. An adjacent layer is provided on the cathode side of the electron extraction layer. An electron injection layer is provided on the anode side of the electron extraction layer. The absolute value of the HOMO energy level of the adjacent layer | HOMO (B) | and the absolute value of the LUMO energy level of the electron extraction layer | LUMO (A) | are | HOMO (B) | --LUMO (A) | ≦ 1.5 eV. That is, the LUMO energy level of the electron extraction layer is close to the HOMO energy level of the adjacent layer. For this reason, the electron extraction layer can extract electrons from the adjacent layer. Due to the extraction of electrons from the adjacent layer, holes are generated in the adjacent layer. When the adjacent layer is provided in the first light emitting unit, holes are generated in the first light emitting unit. Further, when the adjacent layer is provided between the electron extraction layer and the first light emitting unit, that is, provided in the intermediate unit, holes generated in the adjacent layer are formed in the first light emitting unit. Supplied. The holes supplied to the first light emitting unit recombine with electrons from the cathode, and thereby the first light emitting unit emits light.

  On the other hand, the electrons extracted by the electron extraction layer move to the electron injection layer, are supplied from the electron injection layer to the second light emitting unit, and recombine with the holes supplied from the anode, thereby the second light emitting unit. Emits light.

  Therefore, according to the present invention, a recombination region can be formed in each of the first light-emitting unit and the second light-emitting unit, whereby each of the first light-emitting unit and the second light-emitting unit emits light separately. Can be made.

  In the present invention, in order for the electron extraction layer to extract electrons from the adjacent layer, the LUMO energy level of the electron extraction layer is preferably closer to the HOMO energy level of the adjacent layer than the LUMO energy level of the adjacent layer. . That is, it is preferable that the absolute value | LUMO (B) | of the LUMO energy level of the adjacent layer satisfies the following relationship.

│HOMO (B) │-│LUMO (A) │ <│LUMO (A) │-│LUMO (B) │
In addition, since the absolute value of the LUMO energy level of the material used as the electron extraction layer is generally smaller than the absolute value of the energy level of the adjacent layer, the absolute value of each energy level is as follows: It is shown by the relational expression.

0eV <│HOMO (B) │-│LUMO (A) │ ≦ 1.5eV
In the present invention, the absolute value | LUMO (C) | of the LUMO energy level of the electron injection layer or the absolute value | WF (C) | of the work function is the absolute value | LUMO (A ) | For this reason, the electrons extracted from the electron extraction layer move to the electron injection layer and are supplied from the electron injection layer to the second light emitting unit.

  In the present invention, an electron transport layer is preferably provided between the electron injection layer in the intermediate unit and the second light emitting unit. The absolute value of LUMO energy level | LUMO (D) | of the electron transport layer is smaller than the absolute value of LUMO energy level of the electron injection layer | LUMO (C) | or the absolute value of work function | WF (C) | It is preferable. When the electron transport layer is provided, the electrons that have moved to the electron injection layer are supplied to the second light emitting unit through the electron transport layer. Therefore, the intermediate unit supplies the electrons extracted by the electron extraction layer to the second light emitting unit via the electron injection layer and the electron transport layer.

  In the present invention, the thickness of the electron extraction layer is preferably in the range of 8 to 100 nm. By setting it as such a range, it can be set as the organic electroluminescent element excellent in lifetime characteristics and luminous efficiency. When the thickness of the electron extraction layer is less than 8 nm, the life characteristics and the light emission efficiency may be deteriorated. In addition, when the thickness of the electron extraction layer exceeds 100 nm, the life characteristics and the light emission efficiency are deteriorated, and a dark spot may be generated. A more preferable thickness of the electron extracting layer is in the range of 10 to 80 nm, and particularly preferably in the range of 10 to 30 nm.

  Each of the first light emitting unit and the second light emitting unit in the present invention may be formed of a single light emitting layer, or may be configured by laminating a plurality of light emitting layers so as to be in direct contact with each other. However, the present invention is particularly useful when the first light-emitting unit and the second light-emitting unit have a structure in which two light-emitting layers are stacked in direct contact with each other. That is, in such a case, when the first light emitting unit and the second light emitting unit are directly stacked, a structure in which four light emitting layers are directly stacked is formed, and as described above, the recombination region of electrons and holes is expanded. Therefore, the recombination region does not straddle the four light emitting layers. For this reason, recombination occurs at one location in the thickness direction of the four light emitting layers, and high luminous efficiency cannot be obtained. In addition, since the first light emitting unit and the second light emitting unit recombine in a region different from the recombination region in the case where each of the first light emitting unit and the second light emitting unit emits light separately, the light emission colors of the first light emitting unit and the second light emitting unit are different The color emits light.

  According to the present invention, by providing an intermediate unit between the first light emitting unit and the second light emitting unit, recombination can be performed in each of the first light emitting unit and the second light emitting unit. That is, a recombination region can be formed in each of the first light emitting unit and the second light emitting unit, and each of the first light emitting unit and the second light emitting unit can independently emit light. For this reason, while being able to obtain high luminous efficiency, it is possible to emit the same color as the emission colors of the first light emitting unit and the second light emitting unit.

  In the present invention, the adjacent layer is preferably formed of a hole transporting material, and particularly preferably formed of an arylamine-based hole transporting material.

  In the present invention, the adjacent layer may be provided in the first light emitting unit. In particular, when the host material of the light emitting layer located on the intermediate unit side in the first light emitting unit is a hole transporting material suitable as the adjacent layer, the light emitting layer on the intermediate unit side in the first light emitting unit is adjacent. It can be a layer.

  In the present invention, the adjacent layer may be provided in the intermediate unit. In the case where the host material of the light emitting layer on the intermediate unit side in the first light emitting unit is not a hole transporting material suitable as the adjacent layer, it may not function as the adjacent layer. Adjacent layers can be provided in the intermediate unit. In such a case, the adjacent layer is disposed between the electron extraction layer and the first light emitting unit.

  In the present invention, the electron extraction layer can be used without particular limitation as long as the absolute value of the LUMO energy level is 1.5 eV smaller than the absolute value of the HOMO energy level of the adjacent layer. As a specific example, for example, it can be formed from a pyrazine derivative represented by the structural formula shown below.

(Here, Ar represents an aryl group, and R represents hydrogen, an alkyl group having 1 to 10 carbon atoms, an alkyloxy group, a dialkylamine group, or F, Cl, Br, I, or CN.)
In the present invention, more preferably, an electron extraction layer can be formed from a hexaazatriphenylene derivative represented by the structural formula shown below.

(Here, R represents hydrogen, an alkyl group having 1 to 10 carbon atoms, an alkyloxy group, a dialkylamine group, or F, Cl, Br, I, or CN.)
In the present invention, the electron injection layer in the intermediate unit may be formed from, for example, alkali metals such as Li and Cs, alkali metal oxides such as Li ₂ O, alkaline earth metals, alkaline earth metal oxides, and the like. preferable.

  In the present invention, the electron transport layer in the intermediate unit can be formed of a material generally used as an electron transport material in the organic EL element.

  In a preferred embodiment according to the present invention, the first light emitting unit and the second light emitting unit are units that emit substantially the same color. In this case, it is preferable that substantially the same material is used to form the same structure.

  The light emitting layer constituting the first light emitting unit and the second light emitting unit in the present invention is preferably formed of a host material and a dopant material. If necessary, a carrier-transporting second dopant material may be contained. The dopant material may be a singlet light-emitting material or a triplet light-emitting material (phosphorescent material).

  The organic electroluminescent display device of the present invention supplies an organic electroluminescent element having an element structure sandwiched between an anode and a cathode and a display signal corresponding to each display pixel to the organic electroluminescent element. An active matrix driving substrate provided with the active element and a transparent sealing substrate provided opposite to the active matrix driving substrate, and the organic electroluminescent element is disposed between the active matrix driving substrate and the sealing substrate. A top emission type organic electroluminescent display device in which an electrode provided on the sealing substrate side of the cathode and the anode is a transparent electrode, wherein the organic electroluminescent element comprises a cathode, an anode, An intermediate unit disposed between the cathode and the anode and between the cathode and the intermediate unit A first light-emitting unit disposed and a second light-emitting unit disposed between the anode and the intermediate unit, and an electron extraction layer for extracting electrons from an adjacent layer adjacent to the cathode side in the intermediate unit; An electron injection layer adjacent to the anode side of the electron extraction layer, and an absolute value | LUMO (A) | of the lowest unoccupied molecular orbital (LUMO) energy level of the electron extraction layer and the maximum coverage of the adjacent layer. The absolute value of the energy level of the molecular orbital (HOMO) | HOMO (B) | is in the relationship of | HOMO (B) |-| LUMO (A) | ≦ 1.5 eV, and the lowest unoccupied molecular orbital of the electron injection layer (LUMO) energy level absolute value | LUMO (C) | or work function absolute value | WF (C) | is smaller than | LUMO (A) | and the intermediate unit is adjacent to the electron extraction layer. Supplies the holes generated by the extraction of electrons from the layer to the first light-emitting unit, and electrons withdrawn to and supplying to the second light-emitting unit through the electron injection layer.

  When the organic electroluminescent element is a white light emitting element, it is preferable to dispose a color filter between the sealing substrate and the organic electroluminescent element.

  Since the organic electroluminescent display device of the present invention is a top emission type display device, the light emitted by the organic electroluminescent element is emitted from the sealing substrate on the side opposite to the side where the active matrix is provided. Emitted. In general, an active matrix circuit is formed by laminating a large number of layers. In the case of a bottom emission type, emitted light is attenuated by the presence of such an active matrix circuit, but the organic electroluminescent display device of the present invention. Can emit light without being affected by such an active matrix circuit. In particular, since the organic electroluminescent device of the present invention has a plurality of light emitting units, the number of films through which emitted light passes is smaller in the case of the top emission type than in the case of the bottom emission type, so that the light interference. The degree of freedom of design for controlling the attenuation of the emitted light or the attenuation of the viewing angle of the emitted light can be increased.

  The organic EL element and the organic EL display device of the present invention are an organic EL element including at least two light emitting units, and are an organic EL element and an organic EL display device that can be driven at a low voltage and have high luminous efficiency.

  FIG. 1 is a schematic cross-sectional view showing an organic EL element according to the present invention. As shown in FIG. 1, a first light emitting unit 41 and a second light emitting unit 42 are provided between the cathode 51 and the anode 52. An intermediate unit 30 is provided between the first light emitting unit 41 and the second light emitting unit 42. The first light emitting unit 41 is provided on the cathode 51 side with respect to the intermediate unit 30, and the second light emitting unit 42 is provided on the anode 52 side with respect to the intermediate unit 30. An electronic extraction layer is provided in the intermediate unit 30. An adjacent layer is provided on the cathode 51 side of the electron extraction layer. As described above, the adjacent layer may be provided in the first light emitting unit 41 or may be provided in the intermediate unit 30.

  FIG. 2 is a diagram showing an energy diagram around the intermediate unit. The intermediate unit 30 includes an electron extraction layer 31, an electron injection layer 32, and an electron transport layer 33. An adjacent layer 40 is provided on the cathode side of the electron extraction layer 31. A second light emitting unit 42 is provided on the anode side of the intermediate unit 30. In FIG. 2, only the layer on the intermediate unit 30 side of the second light emitting unit 42 is shown.

  As shown in FIG. 2, an electron injection layer 32 is provided between the electron extraction layer 31 and the second light emitting unit 42. Further, an electron transport layer 33 is provided between the electron injection layer 32 and the second light emitting unit 42.

  In the embodiment shown in FIG. 2, the electron extraction layer 31 is formed of hexaazatriphenylene hexacarbonitrile (hereinafter referred to as “HAT-CN6”) represented by the following structural formula. HAT-CN6 can be manufactured by the method described in the nonpatent literature 2, for example.

  The electron injection layer 32 is formed of Li (metallic lithium).

  The electron transport layer 33 is made of BCP (2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline) having the following structure.

  In the present invention, the thickness of the electron extraction layer 31 is preferably in the range of 1 to 150 nm, more preferably in the range of 5 to 100 nm. The thickness of the electron injection layer 32 is preferably in the range of 0.1 to 10 nm, more preferably in the range of 0.1 to 1 nm. The thickness of the electron transport layer 33 is preferably in the range of 1 to 100 nm, and more preferably in the range of 5 to 50 nm.

  In the embodiment shown in FIG. 2, the adjacent layer 40 is formed of NPB (N, N′-di (naphthacene-1-yl) -N, N′-diphenylbenzidine) having the following structure.

  In the embodiment shown in FIG. 2, the layer shown as the second light emitting unit 42 is formed from TBADN (2-tertiary-butyl-9,10-di (2-naphthyl) anthracene) having the following structure. ing.

  As shown in FIG. 2, the difference between the absolute value (4.4 eV) of the LUMO energy level of the electron extraction layer 31 and the absolute value (5.4 eV) of the HOMO energy level of the adjacent layer 40 is within 1.5 eV. is there. The absolute value of the LUMO energy level (work function) of the electron injection layer 32 is smaller than the absolute value of the LUMO energy level of the electron extraction layer 31, and the absolute value of the LUMO energy level of the electron transport layer 33 is the electron injection layer. Less than the absolute value of 32 LUMO energy levels.

  Therefore, the electron extraction layer 31 can extract electrons from the adjacent layer 40 when a voltage is applied to the anode and the cathode. The extracted electrons pass through the electron injection layer 32 and the electron transport layer 33 and are supplied to the second light emitting unit 42.

  In the adjacent layer 40, holes are generated because electrons are extracted. This hole is supplied to the first light emitting unit and recombines with electrons supplied from the cathode. As a result, light is emitted in the first light emitting unit.

  The electrons supplied to the second light emitting unit recombine with the holes supplied from the anode in the second light emitting unit 42. As a result, light is emitted in the second light emitting unit 42.

  As described above, according to the present invention, recombination regions can be formed in the first light emitting unit and the second light emitting unit, respectively, and light can be emitted. As a result, the light emission efficiency can be increased, and light can be emitted with the light emission colors of the first light emission unit and the second light emission unit.

<Experiment 1>
(Examples 1-5 and Comparative Examples 1-2)
The organic EL elements of Examples 1 to 5 and Comparative Examples 1 and 2 having the anode, hole injection layer, second light emitting unit, intermediate unit, first light emitting unit, electron transport layer, and cathode shown in Table 1 were prepared. did. In the following table, the numbers in () indicate the thickness (nm) of each layer.

The anode was prepared by forming a fluorocarbon (CF _x ) layer on a glass substrate on which an ITO (indium tin oxide) film was formed. The fluorocarbon layer was formed by plasma polymerization of CHF ₃ gas. The thickness of the fluorocarbon layer was 1 nm.

  A hole injection layer, a second light emitting unit, an intermediate unit, a first light emitting unit, an electron transport layer, and a cathode were sequentially deposited on the anode manufactured as described above by an evaporation method.

  The hole injection layer was formed from HAT-CN6.

  The first light emitting unit and the second light emitting unit are formed by stacking an orange light emitting layer (NPB + 3.0% DBzR) and a blue light emitting layer (TBADN + 2.5% TBP). In any light emitting unit, the orange light emitting layer is located on the anode side and the blue light emitting layer is located on the cathode side. % Is% by weight unless otherwise specified.

  In the orange light emitting layer, NPB is used as a host material and DBzR is used as a dopant material. DBzR is 5,12-bis {4- (6-methylbenzothiazol-2-yl) phenyl} -6,11-diphenylnaphthacene and has the following structure.

  The blue light emitting layer uses TBADN as a host material and TBP as a dopant material.

  TBP is 2,5,8,11-tetra-tertiary-butylperylene and has the following structure.

About each produced organic EL element, chromaticity (CIE (x, y)) and luminous efficiency were measured, and the measurement result was shown in Table 2 with the drive voltage. The luminous efficiency is a value at 10 mA / cm ² .

  As is apparent from the results shown in Table 2, each organic EL element includes a light emitting unit having an orange light emitting layer and a blue light emitting layer, and it can be seen from the measurement result of chromaticity that white light is emitted.

  As is clear from the comparison between Examples 1 to 5 and Comparative Example 2, Examples 1 to 5 including “HAT-CN6” which is an electron extracting layer are compared with Comparative Example 2 including no electron extracting layer. High luminous efficiency is obtained. Moreover, it turns out that the organic EL element of Examples 1-5 has shown the luminescent color which the light emission unit has originally compared with the comparative example 2. FIG.

  The reason why the organic EL elements of Examples 1 to 5 exhibit high luminous efficiency is considered as follows. That is, in the organic EL elements of Examples 1 to 5, since the second light emitting unit is located on the anode side, the number of holes is relatively large. Therefore, when there is no intermediate unit, there is a shortage of electrons. On the other hand, since the first light emitting unit is located on the cathode side, it has a relatively large number of electrons, and if there is no intermediate unit, the hole is insufficient.

  As described above, when there is no intermediate unit, the four light emitting layers are in continuous and direct contact with each other, so that carriers recombine in one region of the four light emitting layers. According to the present invention, by providing an intermediate unit in the middle of the four light emitting layers, the shortage of electrons in the second light emitting unit on the anode side can be compensated, and the shortage of holes in the first light emitting unit on the cathode side can be compensated. . As described with reference to FIG. 2, when a voltage is applied to the anode and the cathode, electrons are extracted from the adjacent layer in the first light emitting unit to the electron extraction layer, and the LUMO of the electron extraction layer is detected. The extracted electron enters into. Further, as a result of electrons being extracted, holes are generated in the HOMO of the adjacent layer. The electrons of the LUMO of the electron extraction layer enter the LUMO of the electron transport layer through the electron injection layer in the intermediate unit, and then enter the second light emitting unit and recombine with the holes injected from the anode. At this time, in addition to the electrons from the intermediate unit, electrons injected from the cathode and not consumed by the first light emitting unit are considered to contribute to recombination at the same time. Thereby, the orange light emitting layer and the blue light emitting layer in the second light emitting unit emit light at the same time, and complementary color white light emission occurs.

  On the other hand, holes generated in the HOMO of the adjacent layer of the first light emitting unit and holes from the anode that are not consumed in the second light emitting unit move to the first light emitting unit in a high electric field, In the light emitting unit, it recombines with electrons injected from the cathode. Thereby, the orange light emitting layer and the blue light emitting layer of the first light emitting unit emit light at the same time, and complementary color white light emission is generated.

As described above, since white light emission occurs at two locations of the first light emitting unit and the second light emitting unit, the light emission efficiency is doubled. In the case of a conventional organic EL element in which a plurality of light emitting units are combined with an inorganic semiconductor layer such as V ₂ O ₅ interposed, a carrier originally present in the inorganic semiconductor layer is used. On the other hand, in the present invention, carriers are separated from a neutral organic layer in which no carrier exists, that is, an adjacent layer, and light is emitted using this carrier. Therefore, the organic EL device of the present invention can be driven at a lower drive voltage than conventional devices. That is, light can be emitted with an energy for extracting electrons (difference between LUMO of the electron extraction layer and HOMO of the adjacent layer) and an energy difference for injecting the generated electrons into the light emitting layer on the anode side.

In the present invention, the luminous efficiency can be doubled, so that the reliability of the element can be improved. For example, when continuous light emission is performed at an initial luminance of 5000 cd / m ² , a normal organic EL element must emit light as it is at a luminance of 5000 cd / m ² . In contrast, in the organic EL device of the present invention, the emission efficiency is doubled, one light-emitting unit in the device if caused to emit light at a luminance of 2500 cd / m ^2, which is half of 5000 cd / m ² Good. Therefore, the amount of current flowing through the element may be half, and the load on the element is reduced. Since the lifetime of the element in continuous light emission is affected by the value of the flowing current, the lifetime of the element can be improved according to the present invention.

  As described above, according to the present invention, it can be seen that by providing the electron extraction layer in the intermediate unit, an organic EL element that can be driven at a low voltage, has high emission efficiency, and exhibits a desired emission color can be obtained. .

<Experiment 2>
(Example 6 and Comparative Example 3)
The organic EL device of Example 6 provided with the anode, hole injection layer, second light emitting unit, intermediate unit, first light emitting unit, electron transport layer, and cathode shown in Table 3 was the same as in Experiment 1 above. Produced. Moreover, the organic EL element of Comparative Example 3 having the structure shown in Table 3 was prepared in the same manner as the organic EL element of Example 6 except that the intermediate unit and the first light emitting unit were not included.

  In this embodiment, an adjacent layer made of NPB is formed between the “HAT-CN6” layer of the intermediate unit and the first light emitting unit. In the present embodiment, the first light emitting unit and the second light emitting unit are composed of a blue single light emitting layer. Thus, in the layer on the anode side of the first light emitting unit, when an arylamine-based hole transporting material such as NPB is not used as the host material, it is preferable to provide an adjacent layer in the intermediate unit.

  For the organic EL elements of Example 6 and Comparative Example 3, the chromaticity and light emission efficiency were measured in the same manner as in Experiment 1, and the measurement results are shown in Table 4 together with the drive voltage.

  As is clear from the results shown in Table 4, the organic EL device of Example 6 according to the present invention has the same chromaticity as Comparative Example 3 having a single light emitting unit, and each light emitting unit is used alone. It can be seen that the same emission color as that obtained was obtained. Moreover, the luminous efficiency of Example 6 is about 1.6 times the luminous efficiency of Comparative Example 3, and it can be seen that high luminous efficiency is obtained.

<Experiment 3>
The organic EL device of Example 7 having the anode, hole injection layer, second light emitting unit, intermediate unit, first light emitting unit, electron transport layer, and cathode shown in Table 5 was prepared in the same manner as in Experiment 1 above. did.

  In the present embodiment, a blue single light emitting layer similar to that of the fourth embodiment is used as the first light emitting unit and the second light emitting unit. In this embodiment, an adjacent layer made of TPD is provided in the intermediate unit. A hole transport layer made of NPB is provided between the adjacent layer made of TPD and the first light emitting unit.

  In this embodiment, TPD is also used for the hole injection layer provided between the anode and the second light emitting unit. As shown in Table 5, a layer made of TPD is provided between the “HAT-CN6” layer and the NPB layer.

  TPD is N, N′-bis- (3-methylphenyl) -N, N′-bis- (phenyl) -benzidine and has the following structure.

  The TPD has a HOMO energy level of −5.3 eV and a LUMO energy level of −2.5 eV, which is almost the same as NPB (HOMO energy level = −5.4 eV, LUMO energy level = −2.6 eV). .

  The organic EL element of Example 7 was measured for chromaticity and luminous efficiency in the same manner as in Experiment 1, and the measurement results are shown in Table 6 together with the driving voltage.

  As shown in Table 6, even when the adjacent layer made of TPD is formed, high luminous efficiency can be obtained as in the case of the adjacent layer made of NPB. This is because, as described above, the HOMO energy level and the LUMO energy level are similar to those of NPB, so that electrons are easily extracted from the adjacent layer, and the holes generated in the adjacent layer move to the first light emitting unit. This is considered to be because it is easy to do.

<Experiment 4>
Organic EL elements of Examples 6 to 9 including an anode, a hole injection layer, a second light emitting unit, an intermediate unit, a first light emitting unit, an electron transport layer, and a cathode shown in Table 7 were produced.

  In Example 8, as in Example 7, an adjacent layer made of TPD was formed, and a layer made of TPD was also provided in the hole injection layer.

  In Example 9, an adjacent layer made of CuPc is formed, and a CuPc layer is also provided in the hole injection layer. CuPc is copper phthalocyanine and has the structure shown below.

  In Example 10, an adjacent layer made of CBP is formed, and a CBP layer is also provided in the hole injection layer. CBP is 4,4'-N, N'-dicarbazole-biphenyl and has the following structure.

  In Example 11, NPB is used as the adjacent layer.

  In the organic EL elements of Examples 8 to 11, as in Example 5, a blue single light emitting layer is used as the first light emitting unit and the second light emitting unit.

  About each organic EL element of Examples 8-11, it carried out similarly to the said experiment 1, and measured chromaticity and luminous efficiency, and showed the measurement result in Table 8 with the drive voltage.

  As shown in Table 8, in any of the organic EL elements of Examples 8 to 11, high luminous efficiency was obtained, and substantially the same emission color as that of the blue light emitting layer used in the light emitting unit was obtained. ing.

[Measurement of HOMO and LUMO energy levels of adjacent layer material and electron extraction layer material]
With respect to the material used for the adjacent layer and the material used for the electron extraction layer, the value of each energy level of HOMO and LUMO was calculated by cyclic voltammetry (CV) as follows.

1. CV measurement (1) Oxidation side measurement Using dichloromethane as a solvent, the supporting electrolyte tert-butylammonium perchlorate is added at a concentration of 10 ⁻¹ mol / l, and the measurement material becomes 10 ⁻³ mol / l. A sample was prepared. The measurement atmosphere was air and the measurement was performed at room temperature.

(2) Measurement on the reduction side Using tetrahydrofuran as a solvent, the supporting electrolyte tert-butylammonium perchlorate was added to a concentration of 10 ⁻¹ mol / l, and the measurement material was added to a concentration of 10 ⁻³ mol / l. Samples were prepared. The measurement atmosphere was a nitrogen gas atmosphere, and the measurement was performed at room temperature.

2. Calculation of HOMO and LUMO (1) The ionization potential in a standard sample NPB thin film is measured in advance using an ionization potential measurement device (“AC-2” manufactured by Riken Keiki Co., Ltd.). The measurement principle of AC-2 is as follows. The sample is irradiated with the dispersed ultraviolet rays emitted from the light source unit, and the ultraviolet energy (wavelength) is increased (shortened). When the sample is a semiconductor, when the energy of ultraviolet rays exceeds the ionization potential, photoelectrons begin to be emitted from the surface of the sample. The photoelectrons are counted using a detector (open counter).

  The relationship between the energy of ultraviolet rays and the square root of the photoelectron count value (Yield) is graphed, and an approximate straight line is drawn on this graph by the method of least squares to obtain the threshold energy of photoelectron emission. This threshold energy is interpreted as an ionization potential when the sample is a semiconductor. If the sample is metal, it is the work function. The ionization potential of NPB measured with AC-2 is -5.4 eV.

  (2) Next, NPB is measured by CV and the redox potential is measured. The oxidation potential of NPB is -0.5V, and the reduction potential is -2.3V. Therefore, HOB of NPB is −5.4 eV, and LUMO is −2.6 eV (5.4− (0.5 + 2.3) = 2.6). In measurement of other materials, for example, in the case of Alq, the oxidation potential is + 0.8V and the reduction potential is −2.0V. Therefore, when NPB is used as a reference, the HOMO of Alq is −5.7 eV (5.4- (0.8−0.5) = 5.7), and the LUMO is −2.9 eV (5.7−). (0.8 + 2.0) = 2.9).

  The energy levels of HOMO and LUMO of TPD, CuPc, CBP, NPB, and HAT-CN6 were calculated by the above measurement method, and the results are shown in Table 9. Table 9 also shows the light emission efficiency (the light emission efficiency of Examples 6 to 9) when each material is used as the material of the adjacent layer.

  As is clear from the results shown in Table 9, the difference between the absolute value of the HOMO energy level of the material of the adjacent layer and the absolute value of the LUMO energy level of the material of the electron extraction layer is in the range of 0 to 1.5 eV. It can be seen that an organic EL element with high luminous efficiency can be obtained.

<Experiment 5>
Table 10 has an anode, a hole injection layer, a second light emitting unit, an intermediate unit, a first light emitting unit, an electron transport layer, and a cathode, and the thickness x of the Li ₂ O layer in the intermediate unit is 0. 1
Organic EL elements with different thicknesses of nm, 0.2 nm, 0.3 nm, 0.5 nm, 1 nm, and 3 nm were prepared.

  The orange light emitting layer in the first light emitting unit and the second light emitting unit is the same as the orange light emitting layer in Experiment 1. The blue light-emitting layer uses 80% by weight TBADN as a host material, 2.5% by weight TBP as a first dopant material, and 20% by weight NPB as a second dopant material.

With respect to each organic EL element in which the thickness of the Li ₂ O layer was changed, the light emission efficiency at 10 mA / cm ² was measured, and the result is shown in FIG.

As is clear from the results shown in FIG. 3, it can be seen that light emission is possible when the Li ₂ O film thickness is in the range of 0.1 nm to 10 nm. It can also be seen that the luminous efficiency is particularly high when the Li ₂ O film thickness is in the range of 0.1 nm to 3 nm.

<Experiment 6>
The organic EL element shown in FIG. 6 was produced. In the organic EL element shown in FIG. 6, an anode 52 is formed on a glass substrate 50, and a hole injection layer 44 made of HAT-CN 6 is formed on the anode 52. On the hole injection layer 44, a second light emitting unit 42 including a blue light emitting layer 42a and an orange light emitting layer 42b is formed. An intermediate unit 30 is formed on the first light emitting unit unit 42. The intermediate unit 30 includes an electron extraction layer 31, an electron injection layer 32, and an electron transport layer 33. On the intermediate unit 30, a first light emitting unit 41 including a blue light emitting layer 41a and an orange light emitting layer 41b is formed. An electron transport layer 43 made of BCP is formed on the first light emitting unit 41. A cathode 51 is formed on the electron transport layer 43.

  As shown in Table 11, in the element structure shown in FIG. 6, the thickness of the electron extraction layer (HAT-CN6) of the intermediate unit was changed within the range of 5 to 150 nm.

The characteristics of the organic EL elements of Examples 12 to 17 were evaluated. The voltage, chromaticity, and efficiency are values when driven with a current of 10 mA / cm ² , and the luminance half-life is a value when driven with a current of 40 mA / cm ² . The evaluation results are shown in Table 12.

  As is apparent from the results shown in Table 12, it can be seen that Examples 13 to 16 have a luminance half-life of 900 hours or more and excellent life characteristics. Moreover, it is excellent also in power efficiency. In particular, Examples 13 and 14 have a luminance half-life of 1000 hours or more, a power efficiency of 10 lm / W or more, and good lifetime characteristics and luminous efficiency.

  On the other hand, Example 12 has a low luminance half-life and is inferior in lifetime characteristics. This is because the thickness of the electron extraction layer is too thin, Li diffuses from the electron injection layer toward the cathode, and the diffused lithium reaches the light emitting layer of the first light emitting unit, suppressing recombination of holes and electrons. Probably because.

  Moreover, in Example 17, it turns out that the brightness | luminance half life falls and electric power efficiency is also low, and it has fallen in a lifetime characteristic and luminous efficiency. In Example 17, dark spots were generated.

  From the above, it is understood that the thickness of the electron extraction layer is preferably in the range of 8 to 100 nm, more preferably in the range of 10 to 80 nm, and particularly preferably in the range of 10 to 30 nm. .

  FIG. 4 is a cross-sectional view showing an organic EL display device including an organic EL element according to an embodiment of the present invention. In this organic EL display device, light emission in each pixel is driven using a TFT as an active element. A diode or the like can be used as the active element. In this organic EL element, a color filter is provided. In this organic EL display device, a color filter is provided. This organic EL display device is a bottom emission type display device that emits and displays light below the substrate 1 as indicated by arrows.

Referring to FIG. 4, a first insulating layer 2 is provided on a substrate 1 made of a transparent substrate such as glass. The first insulating layer 2 is made of, for example, SiO ₂ and SiN _x . A channel region 20 made of a polysilicon layer is formed on the first insulating layer 2. A drain electrode 21 and a source electrode 23 are formed on the channel region 20, and a gate electrode 22 is provided between the drain electrode 21 and the source electrode 23 via the second insulating layer 3. Yes. A fourth insulating layer 4 is provided on the gate electrode 22. The second insulating layer 3 is made of, for example, SiN _x and SiO ₂ , and the third insulating layer 4 is made of SiO ₂ and SiN _x .

A fourth insulating layer 5 is formed on the third insulating layer 4. The fourth insulating layer 5 is made of, for example, SiN _x . A color filter layer 7 is provided in the pixel region on the fourth insulating layer 5. As the color filter layer 7, color filters such as R (red), G (green), and B (blue) are provided. A first planarizing film 6 is provided on the color filter layer 7. A through hole portion is formed in the first planarization film 6 above the drain electrode 21, and a hole injection electrode 8 made of ITO (indium oxide) formed on the first planarization film 6 is through. It is introduced in the hall. A hole injection layer 10 is formed on the hole injection electrode (anode) 8 in the pixel region. A second planarizing film 9 is formed in a portion other than the pixel region.

  On the hole injection layer 10, a light emitting element layer 11 laminated according to the present invention is provided. The light emitting element layer 11 has a structure according to the present invention in which the first light emitting unit is laminated on the second light emitting unit via an intermediate unit. An electron transport layer 12 is provided on the light emitting element layer 11, and an electron injection electrode (cathode) 13 is provided on the electron transport layer 12.

  As described above, in the organic EL element of this example, the hole injection electrode (anode) 8, the hole injection layer 10, the light emitting element layer 11 having the structure according to the present invention, and the electron transport are formed on the pixel region. The layer 12 and the electron injection electrode (cathode) 13 are laminated to constitute an organic EL element.

  In the light emitting element layer 11 of the present embodiment, a light emitting unit in which an orange light emitting layer and a blue light emitting layer are stacked is used, so that the light emitting element layer 11 emits white light. This white light emission is emitted to the outside through the substrate 1, but since the color filter layer 7 is provided on the light emission side, R, G, or B color is emitted according to the color of the color filter layer 7. The

  FIG. 5 is a sectional view showing an organic EL display device of an embodiment according to the present invention. The organic EL display device according to this embodiment is a top emission type organic EL display device that emits light above the substrate 1 for display as shown by arrows.

  The portion from the substrate 1 to the anode 8 is fabricated in substantially the same manner as the embodiment shown in FIG. However, the color filter layer 7 is not provided on the fourth insulating layer 5 and is disposed above the organic EL element. Specifically, the color filter layer 7 is attached on a transparent sealing substrate 10 made of glass or the like, and an overcoat layer 15 is coated thereon, and this is applied to the anode 8 via the transparent adhesive layer 14. It is attached by sticking to. In this embodiment, the positions of the anode and the cathode are reversed from those in the embodiment shown in FIG.

  A transparent electrode is formed as the anode 8, and is formed, for example, by laminating ITO having a thickness of about 100 nm and silver having a thickness of about 20 nm. As the cathode 13, a reflective electrode is formed. For example, an aluminum, chromium, or silver thin film having a thickness of about 100 nm is formed. The overcoat layer 15 is formed with an acrylic resin or the like to a thickness of about 1 μm. The color filter layer 7 may be a pigment type or a dye type. Its thickness is about 1 μm.

  White light emitted from the light emitting element layer 11 is emitted to the outside through the sealing substrate 16, but since the color filter layer 7 is provided on the light emitting side, R, G or B color is emitted. Since the organic EL display device of this embodiment is a top emission type, the region where the thin film transistor is provided can also be used as the pixel region, and the color filter layer 7 is provided in a wider range than the embodiment shown in FIG. ing. The light emitting element layer 11 is formed of an organic EL element according to the present invention and is a light emitting element layer having high light emission efficiency. However, according to this embodiment, a wider area can be used as a pixel area, and therefore, The advantages of a high light emitting element layer can be fully utilized. Further, since the light emitting element layer having a plurality of light emitting units can be formed without considering the influence of the active matrix, the degree of freedom in design can be increased.

In the above embodiment, a glass plate is used as the sealing substrate. However, the sealing substrate is not limited to the glass plate in the present invention. For example, an oxide film such as SiO ₂ or a nitride film such as SiN _{x is used.} A film-like material can also be used as a sealing substrate. In this case, since a film-like sealing substrate can be directly formed on the element, there is no need to provide a transparent adhesive layer.

  In each of the above embodiments, an organic EL element in which two light emitting units (a first light emitting unit and a second light emitting unit) are arranged between an anode and a cathode is illustrated, but the number of light emitting units in the present invention is not limited. Is not limited to two, and three or more light emitting units may be provided, and an intermediate unit may be provided between the respective light emitting units.

The typical sectional view showing the organic EL element of one example according to the present invention. The figure which shows the energy diagram of an intermediate unit periphery. Shows the relationship between the thickness of li ₂ O layer and the light emitting efficiency. Sectional drawing which shows the bottom emission type organic electroluminescence display using the organic electroluminescent element of the Example according to this invention. Sectional drawing which shows the organic electroluminescence display of the Example according to this invention. Typical sectional drawing which shows the organic EL element of the other Example according to this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Substrate 2 ... 1st insulating layer 3 ... 2nd insulating layer 4 ... 3rd insulating layer 5 ... 4th insulating layer 6 ... 1st planarization film 7 ... Color filter layer 8 ... Hole injection electrode 9 DESCRIPTION OF SYMBOLS ... 2nd planarizing film 10 ... Hole injection layer 11 ... Light emitting element layer 12 ... Electron transport layer 13 ... Electron injection electrode 14 ... Transparent adhesive layer 15 ... Overcoat layer 16 ... Sealing substrate 20 ... Channel region 21 ... Drain Electrode 22 ... Gate electrode 23 ... Source electrode 30 ... Intermediate unit 31 ... Electronic extraction layer 32 ... Electron injection layer 33 ... Electron transport layer 40 ... Adjacent layer 41 ... First light emitting unit 42 ... Second light emitting unit 51 ... Cathode 52 …anode

Claims (13)

A cathode, an anode, an intermediate unit disposed between the cathode and the anode, a first light emitting unit disposed between the cathode and the intermediate unit, and disposed between the anode and the intermediate unit A second light emitting unit to be
The intermediate unit is provided with an electron extraction layer for extracting electrons from an adjacent layer adjacent to the cathode side, and an electron injection layer adjacent to the anode side of the electron extraction layer. The absolute value of the energy level of the molecular orbital (LUMO) | LUMO (A) | and the absolute value of the energy level of the highest occupied molecular orbital (HOMO) of the adjacent layer | HOMO (B) | | − | LUMO (A) | ≦ 1.5 eV, and the absolute value of the energy level of the lowest unoccupied molecular orbital (LUMO) of the electron injection layer | LUMO (C) | or the absolute value of the work function | WF ( C) | is smaller than | LUMO (A) |
The intermediate unit supplies holes generated by extraction of electrons from the adjacent layer by the electron extraction layer to the first light emitting unit, and extracts the extracted electrons through the electron injection layer to the second light emission. An organic electroluminescent device characterized by being supplied to a unit. The electron transport layer is provided in the intermediate unit between the electron injection layer in the intermediate unit and the second light emitting unit, and the absolute value of the energy level of the lowest unoccupied molecular orbital in the electron transport layer | LUMO (D) | is smaller than | LUMO (C) | or absolute value of work function | WF (C) |
2. The organic electroluminescence according to claim 1, wherein the intermediate unit supplies the electrons extracted by the electron extraction layer to the second light emitting unit through the electron injection layer and the electron transport layer. Cent element.   The organic electroluminescent device according to claim 1 or 2, wherein the thickness of the electron extraction layer is in a range of 8 to 100 nm.   The organic electroluminescence according to any one of claims 1 to 3, wherein the second light emitting unit is a light emitting unit that emits light having substantially the same color as the first light emitting unit. Cent element.   5. The organic electroluminescent device according to claim 1, wherein the first light emitting unit and the second light emitting unit have a structure in which two light emitting layers are stacked so as to be in direct contact with each other. Nescent element.   The organic electroluminescent element according to claim 1, wherein the adjacent layer is provided in the first light emitting unit.   The organic electroluminescent element according to claim 1, wherein the adjacent layer is provided in the intermediate unit.   The organic electroluminescent device according to claim 1, wherein the adjacent layer is made of a hole transporting material.   The organic electroluminescent element according to any one of claims 1 to 7, wherein the adjacent layer is formed of an arylamine-based hole transporting material. The organic electroluminescent device according to any one of claims 1 to 9, wherein the electron extraction layer is formed of a pyrazine derivative represented by the structural formula shown below.

(Here, Ar represents an aryl group, and R represents hydrogen, an alkyl group having 1 to 10 carbon atoms, an alkyloxy group, a dialkylamine group, or F, Cl, Br, I, or CN.) 10. The organic electroluminescent device according to claim 1, wherein the electron extraction layer is formed of a hexaazatriphenylene derivative represented by a structural formula shown below.

(Here, R represents hydrogen, an alkyl group having 1 to 10 carbon atoms, an alkyloxy group, a dialkylamine group, or F, Cl, Br, I, or CN.) An active matrix driving substrate provided with an organic electroluminescent element having an element structure sandwiched between an anode and a cathode, and an active element for supplying a display signal corresponding to each display pixel to the organic electroluminescent element And a transparent sealing substrate provided opposite to the active matrix driving substrate, the organic electroluminescent element is disposed between the active matrix driving substrate and the sealing substrate, the cathode and the A top emission type organic electroluminescent display device in which an electrode provided on the sealing substrate side of the anode is a transparent electrode,
The organic electroluminescent device includes the cathode, the anode, an intermediate unit disposed between the cathode and the anode, and a first light emitting unit disposed between the cathode and the intermediate unit. A second light emitting unit disposed between the anode and the intermediate unit,
The intermediate unit is provided with an electron extraction layer for extracting electrons from an adjacent layer adjacent to the cathode side, and an electron injection layer adjacent to the anode side of the electron extraction layer. The absolute value of the energy level of the molecular orbital (LUMO) | LUMO (A) | and the absolute value of the energy level of the highest occupied molecular orbital (HOMO) of the adjacent layer | HOMO (B) | | − | LUMO (A) | ≦ 1.5 eV, and the absolute value of the energy level of the lowest unoccupied molecular orbital (LUMO) of the electron injection layer | LUMO (C) | or the absolute value of the work function | WF ( C) | is smaller than | LUMO (A) |
The intermediate unit supplies holes generated by extraction of electrons from the adjacent layer by the electron extraction layer to the first light emitting unit, and extracts the extracted electrons through the electron injection layer to the second light emission. An organic electroluminescent display device characterized by being supplied to a unit. The organic electroluminescent device according to claim 12, wherein the organic electroluminescent device is a white light emitting device, and a color filter is disposed between the organic electroluminescent device and the sealing substrate. Electroluminescent display device.

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