JP2007200776A - Manufacturing method of organic led element - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for manufacturing an organic LED element operable with a lower driving voltage than that of conventional ones. <P>SOLUTION: This method for manufacturing an organic LED element forms layers of a hole blocking layer 7, electron transportation layer 8, LiF layer 9, electron injection layer 10, and cathode 11 in this order on a light-emitting layer 6. Operations are carried out in the combination of a forward direction aging and a reverse direction aging. In the forward direction aging, current is made to flow to the light-emitting layer 6 under the condition that a potential of the anode 3 is higher than that of the cathode 11. In the reverse direction aging, current is made to flow to the light-emitting layer 6 under the condition that a potential of the cathode 11 is higher than that of the anode 3, and an integration value of applied voltages with respect to a voltage application time in the reverse direction aging is at least twice as much as an integration value of applied voltages with respect to a voltage application time in the forward direction aging. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a method for manufacturing an organic LED (Light-Emitting Diode) element.

  The organic LED element is also referred to as an organic EL (Electro Luminescence) element, and is an element that utilizes a phenomenon in which light is emitted by excitons generated by recombination of electrons and holes injected into an organic substance.

  In recent years, displays using this organic LED element have been actively developed. This is because the organic LED display has a wide viewing angle, a fast response speed, a high contrast, and the like as compared with the liquid crystal display.

  In general, an organic LED element has a structure in which an organic layer is sandwiched between a cathode and an anode. When a voltage is applied, electrons are injected as carriers from the cathode and holes are injected from the anode, respectively. When these carriers recombine inside the organic layer, excitons are generated to emit light.

  In such an organic LED element, when an alkali metal compound having a small work function such as LiF is provided at the interface between the cathode and the organic layer, the energy barrier between them can be lowered to reduce the driving voltage. (For example, see Non-Patent Document 1 and Non-Patent Document 2).

T. Wakimoto et al., IEEE Trans. Electron Devices, 1997, 44, p. 1,245 L. S. Hung et al., Applied Physics Letter, 1997, 70, p. 152

  However, there is a high demand for further lowering the voltage of the organic LED element. Then, an object of this invention is to provide the manufacturing method of the organic LED element which can reduce a drive voltage conventionally.

  Other objects and advantages of the present invention will become apparent from the following description.

The present invention relates to a method for producing an organic LED element having a light emitting layer between an anode and a cathode,
Forming an electron transport layer on the light emitting layer;
Forming an insulating layer made of an alkali metal compound or an alkaline earth metal compound in contact with the electron transport layer;
Forming an electron injection layer in contact with the insulating layer;
Forming the cathode in contact with the electron injection layer;
Forward aging in which the light emitting layer is energized under a condition in which the potential of the anode is higher than that of the cathode, and reverse aging in which the light emitting layer is energized in a condition in which the potential of the cathode is higher than the potential of the anode. And performing the process in combination.
A method for producing an organic LED element, characterized in that an integrated value relating to the application time of the applied voltage in the reverse aging is at least twice an integral value relating to the application time of the applied voltage in the forward aging. About.

  The present invention may further include a step of forming a hole blocking layer between the light emitting layer and the electron transport layer.

  In the present invention, the insulating layer is preferably a LiF layer.

In the present invention, both the electron transport layer and the electron injection layer are preferably Alq ₃ layers.

  According to the present invention, an organic LED element having a lower driving voltage than conventional ones is provided by providing an insulating layer between an electron transport layer and an electron injection layer, and performing forward aging and reverse aging in combination. Can be manufactured.

  In the present invention, the organic LED element has a multilayer structure in which an anode, a hole injection layer, a hole transport layer, a light emitting layer, a hole blocking layer, an electron transport layer, an electron injection layer, and a cathode are laminated, A first feature is that an insulating layer made of an alkali metal compound or an alkaline earth metal compound is provided between the electron transport layer and the electron injection layer. Here, the insulating layer is preferably a LiF (lithium fluoride) layer. The hole blocking layer may not be provided. Further, both the hole injection layer and the hole transport layer may be omitted, or only the hole injection layer may be omitted. Furthermore, the hole transport layer and the electron transport layer may be composed of a plurality of layers.

  Further, the present invention is directed to the organic LED element described above, in which forward aging is applied to the light emitting layer when the anode potential is higher than the cathode potential, and the light emitting layer is applied under conditions where the cathode potential is higher than the anode potential. The second feature is that it is performed in combination with reverse aging that is energized. In this case, the integral value related to the application time of the applied voltage in the aging in the reverse direction is set to be twice or more the integral value related to the application time of the applied voltage in the aging in the forward direction.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. In this specification, the hole injection property refers to the ability to inject holes from the anode, and the hole transport property refers to the ability to transport the injected holes to the light emitting layer. The electron injecting property refers to the ability to inject electrons from the cathode, and the electron transporting property refers to the ability to transport the injected electrons to the light emitting layer.

  FIG. 1 is a schematic cross-sectional view of an organic LED element 1 in the present embodiment. In the figure, on a substrate 2, an anode 3, a hole injection layer 4, a hole transport layer 5, a light emitting layer 6, a hole blocking layer 7, an electron transport layer 8, a LiF layer 9, an electron injection layer 10 and a cathode 11 is formed. Thus, the organic LED element manufacturing method of the present invention has a first feature that the LiF layer 9 is provided between the electron transport layer 8 and the electron injection layer 10.

  For the substrate 2, a material having a high transmittance with respect to visible light is used. Specifically, in addition to inorganic glass such as alkali glass, alkali-free glass, and quartz glass, polyester, polycarbonate, polyether, polysulfone, polyethersulfone, polyvinyl alcohol, and fluorine-containing polymers such as polyvinylidene fluoride and polyvinyl fluoride. Transparent materials such as

The anode 3 is made of a transparent metal having a high work function, an alloy thereof, or another conductive compound. For example, ITO (Indium Tin Oxide), SnO ₂ or ZnO can be used as the anode material. For example, after depositing these films on the substrate 2 by vapor deposition or sputtering, the anode 3 can be formed by patterning using photolithography.

  The hole injection layer 4 and the hole transport layer 5 are both made of a hole transporting organic material. That is, since the difference in ionization potential with the anode 3 is small, a material with a low hole injection barrier from the anode 3 and a high hole mobility is used.

  The hole injection layer 4 can also be referred to as an anode buffer layer. In the present embodiment, for example, the hole injection layer 4 can be formed by vapor-depositing copper phthalocyanine on the anode 3.

  Examples of the hole transport layer 5 include N, N′-bis (1-naphthyl) -N, N′-diphenylbenzidine (α-NPD), N, N′-diphenyl-N, N′-bis [N -Phenyl-N- (2-naphthyl) -4'-aminobiphenyl-4-yl] -1,1'-biphenyl-4,4'-diamine (NPTE), 1,1-bis [(di-4- Tolylamino) phenyl] cyclohexane (HTM2), N, N′-diphenyl-N, N′-bis (3-methylphenyl) -1,1′-diphenyl-4,4′-diamine (TPD) and the like are used. it can. In the present embodiment, the hole transport layer 5 can be formed, for example, by depositing α-NPD on the hole injection layer 4.

The light emitting layer 6 is made of a material that provides a field where injected electrons and holes can be recombined and has high light emission efficiency. Specifically, tris (8-quinolinolato) aluminum complex (Alq ₃ ), bis (8-hydroxy) quinaldine aluminum phenoxide (Alq ′ ₂ OPh), bis (8-hydroxy) quinaldine aluminum-2,5- Dimethylphenoxide (BAlq), mono (2,2,6,6-tetramethyl-3,5-heptanedionate) lithium complex (Liq), mono (8-quinolinolato) sodium complex (Naq), mono (2, 2,6,6-tetramethyl-3,5-heptanedionate) lithium complex, mono (2,2,6,6-tetramethyl-3,5-heptanedionate) sodium complex and bis (8-quinolinolate) metal complexes of quinoline derivatives such as calcium complex (CAQ _2), tetraphenyl butadiene, Feniruki Kudrin (QD), anthracene, and a fluorescent substance such as perylene and coronene.

  The fluorescent material is preferably used as a dopant in combination with a host material capable of emitting light by itself. As a result, the emission wavelength characteristic of the host material can be changed, light emission shifted to a long wavelength can be achieved, and the light emission efficiency and stability of the device can be improved.

  As the host substance, a quinolinolato complex is preferable, and an aluminum complex having 8-quinolinol and a derivative thereof as a ligand is particularly preferable.

  In the present embodiment, for example, the light emitting layer 6 can be formed by simultaneously depositing a host material and a dopant on the hole transport layer 5.

  In addition to the light emitting material, the light emitting layer 6 may contain a hole injecting material or an electron injecting material that transports holes injected from the anode or electrons injected from the cathode to the light emitting material.

  For example, the light emitting layer 6 includes a hole transporting light emitting layer (not shown) formed on the hole transporting layer 5 and an electron transporting light emitting layer (illustrated) formed on the hole transporting light emitting layer. (Not shown). The hole transporting light emitting layer may be made of an organic material that functions as a red light emitting layer, for example. On the other hand, the electron transporting light emitting layer can be made of, for example, an organic material that functions as a blue light emitting layer. Specifically, these can be formed by using a hole transporting or electron transporting organic material as a base material and adding a fluorescent substance thereto.

  When a direct current is applied between the anode 3 and the cathode 11, holes are injected from the anode 3 into the hole transporting light emitting layer through the hole injection layer 4 and the hole transport layer 5. Further, electrons are injected from the cathode 11 into the electron transporting light emitting layer through the electron injection layer 10, the LiF layer 9, the electron transport layer 8 and the hole blocking layer 7. Furthermore, when holes and electrons move at the interface of these light emitting layers, the holes and electrons recombine inside each light emitting layer to generate excitons. And the excitation energy of an exciton moves to the fluorescent substance in a light emitting layer, and emission of the intrinsic | native color according to the emission peak wavelength of a fluorescent substance occurs. In this case, the viewer recognizes the color mixture of the light emission colors of the respective light emitting layers as the light emission color of the organic LED element 1.

  Thus, electrons are injected into the hole transporting light emitting layer from the electron transporting light emitting layer. On the other hand, holes are injected into the electron transporting light emitting layer from the hole transporting light emitting layer. When a region having a predetermined thickness from the interface of these light emitting layers is used as a light emission band, red light is emitted from the hole transporting light emitting layer, and at the same time, blue light is emitted from the electron transporting light emitting layer. The eyes appear to emit white light.

  The hole blocking layer 7 is a layer that blocks the movement of holes from the light emitting layer 6 to the electron transport layer 8. For this reason, an electron transporting organic material having a larger ionization potential than the light emitting layer 6 is used for the hole blocking layer 7. In the present embodiment, for example, the hole blocking layer 7 can be formed by evaporating oxadiazole on the light emitting layer 6. However, in this embodiment, the hole blocking layer 7 may not be provided.

  For example, when a compound having a carbazole skeleton is used in the light-emitting layer, this compound has a hole transport property, but has a problem that injected holes are likely to escape to the cathode side due to a large ionization potential. is there. Therefore, as in this embodiment, by providing a hole blocking layer on the cathode side, it is possible to suppress the movement of holes to the cathode side and to efficiently recombine with electrons in the light emitting layer. It becomes possible.

Both the electron transport layer 8 and the electron injection layer 10 are made of an electron transporting organic material. In the present embodiment, a LiF layer 9 is provided between these layers. In this case, an Alq ₃ layer is preferably used for the electron transport layer 8 and the electron injection layer 10.

In the present embodiment, for example, an Alq ₃ layer and a LiF layer are sequentially deposited on the hole blocking layer 7, and further an Alq ₃ layer is deposited again, whereby the electron transport layer 8, the LiF layer 9, and the electrons are deposited. An injection layer 10 can be formed.

Non-Patent Document 1 described above discloses an organic LED element in which an alkali metal compound is provided between an Alq ₃ layer constituting an organic layer and an Al layer as a cathode.

On the other hand, the present inventor has found that an organic LED element having a lower driving voltage than the conventional one can be obtained by providing a LiF layer between two Alq ₃ layers and further performing aging described later. Although the reason for this is not necessarily clear, it is expected that the LiF layer has some influence on the interface between the Alq ₃ layer and the cathode. Therefore, it is considered preferable that the thickness of the Alq ₃ layer as the electron injection layer is not so thick. Since LiF is an insulator, the thickness of the LiF layer 9 is preferably 10 nm or less, and more preferably 5 nm or less. For example, the thickness of the electron transport layer made of Alq ₃ layer is about 7.5 nm, the thickness of the LiF layer is about 0.5 nm, and the thickness of the electron injection layer made of Alq ₃ layer is about 2.0 nm. be able to.

In the present embodiment, the insulating layer provided between the electron transport layer and the electron injection layer may be other than the LiF layer. For example, other alkali halides such as MgF ₂ (magnesium fluoride), NaCl (sodium chloride), KI (potassium iodide), CsF (cesium fluoride) and CsI (cesium iodide) are listed as preferred insulating layers. Can do. In some cases, Al ₂ O ₃ (alumina), PMMA (polymethyl methacrylate), or the like can be used. Since these are sublimable insulating materials, they can be formed into a thin film using a vapor deposition method in the same manner as LiF. It is presumed that the placement of the alkali metal compound or alkaline earth metal compound described above lowers the vacuum order and facilitates movement of electrons.

  For the cathode 11, a metal having a small work function or an alloy thereof is used. Specific examples include alkali metals, alkaline earth metals, and metals of Group 3 of the periodic table. Among these, since it is an inexpensive material with good chemical stability, Al (aluminum), Mg (magnesium), Mg and Al, Mg and Ag (silver), Mg and In (indium), or Al and Li An alloy such as (lithium) can be used. For example, the cathode 11 can be formed by depositing these films on the electron injection layer 10 by vapor deposition and then patterning using photolithography.

  By the way, in general, in the organic LED element, the luminance per current density is changed with the lapse of the light emission time. For this, it is effective to perform aging according to a predetermined condition in the manufacturing process of the organic LED element. In particular, Li is a chemically unstable material with high reactivity to oxygen and water due to its low work function. For this reason, in the manufacturing method of the organic LED element of this invention, it suppresses the change of a brightness | luminance by performing aging as the 2nd characteristic.

  Specifically, the aging includes forward aging in which the anode potential is applied to the light emitting layer under the condition of the cathode potential and reverse aging in which the cathode potential is applied to the light emitting layer under the condition of the anode potential being higher than the anode potential. It carries out in combination. At this time, the integral value related to the application time of the applied voltage in the reverse aging is set to be twice or more the integral value related to the application time of the applied voltage in the forward aging. The "integrated value related to the application time of the applied voltage" is the integrated value of the applied voltage related to the applied time when the applied voltage is plotted on the horizontal axis and the applied voltage is plotted on the vertical axis. Say.

  The forward aging has an effect of causing the organic LED element to emit light and promoting the initial deterioration of the element. On the other hand, aging in the reverse direction has the effect of reducing the part that greatly contributes to deterioration by passing a small current through the element when the element does not emit light, and the rearrangement of molecules in the light emitting layer to stabilize brightness. Has the effect of In addition, both forward aging and reverse aging both have the effect of repairing the leak site of the organic LED element.

  Forward aging and reverse aging can be performed in any combination. For example, after aging in either direction is performed for a predetermined time, aging in the opposite direction can be performed for a predetermined time. Also, forward aging and reverse aging may be switched multiple times. In this case, there is no particular limitation on the number of times of switching. The applied current may be either direct current or alternating current. Furthermore, direct current and alternating current may be combined, and a rectangular wave may be sufficient.

During forward aging, a larger current usually flows than when the organic LED element is in operation. On the other hand, almost no current flows during aging in the reverse direction. In the present embodiment, it is preferred that the voltage in the forward direction of the aging is 5V～40V, it is preferred current is ^{5mA / cm 2 ~1,000mA / cm 2} . On the other hand, the voltage during aging in the reverse direction is preferably 10V to 50V.

  Aging can be performed in the final stage of the manufacturing process of an organic LED element, for example. Further, as long as the conditions for energizing the light emitting layer are satisfied, the process can be performed at any stage of the manufacturing process. The atmosphere in which aging is performed may be either a nitrogen atmosphere or air.

  A higher aging temperature is preferred. However, if the temperature is too high, damage to each component constituting the organic LED element, particularly the organic material, is increased. Therefore, it is preferable to set an appropriate temperature according to the material to be used. Specifically, the temperature is preferably lower than the glass transition point of the organic material used.

  Aging is preferably performed until the luminance per unit current density of the organic LED element falls within a predetermined range. For example, when the temporal change in luminance is monotonously decreasing, aging can be performed until the luminance reaches 90% of the initial value. If the change in luminance is not monotonous, aging may be performed until the change is monotonous.

  As described above, the LiF layer is provided between the electron transport layer and the electron injection layer, and the interface between the electron transport layer and the LiF layer and the interface between the LiF layer and the electron injection layer are obtained by performing predetermined aging. Since an effect | action becomes favorable and an electron becomes easy to move, it seems that the organic LED element with a low drive voltage can be obtained compared with the past.

  Examples of the present invention and comparative examples will be described below.

Example.
ITO was formed on a glass substrate to form an anode. Next, this was put in a vacuum deposition machine, and the inside of the vacuum deposition machine was depressurized to 1 × 10 ⁻⁶ Torr. Next, the substance represented by the formula (1) was vapor-deposited at a rate of 2 Å / sec to a thickness of 40 nm to form a hole injection layer on the anode. Further, a substance represented by the formula (2) was vapor-deposited at a rate of 2 Å / sec to a thickness of 20 nm, and a hole transport layer was formed on the hole injection layer.

Formula (1)
CuPc

Formula (2)
α-NPD

  By simultaneously depositing the substance shown in Formula (3) and the substance shown in Formula (4) as a host substance and using the substance shown in Formula (5) as a dopant on the hole transport layer, a thickness of 20 nm is obtained. A hole transporting light emitting layer was formed. The deposition rate of the host material was 2 kg / second. The deposition rate of the dopant was 0.02 liter / second.

Formula (3)

Formula (4)

Formula (5)

  Subsequently, by simultaneously depositing the substance shown in Formula (4) and the substance shown in Formula (3) on the hole transporting light emitting layer using the substance shown in Formula (6) as a dopant and the substance shown in Formula (6) as a dopant. An electron transporting light emitting layer having a thickness of 40 nm was formed. The deposition rate of the host material was 2 kg / second. The deposition rate of the dopant was 0.02 liter / second.

Formula (6)
Perylene

  Subsequently, a substance represented by the formula (4) was deposited on the electron transporting light emitting layer at a rate of 2 Å / sec to form a hole blocking layer.

  Subsequently, the substance shown in Formula (7) was vapor-deposited at a rate of 2 kg / sec to a thickness of 7.5 nm to form an electron transport layer on the hole blocking layer. Subsequently, after depositing LiF to a thickness of 0.5 nm at a rate of 0.5 Å / sec, the material shown in formula (7) is again deposited to a thickness of 2.0 nm at a rate of 2 Å / sec, A LiF layer and an electron injection layer were formed.

Formula (7)
Alq ₃

  Finally, Al was vapor-deposited at a rate of 5 liters / second to a thickness of 100 nm to form a cathode on the electron injection layer.

  Aging was performed on the obtained organic LED element. Specifically, at a temperature of 125 ° C., aging in which a forward bias voltage of 7 V is applied for 10 milliseconds and aging in which a reverse bias voltage of 35 V is applied for 90 milliseconds are alternately repeated (frequency 20 Hz). For 2 hours.

When the light emission characteristics of the organic LED element after aging were evaluated, the current efficiency when obtaining a luminance of 500 cd / m ² was 10.5 cd / A, and the drive voltage was 7.8 V.

Comparative example.
ITO was formed on a glass substrate to form an anode. Next, this was put in a vacuum deposition machine, and the inside of the vacuum deposition machine was depressurized to 1 × 10 ⁻⁶ Torr. Next, the substance represented by the formula (1) was vapor-deposited at a rate of 2 Å / sec to a thickness of 40 nm to form a hole injection layer on the anode. Further, a substance represented by the formula (2) was vapor-deposited at a rate of 2 Å / sec to a thickness of 20 nm, and a hole transport layer was formed on the hole injection layer.

  Next, the substance shown in Formula (3) and the substance shown in Formula (4) are used as a host material, and the substance shown in Formula (5) is used as a dopant. A hole transporting light emitting layer having a thickness of 20 nm was formed. The deposition rate of the host material was 2 kg / second. The deposition rate of the dopant was 0.02 liter / second.

  Subsequently, by simultaneously depositing the substance shown in Formula (4) and the substance shown in Formula (3) on the hole transporting light emitting layer using the substance shown in Formula (6) as a dopant and the substance shown in Formula (6) as a dopant. An electron transporting light emitting layer having a thickness of 40 nm was formed. The deposition rate of the host material was 2 kg / second. The deposition rate of the dopant was 0.02 liter / second.

  Subsequently, a substance represented by the formula (4) was deposited on the electron transporting light emitting layer at a rate of 2 Å / sec to form a hole blocking layer.

  Next, the substance represented by the formula (7) was vapor-deposited at a rate of 2 Å / sec to a thickness of 20 nm to form an electron transport layer on the hole blocking layer. Subsequently, LiF was vapor-deposited at a rate of 0.5 Å / second to a thickness of 5 nm to form a LiF layer as an electron injection layer.

  Finally, Al was vapor-deposited at a rate of 5 liters / second to a thickness of 100 nm to form a cathode on the electron injection layer.

  Aging was performed on the obtained organic LED element. Specifically, at a temperature of 125 ° C., aging in which a forward bias voltage of 7 V is applied for 10 milliseconds and aging in which a reverse bias voltage of 35 V is applied for 90 milliseconds are alternately repeated (frequency 20 Hz). For 2 hours.

When the light emission characteristics of the organic LED element after aging were evaluated, the current efficiency when obtaining a luminance of 500 cd / m ² was 10.2 cd / A, and the drive voltage was 8.8V.

  FIG. 2 shows the results of evaluating the luminance change with respect to the voltage for the above-described examples and comparative examples. As is apparent from this figure, the voltage of the example is lower than that of the comparative example in order to obtain the same luminance. Therefore, according to this invention, it turned out that a drive voltage can be made lower than a prior art example.

  The present invention is not limited to the above-described embodiment, and various modifications can be made without departing from the spirit of the present invention.

It is typical sectional drawing of the organic LED element in this Embodiment. It is the result of having evaluated the brightness | luminance change with respect to a voltage about an Example and a comparative example.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Organic LED element 2 Board | substrate 3 Anode 4 Hole injection layer 5 Hole transport layer 6 Light emitting layer 7 Hole blocking layer 8 Electron transport layer 9 LiF layer 10 Electron injection layer 11 Cathode

Claims (4)

In the method of manufacturing an organic LED element having a light emitting layer between an anode and a cathode,
Forming an electron transport layer on the light emitting layer;
Forming an insulating layer made of an alkali metal compound or an alkaline earth metal compound in contact with the electron transport layer;
Forming an electron injection layer in contact with the insulating layer;
Forming the cathode in contact with the electron injection layer;
Forward aging in which the light emitting layer is energized under a condition in which the potential of the anode is higher than that of the cathode, and reverse aging in which the light emitting layer is energized in a condition in which the potential of the cathode is higher than the potential of the anode. And performing the process in combination.
A method for producing an organic LED element, characterized in that an integrated value relating to the application time of the applied voltage in the reverse aging is at least twice an integral value relating to the application time of the applied voltage in the forward aging. .   The manufacturing method of the organic LED element of Claim 1 which further has the process of forming a hole blocking layer between the said light emitting layer and the said electron carrying layer.   The method for manufacturing an organic LED element according to claim 1, wherein the insulating layer is a LiF layer. The said electron carrying layer and the said electron injection layer are both Alq ₃ layers, The manufacturing method of the organic LED element of any one of Claims 1-3.

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