JP2013168501A - Organic electroluminescent element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an organic electroluminescent element capable of achieving an improvement in efficiency by applying zinc sulfide as a hole injecting material.SOLUTION: In an organic electroluminescent element including at least one organic layer between a pair of electrodes, a hole injection layer comprising zinc sulfide or a mixture of zinc sulfide and a metal oxide is formed. In another organic electroluminescent element which includes a plurality of light emitting units including at least one luminous layer between a pair of electrodes, and has a multi-photon emission structure where each of the light emitting units is partitioned by a charge generation layer, the charge generation layer includes a layer comprising zinc sulfide, a mixture of zinc sulfide and an alkali metal compound, or a mixture of zinc sulfide and a metal oxide.

Description

  The present invention relates to an organic electroluminescence element (hereinafter, abbreviated as an organic EL element) that can achieve low voltage and high efficiency.

An organic EL element is a self-luminous element that uses an organic compound as a luminescent material, and can emit light at a high speed. Therefore, it is suitable for displaying moving images, and the element structure is simple and the display panel is thinned And has such characteristics as being flexible. Due to such excellent characteristics, organic EL elements are becoming popular in daily life as mobile phones and in-vehicle displays.
Furthermore, in recent years, it has been attracting attention as a next-generation illumination by taking advantage of the above-described thin surface light emission.

The basic element structure of an organic EL element is a structure in which an organic layer is sandwiched between a pair of electrodes. When an electric current is applied, the organic layer emits light, and at least one of the electrodes is made light transmissive. It is a mechanism to take out the outside.
Each layer of such an organic EL element is basically composed of an organic material, but has translucency for the purpose of compensating for the disadvantages of organic compounds that are likely to deteriorate at high temperatures and improving luminous efficiency. Inorganic semiconductor materials are also used.

Examples of the inorganic semiconductor material as described above include zinc oxide and zinc sulfide.
There are many examples where zinc oxide is used as a transparent electrode not only in an organic EL element but also in an organic thin film solar cell or the like, but there are few examples in which zinc sulfide is used as a material for an organic EL element.
As an organic EL element using zinc sulfide, for example, Patent Documents 1 and 2 describe that application to an electron transport layer can improve electron injection and response speed. .
Patent Document 3 describes that zinc sulfide is applied as a semiconductor buffer layer for the purpose of alleviating damage to an organic layer when forming an electrode layer.

JP 2000-215984 A JP 2008-277799 A Japanese Patent Laying-Open No. 2005-243411

As described above, in the conventional organic EL element, although zinc sulfide is used as an electron transport layer or a protective layer, there is no example applied for other purposes.
The inventors of the present invention focused on zinc sulfide, which is such an inorganic semiconductor, and studied application of the organic EL element to a hole injection layer. As a result, the inventors found that the efficiency of the organic EL element can be improved. It came to complete.

  That is, an object of the present invention is to provide an organic EL device capable of improving efficiency by applying zinc sulfide as a hole injecting material.

The organic EL device according to the present invention is an organic EL device including at least one organic layer between a pair of electrodes, and includes a hole injection layer made of zinc sulfide or a mixture of zinc sulfide and a metal oxide. It is characterized by being.
Thus, by using zinc sulfide as the hole injection layer, the voltage of the organic EL element can be reduced.

In addition, another organic EL device according to the present invention includes a plurality of light emitting units including at least one light emitting layer between a pair of electrodes, and each light emitting unit is partitioned by a charge generation layer. An organic EL element (hereinafter abbreviated as MPE element), wherein the charge generation layer includes a layer made of zinc sulfide, a mixture of zinc sulfide and an alkali metal compound, or a mixture of zinc sulfide and a metal oxide. It is characterized by.
Also in the MPE element, by using zinc sulfide for the charge generation layer between the light emitting units as described above, the voltage of the organic EL element can be reduced and the efficiency can be improved.

  In the organic EL element, the metal oxide is preferably molybdenum oxide, vanadium oxide, or tungsten oxide.

  According to the present invention, by applying zinc sulfide, hole injection characteristics are improved, a driving voltage is reduced, and an organic EL element with improved efficiency can be configured.

1 is a schematic cross-sectional view schematically showing a layer structure of an organic EL element according to Example 1. FIG. 6 is a schematic cross-sectional view schematically showing a layer structure of a two-unit MPE element according to Example 2. FIG. 4 is a graph showing a current density-voltage curve of each organic EL element according to Samples 1 to 4 of Example 1. FIG. 4 is a graph showing current density-voltage curves of organic EL elements according to Samples 4 to 8 of Example 1. FIG. 6 is a graph showing an external quantum efficiency-light emission luminance curve of each organic EL element according to Example 2. 10 is a graph showing a current density-voltage curve of each organic EL element according to Example 3.

Hereinafter, the present invention will be described in more detail.
The organic EL device according to the present invention is an organic EL device including at least one organic layer between a pair of electrodes, and includes a hole injection layer made of zinc sulfide or a mixture of zinc sulfide and a metal oxide. It is characterized by being.
As described above, by using zinc sulfide as the hole injection layer, an organic EL element driven at a low voltage can be configured.

  The layer structure of the organic EL device according to the present invention including the hole injection layer as described above includes a pair of electrodes on a substrate and at least one organic layer between the electrodes. Specific examples of these layer structures include anode / hole injection layer / light emitting layer / cathode, anode / hole injection layer / light emitting layer / electron transport layer / cathode, anode / hole injection layer / hole transport layer / light emitting layer / Examples of the structure include electron transport layer / electron injection layer / cathode, anode / hole injection layer / hole transport layer / light emitting layer / hole blocking layer / electron transport layer / electron injection layer / cathode. Furthermore, a known laminated structure including a hole transport light emitting layer, an electron transport light emitting layer, and the like may be used.

The hole injection layer may be composed only of zinc sulfide, or may be composed of a mixture of zinc sulfide and metal oxide. The mixture layer of zinc sulfide and metal oxide tends to provide better hole injection properties.
As the metal oxide, for example, molybdenum oxide, vanadium oxide, tungsten oxide, or the like is preferably used.
Moreover, when it is set as the mixture layer of a zinc sulfide and a metal oxide, it is preferable to mix by equal amounts.

In addition, another organic EL element according to the present invention includes a plurality of light emitting units including at least one light emitting layer between a pair of electrodes, wherein each light emitting unit is partitioned by a charge generation layer. The charge generation layer includes a layer made of zinc sulfide, a mixture of zinc sulfide and an alkali metal compound, or a mixture of zinc sulfide and a metal oxide.
As described above, also in the MPE element, since the charge generation layer between the light emitting units includes the layer containing zinc sulfide, the voltage of the organic EL element can be reduced and the efficiency can be improved. be able to.

FIG. 2 shows an example of the layer structure of the MPE element according to the present invention. This element is an MPE element including two light emitting units, but the number of light emitting units is not particularly limited.
This MPE element has a structure in which two light emitting units including a light emitting layer 6 are provided between a pair of electrodes of an anode 1 and a cathode 5, and each light emitting unit is partitioned by a charge generation layer 7. A hole injection layer 2 is provided in contact with the anode 1, a hole transport layer 3 is provided thereon, and an electron injection layer 8 is provided in contact with the anode side of the charge generation layer 7 and the cathode 5. The configuration of the light emitting unit is not necessarily limited to this.

  The MPE element according to the present invention is characterized in that the charge generation layer 7 between the light emitting units includes a layer made of zinc sulfide, a mixture of zinc sulfide and an alkali metal compound, or a mixture of zinc sulfide and a metal oxide. .

The layer containing zinc sulfide in the charge generation layer may be composed only of zinc sulfide, or a mixture of zinc sulfide and an alkali metal compound, or zinc sulfide and metal oxide, similar to the hole injection layer. It may be comprised by the mixture of these. Rather than using zinc sulfide alone, a mixture of zinc sulfide and an alkali metal compound or a mixture layer of zinc sulfide and a metal oxide tends to obtain better hole injection properties.
As the alkali metal compound, LiF, 8-hydroxyquinolinolatolithium (Liq), NaF, KF, Cs ₂ CO _{3 and the} like are preferably used.
As the metal oxide, for example, molybdenum oxide, vanadium oxide, tungsten oxide or the like, which is a known charge generation material used in a conventional charge generation layer, is preferably used.

Note that, among the constituent layers of the organic EL element, film forming materials used for layers other than the layer containing zinc sulfide in the hole injection layer or the charge generation layer according to the present invention are not particularly limited, It can be appropriately selected from known ones, and may be either a low molecular weight type or a high molecular weight type.
The film thickness of each of the layers is appropriately determined depending on the situation in consideration of adaptability between the layers and the required total layer thickness, but is usually preferably in the range of 5 nm to 5 μm.

  The formation method of each of the above layers is a method using a nanoparticle dispersion liquid in a drive process such as a vapor deposition method or a sputtering method, an ink jet method, a casting method, a dip coating method, a bar coating method, a blade coating method, a roll coating method, a gravure. It may be a wet process such as a coating method, a flexographic printing method, or a spray coating method.

  Moreover, a well-known material and structure may be sufficient as an electrode in each device, and it does not specifically limit. For example, in the case of an organic EL device, a transparent conductive thin film formed on a transparent substrate made of glass or polymer is used, and an indium tin oxide (ITO) electrode is formed on the glass substrate as an anode, so-called An ITO substrate is common. On the other hand, the cathode is composed of a metal, alloy, or conductive compound having a small work function (4 eV or less) such as Al.

EXAMPLES Hereinafter, although this invention is demonstrated further more concretely based on an Example, this invention is not limited to these Examples.
Example 1
An organic EL element having a layer structure as shown in FIG. 1 was produced.
The anode 1 is a transparent electrode made of a glass substrate provided with ITO, on which a hole injection layer ([HIL]) 2 is formed, NPD (film thickness 40 nm) is used as the hole transport layer 3, and Alq ₃ ( The film thickness was 60 nm. On top of that, LiF (film thickness 0.5 nm) and Al (film thickness 80 nm) were sequentially laminated as the cathode 5.
In a simplified manner, the layer structure of the organic EL device produced as described above is ITO / [HIL] / NPD (40) / Alq ₃ (60) / LiF (0.5) / Al (80). .
In the above layer structure, the hole injection layer 2 was changed as shown in Table 1 below, and various element samples were prepared. The hole injection layers of Samples 2 and ₃ are obtained by stacking zinc sulfide (ZnS) and molybdenum oxide (MoO ₃ ), and the hole injection layers of Samples 4 to 9 were co-deposited at a predetermined ratio of ZnS and MoO ₃ . Is.

3 and 4 show current density-voltage curves for each of the above element samples.
As can be seen from the curves shown in FIGS. 3 and 4, the co-deposited film (sample 4) of ZnS and MoO _{3 having} a mixing ratio of 1: 1 has the lowest voltage, and the higher the mixing ratio is from 1: 1, the higher the voltage is. It was recognized that it became voltage.

(Example 2)
An MPE element (sample 10) including two light emitting units having a layer structure as shown in FIG. 2 was produced.
The anode 1 is a transparent electrode made of a glass substrate provided with ITO, on which a co-deposited film (thickness 10 nm) of ZnS and MoO ₃ is mixed as a hole injection layer 2 and an NPD (hole transport layer 3 is NPD). forming a film thickness 65 nm), the light-emitting unit ([EL]) Alq ₃ (film thickness 35nm of the green light-emitting dopant C545T were 1 wt% dope as 6) and sequentially laminating Alq ₃ (thickness 35nm).
The electron injection layer 8 is ZnS (film thickness 5 nm), LiF (film thickness 1 nm) and Al (film thickness 1 nm), and the charge generation layer 7 is a co-deposited film (film thickness) with a mixture ratio of ZnS and MoO ₃ of 1: 1. 10 nm) and NPD (film thickness 65 nm) were sequentially laminated.
Again, a light emitting unit 6 similar to the above is laminated, and ZnS (film thickness 5 nm) and LiF (film thickness 0.5 nm) are sequentially stacked thereon as the electron injection layer 8, and Al (film thickness 80 nm) is sequentially stacked as the cathode 5. did.

  For comparison, among the layer configurations of the element of the sample 10, one light emitting unit and one unit element (sample 9) that does not form the charge generation layer 7 and the electron injection layer 8 between the light emitting units were also produced. .

Further, on the basis of the sample 10, an MPE element (sample 12) provided with one unit element (sample 11) and two light emitting units without using ZnS was produced.
The film thickness was adjusted so that the distance from the cathode to the light emitting region was 76.47 nm per unit and 229.41 nm per unit.
Table 2 summarizes the layer structure of each organic EL element produced above.

FIG. 5 shows an external quantum efficiency-luminescence luminance curve for each of the above element samples.
As can be seen from the curve shown in FIG. 5, it was confirmed that the element (sample 10) using ZnS was more efficient when the unit was a two-unit MPE element.

(Example 3)
Without considering the optical distance, a 1-unit element and 2-unit MPE element having a layer structure as shown in Samples 13 to 17 in Table 3 below were prepared with a constant total film thickness. Two-unit MPE elements were prepared with the electron injection layer made of LiF and Liq, respectively. In addition, a comparison was made between an element not using ZnS and an element using it.

  In Table 2, Rub is rubrene, and B3PyMPM is a compound shown in the following (Chemical Formula 1).

FIG. 6 shows a current density-voltage curve for each of the above element samples.
As can be seen from the curve shown in FIG. 6, in the case of a two-unit MPE element, the element (sample 14) using LiF for the electron injection layer was increased in voltage, but Liq was used for the electron injection layer. The element (sample 17) in contact with which a charge generation layer formed of a mixed vapor deposition film of ZnS and MoO ₃ was driven at the lowest voltage. The rising voltage was twice as high as that of one unit element, but thereafter, the current density-voltage characteristics were the same as those of one unit element.

DESCRIPTION OF SYMBOLS 1 Anode 2 Hole injection layer 3 Hole transport layer 4 Light emitting layer 5 Cathode 6 Light emitting unit 7 Charge generation layer 8 Electron injection layer

Claims (3)

  An organic electroluminescence device comprising at least one organic layer between a pair of electrodes, wherein the organic electroluminescence device comprises a hole injection layer made of zinc sulfide or a mixture of zinc sulfide and a metal oxide. Luminescence element.   A plurality of light emitting units each including at least one light emitting layer between a pair of electrodes, wherein each light emitting unit is an organic electroluminescence element having a multi-photon emission structure partitioned by a charge generating layer, wherein the charge generating layer Includes an organic electroluminescent element characterized by including a layer made of zinc sulfide, a mixture of zinc sulfide and an alkali metal compound, or a mixture of zinc sulfide and a metal oxide.   The organic electroluminescence device according to claim 1 or 2, wherein the metal oxide is molybdenum oxide, vanadium oxide, or tungsten oxide.

Images