JP2005100741A - Light emitting device, and image display device, light source, and photosensitive exposure light source using the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a phosphorescence light emitting device having a light emitting layer formed of a single light emitting material. <P>SOLUTION: The light emitting device comprises an exciplex suppression layer 8 arranged between a light emitting layer 2 and a positive electrode 4. The HOMO level of the phosphorescent luminescent metal coordination compound is -5.5 ev or less, and the difference between the HOMO level of the exciplex suppression layer 8 and the HOMO level of the phosphorescent luminous metal coordination compound is 0.5 ev or less. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a light-emitting element having an anode and a cathode, and an organic light-emitting layer disposed between the anode and the cathode and made only of a phosphorescent metal coordination compound. Furthermore, the present invention relates to an image display device, a light source, and a photoreceptor exposure light source using the light emitting element.

  An organic electroluminescence element (organic EL element) is a light emitting element. Research on organic EL devices has been vigorously conducted (Non-Patent Document 1).

  Further, devices utilizing phosphorescence emission via triplet excitons have been studied (Non-Patent Documents 2 to 3 and Patent Documents 1 to 3).

  In addition, in the study of a device using phosphorescence emission via triplet excitons, there is a description that the light emitting layer is composed of two kinds of materials such as a host and a guest. This uses energy transfer between two types of light-emitting materials to transfer energy from short-wavelength light-emitting materials (green light-emitting) to long-wavelength light-emitting materials (yellow or red light-emitting), and only long-wavelength light-emitting materials. Is emitted (Non-patent Document 4).

  There is also a presentation on organic EL elements using a single material (phosphorescent material). This is a phosphorescent light emitting device using an iridium complex, and has a structure in which a hole blocking layer is provided on the cathode side of the light emitting layer, and from this, light emission is considered to be performed on the electron transport layer side of the light emitting layer. (Non-Patent Document 5).

  A phosphorescent light emitting device using a rhenium complex as a light emitting layer has been announced. According to this, a hole transport layer is disposed between the light emitting layer and the anode. This hole transport layer functions as an electron blocking layer. And the difference of EL spectrum and PL spectrum of this light emitting element is 10 nm (nonpatent literature 6).

JP 11-329739 A Japanese Patent Laid-Open No. 11-256148 JP-A-8-319482 Macromol. Symp. 125,1-48 (1997) D. F. O'Brien et al., "Improved energy transfer in electrophoretic device," Applied Physics Letters 74, Applied Physics 74. , No. 3, p422 " M.M. A. Bardo et al., “Very high-efficiency green organic-lightening devices bases”. “Very high-efficiency green organic light-emitting” “Applied Physics Letters” 1999, Vol. 75, no. 1, p. 4 Yoshiharu Sato, “Material aspects of triplet devices” (Applied Physics Society, Organic Molecules and Bioelectronics Subcommittee, 9th Seminar, Challenges for Next Generation Organic EL) IDW'02 NHK OELp-17 1207-1209 Inorg. Chem. 2002, 41, 3353-3358

  A phosphorescent light-emitting element having a light-emitting layer composed of a host and a guest is expensive in terms of production and other costs. In the case where the light emitting layer is an element described in Non-Patent Document 6, the difference between the EL spectrum and the PL spectrum cannot be kept below a certain level.

  An object of the present invention is to provide a light-emitting element that has a single-material light-emitting layer that does not depend on a host-guest type light-emitting layer, and that can make the difference between an EL spectrum and a PL spectrum less than or equal to a certain level.

The present invention
In a light-emitting element having an anode and a cathode, and an organic light-emitting layer that is disposed between the anode and the cathode and is made only of a phosphorescent metal coordination compound,
Having an exciplex suppression layer disposed between the organic light emitting layer and the anode;
The HOMO level of the phosphorescent metal coordination compound is −5.5 eV or less, and the difference between the HOMO level of the exciplex suppression layer and the HOMO level of the phosphorescent metal coordination compound is 0. Provided is a light emitting element characterized by being within 5 eV.

  According to the present invention, it is possible to provide a phosphorescent light-emitting element in which the difference between the EL spectrum and the PL spectrum is within 10 nm and the light-emitting layer is made of a single material.

  In the light emitting device of the present invention, an exciplex suppressing layer is provided between the anode and the light emitting layer. The light-emitting layer is a layer in which a material that emits phosphorescence alone is present, and is a layer that emits light even without a material intended for energy transfer such as a host guest. Therefore, the light emitting layer may contain another purpose, for example, a material having an action of protecting the light emitting material from moisture or oxygen, for example, an inorganic oxide compound of calcium oxide or a metal such as Mg.

  The exciplex suppression layer is a layer for suppressing the occurrence of exciplex. Exciplex is considered not to emit light in most cases, but even if it emits light, the light emitted from the element has an emission wavelength that is greatly shifted from the emission wavelength of the light emitting material itself (λmax, peak of emission wavelength) to the longer wavelength side. Light. It can be said that the occurrence of exciplex can be suppressed when the difference between the EL spectrum and the PL spectrum is within 10 nm.

  The reason why the exciplex suppressing layer is provided between the anode and the light emitting layer is that the phosphorescent metal coordination compound used in the present invention has an electron transporting property. Since an exciplex occurs on the anode side of the light emitting layer, an exciplex suppression layer is disposed between the anode and the light emitting layer. The exciplex suppression layer may be disposed between the hole transport layer and the light emitting layer. Of course, the hole transport layer is a layer disposed on the anode side as viewed from the light emitting layer. The hole transport layer can also serve as an exciplex suppression layer.

  The central metal of the phosphorescent metal coordination compound constituting the light emitting layer is preferably rhenium from the viewpoint of high electron transport properties. A metal coordination compound having rhenium as a central metal has a lower LUMO level than other metal coordination compounds. Therefore, when the phosphorescent metal compound having a high electron transport property used in the present invention is used as the light emitting layer, it is considered that electrons injected from the cathode easily reach the anode side of the light emitting layer.

  From these findings, the present inventors indicate that the difference between the HOMO level of the exciplex suppression layer and the HOMO level of the light emitting layer is preferably 0.5 eV or less in order to suppress the occurrence of the exciplex. I found it. In order to establish such a relationship, it has been found that the HOMO level of a light emitting layer made of a phosphorescent metal coordination compound needs to be −5.5 eV or less. Note that the lower limit of the HOMO level of the light-emitting layer made of the phosphorescent metal coordination compound is 6.22 eV. Below this value, normal light emission of the phosphorescent metal coordination compound cannot be obtained.

  In the light-emitting element that satisfies these conditions, a phosphorescent light-emitting element in which the difference between the EL spectrum and the PL spectrum is within 10 nm and the light-emitting layer is made of a single material can be provided.

  Next, the layer structure of the light emitting device of the present invention will be described. FIG. 1 is a schematic sectional view of an embodiment of the present invention. In the figure, 1 is a cathode, 2 is a light emitting layer, 3 is a hole transport layer, 4 is an anode, 5 is a substrate, 6 is an electron transport layer, and 8 is an exciplex suppression layer. In the present embodiment, the cathode 1 is a reflective electrode, the anode 4 is a transparent electrode, and the substrate 5 is a transparent substrate. Of course, the cathode 1 may be a transparent electrode and the anode 4 may be a reflective electrode. In that case, in particular, a driving element for controlling lighting of the light emitting element, for example, a thin film transistor (TFT) may be arranged on the substrate. In that case, as a so-called top emission type light emitting element, it is a preferable form in the sense of improving the aperture ratio of the light emitting surface.

  Although an electron injection layer and a hole injection layer are not shown in the drawing, they may be provided as appropriate. Alternatively, the electron transport layer 6 and the hole transport layer 3 are individually replaced with the electron injection layer and the hole injection layer, respectively, or the electron transport layer 6, the hole transport layer 3, the electron injection layer, and the hole injection layer are not provided. Alternatively, at least one of these four layers may be provided as appropriate.

  Further, at least one of the anode 4 and the cathode 1 may have a multilayer structure.

  As described above, it is preferable that the central metal is rhenium as the phosphorescent metal coordination compound used in the present invention. Specific examples are given below as structural formulas (Re (dmphen) (btpz), Re (Dmphen) (pz)). Moreover, the structural formula of the compound used as an exciplex suppression layer is also illustrated below (TFB3, TFB4). Further, structural formulas of compounds that become the hole transport layer and the electron transport layer are also exemplified below (FL03, Bphen).

  The light-emitting element of the present invention can be used for a display device such as a display. For example, it can be used for an image display device used in a pixel portion or a sub-pixel portion of a display. The display is a display device mounted on a television, a personal computer, a digital camera, a camcorder, or the like, or a display device mounted on a vehicle body. The light-emitting element of the present invention may be used as a light source for illumination, or as an exposure light source for a display unit or a photoreceptor of an electrophotographic image forming apparatus.

  The light emitting element of the present invention may be used singly or in plural. In a plurality of cases, for example, light may be emitted by passive driving or active matrix driving. Further, when a plurality of light emitting elements are used, each may be a single color or a different color. In the case of different colors, full color light emission can be performed.

(Example 1, Example 2, Comparative Example 1)
An anode (ITO) having a film thickness of 100 nm was formed on the substrate so as to have an electrode area of 3 mm ² . The layers formed thereon and the film thicknesses are as follows for each example and comparative example. In forming each element, the respective layers were continuously formed by resistance heating vacuum deposition in a vacuum chamber of 10 ⁻⁴ Pa until a cathode serving as a counter electrode with respect to the anode was formed. In each example, each layer is placed on the anode in the order listed.

Example 1
Exciplex suppression layer (25 nm): TFB4
Light emitting layer (40 nm): Re (dmphen) (btpz)
Electron transport layer (60 nm): Bphen
Metal electrode 1 (1 nm): KF
Metal electrode 2 (150 nm): Al

Example 2
Hole transport layer (25 nm): FL03
Exciplex suppression layer (25 nm): TFB3
Light emitting layer (40 nm): Re (dmphen) (btpz)
Electron transport layer (60 nm): Bphen
Metal electrode 1 (1 nm): KF
Metal electrode 2 (150 nm): Al

Comparative example 1
Hole transport layer (40 nm): FL03
Light emitting layer (40 nm): Re (dmphen) (btpz)
Electron transport layer (60 nm): Bphen
Metal electrode 1 (1 nm): KF
Metal electrode 2 (150 nm): Al

  Table 1 shows the characteristic evaluation of each element.

  [1] in Table 1 is the HOMO level of the exciplex suppression layer. For the measurement, AC-1 manufactured by Riken Keiki Co., Ltd. was used. The measurement conditions are air and room temperature.

  [2] is the HOMO level of the light emitting layer (phosphorescent light emitting metal coordination compound). For the measurement, AC-1 manufactured by Riken Keiki Co., Ltd. was used. The measurement conditions are room temperature in the atmosphere.

Further, cd / A [600 cd / cm ² ] means current efficiency at a luminance of 600 cd / m ² . A BM-7 manufactured by TOPCON was used for the measurement of the amount of luminescence. The measurement condition is room temperature.

T _1/2 (h) means a luminance half time at the time of low current driving. For measuring the luminance half-life, a luminance measuring device using a photodiode was used. The measurement condition is room temperature.

  Next, the difference between the EL spectrum and the PL spectrum was measured in each example and comparative example. The measuring apparatus is a fluorescence measuring instrument manufactured by HITACHI, and the measurement condition is room temperature. The results are shown in Table 1. [3] indicates the difference in λmax between the EL spectrum and the PL spectrum.

  As a result, in Example 1 and Example 2, the difference between the EL spectrum and the PL spectrum was within 10 nm as compared with Comparative Example 1, and light emission derived from the phosphorescent metal coordination compound could be obtained. On the other hand, Comparative Example 1 has the following disadvantages as compared with Example 1 and Example 2. That is, the difference between the HOMO levels of FL03 and Re (dmphen) (btpz) is 0.5 eV or more, and the current efficiency is low because of the formation of an exciplex with the phosphorescent metal coordination compound in the light emitting layer. The half time is shortened.

(Example 3, Comparative Example 2)
An anode (ITO) having a film thickness of 100 nm was formed on the substrate so as to have an electrode area of 3 mm ² . The layers formed thereon and the film thicknesses are as follows for each example and comparative example. In forming each element, each layer was continuously formed by vacuum deposition by resistance heating in a vacuum chamber of 10 ⁻⁴ Pa until the formation of a cathode that was a counter electrode with respect to the anode. In each example, each layer is placed on the anode in the order listed.

Example 3
Exciplex suppression layer (25 nm): TFB4
Light emitting layer (40 nm): Re (dmphen) (btpz)
Electron transport layer (60 nm): Bphen
Metal electrode 1 (1 nm): KF
Metal electrode 2 (150 nm): Al

Comparative example 2
Exciplex suppression layer (25 nm): TFB4
Light emitting layer (40 nm): Re (dmphen) (pz)
Electron transport layer (60 nm): Bphen
Metal electrode 1 (1 nm): KF
Metal electrode 2 (150 nm): Al

  Table 2 shows the characteristic results of each light-emitting element.

[1], [2], cd / A [@ 600 cd / cm ² ], and [3] in Table 1 are the same as in Table 1.

  As a result, it can be seen that Example 3 is superior to Comparative Example 2 in terms of current efficiency and luminance half time.

  It can be seen from the description in [3] of Table 2 that the difference between the EL spectrum and the PL spectrum is within 10 nm only for the complex having a HOMO level of −5.5 eV or less with the phosphorescent metal coordination compound.

It is a cross-sectional schematic diagram of one Embodiment of the light emitting element of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Cathode 2 Light emitting layer 3 Hole transport layer 4 Anode 5 Substrate 6 Electron transport layer 8 Exciplex suppression layer

Claims (5)

In a light-emitting element having an anode and a cathode, and an organic light-emitting layer that is disposed between the anode and the cathode and is made only of a phosphorescent metal coordination compound,
Having an exciplex suppression layer disposed between the organic light emitting layer and the anode;
The phosphorescent metal coordination compound has a HOMO level of −5.5 eV or less, and the difference between the HOMO level of the exciplex suppression layer and the HOMO level of the phosphorescent metal coordination compound is 0. A light emitting element characterized by being within 5 eV. The light emitting device according to claim 1, wherein a central metal of the phosphorescent metal coordination compound is rhenium. An image display apparatus comprising the light emitting element according to claim 1 as a pixel of an image display unit. A light source comprising the light emitting device according to claim 1. A photoconductor exposure light source of an electrophotographic image forming apparatus having the light emitting device according to claim 1 as a light source.

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