JP2005294187A - Display element - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a display element, giving stable light emission over a long period of time by suppressing initial deterioration in the display element and speed of steady deterioration after that, without large changes in element structure from a conventional element structure and which does not depend on emitted color. <P>SOLUTION: In the display element 11 consisting of an organic layer 14, in which a positive hole transport layer and a light-emitting layer 14d are laminated in the order from a positive electrode 13 side, is sandwitched in between a negative electrode 15 and the positive electrode 13; the positive hole transport layer is constituted from a laminated structure of the positive hole transport layer 14c and a positive hole transport intermediate layer 14b located on the positive hole 13 side of 14c. This positive hole transport intermediate layer 14b is either constituted from material provided with arylamine frame, from material with high positive hole transport rate than the material constituting the positive hole transport layer 14c located on the light-emitting layer 14d side, or from a material in which the highest occupied molecular orbit (HOMO) is at an energy level lower than that of the host material of the light-emitting layer 14d. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a display element used for a color display and the like, and more particularly to a self-luminous display element provided with an organic layer.

  In recent years, the importance of human-machine interfaces has been increasing, including multimedia-oriented products. In order for humans to operate the machine more comfortably and efficiently, it is necessary to extract a sufficient amount of information from the machine to be operated without error, in a concise and instantaneous manner. Research has been conducted on display elements.

  In addition, with the miniaturization of machines, the demand for miniaturization and thinning of display elements is increasing day by day. For example, downsizing of laptop information processing devices that are integrated with display elements, such as notebook personal computers and notebook word processors, has made remarkable progress, and as a result, technologies related to liquid crystal displays that are display elements Innovation is also great. Liquid crystal displays are used as interfaces for various products, and are often used in products we use daily, such as laptop TVs, watches, and calculators, as well as laptop information processing equipment.

  However, since a liquid crystal display is not self-luminous, it requires a backlight, and this backlight drive requires more power than driving a liquid crystal. Further, since the viewing angle is narrow, it is not suitable for a large display element such as a large display. Furthermore, since the display method is based on the alignment state of the liquid crystal molecules, the contrast changes depending on the angle even within the viewing angle. In addition, since the liquid crystal displays using the change in the molecular conformation in the ground state, the dynamic range cannot be widened. This is one of the reasons why the liquid crystal display is not suitable for displaying moving images.

  On the other hand, plasma display elements, inorganic electroluminescent elements, organic electroluminescent elements and the like have been studied as self-luminous display elements.

  A plasma display element uses plasma light emission in a low-pressure gas for display and is suitable for an increase in size and capacity, but has problems in terms of thickness reduction and cost. In addition, a high voltage AC bias is required for driving, which is not suitable for portable devices.

  As for the inorganic electroluminescent element, a green light emitting display or the like has been commercialized. However, like the plasma display element, it is an AC bias drive and requires several hundred volts for the drive, and is not accepted by the user. However, due to technological development, the three primary colors of RGB required for color display display have been successfully emitted today, but no blue light-emitting material can emit light with high brightness and long life. Therefore, it is difficult to control the emission wavelength by molecular design.

  In 2000, a full-color display using inorganic electroluminescent elements was announced, but it uses a color conversion method, making it difficult to create a device with the ideal independent three primary color drive method.

  On the other hand, electroluminescence due to organic compounds has been studied for a long time since the discovery of light emission due to carrier injection into anthracene single crystals that generate strong fluorescence by Helfrich et al. In the early 1960s. Since it was monochromatic and single crystal, it was conducted as a basic study of carrier injection into organic materials.

  However, since Tang et al. Of Eastman Kodak in 1978 announced an organic electroluminescent device with a laminated structure having an amorphous light emitting layer that can drive at low voltage and emit high brightness, light emission and stability of RGB primary colors in various directions Research and development on brightness increase, laminated structure, manufacturing method, etc. are actively conducted. Hole transport as described in a research report published in Japanese Journal of Applied Physics Vol. 27, No. 2, pages L269-L271 (1988) by C. Adachi, S. Tokito, T. Tsutsui, S. Saito, etc. A three-layer structure (double heterostructure organic EL device) of materials, light-emitting materials, and electron transport materials has been developed. Furthermore, Journal of Applied Physics Vol. 65, No. 9, pages 3610-3616, such as CW Tang, SA Van Slyke, and CH Chen. As described in a research report published in (1989), an element structure in which a light emitting material is included in an electron transport material has been developed.

  In addition, various new materials have been invented by molecular design, which is a characteristic of organic materials, and application research for color displays of organic electroluminescent elements having excellent characteristics such as direct current low voltage driving, thinness, and self-luminous properties has also been actively conducted. Has begun to be done.

  FIG. 3 shows a structural example of such a display element (organic electroluminescent element). The display element 1 shown in this figure is provided on a transparent substrate 2 made of, for example, glass. The display element 1 includes an anode 3 made of ITO (Indium Tin Oxide: transparent electrode) provided on a substrate 2, an organic layer 4 provided on the anode 3, and a cathode 5 provided thereon. It is configured. The organic layer 4 has a configuration in which, for example, a hole injection layer 4a, a hole transport layer 4b, and an electron transporting light emitting layer 4c are sequentially stacked from the anode 3 side. In the display element 1 configured as described above, light generated when electrons injected from the cathode 5 and holes injected from the anode 3 are recombined in the light emitting layer 4c is extracted from the substrate 2 side.

  In addition to such a configuration, the cathode 5, the organic layer 4, and the anode 3 are sequentially stacked from the substrate 2 side, and the upper electrode (upper electrode) is made of a transparent material. There is also a so-called top emission type display element in which light is extracted from the side opposite to the substrate 2. In particular, in an active matrix type display device in which a thin film transistor (hereinafter referred to as TFT) is provided on a substrate, a so-called top surface in which a top emission type display element is provided on the substrate on which the TFT is formed. The light emitting element structure is advantageous in improving the aperture ratio of the light emitting part.

In the display device having such a top light emitting element structure, when the upper electrode is a cathode, the upper electrode is constituted by an injection electrode using a metal fluoride or oxide layer such as LiF, Li ₂ O, or CsO. Is done. In some cases, an MgAg layer is laminated on these injection electrodes.

  In addition, in the top light emitting element structure, light can be extracted from both sides by using a transparent electrode such as ITO as an anode, but generally an opaque electrode is used to form a cavity structure. The organic layer thickness of the cavity structure is defined by the emission wavelength and can be derived from multiple interference calculations. In the top surface light emitting element structure, it is possible to improve the light extraction efficiency to the outside and control the emission spectrum by actively using the cavity structure.

  In the display element (organic electroluminescent element) having the above-described configuration, the laminated structure of the organic layer 4 can improve luminance, improve luminous efficiency, and improve color purity of emitted light. For example, in a blue light emitting device, an aluminum complex is used for the electron transport layer, and an exciton generation promoting layer is provided in the laminated structure of the organic EL device to form an energy confinement structure of holes and electrons. It is disclosed that holes and electrons are efficiently combined with each other to obtain high luminance and blue light emission unique to the light emitting material (see Patent Documents 1-4 below).

JP-A-10-79297, JP-A-11-204258, JP-A-11-204264, JP-A-11-204259

In addition, even in the case of an element in which the energy is diffused from the light emitting layer by energy transfer and the efficiency is reduced even in light emission colors other than blue, a layer called a hole blocking layer is provided with high efficiency by providing it between the light emitting layer and the electron transport layer It is known that the following light emission can be obtained (see Patent Documents 5-6 below).

JP 2001-237079, JP 2001-237080 A

  By the way, when a display device is configured using a self-luminous display element as described above, in particular, a light-emitting element having an organic layer, it is one of the most important issues to increase the life of the display element and to ensure reliability. One.

  In general, the lifetime of a display element is determined by the initial deterioration accompanied by a decrease in luminance and the rate of steady deterioration thereafter. That is, in order to achieve a long lifetime of the display element, it is important to suppress the initial deterioration rate of the display element and the subsequent steady deterioration rate.

In order to improve the reliability of the light emitting device, it is preferable to widen the recombination region of holes and electrons and generate excitons in a wide region, but in an actual device, light is emitted from the hole transport layer and the light emitting layer interface. In most cases, the center is localized, which is considered to be one of the factors causing long-term deterioration. Therefore, it is considered effective to suppress long-term local deterioration of the luminescent material in order to suppress long-term deterioration. Thus, for example, in a green light emitting element, it has been reported that the reliability is greatly improved by doping a hole transport material into an electron transporting light emitting layer (see Non-Patent Documents 1 and 2 below).

Applied Physics Letters Vol.75, No.2, pp.172-174 (1999) Applied Physics Letters Vol. 80, No. 5, 725-727 (2002)

  However, when the same configuration as in Non-Patent Documents 1 and 2 described above is applied to a blue display element with a wide band gap, Alq3 or the like in which holes reach the electron transport layer and are stacked as the electron transport layer Emits green light and emits green light.

  Therefore, the present invention can suppress the initial deterioration of the display element and the subsequent steady deterioration without changing the large element structure from the conventional element structure and without depending on the light emission color. Accordingly, an object of the present invention is to provide a display element capable of providing stable light emission for a long time.

  In order to achieve such an object, the present invention provides a display element in which an organic layer in which a hole transport layer and a light emitting layer are stacked in this order from the anode side is sandwiched between a cathode and an anode. The hole transport layer has a laminated structure. Of the plurality of layers constituting such a hole transport layer, the layer disposed on the anode side is either (1) composed of a material having an arylamine skeleton, or (2) disposed on the light emitting layer side. It is made of a material having a higher hole mobility than the material constituting the layer, or (3) the highest occupied molecular orbital (HOMO) level is lower than the host material of the light emitting layer. It is made of a material that is In the configurations of (1) to (3) above, the layer disposed on the anode side constituting the hole transport layer can be exemplified by a layer made of an α-NPD derivative.

  In the display element having such a configuration, the hole transport layer between the light emitting layer and the anode is laminated so that the hole transport layer is positioned closer to the anode than the interface between the hole transport layer and the light emitting layer. An interface will be provided. Thereby, holes having a higher mobility than electrons are trapped at this interface, and the amount of holes reaching the interface between the hole transport layer and the light emitting layer is limited. Accordingly, deterioration of the light emitting layer due to accumulation of excess holes at the light emitting layer interface is prevented.

  At this time, the layer constituting the anode side of the hole transport layer is preferably made of the materials as described in (1), (2) and (3) above.

  As described above, according to the display element of the present invention, of the plurality of layers constituting the hole transport layer, the layer disposed on the anode side is composed of (1) a material having an arylamine skeleton. 2) It is made of a material having a higher hole mobility than the material constituting the layer arranged on the light emitting layer side, or (3) the highest occupied molecular orbital (HOMO) is more energetically than the host material of the light emitting layer. By using a material having a very low level, accumulation of excess holes at the light emitting layer interface is prevented, and deterioration of the light emitting layer can be prevented. Accordingly, it is possible to suppress the initial deterioration of the display element and the subsequent steady deterioration speed regardless of the light emission color, thereby improving the life characteristics of the display element.

  FIG. 1 is a cross-sectional view showing one structural example of the display element of the present invention. The display element 11 shown in this figure is a stack type display element 11 formed by laminating light emitting units, and includes an anode 13 provided on a substrate 12, an organic layer 14 provided on the anode 13 in an overlapping manner, A cathode 15 provided on the organic layer 14 is provided.

  In the following description, the emitted light generated when the holes injected from the anode 13 and the electrons generated in the charge generation layer 15 are combined in the light emitting layer 14 is extracted from the cathode 15 side opposite to the substrate 12. A structure of a top emission type display element will be described.

  First, the substrate 12 on which the display element 11 is provided is appropriately selected from a transparent substrate such as glass, a silicon substrate, and a film-like flexible substrate. Further, when the driving method of the display device configured using the display element 11 is an active matrix method, a TFT substrate in which a TFT is provided for each pixel is used as the substrate 12. In this case, the display device has a structure in which the top emission type display element 11 is driven using a TFT.

The anode 13 provided as the lower electrode on the substrate 12 has a high work function from the vacuum level of the electrode material in order to efficiently inject holes, for example, chromium (Cr), gold (Au), An alloy of tin oxide (SnO ₂ ) and antimony (Sb), an alloy of zinc oxide (ZnO) and aluminum (Al), and oxides of these metals and alloys are used alone or in a mixed state. be able to.

  When the display element 11 is a top emission type, it is possible to improve the light extraction efficiency to the outside by the interference effect and the high reflectance effect by configuring the anode 13 with a high reflectance material. As the electrode material, it is preferable to use an electrode mainly composed of Al, Ag, or the like. It is also possible to increase the charge injection efficiency by providing a transparent electrode material layer having a large work function such as ITO on these high reflectivity material layers.

  When the driving method of the display device configured using the display element 11 is an active matrix method, the anode 13 is patterned for each pixel provided with a TFT. An insulating film (not shown) is provided on the upper layer of the anode 13, and the surface of the anode 13 of each pixel is exposed from the opening of the insulating film.

  The organic layer 14 is formed by laminating a hole injection layer 14a, a hole transporting intermediate layer 14b, a hole transport layer 14c, a light emitting layer 14d, and an electron transport layer 14e in this order from the anode 13 side. In such an organic layer 14, the hole transporting intermediate layer 14b and the hole transporting layer 14c are stacked, so that the hole transporting layer is substantially configured to have a stacked structure.

  Each of these layers is composed of an organic layer formed by, for example, a vacuum deposition method or another method such as a spin coating method. In addition, it does not prevent that each of these layers 14a-14e has other requirements, for example, the light emitting layer 14d can be an electron transporting light emitting layer that also serves as the electron transporting layer 14e, and the light emitting layer 14d. May be the hole transporting light-emitting layer 14d, and each layer may have a laminated structure. For example, the light emitting layer 14d may be a white light emitting element formed of a blue light emitting part, a green light emitting part, and a red light emitting part.

  Of these layers, there are no limitations on the materials constituting the hole injection layer 14a, the hole transport layer 14c, the light emitting layer 14d, and the electron transport layer 14e except for the hole transporting intermediate layer 14b.

  For example, in the case of the hole transport layer 14c, a hole transport material such as a benzidine derivative, a styrylamine derivative, a triphenylmethane derivative, or a hydrazone derivative can be used. There was a case of a triphenylamine derivative, and the effect was remarkable.

  The light emitting layer 14d may be an organic thin film containing a trace amount of an organic substance such as a berylene derivative, a coumarin derivative, a pyran-based dye, or a triphenylamine derivative. It is formed by co-deposition of trace molecules. In particular, a luminescent center having a tertiary amine having a feature of hole transport in the molecular structure can be doped at a high concentration if the intermolecular interaction is small and concentration quenching is difficult. It functions as one of the optimal dopants.

  Next, the hole transporting intermediate layer 14b, which is a feature of the present invention, will be described.

  The hole transporting intermediate layer 14b, that is, the layer disposed on the anode 13 side in the laminated structure constituting the hole transport layer is either (1) composed of a material having an arylamine skeleton, or (2) It is made of a material having a higher hole mobility (μh) than the material constituting the hole transport layer 14c arranged on the light emitting layer 14d side, or (3) than the host material constituting the light emitting layer 14d. Are made of a material having the highest occupied molecular orbital (HOMO) at a low energy level.

  Here, the purpose of the hole transporting intermediate layer 14b is to positively inject holes that are injected from the anode 13 and accumulated locally at the hole transport layer 14c side of the interface between the hole transport layer 14c and the light emitting layer 14d. By accumulating at the interface between the hole transporting intermediate layer 14b and the hole transporting intermediate layer 14b at the interface between the hole transporting intermediate layer 14b and the hole transporting layer 14c, necessary and sufficient holes are efficiently injected into the light emitting layer 14c. By extending the injection balance of electrons and holes into the layer 14d) γ close to 1, a longer life is achieved.

  Accordingly, in terms of energy level, the highest occupied molecular orbital (HOMO) of the hole transporting intermediate layer 14b is preferably lower in energy than the HOMO of the host material (main material) of the light emitting layer 14d. Further, it is more preferable that the HOMO of the hole transport layer 14c is shallower than the hole transport intermediate layer 14b (small in absolute value: that is, high in energy level). However, even if the hole transporting layer 14c and the hole transporting intermediate layer 14b are materials having the same level of HOMO, the difference in properties of the material due to hole mobility (μh) and substituents, Since there is a barrier against injection due to a difference or the like, it is not essential that the HOMO level of the hole transport layer 14c is shallower (smaller in absolute value) than the hole transport intermediate layer 14b.

  The material of the hole transporting intermediate layer 14b is considered to be effective as long as it is a material that transports holes, and includes materials used for the hole injection layer 14a and the hole transport layer 14c. In particular, it is preferable to use a material having an arylamine skeleton generally having a hole transporting property.

  In addition, as a material constituting the hole transporting intermediate layer 14b, a material having a high hole mobility (μh) is preferably used, and examples of such a material include an arylamine skeleton. it can. Especially for dimer materials, there are many materials with high hole mobility, and α-NPD (α-naphtyl phenil diamine) has hole mobility of the order of 10E-3 [cm2 / V · S]. It works well as the transportable intermediate layer 14b.

  And as a constituent material of the positive hole transport intermediate | middle layer 14b provided with the above properties, what consists of an alpha-NPD derivative can be illustrated.

  The organic layer 14 is not limited to such a layer structure. If the hole transport layer has a layered structure including the above-described hole transporting intermediate layer 14b, the layered structure according to need may be used. You can choose. For example, the light emitting layer 14d may be a hole transporting light emitting layer 14d, or may have a configuration in which an electron transporting layer is further provided on the light emitting layer 14d. In addition, each of the above organic layers, for example, the hole injection layer 14a and the hole transport layer 14c, may have a laminated structure including a plurality of layers.

  Next, the cathode 15 has a three-layer structure in which a first layer 15a, a second layer 15b, and in some cases a third layer 15c are stacked in this order from the anode 13 side.

The first layer 15a is made of a material having a small work function and good light transmittance. As such a material, for example, Li ₂ O which is an oxide of lithium (Li), Cs ₂ O which is an oxide of cesium (Cs), and a mixture of these oxides can be used. Further, the first layer 15a is not limited to such a material. For example, alkaline earth metals such as calcium (Ca) and barium (Ba), and alkali metals such as lithium (Li) and cesium (Cs). Further, metals having a small work function such as indium (In), magnesium (Mg), silver (Ag), and further fluorides and oxides of these metals alone or these metals and fluorides, oxidation You may use it, improving stability as a mixture or alloy of a thing.

  The second layer 15b is composed of an electrode composed of an alkaline earth metal such as MgAg or an electrode composed of Al. When the cathode 15 is formed of a semi-transmissive electrode like a top light emitting device, light can be extracted by using a thin MgAg electrode or Ca electrode. By configuring the light-transmitting material with good conductivity, the display element 11 emits light from the top surface, which has a cavity structure that resonates and extracts emitted light between the anode 13 and the cathode 15 in particular. In the case of an element, the second layer 15b is formed using a semi-transmissive reflective material such as Mg-Ag. As a result, light emission is reflected at the interface of the second layer 15b and the interface of the anode 13 having light reflectivity to obtain a cavity effect.

  Furthermore, the third layer 15c can also be formed as a sealed electrode that can extract light emission by providing a transparent lanthanoid-based oxide for suppressing deterioration of the electrode.

  The first layer 15a, the second layer 15b, and the third layer 15c described above are formed by a technique such as a vacuum deposition method, a sputtering method, or a plasma CVD method. Further, when the driving method of the display device configured using this display element is an active matrix method, the cathode 15 is formed by an insulating film and an organic layer 14 covering the periphery of the anode 13 (not shown here). 13 is formed in a solid film shape on the substrate 12 in a state of being insulated from the substrate 13, and may be used as a common electrode for each pixel.

  The electrode structure of the cathode 15 shown here is a three-layer structure. However, the cathode 15 may be formed of only the second layer 15b or between the first layer 15a and the second layer 15b as long as it is a laminated structure necessary when the functions of the layers constituting the cathode 15 are separated. In addition, it is possible to form a transparent electrode such as ITO, and it is needless to say that an optimal combination and laminated structure may be taken for the structure of the device to be produced.

  As described above, in the display element 11 of the present embodiment, the hole transport layer disposed between the light emitting layer 14d and the anode 14a is the hole transport layer 14c and the hole transport intermediate layer on the anode 14a side. 14b. Thus, the interface between the hole transport layer 14c and the hole transporting intermediate layer 14b is added to the anode 13 side from the interface between the hole transport layer 14c and the light emitting layer 14d. For this reason, holes having higher mobility than electrons are usually trapped at the added interface, and the amount of holes reaching the interface between the hole transport layer 14c and the light emitting layer 14d is limited. Therefore, deterioration of the light emitting layer 14d due to accumulation of excess holes at the interface of the light emitting layer 14d is prevented.

  As a result, regardless of the emission color of the display element 11, it is possible to suppress the initial deterioration rate and subsequent steady deterioration rate of the display element 11 to be small, thereby improving the life characteristics of the display element 11. It becomes possible.

  The display element of the present invention is not limited to a display element used for an active matrix type display device using a TFT substrate, and can also be applied as a display element used for a passive type display device. An effect (improvement of long-term reliability) can be obtained.

  In the above embodiment, the case of the “top emission type” in which light emission is extracted from the side of the cathode 15 provided on the side opposite to the substrate 12 has been described. However, the present invention is also applied to a “transmission type” display element in which the substrate 12 is made of a transparent material to extract emitted light from the substrate 12 side. In this case, in the laminated structure described with reference to FIG. 1, the anode 13 on the substrate 12 made of a transparent material is configured using a transparent electrode material having a high work function such as ITO. Thereby, emitted light is extracted from both the substrate 12 side and the opposite side of the substrate 12. In such a configuration, by forming the cathode 15 from a reflective material, emitted light is extracted only from the substrate 12 side. In this case, a sealing electrode such as AuGe, Au, or Pt may be attached to the uppermost layer of the cathode 15.

  Furthermore, the “transmission type” in which emitted light is extracted from the substrate 12 side even when the laminated structure described with reference to FIG. 1 is stacked up from the substrate 12 side made of a transparent material and the anode 13 is the upper electrode. The display element can be configured. Also in this case, by changing the anode 13 serving as the upper electrode to a transparent electrode, emitted light is extracted from both the substrate 12 side and the opposite side of the substrate 12.

  Further, the display element of the present invention described in the above embodiment can be applied to a stack type display element in which units of an organic layer having a light emitting layer are stacked. Here, the stack type is a multi-photon emission element (MPE element). For example, in Japanese Patent Application Laid-Open No. 11-329748, a plurality of organic light emitting elements are electrically connected in series via an intermediate conductive layer. The device is characterized by being described.

  Japanese Unexamined Patent Application Publication No. 2003-45676 and Japanese Unexamined Patent Application Publication No. 2003-272860 disclose disclosure of element configurations and detailed examples for realizing a multiphoton emission element (MPE element). According to these, when two units of organic layers are stacked, ideally, when cd / A is doubled and three layers are stacked without changing lm / W, ideally, It is stated that lm / W can triple cd / A without changing.

  Therefore, when the present invention is used in a stack type, a long life due to efficiency improvement by the stack type and a long life effect in the present invention have a synergistic effect, and an extremely long element is obtained. It becomes possible.

  Next, specific examples 1 to 3 of the present invention, and manufacturing procedures of display elements of comparative examples 1 to 3 for these examples, and evaluation results thereof will be described.

<Examples 1-3>
In each of Examples 1 to 3, each display element 11 in which the hole transporting intermediate layer 14b is configured using each material in the configuration of the display element 11 described in the above-described embodiment with reference to FIG. 1 is manufactured. did. Below, the manufacturing procedure of the display element 11 of Examples 1-3 is demonstrated first.

An Ag alloy (film thickness of about 100 nm) was formed as the anode 13 on the substrate 12 made of a glass plate of 30 mm × 30 mm, and a light emitting region other than 2 mm × 2 mm was masked with an insulating film (not shown) by SiO ₂ vapor deposition. A cell for an organic electroluminescence device was produced.

Next, a hole injection layer 14a, a hole transporting intermediate layer 14b, and a hole transport layer 14c were sequentially formed in a stacked structure in which the materials shown in Table 1 below were formed in respective film thicknesses.

  That is, as the hole injection layer 14a, HI-406 (made by Idemitsu Kosan Co., Ltd .: trade name) was formed to a thickness of 10 nm (deposition rate: 0.2 to 0.4 nm / sec) by a vacuum evaporation method. HI-406 is a hole injecting material.

Next, as the hole transporting intermediate layer 14b, α-NPD represented by the following formula (1) in Example 1, β-NPD represented by the following formula (2) in Example 2, and the following formula ( Each TPD shown in 3) was formed with a film thickness of 10 nm (deposition rate: 0.1 nm / sec) by vacuum evaporation.

Finally, HT-320 (Idemitsu Kosan Co., Ltd .: trade name) was formed to a thickness of 10 nm (deposition rate: 0.2 to 0.4 nm / sec) as the hole transport layer 14c. HT-320 is a hole injecting material.

Further, as the light emitting layer 14d, ADN shown in the following formula (4) is used as a host, BD-052x (Idemitsu Kosan Co., Ltd .: trade name) is used as a dopant, and the dopant concentration is 5% in film thickness ratio. These materials were formed into a total film thickness of 32 nm by a vacuum deposition method.

Finally, as the electron transport layer 14e, Alq3 (8-hydroxy quinorine alminum) represented by the following formula (5) was deposited by vacuum deposition to a thickness of 18 nm.

  After forming the organic layer 14 from the hole injection layer 14a to the electron transport layer 14e as described above, LiF is deposited as a first layer 15a of the cathode 15 by about 0.3 nm (deposition rate ~ .0. Then, MgAg was formed as a second layer 15b with a thickness of 10 nm by a vacuum evaporation method, and a cathode 15 having a two-layer structure was provided.

<Comparative Examples 1-3>
In the structure of the display element described with reference to FIG. 1, a display element having a structure in which the hole transport layer 14c is directly provided on the hole injection layer 14a without forming the hole transport intermediate layer 14b was manufactured. That is, a display element having a hole transport layer having a single layer structure was produced. The materials and film thicknesses of the hole injection layer 14a and the hole transport layer 14c in Comparative Examples 1 to 3 are as shown in Table 1 above. In addition, the manufacturing procedure is a procedure in which only the step of forming the hole transport layer 14c directly on the hole injection layer 14a in the manufacturing procedure of the above-described embodiment is changed. In addition, the hole transport layer 14c in the comparative example 3 made (alpha) -NPD and HT-320 into the ratio of 1: 1 by the film thickness ratio.

<Evaluation results>
FIG. 2 is a graph showing the change over time of the relative luminance in the display elements manufactured in Example 1 and Comparative Examples 1 to 3, and the relative luminance is shown with the initial luminance in each display element being 1. The driving condition was a constant current drive of 90 mA / cm ² and a duty of 50. In addition, Example 2 and 3 are the same results as Example 1, and in FIG. 2, the result of Example 1 is shown on behalf of Examples 1-3.

  From this result, the display element of Example 1 in which the hole transporting intermediate layer 14b is provided between the hole injection layer 14a and the hole transporting layer 14c and the hole transporting layer is laminated has the hole injecting layer. 14a and the hole transport layer 14c are not provided with the hole transport intermediate layer 14b (the hole transport layer has a single layer structure). The rate of reduction to 0.5 (ie half-life) is extended, and it is effective to improve the long-term reliability of the display element by providing the hole transporting intermediate layer 14b with the hole transporting layer as a laminated structure. Was confirmed.

  Further, Comparative Example 3 in which the mixed layer of the constituent material of the hole transporting intermediate layer 14b and the constituent material of the hole transport layer 14c is a hole transport layer 14c is a single layer of α-NPD or HT-320. The half-life was further deteriorated as compared with Comparative Examples 1 and 2 constituting the hole transport layer 14c. From this result, it was confirmed that the hole transport layer is not a mixed layer of each material but a laminated structure of each material.

It is sectional drawing which shows one structural example of the display element of this invention. It is a graph which shows the time-dependent change of the relative luminance of the display element in Example 1 and Comparative Examples 1-3. It is sectional drawing which shows the structure of the conventional display element.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 11 ... Display element, 13 ... Anode, 14 ... Light emitting layer, 14a ... Hole injection layer, 14b ... Hole transportable intermediate layer, 14c ... Hole transport layer, 14d ... Light emitting layer, 15 ... Cathode

Claims (6)

In a display element formed by sandwiching an organic layer in which a hole transport layer and a light emitting layer are laminated in order from the anode side between the cathode and the anode,
The hole transport layer has a laminated structure, and a layer disposed on the anode side among a plurality of layers constituting the hole transport layer is composed of a material having an arylamine skeleton. Display element to be used. The display element according to claim 1,
The display element, wherein a layer arranged on the anode side among a plurality of layers constituting the hole transport layer is made of an α-NPD derivative. In a display element formed by sandwiching an organic layer in which a hole transport layer and a light emitting layer are laminated in order from the anode side between the cathode and the anode,
The hole transport layer has a laminated structure, and a layer disposed on the anode side of a plurality of layers constituting the hole transport layer is more than a material constituting a layer disposed on the light emitting layer side. A display element comprising a material having high hole mobility. The display element according to claim 3, wherein
The display element, wherein a layer arranged on the anode side among a plurality of layers constituting the hole transport layer is made of an α-NPD derivative. In a display element formed by sandwiching an organic layer in which a hole transport layer and a light emitting layer are laminated in order from the anode side between the cathode and the anode,
The hole transport layer has a laminated structure, and the layer disposed on the anode side among the plurality of layers constituting the hole transport layer is the highest occupied molecular orbital (HOMO) than the host material of the light emitting layer. A display element characterized by being made of a material having a low energy level. The display element according to claim 5, wherein
The display element, wherein a layer arranged on the anode side among a plurality of layers constituting the hole transport layer is made of an α-NPD derivative.

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