JP5306038B2 - Organic electroluminescence device - Google Patents

Description

  The present invention relates to an electroluminescence element (electroluminescence element) that emits light by application of an electric field, and particularly relates to an organic electroluminescence element including a light emitting layer that emits phosphorescence.

  An organic electroluminescence (EL) element is formed by laminating a plurality of layers such as an electron transport layer, a light emitting layer, and a hole transport layer between a cathode and an anode, and by applying a voltage between the electrodes, electrons from each electrode, Holes are injected and they combine in the light emitting layer to emit light.

  In the organic EL element, the light emitting material of the light emitting layer is excited by recombination of holes and electrons, and excitons are generated. Theoretically, the exciton generation efficiency of the first excited singlet energy level (hereinafter referred to as “S1 level”) is 25%, and the first excited triplet energy level (hereinafter referred to as “T1 level”) is excited. The child generation efficiency is 75%. The light emitted when returning from the S1 level to the ground state is fluorescence, and the light emission when returning from the T1 level to the ground state is phosphorescence. Since the T1 level is lower than the S1 level, in principle, the generation efficiency of T1 can be increased from 75% of the theoretical limit to 100% by promoting inter-term crossing.

  That is, theoretically, it is possible to dramatically increase the luminous efficiency by using phosphorescence rather than fluorescence. However, at present, fluorescent EL elements have been put into practical use, but phosphorescent EL elements have not been put into practical use.

  In the organic EL element, the external quantum yield is represented by the product of the PL quantum yield, the hole electron balance factor, and the extraction efficiency. Therefore, in order to increase the external quantum yield, it is required to design the element configuration so as to increase the PL quantum yield, the hole electron balance factor, and the extraction efficiency.

  An organic EL element has a configuration in which an electrode layer, a light emitting layer, and the like are laminated on a substrate, and generally, light emitted from the light emitting layer is extracted through a transparent electrode. At that time, due to the influence of the refractive index of each layer, light incident at a critical angle or more at the layer interface on the light extraction side is totally reflected and confined in the element and cannot be extracted outside. For this reason, it is difficult to extract emitted light with high efficiency, and it is said that the upper limit of the extraction efficiency is about 20% in the case of the refractive index of a transparent electrode that is often used at present such as ITO.

  On the other hand, regardless of fluorescent EL or phosphorescent EL, as a practical EL element, a material having a PL quantum yield close to 1 (PL emission efficiency 100%) is generally used as a light emitting material.

  On the other hand, in order to make EL emission highly efficient, it is required that the excitation energy in the light emitting layer is not dissipated to the layer adjacent to the light emitting layer (energy does not move). This is because the light emission efficiency decreases due to the dissipation of energy from the light emitting layer to the adjacent layer.

  Therefore, in order to obtain high-efficiency light emission in the fluorescent EL element, the materials constituting the light emitting layer and the adjacent layer can be selected so that the S1 level of the light emitting material is smaller than the lowest S1 level of the adjacent layer. It becomes a condition.

  Similarly, in order to obtain high-efficiency light emission in the phosphorescent EL element, the materials constituting the light emitting layer and the adjacent layer are selected so that the T1 level of the light emitting material is smaller than the lowest T1 level of the adjacent layer. It is a condition to do.

  In fluorescent EL devices that are currently in practical use, the condition that the excitation level of the light emitting material is lower than the excitation level of the adjacent layer can be satisfied relatively easily, and the charge transport that constitutes the adjacent layer of the light emitting layer can be achieved. As a material suitable for the above, a material having an S1 level higher than that of the fluorescent material is used.

  In the fluorescent EL element, in order to increase the light emission efficiency, a plasmon enhancement method for enhancing light emission by arranging a metal in the vicinity of the fluorescent light emitting layer (for example, several tens of nm) is disclosed in Non-Patent Documents 1 and 2, etc. Proposed. This enhancement of light emission is accompanied by the addition of new light that is re-emitted after dipole radiation from the light-emitting element induces plasmons (or localized plasmons) on the metal surface and absorbs energy. However, as described above, in the current practical fluorescent EL element, it is common to use a fluorescent light-emitting material having a PL quantum yield close to 1, which satisfies the conditions for increasing the hole-electron balance factor. Therefore, even if the plasmon enhancement method is applied, the effect of increasing the luminous efficiency is hardly obtained.

  On the other hand, in phosphorescent EL elements, particularly blue and green phosphorescent EL elements, the T1 level of the material of the hole transport layer and the electron transport layer, which are highly practical from the viewpoints of driving voltage and durability, is low (light wavelength). Therefore, it is difficult to satisfy the condition that the T1 level of the phosphorescent material is lower than the T1 level of the adjacent layer.

Journal of Modern Optics (USA) vol. 45, pp.661-699, 1998 Proc. SPIE (USA) Vol.7032, 703224 (2008)

  As described above, in the phosphorescent EL element, it is difficult to satisfy the condition that the T1 level of the phosphorescent light emitting material is lower than the T1 level of the adjacent layer. Therefore, high luminous efficiency can be obtained. This is a barrier to the practical application of phosphorescent EL elements.

  This invention is made | formed in view of the said situation, Comprising: It aims at providing the organic EL element which produces phosphorescence which can implement | achieve high luminous efficiency.

The organic electroluminescence device of the present invention includes a light emitting layer containing a phosphorescent material that emits phosphorescence, and a plurality of first and second adjacent layers disposed adjacent to the light emitting layer with the light emitting layer interposed therebetween. Is an organic electroluminescence element formed by laminating between the electrodes,
A first excited triplet energy level of the light emitting material is higher than a lowest energy level of first excited triplet energy levels of a plurality of materials constituting the first and second adjacent layers;
In the vicinity of the light emitting layer, a metal member that causes plasmon resonance by light emitted from the light emitting layer to be generated on the surface is provided.

  The distance between the metal member and the light emitting region is preferably 30 nm or less.

  The metal member is preferably a metal film disposed between the plurality of layers. The metal film may be a solid film or a granular film (a film having a concavo-convex structure smaller than the wavelength of emitted light), but in particular, metal fine particles having a particle diameter of 5 nm or more are randomly selected. Alternatively, an island structure film that is dispersed in the form of a film in a periodic array pattern is particularly desirable. Here, the particle diameter refers to the maximum length of the fine particles. That is, when the fine particles are spherical, the diameter is referred to, and when the fine particles have an elongated shape (rod shape) in which the aspect ratio between the long diameter and the short diameter perpendicular to the fine particles is greater than 1, the long diameter is referred to.

The material of the metal film may be any material that causes plasmon resonance by the emitted light, such as Ag (silver), Au (gold), Cu (copper), Al (aluminum), Pt (platinum), An alloy containing these metals as a main component can be used. Here, the “main component” is defined as a component having a content of 80% by mass or more.
Of these materials, Ag or Au is particularly desirable.

  Further, it is desirable that at least one surface of the metal film is surface-modified with a terminal group having a polarity that brings the work function of the metal film close to the work function of a layer adjacent to the metal film. . When the work function of the metal film is smaller than the work function of the adjacent layers on both sides of the metal film (cathode side), the end group is an electron donating group, and the work functions of the adjacent layers on both sides of the metal film are When the work function is larger than the work function (on the anode side), the terminal group becomes an electron-withdrawing group.

  The polar end group means an electron donating group having an electron donating property or an electron donating group having an electron withdrawing property. Examples of the electron donating group include an alkyl group such as a methyl group, an amino group, and a hydroxyl group. Examples of the electron withdrawing group include a nitro group, a carboxyl group, and a sulfo group.

  The metal member may be a core-shell type fine particle including at least one metal fine particle core and an insulator shell covering the metal fine particle core, and a large number of core-shell type fine particles are dispersed in a layer near the light emitting region. It is preferable that Here, the core-shell type fine particles may be present in the light emitting region. The particle diameter of the metal fine particle core of the core-shell type fine particle is preferably 10 nm or more and 1 μm or less, and the thickness of the insulator shell is preferably about 30 nm or less. Here, the “particle diameter” is the maximum diameter of the fine particles.

In addition, when the core-shell type fine particles or the metal fine particles are elongated (rod-shaped) particles whose aspect ratio between the major axis of the fine particles and the minor axis perpendicular to the fine particles is larger than 1, a large number of the elongated particles are formed. In addition, it is preferable that the minor axis of the fine particles is arranged with orientation in a direction substantially perpendicular to the electrode surface.
A plurality of fine metal particle cores may be provided in the insulator shell.

The metal fine particle core is preferably made of any one of Au, Ag, Al, Cu, and Pt, or an alloy containing these as a main component.
As a material for the insulator shell, an insulator such as SiO ₂ , Al ₂ O ₃ , MgO, ZrO ₂ , PbO, B ₂ O ₃ , CaO, or BaO can be preferably used.

  Further, in the organic EL device of the present invention, in the plurality of layers, a region where the electric field intensity of the standing wave due to the light emitted from the light emitting layer is maximum coincides with the light emitting layer. It is desirable to have a layer thickness and a refractive index that satisfy the resonance condition.

  In the organic EL device that emits phosphorescence according to the present invention, the first excitation triplet energy level of the light emitting material is the first excitation 3 of the plurality of materials constituting the first and second adjacent layers adjacent to the light emitting layer. Although it is configured higher than the lowest energy level among the multiplet energy levels, a metal member that causes plasmon resonance due to light emitted from the light emitting layer on the surface is provided in the vicinity of the light emitting layer. This dipole radiation induces plasmons (or localized plasmons) on the metal surface and absorbs energy, and then the emission efficiency can be improved with the addition of new light emission. In other words, the light emission process of the light emitting element is added with a light emission transition due to a new plasmon, and the effect of shortening the exciton lifetime can be expressed, before the excitation energy in the light emitting layer is dissipated to the adjacent layer. Therefore, it can be extracted as phosphorescent light emission, and the luminous efficiency can be improved.

  Therefore, the first excited triplet energy level of the luminescent material such as green or blue phosphorescence is the first excited triplet energy level of the plurality of materials constituting the first and second adjacent layers adjacent to the light emitting layer. Among them, in an element that must be higher than the lowest energy level, it is possible to emit light with higher efficiency than before, and green and blue phosphorescence can be put to practical use.

  In addition, by suppressing the dissipation of the excitation energy to the adjacent layer, heat generation can be suppressed at the same time and the durability of the element can be improved.

The schematic diagram which shows the structure of the organic EL element concerning 1st Embodiment of this invention. The schematic diagram which shows the structure of the organic EL element concerning 2nd Embodiment of this invention. The figure for demonstrating the work function adjustment film | membrane of the organic EL element of FIG. The schematic diagram which shows the structure of the organic EL element concerning 3rd Embodiment of this invention. The schematic diagram which shows the structure of the organic EL element concerning 4th Embodiment of this invention.

  Embodiments of the present invention will be described with reference to the drawings.

<First Embodiment>
FIG. 1 is a diagram schematically showing the structure of an organic electroluminescence element (EL element) 1 according to the first embodiment of the present invention.
The organic EL element 1 of this embodiment is basically a standard EL element including an anode 11, a hole injection layer 12, a hole transport layer 13, a light emitting layer 14, an electron transport layer 15, an electron injection layer 16, and a cathode 17. It has a structure. The light emitting layer 14 includes a light emitting material that emits phosphorescence, and emits phosphorescence when holes and electrons injected from the anode 11 and the cathode 17 recombine with the light emitting layer 14.

The light emitting material contained in the light emitting layer 14 may be any material that emits phosphorescence, but it is particularly preferable to use a material having a PL quantum yield close to 1 at room temperature. Here, the PL quantum yield of 1 to 0.8 is assumed to be close to 1. Specifically, a metal complex having Ir or Pt can be suitably used, and examples thereof include the following compounds.

  In the element 1, the first excited triplet energy level (T 1 level) of the light emitting material of the light emitting layer 14 is the material constituting the hole transport layer 13 and the electron transport layer 15 adjacent to the light emitting layer 14. It is assumed that the T1 level is higher than the lowest T1 level. In general, the hole transport layer and the electron transport layer are each composed of one or more materials, and each material has a unique T1 level. If there is at least one material having a T1 level lower than the T1 level of the light emitting material in the adjacent layer, it can be said that the excitation energy of the light emitting layer is easily dissipated to the adjacent layer. The T1 level of the light emitting layer may be larger than the T1 level of all the materials constituting the adjacent layers 13 and 15.

NPD, 2TNATA, or the like can be used as a practical material for the hole transport layer 13 that satisfies the above conditions, and BAlq, Alq3, or the like can be used as a material for the practical electron transport layer 15 ( (See chemical formulas in Examples below).
The “practical material” as used herein refers to a material that is not significantly inferior in all aspects of driving voltage, luminous efficiency, and driving durability of the element when applied to an EL element. This is a material that has been found in the accumulated knowledge of organic EL research and development.

  Further, in the element 1, a metal film 20 as a metal member that causes plasmon resonance by emitted light is disposed in the vicinity of the light emitting region (light emitting layer 14). In order for the metal film 20 not to be a reflective material, the thickness is preferably thin. Further, if the metal film 20 is in contact with the light emitting layer 14 or close to the light emitting layer 14 at a distance d of less than 5 nm, charge transfer directly occurs from the light emitting layer 14 and light emission is attenuated. It is desirable that the distance is 5 nm or more. On the other hand, if the distance from the light emitting layer 14 is too far, plasmon resonance due to the emitted light does not occur and the light emission enhancing effect cannot be obtained. Therefore, the distance d to the light emitting layer 14 is preferably 30 nm or less.

  The metal film 20 may be a flat film, but a film having a concavo-convex structure smaller than the wavelength of emitted light, that is, a granular film having a granular surface, or metal fine particles having a particle diameter of 5 nm or more are randomly formed in a film shape. Alternatively, an island (island-like) structure film in which voids exist between fine particles, which are dispersed in a periodic arrangement pattern, is preferable. When the metal film is a flat film, surface plasmon is induced on the surface of the metal film by the emitted light, but recombination to the radiation mode is difficult to occur, and as a non-radiation process, it eventually disappears as heat. The ratio is large. On the other hand, when the metal film is an island structure film, the surface plasmon induced on the film surface by the emitted light is coupled again to the radiation mode, and the efficiency of emitting the radiation light is high.

  The material of the metal film is not particularly limited as long as plasmon resonance is generated by the emitted light, and Ag (silver), Au (gold), Cu (copper), Al (aluminum), and one of these metals as a main component. An alloy of 80% or more is applicable. In particular, silver is desirable if the emitted light is a visible wavelength. This is because silver can cause surface plasmon resonance in the visible region from the plasma frequency. If the emitted light has a wavelength other than the visible range, for example, infrared, gold is desirable.

  Note that the above-described EL element is configured, for example, so that it is sequentially stacked on the substrate from the cathode side, and light is extracted from the anode side. About each layer other than a metal film, it can form by the lamination | stacking method currently used in manufacture of the conventional organic EL element. The metal film can be formed using, for example, a sputtering method, a vacuum deposition method, or the like.

  In the element 1, the T1 level of the light emitting material of the light emitting layer 14 is larger than T1 of the material having the lowest T1 level among the materials constituting the hole transport layer 13 and the electron transport layer 15 adjacent to the light emitting layer 14. Therefore, the layer structure in which the excitation energy in the light-emitting layer 14 is easily dissipated from the T1 level of the light-emitting material to the adjacent layer (the hole transport layer 13 and / or the electron transport layer 15) containing a T1 level material lower than the T1 level. It is. However, since the element 1 includes a metal film that generates plasmon resonance, the lifetime of excitons can be shortened and light emission as phosphorescence can be promoted, and excitation energy can be prevented from being dissipated to an adjacent layer. be able to. That is, by providing the metal film 20, it is possible to suppress the dissipation of excitation energy and improve the light emission efficiency. At the same time, it can be expected to improve durability by shortening the excitation life.

<Second Embodiment>
The configuration of the organic EL element 2 of the second embodiment is schematically shown in FIG. FIG. 2 also shows the potential energy of each layer. In the following embodiment, the same reference numerals are assigned to the same layers as those of the organic EL element 1 of the first embodiment, and detailed description thereof is omitted.

  The organic EL element 2 of the present embodiment is different from the organic EL element 1 of the first embodiment in that the electron injection layer 16 is not provided and the work function adjusting layer 25 is provided on one side of the metal film 20. . The work function adjusting layer 25 is a surface modification layer having a polar end group that brings the work function of the metal film 20 close to the work function of a layer adjacent to the metal film 20 (here, the electron transport layer 15).

  In FIG. 2, black circles (●) indicate electrons e and white circles (◯) indicate holes h. As shown in FIG. 2, each layer is generally arranged so that the work function continuously changes from the anode 11 side and the cathode 17 side toward the light emitting layer 14, but is inserted into the electron transport layer 15. The metal film 20 has a work function larger than that of the electron transport layer 15 (low potential energy), and becomes an electron trap when an electric field is applied, so that the electron flow is inhibited and recombination in the light emitting layer 14 cannot be performed. , There is a risk that light emission does not occur well.

Here, the work function adjusting layer 25 has a function of suppressing the metal film 20 from trapping electrons. The work function adjustment layer 25 reduces the effective work function of the metal film 20 (increases the potential energy), that is, in FIG. 2, the original energy level E ₀ of the metal film 20 is effectively reduced to the energy level E. _The electron e is moved to the light emitting layer without being trapped by the metal film 20.

  FIG. 3 shows an example of the work function adjustment layer 25. Here, the metal film 20 is made of Au. As shown in FIG. 3, the work function adjusting layer 25 is a SAM film (self-assembled monomolecular film) formed by Au reaction with a thiol or disulfide having a terminal group having polarity and bonding to the Au film surface. It is. FIG. 3 shows an example of a SAM film of benzenethiol (thiophenol) having a methyl group at the para position of the thiol group.

  An alkyl group such as a methyl group is an electron donating group, and when it has such a terminal group, the potential energy of Au can be increased by the electron donating property, and the work function can be reduced. Examples of the electron donating group include an alkyl group such as a methyl group, an amino group, and a hydroxyl group.

  The work function adjusting layer 25 can be formed by a general SAM manufacturing method after the Au film is formed. In particular, a liquid phase method such as a coating method, a vapor deposition method, or a sputtering method can be used. The work function adjusting layer may be provided not only on one side of the metal film 20 but also on both sides.

  Here, an example in which the metal film 20 is provided in the electron transport layer 15 is described, but the metal film 20 may be provided in the positive hole transport layer 13. In this case, since the work function of the metal film 20 is smaller than the work function of the hole transport layer 13 (potential energy is high), the work for reducing the potential energy so as to approach the work function of the hole transport layer 13. The function adjusting layer may be provided on at least one side of the metal film. In this case, the work function adjusting layer has an electron withdrawing group instead of the electron donating group shown in FIG. 3 as its end group, so that the effective potential energy can be lowered, and the work of the metal film 20 can be reduced. The function can be approximated to that of the hole transport layer 13. Examples of the electron withdrawing group include a nitro group, a carboxyl group, and a sulfo group.

  As described above, since the work function adjusting layer (polar molecular film) 25 for adjusting the work function of the metal film is provided, it is possible to suppress the adverse effect of the metal film on the movement of charges when an electric field is applied. Therefore, it is possible to more effectively realize improvement in phosphorescence emission efficiency and improvement in element durability due to plasmon enhancement.

In organic LEDs, in order to adjust the work function between the metal electrode and the organic polymer that forms the Schottky barrier, modifying the metal surface with SAM having an electron-donating group is “Tuning the Work Function of Gold with Self-Assembled Monolayers Derived from X- [C ₆ H ₄ -C≡C-] nC ₆ H ₄ -SH (n = 0,1,2; X = H, F, CH ₃ , CF ₃ , and OCH ₃ ) ”, Robert W. Zehner et al, Langmuir 1999, 15, p. 1121-1127. In addition, Toru Toda et al., Journal of the Japan Photography Society, 70, 38 (2007), the energy level of gold or silver is adjusted by modifying the metal surface with an electron donating group or an electron withdrawing group. Controlling the flow of electrons is described.

  If the energy level of the metal film is only adjusted, the technique described in the above document may be applied to the metal film, but if applied as it is, the effect of improving the light emission efficiency by plasmon resonance can be hindered. There is sex. The present inventors have found a configuration for adjusting the energy level of a metal film while fully utilizing the effect of improving the luminous efficiency by plasmon resonance, and an electroluminescent device that realizes high luminous efficiency without reducing durability. It was realized.

<Third Embodiment>
FIG. 4 schematically shows the configuration of the organic EL element 3 according to the third embodiment of the present invention.
As shown in FIG. 4, the organic EL element 3 of the present embodiment includes an anode 11, a hole transport layer 13, a light emitting layer 14, an electron transport layer 15, and a cathode 17 on a translucent substrate 10. Here, a large number of core-shell type fine particles 40 comprising a metal fine particle core 42 and an insulator shell 41 covering the metal fine particle core 42 are dispersed in the hole transport layer 13 as a metal member that causes plasmon resonance by emitted light. Has been. Here, the insulator shell 41 is made of a material having translucency with respect to emitted light. Here, translucency means that the transmittance of emitted light is 70% or more.

  The translucent substrate 10 is not particularly limited, and a flexible substrate such as glass, quartz, or polymer can be used.

  The core-shell type fine particles 40 are arranged in the vicinity of the light emitting region (light emitting layer 14) so that plasmon resonance can be generated by the emitted light. The core-shell type fine particle 40 is not particularly limited as long as the metal fine particle core 42 contained therein is arranged in a region where plasmon resonance due to emitted light occurs. However, if the distance from the light emitting layer 14 to the metal fine particle core 42 is too large, plasmon resonance due to the emitted light is difficult to occur, and an effective light emission enhancing effect cannot be obtained. The distance d to 14 is preferably 30 nm or less. In the organic EL element of the present embodiment, the metal fine particle core 42 is covered with the insulator shell 41, and therefore can be disposed in the light emitting layer 14.

  The material of the metal fine particle core 42 is not limited to Ag (silver) as long as plasmon resonance is generated by the emitted light, and Au (gold), Cu (copper) as in the metal film of the first embodiment. ), Al (aluminum), Pt (platinum), and alloys containing any of these metals as a main component (80% or more) are applicable.

On the other hand, as the material of the insulator shell 41, an insulator such as SiO ₂ , Al ₂ O ₃ , MgO, ZrO ₂ , PbO, B ₂ O ₃ , CaO, or BaO can be suitably used.

As described above, when a metal member is inserted into the laminate, the metal member may hinder the movement of electric charges. In the present embodiment, in order to prevent this, the core-shell type fine particles 40 are used as the metal member. The core-shell type fine particle 40 is formed using, for example, an Ag fine particle as the metal fine particle core 42 and a dielectric such as SiO ₂ as the insulator shell 41. Since the metal fine particle core 42 contributing to plasmon resonance is covered with the insulator shell 41, even when an electric field is applied between both electrodes, charges (electrons or holes) are trapped by Ag which is a conductor. Therefore, the flow of charge is not hindered, and the light emitting layer can obtain the effect of plasmon resonance. Therefore, it is possible to more effectively realize improvement in phosphorescence emission efficiency and improvement in element durability due to plasmon enhancement.

An example of a method for manufacturing the EL element 3 of this embodiment will be briefly described.
The anode 11 made of a transparent electrode such as ITO is formed on the transparent substrate 30 by vapor deposition. As the core-shell type fine particles 40, those obtained by coating Ag fine particles 42 having a particle diameter of 50 nm with SiO ₂ 41 having a thickness of 10 nm are used. Next, the core-shell type fine particles 40 are dispersed by dispersing the core-shell type fine particles 40 in dichloromethane in which the trifoldiamine derivative (TPD), which is a hole transporting material, is dissolved, and coating the anode 11 by spin coating. The formed hole transport layer 13 is formed. Next, the light emitting layer 14 and the electron transport layer 15 are formed by vapor deposition, and finally the cathode 17 is formed.

  In the above embodiment, the core-shell type fine particles 40 are elements dispersed in the hole transport layer 13. However, the core-shell type fine particles 40 may be any layer as long as it is a region where plasmon resonance due to emitted light occurs between the electrodes. It may be placed inside. In particular, plasmon resonance can be generated more effectively by being present in the light emitting layer, which is preferable.

  Although FIG. 4 shows the case where a large number of core-shell type fine particles 40 exist, even if the number is one, the effect of enhancing the luminous efficiency by plasmon resonance can be obtained.

  4 shows a configuration in which one metal fine particle core 42 is provided in the insulator shell 41, but core-shell type fine particles in which a plurality of metal fine particle cores 42 are provided in the insulator shell 41 may be used.

  The particle diameter of the metal fine particle core 42 of the core-shell type fine particle 40 is not particularly limited as long as it is a size capable of inducing localized plasmons, but is preferably a size equal to or smaller than the wavelength of the emitted light, particularly 10 nm or more, 1 μm or less is preferable.

  The thickness of the insulator shell 41 is preferably a thickness that does not inhibit the induction of localized plasmons in the metal fine particle core 42 by the emitted light. In order to effectively induce induction of localized plasmons by emitted light in the light emitting layer 14, it is preferable that the distance between the light emitting layer 14 and the surface of the metal fine particle core is 30 nm or less. It is desirable that the position, the layer structure, and the thickness of the insulator shell 41 are designed so that more effective plasmon resonance can be obtained. Here, the thickness of the insulator shell 41 is an average distance between the surface of the insulator shell 41 and the surface of the metal fine particle core in a configuration in which only one metal fine particle 41 is included in the insulator shell 41. When a plurality of metal fine particle cores are provided in the insulator shell, the average value of the shortest distances between the surface of the insulator shell and the surface of each metal fine particle core is defined as the thickness of the insulator shell.

As another preferred embodiment, as the core-shell type fine particles 40, a large number of so-called elongated fine particles having an aspect ratio of the long diameter of the fine particles and the short diameter perpendicular to the fine particles larger than 1 are used. Examples thereof include organic EL elements in which mold fine particles are arranged such that the minor axis of the fine particles has orientation in a direction substantially perpendicular to the electrode surface.
With such a configuration, strong scattered light can be obtained on the light extraction surface side due to the shape anisotropy of the elongated core-shell type fine particles, and higher extraction efficiency can be obtained.

  Another preferred embodiment includes a configuration in which core-shell type fine particles 40 having a large diameter are arranged so that the metal fine particle core 42 enters the light emitting layer 14. As already described, the emission transition by plasmon resonance can be obtained more effectively by making the metal fine particle core 42 exist in the light emitting layer 14.

<Fourth Embodiment>
The structure of the organic EL element 4 of the 4th Embodiment of this invention is typically shown in FIG.
The organic EL element 4 of the present embodiment has substantially the same layer configuration as the organic EL element 1 of the first embodiment, but the cathode 17 and the anode 11 are reflection portions that reflect emitted light, and light is transmitted between both ends. It differs in that it also plays a role of constituting a resonator and that the metal film 20 is provided in the hole transport layer 13.

In the element of this embodiment, the cathode 17 is made of Ag (silver) and the anode 11 is made of Cu (copper), whereby a standing wave 19 is generated between the electrodes 11 and 17 and the directivity of emitted light is improved. An improvement effect can be produced. Further, by matching the light emitting layer 14 with the antinode 19a of the standing wave, the electric field intensity can be maximized in the light emitting layer 14, and the light emission efficiency can be maximized. The resonance condition for producing such a microcavity effect is given by the following formula, and each of the layers 12 to 16 is designed to have a refractive index and a thickness that satisfy the following formula. In the following equation, λ ₀ is the wavelength of the emitted light, n _i is the refractive index in each layer, d _i is the thickness of each layer, φ ₁ and φ ₂ are the phase differences due to reflection at the anode 11 and the cathode 17, m Is the cavity order.

In the microcavity type organic EL device as shown in FIG. 5, the light emitting layer 14 is separated from the light emitting layer 14 by about 20 nm at a position within 10% from the antinode (peak) of the standing wave where the electric field intensity becomes maximum. The metal film 20 is disposed at the position. As described above, the metal film is also preferably disposed at a position close to the antinode of the standing wave, that is, a position where the electric field intensity due to the standing wave is large, so that the effect of plasmon resonance can be obtained more efficiently. In the design of this embodiment, the cavity order m = 1 is adopted.
With such a configuration, the microcavity effect and the effect of plasmon enhancement are superimposed, and the effect of improving luminous efficiency, improving directivity, and further improving durability can be obtained more remarkably.

  As described above, in the element 4 of the present embodiment, the electrodes 11 and 17 are made of metal, and a cavity is formed between the electrodes to form a standing wave inside the element. The reflectance with respect to the emitted light of the electrodes 11 and 17 constituting the reflection part of the cavity may be sufficient as long as a standing wave is formed. In the present embodiment, the thickness of the electrode from which light is extracted (here, the Cu anode 11) is adjusted so that the reflectance is, for example, about 30%. The reflectance on the Ag cathode 17 side may be a high reflectance of 90% or more. Moreover, a transparent electrode may be provided as an electrode, and a reflective layer may be further provided outside the electrode. The reflective layer can be composed of a metal having an appropriate reflectance or a dielectric multilayer film.

  Note that the EL elements of the first to third embodiments described above are also configured such that a standing wave is generated in the element with the space between the electrodes 11 and 17 as a cavity, and the antinodes of the standing wave and the light emitting layer 14 are formed. The effects of plasmo enhancement and the microcavity effect can be obtained in a superimposed manner by designing the layers to have substantially the same refractive index and thickness that satisfy the above-described resonance condition.

  Examples and comparative examples of the organic EL element 1 (see FIG. 1) of the first embodiment will be described.

"Example 1"
The EL device of Example 1 includes an ITO anode 11, 2TNATA + 0.3% F4TCNQ (120 nm) hole injection layer 12, NPD (10 nm) hole transport layer 13, CBP-10% Ir (ppy) 3 (30 nm) light emitting layer 14, a BAlq (10 nm) electron transport layer 15, an Ag (14 nm) metal film 20, a BAlq (20 nm) electron transport layer 15, a LiF (1 nm) electron injection layer 16, and an Al cathode 17 are sequentially stacked.
The light emitting material of the light emitting layer 14 is Ir (ppy) 3, and its T1 level is 2.5 eV. The metal film 20 is disposed in the electron transport layer 15. The T1 level of NPD constituting the hole transport layer 13 is 2.3 eV, and the T1 level of BAlq constituting the electron transport layer 15 is 2.3 eV, both of which are lower than the T1 level of the light emitting material.

"Comparative Example 1"
The EL element of Comparative Example 1 is the same as the element of Example 1 except that the Ag metal film 20 is not provided.
That is, the device of Comparative Example 1 has an ITO anode 11, 2TNATA + 0.3% F4TCNQ (120 nm) hole injection layer 12, NPD (10 nm) hole transport layer 13, CBP-10% Ir (ppy) 3 (30 nm) light emission. The layer 14, the BAlq (30 nm) electron transport layer 15, the LiF (1 nm) electron injection layer 16, and the Al cathode 17 were sequentially laminated.

"Example 2"
The EL device of Example 2 has an ITO anode 11, 2TNATA + 0.3% F4TCNQ (120 nm) hole injection layer 12, NPD (10 nm) hole transport layer 13, mCP + 15% Pt1 (30 nm) light emitting layer 14, BAlq (10 nm). The electron transport layer 15, the Ag (14 nm) metal film 20, the BAlq (20 nm) electron transport layer 15, the LiF (1 nm) electron injection layer 16, and the Al cathode 17 are sequentially stacked.
The light emitting material of the light emitting layer 14 is Pt1, and its T1 level is 2.7 eV. The metal film 20 is disposed in the electron transport layer 15. The T1 level of NPD constituting the hole transport layer 13 is 2.3 eV, and the T1 level of BAlq constituting the electron transport layer 15 is 2.3 eV, both of which are lower than the T1 level of the light emitting material.

"Comparative Example 2"
The EL element of Comparative Example 2 is the same as the element of Example 2 except that the Ag metal film 20 is not provided.

"Example 3"
The EL device of Example 3 includes an ITO anode 11, 2TNATA + 0.3% F4TCNQ (120 nm) hole injection layer 12, NPD (10 nm) hole transport layer 13, mCP + 15% Pt2 (30 nm) light emitting layer 14, BAlq (10 nm). The electron transport layer 15, the Ag (14 nm) metal film 20, the BAlq (20 nm) electron transport layer 15, the LiF (1 nm) electron injection layer 16, and the Al cathode 17 are sequentially stacked.
The light emitting material of the light emitting layer 14 is Pt2, and its T1 level is 2.5 eV. The metal film 20 is disposed in the electron transport layer 15. The T1 level of NPD constituting the hole transport layer 13 is 2.3 eV, and the T1 level of BAlq constituting the electron transport layer 15 is 2.3 eV, both of which are lower than the T1 level of the light emitting material.

“Comparative Example 3”
The EL element of Comparative Example 3 is the same as the element of Example 3 except that the Ag metal film 20 is not provided.

Detailed chemical formulas and T1 levels of the respective materials used in the above examples and comparative examples are as follows. In addition, T1 level of each material was calculated | required from the peak wavelength of the phosphorescence spectrum of the vapor deposition film in 77K.

About the element of each said Example and comparative example, the light emission peak wavelength, the light emission lifetime, EL external quantum yield, and EL drive half life were measured, respectively. Moreover, PL quantum yield of the film | membrane of only the light emitting layer of Example (comparative example) 1-3 was measured, respectively. The measurement method for each is as follows.
<Luminescent life>
Each element was irradiated with nitrogen laser light (wavelength 337 nm, pulse width 1 ns) as excitation light, and the light emission lifetime from each light emitting material was measured with a streak camera (C4334 manufactured by Hamamatsu Photonics).
<Emission peak wavelength and EL external quantum yield>
Using a source measure unit 2400 manufactured by Toyo Technica Co., Ltd., direct current was applied to each element to emit light. The emission spectrum at that time was measured using a spectral radiance meter SR-3 manufactured by Topcon Corporation, and the emission peak wavelength was determined. Further, based on the obtained spectrum, the external quantum efficiency at the device current density of 0.25 mA / cm ² and 25 mA / cm ² was calculated by the intensity conversion method for each wavelength.
<EL drive half life>
A direct current at which each element has a luminance of 2000 cd / m ² was measured, and each element was continuously driven with the current value, and the time until the luminance became 1000 cd / m ² was measured.
<PL quantum yield of light emitting layer film>
Example (Comparative example) Films of only the light emitting layer of 1 to 3 were formed on a quartz substrate, and PL quantum yield of each film was measured using an absolute PL quantum yield measuring apparatus (C9920-02, manufactured by Hamamatsu Photonics). It measured using.

The measurement results are shown in Table 1 below.

  As shown in Table 1, each example has a shorter emission lifetime, an improved external quantum efficiency, and a longer driving half-life of the device than the corresponding comparative example. It was confirmed that it was improved.

  The EL element of the present invention can be suitably used for display elements, displays, backlights, electrophotography, illumination light sources, recording light sources, exposure light sources, reading light sources, signs, signboards, interiors, optical communications, and the like.

1, 2, 3, 4 Organic EL device 11 Cathode 12 Electron injection layer 13 Electron transport layer 14 Light emitting layer 15 Hole transport layer 16 Hole injection layer 17 Anode 19 Standing wave 19a Abdominal 20 Metal film (metal member)
25 Work function adjustment layer (surface modification)
40 Core-shell type fine particles (metal member)
41 Insulator shell 42 Metal fine particle core

Claims (7)

A plurality of layers including a light-emitting layer including a light-emitting material that emits phosphorescence and first and second adjacent layers disposed adjacent to the light-emitting layer with the light-emitting layer interposed therebetween are stacked between the electrodes. An organic electroluminescence device comprising:
A first excited triplet energy level of the luminescent material is higher than a lowest energy level of the first excited triplet energy levels of the plurality of materials constituting the first and second adjacent layers;
In the vicinity of the light-emitting layer, a metal member that causes plasmon resonance due to light emitted from the light-emitting layer on the surface is provided at a position separated by 5 nm or more ,
The organic electroluminescent element , wherein the metal member is Au, Ag, or an alloy containing any one of these metals as a main component . The organic electroluminescence element according to claim 1, wherein a distance between the metal member and the light emitting layer is 30 nm or less.   The organic electroluminescence element according to claim 1, wherein the metal member is a metal film disposed between the plurality of layers.   4. The organic electroluminescence device according to claim 3, wherein the metal film is an island structure film in which a large number of metal fine particles having a particle diameter of 5 nm or more are dispersed in a film shape.   At least one surface of the metal film is surface-modified with a terminal group having a polarity that brings the work function of the metal film close to the work function of a layer adjacent to the metal film. The organic electroluminescent element according to claim 3 or 4.   3. The organic electroluminescence device according to claim 1, wherein the metal member is a core-shell type fine particle comprising a metal fine particle core and an insulator shell covering the metal fine particle core.   The plurality of layers have a layer thickness and a refractive index satisfying a resonance condition such that a region where the electric field intensity of the standing wave by the light emitted from the light emitting layer is maximum in the element substantially coincides with the light emitting layer. The organic electroluminescent element according to claim 1, wherein

Images