JP4785034B2 - EL element - Google Patents

Description

  The present invention is also referred to as injection-type electroluminescence (EL), and relates to an EL element that emits light by injecting electrons and holes into a thin-film EL material and recombining them.

  An organic EL element is a self-luminous display element that converts electric energy into light energy by injecting a current into an organic light emitting layer, and research and development have been actively promoted in recent years. In particular, an organic EL device provided with an organic hole transport layer made of an aromatic diamine and an organic light emitting layer made of an aluminum complex of 8-hydroxyquinoline has improved luminous efficiency as compared with an electroluminescent device using anthracene or the like. Since then, active efforts have been made in research and development (for example, see Non-Patent Document 1).

  A general structure of the organic EL element is shown in FIG. A transparent electrode layer (anode) is provided on a transparent substrate, and a hole transport layer, an organic light emitting layer, and a cathode layer are sequentially formed thereon by a vacuum deposition method. When a DC voltage is applied between the transparent electrode layer serving as the anode and the cathode layer, the holes injected from the transparent electrode layer and the electrons injected from the cathode layer are converted into organic layers (hole transport layer and organic light emitting layer). ) And electrons and holes are recombined. At that time, electric energy is converted into light energy, and light is emitted from the organic light emitting layer.

  In this case, the organic layer provided between the anode and the cathode has a two-layer structure of a hole transport layer / organic light emitting layer in order from the anode side, but in addition, a hole injection layer / hole transport layer / organic light emitting layer. / Having a structure of electron transport layer, having a structure of hole injection layer / hole transport layer / organic light emitting layer /, having a structure of hole transport layer / organic light emitting layer / electron transport layer, etc. Some organic light-emitting layers are laminated as a multilayer.

  The hole transport layer has a function of easily injecting holes from the anode and a function of blocking electrons. The electron injection layer has a function of facilitating injection of electrons from the cathode.

Materials for forming the anode layer include metals having a high work function, alloys and compounds thereof, such as nickel, gold, platinum, palladium, alloys thereof, tin oxide (SnO ₂ ), copper iodide, and polypyrrole. In general, a transparent electrode layer made of ITO is often used.

  Since it is necessary to form the cathode layer with a material effective for electron injection, it is preferable to use a metal having a low work function (low work function metal material) that can improve the electron injection efficiency. Aluminum, magnesium, magnesium The indium alloy, the magnesium aluminum alloy, the magnesium silver alloy, the aluminum lithium alloy, and the like are used to form a film by a dry process such as a vacuum evaporation method or a sputtering method.

The organic layer formed by the organic material includes tris (8-hydroxyquinalinato) aluminum (hereinafter abbreviated as Alq ₃ ) and N, N′-diphenyl-N, N′-bis (3-methylphenyl) 1. , 1′-biphenyl-4,4′-diamine (hereinafter abbreviated as TPD) and high molecular materials such as polyparaphenylene vinylene (PPV) have been tried. In addition, research and development for high brightness and multi-color has been actively conducted, and practical application has begun.

  In the organic EL element having the above structure, light emitted from the organic light emitting layer and emitted from the transparent substrate to the outside (in the atmosphere) is emitted from the organic light emitting layer toward the transparent electrode and passes through the transparent electrode and the transparent substrate. Outgoing light, emitted from the organic light emitting layer toward the cathode, reflected from the surface of the cathode and emitted through the transparent electrode and the transparent substrate, emitted from the organic light emitting layer, transparent substrate, transparent electrode and multilayered There is light emitted from the transparent substrate while repeating reflection and refraction at each interface of the organic compound material layer (hole transport layer and organic light emitting layer). Each light is emitted from the organic light-emitting layer and has a different optical path length from when it is emitted to the outside through the transparent substrate. Therefore, it is known that an optical path difference occurs, and interference caused by the influence affects the output. It has been.

  Therefore, in response to such a phenomenon, each layer of the organic compound material layer excluding the organic light emitting layer of the organic EL element is set to have a different film thickness corresponding to the light emission color, and each light emission is performed by utilizing the reflection interference phenomenon. Proposals have been made to improve the extraction efficiency of colored light.

Specifically, control of the film thickness of the positive hole transport layer (hole injection layer and hole transport layer) on the anode side and the electron transport layer (electron injection layer and electron transport layer) on the cathode side with the organic light emitting layer interposed therebetween Is done. In the case of the element structure of the organic EL element shown in FIG. 21, the optical distance ((n _org · d _org ) + (n _ITO · d _ITO )) from the light emitting interface of the organic light emitting layer to the boundary plane between the transparent electrode and the transparent substrate is internally desired by depositing the hole transport layer so as to be substantially even multiple of 1/4 of the peak emission wavelength lambda _p that is emitted to the transparent electrode direction from the light-emitting surface and the transparent electrode and the transparent substrate of the interface plane The light reflected and returning to the light emitting interface interferes with the emitted light, strengthening it, and maximizing the brightness.

Further, the organic light emitting layer is formed so that the optical distance (n _org · d ₀ ) from the light emitting interface to the cathode layer is substantially an odd multiple of 1/4 of the desired peak light emission wavelength λ _p , thereby forming the light emitting interface. The light emitted from the cathode toward the cathode, reflected by the surface of the cathode and returned to the light emitting interface interferes with the emitted light, and the brightness is maximized (see, for example, Patent Document 1).

  Similarly, there is one in which emission light having a desired peak wavelength is obtained by controlling the optical distance between the transparent electrode and the organic compound material layer (hole transport layer and organic light emitting layer) (for example, Patent Documents). 2 and 3).

Further, an MPE structure in which a plurality of light emitting positions exist in a discrete manner (an organic layer sandwiched between a pair of opposing electrodes has a plurality of light emitting units each including at least one light emitting layer, In the organic EL element having a structure in which the light emitting unit is partitioned by at least one charge generation layer, by making all the optical film thicknesses from the respective light emitting positions to the light reflecting electrodes substantially odd times the quarter wavelength, Some have achieved high luminous efficiency (for example, see Patent Documents 4 and 5).
Applied Physics Letters Vol. 51, p. 913, 1987 JP 2000-323277 A Japanese Patent No. 2846571 JP 2003-142277 A JP 2003-272860 A JP 2003-45676 A

  However, in the inventions described in “JP 2000-323277 A”, “Patent No. 2846571” and “JP 2003-142277 A”, the organic layer or the transparent electrode layer film constituting the organic EL element By controlling the thickness, emission light of the desired peak wavelength is obtained. However, if the film thickness of the transparent electrode layer is only controlled, light is emitted from the light emitting interface toward the cathode and reflected from the surface of the cathode to emit light. The interference between the light returning to the interface and the emitted light cannot be controlled.

  In addition, the original purpose of controlling the film thickness of the organic layer is to efficiently transport the injected carriers, recombine, and emit light in view of the light emission mechanism, focusing only on the interference phenomenon. If the film thickness is controlled, the voltage-luminance characteristics are deteriorated, and the light emission efficiency is lowered.

  In the inventions described in “JP-A 2003-272860” and “JP-A 2003-45676”, in an organic EL element in which a plurality of light emission positions are present in a discrete manner, light is emitted from each light emission position. It is said that the light emission efficiency can be improved by making the optical film thickness up to the reflective electrode all approximately odd multiples of ¼ wavelength.

  However, in this case, since the film thickness is thick, the interference effect is remarkable, and even a slight deviation in the film thickness control is largely offset. As a result, the emission spectrum distribution changes to cause a color shift and a decrease in light emission efficiency.

  Therefore, it is required to strictly control the optical film thickness from each light emitting position to the light reflecting electrode. However, even in the structure of this organic EL element, the transport, recombination, and light emission of injected carriers are efficiently performed. It is of course necessary to set such a film thickness, and it is extremely difficult to set the film thickness for both interference conditions.

  Further, even if the optical film thickness from each light emitting position to the light reflecting electrode can be strictly controlled, the number of light emitting units (organic film structure including at least one light emitting layer) constituting the organic EL element is increased. When the thickness becomes thicker, the peak emission wavelength does not change, but the spectral half-value width becomes smaller, resulting in deviation from the original emission spectrum distribution of the luminescent material. Furthermore, there also arises a problem that the color tone of light appears to change depending on the viewing angle of the organic EL element.

  By the way, at present, the film thickness is set so as to obtain an optimal carrier balance so that the transport, recombination, and light emission of the injected carriers are efficiently performed in a normal organic EL element (organic EL element having a single light emitting unit structure that is not MPE). However, it is inappropriate to apply it to MPE as it is, and MPE is performed with some sacrifice of optical and electrical characteristics. In addition, when organic EL elements having a plurality of light emitting units in one light emitting unit configuration (for example, an element that emits white light by additive color mixture of blue and orange light emission) are stacked, adjacent light emission Since there are a plurality of target wavelengths in the part, even a design for realizing an element that emits light of a desired color tone is difficult.

  Therefore, as a method capable of suppressing the influence of interference without performing strict film thickness control as described above, the cathode that has been a conventional light reflecting electrode is blackened to become a non-reflecting electrode, or a light emitting layer and It has been proposed that at least one layer present between the cathodes functions as a light absorption layer. However, such a method has a problem that the internal reflection light of the organic EL element cannot be used, and the luminance is lower than that of the conventional organic EL element.

  Therefore, the present invention was devised in view of the above problems, and the object thereof is to prevent light emitted from the light emitting interface of the light emitting layer and reflected inside the EL element from causing interference. In addition, the present invention provides an EL element that can effectively utilize light emitted from the light emitting interface of the light emitting layer and reflected inside the EL element, and can efficiently extract light that is free from the influence of interference. is there.

In order to solve the above problems, the invention described in claim 1 of the present invention is an EL element in which a thin film multilayer structure including at least one light emitting layer is sandwiched between a pair of opposing electrodes,
The optical distance from the light emitting interface of the light emitting layer is expressed by the following formula:
Lc = λp ² / Δλ
(Where λp is the peak wavelength of the emission spectrum of light emitted from the light emitting layer, Δλ is the half width of the emission spectrum of light emitted from the light emitting layer)
The reflection mirror is disposed at a distance greater than or equal to the coherence distance Lc of the light emitted from the light emitting interface , and any one of the direction of the reflection mirror and the direction opposite to the reflection mirror from the light emitting layer. Also in the direction, within the range where the optical distance from the light emitting interface of the light emitting layer is less than the coherence distance of the light emitted from the light emitting interface, the refractive index difference of all two adjacent layers excluding the reflecting mirror is It is characterized by being 0.6 or less.

  Further, in the invention described in claim 2 of the present invention, in claim 1, the pair of electrodes are both constituted by transparent electrodes, and are formed on the outer surface of one of the transparent electrodes. A reflection mirror is disposed so as to be in contact with or to face the transparent electrode at a certain distance from the outer surface.

Moreover, the invention described in claim 3 of the present invention is the buffer layer according to claim 2 , wherein the buffer layer is outside the transparent electrode and / or between the transparent electrode and the reflecting mirror. It is characterized by forming.

  According to a fourth aspect of the present invention, in the third aspect, the buffer layer is made of one selected from the group consisting of a light-transmitting material, a gas, and a vacuum. Is.

  Further, in the invention described in claim 5 of the present invention, in claim 1, the pair of electrodes is such that one of the electrodes is a transparent electrode and the other is a reflective electrode. It is the reflection mirror.

Moreover, the invention described in claim 6 of the present invention is the thin film multilayer structure according to any one of claims 1 to 5, wherein the thin film multilayer structure includes a plurality of light emitting units including at least one light emitting layer. have a charge generation layer formed between the light emitting unit, the reflection mirror is kept the coherence length obtained from the PL spectrum of the light emitted from the light emitting layer in each light-emitting unit from each of the light emitting surface Of the light emitting layer, the farthest from the electrode on the reflective mirror side, or a position further away from the electrode, and from each light emitting layer in the direction of the reflective mirror and in the direction opposite to the reflective mirror In any direction, the optical distance from the light emitting interface of each light emitting layer is within a range less than the coherent distance obtained from the PL spectrum of the light emitted from the light emitting surface of each light emitting layer, Serial two layers all refractive index difference of adjacent excluding the reflecting mirror is characterized in that it is 0.6 or less.

  The EL element of the present invention is an EL element in which a thin film multilayer structure including at least one light emitting layer is sandwiched between a pair of opposing transparent electrodes, and the outer surface of one of the pair of transparent electrodes A reflecting mirror is disposed so as to face the transparent electrode at a certain distance from the outer surface or at an optical distance from the light emitting interface of the light emitting layer to the reflecting mirror. The coherence distance of light emitted from the interface was set to be longer than the coherence distance.

  The thin film multilayer structure including at least one light emitting layer is an EL element sandwiched between a pair of opposed transparent electrodes, and is in contact with the outer surface of one of the pair of transparent electrodes, Alternatively, a reflecting mirror is disposed so as to face the transparent electrode at a certain distance from the outer surface, and an optical distance from the light emitting interface of the light emitting layer to the reflecting mirror is emitted from the light emitting interface. When the distance is less than the coherent distance of light, the refractive index difference between all adjacent two layers of the thin film layer, the first buffer layer, and the transparent electrode constituting the EL element is set to 0.6 or less. .

  In addition, an EL element in which a thin film multilayer structure including at least one light emitting layer is sandwiched between a transparent electrode and a reflective electrode facing each other, and a second buffer layer is formed between the light emitting unit and the reflective electrode in the final stack. Thus, the optical distance from the light emitting interface of the light emitting layer to the reflective electrode is set to be equal to or greater than the coherence distance, and the refractive index difference of all interfaces existing from the light emitting interface to the reflective electrode is set to 0.6 or less.

  As a result, the light emitted from the light emitting interface of the light emitting layer and reflected inside the EL element does not cause interference.

  In addition, the light emitted from the light emitting interface of the light emitting layer and reflected inside the EL element can be effectively used, and the light from which the influence of interference is eliminated can be efficiently extracted to the outside.

  In particular, for the organic EL element, it is not necessary to take into account the influence of interference when setting the film thickness of each transparent electrode layer and each organic thin film layer constituting the organic EL element, so that carrier transport efficiency, recombination and light emission are reduced. Since the film thickness can be set with emphasis only on efficiency etc., the film thickness can be optimized.

  Further, even if the film thickness changes, the emission spectrum distribution does not change, and the color tone of the emitted light does not change.

Further, even when the viewing angle with respect to the light emitting surface of the organic EL element was changed, there was almost no change in the spectral distribution, and no change in the color tone of the emitted light was observed from any direction.

  In this embodiment, an organic EL element is described as an example as follows, but the present invention may be an inorganic EL element that is common in that it has a thin film laminated structure between both electrodes. Accordingly, embodiments of the present invention will be described in detail below with reference to FIGS. The embodiments described below are preferable specific examples of the present invention, and thus various technically preferable limitations are given. However, the scope of the present invention particularly limits the present invention in the following description. Unless stated to the effect, the present invention is not limited to these embodiments.

  FIG. 1 is a cross-sectional view showing the structure of an embodiment relating to the organic EL element of the present invention. A first transparent electrode layer (anode), a hole transport layer, an organic light emitting layer, and a second transparent electrode layer (cathode) are sequentially formed on the transparent substrate. Further, a reflection mirror is disposed outside the second transparent electrode layer (cathode) at a position spaced apart from the second transparent electrode layer so that the reflection surface faces the second transparent electrode layer. .

  The position where the reflecting mirror is disposed is a position set so that the optical distance from the light emitting interface of the organic light emitting layer to the reflecting mirror is equal to or greater than the “coherent distance of light emitted from the light emitting layer”. The term “coherence distance” used in this specification is defined as follows.

  The light wave emitted from one light emitter takes the form of a damped oscillation as shown in FIG. 2. In this case, the light wave can be regarded as a wave effectively because the amplitude is 1 / e (e is a natural value). It is considered to be up to a finite length l which is the base of the logarithm).

  One light emitter emits such a wave and then emits the next wave of the same frequency, but the phase of the preceding wave and the following wave are different. Therefore, even if the wave emitted from the same light source is divided again after being divided into two, if the optical path difference of the optical path followed by each wave exceeds 1, wavefronts of different phases will be superimposed, and the interference phenomenon Does not happen.

The present invention utilizes such a principle and eliminates the influence of interference on the optical system until the light emitted from the light emitting layer of the organic EL element is emitted to the outside. Therefore, the “coherence distance” of light corresponds to the above-mentioned finite length l of the wave and is generally represented by the following relational expression (1).
Lc = λ _p ² / Δλ
(However, Lc is the coherence distance, λ _p is the peak wavelength (peak emission wavelength) of the emission spectrum, and Δλ is the half width of the spectrum)

  In a conventional organic EL element, for example, interference occurs between light emitted from the light emitting interface toward the cathode, reflected by the surface of the cathode and returned to the anode, and light emitted from the light emitting interface to the anode. It was. The optical path difference of the optical path followed by each light is twice the optical distance from the light emitting interface to the reflecting surface of the cathode. Therefore, if this optical path difference is larger than the coherent distance Lc, no interference occurs. In other words, it is considered that the influence of interference can be suppressed if the optical distance from the light emitting interface to the reflecting surface of the cathode is larger than Lc / 2.

  However, in an actual organic EL element, even if the optical distance from the light emitting interface to the reflecting surface of the cathode is made larger than Lc / 2, the influence of interference is reduced, but the film thickness shifts due to imperfection. It was found that the chromaticity also shifted. In practice, in order to completely suppress the influence of interference on chromaticity and the like, it is desirable that the optical distance from the light emitting interface to the reflecting surface of the cathode is equal to or greater than the coherent distance Lc.

In calculating the coherence distance Lc of the emitted light in the organic EL element, the peak wavelength λ _p and the spectral half width Δλ used in the above relational expression are read from the PL (photoluminescent) spectrum of the light emitting layer. be able to.

For example, the coherence distance of Alq ₃ having the PL spectrum shown in FIG. 3 is obtained from the PL spectrum as λ _p = 523 nm and Δλ = 105 nm, and when this is substituted into the above relational expression, it becomes 2605 nm. Accordingly, when it is assumed that the light emitting layer is formed of Alq ₃ in the organic EL element of the present invention shown in FIG. 1, the optical distance from the light emitting interface to the reflecting mirror is set to 2605 nm or more.

Therefore, in this case, the film thickness of each layer from the light emitting interface to the reflection mirror is set so as to satisfy the following relational expression (2).
Lc ≦ d ₀ · n ₀ + d _{TO 2} · n _TO 2 + d _B · n _B
There is no particular upper limit on the optical distance from the light emitting interface to the reflecting mirror, but it is preferably within 1000 μm in order not to lose the advantage of the thinness of the organic EL element. On the other hand, the lower limit is preferably equal to or longer than the optical distance Lc, but is more preferably about twice Lc in order to completely eliminate the influence of interference. Here, although the light emitting region is considered to have a certain width in the light emitting layer, a predetermined position assumed to have high light emission intensity is set as the light emitting interface. For example, in the configuration composed of the hole transport layer and the organic light emitting layer as shown in FIG. 1, it is assumed that the emission intensity is maximized on the hole transport layer side of the organic light emitting layer. The interface with the light emitting layer can be considered as the light emitting interface.

  Further, when one light emitting layer is formed of two or more kinds of light emitting materials, the longest coherence distance calculated from the PL spectrum of each light emitting material is adopted.

  FIG. 4 is a cross-sectional view showing the structure of another embodiment relating to the organic EL element of the present invention. In this embodiment, a plurality of light emitting positions are separated from each other, and an MPE structure (an organic layer sandwiched between a pair of opposing electrodes has a plurality of light emitting units including at least one light emitting layer, Each light emitting unit is divided by a charge generation layer composed of at least one layer.

The light emitting unit has a layer structure including at least one light emitting layer mainly composed of an organic compound, and refers to an element excluding an anode and a cathode among components of a general organic EL element. . The charge generation layer is made of a transparent conductive material such as ITO, IZO, SnO ₂ , ZnO ₂ , V ₂ O ₅ and 4F-TCNQ, etc., a hole transport material formed thereon, and a charge transfer complex by oxidation-reduction reaction. A material capable of forming a hole is used, and holes are injected into the light emitting unit in contact with the cathode side of the charge generation layer, and electrons are injected into the light emitting unit in contact with the anode side.

  The specific structure of the organic EL element shown in FIG. 4 is that a first transparent electrode layer (anode) is formed on a transparent substrate, and a first light emitting unit and a first charge generation layer including a first light emitting interface thereon. The second light emitting unit including the second light emitting interface, the second charge generation layer, the third light emitting unit including the third light emitting interface, and the second transparent electrode layer (cathode) are sequentially formed. Further, a reflection mirror is disposed outside the second transparent electrode layer (cathode) at a position spaced apart from the second transparent electrode layer so that the reflection surface faces the second transparent electrode layer. .

In the organic EL element having such a structure, in order to dispose the reflection mirror so as not to cause interference between the light emitted from the light emitting interfaces of the three light emitting units, the light emitting interfaces of the respective light emitting units are arranged. Each coherence distance is obtained from the PL spectrum of the emitted light using the above relational expression (1), and the distance from the second transparent electrode layer is the farthest from the positions where the coherence distance is maintained from each light emitting interface. Find the position. As a result, as shown in FIG. 4, Lc ₁ is located farthest from the second transparent electrode layer, so that the reflection mirror is disposed at this position, or even further with respect to the second transparent electrode layer. Set it apart. Thereby, it is possible to eliminate all the influences of interference of light emitted from each light emitting unit.

  1 and 4, the space between the second transparent electrode layer and the reflection mirror may be filled with vacuum or gas, or may be filled with liquid. Moreover, it is good also as a reflective mirror by forming the transparent 1st buffer layer which consists of a translucent material on a 2nd transparent electrode layer, and performing metal vapor deposition on the upper surface of a transparent 1st buffer layer.

There are no particular restrictions on the gaseous substance that fills the space between the second transparent electrode layer and the reflecting mirror, but an inert gas such as N ₂ gas or Ar gas that is mainly dehumidified is desirable. Examples of the transparent liquid substance include dehydrated silicone oil and fluorine oil.

As the first buffer layer, TiO ₂ , SiO ₂ , SiNx, Ta ₂ O ₅ , SiO, Al ₂ O ₃ , ZrO ₂ , Sb ₂ O ₃ , TiO, HfO ₂ , Y ₂ O ₃ , MgO, CeO ₂ , Transparent metal oxides such as Nb ₂ O ₅ , MgF, SrF ₂ , BaF ₂ , nitrides, fluorides or low molecular materials, high molecular materials such as transparent epoxy, acrylic, nylon, etc. Any material can be used as long as it can be formed on the order of μm.

  In any case, it is provided between the second transparent electrode layer and the reflecting mirror in order to keep the optical distance from the light emitting interface to the reflecting mirror to be equal to or greater than the coherent distance Lc. The purpose is fulfilled.

  However, the difference between the refractive index of the transparent material and the refractive index of the second transparent electrode layer may cause interference. This is because when the boundary plane is formed by two substances having different refractive indexes, the greater the difference in the refractive index between the two substances forming the boundary plane, the greater the reflectance of the incident light.

  As a result, since the optical distance of light from the light emitting interface to the boundary plane between the transparent material provided between the second transparent electrode layer and the reflecting mirror and the second transparent electrode layer is smaller than the coherent distance Lc, interference is caused. It will be a cause. The greater the difference in refractive index between the two materials that form this boundary plane, the higher the reflectance and the greater the degree of interference that occurs.

  Therefore, the difference in refractive index between the transparent material provided between the second transparent electrode layer and the reflecting mirror and the second transparent electrode layer is preferably 0.6 or less so as not to cause interference. It is more desirable to approach zero.

  For example, in FIG. 1 showing the present embodiment, the boundary plane existing between the light emitting interface and the reflecting mirror is the boundary plane of the organic layer / organic layer, the boundary plane of the organic layer / second transparent electrode layer, and the second It is a boundary plane of the transparent material on the transparent electrode layer / second transparent electrode layer. Each of these boundary planes exists as a reflection surface at a position where the optical distance from the light emitting interface may be less than the coherence distance Lc, and thus causes a strong interference when the reflectance increases.

Among these, in particular, the boundary plane of the transparent material on the second transparent electrode layer / second transparent electrode layer may be a reflective surface having a relatively high reflectance. For example, when the space between the second transparent electrode layer and the reflecting mirror is filled with vacuum or gas (refractive index: about 1.0), ITO, IZO, ZnO, SnO ₂ etc. used as the second transparent electrode layer ( (Refractive index: approximately 1.95) and the difference in refractive index is approximately 0.95. On the reflective surface of this refractive index step 0.95, the transparent material on the second transparent electrode layer / second transparent electrode layer from the light emitting interface When the optical distance to the boundary plane is about 300 nm, the influence of interference appears to some extent.

  However, as shown in FIG. 4 showing another embodiment of the present invention, the transparent material on the second transparent electrode layer / second transparent electrode layer has a plurality of light emitting positions separated from each other at a light emitting interface. In an organic EL element having a structure in which the optical distance to the boundary plane is very long, the influence of interference appears remarkably.

  Therefore, in order to suppress the occurrence of interference, the material forming the second transparent electrode layer according to the optical distance from the light emitting interface to the boundary plane of the transparent material on the second transparent electrode layer / second transparent electrode layer It is necessary to set the difference between the refractive indexes of the two by appropriately selecting the material of the transparent material on the second transparent electrode layer.

  Therefore, in an organic EL device having an optical distance of about 300 nm from the light emitting interface to the boundary plane of the transparent material on the second transparent electrode layer / second transparent electrode layer as shown in FIG. It is preferable to set the difference in refractive index between the layer and the transparent material on the second transparent electrode layer to be 0.6 or less, since this substantially suppresses the occurrence of interference.

  On the other hand, in an organic EL element having a structure in which a plurality of light emitting positions are separated and exist as shown in FIG. 4, light emission occurs as the number of light emitting units including at least one light emitting layer increases. Because the optical distance from the interface to the boundary plane of the second transparent electrode layer / transparent material on the second transparent electrode layer increases, the difference in refractive index between the second transparent electrode layer and the transparent material on the second transparent electrode layer. Needs to be further reduced.

As a reflection mirror, Ag, Al, Au, Pt, W, Mg, Ni, Rh and other metal single-piece or alloy vapor-deposited films, two kinds of oxides, nitrides, or semiconductor layers having different refractive indexes are alternately arranged. Laminated multilayer mirrors can be used. Examples of such combinations include TiO ₂ and SiO ₂ , SiNx and SiO ₂ , Ta ₂ O ₅ and SiO _{2, and} GaAs and GaInAs. In addition, SiO, Al ₂ O ₃ , ZrO ₂ , and Sb _{2 are used.} A material having a different refractive index from O ₃ , TiO, HfO ₂ , Y ₂ O ₃ , MgO, CeO ₂ , Nb ₂ O ₅ , MgF, SrF ₂ and BaF ₂ may be appropriately selected and used in combination. Is possible.

  Up to this point, the interference phenomenon related to the light emitted from the light emitting interface of the organic EL element toward the reflecting mirror has been studied from various angles. The same applies to the light emitted from the light emitting interface toward the transparent substrate. This is related to the interference phenomenon.

  In the structure of the organic EL device shown in FIG. 1, the organic layer / organic layer boundary plane, the organic layer / first transparent electrode layer boundary plane, and the first transparent electrode layer / transparent substrate are arranged in the direction from the light emitting interface to the transparent substrate. There is a possibility that the optical distance from the light emitting interface to each boundary plane is less than the coherence distance Lc. Therefore, interference may occur between the light emitted from the light emitting interface in the direction of the transparent substrate and reflected by these boundary planes and the light emitted from the light emitting interface in the direction of the second transparent electrode layer. It is desirable to suppress the occurrence of interference by minimizing the difference in refractive index between the two constituting the boundary plane.

Among these, in particular, the boundary plane of the first transparent electrode layer / transparent substrate has the largest refractive index difference among the three boundary planes. For example, when soda glass (refractive index: about 1.55) is used for the transparent substrate and general ITO, IZO, ZnO, SnO ₂ or the like (refractive index: about 1.95) is used for the first transparent electrode, The difference in refractive index is about 0.4. On the reflecting surface having a refractive index difference of about 0.4, the influence of interference hardly appears when the optical distance from the light emitting interface to the transparent substrate is about 400 nm.

  However, as shown in FIG. 4 showing another embodiment of the present invention, in an organic EL element having a structure in which the optical distance from the light emitting interface to the transparent substrate is very long such that a plurality of light emitting positions are separated and existed. The effect of interference appears remarkably.

  Therefore, in order to suppress the occurrence of interference, the refractive index of both is determined by appropriately selecting the material forming the first transparent electrode layer and the material of the transparent substrate according to the optical distance from the light emitting interface to the transparent substrate. It is necessary to set the difference.

  Specifically, between the first transparent electrode layer and the transparent substrate, the first buffer layer having a very small difference in refractive index from the first transparent electrode layer is used, and the optical distance from the light emitting interface to the transparent substrate is from the light emitting layer. The film is formed so as to have a thickness equal to or longer than the coherence distance Lc of the emitted light. If a transparent substrate having a small refractive index difference with respect to the first transparent electrode layer is used, the first buffer layer can be dispensed with.

  As a material used for the first buffer layer, the same material as the first buffer layer provided between the second transparent electrode layer and the reflection mirror can be used. Moreover, although it does not specifically limit as a material of a transparent substrate with a very small difference in refractive index with respect to a 1st transparent electrode layer, if an example is given, LaSFN9 (made by SCHOTT GLAS) (refractive index: 1. 85) can be used.

  The organic EL element can also have a structure in which a reflection mirror is provided outside the transparent substrate and light emitted from the light emitting interface is emitted to the outside through the second transparent electrode. In that case, a metal film may be formed on the outer surface of the transparent substrate to form a reflection mirror, or an organic EL element may be formed thereon using the reflection mirror as a substrate.

As a material for the transparent substrate, glass, PET, polycarbonate, amorphous polyolefin, or the like is used. The first transparent electrode and the second transparent electrode are preferably formed as a transparent conductive film such as ITO, IZO, SnO ₂ , and ZnO, and the film thickness is preferably in the range of 10 to 500 nm.

  In addition to the structure of the organic EL element shown in FIGS. 1 and 4, the organic layer from the first transparent electrode side on the transparent substrate is formed as a hole injection layer / hole transport layer / light emitting layer / electron transport layer. /, A hole injection layer / a hole transport layer / a light-emitting layer, a hole transport layer / a light-emitting layer / an electron transport layer, a multilayer of the light-emitting layer, etc. Is possible.

  The hole transport layer has a function of facilitating injection of holes from the first transparent electrode layer and a function of blocking electrons, and preferably has a high hole mobility, is transparent and has good film formability. Triphenylamine derivatives such as TPD, polyolefin compounds such as phthalocyanine and copper phthalocyanine, hydrazone derivatives and arylamine derivatives are used.

  The hole transport layer is divided into a hole injection layer having a function of facilitating the injection of holes from the anode and a hole transport layer having a function of transporting holes and a function of blocking electrons. May be. In that case, it is desirable that the film thicknesses of the hole injection layer and the hole transport layer are both in the range of 10 to 200 nm.

  The electron injection layer may be formed on the organic light emitting layer or the electron transport layer so that electrons are easily injected from the second transparent electrode layer. As a material for forming the electron injection layer, a metal having a low work function such as Li, Ca, Sr, and Cs is mainly used, and a very small amount is deposited on the organic layer.

In addition, it is desirable that the light-emitting layer has high emission efficiency due to recombination of electrons and holes, has good thin film properties, and does not have a strong interaction at the boundary plane with the transport material, such as an aluminum chelate complex (Alq ₃ ) Materials such as styrylarylene (DSA) derivatives such as styryl biphenyl derivative (DPVBi), quinacridone derivatives, rubrene, coumarin, and perylene are used. These materials are used alone or in combination of two or more to form a light emitting layer. The light emitting layer may be laminated as a multilayer. The thickness of the light emitting layer is preferably in the range of 10 to 200 nm.

As the material for forming the electron transport layer, an aluminum chelate complex (Alq ₃ ), a distyryl biphenyl derivative (DPVBi), an oxadiazole derivative, a bistylylanthracene derivative, a benzoxazole thiophene derivative, and the like are used. It is desirable to be in the range of 200 nm.

  Another embodiment relating to the organic EL device of the present invention is an EL device comprising a transparent substrate / first transparent electrode / a plurality of thin film layers including a light emitting layer / second transparent electrode, outside the second transparent electrode. A first buffer layer made of a translucent material, and a reflection mirror formed by forming a metal film thereon. The optical distance from the light emitting interface of the light emitting layer to the reflecting mirror, and the optical distance from the light emitting interface of the light emitting layer to the outer surface of the transparent substrate are equal to or greater than the coherence distance of light emitted from the light emitting interface, Furthermore, the refractive index difference between the transparent material forming the first buffer layer and the second transparent electrode adjacent to the transparent material, and the refractive index difference between the transparent substrate and the first transparent electrode adjacent to the transparent substrate. Is 0.6 or less. The first buffer layer may be formed of a plurality of layers made of different materials.

  Another embodiment relating to the organic EL device of the present invention is an EL device comprising a transparent substrate / first transparent electrode / a plurality of thin film layers including a light emitting layer / second transparent electrode, wherein the second transparent electrode A reflection mirror disposed so as to face the second transparent electrode with a certain distance outward from the first buffer layer, and a first buffer layer filling the space between the second transparent electrode and the reflection mirror facing each other. The optical distance from the light emitting interface of the light emitting layer to the reflecting mirror, and the optical distance from the light emitting interface of the light emitting layer to the outer surface of the transparent substrate are equal to or greater than the coherence distance of light emitted from the light emitting interface, And the refractive index difference of the said transparent substrate and the 1st transparent electrode adjacent to this transparent substrate shall be 0.6 or less. The first buffer layer is made of a translucent material having a refractive index difference with respect to the second transparent electrode of 0.6 or less, may be a layer made of gas or liquid, or may be a vacuum. The reflection mirror may be configured by forming a reflection surface on the substrate surface, or may be configured by a metal plate.

  Another embodiment relating to the organic EL device of the present invention is an EL device comprising a transparent substrate / a first transparent electrode / a plurality of thin film layers including a light emitting layer / a second transparent electrode, and the outer side of the transparent substrate. A reflection mirror formed by forming a metal film on the surface, and a first buffer layer made of a translucent material formed on the second transparent electrode. The optical distance from the light emitting interface of the light emitting layer to the reflecting mirror, and the optical distance from the light emitting interface of the light emitting layer to the outer surface of the first buffer layer are equal to or greater than the coherence distance of light emitted from the light emitting interface. Furthermore, the refractive index difference between the translucent material forming the first buffer layer and the second transparent electrode adjacent to the translucent material, and the refraction of the transparent substrate and the first transparent electrode adjacent to the transparent substrate. The rate difference is set to 0.6 or less.

  Next, Examples 1 to 13 and Comparative Examples 1 to 5 relating to the organic EL element of the present invention will be described. First, Examples 1-12 employ | adopt as a basic structure the structure of FIG. 1 which shows embodiment concerning the organic EL element of this invention. First, common components in the structures of the organic EL elements of Examples 1 to 9 are described below.

An ITO transparent electrode serving as a first transparent electrode layer (anode) was formed on a glass substrate serving as a transparent substrate by sputtering. The sheet resistance of ITO is 10Ω / □. Then, the ITO transparent electrode was etched into a predetermined shape, subjected to ultrasonic cleaning with acetone and isopropyl alcohol, and then dried. Further, after cleaning with UV-O ₃ , it was set in a vacuum vapor deposition tank and the pressure in the tank was reduced to about 1 × 10 ⁻⁵ Torr, and a hole transport layer was deposited on the ITO transparent electrode to a film thickness of 100 nm. Next, an organic light emitting layer to which a blue light emitting material (hereinafter referred to as a blue light emitting dopant) emitting light having an emission spectrum shown in FIG. 5 is added at a concentration of 1% by weight is co-evaporated, and an electron injection layer is formed thereon. A micro vapor deposition film was formed, and an IZO transparent electrode as a second transparent electrode (cathode) was formed into a film thickness of 50 nm by sputtering, and a SiO film was formed thereon as a first buffer layer. On top of this, Al was deposited to a thickness of 200 nm as a reflection mirror. The refractive index of the SiO layer was 1.90 with respect to light having a wavelength of 450 nm.

In Examples 1 to 9, the thickness of the SiO layer formed as the first buffer layer is different, and accordingly, the optical distance from the light emitting interface to the Al reflecting mirror is different. These elements differing in Examples 1-9 are shown in Table 1 below.

  In Example 10, the configuration from the glass substrate to the IZO transparent electrode is the same as in Examples 1 to 9, but the thickness of about 0.5 mm is formed on the IZO transparent electrode instead of the first buffer layer and the reflecting mirror. The structure is provided with an oil layer. The refractive index of the oil was 1.55 for light having a wavelength of 450 nm.

Example 11 has a structure in which N ₂ gas is filled in an 18 μm gap formed by an IZO transparent electrode and a reflecting mirror instead of the oil layer of Example 10 above. The reflection mirror is formed by vapor-depositing Ag on a substrate, and the reflection mirror is arranged so that the reflection surface faces the IZO transparent substrate,
A UV curable sealant in which a gap agent of 18 μm is dispersed is applied to the outer periphery of the gap between the reflection mirror and the IZO transparent electrode, and bonded and fixed, and N ₂ is formed in a space surrounded by the reflection mirror, the ITO transparent electrode, and the sealant. It is filled with gas.

In Example 12, instead of the N ₂ gas of Example 11, oil having a refractive index of 1.55 is filled with light having a wavelength of 450 nm.

Example 13 employs, as a basic structure, the structure of FIG. 4 showing another embodiment relating to the organic EL element of the present invention. However, in this case, there are two light emitting positions. The concrete structure formed the ITO transparent electrode used as a 1st transparent electrode layer (anode) on the glass substrate used as a transparent substrate by the sputtering method. The sheet resistance of ITO is 10Ω / □. Then, the ITO transparent electrode was etched into a predetermined shape, subjected to ultrasonic cleaning with acetone and isopropyl alcohol, and then dried. Further, after cleaning with UV-O ₃ , it was set in a vacuum vapor deposition tank and the pressure in the tank was reduced to about 1 × 10 ⁻⁵ Torr, and a hole transport layer was formed on the ITO transparent electrode. Next, an organic light emitting layer to which a blue light emitting dopant that emits light having an emission spectrum shown in FIG. 5 was added at a concentration of 1% by weight was co-evaporated. The stacked structure of the hole transport layer and the organic light emitting layer was used as a light emitting unit, and after forming a charge generation layer on the light emitting unit, another light emitting unit having an organic light emitting layer added with a yellow light emitting dopant was formed. Further, a small amount of an electron injection layer was deposited thereon, and an IZO transparent electrode was formed thereon. Then, a reflective mirror with Ag deposited on the substrate is disposed so that the reflective surface faces the IZO transparent substrate, and a UV curable sealant in which a gap agent of 18 μm is dispersed in the outer periphery of the gap between the reflective mirror and the IZO transparent electrode. It is applied, bonded and fixed, and filled with oil having a refractive index of 1.6 with respect to light having a wavelength of 450 nm in a space surrounded by a reflection mirror, an ITO transparent electrode, and a sealant.

Next, the structures of Comparative Examples 1 to 5 will be described. Comparative Examples 1 to 3 adopt the structure of the conventional organic EL element shown in FIG. 21 as the basic structure. The structure of the comparative example 1 formed the ITO transparent electrode used as a 1st transparent electrode layer (anode) on the glass substrate used as a transparent substrate by the sputtering method. The sheet resistance of ITO is 10Ω / □. Then, the ITO transparent electrode was etched into a predetermined shape, subjected to ultrasonic cleaning with acetone and isopropyl alcohol, and then dried. Further, after cleaning with UV-O ₃ , it was set in a vacuum vapor deposition tank and the pressure in the tank was reduced to about 1 × 10 ⁻⁵ Torr, and a hole transport layer was deposited on the ITO transparent electrode to a film thickness of 100 nm. Next, an organic light-emitting layer to which a blue light-emitting dopant that emits light having an emission spectrum shown in FIG. 5 was added at a concentration of 1% by weight was co-evaporated to a thickness of 62 nm. Further, a minute amount of electron injection layer was deposited thereon, and Al was deposited thereon to a thickness of 200 nm as a cathode. The refractive index of the organic light emitting layer was 1.80 for light having a wavelength of 450 nm.

  In this case, the optical distance from the light emitting interface to the Al cathode was 112 nm, which was almost ¼ of the peak emission wavelength of 450 nm.

  The structure of Comparative Example 2 is the same as that of Comparative Example 1 described above, but the film thickness of the organic light emitting layer co-deposited by adding a blue dopant is 105 nm, and the optical distance from the light emitting interface to the Al cathode is 189 nm. The difference is that the peak emission wavelength deviates from an odd multiple of 450 nm.

  The structure of Comparative Example 3 is the same as that of Comparative Example 2 from the glass substrate to the organic light-emitting layer co-deposited by adding the blue dopant, but above the organic light-emitting layer co-deposited by adding the blue dopant. The electron injection layer is deposited in a small amount by vapor deposition, and an IZO transparent electrode is formed thereon by sputtering to a thickness of 50 nm, and further, Al is used as a reflection mirror to form a thickness of 200 nm. The refractive index of the IZO transparent electrode was 1.95 for light having a wavelength of 450 nm. As a result, the optical distance from the light emitting interface to the Al reflecting mirror was 286 nm, which was different from the odd multiple of the peak emission wavelength of 450 nm as in Comparative Example 2.

In Comparative Example 4, the Al reflecting mirror is removed from Comparative Example 3, and the IZO transparent electrode is placed in an N ₂ atmosphere.

In Comparative Example 5, the structure of FIG. 4 showing another embodiment relating to the organic EL element of the present invention is adopted as a basic structure. However, in this case, there are two light emitting positions. The concrete structure formed the ITO transparent electrode used as a 1st transparent electrode layer (anode) on the glass substrate used as a transparent substrate by the sputtering method. The sheet resistance of ITO is 10Ω / □. Then, the ITO transparent electrode was etched into a predetermined shape, subjected to ultrasonic cleaning with acetone and isopropyl alcohol, and then dried. Further, after cleaning with UV-O ₃ , it was set in a vacuum vapor deposition tank and the pressure in the tank was reduced to about 1 × 10 ⁻⁵ Torr, and a hole transport layer was formed on the ITO transparent electrode. Next, an organic light emitting layer to which a blue light emitting dopant that emits light having an emission spectrum shown in FIG. 5 was added at a concentration of 1% by weight was co-evaporated. The stacked structure of the hole transport layer and the organic light emitting layer was used as a light emitting unit, and after forming a charge generation layer on the light emitting unit, another light emitting unit having an organic light emitting layer added with a yellow light emitting dopant was formed. Furthermore, a small amount of electron injection layer is deposited thereon,
Finally, an Al cathode was deposited to a thickness of 200 nm.

  As mentioned above, although Examples 1-13 and Comparative Examples 1-5 regarding the organic EL element of this invention were demonstrated, those characteristics are demonstrated below.

FIG. 6 shows the emission spectra of the organic EL elements having the conventional structures of Comparative Examples 1 to 3. The organic EL element of Comparative Example 1 is an element in which the optical distance from the light emitting interface to the Al electrode is approximately λ _p / 4 (peak wavelength of emitted light λ _p = 450 nm), and the emission spectrum distribution is blue light emission shown in FIG. It has almost the same form as the PL spectrum distribution of the dopant.

On the other hand, when the film thickness of the organic light emitting layer is 105 nm as in Comparative Example 2, the emission spectrum distribution is significantly different from the PL spectrum distribution of the blue light emitting dopant. This is because the optical distance from the light emitting interface to the Al electrode deviates from an odd multiple of λ _p / 4 (peak wavelength of emitted light λ _p = 450 nm). Similarly, in Comparative Example 3, the optical distance from the light emitting interface to the Al electrode is shifted from an odd multiple of λ _p / 4 (peak wavelength of emitted light λ _p = 450 nm). What is the distribution of the PL spectrum of the blue dopant? It is different.

  7 shows the emission spectrum of Example 1, FIG. 8 shows the emission spectrum of Example 4, FIG. 9 shows the emission spectrum of Example 6, and FIG. 10 shows the emission spectrum of Example 9.

  The emission spectrum distribution of the organic EL element of Example 1 shown in FIG. 7 is greatly different from the PL spectrum of the blue light-emitting dopant due to the influence of interference, but as in Examples 2 to 9, the first buffer ( When the optical distance to the Al reflecting mirror at the light emitting interface is increased by increasing the thickness of the (SiO) layer, the emission spectrum distribution gradually approaches the PL spectrum distribution of the blue light emitting dopant as shown in FIGS. go.

Here, the concept of “degree of modulation” was introduced as a method for objectively expressing the degree of approximation of the emission spectrum distribution. First, at each wavelength, the modulation spectrum distribution is calculated by dividing the relative luminous intensity of the emission spectrum distribution of the light to be compared by the relative luminous intensity of the emission spectrum distribution of the reference light. Then, in the modulation spectrum distribution, the “degree of modulation (V)” is calculated from the maximum value (I _MAX ) and the minimum value (I _MIN ) existing within the light emission wavelength range for which the degree of approximation is to be obtained using the following equation. Is.
V = (I _{MAX -IMIN} ) / (I _MAX + I _MIN )

  FIG. 11 shows, as an example, a modulation spectrum distribution when the light emitted from the organic EL element of Example 1 is used as comparative light and the light emitted from the blue light emitting dopant is used as reference light. Then, the degree of modulation is calculated from this modulation spectrum distribution and the above equation.

  FIG. 12 shows the light emitted from the organic EL elements of Examples 1 to 9 and Comparative Example 2 as the comparative light, and the light emitted from the blue light emitting dopant as the reference light. This shows the relationship between the “optical distance from the light emitting interface to the reflecting mirror” and the “degree of modulation” in the organic EL element. The degree of modulation in an organic EL element having an optical distance from the light emitting interface to the cathode (reflection mirror) of about several tens to several hundreds of nanometers is a value close to 1. For example, the degree of modulation in the organic EL element of Comparative Example 2 was 0.94. On the other hand, the degree of modulation at a value (4218 nm) equivalent to the coherence distance Lc of the light emitted from the blue light emitting dopant indicated by the dotted line in the figure is about 0.36, although some interference occurs. It was found that the effect was greatly reduced. Therefore, in order to minimize the influence of interference in the organic EL element, it has been confirmed that the “degree of modulation” required by the above method is about 0.36 or less as one criterion.

  In addition, the organic EL element is characterized by being a self-luminous element, and thus the emission color is one of the important requirements. FIG. 13 and FIG. 14 show the relationship between the “optical distance from the light emitting interface to the reflecting mirror” and the “chromaticity” of the emitted light in the organic EL elements of Examples 1 to 9 and Comparative Examples 1 to 4. 13 shows chromaticity coordinates (x), and FIG. 14 shows chromaticity coordinates (y).

  In an organic EL element having an optical distance from the light emitting interface to the cathode (reflection mirror) of about several tens to several hundreds of nm as in Comparative Examples 1 to 3, the chromaticity of the emitted light (even when the film thickness is slightly changed) It can be seen that x, y) changes significantly. In contrast, the optical distance from the light emitting interface to the cathode (reflection mirror) is increased as in Examples 1 to 9, and the coherence distance Lc of the light emitted by the blue light emitting dopant indicated by the dotted line in the figure is It can be seen that when the value is equal to or greater than 4218 nm, the difference with respect to the chromaticity of light originally emitted by the blue light-emitting dopant (during PL light emission) converges to 0.01 or less for both x and y.

  In other words, the greater the optical distance from the light emitting interface to the cathode (reflection mirror), the greater the effect of suppressing interference. However, even when the distance is small, the configuration of the present invention makes interference more effective than the configuration of the comparative example. It can be confirmed that there is an effect of suppressing the above. In particular, by making the optical distance equal to or greater than the coherence distance Lc of light emitted by the blue light emitting dopant (4218 nm), the chromaticity of the light originally emitted by the blue light emitting dopant can be substantially reproduced, and the film thickness can be increased. It can be seen that the chromaticity hardly changes even if it changes slightly.

  Up to this point, the characteristics of an organic EL device having an organic light emitting layer to which a blue light emitting dopant is added have been described. However, a green light emitting dopant and a red light emitting dopant that emit light having a PL spectrum distribution as shown in FIG. 15 are added. The characteristics of the organic EL device having the organic light emitting layer were also confirmed. In the structure similar to the organic EL element of Examples 1-9, the dopant added to the organic light emitting layer is a green light emitting dopant and a red light emitting dopant, and a value equivalent to the coherence distance Lc of light emitted from each dopant ( The degree of modulation at 4992 nm and 5362 nm) is shown in Table 2 below. The same results as in the case of the organic EL element emitting blue light were confirmed in the organic EL elements emitting light other than blue.

FIG. 16 shows emission spectra of light emitted from the glass substrates of the organic EL elements of Comparative Example 4 and Example 10. The difference between the structures of Comparative Example 4 and Example 10 is that Comparative Example 4 has an N ₂ atmosphere on the IZO transparent electrode having a refractive index of 1.95 with respect to light having a wavelength of 450 nm. No. 10 has a structure in which an oil layer having a refractive index of 1.55 with respect to light having a wavelength of 450 nm is provided on the IZO transparent electrode in a thickness of about 0.5 mm.

  Therefore, the refractive index difference between the IZO transparent electrode and the N2 atmosphere in Comparative Example 4 is 0.95, and the refractive index difference between the IZO transparent electrode and the oil layer in Example 10 is 0.4. By the way, comparing the emission spectrum distributions of Comparative Example 4 and Example 10, the emission spectrum distribution of Comparative Example 4 is different from the spectrum distribution of the light emitted by the blue light emitting dopant shown in FIG. In contrast, the emission spectrum distribution of Example 10 was almost the same as the spectrum distribution of the light emitted by the blue light-emitting dopant.

  Thus, it was confirmed that even if the refractive index difference between the two layers forming the boundary plane is about 0.95, it causes interference. At the same time, it was confirmed that the interference problem can be solved by reducing the difference in refractive index.

FIG. 17 shows an emission spectrum of light emitted from the glass substrate of the organic EL elements of Example 11 and Example 12. The difference between the structures of Example 11 and Example 12 is that a reflecting mirror having Ag deposited on the substrate is arranged so that the reflecting surface faces the IZO transparent substrate, and the outer periphery of the gap between the reflecting mirror and the IZO transparent electrode is 18 μm. In the case of the organic EL device of Example 11, this is a structure in which a gap surrounded by a reflective mirror, an IZO transparent electrode, and a sealing agent is formed by applying and fixing and fixing a UV curable sealing agent in which a gap agent is dispersed. The gap is filled with N ₂ gas, and the organic EL device of Example 11 has a structure filled with oil having a refractive index of 1.55 with respect to light having a wavelength of 450 nm.

Emission spectrum distribution (FIG. 16) of Comparative Example 4 having a structure in which the top of the IZO transparent electrode is simply an N ₂ atmosphere, and a structure in which a reflection mirror having an N ₂ atmosphere on the IZO transparent electrode and vapor-deposited Ag thereon is provided. When the emission spectrum distribution of Example 11 is compared, the emission spectrum distribution of Example 11 is closer to the PL spectrum distribution of the blue-emitting dopant. That is, it was found that the influence of interference can be suppressed by the reflection mirror provided so as to face the IZO transparent electrode.

  Furthermore, the emission spectrum distribution of Example 12 in which the gap between the IZO transparent electrode and the reflection mirror is filled with oil having a refractive index of 1.55 with respect to light having a wavelength of 450 nm is the PL spectrum distribution of the blue light-emitting dopant. It is almost equivalent.

  18 and 19 each have a structure in which two light emitting portions of a light emitting unit having an organic light emitting layer co-deposited with a blue light emitting dopant and a light emitting unit having an organic light emitting layer co-deposited with a yellow light emitting dopant are present in a discrete manner. It is an emission spectrum of Comparative Example 5 and Example 13 which are organic EL elements.

  The difference between the structures of Comparative Example 5 and Example 13 is that Comparative Example 5 has a structure in which Al serving as a cathode is directly formed on the electron injection layer to a film thickness of 200 nm, whereas that of Example 13 is different. In this case, an IZO transparent electrode is formed on the electron injection layer, and a reflecting mirror having a vapor-deposited Ag is disposed on the substrate so that the reflecting surface faces the IZO transparent substrate, and a gap between the reflecting mirror and the IZO transparent electrode is formed. A UV curable sealant in which a gap agent of 18 μm is dispersed is applied to the outer periphery to adhere and fix, and a refractive index with respect to light having a wavelength of 450 nm is formed in a space surrounded by the reflection mirror, the ITO transparent electrode, and the sealant. It has a structure filled with 1.6 oil.

  The emission spectrum of the organic EL device having the structure of Comparative Example 5 shown in FIG. 18 shows the PL spectrum distribution of the blue light emitting dopant and the yellow light emitting dopant when the light emitting surface of the organic EL device is viewed from the normal direction (0 deg) of the light emitting surface. Both PL spectral distributions of the above are not reproduced, and the spectral distribution changes remarkably depending on the viewing angle (intersection angles with respect to the normal of the light emitting surface are 30 deg and 60 deg).

  On the other hand, the emission spectrum of the organic EL device having the structure of Example 13 shown in FIG. 19 shows the PL spectrum distribution of the blue emission dopant and the PL spectrum distribution of the yellow emission dopant when the emission surface is viewed from the normal direction as described above. Both are substantially reproduced, and even if the viewing angle changes, there is almost no change in the spectral distribution.

  As a result, in the organic EL device of the present invention, the structure of the organic EL device has a plurality of light emitting units in which the organic layer sandwiched between two opposing transparent electrodes includes at least one light emitting layer. Thus, it was found that even in the organic EL element in which each light emitting unit is partitioned by at least one charge generation layer, the same effect as that of the organic EL element having one light emitting layer can be obtained.

Here, the effect concerning embodiment of this invention is demonstrated. First, an organic EL element structure is formed by sequentially forming each layer of a first transparent electrode layer (anode), a hole transport layer, an organic light emitting layer and a second transparent electrode layer (cathode) on a transparent substrate, and further a second transparent A reflection mirror was placed outside the electrode layer (cathode) at a predetermined distance. The position of the reflecting mirror was set so that the optical distance from the light emitting interface of the organic light emitting layer to the reflecting mirror was not less than the coherence distance Lc of the light emitted from the light emitting layer. Note that the coherence distance Lc is calculated by an equation of Lc = λ _p ² / Δλ (where λ _p is the peak wavelength of the emission spectrum and Δλ is the half width of the spectrum).

  By making the organic EL element have the above structure, the light emitted from the light emitting interface of the organic light emitting layer and reflected inside the organic EL element can be prevented from causing interference.

  As a result, the light emitted from the light emitting interface of the organic light emitting layer and reflected inside the organic EL element can be effectively used, and the light from which the influence of interference is eliminated can be efficiently extracted to the outside.

  In addition, it is no longer necessary to take into account the influence of interference when setting the film thickness of each transparent electrode layer and each organic layer constituting the organic EL element. Therefore, the film thickness can be set with an emphasis only on the carrier transport efficiency, recombination and light emission efficiency, and the film thickness can be optimized.

  Further, even if the film thickness changes, the emission spectrum distribution does not change, and the color tone of the light emitted from the organic EL element does not change.

  Further, even if the viewing angle with respect to the light emitting surface of the organic EL element is changed, there is almost no change in the spectral distribution, and there is no change in color tone when the light emitted from the organic EL element is viewed from any direction.

  Furthermore, a first transparent electrode layer (anode) is formed on a transparent substrate, and a plurality of light emitting units including a light emitting layer are provided on the transparent substrate, with a charge generation layer interposed therebetween, and a second transparent electrode formed thereon It was confirmed that the organic EL element of another embodiment of the present invention having a structure in which a reflecting mirror was arranged at a predetermined distance above the layer (cathode) had the same effect as described above.

Next, Examples 14 to 20 and Comparative Example 6 relating to the organic EL element of the present invention will be described. First, in Example 14, in place of the N ₂ gas in Comparative Example 4, the refractive index is in a gap of 18 μm formed of an IZO transparent electrode and a sealing glass substrate disposed so as to face the IZO transparent electrode. It has a structure filled with 1.55 oil. The 18 μm gap between the IZO transparent electrode and the sealing glass substrate is a UV curable sealant in which a gap agent is dispersed in a glass substrate on which an element is formed and a sealing glass substrate disposed so as to seal the element. Is secured by bonding and fixing.

  In Example 15, the gap between the IZO transparent electrode and the sealing glass substrate was filled with oil having a refractive index of 1.33. The other conditions were the same as in Example 14.

  In Example 16, the gap between the IZO transparent electrode and the sealing glass substrate was filled with oil having a refractive index of 1.37. The other conditions were the same as in Example 14.

  In Example 17, the gap between the IZO transparent electrode and the sealing glass substrate was filled with oil having a refractive index of 1.43. The other conditions were the same as in Example 14.

In Example 18, instead of the N ₂ gas of Comparative Example 5, the refractive index is 1. in a gap of 18 μm formed of an IZO transparent electrode and a sealing glass substrate disposed so as to face the IZO transparent electrode. It has a structure filled with 55 oil. The 18 μm gap between the IZO transparent electrode and the sealing glass substrate is a UV curable sealant in which a gap agent is dispersed in a glass substrate on which an element is formed and a sealing glass substrate disposed so as to seal the element. Is secured by bonding and fixing.

  In Example 19, the gap between the IZO transparent electrode and the sealing glass substrate was filled with oil having a refractive index of 1.33. The other conditions were the same as in Example 18.

  In Example 20, the gap between the IZO transparent electrode and the sealing glass substrate was filled with oil having a refractive index of 1.43. The other conditions were the same as in Example 18.

Next, Comparative Example 6 will be described. In the structure of Comparative Example 6, an ITO transparent electrode was formed by sputtering on a glass substrate having a refractive index of 1.53 with respect to light having a wavelength of 560 nm to be a transparent substrate. The film thickness of ITO is 125 nm, the sheet resistance is 10Ω / □, and the refractive index is 1.90 for light having a wavelength of 560 nm. Then, the ITO transparent electrode was etched into a predetermined shape, subjected to ultrasonic cleaning with acetone and isopropyl alcohol, and then dried. Further, after cleaning with UV-O ₃ , it was set in a vacuum vapor deposition tank and the pressure in the tank was reduced to about 1 × 10 ⁻⁵ Torr, and a hole transport layer was deposited on the ITO transparent electrode to a film thickness of 100 nm. Next, an organic light-emitting layer to which a yellow light-emitting dopant that emits light having an emission spectrum shown in FIG. 22 was added at a concentration of 1% by weight was co-evaporated to a thickness of 80 nm. Further, an electron injection layer was deposited on the electrode in a small amount, and IZO was deposited thereon to a thickness of 50 nm as a cathode. The refractive index of the organic light emitting layer was 1.75 with respect to light having a wavelength of 560 nm, and the refractive index of the IZO transparent electrode was 1.90 with respect to light having a wavelength of 560 nm. An N ₂ atmosphere was formed on the IZO surface.

  As described above, Examples 14 to 20 and Comparative Example 6 related to the organic EL element of the present invention have been described. Hereinafter, these characteristics will be described together with Comparative Example 4.

FIG. 23 shows emission spectra of light emitted from the glass substrates of the organic EL elements of Comparative Example 4 and Example 14. The difference between the structures of Comparative Example 4 and Example 14 is that Comparative Example 4 has an N ₂ atmosphere above the IZO transparent electrode having a refractive index of 1.95 with respect to light having a wavelength of 450 nm. No. 14 has a structure in which an oil layer having a refractive index of 1.55 is provided for light having a wavelength of 450 nm on the IZO transparent electrode.

Therefore, the refractive index difference between the IZO transparent electrode and the N ₂ atmosphere in Comparative Example 4 is 0.95, and the refractive index difference between the IZO transparent electrode and the oil layer in Example 14 is 0.4. By the way, comparing the emission spectrum distribution of Comparative Example 4 and Example 14, the emission spectrum distribution of Comparative Example 4 is different from the spectrum distribution of the light emitted by the blue light emitting dopant shown in FIG. This optical distance from the emission interface is the refractive index difference between the IZO and _{N 2} atmosphere in a range of less than the coherent distance Lc obtained from the PL spectrum of the blue light-emitting dopant in FIG. 5 (about 4.2 .mu.m) 0.95 It is considered that the reflection surface is greatly affected by interference.

On the other hand, the emission spectrum distribution of Example 14 was almost the same as the spectrum distribution of the light emitted by the blue light emitting dopant shown in FIG. This is considered to be because oil of refractive index 1.55 was filled instead of the N ₂ atmosphere of Comparative Example 4 and the refractive index difference from IZO was set to 0.4.

  From the above results, it was found that even when the value of the refractive index step existing at an optical distance from the light emitting interface less than the coherent distance Lc (about 4.2 μm) is about 0.95, it causes interference. It was also found that the influence of interference on light emission can be suppressed by making the value of the refractive index step as small as possible.

  FIG. 24 shows the color of light emitted from the glass substrate side with respect to the refractive index difference between the IZO transparent electrode and the material forming the interface with the IZO transparent electrode in Examples 14 to 17 and Comparative Example 4. Degrees (x, y) are shown. As the refractive index difference becomes smaller, the influence of interference on the light emitted from the glass substrate side becomes smaller, and the chromaticity (x, y) of the light emitted from the glass substrate side is PL specific to the blue light emitting dopant. It can be seen that the chromaticity (x, y) obtained from the spectrum approaches.

  When the refractive index difference is 0.6 or less, the deviation from the chromaticity (x, y) of the blue light emitting dopant is 0.01 or less in both x and y, and the chromaticity deviation is reduced to a region that cannot be identified by human color discrimination ability. It turned out that it becomes possible to suppress.

  From the above results, even if the refractive index difference is about 0.95, the distance from the light emitting interface of the organic light emitting layer is an optical distance that is less than the coherence distance Lc of light emitted from the light emitting interface. If present, it was found to cause interference. It was also found that the influence of interference can be suppressed by reducing this refractive index difference.

  In addition, the above results show that in the light emission of the organic EL element having the reflection mirror, the original color of the blue light-emitting dopant can be obtained even if the optical distance from the emission interface to the reflection mirror is a value equal to or greater than the “coherence distance Lc” (4.2 μm). The degree may not be reproduced. That is, if an optical distance from the light emitting interface is less than the coherent distance (4.2 μm) between the light emitting interface and the reflecting mirror and there is an interface with a large refractive index step, this interface acts as a reflecting surface, and the glass substrate surface It will affect the light emission from. Therefore, in order to reproduce the original chromaticity of the blue light-emitting dopant in the light emission of the element having the reflection mirror, the optical distance from the light emission interface to the reflection mirror is set to “coherence distance Lc” (4.2 μm) or more. It is necessary to make the refractive index step existing at an optical distance from the light emitting interface less than the coherence distance (4.2 μm) as small as possible (at least adjusting the refractive index step to 0.6 or less). Thereby, it was found that the deviation from the original chromaticity of the blue dopant can be within 0.01 in both x and y.

FIG. 25 shows emission spectra of light emitted from the glass substrates of the organic EL elements of Comparative Example 6 and Example 18. The difference between the structures of Comparative Example 6 and Example 18 is that Comparative Example 6 has an N ₂ atmosphere on the IZO transparent electrode having a refractive index of 1.90 with respect to light having a wavelength of 560 nm. No. 18 has a structure in which an oil layer having a refractive index of 1.55 is provided for light having a wavelength of 560 nm on the IZO transparent electrode.

Therefore, the refractive index difference between the IZO transparent electrode and the N ₂ atmosphere in Comparative Example 6 is 0.90, and the refractive index difference between the IZO transparent electrode and the oil layer in Example 18 is 0.35. By the way, comparing the emission spectrum distribution of Comparative Example 6 and Example 18, the emission spectrum distribution of Comparative Example 6 is different from the spectrum distribution of the light emitted by the yellow light-emitting dopant shown in FIG. This optical distance from the emission interface is the refractive index difference between the IZO and _{N 2} atmosphere in a range of less than the coherent distance Lc obtained from the PL spectrum of the yellow emitting dopant in FIG 22 (about 4.8 .mu.m) 0.90 It is considered that the reflection surface is greatly affected by interference.

In contrast, the emission spectrum distribution of Example 18 was almost the same as the spectrum distribution of the light emitted by the yellow light-emitting dopant shown in FIG. This is considered to be because oil of refractive index 1.55 was filled instead of the N ₂ atmosphere of Comparative Example 6 and the refractive index difference from IZO was set to 0.35.

  From the above results, it was found that even if the value of the refractive index step existing at a distance from the light emitting interface less than the coherent distance Lc (about 4.8 μm) is about 0.90, it causes interference. It was also found that the influence of interference on light emission can be suppressed by making the value of the refractive index step as small as possible.

  FIG. 26 shows the color of light emitted from the glass substrate side with respect to the refractive index difference between the IZO transparent electrode and the material forming the boundary surface with the IZO transparent electrode in Examples 18 to 20 and Comparative Example 6. Degrees (x, y) are shown. As the refractive index difference becomes smaller, the influence of interference on the light emitted from the glass substrate side becomes smaller, and the chromaticity (x, y) of the light emitted from the glass substrate side is PL specific to the yellow light emitting dopant. It can be seen that the chromaticity (x, y) obtained from the spectrum approaches.

  When the refractive index difference is 0.6 or less, the deviation from the chromaticity (x, y) of the yellow light-emitting dopant becomes 0.01 or less for both x and y, and the chromaticity deviation is reduced to a region that cannot be identified by human color discrimination ability. It turned out that it becomes possible to suppress.

  From the above results, even if the refractive index difference is about 0.90, the distance from the light emitting interface of the organic light emitting layer is an optical distance that is less than the coherence distance Lc of light emitted from the light emitting interface. If present, it was found to cause interference. It was also found that the influence of interference can be suppressed by reducing this refractive index difference.

In addition, the above results show that in the light emission of the organic EL element having the reflection mirror, the original color of the yellow light-emitting dopant can be obtained even if the optical distance from the light emission interface to the reflection mirror is set to a value equal to or greater than the “coherence distance Lc” (4.8 μm) The degree may not be reproduced. That is, if an optical distance from the light emitting interface is less than the coherence distance (4.8 μm) between the light emitting interface and the reflecting mirror and there is a large refractive index step, this interface acts as a reflecting surface, and the glass substrate surface It will affect the light emission from. Therefore, in order to reproduce the original chromaticity of the yellow light emitting dopant in the light emission of the element having the reflection mirror, the optical distance from the light emission interface to the reflection mirror is set to “coherence distance Lc” (4.8 μm) or more. It is necessary to reduce the existing refractive index step as much as possible (adjust the refractive index step to 0.6 or less) because the optical distance from the light emitting interface is less than the coherence distance (4.8 μm). Thereby, it turned out that the shift | offset | difference with respect to the original chromaticity of a yellow dopant can be less than 0.01 in both x and y.

  When the optical distance from the light emitting interface to the refractive index step is compared as an optical distance of substantially the same level, the chromaticity shift can be reduced even if the refractive index step is slightly larger as the emission wavelength is longer. In addition, human eyes can detect even a slight chromaticity difference near blue, but cannot detect a large chromaticity difference near green. Even yellow and red cannot be detected unless there is a greater chromaticity difference than blue. Therefore, when there is a refractive index step within the range where the optical distance from the light emitting interface is less than the coherence distance, the step is set to 0.6 or less, and the chromaticity shift is suppressed to 0.01 or less. Also in the color tone, it is possible to obtain light emission of a color tone that can be recognized as being equivalent to the original emission color of the light emitting material.

  Next, another embodiment according to the organic EL element of the present invention will be described. An EL device comprising a transparent substrate / first transparent substrate / a plurality of thin film layers including a light emitting layer / a reflective electrode, and having a second buffer layer formed between the light emitting layer and the reflective electrode. The optical distance from the light emitting interface of the light emitting layer to the reflective electrode is set to be equal to or greater than the coherence distance of light emitted from the light emitting interface, and the optical distance from the light emitting interface of the light emitting layer is allowed to be emitted from the light emitting interface. Within a range shorter than the interference distance, the refractive index difference of all two adjacent layers excluding the reflective electrode is set to 0.6 or less.

  FIG. 27 is a cross-sectional view showing one of the structures of this embodiment. In the present embodiment, a plurality of MPE structures in which a plurality of light emission positions are separated from each other (an organic layer sandwiched between a pair of opposed electrodes has at least one light emission unit including at least one light emission layer) Is a structure in which each light emitting unit is partitioned by at least one charge generation layer). In addition, various combinations of light emitted from each light emitting unit are conceivable, such as a stack of monochromatic light emitting units of the same light emitting color, a stack of monochromatic light emitting units of different light emitting colors, and a stack of mixed color light emitting units such as white. Can be considered.

  The specific structure of the organic EL element shown in FIG. 27 is that a transparent electrode layer (anode) is formed on a transparent substrate, and a first light emitting unit, a first charge generating layer, A second light emitting unit including a second light emitting interface, a second charge generation layer, a third light emitting unit including a third light emitting interface, and a second buffer layer are sequentially formed. Furthermore, an electrode (reflecting electrode (cathode)) also serving as a reflecting mirror is disposed outside the second buffer layer so that the reflecting surface faces the third light emitting unit.

  Then, a second buffer layer is formed between the light emitting unit (third light emitting unit) in the final stack and the reflective electrode so that the optical distance from the light emitting interface of the light emitting layer to the reflective electrode is equal to or greater than the coherence distance, and the light emitting interface. The refractive index difference of all the interfaces existing from to the reflective electrode was made to be 0.6 or less.

Therefore, in manufacturing the element, it is possible to configure the light emitting unit with the optimum film thickness of the carrier balance without depending on the influence of optical interference. The film thickness adjustment in the conventional technique is performed such that the optical distance from each light emitting interface to the cathode is an odd multiple of λ / 4 by using the electron transport layer film thickness and the transparent electrode film thickness, and the conditions for strengthening the optical interference effect are used. In other words, it is possible to obtain emitted light substantially the same as the peak wavelength of light emitted from the light emitting layer.
In particular, in manufacturing an MPE element, even if a light emitting unit configured with an optimum film thickness is simply stacked and converted to MPE, the change in optical interference conditions accompanying MPE is corrected by the second buffer layer. This eliminates the need to adjust the optical length from each light emitting interface to the cathode to an odd multiple of λ / 4. That is, it is possible to stack light emitting units having optimum carrier balance characteristics while maintaining the original light emission color of a desired material, and it is possible to manufacture an ideal MPE element with higher efficiency.

The second buffer layer plays the same role as the first buffer layer described above, which adjusts the optical distance from each light emitting interface to the cathode to a coherent distance or more without affecting the carrier balance. It differs from one buffer layer in that an insulator cannot be used.
In addition, when a film is formed on the upper surface of the electron transport layer by sputtering or the like in order to form a transparent electrode such as IZO on the cathode, the electron transport layer is damaged and the device is deteriorated. However, in the organic EL element of this configuration, the second buffer layer on the electron transport layer also plays a role of protecting the electron transport layer from damage during sputtering film formation.

The material used for the second buffer layer is required not to absorb light in the visible light region, to have high conductivity (particularly, electron transport properties), and to be a non-light emitting material. Specifically, a transparent electrode material typified by IZO, ZnO or the like, a charge generation material (CGL material) typified by V ₂ O ₅ or the like (note that the charge generation material used here is OPC, etc. As described in "Japanese Patent Laid-Open No. 2003-272860", a plurality of light emission existing between the facing anode electrode and the cathode electrode is not generated by photoexcitation as used in Since the unit has a structure in which the unit is partitioned and stacked by a charge generation layer having a specific resistance of 1.0 × 10 ² Ω · cm or more, it is as if a plurality of units are applied when a predetermined voltage is applied between both electrodes. These light emitting units can emit light simultaneously such that they are electrically connected in series.), And a thin film multilayer film obtained by mixing or stacking them is a candidate. Preferable examples include a laminated film (Li / CGL / Li / CGL /.../ Li / CGL) of the aforementioned CGL material and a low work function metal (Cs, Li, etc.).

In addition, as the second buffer layer, an electron transport material or a hole block material can be considered as candidates. In these cases, since these materials that are generally used have a remarkable increase in driving voltage, an electron transport material or a hole block material having a high mobility of a specific resistance of 1.0 × 10 ² Ω · cm or more is preferable.

  Further, the second buffer layer is not a layer independent of the cathode and the electron transport layer as in the above structure, but may be a layer sharing these. That is, it is a case where it becomes an electron transport layer / second buffer layer or a cathode / second buffer layer. As described above, a material having a high mobility is preferable for the electron transport layer / second buffer layer. In any case, unless the second buffer layer also functions as the other layers, it is desirable that the second buffer layer has no optical or electrical influence on the light emitting unit other than eliminating the influence of optical interference. .

  FIG. 28 shows an emission spectrum of a blue organic EL element of one light emitting unit, a blue organic EL element of three light emitting units, and a blue organic EL element of three light emitting units into which a second buffer layer is introduced. The vertical axis represents the EL intensity of each element, and the horizontal axis represents the emission wavelength. The CIE chromaticity of each element was (0.15, 0.14), (0.14, 0.23) (0.15, 0.15), respectively.

The element that emits light of the emission spectrum (same as the PL spectrum) indicated by the black line in the figure is a transparent electrode (material: ITO * film thickness: 160 nm) / hole injection layer (starburst material * 60 nm) / hole. It is a one light emitting unit blue organic EL element composed of a transport layer (α-NPD * 20 nm) / blue light emitting part (35 nm) / electron transport layer (Alq ₃ * 40 nm) / cathode (Al * 100 nm). The unit (nm) indicates the unit thickness of the layer structure, and the unit of the optical film thickness for Lc and the like.

An element that emits light having an emission spectrum indicated by continuous black triangles in the figure is an MPE formed by simply laminating the one light-emitting unit blue organic EL element, and its structure is a transparent electrode (ITO * 160 nm) / hole injection layer (starburst material * 60 nm) / hole transport layer (α-NPD * 20 nm) / blue light emitting part (35 nm) / electron transport layer (Alq ₃ * 40 nm) / charge generation layer (five Vanadium oxide * 30 nm) / hole injection layer (starburst material * 60 nm) / hole transport layer (α-NPD * 20 nm) / blue light emitting part (35 nm) / electron transport layer (Alq ₃ * 40 nm) / charge generation Layer (vanadium pentoxide * 30 nm) / hole injection layer (starburst material * 60 nm) / hole transport layer (α-NPD * 20 nm) / blue light emitting part (35 nm) / electron transport layer ( It is a blue organic EL element composed of Alq ₃ * 40 nm) / cathode (Al * 100 nm).

In the figure, an element that emits light having an emission spectrum indicated by continuous white circles is formed by laminating the above-described one light emitting unit blue organic EL element into MPE, and further a second buffer layer (refractive index for light having a wavelength of 450 nm). 1.95) is inserted at 5000 nm, and the structure is transparent electrode (ITO * 160 nm) / hole injection layer (starburst material * 60 nm) / hole transport layer (α-NPD * 20 nm) / blue Light emitting part (35 nm) / electron transport layer (Alq ₃ * 40 nm) / charge generation layer (vanadium pentoxide * 30 nm) / hole injection layer (starburst material * 60 nm) / hole transport layer (α-NPD * 20 nm) ) / blue light emitting unit (35 nm) / electron-transporting layer _(Alq 3 * 40 nm) / charge generation layer (vanadium pentoxide * 30 nm) / hole injection layer (starburst material * 6 nm) / hole transport layer (α-NPD * 20nm) / blue light emitting portion from (35 nm) / electron-transporting layer _(Alq 3 * 40nm) / second buffer layer (zinc oxide * 5000 nm) / cathode (Al * 100 nm) This is a blue organic EL element.

The blue organic EL element of 3 light emitting units (black triangle mark in the figure) shows a different emission spectrum shape that is greatly affected by interference compared to the blue organic EL element of 1 light emitting unit (black line in the figure) In addition, the chromaticity was greatly different.
On the other hand, the organic EL element of the three light-emitting units (a white circle in the figure) in which the second buffer layer is added to the element configuration of the blue organic EL element of the three light-emitting units is a blue organic EL element (black in the figure) of the one light-emitting unit. The emission spectrum shape was different from that of the line, but almost the same chromaticity. In other words, it can be seen from the spectrum distribution that the optical interference has not been completely eliminated, but with respect to CIE chromaticity, a blue light emitting color comparable to that of a blue organic EL element (black line in the figure) consisting of one light emitting unit. Obtained.
That is, by forming a blue organic EL element of three light emitting units into which a second buffer layer has been introduced, three layers of one light emitting unit blue organic EL element are simply laminated to indicate MPE, which are indicated by continuous black triangle marks in the figure. Compared to the blue-green light emission of the blue organic EL element that emits light having a light emission spectrum, it can be used as an independent monochromatic light-emitting element.

  There are two main methods for adjusting the emission color when one light emitting unit is stacked to form an MPE element, depending on the “optical distance from the light emitting interface to the reflecting mirror”. In the organic EL element having a plurality of light emitting units, by making the value of “optical distance from the light emitting interface to the reflecting mirror” equal to or greater than the coherence distance Lc of the emitted light, the influence of interference is eliminated, The CIE chromaticity of the emission color can be made substantially the same as the emission of the organic EL element of one light emitting unit. In addition, by setting the value of the “optical distance from the light emitting interface to the reflecting mirror” to 2Lc or more, the influence of interference is completely eliminated, and not only the CIE chromaticity of the emitted color but also the shape of the emission spectrum. Also, it can be made substantially the same as the light emission of the organic EL element of one light emitting unit.

  In this embodiment, the refractive index of any organic layer constituting the element is 1.8 with respect to light having a wavelength of 450 nm, and there is a difference in refractive index at the interface other than the organic layer / cathode or the second buffer layer / cathode. There are no interfaces greater than 0.6. The optical distance from the light emitting part to the reflecting mirror / electrode (reflecting electrode) is 5075 nm for the MPE element having the second buffer layer, and for the MPE element simply having three layers laminated without the second buffer layer. 75 nm. Since Lc is calculated to be 4218 nm and 2Lc is 8436 from the PL spectrum of the blue light-emitting dopant, the MPE element having the second buffer layer is in a state of not less than Lc and less than 2Lc in the above conditions. That is, this condition is a condition in which only the chromaticity coincides with the light emitting material emission, and only the MPE element having the second buffer layer has the same chromaticity as the PL of the light emitting material.

  From the above results, in the MPE element of the organic LED element, in the MPE element having a structure in which one light emitting unit element is simply stacked and the second buffer layer is inserted without changing the film thickness of each light emitting unit to be stacked. It can also be seen that the emission color similar to that of one light emitting unit element can be obtained.

  Further, the same effect can be expected even when the second buffer layer / cathode in the present embodiment is configured as a thick film transparent cathode / reflection mirror. Specifically, IZO (5000 nm) / Al or the like. That is, the second buffer layer may be provided on the light emitting layer side from the transparent cathode.

It is a schematic sectional drawing which shows the organic EL element of embodiment concerning this invention. It is a wave form diagram which shows the damped oscillation of the light wave emitted from a light-emitting body. It is a graph showing the photoluminescent (PL) spectra of Alq _3. It is a schematic sectional drawing which shows the organic EL element of other embodiment concerning this invention. It is a graph which shows PL spectrum of a blue light emission dopant. It is a graph which shows the PL spectrum of Comparative Examples 1-3. 3 is a graph showing a PL spectrum of Example 1. It is a graph which shows PL spectrum of Example 4. 10 is a graph showing a PL spectrum of Example 6. 10 is a graph showing a PL spectrum of Example 9. 3 is a graph showing a modulation spectrum of Example 1. It is a graph which shows the relationship between the optical distance from the light emission interface of Examples 1-9 and Comparative Example 2 to a reflective mirror, and the degree of modulation. It is a graph which shows the relationship between the optical distance from the light emission interface of Examples 1-9 and Comparative Examples 1-4 to a reflective mirror, and the chromaticity coordinate (x) of emitted light. It is a graph which shows the relationship between the optical distance from the light emission interface of Examples 1-9 and Comparative Examples 1-4 to a reflective mirror, and the chromaticity coordinate (y) of emitted light. It is a graph which shows the PL spectrum of a green light emission dopant and a red light emission dopant. It is a graph which shows PL spectrum of Example 10 and Comparative Example 4. It is a graph which shows the PL spectrum of Example 11 and Example 12. FIG. 10 is a graph showing a PL spectrum of Comparative Example 5. 14 is a graph showing a PL spectrum of Example 13. It is a schematic structure figure of a general organic EL element. Similarly, it is a schematic structural diagram of a general organic EL element. It is a graph which shows PL spectrum of a yellow light emission dopant. It is a graph which shows PL spectrum of Example 14 and Comparative Example 4. It is a graph which shows the relationship between the refractive index level | step difference of Examples 14-17 and the comparative example 4, and the chromaticity coordinate of emitted light. It is a graph which shows PL spectrum of Example 18 and Comparative Example 6. It is a graph which shows the relationship between the refractive index level | step difference of Examples 18-20 and the comparative example 6, and the chromaticity coordinate of emitted light. It is a schematic sectional drawing which shows the organic EL element of other embodiment concerning this invention. It is a graph which shows each PL spectrum of the organic EL element which consists of three types of structures.

Claims (6)

An EL element in which a thin film multilayer structure including at least one light emitting layer is sandwiched between a pair of opposing electrodes,
The optical distance from the light emitting interface of the light emitting layer is expressed by the following formula:
Lc = λp ² / Δλ
(Where λp is the peak wavelength of the emission spectrum of light emitted from the light emitting layer, Δλ is the half width of the emission spectrum of light emitted from the light emitting layer)
In represented, a reflecting mirror disposed apart coherence length Lc or more of the light emitted from the light emitting surface,
In either direction of the reflecting mirror and the direction opposite to the reflecting mirror from the light emitting layer,
Within the range where the optical distance from the light emitting interface of the light emitting layer is less than the coherence distance of the light emitted from the light emitting interface, the refractive index difference of all two adjacent layers excluding the reflecting mirror is 0.6 or less. An EL element characterized by the above.   Each of the pair of electrodes is formed of a transparent electrode, and is in contact with the outer surface of one of the transparent electrodes or opposed to the transparent electrode at a certain distance from the outer surface. The EL element according to claim 1, further comprising a reflection mirror. The EL element according to claim 2 , wherein a buffer layer is formed on the outside of one of the transparent electrodes and / or between the transparent electrode and the reflecting mirror.   The EL device according to claim 3, wherein the buffer layer is made of one selected from the group consisting of a light transmissive material, a gas, and a vacuum.   2. The EL element according to claim 1, wherein one of the electrodes is a transparent electrode, the other is a reflective electrode, and the reflective electrode is the reflective mirror. The thin-film multilayer structure has a plurality of light emitting units including at least one layer of the light-emitting layer, it has a charge generation layer formed between the light emitting unit,
The reflection mirror is farthest from the electrode on the reflection mirror side among the positions where the coherence distance determined from the PL spectrum of the light emitted from each light emitting layer in each light emitting unit is maintained from each light emitting interface. Is disposed at a position further away from the light emitting layer, and in each direction from the light emitting layer to the reflecting mirror and the direction opposite to the reflecting mirror,
Two adjacent layers excluding the reflecting mirror are within the range where the optical distance from the light emitting interface of each light emitting layer is less than the coherence distance obtained from the PL spectrum of the light emitted from the light emitting surface of each light emitting layer. All the refractive index differences are 0.6 or less, The EL element of any one of Claims 1-5 characterized by the above-mentioned.

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