JP2016035830A - Organic electroluminescence element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an organic electroluminescence element that suppresses an optical loss caused by metal and is excellent in light extraction efficiency.SOLUTION: An organic electroluminescence element has a board 1, a light transmissible electrode 2, a light reflection electrode 4, and a light emission layer 3. The light reflection electrode 4 is formed of metal. The light emission layer 3 is disposed between the light transmissible electrode 2 and the light reflection electrode 4. The organic electroluminescence element has an optical adjusting layer 5 having lower refractivity than the light emission layer 3 and a charge transport layer 6 between the light emission layer 3 and the light reflection electrode 4.SELECTED DRAWING: Figure 1

Description

  An organic electroluminescent device is disclosed. More specifically, an organic electroluminescence element including a light reflective electrode made of metal is disclosed.

  As an organic electroluminescence element (hereinafter also referred to as “organic EL element”), a light emitting body comprising a laminate of an anode, a hole transport layer, a light emitting layer, an electron transport layer, and a cathode is provided on the surface of a substrate. Is generally known. In the organic EL element, by applying a voltage between the anode and the cathode, light emitted from the light emitting layer is extracted to the outside through an electrode having optical transparency.

  In an organic EL element, many attempts have been made to increase the light extraction efficiency. In the organic EL element, light generated in the light emitting layer may not be emitted to the outside due to total reflection due to a difference in refractive index, absorption of light by the material, and the like. Therefore, the efficiency of the organic EL element can be improved by improving the light extraction efficiency. For example, when a light-reflective electrode is provided, light emitted from the light-emitting layer can be reflected by the light-reflective electrode and travel to the opposite side (light-transmissive electrode side), collecting light in one direction. It becomes easy to emit. Thereby, the light extraction efficiency can be increased.

  Here, there is a light loss due to metal as a cause of a decrease in light extraction efficiency of the organic EL element. Light loss is seen when the light reflective electrode is made of metal. One of the causes of light loss is surface plasmon. When light emitted from the light emitting layer reaches the metal, plasma vibration is induced on the surface of the metal. This plasma oscillation is called surface plasmon (or simply plasmon). In surface plasmons, light energy is converted into vibration energy on the metal surface, and light is lost due to this energy transfer. In addition, light absorption is one of the causes of light loss. Light is lost when the metal absorbs light. As described above, when a metal is used as a light reflective electrode, an improvement in light extraction efficiency due to light reflection can be expected, but light loss may occur. Therefore, a further structure that increases the light extraction efficiency is important.

  In Patent Document 1, in order to increase light extraction efficiency, attention is paid to surface plasmon, and the structure of the electrode on the light reflection side is a lamination of a light transmissive electrode, a layer having a low refractive index, and a metal layer having light reflectivity. An organic EL element having a structure is disclosed. This structure is expected to suppress light loss due to plasmons by providing a light-reflective metal layer separately from the electrodes. However, the laminated structure of the light transmissive electrode, the low refractive index layer, and the light reflective metal layer facilitates complicated interference action, so that the design of optical interference becomes complicated, and the light extraction efficiency is easy. May not be able to be increased. In addition, since this structure causes a complicated interference action, the reflectance at a specific angle is likely to be lowered, and there is a high possibility that uniform light emission with little angle dependency cannot be obtained.

JP 2013-145700 A

  An object of the present disclosure is to provide an organic electroluminescence device that suppresses light loss due to a metal constituting a light reflective electrode and has excellent light extraction efficiency.

  An organic electroluminescence element of the present disclosure includes a substrate, a light transmissive electrode, a light reflective electrode made of a metal, and a light emitting layer disposed between the light transmissive electrode and the light reflective electrode. I have. The organic electroluminescence element includes an optical adjustment layer having a refractive index lower than that of the light emitting layer and a charge transport layer between the light emitting layer and the light reflective electrode.

  The organic electroluminescence element according to the present disclosure can suppress light loss due to the metal constituting the light reflective electrode, and can improve the light extraction efficiency.

It is typical sectional drawing which shows an example of an organic electroluminescent element. It is typical sectional drawing which shows an example of an organic electroluminescent element. It is a graph which shows an example of the relationship between the refractive index of an optical adjustment layer, and the mode of light. It is typical sectional drawing which shows an example of an organic electroluminescent element. It is a graph which shows an example of the relationship between the incident angle of the light in an optical adjustment layer, and a reflectance. It is a graph which shows an example of the relationship between the thickness of an optical adjustment layer, and light reflectance. It is typical sectional drawing which shows an example of an organic electroluminescent element. It is typical sectional drawing which shows an example of an organic electroluminescent element. It is a graph which shows an example of the relationship between the wavelength of the light in a distributed Bragg reflection structure, and a reflectance. It is a graph which shows an example of the relationship between the incident angle of light and a reflectance. It is typical sectional drawing which shows an example of an organic electroluminescent element.

  An organic electroluminescence element (organic EL element) is disclosed. FIG. 1 shows an example of an organic EL element. The organic EL element includes a substrate 1, a light transmissive electrode 2, a light emitting layer 3, and a light reflective electrode 4. The light reflective electrode 4 is made of metal. The light emitting layer 3 is disposed between the light transmissive electrode 2 and the light reflective electrode 4. The organic EL element includes an optical adjustment layer 5 and a charge transport layer 6 between the light emitting layer 3 and the light reflective electrode 4. The optical adjustment layer 5 is a layer having a refractive index lower than that of the light emitting layer 3. The introduction of the optical adjustment layer 5 can suppress plasmons on the surface of the light reflective electrode 4 made of metal. Therefore, the light extraction efficiency is improved.

  In the form of FIG. 1, the substrate 1 supports an organic light emitter. The organic light emitter is a laminate including the light transmissive electrode 2, the light emitting layer 3, and the light reflective electrode 4. The organic light emitter has a plurality of layers stacked in the thickness direction DT. In the subsequent figures, the thickness direction is indicated by an arrow indicated by DT. The substrate 1 functions as a formation substrate. A formation substrate means the base material for forming a laminated body. Due to the presence of the substrate 1, the organic light emitter is formed well. In FIG. 1, the light emission direction is indicated by a white arrow (the same applies to the following drawings). The light emitted from the light emitting layer 3 is emitted to the outside through the substrate 1. This structure is called a bottom emission structure. The substrate 1 is preferably light transmissive. Note that a top emission structure in which light generated in the light emitting layer 3 is emitted from a direction opposite to the substrate 1 may be employed. In the top emission structure, the substrate 1 can be disposed on the outer surface of the light reflective electrode 4 (surface opposite to the light emitting layer 3).

  The substrate 1 can be formed from glass, resin, or the like. The glass substrate can easily suppress the intrusion of moisture and can improve the reliability. The resin substrate can impart flexibility. The substrate 1 may be formed of a composite material of glass and resin.

  The light transmissive electrode 2 is provided on the surface of the substrate 1. The light transmissive electrode 2 may be provided directly on the substrate 1 or may be provided on the substrate 1 via another layer. A light extraction structure exemplified by a light scattering layer can be provided between the substrate 1 and the light transmissive electrode 2. The light extraction efficiency is further improved by forming the light extraction structure.

  The light transmissive electrode 2 is formed of an appropriate electrode material capable of transmitting light. Examples of the light transmissive electrode 2 include a transparent conductive layer exemplified by a metal thin film and a transparent metal oxide layer. An auxiliary wiring for assisting electrical conductivity may be provided on the surface of the transparent conductive layer. The light transmissive electrode 2 can be formed of, for example, ITO.

  The light reflective electrode 4 is formed of an appropriate electrode material capable of reflecting light. The light reflective electrode 4 is made of metal. Since metal has high light reflectance, an electrode having excellent reflectivity can be obtained. In addition, since metal has high conductivity, an electrode having excellent electrical conductivity can be obtained. Examples of the metal constituting the light reflective electrode 4 include silver (Ag), aluminum (Al), platinum (Pt), rhodium (Rh), gold (Au), and alloys of metals selected from them, and An alloy containing a metal selected from them and another metal can be mentioned. Examples of the alloy include MgAg, AlLi, and MgIn. The light reflective electrode 4 may have a multilayer structure in which a plurality of metal layers are stacked.

  The light transmissive electrode 2 and the light reflective electrode 4 are electrically paired electrodes. One of the light transmissive electrode 2 and the light reflective electrode 4 constitutes an anode, and the other constitutes a cathode. In a preferred embodiment, the light transmissive electrode 2 is an anode and the light reflective electrode 4 is a cathode. In that case, a structure with high light extraction efficiency can be easily formed. Of course, the light transmissive electrode 2 may be a cathode, and the light reflective electrode 4 may be an anode.

  The light emitting layer 3 is a layer containing a light emitting material. The light emitting layer 3 may be a layer containing a light emitting dopant and a host material. The luminescent dopant can be doped into the host material. In the organic EL element, holes and electrons enter from the two electrodes toward the light emitting layer 3, respectively, and are combined in the light emitting layer 3, thereby generating light emission. The light emitting layer 3 may be a single layer or a multilayer.

  The charge transport layer 6 is a layer that transports charges from the electrode toward the light emitting layer 3. An electric charge means an electron or a hole (hole). The charge transport layer 6 can be disposed between the electrode and the light emitting layer 3. In the example of FIG. 1, the charge transport layer 6 </ b> A is disposed between the light emitting layer 3 and the light reflective electrode 4. In addition, a charge transport layer 6 </ b> B is disposed between the light emitting layer 3 and the light transmissive electrode 2. The charge transport layer 6 </ b> A is a layer that transports charges from the light reflective electrode 4 toward the light emitting layer 3. For example, when the light reflective electrode 4 is a cathode, the charge transport layer 6A is an electron transport layer. The charge transport layer 6 </ b> B is a layer that transports charges from the light transmissive electrode 2 toward the light emitting layer 3. For example, when the light-transmissive electrode 2 is an anode, the charge transport layer 6B is a hole transport layer.

  A charge injection layer may be disposed between the electrode and the charge transport layer 6. Thereby, the charge injection property is enhanced. For example, when the light reflective electrode 4 is a cathode, an electron injection layer may be provided between the charge transport layer 6 </ b> A that is an electron transport layer and the light reflective electrode 4. For example, when the light transmissive electrode 2 is an anode, a hole injection layer may be provided between the charge transport layer 6 </ b> B that is a hole transport layer and the light transmissive electrode 2.

  The optical adjustment layer 5 is disposed between the light emitting layer 3 and the light reflective electrode 4. The optical adjustment layer 5 is a layer having optical transparency. The optical adjustment layer 5 can optically adjust the light generated in the light emitting layer 3.

  The refractive index of the optical adjustment layer 5 is smaller than the refractive index of the light emitting layer 3. For this reason, the light traveling direction of the light emitted from the light emitting layer 3 can change due to the difference in refractive index between the light emitting layer 3 and the optical adjustment layer 5. Here, as described above, on the surface of the metal, a phenomenon occurs in which light energy is converted into vibration energy by surface plasmons and light is lost. Therefore, in the light reflective electrode 4, plasmons are a problem. However, when the optical adjustment layer 5 exists, the light from the light emitting layer 3 is optically adjusted by the optical adjustment layer 5 and becomes light in which plasmons are hardly generated. Therefore, plasmon is suppressed. As described above, the light reaching the light-reflecting electrode 4 from the light-emitting layer 3 due to plasmon suppression is suppressed in light loss, and is reflected at a high rate and becomes light directed toward the substrate 1. Therefore, more light can be extracted.

  Further, by introducing the optical adjustment layer 5, light absorption by the light reflective electrode 4 made of metal can be suppressed. One factor of light loss is light absorption by metal. A metal that is light reflective and suitable for the electrode can have the property of absorbing light. Light is easily lost due to the light absorption of the metal. However, when the optical adjustment layer 5 exists, the light from the light emitting layer 3 is optically adjusted by the optical adjustment layer 5 and becomes light that is not easily absorbed by the light reflective electrode 4. Therefore, light absorption is suppressed. As described above, the light reaching the light-reflecting electrode 4 from the light-emitting layer 3 due to the suppression of light absorption suppresses the loss of light, and is reflected at a high rate and becomes light directed toward the substrate 1. Therefore, more light can be extracted.

The optical adjustment layer 5 may have a charge transport property. As a result, charge transport between the electrode and the light emitting layer 3 can be facilitated. The optical adjustment layer 5 is preferably capable of transporting the same charge as the charge transport layer 6A. For example, when the charge transport layer 6A is an electron transport layer, the optical adjustment layer 5 may have an electron transport property. For example, the electron mobility of the optical adjustment layer 5 may be 1 × 10 ⁻⁷ cm ² / Vs or higher.

The optical adjustment layer 5 may be composed of a transparent conductive layer. The optical adjustment layer 5 may be made of a transparent electrode material. The optical adjustment layer 5 may have both electron transport properties and hole transport properties. The optical adjustment layer 5 may have lower transportability for the same charge as the charge transport layer 6A than the charge transport layer 6A. In addition to the optical adjustment layer 5, a charge transport layer 6 </ b> A is provided between the light reflective electrode 4 and the light emitting layer 3. Therefore, charge transport is mainly performed by the charge transport layer 6A, and the optical adjustment layer 5 can adopt a configuration suitable for optical adjustment. For example, in the optical adjustment layer 5 having a relatively low electron transport property, the electron mobility of the optical adjustment layer 5 may be 1 × 10 ⁻⁷ cm ² / Vs or less. The electron mobility in this case can be lower than that of Alq ₃ (about 1 × 10 ⁻⁶ cm ² / Vs).

  The optical adjustment layer 5 can be formed of a material containing an organic substance. The optical adjustment layer 5 can be composed of an organic layer. The optical adjustment layer 5 may contain a metal. An example of the optical adjustment layer 5 includes an organic layer containing metal particles. As the metal particles, metal nanowires are preferable. Metal nanowires can be doped into the organic layer. The material of the organic layer may be insulative or have charge transport properties. The organic layer can be formed of a material having a low refractive index. An organic layer containing metal nanowires can transfer charges. Therefore, charge transportability can be imparted to the optical adjustment layer 5. An example of the optical adjustment layer 5 is a conductive organic material. For example, PEDOT: PSS is mentioned as the example. This material has charge transport properties. In addition, it is easy to reduce the refractive index. Therefore, an excellent optical adjustment layer 5 is obtained. An example of the optical adjustment layer 5 is an organic metal material. For example, an organic indium compound is mentioned as the example. An organic indium compound easily has electric conductivity and can move electric charge. In addition, organic indium compounds can be easily reduced in refractive index. Therefore, an excellent optical adjustment layer 5 is obtained. The optical adjustment layer 5 may contain low refractive index particles. By including the low refractive index particles, the refractive index of the optical adjustment layer 5 is lowered. Examples of the low refractive index particles include hollow silica and mesoporous silica. The above only exemplifies materials that can be used for the optical adjustment layer 5, and the material of the optical adjustment layer 5 is not limited to this.

  The optical adjustment layer 5 only needs to be disposed between the light emitting layer 3 and the light reflective electrode 4. Unlike FIG. 1, the charge transport layer 6 </ b> A and the optical adjustment layer 5 are sequentially formed from the light emitting layer 3 side. , May be arranged. Even in that case, the optical loss is suppressed by the action of the optical adjustment layer 5. In a preferred embodiment, as in the example of FIG. 1, the optical adjustment layer 5 and the charge transport layer 6A are arranged in this order from the light emitting layer 3 side. The optical adjustment layer 5 is preferably in contact with the light emitting layer 3. When the optical adjustment layer 5 is in contact with the light emitting layer 3, the light generated in the light emitting layer 3 is immediately adjusted by the optical adjustment layer 5. Therefore, the plasmon suppressing effect can be further improved.

  FIG. 2 shows another example of the organic EL element. The same components as those in FIG. 1 are denoted by the same reference numerals and description thereof is omitted. The configurations with the same reference numerals are applicable to other forms unless otherwise specified (the same applies to the following embodiments). In FIG. 2, the configuration between the light transmissive electrode 2 and the light emitting layer 3 is different from that in FIG. 1. About the other than that, it may be the same as the structure of FIG.

  In the form of FIG. 2, an additional optical adjustment layer 7 having a refractive index lower than that of the light emitting layer 3 is provided between the light transmissive electrode 2 and the light emitting layer 3. The additional optical adjustment layer 7 may have the same configuration as the optical adjustment layer 5. The contents described in the optical adjustment layer 5 can be applied to the additional optical adjustment layer 7. For example, the additional optical adjustment layer 7 can be formed of a transparent conductive layer. The additional optical adjustment layer 7 can be considered as an added optical adjustment layer.

  In the plasmon loss, the plasmon in the light reflective electrode 4 is mainly a problem, but the plasmon phenomenon can also occur in the light transmissive electrode 2. Therefore, if the plasmon in the light transmissive electrode 2 is suppressed, further improvement in light extraction efficiency is expected. Here, since the refractive index of the additional optical adjustment layer 7 is smaller than the refractive index of the light emitting layer 3, the light emitted from the light emitting layer 3 is reflected by the difference in refractive index between the light emitting layer 3 and the additional optical adjustment layer 7. The direction of travel can change. When the additional optical adjustment layer 7 is present, the light from the light emitting layer 3 is optically adjusted by the additional optical adjustment layer 7 and becomes light that hardly causes plasmons on the surface of the light transmissive electrode 2. Therefore, plasmon can be suppressed. Further, as in the case of the light reflective electrode 4, light absorption by the light transmissive electrode 2 can also be suppressed. Thus, the light reaching the light transmissive electrode 2 from the light emitting layer 3 is suppressed to light loss and becomes light entering the substrate 1 at a higher rate. Therefore, more light can be extracted.

  The additional optical adjustment layer 7 can be provided as a layer different from the charge transport layer 6B. Thereby, a layer suitable for optical adjustment can be easily formed. The additional optical adjustment layer 7 is preferably in contact with the light emitting layer 3. Thereby, plasmon can be suppressed higher. The additional optical adjustment layer 7 can be formed of the same material as the optical adjustment layer 5.

  The operation of the optical adjustment layer will be described with reference to FIG. The graph of FIG. 3 represents an example of the relationship between the refractive index of the optical adjustment layer and the light mode. The light mode means a relative average light contribution (relative average contribution), and indicates the distribution of light energy. In FIG. 3, plasmon loss, absorption loss, and substrate arrival are shown as light modes. The absorption loss here includes not only the light reflective electrode but also the absorption up to the substrate.

  A simulation will be described. In an organic EL element including a light transmissive electrode, a light emitting layer, and a light reflective electrode, a layer having a refractive index adjustable (defined as an introduction layer) is inserted between the light emitting layer and the light reflective electrode. . The thickness of the introduction layer is 60 nm. Then, the refractive index is changed to examine the light distribution. Thereby, the graph of FIG. 3 is obtained.

  As shown in FIG. 3, it can be seen that the plasmon loss is large when the refractive index of the introduction layer is large. However, it can be seen that the plasmon loss decreases as the refractive index of the introduction layer decreases. When the refractive index of the introduction layer is smaller than the refractive index of the light emitting layer, the introduction layer can be an optical adjustment layer. In the case of an optical adjustment layer, plasmon loss can be further suppressed. For example, the refractive index of the introduction layer (optical adjustment layer) is 1.60 or less, 1.55 or less, 1.50 or less, 1.45 or less, 1.40 or less, 1.35 or less, 1.30 or less. Accordingly, the plasmon loss is effectively reduced. Thus, it is understood that introduction of a layer (optical adjustment layer) having a refractive index lower than that of the light emitting layer is effective for suppressing plasmons. It is effective that the refractive index of the optical adjustment layer is lower. The lower limit of the refractive index of the optical adjustment layer is not particularly limited, but may be, for example, 1.1 or more in consideration of easy formation. In addition, although the optical adjustment layer was demonstrated here, the same effect | action arises also about an additional optical adjustment layer.

  FIG. 4 is a schematic cross-sectional view showing an example of an organic electroluminescence element. The organic EL element may have the same layer structure as that shown in FIG. In FIG. 4, the optical adjustment layer 5 is optimized, and the progress of light in that case is indicated by arrows.

  The refractive index of the optical adjustment layer 5 is preferably 0.9 times or less than the refractive index of the light emitting layer 3. Thereby, the suppression effect of plasmon increases. As described with reference to FIG. 3, the plasmon suppression effect increases as the refractive index of the optical adjustment layer 5 decreases. For example, when the refractive index of the light emitting layer 3 is 1.8, the refractive index of the optical adjustment layer 5 is preferably 1.62 or less. The refractive index of the optical adjustment layer 5 is more preferably 0.85 times or less of the refractive index of the light emitting layer 3, and further preferably 0.8 times or less of the refractive index of the light emitting layer 3.

  Furthermore, if the refractive index of the optical adjustment layer 5 is low, the total reflection component can be increased. In the light source of the light emitting layer 3, light can be emitted in all directions. That is, the light generated in the light emitting layer 3 includes light traveling in an oblique direction inclined from the thickness direction DT. When the difference in refractive index between the light emitting layer 3 and the optical adjustment layer 5 increases, light traveling in the oblique direction is refracted more greatly by the optical adjustment layer 5 (this refraction also contributes to plasmon suppression). When light enters a low refractive index region from a high refractive index region, the light entering at a low angle can be reflected at the interface if the refractive index difference is large. That is, total reflection can occur. When light is reflected on the surface of the optical adjustment layer 5 based on this principle, the light traveling toward the light reflective electrode 4 becomes light traveling toward the substrate 1 before reaching the light reflective electrode 4. Therefore, more light can be extracted (see arrow P2 in FIG. 4).

Preferred relationships of the refractive index of the above, the refractive index of the optical adjustment layer 5 is n, when the refractive index of the light emitting layer 3 was set to n _1,
n ≦ 0.9 × n ₁
Can be expressed as

The thickness of the optical adjustment layer 5 is λ _w / 2n or more when the weighted average wavelength of light emitted from the light emitting layer 3 represented by the following formula is λ _w and the refractive index of the optical adjustment layer 5 is n. It is preferable.

  In the above formula, P (λ) indicates the spectral intensity at each wavelength. Λ represents a wavelength and is a variable from 380 (nm) to 780 (nm).

When the difference in refractive index is large as described above, reflection tends to occur on the surface of the optical adjustment layer 5, but the thickness of the optical adjustment layer 5 is related to the reflection of light. When the thickness of the optical adjustment layer 5 is large, the light reflectivity of the wide-angle component (component with a large incident angle) becomes higher. In addition, when the thickness of the optical adjustment layer 5 is large, the range of incident angles that allow total reflection can be increased. When the thickness of the optical adjustment layer 5 is λ _w / 2n or more as described above, the total reflected light is likely to increase, and the wide-angle component light can be effectively reflected.

The thickness of the optical adjustment layer 5 is indicated by D1 in FIG. Therefore, the above preferred relationship is
D1 ≧ λ _w / 2n
Can be expressed as

  Although the upper limit of the thickness of the optical adjustment layer 5 is not specifically limited, For example, the thickness of the optical adjustment layer 5 may be 1000 nm or less, Furthermore, 800 nm or less, Furthermore, 600 nm or less may be sufficient. By preventing the thickness of the optical adjustment layer 5 from becoming unnecessarily large, it is possible to effectively reduce light loss and improve light extraction efficiency. In addition, it is advantageous in interference design that the thickness of the optical adjustment layer 5 does not become too large.

  When the optical adjustment layer 5 is optimized in one or both of the refractive index and the thickness as described above, the light generated in the light emitting layer 3 is efficiently reflected as shown in FIG. Of the light generated in the light emitting layer 3, the light passing through the optical adjustment layer 5 is efficiently reflected by the light reflective electrode 4 and travels to the opposite side, and can be emitted to the outside through the substrate 1 ( Arrow P1) in FIG. In addition, of the light generated in the light emitting layer 3, light having a wide angle component is reflected by the optical adjustment layer 5 and travels to the opposite side, and can be emitted to the outside through the substrate 1 (arrow P2 in FIG. 4). ). For this reason, the light extraction efficiency is effectively improved. The drawings including FIG. 4 schematically show the layer structure. In an actual organic EL element, the thickness of the layer is considerably smaller than the size of the light emitting surface. Therefore, most of the reflected light in the oblique direction reaches the substrate 1 without reaching the side end portion of the light emitter, so that the light is efficiently extracted.

  The optimization of the optical adjustment layer 5 is further illustrated by FIGS.

FIG. 5 shows the relationship between the incident angle (incident angle) and the reflectance (reflectance) when the optical adjustment layer is changed. In the graph of FIG. 5, the refractive index n and the thickness D1 of the optical adjustment layer are changed. The reflectance is shown as a relative value where the maximum value (reflectance 100%) is 1.00. In the graph, (a) shows an example in which the refractive index n is 1.3 and the thickness D1 is 300 nm. (B) shows an example in which the refractive index n is 1.3 and the thickness D1 is 60 nm. (C) shows an example in which the refractive index n is 1.5 and the thickness D1 is 60 nm. (D) shows an example in which the refractive index n is 1.8 and the thickness D1 is 60 nm. The refractive index n ₁ of the light emitting layer is 1.8. Although (d) does not exactly satisfy the conditions of the optical adjustment layer, it is treated here as an optical adjustment layer for easy understanding (may be considered as the introduction layer described above).

  As shown in FIG. 5, as the refractive index decreases (d), (c), and (b), the reflectance is improved in all directions (all incident angles). Therefore, reducing the refractive index of the optical adjustment layer is effective for improving the light extraction efficiency. Further, as shown in (a), when the thickness of the optical adjustment layer is increased, the light of the wide-angle component has a reflectance of 1.0 and total reflection occurs. As described above, when totally reflected, the wide-angle component light is reflected by the optical adjustment layer and does not reach the light-reflecting electrode as indicated by an arrow P2 in FIG. 4, so that plasmon and absorption do not occur. Therefore, increasing the thickness of the optical adjustment layer is particularly effective for suppressing plasmon and light absorption. In this way, light loss at the light reflective electrode is suppressed, and light extraction efficiency is improved.

FIG. 6 shows the relationship between the thickness of the optical adjustment layer and the reflectance. In the graph of FIG. 6, the refractive index n of the optical adjustment layer is 1.3, the refractive index of the light-emitting layer was 1.8, and the wavelength lambda _w of the light and 550 nm. The light reflectance at an incident angle of 60 degrees when the thickness of the optical adjustment layer is changed is shown.

As shown in FIG. 6, the reflectance is about 94% until the thickness of the optical adjustment layer is about 100 nm. When the thickness of the optical adjustment layer exceeds 100 nm, the reflectance increases as the thickness increases. Furthermore, when the thickness of the optical adjustment layer exceeds 200 nm, the reflectance exceeds 99%. Here, under the conditions described in FIG. 6, λ _w is 550 nm and the refractive index n is 1.3, so that λ _w / 2n is about 211 nm. In other words, it is understood that when the thickness D1 of the optical adjustment layer is λ _w / 2n or more, the reflectance is effectively improved, and the thickness relationship described above is preferable.

By the way, the preferable configuration of the optical adjustment layer 5 described above can also be applied to the additional optical adjustment layer 7. For example, the refractive index of the additional optical adjustment layer 7 is preferably 0.9 times or less than the refractive index of the light emitting layer 3. For example, the thickness of the additional optical adjustment layer 7, the weight average wavelength of light emitted from the light emitting layer 3 and lambda _w, is the refractive index of the additional optical adjustment layer 7 is taken as n _a, λ _{w /} 2n _a more It is preferable. Those reasons are the same as those of the optical adjustment layer 5. The further description of the optical adjustment layer 5 can be applied to the additional optical adjustment layer 7 as well.

The charge transport layer 6 </ b> A (the charge transport layer 6 between the light reflective electrode 4 and the light emitting layer 3) may have a refractive index lower than that of the light emitting layer 3. In that case, the charge transport layer 6 </ b> A can perform optical adjustment by supplementing the function of the optical adjustment layer 5. A preferable range of the refractive index of the charge transport layer 6A is the same as that of the optical adjustment layer 5 described above. That is, the refractive index of the charge transport layer 6A is preferably 0.9 times or less than the refractive index of the light emitting layer 3. The thickness of the charge transport layer 6A is preferably λ _w / 2n or more. Further, the sum of the thickness of the optical adjustment layer 5 and the thickness of the charge transport layer 6A may be λ _w / 2n or more. Even in that case, the reflectivity can be increased.

  FIG. 7 shows an example of the organic EL element. About the same structure as the above, the same code | symbol is attached | subjected and description is abbreviate | omitted. In FIG. 7, the configuration between the light reflective electrode 4 and the light emitting layer 3 is different from that in FIG. 1. About the other than that, it may be the same as the structure of FIG.

  The organic EL element preferably includes a distributed Bragg reflection structure 8 between the optical adjustment layer 5 and the light reflective electrode 4. When the distributed Bragg reflector structure 8 is present, light is reflected by the distributed Bragg reflector structure 8. Then, since light is reflected before reaching the light reflective electrode 4, it becomes difficult to reach the light reflective electrode 4. Therefore, plasmons and light absorption by the light reflective electrode 4 can be suppressed, and light extraction efficiency can be improved. The distributed Bragg reflector structure 8 utilizes the principle of a diffraction grating. The distributed Bragg reflection structure 8 is an advantageous structure for reflecting light of a specific wavelength.

  In the example of FIG. 7, the distributed Bragg reflection structure 8 is introduced as a layer different from the charge transport layer 6 </ b> A (the charge transport layer 6 disposed between the light reflective electrode 4 and the light emitting layer 3). . A layer having the distributed Bragg reflection structure 8 is defined as a distributed Bragg reflection layer 8A. The distributed Bragg reflector structure is also called a distributed Bragg reflector, or DBR for short.

  The distributed Bragg reflection structure 8 may be disposed between the optical adjustment layer 5 and the charge transport layer 6A, or may be disposed between the charge transport layer 6A and the light reflective electrode 4. Preferably, as shown in FIG. 7, the distributed Bragg reflector structure 8 is disposed between the optical adjustment layer 5 and the charge transport layer 6A. The distributed Bragg reflection structure 8 is more preferably in contact with the optical adjustment layer 5. The closer the distributed Bragg reflection structure 8 is to the optical adjustment layer 5, the more light can be reflected near the light emitting layer 3, and thus the reflection efficiency can be increased.

  FIG. 8 shows an example of the organic EL element. The same components as those in FIG. 1 are denoted by the same reference numerals and description thereof is omitted. In FIG. 8, the structure between the light reflective electrode 4 and the light emitting layer 3 is different from FIG. About the other than that, it may be the same as the structure of FIG.

  In the example of FIG. 8, the organic EL element has a distributed Bragg reflection structure 8 as in FIG. 7. However, in the example of FIG. 8, the charge transport layer 6 </ b> A includes the distributed Bragg reflection structure 8. When the charge transport layer 6 </ b> A is an electron transport layer, the electron transport layer has the distributed Bragg reflection structure 8. If the charge transport layer 6A has the distributed Bragg reflection structure 8 as shown in FIG. 8, the number of layers is reduced as compared with the case of FIG. Further, when the charge transport layer 6A also serves as the distributed Bragg reflection structure 8, the reflectivity is more efficiently improved.

  The distributed Bragg reflector structure 8 shown in FIGS. 7 and 8 will be further described.

  The distributed Bragg reflection structure 8 may be a structure in which layers having different refractive indexes are alternately stacked with a predetermined thickness. With such a structure, it is possible to reflect light of a specific wavelength and a specific direction with approximately 100%. The predetermined thickness of the layers having different refractive indexes can be, for example, about one quarter of the wavelength of the target light.

  The distributed Bragg reflection structure 8 may be a structure in which low refractive index layers and high refractive index layers are alternately arranged in the thickness direction DT. In this case, the distributed Bragg reflection structure 8 includes a plurality of low refractive index layers and a plurality of high refractive index layers. The high and low refractive indexes in the low refractive index layer and the high refractive index layer are relative to each other. That is, the high refractive index layer is a layer having a higher refractive index than the low refractive index layer. The difference in refractive index between the high refractive index layer and the low refractive index layer is preferably 0.1 or more. Thereby, the reflectivity is improved. The refractive index difference between the high refractive index layer and the low refractive index layer is more preferably 0.15 or more, and further preferably 0.2 or more. Thereby, the reflectivity is further improved. The upper limit of the refractive index difference between the high refractive index layer and the low refractive index layer is not particularly limited, but for example, the refractive index difference is preferably 0.8 or less for ease of production. More preferably, it is 5 or less. The high refractive index layer may have a higher refractive index than the light emitting layer 3. The low refractive index layer may have a refractive index lower than that of the light emitting layer 3. The low refractive index layer may have a refractive index lower than that of the optical adjustment layer 5. The total number of the low refractive index layer and the high refractive index layer is preferably 10 or more (each 5 or more). Thereby, the reflectivity is improved. The total number of the low refractive index layer and the high refractive index layer is preferably 100 or less (50 or less each). As a result, manufacturing is less likely to be complicated. The number of low refractive index layers and high refractive index layers may be the same, or one of them may be one more. The thickness of the low refractive index layer and the thickness of the high refractive index layer may be substantially the same, but preferably the thickness is appropriately adjusted according to each refractive index. The plurality of low refractive index layers may have the same thickness. The plurality of high refractive index layers may have the same thickness.

The distributed Bragg reflector structure 8 can efficiently reflect light having a small incident angle. Therefore, the aspect having the distributed Bragg reflection structure 8 is particularly effective when the thickness D1 of the optical adjustment layer 5 is λ _w / 2n or more as described above. This is because light of a wide angle component can be reflected by the optical adjustment layer 5 and light of a component having a small incident angle can be reflected by the distributed Bragg reflection structure 8. Thereby, the reflectivity can be enhanced at a wide angle. In FIG. 7, reflection of light having a wide angle component is indicated by an arrow P2, and reflection of light having a small incident angle is indicated by an arrow P3. As indicated by the arrow P3, when the distributed Bragg reflector structure 8 exists, the light is reflected by the distributed Bragg reflector structure 8 before reaching the light reflective electrode 4.

  The distributed Bragg reflection structure 8 preferably has a property of transporting charges moving from the light reflective electrode 4 to the light emitting layer 3. Thereby, the movement of electric charges becomes smooth and light emission is not hindered. When the light reflective electrode 4 is a cathode, the distributed Bragg reflection structure 8 preferably has an electron transport property.

FIG. 9 shows the reflection characteristics of the distributed Bragg reflection structure. This distributed Bragg reflection structure has a plurality of low refractive index layers and a plurality of high refractive index layers. Low refractive index layer has a refractive index _{n L} are 1.6, thickness _{D L} is in the 85.9Nm. The high refractive index layer has a refractive index n _H of 1.85 and a thickness _DH of 74.3 nm. Eight low-refractive index layers and eight high-refractive index layers are alternately stacked to form a distributed Bragg reflection structure with a total of sixteen layers. The design wavelength of light is 550 nm. The thickness of each layer is obtained by dividing a quarter of the light wavelength λ by the refractive index of the layer. It is preferable to set the thickness of the layer in such a relationship in order to obtain high reflectivity. In the low refractive index layer, D _L = λ / 4n _L. The high refractive index layer, and has a _{D H} = _λ / 4n _H. The wavelength lambda can be a weighted average wavelength lambda _w described above.

  FIG. 9 shows the relationship between the wavelength and the reflectance of light (light traveling along the thickness direction DT) that reaches the distributed Bragg reflector structure at an incident angle of 0 degrees. As shown in FIG. 9, at a target wavelength of 550 nm, a reflectance of about 100% is obtained. Moreover, it has been confirmed that it has reflectivity in the wavelength range of 380 to 780 nm including the visible light region. Thus, it can be seen that the distributed Bragg reflection structure can effectively reflect light with a small incident angle.

FIG. 10 shows the relationship between the incident angle of light and the reflectance when the thickness D1 of the optical adjustment layer is set to λ _w / 2n or more and the distributed Bragg reflection structure is provided. The distributed Bragg reflection structure is the same as that described in FIG. As shown in FIG. 10, in the range where the incident angle is small (less than 44.5 degrees), the action of the distributed Bragg reflection structure works greatly, the reflectivity is about 100%, and a high reflectivity is obtained. Further, in the range where the incident angle is large (62.3 degrees or more), the effect of total reflection of the optical adjustment layer is large, the reflectance is close to 100%, and the reflectance is high. Also in the range of 44.5 degrees to 62.3 degrees, the optical adjustment layer and the distributed Bragg reflection structure complement each other, and the reflectance is high. Thus, high reflectivity can be obtained in a wide angle by the synergistic effect of the optical adjustment layer and the distributed Bragg reflection structure.

The distributed Bragg reflection structure can be formed of an appropriate material. The distributed Bragg reflection structure is preferably formed of an organic material. Examples of the material for the low refractive index layer include PEDOT and organic indium compounds. An example of the material for the high refractive index layer is an electron-transporting organic material. Specific examples of the material for the high refractive index layer include an organometallic complex, and a representative example is Alq ₃ .

  By the way, although the example which introduce | transduced the distribution type Bragg reflection structure 8 to the organic EL element of FIG. 1 was shown above, the distribution type Bragg reflection structure 8 was shown in the organic EL element (refer FIG. 2) provided with the additional optical adjustment layer 7. FIG. May be introduced.

  FIG. 11 shows an example of the organic EL element. The same components as those in FIG. 1 are denoted by the same reference numerals and description thereof is omitted. In FIG. 11, the configuration between the light transmissive electrode 2 and the light emitting layer 3 is different from that in FIG. 1. About the other than that, it may be the same as the structure of FIG.

  The organic EL element may include at least one additional light emitting layer 9 between the light transmissive electrode 2 and the light emitting layer 3. Even in that case, by providing the optical adjustment layer 5, light loss can be suppressed and light extraction efficiency can be improved. The organic EL element of FIG. 11 has an additional light emitting layer 9. The additional light emitting layer 9 may be a single layer or a multilayer.

  The organic EL element of FIG. 11 has a multi-unit structure including a plurality of light emitting units 10. The light emitting unit 10 means a structure that emits light when electricity is passed between an anode and a cathode. A structure in which the light emitting units 10 are arranged in a plurality of thickness directions DT is a multi-unit structure. In the multi-unit structure, it becomes easier to efficiently obtain a target color (particularly white).

  As shown in FIG. 11, the laminated structure of the charge transport layer 6B, the light emitting layer 3, the optical adjustment layer 5, and the charge transport layer 6A constitutes the first light emitting unit 10a. The stacked structure of the charge transport layer 6D, the additional light emitting layer 9, and the charge transport layer 6C constitutes the second light emitting unit 10b. When the light-transmissive electrode 2 is an anode, the charge transport layer 6D can be a hole transport layer, and the charge transport layer 6C can be an electron transport layer. An intermediate layer 11 is disposed between the adjacent light emitting units 10. The intermediate layer 11 has a function of imparting electric charges to the light emitting units 10 on both sides. The intermediate layer 11 preferably has light transmittance. The intermediate layer 11 facilitates the movement of charges.

The organic EL element having a multi-unit structure shown in FIG. 11 can also be optimized as described above. The refractive index of the optical adjustment layer 5 is preferably 0.9 times or less than the refractive index of the light emitting layer 3. The thickness of the optical adjustment layer 5 is preferably λ _w / 2n or more. The distributed Bragg reflection structure 8 described above may be provided between the optical adjustment layer 5 and the light reflective electrode 4. The distributed Bragg reflection structure 8 may be introduced as a layer different from the charge transport layer 6 </ b> A, or the charge transport layer 6 </ b> A may include the distributed Bragg reflection structure 8. Further, in the organic EL element of FIG. 11, the additional optical adjustment layer 7 described above may be provided. At this time, the additional optical adjustment layer 7 can be preferably disposed between the additional light emitting layer 9 and the light transmissive electrode 2. The refractive index and thickness of the additional optical adjustment layer 7 may be set based on the refractive index of the additional light emitting layer 9.

  Although FIG. 11 shows the case where there are two light emitting units 10, the number of light emitting units 10 may be three or more. The added light emitting unit 10 is inserted between the first light emitting unit 10a and the second light emitting unit 10b. At this time, the intermediate layer 11 may be further inserted between the adjacent light emitting units 10. The light emitting layer 3 is defined as the light emitting layer closest to the light reflective electrode 4. The additional light emitting layer 9 is defined as the light emitting layer closest to the light transmissive electrode 2.

  The organic EL element demonstrated above can be formed using a lamination process. Layers are sequentially formed on the surface of the substrate 1 by an appropriate lamination method. The lamination method may be selected from sputtering, vapor deposition, coating, and the like. The lamination method is appropriately selected depending on the layer. For example, the light emitting layer 3 and the charge transport layer 6 can be formed by vapor deposition. The optical adjustment layer 5 can be formed by vapor deposition or coating. The additional optical adjustment layer 7 can also be formed by vapor deposition or coating. The distributed Bragg reflection structure 8 can be formed by alternately depositing or applying a low refractive index layer and a high refractive index layer. In addition, it is preferable that the laminated body containing the light emitting layer 3 is sealed with an appropriate sealing material and blocked from the outside. Thereby, deterioration of the light emitting layer 3 is suppressed. For example, sealing can be performed by adhering a sealing material to the substrate 1 so as to cover the laminate including the light emitting layer 3.

DESCRIPTION OF SYMBOLS 1 Substrate 2 Light transmitting electrode 3 Light emitting layer 4 Light reflective electrode 5 Optical adjustment layer 6 Charge transport layer 7 Additional optical adjustment layer 8 Distributed Bragg reflection structure 9 Additional light emitting layer

Claims (7)

A substrate,
A light transmissive electrode;
A light reflective electrode made of metal;
A light emitting layer disposed between the light transmissive electrode and the light reflective electrode,
An organic electroluminescence device comprising an optical adjustment layer having a refractive index lower than that of the light emitting layer and a charge transport layer between the light emitting layer and the light reflective electrode.   The organic electroluminescence element according to claim 1, wherein the optical adjustment layer is in contact with the light emitting layer.   The organic electroluminescent element according to claim 1, further comprising an additional optical adjustment layer having a refractive index lower than that of the light emitting layer between the light transmissive electrode and the light emitting layer.   The organic electroluminescent element according to any one of claims 1 to 3, wherein a refractive index of the optical adjustment layer is 0.9 times or less of a refractive index of the light emitting layer. The thickness of the optical adjustment layer, the weight average wavelength of light emitted from the light emitting layer of the following formula as a lambda _w, the refractive index of the optical adjustment layer is n, is a lambda _{w /} 2n or more The organic electroluminescent element of any one of Claims 1 thru | or 4.

In the above formula, P (λ) indicates the spectral intensity at each wavelength.   The organic electroluminescence device according to claim 1, further comprising a distributed Bragg reflection structure between the optical adjustment layer and the light reflective electrode.   The organic electroluminescent element according to claim 1, further comprising at least one additional light emitting layer between the light transmissive electrode and the light emitting layer.

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