JP2013012377A - Light-emitting device and display device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a light-emitting device and a display device, using an organic EL element, capable of improving light extraction efficiency by suppressing attenuation of guided light caused by absorption in a metal film.SOLUTION: The light-emitting device and the display device comprise: a transparent electrode 4, having refractive index higher than that of an organic compound layer 3 and disposed in the opposite side of a light extraction side of an organic EL element 5; a low refractive index layer 6, touching the electrode 4 and having refractive index lower than that of the organic compound layer 3 and thickness thicker than or equal to an emission peak wavelength; and a metal reflection film 7, so that a guided light propagating within the organic EL element 5 is totally reflected between the electrode 4 and the low refractive index layer 6, resulting in reduction in absorption of the guided light in the metal film.

Description

  The present invention relates to a light emitting device and a display device including an organic EL element (organic electroluminescence element).

  The organic EL light-emitting device is a new type of light-emitting device using, as a pixel, an organic EL element that is a thin film and is characterized by self-light emission. One of the problems is an improvement in luminous efficiency. Since light emission of the organic EL element occurs in the organic light emitting layer having a refractive index higher than that of air, most of the light emitted from the organic EL element is propagated in a direction horizontal to the substrate due to total reflection caused by the difference in refractive index. It becomes wave light. As a result, only about 20% to 30% of the light can be extracted and used in the air. In order to increase the extraction efficiency of the organic EL element, it is necessary to provide a structure for extracting the guided light into the air. In Patent Literature 1, guided light is extracted into the air by diffraction by using a periodic structure (diffraction grating) as a component of the organic EL element. Further, in Patent Document 2, by arranging a low refractive index material on the end face of the organic EL element, guided light propagating in the organic EL element is emitted from the end face into the low refractive index material, and is emitted into the low refractive index material. Part of the emitted light is taken out into the air.

JP 2006-85985 A US Patent Application Publication No. 2008/0238310

  However, when the reflected light propagating in the organic EL light emitting device is reflected by the reflective film made of metal, a part of the guided light is absorbed and attenuated by the reflective film. I cannot expect improvement.

  An object of the present invention is to provide a light emitting device and a display device in which light extraction efficiency is improved by suppressing attenuation due to absorption of guided light by a metal film.

The first of the present invention is an organic EL device having a pair of transparent electrodes and an organic compound layer including a light emitting layer disposed between the transparent electrodes,
The transparent electrode opposite to the light extraction surface of the organic EL element has a refractive index higher than that of the organic compound layer,
On the side opposite to the light extraction surface of the organic EL element, a low refractive index layer having a refractive index lower than that of the organic compound layer, and a metal reflective film in this order from the organic compound layer side,
The film thickness of the low refractive index layer has a thickness that causes total reflection of light incident at a critical angle or more from the low refractive index layer at the interface with the transparent electrode,
The light-emitting device is provided with a light extraction structure for extracting guided light.
A second aspect of the present invention is a display device including an organic EL element that emits red, an organic EL element that emits green, and an organic EL element that emits blue.
The organic EL element has a pair of transparent electrodes and an organic compound layer including a light emitting layer disposed between the transparent electrodes,
The transparent electrode opposite to the light extraction surface of the organic EL element has a refractive index higher than that of the organic compound layer,
On the side opposite to the light extraction surface of the organic EL element, a low refractive index layer having a refractive index lower than that of the organic compound layer, and a metal reflective film in this order from the organic compound layer side,
The film thickness of the low refractive index layer has a thickness that causes total reflection of light incident at a critical angle or more from the low refractive index layer at the interface with the transparent electrode,
A light extraction structure for extracting guided light is provided,
The display device is characterized in that the thickness of the low refractive index layer is common to all organic EL elements and is equal to or greater than the emission peak wavelength of red light emission.

  According to the present invention, the electrode opposite to the light extraction side of the organic EL element is used as a transparent electrode, and a low refractive index layer having a predetermined thickness and a metal reflection film are provided on the transparent electrode side, thereby providing a metal reflection. Attenuation due to absorption of guided light in the film can be suppressed. Therefore, a light emitting device and a display device with improved guided light extraction efficiency and improved light emission efficiency are provided.

It is sectional drawing which shows typically the structure of one Embodiment of the light-emitting device of this invention. It is a figure which shows the relationship between the film thickness of a low-refractive-index layer, and the light absorption rate of a metal reflective film. It is a figure which shows the angle dependence of the reflectance of the light in the metal reflective film of the light-emitting device of this invention. It is a figure which shows the angle dependence of what raised the reflectance of FIG. 3 to the 5th power. It is sectional drawing which shows typically the structural example of the light-emitting device of this invention. It is sectional drawing which shows the other structural example of the light-emitting device of this invention typically. It is sectional drawing which shows the other structural example of the light-emitting device of this invention typically.

  The light emitting device and the display device of the present invention include a low refractive index layer and a metal reflective film on the side opposite to the light extraction side of the organic EL element, and the electrode on the light extraction side has a refractive index higher than that of the organic compound layer. Is expensive. Therefore, a part of the guided light propagating in the organic EL element is totally reflected between the light extraction side electrode and the low refractive index layer, and the light absorption is reduced as compared with the case where it is reflected by the conventional metal electrode. . Furthermore, in the present invention, by adjusting the low refractive index layer to a predetermined thickness, when the guided light is totally reflected between the electrode on the light extraction side and the low refractive index layer, the low refractive index layer side is changed. Absorption of the evanescent light that protrudes into the metal reflective film is prevented, and attenuation of the guided light is further reduced.

  Embodiments of a light emitting device according to the present invention will be described below with reference to the drawings. In addition, the well-known or well-known technique of the said technical field is applied regarding the part which is not illustrated or described in particular in this specification. The embodiment described below is one embodiment of the present invention and is not limited thereto.

  FIG. 1 is a cross-sectional view schematically showing a configuration of an embodiment of a light emitting device of the present invention. The light emitting device of the present embodiment is a bottom emission type that extracts light to the substrate side on which the organic EL element is formed.

  In the present embodiment, the organic EL element 5 is disposed on the transparent insulating substrate 1. The organic EL element 5 includes an organic compound layer (organic EL layer) 3 including a light emitting layer sandwiched between a pair of transparent electrodes 2 and 4. Specifically, the substrate 1 has an anode electrode 2, an organic compound layer 3 provided on the anode electrode 2, and a cathode electrode 4 provided on the organic compound layer 3. In the present invention, the anode electrode 2, the organic compound layer 3, and the cathode electrode 4 are combined to form an organic EL element 5.

  As the insulating substrate 1, a glass substrate having a refractive index of about 1.5 is generally used. As the anode electrode 2, an electrode having a high work function is preferable from the viewpoint of hole injection properties, and transparent electrodes such as ITO and IZO are exemplified. When the refractive index of the insulating substrate 1 is lower than the refractive index of the organic compound layer 3, total reflection occurs at the interface between the anode electrode 2 and the insulating substrate 1. In addition to the light emitting layer, the organic compound layer 3 can include a hole injection layer, a hole transport layer, an electron transport layer, and an electron injection layer, and each layer can be made of a known material. Also, a known film forming method such as vapor deposition or transfer can be used. As the cathode electrode 4, it is preferable to use a transparent electrode such as ITO or IZO, and to ensure the electron injection property by using a known electron injection layer as required.

  Here, in the conventional organic EL element, the cathode electrode 4 is made of a highly reflective metal film. Therefore, the guided light is repeatedly reflected between the interface between the insulating substrate 1 and air and the cathode electrode 4 or between the interface between the anode electrode 2 and the insulating substrate 1 and the cathode electrode 4. Will propagate. However, the reflectance of Al or Ag, which is generally called a highly reflective metal, is about 90% to 95%, and about 5% to 10% is absorbed by the metal film. Therefore, when the inside of the organic EL element is reciprocated five times, the absorption in the metal film becomes about 20% to 40%, and even if a structure for extracting guided light is provided, a large improvement in light extraction efficiency cannot be expected.

  Therefore, in the present invention, a low refractive index layer 6 made of a material having a refractive index lower than that of the organic compound layer 3 in contact with an electrode (cathode electrode 4 in the present embodiment) located on the side opposite to the light extraction side, a metal The reflective film 7 is provided in this order. The electrode opposite to the light extraction side (the cathode electrode 4 in this embodiment) is a transparent electrode having a higher refractive index than the organic compound layer 3. Thereby, the guided light propagating in the organic EL element 5 is totally reflected at the interface between the cathode electrode 4 and the low refractive index layer 6, and the guided light does not enter the metal reflecting film 7. Absorption of guided light is suppressed.

  Here, there are two conditions for totally reflecting the guided light at the interface between the cathode electrode 4 and the low refractive index layer 6, and the refractive index of the low refractive index layer 6 is smaller than the refractive index of the organic compound layer 3. In other words, the low refractive index layer 6 has a sufficient thickness to cause total reflection of the guided light. When guided light is incident at an incident angle greater than the critical angle at the interface between the cathode electrode 4 having a refractive index higher than that of the organic compound layer 3 and the low refractive index layer 6, an evanescent wave as near-field light is generated. When the film thickness of the low refractive index layer 6 is thin, an evanescent wave is incident on the metal reflection film 7, so that the reflectivity decreases due to metal absorption or surface plasmon. Here, FIG. 2 shows the relationship between the film thickness d of the low refractive index layer 6 and the absorptance, assuming that all the evanescent waves incident on the metal reflecting film 7 are absorbed. The horizontal axis is the incident angle of guided light in the organic compound layer 3 when the direction perpendicular to the substrate is 0 °, and the vertical axis is the thickness of the low refractive index layer 6 normalized by wavelength (d / λ). It is a thing. As calculation conditions, the refractive index of the organic compound layer 3 was 1.8, the refractive index of the cathode electrode 4 was 2.0, and the refractive index of the low refractive index layer 6 was 1.4. As can be seen from FIG. 2, under this condition, the film thickness d of the low refractive index layer 6 may be set to the incident wavelength λ or more in order to make the absorption rate of the guided light having an incident angle of 60 ° or more 5% or less. Furthermore, the film thickness d of the low refractive index layer 6 may be 1.5 times or more of the incident wavelength λ in order to make the absorption rate of the guided light having an incident angle of 60 ° or more 1% or less. Actually, not all evanescent waves incident on the metal reflection film 7 are absorbed, so if the film thickness d of the low refractive index layer 6 is set to a wavelength λ or more, the absorption by the metal film is suppressed as compared with the conventional organic EL element. be able to.

  Note that light having an incident angle of about 35 ° to 90 ° is normally confined in the organic EL element 5 by total reflection among the light emitted from the organic compound layer 3, so that light having an incident angle of 60 ° or more is included in the guided light. Accounts for more than half. Most of the guided light having an incident angle of less than 60 ° is within the critical angle at the interface between the cathode electrode 4 and the low refractive index layer 6, so that the metal reflection film 7 and the interface between the insulating substrate 1 and the air The reflection will be repeated between them. However, the effect of the present invention can be expected if the absorption by the metal film is suppressed in the guided light having an incident angle of 60 ° or more.

  The effect of the present invention will be described in more detail. FIG. 3 shows the reflectance of light incident on the metal reflective film 7 from the organic compound layer 3 in the present invention. The film thickness of the low refractive index layer 6 was set to 520 nm which is the same as the wavelength, and the reflectance of light having a wavelength of 520 nm was calculated with the metal reflecting film 7 being Al. As a comparative example, the result when light is directly incident on the metal reflection film from the organic compound layer 3 in the conventional organic EL element is also described. The refractive index of the low refractive index layer 6 is calculated in two ways of 1.2 and 1.4. In either case, the reflectance is higher than the conventional configuration, and absorption in the metal reflective film can be suppressed. I understand that In general, the film thickness from the cathode electrode to the anode electrode in an organic EL element is about 200 nm to 300 nm. Therefore, when guided by 5 μm, the guided light travels between the cathode electrode and the anode electrode about 5 to 10 times. . FIG. 4 shows the fifth power of the reflectance in FIG. 3 on the assumption that the guided light has made five round trips, but it can be seen that the guided light in the conventional configuration is halved after five round trips. On the other hand, in this configuration, when the refractive index of the low refractive index layer 6 is 1.4, it is about 1.5 times that of the conventional configuration, and when the refractive index of the low refractive index layer 6 is 1.2, the conventional configuration. It can be seen that there is approximately 2.0 times as much guided light as. Although the organic material constituting the organic EL element and the transparent electrode such as ITO and IZO have some absorption in the short wavelength region, the absorption is almost negligible in most wavelength bands of visible light. Therefore, most of the attenuation of the guided light is absorption by the metal reflection film, and the present invention for suppressing such absorption is effective.

  Further, when the guided light whose absorption by the metal reflecting film 7 is suppressed is provided in the air by providing a light extraction structure, the light extraction efficiency can be improved. In FIG. 1, a light extraction layer 8 is provided as a light extraction structure. In the present invention, examples of the light extraction structure include a structure in which guided light is extracted into the air using reflection, diffraction, scattering, and the like, and a structure in which a low refractive index material is disposed on the end face of the organic EL element. If the light extraction structures are arranged at a narrow pitch, the aperture ratio of the light emitting pixels is lowered and the manufacturing method becomes difficult, and therefore, the light extraction structures are often arranged at a pitch of several μm.

  As specific examples of the light extraction structure, in the following examples, a structure in which a low refractive index material is disposed on the end face of an organic EL element (Example 1), a diffraction grating structure (Example 2), and a reflection mirror structure (Example 3) The present invention can be applied to other light extraction structures. For example, the method of extracting guided light by scattering is effective for adjusting the chromaticity according to the present invention because it is extracted to the outside by scattering while reciprocating in the light emitting device as in the diffraction grating.

  The above light-emitting device can also be applied to a display device. In the display device of the present invention, the organic EL element that emits red, the organic EL element that emits green, and the organic EL element that emits blue, and a common low refractive index layer is provided for these organic EL elements, The thickness is preferably equal to or greater than the peak wavelength of red light emission.

Example 1
An embodiment in which a low refractive index material is disposed on the end face (end portion in the in-plane direction) of a blue light emitting organic EL element as a light extraction structure will be described with reference to FIG. Porous silica, which is a low refractive index material (n = 1.2), is formed as a low refractive index structure 18 on a transparent insulating substrate 1 on which ITO is formed as the anode electrode 2. Then, a grid having a width of 1 μm and a height of 0.3 μm is produced by a normal photolithography process. At this time, an opening of 5 μm × 5 μm is provided as the organic EL light emitting region.

  Further, an organic compound layer 3 including a hole injection layer, a hole transport layer, a blue light emitting organic light emitting layer, an electron transport layer, and an electron injection layer is formed by vapor deposition, and IZO is sputtered as the cathode electrode 4. In addition, since a well-known thing can be used as a material of an organic compound layer, it abbreviate | omits for details.

  The low refractive index layer 6 was formed by LiF (n = 1.4) vapor deposition with a film thickness approximately the same as the emission peak wavelength of the organic EL element, and the metal reflective film 7 was formed by vapor deposition. Finally, sealing was performed with a cap can.

  By performing the above steps consistently in a vacuum, guided light with improved color purity can be taken out into the air from the low refractive index structure 18 disposed on the end face of the organic EL element 5.

(Example 2)
FIG. 6 is a cross-sectional view schematically showing the configuration of this example. The light emitting device of this example is a top emission type that extracts light on the side opposite to the substrate on which the organic EL element is formed. As a light extraction structure, a diffraction grating is provided on the light extraction side of the organic EL element. Since the diffraction efficiency of the diffraction grating is usually as low as 5% to 20%, the guided light travels back and forth within the light emitting device before extraction by diffraction. Therefore, the present invention for adjusting the chromaticity of the guided light is effective even when a diffraction grating is provided as the light extraction structure. This will be specifically described below.

  In this embodiment, a metal reflection film 7, a low refractive index layer 6, an anode electrode 2, an organic compound layer 3, and a cathode electrode 4 are formed on an insulating substrate 1. Above the cathode electrode 4 is a protective film 28 that protects the organic EL element 5 from moisture and oxygen and a diffraction grating layer 29 that functions as a light extraction structure.

  The protective film 28 is preferably a member having high light transmittance and excellent moisture resistance, and is preferably a silicon nitride film or a silicon oxynitride film. A known method such as a CVD method can be applied as a film forming method. The diffraction grating layer 29 can be produced by applying a nanoimprint method to a resin material. As the resin material, a thermosetting resin, a thermoplastic resin, or a photocurable resin can be used. As an example, the mold may be peeled by being cured by heat while pressing a mold for forming a desired periodic structure against the applied thermosetting resin. The period of the diffraction grating is preferably about 0.5 to 1.5 times the emission peak wavelength because the diffraction efficiency is high.

(Example 3)
FIG. 7 is a cross-sectional view schematically showing the configuration of this example. The light emitting device of this example is a top emission type in which light is extracted to the side opposite to the substrate on which the organic EL element is formed. The light extraction structure includes a reflection mirror 38 that extracts guided light into the air by reflection.

  The organic EL element in this example is obtained by forming a metal reflective film 37, a low refractive index layer 6, an anode electrode 2, an organic compound layer 3, and a cathode electrode 4 on an insulating substrate 1. In this embodiment, in order to suppress reflection of external light, a metal reflection film 37 which is a low reflection film is provided on the side opposite to the light extraction surface of the organic EL element 5. The definition of the low reflection film is that the reflectance in visible light is 20% or less, more preferably 10% or less. The guided light repeats total reflection between the interface between the anode electrode 2 and the low refractive index layer 3 and the interface between the cathode electrode 4 and air and is taken out into the air by the reflection mirror 38. By making the film thickness of the low refractive index layer 6 equal to or greater than the emission peak wavelength of the organic EL element 5, the guided light is incident on the reflection mirror without being absorbed by the metal reflection film 7 which is a low reflection film. . Therefore, in the present embodiment, both reduction of external light reflection and improvement of light extraction efficiency are achieved.

  2, 4: Transparent electrode, 3: Organic compound layer, 5: Organic EL element, 6: Low refractive index layer, 7: Metal reflection film

Claims (6)

An organic EL element having a pair of transparent electrodes and an organic compound layer including a light emitting layer disposed between the transparent electrodes;
The transparent electrode opposite to the light extraction surface of the organic EL element has a refractive index higher than that of the organic compound layer,
On the side opposite to the light extraction surface of the organic EL element, a low refractive index layer having a refractive index lower than that of the organic compound layer, and a metal reflective film in this order from the organic compound layer side,
The film thickness of the low refractive index layer has a thickness that causes total reflection of light incident at a critical angle or more from the low refractive index layer at the interface with the transparent electrode,
A light-emitting device having a light extraction structure for extracting guided light.   The light emitting device according to claim 1, wherein a film thickness of the low refractive index layer is equal to or greater than a light emission peak wavelength of the organic EL element.   The said light extraction structure is a low-refractive-index structure body with a refractive index lower than the organic compound layer arrange | positioned in the in-plane direction edge part of the said organic EL element, The Claim 1 or 2 characterized by the above-mentioned. Light emitting device.   The light-emitting device according to claim 1, wherein the light extraction structure is a structure that extracts guided light by reflecting, diffracting, or scattering the guided light.   5. The light emitting device according to claim 1, wherein the metal reflection film is a low reflection film having a reflectance in visible light of 20% or less. A display device comprising an organic EL element that emits red, an organic EL element that emits green, and an organic EL element that emits blue,
The organic EL element has a pair of transparent electrodes and an organic compound layer including a light emitting layer disposed between the transparent electrodes,
The transparent electrode opposite to the light extraction surface of the organic EL element has a refractive index higher than that of the organic compound layer,
On the side opposite to the light extraction surface of the organic EL element, a low refractive index layer having a refractive index lower than that of the organic compound layer, and a metal reflective film in this order from the organic compound layer side,
The film thickness of the low refractive index layer has a thickness that causes total reflection of light incident at a critical angle or more from the low refractive index layer at the interface with the transparent electrode,
A light extraction structure for extracting guided light is provided,
A display device characterized in that the thickness of the low refractive index layer is common to all organic EL elements and is equal to or greater than the emission peak wavelength of red light emission.

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