Light Emitting Element Having a Capping Layer on an Electrode, Light Emitting Device Having the Same and Method for Manufacturing the Same

A light emitting element comprises a first electrode, a second electrode configured to transmitting light, an organic layer arranged between the first and the second electrodes, comprising a light emitting layer, and a capping layer arranged on the second electrode and made of a material with a higher refractive index than the refractive index of the material constituting the second electrode. The material constituting the capping layer comprises at least one selected from the group consisting of triarylamine derivative, carbazole derivative, benzimidazole derivative and triazole derivative.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a light emitting element which is used in organic electroluminescent display devices, a light emitting device comprising the light emitting element, and a method for manufacturing the same.

2. Description of the Related Art

An organic electroluminescent (EL) display device has properties such as thin type, wide viewing angle, low power consumption and excellent moving picture display and has been expected to be an image display device of the next generation. An organic electroluminescent element (hereinafter, referred to as “light emitting element”) which is used for the organic EL display device, has at least one light emitting layer between two electrodes such that the light emitting layer emits the light by applying the voltage between the electrodes and can display an image.

Until now, a light emitting element with a bottom emission structure emitting light from the bottom part has been produced by laminating a transparent electrode as an anode such as ITO (Indium Tin Oxide), a charge transport layer, a light emitting layer comprising organic compounds, and a cathode comprising metal such as Al sequentially onto a glass substrate.

Recently, a light emitting element with a top emission structure emitting light from the upper part has become more commonly used. The top emission structure utilizes metal having high work function for the anode. Unlike the light emitting element with the bottom emission structure of which the area of the light emitting part is limited depending on the pixel circuit, the light emitting element with top emission structure has advantage in widening the area of the light emitting part. As for the light emitting element with the top emission structure, a semi-transparent electrode such as LiF/Al/Ag (Applied Physics Letters, United States, 2001, vol. 78, pp. 544-546, hereinafter referred to as ‘Non-patent literature 1’), Ca/Mg (Applied Physics Letters, United States, 2003, vol. 82, pp 466-468, hereinafter referred to as ‘Non-patent literature 2’) or LiF/MgAg is used for the cathode. The contents of these publications are incorporated herein by reference in their entirety. When the emitted light from the light emitting layer enters the other layer with an incident angle larger than a certain value in this light emitting device, the light is totally reflected on the interface between the light emitting layer and the other layer thickness. For this reason, it cannot be avoided that only a part of the emitted light is used. Recently, it has been proposed the light emitting device which is provided with a capping layer having a high refractive index, on the outside of the semi-transparent electrode having a low refractive index so as to improve the light extraction efficiency (Non-patent literature 1, 2).

Non-patent literature 2 discloses the effect of the capping layer in the light emitting element with a top emission structure. According to the Non-patent literature 2, a light emitting element using Ir(ppy)3(fac-tris-(2-phenylpyridinato)indium) as a light emitting material, showed 38 cd/A of current efficiency when the light emitting element was not provided with a capping layer, while the light emitting element provided with a capping layer made of ZnSe (layer thickness 60 nm), showed 64 cd/A of current efficiency which is about 1.7 times higher.

Furthermore, the Non-patent literature 2 discloses that the maximum point of transmittance of the semi-transparent electrode and capping layer is not necessarily coincident with the maximum point of light extraction efficiency of the semi-transparent electrode and capping layer, thus it is indicated that the maximum point of light extraction efficiency is determined by an interference effect.

It has been proposed that a metal mask with a high accuracy is used for forming a capping layer, however the metal mask has a problem of being inferior in alignment accuracy due to the deformation by heat. That is, ZnSe used in the Non-patent literature 2 has a high melting point as 1100° C. or more, so it is difficult to be deposited on the accurate position with the mask having a high accuracy. Most of inorganic compounds are not suitable when using the mask having a high accuracy and also may inflict damage on the light emitting element itself, due to the high deposition temperature. Moreover, since forming an inorganic layer by the sputter method may inflict damage on the light emitting element, the inorganic compounds are difficult to be applied for a capping layer.

According to the Non-patent literature 1, Tris-(8-hydroxyquinolinato)aluminum (Alq3) is used as a capping layer for adjusting the refractive index. Alq3is known for an organic EL material showing green light emitting and weakly absorbs light having a wavelength of about 450 nm which is the wavelength of general blue light used for a general blue light emitting element, as shown inFIG. 8. For this reason, both color purity and light extraction efficiency decrease in the blue light emitting element.FIG. 8is a graph showing the relation of refractive index and extinction coefficient to wavelength in the capping layer using Alq3. InFIG. 8, the solid line depicts refractive index and the broken line depicts extinction coefficient.

SUMMARY OF THE INVENTION

According to one aspect of the invention, a light emitting element comprises a first electrode; a second electrode configured to transmit light, an organic layer arranged between the first and the second electrodes, comprising a light emitting layer; and a capping layer arranged on the second electrode and made of a material with a higher refractive index than the refractive index of the material constituting the second electrode. The material constituting the capping layer comprises at least one selected from the group consisting of triarylamine derivative, carbazole derivative, benzimidazole derivative and triazole derivative.

According to another aspect of the invention, a first light emitting device comprises a plurality of the light emitting elements which are at least classified into a first light emitting element, a second light emitting element which emits light in a different color from the first light emitting element, and a third light emitting element which emits light in a different color from the first and the second light emitting elements. The capping layers of the first, the second, and the third light emitting elements have respectively different thickness from each other.

According to another aspect of the invention, a first method for manufacturing the first light emitting device in which the capping layers of the light emitting elements are made of a homogeneous material comprises depositing the material constituting the capping layer onto the second electrodes of the first and the second light emitting elements, to form a first layer onto the second electrodes of the first light emitting element, and to form the capping layer onto the second electrode of the second light emitting element, respectively. The first method also comprises depositing the material constituting the capping layer onto the first layer and onto the second electrode of the third light emitting element, respectively, to form the capping layer onto the second electrodes of the first and the third light emitting elements.

According to another aspect of the invention, a second method for manufacturing a light emitting device comprises providing a substrate and a first, a second, and a third light emitting elements on the substrate. Each of the light emitting elements comprises a first electrode, a second electrode transmitting the light, and an organic layer arranged between the first and second electrodes and comprising a light emitting layer. Each of the light emitting elements also comprises a capping layer arranged on the second electrode and made of a material with a higher refractive index than the refractive index of the material constituting the second electrode and with a bandgap of 3.2 eV or higher. The second method also comprises depositing the material constituting the capping layer onto the second electrode of the first and the second light emitting elements, to form a first layer onto the second electrode of the first light emitting element and to form the capping layer onto the second electrode of the second light emitting element respectively. The second method further comprises depositing the material constituting the capping layer onto the first layer of the first light emitting element and onto the second electrode of the third light emitting element respectively, to form the capping layer onto the second electrode of the first and the third light emitting elements.

According to one aspect of the invention, a second light emitting device comprises a plurality of light emitting elements. Each of the light emitting elements comprises a first electrode, a second electrode transmitting the light, an organic layer arranged between the first and the second electrodes and comprising a light emitting layer. Each of the light emitting elements also comprises a capping layer arranged on the second electrode and made of a material with a higher refractive index than the refractive index of the material constituting the second electrode and with a bandgap of 3.2 eV or higher. The light emitting elements are classified into a first light emitting element emitting red light, a second light emitting element emitting green light, and a third light emitting element emitting blue light. When the thickness of the capping layer of the first light emitting element (dR), the thickness of the capping layer of the second light emitting element (dG), and the thickness of the capping layer of the third light emitting element (dB) are different from each other.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Hereinafter, exemplary embodiments of a light emitting element, a light emitting device comprising the light emitting element, and a method for manufacturing the same according to the present invention will be explained in detail with reference to the drawings. However, the present invention is not limited by the embodiments described below.

First, a light emitting element of the first embodiment will be described.FIG. 1is a side sectional view for illustrating a light emitting element according to an embodiment of the present invention. InFIG. 1, a light emitting element1has a top emission structure, where an anode11as a first electrode made of metal, a hole transport layer12, a light emitting layer13, a electron transport layer14, a semi-transparent cathode15as a second electrode, and a capping layer16are sequentially laminated onto a substrate such as glass substrate10.

In the light emitting element1with a top emission structure, the glass substrate10may not be necessarily transparent. The hole transport layer12transports holes from the anode11to the light emitting layer13. The electron transport layer14transports electrons from the semi-transparent cathode15to the light emitting layer13. When holes from the hole transport layer12and electrons from the electron transport layer14are recombined in the light emitting layer13, the electrons of the organic material in the light emitting layer13transit to the excited state such as the singlet excited state and/or triplet excited state, and then when being returned to the stable ground state from the excited state, the light emitting layer13emits fluorescent or phosphorescent light.

The capping layer16is made of a material which can form a layer reliably at a lower temperature and is composed of the material which does not show a major absorption at a wavelength range of each of blue, green, and red. The material constituting the capping layer16will be described later.

The sum of thickness of the above mentioned anode11, the hole transport layer12, the light emitting layer13, the electron transport layer14, and the semi-transparent cathode15is in the range of about 200 nm to 600 nm. And the thickness of the capping layer16is preferably in the range of 30 nm to 120 nm, for example. When the capping layer16has the thickness of the above mentioned range, high light extraction efficiency can be obtained. Further, the thickness of the capping layer16can be properly changed depending on the kind of the light emitting material used for light emitting element, the thickness of the layers constituting light emitting element other than the capping layer16, and so on. Materials for the semi-transparent cathode16include Ca/Mg, MgAg, MgAl, ITO, IZO, and so on.

From the aspect of assuring density of a layer and high step coverage, chemical methods such as CVD (Chemical Vapor Deposition) method are preferably used for forming a layer. Also, other than CVD method, for example, vacuum deposition method can be applied. Also, thermal deposition method, inkjet method or gravure printing, and the like can be applied.

Further, the present invention is not limited to the light emitting element1with top emission structure described inFIG. 1, but can be applied to a light emitting element with bottom emission structure or a light emitting element with dual emission structure which emits the light from both the upper part and the bottom part. In these cases, an electrode which is disposed in the direction of extracting light from the light emitting element to the outside, need to be transparent or semi-transparent.

Also,FIG. 1describes an example having a three-layer structure comprising a hole transport layer12, a light emitting layer13, and an electron transport layer14, which are interposed between an anode11and a semi-transparent cathode15. However, the structure between the anode11and the semi-transparent cathode15is not limited to the three layer structure. Various structures such as a five-layer structure comprising hole injection layer and electron injection layer additionally, a four-layer structure, a two-layer structure, and a one-layer structure comprising only light emitting layer, can also be applied to the light emitting element, depending on various conditions.

Preferably, the refractive index of the material constituting the capping layer16is higher than the refractive index of the electrode adjacent to the capping layer. That is, although the light extraction efficiency of the light emitting element1is improved by the capping layer16, it is more effective when a reflectance is higher at the interface between the capping layer16and the material adjacent to the capping layer16due to the large light interference effect. For this reason, the refractive index of the material constituting the capping layer16is preferably higher than the adjacent electrode and more preferably 1.5 or higher.

FIG. 2is a graph for showing the relation of refractive index and extinction coefficient to wavelength in capping layer using CBP represented by formula (7) described below. A solid line depicts the refractive index and a broken line depicts the extinction coefficient in the drawing. The refractive index means the value of real part of complex refractive index, and extinction coefficient means the value of imaginary part of complex refractive index. For the determination of refractive index and extinction coefficient, n & k method is used, which is well-known. As shown in the drawing, the refractive index of CBP is about 1.75 or higher at every wavelength. Major extinction does not occur at about 450 nm which is used for blue light emitted by blue light emitting element. Also, major extinction does not occur at other wavelength and the extinction coefficient of the capping layer is 0.12 or lower at 380 nm to 780 nm of wavelength range of the light which transmits the capping layer. Accordingly, major absorption does not occur at wavelength range of each of blue, green, and red, and the light emitting element having the capping layer is applied to full color displays desirably.

A bandgap of the material constituting the capping layer16is preferably 3.2 eV or higher. A bandgap of 3.2 eV corresponds to about 387 nm of light wavelength and 3.1 eV corresponds to about 400 nm. Accordingly, in order to keep the capping layer16transparent within almost full range of visible part (that is, 380 nm to 780 nm,) a bandgap of the material constituting the capping layer16is preferably 3.2 eV or higher. A bandgap below 3.2 eV has influence on the wavelength of blue color.

More particularly, the higher a bandgap of the material constituting the capping layer16is, the closer an optical absorption edge of the electron transiting within the bandgap moves to short wavelength side. Accordingly, the higher the bandgap of the material is, the more the deviation of the optical absorption edge from the visible part is, thereby the optical absorption of the material lessens to be transparent. Since the material constituting the capping layer16is desirably transparent within almost full wavelength range of the visible part, a higher bandgap of the material is preferable. It is more preferable that a bandgap of the material be 3.2 eV or higher since the optical absorption edge is out of the visible part.

The bandgap of the material constituting the capping layer16can be obtained by measuring the wavelength of absorption edge in the optical absorption spectrum where the absorption edge is positioned on long wavelength side

As the material constituting the capping layer16, triarylamine derivatives can be used. Examples of the triarylamine derivatives include N,N′-Bis(3-methylphenyl)-N,N′-diphenylbenzidine (hereinafter, referred to as “TPD”) represented by formula (1), 4,4′,4″-tris[(3-methylphenyl)phenylamino]triphenylamine (hereinafter, referred to as “m-MTDATA”) represented by formula (2), 1,3,5-tris[N,N-bis(2-methylphenyl)-amino]-benzene (hereinafter, referred to as “o-MTDAB”) represented by formula (3), 1,3,5-tris[N,N-bis(3-methylphenyl)-amino]-benzene (hereinafter, referred to as “m-MTDAB”) represented by formula (4), 1,3,5-tris[N,N-bis(4-methylphenyl)-amino]-benzene (hereinafter, referred to as “p-MTDAB”) represented by formula (5), and 4,4′-bis[N,N-bis(3-methylphenyl)-amino]-diphenylmethane (hereinafter, referred to as “BPPM”) represented by formula (6). These materials can be used alone or in combination with more than one material.

For the capping layer16, triphenylamine derivatives can be used like triarylamine derivatives.

As another material constituting the capping layer16, carbazole derivatives can be used. Examples of the carbazole derivatives include 4,4′-N,N′-dicarbazol-biphenyl (hereinafter, referred to as “CBP”) represented by formula (7) and 4′4′,4″-tris(carbazol-9-yl)-triphenylamine (hereinafter, referred to as “TCTA”) represented by formula (8). These materials can be used alone or in combination with more than one material.

As another material constituting the capping layer16, benzimidazole derivatives can be used. Examples of the benzimidazole derivatives include 2,2′,2″-(1,3,5-phenylene)tris-[1-phenyl-1H-benzimidazole] (hereinafter, referred to as “TPBI”) represented by formula (9).

As another material constituting the capping layer16, triazole derivatives can be used. Examples of the triazole derivatives include 3-(4-biphenyl)-4-phenyl-5-t-butylphenyl-1,2,4-triazole (hereinafter, referred to as “TAZ”) represented by formula (10).

Also, the materials constituting the capping layer16with a bandgap of 3.2 eV or higher, include TPD (3.2 eV), m-MTDATA (3.2 eV), TAZ (4.0 eV), and so on. As for a material constituting the capping layer16with a bandgap below 3.2 eV, Alq3, Almq3, CuPc, and so on are enumerated.

According to the first embodiment, a light emitting element comprises a capping layer which is provided on the outside of a transparent or semi-transparent electrode, with higher refractive index than the semi-transparent electrode. Therefore, the light emitting element can improve light extraction efficiency. Moreover, since the capping layer can be formed at temperature of 500° C. or lower by using compounds such as triarylamine derivative and/or carbazole derivative for the capping layer, the light extraction efficiency of each color can be optimized without the major damage to the light emitting element by using a mask with high accuracy. The light emitting element can be applied to full-color displays desirably, and a bright image with high color purity and high resolution can be displayed.

Next, a light emitting device of the second embodiment of the present invention will be described.FIG. 3is a side sectional view for illustrating a light emitting device according to the second embodiment of the present invention. InFIG. 3, a light emitting device50comprises a substrate10, a first light emitting element20, a second light emitting element30, and a third light emitting element40. These light emitting elements20,30and40are provided on the substrate10.

Each of the first light emitting element20, the second light emitting element30, and the third light emitting element40comprises an anode11as a first electrode, a hole transport layer12, a light emitting layer13, a electron transport layer14, a semi-transparent cathode15A,15B, and15C, respectively, as a second electrode, and a capping layer16A,16B, and16C, respectively, which are sequentially laminated in the same way as the light emitting element1described inFIG. 1.

The first light emitting element20, the second light emitting element30, and the third light emitting element40emit red light, green light, and blue light, respectively. The capping layers16A,16B, and16C may be made of a homogeneous material or heterogeneous material. If the capping layers16A,16B, and16C are made of a homogeneous material, time and cost for forming the capping layers16A,16B, and16C can be reduced, thereby contributing to productivity improvement or cost reduction of the light emitting device50.

The thickness of the capping layers16A,16B, and16C may be same or different. When the thickness of the capping layer16A of the first light emitting element20is dR, the thickness of the capping layer16B of the second light emitting element30is dG, and the thickness of the capping layer16C of the third light emitting element40is dB, it is preferable that the formula of dR>dG>dBbe satisfied. Red color has the longest wavelength and blue color has the shortest wavelength among red, blue, and green color. Therefore, the thickness of the capping layers16A,16B, and16C may be determined depending on the wavelength of the light they emit, to improve light extraction efficiency of the first light emitting element20to the third light emitting element40corresponding to each color more.

Also, the description about the light emitting element in the first embodiment including materials for the capping layer, layer thickness or bandgap, and refractive index are equally applied to the first light emitting element20, the second light emitting element30, and the third light emitting element40in the light emitting device50. Further, the number of the light emitting element is not limited to three, and a plurality of light emitting elements can be provided additionally on the same substrate, or on the different substrate.

[Method for Manufacturing Light Emitting Device]

Next, a method for manufacturing a light emitting device of third embodiment of the present invention will be described.FIG. 4toFIG. 6are side sectional views for explaining a method for manufacturing a light emitting device according to the third embodiment of the present invention. In these figures, the illustration between semi-transparent cathode15A,15B, and15C and anode11is simplified, to make the explanation easier to understand.

InFIG. 4, a light emitting device50comprises a substrate10, a first light emitting element20, a second light emitting element30, and a third light emitting element40. These light emitting elements20,30, and40are provided on the same substrate10. Each of the first light emitting element20, the second light emitting element30, and the third light emitting element40comprises an anode11as a first electrode, a hole transport layer12, a light emitting layer13, a electron transport layer14, and a semi-transparent cathode15A,15B, and15C, respectively, as a second electrode, which are sequentially laminated in the same way as the light emitting element1described inFIG. 1.

When the capping layers16A,16B, and16C are formed of a homogeneous material to have different thickness, as illustrated inFIG. 5, the material for constituting a capping layer is laminated onto the semi-transparent cathodes (15A and15B) in the first light emitting element20and the second light emitting element30, respectively, by mask deposition method using a deposition mask19A such as shadow mask, and then a first layer17made of the material is formed to have layer thickness d1. In this case, the first layer17is formed as a semi-formed layer17of the capping layer, since in the next step, the same material is laminated onto the first layer17which is made of the material for constituting capping layer and formed onto the semi-transparent cathode15A in the first light emitting element20. On the other hand, the first layer17formed on the semi-transparent cathode15B in the second light emitting element30is the capping layer16B itself.

Next, as illustrated inFIG. 6, a second layer18made of the material constituting capping layer is formed on the first layer17in the first light emitting element20and on the semi-transparent cathode15C in the third light emitting element40, respectively, by mask deposition method using a deposition mask19B. The second layer18is formed to have a layer thickness d2. In the first light emitting element20, the capping layer16A consists of the first layer17and the second layer18formed thereon. On the other hand, in the third light emitting element40, the second layer18formed onto the semi-transparent cathode15C, is the capping layer16C itself.

Since the capping layer16A in the first light emitting element20consists of the first layer17and the second layer18formed thereon, when the thickness of the capping layer16A is dR, the formula of dR=d1+d2is satisfied. In other words, the thickness of the capping layer16A in the first light emitting element20is substantially the same as the sum of the thickness d1of the capping layer16B in the second light emitting element30and the thickness d2of the capping layer16C in the third light emitting element40. However, the sum of the thickness may not be the same value as the formula depending on the condition of deposition, but may be within 10% with respect to the thickness dRof the capping layer16A.

In this way, the capping layers16A,16B, and16C having different layer thickness are formed through two steps. In the case that the capping layers16A,16B, and16C are made of a homogeneous material and have different layer thickness, the two-step method is more simplified in comparison with the three-step method where each of the capping layers16A,16B, and16C is formed by a separate step. As a result, the two-step method can contribute to the productivity improvement of the light emitting device. Also, the descriptions about the light emitting element in the first embodiment and light emitting device in the second embodiment are equally applied to the light emitting device described on the basis ofFIG. 4throughFIG. 6.

EXAMPLE

The example of the present invention will be explained more specifically. A red light emitting element of the present invention has been manufactured as below. First, Ni anode with layer thickness of 100 nm was laminated by sputtering method onto the substrate and then copper phthalocyanine (CuPc) with layer thickness of 20 nm as a hole injection layer was laminated through deposition method onto the Ni anode. Subsequently, N,N′-di(naphthalene-1-yl)-N,N′-diphenyl-benzidine (NPB) with layer thickness of 40 nm as a hole transport layer was laminated through deposition method. And then, Alq3and 4-(dicyanomethylene)-2-t-butyl-6-(1,1,7,7,-tetramethyljulolidyl-9-enyl)-4H-pyran (DCJTB) as a light emitting layer were deposited by dual deposition. The layer thickness of Alq3was 30 nm and 1% of DCJTB as the volume ratio was laminated simultaneously.

Then, Alq3with layer thickness of 40 nm as an electron transport layer, and Ca with layer thickness of 12 nm and Mg with layer thickness of 12 nm as a cathode were laminated sequentially through deposition method. Finally, CBP as a capping layer was laminated through deposition method, changing the layer thickness in the range of from 0 nm to 100 nm. The method for forming red pixel is described above, however, green and blue pixel can be formed by substantially the same method except for the thickness and material of the layer. To form a red, green, and blue pixel of which the position is selected respectively for the forming, a metal mask having an opening at the position of laminating is aligned precisely to a substrate, and then the metal mask is contacted to the side of the main surface of the substrate which is to be deposited, and finally the layers are deposited.

The light extraction efficiency of the red light emitting elements which have been manufactured to have capping layers with different thickness, were determined.FIG. 7shows the result. As clear fromFIG. 7, the capping layer with layer thickness of 70 nm showed the highest light extraction efficiency, which is 1.9 times higher than the light extraction efficiency in the case of a light emitting element without the capping layer. The capping layers with layer thickness between 50 nm and 90 nm showed light extraction efficiency is 1.6 times or more higher than the light extraction efficiency in the case of a light emitting element without the capping layer, and the capping layers with layer thickness between 60 nm and 80 nm showed light extraction efficiency is 1.8 times or more higher than the light extraction efficiency in the case of a light emitting element without the capping layer. These results showed great improvement.

As described above, the light emitting element, the light emitting device comprising the light emitting element, and the method for manufacturing the same according to the embodiments, comprise the capping layer made of the material which can form the layer at a lower temperature reliably and does not show an major absorption at a wavelength range of blue, green, and red respectively. Therefore, they can be preferably applied to full color displays, to display a bright image with high color purity and high resolution.