JPH11154596A - Organic electroluminescent element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an organic EL element by which luminance is not reduced even if it is continuously used for many hours and which can maintain the high service life and can expand its application range. SOLUTION: At least a transparent electrode 2, an organic thin film layer and a cathode 5 are laminated in order on a glass substrate 1, and the organic thin film layer is formed of a hole transport layer 3 laminated/formed on the transparent electrode 2 and a light emitting layer 4 laminated/formed on it, and the hole transport layer 3 is formed of a laminated structure of a lower layer 3a having a characteristic of a low glass transition point and of a small energy barrier between it and an anode and an upper layer 3b having a characteristic of a glass transition point higher than this lower layer 3a, and attenuation of light emitting luminance with the lapse of time can be restrained.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an organic EL device (hereinafter referred to as "organic EL device"), and more particularly to an organic EL device which can achieve a long life by improving a hole transport layer laminated on a substrate. Related to the element.

[0002]

2. Description of the Related Art For example, an organic EL element used as a backlight of a liquid crystal display or a light source for display and optical communication of various displays is difficult to use with a conventional inorganic EL element by changing a light emitting material and a layer structure. Since various emission wavelengths including blue emission can be obtained, it has been widely used in various light emitting devices and color display fields.

The basic structure of an organic EL device is that various thin film layers necessary for light emission are laminated on the surface of a glass substrate, and a schematic sectional view thereof is shown in FIG.

Referring to FIG. 4, a transparent conductive film, for example, an ITO film is formed on the surface of a glass substrate 51 to form an anode 52, which is supplied to a resistance heating vacuum evaporation apparatus to form an organic layer. Are laminated. As this organic layer, a hole transport layer 53 made of, for example, TPD and a light emitting layer 54 made of, for example, Alq ₃ are sequentially deposited, respectively. Then, the cathode 5 is formed on the light emitting layer 54.
5 is deposited, and these deposited layers are covered with a protective film (not shown) because they are sensitive to water, and the product is obtained by removing the deposited layer from the apparatus.

In the organic EL device having the above structure, when a DC voltage or a DC current is applied with the anode 52 as a positive electrode and the cathode 55 as a negative electrode, the anode 52 is connected to the light emitting layer 54 via the hole transport layer 53. Holes are injected, and electrons are injected from the cathode 55 into the light emitting layer 54. Then, in the light-emitting layer 54, recombination of holes and electrons occurs, and a light-emitting phenomenon occurs when the exciton generated accordingly shifts from the excited state to the ground state.
Is emitted as a light extraction surface.

In order to improve the light emission characteristics of an organic EL device based on such a light emission principle, the structure of an organic thin film layer composed of a laminate of a hole transport layer 53 and a light emitting layer 54 and the improvement of an organic material used for the same have conventionally been performed. And the anode 52 and the cathode 5
For No. 5, the search for the most suitable material is in progress.

[0007] As one of the improvement of the light emission characteristics, improvement of the life of the organic EL element has been a major problem.
That is, it is known that an organic EL element has a large number of non-light-emitting portions on its light-emitting surface, making it difficult to obtain a uniform surface-emitting element. In addition, it is considered that the luminance is significantly deteriorated with time with respect to a decrease in luminance. For this reason, especially when driving is continued continuously for a long period of time, it is usually unavoidable that the luminance is reduced, and it is important for the future development to have little change over time and a long life.

Various improvements have conventionally been made to the hole transport layer 53 in the example shown in FIG. 4 to improve the life of the organic EL element. That is, the hole transport layer 53 has a function of injecting holes and transporting carriers (holes) to the light-emitting layer 54. The characteristics regarding such hole injection and carrier transport include ionization potential and ionization potential. It is greatly affected by the carrier moving speed. Since the hole transport layer 53 has an amorphous structure in many cases, if crystallization occurs when subjected to a thermal load, the values of the ionization potential and the carrier moving speed are deteriorated. Therefore, it can be said that the hole transport layer 53 is strongly related to the reduction of the light emission luminance, and is considered to be more effective for improving the lifetime of the organic EL element than for other elements such as the light emitting layer 54. Can be

On the other hand, compared to the hole transport layer 53, the light emitting layer 5
4 has a property closer to crystalline (microcrystalline) rather than amorphous, and thus is more affected by decomposition and electrochemical deterioration caused by photo-oxidation by light emission than by the concept of thermal deterioration.

[0010] From such a background, the conventional general guidelines for the material design of the hole transport layer 53 are based on the glass transition point Tg of the organic thin film forming the hole transport layer 53.
Is operated in the direction in which is raised. Glass transition in the field of organic chemistry refers to the change from a viscous or elastic state to a hard and relatively brittle state in the amorphous region of a partially crystallized polymer, usually caused by a temperature change. It is. Then, the hole transport layer 53 of the organic EL element efficiently injects holes from the anode with respect to the emission luminance and maintains the emission luminance, and furthermore, inserts the holes into the light emitting layer 5.
4, and must be stably maintained without any change over time. Therefore, it is effective to increase the glass transition point Tg.

[0011]

However, according to the findings of the present inventors, the glass dislocation point Tg of the hole transport layer 53 has been found.
Is not necessarily a condition for suppressing a decrease in emission luminance and improving the lifetime, and the difference between the light emitting layer 54 and the hole transport layer 53 occurring at an extremely thin interface (about several nm) with the light emitting layer 54 is not always possible. It became clear that the deterioration phenomenon greatly affected the life.

In other words, the hole transport layer 53 can be divided into a single type of organic thin film of a single layer and a layered structure of multiple organic thin films. In either case, as shown in FIG. Since it is laminated on the anode 54 using an ITO film, it is required that the heat resistance is high and the energy barrier between the anode 52 is small. At present, a material that satisfies such conditions has not yet been found. Therefore, in the case where the hole transport layer 53 is a single layer as in the example shown in the figure, the glass dislocation point Tg is increased. Thus, it is only possible to impart high heat resistance, and it cannot be said that sufficient efficacy can be achieved.

On the other hand, the concept of multi-layering the hole transport layer 53 has already been known. For example, as shown in US Pat. No. 4,720,432, copper phthalocyanine and low molecular weight Polymerized layer of aromatic tertiary amine, 40th JSAP Related Conference, 30a
-SZK-14 (1993) and a polymer layer of a star bust amine-based and TPD.

However, these are all anodes (I
This is merely for the purpose of enhancing the contact property with TO), and has a structure in which a copper phthalocyanine or star bust amine organic material is disposed as a buffer layer. Further, copper phthalocyanine and starburst amine-based compounds have a higher Tg than low molecular weight aromatics or TPD formed thereon, and therefore, an improvement in the life of the device cannot be expected. In addition, copper phthalocyanine and starburst amine have a structure in which the number of molecules is larger than that of a low molecular aromatic or TPD formed thereon. Therefore, similarly, improvement in the life of the element cannot be expected.

As described above, in the conventional organic EL device, optimization of the hole transport layer, which has the possibility of suppressing the decrease in luminance and improving the life, has not yet been achieved. However, the application range of the element is also restricted due to the limitation of the life improvement.

The problem to be solved in the present invention is to provide an organic EL device which can maintain a long life without a decrease in luminance even when used continuously for a long time and can expand its application range.

[0017]

According to the present invention, there is provided a hole transport layer in which at least an anode, an organic thin film layer, and a cathode are sequentially stacked on a substrate, and an organic thin film layer is formed on the anode, and the hole transport layer is formed thereon. An organic electroluminescent device as a light emitting layer, wherein the hole transport layer has a lower layer having a characteristic that a glass dislocation point is low and an energy barrier between the anode and the anode is small, and a glass dislocation point higher than this lower layer. It is characterized in that it has a laminated structure with an upper layer of characteristics.

With this configuration, it is possible to suppress the decay of light emission luminance over time and to develop a field of application as a light emitting element having a long life.

[0019]

DETAILED DESCRIPTION OF THE INVENTION The invention according to claim 1 provides a hole transport layer in which at least an anode, an organic thin film layer, and a cathode are sequentially laminated on a substrate, and the organic thin film layer is formed on the anode, and An organic electroluminescent device having a light emitting layer laminated on a hole transport layer, wherein the hole transport layer has a lower glass dislocation point and a smaller energy barrier between the anode and the anode, and a glass dislocation higher than the lower layer. It has a layered structure with the upper layer having characteristics with points, the lower layer on the anode side efficiently injects and transports holes, the upper layer with a high glass transition point maintains a long life, and has excellent characteristics and reliability. Has the effect of increasing.

According to a second aspect of the present invention, there is provided the organic electroluminescent device according to the first aspect, wherein the hole transport layer has a laminated structure of a lower layer and an upper layer having a larger number of molecules than the lower layer. The lower layer on the anode side has a function of efficiently injecting and transporting holes, and has a function of maintaining a long life by an upper layer having a high molecular number, and has excellent characteristics and reliability.

Hereinafter, specific examples of the embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a longitudinal sectional view schematically showing an organic EL device according to one embodiment of the present invention.

In FIG. 1, a transparent electrode 2 made of ITO (indium tin oxide), which is conventionally known as a transparent conductive film, is laminated on a transparent glass substrate 1, and this transparent electrode 2 is formed. A hole transport layer 3 and a light emitting layer 4 made of a tris (8-quinolinol) aluminum complex (Alq ₃ ) are sequentially stacked on the electrode 2 to form an organic thin film. A material cathode 5 is formed.

The transparent electrode 2 is formed by patterning using a conventionally well-known mask pattern.
Similarly, the light emitting layer 4 and the cathode 5 are formed by a vacuum deposition method widely used in the related art. The transparent electrode 2 is about 200 nm, the hole transport layer 3 is about 50 to 100 nm, the light emitting layer 4 is about 50 to 100 nm, and the cathode 5 is about 20 nm.
It is about 0 nm.

The hole transport layer 3 includes a lower layer 3a laminated on the transparent electrode 2 and an upper layer 3b further laminated thereon.
The glass transition point Tg is about 60 ° C. and about 130 ° C., respectively.

As described above, the light emitting layer 4 is a polymerized layer of the lower layer 3a and the upper layer 3b having different values of the glass dislocation point Tg, and the glass dislocation point Tg is smaller in the lower layer 3a and relatively larger in the upper layer 3b. If the material of the lower layer 3a is selected so that the energy barrier between the lower layer 3a and the transparent electrode 2 is reduced, it is possible to suppress the deterioration of the light emission luminance when the light from the light emitting layer 4 is extracted from the bottom surface of the glass substrate 1. it can.

Here, the hole transport layer 3 has an ionization potential for efficiently injecting holes from the transparent electrode 2 formed as an anode and for efficiently injecting holes into the light emitting layer 4. The value of is important. Further, the value of the electron affinity for blocking electrons injected from the cathode 5 into the light emitting layer 4 so as not to enter the transparent electrode 2 is also important. Further, the carrier mobility for efficiently transporting the injected holes to the light emitting layer 4 is also an indispensable factor for causing the device to emit light efficiently. It is very difficult at this stage to find a hole transport layer 3 that satisfies all these characteristics and has a high Tg for obtaining a longer life.

On the other hand, the hole transport layer 3 is multi-layered to separate the respective functions, or as described in the aforementioned US Pat. No. 4,720,432, an ITO film for forming the transparent electrode 2 is used. Attempts have already been made to add an improvement in another sense of improving the adhesion to the steel.
The present inventors have found that various deteriorations caused at the interface between the light emitting layer 4 and the hole transport layer 3 greatly affect the continuous operating life of the device, and the deteriorated layer is a very thin layer of about several nm. Confirmed that there is. If the hole transport layer 3 is a very thin layer having a thickness of about several nm, even if the hole transport layer 3 has a poor carrier transport ability, the light emitting characteristics of the organic EL element do not have a significant effect. Therefore, even if the value of the ionization potential necessary for efficient hole injection from the transparent electrode 2 is not suitable, and even if the carrier transporting ability is inferior, the hole injection into the light emitting layer 4 is performed. A hole transporting material having a high electron affinity necessary for efficiently performing the above-described step is suitable for the light emitting layer 4 and has a high Tg, and is disposed at the interface with the light emitting layer 4;
Although the Tg is low on the transparent electrode 2 side, the value of the ionization potential is suitable for the transparent electrode 2 and the carrier mobility for efficiently transporting holes is high, and the value of the electron affinity for blocking electrons is low ( (Absolute value) If the hole transport layer is used, it is possible to obtain an organic EL device having good luminous characteristics, long life, and high reliability.

The hole transport layer 3 provided on the light emitting layer 4 side
Does not need to be very thin if it has excellent carrier transporting ability.

As described above, although Tg is high and excellent in reliability, there is a problem in characteristics. However, the present invention provides various organic materials which could not be used as a hole transport material of an organic EL device. It also brings benefits in terms of material options that can be applied. Further, there is a possibility that an organic EL element having a very long life can be obtained by using a very reliable material which has not been considered to be applied to the organic EL element.

It is apparent that the work function and the electron affinity of the material of the hole transport layer 3 can be factors that determine the light emission characteristics. Therefore, lower layer 3
As described above, even if a has poor life characteristics, good light emission characteristics can be ensured only by selecting a material having a work function and an electron affinity satisfying suitable conditions for light emission characteristics. Based on the knowledge of the present inventors that the state of the interface between the hole transport layer 3 and the light emitting layer 4 is dominant in the lifetime of the device, injection of holes into the light emitting layer 4 is efficiently performed in the upper layer 3b. If a material that satisfies the work function that can be obtained and has a high glass transition point Tg is selected, it is possible to obtain an element having good emission characteristics and a long life.

As described above, in the present invention, the lowering of the light emission luminance can be prevented by the polymerization of the lower layer 3a and the upper layer 3b of the light emitting layer 3. For example, as shown in FIG. The lower layer 3c is disposed on the lower side as the first layer 3c.
When the same composition as a is polymerized as the second layer 3d, the life is shortened. The reason is as follows.

That is, the decrease in luminance during driving of the organic EL element is dominated by the deterioration occurring at the interface between the hole transport layer 3 and the light emitting layer 4. Organic materials having a low glass transition point Tg are more likely to undergo structural changes due to heat than materials having a high glass transition point Tg. On the other hand, in the structure arranged on the light emitting layer 4 side, molecules become easy to move due to Joule heat due to energization or heat of the external environment, which causes a reaction with the light emitting layer 4 and lowers brightness during driving. It is thought to be.

[0033]

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the structure of the device shown in FIG. 1, the thickness of the glass substrate 1 is 1 mm, the thickness of the transparent electrode 2 is 200 nm,
The layer thickness of the hole transport layer 3 is 50 nm and the lower layer 3a: 4
0 nm and upper layer 3b: 10 nm, layer thickness of light emitting layer 4: 75
m and the thickness of the cathode 5: 200 nm. Then, as shown in (Table 1), the upper layer 3b was made of TPOTA and the lower layer 3a was made of TPD,
Example 2 is a case where a is TPD and the upper layer 3b is triphenylamine tetramer.

As a comparative example, in the device configuration shown in FIG. 2, the first layer 3c and the second layer 3d have the same composition and the same layer thickness as the upper layer 3b and the lower layer 3a in the present invention, respectively. Made. That is, as shown in (Table 1), in each of Example 1 and Example 2 of the present invention, the lower layer 3a and the upper layer 3b are turned upside down to be Comparative Examples 1 and 2.

Table 1 shows the life time of each of Examples and Comparative Examples of the present invention as a half-life, and Table 2 shows Tg of TPD, TPOTA and triphenylamine tetramer. ° C). FIG. 3 is a plot of the change over time in luminance of each of the examples and the comparative examples obtained by experiments. The values of the half-life periods shown in Table 1 are based on these plots.

[0036]

[Table 1]

[0037]

[Table 2]

As is apparent from the plot of FIG.
In the configurations of Examples 1 and 2 of the present invention, the rate of decay of the emission luminance over time tends to be clearly smaller than those of Comparative Examples 1 and 2.

The difference between the embodiment of the present invention and the comparative example is only that the relationship between the lower layer 3a and the upper layer 3b in the examples 1 and 2 is a vertically inverted positional relationship in the comparative examples 1 and 2. The relationship between the values of the glass transition points Tg is also reversed. Therefore, the hole transport layer 3 is formed by
When the layers are polymerized, the relation that the glass dislocation point Tg of the lower layer 3a on the transparent electrode 2 side of the ITO is small and the glass dislocation point Tg of the upper layer 3b is large is a factor for suppressing the reduction of the emission luminance. You can see that.

[0040]

According to the first aspect of the present invention, by disposing a hole transport layer having a high glass transition point on the light emitting layer side, it is possible to suppress a diffusion reaction with the light emitting layer which causes a luminance decay during continuous driving. Therefore, the decay of the light emission luminance over time can be suppressed, and the life of the light emitting element can be significantly extended. Therefore, conventionally, the application field of the element has tended to be limited. However, the improvement of the service life enables application to various display devices and the like.

According to the second aspect of the present invention, by disposing a hole transporting layer having a large number of molecules on the side of the light emitting layer, it is possible to suppress a diffusion reaction with the light emitting layer which causes a decrease in luminance during continuous driving. The decay of the light emission luminance over time can be suppressed, and the life of the light emitting element can be significantly extended.

[Brief description of the drawings]

FIG. 1 is a longitudinal sectional view schematically showing an organic EL device according to an embodiment of the present invention.

FIG. 2 is a longitudinal sectional view schematically showing a case where a lower layer and an upper layer of a hole transport layer are polymerized upside down in the organic EL device of FIG. 1;

FIG. 3 is a plot diagram showing a change over time in light emission luminance of an example of the present invention and a comparative example.

FIG. 4 is a schematic longitudinal sectional view of an organic EL device of a conventional example in which a hole transport layer is a single layer.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Glass substrate 2 Transparent electrode 3 Hole transport layer 3a Lower layer 3b Upper layer 3c First layer 3d Second layer 4 Light emitting layer 5 Cathode

Claims (2)

[Claims] 1. An organic electroluminescent device comprising: a hole transport layer in which at least an anode, an organic thin film layer, and a cathode are sequentially laminated on a substrate, and an organic thin film layer formed on the anode and a light emitting layer formed thereon. In the sense element, the hole transport layer has a laminated structure of a lower layer having a low glass dislocation point and a small energy barrier between the anode and an upper layer having a glass dislocation point higher than the lower layer. An organic electroluminescent device, comprising: 2. The organic electroluminescence device according to claim 1, wherein said hole transport layer has a lower hole transport layer and an upper layer having a larger number of molecules than the lower hole transport layer. An organic electroluminescence device characterized by having a laminated structure of: