GB2352327A - Organic electroluminescence device with a metal oxide anode having a surface coated with nitrogen oxide - Google Patents

Abstract

An organic electroluminescent device has transparent substrate (1), anode (2) containing a metal oxide, such as indium-tin oxide or indium-zinc oxide as a major component, an organic electroluminescent layer, which may be composed of hole injection layer (3), hole transport layer (4), light-emitting layer (5), and cathode (6). The anode (2) is subjected to nitrogen oxide (e.g. NO<SB>2</SB>) decoration after deposition on substrate (1), which improves filling rate, adhesivity and thermal stability at the interface between the anode (2) and hole injection layer (3), resulting in lower driving voltage and longer lifetimes in the device. The construction may include seal-off plate (7). The electroluminescent layer may include a layer containing a phthalocyanine compound as a major component.

Description

2352327 ORGANIC ELECTROLUMINESCENCE DEVICE AND METHOD FOR MANUFACTURING

SAME The present invention relates to an organic electroluminescence device (also referred to as an organic EL device) used as various display units such as a monochromatic or multicoloured display in information apparatuses, a signal, and a television set. The invention also relates to a method of manufacturing such an organic EL device.

In recent years, an organic EL device, a light-emitting device having an organic substance layer, has been receiving wide attention, and studies are being conducted towards applications to display units. The organic light-emitting device being a full solid-state and self-light-emitting device, is attracting notices as a display exhibiting excellent visibility, which arises from features of high luminance, low operating voltage, and fast response in light-emission. An organic EL device commonly has a structure at least provided with layers of a first electrode on a substrate, an organic EL medium and a second electrode. Desired ligbt-emission is obtained by applying dc voltage between the first and the second electrode, and passing dc current through the organic EL medium.

Because transparency in visible light region and electric conductivity are required for an anode of the organic EL device, primary material selected for the anode may be metal oxide such as indium-tin oxide, indium-zinc oxide, or tin oxide. The anode is formed in a desired shape on a transparent substrate of glass, for example.

It is an important technology to control the boundary surface between the anode surface and the organic EL medium layer laminated thereon, particularly when the organic EL medium layer is formed with a thin evaporation film of low molecular substance. An organic light-emitting device of a laminate type involving low molecular substance is described as an example, which has a typical construction of an anode/ hole injection-and-transport layer/ an electron injection-and-transport and light-emitting layer/ a cathode. The initial performances such as a lightemission initiating voltage and a light-emission quantum efficiency are much affected by hole injection performance at the boundary surface between the anode and the hole injection-and-transport layer formed thereon when the hole injection-and- transport layer is formed of a phthalocyanine compound such as copper phthalocyanine, titanyloxyphthalocyanine, or metal-free phthalocyanine, or an aryldiamine compound or an aryltriamine compound such as tetraphenylbenzidine or N,N'-dinaphthyl-N,N'-diphenylbenzidine. The boundary region between the anode and the hole injection-and-transport layer is composed of metal oxide and organic low molecular substance which are thermally different from each other. Consequently, it is readily presumed that stability in preservation and stability at driving operation of an organic EL device are related to adhesivity, filling rate, and thermal stability of a crystalline or amorphous state in the hole injection-and-transport layer adjacent to the boundary surface.

An effective method for controlling the boundary surface in the device fabrication process is a dry cleaning of the anode surface. Ultraviolet light cleaning and plasma treatment are generally known as the methods for dry cleaning of a substrate surface undergone a photolithography process. The methods are practically used in the processes for manufacturing semiconductor devices and liquid crystal devices. The methods are also applied to cleaning of a substrate of an organic EL device as disclosed in Japanese Unexamined Patent Application Publication (KOKAI) Nos. H7-142168, H9-232075, H10-261484 and H10-255972.

Japanese Unexamined Patent Application Publication (KOKAI) No. H10-261484, for example, discloses the effect of the above-mentioned known method in the case in which cleaning of the anode surface is carried out by ultraviolet light cleaning. According to the disclosure, the cleaning effect arises from ozone and a singlet excitation oxygen atom which are generated by excitation of oxygen in the treatment gas by ultraviolet radiation. The ozone and the singlet excitation oxygen atom decompose and remove hydrocarbons, such as resist residues attached on the anode surface. Thus, the anode surface is cleansed, which in turn, gives rise to change of ionization potential of the anode surface.

Although the above described conventional technology applied to organic EL devices has brought about reduction in operating voltage and retardation of luminance decay caused by driving operation in some extent, the conventional technology is not enough for organic EL devices to be practically applied to large extent of areas. So, further improvements in 3 luminance stability and further reduction of operating voltage and power dissipation have been expected.

In view of the foregoing, it is an object of the invention to solve the above problem and develop a method for surface reforming to lead to high performance and stable anode substrate.

It is another object of the present invention to improve the performances of an organic EL device, in particular, a driving voltage, a driving operation life and a storage life.

The inventors of the present invention have conducted extensive studies for solving the above problems and have found that treating an anode substrate with nitrogen oxide decoration leads to attain the above objects, and have achieved the present invention. That is, an organic electroluminescence device of the present invention comprises a transparent substrate, an anode on the substrate, the major component thereof being metal oxide, an organic electroluminescent medium layer having at least a molecular evaporation film layer, and a cathode, each laminated in this order, wherein the anode surface is subjected to nitrogen oxide decoration.

Advantageously, the molecular evaporation film layer contains a phthalocyanine compound as a major component.

A manufacturing method of the present invention comprises a step of subjecting the anode surface to nitrogen oxide decoration prior to forming a molecular evaporation film layer.

Advantageously, the nitrogen oxide decoration in the manufacturing method of the invention can be suitably performed by irradiation of ultraviolet light in a mixed-gas atmosphere of nitrogen and oxygen, or by irradiation of ultraviolet light in oxygen gas atmosphere followed by replacing the oxygen gas atmosphere with a nitrogen-containing gas atmosphere.

The present invention makes it possible to control adhesivity, filling rate, and crystalline state or amorphous state of the hole injection-andtransport layer in the boundary region composed of metal oxide and organic low molecular substance by forming an evaporation film of the organic low molecular substance on a surface of the anode of metal oxide subjected to nitrogen oxide decoration. Although detailed mechanism is not clear yet, it can be considered that a denser and higher 4 filling-rate thin-film of organic low molecular substance is formed on the metal oxide with nitrogen oxide decoration, in comparison with the case where the organic thin-film of organic low molecular substance is directly evaporated on a metal oxide surface. The high density of the organic thin-film in the boundary region may be attributed to relatively strong inter-molecular interaction between nitrogen oxide, such as N02, decorated by chemical bond on the metal oxide and organic low molecular substance evaporated on the nitrogen oxide.

Now the aspects of the embodiments of the present invention will be explained hereinafter with reference to the accompanying drawings. However, the scope of the present invention is not limited to the specific constitution of the organic EL devices described below.

Figure 1 is a cross sectional view that schematically shows an example of a construction of an organic EL device of the invention; and Figure 2 is a graph showing voltage-luminance characteristics of Example 1 and Example 2.

Referring to the Figures, the construction of the organic EL device shown in Figure 1 is provided with a substrate 1, a transparent anode 2 laminated on the substrate 1, a hole injection layer 3, which is a film of organic substance laminated on a transparent electrode film consisting of the substrate 1 and the anode 2, a hole transport layer 4 laminated on the hole injection layer 3, a light-emitting layer 5, which is a film of organic substance laminated on the hole transport layer 4, and a cathode 6 laminated on the light emitting layer 5, each of the layers being sequentially laminated in this order. A construction of an organic lightemitting device is finished by sealing-off with a seal-off plate 7.

The substrate 1 is made of glass or transparent polymer. A major component of the anode 2 is metal oxide. An indium-tin oxide film or an indium-zinc oxide film may be used for the anode 2.

In the present invention, the surface of the anode 2 is subjected to nitrogen oxide decoration after the anode 2 is formed on the substrate 1. It is known by the inventors that the nitrogen oxide decoration improves filling rate, adhesivity, and thermal stability in the boundary region between the nitrogen oxide, such as N02, decorated on the anode of metal oxide and the low molecular substance evaporated on the nitrogen oxide. In particular, when the molecular evaporation film layer is an evaporation film, a major component of which is a crystalline molecule often used in an organic EL device, a phthalocyanine compound, such as copper phthalocyanine, titanyloxyphthalocyanine, or metal-free phthalocyanine, remarkable improvement in epitaxy in initial film lamination layer of 2 to 10 nm thick has been revealed by X-ray analysis and AFM (interatomic force microscopy).

A means for performing nitrogen oxide decoration of an anode surface in the present invention may be a method using reactive gas and ultraviolet light irradiation or a method using reactive gas and heat, for example. Of the methods, the method using reactive gas and ultraviolet light is preferable from the viewpoints of economy, homogeneity of the treatment effect, and controllability.

Although ultraviolet light source in the present invention is not limited to any specific source, a low-pressure mercury lamp, for example, is preferable from the viewpoints of stability, radiation intensity, and economy.

In the first specific method that uses reactive gas and ultraviolet light radiation, nitrogen oxide decoration on an anode surface is performed by an operation of irradiating ultraviolet light in a pure oxygen gas atmosphere followed by an operation of replacing the atmosphere with nitrogen-containing gas. Nitrogen oxide decoration is performed by a reaction between nitrogen molecules and the anode surface chemically activated by ozone and singlet excitation oxygen which are generated by the ultraviolet light irradiation in an oxygen atmosphere.

The second specific method using reactive gas and ultraviolet light radiation includes at least an operation of irradiating ultraviolet light in a mixed-gas atmosphere of nitrogen and oxygen. As described in the first specific method, activation of metal oxide surface must contain oxygen gas in a certain amount in the mixed-gas. At the same time, the operation also has the effect of removing organic substances such as residues of resist attached on the anode surface in a photolithography process. Considering the above points, oxygen content in the mixed-gas of nitrogen and oxygen ranges preferably from 1 to 90 volume %, more preferably, from 5 to 20 volume % which is a same order of magnitude as the composition of the atmospheric air and desirable from the standpoint of safety.

6 Identification of the decoration of the anode boundary surface by the nitrogen oxide involving physical adsorption or chemical bond can be confirmed by ultra-high vacuum thermal desorption spectroscopy (UHV-TDS), time-of-flight type secondary ion mass spectroscopy (TOF-SIMS), or X-ray photoelectron spectroscopy (XPS).

After the process of nitrogen oxide decoration using ultraviolet light radiation, it is favourable to immediately transfer the substrate into an evaporation apparatus in which the process for forming the evaporation film of low molecular substance is performed, so as to minimize repollution with hydrocarbons and adsorption of moisture. However, since the decoration continues to be effective during exposure to an environment of a common clean room of about class 1000, any special means, for example, load-locking between the ultraviolet light irradiation system and the evaporation system by means of a vacuum vessel, is unnecessary.

An important point for maintaining the decoration effect is that evacuation systems in the vacuum system for load-locking (if adapted) and in the evaporation system must employ an oil-free type vacuum pump such as a cryopump, a turbornolecular pump, or a dry pump. That is because partial pressure of organic substances or moisture existing in the vacuum system involving oil-relying vacuum pump is unfavourable for maintaining the decoration effect and for maintaining a clean substrate surface.

Considering the above, when the substrate is transported through atmospheric air into an evaporation apparatus equipped with an evacuating pump system consisting of a turbomolecular pump and a drypump immediately after finish of the nitrogen oxide decoration step using ultraviolet light irradiation, it has been confirmed by analysis that the decoration effect maintains for several to about ten hours and that the substrate surface is kept clean.

Material for the hole injection layer 3 may be selected from a phthalocyanine compound, such as copper phthalocyanine, titanyloxyphthalocyanine, or metal-free phthalocyanine, and an aryldiamine compound and aryltriamine compound represented by general formulas (1) and (2), namely tetraphenylbenzidine or N,N'-dinaphthyl-N, N'diphenylbenzidine, for example.

7 A2 A 3 4 A B A wherein each of A' to A4 independently represents an optionally substituted aryl group of six or more carbon atoms, and B represents one or more optionally substituted arylene groups of six or more carbon atoms.

AB 7 N N Jk 5/ B A 8 1 (2) 10/ N A wherein each of A5 to A10 independently represents an optionally substituted aryl group of six or more carbon atoms, and B represents one or more optionally substituted arylene groups of six or more carbon atoms.

Material for hole transport layer 4 may be selected from an ary1diamine compound and an aryltriamine compound represented by general formulas (1) and (2), namely tetraphenylbenzidine and N,N'-dinaphthyl-N,N'diphenylbenzidine, for example.

Material for light-emitting layer 5 may be selected from a film of tris(8hydroxyquinoline) aluminium (so-called Alq3) alone, or a mixture film of Alq3 with dispersed fluorescence dye.

For the cathode 6, aluminium, an alloy of aluminiurn and another metal of low work function, or an alloy of magnesium and another metal of low work function may be used.

For the seal-off plate 7, material of very low transmission rate for oxygen and water is used. Adhesive for sealing in liquid crystal display is favourably used for joining substrate 1 and seal-off plate 7.

Examples of some of the preferred embodiments of the invention will be described hereinafter in detail. Two samples were fabricated for each of Example 1 and Example 2: one for analytical evaluation, the other for device performance evaluation.

8 Example 1

An anode of a transparent conductive film abo t 100 nm thick was r formed on a transparent glass substrate (Cornin(g 1F31) by sputtering A indium-zinc oxide (IDIXO, manufactured by Idemitu Kosan Co. Ltd.) at room (R"rm) temperature. Then, positive-type photoresist (OFPR-800, manufactured by A Tokyo Ohka Kogyo Co., Ltd.) was applied using spincoater to about 1 jkm thickness and pre-baked in a hot-air-circulating oven, followed by exposure using a photomask having a pattern with 4 mm pitch and 2 mm line width on a display surface. This was then developed with a developer (NMD-3, manufactured by Tokyo Ohka Kogyo Co., Ltd.) and etched with oxalic acid. Removing the photoresist, a patterned electrode was formed.

The substrate with electrode was transported into an ultraviolet light irradiation apparatus, and surface decoration treatment was carried out using low pressure mercury lamp for ultraviolet irradiation in oxygen/nitrogen mixed-gas (oxygen content was 20 volume %) at an atmospheric pressure.

The decorated substrate with electrode was put into an entrance room of an evaporation apparatus. The evaporation apparatus used for preparing samples of the embodiments is of the structure that allows a substrate to be transferred between a cleaning room and an evaporation room through a central chamber. Therefore, the substrate put in the entrance room can be brought to formations of an organic EL medium layer and a cathode layer without exposure to atmospheric air.

Immediately after the samples were put in the entrance room, evacuation started and continued until the pressure reached about 5 x 10-5 Pa. The processes up to this step were conducted with both of the two samples. One of the two was instantly transported within vacuum to the evaporation room and evaporation was initiated. The other was taken out for surface analysis.

The structure of the laminate of the samples is shown in Figure 1. Each of the film layers was formed by means of resistance heating evaporation in each of separate rooms with the following material, thickness and order.

The hole injection layer 3 was formed by depositing copper phthalocyanine (CuPc) represented by formula (3) in 100 mn thickness; 9 N (3) _C N N N Z the hole transport layer 4, by depositing N,N'-dinaphthyl-N, N'diphenylbenzidine (a-NPD) represented by formula (4) in 15.nm thickness; q - P N (4) the light-emitting layer 5, by depositing tris(8-hydroxyquinoline) aluminium (Alq3), represented by formula (5), with an additive of a trace amount of fluorescent dye, coumarin 6, in 40 nm thickness; N 0' A I the cathode 6, by depositing an Al-Li alloy (Li 0.5 at. %) in 200 nrn thickness.

The thus laminated substrate was taken out from the evaporation apparatus and transferred to an apparatus for sealing process with moisture content below 10 ppm in which sealing-off was performed by applying roomtemperature-setting adhesive on the periphery of the substrate and joining the substrate with a seal-off plate 7 of glass. Example 2 After the pressure of the entrance room containing the substrate with electrode reached about 5 x 10-5 Pa, the substrate was continued to be stored in the entrance room held at pressure in the range from 8 x 10- 6 to 5 x 10-5 Pa for 96 hours. The processes up to this step were conducted with both of the two samples. One of the two was instantly transported within vacuum to the evaporation room and evaporation was initiated. The other was taken out for surface analysis.

Excepting the above-described alteration, the samples of Example 2 were prepared in the same manner as Example 1.

Surface analyses were conducted with the samples of Example 1 and Example 2 by means of ultra high vacuum TDS, TOF-SIMS and XPS. The analyses showed that proportions of hydrocarbons on the substrate surface of samples of Examples 1 and 2 were minute and not significantly different between Example 1 and Example 2. However, the analyses also revealed that the proportions of N02 and N03 of Example 2 were decreased to 1/3 of those of Example 1.

The result of the analyses confirmed that storage for a long time in the entrance room under high vacuum subjected to the samples of Example 2 did not cause the anode substrate pollution with hydrocarbons and only brought about decrease in quantity of nitrogen oxide decoration as compared with the samples of Example 1. It means that a control test was established with respect to quantity of nitrogen oxide decoration for observing its effect to device performances.

The evaluation method and the results of the performance evaluation on the devices of Examples 1 and 2 are described in the following. (1) Luminance-voltage characteristics: Peak luminance was measured by intermittent lightning with frequency at 60Hz and with duty of 1/100. The resulted voltage-luminance characteristics are illustrated in Figure 2. In Figure 2, the unit for luminance 'arb. unit' means arbitrary unit of light intensity. Figure 2 indicates that voltage-luminance characteristics were significantly improved in Example 1 which contained large amounts of nitrogen oxide decoration in comparison with Example 2 which contained less decoration. (2) Operation life in constant current driving: Half value period of luminance in continuous lightning was measured and compared in a constant temperature and humidity chamber at temperature 20'C and relative humidity 11 40% under acceleration condition of current density 10 mA/cM2. In the test, the initial luminance was about 1200 cd/rft2 for both Examples 1 and 2. The result is shown in Table 1.

Table 1 luminance half-life (h) Example 1 2,200 Example 2 900

Table 1 indicates that the operation life in constant-current driving was significantly improved in Example 1 with larger amount of nitrogen oxide decoration as compared with Example 2 with less decoration.

The above-described test results clearly demonstrate that the nitrogen oxide decoration according to the invention on the anode surface significantly improves performances of an organic EL device and that the larger amount of the decoration brings about the more favourable effect.

According to the present invention wherein nitrogen oxide decoration is subjected to an anode surface of an organic EL device prior to forming a molecular evaporation film layer, an organic EL device with practical drive stability and operation voltage has been provided as wen as a manufacturing method of such an organic EL device.

12

Claims (6)

1. An organic electroluminescence device comprising: a substrate, an anode containing metal oxide as a major component, an organic electroluminescence medium layer at least including a molecular evaporation layer, and a cathode, sequentially laminated on the substrate in this order; wherein a surface of the anode is decorated with nitrogen oxide.

2. An organic electroluminescence device according to claim 1, wherein the molecular evaporation layer contains a phthalocyanine compound as a major component.

3. A method for manufacturing an organic electroluminescence device comprising steps oflaminating an anode containing metal oxide as a major component on a substrate; decorating the surface of the anode with nitrogen oxide; laminating an organic electroluminescence medium layer at least including a molecular evaporation layer on the decorated surface of the anode; and laminating a cathode on the organic electroluminescence medium layer.

4. A method for manufacturing an organic electroluminescence device according to claim 3, wherein the molecular evaporation layer contains a phthalocyanine compound as a major component.

5. A method for manufacturing an organic electroluminescence device according to claim 3, wherein the decoration of the surface of the anode is performed by means of ultraviolet light irradiation under an atmosphere of mixed-gas of nitrogen and oxygen.

6. A method for manufacturing an organic electroluminescence device according to claim 3, wherein the decoration of the surface of the anode is performed by means of ultraviolet light irradiation under an atmosphere of oxygen gas followed by replacing the atmosphere of oxygen gas with an atmosphere of nitrogen-containing gas.