WO2002054834A2

WO2002054834A2 - Electroluminescent element

Info

Publication number: WO2002054834A2
Application number: PCT/US2001/050287
Authority: WO
Inventors: Takashi Yamasaki; Kazumi Matsumoto
Original assignee: 3M Innovative Properties Company
Priority date: 2000-12-28
Filing date: 2001-12-19
Publication date: 2002-07-11
Also published as: JP2002216967A; WO2002054834A3

Abstract

An electroluminescent element (1) includes an emission layer (4) inserted between a pair of electrodes (2, 5), said emission layer (4) contains phosphor. The amount of chloride ion eluted from the phosphor is not more than 10 ppm. The electroluminescent element (1) improves lifetime of the device and reduces power comsumption.

Description

ELECTROLUMINESCENT ELEMENT

BACKGROUND

The present invention relates to an electroluminescent element, also referred to as an EL element.

Electroluminescence means a phenomenon wherein light is emitted in the electric field by applying a voltage. EL elements comprising an emission layer made of phosphor, which exhibits such an electroluminescent behavior, inserted between a pair of electrodes are widely used as a back light for liquid crystal display. EL elements have had difficulty meeting the requirements of low power consumption and long life.

To satisfy these requirements, it has been proposed to increase lifetime of the EL element and to reduce power consumption by reducing the content of an alkali metal in an emission layer and an insulation layer of the EL element (Japanese Unexamined Patent Publication No. 3-8295) to 20 ppm. However, satisfactory results could not necessarily be obtained even if the content of the alkali metal in the emission layer and insulation layer of the EL element is reduced.

Sulfides of the group Ila, for example, ZnS and ZnCdS have conventionally been used as the phosphor for EL element, while Cu and Cl have often been used as one of auxiliary components for doping the phosphor. Particles of the phosphor are sometimes coated with glass in order to reduce the adverse effects of moisture (for example, Japanese Unexamined Patent Publication No. 62-195894). In this case, the coating film sometimes contains chloride ions which originate in silicon tetrachloride used in the formation of the coating film. When water penetrates into an emission layer of an EL element made by using phosphor containing large amounts of chloride ions, chloride ions dissolve in water to generate an acid and the acid corrodes an electrode layer. As a result, the resistance of the electrode layer increases, lowering the luminance of the EL element with a lapse of time, thus reducing the lifetime.

SUMMARY

The present disclosure is directed to an EL element that has long lifetime and achieves low power consumption. According to the present disclosure, there is provided an electroluminescent element comprising an emission layer inserted between a pair of electrodes, said emission layer contains phosphor, and an amount of chloride ion eluted from the phosphor is not more than 10 ppm.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a schematic cross sectional view showing the structure of an EL element of the present invention.

Fig. 2 is a schematic cross sectional view showing the specific structure of an emission layer in the EL element of the present invention.

DESCRIPTION

In the EL element of the present disclosure, an emission layer is inserted between a pair of electrodes and at least one^' electrode of the EL element is a transparent electrode. In cases where only one electrode is a transparent electrode, the other electrode is a back electrode. One example is shown in Fig. 1. In the EL element

1 an emission layer 4 is formed between a back electrode layer 2 and a transparent electrode layer 5, while an insulation layer 3 is formed between the back electrode layer

2 and the emission layer 4. This prevents dielectric breakdown of the whole EL element 1 and stably applies a high electric field required to electroluminesce the emission layer 4. A back insulation layer (not shown) may be formed on the surface of the back electrode layer 2 and a transparent substrate (not shown) may be formed on the surface of the transparent electrode layer 5.

In the EL element, the back electrode layer 2, the insulation layer 3 and the transparent electrode layer 5 can be formed with the same construction as that used in a conventional EL element. The transparent electrode layer 5 can be preferably made of ITO (indiumtin oxide) and this ITO electrode can be formed by sputtering. The thickness of the transparent electrode layer is usually within a range from 0.01 to 1,000 μm and the surface resistance is usually 500 Ω/square or less, and preferably from 1 to 300 Ω/square. The light transmittance of the transparent electrode layer is usually 70% or more, and preferably 80% or more. The back electrode layer 2 in which the emission layer 4 is inserted between it and the transparent electrode layer 5 can be made of a carbon-containing resin or a silver paste. The thickness of the back electrode layer is preferably within a range from 5 nm to 1 mm.

These electrodes are provided with an electric connection so as to apply a requisite potential to the emission layer inserted between these electrodes. Specifically, it is preferred that a pin terminal is attached to a predetermined portion and a lead wire for connecting to the pin terminal is connected.

The insulation layer 3 can be formed, for example, by covering a foil, sheet or film, which is formed by coating a mixed solution of a fluororesin and barium titanate and drying the mixed solution or of a mixture of a fluororesin and barium titanate. Preferred fluororesin includes polyvinylidene fluoride. The thickness of the insulation layer is preferably within a range from 2 to 1,000 μm.

The back insulation layer, which is optionally formed on the surface of the back electrode layer, can be formed, for example, by coating insulation ink or an insulation resin and drying it or covering an insulating foil, sheet or film. The transparent substrate, which can be formed on the surface of the surface electrode layer, can be made of various transparent resins such as polyester resin, acrylic resin, polycarbonate resin, and vinyl chloride resin.

The emission layer 4 can be formed by using various phosphors which have conventionally been used in the EL element. Preferred phosphor is, for example, made of a zinc sulfide compound or a zinc selenide compound, such as ZnS, CdZnS,

ZnSSe, or CdZnSe. An auxiliary component such as Cu, I, Cl, Al, Mn, NdF₃, Ag or B may be added to the phosphor, thereby making it possible to obtain a desired emission color. Alternatively, the phosphor may be replaced by particles made by encapsulation of the phosphor. As shown in Fig. 2, the emission layer can be formed, for example, by coating a binder resin 6, 8 on the surface of the insulation layer 3 or the transparent electrode layer 5, scattering particles 7 of the phosphor over the binder resin, and coating the phosphor particles 7 with the binder resin 6, 8. As the binder resin, there can be used various transparent resins which are similar to the transparent substrate described above, for example, polyester resin, acrylic resin, polycarbonate resin, and vinyl chloride resin. The emission layer can also be formed, for example, by coating a resin solution, which is prepared by dispersing the phosphor in the binder resin, on the surface of the transparent electrode layer using a screen printing method, or by coating the phosphor using a roll coater. The thickness of the emission layer is preferably within a range from 5 to 500 μm.

As described above, in cases where the EL element has phosphor containing large amounts of chloride ions, the chloride ions dissolve in water in the element to generate an acid. The acid corrodes an electrode layer. As a result, the resistance of the electrode layer increases thereby reducing the lifetime of the element. When pure water is added to the phosphor containing large amounts of chloride ions, the water exhibits strong acidity. Also when the EL element is made by using this phosphor and the resistance of the electrode layer was measured after drying, the resistance drastically increased.

Phosphor in which an amount of chloride ion eluted is not more than 10 ppm is used in the emission layer. When the amount of chloride ion eluted from the phosphor contained in the emission layer is not more than 10 ppm, the cause for reduction of lifetime is removed.

In the present disclosure, the amount of chloride ion eluted from the phosphor is measured by the following method. A measuring sample is made by adding 10.0 g of ultra-pure water to 0.5 g of phosphor, followed by stirring with vibrating at 100 rpm using a vibrator and further standing for 12 hours. The amount of chloride ion in this sample is measured by ion chramatography using IonPacAS4A manufactured by Nippon Dionex K.K. as a column (chloride ion) for anion. The amount of chloride ion is reduced into a value per unit weight of the phosphor, which is taken as an amount of chloride ion eluted from the phosphor. Ultra-pure water having a resistivity of 18.2 Ω/square is used and neither cation nor anion is detected from this ultra-pure water by ion chromatography.

As described above, the present inventors have found that reduction of lifetime of the EL element is caused by the chloride ion eluted from the phosphor. Therefore, the chloride ion eluted is previously removed by an optional means such as method of washing the phosphor with pure water before introducing the phosphor into the emission layer. Alternatively, the phosphor may be coated by a material containing no chloride ions. The phosphor itself may contain chloride ion as far as chloride ion is not eluted in the EL element in any significant amount. EXAMPLES

Example 1 A polymer with high dielectric constant (tetrafluoroethylene- hexafluoroethylene-vinylidene fluoride copolymer THV200G, manufactured by Sumitomo 3M. Ltd.), ethyl acetate and methyl ethyl ketone were mixed in a weight ratio of 15:42.5:42.5 and then uniformly dissolved to prepare a binder resin. Using a notch bar set at a gap setting of 75 μm, the binder resin was coated on the ITO surface of a polyethylene terephthalate film with ITO (indium-tin-oxide) (KPC300-75(A), manufactured by Oike Industrial Co., Ltd.) as a transparent electrode. After about 2 minutes have passed after the completion of the coating before drying the binder resin, ZnS phosphor particles, in which an amount of chloride ion eluted from phosphor as measured by the above method is 7.2 ppm, were scattered and then dried at about 100°C for 4 minutes.

Using a notch bar set at a gap setting of 100 μm, the same binder resin as that described above was coated on the phosphor particles and then dried at about 100°C for 4 minutes to form an emission layer.

Using a notch bar set at a gap setting of 100 μm, a resin for forming insulator was coated on the emission layer and then dried at about 100°C for 4 minutes to form an insulation layer. This resin for forming insulator is prepared by uniformly dispersing 36 g of insulator particles (barium titanate HPBT-1 manufactured by Fuji Titanium Industry Co., Ltd., Na content: 7.4 ppm) in 100 g of the same binder resin as that described above. Subsequently, an Al coating film was formed on the insulation layer under the film-forming conditions of a vacuum degree of 0.67 Pa (5 mTorr) and a discharge current of 1.0 A by a sputtering method using an Al target to form a back electrode layer, thus obtaining an EL element.

A power supply (PCR5001, manufactured by Kikusui Electronics Corp.) was connected between the transparent electrode layer and the back electrode layer of this EL element and a sinusoidal voltage of 400 Hz was applied in an effective voltage of 100 V. As a result, uniform light emission was obtained on the whole emission surface defined by the back electrode layer. Using a power meter (WT-110E, manufactured by Yokogawa Electric Corporation), power consumption (unit: W) was measured and power consumption per unit volume of the EL element was calculated. Using a luminance meter (BM-8, manufactured by Topcon Corporation), the luminance was measured with a lapse of time and lifetime was defined by the time required for luminance to be reduced to half of the initial luminance. These results are shown in Table 1 below.

Example 2 In the same manner as in Example 1 , except that 0.2 g of an alkali metal-containing solution (20% sodium ethoxide-ethanol solution, manufactured by Wako Pure Chemicals Industries, Ltd.) was added to 120 g of the binder resin, an EL element was produced. The sodium ion content of this EL element was 770 ppm in the binder resin of the emission layer, while it was 222 ppm in the insulation layer. In the same manner as in Example 1, the power consumption, lifetime and alkali metal ion content of the EL element were measured. The results are shown in Table 1.

Example 3

In the same manner as in Example 1, except that the same phosphor as that used in Example 1 was washed before use, an EL element was produced. This phosphor was washed with water and then sufficiently dried at about 100°C. As a result, the amount of chloride ion eluted from the phosphor, which was measured after washing, was 1.2 ppm. In the same manner as in Example 1, the power consumption, lifetime and alkali metal ion content of the EL element were measured. The results are shown in Table 1.

Comparative Example 1

In the same manner as in Example 1, except for using, as the phosphor, ZnS phosphor particles in which the amount of chloride ion eluted, which was measured by the above method, is 620 ppm, an EL element was produced. In the same manner as in Example 1, the power consumption, lifetime and alkali metal ion content of the EL element were measured. The results are shown in Table 1. Comparative Example 2

In the same manner as in Example 2, except that 0.2 g of the alkali metal-containing solution (20% sodium ethoxide-ethanol solution, manufactured by Wako Pure Chemicals Industries, Ltd.) was added to 120 g of the binder resin and ZnS phosphor particles in which the amount of chloride ion eluted is 620 ppm, which are the same as those in Comparative Example 1, were used, an EL element was produced. In the same manner as in Example 1, the power consumption, lifetime and alkali metal ion content of the EL element were measured. The results are shown in Table 1.

Comparative Example 3

In the same manner as in Example 1, except for using, as the phosphor, ZnS phosphor particles in which the amount of chloride ion eluted is 1000 ppm, an EL element was produced. In the same manner as in Example 1, the power consumption, lifetime and alkali metal ion content of the EL element were measured. The results are shown in Table 1.

Table 1

As is apparent from the results shown in Table 1, lifetime of the EL element is prolonged and power consumption is reduced by using phosphor, an amount of chloride ion eluted therefrom is not more than 10 ppm. Furthermore, as is apparent from Example 2 and Comparative Example 2, only the reduction of the alkali metal content in the phosphor neither improves lifetime of the element nor reduces power consumption, satisfactorily, and the amount of chloride ion eluted from the phosphor must be reduced to 10 ppm or less.

By reducing the amount of chloride ion eluted from a phosphor contained in an emission layer of the EL element of the present invention to 10 ppm or less, it is made possible to improve lifetime of the EL element and to reduce power consumption.

Claims

CLAIMS:

1. An electroluminescent element comprising an emission layer inserted between a pair of electrodes, said emission layer contains phosphor, and an amount of chloride ion eluted from the phosphor is not more than 10 ppm.

2. The electroluminescent element according to claim 1, wherein said emission layer contains particles of phosphor dispersed within a binder resin.