CN1155226A

CN1155226A - Middle layer of electroluminescent device containing fine inorganic particles

Info

Publication number: CN1155226A
Application number: CN 96121100
Authority: CN
Inventors: A·埃尔斯纳; R·韦尔曼; R·比杰
Original assignee: Bayer AG
Current assignee: Bayer AG
Priority date: 1995-11-20
Filing date: 1996-11-20
Publication date: 1997-07-23

Abstract

The invention relates to an inorganic middle layer of electroluminescent device containing fine inorganic particles dispersing in the polymer adhesive.

Description

Intermediate layer in electroluminescent devices containing finely divided inorganic particles

The invention relates to an intermediate layer of inorganic material in an electroluminescent device or arrangement, respectively, containing finely divided inorganic particles (fibres) dispersed in a polymeric binder.

In our photovoltaic age, the commonly used light emitting devices take a very important role. The light display element is developed towards the goals of being flat, being operable over a large area, being efficient and having low production costs. There are many different approaches for making electrically operated light emitting devices. However, only two types of systems are known which simultaneously satisfy the requirements of large area flatness and low cost.

1) Incorporation of ZnS particles in a binder matrix

ZnS particles are mixed into a thin layer and emit light under application of a high frequency electric field. The ZnS system has low luminosity. Another disadvantage is the high operating voltages required at high frequencies.

2) Electroluminescent polymers

Friend et al (US P5247190) describe for the first time polymer Light Emitting Diodes (LEDs) based on poly (phenylene) vinyl. In the simplest configuration, the polymer is placed between a light-transmissive anode (e.g., ITO) and a vapor-deposited cathode (e.g., Ca, Al). With the application of a voltage, the device emits light.

The most common example of ｃA layered construction in organic LEDs is described in EP- ｃA-0637899, where additional layers are also described which exert ｃA positive influence on the function of the light-emitting device. The properties of LEDs constructed from organic components can be significantly improved by the introduction of additional organic layers (Tang et al, appl. phys. lett.51(1987) 913). For this purpose, one to two filling layers are introduced between the electroluminescent layer and the two electrodes. These priming layers may include vapor deposited monomers (Adachi et al, Appl Phys. Lett.57(1990) 531). Polymers (US P5231329) or monomers dispersed in polymers (Brow et al, appl. Phys. Lett.61(1992) 2739). They are, for example, oxadiazole or tertiary amine compounds, polymers having an oxadiazole structure are also described. These molecules are always referred to as organic molecules.

The present invention relates to interlayers comprising finely divided inorganic particles (fibrils) dispersed in a polymeric binder for use in light emitting devices or Light Emitting Diodes (LEDs) and to the use of fibrils in the preparation of interlayers of LEDs and light emitting (field emission) devices comprising one or more layers of these interlayers.

The particles generally have a particle size of<1 μm, preferably from 1 to 100 nm. Particle size is determined by using high resolution transmission electron microscopy or ultracentrifugation. Such particles are preferably of semiconducting material composition having a band gap, i.e. an energy difference between the valence and conduction bands, of at least 1 eV. Suitable finely divided inorganic (finely divided) particles are, for example, metal oxides such as TiO₂、SnO₂、ZnO、WO₃、Al₂O₃、Fe₂O₃Or semiconductor compounds of the III and V main groups of the periodic Table (III-V semiconductors) such as GaP, AlN, or semiconductor compounds of the II and VI main groups of the periodic Table (II-VI semiconductors) such as ZnSe, CdSe, CdS (Mendeleev's periodic Table).

Particular preference is given to the granules consisting of titanium dioxide.

The concentration of the fiber particles in the polymer binder is from 0.1 to 90% by weight, preferably from 1 to 70% by weight, particularly preferably from 5 to 60% by weight, based on the weight of the intermediate layer.

For subsequent layering, the fiber particles are dispersed in a polymeric binder. Amorphous polymers which can be processed to transparent films are preferably suitable as binders. Examples of such polymers are polycarbonate, polystyrene and copolymers of polystyrene such as SAN. Cyclic polyolefins, polysulfones, (meth) acrylate polymers and/or copolymers, polyvinyl alcohol, polyvinylcarbazole, vinyl acetate copolymers or polyvinylpyrrolidone.

The invention also provides for the use of finely divided inorganic particles dispersed in a polymeric binder for the preparation of interlayers in electroluminescent devices.

The invention also provides electroluminescent devices comprising one or more, typically one or two, of the above-described intermediate layers and the use of such electroluminescent devices in the preparation of light-emitting displays.

An example of the arrangement of the LEDs is briefly described as follows:

A) cathode (e.g. Al, Ca, Mg), intermediate layer containing fine particles, electroluminescent layer, transparent anode

(In-Sn-oxide)/transparent substrate (such as glass or film),

B) cathode/electroluminescent layer/intermediate layer containing fiber particles/transparent anode/transparent substrate,

C) cathode/intermediate layer containing fiber particles/electroluminescent layer/intermediate layer containing fiber particles/transparent anode/transparent

A substrate.

The intermediate layer is arranged as follows:

A) as an intermediate layer between the electroluminescent layer and the cathode

Or

B) As an intermediate layer between the electroluminescent layer and the anode

Or

C) As between cathode and electroluminescent layer and between anode and electroluminescent layer

An intermediate layer.

The system of the particles in the polymer binder can be applied by spray dip coating and blade coating onto a large piece of film in the form of a thin film as an intermediate layer in the polymer LED. It retains the advantages of low cost and large-area production of polymer LED.

The performance of polymer LEDs made from polymers (corresponding to US P5247190) is significantly improved by these intermediate layers. The efficiency of electroluminescence is thus significantly increased. With the same voltage, the direct current through the device is significantly reduced, but the electroluminescent intensity remains unchanged or even increases.

The layer according to the invention is characterized not only by an increased efficiency but also by an increased long-term stability. As it operates in the case of organic systems comprising monomers in a binder or vapor deposition monomers; the components in the system do not crystallize during operation. While the electroluminescent arrangement is more stable to chemical changes such as oxidation.

Electroluminescent devices made from polymers having interlayers of the present invention operate at low dc voltages. In addition to being simple and inexpensive to produce by knife coating of the layer, the emission color can be controlled by varying the color development modulus.

The intermediate layer has a thickness of less than 2 μm, typically 10-15 nm. Thus high electric field-induced strength can be obtained even at low voltage. The intermediate layer according to the invention is characterized by a uniform layer thickness and even in the case of a thickness of generally>10⁶The electroluminescent arrangement is also resistant to dielectric breakdown at high electric field strengths of V/cm.

Suitable inorganic fibre particles having a particle size of<1 μm can be prepared in different ways:

by precipitation reaction (E.Matijevic, chem.Mater.5(1993)421-

TiO₂In the case of, for example, hydrolysis by titanyl sulfate and washing in monovalent acid to sulfur-free

Peptization of the precipitate of the acid salt (EP-A261560) by TiCl₄Hydrolysis of the solution (W.

Bauer and G.Tomandl，Ceram.Int.20(1994)189-93)；

According to the sol method by alkoxide hydrolysis, such as in j.livage, Mat.

Sci.Forum 152-; g.w.koebrugge et al.

Mater.chem.3(1993), 1095-; PCT-WO 93/05875 and PCT-

The method described in WO 91/16719;

by passing a vaporizable metal compound (e.g. alkoxide, halide) and excess oxygen

Or the reaction of hydrogen and CVR process for pyrolysis in the gas phase, see US P4842832

And k.y.kim et al, j.chem.engng.jpn.27(1994), 657-61;

by plasma synthesis, see z. chen et al, Gongneng cailiano 23

(1992)，83-87，105(Chem.Abstr.119：277086C)；

By means of, for example, TiCl₄Flame hydrolysis of (2); this product has the trade name Deyusas P

25, a commercial product;

by reaction of titanyl sulfate solution with a deficiency of alkali (see below).

The granules consisting of titanium oxide obtained from titanyl sulfate solution are particularly suitable. The process for preparing finely divided titanium dioxide which is particularly suitable is characterized in that:

a) adding titanyl sulfate solution containing excess sulfuric acid to the alkaline solution at elevated temperature until such time as

The mixture of (a) is in an acidic reaction, i.e. excess sulfuric acid is present,

b) cooling the mixture obtained in a) and,

c) thenadding a monovalent acid to the mixture obtained in b), a formation of the bis according to a) taking place

Flocculation of the titanium oxide fiber particles is carried out,

d) filtering the flocculated precipitate formed in c) and washing it with the same monovalent acid as described in c)

And (6) washing.

To obtain the titanium dioxide sol according to the invention, the precipitate obtained after reaction step d) is dissolved in water or an alcohol having 1 to 10 carbon atoms and 1 or more hydroxyl groups in the molecule.

Finely divided TiO according to the invention₂The preparation of (A) can also be successfully carried out within the framework of large-scale production, i.e. TiO according to the sulphate process₂The production of the pigments is carried out in a very simple and economical manner.

The filter residue obtained in accordance with the method of the invention (after step d) can be subjected to an inorganic and/or organic post-treatment.

In principle any titanyl sulfate solution containing excess sulfuric acid is suitable as educt (titanyl sulfate solution). Metallic impurities such as iron, magnesium, aluminum and alkali metals which form soluble sulfates and oxides are in principle not disturbing in the preparation process, unless these elements, even in trace amounts, are favorable for their respective use purposes. The method according to the invention can therefore be used to a large extent. Educts such as black liquor may be used, which may be prepared by digesting chromite and/or titanium slag with sulphuric acid, the digested cake being dissolved in water and filtered followed by the sulphate process.

However, the preparation method is not limited to black liquor as educt. Examples of other methods for preparing titanyl sulfate solutions suitable for use as educts are:

a) titanium dioxide and TiO in excess sulfuric acid₂Dissolving hydrates such as orthotitanic acid and metatitanic acid

Performing digestion/digestion;

b) dissolution of basic titanates and magnesium titanates, also in aqueous form, in the presence of excess sulfuric acid

Performing digestion/digestion;

c) TiCl as described in DE-A4216122₄And excess of H₂SO₄Formation of TiOSO₄And

reaction of HCl.

When it is undesirable to have traces of foreign metals, such as iron, in the product (finely divided titanium dioxide), it is preferred that the product, in particular a) and c), be used as the titanyl sulfate solution.

In order to operate in a cost-effective manner, according to the inventionThe titanyl sulfate solution used preferably contains TiO₂The amount of 100-260gTi/l, particularly preferably 170-230 gTi/l. The excess amount of acid is preferably per mole of TiOSO₄0.3-4.0 mol H₂SO₄Particularly preferably 0.5 to 1.5 mol of H₂SO₄。

Preferably an aqueous solution of sodium hydroxide, potassium hydroxide or ammonia is used as the lye; although CO is produced because of its strong production₂The situation is not so applicable, but in principle also sodium, potassium and ammonia carbonates can be used. Sodium hydroxide solution is particularly preferred and the process will be carried out according to the use of sodium hydroxideThe case of the sodium solution is explained in detail.

The amount of sodium hydroxide used is stoichiometrically less than the corresponding amount of "free sulfuric acid" according to reaction step a). Preparation of TiO by sulfuric acid process₂As used herein, the term "free sulfuric acid" is understood by those skilled in the art to mean the total amount of sulfuric acid minus extraneous metal sulfates (primarily FeSO)₄) Form-bound moieties, i.e. H₂SO₄And as TiOSO₄The sum of the bound sulfuric acids; the last-mentioned component is hydrolyzed by H₂SO₄In the form of a salt.

The quantity of sodium hydroxide is stoichiometrically less than the quantity required for two reactions

The deficiency is preferably such that the pH at the end of reaction step a) is less than 2.

The concentration of the aqueous sodium hydroxide solution is preferably about 5-10% NaOH (by weight).

The reaction of the stoichiometrically deficient sodium hydroxide solution and the sulfuric acid/titanyl sulfate solution is preferably carried out as follows: the sodium hydroxide solution is first placed in a flash vessel or container and heated to about 60-100 c, and the sulfuric acid/titanyl sulfate solution is then added to the sodium hydroxide solution.

The reaction step a) is preferably carried out with vigorous stirring and at a temperature of from 60 to 100 ℃.

The time during which the pH of the mixture is basic is to be passed as quickly as possible (preferably less than 5 minutes).

After reaction step a) the mixture is preferably cooled to below 60 ℃ and then maintained at this temperature 1/2 for 2 hours, optionally with stirring.

The mixture thus obtained was more or less turbid (cloudy sol). The mixture is in TiO₂The sulfate process is used as so-called hydrolysis seed. They are not suitable for use as transparent sols.

After cooling, it is flocculated with a monovalent acid and the flocculated precipitate is separated by filtration. A finely divided titanium dioxide having a particle size of from 1 to 10nm, containing less than 0.1% by weight of carbon and having a transparency of at least 99% is obtained (see below).

Clarification is preferably carried out before the first-order acid is added. This can be done in a simple manner by settling, i.e. decanting after standing for at least 12 hours. But may also be centrifuged or filtered if filter assistance is required.

The result of the addition of the first-order mineral acid is a reversible flocculation of the fibre particles formed in reaction step a). Due to their size (preferably 1-10 μm), the resulting numerous sheet-like flocs are well centrifuged and filtered. The preferred monobasic acid is hydrochloric acid and the continuous process will be described in detail with reference to the use of hydrochloric acid. When other monovalent mineral acids are used, the corresponding steps are also carried out accordingly.

The final HCl concentration in the mixture is preferably not less than 1 mol/l; preferably to 1 to 6 mol/l, particularly preferably 1 to 4 mol/l, for the precipitation of the fibre particles.

A preferred filter cloth is one made of an acid resistant material such as polypropylene. Particularly suitable filter cloths are also used for separating TiO₂Acid-proof filter cloths, which are known to the person skilled in the art and are used in the sulphate process of hydrolysates, and membrane filters based on cellulose bases, cellulose ethers or cellulose esters.

The precipitate is then preferably washed with the same monovalent acid as used for flocculation. In the case of hydrochloric acid, 3 to 6 mol/l of an acid salt are particularly suitable as washing liquid.

Depending on the filter device and the starting material, the resulting (acid) salt precipitate (slurry)Containing 20-40% (generally about 30%) TiO₂(weight), residual washing acid and small amounts of impurities that may be present.

The supply "solution" (sol) from which the light opalescence (Tyndall effect) was removed was clear, transparent and colorless or substantially colorless, as added to the precipitate. Of these sols, TiO₂Is completely in the form of fiber particles with a particle size of 1-10 nm.

This makes it possible to prepare TiO in amounts of up to about 20% by weight₂Is strongly acidic and is virtually completely transparent (as clear water). TiO 2₂The transparency of this sol at a concentration of 5% by weight is greater than 99% over the entire visible spectral region (measured on a 180 DEG/d-geometry).

The fiber particles can be reflocculated, filtered and washed by adding a monovalent mineral acid such as HCl. So that they can be kept at a temperature of about 0c for several weeks.

Similar sols can also be prepared in polar organic solvents, which are predominantly monohydric or polyhydric short-chain alcohols such as ethanol and butanediol (1, 4). Alcohols having 1 to 10 carbon atoms in the molecule are preferred.

A glassy xerogel is obtained from the slurry by evaporating off the liquid and contained acid under vacuum or with NaOH (room temperature, freeze drying) at as low a temperature as possible. Unless H is removed₂The amounts of O and HCl are too high and the xerogel can be completely dispersed in water.

The weight of metal ions that may be present can be removed by dialysis with a dilute monovalent mineral acid.

In applications where an excess of acid is disturbing, the particles according to the invention can be stabilized in a substantially known manner, for example using acetylacetone (WO93/05875) or hydroxy acids (EP-A518175), in the neutral pH range.

Examples

Microcrystalline TiO from polyvinyl alcohol₂The preparation of interlayers and electroluminescent arrangements comprising such interlayers.

A) ITO-coated glass (produced by Balzer) was cut into 20X 30mm pieces²Of a substrateIt is purified. The following steps are carried out continuously here:

1. washed in an ultrasonic bath for 15 minutes with distilled water and Fatterol (rinse),

2. washing with fresh distilled water in an ultrasonic bath for 2X 15 minutes (fresh steam for each time)

Distilled water),

3. washed with ethanol in an ultrasonic bath for 15 minutes,

4. washing with fresh acetone in an ultrasonic bath for 2X 15 minutes (fresh acetone for each time)

A ketone),

5. dry with a hairless prism cloth.

Electroluminescent polymers (poly (2-methoxy, 5- (2' -ethylhexyloxy) -1, 4-phenylene vinylene), abbreviated MEH-PPV) were prepared as described by D.Braun et al, App.Phys.Lett.58(1991) 1892.

A chloroform solution containing 1% by weight of the electroluminescent polymer MEH-PPV was filtered (0.2 μm filter produced by Sartorius). The filtrate was distributedonto the ITO layer using a lacquer ion machine at 1000 rpm. The thickness of the film after drying was 105 nm. The surface roughness was 5nm (stylus profilometer Alpha-Step 200 from Tencor Inst).

B) The procedure used is as follows, the titanyl sulfate solution is precipitated with sodium hydroxide solution

TiO₂Fiber particle:

preparation of fine TiO from so-called black liquor₂：

In a double-walled, heatable 6-liter flask equipped with a mechanical stirrer, thermometer, reflux condenser and bottom valve for discharging the product, 1400ml of 7.5% by weight sodium hydroxide solution were heated to 85 ℃. FeSO-containing material prepared by the sulfate process was placed in a 1 liter three-necked flask equipped with a stirrer, reflux condenser, electric heater and bottom discharge valve₄804ml of black liquor (d)_60℃1.566 g/ml; 13.83% by weight TiO, corresponding to 21.7g/l₂) (ii) a 28.39% by weight of free H₂SO₄) Heating to 60 ℃. The black liquor was run into sodium hydroxide solution over 3 minutes with vigorous stirring. A dark precipitate formed temporarily. The temperature of the mixture rose to 92 ℃ as a result of the neutralization. Stirring was continued for about 5 minutes until the mixture was only slightly cloudy. Then cooled to 30 ℃ over 20 minutes with continued stirring.

244ml of half-concentrated hydrochloric acid [ 20.6% by weight, ca.6.2 mol/l]were added at room temperature over a period of 5 minutes]To 244ml of the mixture thus prepared was added dropwise. A white precipitate formed. After standing for 1 hour, the precipitate was precipitated, filtered through a cellulose nitrate filter and washed with a total of 900ml of the above hydrochloric acid in portions. 32.5g of a powder containing 34.5% by weight of TiO were obtained₂(corresponding to 58% of theory), 14.7% by weight of HCl, 2.7% by weight of SO₄ ^2-And 170ppm Fe.

10.8g of the slurry was dissolved in 32.1g of distilled water. The "solution" contains about 8.3% by weight TiO₂And 3.6% by weight HCl, and is substantially clear and transparent.

TiO, determined by ultracentrifuge (solvent: Water/HCl)₂The fiber particle size was 4.5mm, which for stabilization purposes approximately corresponds to TiO₂The amount of 50% by weight of HCl.

The polyvinyl alcohol used was Moviol 8-88(Hoechst AG). C) Preparation of an electroluminescent device comprising an interlayer according to the invention

Preparation from 1 part by weight of 8% by weight TiO₂An aqueous solution of the fibre granules (corresponding to example B) and 2 parts by weight of a 2.5% by weight aqueous solution of Moviol.

The solution was filtered (using a 0.2 μm filter manufactured by Sartorius). The filtrate was applied to a MEH-PPV membrane and distributed through a paint centrifuge at 3000 rpm. The total layer thickness after drying of the 2-layer construction was 159 nm. The surface roughness was 8nm (stylus profilometer Alpha Step 200 from Tencor Inst).

The bilayer system thus produced is then vapor deposited with an Al-electrode. For this purpose, separate Al dots with a diameter of 3mm are gasified to TiO using a perforated hood₂On the surface of the Moviol. In the vapour-depositing process, the pressure in the vapour-depositing device (Legbold) is less than 10^-5Millibar.

The ITO layer and the Al-electrode are connected to a voltage source via supply lines. When the voltage is raised, a current flows through the double layer. Electroluminescence was detected at a voltage of 5V. The color of the electroluminescence is yellowish red.

The electroluminescence as a function of the current flowing is illustrated in fig. 2.

Fig. 2 illustrates the dependence of the electroluminescence intensity on the direct current flowing through the device at variable voltages. Having a two-layer structure, i.e. with TiO between the electroluminescent polymer layer (MEH-PPV) and the cathode₂The LED of the/Meviol layer has obviously stronger luminous intensity under the same current than the LED of a layer structure.

With TiO₂Electroluminescent device with/Moviol interlayer has an electroluminescent intensity at the same current that is significantly higher than without TiO₂The system of/Moviol interlayers (analogous to the comparison of example C, but without interlayers) is high. At 20V, the luminance of both electroluminescent devices was 10cd/m². However, with TiO₂In the case of the/Moviol intermediate layer, the flow current is only 0.5mA, whereas in the absence of TiO₂The current in the case of the/Moviol intermediate layer was 30mA at 20V. If the intermediate layer consists only of Moviol, no current passes and no electroluminescence is detected.

Claims

1. An intermediate layer in an electroluminescent device comprising finely divided inorganic particles (fibrils) dispersed in a polymeric binder.

2. An interlayer according to claim 1, which comprises as inorganic particles a metal oxide or a semiconductor compound of main groups III and V of the periodic Table of the elements (III-V semiconductor) or a semiconductor compound of main groups II and VI of the periodic Table of the elements (II-VI semiconductor).

3. An interlayer according to claim 2, comprising TiO₂、SnO₂、ZnO、WO₃、Al₂O₃、Fe₂O₃GaP, AlN, ZnSe, CdSe or CdS as inorganic particles.

4. An interlayer according to claim 1, comprising titanium dioxide as inorganic particles.

5. An interlayer according to claim 1, wherein the concentration of inorganic particles in the polymer binder is from 0.1 to 90% by weight (relative to the total interlayer).

6. An interlayer according to claim 1, characterized in that the inorganic particles of semiconductor material have a band gap of at least 1 eV.

7. Use of finely divided inorganic particles dispersed in a polymeric binder for the preparation of an intermediate layer of an electroluminescent device.

8. An electroluminescent device comprising one or more intermediate layers according to claim 1.

9. Use of the electroluminescent arrangement of claim 8 for the production of a light-emitting display.