JPH04349388A - Driving method for organic emission element - Google Patents

Abstract

PURPOSE:To realize high intensity emission for a long time by applying AC voltage between the electrodes of an organic emission element which has an emission layer made up of fluorescent material and a positive hole and/or a mobile donating agent between two electrodes opposed to each other. CONSTITUTION:An emission layer whose thickness is 1000Angstrom is formed by dip coating from a solution of 1.2-dichloroethane containing one part by weight of poly (N-vinylcarbazole) as a positive hole mobil donating agent, one part by weight of 2,5-vis(1-naphtyl)-1,3,4-oxadiazol. as an electron mobil donating agent and 0.02 parts by weight of 3-(2'-benzothiazole)-7-diethyl amino coumarin (coumarin 6) as fluorescent material on an ITO glass substrate to which ultrasonic cleaning and ultraviolet rays cleaning are applied, and further, vacuum deposition of magnesium is applied to the substrate in a thickness of 2000Angstrom . Green light of 300cd/m<2> can be obtained for 200 hours at a luminance retention of 0.92 after emission of light from the side of the substrate when a sine-wave voltage of effective voltage of 20V 50Hz is applied to this element.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of driving an organic electroluminescent device capable of emitting high-intensity light for a long period of time.

0002

[Prior Art] Organic electroluminescent devices are
It consists of an organic light emitter sandwiched between opposing electrodes, with electrons being injected from one electrode and holes being injected from the other electrode. Light is emitted when the injected electrons and holes recombine within the light emitting layer.

[0003] In such devices, a single crystal material such as single crystal anthracene has been used as a light emitter, but single crystal materials are expensive to manufacture and have many problems in terms of mechanical strength. Furthermore, it is not easy to reduce the thickness, and a single crystal of about 1 mm emits weak light;
Drive voltages of 100 volts or more are often required, making them impractical.

[0004] Therefore, an attempt was made to obtain a film of 1 μm or less of anthracene, for example, by using a vapor deposition method [Thin Sol].
id Films, 94 P. 171 (1982)
] has been attempted. However, in order to obtain sufficient performance, it is necessary to form a thin film of several thousand angstroms under strictly controlled film-forming conditions, and even though the light-emitting layer is formed as a thin film with high precision, the carrier −
Because the density of holes or electrons is very low, and the probability of excitation of functional molecules due to carrier movement or recombination is low, efficient light emission cannot be obtained, and in particular, the power consumption and brightness are unsatisfactory. The current situation is that this is not possible.

Furthermore, Japanese Patent Laid-Open No. 57-51781 shows that high luminous efficiency can be obtained by providing a hole injection layer between the anode and the luminescent layer to increase the density of holes, which are carriers.
No. 194393, Japanese Patent Application Laid-Open No. 1983-19439
It is known from the publication No.-295695.

[0006] In these devices, it is necessary to use a substance that has electron injection and transport properties and high fluorescence efficiency for the light emitting layer. Alternatively, a compound in which a compound with hole injection and transport properties and high fluorescence efficiency is used as a light emitting layer, and an electron injection and transport layer is laminated thereon, or a hole injection and transport layer, a light emitting layer, and an electron injection and transport layer are laminated in this order. is proposed.

In addition to such a stacked organic electroluminescent device, Japanese Patent Application No. 3-51106 discloses
A compound containing a fluorescent substance between two opposing electrodes, at least one of which is transparent, and which transfers holes injected from the electrodes and provides them to the fluorescent substance (hereinafter referred to as a hole transfer donor);
A dispersion type electroluminescent device is disclosed which has a light-emitting layer comprising a compound (hereinafter referred to as an electron transfer donor) that transfers electrons injected from an electrode and provides them to the fluorescent substance.

In any of these conventional organic electroluminescent devices, a material with a small work function that can efficiently inject electrons into the electron injection transport layer and the light emitting layer is used for the cathode. Generally, a material with a large work function that can efficiently inject holes into the hole injection transport layer and the light emitting layer is used for the anode, and DC drive is performed.

However, when such an organic electroluminescent element is driven with a constant voltage DC power supply, the luminance decreases as the current decreases over time.
To prevent this, if the device is driven with a constant current DC power source, the voltage will increase over time, eventually leading to electrode breakdown. Therefore, an organic electroluminescent device capable of emitting high-intensity light for a long time has not been obtained.

0010

SUMMARY OF THE INVENTION The present invention provides an organic electroluminescent device capable of emitting high-intensity light for a long period of time.

[0011]

[Means for Solving the Problems] The present inventors have intensively studied various device structures, electrode materials, and driving methods in an organic electroluminescent device having a light emitting layer made of an organic substance between two opposing electrodes. result,
In an organic electroluminescent element having a luminescent layer containing a fluorescent substance between two opposing electrodes, at least one of which is transparent, and consisting of a hole transfer donor and/or an electron transfer donor, The present inventors have discovered that by applying an alternating current voltage to the device, it is possible to emit high-intensity light for a long period of time, and have completed the present invention.

That is, the present invention provides an organic electroluminescent device having a luminescent layer containing a fluorescent substance and consisting of a hole transfer donor and/or an electron transfer donor between two opposing electrodes, at least one of which is transparent. This is a method of driving an organic light-emitting device, characterized in that the device emits light by applying an alternating current voltage to two opposing electrodes.

The present invention will be explained in detail below. The present invention provides an organic electroluminescent device having a luminescent layer containing a fluorescent substance and consisting of a hole transfer donor and/or an electron transfer donor between two opposing electrodes, at least one of which is transparent. This is based on the discovery that by applying an AC voltage to two opposing electrodes, the brightness decreases over time less than when applying a DC voltage, and furthermore, it is possible to maintain high brightness light emission for a long time without causing electrode breakdown. .

[0014] In conventional driving methods, it is assumed that the cause of deterioration of luminescent properties over time is due to charge accumulation at the electrode-emitting layer interface, and the method of the present invention has made it possible to emit high-intensity light for a long time. We speculate that this is because applying voltage in both directions prevented charge accumulation at the interface.

[0015] Electrode materials used in the organic electroluminescent device of the present invention include metals, metalloid elements, alloys thereof, intermetallic compounds, oxides, composite oxides, carbides, sulfides, silicides, iodides, and It can be freely selected from inorganic conductive substances such as composite materials thereof, or conductive polymers such as polyaniline, polypyrrole, polyparaphenylene vinylene, polythiophene, polyacetylene, and polythienylene vinylene.

Furthermore, the electrode material has a work function difference of 0.5e.
When a combination of materials with a voltage lower than V is used, the luminance decreases over time less than when a DC voltage is applied, and furthermore, it is possible to maintain high luminance light emission for a long time without causing electrode breakdown. In order to emit light in either polarity, the electrode material has a work function difference of 0.5.
Higher luminous efficiency can be obtained compared to the case where a combination of materials with eV or more is used.

The work function of the electrode material can be found, for example, from Thin Film Handbook, published by Ohm Co., Ltd. in 1988, page 475. This kind of combination of electrode materials is as follows:
Examples include:

When magnesium is used as the main component for one electrode, magnesium, indium,
Examples include using lead, manganese, etc. as the main component. In addition, when one electrode uses indium as the main component, the counter electrode can be indium, aluminum, silver,
Examples include using iron, tin, chromium, manganese, lead, etc. as main components. Further, when aluminum is used as a main component for one electrode, aluminum, iron, copper, silver, tin, antimony, lead, palladium, chromium, manganese, etc. can be used as a main component for the counter electrode. In addition, when silver is used as the main component for one electrode,
As a counter electrode, iron, copper, silver, tin, antimony, lead, chromium, manganese, etc. may be used as a main component.

Further, when gold is used as a main component for one electrode, palladium, gold, or the like may be used as a main component for the counter electrode. Further, when tin is used for one electrode, tin, copper, antimony, iron, chromium, lead, manganese, etc. may be used as a main component for the counter electrode. Moreover, when copper is used as a main component for one electrode, copper, iron, antimony, chromium, etc. can be used as a main component for the counter electrode. Moreover, when iron is used as a main component for one electrode, iron, antimony, chromium, lead, manganese, etc. can be used as a main component for the counter electrode.

Furthermore, when lead is used as a main component for one electrode, lead, chromium, manganese, etc. may be used as a main component for the counter electrode. Moreover, when antimony is used as a main component for one electrode, antimony, chromium, manganese, etc. can be used as a main component for the counter electrode. Furthermore, when one electrode uses chromium as a main component, the counter electrode may use chromium, manganese, or the like as a main component. Furthermore, when manganese is used as a main component for one electrode, manganese or the like may be used as a main component for the counter electrode. If indium tin oxide (ITO) is used as one electrode, ITO or tin oxide can be used as the counter electrode, or indium, lead, manganese, aluminum, iron, silver,
Examples include using tin, antimony, chromium, copper, etc. as main components.

[0021] When tin oxide is used as one electrode, tin oxide or ITO is used as the counter electrode, or a material mainly composed of indium, lead, manganese, aluminum, iron, silver, tin, antimony, chromium, copper, etc. For example, it can be used.

Other metals may be added to these metal electrodes for the purpose of improving conductivity, adhesion to the organic layer, or stability. In the organic electroluminescent device of the present invention, one of the two opposing electrodes must be transparent or semitransparent. Therefore, it is preferable to make one electrode thin enough to have sufficient transmittance.

[0023] As a method for forming the electrode, a normal metal thin film forming method such as vacuum evaporation or sputtering can be used. When the organic electroluminescent device of the present invention is driven with alternating current, the waveform of the alternating current signal applied to two opposing electrodes can be of course a sine wave, but if necessary, a rectangular wave, a triangular wave, or a combination or synthesis thereof. Any AC waveform can be used. Therefore, although these AC waveforms may be vertically asymmetrical, they are required to have a certain repetition period and change polarity once within one period. The repetition period is 10 seconds to 1 x 10-6 seconds, which is converted into a frequency of 0.
1 Hertz to 1×10 6 Hertz is preferred.

Furthermore, the power source for driving the organic electroluminescent device of the present invention may be a voltage source or a current source. The organic electroluminescent device of the present invention contains a fluorescent substance and has a light-emitting layer made of a hole transfer donor and/or an electron transfer donor as an essential component, and in addition to the light-emitting layer, a positive electrode injected from an electrode as necessary. A hole injection transport layer that moves holes and provides them to the light emitting layer and/or a hole blocking layer that confines holes in the light emitting layer, and an electron injection transport layer that moves electrons injected from the electrode and provides them to the light emitting layer may be provided. good.

The hole injection transport layer, hole blocking layer, and electron injection transport layer are layers provided between the electrode and the light emitting layer, and are formed of a hole transport compound and an electron transport compound, respectively. The light-emitting layer, hole injection transport layer, hole blocking layer, and electron injection transport layer of the organic electroluminescent device of the present invention may be formed by vapor deposition, or if necessary, by using a polymer binder to form a solution. It may also be formed by coating from scratch. When coating from a solution, conventional and well-known coating methods such as casting, blade coating, dip coating, spin coating, spray coating, and roll coating can be used. This can be done using a construction method.

Materials used for the light emitting layer, hole injection layer, hole blocking layer, and electron injection transport layer of the organic electroluminescent device of the present invention include the compounds described in Japanese Patent Application No. 3-51106. Any one of these can be used. The thickness of the light-emitting layer is preferably 50 Å or more and 1 μm or less in any case, and the optimal thickness is 5000 Å or more and 1 μm or less.
Å or less is preferable.

The organic electroluminescent device of the present invention can be used for OA equipment such as color displays, flat panel displays, backlights for liquid crystal displays, static eliminating light sources for copying machines, and light sources for printers, and for automotive displays and stop lights. Many ordinary light-emitting devices are used as automotive parts such as lamps, turn signals and tail lamps, and even as light-emitting devices for toys and night indicators for road construction. Possible uses are: Furthermore, since a variety of luminescent colors can be obtained by selecting the fluorescent substance, application to full-color displays can be expected.

[0028]

[Examples] The present invention will be explained in more detail with reference to Examples below.

[Examples 1 to 4] ITO glass substrates [manufactured by HOYA Co., Ltd.] were ultrasonically cleaned in acetone and air-dried.
110] for 5 minutes. Next, poly(N-vinylcarbazole) [BAS
Manufactured by Company F, Luvican M170] 1 part by weight, 2,5-bis(1-naphthyl)-1 as an electron transfer donor,
3,4-oxadiazole [Lancaster S
[manufactured by Synthesis] 1 part by weight, 3- as a fluorescent substance
A light-emitting layer was formed to a thickness of 1000 Å by dip coating from a 1,2-dichloroethane solution containing 0.02 parts by weight of (2'-benzothiazolyl)-7-diethylaminocoumarin (coumarin 6) [manufactured by Kodak].

Next, magnesium was vacuum-deposited to a thickness of 2000 Å over an area of 0.1 cm 2 through a shadow mask. When a sinusoidal voltage with an effective voltage of 20 V and a frequency of 50 Hz was applied to the device thus manufactured, green light emission of 300 cd/m2 was obtained from the glass substrate side. Regarding the case where the frequency of the sine wave voltage is changed,
Table 1 shows the brightness retention rate obtained by dividing the brightness after 200 hours of light emission by the initial brightness.

[0030]

[Comparative Examples 1-2] 2 as a comparative example for Examples 1-4
0V DC voltage (0 Hertz) and 20V, 3 x 106
Experiments similar to Examples 1 to 4 were conducted except that a Hertzian sine wave voltage was applied. The brightness retention rates after 200 hours of light emission are shown in Table 1 together with Examples 1 to 4.

[0031]

[Example 5] A copper electrode with a thickness of 200 Å was formed as one electrode by vacuum evaporation on a glass substrate that had been cleaned in the same manner as in Example 1.
-Inpropylcarbazole, 2 as an electron transfer donor
-(4'-tert-butylphenyl)-5-(4''-
biphenylyl)-1,3,4-oxadiazole (butyl-PBD) [manufactured by Dojindo Laboratories Co., Ltd.], coumarin 6 was used as the fluorescent material, and the weight ratio of each was 1:1:0.
．． The temperature of each crucible was adjusted under a pressure of 1.5×10 −2 Torr so that the luminescent layer had a thickness of 1000 Å by vacuum evaporation.

[0032] Next, a layer with a thickness of 2000 Å and an area of 0
．． A chromium electrode of 1 cm2 was formed by vacuum deposition. When a triangular wave voltage of 1×10 3 Hz and an effective voltage of 22 V was applied to the device thus fabricated, green light was emitted from the glass substrate side. Furthermore, the brightness retention rate after emitting light for 200 hours was 0.75.

[0033]

[Comparative Example 3] As a comparative example of Example 5, an experiment similar to Example 5 was conducted except that a DC voltage of 22 V was applied. As a result, the brightness retention rate after 200 hours of light emission was 0.38.

[0034]

[Example 6] 200 Å on a glass substrate as one electrode
200 silver electrodes were formed by vacuum evaporation, and 200 silver electrodes were used as counter electrodes.
A device was produced in the same manner as in Example 1, except that a 0 Å silver electrode was formed by vacuum evaporation. When a sine wave voltage of 50 Hz and an effective voltage of 20 V is applied to the device thus fabricated, the initial luminance from the glass substrate side is 410 cd/m.
2 green luminescence was obtained. Furthermore, the brightness retention rate after emitting light for 200 hours was 0.95.

[0035]

[Comparative Example 4] As a comparative example of Example 6, an experiment similar to Example 6 was conducted except that a DC voltage of 20 V was applied. As a result, the brightness retention rate after 200 hours was 0.33.

[0036]

[Table 1]

[0037]

The present invention provides a method for driving an organic electroluminescent device capable of emitting high-intensity light for a long period of time.

Claims (1)

[Claims] 1. A light-emitting layer is provided between two opposing electrodes, at least one of which is transparent, and the light-emitting layer includes a fluorescent substance;
In an organic electroluminescent element containing a compound that moves holes injected from an electrode and provides the fluorescent material, and/or a compound that moves electrons injected from the electrode and provides the fluorescent material, the two opposing electrodes A method for driving an organic light-emitting device, characterized in that it emits light by applying an alternating current voltage to the device.