JPH04308687A - Organic electroluminescence display - Google Patents

Abstract

PURPOSE:To suppress the deterioration of electric property even if DC driving continues by providing a protective means, which applies reverse voltage, between an organic electroluminescence(EL) element, which is driven by the application of DC voltage, and its cathode and anode. CONSTITUTION:A plurality of transparent anodes 2 consisting of ITOs, a positive transport layer 3, an EL layers 4, and a plurality of cathodes 5 are stacked in order on a glass transparent substrate. A driving circuit makes it emit light according to the pigments of a plurality of EL elements by applying positive DC voltage. A protective circuit 7 applies the voltage reverse to the DC voltage from the driving circuit 6 to between electrodes to between the electrodes 2 and 5. The protective circuit 7 is equipped with a reverse voltage power source 8, and applies reverse voltage to the electrodes 2 and 5. This way, it can be made the device wherein the deterioration of the electric property of the EL element is suppressed even if DC driving continues.

Description

[Detailed description of the invention]

0001

TECHNICAL FIELD The present invention relates to an EL display device comprising an EL element that utilizes electroluminescence (hereinafter referred to as EL) of a substance that emits light when an electric field is applied to form a layer of such a substance, and particularly relates to an EL display device comprising an EL element that is equipped with a layer of such a substance. The present invention relates to an organic EL display device made of an organic compound.

0002

[Background technology] When EL elements are classified by the material of the EL layer, there are inorganic types with an EL layer made of inorganic substances, and E elements with an EL layer made of organic substances.
It is divided into an organic type having an L layer. Such inorganic EL
Elements are classified into DC types, which are driven by a DC electric field, and AC types, which are driven by an AC electric field. In addition, organic EL
A DC injection type device is well known, and is a rectifying device similar to an LED.

For example, as shown in FIG. 4, an organic EL element includes a glass transparent substrate 1, a plurality of transparent electrodes 2 (anodes) made of ITO (indium tin oxide), a hole transport layer 3, and an organic EL element. There is an XY matrix type organic EL element 10 in which a plurality of back electrodes 5 (cathode) intersecting a layer 4 and a transparent electrode 2 at right angles are sequentially laminated and formed. For organic EL elements,
A two-layer structure consisting of a hole transport layer 3 and an organic EL layer 4 as shown, or an organic EL layer 4 and a cathode 5 (not shown).
A three-layer structure in which an organic electron transport layer is further arranged between is also known. Further, the cathode 5 is usually coated with a protective layer that protects the cathode 5 and prevents short circuits. Organic EL layer 4 sandwiched between a set of anode 2 and cathode 5 shown in the figure
emits light when DC voltage is applied from the anode 2 and cathode 5.

As shown in FIG. 5, the organic EL display device has the above-described organic EL element and a circuit 6 for driving the organic EL element. Although this drive circuit 6 is omitted in the figure and is shown as a representative of only a DC power source, this drive circuit 6 causes one organic EL element portion in the dot matrix shown in FIG. 4 to correspond to one pixel and emit light. . The display drive circuit 6 includes, for example, a memory (not shown) having storage locations corresponding to each of a plurality of pixels made of organic EL elements, and outputs video data from a video data signal generating means (not shown). Video day
The DC voltage is applied or not applied to each organic EL element portion according to the contents of the memory, the contents of which are updated in accordance with the data signal, to determine whether each pixel of the organic EL element is turned on or off, that is, bright or dark. Thus, such matrix-form organic E
In the L display device, one intersection point of each orthogonal strip-shaped electrode corresponds to one pixel, and the DC voltage applied between any electrodes of the scanning electrode group and the signal electrode group is controlled, and each pixel, that is, the organic EL An image is displayed by changing the light emitting state of the element. For example, matrix type organic E
In the L display device, a positive driving DC voltage is intermittently applied in accordance with a video data signal or the like, as shown in FIG. 6(b), in order to turn on the matrix. In addition, when lighting the organic EL element continuously (static lighting),
As shown in FIG. 6(a), a positive DC voltage is continuously applied.

However, in organic EL devices,
Initially, as shown in FIG. 7A, the dielectric organic molecules 34 forming the hole transport layer 3 and the organic EL layer 4 sandwiched between the anode 2 and cathode 5 are amorphous even near the pn junction 35. However, when the organic EL element continues to be driven with direct current, the organic molecules 34 become oriented near the pn junction 35 due to dielectric polarization caused by the electric field in a certain direction, and the pn junction is destroyed, as shown in FIG. 7(b). As a result, the electrical characteristics of the element deteriorate over time, and eventually the organic EL element itself is damaged in short circuit mode.

As shown in FIG. 8, ITO is deposited on a glass substrate.
anode, TPD hole transport layer 3, Alq3 organic E
When examining the electrical characteristics of an organic EL display device including an organic EL element in which an L layer and a cathode 5 of MgAl are sequentially laminated, it is found that the voltage-current characteristics (broken line) and voltage-luminance characteristics (dotted-dashed line) of the original organic EL element are the same. On the other hand, the voltage-current characteristics (solid line) and voltage-luminance characteristics (two-dot chain line) after long-term DC voltage driving have significantly deteriorated.

[0007]

OBJECTS OF THE INVENTION The present invention was made in view of the above-mentioned problems, and an object of the present invention is to provide an organic EL display device that suppresses deterioration of the electrical characteristics of organic EL elements even when DC driving is continued.

[0008]

[Structure of the Invention] An organic EL display device of the present invention includes an organic EL element having an organic EL layer made of an organic substance disposed between a cathode electrode and an anode electrode facing each other on a substrate, the cathode electrode and the anode electrode. It is characterized by comprising a driving means for applying a DC voltage between the anode electrodes, and a protection means for intermittently applying a reverse voltage between the cathode electrode and the anode electrode.

[0009]

According to the present invention, it is possible to obtain an organic EL display device in which the electrical characteristics of organic EL elements change little over time.

0010

Embodiments Hereinafter, embodiments of the present invention will be described with reference to the drawings. As shown in FIG. 1, the organic EL display device of this embodiment includes an organic EL element 10, a driving means for applying a DC voltage to the organic EL element, that is, a driving circuit 6,
It has a protection means, that is, a protection circuit 7 for applying a reverse voltage to the element.

Although only one pixel portion of the organic EL element is shown in FIG. 1, the organic EL element has a plurality of transparent anodes 2 made of ITO and a hole transport layer on a glass transparent substrate 1, similar to the one described above. 3. An organic EL layer 4 and a plurality of cathodes 5 are laminated and formed in this order. The organic EL element has a two-layer structure consisting of a hole transport layer 3 and an organic EL layer 4 as shown in the figure.
Alternatively, it may be a three-layer structure in which an organic electron transport layer is further disposed between the organic EL layer 4 and the cathode 5, or a two-layer structure in which an electron transport layer is provided between the organic EL layer and the electrode between the cathode 5. A protective layer may be coated on the cathode 5 to protect it and prevent short circuits.

Although the drive circuit 6 is omitted in the figure and is shown as a representative of only the DC power supply as described above, the drive circuit 6 applies a positive DC voltage to correspond to the pixels of the plurality of organic EL elements. Similarly to the above, the DC voltage applied between the electrodes 2 and 5 is controlled in accordance with signals such as video data, and the light emitting state of each pixel of the organic EL element is changed. This causes the organic EL element to display an image.

The protection circuit 7 applies a reverse voltage between the electrodes 2 and 5, which is in the opposite direction to the DC voltage applied between the electrodes from the drive circuit 6. The protection circuit 7 includes a reverse voltage power supply 8 , a switch 9 that switches between applying the reverse voltage between the electrodes 2 and 5 from the reverse voltage power supply 8 or applying it from the drive circuit 6 .
The oscillator 11 controls the connection time of each of the switching devices 9 and drives the switching device 9. The reverse voltage power supply 8 can also have its output potential variable, and the protection circuit prevents a reverse voltage of 3 V or higher and lower than the reverse withstand voltage of the organic EL element from being applied to the electrode 2.
, 5.

For example, as shown in FIG. 2(a), when the protection circuit 7 applies a reverse voltage to the organic EL element, in the case of static lighting of the organic EL element, the total time during which the DC voltage is applied is By applying a reverse voltage for 1/100 of the time, deterioration of the organic EL element can be suppressed. Further, in this case, the protection circuit 7 applies a reverse voltage for a time t of 20 milliseconds or less during the period when the DC voltage is applied and the light should be turned on, thereby eliminating flickering to the viewer. The oscillator 11 is shown in FIG.
), the switch 9 is operated to produce pulses of reverse voltage at regular intervals.

Further, for example, as shown in FIG. 2(b),
In the case of matrix lighting of organic EL elements, deterioration of the organic EL elements can be prevented by not applying a reverse voltage while a DC voltage is applied and lighting is to be performed, but by continuously applying a reverse voltage only during non-lighting periods. is suppressed. In addition, as shown in FIG. 2(c), the reverse voltage is not applied while the DC voltage is applied and the light should be lit, and the reverse voltage is applied only during the non-lighting period by the oscillator 11, which pulses the reverse voltage at regular intervals. So that
A similar effect can be obtained by applying

By controlling the oscillator 11 to make the width and period of the constant interval pulses of the reverse voltage variable, and by controlling the output potential of the reverse voltage power source 8 to be variable, the total time during which the DC voltage is applied can be changed. The total application time of the reverse voltage can be maintained for 1/100 of the time. For example, when examining the electrical characteristics of an organic EL display device that includes an organic EL element in which an ITO anode, a TPD hole transport layer 3, an Alq3 organic EL layer, and a MgAl cathode 5 are laminated in this order on a glass substrate, As shown in Figure 3,
The voltage-current characteristics (solid line) and voltage-luminance characteristics (double-dashed line) after long-term DC voltage driving, compared to the original voltage-current characteristics (broken line) and voltage-luminance characteristics (dotted-dash line) of the organic EL element. has little deterioration.

[0017]

As described above, the organic EL display device of the present invention includes an organic EL element that is driven by applying a DC voltage and a protection means that applies a reverse voltage between its cathode and anode. An organic EL element with less deterioration of electrical characteristics over time can be obtained.

[Brief explanation of the drawing]

FIG. 1 is a schematic diagram of an organic EL display device according to an embodiment of the present invention.

FIG. 2 is a graph showing the state of application of a driving DC voltage and a reverse voltage to an organic EL element in an example according to the present invention.

FIG. 3 is a graph showing voltage-current characteristics and voltage-luminance characteristics of an organic EL element in an example according to the present invention.

FIG. 4 is an enlarged perspective view of a cutaway portion of an organic EL element.

FIG. 5 is a schematic diagram of a conventional organic EL display device.

FIG. 6 is a graph showing the state of application of a driving DC voltage to a conventional organic EL element.

FIG. 7 is a schematic diagram illustrating the state of organic molecules of a dielectric material forming a hole transport layer and an organic EL layer when a driving DC voltage is applied to an organic EL element.

FIG. 8 is a graph showing voltage-current characteristics and voltage-luminance characteristics of an organic EL element in a conventional example according to the present invention.

[Explanation of symbols]

1...Transparent substrate 2...Transparent anode 3...Hole transport layer 4...Organic EL layer 5...Cathode 6...Cathode 7...Protection circuit 8... Reverse voltage power supply 9...Switcher 10...Organic EL element 11...oscillator

Claims (7)

[Claims] 1. An organic electroluminescent element having an organic electroluminescent layer made of an organic substance disposed between a cathode electrode and an anode electrode facing each other on a substrate, and a DC voltage applied between the cathode electrode and the anode electrode. 1. An organic electroluminescent display device comprising: a drive means for applying a voltage; and a protection means for intermittently applying a reverse voltage between the cathode electrode and the anode electrode. 2. The organic electroluminescent display device according to claim 1, further comprising a hole transport layer made of an organic compound between the anode electrode and the electroluminescent layer. 3. The organic electroluminescent display device according to claim 1, further comprising an electron transport layer made of an organic compound between the electroluminescent layer and the cathode electrode. 4. A hole transport layer made of an organic compound is provided between the anode electrode and the electroluminescent layer, and an electron transport layer made of an organic compound is provided between the electroluminescent layer and the cathode electrode. The organic electroluminescent display device according to claim 1, characterized in that: 5. The organic electroluminescent display device according to claim 1, wherein the reverse voltage is a pulse having a pulse width that is 1/100 of the time during which the DC voltage is applied. 6. The organic electroluminescent display device according to claim 1, wherein the reverse voltage is a pulse having a pulse width of 20 milliseconds or less. 7. The organic electroluminescent display device according to claim 1, wherein the reverse voltage has a voltage of 3 V or higher and lower than the reverse withstand voltage of the organic electroluminescent element.