JP2001085159A - Organic electroluminescent element driving method, driving device, and display device using it - Google Patents

Abstract

PROBLEM TO BE SOLVED: To ensure high light-emitting efficiency over a long period of time and obtain long light-emitting life by applying driving pulse in which first voltage which is light emitting start voltage or higher and second voltage which is higher than a specified voltage but lower than light emitting start voltage are alternately varied to between electrodes of an organic electroluminescent element. SOLUTION: A driving pulse applying circuit 10 connected between a hole injection electrode 2 and an electron injection electrode 6 of an organic EL element 20 applies driving pulse to the organic EL element 20. An organic light-emitting layer 4 of the organic EL element 20 emits light, and light 100 is emitted from the backside of a glass substrate 1. When first voltage is applied, light emitting intensity is quickly raised to a certain level, and since light emission is continued to the beginning of the application of second voltage of 0 V or higher, the total light emitting time is made long, and high light-emitting efficiency can be obtained. By alternately applying the first voltage and the second voltage, since the organic EL element 20 intermittently emits light, the drop amount of luminance is decreased over a long period of time and long life can be obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for driving an organic electroluminescence element, a driving apparatus for an organic electroluminescence element, and a display device using the same.

[0002]

2. Description of the Related Art In recent years, with the diversification of information equipment, there has been an increasing demand for a flat display element which consumes less power than a generally used CRT (cathode ray tube).
Research and development of a display using an organic electroluminescence element (hereinafter, referred to as an organic EL element) as one of such flat display elements has been actively conducted.
A display using an organic EL element has characteristics such as medium or high efficiency, thinness and light weight, and no dependence on viewing angle.

In an organic EL device, electrons and holes are injected into a light-emitting portion from an electron injection electrode and a hole injection electrode, respectively, and these electrons and holes are recombined at a light emission center to bring the organic molecules into an excited state. Generates fluorescence when returning from the excited state to the ground state. This organic EL element has a structure in which a plurality of light emitting elements are arranged in a matrix on a substrate.

[0004] Such an organic EL element has a voltage of 5V to 20V.
It has the advantage that it can be driven at a voltage as low as possible. Further, in the organic EL element, a light emitting element which emits light of an appropriate color can be obtained by selecting a fluorescent substance which is a light emitting material, and is expected to be used as a multi-color or full-color display device. Furthermore, organic E
Since the L element can emit light at low voltage, it can be used as a backlight for a display device such as a liquid crystal display device.

[0005] Reliability is an important factor in putting an organic EL device into practical use. Many organic light-emitting materials and doping materials have been studied to improve the light-emitting life of the organic EL device. Recently, the life of the organic EL device has been extended by controlling the current supplied to the organic EL device. The method of trying is examined.

For example, JP-A-11-3060 and JP-A-11-8064 disclose that a forward bias voltage and a reverse bias voltage are sequentially applied to an organic EL device.

[0007]

However, the organic E
In order to apply the L element to a display device, it is necessary to extend the life and improve the luminous efficiency (the ratio of the luminance to the applied power (unit: lm / W)). Therefore, there is a demand for achieving a long luminous life while securing high luminous efficiency.

An object of the present invention is to provide a driving method and a driving apparatus of an organic electroluminescence element capable of achieving a long luminous life while securing high luminous efficiency over a long period of time, and a display apparatus using the same. is there.

[0009]

According to a first aspect of the present invention, there is provided a driving method of an organic electroluminescence element, wherein a first voltage equal to or higher than a light emission starting voltage and higher than 0 V is applied between electrodes of the organic electroluminescence element. In addition, a drive pulse that alternately changes to a second voltage equal to or lower than the light emission start voltage is applied.

In the driving method according to the present invention, when a first voltage equal to or higher than the light emission start voltage is applied between the electrodes of the organic electroluminescence element, a current flows through the organic electroluminescence element, and the organic electroluminescence element emits light. I do.

At this time, when the voltage rises to the first voltage, a charging action occurs due to the influence of the capacitance of the organic electroluminescence element itself, and the current instantaneously increases and then stabilizes at a constant value. In this case, since the second voltage higher than 0 V and equal to or lower than the light emission start voltage is applied between the electrodes of the organic electroluminescence element before rising to the first voltage, the charging time is shortened.
Therefore, the emission intensity increases to a certain level in a shorter time.

When a second voltage is applied between the electrodes of the organic electroluminescent element, the electric charge accumulated in the organic electroluminescent element when the first voltage is applied does not flow out to the outside, and the organic electroluminescent element is kept for a while. Flows through the element. Thereby, recombination of holes and electrons occurs and contributes to light emission. As a result, the light emission continues for a certain period of time even after the fall to the second voltage, and then stops.

As described above, since the light emission intensity rises to a certain level in a short time when the first voltage is applied, and the light emission continues at the initial stage when the second voltage is applied, the entire light emission time becomes longer. . Further, it is not necessary to apply a high voltage to obtain a predetermined average luminance. As a result, high luminous efficiency is obtained.

Further, the first voltage and the second voltage are alternately applied to the organic electroluminescent element, so that the organic electroluminescent element emits light intermittently. Long service life is achieved.

According to a second aspect of the present invention, there is provided a driving apparatus for an organic electroluminescence element, wherein a first voltage equal to or higher than a light emission starting voltage and 0 V are applied between electrodes of the organic electroluminescence element.
And a drive pulse applying circuit for applying a drive pulse that alternately changes to a second voltage that is higher than the light emission start voltage.

In the driving device according to the present invention, the light emission intensity rises to a certain level in a short time when the first voltage is applied, and the light emission continues at the initial stage when the second voltage is applied. The light emission time becomes longer. Further, it is not necessary to apply a high voltage to obtain a predetermined average luminance. As a result, high luminous efficiency is obtained.

Further, the first voltage and the second voltage are alternately applied to the organic electroluminescent element, so that the organic electroluminescent element emits light intermittently. Long service life is achieved.

A display device according to a third aspect of the present invention provides a display device comprising: one or more organic electroluminescent elements; and a first voltage higher than a light emission start voltage and higher than 0 V between the electrodes of the one or more organic electroluminescent elements. And a drive device for applying a drive pulse that alternately changes to a second voltage equal to or lower than the light emission start voltage.

In the display device according to the present invention, the light emission intensity rises to a certain level in a short time when the first voltage is applied, and the light emission continues at the initial stage when the second voltage is applied. The light emission time becomes longer. Further, it is not necessary to apply a high voltage to obtain a predetermined average luminance. As a result, high luminous efficiency is obtained.

Further, the first voltage and the second voltage are alternately applied to the organic electroluminescent element, so that the organic electroluminescent element emits light intermittently. Long service life is achieved.

[0021]

FIG. 1 is a schematic diagram showing an example of the structure of an organic electroluminescence device (hereinafter referred to as an organic EL device) and a drive pulse application circuit.

In the organic EL device 20 shown in FIG. 1, a hole injection electrode 2 made of ITO (indium tin oxide), which is a transparent conductive film, is formed on a glass substrate 1.
On the hole injection electrode 2, MTDATA (4,4 ′, 4 ″ -tris (3-methylphenylphenyl) having the molecular structural formula of
A hole transport layer 3 made of amino) triphenyl-amine) having a thickness of 200 mm is formed. On the hole transport layer 3,
TPD (N, N'-diphenyl-) having the molecular structural formula of
N, N '-(3-methylphenyl) -1,1'-biphenyl-4,4'-diamine)
Has a molecular structural formula of (Chemical Formula 3)
Is doped by 5% to form an organic light emitting layer 4 having a thickness of 300 °. On the organic light emitting layer 4, Alq ₃ (tris (8-hydroxy-quinolinate) a having the molecular structural formula of
An electron transport layer 5 of 500 mm thick is formed. On the electron transporting layer 5, a 2000 mm thick M
An electron injection electrode 6 made of gIn is formed.

[0023]

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[0024]

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[0025]

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[0026]

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A drive pulse applying circuit 10 is connected between the hole injection electrode 2 and the electron injection electrode 6 of the organic EL element 20. The drive pulse application circuit 10 applies a drive pulse described later to the organic EL element 20. As a result, organic EL
The organic light emitting layer 4 of the element 20 emits light, and light 100 is emitted from the back surface of the glass substrate 1.

FIG. 2 is a voltage waveform diagram of a driving pulse applied to the organic EL element 20 by the driving pulse applying circuit 10 of FIG.

As shown in FIG. 2, the driving pulse includes a first voltage higher than the light emission start voltage (light emission threshold voltage) V ₀ and a second voltage higher than ₀ V and equal to or lower than the light emission start voltage V _0. And alternately. The frequency of the driving pulse is, for example, in the range of 30 Hz to 10 kHz.

Here, the period during which the drive pulse is at the first voltage is called a light emitting period T1, and the period during which the drive pulse is at the second voltage is called a non-light emitting period T2.

The organic EL element 20 emits light almost in the light emitting period T1, and emits no light almost in the non-light emitting period T2. To be more precise, as described later, there is a time during which the organic EL element 20 does not emit light during the light emitting period T1, and there is a time during which the organic EL element 20 emits light during the non-light emitting period T2.

Here, Examples, Comparative Examples 1 and 2
A driving voltage was applied to the organic EL element 20 by the driving method described above, and a continuous light emission experiment was performed. The voltage waveform, the current waveform, and the light emission intensity were analyzed, and the luminance and the light emission efficiency were changed over time.

In the embodiment, the driving pulse applying circuit 1 shown in FIG.
0, the driving pulse of FIG. 2 is applied to the organic EL element 20. In Comparative Example 1, a constant voltage higher than the light emission start threshold voltage V ₀ is applied to the organic EL element 20, and in Comparative Example 2, the organic EL element 20 is driven. A forward bias voltage and a reverse bias voltage were alternately applied to the device 20.

FIG. 3 is a diagram showing the driving voltage applied to the organic EL element 20, the current flowing through the organic EL element 20, and the emission intensity of the organic EL element 20 in the embodiment. FIG.
6 is a diagram showing a driving voltage applied to the organic EL element 20, a current flowing through the organic EL element 20, and an emission intensity of the organic EL element 20 in Comparative Example 1. FIG. FIG. 5 shows the driving voltage applied to the organic EL element 20 and the organic EL in Comparative Example 2.
FIG. 3 is a diagram showing a current flowing through the element 20 and the emission intensity of the organic EL element 20. In FIG. 5, a period in which the drive voltage is set to the forward bias voltage is called a light emission period T1,
A period in which the drive voltage is set to the reverse bias voltage is called a non-light emitting period T2.

In the present continuous light emission experiment, the driving voltage was set so that the average value of the current flowing through the organic EL element 20 was always constant from the start of light emission to the end of the experiment.
0 was driven at a constant current. Therefore, when the organic EL element 20 is deteriorated due to continuous light emission and the internal resistance increases, the drive voltage gradually increases.

The experimental conditions of the example, comparative examples 1 and 2 are shown in (Table 1).

[0037]

[Table 1]

In this continuous light emission experiment, the driving voltage was adjusted so that the same average luminance was obtained in the example, comparative example 1 and comparative example 2. The initial average luminance of the example, comparative example 1 and comparative example 2 was 300 cd / m ^2, which was the same.

As shown in FIG. 3, in the embodiment, the driving voltage during the light emission period T1 is set to a first voltage V ₃ is higher than the light emission starting voltage V _0. Further, in the non-light emitting period T2, the driving voltage is set to the second voltage equal to the light emitting start voltage V ₀ .

When the drive voltage is set to the first voltage V ₃ during the light emission period T1, a current flows through the organic EL element 20, and the organic EL element 20 emits light. When the driving voltage rises, a charging action occurs due to the influence of the capacitance of the organic EL element 20 itself, and after the current increases instantaneously, the current stabilizes to a constant value. In this case, since the driving voltage is set to the light emission threshold voltage V ₀ in the previous non-light emission period T2, the charging time t1 is shortened. Therefore, the light emission intensity increases to a certain level in a shorter time t2.

In the non-light emitting period T2, when the driving voltage is set to the light emitting threshold voltage V ₀ , the current flowing through the organic EL element 20 becomes zero. At this time, the charges accumulated in the organic EL element 20 during the light emission period T1 do not flow out and flow in the organic EL element 20 for a while. Thereby,
Recombination of holes and electrons occurs and contributes to light emission. As a result, light emission continues at the initial time t3 of the non-light emission period T2. After that, light emission of the organic EL element 20 stops.

As shown in FIG. 4, in Comparative Example 1, the drive voltage is set to a constant voltage higher than the light emission start voltage V ₀ . As a result, a constant current continues to flow through the organic EL element 20, and accordingly, the organic EL element 20 continues to emit light with a constant light emission intensity.

As shown in FIG. 5, in Comparative Example 2, the driving voltage is set to the forward bias voltage V ₂ higher than the light emission start voltage V _{0 in the} light emission period T 1. In addition, the non-light emitting period T
Driving voltage in the two is set to a reverse bias voltage -V _1. In this case, in order to make the average luminance the same as in the example and the comparative example 1, the driving voltage in the light emitting period T1 is set to the first voltage in the example.
It is set high as compared with the driving voltage of the voltage V ₃ and Comparative Example 1.

In the light emitting period T1, the driving voltage is turned forward.
Ass voltage V_TwoWhen the current is set to
Flows, and the organic EL element 20 emits light. Organic EL device 2
The light emission period is affected by the capacitance of the zero itself.
At the beginning of T1, a charging action occurs, and the organic EL element 20 is greatly charged.
Current flows, and then the current stabilizes at a constant value. did
Accordingly, the organic EL element 20 is activated in the early part of the light emitting period T1.
No light emission, forward bias voltage V_TwoA predetermined time delay from the application of
Emission intensity starts to increase and then stabilizes at a constant emission intensity
I do. In this case, the driving voltage is reversed during the previous non-emission period T2.
Ias voltage -V ₁, The charging time t4
Becomes longer. As a result, the emission intensity reaches a certain level.
The time t6 required to go up increases.

[0045] In the non-emitting time period T2, the drive voltage is set to a reverse bias voltage -V _1, after first large reverse current flows by the release of electric charge accumulated in the organic EL element 20, the organic EL element 20 The current approaches 0 due to the rectification, and becomes 0 at time t5. In this case, the organic EL element 20
Since the accumulated charge is discharged in the reverse direction immediately, emission stops application of the reverse bias voltage -V ₁ and at the same time.

FIG. 6 is a diagram showing the relationship between the luminous efficiency of the organic EL element 20 and the driving voltage. As shown in FIG. 6, the luminous efficiency of the organic EL element 20 is such that the driving voltage is the luminescence starting voltage V
It becomes highest at ₀ and becomes lower as the driving voltage becomes higher. Therefore, when the organic EL element 20 is driven at a high voltage, the luminous efficiency decreases.

In the driving method of the embodiment, as shown in FIG. 3, the light emission intensity rises to a predetermined level in a short time in the light emission period T1, and the light emission of the organic EL element 20 continues in the early part of the non-light emission period T2. Therefore, the entire light emission time becomes longer. In addition, in order to obtain a predetermined average luminance, the drive voltage can be set lower than in Comparative Example 2.
As a result, the luminous efficiency increases.

On the other hand, in the driving method of Comparative Example 2,
As shown in FIG. 5, since the time required for the light emission intensity to rise to a predetermined level in the light emission period T1 is long and the organic EL element 20 does not emit light at all in the non-light emission period T2, the entire light emission time is short. Become. Further, it is necessary to set a high driving voltage in order to obtain a predetermined average luminance. Therefore, the luminous efficiency decreases.

[0049] Incidentally, when set to 0V without setting the drive voltage in the reverse bias voltage -V ₁ in the non-emission period T2, although luminous efficiency is somewhat improved compared to Comparative Example 2, and FIG. 5 It shows a similar tendency.

FIG. 7 is a diagram showing a temporal change in luminance in a continuous light emission experiment of the example, comparative examples 1 and 2.
Here, the half life of the luminance is defined as the life.

In Comparative Example 1, the life was about 3,000 hours, whereas in Comparative Example 2, the life was about 10,000 hours. On the other hand, in the example, the life was about 7000 hours, which was twice or more that of the comparative example 1.

FIG. 8 is a diagram showing a time change of the luminous efficiency in the continuous light emission experiment of the example, the comparative examples 1 and 2.

As shown in FIG. 8, in Comparative Example 2, the initial luminous efficiency was as low as 3.8 (lm / W), and the luminous efficiency gradually decreased with continuous light emission. In Comparative Example 1, the initial luminous efficiency was the highest at 5.0 (lm / W), but the luminous efficiency sharply decreased with continuous light emission. After 3000 hours, the luminous efficiency of Comparative Example 2 was reduced. It was about the same. on the other hand,
In this embodiment, the initial luminous efficiency is 4.6 (lm / W), which is slightly smaller than that of Comparative Example 1, but the amount of decrease in luminous efficiency due to continuous light emission is small, and after several hundred hours, Comparative Example 1
Luminous efficiency. In the embodiment,
The luminous efficiency was higher than that of Comparative Example 2.

As described above, according to the driving apparatus and the driving method of the present embodiment, a long luminous life can be achieved while securing high luminous efficiency, and a decrease in luminous efficiency during continuous light emission for a long time is small. Become.

In the driving method of the present invention, the second voltage of the driving pulse is set within a range of 70% or more of the light emission threshold voltage V ₀ and the light emission start voltage V ₀ to obtain higher light emission efficiency. It is preferable that the light emission start voltage V ₀ is 8
It is more preferable to set within the range of the light emission starting voltage V ₀ below 0% or more, more preferably set within a range of light emission starting voltage V ₀ or less at 90% of the emission starting voltage V _0, the emission start It is most preferable to set the voltage to V ₀ .

FIG. 9 is a block diagram showing an example of the configuration of a display device using the driving device of the present invention.

In the display device of FIG. 9, a plurality of organic EL elements 20 having the structure of FIG. 1 are arranged in a matrix on n rows and m columns on a common substrate. Where n
And m is an arbitrary integer of 2 or more.

The hole injection electrodes 2 (see FIG. 1) of the plurality of organic EL elements 20 in each row are connected to a drive signal line 31. A plurality of drive signal lines 31 corresponding to a plurality of rows are connected to the row driver 30. A drive signal is applied to each of the plurality of drive signal lines 31 by the row driver 30.

The electron injection electrodes 6 (see FIG. 1) of the plurality of organic EL elements 20 in each column are connected to a column selection signal line 41. A plurality of column selection signal lines 41 corresponding to a plurality of columns are connected to a column driver 40. A column driver 40 sequentially applies a column selection signal to a plurality of column selection signal lines 41.

The drive signal applied to the drive signal line 31 by the row driver 30 and the column selection signal applied to the column signal selection line 41 by the column driver 40 are supplied to the organic EL element 20 to emit light from the light emission start voltage V ₀ . The first voltage V ₃ is set to be higher than the first voltage V _3, and the second voltage equal to the light emission start voltage V ₀ is applied to the non-light emitting organic EL element 20.

FIG. 10 is a waveform diagram showing an example of drive pulses applied to the organic EL elements 20 in the first to n-th rows of the display device of FIG. As shown in FIG.
A first voltage higher than the light emission start voltage V ₀ is applied to the L element 20, and a second voltage equal to the light emission start voltage V ₀ is applied to the organic EL element 20 when no light is emitted. The luminance changes according to the level of the first voltage applied to the organic EL element 20 during light emission.

The organic EL element 20 shown in FIG.
Although the organic light-emitting layer 4 obtained by doping rubrene is used, the driving device and the driving method of the present invention can be easily applied to various organic EL devices using other organic materials.

[Brief description of the drawings]

FIG. 1 is a schematic diagram illustrating an example of the structure of an organic EL element and a drive pulse application circuit.

FIG. 2 is a voltage waveform diagram of a drive pulse applied to an organic EL element by the drive pulse application circuit of FIG.

FIG. 3 is a diagram showing a driving voltage applied to an organic EL element, a current flowing through the organic EL element, and an emission intensity of the organic EL element in Examples.

FIG. 4 is a diagram showing a drive voltage applied to an organic EL element, a current flowing through the organic EL element, and an emission intensity of the organic EL element in Comparative Example 1.

FIG. 5 is a diagram showing a driving voltage applied to an organic EL element, a current flowing through the organic EL element, and an emission intensity of the organic EL element in Comparative Example 2.

FIG. 6 is a diagram showing the relationship between the luminous efficiency of an organic EL element and a driving voltage.

FIG. 7 is a diagram showing a change over time in luminance in a continuous light emission experiment of Example, Comparative Example 1 and Comparative Example 2.

FIG. 8 is a diagram showing a temporal change in luminous efficiency in a continuous light emission experiment of an example, comparative examples 1 and 2.

FIG. 9 is a block diagram illustrating an example of a configuration of a display device using the driving device of the present invention.

10 is a waveform diagram showing an example of a driving pulse applied to the organic EL elements in each row of the display device of FIG.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 glass substrate 2 hole injection electrode 3 hole transport layer 4 organic light emitting layer 5 electron transport layer 6 electron injection electrode 10 drive pulse applying circuit 20 organic EL element 30 row driver 31 drive signal line 40 column driver 41 column select signal line

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 3K007 AB00 AB03 CA01 CB01 DA00 DB03 EB00 GA00 GA02 5C080 AA06 BB05 DD26 DD29 JJ02 JJ04 JJ05

Claims (3)

[Claims] 1. A driving pulse which alternately changes between a first voltage equal to or higher than a light emission start voltage and a second voltage higher than 0 V and equal to or lower than the light emission start voltage is applied between electrodes of an organic electroluminescence element. A method for driving an organic electroluminescence device, comprising: 2. A driving device for driving an organic electroluminescence element, comprising: a first voltage higher than a light emission start voltage and a second voltage higher than 0 V and lower than the light emission start voltage between electrodes of the organic electroluminescence element. A driving device for an organic electroluminescence element, comprising: a driving pulse application circuit that applies a driving pulse that alternates with a voltage. 3. One or more organic electroluminescence elements, and a first voltage higher than a light emission start voltage between electrodes of the one or more organic electroluminescence elements and a voltage higher than 0 V and equal to or lower than the light emission start voltage. And a drive device for applying a drive pulse that alternates with the second voltage.