JP3063453B2 - Driving method of organic thin film EL element - Google Patents

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 thin film EL device utilizing the electroluminescence (hereinafter referred to as "EL") phenomenon of an organic thin film, and particularly to preventing a half-excited half-light emission state in a non-selected single device. And a method for driving an organic thin-film EL element capable of improving the stability of gradation display.

[0002]

2. Description of the Related Art In recent years, an organic thin film EL device using the EL phenomenon of an organic thin film has been developed by Eastman Kodak C.W.T.
Ang et al.

This organic thin film EL device is disclosed in, for example,
JP-A-9-194393, JP-A-63-264692, JP-A-63-295695, Applied
Physics Letter Vol. 51, No. 12, page 913 (1
987), and Journal Applied Physics, Vol. 65, No. 9, p. 3610 (1989).

Such an organic thin-film EL device is a self-luminous display device that can be driven at a low DC voltage, and surpasses a liquid crystal display in that it has a wide viewing angle, a bright display surface, and a thin and light body. Has advantages. For this reason, it is greatly expected as a large-capacity display element such as a display or a wall-mounted television that requires high reliability.

FIG. 6 is a sectional view showing the structure of this type of organic thin film EL device.

In this organic thin film EL device, a hole injection / transport layer 3, an electron transport / emission layer 4 and a cathode 5 are sequentially laminated on an anode 2 provided on a glass substrate 1.

The anode 2 is a transparent electrode for increasing the transmittance of light emission. For example, various transparent conductive materials such as ITO (indium tin oxide), stannic oxide, and indium oxide can be used.

The hole injecting / transporting layer 3 is an organic substance having a high hole injecting efficiency, hole mobility, and carrier density from the anode 2, such as a triphenylamine derivative, an arylamine derivative, a hydrazone derivative, and a phthalocyanine compound. Can be used as appropriate. The hole injecting / transporting layer 3 is provided on the anode 2 by a vacuum evaporation method in order to form a thin film having a thickness of several tens nm that is uniform, smooth, free of pinholes, and is not contaminated by foreign substances. In addition,
The hole injection / transport layer 3 can be formed by spin coating, casting, or the like in addition to the vacuum evaporation method.

The electron transporting light emitting layer 4 is provided on the hole injection transporting layer 3 and is an organic substance having a high electron injection efficiency from the cathode 5, for example, 8-hydroxyquinoline aluminum complex, phthaloperinone derivative, oxadiazole derivative ,
Those appropriately selected from coumarin derivatives, distyrylbenzene derivatives and the like can be used.

The cathode 5 is a metal electrode having good adhesion to the electron transporting light emitting layer 4 and high electron injection efficiency.
Various metals, such as g, In, Ag, Li, and Al, and their alloys can be used as a single layer or a laminate. The cathode 5 is formed on the electron transporting light emitting layer 4 by a vacuum evaporation method.

Further, such an organic thin film EL device has a work function of the anode 2 and a HOMO (Highest occupied molecul) of the hole injection / transport layer 3 for high luminous efficiency and low voltage driving.
ar orbital level and the work function of the cathode 5 and the LUMO (Lowest unoccupied molecular orb) of the electron transporting luminescent layer 4
ital) Materials close in level are selected.

FIG. 7 is a diagram showing the voltage dependence of luminance and current in an organic EL device of this type. In addition,
This organic thin film EL device uses ITO as the anode 2, a triphenylamine derivative as the hole injection / transport layer 3, an 8-hydroxyquinoline aluminum complex as the electron transport / emission layer 4, and an Mg / Li alloy as the cathode 5.

Here, when a forward bias drive voltage VB is applied between the anode 2 and the cathode 5, voltage-luminance and voltage-current characteristics having strong nonlinearity are observed. here,
By applying a low voltage of about 10 V, a current density of about 40 mA / cm ² and a luminance of 1000 cd / m ² are obtained.

By the way, in such an organic thin film EL element, the anode 2 and the cathode 5 are formed in a stripe shape, and a switching element for applying a driving voltage and a control unit thereof are provided. Matrix driving becomes possible.

For example, the principle aspect of such matrix driving is described in “Development Strategy of Organic EL Device” (page 69,
Science Forum, published in 1992), etc., and a light emitting diode (LED)
This is explained by using an equivalent circuit using.

FIG. 8 is a diagram showing an equivalent circuit for matrix driving in this type of organic thin film EL device. This organic thin-film EL element has a plurality of stripe-shaped data electrodes Y _{1 to} Y _m and a scanning electrode X composed of a plurality of unit electrodes each corresponding to a plurality of light emitting diodes D as a light emitting layer made of an organic substance.
_{1 to} X _n (first and second stripe electrodes) are sandwiched in a matrix. Each data electrode Y _{1 to} Y _m
(First unit electrode) is connected to a driving power supply system having the drive voltage VB via a row selection switch section 6 having the electrode switching unit for each of the data electrodes Y ₁ to Y _m. Each of the scanning electrodes X _{1 to} X _n (second unit electrode) is connected to each of the scanning electrodes X _{1 to} X _n.
And it is connected to the column selection switch section 8 having a MOSFET 7 ₁ to _7-n of the normally-off in each. The row and column selection switching units 6 and 8 are controlled by a control unit (not shown) based on a semiconductor memory in which display data to be displayed is stored.

Here, each electrode switching section includes an electrode switching circuit 9
_{1 to} 9 _m , npn-type bipolar transistor 10 ₁ to
10 _m and normally on MOSFET 11 ₁ to 11 _m
Made, the collector terminal of the bipolar transistor 10 ₁ to 10 _m is connected to a driving power supply system, and the bipolar transistor 10 ₁ to 10 _m of the emitter terminal and MO
SFET11 source terminal of ₁ to 11 _m is connected to the corresponding data electrodes Y ₁ to Y _m. Also, this MOSF
ET11 ₁ to 11 the drain terminal of the _m is connected to ground, and the gate terminal of the base terminal and MOSFET 11 ₁ to 11 _m of the bipolar transistor 10 ₁ to 10 _m is electrode switching circuit 91 _to 93 the first and second _m Is connected to the output unit of.

The MOSFETs 7 _{1 to} 7 _n of the column selection switching unit 8
The corresponding scanning electrodes X ₁ to X _n is connected to the source terminal, and can be connected to ground via the drain terminal of the scanning electrodes by a signal input to the gate terminal.

Therefore, the data electrodes Y _{1 to} Y _m are forward biased in the excited state (light emission) and are connected to the ground in the non-excited state, and the scan electrodes X _{1 to} X _n are held at the ground when selected and floated when not selected. ing.

Next, matrix driving of such an organic thin film EL device will be described.

First, the control unit sends an electrode switching signal indicating a pixel (X _i , Y _l ) corresponding to each position to be displayed to the row and column selection switching units 6 and 8. This electrode switching signal, and the gate of the MOSFET 7 _i corresponding to the scanning electrodes _{X i,} respectively to the gate of the base and MOSFET 11 _l of the bipolar transistor 10 _l which corresponds to the data electrodes Y _l s synchronism with the high-level pulse is input If that, the scanning electrodes X _i is grounded, the data electrodes Y _l is latched in the forward bias voltage VB.

At this time, if the forward bias driving voltage VB is a sufficient voltage equal to or higher than the light emission threshold, the pixel (X _i , Y
_A current flows into the light emitting diode D corresponding to _l ), and the electron transporting light emitting layer is excited to emit light.

On the other hand, unselected data electrodes Y _{k (k ≠ l)}
Is grounded, no voltage is applied to the light emitting diode D on the _Yk electrode, and no light is emitted. _Unselected scan electrodes _{Xj (j ≠ i)} are latched in a floating state, and no current flows in a steady state. Thus, the light emitting on the X _i electrodes, the non-light emitting state is formed, the light-emitting, by repeatedly displayed while shifting the selected electrodes non-emission state, displaying the desired image.

[0024]

[SUMMARY OF THE INVENTION However, in the above organic thin film EL element as is, Y _l drive voltage VB of the forward bias applied to the scanning electrodes X _i other scanning electrodes X
_j since it surely applied also to _{(j ≠ i),} 8 to the data electrodes Y of the non-selected a grounded other than the data electrodes Y _l
There is a problem that the leak current shown by the broken line flows.

The pixel (X _k ,
The voltage applied to Y _l ) is about half or less of VB (see “Development Strategy of Organic EL Device”, p. 209). A crosstalk phenomenon occurs in which the pixel (X _k , Y _l ) responds due to the leak current to be in a half-excitation half-emission state.

Further, such a crosstalk phenomenon tends to become more remarkable as the luminance is increased by increasing the forward bias driving voltage VB.

On the other hand, Japanese Patent Laying-Open No. 2-148667 discloses a method of adjusting a current injected into a pixel to change luminance. However, the circuit configuration of this method is very complicated and impractical.

The present invention has been made in view of the above circumstances, and can eliminate the transition to the semi-excited semi-luminous state by the non-selective electrode and the state due to the increase of the forward bias voltage, thereby improving the stability of the display image quality. It is another object of the present invention to provide a practical method of driving an organic thin-film EL element with a simple configuration.

[0029]

According to a first aspect of the present invention, there is provided a light-emitting layer comprising at least one organic material, wherein at least one of the light-emitting layers is transparent and comprises first and second stripes comprising a plurality of unit electrodes. In the method for driving an organic thin-film EL element sandwiched in a matrix by electrodes, the first light-emitting layer emits light when displaying an object to be displayed.
And selecting a first unit electrode and a second unit electrode corresponding to each position in the display target from among the second stripe electrodes, and exciting the light emitting layer between the selected electrodes. a forward bias voltage is applied so as to form a light emitting state, one of the first and second stripe electrode, whereas
Is a method for driving an organic thin-film EL element in which a voltage between zero and a light emission threshold or less is applied between non-selected electrodes that are not selected so as to prevent the formation of a half-excited semi-light emitting state of the light emitting layer.

[0030] The invention corresponding to claim 2 is the same as the above.
The pulse width corresponding to the bias voltage corresponding to the gradation to be displayed
2. The organic thin film E according to claim 1, which has a forward bias voltage of
This is a method for driving the L element.

[0031]

[0032]

According to the present invention, when the display object is displayed by emitting light from the light emitting layer, the first and second stripe electrodes of the first and second stripe electrodes are provided with the above means. A first certain unit electrode and a second certain unit electrode are selected in accordance with the position, and a forward bias voltage corresponding to the gray scale to be displayed is applied between the selected electrodes, and the first certain unit electrode and the second certain unit electrode are applied.
And of the second stripe electrode, a voltage of less emission threshold from zero to prevent the semi-excited half-emission state formation of one of the light-emitting layer between selected Tei <br/> no unselected electrodes Apply.

As described above, the half-excitation and half-emission state due to the non-selection electrode is prevented, and the gradation is controlled by changing the pulse width without increasing the forward bias voltage. And the organic thin-film EL element can be driven practically with a simple configuration.

[0034]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is a perspective view showing a schematic configuration of an organic thin-film EL device according to a first embodiment of the present invention. This organic thin-film EL element is provided on a glass plate 21 as a transparent insulating substrate having a thickness of 1.1 mm on ITO electrodes 22 ₁ to 22 ₁ as anodes.
22 ₆₄ are formed at 0.7mm pitch 64 striped 0.5mm width thickness 240 nm. In addition,
Each of these 64 electrodes is a first unit electrode, and is formed by, for example, a photolithography method.

On the ITO electrodes 22 _{1 to} 22 ₆₄ , N, N′-bis (3-methylphenyl) -N, N′-diphenyl-1,1′-biphenyl is formed as an organic hole injection / transport layer 23. -4,4′-diamine is deposited to a thickness of 50 nm, and tris (8-quinolinol) aluminum is deposited on the organic hole injection layer 23 as an organic electron transporting / emitting layer 24.
nm.

The metal electrodes 25 _{1 to} 25 ₆₄ as cathodes are
It has the same 64 stripe patterns as the ITO electrodes 22 _{1 to} 22 _64, and Mg and Li having a composition ratio of 4: 1 are co-evaporated on the organic electron transporting light emitting layer so as to have a thickness of 220 nm. The ITO electrode 22 ₁
It is formed so as to be substantially perpendicular to to 22 ₆₄ of the pattern. In addition, each of the 64 units is a second unit electrode.

In this configuration, the light emitting area is 0.25 mm
Single element ^2, the capacity is 0.1 nF, the resistance is a more 100 k.OMEGA (frequency 5 kHz), voltage dependent characteristics of the luminance is the same as FIG. That is, this single element obtains a luminance of 2000 cd / m ^{2 at} DC10V.

FIG. 2 is a diagram showing an equivalent circuit for matrix driving in such an organic thin-film EL element. The same parts as those in FIG. 8 are denoted by the same reference numerals, and detailed description thereof will be omitted. Only mentioned.

That is, in the apparatus applied to the present invention, the scanning electrodes X _{1 to} X _n corresponding to the cathodes are used in order to prevent a current other than the selected electrodes to prevent a half-excitation half-light emission state.
And the drive power supply system are individually connected by a pull-up resistor Rc.

Next, a method of driving such an organic thin film EL device will be described with reference to FIG.

The control section sends an electrode switching signal for selecting a pixel (X _i , Y _l ) corresponding to each position to be displayed to the row and column selection switching sections 6 and 8. By this electrode switching signal,
A gate of the MOSFET 7 _i corresponding to the scan electrode X _i ,
Bipolar transistor 10 corresponding to the data electrodes Y _l
When the base and MOSFET 11 _l and respectively in synchronization with the high-level pulse of _l is inputted, the scanning electrodes X _i is grounded, the data electrodes Y _l is latched to the drive voltage VB of the forward bias, the pixel (X _i , _Yl ) emit light.

[0043] At this time, the scanning electrodes X _i other unselected scanning electrodes X _{j (j ≠ i)} is latched to the drive voltage VB of the reverse bias via a pull-up resistor Rc. Therefore, the data electrode Y _l latched to the forward bias voltage VB
And the scan electrode X latched at the reverse bias voltage VB.
The potential difference with _{j (j ≠ i)} is small or zero. As described above, since the potential difference is small between the non-selected electrodes, only a voltage equal to or lower than the threshold is applied to the light emitting diode D, and the above-described half-excitation half-light emission state is prevented, and the crosstalk phenomenon is prevented.

A non-selected data electrode _{Yk (k ≠ l)} facing scan electrode _{Xj (j ≠ i)} latched at reverse bias voltage VB is connected to a corresponding bipolar transistor 1
The emitter potential of 0 is zero. Therefore, the light emitting diode D between the scan electrode _Xj and the unselected data electrode _Yk is reliably kept in the non-excited state by the application of the reverse bias voltage VB.

[0045] The driving voltage VB of the reverse bias applied to the scan electrode X _i by the pull-up resistor Rc, the pixel (X _i, Y _l) the scanning electrodes X _i is when emits light is grounded Only the current flows to the ground via the resistor Rc, and does not affect the EL element.

Next, a driving method for adjusting the gradation of such an organic thin film EL element will be described.

Here, the present invention differs from the prior art in that the drive voltage VB is not varied to adjust the luminance, but the drive voltage VB is fixed and the application time to the EL element is varied, so that the time averaging is performed. The adjusted luminance (average luminance) is adjusted.

That is, for example, as shown in FIG. 3, the width of the selection time is τ, the gate pulse width of the MOSFET 7 on the scan electrode X side is τ, and the gate pulse width of the bipolar transistor 10 on the data electrode Y side is τ / α. (Α> 1) is maintained at a high level.

Here, the time during which the current flows is only during τ / α when the forward bias driving voltage VB is applied. Since the average luminance observed is approximately proportional to the time during which the current flows,
It can be adjusted by the selection time width τ. Therefore, by changing α for each pixel on the data electrode, the average luminance is adjusted and gradation display is performed. Α is preferably an integer of 2 or more.

For example, with a single organic thin film EL element,
The pulse waveform shown in FIG. 4 was applied to the base terminal of the bipolar transistor 10 from the electrode switching circuit, and the pulse width (response speed) required for sufficient light emission, the frame period f without flicker, and the gate pulse width τ were obtained. As a result, response speed = 20 μs (DC = 10 V), τ was 40 μs or more, and f = 20 ms.

That is, when light emission having a width of 40 μs or more occurs once within the frame period f = 20 ms, a stable light emission state without flicker was observed. This is because τ = f /
It is suggested that 200 = 100 μs or less, and that no gradation display is performed, that N = 200 or more can be used. On the other hand, when τ is 100 μs or less, flicker is observed when the frame period f is 20 ms or more.

The average luminance depends on the voltage VB and the number N of electrodes. When N = 64, VB = 15 V and τ = 300 μs
, A value of 80 cd / m ² is obtained.

Next, a similar experiment is performed using a matrix-like organic thin film EL device.

This organic thin-film EL element is an integrated circuit D8749HD (Japan) as a column selection switching unit 8 in which a plurality of MOSFETs are built in parallel to the circuit shown in FIG. 2 provided on a 64 × 64 dot matrix driving substrate. Electric Co., Ltd.
HD74H as a bipolar transistor
C4514P (manufactured by Hitachi, Ltd.). Further, a control unit (not shown) for controlling the integrated circuit so as to output the display data stored in the nonvolatile memory for each scanning line is provided.

Here, the frame period of the gate pulse is set to 19.2 ms based on the data of the single element described above.
A forward bias driving voltage VB is applied to the pixel (X _i , Y _l ) with a gate pulse width τ of 300 μs.

In the conventional case without Rc, the driving voltage VB
Is raised to about 10 V, the pixel (X _k , Y _l ) (k ≠
The pixel i), which should not emit light, enters a half-excitation half-emission state.

On the other hand, in the case where the pull-up resistor Rc and the resistor Re according to the present invention are connected, even if the driving voltage VB is increased, a half-excitation half-light emission state does not occur at all, and an average luminance almost proportional to the driving voltage VB is obtained. And stable image quality was obtained. Note that pull-up resistance Rc = 1 to 3 kΩ and resistance Re = 1 to 3 kΩ are preferable conditions. DC15
At V, an average luminance of 60 cd / m ² was obtained.

Next, when the frame period f = 19.2 ms and the driving voltage VB = 15 V are kept constant and the gate pulse width is reduced to １ ／, the average luminance is reduced to almost 、, and the gate pulse is similarly reduced. When the width is reduced to 1/4, the average luminance is almost 1
/ 4 was observed.

That is, by varying the pulse width applied to the bipolar transistor 10, the display gradation can be easily controlled without increasing or decreasing the drive voltage VB. The pulse width １ ／ corresponds to the number of electrodes N = 128, and the pulse width ４ corresponds to the number of electrodes N = 256.

As described above, according to the first embodiment,
When the display object is displayed by causing the light emitting layer to emit light at a predetermined cycle, each data electrode Y _{1 to} Y _m and each scan electrode X _{1 to} X _n
Of the data electrodes Y corresponding to the respective positions to be displayed.
_l as well as selecting the phrase scanning electrodes X _i, is between the selected electrodes by applying a drive voltage VB of the forward bias of the pulse width τ corresponding to the gradation to be displayed, each of the data electrodes Y ₁ to Y _m
A voltage equal to or lower than the light emission threshold is applied between the non-selected electrodes where one or both of the scan electrodes X _{1 to} X _n are not selected so as to prevent the light-emitting layer from being half-excited and half-emitted.

As described above, the half-excitation and half-emission state by the non-selection electrode is prevented, and the gradation is controlled by changing the pulse width τ without increasing the forward bias driving voltage VB. The stability of image quality can be improved, and the organic thin-film EL element can be driven practically with a simple configuration.

In addition, a stable image without flicker can be displayed with high luminance regardless of the threshold characteristics of the organic thin film EL element.

Further, a simple circuit configuration using pulse width control that is easier than voltage control enables digital gray scale display in one frame, thereby achieving full color organic thin film EL elements. it can.

Next, a second embodiment of the present invention will be described with reference to the drawings.

FIG. 5 is a diagram showing an equivalent circuit for matrix driving in the organic thin film EL device. The same parts as those in FIG. 2 are denoted by the same reference numerals, and detailed description thereof will be omitted. Only different parts will be described here. .

That is, the device of this embodiment is different from the device shown in FIG. 2 in that the npn-type first transistor 31 is provided for each of the data electrodes Y _{1 to} Y _m corresponding to the anodes, instead of the row selection switching section 6. It is provided with a row selection switch circuit 33 for driving the second transistor 32 ₁ to 32 _m of the pnp by ₁ to 31 _m. Among the collector terminals, base terminals and emitter terminals of the first transistors 31 _{1 to} 31 _m , the collector terminal is connected to a drive power supply system via first and second resistors,
The base terminal is connected to a control unit (not shown), and the emitter terminal is connected to ground. The emitter terminal, the base terminal and the collector terminal of the second transistors 32 _{1 to} 32 _m are connected to the drive power supply system, and the base terminal is connected between the first and second resistors.
The emitter terminal is connected to ground via Re.

Here, the control unit sends an electrode switching signal for selecting a pixel (X _i , Y _l ) corresponding to each position to be displayed to the row selection switching circuit 33 and the column selection switching unit 8 as described above. I do.

The first transistor 31 _l in the row selection switching circuit 33 is turned on by the electrode switching signal to drive the second transistor 32 _l . Second transistor 32 _l is driven by a first transistor, applying a drive voltage VB to the data electrodes Y _l.

On the other hand, the column selection switching section 8 includes a MOSFET 7 _i
The in the ON state, connects the scan electrode X _i to the ground.

Therefore, the pixel (X _i , Y _l ) emits light.
Further, in the resistor Re, a leak current (an example is shown by a broken line in FIG. 5) occurs due to a ground failure of the unselected data electrode _Yk in the conventional case without the pull-up resistor Rc and the resistor Re, and the pixel (X _i , Y _i ) to prevent a pixel which should not emit light in a cross shape around the center from emitting a light, and discharge a collector current accompanying charging / discharging of the corresponding second transistor 32 _k to the ground. This prevents leakage to the pixel side and discharges the accumulated charge of the light emitting diode D to the ground.

Hereinafter, the apparatus of this embodiment is driven by the same driving method as that of the first embodiment.

As described above, according to the second embodiment,
Since a driving circuit including two transistors is realized for each of the data electrodes Y _{1 to} Y _m , the same effect as that of the first embodiment can be obtained by a simple configuration in which the electrode switching circuit is omitted as compared with the first embodiment. Can be obtained.

Further, since the accumulated charge of the second transistors 32 _{1 to} 32 _m and the light emitting diode D is discharged to the ground by the resistor Re, the operation due to the accumulated charge at the time of switching between the excited state and the non-excited state is uneasy. Switching characteristics can be improved by eliminating qualitative characteristics.

In the first embodiment, the case where a drive voltage VB is applied to the non-selected scanning electrodes via the pull-up resistor Rc to apply a voltage equal to or lower than the light emission threshold value between the non-selected electrodes. However, the present invention is not limited to this, and a configuration in which a reverse bias voltage VB ′ larger than the driving voltage VB is applied to the non-selected scanning electrodes via the pull-up resistor Rc to apply a reverse bias voltage between the non-selected electrodes. The same effect can be obtained by implementing the present invention in the same manner.

In the first embodiment, the case where the drive voltage VB is applied to the non-selected scanning electrodes by the pull-up resistor Rc and a voltage equal to or lower than the light emission threshold is applied between the non-selected electrodes has been described. However, the present invention is not limited to this. A driving voltage VB is applied to a non-selected scanning electrode by a switching element such as a transistor in synchronization with the selection of a certain electrode, and a voltage equal to or lower than the light emission threshold is applied between the non-selected electrodes. Even with the configuration, the present invention can be similarly implemented to obtain the same effect.

Further, in the first embodiment, the case where the gray scale display is performed for each pixel has been described. However, the present invention is not limited to this, and the gray scale display for each pixel in one frame and the gray scale display for each frame may be performed. Even if the display is combined with the display, the present invention can be implemented in the same manner and the same effect can be obtained.

In the first embodiment, the case where the hole transport injection layer and the electron transport light-emitting layer are made of an organic material has been described. However, the present invention is not limited to this. The same effects as those of the present invention can be obtained by using an inorganic thin film or a composite thin film of an inorganic material and an organic material having the same electrical characteristics.

Further, in the first embodiment, the case where the voltage application is controlled by the unipolar pulse has been described. However, the present invention is not limited to this.
Even if the voltage application is controlled by repeating the above and slightly modifying them, the same effect can be obtained by implementing the present invention in the same manner.

Further, an element composed of red, blue and green light emitting layers is arranged in each pixel, or a red, blue and green filter is inserted between the glass substrate and the ITO electrode of each pixel of the white light emitting element. According to the driving method of the present invention, full-color display can be performed.

In addition, the present invention can be variously modified and implemented without departing from the gist thereof.

[0081]

As described above, according to the present invention, a forward bias voltage having a pulse width corresponding to the gradation to be displayed is applied between the selection electrodes of the first and second stripe electrodes, By applying a reverse bias voltage or a voltage equal to or lower than the light emission threshold between the non-selected electrodes, the half-excited and semi-light-emitting state by the non-selected electrodes is prevented, and the pulse width can be varied without increasing the forward bias voltage. Since the gradation is controlled in this way, the stability of display image quality can be improved, and the organic thin-film EL element can be practically driven with a simple configuration.

[Brief description of the drawings]

FIG. 1 is a perspective view showing a schematic configuration of an organic thin film EL device according to a first embodiment of the present invention.

FIG. 2 is a diagram showing an equivalent circuit of matrix driving of the organic thin film EL element in the embodiment.

FIG. 3 is a view for explaining pulse width control in the embodiment.

FIG. 4 is a pulse waveform chart in the embodiment.

FIG. 5 is a diagram showing an equivalent circuit of matrix driving of an organic thin film EL device according to a second embodiment of the present invention.

FIG. 6 is a cross-sectional view showing a configuration of a conventional organic thin film EL element.

FIG. 7 is a diagram showing voltage dependence of luminance and current in a conventional organic thin film EL element.

FIG. 8 is a diagram showing an equivalent circuit of matrix driving in a conventional organic thin film EL element.

[Explanation of symbols]

6 ... row selection switching section, 7 _{1 to} 7 _n ... MOSFET (normally off), 8 ... column selection switching section, 9 _{1 to} 9 _m ... electrode switching circuit, 10 ₁ to 10 _m ... bipolar transistor, 11 ₁
To 11 _m ... MOSFET (normally), 21 ... glass plate, 22 ₁ through 22 ₆₄ ... ITO electrode, 23 ... organic hole injection transport layer, 24 ... organic electron-transporting light-emitting layer, 25 _to 253 ₆₄
... metal electrodes, VB ... driving voltage, Y ₁ to Y _m ... data electrodes, X ₁ to X _n ... scan electrodes, D ... light-emitting diode, Rc
... pull-up resistance, Re ... resistance.

Continuation of front page (56) References JP-A-4-308687 (JP, A) JP-A-2-135421 (JP, A) JP-A-62-126993 (JP, A) JP-A-61-165749 (JP) , A) JP-A-54-124998 (JP, A) JP-A-54-92081 (JP, A) JP-A-52-109896 (JP, A) JP-A-63-295695 (JP, A) Science Forum , New Trigger Series, 1992 "Organic EL Device Development Strategy" (58) Fields investigated (Int. Cl. ⁷ , DB name) G09C 3/30 H05B 33/14 G09F 9/30

Claims (2)

(57) [Claims] 1. An organic thin-film EL device having at least a light-emitting layer made of an organic substance, wherein at least one of the light-emitting layers is transparent and sandwiched in a matrix between first and second stripe electrodes made up of a plurality of unit electrodes. In the driving method, when the display object is displayed by causing the light emitting layer to emit light, a first unit electrode corresponding to each position in the display object among the first and second stripe electrodes, with selecting a second, a unit electrode, a forward bias voltage is applied as the inter the selected electrodes forming excited light emission state to the light-emitting layer, of the first and second stripe electrode, whereas organic There are between no Tei selected unselected electrodes and applying a semi-excitation voltage follows emitting <br/> light threshold from zero to prevent the formation of a semi-light-emitting state of the light-emitting layer Thin film EL element Method of driving a. 2. The method according to claim 1, wherein the forward bias voltage is a forward bias voltage having a pulse width corresponding to the gradation to be displayed.