JP2001296837A - Driving method for current controlled type display device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve the reproducibility of gradation expression and the luminance of a display picture by suppressing effect of stray capacity to be generated at electrodes, light emitting elements, wirings or the like. SOLUTION: This driving method of a display device is the driving method of a current controlled type display device in which light emitting elements are arranged in a matrix shape and in the driving method, pre-charging is performed prior to the supplying of a signal current and the amount of the pre-charging is different in accordance with gradation.

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 a current control type display device which can be used in fields such as display elements, flat panel displays, backlights, and interiors.

[0002]

2. Description of the Related Art In recent years, an organic electroluminescent device has attracted attention as one of new display devices. This element emits light when holes injected from the anode and electrons injected from the cathode are recombined in the organic light emitting layer sandwiched between both electrodes, and emits light at a low voltage with high luminance. Is Kodak
First shown by CW Tang et al. (Appl. Phys. L
ett. 51 (12) 21, pp. 913, 1987).

FIG. 5 is a sectional view showing a typical structure of an organic electroluminescent device. A hole transport layer 3, an organic light emitting layer 4, and a cathode 5 are laminated on a transparent anode 2 formed on a glass substrate 1, and light emission generated by driving by a driving source 6 is extracted outside through the anode and the glass substrate. It is. This light-emitting element has a rectifying property in which current flows when the anode has a positive polarity (forward bias direction) and emits light, and almost no current flows when the cathode has a positive polarity (reverse bias direction). Is common.

[0004] Such an organic electroluminescent device is thin, can emit high-brightness light under low-voltage driving, and can emit multicolor light by selecting an organic fluorescent material, and its application to display devices and displays is actively studied. It is.

FIG. 4 is an example of an equivalent circuit showing a conventional simple matrix type display device using an organic electroluminescent element. m × n organic electroluminescent elements 10 (EL) are arranged at electrical intersections of n signal lines 11 and m scanning lines 12. The signal line 11 and the scanning line 12 are connected to a driving source 15, a reverse bias voltage source 16 or a reference potential (earth) via a signal line switch 13 (DSW) and a scanning line switch 14 (SSW), respectively. The signal line 11 corresponds to the anode of the light emitting element, and the scanning line 12 corresponds to the cathode.

In such a display device, each light-emitting element can emit light in a desired pattern by line-sequential driving. For example, in FIG. 4, the light emitting elements ELi, j (1 ≦ i ≦
(m, 1 ≦ j ≦ n), only the scanning line SSWi is connected to a low potential (earth), and all other scanning lines are connected to a high potential (reverse bias Vs). At this time, DS
A signal current is input from Wj in synchronization with the scanning line. The signal current flows only through the light emitting element ELi, j in the forward direction due to the reverse bias of the scanning line, and emits light. To emit light from a plurality of elements on the selected scanning line, signal currents are supplied from a plurality of signal lines simultaneously. If this operation is repeated at high speed for other scanning lines, an image can be displayed by causing any combination of light emitting elements to emit light. Note that the timing at which the scanning lines are switched is determined by the frame rate, the display device, the number of scanning lines, and the like.

The gradation expression in the current control type display device is as follows.
It is performed by modulating the magnitude of the signal current applied to the element or the time during which the signal is applied, and several methods have been considered. In the pulse width modulation method (PWM), FIG.
As shown in (1), during a predetermined scanning period in which a pixel is selected, a signal current is constant, and gradation control is performed depending on the time ratio of the pulse width. On the other hand, in pulse amplitude modulation (PAM), as shown in FIG. 7, gradation control is performed according to the magnitude of a current applied during a scanning period.
In addition, a sub-field display method in which one frame is divided into sub-fields having a time width corresponding to a power of 2 and gradation control is performed by a combination of the sub-fields, and gradation control is performed based on the presence or absence of frame display with a constant applied voltage. There is a frame extraction method to be performed.

In a current control type display device, driving is usually performed by passing a constant current through an element to emit light. However, when driving with a constant current, the delay phenomenon of the signal current flowing into the element has become a serious problem. The delay phenomenon of the signal current is caused not only by the resistance component of the wiring and the like but also by the capacitance component parasitic on the electrode, the light emitting element, the wiring and the like as shown in FIG. Here, the capacitance 21 is the stray capacitance existing between the signal line and the substrate, etc., and the capacitance 2
2 is a stray capacitance existing in the element. The light-emitting element emits light when a signal current flows in the forward direction, but a signal output from the drive circuit does not flow to the light-emitting element instantaneously due to the presence of these stray capacitances. Charging of the stray capacitance is performed first, and a current is supplied after reaching a predetermined potential. Therefore, the drive waveform has a charging time until the signal pulse rises as shown in FIG.

This is not a problem if it is an ideal organic electroluminescent device having a small stray capacitance, but in reality, this charging time cannot be ignored. In display applications, stray capacitance and wiring resistance increase as the size increases and the length of wiring increases and the number of pixels increases. Along with that, the charging time becomes longer,
The effective duty ratio decreases.

As a method for accelerating the charging of the electric charge of the floating capacitance, there is a driving method disclosed in Japanese Patent Application Laid-Open No. 9-232074. This is a method in which, when switching to the next scanning line, all the scanning lines are once connected to a reset potential having the same potential, and charging is accelerated by a reverse bias voltage of the scanning line.

A driving system as disclosed in JP-A-11-45071 has also been considered. These are connected to a first drive source that outputs a constant potential (current) for a certain period after scanning of a scanning line starts, and precharges a stray capacitance. After that, it is connected to the second constant current source to input a signal.

[0012]

However, when gradation display is performed by pulse amplitude modulation or a combination of pulse amplitude modulation and pulse width modulation in the conventional method, the precharge amount is constant, so that the floating capacitance is reduced. In some cases, charging becomes excessive or insufficient. If the stray capacitance is insufficiently charged, more signal current must be passed to obtain the same luminance. If the charge amount of the precharge is too large, it becomes difficult to output a low-luminance gradation clearly, and there has been a problem that image display characteristics are deteriorated.

The present invention solves such a problem, and in a current control type display device, it is possible to suppress the influence of stray capacitance generated on electrodes, light emitting elements, wirings, etc., and to improve the reproducibility of gradation expression and luminance. It is an object to provide a driving method.

[0014]

That is, the present invention relates to a method of driving a current control type display device in which light-emitting elements are arranged in a matrix, wherein a precharge is performed prior to the supply of a signal current, and the precharge amount is controlled. Are different depending on the gray scale.

[0015]

FIG. 1 shows an example of a current control type display device according to the present invention. As in FIG. 4, m × n organic electroluminescent elements 10 (EL) are arranged at electrical intersections of n signal lines 11 and m scanning lines 12. The signal line 11 is
Drive source 15 via signal line switch 13 (DSW)
(D), 17 (C) or the reference potential, and the scanning line 12 is connected to the constant voltage source 16 or the reference potential via the scanning line switch 14 (SSW). The two driving sources for the signal line are a first driving source 17 (C) for performing precharge and a second driving source 15 (D) for inputting a signal current. Usually, a constant current source is used as the second drive source 15 (D).

The operation of pattern display by emitting ELi, j by line-sequential driving according to the present driving method will be described with reference to FIGS. First, it is assumed that both the signal line switch 13 (DSW) and the scanning line switch 14 (SSW) are connected to a reference potential (earth) (FIG. 1). Next, consider scanning DSWj to emit light from ELi, j, which is the emission target.

First, the scanning lines of the light emitting elements ELi, j are connected to the reference potential, and the other scanning lines are connected to the reverse bias voltage Vs (FIG. 2). At the same time, the signal line switch 13
(DSW) to connect to the drive source 17 (C) for precharging the signal line. At this time, in addition to the electric charge due to the reverse bias, the driving source 1
7 (Cj) is precharged. The term “floating capacitance” used herein includes not only those parasitic on the signal lines on the panel and all the light emitting elements sandwiched between the anode and the cathode, but also all the capacitance components existing in the driving circuit and the junction of the signal lines. In the case of pulse width modulation, since the signal current is constant, a constant precharge amount may be used regardless of the gradation. However, when driving by pulse amplitude modulation or a combination of pulse amplitude modulation and pulse width modulation, the signal current differs depending on the gray level, and the amount of charge required for precharge changes almost in proportion to the signal current. I do.

Therefore, the voltage, current or time of the driving source 17 (Cj) is set immediately after the precharging in order to precharge an appropriate charge amount for each gradation.
(Dj) is changed in proportion to the magnitude of the signal current output. In order to charge the electric charge instantaneously, it is preferable to generate a voltage output equal to the signal voltage to emit light by the precharge driving source 17 (C) or a current output larger than the signal current. For example, when expressing 16 gradations (0.01 mA, 0.02 mA,..., 0.16 mA) by pulse amplitude modulation, the precharge current is also increased by 16 correspondingly.
Provide gradation (0.1mA, 0.2mA, ..., 1.6mA).
After the predetermined charge is charged, the signal line switch 13 (DSW) is switched to the driving source 15 (D) while the scanning line is kept as it is, and a signal current corresponding to the gradation is input (FIG. 3). By repeating the above operation, a bright and clear pattern display can be performed at any gradation.

FIG. 10 shows an example of a drive waveform of the present invention in which precharge is set for each gradation, and FIG. 11 shows an example of a conventional drive waveform in which precharge is set to be constant. In these drawings, a driving current waveform when a constant current source is used as the driving source 17 (C) and a voltage waveform actually applied to the light emitting element are shown together. When the precharge is set appropriately (FIG. 10), a pulse waveform with a rapid rise and high luminous efficiency is obtained for any gradation.
On the other hand, when the precharge is constant (FIG. 11), the precharge amount may become excessive or insufficient depending on the magnitude of the signal current. In particular, the effect of the present invention is great when displaying subtle expressions of multiple gradations and color expressions of colors.

The charging time is not particularly limited,
From the viewpoint of not reducing the effective duty ratio, it is desirable that the precharge period is short. The precharge period is determined by the magnitude of the precharge current and a time constant such as wiring resistance, on-resistance of the light emitting element, and stray capacitance. When a current source is used as the driving source 17 (C),
The amount of precharge is proportional to the magnitude of the current and time.

In the driving method illustrated in FIGS. 1 to 3 of the present invention, a voltage source or a current source can be used as the driving source 17 (C), but the charging means is not limited. Further, a voltage limiter or a current limiter may be provided in order to temporarily prevent an excessive current or voltage from being applied to the panel.

In the above example, a description has been given of a simple matrix type display device using an organic electroluminescent element. The method of driving the current-type display device of the present invention can be applied not only to the above-described example but also to general devices having a display function by supplying power, and further preferably to an organic electroluminescent device. When an organic electroluminescent device is used, the configuration of the light emitting element or the electrode is not limited. Further, the current type display device may be a monochrome display device or a color display device. In the color display, the precharge amounts may be different for the red, blue and green light emitting elements.

[0023]

Hereinafter, the present invention will be described with reference to examples.
The present invention is not limited by these examples.

Embodiment 1 FIG. 12 schematically shows a structural example of an organic electroluminescent device. The manufacturing procedure is as follows.

Glass substrate 31 with ITO transparent electrode film
Was cut into a size of 120 × 100 mm. ITO was patterned by a normal photolithography method to have a length of 90 mm and a pitch of 300 μm (ITO width 270 μm) × 272 to obtain a striped first electrode 32 (anode).

This substrate is washed, subjected to UV-ozone treatment, and then fixed to a vacuum evaporation machine, and the degree of vacuum in the apparatus is set to 2 ×
Evacuation was performed until the pressure became 10 ⁻⁴ Pa or less. While rotating the substrate, copper phthalocyanine was deposited to a thickness of 15 nm and bis (m-methylphenylcarbazole) was deposited to a thickness of 60 nm to form a hole transport layer 33. Further, an 8-hydroxyquinoline-aluminum complex (Alq3) was deposited to a thickness of 60 nm to form an organic light-emitting layer 34, and this organic layer was exposed to lithium vapor and doped (0.5 nm in terms of thickness). Next, a shadow mask made of a magnetic material is placed in front of the substrate, and a magnet is placed behind the substrate to fix them.
100mm length, 300μm pitch
(Al width 250 μm) × 200 stripe-shaped second electrodes 35 (cathodes) were formed.

First stripe-shaped electrodes 32 orthogonal to each other
This is a typical simple matrix type display device in which organic layers 33 and 34 are sandwiched between a first electrode and an organic electroluminescent element (one pixel) at the intersection of both electrodes. The size of the pixel is 270 μm × 250 μm, and the number of pixels is 272 × 200. The light emission starting voltage of the organic electroluminescent device was about 5 V in DC driving.

Pattern display was performed on the display device shown in FIGS. 1 to 3 by using the first electrode of the display device as the signal line 11 and the second electrode as the scanning line 12. The drive source 15 (D) is a constant current source, and the precharge drive source 17 (C) includes a constant current source whose output can be changed in proportion to the magnitude of the signal current of the drive source 15 (D). Was used. It should be noted that a control signal generation portion and the like are not shown in FIG. Line-sequential driving conditions were a frame frequency of 60 Hz (interlace) and a duty ratio of 1/200. Of the 166.5 μs allocation time for one scanning line, the precharge time width is set to 2
0 μs and the last 10 μs were set as the reset time. Also,
The magnitude of the precharge current is 10
Fine adjustment was made with reference to double. When pattern display of 16 gradations was actually performed by pulse amplitude modulation, bright and good display characteristics were obtained for all gradations. The luminance was 70 cd / m ² at a signal output of 0.1 mA.

Example 2 A display device was driven in the same manner as in Example 1 except that a 64-gradation pattern was displayed. Even if the number of gradations increases, the linearity of the gradation can be maintained by an appropriate precharge effect, and good display characteristics can be obtained.

Example 3 A display device was driven in the same manner as in Example 1 except that a 256 gradation pattern was displayed. Even when a delicate gradation expression such as 256 gradations is required, color reproducibility can be maintained by an appropriate precharge effect, so that better display characteristics were obtained.

Example 4 A display device was driven in the same manner as in Example 3 except that a voltage source was connected as the driving power supply 17 (C) for precharge. The precharge voltage value was set to be equal to the light emission voltage corresponding to each gradation. A pattern display of 256 gradations was performed, and bright and favorable display characteristics were obtained for all gradations, similarly to the third embodiment.

Embodiment 5 256 combinations of pulse amplitude modulation and pulse width modulation
A gradation pattern was displayed. By expressing 16 gradations with a pulse amplitude and 16 gradations between them with a pulse width, 256 gradations were realized. The magnitude of the precharge current is
For 16 gradations by the pulse amplitude modulation, fine adjustment was made with reference to 10 times the signal current, and the pulse width was fixed. Otherwise, the display device was driven in the same manner as in Example 1. Even in this case, the color reproducibility can be maintained by the effect of an appropriate precharge for each gradation,
Good display characteristics were obtained.

Example 6 A glass substrate provided with an ITO transparent electrode film was made 120 × 100
It was cut to the size of mm. ITO was patterned into a stripe shape having a length of 90 mm and a width of 80 μm by ordinary photolithography. The 816 (3 × 272) stripe-shaped first electrodes are arranged at a pitch of 100 μm.

Next, a positive photoresist (OFPR-800, manufactured by Tokyo Ohka Kogyo Co., Ltd.) was applied to the substrate on which the first electrode was formed so as to have a thickness of 3 μm by spin coating.
The coating film was exposed to a pattern through a photomask, developed, and patterned to form a photoresist. After development, the coating was cured at 160 ° C. In the photomask used for this patterning, 816 openings of an insulating layer having a width of 65 μm and a length of 235 μm are arranged at a pitch of 100 μm in the width direction and 200 at a pitch of 300 μm in the length direction.

Next, the apparatus was fixed to a vacuum evaporation machine and evacuated until the degree of vacuum in the apparatus became 2 × 10 ⁻⁴ Pa or less. While rotating the substrate, 15 nm of copper phthalocyanine and bis (N
-Ethylcarbazole) was deposited in the order of 60 nm to form a hole transport layer.

For light emission patterning, a shadow mask having a mask portion and a reinforcing line formed in the same plane was used. The outer shape of the shadow mask is 120 × 84 mm, the thickness of the mask portion is 25 μm, the length is 64 mm, and the width is 10 mm.
27 μm stripe-shaped openings with a pitch of 300 μm
Two are arranged. In each stripe-shaped opening, a reinforcing line having a width of 20 μm and a thickness of 25 μm orthogonal to the opening is provided.
It is formed at intervals of 8 mm. The shadow mask is fixed to a stainless steel frame having the same outer shape and a width of 4 mm.

A light-emitting layer shadow mask was arranged in front of the substrate so that both adhered to each other, and a ferrite plate magnet was arranged behind the substrate. At this time, the stripe-shaped first electrode is located at the center of the stripe-shaped opening of the shadow mask, the reinforcing line is located on the insulating layer, and the reinforcing line and the insulating layer are arranged to be in contact with each other. In this state, 0.3% by weight of 1,3,5,7,
8-Pentamethyl 4,4-difluoro-4-bora-3a, 4
Alq3 doped with a-diaza-s-indacene- (PM546) was deposited to a thickness of 21 nm, and the G light emitting layer was patterned.

Next, the shadow mask is aligned with the first electrode pattern at a position shifted by one pitch, and
Alq3 doped with 4- (dicyanomethylene) -2-methyl-6- (julolidylstyryl) pyran (DCJT) was deposited to a thickness of 15 nm to pattern the R light emitting layer.

Further, the shadow mask is aligned with the first electrode pattern at a position shifted by one pitch, and
Bis (2,2'-diphenylvinyl) diphenyl (DPV
Bi) was deposited to a thickness of 20 nm to pattern the B light-emitting layer. The R, G, and B light emitting layers are disposed for every three stripe-shaped first electrodes, and completely cover the B-exposed portions of the first electrode.

Next, 35 nm of DPVBi and 10 nm of Alq3 were deposited on the front surface of the substrate. Thereafter, the thin film layer is exposed to lithium vapor for doping (0.5 nm in equivalent film thickness).
did.

The second electrode was formed by a vacuum evaporation method using a resistance wire heating method. The degree of vacuum at the time of vapor deposition was 3 × 1
0 ^-4 and a Pa or less, during the deposition is rotated substrate for the two deposition sources. Similarly to the patterning of the light-emitting layer, a shadow mask for the second electrode was arranged in front of the substrate so that both were brought into close contact with each other, and a magnet was arranged behind the substrate. At this time, both are arranged so that the insulating layer coincides with the position of the mask portion. In this state, aluminum is deposited to a thickness of 240 nm,
Length 100mm, pitch 300μm (Al width 250μ
m) × 200 striped second electrodes were patterned.

Thus, a width of 80 μm and a pitch of 100
A patterned thin film layer including an R light emitting layer, a G light emitting layer, and a B light emitting layer is formed on a 816 ITO stripe-shaped first electrode having a width of 250 μm and a pitch of 300 μm so as to be orthogonal to the first electrode. A simple matrix type color organic electroluminescent device in which 200 stripe-shaped second electrodes were arranged was manufactured. R, G, and B light emitting areas are 1
Since the pixels are formed, the present organic electroluminescent device has a
It has 272 × 200 pixels at m pitches. Since one light emitting region is regulated by the opening of the insulating layer, a width of 65 μm is required.
m and length 235 μm.

The organic electroluminescent device was taken out of the evaporator and transferred to an argon atmosphere having a dew point of −70 ° C. or less. Under this low-temperature atmosphere, the substrate and the glass plate for sealing were bonded together by using a curable epoxy resin.

The organic electroluminescent device was driven in the same manner as in Example 1 except that a full-color 256 gradation pattern was displayed. Even when a fine gradation expression of 256 gradations of a full-color image is required, color reproducibility can be maintained by an appropriate precharge effect, so that good display characteristics were obtained.

Comparative Example 1 A display device was driven without providing a precharge period.
Otherwise, the procedure was the same as in Example 1. The driving pulse waveform did not rise sufficiently, and an image could not be displayed.

Comparative Example 2 A display device was driven in the same manner as in Example 1 except that the precharge current value was set to be constant for any gradation. The precharge current was set equal to the maximum value of the signal current. As a result, image display was possible, but the luminance was 0.1.
It was dark at 5 cd / m ^{2 at} mA, and the gradation was unnatural, and good display characteristics could not be obtained.

[0047]

According to the driving method of the current control type display device of the present invention, the precharge of the floating capacitance can be appropriately performed for any gradation expression when the scanning line is switched. This makes it possible to optimally control the voltage (current) supplied to cause each light emitting element to emit light, thereby improving the brightness and enabling clear gradation and color display, especially in a full color display. become.

[Brief description of the drawings]

FIG. 1 is an equivalent circuit showing an operation example of a current control type display device according to the present invention.

FIG. 2 is an equivalent circuit showing an operation example of the current control type display device according to the present invention.

FIG. 3 is an equivalent circuit showing an operation example of the current control type display device according to the present invention.

FIG. 4 is an equivalent circuit showing an example of a conventional simple matrix type display device.

FIG. 5 is a cross-sectional view illustrating a structural example of an organic electroluminescent element.

FIG. 6 is a diagram showing a driving waveform of a pulse width modulation method.

FIG. 7 is a diagram showing a driving waveform of a pulse amplitude modulation method.

FIG. 8 is an electrical equivalent circuit showing a stray capacitance parasitic on a light emitting element and a wiring.

FIG. 9 is a diagram showing a charging time of a floating capacitance.

FIG. 10 is a driving waveform of the present invention in which precharge is set for each gradation.

FIG. 11 shows a conventional driving waveform when the amount of charge of precharge is set to be constant.

FIG. 12 illustrates a structure of an organic electroluminescent device.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1, 31 Glass substrate 2, 32 Anode 3, 33 Hole transport layer 4, 34 Organic light emitting layer 5, 35 Cathode 6, 15, 17 Drive source 10 Organic electroluminescent element 11 Signal line 12 Scanning line 13 Signal line switch 14 Scan Line switch 16 Reverse bias voltage source 21, 22 Capacitance (floating capacitance)

Claims (6)

[Claims] 1. A method of driving a current control type display device in which light emitting elements are arranged in a matrix, wherein precharge is performed prior to supply of a signal current, and the amount of precharge differs according to gradation. A method for driving a current control type display device. 2. The drive of the current control type display device according to claim 1, wherein the current control type display device is a simple matrix type in which light emitting elements are connected to electrical intersections of signal lines and scanning lines. Method. 3. The method according to claim 1, wherein the magnitude of the voltage or current of the precharge differs according to the magnitude of the signal current for giving each gradation. 4. The method according to claim 1, wherein the time width of the precharge differs according to the magnitude of the signal current for giving each gradation. 5. The precharge voltage or current magnitude,
2. The method according to claim 1, wherein the time and / or time is proportional to the magnitude of the signal current. 6. The method according to claim 1, wherein the current control type display device is an organic electroluminescent device.