JP2003323157A - Driving method of light emitting device and electronic equipment - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a driving method in which a duty ratio is improved, good picture quality is realized by securing a sufficient sustain (lighting) period even though gradation is increased and long service life is provided for light emitting elements. <P>SOLUTION: One frame period is provided with subframe periods SF<SB>1</SB>, SF<SB>2</SB>, ..., SF<SB>m</SB>having m (where m is a natural number equal to or greater than two) different lengths. The periods SF<SB>1</SB>, SF<SB>2</SB>, ..., SF<SB>m</SB>respectively have address periods and sustain periods. In the address periods, analog signals are inputted into corresponding light emitting elements. Gradation is provided to a plurality of light emitting elements by light emission with n different luminance (where n is a natural number equal to or greater than two) of the analog data signals during the sustain periods. <P>COPYRIGHT: (C)2004,JPO

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 light emitting device formed by forming a light emitting element on a substrate. In particular, the present invention relates to a method for driving a light emitting device which controls the operation of a light emitting element by using a semiconductor element (an element using a semiconductor thin film).

[0002]

2. Description of the Related Art In recent years, a technique for forming a TFT on a substrate has made great progress, and application development to an active matrix type display device has been advanced. In particular, a TFT using a polysilicon film is a TFT using a conventional amorphous silicon film.
Since the field effect mobility (also referred to as mobility) is higher than that of FT, high speed operation is possible. Therefore, it is possible to control a pixel, which has been conventionally performed by a drive circuit outside the substrate, by a drive circuit formed on the same substrate as the pixel.

Such an active matrix type display device has various advantages such as reduction of manufacturing cost, miniaturization of display device, increase of yield, reduction of throughput, etc. by forming various circuits and elements on the same substrate. Is obtained.

Further, research on active matrix type light emitting devices having pixels provided with light emitting elements has been activated. The light emitting device is an organic light emitting device (OELD: Orga).
It is also called a nicEL display) or an organic light emitting diode (OLED).

Further, in the present specification, an EL element which is a light emitting element provided in a pixel of an organic light emitting device will be described as a typical light emitting element.

Unlike a liquid crystal display device, the light emitting device is a self-luminous type. The EL element has a structure in which a light emitting layer is sandwiched between a pair of electrodes, but the light emitting layer usually has a laminated structure. A typical example is a laminated structure of "hole transport layer / organic compound layer / electron transport layer" proposed by Tang et al. Of Eastman Kodak Company. This structure has a very high luminous efficiency, and most of the light-emitting devices currently being researched and developed employ this structure.

In addition, a hole injection layer / hole transport layer / organic compound layer / electron transport layer or a hole injection layer / hole transport layer / organic compound layer / electron transport layer / electrons on the pixel electrode. A structure in which the injection layers are laminated in this order may be used. The organic compound layer may be doped with a fluorescent dye or the like.

In this specification, all layers provided between a pair of electrodes are collectively referred to as a light emitting layer. Therefore, the hole injection layer, the hole transport layer, the organic compound layer, the electron transport layer, the electron injection layer and the like described above are all included in the light emitting layer.

A predetermined voltage is applied to the light emitting layer having the above structure from a pair of electrodes, whereby recombination of carriers occurs in the light emitting layer to emit light. Note that in this specification, emission of an EL element is referred to as driving of the EL element.

In addition, as a gradation display method of the light emitting device,
An analog gray scale display and a digital time division gray scale display are known.

The gradation display is disclosed in Patent Document 1 which discloses the structure of an active matrix type display device.

[0012]

[Patent Document 1] Japanese Patent Laid-Open No. 2001-343933

First, analog gradation display of the light emitting device will be described with reference to FIGS.

FIG. 12 shows the structure of the pixel portion 1800 of the light emitting device. The pixel portion is composed of (x × y) pixels (x and y are natural numbers of 1 or more) arranged in a matrix.
Y gate signal lines (G ₁ ~
G _y ) is a switching TFT 180 included in each pixel
1 is connected to the gate electrode. One of a source region and a drain region of the switching TFT 1801 included in each pixel has x source signal lines (also referred to as data signal lines) to which an analog video signal is input (S _{1 to} S _x ).
The other is connected to the gate electrode of the driving TFT 1804 included in each pixel.

One of the source region and the drain region of the driving TFT 1804 included in each pixel is connected to the power supply line 1810 and the other is connected to the EL element 1806. The power supply line 1810 is always kept at a constant potential, and the potential is represented by V _D.

Further, a capacitor 1808 may be provided between the gate electrode of the driving TFT 1804 and the power supply line 1810 and used as a storage capacitor for holding the gate-source voltage of the driving TFT 1804.

The EL element 1806 comprises an anode, a cathode, and a light emitting layer provided between the anode and the cathode. When the anode is connected to the source region or the drain region of the driving TFT 1804, the cathode is connected to the counter electrode 1809. On the contrary, when the cathode is connected to the source region or the drain region of the driving TFT 1804, the counter electrode 1
An anode is connected to 809.

Although not shown in FIG. 12, the counter electrodes 1809 of the respective pixels are connected so as to have the same potential, and the potential is represented by V _C.

FIG. 13 shows a timing chart when the light emitting device is driven by an analog method. The one frame period (F) is a period necessary for writing from one video signal for one screen to display, and a period in which one gate signal line is selected is referred to as one line period (L). Call. Figure 1
In the case of the light emitting device of 2, since there are y gate signal lines, 1
Y line periods (L _{1 to} L _y ) are provided in the frame period. In addition, a period from the end of selection of the gate signal line in the last row in a certain frame period to the start of selection of the gate signal line in the first row in the next frame period is referred to as a vertical blanking period. .

In a normal light emitting device, 60 or more frame periods are provided per second, and an image is displayed 60 times or more per second. The number of images displayed per second is 6
When the number is less than 0, the flicker of the image such as flicker is visually noticeable.

As y increases, the number of line periods in one frame period also increases, and the drive circuit must be driven at a high frequency.

Next, a method of operating the analog driven light emitting device shown in FIG. 12 will be described with reference to FIG.

In the line period 1 (L ₁ ), the selection signal from the gate signal line drive circuit is input to the gate signal line G ₁ . Then, analog data signals are sequentially input to the source signal lines (S _{1 to} S _x ).

Since all the switching TFTs 1801 connected to the gate signal line G ₁ are turned on, the analog video signal input to the source signal lines (S _{1 to} S _x ) passes through the switching TFT 1801. Drive T
It is input to the gate electrode of FT1804.

The switching TFT 1801 is turned on, and the potential of the gate electrode of the driving TFT 1804 changes according to the potential of the analog video signal input in the pixel. At this time, the drain current is uniquely determined with respect to the gate-source voltage according to the voltage-current characteristics of the driving TFT 1804. As a result, a desired drain current is supplied to the EL element 1806, and the EL element 1806 emits light with a brightness corresponding to the amount of the current.

By repeating the above-mentioned operation, the source signal line
When the input of the analog video signal to (S _{1 to} S _x ) ends, the line period 1 (L ₁ ) ends. The period until the input of the analog video signal to the source signal lines (S _{1 to} S _x ) and the horizontal retrace line period may be combined to form one line period. Then, in the line period 2 (L ₂ ), the selection signal is input to the gate signal line G ₂ . Then, similarly to the line period 1 (L ₁ ), analog video signals are sequentially input to the source signal lines (S _{1 to} S _x ).

When the selection signals are input to all the gate signal lines (G _{1 to} G _y ), all the line periods (L _{1 to} L _y ) are completed. When all line periods (L _{1 to} L _y ) are completed,
One frame period ends. All pixels display during one frame period and one image is formed. Note that all the line periods (L _{1 to} L _y ) and the vertical blanking period may be combined to form one frame period.

The electric potential V _D of the power supply line 1810 and the electric potential V _C of the counter electrode of each pixel may be set to values which can normally perform the above operation.

As described above, the emission brightness of the EL element is controlled by the analog data signal, and gradation display is performed. This method is a driving method called a so-called analog gradation display method, and gradation display is performed by a difference in potential of an analog video signal.

By the way, Id- of the driving TFT of each pixel
If there is variation in the Vg characteristics, it is assumed that the driving TF of each pixel is
Even if a gate-source voltage equal to T is applied,
It cannot output the same drain current. In such a case, due to a slight variation in the Id-Vg characteristic, the light emission amount of the EL element greatly differs between adjacent pixels even if the signal of the same voltage is input.

As described above, the analog gradation display is T
It is extremely sensitive to variations in the characteristics of the FT, which has been an obstacle to displaying high gradation of the light emitting device.

Next, problems in the case of performing gradation display by the digital time division method will be described. Note that Patent Document 2 discloses that gradation display is performed by a digital time division method.

[0033]

[Patent Document 2] Japanese Patent Laid-Open No. 2001-5426

In order to increase the number of gradations without changing the length of one frame period, it is necessary to provide more subframe periods within one frame period, so that a circuit for sending a signal to a pixel operates at a higher speed. There is a need. Therefore, power consumption increases. Further, since the number of address (writing) periods (Ta) increases, the ratio of the display period in the entire one frame period (this is called the duty ratio) becomes small. If the ratio of the total time of the sustain (lighting) period (Ts) is half in one frame period, that is, if the duty ratio is 50%, only half the brightness as when the duty ratio is 100% is obtained. Absent. If it is desired to obtain the same brightness as that of 100%, it is necessary to double the brightness when shining in the sustain (lighting) period, that is, the instantaneous brightness. For that purpose, it is necessary to flow twice the current through the EL element.

[0035]

SUMMARY OF THE INVENTION The present invention has been made in view of the problems of the gradation display method of the light emitting device as described above, and by using a novel driving method,
An object of the present invention is to improve the duty ratio and to secure a sufficient sustain (lighting) period even when the number of gradations is increased to realize good image quality.

[0036]

In the course of reaching the present invention, the problem of analog gray scale display is that the amount of current flowing through the EL element is sensitive to variations in Id-Vg characteristics of the controlled TFT. I thought it was the cause.

Further, it is considered that the problem of the digital time-division gray scale display is due to insufficient brightness due to the reduction of the duty ratio, which occurs when the gray scale is increased.

The present invention has been made in view of the above problems, and the gradation display is performed by combining the brightness of the EL element with the control of the amount of current flowing through the EL element and the time for which the EL element emits light. By performing the gradation display by combining the gradation display of the analog method and the time division gradation display of the digital method, the influence of the Id-Vg characteristic variation of the TFT is less likely to occur, and the operation speed and duty of the circuit are reduced. It is intended to provide a method for performing high gradation display without changing the ratio.

With the above structure, in the present invention, even if there is some variation in the Id-Vg characteristics of the TFTs, it is possible to suppress variations in the amount of current output when equal gate-source voltages are applied. Therefore Id-V
It is possible to avoid a situation in which the light emission amount of the EL element greatly differs between adjacent pixels even if signals of the same voltage are input due to variations in g characteristics.

Further, when displaying the same number of gradations, the above-mentioned configuration can reduce the number of sub-frame periods as compared with the case where gradation display is performed by the digital time division method, so the duty ratio should be set high. Since it is not necessary to operate the circuit at high speed, power consumption can be reduced.

In the text, the m-th power of n is described as "n ^m " or "n ^ m".

The structure of the present invention is shown below.

According to the present invention, there is provided a method for driving a light emitting device, characterized in that gradation display is performed by combining control of the amount of current flowing through the light emitting element and control of the time during which the light emitting element emits light. It

According to the present invention, one frame period is m (m
Has 2 or more natural number) different length subframe periods, and the m different length subframe periods each have an address period and a sustain period,
In the address period, an analog data signal is input to the corresponding light emitting element, and in the sustain period,
A method for driving a light emitting device is provided, wherein the plurality of light emitting elements emit light with n kinds of brightness (n is a natural number of 2 or more) according to the analog data signal.

According to the present invention, one frame period is m (m
Is a natural number of 2 or more) sub-frame periods, and the m sub-frame periods each have an address period and a sustain period, and an analog data signal is input to a corresponding light emitting element in the address period. Then, during the sustain period, the plurality of light emitting elements emit light with n kinds of luminance (n is a natural number of 2 or more) by the analog data signal and express n ^m kinds of gradations. A method of driving a device is provided.

In the method of driving the light emitting device of the present invention,
The light emitting device has a pixel portion provided with a plurality of light emitting elements, and the plurality of light emitting elements each have a first electrode and a second electrode, and the light emitting luminance of the light emitting element in a sustain period. Are controlled by an ON-state light emitting element drive current flowing between the first electrode and the second electrode.

In the method for driving the light emitting device of the present invention,
It is characterized in that the one frame period has a period in which a bias voltage is applied to the light emitting element in a polarity opposite to the forward direction.

The present invention provides an electronic device.

[0049]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, a method for driving a light emitting device according to the present invention will be described in detail in the embodiments. However, the driving method of the light emitting device of the present invention is not limited to the following embodiments.

(Embodiment 1) In the present embodiment, by using FIG. 1 and FIG. 2, two digital data signals of (a + b) bits (a and b are natural numbers of 2 or more) are used.
^A case of displaying ^{(a + b)} gradation will be described.

FIG. 1 shows a structure of a pixel portion 100 of a light emitting device which operates by the driving method of the present invention. The pixel portion is composed of (x × y) pixels (x and y are natural numbers of 1 or more) arranged in a matrix. Gate signal lines (G _{1 to} G _y ) to which a gate signal is input are connected to the gate electrode of the switching TFT 101 included in each pixel.
One of a source region and a drain region of the switching TFT 101 included in each pixel is a source signal line (also referred to as a data signal line) to which a video data signal is input (S ₁
~ S _x ) and the other has a driving T that each pixel has
Each of them is connected to the gate electrode of the FT 104. One of the source region and the drain region of the driving TFT 104 is the power supply line 110, and the other is the EL element 1.
It is connected to 06. The potential of the power supply line 110 is always kept at a constant potential, and the potential is V _D.

Further, a capacitor 108 may be provided between the gate electrode of the driving TFT 104 and the power supply line 110 and used as a storage capacitor for holding the gate-source voltage of the driving TFT 104.

The video data signal is a signal obtained by converting a (a + b) -bit digital data signal for performing digital time-division gradation and analog gradation display. Ve ₁ , Ve ₂ , ... , Ve _{2 ^ b} , which is a signal including image information.

The EL element 106 is composed of an anode, a cathode, and an EL layer provided between the anode and the cathode. When the anode is connected to the source region or the drain region of the driving TFT 104, the cathode is connected to the counter electrode 109. Conversely, when the cathode is connected to the source region or the drain region of the driving TFT 104, the anode is connected to the counter electrode 109.

Although not shown in FIG. 1, the counter electrodes 109 of the pixels are all connected so as to have the same potential, and the potential is called a steady potential and is represented by V _C.

FIG. 2 shows a timing chart for carrying out the present invention. First, in one frame period (F), a sub-frame periods (SF ₁
~ SF _a ) are provided one by one. Note that a period in which all the pixels in the pixel portion display one image is referred to as one frame period (F).

In a normal light emitting device, 60 or more frame periods are provided in one second, and 60 or more images are displayed in one second. The number of images displayed per second is 60
When the number becomes smaller, image flicker such as flicker becomes visually noticeable.

A plurality of periods forming one frame period are called subframe periods (SF).

The sub-frame period is the address period (Ta)
And a sustain period (Ts). The address period is a time required to input a digital data signal to a pixel in one subframe period, and the sustain period (also referred to as a lighting period) is a period in which an EL element emits light.

[0060] sub-frame periods SF ₁ - SF _a is the Ta ₁ to Ta _a respective address period (Ta) having respectively. The lengths of the address periods Ta _{1 to} Ta _a are all equal. SF ₁ - SF _a is a sustain period, each having a (Ts), respectively, and Ts ₁ ~Ts _a. Sustain period Ts ₁ is the longest, so Ts _a is the shortest, is set as follows the ratio of the period length of the sustain period. Ts ₁ : Ts ₂ : ...: Ts _a-1 : Ts _a = 2 ^(a-1) :
2 ^(a-2) : ...: 2 ¹ : 2 ⁰ By setting in this way, it is possible to express 2 ^a gradations when the emission brightness of the EL elements in each sustain period is equal.

However, in the present embodiment, the ratio of each sustain period is set so as to be a power of 2, however, even if the length of the sustain period is not a power of 2, the gradation Display is possible. By setting the sustain periods of the sub-frame periods to be different from each other, 2 ^a gray levels can be expressed.

In the address period (Ta), a period in which a certain row of gate signal lines is selected is called a 1-line period (L). In the case of the light emitting device of FIG. 1, since there are y gate signal lines, y number of line periods (L _{1 to} L ₁ ) are included in one address period.
_y ) is provided.

Address period T of subframe period SF ₁
At a ₁ , the potential V _C of the counter electrode of the EL element 106 is
It is kept at the same height as the potential V _D of the power supply line 110. In this embodiment mode, the potential V _C of the counter electrode in the address period is called an off steady potential. Note that the off steady-state potential may be in the range in which the EL element 106 does not emit light.

[0064] In the first line period (L _1), a gate signal is input to the gate signal line G _1, the gate signal lines G ₁
All the switching TFTs 101 connected to are turned on.

Then, with the switching TFT 101 connected to the gate signal line G ₁ turned on, the video data signals are simultaneously input to all the source signal lines (S _{1 to} S _x ). The potential of each source signal line is Ve ₁ , V
The potential is one of e ₂ , ..., Ve _{2 ^ b} .

Then, the video data signal input to the source signal lines (S _{1 to} S _x ) is input to the gate electrode of the driving TFT 104 via the switching TFT 101 in the ON state. Further, the video data signal is also input to the capacitors 108 of all the pixels connected to the gate signal line G ₁ and the charges thereof are held.

Next, in the second line period (L ₂ ),
The gate signal is input to the gate signal line G _2, all switching TFT101 is turned on, which is connected to the gate signal line G _2. Then, with the switching TFT 101 connected to the gate signal line G ₂ turned on, video data signals are simultaneously input to all the source signal lines (S _{1 to} S _x ). The video data signal input to the source signal lines (S _{1 to} S _x ) is a switching TFT.
It is input to the gate electrode of the driving TFT 104 via 101. The video data signal is also input to the capacitors 108 of all the pixels connected to the gate signal line G ₂ .
The charge is retained.

Then, the above-described operation is repeated in order, and the game is
Signal line (G₁~ G_y), The gate signal is input to all
Line period (L₁~ L_y) Ends. All line periods
(L ₁~ L_y) Is completed, the video data is
A signal is input. Video data signal is input to all pixels
The period until address is the address period.

At the same time when the address period Ta ₁ ends, the sustain period Ts ₁ starts. In the sustain period,
The potential V _{C of the} counter electrode changes from an off steady potential to an on steady potential. In the present embodiment, the steady potential in the sustain period is called the on steady potential. The on-state steady potential may have a potential difference from the power source potential to the extent that the EL element emits light.

Then, during the sustain period Ts ₁ , all the switching TFTs 101 are turned off. Then, the video data signal held in the capacitor 108 is input to the gate electrode of the driving TFT 104.

V is applied to the gate electrode of the driving TFT 104.
e₁, Ve₂, ..., Ve_{2 ^ b}Any one of the potentials is applied
At this time, the source / drain of the driving TFT 104
The current flowing between each is Ie₁, Ie₂, ..., Ie_{2 ^ b}
And A current Ie is applied to the EL element 106.₁When E flows
The L element 106 emits the brightest light, and the current Ie_{2 ^ b}Flows
In this case, the EL element 106 emits the darkest light. E
The current Ie is applied to the L element 106.₁, Ie₂, ..., Ie_{2 ^ b}Flows
The brightness of the EL element when₁, C₂, ..., C
_{2 ^ b}Then, these ratios are set as follows. C₁: C₂: ...: C_{(2 ^ b) -1}: C_{2 ^ b}= 2^{(2 ^ b)}:
Two^{(2 ^ b) -1}:…: 2²: 2¹ By setting like this, 2^bShow the gradation of the street
It is possible to reveal.

However, gradation display is possible even when the ratio of luminance obtained when a current flows through the EL element is not necessarily a power of 2.

In this embodiment, the EL element 106 may not emit light when the video data signal has a potential of Ve _{2 ^ b} , that is, C ₁ = 0.

When the sustain period Ts ₁ ends, the address period Ta ₂ of the next sub-frame period SF ₂ again starts,
Similar to the address period Ta ₁ , when the video data signal is input to all the pixels, the sustain period Ts ₂ starts.

Thereafter, the same operation is repeated for the remaining (a-2) sub-frame periods, and Ts ₃ , T
It is assumed that s ₄ , ..., Ts _a and the sustain period are set, and a predetermined pixel is turned on in each subframe.

When the a sub-frame period ends, 1
The frame period ends. 2 ^a by the above driving method
It is possible to express 2 ^{(a + b)} gradations by combining the sustain periods of the same length and the brightness of 2 ^b kinds of EL elements.

In the present embodiment, as shown in FIG. 2, the case where the subframe periods are provided in the order of SF ₁ , SF ₂ , ..., SF _a has been described, but the setting order of the subframe periods is not necessarily this. Not limited. That is, the order of appearance of subframe periods may be random within one frame period.

Further, in the above-described structure of this embodiment, the potential V _D of the power supply line 110 is always kept constant and the potential V _C of the counter electrode 109 is changed between the address period and the sustain period. However, the present embodiment is not limited to this configuration. On the contrary, the potential V _C of the counter electrode 109 is always kept constant,
The potential V _D of the power supply line 110 may be changed between the address period and the sustain period. Further, both the potential V _D of the power supply line 110 and the potential V _C of the counter electrode 109 may be changed between the address period and the sustain period.

The following conditions are required to express 2 ^{(a + b)} gray levels in the driving method of the present invention. When the first gradation is the darkest and the 2 ^{(a + b)} th gradation is the brightest, the brightness representing the xth gradation is smaller than the brightness representing the (x + 1) th gradation. 2 flowing into the EL element
^It is necessary to set ^b different currents (x is 1 or more 2 ^{(m + n)}
-1 or less natural number).

As an example, of 4-bit video data input from the outside, 2-bit data is used as time division gradation information (a = 2), and 2-bit data is analog gradation data. A case where a video data signal is input to the pixel portion as information (b = 2) will be described. Table 1 shows a gradation level to be expressed, a magnitude relationship of luminance, and a current value flowing through the EL element during the sustain period corresponding to each bit.

[0081]

[Table 1]

For example, during a certain frame period, the gate signal line G is applied to the gate electrode of the switching TFT 101.
_2, of the source region and the drain region, one of the pixels connected to the source signal line S _2, and causes the EL element to emit light at 7 th gradation. In this case, the potential of the source signal line S ₂ is set to Ve ₃ while the gate signal is being input to the gate signal line G ₂ in the address period Ta ₁ .
During the period in which the gate signal to the gate signal line G ₂ in the address period Ta ₂ is input, the potential of the source signal line S ₂ and Ve _2. As a result, a current of Ie ₃ flows through the EL element during the sustain period Ts ₁ and a current of Ie ₂ flows through the EL element during the sustain period Ts ₂ , so that the EL element emits light at the seventh gradation.

When expressing the gradations shown in Table 1, the first gradation is the darkest, the brightness increases as the number increases, and the 16th gradation becomes the brightest.
Currents Ie ₁ , Ie ₂ , Ie ₃ , and Ie ₄ flowing through the L element 106
Need to be set.

According to the configuration of the present invention, when displaying 2 ^{(a + b )} gray scales with ^a digital data signal of (a + b) bits, it is possible to display sub-frames rather than performing gray scale display by a digital time division method. It is possible to reduce the number by b. When the address period is the same in the configuration of the present invention and the digital time division method, the duty ratio becomes larger in the configuration of the present invention because the number of subframes decreases.
It is possible to obtain a light emitting device which emits brighter light and has a good image quality. Further, in the case where the circuit which sends a signal to the pixel is operated so that the duty ratio becomes equal between the configuration of the present invention and the digital time division method, the configuration of the present invention can delay the operation of the circuit, so that the power consumption is small. A light emitting device can be obtained.

In this embodiment, one subframe period of different length a is provided in each one frame period, and two a-bit digital video signals are used.
^Although an example of displaying a number of gradations is shown, the present invention is not limited to this. For example, in one frame period a
It is possible to provide a plurality of some subframe periods of the subframe periods of different lengths.
For example, a number of different subframe periods SF ₁
Two SF ₁ out of SF _a may be provided, and one subframe period other than SF ₁ may be provided. Further, as described above, the order in which the subframe periods are provided in one frame period does not necessarily have to be provided in order from the subframe period having the longest period length as shown in FIG. 2, and may be random.

(Embodiment 2) In Embodiment 1, one sub-frame period having different lengths a is provided in the frame period F, and 2 ^b EL elements are provided in the sustain period included in each sub-frame period. Although it is described that 2 ^{(a + b)} gradations can be expressed by emitting light with the luminance of, the gradation that can be expressed is not always a power of 2. That is, m sub-frame periods SF _{1 to} SF _m having different lengths (m is a natural number of 2 or more) are provided in the frame period (F), and the EL element is provided in the sustain period included in each sub-frame period. It is also possible to express n ^m different gradations by emitting light with n different luminances (n is a natural number of 2 or more). However, darkest first gradation, when the n ^m-th gradation is the brightest, x
It is necessary to set n kinds of currents flowing through the EL element so that the luminance representing the th gradation (x is a natural number of 1 or more and n ^m -1 or less) is smaller than the luminance representing the (x + 1) th gradation. There is.

Also in this embodiment, the order of providing the subframe periods within one frame period does not necessarily have to be the order of the subframe periods having the longest period as shown in FIG. 2, and may be random. .

In the present embodiment, an example has been shown in which one subframe period of m different lengths is provided in one frame period, but the present invention is not limited to this. For example, a plurality of subframe periods out of m different length subframe periods may be provided in one frame period. For example, out of m different subframe periods SF _{1 to} SF _m , two SF ₁ may be provided, and the other subframe periods may be provided one by one. Also in this case, the order of providing the subframe periods does not necessarily have to be provided in order from the subframe period having the longest length, and may be random.

(Embodiment 3) Embodiments 1 and 2
1, the pixel portion is shown in FIG. 1, but the pixel structure of the light emitting device of the present invention is not limited to this. For example, the power supply line 109 in FIG. 1 may be independently provided not for all pixels but for each source signal line. Further, as shown in FIG. 3, a pixel structure in which an erasing TFT 305 is added to the pixel structure of FIG. 1 may be used in order to release the charge accumulated in the storage capacitor 108.

(Embodiment 4) In an EL element, when a potential higher than that of the cathode is applied to the anode, a current flows from the anode to the cathode to emit light, and conversely, a potential higher than that of the anode is applied. It is known that when EL is applied to the cathode, no current flows in the EL element, but by setting the EL element in this state, the element life can be extended. At this time, as usual, a voltage at which a current flows through the EL element is called a forward bias voltage, and the reverse voltage is called a reverse bias voltage.

In the first, second and third embodiments, a period during which the EL element does not emit light may be provided within the frame period, and a reverse bias voltage may be applied to the EL elements included in all pixels during that period. In addition, Japanese Patent Laid-Open No. 2001-109
In the case of a pixel structure in which the technique disclosed in No. 432 can be used, a reverse bias voltage may be applied to the EL element during the address period in which the EL element does not emit light.

[0092]

EXAMPLES Examples of the present invention will be described below.

[Embodiment 1] A light emitting device which operates by the driving method of the present invention will be described with an example of the circuit configuration shown in FIG.

The light emitting device shown in FIG. 4A includes a pixel portion 401,
It has a source signal line driver circuit 402 and a gate signal line driver circuit 403 which are arranged around the pixel portion 401,
Both are composed of TFTs formed on the substrate. Note that in the example illustrated in FIG. 4A, the light-emitting device includes one source signal line driver circuit and one gate signal line driver circuit, but the number of source signal line driver circuits is arbitrary. May be. Further, the number of gate signal line driving circuits may be arbitrary.

Externally input (a + b) bits (a
Is a natural number greater than or equal to 2) (b is a natural number greater than or equal to 2) and the control signal is converted in the video signal processing circuit 421 and input to the source signal line driver circuit and the gate signal line driver circuit. Generate an analog data signal.

The video signal processing circuit 421 has means for providing a plurality of sub-frame periods corresponding to a-bit gradation in one frame period, means for outputting a b-bit gradation as an analog data signal, and source signal line. Drive circuit 40
2 and means for outputting a control signal for operating the gate signal line drive circuit 403. The sub-frame period is divided into an address period and a sustain period.
It is assumed that the sustain period is set according to the bit gradation.

The video signal processing circuit 421 may be provided outside the light emitting device that operates according to the driving method of the present invention, and the data signal generated there may be input to the light emitting device. In this case, the electronic device having the light emitting device that operates by the driving method of the present invention as a display includes the light emitting device that operates by the driving method of the present invention and the video signal processing circuit as separate components.

Further, the video signal processing circuit 421 is mounted in the form of an IC chip or the like on a light emitting device which operates by the driving method of the present invention, and a digital data signal formed by the IC chip operates by the driving method of the present invention. It may be configured to be input to the light emitting device. In this case, an electronic device having a light emitting device that operates by the driving method of the present invention as a display is
A light emitting device mounted with an IC chip including a video signal processing circuit is included as a component.

Finally, the video signal processing circuit 42
1 can be formed using TFTs on the same substrate as the pixel portion 401, the source signal line driver circuit 402, and the gate signal line driver circuit 403. In this case, if a video signal including image information, a control signal, and a power supply voltage are input to the light emitting device, all can be processed on the substrate. In this case, the video signal processing circuit may be formed by a TFT having a polysilicon film as an active layer. Further, in an electronic device including a light emitting device which operates by the driving method of the present invention as a display portion, a video signal processing circuit is incorporated in the light emitting device itself, so that the electronic device can be downsized.

FIG. 5 is a schematic diagram of a source signal line drive circuit used in this embodiment. A shift register 501 including a plurality of stages of flip-flops 510, a first latch circuit 502, a second latch circuit 503, D / A conversion circuit 50
It has 4 mag. In the example of FIG. 5, the circuit configuration is when b = 3, and the digital video signal is input bit by bit.
Here, a 3-bit digital video signal is input from the three signal lines. The signals input from the outside are the clock signal (S-CLK) and the inverted clock signal (S-CLK).
b), start pulse (S-SP), digital video signal (Di
gital Data 1 to 3) and Latch Pulse.

First, a clock signal, a clock inversion signal,
The sampling pulse is sequentially output from the shift register 501 in accordance with the timing of the start pulse. After that, the sampling pulse is input to the first latch circuit 502, and in the first latch circuit 502, at the timing when the sampling pulse is input,
Captures and holds digital video signals for each bit. This operation is sequentially performed from the first column.

When the holding of the digital video signal in the final first latch circuit is completed, a latch pulse is input, and at this timing, the digital video signals held in the first latch circuit 502 are simultaneously output to the second video signal. Is transferred to the latch circuit 503.

After that, the digital video signal for each bit is
After being input to the D / A conversion circuit 504 and converted into an analog video signal, each source signal line (S ₁ , S ₂ , ...
.., S _x ).

On the other hand, FIG. 6 is a schematic diagram of a gate signal line drive circuit used in this embodiment, in which a shift register 611 and a buffer 61 each including a plurality of flip-flops 614 are used.
2. A pulse width control circuit 613 including a plurality of NANDs 615 and the like is included. The signals input from the outside are the clock signal (G-CLK) and the inverted clock signal (G-CL).
Kb), start pulse (G-SP), pulse width control signal
(PWC).

First, a clock signal, a clock inversion signal,
And the start pulse timing
Pulses are sequentially output from the register 611. this
The pulse is amplified by the buffer 612 or the like,
After that, it is continuously output by the pulse width control circuit 613.
The pulse width is adjusted so that the pulses that are generated do not overlap.
To be done. Then, if necessary, pass through a buffer etc.
Gate signal line (G₁, G ₂・ ・ ・, G_y) Are sequentially selected
Pulse to be output to each gate signal line.
Be done. The gate signal lines in the first row are selected in order, and
Gate signal line (G_y), The vertical blanking period
After a lapse of time, a pulse is again output from the shift register 611.
Is output to select the gate signal line.

A plurality of pixels 404 are arranged in a matrix in the pixel portion 401 of FIG. 4A. An enlarged view of the pixel 404 is shown in FIG. In FIG. 4B, 405 is a switching TFT. Switching TFT4
The gate electrode of 05 is connected to a gate signal line 406 to which a gate signal is input. Switching TFT4
One of the source region and the drain region of 05 is connected to the source signal line 407 to which a digital data signal is input, and the other is connected to the gate electrode of the driving TFT 408.

One of the source region and the drain region of the driving TFT 408 is connected to the power supply line 411,
The other side is connected to the EL element 410. In addition, a capacitor 413 is provided between the gate electrode of the driving TFT 408 and the power supply line to hold the gate-source voltage of the driving TFT 408 when the switching TFT 405 is in the non-selected state (OFF state). You may use.

The EL element 410 is composed of an anode, a cathode, and an EL layer provided between the anode and the cathode. When the anode is connected to the source region or the drain region of the driving TFT 408, the cathode is connected to the counter electrode 412. On the contrary, when the cathode is connected to the source region or the drain region of the driving TFT 408, the anode is connected to the counter electrode 412.

The power supply line 411 is kept at a constant potential.

Note that a resistor may be provided between the drain region or the source region of the driving TFT 408 and the EL element 410. By providing a resistor, the driving T
It is possible to control the amount of current supplied from the FT to the EL element and prevent the influence of variations in the characteristics of the driving TFT. The resistor is not limited to a structure and the like as long as it is an element having a resistance value sufficiently larger than the on resistance of the driving TFT 408. Note that the on-resistance is a value obtained by dividing the drain voltage of the TFT by the drain current flowing at that time when the TFT is in the on state. The resistance value of the resistor is 1k
It may be selected from the range of Ω to 50 MΩ (preferably 10 kΩ to 10 MΩ, more preferably 50 kΩ to 1 MΩ). It is preferable to use a semiconductor layer having a high resistance value as the resistor because it is easy to form.

FIG. 7A shows an example of the layout of elements when the pixel having the structure of FIG. 5 is actually manufactured. Further, FIG. 7B is a cross-sectional view of a portion indicated by XX ′ in FIG.

In FIG. 7B, 419 is a substrate having an insulating surface. The driving TFT 4 is provided on the substrate 419.
08 and the like are provided, source / drain electrodes made of a wiring material are formed so as to be connected to the impurity regions forming the source / drain regions of the driving TFT 408, and one of them is overlapped with the pixel electrode 415. It is provided to connect with. An organic conductor film 417 is provided on the pixel electrode 415, and an organic thin film (organic compound layer) 418 is further provided. A counter electrode 412 is provided on the organic thin film (organic compound layer) 418. The counter electrode 412 is formed in a solid shape so as to be commonly connected to all pixels.

The light emitted from the organic thin film (organic compound layer) 418 passes through either the pixel electrode 415 or the counter electrode 412 and is emitted. At this time,
In (B), the case where light is emitted to the pixel electrode side, that is, the side where the TFT and the like are formed is called bottom emission, and the case where light is emitted to the counter electrode side is called top emission.

For bottom emission, the pixel electrode 415 is formed of a transparent conductive film. Conversely, in the case of top emission, the counter electrode 412 is formed of a transparent conductive film.

In a color display light emitting device,
EL elements having respective R, G, and B emission colors may be separately coated, or single-color EL elements may be coated in a solid form to obtain R, G, and B emission by a color filter. .

The present invention can be carried out only in the structure of the light emitting device of FIG. 4, and the structure of FIG. 4 is only one of the preferable modes for carrying out the present invention. Further, the structure shown in this embodiment is merely an example, and the pixel layout, the sectional structure, the stacking order of the electrodes of the EL element, and the like are not limited to this.

[Embodiment 2] As shown in FIG. 8A, when a light emitting device is used as a display portion of an electronic device such as a mobile phone, it is incorporated in the form of a module 801. Here, the module 801 refers to a form in which a light emitting device and a substrate on which a signal processing LSI for driving the light emitting device, a memory, and the like are mounted are connected.

The module 801 is shown in FIG. 8B as a block diagram. The module 801 includes a power supply unit 811, a signal control unit 812, an FPC 813, and a light emitting device 814. The power supply unit 811 receives a source signal line drive circuit, a gate signal line drive circuit, an E signal from a power source such as an external battery 815.
A power supply having a plurality of desired voltage values is generated and supplied to the L element and the like. A video signal and a synchronization signal 816 are input to the signal control unit 812, which converts various signals so that the light emitting device 814 can process the signals and also drives a source signal line driver circuit and a gate signal line driver circuit. To generate a clock signal and the like.

The module 801 shown in this embodiment is
Light emitting device 814, power supply unit 811, and signal control unit 81
Although it is prepared independently of 2, it may be formed integrally on a substrate.

Next, FIG. 9 shows a detailed structure of the light emitting device 814 included in the module 801 shown in FIG.

The light emitting device includes a pixel portion 90 on a substrate 901.
3, a source signal line drive circuit 904, gate signal line drive circuits 905 and 906, an FPC 907, and the like. The counter substrate 902 may be a transparent material such as glass or a metal material. A space between the substrate 901 and the counter substrate 902 may be sealed with a filler or the like, and a desiccant or the like may be sealed in order to prevent deterioration of the EL element due to moisture.

FIG. 9B shows a top view. A pixel portion 903 is arranged in the central portion of the substrate, and a source signal line driving circuit 904, a gate signal line driving circuit 905,
906 is arranged. Source signal line driver circuit 904
The current supply line 911 and the counter electrode contact 9 are provided around the
13 etc. are arranged. The counter electrode of the EL element is formed on the entire surface of the pixel portion, and the counter electrode contact 91 is formed.
The counter potential is applied from the outside through the FPC 907 by 3. The signals for driving the source signal line driver circuit 904 and the gate signal line driver circuits 905 and 906, and power supply are externally supplied through the FPC 907.

Further, as shown in FIG. 9B, the sealant 914 for bonding the substrate 901 and the counter substrate 902 is part of the source signal line driver circuit 904 and the gate signal line driver circuits 905 and 906. It may be formed so as to overlap. By doing so, a narrower frame of the light emitting device can be expected.

Example 3 In this example, a high molecular compound was applied as the organic compound layer, and a buffer layer made of a conductive high molecular compound was provided between the anode and the organic compound layer. The following describes the results of measurement of luminance deterioration when DC driving (always applying a forward bias) and AC driving (alternately applying a forward bias and a reverse bias at a constant cycle) were performed.

10A and 10B show forward bias: 3.
The result of the reliability test when AC drive is performed at 7 V, reverse bias: 1.7 V, duty 50%, and AC frequency 60 Hz is shown. The initial brightness was about 400 cd / cm ² . DC drive (forward bias: 3.65V) for comparison
The results of the reliability test when the above were performed are also shown at the same time. result,
In the DC drive, the brightness was reduced to half after about 400 hours, whereas in the AC drive, the brightness was not reduced to half after about 700 hours.

10C and 10D show forward bias: 3.
The result of the reliability test when AC driving is performed at 8 V, reverse bias: 1.7 V, duty of 50%, and AC frequency of 600 Hz is shown. The initial brightness was about 300 cd / cm ² . For comparison, DC drive (forward bias: 3.65)
The results of the reliability test when V) was performed are also shown at the same time.
As a result, in the DC drive, the brightness was reduced to half after about 500 hours, while in the AC drive, about 60% of the initial brightness was maintained even after about 700 hours.

Example 4 Since the light emitting device using the light emitting element is a self-luminous type, it has better visibility in a bright place and a wider viewing angle than a liquid crystal display. Therefore, it can be used for a display portion of various electronic devices.

As the electronic equipment using the light emitting device which operates by the driving method of the present invention, there are a video camera, a digital camera,
Goggles type display (head mounted display), navigation system, sound reproduction device (car audio, audio component system, etc.), notebook type personal computer, game device, personal digital assistant (mobile computer, mobile phone, portable game console or electronic book etc.) ), An image reproducing device equipped with a recording medium (specifically, Digita
A device provided with a display capable of reproducing a recording medium such as a Versatile Disc (DVD) and displaying an image thereof. In particular, for a portable information terminal that often sees the screen from an oblique direction, since a wide viewing angle is important, it is preferable to use a light emitting device. Specific examples of these electronic devices are shown in FIGS.

FIG. 11A shows an EL display, which includes a housing 3001, a voice output portion 3002, a display portion 3003, and the like. The light emitting device of the present invention can be used for the display portion 3003. Since the light emitting device is a self-luminous type, no backlight is required, and the display portion can be thinner than a liquid crystal display. The light emitting element display device includes all information display devices for personal computers, TV broadcast reception, advertisement display, and the like.

FIG. 11B shows a mobile computer, which includes a main body 3011, a stylus 3012, and a display section 301.
3, operation button 3014, external interface 301
Including 5 etc. The light emitting device of the present invention can be used for the display portion 3013.

Further, FIG. 11C shows a large-sized EL display, which includes a housing 3021, a voice output portion 3022, and a display portion 3023 like FIG. 11A. The light emitting device of the present invention can be used for the display portion 3023.

FIG. 11D shows a game machine, which is a main body 303.
1, a display unit 3032, operation buttons 3033, and the like. The light emitting device of the present invention can be used for the display portion 3032.

FIG. 11E shows a mobile phone, which is a main body 304.
1, a voice output unit 3042, a voice input unit 3043, a display unit 3044, an operation switch 3045, an antenna 3046 and the like. The light emitting device of the present invention can be used for the display portion 3044. Note that the display portion 3044 can suppress current consumption of the mobile phone by displaying white characters on a black background.

If the emission brightness of the organic light emitting material becomes higher in the future, it becomes possible to magnify and project the output light including image information with a lens or the like and use it for a front type or rear type projector.

Further, the above-mentioned electronic equipment is the Internet or C
Information distributed through electronic communication lines such as ATV (cable television) is often displayed, and in particular, opportunities for displaying moving image information are increasing. Since the response speed of the organic light emitting material is very high, the light emitting device is suitable for displaying moving images.

Since the light emitting device consumes power in the light emitting portion, it is desirable to display information so that the light emitting portion is as small as possible. Therefore, when a light emitting device is used in a display unit mainly for character information such as a mobile information terminal, a mobile phone or a sound reproducing device, it is driven so that the character information is formed in the light emitting portion with the non-light emitting portion as the background. It is desirable to do.

As described above, the applicable range of the present invention is extremely wide, and the present invention can be applied to electronic devices in all fields. Further, the electronic apparatus of this embodiment may use the light emitting device having any of the configurations shown in Embodiments 1 and 2.

[0138]

According to the present invention, Id-V can be obtained by the TFT.
Even if there is some variation in the g-characteristic, it is possible to suppress variation in the amount of current output when an equal gate-source voltage is applied. Therefore, it is possible to avoid a situation in which the light emission amount of the EL element is significantly different between adjacent pixels even if signals of the same voltage are input due to variations in the Id-Vg characteristics.

Further, when displaying the same number of gradations, the driving method of the present invention can reduce the number of sub-frame periods as compared with the case where gradation display is performed by the digital time division method, and therefore the duty ratio is increased. Can be set,
Since it is not necessary to operate the circuit at high speed, power consumption can be reduced.

[Brief description of drawings]

FIG. 1 illustrates a circuit example of a pixel portion of a light emitting device of the present invention.

FIG. 2 is a timing chart of digital time-division gray scale display of the present invention.

FIG. 3 is a diagram showing a circuit example of a pixel portion of a light emitting device of the invention.

FIG. 4 is a diagram showing a circuit configuration example of a light emitting device of the invention.

5 is a diagram showing an example of a source side driving circuit in FIG.

FIG. 6 is a diagram showing an example of a gate side driving circuit in FIG.

FIG. 7 is a diagram showing a layout example of a pixel portion in FIG.

FIG. 8 is a diagram showing an example in which a light emitting device and peripheral circuits are modularized and used in an electronic device.

FIG. 9 is a diagram showing an outline of a light emitting device.

FIG. 10 is a diagram showing luminance deterioration when an EL element is subjected to direct current driving (always applying a forward bias) and alternating current driving (alternately applying a forward bias and a reverse bias at a constant cycle).

FIG. 11 is an electronic device using the light emitting device of the present invention.

FIG. 12 is a circuit diagram of a pixel portion of an analog light emitting device.

FIG. 13 is a timing chart of an analog light emitting device.

[Explanation of symbols]

100 pixels 101 Switching TFT 104 driving TFT 106 EL element 108 capacitor

Front page continuation (51) Int.Cl. ⁷ Identification code FI theme code (reference) G09G 3/20 G09G 3/20 641D 641E 641K 642 642D H05B 33/14 H05B 33/14 A (72) Inventor Iwabuchi Tomoyuki Kanagawa Kanagawa 398 Hase, Atsugi, Atsugi, Japan (72) Inventor, Satoshi Seo 398, Hase, Atsugi, Atsugi, Kanagawa Prefecture F term in the company's semiconductor energy laboratory (reference) 3K007 AB02 AB05 AB17 BA06 DB03 GA04 5C080 AA06 BB05 CC03 DD05 DD06 DD07 DD26 DD29 EE29 FF11 HH09 JJ02 JJ03 JJ04 JJ05 JJ06 KK04 KK07 KK43

Claims (6)

[Claims] 1. A method of driving a light emitting device, wherein gradation display is performed by combining control of the amount of current flowing through the light emitting element and control of the time during which the light emitting element emits light. 2. One frame period has m (m is a natural number of 2 or more) different length subframe periods, and the m different length subframe periods are the address period and the sustain period, respectively. In the address period, an analog data signal is input to a corresponding light emitting element, and in the sustain period, the plurality of light emitting elements have n kinds of brightnesses (n is a natural number of 2 or more) according to the analog data signal. A method for driving a light-emitting device, comprising: 3. One frame period has m (m is a natural number of 2 or more) subframe periods, and each of the m subframe periods has an address period and a sustain period. In the above, the analog data signal is input to the corresponding light emitting element, and during the sustain period, the plurality of light emitting elements are caused to have n kinds of brightness (n is 2) by the analog data signal.
A method for driving a light-emitting device, which emits light with the above natural numbers and expresses n ^m gray scales. 4. The light emitting device according to claim 2, wherein the light emitting device includes a pixel portion provided with the plurality of light emitting elements, and the plurality of light emitting elements include a first electrode and a second electrode. And a light emission luminance of the light emitting element during a sustain period is controlled by an ON light emitting element drive current flowing between the first electrode and the second electrode. Driving method. 5. The method according to any one of claims 2 to 4,
A method for driving a light emitting device, comprising a period in which a bias voltage is applied to the light emitting element in a polarity opposite to a forward direction within the one frame period. 6. An electronic device using the method for driving a light emitting device according to claim 1. Description: