JP2003330418A - Display device and its driving method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a display device capable of correcting the change of luminance caused by secular change with a simple circuit configuration without using an photo-conductive element, and to provide its driving method. <P>SOLUTION: In the display device, the change of luminance of the thin film light emitting element 11 caused by the secular change is corrected by detecting the voltage or the current of a circuit including the thin film light emitting element 11 whose luminance depends on a driving current with a detection circuit 12 and by controlling the ratio of a light emission period (Duty) or the current value of the element 11 by a correction circuit 13 based on the detection signal of the detection circuit 12. <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 display device and a driving method thereof, and more particularly to a display device having a light emitting element whose brightness depends on a driving current and a driving method thereof.

[0002]

2. Description of the Related Art A light emitting device whose brightness depends on a driving current, for example, electroluminescence of an organic material.
nescence) element (hereinafter referred to as an organic EL element),
As a change with time, there is an increase in drive voltage and a decrease in luminous efficiency (luminance / current) when constant current driving is performed. As a method of compensating for this, a method of using an organic EL element using transparent electrodes on both sides and monitoring the emission brightness based on the light emitted from one of the transparent electrodes to give feedback (Japanese Patent Laid-Open No. 2001-76882). (See the official gazette),
Make a separate organic EL element for the monitor that is not used for display,
A method of monitoring the emission brightness with this organic EL element for monitoring and giving feedback (International Publication No. 98/408)
No. 71 pamphlet) and the like are known.

[0003]

However, in the method according to the above-mentioned prior art, since the brightness of the organic EL element is directly detected, a photoconductive element or a complicated circuit is required for the detection. Therefore, the cost will increase accordingly. In addition, since the circuit scale of the peripheral drive circuit also becomes large, the area of the pixel region (display region) must be narrowed by that amount, and the aperture ratio must be reduced. Along with this, the brightness is lowered, and a large current needs to be passed in order to maintain the same level of brightness, which causes an adverse effect such as shortening the life of the light emitting element.

The present invention has been made in view of the above problems, and it is an object of the present invention to compensate for changes in luminance due to aging with a simple circuit configuration without using a photoconductive element. An object of the present invention is to provide a display device capable of driving and a driving method thereof.

[0005]

In order to achieve the above object, in the present invention, the voltage or current of a circuit including a light emitting element whose brightness depends on a driving current is detected, and the light emitting element is detected based on the detection result. It adopts a configuration that corrects the brightness change of.

The light emitting element has a current-
The voltage characteristic moves to the high voltage side. On the other hand, when the light emitting element is driven with a constant current, the voltage and luminance of the light emitting element are expressed by a function having a certain correlation. Therefore, the luminance of the light emitting element can be estimated from the current-voltage characteristics of the light emitting element. Focusing on this point, by detecting the voltage or current of the circuit including the light emitting element, the brightness of the light emitting element can be known from the detection result without using the photoconductive element. Then, based on the detected voltage or current, it is possible to correct the change in luminance of the light emitting element due to the change over time.

[0007]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

FIG. 1 is a conceptual diagram showing a display device according to an embodiment of the present invention. As is apparent from FIG. 1, the display device according to the present embodiment is a light-emitting element whose brightness depends on a driving current, for example, a thin-film light-emitting element 11, and a detection circuit that detects voltage or current of a circuit including the thin-film light-emitting element 11. Circuit 1
2 and a correction circuit 13 for correcting the luminance change of the thin film light emitting element 11 based on the detection signal of the detection circuit 12.

Here, the thin film light emitting device 11 is directly connected between the scanning lines and the data lines arranged in one or more sets to form a unit pixel (passive matrix), or
Alternatively, it is connected in series with a driving transistor (pixel transistor) to form a unit pixel (active matrix). As the thin film light emitting device 11, for example, organic E
Examples include L elements and inorganic light emitting diodes. Organic EL
The element also includes an EL element used as a backlight of liquid crystal in a liquid crystal display device using a liquid crystal cell as a light emitting element of a pixel.

Here, the current (I) -voltage (V) characteristic of the thin film light emitting device 11 shifts to the high voltage side as shown in FIG. On the other hand, when the thin film light emitting device 11 is driven with a constant current, the voltage (V) of the thin film light emitting device 11
And luminance (L) are represented by a function having a certain correlation, as shown in FIG. Therefore, the luminance of the light emitting element can be estimated from the current-voltage characteristics of the thin film light emitting element 11. In FIG. 3, Vo is an initial voltage, Lo is an initial luminance,
V represents the voltage after long-term use, and L represents the luminance after long-term use.

Focusing on the characteristics of the thin film light emitting element 11 described above, in the present invention, by detecting the voltage or current of the circuit including the thin film light emitting element 11 by the detection circuit 12, the thin film light emitting element 11 is formed by using a photoconductive element. Even if the luminance of the thin film light emitting element 11 is not directly detected, the luminance of the thin film light emitting element 11 is electrically detected, and the detection result is fed back to the thin film light emitting element 11, so that the luminance change with the aging of the thin film light emitting element 11 can be suppressed. It adopts a configuration to correct. The detection circuit 12 that detects the voltage or current of the circuit including the thin film light emitting element 11 may have the following configuration.

FIG. 4 is a circuit diagram showing the structure of a pixel portion of a passive matrix type display device. In FIG. 4, the scanning lines 21 and the data lines 22 are arranged in a matrix, and the thin film light emitting element 11 is directly connected between the scanning lines 21 and the data lines 22. In this passive matrix display device, by connecting a voltmeter 23 between the data line 22 and a reference potential (for example, ground), the voltage of the thin film light emitting element 11 (in this example, the anode voltage) is connected by the voltmeter 23. ) Can be detected directly. That is, in this example, the voltmeter 23 corresponds to the detection circuit 12.

FIG. 5 is a circuit diagram showing a configuration of a unit pixel of the active matrix type display device. In FIG.
A thin film light emitting element 11 and a transistor (pixel transistor) 24 for driving the thin film light emitting element 11 are connected in series between a first power source (eg, ground) and a second power source (eg, positive power source Vdd) to form a unit pixel. is doing. Here, only the configuration of the main part of the unit pixel is shown, and the configuration is not limited to this. In this active matrix type display device, by connecting a voltmeter 25 between one end of the thin film light emitting element 11, which is an anode end in this example, and a reference potential (for example, ground), the thin film light emitting element 11 is connected by the voltmeter 25. Voltage of light emitting element 11 (anode voltage in this example)
Can be detected directly. That is, in this example, the voltmeter 25 corresponds to the detection circuit 12.

The above two detection methods are used in the thin film light emitting device 1
This is a method of detecting the element voltage in order to determine the current-voltage characteristic of No. 1. On the other hand, to detect the device current in order to determine the current-voltage characteristics of the thin film light emitting device 11,
It suffices to detect the current value flowing through the thin film light emitting element 11 when a certain voltage is applied to the thin film light emitting element 11.

However, in the active matrix type display device shown in FIG. 5, the gate voltage V of the transistor 24 is
When G is low, it is difficult to read the change in the current value due to the change in the characteristics of the thin film light emitting device 11. Because, when the gate voltage VG is low, the characteristic of the transistor 24 is that, as shown in FIG. 6, the drain voltage Vd is low when the drain current Id starts to become constant. Thereby, in FIG. 7, even when the current-voltage characteristic of the thin film light emitting element 11 changes, the current value does not change as in the case of the current I ′.

To solve this problem, the gate voltage VG of the transistor 24 may be temporarily increased when reading the current value. Since the transistor 24 operates in the linear region by increasing the gate voltage VG of the transistor 24, the change in the current value due to the characteristic change of the thin film light emitting element 11 is read as in the case of the currents I0 and I1 in FIG. be able to. Further, as another method for solving the above problem, it is conceivable to decrease the power supply voltage Vdd or increase the potential of the cathode of the thin film light emitting element 11.

In order to realize the above current detection in the active matrix type display device shown in FIG. 5, as shown in FIG. 8, an ammeter 26 is connected in series to the thin film light emitting element 11 on the cathode side, for example. Try to build a circuit. Then, the gate voltage VG is set high to operate the transistor 24 in the linear region, and the current value is measured by the ammeter 2
By reading with 6, it is possible to detect the current value (cathode current in this example) flowing through the thin film light emitting element 11.
That is, in this example, the ammeter 26 corresponds to the detection circuit 12.

In the following, as the detection method in the detection circuit 12, the method of detecting the current value flowing in the thin film light emitting element 11 (FIG. 8) is used, and the change over time of the thin film light emitting element 11 is based on the detected current value. A specific example of correcting the accompanying luminance change will be described. As described above, when the current detection method is used, the transistor 24 connected to the thin film light emitting element 11 in series is not used.
4 is applied with a gate voltage VG capable of operating in the linear region. The operation of detecting the value of the current flowing through the thin film light emitting element 11 is performed in the non-display period, for example, immediately after the system power supply is turned on.

[First Specific Example] FIG. 9 shows an organic E according to a first specific example in which, for example, an organic EL element is used as a thin film light emitting element.
FIG. 1 is a schematic configuration diagram showing an L display device, and shows an example in which the L display device is applied to a monochrome organic EL display device. Here, for simplification of the drawing, a case of a pixel array of 6 rows and 37 columns is shown as an example.

In FIG. 9, the organic EL panel 31 has a structure in which a large number of organic EL elements 32 are arranged in a matrix on a substrate such as transparent glass. Specifically, on the substrate,
A first electrode (for example, an anode) made of a transparent conductive film is formed, and a hole transport layer, a light emitting layer, an electron transport layer and an electron injection layer are sequentially deposited on the first electrode to form an organic layer, A second electrode (for example, a cathode) made of a metal having a low work function is further formed on this organic layer to form an organic E
The L element 32 is formed.

In this organic EL element 32, by applying a DC voltage between the first electrode and the second electrode, holes pass from the first electrode (anode) through the hole transport layer,
Electrons are injected into the light emitting layer from the second electrode (cathode) via the electron transport layer, and the injected positive and negative carriers cause the fluorescent molecules in the light emitting layer to be in an excited state and emit light in the relaxation process of the excited molecules. Is obtained.

In a pixel circuit including the organic EL element 32, a thin film transistor (Thin Film Transistor) is generally used as a driving transistor for driving the organic EL element 32.
r; TFT) is used. The pixel circuit is usually TF
In addition to having a plurality of Ts, a capacitor for holding pixel information (luminance information) is provided. However,
Here, for simplification of the drawing, the pixel circuit is connected in series to the organic EL element 32 and is connected to the element 32.
Only the TFT 33 for driving is shown as a representative.

A drive voltage is selectively applied to the organic EL element 32 through the TFT 33. This allows organic E
The L element 32 is driven. Here, the organic EL element 3
If 2 is, for example, a type of element whose brightness changes depending on the flowing current, the current flowing through the organic EL element 32 of each pixel is controlled by a TFT (not shown) according to the brightness information of each pixel.

One end (cathode end in this example) of the organic EL element 32 is commonly connected to all pixels, and further connected to an input end of the current detection circuit 34. The current detection circuit 34 is
Corresponding to the ammeter 26 of FIG. 8, the current of the organic EL element 32 −
In order to judge the voltage characteristics, in the non-display period, TF
A high gate voltage, specifically, an average current flowing through the TFTs 33 of all pixels is detected with a high gate voltage applied to each T33, specifically, a gate voltage that allows the TFT 33 to operate in a linear region.

The current value detected by the current detection circuit 34 is
It is converted into a voltage value by the current-voltage conversion circuit 35 and output as the detection voltage Vdet. As described above, since the luminance of the light emitting element can be estimated from the current-voltage characteristics of the thin film light emitting element (organic EL element in this example), the voltage value of the detection voltage Vdet corresponds to the luminance of the organic EL element 32. ing. The detection voltage Vdet is supplied to the comparator 36,
The comparator 36 compares it with the reference voltage Vref.

Here, a voltage value corresponding to the initial brightness of the organic EL element 32 is set as the comparison voltage Vref. Then, the comparator 36 detects the difference by comparing the detection voltage Vdet with the reference voltage Vref, and supplies it to the duty control circuit 37 as the duty setting information. The duty control circuit 37 includes a comparator 36.
By holding the duty setting information given from, and controlling the ratio of light emission / non-light emission per frame of the organic EL element 32, that is, the duty ratio, based on the duty setting information in the display period, the change over time can be achieved. The accompanying luminance change of the organic EL element 32 is corrected.

That is, the current-voltage conversion circuit 35, the comparator 36, and the duty control circuit 37, based on the detected current value of the current detection circuit 34, change the brightness of the organic EL element 32, specifically, the brightness accompanying a change over time. A correction unit (corresponding to the correction circuit 13 in FIG. 1) that corrects the change is configured.

Here, if the light emitting period ratio of the organic EL element 32 is Duty and the peak luminance is Lpeak, the organic E element is
The average luminance Lave of the L element 32 is represented by Lave = Duty · Lpeak.

If the current-voltage characteristic of the organic EL element 32 changes with time, only the peak luminance Lpeak becomes small. Therefore, the light emission period ratio Duty of the organic EL element 32 is controlled according to the change of the current-voltage characteristic with the aging as shown in FIG.
Is Lo, the voltage of the organic EL element 32 after long-term use is V, and the initial voltage is Vo, the organic EL element 3
The luminance L of 2 is corrected so that L≈Lo × V / Vo. As a result, even if the current-voltage characteristic changes with time, the organic EL element 32
The peak brightness Lpeak of can be kept constant, that is, the initial peak brightness value.

In general, since the current-voltage characteristics of the organic EL element 32 deteriorate with the lapse of time, the emission period ratio Duty of the organic EL element 32 in FIG. The light emission period ratio Duty can be set to a wide range by increasing the light emission period ratio Duty in accordance with the change of the current-voltage characteristics with the lapse of time.

As described above, in the monochrome type organic EL display device in which the light emitting pixels including the organic EL elements 32 are arranged in a matrix, the current of the circuit including the organic EL elements 32 is detected, and the detection result is shown. Based on organic EL element 3
By controlling the light emission period ratio Duty of 2, the organic EL device has a simple circuit configuration without using a photoconductive element.
It is possible to correct the luminance change of the element 32 with the passage of time. Therefore, the peak luminance Lpe of the organic EL element 32 is
Since ak can be kept constant, the optimum display state can always be maintained without being affected by the change over time.

[Second Embodiment] FIG. 11 is a schematic structural view showing an organic EL display device according to a second specific example, and shows the case of application to a color type organic EL display device as an example. .

In FIG. 11, each organic EL element 42 of R (red) G (green) B (blue) is formed on the organic EL panel 41.
R, 42G, and 42B are repeatedly arranged in each row in the order of RGB, for example. Then, the organic EL elements 42R, 4
One end (cathode end in this example) of 2G and 42B is commonly connected to each color, and further connected to the input end of the current detection circuit 24.

The current detection circuit 44 includes TFTs 43R and 43
An average current flowing through the organic EL elements 42R, 42G, 42B is detected for each color with a gate voltage for operating these TFTs in the linear region applied to each of G, 43B, and the detected current value is used as the current. -Supply to the voltage conversion circuit 45. The current-voltage conversion circuit 45 has a circuit portion corresponding to each color of RGB, converts the current value of each color detected by the current detection circuit 44 into a voltage value, and the voltage value is supplied to the comparator 46 for each color. Supply.

The comparator 46 has a circuit portion corresponding to each color of RGB, and the detection voltages VdetR, VdetG, VdetB output from the current-voltage conversion circuit 45 for each color.
Is compared with, for example, reference voltages Vrefr, Vrefg, and Vrefb set corresponding to the initial luminance for each color,
The difference is given to the duty control circuit 47 as the duty setting information.

The duty control circuit 47 has a circuit portion corresponding to each color of RGB, holds the duty setting information given from the comparator 76, and based on the duty setting information in the display period, the organic EL for each color. Element 42
By controlling the emission / non-emission ratio of R, 42G, and 42B per frame, that is, the duty ratio, the change in luminance of the organic EL elements 42R, 42G, and 42B due to change over time is corrected.

In this color type organic EL display device, when performing a series of brightness corrections, first the correction process is performed on R, then the correction process is performed on G, and finally the correction process is performed on B. Then, the correction process is sequentially performed for each color. However, the order is not limited to the order of R → G → B and is arbitrary.

By the way, in the case of an organic EL element, unlike the case where a color filter is used in a liquid crystal cell, for example, each color of RGB is emitted by changing the material of the light emitting layer. Therefore, the change in the voltage (V / Vo) -luminance (L / Lo) characteristic of the organic EL element with the aging is shown in FIG.
2, the RGB organic EL elements 42R, 42
It will be different for each G and 42B. As a result, when the current-voltage characteristics of the organic EL elements 42R, 42G, and 42B change with the lapse of time, the organic EL elements 42R and 4R change.
Since the peak luminance Lpeak of 2G and 42B varies, the chromaticity balance is lost.

On the other hand, in the color type organic EL display device according to this example, the organic EL elements 42R and 42R are formed.
The respective currents of the circuits including G and 42B are detected, and the organic EL elements 42R, 42G, and 42B are detected based on the detected current values.
By controlling each of the light emitting period ratios Duty, the organic EL element 42 has a simple circuit configuration without using a photoconductive element, as in the case of the previous specific example.
It is possible to correct the luminance change of R, 42G, and 42B with the lapse of time.

In addition to this, the organic EL is changed by aging.
The change in the voltage-luminance characteristic of the element is caused by the organic EL elements 42R, 4R.
Even if different for each color of 2G and 42B, the organic EL element 4 is corrected by the above-described correction operation of the luminance change for each color.
Since the chromaticity balance can be maintained by keeping the respective peak luminances Lpeak of 2R, 42G, and 42B constant, it is possible to always maintain the display state in the optimum chromaticity balance without being affected by the change over time. Become.

In the above embodiment, the case where the luminance change of the thin film light emitting element due to the change with time is corrected by controlling the light emitting period ratio Duty of the thin film light emitting element has been described as an example, but the present invention is not limited to this. However, since the peak luminance Lpeak of the thin film light emitting element is a function of the input data DATA, the peak luminance Lpeak of the thin film light emitting element can be controlled by controlling the current value of the thin film light emitting element as in the above case.
It is possible to correct so that the peak is kept constant.

In the above embodiment, the detection voltage Vdet and the reference voltage Vref are compared, and the light emission period ratio Duty of the thin film light emitting element is determined based on the difference between them. However, a certain function L = F ( It is also possible to calculate the luminance and the light emitting period ratio Duty or the current value from the voltage in V). That is, when the initial luminance and the initial voltage are Lo and Vo, respectively, and the luminance and the voltage after long-term use are L and V, respectively, as shown in FIG.
Since Lo and Vo / V have a correlation, with a certain fixed function L = F (V), the voltage changes to the luminance and the light emission period ratio Dut.
By calculating y or the current value, it is possible to correct the change in luminance of the thin film light emitting element due to change over time.

Further, in addition to determining the light emission period ratio Duty or current value by comparison or calculation, a correspondence table of voltage and luminance, brightness and light emission period ratio Duty or current value is prepared in advance, and as shown in FIG. This is stored in the memory 14 as a look-up table (LUT), and the correction circuit 13 refers to the look-up table of the memory 14 based on the detection signal of the detection circuit 12 to refer to the light emission period ratio Duty or the current of the thin film light emitting element. It is also possible to decide the value.

[0044]

As described above, according to the present invention,
By detecting the voltage or current of the circuit including the light emitting element whose brightness depends on the drive current and correcting the brightness change of the light emitting element based on the detection result, a simple circuit configuration can be realized without using a photoconductive element. As a result, it is possible to compensate for the change in luminance of the light emitting element due to the change over time.

[Brief description of drawings]

FIG. 1 is a conceptual diagram showing a display device according to an embodiment of the present invention.

FIG. 2 is a characteristic diagram showing current-voltage characteristics of a light emitting element.

FIG. 3 is a characteristic diagram showing a voltage-luminance characteristic of a light emitting element.

FIG. 4 is a circuit diagram showing a configuration of a pixel portion of a passive matrix display device.

FIG. 5 is a circuit diagram showing a configuration of a unit pixel of an active matrix display device.

FIG. 6 is a characteristic diagram showing drain voltage-drain current characteristics of a transistor.

FIG. 7 is a characteristic diagram showing a relationship when a voltage-luminance characteristic of a light emitting element changes with respect to a drain voltage-drain current characteristic of a transistor.

FIG. 8 is a circuit diagram for detecting a current flowing through a light emitting element.

FIG. 9 is a schematic configuration diagram showing an organic EL display device according to a first specific example, and shows a case where it is applied to a monochrome system as an example.

FIG. 10 is a waveform diagram showing a relationship between a light emitting period and a non-light emitting period under duty control.

FIG. 11 is a schematic configuration diagram showing an organic EL display device according to a second specific example, and shows a case where it is applied to a color system as an example.

FIG. 12 is a characteristic diagram showing voltage-luminance characteristics of a light emitting element for each of RGB colors.

FIG. 13 is a conceptual diagram showing a display device according to another embodiment of the present invention.

[Explanation of symbols]

11 ... Thin film light emitting element, 12 ... Detection circuit, 13 ... Correction circuit, 31, 41 ... Organic EL panel, 32, 42R, 42
G, 42B ... Organic EL element, 33, 43R, 43G, 4
3B ... TFT (thin film transistor), 34, 44 ... Current detection circuit, 35, 45 ... Current-voltage conversion circuit, 36, 4
6 ... Comparator, 37, 47 ... Duty control circuit

Continuation of front page (51) Int.Cl. ⁷ identification code FI theme code (reference) G09G 3/20 642 G09G 3/20 642L 642P 670 670J (72) Inventor Hiroshi Hasegawa 3-9, Nishigotanda, Shinagawa-ku, Tokyo No. 17 Inside Sony Engineering Co., Ltd. (72) Inventor Tatsuya Sasaoka 6-35 Kita-Shinagawa, Shinagawa-ku, Tokyo F-Term inside Sony Co., Ltd. (reference) 5C080 AA06 BB05 DD03 DD29 EE28 FF11 JJ02 JJ03 JJ04 JJ05

Claims (14)

[Claims] 1. A light emitting element whose brightness depends on a drive current, a detecting means for detecting a voltage or a current of a circuit including the light emitting element, and a brightness change of the light emitting element is corrected based on a detection signal of the detecting means. A display device comprising: 2. The detecting operation of the detecting means and the correcting operation of the correcting means are performed for each light emitting element provided corresponding to each emission color of R (red) G (green) B (blue). The display device according to claim 1, wherein the display device is a display device. 3. The display device according to claim 1, wherein the detecting operation of the detecting means is performed in a non-display period. 4. A passive matrix display device in which the light emitting element is directly connected between a scanning line and a data line, wherein the detecting means directly detects the potential of the light emitting element. 1. The display device according to 1. 5. In an active matrix type display device in which the light emitting element and a transistor for driving the light emitting element are connected in series to form a unit pixel, the detection means directly detects the potential of the light emitting element. The display device according to claim 1. 6. In an active matrix type display device in which the light emitting element and a transistor for driving the light emitting element are connected in series to form a unit pixel, the detection means detects one current or a plurality of currents flowing in the light emitting element. The display device according to claim 1, wherein a total is detected. 7. The display device according to claim 6, wherein the transistor operates in a linear region. 8. The display device according to claim 1, wherein the correction unit changes a light emission period of the light emitting element according to a detection signal of the detection unit. 9. The display device according to claim 1, wherein the correction unit changes a value of a current supplied to the light emitting element according to a detection signal of the detection unit. 10. The correcting means sets the initial luminance of the light emitting element to Lo, and the voltage of the light emitting element after long-time use is V,
The display device according to claim 1, wherein when the initial voltage is Vo, the luminance L of the light emitting element is L≈Lo × V / Vo. 11. The display device according to claim 1, wherein the correction unit estimates and corrects the brightness from the voltage detected by the detection unit based on a correspondence table between the brightness and the voltage. 12. The display device according to claim 1, wherein the light emitting element is an organic electroluminescence element having first and second electrodes and an organic layer including a light emitting layer between these electrodes. 13. A method of driving a display device, comprising detecting a voltage or a current of a circuit including a light emitting element, the luminance of which depends on a driving current, and correcting the luminance change of the light emitting element based on the detection result. . 14. The detection operation of the voltage or current and the correction operation of the brightness change are performed by R (red) G (green) B (blue).
14. The method for driving a display device according to claim 13, wherein the method is performed for each light emitting element provided corresponding to each emission color.