JP2009042788A - Display device and driving method thereof - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To compensate luminance changes with time with a simple circuit constitution without using a photoconductive element. <P>SOLUTION: In a unit pixel which includes a thin film light emitting element 11 whose luminance depends upon a drive current and a transistor 24 which drives the thin film light emitting element 11 in a region where the drain current changes constantly against the changes of a drain voltage, the voltage at one end of the thin film light emitting element 11 which is connected to the transistor 24 is detected at a voltage indicator 25. Then, based on the detected result, the luminance changes of the thin film light emitting element 11 is corrected by estimating the luminance of the thin film light emitting element 11 when driven at a constant current from the current-voltage characteristics of the thin film light emitting element 11, and by controlling the amount of current flowing in the thin film light emitting element 11 or the light emitting period of the thin film light emitting element 11 corresponding to the estimated luminance. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  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 luminance depends on a driving current and a driving method thereof.

In a light-emitting element whose luminance depends on the driving current, for example, an organic electroluminescence element (hereinafter referred to as an organic EL element), as a change with time, the driving voltage increases when constant current driving is performed. There is also a decrease in luminous efficiency (luminance / current). As a method for compensating for this, a method of using an organic EL element using transparent electrodes on both sides and monitoring the emission luminance based on the emitted light from one transparent electrode side and applying feedback (see, for example, Patent Document 1) ) , A method of separately producing a monitor organic EL element that is not used for display, and monitoring the light emission luminance with this monitor organic EL element to provide feedback (for example, see Patent Document 2) .

Japanese Patent Laid-Open No. 2001-76882 International Publication No. 98/40871 Pamphlet

  However, since the method according to the prior art employs a configuration for directly detecting the luminance of the organic EL element, a photoconductive element or a complicated circuit is required for the detection, and the cost increases accordingly. End up. Further, since the circuit scale of the peripheral driving circuit is increased, the area of the pixel region (display region) must be reduced by that amount, and the aperture ratio is inevitably lowered. As a result, the luminance decreases, and in order to maintain the same level of luminance, it is necessary to flow a large current.

  The present invention has been made in view of the above problems, and an object of the present invention is to provide a display that can compensate for a change in luminance with time with a simple circuit configuration without using a photoconductive element. An apparatus and a driving method thereof are provided.

To achieve the above object, the present invention, the unit comprising a light emitting element Brightness is dependent on the drive current, and a transistor which changes in the drain current with respect to a change in the drain voltage for driving the light emitting element in a region to be a constant in the pixel, the detecting a voltage at one end of the light emitting element connected to the transistor, the current of the light emitting element based on the detection result - estimate the luminance in the case of driving voltage characteristics at a constant current of the light emitting element, The light emitting period of the light emitting element or the amount of current flowing through the light emitting element is controlled so that the estimated brightness is obtained.

The current-voltage characteristic of the light emitting element moves to the high voltage side with time. 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 constituting the unit pixel. Focusing on this point, by detecting the voltage at one end of the light emitting element connected to the transistor, the luminance of the light emitting element can be known from the detection result without using a photoconductive element. Then, based on the detected voltage, the brightness when the light emitting element is driven at a constant current is estimated from the current-voltage characteristics of the light emitting element, and the light emission period of the light emitting element or the light emitting element is caused to flow according to the estimated brightness. By controlling the amount of current, it is possible to correct a luminance change accompanying a change with time of the light emitting element.

According to the present invention, a unit pixel including a light-emitting element whose luminance depends on a drive current and a transistor that drives the light-emitting element in a region where a change in drain current with respect to a change in drain voltage is constant is connected to the transistor. that detects the end of the voltage of the light-emitting element, current of the light emitting element based on the detection result - estimate the luminance when driven at a constant current of the light emitting element voltage characteristics, the light-emitting element in response to the estimated luminance By correcting the luminance change of the light-emitting element by controlling the light emission period or the amount of current flowing to the light-emitting element, the luminance change of the light-emitting element with time change can be achieved with a simple circuit configuration without using a photoconductive element. Can be supplemented.

  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 has a light emitting element whose luminance depends on a drive current, for example, a thin film light emitting element 11, and a detection for detecting a voltage or current of a circuit including the thin film light emitting element 11. The circuit 12 and a correction circuit 13 that corrects the luminance change of the thin film light emitting element 11 based on the detection signal of the detection circuit 12 are provided.

  Here, the thin-film light emitting element 11 is directly connected between a scanning line and a data line wired in a matrix form in one or more sets to constitute a unit pixel (passive matrix), or a driving transistor (pixel transistor). Unit pixels are connected in series to form an active matrix. Examples of the thin film light emitting element 11 include an organic EL element and an inorganic light emitting diode. The organic EL element includes an EL element used as a liquid crystal backlight 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) characteristics of the thin-film light emitting element 11 shift to the high voltage side as shown in FIG. On the other hand, when the thin film light emitting element 11 is driven with a constant current, the voltage (V) and the luminance (L) of the thin film light emitting element 11 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 represents an initial voltage, Lo represents an initial luminance, V represents a voltage after long-time use, and L represents a luminance after long-time use.

  Focusing on the characteristics of the thin film light emitting element 11 described above, in the present invention, the voltage or current of the circuit including the thin film light emitting element 11 is detected by the detection circuit 12, so that the luminance of the thin film light emitting element 11 is increased using the photoconductive element. Even if it does not detect directly, the brightness | luminance of the thin film light emitting element 11 is electrically detected, The detection result is fed back to the thin film light emitting element 11, and the brightness | luminance change accompanying the time-dependent change of the said thin film light emitting element 11 is correct | amended. Is adopted. As the detection circuit 12 for detecting the voltage or current of the circuit including the thin film light emitting element 11, the following configuration can be considered.

  FIG. 4 is a circuit diagram illustrating a configuration of a pixel portion of the passive matrix display device. In FIG. 4, scanning lines 21 and data lines 22 are wired 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 illustrating a configuration of a unit pixel of the active matrix display device. In FIG. 5, 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 (for example, ground) and a second power source (for example, positive power source Vdd). It constitutes a unit pixel. Here, the configuration of the main part of the unit pixel is only shown, and the configuration is not limited to this configuration. In this active matrix type display device, a voltmeter 25 is connected between one end of the thin film light emitting element 11, in this example, the anode end and a reference potential (for example, ground). The voltage of the light emitting element 11 (in this example, the anode voltage) can be directly detected. That is, in this example, the voltmeter 25 corresponds to the detection circuit 12.

  The above two detection methods are methods for detecting the element voltage in order to determine the current-voltage characteristics of the thin-film light emitting element 11. On the other hand, in order to detect the device current in order to determine the current-voltage characteristic of the thin film light emitting device 11, the value of the current flowing through the thin film light emitting device 11 when a voltage is applied to the thin film light emitting device 11 is detected. It ’s fine.

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

  In order 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. As another method for solving the above problem, it is conceivable to decrease the power supply voltage Vdd or increase the cathode potential of the thin film light emitting element 11.

  In the active matrix display device shown in FIG. 5, in order to realize the above-described current detection, as shown in FIG. 8, a circuit in which an ammeter 26 is connected in series to the cathode side of the thin film light emitting element 11 is provided. Try to assemble. Then, the gate voltage VG is set high, the transistor 24 is operated in the linear region, and the current value is read by the ammeter 26, thereby detecting the current value flowing through the thin-film light emitting element 11 (in this example, the cathode current). be able to. That is, in this example, the ammeter 26 corresponds to the detection circuit 12.

  Hereinafter, as a detection method in the detection circuit 12, a method of detecting a current value flowing in the thin film light emitting element 11 (FIG. 8) is used, and a luminance change accompanying a change with time of the thin film light emitting element 11 based on the detected current value. A specific example of correcting the will be described. As described above, when the current detection method is used, a state in which a gate voltage VG that can operate the transistor 24 in a linear region is applied to the transistor 24 connected in series to the thin film light emitting element 11. Done in The operation for detecting the value of the current flowing through the thin film light emitting element 11 is performed during a non-display period, for example, immediately after the system power supply is turned on.

[First example]
FIG. 9 is a schematic configuration diagram illustrating an organic EL display device according to a first specific example using, for example, an organic EL element as a thin-film light emitting element, and shows an example of application to a monochrome organic EL display device. ing. Here, for simplification of the drawing, a pixel array of 6 rows and 37 columns is taken as an example.

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

  In this organic EL element 32, by applying a DC voltage between the first electrode and the second electrode, holes are transferred from the first electrode (anode) through the hole transport layer, and electrons are transferred to the second electrode. Each of the electrodes (cathode) is injected into the light-emitting layer through the electron transport layer, and the injected positive and negative carriers cause the fluorescent molecules in the light-emitting layer to be excited, and light emission is obtained in the relaxation process of the excited molecules. It has become.

  In a pixel circuit including the organic EL element 32, a thin film transistor (TFT) is generally used as a drive transistor for driving the organic EL element 32. The pixel circuit usually includes a plurality of TFTs and a capacitor that holds pixel information (luminance information). However, for simplification of the drawing, only the TFT 33 that is connected in series to the organic EL element 32 and drives the element 32 is representatively shown as a pixel circuit.

  A driving voltage is selectively given to the organic EL element 32 through the TFT 33. Thereby, the organic EL element 32 is driven. Here, if the organic EL element 32 is an element whose luminance changes depending on, for example, a flowing current, the current flowing in the organic EL element 32 of each pixel is controlled by a TFT (not shown) according to the luminance information of each pixel. Will be.

  One end (the cathode end in this example) of the organic EL element 32 is connected in common to all pixels, and is further connected to the input end of the current detection circuit 34. The current detection circuit 34 corresponds to the ammeter 26 of FIG. 8, and in order to determine the current-voltage characteristics of the organic EL element 32, a high gate voltage, specifically to each of the TFTs 33 during the non-display period. An average current flowing in the TFTs 33 of all the pixels is detected in a state where a gate voltage that can operate the TFTs 33 in the linear region is applied.

  The current value detected by the current detection circuit 34 is converted into a voltage value by the current-voltage conversion circuit 35 and output as a 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 (in this example, the organic EL element), 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 and is compared with the reference voltage Vref in the comparator 36.

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

  That is, the current-voltage conversion circuit 35, the comparator 36, and the duty control circuit 37 correct the luminance change of the organic EL element 32, specifically, the luminance change accompanying the change with time, based on the detected current value of the current detection circuit 34. The correction means (corresponding to the correction circuit 13 in FIG. 1) is configured.

Here, when the light emission period ratio of the organic EL element 32 is Duty and the peak luminance is Lpeak, the average luminance Level of the organic EL element 32 is
Lave = Duty / Lpeak
It is represented by

If the current-voltage characteristic of the organic EL element 32 changes with time, only the peak luminance Lpeak is reduced. Therefore, the light emission period ratio Duty of the organic EL element 32 is controlled in accordance with the change of the current-voltage characteristic with the change with time, as shown in FIG. When the voltage of the organic EL element 32 is V and the initial voltage is Vo, the luminance L of the organic EL element 32 is
L ≒ Lo × V / Vo
And the correction is made in accordance with the estimated brightness. Thereby, even if the current-voltage characteristics change with time, the peak luminance Lpeak of the organic EL element 32 can be kept constant, that is, the initial peak luminance value.

  In general, since the current-voltage characteristics of the organic EL element 32 change with deterioration with time, the light emission period ratio Duty of the organic EL element 32 in FIG. In addition, the control range of the light emission period ratio Duty can be set wide by increasing the light emission period ratio Duty according to the change of the current-voltage characteristic with the change with time.

  As described above, in the monochrome 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 organic current is detected based on the detection result. By controlling the light emission period ratio Duty of the EL element 32, it is possible to correct a luminance change accompanying a change with time of the organic EL element 32 with a simple circuit configuration without using a photoconductive element. Therefore, since the peak luminance Lpeak of the organic EL element 32 can be kept constant, the optimum display state can always be maintained without being affected by changes with time.

[Second Embodiment]
FIG. 11 is a schematic configuration diagram showing an organic EL display device according to a second specific example, and shows an example of application to a color organic EL display device.

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

  The current detection circuit 44 detects an average current flowing through the organic EL elements 42R, 42G, and 42B for each color in a state where a gate voltage capable of operating these TFTs in a linear region is applied to each of the TFTs 43R, 43G, and 43B. The detected current value is supplied to the current-voltage conversion circuit 45. The current-voltage conversion circuit 45 has a circuit portion corresponding to each color of RGB, converts the current value for each color detected by the current detection circuit 44 into a voltage value, and converts the voltage value 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 for each color from the current-voltage conversion circuit 45 are set corresponding to the initial luminance for each color, for example. Are compared with the reference voltages Vrefr, Vrefg, and Vrefb, and 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 element 42R, By controlling the ratio of light emission / non-light emission per frame of 42G and 42B, that is, the duty ratio, the luminance change of the organic EL elements 42R, 42G, and 42B accompanying the change with time is corrected.

  In this color-type organic EL display device, in a series of brightness corrections, first, correction processing is performed for R, then correction processing is performed for G, and finally correction processing is performed for B. The correction processing is performed in order. However, the order is not limited to the order of R → G → B, and is arbitrary.

  In the case of an organic EL element, for example, in a liquid crystal cell, each color of RGB is emitted by changing a material of a light emitting layer, unlike using a color filter. For this reason, the change in the voltage (V / Vo) -luminance (L / Lo) characteristics of the organic EL element with time change is different for each of the RGB organic EL elements 42R, 42G, and 42B as shown in FIG. become. Accordingly, when the current-voltage characteristics of the organic EL elements 42R, 42G, and 42B change with time, the peak luminances Lpeak of the organic EL elements 42R, 42G, and 42B vary, and thus the chromaticity balance. Will collapse.

  On the other hand, in the color-type organic EL display device according to this specific example, each current of the circuit including the organic EL elements 42R, 42G, and 42B is detected, and the organic EL elements 42R, 42R, 42, By controlling the light emission period ratio Duty of each of 42G and 42B, the organic EL elements 42R, 42G, etc. can be formed with 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 accompanying the change with time of 42B.

  In addition to this, even if the change in the voltage-luminance characteristics of the organic EL element varies with each color of the organic EL elements 42R, 42G, and 42B due to a change with time, the organic EL element is corrected by the above-described luminance change correction operation for each color. Since the chromaticity balance can be maintained by keeping the peak luminances Lpeak of 42R, 42G, and 42B constant, the display state with the optimal chromaticity balance can always be maintained without being affected by changes over time. Become.

  In the above embodiment, the case where the change in luminance of the thin film light emitting element with the change with time is corrected by controlling the light emission 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. 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 is kept constant by controlling the current value of the thin film light emitting element as in the above case. It is possible to correct as follows.

  Further, 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, but with a certain function L = F (V). It is also possible to calculate the luminance and the light emission period ratio Duty or the current value from the voltage. That is, assuming that the initial luminance and initial voltage are Lo and Vo, respectively, and the luminance and voltage after long-time use are L and V, respectively, as shown in FIG. 3, L / Lo and Vo / V have a correlation. By calculating the luminance and the light emission period ratio Duty or the current value from the voltage with a certain function L = F (V), it is possible to correct the luminance change accompanying the change with time of the thin film light emitting element.

  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, luminance and light emission period ratio Duty or current value is prepared in advance, and this is looked up as shown in FIG. The correction circuit 13 determines the light emission period ratio Duty or the current value of the thin-film light emitting element based on the detection signal of the detection circuit 12 with reference to the lookup table of the memory 14 based on the detection signal of the detection circuit 12. It is also possible to do so.

It is a conceptual diagram which shows the display apparatus which concerns on one Embodiment of this invention. It is a characteristic view which shows the current-voltage characteristic of a light emitting element. It is a characteristic view which shows the voltage-luminance characteristic of a light emitting element. It is a circuit diagram which shows the structure of the pixel part of a passive matrix display device. It is a circuit diagram which shows the structure of the unit pixel of an active matrix type display apparatus. It is a characteristic view which shows the drain voltage-drain current characteristic of a transistor. FIG. 10 is a characteristic diagram showing a relationship when the voltage-luminance characteristic of the light emitting element changes with respect to the drain voltage-drain current characteristic of the transistor. It is a circuit diagram in the case of detecting the electric current which flows into a light emitting element. It is a schematic block diagram which shows the organic electroluminescent display apparatus which concerns on a 1st specific example, and has shown taking the case where it applies to a monochrome system as an example. It is a wave form diagram which shows the relationship between the light emission period by non-light emission period by duty control. It is a schematic block diagram which shows the organic electroluminescence display which concerns on a 2nd specific example, and has shown taking the case where it applies to a color system as an example. It is a characteristic view which shows the voltage-luminance characteristic of the light emitting element for every RGB color. It is a conceptual diagram which shows the display apparatus which concerns on other embodiment of this invention.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 11 ... Thin film light emitting element, 12 ... Detection circuit, 13 ... Correction circuit, 31, 41 ... Organic EL panel, 32, 42R, 42G, 42B ... Organic EL element, 33, 43R, 43G, 43B ... TFT (thin film transistor), 34 , 44 ... current detection circuit, 35, 45 ... current-voltage conversion circuit, 36, 46 ... comparator, 37, 47 ... duty control circuit

Claims (7)

A unit pixel including a light emitting element whose luminance depends on a driving current, and a transistor for driving the light emitting element in a region where a change in the drain current with respect to a change in the drain voltage is constant ;
Detecting means connected to one end of the light emitting element connected to the transistor to detect the voltage at the one end ;
Based on the detection signal of the detection means, the brightness when the light emitting element is driven at a constant current is estimated from the current-voltage characteristics of the light emitting element, and the light emission period of the light emitting element or the light emission corresponding to the estimated brightness And a correction means for controlling the amount of current flowing through the element. The display device according to claim 1, wherein the detection operation of the detection unit and the correction operation of the correction unit are performed for each light emitting element provided corresponding to each emission color of R (red), G (green), and B (blue). . The display device according to claim 1, wherein the detection unit performs the detection operation during a non-display period. The display device according to claim 1, wherein the correction unit estimates and corrects the luminance from the voltage detected by the detection unit based on a correspondence table of luminance and voltage. 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 the electrodes. A light-emitting element whose luminance is dependent on the drive current, the unit pixel including a transistor change in the drain current for driving the light emitting element in a region to be a constant with respect to a change in the drain voltage, of the light emitting element connected to the transistor Detect the voltage at one end
Based on the detection result, from the current-voltage characteristics of the light emitting element, estimate the luminance when driving with a constant current of the light emitting element,
A method for driving a display device, comprising: controlling a light emission period of the light emitting element or an amount of current flowing through the light emitting element corresponding to the estimated luminance. The display according to claim 6, wherein the voltage or current detection operation and the luminance change correction operation are performed for each light emitting element provided corresponding to each emission color of R (red), G (green), and B (blue). Device driving method.

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