JP3877049B2 - Image display apparatus and driving method thereof - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an active matrix image display apparatus controlled by a switching element such as a thin film transistor for controlling the light emitting operation of an electro-optical element that emits light by passing a driving current through a light emitting thin film such as an organic semiconductor film, and a driving method thereof.
[0002]
[Prior art]
In recent years, with the advent of the advanced information society, the demand for personal computers, car navigation systems, portable information terminals, information communication devices, or composite products of these has increased. For these products, thin, light, and low power consumption displays are suitable, and liquid crystal display devices or display devices using electro-optic elements such as self-luminous EL (electroluminescence) elements or LED (light emitting diode) elements. It is used.
[0003]
The latter display device using a self-luminous electro-optic element has features such as good visibility, wide viewing angle characteristics, and high-speed response and suitable for video display. It is considered particularly suitable in the future. In particular, the rapid improvement of the luminous efficiency of organic EL elements or organic LED elements (hereinafter collectively referred to as OLEDs) using organic substances as the light emitting layer, and the progress of network technology enabling video communication. Together, the expectations for OLED displays are only rising.
[0004]
In order to increase the power efficiency in the OLED display, active matrix driving by a thin film transistor (hereinafter referred to as TFT) is effective as will be described later. Techniques for manufacturing and driving an OLED display as an active matrix structure are described in, for example, Japanese Patent Laid-Open Nos. 4-328791 and 8-24048 and US Pat. No. 5,555,0066. It is disclosed in the patent publication WO98 / 36407.
[0005]
A typical pixel of the OLED display is one in which the emission luminance of the OLED is controlled by an active element driving circuit composed of two TFTs (switch transistor and driver transistor) and one storage capacitor. In the pixel, n data lines to which image signals are supplied and m scanning lines (gate lines) to which scanning signals are supplied form a matrix of m rows and n columns, and the pixels are arranged in the vicinity of their intersections.
[0006]
In order to drive the pixels, a scanning signal (gate voltage) is sequentially applied to the m rows of gate lines to turn on the switching transistors, complete one vertical scan within one frame period Tf, and then again to the first row. A turn-on voltage is applied to the gate line.
[0007]
In this driving scheme, the time during which the turn-on voltage is applied to one gate line is Tf / m or less. Generally, about 1/60 second is used as the value of one frame period Tf. When a turn-on voltage is applied to a certain gate line, all the switching transistors connected to that gate line are turned on, and an image signal (data voltage) is simultaneously applied to the n columns of data lines in synchronization therewith. The This is called a so-called line-sequential scanning method and is generally used in active matrix liquid crystal.
[0008]
The data voltage is stored in the storage capacitor (capacitor) while the turn-on voltage is applied to the gate line, and is maintained at these values for one frame period. The voltage value of the storage capacitor defines the gate voltage of the driver transistor. Therefore, the current value flowing through the driver transistor is controlled, and a constant current flows through the OLED to cause light emission. When a voltage is applied to the OLED, the response time until light emission starts is usually 1 μs or less, and it is possible to follow a fast moving image (moving image).
[0009]
In the active matrix driving, high efficiency is realized by emitting light over one frame period. The difference is clear when comparing this with the efficiency of the simple matrix driving in which the OLED diode electrode is directly connected to the vertical scanning line and the horizontal scanning line without providing the TFT.
[0010]
In simple matrix driving, current flows in the OLED only during the period when the vertical scanning line is selected. Therefore, in order to obtain the same luminance as light emission in one frame period only by light emission in that short period, compared with active matrix drive. The light emission luminance approximately the number of vertical scanning lines is required. In order to do so, the drive voltage and drive current must be increased, and it is inevitable that power consumption loss such as heat generation will increase and power efficiency will decrease.
[0011]
Thus, active matrix driving is considered to be superior to simple matrix driving from the viewpoint of low power consumption.
[0012]
[Problems to be solved by the invention]
The prior art has been considered that OLED is suitable for moving images because of its high-speed response. However, the active matrix driving of the OLED according to the prior art is the same as the driving method of the liquid crystal display (LCD), and the pixels are displayed over one frame period, that is, a hold type display method in which the OLED emits light.
[0013]
As described in “Electronic Information and Communication Engineers Technical Research Report” EID96-34, pages 19 to 26 (1996, June), LCDs are displayed at the time of video display due to the hold-type display method. The phenomenon that the edge of the animal body is blurred is inevitable.
[0014]
The problem of moving image edge blurring has been pointed out for LCDs, but the cause is due to hold display. Therefore, when the OLED is hold-displayed by active matrix driving, the edge blur of the moving image becomes a problem as well.
[0015]
As described above, the conventional technique has a problem that image quality deteriorates because no consideration is given to edge blurring during moving image display when an electro-optic element such as an OLED is driven in an active matrix.
[0016]
SUMMARY OF THE INVENTION An object of the present invention is to provide an image display apparatus capable of suppressing edge blur at the time of moving image display and improving image quality when driving an electro-optic element in an active matrix, and a driving method thereof.
[0017]
[Means for Solving the Problems]
A feature of the present invention is that, in order to display one image, a pixel is driven by forming an extinction period in which the electro-optic element is extinguished after scanning a plurality of gate lines. In other words, the present invention is to drive the pixels by forming an extinction period in which the electro-optical element is extinguished between one frame and the next one frame.
[0018]
According to a preferred embodiment of the present invention, a pixel is driven by forming a quenching period in which the electro-optic element is quenched within one frame period for displaying one image.
[0019]
In the present invention, in order to display one image, a pixel is driven by forming an extinction period in which the electro-optic element is extinguished after scanning a plurality of gate lines. Integration is lost and blurring of edges, that is, the display characteristics of moving images is greatly improved.
[0020]
DETAILED DESCRIPTION OF THE INVENTION
Embodiments of the present invention will be described with reference to the drawings. First, the configuration of the image display device will be described, and then the driving method will be described.
[0021]
FIG. 1 is a block diagram schematically showing the overall layout of the image display device 1, and FIG. 2 is an equivalent circuit diagram of an active matrix configured in the display unit of FIG.
[0022]
In FIG. 1, the image display device 1 includes a display unit 2 at a substantially central portion of a substrate 6. A data driving circuit 3 that supplies an image signal to the data line 7 is provided above the display unit 2, and a scanning driving circuit 4 that supplies a scanning signal (gate voltage) to the gate line 8 is provided on the left side. ing. On the right side, a current supply drive circuit 5 is provided. These drive circuits 3, 4, and 5 include a shift register circuit, a level shifter circuit, an analog switch circuit, and the like that are configured by complementary circuits composed of N-channel and P-channel TFTs.
[0023]
Similar to the active matrix of the liquid crystal display device, the image display device 1 includes a plurality of gate lines 8 and a plurality of data lines 7 extending in a direction intersecting the extending direction of the gate lines 8 on a substrate 6. Is provided. As shown in FIG. 2, pixels 20 are arranged in a matrix at the intersections of the gate lines 8 (G1, G2,..., Gm) and the data lines 7 (D1, D2,..., Dn).
[0024]
In the pixel 20, as shown in an enlarged view in FIG. 3, the gate electrode of the switch transistor 21 made of an N-channel TFT is connected to the gate line 8, and one of the source electrode and drain electrode of the switch transistor 21 is connected to the data line 7. The other is connected to one end of the storage capacitor 23. One end of the storage capacitor 23 is also connected to the gate electrode of the driver transistor 22 made of an N-channel TFT.
[0025]
The source electrode of the driver transistor 22 is connected to the common potential line 9 extending in the same direction as the data line 7, and the drain electrode is connected to one electrode of the OLED 24. The other electrode of the OLED 24 is connected to the current supply line 10 common to all the pixels 20 and is kept at the potential Va. The OLED 24 has a structure in which an anode is usually formed of a transparent electrode, and light emitted from the OLED layer is extracted outside through a glass substrate on which a TFT is formed.
[0026]
In this configuration, when the switch transistor 21 is turned on by a scanning signal applied to the gate line 8 (G1, G2,..., Gm) , an image signal is written from the data line 7 to the storage capacitor 23 via the switch transistor 21. . Therefore, the gate electrode of the driver transistor 22 is held at a potential corresponding to the image signal by the storage capacitor 23 even when the switch transistor 21 is turned off.
[0027]
The driver transistor 22 is kept in the driving state in the source grounding mode having excellent constant current characteristics, and the current from the current supply line 10 flows to the OLED 24. The OLED 24 is maintained in a light emitting state. The light emission luminance at this time depends on the image data written in the storage capacitor 23. The light emission of the OLED 24 is stopped by turning off the driver transistor 22.
[0028]
Next, a method for driving the image display apparatus will be described with reference to FIGS.
[0029]
FIG. 4 shows a configuration of a driving device for driving the image display device according to the present invention.
[0030]
4, the same reference numerals as those in FIG. 1 denote equivalents, and a timing control signal (clock signal) is given to the scanning drive circuit 4 and the data drive circuit 3 from the display controller 11. The data driving circuit 3 also receives an image signal from the display controller 11.
[0031]
The timing (clock frequency) of the display controller 11 is adjusted by the timing adjustment circuit 12. The timing adjustment circuit 12 sets the clock frequency that is four times the basic frequency of one frame. As a result, the display controller 11 quadruples the data reading from the image memory 13 and sets the timing of the frame start control signal of the shift register in both the drive circuits 3 and 4 to t = 0, Tf / 4, Tf, 5Tf / 4. ...
[0032]
In this configuration, gate voltages VG1, VG2,..., VGm for sequentially turning on the switch transistors 21 are applied to the gate lines G1, G2,. The gate voltages VG1, VG2,..., VGm change from the voltage value (low voltage level) VGL to the voltage value (high voltage level) VGH.
[0033]
On the other hand, data voltages VD1, VD2,..., VDn of image signals are applied to the data lines D1, D2,..., Dn from the data driving circuit 3 in synchronization with the gate voltages VG1, VG2,. The image signal voltages VD1, VD2,..., VDn are set to values between the voltage value (high voltage level) VDH and the voltage value (low voltage level) VDL. The voltage value VDL is usually equal to or lower than the voltage of the common potential line 9. The voltage Va of the current supply line 10 and the voltage of the common potential line 9 are kept constant.
[0034]
The driving is performed in this way, and this driving method is line sequential scanning similar to the conventional technique.
[0035]
In the present invention, the period required for scanning one screen (one image) is shortened to 1/4 of one frame period Tf. Therefore, the selection time per gate line 8 is as short as Tf / 4m. When the scanning of one screen is completed and the gate line G1 is next selected, the data voltages VD1, VD2,..., VDn of the voltage value VDL for turning off the driver transistor 22 are all data lines D1, D2, ..., applied to Dn.
[0036]
In such a voltage scheme, about 1/4 of one frame period is a light emission period, and the remaining 3/4 is an extinction period (non-light emission period). In order to prevent the effective light emission time of the OLED 24 from being shortened and the display image from becoming dark, the peak value of the data voltage is set to be four times the current. Since the value of one frame period Tf is about 16 ms, the period of light emission is about 4 ms. However, because of the high-speed response of the OLED 24, light can be emitted over almost all of this period.
[0037]
Although the image display device is driven in this way, the fact that edge blur of a moving image can be suppressed will be described.
[0038]
First, the occurrence of edge blurring of a moving image will be described with reference to FIG. 6 for easy understanding.
[0039]
As shown in FIG. 6A, consider an image in which a black rectangle moves as a moving image at a constant speed in the arrow direction from the left to the right in the drawing. When this moving image is displayed in a hold type, the display content is rewritten every one frame interval, and the display content is held for one frame period. Focus on expanding.
[0040]
FIG. 6B schematically shows the time change of the enlarged portion for each Tf. As shown in FIG. 6B, the rectangular edge portion is displayed while moving in a staircase pattern over time. FIG. 6B shows an example in which the edge moves 4 pixels per frame.
[0041]
The user's eyes who see this display screen continuously move his / her line of sight following the moving image as indicated by an arrow A in FIG. Since the white background is also recognized during the movement of the line of sight, the luminance signal of the moving image perceived by the user is an integrated value of the white signal and the black signal. That is, the black rectangular edge is blurred.
[0042]
On the other hand, FIG. 7 shows the appearance of an image by driving according to the present invention for the pixels in one row of FIG. 6B.
[0043]
In FIG. 7, for example, the light is extinguished from time t = t0 + Tf / 4 to t = t0 + Tf. However, when the line of sight moves during this extinction period, the white background is not integrated, so that the blurring of the edge, that is, the moving image display characteristic is It will be greatly improved.
[0044]
In the above-described embodiment, the ratio of the light emission period to the quenching period is set to 1: 3. However, since there is no image quality deterioration in the display characteristics of the CRT that observes the afterglow (<˜3 ms) of the phosphor, the timing adjustment is performed. The circuit 12 can further increase the effect of the present invention by further shortening the light emission period.
[0045]
FIG. 8 shows another embodiment of the present invention.
[0046]
8 is different from the embodiment of FIG. 4 in that a current supply line driving circuit 15 is provided and placed under the control of the display controller 11. Each current supply line 10 is provided with a switch 16 for switching the supply voltage of the current supply line 10 in conjunction with the gate voltage for extinction as shown in FIG.
[0047]
FIG. 10 shows a pixel matrix diagram of the display unit 2. 10 is different from FIG. 2 in that current supply lines A1, A2,... Am in which anode electrodes of the OLED 24 are bundled for each row are provided, and voltages VA1, VA2,. That is, VAm is not constant but a plurality of values.
[0048]
The operation of this configuration will be described with reference to the time chart shown in FIG. Also in this embodiment, providing the light emission period and the extinction period is the same as the embodiment shown in FIG.
[0049]
After the end of the light emission period of one frame, a voltage for once turning on the driver transistor 22 in the non-saturation region is applied at the timing of reselecting the gate line 8, and at the same time, the current supply lines A1, A2,. The voltages VA1, VA2,... VAm applied to Am are lowered to the low voltage level VAL. The value of the low voltage level VAL is set lower than the voltage level of the common potential line 9.
[0050]
When the voltages VA1, VA2,... VAm of the current supply lines A1, A2,... Am are set to the low voltage level VAL, the potential of the pixel electrode of the OLED 24 becomes substantially the voltage level of the common potential line 9. This is the reverse of the direction of the bias when emitting light. At this time, if the driver transistor 22 is turned off, this reverse bias application state is maintained over the extinction period. Such voltage application can be realized by connecting the current supply line 10 in a stripe shape in parallel with the gate line 8.
[0051]
In the OLED 24, when a forward bias DC is continuously applied, a space charge or the like is gradually generated and the luminance is lowered. However, if a reverse bias is applied as in the present embodiment, the space charge generation can be prevented, and thus a long lifetime is achieved. Can be.
[0052]
The formation of the stripe-shaped current supply line 10 in the embodiment shown in FIGS. 8 and 10 will be described with reference to FIGS. FIG. 12 shows a planar structure of the pixel portion of the image display device 1, and FIG. 13 shows a cross-sectional structure taken along line AA 'of FIG.
[0053]
An island-like silicon film for forming the switch transistor 21 and the driver transistor 22 is formed on the glass substrate 6, and a gate insulating film is formed on the surface thereof. On the gate insulating film, a gate electrode, a gate line 8, and an electrode for a storage capacitor 23 are formed, and then a source / drain region is formed in a self-aligned manner on the gate electrode. Thereafter, the first interlayer insulating film 30 is provided, and the data line 7, the common potential line 9, and the electrode for the storage capacitor 23 are formed through the contact holes.
[0054]
Furthermore, after providing the cathode 24K and the organic layer 24O of the OLED 24 that is the pixel electrode through the contact hole of the second interlayer insulating film 31, the transparent anode 24A that is the counter electrode is connected to this (connected cover) Current supply line 10 is provided. The current supply line 10 extends in the row direction, that is, the direction in which the gate line 8 extends.
[0055]
The OLED light emitting device 24 includes a cathode 24K made of a metal film such as lithium-containing aluminum or calcium connected on a metal layer connected to the drain of the driver transistor 22, an organic semiconductor layer 24O, and a transparent anode 24A made of an indium-containing oxide film. It has a laminated structure.
[0056]
FIG. 14 shows another example of driving in the embodiment shown in FIG. 14 clearly shows that the drive voltage waveform has a light emission period and a quenching period of 1: 3, and that the voltages VA1, VA2,... VAm of the current supply line 10 are in the quenching period. It is the same that the low voltage level VAL is inside.
[0057]
In the data drive voltage according to the form of FIG. 14, the voltage level when turning off the driver transistor 22 from the application of the turn-on voltage of the driver transistor 22 synchronized with the gate reselection pulse is made lower than VDL. 9 or lower than the potential of the pixel electrode.
[0058]
If the voltage of the gate line 8 is set to a non-selected state in this state, the gate voltage of the driver transistor 22 is kept lower than the source / drain voltage. That is, the driver transistor 22 driven with a positive gate voltage at the time of light emission is applied with a negative gate voltage at the time of extinction, and a characteristic shift caused by charge injection into the gate insulating film of the transistor and a display accompanying it. Image quality deterioration can be prevented.
[0059]
As described above, the image display apparatus of the present invention drives the pixels by forming a quenching period in which the electro-optic element is quenched after scanning a plurality of gate lines in order to display one image. When the line of sight moves during the period, the integration of the white background is lost, so the blurring of the edge, that is, the display characteristics of the moving image is greatly improved.
[0060]
In the above-described embodiment, since the extinction scan is performed after the light emission scan is completed, the moving image display characteristics can be improved without changing the configuration of pixels generally used for active matrix driving.
[0061]
Furthermore, since a reverse bias is applied to the OLED 24, space charge generation can be prevented, so that a long life can be obtained and an image display device with little display luminance degradation can be obtained.
[0062]
The present invention is not limited to the above-described embodiment. For example, the present invention can be applied to a structure in which the driver transistor 22 is a P-channel type and light is extracted from the substrate 6 side as shown in FIG. Of course.
[0063]
In the above-described embodiment, the extinction period in which the electro-optic element is extinguished is formed within one frame period for displaying one image. In short, in order to display one image, the electric light is scanned after scanning a plurality of gate lines. It is obvious that a quenching period for quenching the optical element may be formed.
[0064]
【The invention's effect】
As described above, in the present invention, the pixel is driven by forming an extinction period in which the electro-optic element is extinguished after scanning a plurality of gate lines in order to display one image. When moving the line of sight, the integration of the white background is eliminated, so that the blurring of the edge, that is, the display characteristics of the moving image can be greatly improved.
[Brief description of the drawings]
FIG. 1 is a block diagram illustrating an example of an image display device of the present invention.
FIG. 2 is an equivalent circuit diagram of an active matrix.
FIG. 3 is an example of an active matrix driving pixel;
FIG. 4 is a block diagram showing an embodiment of the present invention.
FIG. 5 is a time chart for explaining a driving operation of the present invention.
FIG. 6 is a diagram for explaining edge blur of a moving image in conventional driving.
FIG. 7 is a diagram for explaining edge blurring of a moving image according to the present invention.
FIG. 8 is a block diagram showing another embodiment of the present invention.
FIG. 9 is a diagram for explaining the configuration of FIG. 8;
FIG. 10 is an equivalent circuit diagram of an active matrix in another embodiment of the present invention.
FIG. 11 is a time chart for explaining a driving operation of another embodiment of the present invention.
FIG. 12 is a diagram illustrating a planar structure of a pixel portion of an image display device according to the present invention.
FIG. 13 is a diagram for explaining a cross-sectional structure of a pixel portion of an image display device according to the present invention.
FIG. 14 is a time chart for explaining another driving operation in another embodiment of the present invention.
FIG. 15 is a diagram illustrating another cross-sectional structure example of a pixel portion of an image display device to which the present invention is applied.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 ... Image display apparatus, 2 ... Display part, 3 ... Data drive circuit, 4 ... Scan drive circuit, 5 ... Current supply drive circuit, 6 ... Substrate, 7 ... Data line, 8 ... Gate line, 9 ... Common potential line, DESCRIPTION OF SYMBOLS 10 ... Current supply line, 20 ... Pixel, 21 ... Switch transistor, 22 ... Driver transistor, 23 ... Storage capacity, 24 ... OLED light emitting element.

Claims (6)

An image display device comprising a plurality of gate lines to which scanning signals are supplied and a plurality of data lines to which image signals are supplied crossing each other and formed in a matrix, and comprising pixels including electro-optic elements and thin film transistors. The pixel includes a first thin film transistor to which a scanning signal is supplied through the gate line, a storage capacitor for holding an image signal supplied from the data line through the first thin film transistor, and a storage capacitor. An N-channel second thin film transistor to which the image signal is supplied, and the pixel electrode and the counter electrode when the pixel electrode is electrically connected to the common potential line through the second thin film transistor. An electro-optic element that emits light by a driving current flowing between them,
After supplying the scanning signal to the plurality of gate lines to display one image, the scanning signal is supplied to the plurality of gate lines and the negative image signal is supplied to the plurality of data lines. An image display device, wherein the pixel is driven by forming a quenching period for quenching the electro-optic element, and a negative voltage is applied to the gate of the second thin film transistor during the quenching period. An image display device that displays a moving image including a plurality of gates to which scanning signals are supplied and a plurality of data lines to which image signals are supplied intersects to form a matrix and includes pixels including electro-optic elements and thin film transistors The pixel includes a first thin film transistor to which a scanning signal is supplied via the gate line, a storage capacitor for holding an image signal supplied from the data line via the first thin film transistor, An N-channel second thin film transistor to which the image signal held by the storage capacitor is supplied and the pixel electrode when the pixel electrode is electrically connected to a common potential line through the second thin film transistor And an electro-optic element that emits light by a driving current flowing between the counter electrode and the counter electrode,
An extinction period for extinguishing the electro-optic element is formed between one frame period for displaying one image and one frame period for displaying the next one image. In the extinction period, the scanning signal is applied to the plurality of gate lines. And the pixel is driven so that a negative voltage extinction image signal of the electro-optic element is supplied to the plurality of data lines in synchronization with the scanning signal. An image display device, wherein a negative voltage is applied to the gate of the second thin film transistor during the period.   3. The method according to claim 1, wherein the gate line, the data line, the first and second thin film transistors, the storage capacitor, and the electro-optic element are mounted on the same substrate. Image display device. A plurality of data lines to which image signals are supplied intersect with a plurality of gate lines to which scanning signals are supplied to form a matrix, and include a pixel including an electro-optic element and a thin film transistor, and the pixels include the gate lines. A first thin film transistor to which a scanning signal is supplied via, a storage capacitor for holding an image signal supplied from the data line via the first thin film transistor, and the image signal held by the storage capacitor is supplied a second thin film transistor of N channel type that is, light emission by the driving current flowing between the pixel electrode and the counter electrode when the pixel electrode is electrically connected to the common potential line through said second thin film transistor In the driving method of the image display device comprising the electro-optic element
After supplying the scanning signal to the plurality of gate lines to display one image, the scanning signal is supplied to the plurality of gate lines and the negative image signal is supplied to the plurality of data lines. A driving method of an image display device, wherein the pixel is driven by forming a quenching period for quenching the electro-optic element, and a negative voltage is applied to the gate of the second thin film transistor during the quenching period. . A plurality of data lines to which image signals are supplied intersect with a plurality of gate lines to which scanning signals are supplied to form a matrix, and include a pixel including an electro-optic element and a thin film transistor, and the pixels include the gate lines. A first thin film transistor to which a scanning signal is supplied via, a storage capacitor for holding an image signal supplied from the data line via the first thin film transistor, and the image signal held by the storage capacitor is supplied a second thin film transistor of N channel type that is, light emission by the driving current flowing between the pixel electrode and the counter electrode when the pixel electrode is electrically connected to the common potential line through said second thin film transistor In the driving method of the image display device comprising the electro-optic element
An extinction period for extinguishing the electro-optic element is formed within one frame period for displaying one image. In the extinction period, the scanning signal is supplied to the plurality of gate lines and the plurality of the scanning signals are synchronized with the scanning signal. A negative voltage extinction image signal of the electro-optic element is supplied to the data line to drive the pixel, and a negative voltage is applied to the gate of the second thin film transistor during the extinction period. Method for driving an image display device.   6. The method for driving an image display device according to claim 4, wherein a reverse bias voltage is applied to the quenched electro-optical element.

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