JP2007101950A - Active matrix type display device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an active matrix type display device whose image quality is improved by preventing the occurrence of a display fluctuation. <P>SOLUTION: The display device includes a plurality of display pixels arranged in a matrix. The display pixel has a display element 16, a driving transistor 22 which outputs a current quantity corresponding to a control voltage to the display element, a capacitor Cs which is connected between a control terminal and a first terminal of the driving transistor, and holds the control voltage, a switch 24 which is connected between the control terminal and a second terminal of the driving transistor and is on-off controlled by a control signal from a control signal line Sa, and a pixel switch 20 which is on-off controlled by the control signal from the control signal line and captures the video signal from the signal line. After the M-th pixel switch is turned off by the control signal output circuit, the M+1th line pixel switch is turned on after the time lag of ≥0.6 μs. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an active matrix display device in which a display screen is configured by arranging display pixels including display elements such as organic electroluminescence (hereinafter referred to as EL) elements in a matrix.

  2. Description of the Related Art Planar active matrix display devices are widely used as display devices for personal computers, portable information terminals, and televisions. In recent years, as such a flat-type active matrix display device, an organic EL display device using a self-luminous element such as an organic EL element has attracted attention and has been actively researched and developed. This organic EL display device does not require a backlight that obstructs the reduction in thickness and weight, is suitable for moving image reproduction because of its high-speed response, and further has a feature that it can be used even in cold regions because the luminance does not decrease at low temperatures. .

  In general, an organic EL display device includes a plurality of display pixels arranged in a plurality of rows and a plurality of columns and constituting a display screen, a plurality of scanning lines extending along each row of display pixels, and a column of display pixels. A plurality of extended signal lines, a scanning line driving circuit for driving each scanning line, a signal line driving circuit for driving each signal line, and the like are provided (for example, Patent Document 1).

  Each display pixel includes an organic EL element that is a self-light-emitting element and a pixel circuit that supplies a drive current to the organic EL element. Each pixel circuit includes a pixel switch arranged in the vicinity of the intersection of the scanning line and the signal line, a driving transistor configured by a thin film transistor connected in series with an organic EL element between a pair of power supply lines, and a gate control voltage of the driving transistor Has a holding capacity. The pixel switch is turned on in response to the scanning signal supplied from the corresponding scanning line, and takes in the video signal supplied from the corresponding signal line. The gate-to-source potential of the drive transistor corresponding to this video signal is written as a gate control voltage in the storage capacitor and held for a predetermined period. Then, the driving transistor supplies a current amount corresponding to the gate control voltage written in the storage capacitor to the organic EL element to perform a light emitting operation.

An organic EL device has a structure in which a light emitting layer, which is a thin film containing a luminescent organic compound, is sandwiched between a cathode and an anode, and excitons are generated by injecting electrons and holes into the light emitting layer and recombining them. It emits light by light emission generated and generated when the exciton is deactivated. The organic EL element emits light with a luminance corresponding to the amount of supplied current, and a luminance of about 100 to 100,000 cd / m ² can be obtained even with an applied voltage of 10 V or less.
US Pat. No. 6,373,454

  In the display device described above, the display pixels in each column are turned on in the pixel switches of the display pixels in the Mth row, and a video signal is supplied between the gate and source of the drive transistor. A driving method is generally known in which the pixel switch of the display pixel of the (M + 1) th row is turned on at the same time. Thereby, the supply of the video signal to the display pixels in the Mth row is completed, and the supply of the video signals to the display pixels in the M + 1th row is started.

  However, if the pixel switch of the Mth row is turned off and the pixel switch of the M + 1th row is simultaneously turned on, the parasitic switch between the control line and the signal line affects the turnoff of the Mth row pixel switch. The off timing may be delayed. In this case, the pixel switch of the (M + 1) th row is turned on before the pixel switch of the (M) th row is turned off, and a video signal is simultaneously supplied to the drive transistor of the Mth row and the drive transistor of the (M + 1) th row. At this time, an undesired current flows to the organic EL element to the display pixel, and display fluctuation such as display unevenness may occur.

  The present invention has been made in view of the above points, and an object of the present invention is to provide an active matrix display device that prevents display fluctuations and improves display quality.

In order to achieve the above object, an active matrix display device according to an aspect of the present invention includes a plurality of display pixels arranged in a matrix, a plurality of control signal lines provided for each row of the display pixels, A plurality of signal lines provided for each column of display pixels,
Each display pixel is connected between a display element, a drive transistor connected to the display element and outputting a current amount corresponding to a control voltage to the display element, a control terminal of the drive transistor, and a first terminal to hold the control voltage A capacitor formed by a transistor, connected between a control terminal and a second terminal of the drive transistor, and a switch that is turned on and off by a control signal from the control signal line, and the drive transistor formed by a transistor And a pixel switch that is connected between the second terminal and the signal line, and that is controlled to be turned on and off by a control signal from the control signal line and captures a video signal from the signal line,
A control signal output circuit for turning on the pixel switch in the (M + 1) th row after a time difference of 0.6 μs or more after turning off the pixel switch in the Mth row is further provided.

  According to the present invention, after the pixel switch of the Mth row is turned off, the pixel switch of the M + 1th row is turned on after a time difference of 0.6 μs or more, thereby preventing display fluctuations and improving display quality. An active matrix display device can be provided.

Hereinafter, an active matrix organic EL display device according to a first embodiment of the present invention will be described in detail with reference to the drawings.
As shown in FIG. 1, the organic EL display device includes an organic EL panel 10 and a controller 12 that controls the organic EL panel 10.

  The organic EL panel 10 is arranged in a matrix on a light-transmitting insulating substrate 8 such as a glass plate, and is connected to each row of display pixels, each of which is connected to each row of display pixels mx that constitutes a display region 11. A first control signal line Sga (1 to m) and a second control signal line Sgb (1 to m) provided independently for each m lines, and n signal lines X respectively connected to each column of display pixels. (1 to n), a control signal output circuit 14 for sequentially driving the first and second control signal lines Sga and Sgb for each row of display pixels, and a signal line driving circuit 15 for driving the plurality of signal lines X1 to Xn. I have.

  Each display pixel PX includes an organic EL element 16 that is a self-light-emitting element and a pixel circuit 18 that supplies a drive current to the organic EL element. FIG. 2 shows an equivalent circuit of the display pixel PX. The pixel circuit 18 is a current signal type pixel circuit that controls light emission of the organic EL element 16 in accordance with a video signal including a current signal, and includes a pixel switch 20, a drive transistor 22, a first switch 24, a second switch 26, and A holding capacitor Cs is provided. Here, the pixel switch 20, the driving transistor 22, the first switch 24, and the second switch 26 are configured by the same conductivity type, for example, a P-channel type thin film transistor. In this embodiment, the thin film transistors constituting the pixel circuit are all formed in the same process and the same layer structure, and are top gate thin film transistors using polysilicon as a semiconductor layer.

  The drive transistor 22, the second switch 26, and the organic EL element 16 are connected in series between the first voltage power line Vss and the second voltage power line Vdd. The first and second voltage power supply lines Vss and Vdd are set to potentials of 0 V and +10 V, for example. The drive transistor 22 has its first terminal, here the source, connected to the second voltage power supply line Vdd, and the organic EL element 16 has one electrode, here the cathode, connected to the first voltage power supply line Vss. The source of the second switch 26 is connected to the second terminal of the driving transistor 22, here the drain. The second switch 26 has a drain connected to the anode of the organic EL element 16 and a gate connected to the second control signal line Sgb.

  The drive transistor 22 outputs a current having a magnitude corresponding to the video signal to the organic EL element 16. The second switch 26 is ON (conductive state) and OFF (non-conductive state) controlled by the control signal Sb from the second control signal line Sgb, and controls connection / disconnection between the driving transistor 22 and the organic EL element 16. .

  The holding capacitor Cs is connected between the constant potential terminal and the control terminal of the driving transistor 22, here, between the source and gate of the driving transistor 22, and holds the gate control voltage of the driving transistor 22 determined by the video signal. The pixel switch 20 is connected between the corresponding signal line X and the drain of the driving transistor 22, and its gate is connected to the first control signal line Sga. The pixel switch 20 takes in the video signal from the corresponding signal line X in response to the control signal Sa supplied from the first control signal line Sga.

  The first switch 24 is connected between the drain and gate of the driving transistor 22 and the gate thereof is connected to the first control signal line Sga. The first switch 24 is turned on / off according to the control signal Sa from the first control signal line Sga, controls connection / disconnection between the gate and drain of the driving transistor 22 and leaks current from the storage capacitor Cs. To regulate.

  On the other hand, the controller 12 shown in FIG. 1 is formed on a printed circuit board disposed outside the organic EL panel 10 and controls the control signal output circuit 14 and the signal line driving circuit 15. The controller 12 receives a digital video signal and a synchronization signal supplied from the outside, and generates a vertical scanning control signal for controlling the vertical scanning timing and a horizontal scanning control signal for controlling the horizontal scanning timing based on the synchronizing signal. The scanning control signal and the horizontal scanning control signal are supplied to the control signal output circuit 14 and the signal line driving circuit 15, respectively, and the digital video signal is supplied to the signal line driving circuit 15 in synchronization with the horizontal and vertical scanning timings.

  The signal line driving circuit 15 converts the video signals Data1 to Dataan sequentially obtained in each horizontal scanning period under the control of the horizontal scanning control signal into an analog format, and supplies them in parallel to the plurality of signal lines X as current signals. The control signal output circuit 14 includes a shift register, an output buffer, and the like. The control signal output circuit 14 sequentially transfers an externally supplied vertical scanning start pulse to the next stage, and outputs two types of control signals to the display pixels PX in each row via the output buffer. That is, the control signals Sa and Sb are supplied. Accordingly, the first and second control signal lines Sga and Sgb are driven by the control signal Sa and the control signal Sb, respectively, in one horizontal scanning period different from each other.

The operation of the pixel circuit 18 based on the output signals of the control signal output circuit 14 and the signal line drive circuit 15 will be described with reference to the timing chart shown in FIG.
For example, the control signal output circuit 14 generates a pulse having a width corresponding to each horizontal scanning period from the start signal and the clock, and outputs the pulse as the control signal Sa and the control signal Sb.

  The operation of the pixel circuit 18 is divided into a video signal writing operation and a display operation. At time t11 in FIG. 3, the control signal SaM (M = 1, 2,...) For the display pixels in the Mth row is at a level (on potential) at which the pixel switch 20 and the first switch 24 are turned on. Is at a low level, and the control signal SbM is at a level (off potential) at which the second switch 26 is turned off, in this case at a high level. Thereby, the pixel switch 20 and the first switch 24 are turned on (conductive state), the second switch 26 is turned off (non-conductive state), and the video signal writing operation is started.

  In the video signal writing period (t11 to t12) of the M-th row display pixel, the driving transistor 22 is in a diode connection state, and the video signal DataN (N = 1, 2,...) From the corresponding signal line X through the pixel switch 20.・) Is captured. That is, the current flowing between the source and drain of the drive transistor 22 is set to a desired constant current by the signal line X and the pixel circuit 18 connected to the analog conversion circuit which is a constant current source. Then, the gate control voltage of the drive transistor 22 corresponding to this amount of current is written to the storage capacitor Cs.

  Next, at time t12, the control signal SaM becomes high level (off potential), and the pixel switch 20 and the first switch 24 are turned off. Thereby, the video signal writing operation is completed.

  Subsequently, at time t13, the control signal SbM becomes low level, and the second switch 26 is turned on. Thereby, the display operation starts. In the display period, the drive transistor 22 supplies a current amount corresponding to the video signal to the organic EL element 16 by the gate control voltage written in the storage capacitor Cs. Thereby, the organic EL element 16 emits light, and the display operation is started. The organic EL element 16 maintains the display state until the control signal SbM becomes the off potential again after one frame period.

  Similarly to the display pixels in the Mth row, the display pixels in the M + 1th row perform the video signal writing operation and the display operation in accordance with the control signal SaM + 1 and the control signal SbM + 1. That is, at time t21, the pixel switch 20 and the first switch 24 are turned on by the control signal SaM + 1, and the second switch 26 is turned off by the control signal SbM + 1, and the video signal writing operation is started. At time t22, the pixel switch 20 and the first switch 24 are turned off by the control signal SaM + 1, and the video signal writing operation is completed. Subsequently, at time t23, the control signal SbM + 1 becomes a low level, and the second switch 26 is turned on. Thereby, the display operation starts.

  Here, the time point t12 at which the pixel switch 20 in the Mth row is turned off and the time point t21 at which the pixel switch 20 in the M + 1th row is turned on are not set at the same time but are set to have a time difference td. That is, the control signal output circuit 14 is configured to output a control signal for turning on the pixel switch 20 in the (M + 1) th row after the time difference td after turning off the pixel switch 20 in the Mth row. The time difference td is set to 0.6 μs or more, for example, 1 μs.

  In this way, after the pixel switch 20 in the Mth row is turned off, the pixel switch 20 in the M + 1th row is turned on after the time difference td. Even when this occurs, the transition period from on to off of the control signal SaM does not overlap with the on period of the control signal SaM + 1. Therefore, the potential fluctuation due to the capacitive coupling via the signal line is eliminated, and a current having a desired magnitude can be supplied to the organic EL element 16. Thereby, occurrence of display fluctuations such as display unevenness can be reduced.

  The inventor investigated the occurrence of display defects by variously changing the time difference td between the pixel switch 20 in the Mth row turned off and the pixel switch 20 in the M + 1th row turned on. Here, a display panel having a screen size of 3.5 inches and a screen resolution of QVGA was used as the organic EL display device. The survey results are shown in FIG. As can be seen from this figure, when the time difference td was 0 to 0.5 μs, display unevenness, streaks, gradation, etc. occurred in the display image, and the display defect occurrence rate was 100%. On the other hand, when the time difference td is set to 0.6 μs, the display defect occurrence rate is greatly reduced to about 25%. Further, when the time difference td was set to 0.9 μs or more, the display defect occurrence rate was 0%.

  From the above, by setting the time difference td between the control signal Sam off and the control signal Sam + 1 on to 0.6 μs or more, more preferably 0.9 μs or more, display unevenness, streaks, gradation Therefore, an active matrix organic EL display device with improved display quality can be obtained. The time difference td is preferably set to 3 μs or less, although it varies depending on the screen size and other specifications.

  In the above-described embodiment, the pixel switch 20 and the first switch 24 are controlled to be turned on / off by the common first control signal line Sga and the common control signal Sa. However, the present invention is not limited to this, and as shown in FIG. In addition, the pixel switch 20 and the first switch 24 may be separately controlled by the independent first control signal line Sga, the third control signal line Sgc, and the independent control signals Sa and Sc. In this case, the pixel switch 20 and the first switch 24 do not necessarily need to be simultaneously turned on / off, and may be turned on / off at different timings.

  In the embodiment shown in FIG. 5, other configurations are the same as those of the above-described embodiment, and the same reference numerals are given to the same portions, and detailed description thereof is omitted. In the embodiment configured as described above, it is possible to obtain the same operational effects as those of the above-described embodiment.

  In addition, the present invention is not limited to the above-described embodiments as they are, and can be embodied by modifying the constituent elements without departing from the scope of the invention in the implementation stage. In addition, various inventions can be formed by appropriately combining a plurality of constituent elements disclosed in the embodiment. For example, some components may be deleted from all the components shown in the embodiment. Furthermore, constituent elements over different embodiments may be appropriately combined.

  In the above-described embodiment, the case where all the thin film transistors constituting the pixel circuit are formed of the same conductivity type, here, the P channel type is described. However, the present invention is not limited to this, and all the thin film transistors are formed of N channel type thin film transistors. Is also possible. It is also possible to form a pixel circuit with a mixture of thin film transistors of different conductivity types, for example, the pixel switch, the first switch is composed of an N-channel thin film transistor, the drive transistor and the second switch are composed of a P-channel thin film transistor It is.

  Furthermore, the semiconductor layer of the thin film transistor is not limited to polysilicon, but may be composed of amorphous silicon. The self-luminous elements constituting the display pixels are not limited to organic EL elements, and various display elements capable of self-luminance are applicable.

1 is a circuit diagram showing a configuration of an organic EL display device according to an embodiment of the present invention. The figure which shows the equivalent circuit of the display pixel in the said organic EL display apparatus. 3 is a timing chart for explaining the operation of the display pixel shown in FIG. 2. The figure which shows the relationship between the time difference of the display pixel OFF of the display pixel of the M line, and the display pixel ON of the M + 1 line, and a display defect occurrence rate. The figure which shows the equivalent circuit of the display pixel in the organic electroluminescence display which concerns on other embodiment of this invention.

Explanation of symbols

12 ... Controller, 14 ... Control signal output circuit,
15 ... signal line drive circuit, 16 ... organic EL element, 18 ... pixel circuit,
20 ... Pixel switch, 22 ... Drive transistor,
24 ... 1st switch, 26 ... 2nd switch,
PX ... display pixel, Vdd ... first voltage power line, Vss ... second voltage power line,
Sga: first control signal line, Sgb: second control signal line,
Sgc: third control signal line, X: signal line, Cs: holding capacitor.

Claims (5)

A plurality of display pixels arranged in a matrix, a plurality of control signal lines provided for each row of the display pixels, and a plurality of signal lines provided for each column of the display pixels,
Each display pixel is connected between a display element, a drive transistor connected to the display element and outputting a current amount corresponding to a control voltage to the display element, a control terminal of the drive transistor, and a first terminal to hold the control voltage A capacitor formed by a transistor, connected between a control terminal and a second terminal of the drive transistor, and a switch that is turned on and off by a control signal from the control signal line, and the drive transistor formed by a transistor And a pixel switch that is connected between the second terminal and the signal line, and that is controlled to be turned on and off by a control signal from the control signal line and captures a video signal from the signal line,
An active matrix display device further comprising a control signal output circuit for turning on the pixel switch in the (M + 1) th row after a time difference of 0.6 μs or more after turning off the pixel switch in the Mth row .   The active matrix display device according to claim 1, wherein the time difference is set to 0.6 μs or more and 3 μs or less.   The active matrix display device according to claim 1, wherein the time difference is set to 0.9 μs or more.   2. The active matrix display device according to claim 1, wherein the driving transistor and the switch are formed of a thin film transistor using polysilicon for a semiconductor layer.   5. The active matrix display device according to claim 1, wherein the display element is a light-emitting element having an organic light-emitting layer between opposed electrodes.

Images