JP2006251455A - Active matrix type display device and method for driving the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To suppress a persistence phenomenon and to enhance a display grade in an active matrix type display device. <P>SOLUTION: A TFT 220 for precharge is turned on to make gate potential and source potential of a TFT 214 to the same potential. Next, after turning the TFT 220 for precharge off, a high-level retention volume control signal SC is applied to a retention volume line 217. By a capacity coupling effect of retention volume 218, potential of a gate of the TFT 214 for drive rises. Thus, the gate potential of the TFT 214 for drive becomes higher than its source potential. When a hole is trapped to a gate insulating film of the TFT 214 for drive by writing of a display signal D in the previous time, the hole is extracted from the gate insulating film to a source or a drain. Thus, electric characteristics of the TFT 214 for drive are initialized. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an active matrix display device and a driving method thereof.

  In recent years, an organic EL display device using an organic electroluminescent device (hereinafter referred to as “organic EL device”) has been developed as a display device that replaces a CRT or LCD. In particular, an active matrix organic EL display device having a thin film transistor (hereinafter referred to as “TFT”) as a switching element for driving the organic EL element has been developed.

  Hereinafter, an active matrix organic EL display device will be described with reference to the drawings. FIG. 5 is an equivalent circuit diagram of the organic EL display device. FIG. 5 shows only one display pixel 210 from among a plurality of display pixels arranged in a matrix on the display panel. An N-channel pixel selection TFT 213 is disposed in the vicinity of the intersection of the pixel selection signal line 211 extending in the row direction and the display signal line 212 extending in the column direction. The gate of the pixel selection TFT 213 is connected to the pixel selection signal line 211, and the drain thereof is connected to the display signal line 212. A high-level pixel selection signal G output from the vertical drive circuit 301 is applied to the pixel selection signal line 211, and the pixel selection TFT 213 is turned on accordingly. A display signal D is output from the horizontal drive circuit 302 to the display signal line 212.

  The source of the pixel selection TFT 213 is connected to the gate of a P-channel type driving TFT 214. A power supply line 223 that supplies a positive power supply voltage PVdd is connected to the source of the driving transistor 214. The drain of the driving transistor 214 is connected to the anode of the organic EL element 216. A negative power supply voltage CV is supplied to the cathode of the organic EL element 216.

  A storage capacitor 218 is connected between the gate of the driving TFT 214 and the storage capacitor line 217. The storage capacitor line 217 is fixed at a constant potential. The storage capacitor 218 holds the display signal D applied to the gate of the driving TFT 214 through the pixel selection TFT 213 for one horizontal period.

  Next, the operation of the above-described organic EL display device will be described. When the high-level pixel selection signal G is applied to the pixel selection signal line 211 over one horizontal period, the pixel selection TFT 213 is turned on. Then, the display signal D output from the display signal line 212 is applied to the gate of the driving TFT 214 through the pixel selection TFT 213 and held by the holding capacitor 218. That is, the display signal D is written to the display pixel 210.

  When the conductance of the driving TFT 214 changes according to the display signal D applied to the gate of the driving TFT 214 and the driving transistor 214 is turned on, a current corresponding to the conductance is driven. The organic EL element 216 is supplied to the organic EL element 216 through the TFT 214, and the organic EL element 216 is lit at a luminance corresponding to the organic EL element 216. On the other hand, when the driving TFT 214 is turned off in accordance with the display signal D supplied to the gate, no current flows through the driving TFT 214, so that the organic EL element 216 is turned off.

  A desired image can be displayed on the entire display panel by performing the above-described operation on the display pixels 210 in all rows over one field period.

In addition, the following patent documents are mentioned as technical documents relevant to the present application.
JP 2004-341435 A

  However, in the above-described organic EL display device, an afterimage due to light emission of the organic EL element 216 may be generated in a part of the display panel, which causes a problem that display quality deteriorates. For the same display pixel, depending on the conduction state (on state or off state) of the driving TFT 214 when the display signal D was written last time, the current state of the driving TFT 214 when it was written this time This is because a current having a current value different from that expected in accordance with the display signal D flows. That is, there is a phenomenon that the current flowing through the driving TFT 214 has hysteresis. This phenomenon is particularly prominent when the display signal D is an intermediate level signal between a high level and a low level.

  According to the study of the present invention, this hysteresis phenomenon is caused by the carrier (hole) flowing in the driving TFT 214 being trapped in the gate insulating film at the time of the previous writing of the display signal D, and the trapped carrier being used for driving. This is probably because the threshold value of the TFT 214 is changed.

  Accordingly, the present invention provides an active matrix type organic EL display device which suppresses the afterimage of the display panel as described above and improves display quality.

  The present invention includes a plurality of display pixels arranged in a matrix, each display pixel being turned on in response to a pixel selection signal, a light emitting element, and a display signal applied through the pixel selection transistor. And a driving transistor for driving the light emitting element, connected between a gate of the driving transistor and a holding capacitor line, holding the display signal, and turned on in response to a precharge signal. A precharging transistor that short-circuits the source and gate of the transistor for driving, and after the precharging transistor is turned off, by applying a storage capacitor control signal that raises the potential of the storage capacitor line, The gate potential of the transistor is made higher than the source potential.

  According to this configuration, since carriers trapped in the gate insulating film of the driving transistor can be extracted to the source or drain, before writing the display signal to the display pixel, the electrical characteristics of the driving transistor (particularly, Threshold voltage) can be restored to the original state. As a result, a current having an appropriate current value corresponding to the display signal always flows through the driving transistor, and the afterimage phenomenon of the display panel can be suppressed.

The driving method of the active matrix display device of the present invention includes a plurality of display pixels arranged in a matrix, each display pixel being turned on in response to a pixel selection signal, a light emitting element, A driving transistor for driving the light emitting element in accordance with a display signal applied through the pixel selection transistor; a storage capacitor connected between the gate of the driving transistor and a storage capacitor line; In a driving method of an active matrix display device comprising: a precharging transistor that is turned on in response to a precharge signal and short-circuits a source and a gate of the driving transistor;
In response to a precharge signal, the precharge transistor is turned on to short-circuit the source and gate of the drive transistor,
After turning off the precharge transistor, by applying a storage capacitor control signal that raises the potential of the storage capacitor line, the gate potential of the driving transistor is made higher than the source potential,
Thereafter, the pixel selection transistor is turned on in accordance with the pixel selection signal, and the display signal is applied to the gate of the driving transistor through the pixel selection transistor.

  According to the present invention, in an active matrix display device, afterimages of the display panel can be suppressed and display quality can be improved.

  Next, an active matrix organic EL display device and a driving method thereof according to the first embodiment of the present invention will be described with reference to the drawings. FIG. 1 is an equivalent circuit diagram of the organic EL display device. In FIG. 1, only one display pixel 210A is shown from the plurality of display pixels arranged in a matrix on the display panel. In FIG. 1, the same components as those in FIG. 5 are denoted by the same reference numerals, and the description thereof is omitted.

  In this organic EL display device, a precharge TFT 220 is connected between the source and gate of the drive TFT 214 in the display pixel 210A. The gate of the precharge TFT 220 is connected to the precharge signal line 221. Since the precharge signal line 221 is supplied with the precharge signal PCG, the precharge TFT 220 is switched according to the precharge signal PCG. When the precharging TFT 220 is turned on, the source and gate of the driving TFT 214 are short-circuited, and when the precharging TFT 220 is turned off, the source and gate of the driving TFT 214 are electrically insulated.

  The storage capacitor line 217 is not supplied with a fixed potential but is supplied with a storage capacitor control signal SC that becomes a high level during a predetermined period to be described later. The precharge signal PCG and the storage capacitor control signal SC are supplied from the control LSI 400 outside the display panel, but the pixel selection signal G output from the vertical drive circuit 301 is used as in the second embodiment to be described later. It can also be used.

  Next, a method for driving the above-described organic EL display device will be described with reference to the drawings. FIG. 2 is a timing diagram illustrating a method for driving the display device according to the present embodiment.

  As shown in FIG. 2, first, a high-level precharge signal PCG is applied to the precharge signal line 221. Then, the precharging TFT 220 is turned on, and the gate potential and the source potential of the driving TFT 214 become the same potential. Next, after the precharge signal PCG falls to the low level and the precharge TFT 220 is turned off, the high level storage capacitor control signal SC is applied to the storage capacitor line 217.

  Then, due to the capacitive coupling effect of the storage capacitor 218, the potential of the gate of the driving TFT 214 rises according to a voltage change ΔV (for example, about 10 V) from the low level to the high level of the storage capacitor control signal SC. As a result, the gate potential of the driving TFT 214 becomes higher than its source potential. At this time, if carriers (holes) are trapped in the gate insulating film of the driving TFT 214 by the previous writing of the display signal D, the carriers (holes) are tunneled by an electric field from the gate to the source or drain. A current is drawn from the gate insulating film to the source or drain. As a result, the electrical characteristics of the driving TFT 214 are initialized.

  Thereafter, a high-level pixel selection signal G is output from the vertical drive circuit 301, and the pixel selection TFT 213 is turned on for one horizontal period in response thereto. Then, during this one horizontal period, the display signal D is output from the horizontal drive circuit 302 to the display signal line 212, and this display signal D is applied to the gate of the drive TFT 214 through the pixel selection TFT 213 and the storage capacitor. 218. Then, a current corresponding to the display signal D is supplied from the driving TFT 214 to the organic EL element 216, and the organic EL element 216 emits light.

  As described above, according to the present embodiment, before writing the display signal D to the display pixel 210A, the carriers (holes) in the gate insulating film of the driving TFT 214 are extracted and the electrical characteristics thereof are initialized. The afterimage phenomenon of the display panel can be suppressed and the display quality can be improved.

  Next, a driving method of the display device according to the second embodiment of the present invention will be described with reference to the drawings. FIG. 3 is an equivalent circuit diagram of an organic EL display device according to the second embodiment of the present invention. In FIG. 3, among a plurality of display pixels arranged in a matrix, three display pixels, that is, a display pixel 210A in the m-th row in a certain column, and a display pixel in the (m−1) -th row in the same column. 210B, the display pixel 210C in the (m-2) th row of the same column is shown.

  In the organic EL display device of this embodiment, the pixel selection signal G output from the vertical drive circuit 301 is used for the precharge signal PCG and the storage capacitor control signal SC. That is, for the display pixel 210A in the m-th row, the pixel selection signal Gm-2 supplied to the display pixel 210C in the (m-2) -th row is used as the precharge signal PCGm, and (m− 1) The pixel selection signal Gm-1 supplied to the display pixel 210B in the row is used. Therefore, the m-th row precharge signal line 211 is connected to the m-th row pixel selection signal line 211 via the first wiring 31, and the m-th row storage capacitor line 217 is connected to the second wiring. 32 is connected to the pixel selection signal line 211 of the (m−1) th row. The display pixel 210B in the (m-1) th row and the display pixel 210C in the (m-2) th row are configured in the same manner.

  With this configuration, the organic EL display device of this embodiment operates equivalent to that of the first embodiment. FIG. 4 is a timing diagram illustrating a method for driving the organic EL display device according to the present embodiment. Here, the display pixel 210A in the m-th row will be described as an example, but the same applies to the display pixels in other rows.

  First, when the pixel selection signal Gm-2 in the (m-2) -th row rises to a high level, a high-level precharge signal PCGm is applied to the precharge signal line 221. Then, the precharging TFT 220 is turned on, and the gate potential and the source potential of the driving TFT 214 are set to the same potential. Next, when the pixel selection signal Gm-2 falls to the low level, the precharge signal PCGm also falls to the low level, and the precharging TFT 220 is turned off.

  Thereafter, the pixel selection signal Gm-1 in the (m−1) th row rises to a high level, and accordingly, a high level storage capacitor control signal SCm is applied to the storage capacitor line 217. Then, due to the capacitive coupling effect of the storage capacitor 218, the potential of the gate of the driving TFT 214 rises according to a voltage change ΔV (for example, about 10 V) from the low level to the high level of the storage capacitor control signal SC. As a result, the gate potential of the driving TFT 214 becomes higher than its source potential. At this time, if carriers (holes) are trapped in the gate insulating film of the driving TFT 214 by the previous writing of the display signal D, the carriers (holes) are tunneled by an electric field from the gate to the source or drain. A current is drawn from the gate insulating film to the source or drain. As a result, the electrical characteristics of the driving TFT 214 are initialized.

  Thereafter, the pixel selection signal Gm in the m-th row rises to a high level, and accordingly the pixel selection TFT 213 is turned on for one horizontal period. Then, during this one horizontal period, the display signal D is output from the horizontal drive circuit 302 to the display signal line 212, and this display signal D is applied to the gate of the drive TFT 214 through the pixel selection TFT 213 and the storage capacitor. 218. Then, a current corresponding to the display signal D is supplied from the driving TFT 214 to the organic EL element 216, and the organic EL element 216 emits light.

  As described above, according to the present embodiment, in the same manner as in the first embodiment, before writing the display signal D to the display pixels 210A, 210B, and 210C, the carriers in the gate insulating film of the driving TFT 214 (positive Since the electrical characteristics are initialized, the afterimage phenomenon of the display panel can be suppressed and the display quality can be improved. Further, since the pixel selection signal G output from the vertical drive circuit 301 is used for the precharge signal PCG and the storage capacitor control signal SC, there is an advantage that the circuit configuration is simplified.

  In the first and second embodiments described above, the organic EL element 216 is used as the light emitting element. Instead, a light emitting element other than the organic EL element, for example, an inorganic EL element or a light emitting diode is used. Also good. In the first and second embodiments described above, the pixel selection TFT 213 and the precharge TFT 220 are, for example, N-channel TFTs, and the driving TFT 214 is, for example, a P-channel TFT. May be other conductive channel types.

1 is an equivalent circuit diagram of an organic EL display device according to a first embodiment of the present invention. FIG. 3 is a timing diagram illustrating a method for driving the organic EL display device according to the first embodiment of the present invention. It is the equivalent circuit schematic of the display apparatus which concerns on the 2nd Embodiment of this invention. It is a timing diagram explaining the drive method of the display apparatus which concerns on the 2nd Embodiment of this invention. It is an equivalent circuit diagram of an organic EL display device according to a conventional example.

Explanation of symbols

210, 210A, 210B, 210C Display pixel 211 Pixel selection signal line 212 Display signal line 213 Pixel selection TFT
214 Driving TFT 215 Power Line 216 Organic EL Element 217 Retention Capacitor Line 218 Retention Capacitor 220 Precharge TFT
221 Precharge signal line 301 Vertical drive circuit 302 Horizontal drive circuit 400 Control LSI

Claims (8)

A plurality of display pixels arranged in a matrix;
Each display pixel has a pixel selection transistor that is turned on in response to a pixel selection signal, a light emitting element, a driving transistor that drives the light emitting element in response to a display signal applied through the pixel selection transistor, and the driving pixel A storage capacitor connected between a gate of the transistor and a storage capacitor line and holding the display signal; and a precharging transistor that is turned on in response to a precharge signal and short-circuits the source and gate of the driving transistor. The active transistor is characterized in that after the precharge transistor is turned off, the gate potential of the driving transistor is made higher than the source potential by applying a storage capacitor control signal for increasing the potential of the storage capacitor line. Matrix type display device. 2. The active matrix display device according to claim 1, wherein a pixel selection signal corresponding to a pixel preceding two rows is used as the precharge signal. 3. The active matrix display device according to claim 1, wherein a pixel selection signal corresponding to a previous pixel is used as the storage capacitor control signal. The active matrix display device according to claim 1, wherein the light emitting element is an organic electroluminescence element. A plurality of display pixels arranged in a matrix;
Each display pixel has a pixel selection transistor that is turned on in response to a pixel selection signal, a light emitting element, a driving transistor that drives the light emitting element in response to a display signal applied through the pixel selection transistor, and the driving pixel A storage capacitor connected between a gate of the transistor and a storage capacitor line and holding the display signal; and a precharging transistor that is turned on in response to a precharge signal and short-circuits the source and gate of the driving transistor. In the driving method of the active matrix display device provided,
In response to a precharge signal, the precharge transistor is turned on to short-circuit the source and gate of the drive transistor,
After turning off the precharge transistor, by applying a storage capacitor control signal that increases the potential of the storage capacitor line, the gate potential of the driving transistor is made higher than the source potential,
Thereafter, the pixel selection transistor is turned on in response to the pixel selection signal, and the display signal is applied to the gate of the driving transistor through the pixel selection transistor. Method. 6. The method of driving an active matrix display device according to claim 5, wherein a pixel selection signal corresponding to a pixel preceding two rows is used as the precharge signal. 7. The driving method of an active matrix display device according to claim 5, wherein a pixel selection signal corresponding to a previous pixel is used as the storage capacitor control signal. The method of driving an active matrix display device according to claim 5, wherein the light emitting element is an organic electroluminescence element.

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