JP2006308863A - Active matrix type display device and its driving method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To improve the display quality of an active matrix type display device by reducing an afterimage phenomenon. <P>SOLUTION: The active matrix type display device which switches the potential of a retention volume line 217 from a first potential Vsc1 to a second potential Vsc2 higher than the first potential Vsc1 to suppress an afterimage when the organic EL element 216 is turned off is subjected to designated voltage application processing before being shipped. Namely, the potential PVdd of a power line 215 and the potential of a cathode 216C of the organic EL element 216 are lowered for a designated period and also a display signal D having a designated potential and a pixel select signal G having a higher designated potential than the display signal D are applied for a designated period so that the potential difference Vgs between the gate and source of a driving TFT 214 and the potential difference between the drain and source are made larger than when the organic EL element 216 is turned off. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

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

  In recent years, organic EL display devices using organic electroluminescent devices (hereinafter referred to as “organic EL devices”) elements have been developed as display devices to replace CRTs and LCDs. 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 215 that supplies a positive power supply potential 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 potential 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 by the present inventor, 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 driven. This is presumably because the threshold value of the TFT 214 for use is changed.

  Therefore, the present invention provides an active matrix type display device that suppresses the afterimage of the display panel as described above and improves display quality.

  An 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 having an anode and a cathode, A drive transistor connected to the power supply line and the anode and driving the light emitting element in response to a display signal applied through the pixel selection transistor; and connected between the gate of the drive transistor and the storage capacitor line; The storage capacitor and the storage capacitor line are switched from the first potential to a second potential higher than the first potential, the driving transistor is turned off, the light emitting element is turned off, and then the storage capacitor A storage capacitor line potential switching circuit for switching the potential of the line to return from the second potential to the first potential, and a gate of the driving transistor; A power source potential switching circuit that lowers the potential of the power source line and the potential of the cathode over a predetermined period so that the potential difference between the source and the drain and the source is larger than when the light emitting element is turned off. It is characterized by providing.

  The present invention also 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 having an anode and a cathode, a power line, A driving transistor that is connected to the anode and drives a light emitting element in accordance with a display signal applied through a pixel selection transistor, and is connected between a gate of the driving transistor and a storage capacitor line and holds a display signal In a driving method of an active matrix display device including a capacitor, a power source is formed so that the potential difference between the gate and the source of the driving transistor and the potential difference between the drain and the source are larger than when the light emitting element is turned off. The potential of the line and the potential of the cathode are lowered over a predetermined period, and the display signal of the predetermined potential is higher than the display signal. And applying a pixel selection signal of a constant potential across the predetermined time period.

  According to the present invention, in an active matrix display device, an afterimage of a display panel can be suppressed, and a bright spot defect that occurs when the afterimage is suppressed can be suppressed to improve display quality. It becomes possible.

  Next, an active matrix organic EL display device and a driving method thereof according to an 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 according to the present embodiment. In FIG. 1, only one display pixel 210 </ b> A is shown from among a plurality of display pixels arranged in a matrix in the display panel 100. In FIG. 1, the same components as those in FIG. 5 are denoted by the same reference numerals, and the description thereof is omitted.

  As shown in FIG. 1, the organic EL display device includes a storage capacitor line potential switching circuit 101 connected to the storage capacitor line 217 of the display pixel 210A. The storage capacitor line potential switching circuit 101 switches the potential of the storage capacitor line 217 from the first potential Vsc1 to the second potential Vsc2 higher than the first potential Vsc1 to turn off the driving TFT 214, and again. The storage capacitor line 217 is switched so as to return from the second potential Vsc2 to the first potential Vsc1.

  The power supply line 215 has a terminal T1 for applying a voltage from the outside, and the cathode 216C of the organic EL element 216 has a terminal T2 for applying a voltage from the outside. Yes.

  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, the storage capacitor line potential switching circuit 101 outputs the first potential Vsc1, but at the predetermined timing, the storage capacitor line is switched to the second potential Vsc2 by switching the first potential Vsc1. The potential of 217 is raised to the second potential Vsc2.

  Then, due to the capacitive coupling effect of the storage capacitor 218, the gate potential Vg of the driving TFT 214 rises according to the voltage change ΔV from the first potential Vsc1 to the second potential Vsc2. As a result, the gate potential Vg of the driving TFT 214 becomes higher than the threshold Vtp of the driving TFT 214 with respect to the source potential PVdd, and the driving TFT 214 is turned off.

  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 caused by an electric field from the gate toward the source or drain. A tunnel 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. That is, the organic EL element 216 is turned off while its afterimage is suppressed.

  Next, after the electrical characteristics of the driving TFT 214 are initialized, the storage capacitor line potential switching circuit 101 switches the storage capacitor line 217 so as to return the potential of the storage capacitor line 217 from the second potential Vsc2 to the first potential Vsc1. As a result, the gate potential Vg of the driving TFT 214 returns to the original state, and the original display signal D is held in the holding capacitor 218.

  Note that in order to initialize the electrical characteristics of the driving TFT 214, the period Tsc2 in which the potential of the storage capacitor line 217 is the second potential Vsc2 is the period in which the potential of the storage capacitor line 217 is the first potential Vsc1. It is set to 1/300 or more of Tsc1.

  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. During this one horizontal period, the display signal D is output from the horizontal driving circuit 302 to the display signal line 212 of the display pixel 210, and this display signal D is applied to the gate of the driving TFT 214 through the pixel selection TFT 213. At the same time, it is held in the holding 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 the light emission of the organic EL element 217 corresponding to the display signal D, carriers (holes) in the gate insulating film of the driving TFT 214 are extracted, and the electrical characteristics thereof are initialized. Therefore, the afterimage phenomenon on the display panel 100 can be suppressed and the display quality can be improved.

  However, during the extinguishing period of the organic EL element 216, that is, the period in which the potential of the storage capacitor line 218 becomes the second potential Vsc2, a leak current is generated between the source and the drain of the driving TFT 214. This leakage current is generated by the gate potential Vg of the driving TFT 214 that increases in accordance with the voltage change ΔV from the first potential Vsc1 to the second potential Vsc2, and the P-type region and the N-type region that constitute the driving THT214. This is considered to occur because a reverse bias is applied to the PN junction.

  This leakage current flows to the drain of the driving TFT 214, that is, the anode of the organic EL element 216, and the organic EL element 216 is turned on during the extinguishing period where the afterimage should be suppressed. For this reason, display pixels serving as luminescent spots exist on the display panel 100, and the display quality has been lowered.

  In order to cope with this problem, in the present embodiment, before shipping the organic EL display device to the user, the following voltage application process is performed on the organic EL display device. The user drives the organic EL display device after the voltage application process is performed.

  That is, in this voltage application process, the potential PVdd of the power source line 215 is set so that the potential difference Vgs between the gate and source of the driving TFT 214 and the potential difference Vds between the drain and source thereof are larger than when the organic EL element 216 is turned off. The potential CV of the cathode 216C is lowered over a predetermined period. At the same time, a display signal D having a predetermined potential and a pixel selection signal G having a predetermined potential higher than the display signal D are applied over the predetermined period.

  When the potential PVdd of the power supply line 215 and the potential CV of the cathode 216C are lowered, a predetermined voltage is applied to the terminals T1 and T2 from the outside. Further, when the display signal D and the pixel selection signal G having the predetermined potential are applied, voltages supplied from the vertical drive circuit 301 and the horizontal drive circuit 302 to which they are connected are used.

  Here, the potential difference Vgs between the gate and the source of the driving TFT 214 and the potential difference Vds between the drain and the source thereof must be about 10 V or more, preferably about 15 V, respectively. In order to realize these potential differences, the potential PVdd of the power supply line 215 is about -5V, the potential CV of the cathode 216C is about -20V, the predetermined potential of the display signal D is about 10V, and the predetermined potential of the pixel selection signal G is It is preferably about 12V. Alternatively, the potentials other than those described above may be used as long as the potential difference Vgs between the gate and the source of the driving TFT 214 and the potential difference Vds between the drain and the source are larger than when the organic EL element 216 is turned off. It may be. Further, the period for performing the voltage processing (the period for maintaining the potential) is not particularly limited, but is, for example, about 1 μsec to about 10 seconds.

  According to the experiment by the present inventor, it has been clarified that the leakage current at the drain of the driving TFT 214 can be suppressed to be lower than that in the case where the voltage application process is not performed by the voltage application process described above. Next, the reduction in leakage current will be described with reference to the drawings.

  FIG. 3 is a characteristic diagram showing the relationship between the leakage current Id, which is the drain current of the driving TFT 214, and the gate potential Vg. Note that the vertical axis in FIG. 3 indicates the leakage current Id, and the horizontal axis indicates the gate potential Vg. 3A shows a characteristic diagram before the voltage application process is performed, and FIG. 3B shows a characteristic diagram after the voltage application process is performed for about 1 μsec to about 10 seconds. Yes.

  As shown in FIG. 3A, when the voltage application process described above is not performed, the leakage current Id decreases as the gate potential Vg of the driving TFT 214 approaches 0 V from the negative potential, but the gate potential Vg decreases to 0 V. If it exceeds, the leakage current Id tends to increase with a predetermined rate of change.

  On the other hand, as shown in FIG. 3B, when the voltage application process described above is performed, the leakage current Id does not show a tendency to increase at a predetermined rate of change even if the gate potential Vg of the driving TFT 214 exceeds 0V. It tends to fall below 1 pA. In this case, the leakage current Id is a sufficiently low value that does not cause the organic EL element 216 to emit light as a bright spot of the display panel 100.

  Therefore, when the user uses the organic EL display device after the voltage application process, even if the potential of the storage capacitor line 217 is switched to the second potential Vsc2 during the extinction period of the organic EL element 216, the afterimage is suppressed. Further, it is possible to suppress a bright spot defect caused by a leakage current that is generated secondaryly when suppressing the afterimage.

  In the above-described embodiment, the voltage application process is performed on the organic EL display device before shipping it, but the present invention is not limited to this. That is, as another embodiment, the organic EL display device of the present invention has a potential PVdd of the power supply line 215 of the display pixel 210B and a cathode 216C of the organic EL element 216 outside the display panel 100 as shown in FIG. A power supply potential switching circuit 102 that lowers the potential CV of the power supply may be incorporated.

  In this case, every time the user turns on the power of the organic EL display device, a predetermined voltage (for example, the power line 215 is supplied from the power supply potential switching circuit 102 built in the organic EL display device). About -5V and about -20V for the cathode 216C).

  In addition, the display signal D and the pixel selection signal G having a predetermined potential for performing the voltage application process are performed using voltages supplied from the vertical drive circuit 301 and the horizontal drive circuit 302 to which the display signal D and the pixel selection signal G are connected. Is called.

  Note that the period during which the leakage current Id can be suppressed as described above by a single voltage application process is finite (for example, about 1000 hours to about 1500 hours), but the voltage is applied each time the organic EL display device is turned on. By performing the application process, it is possible to substantially eliminate the necessity for the user to be aware of the limit of the period during which the leakage current can be suppressed.

  In the above-described embodiment, 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 may be used.

  In the above-described embodiment, the source of the driving TFT 214 is connected to the anode of the organic EL element 216, and the drain of the driving TFT 214 is connected to the cathode 216C of the organic EL element 216. It is not limited to. That is, the present invention is also applied to the case where the source of the driving TFT 214 is connected to the cathode 216C of the organic EL element 216 and the drain of the driving TFT 214 is connected to the anode of the organic EL element 216.

1 is an equivalent circuit diagram of an organic EL display device according to an embodiment of the present invention. It is a timing diagram explaining the drive method of the organic electroluminescence display which concerns on embodiment of this invention. It is a characteristic view which shows the relationship between leakage current and gate potential. FIG. 6 is an equivalent circuit diagram of an organic EL display device according to another embodiment of the present invention. It is an equivalent circuit diagram of an organic EL display device according to a conventional example.

Explanation of symbols

100 display panel 101 storage capacitor line switching circuit 102 power supply potential switching circuit 210, 210A, 210B 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 216C Cathode 217 Holding Capacitance Line 218 Holding Capacitance 219 Parasitic Capacitor 301 Vertical Driving Circuit 302 Horizontal Driving Circuit T1, T2 Terminal

Claims (9)

A plurality of display pixels arranged in a matrix;
Each display pixel is connected to a pixel selection transistor which is turned on in response to a pixel selection signal, a light emitting element having an anode and a cathode, a power supply line and the anode, and is applied to the display signal applied through the pixel selection transistor. The driving transistor for driving the light emitting element, the storage capacitor connected between the gate of the driving transistor and the storage capacitor line, and holding the display signal, and the potential of the storage capacitor line from the first potential The driving transistor is turned off by switching to a second potential higher than the first potential, the light emitting element is turned off, and then the potential of the storage capacitor line is returned from the second potential to the first potential. A holding capacitor line potential switching circuit for switching,
Further, the potential of the power supply line and the potential of the cathode are increased over a predetermined period so that the potential difference between the gate and the source of the driving transistor and the potential difference between the drain and the source are larger than when the light emitting element is turned off. An active matrix display device comprising: a power supply potential switching circuit for lowering the potential. 2. The active matrix display device according to claim 1, wherein the predetermined period is not less than 1 microsecond and not more than 10 seconds. 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 is connected to a pixel selection transistor which is turned on in response to a pixel selection signal, a light emitting element having an anode and a cathode, a power supply line and the anode, and is applied to the display signal applied through the pixel selection transistor. In a driving method of an active matrix display device, comprising: a driving transistor for driving the light emitting element; and a storage capacitor connected between a gate of the driving transistor and a storage capacitor line and storing the display signal. ,
The potential of the power supply line and the potential of the cathode are set over a predetermined period so that the potential difference between the gate and the source of the driving transistor and the potential difference between the drain and the source are larger than when the light emitting element is turned off. A method for driving an active matrix display device, wherein 5. The method for driving an active matrix display device according to claim 4, wherein the potential of the power supply line and the potential of the cathode are lowered over the predetermined period before shipping the active matrix display device. 5. The method of driving an active matrix display device according to claim 4, wherein when the power supply of the active matrix display device is turned on, the potential of the power supply line and the potential of the cathode are lowered over the predetermined period. The display signal at a predetermined potential and the display signal are higher so that the potential difference between the gate and the source of the driving transistor and the potential difference between the drain and the source are larger than when the light emitting element is turned off. 7. The method of driving an active matrix display device according to claim 4, wherein the pixel selection signal having a predetermined potential is applied over the predetermined period. 8. The method of driving an active matrix display device according to claim 4, wherein the predetermined period is not less than 1 microsecond and not more than 10 seconds. 9. The driving method of an active matrix display device according to claim 4, wherein the light emitting element is an organic electroluminescence element.

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