JP2009103772A - Display device and method of driving the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To appropriately correct variations in the mobility, in a transistor which drives a light-emitting element, even if the emission luminances differ, and effectively avoid shading due to the correction of the variations in the mobility, by applying the present invention to a display device or a display device driving method, for example, an active matrix-type display device having organic EL elements that use polysilicone TFTs. <P>SOLUTION: The signal level of a signal line is changed sequentially into an intermediate gradation voltage Vofs2 corresponding to intermediate gradation; a fixed voltage Vofs makes a driving transistor execute off-operation, and a gradation voltage Vsig which indicates the gradation of a pixel; and after changing the signal level to the intermediate gradation voltage Vofs2 and the gradation voltage Vsig, and prescribed time periods T1, T2 have passed, a writing transistor (WS) is made to operate on-operation and correct the variations in the mobility. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a display device and a display device driving method, and can be applied to, for example, an active matrix display device using an organic EL (Electro Luminescence) element. According to the present invention, the signal level of the signal line is sequentially switched to the intermediate gradation voltage corresponding to the intermediate gradation, the fixed voltage for turning off the driving transistor, and the gradation voltage indicating the gradation of the pixel. After switching to the grayscale voltage, the grayscale voltage is turned on for a certain period of time, and the writing transistor is turned on to correct the variation in mobility. By appropriately correcting the mobility variation in the transistor, it is possible to effectively avoid the occurrence of shading due to the mobility variation correction.

  Conventionally, various devices have been proposed for display devices using organic EL elements, for example, in US Pat. No. 5,684,365 and Japanese Patent Laid-Open No. 8-234683.

  FIG. 4 is a block diagram showing a so-called active matrix type display device using a conventional organic EL element. In the display device 1, the display unit 2 is formed by arranging pixels (PX) 3 in a matrix. In the display unit 2, the scanning lines SCN are provided in the horizontal direction in units of lines for the pixels 3 arranged in a matrix, and the signal lines SIG are provided for each column so as to be orthogonal to the scanning lines SCN.

  Here, as shown in FIG. 5, each pixel 3 includes an organic EL element 8 that is a current-driven self-luminous element, and a drive circuit (hereinafter referred to as a pixel circuit) of each pixel 3 that drives the organic EL element 8. ) And formed.

  In the pixel 3, one end of the signal level holding capacitor C1 is held at a constant potential, and the other end of the signal level holding capacitor C1 is connected to the signal line SIG via the transistor TR1 that is turned on / off by the write signal WS. . Thus, in the pixel 3, the transistor TR1 is turned on by the rising of the write signal WS, the other end potential of the signal level holding capacitor C1 is set to the signal level of the signal line SIG, and the transistor TR1 is switched from the on state to the off state. At the switching timing, the signal level of the signal line SIG is sampled and held at the other end of the signal level holding capacitor C1.

  In the pixel 3, the other end of the signal level holding capacitor C1 is connected to the gate of a P-channel transistor TR2 whose source is connected to the power supply Vcc, and the drain of the transistor TR2 is connected to the anode of the organic EL element 8. . Here, the pixel 3 is set so that the transistor TR2 always operates in a saturation region, and as a result, the transistor TR2 forms a constant current circuit using a drain-source current Ids expressed by the following equation. Here, Vgs is the gate-source voltage of the transistor TR2, and μ is the mobility. W is the channel width, L is the channel length, Cox is the capacity of the gate insulating film per unit area, and Vth is the threshold voltage of the transistor TR2. Thereby, each pixel 3 drives the organic EL element 8 by the drive current Ids according to the signal level of the signal line SIG sampled and held by the signal level holding capacitor C1.

  In the display device 1, a write signal WS that is a timing signal for instructing writing to each pixel 3 is generated by sequentially transferring predetermined sampling pulses by a write scan circuit (WSCN) 4 </ b> A of the vertical drive circuit 4. A horizontal selector (HSEL) 5A of the horizontal drive circuit 5 sequentially transfers predetermined sampling pulses to generate a timing signal, and sets each signal line SIG to the signal level of the input signal S1 with reference to the timing signal. . Thereby, the display device 1 sets the terminal voltage of the signal level holding capacitor C1 provided in the display unit 2 according to the input signal S1 in a dot sequence or a line sequence, and displays an image based on the input signal S1.

  Here, as shown in FIG. 6, the current-voltage characteristics of the organic EL element 8 change with time in a direction in which current hardly flows when used. In FIG. 6, symbol L1 indicates initial characteristics, and symbol L2 indicates characteristics due to changes over time. However, when the organic EL element 8 is driven by the P-channel transistor TR2 with the circuit configuration shown in FIG. 5, the transistor TR2 is driven by the gate-source voltage Vgs set according to the signal level of the signal line SIG. By driving, it is possible to prevent a change in luminance of each pixel due to a change in current-voltage characteristics over time.

  By the way, if all of the transistors constituting the pixel circuit, horizontal drive circuit, and vertical drive circuit are composed of N-channel transistors, these circuits can be collectively formed on an insulating substrate such as a glass substrate by an amorphous silicon process. A display device can be easily created.

  However, as shown in FIG. 7 in comparison with FIG. 5, when each pixel 13 is formed by applying the N-channel type to the transistor TR2, and the display device 11 is configured by the display unit 12 by this pixel 13, the transistor TR2 By connecting the source to the organic EL element 8, the gate-source voltage Vgs of the transistor TR2 changes due to the change in the current-voltage characteristics shown in FIG. Thereby, in this case, the current flowing through the organic EL element 8 is gradually reduced by use, and the light emission luminance of the organic EL element 8 is gradually lowered. In the configuration shown in FIG. 7, the light emission luminance varies from pixel to pixel due to variations in the characteristics of the transistor TR2. Note that this variation in light emission luminance disturbs the uniformity of the display screen and is perceived by unevenness and roughness of the display screen.

  For this reason, it is conceivable to configure each pixel as shown in FIG. 8, for example, as a device for preventing a decrease in emission luminance due to a change with time of the organic EL element and a variation in emission luminance due to a variation in characteristics.

  Here, in the display device 21 shown in FIG. 8, the display unit 22 is formed by arranging the pixels 23 in a matrix. In the pixel 23, one end of the signal level holding capacitor C1 is connected to the anode of the organic EL element 8, and the other end of the signal level holding capacitor C1 is connected to the signal via the transistor TR1 that is turned on / off in response to the write signal WS. Connected to line SIG. Thus, in the pixel 23, the voltage at the other end of the signal level holding capacitor C1 is set to the signal level of the signal line SIG in accordance with the write signal WS.

  In the pixel 23, both ends of the signal level holding capacitor C1 are connected to the source and gate of the transistor TR2, and the drain of the transistor TR2 is connected to the scanning line SCN for power supply. Thereby, the pixel 23 drives the organic EL element 8 by the transistor TR2 having the source follower circuit configuration in which the gate voltage is set to the signal level of the signal line SIG. Here, Vcat is the cathode potential of the organic EL element 8.

  The display device 21 outputs a write signal WS and a power supply drive signal DS to the scan line SCN by the write scan circuit (WSCN) 24A and the drive scan circuit (DSCN) 24B of the vertical drive circuit 24. A drive signal Ssig is output to the signal line SIG by the horizontal selector (HSEL) 25A, thereby controlling the operation of the pixel 23.

  Here, FIG. 9 is a time chart showing the operation of the pixel 23. As shown in FIG. 10, in the pixel 23, the transistor TR1 is set in the OFF state by the write signal WS and the power supply voltage Vcc is applied to the transistor TR2 by the drive signal DS, as shown in FIG. Is supplied (FIGS. 9A and 9B). Thereby, in the pixel 23, the gate voltage Vg and the source voltage Vs (FIGS. 9D and 9E) of the transistor TR2 are held at the voltage across the signal level holding capacitor C1, and the gate voltage Vg and the source voltage Vs. The organic EL element 8 is driven by the drive current Ids. This drive current Ids is expressed by equation (1).

  In the pixel 23, when the light emission period ends, as shown in FIG. 11, the drain voltage of the transistor TR2 is lowered to the predetermined voltage Vss by the drive signal DS. Here, the voltage Vss is set to a voltage lower than a voltage obtained by adding the cathode voltage Vcat of the organic EL element 8 to the threshold voltage VthEL of the organic EL element 8. Thus, in the pixel 23, the drive signal DS side of the driving transistor TR2 functions as a source, the anode voltage of the organic EL element 8 (which is the voltage Vs in FIG. 9) falls, and the organic EL element 8 stops emitting light. .

  At this time, in the pixel 23, as indicated by an arrow in FIG. 11, the accumulated charge is discharged from the organic EL element 8 side of the signal level holding capacitor C1, whereby the anode voltage of the organic EL element 8 falls to the voltage Vss. Is set.

  Subsequently, as shown in FIG. 12, in the pixel 23, the signal line SIG is lowered to the predetermined voltage Vofs by the drive signal Ssig, and the transistor TR1 is turned on by the write signal WS (FIGS. 9A and 9C). )). Thereby, in the pixel 23, the gate voltage Vg of the transistor TR2 is set to the voltage Vofs of the signal line SIG, and the gate-source voltage Vgs of the transistor TR2 is set to Vofs−Vss. Here, when the threshold voltage of the transistor TR2 is Vth, the voltage Vofs is set so that the gate-source voltage Vgs (Vofs−Vss) of the transistor TR2 is larger than the threshold voltage Vth of the transistor TR2.

  Subsequently, as shown in FIG. 13, the pixel 23 maintains the transistor TR1 in the on state for the period indicated by the symbol Tth1 in FIG. 9, and the drain voltage of the transistor TR2 is changed to the power supply voltage Vcc by the drive signal DS as shown in FIG. To be launched. As a result, when the voltage between the terminals of the signal level holding capacitor C1 is larger than the threshold voltage of the transistor TR2, the pixel 23 receives the signal level holding capacitor from the power source Vcc via the transistor TR2, as indicated by an arrow in FIG. A charging current flows through the organic EL element 8 side end of C1, and the voltage Vs at the organic EL element 8 side end gradually increases. Here, the organic EL element 8 has an equivalent circuit represented by a parallel circuit of a diode and a capacitor Cel. Here, in the state shown in FIG. 13, a current also flows into the organic EL element 8 by the power source Vcc through the transistor TR2, but the voltage between the terminals of the organic EL element 8 is increased by the increase in the source voltage of the transistor TR2. Since the leakage current of the organic EL element 8 is considerably smaller than the current of the transistor TR2 unless the threshold voltage is exceeded, the current flowing into the organic EL element 8 is caused by the signal level holding capacitor C1 and the organic EL element 8. Used to charge the capacitor Cel. Therefore, in the pixel 23, only the source voltage of the transistor TR2 rises without the organic EL element 8 emitting light.

  In the pixel 23, the transistor TR1 is subsequently turned off by the write signal WS, and the signal level of the signal line SIG is set to the signal level Vsig indicating the gradation of the corresponding pixel on the adjacent line. Thereby, in the pixel 23, the charging current from the power source Vcc through the transistor TR2 continuously flows into the organic EL element 8 side end of the signal level holding capacitor C1, and the source voltage Vs of the transistor TR2 continues to rise. In this case, the gate voltage Vg of the transistor TR2 increases following the increase in the source voltage Vs. Note that the signal level Vsig of the signal line SIG during this period is used to set the gradation of the corresponding pixel on the adjacent line.

  In the pixel 23, the signal level of the signal line SIG is switched to the voltage Vofs again after a certain time has elapsed, and thereby the signal level SIG side potential of the signal level holding capacitor C1 is changed during the period indicated by the symbol Tth2 in FIG. When the voltage between the terminals of the signal level holding capacitor C1 is larger than the threshold voltage of the transistor TR2 while being held at the voltage Vofs, the organic EL element 8 side of the signal level holding capacitor C1 is supplied by the power source Vcc via the transistor TR2. A charging current flows to the end, and the source voltage Vs of the transistor TR2 gradually increases. As a result, as shown in FIG. 14, the source voltage Vs of the transistor TR2 gradually increases so that the gate-source voltage Vgs of the transistor TR2 approaches the threshold voltage Vth of the transistor TR2, and the gate-source voltage of the transistor TR2 increases. When Vgs becomes the threshold voltage Vth of the transistor TR2, the inflow of the charging current through the transistor TR2 is stopped.

  In the pixel 23, the charging current flows into the organic EL element 8 side end of the signal level holding capacitor C1 via the transistor TR2, and the gate-source voltage Vgs of the transistor TR2 is equal to the threshold voltage Vth of the transistor TR2. This is repeated a sufficient number of times (in the example of FIG. 9, it is three times indicated by the symbols Tth1, Tth2, and Tth3). As a result, the threshold voltage Vth of the transistor TR2 is used for holding the signal level as shown in FIG. Set to capacitor C1. In the pixel 23, the voltages Vofs and Vcat are set so that Vel = Vofs−Vth ≦ Vcat + Vthel when the threshold voltage Vth of the transistor TR2 is set in the signal level holding capacitor C1. Thus, the organic EL element 8 is set not to emit light. Here, Vthel is a threshold voltage of the organic EL element 8, and Vel is a voltage at the end of the organic EL element 8 on the transistor TR2 side.

  Thereafter, the pixel 23 cancels the threshold voltage Vth of the transistor TR2 by setting the potential on the signal line SIG side of the signal level holding capacitor C1 to the voltage Vsig indicating the light emission luminance of the organic EL element 8. In this way, a voltage indicating a gradation is set in the signal level holding capacitor C1, thereby preventing variations in light emission luminance due to variations in the threshold voltage Vth of the transistor TR2.

  That is, as shown in FIG. 16, in the pixel 23, after the elapse of the period Tth3, the signal level of the signal line SIG is set to the signal level Vsig indicating the light emission luminance of the pixel 23, and then the writing is performed as indicated by the period Tμ. The transistor TR1 is set to an on state by the signal WS. Thereby, in the pixel 23, the signal level SIG side end of the signal level holding capacitor C1 is set to the signal level Vsig of the signal line SIG, and the current corresponding to the gate-source voltage Vgs by the voltage between the terminals of the signal level holding capacitor C1. Flows from the power source Vcc to the signal level holding capacitor C1 side end of the organic EL element 8 via the transistor TR2, and the source voltage Vs of the transistor TR2 gradually increases. At this time, Vcath is set so that Vel ≦ Vcath + VthEL, and the leakage current of the organic EL element 8 is considerably smaller than the current of the transistor TR2, so that the current flowing into the organic EL element 8 is It is used for charging the level holding capacitor C1 and the capacitance Cel of the organic EL element 8.

  Here, the current flowing through the transistor TR2 changes according to the mobility of the transistor TR2, and as a result, the source voltage Vs of the transistor TR2 increases as the mobility of the transistor TR2 increases as shown in FIG. Increases speed. Even if the current is in the transistor TR2 that drives the organic EL element 8, the current increases in accordance with the mobility. Here, this type of transistor TR2 is a polysilicon TFT or the like, and has a drawback that variations in threshold voltage Vth and mobility μ are large.

  As a result, the pixel 23 turns on the transistor TR2 in a state where the voltage on the signal line SIG side of the signal level holding capacitor C1 is held at the signal level Vsig of the signal line SIG for a certain period of time indicated by the symbol Tμ. A charging current is allowed to flow into the organic EL element 8 side end of the holding capacitor C1, thereby reducing the voltage between the terminals of the signal level holding capacitor C1 by the amount of the mobility of the transistor TR2, and the variation in the mobility of the transistor TR2. Variations in light emission luminance due to light are prevented.

  In the pixel 23, when the predetermined period Tμ elapses, the transistor TR1 is turned off by the write signal WS, the signal level Vsig of the signal line SIG is held by the signal level holding capacitor C1, and the light emission period starts. From these facts, the driving signal Ssig of the signal line SIG is repeated with the signal level Vsig sequentially indicating the gradation of each pixel 23 connected to one signal line with the fixed voltage Vofs interposed therebetween.

  However, with the configuration shown in FIG. 8, the organic EL element 8 is driven by the transistor TR2 while the signal level holding capacitor C1 remains connected to the signal line SIG for a certain period Tμ, and the mobility of the transistor TR2 is increased. When the variation is corrected, there is a problem that the mobility variation is excessively or insufficiently corrected in accordance with the signal level of the signal line SIG, thereby degrading the image quality.

  That is, as shown in FIG. 18, when displaying a white gradation, the signal level of the signal line SIG is maintained at a relatively high signal level as compared with the case of displaying a gray gradation. As compared with the case where the key is displayed, the rising speed of the source voltage Vs is faster, and as shown by the period TW, the variation in mobility of the transistor TR2 can be corrected in a short period. FIG. 18 shows changes in the source voltage Vs when the mobility is large and small, respectively, at L3 and L4.

  On the other hand, when displaying gray gradation, the signal level of the signal line SIG is held at a relatively low signal level as compared with displaying white gradation, and the white gradation is displayed. As compared with the case, the rate of increase of the source voltage Vs is slow, and as a result, the period necessary for correcting the variation in mobility of the transistor TR2 becomes longer as indicated by the period TG.

  As a method for solving this problem, as shown in FIG. 19 in comparison with FIG. 9, in a period Tμ for correcting the variation in mobility, the signal level of the signal line SIG with a predetermined voltage Vofs2 interposed therebetween. Can be considered to rise from a fixed voltage Vofs to a signal level Vsig corresponding to the emission luminance. The fixed voltage Vofs2 is set to a signal level of an intermediate gradation between the white gradation and the black gradation. In the configuration of FIG. 19, the signal waveform of the signal line SIG is set to be the same as the period Tμ for correcting the variation in mobility in the periods Th1, Th2, and Th3 for correcting the variation in threshold value. The configuration of the horizontal drive circuit is simplified.

  In this way, as shown in FIG. 20, when displaying a white gradation, the time t1 required for correcting the variation in mobility of the transistor TR2 can be made longer than in the case of the example of FIG. In FIG. 20, the change of the source voltage Vs in the configuration of FIG. FIG. 21 shows changes in the source voltage Vs and the gate voltage Vg in the configuration of FIG. 9 in comparison with FIG.

  As shown in FIG. 22, when displaying gray gradation, the time t2 required for correcting the variation in mobility of the transistor TR2 can be shortened as compared with the case of the example of FIG. In FIG. 22, the change of the source voltage Vs in the configuration of FIG. Further, FIG. 23 shows changes in the source voltage Vs and the gate voltage Vg in the configuration of FIG. 9 in comparison with FIG.

  Accordingly, if the variation in mobility is corrected by raising the signal level of the signal line SIG from the fixed voltage Vofs to the signal level Vsig corresponding to the emission luminance with the predetermined voltage Vofs2 interposed therebetween, various emission luminances can be obtained. Even when they are different from each other, variation in mobility can be appropriately corrected.

  However, this method has a problem that it cannot be applied directly to a large-screen display device. That is, in the driving of each pixel 23 by the method of FIG. 19, since power is supplied by the drive signal DS for power supply, as shown in FIG. 24, the scanning line SCN that supplies this drive signal DS has the widest width. A scanning line SCN that transmits the write signal WS is provided in parallel with the scanning line SCN that is formed by the wiring pattern and supplies the drive signal DS. Further, a signal line SIG is provided so as to be orthogonal to these scanning lines SCN.

  As a result, the signal line SIG functions as a transmission line having a distributed constant due to the capacitances CA and CB between the two scanning lines SCN and the resistance of the wiring pattern (indicated by a symbol of resistance in FIG. 24). As shown in FIG. 25A, the signal waveform of the drive signal Ssig becomes distorted as the distance from the input end of the drive signal Ssig increases. In FIG. 25, reference signs LN and LF indicate a signal waveform on the side closer to the input end of the drive signal Ssig and a signal waveform on the side farther from the input end.

As a result, at the rise from the half-tone voltage Vofs2 to the signal level Vsig corresponding to the light emission luminance, the pixel 23 far from the input end is compared with the pixel 23 near the input end of the drive signal Ssig. The rise of the gate voltage Vg and the source voltage Vs of the transistor TR1 is delayed (FIGS. 25B to 25D), and eventually the pixel 23 near the input end of the drive signal Ssig and the far side from the input end of the drive signal Ssig. Excess or deficiency occurs in correcting the mobility variation with the pixel 23. Accordingly, there is a problem that shading occurs in the example of FIG.
USP 5,684,365 JP-A-8-234683

  The present invention has been made in consideration of the above points. Even when the light emission luminance is variously changed, the mobility variation in the transistor driving the light emitting element is appropriately corrected, thereby correcting the mobility variation. An object of the present invention is to propose a display device and a display device driving method capable of effectively avoiding the occurrence of shading.

  In order to solve the above-described problems, the invention of claim 1 is directed to a display unit formed by arranging pixels in a matrix, and signals are sent to signal lines and scanning lines of the display unit by a horizontal drive circuit and a vertical drive circuit. By applying a line drive signal and a write signal, the display unit displays a desired image, and the pixel is turned on by a light emitting element, a signal level holding capacitor, and the write signal. Operates and sets a terminal voltage of the signal level holding capacitor to the signal level of the signal line, and connects a gate and a source to both ends of the signal level holding capacitor to hold the signal level A driving transistor that drives the light emitting element to emit light according to a voltage between terminals of the capacitor, and the horizontal driving circuit and the vertical driving circuit are: In a non-light emission period in which light emission of the light emitting element is stopped, the signal level of the signal line driving signal is set to an intermediate gradation voltage corresponding to the intermediate gradation of the light emitting element, and a fixed voltage for turning off the driving transistor. The gradation voltage corresponding to the gradation that causes the light emitting element to emit light is sequentially set, and when the signal level of the signal line drive signal is set to the intermediate gradation voltage and a predetermined time elapses, the write signal The writing transistor is turned on, the voltage at one end of the signal level holding capacitor is set to the intermediate gradation voltage, and the other end of the signal level holding capacitor is charged by the driving transistor. Thus, the variation in mobility in the driving transistor is corrected by the halftone voltage, and the signal level of the signal line driving signal is corrected. Is set to the gradation voltage, and after a certain time has passed, the writing transistor is turned on by the write signal, the voltage at one end of the signal level holding capacitor is set to the gradation voltage, By charging the other end of the signal level holding capacitor by the driving transistor, the variation in mobility in the driving transistor is corrected by the gradation voltage, and then the writing transistor is turned off.

  According to a fourth aspect of the present invention, a signal line drive signal and a write signal are written to a signal line and a scanning line of the display portion by a horizontal drive circuit and a vertical drive circuit for a display portion formed by arranging pixels in a matrix. The pixel is applied to a driving method of a display device that displays a desired image on the display unit by outputting a signal, and the pixel is turned on by a light emitting element, a signal level holding capacitor, and the writing signal. A transistor for writing that sets a terminal voltage of the signal level holding capacitor to a signal level of the signal line; a gate and a source connected to both ends of the signal level holding capacitor; and a terminal of the signal level holding capacitor A driving transistor that drives the light emitting element to emit light according to an inter-voltage, and the driving method stops light emission of the light emitting element. In the light emission period, the signal level of the signal line driving signal is set to an intermediate gradation voltage corresponding to the intermediate gradation of the light emitting element, a fixed voltage for turning off the driving transistor, and a gradation for emitting the light emitting element. When the signal level of the signal line drive signal is set to the intermediate gradation voltage and a certain period of time elapses, the write transistor is turned on by the write signal. The voltage at one end of the signal level holding capacitor is set to the intermediate gradation voltage, and the other end of the signal level holding capacitor is charged by the driving transistor, whereby the intermediate gradation voltage causes the Correct the mobility variation in the driving transistor and set the signal level of the signal line driving signal to the gradation voltage to be constant When the time elapses, the write transistor is turned on by the write signal, the voltage at one end of the signal level holding capacitor is set to the gradation voltage, and the signal level holding signal is set by the driving transistor. By charging the other end of the capacitor, the variation in mobility in the driving transistor is corrected by the gradation voltage, and then the writing transistor is turned off.

  According to the structure of claim 1 or claim 4, when the intermediate gradation voltage is set and a predetermined time elapses, the writing transistor is turned on by the writing signal, and the mobility in the driving transistor is determined by the intermediate gradation voltage. If the signal waveform of the signal line drive signal is rounded in the pixel far from the input end of the signal line drive signal, the signal line drive signal is changed to a sufficient voltage. After waiting, the variation in mobility in the driving transistor can be corrected by the intermediate gradation voltage. Therefore, excess and deficiency due to the distance from the input end of the signal line drive signal can be prevented with respect to the correction of the variation in mobility due to the intermediate gradation voltage. Also, when the signal level of the signal line drive signal is set to the gradation voltage and a certain time elapses, the writing transistor is turned on by the writing signal, and the mobility in the driving transistor is varied by the gradation voltage. If corrected, even if the signal waveform of the signal line drive signal is rounded in the pixel far from the input end of the signal line drive signal, wait until the signal line drive signal changes to a sufficient voltage, The variation in mobility in the driving transistor can be corrected by the gradation voltage. Therefore, excess and deficiency due to the distance from the input end of the signal line drive signal can be prevented with respect to the correction of the mobility variation due to the gradation voltage. If the signal level of the signal line drive signal is sequentially set to the intermediate gradation voltage, the fixed voltage for turning off the driving transistor, and the gradation voltage, the mobility of the driving transistor varies depending on the intermediate gradation voltage. After the correction, the signal line drive signal waits for the voltage to change to a sufficient voltage, and until the variation correction of mobility in the drive transistor is started by the grayscale voltage, the drive for the intermediate grayscale voltage is used. Thus, it is possible to prevent the variation in mobility of the transistors in the transistor from affecting the correction of the variation in mobility. The occurrence of shading due to mobility variation correction can be effectively avoided.

  According to the present invention, even when the light emission luminance varies, it is possible to appropriately correct the mobility variation in the transistor that drives the light emitting element, and effectively avoid the occurrence of shading due to the mobility variation correction. Can do.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings as appropriate.

(1) Configuration of Example FIG. 2 is a time chart showing the driving timing of each pixel in the display device of Example 1 of the present invention in comparison with FIG. The display device of this embodiment is configured in the same manner as the display device described above with reference to FIG. 19 except that the driving of each pixel in the non-light emitting period by the horizontal drive circuit and the vertical drive circuit is different. Therefore, in the following description, the structure of the display device using the above-described organic EL element will be used as appropriate.

  In this display device, as shown in FIG. 2C, a predetermined one horizontal scanning period 1H is assigned to a period Tμ for correcting the mobility, and the transistor TR2 in each pixel connected to one signal line SIG. The variation in mobility is sequentially corrected. Here, in the period Tμ for correcting the mobility, the driving signal Ssig outputs the intermediate gradation voltage Vofs2 corresponding to the intermediate gradation between the highest light emission luminance and the lowest light emission luminance for a certain period, and the transistor TR2. The fixed voltage Vofs to be turned off and the gradation voltage Vsig corresponding to the gradation of each pixel 23 are sequentially set. Accordingly, the signal level of the drive signal Ssig is sequentially switched by the intermediate gradation voltage Vofs2, the fixed voltage Vofs, and the gradation voltage Vsig in this one horizontal scanning period.

  In this embodiment, immediately before the period Tμ for correcting the mobility, the drive signal DS (FIG. 2B) falls to the predetermined voltage Vss in the three horizontal scanning periods, and then the signal level holding capacitor C1. When a sufficient time has elapsed for the end of the organic EL element 8 to fall to the predetermined voltage Vss, the drive signal DS is raised to the power supply voltage Vcc. In addition, the drive signal DS is once lowered to the predetermined voltage Vss and then raised to the power supply voltage Vcc, and the write signal WS (see FIG. 2) during the period in which the drive signal Ssig is set to the fixed potential Vofs. (A)) is selectively raised and the transistor TR1 is set to an on state, whereby the threshold voltage Vth of the transistor TR2 is a signal during the periods Tth1, Tth2, and Tth3 during which the write signal WS is raised. The variation in the threshold voltage Vth of the transistor TR2 is corrected by setting the level holding capacitor C1 (FIGS. 2D and 2E). The relationship between the drive signal Ssig and the write signal WS relating to the correction of the threshold voltage variation is shown in detail in FIG.

  In contrast, in the mobility correction period Tμ, as shown in FIG. 1, the signal level of the drive signal Ssig is raised to the intermediate gradation voltage Vofs2 with the drive signal DS raised to the power supply voltage Vcc. Thereafter, when the predetermined time T1 has elapsed, the write signal WS is raised, and after this, the write signal WS rises, and then the transistor TR2 is driven by the intermediate gradation until the signal level of the drive signal Ssig falls to the fixed voltage Vofs. Variation in mobility is corrected.

  Here, the time T1 from when the signal level of the drive signal Ssig is raised to the intermediate gradation voltage Vofs2 to when the write signal WS is raised is the waveform rounding of the drive signal Ssig farthest from the input end of the drive signal Ssig. In the most intense pixel, after the elapse of the mobility correction period Tμ, the time required for the signal level of the signal line SIG to rise to the intermediate gradation voltage Vofs2 is set to such an extent that shading cannot be perceived sufficiently practically. . As a result, in this embodiment, the variation in mobility of the transistor TR2 is corrected by the intermediate gray level under the same conditions between the pixel on the input end side of the drive signal Ssig and the pixel far from the input end.

  Subsequently, in the display device, the signal level of the drive signal Ssig is switched to the fixed voltage Vofs. In the period in which the signal level of the drive signal Ssig is the fixed voltage Vofs, the transistor TR2 is set to the off state in the pixel, and thus the transistor TR2 is held at the source voltage set by the mobility variation correction by the intermediate gradation. Is done. In this embodiment, in the non-light emitting period, the variation in the threshold voltage in the transistor TR2 is corrected using the fixed potential Vofs as described above, whereby the source of the transistor TR2 is corrected at the start of the mobility correction period Tμ. Since the voltage Vs is set to the voltage Vofs−Vth, and then the source voltage of the transistor TR2 is increased by correcting the variation in mobility due to the intermediate gradation, the drive signal Ssig is set to the fixed potential Vofs. The transistor TR2 can be cut off. Note that the fixed potential for turning off the transistor TR2 is not limited to the fixed potential Vofs, and for example, a voltage equal to or lower than the fixed potential Vofs and a voltage equal to or higher than the fixed potential Vofs can be applied.

  Further, in the display device, the signal waveform when the drive signal Ssig is set to the fixed voltage Vofs is blunted and output to the signal line SIG. Here, the process of dulling the signal waveform is performed by removing in advance the high-frequency component that deteriorates due to the propagation of the signal line SIG, and by using a pixel closer to the input end of the signal line Ssig and a pixel farther from the input end. In this process, the drive signal Ssig is set to have substantially the same signal waveform when falling to the fixed voltage Vofs. For example, the display device sets the drive signal Ssig to the voltage Vofs2, and then inserts a low-pass filter in the supply path of the signal line SIG, and then lowers the drive signal Ssig to the fixed voltage Vofs, whereby the drive signal Ssig is set to the fixed voltage. The signal waveform when falling to Vofs is blunted.

  Subsequently, in this display device, after the drive signal Ssig is raised from the fixed voltage Vofs to the gradation voltage Vsig, when the predetermined time T2 has elapsed, the write signal WS is raised only for a certain period, whereby the write signal WS is changed. During the rising period, the variation in mobility of the transistor TR2 is finally corrected by the gradation voltage.

  Here, the waveform rounding of the drive signal Ssig farthest from the input end of the drive signal Ssig is the most severe even during the time T2 from when the drive signal Ssig is set to the gradation voltage Vsig until the write signal WS rises. In the pixel, after the mobility correction period Tμ has elapsed, the time required for the signal level of the signal line SIG to rise to the gradation voltage Vsig is set to such an extent that shading cannot be perceived sufficiently practically. Thus, in this embodiment, the variation in mobility of the transistor TR2 is finally corrected under the same conditions between the pixel on the input end side of the drive signal Ssig and the pixel far from the input end.

(2) Operation of Embodiment In the above configuration, in the display device of this embodiment (see FIGS. 8 to 16), the signal line SIG and the scanning line SCN are driven by the horizontal drive circuit and the vertical drive circuit sequentially in line units. The signal level Vsig of the signal line SIG is set to the pixel 23 of the display unit 22, and the organic EL element 8 of each pixel 23 emits light by the set signal level Vsig, and a desired image is displayed on the display unit 22. The

  That is, in this display device, one end of the signal level holding capacitor C1 is set to the signal level Vsig of the signal line SIG in the non-light emitting period, and the gate source is generated by the voltage between the terminals of the signal level holding capacitor C1 in the light emitting period. The organic EL element 8 is driven by the transistor TR2 by the inter-voltage Vgs. Thereby, in this display device, the organic EL element 8 of each pixel 23 emits light with the light emission luminance corresponding to the signal level Vsig of the signal line SIG.

  In the display device, during this non-emission period, first, the voltage across the signal level holding capacitor C1 is set to the predetermined fixed voltages Vofs and Vss, and then discharged through the transistor TR2 that drives the organic EL element 8. The threshold voltage Vth of the transistor TR2 is set in the signal level holding capacitor C1 (FIG. 2, periods Tth1, Tth2, Tth3), thereby correcting the variation in emission luminance due to the variation in the threshold voltage Vth of the transistor TR2. The

  Thereafter, the transistor TR1 is turned on by the write signal WS, and the signal level holding capacitor C1 is connected to the signal line SIG while the signal level holding capacitor C1 is connected to the signal line SIG. The other end of the capacitor C1 is charged (FIG. 2, period Tμ), whereby the variation in light emission luminance due to the variation in mobility of the transistor TR2 is corrected.

  After correcting the variation in mobility, the display device switches the operation of the transistor TR1 to the OFF state by the write signal WS, whereby the signal level Vsig of the signal line SIG is sampled and held in the signal level holding capacitor C1, and the organic EL element A light emission luminance of 8 is set.

  However, when the variation in mobility of the transistor TR2 is corrected by simply setting the gradation voltage Vsig indicating the gradation of the pixel to the signal line SIG, the time required for correcting the variation in mobility is short when the emission luminance is high. On the other hand, when the emission luminance is low, the time required for correcting the variation in mobility becomes long, and accordingly, the variation in mobility variation over a certain period of time is insufficient or insufficient for correcting the variation in mobility according to the emission luminance. Will occur and the image quality will deteriorate (FIG. 18).

  Therefore, in this embodiment, first, after correcting the mobility variation with the intermediate gradation voltage Vofs2 corresponding to the intermediate gradation between the highest emission luminance and the lowest emission luminance, the gradation voltage according to the final setting is set. The variation in mobility is corrected by Vsig (see FIGS. 2 and 19 to 23), thereby preventing excessive and insufficient mobility variation correction according to light emission luminance and preventing deterioration of image quality.

  However, when the mobility of the transistor TR2 is corrected by simply varying the intermediate gradation voltage Vofs2 and the gradation voltage Vsig, the waveform rounding of the drive signal Ssig becomes more severe as the pixel is farther from the input end of the drive signal Ssig. It takes time to correct the mobility variation (FIGS. 24 and 25). As a result, the mobility variation is excessively or insufficiently corrected depending on the distance from the input end of the drive signal Ssig. Shading will occur.

  Therefore, in this embodiment, the mobility variation correction by the intermediate gradation voltage Vofs2 and the mobility variation correction by the gradation voltage Vsig of each pixel are respectively switched after the signal level of the drive signal Ssig is switched, and then for a certain time T1. When the signal level of the drive signal Ssig sufficiently changes to the grayscale voltage Vofs2 and the grayscale voltage Vsig even in a pixel far from the input end of the drive signal Ssig after a lapse of T2, the variation correction of the mobility due to the halftone is performed. Then, the correction of the mobility variation by the final gradation voltage is started (FIG. 1).

  Further, in the drive signal Ssig, a fixed voltage Vofs for turning off the transistor TR2 is provided between the intermediate gradation voltage Vofs2 and the gradation voltage Vsig, and the mobility varies due to the intermediate gradation due to the fall to the fixed voltage Vofs. After the correction is finished, the source voltage Vs of the transistor TR2 set by the correction of the variation in mobility due to the intermediate gradation is held, thereby switching the signal level of the drive signal Ssig, and then for a certain time T2. Even when the mobility variation correction based on the final gradation voltage has been executed, the influence of the fixed time T2 on the mobility variation correction is prevented.

  As a result, in this embodiment, even when the light emission luminance is variously varied, the variation in mobility in the transistor driving the light emitting element is appropriately corrected to effectively avoid the occurrence of shading due to the variation in mobility variation. be able to.

  By the way, after switching the signal level of the drive signal Ssig to the intermediate gradation voltage Vofs2 and the gradation voltage Vsig as described above, the writing transistor is turned on after a certain period of time T1 and T2 has elapsed, thereby causing variations in mobility. When correcting, the waveform rounding when the signal level of the drive signal Vsig is set to the intermediate gradation voltage Vofs2, and the influence of the waveform rounding upon the mobility level correction when setting the signal level of the drive signal Vsig to the gradation voltage Vofs. Can be effectively avoided.

  However, when the transistor TR2 is turned off by lowering the signal level of the drive signal Vsig from the intermediate gradation voltage Vofs2 to the voltage Vofs, due to the influence of waveform rounding, the pixels near the input end of the signal line Ssig and the input end The timing at which the transistor TR2 is turned off varies between the pixels farther from the pixel and the pixel nearer to the input end of the signal line Ssig and the pixels farther from the input end than the pixel TR2. There will be a shortage.

  Therefore, in this embodiment, the signal waveform when the drive signal Ssig is lowered to the fixed potential Vofs is almost the same between the pixel near the input end of the signal line Ssig and the pixel far from the input end. A high frequency component that deteriorates due to propagation of the signal line SIG is removed in advance, and the signal waveform of the drive signal Ssig is set to be rounded, and the drive signal Ssig is supplied to the signal line SIG.

  Thus, in this display device, when the signal level of the drive signal Ssig is lowered from the intermediate gradation voltage Vofs2 to the voltage Vofs to turn off the transistor TR2, the pixel and the input terminal near the input terminal of the signal line Ssig The pixels on the far side can be turned off at substantially the same timing, which can more reliably avoid the occurrence of shading due to mobility variation correction.

(3) Effects of Embodiments According to the configuration described above, the signal line is sequentially applied to the intermediate gradation voltage corresponding to the intermediate gradation, the fixed voltage for turning off the driving transistor, and the gradation voltage indicating the gradation of the pixel. After switching to the intermediate grayscale voltage and grayscale voltage by switching the signal level, the write transistor is turned on after a certain period of time, and the variation in mobility is corrected by correcting the light emission luminance. Even when they are different, it is possible to effectively avoid the occurrence of shading due to the mobility variation correction by appropriately correcting the mobility variation in the driving transistor.

  Further, by reducing the signal level of the drive signal and smoothing the signal waveform of the drive signal when the transistor is turned off, it is possible to more reliably avoid the occurrence of shading due to mobility variation correction.

  In the above-described embodiment, the case where the signal waveform of the drive signal Ssih is smoothed only when the signal level of the drive signal Ssig falls from the intermediate gradation voltage Vofs2 to the voltage Vofs has been described. However, the present invention is not limited thereto. Instead, the signal waveform of the drive signal Ssig may be blunted by switching all the signal levels of the drive signal Ssig.

  In the above-described embodiment, the case where one signal line SIG is driven by one system drive signal Ssig has been described. However, the present invention is not limited to this, and a plurality of signal lines are time-divided by one system drive signal Ssig. The present invention can also be widely applied to the case of driving with.

  In the above-described embodiments, the case where an organic EL element is used as a light-emitting element has been described. However, the present invention is not limited to this, and can be widely applied to cases where various current-driven light-emitting elements are used.

  The present invention can be applied to an active matrix display device using an organic EL element using, for example, a polysilicon TFT.

It is a time chart with which it uses for description of the drive of each pixel in the display apparatus of Example 1 of this invention. It is a time chart with which it uses for description of correction | amendment of the threshold voltage by the time chart of FIG. 2 is a time chart for explaining mobility variation correction by the time chart of FIG. 1. It is a block diagram which shows the conventional display apparatus. It is a block diagram which shows the display apparatus of FIG. 3 in detail. It is a characteristic curve figure which shows a time-dependent change of an organic EL element. FIG. 6 is a block diagram showing a case where an N-channel transistor is used in the configuration of FIG. It is a block diagram which shows the display apparatus considered using an N channel type transistor. It is a time chart of the display apparatus of FIG. FIG. 10 is a connection diagram illustrating pixel settings in the light emission period of FIG. 9. FIG. 11 is a connection diagram illustrating a continuation of FIG. 10. FIG. 12 is a connection diagram illustrating a continuation of FIG. 11. FIG. 13 is a connection diagram illustrating a continuation of FIG. 12. It is a characteristic curve figure used for description of correction | amendment of a threshold voltage. FIG. 14 is a connection diagram showing a continuation of FIG. 13. FIG. 16 is a connection diagram illustrating a continuation of FIG. 15. It is a characteristic curve figure with which it uses for description of correction | amendment of a mobility. It is a characteristic curve figure with which it uses for description of the time required for correction | amendment of the dispersion | variation in a mobility. It is a time chart concerning the correction | amendment of the dispersion | variation in mobility using the voltage of an intermediate gradation. FIG. 5 is a signal waveform diagram for explaining correction of mobility variation using a voltage of an intermediate gradation when displaying a white gradation. FIG. 21 is a signal waveform diagram for explaining correction of mobility variation when a grayscale voltage is not used in comparison with FIG. 20. FIG. 6 is a signal waveform diagram for explaining correction of mobility variation using a gray level voltage when displaying gray levels. FIG. 23 is a signal waveform diagram for explaining correction of mobility variation when a grayscale voltage is not used in comparison with FIG. 22. It is a basic diagram which shows the structure of a pixel. It is a time chart with which it uses for description of the waveform rounding of a drive signal.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1, 11, 21 ... Display apparatus, 2, 12, 22 ... Display part, 3, 13, 23 ... Pixel, 4, 24 ... Vertical drive circuit, 44, 24A ... Light scan circuit, 5, 25 ... Horizontal drive circuit, 54, 25A ... Horizontal selector

Claims (4)

By outputting a signal line drive signal and a write signal to a signal line and a scanning line of the display unit by a horizontal drive circuit and a vertical drive circuit with respect to a display unit formed by arranging pixels in a matrix form, In a display device that displays a desired image on a display unit,
The pixel is
A light emitting element;
A signal level holding capacitor;
A write transistor that is turned on by the write signal and sets the terminal voltage of the signal level holding capacitor to the signal level of the signal line;
A gate and a source are connected to both ends of the signal level holding capacitor, and a driving transistor for driving the light emitting element according to a voltage between terminals of the signal level holding capacitor to emit light,
The horizontal drive circuit and the vertical drive circuit are:
In a non-light emission period in which the light emission of the light emitting element is stopped, the signal level of the signal line driving signal is set to an intermediate gradation voltage corresponding to the intermediate gradation of the light emitting element, and a fixed voltage for turning off the driving transistor. , Sequentially setting the gradation voltage corresponding to the gradation that causes the light emitting element to emit light,
When the signal level of the signal line drive signal is set to the intermediate gradation voltage and a certain time has elapsed, the write transistor is turned on by the write signal, and the voltage at one end of the signal level holding capacitor is turned on Is set to the intermediate gradation voltage, and the other end of the signal level holding capacitor is charged by the driving transistor, thereby correcting the variation in mobility in the driving transistor by the intermediate gradation voltage. And
When the signal level of the signal line drive signal is set to the gradation voltage and a certain time has elapsed, the write transistor is turned on by the write signal, and the voltage at one end of the signal level holding capacitor is set to After setting the gradation voltage and charging the other end of the signal level holding capacitor by the driving transistor, the variation in mobility in the driving transistor is corrected by the gradation voltage; A display device, wherein the writing transistor is turned off. The horizontal drive circuit and the vertical drive circuit are:
The display device according to claim 1, wherein after the signal level of the signal line drive signal is set to the fixed voltage, the write transistor is turned off by the write signal. The horizontal drive circuit includes:
The display device according to claim 1, wherein a signal waveform of the signal line drive signal is blunted and output to the signal line. By outputting a signal line drive signal and a write signal to a signal line and a scanning line of the display unit by a horizontal drive circuit and a vertical drive circuit with respect to a display unit formed by arranging pixels in a matrix form, In a driving method of a display device that displays a desired image on a display unit,
The pixel is
A light emitting element;
A signal level holding capacitor;
A write transistor that is turned on by the write signal and sets the terminal voltage of the signal level holding capacitor to the signal level of the signal line;
A gate and a source are connected to both ends of the signal level holding capacitor, and a driving transistor for driving the light emitting element according to a voltage between terminals of the signal level holding capacitor to emit light,
The driving method is:
In the non-light emitting period in which the light emitting element stops light emission, the signal level of the signal line driving signal is set to an intermediate gray level voltage corresponding to the intermediate gray level of the light emitting element, and a fixed voltage for turning off the driving transistor. , And sequentially setting the gradation voltage corresponding to the gradation that causes the light emitting element to emit light,
When the signal level of the signal line drive signal is set to the intermediate gradation voltage and a certain time has elapsed, the write transistor is turned on by the write signal, and the voltage at one end of the signal level holding capacitor is turned on Is set to the intermediate gradation voltage, and the other end of the signal level holding capacitor is charged by the driving transistor, thereby correcting the variation in mobility in the driving transistor by the intermediate gradation voltage. And
When the signal level of the signal line drive signal is set to the gradation voltage and a certain time has elapsed, the write transistor is turned on by the write signal, and the voltage at one end of the signal level holding capacitor is set to After setting the gradation voltage and charging the other end of the signal level holding capacitor by the driving transistor, the variation in mobility in the driving transistor is corrected by the gradation voltage; A driving method of a display device, wherein the writing transistor is turned off.

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