Abstract:
When a forward bias is applied between a selected unit electrode of scanning electrodes and a selected unit electrode of data electrodes to cause a selected pixel concerning both of the selected unit electrodes to emit light, and a reverse bias is applied between nonselected unit electrodes of the scanning electrodes and nonselected unit electrodes of the data electrodes, thereby preventing crosstalk caused by a semi-excited state of the nonselected pixels, all of the scanning electrodes and all of the data electrodes are short-circuited once, immediately before a predetermined unit electrode of the data electrode, which should be selected in accordance with selection of each of the unit electrodes of the scanning electrodes, is selected, to set all of the pixels at a zero bias.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method of driving an organic thin-film EL device which has an organic thin-film EL structure and in which pixels are arranged in a matrix. 
     2. Description of the Prior Art 
     An example of a conventional organic thin-film EL display device is disclosed in, e.g., Japanese Unexamined Patent Publication No. 6-301355. 
     FIG. 1 shows an equivalent circuit of matrix driving of organic thin-film EL elements disclosed in Japanese Unexamined Patent Publication No. 6-301355. 
     In this reference, the organic multilayered thin film including an emission layer is sandwiched between scanning electrodes X 1  to X n  serving as cathodes and data electrodes Y 1  to Y m  serving as anodes. Pixels each having an organic thin-film EL structure are arranged in a matrix. The scanning electrodes X 1  to X n  are scanned, i.e., transistors  7   1  to  7   n  are sequentially turned on one by one to sequentially select the unit electrodes of the scanning electrodes X 1  to X n  and to set them at the ground potential. In accordance with this, a current is supplied to a predetermined unit electrode which should be selected from the data electrodes Y 1  to Y m  in accordance with display data. In other words, a predetermined transistor and a predetermined current supply means which should be selected from transistors  11   1  to  11   m  and from current supply means  10   1  to  10   m , respectively, in accordance with the display data, are turned off and set in an operative state, respectively. Hence, a forward bias is applied to selected pixels, concerning the selected unit electrodes of both the scanning electrodes and data electrodes, to cause them to emit light. The nonselected unit electrodes of the scanning electrodes X 1  to X n  are set at a power supply potential V B  by pull-up means R c  comprising resistors and the like. The nonselected unit electrodes of the data electrodes Y 1  to Y m  are set at the ground potential by pull-down means R e  comprising resistors and the like. A reverse bias is applied to the nonselected pixels concerning the nonselected unit electrodes of both the scanning electrodes and data electrodes, and a zero bias or a bias equal to or lower than an emission threshold is applied to the nonselected pixels concerning the selected and nonselected unit electrodes. In this manner, crosstalk caused by the semi-excited state of the nonselected pixels is prevented. 
     In the prior art shown in FIG. 1, for the sake of simplicity, each of the current supply means  10   1  to  10   m  is constituted by one transistor. In fact, a higher-precision constant-current circuit is often employed as the current supply means so that a difference in luminance does not occur among pixels due to a voltage drop caused by the interconnection resistance of the scanning electrodes and data electrodes. 
     The problem of the conventional method of driving an organic thin-film EL display device described above is that the response speed from selection of a pixel to emission of the selected pixel is low. 
     The reason for this will be described hereinafter. 
     FIG. 2 is an equivalent circuit diagram of a drive circuit concerning an organic thin-film EL display device and a conventional driving method. 
     Scanning electrodes X 1  to X n  are connected, through switches  7   1  to  7   n , to ground when they are selected and to a power supply voltage V B  when they are not selected. Data electrodes Y 1  to Y m  are connected, through switches  11   1  to  11   m , to corresponding current supply means  10   1  to  10   m  when they are selected and to ground when they are not selected. Each pixel D(x:1 to n, y:1 to m) having an organic thin-film EL structure is indicated by a diode and a parallel capacitance. As an example, a case will be described wherein a certain unit electrode X i  of the scanning electrodes is selected, and in accordance with this a certain unit electrode Y j  of the data electrodes is selected, so that a pixel D(i, j) concerning the both unit electrodes is caused to emit light. 
     FIG. 3 is a timing chart showing the conventional method of driving an organic thin-film EL display device. FIG. 3 shows the switching operations of the switches  7   i−1 ,  7   i ,  7   i+1 , and  11   j  of FIG. 2 and a change over time of the potential of each of the unit electrode X i  of the scanning electrodes and of the unit electrode Y j  of the data electrodes caused by these switching operations. 
     Immediately before a time period t i  during which the unit electrode X i  of the scanning electrodes is selected by the switch  7   i  and set at the ground potential, the unit electrode X i−1  of the scanning electrodes is selected by the switch  7   i−1  and set at the ground potential, or all the scanning electrodes X 1  to X n  are in the nonselected state. Hence, at least the unit electrodes of the (n−1) scanning electrodes are at the power supply potential V B . At this time, if the unit electrode Y j  of the data electrodes is not selected by the switch  11   j , as indicated by a solid line, the unit electrode Y j  of the data electrodes is at the ground potential, so that a reverse bias is applied to at least (n−1) pixels of pixels D(1, j) to D(n, j) concerning the scanning electrodes X 1  and X n  and the unit electrode Y j  of the data electrodes, and that the respective parallel capacitances of these (n−1) pixels are charged in the reverse bias direction. Thereafter, during the time period t i , the unit electrode X i  of the scanning electrodes is selected by the switch  7   i , and the unit electrode Y j  of the data electrodes is selected by the switch  11   j . Then, the potential of the unit electrode X i  of the scanning electrodes is quickly set at the ground potential. However, the current from the current supply means  10   i  connected to the unit electrode Y j  of the data electrodes through the switch  11   j  is used to cancel the storage capacitance in the reverse bias direction of at least (n−1) pixels described above. Hence, the potential of the unit electrode Y j  of the data electrodes does not increase at once, and accordingly a delay time t d  occurs until a forward bias is applied to the pixel D(i, j) to cause it to emit light. In particular, if the current supply means  10   j  is a constant-current circuit, the potential of the unit electrode Y j  of the data electrodes increases only as a linear function of time elapsed since the unit electrode Y j  is selected. As a result, the delay time t d  described above increases further. 
     SUMMARY OF THE INVENTION 
     The present invention has been made in consideration of the above problems in the prior art, and has as its object to provide a method of driving an organic thin-film EL display device wherein, when a forward bias is applied between a selected unit electrode of scanning electrodes and a selected unit electrode of data electrodes to cause a selected pixel concerning the both selected unit electrodes to emit light, and a reverse bias is applied between the nonselected unit electrodes of the scanning electrodes and the nonselected unit electrodes of the data electrodes, thereby preventing crosstalk caused by a semi-excited state of the nonselected pixels, a large delay is not caused in emission of the selected pixel, and large-capacity display can be coped with. 
     In order to achieve the above object, according to the first aspect of the present invention, there is provided a method of driving an organic thin-film EL display device, wherein when a forward bias is applied between a selected unit electrode of scanning electrodes and a selected unit electrode of data electrodes to cause a selected pixel concerning both of the selected unit electrodes to emit light, and a reverse bias is applied between nonselected unit electrodes of the scanning electrodes and nonselected unit electrodes of the data electrodes, thereby preventing crosstalk caused by a semi-excited state of the nonselected pixels, all of the scanning electrodes and all of the data electrodes are short-circuited once, immediately before a predetermined unit electrode of the data electrode, which should be selected in accordance with selection of each of the unit electrodes of the scanning electrodes, is selected, to set all of the pixels at a zero bias. 
     According to the second aspect of the present invention, there is provided a method of driving an organic thin-film EL display device, wherein when a forward bias is applied between a selected unit electrode of scanning electrodes and a selected unit electrode of data electrodes to cause a selected pixel concerning both of the selected unit electrodes to emit light, and a reverse bias is applied between nonselected unit electrodes of the scanning electrodes and nonselected unit electrodes of the data electrodes, thereby preventing crosstalk caused by a semi-excited state of the nonselected pixels, all of the scanning electrodes and a predetermined unit electrode of the data electrodes are short-circuited once, immediately before the predetermined unit electrode of the data electrode, which should be selected in accordance with selection of each of the unit electrodes of the scanning electrodes, is selected, to set pixels concerning all of the scanning electrodes and the predetermined unit electrode of the data electrodes at a zero bias. 
     According to the third aspect of the present invention, there is provided a drive circuit for an organic thin-film EL display device, wherein when a forward bias is applied between a selected unit electrode of scanning electrodes and a selected unit electrode of data electrodes to cause a selected pixel concerning both of the selected unit electrodes to emit light, and a reverse bias is applied between nonselected unit electrodes of the scanning electrodes and nonselected unit electrodes of the data electrodes, thereby preventing crosstalk caused by a semi-excited state of the nonselected pixels, all of the scanning electrodes and all of the data electrodes are short-circuited once, immediately before a predetermined unit electrode of the data electrode, which should be selected in accordance with selection of each of the unit electrodes of the scanning electrodes, is selected, to set all of the pixels at a zero bias. 
     According to the fourth aspect of the present invention, there is provided a drive circuit for an organic thin-film EL display device, wherein when a forward bias is applied between a selected unit electrode of scanning electrodes and a selected unit electrode of data electrodes to cause a selected pixel concerning both of the selected unit electrodes to emit light, and a reverse bias is applied between nonselected unit electrodes of the scanning electrodes and nonselected unit electrodes of the data electrodes, thereby preventing crosstalk caused by a semi-excited state of the nonselected pixels, all of the scanning electrodes and a predetermined unit electrode of the data electrodes are short-circuited once, immediately before the predetermined unit electrode of the data electrode, which should be selected in accordance with selection of each of the unit electrodes of the scanning electrodes, is selected, to set pixels concerning all of the scanning electrodes and the predetermined unit electrode of the data electrodes at a zero bias. 
     As is apparent from the aspects described above, the effect of the present invention resides in that, even when a forward bias is applied between the selected unit electrode of the scanning electrodes and the selected unit electrode of the data electrodes to cause the selected pixel concerning both of the selected unit electrodes to emit light, and a reverse bias is applied between the nonselected unit electrodes of the scanning electrodes and the nonselected unit electrodes of the data electrodes, thereby preventing crosstalk caused by a semi-excited state of the nonselected pixels, a large delay does not occur in emission of the selected pixel. 
     The reason for this is as follows. All of the scanning electrodes and all of the data electrodes, or all of the scanning electrodes and the unit electrodes of a data electrode, which should be selected next, are short-circuited once, immediately before a predetermined unit electrode of the data electrode, which should be selected in accordance with selection of each of the unit electrodes of the scanning electrodes, is selected, to set all the pixels, or a pixel concerning the unit electrode of the data electrode, which should be selected next, at a zero bias. Therefore, a forward bias is quickly applied to the selected pixel without accompanying discharge of the storage capacitance of the pixel which has been reverse-biased immediately before the zero bias operation. 
     The above and many other objects, features and advantages of the present invention will become manifest to those skilled in the art upon making reference to the following detailed description and accompanying drawings in which preferred embodiments incorporating the principles of the present invention are shown by way of illustrative examples. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows an equivalent circuit of matrix driving of organic thin-film EL elements disclosed in Japanese Unexamined Patent Publication No. 6-301355; 
     FIG. 2 is an equivalent circuit diagram of a drive circuit concerning an organic thin-film EL display device and a conventional driving method; 
     FIG. 3 is a timing chart showing the conventional drive method of the organic thin-film EL display device; 
     FIG. 4 is an equivalent circuit diagram of a drive circuit concerning an organic thin-film EL display device and a drive method according to the first embodiment of the present invention; 
     FIG. 5 is a timing chart showing the drive method of the drive circuit shown in FIG. 4; 
     FIG. 6 shows the schematic arrangement of one embodiment of the organic thin-film EL display device; 
     FIG. 7 shows the equivalent circuit of the organic thin-film EL display device and a drive circuit that realizes one embodiment of the present invention; 
     FIG. 8 is a timing chart of pulses that control the drive circuit shown in FIG. 7; 
     FIG. 9 is a circuit diagram constituting one of current supply means; 
     FIG. 10 is a timing chart showing a method of driving an organic thin-film EL display device according to the second embodiment of the present invention; and 
     FIG. 11 is a timing chart showing a method of driving an organic thin-film EL display device according to the third embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Several preferred embodiments of the present invention will be described with reference to the accompanying drawings. 
     FIG. 4 is an equivalent circuit diagram of a drive circuit concerning an organic thin-film EL display device and a drive method according to the first embodiment of the present invention. 
     Scanning electrodes X 1  to X n  are respectively connected to switches  7   1  to  7   n , so that they are are connected to ground when they are selected and to a power supply voltage V B  when they are not selected. 
     Data electrodes Y 1  to Y m  are respectively connected to switches  11   1  to  11   m , so that they are connected to the corresponding current supply means  10   1  to  10   m  when they are selected and to ground when they are not selected. 
     The respective current supply means  10   1  to  10   m  are connected in parallel to switches  12   1  to  12   m  that short-circuit them. As an example, a case will be described wherein a pixel D(i, j) is selected to emit light. 
     FIG. 5 is a timing chart showing the drive method of the drive circuit shown in FIG.  4 . FIG. 5 shows the switching operations of the switches  7   i−1 ,  7   i ,  7   i+1 ,  11   j , and  12   j  shown in FIG. 4, and a change over time of the potential of a unit electrode X i  of the scanning electrodes and of a unit electrode Y j  of the data electrodes caused by the switching operations. 
     In a time period t i−1  during which a unit electrode X i−1  of the scanning electrodes is selected by the switch  7   i−1  and connected to ground, the switch  11   j  connects the unit electrode Y j  of the data electrodes to either the current supply means  10   j  or ground in accordance with a display data. At this time, if the unit electrode Y j  of the data electrodes is connected to ground, as indicated by solid lines, a zero bias is applied to a pixel D(i−1, j), and a reverse bias is applied to pixels D(1, j) to D(i−2, j), and pixels D(i, j) to D(n, j), to charge the parallel capacitances of these pixels in the reverse bias direction. Then, a time period t B  follows during which the switches  7   1  to  7   n  connect all the scanning electrodes X 1  to X n  to the power supply voltage V B . In the time period t B , the switches  11   1  to  11   m  connect all the data electrodes Y 1  to Y m  to the corresponding current supply means  10   1  to  10   m . Simultaneously, the switches  12   1  to  12   m  are closed, and all the data electrodes Y 1  to Y m  are short-circuited to all the scanning electrodes X 1  to X n . Accordingly, the storage capacitances of the pixels that have been charged in the reverse bias direction in the time period t i−1  are discharged quickly regardless of the current supply means  10   j , and all the pixels are zero-biased. Thereafter, during a time period t i , when the unit electrode X i  of the scanning electrodes is selected by a switch  7   i  and the switch  11   j  connects the unit electrode Y j  of the data electrodes to the current supply means  10   j , the potential of the unit electrode Y j  of the data electrodes increases immediately, and no delay occurs in emission of the pixel D(i, j). 
     FIG. 6 shows the schematic arrangement of one embodiment of the organic thin-film EL display device. 
     An ITO film having a thickness of 120 [nm] was formed on a glass substrate  20  by sputtering, and 256 transparent stripe electrodes  21   1  to  21   256  each having a width of 0.3 mm were formed on the ITO film with a pitch of 0.33 mm by photolithography. A hole injection layer  22 , a hole transport layer  23 , an emission layer  24 , and an electron transport layer  25  each constituted by an organic thin film were formed on the stripe electrodes  21   1  to  21   256  by vacuum deposition, and 300-[nm] thick stripe electrodes  26   1  to  26   64  made of an Al—Li alloy were formed on the resultant structure by vacuum deposition to perpendicularly intersect the transparent stripe electrodes. This organic thin-film EL display device was driven by the prior art by using the stripe electrodes  26   1  to  26   64  as the scanning electrodes. The turn-on delay time of a selected pixel was 150 to 200 [μs]. 
     FIG. 7 shows the equivalent circuit of the organic thin-film EL display device and a drive circuit that realizes one embodiment of the present invention. FIG. 8 is a timing chart of pulses that control the drive circuit shown in FIG.  7 . 
     An X-driver  30  is a 64-stage shift resistor that generates a pulse having a width of 90 [μs] at a shift interval of 104 [μs]. Upon reception of this shift pulse, transistors  31   1  to  31   64  and transistors  32   1  to  32   64  sequentially switch the stripe electrodes  26   1  to  26   64 . More specifically, when the ith shift pulse is input, a transistor  31   i  is turned on and a transistor  32   i  is turned off to ground a stripe electrode  26   i . Other stripe electrodes  26   1  to  26   i−1  and  26   i+1  to  26   64  are connected to the power supply voltage V B  since the transistors  31   1  to  31   i−1  and  31   i+1  to  31   64  are turned on and the transistors  32   1  to  32   i−1  and  32   i+1  to  32   64  are turned off. 
     In synchronism with the rise of the shift pulse of the X-driver  30 , a Y-driver  40  generates 256 parallel pulses in accordance with display data, and the inverted pulses of these parallel pulses are input to the bases of transistors  33   1  to  33   256 , respectively. For example, when the base of a transistor  33   j  goes low, a transistor  33   j  is turned off. A current from a current supply means  60   j  is supplied to the transparent stripe electrode  21   j . When the base of the transistor  33   j  goes high, the transistor  33   j  is turned on to ground the transparent stripe electrode  21   j . A pulse generator  50  generates a pulse that falls and rises in synchronism with the fall and rise, respectively, of any shift pulse from the X-driver  30 . The pulse from the pulse generator  50  is input to the bases of transistors  34   1  to  34   256  simultaneously. In a time period t B  during which this pulse is kept low, all the transistors  31   1  to  31   64  are turned off, all the transistors  32 , to  32   64  are turned off, all the transistors  33   1  to  33   256  are turned off, and all the transistors  34   1  to  34   256  are turned on. Hence, the potential of the transparent stripe electrodes  21   1  to  21   256  and the potential of the stripe electrodes  26   1  to  26   64  are all set at the power supply voltage V B , and all the organic thin-film EL pixels are set in the zero-bias state. 
     FIG. 9 is a circuit diagram constituting one of current supply means  60   1  to  60   256 . 
     The turn-on delay time of a selected pixel in this embodiment was equal to or less than 5 [μs]. 
     FIG. 10 is a timing chart showing a method of driving an organic thin-film EL display device according to the second embodiment of the present invention. FIG. 10 shows the switching operations of switches  7   i−1 ,  7   i ,  7   i+1 , and  11   j  of the arrangement similar to that of FIG. 2 showing the conventional drive circuit, and a change over time of the potential of each of a unit electrode X i  of the scanning electrodes and of a unit electrode Y j  of the data electrodes caused by the switching operations. 
     As an example, a case will be described wherein a pixel D(i, j) is selected to emit light. 
     In a time period t i−1  during which a unit electrode X i −1  of the scanning electrodes is selected by the switch  7   i−1  and connected to ground, the switch  11   j  connects the unit electrode Y j  of the data electrodes to either the current supply means  10   j  or ground in accordance with display data. At this time, if the unit electrode Y j  of the data electrodes is connected to ground, as indicated by solid lines, a zero bias is applied to a pixel D(i−1, j), and a reverse bias is applied to pixels D(1, j) to D(i−2, j), and pixels D(i, j) to D(n, j), to charge the parallel capacitances of these pixels in the reverse bias direction. 
     Then, a time period t B  follows during which the switches  7   1  to  7   n  connect all the scanning electrodes X 1  to X n  to the power supply voltage V B . In the time period t B , the switches  11   1  to  11   m  connect all the data electrodes Y 1  to Y m  to ground. Hence, all the data electrodes Y 1  to Y m  and all the scanning electrodes X 1  to X n  are short-circuited. Accordingly, the storage capacitances of the pixels that have been charged in the reverse bias direction in the time period t i−1  are discharged quickly regardless of the current supply means  10   j , and all the pixels are zero-biased. 
     Thereafter, during a time period t i , when a unit electrode X i  of the scanning electrodes is selected by a switch  7   i  and the switch  11   j  connects the unit electrode Y j  of the data electrodes to the current supply means  10   j , the potential of the unit electrode Y j  of the data electrodes increases immediately, and no delay occurs in emission of the pixel D(i, j). 
     FIG. 11 is a timing chart showing a method of driving an organic thin-film EL display device according to the third embodiment of the present invention. FIG. 11 shows the operations of switches  7   i−1 ,  7   i ,  7   i+1 ,  11   j ,  11   j+1 , and  12   j  in the arrangement similar to that shown in FIG. 4, and a change over time of the potential of the unit electrodes X i−1 , X i , and X i+1  of the scanning electrodes and of the unit electrodes Y j−1 , Y j , and Y j+1  of the data electrodes caused by the switching operations. 
     As an example, a case will be described wherein a pixel D(i, j) is selected to emit light. 
     In a time period t i−1  during which the unit electrode X i−1  of the scanning electrodes is selected by the switch  7   i−1  and connected to ground, the switches  11   j−1 ,  11   j , and  11   j+1  connect the unit electrodes Y j−1 , Y j , and Y j+1  of the corresponding data electrodes to either the corresponding current supply means  10   j−1 , Y j , and  10   j+1  or ground in accordance with display data. At this time, if the unit electrodes Y j−1 , Y j , and Y j+1  of the data electrodes are connected to ground, as indicated by solid lines, a zero bias is applied to pixels D(i−1, j−1), D(i−1, j), and D(i−1, j+1), and a reverse bias is applied to pixels D(1, j−1) to D(i−2, j−1), pixels D(1, j) to D(i−2, j), pixels D(1, j+1) to D(i−2, j+1), pixels D(i, j−1) to D(n, j−1), pixels D(i, j) to D(n, j), and pixels D(i, j+1) to D(n, j+1), to charge the parallel capacitances of these pixels in the reverse bias direction. 
     Then, a time period t B  follows during which the switches  7   1  to  7   n  connect all the scanning electrodes X 1  to X n  to the power supply voltage V B . In the time period t B , of the switches  11   1  to  11   m , only a switch concerning the unit electrode of a data electrode which should be selected in the time period t 1 , during which the unit electrode X i  of the scanning electrode is to be selected later, is connected to the corresponding current supply means. Simultaneously, the switches  12   1  to  12   m  are closed, and only the data electrode of the data electrodes Y 1  to Y m  which is selected in the time period t i , and all the scanning electrodes X 1  to X n  are short-circuited. As an example, a case wherein only the unit electrode Y j  of the data electrodes is selected in the time period t i  was indicated by a solid line. Accordingly, the storage capacitance of only a pixel, of the pixels that have been charged in the reverse bias direction in the time period t i−1 , which should be selected in the period time t i  is discharged quickly regardless of the current supply means  10   j , and is set at the zero bias. 
     In this manner, the charging/discharging loss, which occurs when a pixel which is not selected in the time period t i  is reverse-biased again, can be decreased.