Patent Publication Number: US-7212179-B2

Title: Light emitting display device, electronic equipment into which the same device is loaded, and drive method of the light emitting display device

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a light emitting display device in which light emitting elements constituting pixels are actively driven for example by TFTs (Thin Film Transistors) and in particular to a light emitting display device which can effectively restrain intensity non-uniformity among respective pixels which occurs due to variations in characteristics of light emission drive transistors that give drive current to the respective light emitting elements. 
   2. Description of the Related Art 
   Demand for a display panel which has a high definition image display function and which can realize a thin shape and low power consumption has increased due to popularity of cellular telephones, personal digital assistants (PDAS), and the like, and conventionally a liquid crystal display panel has been adopted in many products as the one which meets the needs thereof. Meanwhile, these days a self light emitting display panel utilizing an organic EL element whose characteristic as a self light emitting type display element is best used has been manufactured, and this have attracted attention as a next generation display panel in place of the conventional liquid crystal display panel. A background thereof is that by employing, in a light emission functional layer of the element, an organic compound which enables an excellent light emission characteristic to be expected, a high efficiency and a long life which can be equal to practical use have been advanced. 
   The organic EL element is constructed by laminating a transparent electrode for example by ITO, a light emission functional layer, and a metallic electrode one by one basically on a transparent substrate such as glass or the like. The light emission functional layer may be a single layer of an organic light emitting layer, or a double layer structure composed of an organic positive hole transport layer and an organic light emitting layer, or a triple layer structure composed of an organic positive hole transport layer, an organic light emitting layer, and an organic electron transport layer, or a multilayer structure in which an injection layer of electron or positive hole is inserted into an appropriate portion among these layers. 
   The organic EL element can be electrically replaced by a structure composed of a light emitting component having a diode characteristic and a parasitic capacitance component which is connected in parallel to this light emitting component, and thus the organic EL element can be said to be a capacitive light emitting element. When a light emission drive voltage is applied to this organic EL element, at first, electrical charges corresponding to the electric capacity of this element flow into the electrode as a displacement current and are accumulated. It can be considered that when the drive voltage then exceeds a determined voltage (light emission threshold voltage=Vth) peculiar to this element, current begins to flow from one electrode (anode side of the diode component) to an organic layer constituting the light emitting layer so that the element emits light at an intensity proportional to this current. 
   As a display panel employing such organic EL elements, a passive matrix type display panel in which EL elements are simply arranged in a matrix pattern and an active matrix type display panel in which active elements for example constituted by TFTs are added to respective EL elements arranged in a matrix pattern have been proposed. The latter active matrix type display panel can realize low power consumption compared to the former passive matrix type display panel and has a characteristic that crosstalk among pixels is small, whereby it is particularly suitable for a high definition display constituting a large screen. 
     FIG. 1  shows an example of a circuit structure corresponding to one pixel  10  in an active matrix type display panel already proposed. The circuit structure of the pixel  10  shown in this  FIG. 1  shows an example in which a lighting drive method called the SES (Simultaneous Erasing Scan) method which realizes time division gradation expression is adopted. 
   In the structure of this pixel  10 , a data signal Vdata corresponding to a video signal supplied from a data driver  11  is supplied to source S of a scan selection transistor, that is, a data write transistor Tr 2 , via a data line arranged in a display panel. A data write signal Write is supplied from a scan driver  12  to the gate G of the data write transistor Tr 2  via a scan selection line. 
   The drain D of the data write transistor Tr 2  is connected to gate G of a light emission drive transistor Tr 1  and to one terminal of a light emission maintaining capacitor C 1 . The source S of the light emission drive transistor Tr 1  is connected to the other terminal of the capacitor C 1  and to an anode side drive power source Va. Further, the drain D of the light emission drive transistor Tr 1  is connected to anode terminal of an organic EL element E 1  provided as a light emitting element, and cathode terminal of this organic EL element E 1  is connected to a cathode side drive power source Vc. 
   The pixel structure shown in  FIG. 1  is provided with an erase transistor Tr 3 , and an erase signal Erase is supplied from an erase driver  13  to the gate of this erase transistor Tr 3  via an erase signal line. The source S and drain D of the erase transistor Tr 3  are connected to end portions of the light emission maintaining capacitor C 1 , respectively. 
   In the pixel  10  shown in  FIG. 1 , only the light emission drive transistor Tr 1  is constituted by p-channel type TFT, and other transistors are constituted by n-channel type TFTs. A large number of the pixels  10  of the above-described structure are arranged in a matrix pattern in a row direction and a column direction to construct the display panel. 
   In the structure of the pixel  10  shown in  FIG. 1 , an ON voltage Write as a scan signal is supplied from the scan driver  12  to the gate of the data write transistor Tr 2  during an address period. Thus, current corresponding to the data signal Vdata supplied from the data driver  11  flows in the light emission maintaining capacitor C 1  via the source and drain of the data write transistor Tr 2  so that the capacitor C 1  is charged. Its charge voltage is supplied to the gate of the light emission drive transistor Tr 1 , and the transistor Tr 1  allows drain current Id corresponding to a gate-to-source voltage (Vgs) which is based on the gate voltage thereof and on the drive power source Va supplied to the source to flow in the EL element E 1 , whereby the EL element E 1  emits light. 
   When the gate of the data write transistor Tr 2  becomes an OFF voltage after the address period elapses, the transistor Tr 2  becomes in a so-called cut-off state. However, the gate voltage of the light emission drive transistor Tr 1  is maintained by electrical charges accumulated in the capacitor C 1 , and thus drive current to the EL element E 1  is retained. Accordingly, a lighting state of the EL element E 1  which corresponds to the data signal Vdata can be continued until a period to a next address operation (for example, a next one frame period or a next one subframe period). 
   Meanwhile, in the middle of the lighting period of the EL element E 1  (for example, in the middle of one frame period or one subframe period), the erase signal Erase which turns the erase transistor Tr 3  on is supplied from the erase driver  13 . In the case where this erase transistor Tr 3  is turned on, electrical charges charged in the capacitor C 1  are erased (charged) instantly, and as a result, the, light emission drive transistor Tr 1  becomes in the cut-off state, whereby the EL element E 1  is instantly extinguished. 
   In other words, by controlling output timing of the gate-on voltage supplied from the erase driver  13 , the lighting period of the EL element E 1  for example during one frame or one subframe period is controlled, and thus multi-gradation expression can be realized. The structure of a pixel provided with the erase transistor Tr 3  in addition to the data write transistor Tr 2  and the light emission drive transistor Tr 1  as described above is disclosed in the following Japanese Patent Application Laid-open No. 2001-343933. 
   In many of these types of light emitting elements represented by the organic EL element have a current dependency that the light emission intensity is determined in response to the drive current. Meanwhile, regarding the light emission drive transistor employed in the above-described pixel structure, variations occurs in the characteristic of drain current Id with respect to the gate-to-source voltage Vgs, particularly in the characteristic of the gate-to-source voltage Vgs, that is, of the threshold, at which the drain current Id begins to flow. Thus, even though the same level of data signal Vdata is supplied, variations in light emission intensities occur among pixels. 
   Such variations in light emission intensities among pixels allow, particularly when an animation image and the like is reproduced, a vague stripe pattern or a phenomenon resembling flicker to be generated, thereby causing a problem that display quality is considerably degraded. Thus, in order to solve the problem, it is necessary to make the characteristic of TFTs formed in a display panel uniform, and regarding this, conventionally, various discussions and developments have been carried out. However, regarding this, there exist a number of technical problems including a problem of selection of a semiconductor material or other materials, a problem of a manufacturing process, a problem of a manufacturing environment, and the like, and it is difficult to pursue a fundamental solution. 
   SUMMARY OF THE INVENTION 
   The present invention has been developed based on the above-described technical viewpoint, and it is an object of the present invention to provide a light emitting display device equipped with pixel circuits which can suppress variations of light emission intensities generated among pixels by correcting threshold characteristics of light emission drive transistors constituting light emitting pixels within respective pixels, and particularly to provide a light emitting display device equipped with the pixel circuits which can be appropriately adopted in a structure of the SES method already described and the drive method thereof. 
   A light emitting display device according to the present invention which has been developed in order to solve the problem is a light emitting display device in which a large number of pixels are arranged, said pixel having a structure comprising a data write transistor which charges a light emission maintaining capacitor in accordance with a data signal, a light emission drive transistor which supplies drive current to a light emitting element based on the voltage charged in said light emission maintaining capacitor, and an erase transistor which can discharge the charge voltage charged in said light emission maintaining capacitor, characterized by further comprising a correction voltage write transistor which short circuits a gate and a drain of said light emission drive transistor at a timing at which said erase transistor is turned on and a correction voltage retaining capacitor in which a threshold voltage generated between the gate and a source of said light emission drive transistor is written by a short circuit operation of said correction voltage write transistor, and characterized by being constructed in such a way that said light emission drive transistor supplies the drive current to said light emitting element based on both charge voltages charged in said light emission maintaining capacitor and said correction voltage retaining capacitor. 
   A drive method of a light emitting display device according to the present invention which has been developed in order to solve the problem is a drive method of a light emitting display device in which a large number of pixels are arranged, said pixel having a structure comprising a data write transistor which charges a light emission maintaining capacitor in accordance with a data signal, a light emission drive transistor which supplies drive current to a light emitting element based on the voltage charged in said light emission maintaining capacitor, and an erase transistor which can discharge the charge voltage charged in said light emission maintaining capacitor, characterized by sequentially repeatedly performing a first step of short-circuiting a gate and a drain of said light emission drive transistor at a timing at which said erase transistor is turned on so as to allow a threshold voltage generated between the gate and a source of said light emission drive transistor to be written in a correction voltage retaining capacitor, and a second step of supplying light emission drive current to said light emitting element by said light emission drive transistor based on said threshold voltage written in said correction voltage retaining capacitor and a voltage charged in the light emission maintaining capacitor in accordance with said data signal. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a circuit structure diagram showing a pixel structure of a conventional SES drive method. 
       FIG. 2  is a circuit structure diagram showing an example of a pixel structure adopted in a light emitting display device according to the present invention. 
       FIG. 3  is a view explaining a state in which a correction voltage is written in the pixel structure shown in  FIG. 2 . 
       FIG. 4  is a view explaining a state in which data writing is performed in the pixel structure shown in  FIG. 2  similarly. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
   A light emitting display device and the drive method thereof according to the present invention will be described below based on an embodiment shown in the drawings. Any of  FIGS. 2–4  show an embodiment of a light emitting display device according to the present invention, and  FIG. 2  explains a structure of one pixel  10 .  FIG. 3  explains a state in which a correction voltage is written in the same pixel structure, and further  FIG. 4  shows a state in which data writing is performed in the same pixel structure and shows a state which follows this and in which an EL element as a light emitting element is driven to emit light. 
   First, in the structure shown in  FIG. 2 , the respective data driver  11 , scan driver  12 , erase driver  13 , light emission drive transistor Tr 1 , data write transistor Tr 2 , erase transistor Tr 3 , light emission maintaining capacitor C 1 , organic EL element E 1  as a light emitting element, anode side drive power source Va, and cathode side drive power source Vc perform the same functions as those of the respective constituent components shown in  FIG. 1  already described. Therefore, respective description thereof will be omitted. 
   In the structure of the pixel  10  shown in  FIG. 2 , a correction voltage retaining capacitor C 2  lies between the drain of the erase transistor Tr 3 , that is, one terminal of the light emission maintaining capacitor C 1 , and the gate of the light emission drive transistor Tr 1 . In the structure shown in  FIG. 2 , drain and source of a correction voltage write transistor Tr 4  constituted by an n-channel type TFT are connected between the gate and the drain of the light emission drive transistor Tr 1 . 
   Further, in the pixel structure shown in  FIG. 2 , source and drain of a switching transistor Tr 5  constituted by a p-channel type TFT are connected in series between the drain of the light emission drive transistor Tr 1  and the anode terminal of the EL element. The gate of the correction voltage write transistor Tr 4  and the gate of the switching transistor Tr 5  are commonly connected to the gate of the erase transistor Tr 3  preferably as shown by the broken line, and are respectively given ON/OFF control by the erase signal Erase provided from the erase driver  13 . 
   That is, the erase transistor Tr 3  and the correction voltage write transistor Tr 4  constituted by n-channel type TFTs are both brought to an ON state in a state in which the erase signal Erase provided from the erase driver  13  is at level “H.” At this time, the switching transistor Tr 5  constituted by a p-channel type TFT is brought to an OFF state. In a state in which the erase signal Erase is at level “L,” the erase transistor Tr 3  and the correction voltage write transistor Tr 4  are both brought to the OFF state, and the switching transistor Tr 5  is brought to the ON state. 
     FIG. 3  explains a state in which a correction voltage is written as described earlier, and in this case, the erase signal Erase from the erase driver is brought to the level “H.” Further, in this state, the data write signal Write from the scan driver is brought to the state of the level “L.” Accordingly, the erase transistor Tr 3  and the correction voltage write transistor Tr 4  are both brought to the ON state, and the data write transistor Tr 2  and the switching transistor Tr 5  are both brought to the OFF state. 
   Thus, the end portions of the light emission maintaining capacitor C 1  are short circuited by the erase transistor Tr 3 , so that one end of the correction voltage retaining capacitor C 2  is connected to the anode side drive power source Va. In other words, the respective terminals of the correction voltage retaining capacitor C 2  are connected to the gate and source of the light emission drive transistor Tr 1 , respectively. At this time, the gate and the drain of the light emission drive transistor Tr 1  are short circuited by the correction voltage write transistor Tr 4 . Accordingly, the correction voltage retaining capacitor C 2  and the light emission drive transistor Tr 1  are brought to the state of an equivalent circuit enclosed by the broken line on a lower left of  FIG. 3 . 
   As can be understood from the equivalent circuit, the threshold voltage (Vth) in the light emission drive transistor Tr 1  is written in the correction voltage retaining capacitor C 2 . In this state, since the light emission drive transistor Tr 1  is brought to a bias state in which slightly drain current starts to flow therein, the switching transistor Tr 5  is brought to OFF as described above so as to prevent the drain current from being supplied to the EL element E 1  provided as a light emitting element. 
     FIG. 4  shows a state in which data writing is performed, and in this state the erase signal Erase from the erase driver is brought to the level “L.” Further, in this state, the data write signal Write from the scan driver is brought to the state of the level “H.” Accordingly, the erase transistor Tr 3  and the correction voltage write transistor Tr 4  are both brought to the OFF state, and the data write transistor Tr 2  and the switching transistor Tr 5  are both brought to the ON state. 
   At this time, in the case where pixels are controlled for being lit, the level “H” is supplied from the data driver as the data signal Vdata, and in the case where pixels are controlled for being extinguished, the level “L” is supplied. Accordingly, the terminal voltage of the maintaining capacitor C 1 , that is, the electrical potential of the connection point between the capacitor C 1  and the correction voltage retaining capacitor C 2 , is brought to the level “L” in the case where the pixels are lit and is brought to the level “H” in the case where the pixels are extinguished. 
   Here, for example, in the case where the pixels are lit, the threshold voltage (Vth) of the light emission drive transistor written in the correction voltage retaining capacitor C 2  is added to the terminal voltage of the level “L” written in the light emission maintaining capacitor C 1 , so that this is supplied between the gate and source of the light emission drive transistor Tr 1 . 
   That is, by allowing the threshold voltage (Vth) to be added to a data voltage written in the light emission maintaining capacitor C 1 , a data voltage through which variations in respective threshold voltages of the light emission drive transistors Tr 1  are corrected is supplied between the gate and source of the light emission drive transistor Tr 1 . 
   Accordingly, even when there exist variations in the threshold voltages of the light emission drive transistors Tr 1 , respective pixels perform lighting operations of respective EL elements E 1  in accordance with the data voltages written in the light emission maintaining capacitors C 1 . Thus, the lighting operations are performed in a state in which variations in light emission intensities caused by variations in the threshold voltages of the light emission drive transistors are suppressed among respective pixels. 
   In addition to this, in this embodiment, as already described, for example in the middle of one frame period or one subframe period, the erase signal Erase for turning the erase transistor Tr 3  on is supplied from the erase driver, and thus multi-gradation expression can be realized. In this manner, at the timing at which the erase transistor Tr 3  is turned on, as described with reference to  FIG. 3 , the operation of writing the threshold voltage of the light emission drive transistor in the correction voltage retaining capacitor C 2  is performed once again. Then, by repeating of the operations described with reference to  FIGS. 3 and 4 , the lighting operation for the pixels is continued. 
   As is apparent from the description above, with the light emitting display device and the drive method thereof according to the present invention, since variations in the threshold voltages of the light emission drive transistors can be corrected, occurrences of variations of light emission intensities among respective pixels can be effectively restrained. Thus, a problem that a vague stripe pattern or a phenomenon resembling flicker is generated, causing considerable deterioration in display quality, can be resolved. 
   In the case where the present invention is adopted in the pixel structure of the SES drive method shown as an embodiment, it is not necessary to specially dispose a signal line for controlling the gate voltages of the correction voltage write transistor and the switching transistor. Further, since the writing operation of the threshold voltage can be performed for the correction voltage retaining capacitor at the same time as an erasing operation of electrical charges of the light emission maintaining capacitor by the erase transistor, the operation is rational, and the lighting control of the pixels can be performed easily. 
   Although organic EL elements are employed as light emitting elements in the embodiment described above, this light emitting element is not limited to the organic EL element, and even in a case where a current-dependent light emitting element whose light emission intensity is determined in response to drive current is utilized, operations and effects similar to those in the above can be obtained. By adopting the above-described light emitting display device in a variety of electronic equipment calling for this type of display device other than the cellular telephones or portable information terminal described in the opening, the operations and effects already described can be produced as they are.