Abstract:
The present invention relates to an uniformly active light emitting diode drive circuit. This invention provides a 3T1C circuit structure in the emitting pixel and an additional data capacitors connecting to all the pixels are picked out and located on one side of the display panel. In addition, the connecting lines to the OLED on every pixel are all collected to one end of a transistor Moc on the other side of the display panel. Through the arrangement, it is intended that the aperture ratio of the organic electroluminescent (OLED) device can be largely improved. Moreover, an additional by-pass current transistor in parallel with data capacitor (Cd) in a data generator region outside of the pixel array can by-pass the previous left current in the circuit and thus enhance the contrast ratio of the emitting pixel.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention pertains to organic electroluminescent device, and more specifically to an active organic electroluminescent drive circuit structure for improving the uniformities and contrast in organic electroluminescent devices.  
         BACKGROUND OF THE INVENTION  
         [0002]    Matrix displays are well known in the art, where pixels are illuminated using matrix addressing as illustrated in FIG. 1. A typical passive matrix organic electroluminescent display  100  comprises a plurality of picture or display elements (pixels)  160  that are arranged in rows and columns. The display incorporates a column data generator  110  and a row select generator  120 . In practical operation, each row is sequentially activated via row line  130 , where the corresponding pixels are activated using the corresponding column lines  140 . In a passive matrix display, each row of pixels is illuminated sequentially one by one, whereas in an active matrix display, each row of pixels is first loaded with data sequentially. Namely, each row in the passive matrix display is only “active” for a fraction of the total frame time, whereas each row in the active matrix display can be set to be “active” for almost the entire total frame time.  
           [0003]    A typical active matrix organic electroluminescent display has been achieved in the prior art (See U.S. Pat. No. 6,157,356). In this patent, as shown in FIG. 2, the emitting pixel includes a switch transistor M 1   230 , a drive transistor  240 , a data line  210 , a select line  220 , a source capacitor Cs  250 , a power supply Vdd  270  and an OLED  260 . With use, the gate to source voltage (threshold voltage) of the “drive transistor” M 2  may vary, thereby causing a change in the current passing through the LED. This varying current contributes to the nonuniformity in the intensity of the display. Sometimes, in this scheme due to the production quality of the drive transistor M 2   240 , it would lead to a consequence that ultimately produces threshold voltage variations in the pixels.  
           [0004]    Another contribution to the nonuniformity in intensity of the display can be found in the manufacturing of the “drive transistor”. In some cases, the “drive transistor” is manufactured from a material that is difficult to ensure uniformity of the transistors such that variations exist from pixel to pixel.  
           [0005]    Significant improvement in threshold voltage variations has been achieved in the prior art (See U.S. Pat. No. 6,229,506). In this patent, a design of four transistors two capacitors (4T2C) structure to compensate the threshold voltage of the drive transistor in each pixel was demonstrated to improve the uniformity in the intensity of the emitting pixels. However, in this scheme as shown in FIG. 3. The pixel structure adopts a data line  310 , a scan line  320 , a power supply Vdd line  305 , an AZ line  390  and AZb control line  395 , four transistors  330 ,  340 ,  370 ,  350 , auto-zero capacitors  355 ,  380  and an OLED  360 . In this scheme, the addition of the transistor is used to compensate the threshold voltage of the drive transistor M 2   340  in order to improve the uniformity of the emitting pixel. However, the addition of the device components occupy too much space in the tiny pixel structure and brings aperture ratio loss, moreover, there is always accompanying a contrast problem when conducting the auto-zero period, a slight current will run through organic electroluminescent (OLED) devices thus tends to reduce the contrast of the emitting pixel.  
           [0006]    [0006]FIG. 4 depicts a schematic diagram of a time domain of the control signal in accordance with the FIG. 3, the time domain is separated as the auto-zero threshold voltage period and write data period. Before conducting the auto-zero period, M 1   330 , M 3   370  are off, M 2   340  and M 4   350  are on, during this time period, the current running through OLED is the current of the previous frame time. Then after a while, M 1   330  is on, then M 3   370  is on sequentially, thus a connection of the drain and the gate of M 2   340  can be conducted as a diode. Then after a while, M 4   350  is off, then the current voltage of gate of M 2   340  raise to a value of Vdd-Vth (threshold voltage). At this instant, the M 3   370  is off, then the threshold voltage of M 2   340  will be recorded in the capacitor C 2   355 , thus fulfill the auto-zero action.  
           [0007]    It is a purpose of this invention to provide a new method to improve the uniformity of the emitting pixel and meanwhile to improve the aperture ratio of the organic electroluminescent (OLED) device.  
           [0008]    It is another purpose of this invention to provide a new organic electroluminescent (OLED) device for display with improved contrast problem.  
         SUMMARY OF THE INVENTION  
         [0009]    The above problems and others are at least partially solved and the above purposes and others are realized in an organic electroluminescent device shown as follow:  
           [0010]    According to the present invention, there is first obtained a three transistor one capacitor (3T1C) structure in every single pixel and the data capacitors (Cd) connecting to the three transistor one capacitor (3T1C) structure of every pixel are picked out and collected into a data generator region on one side of the display panel. In addition, the connecting lines to the OLED of every pixel are all collected to one end of a transistor Moc on the other side of the display panel.  
           [0011]    Through the arrangement mentioned above, it is intended that the uniformity of the emitting pixels and the aperture problems of the organic electroluminescent (OLED) device can be largely improved.  
           [0012]    In another preferred embodiment, through another arrangement in the design of the circuit, the addition of a by-pass current transistor Mby which is in parallel with data capacitor (Cd) in the data generator region, the function of the by-pass current transistor Mby device can easily reduce the current when conducting the auto-store threshold voltage period and enhance the contrast of the emitting pixel during their operation. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]    The foregoing aspects and many of the attendant advantages of this invention will become more readily appreciated as the same becomes better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein:  
         [0014]    [0014]FIG. 1 depicts a block diagram of a passive-matrix addressing display;  
         [0015]    [0015]FIG. 2 depicts a two transistors and a storage capacitor circuit structure (prior art);  
         [0016]    [0016]FIG. 3 depicts a four transistors and two capacitors circuit structure (prior art);  
         [0017]    [0017]FIG. 4 depicts a drive circuit time domain diagram of a four transistors and two capacitors circuit structure (prior art);  
         [0018]    [0018]FIG. 5 depicts a three transistors one capacitor and the data capacitor embedded in a data generator pixel structure of the present invention;  
         [0019]    [0019]FIG. 6 depicts the display panel structure of a three transistors one capacitor and data capacitor embedded in a data generator of the display panel in the present invention;  
         [0020]    [0020]FIG. 7 depicts a drive circuit time domain diagram of the three transistors one capacitor circuit structure of the present invention;  
         [0021]    [0021]FIG. 8 depicts the drive circuit structure of the display panel with the addition of a by-pass transistor in parallel with the data capacitor in the data generator region of the present invention;  
         [0022]    [0022]FIG. 9 depicts a drive circuit time domain diagram of the addition of a by-pass transistor in parallel with the data capacitor in the data generator region drive circuit; and  
         [0023]    [0023]FIG. 10 depicts a three transistors one capacitor of pixel structure implemented by NMOS transistors of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0024]    [0024]FIG. 5 depicts a schematic diagram of an active matrix OLED pixel structure  500  of the present invention. In the preferred embodiment, the active matrix OLED pixel is implemented using LPTS thin film transistors OLED or a-Si thin film transistors OLED, e.g., transistors manufactured using amorphous or poly-silicon, or crystal-silicon. Although the present pixel structure is implemented using thin film transistors and an organic illuminescent device, it should be understood that the present invention be implemented using other types of transistors and light emitting diodes.  
         [0025]    Referring to FIG. 5, pixel structure  500  comprises three PMOS transistors  530 ,  540 ,  550 , a storage capacitor Cs  555 , an OLED  560  (light element) and a transistor Moc  565 . A scan line  520  is connected to the gate of transistor  530 . A TW line  590  is connected to the gate of the transistor  550 . A data line  510  is connected to the drain of the transistor  550 ,  530  and a power supply Vdd line  505  is connected to the source of the transistor  540  and one end of the storage capacitor Cs  555  while the other end of Cs  555  line is connected to the source of transistor  530  and the gate of the transistor  540 . One electrode of the OLED  560  is connected to the drain of the transistor  540  and the source of the transistor  550 . A collecting transistor of Moc  565  is connected to the OLED  560  in the pixel.  
         [0026]    [0026]FIG. 6 depicts a schematic diagram of a plane view of an active display panel structure of the present invention. In this preferred embodiment, it is obvious that the drain of the transistor  530  and the drain of the transistor  550  in every emitting pixel and are now extended out to connect to a data capacitor Cd  602  in the data generator region  608 , in which the data generator region is now located on one side of the display panel. Furthermore, the lines connecting to the OLED of every pixel are now all collected to one end of a collecting transistor Moc  565 , in which the Moc  565  is then located on the other side of the display panel and not in the pixel array region.  
         [0027]    In sum, the emitting display plane panel structure can be concluded as being separated in five parts which include the data generator region  608  outside of the pixel, the select generator  520  outside of the pixel array region, the TW control line  590  outside of the pixel array region, the Moc transistor  565  outside of the pixel array region, and a plurality of pixels of arrays in the middle of the display panel.  
         [0028]    [0028]FIG. 7 depicts a schematic diagram of a time domain of the control signal. In this diagram, the time domain is separated as the auto-store threshold voltage period and scan (data in) and display period. In the auto-store threshold voltage period, the scan 1  to scan N are varied once in a frame time which can be easily seen from the diagram that the scan 1 , scan 2 , scan 3 , . . . , scanN are first starts “high” (which makes M 1  ‘off’) and then turns “low” (which makes M 2  ‘on’) after a while in the same instant. During the period of “low” of the scan 1 , scan 2 , scan 3 , . . . , scanN, the TW  705  is “high” then “low” and “high”, while the OC  707  is first started “low” then “high”. During the period when TW  705  is “low” and OC  707  is “low”, a current will flow from Vdd  505  through M 2   540 , OLED  560  to the drain of Moc  565 . Thus Cs capacitor  555  records a voltage in which it depends on the characteristics of M 2   540  and the light element OLED  560 . On the other hand, during the period of TW  705  “low”, and OC “high”, Moc  565  is then switched to “off” and Cs  555  auto-stores the threshold voltage of M 2   540 .  
         [0029]    While during the scan (data in) and display period, every sequence of scanning step of “high” to “low”, the variation of data signal will couple through Cd  602 , M 1   530  to Cs  555  and adds to the former threshold voltage in M 2   540  of every pixels. After scanning all the scan lines, OC  707  is set from “high” to “low”, so Moc  565  is switched “on”. The wanted current flowing from Vdd  505  and running through M 2   540 , OLED  560  will makes the OLED  560  emit light more uniformly. So the current of OLED  560  will not depend on the threshold voltage of M 2   540  and depends on the data signal coupled only.  
         [0030]    The advantage of the scheme mentioned above, i.e., that the data capacitor Cd  602  connecting to all of the emitting pixels are collected in the data generator region  608  and the lines connecting to the OLED  560  of every pixel are now connected to a transistor Moc  565  which is located on the other side of the display panel. Through this arrangement, it can largely improve the aperture ratio of the pixel array. Moreover, the entire pixel array layout of this invention exhibits only the scan line  520 , the data line  510 , the Vdd line  505  and the TW control line  590  which can definitely simplify the display panel pixel control complicacy.  
         [0031]    In another preferred embodiment as shown in FIG. 8 and FIG. 9, the layout of the pixel structure is the same as that in FIG. 5 and FIG. 6, while the only distinction is the addition of a by-pass transistor Mby  808  which is in parallel with the data capacitor Cd  602  located in the data generator region  608 . The function of the additional Mby  808  is to reduce the current of OLED during the auto-store threshold voltage period by switching lower data signal to the anode end of OLED. This kind of scheme can improve the contrast because reducing the current of OLED during the auto-store threshold voltage period would also reduce the unwanted light in the auto-store threshold voltage period.  
         [0032]    [0032]FIG. 9 depicts a schematic diagram of a time domain of the control signal. In this diagram, the time domain is separated as the auto-store threshold voltage period and scan (data in) and display period. In the auto-store threshold voltage, the scan 1  to scan N are varied once in a frame time which can be easily seen from the diagram that the scan 1 , scan 2 , scan 3 , . . . , scanN are first started “high” (which makes M 1  “off”) and then turned “low” (which makes M 2  “on”) after a while in the same instant. During the period of the scan 1 , scan 2 , scan 3 , . . . , scanN “low”, TW  905  is “high” then “low” and “high”, OC  907  is “low” then “high”, BY  909  is “high” then “low” and “high”. During the period of TW  905  “low” and OC  907  is “low”, BY  909  is “low” which makes lower level data signal to the anode end of OLED. So only very low level of current flows from Vdd  505  through M 2   540 , OLED  560  to the drain of Moc  565 . During the period of TW  905  “low”, OC  907  “high” and BY “high”, Moc  565  and Mby  808  are switched “off” and Cs  555  auto-stores the threshold voltage of M 2   540 .  
         [0033]    Although the present invention is described using PMOS transistors, it should be understood that the present invention be implemented using NMOS transistors, wherein the associated relevant voltages are reversed. As referring to FIG. 10, pixel structure  1000  comprises three NMOS transistors  1030 ,  1040 ,  1050 , a storage capacitor Cs  1055 , an OLED  1060  (light element) and a transistor Moc  1065 . The main distinction between FIG. 10 and FIG. 5 is the NMOS M 2   1040  instead of the PMOS M 2  in FIG. 5.  
         [0034]    As will be understood by persons skilled in the art, the foregoing preferred embodiment of the present invention is illustrative of the present invention rather than limiting the present invention. Having described the invention in connection with a preferred embodiment, modification will now suggest itself to those skilled in the art. Thus, the invention is not to be limited to this embodiment, but rather the invention is intended to cover various modifications and similar arrangements included within the spirit and scope of the appended claims, the scope of which should be accorded the broadest interpretation so as to encompass all such modification and similar structure.  
         [0035]    While the preferred embodiment of the invention has been illustrated and described, it will be appreciated that various changes can be made therein without departing from the spirit and scope of the invention.