Patent Publication Number: US-2005140304-A1

Title: Organic electroluminescent device and driving circuit thereof

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
      This Non-provisional application claims priority under 35 U.S.C. § 119(a) on patent application Ser. No(s). 09/213,7854 filed in Taiwan, Republic of China on Dec. 31, 2003, the entire contents of which are hereby incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION  
      1. Field of Invention  
      The invention relates to a flat display device and a driving circuit thereof and, in particular, to an organic electroluminescent (OEL) device and a driving circuit thereof.  
      2. Related Art  
      Technology and type of the display device become diversified day by day as the application scope is widely extended and the transmitted information is increased. The display device began as displaying monochromic pictures, and then is capable of displaying color images and three-dimensional images. It also began as a CRT (cathode ray tube) device, is then a flat display device, and is developed towards a portable, foldable and large-screen display device. Regarding to the progress of the display device, the goals of the relative research are to provide a display device, which is more humanized and more convenient for users.  
      In view of the flat display devices, they generally include LCD (liquid crystal display) devices or organic electroluminescent devices. No matter which display device is concerned, the driving circuit for driving a pixel is necessary. As shown in  FIG. 1 , the driving circuit of a conventional electroluminescent device comprises a plurality of scan lines  11  (Y j , Y j+1 , . . . ), a plurality of data lines  12  (X i , X i+1 , . . . ), a plurality of organic light-emitting units  13 , a plurality of light driving units  14  and a plurality of power lines  15 .  
      The light driving unit  14  comprises a first transistor  141 , a second transistor  142  and a capacitor  143 . The first transistor  141  includes a gate  1411 , a first electrode  1412  and a second electrode  1413 . The gate  1411  of the first transistor  141  connects with the j th  scan line, the first electrode  1412  connects with the i th  data line, and the second electrode  1413  connects with one terminal of the capacitor  143 . The other terminal of the capacitor  143  connects with one power line  15 . The second transistor  142  includes a gate  1421 , a third electrode  1422  and a fourth electrode  1423 . The gate  1421  of the second transistor  142  connects with the second electrode  1413  and one terminal of the capacitor  143 . The third electrode  1422  connects with one organic light-emitting unit  13 , and the fourth electrode  1423  connects with the power line  15 .  
      As mentioned above, regarding to the driving circuit of the conventional organic electroluminescent device, the voltage or current signals output from the data line  12  are provided from the power line  15  having the same direction as that of the data line  12 . In other words, the voltage or current signals output from the data line  12  must follow through the power line  15 . Concerning the organic light-emitting unit  13 , since the voltage or current signals output from the data lines  12  have different path lengths, the resistance thereof are different, which results in the different voltages or currents following into the organic light-emitting unit  13 . Thus, the brightness of the organic light-emitting units  13  is not uniform. Besides, the aperture ratio of the light-emitting area (regarding to the organic light-emitting unit  13 ) becomes smaller since the driving circuit of the conventional organic electroluminescent device includes a plurality of power lines  15 , which must be disposed above the organic electroluminescent panel. As a result, the display effect thereof is poor.  
      It is therefore an important subjective of the invention to uniform the brightness of the organic light-emitting unit and to enlarge the aperture ratio of the light-emitting area of the organic light-emitting unit.  
     SUMMARY OF THE INVENTION  
      In view of the foregoing, the invention is to provide an organic electroluminescent device, which has organic light-emitting units with uniform brightness and light-emitting area with enlarged aperture ratio, and driving circuit thereof.  
      To achieve the above, an organic electroluminescent device of the invention comprises M scan lines, N data lines, a plurality of organic light-emitting units and a plurality of light driving units. The light driving units drive the organic light-emitting units, respectively. The light driving unit comprises a first transistor, a second transistor and a capacitance unit. The first transistor comprises a first gate, a first electrode and a second electrode. The first gate connects with the (M−j+1) th  scan line. The first electrode connects with the i th  data line. Wherein, i is equal to or smaller than N and is equal to or greater than 1, j is smaller than M and is equal to or greater than 1, and M, N, i and j are all positive integrals. The second transistor comprises a second gate, a third electrode and a fourth electrode. The second gate connects with the second electrode of the first transistor, the third electrode connects with the (M−j) th  scan line, and the fourth electrode connects with the organic light-emitting unit driven by the light driving unit. The capacitance unit has a first terminal and a second terminal, wherein the first terminal connects with the (M−j) th  scan line and the second terminal connects with the second electrode and the second gate.  
      In addition, a driving circuit of the organic electroluminescent device of the invention comprises a plurality of light driving units, which drive the organic light-emitting units. The light driving unit comprises a first transistor, a second transistor and a capacitance unit. The first transistor comprises a first gate, a first electrode and a second electrode. The first gate connects with the (M−j+1) th  scan line. The first electrode connects with the i th  data line. Wherein, i is equal to or smaller than N and is equal to or greater than 1, j is smaller than M and is equal to or greater than 1, and M, N, i and j are all positive integrals. The second transistor comprises a second gate, a third electrode and a fourth electrode. The second gate connects with the second electrode of the first transistor, the third electrode connects with the (M−j) th  scan line, and the fourth electrode connects with the organic light-emitting unit driven by the light driving unit. The capacitance unit has a first terminal and a second terminal, wherein the first terminal connects with the (M−j) th  scan line and the second terminal connects with the second electrode and the second gate. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The invention will become more fully understood from the detailed description given herein below illustration only, and thus is not limitative of the present invention, and wherein:  
       FIG. 1  is a schematic view showing the driving circuit of the conventional organic electroluminescent device;  
       FIG. 2  is a schematic view showing a driving circuit of an organic electroluminescent device according to a preferred embodiment of the invention;  
       FIG. 3A  is a timing chart showing a voltage signal inputted into each data line shown in  FIG. 2 ;  
       FIG. 3B  is a timing chart showing a voltage signal inputted into each scan line shown in  FIG. 2 ;  
       FIG. 4  is a schematic view showing a driving circuit of an organic electroluminescent device according to another preferred embodiment of the invention;  
       FIG. 5A  is a timing chart showing a voltage signal inputted into each data line shown in  FIG. 4 ; and  
       FIG. 5B  is a timing chart showing a voltage signal inputted into each scan line shown in  FIG. 4 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      The present invention will be apparent from the following detailed description, which proceeds with reference to the accompanying drawings, wherein the same references relate to the same elements.  
      With reference to  FIG. 2 , an organic electroluminescent device of the invention comprises M scan lines  21 , N data lines  22 , a plurality of organic light-emitting units  23  and a plurality of light driving units  24 . In the embodiment, the scan lines  21 , the data lines  22 , the organic light-emitting units  23  and the light driving units  24  can be disposed above a substrate (not shown).  
      The light driving units  24  drive the organic light-emitting units  23 . The light driving unit  24  comprises a first transistor  241 , a second transistor  242  and a capacitance unit  243 . The first transistor  241  comprises a first gate  2411 , a first electrode  2412  and a second electrode  2413 . The first gate  2411  connects with the (M−j+1) th  scan line  21 . The first electrode  2412  connects with the i th  data line  22 . Wherein, i is equal to or smaller than N and is equal to or greater than 1, j is smaller than M and is equal to or greater than 1, and N, M, i and j are all positive integrals. The second transistor  242  comprises a second gate  2421 , a third electrode  2422  and a fourth electrode  2423 . The second gate  2421  connects with the second electrode  2413  of the first transistor  241 , the third electrode  2422  connects with the (M−j) th  scan line  21 , and the fourth electrode  2423  connects with the organic light-emitting unit  23  driven by the light driving unit  24 . The capacitance unit  243  has a first terminal  2431  and a second terminal  2432 . In this case, the first terminal  2431  connects with the (M−j) th  scan line  21  and the second terminal  2432  connects with the second electrode  2413  and the second gate  2421 .  
      To be noted, in the present embodiment, when j is equal to M, the third electrode  2422  connects with the M th  scan line Y M , and the first gate  2411  connects with the first scan line Y 1 . In other words, the last and first scan lines are correspondingly utilized for driving the transistor.  
      Referring to  FIG. 2 , the light driving units  24  drive the organic light-emitting units  23 . The light driving unit  24  comprises a first transistor  241 , a second transistor  242  and a capacitance unit  243 . In this embodiment, the first transistor  241  comprises a first gate  2411 , a first electrode  2412  and a second electrode  2413 . The first gate  2411  connects with the second scan line Y 2 . The first electrode  2412  connects with the first data line  22 . The second transistor  242  comprises a second gate  2421 , a third electrode  2422  and a fourth electrode  2423 . The second gate  2421  connects with the second electrode  2413  of the first transistor  241 , the third electrode  2422  connects with the first scan line Y 1 , and the fourth electrode  2423  connects with the organic light-emitting unit  23  driven by the light driving unit  24 . The capacitance unit  243  has a first terminal  2431  and a second terminal  2432 . In this case, the first terminal  2431  connects with the first scan line Y 1  and the second terminal  2432  connects with the second electrode  2413  and the second gate  2421 .  
      In the current embodiment, the first transistor  241  and the second transistor  242  are a PMOS arrangement. Accordingly, the first electrode  2412  and the second electrode  2413  are the source and drain of the first transistor  241 , and the third electrode  2422  and the fourth electrode  2423  are the source and drain of the second transistor  242 . The capacitance unit  243  is a capacitor.  
      Hereinafter, the actual driving process of the organic electroluminescent device of the invention is described with reference to  FIGS. 3A and 3B .  
      When the voltage signal V Y1  as shown in  FIG. 3B  is inputted into the first scan line Y 1  as shown in  FIG. 2 , the first scan line Y 1  is charged with a negative bias signal during a period T 1 . Accordingly, the first gate  2411  of the first transistor  241  is ON, and the voltage signals V 1 , V 2 , V 3 , etc., as well as current signals, loaded on the i th  data line X i  can be written into the capacitance unit  243  through the first transistor  241 . In addition, since the M th  scan line Y M  is at a positive bias state during the period T 1 , the second transistor  242  is ON. Thus, the electricity stored in the capacitance unit  243  follows through the organic light-emitting unit  23  so as to drive the organic light-emitting unit  23 . Of course, when the voltage signal V Y2  as shown in  FIG. 3B  is inputted into the second scan line Y 2  as shown in  FIG. 2 , the second scan line Y 2  is charged with a negative bias signal during a period T 2 . Therefore, the first gate  2411  of the first transistor  241  is ON, and the voltage signals V 1 , V 2 , V 3 , etc., as well as current signals, loaded on the i th  data line X i  can be written into the capacitance unit  243  through the first transistor  241 .  
      An organic electroluminescent device according to another embodiment of the invention will be described hereinafter with reference to  FIGS. 4, 5A  and  5 B. In this embodiment, the same elements are described referring to the same references mentioned above, and the first transistor  241 ′ and the second transistor  242 ′ are a NMOS arrangement.  
      The light driving units  24  drive the organic light-emitting units  23 . The light driving unit  24  comprises a first transistor  241 ′, a second transistor  242 ′ and a capacitance unit  243 . The first transistor  241 ′ comprises a first gate  2411 ′, a first electrode  2412 ′ and a second electrode  2413 ′. The first gate  2411 ′ connects with the (M−j+1) th  scan line  21 . The first electrode  2412 ′ connects with the i th  data line  22 . Wherein, i is equal to or smaller than N and is equal to or greater than 1, j is smaller than M and is equal to or greater than 1, and M, N, i and j are all positive integrals. The second transistor  242 ′ comprises a second gate  2421 ′, a third electrode  2422 ′ and a fourth electrode  2423 ′. The second gate  2421 ′ connects with the second electrode  2413 ′ of the first transistor  241 ′, the third electrode  2422 ′ connects with the (M−j) th  scan line  21 , and the fourth electrode  2423 ′ connects with the organic light-emitting unit  23  driven by the light driving unit  24 . The capacitance unit  243  has a first terminal  2431  and a second terminal  2432 . Herein, the first terminal  2431  connects with the (M−j) th  scan line  21  and the second terminal  2432  connects with the second electrode  2413  and the second gate  2421 .  
      With reference to  FIG. 4 , the light driving units  24  drive the organic light-emitting units  23 . The light driving unit  24  comprises a first transistor  241 ′, a second transistor  242 ′ and a capacitance unit  243 ′. In this embodiment, the first transistor  241 ′ comprises a first gate  2411 ′, a first electrode  2412 ′ and a second electrode  2413 ′. When j is equal to M, the third electrode  2422 ′ connects with the M th  scan line Y M , and the first gate  2411 ′ connects with the first scan line Y 1 . In other words, the last and first scan lines are correspondingly utilized for driving the transistor. The first gate  2411 ′ connects with the second scan line Y 2 . The first electrode  2412 ′ connects with the first data line  22 . The second transistor  242 ′ comprises a second gate  2421 ′, a third electrode  2422 ′ and a fourth electrode  2423 ′. The second gate  2421 ′ connects with the second electrode  2413 ′ of the first transistor  241 ′, the third electrode  2422 ′ connects with the first scan line Y 1 , and the fourth electrode  2423 ′ connects with the organic light-emitting unit  23  driven by the light driving unit  24 . The capacitance unit  243  has a first terminal  2431  and a second terminal  2432 . In this case, the first terminal  2431  connects with the first scan line Y 1  and the second terminal  2432  connects with the second electrode  2413 ′ and the second gate  2421 ′.  
      Hereinafter, the actual driving process of the organic electroluminescent device of the invention is described with reference to  FIGS. 5A and 5B .  
      When the voltage signal V Y1  as shown in  FIG. 5B  is inputted into the first scan line Y 1  as shown in  FIG. 4 , the first scan line Y 1  is charged with a positive bias signal during a period T 1 . Accordingly, the first gate  2411 ′ of the first transistor  241 ′ is ON, and the voltage signals V 1 , V 2 , V 3 , etc., as well as current signals, loaded on the i th  data line X i  can be written into the capacitance unit  243  through the first transistor  241 ′. In addition, since the M th  scan line Y M  is at a negative bias state during the period T 1 , the second transistor  242 ′ is ON. Thus, the electricity stored in the capacitance unit  243  follows through the organic light-emitting unit  23  so as to drive the organic light-emitting unit  23 .  
      The driving circuit of the organic electroluminescent device of this embodiment is similar to the driving circuit described in the previous embodiment, so the detailed descriptions are omitted for concise purpose.  
      Briefly described, the conventional power lines are not disposed in the organic electroluminescent device and driving circuit of the invention, and the driving unit is provided with the voltage or current source from the two scan lines, including the previous one and next one. Thus, the paths through which the voltage or current signals pass can have the same resistance. Accordingly, the brightness of the organic light-emitting unit is uniform. In addition, since the conventional power lines are not disposed in the organic electroluminescent device and driving circuit of the invention, regarding to the organic light-emitting unit, the aperture ratio of the light-emitting area is enlarged, resulting in the increase of display effect.  
      Although the invention has been described with reference to specific embodiments, this description is not meant to be construed in a limiting sense. Various modifications of the disclosed embodiments, as well as alternative embodiments, will be apparent to persons skilled in the art. It is, therefore, contemplated that the appended claims will cover all modifications that fall within the true scope of the invention.