Abstract:
An organic electroluminescence (OEL) matrix-type single-pixel driver, which comprises: an OEL device, a first transistor, and a second transistor. The first transistor and the second transistor form a complementary structure so that when the data line uses the first transistor to drive an organic light-emitting diode (OLED) device, the second transistor is in the OFF state, causing no power consumption. When the data line is in the LOW state, the first transistor is in the OFF state. The second transistor is in a sub-threshold state after getting rid of extra charges.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     The present invention relates to a single-pixel driver and, in particular, to an organic electroluminescence matrix-type single-pixel driver. 
     2. Related Art 
     The organic electroluminescence (OEL) structure usually consists of a glass substrate, a transparent indium-tin-oxide (ITO) anode, HTL&amp;EML, and a metal cathode. When a voltage is imposed on such an OEL display, electrons and holes flow into the HTL&amp;EML through the anode and the cathode, respectively. The annihilation of electrons and holes produces excitons and radiate photons. The OEL displays can be roughly classified into two different systems according to the material. The molecule-based device using dye or color materials is called an organic light-emitting diode (OLED), and the polymer-based device using conjugate polymers is called a polymer light-emitting diode (PLED). OEL displays have many advantages such as self-luminescence, back-light source free, high illumination efficiencies, low operation voltages, quick responses, no view angle limitations, wide operation temperature ranges, low power consumption, low manufacturing costs, being able to produce true colors, and extremely small thickness. They satisfy all the requirements for multimedia and will be the most favorable devices for modern displays. 
     Recently, due to the need in high resolutions in display panels, the pixel rate also increases. OLED devices  10 , however, are limited by its material characters and parasite capacitance and thus cannot readily turn off pixels when the operation frequency increases accordingly (around 50 KHz). As shown in FIG. 1, VEE can connect to a low potential or negative pulse. A scan line  20  provides scan signals and a data line  30  controls the switch of transistors  40  so as to make the OLED device  10  emit light. The brightness can be further changed by adjusting the pulse width and amplitude imposed on the data line  30 . Its drawback is that when the operation frequencies of both the scan line  20  and the data line  30  increase, the charge/discharge time is greater than the width of the pulse because of the OLED parasite capacitance effect. Thus, some pixels cannot become dark readily; that is, the OLED devices cannot easily turn off the pixels. For a conventional circuit as shown in FIG. 1A, where the transistor  40  is replaced by an NPN transistor  41 , the OLED device still cannot readily turn off the pixel. 
     Accordingly, designing an OLED driver that can increase the operation frequency of the OLED and at the same time satisfy the requirements for high resolutions has become an important subject. 
     SUMMARY OF THE INVENTION 
     It is a primary objective of the present invention to provide a single-pixel driver, whose driving method is to use a transistor to control and accelerate the charge/discharge work speed of OLED devices so as to reach the needed work frequency (1 MHz). 
     The present invention adds a bypass transistor for discharging in a conventional driver so as to solve the response delay due to the parasite capacitance effect and to speed up charge removal. The circuit includes at least: an organic electroluminescence (OEL) device, a first transistor, and a second transistor. The first transistor and the second transistor form a complementary structure so that when the data line uses the first transistor to drive the OLED device, the second transistor is in the OFF state, causing no power consumption. When the data line is in the LOW state, the first transistor is in the OFF state. The second transistor is in a sub-critical state after getting rid of extra charges. Therefore, the only power loss in the whole circuit is due to the leakage current of the first transistor. The power loss is in the order of pico-watts. 
     The first transistor and the second transistor proposed herein can be replaced by an NPN transistor, a PNP transistor, an NMOS or a PMOS. 
     The driver disclosed herein can be accompanied by a resistor so as to linearly control the voltage. The resistor can be replaced by an active transistor load. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will become more fully understood from the detailed description given hereinbelow illustration only, and thus are not limitative of the present invention, and wherein: 
     FIGS. 1 and 1A are circuits of conventional organic EL matrix-type single-pixel drivers; 
     FIGS. 2,  2 A,  2 B, and  2 C are circuits of the organic EL matrix-type single-pixel drivers according to the first embodiment of the invention; 
     FIGS. 3 and 3A are circuits of the organic EL matrix-type single-pixel drivers according to the second embodiment of the invention; 
     FIGS. 4 and 4A are circuits of the organic EL matrix-type single-pixel drivers according to the third embodiment of the invention; and 
     FIG. 5 is a schematic view of the driving voltages of the scan line and the data line in the disclosed organic EL matrix-type single-pixel driver; 
    
    
     In the various drawings, the same references relate to the same elements. 
     DETAILED DESCRIPTION OF THE INVENTION 
     An organic light-emitting diode (OLED) display is a matrix of OLED devices, each of which forms a pixel, and each column in the matrix has a scan line and each row has a data line. The light-emitting behavior of the OLED devices is controlled by manipulating the potentials on the scan line and the data line. 
     To solve the problem of the inability to readily turn off pixels in conventional organic electroluminescence (OEL) matrix-type single-pixel drivers, the present invention controls the OLED devices by controlling the scan line and utilizing VDD. The invention further proposes to add a bypass transistor for discharging to a conventional driver so as to eliminate the response delay effect due to parasite capacitance and to speed up charge removal. With reference to FIG. 2, VDD is a voltage source and the scan line  20  is used to selectively scan. When the scan line  20  is at LOW, it is enabled; while when the scan line  20  is at HIGH, it is disenabled. The data line  30  controls the switch of an NPN transistor  41  so as to make the OLED device  10  emit light. To increase the switch frequency of the OLED device  10 , a PNP transistor  42  is employed to solve the response delay effect caused by the parasite capacitance and to speed up charge removal. The brightness is adjusted by further varying the voltage amplitude imposed on the data line  30 . When the data line  30  is at LOW, the NPN transistor  41  is in the OFF state. The PNP transistor  42  enters the sub-critical state after discharging extra charges. Therefore, the only power consumption is caused by the leakage current of the NPN transistor  41  and is on the order of pico-watts. 
     The collector of the NPN transistor  41  couples to the voltage source VDD. The emitter of the NPN transistor  41  and the emitter of the PNP transistor  42  couple together to the anode of the OLED device  10 . The base of the NPN transistor  41  and the base of the PNP transistor  42  couple together to the data line  30 . The cathode of the OLED device  10  couples to the scan line  20 . The collector of the PNP transistor  42  couples to the ground (GND). 
     FIGS. 2A,  2 B and  2 C show variations of the OEL matrix-type single-pixel driver according to the first embodiment. 
     FIG. 2A illustrates that the NPN transistor  41  can be replaced by an NMOS  43  and the PNP transistor  42  can be replaced by a PMOS  44 . FIG. 2B says that the PNP transistor  42  can be replaced by a PMOS  44 . FIG. 2C shows that the NPN transistor  41  is replaced by an NMOS  43 . These variations, however, still share the same functions and characters of that in FIG.  2 . 
     In FIG. 2A, the drain of the NMOS  43  couples to VDD. The source and the base of the NMOS  43  and the source and the base of the PMOS  44  couple together to the anode of the OLED device  10 . The gate of the NMOS  43  and the gate of the PMOS  44  couple together to the data line  30 . The cathode of the OLED device  10  couples to the scan line  20 . The drain of the PMOS  44  couples to GND. 
     In FIG. 2B, the collector of the NPN transistor  41  couples to VDD. The emitter of the NPN transistor  41  and the source and the base of the PMOS  44  couple together to the anode of the OLED device  10 . The base of the NPN transistor  41  and the gate of the PMOS  44  couple together to the data line  30 . The cathode of the OLED device  10  couples to the scan line  20 . The drain of the PMOS  44  couples to GND. 
     In FIG. 2C, the drain of the NMOS  41  couples to VDD. The source and the base of the NMOS  43  and the emitter of the PNP transistor  42  couple together to the anode of the OLED device  10 . The gate of the NMOS  43  and the base of the PNP transistor  42  couple together to the data line  30 . The cathode of the OLED device  10  couples to the scan line  20 . The collector of the PNP transistor  42  couples to GND. 
     With reference to FIG. 3, VDD is a tunable voltage source. The scan line  20  is used to selectively scan. When the scan line  20  is at LOW, it is enabled; when the scan line  20  is at HIGH, it is disenabled. The data line  30  controls the switch of an NMOS  43  and adjusts the voltage, thus controlling the brightness of the OLED device  10 . Assisted by a resistor  45 , a linear control on the voltage can be achieved. To increase the switch frequency of the OLED device  10 , a PMOS  44  is similarly employed to solve the response delay effect caused by parasite capacitance and to speed up charge removal. The drain of the NMOS  43  couples to VDD through the resistor  45 . The source and the base of the NMOS  43  and the source and the base of the PMOS  44  couple together to the anode of the OLED device  10 . The gate of the NMOS  43  and the gate of the PMOS  44  couple together to the data line  30 . The cathode of the OLED device  10  couples to the scan line  20 . The drain of the PMOS  44  couples to GND. 
     With reference to FIG. 3A, the NMOS  43  and the PMOS  44  in the second embodiment of the invention are replaced by a PMOS  44  and an NMOS  43 , respectively. The source and the base of the PMOS  44  couple together to VDD through the resistor  45 . The drain of the PMOS  44  and the drain of the NMOS  43  couple together to the anode of the OLED device  10 . The gate of the PMOS  44  and the gate of the NMOS  43  couple together to the data line  30 . The cathode of the OLED device  10  couples to the scan line  20 . The source and the base of the NMOS  43  couple together to GND. 
     With reference to FIG. 4 for a third embodiment of the invention, the resistor  45  in FIG. 3 is replaced by an active NMOS  43  load. The new driver still has the same functions and characters as that in FIG.  3 . FIG. 4A is a variation circuit of the OEL matrix-type single-pixel driver according to the third embodiment of the invention. The resistor  45  in FIG. 3A is replaced by an active NMOS  43 . The new driver still has the same functions and characters as that in FIG.  3 A. 
     FIG. 5 is a schematic view of the driving voltages of the scan line and the data line in the disclosed organic EL matrix-type single-pixel driver. 
     ADVANTAGES OF THE INVENTION 
     The present invention proposes to add a bypass transistor for discharging in a conventional driver to solve the response delay effect caused by parasite capacitance and to speed up charge removal. It has the advantages of: 
     1. high resolutions under high speed; 
     2. energy saving in practical applications; 
     3. achieving gray scale effects by adjusting the work voltage; and 
     4. having a longer lifetime. 
     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.