Patent Publication Number: US-10311788-B2

Title: Organic light-emitting display and driving method thereof

Description:
This application claims the priority benefit of Korean Patent Application No. 10-2016-0160638 filed on Nov. 29, 2016, which is hereby incorporated herein by reference for all purposes as if fully set forth herein. 
     BACKGROUND 
     Field of the Invention 
     The present invention relates to an active-matrix organic light-emitting display and a driving method thereof. 
     Discussion of the Related Art 
     Flat-panel displays (FPDs) are widely used for monitors for desktop computers, portable computers such as laptops and personal digital assistants (PDAs), mobile phones, and so on, because they offer advantages in terms of slimness and light-weightness. Such flat-panel displays include liquid-crystal displays (LCDs), plasma display panels (PDPs), field emission displays (FEDs), and organic light-emitting diode (OLED) displays. 
     An OLED, which is a self-luminous device, comprises an anode and a cathode, and an organic compound layer formed between the anode and cathode. The organic compound layer comprises a hole transport layer HTL, an emission layer EML, and an electron transport layer ETL. When an operating voltage is applied to the anode and the cathode, a hole passing through the hole transport layer HTL and an electron passing through the electron transport layer ETL move to the emission layer EML, forming an exciton. As a result, the emission layer EML generates visible light. 
     An active matrix organic-light emitting display comprises a plurality of OLEDs each emitting light itself, and is used in various applications due to their advantages of fast response time, high luminous efficiency, high luminance, and wide viewing angle. 
     In the organic light-emitting display, pixels each comprising an OLED are arranged in a matrix, and the luminance of the pixels is adjusted depending on the gray levels of their video data. Each pixel further comprises a driving transistor that controls drive current flowing through the OLED depending on a gate-source voltage, and at least one switching transistor that programs the gate-source voltage of the driving transistor. The drive current is determined by the gate-source voltage of the driving transistor relative to a data voltage and the threshold voltage of the driving transistor, and the luminance of each pixel is proportional to the amount of drive current flowing through the OLED. 
     Meanwhile, a technology for inserting a black image was proposed to reduce the motion picture response time (MPRT) of an organic light-emitting display. That is, this technology is to eliminate an image of a previous frame by displaying a black image in between image frames. However, the general techniques of displaying a black image require twice as high video frame rates, which creates a problem of making the data charging time shorter. 
     SUMMARY OF THE INVENTION 
     An exemplary embodiment of the present invention provides an organic light-emitting display comprising a driving transistor that drives an organic light-emitting diode; a first transistor connected between a data line supplying a data voltage and a gate electrode of the driving transistor; a second transistor connected between a reference voltage input line and a source electrode of the driving transistor; and a third transistor that directly charges the gate electrode of the driving transistor with a reference voltage supplied from the reference voltage input line, in response to a black data control signal, wherein the third transistor directly receives the reference voltage by being turned on for a certain period of time after the gate electrode of the driving transistor is programmed with the data voltage in one frame before receiving a data voltage in the next frame. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and together with the description serve to explain the principles of the invention. In the drawings: 
         FIG. 1  is a view showing an organic light-emitting display according to an exemplary embodiment of the present invention; 
         FIG. 2  is an equivalent circuit diagram of a pixel of an organic light-emitting display according to a first exemplary embodiment of the present invention; 
         FIG. 3  is a view showing an example of display and non-display periods for the organic light-emitting display according to the present invention; 
         FIG. 4  is a view showing an example of driving signals in a sensing period for the organic light-emitting display according to the present invention; 
         FIG. 5  is a view showing an example of the timings of driving signals in a display interval of the organic light-emitting display according to the first exemplary embodiment; 
         FIG. 6  is a view showing an example of a light-emission period and black data display period for each pixel line on a display panel according to an embodiment of the present invention; 
         FIG. 7  is a view showing an example of a variation of luminance with black data display period according to an embodiment of the present invention; 
         FIG. 8  is an equivalent circuit diagram of a pixel of an organic light-emitting display according to a second exemplary embodiment of the present invention; and 
         FIG. 9  is a view showing the timings of driving signals in a display interval for an organic light-emitting display according to the second exemplary embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     Advantages and features of the present disclosure and methods of accomplishing the same may be understood more readily by reference to the following detailed description of preferred embodiments and the accompanying drawings. The present invention may, however, be embodied in many different forms and should not be construed as being limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete and will fully convey the concept of the invention to those skilled in the art, and the present invention will only be defined by the appended claims. 
     Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings. Throughout the specification, the same reference numerals indicate substantially the same components. In describing the present invention, when it is deemed that a detailed description of known functions or configurations may unnecessarily obscure the subject matter of the present invention, the detailed description will be omitted. 
     Although the embodiments of the present invention disclose transistors of a pixel that are all implemented as P-type, the technical idea of the present invention is not limited thereto and may be applicable to N-type transistors. 
       FIG. 1  is a view showing an organic light-emitting display according to an exemplary embodiment of the present invention. All the components of the organic light-emitting display according to all embodiments of the present invention are operatively coupled and configured. 
     Referring to  FIG. 1 , the organic light-emitting display according to the present invention comprises a display panel  10  with a plurality of pixels P, a timing controller  11 , a data driver  12 , and a gate driver  13 . 
     The pixels P are arranged in a matrix on the display panel  10 . Each pixel P is connected to a data line region  14  and a gate line region  15 . The data line region  14  comprises data lines  14 A and reference voltage lines  14 B (see  FIG. 2 ). The gate line region  15  comprises a plurality of gate lines. 
     A semiconductor layer of transistors of each pixel P may be an oxide semiconductor layer, amorphous silicon (a-Si), polycrystalline silicon (poly-Si), or organic semiconductor. 
     The timing controller  11  generates a data control signal DDC for controlling the operation timing of the data driver  12  and a gate control signal GDC for controlling the operation timing of the gate driver  13 , based on timing signals such as a vertical synchronization signal Vsync, a horizontal synchronization signal Hsync, a dot clock signal DCLK, and a data enable signal DE. Also, the timing controller  11  generates digital compensation data MDATA for compensating for changes in the threshold voltage and mobility of a driving transistor by modulating input digital video data DATA with reference to a digital sensing voltage value supplied from the data driver  12 , and then supplies the digital compensation data MDATA to the data driver  12 . 
     In a sensing operation, the data driver  12  supplies the pixels P with a sensing data voltage synchronized with a first scan signal for sensing, based on a data control signal DDC from the timing controller  11 , and converts sensing voltages input from the display panel  10  through the reference voltage lines ( 14 B of  FIG. 2 ) into digital values and supplies them to the timing controller  11 . In an image display operation, the data driver  12  converts digital compensation data MDATA input from the timing controller  11  into a data voltage for image display, and then synchronizes the data voltage for image display with a first scan signal for image display and supplies it to the data lines ( 14 A of  FIG. 2 ). 
     The gate driver  13  generates a gate pulse based on a gate control signal GDC from the timing controller  11 . The gate pulse comprises a scan signal, a sense signal, and a black data control signal. The black data control signal maintains gate-off voltage in the sensing operation. The timings of the scan signal and sense signal may vary for sensing and display operations. The gate driver  13  may be provided directly on the display panel  10  by GIP (gate-driver in panel) technology. 
       FIG. 2  is a view showing an example of a pixel structure and a data driver according to a first exemplary embodiment of the present invention.  FIG. 2  depicts a first pixel P arranged on an nth pixel line HLn. 
     Referring to  FIGS. 1 and 2 , the pixels arranged on the nth pixel line are connected to the data driver  12  through a first data line  14 A and a first reference voltage line  14 B. A sensing capacitor Cx for storing the source voltage of a second node N 2  as a sensing voltage Vsen may be formed on the reference voltage line  14 B. 
     The data driver  12  comprises a digital-analog converter DAC, an analog-digital converter ADC, a reset switch SW 1 , and a sampling switch SW 2 . 
     In a sensing operation, the digital-analog converter DAC may generate a data voltage Vdata for sensing and output it to the data line  14 A, under control of the timing controller  11 . In an image display operation, the DAC may convert digital compensation data into a data voltage Vdata for image display and output it to the data line  14 A, under control of the timing controller  11 . 
     The reset switch SW 1  switches the current flow between an input terminal of reset voltage Vpre and the reference voltage line  14 B, in response to a reset control signal SPRE. In the sensing operation, the sampling switch SW 2  switches the current flow between the reference voltage line  14 B and the analog-digital converter ADC, in response to a sampling control signal SSAM, and supplies the ADC with a sensing voltage Vsen, which is the source voltage of the driving transistor DT stored in the sensing capacitor Cx on the reference voltage line  14 B for a certain period of time. In the display operation, the sampling switch SW 2  continues to remain turned off in response to a sampling control signal SSAM. 
     Each pixel P comprises an organic light-emitting diode OLED, a driving transistor DT, first to third transistors T 1  to T 3 , and a capacitor Cst. 
     The organic light-emitting diode OLED emits light by a drive current supplied from the driving transistor DT. Multiple layers of organic compounds are formed between an anode and cathode of the organic light-emitting diode OLED. The organic compound layers may comprise at least one hole transfer layer, at least an electron transfer layer, and an emission layer EML. The hole transfer layer is a layer that injects or transfers a hole into the emission layer—for example, a hole injection layer HIL, a hole transport layer HTL, and an electron blocking layer EBL. The electron transfer layer is a layer that injects or transfers an electron into the emission layer—for example, an electron transport layer ETL, an electron injection layer EIL, and a hole blocking layer HBL. The anode of the organic light-emitting diode OLED is connected to the second node N 2 , and the cathode of the organic light-emitting diode OLED is connected to an input terminal of low-potential driving voltage EVSS. 
     The driving transistor DT controls the drive current Ioled flowing through the organic light-emitting diode OLED according to a gate-source voltage Vgs. The driving transistor DT comprises a gate electrode connected to a first node N 1 , a drain electrode connected to an input terminal of high-potential driving voltage EVDD, and a source electrode connected to the second node N 2 . 
     The storage capacitor Cst is connected between the first node N 1  and the second node N 2 . 
     The first transistor T 1  comprises a gate electrode connected to an input terminal that supplies a scan signal SCAN, a drain electrode connected to the data line  14 A, and a source electrode connected to the first node N 1 . 
     The second transistor T 2  comprises a gate electrode connected to an input terminal that supplies a sense signal SEN, a drain electrode connected to the second node N 2 , and a source electrode connected to the reference voltage line  14 B. 
     The third transistor T 3  comprises a gate electrode connected to an input terminal that supplies a black data control signal BS, a drain electrode connected to the reference voltage line  14 B, and a source electrode connected to the first node N 1 . 
     A reference voltage Vref applied through the reference voltage line  14 B may be well below an operating range of the organic light-emitting diode OLED so as to keep the organic light-emitting diode OLED from emitting light during a programming period. For example, the reference voltage Vref may be set to be equal to or lower than the low-potential driving voltage EVSS. 
       FIG. 3  is a view showing an example of a driving period for the organic light-emitting display according to the present invention. 
     Referring to  FIG. 3 , the driving period for the organic light-emitting display according to the present invention comprises first and second non-display periods X 1  and X 2  and an image display period X 0 . 
     The first non-display period X 1  may be defined as the time from power on PON until several tens or hundreds of frames later, and the second non-display period X 2  may be defined as the time from power off POFF until several tens or hundreds of frames later. 
     The image display period X 0  comprises display intervals DF during which a data voltage is written to the pixels P and vertical blanking intervals VB during which no video data is written. 
     A compensation period is positioned in a part other than the display intervals DF. The compensation period may belong to the first and second non-display periods X 1  and X 2  or to the vertical blanking intervals VB. During the compensation period, the data driver  12  generates a compensation data voltage by sensing the threshold voltage Vth of the driving transistor DT and calculating the variation in threshold voltage Vth based on the sampled threshold voltage Vth. 
     The compensation period comprises a programming period Tpg, a sensing period Tsen, and a sampling period Tsam. 
       FIG. 4  is a view showing an example of the timings of driving signals in a compensation period according to an embodiment of the present invention. An operation of the compensation period will be described below with reference to  FIGS. 2 and 4 . 
     In the programming period Tpg, the gate-source voltage of the driving transistor DT is set to turn on the driving transistor DT. To this end, a scan signal SCAN, a sense signal SEN, and a reset control signal SPRE are input at gate-on level, and a sampling control signal SSAM is input at gate-off level. Accordingly, the first transistor T 1  is turned on to supply the first node N 1  with a data voltage Vdata for sensing that is output from the digital-analog converter DAC, and the reset switch SW 1  and the second transistor T 2  are turned on to supply a reference voltage Vref to the second node N 2 . In this case, the sampling switch SW 2  is in the off state. 
     In the sensing period Tsen, the source voltage of the driving transistor DT rises due to the current Ids flowing through the driving transistor DT, and is detected as a first sensing voltage Vsen 1  when it is saturated. In the sensing period Tsen, the gate-source voltage of the driving transistor DT should be kept constant for accurate sensing. To this end, scan signal SCAN for sensing and a sense signal SEN for sensing is input at gate-on level, and the reset control signal SPRE and the sampling control signal SSAM are input at gate-off level. In the sensing period Tsen, the voltage at the second node N 2  increases due to the current Ids flowing through the driving transistor DT, and the voltage at the first node N 1  increases with the increasing voltage at the second node N 2 . 
     In the sampling period Tsam, the source voltage of the driving transistor DT stored in the sensing capacitor Cx for a certain period of time is supplied as the first sensing voltage Vsen 1  to the ADC. To this end, sense signal SEN and the sampling control signal SSAM are input at gate-on level, and the reset control signal SPRE is input at gate-off level. 
       FIG. 5  is a view showing an example of the timings of gate signals for display operation according to the first exemplary embodiment of the present invention depicted in  FIG. 2 . 
     Referring to  FIGS. 2 and 5 , an operation for a display interval according to the first exemplary embodiment will be described below. 
     During the display interval DF, the reset control signal SPRE is kept at gate-on voltage, and the sampling control signal SSAM is kept at gate-off voltage. As a result, the reference voltage line  14 B supplies the reference voltage Vref to the second node N 2 . 
     In this example, 1 frame comprises the period of time from a first horizontal period 1st H in which data is written to a first pixel line HL 1  to an nth horizontal period n-th H in which data is written to an nth pixel line HLn. That is, within 1 frame, the pixels P arranged on the first pixel line HL 1  to nth pixel line HLn are sequentially programmed. 
     The driving period for each pixel line HL comprises a programming period, a light-emission period, and a black data display period. 
     During the programming period, the gate-source voltage of the driving transistor DT of each pixel P is programmed to a voltage value that reflects a data voltage. During the light-emission period, the driving transistor DT of each pixel P generates a drive current that is proportional to the programmed voltage value, and the organic light-emitting diode OLED emits light by this drive current. The black data display period Tbdi is a phase in which the gate electrode and source electrode of the driving transistor DT are reset to stop the organic light-emitting diode OLED from emitting light. The black data display period Tbdi is performed for a certain period of time after the organic light-emitting diode OLED emits light, and the start time of the black data display period Tbdi may be determined by a black data controller  100  to be described later. 
     During the first horizontal period 1st H, the pixels P arranged on the first pixel line HL 1  receive a first scan signal SCAN( 1 ) and a first sense signal SEN( 1 ). As a result, the first node N 1  of each of the pixels P arranged on the first pixel line HL 1  is charged with a data voltage from the data line  14 A, and the second node N 2  is charged with the reference voltage Vref. That is, during the first horizontal period 1st H, the gate-source voltage Vgs of the driving transistor of each of the pixels P arranged on the first pixel line HL 1  is programmed to a desired voltage level that reflects the data voltage. 
     After completion of the first horizontal period 1st H, the first scan signal SCAN( 1 ) and the first sense signal SEN( 1 ) are inverted to gate-off voltage, and the first node N 1  and second node N 2  of each of the pixels P arranged on the first pixel line HL 1  go to a floating state. Accordingly, the driving transistor DT of each of the pixels P arranged on the first pixel line HL 1  generates a drive current Ids in proportion to the programmed voltage level and applies it to the organic light-emitting diode OLED. After the end of the first horizontal period 1st H, the organic light-emitting diode OLED emits light at a brightness corresponding to the drive current Ids to represent a gray level. 
     During the second horizontal period 2nd H, the gate-source voltage of the driving transistor DT of each of the pixels P arranged on the second pixel line HL 2  is programmed in response to a second scan signal SCAN( 2 ) and a second sense signal SEN( 2 ). Likewise, after completion of the second horizontal period 2nd H, the pixels P arranged on the second pixel line HL 2  emit light in response to the programmed voltage. 
     During a kth horizontal period (k)th H, the gate-source voltage of the driving transistor DT of each of the pixels P arranged on the kth pixel line is programmed in response to a kth scan signal SCAN(k) and a kth sense signal SEN(k). 
     During the kth horizontal period (k)th H, the first sense signal SEN( 1 ) and a first black data control signal BS( 1 ) are inverted to gate-on voltage. As a result, the first and second nodes N 1  and N 2  of each of the pixels P arranged on the first pixel line HL 1  are charged with the reference voltage Vref. Accordingly, after the kth horizontal period (k)th H, the driving transistor DT of each of the pixels P arranged on the first pixel line HL 1  generates no drive current, and the organic light-emitting diode OLED is turned off. The organic light-emitting diode OLED of each of the pixels P arranged on the first pixel line HL 1  remains turned off until the first horizontal period 1st H of the next frame. That is, the black data display period for the pixels P arranged on the first pixel line HL 1  continues from the kth horizontal period (k)th H until the first horizontal period 1st H of the next frame. 
     Likewise, during a (k+1)th horizontal period (k+1)th H, the pixels P arranged on a (k+1)th pixel line are programmed, and the pixels P arranged on the second pixel line HL 2  are charged with black data and thereby stop emitting light. 
     As described above, the organic light-emitting display according to the present invention may improve motion picture response time by using a black data display period. Particularly, the organic light-emitting display according to the present invention may display black data without changing driving frequency. That is, it is possible to improve motion picture response time by inserting black data without reducing the length of a programming period. 
     Moreover, the black data display period according to the present invention may vary in real time. 
       FIG. 6  is a view showing an example of a light-emission period and black data display period for each pixel line on a display panel according to an embodiment of the present invention.  FIG. 7  is a view showing an example of the relationship between the black data display period and luminance according to an embodiment of the present invention. 
     Referring to  FIGS. 6 and 7 , a black data display period Tbdi for each pixel line starts after a certain period of time from the start of a programming period. 
     A duty cycle, which is defined as the ratio of a light-emission period to 1 frame, is proportional to k. Since the luminance of each pixel P during 1 frame is proportional to the light-emission period, the value of k determines the luminance of each pixel P. 
     A first line gr 1  on the graph of  FIG. 7  shows the luminance relative to data voltage Vdata when there is no black data display period Tbdi. A second line gr 2  shows the luminance relative to data voltage Vdata when the percentage of a light-emission period Te in one frame is 50% (k=n/2). A third line gr 3  shows the luminance relative to data voltage Vdata when the percentage of the light-emission period Te in one frame is 25% (k=n/4). 
     The black data controller  100  controls the black data display period Tbdi using the relationship between the black data display period Tbdi and luminance shown in  FIG. 7 . Particularly, the black data controller  100  controls the black data display period Tbdi by controlling the timings of scan signals SCAN and black data control signals BS, in order to improve the luminance characteristics of a low grayscale region. 
     [Table 1] below shows an example in which the black data controller  100  sets a duty cycle. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Average value of video data 
                 Duty cycle 
               
               
                   
                   
               
             
            
               
                   
                 DATA_avg ≥ DATA_ref 
                 50% 
               
               
                   
                 DATA_avg &lt; DATA_ref 
                 25% 
               
               
                   
                   
               
            
           
         
       
     
     Referring to [Table 1], an operation of the black data controller  100  will be described below. 
     The black data controller  100  receives video data DATA for each pixel line, and calculates the average video data DATA_avg for each pixel line. For example, for m pixels P (m is a natural number) arranged on each pixel line HL, the black data controller  100  calculates the average value of m video data sets DATA. The black data controller  100  compares the average video data DATA_avg with a preset threshold value DATA_ref. If the average video data DATA_avg is at or above the threshold value DATA_ref, the black data controller  100  sets the duty cycle to 50%. If the average video data DATA_avg is below the threshold value DATA_ref, the black data controller  100  sets the duty cycle to 25%. That is, the black data controller  100  sets the duty cycle to a low percentage when the average video data DATA_avg is low, so as to decrease the luminance of the pixels P. The black data controller  100  may set the duty cycle by varying the value of “k” shown in  FIGS. 5 and 7 . That is, the black data controller  100  controls the output timing of a black data control signal BS and the timing of a sense signal SEN synchronized with the black data control signal BS. The duty cycle set by the black data controller  100  is not limited to the examples given in [Table 1]. The threshold value DATA_ref is a reference luminance for making low gray levels distinctive. 
     In the organic light-emitting display, when the black data display period Tbdi is inserted, the overall luminance of the pixels P decreases. Due to this, the luminance differences in low grayscale display areas decrease depending on video data. The black data controller  100  according to the present invention may decrease luminance by reducing the light-emission period Te of pixels P representing low gray levels, thereby providing a highlighted low grayscale representation. 
       FIG. 8  is a view showing a pixel array of an organic light-emitting display according to a second exemplary embodiment.  FIG. 9  is a view showing driving signals for the pixel array of  FIG. 8 . Regarding the components in  FIGS. 8 and 9 , a detailed description will be omitted or brief if they are substantially the same as those in the foregoing exemplary embodiments. 
     Referring to  FIGS. 8 and 9 , each pixel according to the second exemplary embodiment comprises a first transistor T 1  connected to a first node N 1  and a second transistor T 2  connected to a second node N 2 . The first transistor T 1  comprises a gate electrode connected to an input terminal that supplies a scan signal SCAN, a drain electrode connected to a data line  14 A, and a source electrode connected to the first node N 1 . The second transistor T 2  comprises a gate electrode connected to the input terminal that supplies a scan signal SCAN, a drain electrode connected to the second node N 2 , and a source electrode connected to a reference voltage line  14 B. That is, the first and second transistors T 1  and T 2  are all turned on in response to a scan signal SCAN. 
     Like in the first exemplary embodiment, an operation of the organic light-emitting display according to the second exemplary embodiment comprises a programming period, a light-emission period, and a black data display period. The programming period, light-emission period, and black data display period operate in the same way as the first exemplary embodiment, except that the first transistor T 1  and the second transistor T 2  are all controlled by scan signals SCAN. Since the timings at which the first to third transistors are turn-on and turn-off are substantially the same as in the first exemplary embodiment, the pixels P work in the same way in the first and second exemplary embodiments. 
     Although embodiments have been described with reference to a number of illustrative embodiments thereof, it should be understood that numerous other modifications and embodiments can be devised by those skilled in the art that will fall within the scope of the principles of this disclosure. More particularly, various variations and modifications are possible in the component parts and/or arrangements of the subject combination arrangement within the scope of the disclosure, the drawings and the appended claims. In addition to variations and modifications in the component parts and/or arrangements, alternative uses will also be apparent to those skilled in the art.