Patent Publication Number: US-10319296-B2

Title: Display device correction method and display device correction device

Description:
TECHNICAL FIELD 
     The present disclosure relates to a display device correction method and a display device correction device. 
     BACKGROUND ART 
     In recent years, organic electroluminescent (EL) displays which make use of organic electroluminescence have been the focus of attention as one of next-generation flat panel displays to replace liquid-crystal displays. 
     The organic EL displays include an organic EL panel in which a plurality of display pixels are disposed in a matrix. The display pixel includes an organic EL element and a drive transistor which supplies drive current according to a pixel signal to the organic EL element. 
     In active-matrix display devices such as organic EL displays, thin-film transistors (TFTs) are used as drive transistors. In a TFT, a threshold voltage of the TFT shifts over time due to stress caused by, for example, a gate-source voltage when the TFT is powered up. The shift of the threshold voltage with the passage of time may cause variation in the amount of current supplied to an organic EL element, and thus affects luminance control of the display device, leading to deterioration of the display quality. 
     In the organic EL display, in order to prevent deterioration of the display quality, a cumulative value of a pixel signal (hereinafter referred to as “cumulative value” as appropriate) is calculated, and a pixel signal is corrected using the cumulative value. The pixel signal is a signal included in a video signal indicating an image of one frame, and includes chromaticity, saturation, a gradation value, etc., of one pixel. 
     CITATION LIST 
     Patent Literature 
     
         
         [PTL 1] Japanese Unexamined Patent Application Publication No. 2004-145257 
       
    
     SUMMARY OF INVENTION 
     Technical Problem 
     However, the conventional display devices pose a problem that accuracy of correction of the gradation value is not sufficient, and thus further improvement in the display quality is required. 
     The present disclosure provides a display device correction method and a display device correction device which are capable of improving the display quality. 
     Solution to Problem 
     A display device correction method according to the present disclosure is a display device correction method performed by a control unit that performs display control on a display panel including a plurality of display pixels, in a display device which includes the display panel and the control unit. The display device correction method includes: obtaining a cumulative value of a pixel signal supplied to a drive transistor which is included in a current pixel to be processed among the plurality of display pixels and supplies drive current according to the pixel signal to a light emitting element; calculating a shift amount of a threshold voltage of the drive transistor, using the cumulative value; calculating an amount of change in mobility, using the shift amount; and calculating a correction parameter for correcting a gradation value of the pixel signal, using the amount of change in mobility. 
     A display device correction device according to the present disclosure is a display device correction device including: a display panel including a plurality of display pixels; and a control unit configured to perform display control on the display panel. In the display device correction device, each of the plurality of display pixels includes a light emitting element, and a drive transistor which supplies drive current according to a pixel signal to the light emitting element, and the control unit is configured to: obtain a cumulative value of the pixel signal supplied to the drive transistor which is included in a current pixel to be processed among the plurality of display pixels; calculate a shift amount of a threshold voltage of the drive transistor, using the cumulative value; calculate an amount of change in mobility, using the shift amount; and calculate a correction parameter for correcting a gradation value of the pixel signal, using the amount of change in mobility. 
     Advantageous Effects of Invention 
     The display device correction method and the display device correction device according to the present disclosure are capable of improving the display quality. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is an appearance diagram which illustrates an external view of an organic EL display according to an embodiment. 
         FIG. 2  is a block diagram which illustrates an example of a configuration of the organic EL display according to the embodiment. 
         FIG. 3  is a block diagram which illustrates an example of a configuration of a control unit according to the embodiment. 
         FIG. 4  is a flowchart which illustrates a procedure of stress correction according to the embodiment. 
         FIG. 5  is a graph which illustrates a result of measuring a shift amount of a threshold voltage with respect to a cumulative value, for each design value denoted by (V gs −V th ). 
         FIG. 6  is a graph which illustrates a result of measuring a change amount in mobility with respect to a shift amount of the threshold voltage. 
         FIG. 7  is a graph which illustrates a value of gain with respect to the change amount of the mobility. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     [Details of the Problem] 
     Correction performed on a pixel signal includes, for example, (1) correction of a gradation value with respect to a shift of a threshold voltage, (2) correction of a gradation value using mobility of electric charges in a drive transistor, etc. 
     (1) the correction of a gradation value with respect to a shift of a threshold voltage is carried out in order to prevent reduction of luminance of an organic EL panel due to deterioration of a drive transistor which results from application of a voltage across a gate and a source of the drive transistor. When a voltage is applied across the gate and source of the drive transistor, the drive transistor deteriorates over time, and the threshold voltage shifts. When the threshold voltage shifts, the amount of drive current flowing across the source and the drain of the drive transistor decreases even in the case where the same voltage is applied to the gate. With this, the amount of the drive current supplied to the organic EL element decreases, leading to reduction in luminance of the organic EL element. In correcting of a gradation value using the shift amount of a threshold voltage, the relationship between a cumulative value of a pixel signal and a shift amount of a threshold voltage is used. The cumulative value of the pixel signal is calculated according to the relationship, thereby obtaining the shift amount of the threshold voltage. 
     (2) In the correction using mobility of electric charges in a drive transistor, mobility is obtained according to the amount of current flowing through the drive transistor, and a gradation value is corrected using the mobility. 
     Conventionally, the above-described two corrections are separately carried out. 
     Here, the inventors have found that there is a correlation between the above-described shift amount of the threshold voltage and the amount of change in mobility. It is considered that performing of the correction of a gradation value using the correlation allows correcting the gradation value with higher accuracy, and improving the display quality. 
     Hereinafter, embodiments shall be discussed in detail with reference to the drawings as necessary. However, description that is too detailed will be omitted in some cases. For example, there are instances where detailed description of well-known matter and redundant description of substantially identical components are omitted. This is for the purpose of preventing the following description from being unnecessarily redundant and facilitating understanding of those skilled in the art. 
     It should be noted that the accompanying Drawings and subsequent description are provided by the inventors to allow a person of ordinary skill in the art to sufficiently understand the present disclosure, arid are thus not intended to limit the scope of the subject matter recited in the Claims. 
     (Embodiment) 
     Hereinafter, a display device correction method and a display device correction device according to an embodiment is described with reference to  FIG. 1  to  FIG. 7 . 
     [1-1. Configuration] 
       FIG. 1  is an appearance diagram which illustrates an external view of an organic EL display  10  according to the present embodiment.  FIG. 2  is a block diagram which illustrates an example of a configuration of the organic EL display  10  according to the present embodiment. 
     As illustrated in  FIG. 2 , the organic EL display  10  includes an organic EL panel  11 , a data line drive circuit  12 , a scanning line drive circuit  13 , a memory  14 , and a control unit  20 . 
     (1-1-1. Organic EL Panel, Drive Circuit, and Memory) 
     The organic EL panel  11  is an example of a display panel including a plurality of display pixels. The organic EL panel  11  includes a plurality of display pixels P which are disposed in a matrix, a plurality of scanning lines GL connected to the plurality of display pixels P, and a plurality of data lines SL. 
     According to the present embodiment, the plurality of display pixels P each include an organic EL element OEL, a selection transistor T 1 , a drive transistor T 2 , and a capacitative element C 1 . 
     The selection transistor Ti switches between selection and non-selection of the display pixel P according to a voltage of the scanning line GL. The selection transistor T 1  is a thin-film transistor, and includes a gate terminal connected to the scanning line GL, a source terminal connected to the data line SL, and a drain terminal connected to the node N 1 . 
     The drive transistor T 2  supplies drive current according to a voltage of the data line SL to the organic EL element OEL. The drive transistor T 2  is a thin-film transistor. More specifically, the drive transistor T 2  is an oxide semiconductor element. For example, the drive transistor T 2  is formed using an oxide semiconductor such as a transparent amorphous oxide semiconductor (TAOS). The drive transistor T 2  includes a gate terminal connected to the node N 1 , a source terminal connected to an anode electrode of the organic EL element OEL, and a drain terminal to which a voltage VTFT is supplied. 
     The organic EL element OEL is a light emitting element that emits light according to drive current. The drive current is supplied from the drive transistor T 2 . The organic EL element OEL includes an anode electrode connected to the source terminal of the drive transistor T 2 , and a cathode electrode which is grounded. 
     The capacitative element C 1  is a capacitative element in which an electric charge according to the voltage of the data line SL is accumulated. The capacitative element C 1  has one end connected to the node N 1  and the other end connected to the source terminal of the drive transistor T 2 . 
     The data line drive circuit  12  supplies, to the plurality of data lines SL, a voltage according to a correction signal provided by the control unit  20 . 
     The scanning line drive circuit  13  supplies, to the plurality of scanning lines GL, a voltage according to a drive signal provided by the control unit  20 . 
     It should be noted that, although the case where the selection transistor T 1  and the drive transistor T 2  are n-type TFTs is described as an example in the present embodiment, the selection transistor T 1  and the drive transistor T 2  may be p-type TFTs. The capacitative element C 1  is connected between the gate and the source of the drive transistor T 2 , in this case as well. 
     The memory  14  includes a volatile memory and a non-volatile memory according to the present embodiment. The volatile memory is, for example, a dynamic random access memory (DRAM) or a static random access memory (SRAM). The non-volatile memory is, for example, a flash memory. In the memory  14 , a correction parameter for correcting a video signal, a result of calculation, etc., are stored, 
     (1-1-2. Control Unit) 
     The control unit  20  is a circuit which controls video display on the organic EL panel  11 , and is configured using, for example, timing controller (TCOM) or the like. It should be noted that the control unit  20  may be configured using a computer system including a micro controller, a system large scale integration (LSI), or the like. 
     The control unit  20  controls correction processing on a video signal provided from outside, writing processing using the corrected video signal, etc. The video signal is a signal for displaying an image including one frame on the organic EL panel  11 . The video signal includes pixel signals corresponding one to one to a plurality of pixels included in the image indicated by the video signal. The pixel signal includes chromaticity, saturation, a gradation value, etc. 
     The correction processing performed on the video signal includes correction of a gradation value of the pixel signal, as described above. The correction of the gradation value of the pixel signal is performed to address deterioration of the drive transistor. The control unit  20  generates a correction signal resulting from correcting the gradation value, and outputs the correction signal to the data line drive circuit  12 . 
       FIG. 3  is a block diagram which illustrates an example of a configuration of the control unit  20  according to the present embodiment.  FIG. 3  illustrates part of structural components of the control unit  20 , which is a portion related to the stress correction. Although the control unit  20  includes, in addition to the structural components illustrated in  FIG. 3 , a circuit which generates a drive signal, etc., illustration for those structural components is omitted. 
     As illustrated in  FIG. 3 , the control unit  20  includes an input unit  21  and a stress correction unit  22 . The control unit  20  corresponds to the correction device according to the present embodiment. 
     The input unit  21  receives a video signal provided from outside, and performs adjustment of an image size, etc. The input unit  21  sequentially obtains a gradation value of each of the plurality of display pixels P included in the organic EL panel  11 , and outputs the obtained gradation value to an additional value calculating unit  23  and a correction unit  29 . 
     The stress correction unit  22  performs stress correction using a cumulative value of stress of the drive transistor T 2 . As illustrated in  FIG. 3 , the stress correction unit  22  includes the additional value calculating unit  23 , an adder  24 , a shift amount calculating unit  25 , a correction parameter calculating unit  26 , and a correction unit  29 . 
     The additional value calculating unit  23  calculates a stress value of the drive transistor included in the display pixel P, on the basis of the gradation value of the pixel signal. The stress value of the drive transistor T 2  is a value corresponding to the gradation value of the pixel signal and the cumulative value stored in the memory  14 . The additional value calculating unit  23  calculates, as a stress value, a time conversion value under the assumption that a voltage of a constant value is continuously applied. 
     The adder  24  rewrites, on the memory  14 , a value resulting from adding the stress value to the cumulative value stored in the memory  14 , as a new cumulative value. 
     The shift amount calculating unit  25  calculates a shift amount of a threshold voltage of the drive transistor T 2 , using the cumulative value stored in the memory  14 . 
     The correction parameter calculating unit  26  calculates a correction parameter for correcting a gradation value of the pixel signal. The correction unit  29  which will be described later corrects a gradation value using an expression; that is, gradation value×gain A+offset B. The correction parameter calculating unit  26  includes a gain calculating unit  27  and an offset calculating unit  28 . 
     The gain calculating unit  27  calculates a change amount in mobility using a shift amount, and calculates a gain A using the mobility. The gain calculating unit  27  includes two look-up tables of Δμ LUT  27   a  and gain LUT  27   b.  The details of the look-up tables will be described later. 
     The offset calculating unit  28  calculates an offset B using the shift amount. 
     The correction unit  29  corrects the gradation value using the expression; that is, gradation value×gain A+offset B, as described above, and outputs the corrected gradation value as a correction signal. 
     (1-2. Operation) 
       FIG. 4  is a flowchart which illustrates a procedure of stress correction according to the present embodiment. 
     (1-2-1. Calculation of Shift Amount of Threshold) 
     The shift amount calculating unit  25  calculates a shift amount ΔV th  of a threshold voltage of the drive transistor T 2 , using the cumulative value stored in the memory  14  (S 11 ). The shift amount ΔV th  of a threshold voltage is obtained using Expression 1 below. 
     [Math. 1]
 
Δ V   th   =A   1 ( V   gs   −V   th   +V   offset ) α   t   ref   β   Expression 1
 
     V gs  denotes a gate-source voltage of the drive transistor T 2 , V th  denotes a threshold voltage of the drive transistor T 2  and a design value. In addition, t ref  denotes a time conversion value (i.e., cumulative value) of stress. 
       FIG. 5  is a graph which illustrates a result of measuring a shift amount ΔV th  of a threshold voltage with respect to a cumulative value t ref  (denoted as a stress period in  FIG. 5 ), for each design value denoted by (V gs −V th ). A 1 , α, β, and V offset  of Expression 1 are obtained by performing fitting according to the least-square technique, for the graph in  FIG. 5 . A 1 , α, β, and V offset  according to the design value are stored in advance in the memory  14  of the organic EL display  10 . The shift amount calculating unit  25  calculates a shift amount ΔV th  of a threshold voltage, by assigning the cumulative value T ref  to Expression 1. 
     (1-2-2. Calculation of Mobility) 
     The gain calculating unit  27  of the correction parameter calculating unit  26  calculates a change amount Δμ in mobility, using the shift amount ΔV th  of the threshold (S 12 ). The change amount Δμ in mobility is obtained using Expression 2 below. 
     [Math. 2]
 
Δμ= C ×(Δ V   th ) γ   Expression 2
 
       FIG. 6  is a graph which illustrates a result of measuring a change amount Δμ in mobility with respect to a shift amount ΔV th  of the threshold voltage (an example of Δμ LUT  27   a ). It is possible to obtain C and γ using  FIG. 6 . C and γ are stored in advance in the memory  14  of the organic EL display  10 . The gain calculating unit  27  assigns the shift amount ΔV th  to Expression 2, thereby calculating the change amount Δμ in mobility. It should be noted that an expression Δμ=C 1 ×(ΔV th ) γ +C 2  may be used. Expression 2 is used in the case where C 2 =0. 
     (1-2-3. Calculation of Correction Parameter  1 ; Calculation of Gain A) 
     The gain calculating unit 27 calculates the gain A, using the change amount Δμ in mobility (S 13 ). 
     The gain A is obtained according to Expression 3 below. 
     
       
         
           
             [ 
             
               Math 
               . 
               
                   
               
               ⁢ 
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             ] 
           
         
       
       
         
           
             
               
                 
                   A 
                   = 
                   
                     
                       1 
                       
                         ( 
                         
                           1 
                           + 
                           Δμ 
                         
                         ) 
                       
                     
                     δ 
                   
                 
               
               
                 
                   Expression 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   3 
                 
               
             
           
         
       
     
       FIG. 7  is a graph which illustrates a value of a gain A with respect to the change amount Δμ in mobility (an example of the gain LUT  27   b ). For example, the graph illustrated in  FIG. 7  is obtained by measuring a value of the gain A with respect to the change amount Δμ in mobility, for the first one of a lot. It is possible to obtain δ, using the graph in  FIG. 7 . In  FIG. 7 , δ=1. 
     The gain calculating unit  27  assigns the change amount Δμ in mobility to Expression 3, thereby calculating the gain A. 
     (1-2-4. Calculation of Correction Parameter  2 : Calculation of Offset B) 
     The offset calculating unit  28  calculates an offset B using the shift amount of a threshold (S 14 ). The offset B is obtained according to Expression 4 below, using a constant a. 
     [Math. 4]
 
 B=ΔV   th ×α  Expression 4
 
     (1-2-5. Correction of Gradation Value) 
     The correction unit  29  corrects a gradation value V data  of a pixel signal, using the gain A and the offset B (S 15 ). A corrected gradation value V data ′ is obtained according to Expression 5 below. 
     [Math. 5]
 
 V   data   ′=A×V   data   +B    Expression 5
 
     (1-3. Advantageous Effects, etc.) 
     With the correction device and the correction method according to the present embodiment, a gradation value is corrected using a relationship between the shift amount ΔV th  of the threshold voltage and the change amount Δμ in mobility. In this manner, with the correction device and the correction method according to the present embodiment, it is possible to correct a gradation value with higher accuracy. 
     (Other Embodiments) 
     As described above, the embodiment is described as an exemplification of the technique according to the present disclosure. The accompanying drawings and detailed description are provided for this purpose. 
     Therefore, the structural components described in the accompanying drawings and detailed description include, not only the structural components essential to solving the problem, but also the structural components that are not essential to solving the problem but are included in order to exemplify the aforementioned technique. As such, description of these non-essential structural components in the accompanying drawings and the detailed description should not be taken to mean that these non-essential structural components are essential. 
     Furthermore, since the foregoing embodiment is for exemplifying the technique according to the present disclosure, various changes, substitutions, additions, omissions, and so on, can be carried out within the scope of the Claims or its equivalents. 
     INDUSTRIAL APPLICABILITY 
     The present disclosure can be applied to display devices such as organic EL displays.