JP4883933B2 - Display device - Google Patents

Description

  The present invention relates to a display device, and more particularly to a display device including a plurality of display color pixels.

  In recent years, organic EL displays have been developed, and for example, adopting an organic EL display in a mobile phone is being studied. As a driving method of the organic EL display, a passive matrix driving method in which time-division driving is performed using scanning electrodes and data electrodes, and TFTs are arranged in each pixel, and light emission of each pixel is maintained over one vertical scanning period. There is an active matrix driving method.

  By the way, in the organic EL display, the substantial luminous efficiency of each color organic EL element is different, and it is known that the lifetime of the element depends on the current density applied to the element. As described above, in the organic EL display, since the light emission efficiency of each color element is different, when the light emission area of each pixel is the same, in order to obtain a predetermined luminance in a pixel having a low light emission efficiency, other light emission efficiency There is a problem that a current larger than that of a good pixel has to be passed, which shortens the lifetime of the element of the pixel and thus the lifetime of the entire display device.

In order to solve such a problem, a method has been proposed in which the lifetime of the pixels of each color is made substantially uniform by extending the emission areas of the pixels of each color in accordance with the luminous efficiency ( For example, see Patent Document 1).
JP 2001-290441 A JP 2003-108036 A

  However, in order to apply an organic EL display to a mobile phone or a portable information terminal that frequently uses fixed pattern display, it is necessary to further extend the life. The present invention has been made in view of such circumstances, and an object of the present invention is to provide a technique for realizing a long life of a display device.

  In order to solve the above-described problem, a display device according to an aspect of the present invention includes a plurality of pixels of display colors, and the pixel captures a luminance signal for determining the luminance of the pixel into the pixel, and the luminance Holding means for holding signals, and the holding ability of the holding means is different for each color. The holding capacity of the holding means is made different depending on the difference between the motion signal at the maximum luminance and the minimum luminance, the relaxation time of the afterimage phenomenon, etc., not the convenience of the pixel layout, and the electrode area of the holding capacitor is reduced as a whole. As a result, the pixel aperture ratio can be improved and the lifetime of the pixel element can be extended.

  Among the plurality of display color pixels, the holding capability of the holding unit may be the highest for the pixel having the smallest difference between the luminance signal at the maximum luminance and the luminance signal at the minimum luminance. In addition, the holding capability of the holding unit may be higher as the difference between the luminance signal at the maximum luminance and the luminance signal at the minimum luminance is smaller. That is, the order of the luminance signal difference and the order of the holding ability may be the same. As a result, the influence of the leakage current or the like in the means for taking the luminance signal into the pixel can be smoothed with each color, and the display quality of the display device can be improved.

  Of the plurality of display color pixels, the holding ability of the holding means of the pixel having the longest relaxation time of the afterimage phenomenon may be highest. In addition, the holding capability of the holding unit may be higher as the pixel has a longer afterimage phenomenon relaxation time. That is, the order of the afterimage phenomenon relaxation time and the order of the retention ability may be the same. Thereby, the relaxation time of the afterimage phenomenon can be smoothed with each color, and the display quality of the display device can be improved.

  The plurality of display colors may be four colors of blue, green, red and white, and the holding capability of the holding unit for white pixels may be the highest. In general, since the white pixel has the smallest difference between the luminance signal at the maximum luminance and the luminance signal at the minimum luminance, the holding capability of the white pixel holding means is maximized, and the pixels of other colors are held. By reducing the holding capacity of the means, the pixel aperture ratio can be improved and the lifetime of the element can be extended. Further, by providing white pixels, power consumption of the display device can be reduced.

  The pixel may include a self-luminous element that emits white light, and display colors other than white may be converted using a color filter. Thereby, since the vapor deposition process of a self-light-emitting element can be simplified, a yield can be improved. In addition, since the margin area between the pixels can be narrowed, the resolution can be improved.

  According to the present invention, the lifetime of the display device can be improved.

  FIG. 1 shows a pixel layout of an organic EL display device according to an embodiment. As shown in FIG. 1, the organic EL display device 10 according to the present embodiment includes four color pixels, which are a red pixel A1, a green pixel A2, a blue pixel A3, and a white pixel A4. An image constituent unit (pixel) 12 is represented by four pixels A1, A2, A3, and A4 arranged side by side. By providing the white pixel A4, when displaying white, not all red, green, and blue pixels are lit, but only white pixels need to be lit, thereby reducing power consumption. Can do.

  Each pixel A1, A2, A3, and A4 includes a power supply line PVDD1, a gate line SL1, a data line DL1, a storage capacitor line SCL, an organic EL element OEL1, and a driving transistor MN2 that controls current supply to the organic EL element OEL1. In accordance with the application of the selection signal of the gate line SL1, the conductive state is established, and the luminance signal for determining the luminance of the organic EL element OEL1 is taken into the pixel, and between the gate electrode of the driving transistor MN2 and the storage capacitor line SCL. Is formed of a storage capacitor SC that holds a data voltage. FIG. 2 is an equivalent circuit diagram of one pixel A1, A2, A3, or A4 in the organic EL display device 10 shown in FIG.

  FIG. 3 schematically shows a cross section of the organic EL display device 10 of the present embodiment. As shown in FIG. 3, the three colors of red, green, and blue are obtained by color-converting white light from the organic EL light emitting layer 17 that emits white light using the red, blue, and green color filters 16. For white, white light emitted from the organic EL light emitting layer 17 is used as it is. The organic EL display device 10 performs full color display using these four colors. Here, the organic EL light emitting layer 17 that emits white light is configured by a laminated structure of a light emitting layer that emits blue light and a light emitting layer that emits orange light. When four color pixels are formed by four types of organic EL light emitting layers that emit red, green, blue, and white, respectively, the distance between the pixels is relatively large when the light emitting layer is deposited using a metal mask. The resolution drops because it is necessary to take. In addition, when each color is applied separately, the number of vapor deposition steps for the light emitting element increases, resulting in poor yield. In the organic EL display device 10 according to the present embodiment, the organic EL light emitting layer 17 may be deposited on the entire surface, so that the above-described problems can be avoided and the resolution and yield can be improved.

  FIG. 4 shows the relationship between the data voltage of each color pixel and the storage capacitor. According to the simulations of the present inventors, when the organic EL display device 10 of the present embodiment and an organic EL material are combined, white is determined from the effective luminous efficiency and chromaticity after passing through the color filter 16 of each color. In consideration of the balance, the current values at the maximum luminance per pixel in each color are 0.95 μA, 1.02 μA, 1.30 μA, and 0.85 μA for red, green, blue, and white, respectively. If the ratio of the gate width GW to the gate length GL of the drive transistor MN2 is 5 μm / 60 μm, the difference in data voltage between white display and black display for each color is 2.9 V for red, green, blue and white, respectively. 3.0V, 3.2V, 2.7V. At this time, assuming that the change in the gradation of the data voltage is linear in consideration of the gamma characteristic, the data voltage per gradation in the case of 64 gradations is red, green, blue, and white, respectively. 46 mV, 48 mV, 51 mV, and 43 mV. That is, white having the smallest voltage difference has the smallest data voltage per gradation, and blue having the largest voltage difference has the largest data voltage per gradation.

  The storage capacitor SC is preferably set so that the fluctuation of the data voltage due to the leakage current of the write transistor MN1 is smaller than the data voltage per gradation. Thereby, even if a current leaks in the write transistor MN1, the data voltage is maintained by the storage capacitor SC, and the fluctuation of the data voltage exceeding one gradation can be prevented. In addition, it is desirable that the voltage fluctuation ratio of each color be set to be the same as the data voltage ratio per gradation. As a result, the luminance (gradation) fluctuation due to the leakage current of the writing transistor MN1 can be smoothed so as to be the same in each color.

  From the viewpoint of improving the pixel aperture ratio, it is preferable that the area of the electrode of the storage capacitor SC is small. However, from the viewpoint of suppressing the influence of the leakage current, it is preferable that the storage capacitor SC is large. In the present embodiment, in consideration of such conflicting demands, the storage capacitor SC of the white pixel with the smallest voltage difference is set to 0.2 pF, and the storage capacitors SC of the pixels of red, green, and blue are Considering the data voltage per gradation, the values are 0.18 pF, 0.17 pF, and 0.16 pF, respectively. That is, the storage capacitor SC is made different for each color, and the storage capacitor SC is increased as the pixel has a smaller difference between the luminance signal at the maximum luminance and the luminance signal at the minimum luminance. As a result, the influence of the leakage current can be suppressed while minimizing the area of the electrode of the storage capacitor SC. Further, the influence of the leakage current can be smoothed for each color.

  Like a general organic EL display device, for example, the organic EL display device described in FIG. 2 of Patent Document 2 (Japanese Patent Laid-Open No. 2003-108036), all the storage capacitors SC of the respective colors are the same, in this case 0.2 pF. Compared to the case where the layout is adopted, when the value of the storage capacitor described above is adopted, the area is reduced by about 10%. This corresponds to an increase of about 1% in aperture ratio when the pixel pitch of each color of red, green, blue, and white is 51 μm (horizontal direction) × 137 μm (vertical direction). By increasing the aperture ratio, the current density flowing through the organic EL element can be reduced, so that the lifetime can be extended. It has been empirically found that an increase in aperture ratio of about 1% corresponds to an increase of about 4% in life.

  As described above, the electrode area of the storage capacitor SC is reduced as a whole by reducing the electrode area of the storage capacitor SC in accordance with the difference in data voltage between the maximum luminance and the minimum luminance, thereby improving the pixel aperture ratio. be able to. Therefore, deterioration of the organic EL element can be suppressed and the lifetime as a display device can be extended.

  In addition, when a p-type FET is used as the driving transistor MN2, hysteresis occurs in the characteristics due to the influence of the hole trap, and even after the gate voltage of the driving transistor MN2 is changed, light emission with luminance corresponding to the previous data voltage is emitted. There may be a so-called afterimage phenomenon that remains. In order to alleviate the afterimage phenomenon, there is a method of changing the potential of the gate electrode of the driving transistor MN2 by changing the potential of the other electrode of the storage capacitor SC. In this case, by changing the holding capacity SC in accordance with the afterimage phenomenon relaxation time, the afterimage phenomenon relaxation time can be smoothed for each color. In addition, instead of unifying the storage capacitor SC according to the pixel of the color having the longest relaxation time of the afterimage phenomenon, the electrode area of the storage capacitor SC can be reduced by making the storage capacitor SC different for each color. The pixel aperture ratio can be improved. The order of the electrode areas of the storage capacitor SC may be, for example, white, green, red, blue from the largest.

  As described above, the present embodiment is characterized in that the storage capacitor SC is different for each color in accordance with the data voltage per gradation, the relaxation time of the afterimage phenomenon, and the like, not simply in terms of pixel layout. There is. In order to smooth the influence of the leakage current of the writing transistor MN1 for each color or to smooth the afterimage phenomenon relaxation time for each color, the storage capacitor SC is made to have a different meaning. Accordingly, the life of the display device can be extended by smoothing the influence of the leakage current and the relaxation time of the afterimage phenomenon with each color and suppressing the electrode area of the storage capacitor to improve the pixel aperture ratio.

  The present invention has been described based on the embodiments. This embodiment is an exemplification, and it will be understood by those skilled in the art that various modifications can be made to combinations of the respective constituent elements and processing processes, and such modifications are also within the scope of the present invention. is there.

It is a figure which shows the pixel layout of the organic electroluminescence display which concerns on embodiment. FIG. 2 is an equivalent circuit diagram of a pixel in the organic EL display device shown in FIG. 1. It is a figure which shows typically the cross section of the organic electroluminescence display which concerns on embodiment. It is a figure which shows the relationship between the data voltage of the pixel of each color, and a retention capacity.

Explanation of symbols

  10 display device, 16 color filter, 17 organic EL light emitting layer, A1, A2, A3, A4 pixels, DL1 data line, MN1 write transistor, MN2 drive transistor, OEL1 organic EL element, PVDD1 power supply line, SC storage capacitor, SCL Storage capacitor line, SL1 gate line.

Claims (4)

An organic EL display device having pixels of a plurality of display colors,
Means for controlling the current supply to the organic EL element in the pixel;
Means for taking a luminance signal for determining the luminance of the organic EL element into the pixel;
Holding means for holding the luminance signal, respectively,
An organic EL display device, wherein the holding means of the pixel having the smallest difference between the luminance signal at the maximum luminance and the luminance signal at the minimum luminance among the plurality of display color pixels has the highest holding capability. The display device according to claim 1, wherein a pixel having a smaller difference between a luminance signal at the maximum luminance and a luminance signal at the minimum luminance has a higher holding capability of the holding unit. 3. The display device according to claim 1, wherein the plurality of display colors are four colors of blue, green, red, and white, and the holding capacity of the white pixel or the holding capacity of the holding capacitor is the highest. The organic EL display device described in 1. 4. The organic EL according to claim 1 , wherein the pixel includes a self-luminous element that emits white light, and display colors other than white are converted using a color filter. 5.
Display device.

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