JP2004046218A - Method for determining duty ratio of driving of light emitting device and driving method using same duty ratio - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a determining method for the duty ratio of driving of a light emitting device which can improve reliability by suppressing deterioration of a light emitting element and a driving method using the duty ratio. <P>SOLUTION: Disclosed is the determining method for the duty ratio of the light emitting device which makes display with an analog video signal and the method is characterized in that the range of the duty ratio at which luminance which is a little larger than 0.8 time as large as a maximum value is obtained is determined as an optimum range as to the characteristic obtained by integrating a characteristic of luminance an X hours later which is value of current density and a characteristic of luminance the X time later for the duty ratio when the total quantity of electricity flowing to the light emitting element in one frame period is specified. <P>COPYRIGHT: (C)2004,JPO

Description

The present invention relates to a method for determining a duty ratio and a driving method using the duty ratio in a light emitting device in which a means for supplying current to a light emitting element and a light emitting element are provided in a plurality of pixels. . Note that a light-emitting device includes a panel in which a light-emitting element is sealed, and a module in which an IC or the like including a controller is mounted on the panel.

(4) In an active matrix type light emitting device, gradation is controlled by a video signal written to each pixel. Hereinafter, a method for driving a light emitting device using an analog video signal (referred to as an analog video signal) will be described.

An example will be described in which the frame frequency is k. As shown in FIG. 12, when the frame frequency is k, k frame periods are provided in one second. Note that a frame period corresponds to a period in which a video signal is written to all pixels and display for one screen is performed.

(4) When an analog video signal is written to each pixel in each frame period, the brightness of a light emitting element of each pixel is controlled according to image information of the analog video signal, so that a gray scale is displayed. Note that writing of an analog video signal to a pixel includes a method called dot sequential which is sequentially performed for each pixel, and a type called line sequential which is performed for each pixel in each row. In any of the formats, a period until an analog video signal is written to all pixels corresponds to a writing period Ta.

{After that, after the writing of the analog video signal is completed, the holding period Ts is started, and the luminance of the light emitting element is maintained in each pixel until the frame period ends.

When the above driving method is used, the pixel performs display in any of the writing period Ta and the holding period Ts. Therefore, depending on the image information included in the analog video signal, the pixel is always turned on, in other words, The light-emitting element of the pixel emits light. Note that a period in which display is actually performed is called a display period. In the case of the driving method shown in FIG. 12, a period obtained by adding the writing period Ta and the holding period Ts corresponds to a display period.

In FIG. 12, one frame period is divided into the writing period Ta and the holding period Ts, but the holding period Ts may be omitted and only the writing period Ta may be used. That is, immediately after the analog video signal is written to each pixel, the next frame period starts immediately, and the writing of the analog video signal to each pixel starts again. Also in this case, depending on the image information included in the analog video signal, the pixel is always in a lit state. Therefore, in the case of this driving method, the writing period Ta directly corresponds to the display period.

In FIG. 12, the display is performed during both the writing period Ta and the holding period Ts. However, the display may be performed only during the holding period Ts without performing the display during the writing period Ta. In this case, regardless of the image information included in the analog video signal, all the pixels are turned off during the writing period, in other words, the light emitting elements of all the pixels are set not to emit light. Then, the luminance of the light emitting element is controlled according to the analog video signal during the holding period. Therefore, in the case of this driving method, only the holding period Ts directly corresponds to the display period.

By the way, a problem in putting the light emitting device to practical use is that the life of the light emitting element is short due to deterioration of the electroluminescent layer. FIG. 13 shows a temporal change in luminance with respect to a current flowing through the light-emitting element. As shown in FIG. 13, when the electroluminescent material deteriorates with time, the luminance of the light emitting element becomes lower with respect to the flowing current.

劣化 The deterioration of the electroluminescent material is promoted by moisture, oxygen, light and heat. Specifically, the speed of deterioration depends on the structure of a device for driving the light-emitting device, characteristics of an electroluminescent material, material of an electrode, conditions in a manufacturing process, a driving method of the light-emitting device, and the like.

Especially, as the amount of current flowing through the light emitting element increases, the deterioration of the light emitting element progresses faster. When the light-emitting element deteriorates, the luminance of the light-emitting element decreases even when the voltage applied to the electroluminescent layer is constant, and the displayed image becomes unclear.

In view of the above-described problems, an object of the present invention is to improve reliability such that a constant luminance can be obtained by suppressing deterioration of a light emitting element.

The present inventors have found that there is a difference in the reliability of the light emitting device depending on the existence ratio (duty ratio) in one frame period of the display period in which each pixel performs display, and at the same time, secure high reliability. Of how to calculate the optimum duty ratio for a computer.

(1) FIG. 1 shows, for each duty ratio, the amount of change in the actual measured value (normalized luminance) of the apparent luminance over time when the initial light emission is set to 100%. The luminance of the screen at a duty ratio of 100% is 1000 cd. Then, at all the duty ratios, the apparent luminance at the beginning of light emission was all the same. That is, at all the duty ratios, the sum total (total electricity amount) of the current flowing to the light emitting element in one frame period can be regarded as equal.

Note that in this specification, a light-emitting element (OLED: Organic Light Emitting Diode) includes a layer containing an electroluminescent material capable of obtaining luminescence (Electroluminescence) generated by applying an electric field (hereinafter, referred to as an electroluminescent layer), and an anode layer. And a cathode layer. The electroluminescent layer is provided between the anode and the cathode, and is composed of a single layer or a plurality of layers. These layers may contain an organic compound or may contain an inorganic compound. The luminescence in the electroluminescent layer includes light emission when returning from a singlet excited state to a ground state (fluorescence) and light emission when returning from a triplet excited state to a ground state (phosphorescence).

(1) At each duty ratio, the normalized luminance is greater than 100% in the initial stage of light emission, but after that, the light-emitting element deteriorates with time and the normalized luminance decreases, as can be seen from FIG. The most significant decrease in luminance occurs when the duty ratio is 2.66%, followed by 100% and 72.6%, and the smallest decrease in normalized luminance occurs when the duty ratio is 36%. .

{Circle around (2)} FIG. 2 shows data showing normalized luminance values for different duty ratios for each elapsed time using the data shown in FIG. Here, in order to make the graph easy to see, among the data shown in FIG. 1, the data especially after 2 hours, 20 hours, 44.3 hours, 68.5 hours, and 95.5 hours are representatively shown. .

As shown in FIG. 2, when the duty ratio is 36%, the normalized luminance is reduced the least and the luminance is kept high. Even if the duty ratio is too large, 72.6% or 100%, conversely, 2. It can be seen that the reliability is inferior even if it is as small as 66%. From this, it is considered that the optimum duty ratio for obtaining high reliability exists in the range of about 36% in this case.

The present inventors have considered that the reason why there is an optimum duty ratio with high reliability is that two phenomena occur with the optimum range of the duty ratio interposed therebetween.

Consider a case where the duty ratio is smaller than the optimal range.

(4) In order to keep the apparent brightness on the screen constant, it is necessary to keep the total amount of electricity flowing to the light emitting element during one frame period constant irrespective of the duty ratio. Therefore, as shown in FIG. 3A, when the duty ratio is reduced and the display period is shortened, the amount of electricity passing per unit time per unit area, that is, the current density is increased.

FIG. 4A shows the relationship between the current density and the luminance after X hours when the duty ratio is kept constant. As shown in FIG. 4A, the luminance of the light-emitting element decreases as the current density increases. This is considered because the total amount of electricity flowing through the light emitting element increases as the current density increases. However, as can be seen from FIG. 4A, when the current density exceeds a certain value, the luminance of the light emitting element decreases in a steep curve. This cannot be explained simply by increasing the total amount of electricity. From the above phenomenon, the present inventors suggest that, even if the total amount of electricity is kept constant, if the current density is too large, the deterioration of the light emitting element is promoted, and the luminance may be reduced. Thought. Therefore, it is considered that if the duty ratio is too small while keeping the apparent luminance on the screen constant, the current density increases, and the decrease in the luminance of the light emitting element may increase.

Next, consider the case where the duty ratio is larger than the optimum range.

When the total amount of electricity in one frame period is fixed in order to keep the apparent brightness on the screen constant, when the duty ratio increases, the current density decreases as shown in FIG. 3B, and the display period increases instead. Become.

FIG. 4B shows the relationship between the duty ratio and the luminance after X hours when the current density is kept constant. As shown in FIG. 4B, the luminance of the light emitting element decreases as the duty ratio increases. This is considered to be because the total amount of electricity flowing to the light emitting element increases as the duty ratio increases. However, as can be seen from FIG. 4B, when the duty ratio exceeds a certain value, the brightness of the light emitting element decreases in a steep curve. This cannot be explained simply by increasing the total amount of electricity. From the above-mentioned phenomenon, the present inventors suggest that, even if the total amount of electricity is kept constant, if the duty ratio becomes larger than a certain value, the deterioration of the light emitting element is promoted, and the luminance may decrease. Thought. Therefore, if the duty ratio is made too large while keeping the apparent luminance on the screen constant, it is considered that the continuous display period becomes longer, and the luminance of the light emitting element is greatly reduced.

(4) There are various reasons why deterioration of the light-emitting element is promoted when the period of continuous display is long. One of the reasons is that deterioration caused by heat generated in the light-emitting element is promoted. In addition, ionic impurities present in the electroluminescent layer move toward one of the electrodes, so that a part of the electroluminescent layer has a region having a lower resistance than the other, and the region having a lower resistance is positively active in the region having the lower resistance. It is also considered that the deterioration of the current is promoted by the current flowing.

It is considered that at least the above two phenomena are involved in the decrease in the luminance of the light emitting element.

総 The total amount of electricity flowing to the light emitting element of one pixel in one frame period corresponds to the product of the display period determined by the duty ratio and the current density. Therefore, when the frame frequency is fixed, the value of the current density shown on the horizontal axis of FIG. 4A and the value of the duty ratio shown on the horizontal axis of FIG. , And the product of the luminance after each X time is obtained. FIG. 4C shows the luminance after X hours with respect to the duty ratio and the current density when the total amount of electricity is constant. The vertical axis in FIG. 4C corresponds to the product of the luminance after X time in FIGS. 4A and 4B.

積 As shown in FIG. 4C, when the total amount of electricity is constant, the product of the luminance after the X-time and the duty ratio or the current density has one maximum value. It is considered that the highest reliability can be obtained by driving at a duty ratio at which the maximum value is obtained. Note that the range of the duty ratio at which high reliability is obtained may be any range as long as a luminance of about 0.8 times the maximum value is obtained. For example, assuming that the maximum value of the luminance is 50% of the initial luminance after 95.5 hours, the range of the optimal duty ratio can be included in the range of the luminance exceeding about 40% of the initial luminance.

The graph shown in FIG. 4A is still a graph rising to the right, but as the duty ratio decreases, the amount of decrease in luminance decreases, and the shape of the graph changes accordingly. Although the graph shown in FIG. 4B is still a graph falling to the right, the larger the current density is, the larger the amount of decrease in luminance is, and the shape of the graph changes.

However, if the total amount of electricity corresponding to the product of the display period determined by the duty ratio in the graph shown in FIG. 4A and the current density in the graph shown in FIG. One particular property as shown in (C) can be obtained.

{Circle around (2)} By driving using an optimum duty ratio, deterioration of the light emitting element is suppressed, a constant luminance is obtained, and the reliability of the light emitting device can be improved.

The optimum value of the duty ratio also varies depending on the configuration of the light emitting element. However, each time, when the total amount of electricity is constant, the optimum duty ratio range can be determined from the product of the duty ratio or the current density and the luminance after X hours.

For example, from the data shown in FIG. 1, in the case of the light emitting element used to obtain the data in FIG. 1, when the initial luminance is set to 1000 cd and the reliability after 95.5 hours is set as a reference, the duty ratio is, for example, A range greater than 20% and less than 50% can be used as the optimal value.

(4) The total amount of electricity flowing to one pixel during one frame period also changes depending on the image information of the analog video signal. Since the greater the total amount of electricity is, the more the light emitting element deteriorates, it is desirable to determine the optimum range of the duty ratio based on the case where the total amount of electricity is the largest, regardless of the image information.

駆 動 Thus, by driving using the optimum duty ratio, deterioration of the light emitting element can be suppressed, constant luminance can be obtained, and reliability of the light emitting device can be improved.

In this embodiment, a driving method using an optimal duty ratio will be described.

(Embodiment 1)
In this embodiment mode, a driving method of the present invention is described using a light-emitting device in which light emission of a light-emitting element is controlled using two thin film transistors (TFTs) provided in each pixel as an example.

FIG. 5A is a circuit diagram of a pixel portion of a light emitting device using the driving method of the present invention. The pixel portion 401 is provided with signal lines (S1 to Sx), power supply lines (V1 to Vx), and scanning lines (G1 to Gy).

In the case of this example, a region including any one of the signal lines (S1 to Sx), any one of the power supply lines (V1 to Vx), and any one of the scan lines (G1 to Gy) is a pixel 404. Is equivalent to In the pixel portion 401, a plurality of pixels 404 are arranged in a matrix.

FIG. 5B is an enlarged view of the pixel 404. In FIG. 5B, reference numeral 405 denotes a switching TFT. The gate of the switching TFT 405 is connected to the scanning line Gj (j = 1 to y). One of the source and the drain of the switching TFT 405 is connected to the signal line Si (i = 1 to x), and the other is connected to the gate of the driving TFT 406.

接 続 In this specification, the term “connection” means an electrical connection unless otherwise specified.

{Circle around (1)} One of the source and the drain of the driving TFT 406 is connected to the power supply line Vi (i = 1 to x), and the other is connected to the pixel electrode of the light emitting element 407.

(4) The light-emitting element 407 includes an anode and a cathode, and an electroluminescent layer provided between the anode and the cathode. When the anode is connected to the source or drain of the driving TFT 406, the anode serves as a pixel electrode and the cathode serves as a counter electrode. Conversely, when the cathode is connected to the source or drain of the driving TFT 406, the cathode serves as a pixel electrode and the anode serves as a counter electrode.

Note that when the source or the drain of the driving TFT 406 is connected to the anode of the light-emitting element 407, the driving TFT 406 is preferably a p-channel TFT. When the source or the drain of the driving TFT 406 is connected to the cathode of the light-emitting element 407, the driving TFT 406 is preferably an n-channel TFT.

(4) A voltage is applied to the opposing electrode of the light emitting element 407 and the power supply line Vi from a power supply. Note that in this specification, a voltage means a potential difference from a ground voltage unless otherwise specified.

One of the two electrodes of the storage capacitor 408 is connected to the power supply line Vi, and the other is connected to the gate of the driving TFT 406. The storage capacitor 408 is provided to hold the gate voltage of the driving TFT 406 when the switching TFT 405 is in a non-selected state (off state). Note that FIG. 5B illustrates the structure in which the storage capacitor 408 is provided; however, the present invention is not limited to this structure, and a structure in which the storage capacitor 408 is not provided may be employed.

Next, a driving method of the present invention used for the light emitting device shown in FIG. 5 will be described with reference to FIG.

と き As shown in FIG. 6, when the frame frequency is k, there are k frame periods per second. In order to suppress flickering of the screen such as flicker, it is desirable that k is 60 or more.

(6) In the driving method shown in FIG. 6, a writing period Ta, a holding period Ts, and a non-display period Te are provided within one frame period. FIG. 6 representatively shows the timings at which a display period and a non-display period appear for pixels in a row to which an analog video signal is first input (first row pixels) and for pixels in a row to be input last. .

具体 Specific operation of the pixel will be described. In the writing period Ta, a voltage having the same height as the power supply line is applied to the opposite electrode of the light-emitting element 407. Alternatively, the voltage difference between the counter electrode and the power supply line may be controlled so that a reverse bias voltage is applied to the light emitting element.

In the writing period Ta, the scanning lines G1 to Gy are sequentially selected. The periods in which each scanning line is selected do not overlap each other. For example, when the scanning line Gj (1 to y) is selected, all the switching TFTs 405 whose gates are connected to the scanning line Gj are turned on. Then, the analog video signals sequentially input to the signal lines S1 to Sx are input to the gate of the driving TFT 406 via the switching TFT 405. Although FIG. 6 shows a case where analog video signals are sequentially input to the signal lines S1 to Sx, they may be input to the signal lines S1 to Sx at the same time.

(4) The gate voltage of the driving TFT 406 determined by the analog video signal is stored in the storage capacitor 408. In the present embodiment, the voltage between the common electrode and the power supply line is set such that the same voltage as that of the power supply line is applied to the common electrode in the writing period Ta, or the reverse bias voltage is applied to the light emitting element. Since the difference is controlled, the light emitting elements 407 of all pixels do not emit light regardless of the switching of the driving TFT 406.

(4) When the selection of all the scanning lines G1 to Gy is completed, the writing period Ta ends, and the holding period Ts starts.

(4) In the holding period Ts, a predetermined voltage difference is provided between the counter electrode and the power supply line so that a forward bias voltage is applied to the light emitting element when the driving TFT is turned on. Then, in all the pixels, the ON voltage of the driving TFT is controlled by the gate voltage stored in the storage capacitor 408 all at once, and the light emission of the light emitting element 407 is controlled by the ON current.

(4) When the holding period Ts ends, the non-display period Te starts. In the non-display period Te, a voltage having the same height as that of the power supply line is applied to the opposite electrode of the light-emitting element 407 as in the writing period Ta. Alternatively, the voltage difference between the counter electrode and the power supply line may be controlled so that a reverse bias voltage is applied to the light emitting element. Therefore, the light emitting elements 407 of all the pixels do not emit light at the same time, and all the pixels are turned off.

When the non-display period Te ends, one frame period ends, and one screen can be displayed. Then, the next frame period starts, and the writing period Ta, the holding period Ts, and the non-display period Te appear again.

(6) In the driving method shown in FIG. 6, in the writing period Ta and the non-display period Te, all pixels are in a state in which light emission is not forcibly performed, and no display is performed. The pixels perform display only during the holding period, and this period corresponds to a display period.

駆 動 In the driving method of the present invention, it is necessary to keep the duty ratio in an optimum range. In the case of the driving method shown in FIG. 6, by adjusting the length of the writing period Ta or the non-display period Te, it is possible to control the duty ratio to be within an optimum range. In this manner, by driving using the optimum duty ratio, deterioration of the light-emitting element is suppressed, constant luminance is obtained, and reliability of the light-emitting device can be improved.

The optimum duty ratio depends on the apparent luminance of the light emitting element at the beginning of light emission, that is, the value of the total amount of electricity flowing to one pixel in one frame period. The total amount of electricity flowing in one frame period may be based on a state where the gray level of the pixel is the highest, or may be based on a gray level determined by the practitioner. Further, an optimum duty ratio may be found each time according to the configuration of the light emitting element.

(Embodiment 2)
In this embodiment mode, a driving method of the present invention will be described with an example of a light-emitting device that controls light emission of a light-emitting element using three TFTs provided in each pixel.

FIG. 7A is a circuit diagram of a pixel portion of a light-emitting device using the driving method of the present invention. In FIG. 7A, a signal line (S1 to Sx), a power supply line (V1 to Vx), a first scan line (Ga1 to Gay), and a second scan line (Ge1 to Gey) are provided in a pixel portion 501. I have.

One of the signal lines (S1 to Sx), one of the power supply lines (V1 to Vx), one of the first scanning lines (Ga1 to Gay), and one of the second scanning lines (Ge1 to Gay) are provided. The region corresponds to the pixel 505. In the pixel portion 501, a plurality of pixels 505 are arranged in a matrix.

FIG. 7B is an enlarged view of the pixel 505. In FIG. 7B, reference numeral 507 denotes a switching TFT. The gate of the switching TFT 507 is connected to the first scanning line Gaj (j = 1 to y). One of the source and the drain of the switching TFT 507 is connected to the signal line Si (i = 1 to x), and the other is connected to the gate of the driving TFT 508.

(4) The gate of the erasing TFT 509 is connected to the second scanning line Gej (j = 1 to y). One of the source and the drain of the erasing TFT 509 is connected to the power supply line Vi (i = 1 to x), and the other is connected to the gate of the driving TFT 508.

One of the source and the drain of the driving TFT 508 is connected to the power supply line Vi, and the other is connected to the pixel electrode of the light emitting element 510.

(5) The light emitting element 510 includes an anode, a cathode, and an electroluminescent layer provided between the anode and the cathode. When the anode is connected to the source or drain of the driving TFT 508, the anode serves as a pixel electrode and the cathode serves as a counter electrode. Conversely, when the cathode is connected to the source or drain of the driving TFT 508, the cathode serves as a pixel electrode and the anode serves as a counter electrode.

(4) When the anode is a pixel electrode, the driving TFT 508 is preferably a p-channel TFT. When the cathode is a pixel electrode, the driving TFT 508 is preferably an n-channel TFT.

(4) A voltage is applied to the opposite electrode of the light emitting element 510 and the power supply line Vi from a power supply. The voltage difference between the counter electrode and the power supply line is maintained at such a value that a forward bias voltage is applied to the light emitting element when the driving TFT is turned on.

One of the two electrodes of the storage capacitor 512 is connected to the power supply line Vi, and the other is connected to the gate of the driving TFT 508. The storage capacitor 512 is provided to hold the gate voltage of the driving TFT 508 when the switching TFT 507 is in a non-selected state (OFF state). Note that FIG. 7B illustrates the structure in which the storage capacitor 512 is provided; however, the present invention is not limited to this structure, and a structure in which the storage capacitor 512 is not provided may be employed.

Next, a driving method of the present invention used for the light emitting device shown in FIG. 7 will be described with reference to FIG. The horizontal axis indicates time, and the vertical axis indicates the positions of the first and second scanning lines. Within each frame period, a writing period Ta, a holding period Ts, and a non-display period Te appear.

(1) In the writing period Ta, the first scanning lines Ga1 to Gay are sequentially selected so that the selected periods of the first scanning lines do not overlap with each other. For example, when the first scanning line Gaj (1 to y) is selected, all the switching TFTs 507 whose gates are connected to the first scanning line Gaj are turned on. Then, the analog video signals sequentially or simultaneously input to the signal lines S1 to Sx are input to the gate of the driving TFT 508 via the switching TFT 507.

(4) The on-current of the driving TFT 508 is controlled according to the image information included in the analog video signal, and the luminance of the light-emitting element is controlled by the on-current. As described above, in the present embodiment, the holding period Ts starts sequentially from the pixel to which the analog video signal is written, and the display starts.

The writing period Ta corresponds to a period until the selection of all the first scanning lines Ga1 to Gay is completed. Since the holding period Ts is started when the writing of the analog video signal ends for each pixel, in this embodiment, the writing period Ta overlaps with the holding period Ts of each pixel as shown in FIG. ing.

When the holding period Ts ends, the non-display period Te starts next. When the non-display period Te starts, the second scanning lines Ge1 to Gey are sequentially selected.

(4) When the second scanning line Gej is selected, all the erasing TFTs 509 whose gates are connected to the second scanning line Gej are turned on. Then, the voltages of the power supply lines V1 to Vx are applied to the gate of the driving TFT 508 via the erasing TFT 509.

(4) When the voltage of the power supply line is applied to the gate of the driving TFT 508, the gate and the source of the driving TFT 508 conduct, so that the gate voltage becomes 0 V and the driving TFT 508 is turned off. Accordingly, the light emitting element 510 is in a non-light emitting state, and the display of the pixels in the row is forcibly terminated.

(4) When all display periods have ended, one frame period ends, and one image can be displayed. The pixel described in this embodiment can be driven at a desired duty ratio by adjusting the length of the non-display period.

In the driving method shown in FIG. 8, all pixels do not emit light during the writing period Ta and the non-display period Te, and no display is performed. The pixel performs display only in the holding period Ts, and this period corresponds to a display period.

In the driving method shown in FIG. 8, if the writing periods Ta do not overlap between adjacent frame periods, the length of the display period can be shorter than the writing period Ta. The non-display periods Te may overlap each other, or may not overlap each other.

By using the driving method described in Embodiment Modes 1 and 2, driving at an optimum duty ratio can be performed, so that deterioration of a light-emitting element can be suppressed and reliability of a light-emitting device can be improved.

Note that the light-emitting device using the driving method of the present invention is not limited to the structures described in Embodiment 1 and Embodiment 2 as long as the duty ratio can be kept in an optimum range.

In this example, a driving method of the light emitting device illustrated in FIG. 5 other than the driving method described in Embodiment 1 will be described.

１ One of the driving methods of this embodiment will be described with reference to FIG. The horizontal axis indicates time, and the vertical axis indicates the position of the scanning line. In the driving method illustrated in FIG. 9A, a writing period Ta, a holding period Ts, and a non-display period Te are provided in one frame period, and the order in which they appear appears in Embodiment 1. Driving method.

具体 Specific operation of the pixel will be described. In the writing period Ta, as in Embodiment 1, a voltage having the same height as the power supply line is applied to the opposite electrode of the light-emitting element 407. Alternatively, the voltage difference between the counter electrode and the power supply line may be controlled so that a reverse bias voltage is applied to the light emitting element.

Then, the scanning lines G1 to Gy are sequentially selected, all the switching TFTs 405 whose gates are connected to the respective scanning lines are turned on, and the driving TFTs 405 are driven by analog video signals sequentially or simultaneously input to the signal lines S1 to Sx. The gate voltage of the TFT 406 is determined and stored in the storage capacitor 408.

In the writing period Ta, the voltage difference between the common electrode and the power supply line is controlled so that the same voltage as that of the power supply line is applied to the common electrode or the reverse bias voltage is applied to the light emitting element. Regardless of the switching of the driving TFT 406, the light emitting elements 407 of all the pixels do not emit light.

When the selection of all the scanning lines G1 to Gy is completed, the non-display period Te starts in the driving method shown in FIG.

In the non-display period Te, the gate voltage of the driving TFT 406 determined by the analog video signal is stored in the storage capacitor 408. Then, similarly to the writing period Ta, the voltage difference between the common electrode and the power supply line is controlled such that the same voltage as that of the power supply line is applied to the common electrode or the reverse bias voltage is applied to the light emitting element. Therefore, regardless of the switching of the driving TFT 406, the light emitting elements 407 of all the pixels do not emit light, and all the pixels are turned off.

When the non-display period Te ends, the holding period Ts starts next.

(4) In the holding period Ts, a predetermined voltage difference is provided between the counter electrode and the power supply line so that a forward bias voltage is applied to the light emitting element 407 when the driving TFT 406 is turned on. Then, in all the pixels, the ON voltage of the driving TFT is controlled by the gate voltage stored in the storage capacitor 408 all at once, and the light emission of the light emitting element 407 is controlled by the ON current.

(4) When the holding period Ts ends, one frame period ends, and one screen can be displayed. Then, the next frame period starts, and the writing period Ta, the non-display period Te, and the holding period Ts appear again.

In the driving method shown in FIG. 9A, in the writing period Ta and the non-display period Te, all pixels are in a state in which light emission is not forcibly performed, and no display is performed. The pixel performs display only in the holding period Ts, and this period corresponds to a display period.

駆 動 In the driving method of the present invention, it is necessary to keep the duty ratio in an optimum range. In the case of the driving method illustrated in FIG. 9A, by controlling the length of the writing period Ta, the holding period Ts, or the non-display period Te, it is possible to control the duty ratio to be within an optimal range. In this manner, by driving using the optimum duty ratio, deterioration of the light-emitting element is suppressed, constant luminance is obtained, and reliability of the light-emitting device can be improved.

１ One driving method of this embodiment will be described with reference to FIG. The horizontal axis indicates time, and the vertical axis indicates the position of the scanning line. In the driving method shown in FIG. 9B, the method of controlling the voltage difference between the common electrode and the power supply line during the writing period Ta is different from the driving method shown in the first embodiment.

(4) In the writing period Ta, a predetermined voltage difference is provided between the counter electrode and the power supply line so that a forward bias voltage is applied to the light emitting element when the driving TFT is turned on. Then, the scanning lines G1 to Gy are sequentially selected, all the switching TFTs 405 whose gates are connected to the respective scanning lines are turned on, and the driving TFTs 405 are driven by analog video signals sequentially or simultaneously input to the signal lines S1 to Sx. The gate voltage of the TFT 406 is determined and stored in the storage capacitor 408.

(4) The on-current of the driving TFT 406 is controlled according to the image information included in the analog video signal, and the luminance of the light-emitting element 407 is controlled by the on-current. As described above, in the driving method illustrated in FIG. 9B, the holding period Ts starts in order from the pixel to which the analog video signal is written, and the display starts.

(4) When the selection of all the scanning lines G1 to Gy is completed, the writing period Ta ends. Since the holding period is started at the time when the writing of the analog video signal is completed for each pixel, in the driving method shown in FIG. 9B, the writing period Ta overlaps with the holding period Ts of each pixel. ing.

When the holding period Ts ends, the non-display period Te starts next. In the non-display period Te, a voltage having the same height as the power supply line is applied to the opposite electrode of the light emitting element 407. Alternatively, the voltage difference between the counter electrode and the power supply line may be controlled so that a reverse bias voltage is applied to the light emitting element. Therefore, the light emitting elements 407 of all the pixels do not emit light at the same time, and all the pixels are turned off.

When the non-display period Te ends, one frame period ends, and one screen can be displayed. Then, the next frame period starts, and the writing period Ta, the holding period Ts, and the non-display period Te appear again.

In the driving method shown in FIG. 9B, all pixels do not emit light in the non-display period Te, and no display is performed. The pixel performs display only in the writing period Ta and the holding period Ts, and this period corresponds to a display period.

Note that in the case of the driving method illustrated in FIG. 9B, the writing period Ta needs to be shorter than the holding period Ts in a pixel in which an analog signal is first written. Further, in the case of the driving method shown in FIG. 9B, in order to reduce the difference in the length of the holding period Ts between the pixel in which the analog signal has been written first and the pixel in which the analog signal has been written last, The shorter the length of the period Ta, the more desirable.

駆 動 In the driving method of the present invention, it is necessary to keep the duty ratio in an optimum range. In the case of the driving method illustrated in FIG. 9B, by controlling the length of the writing period Ta, the holding period Ts, or the length of the non-display period Te, it is possible to control the duty ratio to be within an optimal range. In this manner, by driving using the optimum duty ratio, deterioration of the light-emitting element is suppressed, constant luminance is obtained, and reliability of the light-emitting device can be improved.

In the case of the driving method illustrated in FIG. 9B, a difference occurs in the length of the holding period Ts between a pixel in which an analog signal is written first and a pixel in which the analog signal is written last. Naturally, a difference also occurs in the duty ratio. Therefore, in the pixel shown in FIG. 9B, it is necessary to make the duty ratio fall within the optimum range in all the pixels.

In this embodiment, a detailed structure of a signal line driver circuit used for driving the light emitting device shown in FIG. 5 or FIG. 7 and a scan line driver circuit will be described.

FIG. 10 is a block diagram showing an example of a driving circuit of a light emitting device.

{The signal line driver circuit 601 illustrated in FIG. 10A includes a shift register 602, a level shifter 603, and a sampling circuit 604. Note that the level shifter may be used as needed, and may not necessarily be used. Although the level shifter 603 is provided between the shift register 602 and the sampling circuit 604 in this embodiment, the present invention is not limited to this configuration. A configuration in which the level shifter 603 is incorporated in the shift register 602 may be employed.

When the clock signal (CLK) and the start pulse signal (SP) are supplied to the shift register 602, the shift register 602 generates a timing signal for controlling the timing of sampling a video signal. The amplitude of the voltage of the generated timing signal is amplified by the level shifter 603 and input to the sampling circuit 604. The video signal input to the sampling circuit 604 is sampled in synchronization with the timing signal input to the sampling circuit 604, and is input to a corresponding signal line.

{The scanning line driver circuit 605 illustrated in FIG. 10B includes a shift register 606 and a buffer 607, respectively. In some cases, a level shifter may be provided.

In the scan line driver circuit 605, the timing signal from the shift register 606 is supplied to the buffer 607 and supplied to the corresponding scan line (or the first scan line or the second scan line). The gate of the switching TFT (or the erasing TFT) of the pixel for one line is connected to the scanning line. Since the switching TFTs (or erasing TFTs) of the pixels for one line must be turned on all at once, a buffer capable of flowing a large current is used.

This embodiment can be implemented by being freely combined with Embodiment 1.

In this example, a structure and a manufacturing method of a light-emitting element used for obtaining data shown in FIGS. 1 and 2 will be described.

FIG. 11 shows the configuration of the light emitting element used in the measurement performed to obtain the data in FIGS. The light emitting element shown in FIG. 11 uses ITO as a pixel electrode, and uses a cathode made of a laminate of a conductive film made of Ca and a conductive film made of Al as a cathode. The electroluminescent layer is formed by applying a light emitting layer made of a PPV-based electroluminescent material that emits yellow light and a poly (ethylenedioxythiophene) / poly (styrenesulfonic acid) aqueous solution (PEDOT / PSS). A hole injection layer.

To explain a specific manufacturing method, on a transparent conductive film made of ITO, a PEDOT / PSS aqueous solution is spin-coated at 1500 rpm, baked at 100 ° C. for 10 minutes at normal pressure, and then 80 ° C. in a vacuum atmosphere. By baking at 10 ° C. for 10 minutes, a 30 nm PEDOT / PSS layer (hole injection layer) is obtained.

Next, a toluene solution of a PPV derivative emitting yellow light (corresponding to 4 g / l) is prepared and spin-coated at 1300 rpm under a nitrogen atmosphere. Thereafter, vacuum baking is performed at 80 ° C. for 10 minutes to obtain an 80-nm PPV derivative layer (light-emitting layer).

After that, Ca is vacuum-deposited at 20 nm, and Al is further vacuum-deposited at 100 nm to form a cathode.

A measured value of a change in luminance of the light emitting element over time. An actually measured value indicating the relationship between the duty ratio and the change in luminance. The figure which shows the relationship between a duty ratio and a current density. 9 is a graph showing luminance of a light-emitting element after X hours under each condition. FIG. 4 is a circuit diagram of a pixel and a pixel portion of a light-emitting device. FIG. 6 is a diagram showing a driving method of the present invention in the light emitting device shown in FIG. 5. FIG. 4 is a circuit diagram of a pixel and a pixel portion of a light-emitting device. FIG. 8 is a diagram showing a driving method of the present invention in the light emitting device shown in FIG. 7. FIG. 6 is a diagram showing one embodiment of a driving method of the present invention in the light emitting device shown in FIG. 5. FIG. 4 is a block diagram of a driving circuit. FIG. 4 illustrates a structure of a light-emitting element used for measurement. FIG. 4 is a diagram illustrating a driving method of a general light-emitting device when an analog video signal is used. FIG. 7 illustrates deterioration of luminance of a light-emitting element.

Claims (5)

A driving method of a light emitting device having a light emitting element,
Controlling the brightness of the light emitting element with an analog video signal,
A method for driving a light emitting device, wherein the light emitting device is driven at a duty ratio within a range of higher than 20% and lower than 50%. A driving method of a light emitting device having a light emitting element,
Controlling the brightness of the light emitting element with an analog video signal,
When the total amount of electricity supplied to the light emitting element of one pixel in one frame period is constant, the luminance after X hours for different duty ratios exceeds 0.8 times the maximum value of the luminance after X hours. A method for driving a light emitting device, wherein the light emitting device is driven at a duty ratio within a range where luminance can be obtained. A driving method of a light emitting device having a light emitting element,
Controlling the brightness of the light emitting element with an analog video signal,
When the total amount of electricity supplied to the light emitting element of one pixel in one frame period is constant, the luminance after X hours for different duty ratios is 0.8 times the maximum value of the luminance after X hours. Driving with a duty ratio within the range that can obtain more brightness,
The duty ratio within the predetermined range is a ratio of a period in which the same voltage is applied to the anode and the cathode or a period in which a reverse bias voltage is applied in one frame period in the light emitting element. A method for driving a light-emitting device, characterized by being controlled by changing. A method for determining a duty ratio in driving a light emitting device having a light emitting element,
The brightness of the light emitting element is controlled by an analog video signal,
When the total amount of electricity supplied to the light emitting element during one frame period is constant, the characteristic of the luminance after X hours for different duty ratios exceeds 0.8 times the maximum value of the luminance after X hours. Find the range of the duty ratio at which the luminance can be obtained,
A method for determining a duty ratio, wherein a period during which a current corresponding to the analog video signal is supplied to the light emitting element in one frame period is determined so that the duty ratio falls within the range. A method for determining a duty ratio in driving a light emitting device having a light emitting element,
The brightness of the light emitting element is controlled by an analog video signal,
The frame frequency of the light emitting element, the total amount of electricity flowing to the light emitting element during one frame period is constant,
Under a constant duty ratio, the characteristic of the luminance after X hours with respect to the current density of the light emitting element, and under the constant current density, the characteristic of the luminance after X hours with respect to different duty ratios of the light emitting element. , To obtain the characteristic of the luminance after X hours for different duty ratios when the total amount of electricity is constant,
From the obtained characteristics, a range of a duty ratio at which a luminance exceeding 0.8 times the maximum value of the luminance after the X time is obtained,
A method for determining a duty ratio, wherein a period during which a current corresponding to the analog video signal is supplied to the light emitting element in one frame period is determined so that the duty ratio falls within the range.

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