JP2007206471A - Electrophoretic display device and electronic apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To reduce afterimages of an electrophoresis display device although the afterimages of the electrophoresis display device include an erasure-time afterimage generated during image switching and a temporal afterimage generated as a voltage is repeatedly applied to an electrophoresis display element. <P>SOLUTION: The electrophoresis display device is equipped with a display section having a plurality of pixels each having an electrophoresis element having a dispersion system including electrophoresis particles between a common electrode and pixel electrodes and a control section which controls a voltage applied to a common electrode and pixel electrodes to move the electrophoresis particles and thus form an image. In a switching period wherein the image displayed at the display section is changed from a first image to a second image, the control section provides a first period wherein the whole display section has a second gradation by giving a potential difference only between the pixel electrodes in a region where display when the first image is displayed has the first gradation (white) and the common electrode and no potential difference between the pixel electrodes in a region where the display section has a second gradation (black) and the common electrode, and a second period wherein the whole display section is set to a first gradation by giving a potential difference between the pixel electrodes and common electrode in the whole region of the display section. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an electrophoretic display device and an electronic apparatus.

  An electrophoretic display device is configured by sealing an electrophoretic dispersion liquid containing one or more kinds of electrophoretic particles and an electrophoretic dispersion medium between a pair of counter electrode plates, at least one of which is transparent. The When a voltage is applied between the two electrodes, the electrophoretic particles move in the electrophoretic dispersion medium, and the distribution thereof changes, so that the optical reflection characteristics change and information can be displayed.

  In Patent Document 1, in an active matrix electrophoretic display device using electronic ink, when changing display contents, all pixel electrodes are set to the same potential, and a voltage is applied between the common electrode and the pixel electrode. A driving method that can suppress the drive voltage and prevent erroneous rewriting by erasing all the display area that has been displayed by applying the voltage and then displaying new display contents thereafter is disclosed. Has been.

JP 2002-149115 A

  However, according to the driving method described above, an afterimage tends to remain on the display unit. The afterimage in this case includes an afterimage at the time of erasing that occurs at the time of image switching and a temporally afterimage that occurs by repeatedly applying a voltage to the electrophoretic display element.

  Therefore, an object of the present invention is to reduce the afterimage of the electrophoretic display device.

  The electrophoretic display device according to the present invention includes a display unit including a plurality of pixels including an electrophoretic element having a dispersion system including electrophoretic particles between the common electrode and the pixel electrode, the common electrode, and the pixel electrode. A control unit configured to move the electrophoretic particles to form an image by controlling a voltage applied to the display unit, and the control unit converts an image displayed on the display unit from a first image to a second image. In the switching period in which the display is changed, the potential difference is given only between the pixel electrode and the common electrode in the region where the display is the first gradation while the first image is being displayed, In the first period in which the entire display portion is set to the second gradation without applying a potential difference between the pixel electrode and the common electrode in the region having the gray scale, and in the entire region of the display portion, By applying a potential difference between the electrode and the common electrode, The entire section is intended to provide a second period to the first gradation.

  Thereby, in the switching period, it is relieved that the same voltage is repeatedly applied to the electrophoretic element and the same gradation is continuously displayed, and an afterimage due to a decrease in retention characteristics after the voltage interruption of the electrophoretic particles can be reduced. .

Further, the control unit moves the electrophoretic particles for setting the display unit to the second gradation in the switching period, and the electrophoresis for setting the display unit to the first gradation. It is desirable to control the length of the second period so that the amount of particle movement is equal.
Further, the control unit moves the electrophoretic particles for setting the display unit to the second gradation in the switching period, and the electrophoresis for setting the display unit to the first gradation. It is desirable to control a potential difference applied between the pixel electrode and the common electrode in the second period so that the amount of particle movement becomes equal.

In addition, the control unit further includes a third period in which the display unit is alternately set to the first gray level and the second gray level in a cycle of 1 to 10 milliseconds in the switching period. It is desirable.
As a result, the electrophoretic particles are well stirred in the dispersion system, and the afterimage can be reduced.

  The electronic device of the present invention includes all devices including the above-described electrophoretic display device as a display unit, and includes a display device, a television device, an electronic book, an electronic paper, a clock, a calculator, a mobile phone, and a portable information terminal. Etc. Also, things that deviate from the concept of “equipment”, for example, flexible paper / film-like objects, belonging to real estate such as wall surfaces to which these objects are attached, moving objects such as vehicles, flying objects, ships, etc. Including those belonging to.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
Embodiment 1 FIG.
FIG. 1 is a diagram showing an overall electrical configuration of the electrophoretic display device 10 according to the first embodiment. The electrophoretic display panel A (display unit) includes a plurality of pixels, and these pixels include a TFT (Thin Film Transistor) 103 as a switching element, which will be described later, and a pixel electrode 104 connected to the TFT 103. It consists of On the other hand, a scanning line driving circuit 130 and a data line driving circuit 140 are formed in the peripheral region of the element substrate 100. In the electrophoretic display panel A of the element substrate 100, a plurality of scanning lines 101 are formed in parallel along the X direction shown in the drawing. In addition, a plurality of data lines 102 are formed in parallel along the Y direction orthogonal thereto. Each pixel is arranged in a matrix corresponding to the intersection of the scanning line 101 and the data line 102.

  A controller (control unit) 300 is provided in the peripheral circuit of the electrophoresis apparatus. The controller 300 includes an image signal processing circuit and a timing generator. Here, the image signal processing circuit generates image data and a counter electrode control signal, and inputs them to the data line driving circuit 140 and the counter electrode modulation circuit 150, respectively. The counter electrode modulation circuit 150 supplies a bias signal Vcom and a power supply voltage Vs to the common electrode of the pixel and the counter electrode of the storage capacitor, respectively. For example, image reset is set by a positive or negative high level bias signal Vcom (reset signal). The reset signal is output for a predetermined period before the data line driving circuit 140 outputs image data. The reset is used to attract the electrophoretic particles migrating in the dispersion medium to the pixel electrode or the common electrode to initialize the spatial state. The timing generator generates various timing signals for controlling the scanning line driving circuit 130 and the data line driving circuit 140 when reset settings and image data are output from the image signal processing circuit.

  FIG. 2 is a diagram illustrating the structure of each pixel of the electrophoretic display device 10. The pixel (i, j) in the i-th row and the j-th column includes the TFT 103, the pixel electrode 104, and the storage capacitor Cs. A gate terminal of the TFT 103 is connected to the scanning line 101, and a source terminal thereof is connected to the data line 102. Further, the drain terminal of the TFT 103 is connected to the pixel electrode 104 and the storage capacitor Cs. The holding capacitor Cs holds the voltage applied to the pixel electrode 104 by the TFT 103. Since the pixel is configured by sandwiching an electrophoretic layer between the pixel electrode 104 and the common electrode Com, a pixel capacitance Cepd is formed according to the electrode area, the distance between the electrodes, and the dielectric constant of the electrophoretic layer. is doing. The common electrode Com is connected to the counter electrode modulation circuit 150 via the wiring 201. The other side of the storage capacitor Cs is connected to the storage capacitor line 106. The storage capacitor line 106 is connected to the power source Vs by the counter electrode modulation circuit 150.

Regarding the driving of the electrophoretic display device 10, the reset operation will be described first. At the reset timing, when the scanning line driving circuit 130 outputs a selection signal to all the scanning lines 101 and all the scanning line signals become active, the TFTs 103 connected to all the pixels connected to these scanning lines 101 are turned on. It becomes a state. At this time, the data line driving circuit 140 outputs a high level or a low level to all data lines. This signal is supplied to all the pixel electrodes. The counter electrode modulation circuit 150 supplies the common electrode Com with a low level when a high level is supplied to all the data lines, and supplies a high level signal when a low level is supplied to all the data lines. To do. At this time, since the same potential difference is applied between the pixel electrode and the common electrode of all the pixels, the electrophoretic display element is reset to the first gradation or the second gradation in all the pixel regions. The
Next, an image writing operation will be described. During the image writing operation, the scanning line driving circuit 130 sequentially supplies a selection signal to the scanning line 101. When the selection signal is supplied to the j-th scanning line 101 and the selection is made, the TFT 103 connected to the scanning line 101 is turned on. At this time, the data signal Xi (image signal) supplied from the data line driving circuit 140 is written to the pixel electrode 104 in synchronization with the scanning line selection. At this time, the storage capacitor Cs is also charged at the voltage level of the data signal Xi, and even after the TFT 103 is cut off, the charge of the pixels (pixel electrode and common electrode) is maintained to maintain the image by the electrophoretic particles. Each pixel performs display in accordance with the voltage level of the data signal, thereby displaying an image.

Next, a detailed operation when changing the display image during the operation of the electrophoretic display device 10 will be described. First, a mechanism for generating an afterimage in the electrophoretic display device of the comparative example will be described.
FIG. 3 is a diagram illustrating an afterimage of the electrophoretic display device of the comparative example. FIG. 4 is a timing chart showing a method of controlling the voltage applied to the common electrode and the pixel electrode by the controller of the electrophoretic display device of the comparative example. FIG. 5 is a diagram schematically showing the operation when changing the display image of the electrophoretic display device of the comparative example.

The afterimage at the time of erasure and the afterimage over time will be described with reference to FIGS.
Here, the electrophoretic particles include negatively charged white electrophoretic particles (first particles) and positively charged black electrophoretic particles (second particles). Also, let white be the first gradation and black be the second gradation.

  In the state of FIG. 3A, the character “H” in the second gradation (black) is displayed on the display section on the background of the first gradation (white). Here, the region of the character “H” is defined as region a, and the other background region is defined as region b. Immediately before “H” is written, the entire display section is displayed in white. When “H” is written, a second voltage (low level) is applied to the common electrode as shown in FIG. Further, the first voltage (high level) is applied only to the pixel electrode corresponding to the region a, and the second voltage (low level) is applied to the pixel electrode corresponding to the background region b. As a result, as shown in FIG. 5A, the positively charged black electrophoretic particles move only in the region a to the common electrode side, and the letter “H” is displayed. After writing the letter “H”, the second voltage (low level) is applied to the common electrode and all the pixel electrodes, and the display content is retained.

  Next, in the state of FIG. 3B, before changing the display image, the entire display unit is displayed in white to erase the image. When erasing an image, as shown in FIG. 4, a first voltage (high level) is applied to the common electrode, and a second voltage (low level) is applied to all the pixel electrodes. As a result, as shown in FIG. 5B, the black electrophoretic particles move to the pixel electrode side in the region a, and the entire display portion becomes white.

  At this time, as shown in FIG. 3B, the region a is white with a lower reflectance than the region b, that is, white near gray, forming an afterimage. This is because, although the initial distribution state of the electrophoretic particles is different between the region a and the region b, the same electric field is applied to the region a and the region b for the same time, so that there is a difference in the amount of movement of the electrophoretic particles. This is because there is a slight difference in gradation.

電気泳動粒子の移動度とは、電気泳動粒子にある強さの電界を一定時間印加したときの電気泳動粒子の移動距離である。電気泳動粒子の電界中での移動度は、電気泳動粒子の持つ電荷や、重さ、電気泳動粒子が分散している溶媒の粘度等によって決まる。 Here, the migration amount D of the electrophoretic particles is the mobility of the black electrophoretic particles is μ _B , the mobility of the white electrophoretic particles is μ _W , the electric field strength is E, and the black electrophoretic particles are displayed on the display side (common here) Assuming that the voltage application time for moving the electrode side) is T _B and the voltage application time for moving the white electrophoretic particles to the display side is T _W , it can be expressed by the following equation.

The mobility of the electrophoretic particles is a moving distance of the electrophoretic particles when an electric field having a certain strength is applied to the electrophoretic particles for a certain period of time. The mobility of electrophoretic particles in an electric field is determined by the charge, weight, and viscosity of the solvent in which the electrophoretic particles are dispersed.

  Next, in the state of FIG. 3C, the letter “I” is displayed on the display unit with the second gradation (black) on the first gradation (white) background. Here, the area of the character “I” is defined as area c. When "I" is written, the second voltage (low level) is applied to the common electrode as shown in FIG. Further, the first voltage (high level) is applied to the pixel electrode corresponding to the region c, and the second voltage (low level) is applied to the pixel electrodes corresponding to the other regions. As a result, as shown in FIG. 5C, the black electrophoretic particles positively charged only in the region c move to the common electrode side, and the letter “I” is displayed.

  At this time, as shown in FIG. 3C, the portion of the character “H” displayed before remains as a light gray afterimage. Such an afterimage generated at the time of image switching is called an afterimage at the time of erasure.

After writing the letter “I”, the second voltage (low level) is applied to the common electrode and all the pixel electrodes as shown in FIG. 5D, and the display contents are displayed as shown in FIG. Is retained.
At this time, in the state of FIG. 3D, the region b is white with a low reflectance compared to the state of FIG. This is due to a temporal afterimage that occurs when a voltage is repeatedly applied to the electrophoretic element.

If the same voltage is repeatedly applied to a certain part of the electrophoretic element to continuously display the same gradation, the retention characteristics of the electrophoretic particles when the voltage application is interrupted are deteriorated. If the retention property of the electrophoretic particles is lowered, as shown in FIG. 5D, the electrophoretic particles may move slightly when an electric field is not applied to the electrophoretic element, and the contrast of the image may deteriorate.
That is, if a voltage for writing white is continuously applied in an area displaying white, when the voltage is not applied, white having a low reflectivity, that is, white close to gray is obtained.

  In the state of FIG. 5B, the voltage applied to write white is excessive in the electrophoretic element corresponding to the region b compared to the region a. When such voltage application is repeated, the retention characteristics of the electrophoretic particles in the region b when the voltage application is interrupted are lowered, and an afterimage with time is generated in the region b as shown in FIG. become.

  Such a temporal afterimage is eliminated by applying a voltage for refreshing every several tens of minutes to several hours, but frequent refreshing operation has a problem of increasing power consumption.

Next, the operation when changing the display image of the electrophoretic display device 10 according to the present invention will be described.
FIG. 6 is a diagram for explaining an afterimage of the electrophoretic display device according to the first embodiment of the present invention. FIG. 7 is a timing chart showing a method of controlling the voltage applied to the common electrode and the pixel electrode by the controller of the electrophoretic display device according to the first embodiment of the present invention. FIG. 8 is a diagram schematically showing the operation when changing the display image of the electrophoretic display device according to the first embodiment of the present invention.
The difference from the comparative example is the control method in the erasing time (switching period) during the rewriting from the image (first image) “H” before the change to the image (second image) “I” after the change. .

  In the electrophoretic display device 10 according to the first embodiment, during the erasing time for changing the display from “H” to “I”, first, as shown in FIG. 6B and FIG. A display period (first period) is provided, and then, as shown in FIGS. 6C and 8C, a period (second period) in which the entire display portion is displayed in white is provided.

  In the first period, as shown in FIGS. 7 and 8, the second voltage (low level) is applied to the common electrode, and the second voltage is applied to the pixel electrode in the region a. The first voltage (high level) is applied to the pixel electrode in the region b and the pixel electrode in the region c.

Here, the first voltage can be applied to all the pixel electrodes so that the entire region can be displayed in black. However, at this time, since the area a is displayed in black from the “H” display period, if voltage application for further black display is performed at the time of erasure, the voltage application for black display becomes excessive. End up. As described above, when such voltage application is repeated, the retention characteristics of the electrophoretic particles in the region a when the voltage application is interrupted are lowered, and afterimages are generated over time.
Therefore, in the first embodiment, in the region a, the pixel electrode is set to the same potential as the common electrode, and a voltage is applied so that a potential difference is generated only in the region b and the region c. Afterimages due to a decrease in retention characteristics are reduced.

  In the second period, the first voltage is applied to the common electrode and the second voltage is applied to all the pixel electrodes, thereby displaying the entire display portion in white. This resets the display unit before displaying “I”. At this time, since the entire display unit shifts from black display to white display, an afterimage as shown in FIG.

  In addition, since each pixel performs one black write and one white write, the number of white and black writes becomes equal, and excessive writing is performed on a specific electrophoretic element. Can be prevented. Here, the length of the second period is controlled so that the amount of movement of the electrophoretic particles for displaying the display portion in black is equal to the amount of movement of the electrophoretic particles for displaying the display portion in white. It is preferable to do. As a result, it is possible to prevent the voltage from being excessively applied to the electrophoretic element to make the display white or black, thereby preventing overwriting, and the afterimage over time due to a decrease in retention of electrophoretic particles when the voltage application is cut off. Can be reduced. Hereinafter, the calculation method of the second period will be described.

FIG. 9 is a diagram for explaining in detail the voltage applied to the pixel electrode when driving as shown in FIG. 7 is performed during the erase time using the pixel circuit shown in FIG. Drawing, COM voltage of the common electrode, Scan, Data Each scanning line signal, and a data line signal, v _a, v _b the voltage of the pixel electrode of each region a and region _{b, v a -COM, v b} - COM is the potential of the pixel electrodes in the region a and the region b as viewed from the common electrode.

ここで、ＶはＴＦＴ１０３がオン状態の時に画素電極にかかる電圧、Ｃepdは画素容量、Ｃｓは保持容量である。 Specifically, the second period is determined so that the time integration of the potential of the pixel electrode viewed from the common electrode during the erasing time becomes zero.
Here, since the relationship between the on-current I _on and the off-current I _off of the TFT 103 is I _on >> I _off , the charging time to the storage capacitor Cs is negligibly small compared to the charge holding time. Here, when I _on is constant, the time integration of the pixel potential v in the first period is expressed by the following equation.

Here, V is a voltage applied to the pixel electrode when the TFT 103 is on, Cepd is a pixel capacitor, and Cs is a storage capacitor.

よって、次の式が成り立つ。
（Ｃｓ＋Ｃepd／２Ｉ_off）・Ｖ²−ＶＴ＝０
これにより、Ｔは以下のように設定できる。
Ｔ＝（Ｃｓ＋Ｃepd／２Ｉ_off）・Ｖ
なお、以上の方法で求めたＴは概算値であり、計算方法としては回路シミュレータ等を使って厳密な過渡解析を行うことがより望ましい。 If the length of the second period is T, the time integration of the pixel potential v in the second period is expressed by the following equation.

Therefore, the following equation holds.
(Cs + Cepd / 2I _off ) · V ² −VT = 0
Thereby, T can be set as follows.
T = (Cs + Cepd / 2I _off ) · V
Note that T obtained by the above method is an approximate value, and as a calculation method, it is more desirable to perform strict transient analysis using a circuit simulator or the like.

  10 and 11 are other examples of timing charts showing a method of controlling the voltage applied to the common electrode and the pixel electrode by the controller 300 of the electrophoretic display device 10 according to the first embodiment of the present invention.

In the example of FIG. 10, in the erasing time, a period for performing all white display and a period for performing all black display are provided in addition to the first period and the second period.
As in the example shown in FIG. 10, by performing all white display and all black display in addition to the first period and the second period, the white and black electrophoretic particles are agitated in the dispersion medium, and the uniformity Therefore, the afterimage at the time of erasure and the afterimage after time can be reduced.

  Furthermore, in the example of FIG. 11, a third period in which all white display and all black display are repeated in a short time is provided in the erasing time. It is appropriate to repeat the all white display and the all black display with a period of about 1 to 10 milliseconds.

  As in the example shown in FIG. 11, by repeating all white display and all black display in a short time, the electrophoretic particles are well stirred in the dispersion medium, and the afterimage after erasure and the afterimage over time can be reduced. it can.

Electronic Device FIG. 12 is a perspective view illustrating a specific example of an electronic device to which the electrophoretic display device of the present invention is applied. FIG. 12A is a perspective view illustrating an electronic book that is an example of the electronic apparatus. The electronic book 1000 includes a book-shaped frame 1001, a cover 1002 that can be rotated (opened and closed) with respect to the frame 1001, an operation unit 1003, and the electrophoretic display device of the present invention. Display unit 1004.

  FIG. 12B is a perspective view illustrating a wrist watch that is an example of an electronic apparatus. The wristwatch 1100 includes a display unit 1101 configured by the electrophoretic display device of the present invention.

  FIG. 12C is a perspective view illustrating electronic paper which is an example of an electronic device. This electronic paper 1200 includes a main body portion 1201 formed of a rewritable sheet having the same texture and flexibility as paper, and a display portion 1202 formed of an electrophoretic display device of the present invention.

For example, electronic books, electronic papers, and the like are supposed to be used for repeatedly writing characters on a white background, and therefore it is necessary to eliminate afterimages at the time of erasure and afterimages over time.
Note that the range of electronic devices to which the electrophoretic display device of the present invention can be applied is not limited to this, and includes a wide range of devices that utilize changes in visual color tone accompanying the movement of charged particles.

FIG. 1 is a diagram showing an overall electrical configuration of an electrophoretic display device according to Embodiment 1 of the present invention. FIG. 2 is a diagram illustrating the structure of each pixel of the electrophoretic display device. FIG. 3 is a diagram illustrating an afterimage of the electrophoretic display device of the comparative example. FIG. 4 is a timing chart illustrating a method for controlling the voltage applied to the common electrode and the pixel electrode of the electrophoretic display device of the comparative example. FIG. 5 is a diagram schematically showing the operation when changing the display image of the electrophoretic display device of the comparative example. FIG. 6 is a diagram for explaining an afterimage of the electrophoretic display device according to the first embodiment of the present invention. FIG. 7 is an example of a timing chart showing a method for controlling a voltage applied to the common electrode and the pixel electrode of the electrophoretic display device according to the first embodiment of the present invention. FIG. 8 is a diagram schematically showing an operation when changing the display image of the electrophoretic display device according to the first embodiment of the present invention. FIG. 9 is a diagram for explaining in detail the voltage applied to the pixel electrode when driving as shown in FIG. 7 is performed during the erase time. FIG. 10 is an example of a timing chart showing a method for controlling the voltage applied to the common electrode and the pixel electrode of the electrophoretic display device according to the first embodiment of the present invention. FIG. 11 is an example of a timing chart showing a method for controlling the voltage applied to the common electrode and the pixel electrode of the electrophoretic display device according to the first embodiment of the present invention. 12A to 12C are diagrams illustrating examples of electronic devices according to the present invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Electrophoretic display apparatus, 100 element board | substrate, 101 scanning line, 102 data line, 103 TFT, 104 pixel electrode, 106 holding capacity line, 130 scanning line drive circuit, 140 data line drive circuit, 150 counter electrode modulation circuit, 201 wiring 300 controller

Claims (5)

A display unit including a plurality of pixels including an electrophoretic element having a dispersion system including electrophoretic particles between the common electrode and the pixel electrode;
A controller for moving the electrophoretic particles to form an image by controlling a voltage applied to the common electrode and the pixel electrode;
The controller is
In the switching period in which the image displayed on the display unit is changed from the first image to the second image, the pixels in the region where the display during the display of the first image is the first gradation A potential difference is applied only between the electrode and the common electrode, and no potential difference is applied between the pixel electrode and the common electrode in the region where the display portion has the second gradation, and the entire display portion is moved to the second floor. And a second period in which the entire display portion is set to the first gradation by applying a potential difference between the pixel electrode and the common electrode in the entire region of the display portion. An electrophoretic display device.   The control unit includes a movement amount of the electrophoretic particles for setting the display unit to the second gradation in the switching period, and an amount of the electrophoretic particles for setting the display unit to the first gradation. The electrophoretic display device according to claim 1, wherein the length of the second period is controlled so that movement amounts are equal.   The control unit includes a movement amount of the electrophoretic particles for setting the display unit to the second gradation in the switching period, and an amount of the electrophoretic particles for setting the display unit to the first gradation. 2. The electrophoretic display device according to claim 1, wherein a potential difference applied between the pixel electrode and the common electrode in the second period is controlled so that movement amounts are equal.   In the switching period, the control unit further includes a third period in which the display unit is alternately set to the first gradation and the second gradation in a cycle of 1 to 10 milliseconds. The electrophoretic display device according to any one of claims 1 to 3, wherein the electrophoretic display device is characterized in that: An electronic apparatus comprising the electrophoretic display device according to claim 1.

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