Driving method for display apparatus

The present invention relates to a method of driving a display apparatus of the metal deposition type in which display is conducted through deposition and dissolution of a metal, and particularly to a method of driving a display apparatus which is suited to the so-called electronic paper or the like.

In recent years, attendant on the spread of networks, documents hitherto distributed in the form of printed matter have come to be transmitted in the form of the so-called electronic documents. Further, books and magazines have come to be often provided in the form of the so-called electronic publishing. In order to read these pieces of information, reading from CRTs (cathode ray tubes) and liquid crystal displays of computers has conventionally been widely conducted.

However, in a light emission type display such as the CRT, it has been pointed out that the display causes conspicuous wearing on an ergonomic ground and, therefore, is unsuited to long-time reading. In addition, even a light reception type display such as a liquid crystal display is said to be similarly unsuited to reading, because of the flickering which is intrinsic of fluorescent tubes. Furthermore, both of the types have the problem that the reading place is limited to the places where computers are disposed.

In recent years, reflection type liquid crystal displays not using a backlight have put to practical use. However, the reflectance in non-display (display of white color) of the liquid crystal is 30 to 40%, which means a considerably bad visibility, as compared with the reflectance of printed matter printed on papers (the reflectance of OA papers and pocket books is 75%, and the reflectance of newspapers is 52%). In addition, the glaring due to reflectors and the like are liable to cause wearing, which also is unsuited to long-time reading.

In order to solve these problems, the media so-called paper-like displays or electronic papers have been being developed. These media mainly utilize coloration by moving colored particles between electrodes through electrophoresis or by rotating dichroic particles in an electric field. In these methods, however, the gaps between the particles absorb light, with the result that contrast is worsened and that a practical-use writing speed (within 1 sec) cannot be obtained unless the driving voltage is 100 V or higher.

As compared with the displays of these systems, electrochemical display devices for color development based on an electrochemical action (electrochromic display: ECD) are better in contrast, and they have already been put to practical use as light control glass or timepiece displays. It should be noted here that the light control glass and timepiece displays are not directly suited to the electronic paper or the like uses, since it is intrinsically unnecessary to perform matrix driving. Besides, they are generally poor in quality of black color, and the reflectance thereof remains at a low level.

In addition, in such displays as electronic papers, they are continuedly exposed to solar light or room light on a use basis. In an electrochemical display device put to practical use in the light control glass and timepiece displays, an organic material is used for forming black-colored portions. This leads to a problem concerning light resistance. In general, organic materials are poor in light resistance, and the black color density thereof is lowered through fading when used for a long time.

In order to solve these technical problems, there has been proposed an electrochemical display device using metal ions as a material for color change. In the electrochemical display device, the metal ions are preliminarily mixed into a polymer electrolyte layer, the metal is deposited and dissolved by electrochemical redox reactions, and the change in color attendant on this is utilized to perform display. Here, for example, when the polymer electrolyte layer contains a coloring material, it is possible to enhance the contrast in the case where the color change occurs.

Meanwhile, in the metal deposition type electrochemical display device based on deposition and dissolution of the metal, a threshold voltage which is the deposition overvoltage is utilized to achieve display. In each pixel, the metal is deposited when a minus voltage in excess of the threshold voltage is impressed between electrodes, whereas the metal is dissolved when a plus voltage is impressed between the electrodes.

In such a metal deposition type electrochemical display device, it is difficult to control the colored state, which is a great problem in enhancing the quality of display. For example, according to the conventional thought, the metal deposition type display device is basically designed for black-and-white display, and few attempts have been made to achieve display of gradation. This is due to the fact that it is difficult to control the colored state to an intermediate colored state with good reproducibility.

Besides, in the metal deposition type electrochemical display device, it takes a certain period of time to deposit or dissolve the metal and, therefore, problems are left as to response speed and power consumption. For example, when it is tried to once erase all pixels and to perform new writing at the time of rewriting the screen, it takes a considerable length of time and the power consumption is great.

Furthermore, in the metal deposition type electrochemical display device, there is the tendency that the display contrast would be lowered with the lapse of time, and a countermeasure against this problem is desired.

The present invention has been proposed in consideration of the above-mentioned circumstances of the related art. Accordingly, it is an object of the present invention to provide a method of driving a display apparatus by which it is possible to control the colored state of each pixel to an appropriate state, for example, a uniform state in a metal deposition type display apparatus. It is another object of the present invention to provide a method of driving a display apparatus by which it is possible to achieve display of gradations and display with high definition. It is a further object of the present invention to provide a method of driving a display apparatus by which it is possible to contrive a higher display speed and a lower power consumption of the apparatus. It is yet another object of the present invention to provide a driving method by which it is possible to sustain images for a long time in a metal deposition type display apparatus.

In order to attain the above objects, the method of driving a display apparatus according to the present invention is firstly characterized in that the driving method is a method of driving a display apparatus for displaying an image through deposition and dissolution of a metal by impressing a predetermined voltage on each pixel, wherein a voltage pulse or AC voltage having an amplitude of not more than a threshold for deposition of the metal is impressed, and, according to the resulting current, a writing pulse or an erasing pulse is controlled.

A metal deposition type display apparatus has a hysteresis in current-voltage characteristic such that when a negative impressed voltage exceeds a threshold, a current begins to flow to cause deposition of a metal, and, thereafter, the current flowing condition is maintained even if the voltage is reduced to or below the threshold. In the display apparatus having such a current-voltage characteristic, when voltage scanning is conducted by limiting the negative voltage range to the range of not more than the threshold, the resistance in the vicinity of zero voltage is gradually raised as the colored density is lowered.

Where a pulse voltage or AC voltage having a voltage of less than the threshold is impressed and the resulting current (namely, the resistance) is measured by utilizing the above-mentioned phenomenon, it is possible to detect the colored state. In the present invention, the negative writing voltage or the positive erasing voltage which is impressed is controlled according to the colored state detected, whereby density control is performed. By this operation, the colored density of each pixel is accurately controlled to a predetermined level.

The method of driving a display apparatus according to the present invention is secondly characterized in that the driving method is a method of driving a display apparatus for forming an image through deposition and dissolution of a metal by impressing a predetermined voltage on each pixel, wherein, after writing is conducted by impressing a voltage of not less than a threshold for deposition of the metal, a voltage pulse or AC voltage having an amplitude of not more than a threshold for deposition of the metal is impressed, and, when the resulting current is decreased to or below a predetermined value, an additional writing pulse is impressed.

As has been described above, the colored state can be grasped by measuring the above-mentioned current value, and the colored state (the density of the pixel) is maintained when additional writing by the additional writing pulse is conducted when the current value is decreased attendant on the progress of the dissolution of the metal.

The method of driving a display apparatus according to the present invention is thirdly characterized in that the driving method is a method of driving a display apparatus for displaying an image through deposition and dissolution of a metal by impressing a predetermined voltage on each pixel, wherein a voltage pulse or AC voltage having an amplitude of not more than a threshold for deposition of the metal is impressed, the writing condition of each pixel is detected according to the resulting current, and a rewriting operation is selected according to the detection results.

In the case of the third-named invention, first, by measuring the above-mentioned current value, it is detected whether the pixel state is the colored state (black) or the non-colored state (white) on a pixel basis. Then, according to the detection results and the black/white state of the pixels after rewriting, a selection is made among no operation, erasure, writing, density additional writing, etc. For example, where a pixel in the black state is in the black state even after rewriting or where a pixel in the white state is in the white state even after rewriting, no operation is conducted for the relevant pixel. Where a pixel in the black state should be in the white state after rewriting or where a pixel in the white state should be in the black state after rewriting, an erasing operation or a writing operation is conducted for the relevant pixel. By this method, it suffices to perform the driving a minimum required number of times, whereby the rewriting can be achieved in a minimum time and with a minimum power consumption.