Various types of liquid crystal devices are known in the art. They include a reflective liquid crystal device which displays an image by reflecting ambient light through a liquid crystal by reflecting means provided in the device, a transmissive type liquid crystal device in which light emitted from a light source provided in the device is passed through a liquid crystal and output to the outside thereby forming an image, and a transflective liquid crystal device capable of switching its displaying mode between reflective and transmissive modes.
Of these liquid crystal devices, reflective liquid crystal devices can operate with very low power consumption because they need no light source. Because of such an advantage, the reflective liquid crystal device is widely used as a display unit in portable devices and other various systems.
In the transflective liquid crystal device, an image is displayed in the transmissive mode using a light source when used in a dark environment. However, when used in a light environment, an image is displayed using ambient light as in the reflective liquid crystal device, and thus it needs low power consumption. Because of such an advantage, the transflective liquid crystal device is widely used as a display unit in portable devices and other various systems. A typical transflective liquid crystal device is, as disclosed in, for example, Japanese Utility Model Publication No. 57-049271, composed of a polarizer, a transflector, and a backlight which are successively disposed on the outer surface, opposite to the viewing side, of a liquid crystal panel. A transflective liquid crystal device with improved brightness is disclosed in Japanese Unexamined Patent Publication No. 8-292413. In this liquid crystal device, a transflector, a polarizer, and a backlight are successively disposed on the outer surface, opposite to the viewing side, of a liquid crystal panel. Because there is no polarizer between the liquid crystal cell and the transflector, an image with improved brightness can be displayed.
With recent advances in portable devices and office automation devices, there is an increasing need for color liquid crystal devices. In many cases, the capability of displaying a color image is also required in systems or devices using a reflective or transflective liquid crystal device. To realize a liquid crystal device having the capability of displaying a color image in the reflective or transflective mode, a color filter having a large number of colored areas of R (red), G (green), and B (blue) is disposed on one of a pair of substrates between which a liquid crystal is disposed. To avoid mixing among different colored areas of the color filter, and to avoid a reduction in the contrast ratio due to light-struck (white defects) caused by the spaces between adjacent colored areas, a light shielding film generally called a black mask or a black matrix is disposed in the spaces between adjacent colored areas.
However, an essential problem of the reflective liquid crystal device described above is that because an image is displayed using ambient light, it is difficult or impossible to see the image in a dark environment. To obtain better viewability, it is important to increase the reflectance to ambient light incident on the liquid crystal device, and also to increase the ratio of light which is reflected and output from the liquid crystal device to the outside as display light which makes contribution to display contrast relative to the total ambient light input to the device. However, in the reflective liquid crystal device described above, the reflectance and the ratio of output light to input light are not sufficiently high. In the case of the reflective liquid crystal device in which a transparent substrate is disposed between the liquid crystal layer and the reflector, the problem is that double images or dull images occur in an image displayed. If a color filter is combined with this liquid crystal device, parallax makes it difficult to obtain desired colors. When the liquid crystal device includes a color filter, if a structure including no black matrix is employed to avoid absorption of light by the black matrix thereby increasing the image brightness, then light passing through spaces between adjacent colored areas is reflected by the reflector. This causes a greater part of ambient light to be output from the liquid crystal device without making any contribution to the image contrast or formation of an image, and thus causes a reduction in the contrast ratio. Japanese Unexamined Patent Publication No. 9-258219 discloses a reflective color liquid crystal device in which a reflector is disposed in contact with a liquid crystal layer. Also in this liquid crystal device, if spaces between adjacent colored areas of the color filter are not covered with a black mask so as to increase the image brightness as described above, some ambient light, which passes through the spaces located between adjacent colored areas and covered with no black mask, is reflected by the reflector and output in mixture with imaging light to the outside of the liquid crystal device. This results in mixing colors, and it makes the colors dull or faded. Further, a contrast ratio is reduced.
As described above, the problem of the conventional reflective liquid crystal device has difficulty of displaying an image with a high brightness and a high contrast.
The transflective liquid crystal device disclosed in Japanese Unexamined Patent Publication No. 8-292413 cited above also has similar problems associated with double images and dull images because a transparent substrate is disposed between a liquid crystal layer and a transflector. Also in this case, if a color filter is combined with this liquid crystal device, parallax makes it difficult to obtain desired colors. Japanese Unexamined Patent Publication No. 7-318929 discloses a transflective liquid crystal device in which pixel electrodes which also serve as transflective films are formed on the inner surface of a liquid crystal cell. This patent cited herein also discloses a structure in which pixel electrodes formed of ITO (indium tin oxide) are laminated on a transflective film formed of a metal film via an insulating film. However, in this liquid crystal device, it is required to form a large number of very small defects such as hole defects or recessed defects or very small openings in the pixel electrodes which also serve as the transflective films or in the transflective films on which the pixel electrodes are formed. This results in an increase in the complexity of the device. Furthermore, a special process is additionally required to produce the openings. This makes it difficult to produce the pixel electrodes or transflective films with high reliability. In particular, when the pixel electrodes are formed such that they also serve as the transflective films, it is required that, in the transmissive displaying mode, portions of the liquid crystal, through which light emitted from a light source passes after passing through openings, be driven by oblique electric fields generated by pixel electrodes located in non-opening areas. As a result, degradation in image quality occurs due to variations in orientation of the liquid crystal compared with the case where the liquid crystal is driven by vertical electric fields.
On the other hand, in the case where the pixel electrodes are formed via an insulating film on the respective transflective films formed of metal, adjacent pixel electrodes are capacitively coupled with each other via capacitors formed of the respective pixel electrode, the insulating films, and the transflective films, and further via the transflective films. As a result, signals such as image signals supplied to the plurality of pixel electrodes are mixed with one another or have cross-talk with one another. Hence, the signals have distortion in the waveforms, which results in degradation in quality of the image displayed. In particular, when the pixel electrodes are also used as the data lines or segment electrodes via which image signals having complicated waveforms and having a high driving frequency compared with the scanning electrodes are supplied, the degradation in quality becomes more serious.
As described above, the conventional transflective liquid crystal device has the problem that it is difficult to display a high-brightness and high-contrast image.