A transflective LCD unit which uses a lateral-electric-field mode such as an IPS (in-plane-switching) mode or a FFS (fringe-field-switching) mode is known to have an excellent viewing angle characteristic in the transmissive area. This type of transflective LCD unit is described in Patent Publication-1, for example. The transflective LCD unit described in Patent Publication-1, however, has a problem in that a satisfactory black color or dark state cannot be obtained in the reflective area even if the optical axis is adjusted therein.
Another LCD unit which uses a longitudinal-electric-field mode in the reflective area and a lateral-electric-field mode in the transmissive area is described in a Non-Patent Publication-1. In this LCD unit, the reflective area includes a built-in retardation film having a function of a λ/2 film and a λ/4 film, and a LC layer having a function of a λ/2 film, which are consecutively arranged as viewed from the light incident side toward the light emitting side of the LCD unit. In operation of the LCD unit, the light incident thereto is converted into a circularly-polarized light by the built-in retardation film, and then passes through the LC layer upon display of a dark state while maintaining the circularly-polarized state thereof, to reach a reflection film provided in the reflective area. At this stage, the LC layer is oriented in a direction normal to the substrate due to the longitudinal electric field, whereby the polarization of the light incident onto the LC layer is not changed. This prevents occurring of an insufficient black color in the reflective area, which is a significant problem in the reflective area driven by a lateral electric field. Thus, the LCD unit exhibits an excellent black color.
In the LCD unit including a reflective area driven by a longitudinal electric field, display of a dark state (black color) requires application of a voltage so that the orientation of the LC layer in the reflective area is raised by the longitudinal electric field up to a direction normal to the substrate. On the other hand, since the transmissive area has an arrangement of optical axes similar to the arrangement of optical axes in a transmissive LCD device using a lateral-electric-field mode, the transmissive area is not applied with the voltage. For achieving application of a voltage to the reflective area and absence of the applied voltage in the transmissive area, it is effective to provide two separate common electrodes in each unit pixel: one for the reflective area and the other for the transmissive area, the separate common electrodes being driven by respective drive signals having an inversion relationship therebetween. This technique is hereinafter referred to as an inverting-COM technique.
FIG. 15 shows a sectional view of a unit pixel in a transmissive LCD unit driven using the inverting-COM technique, wherein the unit pixel includes a reflective area 210 driven by a longitudinal electric field and a transmissive area 211 driven by a lateral electric field. A reflective-area pixel electrode 205 is formed on an insulating film 204 in the reflective area 210 of a TFT (thin-film-transistor) substrate 201, shown at the bottom side. On the same insulating film 204, there is provided at least one of a transmissive-area pixel electrode 206 and a transmissive-area common electrode 207 in the transmissive area 211 of the TFT substrate 201. On a counter substrate 202 opposing the TFT substrate 210 with an intervention of a LC layer 203, a reflective-area common electrode 209 is provided at the location corresponding to the location of the reflective-area pixel electrode 205, thereby overlapping the same as viewed in the direction normal to the substrate.
The LC layer 203 sandwiched between the TFT substrate 201 and the counter substrate 202 is homogeneously oriented. The reflective-area common electrode 209 and transmissive-area common electrode 206 are applied with a voltage of a rectangular waveform having a higher potential at 5V and a lower potential at zero volt, for example. The reflective-area pixel electrode 205 and transmissive-area pixel electrode 207 are connected to a data line, which supplies a signal for providing a desired electric field to the LC layer 203 of the unit pixel, via a switching device or TFT.
In the LCD unit shown in FIG. 15, the orientation of LC molecules in the reflective area must be aligned to a direction normal to the substrate, requiring application of a potential difference between the reflective-area common electrode 209 and the reflective-area pixel electrode 205. Thus, when a 0-volt signal is applied to the reflective-area common electrode 209, a 5-volt signal, for example, is applied to the reflective-area pixel electrode 205. On the other hand, an inverted signal of the signal applied to the reflective-area common electrode 209, i.e., a 5-volt signal is applied to the transmissive-area common electrode 206, due to the inverting-COM technique, and a 5-volt signal is applied to the transmissive-area pixel electrode 207 similarly to the reflective-area pixel electrode 205. Thus, there is no potential difference applied between the transmissive-area common electrode 206 and the transmissive-area pixel electrode 207, to allow the LC molecules to stay in the original orientation, whereby the transmissive area displays a dark state.
Patent Publication-1 as describe above is JP-2005-338256A (refer to paragraphs 0013-0022)
Non-Patent Publication-1 as described above is “SID INTERNATIONAL SYMPOSIUM DIGEST OF TECHNICAL PAPERS, issued by SOCIETY FOR INFORMATION DISPLAY in 2007; VOL. 38; No. 2, pp. 1270-1273.
In the LCD unit shown in FIG. 15, there is a possibility that a leakage light occurs due to disturbance of the orientation of LC molecules in the vicinity of the boundary between the reflective area and the transmissive area during display of a dark state. This is because an electric field is generated at any time between the reflective-area common electrode 209 formed on the counter substrate 202 in an area corresponding to the reflective area and the transmissive-area common electrode 206 or transmissive-area pixel electrode 207 formed on the TFT substrate 201.
FIG. 16 shows results of simulation conducted for obtaining the distribution of electric field in the LCD unit shown in FIG. 5. Reference numerals 212 and 213 denote LC molecules and isoelectric lines 213, respectively. Since the reflective-area common electrode 209 opposes the transmissive-area common electrode 206 or transmissive-area pixel electrode 207 in a slanted direction in the vicinity of the boundary between the reflective area 210 and the transmissive area 211 with an intervention of the LC layer 203, LC molecules 212 in the LC layer 203 near the reflective-area common electrode 209 are applied with an electric field slanted with respect to the substrate, i.e., slanted in the direction to downward right as shown by the optical axis of LC molecules 212 in FIG. 16. The slanted electric field causes a reverse tilt in a portion of the reflective area 210 in the vicinity of the boundary, thereby generating a leakage light. The leakage light raises the luminance (black luminance) in the reflective area 210 during display of a dark state, thereby degrading the contrast ratio and visibility of the LCD unit.