Display device and driving method thereof

The present invention relates to display devices such as liquid crystal display devices, etc., and driving methods thereof.

Liquid crystal display devices are in wide use as thin and light flat displays for use in various electronic machines. There are several display schemes used in liquid crystal display devices. Among those, a scheme known as IPS (In-Plane Switching), in which an electric field is applied to liquid crystal in parallel to a substrate for obtaining a wide viewing angle, is suitably used for monitor displays for use in personal computers, liquid crystal TV sets or the like because of its excellent image properties.

Liquid crystal display devices using IPS are disclosed in Japanese Unexamined Patent Publication No. 10-10556, for example. A plan view of a pixel portion thereof is shown in FIG. 47. Such a liquid crystal display device comprises an array substrate and an opposing substrate parallel to each other, and liquid crystal held between the array substrate and the opposing substrate. As shown in FIG. 47, in the array substrate, gate wirings 101 feeding scanning signals and source wirings 102 feeding image signals are arranged so as to intersect at approximately right angles. Nearby each intersection of the gate wiring 101 and the source wiring 102, a thin-film transistor (TFT) 104 having a semiconductor layer is formed as a switching element. To the source wiring 102, a comb-like pixel electrode 115 is connected via the TFT 104. Opposing electrodes 116 functioning as a standard potential are arranged so as to mesh with the pixel electrode 115. The opposing electrodes 116 are electrically connected to a common wiring 103 parallel to the gate wiring 101 through a contact hole 108. At the intersection of the common wiring 103 and the pixel electrode 115, with an insulating layer (not shown) in between, a storage capacitor region 107 is formed.

According to such a liquid crystal display device, an electric field substantially parallel to the substrates is generated by the difference between the voltage applied to the pixel electrode 115 and that of the opposing electrode 116, to which a standard potential is applied, and thereby the liquid crystal (not shown) held between the electrodes is driven. By storing electric charge in the storage capacitor region 107 while the TFT 104 is in an on-status, the liquid crystal remains actuated while the TFT 104 is in an off-status.

In prior art IPS style liquid crystal display devices, pixel electrodes and opposing electrodes are generally made of aluminum or the like metals. Therefore, the pixel electrodes and opposing electrodes do not transmit light, leading to the drawback of an unsatisfactory pixel aperture ratio. Japanese Unexamined Patent Publication No. 10-10556 proposes a way to enhance the aperture ratio by forming either or both of the pixel electrode 115 and the opposing electrode 116 out of a transparent conductive film.

In the case where both the pixel electrode 115 and the opposing electrode 116 are made of transparent electrodes, it is preferable that both the electrodes be formed as a same layer in order to avoid a more complicated production process and increased manufacturing costs. However, this arrangement may lower the manufacturing yield by causing short-circuits between the pixel electrode 115 and the opposing electrode 116. Therefore, it is more practical that either the pixel electrode or the opposing electrode be made of a transparent electrode.

However, forming only one of the pixel electrode and the opposing electrode out of a transparent electrode and forming the other out of metal or a like material may cause flicker due to the difference in the optical properties of the two materials.

In order to apply a sufficient voltage to liquid crystal molecules while preventing decomposition or deterioration thereof, liquid crystal display devices are driven by the alternating current drive method, where an electric potential alternately positive and negative relative to that of the opposing electrode is applied to the pixel electrode at a regular interval (for example, once every sixtieth seconds). When the alternating current drive method is employed in a liquid crystal display device in which only one of the pixel electrode and the opposing electrode is a transparent electrode, its transmittance changes cyclically between the period when an electric potential positive relative to that of the opposing electrode (positive frame) is applied to the pixel electrode and the period when an electric potential negative relative to that of the opposing electrode (negative frame) is applied to the pixel electrode, causing observable differences in brightness.

The present invention aims to overcome the drawbacks described above. An object of the invention is to prevent flicker of a display device in which an electro-optic material is driven by applying a voltage between two electrodes having different transmittances.

The inventors conducted research into the causes of the flicker described above and found that the following two factors greatly affect the occurrence of flicker. A first factor is the flexoelectric effect. The flexoelectric effect is a polarization phenomenon brought about by splay deformation (orientation deformation) of liquid crystal. Regarding the relationship between the flexoelectric effect and IPS, “Manuscripts of Lectures at the 1999 Japanese Liquid Crystal Conference” (page 514, lecture number 3D06) explains the occurrence of domain reversal in connection with the positive and negative electrodes and rubbing direction.

How the flexoelectric effect influences flicker will be explained below with reference to FIGS. 44(a), 44(b), 44(c) and 44(d). In FIG. 44(a), when a positive voltage is applied to an electrode 21 and a negative voltage is applied to an electrode 22 in a liquid crystal display device using IPS or the like where a lateral electric field is applied, a solid line 26 represents a line of electric force, when the shape effect of the liquid crystal molecules is left out of consideration. On the electrodes 21 and 22, the lines of electric force splay out. In this figure, 23 represents a liquid crystal layer, 24 represents an opposing substrate, and 25 represents an array substrate. Liquid crystal display devices are driven by the alternating current drive method. Therefore, the direction of the electric field reverses, for example, once every sixtieth of seconds.

FIG. 44(b) shows an array of liquid crystal molecules 27 formed out of this splay electric field. To the end of each of the liquid crystal molecules, a cyano group, a fluorine atom or the like is introduced to give dielectric anisotropy. These parts function as negative electrodes of a dipole moment and compose the larger part of the molecular skeleton. As shown in an enlarged view of FIG. 44(b) (in the circle), the molecule has a wedge-like shape opening to the negative electrode side. Because of the shape effect (excluded volume effect), when a splay shape alternating electric field is applied to the liquid crystal molecules, they will tend to be arranged so as to direct the narrower end of the wedge to the electrode side and the wider end to the center of the liquid crystal layer. The liquid crystal molecules 27 are uniformly aligned as described above and this generates an electric field 28 attributable to the liquid crystal molecules. This phenomenon is known as the flexoelectric effect.

FIG. 44(c) illustrates a composite electric field 29 shown by broken lines which is generated by the original electric field 26 and the electric field 28 attributable to the flexoelectric effect in the liquid crystal molecules. The composite electric field 29 exhibits a stronger vertical electric field on the positive electrode 21 side and a weaker vertical electric field on the negative electrode 22 side.

As a result, its distribution of transmittance varies depending on the polarity (i.e., positive or negative) of the applied voltage. FIG. 44(d) shows the transmittance distribution when both electrodes 21 and 22 are transparent. Here, the solid line shows the transmittance distribution when the electrode 21 has a positive electric potential (positive frame), and the dash-and-dot line shows the transmittance distribution when the electrode 21 has a negative electric potential (negative frame). Both electrodes are symmetric with respect to a longitudinal axis passing through the midpoint thereof. Therefore, when both electrodes 21 and 22 are transparent or both electrodes 21 and 22 have opaque properties, very little variance in the transmittance between the positive and negative frames is observed. When one of the electrodes transmits light and the other blocks light or the transmittances of the two electrodes 21 and 22 are significantly different, the transmittance of the pixel differs between the positive and negative frames due to the difference of their optical contribution ratios, causing flicker.

A second main factor causing flicker is influence by a peripheral electric potential. FIG. 45(a) shows equipotential lines when, out of the three electrodes 32, 33 and 34 disposed on an array substrate 36, a voltage of −5 volts (V) is applied to the end electrodes 32 and 34 and a voltage of +5 V is applied to the middle electrode 33. When the electric potential of the interface of opposing substrate 35 is assumed to be the average of the two voltages (i.e. 0 V), equipotential lines of 0 V exist on the lines normal to the substrate passing through points equidistant to any two adjacent electrodes among 32, 33 and 34. Therefore, when the flexoelectric effect is left out of consideration, the three electrodes 32, 33 and 34 are equivalent. Therefore, when the electrode 33 has a positive electric potential and when it has a negative electric potential, its transmittance distribution is shown by the solid line in FIG. 45(b), and this enables the transmittance of the pixel to remain stable, even when some of the plurality of electrodes 32, 33 and 34 is/are made transparent, resulting in no occurrence of flicker.

However, in an IPS style liquid crystal display device, there is no electrode on the surface of the opposing substrate and this makes it difficult to form a desirable electric potential on the interface 35. Therefore, if the electric potential of the interface 35 of the opposing substrate is assumed to be −5 V, in cases where the electrode 33 has a positive electric potential, as shown in FIG. 46(a), equipotential lines of −5 V form above electrodes 32 and 34 along the direction normal to the substrate. In this case, the transmittance distribution is as shown by the solid line in FIG. 46(b), i.e., the transmittances on the end of electrodes (negative electrodes) 32 and 34 are higher than that on the middle electrode (positive electrode) 33. On the other hand, when the electrode 33 has a negative electric potential, as shown by the broken line in FIG. 46(b), the transmittances on the end electrodes (positive electrodes) 32 and 34 become lower than that on the middle electrode (negative electrode) 33. Therefore, when some of the plurality of electrodes 32, 33 and 34 is/are made transparent, frames where the transparent electrode(s) have a negative electric potential become brighter than frames where the transparent electrode(s) have a positive electric potential, causing flicker.

Taking these phenomena, which are key causes of flicker, into consideration, transmittances of individual pixels in prior art display devices are not even but exhibit a certain distribution, i.e., the transmittance distribution varies between when a pixel electrode has a positive electric potential relative to the opposing electrode (positive frame) and when the pixel electrode has a negative electric potential relative to the opposing electrode (negative frame). Therefore, for example, when the pixel electrode is made of a transparent material and the opposing electrode is made of an opaque material, the transmittance of the pixel electrode in either the positive or negative frame becomes higher than that of the other frame. On the other hand, the opposing electrode does not transmit light and therefore the transmittance of the opposing electrode does not change between a positive frame and a negative frame. As a result, the transmittance variance between frames of the pixel electrode is observed as a variance in the brightness of the whole pixel.

Such a flicker phenomenon is not limited to IPS style liquid crystal display devices but occurs when display devices comprising two electrodes having different light transmittances are driven by the alternating current drive method.

To achieve the above object, the display device of the invention comprises an array substrate, an opposing substrate facing the array substrate and an electro-optic substance held between the array substrate and the opposing substrate. The array substrate is provided with a plurality of gate wirings and a plurality of source wirings intersecting each other, a pixel electrode disposed in each region defined by two adjacent gate wirings and two adjacent source wirings, a switching element for switching a voltage applied to the pixel electrode from the source wiring based on a signal voltage supplied from the gate wiring, a common wiring formed between the two adjacent gate wirings and an opposing electrode being electrically connected to the common wiring and generating an electric field for driving the electro-optic substance between the opposing electrode and the pixel electrode whereto a voltage is applied. The pixel electrode comprises a first pixel electrode and a second pixel electrode, and the opposing electrode comprises a first opposing electrode and a second opposing electrode. A first region is formed in which an electric field is generated between the first pixel electrode and the first opposing electrode whose light transmittance is lower than that of the first pixel electrode. A second region is also formed in which an electric field is generated between the second pixel electrode and the second opposing electrode whose light transmittance is higher than that of the second pixel electrode. According to this display device, flicker can be reduced because the flicker polarities caused by the variance in transmittance between the pixel electrode and the opposing electrode can be cancelled between the first region and the second.

In the display device, it is preferable that the first region and the second region be adjacent to each other.