JP2009036803A - Transflective liquid crystal display element - Google Patents

Abstract

An object of the present invention is to obtain a transflective liquid crystal display element having a high contrast by preventing light leakage caused by a step portion between a transmissive display portion and a reflective display portion, and enabling a setting margin for black display conditions to be expanded. To do.
A transflective liquid crystal display element 20 according to the present invention includes a liquid crystal layer 11 made of a liquid crystal material sandwiched between two electrode substrates, and includes a transmissive display area and a reflective display area within the same display surface. A liquid crystal display element having a step structure between a portion corresponding to the transmissive display region and a portion corresponding to the reflective display region in the liquid crystal layer 11, wherein the liquid crystal material has a dielectric anisotropy of 10 to 15, The retardation value of the liquid crystal layer 11 in the transmissive display area is 170 to 300 nm.
[Selection] Figure 1

Description

  The present invention relates to a transflective liquid crystal display element having a reflective display portion and a transmissive display portion in one pixel.

  Currently, liquid crystal display elements are applied to a very wide range of application fields, and those with relatively large display areas such as TVs and PC monitors are increasing, while taking advantage of their thin, lightweight, and low power consumption characteristics. It is also widely used as a monitor for mobile devices.

  When used in monitors for mobile devices such as mobile phones, it can be said that this is a very suitable application field because it can take advantage of the inherent characteristics of the liquid crystal display element as described above. However, when used in mobile devices, the usage environment changes variously, and it is necessary to visually recognize the display from a very bright environment outdoors to a very dark environment at night. It is required to show good display quality.

  In order to solve such a problem, a transflective liquid crystal display element has been proposed. The transflective liquid crystal display element has a structure having a transmissive display portion and a reflective display portion in one pixel. Therefore, a transmissive display is observed in a dark environment, and conversely, a reflective display using external light is observed in an environment in which external light is bright and a transmitted display cannot be observed. With this configuration, it is possible to obtain a good display at all brightnesses from an environment such as an extremely bright daytime environment to an environment such as darkness at night.

  When producing a transflective liquid crystal display element, the polarizing plate on the surface is common to the reflective display portion and the transmissive display portion. That is, it is necessary to achieve both transmissive display and reflective display by transmitting through the same optical film. Under such restrictions, a liquid crystal display mode (ECB mode: Electrically Controlled Birefringence) in which liquid crystal materials are aligned in parallel is generally applied to a transflective liquid crystal display element.

  In ECB mode using parallel alignment, the retardation of the liquid crystal layer is proportional to its thickness. Further, the transmissive part is only passed through the liquid crystal layer from the back to the front only once, and the reflective part is incident from the front, and is reflected twice by the reflection plate to return to the front. Therefore, the optimum retardation required for each of the transmission part and the reflection part is different. In general, the transmission portion needs to have a retardation of about λ / 2, and the liquid crystal layer in the reflection portion needs to have a retardation of λ / 4. As described above, in the ECB mode using parallel alignment, the retardation of the liquid crystal layer is proportional to the thickness of the liquid crystal layer, that is, the panel gap, so that the panel gap between the transmission portion and the reflection portion is twice as large. If the refractive index anisotropy of the liquid crystal material is assumed to be 0.07, the transmission part is about 4 μm and the reflection part is 2 μm. A step structure is created using an organic film on the TFT substrate, and the same pixel. Different gaps are formed in the transmissive part and the reflective part.

  By applying the ECB mode using parallel alignment created in this way to a transflective liquid crystal display device, the image can be viewed through transmissive display in a dark environment, while the external light is bright and the transmissive display is viewed. Under difficult circumstances, an image can be visually recognized by reflective display using external light (see Patent Document 1 below).

JP 2000-187220 A

  In the case of the transflective liquid crystal display element described in Patent Document 1, since the transmissive part is 4 μm and the panel gap of the reflective display part is 2 μm, an organic film in which all the panel gap differences are formed on the TFT substrate. In order to cover a step structure of 2 μm, a 2 μm step must be created.

  However, this step portion has a problem that the alignment state of the liquid crystal material is deteriorated because unevenness is easily generated in the rubbing treatment of the alignment film and the liquid crystal alignment is easily distorted. In the ECB mode using parallel alignment, normally white mode display is generally performed. Therefore, even if a large alignment defect occurs in the stepped part when no voltage is applied, it is recognized as a display defect. No problem occurs when white is displayed. On the other hand, when black display is performed by applying a voltage, the liquid crystal molecules change the alignment direction to the electric field direction while maintaining the alignment state, and black display is performed. Although the alignment state is improved, it is not a sufficiently good parallel alignment as in other regions, so that light leakage occurs even in black display, and the contrast ratio is greatly reduced.

  Therefore, the present invention has been made to solve such a problem, by preventing light leakage due to the stepped portion of the transmissive display portion and the reflective display portion, and enabling the setting margin of black display conditions to be expanded, An object is to obtain a transflective liquid crystal display element with high contrast.

  The transflective liquid crystal display element in the present invention includes a liquid crystal layer made of a liquid crystal material sandwiched between two electrode substrates, and includes a transmissive display region and a reflective display region in the same display surface, A liquid crystal display element having a step structure between a portion corresponding to a transmissive display region and a portion corresponding to the reflective display region, wherein the liquid crystal material has a dielectric anisotropy of 10 to 15, and the transmissive display The retardation value of the liquid crystal layer in the region is 170 to 300 nm.

  According to the transflective liquid crystal display element of the present invention, since the response state of the liquid crystal molecules at the time of black display is a state in which the liquid crystal molecules sufficiently respond by voltage application, a step portion between the transmissive display portion and the reflective display portion. The light leakage due to the liquid crystal alignment defect in the display can be reduced, and the voltage setting margin for black display can be widened. Thereby, it is possible to obtain a high-contrast transflective liquid crystal display element that does not increase black luminance due to voltage setting deviation.

<Embodiment 1>
FIG. 1 is a diagram showing a configuration of a transflective liquid crystal display element 20 according to Embodiment 1 of the present invention. The transflective liquid crystal display element 20 includes a liquid crystal layer 11 made of a liquid crystal material sandwiched between two electrode substrates. The transflective liquid crystal display element 20 includes a transmissive display region and a reflective display region in the same display surface. A step structure is provided between the portion 13 corresponding to the region and the portion 14 corresponding to the reflective display region. The thickness of the liquid crystal layer 11 in the transmissive display area 13 and the reflective display area 14 varies depending on the step structure, and the transmissive display area 13 is dt and the reflective display area 14 is dr. This step structure is provided in order to make the optical path lengths of light contributing to display substantially equal, and dt = 2dr is preferable. Further, optical films 1 to 4 are provided on the surface of the glass substrate 12 opposite to the liquid crystal layer 11, and polarizing plates 5 and 6 are provided so as to sandwich them.

  Table 1 shows a simple film configuration, and the concept of the present invention will be specifically described on the assumption that this film is used. FIG. 2 is a diagram showing the relationship between retardation of the transmissive display region of the liquid crystal layer 11 and voltage application. As shown in the figure, the retardation of the transmissive display area of the liquid crystal layer 11 tends to decrease with voltage application. In general, in the ECB mode transflective liquid crystal display element 20 using parallel alignment, optical films 1 to 4 are attached to the front and back surfaces of the liquid crystal layer 11 (FIG. 1). When transmissive display is performed, the optical films 2 and 3 are applied to the liquid crystal layer 11 so that a voltage is applied to reduce the retardation of the transmissive display region of the liquid crystal layer 11 and black display is performed at a portion where the retardation is apparently zero. A part of the retardation is canceled.

  As a commonly used value, when the refractive index anisotropy Δn of the liquid crystal material is about 0.07 and the panel gap is 4.4 μm, the retardation of the transmissive display region of the liquid crystal layer 11 when no voltage is applied is about 310 nm. is there. From Table 1, the total retardation of the optical films 2 and 3 is 110 nm. When a voltage is applied to the liquid crystal layer 11 and the retardation of the transmissive display area of the liquid crystal layer 11 becomes 110 nm, the apparent retardation becomes 0. Black display. In addition, the optical film 1 takes reflection display into consideration, and becomes λ / 4 so that a so-called normally white mode (NW mode) display becomes white display when no voltage is applied as in the transmissive display. The optical film 1 is used. Since this film is unnecessary for transmissive display, the optical film 4 having the same size retardation in the direction orthogonal to the optical film 1 is used on the back surface side.

  Regarding the reflective display area, basically, half the retardation of the transmissive display area is required. Specifically, black display is performed by changing the voltage by about 140 nm by voltage application, but retardation of about 20 to 30 nm remains even during black display. Therefore, it is appropriate that the retardation of the reflective display region is about 170 to 180 nm. In this embodiment, this condition is used for the reflective display unless otherwise specified.

  Next, the liquid crystal material is examined using two types of liquid crystal materials having substantially the same refractive index anisotropy and dielectric anisotropy Δε of 12.2 and 6.0. FIG. 3 is a diagram showing the voltage dependence of retardation of a transmissive display region of a liquid crystal panel using two types of liquid crystal materials. From the figure, it can be seen that the retardation of the transmissive display region rapidly decreases as the dielectric anisotropy Δε increases due to the application of voltage. FIG. 4 is a diagram showing the voltage dependence of the transmittance of these liquid crystal panels. In the panel using high Δε liquid crystal and low Δε liquid crystal, the retardation of the transmissive display area of the liquid crystal layer 11 becomes 110 nm at 2.2 V and 3 V, respectively, which is canceled by the optical films 2 and 3, and the retardation of the apparent transmissive display area is Since it becomes 0, the transmittance becomes minimum (black display) at that voltage.

  As shown in FIG. 3, when a liquid crystal material having a large dielectric anisotropy Δε is used, the liquid crystal molecules are more responsive when a voltage is applied. In the transflective liquid crystal display element 20, the liquid crystal alignment is likely to be disturbed at the step portion between the transmissive portion and the reflective portion. Therefore, even when a voltage is applied and the liquid crystal molecules respond to the electric field direction, the effect of the alignment failure remains. , Light leakage and reduced contrast. When high Δε liquid crystal is used, the liquid crystal molecules are more responsive, and when voltage is applied and the liquid crystal molecules are responsive in the direction of the electric field, the effects of alignment defects due to steps when no voltage is applied are less likely to occur, leading to a decrease in contrast. Disappear.

  However, under the assumptions in Table 1, both the panel using the high Δε liquid crystal and the panel using the low Δε liquid crystal display black when the retardation of the transmissive display region of the liquid crystal layer 11 becomes 110 nm. . As shown in FIG. 3, when the retardation of the transmissive display region of the liquid crystal layer 11 is 110 nm, the liquid crystal molecules cannot be said to be in a state in which the response state in the electric field direction is further advanced. FIG. 5 is a diagram showing the rising angle of the liquid crystal molecules when displaying black. Under the assumption of Table 1, both the high Δε liquid crystal and the low Δε liquid crystal are about the same number of 70 degrees. It can be seen that the liquid crystal molecules cannot be said to have a more advanced response state in the electric field direction.

  As described above, even if the dielectric anisotropy of the liquid crystal material is increased and the liquid crystal molecules respond more easily in the direction of the electric field when voltage is applied, the liquid crystal molecules A state in the middle of a sharp change in the direction of the electric field due to voltage application is used as black display, and the black display is deviated from a slight voltage setting change. That is, simply improving the responsiveness by applying a voltage by increasing Δε of the liquid crystal material has a narrow voltage margin and cannot always provide a good black display. Although the voltage for simply obtaining the response state of the same liquid crystal molecules in the electric field direction is lowered, basically the retardation of the optical films 2 and 3 and the state where the transmissive display area has the same retardation is set to black display. In the response state in the middle of sudden change in the electric field direction, black is displayed. As described above, in the transflective liquid crystal display device using the optical films 2 and 3 having a retardation of 110 nm, the use of the high Δε liquid crystal cannot reduce the effect of the alignment defect in the step portion during black display. The contrast cannot be improved.

  From the above, it can be seen that in order to obtain the effect of high contrast by using the high Δε liquid crystal, it is necessary to make the response state of the liquid crystal molecules in the electric field direction during black display more advanced. Therefore, the optical films 2 and 3 are changed so that when the retardation of the transmissive display area of the liquid crystal layer 11 becomes smaller than the example of displaying black when the retardation of the liquid crystal layer 11 becomes 110 nm, the black is displayed. There is a need to do.

  FIG. 6 is a diagram showing the electrical characteristics of the liquid crystal panel when the retardation of the optical films 2 and 3 is changed in four stages using a liquid crystal of Δε = 12.2. As the retardation value of the optical films 2 and 3 is larger, the apparent retardation can be reduced to 0 while a large retardation of the transmissive display region of the liquid crystal layer 11 remains, so that the voltage at which black display can be obtained is lower. became. In these liquid crystal panels, although the liquid crystal layer 11 is the same and the optical films 2 and 3 are different, it can be considered that in any liquid crystal panel, the liquid crystal molecules respond in the same manner when voltage is applied. The smaller the amount of canceling the retardation of the transmissive display area of the liquid crystal layer 11, the more liquid crystal molecules need to respond in the direction of the electric field in order to obtain a black display. In other words, when the retardation of the optical films 2 and 3 is reduced, the voltage for black display is increased and the response of the liquid crystal molecules is further advanced. It is possible to obtain a good black display without any problem. From the above, it can be seen that the best black display can be obtained when the high Δε liquid crystal is used and the retardation of the optical films 2 and 3 is reduced.

  Next, the optimum range of dielectric anisotropy when obtaining a good black display will be examined. When Δε is 15 or more, there is a possibility that a problem occurs in the reliability of the liquid crystal. When Δε is 10 or less, it is considered that the drive voltage becomes too high, causing a problem in power consumption. Therefore, in order to satisfy both display characteristics and reliability, it is optimal to use a liquid crystal material having Δε10-15.

  As can be seen from FIG. 6, when the retardation of the optical films 2 and 3 is reduced, the driving voltage is increased, and the state in which the liquid crystal molecules respond to the electric field direction is used as black display, so that a high contrast effect is obtained. However, at the same time, the retardation of the optical films 2 and 3 also affects the setting margin of the black display voltage and the limit of the driving voltage of the liquid crystal panel. It is necessary to consider including this.

  FIG. 7 is a diagram showing the relationship between the retardation of the optical films 2 and 3 and the black display voltage margin. Here, a voltage setting margin for obtaining a contrast of 100 or more was selected as an object to be examined. From the figure, it can be seen that the smaller the retardation, the wider the voltage setting margin and the easier the voltage setting. More specifically, it can be seen that the effect is large at 50 nm or less, and is not so effective at a range exceeding 50 nm. This characteristic is similar even when the dielectric anisotropy of the liquid crystal layer 11 is changed in the range of Δε10-15.

  FIG. 8 is a diagram showing the retardation dependency of a transmissive display region of a voltage for obtaining a contrast of 100 or more. From the figure, it is shown that the driving voltage becomes lower as the retardation is larger, and the TFT driving becomes easier. In particular, it can be seen that a low drive voltage can be stably obtained at 20 nm or more, but a drive voltage becomes high at 20 nm or less, and a stable operation cannot be obtained. This characteristic is similar even when the dielectric anisotropy of the liquid crystal layer 11 is changed in the range of Δε10-15.

  From the results shown in FIGS. 7 and 8, the retardation of the optical films 2 and 3 (in other words, the retardation of the transmissive display area of the liquid crystal layer 11 at the time of black display) is in an optimum range for obtaining a good black display when the thickness is 20 to 50 nm. I know that there is.

  In addition, when the voltage is not applied (white display) to the voltage applied (black display), the retardation of the transmissive display region of the liquid crystal layer 11 generally requires a change of 150 to 250 nm. From the above, since the retardation of the transmissive display area when voltage is applied (black display) is 20 to 50 nm, the retardation of the transmissive display area of the liquid crystal layer 11 when no voltage is applied (white display) is 170 to 300 nm. Is appropriate.

  Next, since the retardation of the transmissive display area of the liquid crystal layer 11 affects the chromaticity and the white transmittance, it is necessary to examine whether the above-described retardation of 170 to 300 nm including these is optimal. FIG. 9 is a diagram showing the relationship between chromaticity and retardation. From the figure, the white chromaticity rapidly changes to yellow from the portion where the retardation of the transmissive display region of the liquid crystal layer 11 exceeds 300 nm. Therefore, it can be seen that the retardation of 300 nm or less is optimal. FIG. 10 is a diagram showing the relationship between the white transmittance and the retardation of the transmissive display area. From the figure, it can be seen that the white transmittance is greatly improved from 170 nm. 9 and 10 show the same characteristics even when the dielectric anisotropy of the liquid crystal layer 11 is changed in the range of Δε10-15. Also from this point, it is understood that the retardation of the transmissive display region of the liquid crystal layer 11 when no voltage is applied (white display) is in an optimal range of 170 to 300 nm.

  FIG. 11 is a diagram comparing the voltage dependency of the retardation of the liquid crystal layer 11 in the present invention with the voltage dependency of the retardation of the transmissive display region of the conventional liquid crystal layer 11. As described above, in the transflective liquid crystal display element 20, the retardation of the optical films 2 and 3 for canceling the retardation of the transmissive display region of the liquid crystal layer 11 is set so that the dielectric anisotropy Δn of the liquid crystal material used is in the range of 10 to 15. 20 to 50 nm (the retardation of the transmissive display region of the liquid crystal layer 11 during black display is 20 to 50 nm), and the retardation of the transmissive display region of the liquid crystal layer 11 when no voltage is applied is preferably 170 to 300 nm. Is the best range to get. At this time, it is possible to reduce the light leakage due to the liquid crystal alignment failure at the step portion between the transmission portion and the reflection portion, and it is possible to widen the voltage setting margin for black display. Thereby, it is possible to obtain a high-contrast transflective liquid crystal display element that does not increase black luminance due to voltage setting deviation.

<Embodiment 2>
In this embodiment mode, a semiconductor liquid crystal display element which obtains high contrast will be described using a specific example. A transflective liquid crystal cell with a panel gap of the transmission part of 3.8 μm and a panel gap of the reflection part of 2 μm was prepared. Dielectric anisotropy + 12.2 (ε‖ = 16.5, ε⊥ = 4.2) A liquid crystal material having a refractive index anisotropy of 0.0702 (ne = 1.5372, no = 1.4670) was injected. The alignment film made of the polyimide film formed on each substrate surface was rubbed so that the liquid crystal alignment on the upper and lower substrate surfaces was antiparallel. The retardation of the transmissive display area of the liquid crystal layer 11 in the transmissive part was about 270 nm. Further, the size of the step structure was 1.8 μm.

  The optical films 1 to 4 shown in Table 2 were attached to the transflective liquid crystal cell, and the contrast was evaluated. Note that the voltage dependency of retardation of the transmissive display region in this cell is as shown in FIG. 12, and the voltage dependency of transmittance is as shown in FIG. As the voltage was applied, the retardation of the transmissive display region was reduced, and when the voltage of about 3.5 V was applied, the retardation of the transmissive display region of the liquid crystal layer 11 was 50 nm. Further, black display was obtained by applying about 3.5 V at which the retardation of the transmissive display region of the liquid crystal layer 11 was 50 nm. As described above, it is within the optimum range shown in the first embodiment (dielectric anisotropy Δn is in the range of 10 to 15, and the retardation of the transmissive display region of the liquid crystal layer 11 at the time of black display is in the range of 20 to 50 nm). I understand that.

  The contrast of the transmissive display of this liquid crystal panel is 200, and the normal contrast of the liquid crystal panel in which light leakage at a step is seen in the transflective liquid crystal display element 20 having the same structure is 80 to 100. The liquid crystal panel can obtain a very high contrast.

It is the figure which showed the structure of the transflective liquid crystal display element in Embodiment 1 of this invention. It is the figure which showed the voltage dependence of the retardation of the transmissive display area | region of a liquid crystal layer in a prior art. FIG. 4 is a diagram illustrating voltage dependency of retardation of a transmissive display region of liquid crystals having different dielectric anisotropies in the first embodiment. FIG. 4 is a diagram illustrating voltage dependence of transmittance of liquid crystals having different dielectric anisotropies in the first embodiment. FIG. 3 is a diagram illustrating voltage dependence of rising angles of liquid crystals having different dielectric anisotropies in the first embodiment. It is the figure which showed the voltage dependence of the transmittance | permeability of a liquid crystal when the retardation of the optical film in Embodiment 1 differs. It is the figure which showed the retardation dependence of the transmission display area | region of the voltage setting width | variety which obtains the contrast of 100 or more in Embodiment 1. FIG. It is the figure which showed the retardation dependence of the transmissive display area | region of the setting voltage value which obtains the contrast of 100 or more in Embodiment 1. FIG. FIG. 4 is a diagram showing retardation dependency of a transmissive display area of white chromaticity in the first embodiment. 6 is a diagram showing retardation dependency of a transmissive display area of white transmittance in the first embodiment. FIG. It is the figure which compared the voltage dependence of the retardation of the transmissive display area | region of the liquid crystal layer in Embodiment 1, and the retardation of the transmissive display area | region of the liquid crystal layer in a prior art. FIG. 10 is a diagram illustrating voltage dependency of retardation of a transmissive display region of a liquid crystal layer in the second embodiment. 6 is a diagram illustrating voltage dependency of transmittance of a liquid crystal layer in Embodiment 2. FIG.

Explanation of symbols

  1-4 Optical film, 5, 6 Polarizing plate, 11 Liquid crystal layer, 12 Glass substrate, 13 Portion corresponding to transmissive display region, 14 Portion corresponding to reflective display region, 20 Transflective liquid crystal display element.

Claims (3)

A liquid crystal layer made of a liquid crystal material sandwiched between two electrode substrates; a transmissive display region and a reflective display region in the same display surface; and a portion corresponding to the transmissive display region in the liquid crystal layer and the reflective display A transflective liquid crystal display element having a step structure between a portion corresponding to a region,
A transflective liquid crystal display element, wherein the liquid crystal material has a dielectric anisotropy of 10 to 15, and a retardation value of a liquid crystal layer in the transmissive display region is 170 to 300 nm. An optical film is provided on the front and back surfaces of the liquid crystal layer,
The transflective liquid crystal display element according to claim 1, wherein the optical film is an optical film that cancels retardation of a liquid crystal layer in a transmissive display region when performing transmissive display, and has a retardation value of 20 to 50 nm.   2. The transflective liquid crystal display element according to claim 1, wherein the liquid crystal layer has a retardation value of 20 to 50 nm in a transmissive display region when black display is performed by voltage application during transmissive display.

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