KR20120007315A - Trans-reflective liquid crystal display device - Google Patents

Abstract

PURPOSE: A transflective liquid crystal display device is provided to obtain a maximum light transmitting rate at a light transmitting mode and a light reflecting mode. CONSTITUTION: A pixel area includes a reflecting area(RA) with a first cell gap and a transmitting area(TA) with a second cell gap. A lower substrate includes a gate line, a data line, and a pixel electrode. The gate and data lines define the pixel area. The pixel electrode is placed in the pixel area. An upper substrate includes a black matrix, a color filter, and a common electrode. A liquid crystal layer is interposed between the upper substrate and a lower substrate.

Description

Transflective Liquid Crystal Display Device

The present invention relates to a transflective liquid crystal display device. In particular, the present invention relates to a liquid crystal display device capable of switching between a transmission mode and a reflection mode and having a maximum transmittance in both the transmission mode and the reflection mode.

The liquid crystal display of the active matrix driving method displays an image using a thin film transistor (TFT) as a switching element. Liquid crystal displays can be miniaturized compared to cathode ray tubes (CRTs), which are used in displays in portable information devices, office equipment, computers, etc., as well as in televisions, and are rapidly replacing cathode ray tubes.

Since the liquid crystal display is not a self-light emitting device, a separate light source is required. The liquid crystal display may be represented by a transmissive type or a reflective type depending on a light source. In the transmissive liquid crystal display, a backlight unit is disposed to face a lower substrate of two upper and lower substrates into which liquid crystal is injected, and transmits light incident from the backlight unit toward the projection surface. On the other hand, the reflective liquid crystal display device forms a reflective surface on the lower substrate of the two upper and lower substrates into which the liquid crystal is injected, thereby preventing external light incident to the lower substrate via the upper substrate or a separate auxiliary light. Reflect toward the display surface. Recently, research on semi-transmissive liquid crystal display devices, which can have advantages of transmissive and reflective, has been activated.

1 is a schematic view showing a transflective liquid crystal display device according to the prior art. FIG. 2 is a cross-sectional view illustrating a structure of a transflective liquid crystal display device according to the related art, taken by cutting line II ′ in FIG. 1.

A transflective liquid crystal display device according to the prior art will be described in detail with reference to FIGS. 1 and 2. The transflective liquid crystal display device is divided into a reflection area RA and a transmission area TA. A substantially rectangular pixel area is defined while the data line DL running in the vertical direction and the gate line GL running in the horizontal direction cross each other. The pixel electrode PXL is formed in the pixel area. In addition, a reflective layer RFIL made of an opaque metal is formed in the reflective region RA, and a transparent layer TFIL is formed of a transparent conductor such as ITO in the reflective region RA. In some cases, nothing may be formed in the transmission area TA.

A gate electrode G branched from the gate line GL on the lower substrate SUBL, a reflective layer RFIL formed in the reflective region RA, and a transparent layer TFIL formed in the transmission region TA are included. A gate insulating film GI covering the gate line GL, the gate electrode G, the reflective layer RFIL, and the transparent layer TFIL is coated on the entire surface. The semiconductor channel layer A and the gate line GL are orthogonal to the portion overlapping the gate electrode G on the gate insulating layer GI and branched from the data line DL and the data line DL defining the pixel area. A source electrode S in contact with one side of the semiconductor channel layer A and a drain electrode D in contact with the other side of the semiconductor channel layer A are disposed. The thin film transistor TFT formed as described above is protected by applying a protective film PAS. The pixel electrode PXL is formed on the passivation layer PAS to contact the drain electrode D through the contact hole. The lower alignment layer ALGL is coated on the entire surface of the lower substrate SUBL on which the pixel electrode PXL is formed.

The black matrix BM is disposed at a position corresponding to the gate line GL, the data line DL, and the thin film transistor TFT in the upper substrate SUBU. The color filter CF is disposed in a pixel area occupying between the black matrices BM. The color filter CF may be formed to have an RGB arrangement in accordance with the arrangement of the pixels. The common electrode COM is formed on the black matrix BM and the color filter CF. The upper alignment layer ALGL is coated on the entire surface of the upper substrate SUBU on which the common electrode COM is formed.

The upper substrate SUBU and the lower substrate SUBL are spaced apart from each other to maintain a predetermined distance d with the liquid crystal layer LC therebetween. The upper polarizer POLU and the lower polarizer POLL are attached to the outer upper surface of the upper substrate SUBU and the outer lower surface of the lower substrate SUBL, respectively. At this time, the light transmission axes of the upper polarizing plate POLU and the lower polarizing plate POLL are attached so as to be perpendicular to each other. On the other hand, the light transmission axis of the upper polarizing plate POLU and the liquid crystal alignment direction of the upper alignment layer ALGU are formed to be parallel to each other. Similarly, the light transmission axis of the lower polarizing plate POLL and the liquid crystal alignment direction of the lower alignment layer ALGL are formed to be parallel to each other.

In the transflective liquid crystal display device having such a structure, the light transmittance for determining the brightness of the display device is determined by Equation 1 below.

In this case, an element that determines the transmittance when passing through the liquid crystal layer LC is a sin ² (Δndπ / λ) value, and in order to have the maximum value, Δnd = λ / 2 must be satisfied. That is, it is preferable that the gap between the upper substrate SUBU and the lower substrate SUBL is maintained and the cell gap filled with the liquid crystal layer LC is determined to satisfy Δnd = λ / 2.

However, this condition is a value for having the maximum light transmittance when the transflective liquid crystal display is operated in the transmissive mode. When the transflective liquid crystal display is operated in the reflection mode in this state, the light path through which the reflected light passes is twice as large as the transmission mode. Therefore, the condition of Δnd = λ is satisfied. The light transmittance then has a minimum value. That is, the transflective liquid crystal display device designed to have the maximum transmittance in the transmissive mode has a problem of having the minimum transmittance in the reflective mode.

In order to solve this problem, the cell gap may be designed to satisfy Δnd = λ / 4 in order to have the maximum light transmittance in the transmission mode. In this case, in the reflection mode, the optical path passes through the cell gap twice, and in fact, satisfies Δnd = λ / 2 and has the maximum light transmittance. On the other hand, since the cell gap satisfies Δnd = λ / 4 in the transmission mode, the cell gap has a light transmittance corresponding to 1/2 of the maximum light transmittance. That is, it has a maximum transmittance in the reflection mode and a light transmittance of 50% in the transmission mode. The problem can be solved because the minimum transmittance does not come out under either the reflection mode or the transmission mode, but the problem remains that the light transmittance is poor in the transmission mode.

SUMMARY OF THE INVENTION An object of the present invention is to provide a transflective liquid crystal display device in which light transmittance ensures a maximum value in a transmissive mode or a reflective mode. Another object of the present invention is to provide a transflective liquid crystal display device which further improves light transmittance in a transmissive mode.

In order to achieve the objects and advantages of the present invention, the transflective liquid crystal display according to the present invention comprises: a pixel region having a reflective region having a first cell gap and a transmissive region having a second cell gap; A lower substrate including a gate wiring and a data wiring defining the pixel region and a pixel electrode formed in the pixel region; An upper substrate including a black matrix defining the pixel region, a color filter filling the inside of the pixel region, and a common electrode facing the pixel electrode; And a liquid crystal layer interposed between the lower substrate and the upper substrate.

A selective reflection lower polarizer plate attached to a rear surface of the lower substrate and having a first light transmission axis and having a reflection function; And an upper polarizing plate attached to the front surface of the upper substrate and having a second light transmission axis.

The lower substrate further includes a lower alignment layer in contact with the liquid crystal layer and having an alignment direction parallel to the first light transmission axis; The upper substrate may further include an upper alignment layer in contact with the liquid crystal layer and having an alignment direction parallel to the second light transmission axis.

The first light transmission axis and the second light transmission axis may be arranged to be perpendicular to each other.

The color filter may further include a transparent hole.

The transparent hole is formed only in the reflective region of the color filter.

The ratio of the second cell gap to the first cell gap is 1: 2.

At least one of the lower substrate and the upper substrate further comprises a reflective layer in the reflective region.

The lower substrate may further include a thin film transistor formed at a portion where the gate line and the data line cross each other and connected to the pixel electrode.

The liquid crystal layer is characterized in that it comprises a twisted nematic mode liquid crystal material.

The semi-transmissive liquid crystal display according to the present invention has an advantage of having a maximum light transmittance in both the transmission mode and the reflection mode by forming a cell gap of 2: 1 in the transmission region and the reflection region. In addition, even in the transmissive region in which the relative luminance may decrease, the luminance may be further improved by reflecting the external light source by the selective reflective polarizer. Further, by further including a transparent hole in a portion corresponding to the reflective region of the color filter, it is possible to further improve the luminance in the reflective mode.

1 is a schematic view showing a transflective liquid crystal display device according to the prior art.
FIG. 2 is a cross-sectional view illustrating a structure of a transflective liquid crystal display device according to the prior art, taken along the line II ′ of FIG. 1.
3 is a cross-sectional view of a transflective liquid crystal display device according to a first embodiment of the present invention.
4 is a cross-sectional view of a transflective liquid crystal display device according to a second embodiment of the present invention.
5 is a cross-sectional view of a transflective liquid crystal display device according to a third embodiment of the present invention.

Hereinafter, exemplary embodiments of the present invention will be described with reference to the accompanying drawings. 3 is a cross-sectional view illustrating a transflective liquid crystal display device according to a first embodiment of the present invention.

Referring to FIG. 3, the transflective liquid crystal display according to the first embodiment of the present invention is divided into a reflection area RA and a transmission area TA. A substantially rectangular pixel area is defined while the data line DL running in the vertical direction and the gate line GL running in the horizontal direction cross each other. The pixel electrode PXL is formed in the pixel area. In addition, a reflective layer RFIL made of an opaque metal is formed in the reflective region RA and a transparent layer TFIL is formed of a transparent material in the transmissive region TA for the semi-transmissive characteristic. In some cases, nothing may be formed in the transmission area TA.

A gate electrode G branched from the gate line GL on the lower substrate SUBL, a reflective layer RFIL formed in the reflective region RA, and a transparent layer TFIL formed in the transmission region TA are included. A gate insulating film GI covering the gate line GL, the gate electrode G, the reflective layer RFIL, and the transparent layer TFIL is coated on the entire surface. The semiconductor channel layer A and the gate line GL are orthogonal to the portion overlapping the gate electrode G on the gate insulating layer GI and branched from the data line DL and the data line DL defining the pixel area. A source electrode S in contact with one side of the semiconductor channel layer A and a drain electrode D in contact with the other side of the semiconductor channel layer A are disposed. The thin film transistor TFT formed as described above is protected by applying a protective film PAS. The pixel electrode PXL is formed on the passivation layer PAS to contact the drain electrode D through the contact hole. The lower alignment layer ALGL is coated on the entire surface of the lower substrate SUBL on which the pixel electrode PXL is formed.

The black matrix BM is disposed at a position corresponding to the gate line GL, the data line DL, and the thin film transistor TFT in the upper substrate SUBU. The color filter CF is disposed in a pixel area occupying between the black matrices BM. The color filter CF may be formed to have an RGB arrangement in accordance with the arrangement of the pixels. The common electrode COM is formed on the black matrix BM and the color filter CF. The transparent insulating layer INS is formed in a portion of the entire surface of the upper substrate SUBU on which the common electrode COM is formed corresponding to the reflective region RA. It is preferable that the thickness of the transparent insulating film INS is set so that the cell gap d1 of the reflection area RA is 1/2 of the cell gap d2 of the transmission area TA. The upper alignment layer ALGL is coated on the transparent insulating layer INS and the common electrode COM.

The upper substrate SUBU and the lower substrate SUBL are bonded to each other with the liquid crystal layer LC interposed therebetween. In this case, the reflective region RA further includes the transparent insulating layer INS as compared with the transmission region TA. Therefore, the cell gap d1 of the reflection area RA is smaller than the cell gap d2 of the transmission area TA. In particular, the cell gap d1 in the reflection area RA is determined to satisfy Δnd = λ / 4, and the cell gap d2 in the transmission area TA is determined to satisfy Δnd = λ / 2. In the reflection area RA, the physical cell gap d1 satisfies the condition Δnd = λ / 4, but the actual cell gap considering the optical path becomes λ / 2. In the end, it is preferable to determine the ratio of d1: d2 = 1: 2.

In describing the first embodiment, the transparent insulating layer INS is further added to the reflective region RA of the upper substrate SUBU to maintain the cell gap of the reflective region RA and the transparent region TA at 1: 2. The structure to add was taken as an example. However, although not shown in the drawings, if necessary, an insulating film may be further added to the reflective region RA of the lower substrate SUBU.

The upper polarizer POLU and the lower polarizer POLL are attached to the outer upper surface of the upper substrate SUBU and the outer lower surface of the lower substrate SUBL, respectively. At this time, the light transmission axes of the upper polarizing plate POLU and the lower polarizing plate POLL are attached so as to be perpendicular to each other. For example, the optical axis of the upper polarizing plate POLU may be set to the x-axis, and the optical axis of the lower polarizing plate POLL may be set to the y-axis. On the other hand, the light transmission axis of the upper polarizing plate POLU and the liquid crystal alignment direction of the upper alignment layer ALGU are formed to be parallel to each other. Similarly, the light transmission axis of the lower polarizing plate POLL and the liquid crystal alignment direction of the lower alignment layer ALGL are formed to be parallel to each other. The liquid crystal layer LC uses a liquid crystal of twisted nematic (TN) mode. Then, among the liquid crystal molecules constituting the liquid crystal layer LC, the liquid crystal molecules contacting the upper alignment layer ALGU are arranged in accordance with the alignment direction of the upper alignment layer ALGU, and the liquid crystal molecules contacting the lower alignment layer ALGL are disposed in the lower alignment layer ( ALGL) is arranged along the direction of orientation. On the other hand, the liquid crystal molecules between the upper alignment layer ALGU and the lower alignment layer ALGL are sequentially arranged in a twisted form. For example, when the upper alignment layer ALGU is aligned on the x-axis and the lower alignment layer ALGL is aligned on the y-axis, the liquid crystal molecules forming the liquid crystal layer LC go from the upper layer to the lower layer, where the x-axis arrangement is performed. Has a structure arranged like twisting sequentially in the y-axis array.

A case where the transflective liquid crystal display device having such a structure is operated in the reflective mode is as follows. Reflection mode uses light incident from outside to implement video data. As light is incident from the outside through the upper polarizer POLU, the light is linearly polarized to be parallel to the x-axis. In the state in which the liquid crystal is not driven, the light polarized on the x-axis incident to the reflection area RA passes through the liquid crystal layer LC, and the phase is changed by λ / 4 so that the polarization state becomes 45 °. As the light is reflected by the reflective layer RFIL and passes through the liquid crystal layer LC, the phase is changed again by λ / 4 so that the polarization state is changed to 90 °, that is, the linear polarization state is parallel to the y axis. As a result, the light reflected in the reflection area RA does not transmit because the polarization state becomes orthogonal to the upper polarizer POLU. That is, it operates in a normally black (NB) mode that exhibits black gradation when no electric field is applied. Since the phase change by the liquid crystal layer LC does not occur in the state in which the liquid crystal is driven, the light reflected by the reflective layer RFIL passes through the upper polarizing plate POLU and exhibits white gradation.

Next, look at the case of operating in the transmission mode. In the transmissive mode, video data is implemented using a backlight positioned under the liquid crystal display panel. Light incident from a backlight (not shown) positioned below the lower polarizer POLL is linearly polarized to be parallel to the y axis, which is a light transmission axis of the lower polarizer POLL. When the liquid crystal is not driven, the light polarized on the y axis incident to the transmission area TA passes through the liquid crystal layer LC, and the phase is changed by λ / 2 so that the polarization state is rotated by 90 °. The line polarization state changes in parallel. As a result, the light passing through the transmission area TA is transmitted while the polarization state is parallel to the upper polarizing plate POLU. That is, when no electric field is applied, the device operates in a normally white (NW) mode indicating white gradation. Since the phase change by the liquid crystal layer LC does not occur in the state where the liquid crystal is driven, the light passing through the transmission area TA does not pass through the upper polarizing plate POLU, resulting in black gradation.

In the reflective liquid crystal display having the above structure, the actual cell gap considering the optical path has the same lambda / 2 in both the reflection area RA and the transmission area TA. Therefore, by using Equation 1, the maximum light transmittance is ensured in both the reflection area RA and the transmission area TA. However, since the mode implementing the color gradation in the reflection mode and the transmission mode operates in reverse, it is preferable to select and use the reflection mode or the transmission mode.

When the transflective liquid crystal display according to the first embodiment of the present invention is used in the transmissive mode, the surrounding environment is mainly dark. Therefore, the luminance of the backlight can be adjusted to obtain a luminance sufficient for the user to read and view. However, when used in the reflection mode, the external light is used, that is, the solar environment. In this case, the luminance of the liquid crystal display device is determined in proportion to the area of the reflective area TA. Since the area occupied by the reflective area TA in the pixel area is about 1/2, the user may not obtain sufficient luminance to read and view in a bright sunlight environment.

In the second embodiment of the present invention, a structure for improving luminance in the reflection mode is proposed. 4 is a cross-sectional view of a transflective liquid crystal display according to a second exemplary embodiment of the present invention. Referring to FIG. 4, the semi-transmissive liquid crystal display according to the second embodiment has the same structure as the semi-transmissive liquid crystal display according to the first embodiment. Therefore, detailed description of the same components will be omitted. The difference between the first embodiment and the second embodiment is that the selective reflective lower polarizer SPOLL is used in place of the lower polarizer.

According to the second exemplary embodiment of the present invention, the reflectance may be improved by allowing the selective reflection lower polarizer SPOLL to reflect the external light even in the transmission area TA in which the external light is not reflected in the reflection mode. Specifically, it is as follows.

Reflection mode uses light incident from outside to implement video data. As light is incident from the outside through the upper polarizer POLU, the light is linearly polarized to be parallel to the x-axis. Description of light incident to the reflective region RA is the same as in the first embodiment, and thus the description thereof is omitted. In the state where the liquid crystal is not driven, the light polarized on the x-axis incident on the transmission area TA passes through the liquid crystal layer LC, and the phase is changed by λ / 2 so that the polarization state is rotated by 90 ° and parallel to the y-axis. It becomes a state. A considerable amount of light is reflected by the selective reflection lower polarizer SPOLL, and the phase is changed again by λ / 2 while passing through the liquid crystal layer LC. The polarization state is rotated by 90 °, and the linear polarization state is changed parallel to the x-axis. . As a result, the light reflected by the selective reflection lower polarizer SPOLL in the transmission area TA is transmitted because the polarization state becomes parallel to the upper polarizer POLU. However, the cell gap along the actual optical path satisfies the condition of Δnd = λ. In this case, light transmittance becomes a minimum value based on Equation (1). That is, in the transmission area TA in the state where no electric field is applied, it operates in a normally black (NB) mode showing black gradation, thereby realizing the same gradation as the reflection region RA. Since the phase change by the liquid crystal layer LC does not occur in the state in which the liquid crystal is driven, the light reflected by the selective reflective lower polarizer SPOLL transmits the upper polarizer POLU to show white gray scale.

As described above, the transflective liquid crystal display according to the second exemplary embodiment of the present invention includes a selective reflective lower polarizer SPOLL, which may help to implement video data in the transmission area TA when the reflection mode is operated. . Therefore, the luminance can be further improved in the reflection mode.

In the third embodiment of the present invention, another structure for improving the luminance in the reflection mode is proposed. 5 is a cross-sectional view of a transflective liquid crystal display device according to a third exemplary embodiment of the present invention. Referring to FIG. 5, the transflective liquid crystal display according to the third embodiment has the same structure as the transflective liquid crystal display according to the first embodiment. Therefore, detailed description of the same components will be omitted.

The difference from the first embodiment is that in the third embodiment, the porous color filter HCF is used instead of the color filter CF. The porous color filter HCF further includes a transparent hole H in a portion of the pigment layer constituting the color filter. Since the transparent hole H is not filled with the pigment, light passes through the transparent hole H, thereby increasing the luminance of the pixel. The hole color filter HCF may be selectively included only in the reflection area RA having a relatively low luminance.

Those skilled in the art will appreciate that various changes and modifications can be made without departing from the technical spirit of the present invention. Therefore, the technical scope of the present invention should not be limited to the contents described in the detailed description of the specification but should be defined by the claims.

SUBL: lower substrate SUBU: upper substrate
POLL: Lower Polarizer POLU: Upper Polarizer
SPOLL: Selected Bottom Polarizer TFT: Thin Film Transistor
GL: gate wiring DL: data wiring
G: gate electrode A: semiconductor channel layer
S: source electrode D: drain electrode
PXL: pixel electrode GI: gate insulating film
PAS: passivation layer ALGL: lower alignment layer
ALGU: upper alignment layer INS: transparent insulation layer
COM: common electrode BM: black matrix
CF: color filter HCH: porous color filter
H: transparent hole
RFIL: Reflective Layer TFIL: Transparent Layer
RA: reflection area TA: transmission area

Claims (10)

A pixel region having a reflection region having a first cell gap and a transmission region having a second cell gap;
A lower substrate including a gate wiring and a data wiring defining the pixel region and a pixel electrode formed in the pixel region;
An upper substrate including a black matrix defining the pixel region, a color filter filling the inside of the pixel region, and a common electrode facing the pixel electrode; And
And a liquid crystal layer interposed between the lower substrate and the upper substrate.
The method of claim 1,
A selective reflection lower polarizer plate attached to a rear surface of the lower substrate and having a first light transmission axis and having a reflection function; And
A semi-transmissive liquid crystal display device further comprising an upper polarizing plate attached to the front surface of the upper substrate and having a second light transmission axis.
The method of claim 2,
The lower substrate further includes a lower alignment layer in contact with the liquid crystal layer and having an alignment direction parallel to the first light transmission axis;
The upper substrate further includes an upper alignment layer in contact with the liquid crystal layer and having an alignment direction parallel to the second light transmission axis.
The method of claim 2,
The first transmissive axis and the second transmissive axis are arranged to be perpendicular to each other.
The method of claim 1,
The color filter is a transflective liquid crystal display further comprises a transparent hole.
The method of claim 5, wherein
And the transparent hole is formed only in the reflective region of the color filter.
The method of claim 1,
The ratio of the second cell gap to the first cell gap is 1: 2, the transflective liquid crystal display device.
The method of claim 1,
At least one of the lower substrate and the upper substrate further comprises a reflective layer in the reflective region.
The method of claim 1,
The lower substrate may further include a thin film transistor formed at a portion where the gate line and the data line cross each other and connected to the pixel electrode.
The method of claim 1,
The liquid crystal layer is a transflective liquid crystal display, characterized in that it comprises a twisted nematic mode liquid crystal material.

Images