Vertically aligned liquid crystal display device having an optimized viewing angle

A method of improving the viewing angle of a vertically-aligned liquid crystal display device is presented. The method involves designing a uniaxial compensation film to provide a retardation value of 200 nm or less for light having a wavelength of about 550 nm. Using this uniaxial compensation film, a display device can be built by obtaining a liquid crystal panel with liquid crystal molecules contained between glass substrates, coupling the uniaxial compensation film to at least one of the glass substrates, and coupling a polarization film and electrodes to the compensation film. Preferably, the uniaxial compensation film has a thickness less than or equal to 50 microns. Where there are multiple compensation films, the total thickness and the total retardation values should be considered.

BACKGROUND OF THE INVENTION

1. Field of the Invention

2. Description of Related Art

A liquid crystal display (“LCD”) device includes upper and lower panels provided with field-generating electrodes thereon, a liquid crystal layer interposed therebetween, a pair of a polarizer and an analyzer, compensation films, etc. The LCD generates electric field in the liquid crystal layer by applying electric voltages to the field-generating electrodes and adjusts the intensity of the electric field to control the transmittance of light passing through the liquid crystal layer, thereby displaying desired images.

One of the most widely used types of LCD has a common electrode and a plurality of pixel electrodes provided on respective panels and a plurality of thin film transistors (“TFT”) for switching voltages applied to the pixel electrodes, which is provided on the panel having the pixel electrodes.

LCDs may operate in one of several modes. An LCD operating in a vertically-aligned (“VA”) mode contains liquid crystal molecules aligned perpendicular to two panels. VA-mode LCDs are sometimes preferred for their high contrast ratio and wide viewing angle.

LCDs often suffer from light leakage, the severity of which increases with viewing angle. The light leakage, which causes poor visibility from the sides and a narrow viewing angle, is caused by variations in light path and in the effective angle made by the polarizer and the analyzer depending on the viewing directions.

Compensation films are sometimes used to neutralize the effect of these variations. However, use of compensation films usually significantly increases the cost of the LCD because they are expensive and there is no efficient way to select the compensation film that yields optimal results. A method of determining the optimal parameters of a compensation film without the costly trial-and-error process is needed in order to allow more LCD applications to take advantage of compensation films.

SUMMARY

The invention is a display device that includes a first substrate and a second substrate positioned parallel to each other and a liquid crystal layer interposed between the first glass substrate and the second glass substrate. The liquid crystal layer includes liquid crystal molecules whose long axes are oriented substantially orthogonal to the first and second substrates in the absence of an electrical field in the liquid crystal layer. A first compensation film is coupled to the first substrate. The first compensation film has a retardation value of less than or equal to 200 nm for light having a wavelength of about 550 nm and is negatively birefringent. A polarization film is coupled to the first compensation film, and a first electrode is formed on one of the first and second substrates.

In another aspect, the invention is a display device that includes a first substrate and a second substrate positioned parallel to each other and a liquid crystal layer interposed between the first substrate and the second substrate. A polarization film is coupled to the first substrate, and a reverse-dispersion phase difference film is positioned between the first substrate and the polarization film. An electrode is formed on one of the first and second substrates.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In the drawings, the thickness of layers and regions are exaggerated for clarity. Like numerals refer to like elements throughout. It will be understood that when an element such as a layer, film, region, substrate or panel is referred to as being “on” another element, it can be directly on the other element or on one or more intervening elements. In contrast, when an element is referred to as being “directly on” another element, there are no intervening elements present.

Then, LCDs according to embodiments of the present invention will be described in detail with reference to the drawings.

FIG. 1is a sectional view of a transmissive type LCD according to an embodiment of the present invention.

An LCD according to this embodiment includes a TFT array panel1and a color filter array panel2facing each other, and a liquid crystal layer3interposed between the two panels1and2. The LCD also includes first and second polarization films12and22having nonparallel polarization axes, and first and second protective films13and23preferably made of TAC (triacetate cellulose) films and attached on the first and the second polarization films12and22for protecting the polarization films12and22, respectively. The LCD further includes a first uniaxial (C-plate) compensation film14inserted between the TFT array panel1and the first protective film13, and a second uniaxial compensation film24inserted between the color filter array panel2and the second protective film23.

The LCD is in a vertically-aligned (VA) mode. That is, the liquid crystal layer3of the LCD includes liquid crystal molecules aligned to make their long axes substantially perpendicular to the two panels1and2.

The first and the second protective films13and23generate slight retardation. In addition, the uniaxial compensation films14and24have negativity and generate retardation in a range between about 0 nm and about 200 nm for the light having a wavelength of 550 nm. Here, the uniaxiality means that nx=ny≠nz and the negativity means that nx=ny>nz, where nx, ny and nz denote the refractive indices of x, y and z directions, respectively.

The first uniaxial compensation film14may be omitted.

Now, a TFT array panel and a color filter array panel of an LCD according to embodiments are described in more detail.

FIG. 2is a layout view of a TFT array panel for an LCD according to an embodiment of the present invention, andFIG. 3is a sectional view of the TFT array panel shown inFIG. 2taken along the line III-III′.

As shown inFIGS. 2 and 3, a gate wire121,123and125preferably made of a metal having low resistivity such as aluminum, silver, etc. is formed on a transparent insulating substrate110. The gate wire121,123and125includes a plurality of gate lines121extending in a transverse direction and a plurality of gate electrodes123connected to the gate lines121. An end portion125of each gate line121is widened for connection with an external circuit.

A gate insulating layer140is formed on the entire surface of the substrate including the gate wire121,123and125.

A plurality of semiconductor stripes151,153and159preferably made of amorphous silicon are formed on the gate insulating layer140, and a plurality of ohmic contacts161,162,163and165preferably made of amorphous silicon heavily doped with n-type impurity are formed on the semiconductor stripes151,153and159.

A data wire171,173,175,177and179preferably made of a metal having low resistivity such as aluminum, silver, etc. is formed on the ohmic contacts161,162,163and165and the gate insulating layer140.

The data wire171,173,175,177and179includes a plurality of data lines171intersecting the gate lines121to define a plurality of pixel areas, a plurality of source electrodes173which are branches of the data lines171and connected to the ohmic contacts163, a plurality of drain electrode175separated from the source electrodes173and formed on the ohmic contacts165opposite to the source electrodes173with respect to the gate electrodes123, and a plurality of storage electrodes177overlapping the gate lines121to form storage capacitors. An end portion179of each data line171is widened for connection with an external circuit.

A passivation layer180is formed on the data wire171,173,175,177and179. The passivation layer180has a plurality of first contact holes181exposing the drain electrodes175, a plurality of second contact holes182exposing the end portions125of the gate lines, a plurality of third contact holes183exposing the end portions179of the data lines171, and a plurality of fourth contact holes184exposing the storage electrodes177.

A plurality of pixel electrodes190and a plurality of contact assistants95and97are formed on the passivation layer180. The pixel electrodes190are connected to the drain electrodes175and the storage electrodes177via the first and the fourth contact holes181and184, respectively, and the contact assistants95and97are connected to the exposed end portions125of the gate lines121and the exposed end portions179of the data lines171via the second and the third contact holes182and183, respectively. The pixel electrodes190and the contact assistants95and97are preferably made of transparent material such as ITO (indium tin oxide) or IZO (indium zinc oxide).

FIG. 4is a layout view of a color filter array panel of an LCD according to an embodiment of the present invention.

A black matrix220is formed on an insulating substrate210, a plurality of color filters230are formed on the black matrix220, and a common electrode270is formed on the color filters230. The common electrode270is preferably made of a transparent conductive material such as ITO or IZO.

FIG. 5is a sectional view of a reflective type LCD without separate light source according to another embodiment of the present invention.

An LCD according to this embodiment includes a TFT array panel1and a color filter array panel2facing each other, and a liquid crystal layer3interposed between the two panels1and2. The LCD further includes a polarization film22and a protective film23attached on the polarization film22for protecting the polarization film22. The LCD also includes a uniaxial compensation film24and a reverse dispersion phase difference film25inserted between the color filter array panel2and the protective film23.

The LCD is in a VA mode. The protective film23generates slight retardation, and the uniaxial compensation film24has negativity and generates retardation ranging 0 nm to 200 nm for the light with 550 nm wavelength.

FIG. 6is a sectional view of a TFT array panel of an LCD according to another embodiment of the present invention.

Referring toFIG. 6, a gate wire121,123and125, a gate insulating layer140, a plurality of semiconductor stripes151and153, a plurality of ohmic contacts161,162,163and165, a data wire171,173,175,177and179, a passivation layer180, a plurality of pixel electrodes190, and a plurality of contact assistants95and97are formed on a substrate110.

The surface of the passivation layer180has embossment including prominences/protrusions and depressions, and the pixel electrodes190are preferably made of a metal having good reflectance such as aluminum.

FIG. 7is a sectional view of a transflective LCD according to another embodiment of the present invention.

An LCD according to this embodiment includes a TFT array panel1and a color filter array panel2facing each other, and a liquid crystal layer3interposed between the two panels1and2. The LCD also includes a pair of first and second polarization films12and22, and a pair of first and second protective films13and23attached on the polarization films12and22, respectively. The LCD further includes a first uniaxial (C-plate) compensation film14and a first reverse dispersion phase difference film15inserted between the TFT array panel1and the first protective film13, and a second uniaxial (C-plate) compensation film24and a second reverse dispersion phase difference film25inserted between the color filter array panel2and the second protective film23.

The LCD is in a VA mode. The first and the second protective films13and23generate slight retardation, and the first and the second uniaxial compensation films14and24have negativity and generate retardation in a range from 0 nm to 200 nm for the light with 550 nm wavelength. The first uniaxial compensation film14may be omitted.

FIG. 8is a layout view of a TFT array panel of an LCD according to an embodiment of the present invention, andFIG. 9is a sectional view of the TFT array panel shown inFIG. 8taken along the line IX-IX′.

Referring toFIG. 8, a gate wire121,123and125, a gate insulating layer140, a plurality of semiconductor stripes151and153, a plurality of ohmic contacts161,162,163and165, a data wire171,173,175,177and179, and a passivation layer801are formed on a substrate110.

A plurality of transparent electrodes90and a plurality of contact assistants95and97preferably made of ITO or IZO are formed on the passivation layer801. An interlayer insulating layer802having an embossed surface is formed on the transparent electrodes90. A plurality of reflecting electrodes80are formed on the interlayer insulating layer802, and each reflecting electrodes80has a window82for light transmission.

Various characteristics of various types of LCDs with various types of compensation films were measured.

The LCDs used for the measurement have conditions shown in TABLE 1 and TABLE 2.

Here, K11, K22 and K33 are elastic coefficients of spreading, twisting and bending measured in pico-newton (pN) and ε∥ and ε⊥ are permittivity parallel to and perpendicular to the director, respectively.

Here, ne is the refractive index parallel to the director (i.e. for extraordinary ray) and no is the refractive index perpendicular to the director (i.e. for ordinary ray), while Δn=ne−no. In addition, the dispersion relation is given by:

n⁡(λ)=n∞+Aλ2,
where n∞is the refractive index for infinite wavelength and A is a constant.

FIGS. 10 and 11are graphs respectively showing reflectance and transmittance as function of applied voltage for a transflective LCD with and without uniaxial (C-plate) compensation films.

The curves show that the presence of the uniaxial compensation films hardly affects the reflectance and the transmittance of the LCD.

FIGS. 12A to 12Fare graphs showing isocontrast curves of a reflective type LCD without and with one C-plate attached to the upper panel.FIGS. 12A to 12Fshow the isocontrast curves for the cases 2 to 7 in the TABLE 3, respectively.

FIGS. 13A to 13Fare graphs showing isocontrast curves of a transmissive type LCD without and with one C-plate attached to the upper panel.FIGS. 13A to 13Fshow the isocontrast curves for the cases 2 to 7 in TABLE 4, respectively.

FIGS. 14A to 14Eare graphs showing isocontrast curves of a reflective type LCD with two C-plates respectively attached to upper and lower panels.FIGS. 14A to 14Eshow the isocontrast curves for Cases 8 to 12, respectively.

FIGS. 15A to 15Eare graphs showing isocontrast curves of a transmissive type LCD with two C-plates respective attached to upper and lower panels.FIGS. 15A to 15Eshow the isocontrast curves for Cases 8 to 12 in TABLE 6, respectively.

In TABLES 3 to 6, the areal isocontrast ratio is an isocontrast area for the contrast ratio of 10:1 divided by that in Case 3 of the reflective type LCD. White and black voltages in VA mode are 3.5V and 1.8V for the reflective type and 4.5V and 1.8V for transmissive type, respectively. The abbreviation “CR” stands for contrast ratio.

The ratios such as 2:1, 5:1, 10:1, 20:1 and 22:1 in the legends ofFIGS. 12A to 15Eindicate contrast ratios and the values (for example, 68/68/51/51 inFIG. 12A) at the bottom ofFIGS. 12A to 15Eindicate upper/lower/left/right side viewing angles giving the contrast ratio of 2:1.

The measurement values of TABLES 3 to 6 shown inFIGS. 12A to 12F,13A to13F, and14A to15E can be summarized as follows.

Without uniaxial compensation film (C-plate), the transmittance, the reflectance, the contrast ration at the front view, and the viewing angle of the VA mode are superior to those of the TN mode.

Compared with Case 2, which does not include a uniaxial compensation film, Cases 3 and 4 of the VA mode show both improved viewing angle and improved isocontrast curve, and Cases 3 and 4 of the transmissive mode show improved isocontrast curves. In contrast, Cases 5, 6 and 7, each of which includes a compensation film causing a retardation greater than 160 nm, show deteriorated viewing angle and deteriorated isocontrast curves. Data indicates that a uniaxial compensation film providing a retardation value larger than 160 nm has a detrimental effect on the LCD device.

With uniaxial compensation films attached to both upper and lower panels, the isocontrast curves for the transmissive-type LCDs are improved until the sum of the retardation values of the two compensation films equals to 160 nm. When the combined retardation value exceeds 160 nm, both the isocontrast curves and the viewing angles become worse. For the reflective-type LCDs, only one of the two compensation films contributes to the total retardation since light is not transmitted through both of films. Therefore, the actual retardation values of the compensation films for the cases 8 to 11 are equal to or smaller than 160 nm. This explains why case 12 for the reflective-type LCD shows improved isocontrast curves in spite of having a retardation value of 200 nm. For the transmissive-type LCD, although there is no experimental example for the case of 200 nm retardation, the results shown in TABLES 3 to 6 suggest that the isocontrast curves will be improved where retardation values are equal to or less than 200 nm.

The above-described experimental results show a correlation between the total thickness of the uniaxial compensation film(s) and the viewing angle and/or the contrast ratio. It appears that the total thickness of the uniaxial compensation film(s) affects the viewing angle and/or the contrast ratio more than the number or the physical arrangement of the uniaxial compensation film(s).

Of the cases above, Case 3 has optimal characteristics both for the transmissive type and the reflective type LCDs. Although some cases show better contrast ratio at the front view than Case 3, the difference is small. Overall, Case 3 resulted in a better viewing angle than the other cases. Therefore, it can be said that Case 3 is optimized.

In conclusion, the total retardation of the uniaxial compensation film(s) equal to or under 200 nm improves the isocontrast curve and/or the viewing angle. This improvement is irrelevant to the number of uniaxial compensation films used and the type (reflective or the transmissive) of LCD.

According to the present invention, uniaxial compensation film(s) generating a predetermined retardation is used to improve the viewing angle of the LCD.