KR20010093211A - Liquid crystal display with improved viewing angle - Google Patents

Abstract

To improve the viewing of the liquid crystal display, a compensation layer is provided which consists of a crosslinked cholesteric liquid polymer layer having a very short spiral pitch and a spiral axis perpendicular to the layer.

Description

Liquid crystal display with improved viewing angle

The present invention relates to liquid crystal displays, in particular compensators for visual enhancement of displays.

Liquid crystal displays (LCDs) typically exhibit a unique visual dependence of contrast. The arrangement in which the dark state is captured by the liquid crystal indicator perpendicular to the cell plane is particularly affected. This is the case, for example, in the case of vertically arranged nematic (VAN) cells, hybridly arranged nematic (HAN) cells and typically bright state twisted nematic (TN) cells. The contrast of these cell types is very high in the case of vertical incident light and decreases with increasing time (representing the vertical field of view as "time" 0). Contrast can be reversed for large times. The switching state of the LCD where the liquid crystal indicator is inclined relative to the cell standard has an asymmetrical visual dependence.

Undesirable visual dependence can be reduced by disposing a compensation plate comprising a layer with negative uniaxial optical anisotropy in the cell. In the case of VAN-cell, non-driven state of the optical anisotropy is less than the refractive index n _z in a direction perpendicular to the refractive index n _x, n _y plane in the cell plane, that is, the n _z> n _x = n _y After negative optical anisotropy Having, i.e., n _z <n _x = n _y , can be compensated for. In the same way, the driving state of the HAN cell or TN cell in the normally bright state can be compensated.

The present invention relates to such a compensation plate.

According to the invention, a cholesteric liquid crystal polymer (LCP) having a spiral axis perpendicular or generally or essentially or substantially perpendicular to the plane of the layer, the selective reflection range having a spiral pitch sufficiently shorter than the wavelength of visible light Provide a compensation plate comprising a layer. Thus, a suitable pitch is less than 300 nm, preferably less than 200 nm, for example less than 150 nm.

Or a suitable pitch of 350 nm / May be less than, where Is the average refractive index of the polymer.

Preferably, the cholesteric liquid crystal is applied in situ and polymerized, for example crosslinked.

Advantageously, the optical anisotropy Δn of the liquid crystal polymer is greater than 0.25 because high optical anisotropy allows for a thinner compensation layer.

Preferably, the layer has a cholesteric arrangement over at least a portion of its region.

The polymer layer is photo oriented and conveniently adopts the orientation of the linear photopolymerized base layer.

The invention extends to a liquid crystal display (LCD) device comprising a compensation plate as described above, which preferably extends over the entire field of view of the device.

The LCD device may have an indicator of switching liquid crystal material arranged essentially perpendicular to the compensating plate in the bright or dark state.

The liquid crystal cell of the LCD device may be a vertically arranged nematic, hybridly arranged nematic or twisted nematic (VAN, HAN or TN), and the liquid crystal class already pointed out to be affected by visual dependence may be somewhat compensated. .

Thus, a typical compensating plate according to the invention has a spiral axis parallel to the cell standard (perpendicular to the cell plane) and a small spiral pitch such that visible light (λ> 400 nm) is on the long wavelength side of the selective reflection range (λ ₀ <350 nm). And a crosslinked cholesteric liquid crystal polymer layer having λ ₀ is the center wavelength of the optional reflection band. Due to these conditions of the cholesteric arrangement, the light passing vertically causes the average refractive index of the cholesteric layer to experience = (n _o + n _e ) / 2 The cholesteric layer is a negative uniaxial layer for visible light since the optical axis is perpendicular to the layer and has a refractive index n _o , while the effective refractive index in the plane is (n _o + n _e ) / 2> n _o . n _e and n _o are each local abnormality and normal refractive index of the cholesteric layer.

The anisotropy of the positive uniaxial liquid crystal cell can be compensated by selecting a suitable thickness for the compensation layer to be incorporated into the liquid crystal device.

To prepare the LCPs used, monomers or prepolymers in the solution are preferably applied over the alignment layer. The viscosity is preferably arranged low so that the orientation occurs within a short period of time. The cholesteric arrangement can be induced by chiral dopants with high helical twisting power (HTP), whereby a pitch of less than 250 nm can be easily reached. The hardening or crosslinking of the layer is mechanically robust and its optical properties are thermally stable. Due to the large anisotropy of the LCP material, a layer thickness of several μm is sufficient to compensate for a typical VAN cell having an optical stop Δn · d of 250 to 500 nm (optical anisotropy Δn, cell thickness d).

To orient the cholesteric LCP, one of the known orientation layer techniques can in principle be used. Light alignment methods (usually using linear polarized light) are particularly suitable, and particularly good orientation properties can be achieved by linear light polymerization (LPP) alignment layers. These methods advantageously also rule out possible optical defects, for example grooves or scratches caused by friction.

In this way cholesteric LCP, ie cured or crosslinked cholesteric liquid crystal compositions based on monomers or prepolymers, can be used to achieve high quality fast orientation useful for large scale manufacturing.

The compensation layer according to the invention can be easily incorporated into the liquid crystal polymer multilayer.

The invention will now be described by way of example with reference to the accompanying drawings which show the visual characteristics of the cell in the presence or absence of a compensation layer according to the invention described hereinafter.

An example of a compensation layer according to the invention is as follows.

In a first step, a linear photopolymerizable (LPP) alignment layer is applied to the quartz substrate. To this end, 1 wt% solution S _LPP of _photoarray polymerizable photopolymer material A is prepared using cyclopentanone as solvent.

Photopolymer A

Solution S _LPP was spin coated onto the substrate at 2000 rpm at 23 ° C. for 2 minutes. The layer about 50 nm thick is annealed in air at 150 ° C. for 30 minutes. The coated substrate is exposed to linear polarization of the mercury lamp for 10 minutes to photoarray and polymerize.

In a second step, the cholesteric LCP layer is spin coated on the alignment layer. To this end, the three liquid crystal diacrylate monomers Mon 1, Mon 2, Mon 3, chiral component Ch 1 and also the photoinitiator IRGACURE 369 (Ciba SC), all dissolved in anisole, and BHT (2, A solution S _LCP containing 6-di-tert-butyl-4-methylphenol / "butyl hydroxytoluene") is prepared.

Mon 1

Mon 2

Mon 3

Ch 1

Thus, the composition of solution S _LCP is as follows:

Mon 1 24% by weight

Mon 2 4.5 wt%

Mon 3 1.5 wt%

Ch 1 3% by weight

Irgacure 369 0.5 wt%

BHT 0.5 wt%

Anisole 66% by weight

The layer is tempered at 23 ° C. for several minutes, forms a cholesteric mono-domain layer and then crosslinked by exposure to unpolarized mercury light for 5 minutes under nitrogen atmosphere. The LPP / LCP layer is tempered at 200 ° C. in air for 6 minutes.

The transmission spectrum of the coated quartz plate shows the selective reflection band of the cholesteric layer at the center wavelength λ ₀ = 350 nm. The thickness of the layer is 3.2 μm.

The second LCP layer is spin coated, oriented and crosslinked so that the overall thickness of the multilayer is 6.5 μm in the same manner as described above. The central wavelength of selective reflection of the LCP bilayer is 350 nm.

Each dependent reflection measurement in an elliptical polarimeter ("WVASE", JA Woollam Co.) is characterized by the fact that the LCP bilayer is a negative uniaxial bireflective layer with optical axis and optical anisotropy parallel to the layer standard in the range of visible light (400 to 800 nm). Show characteristics. Δn = n _o- (n _o + n _e ) / 2 = -0.07

In a further experiment, this compensation layer is bonded to a VAN cell with optical anisotropy Δn · d = 420 nm. Visual dependency measurements using a spatial photometer (“EZ-Contrast”, ELDIM) demonstrate significantly better visual characteristics of VAN cells with compensation layers compared to uncompensated cells. This can be seen from FIGS. 1-3, where FIG. 1 shows the visual characteristics of the LPP oriented two-domain VAN-LCD in the off state, FIG. 2 shows the same cell with an additional compensation layer according to the invention, 3 is for comparing empty cells without compensation layer between cross polarizers.

Claims (16)

A compensator comprising a layer of cholesteric liquid crystal polymer having a spiral axis that is essentially perpendicular to the plane of the layer and an optional reflection range that is sufficiently shorter than the wavelength of visible light. The compensation plate of claim 1 wherein the pitch is less than 300 nm. The compensation plate of claim 2 wherein the pitch is less than 200 nm. The compensation plate of claim 3 wherein the pitch is less than 150 nm. The method of claim 1, wherein the pitch is 350 nm / (here, Is the average refractive index of the polymer). The compensation plate according to any one of claims 1 to 5, wherein the cholesteric liquid crystal is applied and polymerized in situ. The compensation plate according to any one of claims 1 to 6, wherein the optical anisotropy Δn of the liquid crystal polymer is greater than 0.25. 8. The compensating plate according to any one of claims 1 to 7, wherein the layer has a cholesteric arrangement over at least a portion of its region. The compensating plate according to claim 1, wherein the polymer layer is photo-oriented. The compensation plate of claim 9 wherein the polymer layer adopts the orientation of the linear photopolymerized base layer. A liquid crystal display device comprising the compensation plate according to any one of claims 1 to 10. 12. A liquid crystal display device as claimed in claim 11, wherein the compensation plate extends over the entire field of view of the device. 13. A liquid crystal display device as claimed in claim 11 or 12, having an indicator of a switching liquid crystal material arranged essentially perpendicular to the compensation plate in the bright or dark state. The liquid crystal display device according to any one of claims 11 to 13, wherein the liquid crystal cells are nematically arranged vertically. The liquid crystal display device according to any one of claims 11 to 13, wherein the liquid crystal cell is a nematic hybrid array. The liquid crystal display device according to any one of claims 11 to 13, wherein the liquid crystal cell is twisted nematic.