KR101043248B1 - Cholesteric liquid crystal display - Google Patents

Abstract

< 1의 조건을 만족하고, 나선 축이 기판에 수직하도록 평면 배향된 콜레스테릭 액정 조성물; 상기 기판에 평행한 횡단 전기장을 인가하는 전극; 및 상기 기판의 양단에 서로 수직하도록 각각 설치되는 편광자 및 검광자;로 구성되는 것을 특징으로 하는 콜레스테릭 액정 디스플레이에 관한 것이다. 이때, e는 플렉소전기분극 계수(flexoelectric coefficient)의 평균, Δε는 유전이방성(dielectric anisotropy), K는 탄성 상수(elastic constant)의 평균을 나타낸다. The present invention is a pair of substrate having at least one light-transmitting; Sandwiched between the substrates, 0 ≦

A cholesteric liquid crystal composition which satisfies the condition of <1 and is oriented in a plane such that the spiral axis is perpendicular to the substrate; An electrode applying a transverse electric field parallel to said substrate; And a polarizer and an analyzer respectively installed at both ends of the substrate so as to be perpendicular to each other. In this case, e is an average of the flexoelectric coefficient, Δε is the dielectric anisotropy, K is the average of the elastic constant (elastic constant).

The liquid crystal display of the present invention uses a dielectric reaction of a planar cholesteric liquid crystal, such as a nematic liquid crystal display, has an excellent optical efficiency and a fast response speed, and is independent of the polarity of an applied electric field. It can be widely used.

Cholesteric liquid crystal, display, dielectric anisotropy, pitch, electric field

Description

Cholesteric liquid crystal display

TECHNICAL FIELD The present invention relates to a liquid crystal display, and more particularly, to a liquid crystal display having a fast response speed and irrespective of the polarity of an applied electric field by using a dielectric reaction of a planar cholesteric liquid crystal.

A liquid crystal display (LCD) is a display in which a liquid crystal display displays an image by injecting a liquid crystal, which is an intermediate material between a solid and a liquid, between two substrates and changing an arrangement of liquid crystal molecules by an electric field formed between electrodes disposed on the two substrates. Among the various display elements, the device is thin, lightweight, and low in power consumption, and is used for image display devices such as televisions and monitors, OA (Office Automation) devices such as word processors and personal computers, and information terminals such as cameras and mobile phones. It is used to display information in various fields. In addition, the development and manufacture of new flat panel displays such as organic light emitting diodes (OLEDs) and plasma display panels (PDPs) is accelerating, and the introduction of new applications is driving new and higher performance to meet their requirements. Requires.

TN (Twisted Nematic) display method as a liquid crystal display device applied in a small and medium-sized display, despite the advantage of high transmittance, it is a barrier to surpass the CRT (cathod ray tube) due to defects in viewing angle characteristics, contrast, and response characteristics. In order to improve the narrow viewing angle characteristics, an in-plane switching (IPS) mode using a horizontal electric field has been developed. However, liquid crystal molecules do not rotate in the upper part of the electrode. In recent years, the liquid crystal display using the blue phase is attracting attention because it is expected that a quick reaction that cannot be obtained in the current nematic liquid crystal mode is possible. As described above, liquid crystal displays of various modes are widely used, but each liquid crystal display still has limitations with unique advantages, and efforts to find a mode capable of overcoming them continue. In particular, in order to realize the full-color moving picture required in recent displays, it is necessary to greatly improve the response time.

Cholesteric liquid crystals were first discovered in compounds made of natural cholesterol, which is why they are called cholesteric liquid crystals. Since then, chiral dopants are mixed with nematic liquid crystals to form liquid crystals having the same molecular structure, which is called chiral nematic liquid crystal. Since the two liquid crystal phases have the same molecular structure, the two words have the same meaning.

Cholesteric LC (CLC) exhibits chirality, polarization and color selectivity for light with wavelengths comparable to the pitch of CLC, and many attempts have been made to produce beneficial optical devices using these properties. Planar arrangements of cholesteric liquid crystal polarizers are expected to increase the efficiency compared to conventional linearly polarized sheet polarizers by recycling the reflected light. The pitch extension by the dielectric reaction can be used as a tool for controlling the wavelength of reflected light in the visible light and near-IR region by controlling the dynamically reflected wavelength. Displays using bistable phase transitions between focal conic and planar arrays have different states of the liquid crystal in each state, resulting in different light intensities and also applying an electric field in each state. It can be used in energy-saving bistable devices by keeping them stable even if they are not. The flexoelectric effect of uniformly twisted cholesteric liquid crystals is known to cause inclination of the helical axis, which can be used to apply cholesteric liquid crystals to fast electro-optical devices. However, nothing has yet reached the level of commercialization.

The present invention is to solve the problems of the prior art, to provide a liquid crystal display having a high optical efficiency and a fast response response speed, like a nematic liquid crystal display.

< 1의 조건을 만족하고, 나선 축이 기판에 수직하도록 평면 배향된 콜레스테릭 액정 조성물; 상기 기판에 평행한 횡단 전기장을 인가하는 전극; 및 상기 기판의 양단에 서로 수직하도록 각각 설치되는 편광자 및 검광자;로 구성되는 것을 특징으로 하는 콜레스테릭 액정 디스플레이에 관한 것이다. 이때, e는 플렉소전기분극 계수(flexoelectric coefficient)의 절대값 평균, Δε는 유전이방성(dielectric anisotropy), K는 탄성 상수(elastic constant)의 평균을 나타낸다. The present invention for achieving the above object is a pair of substrate having at least one light-transmitting; Sandwiched between the substrates, 0 ≦

A cholesteric liquid crystal composition which satisfies the condition of <1 and is oriented in a plane such that the spiral axis is perpendicular to the substrate; An electrode applying a transverse electric field parallel to said substrate; And a polarizer and an analyzer respectively installed at both ends of the substrate so as to be perpendicular to each other. In this case, e represents an absolute value of the flexoelectric coefficient, Δε represents a dielectric anisotropy, and K represents an average of the elastic constants.

1 is a schematic diagram showing the structure of a cholesteric liquid crystal display of the present invention. When no electric field is applied, the directors of the cholesteric liquid crystal are helically aligned so that the helix axis is perpendicular to the substrate while changing the direction from the cell to the cell spacing direction to some extent. In this display, since the polarization change of the light in the liquid crystal layer is very small and the polarizer and the analyzer are in the vertical direction, the dark image is maintained because no light is transmitted in the state in which no light is irradiated. When the electric field is applied thereto, the liquid crystal directors line up in the electric field direction according to the strength of the electric field. As a result, the polarization of the transmitted light is changed to show a bright image. At this time, the direction of the liquid crystal director and the direction of the electric field close to the vertical direction is difficult to change the direction regardless of the electric field, so maintain the original direction. Increasing the electric field increases the pitch to stabilize the entire system. As the pitch increases, the directors line up in the same direction as the electric field. However, the change in pitch contributes slightly to the change in transmittance of the entire liquid crystal cell. Due to the short pitch, the change in the direction of the liquid crystal within a constant body as a whole, acts like a very small cell spacing and reacts very quickly.

The cholesteric liquid crystal display of the present invention as described above can be widely used for displaying information in a wider variety of fields because the response speed is very fast compared to the conventional twisted nematic liquid crystal display. In addition, since the polarity of the electric field is applied, the quality of the display can be prevented from being deteriorated by the ions present in the liquid crystal.

Hereinafter, the present invention will be described in detail with reference to the accompanying drawings and specific examples. However, these examples are illustrative only and the present invention is not limited thereto.

Example 1 Preparation of Cholesteric Liquid Crystal Display

Two glass substrates were prepared to produce a liquid crystal cell. One coated two parallel Cr electrodes at intervals of about 120 or 10 μm. No electrode was formed on the other. Both substrates were spin coated with polyimide AL-3046 (JSR) and then fired at 220 ° C. Rubbing was performed uniformly and sufficiently by conventional rubbing techniques. The pretilt angle at the rubbed surface does not appear to be greater than the order of magnitude seen in a typical liquid crystal. Both substrates were fixed using an epoxy resin so that the cell spacing was about 10 mu m. Before fixing the cell, the rubbing direction of the two substrates was adjusted in parallel or vertically. As a result of the following experiment, the rubbing direction had little effect on the experimental results.

The liquid crystal composition (Chisso) includes 44.0 wt% of 4-cyano-t'-pentylbiphenyl (5CB), 48.2 wt% of JC-1041XX and 2, 5-bis- [4 '-(hexyloxy) -phenyl-4-carbonyl] -1 7.8 wt% of a mixture of 4,3,6-dianyhydride-D-sorbitol [ISO- (C6OBA) ₂ ] was used. The composition sequentially shows the isotropic phase, blue phase and cholesteric phase as it cools from high temperature, and shows cholesteric phase at room temperature at which the experiment was conducted. The liquid crystal composition was injected into the cell in an isotropic phase and the temperature was slowly cooled to room temperature.

As a result of cano cell measurement, the cholesteric liquid crystal has a pitch of 0.22 μm and a cell gap of 10.0 μm at room temperature. Under a non-system, the half pitch number of the cell is 91.

As shown in FIG. 1, polarizers and analyzers are provided at both ends of the substrate. The polarizer and the analyzer were adjusted perpendicular to each other, and the direction of the electric field was in the π / 4 direction with respect to the polarizer. The electric field was applied up to 10V / μm through function generators and amplifiers. A sine or square wave of 1 kHz was applied to measure transmittance or texture, and a pulsed bipolar square electric field was applied to measure response time. In the following, all electric fields are expressed as root mean squared values.

Example 2: Characterization of cholesteric liquid crystal display

The texture and transmittance characteristics of the cholesteric liquid crystal display prepared in Example 1 were measured.

On cholesteric, except for some oily strikes, the texture showed a uniform dark image. The rotation of the sample had little effect on the brightness and very dark states remained in the crossed polarizers. As the electric field was increased, the image of the polarizer showed an symmetric pattern of isotropic array and an increase in capacitance. From these results, it can be seen that the direction of the cholesteric phase has a spiral arrangement in which the spiral axis is perpendicular to the substrate.

The brightness of the cell was increased as the in-plane field strength increased as shown in FIG. 2 (b) is a graph showing the change in transmittance of white light and red light according to the intensity of the electric field. White light used a microscope light source of 580 nm (half width 180 nm), and red light 640 nm (half width 80 nm). The electric field is a sinusoidal wave of 1 kHz, and the intensity described on the figure is the square mean value. When the electric field is low, the change in transmittance is very gentle and gradually becomes steep. At an electric field of 8 V / µm, the transmittance reaches its peak and gradually decreases. The contrast ratio of the white light was about 10, and the red light increased to about 80. There was little hysteresis with increasing and decreasing electric field size.

Figure 3 is a graph showing the change in transmittance with time when a square electric field is applied, the intensity of the electric field is 9V / ㎛. It can be seen from the graph of FIG. 3 that the response time of the cholesteric liquid crystal display is about 0.1 ms. There was no noticeable difference in on / off time. Although not shown as separate data, the response time showed similar values in the strength and polarity of other electric fields within the experimental range.

Example 3 Calculation of Characteristics of Cholesteric Liquid Crystal Display

The director was fixed by an infinite anchoring on an unslanted substrate, and under the field of vision, the pitch of the liquid crystal in the cell is about the same as the pitch of the original liquid crystal itself, assuming that the electric field is uniform for the display of the present invention. It can be calculated as follows.

Under the field system, transmission I depends on the number of pitches, birefringence and the wavelength of light. When talking through planar oriented cholesteric liquid crystals, light causes phase retardation. The transmittance of light scanned perpendicular to the substrate can be expressed by the following equation in a structure in which the polarizer and the photosensitive member are vertical (Optical Waves in Crystal, Wiley, New Jersey, 2003).

식 (1)

Formula (1)

Here, the half-pitch M may be expressed as d = Mpo / 2 when po is a pitch as the number of half pitches existing in the cell gap d. j is the angle between the polarizer and the director direction of the alignment layer, and Γ represents the phase delay of the light. The phase delay is defined as Γ = 2Δnd / λ. Δn means the anisotropy of the refractive index.

In the above formula, j = π / 4, and M »Γ / 2, that is, 2λ / Δn >> po, it can be estimated as transmittance I ∝ (Γ ² / 8πM) ² . From this equation, the transmittance decreases as the M value increases, so for an appropriate M value, the transmittance is expected to be sufficiently dark. Figure 4 (a) is a graph showing the result of the transmittance calculation according to the change in half pitch at a fixed cell interval for the display fabricated in Example 1. The cell gap was 10 mu m, the wavelength of light was 0.58 mu m, which is the central spectral line of the lamp, and the refractive index anisotropy was 1.702-1.539 = 0.163, which is the refractive index anisotropy of 5CB.

If the contrast ratio between when the electric field is applied and when it is not applied is 2.5 or more, it can be identified on the reflective display. As can be seen in FIG. 4A, when the electric field is not applied, the transmittance is 0.4 when the pitch number is 10 (the half pitch number is 20). When the electric field is applied, the transmittance is theoretically 1, so the contrast ratio is 2.5. Therefore, the cell spacing (d) / pitch (po) should be approximately 10 or more. Although the numerical value is a calculated value for the liquid crystal material of Example 1, the variation in refractive index anisotropy of the liquid crystal material is not large, and the difference in transmittance due to the variation in refractive index anisotropy may be adjusted by adjusting the cell interval. In FIG. 4A, when the half pitch is large, the transmittance under the non-electric field is close to zero, and the half pitch may be any value as long as 10 or more. However, the gap of the liquid crystal cell is not infinite and is typically less than 10 μm, and the pitch Numbers also come with some limitations, so it will be difficult to actually exceed 1000. In the case of newspapers, the contrast ratio is about 10, and since the reflection type display usually pursues the contrast ratio of newspapers or more, it is more preferable that d / po is 20 or more (half pitch 40) which shows a transmittance of 0.1 under an electric field. The contrast ratios of the two curves with respect to the white light and the red light of FIG. 2 (b) may be partially explained by the above transmittance equation because the phase delay Γ is inversely proportional to the wavelength of light when the field strength is zero.

In the display of the present invention, the pitch is preferably smaller than 380 / n (average refractive index) nm. In the planar state of the cholesteric liquid crystal, light of a specific wavelength is reflected to incident light, which is called Bragg reflection. The wavelength of the reflected light is represented by the product of the average refractive index n and the pitch po, which is the average value of the refractive index of the long axis and the short axis of the cholesteric liquid crystal (λ = n × po). In order for a display to be efficient, the visible light transmittance must be low under a vision system, so the wavelength of the reflected light must be avoided in the visible light region. Usually, the wavelength corresponding to visible light is in the range of 380 to 750 nm, therefore, in the display of the present invention, the pitch is preferably smaller than 380 nm / n.

에서 디렉터가 필드의 방향으로 배열할 것이다. 그러나 실제로는 셀 간격에서의 피치수는 유한값이며 전기장을 증가시킴에 따라 나선의 풀림은 이산된다.Applying a horizontal electric field and ignoring the flexoelectric reaction increases the pitch as the helix is released. For very long cell gaps, the pitch continues to increase as the field strength increases and finally

In the director will arrange in the direction of the field. In practice, however, the number of pitches in the cell gap is finite and the helix loosening is discrete as the electric field increases.

Using the free energy (F) including elastic deformation, flexoelectric reaction and dielectric energy, the change of pitch and transmittance according to the electric field is calculated as follows (Appl. Opt. 43, 5006 (2004)). In the case of the planar arrangement, the change of director is made along the direction of the cell thickness z.

식 (2)

Formula (2)

Since the LC structure is repeated according to the periodicity of the half pitch, the integral range becomes the length of the half pitch. Where M is the half pitch number, n is the director vector, K1, K2, K3 is the elastic constant, qo is 2π / po, po is the pitch of LC in the absence of an electric field, P is the polarization vector caused by flexoelectricity , E is the electric field vector.

For simplicity, one elastic constant (K) approximation and average flexoelectric coefficient (e) are used. E represents (e1 + e2) / 2 and is known to express flexoelectric effects in small cholesteric liquid crystals. By comparing the free energy of the deformed state (half pitch number L) and the deformed and unrolled state (half pitch number L-1), it is possible to easily determine the appropriate pitch number for a given field strength.

사이의 관계를 보여준다. 피치의 변화는 electric coherence length가 큰 영역에서 불연속적이다. 즉, 전기장이 약한 범위에서 전기장을 증가시킴에 따라 불연속적인 피치사이의 전기장 간격이 감소하여 포화점 부근에서는 거의 연속적이다. 계산을 위한 액정 물질의 변수로는 액정 조성물 중 5CB의 값을 사용하였다. 5CB의 유전 이방성(Δε)은 8.8X10^-11F/m이며, 굴절율 이방성은 1.702-1.539=0.163, K1, K2, K3의 평균 탄성 상수(K)는 6.5 X 10^-12 N, e1과 e2의 평균 flexoelectric계수(e)는 -23X10^-12 C/m이다. 셀갭은 10㎛, 빛의 파장은 램프의 중심 스펙트럼(spectrum) 라인인 0.58㎛로 간주하였다. Figure 4 (b) is the pitch number and electric coherence length under the conditions of Example 1

Show the relationship between The change in pitch is discontinuous in the region with large electric coherence length. In other words, as the electric field increases in a weak range, the electric field spacing between discontinuous pitches decreases and is almost continuous near the saturation point. A value of 5 CB in the liquid crystal composition was used as a parameter of the liquid crystal material for the calculation. The dielectric anisotropy (Δε) of 5CB is 8.8X10 ^-11 F / m, and the refractive index anisotropy is 1.702-1.539 = 0.163, and the average elastic constant (K) of K1, K2, K3 is 6.5 X 10 ^-12 N, e1 and e2. The average flexoelectric coefficient (e) is -23X10 ^-12 C / m. The cell gap was assumed to be 10 mu m and the wavelength of light was 0.58 mu m, which is the central spectrum line of the lamp.

The light passing through the cell changes as the field strength changes. The transmittance is calculated as a function of the electric field as can be seen in FIG. The calculation of the number was performed by the following method. First, the director direction and the pitch number were calculated using Equation (2). Second, the cell was divided into thin layers and the optical variation calculated in that layer. Third, the total photosynthesis change was calculated by summing all the layers of the cell.

If we extend the calculation range of the electric field, the curve of Fig. 5 (a) with the flexoelectric effect will be close to the measurement of Fig. 2 (b). However, this calculation at high field strengths may be less accurate than at low fields, since the average flexoelctric coefficients, which are basically valid only in low response situations, are used.

에 의해 결정된다. 유전 이방성이 0인 액정의 경우, flexoelectric 효과가 주요한 전기 반응으로 액정이 광학 장치로 기능하도록 한다. 상기 값이 1보다 작은 경우에는 flexoelectric 반응은 유전 이방성에 의한 유전 반응에 비해 미미하다. 본 발명의 유전 이방성과 flexoelectricity 모두가 전기 반응에 기여하나 유전 반응이 주된 기능을 하므로

은 1보다 작아야 한다. 또한, 그 값이 0에 가까울수록 flexoelectric 반응을 무시할 수 있으므로 본 발명의 디스플레이의 콜레스테릭 액정으로써 이상적이다. 따라서, 본 발명의 콜레스테릭 액정물질은 0≤

< 1의 조건을 만족하여야 한다. flexoelectric 효과는 전기장의 극성에 민감하며, 반응은 전기장의 방향에 대칭이다. 유전 반응과 flexoelectric 반응은 서로 직각이다. 복굴절에 대해서, 유전 반응은 주요한 요인이며 flexoelectricity는 복굴절의 증가를 지연시킨다. 만약 flexoelectricity가 없다면, 투과율은 도 5의 (a)에서처럼 더 작은 전계에서 포화될 것이다. The ratio between the dielectric and flexoelectric reactions

Determined by In the case of liquid crystals with zero dielectric anisotropy, the flexoelectric effect is the primary electrical reaction that allows the liquid crystal to function as an optical device. If the value is less than 1, the flexoelectric reaction is insignificant compared to the dielectric reaction due to dielectric anisotropy. Although both the dielectric anisotropy and flexoelectricity of the present invention contribute to the electrical reaction, the dielectric reaction plays a main role

Must be less than 1. In addition, since the value is closer to zero, the flexoelectric reaction can be neglected, making it ideal as a cholesteric liquid crystal of the display of the present invention. Therefore, the cholesteric liquid crystal material of the present invention is 0≤

The condition of <1 must be satisfied. The flexoelectric effect is sensitive to the polarity of the electric field and the reaction is symmetrical to the direction of the electric field. The dielectric and flexoelectric reactions are perpendicular to each other. For birefringence, the dielectric response is a major factor and flexoelectricity delays the increase in birefringence. If there is no flexoelectricity, the transmittance will saturate at the smaller electric field as in FIG.

Despite the increase in pitch, the transmittance is mainly due to the orientation of the director. As shown in (b) of FIG. 5, the change of the transmission due to the extension of the pitch is negligible as the difference is less than 1% in the maximum electric field region.

Since the transmittance in the absence of an external field depends on the wavelength of light as shown in Equation (1), the transmission in the presence of an electric field is affected by the wavelength of light. 5C shows that the cell transmittance in the presence of an electric field depends on the wavelength. As shown in FIG. 2B, when the light wavelength is long, the transmittance is lower at zero field intensity. Therefore, light having a long wavelength may be used to obtain a good contrast ratio. However, strong field strength is essential to obtain sufficient transmittance. The pitch is increased by increasing the electric field, which can cause abnormal changes in transmittance. Although the pitch in the absence of an electric field is smaller than the product of the wavelength of visible light and the refractive index of the cholesteric liquid crystal, the electric field is applied to selectively reflect a specific wavelength of light. It would be possible to reduce the transmission of the selective reflection wavelength for a particular electric field.

For a given pitch, director deformation occurs periodically with a fixed boundary condition of half pitch. The calculation shows that if the field strength is less than the saturation intensity E _H of the sebum extension, the pitch is only a few hundred nanometers over a wide range of electric field strengths. Thus, LC switching is equivalent to switching in very thin cells. The switching time τ is represented by τ∝po ² when the amplitude is approximated as small amplitute approximation. This causes a very quick response as compared to the conventional nematic display as shown in FIG. 3.

1 is a schematic diagram showing the structure of a cholesteric liquid crystal display of the present invention.

2 is a photograph and graph showing changes in texture and transmittance with application of an electric field.

3 is a graph showing a change in transmittance with time when a square electric field is applied.

4 is a graph showing a change in transmittance according to the theoretically calculated number of pitches and a change in pitch number according to application of an electric field.

5 is a graph showing the theoretical value of the change in transmittance according to the intensity of the electric field.

Claims (3)

A pair of substrates having at least one light transmitting property; Sandwiched between the substrates, 0 ≦

A cholesteric liquid crystal composition which satisfies the condition of <1 and is oriented in a plane such that the spiral axis is perpendicular to the substrate; An electrode applying a transverse electric field parallel to said substrate; Polarizers and analyzers respectively installed at both ends of the substrate to be perpendicular to each other; A cholesteric liquid crystal display, characterized in that consisting of. Where e is the average of the absolute values of the flexoelectric coefficient, Δε is the dielectric anisotropy, and K is the average of the elastic constants. The method of claim 1, And a ratio (d / po) of the pitch between the substrates and the pitch of the cholesteric liquid crystal is 10 or more. The method of claim 1, A helical pitch of the cholesteric liquid crystal is less than 380 / n (nm) cholesteric liquid crystal display. N is the average refractive index of the cholesteric liquid crystal.

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