The thesis is organized as following: the principles and the features of the cholesteric LCD will be presented in Chapter 2. In Chapter 3, the LCD fabrication processes including cell process such as polyimide (PI) printing, rubbing, spacer dispense, sealant dispense, assembly, hot press and LC injection will be presented.
Besides, the major measurement equipments used to characterize the fabricated the Ch-LCD are illustrated. In Chapter 4, the simulated results including reflective
spectra, reflectance and color appearances of different cholesteric LCs used to verify and optimize our design will be presented. About the experimental results, several Ch-LCD samples fabricated by simulated results are demonstrated. Based on the measurement results, some optical properties of the samples are discussed in Chapter 5. Some applications of Ch-LCD are discussed in Chapter 6. Finally, the conclusion of the thesis is given in Chapter 7.
Chapter 2
Principle
2.1 Optical Properties of Cholesteric Liquid Crystals
Cholesteric liquid crystals have two stable states at zero electric field: the reflecting planar state and the scattering focal conic state. Microphotographs of the planar state and the focal conic state are shown in Figs. 2-1(a) and (b). When a cholesteric liquid crystal is in the planar texture, there is a periodic structure of the refractive index in the cell normal direction[13]. The liquid crystal exhibits Bragg reflection at the wavelength λ0 = p×n for normally incident light, where p is the pitch of Ch-LC and n is the average refractive index of Ch-LC. The reflection is strong and multiple reflections inside the liquid crystal is important. The bandwidth of the reflected light given by ∆λ = p×∆n[6], where ∆n=ne-no is the birefringence. Circularly polarized light with the same handedness as the helical structure of cholesteric LC is reflected strongly because of the constructive interference of the light reflected from different positions, while circularly polarized light with the opposite handedness to the helical structure is not reflected because of the destructive interference of the light reflected from different positions. If the normally incident light is unpolarized, then the maximum reflection from the cholesteric LC is 50%.
(a) (b) Fig. 2-1. Microphotographs of (a) the planar and (b) the focal conic states.
2.2 Viewing Angle of Cholesteric Displays
When light is obliquely incident at the viewing angle θ on the cholesteric liquid crystal, as shown in Fig. 2-2, the central wavelength of the reflected light is given by λ= p× n ×cosθ[14], where θ is the incident angle with the normal direction. When θ is increased, the reflected light is shifted to a shorter wavelength, the reflection band becomes broader and the peak reflection becomes higher. The shift of reflection band is undesirable for display applications, because the color of the reflected light changes with the viewing angle.
Fig. 2-2. Viewing angle of cholesteric liquid crystal displays.
For the perfect planar state, there is a concern of viewing angle for incident light at an angle θ, the reflected light is only observed at the corresponding viewing angle. These problems can be partially solved by using an alignment layer or dispersing a small amount of polymer in the liquid crystal which gives weak homogeneous anchoring or homeotropic anchoring. The dispersed polymer and the alignment layer produce defects and create a multi-domain structure[15], as shown in Fig. 2-3.
Fig. 2-3. Multi-domain structure of cholesteric liquid crystal displays.
θ
θ
λ = n P cos
In this structure, the helical axis in the domains is no longer exactly parallel to the cell normal but distributed around the normal direction. In this imperfect planar state, for incident light at a given angle, light reflected from different domains are in different directions, as shown in Fig. 2-3. For the ambient light, light reflected from different domains can be observed at one location. Because the observed light is a mixture of different colors, the colors observed at different viewing angle are not very different. Therefore, this multi-domain structure improves the viewing angle of the cholesteric display[16]. Furthermore, the dispersed polymer or alignment layer stabilizes the focal conic state at zero field. The display stabilized by polymer is called the polymer-stabilized cholesteric display and the one stabilized by the alignment layer is called the surface-stabilized cholesteric display[17][18].
2.3 Cell Design of Cholesteric Displays
When a cholesteric liquid crystal is in the planar state, it reflects narrow band light. When it is in the focal conic state, it is scattering light. In order to achieve high contrast ratio, it is desirable that the backward scattering of the focal conic state can be minimized. It is also desirable that the light from the back of the display can be controlled. Therefore, a color absorption layer is required to coat on the back plate of the display[19], as shown in Fig. 2-4.
Fig. 2-4. Cell structure of the cholesteric display.
A black appearance is made with a black absorption layer[20]. The display has a bright color appearance when the liquid crystal is in the planar state and a black appearance when the liquid crystal is in the focal conic state. A white reflective display is made with a color absorption layer[21]. For example, the liquid crystal reflects yellow color light and the absorption layer reflects blue color. When the liquid crystal is in the planar state, yellow light is reflected from the cholesteric liquid crystal and blue light is reflected from the absorption layer. Therefore, the display has a white appearance. When the liquid crystal is in the focal conic state, only blue light is reflected from the absorption layer. Thus, the display has a blue appearance in dark state. Higher contrast can be achieved by putting the absorption layer inside the cell on top of the back plate.
The reflection of a cholesteric liquid crystal display depends on the cell thickness. For perfect planar state, 3um cell thickness is sufficient to obtain the saturated reflection from the liquid crystal with birefringence ∆n≧0.2. In reflective cholesteric displays, because of the defects produced by the alignment layer or dispersed polymer, usually 5um cell thickness is required to obtain a saturated reflection.
In polymer-stabilized bistable cholesteric displays, it is preferable to mix the cholesteric liquid crystal with a few percent of the monomer. Then the cells are irradiated by UV light for photopolymerisation of the monomer. A blacker focal conic state can usually be obtained by polymerizing the monomer in the homeotropic state in the presence of an applied voltage[22].
In surface-stabilized bistable cholesteric displays, either weak homogeneous alignment layers or homeotropic alignment layers can be used. When homeotropic alignment layers are used, the focal conic state appears darker and the response time becomes shorter, but the reflection of the planar state is lower[23].
Liquid crystal materials with large ∆n and ∆ε are always desirable. When ∆n is large, a smaller pitch of liquid crystal can be used, so the cell gap can be reduced.
Therefore, the driving voltage can be reduced. When ∆ε is large, the driving voltage also can be reduced. For display applications, the cholesteric liquid crystal is a mixture of a nematic liquid crystal and chiral dopants. The components should be chosen carefully such that the cholesteric phase temperature range is wide and the pitch does not shift with temperature.
Reflect red light
2.4 Color of Cholesteric Displays
For a conventional reflective cholesteric display, only a single color can be displayed. In order to become multiple color displays, there are several methods proposed to obtain multiple colors: U.S. Patent 6,377,321 by stacking multiple layers of cholesteric liquid crystals with different pitches, U.S. Patent 6,061,107 and U.S.
Patent 5,949,513 by using one layer of cholesteric liquid crystals partitioned in plane, etc.
For the stacking method, multiple color displays can be made by stacking three layers of cholesteric liquid crystals with three different pitches reflecting red, green, blue light[24], as shown in Figs. 2-5.(a) and (b).
(a) Structure (b) Cross section Figs. 2-5.(a) Structure and (b) Cross section of stacked type of Ch-LCD.
One cholesteric layer with the three colors is fabricated first. Then they are glued together. A potential issue of the stacked approach is parallax, i.e. the incident light and reflected light pass through different pixels. Parallax leads to color mixing
which becomes a serious issue for high-resolution displays. In order to reduce parallax, thin substrates, preferably substrates with conducting coating on both sides should be used to decrease the distance between the liquid crystal layers. Because of the high reflection of the electrodes, the stacking order from bottom to top should be red, green and blue. The reflection spectrum of a three-layer cholesteric display is shown in Fig. 2-6. Without using a polarizer, the reflectance is about 30%~35%, and its contrast ratio is in the range of 5-10 within 60° viewing cone. However, the stacked method is still too complex and the cost is high.
Fig. 2-6. Reflection spectrum of the stacked multiple color cholesteric display. Curve a:
all off, curve b: blue on, curve c: green on, curve d: red on, curve e: all on[25].
Absorption layer
reflect red , green, blue Ch-LC
Absorption layer
reflect red , green, blue Ch-LC
Other methods to achieve multiple colors, such as partition LC, the different pitches of LC must be used. The different pitches can be achieved by two methods.
The first method, three cholesteric liquid crystals with different pitches by depositing different twist agents are put into empty cells with partitions[26], as shown in Figs.
2-7.(a) and (b).
(a) Structure (b) Cross section
Figs. 2-7.(a) Structure and (b) Cross section of different pitches of Ch-LCD.
reflect red , green, blue Ch-LC
Absorption layer reflect red , green, blue
Ch-LC
Absorption layer
The second method is photo color tuning[27]. A photo-sensitive chiral dopant is added to the liquid crystal. The dopant undergoes a chemical reaction under UV irradiation and thus its chirality changes, and the pitch of the liquid crystal changes.
By varying the irradiation time, different pitches can be achieved, as shown in Figs.
2-8.(a) and (b).
(a) Structure (b) Cross section
Figs. 2-8.(a) Structure and (b) Cross section of different UV light curing of Ch-LCD.
The main drawback of one layer multiple color displays is that the reflection is low when only one color is on and the other colors are off.
2.5 New Method for Wide Band Reflection- Full Spectrum Reflective Method Current cholesteric displays are utilizing Bragg reflection theory, one of the intrinsic properties of cholesteric liquid crystal. In Bragg reflection, only a portion of the incident light with the same handedness of circular polarization and within the specific wave band which generates a monochrome color display can reflect back to the viewer. In order to improve the image quality of single color cholesteric displays, a new configuration “Full Spectrum Reflective Method” is proposed, as shown in Fig.
2-9.
Fig. 2-9. Configuration of full spectrum reflective method.
R+G+B
The wide band reflective cholesteric LCD consists of a Ch-LC cell, a full band reflector, and two circular polarizers which are composed by a linear polarizer (LP) and λ/4 plate. In bright state, the Ch-LC in planar state reflects the light component with the same handedness as Ch-LC, like green light component and the reflective bandwidth and wavelength are determined by the helical pitch and birefringence of Ch-LC. Besides, the remaining light component out of the selective bandwidth, like red and blue light component passes the LC, and is reflected by a full band reflector without changing the polarization state. Consequently, the two components: one reflected by the Bragg reflection with a center wavelength λ0 of Ch-LC, and the other reflected by the reflector, are compensatory each other and combine together by the viewer to perceive as full band of visible light, as shown in Fig. 2-10.
B G R
Wavelength (nm) Reflection
Reflection of Ch-LC Reflection of reflector
Fig. 2-10. Schematic plot of full spectrum reflective method.
When the Ch-LC is tuned in invisible Bragg reflection wavelength, for example in infrared wavelength range, a full spectrum of visible light will be reflected by the full band reflector. Therefore, the viewer still perceives full visible spectra of white images.
In dark state, the incoming light reaches a circular polarizer with the same handedness of the Ch-LC and is cut more than 50%. The rest of light goes to the Ch-LC with focal conic texture and is depolarized by the scattering effect of the LC material and assume 60% of incoming light can pass through the Ch-LC. Then the unpolarized light passes a linear polarizer is reduced by more than 50%, then is
Ch - LC (Focal Conic State) 100 %
15% 15%
9% 4.5%
Reflector
LP λ/4 λ/4 LP
50% 30% 15%
reflected by the reflector and further passes through a circular polarizer, located between the reflector and the Ch-LC cell. The remaining light passes through the Ch-LC cell again is depolarized by the scattering effect of the focal conic texture, and then is cut by more than 50% again by the front circular polarizer. Finally, only small portion (about 5%) of total light can reach to the viewer. As a result, by use of scattering effect of Ch-LC in focal conic texture and filtration of the polarizers results in dark state of the display, as shown in Fig. 2-11.
Fig. 2-11. Configuration of the wide band Ch-LCD in dark state.
The full spectrum of reflective cholesteric display can be realized both in visible and invisible spectra, for example, infrared wave band. The optical scattering effect in the infrared wavelength is the same as in the visible wavelength range, which is dependent on the pitch of the liquid crystals and refractive index so that the display obtains the same optical “dark” state. On the other hand, the optical “bright” state is still the full gamut of visible light reflected by a full band reflector. A reflective cholesteric display that works in infrared wavelength will have very fast response time and low driving voltage for the reason of lower viscosity and longer helical pitch of liquid crystal.
Using the full spectrum reflective method, both the two optical paths should have the same angular distribution and mutually matching over a wide viewing angle so that the display looks white in the planar texture. There are two approaches to match
the two paths. First, the distribution of Ch-LC itself is designed to single domain planar structure by means of surface treatment such as rubbing the surface of the glass coated with polyimide. Second, the reflector then is designed in the way of a mirror surface. The two reflected lights enable display to exhibit white color within certain viewing angle.
In order to improve the viewing angle of the Ch-LCD, a diffusing layer coated on the front polarizer surface, which is functioned as an anti-glare layer, can realize the large viewing cone. Other approach is to obtain multi-domain planar structure by controlling the surface rubbing condition to assure the wide angle distribution of reflective light. And the reflector is then designed with a specific surface condition, thus, wide reflective distribution can be achieved. In the case of infrared Ch-LC, the angular distribution of the display is entirely dependent upon the reflector because the reflective film will reflect all the visible white light with predetermined angular distribution.
The black and white reflective Ch-LCD introduces a novel way to realize real video display with higher brightness and contrast ratio. Conventional reflective Ch-LCD with video rates does not look bright because of most of the incoming light being absorbed by the back black absorption material. The transition time is limited by the domain size of the Ch-LC so that the total reflection of the display at high switching speed is not as good as low driving speed. By utilizing the full spectrum reflective method, light out of the selective reflection bandwidth can be used.
Therefore, the total brightness of the display is enhanced even in the video rate driving speed. The other parameter that benefit to the video rate is the longer wavelength of the selective reflection and low threshold voltage of the display, for example, the infrared wave band Ch-LC. The black and white reflective Ch-LCD with lower driving voltage and lower viscosity liquid crystal formulation due to the longer
helical pitch of Ch-LCD will result in a faster driving speed. While the red color monochrome Ch-LCD looks poor for conventional display mode although the driving voltage is lower than that of other wave band monochrome displays. Therefore, by using full spectrum reflective method, a reflective black and white Ch-LCD with video speed can be achieved.
color filter
The black and white display can be easily converted to a full color display in the same way as a reflective STN or a reflective TFT display does. By using conventional color filter process to coat the red, green, and blue three color filters on the Ch-LCD, the full color Ch-LCD can be readily achieved, as shown in Figs. 2-12.(a) and (b).
Ch-LCD
(a) Structure (b) Configuration
Figs. 2-12.(a) Structure and (b) Configuration of full color Ch-LCD.
Besides, the full spectrum reflective method also can be used for transflective displays. The configuration of display is shown in Fig. 2-13.
Fig. 2-13. Transflective Ch-LCD with full spectrum reflective method.
The transflective display structure includes a Ch-LC cell, two circular polarizers, a transflective reflector and backlight. The transflective Ch-LCD basically has the same structure as the black and white full spectrum reflective display as
concerned. The difference is an addition of the backlight system and the half-reflection-half-transmission reflector. The backlight can be turned on in a dim ambient light. The transflective display’s principle is described as following. When the display works in the planar state, the light beam emitted by the backlight passes the transflective film and reaches the second circular polarizer and consequently becomes circular polarized. The polarized light then reaches the Ch-LC cell, a portion of the light which is Bragg reflected by the Ch-LC will pass through the second circular polarizer and the rest of it will move toward to pass the first circular polarizer without attenuation. At this time, the viewer can observe a monochromic bright light, which is different from the full spectrum reflective method mentioned above. When the Ch-LC is in the focal conic texture, the light emitted from the backlight passes the transflective film and the second circular polarizer being by cut more than 50%, consequently, becomes circular polarized. The remaining light reaches the Ch-LC cell with focal conic state and is depolarized by the scattering effect of Ch-LC. The depolarized light further passes the first circular polarizer with more than 50% loss.
The light rays out of the display’s front surface are kept a large angle distribution due to the scattering of Ch-LC. Therefore, the viewer’s eye can only collect a small portion of light. Thus, by use of filtration of polarizers and scattering effect of Ch-LC, the display can yield dark state, though the darkness is not as good as the reflective mode because of shorter optical path of backlight.
When the display works in combination of backlight and transflective reflector,
When the display works in combination of backlight and transflective reflector,