Chapter 4 Simulation Results and Discussions
4.3 Simulation of Reflective Spectrum
To simulate the reflective spectrum of green band cholesteric LC, we use the following LC material parameters: no=1.509, ne=1.698, εo=4.1, εe=14.3, pitch=0.343um, cell gap d=5um. DIMOS allows a spatial varying pitch by defining pitch values at certain levels within the layer. We want to define a uniformly twisted cholesteric layer with 15 full turns. Uniform twist is described by constant pitch of Ch-LC.
Next we define the variation range for optics calculations. Define only one variation: the wavelength of reflective light from 400nm to 700nm with 1 nm increment.
The simulated reflective spectrum result is shown in Fig. 4-1. From the simulation, we find that the reflection band begins roughly at 510nm and ends at 590nm. The center wavelength of reflected light is 550nm, the green light band.
Because the birefringence of Ch-LC material is small, the reflected light usually has narrow band reflection. Therefore, it is only a monochromic display. From the simulation, the bandwidth of reflected light is about 80nm.
0
400 450 500 550 600 650 700
Wavelength(nm)
400 450 500 550 600 650 700
Wavelength(nm)
400 450 500 550 600 650 700
Wavelength(nm)
400 450 500 550 600 650 700
Wavelength(nm)
Reflectivity(%)
Δλ~80nm
cell gap=5um
Fig. 4-1. Simulated reflective spectrum of green band Ch-LC. (d=5um)
0
400 450 500 550 600 650 700
Wavelength(nm)
400 450 500 550 600 650 700
Wavelength(nm)
400 450 500 550 600 650 700
Wavelength(nm)
400 450 500 550 600 650 700
Wavelength(nm)
Reflectivity(%)
Δλ~80nm
cell gap=10um
If we change the cell gap to 10um and 15um, the simulated reflective spectrum results are shown in Figs. 4-2. and 4-3.
Fig. 4-2. Simulated reflective spectrum of green band Ch-LC. (d=10um)
0
400 450 500 550 600 650 700
Wavelength(nm)
400 450 500 550 600 650 700
Wavelength(nm)
400 450 500 550 600 650 700
Wavelength(nm)
400 450 500 550 600 650 700
Wavelength(nm)
Reflectivity(%)
Δλ=80nm
cell gap=15um
Fig. 4-3. Simulated reflective spectrum of green band Ch-LC. (d=15um) From these simulations, we find that the reflective spectrum becomes more and more a rectangle as cell gap increases. In other words, the reflected light becomes to focus on central wavelength range. However, bandwidth of reflective spectrum is almost the same. It is still a narrow band reflection. Therefore, increasing cell gap of Ch-LC can not broaden the reflective spectrum greatly.
Afterwards, we try to simulate other reflection band Ch-LC material. We choose reflection band in shorter wavelength range, UV band Ch-LC. We use the following
0
150 160 170 180 190 200 210 220 230 240 250 Wavelength(nm) gap d =5um. We define the variation range for optics calculations: the wavelength of reflective light from 150nm to 250nm with 1 nm increment. The result is shown in Fig.
4-4.
Δλ~20nm
cell gap=5um
1
Fig. 4-4. Simulated reflective spectrum of UV band Ch-LC. (d=5um) From the simulation result, the center wavelength of reflected light is 200nm, the UV light range. The bandwidth of reflected light is about 20nm. For wide band reflection, the performance of UV band Ch-LC is still not suitable.
Thus, we try to simulate other reflection band Ch-LC. We choose reflection band in longer wavelength range, infrared band Ch-LC. We use the following LC material parameters: no=1.517, ne=1.749, εo=6.5, εe=23.3, pitch=0.459um, cell gap d =5um. We define the variation range for optics calculations: the wavelength of reflective light from 600nm to 900nm with 1 nm increment. The result is shown in Fig. 4-5.
0
600 650 700 750 800 850 900
Wavelength(nm)
600 650 700 750 800 850 900
Wavelength(nm)
600 650 700 750 800 850 900
Wavelength(nm)
600 650 700 750 800 850 900
Wavelength(nm)
Reflectivity(%)
Δλ~100nm
cell gap=5um
Fig. 4-5. Simulated reflective spectrum of infrared band Ch-LC. (d=5um)
From the simulation result, the center wavelength of reflected light is 750nm, the infrared light range. The bandwidth of reflected light is about 100nm. The bandwidth is larger than UV band Ch-LC and green band Ch-LC. But for wide band reflection, the performance of infrared band Ch-LC is still not suitable.
Narrow band reflection is the intrinsic property of Ch-LC material. Because Ch-LC only reflects specific band of light, and most of other bands are absorbed.
Therefore, it is usually monochromic appearance. As described in chapter 2, we proposed a new method “Full Spectrum Reflective Method” to broaden the reflective spectrum in order to fabricate black and white display. Thus, we do some simulations to verify the method.
Conventional Ch-LCD usually utilizes black absorption layer to yield dark state.
However, out of selective reflection band light is absorbed. The reflective spectrum is totally determined by Ch-LC. Thus, our new method utilizes reflective layer instead of black absorption layer. Out of selective reflection band light can be reflected by reflective layer to compensate the spectrum of Ch-LC. Besides, the dark state is obtained by the filtration of polarizers. Here we utilize green band Ch-LC material to simulate. The simulation results are shown in Figs. 4-6.(a)(b)(c)(d).
Fig. 4-6(a) is simulated configuration of proposed Ch-LCD.
Fig. 4-6(a). Simulated configuration of proposed Ch-LCD.
Fig. 4-6(b) is the simulated spectrum of reflector.
97.5 98.0 98.5 99.0
400 450 500 550 600 650 700
Wavelength(nm)
Reflectivity(%)
Fig. 4-6(b) The simulated spectrum of reflector.
The reflectivity of the reflector in DIMOS is 98.5%~98% over the visible band range, thus, the reflector can be assumed an ideal wide band reflector.
The spectrum of proposed Ch-LCD is simulated in Fig. 4-6(c). From the result, we can find the spectrum covers three parts: 400nm~500nm (blue light), 500nm~600nm (green light), and 600nm~700nm (red light). But higher reflectivity is in red light range. Therefore, the color of the Ch-LCD is slightly shifted to red light range. The color distribution is illustrated in CIE 1931 color coordinate, as shown in Fig. 4-6(d).
400 450 500 550 600 650 700
Wavelength(nm)
400 450 500 550 600 650 700
Wavelength(nm)
Reflectivity(%)
cell gap=5um
Fig. 4-6(c). Simulated reflective spectrum of green band Ch-LCD. (d=5um)
Fig. 4-6(d). Color profile of green band Ch-LCD.
Then we utilize UV band Ch-LC material to simulate full spectrum reflective method. The simulated Ch-LCD configuration is shown in Fig. 4-7. The simulated spectrum is shown in Fig. 4-8.and the color distribution is illustrated in CIE 1931 color coordinate, as shown in Fig. 4-9.
Fig. 4-7. Simulated configuration of UV band Ch-LCD.
48.0 49.0 50.0
400 450 500 550 600 650 700
Wavelength(nm)
Reflectivity(%)
Fig. 4-8. Simulated reflective spectrum of UV band Ch-LCD.
From the simulation results, the reflective spectrum can cover all visible light range and has high reflectivity. Thus, it can display white images in bright state, as shown in Fig. 4-9. Though the UV band LC reflects invisible light, out of selective reflection band light is reflected by the wide band reflector. In other words, the spectrum is mainly determined by the wide band reflector instead of Ch-LC material.
By this method, the spectrum can be broadened greatly to become black and white reflective Ch-LCD.
Fig. 4-9. Color profile of UV band Ch-LCD.
Besides, we also simulate infrared band Ch-LC with this method. The simulated configuration is shown in Fig. 4-10.and the simulated spectrum is shown in Fig. 4-11.
Besides, the color profile of the display is illustrated in CIE 1931 color coordinate, as shown in Fig. 4-12.
Fig. 4-10. Simulated configuration of infrared band Ch-LCD.
From the simulated reflective spectrum, reflective light covers all visible light range and the reflection peak is in the green light range. Therefore, the color of the display is slightly shifted to the green light range, as shown in Fig. 4-12. Besides, the reflectivity of the infrared band Ch-LCD is about 20%, which is lower than UV band and green band Ch-LCD.
0 5 10 15 20 25
400 450 500 550 600 650 700
Wavelength(nm)
Reflectivity(%)
Fig. 4-11. Simulated reflective spectrum of infrared band Ch-LCD.
Fig. 4-12. Color profile of infrared band Ch-LCD.
4.4 Summary
Finally, we make some comparisons and conclusions about full spectrum reflective method with green light band, UV light band, and infrared light band different Ch-LC materials. Our objective is to have a broad band reflection of Ch-LCD in bright state. In DIMOS simulation, the full spectrum reflective method can actually broaden the reflective spectrum of Ch-LCD. In term of reflective spectrum, the proposed method with UV band Ch-LC material has the widest reflection band, which can cover all visible light bands. As a result, The Ch-LCD can appear white images instead of monochromic images. Therefore, this method can improve image quality of Ch-LCD. In addition, reflective brightness is an important issue for reflective display application. In term of reflectivity, the reflectivity of conventional Ch-LCD is close to 50%. However, full spectrum reflective method utilizes extra polarizers and retardation films to compensate the reflective spectrum.
Thus, the reflectivity of this method is lower than conventional Ch-LCD’s. Among the three Ch-LC materials, UV band Ch-LC material has the highest reflectivity of 49%,
which is much close to reflectivity of conventional Ch-LCD’s. Therefore, full spectrum reflective method with UV band Ch-LC material can achieve broad band reflection and high reflectivity. The comparisons of the new method with the three Ch-LC materials are listed in Tab. 4-1.
Tab. 4-1. Comparisons of three Ch-LC performances.
Green band Ch-LC
UV band Ch-LC
Infrared band Ch-LC
Spectrum Narrow band Full
visible band Broad band
Color
Slight shift to red light
White appearance
Slight shift to green light Reflectivity About 25% About 49% About 20%
Chapter 5
Experimental Results and Discussions
5.1 Introduction
According to previous described fabrication in chapter 3 with conventional LCD fabrication process, reflective Ch-LCD test cells are fabricated. We choose 3 inch in diagonal and 0.7mm thickness glass substrate. In term of rubbing process, we utilize four different conditions: no rubbing, top and bottom rubbing (parallel direction), top and bottom rubbing (reverse direction), bottom rubbing. The cell gap of test cell is expected to 3.5um to lower the driving voltage of Ch-LC.
Besides, we utilize different reflection band Ch-LC materials. MDA-00-3461 Ch-LC (central wavelength=550nm, green light band) and MDA-02-3885 Ch-LC (central wavelength=200nm, UV light band) were provided by MERCK company[32]. RDP-95155ChBZ1 Ch-LC (central wavelength=750nm, infrared light band) was provided by DAINIPPON INK AND CHEMICALS company[33]. The detail specifications of the three Ch-LC materials are listed in Tab.5-1.
Tab. 5-1. Specifications of three Ch-LC materials.
Green band
5.2 Measurement Results
We utilize measurement instrument “ConoScope” to measure the electro-optical properties of the fabricated Ch-LCD test cells. The reflectivity, reflective spectrum, color appearance, viewing angle, contrast ratio, and voltage response were measured and will be discussed.
0%
380 431 481 531 580 628 676 722 768 Wavelength(nm)
380 431 481 531 580 628 676 722 768 Wavelength(nm)
5.2.1 Measured Reflective Spectrum and Reflectivity
First, we measured green band Ch-LCD with conventional and full spectrum reflective method, respectively. The test cells are both no rubbing process. Fig. 5-1.
shows the measurement results, (a) is the spectrum of conventional method and (b) is spectrum of full spectrum reflective method. The central wavelength of green band Ch-LCD is 560nm and the bandwidth is about 80nm if 20% reflectivity is acceptable.
The peak reflectivity is about 50%. With the proposed method, the bandwidth can increase to 150nm. However, the peak reflectivity is decreased to 40%, because the absorption effect of extra polarizers and retardation films.
Fig. 5-1. Measured reflective spectrum of (a) conventional and (b) full spectrum reflective method.
Second, we measured UV band Ch-LC with full spectrum reflective method. We use four different rubbing conditions test cells. Fig. 5-2. shows the spectrum of no
rubbing condition. From the measurement, the spectrum with 20% reflectivity covers from 470nm to 690nm. It is a wide band reflection to display white images in bright state. Besides, the reflectivity is about 50%, which is close to the conventional Ch-LCD with high reflectivity.
0%
400 430 461 490 521 550 580 611 641 670 700 Wavelength(nm)
400 430 461 490 521 550 580 611 641 670 700 Wavelength(nm)
Reflectivity
Δλ~ 220nm
Fig. 5-2. Measured reflective spectrum of UV band Ch-LCD.
(no rubbing condition)
Then we measured reflective spectrum of top and bottom glasses with parallel rubbing direction condition, as shown in Fig. 5-3.
0%
400 430 460 490 520 550 580 610 640 670 700 Wavelength (nm)
400 430 460 490 520 550 580 610 640 670 700 Wavelength (nm)
Reflectivity (%)
Δλ~ 250nm
Fig. 5-3. Measured reflective spectrum of UV band Ch-LCD.
(parallel rubbing direction condition)
The reflective spectra of reverse rubbing direction condition and only bottom rubbing condition are shown in Figs. 5-4 and 5-5, respectively.
400 430 460 490 520 550 580 610 640 670 Wavelength (nm)
400 430 460 490 520 550 580 610 640 670 Wavelength (nm)
Reflectivity
Δλ~ 180nm
Fig. 5-4. Measured reflective spectrum of UV band Ch-LCD.
(reverse rubbing direction condition)
Fig. 5-5. Measured reflective spectrum of UV band Ch-LCD.
0%
400 430 460 490 520 550 580 610 640 670 700 Wavelength (nm)
400 430 460 490 520 550 580 610 640 670 700 Wavelength (nm)
Reflectivity
Δλ~ 200nm
(bottom rubbing condition)
Compared with the four rubbing condition, the spectra are all wide band reflection over visible light band. Besides, test cell of both surfaces rubbed with parallel direction has higher reflectivity. Thus, different rubbing conditions can affect the reflective spectrum of the displays. Because the domains in the planar texture of bistable reflective Ch-LCD are created by introduction of defects, which are
introduced from the surfaces of the cell structure. The surface of typical Ch-LCD is usually an unrubbed polyimide (PI) alignment layer. The non-homogeneity of the surface results in non-uniform liquid crystal alignment. Therefore, the planar texture has many defects.
These defects reduce the on-axis brightness of the displays as well as the degree of circular polarization. However, the defects play an important role in the viewing angle and the bistability of the display. The viewing angle is increased because of the wide distribution of the helical axes. The reflection from the unrubbed PI layer of the planar texture is diffuse and is nearly lambertian. Therefore, the appearance of the display is close to printed paper. This is a highly desirable property making this kind of display an ideal choice for electronic paper application.
The distribution of the helical axes and the defects are controlled to increase the brightness near the surface normal while maintaining a good viewing angle. A balance is obtained between the defect density, domain size, and distribution. Thus, the result has a brighter texture than conventional bistable planar texture and has a wide viewing angle. The defects control is achieved by rubbing the PI layer.
There are several methods to enhance the brightness of the display. The display can be made by using hybrid alignment, only one of the two PI surfaces is treated.
Depending on which surface is treated, near the viewer or farther from the viewer, has an impact on brightness and appearance. When the surface near the viewer is treated, the display has a bright and shiny appearance, but the viewing angle is decreased.
However, when the surface away from the viewer is treated, the display is bright and more diffusive appearance.
Rubbing the PI surfaces also can have an impact on the polarization state of the reflected light. Contrary to the conventional thought, the reflected light from a stabilized planar texture, from an unrubbed cell is not actually circularly polarized.
The degree of polarization is quite low. Because there are large number of defects in the planar texture. The scattering from the defects results in reflected light that is not circularly polarized. However, the degree of polarization can be increased by treating the PI layers to reduce the number of defects.
It is instructive to compare the microscopic domain structures that have various rubbing conditions. Three planar texture photographs are shown in Fig. 5-6. Fig. 5-6(a) is for a conventional planar texture with unrubbed surface alignment. Small randomly aligned domains can be clearly seen. Fig. 5-6(b) is the hybrid aligned cell where one surface is a rubbed PI layer and other is an unrubbed PI layer. The photograph is taken from the rubbed side. The small domain structure seems to be better defined. In addition, there are some larger domains spread randomly between the smaller domains. It is these larger domains that increase the degree of circular polarization, and increase the brightness of the display. Fig. 5-6(c) is the photograph for the cell with both surfaces rubbed. It shows large planar domains with very few defects.
(a) (b)
(c)
Fig. 5-6. Microscope texture photographs for the planar texture with various rubbing conditions. (a) both surfaces unrubbed (b) only one surface rubbed and (c) both surfaces rubbed.
Besides, we also measured the reflective spectra of Ch-LCD with infrared band LC material with four rubbing conditions. Fig. 5-7 shows the spectrum of no rubbing condition. The peak reflectivity is about 40% and the spectrum can cover all visible light band to reflect white light in bright state. In addition, compared with UV band Ch-LCD, the bandwidth of infrared band Ch-LCD is narrower than UV band Ch-LCD.
400 430 460 490 520 550 580 610 640 670 700 Wavelength (nm)
400 430 460 490 520 550 580 610 640 670 700 Wavelength (nm)
Reflectivity
Δλ~ 200nm
Fig. 5-7. Measured reflective spectrum of infrared band Ch-LCD.
(no rubbing condition)
Other spectra of rubbing condition are shown in Figs. 5-8, 5-9 and 5-10.
0%
400 430 460 490 520 550 580 610 640 670 700 Wavelength (nm)
400 430 460 490 520 550 580 610 640 670 700 Wavelength (nm)
Reflectivity
Δλ~ 200nm
Fig. 5-8. Measured reflective spectrum of infrared band Ch-LCD.
(parallel rubbing direction condition)
As described in UV band Ch-LCD, the reflectivity of rubbed test cell is larger than unrubbed test cell. Thus, by rubbing process, the brightness of Ch-LCD can be improved.
400 430 460 490 520 550 580 610 640 670 700 Wavelength (nm)
400 430 460 490 520 550 580 610 640 670 700 Wavelength (nm)
Reflectivity
Δλ~170nm
Fig. 5-9. Measured reflective spectrum of infrared band Ch-LCD.
(reverse rubbing direction condition)
400 430 460 490 520 550 580 610 640 670 700 Wavelength (nm)
400 430 460 490 520 550 580 610 640 670 700 Wavelength (nm)
Reflectivity
Δλ~170nm
Fig. 5-10. Measured reflective spectrum of infrared band Ch-LCD.
(bottom rubbing direction condition)
Among the four rubbing conditions, test cell of both side rubbed with parallel direction has higher reflectivity about 45% than others.
Conventional cholesteric displays reflect a narrow bandwidth due to the limited birefringence of Ch-LC. However, by utilizing full spectrum reflective method, the spectrum can be broadened to produce wide band reflection. Based on the
measurement results, Ch-LCDs with UV band or infrared band LC materials have wide band reflection in bright state. Besides, brightness is an important issue for reflective displays. In term of reflectivity, UV band Ch-LC material of both surfaces rubbed with parallel direction has the highest reflectivity of about 50%, in good agreement with the simulation results shown in chapter 4. Thus, it is wide band reflection with high brightness. Therefore, Ch-LCD can be a black and white display instead of a monochromic display.
In addition, we also use four different rubbing conditions to fabricate the test cells. From the measurement results, both surfaces rubbed with parallel direction condition has the highest brightness. The result agrees with the expectation. Because when LC molecules align well by rubbing process, the defects can be reduced greatly in planar texture. Most of ambient light can be reflected by cholesteric LC molecules instead of scattering. Therefore, the brightness can be improved.
5.2.2 Measured Reflectance distribution
Reflectance is important factor of displays. We use diffuse light source to measure reflectance of the test cells. Fig. 5-11 shows the measurement result of conventional Ch-LCD with green band Ch-LC material
Fig. 5-11. Reflectance distribution of conventional Ch-LCD.
0%
5%
10%
15%
20%
25%
30%
-80 -60 -40 -20 0 20 40 60 80
Viewing angle (°)
Reflectance
0%
5%
10%
15%
20%
25%
-80 -60 -40 -20 0 20 40 60 80
Viewing angle (°)
Reflectance
From the measurement results, we find reflectance distribution of conventional Ch-LCD is ±40° and the peak reflectance is 28%. The brightness of light source is about 1900 nits.
We also measured Ch-LCD with full spectrum reflective method. The results are shown in Fig. 5-12.
Fig. 5-12. Reflectance distribution of Ch-LCD with full spectrum reflective method.
The reflectance distribution of Ch-LCD with full spectrum reflective method is
±35° and the peak reflectance is 20%. Compared with the reflectance distribution of two methods, the reflectance of full spectrum reflective method is smaller than conventional method, because new method utilizes polarizers and retardation films to yield dark state. Therefore, some light are absorbed by the optical films.
Besides, we also measured full spectrum reflective method with UV band LC material. The results are shown in Fig. 5-13. The test cell is both surfaces rubbed condition. From the measurement results, the reflectance distribution is ±70° and the peak reflectance is 45%. Compared with reflectance distribution of green band and UV band Ch-LC material, the reflectance distribution of UV band Ch-LC material is
much larger than green band Ch-LC material. The performance is acceptable for reflective displays application. The reflected light angle distribution of Ch-LCD with reflection in invisible band is larger than reflection in visible light band, because the
much larger than green band Ch-LC material. The performance is acceptable for reflective displays application. The reflected light angle distribution of Ch-LCD with reflection in invisible band is larger than reflection in visible light band, because the