Chapter 6 Applications
6.4 Full color cholesteric displays
Full color displays are more appealing to user. Conventional full color Ch-LCD was fabricated by stacking three layers of RGB (Red, Green and Blue) colors of cholesteric cells, as described in chapter 2. However, there are some drawbacks of the method. Stacking three cholesteric cells results in the device thick and heavy and parallax will decrease the resolution.
Our approach to full color application is to achieve a broad band reflection covering the entire visible spectrum, i.e., from 450 to 650 nm. We used full spectrum reflective method to realize a black and white reflective cholesteric display. Since the reflected light is white, so we can pattern conventional color filters for obtaining full color displays, as shown in Fig. 6-3. The fabrication processes are compatible with conventional process.
Figs. 6-3.(a) Structure and (b) Configuration of full color Ch-LCD.
6.5 Summary
Finally, we summarize the advantages of Ch-LCD e-book products.
1. Long-term image memory.
2. Low power consumption.
3. Real static images: No flicker.
4. Lightweight.
5. Wide viewing angle: ±80° can be achieved.
6. Low cost.
7. No image parallax.
8. Excellent sunlight readability.
However, full color ability and low operation voltage are important issues for future application. For full color application, we proposed full spectrum reflective method to achieve a black and white cholesteric display. With wide band reflection in bright state, the reflected light is white. Therefore, by conventional color filters process, full color reflective cholesteric displays can be realized. Besides, high operation voltage about 50V is a drawback of cholesteric displays. From the experimental results, we find the operation voltage of infrared band cholesteric LC can be below 25V, thus the display can be compatible with standard driver IC. As a result, the low operation voltage full color cholesteric displays are more suitable for E-book application.
Chapter 7
Conclusions
As the Internet and the computer related industries growth continues, the electronic information display is an important technology. High brightness, high readability, wide viewing angle, low power consumption and high color saturation are the main concerns. Low power consumption and light weight are main advantages of reflective LCDs. Among all reflective LCDs, bistable reflective Ch-LCDs are best solution to e-book application due to the merits of lower power consumption, low cost, and good readability. However, due to the limitation of cholesteric LC materials, the reflective spectrum is narrow band. Therefore, the display is usually monochromic appearance, which often can not satisfy the user’s requirement. Black and white displays are the least desired for viewers. In order to solve the problem of narrow band reflection, a new method “Full Spectrum Reflective Method” is proposed. The characteristic of this method is to use two reflective spectra: one is the spectrum of cholesteric LC, the other is the spectrum of reflector compensating each other to broaden the spectrum of the display. Wide band reflection can display white images instead of monochromic images. Besides, the dark state is created by cholesteric’s scattering effect in focal conic state and polarizer’s filtration effect. Therefore, black and white reflective cholesteric LCD can be demonstrated. A typical LCD cell process was utilized to carry out the Ch-LCD test cells due to its convenient manufacturing process. Finally, “ConoScope” was utilized to characterize the properties of the fabricated test cells and compared with simulation results.
In the simulation, we established a simulation model by LCD simulation
software “DIMOS” used to characterize the features of the reflective Ch-LCDs. We utilized green band, UV band, and infrared band Ch-LC materials to optimize the optical properties of Ch-LCDs. From the simulation results, Ch-LCD with UV band Ch-LC can be wide band reflection, thus, enabling a black and white display.
In the experiments, “ConoScope” was utilized to measure reflective spectra, reflectance, contrast ratio, viewing angle and voltage response. In term of reflective spectra, UV band Ch-LCD has the widest reflective spectrum and high reflectance of 50%, which agreed with the simulated results. In term of contrast ratio, infrared band Ch-LCD has the highest C.R. of 10, which is much higher than conventional reflective Ch-LCD’s. Therefore, the proposed method can improve the image quality of Ch-LCD. Besides, high operation voltage about 50V is the drawback of conventional Ch-LCD’s. However, by using infrared band Ch-LC material, the operation voltage can be decreased to 25V, which can be implemented by standard STN driver. As a result, the cost and manufacturing yield can be improved.
We used full spectrum reflective method to achieve wide band reflection covering the entire visible spectrum to realize black and white reflective cholesteric displays. For full color applications, since the reflected light is white, so we can pattern conventional color filters for obtaining full color displays.
Finally, it can be concluded that UV band or infrared band Ch-LCDs with full spectrum reflective method can have advantages of wide band reflective spectra, high contrast and low driving voltage. The Ch-LCDs are suitable for E-books applications.
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