Liquid crystal displays (LCD) are well known for their thin, lightweight construction, power-efficient designs, and excellent image reproduction. However, they have inherent problems such as narrow viewing angle, slow response time and high color variation. These problems are caused by the dielectric anisotropy and mechanical viscosity of liquid crystal molecules, which have a confined region and high rigidity.
To solve these problems, several wide viewing angle technologies such as wide view (WV) films, vertical alignment (VA), in-plane switching (IPS), and fringe field switching (FFS) have been developed. Among those proposed and recently developed, FFS and IPS are the only original liquid crystal modes that do notrequire additional optical compensating films. Unlike IPS and VA devices, FFS can give rise to high transmittance due to the homogeneous rotation of aligned LC molecules in co-plane fringe field with high density.
To commercialize high performance LCD applications by single technology, electro-optical characteristics to be achieved are free of color shift and provide authentic color reproduction with wide viewing angle, a fast response time with all grays and a low operating voltage, high transmittance with low power consumption, and low cost. However, it is technically difficult to achieve all these electro-optical performances due to the original limitations of LC molecules, such as the low dielectric anisotropy, high viscosity and use of the crossed polarizer.
This article examines advanced fringe field switching (AFFS) as a new standard technology for future LCD applications.
1-Pixel Design
AFFS offers many advantages such as high transmittance by folded transparency ITO electrodes, fast gray-to-gray response time and free of color shift by the auto compensation of the polarized light. AFFS (V.2) has a 95% transmittance of the TN-LCD (100%), but operation voltage and transmittance of other technologies (IPS, PVA) are larger by 5V and lower 80%, respectively.
In previous devices, the LC with the old FFS pixel concept was used, so the transmittance was lower than 80% of TN-mode; a dark area existed due to the wedge deformation at the pixel edge. LCs with advanced pixels were achieved by removing the wedge deformation and scaling down the black matrix. LC cell parameters are optimized for maximum light efficiency and to obtain about 95% of that of TN LC. Using AFFS technology, an unlimited viewing angle and good color characteristics can be achieved. This was from upgrading the one-pixel design rule for the optimization of electrodynamics in the LC molecules without high aperture resin technology.
Various AFFS Applications
AFFS technology provides improvements to viewing angles and image color in mobile displays; it offers ultra low power consumption and high transmittance, which are ideal for mobile phones. The Fig shows the flower images in a 2.2-inch QVGA LCD by AFFS for the mobile version with a color reproduction of 45% and a wide operation temperature range from -30*C to 80*C.
Contrast ratio was improved to over 600:1 after application of advanced pixel concepts with narrow black matrix and low resistance. The AFFS performs well when the panel is touched or pressed by a pen or a finger as it is free of ripple and pooling, which makes it suitable for a Tablet PC.
To realize CRT-like performance of the display image on LCD-TVs, color variation/contrast ratio with wide viewing angle, fast response time and high transmittance (pure darkness) are crucial. In the viewpoint of dielectric anisotropy, AFFS has a unique approach trend due to the reverse behavior of the positive LC in the fringe field that differs from others (IPS(+LC), VA(-LC)).\
High reliability LC should have characteristics such as wide temperature (Tni>=100*C, Tsn<=-40*C), little change of optical performance and operating range of -30 to 80*C. Advanced FFS panels with various LCs show different electro-optical characteristics. In avionic displays, military devices have to overcome extreme temperatures in operation. In medical displays, absolute darkness and uniformity are crucial.
Technical Improvements
AFFS pixel concept is the result of optimum LC dynamics for conventional FFS mode. The technology has the potential to become a leading-edge mode for displays.
AFFS pixel has realized authentic color with high gradation ratio. In vertical direction to cell gap, LC molecules are tilted upward at over 45 degrees near the surface of the bottom substrates. In longitudinal direction, LC molecules are twisted at over 45 degrees at the midplane and it decreases down to 7 degrees. Polarized lights transmitted through the cell can be auto-compensated by formation of liquid molecules. The improvement is achieved by:
1. Precise control of the LC molecules at the mid-plane and pixel edge of each cell;
2. Reduction of black matrix and edges of pixel electrodes, enlargement of aperture;
3. Auto-compensation of polarized light by LCs of multi-directional aligned with fringe electrode.
New Technology Standard
AFFS technology was defined as an optimization of LC dynamics in the conventional FFS mode. The new device shows high performance for all LCD applications from mobile to large LCD-TV. It also shows an unlimited viewing angle and is color shift free. The AFFS TFT-LCD shows intrinsically high image/color quality comparable to CRT displays and a fast response time.
The device is believed to greatly impact all TFT-LCD applications with high transmittance, very low power consumption and fast response time. AFFS device can also be applied to position and pressure sensitive touch panel systems owing to its intrinsic characteristic of stable dynamics against external pressure such as tablet and smart PCs. Sunlight readability increases with improved ultra low surface reflectance and high transmittance obtained from optimization of the 3-dimensional study for the one-pixel structure of the ultra high ppi panel.
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