ABSTRACT
The nematic phase of DLCs has made very signiƒcant progress over the last three decades since their discovery. It has made its way from a mere scientiƒc curiosity to application in commodities and has emerged as a potential candidate for many technological applications. Although liquid crystals (LCs) have diverse applications, such as temperature sensing, solvents in chemical reactions, in chromatography, in spectroscopy, in holography, etc., they are primarily recognized by their ubiquitous presence in electro-optical display devices such as watches, calculators, telephones, personal organizers, laptops, ¨at panel televisions, etc. [1]. The twisted nematic (TN) and supertwisted nematic (STN) display devices have been dominating commercial displays since their invention [2]. The LC layer in these devices is exclusively composed of a mixture of calamitic LCs [3,4]. Simple TN displays were directly addressed followed by multiplex-addressing for complex information display. However, the LC molecules constituting the display cannot respond fast enough to such addressing, which results in poor contrast. STN displays followed in the mid-1980s to address this problem but suffered from the same limitations along with the inherent generation of interference colors. However, these problems were solved by the development of active matrix (AM) addressing of the twisted nematic device through the use of thin ƒlm transistors (TFTs). Such technology allows the advantages of multiplex-addressing with no loss of contrast but is complex and expensive. Hence, cheaper and easily constructed multiplex-addressed TN and STN devices were dominating in most of the applications in the early 1990s. TN active matrix TFT technology developed steadily, eventually became much cheaper and much more reliable and consistent, and is invaluable in satisfying the needs of small portable devices, such as personal organizers, cameras, mobile telephones, laptops, desktop monitors, and some small televisions. However, TN displays suffered from two major problems: the narrow-viewing-angle characteristics and the slow optical response speed. These are severe limitations for large-area television and fast-moving graphics displays. To improve the viewing angle characteristics and response speed, other LCD formats like in-plane switching (IPS) [5,6], multi-domain vertical alignment (VA) LCDs [7-9], fringe-ƒeld switching [10-12], etc., were introduced into the market. But these are not very cost effective for smaller displays. When there was intense cost competition among the various LCD modes, a negative birefringence optical compensation ƒlm was introduced by Fuji photo ƒlm laboratory to widen the viewing angle characteristics and to increase the contrast ratio (CR) of TN TFT-LCDs owing to the advantages of high light transmittance, good process margin, and cost effectiveness of TN modes [13]. The optical compensation ƒlm is nothing but a ƒlm made from a hybrid alignment of discotic nematic LCs by photopolymerization.
