Electro-optical core technologies for flat panel displays
Pioneer of Liquid Crystal Technology
Visual perception surpasses all other human senses with respect to information transfer capacity and wealth of details. Therefore, displays – being the corresponding man-machine-interface – are of utmost importance in nearly every technical system. For nearly forty years, the nowadays clearly dominating liquid crystal technology experienced a still continuing rise of importance, due to the fact that uniquely among numerous competing technologies it can address all applications starting from simple seven segment displays all the way to large area flat panel television sets.
Liquid crystals are organic substances, whose individual molecules exhibit a common orientation in space like those in a solid crystal, while the molecules center points can easily be dislocated like in a liquid. Due to the orientation order, the macroscopic dielectric and optical properties are found to be dependent on the direction of the applied fields. This anisotropy of dielectric constant and refractive index is instrumental for influencing the molecule orientation by an applied low frequency voltage and can be used for influencing the polarization properties of a light wave propagating through the liquid crystal layer. Typically, the liquid crystal is injected between two substrates coated with transparent electrodes, therefore, the electrical field created by the low frequency driving voltage is perpendicular to the substrate surfaces like in a parallel plate capacitor. The transparency of the electrodes is instrumental for directing a light wave through the liquid crystal cell consisting of the substrates and the light modulating liquid crystal layer.
For nearly seventy years after their discovery by Friedrich Reinitzer in 1888 and the subsequent fundamental investigations and naming by Otto Lehmann, liquid crystal had been a scientific curiosity known and worked on by only a small group of experts. At the beginning of the sixties, the vision of large area flat panel displays, which can be hung on the wall like a picture, continuously gained popularity. Recognizing the fact that this could not be achieved with the well established cathode ray tube, various research laboratories initiated research projects with the objective to develop alternative display technologies. Richard Williams and George Heilmeier of the RCA laboratories in the USA identified liquid crystals as an interesting candidate for such an alternative. During their subsequent investigations, they discovered and demonstrated the first liquid crystal effects that could be used for electro-optical modulation of a light wave (domain formation, guest host, dynamic scattering), which motivated worldwide research efforts to find a more practical liquid crystal technology.
With the twisted nematic cell patent filed on December 4th, 1970, the award winner and his former colleague at the Hoffmann-LaRoche research laboratories, Wolfgang Helfrich, achieved the decisive technological breakthrough that was instrumental for the commercial success of liquid crystal displays. Later it turned out that James Fergason in the USA researched on similar liquid crystal configurations, but filed his patent about two months after the Applied Physics Letters paper of Schadt and Helfrich was published.
Using a so called orientation layer, it is possible to induce a common direction for all elongated liquid crystal molecules that are directly touching the orientation layer. In a twisted nematic cell, the two substrates are assembled in such a way that orientation layers deposited on top of the transparent electrodes are enforcing perpendicular directions for the molecules on the two sides of the liquid crystal layer. Therefore, the liquid crystal molecules within the layer will form a helix like configuration with a continuously increasing twist angle if no driving voltage is applied to the electrodes. Using an appropriately chosen design of the cell, the polarization vector of a passing linearly polarized light wave will follow the twist of the liquid crystal molecules. In the typical case of a total twist angle of ninety degrees the polarization vector will also be rotated by ninety degrees when the light leaves the cell. If a low frequency driving voltage is applied between the transparent electrodes, the elongated molecules within the bulk of the liquid crystal layer will orient themselves parallel to the electrical field. This results in a non-twisted molecule configuration that does not rotate the polarization vector of the passing light wave. A polarizer behind the liquid crystal cell can then be used to convert the rotation of the polarization vector into a corresponding brightness modulation. Compared to previously known liquid crystal effects, this results in significantly higher contrast ratios at lower driving power and much faster switching speeds. In addition, it was possible to use more stable liquid crystal mixtures and the contrast ratio turned out to be relatively independent from the cell thickness which was difficult to control in the infancy of the liquid crystal technology.
Consequently, the twisted nematic liquid crystal effect became the technology of choice immediately after its invention. It was the essential technological prerequisite for the creation and the growth of the liquid crystal display industry with a current world wide turn over of approximately 100 billion US Dollars per year. Until today, twisted nematic liquid crystal cells are dominating the market up to display sizes of approximately 25 inches. Extremely large displays have much higher viewing angle performance requirements, which are addressed by improved liquid crystal effects that were developed in the nineties. Nevertheless, also these effects are critically dependent on an electrically induced modulation of the liquid crystal layer birefringence, which for the first time ever had been used in the twisted nematic cell.
During his long and extremely productive carrier, this years Technology Award winner, Dr. Martin Schadt, has shown a remarkable continuity in inventing extraordinary innovations in the areas of novel electro-optical concepts for flat panel technologies, as well as the corresponding materials and device concepts. Already 1969, before the invention of the twisted nematic cell, he and his colleague D.F. Williams at the National Research Council in Ottawa, Canada, succeeded in developing the first organic light emitting diode with solid electrodes. Therefore, his name is also directly connected with the only remaining flat panel display technology, which one day might become a successor of the liquid crystal technology. Other important milestones of his research career in the Hoffmann-LaRoche liquid crystal research and in its spin-off ROLIC Ltd are the dual frequency addressing of liquid crystal cells (1982), the deformed helix ferroelectric and short pitch bi-stable ferroelectric effects (1989, 1990) and the invention of the linear photo polymerization technology (1991) and its further development towards mass production.
Numerous prizes and awards such as the Roche Research and Development Prize (1986), the IEEE Jun-Ichi Nishizawa Medal (2008) and the Karl Ferdinand Braun Prize (1992), the highest prize award of the Society of Information Display, are documenting the worldwide recognition of Dr. Schadt’s work.
The Eduard Rhein Foundation honors Doctor Martin Schadt as an internationally renowned and distinguished Inventor, who in the course of his entire career has continuously made extraordinary contributions to the realization of flat panel displays, that can be hung on the wall, a vision shared by the founder Eduard Rhein.
Prof. Dr.-Ing. Norbert Frühauf,