Twisted Nematic field effect (TN effect) edit
The main breakthrough in the development of liquid crystal displays (LCDs) was the invention, patenting and publication [1] of the twisted nematic (TN)-effect by M. Schadt and W. Helfrich (Swiss patent No. 532 261 with the priorty date of December 4, 1970). The twisted nematic field effect, developed in the Central Research Laboratories of Hoffmann-La Roche (Switzerland), marks a paradigm change in flat panel display technology. In contrast to displays based on dynamic scattering (dynamic scattering mode LCDs, DSM-LCDs or DS-LCDs) or light emitting diodes (LEDs), which are current-driven and therefore power-consuming devices, the twisted nematic effect is based on the precisely controlled realignment of liquid crystal molecules between different ordered molecular configurations under the action of an applied electric field. This is essentially achieved without power consumption and at low operating voltages. The new effect required the liquid crystal molecules to be twisted in the OFF-state. Moreover, it required two light-absorbing polarizers, a controlled surface alignment and novel liquid crystal materials (LCs) for its operation. The major technological trends of LEDs, DS-LCDs, cathode ray tubes (CRTs), etc. at the time were against the new field-effect display technology.
The illustrations to the right show both the OFF and the ON-state of a single picture element (pixel) of a twisted nematic light modulator in transmissve mode of operation. A twisted configuration of nematic liquid crystal molecules is formed between two glass plates, G, which are separated by several micrometer wide spacers and coated with transparent electrodes, E1, E2. The electrodes themselves are coated with alignment layers (not shown) to assure uniform uniaxial alignment of the elongated birefringent liquid crystal molecules with the directions of alignment on both glass substrates rotated by 90°. As a consequence of this boundary induced twisted alignment and the long-range molecular interactions, a continuous helical twist deformation is achieved in the OFF-state of the liquid crystal layer (left diagram).
If the birefringence is properly chosen, the polarization of an incident linearly polarized light wave is then guided by the liquid crystal helix. The transmitted wave may therefore pass the second, crossed polarizer, P1, causing the modulator to appear transparent. Apart from their special optical properties, the elongated nematic liquid crystal molecules designed for the twisted nematic effect comprise longitudinal (permanent) dipole moments. These act as sensors for electric fields causing their long axes to align parallel to the direction of the electric field. Therefore, if a voltage above a threshold voltage of about 1 volt is applied to the electrodes of the twisted nematic LC-layer, the electrical field forces the long molecular axes to align in the field direction, i.e. perpendicular to the electrodes. As a result, the twist deformation is completely unwound several volts above threshold (right diagram). Now, the polarization of an incident light wave is not affected by the vertically aligned LC-molecules and therefore it cannot not pass the second polarizer. In this ON-state the modulator appears dark (non-transparent). Obviously, the inverse optical response can be obtained with parallel polarizers. Moreover, gray-scale modulation is achieved by varying the voltage between the threshold for helix deformation and the saturation voltage. It is interesting to note that helix deformation and corresponding gray scale response are governed by elastic and dielectric forces of the liquid crystal helix; i.e. by liquid crystal material parameters and aligning anchoring forces.
To display information with a twisted nematic liquid crystal light modulator, the transparent electrodes are structured by photo-lithography. For low information content numerical and alpha-numerical TN-LCDs as they are required for digital watches, pocket calculators or other simple machine-man interfaces, segmented electrodes are sufficient. All segments are placed on one substrate of the display with a common counter electrode at the opposite substrate and are addressed individually. If more complex data or graphics information have to be displayed, a matrix arrangement of electrodes is used. Obviously, addressing of matrix displays, such as in LCD-screens for computer-monitors or flat television screens, is more complex than with segmented electrodes. These matrix LCDs necessitate integration of additional non-linear electronic elements into each picture element of the display (e.g. thin-film diodes, TFDs, or thin-film transistors, TFTs). .
Apart from well controlled 3D surface alignment at the electrode boundaries, the twisted nematic effect requires not only one but two light absorbing linear polarizers that strongly reduce the transmittance (and thus the perceived brightness) of TN-LCDs. In the 1960s the twisted nematic effect was against the dominant trend in the small liquid crystal community which searched for electro-optical effects enabling bright, polarizer-free, high contrast, easy-to-manufacture and cost-effective solutions. A new superior electro-optical effect would have to overcome the limited contrast of the polarizer free dynamic scattering effect (DS, DSM). It should also be unhampered by the poor contrast of guest-host displays which required one polarizer and absorbed more than 50% of the incident light. To avoid polarizers, some researchers suggested in 1968 to dissolve dichroic dyes in a twisted nematic host such that maximum dye absorption would result without a polarizer. They were of the opinion that the incident unpolarized light would traverse the liquid crystal without rotation of the direction of polarization, thus being completely absorbed in a 90°-twisted host. Contrary to Schadt and Helfrich, they failed to realize the complexity of twisted nematic configurations due to wave-guiding and polarization rotation in 1968. Because operation of twisted nematic displays requires liquid crystal molecules with strong dipole moments along their long axes, fast response times, wide temperature ranges, proper elastic and optical constants, new classes of liquid crystal materials had to be invented and developed.
Source: Martin Schadt, personal communication, 2006/2007
References edit
- ^ M. Schadt and W. Helfrich: Voltage-dependent optical activity of a twisted nematic liquid crystal. Appl. Phys. Lett. 18 (1971), pp. 127-128