The twisted nematic effect (TN effect) was a major technological breakthrough that made the manufacture of large, thin liquid crystal displays practical and cost competitive. Unlike earlier flat-panel displays, TN cells did not require a current to flow for operation and used low operating voltages suitable for use with batteries. The introduction of TN effect displays led to their rapid expansion in the display field, quickly pushing out other common technologies like monolithic LEDs and CRTs for most electronics. By the 1990s, TN-effect LCDs were largely universal in portable electronics, although since then, many applications of LCDs adopted alternatives to the TN effect such as in-plane switching (IPS) or vertical alignment (VA). Many monochrome alphanumerical displays without picture information still use TN LCDs. TN displays benefit from fast response times and less smearing than other liquid crystal display technologies, but suffer from poor color reproduction and limited viewing angles, especially in the vertical direction. Colors will shift, potentially to the point of completely inverting, when viewed at an angle that is not perpendicular to the display. Viewing the display from above whitens colors, and viewing the display from below dims colors.
Description 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 achieved with little power consumption and at low operating voltages. The underlying phenomenon of alignment of liquid crystal molecules in applied field is called Fréedericksz transition and was discovered by Russian physicist Vsevolod Frederiks in 1927. To display information with a twisted nematic liquid crystal, transparent electrodes are structured by photolithography to form a matrix or other pattern of electrodes, such as the seven-segment display used in low-information content applications like watches or calculators. Only one of the electrodes has to be patterned in this way, the other can remain continuous (common electrode). If more complex data or graphics information have to be displayed, a matrix arrangement of electrodes is used. Because of this, voltage-controlled addressing of dot-matrix displays, such as in LCD screens for computer monitors or flat television screens, is more complex than with segmented electrodes. For a matrix of limited resolution or for a slow-changing display on even a large matrix panel, a passive grid of electrodes is sufficient to implement passive matrix addressing, provided that there are independent electronic drivers for each row and column. A high-resolution matrix LCD with required fast response (e.g. for animated graphics and/or video) necessitates integration of additional non-linear electronic elements into each picture element (pixel) of the display (e.g., thin-film diodes, TFDs, or thin-film transistors, TFTs) in order to allow active matrix addressing of individual picture elements without crosstalk (unintended activation of non-addressed pixels). The following illustrations show the OFF and ON states of a single pixel (which could instead be a segment of a character) of a twisted nematic light modulator liquid crystal display operating in the "normally white" mode, i.e., a mode in which light is transmitted when no electrical field is applied to the liquid crystal:
OFF state (transparent)
In the OFF state, i.e., when no electrical field is applied, the nematic liquid crystal molecules form a twisted configuration (aka helical structure or helix) between the two glass plates, G in the figure, which are separated by several spacers and coated with transparent electrodes, E1 and E2. The electrodes themselves are coated with alignment layers (not shown) that precisely twist the liquid crystal by 90° when no external field is present. Incoming light is first polarized by the first polarizer, P2. The helical configuration of the liquid crystal rotates the light's polarization by 90°, so the light will be properly polarized to pass through the second polarizer, P1, set at 90° to the first. Because the light passes through the cell, the pixel, I, appears transparent.
ON state (opaque)
In the ON state, i.e., when a sufficient electrical field is applied between the two electrodes, the crystal molecules align in the direction of that field. Without the helical configuration of the liquid crystal to reorient the light's polarization angle, polarized light from polarizer P2 is instead blocked by polarizer P1, so the pixel, I, appears opaque. Current is only needed to charge and discharge the capacitance of the corresponding LC cell, which happens only when the applied voltage changes. Current isn't needed to sustain the electric field, because no current (ideally) flows through the liquid crystal layer. Thus, LCDs require very little power. However, the electric field's direction may need to be periodically reversed during the ON state by using an alternating voltage for "AC operation", because keeping the electric field in only one direction for too long during the ON state (or having a DC component as small as 50 mV in the AC voltage) may cause electrochemical reactions which reduce the cell's life.
Semi-transparent The amount of opacity can be controlled by varying the voltage. Below a threshold voltage, which depends on the liquid crystal's mixture, no visual change occurs. At voltages near the threshold, only some crystals will realign, so the cell will be mostly transparent but just barely visible. As the voltage is increased, more crystals will realign until the cell reaches its maximum opacity. Already in 1972, mixtures were developed with a threshold voltage of only 0.9 V rms and which reached 90% of maximum opacity at 1.4 V rms.
History
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