Liquid Crystal Displays

Liquid crystals were first discovered in 1888, by Austrian botanist Friedrich Reinitzer [7]. Reinitzer observed that when he melted a curious cholesterol-like substance (cholesteryl benzoate), it first became a cloudy liquid and then cleared up as its temperature rose. Upon cooling, the liquid turned blue before finally crystallizing. This experiment showed the intermediate properties, between those of liquid and solids that a LC possesses. Eighty years later, in 1968, the first experimental LCD was synthesized.

How can liquid crystals be used in displays?

The properties of liquid crystals which make them suitable for use in displays are; their ability to affect the path of plane polarized light and their reaction to changes of temperature. Thermotropic liquid crystals are a type of liquid crystal which reacts to changes in temperature. This class of liquid crystals is subdivided into isotropic, where molecules have a random order and nematic liquid crystals where all the axes through the centre of the molecules are aligned. The nematic phase of liquid crystals is the one most often used in LCD displays. The molecules are rod-shaped and can move with respect to each other but with their molecular long axes, n, remaining aligned (Figure 2) [9]. Changes in this alignment of the nematic phase are seen when an external electric field is applied. Twisted versions of these nematic phases are used in the formation of pixels. A twisted nematic liquid crystal is trapped between two parallel glass sheets with polarizers at 90° to each other placed on either side of this sandwich. The nature of the liquid crystal is such that the orientation of a beam of polarized light will be turned through 90° allowing it to pass, unchanged, through the pixel as it will now be at the correct angle to pass through the second polariser. This causes the device to be in an on state and glowing. On application of an external electric field the liquid crystal molecule will realign so that polarized light is not transmitted by the molecule and as such does not pass through the second polarizer (Figure 3) [10] turning that pixel off. A series of transistors is used to turn on and off pixels in order to make up an image.

Passive versus active matrices [10]

An active matrix is a way of arranging transistors such that each pixel can be accessed individually. A pixel gets voltage from transistors within the matrix’s column turning it on. [When the voltage is turned off other rows within the matrix are activated whilst charge remains on the required pixel. After all the rows within a screen have been addressed the display is refreshed. This process occurs very quickly and prevents picture distortions as it reduces the "cross-talk" voltage that causes picture interference. This is in contrast to a passive matrix where, as can be seen in Figure 4, there are no transistors and cross-talk is greater.
The transistors used in active matrices are known as TFTs or thin film transistors. TFTs consist of deposited layers of different materials making up a semiconductor layer, an insulating layer and electrodes.Two terminals of the transistor conduct current and the third switches the device on and off. They can be deposited on virtually any surface including glass; this is favourable due to its relatively low cost. The manufacturing of TFTs involves purifying a glass layer to remove alkali metals then in a process pioneered by Corning the molten glass is poured onto a very thin sheet and a semiconductor layer deposited on top by a plasma process using silane at very low pressure. This involves an electrical discharge ionizing the silane whose fragments condense onto the glass as a hydrogen-rich network of silicon. The hydrogen is important in tying up broken bonds that might otherwise trap electrons and disrupt the semiconductor. P.K. Wemer of RCA first invented TFTs in 1962 but it was not until 1974 that their use as switches in displays was demonstrated and even then, manufacturing costs stood in the way of mass-production. Use of amorphous silicon was pioneered in 1979 giving favourable and cheap results. The combining of LCD and TFT technologies was pioneered in the mid 1980s by several companies, mostly in Japan, and was by the end of the 80s producing excellent image quality in colour displays of 10inch size or more diagonally. As costs fell and technology advanced many companies produced larger and larger sized screens. The first real LCD screens were in laptops but by the early 1990s LCDs became available for desktop PCs and more recently televisions.
The number of pixels present in a screen is a factor in the relative expense of TFT/ LCD monitors. For example in a 20 inch screen there are 2 million pixels and it takes only a few malfunctioning cells to ruin the image. This means there are a higher number of rejected monitors produced in a certain batch, which in turn raises the price of those which pass the quality tests.