There are many types of liquid crystal displays, each with
unique properties. The most common LCD that is used for everyday
items like watches and calculators is called the twisted
nematic (TN) display. This device consists of a nematic
liquid crystal sandwiched between two plates of glass. A special
surface treatment is given to the glass such that the molecules
are homeotropic yet the director
at the top of the sample is perpendicular to the director at the
bottom. This configuration sets up a 90 degree twist into the
bulk of the liquid crystal, hence the name of the display. The
twist is visible in the following animation.
This structure is similar to the cholesteric state, and sometimes a small amount of a chiral material is added to ensure a uniform twist.
The underlying principle in a TN display is the manipulation of polarized light. When light enters the TN cell, the polarization state twists with the director of the liquid crystal material. For example, consider light polarized parallel to the director at the top of the sample. As is travels through the cell, its polarization rotates with the molecules. When the light emerges, it's polarization has rotated 90 degrees from when it entered.
A schematic of a TN cell is shown in the following animation. The black lines represent crossed polarizers that are attached to the top and bottom of the display. As light enters the cell, its polarization rotates with the molecules. When the light reaches the bottom of the cell, its polarization vector has rotated by 90 degrees, and now can pass through the second polarizer. In a reflecting TN display, a mirror is placed at the bottom of the cell to reflect the transmitted light. Once again the polarization twists as the light traverses the sample, and is able to emerge from the top of the cell. The following animation shows how light entering the cell twists along the way.
Light emerging from such a cell appears the familiar silver-gray color. As explained in the Electric and Magnetic Field Effects section, when an electric field of sufficient magnitude is applied to a sample, the molecules undergo a Freederickzs transition. The following animation shows the transition that happened in a twisted nematic cell.
Note that in this state, the twist is destroyed. The director of the bulk liquid crystal is parallel to the field and no longer twisted. When polarized light enters a cell in such a configuration, it is not twisted, and is canceled by the second polarizer. Regions where an electric field is applied appear dark against a bright background. The following animation demonstrates light traversing a cell with an applied electric field.
Liquid Crystal Display Construction
Before going into any further detail about the physics behind displays, it is probably best to step through the various components of a liquid crystal display and briefly mention their functions. As mentioned in the previous section, a liquid crystal display is composed of multiple layers. First, a sheet of glass is coated with a transparent metal oxide film (shown as a blue layer in the animation below) which acts as an electrode. This film can be patterned to form the rows and columns of a passive matrix display or the individual pixels of an active matrix display. These electrodes are used to set up the voltage across the cell necessary for the orientation transition. Next, a polymer alignment layer is applied (shown in red). This layer undergoes a rubbing process which leaves a series of parallel microscopic grooves in the film. These grooves help align the liquid crystal molecules in a preferred direction, with their longitudinal axes parallel to the grooves. (see Surface Preparations) This anchors the molecules along the alignment layers and helps force the molecules between the alignment layers to twist. Two such sheets of glass are prepared and one is coated with a layer of polymer spacer beads (the slightly green glassy layer). These beads maintain a uniform gap between the sheets of glass where the liquid crystals are eventually placed. The two glass sheets are then placed together and the edges are sealed with epoxy. A corner is left unsealed so that the liquid crystal material can be injected under a vacuum. Once the display has been filled with liquid crystals, the corner is sealed and polarizers (the transparent layers with lines) are applied to the exposed glass surfaces. In a TN display (which is shown in the animation below) the alignment layers are positioned with their rubbing directions perpendicular to each other and the polarizers are applied to match the orientation of the alignment layers. In an STN (super-twisted nematic) display (which will be discussed in the next section) the alignment layers are placed with their rubbing directions at a variety of angles to one another to set up a twist from 180 to 270 degrees and the polarizers are not applied parallel to the alignment layers.
The display is finished off by completing the connections to the driving circuitry which controls the voltage applied to various areas of the display (pixels).
Depending on the field strength, twisted nematic displays can switch between light and dark states, or somewhere in between (grayscale.) How the molecules respond to a voltage is the important characteristic of this type of display. The response of a typical twisted nematic cell to an applied voltage is shown in the following diagram (called an electro-distortional curve). The tilt of the molecules out of the plane of the glass slides is measured as a function of the applied voltage.
In the TN display, the electro-distortional response determines the transmission of light through the cell. Percent transmission as a function of voltage is shown in the following diagram. Keep in mind that maximum transmission for a reflective TN device is only 50 percent because polarized light must be used.
The vertical lines represent the voltages at which the cell is OFF or ON. In order to address many pixels with a multiplexing scheme (see Display Addressing), the differences in the OFF and ON voltages must be very small. This was difficult to achieve with the traditional TN structure. This problem was solved with the invention of the super-twisted nematic (STN) display.