1. What is liquid crystal

Matter has three states: solid, liquid and gas. The following LCD manufacturers talk about the difference between STN liquid crystal and TFT liquid crystal. In fact, the so-called three states is only a general distinction, some substances can be subdivided into different properties of the solid state. Similarly, the liquid can also have a different "state", in which the molecular arrangement of the liquid with directionality we call "liquid crystal", referred to as "liquid crystal".

Since general solid crystals are oriented, many of their physical properties are oriented as well. Liquid crystals have the fluidity of liquids while having the orientation of normal crystals. If you want to change the orientation of a solid crystal you must rotate the entire crystal, unlike liquid crystals whose orientation can be controlled by an electric or magnetic field.

Both STN and TFT use a substance called "Nematic" liquid crystal, which is in the form of a filament, and the use of an electric field to control the direction of the "filament" liquid crystal is a common method of application. Components made from liquid crystals are usually made by encasing liquid crystals in two pieces of glass. The surface of the glass is plated with a layer of a substance called a directional agent, and the arrangement of the liquid crystals in the absence of an external electric field can be controlled by its type and processing method.

2. Principle of STN liquid crystal

The world's first liquid crystal display appeared in the early seventies, called the TN type liquid crystal display (Twisted Nematic). In the eighties, STN type LCD (Super Twisted Nematic) appeared, while TFT LCD (Thin Film Transistor) technology was proposed.

We'll start with the principle of TN type liquid crystal, STN LCD and TN LCD display principle is the same, just the liquid crystal molecules of the distortion angle is different.

To the column type liquid crystal sandwiched between two pieces of glass, the surface of this glass is first coated with a layer of transparent and conductive film for the electrode, and then there is a thin film electrode on the glass coated with surface orienting agent, in order to make liquid crystals aligned with a specific and parallel to the direction of the glass surface. The natural state of the liquid crystal has a 90-degree twist, the use of electric field can make the liquid crystal rotation, the refraction coefficient of the liquid crystal with the direction of the liquid crystal and change, the effect of the result is that the polarization of the light through the TN-type liquid crystal after the change. As long as you choose the appropriate thickness so that the polarization of light just change 90 °, you can use two parallel polarizers so that the light can not pass completely. And large enough voltage can make the liquid crystal direction parallel to the direction of the electric field, so that the polarization of light will not change, the light can pass through the second polarizer. Thus, the light and darkness of the light can be controlled. As mentioned earlier, STN-type liquid crystals have the same display principle as TN-type liquid crystals, except that it rotates the incident light by 180 to 270 degrees instead of 90 degrees. Moreover, the simple TN-type LCD display itself has only two variations of light and dark. STN liquid crystals, on the other hand, are dominated by light green and orange colors. However, if the traditional monochrome STN liquid crystal display plus a color filter, and each pixel in the monochrome display matrix is divided into three sub-pixels, respectively, through the color filter to display the red, green and blue primary colors, you can display the color.

3. TFT liquid crystal principle

Due to the limitation of the display principle of TN and STN type liquid crystal, if its display part is bigger and bigger, the electrode reaction time of the center part may be longer. In fact, this is not a big problem for cell phones, because the current cell phone display are relatively small, the impact of the liquid crystal reaction time is relatively small. But for the notebook and other devices that require a large screen LCD, too slow LCD response time will seriously affect the display effect, so TFT LCD technology has attracted the attention of manufacturers. And, the color screen in the cell phone is used more and more, in the new generation of products many support 65536 color display, some even support 160,000 color display, this time the TFT high contrast, colorful advantage is more important.

LCD manufacturers of STN-type liquid crystal belongs to the reflective LCD device, its benefits are small power consumption, but in a darker environment with poor clarity, so it has to be equipped with an external lighting source. TFT liquid crystal using "back" and "reflective" combination of the way, in the back of the liquid crystal set up special light pipe. This is why we see some cell phone display seems to have "lighting", while some cell phone light seems to be the display itself from the reason. Moreover, LCD manufacturers produce LCD backlighting technology is constantly advancing, from monochrome to color, from thick to thin, from side-mounted fluorescent lamps to flat-panel fluorescent lamps.

By the way, the reflective LCD device has a black background white characters (NB) and white background black characters (NN) two kinds of, we recently saw the V70 display is belong to the NB type, of course, this is certainly the integration of the latest technology to enhance the NB. well, we are still to return to the subject, continue to talk about the display principle of the TFT LCD. TFT LCD technology adopts an "active matrix", "the active matrix", "the active matrix", "the active matrix". TFT LCD technology utilizes an "active matrix" method of driving. Method is to use thin-film technology made of transistor electrodes, the use of scanning methods, "active" control of any one display point on and off. When the light source is illuminated, the light is transmitted upward through the lower polarizing plate, and the liquid crystal molecules are used to conduct the light. When the electrodes are turned on, the liquid crystal molecules change their state of arrangement just like TN liquid crystals, and the purpose of the display is achieved by shading and transmitting light. This sounds similar to the display principle of TN-type liquid crystals, and it is. But the difference is that, because the FET transistor has a capacitive effect, can maintain the potential state, has been transmissive liquid crystal molecules will remain in this state, until the next time the FET electrode is charged again to change its arrangement. TN type liquid crystal does not have this characteristic, liquid crystal molecules once no electric field, immediately return to the original state, this is the TFT liquid crystal and TN liquid crystal display principle of the biggest difference.

TFT liquid crystal for each pixel is equipped with a semiconductor switch, the process is similar to large-scale integrated circuits. As each pixel can be directly controlled by the point pulse, therefore, each node is relatively independent, and can be continuously controlled, this design not only improves the response speed of the display, at the same time can accurately control the display of gray scale, so the TFT liquid crystal color is more realistic.