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TFT LCD Structure
For a more detailed explanation of TFT LCD Structure or LCDs in general, please refer to TFT LCD Basic Knowledge or Introduction to LCDs.
The TFT LCD, or thin film transistor liquid-crystal display, is a popular form of display technology often used in computer monitors and other common device screens. This display module, or more specifically LCD module, is composed of three key layers. The deepest layer, closest to the back of the device is composed of, listed from farthest to closest to the surface, the first polarizer, a glass substrate, pixel electrodes, and TFTs. The surface-most layer is similar to this layer, as it also has a glass substrate, a polarizer, and (in some matrixes) electrodes; however, the order of these components are flipped compared to the other layer (the polarizer is the closest to the surface), and there is an RGB color filter in this layer. Between these two layers, a layer of liquid crystal molecules exists and carries charges and energy towards the surface of the TFT LCD. The crystal molecules can be aligned in a variety of ways to alter the viewing properties of the LCD screen.
As an active-matrix LCD device, the TFT LCD’s individual pixels consist of red, green, and blue sub-pixels, each with their own TFT and electrodes beneath them. These sub-pixels are controlled individually and actively, hence the name active-matrix; this then allows for smoother, fast response time. The active-matrix also allows for larger display modes that continue to uphold quality of color, refresh rate, and resolution when aspect ratio is increased.
Within the pixels composing the TFT LCD display, electrodes play a role in conducting the circuit between them. If layered on both insides of the two glass substrates, the electrodes, along with the TFT, create an electrical pathway within the liquid crystal layer. There are also other placements of electrodes besides on the surface and back of the device that change the effect of the electrical pathway between the substrates (to be discussed later in this article). This pathway has an effect on the crystals through its electric field, which is one of the TFT concepts responsible for the low, minimized power consumption of TFTs, making them so efficient and appealing.
When the electric field interacts with the liquid crystal molecules, the molecules can align in a variety of ways, changing how light passes through from the backlight of the device (found behind the backmost polarizer) to the surface. Because LCD screens cannot illuminate themselves, a backlight is needed to provide lighting that the TFT LCD complex then manipulates. The liquid crystals polarize the light to different degrees, and consequently, the surface polarizer passes different levels of light through it, thus controlling the pixel’s color and brightness.
TN (Twisted Nematic) Type TFT LCD
Though there are a variety of ways to align the crystal molecules, using a twisted nematic (TN) to do so is one of the oldest, most common, and cheapest options for LCD technology. It uses the electric field between the electrodes organized with one on the surface substrate layer and the other on the back substrate layer to manipulate the liquid crystals.
When no electric field affects the structure of the crystals, there is a 90 degree twist in the alignment. This twist allows for light to move through the layer, polarizing the light as it passes to then go through the surface polarizer.
If an electric field is applied, the twist in the crystal structure of the molecules can be unwound, straightening them out. When this happens, the light is not polarized and cannot pass through the surface polarizer, displaying a black pixel. It is also possible to create an in between of the fully lit or fully opaque pixel; if the light is partially polarized (the electric field does not fully straighten the crystal alignment), then a middle luminance level of light is emitted from the LED backlights through the polarizer.
Though this is one of the cheapest options for display technology, it has its own problems. The TN TFT LCD does not have top response times compared to other types, and it does not provide for as wide a viewing angle as other TFT LCDs using different alignment methods. A viewing angle is the direction at which a screen can be looked at before the displayed image cannot be seen properly in terms of light and color. TN displays mostly struggle with vertical viewing angles but also have somewhat limited horizontal angles as well. This TN LCDs viewing angle limit is called the gray scale inversion issue.
There are several ways to resolve the gray scale inversion issue.
Generally, when viewing angle is not ideal, image quality as a whole decreases. Things like contrast ratio (the luminance ratio between the brightest white and darkest black) and readability of the screen are not preserved due to this issue.
Among the methods of liquid crystal alignment, TN is only one option for LCD technology. There are various other common ways to align the crystals for a wide viewing angle, such as the multi-domain vertical alignment or in-plane switching. In addition, because of the abundance of TN devices, something called O-film has also been introduced to pair with TN screens so that users do not have to buy whole new devices.
MVA (Multi-domain Vertical Alignment) TFT LCD
Simply put, this method divides the cell beneath each pixel into multiple domains. With the division, molecules in the same cell can be oriented differently, and so as users shift their views of the display, there are different crystal directional alignments that allow for the preservation of the display properties over these angles such as high brightness and high contrast. This solves the problem of what is known as a mono-domain vertical alignment.
Though mostly similar to the TN, the MVA has one notable feature in its cell that TN cells do not have: glass protrusions. Between the sandwiching electrodes, angles glass protrusions reorient the light traveling within the layer so that when exiting the surface polarizer, it travels in a multitude of directions to satisfy the need for a wide viewing angle.
In recent developments of the MVA TFT LCD, contrast ratio, brightness, and response times have all increased in quality. Contrast ratio, being 300:1 when first developed in 1997, has been improved to 1000:1. Similarly, response time, characterized by rising (black to white) and decay (white to black) time, has reached times that are the fastest that human eyes can process, demonstrating the appropriateness of MVA-based displays for moving images.
IPS (In-Panel Switching) TFT LCD
Another solution to the gray-scale inversion issue caused by TN is the IPS LCD. In terms of benefits of the IPS, it is rather similar to the MVA. But structurally, rather than having surface and back electrodes, the IPS places both electrodes on the back substrate. This then forces the molecules to, when the electric field is on, switch orientations, known as plane switching, and align in a parallel manner to the substrates rather than perpendicularly like in TN devices. A brighter backlight is needed in this case, as the light will need more power to produce the same display brightness that the TN may be able to do with less light from the source.
With this type of alignment, viewing angles were preserved in much wider directions compared to the TN. Recently, IPS displays have improved qualities like response time to make the IPS screens more desirable to consumers. However, this type of TFT LCD will tend to cost more than TN devices.
TN vs O-Film vs MVA vs IPS TFT LCD
While the TN TFT LCD has the smallest cost, that is for a reason. O-films, MVAs, and IPS TFT LCDs have greater costs due to their more intricate technologies that improve viewing angle to retain resolution and general display quality.
The O-film specifically is unique because rather than changing the liquid crystal alignment technology and for a relatively low cost, it can swap the surface polarizer of a TN device with a special film to widen the viewing angle. Because it is combined with TN, it can only improve viewing angle slightly.
IPS has the most potential for improved viewing angle, reaching higher possible angles than all the other options. With IPS, though, there is a higher power consumption than the regular TN device due to the need for a brighter backlight in this device.
MVA is close, only slightly less, to the IPS TFT LCD in angle. What it does have, though, is a much faster response time, as stated earlier.
All these technologies are viable options depending on the consumer’s desires and price range. MVA and IPS TFT LCDs tend to be more practical for consumer products like LCD monitors and phone screens, while TN and O-film LCDs can cross over into industrial applications. Nonetheless, with the growth of the IPS and MVA LCDs, their applications are widening.
AFFS (Advanced Fringe Field Switching) TFT LCD
The AFFS is similar to the IPS in concept; both align the crystal molecules in a parallel-to-substrate manner, improving viewing angles. However, the AFFS is more advanced and can better optimize power consumption. Most notably, AFFS has high transmittance, meaning that less of the light energy is absorbed within the liquid crystal layer and more is transmitted towards the surface. IPS TFT LCDs typically have lower transmittances, hence the need for the brighter backlight. This transmittance difference is rooted in the AFFS’s compact, maximized active cell space beneath each pixel.
Since 2004, Hydis, who developed the AFFS, has licensed the AFFS to the Japanese company Hitachi Displays, where people are developing complicated AFFS LCD panels. Hydis has improved display properties like outdoor readability of the screen, making it even more appealing to use for its main application: mobile phones displays.
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