How Voltage-Controlled Liquid Crystals Work in TFT LCDs

Thin-Film-Transistor Liquid Crystal Displays (TFT LCDs) create images by using electrical voltage to manipulate the orientation of liquid crystal molecules, which in turn controls the passage of polarized light through crossed filters. This article explains the fundamental physics and electronics behind this process, detailing how unpolarized backlight is filtered, twisted by nematic liquid crystals, and regulated by microscopic transistors to control pixel brightness and form the images seen on modern screens.

The Role of Crossed Polarizing Filters

At the core of an LCD panel are two polarizing filters placed at a 90-degree angle to one another, known as crossed polarizers. The first filter, located directly in front of the display's backlight, only allows light waves vibrating in a single plane (for example, vertically) to pass through. The second filter, located near the front of the screen, is oriented horizontally.

Under normal circumstances, vertically polarized light from the first filter cannot pass through the horizontal second filter. Without an active mechanism between them to alter the light's orientation, the screen would remain completely black.

The Twisted Nematic Liquid Crystal State

Sandwiched between these two crossed filters is a layer of nematic liquid crystals. The inner surfaces of the substrates holding the liquid crystals are treated with microscopic grooves that align the molecules in a specific direction. Because the grooves on the front and back substrates are perpendicular to each other, the liquid crystal molecules naturally form a helical, 90-degree twisted structure from one side of the layer to the other.

When no electrical voltage is applied, this twisted structure acts as a waveguide. As vertically polarized light enters the liquid crystal layer, the light's polarization plane follows the 90-degree twist of the molecules. By the time the light reaches the second filter, it has been rotated to a horizontal orientation. This matches the alignment of the second filter, allowing the light to pass through and making the pixel appear bright.

Voltage Control via Thin-Film Transistors

To create an image, the display must be able to block this light on a pixel-by-pixel basis. This is achieved using Thin-Film Transistors (TFTs) and transparent electrodes made of Indium Tin Oxide (ITO). Each sub-pixel on a screen has its own tiny transistor that acts as a precise electrical switch.

When a TFT applies a specific voltage to the electrodes flanking the liquid crystal layer, an electric field is generated. Liquid crystal molecules carry a slight electrical charge and are sensitive to this field. As voltage is applied, the molecules abandon their natural twisted structure and untwist to align themselves parallel to the electric field—pointing straight from the back of the display to the front.

Light Blocking and Grayscale Modulation

Once the liquid crystal molecules are straightened out by the electric field, they no longer guide or rotate the light. The vertically polarized light travels straight through the untwisted liquid crystals without changing its orientation. When this vertical light hits the horizontally aligned second filter, it is completely blocked, and the pixel appears dark.

TFT LCDs do not just switch between fully on and fully off; they also produce millions of colors by controlling grayscale levels. By varying the exact amount of voltage applied by the thin-film transistor, the liquid crystals only partially untwist. A partial twist rotates the light only slightly, meaning only a fraction of the light wave aligns with the second polarizer. Precise voltage control allows the electronics to dictate the exact percentage of light that escapes through each red, green, and blue sub-pixel, blending together to form a full-color image.