Liquid Crystal Display Technology (Part 1)

Liquid Crystal Display (LCD) technology, with its relatively lower requirements for device fabrication and high cost-effectiveness, continues to dominate the mainstream market for display panel sales. LCD is a passive-type display, with the device consisting of two main modules: the liquid crystal panel and the backlight module. The backlight module emits a white light source, which is then modulated for color and brightness at each pixel by the liquid crystal panel, ultimately displaying a moving color image.

The backlight module is composed of LED beads and optical films. The point light source from the LEDs is diffused and homogenized by the optical films to form a uniform surface light source that illuminates the liquid crystal panel. The white light source is a mixture of the three primary colors: red, green, and blue. After passing through the liquid crystal panel, specific colors and brightness are formed through the combined action of the liquid crystal switches corresponding to each sub-pixel cell. An entire 4K panel, consisting of approximately 24.88 million sub-pixel cells, composes one frame.

The structure of the liquid crystal panel is shown in the figure below. In addition to the glass substrates carrying various functional films, the layers from bottom to top are: the lower polarizer, the Thin-Film Transistor (TFT) driving circuit, the liquid crystal layer, the Color Filter (CF), and the upper polarizer.

As the name suggests, liquid crystals are crystalline substances with liquid-like properties, possessing both the flow characteristics of liquids and the vector properties of crystals, which grant them unique mechanical, optical, and electrical characteristics. As shown in the following photo, the liquid crystal panel, through the coordination between the liquid crystals and the upper and lower polarizers, performs switching and dimming actions on the incident light from the backlight, creating different brightness levels (grayscale). Combined with the function of the color filter, different colors and brightness levels are formed.

So, what changes occur when backlight enters the liquid crystal panel?

First, the bottom and top layers of the liquid crystal panel are equipped with polarizers oriented perpendicularly to each other. Then, the area is divided into countless cells according to the pixel resolution. Each pixel is subdivided into red, green, and blue sub-pixel cells. Each cell, from bottom to top, is equipped with an independent electrode, liquid crystal layer, and color filter layer. The upper and lower polarizers, together with the liquid crystal, form a "gate" that controls the on/off state of light transmission for that cell, as well as the intensity of the transmitted light. The different light intensities transmitted through the three sub-pixel cells, after passing through the red, green, and blue color filters, mix in varying proportions to form the display color and brightness of the pixel.

So, how does this combined gate work?

Backlight, similar to natural light, is a transverse electromagnetic wave. The vibration direction of the particles is perpendicular to the direction of wave propagation, meaning the light's vibration direction is always within the x-y plane, perpendicular to the propagation direction (z-axis). When backlight rays hit the lower polarizer of the liquid crystal panel, only light vibrating in the same direction as the polarizer's grating (y-axis direction) can pass through, becoming linearly polarized light propagating with a specific vibration direction. As shown in Figure 3, when the pixel electrode on the TFT substrate is not energized, the polarized light passes through the liquid crystal layer and the CF layer without changing its vibration direction, and reaches the upper polarizer. However, the grating direction of the upper polarizer is perpendicular to the y-axis (x-axis direction). Therefore, the polarized light vibrating in the y-axis direction cannot pass through the upper polarizer, resulting in a dark state observed from the front of the screen.

When the TFT circuit electrode is energized, the liquid crystal molecules rotate. The polarized light emerging from the lower polarizer has its polarization direction twisted under the guidance of the liquid crystal molecules. With specific settings for liquid crystal thickness and rotation angle, the vibration direction of the polarized light can be twisted exactly by 90°. At this point, the vibration direction of the polarized light becomes parallel to the x-axis, aligning with the grating direction of the upper polarizer, allowing it to pass through. If the vibration direction forms an angle with the grating direction, the component of the light wave's energy along the grating direction can pass through the polarizer, and the original light intensity is attenuated. By changing the TFT electrode voltage, the rotation angle of the liquid crystals can be adjusted, thereby controlling the intensity of light transmitted through the upper polarizer.

The Electro-Optical Characteristics of Liquid Crystals

Why can an electric field cause liquid crystals to rotate? And why does liquid crystal rotation change the vibration direction of polarized light?

As shown in the figure below, liquid crystal molecules have a rod-like structure and possess the anisotropy of crystals. They exhibit different electro-optical effects, dielectric constants, and refractive indices along their long and short axes. We can utilize these properties to alter the intensity of incident light for each pixel cell, thereby forming color and grayscale.

Dielectric Anisotropy of Liquid Crystals

--- The dielectric constant (ε) of liquid crystal molecules differs along their long and short axes. When the dielectric constant parallel to the long axis is greater than that perpendicular to the long axis (ε// > ε⊥), it is called positive-type liquid crystal with positive dielectric anisotropy, suitable for parallel alignment. When ε// < ε⊥, it is called negative-type liquid crystal with negative dielectric anisotropy, which can only be used in vertical alignment to achieve the desired electro-optical effect. When an external electric field is applied, the turning direction of the liquid crystal molecules—whether parallel or perpendicular to the electric field—is determined by whether the dielectric anisotropy is positive or negative, which in turn determines whether light is transmitted. Currently, VA-type liquid crystals commonly used in TFT-LCDs mostly belong to the negative dielectric anisotropy type. When energized, the liquid crystal molecules are polarized by the external electric field, causing their long axes to tilt to a direction perpendicular to the field. IPS commonly uses positive-type liquid crystals, while some ADS Pro panels use negative-type liquid crystals.

Birefringence of Liquid Crystals

--- Liquid crystal molecules possess birefringence and optical rotation; the refractive index (n) differs along the long and short axes. The refractive index along the long axis is nO, and along the short axis is nE. 

Because the refractive indices are different, the speeds of light are different. When liquid crystals rotate, the light speeds along the long and short axes differ, causing a phase difference between the outgoing ordinary (O) and extraordinary (E) rays compared to their incident state, resulting in a phase retardation phenomenon. When the light beam exits the liquid crystal, the O and E light vectors recombine, producing a new vibration direction that is rotated. Through special design of optical path length and rotation angle, the phase difference between the outgoing O and E rays can be set to 1/2 wavelength. This causes the phase of the outgoing light to rotate by 90° compared to the incident light, allowing it to pass through the upper polarizer. Different rotation angles of the liquid crystals determine different phase differences between the O and E rays, thereby controlling the intensity of the transmitted light.

Types of Liquid Crystal Panels

Based on the rotation mode of liquid crystals, they are further divided into three types: TN (Twisted Nematic), IPS (In-Plane Switching), and VA (Vertical Alignment). Among them, TN-type LCD panels were the earliest used, with the lowest cost. However, due to their native 6-bit color (low color gamut) and very low viewing angles, they have essentially exited the mainstream LCD display panel market. The current mainstream LCD displays primarily use IPS and VA panels.

As shown in the figure, IPS display panels have liquid crystal molecules arranged horizontally. The positive and negative pixel electrodes are on the same horizontal plane on the TFT substrate. When not energized, the polarized light passing through the lower polarizer goes through the liquid crystal without a change in polarization direction. At this time, the vibration direction of the polarized light is perpendicular to the grating direction of the upper polarizer, and light cannot pass through. When energized, due to the presence of the electric field, the liquid crystal molecules rotate within the horizontal plane (in-plane switching). Due to birefringence, the polarized light decomposes into two light rays with different speeds. Upon exiting the liquid crystal, these two polarized lights have a phase difference and recombine into a new type of polarized light. This polarized light then passes through the color filter, with light from each sub-pixel cell presenting red, green, and blue colors. The polarized light that penetrates the color filter reaches the upper polarizer. Having been twisted by the liquid crystal to a new angle, the light from each sub-pixel has a different rotation angle. The components of polarized light that have different angles relative to the upper polarizer's grating pass through, resulting in red, green, and blue colors of different intensities from each sub-pixel cell. These three colors of varying intensity mix to form the desired colored pixel. The colors from 24,883,200 sub-pixel cells (3840 * 2160 * 3) combine to form the colors of 8,294,400 pixels (3840 * 2160), creating one frame of a 4K resolution image. The liquid crystals, changing with the electric field at a specific frequency, continuously alter the vibration angle of the polarized light, forming frame after frame, ultimately achieving video display.

The working principle of VA displays is the same as that of IPS panels. As shown in Figure 6, the arrangement of liquid crystal molecules differs from IPS panels. In VA displays, the liquid crystal molecules are vertically aligned, and the positive and negative pixel electrodes are distributed on the upper and lower planes, creating an electric field in the vertical direction.

When no voltage is applied to the electrodes, the liquid crystal molecules are aligned perpendicularly to the upper and lower substrates. The linearly polarized light passing through the lower polarizer propagates parallel to the long axis of the liquid crystal molecules, so its polarization state does not change and it cannot pass through the upper polarizer. The panel observed by the human eye is in a dark state. When no or low voltage is applied, polarized light passes along the long axis of the liquid crystal molecules. Due to the birefringence of the liquid crystals, polarized light along the long axis basically does not deviate, thus it cannot pass through the upper polarizer, resulting in very black dark-state performance, high contrast ratio, and excellent image sharpness. When voltage is applied to the electrodes, the liquid crystal molecules rotate under the action of the vertical electric field, with their long axes tilting to a direction perpendicular to the electric field. Components of the incident linearly polarized light entering the liquid crystal will experience phase retardation within the liquid crystal layer. After leaving the liquid crystal layer, the components of polarized light recombine, and the polarization state of the light changes. The vibration direction of the polarized light from each sub-pixel cell ultimately completes a偏转 to different angles according to the set voltage. After passing through the color filter and the upper polarizer, sub-pixel colors of different intensities are obtained. The red, green, and blue sub-pixel colors with different intensity ratios mix to form the set pixel color, finally creating a complete frame for display.