Many notebook computers and desktop PC monitors are using thin-film transistor liquid crystal display (TFT-LCD).TFT-LCD is characterized by a fairly high resolution and color moving image display capabilities, but also enough display brightness. TFT-LCD thickness and weight limit its more widely used, for example, it does not meet the requirements of portable applications.


Since the 1990s, people have been trying to explore new TFT-LCDs that are thinner, lighter, and consume less power by eliminating the need for backlighting, which is called reflective TFT-LCD because it uses ambient light to reflect the image. In this article, we take the 3.8-inch reflective TFT-LCD developed by NEC's Functional Devices Research Institute as an example to illustrate how to address specific issues such as reflectors, display performance, color display, and liquid crystal mode.

Reflectors and display performance


The practice has shown that the reflector in the reflective TFT-LCD is a very critical part, improperly done will be like a haha mirror, it is ridiculous. For this reason, the reflector surface needs to be convex and concave, the incident light as a diffuse light reflection (this is like the silhouette of a clearly visible calm lake, the breeze blowing, ripples, the silhouette of the disappearance of the same situation). If the reflector's surface is made of regular convex and concave ripples, when white light is incident on the reflector, the reflected light is again a variety of colors. This surface produces the same phenomenon as a diffraction grating in a spectrometer. Because white light is composed of different wavelengths of light, each wavelength of light, its reflectable angle is different. In order to solve this problem, that is, to eliminate the reflection angle and the wavelength of light-related phenomena, the reflector needs to have an irregular convex and concave surface structure.


In general, when the incident light into an irregularly convex surface of the reflector, the reflected light can be considered to include both positive reflection and diffuse reflection of light components. The above-mentioned practical exploration, although basically clarified the reflector surface should have the structure, the design and manufacture of a practical reflector still have a lot of work to do. Since the light source environment is different between indoor and outdoor, it is necessary to optimize the design and manufacture of the reflectors according to the actual operating conditions.

Color display and liquid crystal mode


LCD panels capable of displaying color images are required for modern portable network information terminals, such as the IMT-2001 mobile phone. For reflective TFT-LCDs, which are used for color display, the outstanding challenge is to balance brightness and color display capability. For transmissive TFT-LCDs, a 3-color filter is placed on the inner wall of the liquid crystal tank. As a result, only one-third of the incident white light in each sub-pixel of the R.G.B. pixel is transmitted for observation as part of the display light that can be seen. However, because the transmitting type TFT-LCD has a back-illuminated light source, it can adequately compensate for the lack of display brightness caused by the low transmittance, so the transmitting type TFT-LCD can balance brightness and color display functions.

However, because reflective TFT-LCDs rely on ambient lighting, balancing display brightness and color display capability is a difficult research challenge. At present, the R.G.B. filter stacking method is being studied, but it is difficult to control the R.G.B. filter alone. At present, the more practical approach, still in the transmission TFT-LCD R.G.B filter structure, based on the special design.


For reflective TFT-LCD, in order to achieve both the display brightness and color display capability, it is necessary to improve the light reflectivity and strive to reduce the optical loss. For example, in a transmissive TFT-LCD, two polarizing plates are usually placed against the two walls of the liquid crystal tank, but in a reflective TFT-LCD, a single polarizing plate is used to increase the display brightness, and NEC's Functional Device Research Institute tries to eliminate the polarizing plate at all, thus eliminating the light absorption effect of the polarizing plate and increasing the display brightness.

A TFT-LCD with a guest-master liquid crystal mode can omit the polarized light plate, as shown in FIG. 1. Wherein, the box of the Guest-Host liquid crystal layer represents the liquid crystal slot, and the transparent electrode and the transparent glass plate in the slot wall are omitted, and only the reflective plate with the irregularly convex and concave surface is drawn. The working principle of the reflective TFT-LCD in the object-master liquid crystal mode is briefly illustrated using this diagram as a model. In the so-called guest-main liquid crystal, the pigment is actually mixed into the liquid crystal, in which both pigment molecules and liquid crystal molecules are also rod-shaped structure molecules. This rod-shaped pigment molecule absorbs a large amount of polarized light in the long-axis direction and a small amount of polarized light in the short-axis direction, which is an important characteristic of pigment molecules. Moreover, when the liquid crystal molecules are deflected by the voltage signal, the pointing of the pigment molecules changes as well.


Therefore, the "bright" and "dark" states can be displayed in the pattern-reflective TFT-LCD model shown in the figure. For example, when the display shows a "dark" state, the liquid crystal molecules are arranged in a twisted pattern around the Z-axis. In other words, the long axis of the liquid crystal molecules in the upper part of the liquid crystal layer is aligned in the X direction, while the long axis of the liquid molecules in the lower part of the liquid crystal layer is aligned in the Y direction. Therefore, when the light emitted from the light source is incident on the LCD panel shown in Figure 1, the linearly polarized light in the X direction at the upper liquid crystal layer is roughly absorbed, and the linearly polarized light in the Y direction at the lower liquid crystal layer is also absorbed. In fact, the liquid crystal layer is twisted to absorb the polarized light on one side and rotate the polarized light on the other. Therefore, the liquid crystal needs to have the best twisting angle around the Z-axis. Embedded Development Network


One can't help but ask, so the guest-master mode reflective TFT-LCD can display a "bright" state. The answer is yes, because when the voltage signal on the liquid crystal is large enough, the long axis of the liquid crystal molecules can be arranged along the Z direction, and the pigment molecules also show a vertical shape. As a result, the linearly polarized light emitted from the light source in the X and Y directions is hardly absorbed. As a result, the incident light passes through the liquid crystal layer and reaches the irregularly convex and concave surface of the reflector, and then is reflected out, displaying a "bright" state.

Overall balance


Throughout the key technologies of reflective TFT-LCDs, there are many constraints, and each technology has its own strengths and weaknesses. For example, the guest master liquid crystal mode reflective TFT-LCD, the guest master liquid crystal contrast and brightness can not be both: in order to get high contrast, must increase the "dark" state of light absorption, an effective way to increase the pigment concentration; however, an increase in pigment concentration, the display "bright In the "pigmented" state, the action of the pigment causes a decrease in brightness. The ideal pigment material is also hard to find in reality.


At present, we should not only pay close attention to new materials and new developments in liquid crystal devices but also to the development of a new product with a reflective TFT-LCD model that is practical for use as a guest master liquid crystal by integrating optical design technologies related to reflectors, color filters, and liquid crystals.


NEC has developed a new reflective TFT-LCD (3.8-inch NL3224ER24-031) that uses an amorphous TFT active matrix drive method, display screen size (76.8 × 57.6 mm2), diagonal length of 9.6 cm (3.8 inches), and display color of 262,144 colors (R.G.B. 64 colors each). (grayscale), number of pixels horizontal (320) × vertical (240) = 76800 (pixels), number of sub-pixels 76800 × 3 = 230400 (1 pixel = 3 RGB sub-pixels), filter RGB as vertical band, pixel pitch (horizontal 0.24mm × vertical 0.24mm), reflectance target value of 40%, contrast ratio Target value is 40:1, RGB brightness signal digitizing interface (6 bits of RGB each), 40mW power consumption (including LCD board, driver, controller and DC/DC converter power consumption).

The new reflective type TFT-LCD is ideal for outdoor portable applications. The difference is especially noticeable when comparing the transmissive TFT-LCD with the reflective TFT-LCD. When transmissive TFT-LCDs are operated outdoors under strong sunlight, the contrast ratio is significantly reduced and visual performance is degraded. This is due to an increase in the proportion of ambient light reflected from the surface of the LCD panel. However, reflective TFT-LCDs are designed with the ambient light factor in mind, so that the contrast does not change significantly.


Therefore, NEC's reflective TFT-LCD is mainly for portable applications, such as PDAs and other portable terminals should use this LCD board. As this new technology is advancing rapidly, it is believed that new reflective TFT-LCD products that can be used both indoors and outdoors will soon emerge, such as reflective TFT-LCDs with auxiliary light sources is one of them.

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Reflective TFT-LCD