Photonic crystal LED LCD TV backlight

The cathode ray tube has been largely eliminated. Now go into any of the electronic stores and you will see a row of TV and computer LCD monitors. Even if the plasma display technology had previously been regarded as a technology better and more competitive rivals have LCD TV new technology based on the challenges, it is more efficient, more bright and clear image.

Different from plasma display, LCD screen need to use the backlight module (BLU) to the LCD panel projection white light. The cold cathode fluorescent lamp (CCFL) is usually selected, but the LED backlight provides another option, which can improve the contrast by local dimming. Because of the higher efficiency of LED backlight, can extend the battery life, and thus popular on the laptop.

The advantages of LED include reducing the weight, thickness and energy efficiency of the backlight module. Solid state light source is more compatible with dynamic contrast control, the technology used to turn off the black area of the display image of the LED, so that the contrast is as high as 10000: 1.

Choice of LED

The white light produced by BLU can be realized by carefully mixing the red, green and blue LED light, but also by the blue LED and yellow fluorescent powder. The two option is based on the GaN Blu ray LED, all different types of such devices have a common drawback: most of the light is bound in the active area, can not be effectively extracted.

This inefficiency means that the backlight module requires more LED, which increases the cost of. In order to improve the efficiency of light extraction, the researchers developed a number of techniques to extract the total internal reflection is bound to the chip in the light.

Commercial LED manufacturers preferred wafer surface roughening technology. This simple and cost-effective random surface structure can increase the angle of escape and improve the efficiency of light extraction.

Because of the lack of directional control of light distribution, the technology is not suitable for backlight module. In fact, the surface roughness of the LED chip has no directional light intensity distribution in the optical emission cone. This feature is suitable for most of the general lighting applications, but it is not suitable for LED backlight module, because it requires a more regular distribution of light intensity, light guide the most appropriate place.

Remove light

How light is bound, how to remove it from the LED? It is good to study these. The calculation and simulation results show that there exists a horizontal bound state in the GaN epitaxial layer and the sapphire substrate, and a regular porous grating structure on the LED end surface can effectively extract the light. The most popular approach is to use a periodic or quasi periodic shallow hole array to form a two-dimensional photonic crystal lattice.

The results show that the LED of the etched photonic lattice can double the surface brightness, and change the spatial light distribution of the device. The Lambertian distribution produced by the plane LED is not suitable for the backlight module, but by optimizing the structure of the photonic crystal, the radiation intensity distribution tends to the characteristics of the special diffuser and the film.

American company Luminus Device Inc. is enjoying the success of photonic crystal PhatLight LED to bring it. The device has been used in some high-end TV, such as Samsung 56 inch rear projection tv. The manufacturer believes that the introduction of such high priced LED flagship product is worth the money. But if the photonic crystal LED to affect the entire market, especially the key LCD TV industry, their manufacturing costs must be significantly reduced to CCFL.

In May 2007, the "quasi photonic crystal" (PQLDI) project was officially launched, and its primary goal was to develop a low-cost production technology.

The project lasted two years, costing 1 million 200 thousand pounds (about $2 million 400 thousand), which is funded by the UK science and technology strategy committee. Project team members from the University of Glasgow Department of electronics and electrical team, at Clyde University (University of Strathclyde) photon research center and SHARP labs europe.

Advantages of quasicrystals

Traditional photonic crystal LED, such as Luminus manufactured products, has attracted a strong patent portfolio. Thus, we avoid the sharp direct is committed to the study of quasi periodic photonic crystal structure. When used as a backlight display, compared with the traditional photonic crystal photonic crystal is the main advantage of this quasi periodic arrangement of hollow structure provides more freedom for the optimization of output light distribution, which in turn simplifies the design.

The formation of photonic crystal structure is a key step in the fabrication of such devices. The modern deep submicron lithography system is a matter of course, but not the best. Although it can produce photonic crystals of deep sub micron scale, most of the LED wafer factories do not have such expensive equipment.

Figure 1 (left) the main steps of the nano imprint lithography lithography: the use of transparent quartz template to suppress the wafer on the resist coating, UV light on it, after etching the formation of photonic crystal structure.

Figure 2 (above) for the display of the illuminating project of the quasi photonic crystal LED is still using the conventional LED epitaxial structure.

A lower cost alternative is electron beam lithography, which has been widely used by researchers in the field of photonic crystal devices. However, we must point out that its writing speed is very slow, which means that the manufacturing cost is high graphics.

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