TV Knowledge Daily News: The Story of Quantum Dots

    Color is a visual effect of light produced by the eyes, brain and our life experiences.

    In the process of the development of human material and spiritual life, colors have always been glowing with magical charm. In the long river of life, people are never satisfied with the single color for a long time. They not only discover, observe, create and appreciate the gorgeous and colorful world, but also deepen their understanding and application of colors through the long-term changes of the times, and discover new color inspirations.

    In the Paleolithic Age, humans used simple colors to paint the hunting life of the time on the rock walls. The primitive colors reflected the spontaneous colors of human life. After the emergence of pottery and wooden structures, creative artists of different races in the world began to paint colorful colors and rich patterns on them. From then on, colors have a wider social function; since Newton used a prism to refract the seven-color spectrum of red, orange, yellow, green, cyan, blue and purple in the sun in 1976, the color perception of modern color painters has made great progress. The colors at this time show the painters' keen observation of colors and unique color personality; and the emergence of light and color modeling art that combines static and dynamic reflects the comprehensive aesthetic color personality of contemporary people and the full possession of the color essence of human development to date.

    All of this is to show the beauty of the world, and technology has never stopped presenting this beauty. 

    In 1925, the British John Lodge Baird successfully invented the world's first television. Although it was a simple black and white display, humans had fully experienced the broadening of vision and the initial understanding of the world brought by technology. With the development of color television, the beauty of nature was restored on the screen for the first time, showing humans a gorgeous color vision, and people seemed to have truly understood the world. 

    Can the color of a TV be quantified? How can we better display the true colors of nature? Of course we can. The color gamut refers to the range of the number of colors that a device can express, that is, the range of colors that various screen display devices, printers or printing devices can express. In different equipment fields, there are some different color gamut standards. In the field of broadcasting and television, the most commonly used is the NTSC color gamut standard, which is the yardstick for the color gamut performance of TV sets.

    Let's go back to 1953, when the National Television Standards Committee (NTSC) defined the x and y values ​​of the three primary colors of RGB based on the CIE 1931 chromaticity diagram and the phosphor technology of the CRT at that time, which was the NTSC color gamut standard. The standard used C light source (corresponding to white is CIII, color temperature 6766K). In 1966, due to the poor luminous efficiency of the RGB phosphors used in the RGB coordinates formulated by NTSC, the European Broadcasting Union (EBU) redefined the coordinate specifications of RGB and white points according to the actual needs at that time. The area of ​​the newly defined color coordinate triangle just covers 72% of the original NTSC standard, so the subsequent color TV systems all follow the NTSC 72% rule. When modern LCD screens appeared, in order to replace the traditional CRT market, the color gamut specifications also followed the original NTSC 72%, which became a shackle that color technology has never broken through over time.

    In fact, the color gamut is related to many indicators of the TV, among which the backlight source is very important. The backlight factor mainly depends on the purity of the three light waves of red, green and blue. The narrower and sharper the spectrum of the three colors on the spectrum graph, the purer the three colors of light are, and the higher the brightness of the mixed white light is, so the color gamut is wider. Since ordinary LED TVs use blue LED chips + yellow phosphors to produce mixed white light, which leads to insufficient backlight purity, most of the blue and green colors in nature and some red cannot be accurately displayed. Therefore, when playing video movies, the color display on the screen will be more or less biased, and therefore, it has been criticized by consumers.

    In order to break through the shackles of color, people are constantly exploring how to improve the LED backlight solution. With the development of technology, researchers have discovered a technical solution that uses RG powder (red and green phosphor) instead of yellow phosphor and adds color filters to assist in enhancement. Through this technical enhancement solution, the color gamut value of LED TVs can reach about 82%, which is 10% higher than that of ordinary LED TVs, and the backlight color purity is increased to 70-85%, so it is called the first generation of high color gamut TVs. However, TVs using this solution are still insufficient in the expression of green and red, which ultimately leads to poor color gamut saturation, poor grayscale and color transitions of the image, so when using this type of high color gamut TV, the picture still looks gray. For this reason, the industry has proposed a new backlight phosphor solution, namely the new RG powder solution. Through this solution, the color gamut value of LED TVs can theoretically reach more than 96%, the brightness can be increased by more than 10%, and energy saving can be more than 15% at the same brightness, so it is called the second generation of high color gamut TVs.

    In fact, after so much introduction, everyone should realize that these methods of improving color gamut values ​​are all phosphor solutions. However, no matter how much phosphor is improved, its ability to improve the color gamut value of LED TVs has reached its limit. Is there a more efficient color solution?

    In 1979, Chinese-American professor Deng Qingyun discovered organic light-emitting diodes, or OLEDs, in the laboratory. Since then, OLEDs have attracted the attention of industry experts. OLED display technology has the characteristics of self-luminescence. It uses a very thin organic material coating and a glass substrate. When an electric current passes through, these organic materials will emit the required colored light. Since it does not require a backlight source, OLEDs are light, thin, have a wide color gamut, and are bendable. According to the display principle, the color gamut value can reach 100%, so it was once considered by the industry to be the next generation of display technology. However, the current OLED yield rate is extremely low; organic materials and metals are very sensitive to oxygen and water vapor, are easily oxidized, and have a short service life; the ability to accurately control color is insufficient, and there is a problem of insufficient color purity; and due to technical limitations, the actual measured color gamut of various existing OLED TVs is only about 89%.

    When other companies in the industry were struggling with the mass production of OLED, TCL took the lead in breaking the myth and officially launched China's first quantum dot TV H9700 using quantum dot display technology on December 15, 2014, leading the global color TV industry into a new quantum dot era.

    So the question is, what is quantum dot TV? Is it a better representative of the next generation of display technology than OLED?

    To answer this question, let us first get to know a group of people. As early as 2011, TCL joined hands with the world-renowned Stanford Research Institute to establish the TCL Silicon Valley Research Institute in Silicon Valley. This institute, which is not well-known in the Chinese industry, actually has a secret mission, which is to find and develop the next generation of world-leading display technology. It was this institute that discovered the unlimited potential of quantum dot technology in TVs.

    Quantum Dot, also known as Quantum Dot in English, is a very cutting-edge nanomaterial with a grain diameter between 2 and 10 nanometers. It is these tiny crystals that are invisible to the naked eye that give televisions the opportunity to present more natural colors.

    Researchers have found that quantum dots have unique photoelectric properties. When stimulated by electricity or light, they emit very pure, high-quality monochromatic light of various colors according to the diameter of the quantum dots. This led to the idea of ​​applying quantum dot display technology to televisions. After continuous development, the TCL R&D team finally achieved the use of pure blue light sources to stimulate quantum dot crystals of different sizes in quantum dot light tubes, thereby releasing pure red and green photons, and projecting them onto the imaging system with the remaining pure blue light. In this way, quantum dots can emit high-quality red/green monochromatic light with a concentrated energy spectrum and very pure light, completely surpassing the phosphor luminescence characteristics of traditional LED backlights and achieving better imaging colors.

    Why can TCL TV+ quantum dot TV surpass ordinary LED and even OLED and trigger a color technology revolution?

    First, the color gamut is wider. On the CIE 1931 chromaticity diagram, the xy coordinates of the red color of the TCL quantum dot TV H9700 reached 0.6901 and 0.2979, the xy coordinates of the green color were 0.2091 and 0.7415, and the xy coordinates of the blue color were 0.1468 and 0.0708, which ultimately constituted the NTSC color gamut value of the H9700 as high as 110%. At present, the color gamut of ordinary LED backlight is 72% NTSC color gamut, and the color gamut of the highly anticipated OLED can reach about 100% NTSC color gamut in principle, while the color gamut of TCL TV+ quantum dot TV has reached the industry's highest 110% NTSC, which is currently the technology with the widest color gamut coverage.

    Secondly, color control is more precise. At present, the industry generally adopts the principle of photoluminescence (PL) in display technology. Traditional phosphors are multi-level energy level structures. When blue light excites phosphors, the spectrum of light emitted by phosphors is not single. In addition to the red/green/blue light required for imaging, there are other stray lights. These stray lights seriously affect the purity and accuracy of color reproduction; quantum dots are single-level structures. The spectrum of light emitted by each quantum dot of a fixed size is unique, that is, the color is unique and pure. Therefore, by adjusting the size of quantum dot grains, the wavelength of the light wave generated can be easily and accurately adjusted to produce light of different colors, so that the color can be controlled more accurately to achieve accurate color reproduction display effects. In August 2012, the International Telecommunication Union (ITU) released the BT2020 standard for 4K*2K/8K*4K ultra-high-resolution televisions. The color gamut defined by BT2020 covers up to 133% of the NTSC color gamut, and the 110% color gamut value of the quantum dot TV H9700 is the technology that is currently closest to this standard. In addition, its red/green/blue vertex coordinates are consistent with the direction of the BT2020 color gamut vertex coordinates, and it is also the only display solution that can meet the BT2020 color gamut standard.

    Finally, the red, green and blue colors are purer. In addition to being affected by factors such as the screen and the driver, the color gamut of a TV also depends on the purity of the backlight source, and the backlight factor mainly depends on the purity of the three light waves of red, green and blue. The narrower and sharper the spectrum of the three colors on the spectrum graph, the purer the three colors of light are, and the higher the brightness of the mixed white light is, so the color gamut is wider. The spectrum peaks corresponding to the red, green and blue primary colors of the TCL quantum dot TV H9700 are clear and distinct, without overlap, and the energy is concentrated and the peaks are narrow, so the red, green and blue displayed by the H9700 are purer.

    In addition, based on quantum dot display technology, TCL's R&D team has also focused on developing true color display technology for the application of quantum dot technology in TVs. This is one of TCL's core image quality improvement technologies, including hybrid dimming technology, precise color gamut matching technology, and natural light technology, which can achieve a comprehensive improvement in image quality parameters such as color gamut value, contrast, energy efficiency, and brightness, thereby improving display quality. 

    First of all, hybrid dimming technology is TCL's exclusive patented technology. Hybrid dimming refers to adjusting the LED current in the range of 100%-30% maximum brightness. At this time, the color temperature consistency is good, the photoelectric efficiency is improved, and it is energy-saving and flicker-free. In the second half of the dimming, the current peak remains unchanged, and the LED current PWM duty cycle is adjusted to keep the color temperature unchanged. Hybrid dimming technology has two major characteristics: healthy and green energy saving: it can achieve continuous backlighting without flickering under family viewing conditions (less than maximum brightness); at family viewing brightness, the backlight power consumption is reduced by 20% relative to PWM dimming (converted to a 5-10% reduction in the power consumption of the entire machine).

    The second is natural light technology. As we all know, our eyes use different parts in low light and high light conditions. The eyes include cones and rods, which operate in opposite conditions. Cones recognize colors and details in bright conditions (photopic) while rods take on the responsibility in dim conditions (scotopic). In bright conditions, our pupils shrink to sense more details, while the depth of field and perceived brightness increase. In low light, the pupils dilate to let in more light. Therefore, through the application of natural light technology, when the backlight is set to the maximum in dark images, the content is clear, and when the image is bright, the backlight is appropriately reduced and not dazzling, so that people can reduce the amount of pupil adjustment changes when watching H9700, thereby improving the comfort and naturalness of viewing.

    Finally, through the application of precise color gamut matching technology, when ordinary film sources are displayed, the scenery is more vivid and the characters are more realistic and natural. It can be said that ordinary pictures can also take advantage of the wide color gamut of quantum dots.

    From 72% to 110%, the emergence of TCL TV+ quantum dot TV has brought the global color TV industry's display technology a big step forward.