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Technology briefing
Quantum dot materials are tiny semiconductor particles with a size of nanometers (Figure 1). Due to their extremely small size, their physical properties are different from those in the bulk state. When the particle size is smaller than the Bohr wavelength (Bohr wavelength) of electrons in the material, the quantum confinement effect (quantum confinement) occurs, resulting in a separated discontinuous electronic energy gap (energy gap), the smaller the particle size, the electronic energy gap is larger. When electrons are stimulated to transition between this energy gap, light waves with a fixed wavelength can be generated.
Fig. 1 ( Picture courtesy : NSM)
In general, the quantum dot materials used for lighting are mainly group III, IV or group II, VI semicondustor. The main reason is that this type of material has a "direct band gap" (Direct band gap), and electrons can directly fall back to the valence band from the conduction band, and all converted into light energy so the quantum efficiency is higher. At present, the research of Cadmium Selenide (CdSe) is the most popular quantum dot material, because it is not only a material with a direct band gap, but also in an appropriate particle size range, its transition energy gap just falls on the wavelength of visible light scope. The preparation of Cadmium Selenide quantum dot material is to use Cadmium oxide (CdO) and Selenium dioxide (SeO2) as raw materials, and synthesize Cadmium Selenide quantum dots by thermal cracking method in an organic solvent. Since Cadmium Selenide, a single material, is easily damaged by Oxygen and moisture, it is usually coated with a shell to protect the quantum dots, and Zinc Sulfide (ZnS) is usually used as the shell material. Cadmium Selenide quantum dots with Zinc Sulfide as the shell layer are also the most successful commercialized quantum dot materials.
Fig. 2 ( Picture courtesy: NSM)
Since the width of the energy gap is related to the particle size, the wavelength of the output light wave can then be adjusted by adjusting the particle size (Figure 2). In case the size of the quantum dot materials are uniform, then the output wavelength bandwidth of the excited light spectrum will be very narrow to provide a very pure and saturated light color. Therefore, the quantum dot material can be used to replace the fluorescent powder of various existing LED lighting, and its spectral quality is better than that of the light generated from a fluorescent material which is with complicated energy level. Quantum dot materials can be widely used in various fields such as bio-medicine, chemical engineering, lighting and display technology. Therefore, related researches have been very vigorous in recent years. Many research institutions, schools and companies around the world have invested much resources in research and development. At present, the most eager and expected market application lies in the liquid crystal display industry, which got the ambition to optimize the color performance of the LCD display.
For the presentation of any high-quality images, the specifications of the image and the hardware platform must be upgraded together in order to display the best image quality. The biggest technical bottleneck of liquid crystal display is that the color is not saturated enough till now. The current high-definition image specification is based on the BT.709 (Rec.709) regulation, which specifies the resolution (1920x1080; 2K), refresh rate (60Hz), color scale (10bits) and color gamut range (color gamut 87%NTSC) of HD images. For the latest LCD panel technology, the first three specifications can be met, but under the current situation of using white LED as the backlighting source, it is difficult to achieve color performance of 87% color gamut index of NTSC. That is to say, the color specifications of most LCD monitors and TV didn't satisfy the standard till now. At present, the next-generation high-end image quality standard; Rec.2020 has been set that the resolution of a ultra-high-definition (UHDTV) TV is 4K or 8K, the gray scale is 12 bits, and the color gamut range should reach 150% NTSC, respectively. Per this regulation, it can be seen that the bottleneck and challenge of the next-generation LCD display lies in the image color. This is also the most urgent bottleneck of liquid crystal display technology which need to break through in the very near future.
Fig. 3, Spectrum of white light LED backlighting
A narrower-band color filters can be used to filter out narrower-band color light to strengthen the color gamut. However, according to the fact that the utilization efficiency of the backlight source is already low, the way will degrade more of light efficiency. Under the trade-off, consequently, the color quality of the current liquid crystal display is very limited. To put it simply, just like the color of the paint is not rich enough to render a high-saturation painting, the poor spectrum of the backlight cannot provide enough color to demonstrate a image with a saturated color of a high-end LCD display. In particular, panel technology is about to enter 4K and 8K resolutions, and the utilization rate of backlight will be even lower, which is not allowed to consume more power to optimize the color performance.
Various policies of strengthening the image color of LCD monitors have been working hard for more than ten years. LCD TVs using three-primary-color LED backlights have set off a trend. Due to the independent three-primary color spectrum with narrow linewidths, it can provide high-quality lighting with an extremely wide color gamut. However, due to cost and light source stability issues, it has not become the mainstream of the application. Recently, Japan's Sharp Co.has made higher-resolution pixel segmentation on the panel, and expanded the color gamut of the image with four-primary-color technology. Samsung even jumped out of LCD technology and plans to use OLED panels to enhance image color. However, these are not feasible and effective solutions under market considerations of cost.
Improving the spectral quality of the backlight is the ultimatest solution to enhance the color performance of LCD displays. Quantum dot film technology can just satisfy this demand. The quantum dot material that can release the red and green spectrums is made into a film and inserted into the existing backlight module. While it is illuminated with blue light, it can stimulate red and green spectrum with a very narrow bandwidth which will construct a optimized RGB illumination for backlighting. Figure 4 is a schematic diagram of the cross-sectional structure of the quantum dot film and the conversion of light. After using the quantum dot film, the actual measured output spectrum is shown in Figure 5. Compared with the current white LED backlight spectrum shown in Figure 3, the backlight with blue LED and quantum dot film has an absolutely excellent RGB independent spectrum . The color gamut that can be covered on the standard color gamut chart, such as the yellow triangle area (Figure 6), can display images with a very rich color range. At present, the actual measured color gamut value of LCD TVs using a quantum dot film manufactured by Efun has reached 118% NTSC, and after a little optimization of the filter, it can reach more than 135% (Figure 7). Another advantage of the quantum dot film is that it does not need to change the structure of the liquid crystal display in use, and is completely suitable for various types of liquid crystal displays.
Fig. 4
Fig.5
In addition to coat QD material on a substrate to form a film type devices, quantum dot materials are also sealed inside glass tubes and applied to side-illuminated backlight modules. The advantage is that the glass tube sealed QD can fully block the invasion of moisture, but the disadvantage is that the light conversion efficiency is poor that make it difficult for application of mainstream direct-lit backlight systems. QD film can be used in various backlight modules. The technical challenge lies in how to package and block water vapor to maintain the luminous quality. The usage structure of the quantum dot film is shown in Figure 8 and Figure 9, which is basically applicable to all current liquid crystal displays without changing any structure, and does not need to change the manufacturing process of the LCD production line.
Fig. 7
Fig. 6
Fig. 9
Fig. 8
The formulations of several common quantum dot materials currently on the market are shown in Table 1. Among them, the widely commercialized material is Cadmium Selenide (CdSe), which has stable performance to pass the strict specifications request of liquid crystal displays. It has good color rendering, as well as wide color gamut. However, because it contains heavy metal Cadmium, it often face to the challenge of environmental pollution. However, theoretically its Cadmium content is extremely low. Taking a quantum dot film for a 75-inch TV as an example, the Cadmium content is only equivalent to a bowl of white rice. Therefore, although major manufacturers have questioned the content of heavy metals, they have widely used them and increased their quantity.
Table 1
At present, this formula is used as a mainstream the market. In the meantime, some manufacturers have also developed low-Cadmium recipe to skip the pollution issue. In the early days, quantum dot films made of Indium Phosphide (InP) materials without heavy metal content entered the market. However, although there is no heavy metal inside, they are with carcinogens substance. And unfortunately, the color rendering is not good, the color gamut is small, as well as the quantum efficiency is low. Therefore, it is not a suitable choice for the market. Another genre is CsPbX3 with Perovskite lattice structure (X can be a Chlorine, Bromine or Iodine (Cl, Br, or I)), which also claims to be a free-of- heavy metals material. But unfortunately it contains lead. Efun also has experience in developing Perovskite recipes. Although the half-width of its output spectrum is extremely narrow to provide an extremely high color purity and wide color gamut. However, it's difficult to extend the green spectrum to long wavelength, so that the color gamut coverage is low. Moreover, it is easy to decay under the influence of environmental conditions, which usually make it difficult to pass the a common RA test of a liquid crystal display regulation.
Efun QD film product
Efun has invested in the R&D and manufacturing of quantum dot films for nearly ten years, and currently uses Cadmium Selenide quantum dot materials from Nano system materials Co. (NSM). NSM is a leading company which has more than ten years experience in developing quantum dot materials. NSM owned many key patents on the quantum dot material technology which dominated the competence of LCD market in either IP and technology know how. Efun got tight partnership to carry on the film fabrication by using the QD materials from NSM for many years. At present time, Efun is familiar of mass producing QD film for the market. NSM also provide water- barrier- film free QD material recipe to provide a QD film with relative low price to the market.
The specifications of the barrier free quantum dot film produced by Efun are shown in Table 2. The center wavelength can be adjusted according to the needs of customers. The maximum size can be as large as 85 inches (16:9), with thickness of 220 and 360 microns. Since the quantum dot material is intrinsically resistance of water and oxygen, consequently, the depth of the edge failure zone is less than 300 microns, which is suitable for application of narrow-frame LCD products.
Figure 10 is a photo of the edge failure area after 1,000 hours of RA testing. The depth of edge failure is only 150 microns, which is much better than the that from other competitors.
Table 2
Fig.10 ( Picture courtesy: NSM)
Research and production
Efun is wiith experience in the development and mass production of quantum dot film products. Over the past few years, we has accumulated the know how of preparation, mixing, and dispersion of quantum dot materials. Also, we established automatic production technology, as well as the setup of mass-production equipment. For now, the coating process is becoming the key competence for making a QD film. It is also a bottleneck challenge to whom want to engage in the manufacturing of QD film product. For instance, the thickness uniformity determines the conversion ratio of blue light into white light. Therefore, to coat the QD material uniformly on the substrate with uniform thickness is a big goal to achieve . Taking a quantum dot film for a 85-inch TV as an example, in order to meet the precise requirements of TV color coordinates, the coating thickness of the entire area is only allowed to have a few micron distortion, which has almost reached the limitation of mechanical equipment.
Efun has been engaged in precision coating technology for many years, and with the dedicated investment in the past few years. We well controlled the mass-production technology of quantum dot films to get actual shipping performance. Figure 11 shows Efun's self-developed roll-to-roll QD film production line, Figure 12 shows the shipment of quantum dot film for 75-inch TVs, and Figure 13 shows the PAD using Efun's thin QD film. Figure 14 shown the application of LCD devices.
Fig. 11
Fig. 12
Fig. 13
Fig. 14
Quantum dot film is currently the best way to improve the color saturation of liquid crystal displays. The color quality of liquid crystal displays using quantum dot film is much higher than that of OLED panels, so the market prospect of quantum dot film is promising. At the critical moment when all liquid crystal display manufacturers are eager to break through the technical difficulties, quantum dot film is undoubtedly the best choice to overcome the bottleneck of liquid crystal color performance to create differences.
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