Chinese scientists design new quantum dot light emitting diodes

In October last year, the Nobel Prize in physics was awarded to a digital Japanese scientist who invented a blue light-emitting diode. "The incandescent lamp lit up in the 20th century, and the LED lit up in the 21st century." According to the prize presentation, the LED is the LED. Recognized as the core device for next-generation display and lighting technology. A few dozen days later, a paper from a Chinese scientist was published in Nature, reporting important research advances made in the field of quantum dot LEDs.

Chinese scientists design new quantum dot light-emitting diodes

Do not see the quantum dots for you to shine

In this study by the Zhejiang University Task Force, scientists designed a new type of high-performance quantum dot light-emitting diode (QLED), and will use a lifetime of 100,000 hours to reach a practical level, which means this The new device is expected to become a strong contender for next-generation display and lighting technologies.

“We have already seen the first quantum dot application with subversive significance, that is, the 'QD LED' with excellent performance,” said Peng Xiaogang, head of the research team and professor at the Zhejiang University High-tech Materials Chemistry Center.

Quantum dots can greatly improve the light-emitting performance of diodes

“The importance of luminescent materials to humans determines that quantum dots will become star material.” Peng Xiaogang believes.

Light is a form of energy. When the electrons in a material transition from a high energy level to a relatively low level of energy, the energy is released - if this energy is expressed in the form of light, it will be seen This substance is shining.

Researchers explain that in semiconductor materials, electrons emit photons if they fall into a hole in the air level, which is called "electron hole recombination." However, complex electrons and holes are not often present in matter, and the recombination process requires electrical excitation or photoexcitation. Light emitting diodes are electrically excited light emitting devices.

When light-emitting diodes are energized, electrons and holes migrate under the influence of an electric field, and they may recombine when they meet, but this process is not easy. They need to have a chance to interact and form an "electron-hole pair" that can eventually be compounded under suitable conditions and give a symbol of happiness - photons.

In order to ensure a high compounding efficiency, researchers often provide a composite medium, which is called “luminescent material”. Arranging electrons and holes in this kind of material is a "kinding" and the chance of success will be greatly improved. The scientific name “soluble inorganic semiconductor nanocrystals”, referred to as quantum dots of solution nanocrystals, is a very excellent luminescent medium. As long as electrons and holes enter the quantum dots one-on-one, they emit light compositely. Efficiency can be as high as 100%.

Peng Xiaogang's task group is to synthesize a quantum dot luminescent material suitable for LED, and then cooperated with the Jin Yizheng Research Group of Zhejiang University to create a novel quantum dot light-emitting diode. At the same time, the structure is delicately designed to allow the electrons to slow down the “step”, and the holes accelerate the pace to facilitate the effective meeting of electrons and holes, which greatly improves the high-efficiency light-emitting performance and stability of the quantum dot light-emitting diodes.

This also precisely solves two key issues that Peng Xiaogang considers to be: to enable quantum dot LEDs to achieve practical application levels, one is how to tailor quantum dot materials that are suitable for LEDs; second is how to design their structures to achieve maximum Electro-optical conversion efficiency.

What kind of material is the most important quantum dot?

Quantum dots of different sizes can express different colors

“Quantum dots are nanometer-sized semiconductor crystals that have a three-dimensional dimension below 100 nanometers. They are put into solution. Since then, humans have a whole new class of materials that have the dual properties of crystals and solutions. It is even a new type of molecule; from the perspective of the material's future, it represents many new possibilities," Peng Xiaogang said.

The size of a quantum dot, which is about one hundred thousandth of the diameter of a hair filament, cannot be seen by the human eye. It is at the nanometer scale that quantum dots exhibit quantum effects - when these semiconductor crystals are made as small as the nanometer scale, different sizes can emit different colors of light, even if the sizes differ by a few or a dozen atoms. By adjusting the size of the quantum dots, light of a desired color can be obtained. For example, semiconductor nanocrystals such as cadmium selenide emit blue light at 2 nanometers, red light at 8 nanometers, and green, yellow, and orange at middle sizes.

“Using quantum dots of different sizes, we will see different colors, and the colors are very bright.” Dr. Zhao Fei of Hangzhou Najing Technology Co., Ltd. who participated in the cooperation of the project said that the name of quantum dots is also derived from semiconductor nanometers. The quantum size effect of crystals.

For a long time, the synthesis of quantum dots has relied on some particularly active and extremely toxic substances. When air is seen to explode, it must be kept in the refrigerator. Peng Xiaogang’s earlier contribution when he was abroad was to find a “green” organic solvent route, as long as there is an ordinary chemical synthesis laboratory that can do simple synthesis of quantum dots. Afterwards, the quantum dot growth mechanism was further systematically explored, and the range of relatively high-quality quantum dots gradually expanded to various types of semiconductors. Soon, this "green" route was promoted all over the world.

“The last way to find out is to replace expensive and unstable raw materials with common chemicals through the understanding of the special mechanism of crystal growth. Science is such a thing, it did not find a headwater, and it was easy to find after finding it.” Peng Xiaogang said.

Expected to play an important role in lighting and display industry

In Nano Crystal Technology, several test tubes and several large or small plastic bottles are filled with green, yellow, and red liquids, respectively. This is the quantum dot solution. After purging a 2000-ml barrel of solution, the crystals were only about the same as the fingers. "But there are 10,000 TVs inside," said Zhao Fei. These quantum dots can be used to create 10,000 new color TVs using quantum dots.

Judging from the presentation screen of the quantum dot TV set, it is also blue or red, and many different levels of vividness can be distinguished. The same is the red lipstick, on the screen is able to present and distinguish 100 lipsticks with different colors.

Peng Xiaogang introduced that the application of quantum dots is very extensive. In the field of biomedicine, quantum dots can be used to fully display the skeleton of cells. Quantum dots can be easily used to detect multiple pathogens or pesticide residues at the same time. Moreover, because the quantum dot absorption ability is very strong, it can greatly increase the sensitivity. Lighting is also a big industry. Light emitting diodes using quantum dots are closer to natural light, and heat generation is greatly reduced.

Scientists believe that quantum dots may bring significant changes to the industry, first of all to show. The current first-generation quantum dot display products are based on light-excited luminescence, and both Nanotech and the United States have entered the commercialization phase. This new type of backlight allows the display color to be very pure and high in color saturation. Quantum dot LEDs, on the other hand, bring quantum dots to the second generation. At present, Zhejiang University and Nano Crystal Technology Corporation are in the leading position in the second-generation quantum dot display technology.

"A series of experimental results verify the practicality of quantum dot light-emitting diodes. This in turn indicates that quantum dot light-emitting diodes are expected to play a more important role in the lighting and display industries." Peng Xiaogang said that both display and lighting require white light. Or red, green and blue light, the research team will continue to develop high-efficiency QLEDs with various wavelengths of light while maintaining a low-cost solution preparation process, allowing the photons generated by the combination of electrons and holes to illuminate thousands of households.