Time: 2019/1/15 8:56:30 Source: China Science Journal Author: Views: 3 times

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The energy problem has always been one of the important problems plaguing mankind.solar energyGreen, pollution-free, and inexhaustible in the foreseeable time frame, is regarded as an important way to solve the energy problem in the future.

Recently, the team of Wu Kaifeng, a researcher at the Dalian Institute of Chemical Physics, Chinese Academy of Sciences, proposed the concept of "quantum tailoring solar concentrator" for the first time based on nanocrystalline materials doped with rare earth metal ytterbium, and based on this concept, a prototype device of high-efficiency solar concentrator was prepared. The research was published in Nano Letters.

Fluorescent solar concentrators have advantages

Since 2011, solar photovoltaic power generation systems have developed rapidly around the world. According to the report released by the International Energy Agency (IEA), my country ranked first in the market in terms of new and cumulative installed capacity in 2017, with the newly installed capacity of 53GW accounting for 54.08% of the world's total new installed capacity; as of the end of 2017 , my country's cumulative installed capacity is 131GW, accounting for 32.57% of the global cumulative installed capacity, ranking first in the world.

One of the important problems faced by urban solar power generation is that, for cities with high-rise buildings, most of the sunlight shines on the side of the building instead of the roof, but the current solar concentrators are mainly installed on the roof. .

Could it be installed on the side of a building and act as a window to absorb solar power at the same time? It not only acts as a window, but also provides electricity to the building, which is in line with the popular concept of "smart building". However, the function of windows is to transmit light, the function of solar concentrators is to concentrate light, and the commonly used silicon-based solar panels are even more opaque. How can we realize the idea of both transmitting and concentrating light?

Wu Kaifeng introduced in an interview with "China Science News" that there are three types of sunlight concentrating technology: focusing type, reflective type and fluorescent concentrating. Among them, focusing and reflective properties belong to geometrical concentrating, which uses the basic principles of geometric optics to condense sunlight. For example, common magnifying glasses can achieve geometrical concentrating; while fluorescent concentrating involves light and matter. Interaction, sunlight excites the luminophore, emits fluorescent light, and then conducts waveguide convergence for the fluorescent photons.

In 1976, fluorescent solar concentrators (Luminescent Solar Concentrators; LSC) were first proposed by WH Weber et al. Regarding the working mode of the fluorescent solar concentrator, Wu Kaifeng said: "LSC is a large-area solar energy capture device with relatively simple structure, which consists of luminophores coated or embedded in transparent substrates (such as glass plates). After absorbing solar photons incident on the plate, fluorescent photons are emitted. Due to the difference in the refractive index of the substrate and the air, about 75% of the photons will enter the total reflection mode and then be waveguided to the edge of the plate to excite the photons attached to the edge. solar cells to convert light energy into electricity."

Compared with the other two concentrating methods, what are the advantages of fluorescent concentrating?

Wu Kaifeng said that there are mainly two advantages. "First, geometric concentrating requires the concentrator to instantly track the incident angle of sunlight to achieve effective concentrating, and such tracking devices are usually expensive; in contrast, fluorescent concentrators can Diffuse and scattered light from various angles can be concentrated without the need to track sunlight. Secondly, the appearance of the fluorescent concentrator looks like a translucent or fully transparent window, which can be integrated into the building, it is possible to Achieving the goal of 'solar windows' production capacity."

LSC is low cost but efficiency needs to be improved

Comparing the two methods of geometric concentrating and fluorescent concentrating, the working principles of the two are completely different, and each has its own advantages and disadvantages and scope of application.

“It’s hard to define which of the two is favored and which is not. For some applications where efficiency rather than cost is the most important consideration, such as photovoltaics used in aerospace, geometric concentrating has significant advantages. For other applications, such as solar windows, fluorescent concentrating obviously has unique advantages." Wu Kaifeng concluded.

According to reports, the disadvantage of fluorescent concentrating is that the concentrating efficiency is much lower than that of geometric concentrating. From the data, traditional LSCs are limited by the low fluorescence efficiency of the luminophore (usually less than 80%) and self-absorption losses, resulting in the internal optical efficiency of the device generally less than 60%. Therefore, the current concentrating photovoltaic devices use geometric concentrators.

However, because the fluorescent solar concentrator is composed of cheap polymers, glass and a small amount of fluorophore materials, the cost is much lower than that of the current mainstream solar panels (such as polysilicon).

If the concentration efficiency is high enough, a single LSC plus a small number of solar cells at the edge is functionally equivalent to a whole large-area solar cell, which will greatly reduce the cost of photovoltaic production. Wu Kaifeng pointed out: "We have made some simple estimates, and the cost may not exceed 1/10. However, what is really important for practical applications is a parameter similar to cost performance. This, due to the current low efficiency of LSC, cost performance compared to solar energy Panels may have only a slight advantage. That's one of the reasons the technology has not yet been commercialized."

Quantum cropped solar concentrators are even more magical

In order to improve the fluorescence efficiency of traditional LSC luminophores, scientists have thought of many methods. For example, for organic dye molecules, group modification can improve the fluorescence efficiency; or for inorganic nanoparticles (quantum dots, etc.), core/shell coating can also significantly improve the fluorescence efficiency. But in any case, the upper limit of the fluorescence efficiency of these traditional luminescent materials is 100%; and Wu Kaifeng's team increased this upper limit to 200% by means of quantum tailoring.

Quantum cutting is a novel optical phenomenon. "Materials based on this effect can absorb one high-energy photon and release two low-energy photons at the same time, satisfying the basic physical law of energy conservation." Wu Kaifeng said, "And we know that general luminescent materials, no matter what kind of energy a photon absorbs ( As long as that photon can excite the material), at most one photon is emitted. Therefore, quantum tailoring can double the luminous efficiency."

LSCs based on the quantum tailoring effect can theoretically achieve doubled fluorescence quantum efficiency (200%), while completely suppressing self-absorption losses. Wu Kaifeng explained: "In general luminophores, due to the large spectral overlap between absorption and emission, fluorescent photons will be lost to the self-absorption process during the waveguide process. For quantum tailored materials, since the emission wavelength is far away from the absorption site of the material, The self-absorption loss can be almost completely suppressed, which is also critical for the improvement of LSC efficiency."

Wu Kaifeng's research team proposes that the internal optical efficiency of LSCs based on the quantum clipping effect can redefine a new theoretical limit of 150%. The research team synthesized CsPbCl3 nanocrystals doped with rare earth metal ytterbium and found that their fluorescence efficiency was as high as 164%, showing typical quantum tailoring characteristics. The CsPbCl3 nanocrystal absorbs a blue light photon to generate excitons, and then transfers the energy to the excited states of two ytterbium atoms, thereby emitting two near-infrared photons. Kinetic tests show that efficient quantum tailoring occurs at the picosecond level. Prototype quantum-tailored LSCs were fabricated using such nanocrystals, achieving a device internal optical efficiency of about 120%. It is expected that by further optimizing the device and improving the solar light absorption ability, the external optical efficiency of 10% can be exceeded in large-area LSCs.

This research innovatively introduced ytterbium-doped nanocrystals into the field of LSC, which was highly praised by peers. Regarding the future research direction, Wu Kaifeng said that the absorption of CsPbCl3 nanocrystals is mainly concentrated in the ultraviolet part, and the utilization efficiency of sunlight is too low. The team is currently trying to modify the material to achieve a wider spectrum of sunlight absorption. Second, CsPbCl3 belongs to the widely studied lead-containing perovskite materials, and their toxicity and stability are urgent problems to be solved.

"It is difficult to predict how long it will take to solve these technical problems, but we have seen that the stability of lead-containing perovskite solar cells has been greatly improved through several years of research, so I think it should be The field is optimistic." Wu Kaifeng said.