ENGINEERING TECHNOLOGY

Flexible monocrystalline silicon solar cells were born


Under the background of “double carbon”, the photovoltaic industry that converts solar energy into electricity has developed rapidly, and monocrystalline silicon solar cells are the main force of it, and its share in the photovoltaic market has risen to more than 95%.

The reason why it is so hot is because monocrystalline silicon has so many advantages. First, silicon is the most abundant semiconductor element on earth, and there is no shortage of materials; Second, the cost is low; Third, many processes such as silicon wafers, monocrystalline silicon, and solar cells have been very mature in the traditional semiconductor field and can be directly referenced.

However, nothing is perfect, and monocrystalline silicon solar cells have a serious flaw. It is “fragile”, mechanically, a slight bending stress on it, or vibration during transportation, can cause it to break up directly. This also makes the application scenarios of monocrystalline silicon solar cells very limited.

A research team from the Shanghai Institute of Microsystems and Information Technology, Chinese Academy of Sciences (hereinafter referred to as the Shanghai Institute of Microsystems) successfully developed flexible monocrystalline silicon solar cells, achieving a milestone leap. At 23:00 on May 24, the research paper was published online in Nature and was chosen as the cover of the current issue. This is also the first long research paper on pure monocrystalline silicon solar cells published in Nature since the invention of monocrystalline silicon solar cells 69 years ago.

The research paper was published online in Nature and was chosen for the cover of the current issue

From “V” to “U”, the transformation of mechanical properties is realized

Making monocrystalline silicon solar cells flexible, where they need to be pasted, is the ideal of many people in the industry.

When he was a student, Liu Wenzhu, the first author of the paper and an associate researcher at the Shanghai Institute of Microsystems, considered this problem. But what really stimulated him to do this was because in many academic conferences, he could always hear a voice: “Our thin-film cells can be very soft, and monocrystalline silicon solar cells are difficult to achieve.” ”

In order to make the impossible possible, Liu Wenzhu and his collaborators opened a long road of experimentation.

Silicon solar cells are originally a flat surface, with serious reflection, and in order to improve their power generation efficiency, it is necessary to reduce light reflection. A common practice is to put silicon wafers in chemical solvents, and many pyramid-shaped micron structures will appear on the surface. When light comes in, it bounces back and forth on these structures, allowing most of the light energy to enter the silicon wafer and contribute to power generation.

“We found that at the junction of the pyramid and the pyramid is a very sharp ‘V’ shaped groove, and with a little force in this place, cracks will develop. Starting from here, it may be possible to change the fragile soul of silicon wafers. Liu Wenzhu said in an interview with China Science News.

The research team made a simple chemical treatment of the silicon wafer, through isotropic chemical corrosion or plasma treatment, the “V” shaped groove into a “U” shape. “This approach allows the bending strain to be effectively dispersed, effectively suppresses the strain fracture behavior, and improves the flexibility of the silicon wafer.” Di Zengfeng, the corresponding author of the paper and researcher of the Shanghai Institute of Microsystems, introduced.

But just pressed this end, cocked that end. Although the mechanical properties have been optimized, the reflection has also increased, and the power generation efficiency has been greatly reduced. How to achieve both mechanical properties and power generation efficiency makes everyone very distressed.

The research team intends to find a breakthrough in the full details of silicon wafer fragmentation. This requires a super-fast camera that can take 1 million consecutive photos per second. Liu Wenzhu sent the sample to Yang Ziqiang, a junior brother who used to be in the fluid mechanics team at King Abdullah University of Science and Technology. Yang Ziqiang used an ultra-high-speed camera to show the process of silicon wafers breaking under bending stress.

“We see that all wafers break under bending stress, starting at the edge. This shows that the most marginal area is the ‘mechanical short board’ of silicon wafers. Liu Wenzhu said, “Then, we only need to deal with a very small area at the edge of the entire large silicon wafer, and the problem can be solved.” ”

In this regard, the research team innovatively developed the edge smoothing processing technology. They stack dozens, hundreds, or more silicon wafers on top of each other, and combined with plasma corrosion, they can treat sharp “V” grooves on the surface and sides of the edge of the silicon wafer into smooth “U” grooves.

The structural design scheme can significantly improve the “flexibility” of silicon wafers, and monocrystalline silicon solar cells with a thickness of 60 microns can be folded like A4 paper, with a minimum bending radius of less than 5 mm; It is also possible to perform repetitive bending at a bending angle of more than 360 degrees.

Flexible monocrystalline silicon solar cells bend at an angle of more than 360 degrees Courtesy of the interviewee

“Since the sleek strategy is only implemented at the edge of the silicon wafer, it basically does not affect the photoelectric conversion efficiency of solar cells, and at the same time can significantly improve the flexibility of solar cells, and has broad application prospects in space applications, green buildings, portable power supplies and other aspects in the future.” Liu Zhengxin, the corresponding author of the paper and a researcher at the Shanghai Institute of Microsystems, said.

The evidence was sufficient to convince the reviewers

In 1954, researchers at Bell Labs in the United States invented monocrystalline silicon solar cells, using monocrystalline silicon wafers to achieve a breakthrough in converting sunlight energy into electrical energy, and successfully used in artificial satellites.

For the next 69 years, no long research paper on monocrystalline silicon solar cells was published in Nature. So when this article was submitted to the journal editors, it caught their eyes, and the article quickly entered the submission process.

In the first round of review, one reviewer gave a high evaluation, believing that “this is a breakthrough discovery in the field of photovoltaic in the future”, and another reviewer did not directly reject the draft, but raised many detailed questions.

“Maybe my English writing was not very good, and I felt that the second reviewer did not fully understand the article, and then I broke up and crumpled a little bit to explain to him in response to his questions.” Liu Wenzhu said.

In the second round of review, the reviewer wrote that “the work itself is top-notch by every measure, and it is fair to publish it in Nature.” ”

After the article was submitted, the relevant macroscopic mechanics experiments were still underway, and when the article was submitted for review and did not receive feedback, the research team made new progress, using the three-point bending method to test the mechanical properties of monocrystalline silicon solar cells and calculate how much its theoretical limit can be.

In the absence of supplementary experiments, the reviewers added the research progress obtained in the later stage to the article, forming sufficient evidence to convince the reviewers. The article was submitted on August 24, 2022 and was accepted in principle after 5 months. But why hasn’t the article been online until now?

“It’s all a disaster!” Liu Wenzhu said, “I have never submitted such a good journal before, just more than a year ago, I submitted to Nature, but I was rejected. I have no experience in making pictures, and the pictures I made are ugly, and after the article notification was received, I went through 5 rounds of detail modifications. ”

Liu Wenzhu was surprised that he was finally selected as the cover of the current issue, and in his opinion, the cover design of “Nature” is very competitive. “At that time, I didn’t know about painting, I didn’t have an aesthetic sense, so I kept thinking about where the concept of flexibility could be used in the future, and I happened to see the street lamp on the street, so I took a picture directly. Then the monocrystalline silicon solar cell is curled and pasted on the street lamp pole, and then do a little hazy effect to highlight the street lamp, just two or three simple elements, but the whole picture is simple and clear. ”

Leverage the huge flexible photovoltaic market

Whether a technology is reliable requires multiple verifications. The research team not only made monocrystalline silicon solar cells, but also considered a lot of back-end things and sent the flexible components of monocrystalline silicon solar cells to a third party for testing.

“There is a company in Jiangsu that specializes in high-speed rail shell vibration testing, we send the flexible components made about one square meter to the test, fix the components around the bottom plate, let the middle hang, after the vibration test starts, the components will shake up and down like a mat.” Liu Wenzhu said.

In such a harsh test environment, they measured 18,000 cycles without losing any power from the flexible components.

In addition to this test, the research team also did a more “ruthless” operation. The international standard for photovoltaics is the IEC standard, which means that photovoltaic materials must meet the requirements of operating between minus 40 °C and 85 °C for 20 or 25 years, and the performance degradation should not exceed 15%.

“Our experiment is more stringent than international standards, allowing monocrystalline silicon solar cells to cycle continuously between minus 70°C and 85°C for 120 hours, and found that the power attenuation is very small.” Liu Wenzhu introduced.

This is the good side of silicon, and the performance is very stable. Liu Wenzhu said that although the multimode battery is very thin and has good flexibility, it is a polycrystalline structure. Under a microscopic microscope, it is something that is put together by grains, which means that there is a grain boundary between each grain and adjacent grains, and if you bend and fold it back and forth, the grain boundary will form mechanical friction, resulting in performance degradation.

“Now, we have made a flexible monocrystalline silicon solar cell, the entire large silicon wafer can be restored no matter how bent it is, it has no grain boundaries, no friction, even if it is folded 1,000 times, the power has not been attenuated at all, and the service life will be long.” And these are difficult to achieve with polycrystalline cells. Liu Wenzhu said.

Client-facing products Photo courtesy of interviewee

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Flexible monocrystalline silicon solar cell modules have been successfully used in adjacent space vehicles, photovoltaic building integration, vehicle photovoltaic and other fields.        Photo courtesy of Shanghai Microsystems

This study verifies the feasibility of mass production in the mass production line, and provides a feasible technical route for the development of lightweight and flexible monocrystalline silicon solar cells. The large-area flexible photovoltaic modules developed by the research team have been successfully used in the fields of adjacent space vehicles, building photovoltaic integration and vehicle-mounted photovoltaics.

Liu Wenzhu said that flexible monocrystalline silicon solar cells will be used in more scenarios in the future, such as street lamps, wearable electronics, mobile communications, aerospace and other fields have a good development space, or can leverage the huge flexible photovoltaic market. (Source: Zhang Qingdan, China Science News)

Related paper information:https://doi.org/10.1038/s41586-023-05921-z



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