Perovskite cells with 26.1% photoelectric conversion efficiency were born

Recently, the team of Pan Xu and Tian Xingyou, researchers of the Institute of Solid State Physics, Hefei Institute of Physical Sciences, Chinese Academy of Sciences (hereinafter referred to as the Institute of Solid State) and the Key Laboratory of Photovoltaic and Energy-saving Materials, Chinese Academy of Sciences, cooperated with Professor Nam-Gyu Park of Sungkyunkwan University in South Korea and Professor Dai Songyuan of North China Electric Power University to find for the first time that the uneven cation distribution is the main reason affecting the performance of perovskite solar cells, and successfully prepared a “homogeneous” perovskite solar cell, obtaining 26.1% The certified efficiency is 25.8%. The research results were recently published online in the journal Nature.

It is worth mentioning that it took just over a week for the paper to be published in the journal from the official acceptance.

“This study provides a clear direction for further improving the efficiency and stability of perovskite solar cells, and is of great significance for promoting their commercial development. Pan Xu, the first corresponding author of the paper, introduced to China Science News.

A reviewer in the journal Nature commented on the results, saying that “this work provides valuable insights into the effective inhibition of ion phase segregation in the field of perovskites, which will help promote the commercialization of perovskite solar cells”.

Xu Pan (right), the first corresponding author of the paper and a researcher at the Institute of Solid State Physics, discusses the problem with the students. Photo by Yao Jie

“Rookie” in the field of solar cells

Solar energy is the main source of energy for life on Earth. It is inexhaustible and cleaner and more environmentally friendly than traditional energy sources such as coal and oil. Researchers have invented solar power generation technology, which converts light energy into electrical energy for production and living needs.

Perovskite solar cells are a “rookie” in the field of solar cells.

“Here we want to popularize the concept that perovskite is not a mineral, but a crystal structure. Perovskite materials used in solar cells have very high absorption and conversion efficiency for visible light, and inherently have the characteristics of producing high-efficiency solar cells. Pan Xu said.

“The increase in the efficiency of perovskite solar cells is unprecedented. Pan Xu made a comparison, it took nearly 80 years for crystalline silicon solar cells to increase from the initial 3% to the current 26%, while it took only more than 10 years for perovskite solar cells to increase from 3.8% to the current 26%.

In addition to excellent efficiency, perovskite solar cells are simple to fabricate and low in cost. At room temperature, several chemicals are mixed in the solution, and then the solution is “brushed” on the substrate like “brushing the wall” to obtain a perovskite film. Finally, with the addition of functional layers such as electron transport layers and metal electrodes, a perovskite solar cell is fabricated.

“A perovskite solar cell is about 1 micron thick, which is equivalent to one hundredth of the thickness of a sheet of A4 paper. Pan Xu explained that thin means that the overall weight of the cell itself is very light and can be superimposed on existing crystalline silicon solar cells, and thin means that it has good light transmittance and can obtain more light energy. Thin, but flexible, it is expected to be used in aerospace and wearable devices in the future.

However, there are still some problems with such a “perfect” perovskite solar cell. For example, the poor stability, the current outdoor service life is only 2 to 3 years, and the improvement rate of photoelectric conversion efficiency has slowed down significantly, which are the core problems restricting the industrialization of perovskite solar cells, and they are also the difficulties for researchers to overcome.

In this work, Pan Xu et al. found for the first time that the cations in the perovskite film are unevenly distributed in the vertical direction, proposed a “homogeneous” cation phase distribution strategy, and successfully prepared a high-efficiency perovskite solar cell, obtained a photoelectric conversion efficiency of 26.1%, and the continuous light stability test reached 2500 hours.

Dr. Jiajiu Ye (right), co-corresponding author of the paper, and Dr. Zheng Liang (left), the first author of the paper, are testing the performance of battery devices. Courtesy of Solids

“Cool high resolution”

After the paper was launched, Pan Xu’s mobile phone information “exploded”, and many industry peers sent congratulatory messages. One of the messages reads: “This high resolution is cool”.

was “pointed out” by his peers as soon as he was “pointed out” the research highlights, and Pan Xu had some small “satisfaction”.

In the past ten years of development, a lot of research work has mainly focused on the properties and optimization of the plane of perovskite films, and the inside of perovskite films is like a “black box”, and people lack in-depth understanding of their crystal growth and component distribution, and basically rely on inference.

“You can’t rely on inference, scientific research needs to be evidence. Pan Xu said bluntly.

Based on years of research on the properties of high-performance perovskite solar cells and perovskite thin films, Pan Xu et al. have tackled key problems.

They first deeply analyzed the X-ray photoelectron spectroscopy, and really clearly observed the internal element distribution of the perovskite film from a microscopic perspective. Then, through high-resolution electron microscopy, the difference in crystal plane spacing was directly “seen”, which showed that cations of different sizes exist in different positions, that is, cation inhomogeneity. Large-sized cations are enriched at the upper interface of the film, and small-sized cations are enriched at the bottom of the film.

“The electron transport channel inside the perovskite film is like a road, and these cations of different sizes are obstacles, which hinder the advance of electrons and naturally cannot improve the efficiency of the cell. The co-corresponding author of the paper, Dr. Ye Jiajiu of the Institute of Solid Solids, introduced.

Furthermore, the team and the Shanghai synchrotron radiation source developed a new test method, i.e., in-situ grazing-incidence wide-angle X-ray diffraction, which monitors the crystal growth inside perovskite films.

It is found that the crystallization rate of cations of different sizes is too different in the process of crystal formation, and the crystallization rate of large-size cations is slow, and the crystallization rate of small-size cations is fast, which leads to the uneven distribution of perovskite films.

Eventually, they devised an additive that synchronized the crystallization rates of different cations so that they were evenly aligned and promoted charge transfer.

“This result shows that excellent cell performance can be obtained by homogenizing the vertical distribution of perovskite cations, opening up a new way to improve the stability of cell devices, and is expected to break the efficiency bottleneck of perovskite solar cells.” ”

It took just over a week from receipt to publication

In this paper, the reporter noticed a message that on October 25, 2023, the paper was officially accepted and published online on November 2. In other words, it took just over a week for the paper to be published.

“In general, there are two forms of publication in academic journals, one is a modest queue, and the other is a fast-track online publication. The reviewers believe that the research results are suitable for the latter form. Pan Xu admitted that they were also pleasantly surprised by such a quick process, and at the same time they were under pressure to prepare various materials within a week, including providing raw data, pictures, etc., to ensure the reliability and rigor of the paper.

On January 1, 2023, the paper was officially submitted. “This date is easy to calculate. Pan Xu said that they have been waiting for the email since the submission.

Soon, on January 20 (Chinese New Year’s Eve), they received a reply from the editor, and made changes to the editor’s comments, returned to the editor in March, and successfully submitted it for review.

Just when they felt that the start of the submission seemed to be going well, they received a new email in April, with nearly 40 comments from reviewers and requests for “major revisions”. So, they worked overtime for two whole months, carefully replied to every comment, and wrote more than 100 pages of content.

On June 19, they submitted for the third time. The reviewers highly praised the content of the responses, saying that “the responses were very patient and professional, and addressed all their comments”.

On October 25, the paper was finally accepted and published online “at an accelerated pace” a week later. Pan Xu said, “The reason for the rapid publication is that the work itself is meaningful and has direct reference value for the industry. ”

“Practical research is to be done”

Pan Xu was one of the earliest engaged in perovskite research in China. “The reason why we chose this direction is that because of the excellent optoelectronic properties of perovskite materials themselves, the potential for initial development and the theoretical limit efficiency of up to 33%, there is an irresistible attraction for researchers who have been engaged in the photovoltaic industry. Pan Xu said with a smile.

However, he was also very unconfident at first. “At that time, we held the perovskite thin-film cell in our hands, and it was visible to the naked eye that it slowly turned from black to yellow, and the light absorbance deteriorated. Pan Xu recalled the early days of perovskite research.

After more than ten years of deep cultivation, Pan Xu and others have made a major breakthrough. On November 2, the research results were published in the journal Nature. He said, “Perseverance is the most important quality in doing scientific research.”

At 1 o’clock in the morning of the same day, Pan Xu, who was sitting in the office, posted such a circle of friends – “The road ahead is still very long, there are still heavy rains, and there are bumps, keep going.” It’s nice to enjoy the scenery along the way, and it’s a treat even if you can’t see the end in sight. ”

“I want to tell my students that doing scientific research is not only for publishing articles, but also for practical research, doing research that has an impact on the national economy and people’s lives, even if it plays a small role. Pan Xu said that this is what he thinks is the greatest significance of doing scientific research.

“This work is just the beginning, and we will continue to explore further in this direction, hoping to develop improved additives to improve the efficiency and stability of perovskite solar cells. When it comes to the development of perovskite solar cells in the future, Pan Xu is full of confidence. (Source: Wang Min, China Science News)

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