Recently, the École Polytechnique Fédérale de Lausanne in Switzerland, in collaboration with teams from North China Electric Power University, Suzhou University, and others, has proposed a unique "dopant-additive" synergistic enhancement mechanism and successfully developed innovative perovskite solar cell modules, forming stable perovskite thin films with high crystallinity, low defects, and excellent orientation.

A certified efficiency of 23.30% was achieved on a perovskite module with an aperture area of 27.22 cm², with a steady-state efficiency of 22.97%. Moreover, after continuous 1000 hours of simulated sunlight exposure, the efficiency was maintained at 94.66% of the initial value at room temperature and 84.53% at a working condition of 65°C.

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This means that many challenges faced in the large-scale production of perovskite solar modules, such as efficiency, stability, consistency, repeatability, and yield, have been effectively addressed.

The research not only provides a solid technical foundation and in-depth theoretical guidance for further improving the performance of perovskite solar cells but also demonstrates the broad application prospects of this technology in the field of new energy, greatly promoting the practical and commercial process of perovskite solar cell technology.

Recently, the related paper was published in Nature with the title "Dopant-additive synergism enhances perovskite solar modules" [1].École Polytechnique Fédérale de Lausanne (EPFL) postdoctoral researcher Dr. Bin Ding, Associate Professor Yong Ding from North China Electric Power University, Professor Jun Peng from Suzhou University, EPFL research assistants Jan Romano-deGea and Lindsey E. K. Frederiksen are the co-first authors.

Honorary Professor Mohammad Khaja Nazeeruddin at EPFL, Professor Paul J. Dyson, Researcher Fei Zhaofu, Professor Xiaohong Zhang from Suzhou University, and Associate Professor Yong Ding from North China Electric Power University serve as the co-corresponding authors.

Significant Improvement in the Stability of Perovskite Solar Modules

Due to the outstanding optoelectronic properties of perovskite solar cells, they are considered the "stars of the future" in photovoltaic technology.However, as people expand this technology from the laboratory to industrial-scale production, a series of challenges have gradually emerged, mainly focusing on the preparation of high-quality perovskite thin films over large areas and ensuring the stability of the films.

In recent years, the field of perovskite photovoltaics has developed rapidly, especially in improving the performance of small-area devices, with significant achievements. However, in comparison, the efficiency and stability of the components have lagged.

Formamidinium-based perovskites (such as FAPbI3) have a better optical bandgap, but the formation energy of its photoactive "black phase (α phase)" is relatively high, which is not conducive to directly obtaining well-crystallized and stable α-FAPbI3.

By introducing dopants such as methylammonium chloride (MACl), the formation energy can be partially adjusted, but it has not been able to completely solve these problems.

On the other hand, perovskite research involves multiple disciplines including materials, chemistry, physics, and semiconductor device design. To fully understand the complex mechanisms behind performance improvement, a clearer understanding of the entire perovskite system is needed.After an in-depth study of the formation process of perovskite precursor solutions and thin films, the research team revealed the fundamental reasons affecting the stability of the precursor solution and the limitations on efficiency improvement. Subsequently, they invested four years of effort in research, aiming to address key bottleneck issues in perovskite photovoltaic technology.

Ding Bin and Ding Yong said: "In our research, we first noticed some previously overlooked phenomena, such as the instability of the perovskite solution, the yellow-phase perovskite that appears during the nucleation and crystallization process, and the wide distribution of thin film grain sizes."

Through in-depth characterization and analysis, they found that the decomposition of methylammonium chloride, the condensation reaction of its decomposition products with methylammonium iodide, and the excessive aggregation of methylammonium chloride are the main causes of these problems.

To address the above issues, the use of additives with strong interactions has become particularly critical. After extensive literature research and screening of a large number of additives, the researchers decided to use ionic liquids as stabilizers for perovskite solutions.

After exploring and comparing more than 30 types of ionic liquids, they added a new Lewis basic ionic liquid additive, 1,3-bis(cyanomethyl)imidazolium chloride ([Bcmim]Cl), to the perovskite precursor solution and used it in conjunction with methylammonium chloride dopants.This strategy of synergistic enhancement by doping agents and additives achieves a "1+1 greater than 2" effect, not only effectively suppressing condensation reactions, but also promoting the uniform distribution of ammonium chloride, significantly improving the stability of the precursor solution, successfully eliminating the yellow perovskite phase, and thus preparing high-quality perovskite thin films with oriented growth and excellent crystallization.

"The stability of the pure perovskite precursor solution can only be maintained for one or two days, but after adding the new ionic liquid, it can maintain stability for at least 10 days," said Ding Bin.

More importantly, the research team has also delved into the intrinsic mechanism of the synergistic effect. On the one hand, by introducing the new ionic liquid, it can undergo proton exchange with ammonium chloride, effectively suppressing the decomposition of ammonium chloride, thereby stabilizing the perovskite precursor solution.

On the other hand, this ionic liquid contains special dicyano cations and strongly electronegative anions, providing multiple sites for hydrogen bonding. This strong interaction promotes the uniform distribution of ammonium chloride, further enhancing the quality and stability of the perovskite thin films during the preparation process.

Ding Bin is a materials scientist by training, and because this research spans multiple disciplines, he needs to start learning many knowledge from scratch. For example, liquid-state nuclear magnetic resonance is one of the commonly used techniques in the field of chemistry, but for him, it is like a "book of heaven".In his research, he found that due to the extreme sensitivity of liquid nuclear magnetic resonance (NMR) to minor changes in the chemical environment, relying solely on simple NMR experiments was not sufficient to deeply explore and solve the problems. He observed that the content of additives, the composition and volume of the solution could easily affect the chemical shift of hydrogen atoms in the NMR spectrum.

Furthermore, other halides also caused extremely similar changes in the chemical shift of hydrogen atoms, making the research process extremely difficult.

"After more than 100 attempts of NMR experiments, I gained a deep understanding of the working principle of nuclear magnetic resonance, and finally designed a more rigorous experiment to reveal the secrets behind the cooperative effect," said Ding Bin.

Paving the way for the large-scale application of perovskite solar cells.Before the stability mechanism of perovskite was studied, the entire field was somewhat like "feeling the stones while crossing the river"; how to make perovskite more stable required a large amount of slow attempts by researchers.

This study conducted a series of explorations from macro to micro, providing guiding strategies for a comprehensive understanding and resolution of the instability dilemma of perovskite, promoting subsequent research to develop more efficiently and rapidly.

"Our developed ionic liquid significantly enhances the stability of perovskite, performing best among all additives I tested, providing a new solution for the commercial application of perovskite," said Ding Bin.

The study explored the essence of perovskite, so its impact is not limited to perovskite solar cells, but also has guiding significance for detectors, perovskite LED light-emitting devices, and other directions.

Ding Bin and Ding Yong pointed out that in the future, proton exchange and strong interactions can be used as guiding mechanisms to more effectively find or design new additives to stabilize perovskite materials.In the study, researchers found that the higher the concentration of ionic liquids, the more stable the perovskite thin films were. However, the use of high concentrations of additives could potentially damage the quality of the thin films, such as causing the formation of pores. Therefore, they plan to continue researching in order to find a solution that can maximize stability without compromising efficiency.

On the other hand, researchers also plan to use advanced methods such as big data and artificial intelligence to screen and design better-performing additives on the basis of this study.

"Double Ding" Doctor's Strong Alliance

The large-scale preparation of perovskite thin films is an inevitable trend in the development of perovskite, so Ding Bin and Ding Yong have taken this as the main direction from the beginning of their research.Bing Ding graduated with a bachelor's and a doctoral degree from Xi'an Jiaotong University, and his undergraduate graduation project was on perovskite. The year 2024 marks his 10th year of research in the field of perovskites.

When he first started researching perovskites, due to his dedication to deepening his understanding of the relevant basic knowledge and continuously honing and accumulating his professional skills, the research progress was relatively slow. As a result, in the first few years of entering this research field, he did not publish many papers.

However, he did not stop moving forward because of this. "What makes me proud is that the several papers I have published in recent years are very instructive and have been highly recognized by many peers and reviewers," said Bing Ding.

During his doctoral studies, under the guidance of his doctoral supervisor, Professor Yang Guanjun, Bing Ding independently invented and designed the prototype of the vacuum extraction method and developed the second-generation air-assisted vacuum extraction technology. During his postdoctoral research at EPFL, he and Ding Yong jointly promoted the development of the third-generation vacuum extraction technology.

This technology successfully manufactured high-quality thin films and high-efficiency solar cell devices by depositing perovskite materials in the form of thin films onto the substrate in a vacuum environment, and has applied for a patent for this technology. It is reported that this method has been adopted by global companies including China, the United States, and Japan, and has become one of their core technologies.In 2022, Ding Bin and Ding Yong set a new world record for the certified steady-state efficiency of perovskite modules with a performance of 22.40%, which has been maintained to date.

This record is currently the highest efficiency recognized for perovskite modules and has been included in the "Solar Cell Efficiency Tables" (Versions 61-63) edited by the "Father of Solar Cells," Martin Green[2].

In recent years, Ding Bin and Ding Yong have co-authored multiple top publications, including Nature, Nature Energy, Nature Nanotechnology, etc.

They reported a key mechanism of oriented nucleation, using in-situ synchrotron radiation technology and fluorescence technology to monitor the nucleation and growth stages of perovskite online, revealing the nucleation and growth mechanism of perovskite, and found that the use of hydrochloric acid pentamidine can effectively suppress the generation of intermediate phases, expanding the preparation window for large-area perovskite thin films[3].

By adopting the strategy of thermal liquid crystal additives, targeting the passivation of defects in perovskite thin films, the quality of perovskite thin films has been greatly improved, and the efficiency of perovskite modules has been increased to 22%, while maintaining good long-term stability[4].Single-crystal rhombic titanium dioxide nanoparticles with excellent electron mobility and low defect density were synthesized using the solvothermal method, achieving a certified efficiency of 22.72% on perovskite modules, setting a new efficiency record [5].

"The twists and turns in the research process have also made me appreciate the meaning of the phrase 'the fragrance of plum blossoms comes from the bitter cold.' Throughout this journey, teamwork has been particularly important, and I sincerely thank all collaborators for their contributions," said Ding Bin with emotion.

It is reported that he is about to return to his home country to take up a position at a university and establish a semi-industrialized laboratory, continuing to focus on the research of large-area perovskite solar cells, to promote the close integration of industry, academia, and research, and to advance the industrialization process of perovskite technology.

Ding Bin expressed his hope that perovskite solar photovoltaic technology will become a powerful driving force in the search for new energy solutions, thereby helping China achieve its goals of carbon peak and carbon neutrality. This technology is not only of great significance to the transformation of the country's energy structure but also provides new perspectives and solutions for the global response to climate change issues.