On November 18, Professor Chen Qi's team from Beijing Institute of Technology published an article onScience, which is the fifth high-level paper published by Beijing Institute of Technology onNatureandSciencethis year.

The article is entitled "Initializing Film Homogenity to Restart Phase Segregation for Stable Perovskite Solar Cells", which reveals the influence of initial homogeneity of perovskite film materials on the stability of films and devices, and on this basis, a highly efficient and stable perovskite solar cell is prepared.

Professor Chen Qi of School of Materials of Beijing Institute of Technology is the corresponding author of the article. Bai Yang, associate professor of School of Materials of Beijing Institute of Technology, Huang Zijian, a doctoral student of Peking University, Zhang Xiao and Lu Jiuzhou, 2018 master's students of Beijing Institute of Technology, and Niu Xiuxiu, 2019 doctoral student are the co first authors of the article. The cooperative units of this work include Peking University, Institute of Automation of Chinese Academy of Sciences, Institute of High Energy Physics of Chinese Academy of Sciences, Institute of Physics of Chinese Academy of Sciences, Beijing University of Aeronautics and Astronautics, Institute of Semiconductors of Chinese Academy of Sciences, and Beijing Yaoneng Technology Co., Ltd.

The new energy technology of photovoltaic power generation is of great significance for achieving the goal of carbon neutrality. In recent years, optoelectronic solar cell devices based on organic-inorganic hybrid perovskite have achieved rapid development. The highest photoelectric conversion efficiency reported at present is close to 26%, and device stability is the key to its industrialization.

Due to its rich component space, the mixed component perovskite has outstanding advantages in regulating the semiconductor performance of materials and improving the efficiency and stability of devices. However, due to the introduction of multi-component, multiphase competition will occur in the process of material growth, resulting in uneven initial component distribution of the film. The development of the inhomogeneity of mixed cation perovskite thin films was studied. It is found that the non-uniform sites of the film at the nanoscale will develop rapidly under external stimulation, leading to more serious component distribution differentiation (Figure 1), and eventually forming a thermodynamic stable phase separation that runs through the entire perovskite film, causing material degradation and device deactivation.

Fig. 1 Initial distribution of components of (A-H) perovskite thin films and their evolution under external stimulation. Phase separation of perovskite thin films driven by (I-N) thermodynamics.

Although there have been relevant reports on the micro nano scale phase separation thermodynamics and atomic scale element migration behavior, there is still a lack of a unified model to systematically study the individual behavior of ions and atoms and the macro behavior of films in a unified context. In order to solve this problem, the author of this paper has developed a mathematical model that can be used to simulate the migration and aggregation of ions in perovskite based on the Schelling model, which studies group behavior in economics and physics. In combination with the prediction of density functional theory and experimental observation data, the film phase separation behavior has been semi quantitatively analyzed. As shown in Figure 2, the simulation results of the model show that the initial homogeneity of the perovskite film has a significant impact on the aging behavior of the film, and the improvement of film homogeneity will significantly slow down its aging rate.

Fig. 2 (A) Theoretical calculation of phase separation of perovskite films. Schelling model simulation of aging process of (B-F) perovskite thin films

Guided by the prediction results of the model, the author effectively regulated the colloidal environment of the solution and improved the film homogeneity by introducing the weakly coordinated additive selenol into the perovskite precursor solution. The experimental results show that the films with improved homogeneity show good stability under thermal and light aging conditions, and no significant phase separation occurs during the experimental period (Fig. 3). At the same time, after further device optimization, the prepared solar cell devices show good photoelectric performance, which is at 1 cm ² On the device, 23.7% of the certification efficiency is obtained. Under different temperature conditions, the device also shows good working stability under continuous illumination of LED light source.

Fig. 3 (A) Theoretical calculation of phase separation of perovskite films. Schelling model simulation of aging process of (B-F) perovskite thin films.

In addition, the conclusions about the initial state of homogeneity can be further extended from mixed cation system to mixed anion system. The author of the paper prepared mixed anion perovskite films with different initial homogeneity, and studied their aging process, expanding the universality of the conclusion that "improving the homogeneity of the composition distribution of perovskite films is conducive to improving the stability of films and devices" (Figure 4). In this paper, Schelling model is developed, which builds a bridge between atomic scale ion movement and micro nano level phase separation, and links the initial homogeneity of materials with device stability. It provides a new idea for improving the stability of perovskite materials and photovoltaic devices, and also provides a theoretical basis for preparing efficient and stable perovskite optoelectronic devices.

Fig. 4 Aging behavior of (A-H) mixed anionic perovskite films.

Author introduction

Chen Qi

Professor, School of Materials, Beijing Institute of Technology.

In 2005, he graduated from the Department of Chemical Engineering of Tsinghua University with a bachelor's degree, in 2007, he graduated from the Department of Chemistry of Tsinghua University with a master's degree, and in 2012, he graduated from the Department of Materials Science and Engineering of the University of California, Los Angeles (UCLA) with a doctor's degree. Later, he worked as a postdoctoral fellow in the California Nanometer Research Center of UCLA. He joined Beijing Institute of Technology in 2016. In 2019, he was selected into the Outstanding Youth Program of Beijing Natural Science Foundation. Mainly engaged in the development and application research of organic-inorganic hybrid and composite materials, which are widely used in energy, photoelectric information and other devices, such as solar cells. He has published more than 100 papers, includingScienceandNat Commun.,Joule,J. Am. Chem. Soc.,Adv. Mater.H-Index 61, with a total of more than 28000 references.

Bai Yang

Associate Professor, School of Materials, Beijing Institute of Technology

In 2013, he graduated from the Department of Applied Chemistry of Beijing University of Aeronautics and Astronautics, and in 2017, he graduated from the Department of Chemistry of Hong Kong University of Science and Technology with a doctor's degree. Later, he carried out research in the Department of Chemistry of Hong Kong University of Science and Technology as a postdoctoral fellow. In 2018, he joined the School of Materials of Beijing University of Technology. Mainly engaged in the research of new energy materials and perovskite solar cells. He has published more than 60 papers, includingScienceandAngelw Chem. Int. Ed.,Adv. Energy Mater.H-Index 36, cited more than 6000 times in total, and undertook research tasks such as general programs of the National Natural Science Foundation of China.

Zhang Xiao

2018 graduate student of School of Materials

2018 graduate student of School of Materials, Beijing Institute of Technology, under the guidance of Professor Chen Qi and Associate Professor Bai Yang, is committed to the research of organic-inorganic hybrid perovskite materials, mainly focusing on the interface design and mechanism research of perovskite photovoltaic devices.

Lu Jiuzhou

2018 graduate student of School of Materials

The 2018 graduate student of School of Materials, Beijing Institute of Science and Technology, under the guidance of Professor Chen Qi and Associate Professor Bai Yang, is committed to the research on the preparation of efficient and stable perovskite solar cells, mainly focusing on the phase separation phenomenon and mechanism of perovskite films.

Niu Xiuxiu

2021 doctoral candidate of School of Materials

He is a 2021 doctoral candidate in the School of Materials, Beijing Institute of Technology, under the guidance of Professor Chen Qi. His main direction is perovskite solar cells, and he focuses on the research on the homogeneity and stability of perovskite materials.