During the past decades, the green renewable energy such as solar energy has attracted great attention due to energy crisis and environmental pollution. Solar cells possess huge market potential with increasing power conversion efficiency (PCE) and decreasing cost. Thin-film photovoltaics using organic-inorganic hybrid lead halide perovskites as light absorption layer have been a rapidly developing technology, offering a sky-rocketing PCE from 3.8% to 22.1% in only seven years. Although remarkable progress in device efficiency has been achieved, however, the in-depth understanding of nucleation kinetics and growth mechanisms of perovskite materials still needs to be further researched. Hence, the comprehensive investigation of perovskites crystallization dynamics and morphology evolution is critically important to further improve material quality and corresponding device performance.

Recently, Prof. Rui Zhu and Prof. Qihuang Gong at Department of Physics, Peking University working with the collaborators, Prof. Thomas P. Russell and Dr. Feng Liu from Lawrence Berkeley National Laboratory, and Dr. Wei Zhang from University of Surrey, made important progress in studies of perovskite crystallization and microstructure evolution.

This research mainly focuses on the crystallization dynamics and growth mechanisms of hybrid lead halide perovskites via utilizing some advanced in-situ characterization techniques. The researchers performed the in-situ investigation of the perovskites crystallization kinetic and morphological evolution in real time from liquid precursor continually to the bulk polycrystal film. In-situ grazing incidence X-ray diffraction (GIXD) and Fourier transform infrared spectroscopy (FTIR) were utilized to reveal the crystallization dynamics and growth mechanism of perovskites over relevant temperature and time scales. The nano-assemble model from perovskite precursor solution, to the intermediated [PbI6]4- cage nanoparticels, and to the final bulk polycrystals was proposed to understand crystal structure and chemical transition at a molecular- or nano-scale level.

The film morphology evolution was obtained through the in-situ compound optical microscope observation and other morphology characterizations. A “crystallization-depletion” mechanism was developed to elucidate the periodic patterns and the kinetically-trapped morphology at a mesoscopic level. Based on these in-situ dynamic studies, the whole process of perovskite formation from molecular to microstructure was successfully demonstrated. The printing perovskite solar cells were also fabricated and optimized, and comparable device performances with traditional spin-coating method were obtained.

This work was published in Nature Communications (Nat. Commun. 2017, 8, 15688). Qin Hu and Lichen Zhao from Prof. Rui Zhu’s group are the co-first authors. Prof. Michael Gratzel in EPFL supported some constructive suggestions for the manuscript. This work was supported by the Ministry of Science and Technology of China, the National Natural Science Foundation of China, State Key Laboratory for Artificial Microstructure and Mesoscopic Physics, 2011 Collaborative Innovation Center of Quantum Matter, China, Collaborative Innovation Center of Extreme Optics, 1000 Talents Program for Young Scientists of China, and Lawrence Berkeley National Laboratory.