Understanding Microstructural Development of Perovskite Crystallization for High Performance Solar Cells.

Solution crystallization in film devices has attracted broad interest from various fields such as perovskite solar cells. However, the detailed perovskite crystallization kinetics remain unclear due to the difficulty of in situ observation of grain cluster growth during annealing. This article presents the development of an in situ laser scanning confocal polarized microscopy with a temperature-controlled stage to observe nucleation and growth of perovskite crystal clusters. It is found that enhanced interactions by a liquid crystal with perovskite form a new intermediate complex that induces diffusion-controlled growth according to Avrami equation. The retarded cluster growth (63 nm s-1 ) originates from enlarged diffusion activation energy 40 kJ mol-1 compared with 152 nm s-1 and 37 kJ mol-1 for the Control film during annealing. Finally, the optimized perovskite films with enhanced crystallographic and optical characteristics are applied in solar cells to achieve a champion efficiency of 24.53% with open circuit voltage of 1.172 V and fill factor of 82.78%. The bare device without any protection maintains 89% of its initial efficiency after 6600 h of aging in ambient environment. This work implies that the in situ observation using fluorescence microscopy is a critical for understanding of crystallization kinetics in film devices.