during the past few years, researchers have focused on developing alternative Pbfree perovskites by replacing Pb with other metals, including tin (Sn), germanium (Ge), bismuth (Bi), stibium (Sb), and copper (Cu). [11][12][13][14][15][16] Among these alternatives, Sn-based perovskites have made an emerging breakthrough, and their PSCs with highest PCE of 14.81% have been recently reported, demonstrating the great potential. [17][18][19] Despite the great progress in performance improvement of Sn-based PSCs, PCE and stability of Sn-based PSCs are still far behind those of their Pb counterparts. The poor device performance mainly stems from the poor quality of solution processed Sn-based perovskite film, which is attributed to the hardly controllable crystallization process of Sn-based perovskites. [12,20] It should be noted that the uncontrollable crystallization process of Sn-based perovskites does not directly lead to poor solar cell performance, but the formed defects, film morphology, crystallinity and orientation, structural distribution, and residual strains induced during crystallization do, which are the direct factors affecting the performance of PSCs. [21][22][23][24][25] For Sn-based perovskite, the uncontrollable crystallization process mainly originates from the unique property of Sn 2+ , including high Lewis acidity and easy oxidation of Sn 2+ . [26,27] On the one hand, due to the higher energy of 5p orbital compared to 6p orbital, the Lewis acidity of Sn 2+ is higher than that of Pb 2+ , resulting in the fast reaction rate of SnI 2 with MAI or FAI. On the other hand, because the two electrons on 5s orbital of Sn 2+ are active and easy to lose, giving rise to the easy oxidation of Sn 2+ , the formation of Sn vacancy Tin-based perovskites show great potential in photovoltaic applications, and the development of the corresponding solar cells (PSCs) has made exciting progress during the past few years. However, owing to the high Lewis acidity and easy oxidation of Sn 2+ , Sn-based perovskite films suffer from fast crystallization and easy formation of vacancy defects with low activation energy during the solution film-forming process, resulting in poor film quality and inferior device performance. Therefore, an in-depth understanding and rational control of film-forming dynamics of Sn-based perovskites is essential to improve the photovoltaic performance of their PSCs. In this review, the state-of-the-art developments in crystallization dynamics control for Sn-based perovskites and their impact on the photovoltaic performance of PSCs are systematically summarized. The review begins with the introduction of fundamentals and key difficulties for the control of the crystallization process of Sn-based perovskites. Then, the advanced strategies that focus on regulating the crystallization process of Sn-based perovskite films are comprehensively reviewed, including solvent engineering, additive engineering, cation engineering, and film-forming technique engineering. Finally, future perspectives and research di...
