Tin perovskite solar cells as the most promising candidate for replacing the toxic lead one have not shown an outstanding performance as same as lead counterparts due to the fast crystallization and ease of oxidation of Sn2+ to Sn4+ . The fast crystallization of tin perovskite which contributes to poor surface morphology and lots of defects such as iodide /tin vacancy in the bulk. Herein, we examine several functional groups and find the N-H bond in the additives play a key role in passivating the iodide which make it difficult to form iodide vacancy as showed from the XPS data. Furthermore, the enhanced hole mobility in the perovskite benefited from the carbazole additive results in a power conversion efficiency improvement from 7.32% to 8.68%.