Chemical bath deposition (CBD) is generally used to fabricate SnOX electron transport layer (ETL) for perovskite solar cells, whereas a lack of understanding of the reaction mechanism hampers further improvement of device performance. Here, we elucidate the reaction mechanism by monitoring the SnOX growth process via a series of advanced X-ray photoelectron spectroscopy studies. For the first time, we successfully modulated the nucleation and crystal growth of SnOX during the CBD process by introducing complexing agent via inherently forming a self-assembled monolayer on the outmost surface of SnOX. The complexing agents can assist the heterogeneous nucleation and crystal growth of SnOX. By a comparison study of different complexing agents, we found that the 2,3-dimercapto-1-propanesulfonate sodium not only assists in achieving a high-quality SnOX ETL, but also serves as a bridge to connect with perovskite via coordination interactions. Leveraging these insights, we achieved a champion efficiency of 24.2 % for perovskite solar cells with ∼ 2300 h stability at ambient condition.