In this work, for the first time, we identify the effect of adding chlorine (Cl) in the dynamic sequential (no stoppage between the sequential steps and the precursor for the second-step is dispensed while the substrate is in motion) solution processing of formamidinium methylammonium lead iodide FA0.3MA0.7PbI3 perovskite solar cells by comparing (i) the roles of two different Cl sources (MACl and FACl) and (ii) the time at which the Cl sources were added: (a) into PbI2-DMSO in the first deposition step or (b) into MAI-FAI in the second deposition step. It is found that Cl in general (regardless of the type of source and at which step it is added) improves the perovskite film with better coverage, better crystallinity, and higher photoluminescence response and longer carrier lifetime due to reduced trap densities. Characterizations for investigating film formation mechanisms reveal that the amount of Cl remaining in the final film is most crucial in determining the film quality. This in turn is controlled by when Cl is added. When Cl is added in the second step, the higher Cl content which is highly diffusive initiates the Dimethyl sulfoxide (DMSO) complex formation and perovskite formation too rapidly producing films with excess PbI2, which are more disorder and have lower crystallinity. FACl exacerbates this while MACl reduces the detrimental effect by slowing down perovskite formation. Therefore, for dynamic sequential processing, it is recommended that MACl is used as the preferred source over FACl and Cl should be added in the 1st step of the process allowing sufficient Cl to leave the film in the 2nd step of the sequential process. The resulting film has better film quality producing a champion device with a 2.8% absolute efficiency increase over the best device fabricated without Cl additives. This work has provided great insights into the film formation mechanism by sequential processing with additive engineering.