Abstract: The development of tin-based devices with low toxicity is critical for the commercial viability of perovskite solar cells. However, because tin halide is a stronger Lewis acid, its crystallization rate is extremely fast, resulting in the formation of numerous defects that affect the device performance of tin-based perovskite solar cells. Herein, propylamine hydrobromide (PABr) was added to the perovskite precursor solution as an additive to passivate defects and fabricate more uniform and dense perovskite films. Because propylamine cations are too large to enter the perovskite lattices, they only exist at the grain boundary to passivate surface defects and promote crystal growth in a preferred orientation. The PABr additive raises the average short-circuit current density from 19.45 to 25.47 mA·cm⁻² by reducing carrier recombination induced by defects. Furthermore, the device’s long-term illumination stability is improved after optimization, and the hysteresis effect is negligible. The addition of PABr results in a power conversion efficiency of 9.35%.