<p>Cesium tin iodide (CsSnI₃) is a promising lead-free alternative to traditional lead-based perovskites due to its superior optoelectronic properties and eco-friendly nature. However, its structural instability and the propensity of tin (Sn) to oxidize pose significant challenges. In this work, we address these issues by fine-tuning the synthesis parameters of CsSnI₃. We adjusted the precursor feed ratio to 1.1:1 (CsI to SnI₂) and incorporated 2% by mass of tin powder. This modification optimizes the crystal structure of CsSnI₃ and maintains its chemical purity without requiring complex procedures. Our method significantly improves stability, by doubling the time of phase transition under vacuum compared to conventional samples. Additionally, the material shows enhanced stability in air, reducing the formation of the undesirable black perovskite phase, Cs₂SnI₆. The increased A-site Cs ions refine the structural framework of CsSnI₃, preventing spontaneous collapse, while the B-site Sn powder mitigates internal Jahn–Teller distortion and affects the formation environment of Cs₂SnI₆. These combined additions result in improved performance. We discuss the underlying mechanisms of structure and phase transition, highlighting this formulation’s potential as an optimal approach for synthesizing CsSnI₃. This study paves the way for developing more stable and efficient lead-free perovskite materials for optoelectronic applications.</p>