The utilization of wide bandgap (WBG) tin halide perovskites (Sn‐HPs) offers an environmentally friendly alternative for multi‐junction Sn‐HP photovoltaics. Nonetheless, rapid crystallization leads to suboptimal film morphology and substantial creation of defect states, which undermine device efficiency. This study introduces 4‐Phenylthiosemicarbazide (4PTSC) as an additive to achieve a densely packed Sn‐HP film with fewer imperfections. The strong chemical coordination between SnI₂ and the functional groups S═C─N (Sn···S═C─N), NH₂, and phenyl conjugation enhances solution stability and supports the delay of perovskite crystallization through adduct formation. This process yields pinhole‐free films with preferred grain growth. 4PTSC acts as a strong coordination complex and a reducing agent to passivate uncoordinated Sn²⁺ and halide ions and reduce the formation of SnI₄, thereby reducing defect formation. The π‐conjugated phenyl ring in the 4PTSC facilitates the preferred crystal growth orientation of perovskite grains. Furthermore, the hydrophobic nature of 4PTSC mitigates Sn²⁺ oxidation by repelling moisture, enhancing stability. The open circuit voltage significantly increased from 0.78 to 0.94 V, resulting in achieving the champion efficiency of 12.22% (certified 11.70%), surpassing all previously reported efficiencies for WBG Sn halide perovskite solar cells. Additionally, the unencapsulated 4PTSC‐1.0 device maintained outstanding stability over 1200 h under ambient atmospheric conditions.