Steric Hindrance‐Assisted Coordination for Efficient and Stable Tin‐Based Perovskite Solar Cells

Tin‐based perovskite solar cells (TPSCs) have recently emerged as a leading eco‐friendly photovoltaic technology owing to rapid efficiency gains. However, their development is limited by the facile oxidation of Sn ²⁺ and high defect density, leading to poor crystallization quality and severe non‐radiative recombination. In this study, we leverage the strong PO···Sn ²⁺ coordination and the steric hindrance of multiple phenyl rings to introduce a series of phosphine additives into the bulk phase of Sn‐based perovskites. Through theoretical calculations and experimental screening, triphenylphosphine oxide (TPPO) is identified as the optimal additive due to its unique tetrahedral geometry and superior coordination capability. The PO group of TPPO acts as a hard Lewis base, forming robust coordination with Sn ²⁺ , while its tetrahedral steric hindrance synergistically enhances the stability of Sn‐based perovskites structure. Moreover, the incorporation of TPPO boosts the PCE of TPSCs to 14.13%, and the optimized devices retain 80% of their initial efficiency after 720 h.