Despite showing great potential in lead-free green energy, tin-based perovskite materials still face challenges such as inherent material instability and energy level misalignment with the hole transport layer (HTL), which limits the advancement of tin-based perovskite solar cells (Sn-PSCs). In this work, a natural antioxidant organic small molecule, thiolactic acid (TA), is used to modify the interface between PEDOT:PSS and the tin-based perovskite film. The TA molecule can cross-link to form a network polymer and regulate the microstructure and photoelectrical characteristics of PEDOT:PSS. Meanwhile, TA contains C═O and C─S groups, which can interact with Sn