Abstract: Inorganic perovskites are considered promising absorbers for the top sub-cell of two-terminal perovskite/silicon tandem solar cells due to their superior optical and thermal stability. However, severe non-radiative recombination within the absorber layer continues to limit further efficiency improvements and practical deployment. In this study, a thin PMMA layer is introduced onto the tin dioxide (SnO₂) electron-transport layer to regulate the buried interface. During annealing, the viscoelastic PMMA layer suppresses abnormal grain coarsening, optimizes grain-size distribution, and promotes the formation of smoother, more compact perovskite films. Furthermore, the carbonyl groups (C=O) in PMMA interact with surface hydroxyl groups (—OH) on SnO₂ via hydrogen bonding and polar interactions, thereby passivating interfacial defects and reducing non-radiative recombination losses. Consequently, the PMMA-treated IPSCs achieve a power conversion efficiency (PCE) of 20.56%, accompanied by enhanced device stability.