Mitigating tin ion migration and reinforcing buried interface via synergistic polymer-modified tin oxide

The buried interface between charge transport layer and the overlying perovskite absorber is critical yet difficult to probe in perovskite solar cells. Because light enters through the transparent substrate and is predominantly absorbed near this region, photo-induced degradation processes are most likely initiated at hidden buried interface. Here, we reveal that instability at the buried SnO2/perovskite interface triggers structural and chemical decomposition of the perovskite layer. To address this issue, we introduce polymeric interlayers that simultaneously reinforce interfacial bonding and suppress tin-ion migration. Poly(1-ethenylpyrrolidine-2,5-dione) (PED) forms the most robust and chemically compatible interface, yielding enhanced phase purity, reduced defect density, and significantly mitigated tin diffusion. These findings identify the buried interface as the primary origin of perovskite degradation and highlight interfacial polymer design as an effective strategy for achieving intrinsically stable perovskite photovoltaics.