Metal halide perovskite (MHP) solar cell technology has already demonstrated power conversion efficiencies comparable to those of well-established silicon solar cells. However, the major challenges hindering its widespread adoption and commercialization stem from its poor long-term stability. Most stability investigations are performed on fully fabricated devices, which often obscures the underlying reaction mechanisms occurring at buried interfaces. In particular, the interfaces between the perovskite absorber and the adjacent charge transport layers (CTLs) play a decisive role in governing both efficiency and stability. Consequently, a fundamental understanding of the interfacial reactions between the perovskite and CTLs is critically important.

SnO_x grown by atomic layer deposition (ALD) is widely employed in standard p-i-n perovskite solar cells (PSCs) deposited on perovskite/C₆₀ stacks. The SnO_x layer not only facilitates efficient electron transport but also protects the underlying perovskite/C₆₀ from sputtering damage during the subsequent deposition of transparent conductive oxides. Nevertheless, the precursors used in ALD processes are often highly reactive toward the underlying surface and can induce the formation of new interfacial species. In our previous work, we demonstrated that SnO_x deposited directly by ALD onto perovskite surfaces leads to the formation of unfavorable interfaces.[1],[2] Specifically, ALD‑grown SnO_x on FAPbX₃ (X = I, Br) results in the degradation of FA⁺ cation. Moreover, iodide‑based perovskites suffer from PbI₂ formation, while bromide‑based perovskites exhibit Sn-Br formation.

Motivated by these findings, the present work further investigates whether an in-between C₆₀ layer fully suppresses the interaction between the perovskite surface and SnO_x precursors by performing in situ SnO_x deposition on different perovskite/C₆₀ architectures. The growth dynamics and interface formation were monitored through in situ SnO_x deposition combined with parallel X‑ray photoelectron spectroscopy measurements. This approach enabled time‑resolved, cycle‑by‑cycle tracking of interfacial interactions and SnO_x evolution on perovskite/C₆₀ structures. The results provide detailed insights into perovskite‑composition‑dependent ion‑migration dynamics during SnO_x ALD, which critically influence the growth behavior and stoichiometry of SnO_x during the initial deposition cycles.