Defect-Passivated Bilayer SnO₂ Electron Transport Layers for Efficient Perovskite Solar Cells

Sputtered SnO2 is an attractive electron transport layer (ETL) for scalable perovskite solar cells (PSCs) due to its excellent uniformity, pinhole-free coverage, and compatibility with vacuum processing. However, its performance remains limited by high interfacial recombination arising from oxygen vacancies and undercoordinated Sn2+ states. In this work, we address this limitation by introducing a defect-passivating SnO2 nanoparticle (NP) overlayer onto optimized sputtered SnO2 films, forming a synergistic bilayer ETL. Structural and spectroscopic analyses reveal that the compact sputtered underlayer ensures conformal coverage on rough FTO substrates, while the NP overlayer selectively passivates interfacial defects and improves band alignment with FA0.88Cs0.12PbI3 perovskite absorbers. Time-resolved photoluminescence and surface recombination velocity analysis demonstrate a pronounced suppression of interfacial non-radiative recombination, with SRVs reduced from >100 cm s-1 to ~25 cm s-1. These interfacial improvements translate into enhanced device performance, yielding PSCs with power conversion efficiencies of up to 24.4% when combined with molecular surface passivation. The bilayer architecture also delivers improved operational stability, establishing a general and scalable strategy for high-efficiency vacuum-processed ETLs compatible with rough device interfaces.