Inverted p–i–n perovskite solar cells (IPSCs) offer promise for next-generation photovoltaics. However, IPSCs utilizing solution-processed PC61BM as the electron transport layer (ETL) remain less interface-optimized than conventional n–i–p configurations, restricting their efficiency, stability, and scalability. In this work, we introduce an ultrathin atomic-layer-deposited SnOx (ALD-SnOx) film, fabricated at a low temperature (80 °C), as a versatile interfacial modifier to address these shortcomings. This scalable, vapor-phase approach directly addresses the core instability in p–i–n architectures, effectively remedies morphological defects such as pinholes and phase segregation in PC61BM, significantly enhancing interfacial contact and suppressing charge recombination. Consequently, the champion IPSC incorporating a 10 nm ALD-SnOx interlayer yields a power conversion efficiency (PCE) of ∼19.2%, representing a remarkable 58% improvement over control devices (PCE ∼11.3%). The ALD-SnOx interlayer effectively enhances moisture resistance, giving the IPSCs excellent environmental stability. Additionally, the redesigned IPSCs show scalability by effectively generating a large-area (∼12.1 cm2) mini-module with a high PCE (∼14.1%). These findings demonstrate the immense potential of this interfacial engineering approach for the commercial production of scalable, stable, and effective IPSCs.