KOH‑Modified SnO ₂ Buried Interfacial Crystallization Engineering for Efficient Flexible Perovskite Solar Cells

Precise control of low‐temperature crystallization of SnO 2 is crucial for high‐performance flexible perovskite solar cells (F‐PSCs). Nevertheless, conventional sol‐gel‐derived SnO 2 nanocrystals (NCs) are plagued by low crystallinity and high defect density due to inherent synthesis limitations, which limit charge transport and interfacial stability. Here, we report a pre‐treatment strategy using aqueous KOH as a hydrolysis regulator to direct the crystallization of SnO 2 NCs at 80°C. The in‐situ generated KCl by‐product simultaneously passivates the buried electron transport layer (ETL)/perovskite interface. This dual‐role strategy yields high‐quality K‐SnO 2 NCs with markedly improved crystallinity and particle morphology, leading to a lower charge transport barrier (73.5 mV vs. 126.2 mV) and superior interfacial adhesion. Thereby, we achieve champion power conversion efficiency (PCE) of 26.13% (rigid) and 25.37% (flexible), along with significantly improved device stability (Active area: 0.048 cm 2 ). This study establishes a robust pretreatment protocol for low‐temperature fabrication of highly quality and stable ETLs for F‐PSCs.