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SnO2-based electron transport layers (ETLs) offer outstanding band alignment, excellent chemical and UV stability, high transmittance, high conductivity, and processability at low temperatures. However, unfortunately, the state-of-the-art SnO2colloid precursor suffered from agglomeration over time and structural defects such as dangling hydroxyl groups and oxygen vacancies, which deteriorate both the morphology and electronic quality of the resulting ETL. Especially, these trap states near the valence band can hinder charge extraction and transport of electrons to couple with non-radiative recombination loss. Here, we introduce a novel WO3@SnO2nanocomposite ETL, which is synthesized by in situ peptizations of WO3in commercial alkaline SnO2colloid nanocrystals. The hydrated (peptized) WO3forms H2WO4(WO42-) to effectively stabilize the SnO2nanocrystals in the dispersion and bind to the defect sites. Intriguingly, the H2WO4converts back to the WO3phase to form nano-heterostructured composite with SnO2particles during the process of film fabrication, further promoting passivation and charge extraction. Through the novel method, we could achieve molecular level passivation of SnO2layer by WO3, and a power conversion efficiency of 23.6% for a 0.1 cm2PSC device with ultra-high FF of 85.8% was demonstrated. Furthermore, a modified detailed balance model was used to verify the drastically lessened surface & bulk defect-induced recombination loss in WO3@SnO2based devices. Finally, the corresponding unencapsulated cell retained ~91% of its initial efficiency after 2000 h of damp exposure. This work provides a promising method to access the Shockley–Queisser limit of fill factor for single-junction PSC.