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 SnO2 colloid 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 @SnO2 nanocomposite ETL, which is synthesized by in situ peptizations of WO3 in commercial alkaline SnO2 colloid nanocrystals. The hydrated (peptized) WO3 forms H2WO4 (WO42-) to effectively stabilize the SnO2 nanocrystals in the dispersion and bind to the defect sites. Intriguingly, the H2WO4 converts back to the WO3 phase to form nanoheterostructured composite with SnO2 particles during the process of film fabrication, further promoting passivation and charge extraction. Through the novel method, we could achieve molecular level passivation of SnO2 layer by WO3, and a power conversion efficiency of 23.6% for a 0.1 cm2 PSC 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 @SnO2 based 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.