Perovskite solar cells (PSCs) have delivered power conversion efficiencies (PCEs) up to 25.5%, rivaling the single crystalline silicon solar cells. SnO₂-based electron transport layers (ETLs) are mostly used in PSC devices due to their outstanding band alignment, excellent chemical and UV stability, high transmittance, high conductivity, and processability at low temperatures. 

However, the scintillating SnO₂ colloid precursor suffered from adverse formation of large agglomerates over time due to interparticle van der Waals interactions, deteriorating both the morphology and electronic quality of the resulting ETL. These structural defects, such as dangling hydroxyl groups and oxygen vacancies near the valence band, can hinder charge extraction and transport of electrons to couple with non-radiative recombination loss. 

In a study published in Nano Energy, the research group led by Prof. GAO Peng from Fujian Institute of Research on the Structure of Matter of the Chinese Academy of Sciences developed a novel WO₃@SnO₂ nanocomposite ETL prepared by in situ peptizations of WO₃ in the alkaline SnO₂ colloid solution in which WO₃ played multiple roles from precursor solution to the film state. 

The researchers first hydrated the WO₃ nanocrystals to form WO₄^2- via the reaction of WO₃+H₂O→WO₃·H₂O↔H₂WO₄ to successively heal the dangling hydroxyl and oxygen vacancy defect over the SnO₂ surface, a precondition to high-quality ETL and excellent charge extraction and transfer. They then returned the WO₄^2- anions into the WO₃ phase after heat treatment and shaped nano-heterostructure with the matrix SnO₂ particles, accelerating interfacial charge transfer. 

The researchers achieved molecular level passivation of the SnO₂ layer by WO₃, and WO₃@SnO₂ ETL delivered an encouraging efficiency of 23.6 %, and an ultrahigh FF of 85.8%, along with good long-term storage and humidity stability. 

Notably, through the loss mechanism analysis by the revised detailed balance model, the researchers ascribed such ultrahigh fill factor to reduced non-radiative recombination losses and distinctly suppressed charge transport losses. 

This study highlights a new perspective of metal oxide incorporation in colloidal solution to form a composite/dual charge selective layer, shedding light on the direction to approach the theoretical fill factor limit of about 91% and boost the PCE to 30%.