Annealing-Free SnO₂ Layers for Improved Fill Factor of Perovskite Solar Cells

Perovskite solar cells (PSCs) have developed rapidly with simplified planar structures, in which the electron transport layer (ETL) is one of the key components for high efficiency. As one of the most widely used ETLs for PSCs, a tin dioxide (SnO₂) ETL is usually obtained by thermal annealing at around 150 °C, which complicates the fabrication process and confines the application of PSCs onto thermally sensitive flexible substrates. Here, we adopted an annealing-free process for the first time, the negative pressure evaporation (NPE) method, to quickly prepare SnO₂ ETLs (NPE-SnO₂) within 1 minute at room temperature from widely used commercial aqueous SnO₂ colloid. The NPE process developed here significantly improves the surface morphology and conductivity of SnO₂ layers compared to the traditional thermally annealed ones (A-SnO₂). Detailed characterizations reveal that increased oxygen vacancies and reduced hydroxyl defects contribute to higher conductivity of NPE-SnO₂ and less interfacial recombination of PSCs. Therefore, a PSC with NPE-SnO₂ delivers an improved fill factor (FF) of 82.33% and a higher power conversion efficiency (PCE) of 23.07%, which is the highest value based on annealing-free SnO₂. To conclude, the NPE process is a universal technique to obtain high-quality semiconductor films from their wet state within 1 min and opens up the possibility of fabricating functional layers of PSCs without thermal annealing.