Humidity-Controlled SnO₂ Aggregation for Reliable Perovskite Solar Cell Fabrication

The SnO₂ electron transport layer (ETL), fabricated by spin-coating from commercially available aqueous colloidal dispersion, is widely used in perovskite solar cells (PSCs). In this study, we demonstrate that the relative humidity (RH) during SnO₂ spin-coating significantly affects PSC performance. Spin-coating at higher RH levels leads to the formation of rougher SnO₂ ETL surfaces with numerous pinholes, which results in reduced PSC performance due to partial contact between the perovskite light absorber and the indium tin oxide (ITO) electrode layer. In contrast, smoother, pinhole-free SnO₂ ETL surfaces are achieved by spin-coating at lower RH levels, reducing ITO/perovskite contact and thereby enhancing PSC performance. A double-layer SnO₂ architecture is proposed to further improve PSC performance. In this architecture, the first SnO₂ layer, fabricated at 0% RH with a smooth surface, minimizes direct ITO/perovskite contact, while the second SnO₂ layer, fabricated at 80% RH with a rougher surface, enhances electron extraction by increasing the SnO₂/perovskite interface area. These findings underscore the importance of controlling RH during SnO₂ spin-coating to achieve PSCs with better reproducibility.