SnO₂–TiO₂ Hybrid Electron Transport Layer for Efficient and Flexible Perovskite Solar Cells

Inorganic electron transport layers (ETLs), such as titanium dioxide (TiO2) and tin dioxide (SnO2), are important in n–i–p structured perovskite solar cells (PSCs). In particular, the ETLs for flexible PSCs (f-PSCs) using a polymer substrate require strong adhesion with a transparent conducting oxide (TCO) and formation of a uniform thin film at a temperature below 150 °C. Hence, SnO2 colloidal nanoparticles are primarily used to meet these demands. Nevertheless, there exist further opportunities for improvement in terms of efficiency, uniform coating, and adhesion on TCO. In this study, we prepared a SnO2–TiO2 hybrid electrode by adding a certain amount of TiO2 nanosol, which functions as an inorganic binder, to a SnO2 colloidal solution. In comparison with the SnO2 colloid alone, f-PSC fabricated with a SnO2–TiO2 hybrid electrode demonstrated not only better mechanical reliability against bending due to strong adhesion to the substrate but also greatly improved efficiency because of improved energy alignment. Eventually, the SnO2–TiO2 hybrid electrode resulted in an efficiency of 21.02% and even an efficiency of over 16% in a mini-module (7 × 7 cm2) due to the uniform coating over a large area. This study provides a new strategy for the ETL of high-efficiency f-PSCs.