TiO2 Colloid‐Spray Coated Electron‐Transporting Layers for Efficient Perovskite Solar Cells

TiO2 is one of the most efficient and widely used materials for electron‐transporting layer (ETLs) in perovskite solar cells (PSCs). The formation of efficient TiO2 layers is generally carried out at high temperature by baking at a temperature >400 °C or by vacuum deposition (e.g., atomic layer deposition and E‐beam). In this study, the preparation of a TiO2 ETL for PSCs is reported with excellent properties at low temperatures based on the synthesis of a stable TiO2 colloidal aqueous solution and spray coating. The prepared TiO2 colloids are able to produce a dense and uniform ETL even if it is simply dried at 100 °C after spray coating. It is believed that this is owing to the peroxo functional group remaining on the surface of the TiO2 colloids. The TiO2 ETLs, combined with the TiO2 underlayer formed by chemical bath deposition, and the sprayed TiO2 colloids allowed the fabrication of PSCs with performance similar to those of PSCs produced by annealing at 450 °C with a TiO2 paste. The PSCs fabricated entirely at 100 °C demonstrated power conversion efficiency of 22.7% in small cells, and 19.0% in mini‐modules. This study reports the deposition of a TiO2 electron transporting layer for perovskite solar cells by spray coating using a stable TiO2 colloidal aqueous solution, which is synthesized via the self‐condensation of a titanium peroxide complex under hydrothermal conditions. Although the whole fabrication process for the cells is performed at 100 °C, 22.7% efficiency is achieved.