Nanostructured TiO₂ Grown by Low-Temperature Reactive Sputtering for Planar Perovskite Solar Cells

Low-temperature nanostructured electron-transporting layers (ETLs) for perovskite solar cells are grown by reactive sputtering at 160 °C with thickness in the range 22-76 nm and further stabilization in air at 180 °C to improve the lattice structure and to consequently reduce charge recombination during solar cell operation. In addition, the post-deposition treatment aims at leveling differences among samples to ensure material reproducibility. Nanostructured TiO₂ has a further added value in promoting the structural coupling with the perovskite layer and establishing conformal interfaces in favor of the charge extraction from the active material. Nanostructuring of the ETLs also allows the shaping of the band gap width and position with a beneficial impact on the electrical parameters, as tested in standard architecture containing methylammonium lead iodide perovskites. A balance among parameters is achieved using a 40-nm-thick TiO₂ ETL with a maximum efficiency of ∼15% reached without surface treatments or additional layers. The proposed growth methodology would be compatible with the use of flexible substrates after appropriated ETL structural adaptation. It can be likewise applied in perovskite/silicon-heterojunction tandem solar cells to fulfill the industrial demand for clean, solvent-free, reproducible, reliable, and high-throughput processes.