Chemical bath deposited rutile TiO₂ compact layer toward efficient planar heterojunction perovskite solar cells

Highlights: • Rutile TiO₂ thin film can be grown on FTO substrate below 100 °C. • 200 mM TiCl₄ precursor solution results in the best PSC performance. • UV/O₃ treatment can reduce the carrier recombination effectively. • Over 12% power conversion efficiency can be achieved for PSCs. - Abstract: TiO₂ is a best choice of electron transport layers in perovskite solar cells, due to its high electron mobility and stability. However, traditional TiO₂ processing method requires rather high annealing temperature (>500 °C), preventing it from application to flexible devices. Here, we show that TiO₂ thin films can be synthesized via chemical bath deposition below 100 °C. Typically, a compact layer of rutile TiO₂ is deposited onto fluorine-doped tin oxide (FTO) coated substrates, in an aqueous TiCl₄ solution at 70 °C. Through the optimization of precursor concentration and ultraviolet-ozone surface modification, over 12% power conversion efficiency can be achieved for CH₃NH₃PbI₃ based perovskite solar cells. These findings offer a potential low-temperature technical solution in using TiO₂ thin film as an effective transport layer for flexible perovskite solar cells.