Low-temperature processed ultrathin TiO₂ for efficient planar heterojunction perovskite solar cells

Highlights: • An ultrathin and discrete TiO₂ (u-TiO₂) was fabricated at low temperature. • High-performance perovskite solar cells based u-TiO₂was realized. • u-TiO₂ between perovskite and FTO functions as a bridge for electron transport. • u-TiO₂ accelerates electron transfer and alleviates charge recombination. - Abstract: A compact TiO₂ (c-TiO₂) layer fabricated by spin coating or spray pyrolysis following a high-temperature sintering is a routine in high-performance planar heterojunction perovskite solar cells. Here, we demonstrate an effective low-temperature approach to fabricate an ultrathin and discrete TiO₂ (u-TiO₂) for enhancing photovoltaic performance of perovskite solar cells. Via hydrolysis of low-concentration TiCl₄ solution at 70 °C, u-TiO₂ was grown on a fluorine doped tin oxide (FTO) substrate, forming the electron selective contact with the photoactive CH₃NH₃PbI₃ film. The perovskite solar cell using u-TiO₂ achieves an efficiency of 13.42%, which is compared to 13.56% of the device using c-TiO₂ prepared by high-temperature sintering. Cyclic voltammetry, steady-state photoluminescence spectroscopy and electrical impedance spectroscopy were conducted to study interface engineering and charge carrier dynamics. Our results suggest that u-TiO₂ functions as a bridge for electron transport between perovskite and FTO, which accelerates electron transfer and alleviates charge recombination.