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

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.