Comprehensive understanding of TiCl4 treatment on the compact TiO2 layer in planar perovskite solar cells with efficiencies over 20%

A uniform and compact hole blocking layer is necessary for high-performance perovskite solar cells, since it not only serves as an electron collector but also suppresses the carrier recombination. So far, high efficient perovskite solar cells have been obtained by using a blocking layer TiO2 that requires an additional TiCl4 treatment. In this paper, a comprehensive understanding of TiCl4 treatment on the TiO2 layer was investigated in planar perovskite solar cells. Scanning electron microscope, atomic force microscope, and Kelvin probe force microscopy were applied to investigate the morphology of the TiCl4 treated TiO2, and steady-state photo luminescence spectroscopy and electrical impedance spectroscopy were conducted to study charge carrier dynamics. The TiCl4 treated TiO2 layer can result in high efficiency over 20% in planar perovskite solar cells. Our results demonstrate that the TiCl4 treatment on the compact TiO2 prior to the perovskite deposition was necessary for achieving high-performance solar cells.