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Abstract 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 TiO 2 that requires an additional TiCl 4 treatment. In this paper, a comprehensive understanding of TiCl 4 treatment on the TiO 2 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 TiCl 4 treated TiO 2 , and steady-state photo luminescence spectroscopy and electrical impedance spectroscopy were conducted to study charge carrier dynamics. The TiCl 4 treated TiO 2 layer can result in high efficiency over 20% in planar perovskite solar cells. Our results demonstrate that the TiCl 4 treatment on the compact TiO 2 prior to the perovskite deposition was necessary for achieving high-performance solar cells. Graphical abstract Image 1 Highlights • TiCl 4 treatment on the TiO 2 layer was investigated comprehensively. • TiCl 4 treatment fills the cracks and pinholes of the TiO 2 layer. • TiCl 4 treatment depresses the surface work function of the TiO 2 layer. • Perovskite solar cells with the TiCl 4 treated TiO 2 result in high efficiency over 20%. [ABSTRACT FROM AUTHOR]