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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]

• a-CsPbI 3 light harvester layers are prepared at low temperature without hydrogen iodide additive. • Lead acetate is used as the lead source of precursor solutions. • Ethylenediaminium iodide can cooperates with the lead acetate precursor. • The grain size of CsPbI 3 layers is increased and their trap density is reduced. • The power conversion efficiency and photo-stability of devices are improved. CsPbI 3 triggers strong interest in the photovoltaic community for their promising photoelectric property and nonvolatility. However, CsPbI 3 layers with a-phase crystal structure are difficult for preparation, and these layers easily degrade to non-photoactive δ-phase at room temperature. Tuning the composition of precursor solutions is an important method for improving the properties of CsPbI 3 layers. Herein, we prepare inorganic a-CsPbI 3 perovskite solar cells using lead acetate as the lead source, which is different from conventional precursors with lead iodide as the lead source. The lead acetate precursor needs neither incorporating hydrogen iodide nor preparing PbI 2 complex, and it is shown to enlarge the grain size, improve the crystallinity and reduce the trap density of CsPbI 3 layers. By cooperating the lead acetate precursor with EDAI 2 additive, the power conversion efficiency (PCE) of devices increases from 9.2% to 12.9% and the photo-stability of these devices is improved simultaneously. [ABSTRACT FROM AUTHOR]