as methylammonium and formamidinium), resulting in the relatively poor long-term stability of PSCs. For another, Au electrodes and organic hole transport layers such as 2,2′,7,7′-Tetrakis(N,N-di-pmethoxyphenylamine)-9,9′-spirobifluorene (spiro-OMeTAD) commonly used in organic-inorganic hybrid PSCs are expensive, which weakens the cost advantage of devices. Aiming at these problems, hole transport layer-free, all-inorganic carbonbased PSCs (C-PSCs) with a device structure of fluorine-doped tin oxide (FTO) or tin-doped indium oxide (ITO)/electron transport layer (ETL)/inorganic perovskite/carbon, would be a potential alternative to achieve low cost, high-efficiency, and high stability, simultaneously. [3][4][5][6][7][8][9][10] Among many inorganic perovskites, CsPbI 3 perovskite has the greatest application potential due to its ideal band gap (≈1.70 eV). [11][12][13][14][15][16][17] To pursue highly efficient and stable CsPbI 3 C-PSCs, developing stable and high-quality CsPbI 3 perovskite film with fewer defects is a prerequisite. Xiang et al. discovered air-stable CsPbI 3 film can be obtained by one-step solution deposition method using HPbI 3 instead of PbI 2 , and a PCE of 9.5% was achieved for CsPbI 3 C-PSCs by employing such perovskite film. [18] Then, by doping Natrium in the CsPbI 3 lattice, they further boosted the PCE of CsPbI 3 C-PSCs to 10.7%. [19] Liang et al. boosted the PCE of CsPbI 3 C-PSCs to 12.04% by developing CsPbI 3 :Br:InI 3 , which possesses the same bandgap as CsPbI 3 but has significantly reduced defect density. [20] Wang et al. systematically studied the crystallization of CsPbI 3 films after dimethylammonium iodide (DMAI) treatment, which boosts the efficiency of CsPbI 3 C-PSCs to 14.6%. [21] Recently, Wang et al. adopted the CsPbI 3 film post-processing strategy to form the energy level gradient and generate a 2D Cs 2 PbI 2 Cl 2 nanosheets structure as the electronic blocking layer, which boosts the efficiency of CsPbI 3 C-PSCs to 15.23%. [22] Even though many efforts have been made to achieve high-quality CsPbI 3 films as well as efficient CsPbI 3 C-PSCs, the relatively poor crystallinity is accompanied by many defects of CsPbI 3 film is still the main obstacle for further boosting the PCE of CsPbI 3 C-PSCs.Hole transport layer-free, carbon-based, all-inorganic CsPbI 3 perovskite solar cells (PSCs) have exhibited great potential in photovoltaic applications owing to their low cost and excellent thermal stability. However, the low power conversion efficiency (PCE) hampers its development, mainly due to the existence of defects inside the CsPbI 3 film or at TiO 2 electron transport layer/CsPbI 3 interface. Herein, these issues were addressed through a facile TiO 2 post-treatment strategy using 1-butyl-3-methylimidazole hexafluorophosphate (BMIMPF 6 ) ionic liquid. First, BMIMPF 6 can passivate TiO 2 /CsPbI 3 interface defects by forming strong bond between the electron-rich N atoms and uncoordinated ions. Second, BMIMPF 6 -modified TiO 2 shows reduced hydrophilicity, inducing decreased he...