The interface closely related to carrier transport plays a vital role in high-performance perovskite solar cells (PSCs). In this work, an ionic liquid (IL) 1-butyl-3-methylimidazole hexafluorophosphate (BMIMPF 6 ) has been introduced to simultaneously modify the TiO 2 and perovskite films, which improves the energy level matching and contact of the TiO 2 / CsPbI 2 Br interface as well as provides perovskite films with larger grain size, leading to more faster charge transfer and lower energy loss. Furthermore, the BMIMPF 6 modifier passivates the perovskite film defects and reduces defect-induced charge trapping and recombination in CsPbI 2 Br PSCs. Under dual-interface modification, the open-circuit voltage of the modified device increased to 1.22 V, and the power conversion efficiency (PCE) increased from 10.65 to 13.19%. Moreover, the unencapsulated modified devices exhibit an enhanced stability and maintain 91% of their initial PCE after 60 days of storage. This work has provided a strategy for efficient and stable PSCs by exploiting an IL to optimize the dual-interface at the same time.

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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...

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Hole transporting layer (HTL)‐free, all‐inorganic CsPbX3 (X: I, Br, or mixed halides), carbon‐based perovskite solar cells (C‐PSCs) show promising prospect for photovoltaic application due to their low cost, excellent stability, and theoretical high efficiency. However, the inefficient hole extraction of the carbon electrode and relatively narrow light absorption range of inorganic perovskite limit the power conversion efficiency (PCE) of this kind of PSCs. Herein, these issues are addressed through in situ constructing of an intermediate energy‐level perovskite transition layer between CsPbI2.2Br0.8 and the carbon electrode via a facile formamidinium iodide (FAI) posttreatment strategy. It is demonstrated that the (CsFA)PbI3–xBrx film is in situ formed atop inorganic perovskite due to the ions exchange between FAI and CsPbI2.2Br0.8, which can not only broaden the light absorption edge of CsPbI2.2Br0.8 from 657 to 680 nm, but also serve as a hole transfer highway between CsPbI2.2Br0.8 and the carbon electrode, mainly due to its suitable intermediate energy‐level and effective defect passivation. Consequently, the optimized HTL‐free C‐PSC achieves a champion PCE of 15.03% with an ultrahigh fill factor of 0.81. Besides, the stability of CsPbI2.2Br0.8 film (especially under humid environment) and corresponding C‐PSC are also improved.