In Situ Constructing Intermediate Energy‐Level Perovskite Transition Layer for 15.03% Efficiency HTL‐Free Carbon‐Based Perovskite Solar Cells with a High Fill Factor of 0.81

Hole transporting layer (HTL)‐free, all‐inorganic CsPbX 3 (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 CsPbI 2.2 Br 0.8 and the carbon electrode via a facile formamidinium iodide (FAI) posttreatment strategy. It is demonstrated that the (CsFA)PbI 3– x Br x film is in situ formed atop inorganic perovskite due to the ions exchange between FAI and CsPbI 2.2 Br 0.8 , which can not only broaden the light absorption edge of CsPbI 2.2 Br 0.8 from 657 to 680 nm, but also serve as a hole transfer highway between CsPbI 2.2 Br 0.8 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 CsPbI 2.2 Br 0.8 film (especially under humid environment) and corresponding C‐PSC are also improved.