<p>Recently, compared to hybrid perovskites, all-inorganic perovskite solar cells have received enormous attention due to their excellent capability to resist heat, moisture, and ultraviolet light. The all-inorganic cesium lead halide perovskite (CsPbX₃, X = Br, I, or their mixture) is considered an alternative light-harvesting material for solar cells. Moreover, CsPbI_3-xBrₓ derivatives have therefore emerged as ideal candidates owing to their tunable bandgap and high absorption coefficient. Nevertheless, interfacial non-radiative recombination and the thermodynamically favored δ-phase transition still impose efficiency and stability bottlenecks. In this work, trimethylammonium chloride (TACl) is employed as a multifunctional interfacial modifier. By integrating this interlayer modification at the TiO₂/CsPbI_2.2Br_0.8 interface, we simultaneously minimized interfacial defects, graded the energy levels, and controlled the crystallographic orientation of the perovskite lattice, thereby suppressing the charge recombination and stabilizing carrier dynamics through multifunctional synergy beyond single-function strategies. In addition, we successfully prepared hole-free carbon-based all-inorganic perovskite solar cells of FTO /TiO₂/ CsPbI_2.2Br_0.8 /C by using low-cost commercial carbon slurry instead of the traditional gold/silver noble metal electrodes and the organic hole-transport layer Spiro-OMeTAD. After surface treatment with trimethylammonium chloride, the optimized device exhibited a peak power conversion efficiency of 12.2%, representing a 14.8% enhancement relative to the pristine device (10.6%).</p>