Carbon-based hole transport material (HTM)-free perovskite solar cells have exhibited a promising commercialization prospect, attributed to their outstanding stability and low manufacturing cost. However, the serious charge recombination at the interface of the carbon counter electrode and titanium dioxide (TiO 2 ) suppresses the improvement in the carbon-based perovskite solar cells' performance. Here, we propose a modified sequential deposition process in air, which introduces a mixed solvent to improve the morphology of lead iodide (PbI 2 ) film. Combined with ethanol treatment, the preferred crystallization orientation of the PbI 2 film is generated. This new deposition strategy can prepare a thick and compact methylammonium lead halide (MAPbI 3 ) film under high-humidity conditions, which acts as a natural active layer that separates the carbon counter electrode and TiO 2 . Meanwhile, the modified sequential deposition method provides a simple way to facilitate the conversion of the ultrathick PbI 2 capping layer to MAPbI 3 , as the light absorption layer. By adjusting the thickness of the MAPbI 3 capping layer, we achieved a power conversation efficiency (PCE) of 12.5% for the carbon-based perovskite solar cells.