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Formamidinium lead iodide (FAPI) represents the most promising perovskite for single junction solar cells, exhibiting an impressive performance when deposited in a controlled nitrogen environment. In order to foster the real-world application of this technology, the deposition of FAPI in ambient air is a highly desirable prospect, as it would reduce fabrication costs. This study demonstrates that the wettability of FAPI precursors on the hole transporting layers (HTL) used to fabricate inverted p - i - n solar cells is extremely poor in ambient air, hampering the realization of a perovskite active layer with good optoelectronic quality. To address this issue, herein, a double compatibilization method is developed, which results in the attainment of remarkable performance, exceeding 21%, representing one of the highest reported efficiencies for FAPI solar cells fabricated in humid ambient air. The incorporation of a small quantity of anionic surfactant, comprising a hydrocarbon tail and a polar headgroup, sodium dodecyl sulfate (SDS), in the perovskite solution and an ultrathin layer of alumina nanoparticles on the HTL, results in a significant improvement in the wettability of the FAPI solution. This enables the reproducible deposition of highly homogeneous perovskite films with complete coverage and excellent optical and optoelectronic quality. Furthermore, devices based on FAPI with SDS exhibit enhanced stability, retaining 98% of their initial efficiency after 40 h of continuous illumination.