Formamidinium lead iodide (FAPbI₃) is the most promising perovskite material for producing efficient perovskite solar cells (PSCs). Here, we develop a facile method to obtain an α-phase FAPbI₃ layer with passivated grain boundaries and weakened non-radiative recombination. For this aim, during the FAPbI₃ fabrication process, cetrimonium bromide + 5% potassium thiocyanate (CTABr + 5% KSCN) vapor post-treatment is introduced to remove non-perovskite phases in the FAPbI₃ layer. Incorporation of CTA⁺ along with SCN⁻ ions induces FAPbI₃ crystallization and stitch grain boundaries, resulting in PSCs with lower defect losses. The vapor-assisted deposition increases the carriers' lifetime in the FAPbI₃ and facilitates charge transport at the interfacial perovskite/hole transport layer via a band alignment phenomenon. The treated α-FAPbI₃ layers bring an excellent PCE of 22.34%, higher than the 19.48% PCE recorded for control PSCs. Besides, the well-oriented FAPbI₃ and its higher hydrophobic behavior originating from CTABr materials lead to improved stability in the treated PSCs.