Realizing efficient FAPbI3-based devices with high open-circuit voltage (VOC) is still challenging, due to severe energy loss between the n-type perovskite and p-type hole-transporting layer (HTL). Here, we developed a strategy involving controlling the formation of iodine vacancies in order to induce formation of p-type perovskite and hence mitigate such energy loss. Post-deposition of n-butylamine iodide was discovered to induce an n-to-p-type transition in the FAPbI3 perovskite and hence form the p-type perovskite/p-type HTL junction. The resultant device realized a VOC of as high as 1.12 V, a value ∼14.3% higher than that of the corresponding n-type FAPbI3 device (0.98 V).