Despite the tremendous progress made in the field of perovskite solar cells, their commercialization remains hindered by several challenges, including scalability, stability, and sustainability. Thermal evaporation is a solvent-free, scalable, and industrially relevant method, yet despite its many advantages, this method is limited by the lack of additive engineering strategies for controlling the growth of perovskite layers. Here, a novel additive engineering strategy is reported that enables the complete conversion of precursors to a perovskite phase during the two-step deposition of formamidinium lead triiodide (FAPbI₃). The approach is based on the co-evaporation of potassium-containing additives (KI and KSCN) alongside PbI₂ during the first deposition step, followed by the evaporation of formamidinium iodide. It is demonstrated that the absence of additives leads to an incomplete conversion with a substantial amount of unconverted PbI₂ remaining at the buried interface. On the other hand, the co-evaporation of the additives improves the conversion process, leading, in the case of KSCN, to phase-pure α-FAPbI₃ with improved microstructure. The additive-engineered p-i-n devices achieve efficiencies up to 18.34%, among the highest reported for evaporated FAPbI₃ solar cells without interfacial passivation. This work highlights the great potential of additive engineering for controlling the film formation of thermally evaporated perovskites.