This work presents an additive engineering approach to prepare structurally stable highly crystalline triple-cation and mixed-anion perovskite thin film by using 5 mol% doping of perovskite solution with phenethylammonium iodide (PEAI). Such an additive provides increase in grain size up to 20 % and preserves surface morphology of non-encapsulated films for at least 6 months at ambient conditions. Stability experiments showed excellent results for films with PEAI additive, which confirmed by X-ray diffraction measurements. The additive strategies have great potential to improve the power conversion efficiency and the long-term stability of the perovskite solar cell suitable for commercialization.